'Zeitgeber' of the endocrine system: Pineal melatonin and pars tuberalis of the hypophysis

The pars tuberalis is the proximal part of the adenohypophysis. It is mainly composed of a yet ill-defined special type of endocrine cells covering the primary capillary plexus of the hypophysial portal system. Dense expression of melatonin receptors and marked changes of morphology and secretory activity during the annual cycle suggest that these cells are involved in the transmission of photoperiodic stimuli to endocrine targets, like the pituitary-gonadal axis. Cloning of the melatonin receptor and approaches to identify mechanisms of G-protein coupled signal transduction in these cells will probably give new insights in the regulatory role of melatonin. The identity of secretory products and their effects within neuroendocrine regulation remains rather enigmatic. Recent results suggest that pars tuberalis-specific cells are pleiotropic cells exerting rather complex modulatory influences on secretory activity of the pars distalis. Whether such changes in gene expression of the pars tuberalis in response to photoperiodic stimuli occur also in humans remains to be established. However, different lines of evidence confirm that alterations of the photoperiod could be mediated by the pars tuberalis as a melatonin regulated 'Zeitgeber' of the endocrine system.