Functional opiate receptors in the guinea-pig ileum: their differentiation by means of selective tolerance...

A differentiation of multiple opiate receptors in the electrically stimulated longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum is described employing the technique of tolerance development of selective opiate receptors. Apparently, mu- and kappa-opiate receptors are clearly differentiated in this tissue, since highly tolerant mu-receptors (80-fold) coexist with kappa-receptors of almost normal sensitivity and vice versa. Attempts to demonstrate delta-receptors failed, as there was complete cross-tolerance between mu- and delta-agonists. It is proposed that delta-agonists bring about their action via mu-receptors in this preparation. Although mu- and kappa-receptors have been demonstrated, neither of them appear to represent a uniform population.