Intracellular mediation of GnRH action on GTH release in tilapia

Abstract

The objective of the present study was to confirm previous results on the mediation of GnRH signal in tilapia by providing evidence from experiments in cultured pituitary cells and from perifusion experiments using a GnRH-antagonist. After 4 days in culture under identical conditions, cells taken from pituitaries of fish maintained at 26°C were more sensitive to GnRHa ([D-Ala(6), Pro(9)-NEt]-LHRH) than those taken from fish maintained at 19°C. Cells from female pituitaries were more responsive than those from males. taGTH release in culture was augmented by Ca(2+) ionophore (A23187; 1-100 μM) or ionomycin (0.02-10 μM). The response of perifused pituitary to GnRH was reduced by nimodipine (1-10 μM) indicating that Ca(2+) influx via voltage-sensitive Ca(2+) channels is involved in the stimulation of GTH release. Activation of protein kinase C by OAG (1-oleyl-2-acetyl glycerol; 0.16-160 μM) or TPA (1-O-tetra-decanoyl phorbol-13-acetate; 1.25-125 nM) resulted in a dose-dependent stimulation of taGTH release from cultured cells. Arachidonic acid (0.33-330 μM) also augmented the release of taGTH from the culture. Four sequential pulses of sGnRH (100 nM) at 2h intervals resulted in surges of taGTH release from perifused pituitary fragments; the surges were similar in magnitude with no signs of desensitization. Sequential stimulation with graded doses of sGnRH (0.1 nM to 1 μM) in the presence of GnRH-antagonist ([Pro(2,6), Trp(3)]-GnRH) resulted in an attenuation of taGTH release. However, the GnRH-antagonist did not alter the pattern of forskolin-stimulated GTH release, indicating that forskolin stimulation is exerted at the level of the adenohypophyseal cells. It is concluded that, as in other vertebrates, the transduction of GnRH stimulation of GTH release involves Ca(2+) influx through voltage-sensitive Ca(2+) channels, mobilization of the ion from intracellular sources, arachidonic acid and activation of PKC. Adenylate cyclase-cAMP system us also involved in the mediation but its relationship with other transduction cascades requires further investigations.

Regulation of Gonadotropin-Releasing Hormone (GnRH)-Receptor Gene Expression in Tilapia: Effect of GnRH and Dopamine1

“…However, unlike in mammals, the piscine GnRH-R counterpart possesses a long cytoplasmic C-terminal tail [9]. In tilapia, GnRH stimulates the secretion of LH from the pituitary both in vivo [10] and in vitro [11,12] and increases its mRNA levels [13]. The GnRH also stimulates the secretion of GH from the pituitary both in vivo and in vitro [14].…”

Second Messengers Involved in the Response of Gonadotropic Hormone (GtH) Cells in Fish: GtH Release and GtH IIβ mRNA Levelsa

“…The GnRH effect on GtH release can be mimicked by DAG analogs such as 1‐oleyl‐2‐acetyl glycerol (OAG), dictanoyl glycerol (DiC8) or the phorbol ester tissue plasminogen activator (TPA). 8,10,13 Conversely, the GnRH‐stimulated GtH release can be blocked by PKC inhibitors (H7, staurosporin or GF109203X) or by downregulating cellular PKC content (Fig. 1).…”

Use of Herbal Extracts for Controlling Reproduction in Tilapia Culture: Trends and Prospects - a Review

“…The interaction between environmental factors, the neuroendocrine system, and reproduction in fish has not yet been fully explored, however, a considerable number of related studies have been published. Similar to other vertebrates, reproduction in fish depends on the neuroendocrine system, which initiates and controls gametogenesis (formation of female oocyte and male spermatozoon) and steroidogenesis (formation of steroids), through the activation of hypothalamus-pituitary-gonad axis (HPG) or the gonadotropic axis (Levavi-Sivan and Yaron, 1993;Kah et al, 1993;Van der Kraak et al, 1998;Fink, 2000;Tena-Sempere and Huhtaniemi, 2003;Kajimura et al, 2004;Marchesan et al, 2005;Pinilla et al, 2012). The initiation and completion of reproductive activity in teleost fish is controlled by photoperiod, which ensures an appropriate reproduction season favorable for offspring survival (Bromage et al, 2001).…”