Optical imaging of stimulation-evoked cortical activity using GCaMP6f and jRGECO1a

Genetically encoded calcium indicators (GECIs), especially the GCaMP-based green fluorescence GECIs have been widely used for in vivo detection of neuronal activity in rodents by measuring intracellular neuronal Ca²⁺ changes. More recently, jRGECO1a, a red shifted GECI, has been reported to detect neuronal Ca²⁺ activation. This opens the possibility of using dual-color GECIs for simultaneous interrogation of different cell populations. However, there has been no report to compare the functional difference between these two GECIs for in vivo imaging. Here, a comparative study is reported on neuronal responses to sensory stimulation using GCaMP6f and jRGECO1a that were virally delivered into the neurons in the somatosensory cortex of two different groups of animals, respectively.

GCaMP6f and jRGECO1a GECI were virally delivered to sensory cortex. After 3-4 weeks, the animals were imaged to capture the spatiotemporal changes of neuronal Ca²⁺ and the hemodynamic responses to forepaw electrical stimulation (0.3 mA, 0.3 ms/pulse, 0.03 Hz). The stimulation-evoked neuronal Ca²⁺ transients expressed with GCaMP6f or jRGECO1a were recorded during the baseline period and after an acute cocaine administration (1 mg/kg, i.v.).

Histology confirmed that the efficiency of jRGECO1a and GCaMP6f expression into the cortical neurons was similar, i.e., 34%±3% and 32.7%±1.6%, respectively. Our imaging in vivo showed that the hemodynamic responses to the stimulation were the same between jRGECO1a and GCaMP6f expressed groups. Although the stimulation-evoked fluorescence change (∆F/F) and the time-to-peak of the neuronal Ca²⁺ transients were not significantly different between these two indicators, the full-width-half-maximum (FWHM) duration of the ∆F/F rise in the jRGECO1a-expressed group (0.16±0.02 s) was ~50 ms or 46% longer than that of the GCaMP6f group (0.11±0.003 s), indicating a longer recovery time in jRGECO1a than in GCaMP6f transients (P<0.01). This is likely due to the longer off rate of jRGECO1a than that of GCaMP6f. After cocaine, the time-to-peak of Ca²⁺ transients was delayed and their FWHM duration was prolonged for both expression groups, indicating that these are cocaine's effects on neuronal Ca²⁺ signaling and not artifacts due to the property differences of the GCEIs.

This study shows that both jRGECO1a and GCaMP6f have sufficient sensitivity for tracking single-stimulation-evoked Ca²⁺ transients to detect neuronal activities from the brain. Since these GECIs are emitted at the different wavelengths, it will be possible to use them together to characterize the activity of different cell types (e.g., neurons and astrocytes) to study brain activation and brain functional changes in normal or diseased brains.