Short photoperiod-dependent down-regulation of thyrotropin-alpha and -beta in hamster pars tuberalis-specific cells is prevented by pinealectomy

Hamster hypophyseal pars tuberalis (PT)-specific cells are characterized by the expression of common alpha-chain and TSH beta. Immuno-reactivity for these subunits and the morphology of these cells are known to exhibit remarkable seasonal changes. The high density of melatonin (Mel) receptors on PT-specific cells leads to the supposition that fluctuations in circulating Mel levels induced by photoperiodic signals are a crucial factor for the morphological alterations. To more closely investigate transcriptional and translational activities in PT-specific cells, we cloned and sequenced hamster alpha and TSH beta complementary DNA fragments and assessed messenger RNA/protein formation by in situ hybridization and immunocytochemistry under short and long photoperiod and in pinealectomized animals kept in short photoperiod. Hamster common alpha-chain and TSH beta exhibited high sequence homology with the corresponding rat hormones [94% (alpha-chain) and 90% (TSH beta) on the nucleotide level and 100% (alpha-chain) and 96% (TSH beta) on the amino acid level]. Immunocytochemical staining with antibodies directed against the common alpha-chain and TSH beta revealed a reduced immunoreactivity of PT-specific cells under short photoperiod, but this was not altered in pinealectomized animals exposed to short photoperiod. In situ hybridization against both hormonal subunits paralleled these changes, with a dramatic decrease in hormonal messenger RNA in short photoperiod. This regulatory influence was also blocked by pinealectomy. Taken together, these results demonstrate that; transcription and translation of hormonal subunits are regulated by photoperiod in hamster PT-specific cells, whereas expression remained unchanged in short photoperiod if pinealectomy was performed. We, therefore, conclude that in hamsters, the Mel signal not the light regimen per se, is a direct or indirect Zeitgeber for the transduction of photoperiodic information to the secretory activity in this pituitary cell type.