Analysis of Claims that the Brain Extracellular Impedance Is High and Non-resistive.

Numerous measurements in the brain of the impedance between two extracellular electrodes have shown that it is approximately resistive in the range of biological interest, <10 kHz, and has a value close to that expected from the conductivity of physiological saline and the extracellular volume fraction in brain tissue. Recent work from Gomes et al. has claimed that the impedance of the extracellular space is some three orders of magnitude greater than these values and also displays a 1/f frequency dependence (above a low-frequency corner frequency). Their measurements were performed between an intracellular electrode and an extracellular electrode. It is argued that they incorrectly extracted the extracellular impedance because of an inaccurate representation of the large confounding impedance of the neuronal membrane. In conclusion, no compelling evidence has been provided to undermine the well-established and physically plausible consensus that the brain extracellular impedance is low and approximately resistive.

Neuronal morphology generates high-frequency firing resonance.

The attenuation of neuronal voltage responses to high-frequency current inputs by the membrane capacitance is believed to limit single-cell bandwidth. However, neuronal populations subject to stochastic fluctuations can follow inputs beyond this limit. We investigated this apparent paradox theoretically and experimentally using Purkinje cells in the cerebellum, a motor structure that benefits from rapid information transfer. We analyzed the modulation of firing in response to the somatic injection of sinusoidal currents. Computational modeling suggested that, instead of decreasing with frequency, modulation amplitude can increase up to high frequencies because of cellular morphology. Electrophysiological measurements in adult rat slices confirmed this prediction and displayed a marked resonance at 200 Hz. We elucidated the underlying mechanism, showing that the two-compartment morphology of the Purkinje cell, interacting with a simple spiking mechanism and dendritic fluctuations, is sufficient to create high-frequency signal amplification. This mechanism, which we term morphology-induced resonance, is selective for somatic inputs, which in the Purkinje cell are exclusively inhibitory. The resonance sensitizes Purkinje cells in the frequency range of population oscillations observed in vivo.

Contribution of postsynaptic T-type calcium channels to parallel fibre-Purkinje cell synaptic responses.

KEY POINTS: At the parallel fibre-Purkinje cell glutamatergic synapse, little or no Ca(2+) entry takes place through postsynaptic neurotransmitter receptors, although postsynaptic calcium increases are clearly involved in the synaptic plasticity. Postsynaptic voltage-gated Ca(2+) channels therefore constitute the sole rapid postsynaptic Ca(2+) signalling mechanism, making it essential to understand how they contribute to the synaptic signalling. Using a selective T-type calcium channel antagonist, we describe a T-type component of the EPSC that is activated by the AMPA receptor-mediated depolarization of the spine and thus will contribute to the local calcium dynamics. This component can amount up to 20% of the EPSC, and this fraction is maintained even at the high frequencies sometimes encountered in sensory processing. Modelling based on our biophysical characterization of T-type calcium channels in Purkinje cells suggests that the brief spine EPSCs cause the activated T-type channels to deactivate rather than inactivate, enabling repetitive activation. ABSTRACT: In the cerebellum, sensory information is conveyed to Purkinje cells (PC) via the granule cell/parallel fibre (PF) pathway. Plasticity at the PF-PC synapse is considered to be a mechanism of information storage in motor learning. The induction of synaptic plasticity in the cerebellum and elsewhere usually involves intracellular Ca(2+) signals. Unusually, postsynaptic Ca(2+) signalling in PF-PC spines does not involve ionotropic glutamatergic receptors because postsynaptic NMDA receptors are absent and the AMPA receptors are Ca(2+) -impermeable; postsynaptic voltage-gated Ca(2+) channels therefore constitute the sole rapid Ca(2+) signalling mechanism. Low-threshold activated T-type calcium channels are present at the synapse, although their contribution to PF-PC synaptic responses is unknown. Taking advantage of 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide, a selective T-type channel antagonist, we show in the mouse that inhibition of these channels reduces PF-PC excitatory postsynaptic currents and excitatory postsynaptic potentials by 15-20%. This contribution was preserved during sparse input and repetitive activity. We characterized the biophysical properties of native T-type channels in young animals and modelled their activation during simulated dendritic excitatory postsynaptic potential waveforms. The comparison of modelled and observed synaptic responses suggests that T-type channels only activate in spines that are strongly depolarized by their synaptic input, a process requiring a high spine neck resistance. This brief and local activation ensures that T-type channels rapidly deactivate, thereby limiting inactivation during repetitive synaptic activity. T-type channels are therefore ideally situated to provide synaptic Ca(2+) entry at PF-PC spines.

Adaptation of granule cell to Purkinje cell synapses to high-frequency transmission.

The mossy fiber (MF)-granule cell (GC) pathway conveys multiple modalities of information to the cerebellar cortex, converging on Purkinje cells (PC), the sole output of the cerebellar cortex. Recent in vivo experiments have shown that activity in GCs varies from tonic firing at a few hertz to phasic bursts >500 Hz. However, the responses of parallel fiber (PF)-PC synapses to this wide range of input frequencies are unknown, and there is controversy regarding several frequency-related parameters of transmission at this synapse. We performed recordings of unitary synapses and combined variance-mean analysis with a carefully adapted extracellular stimulation method in young and adult rats. We show that, although the probability of release at individual sites is low at physiological calcium concentration, PF-PC synapses release one or more vesicles with a probability of 0.44 at 1.5 mm [Ca(2+)](e). Paired-pulse facilitation was observed over a wide range of frequencies; it renders burst inputs particularly effective and reproducible. These properties are primarily independent of synaptic weight and age. Furthermore, we show that the PF-PC synapse is able to sustain transmission at very high frequencies for tens of stimuli, as a result of accelerated vesicle replenishment and an apparent recruitment of release site vesicles, which appears to be a central mechanism of paired-pulse facilitation at this synapse. These properties ensure that PF-PC synapses possess a dynamic range enabling the temporal code of MF inputs to be transmitted reliably to the PC.

Direct whole-cell patch-clamp recordings from small boutons in rodent primary neocortical neuron cultures.

Direct electrical recordings from conventional boutons in the mammalian central nervous system have proven challenging due to their small size. Here, we provide a protocol for direct whole-cell patch-clamp recordings from small presynaptic boutons of primary dissociated cultured neurons of the rodent neocortex. We describe steps to prepare primary neocortical cultures and recording pipettes, followed by identifying boutons and establishing a whole-cell bouton recording. We then provide details on precise pipette capacitance compensation required for high-resolution current-clamp recordings from boutons. For further details on the use and execution of this protocol, please refer to Ritzau-Jost et al.1.

Presynaptic NR2A-containing NMDA receptors implement a high-pass filter synaptic plasticity rule.

The detailed characterization of synaptic plasticity has led to the replacement of simple Hebbian rules by more complex rules depending on the order of presynaptic and postsynaptic action potentials. Here, we describe a mechanism endowing a plasticity rule with additional computational complexity--a dependence on the pattern of presynaptic action potentials. The classical Hebbian rule is based on detection of conjunctive presynaptic and postsynaptic activity by postsynaptic NMDA receptors, but there is also accumulating evidence for the existence of presynaptic NMDA receptors in several brain structures. Here, we examine the role of presynaptic NMDA receptors in defining the temporal structure of the plasticity rule governing induction of long-term depression (LTD) at the cerebellar parallel fiber-Purkinje cell synapse. We show that multiple presynaptic action potentials at frequencies between 40 Hz and 1 kHz are necessary for LTD induction. We characterize the subtype, kinetics, and role of presynaptic NMDA receptors involved in the induction of LTD, showing how the kinetics of the NR2A subunits expressed by parallel fibers implement a high-pass filter plasticity rule that will selectively attenuate synapses undergoing high-frequency bursts of activity. Depending on the type of NMDA receptor subunit expressed, high-pass filters of different corner frequencies could be implemented at other synapses expressing NMDA autoreceptors.

Multiple climbing fibers signal to molecular layer interneurons exclusively via glutamate spillover.

Spillover of glutamate under physiological conditions has only been established as an adjunct to conventional synaptic transmission. Here we describe a pure spillover connection between the climbing fiber and molecular layer interneurons in the rat cerebellar cortex. We show that, instead of acting via conventional synapses, multiple climbing fibers activate AMPA- and NMDA-type glutamate receptors on interneurons exclusively via spillover. Spillover from the climbing fiber represents a form of glutamatergic volume transmission that could be triggered in a regionalized manner by experimentally observed synchronous climbing fiber activity. Climbing fibers are known to direct parallel fiber synaptic plasticity in interneurons, so one function of this spillover is likely to involve controlling synaptic plasticity.

Large, Stable Spikes Exhibit Differential Broadening in Excitatory and Inhibitory Neocortical Boutons.

Presynaptic action potential spikes control neurotransmitter release and thus interneuronal communication. However, the properties and the dynamics of presynaptic spikes in the neocortex remain enigmatic because boutons in the neocortex are small and direct patch-clamp recordings have not been performed. Here, we report direct recordings from boutons of neocortical pyramidal neurons and interneurons. Our data reveal rapid and large presynaptic action potentials in layer 5 neurons and fast-spiking interneurons reliably propagating into axon collaterals. For in-depth analyses, we establish boutons of mature cultured neurons as models for excitatory neocortical boutons, demonstrating that the presynaptic spike amplitude is unaffected by potassium channels, homeostatic long-term plasticity, and high-frequency firing. In contrast to the stable amplitude, presynaptic spikes profoundly broaden during high-frequency firing in layer 5 pyramidal neurons, but not in fast-spiking interneurons. Thus, our data demonstrate large presynaptic spikes and fundamental differences between excitatory and inhibitory boutons in the neocortex.

What can we learn from synaptic weight distributions?

Much research effort into synaptic plasticity has been motivated by the idea that modifications of synaptic weights (or strengths or efficacies) underlie learning and memory. Here, we examine the possibility of exploiting the statistics of experimentally measured synaptic weights to deduce information about the learning process. Analysing distributions of synaptic weights requires a theoretical framework to interpret the experimental measurements, but the results can be unexpectedly powerful, yielding strong constraints on possible learning theories as well as information that is difficult to obtain by other means, such as the information storage capacity of a cell. We review the available experimental and theoretical techniques as well as important open issues.