The pineal hormone, melatonin, is uniquely defined by its role as hormonal time, but the processes whereby cells extract temporal information from the melatonin signal are not understood. Melatonin receptors are expressed in the pars tuberalis (PT) and, during fetal and perinatal life, in the pars distalis (PD). Functional studies suggest that the PT mediates the seasonal effects of melatonin on prolactin secretion, whilst the PD may be involved in photoperiodic programming of the developing gonadotrophic axis. To understand these effects at the cellular level we need to know the phenotype of melatonin-responsive cells. This review summarises current understanding in this area, and highlights present shortcomings. A case is presented for exploring the hypothesis that there is a functional association between melatonin receptor expression and cell differentiation in the anterior pituitary.

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