Change in the phosphorylation of vanilloid receptor 1 in dorsal root ganglion in rats with inflammatory pain- morphine tolerance

Objective To investigate the change in the phosphorylation of vanilloid receptor 1 （VR1） in dorsal root ganglion （DRG） in rats with inflammatory pain （IP）-morphine tolerance. Methods Twenty 8-12 months old male SD rats in which intrathecal （IT） catheters were successfully implanted without complication were randomly divided into 4 groups （n =5 each）： group NS, IP ＋ normal saline （NS） 10 μl IT twice a day ×7 days; group M0 ,intact rats ＋ morphine 10 μg/kg （ 10 μl） IT twice a day × 7 days; group M1 , IP ＋ morphine 10 μg/kg （10 μl） IT once; group M2 ,IP ＋ morphine 10 μg/kg（10 μl） IT twice a day × 7 days. IP 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 3rd day after induction of IP. Paw withdrawal threshold （PWT） and paw withdrawal latency （PWL）were measured after catheterization, before and at 1-7 days of IT consecutive administration. The rats were sacrificed after last pain threshold measurement. The phosphorylated VR1 （p-VR1） protein expression in DRG was determined by Western blot. Results There was no significant difference in the baseline PWL measured before induction of IP among the 4 groups. Morphine tolerance developed in group M0 and M2. The expression of p-VR1 in DRG was highest in group M2. Conclusion The phosphorylation of VR1 in DRG in rats with IP tolerance is increased, which is involved in the development of morphine tolerance. Key words: Drug tolerance;  Morphine;  Inflammation;  Ganglia,spinal;  TRPV cation channels