HISTAMINE DIRECTLY STIMULATES GONADOTROPIN-RELEASING-HORMONE SECRETION FROM GT(1-1) CELLS VIA H1 RECEPTORS COUPLED TO PHOSPHOINOSITIDE HYDROLYSIS

It is unclear whether the central stimulating effect of histamine on GnRH secretion is exerted directly on GnRH neurosecretory neurons or indirectly via multisynaptic pathways, and controversy exists about the nature of the receptors involved. The current studies were undertaken to examine whether GnRH secretion from immortalized GnRH cell lines is directly regulated by histamine and, if so, to determine the identity of the receptors and the signaling pathways coupling this action. Histamine stimulated GnRH release from GT(1-1) cells in a sustained and reversible manner and in a dose-dependent fashion. This effect was blocked by the selective H-1 histamine receptor antagonist, mepyramine, but not by the H-2 or H-3 antagonists, ranitidine or thioperamide, respectively. Saturable and specific binding sites for [H-3]mepyramine were demonstrated in GT(1-1) cells, showing high affinity (apparent K-d, 37.8 nM) and density (apparent binding capacity, 279 fmol/mg protein) comparable to respective values in brain tissue. Competition of [H-3]mepyramine binding was achieved with mepyramine at concentrations 3 orders of magnitude lower than those of ranitidine. Histamine also increased the production of inositol phosphates in GT(1-1) cells in a dose- and time-dependent manner. This response was mimicked by the selective H-1 receptor agonist 2-thiazolylethylamine and blocked by the H-1 antagonists mepyramine, chlorpheniramine, and triprolidine. In contrast, histamine did not alter the formation of cAMP in GT(1-1) cells. The present results show a direct action of histamine on immortalized GnRH neurons, suggesting that histamine may stimulate the reproductive axis by activation of H-1 receptors on the surface of GnRH neurons coupled to the formation of inositol phosphates.