Peptides Derived from the Third Cytoplasmic Loop of the Serotonin Subtype 1B Receptor Selectively Inhibit Transmission of Serotoninergic Signals via Their Homologous Receptors

Abstract

A leading role in the interactions of most serotonin-type hormone receptors with heterotrimeric G proteins is played by their third cytoplasmic loops. Studies in recent years have shown that synthetic peptides corresponding to the membrane-proximal parts of these loops may have selective influences on the transmission of hormone signals via their homologous receptors and trigger the signal cascade in the absence of hormone. We report the first synthesis of peptides derived from the C-terminal region of the third cytoplasmic loop of type 1B serotonin receptors, along with studies of their influences on the serotonin sensitivity of the adenylate cyclase system in the rat brain. Peptides 300–316 and 306–316 (numbered from amino acid positions in the rat serotonin subtype 1B receptor molecule) at micromolar concentrations, in the absence of hormone, stimulated GTP-binding Gi-proteins coupled with subtype 1B serotonin receptors and inhibited forskolin-stimulated adenylate cyclase activity. Studies using selective serotonin receptor agonists and antagonists showed that peptides 300–316 and 306–316 inhibit the transmission of the serotonin signal via the homologous receptor but have weak effects on other types of serotonin receptor. Peptide 300–316 were markedly more active than its shortened analog 306–316 in terms of the selectivity and efficacy of actions on the adenylate cyclase signaling system, which is regulated via subtype 1B serotonin receptors. These data provide evidence that region 300–316 of subtype 1B serotonin receptors is involved in the interaction with Gi proteins and includes the main molecular determinants responsible for transmission of the serotonin signal to adenylate cyclase.