The photoperiodic entrainment and induction of the circadian clock regulating seasonal responses in the migratory blackheaded bunting (Emberiza melanocephala).

The properties of the circadian photoperiodic oscillator have been investigated in detail only in the Japanese quail. While the study of the quail is clearly very important, one cannot simply assume that other species, especially passerines that seem to have a different circadian organization than quail, function the same way. The current set of experiments was conducted to understand the entrainment and photoinduction of the circadian photoperiodic oscillator in a passerine species, the blackheaded bunting (Emberiza melanocephala). The experimental paradigm used skeleton photoperiods with two light periods, the first called the "entraining light pulse" (E-pulse) and the second called the "inducing light pulse" (I-pulse). Three experiments were performed on photosensitive male birds (N = 6-8/group). Experiment 1 investigated the effects of the temporal relationship between E- and I-pulses on photoperiodic induction. Buntings entrained to 8h: 16h L:D for 4 wk were released into constant dim light (LLdim, approximately 1 lux). Beginning on subjective day 8, they received for 8 wk, E- and I-pulses only at alternate cycles. While I-pulse was 1h and always began at zt 11.5, E-pulse varied in duration and timing (the 1h E-pulse beginning either at zt 0, zt 5, or zt 9, the 4h one beginning at zt 0 or zt 6, and the 10 h one at zt 0; zeitgeber time 0 = time of lights-on under 8h: 16h L:D prior to release into LLdim). A photoperiodic response was induced only when the E-pulse began at zt 0, and thus the beginning of E- and 1-pulses were separated by 11.5h. Experiment 2 determined whether the duration of the E-pulse influences the position of the photoinducible phase (phii) of the circadian photoperiodic oscillator. Birds were entrained to 1h:23h L:D or 10h: 14h L:D for 2 wk, and then exposed to 1h I-pulse at zt 11.5, zt 15, or zt 18.5 for another 8 wk. Photoperiodic induction occurred at all 3 zts in birds entrained to 10h but only at zt 11.5 in birds entrained to 1h, which infers the circadian rhythm of photoinducibility (CRP) in buntings was re-entrained when I-pulse fell at zt 15 and after. The last experiment examined the possibility of the re-entrainment of the CRP to light pulses falling at zt 15 and after. Birds received 1h I-pulse for 8wk at zt 15 following 2wk of 2.5h:21.5h L:D or 3.5h:20.5h L:D, or at zt 21.5 or zt 22.5 following 2 wk of 10h: 14h LD. Photoperiodic induction was consistent with the hypothesis of the re-entrainment of the CRP under these light-dark cycles. The I-pulse appeared to be interpreted as a "new dawn", and so the photoperiodic induction was determined by the coincidence of phii with the E-pulse. These results suggest a phase-dependent action of light on the circadian oscillator regulating photoperiodic responses in the blackheaded bunting. This could be a useful strategy for a photoperiodic species to regulate its seasonal responses in nature.