Brain responsiveness and its activation complexity are linked to the level of consciousness (Tononi et al., 2004). However, how these features change across brain states is still not clear. The combination of Transcranial Magnetic Stimulation and hd-EEG recordings represents the standard method to address this issue in humans. A preclinical analogous in lab animals would provide novel mechanistic insights on the brain-state-dependent complexity of the brain.
A powerful technique to study mesoscale cortical connectivity in mice exploits wide-field fluorescence microscopy. This approach provides simultaneous information of neuronal ensemble activity from distributed cortical areas, while optogenetic has been demonstrated to be a powerful tool to activate cortical neuronal clusters. However, all-optical systems that combine these techniques critically suffer from crosstalk between imaging and photostimulation (Emiliani et al. 2015).
Brain responsiveness and its activation complexity are linked to the level of consciousness (Tononi et al., 2004). However, how these features change across brain states is still not clear. The combination of Transcranial Magnetic Stimulation and hd-EEG recordings represents the standard method to address this issue in humans. A preclinical analogous in lab animals would provide novel mechanistic insights on the brain-state-dependent complexity of the brain.
A powerful technique to study mesoscale cortical connectivity in mice exploits wide-field fluorescence microscopy. This approach provides simultaneous information of neuronal ensemble activity from distributed cortical areas, while optogenetic has been demonstrated to be a powerful tool to activate cortical neuronal clusters. However, all-optical systems that combine these techniques critically suffer for crosstalk between imaging and photostimulation (Emiliani et al. 2015).
Here we established an all-optical method combining wide-field fluorescence imaging of the red-shifted calcium indicator jRCaMP1b and transcranial optogenetic stimulation of Channelrhodopsin-2 (ChR2). To achieve a cortex-wide expression of the calcium indicator, an adeno-associated virus (AAV.PHP.eb) carrying jRCaMP1b under the control of the synapsin promoter was injected in the retro-orbital sinus of anesthetized mice. This led to a uniform expression of the functional indicator in the whole cortex, giving the possibility to visualize the neuronal activity propagation in all the cortical areas. Due to the high opsins expression required for effective optogenetic stimulation, AAV9-ChR2 was locally injected in the somatosensory cortex (S1). Results show that in awake mice, optogenetic stimulations at increasing laser power evoke a distributed cortical response in several areas in the two cortical hemispheres, whereas, during anesthesia, stimulation led to a localized reponse limited in space and time. These results suggest that response complexity decrease with the levels of consciousness, as observed in pathological patients affected by disorders of consciousness (Massimini et al., 2009).
To understand the brain computation paradigms and the causal interactions in complex neuronal networks, we need methods and technologies to record and perturb neuronal distributions over large fields of view. In this application, two-photon (2P) imaging has become a cornerstone microscopy technique, widely used for deep optical access in biological samples and selective light targeting with submicrometric resolution. In parallel to structural and functional imaging, 2P optogenetics has represented a game-changer, allowing targeted stimulation of specific neural circuits. However, the long commutation times and refresh rates of traditional scanning methods substantially hinder near-simultaneous multi-site 3D stimulation. Acousto-optic deflectors (AODs), owing to their fastest scanning and refresh rates, can fulfil the temporal requirements for concurrent activation of sparsely distributed neurons. Nevertheless, their applicability to 2P optogenetics in large volumes has been limited so far by the massive efficiency drop along the optical axis during their use in axial scanning. To counteract this drawback, a compensation software module is frequently employed to flatten the power distribution throughout the volume. However, the power threshold is reduced to the minimum intensity value addressable, lowering the peak intensity released in the centre of the axial scan.
Here, we propose a unique approach for overcoming this drawback which provided lifted axial power distribution while maintaining a uniform lateral illumination range. We tested this method by the 2P photoactivation of optogenetic actuators in 3D in zebrafish larvae, showing how the probability of evoking an electrophysiological response and the relative neuronal activity amplitude improved by carefully optimizing the light targeting time on different axial planes.
In conclusion, fast and uniform axial light addressing with AODs enables unprecedented 3D 2P optostimulation, formerly not feasible. Furthermore, this approach can be adopted as an upgrade for existing microscopes designed for volumetric imaging, providing 3D multi-site imaging and random-access illumination.
KEYWORDS: In vivo imaging, Optogenetics, Cortical activation, Calcium, Brain mapping, Organisms, Photostimulation, Radiofrequency ablation, Actuators, Control systems
Neuronal networks in living organisms are highly interconnected. Usually, to study their functional roles in healthy conditions, task-evoked neuronal responses are correlated with the behavioral readout in freely moving or head-fixed animals. Recently, optogenetics proved to be a useful tool to manipulate targeted neuronal circuits using light. Optogenetic photostimulation of different cortical motor areas revealed distinct and reproducible motor movements: Rostral Forelimb Area (RFA) is critically involved in controlling grasping-like movements, while Caudal Forelimb Area (CFA) has a role in tap- or locomotion-like movements. In parallel, the development of red-shifted genetically encoded calcium indicators (red-GECIs) like jRCaMP1a allowed to reduce the spectral overlap with the most common optogenetic actuator, channelrhodopsin-2 (ChR2). Therefore, by combining these optical tools it is possible to develop all-optical systems, which are smart approaches for long-term low-invasive studies of neuronal patterns.
Here, in order to understand the functional role that cortical ensembles play in motor generation and control, we developed a cross-talk free large-scale all-optical system for unraveling cortical neuronal patterns associated with optogenetically-evoked movements. We demonstrated that the motor cortex exhibits precise inter-regional patterns during movement initiation of grasp- or locomotion-evoked movements. Moreover, the cortical activation covers most of the related light-based optogenetic maps, revealing that a strong local neuronal connectivity is associated with optogenetically-evoked complex movements. To confirm the relevance of local connectivity for the generation of complex movements we used both optogenetic interference and pharmacological inhibition, showing that movement disruption is linked to reduced cortico-cortical coactivation.
We developed an all-optical system that couples large-scale cortical imaging with chronic light-based motor mapping in awake mice. By AAV-mediated cortical transfection, we induced the co-expression of the red-shifted genetically encoded calcium indicator and a light-sensitive optogenetic actuator ChR2 over both the rostral and caudal forelimb areas, which was stable over several months. No evidence of cross-talk was detected during illumination of ChR2+ neurons with the light source used for RCaMP1a excitation. Light-based motor mapping coupled with wide-field imaging of neuronal activation in awake mice revealed spatiotemporal patterns of cortical activation specific for movement category.
In the last few years, optogenetic tools and optical functional indicators are increasingly used together to perform simultaneous manipulation and recording of neuronal activity. Nevertheless, this method has still some limitations mainly due to the spectral cross-talk between optogenetic actuators and functional sensors [1;2]. To address this issue, red variants of genetically encoded calcium indicators (red-GECIs) have been recently developed [3;4]. The main goal of this project is to develop a full-optical system that allows effective interrogation of brain circuits. To this aim, we combined a red-shifted calcium indicator (jRCaMP1a), with the most common blue-light activated opsin, Channelrhodopsin II (ChR2). The results presented here show: (I) extended expression of the full-optical system that covers all the motor areas, (II) functional correlation between the laser power and the evoked neuronal activity, (III) segregation of the cortical functional areas of two different forelimb evoked movements. The future perspective of this project concerns the study of the functional areas correlation during optogenetically-evoked forelimb complex movements.
In this study, we developed a wide-field all-optical system based on a red-shifted GECI (RCaMP1a) combined with channelrhodopsin II (ChR2) for simultaneous stimulation and readout of neuronal activity. Our results show that RCaMP1a transfection in primary motor cortex extends all over the cortical motor areas. The RCaMP1a and ChR2 reporter expression largely overlap, thus allowing the stimulation and readout from the same functional areas. Furthermore, we characterized the functional response by performing single pulse optogenetic stimulation and we observed that evoked calcium signals increase at increasing laser power. In order to study the cortical activation underlying a specific motor behavior, we performed optogenetic-stimulation of the Rostral Forelimb Area (RFA) with a train of lasers pulses. We observed that during 1s of 16 Hz train stimulus the animals suddenly start grasping with the contralateral forelimb. Cortical dynamics recorded during the optogenetically-triggered motor task show correlated activity between the RFA and the nearby motor areas. The all-optical system optimization and the possibility to link the neuronal population activity with the animal behavior would be a key point in understanding the network activity underlying a specific behavior.
Neuro-rehabilitative research is developing novel strategies to enhance the effectiveness of therapies after stroke by using a combination of physical and plasticizing treatments 1-3. Previous studies have shown that repeated optogenetics stimulation of neurons in the peri-lesioned area induces a significant improvement in cerebral blood flow and neurovascular coupling response 4-6. Up to now the mechanisms underneath the reshaping of brain circuitry induced by rehabilitation after stroke are widely unknown. To investigate how rehabilitative therapies shape new cortical maps in the peri-infarct region, we induce a photothrombotic stroke in the primary motor cortex and the expression of Channelrhodopsine 2 (ChR2) in the peri-infarct area on Thy1-GCaMP6f mice. To promote functional recovery after stroke we use both an optogenetic strategy to stimulate targeted excitatory neurons in the peri-lesional region and motor training on a robotic platform (M-Platform) 7. A 473 nm laser repeatedly stimulates ChR2-transfected neurons; the optostimulation is performed five days a week. The motor rehabilitation consists in a pulling task: after the forelimb is passively extended by the linear actuator of the M-platform, the animal has to pull back up to the resting position. By analysing the spatio-temporal calcium dynamic and the reshaping of cortical activation area during the movement throughout the treatment period, we found that the combined treatment restores cortical activation profiles during the forelimb movement. Through behavioural experiments, using Schallert test, we also evaluate changes of forelimb functionality during rehabilitation. Our combination of techniques allows obtaining unprecedented views on cortical plasticity induced by rehabilitative therapies.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.