Can Receptor Potentials Be Detected With Threshold Tracking in Rat Cutaneous Nociceptive Terminals?

Sensitized peripheral nociception in experimental diabetes of the rat

Painful neuropathy is a common complication of diabetes. Particularly in the early stage of diabetic neuropathy, patients are characterized by burning feet, hyperalgesia to heat, and mechanical stimuli, as if residual nociceptors were sensitized. Such symptoms are barely explained by common pathophysiological concepts of diabetic neuropathy. Diabetes was induced in Wistar rats by streptozotocin (STZ). After 4 weeks behavioral testing (Plantar test, Randall-Selitto) was conducted. Basal and stimulated release of calcitonin gene-related peptide (CGRP), Substance P (SP) and prostaglandin E(2) (PGE(2)) from isolated skin and sciatic nerve were assessed by enzyme immunoassays. Electrophysiological properties of identified nociceptors under hyperglycemic, hypoxic, and acidotic conditions were investigated using the skin-nerve preparation. The diabetic rats showed hyperalgesia to heat and pressure stimulation. The basal CGRP/SP release was reduced, but chemical stimulation with bradykinin induced greater release of SP, CGRP and PGE(2) than in control animals. In contrast, capsaicin-stimulated CGRP release was reduced in sciatic nerves. Hypoxia per se lowered von Frey thresholds of most C-nociceptors to half. Hyperglycemic hypoxia induced ongoing discharge in all diabetic but not control C-fibers which was further enhanced under acidosis. Sensory and neurosecretory nociceptor functions are sensitized in diabetes. Diabetic C-fibers show exaggerated sensitivity to hyperglycemic hypoxia with and without additional acidosis, conditions that are thought to mimic ischemic episodes in diabetic nerves. Ongoing C-fiber discharge is known to induce spinal sensitization. Together with altered receptor and ion channel expressions this may contribute to painful episodes in diabetic neuropathy.

TRPV1 controls acid- and heat-induced calcitonin gene-related peptide release and sensitization by bradykinin in the isolated mouse trachea

Chronic cough derives from inflammatory hypersensitivity of tracheobronchial nerve endings, most of which express the polymodal capsaicin receptor-channel transient receptor potential vanilloid (TRPV) type 1 and the secretory neuropeptide calcitonin gene-related peptide (CGRP). An isolated mouse trachea preparation was established to measure chemically and thermally stimulated CGRP release as an index for sensory transduction of potential cough-inducing stimuli. TRPV1 knockout mice were employed to assess the TRPV1 contribution to tracheal responsiveness and sensitization. Graded heat-induced CGRP release depended entirely on extracellular calcium and partly on TRPV1; knockout mice showed 60% less CGRP release at 45 degrees C (for 5 min) than wild-types. This heat response was facilitated by the TRPV1 agonist ethanol and the TRPV1-3 agonist 2-aminoethoxydiphenyl borate, effects that were reduced or absent in TRPV1(-/-), respectively. The TRPV1 antagonists ruthenium red and N-(4-t-butylphenyl)-4-(3-chloropyridin-2-yl) tetrahydropyrazine-1(2H)-carboxamide were ineffective on the basal heat response. A step increase of temperature from 22 to 40 degrees C caused a TRPV1-independent CGRP release that was doubled by bradykinin in wild-types but not TRPV1(-/-). Proton stimulation resulted in a bell-shaped concentration-response curve with threshold at pH 6.7 and a maximum at pH 5.7; responses were greatly reduced but not abolished in TRPV1(-/-). Coadministration of amiloride (30 microm), the blocker of acid-sensing ion channels, was ineffective in both TRPV1 genotypes. The data suggest that tracheal acid sensing mainly involves TRPV1 but not acid-sensing ion channels, whereas noxious heat responsiveness partly depends and (inflammatory) sensitization to heat largely depends on the capsaicin receptor in tracheal nerve endings. Lowering of their heat threshold to near body temperature may sustain hypersensitivity and neurogenic inflammation of the upper airways.

Retigabine, the specific KCNQ channel opener, blocks ectopic discharges in axotomized sensory fibres

The M-current has been proposed as a potential target for analgesia under neuropathic pain conditions. M-currents and/or their molecular correlates, KCNQ proteins, have been demonstrated in key elements of the nociceptive system including spinal and dorsal root ganglion neurons. Here we demonstrate that retigabine, a selective KCNQ channel opener, applied at neuromatose endings modulates the excitability of axotomized fibres inhibiting ectopic discharges. Responses to mechanical and chemical stimulation were obtained from intact and previously axotomized Adelta- and C-fibres using in vitro preparations and extracellular electrophysiological recording techniques. Application of retigabine (10 microM) produced an estimated approximately 80% reduction in the number of discharges produced by mechanical and chemical stimulation of most axotomized fibres tested (24/27). The electrical threshold of stimuli applied to the neuroma was found to increase in the presence of retigabine (+17.5+/-2.3%) and to decrease in the presence of a high potassium medium (-16.5+/-3.7%). This indicates that retigabine produces a hyperpolarization and a subsequent reduction of the excitability in aberrant sensory endings. Application of XE-991 (10 microM), a KCNQ channel blocker, had no effect on responses to stimulation of the neuroma but blocked the effects of retigabine indicating a specific involvement of KCNQ channels. In contrast to the strong effects on ectopic discharges, retigabine did not change responses to stimulation recorded from intact receptors. Results indicate that KCNQ channel opening at axotomized endings may constitute a novel and selective mechanism for modulation of some neuropathic pain symptoms.

Mechanisms of potassium- and capsaicin-induced axonal calcitonin gene-related peptide release: involvement of L- and T-type calcium channels and TRPV1 but not sodium channels

We have previously shown that capsaicin, noxious heat, protons and potassium ions (K(+)) induce a graded, calcium- and receptor-dependent increase of immunoreactive calcitonin gene-related peptide (iCGRP) release from isolated rat sciatic axons. Morphological evidence for axonal vesicular exocytosis has also been presented. Here we determine the differential contribution of voltage-gated calcium and sodium channels to high extracellular potassium and capsaicin-induced iCGRP secretion. Blockade of L-type calcium channels significantly decreased the K(+)-induced axonal response (nimodipine (10 microM) by 66% and methoxyverapamil, D600 (50 microM), by 77%). Interestingly, however, D600 was unable to reduce the capsaicin-induced iCGRP release. Omega-Conotoxin GVIA (1 microM), a N-type blocker, and omega-agatoxin TK (0.1 microM), a P/Q-type blocker, had no significant effect. Also the anticonvulsant gabapentin (50 microM and 100 microM), reported to impede calcium channels, was ineffective. Inhibition of low threshold T-type calcium channels by mibefradil (10 microM) significantly reduced potassium (by 47%) but not capsaicin-stimulated iCGRP release. Reduction of total sodium channel conductance by tetrodotoxin (1 microM), lidocaine (10 microM, 50 microM or 500 microM) or by replacement of extracellular sodium with choline-chloride did not result in a reduction of either potassium- or capsaicin-induced axonal iCGRP release. These results suggest that slow depolarization by high extracellular potassium activates axonal low threshold (T-type) as well as high threshold-activated (L-type) voltage-gated calcium channels to mediate iCGRP release, and that capsaicin-induced release is largely dependent on calcium influx through TRPV1. Action potential generation and propagation are not required for axonal release mechanisms.

Post-spike excitability indicates changes in membrane potential of isolated C-fibers

Recording of action potentials from single unmyelinated nerve fibers by microneurography is an important tool to investigate peripheral neural functions in human neuropathies. However, the interpretation of microneurography recordings can be difficult because axonal membrane potential is not revealed by this method. We tested the hypothesis that the recovery cycle of excitability after a single action potential is correlated with changes in the axonal membrane potential. To this end, we used the threshold tracking technique to study how different chemical mediators, with known effects on the membrane potential, influence the post-spike superexcitability of C-fiber compound action potentials in isolated rat sural and vagus nerves. We found that: (1) some chemical mediators (e.g., adenosine 5'-triphosphate) produce a reduction or loss of superexcitability together with increased axonal excitability, indicating membrane depolarization; (2) blockade of axonal hyperpolarization-activated (Ih) currents produces an enhancement of superexcitability together with a decreased excitability, indicating membrane hyperpolarization; and (3) application of calcium produces an increase in membrane threshold without an alteration in superexcitability, indicating a non-specific increase in surface charge and a change in the voltage-dependent activation of sodium channels. In addition, we demonstrated that membrane depolarization and hyperpolarization induce opposite post-spike latency shifts (changes in supernormality) in rat and human nerve segments. Thus, recordings of post-spike excitability and shifts in latency are sensitive techniques for detection of various types of neuromodulation, which are correlated with changes in membrane potential of unmyelinated peripheral axons and may help to understand observations obtained by microneurography in peripheral human neuropathies.

Electrophysiological characterization of vagal afferents relevant to mucosal nociception in the rat upper oesophagus

Emerging evidence indicates a nociceptive role of vagal afferents. A distinct oesophageal innervation in the rat, with muscular and mucosal afferents travelling predominantly in the recurrent (RLN) and superior laryngeal nerve (SLN), respectively, enabled characterization of mucosal afferents with nociceptive properties, using novel isolated oesophagus-nerve preparations. SLN and RLN single-fibre recordings identified 55 and 14 units, respectively, with none conducting faster than 8.7 m s(-1). Mucosal response characteristics in the SLN distinguished mechanosensors (n = 13), mechanosensors with heat sensitivity (18) from those with cold sensitivity (19) and a mechanoinsensitive group (5). The mechanosensitive fibres, all slowly adapting, showed a unimodal distribution of mechanical thresholds (1.4-128 mN, peak approximately 5.7 mN). No difference in response characteristics of C and Adelta fibres was encountered. Mucosal proton stimulation (pH 5.4 for 3 min), mimicking gastro-oesophageal reflux disease (GORD), revealed in 31% of units a desensitizing response that peaked around 20 s and faded within 60 s. Cold stimulation (15 degrees C) was proportionally encoded but the response showed slow adaptation. In contrast, the noxious heat (48 degrees C) response showed no obvious adaptation with discharge rates reflecting the temperature's time course. Polymodal (69%) mucosal units, > 30% proton sensitive, were found in each fibre category and were considered nociceptors; they are tentatively attributed to vagal nerve endings type I, IV and V, previously morphologically described. All receptive fields were mapped and the distribution indicates that the posterior upper oesophagus may serve as a 'cutbank', detecting noxious matters, ingested or regurgitated, and triggering nocifensive reflexes such as bronchoconstriction in GORD.