NEUROENDOCRINE REGULATION OF GROWTH-HORMONE SECRETION AND GROWTH IN FISH

Growth in fish is regulated in large part by the brain neuroendocrine - growth hormone (GH)- insulin-like growth factor axis. GH secretion i s in turn regulated by multiple factors from the brain, with both stim ulatory and inhibitory neurohormones acting on the somatotrophs season ally. Somatostatin is the primary inhibitor of basal and stimulated GH secretion. Norepinephrine and serotonin also have inhibitory actions on GH release. On the other hand, GH secretion is stimulated by a numb er of neuroendocrine factors, including growth hormone-releasing facto r (GRF), dopamine (DA), gonadotropin-releasing hormone (GnRH), neurope ptide Y (NPY), thyrotropin-releasing hormone (TRH), cholecystokinin (C CK), bombesin (BBS), and activin. While GRF and DA are more potent in stimulating GH secretion in sexually regressed fish, GnRH, NPY, and TR H have greater stimulatory effects on GH secretion in sexually mature (i.e., prespawning) fish. Sex steroids, in particular estradiol, influ ence the responsiveness of the somatotrophs to neuroendocrine factors. The integrated action of sex steroids and neuroendocrine factors prov ides a basis for the seasonal regulation of growth hormone secretion. The brain peptide systems regulating food intake are linked to the bra in neuroendocrine regulation of GH secretion. Following a meal, goldfi sh characteristically show a short-term increase in serum GH concentra tions, and then a decrease in serum GH concentrations to below premeal levels. BBS and CCK are involved in satiation and the changes in GH s ecretion following a meal in goldfish. Both the neuroendocrine regulat ion of GH secretion and the brain regulation of feeding are multifacto rial. Understanding the integration of these systems presents a major challenge.