Antiserum to an eye-specific protein identifies photoreceptor and circadian pacemaker neuron projections in Aplysia

The marine gastropod Aplysia has a circadian clock in each eye that generates a circadian rhythm of optic nerve activity. The axons of pacemaker neurons carry the rhythmic activity to the brain where it can be recorded from various ganglionic connectives as it is distributed throughout the CNS. We had previously identified an eye-specific 48-kD protein using an antiserum, anti-S, that recognizes the period gene product of Drosophila. We have now obtained two partial amino acid sequences of the 48-kD protein and raised a polyclonal antiserum using a synthetic peptide with the amino acid sequence of one of them. The antiserum recognizes a family of spots of M(r) 47-48 kD and Pi 5.9-6.0 on 2D immunoblots of eye proteins. The immunoblot staining intensity does not exhibit a circadian rhythm. Used in immunocytochemistry, the antiserum recognizes fibers in the optic nerve and retinal neuropil, pacemaker neurons, certain photoreceptors, and the photoreceptor rhabdom layer. It stains the optic nerve fibers and optic fiber terminals in the cerebral optic ganglion and recognizes the cerebral optic tracts, putative synaptic exchange areas, and optic tract projections from the cerebral ganglion into various head nerves and interganglionic connectives. The function of the 48-kD protein is not known but it could be involved in the maintenance or regulation of the retinal afferent pathways, including the pacemaker neuron axons, known from previous axonal transport and electrical recording studies to be the circadian output pathway.