Abstracts

Rationale: Clarification of the subcellular intraneuronal chloride [Cl^-]_i) distribution is essential to understand the dynamics of GABAergic transmission and mechanisms of epilepsy because the driving force for GABA_A receptor is largely determined by the [Cl^-]_i in the micro-domain of synaptic inputs. In this view, extrasynaptic GABA_A receptor mediated-tonic Cl influx might play an important role for the dynamic spatial modification of the synaptic inhibition by focally changing [Cl^-]_i. Methods: We utilized the high spatial resolution of two-photon microscopy and the transgenically-expressed fluorophore Clomeleon to image intraneuronal chloride in acute hippocampal slices and organotypic slice cultures. Results: The value of dendritic [Cl^-]_i was more broadly distributed and lower than somatic [Cl^-]_i in both CA1 pyramidal neurons and interneurons. Application of 2 uM THIP, which at this concentration is a selective agonist for δ subunit containing extrasynaptic GABAA receptors, did not alter [Cl^-]_i of pyramidal cells. In interneurons, somatic [Cl^-]_i was unchanged but [Cl^-]_i was focally increased in some dendritic branches. In contrast, 2 uM THIP increased [Cl^-]_i more generally dentate granule cells, both at the soma and dendrites. These data do not exclude regional variance in Cl cotransport as the mechanism for the observed subcellular variance in THIP-induced changes in [Cl^-]_i. However, studies of dendritic Cl transport rates have not suggested such heterogeneity, and we will test this possibility more rigorously by repeating the current experiments in the presence of Cl cotransport inhibitors. We currently favor the idea that postsynaptic components of tonic GABA_A recaptor-mediated inhibition, which previously have only been evaluated electrophysiologically at single subcellular recording locations, may exhibit substantial subcellular variance. Conclusions: The subcellular localization of tonic GABA_A receptor-mediated inhibition implies both a richer computational role for this conductance in selected cell types, and the possibility that local postsynaptic plasticity underlies the observed subcellular variance.