Hippocampal slices from Mecp2 null mice are hyperexcitable due to impaired GABAergic inhibition: implications for Rett syndrome

POZZO-MILLER L., AMARAL M., INOUE T., HABLITZ J.& CALFA G

Amsterdam

Congreso; 7th Forum of European Neuroscience (FENS); 2010

Partial or generalized, convulsive or silent seizures are common clinical manifestations in Rett Syndrome (RTT), a neurodevelopmental disorder caused by mutations in the transcriptional regulator MECP2. Generalized myoclonic jerks coupled with recurrent abnormal EEG discharges were also observed in Mecp2 deficient mice. We studied the excitation/inhibition balance in acute hippocampal slices from Mecp2 null mice and wildtype littermates. Voltage-sensitive dye (VSD) signals evoked by single pulse stimulation of afferent fibers revealed a hyperexcitable hippocampal network. When the CA1 region was surgically isolated from CA3 (mini-slices), CA1 VSD signals evoked by Schaffer collateral stimulation were comparable between Mecp2 null and wildtype mini-slices. In addition, the enhancement of VSD signals by the GABAA receptor antagonist bicuculline was comparable between Mecp2 null and wildtype mini-slices, indicating that local GABAergic inhibition in CA1 is not affected by Mecp2 deletion. Quantitative EM and multiphoton imaging of synaptic vesicle recycling with FM1-43 revealed that synaptic structure and function in CA1 could not account for its hyperexcitability in Mecp2 null mice. Considering that surgical isolation increased CA1 VSD signals in wildtype slices making them comparable to Mecp2 null slices, we assessed the level of neuronal activity in area CA3. Indeed, extracellular multiunit recordings from CA3 stratum pyramidale and intracellular whole-cell recordings from CA3 pyramidal neurons revealed a higher frequency of spontaneous excitatory activity in Mecp2 null slices. Together with a reduced frequency of spontaneous IPSCs in CA3 pyramidal cells of Mecp2 null slices, these results indicate that the network hyperexcitability originates from a desinhibited CA3 region. Thus, Mecp2 deletion results in impaired development of GABAergic synapses in CA3 leading to an excitation/inhibition imbalance across the hippocampal network.