Substance P-like immunoreactivity in the retina and optic lobe of the squid.

The distribution of substance P-like immunoreactivity within the squid retina and brain was studied by immunofluorescence. Positive immunoreactivity was observed as a single layer of fibres in the retina. The retina was devoid of tyrosine-hydroxylase, serotonin, gamma-aminobutyric acid, cholecystokinin, neuropeptide Y, somatostatin, enkephalin and vasoactive intestinal peptide immunoreactivities. Substance P immunoreactivity was particularly abundant in the optic lobe. The optic lobe had a distinct layer of substance P fibres near the periphery. Immunoreactive cell bodies, fibres and varicosities were additionally present in various areas of the optic lobe. Substance P immunoreactivity in the other ganglia of the brain was restricted to a few scattered fibres.