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1.  Target-Specific Effects of Somatostatin-Expressing Interneurons on Neocortical Visual Processing 
The Journal of Neuroscience  2013;33(50):19567-19578.
A diverse array of interneuron types regulates activity in the mammalian neocortex. Two of the most abundant are the fast-spiking, parvalbumin-positive (PV+) interneurons, which target the axosomatic region of pyramidal cells, and the somatostatin-positive (SOM+) interneurons, which target the dendrites. Recent work has focused on the influence of PV+ and SOM+ interneurons on pyramidal cells. However, the connections among PV+ and SOM+ interneurons are poorly understood and could play an important role in cortical circuitry, since their interactions may alter the net influence on pyramidal cell output. We used an optogenetic approach to investigate the effect of SOM+ interneurons on pyramidal cells and PV+ interneurons during visual stimulation in mouse primary visual cortex. We find that SOM+ interneuron activation suppresses PV+ cell spiking at least twice as potently as pyramidal cell spiking during visual stimulation. This differential effect of SOM+ cell stimulation is detectable even when only two to three SOM+ cells are activated. Importantly, the remaining responses to oriented gratings in PV+ cells are more orientation tuned and temporally modulated, suggesting that SOM+ activity unmasks this tuning by suppressing untuned input. Our results highlight the importance of SOM+ inhibition of PV+ interneurons during sensory processing. This prominent competitive inhibition between interneuron types leads to a reconfiguration of inhibition along the somatodendritic axis of pyramidal cells, and enhances the orientation selectivity of PV+ cells.
doi:10.1523/JNEUROSCI.2624-13.2013
PMCID: PMC3858626  PMID: 24336721
2.  Synaptically Induced Long-Term Modulation of Electrical Coupling in the Inferior Olive 
Neuron  2014;81(6):1290-1296.
Summary
Electrical coupling mediated by gap junctions is widespread in the mammalian CNS, and the interplay between chemical and electrical synapses on the millisecond timescale is crucial for determining patterns of synchrony in many neural circuits. Here we show that activation of glutamatergic synapses drives long-term depression of electrical coupling between neurons of the inferior olive. We demonstrate that this plasticity is not triggered by postsynaptic spiking alone and that it requires calcium entry following synaptic NMDA receptor activation. These results reveal that glutamatergic synapses can instruct plasticity at electrical synapses, providing a means for excitatory inputs to homeostatically regulate the long-term dynamics of microzones in olivocerebellar circuits.
Highlights
•Chemical synapses trigger long-term depression of inferior olive electrical coupling•Depression of electrical coupling requires NMDAR activation and calcium entry•Plasticity is not triggered by postsynaptic spiking alone and EPSPs remain unchanged•Excitatory inputs can thus homeostatically regulate synchrony patterns in the olive
The interplay between electrical and chemical synapses is crucial for determining patterns of synchrony in neuronal networks. Recording in the inferior olive, Mathy et al. show that coincident excitatory synaptic input and spiking can cause a sustained suppression of electrical coupling.
doi:10.1016/j.neuron.2014.01.005
PMCID: PMC3988996  PMID: 24656251
3.  Structured Connectivity in Cerebellar Inhibitory Networks 
Neuron  2014;81(4):913-929.
Summary
Defining the rules governing synaptic connectivity is key to formulating theories of neural circuit function. Interneurons can be connected by both electrical and chemical synapses, but the organization and interaction of these two complementary microcircuits is unknown. By recording from multiple molecular layer interneurons in the cerebellar cortex, we reveal specific, nonrandom connectivity patterns in both GABAergic chemical and electrical interneuron networks. Both networks contain clustered motifs and show specific overlap between them. Chemical connections exhibit a preference for transitive patterns, such as feedforward triplet motifs. This structured connectivity is supported by a characteristic spatial organization: transitivity of chemical connectivity is directed vertically in the sagittal plane, and electrical synapses appear strictly confined to the sagittal plane. The specific, highly structured connectivity rules suggest that these motifs are essential for the function of the cerebellar network.
Highlights
•Quadruple patch recordings from interneurons reveal highly nonrandom connectivity•Both chemical and electrical interneuron networks exhibit spatial clustering•Chemical synaptic connections are organized in transitive feedforward motifs•The electrical and chemical networks show specific overlap at the triplet level
Interneurons are connected by both chemical and electrical synapses, but the organization of this connectivity is unknown. With quadruple recordings from cerebellar interneurons, Rieubland et al. find that electrical and chemical interneuron networks exhibit highly structured nonrandom connectivity, including clustered and feedforward motifs.
doi:10.1016/j.neuron.2013.12.029
PMCID: PMC3988957  PMID: 24559679
4.  How to build a grid cell 
Neurons in the medial entorhinal cortex fire action potentials at regular spatial intervals, creating a striking grid-like pattern of spike rates spanning the whole environment of a navigating animal. This remarkable spatial code may represent a neural map for path integration. Recent advances using patch-clamp recordings from entorhinal cortex neurons in vitro and in vivo have revealed how the microcircuitry in the medial entorhinal cortex may contribute to grid cell firing patterns, and how grid cells may transform synaptic inputs into spike output during firing field crossings. These new findings provide key insights into the ingredients necessary to build a grid cell.
doi:10.1098/rstb.2012.0520
PMCID: PMC3866442  PMID: 24366132
grid cell; entorhinal cortex; spatial navigation; patch clamp; neural circuit; path integration
5.  Synchronous and asynchronous modes of synaptic transmission utilize different calcium sources 
eLife  2013;2:e01206.
Asynchronous transmission plays a prominent role at certain synapses but lacks the mechanistic insights of its synchronous counterpart. The current view posits that triggering of asynchronous release during repetitive stimulation involves expansion of the same calcium domains underlying synchronous transmission. In this study, live imaging and paired patch clamp recording at the zebrafish neuromuscular synapse reveal contributions by spatially distinct calcium sources. Synchronous release is tied to calcium entry into synaptic boutons via P/Q type calcium channels, whereas asynchronous release is boosted by a propagating intracellular calcium source initiated at off-synaptic locations in the axon and axonal branch points. This secondary calcium source fully accounts for the persistence following termination of the stimulus and sensitivity to slow calcium buffers reported for asynchronous release. The neuromuscular junction and CNS neurons share these features, raising the possibility that secondary calcium sources are common among synapses with prominent asynchronous release.
DOI: http://dx.doi.org/10.7554/eLife.01206.001
eLife digest
Neurons communicate with one another at junctions called synapses. The arrival of an electrical signal known as an action potential at the first (presynaptic) neuron causes calcium ions to flood into the cell. This in turn causes the neuron to release packages of chemicals called neurotransmitters into the synapse. These activate receptors on the second (postsynaptic) neuron, triggering a new action potential that travels down the axon to the next synapse.
The ions that trigger the release of the neurotransmitters are thought to enter the neuron through special calcium channels on or near the synapse. A sudden discrete influx of calcium ions causes the neuron to release many packages of transmitter simultaneously. This is called synchronous release. By contrast, when successive action potentials occur in the same neuron, the ions entering through the calcium channels accumulate inside the cell. This is thought to account for a sustained release of the neurotransmitter that continues even in the absence of nerve action potentials. This is called asynchronous release.
Wen et al. have now obtained evidence that these two forms of release might be triggered by calcium from different sources. The work was performed using a synapse between nerve and muscle cells in zebrafish: it has been shown that channels called P/Q calcium channels control the release of neurotransmitters at this synapse in zebrafish.
Mutant zebrafish with greatly reduced numbers of P/Q channels showed reduced synchronous release, but normal asynchronous release. Blocking the P/Q channels with a specific toxin in normal zebrafish eliminated synchronous release but left asynchronous release intact. Imaging experiments on these toxin-treated zebrafish revealed that a wave of calcium ions that propagated from a distant source coincided with the onset of asynchronous release. This wave of calcium fully accounted for the delayed onset and the persistence of asynchronous release following termination of the action potentials. This study further demonstrates that asynchronous release can be triggered by calcium ions that do not enter through the P/Q calcium channels.
Waves of calcium have been described in the nervous system before, but their significance has always been unclear. The work of Wen et al. offers the first possible explanation for the role of these waves, and further experiments are now needed to determine whether this process happens at other types of synapses.
DOI: http://dx.doi.org/10.7554/eLife.01206.002
doi:10.7554/eLife.01206
PMCID: PMC3869123  PMID: 24368731
synaptopHluorin; calcium indicators; motor neuron; Zebrafish
6.  Inhibition dominates sensory responses in awake cortex 
Nature  2012;493(7430):97-100.
The activity of the cerebral cortex is thought to depend on the precise relationship between synaptic excitation and inhibition1-4. In visual cortex, in particular, intracellular measurements have related response selectivity to coordinated increases in excitation and inhibition5-9. These measurements, however, have all been performed during anaesthesia, which strongly influences cortical state10 and therefore sensory processing7,11-15. The synaptic activity evoked by visual stimulation during wakefulness is unknown. Here, we measured visually evoked responses – and the underlying synaptic conductances – in the visual cortex of anaesthetised and awake mice. Under anaesthesia, responses could be elicited from a large region of visual space16 and were prolonged in time. During wakefulness responses were more spatially selective and much briefer. Whole-cell patch-clamp recordings of synaptic conductances5,17 revealed a surprising difference in synaptic inhibition during the two conditions. Whereas under anaesthesia inhibition tracked excitation in amplitude and spatial selectivity, during wakefulness it was much stronger than excitation and exhibited extremely broad spatial selectivity. We conclude that during wakefulness cortical responses to visual stimulation are dominated by synaptic inhibition, restricting their spatial spread and temporal persistence. These results provide the first direct glimpse of synaptic mechanisms that control visual responses in the awake cortex.
doi:10.1038/nature11665
PMCID: PMC3537822  PMID: 23172139
7.  Changing the responses of cortical neurons from sub- to suprathreshold using single spikes in vivo 
eLife  2013;2:e00012.
Action Potential (APs) patterns of sensory cortex neurons encode a variety of stimulus features, but how can a neuron change the feature to which it responds? Here, we show that in vivo a spike-timing-dependent plasticity (STDP) protocol—consisting of pairing a postsynaptic AP with visually driven presynaptic inputs—modifies a neurons' AP-response in a bidirectional way that depends on the relative AP-timing during pairing. Whereas postsynaptic APs repeatedly following presynaptic activation can convert subthreshold into suprathreshold responses, APs repeatedly preceding presynaptic activation reduce AP responses to visual stimulation. These changes were paralleled by restructuring of the neurons response to surround stimulus locations and membrane-potential time-course. Computational simulations could reproduce the observed subthreshold voltage changes only when presynaptic temporal jitter was included. Together this shows that STDP rules can modify output patterns of sensory neurons and the timing of single-APs plays a crucial role in sensory coding and plasticity.
DOI: http://dx.doi.org/10.7554/eLife.00012.001
eLife digest
Nerve cells, called neurons, are one of the core components of the brain and form complex networks by connecting to other neurons via long, thin ‘wire-like’ processes called axons. Axons can extend across the brain, enabling neurons to form connections—or synapses—with thousands of others. It is through these complex networks that incoming information from sensory organs, such as the eye, is propagated through the brain and encoded.
The basic unit of communication between neurons is the action potential, often called a ‘spike’, which propagates along the network of axons and, through a chemical process at synapses, communicates with the postsynaptic neurons that the axon is connected to. These action potentials excite the neuron that they arrive at, and this excitatory process can generate a new action potential that then propagates along the axon to excite additional target neurons. In the visual areas of the cortex, neurons respond with action potentials when they ‘recognize’ a particular feature in a scene—a process called tuning. How a neuron becomes tuned to certain features in the world and not to others is unclear, as are the rules that enable a neuron to change what it is tuned to. What is clear, however, is that to understand this process is to understand the basis of sensory perception.
Memory storage and formation is thought to occur at synapses. The efficiency of signal transmission between neurons can increase or decrease over time, and this process is often referred to as synaptic plasticity. But for these synaptic changes to be transmitted to target neurons, the changes must alter the number of action potentials. Although it has been shown in vitro that the efficiency of synaptic transmission—that is the strength of the synapse—can be altered by changing the order in which the pre- and postsynaptic cells are activated (referred to as ‘Spike-timing-dependent plasticity’), this has never been shown to have an effect on the number of action potentials generated in a single neuron in vivo. It is therefore unknown whether this process is functionally relevant.
Now Pawlak et al. report that spike-timing-dependent plasticity in the visual cortex of anaesthetized rats can change the spiking of neurons in the visual cortex. They used a visual stimulus (a bar flashed up for half a second) to activate a presynaptic cell, and triggered a single action potential in the postsynaptic cell a very short time later. By repeatedly activating the cells in this way, they increased the strength of the synaptic connection between the two neurons. After a small number of these pairing activations, presenting the visual stimulus alone to the presynaptic cell was enough to trigger an action potential (a suprathreshold response) in the postsynaptic neuron—even though this was not the case prior to the pairing.
This study shows that timing rules known to change the strength of synaptic connections—and proposed to underlie learning and memory—have functional relevance in vivo, and that the timing of single action potentials can change the functional status of a cortical neuron.
DOI: http://dx.doi.org/10.7554/eLife.00012.002
doi:10.7554/eLife.00012
PMCID: PMC3552422  PMID: 23359858
synaptic plasticity; STDP; visual cortex; circuits; in vivo; spiking patterns; rat
8.  A Preferentially Segregated Recycling Vesicle Pool of Limited Size Supports Neurotransmission in Native Central Synapses 
Neuron  2012;76(3-3):579-589.
Summary
At small central synapses, efficient turnover of vesicles is crucial for stimulus-driven transmission, but how the structure of this recycling pool relates to its functional role remains unclear. Here we characterize the organizational principles of functional vesicles at native hippocampal synapses with nanoscale resolution using fluorescent dye labeling and electron microscopy. We show that the recycling pool broadly scales with the magnitude of the total vesicle pool, but its average size is small (∼45 vesicles), highly variable, and regulated by CDK5/calcineurin activity. Spatial analysis demonstrates that recycling vesicles are preferentially arranged near the active zone and this segregation is abolished by actin stabilization, slowing the rate of activity-driven exocytosis. Our approach reveals a similarly biased recycling pool distribution at synapses in visual cortex activated by sensory stimulation in vivo. We suggest that in small native central synapses, efficient release of a limited pool of vesicles relies on their favored spatial positioning within the terminal.
Highlights
► Native hippocampal synapses have a small spatially biased recycling vesicle pool ► Pool size is regulated and positioning near the active zone relies on actin turnover ► In vivo sensory-activated synapses in visual cortex share same organization ► Positional bias ensures effective transmission in central size-limited terminals
Using a functional-ultrastructural approach, Marra et al. demonstrate that recycling vesicles in native synapses form a small subset of the total vesicle population. The spatial bias of this functional pool toward the release site helps to support efficient neurotransmission.
doi:10.1016/j.neuron.2012.08.042
PMCID: PMC3526798  PMID: 23141069
9.  Dendritic Calcium Signaling Triggered by Spontaneous and Sensory-Evoked Climbing Fiber Input to Cerebellar Purkinje Cells In Vivo 
The Journal of Neuroscience  2011;31(30):10847-10858.
Cerebellar Purkinje cells have one of the most elaborate dendritic trees in the mammalian CNS, receiving excitatory synaptic input from a single climbing fiber (CF) and from ∼200,000 parallel fibers. The dendritic Ca2+ signals triggered by activation of these inputs are crucial for the induction of synaptic plasticity at both of these synaptic connections. We have investigated Ca2+ signaling in Purkinje cell dendrites in vivo by combining targeted somatic or dendritic patch-clamp recording with simultaneous two-photon microscopy. Both spontaneous and sensory-evoked CF inputs triggered widespread Ca2+ signals throughout the dendritic tree that were detectable even in individual spines of the most distal spiny branchlets receiving parallel fiber input. The amplitude of these Ca2+ signals depended on dendritic location and could be modulated by membrane potential, reflecting modulation of dendritic spikes triggered by the CF input. Furthermore, the variability of CF-triggered Ca2+ signals was regulated by GABAergic synaptic input. These results indicate that dendritic Ca2+ signals triggered by sensory-evoked CF input can act as associative signals for synaptic plasticity in Purkinje cells in vivo and may differentially modulate plasticity at parallel fiber synapses depending on the location of synapses, firing state of the Purkinje cell, and ongoing GABAergic synaptic input.
doi:10.1523/JNEUROSCI.2525-10.2011
PMCID: PMC3758548  PMID: 21795537
10.  Initiation of simple and complex spikes in cerebellar Purkinje cells 
The Journal of Physiology  2010;588(Pt 10):1709-1717.
Cerebellar Purkinje cells produce two distinct forms of action potential output: simple and complex spikes. Simple spikes occur spontaneously or are driven by parallel fibre input, while complex spikes are activated by climbing fibre input. Previous studies indicate that both simple and complex spikes originate in the axon of Purkinje cells, but the precise location where they are initiated is unclear. Here we address where in the axon of cerebellar Purkinje cells simple and complex spikes are generated. Using extracellular recording and voltage-sensitive dye imaging in rat and mouse Purkinje cells, we show that both simple and complex spikes are generated in the proximal axon, ∼15–20 μm from the soma. Once initiated, simple and complex spikes propagate both down the axon and back into the soma. The speed of backpropagation into the soma was significantly faster for complex compared to simple spikes, presumably due to charging of the somatodendritic membrane capacitance during the climbing fibre synaptic conductance. In conclusion, we show using two independent methods that the final integration site of simple and complex spikes is in the proximal axon of cerebellar Purkinje cells, at a location corresponding to the distal end of the axon initial segment.
doi:10.1113/jphysiol.2010.188300
PMCID: PMC2887989  PMID: 20351049
11.  Parallel processing of visual space by neighboring neurons in mouse visual cortex 
Nature neuroscience  2010;13(9):1144-1149.
Visual cortex exhibits smooth retinotopic organization on the macroscopic scale, but it is unknown how receptive fields are organized at the level of neighboring neurons. This information is crucial for discriminating among models of visual cortex. We used in vivo two-photon calcium imaging to independently map ON and OFF receptive field subregions of local populations of layer 2/3 neurons in mouse visual cortex. We found that receptive field subregions are often precisely shared among multiple neighboring neurons. Furthermore, large subregions appear to be assembled from multiple smaller, non-overlapping subregions of other neurons in the same local population. These experiments provide the first characterization of the diversity of receptive fields in a dense local network of visual cortex, and reveal elementary units of receptive field organization. Our results suggest that a limited pool of afferent receptive fields is available to a local population of neurons, and reveal new organizational principles for the neural circuitry of the mouse visual cortex.
doi:10.1038/nn.2620
PMCID: PMC2999824  PMID: 20711183
12.  Sensitivity to perturbations in vivo implies high noise and suggests rate coding in cortex 
Nature  2010;466(7302):123-127.
It is well known that neural activity exhibits variability, in the sense that identical sensory stimuli produce different responses, but it has been difficult to determine what this variability means. Is it noise, or does it carry important information – about, for example, the internal state of the organism? We address this issue from the bottom up, by asking whether small perturbations to activity in cortical networks are amplified. Based on in vivo whole-cell recordings in rat barrel cortex, we find that a perturbation consisting of a single extra spike in one neuron produces ~28 additional spikes in its postsynaptic targets, and we show, using simultaneous intra- and extra-cellular recordings, that a single spike produces a detectable increase in firing rate in the local network. Theoretical analysis indicates that this amplification leads to intrinsic, stimulus-independent variations in membrane potential on the order of ±2.2 - 4.5 mV – variations that are pure noise, and so carry no information at all. Therefore, for the brain to perform reliable computations, it must either use a rate code, or generate very large, fast depolarizing events, such as those proposed by the theory of synfire chains – yet in our in vivo recordings, we found that such events were very rare. Our findings are consistent with the idea that cortex is likely to use primarily a rate code.
doi:10.1038/nature09086
PMCID: PMC2898896  PMID: 20596024
13.  Quantitative Comparison of Genetically Encoded Ca2+ Indicators in Cortical Pyramidal Cells and Cerebellar Purkinje Cells 
Genetically encoded Ca2+ indicators (GECIs) are promising tools for cell type-specific and chronic recording of neuronal activity. In the mammalian central nervous system, however, GECIs have been tested almost exclusively in cortical and hippocampal pyramidal cells, and the usefulness of recently developed GECIs has not been systematically examined in other cell types. Here we expressed the latest series of GECIs, yellow cameleon (YC) 2.60, YC3.60, YC-Nano15, and GCaMP3, in mouse cortical pyramidal cells as well as cerebellar Purkinje cells using in utero injection of recombinant adenoviral vectors. We characterized the performance of the GECIs by simultaneous two-photon imaging and whole-cell patch-clamp recording in acute brain slices at 33 ± 2°C. The fluorescent responses of GECIs to action potentials (APs) evoked by somatic current injection or to synaptic stimulation were examined using rapid dendritic imaging. In cortical pyramidal cells, YC2.60 showed the largest responses to single APs, but its decay kinetics were slower than YC3.60 and GCaMP3, while GCaMP3 showed the largest responses to 20 APs evoked at 20 Hz. In cerebellar Purkinje cells, only YC2.60 and YC-Nano15 could reliably report single complex spikes (CSs), and neither showed signal saturation over the entire stimulus range tested (1–10 CSs at 10 Hz). The expression and response of YC2.60 in Purkinje cells remained detectable and comparable for at least over 100 days. These results provide useful information for selecting an optimal GECI depending on the experimental requirements: in cortical pyramidal cells, YC2.60 is suitable for detecting sparse firing of APs, whereas GCaMP3 is suitable for detecting burst firing of APs; in cerebellar Purkinje cells, YC2.60 as well as YC-Nano15 is suitable for detecting CSs.
doi:10.3389/fncel.2011.00018
PMCID: PMC3182323  PMID: 21994490
genetically encoded Ca2+ indicators; adenovirus; two-photon imaging; patch-clamp recording; cortical pyramidal cell; cerebellar Purkinje cell; acute brain slice
14.  One Rule to Grow Them All: A General Theory of Neuronal Branching and Its Practical Application 
PLoS Computational Biology  2010;6(8):e1000877.
Understanding the principles governing axonal and dendritic branching is essential for unravelling the functionality of single neurons and the way in which they connect. Nevertheless, no formalism has yet been described which can capture the general features of neuronal branching. Here we propose such a formalism, which is derived from the expression of dendritic arborizations as locally optimized graphs. Inspired by Ramón y Cajal's laws of conservation of cytoplasm and conduction time in neural circuitry, we show that this graphical representation can be used to optimize these variables. This approach allows us to generate synthetic branching geometries which replicate morphological features of any tested neuron. The essential structure of a neuronal tree is thereby captured by the density profile of its spanning field and by a single parameter, a balancing factor weighing the costs for material and conduction time. This balancing factor determines a neuron's electrotonic compartmentalization. Additions to this rule, when required in the construction process, can be directly attributed to developmental processes or a neuron's computational role within its neural circuit. The simulations presented here are implemented in an open-source software package, the “TREES toolbox,” which provides a general set of tools for analyzing, manipulating, and generating dendritic structure, including a tool to create synthetic members of any particular cell group and an approach for a model-based supervised automatic morphological reconstruction from fluorescent image stacks. These approaches provide new insights into the constraints governing dendritic architectures. They also provide a novel framework for modelling and analyzing neuronal branching structures and for constructing realistic synthetic neural networks.
Author Summary
More than a century has passed since Ramón y Cajal presented a set of fundamental biological laws of neuronal branching. He described how the shape of the core elements of the neural circuitry – axons and dendrites – are constrained by physical parameters such as space, cytoplasmic volume, and conduction time. The existence of these laws enabled him to organize his histological observations, to formulate the neuron doctrine, and to infer directionality in signal flow in the nervous system. We show that Cajal's principles can be used computationally to generate synthetic neural circuits. These principles rigorously constrain the shape of real neuronal structures, providing direct validation of his theories. At the same time, this strategy provides us with a powerful set of tools for generating synthetic neurons, as well as a model-based approach for automated reconstructions of neuronal trees from confocal image stacks.
doi:10.1371/journal.pcbi.1000877
PMCID: PMC2916857  PMID: 20700495
15.  Traveling waves in developing cerebellar cortex mediated by asymmetrical Purkinje cell connectivity 
Nature neuroscience  2009;12(4):463-473.
Correlated network activity plays a critical role in the development of many neural circuits. Purkinje cells are among the first neurons to populate the cerebellar cortex, where they sprout exuberant axon collaterals. Here we use multiple patch-clamp recordings targeted with two-photon microscopy to characterize monosynaptic connections between Purkinje cells of juvenile mice. We show that Purkinje cell axon collaterals project asymmetrically in the sagittal plane, directed away from the lobule apex. Based on our anatomical and physiological characterization of this connection, we construct a network model that robustly generates waves of activity traveling along chains of connected Purkinje cells. Consistent with the model, we observe traveling waves of activity in Purkinje cells in sagittal slices from young mice that require GABAA receptor-mediated transmission and intact Purkinje cell axon collaterals. These traveling waves are absent in adult animals, suggesting they play a developmental role in wiring the cerebellar cortical microcircuit.
doi:10.1038/nn.2285
PMCID: PMC2912499  PMID: 19287389
16.  A New Approach for Determining Phase Response Curves Reveals that Purkinje Cells Can Act as Perfect Integrators 
PLoS Computational Biology  2010;6(4):e1000768.
Cerebellar Purkinje cells display complex intrinsic dynamics. They fire spontaneously, exhibit bistability, and via mutual network interactions are involved in the generation of high frequency oscillations and travelling waves of activity. To probe the dynamical properties of Purkinje cells we measured their phase response curves (PRCs). PRCs quantify the change in spike phase caused by a stimulus as a function of its temporal position within the interspike interval, and are widely used to predict neuronal responses to more complex stimulus patterns. Significant variability in the interspike interval during spontaneous firing can lead to PRCs with a low signal-to-noise ratio, requiring averaging over thousands of trials. We show using electrophysiological experiments and simulations that the PRC calculated in the traditional way by sampling the interspike interval with brief current pulses is biased. We introduce a corrected approach for calculating PRCs which eliminates this bias. Using our new approach, we show that Purkinje cell PRCs change qualitatively depending on the firing frequency of the cell. At high firing rates, Purkinje cells exhibit single-peaked, or monophasic PRCs. Surprisingly, at low firing rates, Purkinje cell PRCs are largely independent of phase, resembling PRCs of ideal non-leaky integrate-and-fire neurons. These results indicate that Purkinje cells can act as perfect integrators at low firing rates, and that the integration mode of Purkinje cells depends on their firing rate.
Author Summary
By observing how brief current pulses injected at different times between spikes change the phase of spiking of a neuron (and thus obtaining the so-called phase response curve), it should be possible to predict a full spike train in response to more complex stimulation patterns. When we applied this traditional protocol to obtain phase response curves in cerebellar Purkinje cells in the presence of noise, we observed a triangular region devoid of data points near the end of the spiking cycle. This “Bermuda Triangle” revealed a flaw in the classical method for constructing phase response curves. We developed a new approach to eliminate this flaw and used it to construct phase response curves of Purkinje cells over a range of spiking rates. Surprisingly, at low firing rates, phase changes were independent of the phase of the injected current pulses, implying that the Purkinje cell is a perfect integrator under these conditions. This mechanism has not yet been described in other cell types and may be crucial for the information processing capabilities of these neurons.
doi:10.1371/journal.pcbi.1000768
PMCID: PMC2861707  PMID: 20442875
17.  Predicting the synaptic information efficacy in cortical layer 5 pyramidal neurons using a minimal integrate-and-fire model 
Biological Cybernetics  2008;99(4-5):393-401.
Synaptic information efficacy (SIE) is a statistical measure to quantify the efficacy of a synapse. It measures how much information is gained, on the average, about the output spike train of a postsynaptic neuron if the input spike train is known. It is a particularly appropriate measure for assessing the input–output relationship of neurons receiving dynamic stimuli. Here, we compare the SIE of simulated synaptic inputs measured experimentally in layer 5 cortical pyramidal neurons in vitro with the SIE computed from a minimal model constructed to fit the recorded data. We show that even with a simple model that is far from perfect in predicting the precise timing of the output spikes of the real neuron, the SIE can still be accurately predicted. This arises from the ability of the model to predict output spikes influenced by the input more accurately than those driven by the background current. This indicates that in this context, some spikes may be more important than others. Lastly we demonstrate another aspect where using mutual information could be beneficial in evaluating the quality of a model, by measuring the mutual information between the model’s output and the neuron’s output. The SIE, thus, could be a useful tool for assessing the quality of models of single neurons in preserving input–output relationship, a property that becomes crucial when we start connecting these reduced models to construct complex realistic neuronal networks.
doi:10.1007/s00422-008-0268-3
PMCID: PMC2798051  PMID: 19011927
Synaptic information efficacy; Linear integrate-and-fire model; Predicting every spike; Layer 5 cortical pyramidal neuron
18.  The Origin of the Complex Spike in Cerebellar Purkinje Cells 
Activation of the climbing fiber input powerfully excites cerebellar Purkinje cells via hundreds of widespread dendritic synapses, triggering dendritic spikes as well as a characteristic high-frequency burst of somatic spikes known as the complex spike. To investigate the relationship between dendritic spikes and the spikelets within the somatic complex spike, and to evaluate the importance of the dendritic distribution of climbing fiber synapses, we made simultaneous somatic and dendritic patch-clamp recordings from Purkinje cells in cerebellar slices. Injection of large climbing fiber-like synaptic conductances at the soma using dynamic clamp was sufficient to reproduce the complex spike, independently of dendritic spikes, indicating that neither a dendritic synaptic distribution nor dendritic spikes are required. Furthermore, we found that dendritic spikes are not directly linked to spikelets in the complex spike, and that each dendritic spike is associated with only 0.24 ± 0.09 extra somatic spikelets. Rather, we demonstrate that dendritic spikes regulate the pause in firing that follows the complex spike. Finally, using dual somatic and axonal recording, we show that all spikelets in the complex spike are axonally generated. Thus, complex spike generation proceeds relatively independently of dendritic spikes, reflecting the dual functional role of climbing fiber input: triggering plasticity at dendritic synapses and generating a distinct output signal in the axon. The encoding of dendritic spiking by the post-complex spike pause provides a novel computational function for dendritic spikes, which could serve to link these two roles at the level of the target neurons in the deep cerebellar nuclei.
doi:10.1523/JNEUROSCI.0559-08.2008
PMCID: PMC2730632  PMID: 18650337
climbing fiber; Purkinje cell; cerebellum; dendritic spike; burst; axon
19.  Encoding of Oscillations by Axonal Bursts in Inferior Olive Neurons 
Neuron  2009;62(3):388-399.
Summary
Inferior olive neurons regulate plasticity and timing in the cerebellar cortex via the climbing fiber pathway, but direct characterization of the output of this nucleus has remained elusive. We show that single somatic action potentials in olivary neurons are translated into a burst of axonal spikes. The number of spikes in the burst depends on the phase of subthreshold oscillations and, therefore, encodes the state of the olivary network. These bursts can be successfully transmitted to the cerebellar cortex in vivo, having a significant impact on Purkinje cells. They enhance dendritic spikes, modulate the complex spike pattern, and promote short-term and long-term plasticity at parallel fiber synapses in a manner dependent on the number of spikes in the burst. Our results challenge the view that the climbing fiber conveys an all-or-none signal to the cerebellar cortex and help to link learning and timing theories of olivocerebellar function.
doi:10.1016/j.neuron.2009.03.023
PMCID: PMC2777250  PMID: 19447094
SYSNEURO; SIGNALING
20.  Cerebellar LTD and Pattern Recognition by Purkinje Cells 
Neuron  2007;54(1):121-136.
Summary
Many theories of cerebellar function assume that long-term depression (LTD) of parallel fiber (PF) synapses enables Purkinje cells to learn to recognize PF activity patterns. We have studied the LTD-based recognition of PF patterns in a biophysically realistic Purkinje-cell model. With simple-spike firing as observed in vivo, the presentation of a pattern resulted in a burst of spikes followed by a pause. Surprisingly, the best criterion to distinguish learned patterns was the duration of this pause. Moreover, our simulations predicted that learned patterns elicited shorter pauses, thus increasing Purkinje-cell output. We tested this prediction in Purkinje-cell recordings both in vitro and in vivo. In vitro, we found a shortening of pauses when decreasing the number of active PFs or after inducing LTD. In vivo, we observed longer pauses in LTD-deficient mice. Our results suggest a novel form of neural coding in the cerebellar cortex.
doi:10.1016/j.neuron.2007.03.015
PMCID: PMC1885969  PMID: 17408582
SYSNEURO; MOLNEURO; SYSBIO
21.  Initiation of simple and complex spikes in cerebellar Purkinje cells 
The Journal of Physiology  2010;588(10):1709-1717.
Cerebellar Purkinje cells produce two distinct forms of action potential output: simple and complex spikes. Simple spikes occur spontaneously or are driven by parallel fibre input, while complex spikes are activated by climbing fibre input. Previous studies indicate that both simple and complex spikes originate in the axon of Purkinje cells, but the precise location where they are initiated is unclear. Here we address where in the axon of cerebellar Purkinje cells simple and complex spikes are generated. Using extracellular recording and voltage-sensitive dye imaging in rat and mouse Purkinje cells, we show that both simple and complex spikes are generated in the proximal axon, ∼15–20 μm from the soma. Once initiated, simple and complex spikes propagate both down the axon and back into the soma. The speed of backpropagation into the soma was significantly faster for complex compared to simple spikes, presumably due to charging of the somatodendritic membrane capacitance during the climbing fibre synaptic conductance. In conclusion, we show using two independent methods that the final integration site of simple and complex spikes is in the proximal axon of cerebellar Purkinje cells, at a location corresponding to the distal end of the axon initial segment.
doi:10.1113/jphysiol.2010.188300
PMCID: PMC2887989  PMID: 20351049

Results 1-21 (21)