Green light evokes maximum phase shifts in the locomotor activity rhythm of the field mouse Mus booduga

The effect of pulses of different monochromatic light wavelengths was investigated on the circadian locomotor activity rhythm of the tropical rodent Mus booduga. Blue (480 nm), green (549 nm) and red (649 nm) light were used for pulses at circadian time (CT) 14 and 20, the maximum delay and advance eliciting phases of the white light phase response curve (PRC). All three wavelengths evoked qualitatively similar responses in terms of the signs of the phase shifts. Contrary to the results obtained in rabbits and bats, M. Booduga does not exhibit differential responses to different light wavelengths at various phases. This prompts us to speculate on the existence of a single class of photoreceptors for all light wavelengths. The responses of these animals to three different energy levels (5, 25 and 50 microW cm2) of these light wavelengths were also studied and these showed quantitative differences. Green light evoked larger phase shifts in terms of both delays and advances compared with red light (P < 0.1 at both CTs). However, it failed to exhibit an energy-dependent increase in the magnitude of the phase shifts. In contrast, red light evoked an energy-dependent increase in the magnitude of the phase shifts of both CT 14 and CT 20 and blue light at CT 14 alone. It is speculated that the threshold level for perceiving green light may be lower than the energy levels used and hence saturation would have been reached at 5 microW cm-2. For red light, it is possible that this threshold is at a much higher level. This explains the failure to exhibit an energy-dependent increase by green light and the systematic increase by red light.