Developmental appearance and age related changes in specific 2-[¹²⁵I]iodomelatonin binding sites in the suprachiasmatic nuclei of female Syrian hamsters

In Syrian hamsters, the circadian timing system is sensitive to melatonin during gestation but is not responsive in the adult. In order to further understand this developmental change in melatonin responsiveness, in vitro autoradiography was used to assess the presence of specific 2-[¹²⁵I]iodomelatonin binding sites in the suprachiasmatic nuclei of female hamsters of selected embryonic (E) and postnatal (PN) ages (e.g. E13, E14, E15, PN1, PN2, PN12, PN25, PN112-133). Specific 2-[¹²⁵I]iodomelatonin binding sites were seen in the suprachiasmatic nuclei of some of the E14 hamsters and all the perinatal hamsters (E15, PN1 and PN2) but not in older hamsters. In contrast, specific 2-[¹²⁵I]iodomelatonin binding sites were seen in the pars tuberalis of all hamsters (with the exception of one), regardless of age. The transient expression of specific 2-[¹²⁵I]iodomelatonin binding sites in the suprachiasmatic nuclei suggests that melatonin may have some special functions restricted to early development. The specific 2-[¹²⁵I]iodomelatonin binding sites in the embryonic suprachiasmatic nuclei may represent the substrate for maternal melatonin to set the phase of the developing circadian timing system.