Somatostatin cyclic octapeptide analogs which preferentially bind to SOMa receptors block a calcium current in rat superior cervical ganglion neurons.

To characterize further the somastatin (SOM) receptor mediating Ca2+ current reduction in rat superior cervical ganglion (SCG) neurons, the effects of three synthetic SOM octapeptide analogs, D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2 (IM-4-82), D-Nal-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2 (DC 13-116), and D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-OL (SMS 201-995), which bind preferentially to pituitary SOM receptors (SOMa) were investigated. Ca2+ currents were recorded using the whole-cell variant of the patch-clamp technique from neurons isolated enzymatically from adult rat SCG. Application of the SOM analogs (0.003-3 microM) produced a rapid, reversible, and concentration-dependent decrease in Ca2+ current amplitude in addition to slowing the rising phase of the Ca2+ current. Estimates of the concentration producing half-maximal block (EC50) and maximum attainable block (Bmax) for DC 13-116, IM 4-28, and SMS 201-995 were 196, 67, and 9.5 nM, respectively, and 52, 57, and 48%, respectively. The results suggest that the SOM receptor on SCG neurons more closely resembles the SOMa receptor of the anterior pituitary than the SOMb receptor of cerebral cortical membranes.