Pain after Discontinuation of Morphine Treatment Is Associated with Synaptic Increase of GluA4-Containing AMPAR in the Dorsal Horn of the Spinal Cord

Abstract

Withdrawal from prescribed opioids results in increased pain sensitivity, which prolongs the treatment. This pain sensitivity is attributed to neuroplastic changes that converge at the spinal cord dorsal horn. We have recently reported that repeated morphine administration triggers an insertion of GluA2-lacking (Ca 2 þ -permeable) a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPAR) in the hippocampus. This finding together with the reported involvement of AMPAR in the mechanisms underlying inflammatory pain led us to hypothesize a role for spinal AMPAR in opioid-induced pain behavior. Mice treated with escalating doses of morphine showed hypersensitivity to mechanical stimulation. Intrathecal administration of a Ca 2 þ -permeable AMPAR selective blocker disrupted morphine-induced mechanical sensitivity. Analysis of the expression and phosphorylation levels of AMPAR subunits (GluA1/2/3/4) in homogenates and in postsynaptic density fractions from spinal cord dorsal horns showed an increase in GluA4 expression and phosphorylation in the postsynaptic density after morphine. Co-immunoprecipitation analyses suggested an increase in GluA4 homomers (Ca 2 þ -permeable AMPAR) and immunohistochemical staining localized the increase in GluA4 levels in laminae III-V. The excitatory postsynaptic currents (EPSCs) recorded in laminae III-V showed enhanced sensitivity to Ca 2 þ -permeable AMPAR blockers in morphinetreated mice. Furthermore, current-voltage relationships of AMPAR-mediated EPSCs showed that rectification index (an indicator of Ca 2 þ -permeable AMPAR contribution) is increased in morphine-treated but not in saline-treated mice. These effects could be reversed by infusion of GluA4 antibody through patch pipette. This is the first direct evidence for a role of GluA4-containing AMPAR in morphineinduced pain and highlights spinal GluA4-containing AMPAR as targets to prevent the morphine-induced pain sensitivity.

Inflammation-induced GluA1 trafficking and membrane insertion of Ca2+ permeable AMPA receptors in dorsal horn neurons is dependent on spinal tumor necrosis factor, PI3 kinase and protein kinase A

“…While Ca 2+ permeable (GluA2-containing) AMPA receptors mediate the majority of acute pain sensations, AMPA receptor subunit trafficking resulting in an increased GluA1/GluA2 ratio comes into prominence in a number of painful conditions, including tissue and nerve injury (Katano et al, 2008; Park et al, 2009; Vikman et al, 2008; Wang et al, 2011). The trafficking of GluA4 enriched and GluA2 lacking receptors in spinal cord has been less studied, but the relationship of membrane GluA4 to spinal sensitization and nociception is starting to emerge (Cabañero et al, 2013; Polgár et al, 2010). Interestingly, the nature of the injury may govern the final pattern of the GluA1/GluA2 trafficking and possibly that of GluA4 (Tao, 2012).…”

“…Z. Lee et al, 2012; Malinow and Malenka, 2002; Park et al, 2009; Seidenman et al, 2003). Morphine-induced hypersensitivity appears to be a unique pain state thus far, in that it is associated with trafficking of GluA4, but not GluA1 homomeric AMPA receptors to synaptic membranes in laminae III–V (Cabañero et al, 2013). The present results indicate a predominant inflammation/injury-induced insertion of Ca 2+ permeable AMPA receptors and deletion of GluA2 containing Ca 2+ impermeable AMPA receptors and are thought to reflect the sum of the constitutive balance of AMPA receptors in the plasma membranes plus the activity driven insertion and deletion of GluA1 (or GluA4) enriched and GluA2 containing AMPA receptors, respectively.…”

“…Significantly, there is a small population of nociceptive lamina I projection neurons that lack GluA1, but instead contain GluA4, (Polgár et al, 2008; 2010). Other nociceptive neurons that develop Ca 2+ permeable AMPA receptors with GluA4 have more recently been identified in deep dorsal horn (Cabañero et al, 2013). Plasmalemmal AMPA receptors are normally in dynamic equilibrium with those in the cytosolic compartment (Scannevin and Huganir, 2000).…”

“…Acutely, increased nociceptive drive favors membrane insertion of GluA1 enriched and GluA2 lacking AMPA receptors and removal of GluA2 containing receptors from the membrane of some cell types (Choi et al, 2010; 2012; Galan et al, 2004; Park et al, 2008; Pezet et al, 2008). Analogous membrane trafficking of GluA4 subunit enriched, GluA2 lacking AMPA receptors occurs in deep dorsal horn during morphine-induced hypersensitivity (Cabañero et al, 2013). Following endocytosis, AMPA receptors are either recycled or degraded depending, among other things, on the proportion of AMPA and NMDA receptor activation and the phosphorylation state of GluA1 (Ehlers, 2000; Fernández-Monreal et al, 2012).…”

What Is Being Trained? How Divergent Forms of Plasticity Compete To Shape Locomotor Recovery after Spinal Cord Injury

“…122,[133][134][135][136][137][138][139][140] However, the SCI field has been less focused on balancing homeostatic glutamatergic plasticity, and most neuromodulatory approaches have instead focused on driving locomotor activity through pharmacological and neurostimulator technologies (see section on neuromodulation, above). It is important to note that although locomotor training models that use neurostimulation (e.g., tail stimulation in spinalized rats) to facilitate stepping have yielded promising results, the potential for robust afferent input to detrimentally alter sensory and motor function must be considered.…”