1Center for Brain Science, Harvard University, 2Program in Neuroscience, Harvard University, 3Department of Organismic and Evolutionary Biology, Harvard University
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Otchy, T. M., Ӧlveczky, B. P. Design and Assembly of an Ultra-light Motorized Microdrive for Chronic Neural Recordings in Small Animals. J. Vis. Exp. (69), e4314, doi:10.3791/4314 (2012).
The ability to chronically record from populations of neurons in freely behaving animals has proven an invaluable tool for dissecting the function of neural circuits underlying a variety of natural behaviors, including navigation1 , decision making 2,3, and the generation of complex motor sequences4,5,6. Advances in precision machining has allowed for the fabrication of light-weight devices suitable for chronic recordings in small animals, such as mice and songbirds. The ability to adjust the electrode position with small remotely controlled motors has further increased the recording yield in various behavioral contexts by reducing animal handling.6,7
Here we describe a protocol to build an ultra-light motorized microdrive for long-term chronic recordings in small animals. Our design evolved from an earlier published version7, and has been adapted for ease-of use and cost-effectiveness to be more practical and accessible to a wide array of researchers. This proven design 8,9,10,11 allows for fine, remote positioning of electrodes over a range of ~ 5 mm and weighs less than 750 mg when fully assembled. We present the complete protocol for how to build and assemble these drives, including 3D CAD drawings for all custom microdrive components.
1. Overview of Components
2. Drive Chassis Preparation and Assembly
3. Electrode Shuttle Assembly
4. Guide Tube and Electrode Installation
5. Final Assembly and Pre-Implantation Preparation
6. Representative Results
This protocol requires approximately 5 hr of hands-on assembly time with an additional 6-8 hr interspersed for the epoxy and glue to dry. However, once the microdrive has been assembled the first time, it can be prepared for reuse (i.e., the electrodes, electrode guide tubes, and electrode wires can be replaced) in less than 2 hr. The quality and character of the recordings obtained by following this protocol will, of course, be in part dependent on the recording target, the choice of electrodes, and any processing performed at the headstage or further upstream of the microdrive. This aside, it is possible to obtain stable recordings from a behaving animal over a time-span of more than ten weeks. Examples of single unit recordings from the robust nucleus of the archistriatum (RA) in a behaving songbird is shown in Figure 7A; multi-unit activity from the same region is shown in Figure 7B.
Figure 1. 3D model of the assembled motorized microdrive. The microdrive consists of several major components: (i) the chassis, (ii) motor with threaded output shaft, (iii) electrode shuttle, (iv) shuttle tubes, (v) electrodes, (vi) electrode guide tubes, and (vii) connector.
Figure 2. A) Model and connection diagram for the connector. Motor wires: b, i, l. Electrode wires: c,d,e,f. Ground wire: k. All other pins available for auxiliary instrumentation. B) Photo of the connector attached to the chassis with Torr Seal epoxy. Note the position of both the connector and the underlying wire guide tubes. The cyanoacrylate glue holding the guide tubes in position dries clear and, thus, is not visible in the image.
Figure 3. Electrode shuttle assembly.
Figure 4. A) The electrode guide tubes positioned in the chassis prior to applying Kwik-Cast. B) To ensure that the electrodes exit the bottom of the drive in parallel, bring the bottom end of the guide tubes into a tight bundle before adding the last drop of the Kwik-Cast.
Figure 5. The amount that the electrode guide tubes extend beyond the bottom of the drive is specified by the anatomy of the implantation site. The diagram illustrates the dimensions that are relevant for determining the length of the guide tubes. For implanting in the songbird, 1.5 mm is sufficient.
Figure 6. The finished motorized microdrive with protective covers attached.
Figure 7. Representative recordings from the robust nucleus of the archistriatum (RA) in the behaving zebra finch. A) Single-unit activity recorded with 10MΩ platinum-iridium electrodes. Top: example recording made one week after implantation. Bottom: example recording from the same electrode as above, nine weeks later. B) Multi-unit activity recorded with 1MΩ platinum electrodes. Click here to view larger figure.
Additional Files: The chassis, shuttle, and electrode tubes were designed with SolidWorks 2010 CAD software. These part files are provided in both the proprietary (*.sldprt) and vendor-neutral (*.iges) formats and dimensioned production drawings are provided in PDF format.
The protocol presented here will result in a device capable of high-quality recordings with minimal motion artifacts only if proper care is taken with construction. The fit of the shuttle in the chassis if of critical importance: too tight and the risk of overloading the motor is high; too loose and the risk of significant motion artifact is high. An ideal fit will allow the shuttle to travel the entire length of the threaded shaft without tilting out of position or chattering.
Selection of recording electrodes is similarly important; the choice of material, impedance, insulation, and tip profile may affect tissue reactivity, long-term stability and signal to noise ratio. For our application, we have found that high impedance (5-10MΩ) platinum-iridium microelectrodes produce stable single-unit recordings with a high signal-to-noise ratio (Figure 7); different applications may be better served by other electrodes. Within a relatively large range of electrode choices, a simple change in electrode guide tube sizing is likely the only required modification to adapt this microdrive.
Though recording neural signals can be informative in its own right, much nuanced insight can be gained by merging this with other behavioral or bio-signal data. One advantage of using such an ultra-light design is that it opens up the possibility of adding additional head-mounted sensors or effectors to the animal without fear of overloading. For example, this microdrive may be implanted in conjunction with a microphone, an audio receiver, stimulation electrodes, or microdialysis probes to provide a richer dataset of neural activity in various contexts. The spare contacts on the Omnetics connector (Figure 2A) provide a convenient interface for this additional instrumentation.
Like all electrophysiology devices, the quality of the recordings made with this microdrive are limited by the upstream signal conditioning and data acquisition equipment. Though the specifications for this equipment will be dictated by the requirements of the experiment, it is essential that a headstage preamplifier is employed immediately upstream of the microdrive to amplify the very small currents induced at the tip of the electrode to voltages measurable by standard acquisition instrumentation. There are a variety of commercial products that may be suitable for particular applications, though guidance on custom solutions is available in previously published work.5,12,13
No conflicts of interest declared.
This work was supported by the Ester and Joseph Klingenstein Fund, the McKnight Endowment Fund, and NINDS 1R01NS066408-01A1.
|Chassis||Custom||Cut from PEI|
|Electrode Shuttle||Custom||Cut from PEI|
|Shuttle Tubes||Custom||Cut from Stainless Steel|
|Connector||Omnetics||A7886-001||Mates to A7877-001|
|Motor w/ Gearhead||Faulhaber||0206-A-001-B-021-47:1|
|Wire Guide||Small Parts, Inc.||SWPT-0113-12|
|Electrode Guide||Small Parts, Inc.||SWPT-0045-12|
|10MΩ Pt-Ir electrodes||Microprobes, Inc||PI2PT310.0H3|
|Platinum Wire||A-M Systems||772000||For electrode wires|
|Silver Wire||A-M Systems||786000||For ground wire|
|Tungsten Wire||A-M Systems||797000||For electrode pins|
|Torr Seal||Varian Inc., Agilent||9530001|
|Kwik-Cast||World Precision Instruments, Inc.||KWIK-CAST|
|Light Mineral Oil||Sigma-Aldrich||M5310|
|Drive jig||Custom||Epoxy the mating connector to a syringe or stick|