Variation in melatonin receptors (Mel1a and Mel1b) and androgen receptor (AR) expression in the spleen of a seasonally breeding bird, Perdicula asiatica

Daily variation in the peripheral level of melatonin plays a major role in integrating reproduction and environmental information for seasonally breeding birds. However, the variation in immunity and reproduction has never been assessed in any avian species on a 24 h time scale. Therefore, to understand the relationship between immune function and reproductive phases in a seasonally breeding bird, Perdicula asiatica, the Indian jungle bush quail, we studied the daily variation of melatonin and testosterone levels along with expression of their receptors Mel1a, Mel1b, and androgen receptor in the spleen during the reproductively active phase. Immunocytochemistry for the melatonin receptors Mel1a and Mel1b presented a differential distribution pattern. Western blot of splenic protein suggested a daily rhythm of melatonin receptors, while acrophases for the two melatonin receptors Mel1a and Mel1b differed by 4 h, suggesting that the expression of the receptors may peak at different times, causing more of either Mel1a or Mel1b to be available at a particular time to mediate function. The circulatory melatonin level correlated with percentage stimulation ratio of splenocytes and plasma interleukin-2 level, but did not correlate with testosterone or androgen receptor, suggesting that melatonin could be a major hormone imparting a time-of-day effect on the modulation of immune function in a seasonally breeding bird during the reproductively active phase.