Pre- and post-translational regulation of aromatase by steroidal and non-steroidal aromatase inhibitors

Estrogen receptor alpha, G-protein coupled estrogen receptor 1, and aromatase: Developmental, sex, and region-specific differences across the rat caudate-putamen, nucleus accumbens core and shell

Sex steroid hormones such as 17β-estradiol (estradiol) regulate neuronal function by binding to estrogen receptors (ERs), including ERα and GPER1, and through differential production via the enzyme aromatase. ERs and aromatase are expressed across the nervous system, including in the striatal brain regions. These regions, comprising the nucleus accumbens core, shell, and caudate-putamen, are instrumental for a wide-range of functions and disorders that show sex differences in phenotype and/or incidence. Sex-specific estrogen action is an integral component for generating these sex differences. A distinctive feature of the striatal regions is that in adulthood neurons exclusively express membrane but not nuclear ERs. This long-standing finding dominates models of estrogen action in striatal regions. However, the developmental etiology of ER and aromatase cellular expression in female and male striatum is unknown. This omission in knowledge is important to address, as developmental stage influences cellular estrogenic mechanisms. Thus, ERα, GPER1, and aromatase cellular immunoreactivity was assessed in perinatal, prepubertal, and adult female and male rats. We tested the hypothesis that ERα, GPER1, and aromatase exhibits sex, region, and age-specific differences, including nuclear expression. ERα exhibits nuclear expression in all three striatal regions before adulthood and disappears in a region- and sex-specific time-course. Cellular GPER1 expression decreases during development in a region- but not sex-specific time-course, resulting in extranuclear expression by adulthood. Somatic aromatase expression presents at prepuberty and increases by adulthood in a region- but not sex-specific time-course. These data indicate that developmental period exerts critical sex-specific influences on striatal cellular estrogenic mechanisms.

Acute neuroestrogen blockade attenuates song-induced immediate early gene expression in auditory regions of male and female zebra finches

Neuron-derived estrogens are synthesized by aromatase and act through membrane receptors to modulate neuronal physiology. In many systems, long-lasting hormone treatments can alter sensory-evoked neuronal activation. However, the significance of acute neuroestrogen production is less understood. Both sexes of zebra finches can synthesize estrogens rapidly in the auditory cortex, yet it is unclear how this modulates neuronal cell signaling. We examined whether acute estrogen synthesis blockade attenuates auditory-induced expression of early growth response 1 (Egr-1) in the auditory cortex of both sexes. cAMP response element-binding protein phosphorylation (pCREB) induction by song stimuli and acute estrogen synthesis was also examined. We administered the aromatase inhibitor fadrozole prior to song exposure and measured Egr-1 across several auditory regions. Fadrozole attenuated Egr-1 in the auditory cortex greater in males than females. Females had greater expression and clustering of aromatase cells than males in high vocal center (HVC) shelf. Auditory-induced Egr-1 expression exhibited a large sex difference following fadrozole treatment. We did not observe changes in pCREB expression with song presentation or aromatase blockade. These findings are consistent with the hypothesis that acute neuroestrogen synthesis can drive downstream transcriptional responses in several cortical auditory regions, and that this mechanism is more prominent in males.

Behavioral evidence for sex steroids hypersensitivity in castrated male canaries

In seasonally breeding songbirds such as canaries, singing behavior is predominantly under the control of testosterone and its metabolites. Short daylengths in the fall that break photorefractoriness are followed by increasing daylengths in spring that activate singing via both photoperiodic and hormonal mechanisms. However, we observed in a group of castrated male Fife fancy canaries maintained for a long duration under a short day photoperiod a large proportion of subjects that sang at high rates. This singing rate was not correlated with variation in the low circulating concentrations of testosterone. Treatment of these actively singing castrated male canaries with a combination of an aromatase inhibitor (ATD) and an androgen receptor blocker (flutamide) only marginally decreased this singing activity as compared to control untreated birds and did not affect various measures of song quality. The volumes of HVC and of the medial preoptic nucleus (POM) were also unaffected by these treatments but were relatively large and similar to volumes in testosterone-treated males. In contrast, peripheral androgen-sensitive structures such as the cloacal protuberance and syrinx mass were small, similar to what is observed in castrates. Together these data suggest that after a long-term steroid deprivation singing behavior can be activated by very low concentrations of testosterone. Singing normally depends on the activation by testosterone and its metabolites of multiple downstream neurochemical systems such as catecholamines, nonapeptides or opioids. These transmitter systems might become hypersensitive to steroid action after long term castration as they probably are at the end of winter during the annual cycle in seasonally breeding temperate zone species.

Two aromatase inhibitors inhibit the ability of a third to promote mating in male rats

Aromatase, the enzyme that aromatizes androstenedione (A) to estrone and testosterone (T) to estradiol (E), affects androgen control of male sex behavior in many vertebrates. In male monkeys, rats and quail, E mimics the ability of T to promote mating, and aromatase inhibitors block mating induced by T but not E. Aromatase inhibitors include androgens with different A-rings than T and A, e.g., 1,4,6-androstatriene-3,17-dione (ATD), azoles, e.g., fadrozole, and androgens α-halogenated at carbon 6, e.g., 6α-bromoA, 6α-fluoroA and 6α-fluoroT. 6α-FluoroT is the only 6α-halogenated androgen studied in regard to mating. It promotes mating in male rats and quail and was studied, before it was known to inhibit aromatase, because it cannot be aromatized yet has the same A-ring as T. 6α-FluoroT might promote mating by binding estrogen receptors (ER) directly, i.e., unassisted, or by metabolism to an androgen that binds ER. Since neither process would require aromatase, this study tested both hypotheses by determining how mating induced in castrated male rats by 6α-fluoroT is affected by ATD and fadrozole. Both aromatase inhibitors inhibited the effects of 6α-fluoroT on mating. Thus, 6α-fluoroT does not promote mating by direct ER binding or metabolism to another androgen. Since aromatase underlies a process in which 6α-fluoroT, unlike most nonaromatizable androgens, mimics T effects on male sex behavior, the process must involve a feature that 6α-fluoroT shares with T but not other nonaromatizable androgens. A-ring structure is a candidate. A hypothesis is also offered for how aromatase may participate without aromatizing the androgen.

Sex differences and rapid estrogen signaling: A look at songbird audition

The actions of estrogens have been associated with brain differentiation and sexual dimorphism in a wide range of vertebrates. Here we consider the actions of brain-derived 'neuroestrogens' in the forebrain and the accompanying differences and similarities observed between males and females in a variety of species. We summarize recent evidence showing that baseline and fluctuating levels of neuroestrogens within the auditory forebrain of male and female zebra finches are largely similar, and that neuroestrogens enhance auditory representations in both sexes. With a comparative perspective we review evidence that non-genomic mechanisms of neuroestrogen actions are sexually differentiated, and we propose a working model for nonclassical estrogen signaling via the MAPK intracellular signaling cascade in the songbird auditory forebrain that is informed by the way sex differences may be compensated. This view may lead to a more comprehensive understanding of how sex influences estradiol-dependent modulation of sensorimotor representations.

Neurochemical organization and experience-dependent activation of estrogen-associated circuits in the songbird auditory forebrain

The classic steroid hormone estradiol is rapidly produced by central auditory neurons in the songbird brain and instantaneously modulates auditory coding to enhance the neural and behavioral discrimination of acoustic signals. Although recent advances highlight novel roles for estradiol in the regulation of central auditory processing, current knowledge on the functional and neurochemical organization of estrogen-associated circuits, as well as the impact of sensory experience in these auditory forebrain networks, remains very limited. Here we show that both estrogen-producing and -sensitive neurons are highly expressed in the caudomedial nidopallium (NCM), the zebra finch analog of the mammalian auditory association cortex, but not other auditory forebrain areas. We further demonstrate that auditory experience primarily engages estrogen-producing, and to a lesser extent, estrogen-responsive neurons in NCM, that these neuronal populations moderately overlap and that acute episodes of sensory experience do not quantitatively affect these circuits. Finally, we show that whereas estrogen-producing cells are neurochemically heterogeneous, estrogen-sensitive neurons are primarily glutamatergic. These findings reveal the neurochemical and functional organization of estrogen-associated circuits in the auditory forebrain, demonstrate their activation and stability in response to sensory experience in behaving animals, and highlight estrogenic circuits as fundamental components of central networks supporting sensory processing.

Neuroprotection by estradiol: A role of aromatase against spine synapse loss after blockade of GABAA receptors

Estrogen has been suggested to be pro-epileptic by reducing GABA synthesis, resulting in increased spine density and a decreased threshold for seizures in the hippocampus, which, once they occur, are characterized by a dramatic spine loss in the affected brain areas. As considerable amounts of estradiol are synthesized in the hippocampus, in this study we focused on aromatase, the rate-limiting enzyme in estrogen synthesis in order to examine the role of locally synthesized estrogens in epilepsy. To this end, we first examined the effects of letrozole, a potent aromatase inhibitor, on GABA metabolism in single interneurons of hippocampal dispersion cultures. Letrozole downregulated estradiol release into the medium, as well as glutamate decarboxylase (GAD) expression and GABA synthesis, and decreased the number of GAD positive cells in the cultures. Next, we counted spine synapses and measured estradiol release of hippocampal slice cultures, in which GABA(A) receptors had been blocked by bicuculline, in order to mimic epileptic activity. Treatment of slice cultures with bicuculline resulted in a dramatic decrease in the number of spine synapses and in a significant suppression of estrogen synthesis. The decrease in synapse number in response to bicuculline was restored by combined application of estradiol and bicuculline. Surprisingly, estradiol alone had no effect on either spine synapse number or on GAD expression and GABA synthesis. "Rescue" of synapse number in "epileptic slices" by estradiol and maintenance of GABA metabolism by hippocampus-derived estradiol points to a neuroprotective role of aromatase in epilepsy. Re-filling of estradiol stores after their depletion due to overexcitation may therefore add to therapeutical strategies in epilepsy.

Aromatase destabilizer: novel action of exemestane, a food and drug administration-approved aromatase inhibitor

Using Western blot as the major technique, we studied the effects of the three Food and Drug Administration (FDA)-approved aromatase inhibitors (AI) on aromatase protein stability in the aromatase-overexpressing breast cancer cell line MCF-7aro. We have found that exemestane treatment significantly reduces aromatase protein level. Exemestane induces aromatase degradation in a dose-responsive manner (25-200 nmol/L), and the effect can be seen in as early as 2 hours. Metabolic labeling with S(35)-methionine was used to determine the half-life (t(1/2)) of aromatase protein. In the presence of 200 nmol/L exemestane, the t(1/2) of aromatase was reduced to 12.5 hours from 28.2 hours in the untreated cells. Furthermore, exemestane-induced aromatase degradation can be completely blocked by 10 micromol/L MG132, indicating that the degradation is mediated by proteasome. We also examined the effect of exemestane on aromatase mRNA level using real-time reverse transcription-PCR. No significant changes in mRNA level were detected after 8 hours of treatment with exemestane (200 nmol/L). This is the first report on the evaluation of three FDA-approved AIs on the stability of the aromatase protein. We have found that exemestane, different from letrozole and anastrozole, can destabilize the aromatase protein.

Oestrogen synthesis in the hippocampus: role in axon outgrowth

Ovarian oestrogens have been postulated to be neuroprotective. It has also been shown that considerable amounts of oestrogens are synthesised in hippocampal neurones. In the present study, we focused on a potential role of hippocampus-derived oestradiol compared to gonad-derived oestradiol on axon outgrowth of hippocampal neurones. To address the role of hippocampus-derived oestradiol, we inhibited oestrogen synthesis by treatment of neonatal hippocampal cell cultures with letrozole, a specific aromatase inhibitor. As an alternative, we used siRNA against steroidogenic acute regulatory protein (StAR). Axon outgrowth and GAP-43 expression were significantly down-regulated in response to letrozole and in siRNA-StAR transfected cells. The effects after inhibition of oestrogen synthesis in response to letrozole and in siRNA-StAR transfected cells were reversed by oestrogen supplementation. No difference was found between ovariectomised animals, cycling animals at pro-oestrus and ovariectomised and subsequently oestradiol-treated animals. However, high pharmacological doses of oestradiol promoted axon outgrowth, which was possible to abolish by the oestrogen receptor antagonist ICI 182,780. Our results show that oestradiol-induced neurite outgrowth is very likely mediated by genomic oestrogen receptors and requires higher doses of oestradiol than physiological serum concentrations derived from the gonads.

Inhibition of hippocampal estrogen synthesis causes region-specific downregulation of synaptic protein expression in hippocampal neurons

Previous studies have shown that synapses and expression of synaptic proteins in hippocampal neurons are regulated by hippocampus-derived estradiol. Here, we compared the effects of this paracrine regulation in different hippocampal regions. In tissue sections, immunohistochemistry followed by semiquantitative image analysis revealed a three-fold higher expression of steroidogenic acute regulatory protein (StAR) and aromatase in neurons of the CA3 than that of the CA1 region and in granule cells. Next, we treated hippocampal cell cultures with letrozole, an aromatase inhibitor, which resulted in a dose-dependent decrease in the release of 17beta-estradiol into the medium and in a dose-dependent downregulation of spinophilin and synaptophysin expression in dissociated hippocampal neurons. The downregulation of synaptic protein expression was restored by simultaneous application of letrozole together with estradiol. In response to a defined dose of letrozole, the downregulation of spinophilin expression was significantly stronger in CA1 neurons and in granule cells, than in cells of the CA3 region in slice cultures. With synaptophysin, downregulation was stronger in stratum lucidum of CA3 than in stratum radiatum of CA1. Both region-specific expression of steroidogenic enzymes and region-specific downregulation of synaptic proteins in response to a defined dose of letrozole may suggest different levels of estrogen concentrations within the hippocampus. Varying concentrations of estradiol in the hippocampus in turn may contribute to region-specific differentiation of hippocampal neurons.

Neurosteroid synthesis in the hippocampus: Role in synaptic plasticity

Neurosteroids are still found in the brain after steroidogenic glands were removed, indicating that they are synthesized either de novo or from endogenous precursors by enzymes present in the CNS. In fact, steroidogenic acute regulatory protein, and aromatase, two molecules essential for estrogen synthesis, are expressed in the hippocampus. We recently showed, for the first time, that estrogens are synthesized de novo in hippocampal neurons and that these hippocampus-derived estrogens are essential for synaptic plasticity. Both estrogen receptor isoforms, estrogen receptor alpha and estrogen receptor beta, are expressed in the hippocampus, and estradiol treatment of the cultures leads to an upregulation of estrogen receptor alpha. This finding confirmed the presence of functional estrogen receptors in hippocampal neurons and showed the responsiveness of the cultured hippocampal neurons to estradiol. By using letrozole, an inhibitor of aromatase, estradiol levels in hippocampal dispersion cultures as well as in hippocampal slice cultures were significantly suppressed which in turn led to a downregulation of estrogen receptor alpha. Letrozole treatment was followed by a significant decrease in the density of spines and spine synapses and in the number of presynaptic boutons. Quantitative immunohistochemistry revealed a dose-dependent downregulation of spinophilin, a spine marker, and of synaptophysin, a presynaptic marker, and of growth-associated protein 43 after letrozole treatment. Our data provide strong evidence for estrogens being potent modulators of structural synaptic plasticity and point to a paracrine rather than endocrine mechanism of estrogen action in the hippocampus.

Hippocampal synapses depend on hippocampal estrogen synthesis

Estrogens have been described to induce synaptogenesis in principal neurons of the hippocampus and have been shown to be synthesized and released by exactly these neurons. Here, we have focused on the significance of local estrogen synthesis on spine synapse formation and the synthesis of synaptic proteins. To this end, we reduced hippocampal estrogen synthesis in vitro with letrozole, a reversible nonsteroidal aromatase inhibitor. In hippocampal slice cultures, letrozole treatment resulted in a dose-dependent decrease of 17beta-estradiol as quantified by RIA. This was accompanied by a significant decrease in the density of spine synapses and in the number of presynaptic boutons. Quantitative immunohistochemistry revealed a downregulation of spinophilin, a marker of dendritic spines, and synaptophysin, a protein of presynaptic vesicles, in response to letrozole. Surprisingly, no increase in the density of spines, boutons, and synapses and in spinophilin expression was seen after application of estradiol to the medium of cultures that had not been treated with letrozole. However, synaptophysin expression was upregulated under these conditions. Our results point to an essential role of endogenous hippocampal estrogen synthesis in the maintenance of hippocampal spine synapses.

Prenatal blockade of estradiol synthesis impairs respiratory and metabolic responses to hypoxia in newborn and adult rats

We tested the hypothesis that estradiol modifies respiratory control in pregnant rats and participates in the development of respiratory chemoreflexes in fetuses. Pregnant rats (n = 12) received daily subcutaneous injections of vehicle (Veh, n = 6) or 4-androsten-4-ol-3,17-dione acetate (ATD; inhibitor of estradiol synthesis; n = 6; 5 mg/day in vehicle) from gestational day 16 (G16) to delivery. Baseline ventilation (whole body plethysmography) and metabolic rate [oxygen consumption (Vo(2))] were determined at G14 and G20, in pups [on postnatal day 3 (P3) and P20] and in adult rats (on P70) born to Veh- or ATD-treated mothers. Hypoxic chemoreflex was assessed in P3 rats by acute exposure to 60% O(2) and in P20 or P70 rats by moderate hypoxia (12% O(2), 30 min). ATD treatment reduced circulating estradiol in pregnant dams at G20 without producing changes in the circulating level of estradiol precursors (testosterone and androstenedione). ATD-treated dams showed impaired respiratory adjustment to late gestation. Pups born to ATD mothers had higher resting Vo(2) (+23% at P3, +21% at P20), respiratory frequency (+15% at P3, +12% at P20), and minute ventilation (+11% at P3, +18% at P20) than pups from Veh mothers. Respiratory decrease during acute hyperoxic exposure at P3 was -9.7% in Veh (P < 0.05 vs. room air) and only -2.6% (P = not significant) in ATD pups. In P20 ATD rats, hypoxic ventilatory response was attenuated compared with Veh. In P20 and P70 rats, the drop of Vo(2) in hypoxia (-31% in P70, P < 0.0001) was not observed in ATD rats. We conclude that estradiol secreted during late gestation is necessary for respiratory adjustment to pregnancy and is required for adequate development of respiratory and metabolic control in the offspring.

Aromatase modulation alters gonadal differentiation in developing zebrafish (Danio rerio)

This study investigated whether gonadal sex differentiation of zebrafish (Danio rerio) is susceptible to compounds that interfere with cytochrome P450 aromatase (P450arom). Treatment of zebrafish during the period of gonadal differentiation with either the non-steroidal aromatase inhibitor fadrozole or 17alpha-methyltestosterone (MT) changed gonad morphological differentiation and altered the pattern of P450arom gene (CYP19) expression. Application of fadrozole (500 microg/g of food) between days 35 and 71 post-fertilisation (pf) resulted in 100% masculinisation, i.e. the gonads of all individuals examined ( n = 40) showed testicular morphology. At the same time, fadrozole treatment suppressed gonadal CYP19A (gonad-derived CYP19 gene) mRNA expression, as assessed by means of semi-quantitative RT-PCR. After termination of fadrozole treatment at day 71 pf and subsequent rearing of zebrafish under control conditions until 161 days pf, the gonads of all individuals still displayed testicular morphology. Gonadal CYP19A expression, however, showed a dimorphic pattern, with 14 out of 22 individuals having low CYP19A mRNA levels similar to those found in testes of control fish, while eight fish showed high, ovary-like levels of gonadal CYP19A mRNA. MT treatment (10 microg/l) during the period of gonadal differentiation (days 35 to 71 pf) resulted in phenological feminisation, i.e., all fish examined (n = 28) showed an ovarian gonadal morphology. While gonadal CYP19A expression was suppressed compared to control fish, cerebral CYP19B (brain-derived CYP19 gene) mRNA expression was increased in 71-day-old MT-treated fish. The results from this study provide evidence that exogenous compounds interfering with the P450arom system are able to disrupt, even persistently, gonadal sex differentiation of the protogynic zebrafish.

Prenatal exposure of testosterone prevents SDN-POA neurons of postnatal male rats from apoptosis through NMDA receptor

The role of N-methyl-D-aspartate (NMDA) receptor in mediating the effect of testosterone exposure prenatally on neuronal apoptosis in the sexual dimorphic nucleus of the preoptic area (SDN-POA) of rats was studied. The endogenous testosterone was diminished by prenatal stress (PNS) or simulated by testosterone exposure (TE) to understand the effect of testosterone on NR(1) (a functional subunit protein of NMDA receptor) expression and neuronal apoptosis. To further study whether the testosterone, after being converted into estradiol, modulates NR(1) expression, 4-androstein-4-ol-3,17-dione (ATD; an aromatase inhibitor) was used to block the conversion of estradiol from testosterone. The expressions of the NR(1) mRNA and NR(1) subunit protein were quantified by RT-PCR and western blotting analysis, respectively. In addition, a noncompetitive antagonist of NMDA receptor, MK-801, was used to find out whether blockage of NMDA receptor affects the naturally occurring apoptosis in SDN-POA. The results showed the following. 1) Expression of perinatal NR(1) subunit protein in the central part of the medial preoptic area of male rats was significantly higher than that of females, especially on postnatal days 1 and 3. 2) The testosterone level of male fetuses on embryonic day 18 was significantly higher than that of females, while the testosterone level of TE females or PNS males was similar to that of intact males or intact females, respectively. 3) The apoptotic incidence of intact male rats was significantly less than that of females, and the apoptosis was stimulated by PNS in male or inhibited by TE in female. 4) The expression of NR(1) subunit protein could be inhibited by PNS or ATD-treatment in male, while stimulated by TE in female. 5) NR(1) mRNA showed no significant difference among intact male, PNS male, ATD-treated male, TE female and intact female rats. 6) The low apoptotic incidence of male rats was significantly increased when NMDA receptor was blocked by MK-801. These results suggest that testosterone, after being converted to estradiol, may prevent the SDN-POA neurons of male rats from apoptosis through enhancing the expression of NR(1) at the posttranscriptional level.

Estrogen synthetase (aromatase) immunohistochemistry reveals concordance between avian and rodent limbic systems and hypothalami

During amniote evolution, an early divergence occurred about 300 million years ago between the reptilian lines leading to the appearance of birds (anapsids) and mammals (synapsids). The different functional requirements of these vertebrate groups have involved divergent evolution of their brains. Even superficial examination reveals major anatomical differences between mammalian and avian brains, such as extensive development of the optic lobes and cerebellum in birds and a highly developed cortex in mammals. It has been nearly impossible to identify avian homologs of some mammalian brain regions by standard morphological criteria. This has long frustrated efforts at clarifying hypotheses regarding the anatomical location, field size, and regulation of brain functions shared between these two classes, despite the certainty that the principles of neurobiology apply equally at the cellular level in both groups. In an effort to remove this barrier, we have sought markers of common function that despite apparent anatomical dissimilarity, can allow recognition of homologous brain structures. We illustrate here how comparative analysis of the distribution of the steroid-metabolizing enzyme estrogen synthetase (aromatase) may help to understand the differences and similarities in the limbic system and hypothalamus of birds and mammals.

Aromatase inhibition reduces specifically one display of the ring dove courtship behavior

The courtship behavior of the male ring dove (Streptopelia risoria) combines aggressive displays (chasing, bowing) and nest-oriented displays (nest soliciting). Aggressive displays depend on circulating testosterone, whereas nest soliciting is estrogen-dependent and appears to depend on the aromatization of androgen into estrogen within the brain. The present work tested the hypothesis that aromatase specifically modulates the nest soliciting display in intact male ring doves. Males were tested for courtship behavior with receptive females before and after being implanted with micro-osmotic pumps containing Fadrozole, a nonsteroidal aromatase inhibitor, or saline. Fadrozole at the higher dose reduced estrogen-dependent nest soliciting but did not affect androgen-dependent chasing and bowing. These results support the hypothesis that aromatase modulates nest soliciting in male ring doves, and provide further evidence for separate hormonal control of different courtship displays in this species.

Combined aromatase inhibitor and antiandrogen treatment decreases territorial aggression in a wild songbird during the nonbreeding season

Male song sparrows (Melospiza melodia morphna) defend territories throughout the year in western Washington State. In the nonbreeding season (autumn and winter), aggression and song are robustly expressed but plasma testosterone (T) levels are basal. Also, castration does not decrease nonbreeding territoriality. In this field experiment, we asked whether nonbreeding aggression is independent of T. T can act via androgen receptors or T can be aromatized to 17beta-estradiol (E(2)) and act via estrogen receptors. We treated free-living nonbreeding birds with an aromatase inhibitor (ATD) and an androgen receptor antagonist (flutamide) in combination. We then challenged subjects with a live decoy and playback of tape-recorded songs. ATD+flutamide treatment decreased several aggressive behaviors. However, ATD+flutamide treatment did not affect body condition, suggesting that subjects were healthy and that foraging behavior was not reduced. As expected, ATD+flutamide treatment increased plasma T, probably by blocking negative feedback on luteinizing hormone. Surprisingly, ATD+flutamide treatment increased plasma E(2). Most other studies using aromatase inhibitors have not measured plasma E(2). However, it is possible that ATD+flutamide treatment decreased local E(2) levels in the brain but not in plasma. Finally, ATD+flutamide treatment increased plasma corticosterone, perhaps in response to increased plasma T or E(2). To our knowledge, these are the first data to suggest that nonbreeding territoriality is regulated by endogenous steroid hormones. Nongonadal production of sex steroids may support aggression in the nonbreeding season.

Neuroanatomical distribution and variations across the reproductive cycle of aromatase activity and aromatase-immunoreactive cells in the pied flycatcher (Ficedula hypoleuca)

The anatomical distribution and seasonal variations in aromatase activity and in the number of aromatase-immunoreactive cells were studied in the brain of free-living male pied flycatchers (Ficedula hypoleuca). A high aromatase activity was detected in the telencephalon and diencephalon but low to negligible levels were present in the optic lobes, cerebellum, and brain stem. In the diencephalon, most aromatase-immunoreactive cells were confined to three nuclei implicated in the control of reproductive behaviors: the medial preoptic nucleus, the nucleus of the stria terminalis, and the ventromedial nucleus of the hypothalamus. In the telencephalon, the immunopositive cells were clustered in the medial part of the neostriatum and in the hippocampus as previously described in another songbird species, the zebra finch. No immunoreactive cells could be observed in the song control nuclei. A marked drop in aromatase activity was detected in the anterior and posterior diencephalon in the early summer when the behavior of the birds had switched from defending a territory to helping the female in feeding the nestlings. This enzymatic change is presumably controlled by the drop in plasma testosterone levels observed at that stage of the reproductive cycle. No change in enzyme activity, however, was seen at that time in other brain areas. The number of aromatase-immunoreactive cells also decreased at that time in the caudal part of the medial preoptic nucleus but not in the ventromedial nucleus of the hypothalamus (an increase was even observed), suggesting that differential mechanisms control the enzyme concentration and enzyme activity in the hypothalamus. Taken together, these data suggest that changes in diencephalic aromatase activity contribute to the control of seasonal variations in reproductive behavior of male pied flycatchers but the role of the telencephalic aromatase in the control of behavior remains unclear at present.

Co-localization of estrogen receptor and aromatase enzyme immunoreactivities in adult musk shrew brain

Estrogens are produced by the aromatization of androgens. These steroids exert their actions after binding to their receptors. Past studies have shown that estrogen receptors (ER) and aromatase enzyme (AROM) reside in many of the same brain regions. Few studies, however, have examined the neural co-localization of these important components involved in estrogen-activated behaviors. In the present study we examined the co-localization of ER and AROM immunoreactive (ir) neurons in musk shrew (Suncus murinus) brains. Data were collected from a representative section from three neural regions, the bed nucleus of the stria terminalis (BNST), medial preoptic area (mPOA), and ventromedial nucleus of the hypothalamus (VMN). Here we report a sex difference in the number of ER-ir neurons from the analyzed section of the mPOA and BNST. Females have more ER-ir neurons in the mPOA and males have more in the BNST. In the sections we examined, males tended to have more aromatase containing neurons than females. Although there were no significant differences in the numbers of double-labeled cells, the VMN contains the greatest percentage of these cells in both males and females; followed by the mPOA and the BNST. In addition, in the mPOA of both sexes, a distinct nucleus of aromatase containing neurons which was devoid of ER immunoreactivity was noted. Area measurements of the AROM-ir nucleus showed that it was significantly larger in males than in females. Taken together, these data suggest that there is not extensive cellular co-localization of estrogen receptors and aromatase enzyme in the musk shrew brain. However, the presence of other genomic forms of ER (membrane and/or ERbeta) in AROM containing neurons has not been ruled out by this study. Thus, we hypothesize that estrogens produced in brain affect behavior by binding to ER in neurons other than those that contain aromatase enzyme.

Distribution of aromatase-immunoreactive cells in the forebrain of zebra finches (Taeniopygia guttata): implications for the neural action of steroids and nuclear definition in the avian hypothalamus

Cells immunoreactive for the enzyme aromatase were localized in the forebrain of male zebra finches with the use of an immunocytochemistry procedure. Two polyclonal antibodies, one directed against human placental aromatase and the other directed against quail recombinant aromatase, revealed a heterogeneous distribution of the enzyme in the telencephalon, diencephalon, and mesencephalon. Staining was enhanced in some birds by the administration of the nonsteroidal aromatase inhibitor, R76713 racemic Vorozole) prior to the perfusion of the birds as previously described in Japanese quail. Large numbers of cells immunoreactive for aromatase were found in nuclei in the preoptic region and in the tuberal hypothalamus. A nucleus was identified in the preoptic region based on the high density of aromatase immunoreactive cells within its boundaries that appears to be homologous to the preoptic medial nucleus (POM) described previously in Japanese quail. In several birds alternate sections were stained for immunoreactive vasotocin, a marker of the paraventricular nucleus (PVN). This information facilitated the clear separation of the POM in zebra finches from nuclei that are adjacent to the POM in the preoptic area-hypothalamus, such as the PVN and the ventromedial nucleus of the hypothalamus. Positively staining cells were also detected widely throughout the telencephalon. Cells were discerned in the medial parts of the ventral hyperstriatum and neostriatum near the lateral ventricle and in dorsal and medial parts of the hippocampus. They were most abundant in the caudal neostriatum where they clustered in the dorsomedial neostriatum, and as a band of cells coursing along the dorsal edge of the lamina archistriatalis dorsalis. They were also present in high numbers in the ventrolateral aspect of the neostriatum and in the nucleus taeniae. None of the telencephalic vocal control nuclei had appreciable numbers of cells immunoreactive for aromatase within their boundaries, with the possible exception of a group of cells that may correspond to the medial part of the magnocellular nucleus of the neostriatum. The distribution of immunoreactive aromatase cells in the zebra finch brain is in excellent agreement with the distribution of cells expressing the mRNA for aromatase recently described in the finch telencephalon. This widespread telencephalic distribution of cells immunoreactive for aromatase has not been described in non-songbird species such as the Japanese quail, the ring dove, and the domestic fowl.