Evidence for alpha adrenoceptor modulation of the nociceptive jaw-opening reflex in rats and rabbits.
The jaw-opening reflex (JOR) in anesthetized rats and rabbits was the pain paradigm studied. The JOR was elicited by the electrical intrapulpal (left maxillary) stimulation and quantified by the measurement of threshold values for eliciting electromyograms (dEMGs) from the ipsilateral digastric muscle which served as the experimental nociceptive index. In both species, the threshold for the JOR was significantly elevated by the systemic administration of clonidine (12.5-50 micrograms/kg i.v.) and these JOR thresholds were inversely correlated with the frequency of stimulation. The analgesia elicited by clonidine was antagonized by pre- and postdrug treatment with the alpha-2 adrenoceptor antagonist yohimbine (1 mg/kg i.v.) but not the alpha-1 adrenoceptor antagonist prazosin (1 mg/kg i.v.) or the opiate receptor antagonist naloxone (1 mg/kg i.v.). The lipophilic alpha-1 adrenoceptor agonist St587 (100-400 micrograms/kg i.v.) had no significant effect on dEMG. Yohimbine did not antagonize the increase in dEMG elicited by morphine or pentobarbital. There was no direct correlation between the antinociceptive and cardiovascular effects of clonidine. Our results suggest that in the JOR nociceptive paradigm, clonidine elicits potent analgesia by activation of alpha-2 and not alpha-1 adrenoceptors.
D000698:Analgesia; D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D003782:Dental Pulp / Q000502:physiology; D004576:Electromyography; D007568:Jaw / Q000502:physiology; D008297:Male; D010146:Pain / Q000503:physiopathology*; D011817:Rabbits; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D012018:Reflex / Q000187:drug effects / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology
D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D003000:Clonidine