Distribution of immunoreactive somatostatin (ISRIF) in the nervous system of the squid, Loligo pealei

Abstract Somatostatin (SRIF) is a neuropeptide with a ‐widespread distribution in the mammalian CNS. In the present study we have examined the distribution of immunoreactive‐like SRIF (ISRIF)‐containing elements in the nervous system of the cephalopod mollusk Loligo pealei or the Woods Hole squid. ISRIF was localized by light immunocytochemistry in sections of the squid—optic lobe, circumesophageal ganglia—and in stellate ganglion. In the optic lobe, ISRIF neurons were found in the internal granule cell layer and medulla and immunoreactive fibers were seen throughout the lobe and in the optic tract but were absent from the optic nerve, i.e., the projection between the retina and optic lobe. In the supraesophageal complex, ISRIF neurons were found in all lobes, but primarily in the vertical, subvertical, and frontal. In the subesophageal ganglion, ISRIF neurons were seen mainly following unilateral pallial nerve lesions; these neurons were primarily small‐to‐medium sized. ISRIF fibers were seen in many of the nerves exiting from the brain and in nerves extending between the sub‐ and supra‐esophageal ganglia. In the stellate ganglion, ISRIF was present in many neurons as well as in a plexus of fibers within the ganglion; the peptide was absent from the second‐order fibers and the giant axon. The data suggest that a molecule immunologically similar to vertebrate SRIF may be a major transmitter/modulator in this invertebrate. These results provide a foundation for further studies to evaluate the role of this molecule.