Adaptive changes in withdrawal reflexes after noxious stimulation at the heel and the toes in the decerebrated rabbit

In decerebrated rabbits, reflexes evoked by electrical stimulation of the t oes in the ankle flexor tibialis anterior were enhanced for > 30 min after application of 20% mustard oil to the base of the toes, whereas responses o f the ankle extensor medial gastrocnemius to stimulation of the heel were d epressed for > 20 min by the same stimulus. Applied to the heel, mustard oi l had inconsistent effects on the flexor reflex but potentiated the extenso r response for similar to1 h. Intrathecal co-administration of naloxone (25 mug) with the selective alpha (2)-adrenoceptor antagonist RX 821002 (200 m ug) enhanced both reflexes to more than twice pre-drug values and reduced o r abolished all effects of mustard oil. These data confirm that the locatio n of a noxious stimulus is an important determinant of the subsequent adapt ive changes in reflexes, and indicate roles for endogenous opioids and nora drenaline in these processes. (C) 2001 Elsevier Science Ireland Ltd. All ri ghts reserved.