Neuropharmacologic Studies on the Brain Serotonin_1A Receptor Using the Selective Agonist Osemozotan

Abstract

Alterations in serotonin (5-HT) neurochemistry have been implicated in the etiology of major neuropsychiatric disorders such as anxiety-spectrum disorders, depression, and schizophrenia. The neuromodulatory effects of 5-HT are mediated through 14 receptor subtypes, and those receptors, including the 5-HT_1A receptor, are considered to be potential targets for the treatment of psychiatric disorders. We developed the novel 5-HT_1A receptor agonist MKC-242 (called osemozotan) and characterized its neurochemical and pharmacological profiles. 5-HT_1A receptor agonists modulate the release of amine neurotransmitters through the activation of presynaptic or postsynaptic 5-HT_1A receptors in the brain. The agonist has antianxiety and antidepressant effects and improves abnormal behaviors such as aggressive behavior and deficits of prepulse inhibition in isolation-reared mice. We also demonstrated that spinal 5-HT_1A receptor activation is involved in isolation rearing-induced hypoalgesia. Concerning the mechanism for induction of isolation-induced abnormal behaviors, we have recently found that the raphe-prefrontal 5-HT system plays a key role in encounter stimulation-induced hyperactivity in isolation-reared mice. Furthermore, we showed that osemozotan attenuates psychostimulant-induced behavioral sensitization and that prefrontal dopamine release is enhanced by functional interaction between the 5-HT_1A receptor and other receptors. This review summarizes the neuropharmacology of the 5-HT_1A receptor, focusing on our studies using osemozotan, and suggests that the 5-HT_1A receptor may be a target molecule for the treatment of psychiatric disorders, pain, and drug dependence.

1. INTRODUCTION

Among the 14 known serotonin (5-HT) receptor subtypes, the 5-HT_1A receptor has received a great deal of attention mainly because it is implicated in anxiety and depression.^1–4) 5-HT_1A receptors are located both pre- and postsynaptically. Presynaptic 5-HT_1A receptors (as somadendritic 5-HT_1A autoreceptors) are present on serotonergic neurons in the dorsal and medial raphe nuclei, and postsynaptic 5-HT_1A receptors are found at high density in the limbic regions and in the frontal and entorhinal cortices.^5,6) They couple negatively via G-proteins to adenylate cyclase in the hippocampal tissue and cell lines stably expressing the cloned 5-HT_1A receptor,^7,8) and their activation causes neuronal hyperpolarization, an effect mediated through the G-protein-coupled opening of the K⁺ channels.^9,10) The effects of 5-HT_1A receptor agonists on animal behaviors have been extensively studied, since the agonists show antidepressant-like and anxiolytic-like effects.^1–4) We developed the novel 5-HT_1A receptor agonist (S)-5-[3-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole HCl (MKC-242, called osemozotan)^11,12) (Fig. 1).

Fig. 1. Chemical Structure of Osemozotan (5-[3-{(2S)-(1,4-benzodioxan-2-ylmethyl)amino}propoxy]-1,3-benzodioxol Hydrochloride)

Rearing rats or mice in social isolation produces behavioral changes such as hyperactivity,^13–17) anxiety-like behavior,^18–21) depression,^22,23) aggression,²⁴⁾ deficits of prepulse inhibition,^25–27) and reduced pain sensitivity to noxious stimuli.^28–30) These findings suggest that isolation-reared animals reflect certain aspects of human psychopathologies such as anxiety, depression, and schizophrenia.^31,32) Therefore, this model is useful to study the pathological role of the 5-HT_1A receptor. Using this psychiatric disorder model, we studied the neuropharmacological effects of osemozotan.

Selective 5-HT reuptake inhibitors (SSRIs) are widely used as first-line treatment of depression, but about 30–50% of patients do not initially respond to SSRIs.^33,34) Clinical reports show that combination therapy with an SSRI and antipsychotic drug is effective in patients with treatment-resistant depression,^35–38) but the neurochemical basis for the effectiveness of combination therapy is not known. It is likely that the antidepressant-like effect of SSRIs is mediated by the activation of the 5-HT_1A receptor, since SSRIs increase extracellular levels of 5-HT, which interacts with the 5-HT_1A receptor. On the other hand, fluvoxamine, an SSRI, has an affinity for the σ₁ receptor,³⁹⁾ which may be involved in cognitive function.⁴⁰⁾ First, we studied the effects of combined fluvoxamine and the dopamine (DA)-D₂ antagonist sulpiride on brain monoamine release and second, the involvement of the σ₁ receptor in the effects of fluvoxamine. Subsequent studies using osemozotan showed that prefrontal DA release is enhanced by functional interaction of 5-HT_1A and DA-D₂ or σ₁ receptors. This review summarizes the neuropharmacology of the 5-HT_1A receptor based on studies using the receptor agonist osemozotan.

2. NEUROCHEMICAL AND PHARMACOLOGICAL EFFECTS OF OSEMOZOTAN

Progress in the pharmacology of 5-HT_1A receptors was driven by the early identification of a selective 5-HT_1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the discovery that the 5-HT_1A receptor agonist azapirone compounds were anxiolytic and antidepressant in a clinical setting.^41,42) A number of compounds were developed as 5-HT_1A receptor agonists and antagonists, but so far only a few have been clinically effective.^43,44) 5-HT_1A receptor agonists not only produce anxiolytic, antidepressant, and hypothermic effects,^45–47) but also alter feeding behavior,^48,49) sexual behavior,^50–52) and pain modulation.^53,54) Using the selective 5-HT_1A receptor agonist osemozotan,¹¹⁾ we studied the roles of the 5-HT_1A receptor in the brain. Osemozotan is about 500-fold to more than 1000-fold more active at the 5-HT_1A site than at the 5-HT_2A, 5-HT_1B, 5-HT_2C, 5-HT₃, α₂-adrenergic, and DA-D₁ sites.¹¹⁾ Furthermore, this compound interacts in vivo with 5-HT_1A receptors in the brain.⁵⁵⁾ In vivo administration of osemozotan, unlike azapirone compounds, does not produce the metabolite 1-(2-pyrimidinyl)piperazine, which antagonizes the antidepressant effect of 8-OH-DPAT.¹²⁾ Therefore, it should be noted that osemozotan is a suitable 5-HT_1A receptor agonist for in vivo experiments. The 5-HT_1A receptor is expressed both presynaptically as an autoreceptor by 5-HT-containing neurons and postsynaptically by a variety of other neurons; the effects of 5-HT_1A receptor agonists are then mediated by presynaptic or postsynaptic 5-HT_1A receptors (Fig. 2). Activation of 5-HT_1A receptors affects the release of not only 5-HT but also of other neurotransmitters such as noradrenaline (NA), DA, and acetylcholine (ACh). That is, 5-HT_1A receptors have the effect of inhibitory regulation on the release of 5-HT and stimulatory regulation on the release of NA, DA, and ACh. The regulation of 5-HT^56,57) and ACh⁵⁸⁾ is mediated by presynaptic 5-HT_1A receptors, and that of NA^59,60) or DA⁶¹⁾ release is mediated by postsynaptic 5-HT_1A receptors. It should be noted that the regulation of DA release by 5-HT_1A receptor agonists is region specific; stimulation was observed in the hippocampus and cerebral cortex but not in the striatum.

Fig. 2. Neurochemical Profiles of Presynaptic and Postsynaptic 5-HT_1A Receptor Activation

Activation of the presynaptic 5-HT_1A receptor inhibits 5-HT release and facilitates ACh release, while that of the postsynaptic 5-HT_1A receptor facilitates the release of NA and DA. The presynaptic 5-HT_1A receptor has higher affinities for ligands than the postsynaptic 5-HT_1A receptor.

Osemozotan showed the pharmacological profile of a 5-HT_1A agonist as described above. That is, systemic administration of osemozotan inhibited anxiety-like^12,62) and depression-like behaviors.^12,63,64) Furthermore, it inhibited aggressive^65–67) and obsessive–compulsive⁶⁸⁾ behaviors. These results support the idea that the 5-HT_1A receptor is a potential target for the treatment of psychiatric disorders.

3. DIFFERENCE IN AFFINITY FOR LIGANDS BETWEEN PRESYNAPTIC AND POSTSYNAPTIC 5-HT_1A RECEPTORS

There are differences in the G-protein coupling between pre- and postsynaptic 5-HT_1A receptors. The presynaptic 5-HT_1A receptors are considered to play a role in not only the anxiolytic effect of 5-HT_1A receptor agonists^69,70) but also the augmentation of SSRI therapy by the mixed 5-HT_1A receptor/β-adrenoceptor antagonist, pindolol.^71–73) For this reason, it is important to study the regulation of pre- and postsynaptic 5-HT_1A receptors to understand not only the action of 5-HT_1A receptor agonists but also the antidepressant effect of coadministering an SSRI and a 5-HT_1A receptor antagonist. The role of the presynaptic 5-HT_1A receptors has been studied by chemical lesion with the 5-HT neuronal toxin 5,7-dihydroxytryptamine or the 5-HT synthesis inhibitor p-chlorophenylalanine. Treatment with these drugs decreases 5-HT levels in the brain markedly but not completely. Therefore, the strategy cannot exclude the possibility that serotonergic neurons are still present. It is also likely that treatment upregulates postsynaptic 5-HT receptors. Alternatively, local application of the agonists in discrete regions of the brain such as the dorsal raphe and limbic regions is also used to study the roles of presynaptic and postsynaptic 5-HT_1A receptors, respectively. This strategy, however, is difficult in small animals like mice.

In these circumstances, we found in mice that presynaptic and postsynaptic 5-HT_1A receptors differed in sensitivity to the 5-HT_1A receptor antagonist WAY100635; a low dose of WAY100635 blocked the 5-HT_1A receptor agonist-induced decrease in cortical 5-HT release, a presynaptic 5-HT_1A receptor-mediated response, but not the 5-HT_1A receptor agonist-induced increase in the DA release, a postsynaptic 5-HT_1A receptor-mediated response⁷⁴⁾ (Fig. 2). Using WAY100635, we demonstrated that presynaptic 5-HT_1A receptors play a key role in the hypothermic (unpublished) and anxiolytic effects of osemozotan in mice.⁶²⁾

4. ANIMAL MODELS

To clarify the role of the 5-HT_1A receptor as a potential target for the treatment of psychiatric disorders, we studied the effects of osemozotan in mouse models such as isolation rearing, chronic corticosterone treatment, and psychostimulant-induced behavioral sensitization. These models appear to reflect the conditions of patients with schizophrenia, treatment-resistant depression, and drug dependence, respectively.

4.1. Neurochemical Basis of Isolation Rearing-Induced Abnormal Behaviors

Isolation-reared rodents show abnormal behaviors in adulthood such as hyperlocomotion, aggressive behaviors, deficits of prepulse inhibition, cognitive impairments, decreased social contact, depression- and anxiety-like behaviors, and reduced pain sensitivity, as mentioned above. The results of previous studies indicated that the functions of monoaminergic neurons are altered in isolation-reared animals, although measurements of tissue and extracellular neurotransmitter levels in selected brain regions have produced inconsistent results.^43,75) Furthermore, studies using in vivo microdialysis provide more direct information on monoaminergic neuronal activity in selected brain regions of isolation-reared rats. Isolation rearing increases basal levels of extracellular DA in the nucleus accumbens of rats⁷⁶⁾ and in the prefrontal cortex of mice,⁶⁶⁾ although inconsistent results were also reported in rats.^77,78) In addition, isolation rearing enhances DA release induced by footshock, contextual stimulus, and amphetamine in rats.^76–78) We found that isolation rearing specifically affects dopaminergic neurons in the frontal cortex in mice.⁷⁹⁾ That is, isolation rearing causes a selective enhancement of mesocortical dopaminergic activity, resulting in an imbalance between the monoaminergic neurotransmitter systems. This neurochemical change may contribute to isolation-induced abnormal behavior such as high spontaneous locomotor activity or aggressive behavior. We also showed that isolation rearing not only enhances mesocortical dopaminergic activity but also reduces the response of dopaminergic, but not adrenergic and serotonergic, neurons to 5-HT_1A receptor activation. In other words, the administration of 5-HT_1A agonists activates brain adrenergic and serotonergic, but not dopaminergic, neurons in isolation-reared mice. This observation suggests that the administration of 5-HT_1A receptor agonists improves the impaired balance of mesocortical monoaminergic neurotransmitter systems in isolation-reared mice. It is likely that an imbalance between monoaminergic neurotransmitter systems may be the neurochemical basis for isolation-induced abnormal behavior, and counteracting this imbalance by 5-HT_1A receptor activation partly contributes to the attenuation of isolation-induced abnormal behavior.

In contrast to the DA and NA systems, there is little information on the cholinergic system in isolation-reared rodents.⁵⁰⁾ A disruption of cerebral cholinergic pathways may contribute to the cognitive deficits of schizophrenia, and acetylcholinesterase inhibitors and ACh receptor agonists have the therapeutic potential to improve such deficits.⁸⁰⁾ We have recently reported that galantamine, but not donepezil, improved social isolation-induced prepulse inhibition disruption in mice^81–84) (Fig. 3). To determine whether the isolation-induced prepulse inhibition deficits may be due to dysfunction of the cholinergic system in isolation-reared mice, we examined the effect of isolation rearing on brain cholinergic functions.⁸⁴⁾ There were no significant differences in the choline acetyltransferase and acetylcholinesterase activities and basal extracellular ACh levels between group- and isolation-reared mice. However, by measuring the cholinergic receptor agonist-induced behavioral responses, we found that the muscarinic receptor function is reduced in isolation-reared mice. These observations suggest that muscarinic, especially M₁, but not nicotinic, receptor function is reduced in isolation-reared mice. The proposed mechanism for the effect of galantamine on isolation rearing-induced prepulse inhibition deficits is summarized in Fig. 3. In addition to the muscarinic effect, galantamine has a nicotinic effect that increases hippocampal insulin-like growth factor 2 expression.⁸⁵⁾ We also found that the binding of the metabotropic glutamate 2/3 receptor antagonist [³H]LY341495 in the prefrontal cortex, cerebral cortical layers I–III, and hippocampus was significantly increased by rearing in social isolation and that the antagonists decreased the immobility time of isolation-reared mice in the forced swim test.⁸⁶⁾ These observations suggest that the glutamate system is also involved in depression-like behavior in the isolation-rearing model.

Fig. 3. Proposed Mechanism of the Effect of Galatanamine on Isolation Rearing-Induced Deficits of Prepulse Inhibition in Mice

Galantamine-induced increase in ACh levels is inhibited by the DA-D₁ receptor antagonist SCH23390. Galantamine improves prepulse inhibition deficits in social isolation-reared mice in an M₁ receptor-dependent manner, since the muscarinic receptor antagonist scopolamine and the specific M₁ muscarinic receptor antagonist telenzepine antagonize the effect of galantamine, and the muscarinic receptor agonist oxtremorine and the M₁ muscarinic receptor agonist N-desmethylclozapine improve isolation rearing-induced prepulse inhibition deficits. Rearing in isolation decreases muscarinic, especially M₁, receptor function and the reduced muscarinic receptor function may be involved in prepulse inhibition deficits in isolation-reared mice.

4.2. Effect of Osemozotan on Abnormal Behaviors in Isolation-Reared Mice

Postmortem studies showed that 5-HT_1A receptors are increased in the prefrontal cortex of schizophrenic patients.^87,88) Microdialysis studies found that 5-HT_1A receptor agonists increase DA release in the frontal cortex in rats^61,89) and mice.⁷⁹⁾ In addition, atypical neuroleptics have affinity for 5-HT_1A receptors,^90,91) and risperidone increases ACh release in a 5-HT_1A receptor-mediated mechanism.⁹²⁾ These observations suggest that 5-HT_1A receptors are implicated in the pathophysiology of schizophrenia. In this regard, we showed that osemozotan reversed prepulse inhibition deficits in isolation-reared mice.⁹³⁾ We also observed that the effect of osemozotan on prepulse inhibition deficits was blocked by a low dose of WAY100635, suggesting the involvment of presynaptic 5-HT_1A receptors, since the presynaptic 5-HT_1A receptors are more sensitive to WAY100635 than the postsynaptic 5-HT_1A receptors. Activation of the presynaptic 5-HT_1A receptors results in a reduction in the firing rate of serotonergic neurons and suppression of 5-HT synthesis, 5-HT turnover, and 5-HT release in the projection areas.²⁾ This causes a reduction in signaling via all subtypes of 5-HT receptor at the target cells. Therefore, presynaptic 5-HT_1A receptor activation may improve isolation rearing-induced changes in brain dopaminergic neurons,^61,79) which play a role in prepulse inhibition deficits.¹¹⁾

5-HT_1A receptor agonists reduce not only prepulse inhibition deficits but also aggressive behavior in isolation-reared rodents,^{71,72,93–98)} although the exact mechanism is not known. Since aggressive behaviors and social interaction deficits in isolation-reared rodents are induced by exposure to an intruder, it is likely that an encounter with an intruder may produce neurobiological changes for the induction of abnormal behaviors. In resident-intruder tests, van Erp and Miczek⁹⁹⁾ reported that encounters with the intruder increased prefrontal and accumbal DA and decreased cortical 5-HT levels in the resident aggressive rats. Anstrom et al.,¹⁰⁰⁾ using fast-scan cyclic voltammetry and multiunit recording techniques, reported that an aggressive encounter increases phasic DA transmission in the mesolimbic pathway in defeated rats. These neurochemical changes were observed both during and after the encounters, and it is likely that the changes are related to both psychological and physical stress.

We have recently tried to identify the primary neurochemical changes for induction of abnormal behaviors in isolation-reared mice¹⁰¹⁾ (Fig. 4). This study used a cage that was divided into two compartments (large and small) by a mesh partition, and we examined the effects of intruder encounters on c-Fos expression and the levels of DA and 5-HT in the brain regions of resident mice reared in a group or in isolation. We found that the encounter stimulation increased prefrontal c-Fos expression, DA levels, and 5-HT levels in male isolation-reared mice, while it did not affect the c-Fos expression, DA, or 5-HT levels in group-reared mice. The important finding was that encounter-induced increases in c-Fos expression in the dorsal raphe nucleus and ventral tegmental area, but not nucleus accumbens shell, are much greater in isolation-reared than group-reared mice. Furthermore, osemozotan, the metabotropic glutamate 2/3 receptor agonist MGS0028, and the γ-aminobutyric acid-A receptor agonist diazepam attenuated isolation-induced abnormal behaviors^{65,66,93,102)} and encounter-induced hyperactivity, c-Fos expression in the prefrontal cortex and dorsal raphe nucleus, and increases in prefrontal 5-HT levels.¹⁰⁰⁾ These findings suggest that the prefrontal DA and 5-HT systems are activated by encounter stimulation in isolation-reared mice, and the encounter-induced activation of 5-HT system triggers the induction of some abnormal behaviors in male isolation-reared mice (Fig. 4). Although those results suggested that the glutamate system is also involved in the abnormal behavior of isolation-reared mice, it should be noted that metabotropic glutamate 2/3 receptor agonists attenuate hyperactivity, while the receptor antagonists attenuate depression-like behavior in isolation-reared mice.

Fig. 4. Prefrontal DA and 5-HT Responses to Encounter Stimulation in Isolation-Reared Mice

Male isolation-reared mice show enhanced encounter-induced increases in c-Fos expression in the prefrontal cortex (PFC), dorsal raphe nucleus (DRN), and ventral tegmental area (VTA). Microdialysis study also shows that encounter stimulation increases prefrontal DA and 5-HT release in isolation-reared mice. The increased prefrontal 5-HT levels are blocked by diazepam, osemozotan, and LY379268, whereas increased prefrontal DA levels are only blocked by diazepam and LY379268. The pharmacologic approach suggests that psychologic stress specifically activates the prefrontal DA and 5-HT systems, and the 5-HT system plays a key role in the induction of abnormal behaviors in male isolation-reared mice.

4.3. Isolation Rearing-Induced Hypoalgesia

Previous studies showed that the 5-HT_1A receptor is involved in the antinociceptive effect of the 5-HT-NA reuptake inhibitor venlafaxine¹⁰³⁾ and the antiallodynic effect of the analgesic tramadol.¹⁰⁴⁾ Furthermore, F13640 (befiradol), a novel 5-HT_1A receptor agonist, shows analgesic activity in animal models and is currently developed for human use.^105,106) These results support the utility of 5-HT_1A receptor activation as an antinociceptive strategy. Along this line, isolation rearing from postweaning reduces pain sensitivity to noxious thermal stimuli in rodents.^26–28,107) We have recently demonstrated that isolation rearing causes activation of the anterior cingulate cortex, periaqueductal gray matter, and rostral ventromedial medulla in mice, and that spinal 5-HT_1A receptors are involved in isolation rearing-induced hypoalgesia in capsaicin-induced nociception.¹⁰⁸⁾ The intrathecal injection of WAY100635 attenuated isolation rearing-induced hypoalgesia in capsaicin-induced nociception, and osemozotan caused analgesia in group-reared mice after capsaicin-induced nociception. These findings suggest that spinal 5-HT_1A receptor activation via the descending serotonergic inhibitory pathway contributes to isolation rearing-induced hypoalgesia.

4.4. Corticosterone Model

There are many studies on the effects of antidepressants in normal animals, but a more relevant study is to evaluate the effects of the drugs in paradigms designed to mimic symptoms of human depression.¹⁰⁹⁾ As shown above, mice reared in isolation show increased spontaneous locomotor activity, aggressive behavior, anxiety behavior, and deficits in prepulse inhibition.^{17,65,67,81,93)} We also found that isolation rearing increased immobility time in the forced swim test in mice.¹¹⁰⁾ Animal models of repeated stress exposure simulate the presumed etiology of depression, and this stress may be produced by exogenous corticosterone administration.^111–114) Thus, chronic corticosterone administration, like isolation rearing,¹¹⁰⁾ provides a reliable rodent model of depression for studies on the neurobiological mechanisms of depression or antidepressant-like effects. We found that chronic corticosterone administration and isolation rearing increased immobility time in the forced swim and tail suspension tests in mice, and that the glucocorticoid receptor antagonist RU-43044 reversed the increased immobility time in the forced swim test.¹¹⁰⁾ The role of the glucocorticoid receptor was reported clinically with mifepristone for the treatment of psychotic depression.^115–117) Consistent with the results of behavioral studies, chronic corticosterone administration and isolation rearing enhanced dopaminergic neurotransmission in the prefrontal cortex, and this enhanced neurotransmission was reversed by RU-43044¹¹⁰⁾ (Fig. 5). These findings suggest that the prefrontal dopaminergic system is responsible for the antidepressant effect of RU-43044. This suggestion is in agreement with the recent finding that glucocorticoid acts locally within the prefrontal cortex to modulate mesocortical DA efflux by potentiation of the glutamatergic drive onto DA neurons in the ventral tegmental area.¹¹⁸⁾ Concerning the involvement of the 5-HT_1A receptor in the corticosterone model, Czyrak et al. demonstrated that chronic corticosterone administration decreased the 5-HT_1A receptor binding in the ventral hippocampus and entorhinal cortex.¹¹⁹⁾ Furthermore, Saphier et al. reported that 5-HT_1A agonists attenuated the adrenocortical responses to acoustic stimulation, conditioned fear, interleukin-1α and cocaine administration.¹²⁰⁾ Recently, we have found that chronic corticosterone-treated mice could be used as an animal model of treatment-resistant depression and that mGlu2/3 receptor antagonists had an antidepressant-like effect in this model.¹²¹⁾ This study also suggested that the prefrontal dopaminergic system is involved in the antidepressant-like effect of mGlu2/3 receptor antagonists in the depression model. We also found that prefrontal dopaminergic neurotransmission is involved in glucocorticoid receptor-mediated modulation of methamphetamine (METH)-induced hyperactivity.¹²²⁾

Fig. 5. Involvement of the Prefrontal DA System in Chronic Corticosterone Administration-Induced and Isolation Rearing-Induced Depressive-Like Behaviors in Mice

Chronic corticosterone administration and isolation rearing cause depression-like behavior in mice, and the glucocorticoid receptor (GR) antagonist RU-43044 attenuates the abnormal behavior. Moreover, chronic corticosterone administration and isolation rearing enhance dopaminergic neurotransmission in the prefrontal cortex, and RU-43044 reverses this enhanced neurotransmission. It is likely that the prefrontal DA system plays a key role in glucocorticoid responses.

4.5. Psychostimulant-Induced Behavioral Sensitization

METH is a central nervous system stimulant, and its prolonged use results in dependence and psychosis that are indistinguishable from paranoid-type schizophrenia.¹²³⁾ Repeated administration of METH causes a long-lasting, augmented locomotor response, called behavioral sensitization,^124,125) which is one of the animal models of METH dependence and psychosis. The mesocorticolimbic DA system is widely recognized to play an essential role in sensitization to METH, for which the release and reuptake inhibition of DA are a primary mechanism mediating the behavioral effects of the drug, although the behavioral effects of repeated METH administration have also been suggested to be modulated by other systems including 5-HT.^126,127) However, the neurochemical mechanisms by which 5-HT receptor ligands affect the psychostimulant-induced behavioral effects are unknown. We found that repeated METH administration causes not only behavioral sensitization but also enhances a METH challenge-induced increase in the extracellular 5-HT levels in the prefrontal cortex, and that these effects are reversed by osemozotan¹²⁸⁾ (Fig. 6). We also found that ritanserin, a 5-HT₂ receptor antagonist, attenuated METH-induced behavioral sensitization.¹²⁹⁾ METH-induced behavioral sensitization in mice is an animal model for psychostimulant-induced psychosis and schizophrenia in terms of paranoid psychotic state and relapse liability.^124,130) Therefore, the results of our study imply that 5-HT_1A receptor agonists may have a therapeutic value for the treatment of METH abuse or psychosis.¹³¹⁾ In this relation, we also found that lithium attenuates acute METH-induced hyperactivity and chronic METH-induced behavioral sensitization via modulation of the prefrontal release of DA and 5-HT, respectively.¹³²⁾ This result suggests that a 5-HT_1A receptor-mediated mechanism is involved in the effect of lithium on chronic METH-induced behavioral sensitization. The 5-HT_1A receptor is also involved in cocaine-induced behavioral sensitization.^133,134) With respect to the role of the 5-HT system in METH-induced hyperactivity, we found that the activation of prefrontal mGlu2/3 receptors inhibits the psychomotor stimulant effect of METH in mice, and this effect may be mediated by the prefrontal 5-HT system.¹³⁵⁾ This finding suggests that prefrontal mGlu2/3 receptors are functionally coupled with the serotonergic system. Our recent study has shown that the mGlu2/3 receptor agonist MGS0028 improves abnormal behaviors in pituitary adenylate cyclase activating polypeptide-knockout mice, an experimental model of psychiatric disorders.¹³⁶⁾

Fig. 6. Effect of Chronic METH Administration on Prefrontal Serotonergic Neurons and Locomotor Activity

Chronic METH administration causes behavioral sensitization and an increase in prefrontal 5-HT release. The 5-HT_1A agonist osemozotan inhibits prefrontal 5-HT release and improves METH-induced behavioral sensitization. The 5-HT_1A receptor is involved in METH-induced behavioral sensitization, and this receptor is a potential target for the treatment of METH dependence.

5. MODULATION OF PREFRONTAL DA RELEASE BY INTERACTION OF THE 5-HT_1A AND OTHER RECEPTORS

Clinical studies showed that combined therapy with an SSRI and antipsychotic drug is effective in patients with treatment-resistant depression.^35–38) We found that the combination of sulpiride and fluvoxamine caused a marked increase in extracellular DA levels in the prefrontal cortex, while sulpiride did not affect the fluvoxamine-induced increases in the 5-HT and NA levels.¹³⁷⁾ This combined effect of sulpiride and fluvoxamine was blocked by a low dose of WAY100635, a 5-HT_1A receptor antagonist, but not by local application of the antagonist. On the other hand, local application of sulpiride in the cortex, like systemic sulpiride, with systemic fluvoxamine increased extracellular levels of DA. These findings suggest that a combination of prefrontal DA-D₂/D₃ blockade and 5-HT_1A receptor activation via 5-HT transporter inhibition in regions other than the cortex increases prefrontal DA release (Fig. 7). In agreement with the neurochemical effect, the coadministration of sulpiride and fluvoxamine had an antidepressant-like effect under the conditions that each drug alone had no effect.¹³⁸⁾ Antipsychotic drugs increase prefrontal ACh release, but this effect is not enhanced by fluvoxamine.¹³⁹⁾

Fig. 7. Neurochemical Basis for Combination Therapy with SSRIs and Antipsychotic Drugs

SSRIs increase extracellular levels of 5-HT, which interacts with the 5-HT_1A receptor in brain regions other than the prefrontal cortex, probably the raphe nucleus. An antipsychotic drug blocks the DA-D₂ receptor. Combined activation of the 5-HT_1A receptor and inhibition of the DA-D₂ receptor increases prefrontal DA release, resulting in enhancement of the antidepressant-like effects of SSRIs.

Fluvoxamine has the highest affinity for the σ₁ receptors³⁹⁾ and shows agonistic activity toward the receptors.^140–143) A positron emission tomography study using [¹¹C]SA4503, a selective σ₁ receptor agonist, indicated that high occupancy of σ₁ receptors occurs in the brain following the administration of therapeutic doses of fluvoxamine to healthy male volunteers.¹⁴⁴⁾ This suggests that σ₁ receptors are involved in the clinical effects of fluvoxamine. σ₁ Receptors have a neuromodulatory role on the neurotransmitter system, including the serotonergic, noradrenergic, dopaminergic, glutamatergic, and cholinergic systems, and are related to depression.^145–151) Previous studies using in vivo microdialysis showed that fluvoxamine causes a transient increase in the extracellular levels of DA and/or NA in the prefrontal cortex,^137,152,153) but it is not known whether σ₁ receptors are involved in the neurochemical effect of fluvoxamine.

To investigate the involvement of σ₁ receptors in the neurochemical effects of fluvoxamine, endocrine manipulations were performed.¹⁵²⁾ Mice were adrenalectomized and castrated to remove the peripheral sources of neuroactive steroids, which are endogenous ligands for σ₁ receptors.^154,155) Adrenalectomy/castration potentiated fluvoxamine-induced increases in the extracellular levels of DA, but not of NA and 5-HT, and this effect was blocked by BD1047, a σ₁ receptor antagonist.¹⁵⁶⁾ Moreover, we found that the enhancement of prefrontal dopaminergic neurotransmission by fluvoxamine is mediated by the activation of 5-HT_1A and σ₁ receptor activation under circulating neuroactive steroid-deficient conditions¹⁵⁷⁾ (Fig. 8). It is likely that fluvoxamine exerts its beneficial effect via σ₁ receptor-mediated enhancement of prefrontal dopaminergic neurotransmission under neurosteroid-deficient conditions. SSRIs increase extracellular levels of 5-HT, which interacts with the receptor subtypes including the 5-HT_1A receptor. The two studies on fluvoxamine described above provide novel regulation mechanisms of the prefrontal DA release through the interaction of 5-HT_1A and other receptors.

Fig. 8. Enhancement of Prefrontal DA Levels by Combined Activation of 5-HT_1A and σ₁ Receptors in the Absence of Neurosteroids

Fluvoxamine inhibits the serotonin transporter (SERT) and activates the σ₁ receptor. Under the conditions of lower levels of circulating neurosteroids, combined activation of the 5-HT_1A receptor and σ₁ receptor enhances prefrontal DA release. The 5-HT_1A receptor involved in the interaction is localized in brain regions other than the prefrontal cortex, and the σ₁ receptor is localized in the prefrontal cortex.

6. CONCLUSION

Progress in the pharmacology of 5-HT_1A receptors was made by the identification of a selective 5-HT_1A receptor agonist and the finding that the 5-HT_1A receptor agonist azapirone compounds have anxiolytic and antidepressant effects in clinical cases. We developed the novel 5-HT_1A receptor agonist osemozotan and studied the functional roles of the receptor in the brain using this compound. 5-HT_1A receptors are localized presynaptically or postsynaptically, and their activation affects not only 5-HT release but also DA, NA, and ACh release in the brain. That is, the brain 5-HT system affects other transmitter systems. In this relation, we found that the 5-HT system is also coupled with the glutamate system. Osemozotan improved most abnormal behaviors of psychiatric disorder models such as isolation rearing, chronic corticosterone administration, and chronic psychomotor stimulant models. It should be noted that abnormal behaviors of isolation-reared mice are triggered by transient activation of the raphe-prefrontal 5-HT system. Studies in animal models suggested that the 5-HT_1A receptor is a potential target for the treatment of anxiety, depression, pain, and drug dependence. Studies on fluvoxamine also showed that prefrontal DA release is regulated by a functional interaction between 5-HT_1A and DA-D₂ or σ₁ receptors. Further studies are required to clarify the pharmacologic significance of the novel regulation mechanisms of the DA system.

Acknowledgment

I would like to express many thanks to my supervisor, Professor Akemichi Baba (President of Hyogo University of Health Sciences), for his invaluable guidance and suggestions. I also thank Drs. Hitoshi Hashimoto, Kazuhiro Takuma, Norito Shintani, and Yukio Ago of Osaka University Graduate School of Pharmaceutical Sciences; Dr. Yutaka Koyama of the Faculty of Pharmacy, Osaka Ohtani University; and Drs. Michikazu Abe, Kenichi Saito, Mitsuo Egawa and Akihiro Tobe of Mitsubishi Tanabe Pharma Co. for their kind support; as well as all graduate students who were involved in the studies for their creative work. This work was supported in part by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science.

REFERENCES

• 1)  De Vry J. 5-HT_1A receptor agonists: recent developments and controversial issues. Psychopharmacology (Berl.), 121, 1–26 (1995).
• 2)  Barnes NM, Sharp T. A review of central 5-HT receptors and their function. Neuropharmacology, 38, 1083–1152 (1999).
• 3)  Blier P, Ward NM. Is there a role for 5-HT_1A agonists in the treatment of depression? Biol. Psychiatry, 53, 193–203 (2003).
• 4)  Lucki I, Singh A, Kreiss DS. Antidepressant-like behavioral effects of serotonin receptor agonists. Neurosci. Biobehav. Rev., 18, 85–95 (1994).
• 5)  Pazos A, Palacios JM. Quantitative autoradiographic mapping of serotonin receptors in the rat brain. I. Serotonin-1 receptors. Brain Res., 346, 205–230 (1985).
• 6)  Vergé D, Daval G, Marcinkiewicz M, Patey A, El Mestikawy S, Gozlan H, Hamon M. Quantitative autoradiography of multiple 5-HT₁ receptor subtypes in the brain of control or 5,7-dihydroxytryptamine-treated rats. J. Neurosci., 6, 3474–3482 (1986).
• 7)  Saudou F, Hen R. 5-Hydroxytryptamine receptor subtypes in vertebrates and invertebrates. Neurochem. Int., 25, 503–532 (1994).
• 8)  Albert PR, Lembo P, Storring JM, Charest A, Saucier C. The 5-HT_1A receptor: signaling, desensitization, and gene transcription. Neuropsychopharmacology, 14, 19–25 (1996).
• 9)  Nicoll RA, Malenka RC, Kauer JA. Functional comparison of neurotransmitter receptor subtypes in mammalian central nervous system. Physiol. Rev., 70, 513–565 (1990).
• 10)  Aghajanian GK. Electrophysiology of serotonin receptor subtypes and signal transduction pathways. Psychopharmacolgy: The Fourth Generation of Progress. (Bloom FR, Kupfer DJ eds.), Raven Press, New York, pp. 1451—1459 (1995).
• 11)  Matsuda T, Yoshikawa T, Suzuki M, Asano S, Somboonthum P, Takuma K, Nakano Y, Morita T, Nakasu Y, Kim HS, Egawa M, Tobe A, Baba A. Novel benzodioxan derivative, 5-(3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy)-1,3-benzodioxole HCl (MKC-242), with a highly potent and selective agonist activity at rat central serotonin_1A receptors. Jpn. J. Pharmacol., 69, 357–366 (1995).
• 12)  Abe M, Tabata R, Saito K, Matsuda T, Baba A, Egawa M. Novel benzodioxan derivative, 5-[3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole HCl (MKC-242), with anxiolytic-like and antidepressant-like effects in animal models. J. Pharmacol. Exp. Ther., 278, 898–905 (1996).
• 13)  Einon DF, Morgan MJ. Early isolation produces enduring hyperactivity in the rat, but no effect upon spontaneous alternation. Q. J. Exp. Psychol., 30, 151–156 (1978).
• 14)  Gentsch C, Lichtsteiner M, Feer H. Locomotor activity, defecation score and corticosterone levels during an open-field exposure: a comparison among individually and group-housed rats, and genetically selected rat lines. Physiol. Behav., 27, 183–186 (1981).
• 15)  Gentsch C, Lichtsteiner M, Feer H. Behavioural comparisons between individually- and group-housed male rats: effects of novel environments and diurnal rhythm. Behav. Brain Res., 6, 93–100 (1982).
• 16)  Wright IK, Ismail H, Upton N, Marsden CA. Effect of isolation rearing on 5-HT agonist-induced responses in the rat. Psychopharmacology (Berl.), 105, 259–263 (1991).
• 17)  Heidbreder CA, Weiss IC, Domeney AM, Pryce C, Homberg J, Hedou G, Feldon J, Moran MC, Nelson P. Behavioral, neurochemical and endocrinological characterization of the early social isolation syndrome. Neuroscience, 100, 749–768 (2000).
• 18)  Hall FS, Huang S, Fong GW, Pert A, Linnoila M. Effects of isolation-rearing on locomotion, anxiety and responses to ethanol in Fawn Hooded and Wistar rats. Psychopharmacology (Berl.), 139, 203–209 (1998).
• 19)  Ago Y, Takahashi K, Nakamura S, Hashimoto H, Baba A, Matsuda T. Anxiety-like and exploratory behaviors of isolation-reared mice in the staircase test. J. Pharmacol. Sci., 104, 153–158 (2007).
• 20)  Maisonnette S, Morato S, Brandao ML. Role of resocialization and of 5-HT_1A receptor activation on the anxiogenic effects induced by isolation in the elevated plus-maze test. Physiol. Behav., 54, 753–758 (1993).
• 21)  Schreiber R, De Vry J. 5-HT_1A receptor ligands in animal models of anxiety, impulsivity and depression: multiple mechanisms of action? Prog. Neuropsychopharmacol. Biol. Psychiatry, 17, 87–104 (1993).
• 22)  Hilakivi LA, Ota M, Lister RG. Effect of isolation on brain monoamines and the behavior of mice in tests of exploration, locomotion, anxiety and behavioral ‘despair.’ Pharmacol. Biochem. Behav., 33, 371–374 (1989).
• 23)  Heritch AJ, Henderson K, Westfall TC. Effects of social isolation on brain catecholamines and forced swimming in rats: prevention by antidepressant treatment. J. Psychiatr. Res., 24, 251–258 (1990).
• 24)  Potegal M, Einon D. Aggressive behaviors in adult rats deprived of playfighting experience as juveniles. Dev. Psychobiol., 22, 159–172 (1989).
• 25)  Geyer MA, Wilkinson LS, Humby T, Robbins TW. Isolation rearing of rats produces a deficit in prepulse inhibition of acoustic startle similar to that in schizophrenia. Biol. Psychiatry, 34, 361–372 (1993).
• 26)  Wilkinson LS, Killcross SS, Humby T, Hall FS, Geyer MA, Robbins TW. Social isolation in the rat produces developmentally specific deficits in prepulse inhibition of the acoustic startle response without disrupting latent inhibition. Neuropsychopharmacology, 10, 61–72 (1994).
• 27)  Bakshi VP, Swerdlow NR, Braff DL, Geyer MA. Reversal of isolation rearing-induced deficits in prepulse inhibition by seroquel and olanzapine. Biol. Psychiatry, 43, 436–445 (1998).
• 28)  Coudereau JP, Monier C, Bourre JM, Frances H. Effect of isolation on pain threshold and on different effects of morphine. Prog. Neuropsychopharmacol. Biol. Psychiatry, 21, 997–1018 (1997).
• 29)  Tuboly G, Benedek G, Horvath G. Selective disturbance of pain sensitivity after social isolation. Physiol. Behav., 96, 18–22 (2009).
• 30)  Meng Q, Li N, Han X, Shao F, Wang W. Peri-adolescence isolation rearing alters social behavior and nociception in rats. Neurosci. Lett., 480, 25–29 (2010).
• 31)  Geyer MA, Krebs-Thomson K, Braff DL, Swerdlow NR. Pharmacological studies of prepulse inhibition models of sensorimotor gating deficits in schizophrenia: a decade in review. Psychopharmacology (Berl.), 156, 117–154 (2001).
• 32)  Willner P. The validity of animal models of depression. Psychopharmacology (Berl.), 83, 1–16 (1984).
• 33)  Ferrier IN. Treatment of major depression: is improvement enough? J. Clin. Psychiatry, 60 (Suppl. 6), 10–14 (1999).
• 34)  Nelson JC. A review of the efficacy of serotonergic and noradrenergic reuptake inhibitors for treatment of major depression. Biol. Psychiatry, 46, 1301–1308 (1999).
• 35)  Barbui C, Hotopf M. Amitriptyline v. the rest: still the leading antidepressant after 40 years of randomized controlled trials. Br. J. Psychiatry, 178, 129–144 (2001).
• 36)  O’Connor M, Silver H. Adding risperidone to selective serotonin reuptake inhibitor improves chronic depression. J. Clin. Psychopharmacol., 18, 89–91 (1998), letter.
• 37)  Ostroff RB, Nelson JC. Risperidone augmentation of selective serotonin reuptake inhibitors in major depression. J. Clin. Psychiatry, 60, 256–259 (1999).
• 38)  Shelton RC, Tollefson GD, Tohen M, Stahl S, Gannon KS, Jacobs TG, Buras WR, Bymaster FP, Zhang W, Spencer KA, Feldman PD, Meltzer HY. A novel augmentation strategy for treating resistant major depression. Am. J. Psychiatry, 158, 131–134 (2001).
• 39)  Narita N, Hashimoto K, Tomitaka S, Minabe Y. Interactions of selective serotonin reuptake inhibitors with subtypes of σ receptors in rat brain. Eur. J. Pharmacol., 307, 117–119 (1996).
• 40)  Hayashi T, Su TP. σ-1 Receptor ligands: potential in the treatment of neuropsychiatric disorders. CNS Drugs, 18, 269–284 (2004).
• 41)  Middlemiss DN, Fozard JR. 8-Hydroxy-2-(di-n-propylamino)tetralin discriminates between subtypes of the 5-HT₁ recognition site. Eur. J. Pharmacol., 90, 151–153 (1983).
• 42)  Robinson DS, Rickels K, Feighner J, Fabre LF Jr, Gammans RE, Shrotriya RC, Alms DR, Andary JJ, Messina ME. Clinical effects of the 5-HT_1A partial agonists in depression: a composite analysis of buspirone in the treatment of depression. J. Clin. Psychopharmacol., 10 (Suppl.), 67S–76S (1990).
• 43)  Fone KC, Porkess MV. Behavioural and neurochemical effects of post-weaning social isolation in rodents—relevance to developmental neuropsychiatric disorders. Neurosci. Biobehav. Rev., 32, 1087–1102 (2008).
• 44)  Lacivita E, Di Pilato P, De Giorgio P, Colabufo NA, Berardi F, Perrone R, Leopoldo M. The therapeutic potential of 5-HT_1A receptors: a patent review. Expert Opin. Ther. Pat., 22, 887–902 (2012).
• 45)  Matsuda T, Kanda T, Seong YH, Baba A, Iwata H. p-Chlorophenylalanine attenuates the pituitary-adrenocortical response to 5-HT_1A receptor agonists in mice. Eur. J. Pharmacol., 181, 295–297 (1990).
• 46)  Bill DJ, Knight M, Forster EA, Fletcher A. Direct evidence for an important species difference in the mechanism of 8-OH-DPAT-induced hypothermia. Br. J. Pharmacol., 103, 1857–1864 (1991).
• 47)  Millan MJ, Rivet JM, Canton H, Le Marouille Girardon S, Gobert A. Induction of hypothermia as a model of 5-hydroxytryptamine_1A receptor-mediated activity in the rat: a pharmacological characterization of the actions of novel agonists and antagonists. J. Pharmacol. Exp. Ther., 264, 1364–1376 (1993).
• 48)  Hartley JE, Fletcher A. The effects of WAY-100135 and 8-hydroxy-2-(di-n-propylamino)tetralin on feeding in the rat. Eur. J. Pharmacol., 252, 329–332 (1994).
• 49)  Jhanwar-Uniyal M, Moorjani B, Kahn AH. Indications of pre- and post-synaptic 5-HT_1A receptor interactions in feeding behavior and neuroendocrine regulation. Brain Res., 646, 247–257 (1994).
• 50)  Uphouse L, Caldarola Pastuszka M. Female sexual behaviour following intracerebral infusion of 5-HT_1A agonist, 8-OH-DPAT, into the medal preoptic area. Brain Res., 601, 203–208 (1993).
• 51)  Pomerantz SM, Hepner BC, Wertz JM. Serotonergic influences on male sexual behavior of rhesus monkeys: effects of serotonin agonists. Psychopharmacology (Berl.), 111, 47–54 (1993).
• 52)  Uphouse L, Caldarola Pastuszka M, Montanez S. Intracerebral actions of the 5-HT_1A agonists, 8-OH-DPAT and buspirone, and of the 5-HT_1A partial agonist/antagonist, NAN-190, on female sexual behavior. Neuropharmacology, 31, 969–981 (1992).
• 53)  Colpaert FC. 5-HT_1A receptor activation: new molecular and neuroadaptive mechanisms of pain relief. Curr. Opin. Investig. Drugs, 7, 40–47 (2006).
• 54)  Martel JC, Assié MB, Bardin L, Depoortère R, Cussac D, Newman-Tancredi A. 5-HT_1A receptors are involved in the effects of xaliproden on G-protein activation, neurotransmitter release and nociception. Br. J. Pharmacol., 158, 232–242 (2009).
• 55)  Asano S, Matsuda T, Yoshikawa T, Somboonthum P, Tasaki H, Abe M, Baba A. Interaction of orally administered 5-{3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy}-1,3-benzodioxole (MKC-242) with 5-HT_1A receptors in rat brain. Jpn. J. Pharmacol., 74, 69–75 (1997).
• 56)  Hjorth S, Magnusson T. The 5-HT_1A receptor agonist, 8-OH-DPAT, preferentially activates cell body 5-HT autoreceptors in rat brain in vivo. Naunyn Schmiedebergs Arch. Pharmacol., 338, 463–471 (1988).
• 57)  Sharp T, Bramwell SR, Grahame-Smith DG. 5-HT₁ agonists reduce 5-hydroxytryptamine release in rat hippocampus in vivo as determined by brain microdialysis. Br. J. Pharmacol., 96, 283–290 (1989).
• 58)  Somboonthum P, Matsuda T, Asano S, Sakaue M, Baba A. MKC-242, a novel 5-HT_1A receptor agonist, facilitates cortical acetylcholine release by a mechanism different from that of 8-OH-DPAT in awake rats. Neuropharmacology, 36, 1733–1739 (1997).
• 59)  Suzuki M, Matsuda T, Asano S, Somboonthum P, Takuma K, Baba A. Increase of noradrenaline release in the hypothalamus of freely moving rat by postsynaptic 5-hydroxytryptamine_1A receptor activation. Br. J. Pharmacol., 115, 703–711 (1995).
• 60)  Hajós-Korcsok E, McQuade R, Sharp T. Influence of 5-HT_1A receptors on central noradrenergic activity: microdialysis studies using (±)-MDL 73005EF and its enantiomers. Neuropharmacology, 38, 299–306 (1999).
• 61)  Sakaue M, Somboonthum P, Nishihara B, Koyama Y, Hashimoto H, Baba A, Matsuda T. Postsynaptic 5-hydroxytryptamine_1A receptor activation increases in vivo dopamine release in rat prefrontal cortex. Br. J. Pharmacol., 129, 1028–1034 (2000).
• 62)  Sakaue M, Ago Y, Sowa C, Koyama Y, Baba A, Matsuda T. The 5-HT_1A receptor agonist MKC-242 increases the exploratory activity of mice in the elevated plus-maze. Eur. J. Pharmacol., 458, 141–144 (2003).
• 63)  Matsuda T, Somboonthum P, Suzuki M, Asano S, Baba A. Antidepressant-like effect by postsynaptic 5-HT_1A receptor activation in mice. Eur. J. Pharmacol., 280, 235–238 (1995).
• 64)  Abe M, Saito K. Reduction of wrap restraint stress-induced defecation by MKC-242, a novel benzodioxan derivative, via 5-HT_1A-receptor agonist action in rats. Jpn. J. Pharmacol., 77, 211–217 (1998).
• 65)  Sakaue M, Ago Y, Murakami C, Sowa C, Sakamoto Y, Koyama Y, Baba A, Matsuda T. Involvement of benzodiazepine binding sites in an antiaggressive effect by 5-HT_1A receptor activation in isolated mice. Eur. J. Pharmacol., 432, 163–166 (2001).
• 66)  Matsuda T, Sakaue M, Ago Y, Sakamoto Y, Koyama Y, Baba A. Functional alteration of brain dopaminergic system in isolated aggressive mice. Jpn. J. Neuropsychopharmacol., 21, 71–76 (2001).
• 67)  Sakaue M, Ago Y, Sowa C, Sakamoto Y, Nishihara B, Koyama Y, Baba A, Matsuda T. Modulation of 5-HT_2A receptors of aggressive behavior in isolated mice. Jpn. J. Pharmacol., 89, 89–92 (2002).
• 68)  Harasawa T, Ago Y, Itoh S, Baba A, Matsuda T. Role of serotonin type 1A receptors in fluvoxamine-induced inhibition of marble-burying behavior in mice. Behav. Pharmacol., 17, 637–640 (2006).
• 69)  Traber J, Glaser T. 5-HT_1A receptor-related anxiolytics. Trends Pharmacol. Sci., 8, 432–437 (1987).
• 70)  File SE, Gonzalez LE, Andrews N. Comparative study of pre- and postsynaptic 5-HT_1A receptor modulation of anxiety in two ethological animal tests. J. Neurosci., 16, 4810–4815 (1996).
• 71)  Blier P, de Montigny C. Current advances and trends in the treatment of depression. Trends Pharmacol. Sci., 15, 220–226 (1994).
• 72)  Blier P, Bergeron R. Effectiveness of pindolol with selected antidepressant drugs in the treatment of major depression. J. Clin. Psychopharmacol., 15, 217–222 (1995).
• 73)  Artigas F, Romero L, de Montigny C, Blier P. Acceleration of the effect of selected antidepressant drugs in major depression by 5-HT_1A antagonists. Trends Neurosci., 19, 378–383 (1996).
• 74)  Ago Y, Koyama Y, Baba A, Matsuda T. Regulation by 5-HT_1A receptors of the in vivo release of 5-HT and DA in mouse frontal cortex. Neuropharmacology, 45, 1050–1056 (2003).
• 75)  Brenes JC, Rodríguez O, Fornaguera J. Differential effect of environment enrichment and social isolation on depressive-like behavior, spontaneous activity and serotonin and norepinephrine concentration in prefrontal cortex and ventral striatum. Pharmacol. Biochem. Behav., 89, 85–93 (2008).
• 76)  Hall FS, Wilkinson LS, Humby T, Inglis W, Kendall DA, Marsden CA, Robbins TW. Isolation rearing in rats: pre- and postsynaptic changes in striatal dopaminergic system. Pharmacol. Biochem. Behav., 59, 859–872 (1998).
• 77)  Jones GH, Hernandez TD, Kendall DA, Marsden CA, Robbins TW. Dopaminergic and serotonergic function following isolation rearing in rats. A study of behavioural responses and postmortem and in vivo neurochemistry. Pharmacol. Biochem. Behav., 43, 17–35 (1992).
• 78)  Fulford AJ, Marsden CA. Effect of isolation-rearing on conditioned dopamine release in vivo in the nucleus accumbens of the rat. J. Neurochem., 70, 384–390 (1998).
• 79)  Ago Y, Sakaue M, Baba A, Matsuda T. Selective reduction by isolation rearing of 5-HT_1A receptor-mediated dopamine release in vivo in the frontal cortex of mice. J. Neurochem., 83, 353–359 (2002).
• 80)  Friedman JI. Cholinergic targets for cognitive enhancement in schizophrenia: focus on cholinesterase inhibitors and muscarinic agonists. Psychopharmacology (Berl.), 174, 45–53 (2004).
• 81)  Koda K, Ago Y, Kawasaki T, Hashimoto H, Baba A, Matsuda T. Galantamine and donepezil differently affect isolation rearing-induced deficits of prepulse inhibition in mice. Psychopharmacology (Berl.), 196, 293–301 (2008).
• 82)  Yano K, Koda K, Ago Y, Kobayashi H, Kawasaki T, Takuma K, Matsuda T. Galantamine improves apomorphine-induced deficits in prepulse inhibition via muscarinic ACh receptors in mice. Br. J. Pharmacol., 156, 173–180 (2009).
• 83)  Ago Y, Koda K, Ota Y, Kita Y, Fukada A, Takuma K, Matsuda T. Donepezil, but not galantamine, blocks muscarinic receptor-mediated in vitro and in vivo responses. Synapse, 65, 1373–1377 (2011).
• 84)  Koda K, Ago Y, Yano K, Nishimura M, Kobayashi H, Fukada A, Takuma K, Matsuda T. Involvement of decreased muscarinic receptor function in prepulse inhibition deficits in mice reared in social isolation. Br. J. Pharmacol., 162, 763–772 (2011).
• 85)  Kita Y, Ago Y, Takano E, Fukada A, Takuma K, Matsuda T. Galantamine increases hippocampal insulin-like growth factor 2 expression via α7 nicotinic acetylcholine receptors in mice. Psychopharmacology (Berl.), 225, 543–551 (2013).
• 86)  Kawasaki T, Ago Y, Yano K, Araki R, Washida Y, Onoe H, Chaki S, Nakazato A, Hashimoto H, Baba A, Takuma K, Matsuda T. Increased binding of cortical and hippocampal group II metabotropic glutamate receptors in isolation-reared mice. Neuropharmacology, 60, 397–404 (2011).
• 87)  Hashimoto T, Nishino N, Nakai H, Tanaka C. Increase in serotonin 5-HT_1A receptors in prefrontal and temporal cortices of brains from patients with chronic schizophrenia. Life Sci., 48, 355–363 (1991).
• 88)  Sumiyoshi T, Stockmeier CA, Overholser JC, Dilley GE, Meltzer HY. Serotonin_1A receptors are increased in postmortem prefrontal cortex in schizophrenia. Brain Res., 708, 209–214 (1996).
• 89)  Wȩdzony K, Maćkowiak M, Fijal K, Golembiowska K. Ipsapirone enhances the dopamine outflow via 5-HT_1A receptors in the rat prefrontal cortex. Eur. J. Pharmacol., 305, 73–78 (1996).
• 90)  Mason SL, Reynolds GP. Clozapine has sub-micromolar affinity for 5-HT_1A receptors in human brain tissue. Eur. J. Pharmacol., 221, 397–398 (1992).
• 91)  Newman-Tancredi A, Gavaudan S, Conte C, Chaput C, Touzard M, Verriele L, Audinot V, Millan MJ. Agonist and antagonist actions of antipsychotic agents at 5-HT_1A receptors: a [³⁵S]GTPγS binding study. Eur. J. Pharmacol., 355, 245–256 (1998).
• 92)  Sato M, Ago Y, Koda K, Nakamura S, Kawasaki T, Baba A, Matsuda T. Role of postsynaptic serotonin_1A receptors in risperidone-induced increase in acetylcholine release in rat prefrontal cortex. Eur. J. Pharmacol., 559, 155–160 (2007).
• 93)  Sakaue M, Ago Y, Baba A, Matsuda T. The 5-HT_1A receptor agonist MKC-242 reverses isolation rearing-induced deficit of prepulse inhibition in mice. Psychopharmacology (Berl.), 170, 73–79 (2003).
• 94)  Olivier B, Mos J, van del Heyden J, Hartog J. Serotonergic modulation of social interactions in isolated male mice. Psychopharmacology (Berl.), 97, 154–156 (1989).
• 95)  White SM, Kucharik RF, Moyer JA. Effects of serotonergic agents on isolation-induced aggression. Pharmacol. Biochem. Behav., 39, 729–736 (1991).
• 96)  Sánchez C, Arnt J, Hyttel J, Moltzen EK. The role of serotonergic mechanisms in inhibition of isolation-induced aggression in male mice. Psychopharmacology (Berl.), 110, 53–59 (1993).
• 97)  De Boer SF, Lesourd M, Mocaer E, Koolhaas JM. Selective antiaggressive effects of alnespirone in resident-intruder test are mediated via 5-hydroxytryptamine_1A receptors: a comparative pharmacological study with 8-hydroxy-2-dipropylaminotetralin, ipsapirone, buspirone, eltoprazine, and WAY-100635. J. Pharmacol. Exp. Ther., 288, 1125–1133 (1999).
• 98)  Mendoza DL, Bravo HA, Swanson HH. Antiaggressive and anxiolytic effects of gepirone in mice, and their attenuation by WAY 100635. Pharmacol. Biochem. Behav., 62, 499–509 (1999).
• 99)  van Erp AM, Miczek KA. Aggressive behavior, increased accumbal dopamine, and decreased cortical serotonin in rats. J. Neurosci., 20, 9320–9325 (2000).
• 100)  Anstrom KK, Miczek KA, Budygin EA. Increased phasic dopamine signaling in the mesolimbic pathway during social defeat in rats. Neuroscience, 161, 3–12 (2009).
• 101)  Ago Y, Araki R, Tanaka T, Sasaga A, Nishiyama S, Takuma K, Matsuda T. Role of social encounter-induced activation of prefrontal serotonergic systems in the abnormal behaviors of isolation-reared mice. Neuropsychopharmacology, 38, 1535–1547 (2013).
• 102)  Ago Y, Araki R, Yano K, Kawasaki T, Chaki S, Nakazato A, Onoe H, Hashimoto H, Baba A, Takuma K, Matsuda T. The selective metabotropic glutamate 2/3 receptor agonist MGS0028 reverses isolation rearing-induced abnormal behaviors in mice. J. Pharmacol. Sci., 118, 295–298 (2012).
• 103)  Berrocoso E, Mico JA. Role of serotonin 5-HT_1A receptors in the antidepressant-like effect and the antinociceptive effect of venlafaxine in mice. Int. J. Neuropsychopharmacol., 12, 61–71 (2009).
• 104)  Berrocoso E, De Benito MD, Mico JA. Role of serotonin 5-HT_1A and opioid receptors in the antiallodynic effect of tramadol in the chronic constriction injury model of neuropathic pain in rats. Psychopharmacology (Berl.), 193, 97–105 (2007).
• 105)  Colpaert FC, Tarayre JP, Koek W, Pauwels PJ, Bardin L, Xu XJ, Wiesenfeld-Hallin Z, Cosi C, Carilla-Durand E, Assié MB, Vacher B. Large-amplitude 5-HT_1A receptor activation: a new mechanism of profound, central analgesia. Neuropharmacology, 43, 945–958 (2002).
• 106)  Colpaert FC. 5-HT_1A receptor activation: new molecular and neuroadaptive mechanisms of pain relief. Curr. Opin. Investig. Drugs, 7, 40–47 (2006).
• 107)  Coudereau JP, Monier C, Bourre JM, Frances H. Effect of isolation on pain threshold and on different effects of morphine. Prog. Neuropsychopharmacol. Biol. Psychiatry, 21, 997–1018 (1997).
• 108)  Horiguchi N, Ago Y, Asada K, Kita Y, Hiramatsu N, Takuma K, Matsuda T. Involvement of spinal 5-HT_1A receptors in isolation rearing-induced hypoalgesia in mice. Psychopharmacology (Berl.), 227, 251–261 (2013).
• 109)  Cryan JF, Markou A, Lucki I. Assessing antidepressant activity in rodents: recent developments and future needs. Trends Pharmacol. Sci., 23, 238–245 (2002).
• 110)  Ago Y, Arikawa S, Yata M, Yano K, Abe M, Takuma K, Matsuda T. Antidepressant-like effects of the glucocorticoid receptor antagonist RU-43044 are associated with changes in prefrontal dopamine in mouse models of depression. Neuropharmacology, 55, 1355–1363 (2008).
• 111)  Gregus A, Wintink AJ, Davis AC, Kalynchuk LE. Effect of repeated corticosterone injections and restraint stress on anxiety and depression-like behavior in male rats. Behav. Brain Res., 156, 105–114 (2005).
• 112)  Johnson SA, Fournier NM, Kalynchuk LE. Effect of different doses of corticosterone on depression-like behavior and HPA axis responses to a novel stressor. Behav. Brain Res., 168, 280–288 (2006).
• 113)  Kalynchuk LE, Gregus A, Boudreau D, Perrot-Sinal TS. Corticosterone increases depression-like behavior, with some effects on predator odor-induced defensive behavior, in male and female rats. Behav. Neurosci., 118, 1365–1377 (2004).
• 114)  Zhao Y, Ma R, Shen J, Su H, Xing D, Du L. A mouse model of depression induced by repeated corticosterone injections. Eur. J. Pharmacol., 581, 113–120 (2008).
• 115)  Belanoff JK, Flores BH, Kalezhan M, Sund B, Schatzberg AF. Rapid reversal of psychotic depression using mifepristone. J. Clin. Psychopharmacol., 21, 516–521 (2001).
• 116)  DeBattista C, Belanoff J, Glass S, Khan A, Horne RL, Blasey C, Carpenter LL, Alva G. Mifepristone versus placebo in the treatment of psychosis in patients with psychotic major depression. Biol. Psychiatry, 60, 1343–1349 (2006).
• 117)  Flores BH, Kenna H, Keller J, Solvason HB, Schatzberg AF. Clinical and biological effects of mifepristone treatment for psychotic depression. Neuropsychopharmacology, 31, 628–636 (2006).
• 118)  Butts KA, Phillips AG. Glucocorticoid receptors in the prefrontal cortex regulated dopamine efflux to stress via descending glutamataergic feedback to the ventral tegmental area. Int. J. Neuropsychopharmacol., 16, 1799–1807 (2013).
• 119)  Czyrak A, Maćkowiak M, Chocyk A, Fijał K, Gądek-Michalska A, Wędzony K. 8-OHDPAT-induced disruption of prepulse inhibition in rats is attenuated by prolonged corticosterone treatment. Neuropsychopharmacology, 28, 1300–1310 (2003).
• 120)  Saphier D, Farrar GE, Welch JE. Differential inhibition of stress-induced adrenocortical responses by 5-HT_1A agonists and by 5-HT₂ and 5-HT₃ antagonists. Psychoneuroendocrinology, 20, 239–257 (1995).
• 121)  Ago Y, Yano K, Araki R, Hiramatsu N, Kita Y, Kawasaki T, Onoe H, Chaki S, Nakazato A, Hashimoto H, Baba A, Takuma K, Matsuda T. Metabotropic glutamate 2/3 receptor antagonists improve behavioral and prefrontal dopaminergic alterations in the chronic corticosterone-induced depression model in mice. Neuropharmacology, 65, 29–38 (2013).
• 122)  Ago Y, Arikawa S, Yata M, Yano K, Abe M, Takuma K, Matsuda T. Role of prefrontal dopaminergic neurotransmission in glucocorticoid receptor-mediated modulation of methamphetamine-induced hyperactivity. Synapse, 63, 7–14 (2009).
• 123)  Pierce RC, Kalivas PW. A circuitry model of the expression of behavioral sensitization to amphetamine-like psychostimulants. Brain Res. Brain Res. Rev., 25, 192–216 (1997).
• 124)  Robinson TE, Becker JB. Enduring changes in brain and behavior produced by chronic amphetamine administration: a review and evaluation of animal models of amphetamine psychosis. Brain Res. Rev., 11, 157–198 (1986).
• 125)  Kalivas PW, Stewart J. Dopamine transmission in the initiation and expression of drug- and stress-induced sensitization of motor activity. Brain Res. Brain Res. Rev., 16, 223–244 (1991).
• 126)  Przegaliński E, Siwanowicz J, Baran L, Filip M. Activation of serotonin (5-HT)_1A receptors inhibits amphetamine sensitization in mice. Life Sci., 66, 1011–1019 (2000).
• 127)  Przegalinski E, Siwanowicz J, Nowak E, Papla I, Filip M. Role of 5-HT_1B receptors in the sensitization to amphetamine in mice. Eur. J. Pharmacol., 422, 91–99 (2001).
• 128)  Ago Y, Nakamura S, Uda M, Kajii Y, Abe M, Baba A, Matsuda T. Attenuation by the 5-HT_1A receptor agonist osemozotan of the behavioral effects of single and repeated methamphetamine in mice. Neuropharmacology, 51, 914–922 (2006).
• 129)  Ago Y, Nakamura S, Kajita N, Uda M, Hashimoto H, Baba A, Matsuda T. Ritanserin reverses repeated methamphetamine-induced behavioral and neurochemical sensitization in mice. Synapse, 61, 757–763 (2007).
• 130)  Seiden LS, Sabol KE, Ricaurte GA. Amphetamine: effects on catecholamine systems and behavior. Annu. Rev. Pharmacol. Toxicol., 33, 639–677 (1993).
• 131)  Ago Y, Nakamura S, Baba A, Matsuda T. Neuropsychotoxicity of abused drugs: Effects of serotonin receptor ligands on methamphetamine- and cocaine-induced behavioral sensitization in mice. J. Pharmacol. Sci., 106, 15–21 (2008).
• 132)  Ago Y, Tanaka T, Kita Y, Tokumoto H, Takuma K, Matsuda T. Lithium attenuates methamphetamine-induced hyperlocomotion and behavioral sensitization via modulation of prefrontal monoamine release. Neuropharmacology, 62, 1634–1639 (2012).
• 133)  Ago Y, Nakamura S, Hayashi A, Itoh S, Baba A, Matsuda T. Effects of osemozotan, ritanserin and azasetron on cocaine-induced behavioral sensitization in mice. Pharmacol. Biochem. Behav., 85, 198–205 (2006).
• 134)  Nakamura S, Ago Y, Hayashi A, Itoh S, Kakuda M, Hashimoto H, Baba A, Matsuda T. Modification of cocaine-induced behavioral and neurochemical effects by serotonin1A receptor agonist/antagonist in mice. Synapse, 60, 479–484 (2006).
• 135)  Ago Y, Araki R, Yano K, Hiramatsu N, Kawasaki T, Chaki S, Nakazato A, Onoe H, Hashimoto H, Baba A, Takuma K, Matsuda T. Activation of metabotropic glutamate 2/3 receptors attenuates methamphetamine-induced hyperlocomotion and increase in prefrontal serotonergic neurotransmission. Psychopharmacology (Berl.), 217, 443–452 (2011).
• 136)  Ago Y, Hiramatsu N, Ishihama T, Hazama K, Hayata-Takano A, Shibasaki Y, Shintani N, Hashimoto H, Kawasaki T, Onoe H, Chaki S, Nakazato A, Baba A, Takuma K, Matsuda T. The selective metabotropic glutamate 2/3 receptor agonist MGS0028 reverses psychomotor abnormalities and recognition memory deficits in mice lacking the pituitary adenylate cyclase-activating polypeptide. Behav. Pharmacol., 24, 74–77 (2013).
• 137)  Ago Y, Nakamura S, Baba A, Matsuda T. Sulpiride in combination with fluvoxamine increases in vivo dopamine release selectively in rat prefrontal cortex. Neuropsychopharmacology, 30, 43–51 (2005).
• 138)  Ago Y, Harasawa T, Itoh S, Nakamura S, Baba A, Matsuda T. Antidepressant-like effect of coadministration of sulpiride and fluvoxamine in mice. Eur. J. Pharmacol., 520, 86–90 (2005).
• 139)  Ago Y, Sato M, Nakamura S, Baba A, Matsuda T. Lack of enhanced effect of antipsychotics combined with fluvoxamine on acetylcholine release in rat prefrontal cortex. J. Pharmacol. Sci., 102, 419–422 (2006).
• 140)  Egashira N, Harada S, Okuno R, Matsushita M, Nishimura R, Mishima K, Iwasaki K, Orito K, Fujiwara M. Involvement of the sigma₁ receptor in inhibiting activity of fluvoxamine on marble-burying behavior: comparison with paroxetine. Eur. J. Pharmacol., 563, 149–154 (2007).
• 141)  Hashimoto K, Fujita Y, Iyo M. Phencyclidine-induced cognitive deficits in mice are improved by subsequent subchronic administration of fluvoxamine: role of sigma-1 receptors. Neuropsychopharmacology, 32, 514–521 (2007).
• 142)  Hindmarch I, Hashimoto K. Cognition and depression: the effects of fluvoxamine, a sigma-1 receptor agonist, reconsidered. Hum. Psychopharmacol., 25, 193–200 (2010).
• 143)  Nishimura T, Ishima T, Iyo M, Hashimoto K. Potentiation of nerve growth factor-induced neurite outgrowth by fluvoxamine: role of sigma-1 receptors, IP₃ receptors and cellular signaling pathways. PLoS ONE, 3, e2558 (2008).
• 144)  Ishikawa M, Ishiwata K, Ishii K, Kimura Y, Sakata M, Naganawa M, Oda K, Miyatake R, Fujisaki M, Shimizu E, Shirayama Y, Iyo M, Hashimoto K. High occupancy of sigma-1 receptors in the human brain after single oral administration of fluvoxamine: a positron emission tomography study using [¹¹C]SA4503. Biol. Psychiatry, 62, 878–883 (2007).
• 145)  Bermack JE, Debonnel G. The role of sigma receptors in depression. J. Pharmacol. Sci., 97, 317–336 (2005).
• 146)  Cobos EJ, Entrena JM, Nieto FR, Cendán CM, Del Pozo E. Pharmacology and therapeutic potential of sigma₁ receptor ligands. Curr. Neuropharmacol., 6, 344–366 (2008).
• 147)  Delgado PL. Depression: the case for a monoamine deficiency. J. Clin. Psychiatry, 61 (Suppl. 6), 7–11 (2000).
• 148)  Delgado PL, Moreno FA. Role of norepinephrine in depression. J. Clin. Psychiatry, 61 (Suppl. 1), 5–12 (2000).
• 149)  Kobayashi T, Matsuno K, Murai M, Mita S. σ₁ receptor subtype is involved in the facilitation of cortical dopaminergic transmission in the rat brain. Neurochem. Res., 22, 1105–1109 (1997).
• 150)  Rhodes ME, Li PK, Flood JF, Johnson DA. Enhancement of hippocampal acetylcholine release by the neurosteroid dehydroepiandrosterone sulfate: an in vivo microdialysis study. Brain Res., 733, 284–286 (1996).
• 151)  Zheng P. Neuroactive steroid regulation of neurotransmitter release in the CNS: Action, mechanism and possible significance. Prog. Neurobiol., 89, 134–152 (2009).
• 152)  Bymaster FP, Zhang W, Carter PA, Shaw J, Chernet E, Phebus L, Wong DT, Perry KW. Fluoxetine, but not other selective serotonin uptake inhibitors, increases norepinephrine and dopamine extracellular levels in prefrontal cortex. Psychopharmacology (Berl.), 160, 353–361 (2002).
• 153)  Kitaichi Y, Inoue T, Nakagawa S, Boku S, Kakuta A, Izumi T, Koyama T. Sertraline increases extracellular levels not only of serotonin, but also of dopamine in the nucleus accumbens and striatum of rats. Eur. J. Pharmacol., 647, 90–96 (2010).
• 154)  Phan VL, Su TP, Privat A, Maurice T. Modulation of steroidal levels by adrenalectomy/castration and inhibition of neurosteroid synthesis enzymes affect σ₁ receptor-mediated behaviour in mice. Eur. J. Neurosci., 11, 2385–2396 (1999).
• 155)  Urani A, Roman FJ, Phan VL, Su TP, Maurice T. The antidepressant-like effect induced by σ₁-receptor agonists and neuroactive steroids in mice submitted to the forced swimming test. J. Pharmacol. Exp. Ther., 298, 1269–1279 (2001).
• 156)  Ago Y, Yano K, Hiramatsu N, Takuma K, Matsuda T. Fluvoxamine enhances prefrontal dopaminergic neurotransmission in adrenalectomized/castrated mice via both 5-HT reuptake inhibition and σ₁ receptor activation. Psychopharmacology (Berl.), 217, 377–386 (2011).
• 157)  Hiramatsu N, Ago Y, Hasebe S, Nishimura A, Mori K, Takuma K, Matsuda T. Synergistic effect of 5-HT_1A and σ₁ receptor activation on prefrontal dopaminergic transmission under circulating steroid deficiency. Neuropharmacology, 75, 53–61 (2013).