Post-tetanic modification of the efficiency of excitatory transmission in neural networks including interhemispheric connections.

This is the first report of modifiable reciprocal transcallosal monosynaptic excitatory connections, detected in in vivo experiments in the rat motor cortex by recording of multineuron activity and cross-correlation analysis. High-frequency microstimulation of a small group of cortical neurons in one hemisphere was shown to alter the efficiency of transcallosal excitatory connections, and also altered the efficiency of ipsilateral connections in both hemispheres. Post-tetanic changes consisted of long-term potentiation and depression. Neurons producing spike trains were found to have better conditions (compared with other neurons) for long-term potentiation of inputs converging on them. Synapses formed by axon collaterals of a given callosal cell on several neurons could simultaneously induce both long-term potentiation and long-term depression, while a given callosal neuron could simultaneously show long-term potentiation in some synapses and long-term depression in others. After microstimulation there were increases in the number of background-active callosal neurons, along with increases in the number and efficiency of transcallosal connections, while the number and efficiency of ipsilateral connections decreased. These data lead to the conclusion that ipsilateral inhibition is more effective than transcallosal inhibition. Microstimulation modified the pattern of initially existing connections between the many elements of ensembles including callosal cells in both hemispheres.