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The pars tuberalis is a structurally distinct region of the adenohypophysis, the function of which has been unclear for decades. Recent studies, which demonstrate the localization of melatonin receptors on the pars tuberalis, suggest a photoperiodic function. The principal cell type of the pars tuberalis is morphologically distinct from others in the pituitary and is thought to secrete a specific product. In support of this, evidence is emerging that ovine pars tuberalis cells secrete a factor ('tuberalin') that exerts hormonal control over both gene expression and prolactin release from the pars distalis lactotrophs. These data in conjunction with physiological studies, which show that photoperiodically driven cycles in prolactin secretion can occur in the absence of an intact hypothalamic-pituitary axis, suggest that the function of the pars tuberalis is to act as an endocrine intermediate in the photoperiodic effects of melatonin on prolactin secretion. Studies of the cellular biochemistry of the ovine pars tuberalis suggest that the main function of melatonin is to prevent or terminate transcriptional and translational activation by an unidentified factor (Stim X). On the basis of these physiological and biochemical studies, a hypothetical model is proposed to account for the mechanism of photoperiodic regulation of prolactin secretion by melatonin.
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