Neuroanatomical tract-tracing techniques that did go viral

Abstract

Neuroanatomical tracing methods remain fundamental for elucidating the complexity of brain circuits. During the past decades, the technical arsenal at our disposal has been greatly enriched, with a steady supply of fresh arrivals. This paper provides a landscape view of classical and modern tools for tract-tracing purposes. Focus is placed on methods that have gone viral, i.e., became most widespread used and fully reliable. To keep an historical perspective, we start by reviewing one-dimensional, standalone transport-tracing tools; these including today’s two most favorite anterograde neuroanatomical tracers such as Phaseolus vulgaris-leucoagglutinin and biotinylated dextran amine. Next, emphasis is placed on several classical tools widely used for retrograde neuroanatomical tracing purposes, where Fluoro-Gold in our opinion represents the best example. Furthermore, it is worth noting that multi-dimensional paradigms can be designed by combining different tracers or by applying a given tracer together with detecting one or more neurochemical substances, as illustrated here with several examples. Finally, it is without any doubt that we are currently witnessing the unstoppable and spectacular rise of modern molecular-genetic techniques based on the use of modified viruses as delivery vehicles for genetic material, therefore, pushing the tract-tracing field forward into a new era. In summary, here, we aim to provide neuroscientists with the advice and background required when facing a choice on which neuroanatomical tracer—or combination thereof—might be best suited for addressing a given experimental design.

Anatomy of nerve fiber bundles at micrometer-resolution in the vervet monkey visual system

“…Over the last century, a number of invasive methods has been developed for analyzing cortico-cortical connections through white matter, from Klingler’s dissection, strychnine neuronography, Nauta method, and modern tract tracing methods ( Lanciego and Wouterlood, 2011 ; Lanciego and Wouterlood, 2020 ; Takemura et al, 2019b ). Among these methods, axonal tract tracing has been the most widely used to study the visual system of non-human primates because of the high specificity and greater confidence in identifying cell bodies of neurons which are origins or terminations of axonal connections ( Kennedy et al, 2013 ; Rockland, 2020 ), and the course of fiber tracts through the white matter was analyzed by means of radiolabeled isotopes ( Schmahmann and Pandya, 2006 ) or MR-visible tracers ( Saleem et al, 2002 ).…”

“…Although the trajectories of early visual pathways (optic nerve, optic tract and optic radiation) have been established, the existence and organization of association fibers in the occipital lobe have been controversial among classical neuroanatomists ( Yeatman et al, 2014 ). Later, axonal tract tracing has been widely used to investigate anatomical connections in the visual system of non-human primates ( Lanciego and Wouterlood, 2011 ; Lanciego and Wouterlood, 2020 ; Kennedy et al, 2013 ; Rockland, 2020 ). A comprehensive overview of axonal tracing on major white matter tracts in macaque monkey was provided by Schmahmann and Pandya, 2006 .…”

Dorsal and ventral mossy cells differ in their long-range axonal projections throughout the dentate gyrus of the mouse hippocampus

“…However, these mouse lines are also useful to address the MC axon projections in the adult mouse. This type of investigation is valuable because past studies mainly used rats, and in addition there is excellent identification of membrane processes using viral labeling of proteins that insert into plasma membranes (Lanciego and Wouterlood, 2020).…”

Retrograde Labeling of Different Distribution Features of DRG P2X2 and P2X3 Receptors in a Neuropathic Pain Rat Model

“…Therefore, FG can be considered today's flagship of the retrograde fluorescence tracer repertoire [32]. Among several classic tools that are widely used for retrograde neurodissection tracing purposes, Fluoro-Gold appears to be the best example [33].…”