Until a decade ago the pars tuberalis (PT) had been dismissed as an accessory endocrine gland to the pars distalis (PD) (18). A major re-appraisal of this view was demanded when melatonin receptors were localised to this gland in rodents and sheep, yet these receptors were apparently absent from the pars distalis, at least in the post-neonate (17). This suggested that the pars tuberalis has a function related to photoperiod and melatonin, and one distinct from the pars distalis. Considerable focus and interest in the pars tuberalis was generated also because of all the central target sites for melatonin identified through radioligand binding in mammals, it was the only site labelled by 2-125I-iodomelatonin in each of the species examined (17), suggesting a common function. In some species such as the ferret, it is the only central site of action that has been identified (24). These data, together with its anatomical position, provided strong support for a major role of the PT in the photoperiodic actions of melatonin. This has provided the impetus for studying the pars tuberalis as a target site for melatonin, and it has thereby become an important model gland for understanding the cellular mode of action of melatonin.
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