Electrophysiology in animal models. By Maulik Oza and James Morizio.
Laboratories routinely undertake in vivo neuroscience studies with animal models ranging from mice to non-human primates in different states of consciousness including anesthetized, sleeping, awake or task-driven. For neurological diseases where neuronal networks are affected, electrophysiology experiments cannot be limited to anesthetized head-restrained animals, for neurons are shown to demonstrate different activity patterns as a function of brain state and behavioral engagement. To study neurological dysfunctions that span multiple brain regions, the spine, as well as visceral organs, scientists design electrophysiology protocols that often require “multi-site recordings and stimulation” of various neuronal structures simultaneously. These experiments require electrical systems with capabilities to record neural activity and simultaneously stimulate different anatomical targets in the central and peripheral nervous systems. These devices need to be plug-and-play while allowing the scientist to adapt their scientific protocols from not only studying neural network in association with the behavior it correlates but also to understanding its functional dynamics as they seek to develop new drug treatments in animal models.
Successful in vivo recordings require precise placement of electrodes with suitable conditions at the tissue electrode interface, in addition to proper grounding and signal references of electrical circuits. Newer head-mounted telemetric systems are reported to have mechanical stability in their headstages, electrodes and mounting mechanisms without wire tension and vibration noise that arises from cables and swivel during animal movement. The headstages also require low noise and highly sensitive preamplifiers for effective electrode signal pre-conditioning. Wireless recording and neuromodulation techniques are therefore less stressful and more physiologically relevant as animals do not have to contend with the practical issues arising out of cable twisting, external force and visual distraction, thus increasing animal comfort. Concurrent and often synchronized use of wireless systems along with other platforms that record markers for sensory stimuli or behavioral events require powerful computational capabilities and interoperability between different devices. Commercial hardware and software solutions now record low and high bandwidth signals, multiple stimuli and behavioral markers, video and animal positions simultaneously with millisecond temporal resolution. Constant current electrical and optical stimulation with adaptive pulse timings and amplitude are also available in open or closed loop scenarios. With increasingly complex experimental paradigms being implemented, interoperability amongst different platforms and powerful computational capabilities have made it easier to directly correlate and control neural activity with behavior which is a reflection of that underlying cognitive process. These studies taking place in freely moving animals hold immense significance as one can relate electrophysiology results to anatomical and neurochemical properties of the underlying circuits. Collection of such data across multiple scales and platforms is driving the development of more complex disease models, which in turn generates the need for innovative hardware and higher-performance computational capabilities.
Evolving disease models drive development of new systems
The need to mimic natural conditions in animal models leads the motivation to control and understand all possible relevant variables that may influence the activity being recorded and stimulated even though it may not be possible to accurately predict or control all variables in experiment designs –namely, behavioral state, training differences, selective stimulation and temporal precision. Recent rapid technological developments in integrated electronics have significantly impacted the latter two variables as systems are now capable of imitating and manipulating physiologically relevant neural processes with high temporal precision. The “Smart Ephys Wireless System” (SE-W) from Harvard Bioscience came about in response to these scientific and technological trends, a system with a high degree of flexibility necessary to adapt to evolving experimental paradigms.
With a recording and stimulation system design that incorporates novel technology from its two subsidiaries, Multi Channel Systems (MCS) and Triangle BioSystems (TBSI), powerful microcontrollers and computational chips in the MCS interface board are scaled to record from 5 to 384 channels while stimulating up to 16 channels. Whether experiments call for higher sampling frequency using analog modulation (TBSI headstage) or digitally modulated signals (MCS headstage) in complex noisy environments, the SE-W systems can be implemented in multiple configurations using TBSI or MCS wireless headstages. Headstage miniaturization using tiny electronic packages, RF chip antennae and small batteries reduces overall weight, which in return minimizes inflammation and tear at the site of surgery. In such head-mounted wireless headstages, semiconductor technology acquires and amplifies neural signals from multiple channels simultaneously without multiplexing with one channel for each electrode. This allows innovative architecture and packaging of a hybrid mixed-signal PCB combining radio frequency, record and stimulator electronics and passive chip antennae that results in 60 percent size reduction and micropower consumption per channel. The inductive charging accessory facilitates long-term recording and stimulation while eliminating the need to change batteries. On the receiver, the interface board with on-chip digitization and fast field-programmable gate array (FPGA) chip makes the SE-W a milliseconds closed-loop ecosystem that can be implemented to study efferent and afferent processes in a neural system without human intervention. With scaling flexibility to work with a wide variety of headstage configurations, the SE-W system provides the necessary tool to adapt to evolving needs of a progressive study. Through its additional digital and analog inputs, a versatile software allows the system to successfully synchronize and trigger stimulation pulses with most commercial platforms used in animal electrophysiology studies.
Physiologically active tissue is dynamic, complex and responsive to external stimuli, so it is imperative that the systems used to study and modulate them are equally adaptive and responsive. While it is understood that current electrophysiology studies yield correlative data, researchers continue to push the limits of traditional systems and experimental paradigms by exploring advantages presented by newer desktop and cloud platforms that make radical experimental designs possible.
With new published technology, further miniaturization and improved electrode interfaces are significant accomplishments in efforts to make such experiments less stressful and permanent for animals. Integration of biologically inert materials with fully integrated electronic systems operating on inductive power will improve packaging and tissue-electronic coupling. Bidirectional data flow with high bandwidth and on-board computational capabilities will enable real-time engagement of large-scale neuronal networks in awake animals with translational applications.