DOES UNUSUAL ENTRAINMENT OF THE CIRCADIAN SYSTEM UNDER T36H PHOTOCYCLES REDUCE THE CRITICAL DAYLENGTH FOR PHOTOPERIODIC INDUCTION IN JAPANESE-QUAIL

In photoperiodic species, short daylength resonance cycles of module t + 1/2t (t = 24 h) behave like long days because they entrain the circ adian system so that alternate light pulses coincide with the photoind ucible phase (O-i) in castrated quail. However, while a long-day respo nse after exposure to a single long daylength is readily detected by a rise in plasma LH (photoinduction), long-term exposure to LD 6:30 is ineffective in this respect. To discover whether this occurs because o f unusual entrainment, circadian rhythms in quail and starlings were i nvestigated. Whereas starlings entrained in the expected way with alte rnate pulses falling at different circadian phases, activity bouts in quail appeared to follow 24 h after successive light pulses. Because o f this, activity was examined in free-running conditions to confirm th at the pacemaker in quail was indeed being reset to a constant phase ( reset to circadian time [CT] 0) by successive pulses. Examination of t he circadian rhythms of plasma melatonin secretion under LD 6:30 also showed a resetting to CT 0. The positioning of all light pulses at the same circadian phase in the early subjective day explains the lack of photoinduction in quail since O-i in the early subjective night phase remains unilluminated. A second feature in quail is that when the len gth of the photophase is gradually increased within T36h cycles, there is a progressive increase in the degree of photoinduction although th e photophase length remains well below the critical daylength for indu ction in normal T24h cycles. We therefore tested whether O-i, is reset to a constant phase by successive pulses in LD 6:30, and that this ph ase is also advanced relative to Light onset so that photophases short er than the critical daylength fan interact with O-i, to cause inducti on. Such a reduction in critical daylength relative to successive LD 6 :30 pulses was confirmed by transferring quail to various types of lon g day and measuring the change in LH secretion. When the long-day test was replaced with continuous light, stimulation of LH secretion occur red 5-7 h earlier in quail pretreated with LD 6:30 and LD 6:54 compare d to quail pretreated with LD 6:18 or LD 6:42, implying that O-i, had been markedly phase advanced under resonance cycles.