Hypothalamic regulatory hormones: a review.

It has long been suggested that the secretion of the anterior pituitary hormones is under the control of the hypothalamus (Green and Harris, 1947; Harris, 1955). Early studies of the vasculature of the hypothalamic-hypophyseal system in birds and mammals described a portal system of capillaries in close contact with neurosecretory cells of the median eminence of the hypothalamus and draining to the anterior pituitary (Scharrer and Scharrer, 1954). It was therefore assumed that neurosecretory transmitters from the hypothalamus drained via this portal system to their target cells in the pars anterior. The anatomical relationship between the hypothalamus and posterior pituitary was, however, recognized to be different since the nerve fibres were found to extend from the supraoptic and paraventricular hypothalamic nuclei actually into the posterior pituitary. These nerve endings are closely applied to the capillaries and contain storage granules and direct neurosecretion ofposterior pituitary hormones into the systemic circulation occurs at these endings. The absence of a significant neural connexion but the presence of the portal capillary circulation linking the hypothalamus and anterior pituitary clearly suggested that the control of the secretion of the anterior pituitary hormones was different from that of the posterior, and the alteration in function of the target organs when the pituitary stalk was cut leaving the pituitary intact implied that the hypothalamus secreted regulatory substances. Despite the abundant evidence for the existence of hypothalamic substances controlling pituitary function, it is only recently that the chemical structures of some of these have been established and their effects demonstrated. They are conventionally referred to as factors (F) if the activity of simple extracts of the hypothalamus is being studied, or as hormones (H) if their structures have been elucidated (Schally, Arimura, Bowers, Kastin, Sawano, and Redding, 1968). Since they have been shown to have either stimulatory or inhibitory effects the term 'hypothalamic regulatory hormones' would seem to be appropriate although early work resulted in the discovery mainly of releasing hormones. Each of the hormones will be considered individually and