William J. Spain

Reduced sodium current in GABAergic interneurons in a mouse model of severe myoclonic epilepsy in infancy

Voltage-gated sodium channels (NaV) are critical for initiation of action potentials. Heterozygous loss-of-function mutations in NaV1.1 channels cause severe myoclonic epilepsy in infancy (SMEI). Homozygous null Scn1a−/− mice developed ataxia and died on postnatal day (P) 15 but could be sustained to P17.5 with manual feeding. Heterozygous Scn1a+/− mice had spontaneous seizures and sporadic deaths beginning after P21, with a notable dependence on genetic background. Loss of NaV1.1 did not change voltage-dependent activation or inactivation of sodium channels in hippocampal neurons. The sodium current density was, however, substantially reduced in inhibitory interneurons of Scn1a+/− and Scn1a−/− mice but not in their excitatory pyramidal neurons. An immunocytochemical survey also showed a specific upregulation of NaV1.3 channels in a subset of hippocampal interneurons. Our results indicate that reduced sodium currents in GABAergic inhibitory interneurons in Scn1a+/− heterozygotes may cause the hyperexcitability that leads to epilepsy in patients with SMEI.

Fractional differentiation by neocortical pyramidal neurons

Neural systems adapt to changes in stimulus statistics. However, it is not known how stimuli with complex temporal dynamics drive the dynamics of adaptation and the resulting firing rate. For single neurons, it has often been assumed that adaptation has a single time scale. We found that single rat neocortical pyramidal neurons adapt with a time scale that depends on the time scale of changes in stimulus statistics. This multiple time scale adaptation is consistent with fractional order differentiation, such that the neurons firing rate is a fractional derivative of slowly varying stimulus parameters. Biophysically, even though neuronal fractional differentiation effectively yields adaptation with many time scales, we found that its implementation required only a few properly balanced known adaptive mechanisms. Fractional differentiation provides single neurons with a fundamental and general computation that can contribute to efficient information processing, stimulus anticipation and frequency-independent phase shifts of oscillatory neuronal firing.

Synaptic depression in the localization of sound

Short-term synaptic plasticity, which is common in the central nervous system, may contribute to the signal processing functions of both temporal integration and coincidence detection. For temporal integrators, whose output firng rate depends on a running average of recent synaptic inputs, plasticity modulates input synaptic strength and thus may directly control signalling gain and the function of neural networks. But the firing probability of an ideal coincidence detector would depend on the temporal coincidence of events rather than on the average frequency of synaptic events. Here we have examined a specific case of how synaptic plasticity can affect temporal coincidence detection, by experimentally characterizing synaptic depression at the synapse between neurons in the nucleus magnocellularis and coincidence detection neurons in the nucleus laminaris in the chick auditory brainstem. We combine an empirical description of this depression with a biophysical model of signalling in the nucleus laminaris. The resulting model predicts that synaptic depression provides an adaptive mechanism for preserving interaural time-delay information (a proxy for the location of sound in space) despite the confounding effects of sound-intensity-related information. This mechanism may help nucleus laminaris neurons to pass specific sound localization information to higher processing centres.

Diversity of Gain Modulation by Noise in Neocortical Neurons: Regulation by the Slow Afterhyperpolarization Conductance

Neuronal firing is known to depend on the variance of synaptic input as well as the mean input current. Several studies suggest that input variance, or “noise,” has a divisive effect, reducing the slope or gain of the firing frequency–current (f–I) relationship. We measured the effects of current noise on f–I relationships in pyramidal neurons and fast-spiking (FS) interneurons in slices of rat sensorimotor cortex. In most pyramidal neurons, noise had a multiplicative effect on the steady-state f–I relationship, increasing gain. In contrast, noise reduced gain in FS interneurons. Gain enhancement in pyramidal neurons increased with stimulus duration and was correlated with the amplitude of the slow afterhyperpolarization (sAHP), a major mechanism of spike-frequency adaptation. The 5-HT2 receptor agonist α-methyl-5-HT reduced the sAHP and eliminated gain increases, whereas augmenting the sAHP conductance by spike-triggered dynamic-current clamp enhanced the gain increase. These results indicate that the effects of noise differ fundamentally between classes of neocortical neurons, depending on specific biophysical properties including the sAHP conductance. Thus, noise from background synaptic input may enhance network excitability by increasing gain in pyramidal neurons with large sAHPs and reducing gain in inhibitory FS interneurons.

Two transient potassium currents in layer V pyramidal neurones from cat sensorimotor cortex.

1. Two transient outward currents were identified in large pyramidal neurones from layer V of cat sensorimotor cortex (‘Betz cells’) using an in vitro brain slice preparation and single‐microelectrode voltage clamp. Properties of the currents deduced from voltage‐clamp measurements were reflected in neuronal responses during constant current stimulation. 2. Both transient outward currents rose rapidly after a step depolarization, but their subsequent time course differed greatly. The fast‐transient current decayed within 20 ms, while the slow‐transient current took greater than 10 s to decay. Raised extracellular potassium reduced current amplitude. Both currents were present in cadmium‐containing or calcium‐free perfusate. 3. Tetraethylammonium had little effect on the slow‐transient current at a concentration of 1 mM, but the fast‐transient current was reduced by 60%. 4‐Aminopyridine had little effect on the fast‐transient current over the range 20 microM‐2 mM, but these concentrations reduced the slow‐transient current and altered its time course. 4. Both transient currents were evoked by depolarizations below action potential threshold. The fast‐transient current was evoked by a 7 mV smaller depolarization than the slow‐transient current, but its chord conductance increased less steeply with depolarization. 5. Voltage‐dependent inactivation of the fast‐transient was steeper than that of the slow‐transient current (4 vs. 7 mV per e‐fold change), and half‐inactivation occurred at a less negative potential (‐59 vs. ‐65 mV). The activation and inactivation characteristics of each current overlapped, however, implying the existence of a steady ‘window current’ extending over a range of approximately 14 mV beginning negative to action potential threshold. 6. The fast‐transient current displayed a clear voltage dependence of both its activation and inactivation kinetics, whereas the slow‐transient current did not. Recovery of either current from inactivation took about 1 s near ‐70 mV. The recovery of the slow‐transient current became faster with hyperpolarization. 7. The contribution of each transient current to repolarization of the action potential was assessed from pharmacological responses. Blockade of calcium influx had little or no effect on the rate of action potential repolarization, whereas the selective reduction of either transient current caused significant slowing of repolarization. 8. We conclude that Betz cells possess at least two transient potassium currents, each a member of the rapidly expanding family of voltage‐gated potassium currents that have been identified in various cell types.(ABSTRACT TRUNCATED AT 400 WORDS)

Two-Dimensional Time Coding in the Auditory Brainstem

Avian nucleus magnocellularis (NM) spikes provide a temporal code representing sound arrival times to downstream neurons that compute sound source location. NM cells act as high-pass filters by responding only to discrete synaptic events while ignoring temporally summed EPSPs. This high degree of input selectivity insures that each output spike from NM unambiguously represents inputs that contain precise temporal information. However, we lack a quantitative description of the computation performed by NM cells. A powerful model for predicting output firing rate given an arbitrary current input is given by a linear/nonlinear cascade: the stimulus is compared with a known relevant feature by linear filtering, and based on that comparison, a nonlinear function predicts the firing response. Spike-triggered covariance analysis allows us to determine a generalization of this model in which firing depends on more than one spike-triggering feature or stimulus dimension. We found two current features relevant for NM spike generation; the most important simply smooths the current on short time scales, whereas the second confers sensitivity to rapid changes. A model based on these two features captured more mutual information between current and spikes than a model based on a single feature. We used this analysis to characterize the changes in the computation brought about by pharmacological manipulation of the biophysical properties of the neurons. Blockage of low-threshold voltage-gated potassium channels selectively eliminated the requirement for the second stimulus feature, generalizing our understanding of input selectivity by NM cells. This study demonstrates the power of covariance analysis for investigating single neuron computation.

Kv1 channels control spike threshold dynamics and spike timing in cortical pyramidal neurones

Non‐Technical Summary  Spiking neurones generate action potentials when the transmembrane voltage difference near the spike generating zone reaches a threshold level. Above the threshold, the inward sodium current exceeds the outward potassium current, causing the rapid upstroke of the action potential. In many neurones, including cortical pyramidal cells, the threshold is not constant but responds to a change in voltage with a short delay. The functional effect is equivalent to high‐pass filtering of the voltage response, and a major benefit is enhanced spike timing precision. Two mechanisms that may contribute to a dynamic spike threshold are sodium channel inactivation and potassium channel activation, both caused by a rise in voltage. We found that blocking low‐threshold Kv1 potassium channels greatly reduced threshold changes in pyramidal neurones located in layer 2–3 of the rat motor cortex. Studies using noise stimulation showed that blocking Kv1 impaired the ability of these cells to encode fast components of the input signal with precisely timed spikes. These results demonstrate a key role of Kv1 in cortical spike timing, with possible implications for information coding as well as pathological hypersynchronous discharges in epilepsy.

Conditional bursting enhances resonant firing in neocortical layer 2-3 pyramidal neurons

The frequency response properties of neurons are critical for signal transmission and control of network oscillations. At subthreshold membrane potential, some neurons show resonance caused by voltage-gated channels. During action potential firing, resonance of the spike output may arise from subthreshold mechanisms and/or spike-dependent currents that cause afterhyperpolarizations (AHPs) and afterdepolarizations (ADPs). Layer 2–3 pyramidal neurons (L2–3 PNs) have a fast ADP that can trigger bursts. The present study investigated what stimuli elicit bursting in these cells and whether bursts transmit specific frequency components of the synaptic input, leading to resonance at particular frequencies. We found that two-spike bursts are triggered by step onsets, sine waves in two frequency bands, and noise. Using noise adjusted to elicit firing at ∼10 Hz, we measured the gain for modulation of the time-varying firing rate as a function of stimulus frequency, finding a primary peak (7–16 Hz) and a high-frequency resonance (250–450 Hz). Gain was also measured separately for single and burst spikes. For a given spike rate, bursts provided higher gain at the primary peak and lower gain at intermediate frequencies, sharpening the high-frequency resonance. Suppression of bursting using automated current feedback weakened the primary and high-frequency resonances. The primary resonance was also influenced by the SK channel-mediated medium AHP (mAHP), because the SK blocker apamin reduced the sharpness of the primary peak. Our results suggest that resonance in L2–3 PNs depends on burst firing and the mAHP. Bursting enhances resonance in two distinct frequency bands.

Postsynaptic GABAB Receptors Enhance Extrasynaptic GABAA Receptor Function in Dentate Gyrus Granule Cells

Ambient GABA in the brain tonically activates extrasynaptic GABAA receptors, and activity-dependent changes in ambient GABA concentration can also activate GABAB receptors. To investigate an interaction between postsynaptic GABAB and GABAA receptors, we recorded GABAA currents elicited by exogenous GABA (10 μm) from dentate gyrus granule cells (DGGCs) in adult rat hippocampal slices. The GABAB receptor agonist baclofen (20 μm) enhanced GABAA currents. This enhancement was blocked by the GABAB receptor antagonist CGP 55845 and intracellular solutions containing the GTP analog GDP-β-s, indicating that baclofen was acting on postsynaptic GABAB receptors. Modulation of GABAA currents by postsynaptic GABAB receptors was not observed in CA1 pyramidal cells or layer 2/3 cortical pyramidal neurons. Baclofen reduced the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) but did not alter sIPSC amplitude or kinetics. Thus, GABAA receptors activated at synapses were not modulated by postsynaptic GABAB receptors. In contrast, tonic GABA currents and currents activated by the GABAA receptor δ subunit-selective agonist THIP (10 μm) were potentiated by baclofen. Our data indicate that postsynaptic GABAB receptors enhance the function of extrasynaptic GABAA receptors, including δ subunit-containing receptors that mediate tonic inhibition in DGGCs. The modulation of GABAA receptor function by postsynaptic GABAB receptors is a newly identified mechanism that will influence the inhibitory tone of DGGCs when GABAB and GABAA receptors are both activated.

Post-inhibitory excitation and inhibition in layer V pyramidal neurones from cat sensorimotor cortex

1. The effect of conditioning pre‐pulses on repetitive firing evoked by intracellular current injection was studied in layer V pyramidal neurones in a brain slice preparation of cat sensorimotor cortex. Most cells displayed spike frequency adaptation (monotonic decline of firing rate to a tonic value) for several hundred milliseconds when depolarized from resting potential, but the cells differed in their response when pre‐pulses to other potentials were employed. In one group of cells, the initial firing rate increased as the pre‐pulse potential was made more negative (post‐hyperpolarization excitation). Adaptation was abolished by depolarizing prepulses. In a second group, the initial firing rate decreased as the pre‐pulse potential was made more negative (post‐hyperpolarization inhibition). Hyperpolarizing pre‐pulses caused the initial firing to fall below and accelerate to the tonic rate over a period of several seconds. A third group displayed a mixture of these two responses: the first three to seven interspike intervals became progressively shorter and subsequent intervals became progressively longer as the conditioning pre‐pulse was made more negative (post‐hyperpolarization mixed response). 2. Cells were filled with horseradish peroxidase or biocytin after the effect of pre‐pulses was determined. All cells whose firing patterns were altered by pre‐pulses were large layer V pyramidal neurones. Cells showing post‐hyperpolarization excitation or a mixed response had tap root dendrites, fewer spines on the apical dendrite and larger soma diameters than cells showing post‐hyperpolarization inhibition. 3. Other electrophysiological parameters varied systematically with the response to conditioning pre‐pulses. Both the mean action potential duration and the input resistance of cells showing post‐hyperpolarization excitation were about half the values measured in cells showing post‐hyperpolarization inhibition. Values were intermediate in cells showing a post‐hyperpolarization mixed response. The after‐hyperpolarization following a single evoked action potential was 20% briefer in cells showing post‐hyperpolarization excitation compared to those showing inhibition. 4. Membrane current measured during voltage clamp suggested that two ionic mechanisms accounted for the three response patterns. Post‐hyperpolarization excitation was caused by deactivation of the inward rectifier current (Ih). Selective reduction of Ih with extracellular caesium diminished post‐hyperpolarization excitation, whereas blockade of calcium influx had no effect. Post‐hyperpolarization inhibition was caused by enhanced activation of a slowly inactivating potassium current. Selective reduction of this current with 4‐aminopyridine diminished the post‐hyperpolarization inhibition. 5. Chord conductances underlying both Ih and the slow‐transient potassium current were measured and divided by leakage conductance to control for differences in cell size.(ABSTRACT TRUNCATED AT 400 WORDS)