Effect of depolarizing GABAA-mediated membrane

20 In immature neurons activation of ionotropic GABA receptors induces depolarizing 21 membrane responses due to a high intracellular Cl-concentration ([Cl-] i). However, it 22 is difficult to draw conclusions about the functional consequences of subthreshold 23 GABAergic depolarizations, since GABAergic membrane shunting and additional 24 effects on voltage-dependent ion channels or action potential threshold must be 25 considered. In order to systematically investigate factors that determine the 26 GABAergic effect on neuronal excitability we performed whole-cell patch-clamp 27 recordings from Cajal-Retzius cells in immature rat neocortex using [Cl-] i between 10 28 and 50 mM. The effect of focal GABA application was quantified by measuring 29 various parameters of GABAergic responses including the shift in minimal threshold 30 current (rheobase). The rheobase shift was correlated with other parameters of the 31 GABAergic responses by multiple linear regression analyses with a set of simple 32 mathematical models. Our experiments demonstrate that focal GABA application 33 induces heterogeneous rheobase shifts in Cajal-Retzius cells that could not be 34 predicted reliably from [Cl-] i or the GABAergic membrane depolarization. 35 Implementation of a linear mathematical model, which is taking the GABAergic 36 membrane conductance and the difference between action potential threshold and 37 GABA reversal potential into account, resulted in a close correlation between 38 calculated and experimentally obtained rheobase shifts. Addition of a linear term 39 proportional to the GABAergic membrane depolarization improved the accuracy of 40 correlation. The main advantage of using multiple linear regression with simple 41 models is that direction and strength of GABAergic excitability shifts can be analyzed 42 using only measured parameters of GABAergic responses and with minimal a priori 43 information about cellular parameters.