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Tai YH, Wang YH, Tsai RY, Wang JJ, Tao PL, Liu TM, Wang YC, Wong CS: 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

Graduate Institute of Medical Science, National Defense Medical Center, Taipei, Taiwan.
Pain (Impact Factor: 5.84). 07/2007; 129(3):343-54. DOI: 10.1016/j.pain.2007.01.031
Source: PubMed

ABSTRACT 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 microg/10 microl, 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, PKCalpha, PKCbetaII, and PKCgamma. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor Gö6805 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 EAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF.

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    • "Glutamate transporter activities are decreased by increased arachidonic acid in nerveinjury-induced neuropathic rats (Sung et al., 2007). Studies in cell cultures (Susarla and Robinson, 2008; Garcia-Tardon et al., 2012) and morphine-tolerant animals (Tai et al., 2007) indicate that trafficking of glial glutamate transporters between the cell surface and cytosol is a key post-translational mechanism regulating glial glutamate transporter function. In taxol-induced neuropathic rats, nitration of glial glutamate transporters by peroxynitrite reduces glial glutamate transporter function (Doyle et al., 2012). "
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    • "For example, hyperalgesia induced by nerve injury and the associated deficiency in glutamate uptake, as measured using synaptosome preparations, can be reversed with a cytosolic phospholipase A2 inhibitor (Sung et al., 2007). Prevention of GLT-1 downregulation by amitriptyline in the spinal dorsal horn is accompanied by an attenuation of morphine tolerance in rats (Lim et al., 2005; Tai et al., 2007). Furthermore, selective increased expression of GLT-1 by ceftriaxone treatment (Hu et al., 2010) or gene transfer (Maeda et al., 2008) significantly reduces hyperalgesia induced by nerve injury. "
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    • "Multiple studies have shown that PKC activation can increase EAAT3 activity (Do et al 2002; Huang & Zuo 2005; Huang et al 2006) and amitriptyline can inhibit PKC (Tai et al 2007). Thus, it is possible that the amitriptyline effects on EAAT3 activity may be mediated by PKC. "
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