The in vivo reduction of afferent facilitation induced by low frequency electrical stimulation of the motor cortex is antagonized by cathodal direct current stimulation of the cerebellum

Abstract

Background: Low-frequency electrical stimulation to the motor cortex (LFSMC) depresses the excitability of motor circuits by long-term depression (LTD)-like effects. The interactions between LFSMC and cathodal direct current stimulation (cDCS) over the cerebellum are unknown.

Methods: We assessed the corticomotor responses and the afferent facilitation of corticomotor responses during a conditioning paradigm in anaesthetized rats. We applied LFSMC at a frequency of 1 Hz and a combination of LFSMC with cDCS.

Results: LFSMC significantly depressed both the corticomotor responses and the afferent facilitation of corticomotor responses. Simultaneous application of cDCS over the cerebellum antagonized the depression of corticomotor responses and cancelled the depression of the afferent facilitation.

Conclusion: Our results demonstrate that cDCS of the cerebellum is a potent modulator the inhibition of the motor circuits induced by LFSMC applied in vivo. These results expand our understanding of the effects of cerebellar DCS on motor commands and open novel applications for a cerebellar remote control of LFSMC-induced neuroplasticity. We suggest that the cerebellum acts as a neuronal machine supervising not only long-term potentiation (LTP)-like effects, but also LTD-like effects in the motor cortex, two mechanisms which underlie cerebello-cerebral interactions and the cerebellar control of remote plasticity. Implications for clinical ataxiology are discussed.

Keywords: Cerebellum; Direct current stimulation; In vivo; Motor cortex; Neuromodulation; Plasticity.

Fig. 1

Top panels: example of averaged corticomotor response (MEP: motor evoked potential; averaging of 10 responses) evoked in left gastrocnemius muscle (stimulation of right motor cortex) at baseline (A, T0 min), 10 min after application of low frequency electrical stimulation of the motor cortex (B: post-LFSMC; duration of LFSMC: 10 min from T0 to T10 min), 45 min after baseline recording when the excitability of the motor cortex has returned to basal state (C: T45 min), after application of combined cDCS (cathodal DCS of the cerebellum) and LFSMC from T50 to T60 min (D: T70 min). Bottom panels: superimposition of averaged unconditioned MEPs (thin traces) and averaged MEPs with a conditioning stimulus (thick traces) during the paradigm of afferent facilitation (AF). Values of AF are given near the corresponding duos of traces

Fig. 2

a: box and whisker plots of amplitudes of MEPs expressed in μV. ***: p < 0.001 as compared to T0, T45 and T70 min. b: box and whisker plots of afferent facilitation. Ratios of conditioned (CR)/unconditioned (UR) are shown at T0, T20, T45 and T70 min. Ratios are expressed in arbitrary units (a.u.). Medians (continuous lines), dotted lines (mean values) and outliers are illustrated. *: p < 0.05, **: p < 0.01, ***: p < 0.001