A multimodal micro-optrode

Microelectrodes are powerful tools for in vivo functional studies. However they are limited in the number of information they provide. In January 2011, LeChasseur and colleagues (Nature Methods 8(4), 319-325, 2011) developed a glass microelectrode which was integrating an optical micro-channel for light delivery and fluorescence collection. The probe serves for specific cellular fluorescence optical detection and activation/inhibition. In a recent issue of PLOS ONE, Dufour et al. extended the multimodal aspect of this micro-optrode. They introduce a, aluminum-coated, fibre optic-based glass microprobe (diameter ≤ 10 μm) with multiple electrical and optical detection capabilities. The probe enables optical separation from individual cells in transgenic mice expressing multiple fluorescent proteins in distinct populations of neurons within the same deep brain nucleus. It also enables color conversion of photoswitchable fluorescent proteins, which can be used for post-hoc identification of the recorded cells and finally it enables dual electrical recordings. Figure 1 shows a representation of the microp-optrode and the modalities described in this paper.

Figure 1: Optical and electrical microprobes. Schematic representation of the probe (left) and a metal coated probe adapted to achieve large field recording through the Al coating (middle: 3D representation, right: transverse cut view) and the modalities described. Insets are scanning electron microscopy images of the respective electrode tips (scale bars are 2 µm). Adapted from PloS one 8(2), e57703, 2013.

Figure 1: Optical and electrical microprobes. Schematic representation of the probe (left) and a metal coated probe adapted to achieve large field recording through the Al coating (middle: 3D representation, right: transverse cut view) and the modalities described. Insets are scanning electron microscopy images of the respective electrode tips (scale bars are 2 µm). Adapted from PloS one 8(2), e57703, 2013.

These modalities are in addition to the calcium monitoring and optogenetic cellular activation previously reported (Nature Methods 8(4), 319-325, 2011). In this study, two different excitation sources and detection pathways were used simultaneously to differentiate two different populations of neurons in vivo. Experiments were conducted on BAC transgenic mice with neurons expressing tdTomato and EGFP proteins under the control of the promoters for D1 and D2 dopamine receptors, respectively (see Fig. 2A-E).

Figure 2: A) Striatal section of a transgenic mouse showing cells expressing fluorescent proteins under the control of D1 (red) or D2 (green) receptor (green) promoters. Some cells co-express both fluorescent proteins (see arrow). B) Average density of the different cell types in the striatum (n=5 striatal sections). C) Histogram of the optically detected cells. (D-E) Examples of in vivo simultaneous optical and electrical recordings (inset) as the probe pass by a green (D) or a red (E) cell. F) Images at different time points of a mEOS2 expressing cell during UV-induced photoconversion. UV illumination with the probe causes an increase in red fluorescence. G) Simultaneous recording of field potential oscillations and single unit achieved with the microprobes. Inset: overlay of 10 successive spikes (vertical scale bar: 0.1 mV, horizontal scale bar: 1 ms). Adapted from PloS one 8(2), e57703, 2013.

Figure 2: A) Striatal section of a transgenic mouse showing cells expressing fluorescent proteins under the control of D1 (red) or D2 (green) receptor (green) promoters. Some cells co-express both fluorescent proteins (see arrow). B) Average density of the different cell types in the striatum (n=5 striatal sections). C) Histogram of the optically detected cells. (D-E) Examples of in vivo simultaneous optical and electrical recordings (inset) as the probe pass by a green (D) or a red (E) cell. F) Images at different time points of a mEOS2 expressing cell during UV-induced photoconversion. UV illumination with the probe causes an increase in red fluorescence. G) Simultaneous recording of field potential oscillations and single unit achieved with the microprobes. Inset: overlay of 10 successive spikes (vertical scale bar: 0.1 mV, horizontal scale bar: 1 ms). Adapted from PloS one 8(2), e57703, 2013.

Photoconversion experiments were also conducted in hippocampal mEOS2 transfected cells. Emission properties of mEOS2 were converted using UV illumination provided by the microprobe. Fluorescent red and green signals were image separately (Zeiss filter sets: (BP564/12, FT580, LP590) and (BP450-490, FT510, BP515-565) under a microscope objective to visualize in real time the emission spectral shift of the mEOS2 as it was being exposed to UV light through the microprobe (see Fig. 2F).
This work also demonstrates that a thin reflective aluminum film can be used as a large scale field potential electrode yielding a highly compact microprobe with combined single unit and population recording capabilities. This allows relating activity in single cells to that of the surrounding population of neurons. The hollow core of the optrode was used to perform single unit recordings and a metal film was used to record the activity of a large volume of surrounding cells (See Fig 2G).

In conclusion, optical and electrical multimodalities of this microprobe vastly expand the possibilities for in vivo electrophysiology, in particular, with optical means to monitor the impact of genetic manipulations of individual cells in vivo. The simplicity of the probe, its low cost, ease of use and adaptability make it particularly attractive modality to enhance in vivo electrophysiological recording systems.

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