Reproducibility of the drug effects over time on chronic lumbar epidural catheterization in rats

Chronically implanted epidural catheters lead to a reaction that: impedes drug action. The purpose of this study in a rat model with chronically implanted epidural catheters was to investigate the change in opiate activity and histology over time with this model. A skin incision of 1-2 cm was made at the T 13 level on the back of male Sprague-Dawley rats under halothane anesthesia. Muscles were dissected bluntly from the vertebrae, and the intervertebral ligament was cut to insert an epidural catheter (polyethylene tube, outer diameter of 0.14 mm) 2-cm caudally. The longer portion of the catheter was passed through a trocar subcutaneously to exit the dorsal neck area. One, two, and six days after catheterization, the effects of morphine on thermal stimulation using the hot-box test and histology were investigated. Analgesic effects of morphine 6 days after catheterization were significantly less than those on the first and second days. Histologically, evidence of inflammation around the catheter was noted as early as 4 h after catheterization. Pericatheter fibrosis was severe after 2 days. We conclude that this model of chronic epidural catheterization in the rat evoked a histologically defined, pharmacodynamically significant, local reaction 2 to 6 days after catheter implantation. Implications: A rat model with chronically implanted epidural catheters should be used for testing the analgesic effects of drugs within two days after catheterization.