• Yan-ping Chen, De-quan Cao, Chao-hua Tan, Jun-mei Xu, and Ye-tian Chang.
• Department of Anesthesiology, Second Xiangya Hospital, Central South University, Changsha 410011, China. hershey_diane@yahoo.com
• Zhong Nan Da Xue Xue Bao Yi Xue Ban. 2006 Aug 1;31(4):534-7.

ObjectiveTo explore the inhibitory effect of spinal topical morphine on the dorsal horn projection neurons in nerve-injured rats and its mechanism.MethodsSingle-unit activity of dorsal horn projection neurons was recorded in anesthetized L(5)/L(6) nerve-ligated rats. Allodynia was determined by a behavior test in nerve-injured rats. The evoked neuronal responses to mechanical stimuli applied to the receptive field were determined before and after the spinal topical application of morphine, bicuculline plus morphine, strychnine plus morphine, and both bicuculline and strychnine plus morphine in normal, sham operation, and nerve-injured rats.ResultsSpinal topical application of 10 micromol/L morphine significantly inhibited the evoked responses of dorsal horn projection neurons in normal, sham, operation and nerve-injured rats. However, the inhibitory effect of morphine was significantly reduced in nerve-injured rats compared with that in normal and sham operation rats. Furthermore, the topical application of 20 micromol/L bicuculline had little effect on the inhibitory effect of morphine in nerve-injured rats but it almost abolished the effect of morphine in normal and sham operation rats. The glycine receptor antagonist strychnine at 4 micromol/L significantly decreased the effect of morphine in nerve-injured, normal, and sham operation rats.ConclusionThe loss of tonic GABAergic inhibition contributes to the reduced inhibitory effect of morphine on dorsal horn projection neurons in nerve-injured rats.

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