Direct current stimulation boosts associative Hebbian synaptic plasticity and maintains its pathway specificity

Abstract

Background: There is evidence that transcranial direct current stimulation (tDCS) can improve learning performance. Arguably, this effect is related to long term potentiation (LTP), but the precise biophysical mechanisms remain unknown. Hypothesis: We propose that direct current stimulation (DCS) causes small changes in postsynaptic membrane potential during ongoing endogenous synaptic activity. The altered voltage dynamics in the postsynaptic neuron then modify synaptic strength via the machinery of endogenous voltage-dependent Hebbian plasticity. This hypothesis predicts that DCS should exhibit Hebbian properties, namely pathway specificity and associativity. Methods: We studied the effects of DCS applied during the induction of LTP in the CA1 region of rat hippocampal slices and using a biophysical computational model. Results: DCS enhanced LTP, but only at synapses that were undergoing plasticity, confirming that DCS respects Hebbian pathway specificity. When different synaptic pathways cooperated to produce LTP, DCS enhanced this cooperation, boosting Hebbian associativity. Further slice experiments and computer simulations support a model where polarization of postsynaptic pyramidal neurons drives these plasticity effects through endogenous Hebbian mechanisms. The model is able to reconcile several experimental results by capturing the complex interaction between the induced electric field, neuron morphology, and endogenous neural activity. Conclusions: These results suggest that tDCS can enhance associative learning. We propose that clinical tDCS should be applied during tasks that induce Hebbian plasticity to harness this phenomenon, and that the effects should be task specific through their interaction with endogenous plasticity mechanisms. Models that incorporate brain state and plasticity mechanisms may help to improve prediction of tDCS outcomes.Abbreviations: tDCS (transcranial direct current stimulation); DCS (direct current stimulation); LTP (long term potentiation); TBS (theta burst stimulation); STDP (spike-timing dependent plasticity) Effect of DCS on LTP is pathway specificHebbian synaptic plasticity is classically characterized as a pathway specific process, i.e. only pathways that are coactive with the postsynaptic neuron are strengthened (35). Our proposal that DCS enhances LTP through membrane potential implies that the effects of DCS should follow this pathway specificity. We tested this by monitoring two independent synaptic pathways in CA1 (Figure 2A). During induction, the strong pathway received TBS while the other inactive pathway was not stimulated. As expected, LTP was observed in the strong pathway ( Figure 2B black; 1.377+-0.052, N=16, p=2.8E-6), but not the inactive pathway ( Figure 2B gray; 0.986+-0.031, N=14, p=0.657), demonstrating the wellestablished pathway specificity of LTP (35). When this induction protocol was paired with anodal DCS, LTP was enhanced only in the strong pathway ( Figure 2B red; 1.613+-0.071, N=14, p=0.011 vs. control), while the inactive pathway was unaff...

Efficacy of Noninvasive Brain Stimulation (tDCS or TMS) Paired with Language Therapy in the Treatment of Primary Progressive Aphasia: An Exploratory Meta-Analysis

“…Our hypothesis was based on mechanistic differences between the two NIBS techniques. Because tDCS induces sub-threshold shifts in neuronal resting membrane potentials, its ability to elicit enduring effects on behavior are thought to rely upon and leverage Hebbian principles of neuroplasticity [ 72 ]. For this reason, it has been argued that tDCS ought to be most effective when paired with behavioral approaches that engage relevant networks [ 37 , 40 , 42 ], in this case the language network.…”

Behavioral and neurological effects of tDCS on speech motor recovery: A single-subject intervention study

“…This mechanism is suspected to underlie both novel skill acquisition and recovery of impaired motor skills subsequent to acquired brain injury. Recent in vitro studies have suggested that tDCS can enhance synaptic plasticity in the presence of potentiating neural activity (Kronberg et al, 2017;Kronberg, Rahman, Lafon, Bikson, & Parra, 2019), and this has been recently demonstrated in an in vivo study involving the auditory neurophysiological pathway (Boroda, Sponheim, Fiecas, & Lim, 2020). Thus, given that motor learning involves associative learning and long-term potentiation, and that tDCS may enhance this type of plasticity in the presence of potentiating neural activity, tDCS may be particularly wellsuited to enhancing motor learning.…”

Assessing neurophysiological changes associated with combined transcranial direct current stimulation and cognitive‐emotional training for treatment‐resistant depression

“…To improve therapeutic efficacy in people with treatment-resistant depression, some trials have combined tDCS with cognitive training (Brunoni et al, 2014;Segrave, Arnold, Hoy, & Fitzgerald, 2014). It has been suggested that tDCS interacts with ongoing neuronal activity to boost synaptic plasticity within activated regions (Kronberg, Bridi, Abel, Bikson, & Parra, 2017;Kronberg, Rahman, Lafon, Bikson, & Parra, 2019). The administration of behavioural tasks during tDCS may thus be used to pre-activate relevant cortical regions and augment the neuromodulatory potential of tDCS by improving the functional specificity of stimulation (Bikson & Rahman, 2013).…”