The effects of traumatic brain injury on neuronal GABA(A) receptors in cultured cortical neurons

Approximately 1.5 million Americans sustain a traumatic brain injury (TBI) each year. This research evaluates specific cellular mechanisms related to the GABAA receptor that may mediate neuronal dysfunction following TBI, and may contribute to the development of novel therapies to treat TBI. GABAA currents were found to be similar in control and mildly stretched cortical neurons when examined immediately post-injury. GABA A currents were larger in amplitude in neurons grown on mature astrocytes compared to currents in mixed cultures of neurons and glia.;Zinc and zolpidem neuromodulation of the GABAA receptor were evaluated by measuring macroscopic currents and miniature inhibitory post-synaptic currents (mIPSCs). Zolpidem potentiated GABAA currents in both control and stretched neurons. Zinc inhibited whole cell currents equally in stretched and control neurons, but a trend indicates zinc inhibition of mIPSCs may be decreased by stretch. Zinc block appeared to be mildly voltage sensitive. These results suggest that zinc block of whole cell versus synaptic GABAA receptors may be differentially modulated by injury.;The number of propidium iodide positive cells, an indicator of cell death or injury, was greater in stretched than control cultures. Propidium iodide uptake was further increased in stretched cultures after exposure to the GABA A receptor antagonist bicuculline. This effect of bicuculline was attenuated by the inclusion of sodium channel blocker tetrodotoxin (TTX), suggesting involvement of increased neuronal spiking in the potentiation of neuronal injury. Bicuculline increased action potential frequency, and stretch injury trends toward a reduction in action potential frequency in current-clamped neurons. These results indicate GABAA receptor inhibition attenuates post-injury excitation and is critical for neuronal survival.;Before and after injury the beta2/3 subunit of the GABA A receptor was observed in vitro, and the anchoring protein gephyrin as well as the alpha5 and gamma subunits were expressed in vivo. The intermediate affinity of these neuronal GABAA receptors to zolpidem, and their relatively low sensitivity to zinc are consistent with mediation by alpha2/3beta2/3gamma2 receptor subunits. Compared to earlier results from this laboratory, it appears that the method of cell culture used directly impacts upon the expression, regulation, and response to injury of cultured cortical neurons.