Arnd Roth

Physiology and anatomy of synaptic connections between thick tufted pyramidal neurones in the developing rat neocortex.

1. Dual voltage recordings were made from pairs of adjacent, synaptically connected thick tufted layer 5 pyramidal neurones in brain slices of young rat (14‐16 days) somatosensory cortex to examine the physiological properties of unitary EPSPs. Pre‐ and postsynaptic neurones were filled with biocytin and examined in the light and electron microscope to quantify the morphology of axonal and dendritic arbors and the number and location of synaptic contacts on the target neurone. 2. In 138 synaptic connections between pairs of pyramidal neurones 96 (70%) were unidirectional and 42 (30%) were bidirectional. The probability of finding a synaptic connection in dual recordings was 0.1. Unitary EPSPs evoked by a single presynaptic action potential (AP) had a mean peak amplitude ranging from 0.15 to 5.5 mV in different connections with a mean of 1.3 +/‐ 1.1 mV, a latency of 1.7 +/‐ 0.9 ms, a 20‐80% rise time of 2.9 +/‐ 2.3 ms and a decay time constant of 40 +/‐ 18 ms at 32‐24 degrees C and ‐60 +/‐ 2 mV membrane potential. 3. Peak amplitudes of unitary EPSPs fluctuated randomly from trial to trial. The coefficient of variation (c.v.) of the unitary EPSP amplitudes ranged from 0.13 to 2.8 in different synaptic connections (mean, 0.52; median, 0.41). The percentage of failures of single APs to evoke a unitary EPSP ranged from 0 to 73% (mean, 14%; median, 7%). Both c.v. and percentage of failures decreased with increasing mean EPSP amplitude. 4. Postsynaptic glutamate receptors which mediate unitary EPSPs at ‐60 mV were predominantly of the L‐alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA) receptor type. Receptors of the N‐methyl‐D‐aspartate (NMDA) type contributed only a small fraction (< 20%) to the voltage‐time integral of the unitary EPSP at ‐60 mV, but their contribution increased at more positive membrane potentials. 5. Branching patterns of dendrites and axon collaterals of forty‐five synaptically connected neurones, when examined in the light microscope, indicated that the axonal and dendritic anatomy of both projecting and target neurones and of uni‐ and bidirectionally connected neurones was uniform. 6. The number of potential synaptic contacts formed by a presynaptic neurone on a target neurone varied between four and eight (mean, 5.5 +/‐ 1.1 contacts; n = 19 connections). Synaptic contacts were preferentially located on basal dendrites (63%, 82 +/‐ 35 microns from the soma, n = 67) and apical oblique dendrites (27%, 145 +/‐ 59 microns, n = 29), and 35% of all contacts were located on tertiary basal dendritic branches. The mean geometric distances (from the soma) of the contacts of a connection varied between 80 and 585 microns (mean, 147 microns; median, 105 microns). The correlation between EPSP amplitude and the number of morphologically determined synaptic contacts or the mean geometric distances from the soma was only weak (correlation coefficients were 0.2 and 0.26, respectively). 7. Compartmental models constructed from camera lucida drawings of eight target neurones showed that synaptic contacts were located at mean electrotonic distances between 0.07 and 0.33 from the soma (mean, 0.13). Simulations of unitary EPSPs, assuming quantal conductance changes with fast rise time and short duration, indicated that amplitudes of quantal EPSPs at the soma were attenuated, on average, to < 10% of dendritic EPSPs and varied in amplitude up to 10‐fold depending on the dendritic location of synaptic contacts. The inferred quantal peak conductance increase varied between 1.5 and 5.5 nS (mean, 3 nS). 8. The combined physiological and morphological measurements in conjunction with EPSP simulations indicated that the 20‐fold range in efficacy of the synaptic connections between thick tufted pyramidal neurones, which have their synaptic contacts preferentially located on basal and apical oblique dendrites, was due to differences in transmitter release probability of the projecting neurones and, to a lesser extent, to differenc

Dendritic excitability and synaptic plasticity

Most synaptic inputs are made onto the dendritic tree. Recent work has shown that dendrites play an active role in transforming synaptic input into neuronal output and in defining the relationships between active synapses. In this review, we discuss how these dendritic properties influence the rules governing the induction of synaptic plasticity. We argue that the location of synapses in the dendritic tree, and the type of dendritic excitability associated with each synapse, play decisive roles in determining the plastic properties of that synapse. Furthermore, since the electrical properties of the dendritic tree are not static, but can be altered by neuromodulators and by synaptic activity itself, we discuss how learning rules may be dynamically shaped by tuning dendritic function. We conclude by describing how this reciprocal relationship between plasticity of dendritic excitability and synaptic plasticity has changed our view of information processing and memory storage in neuronal networks.

Submillisecond AMPA Receptor-Mediated Signaling at a Principal Neuron–Interneuron Synapse

Glutamatergic transmission at a principal neuron-interneuron synapse was investigated by dual whole-cell patch-clamp recording in rat hippocampal slices combined with morphological analysis. Evoked EPSPs with rapid time course (half duration = 4 ms; 34 degrees C) were generated at multiple synaptic contacts established on the interneuron dendrites close to the soma. The underlying postsynaptic conductance change showed a submillisecond rise and decay, due to the precise timing of glutamate release and the rapid deactivation of the postsynaptic AMPA receptors. Simulations based on a compartmental model of the interneuron indicated that the rapid postsynaptic conductance change determines the shape and the somatodendritic integration of EPSPs, thus enabling interneurons to detect synchronous principal neuron activity.

Energy-Efficient Action Potentials in Hippocampal Mossy Fibers

Minimizing Brain Energy Consumption How much energy is actually required to generate neuronal activity and information processing? By combining direct recordings at physiological temperatures from mossy fiber axons in rat brain slices with modeling and simulation approaches, Alle et al. (p. 1405; see the Perspective by Magistretti) found that the regenerative action potentials in nonmyelinated axons of mammalian hippocampus are remarkably energy efficient. The data indicate a surprisingly minor contribution of action potentials to the entire energy expenditure of neural information processing. Mammalian neurons have developed highly efficient ways to limit energy consumption while propagating neuronal information. Action potentials in nonmyelinated axons are considered to contribute substantially to activity-dependent brain metabolism. Here we show that fast Na+ current decay and delayed K+ current onset during action potentials in nonmyelinated mossy fibers of the rat hippocampus minimize the overlap of their respective ion fluxes. This results in total Na+ influx and associated energy demand per action potential of only 1.3 times the theoretical minimum, in contrast to the factor of 4 used in previous energy budget calculations for neural activity. Analysis of ionic conductance parameters revealed that the properties of Na+ and K+ channels are matched to make axonal action potentials energy-efficient, minimizing their contribution to activity-dependent metabolism.

Dynamic Receptive Fields of Reconstructed Pyramidal Cells in Layers 3 and 2 of Rat Somatosensory Barrel Cortex

Whole‐cell voltage recordings were made in vivo from subsequently reconstructed pyramidal neurons (n= 30) in layer 3 (L3) and layer 2 (L2) of the barrel cortex of urethane‐anaesthetised rats. Average resting membrane potentials were well below (15−40 mV) action potential (AP) initiation threshold. The average spontaneous AP activity (0.068 ± 0.22 APs s−1) was low. Principal whisker (PW) deflections evoked postsynaptic potentials (PSPs) in almost all cells of a PW column but evoked AP activity (0.031 ± 0.056 APs per PW stimulus 6 deg deflection) was low indicating ‘sparse’ coding by APs. Barrel‐related cells (n= 16) have their soma located above a barrel and project their main axon through the barrel whereas septum‐related cells (n= 8) are located above and project their main axon through the septum between barrels. Both classes of cell had broad subthreshold receptive fields (RFs) which comprised a PW and several (> 8) surround whiskers (SuW). Barrel‐related cells had shorter PSP onset latencies (9.6 ± 4.6 ms) and larger amplitude PW stimulus responses (9.1 ± 4.5 mV) than septum‐related cells (23.3 ± 16.5 ms and 5.0 ± 2.8 mV, respectively). The dendritic fields of barrel‐related cells were restricted, in the horizontal plane, to the PW column width. Their axonal arbors projected horizontally into several SuW columns, preferentially those representing whiskers of the same row, suggesting that they are the major anatomical substrate for the broad subthreshold RFs. In barrel‐related cells the response time course varied with whisker position and subthreshold RFs were highly dynamic, expanding in size from narrow single‐whisker to broad multi‐whisker RFs, elongated along rows within 10‐150 ms following a deflection. The response time course in septum‐related cells was much longer and almost independent of whisker position. Their broad subthreshold RF suggests that L2/3 cells integrate PSPs from several barrel columns. We conclude that the lemniscal (barrel‐related) and paralemniscal (septum‐related) afferent inputs remain anatomically and functionally segregated in L2/3.

Sensitivity to perturbations in vivo implies high noise and suggests rate coding in cortex.

It is well known that neural activity exhibits variability, in the sense that identical sensory stimuli produce different responses, but it has been difficult to determine what this variability means. Is it noise, or does it carry important information—about, for example, the internal state of the organism? Here we address this issue from the bottom up, by asking whether small perturbations to activity in cortical networks are amplified. Based on in vivo whole-cell patch-clamp recordings in rat barrel cortex, we find that a perturbation consisting of a single extra spike in one neuron produces approximately 28 additional spikes in its postsynaptic targets. We also show, using simultaneous intra- and extracellular recordings, that a single spike in a neuron produces a detectable increase in firing rate in the local network. Theoretical analysis indicates that this amplification leads to intrinsic, stimulus-independent variations in membrane potential of the order of ±2.2–4.5 mV—variations that are pure noise, and so carry no information at all. Therefore, for the brain to perform reliable computations, it must either use a rate code, or generate very large, fast depolarizing events, such as those proposed by the theory of synfire chains. However, in our in vivo recordings, we found that such events were very rare. Our findings are thus consistent with the idea that cortex is likely to use primarily a rate code.

Compartmental models of rat cerebellar Purkinje cells based on simultaneous somatic and dendritic patch‐clamp recordings

1 Simultaneous dendritic and somatic patch‐clamp recordings were made from Purkinje cells in cerebellar slices from 12‐ to 21‐day‐old rats. Voltage responses to current impulses injected via either the dendritic or the somatic pipette were obtained in the presence of the selective Ih blocker ZD 7288 and blockers of spontaneous synaptic input. Neurons were filled with biocytin for subsequent morphological reconstruction. 2 Four neurons were reconstructed and converted into detailed compartmental models. The specific membrane capacitance (Cm), specific membrane resistance (Rm) and intracellular resistivity (Ri) were optimized by direct fitting of the model responses to the electrophysiological data from the same cell. Mean values were: Cm, 0.77 ± 0.17 μF cm−2 (mean ±s.d.; range, 0.64‐1.00 μF cm−2), Rm, 122 ± 18 kΩ cm2 (98‐141 kΩ cm2) and Ri, 115 ± 20 Ω cm (93‐142 Ω cm). 3 The steady‐state electrotonic architecture of these cells was compact under the experimental conditions used. However, somatic voltage‐clamp recordings of parallel fibre and climbing fibre synaptic currents were substantially filtered and attenuated. 4 The detailed models were compared with a two‐compartment model of Purkinje cells. The range of synaptic current kinetics that can be faithfully recorded using somatic voltage clamp is predicted fairly well by the two‐compartment model, even though some of its underlying assumptions are violated. 5 A model of Ih was constructed based on voltage‐clamp data, and inserted into the passive compartmental models. Somatic EPSP amplitude was substantially attenuated compared to the amplitude of dendritic EPSPs at their site of generation. However, synaptic efficacy of the same quantal synaptic conductance, as measured by the somatic EPSP amplitude, was only weakly dependent on synaptic location on spiny branchlets. 6 The passive electrotonic structure of Purkinje cells is unusual in that the steady‐state architecture is very compact, while voltage transients such as synaptic potentials and action potentials are heavily filtered.

Dendritic and somatic glutamate receptor channels in rat cerebellar Purkinje cells

1 The properties of glutamate receptor (GluR) channels in outside‐out patches from the dendrites and somata of rat cerebellar Purkinje cells in brain slices were studied using fast agonist application techniques. Dendritic patches were isolated 40–130 μm from the soma. 2 Outside‐out patches from both dendrites and somata of Purkinje cells responded to application of glutamate with a current which desensitized rapidly and nearly completely. Currents evoked by glutamate application were blocked by 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX), were mimicked by l‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA), and were modulated by cyclothiazide. Kainate produced small, non–desensitizing currents. No currents were observed in response to aspartate application. Responses characteristic of NMDA receptor activation were not observed. These findings indicate that glutamate‐activated currents were mediated by the AMPA subtype of GluR. 3 Deactivation of the GluR channels following 1 ms pulses of glutamate occurred with a time constant of 1.23 ± 0.07 ms in dendritic and 1.12 ± 0.04 ms in somatic patches. Desensitization occurred with a time constant of 5.37 ± 0.26 ms in dendritic and 5.29 ± 0.29 ms in somatic patches. The time constant of recovery from desensitization caused by a 1 ms application of 1 mm glutamate was 36 ms in dendritic patches and 33 ms in somatic patches. 4 Half‐maximal activation of the GluR channels was achieved at a glutamate concentration of 432 μm. Deactivation kinetics were not dependent on the glutamate concentration, while desensitization became slower at lower glutamate concentrations. 5 Pre‐equilibration of patches with low concentrations of glutamate reduced the peak current activated by 1 mm glutamate. The IC50 for this effect was 8.7 μm. Equilibrium desensitization did not affect the kinetics of the current activated by 1 mm glutamate. 6 The current–voltage relationship of the peak current was linear in normal Na+‐rich external solution, with a reversal potential near 0 mV. In Ca2+‐rich external solution, the reversal potentials were −51.4 ± 2.9 and −51.5 ± 2.8 mV for dendritic and somatic patches, respectively, indicating that these glutamate channels have a low permeability to Ca2+ (PCa/PCs= 0.053). 7 The mean single‐channel conductance of the GluR channels measured using non–stationary fluctuation analysis was ∼8 pS in dendritic and somatic patches, and the maximum open probability was at least 0.7 with 5 mm glutamate. 8 GluR channel kinetics in patches excised from the soma of neonatal (postnatal day 4; P4) Purkinje cells, before the development of the dendritic arborization of the Purkinje cell, were similar to those in patches excised from more mature (P12–18) Purkinje cells. 9 Dendritic and somatic GluR channels in Purkinje cells appear to be functionally identical, are AMPA‐subtype receptors containing the GluR‐B subunit, and have rapid kinetics and low permeability to Ca2+. A kinetic model was constructed which faithfully reproduces the gating characteristics of the GluR channels.

Determinants of action potential propagation in cerebellar Purkinje cell axons

Axons have traditionally been viewed as highly faithful transmitters of action potentials. Recently, however, experimental evidence has accumulated to support the idea that under some circumstances axonal propagation may fail. Cerebellar Purkinje neurons fire highfrequency simple spikes, as well as bursts of spikes in response to climbing fiber activation (the “complex spike”). Here we have visualized the axon of individual Purkinje cells to directly investigate the relationship between somatic spikes and axonal spikes using simultaneous somatic whole-cell and cell-attached axonal patch-clamp recordings at 200-800 μm from the soma. We demonstrate that sodium action potentials propagate at frequencies up to ∼260 Hz, higher than simple spike rates normally observed in vivo. Complex spikes, however, did not propagate reliably, with usually only the first and last spikes in the complex spike waveform being propagated. On average, only 1.7 ± 0.2 spikes in the complex spike were propagated during resting firing, with propagation limited to interspike intervals above ∼4 msec. Hyperpolarization improved propagation efficacy without affecting total axonal spike number, whereas strong depolarization could abolish propagation of the complex spike. These findings indicate that the complex spike waveform is not faithfully transmitted to downstream synapses and that propagation of the climbing fiber response may be modulated by background activity.

Enhanced Role of Transition Metal Ion Catalysis During In-Cloud Oxidation of SO2

Dust in the Clouds Sulfate aerosols have the greatest radiative impact on climate systems. Harris et al. (p. 727) report that the oxidation of sulfur dioxide gas, catalyzed by natural transition metal ions mostly on the surface of coarse mineral dust, is the dominant pathway for sulfate production in clouds. In view of the growing sulfur dioxide emissions from large, industrializing countries, including this process in climate models should improve the agreement between models and observations. Transition metal ions catalyze most of the oxidation of sulfur dioxide that occurs in clouds. Global sulfate production plays a key role in aerosol radiative forcing; more than half of this production occurs in clouds. We found that sulfur dioxide oxidation catalyzed by natural transition metal ions is the dominant in-cloud oxidation pathway. The pathway was observed to occur primarily on coarse mineral dust, so the sulfate produced will have a short lifetime and little direct or indirect climatic effect. Taking this into account will lead to large changes in estimates of the magnitude and spatial distribution of aerosol forcing. Therefore, this oxidation pathway—which is currently included in only one of the 12 major global climate models—will have a significant impact on assessments of current and future climate.