Modulation of intracellular calcium influences capsaicin-induced currents of TRPV-1and voltage-activated channel currents in nociceptive neurones

TRPA1, TRPV1, and Caffeine: Pain and Analgesia

Caffeine (1,3,7-trimethylxanthine) is a naturally occurring methylxanthine that acts as a potent central nervous system stimulant found in more than 60 different plants and fruits. Although caffeinated beverages are widely and casually consumed, the application of caffeine beyond dietary levels as pharmacologic therapy has been recognized since the beginning of its recorded use. The analgesic and vasoactive properties of caffeine are well known, but the extent of their molecular basis remains an area of active research. There is existing evidence in the literature as to caffeine's effect on TRP channels, the role of caffeine in pain management and analgesia, as well as the role of TRP in pain and analgesia; however, there has yet to be a review focused on the interaction between caffeine and TRP channels. Although the influence of caffeine on TRP has been demonstrated in the lab and in animal models, there is a scarcity of data collected on a large scale as to the clinical utility of caffeine as a regulator of TRP. This review aims to prompt further molecular research to elucidate the specific ligand-host interaction between caffeine and TRP by validating caffeine as a regulator of transient receptor potential (TRP) channels-focusing on the transient receptor potential vanilloid 1 (TRPV1) receptor and transient receptor potential ankyrin 1 (TRPA1) receptor subtypes-and its application in areas of pain.

Transient receptor potential vanilloid 1 (TRPV1)-independent actions of capsaicin on cellular excitability and ion transport

Capsaicin is a naturally occurring alkaloid derived from chili pepper that is responsible for its hot pungent taste. Capsaicin is known to exert multiple pharmacological actions, including analgesia, anticancer, anti-inflammatory, antiobesity, and antioxidant effects. The transient receptor potential vanilloid subfamily member 1 (TRPV1) is the main receptor mediating the majority of the capsaicin effects. However, numerous studies suggest that the TRPV1 receptor is not the only target for capsaicin. An increasing number of studies indicates that capsaicin, at low to mid µM ranges, not only indirectly through TRPV1-mediated Ca²⁺ increases, but also directly modulates the functions of voltage-gated Na⁺ , K⁺ , and Ca²⁺ channels, as well as ligand-gated ion channels and other ion transporters and enzymes involved in cellular excitability. These TRPV1-independent effects are mediated by alterations of the biophysical properties of the lipid membrane and subsequent modulation of the functional properties of ion channels and by direct binding of capsaicin to the channels. The present study, for the first time, systematically categorizes this diverse range of non-TRPV1 targets and discusses cellular and molecular mechanisms mediating TRPV1-independent effects of capsaicin in excitable, as well as nonexcitable cells.

Elevated expression of transient receptor potential vanilloid type 1 in dorsal root ganglia of rats with endometriosis

Pain is the most pronounced complaint of women with endometriosis, however the underlying mechanism is still poorly understood. In the present study, the authors evaluate the effect of transient receptor potential vanilloid type 1 (TRPV1) of dorsal root ganglia (DRG) on endometriosis-associated pain. A total of 36 SD rats were randomly divided into a sham group (n=9) and a Model group (n=27), accepted auto‑transplanted pieces of fat or uterus to the pelvic cavity. At 4 weeks, the Model group was randomly subdivided into the following groups: ENDO group (no treatment, n=9), BCTC group (Model + BCTC, an antagonist of TRPV1, n=9), Vehicle group (Model + cyclodextrin, the vehicle of BCTC, n=9). Tail‑flick test was performed prior to surgery, 1 h prior to and following treatment of BCTC or cyclodextrin. The expression of TRPV1, substance P (SP), calcitonin gene‑related peptide (CGRP) in L1‑L6 DRG was measured via immunohistochemistry, western blotting and RT‑qPCR. The results indicated that the Model group exhibited a significant decrease in tail flick latency compared to pre‑surgical baseline, and the expression of TRPV1, SP, CGRP protein and mRNA in L1‑L6 DRG significantly increased compared to the sham group. BCTC significantly improved tail flick latency, and downregulated the expression of TRPV1, SP and CGRP protein and mRNA levels in L1‑L6 DRG compared to ENDO group. However, there were no significant differences of those in Vehicle group compared with the ENDO group. Taken together, the current study provides evidence that TRPV1 expressed in DRG may serve an important role in endometriosis-associated pain.

Icilin reduces voltage-gated calcium channel currents in naïve and injured DRG neurons in the rat spinal nerve ligation model

Recently, the transient receptor potential (TRP) channels TRPM8 and TRPA1 have been identified as molecular sensors for cold, and it has been suggested that they play a crucial role in allodynia by modulating voltage-gated calcium channel currents (ICa(V)). The aim of this study was to analyze the modulation of ICa(V) by the TRPM8-agonist icilin in vitro and to investigate the analgesic effect of icilin in a neuropathic pain model in vivo. Whole cell patch-clamp recordings were performed on isolated naïve and injured rat dorsal root ganglia (DRG) neurons, and the analgesic efficacy of icilin applied topically to the paws or intrathecally was tested in rats after spinal nerve ligation (SNL). ICa(V) (depolarization from -80 to 0mV) in naïve DRG neurons was reduced dose dependently (0.002-200µM) by icilin (18-80%). Subtype isolation of calcium channels show a marked reduction of L-type channel currents compared to N-type channel currents. The effects of icilin on ICa(V) were not significantly different in non-injured and SNL-injured DRG neurons. In vivo, neither topical (10-200µM) nor intrathecal application of icilin (0.1nM to 1µM) affected tactile allodynia or thermal hyperalgesia after SNL, but it increases cold allodynia 6h after application. We conclude that the icilin-induced modulation of ICa(V) in DRG neurons is unlikely to mediate analgesic effects or contribute directly to the pathogenesis of cold allodynia in the rat SNL model, but it is a potential mechanism for the analgesic effects of icilin in other pain models.

Additive inhibitory effects of calcitonin and capsaicin on voltage activated calcium channel currents in nociceptive neurones of rat

Calcitonin, a peptide hormone expressed in C-cells of the thyreoid gland, as well as capsaicin, isolated from chili, both, modify intracellular signalling in nociceptive neurones. The pathways triggered by the activation of either of these receptors results in a modulation of the intracellular calcium ([Ca(2+)](i)) concentration. While the regulation of [Ca(2+)](i) depends on many factors, voltage activated calcium channels (VACCs) are a major gate for the calcium entry into neurones. Here we describe the changes of voltage gated calcium channel currents (I(Ca(V))) induced by calcitonin and/or capsaicin. Currents were recorded using adequate solutions and voltage protocols with the whole cell patch-clamp technique. When the channels were opened by a depolarisation to 0 mV, both substances reduce the peak I(Ca(V)) (calcitonin (10nM): 29.3 ± 3.9%; capsaicin (0.5 μM): 41.1 ± 7.7%). While the effect of calcitonin was voltage dependent, capsaicin shifted the largest current to the more hyperpolarizing range (peak current from -10 to -20 mV). A subsequent co-application of either of the two substances (with a pre-application of either 3 min or 60 min) results in an additive reduction of the currents, and prevents the capsaicin-induced shift of the current-voltage relation. Therefore, we hypothesize, that the activation of either of the two receptors reduces I(Ca(V)) by different cellular binding sites of the channel protein triggering channel opening. These findings may be useful to understand cellular mechanisms of pain modulation and might help to find better treatments for neuropathic pain.

Sensitization of voltage activated calcium channel currents for capsaicin in nociceptive neurons by tumor-necrosis-factor-alpha

It is known that application of tumor-necrosis-factor-alpha (TNF-alpha) sensitizes neuronal calcium channels for heat stimuli in rat models of neuropathic pain. This study examines whether TNF-alpha modulates the capsaicin-induced effects after transient receptor potential vanilloid (TRPV)-1 receptor activation on voltage activated calcium channel currents (I(Ca(V))). TRPV-1 receptors are activated by heat and play an important role in the pathogenesis of thermal hyperalgesia in neuropathic pain syndromes, while voltage activated channels are essential for transmission of neuronal signals. Eliciting I(Ca(V)) in DRG neurons of rats by a depolarization from the resting potential to 0 mV, TNF-alpha (100 ng/ml) reduces I(Ca(V)) by 16.9+/-2.2%, while capsaicin (0.1 microM) decreases currents by 27+/-4.3%. Pre-application of TNF-alpha (100 ng/ml) for 24h results in a sensitization of I(Ca(V)) to capsaicin (0.1 microM) with a reduction of 42.8+/-4.4% mediated by TRPV-1. While L-type (36.6+/-5.2%) and P/Q-type currents (35.6+/-4.1%) are also sensitized by TRPV-1 activation, N-type channel currents are most sensitive (74.5+/-7.3%). The capsaicin-induced shift towards the hyperpolarizing voltage range does not occur when TNF-alpha is applied. Summarizing, TNF-alpha sensitizes nociceptive neurons for capsaicin.

TNF-alpha differentially modulates ion channels of nociceptive neurons

Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine involved in the development and maintenance of inflammatory and neuropathic pain conditions. The mechanisms by which TNF-alpha elicits pain behavior are still incompletely understood. Numerous studies suggest that TNF-alpha sensitizes primary afferent neurons. Most recently, it was shown that TNF-alpha induced an enhancement of TTX-R Na(+) current in dorsal root ganglion (DRG) cells. In the present study, we have tested the effect of acute application of TNF-alpha on voltage-gated potassium, calcium and sodium channel currents as well as its influence on membrane conductance in isolated rat DRG neurons. We report that voltage-gated potassium channel currents of nociceptive DRG neurons are not influenced by TNF-alpha (100 ng/ml), while voltage-gated calcium channel currents were decreased voltage-dependently by -7.73+/-6.01% (S.D.), and voltage-activated sodium channels currents were increased by +5.62+/-4.27%, by TNF-alpha. In addition, TNF-alpha induced a significant increase in IV ramps at a potential of +20 mV, which did not exist when the experiments were conducted in a potassium-free solution, indicating that this effect is mainly the result of a change in potassium conductance. These different actions of TNF-alpha might help to explain how it sensitizes primary afferent neurons after nerve injury and thus facilitates pain.