A direct dopaminergic control of aromatase activity in the quail preoptic area

24 hour patterning in gene expression of pineal neurosteroid biosynthesis in young chickens (Gallus gallus domesticus L.)

The pineal gland, one of the three equivalent avian biological clock structures, is also the site of intensive neurosteroid synthesis (7α-hydroxypregnenolone and allopregnanolone). Pineal neurosteroid biosynthesis involves six enzymes: cytochrome P450 side-chain cleavage - Cyp11a1 encoded, cytochrome P450_7α - Cyp7b1, 3β-hydroxysteroid dehydrogenase - Hsd3b2, 5α-reductase - Srd5a1, 3α-hydroxysteroid dehydrogenase - Akr1d1, and 5β-reductase - Srd5a3. Regulation of neurosteroid biosynthesis is not fully understood; although it is known that the E4BP4 transcription factor induces activation of biosynthetic cholesterol genes, which are the targets for SREBP (element-binding protein transcription factor). SREBP principal activity in the pineal gland is suppression and inhibition of the Period2 canonical clock gene, suggesting our hypothesis that genes encoding enzymes involved in neurosteroidogenesis are under circadian clock control and are the Clock Control Genes (CCGs). Therefore, through investigation of daily changes in Cyp11a1, Cyp7b1, Hsd3b2, Akr1d1, Srd5a1, and Srd5a3, pineal genes were tested in vivo and in vitro, in cultured pinealocytes. Experiments were carried out on pineal glands taken from 16-day-old chickens in vivo or using in vitro cultures of pinealocytes collected from 16-day-old animals. Both the birds in the in vivo experiments and the pinealocytes were kept under controlled light conditions (LD 12:12) or in constant darkness (DD). Subsequently, materials were prepared for RT-qPCR analysis. Results revealed that three of the six tested genes: Cyp11a1, Cyp7b1, and Srd5a3 demonstrated significant 24-hour variation in in vivo and in vitro. Findings of this study confirm that these genes could be under clock control and satisfy many of the requirements to be identified as CCGs.

Effects of a novel partner and sexual satiety on the expression of male sexual behavior and brain aromatase activity in quail

This study was designed to determine whether changes in sexual motivation acutely regulate brain estrogen synthesis by aromatase. Five experiments (Exp.1-5) were first conducted to determine the effect of recent mating and of the presentation of a new female (Coolidge effect) on sexual motivation. Exp.1-2 showed that 10 min or overnight access to copulation decreases measures of male sexual motivation when male subjects were visually exposed to the female they had copulated with and this effect is not counteracted by the view of a new female. Exp.3 showed that sexual motivation is revived by the view of a new female in previously unmated males only allowed to see another female for 10 min. After mating for 10 min (Exp.4) or overnight (Exp.5) with a female, males showed a decrease in copulatory behavior that was not reversed by access to a new female. Exp.6 and 7 confirmed that overnight copulation (Exp.6) and view of a novel female (Exp.7) respectively decreases and increases sexual behavior and motivation. Yet, these manipulations did not affect brain aromatase activity except in the tuberal hypothalamus. Together these data confirm that copulation or prolonged view of a female decrease sexual motivation but a reactivation of sexual motivation by a new female can only be obtained if males had only seen another female but not copulated with her, which is different in some degree from the Coolidge effect described in rodents. Moreover changes in brain aromatase do not simply reflect changes in motivation and more complex mechanisms must be considered.

Co-localization of mu-opioid and dopamine D1 receptors in the medial preoptic area and bed nucleus of the stria terminalis across seasonal states in male European starlings

In seasonally breeding animals, changes in photoperiod and sex-steroid hormones may modify sexual behavior in part by altering the activity of neuromodulators, including opioids and dopamine. In rats and birds, activation of mu-opioid receptors (MOR) and dopamine D1 receptors in the medial preoptic area (mPOA) often have opposing effects on sexual behavior, yet mechanisms by which the mPOA integrates these opposing effects to modulate behavior remain unknown. Here, we used male European starlings (Sturnus vulgaris) to provide insight into the hypothesis that MOR and D1 receptors modify sexual behavior seasonally by altering activity in the same neurons in the mPOA. To do this, using fluorescent immunohistochemistry, we examined the extent to which MOR and D1 receptors co-localize in mPOA neurons and the degree to which photoperiod and the sex-steroid hormone testosterone alter co-localization. We found that MOR and D1 receptors co-localize throughout the mPOA and the bed nucleus of the stria terminalis, a region also implicated in the control of sexual behavior. Numbers of single and co-labeled MOR and D1 receptor labeled cells were higher in the rostral mPOA in photosensitive males (a condition observed just prior to the breeding season) compared to photosensitive males treated with testosterone (breeding season condition). In the caudal mPOA co-localization of MOR and D1 receptors was highest in photosensitive males compared to photorefractory males (a post-breeding season condition). Seasonal shifts in the degree to which neurons in the mPOA integrate signaling from opioids and dopamine may underlie seasonal changes in the production of sexual behavior.

Differential control of appetitive and consummatory sexual behavior by neuroestrogens in male quail

Contribution to Special Issue on Fast effects of steroids. Estrogens exert pleiotropic effects on multiple physiological and behavioral traits including sexual behavior. These effects are classically mediated via binding to nuclear receptors and subsequent regulation of target gene transcription. Estrogens also affect neuronal activity and cell-signaling pathways via faster, membrane-initiated events. Although the distinction between appetitive and consummatory aspects of sexual behavior has been criticized, this distinction remains valuable in that it facilitates the causal analysis of certain behavioral systems. Effects of neuroestrogens produced by neuronal aromatization of testosterone on copulatory performance (consummatory aspect) and on sexual motivation (appetitive aspect) are described in male quail. The central administration of estradiol rapidly increases expression of sexual motivation, as assessed by two measures of sexual motivation produced in response to the visual presentation of a female but not sexual performance in male Japanese quail. This effect is mimicked by membrane-impermeable analogs of estradiol, indicating that it is initiated at the cell membrane. Conversely, blocking the action of estrogens or their synthesis by a single intracerebroventricular injection of estrogen receptor antagonists or aromatase inhibitors, respectively, decreases sexual motivation within minutes without affecting performance. The same steroid has thus evolved complementary mechanisms to regulate different behavioral components (motivation vs. performance) in distinct temporal domains (long- vs. short-term) so that diverse reproductive activities can be properly coordinated. Changes in preoptic aromatase activity and estradiol as well as glutamate concentrations are observed during or immediately after copulation. The interaction between these neuroendocrine/neurochemical changes and their functional significance is discussed.

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.

Clustered organization and region-specific identities of estrogen-producing neurons in the forebrain of Zebra Finches (Taeniopygia guttata)

A fast, neuromodulatory role for estrogen signaling has been reported in many regions of the vertebrate brain. Regional differences in the cellular distribution of aromatase (estrogen synthase) in several species suggest that mechanisms for neuroestrogen signaling differ between and even within brain regions. A more comprehensive understanding of neuroestrogen signaling depends on characterizing the cellular identities of neurons that express aromatase. Calcium-binding proteins such as parvalbumin and calbindin are molecular markers for interneuron subtypes, and are co-expressed with aromatase in human temporal cortex. Songbirds like the zebra finch have become important models to understand the brain synthesis of steroids like estrogens and the implications for neurobiology and behavior. Here, we investigated the regional differences in cytoarchitecture and cellular identities of aromatase-expressing neurons in the auditory and sensorimotor forebrain of zebra finches. Aromatase was co-expressed with parvalbumin in the caudomedial nidopallium (NCM) and HVC shelf (proper name) but not in the caudolateral nidopallium (NCL) or hippocampus. By contrast, calbindin was not co-expressed with aromatase in any region investigated. Notably, aromatase-expressing neurons were found in dense somato-somatic clusters, suggesting a coordinated release of local neuroestrogens from clustered neurons. Aromatase clusters were also more abundant and tightly packed in the NCM of males as compared to females. Overall, this study provides new insights into neuroestrogen regulation at the network level, and extends previous findings from human cortex by identifying a subset of aromatase neurons as putative inhibitory interneurons.

On the role of brain aromatase in females: why are estrogens produced locally when they are available systemically?

The ovaries are often thought of as the main and only source of estrogens involved in the regulation of female behavior. However, aromatase, the key enzyme for estrogen synthesis, although it is more abundant in males, is expressed and active in the brain of females where it is regulated by similar mechanisms as in males. Early work had shown that estrogens produced in the ventromedial hypothalamus are involved in the regulation of female sexual behavior in musk shrews. However, the question of the role of central aromatase in general had not received much attention until recently. Here, I will review the emerging concept that central aromatization plays a role in the regulation of physiological and behavioral endpoints in females. The data support the notion that in females, brain aromatase is not simply a non-functional evolutionary vestige, and provide support for the importance of locally produced estrogens for brain function in females. These observations should also have an impact for clinical research.

Steroid metabolism in the brain: From bird watching to molecular biology, a personal journey

Since Arnold Adolph Berthold established in 1849 the critical role of the testes in the activation of male sexual behavior, intensive research has identified many sophisticated neurochemical and molecular mechanisms mediating this action. Studies in Japanese quail demonstrated the critical role of testosterone action and of testosterone aromatization in the sexually dimorphic medial preoptic nucleus in the activation of male copulatory behavior. The development of an immunohistochemical visualization of brain aromatase in quail then allowed further refinement in the localization of the sites of neuroestrogens production. Testosterone aromatization is required for the activation of both appetitive and consummatory aspects of male sexual behavior. Brain aromatase activity is modulated by steroid-induced changes in the transcription of the corresponding gene but also more rapidly by phosphorylation processes. Sexual interactions with a female also rapidly regulate brain aromatase activity in an anatomically specific manner presumably via the release and action of endogenous glutamate. These rapid changes in estrogen production modulate sexual behavior and in particular its motivational component with latencies ranging between 15 and 30min. Brain estrogens seem to act in a manner akin to a neurotransmitter or at least a neuromodulator. More recently, assays of brain estradiol concentrations in micropunched samples or in dialysis samples obtained from behaviorally active males suggested that aromatase activity measured ex vivo might not be an accurate proxy to the rapid changes in local neuroestrogens production and concentrations. Studies of brain testosterone metabolism are thus not over and will keep scientists busy for a little longer. Elsevier SBN Keynote Address, Montreal.

Glutamate released in the preoptic area during sexual behavior controls local estrogen synthesis in male quail

Estrogens are known to act rapidly, probably via membrane estrogen receptors, to induce fast effects on physiological and behavioral processes. Engaging in some of these behaviors, such as sexual behavior, results in an acute modulation of the production of estrogens in the brain by regulating the efficiency of the estrogen synthase enzyme, aromatase. We recently demonstrated that aromatase activity (AA) in the male quail brain is rapidly inhibited in discrete brain regions including the medial preoptic nucleus (POM) following exposure to a female. Evidence from in vitro studies point to glutamate release as one of the mechanisms controlling these rapid regulations of the aromatase enzyme. Here, we show that (a) the acute injection of the glutamatergic agonist kainate into the POM of anesthetized male quail inhibits AA and (b) glutamate is released in the POM during copulation. These results provide the first set of in vivo data demonstrating a role for glutamate release in the rapid control of AA in the context of sexual behavior.

Dig1 protects against locomotor and biochemical dysfunctions provoked by Roundup

BACKGROUND

Plant medicinal extracts may be claimed to prevent or cure chemical intoxications. Few of these are tested for their mechanisms of actions in vivo and for their cellular impacts. In 2011, we demonstrated that hepatic cell mortality induced by environmentally realistic levels of the widely used herbicide Roundup (R) in vitro can be almost entirely prevented by plant extracts called Dig1 (D, Digeodren).

METHODS

We tested the in vivo effects of D alone (1.2 ml/kg bw/d), but also prior to and during 8 days of R intoxication (at 135 mg/kg bw/d) in a total of 4 groups of 40 adult Sprague-Dawley male rats each. After treatments, horizontal and vertical locomotor activities of the animals were measured by use of actimeters. Brain, liver, kidneys, heart and testes were collected and weighted. Body weights as well as feed and water consumption were recorded. Proteins, creatinine, urea, phosphate, potassium, sodium, calcium, chloride ions, testosterone, estradiol, AST and ALT were measured in serum. In liver S9 fractions, GST, GGT, and CYP450 (1A2, 2C9, 2C19, 2D6, 3A4) were assessed.

RESULTS

D did not have any physiological or biochemical observable impact alone at 2 %. Out of a total of 29 measured parameters, 8 were significantly affected by R absorption within only 8 days. On these 8 parameters, only 2 were not restored by D (GGT activity and plasmatic phosphate), 5 were totally restored (horizontal and vertical locomotor activities, CYP2D6 activity, plasmatic Na + and estradiol), and the 6th was almost restored (plasmatic K+). The specificities of the toxic effects of R and of the therapeutic effects of D treatment were thus demonstrated, both at the behavioural and biochemical levels.

CONCLUSIONS

D, without any side effect observable in these conditions, presented strong preventive and therapeutic properties in vivo after a short-term intoxication by the widely used pesticide Roundup.

Non-ovarian aromatization is required to activate female sexual motivation in testosterone-treated ovariectomized quail

Although aromatase is expressed in both male and female brains, its functional significance in females remains poorly understood. In female quail, sexual receptivity is activated by estrogens. However it is not known whether sexual motivation is similarly estrogen-dependent and whether estrogens locally produced in the brain contribute to these behavioral responses. Four main experiments were designed to address these questions. In Experiment 1 chronic treatment of females with the anti-estrogen tamoxifen decreased their receptivity, confirming that this response is under the control of estrogens. In Experiment 2 chronic treatment with tamoxifen significantly decreased sexual motivation as treated females no longer approached a sexual partner. In Experiment 3 (a) ovariectomy (OVX) induced a significant decrease of time spent near the male and a significantly decreased receptivity compared to gonadally intact females, (b) treatment with testosterone (OVX+T) partially restored these responses and (c) this effect of T was prevented when estradiol synthesis was inhibited by the potent aromatase inhibitor Vorozole (OVX+T+VOR). Serum estradiol concentration was significantly higher in OVX+T than in OVX or OVX+T+VOR females. Together these data demonstrate that treatment of OVX females with T increases sexual motivation and that these effects are mediated at least in part by non-gonadal aromatization of the androgen. Finally, assays of aromatase activity on brain and peripheral tissues (Experiment 4) strongly suggest that brain aromatization contributes to behavioral effects observed here following T treatment but alternative sources of estrogens (e.g. liver) should also be considered.

Region-specific rapid regulation of aromatase activity in zebra finch brain

Recent studies demonstrate that rapid modulation of the estrogen synthetic enzyme aromatase, regulates hypothalamic (HYP) estrogen production, and subsequent neurophysiology and reproductive behavior. In songbirds, in addition to expression in the HYP, aromatase is expressed at high levels in several brain regions notably in the hippocampus (HP) and caudomedial nidopallium (NCM), where estrogens affect learning and memory and auditory processing, respectively. Previous studies, largely in quail HYP, show that aromatase activity is acutely down-regulated by Ca²⁺ -dependent phosphorylation. Here, using zebra finches (Taeniopygia guttata), we ask if similar mechanisms are at work in the songbird HYP and if there are sex as well as regional differences in aromatase modulation. Using in vitro assays to measure activity in homogenates or in partially purified supernatants containing microsomes and synaptosomes of the HP, HYP, and NCM, we examined effects of Ca²⁺ , Mg²⁺ , ATP, NADPH, and an inhibitor of kinase activity. We report a rapid down-regulation of aromatase activity in the presence of phosphorylating conditions across all three brain regions and both sexes. However, regional differences were seen in response to some phosphorylating factors, some of which were improved by partial purification of the homogenates. Furthermore, while low concentrations of ATP inhibited aromatase activity, unexpectedly, inhibition was no longer seen with high ATP concentrations. These results provide evidence for a regional and temporal specificity in the rapid modulation of aromatase activity that may bear on local neuroendocrine function. Aromatase activity in male and female zebra finch hippocampus, hypothalamus, and caudomedial nidopallium is rapidly regulated by Ca²⁺ -dependent phosphorylation. Low ATP and Mg²⁺ decrease activity, whereas nicotinamide adenine dinucleotide phosphate (NADPH), high ATP, and inhibition of protein kinase C increase activity. Evidence suggests this may occur at the synapse. These results provide a mechanism for rapid regulation of behavior via brain estrogen synthesis.

Local modulation of steroid action: rapid control of enzymatic activity

Estrogens can induce rapid, short-lived physiological and behavioral responses, in addition to their slow, but long-term, effects at the transcriptional level. To be functionally relevant, these effects should be associated with rapid modulations of estrogens concentrations. 17β-estradiol is synthesized by the enzyme aromatase, using testosterone as a substrate, but can also be degraded into catechol-estrogens via hydroxylation by the same enzyme, leading to an increase or decrease in estrogens concentration, respectively. The first evidence that aromatase activity (AA) can be rapidly modulated came from experiments performed in Japanese quail hypothalamus homogenates. This rapid modulation is triggered by calcium-dependent phosphorylations and was confirmed in other tissues and species. The mechanisms controlling the phosphorylation status, the targeted amino acid residues and the reversibility seem to vary depending of the tissues and is discussed in this review. We currently do not know whether the phosphorylation of the same amino acid affects both aromatase and/or hydroxylase activities or whether these residues are different. These processes provide a new general mechanism by which local estrogen concentration can be rapidly altered in the brain and other tissues.

Age-dependent and age-independent effects of testosterone in male quail

Various studies in rodents recently concluded that puberty should be considered as a second period of organization of brain and behavior and that action of sex steroids at that time is long lasting and possibly permanent. We tested this notion in male Japanese quail that had been castrated before 3weeks post-hatch by analyzing whether a similar treatment with exogenous testosterone initiated at 3, 5 or 7weeks post-hatch has a differential influence on the development of testosterone-dependent morphological, behavioral and neural characteristics that are known to be sexually differentiated. The growth of the androgen-dependent cloacal gland was significantly faster when testosterone treatment was initiated later in life indicating that the target tissue is not ready to fully respond to androgens at 3weeks post-hatch. The three groups of birds nevertheless developed a gland of the same size typical of intact sexually mature birds. When adults, all birds expressed copulatory behavior with the same frequencies and latencies and they displayed the same level of aromatase activity and of vasotocinergic innervation in the preoptic area as gonadally intact males despite the fact that they had been treated with testosterone for different durations starting at different ages. Surprisingly, the frequency of cloacal sphincter contractions, a measure of appetitive sexual behavior, was significantly higher when testosterone treatment had been initiated later. Together these data provide no clear evidence for an organizational action of testosterone during sexual maturation of male quail but additional experiments should investigate whether estrogens have such an action in females.

Review: neuroestrogen regulation of socio-sexual behavior of males

It is thought that estrogen (neuroestrogen) synthesized by the action of aromatase in the brain from testosterone activates male socio-sexual behaviors, such as aggression and sexual behavior in birds. We recently found that gonadotropin-inhibitory hormone (GnIH), a hypothalamic neuropeptide, inhibits socio-sexual behaviors of male quail by directly activating aromatase and increasing neuroestrogen synthesis in the preoptic area (POA). The POA is thought to be the most critical site of aromatization and neuroestrogen action for the regulation of socio-sexual behavior of male birds. We concluded that GnIH inhibits socio-sexual behaviors of male quail by increasing neuroestrogen concentration beyond its optimal concentration in the brain for expression of socio-sexual behavior. On the other hand, it has been reported that dopamine and glutamate, which stimulate male socio-sexual behavior in birds and mammals, inhibit the activity of aromatase in the POA. Multiple studies also report that the activity of aromatase or neuroestrogen is negatively correlated with changes in male socio-sexual behavior in fish, birds, and mammals including humans. Here, we review previous studies that investigated the role of neuroestrogen in the regulation of male socio-sexual behavior and reconsider the hypothesis that neuroestrogen activates male socio-sexual behavior in vertebrates. It is considered that basal concentration of neuroestrogen is required for the maintenance of male socio-sexual behavior but higher concentration of neuroestrogen may inhibit male socio-sexual behavior.

Relationships between rapid changes in local aromatase activity and estradiol concentrations in male and female quail brain

Estradiol-17β (E2) synthesized in the brain plays a critical role in the activation of sexual behavior in many vertebrate species. Because E2 concentrations depend on aromatization of testosterone, changes in aromatase enzymatic activity (AA) are often utilized as a proxy to describe E2 concentrations. Utilizing two types of stimuli (sexual interactions and acute restraint stress) that have been demonstrated to reliably alter AA within minutes in opposite directions (sexual interactions=decrease, stress=increase), we tested in Japanese quail whether rapid changes in AA are paralleled by changes in E2 concentrations in discrete brain areas. In males, E2 in the pooled medial preoptic nucleus/medial portion of the bed nucleus of the stria terminalis (POM/BST) positively correlated with AA following sexual interactions. However, following acute stress, E2 decreased significantly (approximately 2-fold) in the male POM/BST despite a significant increase in AA. In females, AA positively correlated with E2 in both the POM/BST and mediobasal hypothalamus supporting a role for local, as opposed to ovarian, production regulating brain E2 concentrations. In addition, correlations of individual E2 in POM/BST and measurements of female sexual behavior suggested a role for local E2 synthesis in female receptivity. These data demonstrate that local E2 in the male brain changes in response to stimuli on a time course suggestive of potential non-genomic effects on brain and behavior. Overall, this study highlights the complex mechanisms regulating local E2 concentrations including rapid stimulus-driven changes in production and stress-induced changes in catabolism.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Dynamic changes in brain aromatase activity following sexual interactions in males: where, when and why?

It is increasingly recognized that estrogens produce rapid and transient effects at many neural sites ultimately impacting physiological and behavioral endpoints. The ability of estrogens to acutely regulate cellular processes implies that their concentration should also be rapidly fine-tuned. Accordingly, rapid changes in the catalytic activity of aromatase, the limiting enzyme for estrogen synthesis, have been identified that could serve as a regulatory mechanism of local estrogen concentrations. However, the precise anatomical localization, time-course, triggering stimuli and functional significance of these enzymatic changes in vivo are not well understood. To address these issues as to where, when and why aromatase activity (AA) rapidly changes after sexual interactions, AA was assayed in six populations of aromatase-expressing cells microdissected from the brain of male quail that experienced varying durations of visual exposure to or copulation with a female. Sexual interactions resulted in a rapid AA inhibition. This inhibition occurred in specific brain regions (including the medial preoptic nucleus), in a context-dependent fashion and time-scale suggestive of post-translational modifications of the enzyme. Interestingly, the enzymatic fluctuations occurring in the preoptic area followed rather than preceded copulation and were tied specifically to the female's presence. This pattern of enzymatic changes suggests that rapid estrogen effects are important during the motivational phase of the behavior to trigger physiological events essential to activate mate search and copulation.

Neurochemical control of rapid stress-induced changes in brain aromatase activity

In the male brain, the medial preoptic nucleus (POM) is known to be a critical relay for the activation of sexual behaviour, with the aromatisation of testosterone into 17β-oestradiol (E2 ) playing a key role. Acute stress has been shown to differentially modulate the aromatase enzyme in this and other brain nuclei in a sex-specific manner. In POM specifically, stress induces increases in aromatase activity (AA) that are both rapid and reversible. How the physiological processes initiated during an acute stress response mediate sex- and nuclei- specific changes in AA and which stress response hormones are involved remains to be determined. By examining the relative effects of corticosterone (CORT), arginine vasotocin (AVT, the avian homologue to arginine vasopressin) and corticotrophin-releasing factor (CRF), the present study aimed to define the hormone profile regulating stress-induced increases in AA in the POM. We found that CORT, AVT and CRF all appear to play some role in these changes in the male brain. In addition, these effects occur in a targeted manner, such that modulation of the enzyme by these hormones only occurs in the POM rather than in all aromatase-expressing nuclei. Similarly, in the female brain, the experimental effects were restricted to the POM but only CRF was capable of inducing the stress-like increases in AA. These data further demonstrate the high degree of specificity (nuclei-, sex- and hormone-specific effects) in this system, highlighting the complexity of the stress-aromatase link and suggesting modes through which the nongenomic modulation of this enzyme can result in targeted, rapid changes in local oestrogen concentrations.

Prostaglandin E2 stimulates estradiol synthesis in the cerebellum postnatally with associated effects on Purkinje neuron dendritic arbor and electrophysiological properties

Prostaglandins (PGs) are ubiquitous membrane-derived, lipid-signaling molecules with wide ranging effects throughout the body. In the brain, PGE(2) is the key regulator of fever after inflammation but is also implicated in neural development and synaptic plasticity. The steroid hormone estradiol is also a key regulator of neural development and synaptic plasticity. Recently, we showed that administering cyclooxygenase (COX) inhibitors to block PGE(2) production increased the total length of Purkinje cell dendrites, the number of dendritic spines, and the level of spinophilin protein, which is enriched in dendritic spines. Correspondingly, PGE(2) administration into the cerebellum decreased spinophilin protein content. We now report that PGE(2) stimulates estradiol synthesis in the immature rat cerebellum via enhanced activity of the aromatase enzyme. Treatment with cyclooxygenase inhibitors reduced cerebellar aromatase activity and estradiol content whereas PGE(2) administration increased both. Treatment with either PGE(2) or estradiol stunted Purkinje neuron dendritic length and complexity and produced a corresponding reduction in spinophilin content. Treatment with formestane to inhibit aromatase activity led to excessive sprouting of the dendritic tree, whereas elevated estradiol had the opposite effect. Electrophysiological measurements from Purkinje neurons revealed novel sex differences in input resistance and membrane capacitance that were abolished by estradiol exposure, whereas a sex difference in the amplitude of the afterhyperpolarization after an action potential was not. Correlated changes in action potential threshold suggest that prolonged alterations in neuronal firing activity could be a consequence of increased estradiol content during the second week of life. These findings reveal a previously unappreciated role for PG-stimulated steroidogenesis in the developing brain and a new potential route for inflammation-mediated disruption of neuronal maturation.

Brain aromatase and circulating corticosterone are rapidly regulated by combined acute stress and sexual interaction in a sex-specific manner

Neural production of 17β-oestradiol via aromatisation of testosterone may play a critical role in rapid, nongenomic regulation of physiological and behavioural processes. In brain nuclei implicated in the control of sexual behaviour, sexual or stressfull stimuli induce, respectively, a rapid inhibition or increase in preoptic aromatase activity (AA). In the present study, we tested quail that were either nonstressed or acutely stressed (15 min of restraint) immediately before sexual interaction (5 min) with stressed or nonstressed partners. We measured nuclei-specific AA changes, corresponding behavioural output, fertilisation rates and corticosterone (CORT) concentrations. In males, sexual interaction rapidly reversed stress-induced increases of AA in the medial preoptic nucleus (POM). This time scale (< 5 min) highlights the dynamic potential of the aromatase system to integrate input from stimuli that drive AA in opposing directions. Moreover, acute stress had minimal effects on male behaviour, suggesting that the input from the sexual stimuli on POM AA may actively preserve sexual behaviour despite stress exposure. We also found distinct sex differences in contextual physiological responses: males did not show any effect of partner status, whereas females responded to both their stress exposure and the male partner's stress exposure at the level of circulating CORT and AA. In addition, fertilisation rates and female CORT correlated with the male partner's exhibition of sexually aggressive behaviour, suggesting that female perception of the male can affect their physiology as much as direct stress. Overall, male reproduction appears relatively simple: sexual stimuli, irrespective of stress, drives major neural changes including rapid reversal of stress-induced changes of AA. By contrast, female reproduction appears more nuanced and context specific, with subjects responding physiologically and behaviourally to stress, the male partner's stress exposure, and female-directed male behaviour.

Meta-type analysis of dopaminergic effects on gene expression in the neuroendocrine brain of female goldfish

Dopamine (DA) is a major neurotransmitter important for neuroendocrine control and recent studies have described genomic signaling pathways activated and inhibited by DA agonists and antagonists in the goldfish brain. Here we perform a meta-type analysis using microarray datasets from experiments conducted with female goldfish to characterize the gene expression responses that underlie dopaminergic signaling. Sexually mature, pre-spawning [gonadosomatic index (GSI) = 4.5 ± 1.3%] or sexually regressing (GSI = 3 ± 0.4%) female goldfish (15-40 g) injected intraperitoneally with either SKF 38393, LY 171555, SCH 23390, sulpiride, or a combination of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and α-methyl-p-tyrosine. Microarray meta-type analysis identified 268 genes in the telencephalon and hypothalamus as having reciprocal (i.e., opposite between agonism and antagonism/depletion) fold change responses, suggesting that these transcripts are likely targets for DA-mediated regulation. Noteworthy genes included ependymin, vimentin, and aromatase, genes that support the significance of DA in neuronal plasticity and tissue remodeling. Sub-network enrichment analysis (SNEA) was used to identify common gene regulators and binding proteins associated with the differentially expressed genes mediated by DA. SNEA analysis identified gene expression targets that were related to three major categories that included cell signaling (STAT3, SP1, SMAD, Jun/Fos), immune response (IL-6, IL-1β, TNFs, cytokine, NF-κB), and cell proliferation and growth (IGF1, TGFβ1). These gene networks are also known to be associated with neurodegenerative disorders such as Parkinsons' disease, well-known to be associated with loss of dopaminergic neurons. This study identifies genes and networks that underlie DA signaling in the vertebrate CNS and provides targets that may be key neuroendocrine regulators. The results provide a foundation for future work on dopaminergic regulation of gene expression in fish model systems.

Human and quail aromatase activity is rapidly and reversibly inhibited by phosphorylating conditions

Besides their slow genomic actions, estrogens also induce rapid physiological responses. To be functionally relevant, these effects must be associated with rapid changes in local concentrations of estrogens. Rapid changes in aromatase activity (AA) controlled by calcium-dependent phosphorylations of the enzyme can alter in a rapid manner local estrogen concentrations, but so far this mechanism was identified only in the avian (quail) brain. We show here that AA is also rapidly down-regulated by phosphorylating conditions in quail ovary homogenates and in various cell lines transfected with human aromatase (HEK 293, Neuro2A, and C6). Enzymatic activity was also rapidly inhibited after depolarization of aromatase-expressing HEK 293 cells with 100 mM KCl, and activity was fully restored when cells returned to control conditions. Western blot analysis demonstrated that the reduction of enzymatic activity is not due to protein degradation. We next investigated by site-directed mutagenesis the potential implication in the control of AA of specific aromatase residues identified by bioinformatic analysis. Mutation of the amino acids S118, S247, S267, T462, T493, or S497 to alanine, alone or in combination, did not block the rapid inhibition of enzymatic activity induced by phosphorylating conditions, but basal AA was markedly decreased in the S118A mutant. Altogether, these results demonstrate that the rapid inhibition of AA is a widespread and fully reversible process and that phosphorylation of specific residues modulate AA. These processes provide a new general mechanism by which local estrogen concentration can be rapidly altered in the brain and other tissues.

Acute stress differentially affects aromatase activity in specific brain nuclei of adult male and female quail

The rapid and temporary suppression of reproductive behavior is often assumed to be an important feature of the adaptive acute stress response. However, how this suppression operates at the mechanistic level is poorly understood. The enzyme aromatase converts testosterone to estradiol in the brain to activate reproductive behavior in male Japanese quail (Coturnix japonica). The discovery of rapid and reversible modification of aromatase activity (AA) provides a potential mechanism for fast, stress-induced changes in behavior. We investigated the effects of acute stress on AA in both sexes by measuring enzyme activity in all aromatase-expressing brain nuclei before, during, and after 30 min of acute restraint stress. We show here that acute stress rapidly alters AA in the male and female brain and that these changes are specific to the brain nuclei and sex of the individual. Specifically, acute stress rapidly (5 min) increased AA in the male medial preoptic nucleus, a region controlling male reproductive behavior; in females, a similar increase was also observed, but it appeared delayed (15 min) and had smaller amplitude. In the ventromedial and tuberal hypothalamus, regions associated with female reproductive behavior, stress induced a quick and sustained decrease in AA in females, but in males, only a slight increase (ventromedial) or no change (tuberal) in AA was observed. Effects of acute stress on brain estrogen production, therefore, represent one potential way through which stress affects reproduction.

Sex differences in the rapid control of aromatase activity in the quail preoptic area

Adult male quail show high levels of aromatase activity in the preoptic area-hypothalamus (POA-HYP), which parallels the high number of aromatase-immunoreactive cells and elevated mRNA concentrations detected in this brain region by in situ hybridisation. Interestingly, females display considerably lower aromatase activity than males but have almost equal numbers of aromatase-immunoreactive cells and express similar levels of aromatase mRNA. Aromatase activity in the male POA-HYP can be rapidly regulated by calcium-dependent phosphorylations, in the absence of changes in enzyme concentration. In the present study, we investigated whether aromatase activity is differentially regulated by phosphorylations in males and females. A linear increase in accumulation of aromatisation products was observed in both sexes as a function of time but the rate of conversion was slower in females. Saturation analysis confirmed the lower maximum velocities (V(max) ) in females but indicated a similar affinity (K(m) ) in both sexes. Aromatase activity in females reacted differentially to manipulations of intracellular calcium. In particular, chelating calcium with ethylene glycol tetraacetic acid (EGTA) resulted in a larger increase of enzymatic activity in males than in females, especially in the presence of ATP. A differential reaction to kinase inhibitors was also observed between males and females (i.e. a larger increase in aromatase activity in females than in males after exposure to specific inhibitors). These findings suggest that the nature of aromatase is conserved between the sexes, although the control of its activity by calcium appears to be different. Additional characterizations of intracellular calcium in both sexes would therefore be appropriate to better understand aromatase regulation.

Organizing effects of sex steroids on brain aromatase activity in quail

Preoptic/hypothalamic aromatase activity (AA) is sexually differentiated in birds and mammals but the mechanisms controlling this sex difference remain unclear. We determined here (1) brain sites where AA is sexually differentiated and (2) whether this sex difference results from organizing effects of estrogens during ontogeny or activating effects of testosterone in adulthood. In the first experiment we measured AA in brain regions micropunched in adult male and female Japanese quail utilizing the novel strategy of basing the microdissections on the distribution of aromatase-immunoreactive cells. The largest sex difference was found in the medial bed nucleus of the stria terminalis (mBST) followed by the medial preoptic nucleus (POM) and the tuberal hypothalamic region. A second experiment tested the effect of embryonic treatments known to sex-reverse male copulatory behavior (i.e., estradiol benzoate [EB] or the aromatase inhibitor, Vorozole) on brain AA in gonadectomized adult males and females chronically treated as adults with testosterone. Embryonic EB demasculinized male copulatory behavior, while vorozole blocked demasculinization of behavior in females as previously demonstrated in birds. Interestingly, these treatments did not affect a measure of appetitive sexual behavior. In parallel, embryonic vorozole increased, while EB decreased AA in pooled POM and mBST, but the same effect was observed in both sexes. Together, these data indicate that the early action of estrogens demasculinizes AA. However, this organizational action of estrogens on AA does not explain the behavioral sex difference in copulatory behavior since AA is similar in testosterone-treated males and females that were or were not exposed to embryonic treatments with estrogens.

Rapid increase in aggressive behavior precedes the decrease in brain aromatase activity during socially mediated sex change in Lythrypnus dalli

In the bluebanded goby, Lythrypnus dalli, removal of the male from a social group results in a rapid behavioral response where one female becomes dominant and changes sex to male. In a previous study, within hours of male removal, aromatase activity in the brain (bAA) of dominant females was almost 50% lower than that of control females from a group in which the male had not been removed. For those females that displayed increased aggressive behavior after the male was removed, the larger the increase in aggressive behavior, the greater the reduction in bAA. To investigate whether decreased bAA leads to increased aggression, the present study used a more rapid time course of behavioral profiling and bAA assay, looking within minutes of male removal from the group. There were no significant differences in bAA between control females (large females from groups with the male still present), females that doubled their aggressive behavior by 10 or 20 min after male removal, or females that did not double their aggressive behavior within 30 min after male removal. Further, individual variation in bAA and aggressive behavior were not correlated in these fish. Whole brain decreases in aromatase activity thus appear to follow, rather than precede, rapid increases in aggressive behavior, which provides one potential mechanism underlying the rapid increase in androgens that follows aggressive interactions in many vertebrate species. For fish species that change sex from female to male, this increase in androgens could subsequently facilitate sex change.

Testosterone recruits new aromatase-imunoreactive cells in neonatal quail brain

It was shown earlier that, in Japanese quail the mechanism controlling the induction by testosterone of aromatase activity develops between embryonic days 10 and 14. The cellular processes underlying this activation have, however, not been investigated in detail. Here, we demonstrate that the increase in aromatase activity observed in neonates treated with testosterone propionate between postnatal days 1 and 3 results from the recruitment of additional populations of aromatase-immunoreactive cells that were not expressing the enzyme at detectable levels before. This recruitment concerns all brain nuclei normally expressing the enzyme even if it is more prominent in the ventromedial hypothalamus than in other nuclei.

Japanese quail as a model system for studying the neuroendocrine control of reproductive and social behaviors

Japanese quail (Coturnix japonica; referred to simply as quail in this article) readily exhibit sexual behavior and related social behaviors in captive conditions and have therefore proven valuable for studies of how early social experience can shape adult mate preference and sexual behavior. Quail have also been used in sexual conditioning studies illustrating that natural stimuli predict successful reproduction via Pavlovian processes. In addition, they have proven to be a good model to study how variation in photoperiod regulates reproduction and how variation in gonadal steroid hormones controls sexual behavior. For example, studies have shown that testosterone activates male-typical behaviors after being metabolized into estrogenic and androgenic metabolites. A critical site of action for these metabolites is the medial preoptic nucleus (POM), which is larger in males than in females. The enzyme aromatase converts testosterone to estradiol and is enriched in the POM in a male-biased fashion. Quail studies were the first to show that this enzyme is regulated both relatively slowly via genomic actions of steroids and more quickly via phosphorylation. With this base of knowledge and the recent cloning of the entire genome of the closely related chicken, quail will be valuable for future studies connecting gene expression to sexual and social behaviors.

Kinetics and pharmacology of the D1- and D2-like dopamine receptors in Japanese quail brain

Although the avian brain dopamine system and its functions have been studied much less than the mammalian one, there is an increasing interest in the role of dopamine and its receptors in a wide variety of motor, cognitive and emotional functions in birds with implications for basic research, medicine or agriculture. Pharmacological characterisation of the avian dopamine receptors has had little attention. In this paper we characterise the two classes of dopamine receptors in Japanese quail brain by radioligand binding techniques using [(3)H]SCH 23390 (D(1)) and [(3)H]spiperone (D(2)). Association, dissociation and saturation analyses showed that the binding of both radioligands is time- and concentration-dependent, saturable and reversible. Apparent dissociation constants determined for [(3)H]SCH 23390 and [(3)H]spiperone from concentration isotherms were 1.07 and 0.302 nM and the maximum binding capacities were 89.3 and 389.3 fmol per mg of protein, respectively. Using competitive binding studies with a spectrum of dopamine and other neurotransmitter receptor agonists/antagonists, the [(3)H]SCH 23390 and [(3)H]spiperone binding sites were characterised pharmacologically. Pharmacological profiles of quail dopamine receptors showed a high degree of pharmacological homology with other vertebrate dopamine receptors. The data presented extend the knowledge of kinetics and pharmacology of D(1)- and D(2)-like dopamine receptors in birds, provide data for avian psychopharmacological and comparative studies and represent an important complement to studies using cell expression systems.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Neurosteroid production in the songbird brain: a re-evaluation of core principles

Concepts of brain-steroid signaling have traditionally placed emphasis on the gonads and adrenals as the source of steroids, the strict dichotomy of early developmental ("organizational") and mature ("activational") effects, and a relatively slow mechanism of signaling through intranuclear receptors. Continuing research shows that these concepts are not inaccurate, but they are certainly incomplete. In this review, we focus on the song control circuit of songbird species to demonstrate how each of these concepts is limited. We discuss the solid evidence for steroid synthesis within the brain ("neurosteroidogenesis"), the role of neurosteroids in organizational events that occur both early in development and later in life, and how neurosteroids can act in acute and non-traditional ways. The songbird model therefore illustrates how neurosteroids can dramatically increase the diversity of steroid-sensitive brain functions in a behaviorally-relevant system. We hope this inspires further research and thought into neurosteroid signaling in songbirds and other animals.

Targeting steroid receptor coactivator-1 expression with locked nucleic acids antisense reveals different thresholds for the hormonal regulation of male sexual behavior in relation to aromatase activity and protein expression

Steroid receptors such as the androgen and estrogen receptors require the presence of several proteins, known as coactivators, to enhance the transcription of target genes. The first goal of the present study was to define the role of SRC-1 on the steroid-dependent expression of the aromatase protein and its activity in male Japanese quail. The second goal was to analyze the rapid plasticity of the POM following antisense treatment interruption. We confirm here that the inhibition of SRC-1 expression by daily intracerebroventricular injections of locked nucleic acid antisense oligonucleotides in the third ventricle at the level of the preoptic area-hypothalamus (HPOA) significantly reduces testosterone-dependent male sexual behavior. In the first experiment, aromatase protein expression in HPOA was inhibited in SRC-1-depleted males but the enzymatic activity remained at the level measured in controls. We observed in the second experiment a recovery of the behavioral response to testosterone treatment after interruption of the antisense injection. However, several morphological characteristics of the POM were not different between the control group, the antisense-treated birds and antisense-treated birds in which treatment had been discontinued 3 days earlier. Antisense was also less effective in knocking-down SRC-1 in the present experiments as compared to our previous study. An analysis of this variation in the degree of knock-down of SRC-1 expression suggests dissociation among different aspects of steroid action on brain and behavior presumably resulting from the differential sensitivity of behavioral and neurochemical responses to the activation by testosterone and/or its estrogenic metabolites.

Rapid effects of aromatase inhibition on male reproductive behaviors in Japanese quail

Non-genomic effects of steroid hormones on cell physiology have been reported in the brain. However, relatively little is known about the behavioral significance of these actions. Male sexual behavior is activated by testosterone partly through its conversion to estradiol via the enzyme aromatase in the preoptic area (POA). Brain aromatase activity (AA) changes rapidly which might in turn be important for the rapid regulation of behavior. Here, acute effects of Vorozole, an aromatase inhibitor, injected IP at different doses and times before testing (between 15 and 60 min), were assessed on male sexual behavior in quail. To limit the risk of committing both types of statistical errors (I and II), data of all experiments were entered into a meta-analysis. Vorozole significantly inhibited mount attempts (P < 0.05, size effect [g] = 0.527) and increased the latency to first copulation (P < 0.05, g = 0.251). The treatment had no effect on the other measures of copulatory behavior. Vorozole also inhibited appetitive sexual behavior measured by the social proximity response (P < 0.05, g = 0.534) or rhythmic cloacal sphincter movements (P < 0.001, g = 0.408). Behavioral inhibitions always reached a maximum at 30 min. Another aromatase inhibitor, androstatrienedione, induced a similar rapid inhibition of sphincter movements. Radioenzyme assays demonstrated that within 30 min Vorozole had reached the POA and completely blocked AA measured in homogenates. When added to the extracellular milieu, Vorozole also blocked within 5 min the AA in POA explants maintained in vitro. Together, these data demonstrate that aromatase inhibition rapidly decreases both consummatory and appetitive aspects of male sexual behavior.

Rapid control of brain aromatase activity by glutamatergic inputs

Estrogens derived from the neural aromatization of testosterone play a key role in the activation of male sexual behavior in many vertebrates and have now been recognized to have rapid membrane effects on brain function. Such changes in aromatase activity and hence in local estrogen concentrations could rapidly modulate behavioral responses. We show here that there is a very rapid (within minutes) decrease in aromatase activity in quail hypothalamic explants exposed to treatments affecting intracellular Ca2+ concentrations, such as the addition of glutamate agonists (kainate, alpha-amino-3-hydroxymethyl-4-isoxazole propionic acid, and, to a much lesser extent, N-methyl-D-aspartate), but not of gamma-aminobutyric acid. The kainate effects, which reduce aromatase activity by 25-50%, are observed within 5 min, are completely blocked in explants exposed to specific kainate antagonists (6-cyano-7-nitroquinoxaline-2,3-dione disodium or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium), and are also rapidly reversible when effectors are washed out. Together, these data support the idea that the synthesis of estrogen can be rapidly regulated in the brain, thus producing rapid changes in local estrogen bioavailability that could rapidly modify brain function with a time course similar to what has previously been described for neurotransmitters and neuromodulators.

Socially induced and rapid increases in aggression are inversely related to brain aromatase activity in a sex-changing fish, Lythrypnus dalli

Social interactions can generate rapid and dramatic changes in behaviour and neuroendocrine activity. We investigated the effects of a changing social environment on aggressive behaviour and brain aromatase activity (bAA) in a sex-changing fish, Lythrypnus dalli. Aromatase is responsible for the conversion of androgen into oestradiol. Male removal from a socially stable group resulted in rapid and dramatic (> or =200%) increases in aggression in the dominant female, which will become male usually 7-10 days later. These dominant females and recently sex-changed individuals had lower bAA but similar gonadal aromatase activity (gAA) compared to control females, while established males had lower bAA than all groups and lower gAA than all groups except dominant females. Within hours of male removal, dominant females' aggressive behaviour was inversely related to bAA but not gAA. These results are novel because they are the first to: (i) demonstrate socially induced decreases in bAA levels corresponding with increased aggression, (ii) identify this process as a possible neurochemical mechanism regulating the induction of behavioural, and subsequently gonadal, sex change and (iii) show differential regulation of bAA versus gAA resulting from social manipulations. Combined with other studies, this suggests that aromatase activity may modulate fast changes in vertebrate social behaviour.

Effects of calmodulin on aromatase activity in the preoptic area

Oestrogens derived from the neural aromatisation of testosterone play a key role in the activation of male sexual behaviour in many vertebrates. Besides their slow action on gene transcription mediated by the binding to nuclear receptors, oestrogens have now been recognised to have more rapid membrane-based effects on brain function. Rapid changes in aromatase activity, and hence in local oestrogen concentrations, could thus rapidly modulate behavioural responses. We previously demonstrated that calcium-dependent kinases are able to down-regulate aromatase activity after incubations of 10-15 min in phosphorylating conditions. In the present study, in quail hypothalamic homogenates, we show that Ca2+ or calmodulin alone can very rapidly change aromatase activity. Preincubation with 1 mM EGTA or with a monoclonal antibody raised against calmodulin immediately increased aromatase activity. The presence of calmodulin on aromatase purified by immunoprecipitation and electrophoresis was previously identified by western blot and two consensus binding sites for Ca2+-calmodulin are identified here on the deduced amino acid sequence of the quail brain aromatase. The rapid control of brain aromatase activity thus appears to include two mechanisms: (i) an immediate regulatory process that involves the Ca2+-calmodulin binding site and (ii) a somewhat slower phosphorylation by several protein kinases (PKC, PKA but also possibly Ca2+-calmodulin kinases) of the aromatase molecule.

Interactions between kinases and phosphatases in the rapid control of brain aromatase

Aromatization of testosterone into oestradiol plays a key role in the activation of male sexual behaviour in many vertebrate species. Rapid changes in brain aromatase activity have recently been identified and the resulting changes in local oestrogen bioavailability could modulate fast behavioural responses to oestrogens. In quail hypothalamic homogenates, aromatase activity is down-regulated within minutes by calcium-dependent phosphorylations in the presence of ATP, MgCl2 and CaCl2 (ATP/Mg/Ca). Three kinases (protein kinases A and C and calmodulin kinase; PKA, PKC and CAMK) are potentially implicated in this process. If kinases decrease aromatase activity in a reversible manner, then it would be expected that the enzymatic activity would increase and/or return to baseline levels in the presence of phosphatases. We showed previously that 0.1 mM vanadate (a general inhibitor of protein phosphatases) significantly decreases aromatase activity but specific protein phosphatases that could up-regulate aromatase activity have not been identified to date. The reversibility of aromatase activity inhibition by phosphorylations was investigated in the present study using alkaline and acid phosphatase (Alk and Ac PPase). Unexpectedly, Alk PPase inhibited aromatase activity in a dose-dependent manner in the presence, as well as in the absence, of ATP/Mg/Ca. By contrast, Ac PPase completely blocked the inhibitory effects of ATP/Mg/Ca on aromatase activity, even if it moderately inhibited aromatase activity in the absence of ATP/Mg/Ca. However, the addition of Ac PPase was unable to restore aromatase activity after it had been inhibited by exposure to ATP/Mg/Ca. Taken together, these data suggest that, amongst the 15 potential consensus phosphorylation sites identified on the quail aromatase sequence, some must be constitutively phosphorylated for the enzyme to be active whereas phosphorylation of the others is involved in the rapid inhibition of aromatase activity by the competitive effects of protein kinases and phosphatases. Two out of these 15 putative phosphorylation sites occur in an environment corresponding to the consensus sites for PKC, PKA (and possibly a CAMK) and, in all probability, represent the sites whose phosphorylation rapidly blocks enzyme activity.

Rapid decreases in preoptic aromatase activity and brain monoamine concentrations after engaging in male sexual behavior

In Japanese quail, as in rats, the expression of male sexual behavior over relatively long time periods (days to weeks) is dependent on the local production of estradiol in the preoptic area via the aromatization of testosterone. On a short-term basis (minutes to hours), central actions of dopamine as well as locally produced estrogens modulate behavioral expression. In rats, a view of and sexual interaction with a female increase dopamine release in the preoptic area. In quail, in vitro brain aromatase activity (AA) is rapidly modulated by calcium-dependent phosphorylations that are likely to occur in vivo as a result of changes in neurotransmitter activity. Furthermore, an acute estradiol injection rapidly stimulates copulation in quail, whereas a single injection of the aromatase inhibitor vorozole rapidly inhibits this behavior. We hypothesized that brain aromatase and dopaminergic activities are regulated in quail in association with the expression of male sexual behavior. Visual access as well as sexual interactions with a female produced a significant decrease in brain AA, which was maximal after 5 min. This expression of sexual behavior also resulted in a significant decrease in dopaminergic as well as serotonergic activity after 1 min, which returned to basal levels after 5 min. These results demonstrate for the first time that AA is rapidly modulated in vivo in parallel with changes in dopamine activity. Sexual interactions with the female decreased aromatase and dopamine activities. These data challenge established views about the causal relationships among dopamine, estrogen action, and male sexual behavior.

Dopamine modulates male sexual behavior in Japanese quail in part via actions on noradrenergic receptors

In rats, dopamine (DA) facilitates male sexual behavior through its combined action on D1- and D2-like receptors, in the medial preoptic area (MPOA) as well as other brain areas. In Japanese quail, systemic injections of dopaminergic drugs suggested a similar pharmacology but central injections have never been performed. Recent electrophysiological experiments demonstrated that DA effects in the MPOA of quail are mediated mainly through the activation of alpha2-noradrenergic receptors. Previous studies of DA action on behavior used specific dopaminergic agonists/antagonists and therefore unintentionally avoided the potential cross-reaction with alpha2-receptors. The present study was thus designed to investigate directly the effects of DA on male sexual behavior and to test whether the interaction of DA with heterologous receptors affects this behavior. Intracerebroventricular (i.c.v.) injection of DA or NE inhibited copulation in a dose-dependent manner. Systemic injections of yohimbine, an alpha2-noradrenergic antagonist, modulated copulation in a bimodal manner depending on the dose injected. Interestingly, a behaviorally ineffective dose of yohimbine markedly reduced the inhibitory effects of DA when injected 15min before. Together, these results show for the first time that i.c.v. injections of DA itself inhibit male sexual behavior in quail and suggest that the interaction of DA with alpha2-receptors has behavioral significance.

Brain estradiol content in newborn rats: sex differences, regional heterogeneity, and possible de novo synthesis by the female telencephalon

Accurate assessment of gonadal steroid levels in the developing brain is critical for understanding naturally occurring steroid-mediated sexual differentiation as well as determining the physiological relevance of exogenous steroid treatments commonly used in the study of this phenomenon. Using RIA, we measured the estradiol (E(2)) content of six regions of the developing brain immediately post partum, 1 d post partum, and after injection of exogenous estradiol benzoate, testosterone propionate, or the aromatase inhibitor formestane. We found sexually dimorphic E(2) content in several regions of the newborn brain. At 2 h of life, there was significantly higher E(2) content in males vs. females in the frontal cortex, hypothalamus and preoptic area but not in the hippocampus, brainstem, or cerebellum. Surprisingly, the female hippocampus had significantly higher E(2) content than all other female regions examined. By d 1 post partum, E(2) levels had decreased precipitously in most brain regions, and only the hypothalamus maintained a sex difference. Injection of female pups with estradiol benzoate raised tissue levels to that of the male in the hypothalamus but 2- to 3-fold higher in the other five regions. Testosterone administration increased E(2) content exclusively in the preoptic area, suggesting local variation in aromatase activity and/or substrate availability. Central administration of formestane decreased estrogen content in the male cortex, hypothalamus, and preoptic area. Formestane treatment also decreased endogenous E(2) in female cortex and hippocampus, suggesting de novo synthesis selectively in these brain regions. These data corroborate and extend previous findings of sex differences in brain E(2) levels perinatally and reveal an unexpected regional heterogeneity in E(2) synthesis and/or metabolism.

Territorial aggression, circulating levels of testosterone, and brain aromatase activity in free-living pied flycatchers

Testosterone (T) is a critical endocrine factor for the activation of many aspects of reproductive behavior in vertebrates. Castration completely eliminates the display of aggressive and sexual behaviors that are restored to intact level by a treatment with exogenous T. There is usually a tight correlation between the temporal changes in plasma T and the frequency of reproductive behaviors during the annual cycle. In contrast, individual levels of behavioral activity are often not related to plasma T concentration at the peak of the reproductive season suggesting that T is available in quantities larger than necessary to activate behavior and that other factors limit the expression of behavior. There is some indication from work in rodents that individual levels of brain aromatase activity (AA) may be a key factor that limits the expression of aggressive behavior, and in agreement with this idea, many studies indicate that estrogens produced in the brain by the aromatization of T may contribute to the activation of reproductive behavior, including aggression. We investigated here in pied flycatcher (Ficedula hypoleuca) the relationships among territorial aggression, plasma T, and brain AA at the peak of the reproductive season. In a first experiment, blood samples were collected from unpaired males holding a primary territory and, 1 or 2 days later, their aggressive behavior was quantified during standardized simulated territorial intrusions. No relationship was found between individual differences in aggressive behavior and plasma T or dihydrotestosterone levels but a significant negative correlation was observed between number of attacks and plasma corticosterone. In a second experiment, aggressive behavior was measured during a simulated territorial intrusion in 22 unpaired males holding primary territories. They were then immediately captured and AA was measured in their anterior and posterior diencephalon and in the entire telencephalon. Five males that had attracted a female (who had started egg-laying) were also studied. The paired males were less aggressive and correlatively had a lower AA in the anterior diencephalon but not in the posterior diencephalon and telencephalon than the 22 birds holding a territory before arrival of a female. In these 22 birds, a significant correlation was observed between number of attacks/min displayed during the simulated territorial intrusion and AA in the anterior diencephalon but no correlation was found between these variables in the two other brain areas. Taken together, these data indicate that the level of aggression displayed by males defending their primary territory may be limited by the activity of the preoptic aromatase, but plasma T is not playing an important role in establishing individual differences in aggression. Alternatively, it is also possible that brain AA is rapidly affected by agonistic interactions and additional work should be carried out to determine whether the correlation observed between brain AA and aggressive behavior is the result of an effect of the enzyme on behavior or vice versa. In any case, the present data show that preoptic AA can change quite rapidly during the reproductive cycle (within a few days after arrival of the female) indicating that this enzymatic activity is able to regulate rapid behavioral transitions during the reproductive cycle in this species.

Multiple mechanisms control brain aromatase activity at the genomic and non-genomic level

Evidence has recently accumulated indicating that aromatase activity in the preoptic area is modulated in parallel by both slow (hours to days) genomic and rapid (minutes to hours) non-genomic mechanisms. We review here these two types of control mechanisms and their potential contribution to various aspects of brain physiology in quail. High levels of aromatase mRNA, protein and activity (AA) are present in the preoptic area of this species where the transcription of aromatase is controlled mainly by steroids. Estrogens acting in synergy with androgens play a key role in this control and both androgen and estrogen receptors (ER; alpha and beta subtypes) are present in the preoptic area even if they are not necessarily co-localized in the same cells as aromatase. Steroids have more pronounced effects on aromatase transcription in males than in females and this sex difference could be caused, in part, by a sexually differentiated expression of the steroid receptor coactivator 1 in this area. The changes in aromatase concentration presumably control seasonal variations as well as sex differences in brain estrogen production. Aromatase activity in hypothalamic homogenates is also rapidly (within minutes) down-regulated by exposure to conditions that enhance protein phosphorylation such as the presence of high concentrations of calcium, magnesium and ATP. Similarly, pharmacological manipulations such as treatment with thapsigargin or stimulation of various neurotransmitter receptors (alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA)) leading to enhanced intracellular calcium concentrations depress within minutes the aromatase activity measured in quail preoptic explants. The effects of receptor stimulation are presumably direct: electrophysiological data confirm the presence of these receptors in the membrane of aromatase-expressing cells. Inhibitors of protein kinases interfere with these processes and Western blotting experiments on brain aromatase purified by immunoprecipitation confirm that the phosphorylations regulating aromatase activity directly affect the enzyme rather than another regulatory protein. Accordingly, several phosphorylation consensus sites are present on the deduced amino acid sequence of the recently cloned quail aromatase. Fast changes in the local availability of estrogens in the brain can thus be caused by aromatase phosphorylation so that estrogen could rapidly regulate neuronal physiology and behavior. The rapid as well as slower processes of local estrogen production in the brain thus match well with the genomic and non-genomic actions of steroids in the brain. These two processes potentially provide sufficient temporal variation in the bio-availability of estrogens to support the entire range of established effects for this steroid.

Calcium-dependent phosphorylation processes control brain aromatase in quail

Increased gene transcription activated by the binding of sex steroids to their cognate receptors is one important way in which oestrogen synthase (aromatase) activity is regulated in the brain. This control mechanism is relatively slow (hours to days) but recent data indicate that aromatase activity in quail preoptic-hypothalamic homogenates is also rapidly (within minutes) affected by exposure to conditions that enhance Ca2+-dependent protein phosphorylation. We demonstrate here that Ca2+-dependent phosphorylations controlled by the activity of multiple protein kinases including PKC, and possibly also PKA and CAMK, can rapidly down-regulate aromatase activity in brain homogenates. These phosphorylations directly affect the aromatase molecule itself. Western blotting experiments on aromatase purified by immunoprecipitation reveal the presence on the enzyme of phosphorylated serine, threonine and tyrosine residues in concentrations that are increased by phosphorylating conditions. Cloning and sequencing of the quail aromatase identified a 1541-bp open reading frame that encodes a predicted 490-amino-acid protein containing all the functional domains that have been previously described in the mammalian and avian aromatase. Fifteen predicted consensus phosphorylation sites were identified in this sequence, but only two of these (threonine 455 and 486) match the consensus sequences corresponding to the protein kinases that were shown to affect aromatase activity during the pharmacological experiments (i.e. PKC and PKA). This suggests that the phosphorylation of one or both of these residues represents the mechanism underlying, at least in part, the rapid changes in aromatase activity.

Dopamine activates noradrenergic receptors in the preoptic area

Dopamine (DA) facilitates male sexual behavior and modulates aromatase activity in the quail preoptic area (POA). Aromatase neurons in the POA receive dopaminergic inputs, but the anatomical substrate that mediates the behavioral and endocrine effects of DA is poorly understood. Intracellular recordings showed that 100 microm DA hyperpolarizes most neurons in the medial preoptic nucleus (80%) by a direct effect, but depolarizes a few others (10%). DA-induced hyperpolarizations were not blocked by D1 or D2 antagonists (SCH-23390 and sulpiride). Extracellular recordings confirmed that DA inhibits the firing of most cells (52%) but excites a few others (24%). These effects also were not affected by DA antagonists (SCH-23390 and sulpiride) but were blocked by alpha2-(yohimbine) and alpha1-(prazosin) noradrenergic receptor antagonists, respectively. Two dopamine-beta-hydroxylase (DBH) inhibitors (cysteine and fusaric acid) did not block the DA-induced effects, indicating that DA is not converted into norepinephrine (NE) to produce its effects. The pK(B) of yohimbine for the receptor involved in the DA- and NE-induced inhibitions was similar, indicating that the two monoamines interact with the same receptor. Together, these results demonstrate that the effects of DA in the POA are mediated mostly by the activation of alpha2 (inhibition) and alpha1 (excitation) adrenoreceptors. This may explain why DA affects the expression of male sexual behavior through its action in the POA, which contains high densities of alpha2-noradrenergic but limited amounts of DA receptors. This study thus clearly demonstrates the existence of a cross talk within CNS catecholaminergic systems between a neurotransmitter and heterologous receptors.

Interactions between aromatase (estrogen synthase) and dopamine in the control of male sexual behavior in quail

In male quail, like in other vertebrates including rodents, testosterone acting especially through its estrogenic metabolites is necessary for the activation of male sexual behavior. Also, the administration of dopamine agonists and antagonists profoundly influences male sexual behavior. How the steroid-sensitive neural network and dopamine interact physiologically, remains largely unknown. It is often implicitly assumed that testosterone or its metabolite estradiol, stimulates male sexual behavior via the modification of dopaminergic transmission. We have now identified in quail two possible ways in which dopamine could potentially affect sexual behavior by modulating the aromatization of testosterone into an estrogen. One is a long-acting mechanism that presumably involves the modification of dopaminergic transmission followed by the alteration of the genomic expression of aromatase. The other is a more rapid mechanism that does not appear to be dopamine receptor-mediated and may involve a direct interaction of dopamine with aromatase (possibly via substrate competition). We review here the experimental data supporting the existence of these controls of aromatase activity by dopamine and discuss the possible contribution of these controls to the activation of male sexual behavior.

Phosphorylation processes mediate rapid changes of brain aromatase activity

The enzyme aromatase (also called estrogen synthase) that catalyzes the transformation of testosterone (T) into estradiol plays a key limiting role in the action of T on many aspects of reproduction. The distribution and regulation of aromatase in the quail brain has been studied by radioenzyme assays on microdissected brain areas, immunocytochemistry, RT-PCR and in situ hybridization. High levels of aromatase activity (AA) characterize the sexually dimorphic, steroid-sensitive medial preoptic nucleus (POM), a critical site of T action and aromatization for the activation of male sexual behavior. The boundaries of the POM are clearly outlined by a dense population of aromatase-containing cells as visualized by both immunocytochemistry and in situ hybridization histochemistry. Aromatase synthesis in the POM is controlled by T and its metabolite estradiol, but estradiol receptors alpha (ERalpha) are not normally co-localized with aromatase in this brain area. Estradiol receptor beta (ERbeta) has been recently cloned in quail and localized in POM but we do not yet know whether ERbeta occurs in aromatase cells. It is therefore not known whether estrogens regulate aromatase synthesis directly or by affecting different inputs to aromatase cells as is the case with the gonadotropin releasing hormone neurons. The presence of aromatase in presynaptic boutons suggests that locally formed estrogens may exert part of their effects by non-genomic mechanisms at the membrane level. Rapid effects of estrogens in the brain that presumably take place at the neuronal membrane level have been described in other species. If fast transduction mechanisms for estrogen are available at the membrane level, this will not necessarily result in rapid changes in brain function if the availability of the ligand does not also change rapidly. We demonstrate here that AA in hypothalamic homogenates is rapidly down-regulated by exposure to conditions that enhance protein phosphorylation (addition of Ca2+, Mg2+, ATP). This inhibition is blocked by kinase inhibitors which supports the notion that phosphorylation processes are involved. A rapid (within minutes) and reversible regulation of AA is also observed in hypothalamic explants incubated in vitro and exposed to high Ca2+ levels (K+-induced depolarization, treatment by thapsigargin, by kainate, AMPA or NMDA). The local production and availability of estrogens in the brain can therefore be rapidly changed by Ca2+ based on variation in neurotransmitter activity. Locally-produced estrogens are as a consequence available for non-genomic regulation of neuronal physiology in a manner more akin to the action of a neuropeptide/neurotransmitter than previously thought.

The control of preoptic aromatase activity by afferent inputs in Japanese quail

This review summarizes current knowledge on the mechanisms that control aromatase activity in the quail preoptic area, a brain region that plays a key role in the control of reproduction. Aromatase and aromatase mRNA synthesis in the preoptic area are enhanced by testosterone and its metabolite estradiol, but estradiol receptors of the alpha subtype are not regularly colocalized with aromatase. Estradiol receptors of the beta subtype are present in the preoptic area but it is not yet known whether these receptors are colocalized with aromatase. The regulation by estrogen of aromatase activity may be, in part, trans-synaptically mediated, in a manner that is reminiscent of the ways in which steroids control the activity of gonadotropic hormone releasing hormone neurons. Aromatase-immunoreactive neurons are surrounded by dense networks of vasotocin-immunoreactive and tyrosine hydroxylase-immunoreactive fibers and punctate structures. These inputs are in part steroid-sensitive and could therefore mediate the effects of steroids on aromatase activity. In vivo pharmacological experiments indicate that catecholaminergic depletions significantly affect aromatase activity presumably by modulating aromatase transcription. In addition, in vitro studies on brain homogenates or on preoptic-hypothalamic explants show that aromatase activity can be rapidly modulated by a variety of dopaminergic compounds. These effects do not appear to be mediated by the membrane dopamine receptors and could involve changes in the phosphorylation state of the enzyme. Together, these results provide converging evidence for a direct control of aromatase activity by catecholamines consistent with the anatomical data indicating the presence of a catecholaminergic innervation of aromatase cells. These dopamine-induced changes in aromatase activity are observed after several hours or days and presumably result from changes in aromatase transcription but rapid non-genomic controls have also been identified. The potential significance of these processes for the physiology of reproduction is critically evaluated.