Circadian Rhythmicity in Photoperiodically Induced Gonadotrophin Release and Gonadal Growth in the Quail

Abstract

IN 1936 Bünning^1,2 suggested that living organisms measured the length of the daily photoperiod by using an endogenous daily (circadian) rhythm which he envisaged as consisting of two half-cycles (the photophil followed by the scotophil) each of approximately 12 h duration². Photoperiodic induction of a physiological process requiring long days occurred only when the duration of the natural photoperiod extended into the scotophil part of the endogenous cycle. A more explicit version of the Bünning hypothesis has been presented by Pittendrigh^3,4, stressing the dual function of light in acting both as an entraining agent (or Zeitgeber) for the circadian oscillation and as the photoperiodic inducer. He has also emphasized that the “scotophil” may not last for a full 12 h and suggests the more restrictive term “photo-inducible phase” to refer to that portion of the daily oscillation which, if illuminated, leads to photoperiodic induction. There is now good evidence available to support Bünning's hypothesis in plants^5,6, and in at least four species of finches where the photoperiodic induction of testicular growth also relies on a circadian rhythm^7–16. The present studies in a Galliform species, the Japanese quail (Coturnix coturnix japonica)^17–20, were undertaken not only to check whether a circadian oscillator was involved in the photo-periodic response mechanism of both sexes but also to measure the duration of the photo-inducible phase. Experiments on the latter seemed possible, for Coturnix shows an extremely high rate of gonadal development¹⁸.