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)

Neurophysiology of male sexual arousal-Behavioral perspective

In the presented review, we analyzed the physiology of male sexual arousal and its relation to the motivational aspects of this behavior. We highlighted the distinction between these processes based on observable physiological and behavioral parameters. Thus, we proposed the experimentally applicable differentiation between sexual arousal (SA) and sexual motivation (SM). We propose to define sexual arousal as an overall autonomic nervous system response leading to penile erection, triggered selectively by specific sexual cues. These autonomic processes include both spinal and supraspinal neuronal networks, activated by sensory pathways including information from sexual partner and sexual context, as well as external and internal genital organs. To avoid misinterpretation of experimental data, we also propose to precise the term "sexual motivation" as all actions performed by the individual that increase the probability of sexual interactions or increase the probability of exposition to sexual context cues. Neuronal structures such as the amygdala, bed nucleus of stria terminalis, hypothalamus, nucleus raphe, periaqueductal gray, and nucleus paragigantocellularis play crucial roles in controlling the level of arousal and regulating peripheral responses via specific autonomic effectors. On the highest level of CNS, the activity of cortical structures involved in the regulation of the autonomic nervous system, such as the insula and anterior cingulate cortex, can visualize an elevated level of SA in both animal and human brains. From a preclinical perspective, we underlie the usefulness of the non-contact erection test (NCE) procedure in understanding factors influencing sexual arousal, including studies of sexual preference in animal models. Taken together results obtained by different methods, we wanted to focus attention on neurophysiological aspects that are distinctly related to sexual arousal and can be used as an objective parameter, leading to higher translational transparency between basic, preclinical, and clinical studies.

Loss of TRPC2 function in mice alters sex differences in brain regions regulating social behaviors

The transient receptor potential cation channel 2 (TRPC2) conveys pheromonal information from the vomeronasal organ (VNO) to the brain. Both male and female mice lacking this gene show altered sex-typical behavior as adults. We asked whether TRPC2, highly expressed in the VNO, normally participates in the development of VNO-recipient brain regions controlling mounting and aggression, two behaviors affected by TRPC2 loss. We now report significant effects of TRPC2 loss in both the posterodorsal aspect of the medial amygdala (MePD) and ventromedial nucleus of the hypothalamus (VMH) of male and female mice. In the MePD, a sex difference in neuron number was eliminated by the TRPC2 knockout (KO), but the effect was complex, with fewer neurons in the right MePD of females, and fewer neurons in the left MePD of males. In contrast, MePD astrocytes were unaffected by the KO. In the ventrolateral (vl) aspect of the VMH, KO females were like wildtype (WT) females, but TRPC2 loss had a dramatic effect in males, with fewer neurons than WT males and a smaller VMHvl overall. We also discovered a glial sex difference in VMHvl of WTs, with females having more astrocytes than males. Interestingly, TRPC2 loss increased astrocyte number in males in this region. We conclude that TRPC2 normally participates in the sexual differentiation of the mouse MePD and VMHvl. These changes in two key VNO-recipient regions may underlie the effects of the TRPC2 KO on behavior.

Functional connectivity of intercalated nucleus with medial amygdala: A circuit relevant for chemosignal processing

Medial amygdala processes social/reproductive chemosensory input, and its projections to preoptic and hypothalamic areas evoke appropriate behavioral and physiological responses. We and others have shown that different chemosensory signals elicit differential responses in medial amygdala subregions and in adjacent main intercalated nucleus (mICN). The largely GABAergic mICN receives no direct chemosensory input but, as we show, mICN has functional circuit connections with medial amygdala that could be responsible both for mICN chemosensitivity and for a feedforward inhibitory effect on posterior medial amygdala; which, in turn would affect chemosignal response. mICN is subject to inhibition by dopamine and is probably regulated by neuropeptides and input from frontal cortex. Thus, mICN is in position to modify chemosensory processing in medial amygdala and behavioral responses to social signals, according to internal brain state. Patch-clamp recordings from neurons in each relevant nucleus in horizontal brain-slices, with electrical stimulation in adjacent nuclei, reveal multiple functional connections between medial amygdala subregions and mICN. We highlight a triangular circuit which may underlie mICN chemosensitivity and its potential for modifying chemosensory information transmitted to basal forebrain. Anterior medial amygdala, which receives most of the chemosensory input, connects to posterior medial amygdala directly and both areas send information on to basal forebrain. Anterior medial amygdala can also modulate posterior medial amygdala indirectly via the mICN side-loop, which also provides a pathway for modulation by cortical input or, when inhibited by dopamine, could allow a more automatic response - as proposed for other amygdala circuits with similar ICN side loops.

Volume of Amygdala Subregions and Plasma Levels of Brain-Derived Neurotrophic Factor and Cortisol in Patients with s/s Genotype of Serotonin Transporter Gene Polymorphism of First-Episode and Drug-Naive Major Depressive Disorder: An Exploratory Study

The amygdala is a prominent region of the brain that plays a critical role in the pathophysiology of major depressive disorder (MDD). The amygdala is formed from a collection of interconnected substructures (nuclei) that relay signals from multiple brain areas, which suggests that the amygdala has different functions depending on its subregion. There are two main alleles of serotonin transporter gene polymorphism (5-HTTLPR): a 44-bp insertion (l-allele) or deletion (s-allele). The transcriptional activity of the l-allele of the gene is twice that of the s-allele. The present study aimed to investigate the association between the volume of the whole amygdala and subregions of the amygdala in 25 first-episode and drug-naive patients with MDD and 46 healthy controls (HCs) with the s/s genotype of 5-HTTLPR and plasma levels of brain-derived neurotrophic factor (BDNF) or cortisol. No significant difference was observed in the amygdala total and subregion volumes between the HC and MDD groups. No significant difference was found in the plasma levels of BDNF and cortisol between the two groups. In addition, no correlations were found between the total and subregion amygdala volume and plasma levels of cortisol or BDNF.

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.

Dietary phytoestrogens modulate aggression and activity in social behavior circuits of male mice

Phytoestrogens comprise biologically active constituents of human and animal diet that can impact on systemic and local estrogen functions in the brain. Here we report on the importance of dietary phytoestrogens for maintaining activity in a brain circuit controlling aggressive and social behavior of male mice. After six weeks of low-phytoestrogen chronic diet (diadzein plus genistein <20 μg/g) a reduction of intermale aggression and altered territorial marking behavior could be observed, compared to littermates on a standard soy-bean based diet (300 μg/g). Further, mice on low-phyto diet displayed a decrease in sociability and a reduced preference for social odors, indicating a general disturbance of social behavior. Underlying circuits were investigated by analysing the induction of the activity marker c-Fos upon social encounter. Low-phyto diet led to a markedly reduced c-Fos induction in the medial as well as the cortical amygdala, the lateral septum, medial preoptic area and bed nucleus of the stria terminalis. No difference between groups was observed in the olfactory bulb. Together our data suggest that dietary phytoestrogens critically modulate social behavior circuits in the male mouse brain.

Activation of basolateral amygdala in juvenile C57BL/6J mice during social approach behavior

There is a strong need to better understand the neurobiology of juvenile sociability (tendency to seek social interaction), a phenotype of central relevance to autism spectrum disorders (ASD). Although numerous genetic mouse models of ASD showing reduced sociability have been reported, and certain brain regions, such as the amygdala, have been implicated in sociability, there has been little emphasis on delineating brain structures and circuits activated during social interactions in the critical juvenile period of the mouse strain that serves as the most common genetic background for these models-the highly sociable C57BL/6J (B6) strain. We measured expression of the immediate early genes Fos and Egr-1 to map activation of brain regions following the Social Approach Test (SAT) in juvenile male B6 mice. We hypothesized that juvenile B6 mice would show activation of the amygdala during social interactions. The basolateral amygdala (BLA) was activated by social exposure in highly sociable, 4-week-old B6 mice. In light of these data, and the many lines of evidence indicating alteration of amygdala circuits in human ASD, future studies are warranted to assess structural and functional alterations in the BLA, particularly at BLA synapses, in various mouse models of ASD.

Pubertal activation of estrogen receptor α in the medial amygdala is essential for the full expression of male social behavior in mice

Testosterone plays a central role in the facilitation of male-type social behaviors, such as sexual and aggressive behaviors, and the development of their neural bases in male mice. The action of testosterone via estrogen receptor (ER) α, after being aromatized to estradiol, has been suggested to be crucial for the full expression of these behaviors. We previously reported that silencing of ERα in adult male mice with the use of a virally mediated RNAi method in the medial preoptic area (MPOA) greatly reduced sexual behaviors without affecting aggressive behaviors whereas that in the medial amygdala (MeA) had no effect on either behavior. It is well accepted that testosterone stimulation during the pubertal period is necessary for the full expression of male-type social behaviors. However, it is still not known whether, and in which brain region, ERα is involved in this developmental effect of testosterone. In this study, we knocked down ERα in the MeA or MPOA in gonadally intact male mice at the age of 21 d and examined its effects on the sexual and aggressive behaviors later in adulthood. We found that the prepubertal knockdown of ERα in the MeA reduced both sexual and aggressive behaviors whereas that in the MPOA reduced only sexual, but not aggressive, behavior. Furthermore, the number of MeA neurons was reduced by prepubertal knockdown of ERα. These results indicate that ERα activation in the MeA during the pubertal period is crucial for male mice to fully express their male-type social behaviors in adulthood.

Social Isolation During Postweaning Development Causes Hypoactivity of Neurons in the Medial Nucleus of the Male Rat Amygdala

Children exposed to neglect or social deprivation are at heightened risk for psychiatric disorders and abnormal social patterns as adults. There is also evidence that prepubertal neglect in children causes abnormal metabolic activity in several brain regions, including the amygdala area. The medial nucleus of the amygdala (MeA) is a key region for performance of social behaviors and still undergoes maturation during the periadolescent period. As such, the normal development of this region may be disrupted by social deprivation. In rodents, postweaning social isolation causes a range of deficits in sexual and agonistic behaviors that normally rely on the posterior MeA (MeAp). However, little is known about the effects of social isolation on the function of MeA neurons. In this study, we tested whether postweaning social isolation caused abnormal activity of MeA neurons. We found that postweaning social isolation caused a decrease of in vivo firing activity of MeAp neurons, and reduced drive from excitatory afferents. In vitro electrophysiological studies found that postweaning social isolation caused a presynaptic impairment of excitatory input to the dorsal MeAp, but a progressive postsynaptic reduction of membrane excitability in the ventral MeAp. These results demonstrate discrete, subnucleus-specific effects of social deprivation on the physiology of MeAp neurons. This pathophysiology may contribute to the disruption of social behavior after developmental social deprivation, and may be a novel target to facilitate the treatment of social disorders.

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.

Afferent projections to the different medial amygdala subdivisions: a retrograde tracing study in the mouse

The medial amygdaloid nucleus (Me) is a key node in the socio-sexual brain, composed of anterior (MeA), posteroventral (MePV) and posterodorsal (MePD) subdivisions. These subdivisions have been suggested to play a different role in reproductive and defensive behaviours. In the present work we analyse the afferents of the three Me subdivisions using restricted injections of fluorogold in female outbred CD1 mice. The results reveal that the MeA, MePV and MePD share a common pattern of afferents, with some differences in the density of retrograde labelling in several nuclei. Common afferents to Me subdivisions include: the accessory olfactory bulbs, piriform cortex and endopiriform nucleus, chemosensory amygdala (receiving direct inputs from the olfactory bulbs), posterior part of the medial bed nucleus of the stria terminalis (BSTM), CA1 in the ventral hippocampus and posterior intralaminar thalamus. Minor projections originate from the basolateral amygdala and amygdalo-hippocampal area, septum, ventral striatum, several allocortical and periallocortical areas, claustrum, several hypothalamic structures, raphe and parabrachial complex. MeA and MePV share minor inputs from the frontal cortex (medial orbital, prelimbic, infralimbic and dorsal peduncular cortices), but differ in the lack of main olfactory projections to the MePV. By contrast, the MePD receives preferential projections from the rostral accessory olfactory bulb, the posteromedial BSTM and the ventral premammillary nucleus. In summary, the common pattern of afferents to the Me subdivisions and their interconnections suggest that they play cooperative instead of differential roles in the various behaviours (e.g., sociosexual, defensive) in which the Me has been shown to be involved.

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.

Differential efferent projections of the anterior, posteroventral, and posterodorsal subdivisions of the medial amygdala in mice

The medial amygdaloid nucleus (Me) is a key structure in the control of sociosexual behavior in mice. It receives direct projections from the main and accessory olfactory bulbs (AOB), as well as an important hormonal input. To better understand its behavioral role, in this work we investigate the structures receiving information from the Me, by analysing the efferent projections from its anterior (MeA), posterodorsal (MePD) and posteroventral (MePV) subdivisions, using anterograde neuronal tracing with biotinylated and tetrametylrhodamine-conjugated dextranamines. The Me is strongly interconnected with the rest of the chemosensory amygdala, but shows only moderate projections to the central nucleus and light projections to the associative nuclei of the basolateral amygdaloid complex. In addition, the MeA originates a strong feedback projection to the deep mitral cell layer of the AOB, whereas the MePV projects to its granule cell layer. The Me (especially the MeA) has also moderate projections to different olfactory structures, including the piriform cortex (Pir). The densest outputs of the Me target the bed nucleus of the stria terminalis (BST) and the hypothalamus. The MeA and MePV project to key structures of the circuit involved in the defensive response against predators (medial posterointermediate BST, anterior hypothalamic area, dorsomedial aspect of the ventromedial hypothalamic nucleus), although less dense projections also innervate reproductive-related nuclei. In contrast, the MePD projects mainly to structures that control reproductive behaviors [medial posteromedial BST, medial preoptic nucleus, and ventrolateral aspect of the ventromedial hypothalamic nucleus], although less dense projections to defensive-related nuclei also exist. These results confirm and extend previous results in other rodents and suggest that the medial amygdala is anatomically and functionally compartmentalized.

Social experience induces sex-specific fos expression in the amygdala of the juvenile rat

To compare the response of the medial amygdala and central amygdala to juvenile social subjugation (JSS), we used unbiased stereology to quantify the immediate early gene product Fos in prepubertal rats after aggressive or benign social encounters or handling. We estimated the overall number of neurons and the proportion of Fos immunoreactive neurons in the posterodorsal (MePD) and posteroventral medial amygdala (MePV) and the central amygdala (CeA). Experience elicited Fos in a sex- and hemisphere-dependent manner in the MePD. The left MePD was selective for JSS in both sexes, but the right MePD showed a specific Fos response to JSS in males only. In the MePV, irrespective of hemisphere or sex, JSS elicited the greatest amount of Fos, benign social experience elicited an intermediate level, and handling the least. None of the experiential conditions elicited significant levels of Fos in the CeA. We found a previously unreported sex difference in the number of CeA neurons (M>F) that was highly significant and a strong trend toward a sex difference (M>F) in the MePD. These data show that the posterior MeA subnuclei are more responsive to JSS than to benign social interaction, that sex interacts with hemispheric laterality to determine the response of the MePD to JSS and that the MePV responds to social experience and JSS. Taken together, these findings support the hypothesis that juvenile rats process JSS in a sex-specific manner.

Pubertal testosterone organizes regional volume and neuronal number within the medial amygdala of adult male Syrian hamsters

The medial amygdala plays a key role in regulating adult social behavior and undergoes structural changes during puberty that may be driven by gonadal hormone secretion during this developmental period. The current study sought to investigate potential organizational effects of testosterone during puberty, activational effects of testosterone in adulthood, and any interactions on regional volume and neuronal number of the medial amygdala. Male Syrian hamsters either did or did not experience endogenous testosterone during pubertal brain development, and then received either testosterone-filled or blank capsules during adulthood 2 weeks before tissue collection. The results show that pubertal testosterone has long-term organizational effects on volume of specific subregions of the medial amygdala such that the presence of pubertal testosterone resulted in 1) decreased volume of the anterior ventral amygdala and, to a lesser extent, the anterior dorsal medial amygdala; and 2) increased volume of the posterior dorsal medial amygdala. Both effects were independent of the presence of testosterone during adulthood. Pubertal testosterone also decreased neuronal number in the anterior dorsal medial amygdala, suggesting a possible mechanism by which pubertal testosterone decreases volume in this subregion. In addition, there was a significant interaction between pubertal and adult testosterone, such that testosterone in adulthood increased the number of neurons in the posterior ventral medial amygdala only in males that did not experience endogenous pubertal testosterone. In conclusion, pubertal testosterone organizes the medial amygdala in a subregion-specific manner, which may contribute to the maturation of adult-typical social behavior.

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.

Both olfactory epithelial and vomeronasal inputs are essential for activation of the medial amygdala and preoptic neurons of male rats

Chemosensory inputs signaling volatile and nonvolatile molecules play a pivotal role in sexual and social behavior in rodents. We have demonstrated that olfactory preference in male rats, that is, attraction to receptive female odors, is regulated by the medial amygdala (MeA), the cortical amygdala (CoA), and the preoptic area (POA). In this paper, we investigated the involvement of two chemosensory organs, the olfactory epithelium (OE) and the vomeronasal organ (VNO), in olfactory preference and copulatory behavior in male rats. We found that olfactory preferences were impaired by zinc sulfate lesion of the OE but not surgical removal of the VNO. Copulatory behaviors, especially intromission frequency and ejaculation, were also suppressed by zinc sulfate treatment. Neuronal activation in the accessory olfactory bulb (AOB), the MeA, the CoA, and the POA was analyzed after stimulation by airborne odors or soiled bedding of estrous females using cFos immunohistochemistry. Although the OE and VNO belong to different neural systems, the main and accessory olfactory systems, respectively, both OE lesion and VNO removal almost equally suppressed the number of cFos-immunoreactive cells in those areas that regulate olfactory preference. These results suggest that signals received by the OE and VNO interact and converge in the early stage of olfactory processing, in the AOB and its targets, although they have distinct roles in the regulation of social behaviors.

Change in number and activation of androgen receptor-immunoreactive cells in the medial amygdala in response to chemosensory input

In many species social behaviors are dependent on integration of chemosensory and hormonal cues. Many chemosensory stimuli are detected by the vomeronasal organ, which projects to many regions that contain steroid receptors, including the medial amygdala. In male hamsters, testosterone is known to acutely increase in response to chemosensory stimulation, and can facilitate sexual behavior by direct action within the medial amygdala. Conspecific stimuli activate the anterior (MeA) and posterior (MeP) medial amygdala, while heterospecific stimuli activate only MeA. Chemosensory stimuli with different social significance differentially activate the dorsal and ventral subdivisions of MeA and MeP. Therefore, it is likely that steroids differentially facilitate stimulation of the medial amygdala by various chemosensory stimuli. We used Fos expression to examine activation of androgen receptor (AR)-containing cells in the medial amygdala by heterospecific and conspecific stimuli in intact male hamsters and castrated males with testosterone (T)-replacement. The number of AR-immunoreactive (-ir) cells was significantly different from control and between stimuli in intact males, but not in T-replaced castrates. Fos activation was similar in all animals. The results are consistent with a change in number of AR-ir cells in intact animals due to acute increases in testosterone caused by chemosignals.

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.

Synaptic reorganisation of the medial amygdala during puberty

The medial amygdala (MeA) is an important site for the gonadal hormone control of several socio-sexual behaviours that emerge during puberty, including aggression, mating and parental behaviour. We have previously shown that rising levels of pubertal androgens increase the regional volume and mean soma size of neurones in the posterodorsal subnucleus of the MeA, the MePD. The present study aimed to determine some of the constituents of pubertal volumetric growth. Using computer-guided unbiased stereology, we compared the regional volume, mean somal volume and the overall number of neurones and glia in 45-day-old male Siberian hamsters (Phodopus sungorus). Half of the hamsters had completed puberty, whereas the remainder were prepubertal as a result of photoinhibition of the hypothalamic-pituitary-gonadal axis. Puberty significantly increased MePD regional volume and mean somal volume, as previously observed. We also compared the number of puncta immunoreactive for vesicular glutamate transporter-2 (vGlut2) and post-synaptic density 95 (PSD-95), which are both markers of glutamatergic pre- and post-synaptic specialisations, as well as glutamic acid decarboxylase 65 (GAD-65), which is a marker of GABAergic terminals. Puberty increased the number of vGlut2 and PSD-95 immunoreactive puncta by two- and three-fold, respectively, whereas the number of GAD-65 immunoreactive puncta was unchanged. These results suggest that numerous excitatory synapses are added to the MeA during puberty. More broadly, they show that the pubertal emergence of sexual behaviour is accompanied by synaptic reorganisation of a key network involved in the expression of sexual behaviour.

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.

Anatomical connections between the anterior and posterodorsal sub-regions of the medial amygdala: integration of odor and hormone signals

In many rodent species, such as Syrian hamsters, reproductive behavior requires neural integration of chemosensory information and steroid hormone cues. The medial amygdala 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 dense populations of androgen and estrogen receptors. Importantly, these sub-regions have considerable reciprocal connections, and previous studies in our laboratory have shown that functional interactions between MeA and MePD are required for the preference to investigate opposite-sex odors in male hamsters. We therefore hypothesized that chemosensory and hormone signals are conveyed directly between MeA and MePD. To test this hypothesis, we injected the retrograde tracer, cholera toxin B (CTB), into either MeA or MePD of male subjects and identified whether retrogradely labeled cells within MePD or MeA, respectively, expressed (1) Fos protein following exposure to female or male odors or (2) androgen receptors (AR). Approximately 36% of CTB-labeled cells within MeA (that project to MePD) also expressed Fos following exposure to either social odor, compared to the only 13% of CTB-labeled cells within MePD (that project to MeA) that also expressed odor-induced Fos. In contrast, 57% of CTB-labeled cells within MePD also contained AR, compared to the 28% of CTB-labeled cells within MeA that were double-labeled for AR/CTB. These results provide the first anatomical evidence that chemosensory and hormone cues are conveyed directly between MeA and MePD. Furthermore, these data suggest that chemosensory information is conveyed primarily from MeA to MePD, whereas hormone information is conveyed primarily from MePD to MeA. More broadly, the interactions between MeA and MePD may represent a basic mechanism by which the brain integrates information about social cues in the environment with hormonal indices of reproductive state.