A versatile and modular tetrode-based device for single-unit recordings in rodent ex vivo and in vivo acute preparations

Highlights

• Developed an inexpensive tetrode device for interchangeable acute brain recordings between in vivo and ex vivo preparations.

• Conducted ex vivo extracellular recordings in acute cerebellar slices.

• Conducted in vivo extracellular recordings in auditory cortex of anaesthetized mice.

• Device can be reused multiple times and easily extended to accommodate optical fiber and cannula.

Abstract

Background

The demand for affordable tools for recording extracellular activity and successfully isolating single units from different brain preparations has pushed researchers and companies to invest in developing and fabricating new recording devices. However, depending on the brain region of interest, experimental question or type of preparation, different devices are required thus adding substantial financial burden to laboratories.

New method

We have developed a simple and affordable tetrode-based device that allows interchangeable extracellular recordings of neuronal activity between in vivo and ex vivo preparations and can be easily implemented in all wet-bench laboratories.

Results

Spontaneous activity from several putative single neurons could be easily recorded and isolated by lowering the device into ex vivo cerebellum brain slices. The same device was also used in vivo, lowered into primary auditory cortex of adult anesthetized transgenic mice expressing channelrhodopsin in cortical neurons. Acoustic stimulation of the contralateral ear or direct laser optogenetic stimulation successfully evoked cortical activity at the recording site. Several isolated putative single neurons presented time-locked activity response to the different stimuli.

Comparison with existing methods

Besides low fabrication cost, our device uses an omnetics connector compatible with the majority of headstages already available at most electrophysiology laboratories. The device allows custom tetrode configuration arrays and extensions for optogenetics and pharmacology, providing experimental flexibility not available in commercial off-the-shelf microelectrode arrays and silicon probes.

Conclusions

We developed an affordable, versatile and modular device to facilitate tetrode extracellular recordings interchangeably between in vivo anaesthetized animals and ex vivo brain slice recordings.

Introduction

Extracellular neuronal activity has been recorded and analyzed in neuroscience research for decades, providing insight into cell- and circuit-level brain computations (Henze et al., 2000; Barthó et al., 2004). The ability to perform spike sorting on extracellular activity recordings has been particularly useful since it allows the isolation of putative single neurons and the analysis of their activity/responses individually (Rey et al., 2015). However, spike sorting is less commonly used in ex vivo brain slice recordings because typical single-wire pipette extracellular recordings are not ideal for this purpose and commercially available options are usually costly, namely multi-electrode arrays (MEA) (Meister et al., 1991; Heuschkel et al., 2002) and especially designed silicon probes (Aivar et al., 2014). Tetrodes are a simple and cost-effective approach that has been routinely used for chronic recordings of extracellular activity in freely-moving rodents (Wilson and McNaughton, 1993). Their long lasting success in neuroscience relies not only on their cost and easy implementation but also on their ability to facilitate spike sorting (Wilson and McNaughton, 1993; Gray et al., 1995). Surprisingly, despite their success in freely-moving electrophysiology, tetrodes have been less frequently used in acute preparations including in vivo anesthetized animals and especially ex vivo slice preparations. One main factor limiting the use of tetrodes for the recording of extracellular spikes and subsequent unit isolation in acute preparations is the non-existence of commercially available devices that can simultaneously work as tetrode interface boards and structurally support/guide tetrodes into slices or whole-brains in anaesthetized animals.

Here, we propose a versatile and modular tetrode-based device that takes inspiration from probe design and freely-moving rodent tetrode microdrives, working for both ex vivo brain slices and in vivo anaesthetized recordings. The device parts can be easily produced by affordable and widespread technologies, many times in-lab, such as printed circuit board (PCB) manufacturing and 3D printing. Additionally, the device can be: quickly assembled; integrated with any amplifier/recording system already available at the lab for extracellular recordings; and used unlimited times, including inter-changeably between ex vivo and in vivo experiments, with multiple tetrodes and configurations. For in vivo preparations it can also be easily extended to be coupled with optical fiber or cannulas for optogenetic and pharmacological experiments. Using this device, we recorded spontaneous and opto- and sound-evoked neuronal activity in transgenic mice, both in vivo and ex vivo. In both configurations it was possible to reliably isolate several good quality single units from short recordings and analyze individual units’ responses to light and sound modulation. This device will facilitate a more widespread use of tetrodes for acute preparations in neuroscience experiments.