Animal models of transcranial direct current stimulation: Methods and mechanisms

Abstract

The objective of this review is to summarize the contribution of animal research using direct current stimulation (DCS) to our understanding of the physiological effects of transcranial direct current stimulation (tDCS). We comprehensively address experimental methodology in animal studies, broadly classified as: (1) transcranial stimulation; (2) direct cortical stimulation in vivo and (3) in vitro models. In each case advantages and disadvantages for translational research are discussed including dose translation and the overarching "quasi-uniform" assumption, which underpins translational relevance in all animal models of tDCS. Terminology such as anode, cathode, inward current, outward current, current density, electric field, and uniform are defined. Though we put key animal experiments spanning decades in perspective, our goal is not simply an exhaustive cataloging of relevant animal studies, but rather to put them in context of ongoing efforts to improve tDCS. Cellular targets, including excitatory neuronal somas, dendrites, axons, interneurons, glial cells, and endothelial cells are considered. We emphasize neurons are always depolarized and hyperpolarized such that effects of DCS on neuronal excitability can only be evaluated within subcellular regions of the neuron. Findings from animal studies on the effects of DCS on plasticity (LTP/LTD) and network oscillations are reviewed extensively. Any endogenous phenomena dependent on membrane potential changes are, in theory, susceptible to modulation by DCS. The relevance of morphological changes (galvanotropy) to tDCS is also considered, as we suggest microscopic migration of axon terminals or dendritic spines may be relevant during tDCS. A majority of clinical studies using tDCS employ a simplistic dose strategy where excitability is singularly increased or decreased under the anode and cathode, respectively. We discuss how this strategy, itself based on classic animal studies, cannot account for the complexity of normal and pathological brain function, and how recent studies have already indicated more sophisticated approaches are necessary. One tDCS theory regarding "functional targeting" suggests the specificity of tDCS effects are possible by modulating ongoing function (plasticity). Use of animal models of disease are summarized including pain, movement disorders, stroke, and epilepsy.

Keywords: Animal models; Galvanotropy; Long term potentiation; Noninvasive brain stimulation; Oscillations; Synaptic plasticity; Transcranial direct current stimulation.

Figure 1

Animal studies on tDCS mechanism allow rapid screening of stimulation parameters and analysis of neurophysiological and molecular changes in ways not possible clinically. Meaningful translation research in animals required replication of electric fields generated clinically in animal brain/tissue. The electric field generated in the brain during tDCS is dependent on the stimulation dose (current intensity, electrode montage) and head anatomy. It is not trivial to relate externally controlled dose with internally generated electric fields (i.e., the current density in the brain is not the same as at the electrodes), but FEM computational models provide a method to do so. In the experimental design of animal studies, the electric field generated should correspond in intensity to that generated clinically; otherwise results should be applied to the clinical case with caution. For the case of in vitro brain slice studies, the replication of clinical electrical fields is experimentally straightforward with the use of two long parallel wires placed across the bath, generating a uniform electric field. The uniform electric field in the chamber can be calibrated using a field-recording electrode in the current applied to the wires. The relative position of the brain slice in the uniform field is not important to control, but the slice’s orientation within the electric field must be carefully controlled; moreover multiple slices can be screened at once.

Figure 2

The quasi-uniform assumption is implicit in the majority of modeling and animal studies of tDCS. The first aspect of the quasi-uniform assumption is based on the electric field generated in the brain to not significantly change (be uniform) on the scale of a single cortical column or neuronal dendritic tree. Only in this way it is meaningful to represent, for a first approximation, neuromodulation by regional electric field. This assumption underpins the rational basis for replicating an electric field of interest in an animal model as described in the text. Shown is a high-resolution finite element model (FEM) computational model of predicted current distribution using tDCS on humans and rats, as well as DCS on brain slices.

Figure 3

Incremental membrane polarization produced by tDCS may significantly affect the timing of action potentials in response to a ramp (synaptic, oscillation) input. Moreover, the amplification of effect (change in timing per change in membrane polarization) increased for more gradual input. A.1 Schematic illustrates the principle of timing amplification from Radman et al. (2007b). A.2 The timing amplification was validated in hippocampal CA1 neurons using intracellular injected current ramp of various slopes. B.1/B.2 The timing change increased with membrane polarization with a sensitivity (amplification) that is the inverse of the input ramp slope. The amplification would function during processing of incoming synaptic input including oscillations. C.1 Demonstration of timing change in response to an incoming EPSP. An incremental depolarization produced by direct current led to significant change in action potential timing in response to a synaptic input.

Figure 4

The principle and quantification of the somatic doctrine. (Top) The somatic doctrine simplifies tDCS design by assuming inward current flow under the anode, leading to somatic depolarization, and a generic increase in excitability and function. Under the cathode, an outward current leads to somatic hyperpolarization and a generic decrease in excitability and function. (Bottom) Modern efforts to quantify somatic polarization in animal models have confirmed some aspects of the somatic doctrine, at least under specific controlled and tested conditions, but indicated that the polarization produced by tDCS would be small.

Figure 5

Further advantages of the brain slice preparation in studying mechanisms of weak DC stimulation. (Top) A discussion in the text, the direction of the applied electric field relative to the somato-dendritic axis can be precisely controlled (adapted from Bikson et al., 2004). The effects of DC current on brain function may vary with orientation. (Bottom) Synaptic function/efficacy is not “one thing”, rather there are multiple distinct synaptic afferent to any brain region which can be evaluated in isolation in brain slices. The effects of DC current on synaptic function may be highly pathway specific.

Figure 6

Modulation of gamma oscillations in brain slice by weak DC fields. Gamma oscillations were induced in the CA3 region by perfusion with carbachol. Negative fields which produce hyperpolarization of CA1 pyramidal neuron soma, attenuated oscillation, but interestingly the attenuation was most pronounced when the fields where turned on, after which oscillation activity partly rebounded even through the field was still on. This suggests homeostatic “adaptation” (arrows) to the DC field by neuronal network system. After the field is turn off, there is an excitatory rebound response consistent with this adaption. An opposite effects is observed for positive fields that would depolarizing the soma of CA3 pyramidal neurons. This adaption at the network level is not expected from single neurons, so reflects an emergent response of an active network to DC fields.