Viral-genetic tracing of the input–output organization of a central noradrenaline circuit

Abstract

Deciphering how neural circuits are anatomically organized with regard to input and output is instrumental in understanding how the brain processes information. For example, locus coeruleus norepinephrine (LC-NE) neurons receive input from and send output to broad regions of the brain and spinal cord, and regulate diverse functions including arousal, attention, mood, and sensory gating1–8. However, it is unclear how LC-NE neurons divide up their brain-wide projection patterns and whether different LC-NE neurons receive differential input. Here, we developed a set of viral-genetic tools to quantitatively analyze the input–output relationship of neural circuits, and applied these tools to dissect the LC-NE circuit in mice. Rabies virus-based input mapping indicated that LC-NE neurons receive convergent synaptic input from many regions previously identified as sending axons to the LC and suggested novel presynaptic partners, including cerebellar Purkinje cells. The TRIO (tracing the relationship between input and output) method enables trans-synaptic input tracing from specific subsets of neurons based on their projection and cell type. We found that LC-NE neurons projecting to diverse output regions receive mostly similar input. Projection-based viral labeling revealed extensive output divergence: LC-NE neurons projecting to one output region also project to all brain regions we examined. Thus, the LC-NE circuit overall integrates information from, and broadcasts to, many brain regions, consistent with its primary role in regulating brain states. At the same time, we uncovered several levels of specificity in certain LC-NE sub-circuits. These viral-genetic tools for mapping output architecture and input–output relationship are applicable to other neuronal circuits and organisms. More broadly, our viral-genetic approaches provide an efficient intersectional means to target neuronal populations based on cell type and projection pattern.

Expanding the power of recombinase-based labeling to uncover cellular diversity

“…Our proof-of-principle application of RC::FLTG and RC::RFLTG to analysis of the central noradrenergic system confirms the value of key features of these alleles. There are no molecular markers known to uniquely label subpopulations of noradrenergic neurons, so investigation of noradrenergic neuron heterogeneity, particularly axon projection patterns, depends either on genetic intersectional labeling (Robertson et al, 2013) or strategies for retrograde labeling by injection of dyes or viruses at target sites (Chandler and Waterhouse, 2012;Chandler et al, 2013Chandler et al, , 2014Schwarz et al, 2015). The ability of RC::FLTG to label axons of the entire noradrenergic system, subdivided between an intersectional and a subtractive population, reveals diversity of noradrenergic projections at target sites.…”

What is CAR doing in the middle of the adult neurogenic road?

“…canine adenovirus type 2 or "CAV-2") that engage CAR as an attachment molecule, circumstantial evidence suggests that CAR is not expressed by all neuron subtypes. We have previously shown that CAV-2 vectors infect motor, cholinergic, dopamine, melanin-concentrating hormone, noradrenergic neurons, 8,9 and all available evidence suggest that CAV-2 vector infection depends on CAR expression. [10][11][12] In contrast to the transcellular interaction in epithelial cells, we showed that when CAR is at the synapse of mature neurons, it is exclusively pre-but not post-synaptic.…”

Projection specificity in heterogeneous locus coeruleus cell populations: implications for learning and memory

“…Although the anatomical studies reveal that distinct LC neurons project to specific brain regions, other studies using a variety of anatomical approaches have found that LC neurons are homogenous and exhibit more collateralization in their efferent connectivity (Nakamura and Iwama 1975;Nagai et al 1981;Room et al 1981;Schwarz et al 2015). It will be important in future work to determine the degree of collateralization and specificity individual populations of LC neurons exhibit in their efferent connectivity.…”

“…For example, it hinges on the idea that different subpopulations of LC neurons project to amygdala and mPFC and that they serve distinct functions during different types of learning. However, there are many studies which report highly collateralized projection patterns of LC neurons (Nakamura and Iwama 1975;Nagai et al 1981;Room et al 1981;Schwarz et al 2015) and it is not clear whether specific subpopulations of cells project to these regions or even, more generally, whether distinct behaviorally functional subclasses of LC neurons exist. Furthermore, this model may be too general.…”