Sexually dimorphic cell groups in the medial preoptic area that are essential for male sex behavior and the neural pathways needed for their effects

Sexual dimorphism in synaptic inputs to the mouse amygdala and orbital cortex

The medial amygdala (MeA) is a sexually dimorphic brain region that regulates fear responses, emotional memories, and social behaviors. It is known to be larger and contains more cells in males. The MeA integrates information through input connections from olfactory regions, bed nucleus of the stria terminalis, ventral hippocampus, and thalamic and hypothalamic structures. We hypothesize that in addition to the size differences, there are differences in regional connectivity between the sexes. In this study, we utilized G-deleted rabies monosynaptic retrograde tracing to compare amygdala presynaptic cells in male and female whole mouse brains. We report differences in connection patterns to the amygdala, with higher overall connectivity (presynaptic per starter) in males and a larger fraction of inputs originating from the bed nucleus of the stria terminalis, lateral septum, and medial preoptic area. Furthermore, we examined input connections to the orbital cortex (ORB), a brain region shown to be larger in volume in females, and found the opposite trend, where females had more total inputs. Together, our findings extend the evidence for sexual dimorphism in the brain to the neuronal wiring pattern, with likely impacts on behavior and disease susceptibility.

Distinct preoptic-BST nuclei dissociate paternal and infanticidal behavior in mice

Paternal behavior is not innate but arises through social experience. After mating and becoming fathers, male mice change their behavior toward pups from infanticide to paternal care. However, the precise brain areas and circuit mechanisms connecting these social behaviors are largely unknown. Here we demonstrated that the c-Fos expression pattern in the four nuclei of the preoptic-bed nuclei of stria terminalis (BST) region could robustly discriminate five kinds of previous social behavior of male mice (parenting, infanticide, mating, inter-male aggression, solitary control). Specifically, neuronal activation in the central part of the medial preoptic area (cMPOA) and rhomboid nucleus of the BST (BSTrh) retroactively detected paternal and infanticidal motivation with more than 95% accuracy. Moreover, cMPOA lesions switched behavior in fathers from paternal to infanticidal, while BSTrh lesions inhibited infanticide in virgin males. The projections from cMPOA to BSTrh were largely GABAergic. Optogenetic or pharmacogenetic activation of cMPOA attenuated infanticide in virgin males. Taken together, this study identifies the preoptic-BST nuclei underlying social motivations in male mice and reveals unexpected complexity in the circuit connecting these nuclei.

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.

Hormones of choice: the neuroendocrinology of partner preference in animals

Partner preference behavior can be viewed as the outcome of a set of hierarchical choices made by an individual in anticipation of mating. The first choice involves approaching a conspecific verses an individual of another species. As a rule, a conspecific is picked as a mating partner, but early life experiences can alter that outcome. Within a species, an animal then has the choice between a member of the same sex or the opposite sex. The final choice is for a specific individual. This review will focus on the middle choice, the decision to mate with either a male or a female. Available data from rats, mice, and ferrets point to the importance of perinatal exposure to steroid hormones in the development of partner preferences, as well as the importance of activational effects in adulthood. However, the particular effects of this hormone exposure show species differences in both the specific steroid hormone responsible for the organization of behavior and the developmental period when it has its effect. Where these hormones have an effect in the brain is mostly unknown, but regions involved in olfaction and sexual behavior, as well as sexually dimorphic regions, seem to play a role. One limitation of the literature base is that many mate or 'partner preference studies' rely on preference for a specific stimulus (usually olfaction) but do not include an analysis of the relation, if any, that stimulus has to the choice of a particular sexual partner. A second limitation has been the almost total lack of attention to the type of behavior that is shown by the choosing animal once a 'partner' has been chosen, specifically, if the individual plays a mating role typical of its own sex or the opposite sex. Additional paradigms that address these questions are needed for better understanding of partner preferences in rodents.

Estrogen receptor-alpha in the bed nucleus of the stria terminalis regulates social affiliation in male prairie voles (Microtus ochrogaster)

Estrogen receptor alpha (ERalpha) typically masculinizes male behavior, while low levels of ERalpha in the medial amygdala (MeA) and the bed nucleus of the stria terminalis (BST) are associated with high levels of male prosocial behavior. In the males of the highly social prairie vole (Microtus ochrogaster), increasing ERalpha in the MeA inhibited the expression of spontaneous alloparental behavior and produced a preference for novel females. To test for the effects of increased ERalpha in the BST, a viral vector was used to enhance ERalpha expression in the BST of adult male prairie voles. Following treatment, adult males were tested for alloparental behavior with 1-3-day-old pups, and for heterosexual social preference and affiliation. Treatment did not affect alloparental behavior as 73% of ERalpha-BST males and 62.5% of control males were alloparental. Increasing ERalpha in the BST affected heterosexual affiliation, with ERalpha-BST males spending significantly less total time in side-by-side contact with females relative to time spent with control males. ERalpha-BST males did not show a preference for either the familiar or novel female. These findings differed significantly from those reported in ERalpha-MeA enhanced males, where ERalpha inhibited alloparental behavior and produced a preference for a novel female. The findings from this study suggest two things: first, that increased ERalpha in the BST decreases social affiliation and second, that altering ERalpha in different regions of the social neural circuit differentially impacts the expression of social behavior.

Intrauterine proximity to male fetuses affects the morphology of the sexually dimorphic nucleus of the preoptic area in the adult rat brain

Previous studies on polytocous rodents have revealed that the fetal intrauterine position influences its later anatomy, physiology, reproductive performance and behavior. To investigate whether the position of a fetus in the uterus modifies the development of the brain, we examined whether the structure of the sexually dimorphic nucleus of the preoptic area (SDN-POA) of rat brains accorded to their intrauterine positions. Brain sections of adult rats gestated between two male fetuses (2M) and between two female fetuses (2F) in the uterus were analysed for their immunoreactivity to calbindin-D28k, which is a marker of the SDN-POA. The SDN-POA volume of the 2M adult males was greater than that of the 2F adult males, whereas the SDN-POA volume of the 2M and 2F adult females showed no significant difference. This result indicated that contiguous male fetuses have a masculinizing effect on the SDN-POA volume of the male. To further examine whether the increment of SDN-POA volume in adulthood was due to exposure to elevated steroid hormones during fetal life, concentrations of testosterone and 17beta-estradiol in the brain were measured with 2M and 2F fetuses during gestation, respectively. On gestation day 21, the concentrations of testosterone and 17beta-estradiol in the brain were significantly higher in the 2M male rats as compared with the 2F male rats. The results suggested that there was a relationship between the fetal intrauterine position, hormone transfer from adjacent fetuses and the SDN-POA volume in adult rat brains.

Sexual dimorphism in the vomeronasal system of the rabbit

Studies have shown that the vomeronasal system (VNS), an olfactory neural network that participates in the control of reproductive physiology and behavior, is sexually dimorphic in the rat. These works have also shown two main characteristics of brain sexual dimorphism: (a) dimorphism appears in neural networks related to reproduction and (b) it can present two morphological patterns: one in which males present greater morphological measures than females (male > female) and another in which the opposite is true (female > male). The present work extends the hypothesis to the rabbit, as a representative species of Lagomorpha. In addition, the locus coeruleus (LC), which is known to send rich noradrenergic projections to VNS structures, was also studied. Sex differences were found in: (a) the number of mitral, and dark and light granule cells (female > male) of the accessory olfactory bulb (AOB); (b) the medial amygdala (Me) and its dorsal (Med) and ventral (Mev) subdivisions, males showing greater values than females in volume and number of neurons, while in the posteromedial cortical amygdala (PMCo or C(3)), females show greater density of neurons than males and (c) the posteromedial division of the bed nucleus of the stria terminalis (BSTMP) in which males have more neurons than females. No sex differences were seen in the bed nucleus of the accessory olfactory tract (BAOT) and the LC. These results evidence that, as it was observed in rodents, sex differences are also seen in the VNS of Lagomorpha and that these sex differences present the two morphological patterns seen in Rodentia. Differences between orders are discussed with respect to the species-specific physiological and behavioral peculiarities.

The bed nucleus of the stria terminalis in the Syrian hamster: subnuclei and connections of the posterior division

The bed nucleus of the stria terminalis is a key part of a ring of cells extending between the centromedial amygdala and bed nucleus of the stria terminalis referred to as the extended amygdala. The present study describes the architecture of the bed nucleus of the stria terminalis and the connections of subnuclei in posterior bed nucleus of the stria terminalis. The hamster bed nucleus of the stria terminalis is readily allotted to anterior and posterior divisions separated by the fibers of the body of the anterior commissure. The anterior division has four subnuclei: anteromedial, anterointermediate, anterolateral, and anteroventral. Within the posterior division, there are three distinct regions: posteromedial, posterointermediate, and posterolateral. In hamsters, the posterior bed nucleus of the stria terminalis contributes to male sexual behavior, particularly chemoinvestigation. Moreover, the posterior bed nucleus of the stria terminalis is part of a neural circuit essential for mating, including the medial amygdaloid nucleus and medial preoptic area. The connections of bed nucleus of the stria terminalis, posteromedial part, bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posterolateral part were visualized by co-injection of anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (cholera toxin B) tract tracers. The bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part have dense bidirectional connections with medial amygdaloid nucleus and cortical amygdala via the stria terminalis and ventral amygdalofugal pathway. These subnuclei also maintain bidirectional connections with steroid-concentrating areas including lateral septum, medial preoptic area, hypothalamus, and periaqueductal gray. The bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part receive projections from the subiculum and send projections to deep mesencephalic nuclei. By contrast, the bed nucleus of the stria terminalis, posterolateral part is connected with the central amygdala, lateral hypothalamus, subthalamic nucleus, nucleus accumbens, substantia innominata, substantia nigra and thalamus. Thus, the bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part have similar connections with areas involved in social behaviors. The bed nucleus of the stria terminalis, posterolateral part maintains connections with areas involved in motivational circuits. This supports the concept of distinct circuits within the extended amygdala which differentially link the centromedial amygdala and bed nucleus of the stria terminalis.

Intraspecific variation in estrogen receptor alpha and the expression of male sociosexual behavior in two populations of prairie voles

Estrogen (E) regulates a variety of male sociosexual behaviors. We hypothesize that there is a relationship between the distribution of estrogen receptor alpha (ERalpha) and the degree of male social behavior. To test this hypothesis, ERalpha immunoreactivity (IR) was compared in prairie voles (Microtus ochrogaster) from Illinois (IL), which are highly social, and Kansas (KN), which are less social. The expression of androgen receptors (AR) in males also was compared between populations. The expression of ERalpha and AR were compared in brains from KN and IL males and females using immunocytochemistry (ICC). There were significant intrapopulational differences, with males expressing less ERalpha-IR than females in the medial preoptic area, ventromedial nucleus, ventrolateral portion of the hypothalamus, and bed nucleus of the stria terminalis (BST). IL males also displayed less ERalpha-IR in the medial amygdala (MeA) than IL females. While IL males expressed significantly less ERalpha-IR in the BST and MeA than KN males, there was no difference in AR-IR. Differences in the pattern of ERalpha-IR between KN and IL males were behaviorally relevant, as low levels of testosterone (T) were more effective in restoring sexual activity in castrated KN males than IL males. The lack of difference in AR combined with lower expression of ERalpha-IR in IL males suggests that behavioral differences in response to T are associated with aromatization of T to E and that reduced sensitivity to E may facilitate prosocial behavior in males.

The MPN mag

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

Sexual differentiation of the human hypothalamus

Functional sex differences in reproduction, gender and sexual orientation and in the incidence of neurological and psychiatric diseases are presumed to be based on structural and functional differences in the hypothalamus and other limbic structures. Factors influencing gender, i.e., the feeling to be male or female, are prenatal hormones and compounds that change the levels of these hormones, such as anticonvulsants, while the influence of postnatal social factors is controversial. Genetic factors and prenatal hormone levels are factors in the determination of sexual orientation, i.e. heterosexuality, bisexuality or homosexuality. There is no convincing evidence for postnatal social factors involved in the determination of sexual orientation. The period of overt sexual differentiation of the human hypothalamus occurs between approximately four years of age and adulthood, thus much later than is generally presumed, although the late sexual differentiation may of course be based upon processes that have already been programmed in mid-pregnancy or during the neonatal period. The recently reported differences in a number of structures in the human hypothalamus and adjacent structures depend strongly on age. Replication of these data is certainly necessary. Since the size of brain structures may be influenced by premortem factors (e.g. agonal state) and postmortem factors (e.g. fixation time), one should not only perform volume measurements, but also estimate a parameter that is not dependent on such factors as, i.e., total cell number of the brain structure in question. In addition, functional differences that depend on the levels of circulating hormones in adulthood have been observed in several hypothalamic and other brain structures. The mechanisms causing sexual differentiation of hypothalamic nuclei, the pre- and postnatal factors influencing this process, and the exact functional consequences of the morphological and functional hypothalamic differences await further elucidation.

Sex differences in the magnocellular subdivision of the medial preoptic nucleus in Syrian hamsters

Exposure to pheromonal cues initiates male mating behavior. Pheromones are processed within a pathway that converges on the magnocellular subdivision of the medial preoptic nucleus. Lesions of this area eliminate male copulatory behavior, but do not affect anogenital investigation. Exposure to pheromones stimulates cells of the magnocellular subdivision of the medial preoptic nucleus in a sex-specific manner. In this study, we hypothesize that sex differences in cell number may underlie sex differences in pheromone-induced neural stimulation. The current study used unbiased stereological methods to identify sexual dimorphisms in the magnocellular subdivision of the medial preoptic nucleus. Sex differences were found in the number and density of neurons, but not in overall volume or neuron volume. Consequently, the total volume is not sexually differentiated because neurons are more densely packed within the male magnocellular subdivision of the medial preoptic nucleus. These results support the hypothesis that additional neurons in the magnocellular subdivision of the medial preoptic nucleus are critical for the expression of male copulatory behaviors in adulthood. Furthermore, they suggest that sexual differentiation of the magnocellular subdivision of the medial preoptic nucleus is an important process that forms the anatomical basis for sex-specific behavioral responses to pheromonal stimulation.

Sex-induced fos in the medial preoptic area: projections to the midbrain

Sexual activity results in cells displaying Fos-like Immunoreactivity (FLI) in the medial preoptic area (MPOA) of male rats. This study combined retrograde tracing techniques with FLI to determine if MPOA cells displaying sex-induced FLI project to known efferent sites of the MPOA. FluoroGold was injected into the dorsal central gray, lateral central gray, ventral tegmental area, medial central tegmental field, or lateral central tegmental field of male rats that later engaged in sexual activity. Examination of FLI and FluoroGold in the MPOA revealed that the lateral region of the MPOA projected to the lateral central gray and contained smaller projections to the other regions. These findings suggest that the lateral MPOA contains secondary sex-relevant projections to the midbrain.

Temporary inactivation of the retrorubral fields decreases the rewarding effect of medial forebrain bundle stimulation

Prior studies indicate that lesioning the retrorubral fields (RRF) decreases the rewarding effect of medial forebrain bundle (MFB) stimulation, although these studies did not make the RRF their primary target. This study directly investigates the role of the RRF in MFB self-stimulation using transient lidocaine-induced inactivation of target tissue rather than permanent lesioning. In 18 rats with MFB stimulation electrodes, inactivation of the RRF via 0. 5 and 1.0 microl of 4% lidocaine produced immediate, substantial upward shifts in the frequency required to maintain half-maximal self-stimulation response rates whereas injecting comparable volumes of saline did not. Bilateral inactivation was particularly effective, especially at medium and high stimulation currents, although unilateral inactivation ipsilateral to the stimulation site was also effective. Contralateral inactivation alone did not substantially change the stimulation's reward value, although contralateral inactivation appeared to contribute to the effectiveness of bilateral inactivation. The frequency required to maintain half-maximal responding returned to baseline levels by 15-20 min after lidocaine infusion. In seven rats whose infusion sites were not in the RRF, lidocaine inactivation did not consistently degrade the stimulation's reward value. These results indicate that some neural elements located in the RRF contribute to the rewarding effect of MFB stimulation. Possible roles for these elements in the anatomical substrate for MFB self-stimulation are discussed.

Castration rapidly decreases hypothalamic gamma-aminobutyric acidergic neuronal activity in both male and female rats

The postcastration LH response is greater and somewhat more rapid in male than female rats. We have previously demonstrated that hypothalamic gamma-aminobutyric acid (GABA)ergic neuronal activity decreases following gonadectomy in male rats. To investigate whether these same hypothalamic GABA neurons decrease their activity postcastration in female rats, and whether more rapid and or greater postcastration decreases occur in male rats, we determined the timing and magnitude of the postcastration decreases in GABA turnover which are associated with the sexually dimorphic postcastration LH response. Adult male and 4-day cycling female rats were castrated between 0800 and 1000 h (females ovariectomized on diestrus day 1). Serum LH levels increased significantly by 12 h postcastration in both males and females with the magnitude of the increases being 6.2-fold in males and 2.8-fold in females. GABA turnover was determined in 16 microdissected brain structures by the GABA transaminase inhibition method at 0 h (sham-operated controls), 6 h, 12 h and 1, 2, 4 and 6 days postcastration. In male rats, in the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis [DBB(ovlt)], the rate of GABA turnover decreased significantly already by 6 h postcastration compared with the 0 h controls, and remained suppressed through 6 days. This rapid down regulation of DBB(ovlt) GABAergic neurons also occurred in female rats, however, the duration of the decrease was not as prolonged as in male rats. Similar changes occurred in the tuberoinfundibular GABAergic (TIGA) neurons projecting to the median eminence in both males and females. Down regulation of these GABAergic neurons precedes or is coincident with increased postcastration LH secretion in both sexes, and the duration of the decreases is consistent with the less robust postcastration LH response in female rats. In addition, the rate of GABA turnover decreased after castration in the interstitial (bed) nucleus of the stria terminalis, ventral aspect (INSTv), the medial preoptic nucleus, dorsomedial aspect (MPNdm) and the ventromedial nucleus, ventrolateral aspect (VMNvl) in male rats, and in the INSTv and VMNvl of female rats, while there was no effect of castration in other hypothalamic regions or control structures. The result in the female VMNvl is consistent with reports that GABA facilitates lordosis behavior in this hypothalamic structure. These findings are consistent with the hypothesis that discrete hypothalamic populations of sex steroid-sensitive GABAergic neurons mediate the postcastration LH responses in both male and female rats, and may underlie other sexually dimorphic adult phenotypes such as sex behavior.

The preoptic area/anterior hypothalamus of different strains of mice: sex differences and development

While sex differences in neural morphology in the preoptic area/anterior hypothalamus (POA/AH) have been demonstrated in many species, their existence in mice have been controversial. Given the increased use of transgenic and gene-disrupted mice, we characterized sex differences using Nissl stains, and the immunocytochemical location of estrogen receptor-alpha (ER-alpha) and galanin in the POA/AH of two widely used strains, C57BL/6 and 129SvEv, and a mixed strain (C57BL/6x129Sv); the wild-type littermates of steroidogenic factor-1 (SF-1) gene-disrupted mice. Cell grouping was not a reliable marker of sex. In adults, cells located beneath the anterior commissure (AC) were reliably larger in females than males in 129SvEv, but not in the other strains. Caudally, cells in a group medial to the medial extension of the bed nucleus of the stria terminalis (BST) were significantly larger in males than females in C57BL/6J and SF-1 gene-disrupted wild-types. Cell groups discernible by embryonic day (E) 18 were not sexually dimorphic for cell size in C57BL/6J mice at E18 or postnatal day (P) 4. The pattern of distribution of cells containing ER-alpha was similar among the strains, reduced in the group medial to the BST; a pattern established by P0. Galanin-containing cells and fibers were seen from E15 to adulthood ventral to the AC. Caudally, a smaller group ventromedial to the BST was found only in 129SvEv adults. Sex differences in neural morphology which develop within the POA/AH depend upon multiple factors, particularly including genetic background.

Neuronal cell death during sexual differentiation and lateralisation of vocal communication

A rodent analogy has been established to investigate the neural mechanisms occurring during sexual differentiation and lateralization. A sexually dimorphic hypothalamic nucleus (SDApc) is closely associated with a stereotyped, courtship vocalisation in male gerbils. Stereological analysis of SDApc cytoarchitecture reveals that neuron number and nuclear volume are asymmetric in male adults. Strikingly, neuron number on the left side of the SDApc correlates significantly with the rate of the courtship call in males. Exogenous testosterone treatment in female neonates masculinises and lateralises SDApc structure and function. Neuronal programmed cell death (apoptosis), manifested in SDApcs of neonates, is more frequent in females. Significantly, apoptosis in males is lateralised, as revealed by lateral asymmetry of neuron number at postnatal day 16. It is concluded that neuroendocrine-dependent, sexual differentiation and lateralization are concurrent and influenced by apoptotic mechanisms. It is suggested that apoptosis is the result of a genetically-driven device, inherent in postmitotic, undifferentiated cells which may have recently migrated into the SDApc. The genomic mechanism inducing lateralised apoptosis is apparently activated only neonatally in males.

Sex differences in the activity of gamma-aminobutyric acidergic neurons in the rat hypothalamus

The rate of GABA turnover was determined in nine microdissected brain regions in adult male and female rats. In the medial preoptic nucleus (central aspect) and ventromedial nucleus (ventrolateral aspect) of the hypothalamus, areas involved in the regulation of gonadotropin secretion and sex behavior, GABAergic neuronal activity was about 2-fold greater in males than females. These results demonstrate a striking sexual dimorphism in the activity of specific populations of hypothalamic GABAergic neurons.

Effects of age and sociosexual experience on the morphology and metabolic capacity of brain nuclei in the leopard gecko (Eublepharis macularius), a lizard with temperature-dependent sex determination

In vertebrates having sex chromosomes, sexual behavior is influenced by steroid hormones throughout life as well as by the cumulative experiences of the individual. Because males and females differ genetically as well as hormonally, it would be valuable to distinguish the contribution of sex-specific genes from hormones. In addition, since animals age as they gain sociosexual experience, but do not necessarily gain sociosexual experience as they age, it is important to separate the effects of age from those attributable to experience. The leopard gecko is a lizard lacking sex chromosomes, depending instead upon the temperature during incubation to establish gonadal sex. This effectively removes sex-specific genetic influences from any study of sexual differentiation. Eggs were incubated at either 26 degrees C or 32.5 degrees C, temperatures that produce only female hatchlings or a male-biased sex ratio, respectively. By raising geckoes in isolation and then housing some animals together in breeding groups at different ages after they attained sexual maturity, it was possible to assess the relative effects of age and sociosexual experience on the volume and metabolic capacity of limbic and non-limbic brain areas. In general, males showed more changes compared to females. For example, there was a decrease with age in the volume of the preoptic area and the ventromedial hypothalamus in males, but not in females. Both age and sociosexual experience influenced cytochrome oxidase activity in these and other brain areas. Experienced animals had greater metabolic capacity in nuclei functionally associated with sociosexual behavior in lizards and other vertebrates. For example, cytochrome oxidase activity was higher in the anterior hypothalamus of males, in the ventromedial hypothalamus of both males and females from the male-biased incubation temperature, and in the preoptic area of females from both incubation temperatures. These differences were not paralleled by differences in circulating levels of sex hormones; only plasma androgen levels differed as a function of experience in males. These data suggest that the volume and metabolic capacity of specific brain regions change as animals age and gain sociosexual experience, but the nature and degree of change depend upon prenatal events.

Postnatal development of a sexually dimorphic, hypothalamic nucleus in gerbils: a stereological study of neuronal number and apoptosis

Steroid-sensitive, vocal courtship behavior is a function of a specific, hypothalamic nucleus, the sexually dimorphic area pars compacta (SDApc) in the male adult gerbil. Gender-related differences in the number of neurons in this nucleus are evident immediately after birth. By using unbiased stereological estimates of cell numbers in Nissl-stained, paraffin-wax sections of brain, we investigated the mechanisms differentiating cell number between the sexes in the SDApc on postnatal days 0, 3, 6, and 15. Cell death, identified by pyknosis, was greatest in the SDApc between days 0-3 in males, whereas in females, maximum values were reached between days 3-6. Similarly, the ratio of pyknotic to normal neurons peaked between days 0-3 in males and 3-6 in females but then declined in both sexes. Pyknotic cells were seldom seen in either sex by day 15. Morphological characteristics of apoptosis including chromatin condensation, cell fragmentation, and ingestion of apoptic bodies by macrophages were all demonstrated by transmission electron microscopy. Macrophages showed specific morphological characteristics of microglia. Cell division (mitosis) was identified in the SDApc during postnatal days 0, 3, and 6 but the numbers of mitotic figures were low, negligible on day 15, and similar between the sexes. These results demonstrate that cell death and proliferation occur simultaneously in the neonatal gerbil brain. The stereological estimates of cell death in the developing SDApc indicated a lower incidence of neuronal death occurring earlier in males than in females.