Researchers at Tsinghua University and Huazhong University of Science and Technology have developed an all-fiber-transmission photometry system that permits optogenetic manipulation and multicolor recording of neuronal activities and neurotransmitter release to take place concurrently. To support the study of brain activity in freely moving animals, the system is composed of a custom-designed, multibranch fiber bundle, through which it delivers all the light required for manipulating and recording brain activity.

The wavelength-independent system — it enables simultaneous manipulation and recording at different wavelengths — supports the need to comprehensively profile neural circuit functions and study neurological disease.

Photometry delivers a measure of visible light in units that are weighted against the brightness sensitivity perception of the human eye. Approaches to the technique based on fiber architectures have recently gained favor, the researchers said, as they provide the simple, stable recording of population activities with cell-type specificity in freely moving animals.

To study neural circuitry and neurological disease more effectively, scientists need the ability to monitor and manipulate neuronal activity at the same time. Most optogenetic techniques used to manipulate neuronal activity work independently from methods used to monitor and record brain activity.

The researchers’ non-wavelength-selective, multichannel fiber bundle is designed to perform the all-fiber transmission of both excitation light, for optogenetic stimulation, and emission light. Using a small multibranch fiber bundle in place of dichroic mirrors and an objective lens, the researchers realized, in the single fiber bundle, three excitation lights and two emission lights. This approach simplified the system and made it more robust and flexible for use in freely moving animal experiments.

The spectral transmittance of the multichannel bundle is about 60% per meter in the range of 400 to 900 nm. To decrease optical transmission loss, the researchers limited the bundle length to about 350 mm, which allows about 84% of the light to be transmitted.

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Multicolor recording and optogenetic manipulation of neuronal activities in nucleus accumbens (NAc) of a freely moving mouse. (a) Simultaneous recording of dopamine dynamics and neuronal calcium signals in a freely moving mouse. (b) Simultaneous multicolor recording and optogenetic manipulation of neuronal activities in a freely moving mouse. Courtesy of Opto-Electronic Advances.

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Artifacts from optogenetic stimulation can be a common occurrence during recording since it is difficult to completely filter out optogenetic stimulation light. To suppress the optogenetic stimulation-induced artifacts and channel crosstalk, the researchers used a laser of narrow linewidth at 660 nm to activate the red light-drivable channelrhodopsin — light-gated ion channels — and inhibited the stimulation-induced artifacts using a lock-in amplification method.

The approach also allowed the researchers to separate the fluorescence signals of the dual-color channels. Further, the researchers used the system was used to extract fluorescence signals of two different colors — green fluorescent protein-based and red fluorescent protein-based — while it suppressed the potential artifacts caused by the optogenetic manipulation.

To characterize the loss in fluorescence signal in the fiber photometry system, the researchers tested the collection efficiency using a series of sodium fluorescein solutions and compared the results to those acquired using a traditional epi-fluorescence system. The results demonstrated that the all-fiber-transmission photometry system effectively excited and collected fluorescence signals, and the collection efficiency of the system outperformed a traditional epi-fluorescence system. 

In additional tests, the researchers recorded dynamic dopamine responses to unexpected rewards in the nucleus accumbens in a freely moving mouse. The team also showed simultaneous dual-color recording of neuronal calcium signals and dopamine dynamics in the nucleus accumbens upon delivery of an unexpected reward and the simultaneous optogenetic activation at dopaminergic terminals in the same location.

The researchers said that the fiber photometry system can be easily modified for other fluorescent probes and opsin-based sensors by changing the light sources and filter positions. It also has the potential to be modified into a closed-loop system that could guide optogenetic manipulation informed by real-time monitoring.

The research was published in Opto-Electronic Advances (www.doi.org/10.29026/oea.2022.210081).