Neuropeptide Y stimulates luteinizing hormone-releasing hormone release from superfused hypothalamic GT1-7 cells

Bisphenol A Alters the Levels of miRNAs That Directly and/or Indirectly Target Neuropeptide Y in Murine Hypothalamic Neurons

The hypothalamus is a vital regulator of energy homeostasis. Orexigenic neuropeptide Y (NPY) neurons within the hypothalamus can stimulate feeding and suppress energy expenditure, and dysregulation of these neurons may contribute to obesity. We previously reported that bisphenol A (BPA), an endocrine disruptor with obesogenic properties, alters Npy transcription in hypothalamic neurons by inducing oxidative stress. We hypothesized that hypothalamic microRNAs (miRNAs), a class of small non-coding RNAs, could directly regulate Npy gene expression by binding the 3' untranslated region (UTR). Five predicted Npy-targeting miRNA candidates were uncovered through TargetScan and were detected in Npy-expressing hypothalamic neuronal cell models and hypothalamic neuronal primary cultures. BPA dysregulated the expression of a number of these hypothalamic miRNAs. We examined the effects of putative Npy-targeting miRNAs using miRNA mimics, and we found that miR-143-3p, miR-140-5p, miR-29b-1-5p, and let-7b-3p altered Npy expression in the murine hypothalamic cell lines. Importantly, miR-143-3p targets the mouse Npy 3' UTR, as detected using a luciferase construct containing the potential 3' UTR binding sites. Overall, this study established the first hypothalamic miRNA that directly targets the 3' UTR of mouse Npy, emphasizing the involvement of miRNAs in the NPY system and providing an alternative target for control of NPY levels.

BPA Differentially Regulates NPY Expression in Hypothalamic Neurons Through a Mechanism Involving Oxidative Stress

Bisphenol A (BPA), a ubiquitous endocrine-disrupting chemical, interferes with reproduction and is also considered an obesogen. The neuropeptide Y (NPY) neurons of the hypothalamus control both food intake and reproduction and have emerged as potential targets of BPA. These functionally diverse subpopulations of NPY neurons are differentially regulated by peripheral signals, such as estrogen and leptin. Whether BPA also differentially alters Npy expression in subpopulations of NPY neurons, contributing to BPA-induced endocrine dysfunction is unclear. We investigated the response of 6 immortalized hypothalamic NPY-expressing cell lines to BPA treatment. BPA upregulated Npy mRNA expression in 4 cell lines (mHypoA-59, mHypoE-41, mHypoA-2/12, mHypoE-42), and downregulated Npy in 2 lines (mHypoE-46, mHypoE-44). This differential expression of Npy occurred concurrently with differential expression of estrogen receptor mRNA levels. Inhibition of G-protein coupled estrogen receptor GPR30 or estrogen receptor β prevented the BPA-mediated decrease in Npy, whereas inhibition of energy sensor 5' adenosine monophosphate-activated protein kinase (AMPK) with compound C prevented BPA-induced increase in Npy. BPA also altered neuroinflammatory and oxidative stress markers in both mHypoA-59 and mHypoE-46 cell lines despite the differential regulation of Npy. Remarkably, treatment with BPA in an antioxidant-rich media, Neurobasal A (NBA), or with reactive oxygen species scavenger tauroursodeoxycholic acid mitigated the BPA-induced increase and decrease in Npy. Furthermore, 2 antioxidant species from NBA-N-acetylcysteine and vitamin B6-diminished the induction of Npy in the mHypoA-59 cells, demonstrating these supplements can counteract BPA-induced dysregulation in certain subpopulations. Overall, these results illustrate the differential regulation of Npy by BPA in neuronal subpopulations, and point to oxidative stress as a pathway that can be targeted to block BPA-induced Npy dysregulation in hypothalamic neurons.

The regulation of reproductive neuroendocrine function by insulin and insulin-like growth factor-1 (IGF-1)

The mammalian reproductive hormone axis regulates gonadal steroid hormone levels and gonadal function essential for reproduction. The neuroendocrine control of the axis integrates signals from a wide array of inputs. The regulatory pathways important for mediating these inputs have been the subject of numerous studies. One class of proteins that have been shown to mediate metabolic and growth signals to the CNS includes Insulin and IGF-1. These proteins are structurally related and can exert endocrine and growth factor like action via related receptor tyrosine kinases. The role that insulin and IGF-1 play in controlling the hypothalamus and pituitary and their role in regulating puberty and nutritional control of reproduction has been studied extensively. This review summarizes the in vitro and in vivo models that have been used to study these neuroendocrine structures and the influence of these growth factors on neuroendocrine control of reproduction.

Direct regulation of GnRH neuron excitability by arcuate nucleus POMC and NPY neuron neuropeptides in female mice

Hypothalamic neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons act to sense and coordinate the brain's responses to metabolic cues. One neuronal network that is very sensitive to metabolic status is that controlling fertility. In this study, we investigated the impact of neuropeptides released by NPY and POMC neurons on the cellular excitability of GnRH neurons, the final output cells of the brain controlling fertility. The majority (∼70%) of GnRH neurons were activated by α-melanocyte-stimulating hormone, and this resulted from the direct postsynaptic activation of melanocortin receptor 3 and melanocortin receptor 4. A small population of GnRH neurons (∼15%) was excited by cocaine and amphetamine-regulated transcript or inhibited by β-endorphin. Agouti-related peptide, released by NPY neurons, was found to have variable inhibitory (∼10%) and stimulatory (∼25%) effects upon subpopulations of GnRH neurons. A variety of NPY and pancreatic polypeptide analogs was used to examine potential NPY interactions with GnRH neurons. Although porcine NPY (Y1/Y2/Y5 agonist) directly inhibited the firing of approximately 45% of GnRH neurons, [Leu(31),Pro(34)]-NPY (Y1/Y4/Y5 agonist) could excite (56%) or inhibit (19%). Experiments with further agonists indicated that Y1 receptors were responsible for suppressing GnRH neuron activity, whereas postsynaptic Y4 receptors were stimulatory. These results show that the activity of GnRH neurons is regulated in a complex manner by neuropeptides released by POMC and NPY neurons. This provides a direct route through which different metabolic cues can regulate fertility.

Leptin differentially regulates NPY secretion in hypothalamic cell lines through distinct intracellular signal transduction pathways

Leptin acts as a key peripheral hormone in distinct neurons in the hypothalamus to modulate both reproductive function and energy homeostasis. The control of neuropeptide Y (NPY) secretion is an example of a process that can be differentially regulated by leptin. In order to further understand these distinct modulatory effects, we have used immortalized, neuronal hypothalamic cell lines expressing NPY, mHypoE-38 and mHypoE-46. We found that these cell lines express the endogenous leptin receptor, ObRb, and secrete detectable levels of NPY. We exposed the neurons to 100nM leptin for 1h and determined that the basal levels of NPY in the cell lines were differentially regulated: NPY secretion was inhibited in mHypoE-46 neurons, whereas NPY secretion was induced in the mHypoE-38 neurons. In order to determine the mechanisms involved in the divergent regulation of NPY release, we analyzed the activity of a number of signaling components using phospho-specific antibodies directed towards specific proteins in the MAP kinase, PI3K, and AMPK pathways, among others. We found that leptin activated a different combination of second messengers in each cell line. Importantly, we could link the regulation of NPY secretion to different signaling pathways, AMPK in the mHypoE-46 and both MAPK and PI3K in the mHypoE-38 neurons. This is the first demonstration that leptin can specifically regulate individual NPY neuron secretory responses through distinct signaling pathways.

Immortalized neurons for the study of hypothalamic function

The hypothalamus is a vital part of the central nervous system: it harbors control systems implicated in regulation of a wide range of homeostatic processes, including energy balance and reproduction. Structurally, the hypothalamus is a complex neuroendocrine tissue composed of a multitude of unique neuronal cell types that express a number of neuromodulators, including hormones, classical neurotransmitters, and specific neuropeptides that play a critical role in mediating hypothalamic function. However, neuropeptide and receptor gene expression, second messenger activation, and electrophysiological and secretory properties of these hypothalamic neurons are not yet fully defined, primarily because the heterogeneity and complex neuronal architecture of the neuroendocrine hypothalamus make such studies challenging to perform in vivo. To circumvent this problem, our research group recently generated embryonic- and adult-derived hypothalamic neuronal cell models by utilizing the novel molecular techniques of ciliary neurotrophic factor-induced neurogenesis and SV40 T antigen transfer to primary hypothalamic neuronal cell cultures. Significant research with these cell lines has demonstrated their value as a potential tool for use in molecular genetic analysis of hypothalamic neuronal function. Insights gained from hypothalamic immortalized cells used in conjunction with in vivo models will enhance our understanding of hypothalamic functions such as neurogenesis, neuronal plasticity, glucose sensing, energy homeostasis, circadian rhythms, and reproduction. This review discusses the generation and use of hypothalamic cell models to study mechanisms underlying the function of individual hypothalamic neurons and to gain a more complete understanding of the overall physiology of the hypothalamus.

Kisspeptin directly regulates neuropeptide Y synthesis and secretion via the ERK1/2 and p38 mitogen-activated protein kinase signaling pathways in NPY-secreting hypothalamic neurons

Kisspeptin is a key component of reproduction that directly stimulates GnRH neurons. However, recent studies indicate that kisspeptin can indirectly stimulate GnRH neurons through unidentified afferent networks. Neuropeptide Y (NPY) is another key reproductive hormone that is an afferent stimulator of GnRH neurons. Herein, we report kisspeptin receptor Kiss1r mRNA expression in native NPY neurons FAC-sorted from NPY-GFP transgenic mice. Thus, we hypothesized that kisspeptin indirectly stimulates GnRH neurons through direct regulation of NPY neurons. Using hypothalamic NPY-secreting cell lines, we determined that kisspeptin stimulates NPY mRNA expression and secretion in the mHypoE-38 cells, but not the mHypoE-42 cells, using quantitative RT-PCR and enzyme immunoassays. Furthermore, agouti-related peptide, ghrelin, neurotensin, or Kiss1r mRNA expression was not changed upon exposure to kisspeptin in either cell line. These results concur with our previous work identifying the mHypoE-38 cell line as a putative reproductive NPY neuron and the mHypoE-42 cell line as a potential feeding-related NPY neuron. In the mHypoE-38 cells, kisspeptin activated the ERK1/2 and p38 MAPK kinases as shown by Western blot analysis. Moreover, inhibiting the ERK1/2 and p38 pathways with U0126 and SB239063, respectively, prevented kisspeptin induction of NPY mRNA expression and secretion. Altogether, we find that kisspeptin directly regulates NPY synthesis and secretion via the ERK1/2 and p38 MAPK pathways in a NPY-secreting cell line, and we propose NPY neurons as an afferent network by which kisspeptin indirectly stimulates GnRH secretion.

Estrogen regulation of gene expression in GnRH neurons

Estrogen plays an essential role in the regulation of the female reproductive hormone axis, and specifically is a major regulator of GnRH neuronal function in the female brain. GnRH neuronal cell lines were used to explore the direct effects of estradiol on gene expression in GnRH neurons. The presence of estrogen receptor (ER) binding sites was established by a receptor-binding assay, and estrogen receptor alpha and beta mRNA were identified in GN11 cells and ERbeta in GT1-7 cells using RT-PCR analysis of mRNA. ERalpha was more abundantly expressed in GN11 cells than ERbeta as assessed by real-time PCR. Additionally, GN11 cells expressed significantly more of both ERalpha and beta than GT1-7 cells. Functional studies in GN11 and GT1-7 demonstrated estrogen down regulation of endogenous mouse GnRH mRNA levels using quantitative real-time PCR (qRT-PCR). Correspondingly, estradiol also reduced secretion of GnRH from both the GN11 and GT1-7 cell lines. Since estradiol has been shown to regulate progesterone receptor (PR) expression; similar studies were performed demonstrating an estradiol mediated increase in PR in both cell lines. Estradiol regulation of ER expression was also explored and these studies indicated that estradiol decreased ERalpha and ERbeta mRNA levels in a dose-dependent manner in GN11 and GT1-7 cells. These effects were blocked by the addition of the estrogen receptor antagonist ICI 182,780. Both PPT, a specific ERalpha agonist, and DPN, a specific ERbeta agonist, inhibited GnRH gene expression in GN11 cells, but only DPN inhibited GnRH gene expression in GT1-7 cells, consistent with their undetectable levels of ERalpha expression. These studies characterize a direct inhibitory effect of estradiol on GnRH in GnRH neurons, and a direct stimulatory effect of estradiol on PR gene expression. In addition, the agonist studies indicate that there is a functional overlap of ERalpha and ERbeta regulation in GnRH neurons. These studies may give insight into the molecular regulation of estrogen negative feedback in the central reproductive axis.

Fasting-induced suppression of LH secretion does not require activation of ATP-sensitive potassium channels

Reproductive hormone secretions are inhibited by fasting and restored by feeding. Metabolic signals mediating these effects include fluctuations in serum glucose, insulin, and leptin. Because ATP-sensitive potassium (K(ATP)) channels mediate glucose sensing and many actions of insulin and leptin in neurons, we assessed their role in suppressing LH secretion during food restriction. Vehicle or a K(ATP) channel blocker, tolbutamide, was infused into the lateral cerebroventricle in ovariectomized mice that were either fed or fasted for 48 h. Tolbutamide infusion resulted in a twofold increase in LH concentrations in both fed and fasted mice compared with both fed and fasted vehicle-treated mice. However, tolbutamide did not reverse the suppression of LH in the majority of fasted animals. In sulfonylurea (SUR)1-null mutant (SUR1(-/-)) mice, which are deficient in K(ATP) channels, and their wild-type (WT) littermates, a 48-h fast was found to reduce serum LH concentrations in both WT and SUR(-/-) mice. The present study demonstrates that 1) blockade of K(ATP) channels elevates LH secretion regardless of energy balance and 2) acute fasting suppresses LH secretion in both SUR1(-/-) and WT mice. These findings support the hypothesis that K(ATP) channels are linked to the regulation of gonadotropin-releasing hormone (GnRH) release but are not obligatory for mediating the effects of fasting on GnRH/LH secretion. Thus it is unlikely that the modulation of K(ATP) channels either as part of the classical glucose-sensing mechanism or as a component of insulin or leptin signaling plays a major role in the suppression of GnRH and LH secretion during food restriction.

Neuropeptide Y and alpha-melanocyte-stimulating hormone: interaction in obesity and possible role in the development of hypertension

AIM

Obesity and hypertension frequently coexist and both represent important risk factors for cardiovascular disease. The mechanisms implicated in the regulation of food intake have not been completely elucidated. Recent data suggests that peripheral and central neuropeptides play an important role in the maintenance of energy balance. More specifically, leptin, neuropeptide Y (NPY) and alpha-melanocyte-stimulating hormone (a-MSH) appear to be implicated in the pathogenesis of obesity and also contribute to the development of hypertension in obesity.

METHODS

Analysis of the pertinent bibliography published in PubMed database.

RESULTS

Leptin is produced in the adipose tissue directly correlated with fat tissue mass. Leptin acts on two distinct neural populations in the hypothalamus: the first expresses the orexigenic peptides NPY and agouti-related protein (AgRP), the second pro-opiomelanocortin (POMC). The activation of POMC neurons increases the production of the anorexigenic hormone a-MSH and inhibits the release of NPY and AgRP. In addition, the hypothalamus integrates the neuroendocrine systems with the autonomic nervous system and controls the activity of the latter. Stimulation of hypothalamic nuclei elicits sympathetic responses including blood pressure elevation. Both NPY and a-MSH appears to be implicated in the hypothalamic regulation of sympathetic nervous system (SNS) activity.

CONCLUSION

Alterations in leptin, NPY and a-MSH are frequently observed in obesity and might stimulate SNS activity, contributing to the development of hypertension in obese patients. These neuropeptides might provide a pathophysiologic link between excess weight and hypertension. However, more research is needed before the pharmacologic manipulation of these complex neuroendocrine systems can be applied in the treatment of obesity and hypertension.

Dependence of 3',5'-cyclic adenosine monophosphate--stimulated gonadotropin-releasing hormone release on intracellular calcium levels and L-type calcium channels in superfused GT1-7 neurons

OBJECTIVE

Immortalized GT1-7 neurons were used to characterize the interactive roles of adenylate cyclase-3',5'-cyclic adenosine monophosphate (cAMP) and L-type calcium channels on gonadotropin-releasing hormone (GnRH) release.

METHODS

Dibutyryl (db)-cAMP was used as an active analog of endogenous cAMP, and forskolin was used to activate adenylate cyclase. Extracellular calcium was chelated using EGTA and L-type calcium channels were blocked using nimodipine. The selective Ca2+ ionophore A23187 was employed to increase intracellular calcium levels. GT1-7 neurons were grown on Cytodex-3 beads (Pharmacia Biotech, Uppsala, Sweden) and placed in special superfusion microchambers. The cells were superfused at a rate of 6.2 mL/h with media 199 (M-199; Gibco, Grand Island, NY; pH 7.35, 37C); effluent fractions were collected at 5-minute intervals for analysis of GnRH concentrations by radioimmunoassay.

RESULTS

Basal GnRH release from superfused GT1-7 neurons ranged from 10 to 62 pg. min(-1). mL(-1). Coexposure of the cells to forskolin and A23187 produced an additive effect on stimulated release of GnRH. Cells exposed to 1 microM of forskolin (an activator of adenylate cyclase) for 5 minutes showed a 2.6-fold increase in GnRH release. Likewise, the addition of 100 microM of db-cAMP to the superfusion for 5 minutes demonstrated a 2.3-fold increase in the amplitude of GnRH secretion. Maintaining the superfused cells in medium containing 5 mM EGTA had no obvious effect on basal GnRH release but blocked the effect of db-cAMP to increase GnRH release. Similarly, the addition of 10 microM nimodipine to the superfusion medium blocked db-cAMP-stimulated GnRH release.

CONCLUSIONS

These findings provide additional evidence that cAMP-mediated GnRH release from GT1-7 neurons is dependent on influx of extracellular calcium via L-type Ca2+ channels.

Estrogen-induced mu-opioid receptor internalization in the medial preoptic nucleus is mediated via neuropeptide Y-Y1 receptor activation in the arcuate nucleus of female rats

The endogenous peptides beta-endorphin (beta-END) and neuropeptide Y (NPY) have been implicated in regulating sexual receptivity. Both beta-END and NPY systems are activated by estrogen and inhibit female sexual receptivity. The initial estrogen-induced sexual nonreceptivity is correlated with the activation and internalization of mu-opioid receptors (MORs), in the medial preoptic nucleus (MPN). Progesterone reverses the estrogen-induced activation/internalization of MOR and induces the sexual receptive behavior lordosis. To determine whether NPY and endogenous opioids interact, we tested the hypothesis that estrogen-induced MOR activation is mediated through NPY-Y1 receptor (Y1R) activation. Retrograde tract tracing demonstrated Y1Ron beta-END neurons that projected to the MPN. Sex steroid modulation of MOR in the MPN acts through NPY and the Y1R. Estradiol administration or intracerebroventricular injection of NPY activated/internalized Y1R in the arcuate nucleus and MOR in the MPN of ovariectomized (OVX) rats. Moreover, the selective Y1R agonist [Leu31, Pro34]-Neuropeptide Y (LPNY) internalized MOR in the MPN of OVX rats. The Y1R antagonist (Cys31, Nva34)-Neuropeptide Y (27-36)2 prevented estrogen-induced Y1R and MOR activation/internalization. NPY reversed the progesterone blockade of estradiol-induced Y1R and MOR internalization in the arcuate nucleus and MPN, respectively. Behaviorally, LPNY inhibited estrogen plus progesterone-induced lordosis, and the MOR-selective antagonist D-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide reversed LPNY-induced inhibition of lordosis. These results suggest that a sequential sex steroid activation of NPY and MOR circuits regulates sexual receptivity.

Comparative distribution of neuropeptide Y Y1 and Y5 receptors in the rat brain by using immunohistochemistry

Neuropeptide Y (NPY) Y1 and Y5 receptor subtypes mediate many of NPY's diverse actions in the central nervous system. The present studies use polyclonal antibodies directed against the Y1 and Y5 receptors to map and compare the relative distribution of these NPY receptor subtypes within the rat brain. Antibody specificity was assessed by using Western analysis, preadsorption of the antibody with peptide, and preimmune serum controls. Immunostaining for the Y1 and Y5 receptor subtypes was present throughout the rostral-caudal aspect of the brain with many regions expressing both subtypes: cerebral cortex, hippocampus, hypothalamus, thalamus, amygdala, and brainstem. Further studies using double-label immunocytochemistry indicate that Y1R immunoreactivity (-ir) and Y5R-ir are colocalized in the cerebral cortex and caudate putamen. Y1 receptor ir was evident in the central amygdala, whereas both Y1- and Y5-immunoreactive cells and fibers were present in the basolateral amygdala. Corresponding with the physiology of NPY in the hypothalamus, both Y1R- and Y5R-ir was present within the paraventricular (PVN), supraoptic, arcuate nuclei, and lateral hypothalamus. In the PVN, Y5R-ir and Y1R-ir were detected in cells and fibers of the parvo- and magnocellular divisions. Intense immunostaining for these receptors was observed within the locus coeruleus, A1-5 and C1-3 nuclei, subnuclei of the trigeminal nerve and nucleus tractus solitarius. These data provide a detailed and comparative mapping of Y1 and Y5 receptor subtypes within cell bodies and nerve fibers in the brain which, together with physiological and electrophysiological studies, provide a better understanding of NPY neural circuitries.

Involvement of NPY Y2 receptor subtype in the control of the spontaneous NO/GnRH release at the rat median eminence

The role of nitric oxide (NO) from vascular endothelium in the control of GnRH release at the median eminence (ME) level is well established. Interactions between NPY receptor/endothelium/nitric oxide are clearly demonstrated. While several studies implicate NPY Y1 receptor in the control of GnRH/LH at the time of the preovulatory LH surge, our results also demonstrate the importance of NPY Y2 receptor in the control of GnRH release via endothelial NO. We conclude that NPY may be one of the elements implicated in the generation of the spontaneous NO/GnRH via Y2 receptor located on endothelium.

Long-term recordings of networks of immortalized GnRH neurons reveal episodic patterns of electrical activity

The CNS controls reproduction through pulsatile secretion of gonadotropin-releasing hormone (GnRH). Episodic increases in the firing rate of unidentified hypothalamic neurons have been associated with downstream markers of GnRH secretion. Whether this episodic electrical activity is intrinsic to GnRH neurons, intrinsic to other "pulse generator" neurons that drive GnRH neurons, or a combination of these is unknown. To determine if GnRH neurons display episodic firing patterns in isolation from other cell types, immortalized GnRH neurons (GT1-7 cells) were cultured on multiple microelectrode arrays. Long-term, multi-site recordings of GT1-7 cells revealed repeated episodes of increased firing rate with an interval of 24.8 +/- 1.3 (SE) min that were completely eliminated by tetrodotoxin, a sodium channel blocker. This pattern was comprised of active units that fired independently as well as coincidentally, suggesting the overall pattern of electrical activity in GT1-7 cells emerges as a network property. The A-type potassium-channel antagonist 4-aminopyridine (1 mM) increased both firing rate and GnRH secretion, demonstrating the presence of A-type currents in these cells and supporting the hypothesis that electrical activity is associated with GnRH release. Physiologically relevant episodic firing patterns are thus an intrinsic property of immortalized GnRH neurons and appear to be associated with secretion. The finding that overall activity is derived from the sum of multiple independent active units within a network may have important implications for the genesis of the GnRH secretory pattern that is delivered to the target organ. Specifically, these data suggest not every GnRH neuron participates in each secretory pulse and provide a possible mechanism for the variations in GnRH-pulse amplitude observed in vivo.

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

Luteinizing hormone-releasing hormone (LHRH) neurons: mechanism of pulsatile LHRH release

Many types of neurons and glia exhibit oscillatory changes in membrane potentials and cytoplasmic Ca2+ concentrations. In neurons and neuroendocrine cells an elevation of intracellular Ca2+ concentration is associated with neurosecretion. Since both oscillatory membrane potentials and intracellular Ca2+ oscillations have been described in primary LHRH neurons and in GT1 cells, it is evident that an endogenous pulse-generator/oscillator is present in the LHRH neuron in vitro. The hourly rhythms of LHRH neurosecretion appear to be the synchronization of a population of LHRH neurons. How a network of LHRH neurons synchronizes their activity, i.e., whether by the result of synaptic mechanisms or electrical coupling through gap junctions or through a diffusible substance(s), remains to be clarified. Even though LHRH neurons themselves possess an endogenous pulse-generating mechanism, they may be controlled by other neuronal and nonneuronal elements in vivo. NE, NPY, glutamate, and GABA are neurotransmitters possibly controlling pulsatile LHRH release, and NO, cAMP, and ATP may be diffusible substances involved in pulsatile LHRH release without synaptic input. Although synaptic inputs to the perikarya of LHRH neurons could control the activity of LHRH neurons, a line of evidence suggests that direct neuronal and nonneuronal inputs, especially those from astrocytes to LHRH neuroterminals, appear to be more important for pusatile LHRH release.

Regulation of hypothalamic neuropeptide Y Y1 receptor gene expression during the estrous cycle: role of progesterone receptors

Neuropeptide Y (NPY) stimulates the release of GnRH in an estrogen (E2)-dependent manner, which is important in generating preovulatory GnRH surges. We tested the hypothesis that E2 up-regulates NPY's actions by stimulating NPY Y1 receptor (Y1r) gene expression through a mechanism mediated by E2's ability to induce progesterone (P) receptors (PRs). In initial experiments, a specific Y1r antagonist BIBP3226 was used to confirm the involvement of Y1r in the stimulatory effects of NPY on in vivo GnRH release. Hypothalamic Y1r messenger RNA (mRNA) levels were then measured using competitive RT-PCR and were found to be significantly increased at 1000, 1200, and 1400 h on proestrus compared with other times of the day or cycle stage. Ovariectomy eliminated these increases, and E2 treatment restored them. Additional P treatment produced even larger increases in Y1r mRNA levels. To assess the role of PRs in stimulating Y1r expression, proestrous rats were treated with PR antagonist or oil vehicle and killed at 1200 h. Treatment with PR antagonist completely blocked the proestrous rise in Y1r gene expression. In parallel experiments, the same in vivo PR antagonist treatments also blocked NPY stimulation of GnRH release in vitro. Together our findings reveal that 1) Y1r mRNA levels are increased during the late morning and afternoon of proestrus; 2) Y1r mRNA levels are similarly increased by E2, and to an even greater extent by additional P; and 3) PR antagonism blocks both increased Y1r mRNA and induction of GnRH responsiveness to NPY. These observations support the idea that E2 up-regulates GnRH neuronal responses to NPY through stimulation of Y1r gene expression, and that E2's actions are mediated by the induction and subsequent activation of PRs.

The role of neuropeptide Y in the progesterone-induced luteinizing hormone-releasing hormone surge in vivo in ovariectomized female rhesus monkeys

Progesterone induces a LHRH surge in estrogen-primed ovariectomized rhesus monkeys, with a concomitant increase in the pulse frequency of neuropeptide Y (NPY) release. However, the role for NPY in the positive feedback action of progesterone on LHRH release in primates is unknown. The present study examines the effect of an antisense oligodeoxynucleotide for NPY messenger RNA (AS NPY) on the progesterone-induced LHRH surge in vivo using push-pull perfusion. The AS NPY was directly infused into the stalk-median eminence (S-ME), whereas perfusates were collected for assessment of LHRH release. For a control, a scrambled oligodeoxynucleotide was infused. The results indicate that 1) the scrambled oligodeoxynucleotide did not interfere with the progesterone-induced LHRH surge, 2) whereas AS NPY blocked the progesterone-induced increase in LHRH release, and 3) no LHRH surges were induced by oil as a control for progesterone, but the AS NPY also reduced LHRH release in oil controls. These data suggest that 1) AS NPY infusion into the S-ME results in reduction in LHRH release; and 2) NPY release in the S-ME is important for the positive feedback effects of progesterone on LHRH release in estrogen-primed ovariectomized monkeys.

Role of the cAMP signaling pathway in the regulation of gonadotropin-releasing hormone secretion in GT1 cells

We studied the signaling pathways coupling gonadotropin-releasing hormone (GnRH) secretion to elevations in cAMP levels in the GT1 GnRH-secreting neuronal cell line. We hypothesized that increased cAMP could be acting directly by means of cyclic nucleotide-gated (CNG) cation channels or indirectly by means of activation of cAMP-dependent protein kinase (PKA). We showed that GT1 cells express the three CNG subunits present in olfactory neurons (CNG2, -4.3, and -5) and exhibit functional cAMP-gated cation channels. Activation of PKA does not appear to be necessary for the stimulation of GnRH release by increased levels of cAMP. In fact, pharmacological inhibition of PKA activity caused an increase in the basal secretion of GnRH. Consistent with this observation activation PKA inhibited adenylyl cyclase activity, presumably by inhibiting adenylyl cyclase V expressed in the cells. Therefore, the stimulation of GnRH release by elevations in cAMP appears to be the result of depolarization of the neurons initiated by increased cation conductance by cAMP-gated cation channels. Activation of PKA may constitute a negative-feedback mechanisms for lowering cAMP levels. We hypothesize that these mechanisms could result in oscillations in cAMP levels, providing a biochemical basis for timing the pulsatile release of GnRH.

Morphological evidence for direct interaction between arcuate nucleus neuropeptide Y (NPY) neurons and gonadotropin-releasing hormone neurons and the possible involvement of NPY Y1 receptors

Neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus (ARH) have been shown to play an important role in modulating LH secretion. One mechanism by which the ARH NPY system may regulate LH secretion is by modulating GnRH neuronal function. Thus, the present study examined whether the ARH NPY system provided direct input to GnRH cell bodies in the preoptic area (POA), as well as to their nerve terminals in the median eminence (ME). The possible involvement of the NPY Y1 receptor subtype in mediating the effects of NPY was also investigated. Lactating rats were used in these studies because they have increased hypothalamic NPY content, especially in the ARH/ME areas, making it easier to detect NPY fibers and terminals. The anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), was iontophoresed into the ARH of lactating rats; and triple-label immunofluorescence was performed, with the aid of confocal microscopy, to visualize NPY, PHA-L, and GnRH. GnRH cell bodies were found scattered throughout the organum vasculosum laminae terminalis (OVLT)/POA region, and NPY/ PHA-L double-labeled fibers were found in very close proximity to numerous GnRH perikarya. In the ME, double-labeled NPY/PHA-L fibers were found in the inner and external zones, and they were found in close proximity to GnRH neuronal fibers. Using a NPY Y1 specific antibody, double-label immunofluorescence was performed to examine whether the Y1 receptor subtype was expressed in GnRH neurons. No convincing Y1-positive staining was found in GnRH cell bodies in the OVLT/POA region. However, abundant Y1-positive fiber and cell staining were observed throughout the region, and Y1-positive fibers were found in close apposition to GnRH cell bodies. In contrast, numerous GnRH nerve fibers and terminals in both the OVLT and ME were colocalized with Y1-positive staining. The results of this study suggest that ARH NPY neurons come in close contact with GnRH neurons and may provide direct input to both GnRH cell bodies in the POA region and to their nerve terminals in the ME. The Y1 receptor subtype may be directly involved in NPY modulation of GnRH secretion from its nerve terminals.

Neuropeptide Y, leptin, galanin and insulin in women with polycystic ovary syndrome

It has been reported that polycystic ovary syndrome (PCOS) is very frequently associated with obesity, insulin resistance and hyperinsulinemia. However, metabolic disorders may lead to suppression of reproductive hormone secretion during undernutrition and in obesity. Some neuropeptides, such as neuropeptide Y (NPY) and galanin, modulate the control of appetite and play an important role in the mechanism of luteinizing hormone-releasing hormone (LHRH) secretion. NPY and galanin regulate appetite via both central and peripheral mechanisms. The interaction between central and peripheral signals for the control of food intake is due to leptin. Leptin can modulate the activity of NPY and other peptides in the hypothalamus that are known to affect eating behavior. In order to evaluate the relationship between NPY, galanin and leptin, 28 women with PCOS, 32 obese women (non-PCOS) and 19 lean healthy women (control group) were investigated. Obese women with PCOS were divided into two groups: PCOS (A) overweight (body mass index, BMI 26-30 kg/m2), and PCOS (B) obese (BMI 31-40 kg/m2). Plasma NPY, galanin and leptin concentrations were measured by radioimmunoassay. Plasma leptin levels in obese women with PCOS (groups A and B) were significantly higher than those in the control group (p < 0.05, p < 0.05, respectively). A significant positive correlation between plasma leptin and BMI in women with PCOS was found (r = 0.427, p < 0.01). A positive correlation was demonstrated between leptin and testosterone in PCOS (r = 0.461, p < 0.01). Plasma galanin concentrations in PCOS were higher than in the control group but the differences were not significant. Plasma NPY levels were significantly elevated in both non-obese (normal) and obese women with PCOS (group A) (p < 0.01, p < 0.005, respectively). However, in obese non-PCOS women plasma NPY levels gradually increased with increase in BMI. No significant correlations were found between galanin, NPY and percentage change in response of LH to LHRH, as well as between NPY and insulin, and galanin and testosterone. Plasma insulin concentrations in women with PCOS (group B) were significantly higher than in the control group (p < 0.001). Increased plasma NPY levels are found in both obese and non-obese women with PCOS. The increase in NPY is independent of the increase in BMI. In obese women with PCOS, plasma leptin is increased compared with control lean women. Serum insulin concentration is increased in obese women with PCOS. A positive correlation exists between leptin and BMI as well as between leptin and testosterone in women with PCOS. These results may suggest that the feedback system in the interaction between leptin and NPY is disturbed in PCOS.

Glucagon-like peptide-1 stimulates luteinizing hormone-releasing hormone secretion in a rodent hypothalamic neuronal cell line

To examine the influence of the putative satiety factor (GLP-1) on the hypothalamo-pituitary-gonadal axis, we used GT1-7 cells as a model of neuronal luteinizing hormone- releasing hormone (LHRH) release. GLP-1 caused a concentration-dependent increase in LHRH release from GT1-7 cells. Specific, saturable GLP-1 binding sites were demonstrated on these cells. The binding of [125I]GLP-1 was time-dependent and consistent with a single binding site (Kd = 0.07+/-0.016 nM; binding capacity = 160+/-11 fmol/mg protein). The specific GLP-1 receptor agonists, exendin-3 and exendin-4, also showed high affinity (Ki = 0.3+/-0.05 and 0.32+/-0.06 nM, respectively) as did the antagonist exendin-(9-39) (Ki = 0.98+/-0.24 nM). At concentrations that increased LHRH release, GLP-1 (0.5-10 nM) also caused an increase in intracellular cAMP in GT1-7 cells (10 nM GLP-1: 7.66+/-0.4 vs. control: 0.23+/-0.02 nmol/mg protein; P < 0.001). Intracerebroventricular injection of GLP-1 at a single concentration (10 microg) produced a prompt increase in the plasma luteinizing hormone concentration in male rats (GLP-1: 1.09+/-0.11 vs. saline: 0.69+/-0.06 ng/ml; P < 0.005). GLP-1 levels in the hypothalami of 48-h-fasted male rats showed a decrease, indicating a possible association of the satiety factor with the low luteinizing hormone levels in animals with a negative energy balance.

Differential effects of basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha, and insulin-like growth factor-I on a hypothalamic gonadotropin-releasing hormone neuronal cell line

Recent studies in several neuronal lineages suggest that extrinsic factors such as polypeptide growth factors regulate various stages of neuronal development, from initial commitment of multipotent progenitors to induction of specific gene expression that is characteristic of terminal neuronal differentiation. In the present study, immortalized hypothalamic neurons of the GT1-1 lineage were used to analyze proliferative, as well as morphological and molecular differentiation actions of basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), and insulin-like growth factor-I (IGF-I). These effects were compared with those induced by specific activators of protein kinase A and C pathways, which potently inhibited cell proliferation and gonadotropin-releasing hormone (GnRH) gene expression, but stimulated morphological neuronal maturation as determined by the length and number of neurite outgrowth. bFGF exerted a broad spectrum of stimulatory effects, increasing the rate of proliferation measured both by the incorporation of 3H-thymidine and by cell number, and parameters of terminal differentiation, such as neurite outgrowth and induction of gene expression. bFGF stimulated the expression of the hybrid transgene-containing portions of the rat GnRH promoter. In contrast, EGF, TGF-alpha, and IGF-I inhibited cell proliferation, while having subtle effects on neurite outgrowth. Thus, GT1-1 cells appear to be differentially responsive to distinct neurotrophic factors, providing a model for studying the specific effects of neurotrophic factors on functional differentiation, migration, and connectivity of hypothalamic neurons.

An alternative gonadotropin-releasing hormone (GnRH) RNA splicing product found in cultured GnRH neurons and mouse hypothalamus

Gonadotropin-releasing hormone (GnRH) is encoded by the proGnRH gene which contains four exons and three introns. In this study, two immortalized GnRH-expressing cell lines (Gn11 and NLT) were characterized. The NLT and Gn11 cells, derived from a same brain tumor in a transgenic mouse, display neuronal morphology and neuron-specific markers. However, NLT cells secrete much higher levels of GnRH than Gn11 cells. To delineate the mechanism underlying this difference, reverse transcriptase-polymerase chain reaction and RNase protection assays were performed to examine proGnRH gene expression. While the mature proGnRH mRNA was predominately expressed in NLT cells, Gn11 cells express an abundant short transcript. Sequence analysis revealed that this short transcript contains exons 1, 3, and 4, but not exon 2, which encodes the GnRH decapeptide. RNase protection assays demonstrated that NLT cells express much higher levels of mature proGnRH mRNA than Gn11 cells. The lower level of GnRH secreting capacity in Gn11 cells is due, in part, to decreased expression of mature proGnRH mRNA. When proGnRH gene expression in the mouse brain was examined, the same short splicing variant was observed in the olfactory area and preoptic area-anterior hypothalamus. But the prevalent transcript in these regions was the mature proGnRH mRNA. In contrast, only the mature proGnRH mRNA was found in the caudal hypothalamus. These results suggest that alternative splicing may be one of the mechanisms regulating proGnRH gene expression in the animal brain.

Evidence showing that beta-endorphin regulates cyclic guanosine 3',5'-monophosphate (cGMP) efflux: anatomical and functional support for an interaction between opiates and nitric oxide

Nitric oxide (NO) is now recognized as a diffusible messenger molecule that normally augments intercellular communication in the central nervous system, but is neurotoxic if released in excessive amounts. NO is synthesized from L-arginine by the Ca2+/calmodulin-dependent neuronal isoform NO synthase (NOS) localized in sub-populations of neurons throughout the brain, including the hypothalamus. In the hypothalamus, NO stimulates the release of GnRH, the primary neurohormone governing reproduction in mammals. Although the excitatory amino acid, glutamate, acting through the N-methyl-D-aspartate (NMDA) receptor is believed to be responsible for stimulation of NO release, the neuronal system(s) that inhibits NO efflux is unknown. As the endogenous opioids, primarily beta-endorphin (betaEND), exert a tonic restraint on GnRH secretion, we sought evidence for a possible functional link between betaEND and NOS pathways in the hypothalamus. We observed that restraining the opioid influence with the opiate receptor antagonist, naloxone, in intact, but not in castrated, rats rapidly augmented extracellular cGMP/NO efflux in the medial preoptic area, where GnRH, NOS, and betaEND immunoreactive pathways are coextensive. Pituitary LH secretion increased in conjunction with this augmented cGMP/NO response and pretreatment with the mu opiate receptor agonist, morphine, suppressed these naloxone-induced responses. Further, visualization of hypothalamic sections immunostained for both betaEND and NOS revealed betaEND-immunoreactive axon terminals in close proximity to NOS-positive cell bodies and dendrites in a number of hypothalamic subdivisions, including the medial preoptic area. These close appositions represented conventional synapses between betaEND nerve terminals and NOS-positive perikarya and dendrites under the electron microscope. Clearly, the experimental data, corroborated by morphological evidence, point to a direct inhibitory control of betaEND on NOS-immunoreactive neurons in monitoring cGMP/NO release. These findings together with the previous observations that the glutamate neurotransmitter acting through NMDA receptors located on NOS-immunopositive cells stimulates cGMP/NO efflux and plasma LH selectively in intact rats document the existence of a dual control comprised of the excitatory NMDA and the inhibitory mu opiate receptors in modulating cGMP/NO release, a response also directed by gonadal steroids. This new knowledge of an inhibitory opioid influence on cGMP/NO release is probably extremely important both in the generation of periodicities in GnRH secretion that underlie hypothalamic control of reproduction and in protecting against neurotoxic overstimulation of NO release by excitatory amino acids.

Regulation of gonadotropin-releasing hormone gene expression in vivo and in vitro

The pulsatile release of gonadotropin-releasing hormone (GnRH) into the portal vasculature is responsible for the maintenance of reproductive function. Levels of GnRH decapeptide available for this process can be regulated at transcriptional, posttranscriptional, and posttranslational levels. In the immortalized neuronal GT1 cell lines which synthesize and secrete GnRH, regulation of GnRH biosynthesis has been studied using activators of the protein kinase A (PKA), protein kinase C (PKC), and calcium second messenger systems. These substances, while stimulating GnRH release, cause a universal inhibition of all biosynthetic indices measured to date, including decreases in transcription of the proGnRH gene, GnRH mRNA levels, mRNA stability, and translational efficiency. In contrast, in the animal, the mechanism for the regulation of GnRH gene expression appears to be primarily posttranscriptional, since changes in GnRH mRNA levels often occur in the absence of changes in GnRH primary transcript levels an index of GnRH gene transcription. For example, GnRH mRNA levels increase in response to stimulation with glutamate analogs, while GnRH primary transcript levels are unchanged. However, parallel changes in GnRH mRNA and primary transcript have been observed on proestrus prior to the LH/GnRH surge, suggesting that the regulation of GnRH mRNA levels in vivo involves a complex interplay of transcriptional and posttranscriptional processes.

Effects of neuropeptide Y (NPY) and NPY agonists on lordosis in the female guinea pig

We have previously shown that an NPY antagonist decreases lordosis behavior and that this decrease can be reversed with NPY administration. The present experiments examined whether intracerebroventricular (icv) administration of NPY would facilitate lordosis behavior and whether it would increase feeding behavior in the female guinea pig. Additionally, we examined whether icv administration of a more specific NPY Y1 and/or Y2 receptor agonist would facilitate lordosis behavior. Although NPY (1 μg) increased feeding behavior when it was administered to the lateral ventricle of ovariectomized (ovx) estrogen (i.e., estradiol benzoate; EB) and progesterone- (P) treated guinea pigs, it had no facilitatory effect on lordosis behavior at any of the doses tested (0.5, 1, 5, or 10μg). In fact, the lower doses had a small, delayed inhibitory effect. NPY also had no effect on lordosis in females treated with EB alone. In contrast, the NPY Y1 agonist (Leu(31)Pro(34)) NPY significantly facilitated lordosis in ovx EB- and P-treated females. It had no effect in ovx females treated with EB alone. The NPY Y2 agonist NPY (13-36) had a slight, delayed inhibitory effect in ovx EB- and P-treated females. These data are consistent with the hypothesis that NPY can act at a number of receptor subtypes to affect lordosis behavior, and that NPY can facilitate lordosis behavior by acting at Y1 receptors. Furthermore, it appears that this facilitatory effect of Y1 receptors is an effect on some progesterone-mediated component of lordosis, as the Y1 agonist facilitated EB- and P-induced lordosis, but not that induced with EB alone.

Cell-specific expression of the human gonadotropin-releasing hormone gene in transgenic animals

We have previously demonstrated that 1131 base pairs (bp) of the human gonadotropin-releasing hormone (hGnRH) gene promoter can target simian virus 40 T antigen expression to GnRH neurons in transgenic mice. In these animals, GnRH neurons were transformed before they migrated to their final location in the rostral hypothalamus, complicating an analysis of cell-specific expression. To localize regions of the hGnRH promoter that are important for cell-specific expression, we created transgenic mice with various 5'-flanking regions of the hGnRH gene fused to the luciferase reporter gene. When 3828 or 1131 bp of the hGnRH promoter 5'-flanking DNA were used (-3828/+5LUC and -1131/+5LUC, respectively), luciferase expression in adult transgenic mice was observed in the rostral hypothalamus and olfactory tissues, regions which have been shown to be loci of GnRH-expressing neurons. Luciferase expression was not observed in other brain or peripheral tissues. Double-labeled in situ hybridization further demonstrated that luciferase expression was invariably colocalized with GnRH expression. When transgenic animals were created with a construct consisting of 484 bp of the hGnRH 5'-flanking DNA fused to the luciferase gene (-484/+5LUC), luciferase expression was not observed in the hypothalamus or in olfactory tissues. This is the first report localizing DNA sequences responsible for cell-specific expression of the GnRH gene in vivo.

Amplitude and frequency modulation of pulsatile luteinizing hormone-releasing hormone release

1. A variety of neuroendocrine approaches has been used to characterize cellular mechanisms governing luteinizing hormone-releasing hormone (LHRH) pulse generation. We review recent in vivo microdialysis, in vitro superfusion, and in situ hybridization experiments in which we tested the hypothesis that the amplitude and frequency of LHRH pulses are subject to independent regulation via distinct and identifiable cellular pathways. 2. Augmentation of LHRH pulse amplitude is proposed as a central feature of preovulatory LHRH surges. Three mechanisms are described which may contribute to this increase in LHRH pulse amplitude: (a) increased LHRH gene expression, (b) augmentation of facilitatory neurotransmission, and (c) increased responsiveness of LHRH neurons to afferent synaptic signals. Neuropeptide Y (NPY) is examined as a prototypical afferent transmitter regulating the generation of LHRH surges through the latter two mechanisms. 3. Retardation of LHRH pulse generator frequency is postulated to mediate negative feedback actions of gonadal hormones. Evidence supporting this hypothesis is reviewed, including results of in vivo monitoring experiments in which LHRH pulse frequency, but not amplitude, is shown to be increased following castration. A role for noradrenergic neurons as intervening targets of gonadal hormone negative feedback actions is discussed. 4. Future directions for study of the LHRH pulse generator are suggested.