Cutaneous nociceptive signaling is carried into the CNS by high threshold primary afferent fibers, or nociceptors, that form glutamatergic/peptidergic synapses with neurons in the superficial dorsal horn. Glutamate released from these terminals acts on multiple receptor types including the ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA receptors. The two main types of Ca2+ permeable glutamate receptors expressed by dorsal horn neurons, the NMDA and Ca2+ permeable AMPA receptors, have different voltage dependent properties making Ca2+ entry through these receptors have different activity dependence. Both of these receptors have been implicated in forms of central sensitization association with hyperalgesia and allodynia. In addition to glutamatergic synaptic transmission, GABA and glycine are also importantly involved in regulating nociception (Willis and Coggeshall, 1991; Dickenson et al, 1997). It is not known whether the critical importance of non-NMDA receptors in the regulation of nociception is due to their action to enhance inhibition by mediating excitation of GABA/glycinergic neurons at postsynaptic sites or even at the nerve terminals of these inhibitory neurons. Understanding the role of Ca2+ permeable glutamate receptors in activation of projection neurons and modulating excitation of inhibitory neurons is key to understanding regulation of information flow through the superficial dorsal horn. We plan to use postnatal rat spinal cord slices to study the role of pre and postsynaptic AMPA receptors in regulating synaptic transmission in the spinal cord dorsal horn. Using electrophysiology, Ca2+ and general fluorescent imaging, and immunocytochemistry, we will clarify the location and function of the Ca2+ permeable non-NMDA receptors in the dorsal horn. We will use co-cultures of nociceptors and dorsal horn neurons grown on microislands to determine the rules for sorting of Ca2+ permeable AMPA receptors and NMDA receptors at individual synaptic sites and how that impacts on the activity dependence of each synapse studied. In addition to developing our understanding of regulation of nociceptive information transmission in the dorsal horn, these studies may also contribute to the development of new drugs for alleviating pain that are targeted to the unexplored cell specific AMPA receptors we propose to study.
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