Neuroendocrine regulation of growth hormone secretion in teleost fishes with emphasis on the involvement of gonadal sex steroids

In teleost fishes, growth hormone (GH) appears to play an important regulat ory role in several, apparently disparate, physiological events, including reproduction, osmotic or ionic regulation, metabolism, growth and developme nt. GH secretion is regulated by hypothalamic neuroendocrine factors that e ither act directly on the somatotrophic cells in the pituitary gland, or mo dulate the secretion or activity of other neuroendocrine factors. In additi on, the degree of the neuroendocrine influence on GH release is influenced by the nutritional and reproductive state of the fish; moreover, there appe ar to be marked species differences in some aspects of this neuroendocrine - physiological condition relationship among fish species. Thus, the neuroe ndocrine control of GH secretion in fishes is complex, and still poorly und erstood. The neuropeptides, gonadotrophin-releasing hormone, growth hormone -releasing hormone, thyrotrophin-releasing hormone, neuropeptide Y, seroton in and pituitary adenylate cyclase-activating polypeptide have all been dem onstrated to stimulate GH in fish, as has the glutamate agonist, N-methyl-D ,L-aspartate. Conversely somatostatin has a potent inhibitory action on GH release in goldfish and carp, but is less effective in salmon and trout spe cies. This review examines the interactive nature of the neuroendocrine control o f GH secretion in fishes, and the manner in which gonadal steroids, directl y or indirectly, modulate GH secretion and/or the release, or the activity, of the neuroendocrine factors.