Trigeminal nerve direct current stimulation causes sustained increase in neural activity in the rat hippocampus

Liyi Chen; Zhengdao Deng; Boateng Asamoah; Myles Mc Laughlin

doi:10.1016/j.brs.2024.05.005

Trigeminal nerve direct current stimulation causes sustained increase in neural activity in the rat hippocampus

Brain Stimul. 2024 May 11;17(3):648-659. doi: 10.1016/j.brs.2024.05.005. Online ahead of print.

Authors

Liyi Chen¹, Zhengdao Deng², Boateng Asamoah¹, Myles Mc Laughlin³

Affiliations

¹ Exp ORL, Department of Neurosciences, The Leuven Brain Institute, KU Leuven, Belgium.
² Experimental Neurosurgery and Neuroanatomy, Department of Neurosciences, KU Leuven, Belgium.
³ Exp ORL, Department of Neurosciences, The Leuven Brain Institute, KU Leuven, Belgium. Electronic address: myles.mclaughlin@kuleuven.be.

PMID: 38740183
DOI: 10.1016/j.brs.2024.05.005

Abstract

Background: Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulation method that can modulate many brain functions including learning and memory. Recent evidence suggests that tDCS memory effects may be caused by co-stimulation of scalp nerves such as the trigeminal nerve (TN), and not the electric field in the brain. The TN gives input to brainstem nuclei, including the locus coeruleus that controls noradrenaline release across brain regions, including hippocampus. However, the effects of TN direct current stimulation (TN-DCS) are currently not well understood.

Hypothesis: In this study we tested the hypothesis that stimulation of the trigeminal nerve with direct current manipulates hippocampal activity via an LC pathway.

Methods: We recorded neural activity in rat hippocampus using multichannel silicon probes. We applied 3 min of 0.25 mA or 1 mA TN-DCS, monitored hippocampal activity for up to 1 h and calculated spikes-rate and spike-field coherence metrics. Subcutaneous injections of xylocaine were used to block TN, while intraperitoneal and intracerebral injection of clonidine were used to block the LC pathway.

Results: We found that 1 mA TN-DCS caused a significant increase in hippocampal spike-rate lasting 45 min in addition to significant changes in spike-field coherence, while 0.25 mA TN-DCS did not. TN blockage prevented spike-rate increases, confirming effects were not caused by the electric field in the brain. When 1 mA TN-DCS was delivered during clonidine blockage no increase in spike-rate was observed, suggesting an important role for the LC-noradrenergic pathway.

Conclusion: These results support our hypothesis and provide a neural basis to understand the tDCS TN co-stimulation mechanism. TN-DCS emerges as an important tool to potentially modulate learning and memory.

Keywords: Hippocampus; Trigeminal nerve stimulation; tDCS.