pH-dependent modulation of TRPV1 by modality-selective antagonists

TRPV1 channel in the pathophysiology of epilepsy and its potential as a molecular target for the development of new antiseizure drug candidates

Identification of transient receptor potential cation channel, subfamily V member 1 (TRPV1), also known as capsaicin receptor, in 1997 was a milestone achievement in the research on temperature sensation and pain signalling. Very soon after it became evident that TRPV1 is implicated in a wide array of physiological processes in different peripheral tissues, as well as in the central nervous system, and thereby could be involved in the pathophysiology of numerous diseases. Increasing evidence suggests that modulation of TRPV1 may also affect seizure susceptibility and epilepsy. This channel is localized in brain regions associated with seizures and epilepsy, and its overexpression was found both in animal models of seizures and in brain samples from epileptic patients. Moreover, modulation of TRPV1 on non-neuronal cells (microglia, astrocytes, and/or peripheral immune cells) may have an impact on the neuroinflammatory processes that play a role in epilepsy and epileptogenesis. In this paper, we provide a comprehensive and critical overview of currently available data on TRPV1 as a possible molecular target for epilepsy management, trying to identify research gaps and future directions. Overall, several converging lines of evidence implicate TRPV1 channel as a potentially attractive target in epilepsy research but more studies are needed to exploit the possible role of TRPV1 in seizures/epilepsy and to evaluate the value of TRPV1 ligands as candidates for new antiseizure drugs.

TRPV1: Receptor structure, activation, modulation and role in neuro-immune interactions and pain

In the 1990s, the identification of a non-selective ion channel, especially responsive to capsaicin, revolutionized the studies of somatosensation and pain that were to follow. The TRPV1 channel is expressed mainly in neuronal cells, more specifically, in sensory neurons responsible for the perception of noxious stimuli. However, its presence has also been detected in other non-neuronal cells, such as immune cells, β- pancreatic cells, muscle cells and adipocytes. Activation of the channel occurs in response to a wide range of stimuli, such as noxious heat, low pH, gasses, toxins, endocannabinoids, lipid-derived endovanilloid, and chemical agents, such as capsaicin and resiniferatoxin. This activation results in an influx of cations through the channel pore, especially calcium. Intracellular calcium triggers different responses in sensory neurons. Dephosphorylation of the TRPV1 channel leads to its desensitization, which disrupts its function, while its phosphorylation increases the channel's sensitization and contributes to the channel's rehabilitation after desensitization. Kinases, phosphoinositides, and calmodulin are the main signaling pathways responsible for the channel's regulation. Thus, in this review we provide an overview of TRPV1 discovery, its tissue expression as well as on the mechanisms by which TRPV1 activation (directly or indirectly) induces pain in different disease models.

Targeting TRP channels for pain relief: A review of current evidence from bench to bedside

Several decades of research support the involvement of transient receptor potential (TRP) channels in nociception. Despite the disappointments of early TRPV1 antagonist programs, the TRP family remains a promising therapeutic target in pain disorders. High-dose capsaicin patches are already in clinical use to relieve neuropathic pain. At present, localized injections of the side-directed TRPV1 agonist capsaicin and resiniferatoxin are undergoing clinical trials in patients with osteoarthritis and bone cancer pain. TRPA1, TRPM3, and TRPC5 channels are also of significant interest. This review discusses the role of TRP channels in human pain conditions.

The detection and modulation of piperine in the human oral cavity

Piperine is an alkaloid that is responsible for the pungency of black pepper and long pepper. This hydrophobic compound causes a spicy sensation when it comes in contact with trigeminal neurons of the oral cavity. Piperine has low solubility in water, which presents difficulties in examining the psychophysical properties of this stimulus by standard aqueous chemosensory tests. This report describes approaches that utilize novel edible film formulations for delivering precise amounts of piperine to the human oral cavity. These films were then used to identify detection thresholds for piperine, and to identify the chemosensory properties of this compound at suprathreshold amounts. When incorporated into edible films, mean detection thresholds for piperine were approximately 35 nanomoles. For suprathreshold studies, edible films that contained 4000 nanomole amounts of piperine yielded variable intensity responses in subjects, with mean intensities in the moderate range. This amount of piperine caused significant self-desensitization, which was partially reversed after 60-90 min. In contrast, edible films that contained lower amounts of piperine yielded mean intensity ratings in the weak range and showed essentially no self-desensitization. The application of piperine to the circumvallate region of the tongue caused moderate intensity responses that were identified as primarily spicy, and rarely bitter. In addition, oral rinses with aqueous sucrose solutions decreased mean intensities for piperine by approximately twenty-five percent over sixty seconds. Blockage of nasal airflow significantly decreased piperine intensities in the oral cavity. These two findings indicate that oral sucrose or blockage of nasal airflow can modulate piperine perception in the human oral cavity. Finally, these results indicate that a variety of excipients can be included in edible film formulations for presenting piperine to the oral cavity at stimulus amounts that cause quantifiable chemosensory responses.