SELECTIVE POTENTIATION OF NMDA-INDUCED ACTIVITY AND RELEASE OF EXCITATORY AMINO-ACIDS BY DYNORPHIN - POSSIBLE ROLES IN PARALYSIS AND NEUROTOXICITY

Selective antagonists of N-methyl-D-aspartate (NMDA) excitatory amino acid (EAA) receptors have been shown to protect against dynorphin-A (DYN)-induced paralysis and neurotoxicity in the spinal cord. To test the hypothesis that either DYN-induced paralysis or neurotoxicity involves an enhanced release of EAAs, we used microdialysis to monitor aspartate (Asp) and glutamate (Glu) release in both the lumbar spinal cord extracellular fluid (ECF) and the spinal cord cerebral spinal fluid (CSF) of conscious rats in response to DYN (1-13). Injection of 5 nmol of DYN produced temporary paralysis in 8 of 10 animals, but did not significantly change Asp or Glu release in either the ECF or the CSF. Injection of 20 nmol of DYN caused permanent paralysis and neuronal cell loss in all animals tested as well as a significant increase of Asp and Glu in both the ECF and the CSF, and a decrease in glutamine (Gln) release only in the ECF. Pretreatment with 1 mg/kg of the NMDA antagonist MK-801 blocked both paralysis and amino acid changes in the ECF. Pretreatment of animals with 5 mg/kg naloxone inhibited glutamate release in the ECF, but did not block paralysis, Asp release or inhibition of Gln release. As MK-801 sensitive paralysis by DYN was not mediated through enhanced EAA release, we examined whether DYN could act through postsynaptic facilitation of NMDA receptors by testing the ability of DYN to alter the magnitude of a behavioral response produced by intrahecal injection of NMDA in mice. At a dose that produces no behavioral effects alone, DYN reversed the behavioral desensitization seen following repeated injections of NMDA. DYN did not enhance the behavior produced by single or multiple injections of kainic acid or substance P. This effect was not blocked by naloxone, but was inhibited by nalmefene. These results support the hypothesis that DYN-induced paralysis does not involve enhanced EAA release, but could (as it does with NMDA-induced behavior in mice) produce facilitation of NMDA receptor activity and reversal of desensitization. At higher doses, DYN does produce an MK-801-sensitive release of Asp which may contribute to the progression from paralysis to neurotoxicity.