Abstracts

Rationale: Mutations in the gene, ARX, the Aristaless Related Homeobox gene, are associated with early-onset epileptic encephalopathy presentation, often as Infantile Spasms Syndrome (ISS). The most common ARX mutations are GCG expansions of the first or second polyalanine tract. A mouse model (Arx^(GCG)7/Y) of this common mutation recapitulates many of the phenotypes seen in humans, including the occurrence of seizures and cognitive impairment. Previous work from the Kitamura lab (2009) demonstrated that hippocampal projecting cholinergic neurons are reduced by ~77% in mice with a GCG mutation. As the hippocampus is often involved in epileptogenesis and cognitive dysfunction, we hypothesized that this alteration in cholinergic input is contributing to the epilepsy and cognitive dysfunction phenotype in the mice and humans. Thus, we investigated how the GCG mutation affects cholinergic modulation of hippocampal CA1 pyramidal neurons in an Arx^(GCG)7/Y mouse model.

Methods: Post-natal day 30 (P30) Arx^(GCG)7/Y mice, containing an additional 7 alanines in the first polyalanine tract in Arx, were used for in vitro patch-clamp studies. Whole-cell current clamp was used to record evoked firing activity and passive membrane properties from CA1 pyramidal cells. Electrophysiological properties were recorded before and during the bath administration of 5 or 20 µM carbachol.

Results: Evoked firing activity of Arx^(GCG)7/Y and WT CA1 pyramidal neurons excitability are similar at P30. Administration of 5 μM carbachol decreased rheobase and increased evoked firing activity in both genotypes. However, carbachol’s enhancement of firing activity was significantly less in Arx^(GCG)7/Y mice. Membrane properties such as resting membrane potential and input resistance showed no differences between genotypes and were slightly affected by carbachol. A minority of cells from WT mice displayed bursting activity in response to carbachol, while no cells from Arx^(GCG)7/Y showed similar activity.

Conclusions: Our study suggests that the polyalanine expansion reduces cholinergic modulation of the hippocampal excitability in mature mice. This reduced cholinergic input to the hippocampus appears to paradoxically contribute to the development of seizures and cognitive impairments by an unknown mechanism. Future work will determine how reduced cholinergic excitability leads to seizures and cognitive dysfunction.

Funding: Please list any funding that was received in support of this abstract.: This work was supported by the NIH grant R01 NS100007-03 and the NIH institutional training grant T32 NS007413.