Rat brain membranes possess two high-affinity binding sites for [ 3H]somatostatin

This report described the first use of [4- 3H-Phe 6]somatostatin-14 to characterize binding sites on rat brain membranes for somatostain-14. This ligand is superior to previously used iodinated analogues and is chemically and biological identical to the natural ligand. Two high-affinity binding sites were found, from Scatchard analysis of competitive displacement experiments, with K d SS 1 = 0.41 and K d SS 2 = 22.9 nM. Specific binding was reversible, and kinetic analysis of the dissociation and association time-course gave an apparent K d of 0.44 nM, in good agreement with the K d of the higher-affinity site. Specific binding of the ligand was enriched in cerebral cortex and hippocampus, with intermediate levels in the striatum, hypothalamus and midbrain, and low levels in the pons/medulla and cerebellum. This ligand should prove to be valuable for elucidating the physiological and pharmacological significance of the two subtypes of somatostatin binding sites we have demonstrated.