Are Altered Excitatory Synapses Found in Neuronal Migration Disorders?
Authors: Dudek F. Edward 
Published: September 2005

Physiological and Morphological Characterization of Dentate Granule Cells in the p35 Knock-out Mouse Hippocampus: Evidence for an Epileptic Circuit

Patel LS, Wenzel HJ, Schwartzkroin PA

J Neurosci 2004;24:9005–9014

There is a high correlation between pediatric epilepsies and neuronal migration disorders. What remains unclear is whether intrinsic features of the individual dysplastic cells give rise to heightened seizure susceptibility, or whether these dysplastic cells contribute to seizure activity by establishing abnormal circuits that alter the balance of inhibition and excitation. Mice lacking a functional p35 gene provide an ideal model in which to address these questions, because these knockout animals not only exhibit aberrant neuronal migration but also demonstrate spontaneous seizures.

Extracellular field recordings from hippocampal slices, characterizing the input–output relation in the dentate, revealed little difference between wild-type and knockout mice under both normal and elevated extracellular potassium conditions. However, in the presence of the GABAA antagonist bicuculline, p35 knockout slices, but not wild-type slices, exhibited prolonged depolarizations in response to stimulation of the perforant path. No significant differences were found in the intrinsic properties of dentate granule cells (i.e., input resistance, time constant, action-potential generation) from wild-type versus knockout mice. However, antidromic activation (mossy fiber stimulation) evoked an excitatory synaptic response in more than 65% of granule cells from p35 knockout slices that was never observed in wild-type slices. Ultrastructural analyses identified morphological substrates for this aberrant excitation: recurrent axon collaterals, abnormal basal dendrites, and mossy fiber terminals forming synapses onto the spines of neighboring granule cells. These studies suggest that granule cells in p35 knockout mice contribute to seizure activity by forming an abnormal excitatory feedback circuit.

Prolonged NMDA-mediated Synaptic Response, Altered Ifenprodil Sensitivity, and Generation of Epileptiform-like Events in a Malformed Hippocampus of Rats Exposed to Methylazoxymethanol in Utero

Calcagnotto ME, Baraban SC

J Neurophysiol 2005 [Epub ahead of print]

Cortical malformations are often associated with refractory epilepsy and cognitive deficit. Clinical and experimental studies have demonstrated an important role for glutamate-mediated synaptic transmission in these conditions. With whole-cell voltage-clamp techniques, we examined evoked glutamate-mediated excitatory postsynaptic currents (eEPSCs) and responses to exogenously applied glutamate on hippocampal heterotopic cells in an animal model of malformation (i.e., rats exposed to methylazoxymethanol [MAM] in utero). Analysis of eEPSCs revealed that the late N-methyl-D-aspartate (NMDA) receptor–mediated eEPSC component was significantly increased on heterotopic cells compared with age-matched normotopic pyramidal cells. At a holding potential of +40 mV, heterotopic cells also exhibited eEPSCs with a slower decay-time constant. No differences in the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) component of eEPSCs were detected. In 23% of heterotopic pyramidal cells, electrical stimulation evoked prolonged burstlike responses. Focal application of glutamate (10 m M) targeted to different sites near the heterotopia also evoked epileptiform-like bursts on heterotopic cells. Ifenprodil (10 μM), an NR2B subunit antagonist, only slightly reduced the NMDA receptor–mediated component and amplitude of eEPSCs on heterotopic cells (MAM) but significantly decreased the late component and peak amplitude of eEPSCs in normotopic cells (Control). Our data demonstrate a functional alteration in the NMDA-mediated component of excitatory synaptic transmission in heterotopic cells and suggest that this alteration may be attributable, at least in part, to changes in composition and function of the NMDAR subunit. Changes in NMDA-receptor function may directly contribute to the hyperexcitability and cognitive deficits reported in animal models and patients with brain malformations.