Nitric oxide mediates glutamate-evoked dopamine release in the medial preoptic area

Dopamine (DA) release in the medial preoptic area (MPOA) of the hypothalamus is an important facilitator of male sexual behavior. The presence of a receptive female increases extracellular DA in the MPOA, which increases further during copulation. However, the neurochemical events that mediate the increase of DA in the MPOA are not fully understood. Here we report that glutamate, reverse-dialyzed into the MPOA, increased extracellular DA, which returned to baseline after the glutamate was removed. This increase was prevented by co-administration of the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester (L-NAME), but not by the inactive isomer, Nw-nitro-d-arginine methyl ester (D-NAME). In contrast, extracellular concentrations of the major metabolites of DA were decreased by glutamate, suggesting that the DA transporter was inhibited. These decreases were also inhibited by L-NAME, but not D-NAME. These results indicate that glutamate enhances extracellular DA in the MPOA, at least in part, via nitric oxide activity. Therefore, glutamatergic stimulation of nitric oxide synthase may generate the female-induced increase in extracellular DA in the MPOA, which is important for the expression of male sexual behavior.     

An NMDA antagonist in the MPOA impairs copulation and stimulus sensitization in male rats

Systemic injections of an NMDA antagonist have been shown to impair mating in male rats. One site where glutamate and its NMDA receptors may contribute to mating is the medial preoptic area (MPOA), which is vital for male sexual behavior. Glutamate is released in the MPOA during copulation, and especially at the time of ejaculation. We report here that the NMDA antagonist MK-801, microinjected into the MPOA, impaired copulatory behavior in sexually na?ve as well as experienced males. In rats tested both as na?ve and after sexual experience, drug treatment produced more profound impairment in na?ve males. In addition, MK-801, microinjected into the MPOA before each of 7 noncopulatory exposures to receptive female rats, resulted in copulatory impairments on a drug-free test on Day 8, relative to aCSF-treated rats; their behavior was similar to that of males that had not been preexposed to females. Therefore, NMDA receptors in the MPOA contribute to the control of copulation and stimulus sensitization. Glutamate, acting via NMDA receptors, regulates many neural functions, including neuronal plasticity. This is the first demonstration that a similar mechanism in the MPOA sensitizes male rats to the stimuli from a receptive female, and thereby enhances their behavior.     

MPOA lesions affect female pacing of copulation in rats

This study examined the effects of electrolytic and ibotenic acid (IA) lesions of the medial preoptic area (MPOA) on the temporal pattern of female sexual behavior in the laboratory rat. Both electrolytic and IA MPOA lesions significantly increased the female's latency to return to the male after an intromission or an ejaculation, thereby decreasing the percentage of time spent with a male. Both types of MPOA lesions significantly increased the percentage of times the female left the male's chamber following intromissions. These results demonstrate that neurons in the MPOA regulate the female's temporal copulatory behavior, and the authors suggest that they do so by virtue of their response to vaginocervical stimulation. Studies of female pacing draw attention to parallels between male and female sexual behaviors, including the possibility that they are regulated by similar neural substrates in the MPOA.     

Dopamine, the medial preoptic area, and male sexual behavior

The medial preoptic area (MPOA), at the rostral end of the hypothalamus, is important for the regulation of male sexual behavior. Results showing that male sexual behavior is impaired following MPOA lesions and enhanced with MPOA stimulation support this conclusion. The neurotransmitter dopamine (DA) facilitates male sexual behavior in all studied species, including rodents and humans. Here, we review data indicating that the MPOA is one site where DA may act to regulate male sexual behavior. DA agonists microinjected into the MPOA facilitate sexual behavior, whereas DA antagonists impair copulation, genital reflexes, and sexual motivation. Moreover, microdialysis experiments showed increased release of DA in the MPOA as a result of precopulatory exposure to an estrous female and during copulation. DA may remove tonic inhibition in the MPOA, thereby enhancing sensorimotor integration, and also coordinate autonomic influences on genital reflexes. In addition to sensory stimulation, other factors influence the release of DA in the MPOA, including testosterone, nitric oxide, and glutamate. Here we summarize and interpret these data.     

Effect of medial preoptic lesions on sexual behavior of female rats is determined by test situation

The sexual behavior of female rats with bilateral lesions in the medial preoptic area (MPOA) was examined in two testing conditions. In the first condition, in which the female could not leave the vicinity of males (no-exit test), lordosis quotients (LQs) were elevated in relation to baseline levels. In the second condition, in which the female could control her proximity to males (exit test), LQs were not different from control levels, and experimental subjects permitted fewer copulatory contacts, exhibited less frequent solicitational behavior, and spent less time with males than the controls did. These findings suggest that the higher LQs seen in no-exit tests as a result of MPOA damage are not due to a lesion-induced potentiation in the females' preference to engage in sexual contacts with males.     

Role of dopamine in anticipatory and consummatory aspects of sexual behavior in the male rat.

The ability of dopamine (DA) receptor antagonists to disrupt anticipatory and consummatory measures of sexual behavior displayed by male rats in bilevel chambers was investigated. In Experiment 1, systemic administration of haloperidol, pimozide, and the D1 antagonist SCH 23390 reduced the number of anticipatory level changes (LC) displayed during a 5-min period before the introduction of a sexually receptive female, increased the mount and intromission latencies (ML and IL), and decreased the number of intromissions before ejaculation (NI) and the total number of ejaculations (NE). The dosages of these drugs required to reduce the LC were lower than those required to increase the ML or IL. Clozapine and the D2 antagonist sulpiride reduced the LC and increased the IL at comparable dosages, although neither drug affected the NI or NE. High dosages of haloperidol, pimozide, and clozapine delayed or abolished level changing and the initiation of copulation. In Experiment 2, bilateral infusions of haloperidol into the nucleus accumbens reduced the LC but did not affect consummatory measures of copulation, whereas bilateral infusions into the dorsal striatum increased the NE. Midline infusions of haloperidol to the medial preoptic area (MPOA) produced nearly all the effects of systemic administration, including a reduced LC, increased ML and IL, a decreased NI, and a decreased NE. These results indicate that both anticipatory and consummatory measures of sexual behavior were disrupted by DA receptor antagonists; however, the measure of anticipatory sexual behavior was more sensitive to disruption than consummatory measures of copulation. DA in the nucleus accumbens and MPOA may be involved in the control of anticipatory sexual behavior, whereas in the MPOA it may also be involved in the initiation of copulation and copulatory rate.     

Characterization of copulatory behavior in female mice: evidence for paced mating

In this study we characterized female mouse sexual behavior using a pacing paradigm similar to that used to evaluate sexual behavior in female rats. A pacing chamber was designed for use with mice and we compared the sexual behavior of female mice that were tested in both pacing and nonpacing paradigms and under different hormone conditions. We found that, like rats, female mice do pace their copulatory behavior by altering the temporal sequence of copulatory events. Female mice take longer to return to the male after an ejaculation, compared to either a mount or intromission. However, it is still unclear if female-paced mating serves the same functions as it does in female rats. More work is needed to confirm that paced mating induces hormonal changes needed for pregnancy as is the case in rats.     

The role of the medial preoptic area in appetitive and consummatory reproductive behaviors depends on sexual experience and odor volatility in male Syrian hamsters

In Syrian hamsters (Mesocricetus auratus), the expression of reproductive behavior requires the perception and discrimination of sexual odors. The behavioral response to these odors is mediated by a network of ventral forebrain nuclei, including the medial preoptic area (MPOA). The role of MPOA in male copulatory behavior has been well-studied, but less is known about the role of MPOA in appetitive aspects of male reproductive behavior. Furthermore, many previous studies that examined the role of MPOA in reproductive behavior have used large lesions that damaged other nuclei near MPOA or fibers of passage within MPOA, making it difficult to attribute post-lesion deficits in reproductive behavior to MPOA specifically. Thus, the current study used discrete, excitotoxic lesions of MPOA to test the role of this nucleus in opposite-sex odor preference and copulatory behavior in both sexually-naïve and sexually-experienced males. Lesions of MPOA eliminated preference for volatile, opposite-sex odors in sexually-naïve, but not sexually-experienced, males. When males were allowed to contact the sexual odors, however, preference for female odors remained intact. Surprisingly, lesions of MPOA caused severe copulatory deficits only in sexually-naïve males, suggesting previous reports of copulatory deficits following MPOA lesions in sexually-experienced males were not due to damage to MPOA itself. Together, these results demonstrate that the role of MPOA in appetitive and consummatory aspects of reproductive behavior varies with the volatility of the sexual odors and the sexual experience of the male.     

Sexual refractoriness and locomotion effects on brain monoamines in the male rat

Serotonin (5-HT), 5-hydroxyindolacetic acid (5-HIAA), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), and norepinephrine (NE) concentrations in seven brain regions were compared in sexually refractory, physically active, and inactive male rats. Also examined were 5-HIAA/5-HT and DOPAC/DA concentration ratios. Sexually refractory males were permitted uninterrupted copulation with successive receptive females until they failed to mount or ejaculate in a set period. Physically active males ran in motor-driven activity wheels except during the postejaculatory refractory periods of experimental animals, and inactive males remained alone in testing arenas. Significant group differences were found only in monoamine concentrations in the medial preoptic area (MPOA) and the medial forebrain bundle (MFB). In the MPOA, 5-HT concentration was elevated in sexually refractory males, as was DA in both refractory and active animals. In the MFB, 5-HT concentration was increased in active males. MFB DOPAC levels of sexually refractory rats significantly correlated inversely with their ejaculation totals, as did MFB and dorsal raphe (DR) NE levels. The results suggest that the MPOA is a forebrain target for inhibitory influences on male rat sexual behavior of ascending serotonergic fibers and that increased MFB 5-HT and MPOA DA may be associated with general activity.     

Dopamine D1 receptor stimulation of the nucleus accumbens or the medial preoptic area promotes the onset of maternal behavior in pregnancy-terminated rats

There is good evidence that interference with the mesolimbic dopamine (DA) system results in impaired maternal responding in postpartum female rats. However, whether activation of the mesolimbic DA system is capable of promoting maternal behavior has not been investigated. This study examined whether increasing DA activity in various brain regions of pregnancy-terminated, naive female rats would stimulate the onset of maternal behavior. Experiments 1 and 2 examined the effects of microinjection of various doses (0, 0.2, or 0.5 microg/0.5 microl/side) of a D1 DA receptor agonist, SKF 38393, or a D2 DA receptor agonist, quinpirole, into the nucleus accumbens (NA) on latency to show full maternal behavior, and Experiment 3 determined the effects of SKF 38393 injection into a control site. Finally, because the medial preoptic area (MPOA) is also important for maternal behavior, receives DA input, and expresses DA receptors, the authors examined whether microinjection of SKF 38393 into MPOA was capable of stimulating the onset of maternal behavior. Results indicated that microinjection of SKF 38393 into either the NA or the MPOA facilitates maternal responding in pregnancy-terminated rats.     

Medial preoptic area oxytocin and female sexual receptivity

Intracerebroventricular (icv) administration of the nonapeptide oxytocin (OXT) increases sexual receptivity in female rats. The medial preoptic area (MPOA) appeared to be the most sensitive brain area to the facilitative effects of OXT. Bilateral infusions of 100 ng of OXT into the MPOA significantly elevated lordosis quotients in ovariectomized (OVX), estrogen-treated rats. This dose of OXT was ineffective when infused icv or into the ventromedial hypothalamus, mesencephalic central gray, or ventral tegmental area. A 500-ng dose of OXT significantly elevated lordosis responding when infused icv, corresponding with our previous findings. Mounting by males significantly increased immunoreactive levels of OXT and decreased the number of OXT immunostaining cells in the MPOA of sexually receptive rats pretreated with estrogen and progesterone. The MPOA is a primary site of the OXT facilitation of sexual receptivity where OXT may be released during mating.     

Electrical stimulation of dorsal and ventral striatum differentially alters the copulatory behavior of male rats

Sexual behavior is a natural reward that activates striatal dopaminergic (DA) circuits, and dopamine exerts a facilitative influence on copulation. Electrical stimulation of the striatum has been shown to be rewarding, but its effect on male sexual behavior display has not been established. The objective of the present work was to assess the effects of low- and high-frequency electrical stimulation of the dorsal and ventral striatum on male rat sexual behavior expression. To this aim, copulatory activity of sexually experienced male rats was recorded during electrical stimulation of the nucleus accumbens (NAcc) or caudate-putamen (CP), at each stimulation frequency, before and after sexual exhaustion. Results showed that electrical stimulation of the NAcc at both frequencies increased the number of ejaculations that male rats were able to show in a 30-min period. By contrast, stimulation delivered to the CP inhibited sexual behavior by slowing its display. Each effect was more pronounced at low than at high stimulation frequencies. In the same rats, once sexually exhausted, electrical stimulation of these brain areas did not reverse the sexual behavior inhibition that characterizes the sexual exhaustion state. It is concluded that dorsal and ventral striatal DA brain regions exert opposite influences on copulatory behavior expression of sexually experienced male rats. Also, that the facilitative effect of NAcc electrical stimulation on sexual activity, with the stimulation parameters used, cannot surmount the sexual behavior inhibition resulting from copulation to satiation.     

Interaction of hypothalamic structures in the mediation of male sexual behavior

A series of studies was conducted in which the influences of medial forebrain bundle (MFB) and medial preoptic area (MPOA) lesions on the copulatory behavior of the male rat were investigated. Previous findings that bilateral electropytic lesions in parafornical MFB or the MPOA abolish sexual behavior were confirmed. Further, it was found that MFB lesions as far anterior as the MPOA, but not rostral to this area, were also effective in abolishing male sexual behavior. Asymmetrical hypothalamic lesions were also employed in an attempt to ascertain the degree of interaction between the MPOA and the MFB in the mediation of male sexual behavior. Unilateral lesions in the MFB combined with contralateral destruction of the MPOA were also found to effectively suppress mating. These findings were interpreted as supporting suggestions that portions of the MFB function as a caudally directed pathway from the MPOA in the mediation of sexual behavior. Attempts to reverse the effects on sexual behavior of MFB lesions by the administration of dl-5-hydroxytryptophan were not successful. On the contrary, this chemical was found to be inhibitory to male sexual behavior.     

Effect of lesion location within the medial preoptic-anterior hypothalamic continuum on maternal and male sexual behaviors in female rats

Lesions of the medial preoptic-anterior hypothalamic continuum (MPOA-AH) are known to disrupt both maternal behavior and male sexual behavior in the rat. In order to test the hypothesis that the two behaviors involve different neural systems in the MPOA-AH, small bilateral lesions were made in different anterior-posterior locations in the MPOA-AH of maternal-sensitized, testosterone-treated female rats, and the effects of these lesions on maternal and male sexual behaviors were assessed. Lesions centering in the MPOA disrupted maternal behavior (pup retrieval, nest building, and nursing), with anterior MPOA lesions being more effective (on pup retrieval and nest building) than posterior MPOA lesions. Lesions centering in the AH had little or no effect on maternal behavior. By contrast, male sexual behavior (mounting) was strongly disrupted by lesions in either the MPOA or the AH, with lesions in the rostral AH being most effective.     

Dopamine release in the medial preoptic area is related to hormonal action and sexual motivation

To help elucidate how general the role of dopamine (DA) release in the medial preoptic area (mPOA) is for the activation of male sexual behavior in vertebrates, we recently developed an in vivo microdialysis procedure in the mPOA of Japanese quail. Using these techniques in the present experiment, the temporal pattern of DA release in relation to the precopulatory exposure to a female and to the expression of both appetitive and consummatory aspects of male sexual behavior was investigated. Extracellular samples from the mPOA of adult sexually experienced male quail were collected every 6 min before, while viewing, while in physical contact with, and after exposure to a female. In the absence of a precopulatory rise in DA, males failed to copulate when the barrier separating them from the female was removed. In contrast, males that showed a substantial increase in mPOA DA during precopulatory interactions behind the barrier, copulated with females after its removal. However, there was no difference in DA during periods when the quail were copulating as compared to when the female was present but the males were not copulating. In addition, we show that precopulatory DA predicts future DA levels and copulatory behavior frequency. Furthermore, the size of the cloacal gland, an accurate indicator of testosterone action, is positively correlated with precopulatory DA. Taken together, these results provide further support for the hypothesis that DA action in the mPOA is specifically linked to sexual motivation as compared to copulatory behavior per se.     

Dopamine D1 receptors and phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein-32 in the medial preoptic area are involved in experience-induced enhancement of male sexual behavior in rats

The medial preoptic area (MPOA) is an integral site for male sexual behavior. Dopamine is released in the MPOA before and during copulation and facilitates male rat sexual behavior. Repeated sexual experience and noncopulatory exposures to an estrous female facilitate subsequent copulation. However, the neurobiological mechanisms that mediate such enhancement remain unclear. Here, we examined the role of dopamine D? receptors in the MPOA in experience-induced enhancement of male sexual behavior in rats. In experiment 1, microinjections of the D? antagonist SCH-23390 into the MPOA before each of seven daily 30-min noncopulatory exposures to a receptive female impaired copulation on a drug-free test on Day 8, compared to vehicle-treated female-exposed animals. Copulatory performance in drug-treated animals was similar to that of vehicle-treated males that had not been preexposed to females. This effect was site specific. There were no group differences in locomotor activity in an open field on the copulation test day. In experiment 2, a separate cohort of animals was used to examine phosphorylation of dopamine- and cAMP-regulated phosphoprotein (DARPP-32) in the MPOA of animals with acute and/or chronic sexual experience. DARPP-32 is a downstream marker of D? receptor signaling and substrate of cAMP-dependent protein kinase (PKA). Western immunoblot analysis revealed that p-DARPP-32 expression was greatest in the MPOA of males that received both acute and chronic sexual experience, compared to all other mated conditions and na?ve controls. These data suggest that D? receptors in the MPOA contribute to experience-induced enhancement of male sexual behavior, perhaps through a PKA regulated mechanism.     

Level searching: a new assay of sexual motivation in the male rat

Mendelson and Gorzalka recently described a bilevel chamber for the evaluation of rodent sexual behavior. In initial studies it was observed that during 5 min adaptation periods prior to the introduction of a sexually receptive female rat, male rats with prior sexual experience in these chambers would move from level to level in apparent search for the female rat. In Experiment 1, we examined the acquisition of this level searching behavior in male rats. Sexually active male rats were given access to either sexually receptive or nonreceptive female rats following a 5 min period alone in the bilevel chamber. Only male rats that pursued and copulated to ejaculation with sexually receptive females in the bilevel chamber significantly increased the number of their level to level movements in subsequent tests during the 5 min periods prior to the introduction of the female rat. In Experiment 2, male rats that had acquired asymptotic rates of level searching showed a significant attenuation of this behavior when the presentation of a female rat into the chamber was discontinued. These findings lead us to conclude that the increase in level to level movement by the male rat represents a sexually motivated search for the female rat. We suggest that the analysis of the acquisition, maintenance, and extinction of level searching behavior might serve as a simple assay of sexual motivation in the male rat.     

The effects of D1 or D2 dopamine receptor antagonism in the medial preoptic area, ventral pallidum, or nucleus accumbens on the maternal retrieval response and other aspects of maternal behavior in rats

The medial preoptic area (MPOA), ventral pallidum (VP), and nucleus accumbens (NA) receive dopaminergic afferents and are involved in maternal behavior. Experiments investigated whether dopamine (DA) receptor antagonism in NA disrupts maternal behavior, determined the type of DA receptor involved, and investigated the involvement of drug spread to VP or MPOA. Injection of SCH 23390 (D1 DA receptor antagonist) into NA of postpartum rats disrupted retrieving at dosage levels that were ineffective when injected into MPOA or VP. Motor impairment was not the cause of the deficit. Injection of eticlopride (D2 DA receptor antagonist) into NA or VP was without effect. Results emphasize the importance of DA action on D1 receptors in NA for retrieval behavior.     

Sexual motivation is demasculinized, but not feminized, in prenatally stressed male rats

Sexual motivation and copulation in male rats are associated with dopamine release in the nucleus accumbens. Demasculinized copulatory behavior has been demonstrated in prenatally stressed adult male rats. We have previously reported that approximately 80% of prenatally stressed male rats do not exhibit copulation and that no significant changes in nucleus accumbens dopamine release are seen during exposure to estrous females. In the present study, we investigated whether prenatal stress affects sexual motivation in these animals as adults. Pregnant Wistar rats were subjected to immobilization stress for two hours daily from day 15-19 of gestation. The prenatally stressed male offspring at the age of 3 months were allowed contact with receptive female rats for a 30 min period per week for 10 weeks; then, between the age of 5 and 6 months, their sexual motivation and copulatory activity were measured. Sexual motivation was measured in terms of sexual partner preference. The number of visits and the duration of each visit to an estrous female (stimulus female) or to a sexually active male rat (stimulus male) were recorded. Compared with control males, prenatally stressed male rats showed a significantly lower number of visits and a shorter duration of each visit to stimulus females. Prenatally stressed males showed no preference for male or female stimulus rats in terms of the number of visits and the duration of each visit, whereas control rats showed a significantly higher number of visits and duration of visits to female stimulus rats than male stimulus rats. A significant decrease in copulatory activity was observed in the prenatally stressed male offspring compared with control male rats, with most of the prenatally stressed males failing to show copulation. In vivo microdialysis experiments were performed on the nucleus accumbens with concurrent observation of sexual behavior. The prenatally stressed rats that did not exhibit copulation showed no significant changes in nucleus accumbens dopamine release during exposure to a stimulus male behind a wire-mesh barrier and the amount of dopamine release remained at the basal levels during actual physical contact. These results, combined with those of our previous report, indicate that sexual motivation in prenatally stressed male rats is demasculinized, but not feminized.     

Role of hypothalamic dopaminergic receptors in the control of lordosis behavior in the female rat.

The role of hypothalamic dopaminergic receptors in lordotic behavior was studied by infusing dopaminergic agents into either the medial preoptic area (MPOA), arcuate-ventromedial area (ARC-VM) or lateral hypothalamic area (LHA). Dopaminergic receptor stimulants, dopamine (DA) and apomorphine (APO), enhanced sexual receptivity when infused into the MPOA or ARC-VM in ovariectomized (OVX) rats primed with low doses of estrone. Under these conditions of low preinfusion receptivity (mean preinfusion $\text{L}\text{M}\text{=0.159}$), haloperidol (HALO), a dopaminergic receptor blocker, had no statistically significant effect upon lordotic behavior. Infusions of DA, APO, or HALO into the LHA also had no effect upon lordotic behavior in this model. A second experiment, in which OVX rats were primed with higher doses of estrone to maintain high preinfusion receptivity (mean preinfusion $\text{L}\text{M}\text{=0.869}$), was used to evaluate the effects of dopaminergic receptor blockade upon elevated sexual behavior. Using the second protocol, dopaminergic blockers, HALO and α flupenthixol, were observed to significantly depress lordotic behavior when infused into the MPOA and ARC-VM. In this model no alterations in sexual behavior were observed following MPOA or ARC-VM infusions of APO or the inactive stereoisomer of α flupenthixol, β fluqenthixol. Thus, the hypothalamic activation of dopaminergic receptors was shown to be stimulatory upon lordotic behavior. A third experiment was designed to evaluate the effects of dopaminergic receptor blockade upon luteinizing hormone-releasing hormone (LRH) enhanced lordotic behavior. In this protocol comparisons were made among the lordotic responses to MPOA and ARC-VM infusions of LRH, LRH with HALO and vehicle. Infusions of LRH into the MPOA and ARC-VM significantly enhanced lordotic behavior, whereas the addition of HALO to the LRH infusates abolished this response. It was proposed that hypothalamic dopaminergic receptor activation may contribute to the stimulatory effects of LRH upon lordotic behavior.