Seasonal neuroplasticity in the songbird telencephalon: A role for melatonin

Neuroplasticity in the vocal control system of songbirds is strongly influe nced by seasonal fluctuations in circulating testosterone. These seasonally plastic telencephalic structures are implicated in the learning and produc tion of song in songbirds. The role of the indoleamine melatonin in seasona l adaptations in birds has remained unclear. In this experiment, European s tarlings were castrated to remove the neuromodulating activity of gonadal s teroids and were exposed to different photoperiods to induce reproductive s tates characteristic of different seasonal conditions. Long days increased the volume of the song-control nucleus high vocal center compared with its volume on short days. Exogenous melatonin attenuated the long-day-induced v olumetric increase in high vocal center and also decreased the volume of an other song-control nucleus, area X. This effect was observed regardless of reproductive state. To our knowledge, this is the first direct evidence of a role for melatonin in functional plasticity within the central nervous sy stem of vertebrates.