In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by α2-adrenergic and μ-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and α2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.
The present study was undertaken to determine whether aprindine, a class Ib antiarrythymic agent, exerts beneficial effects on ischemia/reperfusion-induced cardiac contractile dysfunction and metabolic derangement. Isolated rat hearts were subjected to 35-min global ischemia, followed by 60-min reperfusion, and functional and metabolic alterations of the heart were determined with or without aprindine-treatment. Ischemia induced a cessation of left ventricular developed pressure (LVDP), a rise in left ventricular end-diastolic pressure (LVEDP), and an increase in myocardial sodium content and a decrease in myocardial potassium content. When the hearts were reperfused, little recovery of LVDP and sustained rise in LVEDP and perfusion pressure were observed. Ischemia/reperfusion resulted in a release of ATP metabolites and creatine kinase from perfused hearts, an increase in myocardial sodium and calcium contents, and a decrease in myocardial potassium and magnesium contents. Treatment of the perfused heart with either 10 or 30 μM aprindine for the last 3 min of pre-ischemia improved contractile recovery during reperfusion and suppressed changes in myocardial ion content during ischemia and reperfusion. Treatment with the agent also attenuated the release of ATP metabolites and creatine kinase from the heart. However, treatment with high concentrations of aprindine (70 and 100 μM) improved neither cardiac contractile dysfunction, myocardial ionic disturbance nor the release of ATP metabolites and creatine kinase during reperfusion. Two possible mechanisms for the cardioprotection by the agent have been suggested: suppression of transmembrane flux of substrates and enzymes, and prevention of accumulation of myocardial sodium during ischemia.
Two-dimensional images of cytoplasmic and nuclear free Ca2+ movements in cardiac myocytes were obtained at 67-msec intervals using a Ca2+-sensitive fluorescence probe, indo-1, and a rapid scanning confocal laser microscope, Nikon RCM8000. Isolated guinea pig ventricular cells were loaded with indo-1 and stimulated at 0.5 Hz through patch pipettes. On stimulation, nuclear Ca2+ concentration ([Ca2+]) was observed to rise and fall following cytoplasmic [Ca2+] with an obvious delay. Application of isoproterenol significantly increased the peak [Ca2+] on stimulation in both the cytoplasm and nucleus with no substantial change in the basal [Ca2+]; the increase in peak [Ca2+]produced by application of isoproterenol was larger in the cytoplasm than in the nucleus. Under a low [Na+] condition, the basal [Ca2+] was increased from the control values in both the cytoplasm and nucleus; no difference in basal [Ca2+] was observed between the two regions. The increase in peak [Ca2+] by low [Na+] in the cytoplasm was significantly larger than that in the nucleus. When the cells were voltage clamped at 0 mV for 3 sec, no difference in the steady state [Ca2+] was observed between the cytoplasm and nucleus. Nuclear [Ca2+] was also observed to increase following a Ca2+ wave, a local increase in [Ca2+] propagating within the cytoplasm, with a delay. Thus, we demonstrated in isolated myocardial cells that cytoplasmic Ca2+ movements, although hampered by the nuclear envelope, are propagated into the nucleus, a mechanism through which factors affecting cytoplasmic Ca2+ may influence intranuclear events.
Alteration of hepatic microcirculation and its effects on hepatic metabolism were examined using oxethazaine (OXZ). The infusion of OXZ into isolated perfused livers rapidly increased the portal perfusion pressure (PP) and inhibited oxygen (O2) uptake, which was followed by a decrease in tissue ATP content and an increase in lactate, pyruvate and glucose release into the perfusate. P-450-dependent reductive drug metabolism was enhanced by OXZ, whereas oxidative drug metabolism was suppressed, and this was accompanied by a decrease in substrate uptake. During OXZ infusion, a time delay between the inhibition of O2 uptake and the release of cellular and xenobiotic metabolites was observed. The actions of OXZ required Cat+. It is unlikely that the inhibition of O2 uptake is due to the inhibition of cellular respiration. The PP increase induced by OXZ was inhibited by papaverine, but not by prazosin, sodium nitroprusside and verapamil, whereas all of these vasodilators were effective against norepinephrine. Under retrograde perfusion, the PP increase by OXZ was abolished, but norepinephrine, uridine 5 triphosphate, angiotensin II and endothelin 1 were still effective. The extrahepatic portal vein preparation contracted at high concentrations of OXZ. The results suggest that OXZ acts differently from other known vasoconstrictors and possibly narrows hepatic sinusoids to reduce the rate of substance exchange between the sinusoids and hepatocytes, including a reduction in O2 extraction.
The influence of dosing time on the embryotoxicity of sodium valproate (valproic acid, VPA) was investigated in ICR mice under a light-dark (12 : 12) cycle. A significant circadian rhythm was shown for VPA-induced embryotoxicity, with the highest value at 1700 and the lowest at 0100. A similar pattern of rhythm was also shown for VPA-induced toxicity in pregnant and nonpregnant mice. No significant dosing time-dependent difference between injection at 1700 and 0100 was demonstrated for VPA concentrations in the embryo, plasma and brain. The rhythm in the embryotoxicity seems to be related to the rhythm in the sensitivity of the embryo and dam to the drug. Embryotoxicity and VPA concentrations were significantly higher on gestational day 13 than day 7. The pharmacokinetics of VPA contribute, at least partly, to the gestational stage-dependent embryotoxicity of VPA. The timing of drug administration (i.e., gestational stage and circadian phase) appears to be essential for studies on the embryotoxicity of VPA in mice.
Endothelin (ET) receptors involved in ET-1-induced responses of the longitudinal muscle of the isolated guinea pig ileum were studied. ET-1 caused concentration-dependent contractions, while ET-3 and selective ETB-receptor agonists, IRL1620 and sarafotoxin 6c (S6c), showed little or no effect. The ET1-induced contractions were antagonized by BQ-123, an ETA-receptor antagonist, or PD142893, an ETA/ ETB-receptor antagonist, indicating that the contraction is mediated by the ETA receptor. In preparations precontracted with carbachol, ET-1 elicited relaxations at lower concentrations and contractions at higher concentrations. ET-3, IRL1620 and S6c caused relaxations. These relaxations were little affected by BQ-123, but greatly antagonized by PD142893. The ET-1-induced relaxations were slightly affected by BQ-788, an ETB-receptor antagonist, but were markedly inhibited by the combination of BQ-788 and BQ-123. In ETB receptor-desensitized preparations, ET-1-induced relaxations were antagonized by BQ-123, whereas ET-3, S6c and IRL1620 showed no response. All these relaxations were abolished by apamin. These results indicate that ETA and ETB receptors mediate relaxation of the ileal smooth muscle through activation of apamin-sensitive K+ channels.
By dye leakage in mouse skin, we evaluated the inhibition of proinflammatory stimuli-induced plasma extravasation by a putative inhibitor of inducible nitric oxide synthase, S-ethylisothiourea. A low dose of S-ethylisothiourea (5μg/kg) mimicked aminoguanidine in inhibiting the plasma extravasation elicited by lipopolysaccharide but not by 5-hydroxytryptamine or platelet-activating factor. A higher dose of S-ethylisothiourea (10μg/kg) inhibited the plasma extravasation induced by 5-hydroxytryptamine slightly; however, it increased the basal dye leakage. Thus, S-ethylisothiourea may be used as a relatively specific inhibitor for inducible nitric oxide synthase in vivo.
Contractile responses and fura-2 fluorescence signals were simultaneously recorded in fura-2 loaded longitudinal muscle strips of guinea pig vas deferens to examine the relationship between cooperation of ATP and norepinephrine (NE) in the contractile response and intracellular Ca2+level. ATP or NE induced a rapid or delayed contraction, respectively, with concomitant changes in fura-2 signal. Addition of both agonists potentiated the rapid, but not delayed contraction, while there was no potentiation in the fura-2 signal. In another series of experiments, NE (≥10 μM) contracted the muscle without further concomitant increase in Ca2+ level, indicating “Ca2+ sensitization” of the contractile apparatus. These findings suggest that cooperation of ATP and NE in inducing a rapid contraction of guinea pig vas deferens is mainly due to the “Ca2+ sensitization” effect of NE.
We investigated the effect of KW-4679 ((Z)-11-[(3-dimethylamino)propylidene]-6, 11-dihydrodibenz[b, e]oxepin-2-acetic acid monohydrochloride), an orally active anti-allergic drug, on antigen-induced airway hyperresponsiveness using two different indicators, pulmonary resistance (RL) and dynamic lung compliance (Cdyn), in actively sensitized guinea pigs. Oral administration of KW-4679 (0.1 and 1 mg/kg) 1 hr before aerosolized antigen exposure significantly inhibited the development of airway hyperresponsiveness to inhaled acetylcholine in RL and Cdyn in a dose-dependent manner. Terfenadine (10 mg/kg) also inhibited the development of airway hyperresponsiveness. These results indicate that KW-4679 could be useful in the treatment of bronchial asthma.
Effects of nordihydroguaiaretic acid (NDGA) on the Ca2+-dependent K+ channel (BK channel) were examined by the patch clamp technique in single smooth muscle cells of porcine coronary artery. The open probability of BK channels in inside-out patches increased by about 30 times at the holding potential of 0 mV, when 10μM NDGA was added to the bathing solution (pCa 7.0). The effect of NDGA was concentration-dependent in the range of 1—100μM and partly removed by washout. The enhancement of BK channels by NDGA was not observed when the cytosolic Ca2+ concentration was very low (pCa > 8.5). These results clearly indicate that NDGA possesses a BK channel opening property in coronary arterial myocytes.