Phylogenetic evolution of the neuroendocrine control of growth hormone: contribution from Teleosts
Growth hormone (GH) is involved in many physiological functions such as growth, metabolism, reproduction and immunity. In mammals, it is well established that GH release is under a dual hypothalamic control, stimulatory by somatoliberin (GHRH) and inhibitory by somatostatin (SRIH). Peripheral hormones such as the insulin-like growth factors (IGFs) and the thyroid hormones (TH) can also regulate GH release. In non-mammalian vertebrates, a great variability of hypothalamic neuropeptides involved in GH stimulation seems to occur. This review underlines the important contribution of teleost models in the phylogenetic study of GH control. Teleost somatotrophs show a high autonomous activity of synthesis and release in vitro in a serum-free medium and without any secretagogues. This strong autonomous activity in vitro suggests that these cells are under a dominant inhibitory control in vivo. SRIH and IGF1 have strong inhibitory effects on the release and synthesis of GH in teleosts. Concerning the stimulatory control of GH in teleosts, a great diversity of factors involved exists. Thus, depending on species or physiological stage, thyreoliberin, gonadoliberin, corticoliberin and dopamine are able to stimulate GH release. Contrary to mammals, in teleosts, PACAP, which is encoded by the same gene as GHRH, stimulates GH release, whereas GHRH has lower or no effect. Variable actions of TH in GH control are found among teleosts as well as among other vertebrates. In conclusion, the inhibitory control exerted by SRIH and IGF1, which shows a strong molecular and functional conservation, could represent the basic neuroendocrine control of GH, established early during evolution, at least in a common ancestor of the actinopterygians and the sarcopterygians. In contrast, a great variability is seen among the hypothalamic neuropeptides involved in the stimulatory control of GH release. PACAP may represent the ancestral GH-releasing factor, whereas GHRH would have acquired a major role during tetrapod and especially mammalian evolution.