The mt1 Melatonin Receptor and RORβ Receptor Are Co-localized in Specific TSH-immunoreactive Cells in the Pars Tuberalis of the Rat Pituitary

Abstract

S U M M A R YThe 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 ROR ␤ , 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 ␣ GSU ( ␣ -glycoprotein subunit), ␤ TSH, ␤ FSH, ␤ LH, GH, PRL, and ACTH. Expression of mt1 mRNA was found in small round cells, co-localized with ␣ GSU and ␤ TSH. However, not all ␤ TSH-containing cells expressed mt1 mRNA. The distribution of mt1-and ROR ␤ -positive cells appeared to overlap, although more cells were labeled for ROR ␤ 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.(

Gonadotrophin-releasing hormone drives melatonin receptor down-regulation in the developing pituitary gland

“…Overall, MT 1 mRNA expression in this region was Ϸ4-fold higher in hpg mice, and liquid emulsion autoradiography showed a strong increase in expression around the PT-PD interface. Because gonadotrophs are present throughout this region (17,(38)(39)(40), these data are consistent with the hypothesis that GnRH down-regulates pituitary MT 1 expression in vivo. Based on this result, we suggest that melatonin receptor expression is a feature of a quiescent phase in the development of the hypothalamo-pituitary-gonadal axis, after the gonadotrophic phenotype has become established, but before the GnRH system is in place.…”

“…This result is consistent with the view that the ability of melatonin to inhibit GnRHstimulated LH secretion in gonadotroph-enriched cultures (16) is mediated by MT 1 . We also observed cells that were MT 1 positive but LH␤ negative in the rostral PD͞PT, and suspect that these represent other endocrine cell types, notably the PTspecific thryotroph, which has been previously identified in this region (34)(35)(36) and expresses melatonin receptors (17). The function of this cell type remains unclear.…”

“…The subsequent onset of pulsatile GnRH secretion from the hypothalamus then, through the induction of Egr-1, both activates LH␤ expression and suppresses melatonin receptor expression. The maintained expression of MT 1 in the PT of wild-type animals is probably largely attributable the presence of a population of PT-specific thyrotroph cells in this region that are insensitive to both GnRH and thyrotropin-releasing hormone (17,34,35); indeed, although Klosen and colleagues found LH␤ positive cells in the rat PT, they were unable to detect LH␤ coexpression with MT 1 (17). Although explant experiments suggest that a small population of gonadotroph cells may remain melatonin-sensitive within the PT in adults (38,41), the receptor subtype responsible for this phenomenon has not been defined.…”

“…Furthermore, mice bearing a genetic lesion of the Pitx-1 gene exhibit a selective decrease in ventral PD thyrotroph and gonadotroph cells, and also in ''PT-specific'' thyrotrophs (15). Because these cell types are associated with melatonin receptor expression in rodents (16,17), the possibility arises that Pitx-1 may be an important factor supporting melatonin receptor expression in the adenohypophysis.…”

Melatonin Transmits Photoperiodic Signals through the MT1 Melatonin Receptor

“…Error bars indicate SEM. (Reppert et al, 1994;Gauer et al, 1998;von Gall et al, 2002b); (2) it contains high levels of MT1 mRNA but lacks MT2 mRNA (Reppert et al, 1995); (3) MT1 mRNA and TSHB protein are colocalized in the PT of rat (Klosen et al, 2002), and immunosignals of TSHB in the PT of MT1-deficient C3H mice are constitutively upregulated (C. von Gall, unpublished observation); (4) TSH produced in the PT was shown to regulate Dio2 and Dio3 expression in the EC in Japanese quail (Nakao et al, 2008), sheep (Hanon et al, 2008), and mice .…”