Clonidine and yohimbine modulate the effects of naloxone on novelty-induced hypoalgesia |
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Authors: | Joseph Rochford Patricia Dawes |
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Affiliation: | (1) Department of Psychology, Concordia University, Center for Studies in Behavioral Neurobiology, 1455 deMaisonneuve Boulevard, W., H3G 1M8 Montreal, Quebec, Canada;(2) Douglas Hospital Research Center, 6875 Boulevard LaSalle, H4H 1R3 Verdun, Quebec, Canada |
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Abstract: | Previous research has shown that repeated daily pretreatment with the opiate receptor blocker naloxone retards the development of habituation to novelty-induced hypoalgesia. The present experiments were conducted in order to determine whether noradrenergic substrates mediate this effect. Animals in the NAL condition were administered 10 mg/kg naloxone prior to assessment of pain sensitivity on a 48.5° C hot plate. Control animals (SAL condition) were administered saline prior to pain assessment, and naloxone 2–4 h later. Paw lick latencies declined over repeated tests in SAL animals, suggesting the habituation of novelty hypoalgesia. Naloxone pretreatment attenuated this decline. The longer paw lick latencies observed in NAL condition animals were reduced by administration of 2 µg/kg clonidine, a specific noradrenergic alpha-2 receptor agonist, and enhanced in a dose dependent (0.5–4.0 mg/kg) fashion by the alpha-2 antagonist yohimbine. Clonidine and yohimbine either failed to alter pain reactivity in control animals, or produced less marked effects than those observed in naloxone-exposed animals. These results suggest that noradrenergic substrates mediate naloxone's effects on novelty hypoalgesia. |
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Keywords: | Novelty hypoalgesia Noradrenaline Naloxone Alpha-2 receptor Rat |
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