Amitriptyline preserves morphine's antinociceptive effect by regulating the glutamate transporter GLAST and GLT-1 trafficking and excitatory amino acids concentration in morphine-tolerant rats

The present study was undertaken to examine the effect of amitriptyline on the antinociceptive effect of morphine and its underlying mechanisms in regulating glutamate transporters trafficking in morphine-tolerant rats. Long-term morphine infusion induced antinociceptive tolerance and down-regulation of glutamate transporters (GTs), GLAST, GLT-1, and EAAC1, expression in the rat spinal cord dorsal horn. Acute amitriptyline treatment potentiated morphine's antinociceptive effect, with a 5.3-fold leftward shift of morphine's dose-response curve in morphine-tolerant rats, and this was associated with GLAST and GLT-1 trafficking onto the cell surface. Similar to our previous studies, morphine challenge (10 mu g/10 mu l, i.t.) significant by increased the excitatory amino acids (EAAs) aspartate and glutamate level in the CSF dialysates of morphine-tolerant rats. Acute amitriptyline treatment not only suppressed this morphine-evoked EAA release, but further reduced the EAA concentration than baseline level. Furthermore, long-term morphine infusion up-regulated PKA and PKC protein expression in the spinal cord dorsal horn, while amitriptyline inhibited the increase in expression of phospho-PKA, PKC alpha, PKC beta II, and PKC7. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor G66805 attenuated morphine tolerance and the morphine-induced CSF glutamate and aspartate elevation, and induced trafficking of GLAST and GLT-1 from cytosol onto the cell surface. These results show that acute amitriptyline treatment preserved morphine's antinociceptive effect in morphine-tolerant rats; the mechanisms may be involved in inhibition of phospho-PKA and PKC expression, and thus inducing the GLAST and GLT-1 trafficking onto glial cell surface which enhances the FAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF. (c) 2007 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.