Publications by Pablo alejandro Reyes Velasquez

An in vivo model for transcranial direct current stimulation of the motor cortex in awake mice

• Authors: Carlos Andrés Sánchez-León 1, Pablo Alejandro Reyes-Velasquez 2, Christoph van Thriel 3, Michael A Nitsche 4

• Publication date: 2025 Nov-Dec

• Background: Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique mainly used in humans, in which weak direct currents are applied over the scalp to alter cortical excitability and induce neuroplasticity. Previous studies have demonstrated the value of tDCS for modulating sensory, motor, and cognitive functions, nevertheless, knowledge about how externally applied electric fields affect different components of neuronal networks is still incomplete, and in vivo animal models, which are required for a deeper understanding, are not fully developed. To evaluate the impact of tDCS on cortical excitability, many human experiments assess motor evoked potentials elicited by motor cortex (M1) stimulation.

• Methods: To develop a related in vivo animal model, we recorded electrical activity in M1 of alert mice during and after administration of tDCS over M1. M1 excitability was chronically recorded from layers 2-3, layer 5 and layer 6, evoked by stimulation of the ventral lateral nucleus of the thalamus (VAL). M1-tDCS was applied at 100 and 200 μA for 5 s to test the acute effects on neuronal excitability, and for 15 min to induce after-effects.

• Results: Acute M1-tDCS increased and decreased the amplitude of VAL-evoked potentials in a polarity-, layer- and intensity-dependent manner. For 15 min of anodal or cathodal tDCS, a similar polarity- and intensity-dependent modulation of VAL-evoked potential amplitudes during the 15 min of stimulation was observed. After tDCS was switched off, the highest intensity of anodal stimulation induced a significant excitability enhancement during at least 2 h after stimulation, whereas the after-effects of cathodal tDCS were less pronounced.

• Conclusions: The current study demonstrates the feasibility of a mouse model of M1-tDCS to accomplish similar modulatory effects of tDCS on cortical excitability as observed in human experiments. A proper adjustment of tDCS parameters, as compared to application in humans, is however required to obtain these translational effects since a higher density current was necessary in this study to obtain effects comparable to those achieved in humans.

• Authors: Darya Frank 1, Marta Garo-Pascual 2, Pablo Alejandro Reyes Velasquez 3, Belén Frades 4, Noam Peled 5, Linda Zhang 4, Bryan A Strange 6

• Publication date: 2022 Nov

• Abstract: Memory normally declines with ageing and these age-related cognitive changes are associated with changes in brain structure. Episodic memory retrieval has been widely studied during ageing, whereas learning has received less attention. Here we examined the neural correlates of episodic learning rate in ageing. Our study sample consisted of 982 cognitively healthy female and male older participants from the Vallecas Project cohort, without a clinical diagnosis of mild cognitive impairment or dementia. The learning rate across the three consecutive recall trials of the verbal memory task (Free and Cued Selective Reminding Test) recall trials was used as a predictor of grey matter (GM) using voxel-based morphometry, and WM microstructure using tract-based spatial statistics on fractional anisotropy (FA) and mean diffusivity (MD) measures. Immediate Recall improved by 1.4 items per trial on average, and this episodic learning rate was faster in women and negatively associated with age. Structurally, hippocampal and anterior thalamic GM volume correlated positively with learning rate. Learning also correlated with the integrity of WM microstructure (high FA and low MD) in an extensive network of tracts including bilateral anterior thalamic radiation, fornix, and long-range tracts. These results suggest that episodic learning rate is associated with key anatomical structures for memory functioning, motivating further exploration of the differential diagnostic properties between episodic learning rate and retrieval in ageing.