Meeting Abstract

BERN.1  Monday, Jan. 5  VERTEBRATE PHOTOPERIODIC SIGNALLING SHARP, PJ; EDINBURGH UNIVERSITY peter.sharp@roslin.ed.ac.uk

Photoperiodic time measurement is achieved through a hypothalamic pacemaker that generates a circadian rhythm of photoinducibilty. Coincidence of light and the photoinducible phase of this rhythm stimulates expression of TSH beta in pars tuberalis cells that results in a local increase in TSH in the adjacent medial basal hypothalamus (MBH). This in turn stimulates local metabolism of T4 to T3 in MBH tanycytes. Photoinduced increase in T3 in the MBH may both regulate neuropeptide release into the hypophysial portal vasculature by modulating the plasticity of tanycyte endfeet, and regulate efferent pathways to areas of the hypothalamus synthesizing photoperiodically- regulated neuropeptides. The pars tuberalis TSH beta- T4/T3 tanycyte photoperiodic signalling pathway occurs both in birds, and in long and short day-breeding mammals. It is activated by long days irrespective of whether the animal is reproductively photosensitive or photorefractory. Mammals differ from lower vertebrates in dependence on circulating pineal melatonin signalling for transducing photoperiodic information to pars tuberalis melatonin receptors. Whilst mammals use photoreceptors in their eyes to monitor the presence of light for photoperiodic signalling, birds use extra retinal photoreceptors. The identity of the pathway through which photoperiodic signalling is detected by the avian pars tuberalis is uncertain since circulating melatonin does not control photoperiodically-regulated avian gonadal cycles. However, melatonin appears to be generated locally within the avian MBH in photoresponsive dopaminergic neurones, raising the possibility in birds that photoperiodic signalling within the MBH is transduced by circadian- gated changes in local concentrations of melatonin.