The circadian pacemaker in the Aplysia eye sends axons throughout the central nervous system.

Each eye of Aplysia contains a population of electrically coupled pacemaker neurons whose synchronous activity can be recorded from the optic nerve as a compound action potential (CAP). The CAP frequency continues to show a circadian rhythm even when the eye is isolated from the animal and maintained in constant conditions, and thus it contains an autonomous circadian pacemaker, which may reside in the pacemaker neurons. The pacemaker neurons, along with retinal photoreceptors, send axons out of the optic nerve, which connects to the cerebral ganglion of the central nervous system (CNS). Pacemaker neurons, but not photoreceptors, may contain an aminergic transmitter, possibly dopamine (DA). We describe the central projections of optic nerve fibers using horseradish peroxidase filling of the cut optic nerve, and transport of radiolabeled macromolecules after selective exposure of the eye to [3H] leucine, which labels both pacemaker neurons and photoreceptors. We were able to determine the projections of pacemaker axons by exposing the eye to [3H]-3,4-dihydroxyphenylalanine [( 3H] DOPA and [3H]DA, which is preferentially taken up and transported by the pacemaker neurons. Pacemaker axons project bilaterally to the cerebral, pedal, and pleural ganglia and may extend as far as the abdominal ganglion. We corroborate this anatomical evidence by recording an orthodromic CAP in the optic nerve that had originated in the eye and subsequently recording the CAP in the CNS connectives and nerves that contained [3H]DOPA-labeled fibers. These results suggest that circadian pacemaker information from the eye is widely distributed throughout the CNS, including neural structures known from studies by others to mediate circadian-regulated behaviors, such as locomotion. Thus, Aplysia can now be used as a model system to examine the influence of the central projections of an identified circadian pacemaker on behavior, such as locomotion, at the level of identified central neurons.