Tiago Branco

Dendritic Discrimination of Temporal Input Sequences in Cortical Neurons

Discriminating Dendrites Can dendrites read out spatiotemporal input sequences? Combining two-photon glutamate uncaging and two-photon calcium imaging, electrophysiology, and computational modeling, Branco et al. (p. 1671, published online 12 August; see the Perspective by Destexhe) discovered that single dendrites were indeed sensitive to both the direction and velocity of synaptic inputs. This direction- and velocity-sensitivity was measurable with only a few inputs and should thus be engaged frequently during normal brain function. Dendrites of neurons are sensitive to the sequence of synaptic activation and can implement cortical computation. The detection and discrimination of temporal sequences is fundamental to brain function and underlies perception, cognition, and motor output. By applying patterned, two-photon glutamate uncaging, we found that single dendrites of cortical pyramidal neurons exhibit sensitivity to the sequence of synaptic activation. This sensitivity is encoded by both local dendritic calcium signals and somatic depolarization, leading to sequence-selective spike output. The mechanism involves dendritic impedance gradients and nonlinear synaptic N-methyl-d-aspartate receptor activation and is generalizable to dendrites in different neuronal types. This enables discrimination of patterns delivered to a single dendrite, as well as patterns distributed randomly across the dendritic tree. Pyramidal cell dendrites can thus act as processing compartments for the detection of synaptic sequences, thereby implementing a fundamental cortical computation.

The probability of neurotransmitter release: variability and feedback control at single synapses

Information transfer at chemical synapses occurs when vesicles fuse with the plasma membrane and release neurotransmitter. This process is stochastic and its likelihood of occurrence is a crucial factor in the regulation of signal propagation in neuronal networks. The reliability of neurotransmitter release can be highly variable: experimental data from electrophysiological, molecular and imaging studies have demonstrated that synaptic terminals can individually set their neurotransmitter release probability dynamically through local feedback regulation. This local tuning of transmission has important implications for current models of single-neuron computation.

Local Dendritic Activity Sets Release Probability at Hippocampal Synapses

The arrival of an action potential at a synapse triggers neurotransmitter release with a limited probability, p(r). Although p(r) is a fundamental parameter in defining synaptic efficacy, it is not uniform across all synapses, and the mechanisms by which a given synapse sets its basal release probability are unknown. By measuring p(r) at single presynaptic terminals in connected pairs of hippocampal neurons, we show that neighboring synapses on the same dendritic branch have very similar release probabilities, and p(r) is negatively correlated with the number of synapses on the branch. Increasing dendritic depolarization elicits a homeostatic decrease in p(r), and equalizing activity in the dendrite significantly reduces its variability. Our results indicate that local dendritic activity is the major determinant of basal release probability, and we suggest that this feedback regulation might be required to maintain synapses in their operational range.

The single dendritic branch as a fundamental functional unit in the nervous system

The conventional view of dendritic function is that dendrites collect synaptic input and deliver it to the soma. This view has been challenged in recent years by new results demonstrating that dendrites can act as independent processing and signalling units, performing local computations that are then broadcast to the rest of the neuron, or to other neurons via dendritic transmitter and neuromodulator release. Here we describe these findings and discuss the notion that the single dendritic branch may represent a fundamental unit of signalling in the mammalian nervous system. This view proposes that the dendritic branch is a basic organizational unit for integrating synaptic input, implementing synaptic and homeostatic plasticity, and controlling local cellular processes such as protein translation.

Synaptic Integration Gradients in Single Cortical Pyramidal Cell Dendrites

Cortical pyramidal neurons receive thousands of synaptic inputs arriving at different dendritic locations with varying degrees of temporal synchrony. It is not known if different locations along single cortical dendrites integrate excitatory inputs in different ways. Here we have used two-photon glutamate uncaging and compartmental modeling to reveal a gradient of nonlinear synaptic integration in basal and apical oblique dendrites of cortical pyramidal neurons. Excitatory inputs to the proximal dendrite sum linearly and require precise temporal coincidence for effective summation, whereas distal inputs are amplified with high gain and integrated over broader time windows. This allows distal inputs to overcome their electrotonic disadvantage, and become surprisingly more effective than proximal inputs at influencing action potential output. Thus, single dendritic branches can already exhibit nonuniform synaptic integration, with the computational strategy shifting from temporal coding to rate coding along the dendrite.

Dendritic spikes enhance stimulus selectivity in cortical neurons in vivo

Neuronal dendrites are electrically excitable: they can generate regenerative events such as dendritic spikes in response to sufficiently strong synaptic input. Although such events have been observed in many neuronal types, it is not well understood how active dendrites contribute to the tuning of neuronal output in vivo. Here we show that dendritic spikes increase the selectivity of neuronal responses to the orientation of a visual stimulus (orientation tuning). We performed direct patch-clamp recordings from the dendrites of pyramidal neurons in the primary visual cortex of lightly anaesthetized and awake mice, during sensory processing. Visual stimulation triggered regenerative local dendritic spikes that were distinct from back-propagating action potentials. These events were orientation tuned and were suppressed by either hyperpolarization of membrane potential or intracellular blockade of NMDA (N-methyl-d-aspartate) receptors. Both of these manipulations also decreased the selectivity of subthreshold orientation tuning measured at the soma, thus linking dendritic regenerative events to somatic orientation tuning. Together, our results suggest that dendritic spikes that are triggered by visual input contribute to a fundamental cortical computation: enhancing orientation selectivity in the visual cortex. Thus, dendritic excitability is an essential component of behaviourally relevant computations in neurons.

The site of action potential initiation in cerebellar Purkinje neurons.

Knowledge of the site of action potential initiation is essential for understanding how synaptic input is converted into neuronal output. Previous studies have shown that the lowest-threshold site for initiation of action potentials is in the axon. Here we use recordings from visualized rat cerebellar Purkinje cell axons to localize the site of initiation to a well-defined anatomical structure: the first node of Ranvier, which normally forms at the first axonal branch point.

A Vesicle Superpool Spans Multiple Presynaptic Terminals in Hippocampal Neurons

Summary Synapse-specific vesicle pools have been widely characterized at central terminals. Here, we demonstrate a vesicle pool that is not confined to a synapse but spans multiple terminals. Using fluorescence imaging, correlative electron microscopy, and modeling of vesicle dynamics, we show that some recycling pool vesicles at synapses form part of a larger vesicle “superpool.” The vesicles within this superpool are highly mobile and are rapidly exchanged between terminals (turnover: ∼4% of total pool/min), significantly changing vesicular composition at synapses over time. In acute hippocampal slices we show that the mobile vesicle pool is also a feature of native brain tissue. We also demonstrate that superpool vesicles are available to synapses during stimulation, providing an extension of the classical recycling pool. Experiments using focal BDNF application suggest the involvement of a local TrkB-receptor-dependent mechanism for synapse-specific regulation of presynaptic vesicle pools through control of vesicle release and capture to or from the extrasynaptic pool.

Tonic Inhibition Enhances Fidelity of Sensory Information Transmission in the Cerebellar Cortex

Tonic inhibition is a key regulator of neuronal excitability and network function in the brain, but its role in sensory information processing remains poorly understood. The cerebellum is a favorable model system for addressing this question as granule cells, which form the input layer of the cerebellar cortex, permit high-resolution patch-clamp recordings in vivo, and are the only neurons in the cerebellar cortex that express the α6δ-containing GABAA receptors mediating tonic inhibition. We investigated how tonic inhibition regulates sensory information transmission in the rat cerebellum by using a combination of intracellular recordings from granule cells and molecular layer interneurons in vivo, selective pharmacology, and in vitro dynamic clamp experiments. We show that blocking tonic inhibition significantly increases the spontaneous firing rate of granule cells while only moderately increasing sensory-evoked spike output. In contrast, enhancing tonic inhibition reduces the spike probability in response to sensory stimulation with minimal effect on the spontaneous spike rate. Both manipulations result in a reduction in the signal-to-noise ratio of sensory transmission in granule cells and of parallel fiber synaptic input to downstream molecular layer interneurons. These results suggest that under basal conditions the level of tonic inhibition in vivo enhances the fidelity of sensory information transmission through the input layer of the cerebellar cortex.

Cell type-specific transcriptomics of hypothalamic energy-sensing neuron responses to weight-loss

Molecular and cellular processes in neurons are critical for sensing and responding to energy deficit states, such as during weight-loss. Agouti related protein (AGRP)-expressing neurons are a key hypothalamic population that is activated during energy deficit and increases appetite and weight-gain. Cell type-specific transcriptomics can be used to identify pathways that counteract weight-loss, and here we report high-quality gene expression profiles of AGRP neurons from well-fed and food-deprived young adult mice. For comparison, we also analyzed Proopiomelanocortin (POMC)-expressing neurons, an intermingled population that suppresses appetite and body weight. We find that AGRP neurons are considerably more sensitive to energy deficit than POMC neurons. Furthermore, we identify cell type-specific pathways involving endoplasmic reticulum-stress, circadian signaling, ion channels, neuropeptides, and receptors. Combined with methods to validate and manipulate these pathways, this resource greatly expands molecular insight into neuronal regulation of body weight, and may be useful for devising therapeutic strategies for obesity and eating disorders. DOI: http://dx.doi.org/10.7554/eLife.09800.001