Expression Of Male Sexual Behavior Research Articles

Estrogen-dependent sex difference in microglia in the developing brain of Japanese quail (Coturnix japonica).

Brain sexual differentiation is a developmental process leading to the establishment of stable neural sex differences. In birds and rodents, this process is largely driven by estrogens during a critical period. In rodents, estrogens drive the masculinization of the brain, a process that partly depends on microglia. In contrast, in birds, estrogens produced by females induce demasculinization, but whether microglia are involved in this process is unknown. This study assessed whether microglial number, morphology, and/or activity differ between the sexes in selected regions of the developing quail brain and whether they are influenced by estrogens. We found a robust female-biased sex difference in microglial numbers between embryonic day 9 and 12 in the medial preoptic nucleus (POM), a key region for the expression of male sexual behavior. This difference relies on estrogens produced during the sensitive period. Although most embryonic microglia express iNOS, the expression of iNOS in individual microglia does not differ between sexes. Finally, microglial number and the expression of iNOS were not affected by the microglia inhibitor minocycline. Together, these results revealed an estrogen-dependent sex difference in microglia during the critical period for the sexual differentiation of the quail brain. This difference mirrors the different role of estrogens in the development of birds and rodents and suggests a role for microglia in the sexual differentiation of the brain of birds, as in rodents, thus supporting the hypothesis of a conserved role of the neuroimmune system in the organization of the brain by estrogens.

Test of four hypotheses to explain the function of overmarking in foals of four equid species.

Overmarking occurs when one individual places its scent mark directly on top of the scent mark of another individual. Although it is almost ubiquitous among terrestrial mammals, we know little about the function of overmarking. In addition, almost all studies on mammalian overmarking behaviour dealt with adult individuals. Reports on this behaviour in juveniles are extremely rare, yet may elucidate the function of this behaviour. We tested four mutually non-exclusive hypotheses which might explain this behaviour in juveniles: (1) conceal the individual's scent identity, (2) announcement of association with other group members, especially the mother-i.e., sharing identity with the mother, (3) to prevent the next conception of the mother, i.e., parent-offspring conflict, and (4) an early expression of male sexual behaviour. We observed 43 foals (out of 108 individuals) from all African equid species (Equus africanus, E. grevyi, E. quagga, E. zebra) in five zoos. In total, we recorded 3340 eliminations; 260 of these events were overmarked by 38 individual foals representing all species. This represents one of the highest rates of overmarking ever recorded by mammalian juveniles. Foals of all species except African wild ass overmarked the mother more often than another herdmate: with male foals overmarked at a higher rate than female foals. Mothers preferred to overmark foals, but not exclusively their own foal. Our results provide support for the hypotheses that overmarking serves to share identity between foal and mother, and that it is an early expression of male sexual behaviour.

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.

The Preoptic Area and the RFamide-Related Peptide Neuronal System Gate Seasonal Changes in Chemosensory Processing.

Males of many species rely on chemosensory information for social communication. In male Syrian hamsters (Mesocricetus auratus), as in many species, female chemosignals potently stimulate sexual behavior and a concurrent, rapid increase in circulating luteinizing hormone (LH) and testosterone (T). However, under winter-like, short-day (SD) photoperiods, when Syrian hamsters are reproductively quiescent, these same female chemosignals fail to elicit behavioral or hormonal responses, even after T replacement. It is currently unknown where in the brain chemosensory processing is gated in a seasonally dependent manner such that reproductive responses are only displayed during the appropriate breeding season. The goal of the present study was to determine where this gating occurred by identifying neural loci that respond differentially to female chemosignals across photoperiods, independent of circulating T concentrations. Adult male Syrian hamsters were housed under either long-day (LD) (reproductively active) or SD (reproductively inactive) photoperiods with half of the SD animals receiving T replacement. Animals were exposed to either female hamster vaginal secretions (FHVSs) diluted in mineral oil or to vehicle, and the activational state of chemosensory processing centers and elements of the neuroendocrine reproductive axis were examined. Components of the chemosensory pathway upstream of hypothalamic centers increased expression of FOS, an indirect marker of neuronal activation, similarly across photoperiods. In contrast, the preoptic area (POA) of the hypothalamus responded to FHVS only in LD animals, consistent with its role in promoting expression of male sexual behavior. Within the neuroendocrine axis, the RF-amide related peptide (RFRP), but not the kisspeptin neuronal system responded to FHVS only in LD animals. Neither response within the POA or the RFRP neuronal system was rescued by T replacement in SD animals, mirroring photoperiodic regulation of reproductive responses. Considering the POA and the RFRP neuronal system promote reproductive behavior and function in male Syrian hamsters, differential activation of these systems represents a potential means by which photoperiod limits expression of reproduction to the appropriate environmental context.

Handling stress increases expression of male sexual behaviour in an amphibian with an explosive mating strategy

AbstractExposure to stressors typically suppresses reproductive processes, including the expression of sexual behaviour. However, many animal species breed in extreme conditions where opportunities to mate in a given season are limited. In such species, stress‐induced suppression of reproduction would be maladaptive, especially if they also have short lifespans. Species with longer lifespans, but still limited mating opportunities in a given season, may be able to withstand a few missed breeding opportunities. We tested whether capture and handling would suppress male sexual behaviour in spotted salamanders (Ambystoma maculatum), a species with an explosive mating strategy but a relatively long lifespan. Males were captured as they migrated to breeding sites and were handled in a manner known to induce an increase in plasma corticosterone, a component of the stress response. Thirty min later, each male was paired with a freshly captured female for 60 min. Handled males deposited significantly more spermatophores than did control males. The mechanism for the increased sexual behaviour is unknown but may involve handling‐induced changes in stress hormones or reproductive hormones. Regardless of the mechanism whereby handling increased male sexual behaviour, our results support the hypothesis that stressors do not suppress male mating behaviour in a long‐lived species with limited opportunities to mate.

Hormone replacement with 17β-estradiol plus dihydrotestosterone restores male sexual behavior in rats treated neonatally with clomipramine

Male sexual behavior (MSB) in rodents, in both its consummatory and motivational components, is regulated by hormones such as testosterone, 17β-estradiol and 5-α-dihydrotestosterone. In experiments, neonatal treatment with clomipramine (CMI; a serotonin reuptake inhibitor) reproduces some of the signs of depression in adult age, including reduced sexual behavior manifested in a lower percentage of subjects that mount, intromit and ejaculate, although their testosterone levels were not altered. However, the effect of this treatment on estrogen levels and the consequences of hormone substitution using 17β-estradiol and 5-α-dihydrotestosterone on the expression of male sexual behavior are still unknown. Therefore, the objective of the present study was to analyze the effect of neonatal treatment with CMI on plasma testosterone and 17β-estradiol levels, and the role of testosterone, 17β-estradiol and 5-α-dihydrotestosterone in altering the consummatory and motivational components of sexual behavior in male rats. To this end, it analyzed the copulatory parameters and sexual incentive motivation (SIM) of rats treated with CMI under two conditions: basal and post-hormone replacements. Neonatal treatment with CMI did not affect plasma testosterone or 17β-estradiol concentrations, but did decrease both the consummatory component and sexual motivation according to the results of the SIM test. These aspects were recovered after administering 17β-estradiol +5-α-dihydrotestosterone, but not testosterone.

Dopamine receptors play distinct roles in sexual behavior expression of rats with a different sexual motivational tone.

Dopamine (DA) plays a central role in the expression of male sexual behavior. The effects of DA-enhancing drugs on copulation seem to vary depending on the dose of the agonist used, the type of DA receptor activated, and the sexual condition of the animals. The aim of the present study was to carry out a systematic analysis of the effects of dopaminergic agonists on the expression of male sexual behavior by sexually competent rats in different sexual motivational states, that is when sexually active (sexually experienced) and when temporarily inhibited (sexually exhausted). To this end, the same doses of the nonselective DA receptor agonist apomorphine, the selective D2-like DA receptor agonist quinpirole, and the selective D1-like DA receptor agonist SKF38393 were injected intraperitoneally to sexually experienced or sexually exhausted male rats and their sexual behavior was recorded. Low apomorphine doses induced expression of sexual behavior in sexually satiated rats, but only reduced the intromission latency of sexually experienced rats. SKF38393 facilitated the expression of sexual behavior by sexually exhausted rats, but not that of sexually experienced males and quinpirole did not exert an effect in both types of animal. In line with these results, the apomorphine-induced reversal of sexual exhaustion was blocked by the D1-like receptor antagonist SCH23390. The data suggest that DA receptors play distinct roles in the expression of sexual behavior by male rats depending on their motivational state and that activation of D1-like receptors promotes the expression of sexual behavior in satiated rats.

Sexual experience influences mating-induced activity in nitric oxide synthase-containing neurons in the medial preoptic area

Nitric oxide (NO) acts in the medial preoptic area (mPOA) of the hypothalamus to facilitate the expression of male sexual behavior and has also been widely implicated in mechanisms of experience, learning, and memory. Using immunohistochemistry for Fos, as a marker for neural activity, and nitric oxide synthase (NOS), the enzyme that catalyzes the production of nitric oxide (NO), we examined whether sexual activity and sexual experience influence Fos co-expression in NOS-containing neurons in the mPOA of male rats. Consistent with previous findings, results indicate that mating increased activity in the mPOA, and that sexual experience facilitated the expression of sexual behaviors, together with increased mating-induced Fos and NOS in the mPOA. Results also indicate that mating increased co-expression of Fos in NOS-containing neurons, and that this increase was highest in animals undergoing their first sexual encounter, indicating that initial sexual experience increases NO production in the mPOA of male rats.

Sexually dimorphic nuclei in the spinal cord control male sexual functions.

Lower spinal cord injuries frequently cause sexual dysfunction in men, including erectile dysfunction and an ejaculation disorder. This indicates that the important neural centers for male sexual function are located within the lower spinal cord. It is interesting that the lumbar spinal segments contain several neural circuits, showing a clear sexually dimorphism that, in association with neural circuits of the thoracic and sacral spinal cord, are critical in expressing penile reflexes during sexual behavior. To date, many sex differences in the spinal cord have been discovered. Interestingly, most of these are male dominant. Substantial evidence of sexually dimorphic neural circuits in the spinal cord have been reported in many animal models, but major issues remain unknown. For example, it is not known how the different circuits cooperatively function during male sexual behavior. In this review, therefore, the anatomical and functional significance of the sexually dimorphic nuclei in the spinal cord corresponding to the expression of male sexual behavior is discussed.

Elevated testosterone is required for male copulatory behavior and aggression in Madagascar ground gecko (Paroedura picta)

Elevated levels of gonadal androgens are often required for the expression of male-specific behavioral and morphological traits in all classes of vertebrates, including reptiles. Here, we tested the role of male gonadal androgens in the control of male sexual behavior, aggressive behavior, and size of the hemipenes in the gecko Paroedura picta. We performed hormonal manipulations involving castration with and without testosterone (T) replacement in males and application of exogenous T and ovariectomy in females. Castration suppressed sexual behavior and hemipenes size in males, and these effects were fully rescued by exogenous T. Sexual behavior and growth of the hemipenes were masculinized by male-typical levels of T in females, while ovariectomized females retained female-typical expression of these traits. These results indicate that the development of male sexual behavior in adult females does not require early or pubertal organization. Elevated T increased the likelihood of aggressive behavior directed toward a male intruder, but aggression occurred only rarely. Elevated T is necessary and sufficient for enlargement of the hemipenes and the expression of male sexual behavior in both males and females of Paroedura picta. In contrast to sexual behavior, the expression of aggressive behavior is apparently more dependent on other factors in addition to T itself.

Pro-sexual effects of Turnera diffusa Wild (Turneraceae) in male rats involves the nitric oxide pathway

Ethnopharmacological relevanceTurnera diffusa Wild has been used in folk medicine by its aphrodisiac and tranquilizing properties. Previously we experimentally showed the aphrodisiac effect of a chemically characterized aqueous extract of Turnera diffusa in male rats. However, the mechanism of action underlying such effects has not been studied. Study aimsAs part of our systematic studies of pharmacological properties of Turnera diffusa, we aimed to analyze whether the increased sexual motivation and the augmented sexual performance of sexually sluggish (SL) male rats treated with Turnera diffusa involves the NO pathway. Additionally we analyzed whether such effects were exerted at the level of the brain or the spinal cord. Finally, anxiety levels and ambulatory activity were also evaluated. Material and methodsTurnera diffusa (10–40mg/kg) and sildenafil citrate (10mg/kg) with or without a nonspecific inhibitor of NO synthase, Nω-nitro-l-arginine methyl esther (l-NAME, 12.5mg/kg) were evaluated in SL rats, in a standard sexual behavior test and in the fictive ejaculation model in spinal cord transected and urethane-anaesthetized SL rats. Anxiety levels or ambulation were assessed in the burying behavior and open-field tests. ResultsTurnera diffusa and sildenafil (both at 10mg/kg) facilitated expression of male sexual behavior by shortening mainly ejaculation latency. Treatments also facilitated the number of discharges in the ejaculatory motor pattern as well as the number of ejaculatory motor patterns and its associated penile erections. l-NAME prevented the pro-sexual effects of treatments on both experimental models. Besides, the extract of Turnera diffusa (10mg/kg) produced an anxiolytic-like effect in male rats without affecting ambulation. ConclusionsFindings from the present work support the notion that pro-sexual effect of the aqueous extract of Turnera diffusa in rats involves the participation of NO pathway, mainly at central level. The anxiolytic-like effect of Turnera diffusa is an advantage to its use for improving sexual performance.

Epigenetic mechanisms are involved in sexual differentiation of the brain

Sexual differentiation of the brain can be considered as a process during which effects of sex steroid hormones secreted during early development is maintained into adulthood. Epigenetic regulation is emerging as a potentially important mechanism of conveyance of long-lasting effects of the hormonal and environmental milieu in the developing brain. Evidence has accumulated to show that epigenetic regulation is involved in the control of sexual differentiation of the brain. In the preoptic area (POA), which is important for male sexual behavior, histones associated with the estrogen receptor (ER) α and aromatase (Arom) gene promoters are differentially acetylated between the sexes, and two subtypes of histone deacetylase (HDAC2 and 4) are associated with the same promoters at higher frequencies in males in the early postnatal period. Since ERα and Arom are essential genes in masculinization of the brain, these findings suggest that histone deacetylation in the early postnatal period is involved in masculinization of the brain. Indeed, inhibition of HDAC activity in males during this period abrogates brain masculinization: structural sexual dimorphism of the bed nucleus of the stria terminalis is eliminated and expression of male sexual behavior is reduced in adulthood. Previous reports have demonstrated that ERα gene expression in the POA is higher in females during the developmental and pubertal periods and in adulthood, indicating that sexually dimorphic ERα expression that appears in early postnatal development is maintained until adulthood by epigenetic programming. The ERα promoter is also more sparsely methylated in females, with an inverse correlation with ERα expression. In addition to the hormonal effect, the amount of maternal care received during postnatal development has a lasting effect on ERα expression mediated by DNA methylation of its promoter. Taken together, these results suggest that epigenetic mechanisms play a central role in the transduction and maintenance of early hormonal and social cues to organize sexually differentiated brain functions.

Oxytocin in the medial preoptic area facilitates male sexual behavior in the rat

Oxytocin (OT) is a versatile neuropeptide that is involved in a variety of mammalian behaviors, and its role in reproductive function and behavior has been well established. The majority of pharmacological studies of the effects of OT on male sexual behavior have focused on the paraventricular nucleus (PVN), ventral tegmental area (VTA), hippocampus, and amygdala. Less attention has been given to the medial preoptic area (MPOA), a major integrative site for male sexual behavior. The present study investigated the effects of intra-MPOA administration of OT and (d(CH2)51, Tyr(Me)2, Thr4, Orn8, Tyr-NH29)-vasotocin, an OT antagonist (OTA), on copulation in the male rat. The relationship between OT receptor (OTR) binding levels in the MPOA and sexual efficiency was also explored. Microinjection of OT into the MPOA facilitated copulation in sexually experienced male rats, whereas similar injections of an OTA inhibited certain aspects of copulation but had no significant effect on locomotor activity in an open field. Contrary to expectation, sexually efficient males had lower levels of OTR binding in the rostral MPOA compared to inefficient animals. The present data suggest that OT activity in the MPOA is not necessary for the expression of male sexual behavior but is sufficient to facilitate copulatory behaviors and improve sexual efficiency in sexually experienced male rats. These data also suggest that OTR activity in the MPOA stimulates anogenital investigation, facilitates the initiation of copulation, and plays a role in the sensitization effect of the first ejaculation on subsequent ejaculations.

Distribution of Two Isozymes of 5α-Reductase in the Brains of Adult Male and Female Green Anole Lizards

The 5α-reductase (5αR) enzyme converts testosterone to 5α-dihydrotestosterone. This local metabolism within the brain is important for the full expression of male sexual behavior in many species, including green anole lizards. Two isozymes of 5αR exist and little is known about their specific distributions. We conducted in situ hybridization for both isozymes in intact male and female green anole brains during the breeding (BS) and non-breeding (NBS) seasons. 5αR1 mRNA was only detected in the brainstem, while 5αR2 was expressed in specific areas throughout the brain. As our primary interest was evaluating the potential role of 5αR in forebrain regulation of reproductive behavior, we quantified 5αR2 expression in the preoptic area, amygdala (AMY), and ventromedial hypothalamus (VMH). More 5αR2 cells were detected during the NBS than BS in the AMY, and the density of these cells was greater in females than males. In the VMH, the right side contained more 5αR2 cells than the left, an effect driven by a lateralized increase in the NBS. These data expand understanding of the distribution and potential roles of both isozymes in the adult brain, and differences in expression patterns between mammals and birds suggest that they may have been co-opted for different functions later in evolution.

The differential expression of male sexual behavior in the Lewis, Fischer and Sprague-Dawley rat strains

Several indices of sexual behavior were characterized for male rats of the Lewis (LEW), Sprague-Dawley (SD) and Fischer (F344) strains. Males were permitted access to a receptive female of their own strain for five 50-min sessions every fifth day. LEW males emitted longer latencies (first mount, intromission, ejaculation and post-ejaculatory interval) and lower frequencies (mounts, intromissions, ejaculations, intromission ratio) relative to F344 males, with the SD strain in general, lying intermediate. Following this assessment, males of each strain were randomly assigned to females of their own or another strain to ascertain whether differences were due to differential female receptivity. Although there were no significant differences during this phase indicative of differential receptivity, the sample size may preclude a definitive interpretation of such results. The data were discussed in the context of other behavioral differences between the strains and the possible neurobiological mechanisms underlying these differences.

The differential expression of male sexual behavior in the Lewis, Fischer and Sprague‐Dawley rat strains

Lewis (LEW) and Fischer (F344) rats differ markedly in reactivity to drugs of abuse and mesolimbic dopamine activity, and interestingly sexual behavior is thought to be mediated in part by some common substrates. The following experiment examined sexual behavior of these strains (and outbred Sprague‐Dawley (SD) rats) to assess possible strain differences. Males were given repeated access to a receptive female of their respective strain during which sexual behavior was coded. Males were then exposed to a receptive LEW, F344 or SD female to assess if male behavior was a function of female receptivity. LEW males exhibited fewer mounts and intromissions (frequency and/or percent of subjects) than the F344 and SD strains (which never differed). Latency to first mount or intromission was similar for the F344 and SD strains but both were shorter than the LEW males (an index of sexual motivation). Ejaculation frequency and percent of subjects displaying ejaculation were different among strains (F344> SD>LEW). No LEW male ever ejaculated. For all males, there was no significant change in mounts, intromissions or ejaculations when given access to females of different strains, indicating the initial differences were not a result of strain differences in receptivity. These findings are consistent with a growing literature describing strain differences in motivated behaviors.Supported by a Mellon Foundation grant to ALR.

Postnatal treatment of rats with beta-adrenergic agonists or antagonists influences differentiation of sexual brain functions.

Sexual differentiation of the brain seems to be influenced by postnatal interaction of gonadal steroids with neurotransmitter systems, in particular the adrenergic system. Stimulation or inhibition of alpha-adrenergic receptors during early postnatal development had previously been shown to influence steroid-induced sexual differentiation of brain functions. In the present study newborn male and female rats were treated either with salbutamol, a selective beta 2-adrenergic receptor agonist, isoprenaline, a general beta-adrenergic receptor agonist, or with alprenolol, a general beta-adrenergic receptor antagonist. In adulthood the female animals were ovariectomized and were tested for the capacity to show an LH-surge response and female sexual behaviour after priming with estradiol benzoate (EB) and progesterone (P). Male animals were tested for expression of male sexual behavior and, after gonadectomy and priming with EB + P, for the capacity to show female lordosis behavior. In summary, our results suggest that activation or inhibition of beta-adrenergic receptors during postnatal development permanently impairs the responsiveness of the center for cyclic gonadotropin release to gonadal steroids in female rats and impairs the expression of ejaculatory behavior in male rats. A slight stimulatory effect on the expression of female lordosis behavior was observed in male and female rats after postnatal activation of beta-adrenergic receptors.

Are rapid changes in gonadal testosterone release involved in the fast modulation of brain estrogen effects?

Estradiol facilitates the expression of male sexual behavior in Japanese quail within a few minutes. These rapid behavioral effects of estradiol could result from rapid changes in its local production in the preoptic area by aromatase, the enzyme converting testosterone into estradiol. Alternatively, aromatase activity may remain constant but fluctuations of local estradiol production could arise from rapid changes in the concentration of the enzymatic substrate, namely testosterone. Rapid increases of circulating testosterone levels have been observed in males of various species following social encounters. Surprisingly, in quail, the interaction with a female seems to result in a decrease in circulating testosterone levels. However, in that study conducted in quail, the samples were collected at intervals longer than the recently observed rapid effects of estradiol on sexual behavior. In the present study we investigated whether plasma testosterone concentrations fluctuate on a shorter time-frame. Eleven male were tested 5 min before and 5, 15 or 30 min after being allowed to have visual access to a female or to copulate with a female for 5 min. Both types of interactions resulted in a significant decline in circulating testosterone levels at latencies as short as 5 min. These data demonstrate that the decrease in testosterone levels is initiated shortly after sexual encounters. Because visual interactions with a female did not result in a rapid increase in testosterone concentrations, these findings rule out the possibility that a rapid rise in circulating testosterone levels participates in the rapid increase in brain estrogen synthesis and its facilitatory effects on copulatory behavior.

Behavioral Effects of Brain‐derived Estrogens in Birds

In birds as in other vertebrates, estrogens produced in the brain by aromatization of testosterone have widespread effects on behavior. Research conducted with male Japanese quail demonstrates that effects of brain estrogens on all aspects of sexual behavior, including appetitive and consummatory components as well as learned aspects, can be divided into two main classes based on their time course. First, estrogens via binding to estrogen receptors regulate the transcription of a variety of genes involved primarily in neurotransmission. These neurochemical effects ultimately result in the activation of male copulatory behavior after a latency of a few days. Correlatively, testosterone and its aromatized metabolites increase the transcription of the aromatase mRNA, resulting in an increased concentration and activity of the enzyme that actually precedes behavioral activation. Second, recent studies with quail demonstrate that brain aromatase activity can also be modulated within minutes by phosphorylation processes regulated by changes in intracellular calcium concentration, such as those associated with glutamatergic neurotransmission. The rapid upregulations or downregulations of brain estrogen concentration (presumably resulting from these changes in aromatase activity) affect, by nongenomic mechanisms with relatively short latencies (frequency increases or decreases respectively within 10-15 min), the expression of male sexual behavior in quail and also in rodents. Brain estrogens thus affect behavior on different time scales by genomic and nongenomic mechanisms similar to those of a hormone or a neurotransmitter.

Estradiol, a key endocrine signal in the sexual differentiation and activation of reproductive behavior in quail

In Japanese quail, estrogen's effects on sexual behavior can be divided into three classes based on the underlying mechanisms and time-course of action and release. During embryonic life, the embryonic ovary secretes large amounts of estrogens. In contrast to what is observed in mammals where sexual differentiation essentially proceeds via masculinization of the males, in quail, females are demasculinized by their endogenous ovarian estrogens, an effect that can be blocked by injection of an aromatase inhibitor and mimicked in male embryos by an injection of estradiol. In adulthood, testosterone secreted by the testes is converted into estrogens by the preoptic aromatase. Locally produced estrogens activate male sexual behavior largely through the activation of estrogen receptors resulting in the transcription of a variety of genes, including brain aromatase (genomic effect). Both changes in estrogen production and action are observed within latencies ranging from a few hours to a few days, and are completely reversible. Additionally, brain aromatase activity can be modulated within minutes by calcium-dependent phosphorylations, triggered by variations in glutamatergic neurotransmission. These rapid changes in aromatase activity affect with relatively short latencies (10-15 min) the expression of male sexual behavior in quail and also in mice. Overall, the effects of estrogens on sexual behavior can thus be categorized into three classes: organizational (irreversible genomic action during ontogeny), activational (reversible genomic action during adulthood) and rapid nongenomic effects. Rapid and slower changes in brain aromatase activity match well with the two modes of estrogen action on behavior and provide temporal variations in the estrogens' bioavailability that should be able to support the entire range of established effects for this steroid.