SHORT PHOTOPERIOD-DEPENDENT DOWN-REGULATION OF THYROTROPIN-ALPHA AND THYROTROPIN-BETA IN HAMSTER PARS-TUBERALIS-SPECIFIC CELLS IS PREVENTEDBY PINEALECTOMY

Hamster hypophyseal pars tuberalis (PT)-specific cells are characteriz ed by the expression of common alpha-chain and TSH beta. Immuno-reacti vity for these subunits and the morphology of these cells are known to exhibit remarkable seasonal changes. The high density of melatonin (M el) receptors on PT-specific cells leads to the supposition that fluct uations in circulating Mel levels induced by photoperiodic signals are a crucial factor for the morphological alterations. To more closely i nvestigate transcriptional and translational activities in PT-specific cells, we cloned and sequenced hamster alpha and TSH beta complementa ry DNA fragments and assessed messenger RNA/protein formation by in si tu hybridization and immunocytochemistry under short and long photoper iod 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) o n the nucleotide level and 100% (alpha-chain) and 96% (TSH beta) on th e amino acid level]. Immunocytochemical staining with antibodies direc ted against the common alpha-chain and TSH beta revealed a reduced imm unoreactivity of PT-specific cells under short photoperiod, but this w as not altered in pinealectomized animals exposed to short photoperiod . In situ hybridization against both hormonal subunits paralleled thes e changes, with a dramatic decrease in hormonal messenger RNA in short photoperiod. This regulatory influence was also blocked by pinealecto my. Taken together, these results demonstrate that; transcription and translation of hormonal subunits are regulated by photoperiod in hamst er PT-specific cells, whereas expression remained unchanged in short p hotoperiod 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 informati on to the secretory activity in this pituitary cell type.