Excitatory and Inhibitory Postsynaptic Currents in a Rat Model of Epileptogenic Microgyria

Abstract

Developmental cortical malformations are common in patients with intractable epilepsy; however, mechanisms contributing to this epileptogenesis are currently poorly understood. We previously characterized hyperexcitability in a rat model that mimics the histopathology of human 4-layered microgyria. Here we examined inhibitory and excitatory postsynaptic currents in this model to identify functional alterations that might contribute to epileptogenesis associated with microgyria. We recorded isolated whole cell excitatory postsynaptic currents and GABA(A) receptor-mediated inhibitory currents (EPSCs and IPSCs) from layer V pyramidal neurons in the region previously shown to be epileptogenic (paramicrogyral area) and in homotopic control cortex. Epileptiform-like activity could be evoked in 60% of paramicrogyral (PMG) cells by local stimulation. The peak conductance of both spontaneous and evoked IPSCs was significantly larger in all PMG cells compared with controls. This difference in amplitude was not present after blockade of ionotropic glutamatergic currents or for miniature (m)IPSCs, suggesting that it was due to the excitatory afferent activity driving inhibitory neurons. This conclusion was supported by the finding that glutamate receptor antagonist application resulted in a significantly greater reduction in spontaneous IPSC frequency in one PMG cell group (PMG(E)) compared with control cells. The frequency of both spontaneous and miniature EPSCs was significantly greater in all PMG cells, suggesting that pyramidal neurons adjacent to a microgyrus receive more excitatory input than do those in control cortex. These findings suggest that there is an increase in numbers of functional excitatory synapses on both interneurons and pyramidal cells in the PMG cortex perhaps due to hyperinnervation by cortical afferents originally destined for the microgyrus proper.

Reorganization of barrel circuits leads to thalamically-evoked cortical epileptiform activity

“…Our results in spiny cells in layer IV of the PMG zone agree in some respects with those previously obtained from pyramidal cells in layer V of the epileptogenic area in the microgyrus model (Jacobs and Prince, 2005). In both studies, the conductance of sIPSCs was increased while that of sEPSPs was not.…”

“…Another potential mechanism underlying epileptogenesis associated with microgyri is enhanced excitatory connectivity, as evidenced by the increased frequency of sEPSCs and mEPSCs in subgroups of layer V PMG cells and the marked reductions in sIPSC frequency induced by perfusion with glutamate receptor blockers (Jacobs and Prince, 2005), as well as by increases in sEPSC frequency in the present experiments ( Table 2). The increased density of axons in the immediate paramicrogyral area (Fig.1B), enhanced glutamate immunoreactivity of axonal projections in this area (Humphreys et al, 1991) and increased thalamocortical projections to areas of aberrantly structured barrels (Jacobs et al,1999c) all suggest restructuring of excitatory inputs.…”

“…The cell type, neuronal intrinsic properties and synaptic connections in barrel cortex are well characterized (Feldmeyer et al, 1999;Feldmeyer et al, 2002), making this a useful model in which to examine the relationship between epileptogenesis and the structural reorganization (e.g. Jacobs et al, 1999c;Rosen et al 2000), and alterations in GABAergic inhibitory circuitry (Rosen et al, 1998;Jacobs and Prince, 2005) that are known to be present. Our results identify reductions in inhibitory synaptic strength as one major factor underlying hyperexcitability in barrel cortex adjacent to experimental microgyri.…”

“…This paramicrogyral zone shows grossly normal cytoarchitectonic organization, however alterations in postsynaptic receptors and synaptic strength are present (DeFazio and Hablitz, 1999;DeFazio and Hablitz, 2000;Jacobs and Prince, 2005) as well as abnormal connectivity (Jacobs et al, 1999c;Rosen et al, 2000) and widespread reductions in the density of PV-containing GABAergic interneurons (Rosen et al, 1998; but see Schwarz et al 2000). Apparent loss of inhibitory neurons or decreases in functional inhibition has also been reported in cortical dysplasia produced by fetal irradiation (Chen and Roper, 2003;Roper et al, 1997;Roper, 1998;Zhu and Roper, 2000) and in human dysplastic cortex (Calcagmoto et al, 2005), and is a common finding in other models of chronic epileptogenesis (Ribak et al, 1986;Lowenstein et al, 1992;Buckmaster and Dudek, 1997;Cossart et al, 2001; reviewed in Prince and Jacobs, 1998)..…”

“…In the freeze lesion model of cortical microgyria (Dvorák and Feit, 1977;Dvorák et al 1978), cellular recordings and intracellular staining have shown that the region generating the epileptiform activity (the paramicrogyral zone or PMG) is not congruent with the microgyrus itself, but rather adjacent to it (Jacobs et al, 1999a). This adjacent region shows normal gross cytoarchitecture, however changes in synaptic strength and postsynaptic receptors are present (DeFazio and Hablitz, 1999;DeFazio and Hablitz, 2000;Jacobs and Prince, 2005;Hagemann et al, 2003). Further, there is evidence for structural reorganization including increased density of thalamocortical fibers and gross reorganization of cortical barrel structures in the PMG (Jacobs et al, 1999c) and other abnormalities of afferent and efferent connectivity (Rosen et al, 2000).…”