The mt1 melatonin receptor and RORbeta receptor are co-localized in specific TSH-immunoreactive cells in the pars tuberalis of the rat pituitary.

The pars tuberalis (PT) of the pituitary represents an important target site for the time-pacing pineal hormone melatonin because it expresses a large number of mt1 receptors. Functional studies suggest that the PT mediates the seasonal effects of melatonin on prolactin (PRL) secretion. The aim of this study was the characterization of the phenotype of melatonin-responsive cells. Furthermore, we determined whether RORbeta, a retinoid orphan receptor present in the PT, was co-expressed in the same cells. We combined nonradioactive in situ hybridization (ISH) with hapten-labeled riboprobes for detection of the receptors and immunocytochemistry (ICC) for detection of alphaGSU (alpha-glycoprotein subunit), betaTSH, betaFSH, betaLH, GH, PRL, and ACTH. Expression of mt1 mRNA was found in small round cells, co-localized with alphaGSU and betaTSH. However, not all betaTSH-containing cells expressed mt1 mRNA. The distribution of mt1- and RORbeta-positive cells appeared to overlap, although more cells were labeled for RORbeta than for mt1. Gonadotrophs, as well as other pars distalis cell types, were never labeled for mt1 melatonin receptor. Therefore, this study identifies the "specific" cells of the PT as the mt1 melatonin receptor-expressing cells.