Circadian nature of the photoperiodic clock in Japanese quail

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Abstract
The photoperiodic clock in quail (Coturnix colurnix japonica) is based upon a rhythm of photoinducibility (Øi) but the extent to which this rhythm is circadian remains unclear. Two types of experiment investigated this situation. In the first, gonadectomized quail were adapted to live in periods of darkness by training them on a schedule containing one short day and 3 days of darkness (SD/DD/DD/DD). They were then exposed to a single pulse of 6 or 10 h of light at different times across 3 days of darkness. The photoperiodic response, measured by the increase in LH secretion, showed clear rhythmicity, demonstrating unequivocally the circadian nature of Øi. The second set of experiments employed Nanda-Hamner cycles and varied the length of the photoperiod from 6 to 11 h. Responsiveness in a 36 h or a 60 h cycle was highly dependent upon the length of the photoperiod, something not predicted from theory. For instance, LD 6:30 was not photoperiodically inductive but LD 10:26 was clearly inductive. Close analysis of patterns of LH secretion indicated an unexpected delay before induction occurred and then a rapid rise to a stable level of induction. When LH was measured in every pulse under LD 10:26 there was no evidence that LH levels alternately increased and decreased. This is not consistent with the simplest interpretation of Nanda-Hamner experiments where alternate pulses of light are thought to “entrain” the rhythm or “induce” a photoperiodic response by coinciding with Øi. It is concluded that the quail's photoinducible rhythm is indeed based on a circadian rhythm but one that is only weakly self-sustaining. Possibly as a consequence of this, the rhythm's behaviour under abnormal photoperiodic cycles may be rather different from that found in other species and from other circadian rhythms in quail.