Androgen receptor blockade in the posterodorsal medial amygdala impairs sexual odor preference in male rats

Androgen receptors and sociosexual behaviors in mammals: The limits of generalization

Circulating testosterone is easily aromatized to estradiol and reduced to dihydrotestosterone in target tissues and elsewhere in the body. Thus, the actions of testosterone can be mediated either by the estrogen receptors, the androgen receptor or by simultaneous action at both receptors. To determine the role of androgens acting at the androgen receptor, we need to eliminate actions at the estrogen receptors. Alternatively, actions at the androgen receptor itself can be eliminated. In the present review, I will analyze the specific role of androgen receptors in male and female sexual behavior as well as in aggression. Some comments about androgen receptors and social recognition are also made. It will be shown that there are important differences between species, even between strains within a species, concerning the actions of the androgen receptor on the behaviors mentioned. This fact makes generalizations from one species to another or from one strain to another very risky. The existence of important species differences is often ignored, leading to many misunderstandings and much confusion.

The effects of pubertal exposure to bisphenol-A on social behavior in male mice

Puberty is a crucial developmental period for structural modifications of brain and activation of the neural circuits underlying sex differences in social behavior. It is possible that pubertal exposure to bisphenol-A (BPA), a common EED with a weak estrogenic activity, influences social behavior. After being exposed to BPA at 0.04, 0.4, 4 mg kg^-1 for 18 days, the 7-week-old male mice were tested with social play and three-chamber. The results showed that pubertal BPA exposure decreased social play between adolescent males and sociability of adolescent males. Further, pubertal BPA exposure reduced sociability and inhibited social novel preferences of adult males. BPA inhibited social interactions with opposite sex but improved socio-sexual exploration and the low-intensity mating behavior (mounting) with same sex in adult males. In residential-intruder test, BPA-exposed adult males showed a decrease in aggressiveness and an enhancement in prosocial behavior with intruder. Western blot analysis showed that BPA (especially at 4 mg/kg/d) down-regulated the levels of AR in the amygdala and the striatum but up-regulated the levels of DR1 and DAT proteins in the striatum of adult males. BPA at 4 mg kg^-1 decreased the levels of T in the serum and the brain. These results suggest that pubertal BPA exposure affects social play and sociability of adolescent males and even results in long-term effects on social behavior of adult males. BPA-induced down-regulations of the levels of AR in the amygdala and the striatum and up-regulation of the levels of DR1 and DAT in the striatum may be involved.

The neuroendocrinology of sexual attraction

Sexual attraction has two components: Emission of sexually attractive stimuli and responsiveness to these stimuli. In rodents, olfactory stimuli are necessary but not sufficient for attraction. We argue that body odors are far superior to odors from excreta (urine, feces) as sexual attractants. Body odors are produced by sebaceous glands all over the body surface and in specialized glands. In primates, visual stimuli, for example the sexual skin, are more important than olfactory. The role of gonadal hormones for the production of and responsiveness to odorants is well established. Both the androgen and the estrogen receptor α are important in male as well as in female rodents. Also in primates, gonadal hormones are necessary for the responsiveness to sexual attractants. In males, the androgen receptor is sufficient for sustaining responsiveness. In female non-human primates, estrogens are needed, whereas androgens seem to contribute to responsiveness in women.

Role of the androgen receptor in the central nervous system

The involvement of gonadal androgens in functions of the central nervous system was suggested for the first time about half a century ago. Since then, the number of functions attributed to androgens has steadily increased, ranging from regulation of the hypothalamic-pituitary-gonadal axis and reproductive behaviors to modulation of cognition, anxiety and other non-reproductive functions. This review focuses on the implication of the neural androgen receptor in these androgen-sensitive functions and behaviors.

Regulation of sexual odor preference by sex steroids in the posterodorsal medial amygdala in female rats

Our previous study in male rats demonstrated that bilateral administration of flutamide, an androgen receptor (AR) antagonist, into the posterodorsal medial amygdala (MePD) increased the time sniffing male odors to as high as that sniffing estrous odors, eliminating the preference for estrous odors over male odors. This made us speculate that under blockade of AR in the MePD, testosterone-derived estrogen acting on the same brain region arouses interest in male odors which is otherwise suppressed by concomitant action of androgen. In cyclic female rats, endogenous androgen has been thought to be involved in inhibitory regulation of estrogen-activated sexual behavior. Thus, in the present study, we investigated the possibility that in female rats the arousal of interest in male odors is also normally regulated by both estrogen and androgen acting on the MePD, as predicted by our previous study in male rats. Implantation of either the estrogen receptor blocker tamoxifen (TX) or a non-aromatizable androgen 5α-dihydrotestosterone (DHT) into the MePD of ovariectomized, estrogen-primed female rats eliminated preference for male odors over estrous odors by significantly decreasing the time sniffing male odors to as low as that sniffing estrous odors. The subsequent odor discrimination tests confirmed that the DHT and TX administration did not impair the ability to discriminate between male and estrous odors. These results suggest that in estrous female rats estrogen action in the MePD plays critical roles in the expression of the preference for male odors while androgen action in the same brain region interferes with the estrogen action.

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.

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.

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.

Blockade of androgen receptor in the medial amygdala inhibits noncontact erections in male rats

Our previous work demonstrated that androgens in the medial amygdala (MeA) of castrated male rats maintained noncontact erections (NCEs), which occur during exposure to an inaccessible receptive female, for one week after implantation. The present experiments investigated the effects of implantation into the MeA of either flutamide (F), a blocker of androgen receptors, or of 1,4,6-androstatrien-3,17-dione (ATD), which blocks aromatization of testosterone. One day after implantation of F, fewer males displayed NCEs, and had longer latencies to the first NCE and fewer NCEs, and spent less total time in genital grooming, compared to the control group. ATD had only weak facilitative effects on some measures of NCEs. These results suggest that androgen receptors in the MeA play a major role in the regulation of NCEs and that the MeA is one of the neuronal structures that regulate male sexual arousal. Furthermore, it is sensitive to relatively fast changes in the level of androgen receptors stimulation.