Effects of sequential preoptic and mammillary lesions on male rat sexual behavior

Central inhibition of sexual response in the male: a theoretical perspective

A theoretical model for central inhibition of sexual response is proposed, postulating individual variability in the propensity for such inhibition. Whereas such inhibition is typically adaptive, individuals with high propensity may be vulnerable to sexual dysfunction, and those with low propensity to high risk sexual behavior. Evidence of the existence and localization of such inhibitory mechanisms from both the animal and human literature is reviewed. Evidence of central neurotransmitters with sexual inhibitory effects is substantial, though in most cases the inhibition is not specific to sexual response or behavior. Recent studies have identified centers in the brain stem and lateral hypothalamus which appear to have specific inhibitory effects on sexual response. A variety of adaptive mechanisms involving inhibition of sexual response are considered, some involving perception of threat, others occurring more directly as consequences of previous sexual activity. These different adaptive functions may well involve different inhibitory mechanisms. This theoretical model opens a new agenda for experimental research into adaptive sexual behavior, both human and animal.

Diurnal rhythm in cyclic GMP/nitric oxide efflux in the medial preoptic area of male rats

Since nitric oxide (NO) is implicated in the neuroendocrine control of luteinizing hormone-releasing hormone (LHRH) secretion and sexual behavior which show diurnal variations, we monitored cGMP levels (an index of NO activity) in the extracellular compartment of the medial preoptic area (MPOA) using microdialysis. It was observed that MPOA cGMP levels rose significantly in the afternoon in both castrated and intact male rats, thereby suggesting the existence of a diurnal rhythm in MPOA cGMP/NO efflux which may participate in eliciting the well-known diurnal variations in LHRH neuronal activity and male sexual behavior.

Implications of immediate-early gene induction in the brain following sexual stimulation of female and male rodents

Induction of immediate-early genes (IEGs), such as c-fos, has been widely used to mark the activation of brain regions following different types of sexual stimulation and behavior. A relatively common set of hormone-concentrating basal forebrain and midbrain structures in female and male rodents is activated by copulatory stimulation, in particular, stimulation of sensory nerves that innervate the penis or vagina/cervix, olfactory or pheromonal stimuli, and conditioned sexual incentives. These regions include the preoptic area, lateral septum, bed nucleus of the stria terminalis, paraventricular hypothalamus, ventromedial hypothalamus, medial amygdala, ventral premammillary nuclei, ventral tegmentum, central tegmental field, mesencephalic central gray, and peripeduncular nuclei. Regions that do not contain classic intracellular steroid receptors, such as the ventral and dorsal striatum or cortex, are also activated. IEGs have also been colocalized with cytoplasmic proteins like GnRH and oxytocin, and have been used in conjunction with retrograde tracers to reveal functional pathways associated with different sexual behaviors. Steroid hormones can also alter the ability of sexual stimulation to induce IEGs. Despite the many similarities, some differences in IEG induction between sexes have also been found. We review these findings and raise the question of what IEG induction in the brain actually means for sexual behavior, that is, whether it indicates the perception of sexual stimulation, commands for motor output, or the stimulation of a future behavioral or neuroendocrine event related to the consequences of sexual stimulation. To understand the role of a particular activated region, the behavioral or neuroendocrine effects of lesions, electrical stimulation, drug or hormone infusions, must also be known.

Importance of the medial amygdala in rat penile erection evoked by remote stimuli from estrous females

Effects of medial amygdala lesions (MAL) were examined on rat penile erection in three different experimental situations. Only sexually vigorous males, as identified by preoperative mating tests, were used. Bilateral radiofrequency lesions were confined to the posterior medial amygdala, with little systematic damage to anterior medial amygdala or to adjacent structures. Lesion electrodes were withdrawn without current application in sham-operated animals (SHAM). After recovery for brain surgery, males were tested for (1) noncontact erection (NCE) that occurs when males were placed in proximity to inaccessible estrous females, (2) reflexive erection evoked in supine males by retraction of the penile sheath, and (3) copulatory behaviour with receptive females. In the NCE test, none of the MAL males showed penile erection during the 20 min observation, whereas 70% of the SHAM males showed it (P < 0.001). In contrast, no erectile dysfunction in the MAL males was detected in the other two tests. MAL males displayed more penile-body erections (flips) than SHAM males in the reflexive-erection test (P < 0.05). In the copulation test, most of the MAL males achieved intromission, but their intromission ratio, a partial measure of erectile function, was marginally lower than that of SHAM males (P = 0.051). MAL males had longer intervals between intromissions (P < 0.001); as a result, none of them ejaculated during the 20 min period that followed the first intromission. The results suggest that the posterior medial amygdala plays an essential role in the regulation of NCE, and it may also contribute to the regulation of erection in other contexts.

Facilitation of copulatory behavior by pCPA treatments following stria terminalis transection but not medial amygdala lesion in the male rat

In this study, we examined the effect of p-chlorophenylalanine (pCPA), a serotonin synthesis inhibitor, on masculine sexual behavior in male rats following bilateral medial amygdala lesions (MAL) or bilateral stria terminals cuts (STC). Males with MAL and STC both showed a suppression of sexual behavior, compared to that of sham-operated males, when injected with saline. On the other hand, intraperitoneal injections of pCPA increased mount and intromission frequencies in males with STC. Although pCPA injections also slightly increased mount frequency in males with MAL, the injections failed to affect intromission behavior in MAL males. These suggest that the stria terminalis is insufficient as pathway of a whole output of the medial amygdala in regulating copulatory behavior. The medial amygdala may be involved in some other function, such as erection, than that of the stria terminalis.

The medial preoptic area is involved in both sexual arousal and performance in male rats: re-evaluation of neuron activity in freely moving animals

A total of 74 single unit activities was recorded from the medial preoptic-anterior hypothalamic continuum (MPOA) during free copulatory behavior of male rats. Forty-six units (62.2%) showed changed activities during at least one phase of male copulatory movements; 26 units (35.1%) increased in the firing rate during pelvic thrusting; 32 units (43.2%) increased in activity during backward jumping immediately after intromission; only one unit (1.4%) showed decreased firing rate during thrusting and backward jumping; 12 units (16.2%) increased in activity during pursuit of a female; 19 units (25.7%) were suppressed during genital grooming. Furthermore, 67 units (90.5%) showed a significant change in activity throughout a series of copulatory behavior. From the introduction of a female up to ejaculation, relatively large number of units increased in the firing rate above the value during pre-introduction adaptation period. During postejaculatory interval, however, most units decreased in activity below the level during copulation. These results strongly suggest that the MPOA is involved in both sexual arousal and performance in male rats.

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. 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.

Socio-sexual behavior in male rats after lesions of the medial preoptic area: evidence for reduced sexual motivation

Different hypotheses have been put forward trying to explain the mechanisms associated with the disruption of male sexual behavior after lesions of the medial preoptic area (MPOA). It has been suggested that sexual motivation, motor execution or both are affected by MPOA lesions. In the present experiment, the socio-sexual behavior of male rats bearing extensive MPOA lesions, that abolished sexual behavior, was compared with that of sham-lesioned animals and to prelesion levels. The socio-sexual interactions were recorded for 10 min in one prelesion and two postlesion tests. The frequency and duration of the following behaviors were recorded: rearing, sniffing, self-grooming, grooming partner, genital exploration, pursuit and resting. The analysis of the socio-sexual interactions showed that the frequency and duration of pursuit was reduced in the first and second tests after the lesion in comparison to both prelesion levels and to a sham-lesioned group. There is strong evidence that pursuit is the only precopulatory behavior that can consistently predict the appearance of sexual behavior. When pursuit is reduced the transition from the precopulatory to the copulatory phase is made more difficult. Therefore, it appears that the MPOA lesions reduce the subject's motivation to engage in sexual behavior.

Lesions of the medial amygdala produce severe impairment of copulatory behavior in sexually inexperienced male rats

The effects of amygdaloid lesions on masculine copulatory behavior were examined in male rats. Sexually inexperienced male rats were castrated and subjected to bilateral lesions in one of the following areas: the medial amygdala, the cortical amygdala, or the basolateral amygdala. Three weeks later, all rats received implantation of silastic capsules containing testosterone. Then, four observations of copulatory behavior were carried out every 5 days following the implantation of testosterone. Rats with medial amygdala lesions showed a severe deficit of copulatory behavior, whereas rats with basolateral amygdala lesions showed no change in the performance of copulation. As for rats with cortical amygdala lesions, although their copulatory behavior was impaired, the effect was confined to a deficit in intromission and ejaculation responses. These findings suggest that the medial amygdala plays a critical role in regulating masculine sexual behavior in the rat.

Testosterone in MPOA elicits behavioral but not neuroendocrine responses in ferrets

The amount of time male ferrets were engaged in neck gripping, mounting, and thrusting was quantified in 30-min tests with a receptive female before and after castration. Bilateral cannulae containing a total of approximately 2 mg testosterone propionate (TP) in cocoa butter were then stereotaxically aimed at the medial preoptic area (MPOA). Tests for sexual behavior were conducted on days 3, 7, 14, and 21 postimplantation. Ferrets were histologically categorized as either 1) Miss (implants not in MPOA), 2) Unilateral implant in MPOA, or 3) Bilateral implants in MPOA. The mean amount of time spent neck gripping, mounting, and thrusting increased significantly over castrate levels on postimplantation day 14 in the Bilateral group, but not in the Miss or Unilateral groups. In all groups, mean plasma testosterone concentrations were below or near the lower limit of detectability on the day before TP implantation and on postimplantation test days. In the same plasma samples, luteinizing hormone concentrations were within the normal range of castrated ferrets, and did not significantly decline after TP implantation. These results suggest that the MPOA is a neural site for androgen activation of certain components of reproductive behavior but not for negative feedback on gonadotropin secretion in male ferrets.

Genital responses in noncopulators and rats with lesions in the medical preoptic area or midthoracic spinal cord

To determine whether genital dysfunction participates in the sexual inactivity of noncopulator and MPOA-lesioned rats, penile reflexes and spontaneous seminal emission (SSE) were investigated in such animals. The display of penile reflexes was either the same or enhanced in these animals, relative to controls. Noncopulators and MPOA-lesioned rats also did not differ from controls in SSE. Plasma testosterone levels showed no differences between noncopulators and sexually active controls but tended to be elevated in rats bearing MPOA lesions. Finally, SSE did not differ between rats with midthoracic spinal cord transections and controls, suggesting that SSE is not under supraspinal inhibition.

Effects of two-stage lesions of the medial preoptic area on sexual behavior of male rats

The effects of lesions of the medial preoptic area (MPOA) on sexual behavior of adult male rats were examined. Bilateral lesions were performed in either one or two stages. Bilateral, but not unilateral, destruction of the MPOA virtually eliminated mounts, intromissions, and ejaculation. No sparing of the male rats' sexual behavior was observed after two-stage MPOA lesions.

Intermale social aggression: suppression by medial preoptic area lesions

The intermale social aggressive behavior of male rats cohabiting with a female rat was quantitatively scored weekly in response to the introduction of an unfamiliar intruding male. Resident male rats whose aggressiveness toward an intruder reached a criterion level were subjected to either sham lesions or bilateral lesions in the region of the medial preoptic area. The lesioned rats continued to exhibit levels of piloerection and lateral attack that were not significantly lower than those of sham-lesioned animals. However, the lesioned animals did emit significantly fewer bites and spent significantly less time in the "on-top" position than did sham-lesioned animals. The lesioned animals also displayed significantly less sexual behavior than the sham-lesioned animals but were not different in terms of defensiveness toward the experimenter. It is suggested that bilateral lesions in the region of the medial preoptic area cause a decrease in the intensity of intermale social aggression but do not prevent external stimuli from eliciting the aggression.

Neural control of the daily rhythm of sexual behavior in the male golden hamster

Circadian and neural mechanisms important for the organization of reproductive behavior in the male golden hamster were examined. The sexual behavior of male hamsters exhibits diel variations; males are quicker to initiate copulation and to ejaculate in the dark phase than in the light phase of a daily light-dark cycle. The copulatory rhythm is endogenously generated and persists under constant environmental conditions. Destruction of the suprachiasmatic nuclei (SCN) eliminated the normal diurnal rhythm of sexual behavior without affecting copulation per se. In contrast to SCN lesion effects, damage to the medial preoptic area (MPOA) reduced or eliminated copulation; in those MPOA-ablated animals that continued to copulate, the circadian modulation of sexual behavior remained intact.

Medial preoptic connections with the midbrain tegmentum are essential for male sexual behavior

The medial preoptic area appears to play a major role in the control of sexual behavior. Efferents from the medial preoptic area course through the medial forebrain bundle to pass through and/or terminate in the dorsolateral and ventral tegmentum of the midbrain. Bilateral lesions of the dorsolateral tegmentum eliminate mating behavior in male rats, reproducing the effect of bilateral medial preoptic lesions. Sexual behavior is also eliminated when a preoptic lesion on one side of the brain is combined with a lesion of the dorsolateral tegmentum on the other side of the brain. In other words, asymmetric brain damage which bilaterally destroys the preoptic connections with the dorsolateral tegmentum eliminates male sexual behavior, and we conclude that the connections between these two regions are essential for copulation.

Effects of postweaning rearing condition on recovery of copulatory behavior from lesions of the medial preoptic area in rats

Bilateral lesions of the medial preoptic area (MPOA)-anterior hypothalamus of adult male rats markedly disrupt male copulatory behavior. In contrast, more group--reared then isolated male rats receiving bilateral MPOA lesions as juveniles copulated to ejaculation as adults. The present experiment was designed to analyze the role of rearing condition in promoting copulatory recovery from MPOA lesions in juvenile male rats. Juvenile male rats were given bilateral MPOA lesions or a sham operation and reared in isolation without handling, in isolation with daily handling, across a perforated Plexiglas divider from a male peer, or together with a male peer (social). Socially reared and handled males, but not isolated and divided males, with MPOA lesions showed evidence of copulatory recovery (combined socially reared and handled males vs combined isolated and divided males). These results support previous findings that postweaning rearing condition can affect copulatory recovery following juvenile MPOA lesions, but indicate that play experiences involving physical contact with peers are not necessary for such recovery to occur.

Effects of ibotenic acid-induced neuronal degeneration in the medial preoptic area and the lateral hypothalamic area on sexual behavior in the male rat

It is well known that electrolytic lesions in the medial preoptic area (MPOA) and the lateral hypothalamic area (LHA) seriously impair masculine sexual behavior in the rat. We here report that bilateral infusions of the neurotoxin, ibotenic acid (IBO), in the MPOA were as effective as electrolytic lesions in eliminating copulation whereas no behavioral effects were detected following similar infusions in the LHA. Histological examination of MPOA and LHA following IBO exposure revealed extensive degeneration of neuronal cell bodies with little evidence of non-specific damage. Also, immunohistochemical studies suggested that the serotonergic innervation of the MPOA remained largely intact in spite of IBO treatment; similarly, the damage inflicted by IBO in LHA on tyrosine hydroxylase-immunoreactive fibers in the medial forebrain bundle was insignificant. These data suggest that: (i) the functional integrity of MPOA nerve cell bodies is necessary for the expression of sexual behavior, and (ii) disruption of mating produced by electrolytic LHA lesions is due to disruption of medial forebrain bundle fiber systems. Behavioral observations of non-copulating males suggested that the MPOA injury did not interfere with all aspects of their sexual interaction with the estrous female; rather, they appeared specifically unable to perform the reflexive pelvic thrust pattern normally associated with mounting. We here report, however, that the ability to perform mounts with pelvic thrusts was temporarily restored in the vast majority of MPOA-injured males by the i.p. administration of the ergot derivative, lisuride. About 50% of these MPOA-damaged males even ejaculated, often after a low number of intromissions and short ejaculation latencies. On the other hand, injections of naloxone (an opiate receptor antagonist) failed to activate mounting in MPOA-lesioned or castrated rats. On the basis of these findings the possible ways in which steroid hormone-sensitive brain areas might interact with monoamine-containing pathways are discussed.U

Effects of medial preoptic lesions on male mouse ultrasonic vocalizations and copulatory behavior

In male mice, lesions in the medial preoptic area (MPA) reduced the proportion of animals showing mounting behavior but failed to abolish ano-genital exploration of the female. The lesions did not effect the amount of 70 kHz ultrasound elicited by intact females or by soiled bedding obtained from female-occupied cages. These results suggest components of male sexual behavior are mediated by neural systems outside the MPA.

The effect of medial preoptic--anterior hypothalamic lesions on bisexual behavior of the male rat

The effects of bilateral radiofrequency lesions in various parts of the medial preoptic area (mPOA) and anterior hypothalamus (AH) on masculine and hormone induced feminine sexual behavior was studied in male rats. Anatomical as well as statistical analysis of the extent and location of the lesions and the consequent behavioral changes indicated that different parts of the mPOA-AH continuum are involved in masculine and feminine sexual behavior in different ways. Bilateral destruction at the transition of the mPOA and AH resulted in a large reduction of masculine sexual activity while lesions concentrated in the AH slightly facilitated feminine sexual behavior in males. These results are consistent with the notion that the mPOA-AH continuum is divided into anatomically and functionally different parts.

Differential serotonergic innervation of individual hypothalamic nuclei and other forebrain regions by the dorsal and median midbrain raphe nuclei

Lesions in the midbrain median but not in the dorsal raphe nucleus significantly decreased the serotonin (5-HT) content of the hippocampus (61%), medial preoptic area (49%), suprachiasmatic nucleus (70%) and anterior hypothalamic area (60%). Electrolytic lesions restricted to either the median or dorsal raphe nucleus produced significant reductions in the 5-HT concentration of the caudate-putamen, anterolateral hypothalamic area (45%) and arcuate nucleus (48--58%). The fall in caudate-putamen 5-HT level was significantly greater after the dorsal (66%) than after the median (24%) raphe lesion. Neither lesion significantly affected the 5-HT content of the posterolateral hypothalamic area, the ventromedial hypothalamic nucleus or the dorsomedial hypothalamic nucleus. Thus, like their differential projections to the caudate-putamen and hippocampus, the mesencephalic dorsal and median raphe nuclei appear to innervate different preoptico-hypothalamic nuclei and areas. Whereas the median raphe nucleus seems to be the primary source of 5-HT fibers to the suprachiasmatic nucleus, anterior hypothalamic area and medial preoptic area, the 5-HT inputs to the anterolateral hypothalamic area and arcuate nucleus appear to derive from both the dorsal and median raphe nuclei.

Preoptic knife cuts and sexual behavior in male rats

Knife cuts aimed at severing either the medial-lateral or dorsal fiber connections of the medial preoptic-anterior hypothalamus (MPOAH) produced two clearly distinguishable patterns of copulatory deficits in male rats. On the other hand, transverse cuts, which were designed to destroy longitudinal connections on either the anterior or posterior end of the MPOAH, did not affect sexual behavior. Males with sagittal cuts exhibited deficits primarily in the initiation of copulation. In contrast, those with dorsal cuts showed alterations primarily in the execution of copulatory behavior. Combined behavioral and anatomical evidence was interpreted as suggesting that the MOPAH has at least two regulatory functions related to sex behavior: (1) it is involved in the mediation of the initiation of copulation through its lateral connections with the medial forebrain bundle, and (2) it influences the achievement of ejaculation and the subsequent refractory period through its dorsal connections with the amygdala and the hippocampus.

Plasma levels of testosterone and male sexual behaviour in strains of the house mouse (Mus musculus)

This paper investigates whether differences in measures of sexual behaviour between strains of the house mouse (Mus musculus) are related to differences in the levels of plasma testosterone. There was found to be a significant negative correlation between measures of sexual behaviour and plasma testosterone. This correlation was not behaviourally mediated, and could be demonstrated for some of the behavioural measures in an F2 generation.

Nembutal treatment of the VMH (rat): effects on feeding and sexual behaviour

In male rats two brain cannulae were implanted bilaterally and directed to an area just dorsal of the ventromedial hypothalamic nucleus. The behaviour of these animals was observed before and after the injection through these cannulae of either 1 micronl saline or 1 micronl Nembutal. Injections were performed under three behavioural conditions: (1) animal alone in cage with food, (2) animal in the presence of an oestrous female and (3) animal in cage with food and oestrous female. Following the Nembutal injection, sniffing and feeding are disinhibited temporarily in condition 1, whereas in condition 3 only sniffing, but not feeding, is disinhibited. In conditions 2 and 3 male sexual behaviour is not influenced by the Nembutal except for an increase of sniffing at the female. It is concluded that a temporary elimination of the VMH leads to a disinhibition of feeding only if the external conditions are favourable for feeding. The same elimination of the VMH does not influence the occurrence of male sexual behaviour under the conditions used in these experiments.

Sexual behavior in castrated male rats treated with monoamine synthesis inhibitors and testosterone

Castrated male rats treated daily with the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA, 20 mg/kg) started to display mounts, intromissions and ejaculations more rapidly in response to daily treatment with testosterone propionate (TP, 0.15 mg/kg) than NaCl-treated rats. Daily treatment with the catecholamine (CA) synthesis inhibitor alpha-methyl-p-tyrosine (alpha-MT, 20 mg/kg) had no effect on the behavioral response to subsequent TP treatment. The acceleration of TP-induced sexual behavior by PCPA pretreatment was inhibited by pretreatment with DL-5-HTP (20 mg/kg) but not with L-DOPA (12.5 mg/kg). Analyses of brain monoamines showed that the PCPA treatment reduced brain 5-HT levels and produced a marked inhibition of the 5-HT synthesis. The 5-HTP treatment restored brain 5-HT levels to normal. Daily treatment with PCPA also reduced brain CA levels and inhibited the CA synthesis but these biochemical effects were not related to the effects of PCPA on sexual behavior. Daily treatment with PCPA (40 mg/kg for 12 days) or treatment with 126 mg/kg PCPA for 3 days induced the complete pattern of sexual behavior in 5 of 9 and 19 of 30 castrated rats respectively without concurrent TP treatment. It is suggested that 5-HT exerts a modulating influence on sexual behavior in male rats.

Intracranial cycloheximide: effect on male mouse sexual behavior and plasma testosterone

Cycloheximide (Cyclo), an inhibitor of protein synthesis, infused bilaterally into the preoptic area (POA) of intact B6D2F male mice significantly inhibited male sexual behavior when the males were presented with receptive females 12 hr after treatment. The few males that ejaculated appeared to copulate normally. This finding suggests that Cyclo acts primarily by inhibiting sexual arousal rather than sexual performance. The inhibition of sexual behavior was not observed when the males were tested 84 hr after treatment. After exposure to an estrous female, plasma testosterone levels were measured in males with POA infusions of Cyclo or saline vehicle. No significant difference was found, but both groups had significantly higher levels of plasma testosterone than males not exposed to estrous females. It is suggested that the interference with sexual behavior by Cyclo was not due to interference with the neuroendocrine mechanisms controlling blood andorgen levels, but due to Cyclo acting directly on the neural circuits controlling sexual responsiveness.

Sexual inhibition is reduced by rostral midbrain lesions in the male rat

Electrolytic lesions in a circumscribed area of the rostral midbrain of rats shortened the inhibitory period following ejaculation, thereby increasing the number of ejaculations achieved in 1-hour tests. These lesions also interrupted the dorsal norepinephrine bundle as reflected in a 63 percent reduction in telencephalic or cortical norepinephrine.