Abstracts

Axonal sprouting after traumatic injuries and consequent formation of recurrent excitatory circuits may be a major factor contributing to epileptogenesis. Significant increases in the total length, number of collaterals, and density of synaptic boutons occur in axons of layer V pyramidal neurons of rat partially isolated [quot]undercut[quot] neocortex, a chronic model of post-traumatic epileptogenesis. To further explore the functional consequence of axonal sprouting, we used laser scanning photostimulation combined with whole cell patch clamp recordings to map changes in excitatory and inhibitory synaptic connectivity in the undercut model of epileptogenesis. Partial cortical isolations in rat sensorimotor cortex were made at postnatal day 21. 300 [mu]m-thick coronal slices from control and lesioned brains were cut at least 2 weeks later. Slices were submerged in re-circulating high divalent ACSF containing 100 [mu]M caged glutamate. Photolysis of caged glutamate was accomplished by directing a UV laser in a grid pattern via galvanometer mirrors through a 5X objective. Whole-cell voltage clamp recordings of photolysis-evoked EPSCs and IPSCs were obtained from layer V pyramidal neurons in the sensorimotor cortex. There was a significant increase in the cumulative EPSC amplitude evoked within a radius of 300 [mu]m from the somata of recorded layer V pyramidal neurons (for the areas within 100 [mu]m, 200 [mu]m and 300 [mu]m radius circles, the mean cumulative EPSC amplitude were -11.0 [plusmn] 2.4, -8.8 [plusmn] 1.8 and -6.4 [plusmn] 1.5 pA for control, [ndash]22.5 [plusmn] 3.1, -18.3 [plusmn] 2.2 and [ndash]11.5 [plusmn] 1.7 pA for the undercut respectively, P [lt] 0.005[sim]0.05). The percentages of responsive spots in a given area also increased significantly close to somata (within radii of 100 [mu]m, 200 [mu]m and 300 [mu]m the mean fraction of responsive regions were 0.37 [plusmn] 0.05, 0.35 [plusmn] 0.05 and 0.31 [plusmn] 0.05 for control, 0.58 [plusmn] 0.05, 0.58 [plusmn] 0.05 and 0.50 [plusmn] 0.06 for the undercut respectively, P [lt] 0.005 [sim] 0.05). The larger evoked responses in undercut tissue were most evident within layer V itself, in a region vertically centered [plusmn] [sim]150 [mu]m from the somata, suggesting a layer specific enhancement of excitatory connectivity in layer V. In addition, the amplitude of the individual EPSCs in the undercut group was smaller than the control (15.2 [plusmn] 0.8 pA vs. 12.7 [plusmn] 0.9 pA, P [lt] 0.05). There was no significant change in evoked IPSCs as measured as either cumulative IPSC amplitude or percentage of responsive regions. The significant increases in intra-laminar cumulative EPSC amplitude and the percentage of responsive spots suggest that layer V pyramidal neurons in the undercut neocortex make stronger excitatory synaptic connections with other layer V pyramidal neurons. These changes in excitatory synaptic connectivity may shift the balance of cortical circuits towards increased excitation, and contribute to epileptogenesis. (Supported by NIH grant NS12151 from the NINDS).)