09 April 2008
Specific cells of the hypophyseal pars tuberalis (PT) have been associated with the transmission of photoperiodic stimuli to the endocrine system. However, their principal secretory products have not been identified yet. Therefore we studied the expression of several adenohypophyseal hormones and their subunits (TSH, FSH, LH, common α-chain, GH, ACTH, PRL, α- and γ-MSH, β-lipotropin) by immunocytochemistry, Northern blot analysis and in situ hybridization in the sheep pituitary. Only the common α-chain of glycoprotein hormones could be detected in ovine PT-specific cells by immunocytochemistry while antibodies directed against the β-chains of LH, FSH, TSH and β-lipotropin labeled single cells in the PT but failed to detect these antigens in PT-specific cells. In situ hybridization and Northern blot analysis with antisense oligonucleotides against the common α-chain, β-LH, β-FSH, β-TSH, PRL and POMC revealed the expression of these subunits in the ovine PT. The mRNA of the common α-chain, β-TSH and, to a far lower extent, PRL and POMC were found throughout the entire pars tuberalis while β-FSH and β-LH could only be detected in cells of the caudal PT. Hence, GH-mRNA and GH immunoreactivity were exclusively found in the pituitary pars distalis. Compared to these results – obtained under the short photoperiod (winter) – we found clear ultrastructural signs of altered secretory activity in PT-specific cells of animals exposed to the long photoperiod (summer); the common α-chain immunoreactivity was nearly absent in PT-specific cells of summer animals. However, no seasonal influence on gene transcription or translation for other adenohypophyseal hormone was observed. These findings suggest that ovine PT-specific cells, which are only immunopositive for the common α-chain, are capable to express different mRNAs of adenohypophyseal hormones. Although it remains elusive how gene transcription and translation are related in this cell type, the presence of an mRNA pool for hormone subunits leads to the speculation that – at least in the sheep – hormone synthesis is mainly regulated at the translational level and that secretion of hormones may be primarily constitutive.