Altered opioid-mediated control of the spinal release of dynorphin and met-enkephalin in polyarthritic rats

Previous studies showed that spinal opioidergic neurotransmission is marked ly altered in the polyarthritic rat, a model of chronic inflammatory pain. Present investigations aimed at assessing possible changes in opioid-mediat ed control of the spinal outflow of met-enkephalin (ME) and dynorphin (DYN) in these animals. Intrathecal (i.t.) perfusion under halothane anesthesia showed that polyarthritis was associated with both a 40% decrease in the sp inal outflow of ME-like material (MELM) and a 90% increase in that of DYNLM . Local treatment with the mu-opioid agonist DAGO (10 mu M i.t.) inhibited equally (-30%) the MELM outflow in polyarthritic and control rats, whereas the delta agonist DTLET (10 mu M i.t.) also reduced the peptide outflow in controls (-27%) but enhanced it in polyarthritic animals (+56%). On the oth er hand, both DAGO (10 mu M i.t.) and DTLET (10 mu M i.t.) decreased (-40 a nd -49%) DYNLM outflow in polyarthritic rats, but were inactive in controls . Finally, neither MELM outflow nor that of DYNLM were affected by the kapp a-agonist U50488H (10 mu M i.t.) in both groups of rats. In all cases, the changes due to active agonists could be prevented by specific antagonists w hich were inactive on their own except the kappa antagonist nor-binaltorphi mine (10 mu M i.t.) that decreased (-38%) DYNLM outflow in polyarthritic ra ts. These data indicate that functional changes in spinal opioid receptors may promote enkephalinergic neurotransmission and reduce dynorphinergic neu rotransmission in polyarthritic rats, thereby contributing to the analgesic efficacy of opioids in inflammatory pain, (C) 2000 Wiley-Liss, Inc.