AMPA receptors mediate pain transmission and have been implicated as critical components in initiating some forms of central hyperalgesia and allodynia. Ca2+ is a second messenger often involved in plasticity at central synapses and some of the AMPA receptors are directly permeable to Ca2+. However the role of these Ca2+ permeable receptors in synaptic plasticity, even their role in synaptic transmission in the spinal cord (but see Preliminary studies), is unknown. If Ca2+ entry through synaptic AMPA receptors is responsible for regulating synaptic strength in the dorsal horn pain pathway, then pharmacological intervention with those receptors would provide an entirely new way to control conditions leading to pathological pain states. Joro spider toxin (JSTX), a polyamine purified from the spider Nephila clavata, selectively blocks Ca2+ permeable AMPA receptors without blocking Ca2+ impermeable AMPA receptors, providing a good example of such selective drug action. Furthermore, due to the unique properties of the Ca2+ -permeable AMPA receptors compared to NMDA receptors, synaptically evoked Ca2+ entry through the AMPA receptors should be enhanced by co-incident inhibitory synaptic activation (rather than inhibited as is the case for the NMDA receptors) making the Ca2+- permeable AMPA receptors a novel type of co-incidence detector. Using synthetic JSTX-3, we have evidence establishing that the Ca2+- permeable AMPA receptors are expressed postsynaptically on dorsal horn neurons. We now intend to clarify the role of Ca2+-permeable AMPA receptors in the central transmission and modulation of pain information. We plan to determine the role of Ca2+-permeable AMPA receptors in the modification of neighboring synaptic receptors in a manner dependent on Ca2+ entry through the AMPA channels themselves. We have already demonstrated that Ca2+ entry through the Ca2+-permeable AMPA receptors causes desensitization of the NMDA receptors (Kyrozis et al, 1995) and now we will determine if this happens during synaptic transmission. We will measure the Ca2+ accumulation following entry through Ca2+-permeable AMPA receptors and determine its dependence on membrane potential. We will contrast these observations with those made on synaptically gated Ca2+ accumulation through the NMDA receptors in the presence of Mg2+ as a way of determining if Ca2+-permeable AMPA receptors could function as co- incidence detectors for inhibitory input. Finally, we will identify which subpopulation of neurons in laminae I and II of the dorsal horn pain pathway most prominently express Ca2+-permeable AMPA receptors at synaptic sites which will give us insight into the major role these receptors play in pain transmission.
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