Effects of neostigmine on activation of spinal cord microglia in a rat model of neuropathic pain

Abstract

Objective To investigate the effects of intrathecal neostigmine on the activation of spinal cord microglia in a rat model of neuropathic pain. Methods Male SD rats weighing 200-250 g were used in this study, A PE 10 catheter was inserted at L_(4,5) interspace into the epidural space and advanced for 3 cm cephalad. Spinal nerve injury (SNI) was induced by exposure of sciatic nerve and its three branches and Iigation and transaction of tibial and common fibular nerves. Forty SD rats in which IT catheter was successfully placed without complication were randomly divided into 4 groups ( n = 10 each) : group Ⅰ SNI control ( SNI);group Ⅱ SNI + neostigmine (N);group DI SNI + atropine + neostigmine (A + N) and group IV SNI + mecamylamine + neostigmine (M + N). In group SNI normal saline 20 μl was given IT;in group N neostigmine 10 μ/g (10 μl) was given IT, while in group A + N ( Ⅲ) and group M + N (Ⅳ) atropine 10 μg and mecamylamine 10 μg were given IT at 5 min before neostigmine 10 μg IT respectively. 50% paw withdrawal threshold to von Frey filament stimulation (50% PWT) was measured at 2 d before SNI, at 0, 1, 3 and 5 d after SNI, before and at 15, 30, 45, 60 min after IT drug administration. The animals were anesthetized and killed at the time when analgesia induced by IT drug administration was most satisfactory. The spinal cord was removed. The number of positive microglia cells in the spinal cord was determined using immuno-histochemistry. The cell morphology was also examined. Results In group N (group Ⅱ ) 50% PWT on the operated side was significantly increased after IT neostigmine administration as compared with the baseline value before IT drug administration. The number of positive microglia cells on the operated side was significantly larger than that on the contralateral side in control group, and was significantly decreased by IT neostigmine in group N and by IT atropine + neostigmine in group A + N. There was no significant difference in the number of positive microglia cells on the operated side between group C and group M + N. Conclusion N-cholinergic receptor is invbolved in the IT neostigmine-induced inhibition of the activation of spinal cord microglia in a rat model of neuropathic pain. Key words: Neostigmine; Microglia; Neuralgia