Role of the cAMP Signaling Pathway in the Regulation of Gonadotropin-Releasing Hormone Secretion in GT1 Cells

Abstract

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.

Modeling of Membrane Excitability in Gonadotropin-Releasing Hormone-Secreting Hypothalamic Neurons Regulated by Ca2+-Mobilizing and Adenylyl Cyclase-Coupled Receptors

“…In accordance with the former possibility, we were unable to observe any effect of charybdotoxin or iberiotoxin, two specific blockers of BK-type I KC a , on GnRH-induced electrical activity (Van Goor et al, 1999a Vitalis et al (2000) have reported recently that the olfactory subtype of CNG channels is expressed in GT1 cells. These channels have a preference for cAMP over cGMP (Wei et al, 1998) and are inhibited by [Ca 2ϩ ] i either directly (Finn et al, 1996;Wei et al, 1998) or indirectly via modulation of AC activity (Hurley, 1999).…”

“…Although we have not recorded I d experimentally, the CNG channel recently described by Vitalis et al (2000) in GT1 cells is an obvious candidate. Another, more complex, possibility is that I d could actually represent [Ca 2ϩ ] i -mediated inactivation of I SOC (Lewis, 1999).…”

“…AC-induced cAMP production occurs in response to dopamine and norepinephrine (Jarry et al, 1990;Martínez de la Escalera 1992b,c;Al-Damluji et al, 1993). In general, cAMP can act indirectly, by modulating various ionic currents in a protein kinase A-dependent manner, or directly, via modulation of cyclic nucleotide-gated (CNG) channels, which have been identified in GT1 neurons (Vitalis et al, 2000). The phospholipase C pathway in GT1 cells is activated by at least two receptors, GnRH and endothelin (Krsmanovic et al, 1991, leading to Ca 2ϩ mobilization from endoplasmic reticulum (ER) stores.…”

Secretion of fatty acid binding protein aP2 from adipocytes through a nonclassical pathway in response to adipocyte lipase activity

“…We demonstrate that adipocyte lipase activities, specifi cally ATGL and, to a lesser extent, HSL, are involved in the regulated secretion of aP2, and genetic deletion or chemical inhibition of these enzymes dramatically diminishes aP2 secretion from adipocytes. It has been reported in various systems that cAMP signaling positively regulates classical and nonclassical hormone secretion (50)(51)(52)(53). However, induction of hormone secretion has not previously been associated with specifi c lipase activity or free fatty acid availability in adipocytes and has not been linked to aP2 secretion.…”

A simple integrative electrophysiological model of bursting GnRH neurons

“…In contrast, the activation of the Ca 2+ dependent phosphatase calcineurin causes dephosphorylation of the channels (Cantrell & Catterall, 2001). Furthermore in GT-1 cells, cyclic nucleotide gated non-selective cation channels have been described (Vitalis et al, 2000;Charles et al, 2001), which may also be related to Ca 2+ dependent currents.…”