ATTENUATION OF C-FOS EXPRESSION IN THE RAT LUMBOSACRAL SPINAL-CORD BYMORPHINE OR TRAMADOL FOLLOWING NOXIOUS COLORECTAL DISTENSION

We have previously reported that repetitive, noxious colorectal disten tion (CRD) induces c-Fos in the lumbosacral spinal cord. This study ex amined the effects of the analgesics morphine and tramadol on c-Fos ex pression resulting from noxious CPD in the rat. Pre-treatment (30 min or 1 min, i.v.) with morphine (1.25 mg/kg-5.0 mg/kg) or tramadol (1 mg /kg-20 mg/kg) dose-dependently attenuated c-Fos expression to CRD in a ll areas of the L(6)-S-1 spinal gray matter. The highest dose of morph ine was equipotent to the highest dose of tramadol. Repetitive dosing (1/4 of the greatest dose every 30 min) was as effective as a single b olus dose for both drugs. The visceromotor response to CRD was dose-de pendently attenuated by tramadol and was reversed by naloxone. However , the dose of tramadol that eliminated the visceromotor response (7% o f control) reduced the c-Fos expression to 47% of control. These resul ts demonstrate that these two analgesics attenuate immediate-early gen e expression and the visceromotor response to a noxious visceral stimu lus and suggest that complete attenuation of c-Fos expression is not n ecessary for these compounds to produce analgesia to a noxious viscera l stimulus.