Background: Opioids appear to produce their physiologic effects by binding to at least three types of opioid receptors, the mu (mu), delta (delta), and kappa (kappa) receptors. Muscle rigidity occurs after administration of supra-analgesic doses of potent mu-preferring agonists like alfentanil. The role of different supraspinal opioid receptors in this rigidity has been addressed only recently. To elucidate the contribution of central mu, delta, and kappa receptors to muscle rigidity, the effects of intracerebroventricularly administered opioid receptor-selective agonists and antagonists on alfentanil-induced muscle rigidity were examined in rats.
Methods: Rats in which chronic intracerebroventricular cannulae had been implanted received an intracerebroventricular infusion of either saline or a mu (D-Ala2,N-Me-Phe4-Gly5-olenkephalin; DAMGO), delta(1) (D-Pen2,D-Pen5-enkephalin; DPDPE), or kappa(1) (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl)- cyclohexyl)-benzene-acetamide methane sulfonate; U50,488H) opioid agonist. Ten minutes later, they received either saline or the mu-agonist alfentanil subcutaneously. Muscle rigidity was assessed using hindlimb electromyographic activity. Different groups of animals were pretreated with an intracerebroventricular infusion of either saline or a mu (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2; CTAP), delta (naltrindole), or kappa(1) (norbinaltorphimine) opioid antagonist before administration of either saline or a selective intracerebroventricular agonist.
Results: The mu agonist DAMGO alone dose-dependently induced muscle rigidity. This effect was antagonized by pretreatment with the mu-selective antagonist CTAP. Neither DPDPE nor U50,488H, when administered alone, affected muscle tone. However, both the delta(1) and kappa(1) agonists dose-dependently attenuated alfentanil-induced rigidity. This antagonism of alfentanil rigidity was abolished after pretreatment with the delta (naltrindole) and kappa(1) (nor-binaltorphimine) antagonists, respectively.
Conclusions: The present data demonstrate that whereas systemic opiate-induced muscle rigidity is primarily due to the activation of central mu receptors, supraspinal delta(1) and kappa(1) receptors may attenuate this effect. This finding is consistent with previous demonstrations of functional interactions between different central opioid receptor populations in other opiate effects, and could have important pharmacotherapeutic implications.