Electrophysiological recordings from ensembles of neurons in behaving mice are a central tool in the study of neural circuits. Despite the widespread use of chronic electrophysiology, the precise positioning of recording electrodes required for high-quality recordings remains a challenge, especially in behaving mice. The complexity of available drive mechanisms, combined with restrictions on implant weight tolerated by mice, limits current methods to recordings from no more than 4-8 electrodes in a single target area. We developed a highly miniaturized yet simple drive design that can be used to independently position 16 electrodes with up to 64 channels in a package that weighs ~2 g. This advance over current designs is achieved by a novel spring-based drive mechanism that reduces implant weight and complexity. The device is easy to build and accommodates arbitrary spatial arrangements of electrodes. Multiple optical fibers can be integrated into the recording array and independently manipulated in depth. Thus, our novel design enables precise optogenetic control and highly parallel chronic recordings of identified single neurons throughout neural circuits in mice.
electrode array; electrophysiology; free behavior; microdrive; multi-site; optogenetics.
The flexDrive provides a low-weight and high-yield method for chronic electrophysiology. (A) Isometric view of the flexDrive showing the one-piece spring (blue) that acts as the drive mechanism. (B) Illustration of the flexDrive implanted in a 6 month old C57/bl6 mouse. Due to the low implant weight (~2 g), the impact of the drive on natural behavior is minimal. (C) Cross section of the drive and its placement on the mouse skull. In this example, electrodes target the thalamus. (D) Cortical action potentials recorded from a stereotrode (12 μm nichrome wire, gold plated to ~300 KΩ) on a flexDrive showing eight clusters (color coded clusters, non-clustered spikes in gray) and average and 95% percentiles of the waveforms on the two electrode contacts.
The drive mechanism of the flexDrive. (A) Isometric view of the spring loaded drive mechanism. The pattern of electrodes is defined by an array of guide tubes (blue). Electrodes (black) are fixed inside shuttle tubes (orange) that can move up or down inside the guide tubes. The top of each shuttle tube is glued to a spring arm that is moved up or down by a drive screw. (B) Schematic view of the drive mechanism (not to scale). The static guide tubes (blue, part of the guide tube array) house the mobile shuttle tubes (orange) that are moved by the drive spring. Stabilizer tubes (green) are used to facilitate assembly of the guide tube array. (C) Examples of electrode patterns that can be fabricated by arrangement of the guide tubes and optical fibers.
(A–C) Examples of identified units on stereotrodes, all plots peak/peak. (A) Recording quality sufficient for sorting units can be maintained on an electrode for >100 days by repeated small increments in electrode depth. (B) Example of an electrode that was not penetrating the cortex at surgery, but is lowered into the brain later. (C) Example of an electrode that loses the ability to discriminate units over time, but is “reactivated” by a small depth adjustment ~3 months after surgery.
Variant of the flexDrive in which an optical fiber is lowered in the brain by one of the 16 drive mechanisms. (A) The fiber is inserted through a guide tube and fixed to a drive spring, replacing a shuttle tube and electrode. The remaining 15 drives can be used for electrodes or more fibers. (B) Sketch of the workflow of an experiment made possible through moveable fibers and electrodes. A target area (dashed lines) is localized by slowly lowering a subset of electrodes first, then the fiber can be brought into optimal position for localized activation of the area or for the collection of optical signals.
Example application of the flexDrive for an experiment that require stable optical excitation of neurons. (A) Activation of PV-positive neurons in layer 2/3 of mouse primary somatosenory cortex (SI) with ChR2. An array of 8 tetrodes arranged in a circular pattern around a static 200 μm fiber (see insert) were slowly lowered into layer 2/3 of SI. (B) Example trace of an identified PV neuron on one of the tetrodes for one session.
Example application of the flexDrive for an experiment that requires simultaneous recordings from distributed, small target regions. (A) Experiment in which an array of 16 stereotrodes was used to simultaneously record from SI and the thalamic reticular nucleus (TRN) in awake, behaving mice. The electrode positions are shown for the 3rd day after the first electrodes reached TRN. (B) Example peri-stimulus time histograms of 23 simultaneously recorded single units. A subset of the recorded neurons in SI and TRN are modulated by vibrissa deflections induced with a piezoelectric stimulator. (C) Example voltage trace (bandpass filtered at 1–9000 Hz) from a cortical electrode 290 days after the implant surgery. Colored circles and spike waveforms show spikes from 4 identified single units.
Comparison between existing types of implants and the flexDrive. Our novel design results in a higher number of individually movable electrodes at a reduced implant weight compared to existing methods (Lin et al., ; Yamamoto and Wilson, ; Battaglia et al., ; Kloosterman et al., ; Anikeeva et al., ; Vandecasteele et al., ; Neuralynx-Bozeman MT). The drive weight of ~2 g enables experimenters to either implant two drives per mouse, or to scale the design to 32 driven electrodes per implant.
All figures (7)
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