Introduction

Functional magnetic resonance imaging (fMRI) based on blood oxygen level dependent (BOLD) signals reflects the changes of oxy/deoxy-hemoglobin ratio in vessels. Simultaneous fMRI studies with electrophysiological recordings have demonstrated that BOLD-fMRI signal correlated with neuronal activity¹. In addition, Schulz et al acquired fMRI and calcium (Ca²⁺) signal simultaneously through Ca²⁺ sensitive dyes². Using the similar strategy, we expressed the genetically encoded calcium indicators (GCaMP6f) specifically in either neurons or astrocytes in the rat cortex. The cell-type specific Ca²⁺ signal showed different coupling features to the BOLD fMRI signal. In contrast to the highly correlated neuronal Ca²⁺ signal to the fMRI signal, the evoked astrocytic Ca²⁺ signal remained positively correlated to fMRI signal, but showed an unexpected decoupling outlier event. The outlier event represented higher astrocytic calcium spikes but reduced BOLD signal in comparison to the other evoked events. This work implies a novel neuron-glia-vascular coupling event mediated through the intrinsic astrocytic calcium signaling.

Method

Methods All images were acquired with a 14.1 T/26cm horizontal bore magnet (Magnex), interfaced to an AVANCE III console (Bruker) and equipped with a 12 cm gradient set, capable of providing 100 G/cm with a rise time of 150 us (Resonance Research). A transreceiver surface coil was used to acquire fMRI images. Electrodes were placed on the forepaw (FP) to deliver a 1.0 mA pulse sequence (4s, 300μs duration repeated at 3Hz). GCaMP6f ³was expressed by AAV5 in the FP somatosensory cortex (FP-S1) or barrel cortex (BC) with Syn or GFAP promoter. Fiber optic (200um) was inserted into the area which expressed GCaMP for calcium-based fluorescent signal recording.

Results

Upon electrical stimulation, the evoked neuronal and astrocytic Ca²⁺ signal was simultaneously acquired with either LFP recordings or BOLD-fMRI in the rat forepaw S1 cortex (Fig 1). The neuronal calcium was highly correlated with either the sensory evoked potential (Fig 2a~c) or spontaneous LFP (Fig 2d~f). The astrocytic calcium response was correlated with corresponding LFP at different durations of the FP stimulus (Fig 3).Fig 4 shows that the peak amplitude of the evoked neuronal and astrocytic Ca²⁺ signal is proportional to the peak amplitude of fMRI signal simultaneously acquired at different electrical stimulation intensities. Interestingly, an outlier event demonstrated a negative correlation event between the astrocytic Ca²⁺ signal and fMRI signal (Fig 4e,f). This outlier event may indicate an unexpected decoupling of astrocytic calcium to fMRI signal.

Conclusion

A multi-modal fMRI platform was established to acquire simultaneous fMRI and GCaMP-mediated cell-type specific Ca²⁺ signal. It allowed us to decipher the cellular specific coupling events through the neuron-glia-vessel network contributing to brain state change. This study implied a novel neuron-glia-vascular coupling event mediated through the intrinsic astrocytic calcium signal. More details about the intrinsic astrocytic calcium signal was described in the other abstract (ID 4475).

Acknowledgements

This research was supported by the internal funding from Max Planck Society.

References

1. Logothetis et al. Nature, 412:150-157 (2001). 2. Schulz et al. Nature Method, 9: 597-605 (2012). 3. Chen et al. Nature, 499:295-302 (2013).