Effects of intrathecal morphine, clonidine and baclofen on allodynia afterpartial sciatic nerve injury in the rat

Background: Increased response to mechanical or cold stimulation of hind pa ws was observed in rats with partial sciatic nerve injury as a result of ph otochemically induced ischemia. The present study examined the effects of i ntrathecal morphine, clonidine and baclofen on the allodynia-like responses . Methods: The left sciatic nerves of rats were irradiated for 2 min with an argon ion laser under chloral hydrate anesthesia. The threshold of paw with drawal to mechanical stimulation was determined with a series of monofilame nts (von Frey hairs). The response to cold stimulation was tested by sprayi ng ethyl chloride on the plantar surface of the paw. When rats were exhibit ing stable mechanical and cold allodynia-like behaviors after nerve injury, the effects of i.t. morphine (1, 2, 7 mu g), clonidine (1, 2, 7 mu g) and baclofen (0.1, 0.2, 0.7, 9 mu g) in a cumulative dose regime were investiga ted. Results: Intrathecal morphine dose-dependently alleviated the mechanical an d cold allodynia without inducing motor impairment or sedation. Intrathecal clonidine did not alter the response of kind paws to mechanical stimulatio n, but reduced the cold allodynia. Intrathecal baclofen reduced the respons es of rats to mechanical stimulation only at doses that also induced profou nd motor deficits. Conclusions: The present data suggest that intrathecal morphine, and to som e extent clonidine, but not baclofen, alleviated the abnormal pain-related behaviors in this new rat model of partial peripheral nerve injury. Differe nces in the pharmacological profile between the present model and other mod els of peripheral nerve injury are discussed.