Michael J. Gutnick

Intrinsic firing patterns of diverse neocortical neurons

Neurons of the neocortex differ dramatically in the patterns of action potentials they generate in response to current steps. Regular-spiking cells adapt strongly during maintained stimuli, whereas fast-spiking cells can sustain very high firing frequencies with little or no adaptation. Intrinsically bursting cells generate clusters of spikes (bursts), either singly or repetitively. These physiological distinctions have morphological correlates. RS and IB cells can be either pyramidal neurons or spiny stellate cells, and thus constitute the excitatory cells of the cortex. FS cells are smooth or sparsely spiny non-pyramidal cells, and are likely to be GABAergic inhibitory interneurons. The different firing properties of neurons in neocortex contribute significantly to its network behavior.

Ongoing activity in peripheral nerves: The physiology and pharmacology of impulses originating from a neuroma ☆

Abstract In rats, the sciatic nerve was cut, drawn into a polythene tube with one end sealed, and a neuroma allowed to develop in the chamber. Activity in the dorsal and ventral roots terminating in the neuroma was examined between 9 days and 4 months after the section. A fraction of the small myelinated afferent fibers originating in the neuroma were carrying a steady ongoing barrage of nerve impulses. Many of the fine terminals were excited by slight mechanical distortion. The fine sprouts in the neuroma were electrically excitable. The ongoing afferent barrage was highly dependent on blood flow. It was abolished for long periods of time after a brief antidromic tetanus had invaded the active fibers. This suggests that some of the pain relief obtained from peripheral nerve stimulation may have a peripheral rather than a central mechanism. No signs of excitatory on inhibitory interaction were detected between volleys in one group of nerve fibers and the activity in other groups of fibers in the neuroma. Alpha active sympathetic amines, noradrenaline, excited the ongoing activity while beta agents, isoprenaline, had no excitatory effect. This suggests that alpha blocking agents might be useful to test if the sympathetic system is involved in particular pains.

Extracellular free calcium and potassium during paroxysmal activity in the cerebral cortex of the cat

SummaryExtracellular calcium and potassium activities (aCa and aK) as well as neuronal activity were simultaneously recorded with ion-sensitive electrodes in the somatosensory cortex of cats. Baseline aCa was 1.2–1.5 mM/1, baseline ak 2.7–3.2 mM/1. Transient decreases in aCa and simultaneous increases in aK were evoked by repetitive stimulation of the contralateral forepaw, the nucleus ventroposterolateralis thalami and the cortical surface. Considerable decreases in aCa (by up to 0.7 mM/1) were found during seizure activity. A fall in aCa preceded the onset of paroxysmal discharges and the rise in aK after injection of pentylene tetrazol. The decrease in aCa led also the rise in aK during cyclical spike driving in a penicillin focus. It is concluded that alterations of Ca++ dependent mechanisms participate in the generation of epileptic activity.

Slow inactivation of Na+ current and slow cumulative spike adaptation in mouse and guinea-pig neocortical neurones in slices.

1. Spike adaptation of neocortical pyramidal neurones was studied with sharp electrode recordings in slices of guinea‐pig parietal cortex and whole‐cell patch recordings of mouse somatosensory cortex. Repetitive intracellular stimulation with 1 s depolarizing pulses delivered at intervals of < 5 s caused slow, cumulative adaptation of spike firing, which was not associated with a change in resting conductance, and which persisted when Co2+ replaced Ca2+ in the bathing medium. 2. Development of slow cumulative adaptation was associated with a gradual decrease in maximal rates of rise of action potentials, a slowing in the post‐spike depolarization towards threshold, and a positive shift in the threshold voltage for the next spike in the train; maximal spike repolarization rates and after‐hyperpolarizations were unchanged. 3. The data suggested that slow adaptation reflects use‐dependent removal of Na+ channels from the available pool by an inactivation process which is much slower than fast, Hodgkin‐Huxley‐type inactivation. 4. We therefore studied the properties of Na+ channels in layer II‐III mouse neocortical cells using the cell‐attached configuration of the patch‐in‐slice technique. These had a slope conductance of 18 +/‐ 1 pS and an extrapolated reversal potential of 127 +/‐ 6 mV above resting potential (Vr) (mean +/‐ S.E.M.; n = 5). Vr was estimated at ‐72 +/‐ 3 mV (n = 8), based on the voltage dependence of the steady‐state inactivation (h infinity) curve. 5. Slow inactivation (SI) of Na+ channels had a mono‐exponential onset with tau on between 0.86 and 2.33 s (n = 3). Steady‐state SI was half‐maximal at ‐43.8 mV and had a slope of 14.4 mV (e‐fold)‐1. Recovery from a 2 s conditioning pulse was bi‐exponential and voltage dependent; the slow time constant ranged between 0.45 and 2.5 s at voltages between‐128 and ‐68 mV. 6. The experimentally determined parameters of SI were adequate to simulate slow cumulative adaptation of spike firing in a single‐compartment computer model. 7. Persistent Na+ current, which was recorded in whole‐cell configuration during slow voltage ramps (35 mV s‐1), also underwent pronounced SI, which was apparent when the ramp was preceded by a prolonged depolarizing pulse.

Thalamocortical Relay Neurons: Antidromic Invasion of Spikes from a Cortical Epileptogenic Focus

Thalamocortical relay neurons whose axons project into a penicillininduced cortical epileptogenic focus generate bursts of action potentials during spontaneous interictal epileptiform discharges. These bursts originate in intracortical axons and propagate antidromically into thalamic neurons. Repetitive spike generation in cortical axons and presynaptic terminals could produce a potent excitatory drive and contribute to the generation of the large depolarization shifts which are seen in cortical elements during focal epileptogenesis.

Dye-coupling between glial cells in the guinea pig neocortical slice

Physiologically identified glial cells in guinea pig neocortical slices were injected with the low molecular weight, fluorescent dye Lucifer yellow CH. The stained aggregates which resulted consisted of one brightly stained, central cell surrounded by numerous lightly stained cells. The central cell had well defined feathery processes and resembled a protoplasmic astrocyte. The surrounding cells appeared also to be glial cells but lacked sufficient detail to be further categorized. This first demonstration of dye-coupling between neocortical glial cells strongly suggests that these cells are connected together via low resistance junctions capable of passing ionic current as well as dye.

Na+ imaging reveals little difference in action potential-evoked Na+ influx between axon and soma

In cortical pyramidal neurons, the axon initial segment (AIS) is pivotal in synaptic integration. It has been asserted that this is because there is a high density of Na+ channels in the AIS. However, we found that action potential–associated Na+ flux, as measured by high-speed fluorescence Na+ imaging, was about threefold larger in the rat AIS than in the soma. Spike-evoked Na+ flux in the AIS and the first node of Ranvier was similar and was eightfold lower in basal dendrites. At near-threshold voltages, persistent Na+ conductance was almost entirely axonal. On a time scale of seconds, passive diffusion, and not pumping, was responsible for maintaining transmembrane Na+ gradients in thin axons during high-frequency action potential firing. In computer simulations, these data were consistent with the known features of action potential generation in these neurons.

Functionally Distinct NMDA Receptors Mediate Horizontal Connectivity within Layer 4 of Mouse Barrel Cortex

In sensory areas of neocortex, thalamocortical afferents project primarily onto the spiny stellate neurons of Layer 4. Anatomical evidence indicates that these cells receive most of their excitatory input from other cortical neurons, including other spiny stellate cells. Although this local network must play an important role in sensory processing, little is known about the properties of the neurons and synapses involved. We have produced a slice preparation of mouse barrel cortex that isolates Layer 4. We report that excitatory interaction between spiny stellate neurons is largely via N-methyl-D-aspartate receptors (NMDARs) and that a given neuron contains more than one type of NMDAR, as distinguished by voltage dependence. Thus, spiny stellate cells act as effective integrators of powerful and persistent NMDAR-mediated recurrent excitation.

Voltage-dependent and calcium-dependent inactivation of calcium channel current in identified snail neurones.

1. The dependence of Ca2+ current inactivation on membrane potential and intracellular Ca2+ concentration ([Ca2+]i) was studied in TEA‐loaded, identified Helix neurones which possess a single population of high‐voltage‐activated Ca2+ channels. During prolonged depolarization, the Ca2+ current declined from its peak with two clearly distinct phases. The time course of its decay was readily fitted by a double‐exponential function. 2. In double‐pulse experiments, the relationship between the magnitude of the Ca2+ current and the amount of Ca2+ inactivation was not linear, and considerable inactivation was present, even when conditioning pulses were to levels of depolarization so great that Ca2+ currents were near zero. Similar results were obtained when external Ca2+ was replaced by Ba2+. 3. In double‐pulse experiments, hyperpolarization during the interpulse interval served to reprime a portion of the inactivated Ca2+ current for subsequent activation. The extent of repriming increased with hyperpolarization, reaching a maximum between ‐130 and ‐150 mV. The effectiveness of repriming hyperpolarizations was considerably increased when Ca2+ was replaced by Ba2+. 4. A significant fraction of inactivated Ca2+ channels can be recovered during hyperpolarizing pulses lasting only milliseconds. If hyperpolarizing pulses were applied before substantial inactivation of Ca2+ current, Ca2+ channels remained available for activation despite considerable Ca2+ entry. 5. The relationship between [Ca2+]i and inactivation was investigated by quantitatively injecting Ca2+‐buffered solutions into the cells. The time course of Ca2+ current inactivation was unchanged at free [Ca2+] between 1 x 10(‐7) and 1 x 10(‐5) M. From 1 x 10(‐7) to 1 x 10(‐9) M, inactivation became progressively slower, mainly due to a decrease of the amplitude ratio (fast/slow) of the two components of inactivation, which fell from about unity to near zero at 1 x 10(‐9) M. In double‐pulse experiments, recovery from inactivation was enhanced in neurones that had been injected with Ca2+ chelator. 6. We conclude that inactivation of Ca2+ channels in these neurones depends on both [Ca2+]i and membrane potential. The voltage‐dependent process may serve as a mechanism to quickly recover inactivated Ca2+ channels during repetitive firing despite considerable Ca2+ influx. 7. The results are discussed in the framework of a model which is based on two states of inactivation, INV and INCA, which represent different conformations of the inactivating substrate, and which are both reached from a lumped state of activation (A). Inactivation leads to high occupancy of INV during depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Persistent Sodium Current in Layer 5 Neocortical Neurons Is Primarily Generated in the Proximal Axon

In addition to the well described fast-inactivating component of the Na+ current [transient Na+ current (INaT)], neocortical neurons also exhibit a low-voltage-activated, slowly inactivating “persistent” Na+ current (INaP), which plays a role in determining neuronal excitability and synaptic integration. We investigated the Na+ channels responsible for INaP in layer 5 pyramidal cells using cell-attached and whole-cell recordings in neocortical slices. In simultaneous cell-attached and whole-cell somatic recordings, no persistent Na+ channel activity was detected at potentials at which whole-cell INaP operates. Detailed kinetic analysis of late Na+ channel activity in cell-attached patches at 36°C revealed that somatic Na+ channels do not demonstrate “modal gating” behavior and that the probability of single late openings is extremely low (<1.4 × 10−4 or <0.02% of maximal open probability of INaT). Ensemble averages of these currents did not reveal a sustained component whose amplitude and voltage dependence could account for INaP as seen in whole-cell recordings. Local application of TTX to the axon blocked somatically recorded INaP, whereas somatic and dendritic application had little or no effect. Finally, simultaneous current-clamp recordings from soma and apical dendrite revealed that Na+ plateau potentials originate closer to the axon. Our data indicate that the primary source of INaP is in the spike initiation zone in the proximal axon. The focal axonal presence of regenerative subthreshold conductance with voltage and time dependence optimal to manipulate integration of synaptic input, spike threshold, and the pattern of repetitive firing provides the layer 5 pyramidal neuron with a mechanism for dynamic control of its gain.