UBUKA, T*; BENTLEY, GE; Univ. of California, Berkeley; Univ. of California, Berkeley: Differential Regulation of Receptor Subtypes in The Central Nervous System of European Starlings

Several of the song control nuclei of songbirds, including HVc (higher vocal center) and Area X, contain melatonin receptor (MelR). In laboratory-housed male European starlings, the densities of MelR in Area X change markedly according to reproductive state. MelR are down-regulated when starlings are photostimulated (in full breeding condition) and are subsequently up-regulated when starlings become photorefractory (reproductively quiescent). Within each reproductive state, MelR density in Area X does not differ over the course of a circadian cycle. The functional significance of seasonal regulation of MelR in this song control nucleus remains unclear, but it is likely to involve a release of cellular inhibition by melatonin during photostimulation, with possible consequences for song learning, memory consolidation or regulation of the context of song production. Unlike in laboratory-housed birds, field-collected samples do not show uniform changes in MelR density. For any given photoperiod, birds collected in the field show both high and low MelR activity in Area X. Thus, we believe that there are as yet unidentified fundamental differences between the lab and field environments that cause differential regulation of MelR in this large forebrain area. It is possible that social, physiological or environmental factors other than photoperiod contribute to regulation of MelR in Area X. We have cloned partial Mel1a, Mel1b, and Mel1c cDNA in starlings and used this for in situ hybridization analysis of MelR activity in Area X, along with analysis of differential distribution of MelR subtypes throughout the brain. These data allow us to identify the neuroanatomical specificity of MelR subtypes, and to investigate the probability that different subtypes have different functions and regulatory factors.