Change in the expression of excitatory amino acid transporter 3 in the spinal cord neurons in a rat model of chronic morphine-tolerance

Abstract

Objective To investigate the change in the expression of excitatory amino acid transporter 3 (EAAT3) in the spinal cord neurons in a rat model of chronic morphine tolerance. Methods Forty-five male SD rats were randomly divided into 5 groups ( n = 9 each) : group I sham operation (group S); group II normal saline (group NS); group Ⅰ morphine (group M); group Ⅳ ketamine (group K) and groupV M + K. In group II - V a catheter was placed in the subarachnoid space at L_(3-5) interspace. The animals were observed for 3 days. The animals with motor or sensory paralysis of the hindlimbs were excluded. NS 40 μl,morphine 20 μg, ketamine 30μg,morphine 20μg + ketamine 30μg were injected via intrathecal catheter twice a day for 7 consecutive days. 50% paw withdrawal threshold and latency (PWT, PWL) of the hindpaw to radiant heat were measured before (T_0, baseline) , on day 1, 3, 5, 7 of (T_(1-4)) and 1 day after (T_5 ) IT drug administration. The rats were sacrificed after last pain threshold measurement. The expression of EAAT3 protein in the spinal cord was determined by Western blotting and immuno-histochemistry. Results The sensitivity of the hindpaw to noxious heat stimulation was significantly decreased during (T_(1,2)) and increased after IT administration (T_(4,5)) in group M and was significantly decreased during and after FT administration (T_(1-5)) in group M + K as compared with the baseline values at T_0 and group S and was significant lower in group M + K than in group M. The expression of EAAT3 protein in the spinal cord was significantly decreased in group M and M + K as compared with group S and was significantly lower in group M than in group M + K. Conclusion The down-regulation of the expression of EAAT3 in the spinal dorsal horn neurons is involved in the development of chronic morphine tolerance and the expression of EAAT3 is down-regulated by morphine partly through the activation of NMDA receptor. Key words: Morphine; Drug tolerance; Spinal cord; Excitatory amino acid transporter 3