CLONIDINE HAS COMPARABLE EFFECTS ON SPONTANEOUS SYMPATHETIC ACTIVITY AND AFFERENT A-DELTA-MEDIATED AND C-FIBER-MEDIATED SOMATOSYMPATHETIC REFLEXES IN DOGS

Background: Clonidine, an alpha(2)-adrenergic agonist, has been studie d as an adjunct or alternative to spinal opioids in the management of moderate to severe pain. This study examined the relative effects of c lonidine on efferent spontaneous sympathetic activity and afferent A d elta- and C fiber-mediated somatosympathetic responses. Methods: Spont aneous and evoked sympathetic activity in renal sympathetic nerves, me diated by A delta and C fibers by means of supramaximal electrical sti mulation of the radial and tibial nerves, were observed in anesthetize d dogs. Incremental doses of clonidine were administered intrathecally or intravenously in each of five preparations followed by intravenous naloxone 2 mg and yohimbine 5 mg. Results: Both spontaneous sympathet ic outflow and afferent A delta- and C fiber-mediated somatosympatheti c responses evoked by tibial nerve stimulation were depressed in a sim ilar dose dependent manner by clonidine administered intrathecally or intravenously in a dose ratio of approximately 1:4. Intrathecal clonid ine inhibited and eliminated both local spontaneous sympathetic outflo w and tibial nerve evoked sympathetic responses but had no significant depressant effect on the radial nerve evoked sympathetic reflexes. Wh en administered intravenously clonidine had a similar depressant effec t on both radial and tibial nerve elicited reflexes and spontaneous sy mpathetic activity. Conclusions Clonidine, administered intrathecally or intravenously, has a similar depressant effect on both spontaneous sympathetic outflow and afferent A delta- and C fiber-mediated somatos ympathetic reflexes. When administered intrathecally it has little eff ect on reflexes evoked via the descending pathway by radial nerve stim ulation.