Korehito Yamanouchi

Role of Septum and Preoptic Area in Regulating Masculine and Feminine Sexual Behavior in Male Rats

The effects of septal or preoptic lesions on both masculine and feminine sexual behaviors were examined in castrated adult male rats. Three weeks after brain surgery, animals were implanted with Silastic tubes containing testosterone (T) and observations of masculine sexual behavior were carried out four times every 5 days. T tubes were removed immediately after the end of the masculine behavioral tests. Two weeks later, animals implanted with Silastic tubes containing estradiol-17 beta(E2) were subjected to three feminine sexual behavioral tests at 5-day intervals. The bilateral lateral septal lesion (LSL) and the medial preoptic lesion (MPOL) effectively suppressed the performance of mounts, intromissions, and ejaculations, whereas the medial septal lesion (MSL), the dorsolateral preoptic lesion (DPOL), and the sham operation did not show any significant suppression of these behaviors. In the feminine sexual behavioral tests, intact and sham-operated control males showed only a low lordotic activity. However, the performance of the lordosis reflex was markedly facilitated by LSL or DPOL, while the lordotic activity of MSL and MPOL males was not significantly different from that of control males. These results suggest that the lateral septum exerts not only a facilitatory influence on masculine sexual behavior but also an inhibitory influence on feminine sexual behavior in male rats. On the other hand, the medial preoptic area may play a critical role in regulating masculine sexual behavior in male rats.

Inhibitory and facilitatory neural mechanisms involved in the regulation of lordosis behavior in female rats: Effects of dual cuts in the preoptic area and hypothalamus

Abstract Two types of cuts were performed at a 4–5 week interval in the same ovariectomized rats; horizontal half-circle cut located just above the anterior commissure (ARD) and half-dome cut located anterior to the ventromedial nucleus (AD). Behavioral tests were carried out following the pretreatment with estradiol benzoate for 3 days and progesterone on the fourth day. When females received AD first (Experiment 1) the mean LQ was significantly lower than that of controls without brain surgery. Then, the AD rats were subjected to ARD or ARD sham. At the second test, the mean LQ of AD-ARD rats increased to the control level, but the LQ of AD-ARD sham rats was still low. In the experiment 2, the order of the brain surgery was just reversed. In the first test, all ARD females showed high levels of lordosis and the mean LQ was higher than that of control. Then, these ARD females receiving AD or AD sham were subjected to the second test. The mean LQ of ARD-AD rats decreased to the level of the control, but the LQ of ARD-AD sham rats was still high. Thus, dorsal neural inputs to or through the preoptic area and hypothalamus may exert inhibitory influences on a lordosis mediating system and anterolateral outputs of the medial basal hypothalamus appear to be concerned with a lordosis facilitating mechanism. These two systems seem to have a mutual correlation in regulating lordosis response. However, ARD or AD could not completely reverse the suppressive effect of AD or the facilitatory effect of ARD in the animals with dual cuts. It is suggested that the dorsal extrahypothalamic inhibitory influence and the hypothalamic facilitatory influence can regulate the display of lordosis independently in female rats.

The septum as origin of a lordosis-inhibiting influence in female rats : effect of neural transection

The septum as the origin of a lordosis-inhibiting influence in female rats has been studied. Prior to testing, brain surgery was performed. Some rats were given a ventromedial cut (VMC) just above the anterior commissure, other rats, a posterior transverse cut (PC) or a dorsal horizontal cut (DC) just below the corpus callosum, and still other rats, a bilateral ventrolateral cut (VLC) in the ventrolateral septum and the bed nucleus of the stria terminalis (BST). Additionally, castrated females and those with a VLC sham operation served as castrated controls and sham-operated controls, respectively. All rats were given a daily dose of 0.2 micrograms of estradiol benzoate (EB) for three days and 0.5 mg of progesterone just prior to testing. The castrated controls evidenced no lordotic response. Lordotic activities also were very low in the surgically operated groups, with the exception of the VLC rats who demonstrated significantly high lordotic scores following the administration of 0.2 micrograms of EB. Two further testings, with the EB increased to 0.5 micrograms, were conducted, the interval between these tests being 2 weeks, and the VLC rats again demonstrated significantly higher lordotic activity than did the other rats. These results suggest that the septum exerts a lordosis-inhibiting influence, and that the ventrolateral outputs play a critical role in sending this inhibitory signal.

Forebrain and lower brainstem participation in facilitatory and inhibitory regulation of the display of lordosis in female rats

Neural transection of the dorsal extrahypothalamic descending afferents by means of an L-shaped Halász knife at the anterior commissure (anterior roof deafferentation. ARD) markedly potentiated the display of lordosis and soliciting behaviors. Bilateral lesions of the ventromedial hypothalamus (VMH) attenuated lordotic activity in the ARD sham females but not in the ARD females. In contrast, the lesions in the pontine central gray concurrently with ARD effectively inhibited the display of lordosis but not soliciting behaviors. These results suggest that the VMH may not be a primary focus of the dorsal extrahypothalamic inhibitory influence on lordosis. The influence of this inhibitory system seems to be dominant in regulating the expression of lordosis behavior, compared to that of the hypothalamic lordosis facilitating system. Furthermore, the dorsal extrahypothalamic inhibitors influence which could be removed by ARD must be modified by the neural mechanism in the lower brain stem in which the pontine central gray may be actively involved.

Dissociation between the display of lordosis and soliciting behaviors in female rats with lesions of the dorsomedial pontine tegmentum

Lesions of the dorsomedial tegmentum (DMTL) between the midbrain pontine junction and the middle level of the pons effectively eliminate the induction by estrogen-progesterone of lordosis behavior in ovariectomized rats. However, soliciting behaviors such as ear wiggling and hopping were not inhibited by this type of lesion. The common damaged area in DMTL rats which failed to show lordosis was the medial periventricular gray. The lesions placed in the caudal pontine central gray were not effective in suppressing lordosis response. Lesions of the ventromedial tegmentum (VMTL) were also ineffective in suppressing lordosis. Most of the animals with the VMTL showed soliciting behaviors. In these rats, the incidence of lordosis and lordosis quotient (LQ) were comparable to those of sham operated rats. When bilateral lesions were placed in the lateral tegmentum region, the mean LQ and incidence of soliciting behavior were not significantly different from those of sham operated controls. These results suggest a clear dissociation of the regulatory mechanisms between the display of lordosis and soliciting behaviors at the pontine level.

Recovery of sexual receptivity in female rats with lesions of the ventromedial hypothalamus

Abstract Bilateral lesions of the hypothalamic ventromedial nucleus (VMN) effectively eliminated the induction by estrogen-progesterone of lordosis behaviors 4 to 6 weeks after the brain surgery. However, recovery of the lordosis response was clearly evident in the rats with effective VMN lesions 2 weeks after the first test. This suggests that the VMN may not be essential to the expression of hormonally induced lordosis behavior, but may be involved in a supplemental lordosis mediating mechanism.

Hypothalamic laterality in regulating gonadotropic function: Unilateral hypothalamic lesion and ovarian compensatory hypertrophy

Regardless of the side of hemiovariectomy, unilateral lesion placed in the right-side medial anterior hypothalamus suppressed ovarian compensatory hypertrophy, but the lesion made in the left side failed to suppress it. This suggests the presence of a hypothalamic laterality in regulating gonadotropin secretion.

Neural transection between preoptic area and septum inhibits maternal behavior in female and male rats

Artificially-induced maternal behavior was examined in inexperienced female and male rats following neural transection between the preoptic area and the septum. Anterior roof deafferentation (ARD) of the preoptic area (POA) was performed with an L-shaped microknife. Virgin female and male rats with or without ARD were exposed to the continuous presence of normal pups to induce maternal behavior 3 weeks after surgery. ARD resulted in a significant delay of the onset of maternal behavior in females and a lack of complete development of maternal behavior in males, compared with corresponding sham-operated and intact controls. The results indicate that neural substrates dorsal to the POA play an important role in modulating the expression of maternal behavior in the rat.

The role of mesencephalic tegmentum in regulating female rat sexual behaviors

Feminine sexual behaviors were tested in estradiol benzoate (EB) and progesterone (P) primed ovariectomized rats following four types of radiofrequency lesions in the midbrain tegmentum. The dorsomedial lesion (DML) which destroyed the ventromedial central gray including the dorsal raphe nucleus and adjacent area induced high sexual receptivity in the females primed with low dose (0.5 micrograms) of EB-P. All females with DML exhibited lordosis and ear wiggling, the mean lordosis quotient (LQ) being significantly higher than that of castrated controls or sham operated rats. Sexual receptivity in females with ventromedial tegmental lesion was not significantly different from those of the control and sham groups. In contrast to the medially lesioned groups, the mean LQ was low in the animals with bilateral lateral tegmental lesions even when the dose of EB was increased to 2 micrograms which was sufficient to induce high sexual receptivity in castrated and sham operated control females. In the animals with dorsolateral tegmental lesions (DLL), a much more severe loss of lordosis was seen than in those with ventrolateral tegmental lesions (VLL). None of the DLL females displayed sexual behavior throughout the present experiments. These results lead us to conclude that the midbrain dorsomedial tegmental area (ventral central gray and the adjacent area) is concerned with female sexual behavior inhibiting system, whereas the lateral tegmental area may be involved in the facilitatory system.

P-chlorophenylalanine facilitates copulatory behavior in septal lesioned but not in preoptic lesioned male rats.

Effects of p‐chlorophenylalanine (pCPA), synthesis‐inhibitor of serotonin, on copulatory behavior were examined in testosterone (T) treated castrated male rats with lesions in the lateral septum (LSL) or in the medial preoptic area (MPOL). Three weeks after the operation, all animals were chronically treated with T using silastic capsules. Behavioral tests were carried out at 10 and 20 days after the implantation of T. In half of the males in each group, 100 mg/Kg pCPA was injected daily for 4 days before each test. The other half of the group was injected with saline instead of pCPA. In saline‐treated groups, the males with LSL or MPOL showed extremely lower frequencies of mount and intromission than control males without brain surgery. PCPA‐treated control males showed higher copulatory activities than saline‐treated control males. In contrast, even after the treatment with pCPA, severe impairment of sexual behavior was observed in males with MPOL. On the other hand, pCPA‐treated LSL males displayed copulatory behavior more frequently than saline‐treated LSL males. These results agreed with previous reports that both the medial preoptic area and the lateral septum play an excitatory role in regulating male sexual behavior. Conversely, the serotonergic neuronal system exerts an inhibitory influence for male sexual behavior. Furthermore, these results suggest that the inhibitory function of serotonergic neurons govern dependently on the function of the medial preoptic area, but independently on the function of the lateral septum.