Abstracts

Rationale: GABAergic inhibition, which is responsible for balancing excitability in the mature cortex, is depolarizing early in development and facilitates seizures in neonatal mice (Dzhala et al. 2005). From studies in the mature hippocampus, we know that the majority of GABAergic inhibition to interneurons is supplied by tonic currents that are mediated by extrasynaptically located GABA_ARs (Semyanov et al. 2003). Here we examined fundamental questions regarding GABAergic transmission that may refine pharmacological treatments for epilepsy. Namely, we studied i.) the relative contributions of phasic vs. tonic currents to overall GABAergic transmission in the immature and adult cortex and ii.) the role of tonic currents in modulating cortical excitability when GABA is depolarizing vs. hyperpolarizing. Methods: In the whole-cell voltage clamp configuration, we recorded phasic (spontaneous IPSCs) and tonic GABAergic currents from pyramidal cells and interneurons in layer V of the somatosensory cortex in the presence of 5 µM bath applied GABA. Recordings were obtained from P7-10 (immature) and P30-40 (mature) CD-1 mice. To monitor changes in excitability, cell-attached recordings were used to measure the spontaneous firing frequencies of pyramidal cells and interneurons before and after tonic currents were pharmacologically enhanced or suppressed. Results: The mean tonic current amplitude recorded from cortical pyramidal cells increases during development (P7-10 vs. P30-40) and is paralleled by a developmentally dependent increase in the mean phasic current. Given the increase in both forms of GABAergic transmission during development, tonic current continues to contribute the majority of total GABAergic current as the cortex matures. Further, in the mature cortex tonic currents measured from pyramidal cells are partially mediated by GABA_ARs containing the α5 subunit. Preliminary data suggest that the mean tonic current amplitude of pyramidal cells is greater than that recorded from interneurons. Conclusions: Although the phasic and tonic currents exhibited by immature pyramidal cells are reduced relative to those recorded from mature pyramidal cells, the relative contribution of tonic current to the total GABAergic current remains stable as the cortex matures. This suggests that synaptically released GABA influences the magnitude of tonic GABAergic currents in a developmentally independent manner thereby stabilizing the ratio of phasic to tonic currents. In the future, we will determine whether tonic currents in the immature cortex enhance excitability in an effort to understand the mechanisms underlying neonatal seizures.