The hypothalamic paraventricular nucleus as a central hub for the estrogenic modulation of neuroendocrine function and behavior

Estradiol and hypothalamic paraventricular nucleus (PVN) help coordinate reproduction with body physiology, growth and metabolism. PVN integrates hormonal and neural signals originating in the periphery, generating an output mediated both by its long-distance neuronal projections, and by a variety of neurohormones produced by its magnocellular and parvocellular neurosecretory cells. Here we review the cyto-and chemo-architecture, the connectivity and function of PVN and the sex-specific regulation exerted by estradiol on PVN neurons and on the expression of neurotransmitters, neuromodulators, neuropeptides and neurohormones in PVN. Classical and non-classical estrogen receptors (ERs) are expressed in neuronal afferents to PVN and in specific PVN interneurons, projecting neurons, neurosecretory neurons and glial cells that are involved in the input-output integration and coordination of neurohormonal signals. Indeed, PVN ERs are known to modulate body homeostatic processes such as autonomic functions, stress response, reproduction, and metabolic control. Finally, the functional implications of the estrogenic modulation of the PVN for body homeostasis are discussed.

Early postnatal genistein administration permanently affects nitrergic and vasopressinergic systems in a sex-specific way

Genistein (GEN) is a natural xenoestrogen (isoflavonoid) that may interfere with the development of estrogen-sensitive neural circuits. Due to the large and increasing use of soy-based formulas for babies (characterized by a high content of GEN), there are some concerns that this could result in an impairment of some estrogen-sensitive neural circuits and behaviors. In a previous study, we demonstrated that its oral administration to female mice during late pregnancy and early lactation induced a significant decrease of nitric oxide synthase-positive cells in the amygdala of their male offspring. In the present study, we have used a different experimental protocol mimicking, in mice, the direct precocious exposure to GEN. Mice pups of both sexes were fed either with oil, estradiol or GEN from birth to postnatal day 8. Nitric oxide synthase and vasopressin neural systems were analyzed in adult mice. Interestingly, we observed that GEN effect was time specific (when compared to our previous study), sex specific, and not always comparable to the effects of estradiol. This last observation suggests that GEN may act through different intracellular pathways. Present results indicate that the effect of natural xenoestrogens on the development of the brain may be highly variable: a plethora of neuronal circuits may be affected depending on sex, time of exposure, intracellular pathway involved, and target cells. This raises concern on the possible long-term effects of the use of soy-based formulas for babies, which may be currently underestimated.

Testosterone and estradiol differentially affect cell proliferation in the subventricular zone of young adult gonadectomized male and female rats

Steroid hormones are important players to regulate adult neurogenesis in the dentate gyrus of the hippocampus, but their involvement in the regulation of the same phenomenon in the subventricular zone (SVZ) of the lateral ventricles is not completely understood. Here, in male rats, we tested the existence of activational effects of testosterone (T) on cell proliferation in the adult SVZ. To this aim, three groups of male rats: castrated, castrated and treated with T, and controls were treated with 5-bromo-2'-deoxyuridine (BrdU) and killed after 24h. The density of BrdU-labeled cells was significantly lower in castrated animals in comparison to the other two groups, thus supporting a direct correlation between SVZ proliferation and levels of circulating T. To clarify whether this effect is purely androgen-dependent, or mediated by the T metabolites, estradiol (E2) and dihydrotestosterone (DHT), we evaluated SVZ proliferation in castrated males treated with E2, DHT and E2+DHT, in comparison to T- and vehicle-treated animals, and sham-operated controls. The stereological analysis demonstrated that E2 and T, but not DHT, increase proliferation in the SVZ of adult male rats. Quantitative evaluation of cells expressing the endogenous marker of cell proliferation phosphorylated form of Histone H3 (PHH3), or the marker of highly dividing SVZ progenitors Mash1, indicated the effect of T/E2 is mostly restricted to SVZ proliferating progenitors. The same experimental protocol was repeated on ovariectomized female rats treated with E2 or T. In this case, no statistically significant difference was found among groups. Overall, our results clearly show that the gonadal hormones T and E2 represent important mediators of cell proliferation in the adult SVZ. Moreover, we show that such an effect is restricted to males, supporting adult neurogenesis in rats is a process differentially modulated in the two sexes.

Chronic unpredictable stress and long-term ovariectomy affect arginine-vasopressin expression in the paraventricular nucleus of adult female mice

Arginine-Vasopressin (AVP) may regulate the hypothalamic-pituitary-adrenal axis (HPA) and its effects on depressive responses. In a recent study, we demonstrated that Chronic Unpredictable Stress (CUS) depressive effects are enhanced by long-term ovariectomy (a model of post-menopause). In the present study, we investigated the effects of long-term ovariectomy and CUS on AVP expression in different subdivision of the paraventricular nucleus (PVN) of female mice. Both long-term ovariectomy and CUS affect AVP immunoreactivity in some of the PVN subnuclei of adult female mice. In particular, significant changes on AVP immunoreactivity were observed in magnocellular subdivisions, the paraventricular lateral magnocellular (PaLM) and the paraventricular medial magnocellular (PaMM), the 2 subnuclei projecting to the neurohypophysis for the hormonal regulation of body homeostasis. AVP immunoreactivity was decreased in the PaLM by both the long-term deprivation of ovarian hormones and the CUS. In contrast, AVP immunoreactivity was increased in the PaMM by CUS, whereas it was decreased by ovariectomy. Therefore, present results suggest morphological and functional differences among the PVN's subnuclei and complex interactions among CUS, gonadal hormones and AVP immunoreactivity.

Endocrine disrupters: a review of some sources, effects, and mechanisms of actions on behaviour and neuroendocrine systems

Some environmental contaminants interact with hormones and may exert adverse consequences as a result of their actions as endocrine disrupting chemicals (EDCs). Exposure in people is typically a result of contamination of the food chain, inhalation of contaminated house dust or occupational exposure. EDCs include pesticides and herbicides (such as dichlorodiphenyl trichloroethane or its metabolites), methoxychlor, biocides, heat stabilisers and chemical catalysts (such as tributyltin), plastic contaminants (e.g. bisphenol A), pharmaceuticals (i.e. diethylstilbestrol; 17α-ethinylestradiol) or dietary components (such as phytoestrogens). The goal of this review is to address the sources, effects and actions of EDCs, with an emphasis on topics discussed at the International Congress on Steroids and the Nervous System. EDCs may alter reproductively-relevant or nonreproductive, sexually-dimorphic behaviours. In addition, EDCs may have significant effects on neurodevelopmental processes, influencing the morphology of sexually-dimorphic cerebral circuits. Exposure to EDCs is more dangerous if it occurs during specific 'critical periods' of life, such as intrauterine, perinatal, juvenile or puberty periods, when organisms are more sensitive to hormonal disruption, compared to other periods. However, exposure to EDCs in adulthood can also alter physiology. Several EDCs are xenoestrogens, which can alter serum lipid concentrations or metabolism enzymes that are necessary for converting cholesterol to steroid hormones. This can ultimately alter the production of oestradiol and/or other steroids. Finally, many EDCs may have actions via (or independent of) classic actions at cognate steroid receptors. EDCs may have effects through numerous other substrates, such as the aryl hydrocarbon receptor, the peroxisome proliferator-activated receptor and the retinoid X receptor, signal transduction pathways, calcium influx and/or neurotransmitter receptors. Thus, EDCs, from varied sources, may have organisational effects during development and/or activational effects in adulthood that influence sexually-dimorphic, reproductively-relevant processes or other functions, by mimicking, antagonising or altering steroidal actions.

Milestones on Steroids and the Nervous System: 10 years of basic and translational research

During the last 10 years, the conference on 'Steroids and Nervous System' held in Torino (Italy) has been an important international point of discussion for scientists involved in this exciting and expanding research field. The present review aims to recapitulate the main topics that have been presented through the various meetings. Two broad areas have been explored: the impact of gonadal hormones on brain circuits and behaviour, as well as the mechanism of action of neuroactive steroids. Relationships among steroids, brain and behaviour, the sexual differentiation of the brain and the impact of gonadal hormones, the interactions of exogenous steroidal molecules (endocrine disrupters) with neural circuits and behaviour, and how gonadal steroids modulate the behaviour of gonadotrophin-releasing hormone neurones, have been the topics of several lectures and symposia during this series of meetings. At the same time, many contributions have been dedicated to the biosynthetic pathways, the physiopathological relevance of neurosteroids, the demonstration of the cellular localisation of different enzymes involved in neurosteroidogenesis, the mechanisms by which steroids may exert some of their effects, both the classical and nonclassical actions of different steroids, the role of neuroactive steroids on neurodegeneration, neuroprotection, and the response of the neural tissue to injury. In these 10 years, this field has significantly advanced and neuroactive steroids have emerged as new potential therapeutic tools to counteract neurodegenerative events.

Effects of estrous cycle and sex on the expression of neuropeptide Y Y1 receptor in discrete hypothalamic and limbic nuclei of transgenic mice

In the present study we used a transgenic mouse model, carrying the neuropeptide Y (NPY) Y1 receptor gene promoter linked to the LacZ reporter gene (Y1R/LacZ mice) to test the hypothesis of its up-regulation by gonadal hormones. Y1 receptor gene expression was detected by means of histochemical procedures and quantitative image analysis in the paraventricular nucleus, arcuate nucleus, medial preoptic nucleus, ventromedial nucleus and bed nucleus of stria terminalis of two-month-old female mice at different stages of estrous cycle. Qualitative and quantitative analyses showed that Y1R/LacZ transgene expression was higher in the paraventricular, arcuate, and ventromedial nuclei of proestrus mice as compared to mice in the other stages of the estrous cycle. In addition, we performed a comparison with a group of sexually active males. In this comparison a significant difference (less in males) was observed between males and proestrus females in the same nuclei. In conclusion, these data indicate that fluctuations in circulating levels of gonadal hormones, depending by estrous cycle, are paralleled by changes in the expression of NPY Y1 receptor in the hypothalamic nuclei involved in the control of both energy balance and reproduction.

Neuropeptides and enzymes are targets for the action of endocrine disrupting chemicals in the vertebrate brain

Endocrine-disrupting chemicals (EDC) are molecules that interfere with endocrine signaling pathways and produce adverse consequences on animal and human physiology, such as infertility or behavioral alterations. Some EDC act through binding to androgen or/and estrogen receptors primarily operating through a genomic mechanism regulating gene expression. This mechanism of action may induce profound developmental adverse effects, and the major targets of the EDC action are the gene products, i.e., mRNAs inducing the synthesis of various peptidic molecules, which include neuropeptides and enzymes related to neurotransmitters syntheses. Available immunohistochemical data on some of the systems that are affected by EDC in lower and higher vertebrates are detailed in this review.

Acute exposure to tributyltin induces c-fos activation in the hypothalamic arcuate nucleus of adult male mice

Tributyltin (TBT) is a largely diffused environmental pollutant, banned from paints in the European Union from 2003. However, the level of TBT (and other organotins) in food, particularly fish and shellfish, remains still high. Several studies demonstrated that TBT is involved in the development of obesity, via peripheral action, but currently, there are only a few data illustrating effects of TBT on the nervous system. In the present study, we tested the hypothesis that acute exposure to TBT may directly activate brain cells in particular, in those hypothalamic nuclei regulating the food intake. To this purpose, TBT was orally administered at a single dose (10 mg/kg/body weight) to two groups of adult male mice: regularly fed or fasted for 24 h. Mice were sacrificed 90 min after the TBT administration and perfused by 4% paraformaldehyde. Brains were quickly dissected, frozen and sectioned for immunocytochemical detection of c-fos, a common marker of cell activation. In both, fed or fasted mice, exposure to TBT induced a significant increase of c-fos expression in the arcuate nucleus in comparison to control mice. The other nuclei involved in the control of feeding behavior did not show any significant increase. These data are the first in vivo demonstration that TBT has not only peripheral effects, but also may activate elements in the brain, in particular in a crucial region for the regulation of food intake like the arcuate nucleus.

Integration and sensory experience-dependent survival of newly-generated neurons in the accessory olfactory bulb of female mice

Newborn neurons generated by proliferative progenitors in the adult subventricular zone (SVZ) integrate into the olfactory bulb circuitry of mammals. Survival of these newly-formed cells is regulated by the olfactory input. The presence of new neurons in the accessory olfactory bulb (AOB) has already been demonstrated in some mammalian species, albeit their neurochemical profile and functional integration into AOB circuits are still to be investigated. To unravel whether the mouse AOB represents a site of adult constitutive neurogenesis and whether this process can be modulated by extrinsic factors, we have used multiple in vivo approaches. These included fate mapping of bromodeoxyuridine-labelled cells, lineage tracing of SVZ-derived enhanced green fluorescent protein-positive engrafted cells and neurogenesis quantification in the AOB, in both sexes, as well as in females alone after exposure to male-soiled bedding or its derived volatiles. Here, we show that a subpopulation of SVZ-derived neuroblasts acquires proper neurochemical profiles of mature AOB interneurons. Moreover, 3D reconstruction of long-term survived engrafted neuroblasts in the AOB confirms these cells show features of fully integrated neurons. Finally, exposure to male-soiled bedding, but not to its volatile compounds, significantly increases the number of new neurons in the AOB, but not in the main olfactory bulb of female mice. These data show SVZ-derived neuroblasts differentiate into new functionally integrated neurons in the AOB of young and adult mice. Survival of these cells seems to be regulated by an experience-specific mechanism mediated by pheromones.

Androgen receptors are required for full masculinization of nitric oxide synthase system in rat limbic-hypothalamic region

The neuronal nitric oxide synthase (nNOS) is involved in the control of male and female sexual behavior and its distribution in several regions of the limbic-hypothalamic system, as well as its coexistence with gonadal hormones' receptors, suggests that these hormones may play a significant role in controlling its expression. However, data illustrating the role of gonadal hormones in controlling the nNOS expression are, at present, contradictory, even if they strongly suggest an involvement of testosterone (T) in the regulation of nNOS. The action of T may be mediated through androgen (AR) or, after aromatization to estradiol (E(2)), through estrogen receptors. To elucidate the role of AR on nNOS expression, we compared male and female rats with a non-functional mutation of AR (Tfm, testicular feminization mutation) to their control littermates. We investigated some hypothalamic and limbic nuclei involved in the control of sexual behavior [medial preoptic area (MPA), paraventricular (PVN), arcuate (ARC), ventromedial (VMH) and stria terminalis (BST) nuclei]. In BST (posterior subdivision), VMH (ventral subdivision), and MPA we detected a significant sexual dimorphism in control animals and a decrease of nNOS positive elements in Tfm males compared to their littermate. In addition, we observed a significant increase of nNOS positive elements in BST (posterior) of Tfm females. No significant changes were observed in the other nuclei. These data indicate that, contrary to current opinions, androgens, through the action of AR may have a relevant role in the organization and modulation of the nNOS hypothalamic system.

The endocrine nervous system: source and target for neuroactive steroids

For a long time the endocrine brain was considered to the hypothalamus and to its special relationships with the hypophysis. The discovery of the wide distribution of steroid hormone receptors, as well as that of the possibility of metabolizing or synthesizing steroids by neural cells (neuroactive steroids), suggest, on the contrary, that interactions among steroids and nervous system are key points of the regulatory processes in the central and peripheral nervous system in normal conditions as well as in pathological conditions. In this brief overview we illustrate a few examples of these relationships with major emphasis on papers collected in this special issue.

Activational effects of estradiol and dihydrotestosterone on social recognition and the arginine-vasopressin immunoreactive system in male mice lacking a functional aromatase gene

In rodents, parts of the arginine-vasopressin (AVP) neuronal system are sexually dimorphic with males having more AVP-immunoreactive cells/fibers than females. This neuropeptide neuronal system is highly sensitive to steroids and has been proposed to play an important role in the processing of olfactory cues critical to the establishment of a social memory. We demonstrate here that gonadally intact male aromatase knockout (ArKO) mice, which cannot aromatize androgens into estrogens due to a targeted mutation in the aromatase gene, showed severe deficits in social recognition as well as a reduced AVP-immunoreactivity in several brain regions. To determine whether this reduction is due to a lack of organizational or activational effects of estrogens, we assessed social recognition abilities and AVP-immunoreactivity in male ArKO and wild-type (WT) mice when treated with estradiol benzoate (EB) in association with dihydrotestosterone propionate (DHTP) in adulthood. Adult treatment with EB and DHTP restored social recognition abilities in castrated ArKO males since they showed normal female-oriented ultrasonic vocalizations and were able to recognize an unfamiliar female using a habituation-dishabituation paradigm. Furthermore, adult treatment also restored AVP-immunoreactivity in the lateral septum of ArKO males to levels observed in intact WT males. These results suggest that social recognition in adulthood and stimulation of AVP expression in the adult mouse forebrain depend predominantly on the estrogenic metabolite of testosterone. Furthermore, our results are in line with the idea that the organization of the AVP system may depend on androgen or sex chromosomes rather than estrogens.

Loss of aromatase cytochrome P450 function as a risk factor for Parkinson's disease?

The final step in the physiological synthesis of 17beta estradiol (E(2)) is aromatization of precursor testosterone by a CYP19 gene product, cytochrome P450 estrogen aromatase in the C19 steroid metabolic pathway. Within the central nervous system (CNS) the presence, distribution, and activity of aromatase have been well characterized. Developmental stage and injury are known modulators of brain enzyme activity, where both neurons and glial cells reportedly have the capability to synthesize this key estrogenic enzyme. The gonadal steroid E(2) is a critical survival, neurotrophic and neuroprotective factor for dopaminergic neurons of the substantia nigra pars compacta (SNpc), the cells that degenerate in Parkinson's disease (PD). In previous studies we underlined a crucial role for the estrogenic status at the time of injury in dictating vulnerability to the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Our ongoing studies address the contribution of brain aromatase and extragonadal E(2) as vulnerability factors for PD pathology in female brain, by exposing aromatase knockout (ArKO, -/-) female mice which are unable to synthesize estrogens to MPTP. Our initial results indicate that aromatase deficiency from early embryonic life significantly impairs the functional integrity of SNpc tyrosine hydroxylase-positive neurons and dopamine transporter innervation of the caudate-putamen in adulthood. In addition, ArKO females exhibited a far greater vulnerability to MPTP-induced nigrostriatal damage as compared to their Wt type gonadally intact and gonadectomized counterparts. Characterization of this novel implication of P450 aromatase as determining factor for PD vulnerability may unravel new avenues for the understanding and development of novel therapeutic approaches for Parkinson's disease.

Effects of xenoestrogens on the differentiation of behaviorally-relevant neural circuits

It has become increasingly clear that environmental chemicals have the capability of impacting endocrine function. Moreover, these endocrine disrupting chemicals (EDCs) have long term consequences on adult reproductive function, especially if exposure occurs during embryonic development thereby affecting sexual differentiation. Of the EDCs, most of the research has been conducted on the effects of estrogen active compounds. Although androgen active compounds are also present in the environment, much less information is available about their action. However, in the case of xenoestrogens, there is mounting evidence for long-term consequences of early exposure at a range of doses. In this review, we present data relative to two widely used animal models: the mouse and the Japanese quail. These two species long have been used to understand neural, neuroendocrine, and behavioral components of reproduction and are therefore optimal models to understand how these components are altered by precocious exposure to EDCs. In particular we discuss effects of bisphenol A and methoxychlor on the dopaminergic and noradrenergic systems in rodents and the impact of these alterations. In addition, the effects of embryonic exposure to diethylstilbestrol, genistein or ethylene,1,1-dichloro-2,2-bis(p-chlorophenyl) is reviewed relative to behavioral impairment and associated alterations in the sexually dimorphic parvocellular vasotocin system in quail. We point out how sexually dimorphic behaviors are particularly useful to verify adverse developmental consequences produced by chemicals with endocrine disrupting properties, by examining either reproductive or non-reproductive behaviors.

Neuroactive steroids: old players in a new game

It is now clear that the study of the effects exerted by steroids on the nervous system may be considered as one of the most interesting and promising topics for biomedical research. Indeed, new effects, mechanisms of action and targets are becoming more and more evident suggesting that steroids are not only important key regulators of nervous system function but they may also represent a new therapeutic tool to combat certain diseases of the nervous system. The present review summarizes recent observations on this topic indicating that while the concept of the nervous system as a target for steroid hormones has been appreciated for decades, a promising new era for the study of these molecules and their actions in the nervous system has been initiated in the last few years.

Effects of gonadal hormones on central nitric oxide producing systems

Nitric oxide-containing neurons are widely distributed within the CNS, including regions involved in the control of reproduction and sexual behavior. The expression of neuronal nitric oxide synthase is influenced by testosterone in male rat, and by estrogens in female. Moreover, nitric oxide synthase may co-localize with gonadal hormones' receptors. Gonadal hormones may influence nitric oxide synthase expression in adulthood as well as during the development. In fact, in mice knockout for estrogen receptor alpha, the nitric oxide synthase-expressing population is deeply reduced in specific regions. In physiological conditions, the female in mammalian species is exposed to short-term changes of gonadal hormones levels (estrous cycle). Our recent studies, performed in the rat vomeronasal system and in mouse hypothalamic and limbic systems reveal that, in rodents, the expression of nitric oxide synthase-producing elements within regions relevant for the control of sexual behavior is under the control of gonadal hormones. The expression of nitric oxide synthase may vary according to the rapid variations of hormonal levels that take place during the estrous cycle. This seems in accordance with the hypothesis that gonadal hormone activation of nitric oxide-cyclic guanosine-monophosphate pathway is important for lordosis behavior, as well as that this system is activated during mating behavior. Finally, comparative data available for other vertebrates suggest that class-specific and species-specific differences occur in the nitric oxide synthase system of hypothalamus and limbic structures. Therefore, particular caution is needed to generalize data obtained from studies in rodents.

Sexual differentiation of central vasopressin and vasotocin systems in vertebrates: different mechanisms, similar endpoints

Vasopressin neurons in the bed nucleus of the stria terminalis and amygdala and vasotocin neurons in homologous areas in non-mammalian vertebrates show some of the most consistently found neural sex differences, with males having more cells and denser projections than females. These projections have been implicated in social and reproductive behaviors but also in autonomic functions. The sex differences in these projections may cause as well as prevent sex differences in these functions. This paper discusses the anatomy, steroid dependency, and sexual differentiation of these neurons. Although the final steps in sexual differentiation of vasopressin/vasotocin expression may be similar across vertebrate species, what triggers differentiation may vary dramatically. For example, during development, estrogen masculinizes vasopressin expression in rats but feminizes its counterpart in Japanese quail. Apparently, nature consistently finds a way of maintaining sex differences in vasopressin and vasotocin pathways, suggesting that the function of these differences is important enough that it was conserved during evolution.

Introduction: neurobiological impact of environmental estrogens

This paper provides an introduction to a special issue dedicated to the action of environmental estrogens on neural circuits and behavior. The problem of endocrine disrupting chemicals (EDCs), i.e. chemicals that have the capacity to interfere with the endocrine system, has gained increasing attention as it has become clear that these environmental contaminants may be active in humans, as well as in wildlife and domestic animal species. The majority of the early investigations were aimed at the discovery of the toxicological effects of the EDCs, but biomedical observations were among some of the first indications that estrogenic compounds may exert deleterious effects, even some time after exposure. The data derived from women exposed prenatally to diethylstilbesterol provided powerful evidence for long-term effects and endocrine disruption associated with selected compounds. The examination of wild animal populations exposed to industrial chemicals showed that the chemical exposure, though nonlethal, left the individual impaired or even incapable of reproducing. Among the multiple targets of the action of EDCs, several researches performed in recent years have investigated subtle modifications of the animal behaviors (reproductive, aggressive) that are likely to be related to alterations of specific neural pathways. We have, therefore, focused here on the behavioral studies as one of the more powerful tools to investigate EDCs effects on specific neural circuits.

The vasotocinergic system in the hypothalamus and limbic region of the budgerigar (Melopsittacus undulatus)

We report a morphological and biochemical analysis on the presence, distribution and quantification of vasotocin in the hypothalamus and limbic region of the budgerigar Melopsittacus undulatus, using immunohistochemistry on serial sections and competitive enzyme linked immunoadsorbent assay measurements on tissue extracts. Analysis of the sections showed large vasotocin-immunoreactive neurons in three main regions of the diencephalon, of both male and female specimens. Vasotocinergic cell bodies were located in the ventral and lateral areas of the hypothalamus, dorsal to the lateral thalamus and medial to the nucleus geniculatus lateralis. Immunoreactive neurons were placed also periventricularly, close to the walls of the third ventricle, at the level of the magnocellular paraventricular nucleus. Well evident bundles of immunoreactive fibers were placed ventral to the anterior commissure in the same regions of the hypothalamus and thalamus where vasotocinergic perikarya are localized. Fibers were identified close to the third ventricle, and in the lateral hypothalamic area along the lateral forebrain bundle. In contrast to what reported for other oscine and non-oscine avian species, we were not able to identify immunopositive neurons in any region above the anterior commissure, or detect relevant differences on the distribution of the vasotocin immmunoreactivity between sexes. Competitive enzyme linked immunoadsorption assay and image analysis of the extension of immunoreactivity in the tissue sections were consistent with the qualitative observations and indicated that there is no statistically significant dimorphism in the content of vasotocin or in the location and distribution of vasotocinergic elements in the investigated areas of male and female parrot brains.

Region-specific testosterone modulation of the vasotocin-immunoreactive system in male dark-eyed junco, Junco hyemalis

The nonapeptide vasotocin (VT) is the avian equivalent of the mammalian antidiuretic hormone vasopressin and is believed to control aggressive and reproductive behaviors. Brain VT distribution has been described in several domesticated avian species. We previously demonstrated that VT distribution in the brain of a free-ranging male passerine, the dark-eyed Junco, Junco hyemalis, resembles that in domesticated birds. A preliminary study also suggested that the VT-immunoreactive (VT-ir) system of juncos is regulated by testosterone (T), as is the case of galliforms. To test this hypothesis, we investigated the effects of castration and T replacement on brain VT-ir innervation in adult male juncos. Castration reduced VT-ir innervation in the lateral septum (SL), the medial preoptic nucleus, the nucleus of the stria terminalis and the intercollicularis nucleus. These effects of castration were largely reversed by T treatment at high physiological doses, but significantly so only for the SL. Given the demonstrated behavioral role of the above VT-ir-containing brain regions, the results suggest that these regions may be sites of action of VT on reproductive behaviors.

Expression of neuropeptides and growth-associated protein 43 (GAP-43) in cutaneous and mucosal nerve structures of the adult rat lower lip after mental nerve section

The reinnervation of the adult rat lower lip has been investigated after unilateral section of the mental nerve. Rats were sacrificed at 4, 7, 9, 14, 30, and 90 days after the operation. A further group of animals with section of the mental nerve and block of the alveolar nerve regeneration, was sacrificed at 14 days. Specimens were processed for immunocytochemistry with antibodies against PGP 9.5, GAP-43 or neuropeptides (CGRP, SP and VIP). Four days after nerve section, axonal degeneration seems evident in the mental nerve branches and inside skin and mucosa. GAP-43 immunoreactivity is intense in the mental nerve 7 days after nerve section and it reaches its maximal expression and distribution in peripheral nerve fibres at 14 days. At 30 days, the decline in its expression is associated with the increase of PGP9.5-, SP-, and CGRP immunopositivity. VIP is observed only in perivascular fibres at all times observed. Present results suggest that, after sensory denervation of the rat lip, nerve fibres in skin and mucosa remain at lower density than normal. The different time courses in the expression of neuropeptides and GAP-43 suggest a possible early involvement of GAP-43 in peripheral nerve regeneration.

Changes in the arginine-vasopressin immunoreactive systems in male mice lacking a functional aromatase gene

In male rodents, the arginine-vasopressin-immunoreactive (AVP-ir) neurones of the bed nucleus of the stria terminalis (BNST) and medial amygdala are controlled by plasma testosterone levels (decreased after castration and restored by exogenous testosterone). AVP transcription in these nuclei is increased in adulthood by a synergistic action of the androgenic and oestrogenic metabolites of testosterone and, accordingly, androgen and oestrogen receptors are present in both BNST and medial amygdala. We used knockout mice lacking a functional aromatase enzyme (ArKO) to investigate the effects of a chronic depletion of oestrogens on the sexually dimorphic AVP system. Wild-type (WT) and ArKO male mice were perfused 48 h after an i.c.v. colchicine injection and brain sections were then processed for AVP immunocytochemistry. A prominent decrease (but not a complete suppression) of AVP-ir structures was observed in the BNST and medial amygdala of ArKO mice by comparison with the WT. Similarly, AVP-ir fibres were reduced in the lateral septum of ArKO mice and but not in the medial preoptic area, a region where the AVP system is not sexually dimorphic in rats. No change was detected in the supraoptic and suprachiasmatic nuclei. However, a decrease in AVP-ir cell numbers was however, detected in one subregion of the paraventricular nucleus. These data support the hypothesis that the steroid-sensitive sexually dimorphic AVP system of the mouse forebrain is mainly under the control of aromatized metabolites of testosterone.

Sexually dimorphic neurogenesis is topographically matched with the anterior accessory olfactory bulb of the adult rat

The accessory olfactory bulb (AOB) is a sexually dimorphic structure of the vomeronasal system, which plays a role in the control of sexual behaviors. In adult rats, we have demonstrated previously that the migrating neuroblasts of the subependymal layer (SEL) directed to the main olfactory bulb (MOB) also reach the AOB. To tackle the relation between sexual dimorphism and targeted cell migration, we quantified the neo-neurogenesis in the AOB of adult rats of both sexes. Our results confirm a morphological sexual dimorphism in the AOB granular layer volumes. We showed that the number of newly generated cells reaching the AOB in both sexes was considerable, even if lower than those directed to the MOB. Moreover, we demonstrated that the rate of neurogenesis in the anterior AOB of the two sexes was significantly different.

Estradiol mediates effects of testosterone on vasotocin immunoreactivity in the adult quail brain

In adult male quail, the activation of sexual behavior by testosterone (T) is mediated at the cellular level by the interaction of T metabolites with intracellular steroid receptors. In particular, the aromatization of T into an estrogen plays a key limiting role. Nonaromatizable androgens such 5alpha-dihydrotestosterone (DHT) synergize with estradiol (E2) to activate the behavior. Given that the density of vasotocin (VT) immunoreactive structures is increased by T in adult male quail and that VT injections affect male behavior, we wondered whether the expression of VT is also affected by T metabolites such as E2 and DHT. We analyzed here, in castrated male quail, the effects of a treatment with T, E2, DHT, or E2 + DHT on sexual behavior and brain VT immunoreactivity. The restoration by T of the VT immunoreactivity in the medial preoptic nucleus, bed nucleus striae terminalis, and lateral septum of castrated male quail could be fully mimicked by a treatment with E2. The androgen DHT had absolutely no effect on the VT immunoreactivity in these conditions and, at the doses used here, DHT did not synergize with E2 to enhance the density of VT immunoreactive structures. These effects of T metabolites in the brain were not fully correlated with their effects on the activation of male copulatory behavior, suggesting that the increase in VT expression in the brain does not represent a necessary step for the activation of behavior. Although VT expression in the medial preoptic nucleus and bed nucleus striae terminalis is often tightly correlated with the expression of male copulatory behavior, VT presumably does not represent simply 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.

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

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

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.

GABAergic and NPY-Y(1) network in the medial amygdala: a neuroanatomical basis for their functional interaction

We used Y(1)R/LacZ transgenic mice to investigate the interaction between NPY, GABA and Y(1) receptors in the amygdala. Immunolabeling of GABA and NPY positive neurons and histochemical staining of beta-galactosidase revealed NPY and GABA colocalization and close contacts of NPY-positive fibers with GABAergic neurons also expressing the Y(1)R/LacZ transgene.

Increased expression of the neuropeptide Y receptor Y(1) gene in the medial amygdala of transgenic mice induced by long-term treatment with progesterone or allopregnanolone

The neurosteroid allopregnanolone, a reduced metabolite of progesterone, induces anxiolytic effects by enhancing GABA(A) receptor function. Neuropeptide Y (NPY) and GABA are thought to interact functionally in the amygdala, and this interaction may be important in the regulation of anxiety. By using Y(1)R/LacZ transgenic mice, which harbour a fusion construct comprising the promoter of the mouse gene for the Y(1) receptor for NPY linked to the lacZ gene, we previously showed that long-term treatment with benzodiazepine receptor ligands modulates Y(1) receptor gene expression in the medial amygdala. We have now investigated the effects of prolonged treatment with progesterone or allopregnanolone on Y(1)R/LacZ transgene expression, as determined by quantitative histochemical analysis of beta-galactosidase activity. Progesterone increased both the cerebrocortical concentration of allopregnanolone and beta-galactosidase expression in the medial amygdala. Finasteride, a 5alpha-reductase inhibitor, prevented both of these effects. Long-term administration of allopregnanolone also increased both the cortical concentration of this neurosteroid and transgene expression in the medial amygdala. Treatment with neither progesterone nor allopregnanolone affected beta-galactosidase activity in the medial habenula. These data suggest that allopregnanolone regulates Y(1) receptor gene expression through modulation of GABA(A) receptor function, and they provide further support for a functional interaction between GABA and neuropeptide Y in the amygdala.

Cutaneous innervation in hereditary sensory and autonomic neuropathy type IV

The authors investigated immunocytochemically the innervation of a skin biopsy in a rare case of hereditary sensory and autonomic neuropathy type IV. A few protein gene product 9.5-, growth-associated protein 43-, calcitonin gene-related peptide-, and substance P-immunoreactive nerve fibers were observed in the deeper regions of the dermis. Neuropeptide Y-, nitric oxide-, and vasoactive intestinal polypeptide-immunoreactive fibers were completely absent. Their observations support the hypothesis that the sensory and autonomic defects reported in hereditary sensory and autonomic neuropathy are based on profound developmental alterations of the peripheral nervous system.

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.

Immunocytochemical evidence of plasticity in the nervous structures of the rat lower lip

In this immunocytochemical study we investigated the distribution of nervous structures in the lower lip of adult rats. The region is characterized by a rich cutaneous and mucosal sensory innervation originating from terminal branches of the trigeminal system. Lower lip innervation was investigated by detection of the general neuronal marker protein gene product 9.5 (PGP 9.5) and the growth-associated protein 43 (GAP-43), a neurochemical marker of neuronal plasticity. The entire neural network of both cutaneous and mucosal aspects was stained by the antibody to PGP 9.5. In particular, nerve fibers were observed in the submucosal and the subepithelial plexuses. Thin immunoreactive fibers were observed within the epithelial layers ending as free fibers or as fibers associated with immunopositive Merkel cells. Well-identified anatomical structures receiving sensory or autonomic innervation were also surrounded by PGP 9.5-ir nerve fibers, in particular, hair follicles, vibrissae, glands, and blood vessels. GAP-43-immunostained nerve fibers were observed in all these structures; however, they were generally less numerous than the PGP 9.5-immunoreactive elements. An equal amount of PGP 9.5 and GAP-43 immunoreactivity occurred, in contrast, in the subepidermal and the submucosal plexuses, or in the epidermis and the mucosal epithelium. The present results show that GAP-43 is normally expressed in the mature trigeminal sensory system of the rat. Skin and oral mucosa are characterized by continuous remodeling that may also involve the sensory nervous apparatus. Continuous neural remodeling, regeneration and sprouting may be the reason for the observed expression of GAP-43.

Central vasotocin-immunoreactive system in a male passerine bird (Junco hyemalis)

Previous investigations have identified regions of the avian brain that contain immunoreactive vasotocinergic (VT-ir) cell bodies and fibers. These studies exclusively used domesticated species, and the relevance of the findings for free-living birds has not been established. The present study used immunocytochemistry to determine the neuroanatomical distribution of the VT-ir system in the brain of a well-studied male passerine bird (dark-eyed junco, Junco hyemalis) obtained from a natural population in interior Alaska (65 degrees N, 147 degrees W). VT-ir cell bodies were observed in several brain regions (paraventricular and supraoptic nuclei, nucleus of the stria terminalis), where they have been described in other oscine species. VT-ir fibers were widespread in many brain regions and were especially abundant in the medial preoptic nucleus, the basal region of the septum, and the hypothalamic-neurohypophyseal tract. Fibers were also present in brain regions that are involved in the control of vocal behavior including the ventromedial capsular region of the nucleus robustus archistriatalis and the dorsomedial portion of the mesencephalic nucleus intercollicularis. The widespread brain distribution of VT-ir cell bodies and fibers in juncos generally resembles that of domestic birds and suggests a role for this neuropeptide in the control of reproductive behavior and physiology.

Gonadal steroid-dependent neuronal circuitries in avian limbic and preoptic regions

In the avian limbic and preoptic region, the sexually dimorphic medial preoptic nucleus and nucleus of the stria terminalis are characterized by the presence of a testosterone-dependent aromatase-immunoreactive neuronal population. In situ hybridization studies confirmed that testosterone is modulating the expression of aromatase gene. Both nuclei are also characterized by a sexually dimorphic, testosterone-dependent vasotocin system. Immunocytochemical and in situ hybridization data, demonstrated that dimorphism and testosterone-sensitivity of this system are both dependent by an embryonic organizational effect of the estradiol. Intracerebroventricularly injected vasotocin, has a profound inhibitory effect on the male sexual behaviour, and immunocytochemical investigations revealed close associations among vasotocin fibres and aromatase cell bodies. These data suggest that this neuropeptidergic system could have a key role in the circuitries controlling different aspects of male reproductive behaviour in the Japanese quail.

Organizational effects of estrogens on brain vasotocin and sexual behavior in quail

Reproductive behavior is sexually differentiated in quail: The male-typical copulatory behavior is never observed in females even after treatment with high doses of testosterone (T). This sex difference in behavioral responsiveness to T is organized during the embryonic period by the exposure of female embryo to estrogens. We showed recently that the sexually dimorphic medial preoptic nucleus (POM), a structure that plays a key role in the activation of male copulatory behavior, is innervated by a dense steroid-sensitive network of vasotocin-immunoreactive (VT-ir) fibers in male quail This innervation is almost completely absent in the female POM and is not induced by a chronic treatment with T, suggesting that this neurochemical difference could be organizational in nature. This idea was tested by injecting fertilized quail eggs of both sexes on day 9 of incubation with either estradiol benzoate (EB) (25 microg, a treatment that suppresses the capacity to show copulatory behavior in adulthood) or the aromatase inhibitor R76713 (10 microg, a treatment that makes adult females behaviorally responsive to T), or with the solvents as a control (C). At 3 weeks posthatch, all subjects were gonadectomized and later implanted with Silastic capsules filled with T. Two weeks later, all birds were perfused and brain sections were processed for VT immunocytochemistry. Despite the similarity of the adult endocrine conditions of the subjects (all were gonadectomized and treated with T Silastic implants providing the same plasma level of steroid to all subjects), major qualitative differences were observed in the density of VT-ir structures in the POM of the different groups. Dense immunoreactive structures (fibers and a few cells) were observed in the POM of C males but not females; EB males had completely lost this immunoreactivity (and lost the capacity to display copulatory behavior); and, conversely, R76713 females displayed a male-typical VT-ir system in the nucleus (and also high levels of copulatory behavior). Similar changes in immunoreactivity were seen in the nucleus of the stria terminalis and in the lateral septum (VT-ir fibers only in this case) but not in the magnocellular vasotocinergic system. These neurochemical changes closely parallel the effects of the embryonic treatments on male copulatory behavior. The vasotocinergic system of the POM can therefore be considered an accurate marker of the sexual differentiation of brain circuits mediating this behavior.

Anatomical and neurochemical definition of the nucleus of the stria terminalis in Japanese quail (Coturnix japonica)

This study in birds provides anatomical, immunohistochemical, and hodological data on a prosencephalic region in which the nomenclature is still a matter of discussion. In quail, this region is located just dorsal to the anterior commissure and extends from the level of the medial part of the preoptic area at its most rostral end to the caudal aspects of the nucleus preopticus medialis. At this caudal level, it reaches its maximal elongation and extends from the ventral tip of the lateral ventricles to the dorsolateral aspects of the paraventricular nucleus. This area contains aromatase-immunoreactive cells and a sexually dimorphic population of small, vasotocinergic neurons. The Nissl staining of adjacent sections revealed the presence of a cluster of intensely stained cells outlining the same region delineated by the vasotocin-immunoreactive structures. Cytoarchitectonic, immunohistochemical, and in situ hybridization data support the notion that this area is similar and is probably homologous to the medial part of the nucleus of the stria terminalis of the mammalian brain. The present data provide a clear definition of this nucleus in quail: They show for the first time the presence of sexually dimorphic vasotocinergic neurons in this region of the quail brain and provide the first detailed description of this region in an avian species.

Distribution and effects of testosterone on aromatase mRNA in the quail forebrain: a non-radioactive in situ hybridization study

A number of studies have been devoted to the analysis of the anatomical distribution, control by steroids and functional significance of aromatase (the enzyme metabolizing testosterone into 17beta-estradiol) in the quail brain. In particular, the sexually dimorphic nucleus preopticus medialis has been the main focus of investigation because testosterone aromatization in this structure mediates the activation of male sexual behavior and aromatase activity is itself testosterone-dependent in this nucleus. No information on the anatomical distribution of aromatase gene expression is, however, available so far in this avian species. In the present study we applied a non-radioactive in situ hybridization technique to describe the distribution of aromatase mRNA containing neurons in the quail prosencephalon. We also analyzed, at a neuronal level of resolution, the induction by testosterone of this mRNA in the medial preoptic nucleus. Dense clusters of aromatase gene expressing neurons were observed within the medial preoptic nucleus, the nucleus of the stria terminalis, the ventro-medial hypothalamus and the tuberal region. Scattered neurons expressing lower levels of aromatase mRNA were also found in the dorsal thalamic area and central gray. The specificity of the staining was confirmed by demonstrating the absence of signal in sections that had been hybridized with a sense probe. Moreover, the distribution of the aromatase mRNA containing cells completely overlapped with the distribution of the aromatase-immunoreactive cells. Aromatase-mRNA expression was controlled by testosterone (or its metabolites) in the entire medial preoptic nucleus. Castration resulted in a decrease in the number of aromatase mRNA-containing cells and this effect was totally reversed by testosterone treatment. These data further support the idea that testosterone regulates the rate of its own aromatization by modulating the expression of aromatase rather than by acting at a post transcriptional level.

Neuroendocrine regulation of GnRH and behavior during aging in birds

Avian species exhibit a great variety of life-long patterns in reproduction. Japanese quail are relatively short lived and undergo an age-related loss of reproductive function, making this species an excellent model for the study of the basic biology of aging. Because individuals age at variable rates, sexual behavior has provided a useful index to assess reproductive status of individuals of the same chronological age. Further, exogenous testosterone restores sexual behavior in reproductively senescent male quail, thereby providing evidence for a continued ability of the system to respond. In addition, we have been studying hypothalamic neuroendocrine systems that regulate the endocrine as well as behavioral components of reproduction. Overall, our findings point to the hypothalamic neuroendocrine systems as the site of initial age-related alterations that contribute to the reproductive deterioration. Specifically, we studied adrenergic, opioid peptide, vasotocin, and aromatase systems to understand their relationship to the cGnRH-I system and their potential role in the deterioration of the cGnRH-I system during aging. Our findings provide evidence for qualitative and quantitative alterations in the aromatase enzyme system, which can be partially restored with exogenous testosterone. In addition, other neuronal systems, including the vasotocin system, decline with the loss of gonadal steroids and are restimulated with exogenous testosterone. We will synthesize the data relative to these neuroendocrine systems with attention to the effects of gonadal steroids on these systems during aging.

Anatomically specific colocalization of NADPH-diaphorase and choline acetyltransferase in the quail brainstem

On the basis of previous studies demonstrating a wide colocalization of NADPH-diaphorase (ND) and choline acetyltransferase (ChAT) in mammalian and reptilian brainstem, ND histochemistry and ChAT immunocytochemistry have been combined to study the distribution of both markers in the mesopontine region of an avian species, the Japanese quail (Coturnix japonica). Co-existence of the two neurochemical markers is present only in part of the system, namely, in the nucleus tegmentalis pedunculo-pontinus, in the nucleus tegmentalis latero-dorsalis, in the nucleus mesencephalicus profundus, and in the nucleus reticularis paragigantocellularis. The degree of colocalization varies in these regions from about 50 to 95% of the ChAT population. However, several other nuclei in the same region display only one of the two markers. These results confirm that even if the general distribution of the ND-positive neurons is largely comparable in vertebrates, there exists species-specific differences in the extension of the system and in the degree of colocalization with other neurochemical markers.

Vasotocinergic innervation of areas containing aromatase-immunoreactive cells in the quail forebrain

In the male quail forebrain, aromatase-immunoreactive (ARO-ir) elements are clustered within the sexually dimorphic medial preoptic nucleus (POM), nucleus striae terminalis (nST), nucleus accumbens (nAc), and ventromedial and tuberal hypothalamus. These ARO-ir cells are sensitive to testosterone and its metabolites: Their number and size increase after exposure to these steroids. The POM and lateral septum are also characterized by a dense vasotocinergic innervation that is also sensitive to testosterone. We analyzed here the anatomical relationships between ARO-ir elements and VT-ir fibers in the quail prosencephalon. Sequential staining for vasotocin, aromatase, or vasotocin plus aromatase was performed on adjacent 30-microm-thick cryostat sections. High concentrations of thin VT-ir fibers were observed within the POM, nST, lateral septum, periventricular mesencephalic central gray, and ventromedial and tuberal hypothalamus. There was a close correspondence between the extension of the ARO-ir cells and of VT-ir fibers. In double-labeled sections, all clusters of ARO-ir cells with the exception of those located in the nAc were embedded in a dense network of VT-ir fibers. Many of the VT-ir terminals appeared to end in the neuropile surrounding ARO-ir elements rather than directly on their cell bodies. This study supports the idea that the testosterone-dependent aromatase system is directly innervated by a testosterone-dependent peptidergic system. Aromatase-containing cells could therefore be modulated by steroids both directly and indirectly through the vasotocin system. Alternatively, this neuroanatomical arrangement may mediate the control of vasotocin synthesis or release by steroids. Functional studies demonstrate that both aromatase and vasotocin affect reproductive behavior in quail, and the present data provide anatomical support for the integration of these effects.

The NADPH-diaphorase-containing system in the brain of the budgerigar (Melopsittacus undulatus)

In the present investigation we studied the presence and distribution of histochemically detected neuronal NADPH-diaphorase (ND) in the brain of the budgerigar, Melopsittacus undulatus. Positive neurons are widely distributed throughout the central nervous system. ND-containing neurons are present in the telencephalon and the paleostriatal-parolfactory lobe complex. Positive cells were observed also in the neostriatum, including the main auditory area (field L), in several nuclei of the archistriatum and in the hyperstriatum (accessory, dorsal, and ventral). In the diencephalon, positive neurons were present both in the lateral hypothalamic and periventricular areas, and in a segregate area at the confluence of the anterior commissure and the lateral prosencephalic bundle. A group of positive perikarya was located lateral to the dorsal part of the IIIrd ventricle, and continued laterally into the thalamus. Weakly stained neurons were observed in the thalamic dorsomedial posterior nucleus. In the mesencephalon, ND-containing neurons were scattered in the reticular formation (pars lateralis and pars medialis) and in the optic tecta. A large population of positive neurons was observed in the substantia nigra, the ventral area of Tsai and the nucleus interpeduncularis. Positive neurons extended through the tegmental nuclei to the locus coeruleus. In the cerebellum, the granular neurons were weakly stained and the internal cerebellar nuclei were surrounded by a wide network of positive fibers. In the medulla the number of positive cells was highly reduced, but stained neurons were observed in the cochlear as well in the vestibular nuclei. The data here presented suggest that the distribution of ND-containing neurons in the brain of the budgerigar is different from those of the chicken and quail. The locations of positive neurons suggest also a possible involvement in sound perception and production pathways, and visual perception.

Connections of the dorsomedial part of the nucleus intercollicularis in a male non-songbird, the Grey partridge: a tract-tracing study

Vocal control systems have been poorly investigated in non-songbirds. In this study we describe descending neural pathways to the dorsomedial portion of the nucleus intercollicularis (ICo) in a galliform (male Grey partridges) by means of the DiI in vitro tracing technique. The simple and sex-dimorphic vocalizations of partridges, which have a critical role in sexual selection, favour this species as a model system for the study of vocal control mechanisms. Our data demonstrate that the ICo, an important site mediating the activation of vocal behavior in all birds, receives afferents from several important higher centers: the nucleus pretectalis, the tuberoinfundibular hypothalamic region, the dorsal thalamus, the preoptic region and the paleostriatal region. Efferent connections of the ICo were directed mainly to the hypothalamic area. This complex neural pathway is consistent with a major role of ICo in male courtship and vocal performance control.

Absence of coexistence between NADPH-diaphorase and antidiuretic hormone in the hypothalamus of two galliforms: Japanese quail (Coturnix japonica) and chicken (Gallus domesticus)

Based on previous studies demonstrating coexistence of NADPH-diaphorase (ND) and vasopressin (VP) in the rat hypothalamus. ND histochemistry and vasotocin (VT) immunocytochemistry have been combined in order to study the distribution of both markers in the hypothalamus of the Japanese quail (Coturnix japonica) and chicken (Gallus domesticus). No coexistence was found, however, close anatomical relationships between ND-positive and VT-immunoreactive elements were observed in specific preoptic and hypothalamic locations. These findings indicate interspecies differences in the expression of ND and the antidiuretic hormone with functional implications in osmoregulation.

Coexistence of NADPH-diaphorase and tyrosine hydroxylase in the mesencephalic catecholaminergic system of the Japanese quail

Previous studies have shown the presence of a large number of tyrosine hydroxylase immunoreactive neurons and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase positive elements within the mesencephalic and pontine regions of the Japanese quail. In the present study histochemical and immunohistochemical procedures reveal that cells expressing at least one of these two neurochemical markers coexist throughout a large part of the substantia nigra and of the area ventralis of Tsai. Also about 40% of the neurons in these two regions that contain immunoreactive tyrosine hydroxylase also exhibit NADPH-diaphorase activity. This is not a general property of the quail catecholaminergic system: in the locus coeruleus (the main noradrenergic group) there is a complete separation between these two neuronal populations. The number of neurons expressing either neurochemical marker is not different between males and females in any of the regions that have been investigated. NADPH-diaphorase is known to be an indicator of the enzyme nitric oxide synthase; these results therefore suggest that nitric oxide may play an important role in the regulation of the activity of a significant part of the avian mesencephalic dopaminergic system.