Regulatory Peptides of the Hypothalamus

Abstract

The peptides that are the principal subject of this review were originally detected in extracts of the hypothalamus as a result of their effects on the secretory functions of the anterior pituitary gland. According to the portal-vessel chemotransmitter hypothesis ( 155, 192), neurosecretory cells in the hypothalamus produce factors or hormones which reach the adenohypophysis by way of the hypothalamic-hypo­ physeal portal system. These neural factors act in concert with other blood-borne signals from the periphery to regulate the functions of the various anterior pituitary cells. To date, three peptides have been purified and characterized, and furthermore shown to have unequivocal hypophysiotropic activities by numerous investigators. These are TRF, LRF, and somatostatin (SS) whose alternative names, sequences, and principal actions on the anterior pituitary are shown in Table 1 . All three of these peptides have biological actions on the central nervous system (CNS) in addition to their effects on the anterior pituitary. Furthermore, TRF, LRF, and somatostatin are widely distributed throughout the brain, and somatostatin is found in and can influence the function of the pancreas and gastrointestinal tract as well. In general, the hypothalamic regulatory peptides (HRP) appear to act as local extracellular signals or messengers, as cells containing (producing) the HRP are found in, are adjacent to, or have restrictive vascular or possibly cerebral spinal-fluid links to cells that can respond to these peptides. Peptides with such local actions have been termed members of the paracrine rather than the endocrine system ( 152). The observations indicating that these peptides are exclusively neither hypo­ thalamic nor hypophysiotropic present problems of classification as well as nomen­ clature to neuroendocrinologists. In the interest of historical continuity, we continue to refer to these peptides as hypothalamic and maintain the original releasing­ factor terminology until such time as IUPAC-IUB-approved trivial names gain acceptance. Analyses for the purpose of categorizing the HRP as hormones, neurotransmitters, or a separate class of extracellular messengers reveal the