Photoperiod-dependent and -independent regulation of melatonin receptors in the forebrain of songbirds.

Melatonin was recently identified as playing a role in fine-tuning the effects of gonadal steroids in the regulation of seasonal neuroplasticity within the telencephalic song control system of European starlings. The present study investigated possible seasonal regulation of melatonin receptors (MelR) within the starling song control system, in the presence or absence of gonadal steroids. Brains were sampled from photosensitive starlings exposed to short days, photostimulated starlings exposed to long days and photorefractory starlings also exposed to long days. Each condition contained a group of gonad-intact birds and a group of castrated birds. Melatonin receptor distribution was assessed in vitro by 125-iodomelatonin (IMEL) receptor autoradiography. In general, MelR distribution was similar to that described in other songbird species. However, there was a striking downregulation of MelR in the song control nucleus Area X of intact and castrated photostimulated birds on long days compared to their photorefractory counterparts on the same long days and to the short-day groups. Downregulation of MelR occurred independently of gonadal steroids. Nevertheless, superimposed on this general pattern of MelR downregulation during photostimulation, IMEL binding was observed in a medial subdivision of Area X when gonadal steroids were present. Downregulation of MelR in Area X during the short breeding season has implications for seasonal regulation of the song control system. Subsequent upregulation of MelR as birds become photorefractory, in the absence of any change in photoperiod, gonadal steroids or melatonin signal is the first description of photoperiod-independent regulation of MelR in adults of any vertebrate class.