Circadian rhythm of visual sensitivity inManduca sexta and its development from an ultradian rhythm

Abstract

Larvae and pupae of the sphingid mothManduca sexta were maintained under various photoperiod regimes (L/D, L/L, D/D). Visual sensitivity was monitored after placing an animal in constant darkness, interrupted by short flashes of low intensity light each half hour. Sensitivity was defined as the reciprocal of the light intensity needed to elicit a criterion ERG response (Fig. 2). 1. A circadian rhythm of visual sensitivity change is present in late pupae (one day before eclosion) and in adults; its period is about 22 h (range 18 to 25 h). Sensitivity changes over that period are 103 to 104 or even larger, and the time of peak sensitivity is during the subjective night. 2. The rhythm is entrained by L/D photoperiods (Fig. 3). 3. Animals raised from the egg or maintained as pupae in constant light show the rhythm after being placed in darkness for recording. In such animals, the L/L to D/D transition serves to set the phase of the rhythm (Fig. 5). 4. Pupae maintained in D/D and set up for recording under red light usually do not show a clear-cut rhythm of visual sensitivity change; one exception which showed a weak rhythm may have been influenced by the red light (Fig. 6). 5. Younger pupae (1–5 days before eclosion) show an ultradian rhythm of visual sensitivity change with about four cycles per day. As a pupa nears eclosion, the period of its ultradian rhythm lengthens to the circadian period of about one cycle per day (Figs. 7 and 8). 6. The circadian change in sensitivity is correlated with, and in part is due to, a circadian migration of pigment granules within the secondary pigment cells, lying distal to the retina. After these granules migrate to their most distal ‘darkadapted’ position, the eye continues to increase in sensitivity by a factor of 10 to 100. Furthermore, if the pigment layer and cornea are removed from an animal during the recording period, a sensitivity change of at least 50-fold occurs at the expected point in the cycle. Hence the full circadian change in eye sensitivity may be a result of coupled processes in two sets of adjacent cells — the secondary pigment cells and the visual cells.