Hippocampal hyperexcitability in Mecp2 null mice: a voltage-sensitive study

G. D. CALFA, J. J. HABLITZ, L. POZZO-MILLER

Congreso; Society for Neroscience - Neuroscience 2008; 2008

Some of the most important clinical manifestations in Rett syndrome (RTT) are partial and generalized convulsive or silent (i.e. absence) seizures, with concomitant EEG impairments. Experiments in mouse models of RTT have shown an imbalance of neuronal networks in favor of neuronal inhibition, suggesting that the absence or loss-of-function mutations in Mecp2 specifically impair excitatory synapse number and/or function. To directly estimate the excitation/inhibition balance in a seizure prone region of the brain, we performed voltage-dye imaging in hippocampal slices from symptomatic Mecp2 null mice (5-6 weeks). Acute hippocampal slices (300μm-thick) were stained with RH-414 and imaged with an array of fast photodiodes. The spread of membrane depolarization in CA1 stratum radiatum evoked by a single afferent stimulation (100μsec, 30μA) is significantly larger in Mecp2 null mice compared to wildtype littermates. In addition, bath application of the K+ channel blocker 4-AP caused a much larger, longer lasting and further spreading depolarization in Mecp2 null mice than in wildtype animals. To test whether this hyperexcitability is due to enhanced transmitter release from excitatory presynaptic terminals, we performed multiphoton imaging of the recycling dye FM1-43. Despite the pronounced hyperexcitability of Mecp2 null slices, and the reports of impaired release from  excitatory
synapses in cultured Mecp2 null neurons, the rate of FM1-43 destaining from the total recycling pool of vesicles is similar in CA1 stratum radiatum of Mecp2 null and wildtype slices. Current studies are  addressing the possibilities of differential effects on the total vs. the readily releasable pool, which can be assessed by using sucrose-evoked FM dye loading (Tyler et al. J Physiol 2006), and the release capability of inhibitory synapses, which can be identified by their location and pharmacological properties (Mathew et al. J Neurosci 2008).