Monosynaptic Circuit Tracing with Glycoprotein-Deleted Rabies Viruses.

Introduction Since the introduction of methods using glycoprotein (G)-deleted rabies virus (RVdG) to identify the direct monosynaptic inputs to genetically targeted neurons eight years ago (Wickersham et al., 2007b), this approach has been widely used for sophisticated circuit-tracing studies throughout the central and peripheral nervous systems. Published studies using monosynaptic RV tracing have described the connections to single neurons (Marshel et al., 2010; Rancz et al., 2011; Vélez-Fort et al., 2014), to cell types defined by their projection to particular locations (Stepien et al., 2010; Yonehara et al., 2011; Takatoh et al., 2013; Cruz-Martín et al., 2014; Levine et al., 2014; Ni et al., 2014; Sreenivasan et al., 2015), to cells targeted by their birth date (Arenkiel et al., 2011; Nakashiba et al., 2012; Vivar et al., 2012; Deshpande et al., 2013; Li et al., 2013; Garcia et al., 2014), to neurons with particular cell-surface receptors (Choi and Callaway, 2011), and to genetically identified cell types in Creor tTA-expressing mouse lines (Haubensak et al., 2010; Wall et al., 2010; Weible et al., 2010; Miyamichi et al., 2011; Watabe-Uchida et al., 2012; Wall et al., 2013; Fu et al., 2014; Kohara et al., 2014; Ogawa et al., 2014; Pollak Dorocic et al., 2014; Sun et al., 2014; Weissbourd et al., 2014). The results have led to novel insights into the relationships between the organization and function of neural circuits throughout the nervous system; from muscles or peripheral sensory receptors to the brain; and in nearly every neural system, from vision to motor function and neuromodulatory systems. The fast and extensive adoption of the approach can be attributed to its power and versatility even when needing to generate and use viral tools that have not traditionally been easily accessible to systems neuroscientists. This powerful reagent overcomes the most difficult challenges of precisely identifying presynaptic partners separated by distances ranging from micrometers to centimeters; it directly relates presynaptic neurons to the starter cell(s)—the initial rabies-infected cell(s)—via synaptic connections. This is possible because RV spreads selectively between synaptically connected neurons and then travels exclusively and efficiently in the retrograde direction (except for some sensory neurons, see below) to label synaptic inputs, regardless of distance (Ugolini, 1995, 2008; Kelly and Strick, 2000; Wickersham et al., 2007b; Callaway, 2008). EnvA-pseudotyped RVdG (EnvA RVdG, see below) has the additional property of selectively infecting genetically defined cell types. Finally, because RV is a means of delivering genetic material and any gene of interest can be inserted into the viral genome, these approaches have access to the full range of genetic tools for monitoring or manipulating neuronal activity or gene expression. These systems, therefore, have the versatility to tap into existing mouse lines and other genetic approaches to label the inputs to specific cell types, and to incorporate not only existing technologies but also any other genetic tools that are developed in the future.