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
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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ABSTRACT: Neurons responsible for sensing noxious stimuli and conducting pain signals from periphery to the spinal cord are predominantly glutamatergic. Members of the SLC1A family of high-affinity glutamate transporters (GluTs) are differentially expressed in sensory neurons and surrounding glial cells. These plasma membrane proteins along with glutamate/cystine exchanger, xCT, are responsible for fine tuning of extracellular glutamate concentrations and, thus, for modulation of excitatory signalling in the spinal cord. Emerging data point at key roles of GluTs in molecular mechanisms of chronic pain and analgesia, incl. development of opioid tolerance. Pharmacological inhibition or antisense down-regulation of spinal GluTs can induce/aggravate pain behaviours, whereas increasing of expression of GluTs by viral gene transfer or positive pharmacological modulators can mitigate chronic pain. Furthermore, some drugs, originally introduced for targeting different pathological conditions, but in parallel exhibiting analgesic properties (e.g., anticonvulsants valproate and riluzole, β-lactam- and tetracycline antibiotics, tricyclic antidepressants), can enhance glutamate transport in the spinal cord. Thus, molecular modulation of GluTs may turn into prospective therapeutic approach for the management of chronic pain. However, precise pharmacological targeting of this transport system requires in-depth elucidation of molecular factors and signaling pathways underlying expression and activity of individual GluT subtypes, including their splice variants.This article is protected by copyright. All rights reserved.Journal of Neurochemistry 09/2014; 131(6). DOI:10.1111/jnc.12957 · 4.24 Impact Factor
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ABSTRACT: Excessive activation of glutamate receptors in spinal dorsal horn neurons is a key mechanism leading to abnormal neuronal activation in pathological pain conditions. Previous studies have shown that activation of glutamate receptors in the spinal dorsal horn is enhanced by impaired glial glutamate transporter functions and proinflammatory cytokines including interleukin-1 beta (IL-1β). In this study, we for the first time revealed that spinal glial glutamate transporter activities in the neuropathic animals are attenuated by endogenous IL-1β. Specifically, we demonstrated that nerve injury results in an increased expression of IL-1β and activation of PKC in the spinal dorsal horn as well as suppression of glial glutamate uptake activities. We provided evidence that the nerve-injury induced suppression of glial glutamate uptake is at least in part ascribed to endogenous IL-1β and activation of PKC in the spinal dorsal horn. IL-1β reduces glial glutamate transporter activities through enhancing the endocytosis of both GLT-1 and GLAST glial glutamate transporters. The IL-1β induced trafficking of glial glutamate transporters is through the calcium/PKC signaling pathway, and the dynamin-dependent endocytosis, which is dependent on the integrity of actin filaments. The signaling pathway regulating glial glutamate transporters revealed in this study provides novel targets to attenuate aberrant activation of glutamate receptors in the spinal dorsal horn, which could ultimately help the development of analgesics. GLIA 2014Glia 07/2014; 62(7). DOI:10.1002/glia.22665 · 5.47 Impact Factor