Effect of pinealectomy on the photoperiod-dependent changes of the specific secretory cells and à-subunit mRNA level in the chicken pars tuberalis

Abstract

The effect of pinealectomy on the synthesis of the common alpha-subunit of glycoprotein hormones by the specific secretory cells of the pars tuberalis (PT-specific cells) was examined in male chicks. Expression of melatonin receptor (Mel(1c)) mRNA was demonstrated in the chick pars tuberalis by reverse transcription-polymerase chain reaction analysis. Northern blot analyses revealed that, after pinealectomy, common alpha-subunit mRNA levels were increased in the pars tuberalis of chicks kept under normal lighting (L:D=12 h:12 h), indicating that melatonin inhibits the synthesis of the alpha-subunit of PT-specific cells. Furthermore, alpha-subunit mRNA levels in the pars tuberalis were shown to display similar photoperiod-dependent changes in pinealectomized chicks as in intact animals. Levels of alpha-subunit mRNA in the pars tuberalis were decreased in both pinealectomized and control chicks kept under continuous light (L:D=24 h:0), whereas the levels were enhanced in pinealectomized chicks kept under extended darkness (L:D=1 h:23 h) and under normal lighting. Thus, pinealectomy did not affect the inhibition or stimulation of the alpha-subunit synthesis in the chicken pars tuberalis elicited by continuous light or extended darkness, respectively. Quantitative electron-microscopic analyses showed that, after exposure to continuous light for 30 days, many PT-specific cells were filled with enlarged secretory granules in both pinealectomized and control chicks. Exposure to extended darkness for 30 days caused an increase in the cytoplasmic and nuclear areas of the PT-specific cells. Secretory granules were however larger in pinealectomized than in intact control chicks. These results suggest that the activity of PT-specific cells is mainly regulated by photoperiod.