The pars tuberalis as a target of the central clock

Abstract

The pars tuberalis (PT) of the pituitary has emerged from being a gland of obscure and unknown function to a tissue of central importance to our understanding of how photoperiod regulates seasonal responses. The discovery of melatonin receptors on this gland first pointed to its involvement in seasonal physiology. However, the more recent demonstration of the expression of clock genes in the PT, such as Per1, has heightened interest in the gland. Recent work shows how photoperiod, through the hormone melatonin, affects the timing and amplitude of expression of the Per1 gene, as well as other genes such as Icer. The effect of photoperiod and melatonin on the expression of Per1 in the PT is distinct to its effects on the SCN, and this probably reflects distinct functions of the clock genes in the two tissues - acting as part of the biological clock in the SCN, but as an interval timing system within the PT. The changes in amplitude of Per1 gene expression in response to altered length of photoperiod have provided the first clues as to how the durational melatonin signal is decoded within the neuroendocrine system.

Encoding and Decoding Photoperiod in the Mammalian Pars Tuberalis

“…Indeed, in the PT of MT1 KO mice or melatonin-deficient C57BL mice, Per1 expression is abolished [46,81]. Similar observations have been made in pinealectomised hamsters [97] and melatonin injections can rescue Per1 expression, if given after the surgery [46,90]. Taken together, these observations on Per1 expression show that, in the PT, melatonin acts as a regulator of a complex regulatory network.…”

“…This discovery was followed by studies in hamsters and sheep where the amplitude and timing of expression of Per1 in the PT was shown be dependent on the duration of the melatonin signal (reviewed in [89]). Most clock genes including Clock , Bmal1 , Per1 , Per2 , Cry1 and Cry2 have now been shown to be expressed in the PT, but unlike in the SCN, their expression is not under strict circadian control [90]. In the SCN, the photoperiodic melatonin signal does not affect the phase of expression of clock genes but was shown to have an effect on the amplitude of Per1 and Per2 rhythms.…”

Distribution of transcription factor inducible cyclicAMP early repressor (ICER) in rodent brain and pituitary

“…The strategic position of the PT in close proximity to the portal vein system puts this structure in the spotlight of cyclicAMP-mediated regulation of pituitary function (for review, see Ross and Morgan, 2002). PT cells are thought to function as an interface, mediating central pacemaker output from the SCN to peripheral hormonal signaling.…”

Control of melatonin synthesis in the mammalian pineal gland: the critical role of serotonin acetylation

“…This lag period may be important in limiting melatonin production to precise periods of the night, and may be linked to seasonal regulation of the periods during which melatonin receptor regulated systems are sensitive to melatonin (for review, see Pévet et al 2002;Ross and Morgan 2002;Stehle et al 2002).…”