The Peptides Mimicking the Third Intracellular Loop of 5-Hydroxytryptamine Receptors of the Types 1B and 6 Selectively Activate G Proteins and Receptor-Specifically Inhibit Serotonin Signaling via the Adenylyl Cyclase System

The third intracellular loop (ICL3) of G protein-coupled receptors has, as a rule, a key role in their interaction with heterotrimeric G proteins. We synthesized peptides corresponding to the C-terminal region of the ICL3 (C-ICL3) of 5-hydroxytryptamine receptors of the type 1B (5-HT 1B R) and 6 (5-HT 6 R) and studied their influence on the functional activity of adenylyl cyclase signaling system (ACSS) in synaptosomal membranes isolated from the rat brain. The 5-HT 1B R-peptide ARERKATKTL 307–316 K-amide mimicking agonist-activated 5-HT 1B R reduced forskolin-stimulated adenylyl cyclase (AC) activity and activated pertussis toxin-sensitive G proteins. It lowered inhibitory effects of serotonin and 5-HT 1B R-agonists on forskolin-stimulated AC activity and their stimulating effects on GTP binding. This was not the case in the presence of 5-HT 1B R-antagonists. The 5-HT 6 R-peptides mimicking 5-HT 6 R activated both the basal AC activity and GTP binding of cholera toxin-sensitive G proteins. They lowered the stimulating effect of serotonin and 5-HT 6 R-agonists on AC and G s proteins, but in the presence of 5-HT 6 R-antagonists their action was blocked. Of all the 5-HT 6 R-peptides with linear and dimeric structure we studied the palmitoylated peptide KHSRKALKASL 258–268 K(Pal)A-amide had a most pronounced effect both on the basal and 5-HT 6 R-agonist-stimulated ACSS. The data was obtained indicating that the peptides corresponding to C-ICL3 of 5-HT 1B R and 5-HT 6 R selectively activate G i and G s proteins, respectively, and in a receptor-specific manner reduce signal transduction via serotonin-sensitive ACSS in the rat brain. The results of the study give strong evidence in favor of active participation of C-ICL3 of these 5-HTRs in their coupling with the G proteins.