Abstracts

Multiple animal models of chronic epilepsy are associated with an upregulation of the GABA[sub]A[/sub] receptor (GABAR) [alpha]4 subunit. This subunit subtype, which is expressed in both thalamus and hippocampus, confers unusual pharmacological properties, including benzodiazepine insensitivity. However, little is known about the current kinetic properties of [alpha]4 subunit-containing GABARs, thus limiting understanding of their role in normal and abnormal physiology. We used ultrafast GABA application to recombinant GABARs expressed in HEK 293T cells to compare the kinetic properties of [alpha]4[beta]3[gamma]2L to the more ubiquitous [alpha]1[beta]3[gamma]2L receptors. Both receptor isoforms produced rapidly activating currents, with similar maximal currents and GABA potency. During long GABA application (1 mM for 4 sec), they both had four phases of desensitization with similar time constants. However, [alpha]4[beta]3[gamma]2L currents had more fast desensitization ([tau] = 15-100 ms) and less residual current at the end of 4 sec of GABA. Furthermore, these currents deactivated more slowly following a brief GABA application (5 ms, 1 mM). Repetitive GABA application reduced both peak current and overall charge transfer, more so for [alpha]4[beta]3[gamma]2L than for [alpha]1[beta]3[gamma]2L GABARs. To further explore the effect of desensitization on GABAR function, GABA concentration-response curves were generated following pre-exposure to 1 mM GABA. The responses to higher concentrations of GABA were suppressed for both GABAR isoforms, but much more for [alpha]4[beta]3[gamma]2L receptors. In summary, [alpha]4[beta]3[gamma]2L currents have enhanced desensitization compared to [alpha]1[beta]3[gamma]2L currents. The associated changes in GABAR current kinetics depend of the timing, duration and concentration of GABA being applied. The actual role of [alpha]4 subunit upregulation in epilepsy remains unknown. However, these results would predict that [alpha]4[beta]3[gamma]2L receptors may respond well to low frequency synaptic input but respond poorly during stronger synaptic activity. Furthermore, [alpha]4[beta]3[gamma]2L receptors may not respond well in a perisynaptic environment where they could be exposed to tonic, low levels up GABA (up to 1 mM). Future work in brain slices and intact animals will need to determine whether upregulation of [alpha]4 in epilepsy is associated with the kinetic changes described here. (Supported by K08 NS045122 to AHL R01 NS333000 to RLM.)