Plasma levels of luteinizing hormone in intact and castrated photosensitive blackheaded buntings (Emberiza melanocephala) exposed to stimulatory and nonstimulatory photoperiods.

Experiments were performed to investigate the effects of castration and/or photostimulation on plasma luteinizing hormone (LH) in photosensitive blackhead buntings. Castration evoked a significant rise in plasma LH in birds held under 8 h light: 16 h darkness (8L:16D). On exposure to 18L:6D for 24 d, both the intact birds and the castrates exhibited a significant rise in plasma LH, although the change in LH levels in the castrates was approximately 8 times greater than in the intact birds. When control birds under 8L:16D which showed no change in plasma LH up to d 13 were transferred to a 36-h resonance cycle (8L:28D), plasma LH rose significantly within 4 cycles of the treatment. Furthermore, the intact buntings showed rapid increase and decrease in the plasma LH levels during a 16-wk exposure to long (16L:8D) but not to short (8L:16D) photoperiods. These results suggest that: i) the photoperiodic drive on LH secretion is inhibited by the testes; ii) the blackheaded bunting has a strong photoresponsive system; and iii) a circadian rhythm of photosensitivity is involved in the photoperiodic time measurement in this species.