Role of GnIH in photoperiodic regulation of seasonal reproduction in the Eurasian tree sparrow.

Seasonal reproductive cycles of most birds are regulated by photoperiod via neuroendocrine control. Gonadotropin-inhibitory hormone (GnIH) in the hypothalamus has been reported to act as neuroendocrine integrator of photoperiodic cues. In this study, both captive and field investigations were carried out to understand the effects of photoperiod and seasonality on GnIH expression in subtropical tree sparrows. Monthly observations of GnIH mRNA and peptide expression in wild birds over a year revealed a significant increase in GnIH mRNA level and number of GnIH-ir neurons during the non-breeding season when compared with their expression in the breeding season. GnIH-ir neurons were found primarily in the paraventricular nucleus (PVN) with their fibers projecting into the median eminence and some other areas of the brain. In an 8 month-long experiment, birds exposed to short days had higher GnIH expression compared with birds exposed to long days regardless of sampling month. Long-day birds with regressed testes had similar GnIH levels to short-day birds. Though the number of GnIH peptide-expressing neurons ran almost parallel to the levels of GnIH mRNA, they were inversely related to gonadal size in both sexes under natural and artificial photoperiodic conditions. These results clearly indicate an inhibitory role of GnIH in photoperiodic regulation of seasonal reproduction in the tree sparrow.