An excitatory-inhibitory imbalance in area CA3 causes network hyperexcitability in hippocampal slices from Mecp2 null mice

AMARAL MD, LI Y, CALFA G, POZZO-MILLER L

Leesburg, Virginia

Congreso; 11th Annual Rett Syndrome Symposium, International Rett Syndrome Foundation (IRSF); 2010

IRSF

An imbalance of excitatory and inhibitory synaptic function in the hippocampus has been previously implicated in neurodevelopmental disorders associated with cognitive impairments and mental retardation. In addition, BDNF is a critical factor for the formation and maturation of inhibitory GABAergic synapses during development. These facts led us to conjecture that impaired development of inhibitory GABAergic synapses due to reduced activity-dependent BDNF release from Mecp2-deficient neurons causes an imbalance of excitatory and inhibitory synaptic function in the hippocampus. Based upon voltage-sensitive dye imaging studies in acute hippocampal slices from symptomatic Mecp2 null mice, we have determined that the origin of the observed network hyperexcitability lies upstream of the CA1 region, likely originating within the CA3 network (see Calfa et al. poster). In support of this model, extracellular multi-unit recordings from the somatic cell layer of area CA3 of acute hippocampal slices revealed a higher frequency of spontaneous activity in slices from symptomatic Mecp2 null mice compared to slices from wildtype littermates. Furthermore, voltage-clamp recordings from CA3 pyramidal neurons revealed that spontaneous inhibitory postsynaptic currents (IPSCs) had significantly smaller amplitudes in slices from early symptomatic Mecp2 null mice in comparison to those from wildtype mice. In order to further characterize hippocampal function in Mecp2 null mice, we investigated the effect of endogenous BDNF, released via theta burst stimulation of mossy fibers (MFs), on CA3 pyramidal neurons. In
wildtype mice, endogenous BDNF elicited a slowly developing cationic current, which we henceforth denote MF-IBDNF, and intracellular Ca2+ elevations (Li et al. J Neurophys 2010) with the same pharmacological profile of the TRPC3-mediated IBDNF activated in CA1 neurons by brief localized applications of recombinant BDNF (Amaral & Pozzo-Miller, J Neurosci 2007; Amaral & Pozzo-Miller, J Neurophys 2007). However, the same manipulation performed in Mecp2 null mice yielded significantly reduced MF-IBDNF. Further studies are required to determine whether this reduced MF-IBDNF reflects impaired presynaptic BDNF release from MFs or reduced expression of TRPC3 channels in postsynaptic CA3 pyramidal neurons. To further increase our understanding of direct BDNF actions on GABAergic interneurons, we also conducted simultaneous whole-cell recording and intracellular Ca2+ imaging experiments from interneurons in CA3 stratum lucidum during localized applications of recombinant BDNF via a picospritzer. These preliminary studies in wildtype slices suggest that BDNF elicits a membrane current and dendritic Ca2+ signals in CA3 interneurons similar to those exhibited in CA1 pyramidal neurons (Amaral & Pozzo-Miller, J Neurosci 2007; Amaral & Pozzo-Miller, J Neurophys 2007). Taken altogether, these results support our model that GABAergic inhibition in area CA3 fails to properly develop in Mecp2 null mice, leading to a desinhibited CA3 region that causes network hyperexcitability in hippocampal slices. Future studies will attempt to enhance GABAergic interneuron development and function by increasing BDNF release from mossy fibers.