Partial peripheral nerve injuries often result in neuropathic pain such as causalgia. The pain is characterized by spontaneous burning pain combined with hyperalgesia and allodynia. There have been many attempts to find the pathogenesis. However, even though the disease has been known over a century, we have little knowledge about not only its pathogenesis, but also its treatment. Recently, a neuropathic animal model, produced by incomplete injury of the nerve innervating the rat tail, has been developed. This model elicits behavioral signs of neuropathic pains mechanical, cold and warm allodynia) that are localized in the tail.

In the present study, using this model, we assessed the effects of several drugs (acetaminophen, diclofenac sodium, amitriptyline, carbamazepine, baclofen, clonidine, and morphine) on the manifestations of the neuropathic signs. To test for mechanical allodynia, mechanical sensitivity of the tail was quantified by the tail-withdrawal frequency occurred by the application a von Frey hair(19.6mN; 2gm). Tests for cold and warm sensitivity of the tail were performed using the method of immersing the tail in 4°C and 40°C water with a cut-off time of 15 sec, respectively. Of injected drugs, amitriptyline, carbamazepine, baclofen and morphine alleviated the signs of neuropathic pains, whereas acetaminophen, diclofenac sodium and clonidine didn't. These results suggest that the supraspinal changes take part in the development of the neuropathic pains.