Effects of growth hormone on neuroendocrine function.

Although the role of growth hormone (GH) in the control of reproductive functions is not well understood, there is considerable evidence that the states of both GH deficiency and GH excess are typically associated with reproductive deficits. To identify the possible involvement of functional alterations in the hypothalamic-pituitary system in producing these deficits, we are studying neuroendocrine function related to reproduction in transgenic animals overexpressing GH, in animals with congenital GH deficiency, and in animals with selective immunoneutralization of GH. The results indicate that GH acts on the hypothalamus to alter dopaminergic and noradrenergic control of prolactin and gonadotropin release. Life-long elevation of GH levels outside the physiological ranges disrupts feedback control of luteinizing hormone (LH) release by gonadal streroids. Plasma LH and follicle-stimulating hormone (FSH) levels and feedback control of LH release are also abnormal in GH-deficient animals indicating that physiological levels of endogenous GH are normally involved in the control of gonadotropin release. Differences between the effects of bovine vs. human GH in transgenic mice and differential effects of GH deficiency in mice and rats should facilitate identification of the mechanisms involved in the actions of GH on the hypothalamic-pituitary system.