Abstracts

Since the observation that malformed brain structure is associated with intractable forms of epilepsy, there has been a great deal of interest in cortical dysplasia. Although recent studies suggest that GABAergic innervation may be altered in human dysplastic tissue (Ferrer et al., 1994; Spreafico et al., 1998, 2000), functional consequences of these anatomical observations have not been elucidated. To study synaptic inhibition in a malformed brain, we have been working with tissue slices obtained from patients undergoing surgery for medically intractable epilepsy with focal cortical dysplasia (FCD). Here we examined the kinetic properties of inhibitory postsynaptic currents (IPSCs).
Brain tissue was obtained from epileptic patients with FCD who underwent surgery at UCSF (8-32 yo). Visualized whole-cell voltage-clamp recordings were performed on cortical slices (300 [micro]m thick; room temp.) To isolate GABAergic synaptic currents (h.p. 0 mV), slices were perfused with nACSF containing 10 [micro]M CNQX/50 [micro]M APV. IPSCs were evoked at 0.1 Hz using a monopolar electrode placed in sites adjacent to the dysplasia or in the white matter. 10 [micro]M bicuculline abolished spontaneous and evoked IPSCs (sIPSCs and eIPSCs) confirming a role for GABA[sub]A[/sub] receptors. Recordings were obtained from [quot]dysplastic[quot] pyramidal cells (cortical sections, FCD patients) and [quot]control[quot] pyramidal cells (cortical sections obtained during TLE surgery or non-epileptic patients undergoing brain tumor resection; 17-38 yo). [ldquo]Control[rdquo] tissue is classified as normal (i.e., with neither lesion, tumor nor epileptiform activity) based on pre-surgical neuroimaging, ECoG and histopathologic examination. Studies were performed under guidelines of the UCSF Committee on Human Research.
In cortical slices from FCD, [quot]dysplastic[quot] pyramidal cells exhibited sIPSCs with amplitude similar to [quot]control[quot] neocortical pyramidal cells (25.5 [plusmn] 3.7; [italic]n[/italic] = 18). Interestingly, neocortical pyramidal cells from FCD exhibited sIPSCs with a slow decay time constant (13.2 [plusmn] 2.4 ms) in comparison with control cells (8.3 [plusmn] 0.3 ms; [italic]p [/italic][gt] 0.05) and lower frequency (FCD: 1.3 [plusmn] 0.3 Hz, [italic]n [/italic]= 18; control: 2.7 [plusmn] 0.3 Hz, [italic]n [/italic]= 18; [italic]p [/italic][gt] 0.05). eIPSCs on neocortical pyramidal cells from FCD also exhibited a slow decay time constant (76.9 [plusmn] 9.1 ms; [italic]n [/italic]= 10) in comparison with controls (20.4 [plusmn] 1.1 ms; [italic]n [/italic]= 20).
Here we describe GABA[sub]A[/sub] receptor mediated currents on neocortical pyramidal cells in FCD with low frequency and slow decay kinetics. Observed changes in synaptic inhibition may represent (i) enhanced GABA release, (ii) decreased GABA re-uptake, or (iii) altered GABA receptor function. In an interesting example of how animal model work can generate concepts that may apply to the human condition, these findings are similar to those reported in freeze lesion (reduced IPSC frequency; Roper et al. 2002) and MAM (slow IPSC kinetics; Calcagnotto et al. 2002) rodent models of malformation-associated epilepsy. Thus, there may be unifying principles of how a malformed brain functions.