Abstracts

Rationale: Gamma-frequency oscillations (γ oscillations; 30-120 Hz) are thought to be important for cognitive processing, and similar fast rhythms may also play a role in the transition from physiological to pathological synchrony during ictogenesis. Synaptic GABA_A receptor-mediated inhibition is critical for controlling spike timing during γ oscillations. However, GABA_A receptors are also located peri- and extra-synaptically, where they mediate tonic inhibitory currents that should affect network dynamics. In many neuronal cell types, tonic inhibition is mediated by δ subunit-containing GABA_A receptors (δ-GABA_AR). In the hippocampal CA3, δ-GABA_AR appear to be localized predominantly to GABAergic interneurons, and here we use δ subunit knockout mice to determine the role of these receptors in modulating cellular excitability and γ-frequency network synchronization. Methods: Horizontal hippocampal slices were prepared from C57/Bl6 and Gabrd-/- mice (Jackson Labs, Bar Harbor, ME), and maintained at room temperature in an interface chamber between humidified carbogen gas (95% O₂ / 5% CO₂) and artificial cerebrospinal fluid (aCSF) containing in (mM): NaCl (126), KCl (2.5), CaCl₂ (2), MgCl₂ (2), NaH₂PO₄ (1.25), NaHCO₃ (26) and D-glucose (10) (pH 7.3 - 7.4). Network activity was recorded with aCSF-filled patch pipettes in the CA3 pyramidal layer. GABAergic currents were recorded in the presence of 3 mM kyneurinic acid or 20 µM DNQX, with an internal solution containing (in mM): CsCl (140), MgCl₂ (1), HEPES (10), NaCl (4), EGTA (0.1), Mg-ATP (2), Na-GTP (0.3), QX-314 (5) and biocytin (5 %). Synaptic events during network oscillations were recorded with an internal solution containing (in mM): cesium-methylsulfonate (140), HEPES (10), EGTA (0.2), NaCl (5), MgATP (2), and NaGTP (0.2). All recordings were performed at 32-34 ^oC. Results: We show that the frequency of cholinergically-induced (20 µM carbachol) γ oscillations in the hippocampal CA3 region in vitro is higher in δ subunit knockout mice compared to wildtype controls (64.8 ± 2.7 vs 43.9 ± 2.2 Hz). This frequency modulation results from a selective reduction in the tonic inhibition of CA3 interneurons compared to pyramidal cells, and a subsequent enhancement in their sensitivity to NMDA receptor-mediated excitation. Recordings of synaptic currents during gamma activity show that NMDA receptor-mediated increases in oscillation frequency correlate with a progressive synchronization of phasic excitation and inhibition in the network. Conclusions: These data suggest that the balance between tonic excitation and tonic inhibition of interneurons may modulate the frequency of hippocampal γ oscillations by shaping interneuronal synchronization. High-frequency oscillations that precede seizures may reflect interneuronal hyperexcitability, and we are currently exploring the role interneuronal tonic inhibition in modulating the generation of epileptiform bursts. (Supported by Epilepsy Foundation Postdoctoral Fellowship to EOM and NIH grants NS30549, NS02808, and the Coelho Endowment to IM).