The Origin of Hyperexcitability in Area CA1 of the Hippocampus in Mecp2 Null Mice

CALFA G, HABLITZ JJ AND POZZO-MILLER L

Chicago

Congreso; 10th Annual Rett Syndrome Symposium, International Rett Syndrome Foundation (IRSF); 2009

IRSF

Consistent with potential miswiring of excitatory and inhibitory synaptic networks in seizure-prone regions of the brain, partial and generalized, convulsive or silent (i.e. absence) seizures are common clinical manifestation in RTT patients. Mecp2308 knock-in mice also display repetitive generalized myoclonic jerks coupled with high-amplitude bilateral cortical spike and wave EEG discharges. However, electrophysiological studies described an imbalance of neuronal networks in favor of neuronal inhibition. To directly estimate the excitation/inhibition balance in the hippocampus, seizure-prone region of the brain, we performed voltage-dye imaging in acute slices from symptomatic Mecp2 null mice (5-6 weeks) and age-matched wildtype littermates. Hippocampal slices (300μm-thick) were stained with RH-414 (30mM) and imaged with an array of photodiodes. The spread and duration of membrane depolarization in CA1 stratum radiatum evoked by a single afferent stimulation (100μsec, 30μA) was significantly larger and longer lasting in Mecp2 null slices compared to wildtype littermates. Similar differences were observed at all stimulation intensities tested. After K+ channel blockade with 4-AP (125mM),Mecp2 null slices also showed larger spread and duration of membrane depolarization than wildtype slices. Since 4-AP blocks K+ channels in both glutamatergic and GABAergic presynaptic terminals, we tested the consequences of GABA-AR blockade with bicuculline (Bic; 10mM). Areas CA2-CA3 were surgically isolated from the imaging region in CA1 to avoid generalized neuronal discharges. Intriguingly, isolating CA1 from CA2-CA3 enhanced the voltage-dye signals in wildtype control slices, thus equalizing their responses to those observed in Mecp2 null slices. Furthermore, Bic enhanced the spatial spread and duration of voltage-dye signals in both wildtype and Mecp2 null mice by comparable amounts. These results indicate that Mecp2 deficiency causes an imbalance of excitatory and inhibitory synaptic activities, but favoring network hyperexcitability, as well as enhancing the sensitivity to epileptogenic manipulations. In addition, surgical isolation of area CA1 equalized the responses of wildtype and Mecp2 null slices, suggesting a misregulation of the GABAergic tone in area CA3. Further studies on the state of the GABAergic inhibitory network in CA3 are currently under way.