Distribution of Melatonin Receptor Subtypes in Area X of Sturnus vulgaris

Julia Margulis, Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA Takayoshi Ubuka, Department of Integrative Biology and Helen Wills Neuroscience Institute, UC Ber George E Bentley, Department of Integrative Biology and Helen Wills Neuroscience Institute, UC Ber

Abstract Adult vertebrate neuroplasticity was first discovered in the telencephalic song control system of songbirds, such as the European starling (Sturnus vulgaris). It is now an accepted phenomenon in mammals, including primates. In songbirds, the concentrations of gonadal steroids such as testosterone and estrogen fluctuate annually as a result of the birds undergoing seasonal “puberty” to prepare for mating. Consequently, there are annual changes in the size and structure of components of the steroid-sensitive song control system. This seasonal neuroplasticity is important because a male’s reproductive fitness is strongly correlated with the variety and complexity of his songs. Melatonin has an inhibitory role in these seasonal changes, acting via receptors (MelR) in several song control nuclei. Here we focus on Area X, a basal ganglia structure that is important for song learning and recognition. The present study aims to determine the possibility of varying roles of the three MelR subtypes (Mel 1a, 1b and 1c) in inhibition of song learning as well as differential sensitivities to gonadal steroids. We mapped the potential differential distributions of MelR subtypes using in situ hybridization and polymerase chain reaction (PCR). Specifically, we hoped to localize the cellular distribution of three MelR subtypes within Area X as a first step towards determining the role of melatonin in seasonal neuroplasticity in songbirds. Importantly, the hormones involved in songbird neuroplasticity have the same structures in all vertebrates. Understanding how melatonin interacts with gonadal steroids in the central nervous system of birds thus has implications for understanding neuroplasticity in vertebrates as a whole.

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