Tremendous progress has been made since Neuron published our Primer on genetic dissection of neural circuits 10 years ago. Since then, cell-type-specific anatomical, neurophysiological, and ...perturbation studies have been carried out in a multitude of invertebrate and vertebrate organisms, linking neurons and circuits to behavioral functions. New methods allow systematic classification of cell types and provide genetic access to diverse neuronal types for studies of connectivity and neural coding during behavior. Here we evaluate key advances over the past decade and discuss future directions.
In a sequel to their 2008 Primer on genetic dissection of neural circuits, Luo, Callaway, and Svoboda evaluate key advances over the past decade on cell-type-specific anatomical, neurophysiological, and perturbation studies to link neurons and neural circuits to behavior.
The striatum integrates information from multiple brain regions to shape motor learning. The two major projection cell types in striatum target different downstream basal ganglia targets and have ...opposing effects on motivated behavior, yet differential innervation of these neuronal subtypes is not well understood. To examine whether input specificity provides a substrate for information segregation in these circuits, we used a monosynaptic rabies virus system to generate brain-wide maps of neurons that form synapses with direct- or indirect-pathway striatal projection neurons. We discovered that sensory cortical and limbic structures preferentially innervated the direct pathway, whereas motor cortex preferentially targeted the indirect pathway. Thalamostriatal input, dopaminergic input, as well as input from specific cortical layers, was similar onto both pathways. We also confirm synaptic innervation of striatal projection neurons by the raphe and pedunculopontine nuclei. Together, these findings provide a framework for guiding future studies of basal ganglia circuit function.
•Sensory cortical and limbic structures preferentially target direct-pathway neurons•Motor cortex preferentially innervates indirect-pathway neurons•Inputs from thalamus, substantia nigra, and specific cortical layers was similar•Only a small proportion of dopaminergic input was transsynaptically labeled
Striatal direct- and indirect-pathway projection neurons serve distinct motor functions, but the specificity of their synaptic inputs is not known. Wall et al. use genetically targeted rabies virus to generate brain-wide input maps to these pathways.
To establish the mouse as a genetically tractable model for high-order visual processing, we characterized fine-scale retinotopic organization of visual cortex and determined functional ...specialization of layer 2/3 neuronal populations in seven retinotopically identified areas. Each area contains a distinct visuotopic representation and encodes a unique combination of spatiotemporal features. Areas LM, AL, RL, and AM prefer up to three times faster temporal frequencies and significantly lower spatial frequencies than V1, while V1 and PM prefer high spatial and low temporal frequencies. LI prefers both high spatial and temporal frequencies. All extrastriate areas except LI increase orientation selectivity compared to V1, and three areas are significantly more direction selective (AL, RL, and AM). Specific combinations of spatiotemporal representations further distinguish areas. These results reveal that mouse higher visual areas are functionally distinct, and separate groups of areas may be specialized for motion-related versus pattern-related computations, perhaps forming pathways analogous to dorsal and ventral streams in other species.
► Seven mouse visual areas are functionally specialized to encode unique information ► Fine-scale retinotopy reveals at least nine distinct visual field representations ► Subsets of areas encode high temporal versus high spatial frequency information ► Three areas encode high direction selectivity; most are highly orientation selective
Glycoprotein-deleted (ΔG) rabies virus is a powerful tool for studies of neural circuit structure. Here, we describe the development and demonstrate the utility of new resources that allow ...experiments directly investigating relationships between the structure and function of neural circuits. New methods and reagents allowed efficient production of 12 novel ΔG rabies variants from plasmid DNA. These new rabies viruses express useful neuroscience tools, including the Ca2+ indicator GCaMP3 for monitoring activity; Channelrhodopsin-2 for photoactivation; allatostatin receptor for inactivation by ligand application; and rtTA, ERT2CreERT2, or FLPo, for control of gene expression. These new tools allow neurons targeted on the basis of their connectivity to have their function assayed or their activity or gene expression manipulated. Combining these tools with in vivo imaging and optogenetic methods and/or inducible gene expression in transgenic mice will facilitate experiments investigating neural circuit development, plasticity, and function that have not been possible with existing reagents.
► New methods allowed efficient production of ΔG rabies viruses from plasmid DNA ► Twelve new rabies variants express useful genetically-encoded neuroscience tools ► Encoded genes monitor or control activity or gene expression in defined circuits ► These reagents facilitate studies directly linking circuit structure to function
Dopamine neurons encode the difference between actual and predicted reward, or reward prediction error (RPE). Although many models have been proposed to account for this computation, it has ...been difficult to test these models experimentally. Here we established an awake electrophysiological recording system, combined with rabies virus and optogenetic cell-type identification, to characterize the firing patterns of monosynaptic inputs to dopamine neurons while mice performed classical conditioning tasks. We found that each variable required to compute RPE, including actual and predicted reward, was distributed in input neurons in multiple brain areas. Further, many input neurons across brain areas signaled combinations of these variables. These results demonstrate that even simple arithmetic computations such as RPE are not localized in specific brain areas but, rather, distributed across multiple nodes in a brain-wide network. Our systematic method to examine both activity and connectivity revealed unexpected redundancy for a simple computation in the brain.
•Electrophysiological recording from monosynaptic inputs of dopamine neurons was performed•Rabies virus tracing was combined with optogenetic tagging in awake recording•Information required to compute reward prediction errors (RPEs) was distributed•There are mixed representations of variables and partially computed RPEs in input neurons
Tian et al. combined rabies virus and photo-tagging to record from monosynaptic inputs to dopamine neurons. Information for reward prediction error computations is distributed and already mixed in input neurons, suggesting highly redundant and distributed computation for a simple arithmetic.
Rabies viruses, negative-strand RNA viruses, infect neurons through axon terminals and spread trans-synaptically in a retrograde direction between neurons. Rabies viruses whose glycoprotein (G) gene ...is deleted from the genome cannot spread across synapses. Complementation of G in trans, however, enables trans-synaptic spreading of G-deleted rabies viruses to directly connected, presynaptic neurons. Recombinant rabies viruses can encode genes of interest for labeling cells, controlling gene expression and monitoring or manipulating neural activity. Cre-dependent or bridge protein-mediated transduction and single-cell electroporation via the EnvA-TVA or EnvB-TVB (envelope glycoprotein and its specific receptor for avian sarcoma leukosis virus subgroup A or B) system allow cell type-specific or single cell-specific targeting. These rabies virus-based approaches permit the linking of connectivity to cell morphology and circuit function for particular cell types or single cells. Here we describe methods for construction of rabies viral vectors, recovery of G-deleted rabies viruses from cDNA, amplification of the viruses, pseudotyping them with EnvA or EnvB and concentration and titration of the viruses. The entire protocol takes 6-8 weeks.
We show functional-anatomical organization of motion direction in mouse dorsal lateral geniculate nucleus (dLGN) using two-photon calcium imaging of dense populations in thalamus. Surprisingly, ...the superficial 75 μm region contains anterior and posterior direction-selective neurons (DSLGNs) intermingled with nondirection-selective neurons, while upward- and downward-selective neurons are nearly absent. Unexpectedly, the remaining neurons encode both anterior and posterior directions, forming horizontal motion-axis selectivity. A model of random wiring consistent with these results makes quantitative predictions about the connectivity of direction-selective retinal ganglion cell (DSRGC) inputs to the superficial dLGN. DSLGNs are more sharply tuned than DSRGCs. These results suggest that dLGN maintains and sharpens retinal direction selectivity and integrates opposing DSRGC subtypes in a functional-anatomical region, perhaps forming a feature representation for horizontal-axis motion, contrary to dLGN being a simple relay. Furthermore, they support recent conjecture that cortical direction and orientation selectivity emerge in part from a previously undescribed motion-selective retinogeniculate pathway.
► Mouse LGN encodes direction and axis motion predicting and matching retinal inputs ► Horizontal motion directions are encoded and integrated in the superficial layer ► Random wiring can account for functional segregation and integration in LGN ► Two-photon population calcium imaging is demonstrated in the thalamus in vivo
Marshel et al. use two-photon calcium imaging in mouse dorsal lateral geniculate nucleus (dLGN) to find that motion-selective neurons in superficial dLGN are specialized for anterior and posterior directions, including horizontal axis-selective neurons, which may arise from integration via random wiring.
Spanning about 9 mm2 of the posterior cortex surface, the mouse’s small but organized visual cortex has recently gained attention for its surprising sophistication and experimental tractability 1–3. ...Though it lacks the highly ordered orientation columns of primates 4, mouse visual cortex is organized retinotopically 5 and contains at least ten extrastriate areas that likely integrate more complex visual features via dorsal and ventral streams of processing 6–14. Extending our understanding of visual perception to the mouse model is justified by the evolving ability to interrogate specific neural circuits using genetic and molecular techniques 15, 16. In order to probe the functional properties of the putative mouse dorsal stream, we used moving plaids, which demonstrate differences between cells that identify local motion (component cells) and those that integrate global motion of the plaid (pattern cells; Figure 1A; 17). In primates, there are sparse pattern cell responses in primate V1 18, 19, but many more in higher-order regions; 25%–30% of cells in MT 17 and 40%–60% in MST 20 are pattern direction selective. We present evidence that mice have small numbers of pattern cells in areas LM and RL, while V1, AL, and AM are largely component-like. Although the proportion of pattern cells is smaller in mouse visual cortex than in primate MT, this study provides evidence that the organization of the mouse visual system shares important similarities to that of primates and opens the possibility of using mice to probe motion computation mechanisms.
•Five different visual areas in the mouse were investigated for responses to plaids•Mouse visual cortex cells respond in diverse ways to gratings and plaids•Areas LM and RL contain small proportions of pattern cells; V1, AL, and AM do not•LM and RL may constitute a dorsal, motion-sensitive stream in the mouse
Juavinett et al. expand on the growing interest of the mouse as a model for visual neuroscience, demonstrating that cells in two areas of mouse visual cortex can compute the global motion of a plaid. The report of these pattern direction cells in areas LM and RL, but not V1, AL, or AM, further delineates dorsal and ventral streams in the mouse.
Incoming sensory information is sent to the brain along modality-specific channels corresponding to the five senses. Each of these channels further parses the incoming signals into parallel streams ...to provide a compact, efficient input to the brain. Ultimately, these parallel input signals must be elaborated on and integrated in the cortex to provide a unified and coherent percept. Recent studies in the primate visual cortex have greatly contributed to our understanding of how this goal is accomplished. Multiple strategies including retinal tiling, hierarchical and parallel processing and modularity, defined spatially and by cell type-specific connectivity, are used by the visual system to recover the intricate detail of our visual surroundings.
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