Secretin Regulates Excitatory GABAergic Neurotransmission to GnRH Neurons via Retrograde NO Signaling Pathway in Mice

Abstract

In mammals, reproduction is regulated by a wide range of metabolic hormones that maintain the proper energy balance. In addition to regulating feeding and energy expenditure, these metabolic messengers also modulate the functional performance of the hypothalamic-pituitary-gonadal (HPG) axis. Secretin, a member of the secretin-glucagon-vasoactive intestinal peptide hormone family, has been shown to alter reproduction centrally, although the underlying mechanisms have not been explored yet. In order to elucidate its central action in the neuroendocrine regulation of reproduction, in vitro electrophysiological slice experiments were carried out on GnRH-GFP neurons in male mice. Bath application of secretin (100 nM) significantly increased the frequency of the spontaneous postsynaptic currents (sPSCs) to 118.0 ± 2.64% compared to the control, and that of the GABAergic miniature postsynaptic currents (mPSCs) to 147.6 ± 19.19%. Resting membrane potential became depolarized by 12.74 ± 4.539 mV after secretin treatment. Frequency of evoked action potentials (APs) also increased to 144.3 ± 10.8%. The secretin-triggered elevation of the frequency of mPSCs was prevented by using either a secretin receptor antagonist (3 μM) or intracellularly applied G-protein-coupled receptor blocker (GDP-β-S; 2 mM) supporting the involvement of secretin receptor in the process. Regarding the actions downstream to secretin receptor, intracellular blockade of protein kinase A (PKA) with KT-5720 (2 μM) or intracellular inhibition of the neuronal nitric oxide synthase (nNOS) by NPLA (1 μM) abolished the stimulatory effect of secretin on mPSCs. These data suggest that secretin acts on GnRH neurons via secretin receptors whose activation triggers the cAMP/PKA/nNOS signaling pathway resulting in nitric oxide release and in the presynaptic terminals this retrograde NO machinery regulates the GABAergic input to GnRH neurons.

Gonadal Cycle-Dependent Expression of Genes Encoding Peptide-, Growth Factor-, and Orphan G-Protein-Coupled Receptors in Gonadotropin- Releasing Hormone Neurons of Mice

“…This evidence strengthens the view that the observed differential gene expression is GnRH cell type specific, and it is due to the gonadal cycle. The rationale behind studying the mPSCs is that postsynaptic actions on GnRH neurons modify the frequency of mPSCs by altering the retrograde endocannabinoid and/or NO signaling in the presynaptic terminals (Farkas et al, 2010(Farkas et al, , 2013(Farkas et al, , 2018Balint et al, 2016;Csillag et al, 2019). In this study, the electrophysiological recordings in the presence of intracellular blockers of the IGF-1 and neuronostatin signaling pathways confirm the postsynaptic action of the peptides.…”

“…Secretin activates hypothalamic magnocellular neurons with involvement of noradrenergic signaling mechanisms in the rat (Velmurugan et al, 2010). The involvement of secretin signaling in the regulation of GnRH neurons of the male mouse has recently been shown (Csillag et al, 2019). Our current finding raises the possibility of a direct targeting of GnRH neurons by secretin in the female, too, and the estrus cycle phase-dependent nature of the regulation with a manifest downregulated state of secretin receptors in late proestrus.…”

“…The beaded GnRH axons project-among othersto the median eminence where they discharge their GnRH content into the portal circulation (Merchenthaler et al, 1980) for regulation of the pituitary-gonadal axis (Carmel et al, 1976). The physiological activity, hormone production, and neurohormone release of GnRH neurons are regulated by diverse neuronal circuits of the brain (Spergel, 2019a,b) and by various endocrine hormones and metabolic signals arriving from the periphery (Finn et al, 1998;Smith and Jennes, 2001;Campbell, 2007;Christian and Moenter, 2010;Farkas et al, 2013Farkas et al, , 2016Csillag et al, 2019). The operation of the hypothalamopituitary-gonadal (HPG) axis is cyclic including the physiological performance of GnRH neurons (Plant, 2015).…”

“…In addition to the classic neurotransmitter systems, neuropeptides, growth factors, and their receptors are equally important regulators of GnRH neurons (Gore, 2010). Electrophysiological studies have provided evidence for direct targeting of GnRH neurons by various peptides via G-protein-coupled receptor (GPCR) signaling mechanisms including anti-Mullerian hormone (AMH) (Cimino et al, 2016;Barbotin et al, 2019), secretin (Csillag et al, 2019), adiponectin (Klenke et al, 2014), alpha-MSH (Roa and Herbison, 2012), AgRP (Roa and Herbison, 2012), CART (Roa and Herbison, 2012), cholecystokinin (CCK) (Giacobini and Wray, 2007), CRH (Phumsatitpong and Moenter, 2018), galanin (Todman et al, 2005), ghrelin (Farkas et al, 2013), GnRH (Todman et al, 2005), glucagon-like peptide 1 (GLP-1) (Farkas Abbreviations: Acvr2b, activin receptor IIB; Adipor1, adiponectin receptor 1; Amhr2, anti-Mullerian hormone type 2 receptor; Cckbr, cholecystokinin B receptor; Ednrb, endothelin receptor type B; Egfr, epidermal growth factor receptor; Fgfr1, fibroblast growth factor receptor 1; Gpr1, G-protein-coupled receptor 1; Gpr107, G-protein-coupled receptor 107; Gpr125, G-proteincoupled receptor 125; Gpr18, G-protein-coupled receptor 18; Gpr183, G-protein-coupled receptor 183; Gpr4, G-protein-coupled receptor 4; Gpr62, G-protein-coupled receptor 62; Gpr87, G-protein-coupled receptor 87; Gpr88, G-protein-coupled receptor 88; Grb10, growth factor receptor-bound protein 10; Grb2, growth factor receptor-bound protein 2; Grk6, G-proteincoupled receptor kinase 6; Igf1r, insulin-like growth factor I receptor; Mc3r, melanocortin 3 receptor; Ngfrap1, nerve growth factor receptor (TNFRSF16) associated protein 1; Nmbr, neuromedin B receptor; Nmur1, neuromedin U receptor 1; Npr3, natriuretic peptide receptor 3; Prokr1, prokineticin receptor 1; Ptafr, platelet-activating factor receptor; Rtn4r, reticulon 4 receptor; Sctr, secretin receptor; Tgfbr2, transforming growth factor, beta receptor II; Vmn1r148, vomeronasal 1 receptor 148; Vmn1r172, vomeronasal 1 receptor 172; Vmn2r-ps54, vomeronasal 2, receptor, pseudogene 54. ), kisspeptin (Pielecka-Fortuna et al, 2008Pielecka-Fortuna and Moenter, 2010), neuromedin B (Todman et al, 2005), NPY (Roa and Herbison, 2012), somatostatin (Todman et al, 2005), and orexin (Gaskins and Moenter, 2012), among others.…”

Social boldness correlates with brain gene expression in male green anoles

“…Secretin receptor expression is widespread in the brain and although central secretin signaling is primarily known for its role in regulating neurodevelopment and memory function via effects on synaptic plasticity, secretin has also been shown to regulate anxiety and associated behaviors (Nishijima et al, 2006; Wang et al, 2019), as well as activity of reproductive circuitry (Csillag et al, 2019). Secretin has been shown to regulate GABAergic transmission to gonadotropin-releasing hormone-producing cells (Csillag et al, 2019), as well as affecting the firing rate of over 50% of examined paraventricular hypothalamus neurons in rat in vivo studies (Chen et al, 2013). These studies suggest a role for secretin as a widespread modulator of neural function, and as such, secretin may also regulate social behavior via modulation of neuronal activity within the ventromedial hypothalamus.…”