THE DISTRIBUTION OF SOMATOSTATIN BINDING-SITES IN THE BRAIN OF GYMNOTIFORM FISH, APTERONOTUS-LEPTORHYNCHUS

The neuropeptide somatostatin (SS) and its binding sites display a wid e distribution in the central nervous system of vertebrates. By employ ing semi-quantitative autoradiography, we identified such binding site s in the brain of the weakly electric fish Apteronotus leptorhynchus ( Gymnotiformes, Teleostei). Whereas (SS1) binding sites for the octapep tide analogue Tyr(3)-SMS-201-995 appear to be absent in the gymnotifor m brain, (SS2) binding sites for the analogue [Tyr(0)-D-Trp(8)]-somato statin-14 were found in many brain regions and showed a similar distri bution to that observed by other authors in the amphibian and mammalia n central nervous system. Telencephalon While binding in the ventral t elencephalon was typically low, all cell groups of the dorsal portion displayed a high degree of binding. The highest density of binding sit es was found in the dorsal and caudal subdivision 2 of the dorsomedial telencephalon. Diencephalon Many cell groups of the diencephalon show ed a medium to high degree of binding density. The highest level was s een in the habenula. Mesencephalon All layers of the optic tectum cont ained a medium number of binding sites; except the stratum marginale. In the torus semicircularis, the different layers displayed distinct b inding density. While laminae 7-8 showed the highest degree of binding , the lowest density was found in lamina 6. Rhombencephalon Binding wa s generally low or absent in the tegmentum. Low levels of binding dens ity were observed in the electrosensory lateral line lobe. Cerebellum Extremely high levels of binding were found in the eminentia granulari s medialis and the eminentia granularis posterior. Throughout most reg ions of the brain, the relative density of binding sites and the relat ive amount of somatostatin immunoreactivity in fibres, as determined i n previous studies, were in good agreement.