Season- and context-dependent sex differences in melatonin receptor activity in a forebrain song control nucleus

There are dense populations of melatonin receptors in large areas of the songbird brain, in particular in the visual system and the song control system. Melatonin has therefore been implicated in neuroplasticity of the song control system. Previously we demonstrated large changes in activity of melatonin receptor in Area X, a forebrain song control nucleus involved in song learning and production. In a laboratory environment, melatonin receptor activity was down-regulated in male and female European starlings during photostimulation (a simulated breeding season). The functional significance of this large change in Area X is unclear, so we sought to elucidate it by tracking melatonin receptor activity in male and female starlings housed in a semi-natural environment and permitted to breed. Males and females all exhibited high melatonin receptor activity in Area X during short days at the start of the breeding season, and maintained this high activity during photostimulation until females laid eggs. At this point the females down-regulated melatonin receptor activity in Area X, whereas the males maintained high activity until later on in the breeding season. Mel 1b was the most abundantly expressed of the 3 known melatonin receptor subtypes in Area X. There was a positive correlation between the expression of Mel 1b and the transcription factor ZENK, indicating that high melatonin receptor expression is correlated with high activity of Area X. Overall, we observed a gradual termination of activity in Area X as the breeding season progressed, but the timing of termination was different between the sexes.