Introduction

Dual-color multichannel fiber photometry system is used for measuring in vivo neuronal activity in freely behaving animals. It can be used with genetically encoded calcium ion indicators (GECI) to measure the activity of groups of neuronal cell bodies, dendrites, and axonal terminals. Dual-colour photometry system comprises two excitation light sources that pass through a multimode optical fiber to stimulate neurons. The technique is frequently used to study neural circuit patterns and develop treatments for various neurological diseases, including traumatic brain injuries. Different neurons are simultaneously stimulated using different wavelengths of light. 

Real-time monitoring of the neuronal activity at multiple sites in freely behaving animals is a bit challenging. It is impossible to record axonal terminals’ activity using traditional electrophysiology because of the small sizes and high densities of these terminals, particularly in freely behaving animals. However, the situation becomes different when calcium imaging techniques are employed. For instance, two-photon (Ca+2) imaging allows monitoring in vivo Ca+2 activity at the micrometer scale and neuronal activity in superficial brain regions like the cerebral cortex. However, two-photon imaging is restricted to anesthetized or head-fixed animals. The development of miniaturized microscopes comprising gradient refractive index (GRIN) lenses allowed functional studies of deep brain regions (striatum, amygdala, and hypothalamus) in freely behaving animals. However, this technology also has a restricted recording range. Multichannel fiber photometry is far better than these techniques because one can easily monitor neuronal activity in many deep brain regions of freely behaving animals (Qin et al., 2019). 

Principle

Fiber photometry is an optogenetics technique in which light is coupled into a multimode optical fiber and is directed toward the neurons. The light stimulates the neurons that emit fluorescent signals depicting the occurrence of neuronal activity. These fluorescent signals correspond to in vivo calcium dynamics, return through the same multimode fiber, and are captured by a detector. The fluorescent signal intensity changes from a “genetically encoded calcium indicator (GECI)” are detected. This way, calcium activity in different neuronal regions and neuronal circuit patterns are recorded. In many cases, multichannel fiber photometry systems are used. These systems allow recording neural activity in more than one brain region simultaneously. For instance, dual-channel fiber photometry systems have been developed that help record neural activity in two motor cortical regions or even two different animals simultaneously using a two-sensor photometry system.

Dual-color multichannel fiber photometry system works on the same principle described above, except that it employs two light sources for neuronal excitation. One excitation light source is used as a background signal to ensure the collection of authentic fluorescence data (Guo et al., 2015).

Apparatus and Equipment

Dual-color multichannel fiber photometry system offered by Conduct Science has two excitation light sources, i.e., 470nm and 410nm. The 410nm light source acts as a background signal for accurate real-time monitoring of the fluorescence data, whereas the 470nm light source excites the genetically encoded calcium indicators. The system utilizes nine channels to record neuronal activity simultaneously from multiple brain regions or multiple animals. The exposure time of the instrument ranges between 1-100ms, and its LED power can be adjusted between 0% and 100%.

Applications

Neuronal Population Dynamics Measurement

To understand the pathophysiology of Levodopa-induced dyskinesia (LID) in Parkinson’s disease (PD), Gao et al. (2022) studied the real-time activity of the Striatal D1 Dopamine neuronal population in rats. The researchers used a dual-color multichannel fiber photometry system to study the neuronal population dynamics. Heterozygous Drd1-Cre transgenic rats (age: 8-10 weeks, weight: 240 ±10gm) were selected as model animals. They ensured consistent genetic background, housed the animals on a 12-h light/dark cycle, and provided water and rodent chow ad libitum. After Levodopa administration following stereotaxic surgery of the subjects, the researchers used a dual-color multichannel fiber photometry system to detect and record fluorescent signals of a GECI named GCaMP6m. The scientists used a multimode optical fiber to guide light between the implanted fiber and the fiber photometry system. A light-emitting diode subjected lights of wavelengths 410nm and 470nm to excite Ca+2 -independent and Ca+2 -dependent control measurements, respectively. The emission light traveled through the same optical fiber was bandpass filtered and captured by a semiconductor camera. The scientists used a digital camera to record real-time rat behavioral videos. The data were statistically analyzed, and it was concluded that striatal D1+ neuronal population activity is linked to dyskinetic symptoms.

Measuring Long-Term Behavioral Changes resulting from Time-Restricted Feeding

Zhai et al. (2022) studied the effect of time-restricted feeding (TRF) on the long-term shift in locomotor behavior and the sleep-wake cycle in mice. The researchers used a dual-color multichannel fiber photometry system to compare intracellular Ca+2 signals in the GABAergic neurons of the suprachiasmatic nucleus (SCN) before and after TRF to analyze the activation of these neurons in dawn-TRF mice. The scientists took six to eight-week-old male C57BL/6J mice and housed them in cages with running wheels for 2 weeks under a light-dark cycle with food and water ad libitum. 

After stereotaxic injections, a 200µm diameter optical fiber was implanted with 5 degrees in SCN in mice. The mice were screened for rhythmic Ca+2 signals using a multichannel fiber photometry system. The mice were kept in LD (light-dark cycle) for 3 days, subjected to TRF for 2 weeks, and then released into DD (dark-dark cycle) with ad libitum. Ca+2 signals were again recorded before TRF entrainment. The 470nm light source was used as a signal channel to stimulate neurons, and 430nm light was used as a negative control channel. The fluorescent signals were recorded every 35s at an optical power of 15-20 μw. The scientists concluded that in mice, SCN GABAergic neuron activity is linked to long-term behavioral changes (i.e., sleep-wake cycle and food-hunger responses).

Strengths and Limitations

Dual-color multichannel fiber photometry system has several advantages. The primary advantage of a multichannel photometry system is that it can record neuronal activity at multiple sites and in multiple animals simultaneously. In addition, the optical probe is small and flexible and causes minimum damage to the surrounding tissue (Wang et al., 2021). Furthermore, traditional electrophysiological recordings employ “optical tagging” to record the activity of cell-type-specific neurons. However, a major disadvantage of optical tagging is its sensitivity to noise interference in natural behaviors. On the other hand, the dual-color multichannel fiber photometry system is relatively efficient and not sensitive to noise interference because of utilizing genetically encoded Ca+2 indicators (Li et al., 2019). The technique is highly efficient and considerably less expensive. Other advantages of the system include high temporal resolution, high sensitivity, greater accuracy, and requires lower optical power. 

Despite these advantages, a potential disadvantage of dual-colour multichannel fiber photometry system is that it has a poorer spatial resolution than other imaging techniques (Qin et al., 2019).

Summary

• Dual-color multichannel fiber photometry system is used for measuring in vivo neuronal activity in freely behaving animals. 
• It can be used with calcium ion indicators to measure the activity of groups of neuronal cell bodies, dendrites, and axonal terminals.
• Dual-color multichannel fiber photometry system employs two light sources for neuronal excitation. One excitation light source is used as a background signal to ensure the collection of authentic fluorescence data.
• The system is employed to measure Ca+2 activity in multiple neuronal cell populations and multiple animals simultaneously. 

References

Qin, H., Lu, J., Jin, W., Chen, X., & Fu, L. (2019). Multichannel fiber photometry for mapping axonal terminal activity in a restricted brain region in freely moving mice. Neurophotonics, 6(3), 035011.

Guo, Q., Zhou, J., Feng, Q., Lin, R., Gong, H., Luo, Q., … & Fu, L. (2015). Multichannel fiber photometry for population neuronal activity recording. Biomedical optics express, 6(10), 3919-3931.

Gao, S., Gao, R., Yao, L., Feng, J., Liu, W., Zhou, Y., … & Liu, J. (2022). Striatal D1 Dopamine Neuronal Population Dynamics in a Rat Model of Levodopa-Induced Dyskinesia. Frontiers in Aging Neuroscience, 14.

Zhai, Q., Zeng, Y., Gu, Y., Zhang, T., Yuan, B., Wang, T., … & Xu, Y. (2021). Time-restricted feeding near dawn entrains long-term behavioral changes through the suprachiasmatic nucleus. bioRxiv.

Wang, Y., DeMarco, E. M., Witzel, L. S., & Keighron, J. D. (2021). A selected review of recent advances in the study of neuronal circuits using fiber photometry. Pharmacology Biochemistry and Behavior, 201, 173113.

Li, Y., Liu, Z., Guo, Q., & Luo, M. (2019). Long-term fiber photometry for neuroscience studies. Neuroscience Bulletin, 35(3), 425-433.