Somatostatin receptors.

The neuropeptide somatostatin (SRIF) is a neurotransmitter in the brain that exerts physiological actions including the modulation of Ca2+ and K+ conductances, neuronal cell firing, neurotransmitter release, and certain behaviors such as locomotion and cognitive functions. SRIF induces its biological effects by interacting with cell surface receptors. Recent studies have revealed that subtypes of SRIF receptors exist in the brain and other tissues. The SRIF1 receptor can be distinguished by its high affinity for the agonist MK 678, is coupled to G proteins, and mediates the stimulatory effects of SRIF on a delayed rectifier K+ current in brain neurons. Furthermore, MK 678, when applied to the nucleus accumbens, evokes locomotor activity, and SRIF1 receptors in this brain region selectively mediate the stimulation of this behavioral response to SRIF. SRIF1 receptors are unevenly distributed in the brain, with high levels in the dentate gyrus of the hippocampus, the locus coeruleus, the neostriatum, and the inner layers of the cerebral cortex. This receptor subtype has characteristics similar to the recently cloned SRIF receptor, SSTR2. A second SRIF receptor subtype has been identified in the brain and is referred to as the SRIF2 receptor. It has no affinity for MK 678, can be selectively labeled with smaller structural analogs of the peptide CGP 23996, and has characteristics similar to the recently cloned receptor subtype SSTR1. SRIF2 receptors are not efficiently coupled to G proteins and have a distinct but overlapping distribution in brain with SRIF1 receptors. No clear biological function has been identified for SRIF2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)