Cerebral Cortex Advance Access originally published online on March 17, 2006
Cerebral Cortex 2007 17(2):434-442; doi:10.1093/cercor/bhj160
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Induction of Neocortical Long-Term Depression Results in Smaller Movement Representations, Fewer Excitatory Perforated Synapses, and More Inhibitory Synapses
1 Behavioural Neuroscience Research Group, Department of Psychology and the Hotchkiss Brain Institute, University of Calgary, Calgary, Alberta, Canada T2N 1N4, 2 Canadian Center for Behavioural Neuroscience, University of Lethbridge, Lethbridge, Alberta, Canada T1K 3M4, 3 Department of Neuroscience, University of Florida, Gainesville, FL, USA
Address correspondence to Dr G. Campbell Teskey, Department of Psychology, 2500 University Drive Northwest, Calgary, Alberta, Canada T2N 1N4. Email: gteskey{at}ucalgary.ca.
Long-term depression (LTD) is one of the most widely investigated models of the synaptic mechanisms underlying learning and memory. Previous research has shown that induction of LTD in the neocortex decreases measures of pyramidal cell dendritic morphology in both layers III and V. Here, we investigated the effects of LTD induction on 1) the time course of recovery of synaptic efficacy, 2) movement representations, 3) cortical thickness and layer V neuron density, and 4) the density of excitatory and inhibitory synapses in layer V of sensorimotor neocortex. Rats carried a stimulating electrode in the midline corpus callosum and a recording electrode in the right sensorimotor neocortex. Each rat received either low-frequency stimulation composed of 900 pulses at 1 Hz or handling daily for a total of 20–25 days. Callosal–neocortical evoked potentials were recorded in the right hemisphere before and after stimulation or handling. Our results show that LTD induction lasts for 3 weeks and results in smaller motor maps of the caudal forelimb area. We did not observe any reduction in neocortical thickness or neuron density. There was a reduction in the density of excitatory perforated synapses and an increase in the density of inhibitory synapses in layer V of the sensorimotor neocortex, thereby providing a general mechanism for the reduction in motor map size. This study sheds light on the interaction between an artificial model of learning, receptive field characteristics, and synaptic number in the sensorimotor cortex.
Key Words: caudal forelimb area • long-term depression • motor cortex reorganization • neocortex • perforated synapses