Abstract
Exposure to novel stimuli has been shown to provoke hypoalgesia. The present experiment was conducted to determine whether this hypoalgesia can be influenced by preadministration of the alpha-2 noradrenergic receptor agonist clonidine and the alpha-2 receptor antagonist yohimbine. One group of male Wistar rats (275–300 g) was exposed to a nonfunctional hot-plate apparatus for 90 sec, once a day for 8 days. A second group was not exposed to the apparatus. Both groups were subsequently assessed for pain sensitivity on the functional (48.5°C) hot plate. Animals not previously exposed to the hot plate displayed significantly longer paw-lick latencies than did exposed animals. Preadministration of clonidine (2 µg/kg i.p.) was found to reverse this hypoalgesia, whereas pretreatment with yohimbine (2 mg/kg i.p.) enhanced the effect. These results suggest the importance of noradrenergic substrates in the mediation of novelty-induced hypoalgesia.
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