Abstracts

RATIONALE: Cortical malformations represent one the major clinical findings in pediatric epilepsy. To study the role of disorganized neurons in epilepsy, we have been working with MAM-exposed rats. Hippocampal neuronal heterotopia in MAM rats are comprised, primarily, of glutamatergic pyramidal-like neurons. Although GABAergic innervation of hippocampal heterotopia is prominent [Baraban et al. Epilepsy Res. 39;87-102, 2000], the functional consequences of these inhibitory inputs have not been studied in great detail. To address the influence of GABAergic inhibition in hippocampal neuronal heterotopia, we examined the kinetic properties of inhibitory postsynaptic currents (IPSCs) on heterotopic cells.
METHODS: Pregnant S-D rats were injected with methylazoxymethanol (MAM: 25 mg/Kg, i.p.) on E15. Visualized whole-cell voltage-clamp recordings were performed on hippocampal slices (300 [mu]m thick; P14-P24) at room temperature. To isolate GABAergic synaptic currents (h.p., 0 mV), slices were perfused with nACSF containing 10 [mu]M CNQX/50 [mu]M APV. Spontaneous IPSCs were recorded on heterotopic and normotopic pyramidal (MAM) or CA1 pyramidal cells (Control). 10 [mu]M bicuculline, applied at the conclusion of a recording epoch, abolished sIPSCs confirming that they were mediated by activation of GABA-A receptors. Carbachol (5[mu]M) was applied to test the cholinergic facilitation of GABA release.
RESULTS: In hippocampal slices from MAM-exposed rats, heterotopic and normotopic pyramidal cells exhibited spontaneous IPSCs (frequency range: 0.2 to 3 Hz). sIPSC current amplitude was comparable between heterotopic (22.0 [plusminus] 2.1 pA; n = 14) and normotopic cells (22.7 [plusminus] 4.4 pA; n = 10). Interestingly, heterotopic cells exhibited sIPSCs with a slow decay time constant (26.7 [plusminus] 6.7 ms) in comparison with normotopic cells (8.8 [plusminus] 0.6 ms; p [gt] 0.05). In controls, CA1 pyramidal sIPSC amplitudes (17.9 [plusminus] 2.4 pA) and decay time constants (9.1 [plusminus] 0.9 ms; n = 17) were similar to values reported for normotopic pyramidal cells (MAM). In the presence of carbachol, changes in sIPSC frequency, decay and amplitude were similar for all cell types.
CONCLUSIONS: Here we describe GABA-A receptor mediated currents on heterotopic neurons with slow decay kinetics. Our findings suggest that GABAergic input to heterotopic cells is altered in a manner indicating enhanced inhibition. In light of our recent findings of cellular hyperexcitability associated with K channel defects on heterotopic neurons [Castro et al., J. Neurosci. in press], we propose that increased GABAergic inhibition of malformed brain regions is a compensatory mechanism to prevent the generation of seizure activity. Whether similar changes in synaptic plasticity occur in children with malformation-associated epilepsies remains to be determined.
Support: Parents Against Childhood Epilepsy