Pharmacology, biochemistry, and behavior
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Pharmacol. Biochem. Behav. · Aug 2010
Co-administration of ultra-low dose naloxone attenuates morphine tolerance in rats via attenuation of NMDA receptor neurotransmission and suppression of neuroinflammation in the spinal cords.
Although mechanisms underlying ultra-low dose naloxone-induced analgesia have been proposed, possible interactions with glutamatergic transmission and glial cell activation have not been addressed. In the present study, we examined the effect of ultra-low dose naloxone on spinal glutamatergic transmission and glial cell activity in rats chronically infused with morphine. In male Wistar rats, intrathecal morphine infusion (15microg/h) for 5days induced (1) antinociceptive tolerance, (2) downregulation of glutamate transporters (GTs) GLT-1, GLAST, and EAAC1, (3) increasing of NMDA receptor (NMDAR) NR1 subunit expression and phosphorylation, (4) upregulation of protein kinase C gamma (PKCgamma) expression, and (5) glial cell activation. ⋯ These effects may result in preservation of the antinociceptive effect of acute morphine challenge in chronic morphine-infused rats. Ultra-low dose naloxone infusion alone did not produce an antinociceptive effect. These findings demonstrated that attenuation of glutamatergic transmission and neuroinflammation by ultra-low dose naloxone co-infusion preserves the lasting antinociceptive effect of morphine in rats chronically infused with morphine.