Impaired presynaptic structure and function in area CA1 of symptomatic Mecp2 null mice may not account for hippocampal hyperexcitability

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

San Diego

Congreso; Society for Neuroscience - 40th Annual meeting - Neuroscience 2010; 2010

Imbalances in the excitation/inhibition (E/I) ratio have emerged as common features in disorders associated with intellectual disabilities, autism and seizure activity, all of which are common clinical manifestations of Rett syndrome, a neurodevelopmental disorder caused by mutations in the transcriptional regulator MeCP2. Here, we characterized network excitability by voltage-sensitive dye (VSD) imaging in acute hippocampal slices. VSD signals evoked by a single stimulation pulse to afferent Shaffer collaterals (SC) had larger amplitude, were longer lasting and spread more in area CA1 of slices from symptomatic Mecp2 null mice than in those from their wildtype littermates (P35-P42). Furthermore, these differences were not observed between slices of younger, pre-symptomatic Mecp2 null mice and their wildtype littermates (P16-23). However, all features of VSD signals (i.e. amplitude, duration and spatial spread) were significantly larger in slices from the younger mice of both genotypes. In addition, all features of VSD signals decreased more in wildtype slices between P16-23 and P35-P42 than in Mecp2 null slices, suggesting that this developmental reduction in hippocampal excitability is impaired in Mecp2 deficient mice. Quantitative electron microscopy analyses revealed that asymmetric (excitatory) and symmetric (inhibitory) synapses in CA1 stratum radiatum of symptomatic Mecp2 null mice have fewer synaptic vesicles docked at the active zone than in wildtype littermates, without any other differences in synaptic structure. Multiphoton imaging of FM1-43 revealed that activity-dependent vesicle release in distal (excitatory) and proximal (inhibitory) synapses in CA1 was impaired in acute slices from symptomatic Mecp2 null mice, but only from the readily releasable pool mobilized by hyperosmotic solutions. Finally, blocking GABAergic inhibition with bicuculline in surgically isolated CA1 mini-slices enhanced the duration and spatial spread of SC-evoked VSD signals to a comparable extent in both genotypes. In addition, the amplitude, duration and spatial spread of these VSD signals were similar in surgically isolated mini-slices from symptomatic Mecp2 null and wildtype mice, suggesting that Mecp2 deletion does not affect GABAergic feed-forward inhibition in CA1. Taken together, these observations strongly suggest that the hyperexcitability in CA1 of symptomatic Mecp2 null mice originates from an E/I imbalance in area CA3. Current experiments are directly characterizing the E/I balance in CA3 pyramidal neurons and its developmental progression.