Abstracts

RATIONALE: Deletion of the p35 gene -- that codes for the neuronal-specific activator of cyclin-dependent kinase 5 -- results in brain structural abnormalities including neocortical lamination defects. These animals also exhibit abnormal morphology of principal neurons in the hippocampus and dentate gyrus, as well as spontaneous behavioral and electrographic seizures. Therefore, this animal provides a unique opportunity to determine the electrophysiological correlates and consequences of abnormal morphology that may result in spontaneous seizures. The goal of this investigation is to elucidate the aberrant electrical interactions that underlie the epileptic phenotype in this animal model of cortical dysplasia.
METHODS: Intracellular sharp electrode recordings of dentate granule cells were obtained from 400 micron thick acute hippocampal slices prepared from both wild-type and knockout mice. Electrodes, backfilled with 2% biocytin in 1 M potassium acetate (pH 7.4, 100-220 M[Omega]), were used to measure the physiological responses of granule cells to stimulation (bipolar stimulating electrode) in CA3 stratum orients (to activate mossy fiber [ndash]MF- axons). Electrophysiologically characterized cells were filled with biocytin, fixed, and processed for light and/or electron microscopy (see accompanying poster by H.J. Wenzel, et.al.).
RESULTS: As previously shown, GCs with abnormal axon and/or dendritic morphology exhibit normal responses to intracellular-injected current. Responses to MF stimulation, however, revealed a significant difference between p35 -/- and wildtype granule cells. Whereas no GC in wildtype dentate responded to stimulation with an excitatory synaptic event, 68% of the p35 -/- cells (n=25) generate an excitatory post synaptic potential (EPSP) (z-test reveals a significant difference between the knockout and wild-type animals, with a P value of 0.002); in 76% of these cases, the EPSP triggered an action potential. Furthermore, in most p35 knockout cells in which an EPSP was elicited, stimulation failed to trigger an antidromic spike; Chi-square analysis indicates a significant negative correlation, with a P value of less than 0.001. Finally, initial analyses of early and late inhibitory post-synaptic potentials suggest a weaker inhibitory drive to GCs in p35 knockouts compared to wildtype controls.
CONCLUSIONS: Electrophysiological measurements of synaptic responses from granule cells in the p35-/- mice reveal a functionally aberrant dentate circuitry which may involve recurrent excitatory stimulation of granule cells. These results are consistent with morphological observations of recurrent axon collaterals and abnormal excitatory synaptic connections. Our results also suggest that inhibition in these epileptic mice may be abnormal [ndash] and thus contribute to the hyperexcitability that generates seizures.
[Supported by: NIH NS 18895 and GM 07108]