A rat model of chronic lumbar epidural catheterisation.

Abstract

There are no rodent models of chronic epidural catheterisation which can induce clinically relevant analgesic effects of morphine (potency ratio of intrathecal:epidural = 10 to 30:1). The purpose of this study was to investigate a new rat model of chronic epidural catheterisation by comparing the analgesic effect of epidural and intrathecal morphine. In Sprague-Dawley rats, for epidural catheterisation, a skin incision and muscle dissection were made at T13 level in the midline. The intervertebral ligament was cut to insert an epidural catheter (polyethylene tube; outer diameter 0.14 mm) 2 cm caudally. The distal end of the catheter (0.61 mm) was passed s.c. to exit in the dorsal neck area. For intrathecal catheterisation, the atlanto-occipital membrane was incised to insert a catheter (0.61 mm) 8.5 cm caudally. The effects of morphine (1, 3, 10, 30, or 100 micrograms.10 microliters-1 intrathecally, and 30, 100, or 300 micrograms.5, 10, or 20 microliters-1 epidurally) on thermal escape latency were investigated by a hot box test. Behaviour and motor function were also tested. A volume of 20 microliters induced greater analgesic effect than 5 and 10 microliters epidural administration. The ED50 of epidural morphine (94.1 micrograms) was 30 times more than that of intrathecal morphine (3.1 micrograms). A new rat model of chronic epidural catheterisation is described, which provides a similar analgesic epidural: intrathecal potency ratio for morphine to that in humans when morphine is administered in a volume of 20 microliters.