Fos-immunoreactivity within the extended amygdala is correlated with the onset of sexual satiety

Nicotine and fluoxetine alter adolescent dopamine-mediated behaviors via 5-HT1A receptor activation

Introduction

Abuse or misuse of tobacco, e-cigarettes, or antidepressants may have serious clinical consequences during adolescence, a sensitive period during brain development when the distinct neurobiology of adolescent serotonin (5-HT) and dopamine (DA) systems create unique behavioral vulnerabilities to drugs of abuse.

Methods

Using a pharmacological approach, we modeled the behavioral and neurochemical effects of subchronic (4-day) nicotine (60µg/kg, i.v.) or fluoxetine (1mg/kg, i.v.) exposure in adolescent and adult male rats.

Results

Nicotine and fluoxetine significantly enhance quinpirole-induced locomotor activity and initial cocaine self-administration in adolescents, but not adults. These effects were blocked by serotonin 5-HT1A receptor antagonists, WAY-100,635 (100 µg/kg, i.v.) or S-15535 (300 µg/kg, i.v.). Neurochemical and anatomical autoradiographic analysis of 8-OH-DPAT-stimulated [35S]GTPγS reveal that prior exposure to nicotine and fluoxetine results in both overlapping and distinct effects on regional 5-HT1A receptor activity. Both fluoxetine and nicotine enhance adolescent 5-HT1A receptor activity in the primary motor cortex (M1), whereas fluoxetine alone targets prefrontal cortical neurocircuitry and nicotine alone targets the amygdala.

Discussion

Given their different pharmacological profiles, comparison between WAY-100,635 and S-15535 indicates that postsynaptic 5-HT1A receptors mediate the behavioral effects of prior nicotine and fluoxetine exposure. In addition, within the adolescent M1, maladaptive changes in 5-HT signaling and 5-HT1A activity after nicotine or fluoxetine exposure may potentiate hyper-responsiveness to dopaminergic drugs and prime adolescent vulnerability for future substance abuse.

Hormones and the Coolidge effect

The Coolidge effect is the renewal of sexual behavior after the presentation of a novel sexual partner and possibly occurs as the result of habituation and dishabituation processes. This re-motivation to copulate is well studied in males and is commonly related to sexual satiety, which involves several neurobiological changes in steroid receptors and their mRNA expression in the CNS. On the other hand, there are few reports studying sexual novelty in females and have been limited to behavioral aspects. Here we report that the levels of rat proceptive behavior, a sign of sexual motivation, declines after 4 h of continuous mating, particularly in females that were unable to regulate the time of mating. Such reduction was not accompanied by changes in lordosis, suggesting that they were not due to the vanishing of the endocrine optimal milieu necessary for the expression of both components of sexual behavior in the female rat. These and previous data support important differences between sexual behavior in both sexes that would result in natural divergences in the Coolidge effect expression. We here also review some reports in humans showing peculiarities between the pattern of habituation and dishabituation in women and men. This is a growing research field that needs emphasis in female subjects.

Caenorhabditis elegans male sensory-motor neurons and dopaminergic support cells couple ejaculation and post-ejaculatory behaviors

The circuit structure and function underlying post-coital male behaviors remain poorly understood. Using mutant analysis, laser ablation, optogenetics, and Ca²⁺ imaging, we observed that following C. elegans male copulation, the duration of post-coital lethargy is coupled to cellular events involved in ejaculation. We show that the SPV and SPD spicule-associated sensory neurons and the spicule socket neuronal support cells function with intromission circuit components, including the cholinergic SPC and PCB and the glutamatergic PCA sensory-motor neurons, to coordinate sex muscle contractions with initiation and continuation of sperm movement. Our observations suggest that the SPV and SPD and their associated dopamine-containing socket cells sense the intrauterine environment through cellular endings exposed at the spicule tips and regulate both sperm release into the hermaphrodite and the recovery from post-coital lethargy.

DOI:http://dx.doi.org/10.7554/eLife.02938.001

The nematode worm, C. elegans, is roughly 1 mm long, made up of around 1000 cells and has two sexes: male and hermaphrodite. Hermaphrodite worms produce both eggs and sperm and can self-fertilize to generate around 300 offspring each time. Fertilization by a male, on the other hand, results in three times as many progeny and introduces genetic diversity into the population. However, it also reduces the lifespan of the hermaphrodite.

Mating also incurs a cost for males: it requires a lot of energy, which prevents male works from engaging in other activities, such as feeding, and it also increases their risk of predation. In many species, including C. elegans, the frequency with which a male can mate is limited by a period of reduced mating drive and ability that follows each instance of successful mating. However, the molecular and cellular basis of this ‘refractory period’ remains largely unclear.

Using a range of techniques, LeBoeuf et al. have now identified the circuits that regulate male mating behavior in C. elegans. When male worms were introduced into a Petri dish containing 15 hermaphrodites, most males initiated mating within about 2 min. The length of the refractory period varied between worms, but averaged roughly 12 min. This consisted of a period of disinterest, in which males did not approach hermaphrodites, followed by a period in which males attempted mating but were slower and less efficient, suggesting that the neural circuits controlling mating behaviors had yet to recover completely.

Males with longer refractory periods produced more progeny in their second mating than those with shorter refractory periods, suggesting that the interval also enables males to replenish their sperm levels. Further experiments revealed that a chemical transmitter called dopamine promotes ejaculation and then immediately reduces the worm's activity levels, giving rise to the refractory period.

By enforcing a delay between matings, the refractory period may also increase the likelihood that successive matings will be with different hermaphrodites, helping to maximize the number and diversity of offspring. Some aspects of the neural circuitry that controls the refractory period in C. elegans resemble those seen in mammals, suggesting that insights gained from an animal with 1000 cells could also be relevant to more complex species.

DOI:http://dx.doi.org/10.7554/eLife.02938.002

Impaired GABAB receptor signaling dramatically up-regulates Kiss1 expression selectively in nonhypothalamic brain regions of adult but not prepubertal mice

Kisspeptin, encoded by Kiss1, stimulates reproduction and is synthesized in the hypothalamic anteroventral periventricular and arcuate nuclei. Kiss1 is also expressed at lower levels in the medial amygdala (MeA) and bed nucleus of the stria terminalis (BNST), but the regulation and function of Kiss1 there is poorly understood. γ-Aminobutyric acid (GABA) also regulates reproduction, and female GABAB1 receptor knockout (KO) mice have compromised fertility. However, the interaction between GABAB receptors and Kiss1 neurons is unknown. Here, using double-label in situ hybridization, we first demonstrated that a majority of hypothalamic Kiss1 neurons coexpress GABAB1 subunit, a finding also confirmed for most MeA Kiss1 neurons. Yet, despite known reproductive impairments in GABAB1KO mice, Kiss1 expression in the anteroventral periventricular and arcuate nuclei, assessed by both in situ hybridization and real-time PCR, was identical between adult wild-type and GABAB1KO mice. Surprisingly, however, Kiss1 levels in the BNST and MeA, as well as the lateral septum (a region normally lacking Kiss1 expression), were dramatically increased in both GABAB1KO males and females. The increased Kiss1 levels in extrahypothalamic regions were not caused by elevated sex steroids (which can increase Kiss1 expression), because circulating estradiol and testosterone were equivalent between genotypes. Interestingly, increased Kiss1 expression was not detected in the MeA or BNST in prepubertal KO mice of either sex, indicating that the enhancements in extrahypothalamic Kiss1 levels initiate during/after puberty. These findings suggest that GABAB signaling may normally directly or indirectly inhibit Kiss1 expression, particularly in the BNST and MeA, and highlight the importance of studying kisspeptin populations outside the hypothalamus.

Gene expression in aminergic and peptidergic cells during aggression and defeat: relevance to violence, depression and drug abuse

In this review, we examine how experiences in social confrontations alter gene expression in mesocorticolimbic cells. The focus is on the target of attack and threat due to the prominent role of social defeat stress in the study of coping mechanisms and victimization. The initial operational definition of the socially defeated mouse by Ginsburg and Allee (1942) enabled the characterization of key endocrine, cardiovascular, and metabolic events during the initial response to an aggressive opponent and during the ensuing adaptations. Brief episodes of social defeat stress induce an augmented response to stimulant challenge as reflected by increased locomotion and increased extracellular dopamine (DA) in the nucleus accumbens (NAC). Cells in the ventral tegmental area (VTA) that project to the NAC were more active as indicated by increased expression of c-fos and Fos-immunoreactivity and BDNF. Intermittent episodes of social defeat stress result in increased mRNA for MOR in brainstem and limbic structures. These behavioral and neurobiological indices of sensitization persist for several months after the stress experience. The episodically defeated rats also self-administered intravenous cocaine during continuous access for 24 h ("binge"). By contrast, continuous social stress, particularly in the form of social subordination stress, leads to reduced appetite, compromised endocrine activities, and cardiovascular and metabolic abnormalities, and prefer sweets less as index of anhedonia. Cocaine challenges in subordinate rats result in a blunted psychomotor stimulant response and a reduced DA release in NAC. Subordinate rats self-administer cocaine less during continuous access conditions. These contrasting patterns of social stress result from continuous vs. intermittent exposure to social stress, suggesting divergent neuroadaptations for increased vulnerability to cocaine self-administration vs. deteriorated reward mechanisms characteristic of depressive-like profiles.

Brain structures involved in the sexual behaviour of Ile de France rams with different sexual preferences and levels of sexual activity

Using Fos, as a marker, we analysed the brain structures of rams, with different libidos or sexual preferences that had been activated by contact with males or females. Ile de France rams aged from 1.5 to 7 years were used. Fos immunoreactivity (Fos IR) was analysed in rams with high (HL) or low libido (LL) after 90 min of direct contact with females (HL DirF n=7 or LL DirF n=7) or in rams of high libido having indirect contact through a fence, with females (HL IndF n=6) or males (HL IndM n=5) and finally, in males who preferred other males as partners by indirect contact through a fence with males (MO IndM n=4). Direct or indirect contact with a preferred sexual partner (LL DirF, HL Dir F, HL IndF, MO IndM) induced the appearance of Fos-IR cells in several diencephalic and cortical structures. Conversely, indirect contact with males did not induce Fos-IR in males interested in females (HL IndM). In the medial preoptic area (MPOA), the paraventricular nucleus and the medial bed nucleus of the stria terminalis the cell density of Fos IR cells was higher in HL Dir F than in LL DirF suggesting involvement in sexual motivation whereas only the MPOA seemed involved the consummatory component of sexual behaviour (Fos IR density HL DirF>HL IndF). The enthorinal cortex was the only structure specifically activated by males attracted to other males (Fos IR density MO IndM>HL IndM) whereas Fos IR density did not differ between the HL IndF and HL IndM groups.

Cholinergic control of male mating behavior in hamsters: effects of central oxotremorine treatment

The responses of rats to intracranial injections of cholinergic drugs implicate acetylcholine in the control of male mating behavior and suggest specific brain areas as mediators of these effects. In particular, past work has linked the medial preoptic area (MPOA) to the control of intromission frequency but implicated areas near the lateral ventricles in effects on the initiation and spacing of intromissions. Studies of responses to systemic cholinergic treatments suggest that acetylcholine is even more important for the control of mating behavior in male hamsters but provide no information on the relevant brain areas. To fill this gap, we observed the effects of central injections of the cholinergic agonist oxotremorine that approached the MPOA along contrasting paths. Both studies suggest that increased cholinergic activity in or near the MPOA can facilitate behavior by reducing the postejaculatory interval and possibly affecting other parts of the mechanisms controlling the initiation of copulation and the efficiency of performance early in an encounter. In addition, oxotremorine caused other changes in behavior that could not be tied to the MPOA and may reflect actions at more dorsal sites, possibly including the bed nucleus of the stria terminalis and medial septum. These effects were notably heterogeneous, including facilitatory and disruptive effects on male behavior along with a facilitation of lordosis responses to manual stimulation. These results emphasize the number and diversity of elements of sexual behavior in hamsters that are under the partial control of forebrain cholinergic mechanisms.

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.

Lesions of the posterior bed nucleus of the stria terminalis eliminate opposite-sex odor preference and delay copulation in male Syrian hamsters: role of odor volatility and sexual experience

In Syrian hamsters (Mesocricetus auratus), the expression of reproductive behavior requires the perception of social odors. The behavioral response to these odors is mediated by a network of ventral forebrain nuclei, including the posterior bed nucleus of the stria terminalis (pBNST). Previous studies have tested the role of the pBNST in reproductive behavior, but the use of large, fiber-damaging lesions in these studies make it difficult to attribute post-lesion deficits to the pBNST specifically. Thus, the current study used discrete, excitotoxic lesions of the pBNST to test the role of the pBNST in opposite-sex odor preference and copulatory behavior in both sexually-naive and sexually-experienced males. Lesions of the pBNST decreased sexually-naive males' investigation of volatile female odors, resulting in an elimination of opposite-sex odor preference. This elimination of preference was not due to a sensory deficit, as males with pBNST lesions were able to discriminate between odors. When, however, subjects were given sexual experience prior to pBNST lesions, their preference for volatile opposite-sex odors remained intact post-lesion. Similarly, when sexually-naive or sexually-experienced subjects were allowed to contact the social odors during the preference test, lesions of the pBNST decreased males' investigation of female odors but did not eliminate preference for opposite-sex odors, regardless of sexual experience. Finally, lesions of the pBNST delayed the copulatory sequence in sexually-naive, but not sexually-experienced, males such that they took longer to mount, intromit, ejaculate and display long intromissions. Together, these results demonstrate that the pBNST plays a unique and critical role in both appetitive and consummatory aspects of male reproductive behaviors.

Male and female odors induce Fos expression in chemically defined neuronal population

Olfactory information modulates innate and social behaviors in rodents and other species. Studies have shown that the medial nucleus of the amygdala (MEA) and the ventral premammillary nucleus (PMV) are recruited by conspecific odor stimulation. However, the chemical identity of these neurons is not determined. We exposed sexually inexperienced male rats to female or male odors and assessed Fos immunoreactivity (Fos-ir) in neurons expressing NADPH diaphorase activity (NADPHd, a nitric oxide synthase), neuropeptide urocortin 3, or glutamic acid decarboxylase mRNA (GAD-67, a GABA-synthesizing enzyme) in the MEA and PMV. Male and female odors elicited Fos-ir in the MEA and PMV neurons, but the number of Fos-immunoreactive neurons was higher following female odor exposure, in both nuclei. We found no difference in odor induced Fos-ir in the MEA and PMV comparing fed and fasted animals. In the MEA, NADPHd neurons colocalized Fos-ir only in response to female odors. In addition, urocortin 3 neurons comprise a distinct population and they do not express Fos-ir after conspecific odor stimulation. We found that 80% of neurons activated by male odors coexpressed GAD-67 mRNA. Following female odor, 50% of Fos neurons coexpressed GAD-67 mRNA. The PMV expresses very little GAD-67, and virtually no colocalization with Fos was observed. We found intense NADPHd activity in PMV neurons, some of which coexpressed Fos-ir after exposure to both odors. The majority of the PMV neurons expressing NADPHd colocalized cocaine- and amphetamine-regulated transcript (CART). Our findings suggest that female and male odors engage distinct neuronal populations in the MEA, thereby inducing contextualized behavioral responses according to olfactory cues. In the PMV, NADPHd/CART neurons respond to male and female odors, suggesting a role in neuroendocrine regulation in response to olfactory cues.

Men versus women on sexual brain function: prominent differences during tactile genital stimulation, but not during orgasm

Biological differences in male and female sexuality are obvious in the behavioral domain, but the central mechanisms that might explain these behavioral gender differences remain unclear. In this study, we merged two earlier positron emission tomography data sets to enable systematic comparison of the brain responses in heterosexual men and women during sexual tactile genital (penile and clitoral) stimulation and during orgasm. Gender commonalities were most evident during orgasm, a phase which demonstrated activations in the anterior lobe of the cerebellar vermis and deep cerebellar nuclei, and deactivations in the left ventromedial and orbitofrontal cortex in both men and women. During tactile genital stimulation, deactivations in the right amygdala and left fusiform gyrus were found for both genders. Marked gender differences were seen during this phase: left fronto-parietal areas (motor cortices, somatosensory area 2 and posterior parietal cortex) were activated more in women, whereas in men, the right claustrum and ventral occipitotemporal cortex showed larger activation. The only prominent gender difference during orgasm was male-biased activation of the periaqueductal gray matter. From these results, we conclude that during the sexual act, differential brain responses across genders are principally related to the stimulatory (plateau) phase and not to the orgasmic phase itself. These results add to a better understanding of the neural underpinnings of human sexuality, which might benefit treatment of psychosexual disorders.

A developmental neurobiological model of motivated behavior: anatomy, connectivity and ontogeny of the triadic nodes

Adolescence is the transition period that prepares individuals for fulfilling their role as adults. Most conspicuous in this transition period is the peak level of risk-taking behaviors that characterize adolescent motivated behavior. Significant neural remodeling contributes to this change. This review focuses on the functional neuroanatomy underlying motivated behavior, and how ontogenic changes can explain the typical behavioral patterns in adolescence. To help model these changes and provide testable hypotheses, a neural systems-based theory is presented. In short, the Triadic Model proposes that motivated behavior is governed by a carefully orchestrated articulation among three systems, approach, avoidance and regulatory. These three systems map to distinct, but overlapping, neural circuits, whose representatives are the striatum, the amygdala and the medial prefrontal cortex. Each of these system-representatives will be described from a functional anatomy perspective that includes a review of their connectivity and what is known of their ontogenic changes.

The posteromedial cortical amygdala regulates copulatory behavior, but not sexual odor preference, in the male Syrian hamster (Mesocricetus auratus)

In rodent species, the expression of reproductive behavior relies heavily on the perception of social odors, as well as the presence of circulating steroid hormones. In the Syrian hamster, chemosensory and hormonal cues are processed within an interconnected network of ventral forebrain nuclei that regulates many aspects of social behavior. Within this network, the posteromedial cortical amygdala (PMCo) receives direct projections from the accessory olfactory bulbs and contains a dense population of steroid receptor-containing neurons. Consequently, the PMCo may be important for generating odor-guided aspects of reproductive behavior, yet little is known regarding the role of this nucleus in regulating these behaviors. Thus, the present study tested male hamsters with site-specific electrolytic lesions of the PMCo for their (a) sexual odor preference in a Y-maze apparatus, (b) sexual odor discrimination in a habituation-dishabituation task, and (c) copulatory behavior when paired with a sexually receptive female. PMCo-lesioned males preferred to investigate female odors over male odors and were able to discriminate between these odor sources. However, PMCo lesions were associated with several alterations in the male copulatory pattern. First, PMCo-lesioned males displayed increased investigation of the female's non-anogenital region, suggesting that the PMCo may be involved in directing appropriate chemosensory investigation during mating. Second, PMCo lesions altered the temporal pattern of the mating sequence, as PMCo-lesioned males took longer than Sham-lesioned males to reach sexual satiety, as indicated by the delayed expression of long intromissions. This delayed onset of satiety was associated with an increased number of ejaculations compared with Sham-lesioned males. Importantly, these data provide the first direct evidence for a functional role of the PMCo in regulating male reproductive behavior.

Effects of acute and chronic apomorphine on sex behavior and copulation-induced neural activation in the male rat

Apomorphine is a non-selective dopaminergic receptor agonist. Because of its pro-erectile effects, apomorphine is clinically used for treatment of erectile dysfunction. We investigated the effects of subcutaneous apomorphine administration (0.4 mg/kg rat) on sexual behavior and mating-induced Fos-expression following acute (day 1) or chronic apomorphine treatment (days 8 and 15) in sexually experienced male rats. Consistent facilitatory effects of apomorphine were observed in the reduced numbers of mounts and intromissions over time and an increased ejaculation frequency on day 1. The first post-ejaculatory interval, however, was lengthened, while other behavioral parameters were unaffected. Fos-immunoreactivity induced by acute apomorphine administration (barrel cortex, paraventricular hypothalamic nucleus, central amygdala and locus coeruleus) was strongly reduced after chronic administration. After mating, induction of Fos-immunoreactivity was observed in well-known areas like medial preoptic nucleus and the posterodorsal medial amygdaloid area. Apomorphine, however, reduced mating-induced Fos-immunoreactivity in the nucleus accumbens shell and prevented its occurrence in its core area. This remarkable apomorphine effect was not observed in any other brain area. We conclude that the behavioral (pro-erectile) effects of apomorphine are consistent over time, and that the diminished accumbens-Fos-immunoreactivity and the elongated post-ejaculatory interval may reflect a decreased response to remote cues from the estrus female.

Enhanced neural activation in brain regions mediating sexual responses following exposure to a conditioned stimulus that predicts copulation

Stimuli associated with sexual behavior increase reproductive success if presented prior to copulation. In Japanese quail, inseminations that take place in a context that predicts the arrival of a female are more likely to result in fertilized eggs. We demonstrate here that in male Japanese quail a sexual conditioned stimulus (CS) also enhances activity in two brain regions that mediate sexual behavior, the medial preoptic area and the medial part of the bed nucleus of the stria terminalis. C-fos expression, a marker of neural activation, was higher in these areas in subjects exposed sequentially to a sexual CS and copulation than in subjects exposed to copulation or the CS alone or in subjects exposed to no sexual stimulus, either an identical, untrained CS or an empty arena. These results suggest a link between a proximate result of sexual CS presentation, male brain activation, and a known ultimate outcome, increased fertilizations.

Dendritic pruning of the medial amygdala during pubertal development of the male Syrian hamster

The medial amygdala (Me), a brain region essential for mating behavior, changes in size during puberty. In pre-, mid-, and late pubertal (21, 35, and 49 days of age) male Syrian hamsters, we examined neuronal structure in Me and protein levels of spinophilin and synaptophysin in the amygdaloid complex for evidence of synaptic plasticity coincident with behavioral and physiological development. Body weight, testes weight, and testosterone levels increased during puberty. Mounting behavior, including ectopic, nonintromittive, and intromittive mounts, also increased. Neuronal structure in the posterodorsal medial amygdala (MePD) was assessed in Golgi-impregnated neurons. Pruning occurred during puberty in the number of dendrites emanating from the cell body and in terminal dendritic spine densities. Approximately half of all MePD neurons analyzed had an axon emanating from a dendrite rather than the cell body. However, prepubertal males were more likely to have the axon emanating from a higher order dendritic segment (secondary or tertiary) than were mid- and late pubertal males. Finally, protein levels in the amygdaloid complex varied with pubertal age. Spinophilin decreased, while synaptophysin and GAPDH protein levels increased. These results suggest that puberty is a period of dramatic synaptic plasticity in Me. Specifically, pruning of dendrites and spines, in combination with axonal changes, is likely to modify the afferent influences and electrophysiological properties of Me neurons. Because the Me is an integral component of a social behavior neural network, these changes may be related not only to sexual behavior, but also to other behaviors that mature during puberty, including aggressive, risk-taking, fear-related, and parental behaviors.

c-Fos expression related to sexual satiety in the male rat forebrain

The long term inhibition of masculine sexual behavior after repeated ejaculations is known as sexual satiety. To investigate the brain areas that may regulate sexual satiety, c-Fos expression was studied in different groups of sexually experienced male rats: controls not allowed to copulate, males allowed two or four ejaculations and animals allowed to reach sexual satiety. Interestingly, males that ejaculated two or four times had similar c-Fos densities in all the evaluated brain regions, except for the suprachiasmatic nucleus. Similarly, sexually satiated males had analogous c-Fos densities in all the evaluated brain areas independently of the number of ejaculations required to reach satiety. Sexual activity (evidenced in males that ejaculated two or four times) increased c-Fos levels in the anteromedial bed nucleus of the stria terminalis, claustrum, entorhinal cortex, medial preoptic area, nucleus accumbens core, suprachiasmatic nucleus and supraoptic nucleus; however, sexual satiety did not modify c-Fos expression in these regions. Sexually satiated males had increased c-Fos densities in the ventrolateral septum and the anterodorsal and posteroventral medial amygdala, compared with animals allowed to copulate but that did not reach sexual satiety, and decreased c-Fos density in the piriform cortex. These results suggest that the network that underlies sexual satiety is different from that which regulates copulation.

Rat RFamide-related peptide-3 stimulates GH secretion, inhibits LH secretion, and has variable effects on sex behavior in the adult male rat

A recently described avian neuropeptide, gonadotropin inhibitory hormone (GnIH), has been shown to have seasonal regulatory effects on the hypothalamic-pituitary-gonadotropin axis (HPG) in several avian species. In the bird, GnIH expression is increased during the photorefractory period and has inhibitory effects on the HPG. A recently described mammalian neuropeptide, RF-amide-related peptide-3 (RFRP-3), may be genetically related and functionally similar to this avian neuropeptide. The purposes of this study were to first see if rat RFRP-3 is expressed in the male rat brain and second to determine if ICV injections of RFRP-3 will have effects on feeding and sex behaviors, as well as hormone release from the anterior pituitary. Results confirm other studies in that immunoreactive cell bodies and fibers are observable in areas of the male rat brain known to control the HPG and feeding and sex behaviors. RFRP-3 fibers are also observed in close proximity to GnRH immunoreactive cell bodies. Behavioral tests indicate that high but not low ICV RFRP-3 (500 vs. 100 ng, respectively) significantly (p<0.05) suppressed all facets of male sex behavior while not having any observable effects on their ability to ambulate. Sex behavior was later exhibited when those same male rats received the ICV vehicle. While suppressing sex behavior, ICV RFRP-3 significantly (p<0.05) increased food intake compared to controls. ICV RFRP-3 also significantly reduced plasma levels of luteinizing hormone but increased growth hormone regardless of the time of day; however, at no time did RFRP-3 alter plasma levels of FSH, thyroid hormone, or cortisol. These results indicate that although RFRP-3 has similar effects on LH as observed with GnIH in avian species, in the rat RFRP-3 has additional roles in regulating feeding and growth.

Associations between behavior, hormones, and Fos responses to novelty differ in pre- and post-pubertal grass rats

As animals progress from one developmental stage to the next, the nature of the challenges they face can change in systematic ways, as do the mechanisms that enable them to deal effectively with them. Here we examined the changes in the behavioral patterns and neuroendocrine mechanisms associated with exposure to a novel environment before and after the transition from a pre- to a post-pubertal stage of development in the unstriped Nile grass rat (Arvicanthis niloticus), a murid rodent that appears to live in family groups in the wild. We introduced juvenile (28 days old) and adult (60 days old) grass rats to a novel glass aquarium where they were kept for 1 h; controls were maintained in their home cages during this time. Animals were then killed, blood was sampled, and plasma corticosterone and testosterone were measured. Brains were collected and processed for immunohistochemical detection of Fos. Although behavior in the novel environment did not differ as a function of age, corticosterone secretion and Fos expression in a variety of stress-related brain regions were increased by the manipulation to a greater extent in the juveniles compared to the adults. The data suggest a pattern of development in which a novel environment that elicits the same levels of exploratory behaviors in young and adult animals triggers a greater response in stress-related brain regions as well as corticosterone secretion in the more vulnerable young ones.

Role of the stria terminalis in food intake and body weight in rats

Previous studies have shown that lesions of the posterodorsal amygdala result in hyperphagia and obesity in female rats. In the present study, lesions of the stria terminalis at its most dorsal point (before it separates into dorsal and ventral components) also resulted in hyperphagia and excessive weight gains in female rats compared to female rats with sham lesions, as did coronal knife cuts anterior to the ventromedial hypothalamus. Identical lesions and knife cuts did not elevate food intake or weight gains in male rats compared to male control animals. Examination of the anterograde degeneration with the amino-cupric-silver method in the brains of two female rats with hypothalamic knife cuts revealed degenerating terminals in the capsule of the ventromedial hypothalamus and in the premammillary nuclei, two classic indicators of damage to the dorsal component of the stria terminalis. No degenerating axon terminals were observed in the paraventricular nucleus. Differences from previous studies that used male rats were attributed to a sex difference for the effects of amygdaloid and ventromedial hypothalamic lesions. It is proposed that the posterodorsal amygdala, dorsal component of the stria terminalis, and ventromedial hypothalamus are part of an inhibitory pathway in the regulation of feeding behavior.

Amygdaloid lesion-induced obesity: relation to sexual behavior, olfaction, and the ventromedial hypothalamus

Lesions of the amygdala have long been known to produce hyperphagia and obesity in cats, dogs, and monkeys, but only recently have studies with rats determined that the effective site is the posterodorsal amygdala (PDA)-the posterodorsal medial amygdaloid nucleus and the intra-amygdaloid bed nucleus of the stria terminalis. There is a sex difference; female rats with PDA lesions display greater weight gain than male rats. In the brains of female rats with obesity-inducing PDA lesions, there is a dense pattern of axonal degeneration in the capsule of the ventromedial hypothalamus (VMH) and other targets of the stria terminalis. Transections of the dorsal component of the stria terminalis also result in hyperphagia and obesity in female rats. Similar to rats with VMH lesions, rats with PDA lesions are hyperinsulinemic during food restriction and greatly prefer high-carbohydrate diets. The PDA is also a critical site for some aspects of rodent sexual behavior, particularly those that depend on olfaction, and the pattern of degeneration observed after obesity-inducing PDA lesions is remarkably parallel to the circuit that has been proposed to mediate sexual behavior. Medial amygdaloid lesions disrupt the normal feeding pattern and result in impaired responses to caloric challenges, and there is evidence that these behavioral changes are also due to a disruption of olfactory input. With its input from the olfactory bulbs and connections to the VMH, the PDA may be a nodal point at which olfactory and neuroendocrine stimuli are integrated to affect feeding behavior.

Further studies on the rat posterodorsal medial amygdala: Dendritic spine density and effect of 8-OH-DPAT microinjection on male sexual behavior

The rat posterodorsal medial amygdala (MePD) is a component of the neural network that modulates male sexual behavior. Dendritic spines were counted in Golgi-impregnated bitufted and stellate neurons and from cells located in the medial and lateral MePD subregions. It was also studied the effect of 8-OH-DPAT, a 5-HT1A receptor agonist, microinjected into the MePD on male sexual behavior. There were no significant differences in the dendritic spine density obtained from multipolar bitufted and stellate neurons (n = 48 cells in each group; p > 0.05) or in the data from the medial or the lateral MePD (n = 48 neurons per region; p > 0.05). Rats were stereotaxically microinjected into the MePD with saline (0.2 microl, n = 6) or 8-OH-DPAT (0.1 and 1.0 microg/0.2 microl, n = 6 and 5, respectively). Behavioral recordings prior to surgery and "non-target" microinjections served as additional control data. 8-OH-DPAT 1.0 microg decreased the latencies to intromission and ejaculation, the postejaculatory refractory period and the mount frequency when compared to control pre-surgery data (p < 0.05). When compared among groups, 8-OH-DPAT 1.0 microg promoted the highest percentage reduction in the postejaculatory refractory period. Saline and injections in the vicinity of MePD did not promote relevant effects on ejaculation (p > 0.05). Results indicate that a similar dendritic spine density can be found in morphologically different populations of MePD neurons and, 8-OH-DPAT can facilitate male sexual behavior by acting on postsynaptic 5-HT1A receptors in this brain area.

The rise, fall, and resurrection of the ventromedial hypothalamus in the regulation of feeding behavior and body weight

Early researchers found that lesions of the ventromedial hypothalamus (VMH) resulted in hyperphagia and obesity in a variety of species including humans, which led them to designate the VMH as the brain's "satiety center." Many researchers later dismissed a role for the VMH in feeding behavior when Gold claimed that lesions restricted to the VMH did not result in overeating and that obesity was observed only with lesions or knife cuts that extended beyond the borders of the VMH and damaged or severed the ventral noradrenergic bundle (VNAB) or paraventricular nucleus (PVN). However, anatomical studies done both before and after Gold's study did not replicate his results with lesions, and in nearly every published direct comparison of VMH lesions vs. PVN or VNAB lesions, the group with VMH lesions ate substantially more food and gained twice as much weight. Several other important differences have also been found between VMH and both PVN and VNAB lesion-induced obesity. Concerns regarding (a) motivation to work for food and (b) the effects of nonirritative lesions have also been addressed and answered in many studies. Lesion studies with weanling rats and adult pair-tube-fed rats, as well as recent studies of knockout mice deficient in the orphan nuclear receptor steroidogenic factor 1, indicate that VMH lesion-induced obesity is in large part a metabolic obesity (due to autonomic nervous system disorders) independent of hyperphagia. However, there is ample evidence that the VMH also plays a primary role in feeding behavior. Neuroimaging studies in humans have shown a marked increase in activity in the area of the VMH during feeding. The VMH has a large population of glucoresponsive neurons that dynamically respond to blood glucose levels and numerous histamine, dopamine, serotonin, and GABA neurons that respond to feeding-related stimuli. Recent studies have implicated melanocortins in the VMH regulation of feeding behavior: food intake decreases when arcuate nucleus pro-opiomelanocortin (POMC) neurons activate VMH brain-derived neurotrophic factor (BDNF) neurons. Moderate hyperphagia and obesity have also been observed in female rats with damage to the efferent projections from the posterodorsal amygdala to the VMH. Hypothalamic obesity can result from damage to either the POMC or BDNF neurons. The concept of hypothalamic feeding and satiety centers is outdated and unnecessary, and progress in understanding hypothalamic mechanisms of feeding behavior will be achieved only by appreciating the different types of neural and blood-borne information received by the various nuclei, and then attempting to determine how this information is integrated to obtain a balance between energy intake and energy output.

Do similar neural systems subserve aggressive and sexual behaviour in male rats? Insights from c-Fos and pharmacological studies

It is a common belief that male aggressive and sexual behaviour share many of the underlying neurobiological, neurological, pharmacological and neuroendocrine mechanisms. Therefore, we studied brain activation patterns in male rat after performance of aggressive and sexual behaviour and compared serotonergic pharmacology in the same paradigms to delineate possible similarities and differences. Patterns of Fos-immunoreactivity induced by aggressive and sexual encounters of Wild-type male Brown Norway rats were studied to localise the commonly activated (functionally shared) parts of the circuitry, and the specific (functionally different) parts of the neuronal circuitry. Some brain areas (caudal medial preoptic area and medial amygdala) were commonly activated, but other areas (e.g. posterodorsal parts of the medial amygdala, rostral preoptic and premammillary hypothalamus) showed remarkably specific differences in neural activation. 5-HT(1A) receptor agonists inhibit aggressive, but stimulate male sexual behaviour, whereas 5-HT(1B) receptor agonists inhibit both types of behaviour. Selective serotonin reuptake inhibitors share comparable inhibitory effects in aggression and sexual behaviour, although only at relatively high doses. We propose that separate hard-wired neural systems exist in the brain for aggressive and sexual behaviours, modulated via hierarchically 'higher-level' brain areas that are involved in the integration (gating) of the behavioural outcome of an organism.

Citalopram combined with WAY 100635 inhibits ejaculation and ejaculation-related Fos immunoreactivity

The role of 5-HT (5-hydroxytryptamine, 5-HT)(1A) receptor activation in the sexual side-effects, in particular delayed ejaculation, of selective serotonin reuptake inhibitors (SSRIs) was studied. Male Wistar rats were treated for 15 days with vehicle, the SSRI citalopram (10 mg/kg/day p.o.), the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexane carboxamide 3HCL (WAY 100635, 0.1 mg/kg/ day s.c.), or both drugs combined. Sexual behavior was assessed weekly. One h after the last sexual behavior test, rat brains were processed for Fos-immunohistochemistry. Acute and chronic citalopram mildly inhibited ejaculation, which was strongly augmented by co-administration of WAY 100635. WAY 100635 alone did not alter sexual behavior. Brain sites associated with ejaculation showed reduced Fos-immunoreactivity in rats treated with both citalopram and WAY 100635. Citalopram reduced Fos-immunoreactivity in the arcuate hypothalamic nucleus, an area that might link serotonergic neurotransmission to ejaculation.

Neural control of ejaculation

Ejaculation is the most reinforcing component of sexual behavior. However, the neural substrates mediating ejaculation and processing ejaculation-related signals remain poorly understood. We review the current understanding of central control of ejaculation. Specifically, the recent identification of a candidate spinothalamic pathway involved in relay of ejaculation-specific signals is discussed. In addition, the discovery of a neural population of lumbar interneurons playing an pivotal role in expression of ejaculation is reviewed.

Central regulation of ejaculation

Ejaculation is a reflex mediated by a spinal control center, referred to as a spinal ejaculation generator. This spinal ejaculation generator coordinates sympathetic, parasympathetic and motor outflow to induce the two phases of ejaculation, i.e., emission and expulsion. In addition, the spinal ejaculation generator integrates this outflow with inputs that are related to the summation of sexual activity prior to ejaculation that are required to trigger ejaculation. Recently, a group of spinothalamic neurons in the lumbar spinal cord (LSt cells) were demonstrated to comprise an integral part of the spinal ejaculation generator. Specifically, lesions of LSt cells completely ablate ejaculatory function. Moreover, LSt cells are activated following ejaculation, but not following other components of sexual behavior. Furthermore, based on their relationship with autonomic nuclei, motoneurons and genital sensory inputs, LSt cells are also in the ideal anatomical position to integrate sensory inputs and autonomic and motor outflow. Additionally, the spinal ejaculation generator is under inhibitory and excitatory influence of supraspinal sites, including the nucleus paragigantocellularis (nPGi), the paraventricular nucleus of the hypothalamus (PVN) and the medial preoptic area (MPOA). Finally, sensory information related to ejaculation is processed in the spinal cord and brain, possibly contributing to the rewarding properties of ejaculation. One candidate pathway for relay of ejaculation-related cues consists of LSt cells and their projections to the parvocellular subparafascicular thalamic nucleus. Moreover, neural activation specifically related to ejaculation is observed in the brain and may reflect of processing of ejaculation-related sensory cues.

Aggression and defeat: persistent effects on cocaine self-administration and gene expression in peptidergic and aminergic mesocorticolimbic circuits

The question of how ostensibly aversive social stress experiences in an aggressive confrontation can persistently increase intense drug taking such as cocaine 'bingeing' needs to be resolved. The biology of social conflict highlights distinctive behavioral, cardiovascular and endocrine profiles of dominant and subordinate animals, as seen also in rodents and primates under laboratory conditions. In contrast to continuous subordination stress that produces chronic pathophysiological consequences and often is fatal, animals adapt to brief episodes of social defeat stress, but show enduring functional activation in mesocorticolimbic microcircuits. Uncontrollable episodes of social defeat stress produce long-lasting tolerance to opiate analgesia and, concurrently, behavioral sensitization to challenges with either amphetamine or cocaine. One week after a single social defeat stress, cross-sensitization to cocaine is evident in terms of enhanced motor activity as well as in terms of increased Fos labeling in the periaqueductal grey area, the locus coeruleus, and the dorsal raphe nuclei. When challenged with a low amphetamine dose, the behavioral and neural effects of repeated brief episodes of social defeat stress persist for months. Previous exposure to social defeat stress can (1). significantly shorten the latency to acquire cocaine self-administration, (2). maintain this behavior at low cocaine unit doses, (3). significantly increase the levels of cocaine taking during a 24 h binge of continuous drug availability, (4). dysregulate the timing of consecutive infusions, and (5). abolish the circadian pattern of self-administration. Amygdaloid modulation, especially originating from central and basolateral nuclei, of dopaminergic pathways via peptidergic and glutamatergic neurons appears to be a key mechanism by which social defeat stress affects cocaine self-administration. Social stress alters the feedback from prefrontal cortex and thereby may contribute to the dysregulation of dopaminergic activity that is necessary for cocaine self-administration.

The MPN mag

Mating behavior in male hamsters is regulated by a chemosensory pathway that converges on the bed nucleus of the stria terminalis (BST) and the medial nucleus of the amygdala (Me). Both the BST and the Me project to the lateral part of the medial preoptic area. Lesion studies have identified a small group of large cells referred to as the magnocellular medial preoptic nucleus (MPN mag) whose integrity is required for normal mating behavior. Our data, summarized within, indicate that the MPN mag is a sexually differentiated nucleus in a large steroid-responsive network that relays pheromonal signals from the sensory systems to the motor areas to affect behavior.

GABA and glutamate in mating-activated cells in the preoptic area and medial amygdala of male gerbils

The posterodorsal medial amygdala (MeApd), the posterodorsal preoptic nucleus (PdPN), and the medial cell group of the sexually dimorphic preoptic area (mSDA) contain cells that are activated specifically at ejaculation as assessed by Fos expression. The mSDA also expresses Fos early in the mating context. Because little is known about the neurotransmitters of these activated cells, the possibility that they use gamma-aminobutyric acid (GABA) or glutamate was assessed. Putative glutamatergic cells were visualized with immunocytochemistry (ICC) for glutamate and its neuron-specific transporter. Their distributions were compared with those of GABAergic cells visualized with ICC for the 67-kDa form of glutamic acid decarboxylase (GAD(67)) and in situ hybridization for GAD(67) messenger RNA (mRNA). Colocalization of Fos and GAD(67) mRNA in recently mated males indicated that half of the activated cells in the PdPN, mSDA, and lateral MeApd are GABAergic. Colocalization of Fos and glutamate suggested that a quarter of the activated mSDA and lateral MeApd cells are glutamatergic. The PdPN does not appear to have glutamatergic cells. In the lateral MeApd, the percentage of activated cells that are GABAergic (45%) matches the percentage that project to the principal part of the bed nucleus of the stria terminalis (BST; 43%), and the percentage likely to be glutamatergic (27%) matches the percentage projecting to the mSDA (27%). The latter could help to trigger ejaculation. The distribution of GABAergic and putative glutamatergic cells in the caudal preoptic area, caudal BST, and medial amygdala of male gerbils is also described.

Obesity-inducing amygdala lesions: examination of anterograde degeneration and retrograde transport

Small lesions centered in the posterodorsal region of the medial amygdala resulted in excessive weight gains in female rats. Unilateral lesions were nearly as effective as bilateral lesions in the first 48 h after surgery (+21 to +32 g). Assessment of lesion damage was done by both qualitative evaluation and by a quantitative grid-point counting method. The critical sites for weight gain were the intra-amygdaloid bed nucleus of the stria terminalis and the posterodorsal medial amygdaloid nucleus. Incidental damage to the overlying globus pallidus was negatively related to weight gain. The cupric silver method for demonstrating axonal degeneration was applied to brains with obesity-inducing lesions. A dense pattern of degenerating terminals was found in the lateral septum, amygdala, ventral striatum, and ventromedial hypothalamus. Degeneration in the paraventricular nucleus of the hypothalamus was scarce or absent. Small retrograde tracer injections made in either the intra-amygdaloid bed nucleus of the stria terminalis or in the posterodorsal medial amygdaloid nucleus labeled cells in the amygdala, lateral septum, and hypothalamus, reciprocating the anterograde projections from the amygdala to these areas. The data suggest that subdivisions of the posterodorsal amygdala participate in the regulation of feeding in a manner that is similar to the better-known role of this part of the brain in mediating reproductive behavior. Although topographical differences may exist within the amygdaloid and hypothalamic subdivisions regulating these two sexually dimorphic behaviors, the relays engaged by feeding-related connections and those related to reproduction are remarkably parallel.

Differential effects of chronic escapable versus inescapable stress on male syrian hamster (Mesocricetus auratus) reproductive behavior

Stress decreases sexual activity, but it is uncertain which aspects of stress are detrimental to reproduction. This study used an escapable/inescapable stress paradigm to attempt to dissociate physical from psychological components of stress, and assess each component's impact on reproductive behavior in the male Syrian hamster (Mesocricetus auratus). Two experiments were completed using this protocol where two animals receive the same physical stressor (an electric footshock) but differ in the psychological aspect of control. One group (executive) could terminate the shock for themselves as well as a second group (yoked) by pressing a bar. Experiment 1 demonstrated a significant increase in plasma glucocorticoids at the end of a single 90-min stress session with no difference in glucocorticoid levels between the executive and yoked groups at any time point. Experiment 2 quantified male reproductive behavior prior to and immediately following 12 days of escapable or inescapable stress in executive, yoked, and no-stress control hamsters (n = 12/group). Repeated-measures analysis of variance revealed a number of significant changes in reproductive behavior before and after stress in the three treatment groups. The most striking difference was a decrease in hit rate observed only in the animals that could not control their stress (yoked group). Hit rate in the executive males that received the exact same physical stressor but could terminate the shock by pressing a bar was nearly identical to control animals that never received any foot shock. Therefore, we conclude that coping or control can ameliorate the negative effects of stress on male reproductive behavior.

The perirhinal–entorhinal cortex, but not the hippocampus, is critical for expression of individual recognition in the context of the Coolidge effect

The Coolidge effect is a phenomenon in which males show renewed sexual interest in a novel female following copulation to satiety with another female. In golden hamsters, this phenomenon depends on the ability to recognize conspecifics using chemosensory cues processed through the main olfactory system. Here we tested whether olfactory targets in the hippocampal system support this natural form of recognition memory. Male hamsters received ibotenic acid lesions of the perirhinal-entorhinal cortex (PR-ENT) or hippocampus (H) and were allowed to copulate to satiety with a female conspecific, then were presented with two anesthetized females, the familiar mate and an unfamiliar female that copulated with another male. Sham-operated and H-lesioned subjects preferentially investigated the novel female, indicating intact recognition of individual identity. By contrast, PR-ENT-lesioned males failed to discriminate familiar and novel females, and this deficit could not be attributed to abnormal copulatory behavior during mating. All subjects were able to detect and discriminate between female odors when presented in isolation during a habituation-discrimination test, indicating that behavioral deficits shown by PR-ENT males were not due to anosmia or a general investigatory deficit. Thus, the perirhinal-entorhinal cortex, but not the hippocampus, is critical for the recognition of familiar conspecifics in this naturalistic situation. This study reveals an essential role for the perirhinal-entorhinal cortex, but not the hippocampus, in a natural form of recognition memory within the social behavior of hamsters. The findings show a strikingly similar pattern to the effects of selective damage to the same brain regions on performance in standard recognition memory tasks by rats and monkeys. Therefore, the present data extend our understanding of the differential role of structures of the hippocampal memory system, showing continuity across species and between formal laboratory tests and the function of memory in natural social behavior.

Experience facilitates vomeronasal and olfactory influence on Fos expression in medial preoptic area during pheromone exposure or mating in male hamsters

Chemosensory stimuli are essential for mating in male hamsters but either main olfactory or vomeronasal input is sufficient in sexually experienced males. Activation in central chemosensory pathways and medial preoptic area, after stimulation with female chemosignals or after mating, was estimated by counting neurons expressing Fos protein in experienced and naive males, with or without vomeronasal organ lesions. Regions counted included main and accessory olfactory bulbs, corticomedial amygdala, bed nucleus stria terminalis and medial preoptic area. Chemosensory stimulation was more effective in activating medial preoptic area in experienced than in naive males. In experienced males with vomeronasal organs removed, main olfactory input was as effective in activating medial preoptic area as was the combination of main and accessory input available to intact animals. As previously reported, the main olfactory input remaining after vomeronasal lesions in naive males was poorly effective in activating medial preoptic area, and these animals had impaired mating behavior. The change in access of chemosensory input to medial preoptic area after experience suggests that an experience-dependent synaptic modulation in this pathway, perhaps in the amygdala, may underlie some changes in mating behavior with experience.

Repeated interactions with females elevate metabolic capacity in the limbic system of male rats

The effect of heterosexual social experience on brain metabolic capacity was investigated by measuring the activity of cytochrome oxidase, a rate-limiting enzyme in oxidative metabolism. Male Sprague-Dawley rats were kept naïve or allowed to copulate with receptive females three (3 F males) or 16 times (16 F males). Throughout the vomeronasal system and other limbic areas, 16 F males had elevated metabolic capacity relative to naïve and 3 F males, whereas no significant differences in brain metabolism were found between 3 F and naïve males. Behavioral differences were also found between 3 F and 16 F males. In a second experiment, we assessed differences in brain metabolism between sexually active and inactive males given only one opportunity to copulate and found no significant difference in neural metabolism between these males. This suggests that the differences found in the first experiment were primarily driven by differences in repeated experience rather than by sexual performance between 16 F and 3 F males. We speculate that these changes in brain metabolic capacity could be related to immediate early gene expression during copulation and could underlie the long-term behavioral changes accompanying heterosexual social experience.

Effects of yohimbine and apomorphine on the male sexual behaviour pattern of the golden hamster (Mesocricetus auratus)

It has been reported that the copulatory pattern of male hamsters differs from that displayed by most rodents. Besides mount, intromission and ejaculatory patterns, male hamsters display a peculiar copulatory pattern known as long intromission (LI). This peculiar behavioural pattern emerges after the male has been allowed to ejaculate repeatedly. Although LIs have been linked to sexual exhaustion, their functional meaning and their pharmacological regulation have not yet been elucidated. In this study, the sexual behaviour pattern of male golden hamsters was analysed after the administration of yohimbine and apomorphine, drugs that selectively acts on the noradrenergic and dopaminergic system, respectively. Both drugs have proved effective in inducing facilitation of masculine sexual behaviour in several species, including rodents. Results showed that, as in rats, the administration of yohimbine and apomorphine in male hamsters seems to have a stimulatory effect on masculine sexual behaviour, although their effects differ in characteristics and in intensity. In particular, after yohimbine administration, the onset of LIs appears sooner than in control subjects and it seems that they are linked to the number of ejaculations. In addition, sexual activity seems increased after the onset of LIs, including an increase in ejaculations and in the number of LIs. On the other hand, apomorphine administration induced just a slight stimulatory effect limited to ejaculatory latency and postejaculatory interval. Concerning LIs, apomorphine induced a complete disappearance of LIs in 60% of the subjects. The full significance of these findings remains to be elucidated.

Cell-body lesions of the posterodorsal preoptic nucleus or posterodorsal medial amygdala, but not the parvicellular subparafascicular thalamus, disrupt mating in male gerbils

In gerbils, the posterodorsal preoptic nucleus (PdPN) and the lateral part of the posterodorsal medial amygdala (MeApd) express Fos with ejaculation. In contrast, the medial/central part of the MeApd expresses Fos when a sexually experienced male reenters the environment associated with mating. The parvicellular part of the subparafascicular thalamic nucleus (SPFp) of gerbils expresses Fos under both conditions. To study the role of the PdPN and MeApd in male sex behavior, male gerbils were tested for mating before and after these areas were bilaterally lesioned by infusions of N-methyl-D-aspartate (NMDA). Controls received the vehicle or inactive isomer, NMLA. Lesions in either area reduced mounting, but MeApd lesions, which were more complete than PdPN lesions, also delayed ejaculation when males intromitted. To determine if the MeApd and PdPN affect mating via a common pathway, they were bilaterally disconnected by lesioning them unilaterally, contralateral to each other. Other groups received ipsilateral lesions, NMLA, or bilateral lesions of the PdPN or MeApd. In addition, the SPFp was studied using bilateral lesions. MeApd and PdPN lesions again decreased mounting, and this time both lesions, which were quite complete, delayed ejaculation when males intromitted. Contralateral lesions that bilaterally disconnected these cell groups from each other mimicked both effects. Thus, the MeApd and PdPN affect mounting and ejaculation, at least in part, via their connections with each other. In contrast, SPFp lesions did not affect mating. Thus, SPFp cells activated at ejaculation may react to ejaculation rather than trigger it, possibly initiating preparations for paternity.

Neurobiology of sexual behavior

Recent advances in the neurobiology of sexual behavior have helped to refine our understanding of the neuroanatomical, neuroendocrine and neurochemical systems that modulate responses to sexual stimulation. Both appetitive and consummatory sexual behaviors have been studied in several laboratory species and in humans using traditional and novel behavioral paradigms. New knowledge has emerged concerning the role of hypothalamic, limbic and brainstem structures, neuropeptides, brain monoamines and nitric oxide in the control of partner preference, sexual desire, erection, copulation, ejaculation, orgasm and sexual satiety. Brain imaging of visually evoked sexual arousal in humans has also been examined.

The medial extended amygdala in male reproductive behavior. A node in the mammalian social behavior network

Hormonal and chemosensory signals regulate social behaviors in a wide variety of mammals. In the male Syrian hamster, these signals are integrated in nuclei of the medial extended amygdala, where olfactory and vomeronasal system transmission is modulated by populations of androgen- and estrogen-sensitive neurons. Evidence from behavioral changes following lesions and from immediate early gene expression supports the hypothesis that the medial extended amygdala and medial preoptic area belong to a circuit that functions selectively in male sexual behavior. However, accumulated behavioral, neuroanatomical, and neuroendocrine data in hamsters, other rodents, and other mammals indicate that this circuit is embedded in a larger integrated network that controls not only male mating behavior, but female sexual behavior, parental behavior, and various forms of aggression. In this context, perhaps an individual animal's social responses can be more easily understood as a repertoire of closely interrelated, hormone-regulated behaviors, shaped by development and experience and modulated acutely by the environmental signals and the hormonal milieu of the brain.