Intracerebroventricular physostigmine-induced analgesia: enhancement by naloxone, beta-funaltrexamine and nor-binaltorphimine and antagonism by dynorphin A (1-17)

JM Fujimoto and JJ Rady

Research Service, Clement Zablocki Veterans Administration Medical Center, Wisconsin, Milwaukee.

The antinociceptive action (tail-flick test) of physostigmine given i.c.v. to mice was enhanced by the administration intrathecally (i.t.) of narcotic antagonists. Doses, i.t., as low as 0.1 fentog of naloxone, 0.25 ng of beta-funaltrexamine and 0.1 ng of nor-binaltorphimine enhanced physostigmine, 2 micrograms i.c.v., analgesia. These doses of opioid antagonists did not inhibit spinal mu receptors or kappa receptor agonist-induced analgesia as assessed by absence of effect on Tyr-D-Ala2-N-MePhe4-Gly-ol5 or U50,488H i.t. analgesia. Enhancing effects of the opioid antagonists were interpreted to indicate that i.c.v. physostigmine-induced analgesia was mediated spinally by an endogenous opioid which had an antagonistic effect. This putative opioid antagonist was postulated to be dynorphin A (1-17). Thus, i.t. administration of small doses of less than 10 pg of dynorphin was shown to antagonize the analgesic action of physostigmine, i.c.v. Furthermore, this effect of dynorphin was attenuated by the doses of naloxone, beta-funaltrexamine or nor-binaltorphimine which were effective in enhancing physostigmine-induced analgesia. We concluded that physostigmine given i.c.v. had two actions, the first produced analgesia and the second activated a system which had an antianalgesic effect. Evidence indicated that the latter effect was mediated by dynorphin A (1-17). This concept of dynorphin action may be the basis for some of the unusual findings of the analgesic action of naloxone in other situations and support the concept for a descending dynorphin A (1-17)-mediated antianalgesic system.

Volume 251, Issue 3, pp. 1045-1052, 12/01/1989
Copyright © 1989 by American Society for Pharmacology and Experimental Therapeutics

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