Light microscopic autoradiographic localization of somatostatin receptors in the rat brain

The binding parameters and distribution of somatostatin receptors were determined in the rat brain using in vitro light microscopic autoradiography. The proteolysis resistant somatostatin analog (des-Ala1-, Gly2-desamino-Cys3Tyr11-dicarba3,14-somatostatin; CGP 23,996) radiolabeled with 125iodine proved to be suitable for the localization of somatostatin receptors. Slide mounted tissue sections showed that 125I-CGP 23,996 bound to the somatostatin receptor with a mean Kd value of 4.0 nM. The mean density of receptors (maximum binding) was determined to be 182 fmol/mg of protein. Both somatostatin and unlabeled CGP 23,996 displayed high-affinity binding for somatostatin receptors with IC50 values of 6 and 5 nM, respectively. The areas containing the highest densities of receptors are the basal amygdaloid nucleus, medial habenular nucleus, stratum oriens and radiatum of CA1 and CA2, and the subiculum. High receptor density can also be found in the deep layers of the cingulate cortex and in the deep layers of temporal cortex. Moderate densities occur in the caudate-putamen, the granule cell layer of the cerebellum, CA3 area of the hippocampus, the molecular layer of the dentate gyrus and in the substantia nigra. Brain areas with low specific binding include the molecular layer of the cerebellum and the corpus callosum, a white matter area.