In our brain networks, diverse types of neurons assemble into circuits and participate in computations that support the processing and storage of information. While headway is being made in understanding the scale and identity of these cellular elements and their interconnections, grasping their functional implications for behavior requires the precise identification of these cell types in vivo and the monitoring of their activity profiles in the context of network and behavioral states. In a study published in the journal Cell Reports, Muñoz and colleagues devised a new method facilitating this avenue of questions.
The developed method, channelrhodopsin-assisted patching, combines in vivo optogenetics with juxtacellular recording and labeling technologies. The strategy utilizes patch pipettes containing an optic fiber to record from mice conditionally expressing channelrhodopsin-2 (ChR2) in various genetically defined cell populations. The paper shows that this strategy has numerous advantages.
First, the authors demonstrate that their approach enabled the reliable and efficient targeting and juxtacellular recordings of light-responsive neurons, dramatically improving the yield of targeting specific cell types as compared to traditional blind recording methods. Second, since the method does not depend on visual guidance, the authors were able to target recordings at any depth in the brain. Third, the approach enabled the routine labeling of targeted neurons and thus the determination of their morphological features. Fourth, the authors developed a protocol to target genetically defined neurons for intracellular recordings in whole-cell configuration. In this way, this innovative method allows the efficient in vivo targeted recording of neurons that are labeled and identified by their molecular, electrophysiological, and morphological features, which are fundamental determinants of a neuron’s identity and function. Importantly, such recordings can be carried out in anesthetized and awake preparations.
This approach overcomes critical limitations of present technologies. For example, the need for optical access of imaging-based technologies, such as two-photon targeted patching and calcium imaging, has largely restricted their application to superficial cortical layers. On the other hand, while the combination of conditional expression of channelrhodopsin in genetic groups and extracellular recordings has allowed the “optical tagging” of light-responsive units at any recording depth, this strategy can only provide coarse anatomical position of recorded neurons, does not allow morphological recovery, and cannot grant access to intracellular recording. Channelrhodopsin-assisted patching is not subject to these limitations, attaining higher efficiency and precision in the recording of the activity of individual cellular elements in vivo.
The article presents proof of principle supporting the utility of the method. The authors were able to target recording and labeling of GABAergic interneurons in deep neocortical layers with high efficiency, despite the fact that they represent a minority of cortical neurons. Among the results of the study, the first recordings of morphologically identified somatostatin-expressing interneurons in anesthetized and awake preparations and the first targeted whole-cell recordings of parvalbumin-expressing interneurons in neocortical layer 5 were obtained. The results also provide strong evidence for the need of morphological information to disentangle the heterogeneity present within these genetic groups of neocortical networks. Finally, the authors adapted the strategy for the recording of genetically defined neurons at subcortical depths, including morphologically identified cholinergic neurons of the basal forebrain.
This efficient and robust technique is an exciting and important complement to existing tools. By allowing the dissection of the activity of individual cellular elements in vivo with higher efficiency and precision, it will substantially accelerate our understanding of the computational significance of the cell types composing brain networks and bring about new conceptual advances.
—William Muñoz, PhD
Read the paper “Channelrhodopsin-assisted patching: In vivo recording of genetically and morphologically identified neurons throughout the brain” in Cell Reports, published December 24, 2014.