Effects of nerve injury on the inhibitory system of the spinal cord

Many pain states, including neuropathic pain, involve increased neurotransmission at the level of the spinal cord. Electrophysiological, pharmacological, molecular biological and neuroanatomical studies of gamma-aminobutyric acid (GABA) in the adult central nervous system indicate that GABA inhibits synaptic transmission by either presynaptic or postsynaptic mechanisms through actions exerted at GABAA receptors. Because the GABAA receptor is a chloride ion channel, its function is completely dependent on the chloride gradient established the KCC2 and NKCC1 co-transporters in the dorsal horn and the primary afferent neurons respectively. Alterations in either co-transporter, either in number or function, would result in a diminution of the chloride gradient and subsequent functional antagonism of the GABAA receptor. The objective of this thesis was to understand the effect of nerve injury on the inhibitory system at the level of the spinal cord.;The first aim was to select an appropriate animal model of nerve injury to study. Chapter 2 describes the advantages of blinding and persistence of mechanical sensitivity after injury in the spinal nerve ligation model but not in the spared nerve injury model. The second aim was to determine the effects of nerve injury on the GABAA receptor in the dorsal horn of the spinal cord and the DRG. Chapter 3 presents the results from Western blotting experiments of the spinal cord dorsal horn and DRG showing no change of the beta3 subunit protein. The third aim was to determine the effects of nerve injury on the KCC2 co-transporter in the dorsal horn of the spinal cord. Chapters 4 and 5 present results from Western blotting and immunohistochemical methodologies that demonstrated a regionally specific decrease in KCC2 protein levels in the lateral aspect of the dorsal horn. The fourth aim was to determine the effects of nerve injury on the NKCC1 protein on the terminals of the primary afferent neurons. Chapter 6 utilized Western blotting methodologies to demonstrate a significant change in NTKCC1 protein levels in the central terminals of the primary afferent neurons. Combined, these experiments identified a potential role for the chloride transporters in the development or maintenance of neuropathic pain.