Antinociceptive potency of intrathecal morphine in the rat tail flick test: a comparative study using acute lumbar catheter in rats with or without a chronic atlanto-occipital catheter.

Chronic spinal catheterization via an atlanto-occipital puncture (CAO) has been widely used to study the effects of drugs on spinal nociceptive mechanisms, but this method is associated with spinal cord damage that may change the efficacy of spinally injected analgesics. Using a slight modification of the method of Storkson et al. (J. Neurosci. Methods 65 (1996) 167), the rat spinal cord was acutely catheterized via a lumbar puncture (AL) and the potency of morphine-induced antinociception in the tail flick test was comparatively studied in animals with or without a CAO catheter. The opiate potency via an AL catheter (AD50; 95% confidence limits) was significantly more intense in rats without (0.29 microg; 0.19-0.47) than in rats with a CAO catheter (1.1 microg; 0.87-1.47) and stronger than via a CAO catheter (8.2 microg; 4.6-14.4). The potency of morphine via a CAO catheter was significantly improved in indomethacin-pretreated rats (1 mg/kg, i.p., twice a day for 5 days), thus indicating that inflammatory changes produced by a CAO catheter are at least in part the reason for the lower efficacy of the opiate. The use of an AL catheter minimizes such spinal changes and permits acute experimental protocols in which more than one spinal injection is necessary.