Excitable properties and underlying Na+ and K+ currents in neurons from the guinea-pig jugular ganglion

Model of Impedance Changes in Unmyelinated Nerve Fibers

OBJECTIVE

Currently, there is no imaging method that is able to distinguish the functional activity inside nerves. Such a method would be essential for understanding peripheral nerve physiology and would allow precise neuromodulation of organs these nerves supply. Electrical impedance tomography (EIT) is a method that produces images of electrical impedance change (dZ) of an object by injecting alternating current and recording surface voltages. It has been shown to be able to image fast activity in the brain and large peripheral nerves. To image inside small autonomic nerves, mostly containing unmyelinated fibers, it is necessary to maximize SNR and optimize the EIT parameters. An accurate model of the nerve is required to identify these optimal parameters as well as to validate data obtained in the experiments.

METHODS

In this study, we developed two three-dimensional models of unmyelinated fibers: Hodgkin-Huxley (HH) squid giant axon (single and multiple) and mammalian C-nociceptor. A coupling feedback system was incorporated into the models to simulate direct and alternating current application and simultaneously record external field during action potential propagation.

RESULTS

Parameters of the developed models were varied to study their influence on the recorded impedance changes; the optimal parameters were identified. The negative dZ was found to monotonically decrease with frequency for both HH and C fiber models, in accordance with the experimental data.

CONCLUSION AND SIGNIFICANCE

The accurate realistic model of unmyelinated nerve allows the optimization of EIT parameters and matches literature and experimental results.

Plasticity of the afferent innervation of the airways: The role of ion channels

Neuronal pathways associated with cough exhibit remarkable plasticity that can result in a persistent and uncontrollable urge to cough during disease. Afferent neurones involved in detecting tussive stimuli are polymodal, i.e. they respond to several types of stimuli including acid, inflammatory mediators such as bradykinin and mechanical stimuli. The pattern of action potential discharge following the encounter of the nerve terminal with a tussive stimulus is likely to determine the magnitude of the urge to cough and cough itself. The discharge pattern in sensory neurones is determined by multiple distinct voltage-gated ion channels. The function of many of these channels can be modulated via several signal transduction pathways coupled to receptors for a variety of inflammatory mediators. In particular, a key role of voltage-gated Na(+) and K(+) channel subtypes in shaping action potential discharge patterns in sensory neurones seems apparent. This modulation of transduction pathways may be an underlying mechanism of cough reflex plasticity.

Plasticity of vagal afferent fibres mediating cough

Increased sensitivity of cough pathways has been demonstrated in numerous studies. The underlying mechanisms of this sensitization are largely unknown; however, a burgeoning body of evidence suggests that vagal primary afferent neurones that innervate the airways are likely to be involved. This plasticity includes changes in anatomy, neurochemistry and function. PGE2 is an example of an inflammatory mediator that increases responsiveness to tussive stimuli. Electrophysiological studies of neurone cell bodies isolated from afferent ganglia have revealed that prostanoids modulate the function of a variety of distinct ion channels including those that carry TTX-insensitive voltage-gated sodium currents, slow post-spike hyperpolarizations and a hyperpolarization-activated cation current. Mediator-induced modulation of the function of various voltage-gated currents operating at the peripheral terminals of airway afferent neurons would probably influence input from the airways into the central nervous system and contribute to the urge to cough and increased responsiveness to tussive stimuli.

Bronchopulmonary afferent nerves

Vagal afferent nerves are the primary communication pathways between the bronchopulmonary system and the central nervous system. Input from airway afferent nerves to the CNS is integrated in the brainstem and ultimately leads to sensations and various reflex outputs. Afferent nerves innervating the airways can be classified into various distinct phenotypes. However, there is no single classification scheme that takes all features, including conduction velocity, cell body diameter, ganglionic origin, and stimuli to which they respond (modality) into account. At present, bronchopulmonary afferent nerves are typically considered to belong to one of three general categories, namely C-fibres, rapidly adapting stretch receptors (RARs), and slowly adapting stretch receptors (SARs). As our understanding of bronchopulmonary afferent nerves continues to deepen, we are likely to see more sophisticated classification schemes emerge. It is clear that the function of afferent fibres can be substantively influenced by airway inflammation and remodelling. The perturbations and perversions of afferent nerve function that occur during these states almost certainly contributes to many of the signs and symptoms of inflammatory airway disease. A more lucid characterization of bronchopulmonary afferent nerves, and a better understanding of the mechanisms by which these nerves influence pulmonary physiology during health and disease anticipates future research.

Pharmacology of vagal afferent nerve activity in guinea pig airways

The excitability and activity of vagal afferent nerves innervating the airways can be pharmacologically increased and decreased. Autacoids released as a result of airway inflammation can lead to substantial increases in afferent nerve activity, consequently altering pulmonary reflex physiology. In a manner analogous to hyperalgesia associated with inflammation in the somato-sensory system, increases in vagal afferent nerve activity in inflamed airways may lead to a heightened cough reflex, and increases in autonomic activity in the airways. These effects may contribute to many of the symptoms of inflammatory airway disease. Here we provide a brief overview of some of the mechanisms by which the afferent activity in airway nerves can be pharmacologically modified.

Inhibitory effects of organotin compounds on voltage-dependent, tetrodotoxin-resistant Na+ channel current in guinea pig dorsal root ganglion cells

The effects of organotin compounds on voltage-dependent, tetrodotoxin (TTX)-resistant Na+ channel current (I(Na)) in single cells isolated from guinea pig dorsal root ganglion were investigated using a whole cell patch clamp technique. Extracellular application of tributyltin (TBT) inhibited I(Na) in a concentration-dependent manner with an IC50 of 7.2 microM. TBT (100 microM), when applied intracellularly, was without effect. Triphenyltin (TPT, 100 microM) and dibutyltin (DBT, 100 microM), applied extracellularly, inhibited I(Na) with an efficacy ranking of TBT>TPT>DBT. Monobutyltin (100 microM), whether applied externally or internally, had little effect on I(Na). TBT (30 microM) significantly prolonged both time to peak and half-decay time of I(Na) and shifted the activation curve of I(Na) in the positive direction without changing the slope. No such effect was produced by TPT (100 microM). The results indicate that organotin compounds inhibit voltage-dependent, TTX-resistant Na+ channel activity and suggest that the inhibitory action may account, at least in part, for their neurotoxic effects.

Pharmacology of airway afferent nerve activity

Afferent nerves in the airways serve to regulate breathing pattern, cough, and airway autonomic neural tone. Pharmacologic agents that influence afferent nerve activity can be subclassified into compounds that modulate activity by indirect means (e.g. bronchial smooth muscle spasmogens) and those that act directly on the nerves. Directly acting agents affect afferent nerve activity by interacting with various ion channels and receptors within the membrane of the afferent terminals. Whether by direct or indirect means, most compounds that enter the airspace will modify afferent nerve activity, and through this action alter airway physiology.

Ion channels in airway afferent neurons

Action potentials initiated at the peripheral terminal of an afferent nerve are conducted to the central nervous system therein causing release of neurotransmitters that excite secondary neurons in the brain stem or spinal cord. Various chemicals, extremes in osmolarity and pH as well as mechanical stimuli are sensed by primary afferent nerves that innervate the airways. The processes leading to excitation of afferent nerve endings, conduction of action potentials along axons, transmitter secretion, and neuronal excitability are regulated by ions flowing through channels in the nerve membrane. Voltage-gated ion channels selective for K+ and Na+ ions allow the generation and conduction of action potentials and along with families of ion channels selective for other ions such as Ca2+ or Cl- are thought to play distinctive roles in regulating neuronal excitability and transmitter secretion. Here we discuss, in general terms, the roles played by various classes of ion channels in the activation, neurotransmitter secretion and excitability of primary afferent neurons.

Ion channel modifying agents influence the electrical activity generated by canine intrinsic cardiac neurons in situ

This study was designed to establish whether agents known to modify neuronal ion channels influence the behavior of mammalian intrinsic cardiac neurons in situ and, if so, in a manner consistent with that found previously in vitro. The activity generated by right atrial neurons was recorded extracellularly in varying numbers of anesthetized dogs before and during continuous local arterial infusion of several neuronal ion channel modifying agents. Veratridine (7.5 µM), the specific modifier of Na+-selective channels, increased neuronal activity (95% above control) in 80% of dogs tested (n = 25). The membrane depolarizing agent potassium chloride (40 mM) reduced neuronal activity (43% below control) in 84% of dogs tested (n = 19). The inhibitor of voltage-sensitive K+ channels, tetraethylammonium (10 mM), decreased neuronal activity (42% below control) in 73% of dogs tested (n = 11). The nonspecific potassium channel inhibitor barium chloride (5 mM) excited neurons (47% above control) in 13 of 19 animals tested. Cadmium chloride (200 µM), which inhibits Ca2+-selective channels and Ca2+-dependent K+ channels, increased neuronal activity (65% above control) in 79% of dogs tested (n = 14). The specific L-type Ca2+ channel blocking agent nifedipine (5 µM) reduced neuronal activity (52% blow control in 72% of 11 dogs tested), as did the nonspecific inhibitor of L-type Ca2+ channels, nickel chloride (5 mM) (36% below control in 69% of 13 dogs tested). Each agent induced either excitatory or inhibitory responses, depending on the agent tested. It is concluded that specific ion channels (INa, ICaL, IKv, and IKCa) that have been associated with intrinsic cardiac neurons in vitro are involved in their capacity to generate action potentials in situ.Key words: calcium channels, intrinsic cardiac neuron, potassium channels, sodium channels.

Potassium channel blockade induces action potential generation in guinea-pig airway vagal afferent neurones

Electrophysiological studies of vagal sensory nerves with cell bodies in the nodose ganglion and mechanically sensitive receptive fields in the guinea-pig trachea/bronchus, were performed. Exposure of the mechanically sensitive receptive fields to 4-aminopyridine (100 microM-1 mM) caused pronounced action potential discharge in all fibres studied. Action potential generation was also produced by alpha-dendrotoxin, and in a subset of fibres, by barium. By contrast, neither iberiotoxin, tetraethyl ammonium, glybenclamide, BDS-II, nor apamin caused action potential generation in the vagal afferent nerve fibres. Tetramethylrhodamine dextran was instilled into the trachea to retrogradely label cell bodies within the nodose ganglion. In these cells, 4-aminopyridine caused a large depolarization of the resting membrane potential, concomitant with an increase in input impedance. The data suggest 4-aminopyridine- and alpha-dendrotoxin-sensitive ion channels within the airway afferent nerve membrane hold the resting membrane potential below the threshold for action potential generation. Mechanisms that lead to an inhibition of these channels will likely lead to an increase in excitability of the airway afferent neurones.

Adaptation of guinea-pig vagal airway afferent neurones to mechanical stimulation

1. Intracellular and extracellular electrophysiological recording techniques were employed to examine the mechanisms involved in adaptation of guinea-pig airway sensory neurones to suprathreshold mechanical stimulation in vitro. Extracellular recordings performed using an in vitro airway preparation revealed two unambiguously distinct subsets of mechanically sensitive nerve endings in the trachea/bronchus. In one group of fibres, the mechanical stimulus caused a brief burst of action potentials, after which the nerve rapidly adapted. In the other group of fibres, repetitive action potentials were evoked as long as the stimulus was maintained above threshold. 2. The adaptation response strictly correlated with ganglionic origin of the soma. Those fibres derived from the nodose ganglion adapted rapidly, whereas those derived from the jugular ganglion were slowly or non-adapting. 3. Intracellular recordings from airway-identified neurones in isolated intact ganglia revealed that the majority of neurones within either the nodose or jugular ganglion adapted rapidly to prolonged suprathreshold depolarizing current injections. 4. The electrophysiological adaptation of nodose ganglion-derived neurones following prolonged suprathreshold current steps was greatly reduced by 4-aminopyridine. However, 4-aminopyridine did not affect the adaptation of rapidly adapting nodose ganglion-derived nerve endings in response to mechanical stimuli. 5. The data suggest that ganglionic origin dictates adaptive characteristics of guinea-pig tracheal and mainstem bronchial afferent neurones in response to mechanical stimulation. Also, the rapid adaptation of nodose nerve endings in the trachea observed during a mechanical stimulus does not appear to be related to the adaptation observed at the soma during prolonged suprathreshold depolarizing current injections.

Electrophysiological evidence for tetrodotoxin-resistant sodium channels in slowly conducting dural sensory fibers

A tetrodotoxin (TTX)-resistant sodium channel was recently identified that is expressed only in small diameter neurons of peripheral sensory ganglia. The peripheral axons of sensory neurons appear to lack this channel, but its presence has not been investigated in peripheral nerve endings, the site of sensory transduction in vivo. We investigated the effect of TTX on mechanoresponsiveness in nerve endings of sensory neurons that innervate the intracranial dura. Because the degree of TTX resistance of axonal branches could potentially be affected by factors other than channel subtype, the neurons were also tested for sensitivity to lidocaine, which blocks both TTX-sensitive and TTX-resistant sodium channels. Single-unit activity was recorded from dural afferent neurons in the trigeminal ganglion of urethan-anesthetized rats. Response thresholds to mechanical stimulation of the dura were determined with von Frey monofilaments while exposing the dura to progressively increasing concentrations of TTX or lidocaine. Neurons with slowly conducting axons were relatively resistant to TTX. Application of 1 microM TTX produced complete suppression of mechanoresponsiveness in all (11/11) fast A-delta units [conduction velocity (c.v.) 5-18 m/s] but only 50% (5/10) of slow A-delta units (1.5 <c.v.<5 m/s) and 13% (2/15) of C units (c.v. </=1.5 m/s). The mean TTX concentration that produced complete suppression of mechanoresponsiveness was approximately 270-fold higher in C units than in fast A-delta units. In contrast, no significant difference was found between C and A-delta units in the concentration of lidocaine required for complete suppression of mechanoresponsiveness, indicating that the greater TTX resistance of mechanoresponsiveness in C units is not attributable to differences in safety factor unrelated to channel subtype. These data offer indirect evidence that a TTX-resistant channel subtype is expressed in the terminal axonal branches of many of the more slowly conducting (C and slow A-delta) dural afferents. The channel appears to be present in these fibers, but not in the faster A-delta fibers, in sufficient numbers to support the initiation and propagation of mechanically induced impulses. Comparison with previous data on the absence of TTX resistance in peripheral nerve fibers suggests that the TTX-resistant sodium channel may be a distinctive feature of the receptive rather than the conductive portion of the sensory neuron's axonal membrane.