Production of a prolactin releasing factor by the ovine pars tuberalis

The anterior pituitary hormone prolactin is involved in the regulation of a wide variety of processes including mammary growth, lactation, reproduction and pelage growth (1). Correspondingly, in seasonal breeders, pronounced photoperiod-driven, seasonal changes occur in the levels of prolactin secretion (2). These effects are thought to be mediated by the pineal hormone melatonin, which acts as a humoral indicator of photoperiod (3). Melatonin is thought to act through hypothalamic sites to control the gonadotrophic axis (4-6), but the sites through which melatonin modulates prolactin remain to be established. One possibility is that melatonin acts at the level of the hypothalamus to modulate the release of the hypothalamic prolactin inhibitory factor, dopamine (7). However, recent evidence from hypothalamo-pituitary disconnection experiments performed in the ram suggests that the photoperiodic modulation of prolactin secretion can occur independently of the hypothalamus, presumably due to direct effects of melatonin on the anterior pituitary (8). The only identified site of melatonin receptor expression within the ovine pituitary is the pars tuberalis (PT) (9), the cells of which are almost completely non-lactotrophic (10). Thus the possibility exists that factors released by the PT regulate the activity of lactotrophs in the pars distalis (PD). We have investigated this hypothesis using a range of co-culture and medium-conditioning experiments on primary cultures of ovine PT and PD cells, and here we report that PT cells secrete an unidentified factor that is a potent stimulus of prolactin secretion by PD cells.