Neuropeptide and steroid hormone mediators of neuroendocrine regulation.

To maintain the health and well-being of all mammals, numerous aspects of physiology are controlled by neuroendocrine mechanisms. These mechanisms ultimately enable communication between neurones and glands throughout the body and are centrally mediated by neuropeptides and/or steroid hormones. A recent session at the International Workshop in Neuroendocrinology highlighted the essential roles of some of these neuropeptide and steroid hormone mediators in the neuroendocrine regulation of stress-, reproduction- and behaviour-related processes. Accordingly, the present review highlights topics presented in this session, including the role of the neuropeptides corticotrophin-releasing factor and gonadotrophin-releasing hormone in stress and reproductive physiology, respectively. Additionally, it details an important role for gonadal sex steroids in the development of behavioural sex preference.

Early prenatal androgen exposure reduces testes size and sperm concentration in sheep without altering neuroendocrine differentiation and masculine sexual behavior.

Prenatal androgens are largely responsible for growth and differentiation of the genital tract and testis and for organization of the control mechanisms regulating male reproductive physiology and behavior. The aim of the present study was to evaluate the impact of inappropriate exposure to excess testosterone (T) during the first trimester of fetal development on the reproductive function, sexual behavior, and fertility potential of rams. We found that biweekly maternal T propionate (100 mg) treatment administered from Day 30-58 of gestation significantly decreased (P < 0.05) postpubertal scrotal circumference and sperm concentration. Prenatal T exposure did not alter ejaculate volume, sperm motility and morphology or testis morphology. There was, however, a trend for more T-exposed rams than controls to be classified as unsatisfactory potential breeders during breeding soundness examinations. Postnatal serum T concentrations were not affected by prenatal T exposure, nor was the expression of key testicular genes essential for spermatogenesis and steroidogenesis. Basal serum LH did not differ between treatment groups, nor did pituitary responsiveness to GnRH. T-exposed rams, like control males, exhibited vigorous libido and were sexually attracted to estrous females. In summary, these results suggest that exposure to exogenous T during the first trimester of gestation can negatively impact spermatogenesis and compromise the reproductive fitness of rams.

Neurobiology of gender identity and sexual orientation.

Sexual identity and sexual orientation are independent components of a person's sexual identity. These dimensions are most often in harmony with each other and with an individual's genital sex, although not always. The present review discusses the relationship of sexual identity and sexual orientation to prenatal factors that act to shape the development of the brain and the expression of sexual behaviours in animals and humans. One major influence discussed relates to organisational effects that the early hormone environment exerts on both gender identity and sexual orientation. Evidence that gender identity and sexual orientation are masculinised by prenatal exposure to testosterone and feminised in it absence is drawn from basic research in animals, correlations of biometric indices of androgen exposure and studies of clinical conditions associated with disorders in sexual development. There are, however, important exceptions to this theory that have yet to be resolved. Family and twin studies indicate that genes play a role, although no specific candidate genes have been identified. Evidence that relates to the number of older brothers implicates maternal immune responses as a contributing factor for male sexual orientation. It remains speculative how these influences might relate to each other and interact with postnatal socialisation. Nonetheless, despite the many challenges to research in this area, existing empirical evidence makes it clear that there is a significant biological contribution to the development of an individual's sexual identity and sexual orientation.

Tyrosine hydroxylase in the ventral tegmental area of rams with high or low libido-A role for dopamine.

Dopamine synthesis in the ventral tegmental area (VTA) is necessary for the reinforcement of sexual behavior. The objective of this study determined if sexual stimuli initiates reward, and whether reward is attenuated in sexually inactive rams. Sexually active rams were exposed to urine from estrous (n=4) or ovariectomized (n=3) ewes with inactive rams (n=3) exposed to urine from estrous ewes. Following exposure, rams were exsanguinated and brains perfused. Alternating sections of the VTA were stained for Fos related antigens (FRA), tyrosine hydroxylase, and dopamine beta-hydroxylase activity. Forebrain tissue, mid-sagittal ventral to the anterior corpus callosum, was stained for dopamine D2 receptors. Concentrations of cortisol was determined prior to and following exposure. Exposure to ovariectomized-ewe urine in sexually active rams did not influence (P=0.6) FRA expression, but fewer (P<0.05) neurons were positive for tyrosine hydroxylase in the VTA. Sexually inactive rams had fewer (P<0.05) FRA and tyrosine hydroxylase positive neurons in the VTA than sexually active rams following exposure to estrous ewe urine. VTA neurons staining positive for dopamine beta-hydroxylase did not differ by sexual activity (P=0.44) or urine exposure (P=0.07). Exposure to stimulus did not influence (P=0.46) numbers of forebrain neurons staining positive for dopamine D2 receptors in sexually active rams, but fewer (P=0.04) neurons stain positive in inactive rams. Serum concentrations of cortisol did not differ (P≥0.52) among rams prior to or following stimulus. In conclusion sexual inactivity is unlikely due to stress, but may be partially a result of decreased tyrosine hydroxylase and/or the response to dopamine.

Fos Expression in the Olfactory Pathway of High- and Low-Sexually Performing Rams Exposed to Urine from Estrous or Ovariectomized Ewes.

Exposure to estrous ewe urine stimulates investigation and mounting activity in sexually active but not sexually inactive rams. It was hypothesized sexual indifference may result from an inability to detect olfactory cues or an interruption of the pathway from detection of the olfactory stimulus to the motor response. Sexually active (n=4) and inactive (n=3) rams were exposed to urine from estrous ewes. An additional group of sexually active rams (n=3) were exposed to urine from ovariectomized ewes. Rams were exsanguinated following 1 h of exposure to stimulus. Neural activity was determined in tissues of interest by the presence of fos and fos-related proteins detected by immunohistochemistry procedures. Sexually active rams exposed to urine from ovariectomized ewes had more (P ≤ 0.05) fos-positive cells in the olfactory bulb, but fewer (P = 0.03) fos-positive cells in the cortical amygdala compared to sexually active rams exposed to urine from estrous ewes. Sexually inactive rams had similar (P ≥ 0.13) numbers of fos positive neurons in the olfactory bulb and medial amygdala but fewer (P ≤ 0.04) in the central amygdala, bed nucleus of the stria terminalis and the medial preoptic area compared to sexually active rams exposed to urine from estrous ewes. Sexual inactivity was not associated with decreased hypothalamic function since fos activity was similar (P ≥ 0.14) among groups in the suprachiasmatic and ventral medial nucleus. Sexual inactivity is not likely due to an impaired ability to detect or process olfactory stimuli by the main olfactory bulb and medial-cortical amygdala. Sexually inactive rams may have reduced attentiveness to sexual stimuli and/or decreased responsiveness of regions in the brain which regulate reproductive behaviors.

Developmental and Functional Effects of Steroid Hormones on the Neuroendocrine Axis and Spinal Cord.

This review highlights the principal effects of steroid hormones at central and peripheral levels in the neuroendocrine axis. The data discussed highlight the principal role of oestrogens and testosterone in hormonal programming in relation to sexual orientation, reproductive and metabolic programming, and the neuroendocrine mechanism involved in the development of polycystic ovary syndrome phenotype. Moreover, consistent with the wide range of processes in which steroid hormones take part, we discuss the protective effects of progesterone on neurodegenerative disease and the signalling mechanism involved in the genesis of oestrogen-induced pituitary prolactinomas.

Effects of Long-Term Flutamide Treatment During Development on Sexual Behaviour and Hormone Responsiveness in Rams.

Testosterone exposure during midgestation differentiates neural circuits controlling sex-specific behaviours and patterns of gonadotrophin secretion in male sheep. Testosterone acts through androgen receptors (AR) and/or after aromatisation to oestradiol and binding to oestrogen receptors. The present study assessed the role of AR activation in male sexual differentiation. We compared rams that were exposed to the AR antagonist flutamide (Flu) throughout the critical period (i.e. days 30-90 of gestation) to control rams and ewes that received no prenatal treatments. The external genitalia of all Flu rams were phenotypically female. Testes were positioned s.c. in the inguinal region of the abdomen, exhibited seasonally impaired androgen secretion and were azospermic. Flu rams displayed male-typical precopulatory and mounting behaviours but could not intromit or ejaculate because they lacked a penis. Flu rams exhibited greater mounting behaviour than control rams and, similar to controls, showed sexual partner preferences for oestrous ewes. Neither control, nor Flu rams responded to oestradiol treatments with displays of female-typical receptive behaviour or LH surge responses, whereas all control ewes responded as expected. The ovine sexually dimorphic nucleus in Flu rams was intermediate in volume between control rams and ewes and significantly different from both. These results indicate that prenatal anti-androgen exposure is not able to block male sexual differentiation in sheep and suggest that compensatory mechanisms intervene to maintain sufficient androgen stimulation during development.

Sex differences in expression of oestrogen receptor α but not androgen receptor mRNAs in the foetal lamb brain.

Gonadal steroid hormones play important roles during critical periods of development to organise brain structures that control sexually dimorphic neuroendocrine responses and behaviours. Specific receptors for androgens and oestrogens must be expressed at appropriate times during development to mediate these processes. The present study was performed to test for sex differences in the relative expression of oestrogen receptor (ER)α and androgen receptor (AR) mRNA during the window of time in gestation that is critical for behavioural masculinisation and differentiation of the ovine sexually dimorphic nucleus (oSDN) in the sheep. In addition, we examined whether ERα and AR mRNA expression is localised within the nascent oSDN and could be involved in its development. Using the quantitative real-time polymerase chain reaction, we found that females expressed more ERα mRNA than males in medial preoptic area and medial basal hypothalamus during the mid-gestational critical period for brain sexual differentiation. No sex differences were found for AR mRNA in any tissue examined or for ERα in amygdala and frontal cortex. Using radioactive in situ hybridisation, we found that the distributions of ERα and AR mRNA overlapped with aromatase mRNA, which delineates the boundaries of the developing oSDN and identifies this nucleus as a target for both androgens and oestrogens. These data demonstrate that the transcriptional machinery for synthesising gonadal steroid receptors is functional in the foetal lamb brain during the critical period for sexual differentiation and suggest that possible mechanisms for establishing dimorphisms controlled by gonadal steroids may exist at the level of steroid hormone receptor expression.

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

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

Prenatal exposure to vinclozolin disrupts selective aspects of the gonadotrophin-releasing hormone neuronal system of the rabbit.

Developmental exposure to the agricultural fungicide vinclozolin can impair reproductive function in male rabbits and was previously found to decrease the number of immunoreactive-gonadotrophin-releasing hormone (GnRH) neurones in the region of the organum vasculosum of the lamina terminalis and rostral preoptic area by postnatal week (PNW) 6. In the present study, in an aim to further examine the disruption of GnRH neurones by foetal vinclozolin exposure, pregnant rabbits were dosed orally with vinclozolin, flutamide or carrot paste vehicle for the last 2 weeks of gestation. Offspring were euthanised at birth (males and females), PNW 6 (females), PNW 26 (adult males) or PNW 30 (adult females) of age. At birth and in adults, brains were sectioned and processed for immunoreactive GnRH. The numbers of immunoreactive GnRH neuronal perikarya were significantly decreased in vinclozolin-treated rabbits at birth and in adult littermates. By contrast, there was an increase in GnRH immunoreactivity in the terminals in the region of the median eminence. Analysis of PNW 6 female brains by radioimmunoassay revealed a two-fold increase in GnRH peptide content in the mediobasal hypothalamus in vinclozolin-treated rabbits. This finding was complemented by immunofluorescence analyses, which revealed a 2.8-fold increase in GnRH immunoreactivity in the median eminence of vinclozolin compared to vehicle-treated females at PNW 30. However, there was no difference between treatment groups in the measures of reproduction that were evaluated: ejaculation latency, conception rates or litter size. These results indicate that sub-acute, prenatal vinclozolin treatment is sufficient to create perdurable alterations in the GnRH neuronal network that forms an important input into the reproductive axis. Finally, the effect of vinclozolin on the GnRH neuronal network was not comparable to that of flutamide, suggesting that vinclozolin was not acting through anti-androgenic mechanisms.

The ovine sexually dimorphic nucleus, aromatase, and sexual partner preferences in sheep.

We are using the domestic ram as an experimental model to examine the role of aromatase in the development of sexual partner preferences. This interest has arisen because of the observation that as many as 8% of domestic rams are sexually attracted to other rams (male-oriented) in contrast to the majority of rams that are attracted to estrous ewes (female-oriented). Our findings demonstrate that aromatase expression is enriched in a cluster of neurons in the medial preoptic nucleus called the ovine sexually dimorphic nucleus (oSDN). The size of the oSDN is associated with a ram's sexual partner preference, such that the nucleus is 2-3 times larger in rams that are attracted to females (female-oriented) than in rams that are attracted to other rams (male-oriented). Moreover, the volume of the oSDN in male-oriented rams is similar to the volume in ewes. These volume differences are not influenced by adult concentrations of serum testosterone. Instead, we found that the oSDN is already present in late gestation lamb fetuses (approximately day 135 of gestation) when it is approximately 2-fold greater in males than in females. Exposure of genetic female fetuses to exogenous testosterone during the critical period for sexual differentiation masculinizes oSDN volume and aromatase expression when examined subsequently on day 135. The demonstration that the oSDN is organized prenatally by testosterone exposure suggests that the brain of the male-oriented ram may be under-androgenized during development.

Porcine hypothalamic aromatase cytochrome P450: isoform characterization, sex-dependent activity, regional expression, and regulation by enzyme inhibition in neonatal boars.

Domestic pigs have three CYP19 genes encoding functional paralogues of the enzyme aromatase cytochrome P450 (P450arom) that are expressed in the gonads, placenta, and preimplantation blastocyst. All catalyze estrogen synthesis, but the gonadal-type enzyme is unique in also synthesizing a nonaromatizable biopotent testosterone metabolite, 1OH-testosterone (1OH-T). P450arom is expressed in the vertebrate brain, is higher in males than females, but has not been investigated in pigs, to our knowledge. Therefore, these studies defined which of the porcine CYP19 genes was expressed, and at what level, in adult male and female hypothalamus. Regional expression was examined in mature boars, and regulation of P450arom expression in neonatal boars was investigated by inhibition of P450arom with letrozole, which is known to reprogram testicular expression. Pig hypothalami expressed the gonadal form of P450arom (redesignated the "gonadal/hypothalamic" porcine CYP19 gene and paralogue) based on functional analysis confirmed by cloning and sequencing transcripts. Hypothalamic tissue synthesized 1OH-T and was sensitive to the selective P450arom inhibitor etomidate. Levels were 4-fold higher in male than female hypothalami, with expression in the medial preoptic area and lateral borders of the ventromedial hypothalamus of boars. In vivo, letrozole-treated neonates had increased aromatase activity in hypothalami but decreased activity in testes. Therefore, although the same CYP19 gene is expressed in both tissues, expression is regulated differently in the hypothalamus than testis. These investigations, the first such studies in pig brain to our knowledge, demonstrate unusual aspects of P450arom expression and regulation in the hypothalamus, offering promise of gaining better insight into roles of P450arom in reproductive function.

Prenatal programming of sexual partner preference: the ram model.

In our laboratory, the domestic ram is used as an experimental model to study the early programming of neural mechanisms underlying same-sex partner preference. This interest developed from the observation that approximately 8% of domestic rams are sexually attracted to other rams (male-oriented) in contrast to the majority of rams that are attracted to oestrous ewes (female-oriented). One prominent feature of sexual differentiation in many species is the presence of a sexually dimorphic nucleus (SDN) in the preoptic/anterior hypothalamus that is larger in males than in females. Lesion studies in rats and ferrets implicate the SDN in the expression of sexual preferences. We discovered an ovine SDN (oSDN) in the preoptic/anterior hypothalamus that is smaller in male- than in female-oriented rams and similar in size to the oSDN of ewes. Neurones of the oSDN show abundant aromatase expression that is also reduced in male-oriented compared to female-oriented rams. This observation suggests that sexual partner preferences are neurologically hard-wired and could be influenced by hormones. Aromatase-containing neurones constitute a nascent oSDN as early as day 60 of gestation, which becomes sexually dimorphic by day 135 of gestation when it is two-fold larger in males than in females. Exposure of fetal female lambs to exogenous testosterone from days 30-90 of gestation resulted in a masculinised oSDN. These data demonstrate that the oSDN develops prenatally and may influence adult sexual preferences. Surprisingly, inhibition of aromatase activity in the brain of ram foetuses during the critical period did not interfere with defeminisation of adult sexual partner preference or oSDN volume. These results fail to support an essential role for neural aromatase in the sexual differentiation of sheep brain and behaviour. Thus, we propose that oSDN morphology and male-typical partner preferences may instead be programmed through an androgen receptor mechanism not involving aromatisation.

The volume of the ovine sexually dimorphic nucleus of the preoptic area is independent of adult testosterone concentrations.

The ovine sexually dimorphic nucleus (oSDN) is characterized by high levels of aromatase mRNA expression which can be used to delineate its boundaries. The volume of the oSDN is approximately 2 to 3-fold larger in rams that mate with ewes (female-oriented rams) than in rams that mate with other rams (male-oriented rams) and ewes. The sex difference in oSDN volume is present in late gestation fetuses and can be eliminated before birth by exposing genetic females to exogenous testosterone during midgestation, suggesting that early exposure to androgen masculinizes volume of the oSDN. The present study was performed to determine whether differences in oSDN volume are influenced by the adult hormonal environment. Adult rams, behaviorally characterized as female-oriented or male-oriented, and ewes were gonadectomized and treated with subcutaneous implants of testosterone to achieve physiologic concentrations of serum testosterone. Three weeks after implant placement brain tissue was prepared for histological assessment of oSDN volume using in situ hybridization for detection of aromatase mRNA expression. Quantitative analysis revealed that despite similar serum testosterone levels among the groups, the volume of the oSDN was greater in female-oriented rams than in male-oriented rams and ewes (P<0.05). Differences in oSDN volume were specific and not reflective of differences in preoptic area height or brain size. These results suggest that differences in the size of the oSDN in adult sheep were not influenced by adult exposure to testosterone.

Changes in LH secretion in response to an estradiol challenge in male- and female-oriented rams and in ewes.

Two experiments were conducted to determine whether an estradiol challenge could cause a female-type LH surge in castrated male- and female-oriented rams (MORs and FORs). Administration of 17beta-estradiol to castrated MORs and FORs and ovariectomized ewes caused an initial reduction in LH secretion followed for 12-20 h by a surge release of LH in the ewes. No surge release of LH occurred in the MORs and FORs. The pattern of changes in LH secretion within rams and ewes did not differ between the breeding and nonbreeding seasons. Treatment failed to elicit female-typical receptive sexual behaviors in the rams but did stimulate increased sexual receptivity in the ewes as determined by the measures of responsiveness to the teaser ram. Overall, no differences were found in hypothalamic-hypophyseal function in response to exogenous estradiol between MORs and FORs. These data are interpreted to suggest that in contrast to sexual attraction, the neural mechanisms controlling the LH surge and female receptivity are defeminized in MORs.

Neuronal activity of aromatase enzyme in non-copulating male rats.

There are apparently normal male rats that fail to initiate copulation; these animals are called non-copulating (NC) males. Several research groups have demonstrated that conversion of testosterone to oestradiol (aromatisation) in specific brain areas known to be involved in the control of masculine sexual behaviour is fundamental in the control of masculine sexual behaviour. The aim of the present study was to test the hypothesis that the concentration of aromatase activity (AA) in the brain is lower in NC males than in copulating males (C). We quantified AA in several brain nuclei and also evaluated whether NC rats have altered concentrations of testosterone in their plasma. We found that AA was reduced in the medial preoptic nuclei (MPN) of NC male rats vs C males. In addition, NC and C male rats had similar plasma levels of testosterone. These data suggest that reduced levels of AA in the MPN could be a crucial factor associated with lack of male coital behaviour in rats.

The effect of aromatase inhibition on the sexual differentiation of the sheep brain.

This study tested the hypothesis that aromatization of testosterone to estradiol is necessary for sexual differentiation of the sheep brain. Pregnant ewes (n = 10) were treated with the aromatase inhibitor 1,4,6- androstatriene-3,17-dione (ATD) during the period of gestation when the sheep brain is maximally sensitive to the behavior-modifying effects of exogenous testosterone (embryonic d 50-80; 147 d is term). Control (n = 10) ewes received vehicle injections. Fifteen control lambs (7 males and 8 females) and 17 ATD-exposed lambs (7 males and 10 females) were evaluated for sexually dimorphic behavioral and neuroendocrine traits as adults. Prenatal ATD exposure had no significant effect on serum concentrations of androgen at birth, growth rates, expression of juvenile play behaviors, or the onset of puberty in male and female lambs. Rams exposed to ATD prenatally exhibited a modest, but significant, decrease in mounting behavior at 18 mo of age. However, prenatal ATD exposure did not interfere with defeminization of adult sexual partner preferences, receptive behavior, or the LH surge mechanism. In summary, our results indicate that aromatization is necessary for complete behavioral masculinization in sheep. However, before we can conclude that aromatization does not play a role in defeminization of the sheep brain, it will be necessary to evaluate whether intrauterine exposure of male fetuses to higher doses of ATD for a more extended period of time can disrupt normal neuroendocrine and behavioral development.

Distribution of aromatase activity in brain and peripheral tissues of male sheep: effect of nutrition.

Conversion of testosterone to oestradiol plays a major role in the feedback inhibition of gonadotrophin secretion in male sheep but little is known of the distribution or control of aromatase activity among central and peripheral tissues. Changes in activity at those sites may mediate alterations in the effectiveness of negative feedback following, for example, a change in nutrition. Using a tritiated-water assay, we quantified aromatase in several tissues in mature male sheep, assessed their contribution to oestradiol production, and tested whether activity at each site was affected by a nutritional treatment that stimulates gonadotrophin secretion. Among the brain tissues, the preoptic area had the highest concentration of activity, followed by the hypothalamus, amygdala and cortex. Among the peripheral tissues, liver and testis had the highest activity and, due to their mass, they are the major sources of circulating oestradiol. Pituitary, muscle, kidney and adipose tissues had very low aromatase levels. The nutritional stimulus increased activity in testis but not in liver or brain. We conclude that changes in aromatase activity do not mediate the effects of nutrition on steroid feedback, but aromatisation in testis, liver and brain is important in the endocrine regulation of reproduction in the mature ram.

Role of aromatization in anticipatory and consummatory aspects of sexual behavior in male rats.

The present study was designed to test the hypothesis that aromatization is involved in the maintenance by testosterone of the appetitive component of male sexual behavior. We measured appetitive sexual behavior by administering behavioral tests in bilevel chambers and quantifying anticipatory level changes during a 5-min period prior to introduction of a stimulus female. In addition, we recorded standard measures of consummatory male sexual behavior after the female was introduced. Following 3 weekly tests, level-changing behavior reached a plateau and remained stable for up to 10 weeks. After 10 bilevel tests, rats were given subcutaneous testosterone capsules to clamp circulating androgen at physiological levels. Rats were tested and divided into two groups that were matched for measures of sexual behavior. One group was then treated with the nonsteroidal aromatase inhibitor, Fadrozole (2.5 mg/kg/day), given subcutaneously in beta-cyclodextrin and the other group was treated with vehicle. Within 1 week of Fadrozole treatment, the number of anticipatory levels changes was significantly reduced, but not the latency to begin searching. Fadrozole treatment also significantly reduced all measures of copulatory behavior over the period of treatment and increased latencies to first mount, intromission, and ejaculation. After 8 weeks, both treatment groups were given an additional Silastic capsule filled with estradiol and tested for 4 additional weeks. Estrogen treatment partially restored level-changing behavior, mounts, and intromissions but had little effect on ejaculations. These results support the view that aromatization is important for maintaining both the appetitive and the consummatory aspects of sexual behavior in male rats.

Anatomic relationships between aromatase and androgen receptor mRNA expression in the hypothalamus and amygdala of adult male cynomolgus monkeys.

This study mapped the regional locations of cells expressing cytochrome P450 aromatase (P450AROM) and androgen receptor (AR) mRNAs in the adult male macaque hypothalamus and amygdala by in situ hybridization histochemistry using monkey-specific cRNA probes. High densities of P450AROM and AR mRNA-containing neurons were observed in discrete hypothalamic areas involved in the regulation of gonadotropin secretion and reproductive behavior. P450AROM mRNA-containing neurons were most abundant in the medial preoptic nucleus, bed nucleus of the stria terminalis, and anterior hypothalamic area, whereas AR mRNA-containing neurons were most numerous in the ventromedial nucleus, arcuate nucleus, and tuberomamillary nucleus. Moderate to heavily labeled P450AROM mRNA-containing cells were present in the cortical and medial amygdaloid nuclei, which are known to have strong reciprocal inputs with the hypothalamus. Heavily labeled P450AROM mRNA-containing cells were found in the accessory basal amygdala nucleus, which projects to the cingulate cortex and hippocampus, areas that are important in the expression of emotional behaviors and memory processing. In contrast to P450AROM, the highest density of AR mRNA labeling in the temporal lobe was associated with the cortical amygdaloid nucleus and the pyramidal cells of the hippocampus. All areas that contained P450AROM mRNA-expressing cells also contained AR mRNA-expressing cells, but there were areas in which AR mRNA was expressed but not P450AROM mRNA. The apparent relative differences in the expression of P450AROM and AR mRNA-containing neurons within the monkey brain suggests that T acts through different signaling pathways in specific brain areas or within different cells from the same region.