Viral Transneuronal Tracing Technology: Defining the Synaptic Organisation of Neural Circuits

Abstract Viral transneuronal tracing is a powerful approach that has been widely used to define the cellular organisation of functionally defined systems that underlie brain function. The method exploits the ability of neurotropic viruses to infect neurons and produce infectious progeny that pass through the intimate synaptic connections among neurons to infect polysynaptic pathways. A number of important technological advances in recent years have dramatically improved the ability of this experimental approach to define the synaptology of complex neural circuits. In this chapter we provide an overview of the method as well as mechanisms that contribute to its effectiveness for circuit analysis. We focus on alpha herpesviruses and rabies virus as these pathogens have been most widely applied in transneuronal tracing studies. Key Concepts: Viral transneuronal tracing exploits the invasive properties of neurotropic viruses. Knowledge of the functional organisation of the viral genome is fundamental to an informed application of the viral transneuronal tracing method. Circuit architecture is influential in determining the progression of viral infection through a neural circuit. Isogenic recombinant viruses expressing unique reporters permit analysis of collateralisation of projections within complex neural circuits. Colocalisation of reporters of infection with neurotransmitters and peptides permits neurochemical phenotyping of neurons within labelled circuits.