Thinking about networks in the control of male hamster sexual behavior

Higher Rate of Male Sexual Displays Correlates with Larger Ventral Posterior Amygdala Volume and Neuron Soma Volume in Wild-Caught Common Side-Blotched Lizards, Uta stansburiana

Several areas of the vertebrate brain are involved in facilitating and inhibiting the production of sexual behaviors and displays. In the laboratory, a higher rate of sexual displays is correlated with a larger ventral posterior amygdala (VPA), an area of the brain involved in the expression of sexual display behaviors, as well as larger VPA neuronal somas. However, it remains unclear if individuals in the field reflect similar patterns, as there are likely many more selective pressures in the field that may also modulate the VPA architecture. In this study, we examined variation in VPA volume and neuron soma volume in wild-caught common side-blotched lizards (Uta stansburiana) from two different populations. In a population from Nevada, males experience high predation pressure and have decreased sexual display rates during the breeding season, whereas a population in Oregon has lower levels of predation and higher rates of male sexual displays. We found that wild-caught males from the population with lower display rates also exhibited decreased VPA volume and VPA neuron cell soma volume, which may suggest that decreased display rate, possibly due to increased predation rate, covaries with VPA attributes.

Reducing local synthesis of estrogen in the tubular striatum promotes attraction to same-sex odors in female mice

Brain-derived 17β-estradiol (E2) confers rapid effects on neural activity. The tubular striatum (TuS, also called the olfactory tubercle) is both capable of local E2 synthesis due to its abundant expression of aromatase and is a critical locus for odor-guided motivated behavior and odor hedonics. TuS neurons also contain mRNA for estrogen receptors α, β, and the G protein-coupled estrogen receptor. We demonstrate here that mRNA for estrogen receptors appears to be expressed upon TuS dopamine 1 receptor-expressing neurons, suggesting that E2 may play a neuromodulatory role in circuits which are important for motivated behavior. Therefore, we reasoned that E2 in the TuS may influence attraction to urinary odors which are highly attractive. Using whole-body plethysmography, we examined odor-evoked high-frequency sniffing as a measure of odor attaction. Bilateral infusion of the aromatase inhibitor letrozole into the TuS of gonadectomized female adult mice induced a resistance to habituation over successive trials in their investigatory sniffing for female mouse urinary odors, indicative of an enhanced attraction. All males displayed resistance to habituation for female urinary odors, indicative of enhanced attraction that is independent from E2 manipulation. Letrozole's effects were not due to group differences in basal respiration, nor changes in the ability to detect or discriminate between odors (both monomolecular odorants and urinary odors). Therefore, de novo E2 synthesis in the TuS impacts females' but not males' attraction to female urinary odors, suggesting a sex-specific influence of E2 in odor hedonics.

Animal Models for the Study of Female Sexual Dysfunction

INTRODUCTION

Significant progress has been made in elucidating the physiological and pharmacological mechanisms of female sexual function through preclinical animal research. The continued development of animal models is vital for the understanding and treatment of the many diverse disorders that occur in women.

AIM

To provide an updated review of the experimental models evaluating female sexual function that may be useful for clinical translation.

METHODS

Review of English written, peer-reviewed literature, primarily from 2000 to 2012, that described studies on female sexual behavior related to motivation, arousal, physiological monitoring of genital function and urogenital pain.

MAIN OUTCOMES MEASURES

Analysis of supporting evidence for the suitability of the animal model to provide measurable indices related to desire, arousal, reward, orgasm, and pelvic pain.

RESULTS

The development of female animal models has provided important insights in the peripheral and central processes regulating sexual function. Behavioral models of sexual desire, motivation, and reward are well developed. Central arousal and orgasmic responses are less well understood, compared with the physiological changes associated with genital arousal. Models of nociception are useful for replicating symptoms and identifying the neurobiological pathways involved. While in some cases translation to women correlates with the findings in animals, the requirement of circulating hormones for sexual receptivity in rodents and the multifactorial nature of women's sexual function requires better designed studies and careful analysis. The current models have studied sexual dysfunction or pelvic pain in isolation; combining these aspects would help to elucidate interactions of the pathophysiology of pain and sexual dysfunction.

CONCLUSIONS

Basic research in animals has been vital for understanding the anatomy, neurobiology, and physiological mechanisms underlying sexual function and urogenital pain. These models are important for understanding the etiology of female sexual function and for future development of pharmacological treatments for sexual dysfunctions with or without pain. Marson L, Giamberardino MA, Costantini R, Czakanski P, and Wesselmann U. Animal models for the study of female sexual dysfunction. Sex Med Rev 2013;1:108-122.

Chronic social stress in puberty alters appetitive male sexual behavior and neural metabolic activity

Repeated social subjugation in early puberty lowers testosterone levels. We used hamsters to investigate the effects of social subjugation on male sexual behavior and metabolic activity within neural systems controlling social and motivational behaviors. Subjugated animals were exposed daily to aggressive adult males in early puberty for postnatal days 28 to 42, while control animals were placed in empty clean cages. On postnatal day 45, they were tested for male sexual behavior in the presence of receptive female. Alternatively, they were tested for mate choice after placement at the base of a Y-maze containing a sexually receptive female in one tip of the maze and an ovariectomized one on the other. Social subjugation did not affect the capacity to mate with receptive females. Although control animals were fast to approach females and preferred ovariectomized individuals, subjugated animals stayed away from them and showed no preference. Cytochrome oxidase activity was reduced within the preoptic area and ventral tegmental area in subjugated hamsters. In addition, the correlation of metabolic activity of these areas with the bed nucleus of the stria terminalis and anterior parietal cortex changed significantly from positive in controls to negative in subjugated animals. These data show that at mid-puberty, while male hamsters are capable of mating, their appetitive sexual behavior is not fully mature and this aspect of male sexual behavior is responsive to social subjugation. Furthermore, metabolic activity and coordination of activity in brain areas related to sexual behavior and motivation were altered by social subjugation.

Chemosignals and hormones in the neural control of mammalian sexual behavior

Males and females of most mammalian species depend on chemosignals to find, attract and evaluate mates and, in most cases, these appetitive sexual behaviors are strongly modulated by activational and organizational effects of sex steroids. The neural circuit underlying chemosensory-mediated pre- and peri-copulatory behavior involves the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA) and ventromedial hypothalamus (VMH), each area being subdivided into interconnected chemoreceptive and hormone-sensitive zones. For males, MA-BNST connections mediate chemoinvestigation whereas the MA-MPOA pathway regulates copulatory initiation. For females, MA-MPOA/BNST connections also control aspects of precopulatory behavior whereas MA-VMH projections control both precopulatory and copulatory behavior. Significant gaps in understanding remain, including the role of VMH in male behavior and MPOA in female appetitive behavior, the function of cortical amygdala, the underlying chemical architecture of this circuit and sex differences in hormonal and neurochemical regulation of precopulatory behavior.

Adolescent brain maturation is necessary for adult-typical mesocorticolimbic responses to a rewarding social cue

The interpretation of social cues must change during adolescence in order to promote appropriate social interactions in adulthood. For example, adult, but not juvenile, male Syrian hamsters find female pheromones contained in vaginal sections (VS) rewarding, and only adult hamsters engage in sexual behavior with a receptive female. We previously demonstrated that the rewarding value of VS is both testosterone- and dopamine-dependent. Additionally, VS induces Fos expression throughout the mesocorticolimbic circuit in adult but not juvenile hamsters. In this study, we determined whether or not treatment of juvenile male hamsters with testosterone is sufficient to promote adult-like neural responses to VS. Juvenile and adult male hamsters were gonadectomized and given empty or testosterone-filled subcutaneous capsules for 1 week. Hamsters were then exposed to either clean or VS-containing mineral oil on their nares, and brains were collected 1 h later for immunohistochemistry to visualize Fos and tyrosine hydroxylase immunoreactive cells. Testosterone treatment failed to promote adult-typical patterns of Fos expression in juvenile hamsters; indeed, in some brain regions, juveniles exposed to VS expressed less Fos compared to age-matched controls while, as expected, adults exposed to VS expressed greater Fos compared to age-matched controls. Age-related changes in tyrosine hydroxylase expression were also observed. These data indicate that testosterone cannot activate the adult-typical pattern of Fos expression in response to female social cues in prepubertal males, and that additional neural maturation during adolescence is required for adult-typical mesocorticolimbic responses to female pheromones.

Chemosignals, hormones and mammalian reproduction

Many mammalian species use chemosignals to coordinate reproduction by altering the physiology and behavior of both sexes. Chemosignals prime reproductive physiology so that individuals become sexually mature and active at times when mating is most probable and suppress it when it is not. Once in reproductive condition, odors produced and deposited by both males and females are used to find and select individuals for mating. The production, dissemination and appropriate responses to these cues are modulated heavily by organizational and activational effects of gonadal sex steroids and thereby intrinsically link chemical communication to the broader reproductive context. Many compounds have been identified as "pheromones" but very few have met the expectations of that term: a unitary, species-typical substance that is both necessary and sufficient for an experience-independent behavioral or physiological response. In contrast, most responses to chemosignals are dependent or heavily modulated by experience, either in adulthood or during development. Mechanistically, chemosignals are perceived by both main and accessory (vomeronasal) olfactory systems with the importance of each system tied strongly to the nature of the stimulus rather than to the response. In the central nervous system, the vast majority of responses to chemosignals are mediated by cortical and medial amygdala connections with hypothalamic and other forebrain structures. Despite the importance of chemosignals in mammals, many details of chemical communication differ even among closely related species and defy clear categorization. Although generating much research and public interest, strong evidence for the existence of a robust chemical communication among humans is lacking.

Pubertally born neurons and glia are functionally integrated into limbic and hypothalamic circuits of the male Syrian hamster

During puberty, the brain goes through extensive remodeling, involving the addition of new neurons and glia to brain regions beyond the canonical neurogenic regions (i.e., dentate gyrus and olfactory bulb), including limbic and hypothalamic cell groups associated with sex-typical behavior. Whether these pubertally born cells become functionally integrated into neural circuits remains unknown. To address this question, we gave male Syrian hamsters daily injections of the cell birthdate marker bromodeoxyuridine throughout puberty (postnatal day 28-49). Half of the animals were housed in enriched environments with access to a running wheel to determine whether enrichment increased the survival of pubertally born cells compared with the control environment. At 4 wk after the last BrdU injection, animals were allowed to interact with a receptive female and were then killed 1 h later. Triple-label immunofluorescence for BrdU, the mature neuron marker neuronal nuclear antigen, and the astrocytic marker glial fibrillary acidic protein revealed that a proportion of pubertally born cells in the medial preoptic area, arcuate nucleus, and medial amygdala differentiate into either mature neurons or astrocytes. Double-label immunofluorescence for BrdU and the protein Fos revealed that a subset of pubertally born cells in these regions is activated during sociosexual behavior, indicative of their functional incorporation into neural circuits. Enrichment affected the survival and activation of pubertally born cells in a brain region-specific manner. These results demonstrate that pubertally born cells located outside of the traditional neurogenic regions differentiate into neurons and glia and become functionally incorporated into neural circuits that subserve sex-typical behaviors.

Novel odors affect gene expression for cytokines and proteinases in the rat amygdala and hippocampus

Olfaction in rodents provides an excellent modality for the study of cellular mechanisms of information processing and storage, since a single occurrence of precisely timed stimuli has high survival value. We have followed up preliminary evidence of cytokine and proteinase involvement in normal (as opposed to pathologically-induced) brain plasticity by surveying for the presence of these factors in the olfactory circuitry of the rat. Genes for 25-30 common cytokines and their receptors, and over 30 cell matrix and adhesion molecules were found to be expressed across the olfactory bulb, insular cortex, amygdala, and dorsal hippocampus. We then measured by real-time PCR the transcriptional expression of seven of these genes following a one-time exposure to the novel odor of blueberry bars or cornnuts, in contrast to presentation of the familiar odor of lab chow. In the amygdala significant up-regulation of interleukin-1 receptor 1 (IL1r1), interleukin-4 receptor (IL4r), fibroblast growth factor 13 (FGF13), and cathepsin-H (CtsH) was observed in males in response to the odor of cornnuts only. Changes were less consistent and widespread in the hippocampus, but were again sex specific for three genes: cathepsin-L (CtsL), matrix metalloproteinase-14 (MMP-14) and MMP-16. Our results show that transcription for several specific cytokines, growth factors, and proteinases responds to a one-time exposure to a novel odor, in a manner that tends to be region- and sex-specific. This suggests considerable variation in the way that olfactory information is processed at the cellular level in different brain regions and by the two sexes.

Winter day lengths counteract stimulatory effects of apomorphine and yohimbine on sexual behavior of male Syrian hamsters

Yohimbine and apomorphine selectively act on noradrenergic and dopaminergic neural substrates to augment male sexual behavior (MSB) in several rodent species. The present study assessed whether these drugs can overcome the suppressive effects of short winter-like day lengths on MSB. Yohimbine treatments that markedly increase copulatory behavior of male hamsters in long days were completely ineffective in facilitating MSB when injected after gonadal regression induced by 16 wks of short day lengths and after complete gonadal recrudescence after 32 wks of short days; apomorphine was similarly ineffective. The brain circuit that mediates MSB either may be less responsive to yohimbine and apomorphine in short than long days, or these drugs may not produce equivalent neurotransmitter changes in the two day lengths. After 32 wks of short-day treatment, all males had undergone testicular recrudescence and successfully ejaculated on initial tests with sexually receptive females after a hiatus of at least 4 mo during which they were denied mating opportunities. This suggests that overwintering males in the field are in a state of reproductive readiness at the outset of spring conditions favorable for survival of offspring.

Ontogeny of cytochrome p450 aromatase mRNA expression in the developing sheep brain

The intraneuronal conversion of testosterone to oestradiol constitutes a critical step in the development and sexual differentiation of the brain of many short gestation mammalian species and has been inferred to play a similar role in long gestation sheep. This conversion is catalysed by cytochrome P450 aromatase (CYP19), which is expressed in specific brain structures during foetal development. The present study was undertaken to examine the specific neuroanatomical distribution and relative expression of aromatase mRNA in the developing sheep hypothalamus. The foetal sheep is a highly tractable model system for localising the region-specific expression of aromatase in the brain during prenatal development that can help predict regions where oestrogen acts to shape neural development. Our results, obtained using real time quantitative reverse transcriptase-polymerase chain reaction, revealed that aromatase mRNA was expressed throughout mid to late gestation in the foetal preoptic area and amygdala. In the preoptic area, aromatase expression declined with advancing gestation, whereas, it increased in the amygdala. No sex differences were observed in either brain area. We next investigated the anatomical distribution of aromatase using in situ hybridisation histochemistry and found that the pattern of mRNA expression was largely established by midgestation. High expression was observed in the medial preoptic nucleus, bed nucleus of the stria terminalis and corticomedial amygdala. We also observed substantial expression in the dorsal striatum. These results extend our understanding of the developmental expression of aromatase in the foetal sheep brain and lend support to the view that it plays an essential role in sexual differentiation and maturation of the neuroendocrine, motor and reward control systems.

Dissociated functional pathways for appetitive and consummatory reproductive behaviors in male Syrian hamsters

In many species, including Syrian hamsters, the generation of male reproductive behavior depends critically on the perception of female odor cues from conspecifics in the environment. The behavioral response to these odors is mediated by a network of steroid-sensitive ventral forebrain nuclei including the medial amygdala (MA), posterior bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA). Previous studies have demonstrated that each of these three nuclei is required for appropriate sexual behavior and that MA preferentially sends female odor information directly to BNST and MPOA. It is unknown, however, how the functional connections between MA and BNST and/or MPOA are organized to generate different aspects of reproductive behavior. Therefore, the following experiments used the asymmetrical pathway lesion technique to test the role of the functional connections between MA and BNST and/or MPOA in odor preference and copulatory behaviors. Lesions that functionally disconnected MA from MPOA eliminated copulatory behavior but did not affect odor preference. In contrast, lesions that functionally disconnected MA from BNST eliminated preference for volatile female odors but did not affect preference for directly contacted odors or copulatory behavior. These results therefore demonstrate a double dissociation in the functional connections required for attraction to volatile sexual odors and copulation and, more broadly, suggest that appetitive and consummatory reproductive behaviors are mediated by distinct neural pathways.

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 naïve as well as experienced males. In rats tested both as naïve and after sexual experience, drug treatment produced more profound impairment in naïve 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.

Chemosensory and hormone information are relayed directly between the medial amygdala, posterior bed nucleus of the stria terminalis, and medial preoptic area in male Syrian hamsters

In many rodent species, including Syrian hamsters, the expression of appropriate social behavior depends critically on the perception and identification of conspecific odors. The behavioral response to these odors is mediated by a network of steroid-sensitive ventral forebrain nuclei including the medial amygdala (Me), posterior bed nucleus of the stria terminalis (BNST), and medial preoptic area (MPOA). Although it is well-known that Me, BNST, and MPOA are densely interconnected and each uniquely modulates odor-guided social behaviors, the degree to which conspecific odor information and steroid hormone cues are directly relayed between these nuclei is unknown. To answer this question, we injected the retrograde tracer, cholera toxin B (CTB), into the BNST or MPOA of male subjects and identified whether retrogradely-labeled cells in Me and BNST 1) expressed immediate early genes (IEGs) following exposure to male and/or female odors or 2) expressed androgen receptor (AR). Although few retrogradely-labeled cells co-localized with IEGs, a higher percentage of BNST- and MPOA-projecting cells in the posterior Me (MeP) expressed IEGs in response to female odors than to male odors. The percentage of retrogradely-labeled cells that expressed IEGs did not, however, differ between and female and male odor-exposed groups in the anterior Me (MeA), posterointermediate BNST (BNSTpi), or posteromedial BNST (BNSTpm). Many retrogradely-labeled cells co-localized with AR, and a higher percentage of retrogradely-labeled MeP and BNSTpm cells expressed AR than retrogradely-labeled MeA and BNSTpi cells, respectively. Together, these data demonstrate that Me, BNST, and MPOA interact as a functional circuit to process sex-specific odor cues and hormone information in male Syrian hamsters.

The anterior medial amygdala transmits sexual odor information to the posterior medial amygdala and related forebrain nuclei

In Syrian hamsters, reproductive behavior relies on the perception of chemical signals released from conspecifics. The medial amygdala (MEA) processes sexual odors through functionally distinct, but interconnected, sub-regions; the anterior MEA (MEAa) appears to function as a chemosensory filter to distinguish between opposite-sex and same-sex odors, whereas the posterodorsal MEA (MEApd) is critical for generating attraction specifically to opposite-sex odors. To identify how these sub-regions interact during odor processing, we measured odor-induced Fos expression, an indirect marker of neuronal activation, in the absence of either MEAa or MEApd processing. In Experiment 1, electrolytic lesions of the MEAa decreased Fos expression throughout the posterior MEA in male hamsters exposed to either female or male odors, whereas MEApd lesions had no effect on Fos expression within the MEAa. These results indicate that the MEAa normally enhances processing of sexual odors within the MEApd and that this interaction is primarily unidirectional. Furthermore, lesions of the MEAa, but not the MEApd, decreased Fos expression within several connected forebrain nuclei, suggesting that the MEAa provides the primary excitatory output of the MEA during sexual odor processing. In Experiment 2, we observed a similar pattern of decreased Fos expression, using fiber-sparing, NMDA lesions of the MEAa, suggesting that the decreases in Fos expression were not attributable exclusively to damage to passing fibers. Taken together, these results provide the first direct test of how the different sub-regions within the MEA interact during odor processing, and highlight the role of the MEAa in transmitting sexual odor information to the posterior MEA, as well as to related forebrain nuclei.

Facilitation of male sexual behavior in Syrian hamsters by the combined action of dihydrotestosterone and testosterone

Background

Testosterone (T) controls male Syrian hamster sexual behavior, however, neither of T's primary metabolites, dihydrotestosterone (DHT) and estradiol (E₂), even in highly supraphysiological doses, fully restores sexual behavior in castrated hamsters. DHT and T apparently interact with androgen receptors differentially to control male sexual behavior (MSB), but whether these two hormones act synergistically or antagonistically to control MSB has received scant experimental attention and is addressed in the present study.

Methodology/Principal Findings

Sexually experienced male Syrian hamsters were gonadectomized and monitored 5 weeks later to confirm elimination of the ejaculatory reflex (week 0), at which time they received subcutaneous DHT-filled or empty capsules that remained in situ for the duration of the experiment. Daily injections of a physiological dose of 25 µg T or vehicle commenced two weeks after capsule implantation. MSB was tested 2, 4 and 5 weeks after T treatment began. DHT capsules were no more effective than control treatment for long-term restoration of ejaculation. Combined DHT + T treatment, however, restored the ejaculatory reflex more effectively than T alone, as evidenced by more rapid recovery of ejaculatory behavior, shorter ejaculation latencies, and a greater number of ejaculations in 30 minute tests.

Conclusions/Significance

DHT and T administered together restored sexual behavior to pre-castration levels more rapidly than did T alone, whereas DHT and vehicle were largely ineffective. The additive actions of DHT and T on MSB are discussed in relation to different effects of these androgens on androgen receptors in the male hamster brain mating circuit.

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.

Lesions that functionally disconnect the anterior and posterodorsal sub-regions of the medial amygdala eliminate opposite-sex odor preference in male Syrian hamsters (Mesocricetus auratus)

In many rodent species, such as Syrian hamsters, reproductive behavior requires neural integration of chemosensory information and steroid hormone cues. The medial amygdala (MA) processes both of these signals through anatomically distinct sub-regions; the anterior region (MeA) receives substantial chemosensory input, but contains few steroid receptor-labeled neurons, whereas the posterodorsal region (MePD) receives less chemosensory input, but contains a dense population of steroid receptors. Importantly, these sub-regions have considerable reciprocal connections, and the goal of this experiment was therefore to determine whether interactions between MeA and MePD are required for male hamsters' preference to investigate female over male odors. To functionally disconnect MeA and MePD, males received unilateral lesions of MeA and MePD within opposite brain hemispheres. Control males received either unilateral lesions of MeA and MePD within the same hemisphere or sham surgery. Odor preferences were measured using a 3-choice apparatus, which simultaneously presented female, male and clean odor stimuli; all tests were done under conditions that either prevented or allowed contact with the odor sources. Under non-contact conditions, males with asymmetrical lesions investigated female and male odors equally, whereas males in both control groups preferred to investigate female odors. Under contact conditions, all groups investigated female odors longer than male odors, although males with asymmetrical lesions displayed decreased investigation of female odors compared to sham males. These data suggest that MeA-MePD interactions are critical for processing primarily the volatile components of social odors and highlight the importance of input from the main olfactory system (MOS) to these nuclei in the regulation of reproductive behavior. More broadly, these results support the role of the MA in integrating chemosensory and hormone information, a process that may underlie social odor processing in a variety of behavioral contexts.

The organization of feedback projections in a pathway important for processing pheromonal signals

In most of the mammalian sensory systems there are massive cortical feedback projections to early processing stations. The mammalian accessory olfactory system is considered unique in several aspects. It is specialized for processing pheromonal signals and plays a critical role in regulating sociosexual behaviors. Furthermore, pheromonal signals are believed to bypass cortex and reach the hypothalamic behavioral centers after merely three forward projections. Because the organization of the feedback projections in the accessory olfactory system remains largely unclear, the importance of the feedback projections in the processing of pheromonal signals has been ignored. Here we show that in mice the feedback projections from the bed nucleus of stria terminalis (BST) and the vomeronasal amygdala to the accessory olfactory bulb (AOB) are topographically organized and use different neurotransmitters. By retrograde and anterograde tracing, we find that the feedback projection from the BST terminates in the AOB mitral cell layer, whereas that from the amygdala terminates in the AOB granule cell layer. By combining tracing, genetic labeling of GABAergic neurons, and immunostaining against a marker of glutamatergic synapses, we observe that the BST-to-AOB projection is GABAergic whereas the amygdala-to-AOB projection is glutamatergic. In addition, a substantial number of feedback neurons in the amygdala and BST express estrogen receptors. Thus, the accessory olfactory system, like other sensory systems, possesses extensive feedback projections. Moreover, our results suggest that central hormonal cues may modulate the processing of pheromonal signals at early stations through the precisely organized feedback projections.

Infrequent low dose testosterone treatment maintains male sexual behavior in Syrian hamsters

Testosterone (T) secreted in short pulses several times each day is essential for the maintenance of male sex behavior (MSB) in mammals. Blood T concentrations are relatively low during inter-pulse intervals. Assessment of androgenic influences on MSB of rodents has, with very few exceptions, involved either injections of pure or esterified hormones dissolved in oil or implantation of constant release capsules that generate supraphysiological and/or constantly elevated T concentrations. The minimum daily concentration of T necessary to maintain and restore MSB when T is delivered as a discrete short pulse remains unspecified; nor is it known whether infrequent T pulses in the physiological range sustain MSB. To address these questions, we varied T injection concentrations and frequencies in castrated, sexually-experienced Syrian hamsters. All males injected daily with an aqueous vehicle failed to display the ejaculatory reflex 5 weeks after castration. Once daily 15 microg subcutaneous T injections both maintained and restored MSB, whereas once daily 5 microg T injections resulted in fewer males ejaculating and longer ejaculation latencies. Substantially higher T doses were required to restore MSB in previous studies when T was administered in an oil vehicle. 50 microg T maintained MSB in most hamsters injected once every 4 or 7 days, despite long intervals between injections during which circulating T was undetectable or well below physiological concentrations. Some T regimens that maintained MSB were associated with subnormal seminal vesicle and ventral prostate weights. The demonstration that relatively brief, infrequent elevations of T are sufficient to support MSB provides a useful model to assess the neuroendocrine basis of MSB and raises the possibility that infrequent low dose androgen replacement protocols may restore sex behavior to hypogonadal men without inducing some of the negative side-effects associated with more frequent, higher dose treatments.

A Role for Preoptic Glutamate in the Regulation of Male Reproductive Behavior

Although much progress has been made toward understanding the role of the medial preoptic area (MPOA) in the regulation of male reproductive behaviors, the precise mechanisms responsible for its activation during mating are largely unclear. Several studies implicate glutamate in this response. However, not until recently was there direct evidence supporting this hypothesis. Results obtained using in vivo microdialysis showed that levels of glutamate increased in the MPOA during mating, particularly with ejaculation. Levels then decreased rapidly following ejaculation, during a period of sexual quiescence. The magnitude of this decrease correlated with time spent in quiescence. Additionally, central administration of glutamate uptake inhibitors increased levels of glutamate and facilitated behavior. Glutamate activation of N-methyl-D-aspartate (NMDA) receptors in the MPOA is at least partly responsible for behavioral effects evoked by increase glutamate. This is evidenced by histological analysis of the MPOA, which shows that nearly all cells containing mating-induced Fos also contained NMDA receptors. Mating also increased phosphorylation of NMDA receptors, indicating receptor activation. Finally, bilateral microinjections of NMDA receptor antagonists inhibited copulation. This neurochemical, anatomical, and behavioral evidence points to a key role of preoptic glutamate in the regulation of sexual behavior in males. The implications of these findings are discussed.

Masculinization induced by neonatal exposure to PGE(2) or estradiol alters c-fos induction by estrous odors in adult rats

Processing of relevant olfactory and pheromonal cues has long been known as an important process necessary for social and sexual behavior in rodents. Several nuclei that receive input from the vomeronasal projection pathway are involved in sexual behavior and show changes in immediate early gene expression after stimulation with a variety of sex-related stimuli. The nuclei in this pathway are sexually dimorphic due to the early patterning events induced by estradiol derived from testicular androgens, which developmentally defeminize and masculinize the brain and adult sexual behavior. Masculinization can be induced independently of estradiol via prostaglandin-E(2) (PGE(2)), and therefore assessed separately from defeminization. Here we examined the effects of brain defeminization and masculinization on neuronal response to sex-related odors using Fos, the protein product of the immediate early gene c-fos, as an indicator of activity. Female rat pups treated with a cyclooxygenase-2 inhibitor, to reduce PGE(2), plus estradiol, estradiol alone, and PGE(2) alone were exposed to estrous female odor as adults and the resulting Fos expression was examined in the medial amygdala, preoptic area, and ventromedial nucleus of the hypothalamus. Defeminized and/or masculinized females all showed patterns of Fos activity similar to control males and significantly different from control females. These results suggest that early exposure to estradiol and PGE(2) do not affect olfaction in females, but switch the activity pattern of sex-related nuclei in females to resemble that of males following exposure to sexually-relevant cues.

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.

Olfactory experience and the development of odor preference and vaginal marking in female Syrian hamsters

Rodent reproductive behavior relies heavily on odor processing, and evidence suggests that many odor-guided sexual behaviors are shaped by prior experience. We sought to determine if exposure to male odors during development is required for the adult expression of proceptive sexual behavior toward male odors in female Syrian hamsters. Exposure to male odors was restricted in naïve subjects by removing all male siblings from the litter at three to five days of age. Control litters were also culled, but included equal numbers of male and female pups. As adults, naïve females displayed investigatory preferences toward male odors in a Y-maze that were comparable to control females; this preference was observed whether contact with the odor stimuli was prevented of allowed. In contrast, naïve females vaginal scent-marked equally toward male and female volatile odors, suggesting an inability to target behavior toward sexually relevant odors. However, naïve females marked preferentially toward male odors when allowed to contact the odor stimuli. These results provide evidence for the experience-dependent development of vaginal marking behavior toward volatile components of sexual odors. Furthermore, they suggest that distinct mechanisms regulate the development of odor preferences and vaginal marking behavior in this species.

Short duration testosterone infusions maintain male sex behavior in Syrian hamsters

In most mammalian species, reduced androgen availability is associated with marked reductions in male sexuality; conversely, androgen replacement in castrated males restores sex behavior within a few weeks. Testosterone (T) pulse duration, amplitude, frequency, and inter-pulse interval may be as important as total amount of hormone in determining target tissue responsiveness. We remain ignorant of the number and duration of daily T pulses necessary and sufficient to sustain male mating behavior. An in-dwelling infusion system was employed to vary T-pulse frequencies and durations. Daily 4 h infusions of aqueous T (100 microg/0.064 ml) and twice daily 4 h pulses of T (each 50 microg/0.064 ml) were sufficient to maintain ejaculatory behavior of sexually experienced castrated hamsters for 11 weeks post-castration; castrated hamsters infused with vehicle ceased to display the ejaculatory pattern 3 weeks after gonadectomy. Circulating T concentrations of hormone-infused hamsters declined markedly 7 h after the termination of each infusion. These results establish that male sex behavior can be sustained with infusions of relatively low T concentrations for 4 h/day and suggests that the basal concentrations of T sustained by the gonad during inter-pulse intervals may not be necessary for maintenance of sex behavior. 4 h T infusions were sufficient to maintain penile and seminal vesicles weights, but not ventral prostate weights or flank gland dimensions; the threshold for maintaining male sex behavior is lower than that for some androgen-dependent peripheral structures. Development of effective androgen replacement regimens that sustain sex behavior in castrated animals may be useful in the design of androgen replacement therapy for hypogonadal men.

Androgen receptor expression and morphology of forebrain and neuromuscular systems in male green anoles displaying individual differences in sexual behavior

Investigating individual differences in sexual performance in unmanipulated males is important for understanding natural relationships between behavior and morphology, and the mechanisms regulating them. Among male green anole lizards, some court and copulate frequently (studs) and others do not (duds). To evaluate potential factors underlying differences in the level of these behaviors, morphology and androgen receptor expression in neuromuscular courtship and copulatory structures, as well as in the preoptic area and amygdala, were compared in males displaying varying degrees of sexual function. This study revealed that individual differences in behavior among unmanipulated males, in particular the extension of a throat fan (dewlap) used during courtship, were positively correlated with the size of fibers in the associated muscle and with soma size in the amygdala. The physiological response to testosterone, as indicated by the height of cells in an androgen-sensitive portion of the kidney, was also correlated with male sexual behavior, and predicted it better than plasma androgen levels. Androgen receptor expression was not related to the display of courtship or copulation in any of the tissues examined. The present data indicate that higher levels of male courtship behavior result in (or are the result of) enhanced courtship muscle and amygdala morphology, and that androgen-sensitive tissue in studs may be more responsive to testosterone than duds. However, some mechanism(s) other than androgen receptor expression likely confer this difference in responsiveness.

Effects of season, testosterone and female exposure on c-fos expression in the preoptic area and amygdala of male green anoles

Expression of the immediate early gene, c-fos, was used to investigate changes in neuronal activity in forebrain regions involved in male sexual behavior following social, hormonal and/or seasonal manipulations in the male green anole. These factors all influence behavior, yet it is unclear how they interact to modify neuronal activity in forebrain regions, including the preoptic area (POA) and ventromedial nucleus of the amygdala (AMY). These regions are involved in the display of sexual behaviors in male green anoles as in many other vertebrates. To determine the effects of seasonal, hormonal and social cues on these brain areas, we investigated c-fos under environmental conditions typical of the breeding or non-breeding season in adult male green anoles that were castrated and implanted with either testosterone (T) or blank (Bl) capsules. We also manipulated social cues by exposing only half of the animals in each group to females. T enhanced courtship and copulatory behaviors, but decreased c-fos expression in the AMY. A similar, although not statistically significant, pattern was observed in the POA, and the density of c-fos+ cells was negatively correlated in that region with the number of extensions of a throat fan (dewlap) used during courtship. Therefore, it appears that in the male green anole, T may diminish c-fos expression (likely in inhibitory neurons) in the POA and AMY to create a permissive environment in which the appropriate behavioral response can be displayed.

Amygdala function in adolescents with congenital adrenal hyperplasia: a model for the study of early steroid abnormalities

Early disruption of steroids affects the development of mammalian neural circuits underlying affective processes. In humans, patients with classic congenital adrenal hyperplasia (CAH) can serve as a natural model to study early hormonal alterations on functional brain development. CAH is characterized by congenital glucocorticoid insufficiency, leading to altered hypothalamic-pituitary-adrenal (HPA) function, and hyperandrogenism. Using fMRI, we compared fourteen adolescents with CAH to 14 healthy controls on amygdala response to a face viewing task. In response to negative facial emotions, CAH females activated the amygdala significantly more than healthy females, whereas CAH males did not differ from control males. Furthermore, females with CAH showed a similar pattern of amygdala activation to control males, suggesting virilized amygdala function in females with CAH. These findings suggest a prominent effect of early hyperandrogenism on the development and function of the amygdala in females with CAH, whereas no effects were detected in males with CAH. This study provides data that can be further tested in a model of the neurobiological mechanisms underlying early androgen organizational effects on amygdala function.

Post-castration retention of reproductive behavior and olfactory preferences in male Siberian hamsters: role of prior experience

Reproductive behavior of virtually all adult male rodents is dependent on concurrent availability of gonadal steroids. The ejaculatory reflex is incompatible with long-term absence of testicular steroids and typically disappears within 3 weeks after castration. Male Siberian hamsters are an exception to this rule; mating culminating in the ejaculatory reflex occurs as many as 6 months after castration (persistent copulation). The emergence of persistent copulation many weeks after gonadectomy is here shown not to require repeated post-castration sexual experience. Preoperative sexual experience, on the other hand, significantly increases the percent of males that copulate after gonadectomy, but is not required for the emergence of this trait in 25% of males. Castration prior to puberty prevents persistent copulation in all individuals in adulthood. Persistent copulators, unlike males that cease mating activity after castration, prefer the odors of estrous over non-estrous females when tested 4 months after castration and 7 weeks after the last mating test. Neural circuits of persistent copulators retain the ability to mediate male sex behavior and preferences for female odors in the complete absence of gonadal steroids; they are influenced by preoperative sexual experience and organizational effects of gonadal hormones at the time of puberty.

Chemosensory and steroid-responsive regions of the medial amygdala regulate distinct aspects of opposite-sex odor preference in male Syrian hamsters

In rodent species, such as the Syrian hamster, the expression of sexual preference requires neural integration of social chemosensory signals and steroid hormone cues. Although anatomical data suggest that separate pathways within the nervous system process these two signals, the functional significance of this separation is not well understood. Specifically, within the medial amygdala, the anterior region (MEa) receives input from the olfactory bulbs and other chemosensory areas, whereas the posterodorsal region (MEpd) contains a dense population of steroid receptors and receives less substantial chemosensory input. Consequently, the MEa may subserve a primarily discriminative function, whereas the MEpd may mediate the permissive effects of sex steroids on sexual preference. To test these hypotheses, we measured preference and attraction to female and male odors in males with lesions of either the MEa or MEpd. In Experiment 1, lesions of either region eliminated opposite-sex odor preferences. Importantly, MEpd-lesioned males displayed decreased attraction toward female odors, suggesting decreased sexual motivation. In contrast, MEa-lesioned males displayed high levels of investigation of both male and female odors, suggesting an inability to categorize the relevance of the odor stimuli. In Experiment 2, we verified that both MEa- and MEpd-lesioned males could discriminate between female and male odors, thereby eliminating the possibility that the observed lack of preference reflected a sensory deficit. Taken together, these results suggest that both the MEa and MEpd are critical for the expression of opposite-sex odor preference, although they appear to mediate distinct aspects of this 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.

Olfactory preference and Fos expression in the accessory olfactory system of male rats with bilateral lesions of the medial preoptic area/anterior hypothalamus

In the present study we evaluated if a medial preoptic area/anterior hypothalamus lesion affects the olfactory preference toward soiled bedding from receptive females in comparison to bedding from anestrous females or clean bedding. In the second part of the study we evaluated the accessory olfactory system response to estrous bedding with Fos immunoreactivity to determine if the preoptic lesions modify the processing of sexually relevant olfactory cues. Before medial preoptic area/anterior hypothalamus lesions, male rats spent more time investigating estrous bedding as opposed to anestrous or clean bedding. After the lesion, subjects showed no preference between estrous and anestrous bedding; that is, males spent the same amount of time investigating both types of bedding. These two odors were investigated more than clean bedding. Increments in Fos immunoreactivity neurons were seen in structures of the accessory olfactory system after exposure to soiled estrous bedding [granular layer of the accessory olfactory bulb, anterior-dorsal medial amygdala, posterior-dorsal medial amygdala, bed nucleus of the stria terminalis]. These results suggest that bilateral destruction of the medial preoptic area/anterior hypothalamus modify male olfactory preference in such a way that subjects spend the same time smelling and investigating bedding from estrous and anestrous females. This change in olfactory preference is not associated with alterations in the processing of sexually relevant olfactory cues by the accessory olfactory system.

The effects of mating stimulation on c-Fos immunoreactivity in the female hamster medial amygdala are region and context dependent

During mating in hamsters, both tactile and nontactile sensory stimulation experienced by the female affect sexual behavior and progestational neuroendocrine reflexes. To test the interactions of these types of mating stimulation, c-Fos immunohistochemistry measured brain cellular activity during sexual behavior under conditions that included combinations of tactile and nontactile mating stimulation. Test groups received: (1) mating stimulation from a male, females being either fully mated or mated while wearing a vaginal mask, or (2) experimenter applied manual vaginocervical stimulation (VCS)-with or without males present, or (3) handling similar to VCS but without insertions-with or without males present. Numbers of c-Fos immunoreactive cells were counted in specific subdivisions of the posterior medial amygdala (MeP) and ventromedial hypothalamus (VMH). The medial amygdala dorsal and ventral subdivisions responded differentially to components of mating stimulation. The posterodorsal Me (MePD) cellular activation was greatest during mating conditions that included VCS and/or males present. However, the posteroventral Me (MePV) was sensitive to male exposure and not to VCS. Also, MePV and VMH shell responses mirrored each other, both being primarily sensitive to male exposure. In separate tests, manual VCS induced pseudopregnancy, though the procedure was most effective with additional nontactile stimulation from males present. In summary, contextual cues provided by nontactile male stimulation enhance the effect of vaginocervical and other tactile stimulation on reproductive processes. Furthermore, c-Fos expression in the female hamster medial amygdala is region and context dependent.

Steroid hormone modulation of olfactory processing in the context of socio-sexual behaviors in rodents and humans

Primer pheromones and other chemosensory cues are important factors governing social interactions and reproductive physiology in many species of mammals. Responses to these chemosignals can vary substantially within and between individuals. This variability can stem, at least in part, from the modulating effects steroid and non-steroid hormones exert on olfactory processing. Such modulation frequently augments or facilitates the effects that prevailing social and environmental conditions have on the reproductive axis. The mechanisms underlying the hormonal regulation of responses to chemosensory cues are diverse. They are in part behavioral, achieved through the modulation of chemoinvestigative behaviors, and in part a product of the modulation of the intrinsic responsiveness of the main and accessory olfactory systems to conspecific, as well as other classes, of chemosignals. The behavioral and non-behavioral effects complement one another to ensure that mating and other reproductive processes are confined to reproductively favorable conditions.

Vasopressin V1b receptor knockout reduces aggressive behavior in male mice

Increased aggression is commonly associated with many neurological and psychiatric disorders. Current treatments are largely empirical and are often accompanied by severe side effects, underscoring the need for a better understanding of the neural bases of aggression. Vasopressin, acting through its 1a receptor subtype, is known to affect aggressive behaviors. The vasopressin 1b receptor (V1bR) is also expressed in the brain, but has received much less attention due to a lack of specific drugs. Here we report that mice without the V1bR exhibit markedly reduced aggression and modestly impaired social recognition. By contrast, they perform normally in all the other behaviors that we have examined, such as sexual behavior, suggesting that reduced aggression and social memory are not simply the result of a global deficit in sensorimotor function or motivation. Fos-mapping within chemosensory responsive regions suggests that the behavioral deficits in V1bR knockout mice are not due to defects in detection and transmission of chemosensory signals to the brain. We suggest that V1bR antagonists could prove useful for treating aggressive behavior seen, for example, in dementias and traumatic brain injuries.

Effects of angiotensin II microinjected into medial amygdala on male sexual behavior in rats

Research was undertaken to study the role of central angiotensin in the modulation of male sexual behavior, testing the effect of angiotensin II (Ang II) injections into the medial amygdaloid nucleus (MeA). The sexual behavior of adult male Wistar rats was evaluated, 15 min after bilateral intra-amygdaloid microinjection (0.3 microl) of saline and 5 doses of Ang II: 10; 25; 50; 100, and 150 fmol. The effects of the Ang II receptor blockade were also studied. We tested the effect of coinjection of Ang II (50 fmol) with the AT1 antagonist, losartan (20 pmol) and the AT2 antagonist, CGP 42112 (1 pmol). Ang II inhibited sexual behavior and this inhibition was prevented by the coinjection of AT1 antagonist, losartan, or the AT2 antagonist, CGP 42112. Results show that Ang II has a powerful effect on male sexual behavior, which may be mediated by both AT1 and AT2 receptors.

Vernal changes in the behavioral and endocrine responses to GnRH application in male European ground squirrels

This field study was aimed at examining hypothalamic involvement in the behavioral changes of male European ground squirrels (Spermophilus citellus) before, during, and after the mating season. The effects of exogenous gonadotropin-releasing hormone (GnRH) application on androgen secretion and behavioral patterns were investigated. Animals were captured, bled, and injected intramuscularly with 40 ng/100 g of GnRH. A second plasma sample was collected 40 min after the treatment to document changes in testosterone secretion. Behavioral parameters such as intra-sexual aggression, scent marking, and home range size were compared on the days before and after the stimulation. In the first two phases, before female emergence and during mating, GnRH-injection caused increases in plasma testosterone. In the post-mating phase, initial plasma testosterone levels had decreased and no elevation could be induced. Sham treatment of controls had no effect in any phase. Conditional parameters like emergence body mass and testicular size covaried with androgen increases only in the pre-mating period. Behavioral changes after GnRH administration occurred during the pre-mating period. Intra-sexual aggression, scent marking, and home range size increased significantly in experimental individuals. Later, during mating and post-mating, we found no behavioral changes associated with the GnRH treatment or the testosterone increase. The results demonstrate changes in the endocrine and behavioral sensitivity to GnRH application, according to the phases of the active season. An exogenous pulse of GnRH can apparently release behavior in male European ground squirrels, which is normally context dependent with the emergence of females.

Stimulation of the medial amygdala enhances medial preoptic dopamine release: implications for male rat sexual behavior

Increased dopamine (DA) in the medial preoptic area (MPOA) facilitates male sexual behavior. A major source of innervation to the MPOA is the medial amygdala (MeA). We now report that chemical stimulation of the MeA enhanced levels of extracellular MPOA DA in anesthetized male rats. These results suggest that DA activity in the MPOA can be regulated by input from the MeA to the MPOA.

Distribution of gonadal steroid receptor-containing neurons in the preoptic-periaqueductal gray-brainstem pathway: a potential circuit for the initiation of male sexual behavior

The present study used anterograde and retrograde tract tracing techniques to examine the organization of the medial preoptic-periaqueductal gray-nucleus paragigantocellularis pathway in the male rat. The location of neurons containing estrogen (alpha subtype; ER alpha) and androgen receptors (AR) were also examined. We report here that injection of the anterograde tracer biotinylated dextran amine (BDA) into the medial preoptic (MPO) produced dense labeling within the periaqueductal gray (PAG); anterogradely labeled fibers terminated in close juxtaposition to neurons retrogradely labeled from the nucleus paragigantocellularis (nPGi). Dual immunostaining for Fluoro-Gold (FG) and ER alpha or FG and AR showed that over one-third of MPO efferents to the PAG contain receptors for either estrogen or androgen. In addition, approximately 50% of PAG neurons retrogradely labeled from the nPGi were immunoreactive for either ER alpha or AR. These results are the first to establish an MPO-->PAG-->nPGi circuit and further indicate that gonadal steroids can influence neuronal synaptic activity within these sites. We reported previously that nPGi reticulospinal neurons terminate preferentially within the motoneuronal pools of the lumbosacral spinal cord that innervate the pelvic viscera. Together, we propose that the MPO-->PAG-->nPGi circuit forms the final common pathway whereby MPO neural output results in the initiation and maintenance of male copulatory reflexes.

Regulation by the medial amygdala of copulation and medial preoptic dopamine release

The medial preoptic area (MPOA) is a critical integrative site for male copulatory behavior in most vertebrate species. Extracellular dopamine (DA) is increased in the MPOA of male rats immediately before and during copulation. DA agonists microinjected into the MPOA of male rats facilitate and DA antagonists inhibit sexual behavior. A major source of input to the MPOA is the medial amygdala (MeA), which processes and relays olfactory information to the MPOA. We now report that microinjections of a DA agonist into the MPOA of animals with excitotoxic lesions of the amygdala restored copulatory ability that was lost after the lesions. Moreover, radio-frequency lesions of the MeA impaired copulation and blocked the increases in extracellular DA seen in animals with sham lesions during exposure to a receptive female and during copulation. Thus, both copulatory ability and the MPOA DA response, during exposure to a receptive female and during copulation, are facilitated by input from the MeA to the MPOA.

Oestrogen receptor alpha is essential for female-directed chemo-investigatory behaviour but is not required for the pheromone-induced luteinizing hormone surge in male mice

The expression of normal masculine sexual behaviour requires testosterone. Testosterone can bind to androgen receptors, either in its native form, or after reduction to other androgen metabolites. In addition, testosterone can be aromatized to oestrogen, and bind to oestrogen receptor alpha and/or beta. Male copulatory behaviour is deficient in mice lacking functional oestrogen receptor alpha gene (ERalphaKO mice). We sought to determine which aspect(s) of masculine sexual behaviour is compromised in the ERalphaKOs. Specifically, we asked whether ERalphaKO males have reduced motivation and/or an inability to recognize oestrous females. We found significant differences between mice of different genotypes in the amount of chemo-investigatory behaviour displayed and in the target of their investigation. Wild-type males spent more time investigating ovariectomized, oestradiol-treated females, than either males, or ovariectomized females that had not received hormone priming. ERalphaKO males spent little time investigating any of the stimulus mice and showed no preferences. To test the hypothesis that this lack of chemo-investigatory behaviour is due to the inability of ERalphaKO males to detect and respond to female pheromones, we exposed males to chemosensory cues (soiled bedding) from females. Males resided in clean, or female-soiled, cage bedding for 60 min. Next, blood was collected and plasma luteinizing hormone (LH) assayed. We also assessed Fos-like immunoreactivity (Fos-ir) in several neural regions involved in processing chemosensory cues. Despite the fact that male ERalphaKOs spend little time engaged in chemo-investigation of females, their neuroendocrine responses to female-soiled bedding were similar to those seen in wild-type males. Our data suggest that the normal coupling between the neuroendocrine response to females and the generation of sexual behaviour is disrupted in ERalphaKO mice. Responses to female pheromones do not require ERalpha. However, normal male sexual performance requires the ERalpha gene.

Seasonal specificity of hormonal, behavioral, and coloration responses to within- and between-sex encounters in male lizards (Sceloporus undulatus)

This study reports the gender and seasonal specificity of hormonal, behavioral, and coloration responses displayed by "resident" male lizards (Sceloporus undulatus) exposed to male or female "intruders" during staged encounters in outdoor enclosures. Resident males were engaged in staged encounters with males or females for 1 h per day on 9 consecutive days during the breeding and postbreeding seasons. Male-specific responses occurred during the breeding but not the postbreeding season. These included (1) a transient increase in plasma testosterone (T) that was evident on Day 4 and had subsided by Day 10, (2) behavioral displays of aggression (full shows and chases), and (3) a lightening of dorsal integumental color. Female-specific behavioral responses (nod sets) were displayed in both seasons. Season-specific responses consisted only of a transient increase in plasma corticosterone (B) during the breeding season that was evident on Day 4 and had subsided by Day 10. Pushups were displayed in response to both genders during both seasons, although the frequency of pushups was significantly higher in response to females than to males during the postbreeding season. The coloration of residents did not change in response to male intruders during the postbreeding season or to females during either season. These results define the gender and seasonal specificity of hormonal, behavioral, and coloration responses of resident male S. undulatus in social interactions with conspecifics. Thus, our results clarify the biological significance of these responses in terms of potentially aggressive versus courtship interactions and breeding versus postbreeding contexts.