Changes in expression of CGRP, SP and BDNF in dorsal root ganglion and effects of electroacupuncture on morphine tolerance in rats with chronic inflammatory pain and morphine tolerance

Abstract

Objective To investigate the changes in the expression of calcitonin gene-related peptide (CGRP), substance P (SP) and brain-derived neurotrophic factor (BDNF) in dorsal root ganglion (DRG) and the effect of electroacupuncture (EA) on morphine tolerance in rats with chronic inflammatory pain (CIP) and morphine tolerance. Methods Twenty-five 8-month-old male SD rats weighing 230-250 g in which intrathecal (IT)catheters were successfully implanted without complications were randomly divided into 5 groups ( n = 5 each):groupA CIP + normal saline (NS) 10 μl IT twice a day for 7 consecutive days; group B CIP + morphine 10 μg/kg ( 10 μl) IT twice for the first day only; group C CIP + morphine 10 μg/kg ( 10 μl) IT twice a day for 7 consecutive days; group D CIP + EA (intensity 2 mA, frequency 2 Hz, wave length 0.6 ms) + morphine 10 μ g/kg (10 μl) IT twice a day for7 consecutive days; group E CIP + EA (intensity 2 mA, frequency 15 Hz,wave length 0.4 ms) + morphine 10μg/kg (10 μl) IT twice a day for7 consecutive days. CIP was induced by injecting complete Freund's adjuvant (CFA) into the ankle joint of the left hindlimb. IT morphine or NS was started on the 4th day after induction of CIP. EA of Yanglingquan and Zusanli lasting 30 min was performed once a day after first IT administration of morphine for 7 days. Paw withdrawal latency (PWL) to a thermal nociceptive stimulus was measured before induction of CIP, 1 day before (baseline) and at day 1-7 after administration (T0-8) . The animals were sacrificed after the last PWL measurement. The DRGs of the lumbar segment (L4-6) were removed for determination of CGRP, SP and BDNF mRNA expression using RT-PCR. Results PWL was significantly shorter at T1 than at T0 in all groups, and at T3-8 than at T2 in group B-E, while longer at T2 than at T1 in group B-E ( P ＜0.05). PWL was significantly longer in group B-E and CGRP, SP and BDNF mRNA expression higher in group C than in group A ( P ＜ 0.05). PWL was significantly longer in group C-E than in group B ( P ＜ 0.05). PWL was significantly longer and CGRP, SP and BDNF mRNA expression lower in group D and E than in group C ( P ＜0.05). PWL was significantly lower and CGRP, SP and BDNF mRNA expression higher in group E than in group D ( P ＜ 0.05). Conclusion Up-regulation of the expression of CGRP, SP and BDNF mRNA in DRG is involved in the devepopment of morphine tolerance. EA can inhibit the devepopment of morphine tolerance by inhibiting the expression of CGRP2 SP and BDNF mRNA. Key words: Drug tolerance; Morphine; Inflammation; Electroacupuncture; Calcitonin gene-related peptide; Substance P; Brain-derived neurotrophic factor