Barbara Namer

TRPA1 and TRPM8 activation in humans: effects of cinnamaldehyde and menthol.

The aim of this study was to evaluate the psychophysical effects of both TRPA1 and TRPM8 activation in humans by application of either cinnamaldehyde or menthol. We applied 10% cinnamaldehyde or 40% menthol solutions on the forearm in 10 study participants. Quantitative sensory testing and laser Doppler imaging was performed before and after exposure to the compounds. Cinnamaldehyde evoked significant spontaneous pain and induced heat and mechanical hyperalgesia, cold hypoalgesia and a neurogenic axon reflex erythema. In contrast, TRPM8 activation by menthol produced no axon reflex reaction and resulted in cold hyperalgesia. We conclude that agonists of TRPA1 and TRPM8 channels produce strikingly different psychophysical patterns.

H2S and NO cooperatively regulate vascular tone by activating a neuroendocrine HNO-TRPA1-CGRP signalling pathway.

Nitroxyl (HNO) is a redox sibling of nitric oxide (NO) that targets distinct signalling pathways with pharmacological endpoints of high significance in the treatment of heart failure. Beneficial HNO effects depend, in part, on its ability to release calcitonin gene-related peptide (CGRP) through an unidentified mechanism. Here we propose that HNO is generated as a result of the reaction of the two gasotransmitters NO and H2S. We show that H2S and NO production colocalizes with transient receptor potential channel A1 (TRPA1), and that HNO activates the sensory chemoreceptor channel TRPA1 via formation of amino-terminal disulphide bonds, which results in sustained calcium influx. As a consequence, CGRP is released, which induces local and systemic vasodilation. H2S-evoked vasodilatatory effects largely depend on NO production and activation of HNO–TRPA1–CGRP pathway. We propose that this neuroendocrine HNO–TRPA1–CGRP signalling pathway constitutes an essential element for the control of vascular tone throughout the cardiovascular system.

Abnormal Function of C-Fibers in Patients with Diabetic Neuropathy

The mechanisms underlying the development of painful and nonpainful neuropathy associated with diabetes mellitus are unclear. We have obtained microneurographic recordings from unmyelinated fibers in eight patients with diabetes mellitus, five with painful neuropathy, and three with neuropathy without pain. All eight patients had large-fiber neuropathy, and seven patients had pathological thermal thresholds in their feet, indicating the involvement of small-caliber nerve fibers. A total of 163 C-fibers were recorded at knee level from the common peroneal nerve in the patients (36–67 years old), and these were compared with 77 C-fibers from healthy controls (41–64 years old). The ratio of mechano-responsive to mechano-insensitive nociceptors was ∼2:1 in the healthy controls, whereas in the patients, it was 1:2. In patients, a fairly large percentage of characterized fibers (12.5% in nonpainful and 18.9% in painful neuropathy) resembled mechano-responsive nociceptors that had lost their mechanical and heat responsiveness. Such fibers were rarely encountered in age-matched controls (3.2%). Afferent fibers with spontaneous activity or mechanical sensitization were found in both patient groups. We conclude that small-fiber neuropathy in diabetes affects receptive properties of nociceptors that leads to an impairment of mechano-responsive nociceptors.

High spontaneous activity of C-nociceptors in painful polyneuropathy

Summary Ongoing pain in peripheral neuropathy is linked to spontaneous activity in mechanoinsensitive nociceptors. The molecular mechanism differentiating painful from nonpainful neuropathy is still unclear. ABSTRACT Polyneuropathy can be linked to chronic pain but also to reduced pain sensitivity. We investigated peripheral C‐nociceptors in painful and painless polyneuropathy patients to identify pain‐specific changes. Eleven polyneuropathy patients with persistent spontaneous pain and 8 polyneuropathy patients without spontaneous pain were investigated by routine clinical methods. For a specific examination of nociceptor function, action potentials from single C‐fibres including 214 C‐nociceptors were recorded by microneurography. Patients with and without pain were distinguished by the occurrence of spontaneous activity and mechanical sensitization in C‐nociceptors. The mean percentage of C‐nociceptors being spontaneously active or mechanically sensitized was significantly higher in patients with pain (mean 40.5% and 14.6%, respectively, P = .02). The difference was mainly due to more spontaneously active mechanoinsensitive C‐nociceptors (operationally defined by their mechanical insensitivity and their axonal characteristics) in the pain patients (19 of 56 vs 6 of 43; P = .02). The percentage of sensitized mechanoinsensitive C‐nociceptors correlated to the percentage of spontaneously active mechanoinsensitive C‐nociceptors (Kendall’s tau = .55, P = .004). Moreover, spontaneous activity of mechanoinsensitive C‐nociceptors correlated to less pronounced activity‐dependent slowing of conduction (Kendall’s tau = −.48, P = .009), suggesting that axons were included in the sensitization process. Hyperexcitability in mechanoinsensitive C‐nociceptors was significantly higher in patients with polyneuropathy and pain compared to patients with polyneuropathy without pain, while the difference was much less prominent in mechanosensitive (polymodal) C‐nociceptors. This hyperexcitability may be a major underlying mechanism for the pain experienced by patients with painful peripheral neuropathy.

Role of TRPM8 and TRPA1 for cold allodynia in patients with cold injury.

Abstract Local cold injury often induces hypersensitivity to cold and cold allodynia. Sensitisation of TRPM8 or TRPA1 could be the underlying mechanisms. This was evaluated by psychophysics and axon‐reflex‐flare induction following topical menthol and cinnamaldehyde application in cold injury patients and healthy subjects. The patients had no signs of neuropathy except cold allodynia. We applied 20% cinnamaldehyde and 40% menthol solutions in the cold‐allodynic area of the patients and in a corresponding area in healthy subjects and obtained sensory ratings during application. Thermotesting and Laser Doppler Imaging were performed before and after exposure to the compounds. Menthol did not induce axon‐reflex‐erythema in patients or in controls. After menthol cold pain threshold was decreased in healthy subjects; however, no further sensitisation was observed in the patients moreover in some patients an amelioration of their cold allodynia was observed. Cinnamaldehyde‐induced pain sensation did not differ between patients and controls. Heat pain thresholds following cinnamaldehyde were lowered to a similar extent in patients and controls (43–39.8 and 44–39 °C) and also the axon‐reflex‐flare responses were comparable. No evidence for sensitisation of responses to TRPM8 or TRPA1‐stimulation was found in patients with cold injury‐induced cold allodynia. The lack of TRPM8 induced axon‐reflex indicates that also de‐novo expression of TRPM8 on mechano‐insensitive C‐nociceptors does not underlie cold allodynia in these patients. We conclude from these data that the mechanisms for the induction of cold allodynia in the patients with cold injury are independent of TRPM8 or TRPA1 and differ therefore from neuropathic pain patients.

Microneurographic assessment of C-fibre function in aged healthy subjects

Physiological changes in the nervous system occur with ageing. Both a decline of function and a decrease in the number of C‐fibres in the skin have been reported for healthy aged subjects. With the use of microneurographic recordings from single C‐fibres in humans we have compared the sensory and axonal properties of these neurones in young and aged healthy subjects. A total of 146 C‐fibres were recorded from the common peroneal nerve in young subjects (mean age 24.7 years) and 230 C‐fibres were recorded in aged subjects (mean age 56.2 years). In aged subjects, changes were found in the composition of the C‐fibre population and in sensory and axonal properties. The relative incidence of afferent to efferent C‐fibres was relatively constant independent of the age of subjects. The ratio of mechano‐responsive to mechano‐insensitive nociceptors was approximately 8 : 2 in the young controls while in aged subjects it was 7 : 3. In aged subjects 13% of the fibres showed atypical discharge characteristics, while this was not observed in young subjects. Spontaneous activity, sensitization and loss of sensory function were found regularly. Changes in functions of the conductile membrane were also observed in fibres from aged subjects. The degree of activity‐dependent conduction velocity slowing in response to high frequency stimulation (2 Hz) was more pronounced, while the normalization of conduction velocity subsequent to high frequency stimulation was protracted. We found that both sensitization and desensitization or degeneration of afferent C‐fibres occur with age, but are still rare compared to patients with neuropathy. The changes in the axonal properties of C‐fibres in aged subjects are compatible with hypoexcitability of the fibres. These findings are important for the understanding and differential diagnoses regarding pathological processes and normal ageing.

Catecholamine-induced excitation of nociceptors in sympathetically maintained pain

Abstract Sympathetically maintained pain could either be mediated by ephaptic interactions between sympathetic efferent and afferent nociceptive fibers or by catecholamine‐induced activation of nociceptive nerve endings. We report here single fiber recordings from C nociceptors in a patient with sympathetically maintained pain, in whom sympathetic blockade had repeatedly eliminated the ongoing pain in both legs. We classified eight C‐fibers as mechano‐responsive and six as mechano‐insensitive nociceptors according to their mechanical responsiveness and activity‐dependent slowing of conduction velocity (latency increase of 0.5 ± 1.1 vs. 7.1 ± 2.0 ms for 20 pulses at 0.125 Hz). Two C‐fibers were activated with a delay of several seconds following strong endogenous sympathetic bursts; they were also excited for about 3 min following the injection of norepinephrine (10 μl, 0.05%) into their innervation territory. In these two fibers, a prolonged activation by injection of low pH solution (phosphate buffer, pH 6.0, 10 μl) and sensitization of their heat response following prostaglandin E2 injection were recorded, evidencing their afferent nature. Moreover, their activity‐dependent slowing was typical for mechano‐insensitive nociceptors. We conclude that sensitized mechano‐insensitive nociceptors can be activated by endogenously released catecholamines and thereby may contribute to sympathetically maintained pain. No evidence for ephaptic interaction between sympathetic efferent and nociceptive afferent fibers was found.

Patterns of activity-dependent conduction velocity changes differentiate classes of unmyelinated mechano-insensitive afferents including cold nociceptors, in pig and in human

&NA; Activity‐dependent slowing of conduction velocity (ADS) differs between classes of human nociceptors. These differences likely reflect particular expression and use‐dependent slow inactivation of axonal ion channels and other mechanisms governing axonal excitability. In this study, we compared ADS of porcine and human cutaneous C‐fibers. Extracellular recordings were performed from peripheral nerves, using teased fiber technique in pigs and microneurography in humans. We assessed electrically‐induced conduction changes and responsiveness to natural stimuli. In both species, the group of mechano‐insensitive C‐fibers showed the largest conduction slowing (˜30%) upon electrical stimulation (2 Hz for 3 min). In addition, we found mechano‐insensitive cold nociceptors in pig that slowed only minimally (<10% at 2 Hz), and a similar slowing pattern was found in some human C‐fibers. Mechano‐sensitive afferents showed an intermediate conduction slowing upon 2 Hz stimulation (pig: 14%, human 23%), whereas sympathetic efferent fibers in pig and human slowed only minimally (5% and 9%, respectively). In fiber classes with more pronounced slowing, conduction latencies recovered slower; i.e. mechano‐insensitive afferents recovered the slowest, followed by mechano‐sensitive afferents whereas cold nociceptors and sympathetic efferents recovered the fastest. We conclude that mechano‐insensitive C‐fiber nociceptors can be differentiated by their characteristic pattern of ADS which are alike in pig and human. Notably, cold nociceptors with a distinct ADS pattern were first detected in pig. Our results therefore suggest that the pig is a suitable model to study nociceptor class‐specific changes of ADS.

Impact of Scratching on Itch and Sympathetic Reflexes Induced by Cowhage (Mucuna pruriens) and Histamine

Cowhage and histamine, both applied via spicules, were used to induce itch. The quality and intensity of the sensations, axon reflex flare, sympathetic skin vasoconstrictions and the interference of scratching with itch processing were studied. Axon reflex flare reactions were measured by laser Doppler imaging and reflex vasoconstrictions in the finger were recorded by laser Doppler flowmetry. Magnitude of itch sensations was assessed on an electronic visual analogue scale while the skin was intermittently scratched proximal to the application site. The quality of itch was assessed with a questionnaire. Only histamine produced an axon reflex flare. Histamine itch increased faster, but recovered more slowly after scratching, by which it was more effectively suppressed. Cowhage induced a sharper itch sensation and stronger vasoconstrictor reflexes. These findings support the notion that both agents activate different pathways. The differences in sympathetic reflex induction and in the modulation by scratching indicate differential central nervous processing.

Chemically and electrically induced sweating and flare reaction

Both thin afferent (nociceptors) and efferent (sympathetic sudomotor) nerve fibers can be activated electrically and chemically, resulting in neurogenic erythema and sweating. These reactions have been used before to assess the impairment of sympathetic and nociceptor fibers in humans. In this study, electrically induced sweating and erythema were assessed simultaneously in the foot dorsum and thigh, and were compared to chemically induced activation. Reproducible intensity-response relations (stimulation intensities 0-30 mA, 1 Hz) were obtained from 32 subjects. The steepest increase of the sweat response was induced at lower intensities as compared to that of the erythema (18.3 mA vs. 25.7 mA, p<0.01) and reached a plateau for intensities above 25 mA, suggesting lower electrical thresholds for sudomotor fibers. Maximum flare areas induced electrically with 30 mA were smaller than those evoked chemically (flare size: 4.5 cm2 vs. 10.6 cm2). In contrast, the electrically evoked sweating rate was higher than that evoked chemically (acetylcholine, or ACh; sweating rate 0.31 vs. 0.21 microl/cm2/min, p<0.01), which might be attributed to an increased effectiveness of synchronized discharge in sympathetic fibers upon electrical stimulation.