Voltage-dependent currents prolong single-axon postsynaptic potentials in layer III pyramidal neurons in rat neocortical slices

Data-driven multiscale computational models of cortical and subcortical regions

Data-driven computational models of neurons, synapses, microcircuits, and mesocircuits have become essential tools in modern brain research. The goal of these multiscale models is to integrate and synthesize information from different levels of brain organization, from cellular properties, dendritic excitability, and synaptic dynamics to microcircuits, mesocircuits, and ultimately behavior. This article surveys recent advances in the genesis of data-driven computational models of mammalian neural networks in cortical and subcortical areas. I discuss the challenges and opportunities in developing data-driven multiscale models, including the need for interdisciplinary collaborations, the importance of model validation and comparison, and the potential impact on basic and translational neuroscience research. Finally, I highlight future directions and emerging technologies that will enable more comprehensive and predictive data-driven models of brain function and dysfunction.

The role of negative conductances in neuronal subthreshold properties and synaptic integration

Based on passive cable theory, an increase in membrane conductance produces a decrease in the membrane time constant and input resistance. Unlike the classical leak currents, voltage-dependent currents have a nonlinear behavior which can create regions of negative conductance, despite the increase in membrane conductance (permeability). This negative conductance opposes the effects of the passive membrane conductance on the membrane input resistance and time constant, increasing their values and thereby substantially affecting the amplitude and time course of postsynaptic potentials at the voltage range of the negative conductance. This paradoxical effect has been described for three types of voltage-dependent inward currents: persistent sodium currents, L- and T-type calcium currents and ligand-gated glutamatergic N-methyl-D-aspartate currents. In this review, we describe the impact of the creation of a negative conductance region by these currents on neuronal membrane properties and synaptic integration. We also discuss recent contributions of the quasi-active cable approximation, an extension of the passive cable theory that includes voltage-dependent currents, and its effects on neuronal subthreshold properties.

Memory Takes Time

Memory is an adaptation to particular temporal properties of past events, such as the frequency of occurrence of a stimulus or the coincidence of multiple stimuli. In neurons, this adaptation can be understood in terms of a hierarchical system of molecular and cellular time windows, which collectively retain information from the past. We propose that this system makes various timescales of past experience simultaneously available for future adjustment of behavior. More generally, we propose that the ability to detect and respond to temporally structured information underlies the nervous system's capacity to encode and store a memory at molecular, cellular, synaptic, and circuit levels.

A Negative Slope Conductance of the Persistent Sodium Current Prolongs Subthreshold Depolarizations

Neuronal subthreshold voltage-dependent currents determine membrane properties such as the input resistance (R_in) and the membrane time constant (τ_m) in the subthreshold range. In contrast with classical cable theory predictions, the persistent sodium current (I_NaP), a non-inactivating mode of the voltage-dependent sodium current, paradoxically increases R_in and τ_m when activated. Furthermore, this current amplifies and prolongs synaptic currents in the subthreshold range. Here, using a computational neuronal model, we showed that the creation of a region of negative slope conductance by I_NaP activation is responsible for these effects and the ability of the negative slope conductance to amplify and prolong R_in and τ_m relies on the fast activation of I_NaP. Using dynamic clamp in hippocampal CA1 pyramidal neurons in brain slices, we showed that the effects of I_NaP on R_in and τ_m can be recovered by applying an artificial I_NaP after blocking endogenous I_NaP with tetrodotoxin. Furthermore, we showed that injection of a pure negative conductance is enough to reproduce the effects of I_NaP on R_in and τ_m and is also able to prolong artificial excitatory post synaptic currents. Since both the negative slope conductance and the almost instantaneous activation are critical for producing these effects, the I_NaP is an ideal current for boosting the amplitude and duration of excitatory post synaptic currents near the action potential threshold.

A principled dimension-reduction method for the population density approach to modeling networks of neurons with synaptic dynamics

The population density approach to neural network modeling has been utilized in a variety of contexts. The idea is to group many similar noisy neurons into populations and track the probability density function for each population that encompasses the proportion of neurons with a particular state rather than simulating individual neurons (i.e., Monte Carlo). It is commonly used for both analytic insight and as a time-saving computational tool. The main shortcoming of this method is that when realistic attributes are incorporated in the underlying neuron model, the dimension of the probability density function increases, leading to intractable equations or, at best, computationally intensive simulations. Thus, developing principled dimension-reduction methods is essential for the robustness of these powerful methods. As a more pragmatic tool, it would be of great value for the larger theoretical neuroscience community. For exposition of this method, we consider a single uncoupled population of leaky integrate-and-fire neurons receiving external excitatory synaptic input only. We present a dimension-reduction method that reduces a two-dimensional partial differential-integral equation to a computationally efficient one-dimensional system and gives qualitatively accurate results in both the steady-state and nonequilibrium regimes. The method, termed modified mean-field method, is based entirely on the governing equations and not on any auxiliary variables or parameters, and it does not require fine-tuning. The principles of the modified mean-field method have potential applicability to more realistic (i.e., higher-dimensional) neural networks.

Local connections of excitatory neurons in motor-associated cortical areas of the rat

In spite of recent progress in brain sciences, the local circuit of the cerebral neocortex, including motor areas, still remains elusive. Morphological works on excitatory cortical circuitry from thalamocortical (TC) afferents to corticospinal neurons (CSNs) in motor-associated areas are reviewed here. First, TC axons of motor thalamic nuclei have been re-examined by the single-neuron labeling method. There are middle layer (ML)-targeting and layer (L) 1-preferring TC axon types in motor-associated areas, being analogous to core and matrix types, respectively, of Jones (1998) in sensory areas. However, the arborization of core-like motor TC axons spreads widely and disregards the columnar structure that is the basis of information processing in sensory areas, suggesting that motor areas adopt a different information-processing framework such as area-wide laminar organization. Second, L5 CSNs receive local excitatory inputs not only from L2/3 pyramidal neurons but also from ML spiny neurons, the latter directly processing cerebellar information of core-like TC neurons (TCNs). In contrast, basal ganglia information is targeted to apical dendrites of L2/3 and L5 pyramidal neurons through matrix TCNs. Third, L6 corticothalamic neurons (CTNs) are most densely innervated by ML spiny neurons located just above CTNs. Since CTNs receive only weak connections from L2/3 and L5 pyramidal neurons, the TC recurrent circuit composed of TCNs, ML spiny neurons and CTNs appears relatively independent of the results of processing in L2/3 and L5. It is proposed that two circuits sharing the same TC projection and ML neurons are embedded in the neocortex: one includes L2/3 and L5 neurons, processes afferent information in a feedforward way and sends the processed information to other cortical areas and subcortical regions; and the other circuit participates in a dynamical system of the TC recurrent circuit and may serve as the basis of autonomous activity of the neocortex.

The role of glutamatergic inputs onto parvalbumin-positive interneurons: relevance for schizophrenia

Cognitive impairment, a core feature of schizophrenia, has been suggested to arise from a disturbance of gamma oscillations that is due to decreased neurotransmission from the parvalbumin (PV) subtype of interneurons. Indeed, PV interneurons have uniquely fast membrane and synaptic properties that are crucially important for network functions such as feedforward inhibition or gamma oscillations. The causes leading to impairment of PV neurotransmission in schizophrenia are still under investigation. Interestingly, NMDA receptors (NMDARs) antagonism results in schizophrenia-like symptoms in healthy adults. Additionally, systemic NMDAR antagonist administration increases prefrontal cortex pyramidal cell firing, apparently by producing disinhibition, and repeated exposure to NMDA antagonists leads to changes in the GABAergic markers that mimic the impairments found in schizophrenia. Based on these findings, PV neuron deficits in schizophrenia have been proposed to be secondary to (NMDAR) hypofunction at glutamatergic synapses onto these cells. However, NMDARs generate long-lasting postsynaptic currents that result in prolonged depolarization of the postsynaptic cells, a property inconsistent with the role of PV cells in network dynamics. Here, we review evidence leading to the conclusion that cortical disinhibition and GABAergic impairment produced by NMDAR antagonists are unlikely to be mediated via NMDARs at glutamatergic synapses onto mature cortical PV neurons.

Dense inhibitory connectivity in neocortex

The connectivity diagram of neocortical circuits is still unknown, and there are conflicting data as to whether cortical neurons are wired specifically or not. To investigate the basic structure of cortical microcircuits, we use a two-photon photostimulation technique that enables the systematic mapping of synaptic connections with single-cell resolution. We map the inhibitory connectivity between upper layers somatostatin-positive GABAergic interneurons and pyramidal cells in mouse frontal cortex. Most, and sometimes all, inhibitory neurons are locally connected to every sampled pyramidal cell. This dense inhibitory connectivity is found at both young and mature developmental ages. Inhibitory innervation of neighboring pyramidal cells is similar, regardless of whether they are connected among themselves or not. We conclude that local inhibitory connectivity is promiscuous, does not form subnetworks, and can approach the theoretical limit of a completely connected synaptic matrix.

Glutamate receptor subtypes mediating synaptic activation of prefrontal cortex neurons: relevance for schizophrenia

Schizophrenia may involve hypofunction of NMDA receptor (NMDAR)-mediated signaling, and alterations in parvalbumin-positive fast-spiking (FS) GABA neurons that may cause abnormal gamma oscillations. It was recently hypothesized that prefrontal cortex (PFC) FS neuron activity is highly dependent on NMDAR activation and that, consequently, FS neuron dysfunction in schizophrenia is secondary to NMDAR hypofunction. However, NMDARs are abundant in synapses onto PFC pyramidal neurons; thus, a key question is whether FS neuron or pyramidal cell activation is more dependent on NMDARs. We examined the AMPAR and NMDAR contribution to synaptic activation of FS neurons and pyramidal cells in the PFC of adult mice. In FS neurons, EPSCs had fast decay and weak NMDAR contribution, whereas in pyramidal cells, EPSCs were significantly prolonged by NMDAR-mediated currents. Moreover, the AMPAR/NMDAR EPSC ratio was higher in FS cells. NMDAR antagonists decreased EPSPs and EPSP-spike coupling more strongly in pyramidal cells than in FS neurons, showing that FS neuron activation is less NMDAR dependent than pyramidal cell excitation. The precise EPSP-spike coupling produced by fast-decaying EPSCs in FS cells may be important for network mechanisms of gamma oscillations based on feedback inhibition. To test this possibility, we used simulations in a computational network of reciprocally connected FS neurons and pyramidal cells and found that brief AMPAR-mediated FS neuron activation is crucial to synchronize, via feedback inhibition, pyramidal cells in the gamma frequency band. Our results raise interesting questions about the mechanisms that might link NMDAR hypofunction to alterations of FS neurons in schizophrenia.

Synapses of horizontal connections in adult rat somatosensory cortex have different properties depending on the source of their axons

In somatosensory cortex (S1) tactile stimulation activates specific regions. The borders between representations of different body parts constrain the spread of excitation and inhibition: connections that cross from one representation to another (cross-border, CB) are weaker than those remaining within the representation (noncross border, NCB). Thus, physiological properties of CB and NCB synapses onto layer 2/3 pyramidal neurons were compared using whole-cell recordings in layer 2/3 neurons close to the border between the forepaw and lower jaw representations. Electrical stimulation of CB and NCB connections was used to activate synaptic potentials. Properties of excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials (PSP) were determined using 3 methods: 1) minimal stimulation to elicit single-fiber responses; 2) stimulation in the presence of extracellular Sr(2+) to elicit asynchronous quantal responses; 3) short trains of stimulation at various frequencies to examine postsynaptic potential (PSP) dynamics. Both minimal and asynchronous quantal EPSPs were smaller when evoked by CB than NCB stimulation. However, the dynamics of EPSP and IPSP trains were not different between CB and NCB stimulation. These data suggest that individual excitatory synapses from connections that cross a border (CB) have smaller amplitudes than those that come from within a representation (NCB), and suggest a postsynaptic locus for the difference.

Of mice and men, and chandeliers

How does the human neocortex reliably propagate information through neural circuits? One mechanism appears to involve relying on strong connections from pyramidal neurons to interneurons and a depolarizing action of cortical chandelier cells.

Robust but delayed thalamocortical activation of dendritic-targeting inhibitory interneurons

GABA-releasing cortical interneurons are crucial for the neural transformations underlying sensory perception, providing "feedforward" inhibition that constrains the temporal window for synaptic integration. To mediate feedforward inhibition, inhibitory interneurons need to fire in response to ascending thalamocortical inputs, and most previous studies concluded that ascending inputs activate mainly or solely proximally targeting, parvalbumin-containing "fast-spiking" interneurons. However, when thalamocortical axons fire at frequencies that are likely to occur during natural exploratory behavior, activation of fast-spiking interneurons is rapidly and strongly depressed, implying the paradoxical conclusion that feedforward inhibition is absent when it is most needed. To address this issue, we took advantage of lines of transgenic mice in which either parvalbumin- or somatostatin-containing interneurons express GFP and recorded the responses of interneurons from both subtypes to thalamocortical stimulation in vitro. We report that during thalamocortical activation at behaviorally expected frequencies, fast-spiking interneurons were indeed activated only transiently because of rapid depression of their thalamocortical inputs, but a subset of layer 5 somatostatin-containing interneurons were robustly and persistently activated after a delay, due to the facilitation and temporal summation of their thalamocortical excitatory postsynaptic potentials. Somatostatin-containing interneurons are considered distally targeting. Thus, they are likely to provide delayed dendritic inhibition during exploratory behavior, contributing to the maintenance of a balance between cortical excitation and inhibition while leaving a wide temporal window open for synaptic integration and plasticity in distal dendrites.

Local circuit connections between hamster laminae III and IV dorsal horn neurons

To better understand the role of intrinsic spinal cord circuits in the integration of mechanosensory information, we studied synaptic transmission between neurons in Rexed's laminae III-IV, a major termination zone for cutaneous mechanoreceptor afferents, using dual, simultaneous whole cell electrophysiological recordings in young hamsters. Synaptic connections were detected between 32 of 106 cell pairs (linkage probability of 0.3) and were predominantly unidirectional (91%). Inhibitory connections outnumbered excitatory connections by 2:1. Amplitude of single-axon postsynaptic potentials (PSPs) was independent of postsynaptic cell input resistance. Intracellular labeling suggested that recordings were obtained from local axon interneurons. In connected cell pairs, the percentage of presynaptic action potentials that failed to evoke a postsynaptic response was 44 +/- 29%. Shape indices of PSPs suggested that synaptic contacts were widely distributed along the postsynaptic membrane. Linkage probability was unrelated to intrinsic firing properties, laminar position of the cells or the distance (<160 mum) separating them. However, PSPs in target cells following action potentials in neurons with phasic firing patterns had longer duration and lower failure rates than PSPs activated by neurons with tonic firing patterns. Thus transmission reliability at synapses between lamina III/IV interneurons overall is low, and efficacy of these connections is related to firing properties of the presynaptic cells. The observations also suggest that synaptic organization in LIII-IV is fundamentally different from the superficial dorsal horn (LI-II) where neural circuits may be composed of stereotyped units made from connections between a few functional types of neurons.

Critical analysis of dimension reduction by a moment closure method in a population density approach to neural network modeling

Computational techniques within the population density function (PDF) framework have provided time-saving alternatives to classical Monte Carlo simulations of neural network activity. Efficiency of the PDF method is lost as the underlying neuron model is made more realistic and the number of state variables increases. In a detailed theoretical and computational study, we elucidate strengths and weaknesses of dimension reduction by a particular moment closure method (Cai, Tao, Shelley, & McLaughlin, 2004; Cai, Tao, Rangan, & McLaughlin, 2006) as applied to integrate-and-fire neurons that receive excitatory synaptic input only. When the unitary postsynaptic conductance event has a single-exponential time course, the evolution equation for the PDF is a partial differential integral equation in two state variables, voltage and excitatory conductance. In the moment closure method, one approximates the conditional kth centered moment of excitatory conductance given voltage by the corresponding unconditioned moment. The result is a system of k coupled partial differential equations with one state variable, voltage, and k coupled ordinary differential equations. Moment closure at k = 2 works well, and at k = 3 works even better, in the regime of high dynamically varying synaptic input rates. Both closures break down at lower synaptic input rates. Phase-plane analysis of the k = 2 problem with typical parameters proves, and reveals why, no steady-state solutions exist below a synaptic input rate that gives a firing rate of 59 s(1) in the full 2D problem. Closure at k = 3 fails for similar reasons. Low firing-rate solutions can be obtained only with parameters for the amplitude or kinetics (or both) of the unitary postsynaptic conductance event that are on the edge of the physiological range. We conclude that this dimension-reduction method gives ill-posed problems for a wide range of physiological parameters, and we suggest future directions.

The Blue Brain Project

IBM's Blue Gene supercomputer allows a quantum leap in the level of detail at which the brain can be modelled. I argue that the time is right to begin assimilating the wealth of data that has been accumulated over the past century and start building biologically accurate models of the brain from first principles to aid our understanding of brain function and dysfunction.

Modulation of synaptic transmission in neocortex by network activities

Neocortical neurons integrate inputs from thousands of presynaptic neurons that fire in vivo with frequencies that can reach 20 Hz. An important issue in understanding cortical integration is to determine the actual impact of presynaptic firing on postsynaptic neuron in the context of an active network. We used dual intracellular recordings from synaptically connected neurons or microstimulation to study the properties of spontaneous and evoked single-axon excitatory postsynaptic potentials (EPSPs) in vivo, in barbiturate or ketamine-xylazine anaesthetized cats. We found that active states of the cortical network were associated with higher variability and decrease in amplitude and duration of the EPSPs owing to a shunting effect. Moreover, the number of apparent failures markedly increased during active states as compared with silent states. Single-axon EPSPs in vivo showed mainly paired-pulse facilitation, and the paired-pulse ratio increased during active states as compare to silent states, suggesting a decrease in release probability during active states. Raising extracellular Ca(2+) concentration to 2.5-3.0 mm by reverse microdialysis reduced the number of apparent failures and significantly increased the mean amplitude of individual synaptic potentials. Quantitative analysis of spontaneous synaptic activity suggested that the proportion of presynaptic activity that impact at the soma of a cortical neuron in vivo was low because of a high failure rate, a shunting effect and probably dendritic filtering. We conclude that during active states of cortical network, the efficacy of synaptic transmission in individual synapses is low, thus safe transmission of information requires synchronized activity of a large population of presynaptic neurons.

Interneurons of the neocortical inhibitory system

Mammals adapt to a rapidly changing world because of the sophisticated cognitive functions that are supported by the neocortex. The neocortex, which forms almost 80% of the human brain, seems to have arisen from repeated duplication of a stereotypical microcircuit template with subtle specializations for different brain regions and species. The quest to unravel the blueprint of this template started more than a century ago and has revealed an immensely intricate design. The largest obstacle is the daunting variety of inhibitory interneurons that are found in the circuit. This review focuses on the organizing principles that govern the diversity of inhibitory interneurons and their circuits.

Electrophysiological evidence of monosynaptic excitatory transmission between granule cells after seizure-induced mossy fiber sprouting

Mossy fiber sprouting is a form of synaptic reorganization in the dentate gyrus that occurs in human temporal lobe epilepsy and animal models of epilepsy. The axons of dentate gyrus granule cells, called mossy fibers, develop collaterals that grow into an abnormal location, the inner third of the dentate gyrus molecular layer. Electron microscopy has shown that sprouted fibers from synapses on both spines and dendritic shafts in the inner molecular layer, which are likely to represent the dendrites of granule cells and inhibitory neurons. One of the controversies about this phenomenon is whether mossy fiber sprouting contributes to seizures by forming novel recurrent excitatory circuits among granule cells. To date, there is a great deal of indirect evidence that suggests this is the case, but there are also counterarguments. The purpose of this study was to determine whether functional monosynaptic connections exist between granule cells after mossy fiber sprouting. Using simultaneous recordings from granule cells, we obtained direct evidence that granule cells in epileptic rats have monosynaptic excitatory connections with other granule cells. Such connections were not obtained when age-matched, saline control rats were examined. The results suggest that indeed mossy fiber sprouting provides a substrate for monosynaptic recurrent excitation among granule cells in the dentate gyrus. Interestingly, the characteristics of the excitatory connections that were found indicate that the pathway is only weakly excitatory. These characteristics may contribute to the empirical observation that the sprouted dentate gyrus does not normally generate epileptiform discharges.

Psychostimulant-induced plasticity of intrinsic neuronal excitability in ventral subiculum

Psychostimulant drugs such as amphetamine are prescribed to increase vigilance, suppress appetite, and treat attention disorders, but they powerfully activate the dopamine system and have serious abuse potential. Repeated psychostimulant exposure induces neuronal plasticity within the mesolimbic dopamine system. Here we present evidence that repeated amphetamine exposure results in a suppression of intrinsic neuronal excitability in the ventral subiculum, a hippocampal region that activates dopamine neurotransmission. We used patch-clamp recordings from brain slices obtained at different times after withdrawal from repeated amphetamine exposure to determine the long-term effects of amphetamine on subicular excitability. Using several postsynaptic indices of sodium channel function, our results show that excitability is decreased for days, but not weeks, after repeated amphetamine exposure. The resulting increase in action potential threshold and decrease in postsynaptic amplification of excitatory synaptic input provide the first direct evidence that psychostimulants induce plasticity of hippocampal output and suggest one mechanism by which drug withdrawal may influence limbic dopamine-dependent learning and memory.

Excitatory Connections Between CA1 Pyramidal Cells Revealed by Spike Triggered Averaging in Slices of Rat Hippocampus are Partially NMDA Receptor Mediated

Spike triggered averaging was used to record local circuit connections between pairs of CA1 pyramidal neurons in isolated slices of rat hippocampus. Of 795 pairs of neurons tested, six were connected. These epsps were only partially blocked by 2-amino-5-phosphonovalerate (AP-5), which decreased the amplitude and half width of the epsp, but did not affect the early rising phase. In contrast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked all phases of the epsp and combinations of AP-5 and CNQX blocked the epsp almost entirely. These results indicate that these epsps were mediated by both N-methyl-d-aspartate (NMDA) and non-NMDA excitatory amino acid receptors. Moreover, they exhibited a voltage relation typical of neuronal responses to NMDA, increasing in amplitude and duration as the postsynaptic cell was depolarized. These epsps were brief (10 - 90% rise time < 5 ms, width at half amplitude < 20 ms), indicating a proximal location. Increasing presynaptic firing rate (1 - 4 spikes/s) reduced average epsp amplitude by almost 50%. When epsps were evoked by pairs of spikes (interval 3 - 25 ms), a large response to the first spike precluded a large response to the second. No evidence for selective enhancement of the NMDA receptor component by paired spike activation was found. It is concluded that a significant NMDA receptor mediated input to CA1 is provided by local circuit CA1 - CA1 connections and that these synapses can be demonstrated under control conditions.

Dynamic depolarization fields in the cerebral cortex

Recent physiological evidence shows that in response to stimuli and preceding motor activity, large fields of the upper layers of the cerebral cortex depolarize. It is argued that this finding is a general one and that these dynamic depolarization fields represent the computational elements of the cerebral cortex. Each depolarization field engages many more neurons than do columns and hyper-columns. These fields can be explained by cooperative neuronal computing in layers I-III of the cortex. In these layers, the computing modes might be general for all parts of the cerebral cortex and be sufficiently flexible to handle all sorts of cortical computations, including perception, memory storage, memory retrieval, thought and the production of behavior.

A novel mechanism of response selectivity of neurons in cat visual cortex

The spiking of cortical neurons critically depends on properties of the afferent stimuli. In the visual cortex, neurons respond selectively to the orientation and direction of movement of an object. The orientation and direction selectivity is improved upon transformation of the membrane potential changes into trains of action potentials. To address the question of whether the transformation of the membrane potential changes into spiking of a cell depends on the stimulus orientation and the direction of movement, we made intracellular recordings from the cat visual cortex in vivo during presentation of moving gratings of different orientations. We found that the relationship between the membrane polarization and the firing rate (input-output transfer function) depended on the stimulus orientation. The input-output transfer function was steepest during responses to the optimal stimulus; membrane depolarization of a given amplitude led to generation of more action potentials when evoked by an optimal stimulus than during non-optimal stimulation. The threshold for the action potential generation did not depend on stimulus orientation, and thus could not account for the observed difference in the transfer function. Oscillations of the membrane potential in the gamma-frequency range (25-70 Hz) were most pronounced during optimal stimulation and their strength changed in parallel with the changes in the transfer function, suggesting a possible relationship between the two parameters. We suggest that the improved input-output relationship of neurons during optimal stimulation represents a novel mechanism that may contribute to the final sharp orientation selectivity of spike responses in the cortical cells.

Effects of common excitatory and inhibitory inputs on motoneuron synchronization

We compared the effects of common excitatory and inhibitory inputs on motoneuron synchronization by simulating synaptic inputs with injected current transients. We elicited repetitive discharge in hypoglossal motoneurons recorded in slices of rat brain stem using a combination of a suprathreshold injected current step with superimposed noise to mimic the synaptic drive likely to occur during physiological activation. The effects of common inputs to motoneurons were simulated by the addition of a waveform composed of from 6 to 300 trains of current transients designed to mimic excitatory and/or inhibitory synaptic currents. We compared the discharge records obtained in several trials in which the same "common input" waveform was applied repeatedly in the presence of different background noise waveforms. The effects of the common input on motoneuron discharge probability and discharge rate were determined by compiling a cross-correlation histogram (CCHist) and a perispike frequencygram (PSFreq) between the discharges of the same cell at different times. Both excitatory and inhibitory common inputs induced synchronous discharge that was evident by a large central peak in the CCHist. The CCHists produced by common excitatory inputs were characterized by larger and narrower central peaks than those generated by common inhibitory inputs. The PSFreqs produced by common excitatory inputs indicated an increase in the discharge rate of motoneurons around time 0 that coincided with the narrow and large central peak in the CCHist. On the other hand, inhibitory inputs often generated very little, if any, change in the discharge rate around time 0 corresponding with the small and wide central peak in the CCHist. These results suggest that the CCHist indicates the effective strength of the net common input but not its sign. Although correlated changes in discharge rate are often quite different for net excitatory and inhibitory common input, except in some restricted conditions, the PSFreq analysis also cannot be used to unambiguously distinguish net excitation from net inhibition.

Spike transmission and synchrony detection in networks of GABAergic interneurons

The temporal pattern and relative timing of action potentials among neocortical neurons may carry important information. However, how cortical circuits detect or generate coherent activity remains unclear. Using paired recordings in rat neocortical slices, we found that the firing of fast-spiking cells can reflect the spiking pattern of single-axon pyramidal inputs. Moreover, this property allowed groups of fast-spiking cells interconnected by electrical and gamma-aminobutyric acid (GABA)-releasing (GABAergic) synapses to detect the relative timing of their excitatory inputs. These results indicate that networks of fast-spiking cells may play a role in the detection and promotion of synchronous activity within the neocortex.

NMDA receptor-mediated dendritic spikes and coincident signal amplification

Dendrites of cortical neurons possess active conductances, which contribute to the nonlinear processing of synaptic information. Recently it has been shown that basal dendrites can generate highly localized spikes mediated by NMDA receptor channels. These spikes may serve as a powerful mechanism to detect and amplify synchronously activated spatially clustered excitatory synaptic inputs in individual dendritic segments, and may enable parallel processing in several integrative dendritic subunits.

Impact of active dendrites and structural plasticity on the memory capacity of neural tissue

We consider the combined effects of active dendrites and structural plasticity on the storage capacity of neural tissue. We compare capacity for two different modes of dendritic integration: (1) linear, where synaptic inputs are summed across the entire dendritic arbor, and (2) nonlinear, where each dendritic compartment functions as a separately thresholded neuron-like summing unit. We calculate much larger storage capacities for cells with nonlinear subunits and show that this capacity is accessible to a structural learning rule that combines random synapse formation with activity-dependent stabilization/elimination. In a departure from the common view that memories are encoded in the overall connection strengths between neurons, our results suggest that long-term information storage in neural tissue could reside primarily in the selective addressing of synaptic contacts onto dendritic subunits.

Laminar differences in recurrent excitatory transmission in the rat entorhinal cortex in vitro

Paired intracellular recordings were used to investigate recurrent excitatory transmission in layers II, III and V of the rat entorhinal cortex in vitro. There was a relatively high probability of finding a recurrent connection between pairs of pyramidal neurons in both layer V (around 12%) and layer III (around 9%). In complete contrast, we have failed to find any recurrent synaptic connections between principal neurons in layer II, and this may be an important factor in the relative resistance of this layer in generating synchronized epileptiform activity. In general, recurrent excitatory postsynaptic potentials in layers III and V of the entorhinal cortex had similar properties to those recorded in other cortical areas, although the probabilities of connection are among the highest reported. Recurrent excitatory postsynaptic potentials recorded in layer V were smaller with faster rise times than those recorded in layer III. In both layers, the recurrent potentials were mediated by glutamate primarily acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazole receptors, although there appeared to be a slow component mediated by N-methyl-D-aspartate receptors. In layer III, recurrent transmission failed on about 30% of presynaptic action potentials evoked at 0.2Hz. This failure rate increased markedly with increasing (2, 3Hz) frequency of activation. In layer V the failure rate at low frequency was less (19%), and although it increased at higher frequencies this effect was less pronounced than in layer III. Finally, in layer III, there was evidence for a relatively high probability of electrical coupling between pyramidal neurons. We have previously suggested that layers IV/V of the entorhinal cortex readily generate synchronized epileptiform discharges, whereas layer II is relatively resistant to seizure generation. The present demonstration that recurrent excitatory connections are widespread in layer V but not layer II could support this proposal. The relatively high degree of recurrent connections and electrical coupling between layer III cells may be a factor in it's susceptibility to neurodegeneration during chronic epileptic conditions.

Inhibition-based rhythms: experimental and mathematical observations on network dynamics

An increasingly large body of data exists which demonstrates that oscillations of frequency 12-80 Hz are a consequence of, or are inextricably linked to, the behaviour of inhibitory interneurons in the central nervous system. This frequency range covers the EEG bands beta 1 (12-20 Hz), beta 2 (20-30 Hz) and gamma (30-80 Hz). The pharmacological profile of both spontaneous and sensory-evoked EEG potentials reveals a very strong influence on these rhythms by drugs which have direct effects on GABA(A) receptor-mediated synaptic transmission (general anaesthetics, sedative/hypnotics) or indirect effects on inhibitory neuronal function (opiates, ketamine). In addition, a number of experimental models of, in particular, gamma-frequency oscillations, have revealed both common denominators for oscillation generation and function, and subtle differences in network dynamics between the different frequency ranges. Powerful computer and mathematical modelling techniques based around both clinical and experimental observations have recently provided invaluable insight into the behaviour of large networks of interconnected neurons. In particular, the mechanistic profile of oscillations generated as an emergent property of such networks, and the mathematical derivation of this complex phenomenon have much to contribute to our understanding of how and why neurons oscillate. This review will provide the reader with a brief outline of the basic properties of inhibition-based oscillations in the CNS by combining research from laboratory models, large-scale neuronal network simulations, and mathematical analysis.

Synaptic transmission in the neocortex during reversible cooling

We studied the effects of reversible cooling on synaptic transmission in slices of rat visual cortex. Cooling had marked monotonic effects on the temporal properties of synaptic transmission. It increased the latency of excitatory postsynaptic potentials and prolonged their time-course. Effects were non-monotonic on other properties, such as amplitude of excitatory postsynaptic potentials and generation of spikes. The amplitude of excitatory postsynaptic potentials increased, decreased, or remain unchanged while cooling down to about 20 degrees C, but thereafter it declined gradually in all cells studied. The effect of moderate cooling on spike generation was increased excitability, most probably due to the ease with which a depolarized membrane potential could be brought to spike threshold by a sufficiently strong excitatory postsynaptic potential. Stimuli that were subthreshold above 30 degrees C could readily generate spikes at room temperature. Only at well below 10 degrees C could action potentials be completely suppressed. Paired-pulse facilitation was less at lower temperatures, indicating that synaptic dynamics are different at room temperature as compared with physiological temperatures. These results have important implications for extrapolating in vitro data obtained at room temperatures to higher temperatures. The data also emphasize that inactivation by cooling might be a useful tool for studying interactions between brain regions, but the data recorded within the cooled area do not allow reliable conclusions to be drawn about neural operations at normal temperatures.

A dendritic cable model for the amplification of synaptic potentials by an ensemble average of persistent sodium channels

The persistent sodium current density (I(NaP)) at the soma measured with the 'whole-cell' patch-clamp recording method is linearized about the resting state and used as a current source along the dendritic cable (depicting the spatial distribution of voltage-dependent persistent sodium ionic channels). This procedure allows time-dependent analytical solutions to be obtained for the membrane depolarization. Computer simulated response to a dendritic current injection in the form of synaptically-induced voltage change located at a distance from the recording site in a cable with unequally distributed persistent sodium ion channel densities per unit length of cable (the so-called 'hot-spots') is used to obtain conclusions on the density and distribution of persistent sodium ion channels. It is shown that the excitatory postsynaptic potentials (EPSPs) are amplified if hot-spots of persistent sodium ion channels are spatially distributed along the dendritic cable, with the local density of I(NaP) with respect to the recording site shown to specifically increase the peak amplitude of the EPSP for a proximally placed synaptic input, while the spatial distribution of I(NaP) serves to broaden the time course of the amplified EPSP. However, in the case of a distally positioned synaptic input, both local and nonlocal densities yield an approximately identical enhancement of EPSPs in contradiction to the computer simulations performed by Lipowsky et al. [J. Neurophysiol. 76 (1996) 2181]. The results indicate that persistent sodium channels produce EPSP amplification even when their distribution is relatively sparse (i.e. , approximately 1-2% of the transient sodium channels are found in dendrites of CA1 hippocampal pyramidal neurons). This gives a strong impetus for the use of the theory as a novel approach in the investigation of synaptic integration of signals in active dendrites represented as ionic cables.

Dynamic spike threshold reveals a mechanism for synaptic coincidence detection in cortical neurons in vivo

Cortical neurons are sensitive to the timing of their synaptic inputs. They can synchronize their firing on a millisecond time scale and follow rapid stimulus fluctuations with high temporal precision. These findings suggest that cortical neurons have an enhanced sensitivity to synchronous synaptic inputs that lead to rapid rates of depolarization. The voltage-gated currents underlying action potential generation may provide one mechanism to amplify rapid depolarizations. We have tested this hypothesis by analyzing the relations between membrane potential fluctuations and spike threshold in cat visual cortical neurons recorded intracellularly in vivo. We find that visual stimuli evoke broad variations in spike threshold that are caused in large part by an inverse relation between spike threshold and the rate of membrane depolarization preceding a spike. We also find that spike threshold is inversely related to the rate of rise of the action potential upstroke, suggesting that increases in spike threshold result from a decrease in the availability of Na(+) channels. By using a simple neuronal model, we show that voltage-gated Na(+) and K(+) conductances endow cortical neurons with an enhanced sensitivity to rapid depolarizations that arise from synchronous excitatory synaptic inputs. Thus, the basic mechanism responsible for action potential generation also enhances the sensitivity of cortical neurons to coincident synaptic inputs.

Differential modulatory effects of norepinephrine on synaptically driven responses of layer V barrel field cortical neurons

The effects of norepinephrine (NE) and the alpha-1 agonist phenylephrine (PE) on synaptically evoked responses of electrophysiologically identified pyramidal neurons in layer V of rat somatosensory cortex were studied in brain slices using intracellular recording techniques. When added to the bathing medium NE (10 microM) tended to increase the synaptic responsiveness of regular spiking neurons and decrease the responsiveness of intrinsic burst neurons. NE had mixed effects on layer V cells which were characterized as intermediate types between regular spiking and intrinsic burst neurons. PE exerted a similar spectrum of actions on layer V cortical neurons. For both adrenergic agents the greatest facilitating effect was observed on responses to low intensity synaptic stimulation. These results suggest that NE exerts different modulatory actions on different electrophysiologically-defined classes of layer V sensory cortical neurons.

Thalamic-evoked synaptic interactions in barrel cortex revealed by optical imaging

We used optical imaging of voltage-sensitive dye signals to study the spatiotemporal spread of activity in the mouse barrel cortex, evoked by stimulation of thalamocortical afferents in an in vitro slice preparation. Stimulation of the thalamus, at low current intensity, results in activity largely restricted to a single barrel, and to the border between layers Vb and VI. Low concentrations of the GABA(A) receptor antagonist bicuculline increase the amplitude of the optical signals, without affecting their spatiotemporal propagation. Higher concentrations of bicuculline result in paroxysmal activity, which propagates via intracolumnar and intercolumnar excitatory pathways. Enhancing the activity of NMDA receptors, by removing Mg(2+) from the extracellular solution, dramatically alters the spatiotemporal pattern of excitation: activity spreads to supragranular and infragranular layers and adjacent barrel columns. This enhanced propagation is suppressed by the NMDA receptor antagonist AP5. A similar enhancement of activity propagation can be produced by stimulating the thalamus with a short, high-frequency pulse train. Application of AP5 suppresses the frequency-dependent spread of activity. These findings indicate that the spatiotemporal spread of activity in the barrel cortex is altered by varying the temporal patterns of thalamic inputs, via an NMDA receptor-mediated mechanism, and suggest that a similar process occurs during repetitive whisking activity.

Decoding temporal information: A model based on short-term synaptic plasticity

In the current paper it is proposed that short-term plasticity and dynamic changes in the balance of excitatory-inhibitory interactions may underlie the decoding of temporal information, that is, the generation of temporally selective neurons. Our initial approach was to simulate excitatory-inhibitory disynaptic circuits. Such circuits were composed of a single excitatory and inhibitory neuron and incorporated short-term plasticity of EPSPs and IPSPs and slow IPSPs. We first showed that it is possible to tune cells to respond selectively to different intervals by changing the synaptic weights of different synapses in parallel. In other words, temporal tuning can rely on long-term changes in synaptic strength and does not require changes in the time constants of the temporal properties. When the units studied in disynaptic circuits were incorporated into a larger single-layer network, the units exhibited a broad range of temporal selectivity ranging from no interval tuning to interval-selective tuning. The variability in temporal tuning relied on the variability of synaptic strengths. The network as a whole contained a robust population code for a wide range of intervals. Importantly, the same network was able to discriminate simple temporal sequences. These results argue that neural circuits are intrinsically able to process temporal information on the time scale of tens to hundreds of milliseconds and that specialized mechanisms, such as delay lines or oscillators, may not be necessary.

Membrane properties and spike generation in rat visual cortical cells during reversible cooling

We studied the effects of reversible cooling between 35 and 7 C on membrane properties and spike generation of cells in slices of rat visual cortex. Cooling led to a depolarization of the neurones and an increase of the input resistance, thus bringing the cells closer to spiking threshold. Excitability, measured with intracellular current steps, increased with cooling. Synaptic stimuli were most efficient in producing spikes at room temperature, but strong stimulation could evoke spikes even below 10 C. Spike width and total area increased with cooling, and spike amplitude was maximal between 12 and 20 C. Repetitive firing was enhanced in some cells by cooling to 20-25 C, but was always suppressed at lower temperatures. With cooling, passive potassium conductance decreased and the voltage-gated potassium current had a higher activation threshold and lower amplitude. At the same time, neither passive sodium conductance nor the activation threshold of voltage-dependent sodium channels changed. Therefore changing the temperature modifies the ratio between potassium and sodium conductances, and thus alters basic membrane properties. Data from two cells recorded in slices of cat visual cortex suggest a similar temperature dependence of the membrane properties of neocortical neurones to that described above in the rat. These results provide a framework for comparison of the data recorded at different temperatures, but also show the limitations of extending the conclusions drawn from in vitro data obtained at room temperature to physiological temperatures. Further, when cooling is used as an inactivation tool in vivo, it should be taken into account that the mechanism of inactivation is a depolarization block. Only a region cooled below 10 C is reliably silenced, but it is always surrounded by a domain of hyperexcitable cells.

Increased pyramidal excitability and NMDA conductance can explain posttraumatic epileptogenesis without disinhibition: a model

Partially isolated cortical islands prepared in vivo become epileptogenic within weeks of the injury. In this model of chronic epileptogenesis, recordings from cortical slices cut through the injured area and maintained in vitro often show evoked, long- and variable-latency multiphasic epileptiform field potentials that also can occur spontaneously. These events are initiated in layer V and are synchronous with polyphasic long-duration excitatory and inhibitory potentials (currents) in neurons that may last several hundred milliseconds. Stimuli that are significantly above threshold for triggering these epileptiform events evoke only a single large excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP). We investigated the physiological basis of these events using simulations of a layer V network consisting of 500 compartmental model neurons, including 400 principal (excitatory) and 100 inhibitory cells. Epileptiform events occurred in response to a stimulus when sufficient N-methyl-D-aspartate (NMDA) conductance was activated by feedback excitatory activity among pyramidal cells. In control simulations, this activity was prevented by the rapid development of IPSPs. One manipulation that could give rise to epileptogenesis was an increase in the threshold of inhibitory interneurons. However, previous experimental data from layer V pyramidal neurons of these chronic epileptogenic lesions indicate: upregulation, rather than downregulation, of inhibition; alterations in the intrinsic properties of pyramidal cells that would tend to make them more excitable; and sprouting of their intracortical axons and increased numbers of presumed synaptic contacts, which would increase recurrent EPSPs from one cell onto another. Consistent with this, we found that increasing the excitability of pyramidal cells and the strength of NMDA conductances, in the face of either unaltered or increased inhibition, resulted in generation of epileptiform activity that had characteristics similar to those of the experimental data. Thus epileptogenesis such as occurs after chronic cortical injury can result from alterations of intrinsic membrane properties of pyramidal neurons together with enhanced NMDA synaptic conductances.

Anatomical and functional differentiation of glutamatergic synaptic innervation in the neocortex

Pyramidal neurons are the principal neurons of the neocortex and their excitatory impact on other pyramidal neurons and interneurons is central to neocortical dynamics. A fundamental principal that has emerged which governs pyramidal neuron excitation of other neurons in the local circuitry of neocortical columns is differential anatomical and physiological properties of the synaptic innervation via the same axon depending on the type of neuron targeted. In this study we derive anatomical principles for divergent innervation of pyramidal neurons of the same type within the local microcircuit. We also review data providing circumstantial and direct evidence for differential synaptic transmission via the same axon from neocortical pyramidal neurons and derive some principles for differential synaptic innervation of pyramidal neurons of the same type, of pyramidal neurons and interneurons and of different types of interneurons. We conclude that differential anatomical and physiological differentiation is a fundamental property of glutamatergic axons of pyramidal neurons in the neocortex.

Postsynaptic glutamate receptors and integrative properties of fast-spiking interneurons in the rat neocortex

The glutamate-mediated synaptic responses of neocortical pyramidal cell to fast-spiking interneuron (pyramidal-FS) connections were studied by performing paired recordings at 30-33 degrees C in acute slices of 14- to 35-day-old rats (n = 39). Postsynaptic fast-spiking (FS) cells were recorded in whole cell configuration with a patch pipette, and presynaptic pyramidal cells were impaled with sharp intracellular electrodes. At a holding potential of -72 mV (near the resting membrane potential), unitary excitatory postsynaptic potentials (EPSPs) had a mean amplitude of 2.1 +/- 1.3 mV and a mean width at half-amplitude of 10.5 +/- 3.7 ms (n = 18). Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist D(-)2-amino-5-phosphonovaleric acid (D-AP5) had minor effects on both the amplitude and the duration of unitary EPSPs, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) almost completely blocked the synaptic responses. In voltage-clamp mode, the selective antagonist of AMPA receptors 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3, 4-dihydro-5H-2,3-benzodiazepine (GYKI 53655; 40-66 microM) blocked 96 +/- 1.9% of D-AP5-insensitive unitary excitatory postsynaptic currents (EPSCs), confirming the predominance of AMPA receptors, as opposed to kainate receptors, at pyramidal-FS connections (n = 3). Unitary EPSCs mediated by AMPA receptors had fast rise times (0.29 +/- 0.04 ms) and amplitude-weighted decay time constants (2 +/- 0.8 ms; n = 16). In the presence of intracellular spermine, these currents showed the characteristic rectifying current-voltage (I-V) curve of calcium-permeable AMPA receptors. A slower component mediated by NMDA receptors was observed when unitary synaptic currents were recorded at a membrane potential more positive than -50 mV. In response to short trains of moderately high-frequency (67 Hz) presynaptic action potentials, we observed only a limited temporal summation of unitary EPSPs, probably because of the rapid kinetics of AMPA receptors and the absence of NMDA component in these subthreshold synaptic responses. By combining paired recordings with extracellular stimulations (n = 11), we demonstrated that EPSPs elicited by two different inputs were summed linearly by FS interneurons at membrane potentials below the action potential threshold. We estimated that, in our in vitro recording conditions, 8 +/- 5 pyramidal cells (n = 18) should be activated simultaneously to make FS interneurons fire an action potential from -72 mV. The low level of temporal summation and the linear summation of excitatory inputs in FS cells favor the role of coincidence detectors of these interneurons in neocortical circuits.

Somatic amplification of distally generated subthreshold EPSPs in rat hippocampal pyramidal neurones

1. Intracellular recordings from hippocampal CA1 pyramidal neurones revealed that EPSPs evoked by selective stimulation of the isolated afferent input to the distal third of the apical dendrites were relatively insensitive to changes in dendritic membrane potential (Vm) but amplified by depolarizations of the somatic Vm. The amplification was present at potentials depolarized from resting membrane potential (RMP) but was most marked when the EPSPs were close to threshold for action potential generation. The amplification consisted of a uniform component and a variable component which was only present when the EPSPs were threshold straddling. 2. The somatic amplification was caused by an intrinsic membrane current which was blocked by somatic application of tetrodotoxin (TTX, 10 microM), but was insensitive to bath application of NiCl2 (100-200 microM). We therefore suggest that the amplification of the subthreshold EPSP is due primarily to the activation of a non-inactivating Na+ current (INaP). 3. Injection of 4-aminopyridine (4-AP, 25-50 mM) during intradendritic recordings resulted in amplification of the EPSPs in 37% of the dendrites, which was similar to that observed in somatic recordings. However, in the one case in which somatic application of TTX was tested, dendritic amplification was blocked, suggesting that it is a reflection of the somatic amplification. 4. Because the shift to variable amplification was very abrupt and it is present in only a very narrow voltage range close to threshold, we suggest that the variable component is caused by the regenerative activation of INaP. The variability itself is probably due to the simultaneous activation of different outward K+ currents. 5. The present results indicate that the somatic region of CA1 pyramidal neurones can function as a voltage-dependent amplifier of distally evoked EPSPs and that this is due to the activation of a somatic INaP. The presence of this amplifying mechanism will have important functional consequences for the way in which distally generated EPSPs are integrated.

SI neuron response variability is stimulus tuned and NMDA receptor dependent

Skin brushing stimuli were used to evoke spike discharge activity in single skin mechanoreceptive afferents (sMRAs) and anterior parietal cortical (SI) neurons of anesthetized monkeys (Macaca fascicularis). In the initial experiments 10-50 presentations of each of 8 different stimulus velocities were delivered to the linear skin path from which maximal spike discharge activity could be evoked. Mean rate of spike firing evoked by each velocity (MFR) was computed for the time period during which spike discharge activity exceeded background, and an across-presentations estimate of mean firing rate (MFR) was generated for each velocity. The magnitude of the trial-by-trial variation in the response (estimated as CV; where CV = standard deviation in MFR/MFR) was determined for each unit at each velocity. MFR for both sMRAs and SI neurons (MFRsMRA and MFRSI, respectively) increased monotonically with velocity over the range 1-100 cm/s. At all velocities the average estimate of intertrial response variation for SI neurons (CVSI) was substantially larger than the corresponding average for sMRAs (CVsMRA). Whereas CVsMRA increased monotonically over the range 1-100 cm/s, CVSI decreased progressively with velocity over the range 1-10 cm/s, and then increased with velocity over the range 10-100 cm/s. The position of the skin brushing stimulus in the receptive field (RF) was varied in the second series of experiments. It was found that the magnitude of CVSI varied systematically with stimulus position in the RF: that is, CVSI was lowest for a particular velocity and direction of stimulus motion when the skin brushing stimulus traversed the RF center, and CVSI increased progressively as the distance between the stimulus path and the RF center increased. In the third series of experiments, either phencylidine (PCP; 100-500 microg/kg) or ketamine (KET; 0.5-7.5 mg/kg) was administered intravenously (iv) to assess the effect of block of N-methyl-D-aspartate (NMDA) receptors on SI neuron intertrial response variation. The effects of PCP on both CVSI and MFRSI were transient, typically with full recovery occurring in 1-2 h after drug injection. The effects of KET on CVSI and MFRSI were similar to those of PCP, but were shorter in duration (15-30 min). PCP and KET administration consistently was accompanied by a reduction of CVSI. The magnitude of the reduction of CVSI by PCP or KET was associated with the magnitude of CVSI before drug administration: that is, the larger the predrug CVSI, the larger the reduction in CVSI caused by PCP or KET. PCP and KET exerted variable effects on SI neuron mean firing rate that could differ greatly from one neuron to the next. The results are interpreted to indicate that SI neuron intertrial response variation is 1) stimulus tuned (intertrial response variation is lowest when the skin stimulus moves at 10 cm/s and traverses the neuron's RF center) and 2) NMDA receptor dependent (intertrial response variation is least when NMDA receptor activity contributes minimally to the response, and increases as the contribution of NMDA receptors to the response increases).

Linear summation of excitatory inputs by CA1 pyramidal neurons

A fundamental problem in neurobiology is understanding the arithmetic that dendrites use to integrate inputs. The impact of dendritic morphology and active conductances on input summation is still unknown. To study this, we use glutamate iontophoresis and synaptic stimulation to position pairs of excitatory inputs throughout the apical, oblique, and basal dendrites of CA1 pyramidal neurons in rat hippocampal slices. Under a variety of stimulation regimes, we find a linear summation of most input combinations that is implemented by a surprising balance of boosting and shunting mechanisms. Active conductances in dendrites paradoxically serve to make summation linear. This "active linearity" can reconcile predictions from cable theory with the observed linear summation in vivo and suggests that a simple arithmetic is used by apparently complex dendritic trees.

Voltage-activated sodium channels amplify inhibition in neocortical pyramidal neurons

Inhibitory postsynaptic potentials (IPSPs) in neocortical pyramidal neurons are increased in duration and amplitude at depolarized membrane potentials. This effect was not due to changes in the time course of the underlying synaptic current. The role of postsynaptic voltage-activated channels was investigated by mimicking the voltage change that occurs during an IPSP with current injections. The peak and integral of these 'simulated' IPSPs increased during depolarization of the membrane potential in a tetrodotoxin-sensitive manner. This amplification presumably occurs as the hyperpolarization associated with IPSPs turns off sodium channels that are tonically active at depolarized membrane potentials. IPSP amplification increased the ability of IPSPs to inhibit action potential firing and promoted IPSP-induced action potential synchronization.

Voltage-dependent properties of dendrites that eliminate location-dependent variability of synaptic input

We examined the hypothesis that voltage-dependent properties of dendrites allow for the accurate transfer of synaptic information to the soma independent of synapse location. This hypothesis is motivated by experimental evidence that dendrites contain a complex array of voltage-gated channels. How these channels affect synaptic integration is unknown. One hypothesized role for dendritic voltage-gated channels is to counteract passive cable properties, rendering all synapses electrotonically equidistant from the soma. With dendrites modeled as passive cables, the effect a synapse exerts at the soma depends on dendritic location (referred to as location-dependent variability of the synaptic input). In this theoretical study we used a simplified three-compartment model of a neuron to determine the dendritic voltage-dependent properties required for accurate transfer of synaptic information to the soma independent of synapse location. A dendrite that eliminates location-dependent variability requires three components: 1) a steady-state, voltage-dependent inward current that together with the passive leak current provides a net outward current and a zero slope conductance at depolarized potentials, 2) a fast, transient, inward current that compensates for dendritic membrane capacitance, and 3) both alpha amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid- and N-methyl-D-aspartate-like synaptic conductances that together permit synapses to behave as ideal current sources. These components are consistent with the known properties of dendrites. In addition, these results indicate that a dendrite designed to eliminate location-dependent variability also actively back-propagates somatic action potentials.

Active summation of excitatory postsynaptic potentials in hippocampal CA3 pyramidal neurons

The manner in which the thousands of synaptic inputs received by a pyramidal neuron are summed is critical both to our understanding of the computations that may be performed by single neurons and of the codes used by neurons to transmit information. Recent work on pyramidal cell dendrites has shown that subthreshold synaptic inputs are modulated by voltage-dependent channels, raising the possibility that summation of synaptic responses is influenced by the active properties of dendrites. Here, we use somatic and dendritic whole-cell recordings to show that pyramidal cells in hippocampal area CA3 sum distal and proximal excitatory postsynaptic potentials sublinearly and actively, that the degree of nonlinearity depends on the magnitude and timing of the excitatory postsynaptic potentials, and that blockade of transient potassium channels linearizes summation. Nonlinear summation of synaptic inputs could have important implications for the computations performed by single neurons and also for the role of the mossy fiber and perforant path inputs to hippocampal area CA3.

Postsynaptic pyramidal target selection by descending layer III pyramidal axons: dual intracellular recordings and biocytin filling in slices of rat neocortex

Paired intracellular recordings in slices of adult rat neocortex with biocytin filling of synaptically connected neurons were used to investigate the pyramidal targets, in layer V, of layer III pyramidal axons. The time-course and sensitivity of excitatory postsynaptic potentials to current injected at the soma, and locations of close appositions between presynaptic axons and postsynaptic dendrites, indicated that the majority of contributory synapses were located in layer V. Within a "column" of tissue, radius < or = 250 microm, the probability that a randomly selected layer III pyramid innervated a layer V pyramid was 1 in 4 if the target cell was a burst firing pyramid with an apical dendritic tuft in layers II/I. If, however, the potential target was a regular spiking pyramid, the probability of connectivity was only 1 in 40, and none of the 13 anatomically identified postsynaptic layer V targets had a slender apical dendrite terminating in layers IV/III. Morphological reconstructions indicated that layer III pyramids select target layer V cells whose apical dendrites pass within 50-100 microm of the soma of the presynaptic pyramid in layer III and which have overlapping apical dendritic tufts in the superficial layers. The probability that a layer V cell would innervate a layer III pyramid lying within 250 microm of its apical dendrite was much lower (one in 58). Both presynaptic layer III pyramids and their large postsynaptic layer V targets could therefore access similar inputs in layers I/II, while small layer V pyramids could not. One prediction from the present data would be that neither descending layer V inputs to the striatum or thalamus, nor transcallosal connections would be readily activated by longer distance cortico-cortical "feedback" connections that terminated in layers I/II. These could, however, activate corticofugal pathways to the superior colliculus or pons, both directly and via layer III.

Role of NMDA receptors in the propagation of excitation in rat visual cortex as studied by optical imaging

To examine the role of the N-methyl-D-aspartate (NMDA) type of glutamate receptors in the propagation of information in visual cortex, optical imaging with high spatial and temporal resolution of neuronal activity was used in cortical slices of rats. Single-shock stimulation of the white matter elicited a vertical propagation of excitation toward the cortical surface simultaneously with a horizontal spread of excitation in lower layers. The horizontal spread in upper layers occurred subsequent to the vertical spread reaching these layers. The results from perfusion of Ca2+-free medium and application of an antagonist of non-NMDA receptors indicated that this intracortical propagation of signals is due mostly, if not exclusively, to the postsynaptic excitation of cortical neurons. Blockade of NMDA receptors attenuated the rising and peak phases of the upper horizontal spread, but did not affect those of the lower horizontal or vertical propagation of excitation. Perfusion with Mg2+-free solution enhanced the upper horizontal spread, but in most cases did not significantly change the spread of excitation in the other pathways. These results indicate that NMDA receptors are involved in the flow of information in the upper layers of visual cortex, and further suggest that this propagation of activity is mediated mainly by horizontal connections intrinsic to the upper layers.

Input summation by cultured pyramidal neurons is linear and position-independent

The role of dendritic morphology in integration and processing of neuronal inputs is still unknown. Models based on passive cable theory suggest that dendrites serve to isolate synapses from one another. Because of decreases in driving force or resistance, two inputs onto the same dendrite would diminish their joint effect, resulting in sublinear summation. When on different dendrites, however, inputs would not interact and therefore would sum linearly. These predictions have not been rigorously tested experimentally. In addition, recent results indicate that dendrites have voltage-sensitive conductances and are not passive cables. To investigate input integration, we characterized the effects of dendritic morphology on the summation of subthreshold excitatory inputs on cultured hippocampal neurons with pyramidal morphologies. We used microiontophoresis of glutamate to systematically position inputs throughout the dendritic tree and tested the summation of two inputs by measuring their individual and joint effects. We find that summation was surprisingly linear regardless of input position. For small inputs, this linearity arose because no significant shunts or changes in driving force occurred and no voltage-dependent channels were opened. Larger inputs also added linearly, but this linearity was caused by balanced action of NMDA and IA potassium conductances. Therefore, active conductances can maintain, paradoxically, a linear input arithmetic. Furthermore, dendritic morphology does not interfere with this linearity, which may be essential for particular neuronal computations.