Neuroendocrine Regulation of Growth Hormone Secretion

Abstract

The neuroendocrine regulation of growth hormone (GH) secretion is unique and yet simple to understand. It is unique in that GH secretion is the only pituitary hormone regulated by both a hypothalamic releasing hormone, growth hormone-releasing hormone (GHRH), and a hypothalamic release-inhibiting hormone, somatostatin. Yet in spite of this dual regulatory system, it is easy to understand because GHRH and somatostatin interact in a manner analogous to the accelerator and brake of an automobile, a system with which we are all very familiar (Fig. 1). Thus, when there is a physiological demand for GH, we observe the physiological equivalent of the hypothalamus stepping on the gas (releasing GHRH) and simultaneously releasing the brake (inhibiting somatostatin release). Along these same lines, when GH secretion needs to slow down, we observe the physiological equivalent of the hypothalamus putting on the brake (releasing somatostatin) and taking its foot off the gas (decreasing GHRH release). However, there are questions that should immediately come to mind. For example, does GH secretion ever “coast?” That is to say, if neither GHRH and somatostatin are secreted, what happens to GH secretion? Can the accelerator override the effect of the brake and vice versa? What other factors affect this accelerator/braking system?