Introduction
Since the original study from Renshaw, Forbes & Morrison in the 1940s recording the activity of neurons in the brain of anesthetized cats,1 the electrophysiology technique has always had an essential place in neuroscience research for the understanding of the main central nervous system (CNS) functions (language, motor control, cognition, etc.) as well as related CNS disorders: mental/neurological disorders, substance abuse and alcohol-related issues, neurodegenerative disease related with the aging of the population, spinal cord injuries, etc.
At the beginning of neuroscience research, brain research belonged to many different areas that differed in methodology and targets: the morphological, the physiological and the psychological. Nowadays, scientific and technological researchers, from molecular to behavioral levels, understand the benefits of developing brain research in a really interdisciplinary way. Indeed, research is more and more based on the convergence of different interconnected scientific sectors.
In that context, in vivo electrophysiology research is not an exception. For instance, a parallel electrophysiological recording and behavior monitoring of freely moving animals is essential for a better understanding of the neural mechanisms underlying behavior and intrinsic brain processes. It is worth noting that the increase in importance of such multidisciplinary approaches in today’s research would not be possible without rapid improvements in electrophysiology techniques. As an example, the use of wireless in vivo electrophysiology techniques is one of the innovations that is making it easier to combine electrophysiology and behavior in laboratory animals.
The benefits of a multidisciplinary approach are not only scientific (e.g. direct correlation between the observed phenomena, higher specificity in the collected data in terms of their cause-effect relation) but there is also ethical collateral. Indeed, by improving the data quality and quantity, a multidisciplinary approach assists in the effort of reducing the number of animals needed to obtain scientifically valid data as well as in minimizing stress by promoting the use of less invasive methods allowing the running of experiments in progressively more natural conditions. This is particularly important in the context of the current higher pressure directed toward neuroscience researchers by ethical committees.
With this perspective, it is expected that new demand for electrophysiology instruments in the life science or clinical markets will continue to grow with new application areas, related to a more multidisciplinary/integrated analysis of the CNS functions by itself and of its relation with the whole organism.
Neurological diseases also continue to affect one out of five people in the world, which is also driving the increase of research funding in this space. Continually evolving refinements in in vivo electrophysiology methodology will undoubtedly aid neuroscience researchers in getting high-quality, physiologically relevant data and contribute to ground-breaking discoveries that may, ultimately, lead to new therapeutic strategies.
Wireless in vivo electrophysiology and behavior
The living brain is essentially an electrical organ at the interface of the external world. Consequently, most of our knowledge regarding the neuronal correlates of behavior would come from the concomitant study of the electrical activity of the brain. In this context, the oldest and most frequently used technique to study the living brain is the in vivo extracellular recording.
The combination of electrophysiology with the measurement of behavior in awake/freely moving laboratory animals is gaining more sense with the progressive evolution of techniques, especially in the improvements made in the neurophysiology of extracellular recording with a trend toward miniaturization, increased neuronal sampling and data collection. For these recordings to be “ecologically” significant, animals need to be awake and behaving in small, indoor environments defined by the laboratory context.
In this context, the advent of commercially available wireless technology2 for running in vivo electrophysiology recording in laboratory animals is a significant advancement in neuroscience research. It offers the possibility of running long-term, hands-off multidisciplinary measurements in freely moving animals. The in vivo wireless electrophysiology method provides a great number of benefits with respect to the traditional tethered recording techniques. For instance, wireless recording of many signals (field potential, spike, electroencephalogram, electrocorticogram, electromyogram, temperature, etc.) is demonstrated to be less stressful and more relevant physiologically. Crucially, it is able to circumvent various practical issues inherent with the tethered recordings, which includes cable twisting, the external force and visual distraction arising from the cable itself or, for instance in the particular context of a social interaction study, the entanglement of the cables or ease of chewing damage to the cable by the cage mate.
In addition to their rapid set-up times, wireless systems can be easily utilized and moved between the wide variety of experimental arenas and behavioral tests used in neuroscience in small laboratory animals for the study of the CNS functions and related diseases: locomotor activity/exercise and coordination (open-field test, rotarod, treadmill), pain sensitivity (tail-flick test, hot-plate), anxiety (plus maze), emotion and memory (fear conditioning test, radial maze, T or Y maze, water maze), addiction and cognition (operant boxes, self-administration boxes), social interaction, etc.
Technically, in the wireless in vivo electrophysiology systems, the neural signal emitted by the neurons is collected by brain-implanted electrodes that are associated with a battery alimented head-mounted system (headstage) that is used as a transmitter, driving the signal wirelessly to a special receiver. The signal is then interfaced to data acquisition software for storage, further filtering and analysis (Figure 1).
The current headstages available in the market are already a wonder of electronic engineering; interfacing stimulations and signals from different sensors. For instance, the W2100 wireless system developed in Germany by Multi Channel Systems (MCS) features a level-based construction making custom-built headstages possible. This kind of headstage consists of levels for amplification and Analog/Digital (A/D) conversion, electrical or optical stimulation and many more that are currently under development. Of note is that the recorded signals are converted into digital data already on the headstage. Therefore, the signal-to-noise ratio is much better and most importantly, independent from the distance between sender and receiver. This kind of headstage permits flexible long-term experiments in large environments.
The signal received from the electrodes implanted in the brain can also be transmitted through an analog (radio frequency – RF) signal. This is done with the headstage provided by Triangle BioSystems International (TBSI, US), with the advantage of providing a very high data-rate recording (50 kHz/channel) for maximum precision and time-resolution in the recording of the neuron activity, which is in the submillisecond regime.
All wireless systems depend on the associated battery lifetime, whose duration is proportional to the size of the battery. The average nominal battery life of a wireless system can be several hours. The battery life can be extended by turning off data acquisition at some user-defined period of time, allowing the battery life to be distributed over a longer period. Some solutions also exist for the continuous recording of neural data using far-field inductive power technology, such as the one developed by TBSI. This is particularly important for experiments evaluating the diurnal and nocturnal variations of neural activity such as sleeping, chronobiology or epilepsy studies.
Wireless electrical and optical (optogenetics) stimulation3 of the neurons can also be used to complement the recording techniques and are used to artificially challenge the neuronal activity to answer specific questions raised by the experimental context.
The in vivo wireless electrophysiology technique has been widely validated and is available for use in virtually all laboratory animals from mice to monkeys, even in birds and fish (Figure 2). Of note, its greatest use so far has been in rodents, the most frequently used in neuroscience research.
New perspectives in multi-site recording/stimulation and implantable headstages
What would the next steps be in the improvement of in vivo electrophysiology methods? Different avenues are being explored; among them are multi-site recordings/stimulation and implantable headstages.
“Multi-site recordings/stimulation” refers to the capacity to record neural activity in different locations of the brain and/or to stimulate one region of the brain and simultaneously record in another region. These techniques would provide more flexibility in the study of neural activity in a specific network in association with the behavior correlates.
Implantable wireless devices will be a significant accomplishment for recording and stimulation both in the cortical and peripheral nervous system. The implantable devices can be designed to be packaged in a miniaturized capsule and implanted in the animal’s gut in surgery. After recovery, the animal can proceed to delivilyer recordings or receive stimulation in a manner that is as close as possible to natural behavior. Although there is a risk of the package budging with certain organs, the risk is far less of an issue than having a tether or external device on the animal’s head. This technique may provide an additional step into multidisciplinary research, not only for improving the experimental conditions (e.g., minimizing animal stress) but also in providing new information on CNS functions and their relation to the whole organism.
For instance, implantable stimulation and recording electrophysiology technologies can open up new research markets centered around peripheral nerve applications. These new markets lie beyond the brain or central nerve electrophysiology that is possible using the existing headmounted technologies: for example, new research areas with digestive, bladder and respiration nerve-related research. An additional very important prospect would be related to robotics and new artificial interface developments for amputees.
Overall, the continuing advances in the field of electrophysiology are opening up a host of new opportunities for scientific researchers to learn more about how living organisms work.
References:
1. Renshaw B, Forbes A, Morrison BR. Activity of isocortex and hippocampus: electrical studies with micro-electrodes. J Neurophysiol.
1940;3:74-105.
2. Fan D, Rich D, Holtzman T, et al. A wireless multi-channel recording system for freely behaving mice and rats. PLoS One. 2011;6(7):e22033.
3. Rossi MA, Go V, Murphy T, Fu Q, Morizio J, Yin HH. A wirelessly controlled implantable LED system for deep brain optogenetic
stimulation. Front Integr Neurosci. 2015 Feb 10;9:8.
About the Author:
Jeffrey Duchemin is President and Chief Executive Officer of Harvard Bioscience, Inc., a global developer, manufacturer and marketer of a broad range of solutions to advance life science.
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