Estradiol is concentrated in tyrosine hydroxylase-containing neurons of the hypothalamus

Peering into the Brain's Estrogen Receptors: PET Tracers for Visualization of Nuclear and Extranuclear Estrogen Receptors in Brain Disorders

Estrogen receptors (ERs) play a multitude of roles in brain function and are implicated in various brain disorders. The use of positron emission tomography (PET) tracers for the visualization of ERs' intricate landscape has shown promise in oncology but remains limited in the context of brain disorders. Despite recent progress in the identification and development of more selective ligands for various ERs subtypes, further optimization is necessary to enable the reliable and efficient imaging of these receptors. In this perspective, we briefly touch upon the significance of estrogen signaling in the brain and raise the setbacks associated with the development of PET tracers for identification of specific ERs subtypes in the brain. We then propose avenues for developing efficient PET tracers to non-invasively study the dynamics of ERs in the brain, as well as neuropsychiatric diseases associated with their malfunction in a longitudinal manner. This perspective puts several potential candidates on the table and highlights the unmet needs and areas requiring further research to unlock the full potential of PET tracers for ERs imaging, ultimately aiding in deepening our understanding of ERs and forging new avenues for potential therapeutic strategies.

Chromosomal and gonadal factors regulate microglial sex effects in the aging brain

Biological sex contributes to phenotypic sex effects through genetic (sex chromosomal) and hormonal (gonadal) mechanisms. There are profound sex differences in the prevalence and progression of age-related brain diseases, including neurodegenerative diseases. Inflammation of neural tissue is one of the most consistent age-related phenotypes seen with healthy aging and disease. The pro-inflammatory environment of the aging brain has primarily been attributed to microglial reactivity and adoption of heterogeneous reactive states dependent upon intrinsic (i.e., sex) and extrinsic (i.e., age, disease state) factors. Here, we review sex effects in microglia across the lifespan, explore potential genetic and hormonal molecular mechanisms of microglial sex effects, and discuss currently available models and methods to study sex effects in the aging brain. Despite recent attention to this area, significant further research is needed to mechanistically understand the regulation of microglial sex effects across the lifespan, which may open new avenues for sex informed prevention and treatment strategies.

Elevated prolactin secretion during proestrus in mice: Absence of a defined surge

Throughout the reproductive cycle in rodents, prolactin levels are generally low. In some species, including rats, a prolactin surge occurs on proestrus with peak concentrations coinciding with the preovulatory luteinizing hormone (LH) surge. In mice, however, there are conflicting reports relating to the occurrence and timing of a proestrous prolactin surge. To gain further insight into the incidence and characteristics of this surge in mice, we have used serial tail tip blood sampling and trunk blood collection from both C57BL/6J (inbred) and Swiss Webster (outbred) mouse strains to build a profile of prolactin secretion during proestrus in individual mice. A clearly defined LH surge was detected in most animals, suggesting the blood sampling approach was suitable for detecting patterns of hormone secretion on proestrus. Despite this, levels of prolactin were quite variable between individuals. Overall both mouse strains showed a generalized rise in prolactin levels on the day of proestrus compared with levels seen in diestrus. This pattern is quite distinct from the discreet, circadian-entrained surge observed in rats.

Alterations in corticotropin-releasing factor receptor type 1 in the preoptic area and hypothalamus in mice during the postpartum period

Corticotropin-releasing factor (CRF) signaling through CRF receptor 1 (CRFR1) regulates autonomic, endocrine, and behavioral responses to stress, as well as behavioral changes during the maternal period. Previous work in our lab reported higher levels of CRFR1 in female, compared to male, mice within the rostral anteroventral periventricular nucleus (AVPV/PeN), a brain region involved in maternal behaviors. In this study, we used CRFR1-GFP reporter mice to investigate whether the reproductive status (postpartum vs. nulliparous) of acutely stressed females affects levels of CRFR1 in the AVPV/PeN and other regions involved in maternal functions. Compared to nulliparous, postpartum day 14 females showed increased AVPV/PeN CRFR1-GFP immunoreactivity and an elevated number of restraint stress-activated AVPV/PeN CRFR1 cells as assessed by immunohistochemical co-localization of CRFR1-GFP and phosphorylated CREB (pCREB). The medial preoptic area (MPOA) and paraventricular hypothalamus (PVN) of postpartum mice showed modest decreases in CRFR1-GFP immunoreactivity, while increased CRFR1-GFP/pCREB co-expressing cells were found in the PVN following restraint stress relative to nulliparous mice. Tyrosine hydroxylase (TH) and CRFR1-GFP co-localization was also assessed in the AVPV/PeN and other regions and revealed a decrease in co-localized neurons in the AVPV/PeN and ventral tegmental area of postpartum mice. Corticosterone analysis of restrained mice revealed blunted peak, but elevated recovery, levels in postpartum compared to nulliparous mice. Finally, we investigated projection patterns of AVPV/PeN CRFR1 neurons using female CRFR1-Cre mice and revealed dense efferent projections to several preoptic, hypothalamic, and hindbrain regions known to control stress-associated and maternal functions. Together, these findings contribute to our understanding of the neurobiology that might underlie changes in stress-related functions during the postpartum period.

Hypothalamic dopamine neurons motivate mating through persistent cAMP signalling

Transient neuromodulation can have long-lasting effects on neural circuits and motivational states^1-4. Here we examine the dopaminergic mechanisms that underlie mating drive and its persistence in male mice. Brief investigation of females primes a male's interest to mate for tens of minutes, whereas a single successful mating triggers satiety that gradually recovers over days⁵. We found that both processes are controlled by specialized anteroventral and preoptic periventricular (AVPV/PVpo) dopamine neurons in the hypothalamus. During the investigation of females, dopamine is transiently released in the medial preoptic area (MPOA)-an area that is critical for mating behaviours. Optogenetic stimulation of AVPV/PVpo dopamine axons in the MPOA recapitulates the priming effect of exposure to a female. Using optical and molecular methods for tracking and manipulating intracellular signalling, we show that this priming effect emerges from the accumulation of mating-related dopamine signals in the MPOA through the accrual of cyclic adenosine monophosphate levels and protein kinase A activity. Dopamine transients in the MPOA are abolished after a successful mating, which is likely to ensure abstinence. Consistent with this idea, the inhibition of AVPV/PVpo dopamine neurons selectively demotivates mating, whereas stimulating these neurons restores the motivation to mate after sexual satiety. We therefore conclude that the accumulation or suppression of signals from specialized dopamine neurons regulates mating behaviours across minutes and days.

Patterns of prolactin secretion

Prolactin is pleotropic in nature affecting multiple tissues throughout the body. As a consequence of the broad range of functions, regulation of anterior pituitary prolactin secretion is complex and atypical as compared to other pituitary hormones. Many studies have provided insight into the complex hypothalamic-pituitary networks controlling prolactin secretion patterns in different species using a range of techniques. Here, we review prolactin secretion in both males and females; and consider the different patterns of prolactin secretion across the reproductive cycle in representative female mammals with short versus long luteal phases and in seasonal breeders. Additionally, we highlight changes in the pattern of secretion during pregnancy and lactation, and discuss the wide range of adaptive functions that prolactin may have in these important physiological states.

Diverse actions of estradiol on anorexigenic and orexigenic hypothalamic arcuate neurons

Contribution to Special Issue on Fast effects of steroids. There is now compelling evidence for membrane-associated estrogen receptors in hypothalamic neurons that are critical for the hypothalamic control of homeostatic functions. It has been known for some time that estradiol (E2) can rapidly alter hypothalamic neuronal activity within seconds, indicating that some cellular effects can occur via membrane initiated events. However, our understanding of how E2 signals via membrane-associated receptors and how these signals impact physiological functions is only just emerging. Thus, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell excitability and even gene transcription in hypothalamic neurons. One population of hypothalamic neurons, the anorexigenic proopiomelanocortin (POMC) neurons, has long been considered to be a target of E2's actions based on gene (Pomc) expression studies. However, we now know that E2 can rapidly alter POMC neuronal activity within seconds and activate several intracellular signaling cascades that ultimately affect gene expression, actions which are critical for maintaining sensitivity to insulin in metabolically stressed states. E2 also affects the orexigenic Neuropeptide Y/Agouti-related Peptide (NPY/AgRP) neurons in similarly rapid but antagonistic manner. Therefore, this review will summarize our current state of knowledge of how E2 signals via rapid membrane-initiated and intracellular signaling cascades in POMC and NPY/AgRP neurons to regulate energy homeostasis.

60 YEARS OF NEUROENDOCRINOLOGY: The hypothalamo-prolactin axis

The hypothalamic control of prolactin secretion is different from other anterior pituitary hormones, in that it is predominantly inhibitory, by means of dopamine from the tuberoinfundibular dopamine neurons. In addition, prolactin does not have an endocrine target tissue, and therefore lacks the classical feedback pathway to regulate its secretion. Instead, it is regulated by short loop feedback, whereby prolactin itself acts in the brain to stimulate production of dopamine and thereby inhibit its own secretion. Finally, despite its relatively simple name, prolactin has a broad range of functions in the body, in addition to its defining role in promoting lactation. As such, the hypothalamo-prolactin axis has many characteristics that are quite distinct from other hypothalamo-pituitary systems. This review will provide a brief overview of our current understanding of the neuroendocrine control of prolactin secretion, in particular focusing on the plasticity evident in this system, which keeps prolactin secretion at low levels most of the time, but enables extended periods of hyperprolactinemia when necessary for lactation. Key prolactin functions beyond milk production will be discussed, particularly focusing on the role of prolactin in inducing adaptive responses in multiple different systems to facilitate lactation, and the consequences if prolactin action is impaired. A feature of this pleiotropic activity is that functions that may be adaptive in the lactating state might be maladaptive if prolactin levels are elevated inappropriately. Overall, my goal is to give a flavour of both the history and current state of the field of prolactin neuroendocrinology, and identify some exciting new areas of research development.

Membrane-initiated actions of estradiol that regulate reproduction, energy balance and body temperature

It is well known that many of the actions of estrogens in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there now exists compelling evidence for membrane estrogen receptors in hypothalamic and other brain neurons. But, it is not well understood how estrogens signal via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that estrogens can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, estrogens can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by estrogens in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

Estrogenic regulation of dopaminergic neurons in the opportunistically breeding zebra finch

Steroid-induced changes in dopaminergic activity underlie many correlations between gonadal hormones and social behaviors. However, the effects of steroid hormones on the various behaviorally relevant dopamine cell groups remain unclear, and ecologically relevant species differences remain virtually unexplored. We examined the effects of estradiol (E2) manipulations on dopamine (DA) neurons of male and female zebra finches (Taeniopygia guttata), focusing on numbers of tyrosine hydroxylase-immunoreactive (TH-ir) cells in the A8-A15 cell groups, and on TH colocalization with Fos, conducted in the early A.M., in order to quantify basal transcriptional activity. TH is the rate-limiting enzyme for catecholamine synthesis, and specifically DA in the A8-A15 cell groups. In contrast to other examined birds and mammals, reducing E2 levels with the aromatase-inhibitor Letrozole failed to alter TH-ir neuron numbers within the ventral tegmental area (VTA; A10), while increasing neuron numbers in the central gray (CG; A11) and caudal midbrain A8 populations. Consistent with findings in other birds, but not mammals, we also found no effects of E2 manipulations (Letrozole or Letrozole plus E2 replacement) on TH-Fos colocalization in any location. In accordance with previous observations in both mammals and birds, E2 treatment decreased the number of TH-ir neurons in the A12 population of the tuberal hypothalamus, a cell group that inhibits the release of prolactin. In general, males and females exhibited similar TH-ir neuron numbers, although males exhibited significantly more TH-ir neurons in the A11 CG population than did females. These results suggest partial variability in E2 regulation of DA across species.

Seasonal and hormonal modulation of neurotransmitter systems in the song control circuit

In the years following the discovery of the song system, it was realized that this specialized circuit controlling learned vocalizations in songbirds (a) constitutes a specific target for sex steroid hormone action and expresses androgen and (for some nuclei) estrogen receptors, (b) exhibits a chemical neuroanatomical pattern consisting in a differential expression of various neuropeptides and neurotransmitters receptors as compared to surrounding structures and (c) shows pronounced seasonal variations in volume and physiology based, at least in the case of HVC, on a seasonal change in neuron recruitment and survival. During the past 30 years numerous studies have investigated how seasonal changes, transduced largely but not exclusively through changes in sex steroid concentrations, affect singing frequency and quality by modulating the structure and activity of the song control circuit. These studies showed that testosterone or its metabolite estradiol, control seasonal variation in singing quality by a direct action on song control nuclei. These studies also gave rise to the hypothesis that the probability of song production in response to a given stimulus (i.e. its motivation) is controlled through effects on the medial preoptic area and on catecholaminergic cell groups that project to song control nuclei. Selective pharmacological manipulations confirmed that the noradrenergic system indeed plays a role in the control of singing behavior. More experimental work is, however, needed to identify specific genes related to neurotransmission that are regulated by steroids in functionally defined brain areas to enhance different aspects of song behavior.

Phosphorylation state of tyrosine hydroxylase in the stalk-median eminence is decreased by progesterone in cycling female rats

Progesterone has the capacity to suppress hypothalamic dopaminergic neuronal activity on proestrous afternoon and prolong or amplify the preovulatory prolactin surge in rats. In the present study, we examined enzyme activity and phosphorylation state of tyrosine hydroxylase (TH) in the stalk-median eminence of cycling female rats on proestrus and estrus and related these to circulating progesterone levels. Phospho-TH levels were evaluated by Western blot analysis. TH activity was determined from the rate of 3,4-dihydroxyphenylalanine (DOPA) accumulation. Phospho-TH levels at Ser-19, Ser-31, and Ser-40 were similar at 1100, 1300, and 1500 h on proestrus but declined at 1700, 1900, and 2200 h, coincident with rising serum progesterone levels. Similarly, DOPA accumulation was 30-50% lower at 1700, 1900, and 2200 h as compared with 1100-1500 h on proestrus. Ser-31 and Ser-40 phosphorylation states were increased by 1100 h on estrus to a level similar to 1100 h on proestrus, whereas DOPA accumulation was 30% greater on estrous as compared with proestrous morning. There were no significant differences among the several time points on proestrus and estrus with regard to TH protein or beta-tubulin levels. Exogenous progesterone administration (7.5 mg/kg, sc) before the preovulatory progesterone surge decreased TH activity and phospho-TH at Ser-19, Ser-31, and Ser-40, accompanied by premature increased serum prolactin. Our study suggests that decreased TH phosphorylation at Ser-19, Ser-31, and Ser-40 contributes to the decline in TH activity in the stalk-median eminence on proestrous afternoon and that progesterone may cause this initial cytoplasmic response of TH dephosphorylation.

17-Beta-estradiol directly regulates the expression of adrenergic receptors and kisspeptin/GPR54 system in GT1-7 GnRH neurons

Estradiol plays a critical role in the feedback regulation of reproduction, in part by modulating the neurosecretory activity of gonadotropin-releasing hormone (GnRH) neurons. While indirect effects of estradiol on GnRH neurons have been clearly demonstrated, direct actions are still controversial. In the current study, we examined direct effects of 17beta-estradiol upon the expression of receptors for afferent signals at the level of the GnRH neuron, using immortalized GT1-7 cells. Using RT-PCR, we confirmed the expression of mRNA for the adrenergic receptors (AR) alpha(1)A-, alpha(1)B-, alpha(1)D-, alpha(2)A-, alpha(2)C-, and beta(1)-AR, and showed for the first time that mRNAs for alpha(2)B-, beta(2)- and beta(3)-AR, for kisspeptin and its receptor GPR54 and for the novel estrogenic receptor GPR30 are expressed in GT1-7 cells. After treatment with 10 nM 17beta-estradiol, alpha(1)B-AR mRNA was significantly increased (14-fold) after 6 h as determined by real-time PCR, while alpha(1)B- and alpha(1)D-AR mRNA were significantly increased (19- and 23-fold, respectively) after 24 h. The expression of KiSS-1 and GPR54 mRNAs were also significantly increased (8- and 6-fold, respectively) after 24 h treatment of GT1-7 cells with estradiol. GPR30 mRNA expression was not affected by estradiol. Our data also showed that kisspeptin-10 (1-10 nM) can significantly stimulate GnRH release and GnRH mRNA expression in GT1-7 cells. These results suggest that the complex physiologic effects of estradiol on the function of the reproductive axis could be mediated partly through direct modulation of the expression of receptors for afferent signals in GnRH neurons.

Differential effects of centrally-administered oestrogen antagonist ICI-182,780 on oestrogen-sensitive functions in the hypothalamus

Oestrogen actions within the hypothalamus are essential for a range of reproductive functions. In this study, we sought to develop a method for suppressing central oestrogen action without affecting peripheral oestrogenic effects. We administered the oestrogen receptor antagonist ICI-182,780 (ICI) via crystalline implants into the left lateral ventricle or the arcuate nucleus and measured the effectiveness of this drug on three endpoints known to be regulated by oestrogen: gonadotrophin-releasing hormone (GnRH) pulse frequency, progesterone receptor expression and the generation of a sustained prolactin surge during late pregnancy. To confirm that central ICI administration had no effect on peripheral actions of oestrogen, we monitored changes in uterine weight. Intracerebroventricular ICI treatment reversed the inhibitory effects of oestrogen on GnRH pulse frequency, as measured by plasma luteinising hormone pulse frequency. No effect on the oestrogenic induction of progesterone receptors within the arcuate nucleus or ventromedial hypothalamus was observed; however, a small yet significant reduction in progesterone receptor expression within dopaminergic neurones in the arcuate nucleus was observed. Intracerebroventricular or direct crystalline ICI administration to the arcuate nucleus did not change the serum prolactin level during late pregnancy. Central administration of ICI did not affect uterine weight, and thus did not have a peripheral effect. These data suggest that central administration of ICI can overcome some actions of oestrogen in the brain, such as GnRH pulse frequency, but does not affect other oestrogen mediated actions, including the induction of progesterone receptors or the antepartum prolactin surge. Thus, it appears that there is a differential sensitivity to the inhibition of central oestrogen actions by ICI.

Neuropeptide glutamic acid-isoleucine may induce luteinizing hormone secretion via multiple pathways

Neuropeptide glutamic acid-isoleucine (NEI) is a 14-amino acid peptide processed from prepro-melanin-concentrating hormone (ppMCH). In males, the localization of NEI is almost identical to that of MCH, the cell bodies of both being located primarily in the lateral hypothalamic area and zona incerta, projecting fibers throughout the brain. Although MCH has been widely studied, the role that NEI plays in brain circuitry has been poorly investigated. Recently, we showed that intracerebroventricular injection of NEI increases serum luteinizing hormone (LH) levels. In order to identify the anatomical substrate underlying this effect, we used combined immunohistochemistry methods to analyze the forebrains of females on the diestrus and proestrus days, as well as those of ovariectomized females treated with estradiol benzoate, with estradiol benzoate plus progesterone or with sesame oil (control animals). We found that ovariectomized females with no steroid treatment showed an increased number of NEI-immunoreactive neurons in the medial zona incerta. In addition, we observed dense to moderate NEI innervation of areas related to reproduction, including the organum vasculosum of the lamina terminalis, the anteroventral periventricular nucleus (AVPV) and the median eminence. The NEI fibers were in close apposition with the AVPV and gonadotropin-releasing hormone (GnRH) neurons expressing Fos in the afternoon of the proestrus day or following administration of estradiol benzoate plus progesterone. In the median eminence, NEI varicosities and terminal-like structures were in close proximity to blood vessels and GnRH fibers. Our results suggest that NEI might induce LH secretion in one of the following ways: by direct release into the median eminence, by modulation of GnRH neurons located in the preoptic area, by modulation of the GnRH terminals located in the median eminence or by an additive effect involving other neurotransmitters or neurohormones. Release of NEI might also induce LH secretion indirectly by modulating AVPV neurons.

Expression of mRNA for prolactin receptor (long form) in dopamine and pro-opiomelanocortin neurones in the arcuate nucleus of non-pregnant and lactating rats

Under most conditions, prolactin secretion from the pituitary gland is subject to negative-feedback regulation. Prolactin stimulates dopamine release from tuberoinfundibular (TIDA) neurones in the arcuate nucleus of the hypothalamus, which in turn suppresses the production of prolactin. However, during late pregnancy and continuing into lactation, this feedback mechanism becomes less responsive to prolactin and, as a result, a hyperprolactinaemic state develops. We investigated whether long-form prolactin receptor (PRL-R(L)) mRNA is present on TIDA neurones in nonpregnant and lactating rats. In addition, we examined whether PRL-R(L) mRNA is colocalized on hypothalamic pro-opiomelanocortin (POMC) neurones. Dual-label in situ hybridizations using an (35)S-labelled cRNA probe specific for long-form PRL-R, together with a digoxigenin-labelled RNA probe that encoded either tyrosine hydroxylase (TH) or POMC mRNA, were performed on brain sections. In both nonpregnant and lactating rats, the majority of TH mRNA-positive cells (> 90%) were found to express long-form PRL-R mRNA. In sections from nonpregnant rats, few non-TH positive cells expressed PRL-R(L) mRNA. By contrast, during lactation, the proportion of PRL-R(L) mRNA-positive cells that were not TH mRNA-positive increased to approximately 70%. Only a small number of neurones in this subpopulation of PRL-R(L) mRNA-positive neurones were found to be positive for POMC mRNA. These data show that the loss of responsiveness to prolactin occurring during lactation is not due to down regulation of long-form PRL-R gene expression on TIDA neurones. Moreover, the persistent expression of PRL-R(L) in arcuate neuroendocrine circuits suggests that PRL-R-mediated signalling continues to be important in these neurones during lactation.

Neuroendocrine regulation of prolactin secretion during late pregnancy: easing the transition into lactation

Prolactin is an anterior pituitary hormone critical for maintaining pregnancy and lactation. Under normal conditions, prolactin secretion is tightly regulated by inhibitory dopaminergic neuronal systems within the mediobasal hypothalamus in a process known as short-loop negative feedback. This review focuses on neuroendocrine adaptations to prolactin negative feedback during late pregnancy. It is suggested that, in terms of prolactin regulation, late pregnancy is a transition period into lactation because many of the neuroendocrine adaptations promoting hyperprolactinemia in lactation develop during late pregnancy. As a consequence, the maternal brain is geared to provide unrestrained prolactin release critical for milk production, maternal care and thus survival of the offspring before parturition. The mechanisms responsible for these changes are discussed.

Effects of testosterone metabolites on copulation, medial preoptic dopamine, and NOS-immunoreactivity in castrated male rats

The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. Dopamine (DA) is released in the MPOA of male rats shortly before and during copulation. In a previous study, we identified 17beta-estradiol (E(2)) as the metabolite of testosterone (T) that maintains MPOA basal extracellular DA levels. However, the presence of dihydrotestosterone (DHT), an androgenic metabolite of T, is required for the female-induced increase in MPOA DA observed during copulation. Recently, we reported that assays of MPOA tissue DA content showed that castrates actually had more stored DA than did gonadally intact males. Therefore, the reduction in extracellular levels in castrates was not due to decreased availability of DA; most likely it was due to decreased release. Furthermore, T upregulates neuronal nitric oxide synthase (nNOS) in the MPOA. NO has been implicated in the regulation of DA release in the MPOA. It is not known, however, which metabolite(s) of T regulate(s) tissue stores of DA and/or nNOS in the MPOA of male rats. The present experiments were designed to test the following: (1) whether E(2), DHT, or the combination of the two influences MPOA DA tissue levels, an indication of stored DA, in male rat castrates; and (2) whether E(2), DHT, or the combination of the two influences NOS-ir in the MPOA of castrated male rats. The results indicate that E(2) up-regulates nNOS-ir in the MPOA and maintains tissue content of DA at levels similar to those in T-treated rats. DHT did not influence nNOS-ir, while attenuating the effect of castration on tissue DA content.

Effect of steroids and nitric oxide on pituitary hormone release in ovariectomized, peripubertal rats

The purpose of this study was to determine the effects of the duration of steroid depletion on the steroid-induced luteinizing hormone and prolactin surges in ovariectomized, peripubertal female rats. Additionally, the role of nitric oxide (NO) in mediating the surge responses was determined. Peripubertal, 6-week-old, female Sprague-Dawley rats were ovariectomized. One or three weeks later, animals were injected with 17β-estradiol (50 μg, sc) followed 48 h later by progesterone (2.5 mg, sc). Effects of NO were examined by administeringl-arginine (300 mg/kg, ip). The response of ovariectomized, adult females to steroid treatment was also determined.

One and three weeks after ovariectomy, steroid replacement produced an LH and prolactin surge in peripubertal animals. However, both the magnitude and duration of the LH surge was greater 3 weeks after ovariectomy. Whilel-arginine significantly enhanced the magnitude of the LH surge 1 week after ovariectomy, by 3 weeksl-arginine caused a decrease in the duration, but not the magnitude of the surge. In contrast,l-arginine did not affect either the magnitude or duration of the prolactin surge one week after ovariectomy, but diminished the magnitude after 3 weeks of steroid depletion. In adults, steroids induced significant increases in both LH and prolactin. These results demonstrate that sensitivity to NO stimulation of LH, but not prolactin secretion, is modulated by the duration of gonadal steroid hormone depletion. The differences in the responsiveness of LH and prolactin to steroid-induced stimulation in peripubertal animals demonstrate that these hormones are regulated by NO through different mechanisms.

Sex differences in cytochrome P450 1B1, an estrogen-metabolizing enzyme, in the rhesus monkey telencephalon

The metabolic enzyme CYP1B1 is a recently cloned member of the cytochrome P450 superfamily, expressed widely throughout primate tissue, including the CNS. Although CYP1B1 protein is known to metabolize estradiol to catecholestrogens in the uterus, its localization and function in brain have not yet been described. To better understand CYP1B1 distribution, we have combined in situ hybridization (ISH) for its mRNA with immunohistochemistry (IHC) for the CYP1B1 protein in selected brain regions of male and female adult rhesus monkeys (Macaca mulatta). Blocks of formalin-fixed tissue obtained from the frontal cortex, hippocampus, thalamus, and amygdala were processed and embedded in paraffin. They were then sectioned and stained as described for human tissue [Muskhelishvili, L., Thompson, P.A., Kusewitt, D.F., Wang, C., Kadlubar, F.F., 2001. In situ hybridization and immunohistochemical analysis of cytochrome P450 1B1 expression in human normal tissues. J. Histochem. Cytochem. 49, 229-236]. Results indicated widespread distribution of CYP1B1 mRNA in both male and female monkey frontal cortex, hippocampus, thalamus, and amygdala. In contrast, although CYP1B1 protein was co-localized with its mRNA in the female brains, it was primarily restricted to hippocampal pyramidal neurons in the male brains. These results suggest that CYP1B1 may subserve widespread metabolic functions in the female primate brain but have more restricted actions within the hippocampal pyramidal neurons of the male.

Progesterone receptor and dopamine synthesizing enzymes in hypothalamic neurons of the guinea pig: an immunohistochemical triple-label analysis

Interactions among gonadal steroid hormones and the dopamine synthesizing enzymes, tyrosine hydroxylase (TH) or aromatic L-amino acid decarboxylase (AADC), participate in hypothalamic functions. Several findings suggest that the expression patterns of the progesterone receptor (PR), TH and AADC overlap in the guinea pig brain. However, it remained to be determined whether or not these two enzymes coexist in the same neurons which contain the PR. To test this hypothesis and quantify these colocalization relationships in the hypothalamus, we used a triple-labeling immunofluorescence procedure. Only PR/AADC-immunoreactive cells were seen in the preoptic area but no PR/TH cells and, therefore, no triple immunoreactive cells were found. An occasional colocalization between PR and the two enzymes was observed throughout the rostrocaudal extent of the arcuate nucleus with the greatest concentration of triple-labeled cells in the medial subdivision. In this region, quantitative estimation of cellular immunoreactivity showed that the triple immunoreactive cells represented about 29% of PR/TH cells, 9% of PR/AADC cells and 22% of TH/AADC cells in spite of a very low percentage in relation to total populations of neurons expressing only PR, TH or AADC. Thus, the PR are only present in monoenzymatic AADC expressing neurons in the preoptic area while they can be observed in neurons expressing both enzymes in the arcuate nucleus.

Estrogen deficiency leads to apoptosis in dopaminergic neurons in the medial preoptic area and arcuate nucleus of male mice

The aromatase knockout (ArKO) mouse is unable to synthesize estrogens. Immunohistochemical studies on active caspase-3 and tyrosine hydroxylase (TH) revealed apoptosis of dopaminergic neurons in the medial preoptic area (MPO) and arcuate nucleus (Arc) of the hypothalamus of 1-year-old (1yo) male ArKO mice while no active caspase-3 was detected in wild type (WT). Furthermore, the number of TH-positive cells in the MPO and caudal Arc was significantly decreased in 1yo ArKO compared to WT. RNase protection assays support the presence of apoptosis in 1yo ArKO hypothalamus, revealing an up-regulation of pro-apoptotic genes: FASL, FADD, and caspase-8. Concomitantly, the ratio of bcl-2-related anti-apoptotic genes to pro-apoptotic genes in the hypothalamus of 1yo ArKO mice was significantly down-regulated. Previously, we have reported that no such changes were observed in the hypothalamus of female ArKO mice. Thus, we have provided direct evidence that estrogen is required to maintain the survival and functional integrity of dopaminergic neurons in the MPO and Arc of male, but not female mice.

The activation of birdsong by testosterone: multiple sites of action and role of ascending catecholamine projections

Birdsong is a species-typical stereotypic vocalization produced in the context of reproduction and aggression. Among temperate-zone songbirds, it is produced primarily by males, and its frequency and quality are enhanced by the presence of the gonadal steroid hormone testosterone in the plasma. In the brain, the effects of testosterone on song behavior involve both estrogenic and androgenic metabolites of testosterone that are locally produced and act via their cognate receptors. Androgen, and in some cases estrogen, receptors are present in many specialized forebrain song control nuclei. Testosterone can regulate catecholamine steady-state levels and turnover in these song control regions. Tracing studies combined with immunocytochemistry for tyrosine hydroxylase (a marker of catecholamine synthesis) reveal several catecholamine cell groups that project to forebrain song control nuclei. These brain areas also express the mRNA for either androgen receptors or estrogen receptor alpha, and androgens enhance the expression of tyrosine hydroxylase. Dopaminergic cell groups that project to song nuclei express the protein product of the immediate early gene fos in association with the production of territorial song. Thus, testosterone may be acting on song behavior via these ascending catecholamine cell groups. Chemical lesioning studies suggest that noradrenergic projections to the song system are involved in the latency to produce song and the ability to discriminate conspecific from heterospecific song. The song control circuit may thus be modulated in significant ways via the androgen regulation of forebrain catecholamine systems.

Identification of neurones in the female rat hypothalamus that express oestrogen receptor-alpha and vesicular glutamate transporter-2

Oestrogen exerts its effects in the brain by binding to and activating two members of the nuclear receptor family, oestrogen receptor (ER)-alpha and ER-beta. Evidence suggests that oestrogen-receptive neurones participate in the generation of reproductive behaviours and that they convey the oestrogen message to gonadotropin-releasing hormone (GnRH) neurones. The aim of the present study was to identify the neurochemical phenotype of a subset of oestrogen receptor-expressing neurones. To this aim, we focused on the glutamate neuronal system, which is one of the most important stimulators of GnRH synthesis and release. We used the presence of vesicular glutamate transporter-2 (VGLUT2) mRNA as a specific marker to identify glutamate neurones and employed dual in situ hybridization to localize ERalpha mRNA-(35S-labelling) and VGLUT2 mRNA-(digoxigenin-labelling) expressing neurones within the hypothalamus. The results show that the overall distribution of VGLUT2 mRNA and ERalpha mRNA are consistent with previous data in the literature. Dual-labelled neurones were localized in the ventrolateral part of the ventromedial nucleus where 81.3 +/- 3.4% of the ERalpha mRNA containing neurones expressed VGLUT2 mRNA, in the anteroventral periventricular nucleus (30% colocalization) and in the medial preoptic nucleus (19% colocalization). Only 4.4% of the ERalpha expressing neurones in the arcuate nucleus contained VGLUT2 mRNA. These findings reveal that certain subpopulations of oestrogen-receptive neurones are glutamatergic in select hypothalamic areas that are known to regulate reproductive behaviour and GnRH neurones in the female rat. Thus, the oestrogen signal could be propagated through glutamate neurones to distant sites and influence the activity of the postsynaptic neurones.

Dopamine and melanin-concentrating hormone neurons are distinct populations in the rat rostromedial zona incerta

Zona incerta (ZI) is a controversial diencephalic area with a variety of cytoarchitectonic subdivisions, neurotransmitters and related functions. Medial ZI synthesizes dopamine (A13 group) and tyrosine hydroxylase (TH, a catecholamine synthesizing enzyme), which has been considered a neurochemical marker for this region. The rostromedial ZI also expresses melanin-concentrating hormone (MCH), but it is not known whether dopamine and MCH are colocalized. By using double label immunohistochemistry we analyzed the distribution of TH and MCH in the rat ZI. We found that MCH and TH neurons are intermingled but are not colocalized.

Wired for reproduction: organization and development of sexually dimorphic circuits in the mammalian forebrain

Mammalian reproduction depends on the coordinated expression of behavior with precisely timed physiological events that are fundamentally different in males and females. An improved understanding of the neuroanatomical relationships between sexually dimorphic parts of the forebrain has contributed to a significant paradigm shift in how functional neural systems are approached experimentally. This review focuses on the organization of interconnected limbic-hypothalamic pathways that participate in the neural control of reproduction and summarizes what is known about the developmental neurobiology of these pathways. Sex steroid hormones such as estrogen and testosterone have much in common with neurotrophins and regulate cell death, neuronal migration, neurogenesis, and neurotransmitter plasticity. In addition, these hormones direct formation of sexually dimorphic circuits by influencing axonal guidance and synaptogenesis. The signaling events underlying the developmental activities of sex steroids involve interactions between nuclear hormone receptors and other transcriptional regulators, as well as interactions at multiple levels with neurotrophin and neurotransmitter signal transduction pathways.

A novel mechanism for endocrine-disrupting effects of polychlorinated biphenyls: direct effects on gonadotropin-releasing hormone neurones

Polychlorinated biphenyls (PCBs) cause abnormal development and physiology of the reproductive system. We hypothesized that these effects may be mediated, at least in part, by neuroendocrine cells in the hypothalamus that integrate inputs to and outputs from the central nervous system and reproductive systems. The effects of two PCB mixtures, Aroclor 1221 and Aroclor 1254, were tested on the hypothalamic GT1-7 cells, which synthesize and secrete the key hypothalamic hormone, gonadotropin-releasing hormone (GnRH). GT1-7 cells were treated for 24 h in dose-response experiments and GnRH gene expression and release were quantified. Aroclor 1221 was stimulatory to GnRH gene expression, particularly at post-transcriptional levels (GnRH cytoplasmic mRNA), and increased GnRH peptide levels, suggesting a post-translational regulation of GnRH biosynthesis. It also caused a qualitative increase in GT1-7 neurite outgrowth and cell confluency. Aroclor 1254 had very different effects from Aroclor 1221. It inhibited GnRH nuclear mRNA levels at high dosages, and stimulated GnRH mRNA at low doses, suggesting a post-transcriptional mechanism of regulation. Aroclor 1254 did not alter GnRH peptide levels. Qualitatively, Aroclor 1254 caused a retraction of GT1-7 cell processes and neurotoxicity at high dosages. In order to gauge the involvement of the oestrogen receptor in these responses, the oestrogen receptor antagonist, ICI 182,780 (ICI) was coadministered in other studies with the PCBs. While effects of Aroclor 1221 on GnRH gene expression were not blocked by ICI, its effects on GnRH peptide levels were blocked by ICI, indicating that some but not all of the effects of Aroclor 1221 are mediated by the classical oestrogen receptor alpha and/or beta. The inhibitory effects of Aroclor 1254 on GnRH gene expression were not prevented by ICI, although ICI itself had stimulatory effects on GnRH gene expression that were blocked by cotreatment with Aroclor 1254. These results demonstrate a novel mechanism for effects of the two PCBs directly on GnRH gene expression, and indicate a hypothalamic level for endocrine disruption by these environmental toxicants.

Estrogen receptor-beta immunoreactivity in the midbrain of adult rats: regional, subregional, and cellular localization in the A10, A9, and A8 dopamine cell groups

Estrogen modulates dopamine synthesis, release, and metabolism in corticolimbic and striatal targets of midbrain dopamine neurons. The relevant sites of receptor-mediated action, however, had been elusive, because all available evidence suggested a paucity of intracellular estrogen receptors in the A8, A9, and A10 dopamine regions and their afferent targets. More recent evidence of a relative abundance of the beta isoform of the estrogen receptor (ER) in the substantia nigra and ventral tegmental area (VTA), however, suggests that this newly described receptor may be important in estrogen's stimulation of midbrain DA systems. It is unknown, however, precisely how ERbeta is distributed with respect to the functionally and neurochemically diverse cell populations of the ventral midbrain. To address these issues, this study used single- and double-label immunocytochemistry to detail the regional, subregional, and cellular distributions of ERbeta immunoreactivity in and around midbrain dopamine-containing cell groups in hormonally intact adult male and female rats. These analyses revealed that ERbeta-immunoreactive nuclei were found only in neurons, more specifically, within subsets of both dopaminergic and nondopaminergic neurons in the dorsal VTA, the parabrachial pigmented nucleus, the substantia nigra pars lateralis, the retrorubral fields, and to a lesser extent the linear midline nuclei. These regional and cellular receptor distributions thus place the ERbeta isoform in anatomical register with midbrain dopamine systems known to participate in a spectrum of motor, cognitive, and affective functions.

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

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

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.

Suckling stimulus suppresses messenger RNA for tyrosine hydroxylase in arcuate neurons during lactation

Tyrosine hydroxylase (TH) mRNA in tuberoinfundibular dopamine (TIDA) neurons is suppressed during lactation but rebounds upon pup removal. A time course of TH mRNA changes after pup removal revealed three phases: (1) a nuclear phase (evident 1.5 hours after pup removal, maximal at 3 hours) with TH mRNA appearing in 1 or 2 nuclear loci with little or no change in cytoplasmic mRNA; (2) a cytoplasmic phase (noted 6 hours after pup removal, peaked 12-24 hours) with a significant increase in total TH mRNA levels mainly in the cytoplasm; and (3) a stabilization phase (24-48 hours after pup removal) when nuclear signals were low and cytoplasmic RNA showed a slight decline with extension of RNA clusters into the cell dendrites. In rats whose pups could suckle only on one side, TH was up-regulated only on the side contralateral to nipple blockade. These data indicate that after suckling terminates, TH up-regulation is evident at 1.5 hours, but 6 hours is needed before the cells transport sufficient mRNA into the cytoplasm. The rapid signaling of TH up-regulation stems from the fact that the TIDA neurons respond to neural signals from termination of suckling.

Melanin-concentrating hormone stimulates the release of luteinizing hormone-releasing hormone and gonadotropins in the female rat acting at both median eminence and pituitary levels

The purpose of this study was to investigate whether melanin-concentrating hormone (MCH) acts directly on the median eminence and on the anterior pituitary of female rats regulating LHRH and gonadotropin release. In addition, immunohistochemistry was used to examine the density and distribution of MCH-immunoreactive fibers in the median eminence of proestrous rats. MCH-immunoreactive fibers were found in both the internal and external layers of the median eminence and in close association with hypophysial portal vessels. In the first series of in vitro experiments, median eminences and anterior pituitaries were incubated in Krebs-Ringer bicarbonate buffer containing two MCH concentrations (10(-10) and 10(-8) M). The lowest MCH concentration (10(-10) M) increased (P < 0.01) LHRH release only from proestrous median eminences. Anterior pituitaries incubated with both MCH concentrations also showed that 10(-10) M MCH increased gonadotropin release only from proestrous pituitaries. In the second series of experiments, median eminences and pituitaries from proestrous rats were incubated with graded concentrations of MCH. MCH (10(-10) and 10(-9) M) increased (P < 0.01) LHRH release from the median eminence, and only 10(-10) M MCH increased (P < 0.01) LH and FSH release from the anterior pituitary. The effect of MCH on the stimulation of both gonadotropins from proestrous pituitaries was similar to the effect produced by LHRH. Simultaneous incubation of pituitaries with MCH and LHRH did not modify LH but increased the FSH release induced by LHRH. The present results suggest that MCH could be involved in the regulation of preovulatory gonadotropin secretion.

Rapid changes in monoamine levels following administration of corticotropin-releasing factor or corticosterone are localized in the dorsomedial hypothalamus

Monoaminergic systems are important modulators of the neuroendocrine, autonomic, and behavioral responses to stress-related stimuli. The male roughskin newt (Taricha granulosa) was used as a model system to investigate the effects of corticotropin-releasing factor (CRF) or corticosterone administration on tissue concentrations of norepinephrine, epinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) in microdissected brain areas. Intracerebroventricular infusion of 25 or 50 ng of CRF increased locomotor activity and site-specifically increased dopamine concentrations within the dorsomedial hypothalamus 30 min after treatment when compared to vehicle-treated controls. In further studies, male newts were treated as follows: (1) no injection, no handling, (2) saline injection, or (3) 10 microg corticosterone and then placed in a novel environment. Monoamine and monoamine metabolite concentrations were similar in the unhandled and saline-injected controls 20 min after treatment. In contrast, corticosterone-injected newts had elevated concentrations of dopamine, serotonin, and 5-HIAA in the dorsomedial hypothalamus (a region that contains dopamine- and serotonin-accumulating neuronal cell bodies in representatives of all vertebrate classes) but not in several other regions studied. These site-specific neurochemical effects parallel neurochemical changes observed in the dorsomedial hypothalamic nucleus of mammals following exposure to a variety of physical and psychological stress-related stimuli. Therefore, these changes may reflect highly conserved, site-specific neurochemical responses to stress and stress-related neurochemicals in vertebrates. Given the important role of the dorsomedial hypothalamus in neuroendocrine, autonomic, and behavioral responses to stress, and a proposed role for this region in fast-feedback effects of glucocorticoids on the hypothalamo-pituitary-adrenal axis, these stress-related monoaminergic changes are likely to have important physiological or behavioral consequences.

Neuromodulation in polycystic ovary syndrome

Although central and peripheral factors have been implicated in the neuromodulation of GnRH in PCOS, there are no definitive or conclusive data to establish a primary causal role for any one factor. Because increased GnRH pulse frequency is at least a contributor to the secretion of excess LH and insufficient FSH that are the proximate cause of chronic anovulation in PCOS, strategies to slow the GnRH pulse generator are likely to promote ovulation in women with PCOS. Several pharmacologic agents, such as dopamine agonists and antagonists, have been tried, but the lack of consistent effects in women with PCOS limits their clinical utility. Current treatment strategies include the use of the combined oral contraceptive pills, antiandrogens or androgen receptor blockers, and insulin sensitizers. Oral contraceptive preparations are effective in suppressing ovarian hyperandrogenemia, regulating menstrual cycles, and reducing the risk of endometrial hyperplasia. Androgen blockade and antiandrogens provide symptomatic relief from androgen-induced acne and hirsutism and have been reported to restore ovulation in women with PCOS. Whether this effect is mediated peripherally or centrally remains to be clarified. The most recent class of pharmacologic agents to gain popularity are the "insulin modifiers." With increasing evidence that insulin resistance constitutes a key metabolic element, it seems logical that improving insulin sensitivity and glucose disposal might wholly, or partially, reverse certain features of PCOS, including anovulation. To date, insulin modifiers have proved most promising in improving the clinical features and promoting fertility, but whether this effect is centrally mediated is yet to be elucidated.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Targeting drugs to the brain by redox chemical delivery systems

Chemical delivery systems (CDSs) based on the redox conversion of a lipophilic dihydropyridine to an ionic, lipid-insoluble pyridinium salt have been developed to improve the access of therapeutic agents to the central nervous system. A dihydropyridinium-type CDS or a redox analog of the drug is sufficiently lipophilic to enter the brain by passive transport, then undergoes an enzymatic oxidation to an ionic pyridinium compound, which promotes retention in the CNS. At the same time, peripheral elimination of the entity is accelerated due to facile conversion of the CDS in the body. This review discusses chemical, physicochemical, biochemical, and biological aspects in relation to the principles and practical implementation of the redox brain-targeting approach to various classes of drugs. Representative examples to the brain-enhanced delivery of neurotransmitters, steroids, anticonvulsants, antibiotics, antiviral, anticancer and antidementia agents, and neuropeptides and their analogs are presented in detail. In vivo and in vitro studies and preliminary clinical data of several novel derivatives have been promising, which could lead to a practical use of the redox CDSs after proper pharmaceutical development. The investigations accentuate the need for considering physicochemical, metabolic, and pharmacokinetic properties in designing of carrier systems that are able to target drugs into the central nervous system.

Studies of the neuronal transdifferentiation process in cultured human pheochromocytoma cells: effects of steroids with differing functional groups on catecholamine content and cell morphology

The neuronal differentiation of adrenal pheochromocytoma cells from human subjects was studied in vitro for periods of up to 65 days. Changes with time in culture were observed in both intracellular catecholamine content (progressive decreases in epinephrine, norepinephrine, and dopamine, except for a possible transient early increase in the latter) and in morphology (increases in neurite outgrowth) of cells cultured in control medium; supplementation of cultures with nerve growth factor resulted in a substantial increase in neurite formation. The effects on these changes of the presence in the culture medium of various steroids were examined. The addition of 11-oxygenated steroids (aldosterone, corticosterone, cortisol, or dexamethasone) at 10(-5) M concentrations caused at least 2.5-fold increases in mean intracellular dopamine and norepinephrine levels; with dexamethasone, 9-10-fold increases were observed. Intracellular epinephrine content was also enhanced by 11,17-oxygenated steroids (dexamethasone and cortisol), but not by the other 11-oxygenated compounds studied. These two 11,17-oxygenated glucocorticoids also inhibited the morphologic changes seen with extended periods in culture, decreasing the outgrowth of neurite projections and causing cells to attain a vacuolated and granular appearance; the presence of dexamethasone strongly inhibited the morphologic changes induced by nerve growth factor. 11-Deoxy steroid intermediates (pregnenolone, 11-deoxycorticosterone, and 11-deoxycortisol) had little or no effect on catecholamine content or on morphology. Preliminary observations suggest that C-18 and C-19 sex steroid hormones (17 beta-estradiol and testosterone) may have morphologic effects opposite to those of the 11-oxygenated compounds, showing a slight stimulatory influence on the formation of neurite projections, but no significant effect on catecholamine content.

Co-localization of estrogen receptor and aromatase enzyme immunoreactivities in adult musk shrew brain

Estrogens are produced by the aromatization of androgens. These steroids exert their actions after binding to their receptors. Past studies have shown that estrogen receptors (ER) and aromatase enzyme (AROM) reside in many of the same brain regions. Few studies, however, have examined the neural co-localization of these important components involved in estrogen-activated behaviors. In the present study we examined the co-localization of ER and AROM immunoreactive (ir) neurons in musk shrew (Suncus murinus) brains. Data were collected from a representative section from three neural regions, the bed nucleus of the stria terminalis (BNST), medial preoptic area (mPOA), and ventromedial nucleus of the hypothalamus (VMN). Here we report a sex difference in the number of ER-ir neurons from the analyzed section of the mPOA and BNST. Females have more ER-ir neurons in the mPOA and males have more in the BNST. In the sections we examined, males tended to have more aromatase containing neurons than females. Although there were no significant differences in the numbers of double-labeled cells, the VMN contains the greatest percentage of these cells in both males and females; followed by the mPOA and the BNST. In addition, in the mPOA of both sexes, a distinct nucleus of aromatase containing neurons which was devoid of ER immunoreactivity was noted. Area measurements of the AROM-ir nucleus showed that it was significantly larger in males than in females. Taken together, these data suggest that there is not extensive cellular co-localization of estrogen receptors and aromatase enzyme in the musk shrew brain. However, the presence of other genomic forms of ER (membrane and/or ERbeta) in AROM containing neurons has not been ruled out by this study. Thus, we hypothesize that estrogens produced in brain affect behavior by binding to ER in neurons other than those that contain aromatase enzyme.

Gonadal steroids and neuronal function

Gonadal steroid hormones may affect, simultaneously, a wide variety of neuronal targets, influencing the way the brain reacts to many external and internal stimuli. Some of the effects of these hormones are permanent, whereas others are short lasting and transitory. The ways gonadal steroids affect brain function are very versatile and encompass intracellular, as well as, membrane receptors. In some cases, these compounds can interact with several neurotransmitter systems and/or transcription factors modulating gene expression. Knowledge about the mechanisms implicated in steroid hormone action will facilitate the understanding of brain sexual dimorphism and how we react to the environment, to drugs, and to certain disease states.

Glucoprivic induction of Fos immunoreactivity in hypothalamic dopaminergic neurons

The hypothalamic neurotransmitter dopamine (DA) regulates pituitary secretion of the glucoregulatory hormones, growth hormone (GH) and adrenocorticotropin (ACTH). The glucose antimetabolite, 2-deoxy-D-glucose (2DG), elicits expression of the proto-oncogene product Fos, which is expressed in hypothalamic structures where DA is synthesized. These studies utilized dual-label immunocytochemistry to determine whether discrete DA neuron populations in this region of the brain exhibit Fos immunoreactivity (-ir) in response to glucopenia. Ovariectomized female rats implanted s.c. with exogenous estradiol or vehicle were injected with 2DG (400 mg/kg, i.p.) or saline, and sacrificed 2 h later. Whereas Fos-ir was negligible after saline administration, 2DG induced expression of Fos-ir by TH-ir neurons in the paraventricular (PVN), periventricular (Pe) and arcuate nuclei (ARC), and in the anterior hypothalamic area (AHA). TH-ir neurons in the zona incerta did not express Fos-ir following 2DG. Although mean numbers of co-labeled neurons in the Pe, PVN and AHA did not differ between estradiol- and non-steroid-treated rats, the former group exhibited significantly higher numbers of TH-positive plus Fos-positive neurons in the ARC in response to 2DG. These results reveal the functional responsiveness of discrete DA neuron populations to glucoprivation, and indicate that estradiol enhances cellular accumulation of Fos-ir by ARC DA neurons during this metabolic challenge.

Cytosolic estrogen receptor concentrations in the lumbosacral spinal cord fluctuate during the estrous cycle

Estrogen responsive neurons have been anatomically identified with autoradiographic and immunohistochemical techniques and their distribution mapped in the lumbosacral spinal cord of female rats. Such neurons contain estrogen receptors (ERs). The present study was undertaken to: 1) quantify cytosolic estrogen receptor (ER) concentrations in the lumbosacral spinal cord and 2) determine if there is a relationship between cytosolic ER concentrations and fluctuations in serum estradiol (SE2) levels during the estrous cycle. Lumbosacral spinal segments were removed from intact cycling rats during the morning of proestrus, the afternoon of proestrus, and the morning of estrus, metestrus and diestrus. Trunk blood was collected at euthanasia and SE2 levels were determined using radioimmunoassay. Cytosolic ER concentrations were measured using a dextran-charcoal coated tube method. Concentrations of cytosolic ERs were low during estrus and metestrus, increased during diestrus with maximum concentrations during the afternoon of proestrus. These changes in ER concentrations paralleled SE2 levels measured in intact cycling animals; i.e., during estrus SE2 levels were low, but began to rise during metestrus, diestrus, and during the morning of proestrus with a maximum peak increase during the afternoon of proestrus. These data indicate there are fluctuations of cytosolic ER concentrations during the estrous cycle and that these changes coincide with changing SE2 concentrations suggesting that ER content is influenced by SE2.

Differential regulation of gonadotropin-releasing hormone (GnRH) receptor expression in the posterior mediobasal hypothalamus by steroid hormones: implication of GnRH neuronal activity

The present study is designed to evaluate the relationship between gonadotropin-releasing hormone (GnRH) and GnRH receptor (GnRHR) gene expression during the steroid-induced LH surge. One week after ovariectomy (OVX), a capsule containing 17beta-estradiol (E) or vehicle (V) was implanted into OVX rats, and 2 days later a single injection of progesterone (P) or V was administered s.c. at 10:00 h. Poly(A)-rich RNA samples were isolated from the micropunches of the preoptic area (POA) and the posterior mediobasal hypothalamus (pMBH) from both sides of individual brain slices. Using competitive reverse transcription-polymerase chain reaction (RT-PCR) procedures, three parameters (POA GnRH, pMBH GnRHR) and pituitary GnRHR mRNA levels were simultaneously determined in each individual animal. POA GnRH mRNA and pituitary GnRHR mRNA levels were decreased by treatment with E, but increased by a combination of E and P. In contrast, pMBH GnRHR mRNA levels were clearly augmented by treatment with E, and decreased by the combination of E and P. Temporal changes in such parameters were determined in OVX+E+V- and OVX+E+P-treated rats. P augmented POA GnRH mRNA levels at the time of the LH surge (17:00 h) and the increased GnRH mRNA levels were remained until 22:00 h, while E alone failed to alter POA GnRH mRNA levels. In the pMBH micropunch samples, P substantially decreased E-induced increase in GnRHR mRNA levels at 17:00 h and further lowered those until 22:00 h. Antisense oligonucleotides of GnRHR mRNA administered into the lateral ventricle of OVX+E-treated rats blocked the E-induced increase in pMBH GnRHR mRNA levels. The antisense oligonucleotides also prevented the LH surge as well as the increase in pituitary GnRHR mRNA levels in the OVX+E+P-treated group. However, administration of this antisense oligonucleotides failed to alter POA GnRH mRNA levels. In conclusion, the present study demonstrated that there is an inverse relationship between POA GnRH mRNA levels and pMBH GnRHR mRNA levels in response to E and/or P, and that the blockade of the E-induced increase in pMBH GnRHR mRNA levels effectively nullified the P-induced LH surge. These results indicate that pMBH GnRHR gene expression is involved in synchronizing the GnRH neuronal activity, which is crucial for the generation of the LH surge.

Cell-specific induction of c-fos expression in the pituitary gland by estrogen

Estrogens regulate many functions of pituitary lactotrophs, including PRL gene expression, release, storage, and cellular proliferation. The mechanism by which estrogens exert such a variety of functions is poorly understood. In the uterus, estrogens rapidly and transiently induce the expression of the immediate early genes c-fos and c-jun in specific cell types. The Fos/Jun proteins form the activating protein-1 (AP1) transcription factor that mediates ligand-activated cell proliferation, differentiation, and secretion. Here we used Fischer 344 (F344) rats that develop hyperprolactinemia and prolactinomas in response to estrogens. The objectives were to: 1) determine whether estrogen induces c-fos expression in the pituitary gland and identify the responsive cells; 2) compare the dynamics of c-fos induction in the pituitary and uterus; and 3) examine the temporal relationship between c-fos expression and PRL release. Ovariectomized F344 rats were injected with 1 microg estradiol and killed at different times thereafter. Pituitaries were subjected to in situ hybridization for c-fos and immunostaining for selected pituitary cells. Estradiol stimulated c-fos expression in lactotrophs and folliculo-stellate cells within the anterior lobe without affecting either the intermediate or neural lobes. In a second experiment, c-fos messenger RNA levels were measured by solution hybridization in anterior pituitaries and uteri from estradiol-treated rats. Trunk blood was analyzed for PRL by RIA. The estrogen-induced c-fos rise in the uterus was rapid, robust, and transient, whereas that in the anterior pituitary was delayed, lower, and sustained. The profile of serum PRL levels resembles that of c-fos induction in the anterior pituitary. We conclude that: 1) both lactotrophs and folliculo-stellate cells increase c-fos expression in response to estrogens; 2) induction of c-fos expression may mediate some estrogenic effects on PRL synthesis and release and lactotroph proliferation in F344 rats; and 3) the atypical dynamics of c-fos induction in the pituitary could be due to indirect effects of estrogens on PRL-regulating factors within the hypothalamo-pituitary complex as well as to pituitary-specific estrogen receptor isoforms, coactivators, or repressors.

Selective colocalization of immunoreactivity for intracellular gonadal hormone receptors and tyrosine hydroxylase in the ventral tegmental area, substantia nigra, and retrorubral fields in the rat

Gonadal hormones influence brain functions, including motor and motivational behaviors, transmitter release, and receptor binding in midbrain dopamine systems. Much of this influence suggests genomic hormone action. To identify which midbrain cells may be targets of genomic influence, double label immunocytochemistry was used to map intracellular estrogen and androgen receptors and tyrosine hydroxylase (TH) in the ventral tegmental area (VTA), substantia nigra (SN), and retrorubral fields (RRF) in intact, adult rats. The distribution of estrogen and androgen receptor immunoreactivity was highly selective, similar in males and females, and largely nonoverlapping. Estrogen receptors were present within subpopulations of cells in the ventrolateral paranigral VTA and rostrolateral RRF; of these, only a few cells in the RRF were immunoreactive for TH. Cells immunoreactive for androgen receptors were numerous in the paranigral and parabrachial VTA, SN pars lateralis and dorsomedial pars compacta, and lateral RRF. Nearly every androgen receptor-bearing cell in the VTA and SN pars compacta, roughly half in the SN pars lateralis, and about one-third in the RRF were TH immunopositive. The localization of estrogen receptors approximates the distribution of subsets of cells labeled following neostriatal injections, whereas androgen receptors tend to occupy regions labeled by injections in cortical or limbic targets. These receptor-specific alignments with origins of nigrostriatal, mesolimbic, and mesocortical projections are consistent with identified estrogen influence over motor behaviors and androgen involvement in motivational functions and may hold clues for understanding hormone action in these and other functions and dysfunctions of midbrain dopamine systems.

Intraventricular administration of estradiol modulates rat prolactin secretion and synthesis

The effect of estradiol (E2) on rat tuberoinfundibular dopaminergic (TIDA) neurons was examined in vivo, employing chronic intraventricular (i.c.v.) infusion technique using an osmotic mini-pump. The activity of TIDA neurons was assessed by the release and synthesis of prolactin (PRL) in the rat pituitary gland and by the changes in the 3, 4-dihydroxyphenylacetic acid (DOPAC) and dopamine (DA) levels and in the DOPAC/DA ratio in the rat hypothalamus. We also examined the [3H] E2 binding in the rat hypothalamus. Ovariectomized female Wistar rats with E2 replacement were treated with daily i.c.v. infusion of 1 microM of E2 or saline vehicle for 1, 3, and 7 days using the Alzet osmotic mini-pump and brain infusion kit. At 1 day of i.c.v. infusion of E2, the serum PRL level was significantly decreased compared with that in the vehicle group. Northern blot analysis of the total RNA isolated from the pituitary glands demonstrated a decrease in the PRL gene transcript level in the E2 group. At 3 days of E2 treatment, however, the serum PRL level was significantly increased compared with that of the vehicle-injected group and Northern blot analysis also demonstrated that the PRL gene transcript level was increased in the E2 group. At 7 days of E2 administration, there were no significant differences between the E2 and vehicle groups in either serum PRL or PRL gene transcript levels. There was a significant increase in the DOPAC/DA ratio after 1 day in the E2 group. However, no significant effects of E2 on this ratio were observed at 3 and 7 days of treatment. The DOPAC concentration in the E2 group was significantly increased at day 1 and significantly decreased at day 3, compared with that of the respective time in vehicle group. At day 7 there was no significant change in DOPAC concentration in either groups. The DA concentrations in the hypothalamus was not changed on any day in either group. Specific [3H] E2 binding was observed in the rat hypothalamus. These data suggest that E2 may have a biphasic effect on the accumulation of PRL gene transcripts and on the PRL secretion in the rat pituitary by first stimulating and then inhibiting the TIDA neuronal activity.

Galanin microinjected into the medial preoptic nucleus facilitates female- and male-typical sexual behaviors in the female rat

Galanin (GAL) microinjected within the sexually dimorphic medial preoptic nucleus (MPN) facilitates male-typical sexual behaviors in the male rat, a response that requires the presence of testosterone. As in the male, GAL-immunoreactive cells located within the MPN of the female also concentrate gonadal steroids and become less immunoreactive after gonadectomy. Thus, to investigate sexual behaviors in the female and to determine whether effects are comparable to those obtained in the male, GAL was microinjected unilaterally within the MPN of female rats. We report that GAL stimulated female-typical lordosis behavior after estrogen priming, and that the effect was not due to general arousal as measured by nonspecific locomotor activities. In a separate experiment, GAL microinjected within the MPN dose-responsively increased mount frequencies and decreased mount latencies in testosterone-primed females. A higher dose of testosterone was required in females for this stimulation of male-typical sexual behavior than required in a previous experiment in males.

Progesterone receptor immunoreactivity in aromatic L-amino acid decarboxylase-containing neurons of the guinea pig hypothalamus and preoptic area

A double-labeling immunofluorescence procedure was used to determine whether progesterone receptor (PR)-immunoreactive (IR) neurons in the preoptic area and hypothalamus of female guinea pigs also contained aromatic L-amino acid decarboxylase (AADC), an enzyme involved in the synthesis of both catecholamines and serotonin. Immunostaining was performed on cryostat sections prepared from ovariectomized guinea pigs primed by estradiol to induce PR. The nuclear presence of PR was visualized by a red fluorescence while the AADC-containing perikarya showed a yellow-green fluorescence. The topographic distribution of AADC-IR neurons was investigated by using a specific antiserum obtained by immunization of rabbits with a recombinant protein beta-galactosidase-AADC in the two regions known to contain the densest populations of estradiol-induced PR-IR cells: the preoptic area and the mediobasal hypothalamus. The localization of PR-IR and AADC-IR cell populations showed considerable overlap in these areas, mainly in the medial and periventricular preoptic nuclei and in the arcuate nucleus. A quantitative analysis of double-labeled cells estimated that about 15% to 23% of AADC-IR cells in the preoptic area and about 11% to 21% of AADC-IR cells in the arcuate nucleus possessed PR. This colocalization persisted throughout the rostrocaudal extent of these areas and represented 3% to 9% of the population of PR-IR cells. These findings provide neuroanatomical evidence that a subset of AADC neurons is directly regulated by progesterone. The exact physiological role of this enzyme in target cells for progesterone is not understood. AADC may be involved in functions other than that for the synthesis of the classical neurotransmitters.