Localization and characterization of brain somatostatin receptors as studied with somatostatin-14 and somatostatin-28 receptor radioautography.

The localization and characterization of receptors for somatostatin-14 (S-14) and somatostatin-28 (S-28) were studied in the rat brain using the iodinated agonists [Tyr0,D-Trp8]S-14 and [Leu8,D-Trp22,Tyr25]S-28 as tracers. Slide-mounted frozen sections were used for the radioautographic localization and biochemical characterization of somatostatin receptors. In the latter case counting was performed on scraped off serial sections from rostral regions of the brain. Specificity studies demonstrated that either tracer could be displaced with S-28, S-14 or their agonists. The N-terminus fragment (1-12) of S-28 as well as a number of unrelated peptides were unable to compete with either tracer, indicating that the binding capacity for ligand-receptor recognition is located in the C-terminal portion of S-28. Scatchard analysis of saturation curves gave a one-site interaction with Kd values of 0.42 +/- 0.09 nM and 0.32 +/- 0.04 nM for the S-14 and S-28 iodinated agonists, respectively. By radioautography, the distribution of receptors for both S-14 and S-28 appeared very similar with high levels of binding in the deep layers of the cortex, the cingulate cortex, the claustrum, the locus coeruleus and most structures of the limbic system. Treatment with cysteamine, which caused a somatostatin depletion in the brain, was required to observe labeling in the hypothalamus. In some caudal areas of the brain, especially in the cerebellar nuclei, the solitary tract nucleus and the nucleus of the vagus nerve, only labeling with the S-28 agonist could be detected. This S-28 binding could be displaced by native S-14 (10(-6) M). Generally, there was a correlation between the localization of somatostatin receptors and that of immunoreactive somatostatin, as evaluated by immunocytochemistry. However, in some areas, an inverse correlation between receptor and peptide concentrations was observed. These results are in agreement with previous data suggesting that somatostatin could act as a neurotransmitter or neuromodulator in several brain areas.