[Opioid receptor types and dependence].

Abstract

The existence of mu, delta and kappa opioid receptors in the central nervous system is well documented. The present review focuses on the relationships between opioid receptor types and physical and psychic dependences. Mu and delta, but not kappa opioid receptor agonists produce physical dependence. From behavioral, biochemical and molecular biological studies, it is suggested so far that development of physical dependence on morphine results predominantly from an activation of mu 1 and mu 2 opioid receptors which causes functional changes in Gi/o, adenylate cyclase, protein kinases A and C, beta-adrenoceptor and NMDA receptor in the locus coeruleus. Recently, there have been significant advances in studies on psychic dependence. Mu and delta opioid receptor agonists produce psychic dependence, but kappa opioid receptor agonists rather produce an aversive effect. Activation of the mesolimbic dopamine system may lead to psychic dependence on opioids. Mu and delta 1 opioid receptor agonists activate the mesolimbic dopamine system to induce a rewarding effect, whereas the rewarding effect of delta 2 opioid receptor agonists may be produced through a non-dopaminergic system. There are complicated interactions among opioid receptor types. The activation of kappa opioid receptor suppresses physical and psychic dependences on mu and delta opioid receptor agonists, but the activation of delta opioid receptor potentiates the dependence on mu opioid receptor agonists. The clinical use of morphine in patients with cancer pain won't develop dependence probably due to the balance of the opioid system coming from these interactions.