Mechanical sensitization, increased axonal excitability, and spontaneous activity in C-nociceptors after ultraviolet B irradiation in pig skin

Part I: understanding pain in pigs-basic knowledge about pain assessment, measures and therapy

BACKGROUND

Pigs can suffer from pain due to spontaneously occurring diseases, wounds, injuries, trauma, and physiological conditions such as the farrowing process; however, this pain is often neglected. To increase knowledge and awareness about this phenomenon, the current article presents a scoping review of basic and new approaches for identifying, evaluating, and treating pain in pigs.

METHODS

A scoping review was conducted with results from a search of the electronic database VetSearch and CABI. With regard to eligibility criteria, 49 out of 725 publications between 2015 and the end of March 2023 were included. The findings are narratively synthesized and reported orienting on the PRISMA ScR guideline.

RESULTS

The results of this review showed that practitioners need to consider pain not only as a sign of a disease but also as a critical aspect of welfare. If both the symptoms of pain and the underlying reasons remain unassessed, the longevity and prosperity of pigs may be at risk. In this respect, veterinarians are obliged to know about intricacies of pain and pain mechanisms and to provide adequate treatment for their patients.

CONCLUSION

It is pivotal to increase knowledge about pain mechanisms, the reasons for heterogeneity in behavioural signs of pain, and methods for evaluating whether a pig is experiencing pain. This article will help practitioners update their knowledge of this topic and discuss the implications for everyday practice.

Topically applied novel TRPV1 receptor antagonist, ACD440 Gel, reduces evoked pain in healthy volunteers, a randomized, double-blind, placebo-controlled, crossover study

BACKGROUND

The TRPV1 receptor is a key molecule in pain generation. Previous development of oral TRPV1-antagonists was halted due to systemic heat insensitivity and body temperature alterations. The present Phase 1b study investigated the efficacy, safety and plasma exposure of a topically administered TRPV1-antagonist (ACD440 Gel) in healthy subjects.

METHODS

The study comprised two parts. In part 1, 24 healthy subjects were included in this randomized double-blind, placebo-controlled, crossover trial. ACD440 Gel or Placebo was applied once daily and wiped off after 1 h, for 5 consecutive days. Assessments were done in normal skin, skin optimized for penetration (by stripping and occlusive gel application) and UVB-irradiated skin. Pain induced by thermo-nociceptive CO2 laser impulses generated laser-evoked potentials (LEPs), with readouts of peak-to-peak (PtP) amplitude in vertex-EEG and pain assessments by VAS (0-100). Endpoints include effects at 1 hour post-dose, AUC(Days 1-5) and AUC(0-24, Day 4). In UVB-irradiated skin, also pain on pinprick and skin redness were assessed. Part 2 explored the plasma pharmacokinetics of ACD440.

RESULTS

ACD440 Gel reduced LEP PtP amplitude and VAS pain, p < 0.001, in all skin conditions, versus placebo. In UVB-irradiated skin, pinprick pain was also reduced, p = 0.047. Effects were significant after 1 h, maintaining for at least 9 h. There were no adverse events or drug-induced erythema. Plasma exposures of ACD440 were too low to establish an elimination half-life of ACD400.

CONCLUSIONS

Topical ACD440 Gel demonstrated a significant analgesic effect on LEP, VAS score and pinprick pain, with low systemic exposures, supporting further clinical development.

SIGNIFICANCE

This study demonstrates that the topical administration of a TRPV1-antagonist, ACD440 Gel, has potential as a new treatment for painful conditions affecting the skin, such as chronic peripheral neuropathic pain, without any local or systemic side effects.

Optimized Electrical Stimulation of C-Nociceptors in Humans Based on the Chronaxie of Porcine C-Fibers

Classically, to electrically excite C-nociceptors, rectangular pulses are used with a duration close to the estimated chronaxie of C-fibres (about 2 ms). Recent results using slow depolarizing stimuli suggest longer chronaxies. We therefore set out to optimize C-fiber stimulation based on recordings of single C-nociceptors in-vivo and C-fiber compound-action-potentials (C-CAP) ex-vivo using half-sine shaped stimuli of durations between 1 and 250ms. Single fiber (n = 45) recording in pigs revealed high chronaxie values for C-touch fibers (15.8 ms), polymodal- (14.2 ms) and silent-nociceptors (16.8 ms). Activation thresholds decreased 2 to 3-fold in all fibre classes when increasing the duration of half-sine pulses from 1 to 25 ms (P < .05). C-CAPs strength-duration curves of the pig saphenous nerve (n = 7) showed the highest sensitivity for half-sine durations between 10 and 25 ms. Half-maximum currents for C-CAPS were reduced 3-fold compared to rectangular pulses (P < .01) whereas the opposite was found for A-fiber compound action potentials. Psychophysics in humans (n = 23) revealed that half-sine stimulus durations >10 ms reduced detection thresholds, pain thresholds, and stimulus current amplitudes required to generate a pain rating of 3 on an 11-point Numeric Rating Scale (NRS) as compared to 1 ms rectangular pulses (P < 0.05). Increasing the duration from 1 to 25 ms led to a 4-fold amplitude reduction for pain-thresholds and stimuli caused an axon-reflex flare. Excitability of single polymodal nociceptors in animals paralleled human psychophysics and we conclude optimized half-sine pulses facilitate C-nociceptor activation. PERSPECTIVE: Electrical stimulation with longer lasting half-sine wave pulses preferentially activates C-nociceptors and changes in the strength duration curve may identify nociceptor hyperexcitability in patients with neuropathic pain.