Circadian rhythms in the Limulus visual system.

A circadian clock in the Limulus brain generates efferent optic nerve activity at night. The endogenous activity begins near dusk, continues during the night, and stops near dawn. Approximately 10 to 20 efferent fibers in each lateral optic nerve trunk fire in close synchrony with one another and with the efferent fibers in the opposite nerve trunk producing bursts of nerve impulses at night. The synchronous bursting activity indicates extensive coupling in the brain among the efferent neurons or among the circadian pacemakers that drive them. The efferent optic nerve activity mediates circadian rhythms in retinal responses. Electroretinograms (ERGs) from both the lateral eyes and median ocelli exhibit circadian rhythms of equal phase. Sectioning the optic nerves abolishes the rhythms. When the animal is kept in darkness, the phase of the rhythm can be shifted by illuminating the lateral eyes but not the median ocelli. The endogenous rhythm persists for at least 1 year in darkness without attenuation and without substantial changes in the circadian period. Across a large population of animals the duration of the circadian period ranges from 22.2 to 25.5 hr, with a mean value of 23.9 +/- 0.7 hr (n = 75). The nighttime increase in ERG amplitude represents a 20- to 100-fold increase in retinal sensitivity. Most of the increased sensitivity results from an increase in the number of photons absorbed by the photoreceptors at night. A small component may be due to an increase in gain. Pulses of current delivered to the lateral optic nerve during the day mimic the effects of endogenous efferent activity and transform the eye to the nighttime state. The Limulus visual system has evolved numerous mechanisms for adapting visual sensitivity to daily fluctuations in the photic environment. A key to understanding the organization of the visual system is the circadian clock and its associated neural circuitry.