The present experiment was intended to clarify how the behavioral interaction between the male and female rats corresponds with electrical activities of the limbic system such as the olfactory bulb, the amygdala, and the dorsal hippocampus. Olfactory bulb and amygdala burst activities of the male rat appeared earlier than those of the female rat at the initial stage of copulatory behavior. This pattern continued for some time, but these limbic burst activity patterns tended to become very similar over time in both the male and female rats. In the male rat, these limbic burst activities showed an obvious decrease in amplitude during mounting behavior. Also, during sniffing of female anogenital area, these burst activities disappeared in the male rat. After ejaculation, these limbic burst activities disappeared gradually in both the male and female rats. The correlation of the limbic burst activities between the male and female rats was highest during copulatory behavior. Consequently, we conclude that during copulatory behavior there is an electroencephalographic similarity between the male and female rats.

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We examined the effects of preoptic knife cuts on estrogen-induced facilitation of locomotor activity in ovariectomized rats to study the possible involvement of the preoptic area in the facilitation of activity. Animals housed in a cage with a running wheel received ovariectomy and one of the following brain surgeries : anterior cut (AC), posterior cut (PC), or dorsal cut (DC) of the preoptic area. Running activity was significantly suppressed following the ovariectomy. The activity of animals with DC was significantly lower than that of the ovariectomized controls. One week after the surgery, they were treated with estradiol-17β using silastic capsules. Recovery of activity in the running wheel was found in animals with AC and in the controls, but the activity was still lower in both PC and DC animals. Open field tests were also carried out on 3 consecutive days starting on the 5th day following the treatment of estrogen. Only DC animals showed significantly lower activity in the open field tests than control animals. These results indicate that facilitation of wheel-running and open field activities by estrogen may be related to different neural mechanisms in the rat forebrain, and the preoptic area may be actively involed in estrogen-induced facilitation of wheel-running but not of open field activity.

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Using a CS1-Shock1-CS2-Shock2 paradigm, the attenuation of conditioned suppression during CS2 paired with Shock2 by immediately preceding CS1 + Shock1 or CS1-Alone was studied in four groups of rats in which the Shock1 intensity was varied across groups. Compared with the None condition, in which no stimuli preceded CS2, both CS1 + Shock1 and CS1-Alone attenuated conditioned suppression, the amount of attenuation being greater in the former than in the latter. The dis-suppression ratio, or the relative amount of suppression of CS1 + Shock1 and CS1-Alone conditions to the respective None condition of each group, indicated that the ratio increased as a function of Shock1 intensity in both conditinos. Present results are consistent with the opponent-process theory of emotion.

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It is abunduntly clear that there are major differences between acute and chronic pain. However, many animal studies for pain and nociception employ short-lasting noxious stimuli which may have limited relevance to prolonged or chronic pain states.
In this study, we provide a new rat tonic pain model developed in our laboratory which is characterized by recording rats hind paw flexion responses with the passage of time after dilute formalin injection. The response burst, 15 responses and more per minute, were constantly induced for at least 45 minutes, i. e., from 25 to 70 minutes after formalin injection, and were considered to be due to tonic nociception of the animal because of their high sensitivity to morphine.
A few animals showed poor responses (5 responses or less per minute) consistently. The inhibition of formalin-induced flexion responses observed in these animals was estimated to be mediated by endogenous opioid systems, for those inhibitions were antagonized by naloxone, an opioid antagonist.

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