Aromatization of androgens into estrogens reduces response latency to a noxious thermal stimulus in male quail

Aromatase in the Human Brain

The aromatase cytochrome P450 (P450arom) enzyme, or estrogen synthase, which is coded by the CYP19A1 gene, is widely expressed in a subpopulation of excitatory and inhibitory neurons, astrocytes, and other cell types in the human brain. Experimental studies in laboratory animals indicate a prominent role of brain aromatization of androgens to estrogens in regulating different brain functions. However, the consequences of aromatase expression in the human brain remain poorly understood. Here, we summarize the current knowledge about aromatase expression in the human brain, abundant in the thalamus, amygdala, hypothalamus, cortex, and hippocampus and discuss its role in the regulation of sensory integration, body homeostasis, social behavior, cognition, language, and integrative functions. Since brain aromatase is affected by neurodegenerative conditions and may participate in sex-specific manifestations of autism spectrum disorders, major depressive disorder, multiple sclerosis, stroke, and Alzheimer's disease, we discuss future avenues for research and potential clinical and therapeutic implications of the expression of aromatase in the human brain.

A new clinically-relevant rat model of letrozole-induced chronic nociceptive disorders

Among postmenopausal women with estrogen receptor-positive breast cancer, more than 80% receive hormone therapy including aromatase inhibitors (AIs). Half of them develop chronic arthralgia - characterized by symmetric articular pain, carpal tunnel syndrome, morning stiffness, myalgia and a decrease in grip strength - which is associated with treatment discontinuation. Only a few animal studies have linked AI treatment to nociception, and none to arthralgia. Thus, we developed a new chronic AI-induced nociceptive disorder model mimicking clinical symptoms induced by AIs, using subcutaneous letrozole pellets in ovariectomized (OVX) rats. Following plasma letrozole dosage at the end of the experiment (day 73), only rats with at least 90 ng/ml of letrozole were considered significantly exposed to letrozole (OVX + high LTZ group), whereas treated animals with less than 90 ng/ml were pooled in the OVX + low LTZ group. Chronic nociceptive disorder set in rapidly and was maintained for more than 70 days in the OVX + high LTZ group. Furthermore, OVX + high LTZ rats saw no alteration in locomotion, myalgia or experimental anxiety during this period. Bone parameters of the femora were significantly altered in all OVX rats compared to Sham+vehicle pellet. A mechanistic analysis focused on TRPA1, receptor suspected to mediate AI-evoked pain, and showed no modification in its expression in the DRG. This new long-lasting chronic rat model, efficiently reproduces the symptoms of AI-induced nociceptive disorder affecting patients' daily activities and quality-of-life. It should help to study the pathophysiology of this disorder and to promote the development of new therapeutic strategies.

Neuroendocrine Modulation of Cognitive Performance in the Patients with Fibromyalgia

BACKGROUND

Fibromyalgia (FM) is a chronic widespread pain disorder associated with fatigue, tender points, sleep disturbances, and mood disorders. Symptoms associated with FM also include decreased cognitive function in which the neural basis is poorly understood. Neuroendocrine hormones may be correlated with cognitive performance under some ill conditions. However, we are unaware of current evidence on neuroendocrine hormones as factors influencing cognitive function in adults with FM.

OBJECTIVES

The aim of the study was to assess whether neuroendocrine hormones could affect cognition in the patients with FM.

STUDY DESIGN

This study used a case-control trial design.

SETTING

Study patients were recruited from the neurological outpatient clinics in the Second Affiliated Hospital and Affiliated Chaohu Hospital of Anhui Medical University and met the American College of Rheumatology criteria for FM.

METHODS

Forty-six patients with FM were compared with twenty-nine healthy controls (HCs). Several measures of cognitive performance and serum levels of neuroendocrine hormones were used to make these comparisons, and the patients were also asked to complete questionnaires on depression and sleep quality. Partial correlation analysis was performed to control the confounders and linear regression analysis was used to examine the effects of neuroendocrine hormones on cognitive measures.

RESULTS

The FM patients had worse performance in attention, short-term memory, orientation, object working memory and spatial reference memory, higher depression scores, and worse sleep quality than HCs. The raised level of cortisol and gonadotropin-releasing hormone (GnRH) can protect general cognition, whereas the raised level of cortisol and thyroid-stimulating hormone (TSH) will damage spatial memory.

LIMITATIONS

We did not study the sex hormones comprehensively.

CONCLUSIONS

The FM patients showed significant cognitive impairment in several domains. The altered levels of cortisol, thyrotrophin-releasing hormone (TRH), and GnRH may mediate cognitive changes in FM.

Estrogen modulation of pain perception with a novel 17β-estradiol pretreatment regime in ovariectomized rats

Estrogen plays substantial roles in pain modulation; however, studies concerning sex hormones and nociception often yield confusing results. The discrepancy could be a result of lack of consensus to regard estrogen as a variable when working with animal models; thus, the influence of hormones’ fluctuations on nociception has continually been neglected. In the present study, we designed a novel hormone substitution model to aid us to evaluate the effects of estrogen’s long-term alterations on ovariectomy (OVX)-induced mechanical hyperalgesia and the expression of estrogen receptors(ERs). OVX rats were implanted with slow-release estrogen pellets at differently arranged time points and doses, such that a gradual elevation or decrease of serum estrogen levels following a relatively stable period of estrogen replacement was achieved in rats. Our results demonstrated that gradual estrogen depletion rather than elevation following the stable period of estrogen substitution in OVX rats alleviated OVX-induced mechanical hyperalgesia in a dose-independent manner, and the opposite estrogen increase or decrease paradigms differently regulate the expression of spinal ERs. Specifically, in rats rendered to continuously increased serum estrogen, the early phase estrogen-induced anti-nociception effect in OVX rats was eliminated, which was accompanied by an over-activation of ERα and a strong depression of ERβ, while in the OVX rats subject to gradual decrease of estrogen replacement, both ERα and ERβ increased modestly compared with the OVX group. Thus, the present study demonstrated that estrogen increase or decrease modulate nociception differently through change of spinal ERs.

Neuroactive steroids, nociception and neuropathic pain: A flashback to go forward

The present review discusses the potential role of neurosteroids/neuroactive steroids in the regulation of nociceptive and neuropathic pain, and recapitulates the current knowledge on the main mechanisms involved in the reduction of pain, especially those occurring at the dorsal horn of the spinal cord, a crucial site for nociceptive processing. We will make special focus on progesterone and its derivative allopregnanolone, which have been shown to exert remarkable actions in order to prevent or reverse the maladaptive changes and pain behaviors that arise after nervous system damage in various experimental neuropathic conditions.

Aromatase inhibitors augment nociceptive behaviors in rats and enhance the excitability of sensory neurons

Although aromatase inhibitors (AIs) are commonly used therapies for breast cancer, their use is limited because they produce arthralgia in a large number of patients. To determine whether AIs produce hypersensitivity in animal models of pain, we examined the effects of the AI, letrozole, on mechanical, thermal, and chemical sensitivity in rats. In ovariectomized (OVX) rats, administering a single dose of 1 or 5mg/kg letrozole significantly reduced mechanical paw withdrawal thresholds, without altering thermal sensitivity. Repeated injection of 5mg/kg letrozole in male rats produced mechanical, but not thermal, hypersensitivity that extinguished when drug dosing was stopped. A single dose of 5mg/kg letrozole or daily dosing of letrozole or exemestane in male rats also augmented flinching behavior induced by intraplantar injection of 1000nmol of adenosine 5'-triphosphate (ATP). To determine whether sensitization of sensory neurons contributed to AI-induced hypersensitivity, we evaluated the excitability of neurons isolated from dorsal root ganglia of male rats chronically treated with letrozole. Both small and medium-diameter sensory neurons isolated from letrozole-treated rats were more excitable, as reflected by increased action potential firing in response to a ramp of depolarizing current, a lower resting membrane potential, and a lower rheobase. However, systemic letrozole treatment did not augment the stimulus-evoked release of the neuropeptide calcitonin gene-related peptide (CGRP) from spinal cord slices, suggesting that the enhanced nociceptive responses were not secondary to an increase in peptide release from sensory endings in the spinal cord. These results provide the first evidence that AIs modulate the excitability of sensory neurons, which may be a primary mechanism for the effect of these drugs to augment pain behaviors in rats.

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.

Sex differences and hormonal modulation of deep tissue pain

Women disproportionately suffer from many deep tissue pain conditions. Experimental studies show that women have lower pain thresholds, higher pain ratings and less tolerance to a range of painful stimuli. Most clinical and epidemiological reports suggest female gonadal hormones modulate pain for some, but not all, conditions. Similarly, animal studies support greater nociceptive sensitivity in females in many deep tissue pain models. Gonadal hormones modulate responses in primary afferents, dorsal horn neurons and supraspinal sites, but the direction of modulation is variable. This review will examine sex differences in deep tissue pain in humans and animals focusing on the role of gonadal hormones (mainly estradiol) as an underlying component of the modulation of pain sensitivity.

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.

Context-dependent links between song production and opioid-mediated analgesia in male European starlings (Sturnus vulgaris)

Little is known about the neural mechanisms that ensure appropriate vocal behaviors within specific social contexts. Male songbirds produce spontaneous (undirected) songs as well as female-directed courtship songs. Opioid neuropeptide activity in specific brain regions is rewarding, at least in mammals, and past studies suggest that the opioid met-enkephalin in such areas is more tightly linked to undirected than female-directed song. Recent data using a song-associated place preference paradigm further suggest that production of undirected but not directed song is tightly linked to intrinsic reward. Opioids have analgesic properties. Therefore, if production of undirected song is closely linked to opioid-mediated reward, the production of undirected but not directed song should be associated with analgesia. Consistent with this prediction, in male starlings we identified a positive correlation between analgesia (decreased reactivity to a hot water bath) and undirected song (in non-breeding season condition males in affiliative flocks) but not female-directed song (in breeding season condition males presented with females). When breeding condition males were divided according to social status, a negative correlation was found in subordinate males (i.e. males that failed to acquire a nest box). These data are consistent with the hypotheses 1) that the production of undirected song is facilitated or maintained by opioids (and/or other neuromodulators that also induce analgesia) and 2) that production of female-directed song is not linked in the same way to release of the same neuromodulators. Results also demonstrate a link between analgesia and song in subordinate individuals lacking a nesting territory within the breeding season. Overall, the findings indicate that distinct neural mechanisms regulate communication in different social contexts and support the working hypothesis that undirected but not directed song is tightly linked to opioid release.

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.

Rapid control of male typical behaviors by brain-derived estrogens

Beside their genomic mode of action, estrogens also activate a variety of cellular signaling pathways through non-genomic mechanisms. Until recently, little was known regarding the functional significance of such actions in males and the mechanisms that control local estrogen concentration with a spatial and time resolution compatible with these non-genomic actions had rarely been examined. Here, we review evidence that estrogens rapidly modulate a variety of behaviors in male vertebrates. Then, we present in vitro work supporting the existence of a control mechanism of local brain estrogen synthesis by aromatase along with in vivo evidence that rapid changes in aromatase activity also occur in a region-specific manner in response to changes in the social or environmental context. Finally, we suggest that the brain estrogen provision may also play a significant role in females. Together these data bolster the hypothesis that brain-derived estrogens should be considered as neuromodulators.

Expression of estrogen receptor alpha and beta is decreased in hypospadias

PURPOSE

Estrogenic endocrine disruptors acting via estrogen receptors α and β have been implicated in the etiology of hypospadias. However, the expression and distribution of estrogen receptors α and β in normal and hypospadiac human foreskins is unknown. We characterized the location and expression of estrogen receptors α and β in normal and hypospadiac foreskins.

MATERIALS AND METHODS

We prospectively collected excess foreskin from 35 patients undergoing hypospadias repair and 15 patients undergoing elective circumcision. Hypospadias was classified as severe in 18 patients and mild in 17 based on the ectopic position of the meatus. mRNA expression levels in estrogen receptors α and β were quantified using reverse transcriptase polymerase chain reaction. Receptor location was characterized by immunohistochemical analysis. Additionally immunohistochemical analysis was performed in 4 archived human fetal penises.

RESULTS

Mean ± SD ages were similar for the circumcision (9.5±3 months) and hypospadias repair groups (9±3 months, p=0.75). mRNA expression levels in estrogen receptors α and β were significantly decreased in hypospadiac foreskin cases compared to controls (p<0.001), while no statistically significant differences were seen between foreskins with severe and mild hypospadias. Estrogen receptor β immunostaining was strong in normal foreskin but weak in hypospadiac foreskin. Estrogen receptor β immunoreactivity was most intense in the stratum basale and stratum spinosum. Estrogen receptor α immunostaining was weak in normal and mild hypospadias foreskin, and undetectable in severe hypospadias. Fetal penises expressed strong estrogen receptor β immunopositivity in the urethral plate epithelium, corpus spongiosum, corpora cavernosa and penile skin, while estrogen receptor α immunostaining was not detected.

CONCLUSIONS

These data demonstrate a difference in estrogen receptor α and β expression and location in the foreskin of patients with hypospadias compared to controls. These findings are consistent with aberrant estrogenic effects having a role in the etiology of hypospadias.

Inhibition of injury-induced glial aromatase reveals a wave of secondary degeneration in the songbird brain

Mechanical or anoxic/ischemic brain insult results in reactive gliosis and a pronounced wave of apoptotic secondary degeneration (WSD). Reactive glia express aromatase (estrogen synthase) and glial estrogen synthesis decreases apoptosis and the volume of degeneration. Whether aromatization by glia affects gliosis itself or the initiation/maintenance of the WSD remains unknown. Adult male zebra finches (Taeniopygia guttata) were injured with a needle that contained the aromatase inhibitor fadrozole or vehicle into contralateral hemispheres. Birds were killed at 0, 2, 6, 24, 72h, 2 or 6 weeks postinjury. Gliosis and degeneration were measured with vimentin- and Fluoro-Jade B-expression, respectively. Reactive gliosis was detectable at 6 h, reached asymptote at 72 h, and continued until 6 weeks postinsult. Gliosis extended further around fadrozole-injury than vehicle, an effect driven by a larger area of gliosis around fadrozole- relative to vehicle-injury at 72 h postinsult. Glial aromatase was inhibited for about 2 weeks postinjury since aromatase relative optical density was higher around fadrozole-injury relative to vehicle-injury until this time-point. Degeneration around vehicle-injury reached asymptote at 2 h postinsult, but that around fadrozole-injury peaked 24-72 h postinjury and decreased thereafter. Thus, the injury-induced WSD as described in mammals is detectable in zebra finches only following glial aromatase inhibition. In the zebra finch, injury-induced estrogen provision may decrease reactive gliosis and severely dampen the WSD, suggesting that songbirds are powerful models for understanding the role of glial aromatization in secondary brain damage.

Pain sensations to the cold pressor test in normally menstruating women: comparison with men and relation to menstrual phase and serum sex steroid levels

The role of gonadal hormones on pain sensations was investigated in normally menstruating women (n = 16) using the cold pressor test. Tolerance time, pain threshold, and pain intensity were examined once a week during a 4-wk period, and serum concentrations of 17beta-estradiol and progesterone were determined at each test session, which were classified into the early follicular phase, late follicular phase, early luteal phase, and late luteal phase, as determined by the first day of menses and the actual hormone levels recorded. A group of men (n = 10) of the same age interval was examined for comparison. The data show that pain threshold was reduced during the late luteal phase compared with the late follicular phase, and hormone analyses showed significant positive correlation between the progesterone concentration and lowered pain threshold and increasing pain intensity. Hormone analysis also showed an interaction between S-estradiol and S-progesterone on pain intensity, demonstrating that the increased perceived pain intensity that was associated with high progesterone concentrations was significantly reduced with increasing levels of estradiol. While no statistically significant sex differences in pain measurements were found, women displayed much more pronounced, and statistically significant, session-to-session effects than men, with increased pain threshold and decreased pain intensity with each test session. Hence, these data suggest that the changes in the serum concentration of gonadal hormones that occur during the menstrual cycle influence pain sensations elicited by noxious tonic cold stimulation and show that adaptation to the cold pressor test may be sex dependent.

Finasteride, a 5alpha-reductase inhibitor, potentiates antinociceptive effects of morphine, prevents the development of morphine tolerance and attenuates abstinence behavior in the rat

It has been shown that morphine increases 5alpha-reductase enzyme activity in the rat central nervous system; however importance of this finding on morphine analgesia, tolerance and dependence has not been reported. In the present study, we investigated inhibition of 5alpha-reductase enzyme on morphine effects using finasteride. To determine whether the 5alpha-reductase enzyme interact with morphine analgesia, finasteride (5 mg/kg, i.p.) was administrated with morphine (5 and 7 mg/kg, i.p.). The tail-flick test was used to assess the nociceptive threshold, before and 15, 30, 45, 60 and 90 min after drug administration. In tolerance experiments, morphine 20 mg/kg was injected i.p., twice daily for 4 days. The development and expression of dependence were assessed in the naloxone precipitation test 5 days after the morphine (20-30 mg/kg, i.p.) administration. We found that finasteride could potentiate the antinociceptive effect of morphine. In addition, chronic finasteride administration effectively blocked development of tolerance and dependence to morphine. Following chronic morphine administration, single dose injection of finasteride failed to reverse tolerance but prevented naloxone precipitate withdrawal syndrome. Therefore, it was concluded that there is a functional relationship between 5alpha-reductase enzyme and morphine.

Functional significance of the rapid regulation of brain estrogen action: where do the estrogens come from?

Estrogens exert a wide variety of actions on reproductive and non-reproductive functions. These effects are mediated by slow and long lasting genomic as well as rapid and transient non-genomic mechanisms. Besides the host of studies demonstrating the role of genomic actions at the physiological and behavioral level, mounting evidence highlights the functional significance of non-genomic effects. However, the source of the rapid changes in estrogen availability that are necessary to sustain their fast actions is rarely questioned. For example, the rise of plasma estrogens at pro-estrus that represents one of the fastest documented changes in plasma estrogen concentration appears too slow to explain these actions. Alternatively, estrogen can be synthesized in the brain by the enzyme aromatase providing a source of locally high concentrations of the steroid. Furthermore, recent studies demonstrate that brain aromatase can be rapidly modulated by afferent inputs, including glutamatergic afferents. A role for rapid changes in estrogen production in the central nervous system is supported by experiments showing that acute aromatase inhibition affects nociception as well as male sexual behavior and that preoptic aromatase activity is rapidly (within min) modulated following mating. Such mechanisms thus fulfill the gap existing between the fast actions of estrogen and their mode of production and open new avenues for the understanding of estrogenic effects on the brain.

Estrogen synthesis in the spinal dorsal horn: a new central mechanism for the hormonal regulation of pain

The data summarized here suggest the existence of a new central pathway for the hormonal regulation of pain. These data mainly collected in quail, a useful model in neuroendocrinology, demonstrate that numerous neurons in the superficial laminae of the spinal cord express aromatase (estrogen-synthase). Chronic and systemic blockade of this enzyme in quail alters nociception within days, indicating that the slow genomic effects of sex steroids on nociception classically observed in mammals also occur in birds and require aromatization of androgens into estrogens. However, by contrast with these slow effects, acute intrathecal inhibition of aromatase in restricted spinal cord segments reveals that estrogens can also control nociception much faster, within 1 min, presumably through the activation of a nongenomic pathway and in a manner that depends on an immediate response to fast activation/deactivation of local aromatase activity. This emergent central and rapid paracrine mechanism might permit instantaneous and segment-specific changes in pain sensitivity; it draws new interesting perspectives for the study of the estrogenic control of pain, thus far limited to the classical view of slow genomic changes in pain, depending on peripheral estrogens. The expression of aromatase in the spinal cord in other species and in other central nociception-related areas is also briefly discussed.

Is brain estradiol a hormone or a neurotransmitter?

Mounting evidence indicates that, besides their well-known hormonal mode of action at the genetic level, estrogens such as 17beta-estradiol also influence brain function by direct effects on neuronal membranes. Experimentally induced rapid changes in estradiol bioavailability in the brain have been shown to alter the expression of male sexual behavior significantly within minutes--probably too quickly to be accounted for by conventional genetic mechanisms. In parallel, recent studies indicate that aromatase, the enzyme that converts testosterone to estradiol in the brain, is expressed in presynaptic terminals and modulated within minutes by Ca(2+)-dependent phosphorylation. In this article, we develop the hypothesis that brain estrogens display many, if not all, functional characteristics of neuromodulators or even neurotransmitters.

Rapid changes in production and behavioral action of estrogens

It is well established that sex steroid hormones bind to nuclear receptors, which then act as transcription factors to control brain sexual differentiation and the activation of sexual behaviors. Estrogens locally produced in the brain exert their behavioral effects in this way but mounting evidence indicates that estrogens also can influence brain functioning more rapidly via non-genomic mechanisms. We recently reported that, in Japanese quail, the activity of preoptic estrogen synthase (aromatase) can be modulated quite rapidly (within minutes) by non-genomic mechanisms, including calcium-dependent phosphorylations. Behavioral studies further demonstrated that rapid changes in estrogen bioavailability, resulting either from a single injection of a high dose of estradiol or from the acute inhibition of aromatase activity, significantly affect the expression of both appetitive and consummatory aspects of male sexual behavior with latencies ranging between 15 and 30 min. Together these data indicate that the bioavailability of estrogens in the brain can change on different time-scales (long- and short-term) that match well with the genomic and non-genomic actions of this steroid and underlie two complementary mechanisms through which estrogens modulate behavior. Estrogens produced locally in the brain should therefore be considered not only as neuroactive steroids but they also display many (if not all) functional characteristics of neuromodulators and perhaps neurotransmitters.

Recent advances in behavioral neuroendocrinology: insights from studies on birds

Ever since investigations in the field of behavioral endocrinology were hatched with experiments on roosters, birds have provided original insights into issues of fundamental importance for all vertebrate groups. Here we focus on more recent advances that continue this tradition, including (1) environmental regulation of neuroendocrine and behavioral systems, (2) steroidogenic enzyme functions that are related to intracrine processes and de novo production of neurosteroids, and (3) hormonal regulation of neuroplasticity. We also review recent findings on the anatomical and functional organization of steroid-sensitive circuits in the basal forebrain and midbrain. A burgeoning body of data now demonstrates that these circuits comprise an evolutionarily conserved network, thus numerous novel insights obtained from birds can be used (in a relatively straightforward manner) to generate predictions for other taxa as well. We close by using birdsong as an example that links these areas together, thereby highlighting the exceptional opportunities that birds offer for integrative studies of behavioral neuroendocrinology and behavioral biology in general.

Preoptic aromatase modulates male sexual behavior: slow and fast mechanisms of action

In many species, copulatory behavior and appetitive (anticipatory/motivational) aspects of male sexual behavior are activated by the action in the preoptic area of estrogens locally produced by testosterone aromatization. Estrogens bind to intracellular receptors, which then act as transcription factors to activate the behavior. Accordingly, changes in aromatase activity (AA) result from slow steroid-induced modifications of enzyme transcription. More recently, rapid nongenomic effects of estrogens have been described and evidence has accumulated indicating that AA can be modulated by rapid (minutes to hour) nongenomic mechanisms in addition to the slower transcriptional changes. Hypothalamic AA is rapidly down-regulated in conditions that enhance protein phosphorylation, in particular, increases in the intracellular calcium concentration, such as those triggered by neurotransmitter (e.g., glutamate) activity. Fast changes in brain estrogens can thus be caused by aromatase phosphorylation as a result of changes in neurotransmission. In parallel, recent studies demonstrate that the pharmacological blockade of AA by specific inhibitors rapidly (within 15-45 min) down-regulates motivational and consummatory aspects of male sexual behavior in quail while injections of estradiol can rapidly increase the expression of copulatory behavior. These data collectively support an emerging concept in neuroendocrinology, namely that estrogen, locally produced in the brain, regulates male sexual behavior via a combination of genomic and nongenomic mechanisms. Rapid and slower changes of brain AA match well with these two modes of estrogen action and provide temporal variations in the estrogen's bioavailability that can support the entire range of established effects for this steroid.

Rapid regulation of pain by estrogens synthesized in spinal dorsal horn neurons

In addition to exerting genomic actions via nuclear receptors within hours to days, estrogens also regulate neuronal activity much faster (within seconds) by activating neuronal membrane receptors coupled to intracellular second-messenger pathways. To date, the origin of estrogens inducing rapid effects in the brain remains unclear, although it is often ascribed to the gonads. We report here that an acute blockade of the endogenous synthesis of estrogens in the quail spinal dorsal horn markedly reduced, within 1 min, the behavioral responsiveness to a thermal painful stimulus. Similar rapid effects in the opposite direction were induced by estradiol. This finding identifies a new paracrine and nongenomic mechanism for the regulation of pain by estrogens. Such regulation was assumed previously to result only from slow genomic actions of estrogens arising from the ovaries. Also, quite importantly, this finding suggests that the numerous rapid nongenomic effects of estrogens in the CNS could depend on their immediate local production by the enzyme aromatase, independently from the gonads.

Testosterone reduces responsiveness to nociceptive stimuli in a wild bird

The hormone testosterone (T) is involved in the control of aggressive behavior in male vertebrates. T enhances the frequency and intensity of aggressive behaviors during competitive interactions among males. By promoting high-intensity aggression, T also increases the risk of injury and presumably the perception of painful stimuli. However, perception of painful stimuli during fights could counteract the expression of further aggressive behavior. We therefore hypothesize that one function of T during aggressive interactions is to reduce nociception (pain sensitivity). Here, we experimentally document that T indeed reduces behavioral responsiveness to a thermal painful stimulus in captive male house sparrows (Passer domesticus). Skin nociception was quantified by foot immersion into a hot water bath, a benign thermal stimulus. Males treated with exogenous testosterone left their foot longer in hot water than control birds. Conversely, males in which the physiological actions of testosterone were pharmacologically blocked withdrew their foot faster than control birds. Testosterone might exert its effects on pain sensitivity through conversion into estradiol in the dorsal horn of the spinal cord. Decreased nociception during aggressive encounters may promote the immediate and future willingness of males to engage in high-intensity fights.

Receptors to steroid hormones and aromatase are expressed by cultured motoneurons but not by glial cells derived from rat embryo spinal cord

The aim of this study was to examine the expression of aromatase and receptors to steroid hormones in cultured motoneurons (MNs). We first developed an original method for obtaining rat MN cultures. Dissociated E15 rat spinal cords were purified using metrizamide and bovine serum albumin density gradients, and cells were then seeded on the culture substratum. We optimized the culture parameters and found that simple addition of rat muscle extract (ME) and conditioned culture medium (CM) from glial cell lines (GCL) derived from spinal cord were sufficient to obtain almost pure MN cultures. MNs were characterized by the presence of specific MN markers and electrophysiology. MNs could be kept alive for 2 weeks. We demonstrate that ME and CM are essential for MN development and survival respectively. Immunocytochemistry and aromatase activity assay indicated the presence of androgen and estrogen receptors as well as aromatase in MNs but not in GCL. This is the first report demonstrating the presence of both female and male sex hormone receptors and a key enzyme in steroid hormone metabolism in MNs and its absence in GCL, at least in our culture conditions. This in vitro model appears to be valuable for elucidating the impact of the sex hormone circuit in neuronal maturation. The relevance of this model for the comprehension of neurodegenerative diseases is discussed.