| Summary: | For many species, adaptation of various physiological characteristics to the seasonally changing environment is vital in their survival and reproductive fecundity. Such seasonal alterations in physiology can affect growth, food intake, energy balance and gonadal activity, as well as the secretion of the pituitary hormone prolactin. The current suggested model of seasonal prolactin regulation dictates that the melatonin-responsive pars tuberalis (PT) of the anterior pituitary gland translates the photoperiodic input (measurement of day length) into a seasonal signal, which controls prolactin release. The primary aims of this study were to investigate the downstream decoding mechanisms of the melatonin signal and the currently uncharacterized factors, named ‘tuberalins’, which are considered to be released by the PT and involved in the seasonal entrainment process. The most likely candidates believed to have a key role in the seasonal regulation of prolactin, as the PT-derived tuberalins, are Substance P and Neurokinin A, members of a group of bioactive peptides known as neurokinins. These molecules, which have previously been shown to have an effect on prolactin release, are encoded by the TAC1 (tachykinin, precursor 1) gene, which was recently identified as being strongly activated by ‘summer like’ long photoperiods (LP) within the ovine PT.In the present study, the Fischer 344/NHsd (F344/NHsd) rat, a photoperiod-sensitive laboratory strain that responds to ‘winter like’ short photoperiods (SP) by decreasing its body weight, food intake, testes weight and the secretion of pituitary prolactin, was used as a model species to investigate the potential involvement of neurokinins in the seasonal regulation of prolactin. The action of the neurokinins Substance P and Neurokinin A was tested with the help of a transgenic F344 rat, which expresses the reporter gene luciferase under the control of the prolactin promoter, providing a tool for studying prolactin regulation. An in vitro reporter system was set up using transgenic F344 tissue, and this revealed no effect of the neurokinins on prolactin promoter activity. In situ hybridization studies revealed a lack of TAC1 mRNA expression in the PT region of the F344/NHsd rat. These results do not support a role for neurokinins in the seasonal regulation of prolactin in the rat, suggesting a possible divergence in the seasonal pathway of prolactin regulation between species. Nonetheless, the expression of TAC1 was found to be regulated by photoperiod in other key areas of the brain, including the ependymal layer of the third ventricle of the hypothalamus, which has previously been associated with seasonal adaptations in body weight and reproduction. Furthermore, the in situ hybridization studies reveal the LP-activated expression of Eyes absent 3 (Eya3), a transcription factor-encoding gene, in the PT of the F344/NHsd rat, and this finding is similarly revealed in the Syrian hamster, a strongly seasonal species. Recent studies in seasonal birds and mammals have identified Eya3 as the first common strongly activated gene in the PT, following exposure to LP and suggested that it may have a key role as a master regulator of seasonal responses. The novel finding of Eya3 up-regulation in the rat under LP further highlights the conserved nature of Eya3 expression in photoperiodic species and presents the F344/NHsd laboratory rat as a valuable rodent model in the study of photoperiodism.
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