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The Journal of Neuroscience, May 3, 2006, 26(18):4891-4900; doi:10.1523/JNEUROSCI.4361-05.2006
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Neurobiology of Disease
Enhanced Excitatory Synaptic Connectivity in Layer V Pyramidal Neurons of Chronically Injured Epileptogenic Neocortex in Rats
Xiaoming Jin, David A. Prince, andJohn R. Huguenard Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305
Correspondence should be addressed to John R. Huguenard, Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Room M016, Stanford, CA 94305-5122. Email: john.huguenard{at}stanford.edu
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°C) containing 100 µM 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 µs 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 µm 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.
Key words: epilepsy; neocortex; synaptic transmission; caged compound; electrophysiology; excitability
Received Oct. 12, 2005; revised Feb. 20, 2006; accepted March 25, 2006.
Correspondence should be addressed to John R. Huguenard, Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Room M016, Stanford, CA 94305-5122. Email: john.huguenard{at}stanford.edu
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