Research has shown that spinal cord injury (SCI) can induce neural hyperexcitability within the spinal cord that facilitates nociceptive reflexes. Nociceptive inputs have been shown to sensitize spinal nociceptive systems, inducing a learning deficit and enhanced mechanical reactivity (EMR) in spinally transected rats. Nociceptive sensitization has been linked to abnormal GABA-mediated inhibition of nociceptive neurons within the spinal cord. However, underlying changes remain poorly understood. This dissertation were designed to test the effect of blocking GABA transmission on nociceptive sensitization after spinal cord injury.
Experiment 1 focused on the effect of bicuculline on shock-induced EMR in transected rats, finding blocking effect of bicuculline. Experiments 2 and 4 investigated whether bicuculline blocks inflammation-induced EMR. I found bicuculline pretreatment prevented both LPS and capsaicin-induced EMR. Further, capsaicin-induced EMR was reversed by bicuculline treatment (Experiment 5). Experiment 6 found that other GABA receptor antagonists also blocked the capsaicin-induced EMR.
Of clinical importance, bicuculline blocked indices of capsaicin-induced central sensitization at the mRNA level (Experiment 7) and protein level (Experiment 8). These results suggest that bicuculline blocks central sensitization in spinally transected rats and that GABA has an excitatory effect.
To explore whether a spinal transection alters GABA function, similar experimental manipulations were conducted in intact rats. Experiment 9 found that bicuculline treatment per se induced EMR and failed to block the capsaicin-induced EMR. Experiments 10 and 11 found that bicuculline did not block central sensitization at the cellular level. These results suggest that GABA inhibits nociceptive processing in intact rats, but promotes it after spinal injury.
Experiment 12 explored that spinal transection induced a downregulation of the membrane-bound KCC2, and thereby changed intracellular chloride homeostasis. To test whether drug manipulation targeting chloride co-transporters switch the role of GABA in nociceptive sensitization, channel blockers targeting KCC2 and NKCC1 were tested. Experiment 13 showed that blocking KCC2 in intact rats causes bicuculline to attenuate capsaicin-induced EMR. Conversely, Experiment 14 showed that blocking NKCC1 in transected rats switches how bicuculline affects capsaicin-induced EMR. Taken together, my results suggest that spinal cord injury switches the effect of GABA in nociceptive sensitization by altering the intracellular chloride homeostasis
