Neuroendocrine control of growth hormone secretion.

It is now a tenet of neuroendocrinology that the secretion of growth hormone (GH) is regulated by the central nervous system (CNS) (1). Two specific neuropeptides in the hypothalamus, GH-releasing hormone (GHRH) and somatostatin, exert dual control over GH secretion. These in turn are regulated by numerous neurotransmitters, which may act at the level of the GHRHor somatostatin-producing neurones or both. Both amine and amino acids neurotransmitters play a role in the control of GH release and, in general, they exert a dual influence, either stimulating or inhibiting hormone release. This dual effect on GH secretion results from actions of the same molecule at both GHRHand somatostatin-secreting neurones. This is best exemplified by catecholamines, where aand 0-adrenoceptors exert, respectively, stimulatory and inhibitory influences. Alternatively, the neurotransmitter may pass into the stalk blood to act at a receptor site located on the somatotroph (2). As a result of GH secretion, both GH itself and the GH-dependent insulinlike growth factors (IGFs) exert a feedback inhibitory action on GH secretion via hypothalamic and/or pituitary sites (Fig. 1). In addition to the classical neurotransmitters, a host of neuropeptides stimulate GH release, though their physiological function is unclear. In general, these compounds do not act directly at the level of the pituitary, but via the CNS, particularly with the mediation of classical neurotransmitters (1, 3). This paper focuses on certain aspects of the neuroendocrine control of GH secretion which the authors have investigated recently.