Axonal contribution to subthreshold currents inaplysia bursting pacemaker neurons

The contribution of axonal activity to the ionic currents which generate bursting pacemaker activity was studied by using the two-electrode voltage-clamp technique in Aplysia bursting neuron somata in conjunction with intraaxonal voltage recordings. Depolarizing voltage-clamp pulses applied to bursting cell somata triggered axonal action potentials. The voltage-clamp current recording exhibited transient inward current "notches" corresponding to each of the axonal spikes. The addition of 50 microM tetrodotoxin (TTX) to the bathing medium blocked the fast axonal spikes and current notches, revealing a slower axonal spike which was blocked by the replacement of external Ca2+ with Co2+. The inward current evoked by applying a depolarizing voltage-clamp pulse in the soma is distorted by the occurrence of the axonal Ca2+ spike. Elimination of the axonal spike, by injecting hyperpolarizing current into the axon, changes both the time course and the magnitude of the inward current. The axonal Ca2+ spikes are followed by a series of Ca2+-dependent afterpotentials: a rapid postspike hyperpolarization, a depolarizing afterpotential (DAP) and, finally, a long-lasting postburst hyperpolarization. The long-lasting hyperpolarization is not blocked by 50 mM external tetraethyl ammonium, an effective blocker of Ca2+-activated K+ current [IK(Ca)], and does not appear to reverse at EK. Hence, the axonal long-lasting hyperpolarization may not be due to IK(Ca). Somatic voltage-clamp pulses in bursting neurons are followed by a slow inward tail current, which is sometimes coincident with a DAP in the axon. In some cells, the amplitude of the slow inward tail current is greatly reduced if axonal spikes and DAPs are prevented by hyperpolarization of the axon, while, in other cells, elimination of axonal activity has little effect. Therefore, the slow inward tail current is not necessarily an artifact of poor voltage-clamp control over the axonal membrane potential but probably results from the activation of an ionic conductance mechanism located partly in the axon and partly in the soma.     

Sharpness of Spike Initiation in Neurons Explained by Compartmentalization

In cortical neurons, spikes are initiated in the axon initial segment. Seen at the soma, they appear surprisingly sharp. A standard explanation is that the current coming from the axon becomes sharp as the spike is actively backpropagated to the soma. However, sharp initiation of spikes is also seen in the input–output properties of neurons, and not only in the somatic shape of spikes; for example, cortical neurons can transmit high frequency signals. An alternative hypothesis is that Na channels cooperate, but it is not currently supported by direct experimental evidence. I propose a simple explanation based on the compartmentalization of spike initiation. When Na channels are placed in the axon, the soma acts as a current sink for the Na current. I show that there is a critical distance to the soma above which an instability occurs, so that Na channels open abruptly rather than gradually as a function of somatic voltage.     

Role of A-type K+ channels in spike broadening observed in soma and axon of Hermissenda type-B photoreceptors: A simulation study

In Hermissenda type-B photoreceptors, the spike is generated in the axon and back-propagated to the soma, resulting in smaller somatic spikes. Experimentally, blocking the A-type K⁺ current (I_K,A) results in broadening of somatic spikes. Similarly, in a compartmental model of the photoreceptor, reducing the maximum A-type K⁺ conductance (g_K,Amax) results in broadening of somatic spikes. However, simulations predict that little or no broadening of axonal spikes occurs when g_K,Amax is reduced. The results can be explained by the voltage-dependent properties of I_K,A and the different potential ranges that the somatic and axonal spike traverse. Because of the steeper I-V curve and faster activation of the K⁺ channels at higher potentials, the recruitment of additional K⁺ channels in the axon is able to compensate for the decrease in K⁺ conductance, yielding less spike broadening. These results also support the idea that spike duration in the axon may not be reliably inferred based upon recordings collected from the soma. Action Editor: Jonathan D. Victor     

An axonal spike in an identified fast crab motor neuron has sodium and calcium components

1. High intensity intracellular stimulation of the FBE axon produced a spike discharge. The spikes divided into two components, each with a different frequency. 2. Ion replacement and blocking specific ion channels revealed that the FBE spike has calcium and sodium components. 3. The pronounced depolarizing wave that follows the FBE spike is not produced by changes in calcium conductance.     

Factors affecting slow regular firing in the suprachiasmatic nucleus in vitro

In isolated slices of hypothalamus, suprachiasmatic nucleus (SCN) neurons were recorded intracellularly. Blockade of Ca++ channels increased spike duration, eliminating an early component of the afterhyperpolarization (AHP) that followed evoked spikes. The duration and reversal potential of AHPs were, however, unaffected, suggesting that only an early, fast component of the AHP was Ca(++)-dependent. Unlike other central neurons that exhibit pacemaker activity, therefore, SCN neurons do not display a pronounced, long-lasting Ca(++)-dependent AHP. Extracellular Ba++ and intracellular Cs+ both revealed slow depolarizing potentials evoked either by depolarizing current injection, or by repolarization following large hyperpolarizations. They had different effects on the shape of spikes and the AHPs that followed them, however. Cs+, which blocks almost all K+ channels, dramatically reduced resting potential, greatly increased spike duration (to tens of milliseconds), and blocked AHPs completely. In contrast, Ba++ had little effect on resting potential and produced only a small increase in spike duration, depressing an early Ca(++)-dependent component and a later Ca(++)-independent component of the AHP. The relatively weak pacemaker activity of SCN neurons appears to involve voltage-dependent activation of at least one slowly inactivating inward current, which brings the cells to firing threshold and maintains tonic firing; both Ca(++)-dependent and Ca(++)-independent K+ channels, which repolarize cells after spikes and maintain interspike intervals; and Ca++ channels, which contribute to activation of Ca(++)-activated K+ currents and may also contribute to slow depolarizing potentials. In the absence of powerful synaptic inputs, SCN neurons express a pacemaker activity that is sufficient to maintain an impressively regular firing pattern. Slow, repetitive activation of optic input, however, increases local circuit activity to such an extent that the normal pacemaker potentials are overridden and firing patterns are altered. Since SCN neurons are very small and have large input resistances, they are particularly susceptible to synaptic input.     

A prolonged,voltage-dependent calcium permeability revealed by tetraethylammonium in the soma and axon of Aplysia giant neuron

The soma but not the axon of the giant neuron, R2, of Aplysia can generate an all-or-none Ca spike in Na-free or TTX-containing medium (Junge and Miller, 1974). Extracellular axonal recordings made at several distances from the soma provide evidence that the transition in ability to fire a spike in Na-free medium occurs within the first 250 μm of the axon. Application of 25 mM TEA-Br to the bathing medium causes a more than tenfold increase in the duration of the somatic action potential. The duration of the axonal action potential in TEA decreases with distance from the soma. At distances greater than 3 mm from the soma this concentration of TEA causes little or no increase in the duration of the axon spike. The effect of 25 mM TEA on both the soma and proximal axon is blocked reversibly by 30 mM CoCl₂ or 1 mM CdCl₂. The duration of the somatic action potential in TEA increases with an increase in Ca concentration of the bath. At a constant concentration of Na, the voltage level of the somatic plateau increases with Ca concentration in the manner predicted for a Ca electrode. In the presence of 11 mM Ca²⁺ the potential of the plateau is relatively insensitive to Na concentration. The TEA plateau in R2 reveals a prolonged voltage-dependent permeability to Ca. The duration of the plateau may indicate the degree of Ca activation during a spike.     

Mechanisms underlying burst generation of the pyloric muscle in the mantis,shrimp, Squilla oratoria

The pyloric constrictor muscles of the stomach in Squilla can generate spikes by synaptic activation via the motor nerve from the stomatogastric ganglion. Spikes are followed by slow depolarizing afterpotentials (DAPs) which lead to sustained depolarization during a burst of spikes. 1. The frequency of rhythmic bursts induced by continuous depolarization is membrane voltage-dependent. A brief depolarizing or hyperpolarizing pulse can trigger or terminate bursts, respectively, in a threshold-dependent manner. 2. The conductance increases during the DAP response. The amplitude of DAP decreases by imposed depolarization, whereas it increases by hyperpolarization. DAPs from successive spikes sum to produce a sustained depolarizing potential capable of firing a burst. 3. The spike and DAP are reduced in amplitude by decreasing [Ca]o, enhanced by Sr2+ or Ba2+ substituted for Ca2+, and blocked by Co2+ or Mn2+. DAPs are selectively blocked by Ni2+, and the spike is followed by a hyperpolarizing afterpotential. 4. The spike and DAP are prolonged by intracellular injection of the Ca2+ chelator EGTA. A hyperpolarizing afterpotential is abolished by EGTA and enhanced by increasing [Ca]o. The DAP is diminished in Na(+)-free saline and reduced by tetrodotoxin. 5. It is concluded that the muscle fiber is endowed with endogenous oscillatory properties and that the oscillatory membrane events result from changes of a voltage- and time-dependent conductance to Ca2+ and Na+ and a Ca2+ activated conductance to K+.     

Ca2+-activated K+ currents regulate odor adaptation by modulating spike encoding of olfactory receptor cells

The olfactory system is thought to accomplish odor adaptation through the ciliary transduction machinery in olfactory receptor cells (ORCs). However, ORCs that have lost their cilia can exhibit spike frequency accommodation in which the action potential frequency decreases with time despite a steady depolarizing stimulus. This raises the possibility that somatic ionic channels in ORCs might serve for odor adaptation at the level of spike encoding, because spiking responses in ORCs encode the odor information. Here I investigate the adaptational mechanism at the somatic membrane using conventional and dynamic patch-clamp recording techniques, which enable the ciliary mechanism to be bypassed. A conditioning stimulus with an odorant-induced current markedly shifted the response range of action potentials induced by the same test stimulus to higher concentrations of the odorant, indicating odor adaptation. This effect was inhibited by charybdotoxin and iberiotoxin, Ca2+-activated K+ channel blockers, suggesting that somatic Ca2+-activated K+ currents regulate odor adaptation by modulating spike encoding. I conclude that not only the ciliary machinery but also the somatic membrane currents are crucial to odor adaptation.     

Identification of Active Membrane Areas in the Giant Neuron of Aplysia

Intracellular and extracellular potentials were simultaneously recorded from the soma and different parts of the axon of the giant cell of Aplysia. Evidence was obtained that for all modes of stimulation the spike originates in the axon at some distance from the cell body. The conduction of the spike is blocked at a distance of 200 to 300 µ from the soma for the antidromic spike, closer to the soma for an orthodromic spike. This event is recorded in the soma as a small or A spike. After some delay, a spike is initiated in the resting part of the axon and in the axon hillock; the soma is invaded only afterwards. The response of these three parts of the neuron is recorded in the soma as the big or S spike.     

Gating molecule interactions in K+ channels. A consistent explanation for cole-moore shifts and potential ramp experiments

The enhanced induction period of potassium channel currents in squid giant axon induced by hyperpolarizing prepulses (the Cole-Moore shift) is observed and analyzed for a range of depolarizing step potentials. The induction periods produced when the axon is voltage clamped with ascending potential ramps are also analyzed since both sets of experiments are incompatible with fourth-power dependence proposed by Hodgkin and Huxley for the squid K⁺ channels. When the Hodgkin-Huxley equations are modified to include the effects of interactions between gating molecules within individual channels, both the Cole-Moore and ascending potential ramp data are described with a fourth-power dependence. The constant interaction parameters which provide a consistent fit for all the data are based on the geometrical arrangement of gating molecules within the channel and the total interaction energy which stabilizes the four gating molecules in their closed configuration. A tetrahedral gating molecule geometry and an interaction energy of only 471 cal/mole provide optimal fits of all the data; the modified equations retain the ability to describe data presently described by the Hodgkin-Huxley equations in the depolarizing regime.     

Spike Potentials Recorded from the Insect Photoreceptor

Slow and spike potentials were recorded from single cells in the receptor layer of the compound eye of the drone of the honeybee. From electron microscopic observation of the drone ommatidium, it was concluded that the response had been recorded from the retinula cell. The following hypothesis is suggested for the initiation of spike potentials in the drone compound eye: Photic stimulation results in a decrease in the resistance of all or part of the retinula cell membrane, giving rise to the retinal action potential. The retinal action potential causes outflow of the current through the proximal process of the cell. This depolarizing current initiates spike potentials in the proximal process or axon of the retinula cell which are recorded across the soma membrane of the retinula cell.     

Correlation of Transmitter Release with Membrane Properties of the Presynaptic Fiber of the Squid Giant Synapse

Depolarization of the presynaptic terminal by current produced a postsynaptic potential (PSP) which increased with increasing presynaptic polarization and then reached a plateau. Iontophoretic injection of tetraethylammonium ions (TEA) into the presynaptic axon near the terminal produced a prolonged presynaptic spike. The resulting PSP is increased in size and its time course closely followed that of the presynaptic spike. The presynaptic fiber no longer exhibited rectification and strong depolarizations revealed that the PSP reached a maximum with about 110 mv depolarization. Further depolarization produced a decrease in PSP amplitude and finally transmission was blocked. However, a PSP then always appeared on withdrawal of the depolarizing current. Under the conditions of these experiments, the PSP could be considered a direct measure of transmitter release. Bathing the TEA-injected synapse with concentrations of tetrodotoxin (TTX) sufficient to block spike activity in both pre- and postsynaptic axons did not greatly modify postsynaptic electrogenesis. However, doubling TTX concentration reversibly blocked PSP. Thus the permeability changes to Na and K accompanying the spike do not appear necessary for transmitter release. Some other processes related to the level of presynaptic polarization must be involved to explain the data. The inhibition of transmitter release by strong depolarizations appears to be related to Ca action. A membrane Ca current may also be necessary for normal transmitter release.     

Effects of Lithium on Different Membrane Components of Crayfish Stretch Receptor Neurons

Unlike several other varieties of input membrane, that of the crayfish stretch receptor develops a generator potential in response to stretch when all the Na of the medium is replaced with Li. However, Li depolarizes the receptor neuron, the soma membrane becoming more depolarized than that of the axon. During exposure to Li the cell usually fires spontaneously for a period, and when it becomes quiescent spike electrogenesis fails in the soma but persists in the axon. These effects are seen in the rapidly adapting as well as the slowly adapting cells. The block of spike electrogenesis of the soma membrane is only partly due to the Li-induced depolarization and a significant role must be ascribed to a specific effect of Li.     

Excitability of the soma in central nervous system neurons

The ability of the soma of a spinal dorsal horn neuron, a spinal ventral horn neuron (presumably a motoneuron), and a hippocampal pyramidal neuron to generate action potentials was studied using patch-clamp recordings from rat spinal cord slices, the "entire soma isolation" method, and computer simulations. By comparing original recordings from an isolated soma of a dorsal horn neuron with simulated responses, it was shown that computer models can be adequate for the study of somatic excitability. The modeled somata of both spinal neurons were unable to generate action potentials, showing only passive and local responses to current injections. A four- to eightfold increase in the original density of Na(+) channels was necessary to make the modeled somata of both spinal neurons excitable. In contrast to spinal neurons, the modeled soma of the hippocampal pyramidal neuron generated spikes with an overshoot of +9 mV. It is concluded that the somata of spinal neurons cannot generate action potentials and seem to resist their propagation from the axon to dendrites. In contrast, the soma of the hippocampal pyramidal neuron is able to generate spikes. It cannot initiate action potentials in the intact neurons, but it can support their back-propagation from the axon initial segment to dendrites.     

Regulation of somatic firing dynamics by backpropagating dendritic spikes

Pyramidal cells of the apteronotid ELL have been shown to display a characteristic mechanism of burst discharge, which has been shown to play an important role in sensory coding. This form of bursting depends on a reciprocal dendro-somatic interaction, in which discharge of a somatic spike causes a dendritic spike, which in turn contributes a dendro-somatic current flow to create a depolarizing afterpotential (DAP) in the soma. We review here our recent work showing how the timing of this DAP influences the somatic firing dynamics, and how the degree of inactivation of dendritic Na⁺ currents can cause an increased delay between somatic and dendritic spikes. This ultimately allows the DAP to become more effective at increasing the excitability of the somatic spike generating mechanism. Further, this delay between dendritic and somatic spiking can be regulated by strongly hyperpolarizing GABA_B mediated dendritic inhibition, allowing the burst dynamics to fall under synaptic regulation. In contrast, a weaker, shunting inhibition due to GABA_A mediated dendritic inhibition can regulate the dendritic spike waveform to decrease the dendro-somatic current flow and the resulting DAP. We therefore show that the qualitative behaviour of an individual cell can depend on the degree of synaptic input, and the exact timing of events across the spatial extent of the neuron. Thus, our results serve to illustrate the complex dynamics that can be observed in cells with significant dendritic arborisation, a nearly ubiquitous adaptation amongst principal neurons.     

Axonal spike generation near the giant neuron soma computed by the Hodgkin-Huxley equation

The behavior of the antidromic spike and the origin of the axonal spike evoked by direct stimulation of the soma were studied with the aid of the Hodgkin-Hexley equation. It is suggested that the mechanisms responsible for electrical excitation of the axon are qualitatively and quantitatively similar to those described by Hodgkin and Huxley for the squid axon. The amplitude of the antidromic spike diminishes rapidly close to the soma. In the example studied, only subthreshod changes of membrane potential take place in the soma. During direct stimulation of the soma the site of primary origin of the axonal spike depends on the strength of the stimulating current. With an increase in its strength the site of primary generation of the spike moves closer to the soma.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 7, No. 4, pp. 422–427, July–August, 1975.     

Ionic currents and ion channels of lobster olfactory receptor neurons

The role of the soma of spiny lobster olfactory receptor cells in generating odor-evoked electrical signals was investigated by studying the ion channels and macroscopic currents of the soma. Four ionic currents; a tetrodotoxin-sensitive Na+ current, a Ca++ current, a Ca(++)-activated K+ current, and a delayed rectifier K+ current, were isolated by application of specific blocking agents. The Na+ and Ca++ currents began to activate at -40 to -30 mV, while the K+ currents began to activate at -30 to -20 mV. The size of the Na+ current was related to the presence of a remnant of a neurite, presumably an axon, and not to the size of the soma. No voltage-dependent inward currents were observed at potentials below those activating the Na+ current, suggesting that receptor potentials spread passively through the soma to generate action potentials in the axon of this cell. Steady-state inactivation of the Na+ current was half-maximal at -40 mV. Recovery from inactivation was a single exponential function that was half-maximal at 1.7 ms at room temperature. The K+ currents were much larger than the inward currents and probably underlie the outward rectification observed in this cell. The delayed rectifier K+ current was reduced by GTP-gamma-S and AIF-4, agents which activate GTP-binding proteins. The channels described were a 215-pS Ca(++)-activated K+ channel, a 9.7-pS delayed rectifier K+ channel, and a 35-pS voltage-independent Cl- channel. The Cl- channel provides a constant leak conductance that may be important in stabilizing the membrane potential of the cell.     

Axons Amplify Somatic Incomplete Spikes into Uniform Amplitudes in Mouse Cortical Pyramidal Neurons

Background

Action potentials are the essential unit of neuronal encoding. Somatic sequential spikes in the central nervous system appear various in amplitudes. To be effective neuronal codes, these spikes should be propagated to axonal terminals where they activate the synapses and drive postsynaptic neurons. It remains unclear whether these effective neuronal codes are based on spike timing orders and/or amplitudes.

Methodology/Principal Findings

We investigated this fundamental issue by simultaneously recording the axon versus soma of identical neurons and presynaptic vs. postsynaptic neurons in the cortical slices. The axons enable somatic spikes in low amplitude be enlarged, which activate synaptic transmission in consistent patterns. This facilitation in the propagation of sequential spikes through the axons is mechanistically founded by the short refractory periods, large currents and high opening probability of axonal voltage-gated sodium channels.

Conclusion/Significance

An amplification of somatic incomplete spikes into axonal complete ones makes sequential spikes to activate consistent synaptic transmission. Therefore, neuronal encoding is likely based on spike timing order, instead of graded analogues.