The use of the rat isolated vagus nerve for functional measurements of the effect of drugs in vitro

Innate gut microbiota predisposes to high alcohol consumption

Gut microbiota is known to be transferred from the mother to their offspring. This study determines whether the innate microbiota of rats selectively bred for generations as high alcohol drinkers play a role in their alcohol intake. Wistar-derived high-drinker UChB rats (intake 10-g ethanol/kg/day) administered nonabsorbable oral antibiotics before allowing access to alcohol, reducing their voluntary ethanol intake by 70%, an inhibition that remained after the antibiotic administration was discontinued. Oral administration of Lactobacillus rhamnosus Gorbach-Goldin (GG) induced the synthesis of FGF21, a vagal β-Klotho receptor agonist, and partially re-invoked a mechanism that reduces alcohol intake. The vagus nerve constitutes the main axis transferring gut microbiota information to the brain ("microbiota-gut-brain" axis). Bilateral vagotomy inhibited rat alcohol intake by 75%. Neither antibiotic treatment nor vagotomy affected total fluid intake. A microbiota-mediated marked inflammatory environment was observed in the gut of ethanol-naïve high-drinker rats, as gene expression of proinflammatory cytokines (TNF-α; IL-6; IL-1β) was significantly reduced by nonabsorbable antibiotic administration. Gut cytokines are known to activate the vagus nerve, while vagal activation induces pro-rewarding effects in nucleus accumbens. Both alcoholics and alcohol-preferring rats share a marked preference for sweet tastes-likely an evolutionary trait to seek sweet fermented fruits. Saccharin intake by UChB rats was inhibited by 75%-85% by vagotomy or oral antibiotic administration, despite saccharin-induced polydipsia. Overall, data indicate that the mechanisms that normally curtail heavy drinking are inhibited in alcohol-preferring animals and inform a gut microbiota origin. Whether it applies to other mammals and humans merits further investigation.

Essential Oil of Croton zehntneri Prevents Conduction Alterations Produced by Diabetes Mellitus on Vagus Nerve

Autonomic diabetic neuropathy (ADN) is a complication of diabetes mellitus (DM), to which there is no specific treatment. In this study, the efficacy of the essential oil of Croton zehntneri (EOCz) in preventing ADN was evaluated in the rat vagus nerve. For the two fastest conducting myelinated types of axons of the vagus nerve, the conduction velocities and rheobase decreased, whilst the duration of the components of the compound action potential of these fibers increased. EOCz completely prevented these DM-induced alterations of the vagus nerve. Unmyelinated fibers were not affected. In conclusion, this investigation demonstrated that EOCz is a potential therapeutic agent for the treatment of ADN.

Fibre sub-type specific conduction reveals metabolic function in mouse sciatic nerve

KEY POINTS

We have developed an improved method that enables simultaneous recording of stimulus evoked compound action potentials from large myelinated A fibres and small unmyelinated C fibres in mouse sciatic nerves. Investigations into the ability of fructose to support conduction in sciatic nerve revealed a novel glia-to-axon metabolic pathway in which fructose is converted in Schwann cells to lactate for subsequent shuttling to A fibres. The C fibres most likely directly take up and metabolise fructose. These differences are indicative of fibre sub-type specific metabolic profiles. These results demonstrate that the physiological insights provided by the method can be applied to investigations of peripheral nerve, with a view to understanding the metabolic disruptions that underlie diabetic neuropathy.

ABSTRACT

The stimulus evoked compound action potential (CAP), recorded using suction electrodes, provides an index of the relative number of conducting axons within a nerve trunk. As such the CAP has been used to elucidate the diverse mechanisms of injury resulting from a variety of metabolic insults to central nervous white matter, whilst also providing a model with which to assess the benefits of clinically relevant neuroprotective strategies. In addition the technique lends itself to the study of metabolic cell-to-cell signalling that occurs between glial cells and neurones, and to exploring the ability of non-glucose substrates to support axon conduction. Although peripheral nerves are sensitive to metabolic insult and are susceptible to diabetic neuropathy, there is a lack of fundamental information regarding peripheral nerve metabolism. A confounding factor in such studies is the extended duration demanded by the experimental protocol, requiring stable recording for periods of many hours. We describe a method that allows us to record simultaneously the stimulus evoked CAPs from A and C fibres from mouse sciatic nerve, and demonstrate its utility as applied to investigations into fibre sub-type substrate use. Our results suggest that C fibres directly take up and metabolise fructose, whereas A fibre conduction is supported by fructose-derived lactate, implying there exist unique metabolic profiles in neighbouring fibre sub-types present within the same nerve trunk.

TRPA1 insensitivity of human sural nerve axons after exposure to lidocaine

TRPA1 is an ion channel of the TRP family that is expressed in some sensory neurons. TRPA1 activity provokes sensory symptoms of peripheral neuropathy, such as pain and paraesthesia. We have used a grease gap method to record axonal membrane potential and evoked compound action potentials (ECAPs) in vitro from human sural nerves and studied the effects of mustard oil (MO), a selective activator of TRPA1. Surprisingly, we failed to demonstrate any depolarizing response to MO (50, 250 μM) in any human sural nerves. There was no effect of MO on the A wave of the ECAP, but the C wave was reduced at 250 μM. In rat saphenous nerve fibres MO (50, 250 μM) depolarized axons and reduced the C wave of the ECAP but had no effect on the A wave. By contrast, both human and rat nerves were depolarized by capsaicin (0.5 to 5 μM) or nicotine (50 to 200 μM). Capsaicin caused a profound reduction in C fibre conduction in both species but had no effect on the amplitude of the A component. Lidocaine (30 mM) depolarized rat saphenous nerves acutely, and when rat nerves were pretreated with 30 mM lidocaine to mimic the exposure of human nerves to local anaesthetic during surgery, the effects of MO were abolished whilst the effects of capsaicin were unchanged. This study demonstrates that the local anaesthetic lidocaine desensitizes TRPA1 ion channels and indicates that it may have additional mechanisms for treating neuropathic pain that endure beyond simple sodium channel blockade.

Perivagal antagonist treatment in rats selectively blocks the reflex and afferent responses of vagal lung C fibers to intravenous agonists

The terminals of vagal lung C fibers (VLCFs) express various types of pharmacological receptors that are important to the elicitation of airway reflexes and the development of airway hypersensitivity. We investigated the blockade of the reflex and afferent responses of VLCFs to intravenous injections of agonists using perivagal treatment with antagonists (PAT) targeting the transient receptor potential vanilloid 1, P2X, and 5-HT(3) receptors in anesthetized rats. Blockading these responses via perivagal capsaicin treatment (PCT), which blocks the neural conduction of C fibers, was also studied. We used capsaicin, α,β-methylene-ATP, and phenylbiguanide as the agonists, and capsazepine, iso-pyridoxalphosphate-6-azophenyl-2',5'-disulfonate, and tropisetron as the antagonists of transient receptor potential vanilloid 1, P2X, and 5-HT(3) receptors, respectively. We found that each of the PATs abolished the VLCF-mediated reflex apnea evoked by the corresponding agonist, while having no effect on the response to other agonists. Perivagal vehicle treatment failed to produce any such blockade. These blockades had partially recovered at 3 h after removal of the PATs. In contrast, PCT abolished the reflex apneic response to all three agonists. Both PATs and PCT did not affect the myelinated afferent-mediated apneic response to lung inflation. Consistently, our electrophysiological studies revealed that each of the PATs prevented the VLCF responses to the corresponding agonist, but not to any other agonist. PCT inevitably prevented the VLCF responses to all three agonists. Thus these PATs selectively blocked the stimulatory action of corresponding agonists on the VLCF terminals via mechanisms that are distinct from those of PCT. PAT may become a novel intervention for studying the pharmacological modulation of VLCFs.

Transcompartmental reversal of single fibre hyperexcitability in juxtaparanodal Kv1.1-deficient vagus nerve axons by activation of nodal KCNQ channels

Kv1.1 channels cluster at juxtaparanodes of myelinated axons in the vagus nerve, the primary conduit for parasympathetic innervation of the heart. Kcna1-null mice lacking these channels exhibit neurocardiac dysfunction manifested by atropine-sensitive atrioventricular conduction blocks and bradycardia that may culminate in sudden death. To evaluate whether loss of Kv1.1 channels alters electrogenic properties within the nerve, we compared the intrinsic excitability of single myelinated A- and Aδ-axons from excised cervical vagus nerves of young adult Kcna1-null mice and age-matched, wild-type littermate controls. Although action potential shapes and relative refractory periods varied little between genotypes, Kv1.1-deficient large myelinated A-axons showed a fivefold increase in susceptibility to 4-aminopyridine (4-AP)-induced spontaneous ectopic firing. Since the repolarizing currents of juxtaparanodal Kv1 channels and nodal KCNQ potassium channels both act to dampen repetitive activity, we examined whether augmenting nodal KCNQ activation could compensate for Kv1.1 loss and reverse the spontaneous hyperexcitability in Kv1.1-deficient A-axons. Application of the selective KCNQ opener flupirtine raised A-axon firing threshold while profoundly suppressing 4-AP-induced spontaneous firing, demonstrating a functional synergy between the two compartments. We conclude that juxtaparanodal Kv1.1-deficiency causes intrinsic hyperexcitability in large myelinated axons in vagus nerve which could contribute to autonomic dysfunction in Kcna1-null mice, and that KCNQ openers reveal a transcompartmental synergy between Kv1 and KCNQ channels in regulating axonal excitability.

Effects of high-frequency alternating current on axonal conduction through the vagus nerve

High-frequency alternating current (HFAC) is known to disrupt axonal conduction in peripheral nerves, and HFAC has much potential as a therapeutic approach for a number of pathological conditions. Many previous studies have utilized motor output as a bioassay of effects of HFAC on conduction through medium- to large-diameter motor axons. However, little is known about the effectiveness of HFAC on smaller, more slowly conducting nerve fibres. The present study tested whether HFAC influences axonal conduction through sub-diaphragmatic levels of the rat vagus nerve, which consists almost entirely of small calibre axons. Using an isolated nerve preparation, we tested the effects of HFAC on electrically evoked compound action potentials (CAPs). We found that delivery of charge-balanced HFAC at 5000 Hz for 1 min was effective in producing reversible blockade of axonal conduction. Both Aδ and C components of the vagus CAP were attenuated, and the degree of blockade as well as time to recovery was proportional to the amount of HFAC current delivered. The Aδ waves were more sensitive than C waves to HFAC blockade, but they required more time to recover.

Rosewood oil induces sedation and inhibits compound action potential in rodents

AIM OF THE STUDY

Aniba rosaeodora is an aromatic plant which has been used in Brazil folk medicine due to its sedative effect. Therefore, the purpose of the present study was to evaluate the sedative effect of linalool-rich rosewood oil in mice. In addition we sought to investigate the linalool-rich oil effects on the isolated nerve using the single sucrose-gap technique.

MATERIALS AND METHODS

Sedative effect was determined by measuring the potentiation of the pentobarbital-induced sleeping time. The compound action potential amplitude was evaluated as a way to detect changes in excitability of the isolated nerve.

RESULTS

The results showed that administration of rosewood oil at the doses of 200 and 300 mg/kg significantly decreased latency and increased the duration of sleeping time. On the other hand, the dose of 100 mg/kg potentiated significantly the pentobarbital action decreasing pentobarbital latency time and increasing pentobarbital sleeping time. In addition, the effect of linalool-rich rosewood oil on the isolated nerve of the rat was also investigated through the single sucrose-gap technique. The amplitude of the action potential decreased almost 100% when it was incubated for 30 min at 100 microg/ml.

CONCLUSIONS

From this study, it is suggested a sedative effect of linalool-rich rosewood oil that could, at least in part, be explained by the reduction in action potential amplitude that provokes a decrease in neuronal excitability.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

Neurotoxic activity of Gln49 phospholipase A2 fromGloydius ussuriensis snake venom

A novel neurotoxic protein phospholipase A(2) (PLA(2)), molecular weight 13 881.83 Da, has been isolated from snake venom of Gloydius ussuriensis, named as Gln49-PLA(2), which shows weak lethal toxic, myotoxic and apparent anticoagulant activity, but lacks phospholipase activity. The Gln49-PLA(2) obviously induced an increase of the pain threshold in intoxicated 615 mice compared with the control group, suggesting it is a neurotoxin. Hot-plate tests also showed that its analgesic activity was dose-dependent, and naloxone antagonized the analgesic effect, implying the mechanism of action of Gln49-sPLA(2) is correlated with opioid receptors. Electrophysiology studies revealed decreases in the action potential and the nerve conduction velocity in isolated hoptoad (Bufo bufo gargarizans Cantor) sciatic nerve, indicating Gln49-PLA(2) most probably had effects on ion channels.

Phenotypic characterization of gastric sensory neurons in mice

Recent studies suggest that the capsaicin receptor [transient receptor potential vanilloid (TRPV)1] may play a role in visceral mechanosensation. To address the potential role of TRPV1 in vagal sensory neurons, we developed a new in vitro technique allowing us to determine TRPV1 expression directly in physiologically characterized gastric sensory neurons. Stomach, esophagus, and intact vagus nerve up to the central terminations were carefully dissected and placed in a perfusion chamber. Intracellular recordings were made from the soma of nodose neurons during mechanical stimulation of the stomach. Physiologically characterized neurons were labeled iontophoretically with neurobiotin and processed for immunohistochemical experiments. As shown by action potential responses triggered by stimulation of the upper thoracic vagus with a suction electrode, essentially all abdominal vagal afferents in mice conduct in the C-fiber range. Mechanosensitive gastric afferents encode stimulus intensities over a wide range without apparent saturation when punctate stimuli are used. Nine of 37 mechanosensitive vagal afferents expressed TRPV1 immunoreactivity, with 8 of the TRPV1-positive cells responding to stretch. A small number of mechanosensitive gastric vagal afferents express neurofilament heavy chains and did not respond to stretch. By maintaining the structural and functional integrity of vagal afferents up to the nodose ganglion, physiological and immunohistochemical properties of mechanosensory gastric sensory neurons can be studied in vitro. Using this novel technique, we identified TRPV1 immunoreactivity in only one-fourth of gastric mechanosensitive neurons, arguing against a major role of this ion channel in sensation of mechanical stimuli under physiological conditions.