Calcium signaling and episodic secretion of gonadotropin-releasing hormone in hypothalamic neurons.

Abstract

Gonadotropin-releasing hormone (GnRH) is released episodically into the pituitary portal vessels and from hypothalamic tissue of male and female rats in vitro. Perfused primary cultures of rat hypothalamic neurons, as well as the GT1-1 GnRH neuronal cell line, spontaneously exhibited episodic GnRH secretion of comparable frequency to that observed with perifused hypothalami. Such pulsatile GnRH release from GT1 cells indicates that GnRH neurons generate rhythmic secretory activity in the absence of input from other cell types. In primary hypothalamic cultures, the frequency of GnRH pulses increased with the duration of culture. The spontaneous pulsatility in GnRH release was abolished in Ca2+-deficient medium and was markedly attenuated in the presence of nifedipine, an antagonist of voltage-sensitive Ca2' channels. The basal intracellular Ca2+ level of perifused GT1-1 cells cultured on coverslips was also dose-dependently reduced by nifedipine. Conversely, depolarization with high K+ increased intracellular Ca+ and GnRH release in an extracellular Ca2+-dependent and nifedipine-sensitive manner. The dihydropyridine Ca2+ channel agonist Bay K 8644 increased basal and K+-induced elevations of intracellular Ca2+ concentration and GnRH secretion. These findings demonstrate that pulsatile neuropeptide secretion is an intrinsic property of GnRH neuronal networks and is dependent on voltage-sensitive Ca2+ influx for its maintenance.The pulsatile secretion of gonadotropin-releasing hormone (GnRH) from hypothalamic nerve terminals adjacent to hypophyseal portal vessels in the median eminence is essential for the synthesis and secretion of gonadotropic hormones from the anterior pituitary gland (1). In contrast to other hypophysiotropic neurons, GnRH-producing cells are not concentrated in a discrete region of the hypothalamus but are distributed in the preoptic area and adjacent sites in the rostal portion of the hypothalamus (2). The majority of the GnRH neurons send projections to the median eminence and, despite their scattered locations, are interconnected to form a functional unit termed the GnRH pulse generator (3) that is responsible for the episodic release of GnRH and, consequently, of pituitary gonadotropins. Thus, pulses of luteinizing hormone secretion are elicited by preceding elevations of GnRH concentration in pituitary portal blood (4, 5), and a pulsatile mode of GnRH action is essential for the maintenance and control of normal gonadotropin profiles in the peripheral circulation (6).Changes in GnRH pulse frequency occur at puberty (7) and during the menstrual cycle (8), as well as after castration (9). Alterations in the pulsatile GnRH signal in several reproductive states may reflect a requirement not only for increased gonadotropin secretion but also for changes in the proportions of luteinizing hormone and follicle-stimulating hormone production under various physiological conditions. Thus, the pattern of GnRH secretion can differentially regulate the levels of gonadotropin subunit mRNAs in th...

Confinement of intracellular calcium signaling in secretory and steroidogenic cells

“…This concept of differential coupling of specific channels to different rates of transmitter or hormone release could be also applied to other cell types (44). L-type channels have been also involved in the secretion of gonadotropin-releasing hormone (GnRH) by perifused primary cultures of rat hypothalamic neurons (45). Indeed, some neurons secrete two kinds of neuromediators: (i) fast acting neurotransmitters, such as biogenic amines, acetylcholine, glutamate, GABA or glycine, and (ii) neuropeptides, such as GnRH, neuropeptide Y, substance P, vasoactive intestinal peptide, somatostatin or AngII.…”

Phosphodiesterase expression targeted to gonadotropin-releasing hormone neurons inhibits luteinizing hormone pulses in transgenic rats

“…Further complicating the understanding of these questions is the complexity of the neural connectivity of GnRH neurons. The development of the highly differentiated GT1 GnRH neuronal cell lines (1) and the demonstration that pulsatile release was an intrinsic property of the cells provided a model to study molecular events underlying pulsatile GnRH release (2)(3)(4). However, one is always faced with the problem of translating findings from an immortalized cell line to the physiological setting.…”

Role of the cAMP signaling pathway in the regulation of gonadotropin-releasing hormone secretion in GT1 cells

“…The cells are highly differentiated and express and process GnRH at high levels (1,2). The pulsatile release of GnRH appears to be an intrinsic property of individual or networks of GnRH neurons, since cultures of the GT1 cells release GnRH with a pulse frequency identical to that seen in castrate rodents (3)(4)(5). In vivo numerous efferent inputs to GnRH neurons release neurotransmitters that stimulate GnRH secretion (6).…”

“…The level of secretion of GnRH was proportional to the activation of AC and increases in cAMP levels. These data are all consistent with the hypothesis that the stimulation of AC results in the increased formation of cAMP, activating the signaling cascade responsible for stimulation of GnRH release.Several studies have shown that depolarization followed by the influx of extracellular Ca 2ϩ via voltage-dependent Ca 2ϩ channels is essential for the stimulation of GnRH secretion from GT1 cells (5,13,14). Stimulation of GnRH secretion from GT1 cells by NE was correlated with depolarization of the cells and the influx of extracellular Ca 2ϩ (15).…”

“…Several studies have shown that depolarization followed by the influx of extracellular Ca 2ϩ via voltage-dependent Ca 2ϩ channels is essential for the stimulation of GnRH secretion from GT1 cells (5,13,14). Stimulation of GnRH secretion from GT1 cells by NE was correlated with depolarization of the cells and the influx of extracellular Ca 2ϩ (15).…”