GABA evokes a membrane depolarization in immature neurons due to a high intracellular Cl^- concentration ([Cl^-]_i). We recently demonstrated that the Cl^- uptake in Cajal-Retzius cells is accomplished by a Na⁺ dependent K⁺-2Cl^--cotransport, which operates at a very low capacity (Achilles et al., 2007, J. Neurosci. 27:8616). To investigate the functional consequences of such an inefficient homeostatic process, we performed gramicidin-perforated and whole-cell patch-clamp recordings of visually identified Cajal-Retzius cells in tangential slices of neonatal (P1-3) rats.

While GABA had an excitatory effect on Cajal-Retzius cells under resting conditions, focal application of GABAergic pulses with high frequency or a burst of Carbachol-induced GABAergic postsynaptic currents reduced the excitatory potential of GABAergic depolarizations. To investigate to which extent activity dependent alterations in [Cl^-]_i contribute to the switch from excitatory to inhibitory action, we performed whole-cell recordings using defined pipette solutions containing 10, 15, 20 and 30 mM Cl^-. To quantify the excitatory potential of GABAergic responses (defined as the GABA-mediated reduction in AP threshold currents), subthreshold and suprathreshold current pulses of different amplitudes were paired with subthreshold depolarizations induced by focal GABA applications. These experiments revealed that the excitatory potential of GABA significantly depends on the Cl^- concentration of the pipette solutions. In further experiments we altered the timing between GABA pulse and injected current, which revealed that in Cajal-Retzius cells excitatory GABA responses can be elicited with rather large temporal latencies, while inhibitory responses require a closer temporal resolution. Calculation using a Goldman model in a one compartment neuronal model revealed that at intermediate [Cl^-]_i the GABAergic effect critically depends on the timing between both stimuli. Combined current clamp recordings and impedance measurements demonstrated that the interplay between GABAergic effects and the [Cl^-]_i depends on the balance between membrane depolarization and a decrease in input resistance. In contrast, neither the slope of the depolarization nor the [Cl^-]_i had a direct effect on action potential threshold in Cajal-Retzius cells.

In summary, these results suggest that a [Cl^-]_i decrease within the physiological range will change GABAergic effects in a temporally complex manner. Thus activity-dependent alterations in [Cl^-]_i will modify GABAergic influences on excitability and information processing in the immature neocortex.