Effect of intrathecal injection of dexamethasone on expression of substance P and calcitonin gene-related peptide in spinal cord of rats with chronic constrictive injury-induced neuropathic pain

Objective To investigate the effects of intrathecal dexamethasone injection on substance P(SP) and calcitonin gene-related peptide(CGRP) contents in the spinal cord of rats with chronic constrictive injury(CCI)-induced neuropathic pain.Methods Forty-eight male Sprague-Dawley rats,weighing 200-250 g,were randomly divided into 4 groups(n=12 each): sham operation,CCI+saline treatment(group C),CCI+early dexamethasone treatment(group ET),and CCI+lately dexamethasone treatment(group LT).The paw withdrawal mechnical threshold(PWMT) to mechanical stimulus was measured 1 day before operation.Six animals were killed at 1 day after operation and the other 6 were killed 1 day after intrathecal injection in each group;the lumbar segments of the spinal cord were collected for determination of SP and CGRP contents by immunohistochemistry;and the PWMT values were measured 1 day after operation and 1 day after intrathecal injection,with the baseline PWMT value determined in each group 1 day before operation.Results PWMT values to mechanical stimulus were increased in ET and LT groups 1 day after intrathecal injection of dexamethasone(all P0.05),and were significantly higher than that of group C(all P0.05).One day after intrathecal injection,the SP and CGRP contents in ET and LT groups were significantly decreased(all P0.05),and those in group C and sham operation group were only slightly decreased(all P0.05);while the contents of SP and CGRP in ET and LT groups were significantly lower than those of group C(all P0.05).Conclusion Intrathecal dexamethasone injection can attenuate mechanical hyperalgesia by reducing SP and CGRP contents in the dorsal horn of spinal cord in CCI-induced rat model of neuropathic pain.(Shanghai Med J,2010,33: 528-531)