Implication of testosterone metabolism in the control of the sexually dimorphic nucleus of the quail preoptic area

The role of sex chromosomes and sex hormones in vocal learning systems

Vocal learning is the ability to imitate and modify sounds through auditory experience, a rare trait found in only a few lineages of mammals and birds. It is a critical component of human spoken language, allowing us to verbally transmit speech repertoires and knowledge across generations. In many vocal learning species, the vocal learning trait is sexually dimorphic, where it is either limited to males or present in both sexes to different degrees. In humans, recent findings have revealed subtle sexual dimorphism in vocal learning/spoken language brain regions and some associated disorders. For songbirds, where the neural mechanisms of vocal learning have been well studied, vocal learning appears to have been present in both sexes at the origin of the lineage and was then independently lost in females of some subsequent lineages. This loss is associated with an interplay between sex chromosomes and sex steroid hormones. Even in species with little dimorphism, like humans, sex chromosomes and hormones still have some influence on learned vocalizations. Here we present a brief synthesis of these studies, in the context of sex determination broadly, and identify areas of needed investigation to further understand how sex chromosomes and sex steroid hormones help establish sexually dimorphic neural structures for vocal learning.

Steroid Transport, Local Synthesis, and Signaling within the Brain: Roles in Neurogenesis, Neuroprotection, and Sexual Behaviors

Sex steroid hormones are synthesized from cholesterol and exert pleiotropic effects notably in the central nervous system. Pioneering studies from Baulieu and colleagues have suggested that steroids are also locally-synthesized in the brain. Such steroids, called neurosteroids, can rapidly modulate neuronal excitability and functions, brain plasticity, and behavior. Accumulating data obtained on a wide variety of species demonstrate that neurosteroidogenesis is an evolutionary conserved feature across fish, birds, and mammals. In this review, we will first document neurosteroidogenesis and steroid signaling for estrogens, progestagens, and androgens in the brain of teleost fish, birds, and mammals. We will next consider the effects of sex steroids in homeostatic and regenerative neurogenesis, in neuroprotection, and in sexual behaviors. In a last part, we will discuss the transport of steroids and lipoproteins from the periphery within the brain (and vice-versa) and document their effects on the blood-brain barrier (BBB) permeability and on neuroprotection. We will emphasize the potential interaction between lipoproteins and sex steroids, addressing the beneficial effects of steroids and lipoproteins, particularly HDL-cholesterol, against the breakdown of the BBB reported to occur during brain ischemic stroke. We will consequently highlight the potential anti-inflammatory, anti-oxidant, and neuroprotective properties of sex steroid and lipoproteins, these latest improving cholesterol and steroid ester transport within the brain after insults.

Steroid receptor coactivator 2 modulates steroid-dependent male sexual behavior and neuroplasticity in Japanese quail (Coturnix japonica)

Steroid receptor coactivators are necessary for efficient transcriptional regulation by ligand-bound nuclear receptors, including estrogen and androgen receptors. Steroid receptor coactivator-2 (SRC-2) modulates estrogen- and progesterone-dependent sexual behavior in female rats but its implication in the control of male sexual behavior has not been studied to our knowledge. We cloned and sequenced the complete quail SRC-2 transcript and showed by semi-quantitative PCR that SRC-2 expression is nearly ubiquitous, with high levels of expression in the kidney, cerebellum and diencephalon. Real-time quantitative PCR did not reveal any differences between intact males and females the medial preoptic nucleus (POM), optic lobes and cerebellum. We next investigated the physiological and behavioral role of this coactivator using in vivo antisense oligonucleotide techniques. Daily injections in the third ventricle at the level of the POM of locked nucleic acid antisense targeting SRC-2 significantly reduced the expression of testosterone-dependent male-typical copulatory behavior but no inhibition of one aspect of the appetitive sexual behavior was observed. The volume of POM, defined by aromatase-immunoreactive cells, was markedly decreased in animals treated with antisense as compared with controls. These results demonstrate that SRC-2 plays a prominent role in the control of steroid-dependent male sexual behavior and its associated neuroplasticity in Japanese quail.

Seasonal aromatase activity in the brain of the male red-sided garter snake

We investigated regional and seasonal variations in neural aromatase activity (AA), the enzyme that converts androgens into estrogens, to examine a possible indirect role of testosterone (T) in mediating spring reproductive behavior of red-sided garter snakes, a species exhibiting a dissociated reproductive pattern. Neural AA in male snakes varied significantly among brain regions. Additionally, there were significant interactions between brain region and season. In the spring, actively courting males had greater AA in the olfactory region (O) compared to the septum/anterior-hypothalamus preoptic area (S/AHPOA), nucleus sphericus (NS) and midbrain (Mb). Fall animals collected as they returned to the den prior to winter dormancy had significantly greater AA in the S/AHPOA compared to all other regions. These findings were consistent using either regional (gross) dissection or punch microdissection, which allowed us to separate the S and AHPOA. There were no significant differences in AA production between the S and AHPOA. This study provides the first documentation of seasonal and regional variations in AA in a snake brain and suggests that aromatization of androgens may play a role in regulating reproduction in red-sided garter snakes. During spring mating, elevated AA in the O may activate pathways essential for detection of courtship pheromones, while increased AA in the S and AHPOA of fall animals suggests that circulating androgens play an indirect role in programming critical neural pathways involved in reproduction. Thus, as in many other vertebrates, estrogenic metabolites of testosterone may be a critical hormonal component regulating reproductive behavior in this dissociated breeder.

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.

Sexual behavior activates the expression of the immediate early genes c-fos and Zenk (egr-1) in catecholaminergic neurons of male Japanese quail

We analyzed the expression of the immediate early genes c-fos and Zenk (egr-1) in the brain of male quail that were gonadally intact (I) or castrated and treated (CX+T) or not (CX) with testosterone and had been exposed for 60 min either to a sexually mature female (F), or to an empty arena (EA) or were left in their home cage (HC). Alternate sections in the brains collected 90 min after the start of behavioral interactions were stained by immunocytochemistry for the proteins FOS or ZENK alone or in association with tyrosine hydroxylase (TH), a marker of catecholaminergic neurons. C-fos and Zenk expression was statistically increased in six brain areas of sexually active birds (I+F, CX+T+F) compared with controls (CX+F, CX+T+EA, CX+T+HC), i.e. the preoptic area, bed nucleus striae terminalis, arcopallium, nucleus intercollicularis, periaqueductal gray and the ventral tegmental area. Interestingly, c-fos and Zenk expression was high in the nucleus intercollicularis, a midbrain vocal control nucleus, of I+F and CX+T+F birds that displayed copulatory behavior but emitted few crows but not in the nucleus intercollicularis of CX+T+EA birds that crowed frequently. Increases in c-fos expression were observed in TH-immunoreactive cells in the periaqueductal gray and ventral tegmental area, but not in the substantia nigra, of I+F and CX+T+F birds indicating the activation of dopaminergic neurons during sexual behavior. Together, these data confirm the implication of the steroid-sensitive preoptic area and bed nucleus striae terminalis in the control of copulation and support the notion that dopamine is involved in its control.

The Japanese quail: a model for studying reproductive aging of hypothalamic systems

During aging, the decline of neuroendocrine, endocrine, and behavioral components of reproduction ultimately leads to reproductive failure. These studies considered both neuroendocrine and behavioral aspects of reproductive aging in Japanese quail, using chronological age and reproductive status to separate animals into experimental groups. In Study I, age-related changes in the gonadotropin releasing hormone (GnRH-I) system were investigated and a sharp decrease was observed in GnRH-I concentration in the median eminence of aging animals of both sexes, whereas preoptic-lateral septal region GnRH-I concentrations declined only in aging males. Immunohistochemistry confirmed these findings since aging females retained, whereas males lost GnRH-I cells. Functional changes were assessed by in vitro incubation of parasaggittal hypothalamic slices collected from young and old inactive males and females. Results showed reduced baseline GnRH-I release and diminished response to norepinephrine (NE). Deteriorating fertility also correlated with decreased male sexual behavior and loss of aromatase immunoreactive (AROM-ir) neurons in the medial, but not lateral preoptic nucleus (POA). Sexual behavior and AROM-ir were restored with exogenous testosterone, which was associated with increased cell size in the medial POA. Comparison of cell size and number of AROM-ir cells showed that aged sexually active males had fewer, larger AROM-ir cells when compared to young males, suggesting neuroplasticity of specific neural systems and a critical role of estradiol in maintaining reproductive function.

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.

Modulation of Hormonal Signaling in the Brain by Steroid Receptor Coactivators

Nuclear receptors, such as estrogen, glucocorticoid or thyroid hormone receptors, have been shown to play a critical role in brain development and physiology. The activity of these receptors is modulated by the interaction with several proteins and, in particular, coactivators are required to enhance their transcriptional activity. The steroid receptor coactivators (SRC-1, -2 and -3) are currently the best characterized coactivators and we review here the current knowledge on the distribution and function of these proteins in the brain. Knock-out models and antisense techniques have demonstrated the requirement for SRC-1 and -2 in the brain, focusing mainly on steroid and thyroid hormone-dependent development and behavior. The precise function of SRC-3 in the brain is currently unknown but its presence throughout the brain suggests an important function. Although the molecular biology of SRCs is relatively well known, the in vivo control of their expression, post-translational modifications and time- and cell-specific interactions with the different nuclear receptors remain elusive. A complete understanding of hormone action on brain and behavior will not be attained until a better knowledge of coactivator physiology is achieved.

Steroid receptor coactivator SRC-1 exhibits high expression in steroid-sensitive brain areas regulating reproductive behaviors in the quail brain

The steroid receptor coactivator SRC-1 modulates ligand-dependent transactivation of several nuclear receptors, including the receptors for sex steroid hormones. Reducing the expression of SRC-1 by injection of specific antisense oligonucleotides markedly inhibits the effects of estrogens of the sexual differentiation of brain and behavior in rats and inhibits the activation of female sexual behavior in adult female rats. SRC-1 thus appears to be involved in both the development and activation of sexual behavior. In the Japanese quail brain, we amplified by RT-PCR a 3,411-bp fragment extending from the HLH domain to the activating domain-2 of the protein. The quail SRC-1 is closely related to the mammalian (m) SRC-1 and contains a high proportion of GC nucleotides (62.5%). Its amino acid sequence presents 70% identity with mammalian SRC-1 and contains the three conserved LXXLL boxes involved in the interaction with nuclear receptors. In both males and females, RT-PCR demonstrates a similarly high level of expression in the telencephalon, diencephalon, optic lobes, brain stem, spinal cord, pituitary, liver, kidney, adrenal gland, heart, lung, gonads and gonoducts. Males express significantly higher levels of SRC-1 in the preoptic area-hypothalamus than females. In both sexes, lower levels of expression are observed in the cerebellum and muscles. In situ hybridization utilizing a mixture of four digoxigenin-labeled oligonucleotides confirms at the cellular level the widespread distribution of SRC-1 mRNA in the brain and a particularly dense expression in steroid-sensitive areas that play a key role in the control of male sexual behavior. These data confirm the presence and describe for the first time the SRC-1 distribution in the brain of an avian species. They confirm its broad, nearly ubiquitous, distribution in the entire body including the brain as could be expected for a coactivator that regulates to the action of many nuclear receptors. However this distribution is heterogeneous in the brain and sexually differentiated in at least some areas. The very dense expression of SRC-1 in limbic and mesencephalic nuclei that are associated with the control of male sexual behavior is consistent with the notion that this coactivator plays a significant role in the activation of this behavior.

Steroid-induced plasticity in the sexually dimorphic vasotocinergic innervation of the avian brain: behavioral implications

Vasotocin (VT, the antidiuretic hormone of birds) is synthesized by diencephalic magnocellular neurons projecting to the neurohypophysis. In addition, in male quail and in other oscine and non-oscine birds, a sexually dimorphic group of VT-immunoreactive (ir) parvocellular neurons is located in a region homologous to the mammalian nucleus of the stria terminalis, pars medialis (BSTm) and in the medial preoptic nucleus (POM). These cells are not visible in females. VT-ir fibers are present in many diencephalic and extradiencephalic locations. Quantitative morphometric analyses demonstrate that, in quail, these elements are expressed in a sexually dimorphic manner (males>females) in regions involved in the control of different aspects of reproduction: i.e., the POM (copulatory behavior), the lateral septum (secretion of gonadotropin-releasing hormone [GnRH]), the nucleus intercollicularis (control of vocalizations), and the locus coeruleus (the main noradrenergic center of the avian brain). In many of these regions, VT-ir fibers are closely related to aromatase-ir, GnRH-ir, or estrogen receptor-expressing neurons. This dimorphism has an organizational nature: administration of estradiol-benzoate to quail embryos (a treatment that abolishes male sexual behavior) results in a dramatic decrease of the VT-immunoreactivity in all sexually dimorphic regions of the male quail brain. Conversely, the inhibition of estradiol (E2) synthesis during embryonic life (a treatment that stimulates the expression of male copulatory behavior in adult testosterone (T)-treated females) results in a male-like distribution of VT-ir cells and fibers. Castration markedly decreases the immunoreactivity in both the VT-immunopositive elements of the BSTm and the innervation of the SL and POM, whereas T-replacement therapy restores the VT immunoreactivity to a level typical of intact birds. These changes reflect modifications of VT mRNA concentrations (and probably synthesis) as demonstrated by in situ hybridization and they are paralleled by similar changes in male copulatory behavior (absent in castrated male quail, fully expressed in CX+T males). The aromatization of T into estradiol (E2) also controls VT expression and, in parallel limits the activation of male sexual behavior by T. In castrated male quail, the restoration by T of the VT immunoreactivity in POM, BSTm and lateral septum could be fully mimicked by a treatment with E2, but the androgen 5alpha-dihydrotestosterone (DHT) had absolutely no effect on the VT immunoreactivity in these conditions. At the doses used in this study, DHT also did not synergize with E2 to enhance the density of VT immunoreactive structures. Systemic or i.c.v. injections of VT markedly inhibit the expression of all aspects of male sexual behavior. VT, presumably, does not simply represent one step in the biochemical cascade of events that is induced by T in the brain and leads to the expression of male sexual behavior. Androgens and estrogens presumably affect reproductive behavior both directly, by acting on steroid-sensitive neurons in the preoptic area, and indirectly, by modulating peptidergic (specifically vasotocinergic) inputs to this and other areas. The respective contribution of these two types of actions and their interaction deserves further analysis.

Preoptic aromatase cells project to the mesencephalic central gray in the male Japanese quail (Coturnix japonica)

Previous tract-tracing studies demonstrated the existence of projections from the medial preoptic nucleus (POM) to the mesencephalic central gray (GCt) in quail. GCt contains a significant number of aromatase-immunoreactive (ARO-ir) fibers and punctate structures, but no ARO-ir cells are present in this region. The origin of the ARO-ir fibers of the GCt was investigated here by retrograde tract-tracing combined with immunocytochemistry for aromatase. Following injection of fluorescent microspheres in GCt, retrogradely labeled cells were found in a large number of hypothalamic and mesencephalic areas and in particular within the three main groups of ARO-ir cells located in the POM, the ventromedial nucleus of the hypothalamus, and the bed nucleus striae terminalis. Labeling of these cells for aromatase by immunocytochemistry demonstrated, however, that aromatase-positive retrogradely labeled cells are observed almost exclusively within the POM. Double-labeled cells were abundant in both the rostral and caudal parts of the POM and their number was apparently not affected by the location of the injection site within GCt. At both rostro-caudal levels of the POM, ARO-ir retrogradely labeled cells were, however, more frequent in the lateral than in the medial POM. These data indicate that ARO-ir neurons located in the lateral part of the POM may control the premotor aspects of male copulatory behavior through their projection to GCt and suggest that GCt activity could be affected by estrogens released from the terminals of these ARO-ir neurons.

Sexual dimorphism in the neuronal circuits of the quail preoptic and limbic regions

A sexually dimorphic nucleus is located in the preoptic area of Japanese quail and plays a key role in the activation of male copulatory behavior. The medial preoptic nucleus (POM) is significantly larger in adult male than in adult female quail. Its volume is steroid-sensitive in adulthood and consequently decreases after castration but is restored to normal levels by a treatment with exogenous testosterone. This volumetric difference appears to result only from a sex difference in the adult hormonal milieu and is not affected by embryonic treatments that permanently modify sexual behavior (no organizational effects). In contrast, some cytoarchitectonic features of the POM such as the size of neurons in the dorso-lateral part of nucleus appear to be irreversibly affected by embryonic steroids. The POM is characterized by the presence of a wide variety of neurotransmitters, neuropeptides, and receptors and can be specifically identified by the presence of a dense cluster of aromatase-immunoreactive cells, by a high density of neurotensin-immunoreactive cells and fibers and by a dense vasotocinergic innervation. Some of these neurochemical markers of the dimorphic nucleus are themselves modulated by steroids. Many of these neurochemical changes appear to play a causal role in the control of male sexual behavior. The quail POM thus represents an excellent model for the analysis of steroid-induced brain plasticity in a behaviorally relevant context.

Effects of testosterone on the synaptology of the medial preoptic nucleus of male Japanese quail

The medial preoptic nucleus (POM) of male Japanese quail is a sexually dimorphic testosterone-dependent structure that plays a key role in the activation of male sexual behavior. Both the total volume of the nucleus and the size of the dorsolateral neurons are decreased in gonadectomized males. Immunocytochemical studies have revealed a complex pattern of innervation: immunopositive fibers for several neuropeptides and neurotransmitters have been detected in the POM; some of them (e.g. vasotocin-immunoreactive fibers) are sexually dimorphic and testosterone-dependent To understand the anatomical bases of these testosterone-dependent neurochemical changes, we performed an ultrastructural study of the POM neuropil in intact sexually mature, gonadectomized, or testosterone-treated gonadectomized males. A complex synaptic organization of the POM neuropil was observed in intact male quail reflecting the heterogeneity of the neurotransmitters and neuropeptides present in this nucleus. Changes in this organization were observed after the endocrine manipulations. The number of axosomatic synapses per cell body decreased after gonadectomy and was restored to the level observed in the intact group after the administration of testosterone. By contrast, no significant change was observed in the density of axodendritic and axospinal synapses after hormonal manipulations which suggests that the total number of synapses in the nucleus should be affected by testosterone (constant density in a changing total volume). The cross-sectional area of synaptic boutons was also decreased by castration and restored to intact level by testosterone. The action of testosterone on the activation of male copulatory behavior in gonadectomized birds is hence paralleled by an extensive rearrangement of neuropil in the POM.

Testosterone effects on the neuronal ultrastructure in the medial preoptic nucleus of male Japanese quail

Dorsolateral neurons of the medial preoptic nucleus (POM) of male Japanese quail are sensitive to the plasma levels of testosterone: their volume and optical density in Nissl-stained sections increase in castrated birds treated with testosterone. The present study was performed on castrated male quail treated or not with Silastic implants filled with testosterone to describe the ultrastructural variations induced by testosterone in these neurons. Gonadally intact male birds were included as controls. The ultrastructure of neurons, taken from the dorsolateral portion of the POM, was dramatically affected by the endocrine manipulations. Quantitative evaluations demonstrated a significant decrease in castrated birds of the rough endoplasmic reticulum (RER), of free polyribosomes, of Golgi complexes, and of dense bodies; these changes paralleled the decrease in cell size. The cell size and the percentage of volume occupied by the intracellular organelles in castrated birds treated with testosterone were comparable to values observed in controls. These ultrastructural changes are similar to those observed in neuronal targets for other gonadal hormones, supporting the idea that testosterone stimulates the development of cytoplasmic structures involved in protein synthesis and secretion. In addition, exposure to testosterone affects the synaptic inputs to POM. These ultrastructural changes are presumably related to the physiological effects (e.g., activation of male sexual behavior) exerted by testosterone on this preoptic region.

Ultrastructural characterization of the sexually dimorphic medial preoptic nucleus of male Japanese quail

The medial preoptic nucleus is a sexually dimorphic structure whose cytoarchitecture, afferent and efferent connections, and functions have been previously described. No detailed ultrastructural study has, however, been performed to date. Here we describe the ultrastructural organization of this important preoptic structure of the male quail. Neuronal cell bodies of the medial preoptic nucleus generally show extensive development of protein-synthesis-related organelles (rough endoplasmic reticulum, polysomes), and of secretory structures (Golgi complexes, secretory vesicles, dense bodies). Previous morphometrical studies at the light-microscopical level have demonstrated the presence of a medial and a lateral neuronal population distinguished by the size of their cell bodies (the medial neurons are smaller than the lateral neurons). The present ultrastructural investigation confirms the difference in size, but no difference has been observed in the ultrastructural organization of the neurons. In both the medial and the lateral part, the nucleus is characterized by a large variety of cell bodies, including some that, on the basis of their ultrastructure, can be considered as putative peptidergic neurons. Close contacts are frequently observed between adjacent cell bodies that are normally arranged in clusters. Various types of synaptic endings are also present, suggesting a rich supply of nerve fibers. A few glial cells are scattered within the nucleus. In view of the crucial role of this region in regulating quail sexual behavior, the large heterogeneity of neurons and of afferent nervous fibers suggest that this region might have an important role in the integration of information arriving from different brain regions.

Vasotocinergic innervation of sexually dimorphic medial preoptic nucleus of the male Japanese quail: influence of testosterone

The distribution of vasotocin (VT)-immunoreactive (IR) fibers was described in the preoptic and septal regions of the male quail brain. The density of VT-IR fibers was measured in the sexually dimorphic preoptic nucleus (POM) and lateral septum (SL) of adult male quail (Coturnix japonica) by means of quantitative image analysis. Experimental manipulations of the hormonal environment in the peripubertal period influenced this distribution. In both regions, the VT immunoreactivity was reduced or absent when males were castrated. The immunoreactivity was restored to its original level in castrated males by Silastic implants of testosterone. These changes were anatomically specific as evidenced by the fact that the density of VT fibers did not vary in the hypothalamo-neurohypohysial tract as a function of the endocrine condition of the subjects. No change was also observed in the number of VT-IR cells in the periventricular region close to the POM. Previously published data show that VT or its mammalian homolog, vasopressin are implicated in the control of a wide range of instinctive behaviors. The steroid-dependent VT afferents to the POM, a key area controlling male copulatory behavior in quail could therefore be involved in the control of the sexual behavior in this species. The outputs of the POM which contains steroid-receptors could therefore be modulated by steroids in two different ways: directly through the steroid receptors it contains and indirectly through its steroid-sensitive peptidergic afferents.

Morphometric studies demonstrate that aromatase-immunoreactive cells are the main target of androgens and estrogens in the quail medial preoptic nucleus

The volume and cytoarchitectonic organization of the sexually dimorphic medial preoptic nucleus (POM) of the quail are sensitive to plasma levels of testosterone (T). We previously showed that, in castrated quail, T or its estrogenic metabolite, estradiol (E2), increases the size of the large neurons located in the lateral part of POM. Embryonic treatments with estrogens are also known to affect permanently the size of these large neurons. Since the lateral POM also contains a dense population of aromatase-immunoreactive (ARO-ir) cells, and these are known to be a target for steroids, we hypothesized that the effects of steroids identified in previous experiments were primarily directed to these ARO-ir cells. This idea was tested in two experiments in which the size of these cells was measured in male quail under various endocrine conditions. In experiment 1, a detailed analysis of ARO-ir and of non-immunoreactive cells in the POM of adult, sexually mature males revealed that the immunoreactive perikarya are larger than the non-immunoreactive cells and that they constitute the vast majority of the large cells (area > 50 microns 2) in the POM. In experiment 2, it was shown that T and E2 actually increase the size of ARO-ir cells in the POM while the androgenic metabolite of T, dihydrotestosterone has no effect at this level. Taken together, these data suggest that the sex differences and the steroid-induced changes in cell size previously described in the study of POM sections stained for Nissl material largely concern aromatase-containing cells. Since aromatization of T plays a limiting role in the activation of male copulatory behavior, these changes may represent the morphological signature of the mechanisms causally involved in the control of this behavior.

Afferent and efferent connections of the sexually dimorphic medial preoptic nucleus of the male quail revealed by in vitro transport of DiI

The medial preoptic nucleus of the Japanese quail is a testosterone-sensitive structure that is involved in the control of male copulatory behavior. The full understanding of the role played by this nucleus in the control of reproduction requires the identification of its afferent and efferent connections. In order to identify neural circuits involved in the control of the medial preoptic nucleus, we used the lipophilic fluorescent tracer DiI implanted in aldheyde-fixed tissue. Different strategies of brain dissection and different implantation sites were used to establish and confirm afferent and efferent connections of the nucleus. Anterograde projections reached the tuberal hypothalamus, the area ventralis of Tsai, and the substantia grisea centralis. Dense networks of fluorescent fibers were also seen in several hypothalamic nuclei, such as the anterior medialis hypothalami, the paraventricularis magnocellularis, and the ventromedialis hypothalami. A major projection in the dorsal direction was also observed from the medial preoptic nucleus toward the nucleus septalis lateralis and medialis. Afferents to the nucleus were seen from all these regions. Implantation of DiI into the substantia grisea centralis also revealed massive bidirectional connections with a large number of more caudal mesencephalic and pontine structures. The substantia grisea centralis therefore appears to be an important center connecting anterior levels of the brain to brain-stem nuclei that may be involved in the control of male copulatory behavior.

Sex difference in the neurotensin-immunoreactive cell populations of the preoptic area in quail (Coturnix japonica)

The distribution of neurotensin-immunoreactive cells and fibers was analyzed by immunocytochemistry in the forebrain of male and female Japanese quail (Coturnix japonica) by using an antibody directed against the C-terminal part of the molecule. Immunoreactive perikarya were located almost exclusively in the medial preoptic area with small populations also being present in the nucleus paraventricularis and in the tuberal region. Immunoreactive fibers were observed not only throughout the preoptic area-hypothalamus, but also in the septal region, nucleus intercollicularis, substantia grisea centralis and the classical catecholaminergic areas of the mesencephalon, such as the area ventralis of Tsai and the nucleus tegmenti pedunculo-pontinus, pars compacta. The preoptic neurotensin-immunoreactive cells were exclusively located within the boundaries of the sexually dimorphic medial preoptic nucleus. They were significantly more numerous in females than in males. In females, the number of neurotensin cells varied during the ovulatory cycle: fewer cells were observed in birds that were about to lay an egg (they had a calcified egg in the oviduct) than in those that had already laid or were not going to lay on that day. These data indicate major variations in the expression of neurotensin in response to neurochemical or neuroendocrine changes associated with ovulation.

Steroid-neuropeptide interactions that control reproductive behaviors in an amphibian

Investigations into the neuroendocrine regulation of reproductive behaviors in an amphibian (Taricha granulosa) reveal the same basic repertoire of chemical messengers as regulators of male behaviors in other vertebrates. These studies have identified seasonal neural interactions between gonadal steroids and neuropeptides that facilitate male courtship behavior. In addition, this species has served to elucidate how stress-induced suppression of courtship is mediated by corticosterone action through a neuronal membrane receptor and subsequent, rapid neurophysiological effects. These findings indicate that a principal mechanism by which steroids and neuropeptides control male reproductive behavior is the modulation of neural processing of specific sensory stimuli.