Raffaele Gatti

4-Hydroxynonenal, an endogenous aldehyde, causes pain and neurogenic inflammation through activation of the irritant receptor TRPA1

TRPA1 is an excitatory ion channel expressed by a subpopulation of primary afferent somatosensory neurons that contain substance P and calcitonin gene-related peptide. Environmental irritants such as mustard oil, allicin, and acrolein activate TRPA1, causing acute pain, neuropeptide release, and neurogenic inflammation. Genetic studies indicate that TRPA1 is also activated downstream of one or more proalgesic agents that stimulate phospholipase C signaling pathways, thereby implicating this channel in peripheral mechanisms controlling pain hypersensitivity. However, it is not known whether tissue injury also produces endogenous proalgesic factors that activate TRPA1 directly to augment inflammatory pain. Here, we report that recombinant or native TRPA1 channels are activated by 4-hydroxy-2-nonenal (HNE), an endogenous α,β-unsaturated aldehyde that is produced when reactive oxygen species peroxidate membrane phospholipids in response to tissue injury, inflammation, and oxidative stress. HNE provokes release of substance P and calcitonin gene-related peptide from central (spinal cord) and peripheral (esophagus) nerve endings, resulting in neurogenic plasma protein extravasation in peripheral tissues. Moreover, injection of HNE into the rodent hind paw elicits pain-related behaviors that are inhibited by TRPA1 antagonists and absent in animals lacking functional TRPA1 channels. These findings demonstrate that HNE activates TRPA1 on nociceptive neurons to promote acute pain, neuropeptide release, and neurogenic inflammation. Our results also provide a mechanism-based rationale for developing novel analgesic or anti-inflammatory agents that target HNE production or TRPA1 activation.

Hydrogen sulfide causes vanilloid receptor 1-mediated neurogenic inflammation in the airways

1 Hydrogen sulfide (H2S) is described as a mediator of diverse biological effects, and is known to produce irritation and injury in the lung following inhalation. Recently, H2S has been found to cause contraction in the rat urinary bladder via a neurogenic mechanism. Here, we studied whether sodium hydrogen sulfide (NaHS), used as donor of H2S, produces responses mediated by sensory nerve activation in the guinea‐pig airways. 2 NaHS evoked an increase in neuropeptide release in the airways that was significantly attenuated by capsaicin desensitization and by the transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine. In addition, NaHS caused an atropine‐resistant contraction of isolated airways, which was completely prevented by capsaicin desensitization. Furthermore, NaHS‐induced contraction was reduced by TRPV1 antagonism (ruthenium red, capsazepine and SB366791), and was abolished by pretreatment with the combination of tachykinin NK1 (SR140333) and NK2 (SR48968) receptor antagonists. 3 In anesthetized guinea‐pigs, intratracheal instillation of NaHS increased the total lung resistance and airway plasma protein extravasation. These two effects were reduced by TRPV1 antagonism (capsazepine) and tachykinin receptors (SR140333 and SR48968) blockade. 4 Our results provide the first pharmacological evidence that H2S provokes tachykinin‐mediated neurogenic inflammatory responses in guinea‐pig airways, and that this effect is mediated by stimulation of TRPV1 receptors on sensory nerves endings. This novel mechanism may contribute to the irritative action of H2S in the respiratory system.

Antitussive activity of iodo-resiniferatoxin in guinea pigs

Background: Iodo-resiniferatoxin (I-RTX) has recently been described as an ultra potent antagonist of the transient receptor potential vanilloid-1 (TRPV1). Methods: The ability of I-RTX to inhibit cough induced by inhalation of two putative TRPV1 stimulants (capsaicin and citric acid) was tested in non-anaesthetised guinea pigs. Results: Pretreatment with I-RTX either intraperitoneally (0.03–0.3 µmol/kg) or by aerosol (0.1–3 µM) reduced the number of coughs produced by inhalation of citric acid (0.25 M) and capsaicin (30 µM) in a dose dependent manner. Capsazepine (CPZ) also reduced citric acid and capsaicin induced cough, but the activity of I-RTX was 10–100 times more potent than CPZ in all the experimental conditions tested. Conclusions: I-RTX is a novel and potent antitussive drug which inhibits cough mediated by agents possibly acting via TRPV1 activation.

Transient receptor potential ankyrin receptor 1 is a novel target for pro‐tussive agents

Background and purpose:  The transient receptor potential ankyrin receptor 1 (TRPA1) is a cation channel, co‐expressed with the pro‐tussive transient receptor potential vanilloid type 1 (TRPV1) channel in primary sensory neurons. TRPA1 is activated by a series of irritant exogenous and endogenous α,β‐unsaturated aldehydes which seem to play a role in airway diseases. We investigated whether TRPA1 agonists provoke cough in guinea pigs and whether TRPA1 antagonists inhibit this response.

Ethanol Causes Neurogenic Vasodilation by TRPV1 Activation and CGRP Release in the Trigeminovascular System of The Guinea Pig

Ethanol stimulating transient receptor potential vanilloid 1 (TRPV1) on primary sensory neurons promotes neurogenic inflammation, including calcitonin gene-related peptide (CGRP)-mediated coronary dilation. Alcoholic beverages trigger migraine attacks and activation of trigeminal neurons plays a role in migraine. We have investigated in guinea pigs whether ethanol by TRPV1 stimulation causes neurogenic inflammation in the trigeminovascular system. Ethanolevoked release of neuropeptides from slices of dura mater was abolished by Ca2+ removal, capsaicin pretreatment and the TRPV1 antagonist, capsazepine. Intragastric ethanol increased plasma extravasation in dura mater, an effect abolished by capsazepine and the NK1 receptor antagonist, SR140333, and caused vasodilation around the middle meningeal artery, an effect abolished by capsazepine and the CGRP receptor antagonist, BIBN4096BS. Vasodilation of meningeal vessels by TRPV1 activation and CGRP release may be relevant to the mechanism by which alcohol ingestion triggers migraine attacks.

TRPV1 Antagonists as Analgesic Agents

The last decade (2001 - 2010), declared as the Decade of Pain Control and Research by the United States Con- gress, brought significantly advances in our understanding of pain biology. Unfortunately, this has not translated into ad- ditional effective treatments of chronic pain conditions. Chronic pain is a debilitating and complex clinical state usually associated with diabetic neuropathy, postherpetic neuralgia, low back pathology, fibromyalgia, and neurological disorders. Standard pain drugs, even narcotic opioid analgesic agents, often provide unsatisfactory pain relief while causing impor- tant side-effect such as sedation, tolerance, dependence, respiratory depression and constipation. Furthermore, the effec- tive management of chronic pain needs a multidisciplinary management approach and still represents one of the most ur- gent unmet medical need. Recently, preclinical research has uncovered new molecular mechanisms underlying the genera- tion and transduction of pain, many of which represent new targets for innovative pharmacological interventions. This re- view focuses on Transient Receptor Potential (TRP) Vanilloid Type 1 (TRPV1) channel as a target for treating chronic pain. TRPV1 is a multifunctional ion channel involved in thermosensation (heat) and taste perception. Importantly, TRPV1 also functions as a molecular integrator for a broad variety of seemingly unrelated noxious stimuli. Indeed, TRPV1 is thought to be a major transducer of the thermal hyperalgesia that follows inflammation and/or tissue injury. De- sensitization to topical TRPV1 agonists (e.g. capsaicin creams and patches) has been in clinical use for decades to treat chronic painful conditions like diabetic neuropathy. Most recently, a number of potent, small molecule TRPV1 antago- nists have been advanced into clinical trials for pain relief. Perhaps not unexpectedly given the prominent role of TRPV1 in thermosensation, some of these antagonists showed worrisome adverse effects (hyperthermia and impaired noxious heat sensation) in humans, leading to their withdrawal from clinical trials. However, recent reports of TRPV1 antagonists that do not affect core body temperature in preclinical species suggest a potential opportunity to reduce at least this impor- tant side effect.

Ethanol potentiates the TRPV1-mediated cough in the guinea pig

Ethanol is a chemical irritant able to induce a large variety of effects in the airways. It has been reported that ethanol sensitizes the transient receptor potential vanilloid-1 (TRPV1) to various stimuli and inhalation of ethanol enhances the cough mediated by TRPV1 activation (capsaicin) in patients suffering of airway sensory hyperreactivity. Here, we set out to investigate whether ethanol sensitizes the cough induced by TRPV1 activation in a guinea pig model and the possible mechanism of such exacerbating effect. Aerosolized resiniferatoxin (RTX, 0.5 microM) and hypertonic saline (7%) produced a cough response dependent and independent of TRPV1 activation, respectively. Ethanol (3%, 10 min) inhalation, that per se did not cause any tussive response, significantly increased the number of coughs evoked by RTX inhalation without affecting hypertonic saline (7%) induced cough. Potentiation by ethanol of the tussive response to RTX was prevented by the PKC inhibitor, GF109203X (GFX). In conclusion, ethanol selectively exaggerates, via a PKC-dependent pathway, the cough response evoked by TRPV1 stimulation. The present results may contribute to explain respiratory distresses sometimes associated to alcohol consumption, including cough and asthma.