Abstract
Noradrenaline (NA) is one of the most important neurotransmitters involved in the regulation of gonadotropin-releasing hormone (GnRH) secretion. In this study, the effects of NA on GnRH secretion, intracellular Ca2+ concentrations ([Ca2+]i), and membrane potentials were investigated in immortalized hypothalamic neurons (GT1-7) to determine the direct effects of NA on GnRH cells. Cells were perfused in a plastic minicolumn, and GnRH concentrations of the effluents were measured. NA increased the release of GnRH in a dose-dependent manner. Cells were loaded with 4μM Fura 2-AM, and the ratio of the intensities of fluorescent emission at 510nm with excitation at 340 and 380nm was calculated at 100-ms intervals. NA increased the [Ca2+]i responses of single GnRH cells dose- dependently. The NA-induced [Ca2+]i increase was attenuated in the absence of extracellular calcium and was blocked by the β-adrenergic antagonist propranolol, but not by the α-adrenergic antagonist phentolamine. The cell membrane potential was recorded with a whole-cell patch clamp amplifier with glass-electrodes. NA induced membrane depolarization under current-clamp conditions. The depolarization was also inhibited by propranolol, but not by phentolamine. The results show that NA directly affects the membrane potential of GT1-7 cells via β-adrenergic receptors and induces Ca2+ mobilization; these effects stimulate GnRH secretion.
