Wiring specificity in the direction-selectivity circuit of the retina

The proper connectivity between neurons is essential for the implementation of the algorithms used in neural computations, such as the detection of directed motion by the retina. The analysis of neuronal connectivity is possible with electron microscopy, but technological limitations have impeded the acquisition of high-resolution data on a large enough scale. Here we show, using serial block-face electron microscopy and two-photon calcium imaging, that the dendrites of mouse starburst amacrine cells make highly specific synapses with direction-selective ganglion cells depending on the ganglion cell's preferred direction. Our findings indicate that a structural (wiring) asymmetry contributes to the computation of direction selectivity. The nature of this asymmetry supports some models of direction selectivity and rules out others. It also puts constraints on the developmental mechanisms behind the formation of synaptic connections. Our study demonstrates how otherwise intractable neurobiological questions can be addressed by combining functional imaging with the analysis of neuronal connectivity using large-scale electron microscopy.     

Mechanisms and circuitry underlying directional selectivity in the retina

In the retina, directionally selective ganglion cells respond with robust spiking to movement in their preferred direction, but show minimal response to movement in the opposite, or null, direction. The mechanisms and circuitry underlying this computation have remained controversial. Here we show, by isolating the excitatory and inhibitory inputs to directionally selective cells and measuring direct connections between these cells and presynaptic neurons, that a presynaptic interneuron, the starburst amacrine cell, delivers direct inhibition to directionally selective cells. The processes of starburst cells are connected asymmetrically to directionally selective cells: those pointing in the null direction deliver inhibition; those pointing in the preferred direction do not. Starburst cells project inhibition laterally ahead of a stimulus moving in the null direction. In addition, starburst inhibition is itself directionally selective: it is stronger for movement in the null direction. Excitation in response to null direction movement is reduced by an inhibitory signal acting at a site that is presynaptic to the directionally selective cell. The interplay of these components generates reduced excitation and enhanced inhibition in the null direction, thereby ensuring robust directional selectivity.     

A synaptic memory trace for cortical receptive field plasticity

Receptive fields of sensory cortical neurons are plastic, changing in response to alterations of neural activity or sensory experience. In this way, cortical representations of the sensory environment can incorporate new information about the world, depending on the relevance or value of particular stimuli. Neuromodulation is required for cortical plasticity, but it is uncertain how subcortical neuromodulatory systems, such as the cholinergic nucleus basalis, interact with and refine cortical circuits. Here we determine the dynamics of synaptic receptive field plasticity in the adult primary auditory cortex (also known as AI) using in vivo whole-cell recording. Pairing sensory stimulation with nucleus basalis activation shifted the preferred stimuli of cortical neurons by inducing a rapid reduction of synaptic inhibition within seconds, which was followed by a large increase in excitation, both specific to the paired stimulus. Although nucleus basalis was stimulated only for a few minutes, reorganization of synaptic tuning curves progressed for hours thereafter: inhibition slowly increased in an activity-dependent manner to rebalance the persistent enhancement of excitation, leading to a retuned receptive field with new preference for the paired stimulus. This restricted period of disinhibition may be a fundamental mechanism for receptive field plasticity, and could serve as a memory trace for stimuli or episodes that have acquired new behavioural significance.     

Selective responses of visual cortical cells do not depend on shunting inhibition

Theoretical analyses of the electrical behaviour of the highly branched processes of nerve cells has focused attention on the possibility that single cells perform complex logical operations rather than simply summing their synaptic inputs. In particular, it has been suggested that the orientation and direction selectivity of cells in the visual cortex results from the action of a nonlinear 'shunting' inhibition that emulates an AND-NOT logical operation. The characteristic biophysical feature of this proposed inhibitory mechanism is that it evokes a large and relatively sustained increase in the conductance of the neuronal membrane while leaving the membrane potential unaffected. This shunting mechanism contrasts with linear 'summative' inhibition in which conductance changes are less prominent, and inhibition is achieved by hyperpolarization of the membrane potential. In a direct experimental test of the hypothesis that the selectivity of visual cortical neurons depends on shunting inhibition we found no evidence for the large conductance changes predicted by the theory.     

Spatially asymmetric reorganization of inhibition establishes a motion-sensitive circuit

Spatial asymmetries in neural connectivity have an important role in creating basic building blocks of neuronal processing. A key circuit module of directionally selective (DS) retinal ganglion cells is a spatially asymmetric inhibitory input from starburst amacrine cells. It is not known how and when this circuit asymmetry is established during development. Here we photostimulate mouse starburst cells targeted with channelrhodopsin-2 (refs 6-8) while recording from a single genetically labelled type of DS cell. We follow the spatial distribution of synaptic strengths between starburst and DS cells during early postnatal development before these neurons can respond to a physiological light stimulus, and confirm connectivity by monosynaptically restricted trans-synaptic rabies viral tracing. We show that asymmetry develops rapidly over a 2-day period through an intermediate state in which random or symmetric synaptic connections have been established. The development of asymmetry involves the spatially selective reorganization of inhibitory synaptic inputs. Intriguingly, the spatial distribution of excitatory synaptic inputs from starburst cells is significantly more symmetric than that of the inhibitory inputs at the end of this developmental period. Our work demonstrates a rapid developmental switch from a symmetric to asymmetric input distribution for inhibition in the neural circuit of a principal cell.     

Development of asymmetric inhibition underlying direction selectivity in the retina

Establishing precise synaptic connections is crucial to the development of functional neural circuits. The direction-selective circuit in the retina relies upon highly selective wiring of inhibitory inputs from starburst amacrine cells (SACs) onto four subtypes of ON-OFF direction-selective ganglion cells (DSGCs), each preferring motion in one of four cardinal directions. It has been reported in rabbit that the SACs on the 'null' sides of DSGCs form functional GABA (γ-aminobutyric acid)-mediated synapses, whereas those on the preferred sides do not. However, it is not known how the asymmetric wiring between SACs and DSGCs is established during development. Here we report that in transgenic mice with cell-type-specific labelling, the synaptic connections from SACs to DSGCs were of equal strength during the first postnatal week, regardless of whether the SAC was located on the preferred or null side of the DSGC. However, by the end of the second postnatal week, the strength of the synapses made from SACs on the null side of a DSGC significantly increased whereas those made from SACs located on the preferred side remained constant. Blocking retinal activity by intraocular injections of muscimol or gabazine during this period did not alter the development of direction selectivity. Hence, the asymmetric inhibition between the SACs and DSGCs is achieved by a developmental program that specifically strengthens the GABA-mediated inputs from SACs located on the null side, in a manner not dependent on neural activity.     

Turning on and off recurrent balanced cortical activity

The vast majority of synaptic connections onto neurons in the cerebral cortex arise from other cortical neurons, both excitatory and inhibitory, forming local and distant 'recurrent' networks. Although this is a basic theme of cortical organization, its study has been limited largely to theoretical investigations, which predict that local recurrent networks show a proportionality or balance between recurrent excitation and inhibition, allowing the generation of stable periods of activity. This recurrent activity might underlie such diverse operations as short-term memory, the modulation of neuronal excitability with attention, and the generation of spontaneous activity during sleep. Here we show that local cortical circuits do indeed operate through a proportional balance of excitation and inhibition generated through local recurrent connections, and that the operation of such circuits can generate self-sustaining activity that can be turned on and off by synaptic inputs. These results confirm the long-hypothesized role of recurrent activity as a basic operation of the cerebral cortex.     

Hair cell synaptic ribbons are essential for synchronous auditory signalling

Hearing relies on faithful synaptic transmission at the ribbon synapse of cochlear inner hair cells (IHCs). At present, the function of presynaptic ribbons at these synapses is still largely unknown. Here we show that anchoring of IHC ribbons is impaired in mouse mutants for the presynaptic scaffolding protein Bassoon. The lack of active-zone-anchored synaptic ribbons reduced the presynaptic readily releasable vesicle pool, and impaired synchronous auditory signalling as revealed by recordings of exocytic IHC capacitance changes and sound-evoked activation of spiral ganglion neurons. Both exocytosis of the hair cell releasable vesicle pool and the number of synchronously activated spiral ganglion neurons co-varied with the number of anchored ribbons during development. Interestingly, ribbon-deficient IHCs were still capable of sustained exocytosis with normal Ca2+-dependence. Endocytic membrane retrieval was intact, but an accumulation of tubular and cisternal membrane profiles was observed in ribbon-deficient IHCs. We conclude that ribbon-dependent synchronous release of multiple vesicles at the hair cell afferent synapse is essential for normal hearing.     

Functional asymmetry in Caenorhabditis elegans taste neurons and its computational role in chemotaxis

Chemotaxis in Caenorhabditis elegans, like chemotaxis in bacteria, involves a random walk biased by the time derivative of attractant concentration, but how the derivative is computed is unknown. Laser ablations have shown that the strongest deficits in chemotaxis to salts are obtained when the ASE chemosensory neurons (ASEL and ASER) are ablated, indicating that this pair has a dominant role. Although these neurons are left-right homologues anatomically, they exhibit marked asymmetries in gene expression and ion preference. Here, using optical recordings of calcium concentration in ASE neurons in intact animals, we demonstrate an additional asymmetry: ASEL is an ON-cell, stimulated by increases in NaCl concentration, whereas ASER is an OFF-cell, stimulated by decreases in NaCl concentration. Both responses are reliable yet transient, indicating that ASE neurons report changes in concentration rather than absolute levels. Recordings from synaptic and sensory transduction mutants show that the ON-OFF asymmetry is the result of intrinsic differences between ASE neurons. Unilateral activation experiments indicate that the asymmetry extends to the level of behavioural output: ASEL lengthens bouts of forward locomotion (runs) whereas ASER promotes direction changes (turns). Notably, the input and output asymmetries of ASE neurons are precisely those of a simple yet novel neuronal motif for computing the time derivative of chemosensory information, which is the fundamental computation of C. elegans chemotaxis. Evidence for ON and OFF cells in other chemosensory networks suggests that this motif may be common in animals that navigate by taste and smell.     

基于TMAES对GrC神经元放电活动影响的仿真

经颅磁声电刺激（TMAES）是一种新型无创的脑神经调控技术,具有良好的应用前景.该技术利用静磁场和超声波共同作用所产生的磁声电效应,在神经组织中产生感应电流,进而对神经组织实施刺激.作者基于小脑颗粒细胞模型（GrC模型）,建立了突触连接GrC模型,对TMAES刺激下突触连接GrC模型的动作电位进行仿真,分析了动作电位的传播方向.在TMAES神经元的不同突触连接方式下,对比了兴奋性与抑制性对神经元放电的影响.通过改变抑制点的位置分析了抑制作用在TMAES下对神经元放电模式的影响.仿真结果显示,经颅磁声电刺激对GrC模型神经元放电节律具有重要影响.实现了两个神经元突触连接模型在TMAES下的仿真,对进一步发掘和研究神经元的传导及连接模式具有重要意义.     

GABA regulates synaptic integration of newly generated neurons in the adult brain

Adult neurogenesis, the birth and integration of new neurons from adult neural stem cells, is a striking form of structural plasticity and highlights the regenerative capacity of the adult mammalian brain. Accumulating evidence suggests that neuronal activity regulates adult neurogenesis and that new neurons contribute to specific brain functions. The mechanism that regulates the integration of newly generated neurons into the pre-existing functional circuitry in the adult brain is unknown. Here we show that newborn granule cells in the dentate gyrus of the adult hippocampus are tonically activated by ambient GABA (gamma-aminobutyric acid) before being sequentially innervated by GABA- and glutamate-mediated synaptic inputs. GABA, the major inhibitory neurotransmitter in the adult brain, initially exerts an excitatory action on newborn neurons owing to their high cytoplasmic chloride ion content. Conversion of GABA-induced depolarization (excitation) into hyperpolarization (inhibition) in newborn neurons leads to marked defects in their synapse formation and dendritic development in vivo. Our study identifies an essential role for GABA in the synaptic integration of newly generated neurons in the adult brain, and suggests an unexpected mechanism for activity-dependent regulation of adult neurogenesis, in which newborn neurons may sense neuronal network activity through tonic and phasic GABA activation.     

Functional imaging with cellular resolution reveals precise micro-architecture in visual cortex

Neurons in the cerebral cortex are organized into anatomical columns, with ensembles of cells arranged from the surface to the white matter. Within a column, neurons often share functional properties, such as selectivity for stimulus orientation; columns with distinct properties, such as different preferred orientations, tile the cortical surface in orderly patterns. This functional architecture was discovered with the relatively sparse sampling of microelectrode recordings. Optical imaging of membrane voltage or metabolic activity elucidated the overall geometry of functional maps, but is averaged over many cells (resolution >100 microm). Consequently, the purity of functional domains and the precision of the borders between them could not be resolved. Here, we labelled thousands of neurons of the visual cortex with a calcium-sensitive indicator in vivo. We then imaged the activity of neuronal populations at single-cell resolution with two-photon microscopy up to a depth of 400 microm. In rat primary visual cortex, neurons had robust orientation selectivity but there was no discernible local structure; neighbouring neurons often responded to different orientations. In area 18 of cat visual cortex, functional maps were organized at a fine scale. Neurons with opposite preferences for stimulus direction were segregated with extraordinary spatial precision in three dimensions, with columnar borders one to two cells wide. These results indicate that cortical maps can be built with single-cell precision.     

Balanced inhibition underlies tuning and sharpens spike timing in auditory cortex

Neurons in the primary auditory cortex are tuned to the intensity and specific frequencies of sounds, but the synaptic mechanisms underlying this tuning remain uncertain. Inhibition seems to have a functional role in the formation of cortical receptive fields, because stimuli often suppress similar or neighbouring responses, and pharmacological blockade of inhibition broadens tuning curves. Here we use whole-cell recordings in vivo to disentangle the roles of excitatory and inhibitory activity in the tone-evoked responses of single neurons in the auditory cortex. The excitatory and inhibitory receptive fields cover almost exactly the same areas, in contrast to the predictions of classical lateral inhibition models. Thus, although inhibition is typically as strong as excitation, it is not necessary to establish tuning, even in the receptive field surround. However, inhibition and excitation occurred in a precise and stereotyped temporal sequence: an initial barrage of excitatory input was rapidly quenched by inhibition, truncating the spiking response within a few (1-4) milliseconds. Balanced inhibition might thus serve to increase the temporal precision and thereby reduce the randomness of cortical operation, rather than to increase noise as has been proposed previously.     

Axonal site of spike initiation enhances auditory coincidence detection

Neurons initiate spikes in the axon initial segment or at the first node in the axon. However, it is not yet understood how the site of spike initiation affects neuronal activity and function. In nucleus laminaris of birds, neurons behave as coincidence detectors for sound source localization and encode interaural time differences (ITDs) separately at each characteristic frequency (CF). Here we show, in nucleus laminaris of the chick, that the site of spike initiation in the axon is arranged at a distance from the soma, so as to achieve the highest ITD sensitivity at each CF. Na+ channels were not found in the soma of high-CF (2.5-3.3 kHz) and middle-CF (1.0-2.5 kHz) neurons but were clustered within a short segment of the axon separated by 20-50 microm from the soma; in low-CF (0.4-1.0 kHz) neurons they were clustered in a longer stretch of the axon closer to the soma. Thus, neurons initiate spikes at a more remote site as the CF of neurons increases. Consequently, the somatic amplitudes of both orthodromic and antidromic spikes were small in high-CF and middle-CF neurons and were large in low-CF neurons. Computer simulation showed that the geometry of the initiation site was optimized to reduce the threshold of spike generation and to increase the ITD sensitivity at each CF. Especially in high-CF neurons, a distant localization of the spike initiation site improved the ITD sensitivity because of electrical isolation of the initiation site from the soma and dendrites, and because of reduction of Na+-channel inactivation by attenuating the temporal summation of synaptic potentials through the low-pass filtering along the axon.     

Repeated cocaine exposure in vivo facilitates LTP induction in midbrain dopamine neurons

Drugs of abuse are known to cause persistent modification of neural circuits, leading to addictive behaviours. Changes in synaptic plasticity in dopamine neurons of the ventral tegmental area (VTA) may contribute to circuit modification induced by many drugs of abuse, including cocaine. Here we report that, following repeated exposure to cocaine in vivo, excitatory synapses to rat VTA dopamine neurons become highly susceptible to the induction of long-term potentiation (LTP) by correlated pre- and postsynaptic activity. This facilitated LTP induction is caused by cocaine-induced reduction of GABA(A) (gamma-aminobutyric acid) receptor-mediated inhibition of these dopamine neurons. In midbrain slices from rats treated with saline or a single dose of cocaine, LTP could not be induced in VTA dopamine neurons unless GABA-mediated inhibition was reduced by bicuculline or picrotoxin. However, LTP became readily inducible in slices from rats treated repeatedly with cocaine; this LTP induction was prevented by enhancing GABA-mediated inhibition using diazepam. Furthermore, repeated cocaine exposure reduced the amplitude of GABA-mediated synaptic currents and increased the probability of spike initiation in VTA dopamine neurons. This cocaine-induced enhancement of synaptic plasticity in the VTA may be important for the formation of drug-associated memory.     

Network anatomy and in vivo physiology of visual cortical neurons

In the cerebral cortex, local circuits consist of tens of thousands of neurons, each of which makes thousands of synaptic connections. Perhaps the biggest impediment to understanding these networks is that we have no wiring diagrams of their interconnections. Even if we had a partial or complete wiring diagram, however, understanding the network would also require information about each neuron's function. Here we show that the relationship between structure and function can be studied in the cortex with a combination of in vivo physiology and network anatomy. We used two-photon calcium imaging to characterize a functional property--the preferred stimulus orientation--of a group of neurons in the mouse primary visual cortex. Large-scale electron microscopy of serial thin sections was then used to trace a portion of these neurons' local network. Consistent with a prediction from recent physiological experiments, inhibitory interneurons received convergent anatomical input from nearby excitatory neurons with a broad range of preferred orientations, although weak biases could not be rejected.     

Role of experience and oscillations in transforming a rate code into a temporal code

In the vast majority of brain areas, the firing rates of neurons, averaged over several hundred milliseconds to several seconds, can be strongly modulated by, and provide accurate information about, properties of their inputs. This is referred to as the rate code. However, the biophysical laws of synaptic plasticity require precise timing of spikes over short timescales (<10 ms). Hence it is critical to understand the physiological mechanisms that can generate precise spike timing in vivo, and the relationship between such a temporal code and a rate code. Here we propose a mechanism by which a temporal code can be generated through an interaction between an asymmetric rate code and oscillatory inhibition. Consistent with the predictions of our model, the rate and temporal codes of hippocampal pyramidal neurons are highly correlated. Furthermore, the temporal code becomes more robust with experience. The resulting spike timing satisfies the temporal order constraints of hebbian learning. Thus, oscillations and receptive field asymmetry may have a critical role in temporal sequence learning.     

Distributed synaptic modification in neural networks induced by patterned stimulation.

Activity-dependent changes in synaptic efficacy or connectivity are critical for the development, signal processing and learning and memory functions of the nervous system. Repetitive correlated spiking of pre- and postsynaptic neurons can induce a persistent increase or decrease in synaptic strength, depending on the timing of the pre- and postsynaptic excitation. Previous studies on such synaptic modifications have focused on synapses made by the stimulated neuron. Here we examine, in networks of cultured hippocampal neurons, whether and how localized stimulation can modify synapses that are remote from the stimulated neuron. We found that repetitive paired-pulse stimulation of a single neuron for brief periods induces persistent strengthening or weakening of specific polysynaptic pathways in a manner that depends on the interpulse interval. These changes can be accounted for by correlated pre- and postsynaptic excitation at distant synaptic sites, resulting from different transmission delays along separate pathways. Thus, through such a 'delay-line' mechanism, temporal information coded in the timing of individual spikes can be converted into and stored as spatially distributed patterns of persistent synaptic modifications in a neural network.     

电压门控钠离子通道:律动生命乐章的主音符

生命个体传递神经冲动时扩布的电位变化过程以动作电位发放形式为特质表征.电压门控钠离子通道 (voltage-gated sodium channels,VGSCs)是形成动作电位的核心蛋白构件,在细胞的电兴奋产生和律动中起主角作用.VGSCs决定神经元细胞的兴奋性以及从突触输入到轴突输出的信号传导过程.VGSCs也是众...