Effect of different photoperiods on the ultrastructure of the specific secretory cells and α-subunit mRNA level in the chicken pars tuberalis

The effect of different photoperiods on the specific secretory cells of the pars tuberalis was examined in male chicks. Animals were placed in one of three different photoperiod regimens: (1) normal control (light:dark = 12 h:12 h), (2) continuous light (L:D = 24 h:0), and (3) extended darkness (L:D = 1 h:23 h). The levels of common α-subunit mRNA in the pars tuberalis were examined by Northern blot analysis and compared with those in the pars distalis. In chicks exposed to continuous light for 1 week, α-subunit mRNA level in the pars tuberalis was decreased, although the level in the pars distalis was increased. Exposure to continuous light for 30 days also induced a decrease in α-subunit mRNA level in the pars tuberalis. On the other hand, in chicks exposed to extended darkness for 1 week, the α-subunit mRNA level of the pars tuberalis was markedly increased. In situ hybridization with digoxigenin-labeled common α-subunit cRNA probe also showed that the hybridization signals for α-subunit mRNA in the pars tuberalis cells become weak under continuous light for 30 days but they are very intense under extended darkness. Thus, the synthesis of α-subunits in the chick pars tuberalis was inhibited by continuous light but stimulated by extended darkness. These results were confirmed by semiquantitative electron-microscopic analyses. After exposure to continuous light for 30 days, many pars tuberalis (PT)-specific cells were filled with enlarged secretory granules, showing the reduction of secretory activity. On the contrary, extended darkness for 30 days induced hypertrophy of the PT-specific cells; the areas of cytoplasm and nucleus were significantly increased. In addition, secretory granules became small in size and exocytotic features were more frequent. Mitochondria and lysosomes were also increased in number. Thus, the synthetic and secretory activities of the PT-specific cells were increased under extended darkness. The data indicate that the specific cells of the pars tuberalis are responsive to photoperiodic changes in the chick.