EFFECTS OF INTRATHECAL AND INTRACEREBROVENTRICULAR BUPRENORPHINE ON AC-FIBER REFLEX IN THE RAT

A C-fiber reflex elicited by electrical stimulation within the territo ry of the ipsilateral sural nerve, was recorded from the biceps femori s muscle in anesthetized rats. The temporal evolution of the reflex wa s studied using a constant level of stimulus intensity (3 x threshold) and recruitment curves were built by varying stimulus intensity from 0 to 7 x threshold. Intrathecal doses of buprenorphine in the 0.2 to 1 0 mu g range, elicited a facilitation of the C-fiber reflex in a dose- dependent manner. A large dose (100 mu g) depressed but did not block the reflex. Intracerebroventricular doses of buprenorphine in the 0.1 to 10 mu g range, facilitated the C-fiber reflex. A higher dose (100 m u g) elicited a biphasic effect: depressive when the stimulus intensit y was weak and facilitatory when the stimulus intensity was strong. It is concluded that the antinociceptive properties of buprenorphine can not be related to a direct or indirect depressive spinal effect. In te rms of spinal and supraspinal effects of buprenorphine, it is likely t hat buprenorphine facilitates the C-fiber reflex via a supraspinal mec hanism that acts on sensory and/or motor components of the reflex are although the depression of the reflex involves a spinal mechanism. The lipophilic properties of buprenorphine could explain a substantial di ffusion from its spinal injection site to the brain. From a clinical s tandpoint, this study confirms that intrathecal administration of bupr enorphine is an inadequate way of accessing spinal opioid receptors.