Augmentation of opioid induced antinociception by the atypical antipsychotic drug risperidone in mice.

Risperidone is a novel atypical neuroleptic with a favorable profile of side effects due to its unique pharmacological activity: it exhibits both potent dopamine D2 and 5-HT2 receptor blocking activity, as well as high affinity for alpha1 and alpha2 adrenergic receptors and histamine H1 receptor. We found that risperidone has a potent antinociceptive effect in the tailflick assay with an ED50 of 26.4 mg/kg. This effect of risperidone was antagonized by naloxone (P < 0.05). This sensitivity to naloxone indicates that at least some of the analgesic effects of risperidone are mediated by an opioid mechanism of action. beta-FNA (mu1 mu2-antagonist), naloxonazine (mu1-antagonist) and norbinaltorphamine (nor-BNI; kappa1-analgesia) reversed risperidone antinociceptive effect (P < 0.05). Naltrindole (delta-antagonist) only partially reversed risperidone antinociceptive effect. We found that the sensitivity of risperidone antinociceptive effect to selective antagonists implies involvement of mu1-, mu2- and kappa1-opioid and to a lesser extent delta-opioid mechanisms. These results suggest a possible role for risperidone both in the management of pain and in the management of opiate withdrawal and detoxification.