GABAA receptors are pentamers formed from a family of 19 different subunits. The most common receptor subtype is α1β2γ2. We are interested in receptors containing the γ1 subunit, because of its high expression in emotion-related brain regions, however the role of γ1 in neuronal inhibition is unknown.
We adopted an in-vitro synapse model to investigate functional synapses while varying the γ subunit expression. In this system, HEK293 cells were transfected with GABAA receptor subunits in addition to the synaptic adhesion molecule neuroligin, and plated onto mature cortical cultures, derived from embryonic rats. The cultured neurons formed synapses onto the HEK cells, allowing us to measure spontaneous inhibitory post-synaptic currents. For α2β2γ2 receptors, the decay time constant of the synaptic events (38.7 ± 3.0 ms) aligned well with data from rapid application experiments on the same receptors (44.9 ± 3.9 ms), however, replacing the γ2 subunit with γ1 resulted in significantly slower synaptic decay times (67.1 ± 7.6 ms) compared with rapid application (33.4 ± 4.2 ms), indicating that something other than channel gating determines the time-course of synaptic events from γ1-containing synapses. Mutations in the intracellular portion of the protein can eliminate the effect of γ1 on synaptic kinetics, which suggests a role for this subunit in determining the density of GABAA receptors at synapses. Preliminary results using the Monte Carlo simulation software MCell to model γ1- and γ2- containing synapses are also consistent with differences in receptor density at the post-synapse.