During the past few years, we have provided evidence which indicates that the LHRH pulse generator resides within the LHRH neuronal network. These studies were based on availability of an immortalized neuronal cell line (GT1 cells) which secretes LHRH and, to some extent, has morphological and functional features similar to those of hypothalamic LHRH neurons. Using a dynamic perifusion paradigm, we expanded our initial observations by uncovering a possible mechanism by which GT1 neurons synchronize their secretory activity leading to pulsatile LHRH secretion in vitro. Nitric oxide (NO) has recently been identified as the elusive endothelium- derived relaxing factor. In addition, several studies in 1992 have indicated that NO is a member of a new family of neurotransmitters involved in the control of several neuronal functions. We have demonstrated. using specific blockers of the NO synthase, that, in GT1 cells, NO might be the synchronizing mechanism to set individual cells in phase and, therefore, allows the cells to elicit pulsatile LHRH secretion. In turn, intermittant release of the decapeptide is of paramount importance in governing reproductive events. If this were the case, NO should promote LHRH secretion and activate the soluble guanylate cyclase, an enzyme specifically activated by NO. We have pursued these two aspects in order to provide a more comprehensive view of the topic of the nitrergic control of LHRH secretion. Sodium nitroprusside, an NO donor, was to challenge both arcuate nucleus-median eminence fragments and GT1 cells in vitro. In these paradigms. we established a stimulatory role for NO in LHRH secretion. In GT1 cells, NO-induced LHRH secretion is accompanied by an elevated extra- and intra-cellular accumulation of cGMP, suggesting that NO-evoked increases in cGMP may be responsible for the effects of the neurotransmitter on LHRH. This possibility has been analyzed using an analog of cGMP, Rp-8-Br-cGMPS, which blocks cGMP- dependent protein kinase. Nitric oxide-induced LHRH secretion was blocked in a dose-dependent manner by Rp-8-Br-cGMPS, demonstrating, therefore, that the effects of NO on LHRH secretion are mediated by cGMP. Our observations clearly show that NO modulates the activity of the LHRH neuronal network. Nitric oxide enhances the release of LHRH via a cGMP- dependent mechanism, providing the functional basis for the participation of this unorthodox neurotransmitter in the mechanisms responsible for ultradian rhythms, i.e., pulsatile hormone secretion.
