Endothelin-1 as a neuropeptide: neurotransmitter or neurovascular effects?

Endothelin-1 (ET-1) is an endothelium-derived peptide that also possesses potent mitogenic activity. There is also a suggestion the ET-1 is a neuropeptide, based mainly on its histological identification in both the central and peripheral nervous system in a number of species, including man. A neuropeptide role for ET-1 is supported by studies showing a variety of effects caused following its administration into different regions of the brain and by application to peripheral nerves. In addition there are studies proposing that ET-1 is implicated in a number of neural circuits where its transmitter affects range from a role in pain and temperature control to its action on the hypothalamo-neurosecretory system. While the effect of ET-1 on nerve tissue is beyond doubt, its action on nerve blood flow is often ignored. Here, we review data generated in a number of species and using a variety of experimental models. Studies range from those showing the distribution of ET-1 and its receptors in nerve tissue to those describing numerous neurally-mediated effects of ET-1

Essential role of G protein-gated inwardly rectifying potassium channels in gonadotropin-induced regulation of GnRH neuronal firing and pulsatile neurosecretion.

Activation of the luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor (LHR) in cultured hypothalamic cells and immortalized GnRH (gonadotropin-releasing hormone) neurons (GT1-7 cells) transiently stimulates and subsequently inhibits cAMP production and pulsatile GnRH release. The marked and delayed impairment of cAMP signaling and episodic GnRH release in GT1-7 cells is prevented by pertussis toxin (PTX). This, and the LH-induced release of membrane-bound Galpha(s) and Galpha(i3) subunits, are indicative of differential G protein coupling to the LHR. Action potential (AP) firing in identified GnRH neurons also initially increased and then progressively decreased during LH treatment. The inhibitory action of LH on AP firing was also prevented by PTX. Reverse transcriptase-PCR analysis of GT1-7 neurons revealed the expression of G protein-gated inwardly rectifying potassium (GIRK) channels in these cells. The LH-induced currents were inhibited by PTX and were identified as GIRK currents. These responses indicate that agonist stimulation of endogenous LHR expressed in GnRH neurons activates GIRK channels, leading to suppression of membrane excitability and inhibition of AP firing. These findings demonstrate that regulation of GIRK channel function is a dominant factor in gonadotropin-induced abolition of pulsatile GnRH release. Furthermore, this mechanism could contribute to the suppression of pituitary function during pregnancy

5HT receptor subtypes mediate specific modes of serotonin-induced signaling and regulation of neurosecretion in GnRH neurons.

Serotonin (5-HT), the endogenous nonselective 5-HT receptor agonist, activates the inositol 1,4,5-triphosphate/calcium (InsP3/Ca2+) signaling pathway and exerts both stimulatory and inhibitory actions on cAMP production and GnRH release in immortalized GnRH neurons. The high degree of similarity between the signaling and secretory responses elicited by GnRH and 5-HT prompted us to target specific 5-HT receptor subtypes to deconvolute the complex actions of these agonists on signal transduction and GnRH release. Specific mRNA transcripts for 5-HT1A, 5-HT2C, 5-HT4, and 5-HT7 were identified in immortalized GnRH neurons (GT1-7). The rate of firing of spontaneous action potentials (APs) by hypothalamic GnRH neurons and cAMP production and pulsatile GnRH release in GT17 cells were profoundly inhibited during activation of the Gi-coupled 5-HT1A receptor. Treatment with a selective agonist to activate the Gq-coupled 5-HT2C receptor increased the rate of firing of spontaneous APs, stimulated InsP3 production and caused a delayed increase in GnRH release. Selective activation of the Gs-coupled 5-HT4 receptor also increased the rate of firing of APs, stimulated cAMP production, and caused a sustained and robust increase in GnRH release. The ability of 5-HT receptor subtypes expressed in GnRH neurons to activate single or multiple G proteins in a time- and dose-dependent manner differentially regulates the phospholipase C/InsP3/Ca2+, and adenylyl cyclase/cAMP signaling pathways, and thereby regulates the frequency and amplitude of pulsatile GnRH release. This process, in conjunction with the modulation of spontaneous electrical activity of the GnRH neuron, contributes to the control of the pulsatile mode of neuropeptide secretion that is characteristic of GnRH neuronal function in vivo and in vitro

Converse regulatory functions of estrogen receptor-alpha and -beta subtypes expressed in hypothalamic gonadotropin-releasing hormone neurons.

Estradiol (E(2)) acts as a potent feedback molecule between the ovary and hypothalamic GnRH neurons, and exerts both positive and negative regulatory actions on GnRH synthesis and secretion. However, the extent to which these actions are mediated by estrogen receptors (ERs) expressed in GnRH neurons has been controversial. In this study, Single-cell RT-PCR revealed the expression of both ERalpha and ERbeta isoforms in cultured fetal and adult rat hypothalamic GnRH neurons. Both ERalpha and ERbeta or individual ERs were expressed in 94% of cultured fetal GnRH neurons. In adult female rats at diestrus, 68% of GnRH neurons expressed ERs, followed by 54% in estrus and 19% in proestrus. Expression of individual ERs was found in 24% of adult male GnRH neurons. ERalpha exerted marked G(i)-mediated inhibitory effects on spontaneous action potential (AP) firing, cAMP production, and pulsatile GnRH secretion, indicating its capacity for negative regulation of GnRH neuronal function. In contrast, increased E(2) concentration and ERbeta agonists increase the rate of AP firing, GnRH secretion, and cAMP production, consistent with ERbeta-dependent positive regulation of GnRH secretion. Consonant with the coupling of ERalpha to pertussis toxin-sensitive G(i/o) proteins, E(2) also activates G protein-activated inwardly rectifying potassium channels, decreasing membrane excitability and slowing the firing of spontaneous APs in hypothalamic GnRH neurons. These findings demonstrate that the dual actions of E(2) on GnRH neuronal membrane excitability, cAMP production, and GnRH secretion are mediated by the dose-dependent activation of ERalpha and ERbeta expressed in hypothalamic GnRH neurons