Localisation of gonadotropin-releasing hormone in the circumventricular organs of human brain

Abstract

GONADOTROPIN-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH) are two hypothalamic oligopeptides which regulate the secretion of gonadotropins and thyrotropin, respectively, from the anterior pituitary^1,2. It is generally accepted that these neurohormones are produced by nerve cells, transmitted along axons and stored in nerve endings. When released, they are transferred by way of the hypothalamo–hypophyseal portal system to the anterior pituitary³. We have recently reported⁴ that in the human hypothalamus most of the GnRH is found in the preoptic area while TRH is localised mainly in the posterior nucleus. Exceedingly high concentrations of the two neurohormones were found in the pituitary stalk. The localisation of the two neurohormones in distinctive regions of the hypothalamus and pituitary stalk may represent sites of synthesis in neuronal cell bodies and storage in nerve terminals, respectively. It has recently been found⁵ that significant amounts of GnRH are present in the circumventricular organs (CVO) of the rat brain. These organs consist of specialised ependymal cells, small nerve cells and axon terminals containing small dense core vesicles. The axon terminals end on fenestrated capillaries comprising many capillary loops⁶. Such an anatomical arrangement is characteristic of neurohumoral secretion, and is present also in the human pituitary stalk⁷. In this study we have determined, by radioimmunoassay, the distribution of GnRH and of TRH in the circumventricular organs of the human brain. We report that exceedingly high concentrations of GnRH, but not of TRH, are present in one of the circumventricular organs, the organum vasculosum of the lamina terminalis (OVLT), and discuss the possible physiological function of this structure in the neural control of reproduction.