The ionic requirements for the action of acetylcholine on mammalian non-myelinated fibres

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Characterization of neuronal nicotinic acetylcholine receptors in the membrane of unmyelinated human C-fiber axons by in vitro studies

Application of acetylcholine to peripheral nerve terminals in the skin is a widely used test in studies of human small-fiber functions. However, a detailed pharmacological profile and the subunit composition of nicotinic acetylcholine receptors in human C-fiber axons are not known. In the present study, we recorded acetylcholine-induced changes of the excitability and of the intracellular Ca2+ concentration in C-fiber axons of isolated human nerve segments. In addition, using immunohistochemistry, an antibody of a subtype of nicotinic acetylcholine receptor was tested. Acetylcholine and agonists reduced the current necessary for the generation of action potentials in C fibers by <or=30%. This increase in axonal excitability was accompanied by a rise in the free intracellular Ca2+ concentration. The following rank order of potency for agonists was found: epibatidine >> 5-Iodo-A-85380 > 1,1-dimethyl-4-phenylpiperazinium iodide > nicotine > cytisine > acetylcholine; choline had no effect. The epibatidine-induced increase in axonal excitability was blocked by mecamylamine and, less efficiently, by methyllycacontine and dihydro-beta-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the alpha5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of alpha3, alpha5, and beta4 but not of alpha4/beta2 or of alpha7 subunits in the axonal membrane of unmyelinated human C fibers. In addition, the observations suggest that the axonal membrane of C fibers in isolated segments of human sural nerve can be used as a model for presumed cholinergic chemosensitivity of axonal terminals.

Axon-glia interactions modulate axonal excitability in mammalian unmyelinated nerves

A period of electrical activity in unmyelinated nerve fibers is followed by a post-tetanic hyperpolarization (PTH), generated by the hyperactivity of the electrogenic Na(+)-K(+) pump. In order to protect the membrane potential against these strong hyperpolarizations, different types of axonal inward currents are activated during the PTH. We investigated in the rabbit vagus nerve one of these currents, which was activated by carbamylcholine (CCh). We observed that the effect of CCh on the PTH amplitude could be blocked or reversed with scopolamine. Moreover, the PTH amplitude increased when scopolamine alone was added to the perfusate, indicating that an endogenous muscarinic agonist was liberated in the preparation during the period of electrical activity. This CCh-activated current was TEA but not Ba(2+) or Cs(+) sensitive. It has been shown previously that muscarinic acetylcholine receptors (mAChRs) in the rabbit vagus nerve are located on the axonal but not glial membrane and that Schwann cells express several types of purinergic receptors, which activation evoke Ca(2+) transients in Schwann cells. We hypothesise that during electrical activity axons release a transmitter, presumably ATP. This transmitter evoke in the neighbouring Schwann cells a Ca(2+)-dependent liberation of a endogenous muscarinic agonist, which in turn activates a TEA-sensitive inward current in axons. We suggest that the major purpose of this mechanism is the control of the membrane potential during and after a period of intense electrical activity when the Na(+)-K(+) pump generates a robust PTH.

Origin of 5-hydroxytryptamine-induced hyperpolarization of the rat superior cervical ganglion and vagus nerve

1 5-Hydroxytryptamine (5-HT)-induced membrane potential changes were recorded extracellularly from rat superior cervical ganglia (SCG) and cervical vagus nerves in vitro. 2 On the SCG, low concentrations of 5-HT (1 X 10(-8)-3 X 10(-7) M) induced concentration-related hyperpolarization responses. Higher concentrations of 5-HT (1 X 10(-6) 1 X 10(-4) M) induced complex responses which typically consisted of an initial hyperpolarization, followed by a depolarization and subsequent after-hyperpolarization. The depolarization, but not the initial hyperpolarization, was blocked by metoclopramide (3 X 10(-5) M), quipazine (1 X 10(-6) M) or MDL 72222 (1 X 10(-5) M). 3 5-HT-induced hyperpolarization of the SCG was potentiated when the amount of calcium chloride added to the superfusion medium was reduced from 2.5 to 0.15 mmol l-1. Hyperpolarization responses recorded from SCG preparations superfused with this low-calcium medium were unaffected by the substitution of lithium chloride for sodium chloride and were potentiated by the omission of potassium ions. Ouabain (1 X 10(-3) M) abolished both the hyperpolarization and the depolarization induced by 5-HT. 4 On the vagus nerve, 5-HT (1 X 10(-7) - 3 X 10(-5)M) did not induce initial hyperpolarization in either normal or low-calcium Krebs-Henseleit medium. However, in the latter solution only, depolarization responses induced by 5-HT at concentrations of 1 X 10(-6)M or greater were followed by hyperpolarization. Both the depolarization and the post-5-HT hyperpolarization were blocked by metoclopramide (3 X 10(-5)M) but were unaffected by spiperone (1 X 10(-7)M). 5 On the vagus nerve, post-5-HT hyperpolarization responses were selectively and reversibly inhibited by ouabain, and by superfusion with Krebs-Henseleit medium that was either potassium-free or contained lithium chloride in place of sodium chloride. 7 These results demonstrate the generation in the rat SCG of a 5-HT-induced hyperpolarization response that is not mediated through 5-HT3 receptors and is unlikely to be a consequence of depolarization. In contrast, on the rat vagus nerve, the post-5-HT hyperpolarization observed in the present study had the characteristics expected of depolarization-dependent activation of a sodium ion pump.

Presynaptic acetylcholine receptors at the excitatory amino acid synapse in locust muscle

Acetylcholine (greater than or equal to 10(-5) M) applied in the bathing medium to the excitatory nerve-muscle junction of the locust caused a dose-dependent increase in the frequency of spontaneous miniature potentials and in the mean quantal content of evoked potentials. The statistical characteristics of the spontaneous release process were also modified, high frequency bursts of spontaneous potentials and "giant' miniature potentials occurring in the presence of acetylcholine. The response, which was dependent on extracellular calcium concentration, consisted of two distinct phases; these could be induced or abolished selectively by nicotinic and muscarinic drugs, respectively. The results suggest the presence, in low density, of two types of cholinergic receptors on the excitatory motor nerve terminals of the locust. The acetylcholine-induced channels may admit calcium ions. The possible role of these presynaptic receptors is discussed.

Mechanism of nicotine-induced release of noradrenaline from adrenergic nerve endings

1 A study of the mechanism of release of [(3)H]-noradrenaline ([(3)H]-NA) by nicotine from isolated vas deferens of the rat was made using incubation media of different ionic composition.2 Nicotine (20 mug/ml)-induced release of [(3)H]-NA was significantly potentiated in K(+)-free Krebs solution as compared to that in normal Krebs-Ringer solution.3 Nicotine-induced release of [(3)H]-NA was significantly reduced in Na(+)-deficient Krebs solution (containing only 11 mM Na(+)) and was abolished in Na(+)-free Krebs solution.4 In totally depolarized tissues, nicotine failed to cause an outflow of [(3)H]-NA but Ca(2+) (5 mM) did so.5 Nicotine required the presence of Ca(2+) in the incubation medium to cause release of [(3)H]-NA from adrenergic nerve terminals, the magnitude of release being dependent upon the concentration of Ca(2+).6 Nicotine-induced release of [(3)H]-NA was demonstrated in high Ca(2+), Na(+)-free Krebs solution in which all Na(+) had been replaced with Ca(2+).7 It is concluded that nicotine increases the membrane permeability to both Na(+) and Ca(2+). It is also suggested that the increase in permeability to Ca(2+) alone is not sufficient but a local depolarizing action of nicotine is necessary to cause release of noradrenaline from adrenergic nerve endings.

Acetylcholine receptors

The idea that certain drugs and neurotransmitters produce their effects by combining with specific receptors was first clearly expressed by Langley (1905) on the basis of the selective and localized effect of nicotine on striated muscle fibres. In 1914, Langley published a paper in which the antagonism between &lsquo;curari&rsquo; and nicotine was analysed and measured as the ratio by which the nicotine concentration had to be increased in order to produce a standard response in the presence of tubocurarine. It was clear that Langley had in mind the idea of competition between nicotine and curare for the receptor sites and it was surprising that he did not formulatethe theory quantitatively, particularly since Hill, working in Langley's laboratory in 1909, published a mathematical analysis of the action of nicotine on frog muscle giving kinetic and equilibrium equations based on the law of mass action, which could easily have been extended to give an account of competitive antagonism. Barger & Dale (1910) did not favour the idea of specific receptors for sympathomimetic amines.

A study of chemoreceptor and baroreceptor A and C-fibres in the cat carotid nerve

1. 149 A-fibres and 52 C-fibres from the cat carotid nerve were studied in vivo with single-unit recording techniques. These units subserved chemoreceptor and baroreceptor modalities. In addition, half of the C-fibres were determined to be efferent in origin. The estimated fibre diameter spectrum for chemoreceptor and baroreceptor A-fibres is described.2. The discharge pattern of chemoreceptor A and C-fibres was characteristically irregular both at rest and during activation. However, about 5% of the chemoreceptor A-fibre population exhibited a very regular discharge pattern, even at low rates of firing.3. In comparing A and C-fibres, it was found that chemoreceptor and baroreceptor A-fibres had lower thresholds, shorter response latencies, more rapid acceleration of discharge and higher discharge frequencies than their C-fibre counterparts.4. During strong chemoreceptor or baroreceptor stimulation, interaction of the ;spontaneous' whole nerve activity with the evoked A and C-fibre compound action potentials provided a method of estimating the relative proportions of chemoreceptors and baroreceptors in the A and C-fibre populations of the carotid nerve. The A-fibre population was found to be comprised of approximately 2/3 chemoreceptors, 1/3 baroreceptors. The reverse was true for the C-fibre population, i.e. 2/3 baroreceptors, 1/3 chemoreceptors.5. A stepwise C-fibre response is described which may arise from the several C-fibres within a single Schwann cell.

Resting and action potentials recorded by the sucrose-gap method in the superior cervical ganglion of the rabbit

1. Evoked ganglionic action potentials and drug-induced changes in resting potential were recorded by the sucrose-gap method. The height of the action potential was 10-20 mV.2. Since the amplitude of the P wave is inversely proportional to the external potassium concentration between 0.5 and 12 mM, the P wave is probably caused by a phase of increased permeability to K(+).3. In the absence of an anticholinesterase, the depolarizing actions of choline and acetylcholine on the ganglion are almost equal, whereas acetylcholine is 2-3 times more potent than choline on the fibres of the internal carotid nerve. In the presence of eserine, acetylcholine induces a depolarization of the ganglion of up to 14 mV, while increase in [K(+)](o) causes a depolarization of up to 55 mV.4. Although the post-ganglionic fibres in the distal pole of the ganglion are depolarized by drugs, for example, acetylcholine, they contribute little to drug-induced potential changes and probably little to evoked action potentials.5. The hyperpolarization which occurs after exposure of the ganglion to acetylcholine is not due to a phase of increased permeability to K(+).

Influence of the ionic environment on the membrane potential of adrenal chromaffin cells and on the depolarizing effect of acetylcholine

1. Intracellular recordings were made from chromaffin cells isolated from adrenal medullae of gerbils to examine the effects, on membrane potential, of changes in the ionic environment that are known, from other experiments, to influence the rate of catecholamine secretion.2. Depolarization in response to acetylcholine fell linearly with the logarithm of the extracellular sodium concentration over the range 154-3 mM and reached a value, in sodium-free medium, of about 30% of the control value.3. The depolarizing effect of acetylcholine in sodium-free media increased linearly with the logarithm of the extracellular calcium concentration over the range 1-117 mM. It is concluded that depolarization in response to acetylcholine involves inward movement of both sodium and calcium ions.4. Depolarization was also observed in response to the secretagogues, excess potassium and barium, both in sodium-rich and sodium-free media. The effect of barium was antagonized by calcium, and it is suggested that these two cations interact at the level of the plasma membrane.5. Depolarization does not appear to be tightly coupled to secretion, for acetylcholine or excess potassium still depolarized the chromaffin cells when the environment was calcium-free or contained an excess of magnesium, conditions that inhibit secretion. Furthermore, although acetylcholine had some depolarizing effect in sodium-free media, the level to which the membrane potential fell was not below the control ;resting' potential since the cells in sodium-free medium were hyperpolarized; yet, secretory responses are augmented in such conditions.6. It is proposed that depolarization in response to acetylcholine may be no more than the electrical sign of increased permeability to ions such as sodium and calcium, and that depolarization is not, in itself, a key event in stimulus-secretion coupling. The evidence is held to favour the view that movement of calcium into the chromaffin cells on exposure to acetylcholine is responsible for evoking secretion.

The oxygen consumption of mammalian non-myelinated nerve fibres at rest and during activity

1. A study has been made of the oxygen consumption of non-myelinated nerve fibres of rabbit desheathed cervical vagus nerves at rest and during activity.2. The average resting oxygen consumption (Q(r)) was 0.0924 mumole/g. min at 21 degrees C. Stimulation for 1-3 min at 3/sec caused an extra oxygen consumption (Q(s)) of 816 p-mole/g.shock.3. When the frequency of stimulation was increased, to 10/sec and 30/sec, Q(s) fell. When the frequency was decreased, to 1/sec and 0.3/sec, Q(s) increased slightly.4. When the temperature was decreased, Q(r) fell; when the temperature was increased, Q(s) also increased. Temperature similarly affected Q(s) with high frequencies of stimulation, but had relatively little effect on Q(s) at low frequencies of stimulation.5. An isolated single shock seemed to produce an increase in oxygen consumption of about 1200 p-mole/g, and this value was largely independent of temperature.6. When part of the sodium in the Locke solution was replaced by barium, Q(r) decreased (by 12%) whereas Q(s) increased (by 87%).7. Veratrine (1 mug/ml.) increased both Q(r) (by 142%) and Q(s) (by 361%).8. Acetylcholine (1.7 mM) increased Q(r) (by 32%).9. When nerves were transferred to potassium-free solutions there was little change in Q(r), and Q(s) fell slightly (by 8%).10. When the potassium concentration in the Locke solution was increased 4-fold, Q(r) increased (by 27%).11. Salicylate (1-10 mM) increased Q(r) (by 24%) and abolished Q(s).12. When the sodium of Locke solution was replaced by lithium, Q(r) decreased (by 19%) and Q(s) was abolished.13. In sodium-Locke solution ouabain (100 muM) decreased Q(r) (by 26%) and abolished Q(s). In lithium-Locke solution ouabain also decreased Q(r) (by 28%).14. All or nearly all of the oxygen consumed at rest or during activity seemed to be used to pump potassium ions into, and sodium ions out of, the axoplasm.15. The K/O(2) ratio during pumping was about 5.0.

Lithium's failure to replace sodium in mammalian sympathetic ganglia

Ganglionic responses to electrical stimulation, acetylcholine, and potassium ions were studied in superior cervical ganglia of. cats perfused with media containing lithium chloride instead of sodium chloride. In lithium-Locke, ganglionic transmission and depolarization evoked by acetylcholine were blocked completely but reversibly, while the depolarization produced by potassium ions was unaltered.

The movements of labelled ions in mammalian non-myelinated nerve fibres

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