MODULATION OF SPINAL CORD NEUROPLASTICITY BY OPIOIDS

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APPLICANT'S ABSTRACT: This application is a request for a Scientist Development Award. This award is sought to enable my full time research on the modulatory effects of opiates on spinal cord neurotransmission and synaptic plasticity. These new research experiences under the tutelage of my preceptor and additional consultants will equip me technically and practically to move forward quickly to become an independent researcher. Long term potentiation (LTP), the best studied model, to date, of synaptic plasticity within the CNS, involves a seemingly permanent increase in neuronal excitation following repeated activation of afferent input to that neuron and is thought to be the cellular basis of learning. Recent clinical and behavioral evidence has accumulated suggesting that much the same activity- dependent synaptic changes can take place in neural systems involved in pain perception. Intense painful stimuli can produce a central sensitization to further noxious stimulation in both laboratory animals and man. Behavioral pharmacologic studies of this sensitization suggest numerous similarities to the neurochemistry of LTP, including its reliance on activation of the NMDA class of glutamate receptors and its modulation by certain opiates. The studies proposed here involve the utilization of an in vitro spinal cord slice preparation to study the opioid modulation of primary afferent dorsal horn synapses in regions of the spinal cord known to be involved in nociceptive processing. These studies stem directly from our findings in the hippocampus that LTP induction can be blocked by kappa opioids. Initial experiments using the neonatal rat transverse spinal cord slice will focus on the actions of applied mu, kappa, and delta opioids in modulating both stimulation-evoked neural excitation in lamina I of the dorsal horn and activity-dependent plastic changes in this area. These studies will use single cell extracellular recording techniques first and will benefit from the in vitro preparation in enabling application of known concentrations of agonists and antagonists to the site of action and in allowing easy visualization of the recording electrode position. Positive extracellular findings will be further evaluated using whole cell voltage clamp techniques to study post-synaptic mechanisms of action. Further, retrogradely transported fluorescent markers from rostral spinothalamic tract lesions will be used to target specific presumably nociceptive spinothalamic cell responses. The effects of endogenous mu, delta and kappa opioids on spinal neurotransmission and synaptic plasticity will then also be evaluated. Finally, these results in neonatal spinal cord will be compared and contrasted to studies using an adult spinal cord slice preparation. Thus, using an in vitro model and a variety of electrophysiological techniques, we hope to provide insight into the neurochemical mechanisms of primary afferent neurotransmission and neuroplasticity and how mu, kappa, and delta opiates differentially act to modulate these mechanisms in the adult and neonate. Such information may ultimately lead to better pharmacological means of managing and/or preventing certain acute and chronic pain states.