The expression of the α-subunit of the glycoprotein hormones was studied in the developing rat pituitary by in situ hybridization and immunocytochemical techniques. The α-subunit mRNA labelling was detected in Rathke’s pouch at 12 days’ gestation (E12); at E13, it was exclusively concentrated in the pars tuberalis primordium (the lateral lobes) where its intensity rapidly increased at following stages. The α-subunit was first immunocytochemically detectable in the pars tuberalis at E14. Labelling of the whole pars tuberalis primordium with both techniques indicated that the specific glandular cells were involved. Immunocytochemical detection of bromodeoxyuridine, injected into the pregnant rat 4 h prior to fixation, revealed a total arrest of cell divisions in the pars tuberalis between E13 and E17, which was not the case in the pars distalis or the pars intermedia at any stage. In the pars distalis, the onset of α-subunit expression was detected much later than in the pars tuberalis, at E16–17 by in situ hybridization and at E17–18 by immunocytology. The functional significance of the early secretory differentation of the pars-tuberalis-specific cells remains to be established.

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The nature of the hormone(s) secreted by the pars tuberalis (PT) is still unknown. This pituitary lobe is mainly formed by specific glandular cells that differ in their ultrastructural features from the other adenohypophysial cell types. Data from the literature indicate the presence of thyroid-stimulating hormone immunoreactivity in the PT-specific cells of the rat and the Djungarian hamster but not of other species, including the mouse and guinea-pig. The PT also encloses variable numbers of pars distalis cells, essentially gonadotrophs that are mainly dispersed in its caudal area. We studied the expression of the glycoprotein hormone alpha-subunit in the PT of the rat, mouse and guinea-pig by in situ hybridization and immunocytochemistry. In situ hybridization, using an oligonucleotide probe complementary to rat cDNA sequence 196-237 revealed the expression of the alpha-subunit gene throughout the PT of the rat and the mouse; in the guinea-pig, the probe labelled no pituitary cells. Light- and electron-microscopic immunocytochemistry demonstrated alpha-subunit immunoreactivity in the secretory granules of the PT-specific cells in the three species examined. These cells did not react with a specific antibody against the beta-subunit of luteinizing hormone, an antibody that labelled scattered gonadotrophs. The present data suggest that hormone(s) produced by the PT-specific glandular cells are, at least partly, related to glycoprotein hormones.

No immunoreactive axons were detected with an antiserum against tyrosine hydroxylase in the rabbit intermediate lobe (IL), which thus appears to be devoid of dopaminergic (DA) innervation. Dopamine and its agonists, which classically inhibit α-MSH release have no inhibitory effects on rabbit IL superfused in vitro but, paradoxically, stimulate α-MSH release. D2 type DA receptors, known to mediate inhibitory control of dopamine on melanotropic cells, and detectable by their affinity for (³H)-spiroperidol, were as previously reported absent from the rabbit IL. The absence of (³H)-spiroperidol binding sites in the IL was further confirmed on rabbit pituitary sections by radioautography. The mechanism of DA stimulation is still not clear, but might be tentatively explained by interference with other receptors involved in the stimulation of the gland. The lack of DA inhibitory control over the rabbit IL is an exception among the species so far studied.

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