Sigma receptors regulate contractions of the guinea pig ileum longitudinal muscle/myenteric plexus preparation elicited by both electrical stimulation and exogenous serotonin

Abstract

Sigma receptors are specific, highly localized binding sites in limbic and sensorimotor structures of the brain that interact with many psychotropic drugs. These agents include the psychotomimetic benzomorphan opiates, the psychotomimetic drug phencyclidine and its analogs, as well as numerous typical and atypical antipsychotics such as haloperidol, chlorpromazine, and the novel drugs BMY 14802 and rimcazole. So far, no physiological function has been assigned to these binding sites. We have synthesized a number of novel sigma receptor-active drugs derived from the selective sigma ligand N,N'-di(o-tolyl)guanidine (DTG). DTG and its congeners were found to inhibit contractions of the guinea pig ileal longitudinal muscle/myenteric plexus (LMMP) preparation evoked by electrical stimulation. In addition, the sigma ligands noncompetitively antagonized contractions of the LMMP preparation evoked by serotonin (5-HT). The 5-HT-evoked contractions were found to be largely due to 5-HT's activation of 5-HT3 receptors to release ACh. The activity of DTG congeners in inhibiting electrically or 5-HT-evoked contractions of the LMMP highly correlated with their potency to inhibit binding of both 3H-DTG and (+)3H-3-PPP [3(3-OH-phenyl)-N-(1-propyl)piperidine] to sigma receptors in guinea pig brain homogenates. Two DTG congeners that did not bind to sigma receptors also showed no activity in the bioassay. Many other (but not all) sigma receptor ligands showed a high correlation between their potency to inhibit electrically evoked contractions of the LMMP and their sigma receptor binding affinity. The benzomorphans (+)SKF 10,047 and (+)cyclazocine potentiated electrically evoked contractions of the LMMP. Sigma ligands also inhibited the contractions of the LMMP in the presence of the opiate antagonist naloxone and in preparations in which opioid receptors had been inactivated by treatment with the irreversible opiate antagonist beta-chlornaltrexamine. Control experiments suggested that the sigma ligands act via a neuronal mechanism to inhibit ACh release evoked by electrical stimulation or by stimulation with 5-HT. These results suggest that there are functional sigma receptors on cholinergic nerve terminals or within the myenteric plexus and that these receptors can inhibit stimulated ACh release through an opioid receptor-independent mechanism. However, sigma receptor activation in the ileum has the same effect on ACh release as activation of naloxone-sensitive opioid receptors. The LMMP may be an in vitro bioassay system for characterizing the mechanism of action of sigma receptors and for determining the biological efficacy of drugs known to bind to sigma receptors in radioligand binding assays.