Endogenous opioids support the spinal inhibitory action of an alpha 2-adrenoceptor agonist in the decerebrated, spinalised rabbit.
The present study examined the possible contribution of endogenous opioids to inhibition of spinal reflexes by an alpha(2)-adrenoceptor agonist. In rabbits decerebrated and spinalised under halothane/nitrous oxide anaesthesia, the selective alpha(2)-adrenoceptor agonist dexmedetomidine (3-30 microg intrathecal) induced significant decreases in short- and long-latency reflex responses evoked in medial gastrocnemius (MG) motoneurones by stimulation of the sural nerve. After recovery from dexmedetomidine, the mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA; 100 microg intrathecal) significantly enhanced short-latency but not long-latency MG reflex responses. After beta-FNA, inhibition of all reflexes by dexmedetomidine was significantly weaker than in the control state, whereas the cardiovascular actions of dexmedetomidine were unaffected. These data confirm that activation of spinal alpha(2)-adrenoceptors depresses MG reflexes evoked by all groups of sural nerve afferent fibres, and shows that endogenous opioid tone supports the inhibitory action of alpha(2) agonists, possibly by a synergistic interaction in the spinal cord.
Lo
W Caroline
WC
Division of Animal Physiology, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, LE12 5RD, UK.
Harris
John
J
Clarke
Rob W
RW
D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology* / Q000627:therapeutic use; D000818:Animals; D003655:Decerebrate State / Q000188:drug therapy* / Q000503:physiopathology; D020927:Dexmedetomidine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D008297:Male; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology; D018847:Opioid Peptides / Q000494:pharmacology* / Q000502:physiology / Q000627:therapeutic use; D011817:Rabbits; D018341:Receptors, Adrenergic, alpha-2 / Q000502:physiology; D012026:Reflex, Stretch / Q000187:drug effects / Q000502:physiology; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology
D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists; D018847:Opioid Peptides; D018341:Receptors, Adrenergic, alpha-2; D020927:Dexmedetomidine