Rapidly inducible, genetically targeted inactivation of neural and synaptic activity in vivo

Individual variability of neural computations underlying flexible decisions

The ability to flexibly switch our responses to external stimuli according to contextual information is critical for successful interactions with a complex world. Context-dependent computations are necessary across many domains1-3, yet their neural implementations remain poorly understood. Here we developed a novel behavioural task in rats to study context-dependent selection and accumulation of evidence for decision-making4-6. Under assumptions supported by both monkey and rat data, we first show mathematically that this computation can be supported by three dynamical solutions and that all networks performing the task implement a combination of these solutions. These solutions can be identified and tested directly with experimental data. We further show that existing electrophysiological and modelling data are compatible with the full variety of possible combinations of these solutions, suggesting that different individuals could use different combinations. To study variability across individual subjects, we developed automated, high-throughput methods to train rats on our task and trained many subjects using these methods. Consistent with theoretical predictions, neural and behavioural analyses revealed substantial heterogeneity across rats, despite uniformly good task performance. Our theory further predicts a specific link between behavioural and neural signatures, which was robustly supported in the data. In summary, our results provide an experimentally supported theoretical framework to analyse individual variability in biological and artificial systems that perform flexible decision-making tasks, open the door to cellular-resolution studies of individual variability in higher cognition, and provide insights into neural mechanisms of context-dependent computation more generally.

Beyond the bolus: transgenic tools for investigating the neurophysiology of learning and memory

Understanding the neural mechanisms underlying learning and memory in the entorhinal-hippocampal circuit is a central challenge of systems neuroscience. For more than 40 years, electrophysiological recordings in awake, behaving animals have been used to relate the receptive fields of neurons in this circuit to learning and memory. However, the vast majority of such studies are purely observational, as electrical, surgical, and pharmacological circuit manipulations are both challenging and relatively coarse, being unable to distinguish between specific classes of neurons. Recent advances in molecular genetic tools can overcome many of these limitations, enabling unprecedented control over neural activity in behaving animals. Expression of pharmaco- or optogenetic transgenes in cell-type-specific "driver" lines provides unparalleled anatomical and cell-type specificity, especially when delivered by viral complementation. Pharmacogenetic transgenes are specially designed neurotransmitter receptors exclusively activated by otherwise inactive synthetic ligands and have kinetics similar to traditional pharmacology. Optogenetic transgenes use light to control the membrane potential, and thereby operate at the millisecond timescale. Thus, activation of pharmacogenetic transgenes in specific neuronal cell types while recording from other parts of the circuit allows investigation of the role of those neurons in the steady state, whereas optogenetic transgenes allow one to determine the immediate network response.

A causal perspective on the analysis of signal and noise correlations and their role in population coding

The role of correlations between neuronal responses is crucial to understanding the neural code. A framework used to study this role comprises a breakdown of the mutual information between stimuli and responses into terms that aim to account for different coding modalities and the distinction between different notions of independence. Here we complete the list of types of independence and distinguish activity independence (related to total correlations), conditional independence (related to noise correlations), signal independence (related to signal correlations), coding independence (related to information transmission), and information independence (related to redundancy). For each type, we identify the probabilistic criterion that defines it, indicate the information-theoretic measure used as statistic to test for it, and provide a graphical criterion to recognize the causal configurations of stimuli and responses that lead to its existence. Using this causal analysis, we first provide sufficiency conditions relating these types. Second, we differentiate the use of the measures as statistics to test for the existence of independence from their use for quantification. We indicate that signal and noise correlation cannot be quantified separately. Third, we explicitly define alternative system configurations used to construct the measures, in which noise correlations or noise and signal correlations are eliminated. Accordingly, we examine which measures are meaningful only as a comparison across configurations and which ones provide a characterization of the actually observed responses without resorting to other configurations. Fourth, we compare the commonly used nonparametric approach to eliminate noise correlations with a functional (model-based) approach, showing that the former approach does not remove those effects of noise correlations captured by the tuning properties of the individual neurons, and implies nonlocal causal structure manipulations. These results improve the interpretation of the measures on the framework and help in understanding how to apply it to analyze the role of correlations.

Cell types, circuits, computation

How does the connectivity of a neuronal circuit, together with the individual properties of the cell types that take part in it, result in a given computation? We examine this question in the context of retinal circuits. We suggest that the retina can be viewed as a parallel assemblage of many small computational devices, highly stereotypical and task-specific circuits afferent to a given ganglion cell type, and we discuss some rules that govern computation in these devices. Multi-device processing in retina poses conceptual problems when it is contrasted with cortical processing. We lay out open questions both on processing in retinal circuits and on implications for cortical processing of retinal inputs.

Bridging the gaps between synapses, circuits, and behavior

The decade of the brain may have come and gone, but the final frontier, cracking the neuronal code, still lies ahead. Today, new technologies that allow precise spatiotemporal remote control over the activity of genetically defined populations of neurons within intact neural circuits are providing a means of obtaining a functional wiring diagram of the mammalian brain, bringing us one step closer to understanding precisely how neuronal activity codes for perception, thought, emotion, and action. These technologies and the design principles underlying them are reviewed herein.

Studying Cerebellar Circuits by Remote Control of Selected Neuronal Types with GABA(A) Receptors

Although GABA_A receptor-mediated inhibition of cerebellar Purkinje cells by molecular layer interneurons (MLIs) has been studied intensely at the cellular level, it has remained unclear how this inhibition regulates cerebellum-dependent behaviour. We have implemented two complementary approaches to investigate the function of the MLI-Purkinje cell synapse on the behavioural level. In the first approach we permanently disrupted inhibitory fast synaptic transmission at the synapse by genetically removing the postsynaptic GABA_A receptors from Purkinje cells (PC-Δγ2 mice). We found that chronic disruption of the MLI-Purkinje cell synapse strongly impaired cerebellar learning of the vestibular occular reflex (VOR), presumably by disrupting the temporal patterns of Purkinje cell activity. However, in PC-Δγ2 mice the baseline VOR reflex was only mildly affected; indeed PC-Δγ2 mice show no ataxia or gait abnormalities, suggesting that MLI control of Purkinje cell activity is either not involved in ongoing motor tasks or that the system compensates for its loss. To investigate the latter possibility we developed an alternative genetic technique; we made the MLI-Purkinje cell synapse selectively sensitive to rapid manipulation with the GABA_A receptor modulator zolpidem (PC-γ2-swap mice). Minutes after intraperitoneal zolpidem injection, these PC-γ2-swap mice developed severe motor abnormalities, revealing a substantial contribution of the MLI-Purkinje cell synapses to real time motor control. The cell-type selective permanent knockout of synaptic GABAergic input and the fast reversible modulation of GABAergic input at the same synapse illustrate how pursuing both strategies gives a fuller view.

Shifting the paradigm: new approaches for characterizing and classifying neurons

Efforts to characterize and classify the cellular components of the nervous system have a rich history in modern neuroscience, and closely mirror the development of new techniques to assay cellular properties. Recent advances in high-throughput histology, genetics and neuroinformatics hold great promise for systematic and reproducible measurement and community databasing of cellular properties. In particular, transgenic approaches to reproducibly target and manipulate specific cell types in mice are rapidly advancing. Cre recombinase-based approaches in particular allow the coupling of cell type specificity with a wide variety of genetic tools for visualization, molecular profiling, tract tracing and functional manipulation. The reproducible multimodal characterization allowed by these transgenics provides a means to classify, building cellular taxonomies based on measurement of many phenotypic properties, as well as manipulate, moving beyond classification to understand the functional role of specific cell types and circuits in complex behaviors.

Transgenic silencing of neurons in the mammalian brain by expression of the allatostatin receptor (AlstR)

The mammalian brain is an enormously complex set of circuits composed of interconnected neuronal cell types. The analysis of central neural circuits will be greatly served by the ability to turn off specific neuronal cell types while recording from others in intact brains. Because drug delivery cannot be restricted to specific cell types, this can only be achieved by putting "silencer" transgenes under the control of neuron-specific promoters. Towards this end we have created a line of transgenic mice putting the Drosophila allatostatin (AL) neuropeptide receptor (AlstR) under the control of the tetO element, thus enabling its inducible expression when crossed to tet-transactivator lines. Mammals have no endogenous AL or AlstR, but activation of exogenously expressed AlstR in mammalian neurons leads to membrane hyperpolarization via endogenous G-protein-coupled inward rectifier K(+) channels, making the neurons much less likely to fire action potentials. Here we show that this tetO/AlstR line is capable of broadly expressing AlstR mRNA in principal neurons throughout the forebrain when crossed to a commercially-available transactivator line. We electrophysiologically characterize this cross in hippocampal slices, demonstrating that bath application of AL leads to hyperpolarization of CA1 pyramidal neurons, making them refractory to the induction of action potentials by injected current. Finally, we demonstrate the ability of AL application to silence the sound-evoked spiking responses of auditory cortical neurons in intact brains of AlstR/tetO transgenic mice. When crossed to other transactivator lines expressing in defined neuronal cell types, this AlstR/tetO line should prove a very useful tool for the analysis of intact central neural circuits.

Plasma-mediated ablation: an optical tool for submicrometer surgery on neuronal and vascular systems

Plasma-mediated ablation makes use of high energy laser pulses to ionize molecules within the first few femtoseconds of the pulse. This process leads to a submicrometer-sized bubble of plasma that can ablate tissue with negligible heat transfer and collateral damage to neighboring tissue. We review the physics of plasma-mediated ablation and its use as a tool to generate targeted insults at the subcellular level to neurons and blood vessels deep within nervous tissue. Illustrative examples from axon regeneration and microvascular research highlight the utility of this tool. We further discuss the use of ablation as an integral part of automated histology.

Place cells, grid cells, and the brain's spatial representation system

More than three decades of research have demonstrated a role for hippocampal place cells in representation of the spatial environment in the brain. New studies have shown that place cells are part of a broader circuit for dynamic representation of self-location. A key component of this network is the entorhinal grid cells, which, by virtue of their tessellating firing fields, may provide the elements of a path integration-based neural map. Here we review how place cells and grid cells may form the basis for quantitative spatiotemporal representation of places, routes, and associated experiences during behavior and in memory. Because these cell types have some of the most conspicuous behavioral correlates among neurons in nonsensory cortical systems, and because their spatial firing structure reflects computations internally in the system, studies of entorhinal-hippocampal representations may offer considerable insight into general principles of cortical network dynamics.

Improved expression of halorhodopsin for light-induced silencing of neuronal activity

The ability to control and manipulate neuronal activity within an intact mammalian brain is of key importance for mapping functional connectivity and for dissecting the neural circuitry underlying behaviors. We have previously generated transgenic mice that express channelrhodopsin-2 for light-induced activation of neurons and mapping of neural circuits. Here we describe transgenic mice that express halorhodopsin (NpHR), a light-driven chloride pump that can be used to silence neuronal activity via light. Using the Thy-1 promoter to target NpHR expression to neurons, we found that neurons in these mice expressed high levels of NpHR-YFP and that illumination of cortical pyramidal neurons expressing NpHR-YFP led to rapid, reversible photoinhibition of action potential firing in these cells. However, NpHR-YFP expression led to the formation of numerous intracellular blebs, which may disrupt neuronal function. Labeling of various subcellular markers indicated that the blebs arise from retention of NpHR-YFP in the endoplasmic reticulum. By improving the signal peptide sequence and adding an ER export signal to NpHR-YFP, we eliminated the formation of blebs and dramatically increased the membrane expression of NpHR-YFP. Thus, the improved version of NpHR should serve as an excellent tool for neuronal silencing in vitro and in vivo.