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

Role of aromatase in distinct brain nuclei of the social behaviour network in the expression of sexual behaviour in male Japanese quail

In male Japanese quail, brain aromatase is crucial for the hormonal activation of sexual behaviour, but the sites producing neuro-oestrogens that are critical for these behaviours have not been completely identified. This study examined the function of aromatase expressed in several nuclei of the social behaviour network on a measure of sexual motivation known as the frequency of rhythmic cloacal sphincter movements (RCSM) and on copulatory behaviour. Sexually experienced castrated males chronically treated with testosterone were stereotaxically implanted with the aromatase inhibitor vorozole (VOR), or cholesterol as control, and tested for sexual behaviour. In experiment 1, males were implanted in the medial preoptic nucleus (POM) with VOR, a manipulation known to reduce the expression of copulatory behaviour. This experiment served as positive control, but also showed that VOR implanted in the dorsomedial or lateral portions of the POM similarly inhibits male copulatory behaviour compared to control implants. In experiments 2 to 4, males received stereotaxic implants of VOR in the periaqueductal gray (PAG), the nucleus taeniae of the amygdala (TnA) and the ventromedial nucleus of the hypothalamus (VMN), respectively. Sexual behaviour was affected only in individuals where VOR was implanted in the PAG: these males displayed significantly lower frequencies of cloacal contact movements, the last step of the copulatory sequence. Inhibition of aromatase in the TnA and VMN did not alter copulatory ability. Overall, RCSM frequency remained unaffected by VOR regardless of implantation site. Together, these results suggest that neuro-oestrogens produced in the POM contribute the most to the control of male copulatory behaviour, while aromatase expressed in the PAG might also participate to premotor aspects of male copulatory behaviour.

Neural and Hormonal Basis of Opposite-Sex Preference by Chemosensory Signals

In mammalian reproduction, sexually active males seek female conspecifics, while estrous females try to approach males. This sex-specific response tendency is called sexual preference. In small rodents, sexual preference cues are mainly chemosensory signals, including pheromones. In this article, we review the physiological mechanisms involved in sexual preference for opposite-sex chemosensory signals in well-studied laboratory rodents, mice, rats, and hamsters of both sexes, especially an overview of peripheral sensory receptors, and hormonal and central regulation. In the hormonal regulation section, we discuss potential rodent brain bisexuality, as it includes neural substrates controlling both masculine and feminine sexual preferences, i.e., masculine preference for female odors and the opposite. In the central regulation section, we show the substantial circuit regulating sexual preference and also the influence of sexual experience that innate attractants activate in the brain reward system to establish the learned attractant. Finally, we review the regulation of sexual preference by neuropeptides, oxytocin, vasopressin, and kisspeptin. Through this review, we clarified the contradictions and deficiencies in our current knowledge on the neuroendocrine regulation of sexual preference and sought to present problems requiring further study.

Overcoming neonatal sickness: Sex-specific effects of sickness on physiology and social behavior

Early-life environmental stressors, including sickness, have the potential to disrupt development in ways that could severely impact fitness. Despite what is known about the effects of sickness on reproduction, the precise physiological mechanisms have not yet been determined. The goal of this study was to investigate the effects of a neonatal immune challenge on adult reproductive physiology and opposite-sex social behavior. Male and female Siberian hamster (Phodopus sungorus) pups were administered lipopolysaccharide ([LPS]; a cell wall component of gram-negative bacteria) or saline injections on postnatal days 3 and 5 and body mass, food intake, and measures of reproductive maturity were taken throughout development. In adulthood, hamsters were placed in staged mating pairs with reproductively mature individuals of the opposite sex, during which a series of behaviors were scored. We found that although males and females showed no change in food intake, body mass, or reproductive behaviors, LPS-treated females had abnormal estrous cycles and smaller ovaries. Females also showed increased investigation of and increased aggression towards males in a reproductive context. In contrast, LPS-treated males showed no change in any physiological measures, nor did they show any changes in behavior. The present findings demonstrate that females may be more robustly affected by neonatal sickness than males and that these effects could have potential impacts on reproductive success. Collectively, the results of this study can be used to expand upon what is already known about sickness and reproduction, specifically the importance of social behaviors involved in pre-copulation and information necessary to choose the appropriate mate.

Characteristic Response to Chemosensory Signals in GABAergic Cells of Medial Amygdala Is Not Driven by Main Olfactory Input

Chemosensory stimuli from same species (conspecific) and different species (heterospecific) elicit categorically different immediate-early gene (IEG) response patterns in medial amygdala in male hamsters and mice. All heterospecific stimuli activate anterior medial amygdala (MeA) but only especially salient heterospecific stimuli, such as those from predators activate posterior medial amygdala (MeP). We previously reported that characteristic patterns of response in separate populations of cells in MeA and MeP distinguish between different conspecific stimuli. Both gamma aminobutyric acid (GABA)-immunoreactive (ir) cells and GABA-receptor-ir cells make this distinction. Here, using zinc sulfate lesions of the main olfactory epithelium, we show evidence that main olfactory input does not contribute to the characteristic patterns of response in GABA-ir cells of male hamster amygdala, either for conspecific or heterospecific stimuli. Some GABAergic cells are output neurons carrying information from medial amygdala to behavioral executive regions of basal forebrain. Thus, the differential response to different conspecific signals can lead to differential activation of downstream circuits based on nonolfactory input. Finally, we show that an intact vomeronasal organ is necessary and sufficient to produce the characteristic patterns of response to conspecific and heterospecific chemosensory stimuli in hamster medial amygdala. Although main olfactory input may be critical in species with less prominent vomeronasal input for equivalent medial amygdala responses, work presented here suggests that hamster medial amygdala uses primarily vomeronasal input to discriminate between important unlearned conspecific social signals, to distinguish them from the social signals of other species, and may convey that information to brain circuits eliciting appropriate social behavior.

GABAergic mechanisms contributing to categorical amygdala responses to chemosensory signals

Chemosensory stimuli from conspecific and heterospecific animals, elicit categorically different immediate-early gene response-patterns in medial amygdala in male hamsters and mice. We previously showed that conspecific signals activate posterior (MeP) as well as anterior medial amygdala (MeA), and especially relevant heterospecific signals such as chemosensory stimuli from potential predators also activate MeP in mice. Other heterospecific chemosignals activate MeA, but not MeP. Here we show that male hamster amygdala responds significantly differentially to different conspecific signals, by activating different proportions of cells of different phenotype, possibly leading to differential activation of downstream circuits. Heterospecific signals that fail to activate MeP do activate GABA-immunoreactive cells in the adjacent caudal main intercalated nucleus (mICNc) and elicit selective suppression of MeP cells bearing GABA-Receptors, suggesting GABA inhibition in MeP by GABAergic cells in mICNc. Overall, work presented here suggests that medial amygdala may discriminate between important conspecific social signals, distinguish them from the social signals of other species and convey that information to brain circuits eliciting appropriate social behavior.

Male-typical courtship, spawning behavior, and olfactory sensitivity are induced to different extents by androgens in the goldfish suggesting they are controlled by different neuroendocrine mechanisms

Male-typical reproductive behaviors vary greatly between different species of fishes with androgens playing a variety of roles that appear especially important in the gonochorist cypriniform fishes. The goldfish is an important model for the cypriniformes and while it is clear that male goldfish are fully feminized by prostaglandin F2α(PGF2α), it is not clear whether females will exhibit normal levels of male-typical reproductive behaviors as well as olfactory function when treated with androgens. To answer this question, we exposed sexually-regressed adult female goldfish to several types of androgen and monitored their tendencies to court (inspect females) and mate (spawn, or attempt to release gametes) while monitoring their olfactory sensitivity until changes in these attributes were maximized. Untreated adult males (intact) were included to determine the extent of masculinization. Treatments included the natural androgens, 11-ketotestosterone and testosterone (KT and T), administered via capsules (KT+T-implanted fish); the artificial androgen, methyltestosterone (MT), administered via capsules (MT-C); and MT administered in the fishes' water (MT-B). Male-typical olfactory sensitivity to a pheromone (15keto-PGF2α) increased in all androgen-treated groups and by week 6 was fully equivalent to that of males. Male-typical courtship behavior increased in all androgen-treated groups although slowly, and only MT-B females came to exhibit levels equivalent to those of males after 18weeks. In contrast, male-typical mating activity increased only slightly, with MT-B females reaching levels one-third that of males after 30weeks. We conclude that while androgens fully masculinize olfactory sensitivity and courtship behavior in goldfish, mating behavior is controlled by a different neuroendocrine mechanism(s) that has yet to be fully elucidated.

PSA-NCAM in the posterodorsal medial amygdala is necessary for the pubertal emergence of attraction to female odors in male hamsters

During puberty, attention turns away from same-sex socialization to focus on the opposite sex. How the brain mediates this change in perception and motivation is unknown. Polysialylated neural cell adhesion molecule (PSA-NCAM) virtually disappears from most of the central nervous system after embryogenesis, but it remains elevated in discrete regions of the adult brain. One such brain area is the posterodorsal subnucleus of the medial amygdala (MePD). The MePD has been implicated in male sexual attraction, measured here as the preference to investigate female odors. We hypothesize that PSA-NCAM gates hormone-dependent plasticity necessary for the emergence of males' attraction to females. To evaluate this idea, we first measured PSA-NCAM levels across puberty in several brain regions, and identified when female odor preference normally emerges in male Syrian hamsters. We found that MePD PSA-NCAM staining peaks shortly before the surge of pubertal androgen and the emergence of preference. To test the necessity of PSA-NCAM for female odor preference, we infused endo-neuraminidase-N into the MePD to deplete it of PSAs before female odor preference normally appears. This blocked female odor preference, which suggests that PSA-NCAM facilitates behaviorally relevant, hormone-driven plasticity.

Human and animal research into sex-specific effects of child abuse

Child abuse is the most potent experiential risk factor for developing a mood disorder later in life. The effects of child abuse are also more severe in girls and women than in men. In this review, we explore the origins of this epidemiological sex difference. We begin by offering the hypothesis that a sex-specific risk factor that influences how social cues are perceived and remembered makes girls more susceptible to the effects of child abuse. We then discuss the neural systems that mediate emotion and stress, and, how child abuse and/or mood disorders like anxiety and depression affect them. Drawing upon human and animal research, several candidates for such a risk factor are discussed. They include glucocorticoid receptor trafficking and corticotropin releasing factor receptor binding and signaling. Our own research shows that the morphometry of the prepubertal amygdala is sexually dimorphic, and could contribute to a sex difference in stimulus appraisal. We have also found that the brain of juvenile female rats is less selective than males' for threatening social stimuli. Thus, one way that women may be more vulnerable to the effects of child abuse is that they are more likely to perceive objectively benign stimuli as threatening. This bias in perception could compound with the genuinely traumatic memories caused by child abuse; the burden of traumatic memories and the increasingly reactive stress response systems could then dispose more women than men to develop depression and/or anxiety.

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.

Sexually dimorphic patterns of neural activity in response to juvenile social subjugation

After experiencing juvenile social subjugation (JSS), adult female rats display more severe depression- and anxiety-like behaviors than adult males, suggesting that JSS is encoded in a sex-specific manner. To test this hypothesis, prepubertal rats (P28-33) were subjected to 10 aggressive acts in ≤10 min from an aggressive adult male, a 10 min encounter with a non-aggressive adult male, or to 10 min in an empty, clean cage (handled control) and were sacrificed one hour later. We then used unbiased stereology to estimate the total number and proportion of neurons immunoreactive for the immediate early gene product Fos bilaterally in the basolateral amygdala (BLA), the anterior and posterior subdivisions of the bed nucleus of the stria terminalis, and the paraventricular nucleus of the hypothalamus (PVN). Overall, females' Fos responses were less selective than males'. The BLA in males displayed a selective Fos response to the non-aggressive male, whereas no such selectivity occurred in the BLA of females. Additionally, there were more neurons overall in the left BLA than the right and this lateralization was specific to males. The principal subdivision of the BST (BSTpr) in males responded selectively to JSS, whereas the BSTpr in females responded to both the non-aggressive and aggressive males. We also found that the regional volume and neuron number of the BSTpr is greater in males than in females. Finally, the PVN in males was, like the BLA, selective for the non-aggressive male, whereas none of the experiences elicited a selective response in females. The greater selectivity for non-threatening stimuli in males in three stress-responsive brain regions may be a clue as to why males are less susceptible to the anxiogenic effects of JSS.

Male rats that differ in novelty exploration demonstrate distinct patterns of sexual behavior

High- versus low-novelty exploration predicts a variety of behavioral differences. For example, rats selectively bred for high-novelty exploration (bred-high responders, bHR) exhibit exaggerated aggression, impulsivity, and proclivity to addictive behaviors compared with low-novelty reactive rats (bred-low responders, bLRs), which are characterized by a high anxiety/depressive-like phenotype. Since bHR/bLR rats exhibit differences in dopaminergic circuitry and differential response to rewarding stimuli (i.e., psychostimulants, food), the present study examined whether they also differ in another key hedonic behavior-sex. Thus, adult bHR/bLR males were given five 30-min opportunities to engage in sexual activity with a receptive female. Sexual behavior and motivation were examined and compared between the groups. The bHR/bLR phenotype affected both sexual motivation and behavior, with bLR males demonstrating reduced motivation for sex compared with bHR males (i.e., fewer animals copulated, longer latency to engage in sex). The bHR males required more intromissions at a faster pace per ejaculation than did bLR males. Thus, neurobiological differences that affect motivation for drugs of abuse, aggression, and impulsivity in rats also affect sexual motivation and performance.

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.

The medial preoptic area is necessary for sexual odor preference, but not sexual solicitation, in female Syrian hamsters

Precopulatory behaviors that are preferentially directed towards opposite-sex conspecifics are critical for successful reproduction, particularly in species wherein the sexes live in isolation, such as Syrian hamsters (Mesocricetus auratus). In females, these behaviors include sexual odor preference and vaginal scent marking. The neural regulation of precopulatory behaviors is thought to involve a network of forebrain areas that includes the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). Although MA and BNST are necessary for sexual odor preference and preferential vaginal marking to male odors, respectively, the role of MPOA in odor-guided female precopulatory behaviors is not well understood. To address this issue, female Syrian hamsters with bilateral, excitotoxic lesions of MPOA (MPOA-X) or sham lesions (SHAM) were tested for sexual odor investigation, scent marking, and lordosis. MPOA-X females did not investigate male odors more than female odors in an odor preference test, indicating that MPOA may be necessary for normal sexual odor preference in female hamsters. This loss of preference cannot be attributed to a sensory deficit, since MPOA-X females successfully discriminated male odors from female odors during an odor discrimination test. Surprisingly, no deficits in vaginal scent marking were observed in MPOA-X females, although these females did exhibit decreased overall levels of flank marking compared to SHAM females. Finally, all MPOA-X females exhibited lordosis appropriately. These results suggest that MPOA plays a critical role in the neural regulation of certain aspects of odor-guided precopulatory behaviors in female Syrian hamsters.