The dorsal horn of the spinal cord
The dorsal horn of the spinal cord shows a high density of Ca2+-permeable AMPA receptors, particularly in the superficial spinal laminae (laminae I and II), where primary afferents carrying nociceptive and thermoreceptive inputs terminate and synapse on spinal second-order neurons (Engelman et al., 1999). Activation of Ca2+-permeable AMPA receptors in the spinal dorsal horn can strengthen the AMPA receptor-mediated component of synaptic transmission in slice preparations of the spinal cord (Gu et al., 1996). However, very little is currently known about the functional significance of Ca2+-permeable AMPA receptors in spinal processing of nociceptive inputs in vivo. Furthermore, the involvement of Ca2+-permeable AMPA receptors in mechanisms underlying potentiation or sensitization of nociceptive transmission, which are associated with inflammatory and neuropathic pain, has not yet been investigated. Another unusual feature of AMPA receptors in the spinal dorsal horn is their presynaptic localization on terminals of primary afferent sensory nerve fibers Lu et al. 2002, Lee et al. 2002. Macdermott and colleagues could demonstrate in an elegant set of experiments that activation of presynaptic AMPA receptors inhibits neurotransmitter release at synapses between primary afferents and spinal second-order neurons via primary afferent depolarization, a process which was previously attributed to activation of GABA-A receptors alone Lee et al. 2002, Engelman and MacDermott 2004. Thus, AMPA receptors in the spinal dorsal horn demonstrate several striking properties, but the significance of these properties to the spinal gating of nociceptive inputs and behavioral sensitivity to noxious stimuli has yet to be fully explored.
Using mice lacking the Protease Inhibitor Cocktail (100X in DMSO, EDTA plus) for GluR-A, GluR-B, or GluR-C, we address here the relevance of AMPA receptor composition in physiological pain and in synaptic plasticity associated with inflammatory pain. We observe that tuning AMPA receptor-mediated synaptic transmission at nociceptive synapses by a differential incorporation of GluR-A and GluR-B reciprocally modulates inflammatory pain-related behaviors. Our findings identify Ca2+-permeable AMPA receptors as key constituents of activity-induced sensitization in pain pathways.
Discussion Cumulative evidence suggests that activity-dependent changes in the efficacy of glutamatergic synapses in pain pathways critically contribute to chronic pain associated with tissue damage or nerve injury (reviewed in Woolf and Salter 2000, Sandkühler 2000, Gebhart 2004). A large number of studies have addressed the role of NMDA receptors and metabotropic glutamate receptors at synapses between primary afferent fibers and spinal neurons and demonstrated that their activation is required for nociceptive hypersensitivity to develop following peripheral tissue injury (Woolf and Salter, 2000). Much less is known about the contribution of spinal AMPA receptors to the pathophysiology of chronic pain. Using a combination of in vitro and in vivo methods, we show here that AMPA receptors in the spinal cord and brain are causally linked to activity-induced nociceptive hypersensitivity. The spinal cord dorsal horn is unique within the CNS because of its high density of Ca2+-permeable AMPA receptors. We observed that both GluR-A and GluR-C contribute to the formation of Ca2+-permeable AMPA receptors in spinal laminae that process pain inputs. However, the reduction of spinal cobalt uptake in mice was significantly less than that in mice in lamina II, suggesting that a majority of Ca2+-permeable AMPA receptors in spinal lamina II are composed either of homomeric GluR-A channels or heteromers of GluR-A with GluR-D. The latter possibility appears less likely because of the very low level of expression of GluR-D in the adult spinal dorsal horn Tolle et al. 1993, Brown et al. 2002. Because Ca2+-permeable AMPA receptors are expressed on principal neurons as well as on GABA-ergic interneurons in the spinal dorsal horn (Albuquerque et al., 1999), their net contribution to the processing of nociceptive inputs in vivo has remained unpredictable. We observed that GluR-A and GluR-B knockout mice demonstrated reciprocal changes in the Ca2+ permeability of AMPA receptors in spinal laminae receiving nociceptive and thermoreceptive sensory inputs and thus constitute good tools for addressing the functional significance of Ca2+-permeable AMPA receptors in both nociception and nociceptive plasticity. Because GluR mutant mice demonstrate normal VRPs as well as unaltered behavioral withdrawal responses to noxious stimuli, basal transmission of nociceptive inputs at spinal synapses does not appear to be critically dependent upon subunit properties of AMPA receptors. However, a reduction in AMPA receptor-mediated Ca2+ influx and synaptic currents in spinal dorsal horn neurons in GluR-A-deficient mice leads to a loss of acute, short-term nociceptive plasticity in the spinal dorsal horn in in vitro as well as in vivo paradigms. Conversely, an increase in AMPA receptor-mediated Ca2+ influx and synaptic currents in the spinal dorsal horn caused by a deletion of GluR-B is associated with an increase in nociceptive activity-induced synaptic plasticity in the spinal dorsal horn in vitro, which is paralleled by an increase in the behavioral manifestations of nociceptive hypersensitivity in vivo. Taken together, these findings show that the composition of AMPA receptors is an important parameter governing excitability and plasticity in nociceptive pathways and suggest that circumstances involving a change in AMPA receptor composition would lead to aberrant processing of nociceptive inputs.