Abstracts

GABA-mediated synaptic inhibition is the most targeted pathway among known inherited epilepsies in humans. Recent progress has been made in identifying molecules that control the development of cortical GABAergic neurons. For example, the Dlx family of transcription factors is essential for differentiation of GABAergic neurons in the embryonic ganglionic eminences, and for tangential migration of GABAergic interneurons into cerebral cortex and hippocampus. Here, we use morphological and electrophysiological approaches to study alterations in GABA-mediated inhibition in mice lacking Dlx1 and its implications for hyperexcitability. For immunohistochemistry, staining was performed on 10-40[mu]m brain sections using antibodies against GABA, somatostatin, parvalbumin, calretinin, calbindin and NPY. For electrophysiology, IR-DIC visualized whole-cell voltage-clamp technique was used to record spontaneous and evoked inhibitory postsynaptic currents (sIPSCs-eIPSCs) from neocortical and CA3 pyramidal cells in brain slices from Dlx1^-/-and Dlx^{+/[sub] ([/sub]}control) at 2 different ages (1 and 2 m.o). To isolate GABAergic synaptic currents, slices were perfused with nACSF containing CNQX and APV. IPSCs were recorded at h.p. of 0 mV and evoked at 0.1 Hz using a concentric bipolar electrode. IPSCs were abolished by 10 [mu]M bicuculline. In Dlx1^-/- mice, the number of GABAergic neurons (primarily those immunoreactive for GABA, NPY and calretinin) gradually decreased in all cortical and hippocampal regions starting at 1 month of age. At 2 months of age, an approximately 30% decrease in the number of GABA-IR was noted. Analysis of sIPSC kinetics in Dlx1^-/- mice (1 m.o) revealed a small decrease in frequency and amplitude when compared with controls ([italic]n=[/italic]10). At 2 m.o., decreases in sIPSC frequency and amplitude reached statistical significance ([italic]n=[/italic]20; p[lt]0.05). No significant changes in IPSC decay time constant or 10-90% rise time were observed at either timepoint. Coincident with the loss of GABA interneurons and reductions in synaptic inhibition, seizures were observed in 14 out of 19 Dlx1^-/-mice video monitored at 2 m.o. Adult Dlx1^-/-mice display a time-specific loss of cortical and hippocampal GABAergic interneurons. This reduction in interneuron density leads to impairment of the inhibitory network in regions linked to epileptogenesis (e.g., neocortex and hippocampus) and spontaneous seizures. To our knowledge, this is the first evidence of a requirement for Dlx transcription factor expression in the proper function of cortical interneurons in adult brain. Thus, Dlx mutants may serve as an important new model to study epileptogenesis in an animal with [quot]programmed[quot] loss of interneurons. (Supported by NARSAD (National Alliance for Research on Schizophrenia and Depression) to J.L.R. and CURE (Citizens United for Research in Epilepsy) to S.C.B.)