Saturday,December 2, 2006 Basic Scientist Poster Session 12: p.m.–2:00 p.m.

Abstract

Abstract 1 Daniel L. Jones, 1 Joel S.F. Greenwood, and 1,2 Scott C. Baraban ( 1 Neuroscience Graduate Program, University of California–San Francisco, San Francisco, CA ; and 2 Neurological Surgery, University of California–San Francisco, San Francisco, CA ) Rationale: Type 1 Lissencephaly is a neuronal migration disorder marked by an absence of gyri, disrupted cortical lamination, and epilepsy. Previous studies in a mouse model of Lissencephaly (Lis-1 heterozygotes) reported a severely disorganized and hyperexcitable hippocampus (Fleck et al 2000). Additionally, Lis1 ± mice show impaired neuronal migration. To further examine functional consequences of Lis1 haploinsufficiency, we studied inhibitory and excitatory synapses in Lis-1 heterozygote mice. Methods: We obtained whole-cell current clamp recordings from visually identified interneurons in st. radiatum, lacunosum-moleculare, and oriens of area CA1 in acutely prepared brain slices. Steady-state firing properties were examined in response to current steps of varying amplitude and length. We also recorded spontaneous and evoked excitatory events from CA1 pyramidal neurons in whole-cell voltage clamp configuration. Histology was performed on 40 micron floating sections using the standard diaminobenzidine (DAB) method. Results: In analysis of interneuron firing patterns, we observed all major interneuron classes (e.g., accommodating [AC], nonaccommodating [NAC], and bursting [BST]) in both Lis1 ± and age-matched WT hippocampi. However, we found significantly more NAC cells in Lis1 ± strata radiatum and lacunosum-moleculare (RLM), but fewer in stratum oriens/alveus (OA). Immunostaining for parvalbumin (Pv), a marker for NAC cells, confirmed these electrophysiological observations; Pv-positive somata shifted from OA to RLM in Lis1 mutants. Staining patterns for other interneuron markers appeared similar between WT and mutant animals. Analysis of spontaneous and miniature inhibitory postsynaptic currents is underway. In separate studies, staining for AMPA receptors, NMDA receptors, and glutamate transporter expression appeared normal. In voltage-clamp recordings, spontaneous and miniature EPSCs were similar in frequency, amplitude, and pharmacology between Lis1 mutants and WT controls. Evoked EPSCs were also similar in input/output and decay kinetics. Interestingly, focal application of glutamate via a picospritzer revealed maps of excitatory connectivity that were significantly larger in the disorganized Lis1 mutant hippocampus compared to controls. Conclusions: Our results demonstrate that a population of NAC interneurons migrate to an incorrect location within the Lis1 ± hippocampus; this is likely to have significant functional consequences with respect to hippocampal circuit function and epilepsy. Additionally, dysplastic pyramidal cells, exhibit enhanced excitatory synaptic connectivity consistent with a hyperexcitable phenotype. These mice continue to provide important insights to how a malformed brain develops and functions. (Supported by NIH (to SCB) and EFA Pre-Doctoral Fellowship (to JSFG).)