Transient spinal inhibition induced by electrical stimulation of the rat brain

Intracortical electrical stimulation of the rat brain using single pulse induced motor evoked potentials (MEPs) with shorter onset latencies in the bilateral extremity muscles. The MEPs appeared in the stimulation of cortical areas outside the motor cortex (MI) and subcortical areas. Train-pulse stimulation of the MI at a stimulus intensity just above the threshold induced MEPs with longer onset latency in muscles corresponding to the somatotopy of the MI stimulated. This implies that the potential characterizing shorter onset latency is equal to responses induced via the extrapyramidal tract, and responses with longer onset latency originate in the pyramidal tract. Corresponding to MEPs, we recorded two types of spinal potentials (SPs) via extrapyramidal and pyramidal tracts. In paired-pulse stimulation of the extrapyramidal tract, second MEPs showed a long-lasting inhibition up to 3 s after the first MEPs, while the changes in second SPs were not remarkable. Extrapyramidal tract stimulation inhibited H-reflex in the same manner as MEPs. These results suggest that the electrical stimulation of rat brain has a long-lasting effect in inhibiting lower motoneuron excitabilities. Our method may be a useful experimental model to induce the transient inhibition of spinal motoneuron excitabilities caused by supraspinal structures.