The distribution of neurotransmitter-specific cells and fibers in the anteroventral periventricular nucleus: implications for the control of gonadotropin secretion in the rat

Hypothalamic neurons fully or partially expressing the dopaminergic phenotype: development, distribution, functioning and functional significance. A review

The hypothalamus is a key link in neuroendocrine regulations, which are provided by neuropeptides and dopamine. Until the late 1980 s, it was believed that, along with peptidergic neurons, hypothalamus contained dopaminergic neurons. Over time, it has been shown that besides dopaminergic neurons expressing the dopamine transporter and dopamine-synthesizing enzymes - tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) - the hypothalamus contains neurons expressing only TH, only AADC, both enzymes or only dopamine transporter. The end secretory product of TH neurons is L-3,4-dihydroxyphenylalanine, while that of AADC neurons and bienzymatic neurons lacking the dopamine transporter is dopamine. During ontogenesis, especially in the perinatal period, monoenzymatic neurons predominate in the hypothalamic neuroendocrine centers. It is assumed that L-3,4-dihydroxyphenylalanine and dopamine are released into the neuropil, cerebral ventricles, and blood vessels, participating in the regulation of target cell differentiation in the perinatal period and the functioning of target cells in adulthood.

Serotonin stimulates female preoptic area kisspeptin neurons via activation of type 2 serotonin receptors in mice

Background

The neuroendocrine control of ovulation is orchestrated by neuronal circuits that ultimately drive the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus to trigger the preovulatory surge in luteinizing hormone (LH) secretion. While estrogen feedback signals are determinant in triggering activation of GnRH neurons, through stimulation of afferent kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3VKISS1 neurons), many neuropeptidergic and classical neurotransmitter systems have been shown to regulate the LH surge. Among these, several lines of evidence indicate that the monoamine neurotransmitter serotonin (5-HT) has an excitatory, permissive, influence over the generation of the surge, via activation of type 2 5-HT (5-HT2) receptors. The mechanisms through which this occurs, however, are not well understood. We hypothesized that 5-HT exerts its influence on the surge by stimulating RP3VKISS1 neurons in a 5-HT2 receptor-dependent manner.

Methods

We tested this using kisspeptin neuron-specific calcium imaging and electrophysiology in brain slices obtained from male and female mice.

Results

We show that exogenous 5-HT reversibly increases the activity of the majority of RP3VKISS1 neurons. This effect is more prominent in females than in males, is likely mediated directly at RP3VKISS1 neurons and requires activation of 5-HT2 receptors. The functional impact of 5-HT on RP3VKISS1 neurons, however, does not significantly vary during the estrous cycle.

Conclusion

Taken together, these data suggest that 5-HT2 receptor-mediated stimulation of RP3VKISS1 neuron activity might be involved in mediating the influence of 5-HT on the preovulatory LH surge.

Urinary neonicotinoid concentrations and pubertal development in Chinese adolescents: A cross-sectional study

BACKGROUND

Animal studies suggest that exposure to certain neonicotinoids may interfere with the normal function of endocrine system in mammals. However, evidence from human studies is limited.

OBJECTIVES

This study conducted a cross-sectional analysis to examine urinary neonicotinoids concentrations in Chinese adolescents and its association with pubertal development.

METHODS

774 urine samples from 439 boys (median age: 13.7 years; 25th-75th percentile: 12.7-14.5 years) and 335 girls (median age: 13.7 years; 25th-75th percentile: 12.7-14.5 years) were collected for determination of ten neonicotinoids (imidacloprid, nitenpyram, acetamiprid, thiacloprid, imidaclothiz, thiamethoxam, clothianidin, dinotefuran, flonicamid, sulfoxaflor) and one metabolite (N-desmethyl-acetamiprid). Urinary creatinine was detected for concentration adjustment. Pubertal development including pubic hair, axillary hair, genitalia (boys), testicular volume (boys) and breast (girls) assessed by Tanner stages and others (spermarche, facial hair for boys and menarche for girls) were obtained by physical examination and questionnaire. Logistic and bayesian kernel machine regression were used to investigate the association between neonicotinoids concentrations and pubertal developments.

RESULTS

High detection rates ranged from 72.0% to 100.0% for all neonicotinoids. Boys and girls with thiacloprid concentration at the >75th percentile had lower stage of genitalia development (OR: 0.83, 95% CI: 0.33-0.93) and higher stage of axillary hair development (OR: 1.46, 95% CI: 1.12-3.41), respectively, compared with those at the <25th percentile. The estimate change in genitalia stage was significantly different at or above the 75th percentile concentration of neonicotinoids mixture compared to the 50th percentile concentration. No associations were found between other urinary neonicotinoids and other indicators of puberty.

CONCLUSIONS

Higher thiacloprid concentration was associated with delayed genitalia development in boys and early axillary hair development in girls. Neonicotinoids mixture was negatively associated with genitalia stage in the joint effect. Given the characteristic of the cross-sectional study, our results need further confirmation of the causal relationship.

Vasoactive intestinal peptide exerts an excitatory effect on hypothalamic kisspeptin neurons during estrogen negative feedback

Hypothalamic kisspeptin neurons are the primary modulators of gonadotropin-releasing hormone (GnRH) neurons. It has been shown that circadian rhythms driven by the suprachiasmatic nucleus (SCN) contribute to GnRH secretion. Kisspeptin neurons are potential targets of SCN neurons due to reciprocal connections with the anteroventral periventricular and rostral periventricular nuclei (AVPV/PeN) and the arcuate nucleus of the hypothalamus (ARH). Vasoactive intestinal peptide (VIP), a notable SCN neurotransmitter, modulates GnRH secretion depending on serum estradiol levels, aging or time of the day. Considering that kisspeptin neurons may act as interneurons and mediate VIP's effects on the reproductive axis, we investigated the effects of VIP on hypothalamic kisspeptin neurons in female mice during estrogen negative feedback. Our findings indicate that VIP induces a TTX-independent depolarization of approximately 30% of AVPV/PeN kisspeptin neurons in gonad-intact (diestrus) and ovariectomized (OVX) mice. In the ARH, the percentage of kisspeptin neurons that were depolarized by VIP was even higher (approximately 90%). An intracerebroventricular infusion of VIP leds to an increased percentage of kisspeptin neurons expressing the phospho^Ser133 cAMP-response-element-binding protein (pCREB) in the AVPV/PeN. On the other hand, pCREB expression in ARH kisspeptin neurons was similar between saline- and VIP-injected mice. Thus, VIP can recruit different signaling pathways to modulate AVPV/PeN or ARH kisspeptin neurons, resulting in distinct cellular responses. The expression of VIP receptors (VPACR) was upregulated in the AVPV/PeN, but not in the ARH, of OVX mice compared to mice on diestrus and estradiol-primed OVX mice. Our findings indicate that VIP directly influences distinct cellular aspects of the AVPV/PeN and ARH kisspeptin neurons during estrogen negative feedback, possibly to influence pulsatile LH secretion.

Mammalian circadian networks mediated by the suprachiasmatic nucleus

The brain has a complex structure composed of hundreds of regions, forming networks to cooperate body functions. Therefore, understanding how various brain regions communicate with each other and with peripheral organs is important to understand human physiology. The suprachiasmatic nucleus (SCN) in the brain is the circadian pacemaker. The SCN receives photic information from the environment and conveys this to other parts of the brain and body to synchronize all circadian clocks. The circadian clock is an endogenous oscillator that generates daily rhythms in metabolism and physiology in almost all cells via a conserved transcriptional-translational negative feedback loop. So, the information flow from the environment to the SCN to other tissues synchronizes locally distributed circadian clocks to maintain homeostasis. Thus, understanding the circadian networks and how they adjust to environmental changes will better understand human physiology. This review will focus on circadian networks mediated by the SCN to understand how the environment, brain, and peripheral tissues form networks for cooperation.

Estrogen Regulation of the Molecular Phenotype and Active Translatome of AVPV Kisspeptin Neurons

In females, ovarian estradiol (E2) exerts both negative and positive feedback regulation on the neural circuits governing reproductive hormone secretion, but the cellular and molecular mechanisms underlying this remain poorly understood. In rodents, estrogen receptor α-expressing kisspeptin neurons in the hypothalamic anteroventral periventricular region (AVPV) are prime candidates to mediate E2 positive feedback induction of preovulatory gonadotropin-releasing hormone and luteinizing hormone (LH) surges. E2 stimulates AVPV Kiss1 expression, but the full extent of estrogen effects in these neurons is unknown; whether E2 stimulates or inhibits other genes in AVPV Kiss1 cells has not been determined. Indeed, understanding of the function(s) of AVPV kisspeptin cells is limited, in part, by minimal knowledge of their overall molecular phenotype, as only a few genes are currently known to be co-expressed in AVPV Kiss1 cells. To provide a more detailed profiling of co-expressed genes in AVPV Kiss1 cells, including receptors and other signaling factors, and test how these genes respond to E2, we selectively isolated actively translated mRNAs from AVPV Kiss1 cells of female mice and performed RNA sequencing (RNA-seq). This identified >13 000 mRNAs co-expressed in AVPV Kiss1 cells, including multiple receptor and ligand transcripts positively or negatively regulated by E2. We also performed RNAscope to validate co-expression of several transcripts identified by RNA-seq, including Pdyn (prodynorphin), Penk (proenkephalin), Vgf (VGF), and Cartpt (CART), in female AVPV Kiss1 cells. Given the important role of AVPV kisspeptin cells in positive feedback, E2 effects on identified genes may relate to the LH surge mechanism and/or other physiological processes involving these cells.

Endocrine-disrupting chemicals: Effects on neuroendocrine systems and the neurobiology of social behavior

A contribution to SBN/ICN special issue. Endocrine-disrupting chemicals (EDCs) are pervasive in the environment. They are found in plastics and plasticizers (bisphenol A (BPA) and phthalates), in industrial chemicals such as polychlorinated biphenyls (PCBs), and include some pesticides and fungicides such as vinclozolin. These chemicals act on hormone receptors and their downstream signaling pathways, and can interfere with hormone synthesis, metabolism, and actions. Because the developing brain is particularly sensitive to endogenous hormones, disruptions by EDCs can change neural circuits that form during periods of brain organization. Here, we review the evidence that EDCs affect developing hypothalamic neuroendocrine systems, and change behavioral outcomes in juvenile, adolescent, and adult life in exposed individuals, and even in their descendants. Our focus is on social, communicative and sociosexual behaviors, as how an individual behaves with a same- or opposite-sex conspecific determines that individual's ability to exist in a community, be selected as a mate, and reproduce successfully.

Impact of Proestrus on Gene Expression in the Medial Preoptic Area of Mice

The antero-ventral periventricular zone (AVPV) and medial preoptic area (MPOA) have been recognized as gonadal hormone receptive regions of the rodent brain that-via wiring to gonadotropin-releasing hormone (GnRH) neurons-contribute to orchestration of the preovulatory GnRH surge. We hypothesized that neural genes regulating the induction of GnRH surge show altered expression in proestrus. Therefore, we compared the expression of 48 genes obtained from intact proestrous and metestrous mice, respectively, by quantitative real-time PCR (qPCR) method. Differential expression of 24 genes reached significance (p < 0.05). Genes upregulated in proestrus encoded neuropeptides (kisspeptin (KP), galanin (GAL), neurotensin (NT), cholecystokinin (CCK)), hormone receptors (growth hormone secretagogue receptor, μ-opioid receptor), gonadal steroid receptors (estrogen receptor alpha (ERα), progesterone receptor (PR), androgen receptor (AR)), solute carrier family proteins (vesicular glutamate transporter 2, vesicular monoamine transporter 2), proteins of transmitter synthesis (tyrosine hydroxylase (TH)) and transmitter receptor subunit (AMPA4), and other proteins (uncoupling protein 2, nuclear receptor related 1 protein). Proestrus evoked a marked downregulation of genes coding for adenosine A2a receptor, vesicular gamma-aminobutyric acid (GABA) transporter, 4-aminobutyrate aminotransferase, tachykinin precursor 1, NT receptor 3, arginine vasopressin receptor 1A, cannabinoid receptor 1, ephrin receptor A3 and aldehyde dehydrogenase 1 family, member L1. Immunocytochemistry was used to visualize the proteins encoded by Kiss1, Gal, Cck and Th genes in neuronal subsets of the AVPV/MPOA of the proestrous mice. The results indicate that gene expression of the AVPV/MPOA is significantly modified at late proestrus including genes that code for neuropeptides, gonadal steroid hormone receptors and synaptic vesicle transporters. These events support cellular and neuronal network requirements of the positive estradiol feedback action and contribute to preparation of the GnRH neuron system for the pre-ovulatory surge release.

GnRH Neuron Activity and Pituitary Response in Estradiol-Induced vs Proestrous Luteinizing Hormone Surges in Female Mice

During the female reproductive cycle, estradiol exerts negative and positive feedback at both the central level to alter gonadotropin-releasing hormone (GnRH) release and at the pituitary to affect response to GnRH. Many studies of the neurobiologic mechanisms underlying estradiol feedback have been done on ovariectomized, estradiol-replaced (OVX+E) mice. In this model, GnRH neuron activity depends on estradiol and time of day, increasing in estradiol-treated mice in the late afternoon, coincident with a daily luteinizing hormone (LH) surge. Amplitude of this surge appears lower than in proestrous mice, perhaps because other ovarian factors are not replaced. We hypothesized GnRH neuron activity is greater during the proestrous-preovulatory surge than the estradiol-induced surge. GnRH neuron activity was monitored by extracellular recordings from fluorescently tagged GnRH neurons in brain slices in the late afternoon from diestrous, proestrous, and OVX+E mice. Mean GnRH neuron firing rate was low on diestrus; firing rate was similarly increased in proestrous and OVX+E mice. Bursts of action potentials have been associated with hormone release in neuroendocrine systems. Examination of the patterning of action potentials revealed a shift toward longer burst duration in proestrous mice, whereas intervals between spikes were shorter in OVX+E mice. LH response to an early afternoon injection of GnRH was greater in proestrous than diestrous or OVX+E mice. These observations suggest the lower LH surge amplitude observed in the OVX+E model is likely not attributable to altered mean GnRH neuron activity, but because of reduced pituitary sensitivity, subtle shifts in action potential pattern, and/or excitation-secretion coupling in GnRH neurons.

Sexual dimorphism in the hypophysiotropic tyrosine hydroxylase-positive neurons in the preoptic area of the teleost, Clarias batrachus

BACKGROUND

Dopamine (DA) neurons in the anteroventral periventricular nucleus (AVPV) in the preoptic area (POA) of mammals express estrogen receptors, regulate luteinizing hormone (LH) secretion, and show distinct sexual dimorphism. In teleosts, hypophysiotropic DA neurons of the nucleus preopticus periventricularis (NPP), located in the anteroventral POA, express estrogen receptors, innervate LH cells, and emerged as a neuroanatomical substrate for inhibiting LH cells. Interestingly, the NPP and AVPV seem to share several similarities. Whether DAergic neurons in the NPP show sexual dimorphism is, however, not known. Based on the proposed homology to AVPV and previous studies showing greater tyrosine hydroxylase (TH) mRNA and enzyme activity levels in the brain of female catfish, we hypothesize that females have greater number of DAergic neurons in the NPP and correspondingly more TH-immunoreactive fiber innervation of the pituitary.

METHODS

Adult, male and female Clarias batrachus collected during the prespawning phase of their reproductive cycle were used. Fish were anesthetized and perfused transcardially with phosphate-buffered saline (pH 7.4) and 4 % paraformaldehyde in phosphate buffer. Sections through the rostro-caudal extent of the POA and pituitary were processed for TH immunofluorescence. Using double immunofluorescence, the association between TH-immunoreactive fibers and LH cells in the pituitary was explored. Sections were analyzed using semiquantitative analysis.

RESULTS

NPP in POA of C. batrachus has two distinct subdivisions, viz, anterior (NPPa) and posterior (NPPp), and TH neurons were observed in both the subdivisions. Compared to that in the males, a significantly higher (P < 0.05) number of TH neurons was consistently observed in the NPPa of females. TH neurons in NPPp, however, showed no difference in the number or immunoreactivity. Since DA neurons in NPPa are hypophysiotropic, we compared TH-fiber innervation of the pituitary in both sexes. Compared to males, proximal pars distalis and LH cells in this region of the pituitary in females were densely innervated by TH fibers.

CONCLUSIONS

Neurons of NPPa and their innervation to the pituitary seem to be a distinct sexually dimorphic DAergic system in C. batrachus. The DAergic system may serve as a component of the neural mechanisms controlling the sexually dimorphic LH surge in teleosts. Given the similarities shared by NPPa and AVPV, homology between these two nuclei is suggested.

Distribution of vasotocin- and vasoactive intestinal peptide-like immunoreactivity in the brain of blue tit (Cyanistes coeruleus)

Blue tits (Cyanistes coeruleus) are songbirds, used as model animals in numerous studies covering a wide field of research. Nevertheless, the distribution of neuropeptides in the brain of this avian species remains largely unknown. Here we present some of the first results on distribution of Vasotocine (AVT) and Vasoactive intestinal peptide (VIP) in the brain of males and females of this songbird species, using immunohistochemistry mapping. The bulk of AVT-like cells are found in the hypothalamic supraoptic, paraventricular and suprachiasmatic nuclei, bed nucleus of the stria terminalis, and along the lateral forebrain bundle. Most AVT-like fibers course toward the median eminence, some reaching the arcopallium, and lateral septum. Further terminal fields occur in the dorsal thalamus, ventral tegmental area and pretectal area. Most VIP-like cells are in the lateral septal organ and arcuate nucleus. VIP-like fibers are distributed extensively in the hypothalamus, preoptic area, lateral septum, diagonal band of Broca. They are also found in the bed nucleus of the stria terminalis, amygdaloid nucleus of taenia, robust nucleus of the arcopallium, caudo-ventral hyperpallium, nucleus accumbens and the brainstem. Taken together, these results suggest that both AVT and VIP immunoreactive structures show similar distribution to other avian species, emphasizing evolutionary conservatism in the history of vertebrates. The current study may enable future investigation into the localization of AVT and VIP, in relation to behavioral and ecological traits in the brain of tit species.

Common and divergent psychobiological mechanisms underlying maternal behaviors in non-human and human mammals

Maternal interactions with young occupy most of the reproductive period for female mammals and are absolutely essential for offspring survival and development. The hormonal, sensory, reward-related, emotional, cognitive and neurobiological regulators of maternal caregiving behaviors have been well studied in numerous subprimate mammalian species, and some of the importance of this body of work is thought to be its relevance for understanding similar controls in humans. We here review many of the important biopsychological influences on maternal behaviors in the two best studied non-human animals, laboratory rats and sheep, and directly examine how the conceptual framework established by some of the major discoveries in these animal "models" do or do not hold for our understanding of human mothering. We also explore some of the limits for extrapolating from non-human animals to humans. We conclude that there are many similarities between non-human and human mothers in the biological and psychological factors influencing their early maternal behavior and that many of the differences are due to species-characteristic features related to the role of hormones, the relative importance of each sensory system, flexibility in what behaviors are exhibited, the presence or absence of language, and the complexity of cortical function influencing caregiving behaviors.

Vasoactive intestinal peptide modulation of the steroid-induced LH surge involves kisspeptin signaling in young but not in middle-aged female rats

Age-related LH surge dysfunction in middle-aged rats is characterized, in part, by reduced responsiveness to estradiol (E2)-positive feedback and reduced hypothalamic kisspeptin neurotransmission. Vasoactive intestinal peptide (VIP) neurons in the suprachiasmatic nucleus project to hypothalamic regions that house kisspeptin neurons. Additionally, middle-age females express less VIP mRNA in the suprachiasmatic nucleus on the day of the LH surge and intracerebroventricular (icv) VIP infusion restores LH surges. We tested the hypothesis that icv infusion of VIP modulates the LH surge through effects on the kisspeptin and RFamide-related peptide-3 (RFRP-3; an estradiol-regulated inhibitor of GnRH neurons) neurotransmitter systems. Brains were collected for in situ hybridization analyses from ovariectomized and ovarian hormone-primed young and middle-aged females infused with VIP or saline. The percentage of GnRH and Kiss1 cells coexpressing cfos and total Kiss1 mRNA were reduced in saline-infused middle-aged compared with young females. In young females, VIP reduced the percentage of GnRH and Kiss1 cells coexpressing cfos, suggesting that increased VIP signaling in young females adversely affected the function of Kiss1 and GnRH neurons. In middle-aged females, VIP increased the percentage of GnRH but not Kiss1 neurons coexpressing cfos, suggesting VIP affects LH release in middle-aged females through kisspeptin-independent effects on GnRH neurons. Neither reproductive age nor VIP affected Rfrp cell number, Rfrp mRNA levels per cell, or coexpression of cfos in Rfrp cells. These data suggest that VIP differentially affects activation of GnRH and kisspeptin neurons of female rats in an age-dependent manner.

Prodynorphin and proenkephalin gene expression in the anteroventral periventricular nucleus of the rat: Sexual differentiation and hormonal regulation

Opioid peptides are generally thought to exert hormone-dependent regulatory influences on gonadotropin secretion and the anteroventral periventricular nucleus (AVPv) has been shown to play a critical role in the neural control of this sexually dimorphic function. In the present study we used in situ hybridization to compare the numbers of proenkephalin (PENK) and prodynorphin (PDYN) mRNA-containing neurons in the AVPv of male and female rats and to evaluate the influence of circulating sex steroid hormones on the development and mature regulation of PENK and PDYN gene expression in these neurons. In agreement with earlier immunohistochemical observations, the number of PENK mRNA-containing neurons in the AVPv of male rats was found to be twice that of female animals. In contrast, the AVPv contains over four times the number of PDYN mRNA-containing cells in female rats, relative to intact males. Treatment of newborn female rats with testosterone increases the number of PENK mRNA-containing neurons in the AVPv, but decreases the number of PDYN mRNA-containing neurons in the AVPv compared with untreated females. Treatment of adult ovariectomized female rats with estradiol significantly increased PDYN mRNA levels in the AVPv; however, comparable changes in levels of PENK mRNA were not detected. In adult male rats, neither PDYN, nor PENK mRNA were significantly altered by orchidectomy or testosterone treatment. Thus, the maintenance of enkephalinergic neurons and the loss of hormone-sensitive dynorphin-containing neurons in the AVPv may represent important developmental influences of neonatal androgens on the sexually differentiated neural circuitry controlling gonadotropin secretion.

Physiology of the gonadotrophin-releasing hormone (GnRH) neurone: studies from embryonic GnRH neurones

Gonadotrophin-releasing hormone (GnRH)-secreting neurones are the final output of the central nervous system driving fertility in all mammals. Although it has been known for decades that the efficiency of communication between the hypothalamus and the pituitary depends on the pulsatile profile of GnRH secretion, how GnRH neuronal activity is patterned to generate pulses at the median eminence is unknown. To date, the scattered distribution of the GnRH cell bodies remains the main limitation to assessing the cellular events that could lead to pulsatile GnRH secretion. Taking advantage of the unique developmental feature of GnRH neurones, the nasal explant model allows primary GnRH neurones to be maintained within a micro-network where pulsatile secretion is preserved and where individual cellular activity can be monitored simultaneously across the cell population. This review summarises the data obtained from work using this in vitro model, and brings some insights into GnRH cellular physiology.

Forebrain origins of glutamatergic innervation to the rat paraventricular nucleus of the hypothalamus: differential inputs to the anterior versus posterior subregions

The hypothalamic paraventricular nucleus (PVN) regulates numerous homeostatic systems and functions largely under the influence of forebrain inputs. Glutamate is a major neurotransmitter in forebrain, and glutamate neurosignaling in the PVN is known to mediate many of its functions. Previous work showed that vesicular glutamate transporters (VGluTs; specific markers for glutamatergic neurons) are expressed in forebrain sites that project to the PVN; however, the extent of this presumed glutamatergic innervation to the PVN is not clear. In the present study retrograde FluoroGold (FG) labeling of PVN-projecting neurons was combined with in situ hybridization for VGluT1 and VGluT2 mRNAs to identify forebrain regions that provide glutamatergic innervation to the PVN and its immediate surround in rats, with special consideration for the sources to the anterior versus posterior PVN. VGluT1 mRNA colocalization with retrogradely labeled FG neurons was sparse. VGluT2 mRNA colocalization with FG neurons was most abundant in the ventromedial hypothalamus after anterior PVN FG injections, and in the lateral, posterior, dorsomedial, and ventromedial hypothalamic nuclei after posterior PVN injections. Anterograde tract tracing combined with VGluT2 immunolabeling showed that 1) ventromedial nucleus-derived glutamatergic inputs occur in both the anterior and posterior PVN; 2) posterior nucleus-derived glutamatergic inputs occur predominantly in the posterior PVN; and 3) medial preoptic nucleus-derived inputs to the PVN are not glutamatergic, thereby corroborating the innervation pattern seen with retrograde tracing. The results suggest that PVN subregions are influenced by varying amounts and sources of forebrain glutamatergic regulation, consistent with functional differentiation of glutamate projections.

BAX-dependent and BAX-independent regulation of Kiss1 neuron development in mice

The Kiss1 gene and its product kisspeptin are important regulators of reproduction. In rodents, Kiss1 is expressed in the hypothalamic arcuate (ARC) and anteroventral periventricular (AVPV)/rostral periventricular (PeN) nuclei. In the AVPV/PeN, females have more Kiss1 and tyrosine hydroxylase (TH) neurons than males. We explored the ontogeny of the Kiss1 sex difference, and the role of cell death in establishing Kiss1 and TH cell number. We also determined whether Kiss1 cells in AVPV/PeN coexpress TH. AVPV/PeN Kiss1 neurons were first detected in both sexes on postnatal d 10, but the Kiss1 sex difference did not emerge until postnatal d 12. The role of BAX-mediated apoptosis in generating this sex difference was tested in adult Bax knockout (KO) and wild-type mice. Deletion of Bax did not diminish the sex difference in Kiss1 expression in the AVPV/PeN. TH expression was sexually dimorphic in the AVPV of both wild-type and Bax KO mice but, unlike Kiss1, was not sexually dimorphic in the PeN of either genotype. Double-label analysis determined that most Kiss1 neurons coexpress TH mRNA, but many TH neurons do not coexpress Kiss1, especially in the PeN. These findings suggest that several subpopulations of TH cells reside within the AVPV/PeN, only one of which coexpresses Kiss1. In the ARC, Kiss1 cell number was markedly increased in Bax KO mice of both sexes, indicating that although BAX-dependent apoptosis does not generate the sex difference in either Kiss1 or TH expression in AVPV/PeN, BAX does importantly regulate Kiss1 cell number in the ARC.

Cellular mechanisms of estradiol-mediated sexual differentiation of the brain

Gonadal steroids organize the developing brain during a perinatal sensitive period and have enduring consequences for adult behavior. In male rodents testicular androgens are aromatized in neurons to estrogens and initiate multiple distinct cellular processes that ultimately determine the masculine phenotype. Within specific brain regions, overall cell number and dendritic morphology are the principal targets for hormonal organization. Recent advances have been made in elucidating the cellular mechanisms by which the neurological underpinnings of sexually dimorphic physiology and behavior are determined. These include estradiol-mediated prostaglandin synthesis, presynaptic release of glutamate, postsynaptic changes in glutamate receptors and changes in cell adhesion molecules. Sex differences in cell death are mediated by hormonal modulation of survival and death factors such as TNFalpha and Bcl-2/BAX.

Coming of age in the kisspeptin era: sex differences, development, and puberty

The status of the neuroendocrine reproductive axis differs dramatically during various stages of development, and also differs in several critical ways between the sexes, including its earlier pubertal activation in females than males and the presence of neural circuitry that generates preovulatory hormone surges in females but not males. The reproductive axis is controlled by various hormonal and neural pathways that converge upon forebrain gonadotropin-releasing hormone (GnRH) neurons, and many of the critical age and sex differences in the reproductive axis likely reflect differences in the "upstream" circuits and factors that regulate the GnRH system. Recently, the neural kisspeptin system has been implicated as an important regulator of GnRH neurons. Here I discuss the evidence supporting a critical role of kisspeptin signaling at different stages of life, including early postnatal and pubertal development, as well as in adulthood, focusing primarily on information gleaned from mammalian studies. I also evaluate key aspects of sexual differentiation and development of the brain as it relates to the Kiss1 system, with special emphasis on rodents. In addition to discussing recent advances in the field of kisspeptin biology, this paper will highlight a number of unanswered questions and future challenges for kisspeptin investigators, and will stress the importance of studying the kisspeptin system in both males and females, as well as in multiple species.

The neurobiology of preovulatory and estradiol-induced gonadotropin-releasing hormone surges

Ovarian steroids normally exert homeostatic negative feedback on GnRH release. During sustained exposure to elevated estradiol in the late follicular phase of the reproductive cycle, however, the feedback action of estradiol switches to positive, inducing a surge of GnRH release from the brain, which signals the pituitary LH surge that triggers ovulation. In rodents, this switch appears dependent on a circadian signal that times the surge to a specific time of day (e.g., late afternoon in nocturnal species). Although the precise nature of this daily signal and the mechanism of the switch from negative to positive feedback have remained elusive, work in the past decade has provided much insight into the role of circadian/diurnal and estradiol-dependent signals in GnRH/LH surge regulation and timing. Here we review the current knowledge of the neurobiology of the GnRH surge, in particular the actions of estradiol on GnRH neurons and their synaptic afferents, the regulation of GnRH neurons by fast synaptic transmission mediated by the neurotransmitters gamma-aminobutyric acid and glutamate, and the host of excitatory and inhibitory neuromodulators including kisspeptin, vasoactive intestinal polypeptide, catecholamines, neurokinin B, and RFamide-related peptides, that appear essential for GnRH surge regulation, and ultimately ovulation and fertility.

Gonadal and nongonadal regulation of sex differences in hypothalamic Kiss1 neurones

The brains of males and females differ anatomically and physiologically, including sex differences in neurone size or number, synapse morphology and specific patterns of gene expression. Brain sex differences may underlie critical sex differences in physiology or behaviour, including several aspects of reproduction, such as the timing of sexual maturation (earlier in females than males) and the ability to generate a preovulatory gonadotrophin surge (in females only). The reproductive axis is controlled by afferent pathways that converge upon forebrain gonadotrophin-releasing hormone (GnRH) neurones, but GnRH neurones are not sexually dimorphic. Although most reproductive sex differences probably reflect sex differences in the upstream circuits and factors that regulate GnRH secretion, the key sexually-dimorphic factors that influence reproductive status have remained poorly defined. The recently-identified neuropeptide kisspeptin, encoded by the Kiss1 gene, is an important regulator of GnRH secretion, and Kiss1 neurones in rodents are sexually dimorphic in specific hypothalamic populations, including the anteroventral periventricular nucleus-periventricular nucleus continuum (AVPV/PeN) and the arcuate nucleus (ARC). In the adult AVPV/PeN, Kiss1 neurones are more abundant in females than males, representing a sex difference that is regulated by oestradiol signalling during critical periods of postnatal and pubertal development. By contrast, Kiss1 neurones in the ARC are not sexually differentiated in adult rodents but, in mice, the regulation of ARC Kiss1 cells by gonadal hormone-independent factors is sexually dimorphic during prepubertal development. These various sex differences in hypothalamic Kiss1 neurones may relate to known sex differences in reproductive physiology, such as puberty onset and positive feedback.

Comparison of the spatial distribution of seven types of neuroendocrine neurons in the rat paraventricular nucleus: toward a global 3D model

The paraventricular nucleus of the hypothalamus (PVH) coordinates neuroendocrine, autonomic, and behavioral responses to help maintain energy and body water balance. The rat paraventricular nucleus has three major divisions: descending with axonal projections to somatomotor-behavioral and autonomic circuitry, magnocellular neuroendocrine with projections directly to the posterior pituitary, and parvicellular neuroendocrine with projections to the median eminence for controlling anterior pituitary hormone secretion. The present work was undertaken to provide high-resolution mapping of spatial relationships among the two magnocellular neuroendocrine and five parvicellular neuroendocrine neuron types throughout the nucleus. Double immunohistochemical labeling for two neuron types combined with retrograde labeling to identify neuroendocrine neurons positively was used in individual sections spaced 45 mum apart, along with a grid transfer method for reducing plane of section artifacts when comparing staining pattern data between animals. The results indicate that whereas each neuroendocrine neuron phenotype displays a unique distribution pattern, there is extensive partial overlap in a complex pattern between small "hot spots" with a relatively high density of a particular neuron type and few if any other phenotypes. In addition, the distribution of non-neuroendocrine neurons staining with each of the markers (but not retrogradely labeled) was mapped and compared with each other and with the neuroendocrine neuron populations. This spatial organization raises important questions about the differential functional regulation of individual-and perhaps sets of-neuroendocrine motor neuron populations in the PVH by synaptic mechanisms and by less traditional mechanisms like dendritic neurotransmitter release and gap junctions within and between neuron types.

Estrogen induces caspase-dependent cell death during hypothalamic development

The sexually dimorphic population of dopamine neurons in the anteroventral periventricular nucleus of the preoptic region of the hypothalamus (AVPV) develops postnatally under the influence of testosterone, which is aromatized to estrogen. There are fewer dopaminergic neurons labeled with tyrosine hydroxylase (TH) in the male AVPV than the female, and sex steroids determine this sex difference, yet the role of cell death in specifying numbers of dopaminergic neurons in the AVPV is unknown. Estradiol treatment of the AVPV, in vivo and in vitro, was used to manipulate TH-ir cell number. In vitro, concurrent treatment with the estrogen receptor antagonist ICI 182,780 rescued TH-ir cells. Cyclosporin A, an inhibitor of cell death dependent on the opening of a mitochondrial permeability transition pore also blocked TH-ir cell loss. In vivo, estradiol increased the number of apoptotic profiles, both TUNEL and Hoechst labeled nuclei, in the AVPV. This increased apoptosis was also dependent on the presence of the alpha form of the estrogen receptor. To test for caspase dependent TH-ir cell loss, the pancaspase inhibitor ZVAD (N-benzyloxycabonyl-Val-Ala-Asp-fluoromethylketone) was used to rescue TH-ir cells from estradiol-mediated reduction in number. Together, these data suggest that an intrinsic cell death pathway is activated by estrogen to regulate TH-ir cell number. Thus, in contrast to the more widespread neuroprotective actions of sex steroids in the mammalian nervous system, in the AVPV estrogen regulates dopaminergic neuron number through a caspase-dependent mechanism of apoptotic cell death.

Circadian regulation of Kiss1 neurons: implications for timing the preovulatory gonadotropin-releasing hormone/luteinizing hormone surge

The preovulatory GnRH/LH surge depends on the presence of estradiol (E(2)) and is gated by a circadian oscillator in the suprachiasmatic nucleus (SCN) that causes the surge to occur within a specific temporal window. Although the mechanisms by which the clock times the LH surge are unclear, evidence suggests that the SCN is linked to GnRH neurons through a multisynaptic pathway that includes neurons in the anteroventral periventricular nucleus (AVPV). Recently, Kiss1 neurons in the AVPV have been implicated in the surge mechanism, suggesting that they may integrate circadian and E(2) signals to generate the LH surge. We tested whether Kiss1 neurons display circadian patterns of regulation in synchrony with the temporal pattern of LH secretion. Mice housed in 14 h light, 10 h dark were ovariectomized, given E(2) capsules (or nothing), and transferred into constant darkness. Two days later, the mice were killed at various times of day and their LH and Kiss1 levels assessed. In E(2)-treated females, LH levels were low except during late subjective day (indicative of an LH surge). Similarly, AVPV Kiss1 expression and c-fos coexpression in Kiss1 neurons showed circadian patterns that peaked coincident with LH. These temporal changes in Kiss1 neurons occurred under steady-state E(2) and constant environmental conditions, suggesting that Kiss1 neurons are regulated by circadian signals. In the absence of E(2), animals displayed no circadian pattern in LH secretion or Kiss1 expression. Collectively, these findings suggest that the LH surge is controlled by AVPV Kiss1 neurons whose activity is gated by SCN signals in an E(2)-dependent manner.

The anti-dopaminergic agent, haloperidol, antagonises the feminising effect of neonatal serotonin on sexually dimorphic hypothalamic nuclei and tyrosine hydroxylase immunoreactive neurones

There is a transient fall in hypothalamic serotonin (5-hydroxytryptamine; 5-HT) activity in the second week post partum in male but not female rats. When this fall is masked by administration of the 5-HT(2) agonist (-) 2,5-dimethoxy-4-iodophenyl]-2-aminopropane hydrochloride [(-)DOI], over days 8-16 post partum, males are feminised in adulthood. To investigate whether the effect of 5-HT is mediated by dopamine and whether testosterone exerts its masculinising effect by reducing 5-HT and dopamine activity, male pups were treated with (-)DOI alone or together with the dopamine antagonist, haloperidol, over days 8-16 post partum, whereas females were treated with testosterone propionate on day 2 post partum. In adulthood, the volumes of the anteroventral periventricular nucleus (AVPV), sexually dimorphic nucleus of the preoptic area (SDN-POA) and arcuate nucleus (ARC) were determined, together with the number of tyrosine hydroxylase-immunoreactive (TH-ir) cells and fibres within them. The concentrations of 5-HT, dopamine and their metabolites were also measured. (-)DOI treatment increased the volume of the AVPV, decreased that of the SDN-POA and increased the number of TH-ir cells in the AVPV. These feminising effects were antagonised by concurrent haloperidol treatment. Neonatal testosterone propionate masculinised the volumes of the female AVPV and SDN-POA and reduced the number of TH-ir cells in the AVPV. Dopamine and 5-HT turnover in the AVPV was greater in female compared to male rats and neonatal testosterone propionate reduced dopamine concentration in the female AVPV. Neonatal (-)DOI had no effect on dopamine and 5-HT activity in the AVPV but increased both in the ARC. The findings that TH-ir neurone number and dopamine activity are greater in the female AVPV; the feminising effect of 5-HT is prevented by a haloperidol; and the masculinising effect of testosterone propionate is accompanied by a decrease in TH-ir neurone number and dopamine concentration in the female AVPV, suggest that dopamine is involved in hypothalamic sexual differentiation and may mediate the effect of 5-HT.

Morphology and estrogen receptor alpha mRNA expression in the developing green anole forebrain

Sex differences in forebrain morphology arise during development and are often linked to hormonal changes. These dimorphisms frequently occur in regions related to reproductive behaviors. Little is known about the normal ontogeny of reproductive nuclei in the green anole lizard, including whether steroid hormones influence their development. To address this issue, brain region volume, cell density, soma size, and estrogen receptor alpha (ERalpha) mRNA expression were characterized in the preoptic area (POA), ventromedial amygdala (AMY), and ventromedial hypothalamus (VMH) of late embryonic and early post-hatchling anoles. In adulthood, the POA and AMY are associated with male-specific reproductive behaviors and the VMH is implicated in female receptivity. Although soma size decreased in all brain regions with age, brain region volume diminished only in the POA, with a transient sex difference appearing before hatching. Cell density increased with age only in the female AMY. ERalpha mRNA expression was up to four times greater in the developing VMH than POA and AMY, peaking in the VMH around the day of hatching. These results are consistent with the idea that estradiol may influence differentiation of the VMH in particular. However, other factors are likely important to the development of these three brain regions, some of which exert their effects at later developmental stages.

Comparing histological data from different brains: sources of error and strategies for minimizing them

The recent development of brain atlases with computer graphics templates, and of huge databases of neurohistochemical data on the internet, has forced a systematic re-examination of errors associated with comparing histological features between adjacent sections of the same brain, between brains treated in the same way, and between brains from groups treated in different ways. The long-term goal is to compare as accurately as possible a broad array of data from experimental brains within the framework of reference atlases. Main sources of error, each of which ideally should be measured and minimized, include intrinsic biological variation, linear and nonlinear distortion of histological sections, plane of section differences between each brain, section alignment problems, and sampling errors. These variables are discussed, along with approaches to error estimation and minimization in terms of a specific example-the distribution of neuroendocrine neurons in the rat paraventricular nucleus. Based on the strategy developed here, the main conclusion is that the best long-term solution is a high-resolution 3D computer graphics model of the brain that can be sliced in any plane and used as the framework for quantitative neuroanatomy, databases, knowledge management systems, and structure-function modeling. However, any approach to the automatic annotation of neuroanatomical data-relating its spatial distribution to a reference atlas-should deal systematically with these sources of error, which reduce localization reliability.

Dopaminergic projections to the medial preoptic area of postpartum rats

Dopamine receptor activity in the rodent medial preoptic area (mPOA) is crucial for the display of maternal behaviors, as well as numerous other physiological and behavioral functions. However, the origin of dopaminergic input to the mPOA has not been identified through neuroanatomical tracing. To accomplish this, the retrograde tracer Fluorogold was iontophoretically applied to the mPOA of postpartum laboratory rats, and dual-label immunocytochemistry for Fluorogold and tyrosine hydroxylase later performed to identify dopaminergic cells of the forebrain and midbrain projecting to the mPOA. Results indicate that the number of dopaminergic cells projecting to the mPOA is moderate ( approximately 90 cells to one hemisphere), and that these cells have an unexpectedly wide distribution. Even so, more than half of the dual-labeled cells were found in either what has been considered extensions of the A10 dopamine group (particularly the ventrocaudal posterior hypothalamus and adjacent medial supramammillary nucleus), or in the A10 group of the ventral tegmental area. The rostral hypothalamus and surrounding region also contained numerous dual-labeled cells, with the greatest number found within the mPOA itself (including in the anteroventral preoptic area and preoptic periventricular nucleus). Notably, dual-labeled cells were rare in the zona incerta (A13), a site previously suggested to provide dopaminergic input to the mPOA. This study is the first to use anatomical tracing to detail the dopaminergic projections to the mPOA in the laboratory rat, and indicates that much of this projection originates more caudally than previously suggested.

Fasting reduces KiSS-1 expression in the anteroventral periventricular nucleus (AVPV): effects of fasting on the expression of KiSS-1 and neuropeptide Y in the AVPV or arcuate nucleus of female rats

Changes in metabolic state, such as those induced by fasting, have profound effects on reproduction. In rats, the time-course over which fasting inhibits luteinising hormone (LH) release is reduced to 48 h by the presence of oestradiol-17beta (E(2)). Hypothalamic kisspeptin plays a key role in mediating the actions of E(2) on gonadotrophin-releasing hormone (GnRH) neurones, and thereby promotes LH release. KiSS-1-expressing neurones are found in the anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC). Extensive evidence implicates the AVPV in GnRH release and the ARC in energy balance. The latter nucleus also contains neurones that express neuropeptide Y (NPY), an orexigenic peptide implicated in GnRH control. To elucidate the involvement of kisspeptin and/or NPY in hypothalamic responses to fasting, their expression was quantified by in situ hybridisation histochemistry in ovariectomised rats, with or without E(2) replacement, before and after 48 h of fasting. In the presence of E(2), but not in its absence, the fasting suppressed plasma LH. In the AVPV, the low level of KiSS-1 expression found in the absence of E(2) was unaffected by fasting. By contrast, the elevated level found in the presence of E(2) was suppressed by fasting. Independent of E(2), fasting had no effect on KiSS-1 expression in the ARC, but increased NPY expression at that site. The present study has identified the AVPV as a site at which KiSS-1 expression can be influenced by fasting. The results suggest that inhibition of KiSS-1 expression in the AVPV may be a significant factor in restraining the gonadotrophic axis in response to negative energy balance in the presence of oestrogen. The extent to which the concurrent rise in NPY expression in the ARC may contribute to the suppression of LH release by influencing AVPV kisspeptin neurones, directly or indirectly, or by actions independent of kisspeptin, remains to be established.

Sex differences and effects of neonatal aromatase inhibition on masculine and feminine copulatory potentials in prairie voles

Copulatory behaviors in most rodents are highly sexually dimorphic, even when circulating hormones are equated between the sexes. Prairie voles (Microtus ochrogaster) are monomorphic in their display of some social behaviors, including partner preferences and parenting, but differences between the sexes in their masculine and feminine copulatory behavior potentials have not been studied in detail. Furthermore, the role of neonatal aromatization of testosterone to estradiol on the development of prairie vole sexual behavior potentials or their brain is unknown. To address these issues, prairie vole pups were injected daily for the first week after birth with 0.5 mg of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) or oil. Masculine and feminine copulatory behaviors in response to testosterone or estradiol were later examined in both sexes. Males and females showed high mounting and thrusting in response to testosterone, but only males reliably showed ejaculatory behavior. Conversely, males never showed feminine copulatory behaviors in response to estradiol. Sex differences in these behaviors were not affected by neonatal ATD, but ATD-treated females received fewer mounts and thrusts than controls, possibly indicating reduced attractiveness to males. In other groups of subjects, neonatal ATD demasculinized males' tyrosine hydroxylase expression in the anteroventral periventricular preoptic area, and estrogen receptor alpha expression in the medial preoptic area. Thus, although sexual behavior in both sexes of prairie voles is highly masculinized, aromatase during neonatal life is necessary only for females' femininity. Furthermore, copulatory behavior potentials and at least some aspects of brain development in male prairie voles are dissociable by their requirement for neonatal aromatase.

Cross-talk between estrogen and leptin signaling in the hypothalamus

Obesity, characterized by enhanced food intake (hyperphagia) and reduced energy expenditure that results in the accumulation of body fat, is a major risk factor for various diseases, including diabetes, cardiovascular disease, and cancer. In the United States, more than half of adults are overweight, and this number continues to increase. The adipocyte-secreted hormone leptin and its downstream signaling mediators play crucial roles in the regulation of energy balance. Leptin decreases feeding while increasing energy expenditure and permitting energy-intensive neuroendocrine processes, such as reproduction. Thus, leptin also modulates the neuroendocrine reproductive axis. The gonadal steroid hormone estrogen plays a central role in the regulation of reproduction and also contributes to the regulation of energy balance. Estrogen deficiency promotes feeding and weight gain, and estrogen facilitates, and to some extent mimics, some actions of leptin. In this review, we examine the functions of estrogen and leptin in the brain, with a focus on mechanisms by which leptin and estrogen cooperate in the regulation of energy homeostasis.

PreproThyrotropin-releasing hormone 178-199 affects tyrosine hydroxylase biosynthesis in hypothalamic neurons: a possible role for pituitary prolactin regulation

ProThyrotropin-releasing hormone (proTRH) is a prohormone widely distributed in many areas of the brain. After biosynthesis, proTRH is subjected to post-translational processing to generate TRH and seven non-TRH peptides. Among these non-TRH sequences, we found previously that preproTRH178-199 could regulate the secretion of prolactin in suckled rats by their pups. Dopamine (DA), the main regulator of prolactin secretion, is produced in dopaminergic tyrosine hydroxylase (TH)-positive neurons in the hypothalamic arcuate nucleus (ARC). In this study we investigated whether prolactin release during the estrous sexual cycle is regulated by preproTRH178-199 through its effect on DA neurons of the ARC. We observed that biotinylated preproTRH178-199 bound to neurons in the ARC; this was higher during proestrus than during diestrus. Binding of preproTRH178-199 to DA neurons was seen only during proestrus in the ARC. Using primary neuronal hypothalamic cultures we found that preproTRH178-199 peptide decreased TH levels in a dose-responsive manner, whereas intra-ARC administration of preproTRH178-199 induced a 20-fold increase in plasma prolactin levels. Together, these results suggest a potential role for preproTRH178-199 in regulating dopaminergic neurons involved in the inhibition of pituitary prolactin release.

Sex Differences in Brain and Behavior: Hormones Versus Genes

Sex determination is the commitment of an organism toward male or female development. Traditionally, in mammals, sex determination is considered equivalent to gonadal determination. Since the presence or the absence of the testes ultimately determines the phenotype of the external genitalia, sex determination is typically seen as equivalent to testis determination. But what exactly does sex determine? The endpoint of sex determination is almost invariably seen as the reproductive structures, which represent the most obvious phenotypic difference between the sexes. One could argue that the most striking differences between males and females are not the anatomy of the genitals, but the size of the gametes-considerably larger in females than males. In fact, there could be many different endpoints to sex determination, leading to differences between the sexes: brain sexual differences, behavioral differences, and susceptibility to disease. The central dogma of sexual differentiation, stemming initially from the gonad-transfer experiments of Alfred Jost, is that sexual dimorphisms of all somatic tissues are dependent on the testicular secretion of the developing fetus. In this chapter, we will take the example of sex differences in brain and behavior as an endpoint of sex determination. We will argue that genetic factors play a role in sexually dimorphic traits such as the number of dopaminergic cells in the mesencephalon, aggression, and sexual orientation, independently from gonadal hormones.

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.

Regulation of Kiss1 gene expression in the brain of the female mouse

The Kiss1 gene encodes a family of neuropeptides called kisspeptins, which activate the receptor G protein-coupled receptor-54 and play a role in the neuroendocrine regulation of GnRH secretion. We examined whether estradiol (E2) regulates KiSS-1 in the forebrain of the female mouse by comparing KiSS-1 mRNA expression among groups of ovary-intact (diestrus), ovariectomized (OVX), and OVX plus E2-treated mice. In the arcuate nucleus (Arc), KiSS-1 expression increased after ovariectomy and decreased with E2 treatment. Conversely, in the anteroventral periventricular nucleus (AVPV), KiSS-1 expression was reduced after ovariectomy and increased with E2 treatment. To determine whether the effects of E2 on KiSS-1 are mediated through estrogen receptor (ER)alpha or ERbeta, we evaluated the effects of E2 in OVX mice that lacked functional ERalpha or ERbeta. In OVX mice that lacked functional ERalpha, KiSS-1 mRNA did not respond to E2 in either the Arc or AVPV, suggesting that ERalpha is essential for mediating the inhibitory and stimulatory effects of E2. In contrast, KiSS-1 mRNA in OVX mice that lacked functional ERbeta responded to E2 exactly as wild-type animals. Double-label in situ hybridization revealed that virtually all KiSS-1-expressing neurons in the Arc and AVPV coexpress ERalpha, suggesting that the effects of E2 are mediated directly through KiSS-1 neurons. We conclude that KiSS-1 neurons in the Arc, which are inhibited by E2, may play a role in the negative feedback regulation of GnRH secretion, whereas KiSS-1 neurons in the AVPV, which are stimulated by E2, may participate in the positive feedback regulation of GnRH secretion.

PAC1 receptors mediate pituitary adenylate cyclase-activating polypeptide- and progesterone-facilitated receptivity in female rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts as a feed-forward, paracrine/autocrine factor in the hypothalamic ventromedial nucleus (VMN) for receptivity and sensitizes pituitary hormone release for ovulation. The present study examined receptor(s) and signaling pathway by which PACAP enhances rodent lordosis. PACAP binds to PACAP (PAC1)- and vasoactive intestinal peptide-preferring receptors (VPAC1, VPAC2). Ovariectomized rodents primed with estradiol (EB) were given PACAP or vasoactive intestinal peptide directly onto VMN cells. Only PACAP facilitated receptivity. Pretreatment with VPAC1 and VPAC2 inhibitors blocked both PACAP- and progesterone (P)-induced receptivity. Antisense (AS) oligonucleotides to PAC1 (not VPAC1 or VPAC2) inhibited the behavioral effect of PACAP and P. By real-time RT-PCR, EB, P and EB+P enhanced VMN mRNA expression of PAC1. Within the total PAC1 population, EB and EB+P induced expression of short form PAC1 and PAC1hop2 splice variants. Finally, blocking cAMP/protein kinase A signaling cascade by antagonists to cAMP activity and protein kinase A or by antisense to dopamine- and cAMP-regulated phosphoprotein of 32 kDa blocked the PACAP effect on behavior. Collectively, these findings provide evidence that progesterone receptor-dependent receptivity is, in part, dependent on PAC1 receptors for intracellular VMN signaling and delineate a novel, steroid-dependent mechanism for a feed-forward reinforcement of steroid receptor-dependent reproductive receptivity.

Novel neuronal phenotypes from neural progenitor cells

We report the first isolation of progenitor cells from the hypothalamus, a derivative of the embryonic basal plate that does not exhibit neurogenesis postnatally. Neurons derived from hypothalamic progenitor cells were compared with those derived from progenitor cultures of hippocampus, an embryonic alar plate derivative that continues to support neurogenesis in vivo into adulthood. Aside from their different embryonic origins and their different neurogenic potential in vivo, these brain regions were chosen because they are populated with cells of three different categories: Category I cells are generated in both hippocampus and hypothalamus, Category II cells are generated in the hypothalamus but are absent from the hippocampus, and Category III is a cell type generated in the olfactory placode that migrates into the hypothalamus during development. Stem-like cells isolated from other brain regions, with the ability to generate neurons and glia, produce neurons of several phenotypes including gabaergic, dopaminergic, and cholinergic lineages. In the present study, we extended our observations into neuroendocrine phenotypes. The cultured neural precursors from 7-week-old rat hypothalamus readily generated neuropeptide-expressing neurons. Hippocampal and hypothalamic progenitor cultures converged to indistinguishable populations and produced neurons of all three categories, confirming that even short-term culture confers or selects for immature progenitors with enough plasticity to elaborate neuronal phenotypes usually inhibited in vivo by the local microenvironment. The range of phenotypes generated from neuronal precursors in vitro now includes the peptides found in the neuroendocrine system: corticotropin-releasing hormone, growth hormone-releasing hormone, gonadotropin-releasing hormone, oxytocin, somatostatin, thyrotropin-releasing hormone, and vasopressin.

Distribution of glutamic acid decarboxylase, neurotensin, enkephalin, neuropeptide Y, and cholecystokinin neurons in the septo-preoptic region of male rats

Neurons in the lateral septum (LS) and preoptic area (POA) are known to play an inhibitory role in feminine sexual behavior regulation in male rats. In this study, the distribution of neurons containing glutamic acid decarboxylase (GAD) and of the peptidergic neurotransmitters neurotensin (NT), enkephalin (ENK), neuropeptide Y (NPY), and cholecystokinin (CCK), was examined immunohistochemically in the LS and POA of castrated male rats subcutaneously implanted with estrogen-containing Silastic tubes. Colchicine was injected into the lateral ventricle of the animals. The forebrain sections were immunostained for each substance. A large number of GAD-immunoreactive (ir) cells were found in the LS. Many NT-ir cells were seen in the intermediate and ventral parts of the LS at the rostral and middle levels. A considerable number of ENK-ir cells were scattered over the LS at the rostral and middle levels and were observed in the ventral part of the caudal LS. There were only a few NPY-ir cells in the LS. No CCK-ir cells were observed in the LS. In the POA, GAD-ir cells were observed in abundance. Many NT-ir cells were seen, especially in the medial preoptic nucleus. Some ENK-ir cells and a few NPY-ir cells were found in the medial POA. CCK-ir cells of the POA were restricted to the periventricular and paraventricular hypothalamic nuclei.

Hypothalamic cocaine- and amphetamine-regulated transcript neurons project to areas expressing gonadotropin releasing hormone immunoreactivity and to the anteroventral periventricular nucleus in male and female rats

Cocaine- and amphetamine-regulated transcript (CART) and CART-derived peptides are widely expressed in the hypothalamus. CART is involved in food intake control and is regulated by circulating leptin, a hormone implicated in a variety of endocrine functions. Lack of leptin (ob/ob mice) is associated with obesity, hypogonadism and infertility. In the arcuate nucleus, dorsomedial nucleus of the hypothalamus, and ventral premammillary nucleus, CART neurons also express leptin receptor long-form splice-variant. Recent studies have suggested that the facilitatory effect of leptin on gonadotropin-releasing hormone (GnRH) secretion is mediated by CART. In the present study, using dual- and triple-label immunohistochemistry, we identified CART fibers in close apposition with GnRH neurons expressing Fos in the afternoon of the proestrous day, as well as with GnRH neurons in male rats. In order to investigate the origin of these fibers, we injected the retrograde tracer Fluorogold into areas containing GnRH cell bodies. In male and female rats, the tracer was injected around the vascular organ of lamina terminalis, median preoptic nucleus and medial preoptic nucleus, as well as in the anteroventral periventricular nucleus. We observed retrogradely labeled neurons in various hypothalamic nuclei, including the arcuate, dorsomedial and ventral premammillary. In these areas, dual-label immunohistochemistry/in situ hybridization revealed that part of the retrogradely labeled neurons also express CART mRNA. As a control, we injected the anterograde tracer biotinylated dextran amine into the ventral premammillary nucleus of both males and females. Most projections targeted brain areas related to reproductive behavior and few fibers were closely associated with GnRH neurons. Our findings indicate that ventral premammillary nucleus CART neurons intermingle with brain circuitry involved in reproduction. Therefore, these neurons are well positioned to mediate leptin effect on reproductive control.

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.

Estrogen receptor-alpha is required for estrogen-induced mu-opioid receptor internalization

Endogenous opioid circuits are pivotal for the regulation of sexual receptivity. Treatment of mice with morphine, a preferential mu-opioid receptor (MOR) agonist, severely attenuates lordosis. Estrogen induces internalization of MOR in cell groups of the limbic-hypothalamic lordosis-regulating circuit. Because rapid MOR internalization is mediated by estrogen release of endogenous opioid peptides, internalization has been used as a neurochemical signature of estrogen action in the central nervous system. Together these results indicate that estrogen induces a MOR mediated inhibition of sexual receptivity. To determine which estrogen receptor, estrogen receptor-alpha (ERalpha) or estrogen receptor-beta (ERbeta), mediates MOR internalization, ERalpha knockout (ERalphaKO), ERbeta knockout (ERbetaKO) and wild-type (WT) mice were used in the present study. WT, ERalphaKO and ERbetaKO mice had similar MOR distributions in the limbic-hypothalamic lordosis-regulating circuit. Estrogen treatment internalized MOR in the medial preoptic nucleus of ovariectomized WT and ERbetaKO, but not ERalphaKO mice. Treatment of ERalphaKO mice with the selective endogenous MOR ligand, endomorphin-1, induced levels of MOR internalization similar to WT mice suggesting that MOR in ERalphaKO mice could be activated and were probably functional. The results of the present experiments indicate that ERalpha is required for estrogen-induced MOR internalization and suggest that ERalpha can mediate rapid actions of estrogen.

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.

Differential effects of alpha-, beta- and gamma(2)-melanocyte-stimulating hormones on hypothalamic neuronal activation and feeding in the fasted rat

Hypothalamic pro-opiomelanocortin neurones have an established role in the control of feeding. While pro-opiomelanocortin is the precursor for at least three melanocortin peptides, alpha-, beta- and gamma-melanocyte-stimulating hormone (MSH), it has been widely assumed that alpha-MSH is the predominant ligand involved. We compared the effects of centrally administered alpha-, beta- and gamma(2)-MSH on hypothalamic neuronal activation and on food intake in rats fasted for 48 h. Significant reductions in food intake were seen with alpha-MSH (first hour) and gamma(2)-MSH (second hour) but not with beta-MSH. The pattern of neuronal activation, assessed by the detection of early growth response factor-1 protein, showed considerable overlap; all three melanocortins activated cells in the arcuate, ventromedial, paraventricular, periventricular and supraoptic nuclei, as well as the preoptic area. alpha-MSH and beta-MSH produced activation in the dorsomedial nuclei while gamma(2)-MSH was only weakly active here. Retrograde labelling by systemic Fluorogold injection revealed that many cells activated by MSH compounds in the arcuate, paraventricular, periventricular and supraoptic nuclei (but not dorsomedial or ventromedial) project outside the blood-brain barrier and are therefore likely to include neuroendocrine cells. Desacetyl-alpha-MSH, which has previously been reported to lack effects on feeding, produced no discernible neuronal activation in the hypothalamus. Our finding that both the pattern of neuronal activation and the distribution of neuroendocrine cells activated in response to these closely related peptides show only partial overlap suggests that, in addition to common pathways, there may exist distinct hypothalamic circuits activated by different pro-opiomelanocortin products. The slower time course of gamma(2)-MSH- versus alpha-MSH-induced suppression of feeding provides further support for the notion that the biological responses to individual melanocortin peptides may involve distinct neuronal mechanisms.

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.

Parallel declines in Fos activation of the medial anteroventral periventricular nucleus and LHRH neurons in middle-aged rats

The middle-age decline in reproductive function is manifested by reduced LHRH release, resulting in a decreased magnitude and delay of onset of the LH surge. Earlier studies suggested that the reductions in LHRH neural activation in middle-aged rats resulted from deficits in the afferent drive to the LHRH neurons. One critical afferent to the LHRH neurons lies in the anteroventral periventricular preoptic area (AVPv) nucleus. The neurons of the medial AVPv are synchronously activated to express Fos with LHRH neurons at the time of an LH surge in young adult animals. The present study examined whether, in middle age, reductions in the activation of AVPv neurons accompany the reduction in Fos activation in LHRH neurons. Young (3- to 4-month-old) and middle-aged (10- to 12-month-old) spontaneously cycling and ovariectomized steroid-replaced rats were killed during peak and early descending phase of the LH surge, and their brains were examined for Fos in LHRH and AVPv neurons. Young animals had a characteristic increase in Fos expression in both LHRH and AVPv neurons. In middle-aged rats, the proportion of LHRH neurons expressing Fos at the time of an LH surge was reduced by approximately 50%, irrespective of whether surges were spontaneous or induced by exogenous steroids. A similar reduction in the number of Fos+ cells (by approximately 50%) was noted in the medial AVPv. Linear regression analysis of the relationship between the extent of Fos activation in LHRH and AVPv neurons revealed a strong positive correlation (r(2) = 0.66; P < 0.01), suggesting that changes in the AVPv's drive to LHRH neurons underlie the decrease in LHRH activity in middle age. A second series of experiments examined whether decreased input from the AVPv could account for reduced Fos activation in LHRH neurons seen in middle-aged animals. When the medial AVPv was lesioned, LHRH neurons failed to express Fos on the side ipsilateral to the lesion. Animals with lesioned medial AVPv also had significantly lower LH values than animals with an intact medial AVPv. Taken together, these data suggest that a principal deficit in middle-aged rats is the ability of the medial AVPv to stimulate LHRH neurons.

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

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