Centrifugal neurons of the octopus optic lobe cortex are immunopositive for calcitonin gene-related peptide.

Distribution of calcitonin gene-related peptide (CGRP)-like substance in the optic lobe cortex and retina of the octopus was examined immunohistochemically. Wheat germ agglutinin (WGA), a retrograde-transporting marker, was also used to label the centrifugal neurons. CGRP-immunoreactive (CGRP-IR) somata were seen in the inner granular cell layer, but not in the outer granular cell layer or the retina. CGRP-IR fibers were seen not only in the optic lobe cortex, but also in the retinal nerve plexus. Retrogradely labeled somata were seen in the inner granular cell layer, but not in the outer granular cell layer. Immunohistochemical double staining indicated that WGA-labeled centrifugal neurons were immunopositive for CGRP. These results suggested that the centrifugal neurons in the octopus optic lobe cortex are CGRP-like peptide-containing neurons, and that the peptide may modulate photoreceptor cell functions.