Mesoscale Transcranial Spontaneous Activity Mapping in GCaMP3 Transgenic Mice Reveals Extensive Reciprocal Connections between Areas of Somatomotor Cortex

Abstract

Transgenic mice expressing genetically encoded activity indicators are an attractive means of mapping mesoscopic regional functional cortical connectivity given widespread stable and cell-specific expression compatible with chronic recordings. Cortical functional connectivity was evaluated using wide-field imaging in lightly anesthetized Emx1-creXRosa26-GCaMP3 mice expressing calcium sensor in cortical neurons. Challenges exist because green fluorescence signals overlap with endogenous activity-dependent autofluorescence and are affected by changes in blood volume and oxygenation. Under the conditions used for imaging and analysis (0.1-1 Hz frequency band), autofluorescence and hemodynamic effects contributed 3% and 8% of the SD of spontaneous activity-dependent GCaMP3 fluorescence when signals were recorded through intact bone. To evaluate the accuracy and sensitivity of this approach, the topology of functional connections between somatomotor cortex (primary S1 and secondary S2 somatosensory, and primary motor cortex M1) was estimated. During sequences of spontaneous activity, calcium signals recorded at each location of area S1 were correlated with activity in contralateral area S1, ipsilateral area S2, and bilateral areas M1. Reciprocal results were observed when "seed pixels" were placed in S2 and M1. Coactivation of areas implies functional connections but could also be attributed to both regions receiving common upstream drive. These apparent connections revealed during spontaneous activity coactivation by GCaMP3 were confirmed by intracortical microstimulation but were more difficult to detect using intrinsic signals from reflected red light. We anticipate GCAMP wide-field imaging will enable longitudinal studies during plasticity paradigms or after models of CNS disease, such as stroke, where the weighting within these connectivity maps may be altered.

Resting-state hemodynamics are spatiotemporally coupled to synchronized and symmetric neural activity in excitatory neurons

“…The kinetics of GCaMP3 and GCaMP6f are similar (25). High-speed wide-field imaging of GCaMP fluorescence provides a depthintegrated, ensemble measurement of neural events over a much larger field of view compared with two-photon microscopy (18,19,24,26). However, an essential consideration when using wide-field GCaMP imaging is the spectral overlap between hemoglobin absorption and GCaMP excitation and emission bands (Fig.…”

“…Previous reports using both voltage-sensitive dyes and wide-field GCaMP imaging have noted similar bilaterally symmetric cortical patterns of neural activity, inferring functional connectivity between temporally correlated regions (26,41). Kozberg et al (19) demonstrated the transition of spontaneous cortical neural events from unilateral to bilateral in mice between postnatal day 7 (P7) and P14, a period of postnatal development where transcallosal axonal projections are establishing, suggesting a direct link between neural activity patterns and physical networks.…”

Resolution of High-Frequency Mesoscale Intracortical Maps Using the Genetically Encoded Glutamate Sensor iGluSnFR

“…After locally anesthetizing the scalp with lidocaine (0.1 ml, 0.2%), the skin covering the skull was removed and replaced by transparent dental cement (Parkell C&B Metabond) and a glass coverslip (Fig. 1B;Silasi et al, 2013;Vanni and Murphy, 2014). A 4 -40 metal stud was attached to the skull for future head fixation during recordings.…”

“…In addition, Ca 2ϩ imaging is most appropriate for monitoring suprathreshold events (Chen et al, 2013b). Despite the limiting kinetics of calcium modalities, the current generation of intracellular calcium sensors are more than sufficient for mapping neuronal activity with low time resolution on the order of seconds Chen et al, 2013b;Vanni and Murphy, 2014), but may not directly follow higher-frequency circuit dynamics.…”

Transient neuronal coactivations embedded in globally propagating waves underlie resting-state functional connectivity

“…It is commonly assumed that spatial organization of Hemo-FC and its state-dependent changes reflect large-scale spatiotemporal dynamics of spontaneous neuronal activity that is converted into HemoS (13). However, previous studies supporting the link between neuronal activity and Hemo-FC were limited to observations from small numbers of selected brain areas (14)(15)(16)(17)(18)(19)(20). Moreover, in most studies, neuronal activity and HemoS were recorded in separate experimental sessions (14,15,18,19).…”