Effects of melatonin in blocking the response to a skeleton photoperiod in the blackheaded bunting

This investigation examined whether a long-day photoperiodic response induced in an interrupted-night (skeleton) photoschedule could not be induced if a short-day melatonin signal was maintained by exogenous melatonin. Three experiments, lasting for 30 days, were performed using photosensitive adult male birds. In Experiment 1, two groups (n = 12 each) were exposed to a skeleton photoperiod (6L:6D:1.5L:10.5D) and received daily either melatonin (10 micrograms bird-1 day-1) or saline (controls) in between 0.5 and 0.25 h prior to the onset of 1.5 h light pulse. Experiments 2 and 3 were similar to the Experiment 1, except in that the duration of the light break was reduced to 1 h (i.e., 6L:6D:1L:11D) and the dose of melatonin was increased to 25 micrograms bird-1 day-1 (Exp. 2) or 100 micrograms bird-1 day-1 (Exp. 3). In addition, one group of 12 birds was maintained on 6L:18D and served as noninjected controls. Although all melatonin treatments reduced fattening and weight gain (p < 0.05, compared to vehicle-treated controls), and the suppressive effects progressively increased with increasing dose of melatonin, in none of the groups were these reduced to the values found in short-day controls. Gonadal growth occurred in all groups but in birds that received 100 micrograms of melatonin photostimulated testes were significantly smaller than the vehicle-treated controls. In buntings exogenous melatonin, which encounters the suppressive effects of mid-night light pulse on melatonin peak (elevated nocturnal melatonin levels), seems not to completely block the inductive effects of light pulse. It is suggested that melatonin may not be involved in the circadian system regulating photoperiodic responses in birds, but may have peripheral effects on the neuroendocrine circuitry.