While it is well-known that neuronal activity promotes plasticity and connectivity,
the success of activity-based neural rehabilitation programs remains extremely limited
in human clinical experience because they cannot adequately control neuronal excitability
and activity within the injured brain in order to induce repair. However, it is possible
to non-invasively modulate brain plasticity using brain stimulation techniques such
as repetitive transcranial (rTMS) and transcranial direct current stimulation (tDCS)
techniques, which show promise for repairing injured neural circuits (Henrich-Noack
et al., 2013; Lefaucher et al., 2014). Yet we are far from having full control of
these techniques to repair the brain following neurotrauma and need more fundamental
research (Ellaway et al., 2014; Lefaucher et al., 2014). In this perspective we discuss
the mechanisms by which rTMS may facilitate neurorehabilitation and propose experimental
techniques with which magnetic stimulation may be investigated in order to optimise
its treatment potential.
Since the year of its first application, interest in rTMS has increased exponentially
and it is widely applied as a non-invasive method for brain stimulation in experimental
and clinical settings (Pell et al., 2011; Di Lazzaro et al., 2013). During magnetic
stimulation, an electric coil induces a magnetic field which passes through the skull
to produce an electric field in the brain (Pell et al., 2011; Deng et al., 2013).
As immediate effects of rTMS can be easily visualised in humans, e.g., stimulation
to the motor cortex results in muscle twitches, it is generally accepted that eddy
currents induced in the cortex lead to action potential firing. As the magnetic field
deteriorates only with distance from the central point of stimulation (Deng et al.,
2013), the discrete stimulated brain regions are surrounded by adjacent cortical and
sub-cortical tissue that also receive stimulation albeit at lower intensity (Rodger
et al., 2012; Makowiecki et al., 2014), but whose contribution to the effects of rTMS
remain ill-defined (Ellaway et al., 20214).
However, in the last few years, there has been mounting evidence that rTMS may not
induce reliable and reproducible effects. The high variability within and between
subjects, and often-contradictory outcomes of rTMS experiments in different laboratories,
has made its use somewhat controversial. Thus in recent years, the viability of rTMS
as a therapeutic tool has increasingly come under scrutiny (Di Lazzaro et al., 2013;
Lefaucher et al., 2014). This lack of reproducibility reflects that rTMS has been
used clinically for almost two decades without preceding fundamental animal and in
vitro research to identify the cellular effects beyond inducing action potentials.
Given that human experiments allow limited opportunity to investigate underlying cellular
and molecular mechanisms, developing the stimulation tools to conduct rTMS experiments
in animals and in vitro models is critical to allow an improved understanding of the
primary actions of rTMS on neurons and neural circuits. This fundamental approach
is necessary if we are to successfully manipulate brain stimulation in order to harness
the excitability and plasticity that promote optimal recovery following injury.
What rTMS parameters may promote neural repair?: (1) Activity dependent plasticity–Although
we know that rTMS induces action potentials in cortical neurons, the factors that
determine whether a magnetic pulse will lead to an action potential remain poorly
characterised. Key factors are magnetic field intensity (directly related to distance
from stimulation device) and its focus (Deng et al., 2013). Computational modelling
studies suggest that the likelihood of action potential firing may also depend on
properties of the neuron (Pell et al., 2011), such as intrinsic excitability, morphology
and orientation with respect to the magnetic field, yet these have never been directly
investigated in real neurons. Moreover, the cerebral cortex is a complex heterogeneous
tissue, thus rTMS may stimulate a combination of excitatory, inhibitory and neuromodulatory
neurons that activate internal regulatory circuits (Pell et al., 2011). This, in turn,
will confound interpretation of what any given stimulation paradigm is doing to neural
activity, how this may be altered when that circuit is damaged (Ellaway et al., 2014)
and thus whether such activation may facilitate repair.
Moreover, as magnetic stimulation induces action potentials, rTMS-induced activity
will trigger long term potentiation (LTP) and long term depression (LTD)-like synaptic
plasticity. Evidence for this comes indirectly from human studies with long lasting
post-stimulation changes in cortical excitability (Pell et al., 2011), but also directly
from ex vivo neonatal mouse brain slices in which rTMS induces LTP (Vlachos et al.,
2012). Not surprisingly, the effects of magnetic stimulation frequency match those
of electrical stimulation observed in classical electrophysiological experiments:
low frequency inhibits, and high frequency excites neural circuits via the induction
of LTD or LTP (Pell et al., 2011).
The frequency-specific aspect of long term rTMS outcomes is a major clinical advantage
because treatments can be tailored to specific dysfunctions: low frequency stimulation
has been successful in treating disorders associated with cortical hyper-excitability
such as stroke and spinal cord injury, while high frequency stimulation is effective
in treating depression (Pell et al., 2011). Human studies have hinted at improved
cognitive function and faster reaction times but evidence is patchy and poorly reproducible
(Pell et al., 2011). However, recent animal studies reveal that rTMS may have significant
and lasting impact by reopening developmental critical periods and altering metaplasticity
(Makowiecki et al., 2014; Mix et al., 2015). This is a more powerful outcome than
a simple change in excitability because it has the potential to facilitate long term
structural and functional change, effectively rewiring the brain.
(2) Are action potentials necessary for rTMS effects?–Because human rTMS studies most
commonly measure muscle responses to magnetic fields applied at intensities close
to those required to activate the motor cortex, the effects of rTMS are generally
assumed to be due to induction of action potentials in neurons. However, there is
a significant body of work showing that low intensity magnetic fields, several orders
of magnitude lower than the common rTMS protocols, are also effective at inducing
neural modulation. In humans, low intensity rTMS (LI-rTMS) modulates cortical excitability,
induces analgesia and alleviates depression (Di Lazzaro et al., 2013). In mice, LI-rTMS
induces structural changes in congenitally abnormal brain circuits, resulting in improved
behaviour (Rodger et al., 2012; Makowiecki et al., 2014). In vitro experiments have
shown that such stimulation (LI-rMS) does not trigger action potentials, but nonetheless
increases intracellular calcium within individual neurons, providing the basis for
synaptic plasticity and metaplasticity processes to occur (Grehl et al., 2015). This
finding raises some key questions about the mechanisms underlying rTMS:
Magnetic field or electric field? There is evidence of magneto-reception in all vertebrate
classes (Wiltschko and Wiltschko, 1995), yet in our focus on induced electric field
and neuronal depolarisation, we forget that the magnetic field itself may exert a
direct effect on cells.
Neurons are not the only targets. Given that action potential firing may not be a
pre-requisite for some aspects of rTMS effectiveness (Grehl et al., 2015), other cells
within the brain such as glial cells, vascular endothelial cells, immune cells etc
should be considered potential targets of rTMS.
What next? How to optimise rTMS for neural repair: Although our current knowledge
provides tantalising information about the power of magnetic stimulation to modulate
brain function, improve dysfunction and potentially repair an injured brain, the appropriate
stimulation parameters remain unknown. The current major challenge is how to identify
them. It is known in human research that stimulation devices can deliver slightly
differing waveforms under the same settings, resulting in diverging cortical effects
over-and-above inter-subject variability (Pell et al., 2011). Unfortunately the effects
of magnetic stimulation are based on the combination of several parameters, the impact
of which can only be assessed by systematically acquiring data under highly controlled
and standardized experimental conditions. This is the strength of animal and in vitro
models, which allow manipulation not only of the external environment, but also of
the genetic and pharmacological environment within the brain. However, the stimulation
tools we possess at the moment are tailored for the human brain and we need to develop
devices to extend our investigations to a wide range of stimulation parameters on
a wide range of targets (
Figure 1
).
Figure 1
Summary of available and desired coils to deliver magnetic fields to animals and in
vitro that are equivalent to those applied in human repetitive transcranial magnetic
stimulation (rTMS).
For all panels, coils are shown in black and the approximate location of the induced
current is in grey. For simplicity, the direction of current flow is not shown. A
typical human “figure of eight” coil (A) showing in dark grey the hotspot of maximal
“focal” stimulation normally used to elicit motor evoked potentials (MEPs). When applied
to the human head, the hotspot is positioned over the target brain region, but the
rest of the brain also receives stimulation, albeit at lower intensity. However, when
this human coil is applied in animals or in vitro, the hotspot is no longer focal
relative to the target, but rather stimulates the entire head/culture, with the induced
current no longer being contained within the target (e.g., Vlachos et al., 2012).
This reduces the efficiency of magnetic induction and changes the properties of the
induced current. In some studies, a small figure of eight coil is used, (B) which
improves focality to one hemisphere in rats, but has similar disadvantages relative
to efficiency of induction. To address this problem, custom-made round coils have
been used to deliver focal stimulation in rodents and in culture (Rodger et al., 2012;
Makowiecki et al., 2014; Grehl et al., 2015) (C). Although these deliver low intensity
magnetic fields, the induced current is fully contained within the brain, increasing
efficiency of induction. The coils are small enough to stimulate one hemisphere in
both mice and rats, and a single culture well. In panel D, we propose “ideal” small
“figure of eight” coils which would provide focal stimulation in animals and in culture,
while maintaining a similar coil to target ratio as that used in humans. Although
a limitation of small coils is that they cannot deliver high intensity magnetic fields
without significant heat generation, the small coil to brain distance in rodents and
in culture means that it may not be necessary to deliver a magnetic field of the same
magnitude required in humans in order to stimulate smaller targets at the same intensity
(E).
(1) Coil design–Although rodent models have revealed key molecular changes following
rTMS (Pell et al., 2011; Vlachos et al., 2012; Grehl et al., 2015), most studies use
coils that are larger than the rodent brain, such as small commercially available
figure-of-eight or round coils of at least 50 mm outer diameter. Whilst the use of
such coils allows for stimulation at the high intensities used in humans (1–2 T),
they lack 2 crucial facets: equivalent spatial resolution which confounds correlation
of its outcomes to humans; and similar stimulation fields which are determined by
the coil-to-target size ratio (Deng et al., 2013). Therefore, animal researchers are
increasingly beginning to design small coils tailored to their experimental requirements.
However, with decreasing coil size, it is challenging to maintain high stimulation
intensities, due to thermal and mechanical stress. Strategies to overcome these problems
address the trade-off between stimulus focality and intensity: addition of inbuilt
cooling devices in commercial coils, complex coil shapes to improve focus (Deng et
al., 2013) or use of low intensity stimulation (Rodger et al., 2012; Makowiecki et
al., 2014; Grehl et al., 2015). However, an “ideal” animal coil that accurately reproduces
the physical properties of human rTMS in an animal brain has yet to be built (
Figure 1
). Thus, while a wide range of repetitive magnetic stimulation paradigms can be evaluated
experimentally, if we are to understand the type of electric fields induced in human
subjects during rTMS, there is an urgent need to develop small coils that deliver
focal magnetic stimulation at high intensity in animal models and in culture dishes.
(2) Control of stimulation parameters–In addition to developing appropriate coils,
it will be necessary to construct stimulation devices that can deliver the full range
of rTMS parameters, controlling frequency, rhythm, number of pulses, intensity, waveform,
field orientation, total length of stimulation, etc (Pell et al., 2011). These constraints
are necessary to define the induced electrical field, which is what acts upon the
neuronal tissue. Therefore the current convention of using rTMS intensities of “X
% of motor threshold” or “Y % maximal output of the machine” does not permit valid
comparison between studies because the induced electric field remains unknown.
Conclusion: rTMS presents a unique opportunity to modulate brain excitability and
plasticity in a precisely controlled manner yet its role for neurorehabilitation remains
poorly understood. We propose that rTMS is taken from the bedside back to the bench:
the use of appropriate delivery devices in animal and in vitro models is crucial to
provide a practical and theoretical framework to direct how rTMS can be applied following
neurotrauma to promote regeneration and rehabilitation of neural circuits.
JR is a NHMRC Senior Research Fellow. We are grateful for CNRS PICS support for our
collaboration, scientific discussions with Stephanie Grehl and Alex Tang that are
reflected in this commentary, and figure preparation by Marissa Penrose-Menz.