Abstracts

Previous work has identified the p35 (neuronal specific activator of cyclin-dependent kinase 5) knockout mouse as a useful model in which to study the relationship between dysplasia and spontaneous behavioral and electrographic seizures. One feature identified in the knockout animal that may contribute to seizure activity is recurrent excitation of the dentate granule cells via aberrant mossy fiber collaterals and basal dendrites. However, little is known about the local inhibitory circuit and its contribution to the suppression (or facilitation) of granule cell excitability. We hypothesize that p35-dependent migrational abnormalities affect dentate interneuron (IN) connectivity and function, and that these abnormal circuitry features contribute to seizure susceptibility and influence the net output of granule cells.
Acute hippocampal slice preparations from both wild-type and knockout animals were used for intracellular recordings from dentate granule neurons. Sharp electrodes were used to measure the electrophysiological responses of granule cells to orthodromic (perforant path) stimulation. Electrodes backfilled with 2% biocytin were also used to label intracellularly electrophysiologically-identified interneurons. After filling, slices were fixed and processed for light and ultra-structural analyses. Immunocytochemistry (ICC) of GABA and parvalbumin (Parv) was carried out in parallel studies.
Of 24 electrophysiologically identified granules cells in p35 knockout animals, 13 showed a reduction in the early GABA[sub]A[/sub]-mediated inhibitory post-synaptic potential (IPSP), and 21 demonstrated a reduction in the late GABA[sub]B[/sub]-mediated IPSP following stimulation of the perforant path (compared to wild-type mice, N = 7). ICC revealed a small reduction in Parv positive interneurons in -/- mice, with a significant decrease in immunoreactivity in the axonal plexus around the GCs. GABA and Parv ICC from p35 -/- tissue also revealed displaced (heterotopic) GABA/Parv-positive IN distribution across the granule cell and molecular layers (GCL [amp] ML). Biocytin-labeled GABAergic basket cells were displaced within the GC/MLs. These cells exhibited a normal-appearing axonal arbor within the GCL; however, extensive axonal arborization was also observed in regions where GCs were dispersed (i.e., hilus and ML). Whereas, interneurons in wild-type mice show a characteristic laminar distribution of their axonal arbors, in p35 knockout mice these INs expand their axonal territories across the dentate layers into the hilus and the IML, to [ldquo]follow[rdquo] the dispersed GCs.
These studies identify a feature of the p35 neuronal migration disorder that may contribute to seizure activity [ndash] an abnormal inhibitory circuit. Electrophysiological recordings from granule cells demonstrate a reduction in inhibitory input. ICC and intracellular labeling of individual interneurons from p35 knockout mice confirm abnormalities in soma and axon distribution of inhibitory interneurons.
[Supported by: NIH grant NS18895.]