Abrupt abstinence or withdrawal from opiate drugs causes a series of severe adverse symptoms, which keep drug-dependent individuals craving continued opiates. One of the core of withdrawal symptoms is an increase in pain sensitivity (pain sensitization or hyperalgesia). This pain sensitization is due to synaptic plasticity, particularly in the spinal cord and primary afferents. Recent evidence suggests that mechanisms underlying synaptic plasticity in the hippocampus may also occur in the spinal cord. Although acute exposure to opiates may induce hyperalgesia, chronic or repeated administration of the drug facilitate the magnitude and duration of opiate-induced hyperalgesia, and may expand the anatomical sites at which hyperalgesia is induced. Persistence of chronic opiate-induced hyperalgesia is consistent with cellular mechanisms of opiate dependence that only develop after repeated exposure to the drug. It has been demonstrated that AMPA glutamate receptor trafficking within the spinal cord is involved in the development of pain sensitivity. Similarly, hippocampal glutamatergic systems are thought to be involved in opiate-induced neuronal and behavioral plasticity. We propose to document the parallel changes in AMPA receptor expression occurring in opiate dependence and pain sensitivity at three different levels: 1) hippocampus, 2) spinal cord and 3) primary afferent neurons. The proposed experiments will test the hypothesis that acute withdrawal from repeated morphine administration results in sensory sensitization by altering synaptic expression and composition of AMPA receptors and that this neuroplasticity in AMPA receptors is similar to that underlying chronic inflammatory pain.
In Specific Aim 1 we will characterize the sensory sensitization that develops in a model of morphine withdrawal and compare it to the sensitization that develops in a model of chronic inflammatory pain.
In Specific Aim 2 we will analyze the dynamic changes in expression and composition of AMPA receptor subunits (GluR1/2/3) in morphine withdrawal-associated sensitization and compare them to those occurring in chronic inflammatory pain.
In Specific Aim 3 we will characterize the neuroplasticity in AMPA receptor synapses underlying morphine withdrawal-associated sensory sensitization and compare it to the AMPA receptor neuroplasticity underlying chronic inflammatory pain.
In Specific Aim 4 we will demonstrate that treatment with AMPA antagonists relieves both morphine withdrawal-associated sensory sensitization as well as that associated with chronic inflammatory pain. These studies are significant because they will elucidate key glutamatergic maladaptive changes that opiate addicts and inflammatory pain patients have in common. Overall, these studies will provide insight into the neuroplasticity that may lead to novel approaches for pharmacotherapeutic intervention for pain treatment in opiate addicts.
The experiments proposed in this application will address the question of how drug-induced alterations in the hippocampus, spinal cord and primary afferents contribute to enhanced sensory sensitivity in opiate addicts. These results will enhance our understanding of the synaptic molecular and membrane mechanisms underlying this adverse effect of drug dependence and will help us to improve pain management in opiate addicts.
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