A complex seasonal rhythm controlled by photoperiod

Abstract

1. A rhythm with three peaks and troughs during the year exists for the size of the receptive field for grooming reflexes in cats with pontile lesions (Fig. 5). To determine the role of photoperiod in regulating this rhythm, cats with pontile lesions were studied in two different experiments involving photoperiod manipulations. 2. In the first experiment two groups of cats with pontile lesions were exposed to continuously-varying LD cycles, with one group exposed to a schedule of LD cycles with progressively longer photoperiods and the other group in an adjacent room simultaneously exposed to a schedule of LD cycles with progressively shorter photoperiods. The schedules were controlled by automatic clocks which provided the naturally-occurring changes in sunlight photoperiods. The size of the receptive field for the two groups diverged and then reconverged after the LD cycles passed through opposite solstices (Fig. 1). 3. In the second photoperiod manipulation, the cats were exposed to fixed LD cycles. The size of the receptive field for grooming reflexes was monitored until stable endpoints were obtained (Fig. 2). The stable endpoints, when plotted with their respective photoperiods, indicated an approximately linear trend between LD 10∶14 and LD 15∶9 (decreasing), with reversals of this trend at both extremes (Fig. 3). 4. The data from the photoperiod manipulations were compared with previous longitudinal data. Biweekly measures of the size of the receptive field from three previous longitudinal studies were combined. The 26 biweekly means when plotted with their respective photoperiods exhibited an approximately linearly decreasing trend with reversals at both extremes (Fig. 4), a pattern similar to the one obtained with the photoperiod manipulations. 5. The results were discussed in terms of Bunning's model of photoperiodic control. This model has the versatility to account for complex rhythms during the year, a versatility provided basically by the circadian oscillator and the possibility of phase control by the particular LD cycle. Auto-interaction, a phenomenon well known in enzymology and pharmacology, is a possible mechanism which increases the versatility of the model.