Evidence That Gonadotropin-Releasing Hormone Also Functions as a Growth Hormone-Releasing Factor in the Goldfish*

Abstract

The present study examined the influence of GnRH on the in vivo and in vitro secretion of GH in the goldfish (Carassius auratus). Intraperitoneal injection of several GnRH peptides, including a form native to goldfish, salmon GnRH (sGnRH), elevated circulating GH levels in female goldfish. An analog of mammalian GnRH (mGnRH), [D-Ala6,Pro9-NEt] mGnRH (mGnRH-A), at a dosage of 0.1 microgram/g BW increased serum GH levels for up to 48 h after a single ip injection. Goldfish receiving a series of injections of this dose of mGnRH-A also displayed an increased rate of body growth, indicating that the mGnRH-A-induced increase in the circulating GH level was sufficient to accelerate body growth. In vitro experiments using perifused pituitary fragments found that sGnRH stimulated the secretion of GH from the goldfish pituitary in a potent, dose-dependent, and reversible manner. The time course of response and half-maximally effective dose of sGnRH were very similar for both GH and gonadotropin (GTH) secretion in vitro, suggesting that the mechanism(s) mediating the stimulatory actions of GnRH in the goldfish may be similar for both GH and GTH secretion. However, GnRH-induced GH and GTH secretion from the goldfish pituitary can occur independently of each other, as demonstrated by the finding that somatostatin inhibited the GnRH stimulation of GH secretion in vitro, without influencing the GTH response, whereas the dopamine agonist apomorphine inhibited GnRH-induced GTH secretion in vitro, without influencing the GH response. Furthermore, the dopamine antagonist pimozide did not influence serum GH levels, although pimozide potentiated the stimulatory effect of GnRH on GTH secretion in vivo by blocking the endogenous GTH release inhibitory action of dopamine. Results of the present study suggest that the secretion of GH and GTH in the goldfish are regulated, at least in part, through a common releasing factor, GnRH, whereas somatostatin and dopamine appear to act independently as GH and GTH release inhibitory factors, respectively.

Estradiol Stimulates Growth Hormone Production in Female Goldfish

“…Nevertheless, the mechanism of E 2 action on GH production appears to be different than the effects noted for thyroid hormones on GH transcrip- 5 8-10). GH levels were estimated using a previously characterized antiserum to common carp GH in an established double antibody radioimmunoassay system (Cook et al, 1983b;Marchant et al, 1989). *P , 0.05 versus control.…”

“…Alternatively, E 2 may indirectly stimulate GH synthesis through actions on the multiple neurotransmitter and neuropeptide systems regulating GH in goldfish. For example, E 2 could suppress somatostatin production (Marchant et al, 1987(Marchant et al, , 1989, stimulate dopamine production (Trudeau et al, 1993;Wong et al, 1993), or potentiate intracellular signal transduction pathways (Chang et al, 1994;TenaSempere et al, 1996) in the somatotroph. These possible mechanisms where E 2 may act to enhance GH synthesis and secretion await further study.…”

“…4A and 4B) using anti-gcGH-I in the dot-blot assay, E 2 implantation in vivo was found to increase pituitary content of GH by approximately 150% (P , 0.05). It was important to confirm this in a second independent experiment using an established radioimmunoassay (Marchant et al, 1989) based on anti-common-carp antibodies and standards (Fig. 5).…”

“…Growth hormone release in the goldfish is stimulated by activin-A (Ge and Peter, 1994), bombesin (Himick and Peter, 1995), cholecystokinin (CCK; Himick et al, 1993), dopamine (DA; Chang et al, 1994;Wong et al, 1993), gonadotropin-releasing hormone (GnRH; Marchant et al, 1989), growth hormone-releasing hormone (GHRH; Vaughan et al, 1992), neuropeptide Y (NPY; Peng et al, 1993), and thyrotropin-releasing hormone (TRH; Trudeau et al, 1992). Conversely, GH release is inhibited by glutamate (Trudeau et al, 1996), norepinephrine (Chang et al, 1991), serotonin (Somoza and Peter, 1991), and somatostatin (SRIF; Marchant et al, 1987;Wong et al, 1993).…”

“…Female goldfish (n 5 8-10) at the end of the spawning season were implanted for 5 days with control or E 2 -containing silastic pellets. Blood and pituitaries were collected for determination of GH levels using a previously characterized antiserum to common carp GH in an established double antibody radioimmunoassay system (Cook et al, 1983b;Marchant et al, 1989).…”