Intracortical synchronization of epileptic discharges at different stages of ultrastructural rearrangements in a completely neuronally isolated area of rat neocortex

Reorganization of inhibitory synaptic circuits in rodent chronically injured epileptogenic neocortex

Reduced synaptic inhibition is an important factor contributing to posttraumatic epileptogenesis. Axonal sprouting and enhanced excitatory synaptic connectivity onto rodent layer V pyramidal (Pyr) neurons occur in epileptogenic partially isolated (undercut) neocortex. To determine if enhanced excitation also affects inhibitory circuits, we used laser scanning photostimulation of caged glutamate and whole-cell recordings from GAD67-GFP-expressing mouse fast spiking (FS) interneurons and Pyr cells in control and undercut in vitro slices to map excitatory and inhibitory synaptic inputs. Results are 1) the region-normalized excitatory postsynaptic current (EPSC) amplitudes and proportion of uncaging sites from which EPSCs could be evoked (hotspot ratio) "increased" significantly in FS cells of undercut slices; 2) in contrast, these parameters were significantly "decreased" for inhibitory postsynaptic currents (IPSCs) in undercut FS cells; and 3) in rat layer V Pyr neurons, we found significant decreases in IPSCs in undercut versus control Pyr neurons. The decreases were mainly located in layers II and IV, suggesting a reduction in the efficacy of interlaminar synaptic inhibition. Results suggest that there is significant synaptic reorganization in this model of posttraumatic epilepsy, resulting in increased excitatory drive and reduced inhibitory input to FS interneurons that should enhance their inhibitory output and, in part, offset similar alterations in innervation of Pyr cells.

Enhanced excitatory synaptic connectivity in layer v pyramidal neurons of chronically injured epileptogenic neocortex in rats

Formation of new recurrent excitatory circuits after brain injuries has been hypothesized as a major factor contributing to epileptogenesis. Increases in total axonal length and the density of synaptic boutons are present in layer V pyramidal neurons of chronic partial isolations of rat neocortex, a model of posttraumatic epileptogenesis. To explore the functional consequences of these changes, we used laser-scanning photostimulation combined with whole-cell patch-clamp recording from neurons in layer V of somatosensory cortex to map changes in excitatory synaptic connectivity after injury. Coronal slices were submerged in artificial CSF (23 degrees C) containing 100 microM caged glutamate, APV (2-amino-5-phosphonovaleric acid), and high divalent cation concentration to block polysynaptic responses. Focal uncaging of glutamate, accomplished by switching a pulsed UV laser to give a 200-400 micros light stimulus, evoked single- or multiple-component composite EPSCs. In neurons of the partially isolated cortex, there were significant increases in the fraction of uncaging sites from which EPSCs could be evoked ("hot spots") and a decrease in the mean amplitude of individual elements in the composite EPSC. When plotted along the cortical depth, the changes in EPSCs took place mainly between 150 and 200 microm above and below the somata, suggesting a specific enhancement of recurrent excitatory connectivity among layer V pyramidal neurons of the undercut neocortex. These changes may shift the balance within cortical circuits toward increased synaptic excitation and contribute to epileptogenesis.