The cerebellum modulates rodent cortical motor output after repetitive somatosensory stimulation

To analyze the possible role of the cerebellum in the modulation of cortical motor output associated with repetitive electrical stimulation of the sciatic nerve in the rat.

A sustained somatosensory stimulation induces an increase in the intensity of the response of the rodent motor cortex. Wistar rats were anesthetized for surgical preparation using a continuous infusion of chloral hydrate. We analyzed the response evoked by electrical stimulation of the right motor cortex before (basal condition) and after peripheral electrical stimulation of the left sciatic nerve in rats with no cerebellar intervention (n = 6), and in control rats with Ringer's infusion via a microdialysis probe (n = 8) implanted in the left cerebellar nuclei. In addition, we investigated the effects of 1) the administration of ethanol (20 mmol/L) in the left cerebellar nuclei (n = 5); 2) the administration of tetrodotoxin (10 micromol/L), a sodium channel blocker, in the left cerebellar nuclei (n = 5); 3) electrical stimulation by deep cerebellar stimulation (frequency 100 Hz) on the left side (n = 5); or 4) electrical stimulation of the cerebellar nuclei on the right side (100 Hz; n = 6). For peripheral stimulation, all of the animals received 1 hour of electrical stimulation. Trains of stimulation consisted of five stimuli (duration of 1 stimulus, 1 ms) at a rate of 10 Hz. During stimulation of the motor cortex, peak-to-peak amplitudes in responses of the left calf muscle were analyzed. Motor threshold was defined as the lowest intensity eliciting at least 5 of 10 evoked responses with an amplitude greater than 20 muV. The intensity used was 130% of the motor threshold.

In the basal condition (before repetitive stimulation), amplitudes of motor responses were similar in the six groups of rats (P = 0.40). In rats without cerebellar intervention, peripheral electrical stimulation was associated with an increase of motor response to 147.4 +/- 8.5% of baseline (P < 0.001). In rats with Ringer's infusion, the motor response increased to 141.6 +/- 7.9% of baseline (P < 0.001). The administration of ethanol in the cerebellum prevented the enhancement of the response ipsilaterally. The mean +/- standard deviation (SD) of motor responses was 105.7 +/- 6.2% of baseline measurements after stimulation of the sciatic nerve (P = 0.36). The same observation was made after the infusion of tetrodotoxin (mean +/- SD of motor responses: 107.1 +/- 7.4% after peripheral stimulation [P = 0.19] and after electrical stimulation of the cerebellum on the left side [mean +/- SD of motor responses, 104.3 +/- 8.5% after peripheral stimulation, P = 0.40]). However, electrical stimulation of cerebellar nuclei on the right side did not impair the modulation of cortical motor output by sciatic nerve stimulation (mean +/- SD of motor responses, 148.4 +/- 5.8% after peripheral stimulation, P < 0.001).

Until now, the increase of motor output after peripheral nerve stimulation has been considered as a plasticity directly and solely dependent on cortical structures. We demonstrate that the cerebellum plays a key role in this form of neural plasticity.