The role of protocerebrum in the modulation of circadian rhythmicity in the crayfish visual system

Dark-adapted crayfishes with protocerebrum only, were submitted to continuous recordings of electroretinogram (ERG) and of eye glow area (EGA) during several days. Circadian variations of ERG amplitude similar to that of intact animals, were revealed by means of restrained test light stimuli (0.2 Cd/ft2) bilaterally applied to each eyestalk. The period (24.6–38 hr) and range (40–80%) value of ERG oscillations always resulted quite similar to one another side. As in intact animals retinal shielding pigments (RSP) position as measured as EGA size showed a clear circadian rhythm, and also a clear consensual reflex in these preparations. We found a loss of both: circadian and consensual mobilization of distal RSP in animals with complete removal of cerebral ganglion. Our proposition is that the crayfish protocerebrum plays a major role in the modulation of circadian retinal sensitivity, probably through the control-release of hormonal neurosecretions from the sinus gland along the day.