Cerebellar Transcranial AC Stimulation Produces a Frequency-Dependent Bimodal Cerebellar Output Pattern

Cerebellar transcranial alternating current stimulation (ctACS) has the potential to be an appealing, non-invasive treatment option for psychiatric and neurological disorders. However, realization of this potential has been limited by gaps in our knowledge of how ctACS affects cerebellar output on single cell and population levels. Previously, we showed that AC stimulation applied to the cerebellar surface produced a strong, frequency-dependent modulation of Purkinje cell (PC) and cerebellar nuclear (CN) cell activity. Here, to approximate more closely the ctACS conditions, we investigated how AC stimulation applied to the external skull surface overlying crus 1 altered PC and CN activity in anesthetized adult female Sprague-Dawley rats. PC and CN activity showed a frequency-dependent modulation in response to ctACS at frequencies ranging from 0.5 to 80 Hz. A unimodal response was seen for most PCs across all frequencies, whereas most CN cells transitioned to bimodal patterns as stimulus frequency increased. The frequency-dependence of the phases of the local minima of the CN cell modulation were consistent with CN cells being driven synaptically by PC activity. Furthermore, comparison of responses with ipsilateral and contralateral placement of the stimulus electrode with respect to the recording site showed that the strength and pattern of the entrainment depended on the stimulus electrode location, suggesting that ctACS electrode placement could be used to target specific cerebellar output channels. In sum, the results show that transcranial stimulation of the cerebellar cortex can modulate cerebellar output, which has potential implications for its use in treating neurological and psychiatric disorders.