PROJECT SUMMARY Following spinal cord injury (SCI) at or above thoracic level 6 (T6), descending supraspinal control over the spinal neurons associated with sympathetic function is severed. This, along with plasticity within these neurons themselves, results in exaggerated sympathetic reflexes known as sympathetic hyperreflexia. This increase in sympathetic activity following SCI causes severe dysfunction of organs receiving sympathetic input, such as the spleen and vasculature. This contributes to cardiovascular disease and immune dysfunction, two leading causes of morbidity and mortality in the SCI population. Therefore, increasing understanding of the mechanisms underlying this plasticity may identify a possible therapeutic for sympathetic hyperreflexia that would enormously benefit SCI individuals. SCI-induced reduction of receptor cation-chloride cotransporter type 2 (KCC2) in neuronal membranes disrupts chloride homeostasis to decrease synaptic inhibition and has been implicated in heightened spinal motor reflexes after SCI. Whether KCC2 contributes to sympathetic hyperreflexia after SCI is not known. We theorize that SCI results in loss of KCC2 in the membrane of spinal neurons associated with sympathetic function to contribute to sympathetic hyperreflexia. We also postulate that the loss of KCC2 in the neuronal membrane results from activation of NF-B, a transcription factor that is downstream of multiple pro-inflammatory cytokines known to be increased following SCI. This proposal will focus on the hypothesis that hyperexcitability of spinal, sympathetically-associated neurons after SCI is due to downregulation of KCC2 expression via increased NF-B activation. The primary goals of this proposal are to: 1) investigate KCC2 in the development of sympathetic hyperreflexia following SCI and if enhancing KCC2 function mitigates sympathetic hyperreflexia (Aim 1); 2) determine if SCI-induced KCC2 downregulation occurs via NF-B (Aim 2).