7 Hyperprolactinaemia S. FRANKS H. S. JACOBS NEUROREGULATION OF PROLACTIN SECRETION Prolactin, in contrast to the other anterior pituitary hormones, is under tonic inhibitory control by the hypothalamus. Twenty years ago Pasteels (1962) and Meites, Nicoll and Talwalker (1963) showed that the secretion of prolactin increased when the pituitary gland was removed from intimate contact with the hypothalamus. There was considerable controversy for many years as to the nature of the prolactin inhibitory factor (PIF) but it has now been firmly established that the major PIF is dopamine (MacLeod, 1976). Recent studies have provided direct evidence of the physiological importance of dopamine in the control of prolactin secretion in the primate. Neill et al (1981) measured dopamine concentrations in pituitary stalk blood in the rhesus monkey and showed that intravenous infusion of dopamine to reproduce stalk blood levels caused significant inhibition of prolactin secretion in both intact and stalk-transsectioned animals. In man it is not clear, for obvious reasons, to what extent physiological changes in prolactin concentrations depend upon changes in dopamine release into stalk blood. A recent study of the physiological hyperprolactinaemia of the puerperium in women provides evidence (albeit derived from interpretation of pharma- cological effects of a dopamine receptor antagonist) that regulation of prolactin secretion post partum may be related to changes in hypothalamic secretion of dopamine (Rodriguez-Arnao et ai, 1982). There is a well defined sleep-related rise in prolactin concentrations (reviewed by Franks, 1979). This involves an afferent input to the hypothalamus from higher centres but the precise neuroregulatory mechanism is not clear (Spies, Norman and Buhl, 1979). There is still some disagreement about whether there are significant variations in prolactin concentrations during the menstrual cycle. While some studies have shown no significant changes throughout the cycle (Hwang, Guyda and Friesen, 1971; Jaffe et al, 1973; McNeilly and Chard, 1974), others have pointed to an increase in prolactin at mid-cycle (Franchimont et al, 1976; Veekemans et ai, 1977; Backstrom et al, 1982). Because of the episodic nature of prolactin secretion, these discrepancies Clinics in Endocrinology and Metabolism - Vol. 12, No.3, November 1983 641