PMID: 7932169Sep 1, 1994Paper

Endogenous opioids released by suspending mice by the tail selectively enhance spinal mu opioid analgesia

The Journal of Pharmacology and Experimental Therapeutics
F L SmithW L Dewey

Abstract

In studying the interactions between handling mice and their subsequent analgesic response to an intrathecally (i.t.) administered mu-opioid agonist, DAMGO, it was found that suspending ICR mice by the tail for 1, 5, or 20 sec, 10 min before the tail-flick test, enhanced DAMGO by 5.3-, 7.4- and 23.6-fold, respectively, compared with mice maintained in a level posture. This enhancement was not accompanied by a change in the rostral flow of [3H]-DAMGO (25 ng, i.t.) to the brain (3.7% in 10 min), in its distribution along the neuraxis or in its systemic absorption. However, i.c.v. administration of beta-endorphin (1-27), an antagonist of epsilon opioid receptors, abolished the enhancement of i.t. DAMGO without affecting its basal analgesic potency. Pretreatment with the delta-opioid antagonist naltrindole (5.6 nmol, i.t.,-30 min) also blocked the enhancement of DAMGO without significantly affecting its basal analgesic potency. Alternatively, this same dose of naltrindole injected i.c.v. failed to block the enhancement of DAMGO in suspended mice. A 20-sec suspension failed to enhance i.t. kappa and delta-agonists, but it did enhance i.t. morphine. In mouse strain comparisons, i.t. DAMGO was more potent in C57BL/6J and DBA/2J mice t...Continue Reading

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