Comparison of antiallodynic effect of intrathecal morphine, brimonidine and rilmenidine between neuritis and ligation injury induced neuropathic pain.

Abstract

Mechanical allodynia is generally resulted from nerve damage by direct injury or inflammation. Thus, this study was designed to compare the antiallodynic effect of morphine, brimonidine and rilmenidine in two models of neuropathic pain, that is, induced by nerve ligation and neuritis. Rats were prepared with tight ligation of the L5/L6 spinal nerves (SNL group) or with Freund's complete adjuvant (FCA) administration evoked sciatic inflammatory neuritis (SIN group). Antiallodynic effects by intrathecal morphine, brimonidine and rilmenidine were measured by applying von Frey filaments to the lesioned hind paw. Thresholds for withdrawal response were assessed and converted to % MPE to obtain an effective dose 50% (ED 50) and a dose response curve. Either SNL group or SIN group showed marked mechanical allodynia in the lesioned hind paw. Antiallodynic effects of morphine were different between two groups. That is ED 50 was 0.16 microgram (SIN) and 8.12 microgram (SNL), and dose response curve of the SIN group shifted left from that of the SNL group. The difference between SIN and SNL groups was statistically significant (P < 0.05). With the brimonidine or rilmenidine administration, ED 50 s were 0.12 microgram (SNL) and 0.37 microgram (SIN) and 2.16 microgram (SIN) and 11.46 microgram (SNL), respectively. And the shift to left of dose response curve from the SNL group is more prominent with rilmenidine administration. These results suggest morphine and rilmenidine showed a better effect on reducing the mechanical allodynia induced by FCA administration.