Sara Luz Morales-Lázaro

Background: Anandamide (AEA) is a lipid mediator of the endocannabinoid system that plays a key role in pain perception by regulating multiple molecular targets, including transient receptor potential vanilloid type 1 (TRPV1). This vanilloid receptor is abundantly expressed in nociceptive neurons (nociceptors) and plays a crucial role in mediating pain responses. Methods: We used mouse behavioral tests, cell culture, and Western blot assays to test whether AEA regulates acute pain induced by TRPV1 activation and TRPV1 total protein levels. Results: Our results demonstrate that AEA downregulates the capsaicin-evoked pain response and TRPV1 protein levels in nociceptors. Furthermore, in a transiently heterologous TRPV1 expression system, we found that AEA induces ubiquitination of TRPV1 and its degradation via the proteasome pathway, independent of calcium influx. We also found that AEA induced an upregulation of BiP protein levels, a key endoplasmic reticulum chaperone and stress sensor, without changes in other ER stress sensors. Finally, we explored whether the endocannabinoid 2-arachidonoylglycerol (2-AG) and the phytocannabinoid β-caryophyllene (BCP) modulate capsaicin-evoked pain, finding that 2-AG is less effective than AEA in reducing pain behavior, whereas BCP shows effects similar to AEA in reducing capsaicin-evoked pain. Conclusions: These results highlight AEA and BCP as regulators of TRPV1 protein levels and of the painful response mediated by this molecular pain target.

Capsaicin, the chili-pungent compound, has a peculiar chemical structure that allows it to impact mammalian physiology. Besides its classical effects through activating the Transient Receptor Potential Vanilloid type 1 (TRPV1), growing experimental evidence demonstrates that capsaicin has pleiotropic actions in a TRPV1-independent manner. These effects are achieved by modifying the membrane features or interacting with unclassical putative molecular targets. Here, we will summarize the representative information related to the capsaicin actions through unclassical and TRPV1-independent molecular mechanisms, and we will discuss the impact of these effects on non-neuronal cells and mammalian physiology.

Type 2 diabetes mellitus (T2DM) is a complex chronic disease characterized by decreased insulin secretion and the development of insulin resistance. Previous genome-wide association studies demonstrated that single-nucleotide polymorphisms (SNPs) present in genes coding for ion channels involved in insulin secretion increase the risk of developing this disease. We determined the association of 16 SNPs found in CACNA1D, KCNQ1, KCNJ11, and CACNA1E genes and the increased probability of developing T2DM. In this work, we performed a case-control study in 301 Mexican adults, including 201 cases with diabetes and 100 controls without diabetes. Our findings indicate a moderate association between T2DM and the C allele, and the C/C genotype of rs312480 within CACNA1D. The CAG haplotype surprisingly showed a protective effect, whereas the CAC and CGG haplotypes have a strong association with T2DM. The C allele and C/C genotype of rs5219 were significantly associated with diabetes. Also, an association was observed between diabetes and the A allele and the A/A genotype of rs3753737 and rs175338 in CACNA1E. The TGG and CGA haplotypes were also found to be significantly associated. The findings of this study indicate that the SNPs examined could serve as a potential diagnostic tool and contribute to the susceptibility of the Mexican population to this disease.

Sexual dimorphism among mammals includes variations in the pain threshold. These differences are influenced by hormonal fluctuations in females during the estrous and menstrual cycles of rodents and humans, respectively. These physiological conditions display various phases, including proestrus and diestrus in rodents and follicular and luteal phases in humans, distinctly characterized by varying estrogen levels. In this study, we evaluated the capsaicin responses in male and female mice at different estrous cycle phases, using two murine acute pain models. Our findings indicate that the capsaicin-induced pain threshold was lower in the proestrus phase than in the other three phases in both pain assays. We also found that male mice exhibited a higher pain threshold than females in the proestrus phase, although it was similar to females in the other cycle phases. We also assessed the mRNA and protein levels of TRPV1 in the dorsal root and trigeminal ganglia of mice. Our results showed higher TRPV1 protein levels during proestrus compared to diestrus and male mice. Unexpectedly, we observed that the diestrus phase was associated with higher TRPV1 mRNA levels than those in both proestrus and male mice. These results underscore the hormonal influence on TRPV1 expression regulation and highlight the role of sex steroids in capsaicin-induced pain.

In the spinal cord, attenuation of the inhibitory action of glycine is related to an increase in both inflammatory and diabetic neuropathic pain; however, the glycine receptor involvement in diabetic neuropathy has not been reported. We determined the expression of the glycine receptor subunits (α1–α3 and β) in streptozotocin-induced diabetic Long–Evans rats by qPCR and Western blot. The total mRNA and protein expression (whole spinal cord homogenate) of the α1, α3, and β subunits did not change during diabetes; however, the α2 subunit mRNA, but not the protein, was overexpressed 45 days after diabetes induction. By contrast, the synaptic expression of the α1 and α2 subunits decreased in all the studied stages of diabetes, but that of the α3 subunit increased on day 45 after diabetes induction. Intradermal capsaicin produced higher paw-licking behavior in the streptozotocin-induced diabetic rats than in the control animals. In addition, the nocifensive response was higher at 45 days than at 20 days. During diabetes, the expression of the glycine receptor was altered in the spinal cord, which strongly suggests its involvement in diabetic neuropathy.

Proteins belonging to Transient Receptor Potential (TRP) family are nonselective cation channels that play an essential role in mammalian physiology, functioning as transducers of several environmental signals including those of chemical, thermal and mechanical natures. A subgroup of these receptors is expressed in sensory neurons where they are activated by noxious stimuli and are key players of pain responses in the organism. Some TRP channels are molecular targets for the classical and non-classical effects of sex steroids. This chapter will describe the close relationship between nociceptive TRP channels and sex steroids as well as their impact on nociception and pain-related responses.

The Transient Receptor Vanilloid 1 (TRPV1) or capsaicin receptor is a nonselective cation channel, which is abundantly expressed in nociceptors. This channel is an important transducer of several noxious stimuli, having a pivotal role in pain development. Several TRPV1 studies have focused on understanding its structure and function, as well as on the identification of compounds that regulate its activity. The intracellular roles of these channels have also been explored, highlighting TRPV1′s actions in the homeostasis of Ca²⁺ in organelles such as the mitochondria. These studies have evidenced how the activation of TRPV1 affects mitochondrial functions and how this organelle can regulate TRPV1-mediated nociception. The close relationship between this channel and mitochondria has been determined in neuronal and non-neuronal cells, demonstrating that TRPV1 activation strongly impacts on cell physiology. This review focuses on describing experimental evidence showing that TRPV1 influences mitochondrial function.

Transient Receptor Potential (TRP) channels are a family of ion channels whose members are distributed among all kinds of animals, from invertebrates to vertebrates. The importance of these molecules is exemplified by the variety of physiological roles they play. Perhaps, the most extensively studied member of this family is the TRPV1 ion channel; nonetheless, the activity of TRPV4 has been associated to several physio and pathophysiological processes, and its dysfunction can lead to severe consequences. Several lines of evidence derived from animal models and even clinical trials in humans highlight TRPV4 as a therapeutic target and as a protein that will receive even more attention in the near future, as will be reviewed here.

Transient receptor potential (TRP) channels are remarkable transmembrane protein complexes that are essential for the physiology of the tissues in which they are expressed. They function as non-selective cation channels allowing for the signal transduction of several chemical, physical and thermal stimuli and modifying cell function. These channels play pivotal roles in the nervous and reproductive systems, kidney, pancreas, lung, bone, intestine, among others. TRP channels are finely modulated by different mechanisms: regulation of their function and/or by control of their expression or cellular/subcellular localization. These mechanisms are subject to being affected by several endogenously-produced compounds, some of which are of a lipidic nature such as steroids. Fascinatingly, steroids and TRP channels closely interplay to modulate several physiological events. Certain TRP channels are affected by the typical genomic long-term effects of steroids but others are also targets for non-genomic actions of some steroids that act as direct ligands of these receptors, as will be reviewed here.

The Transient Receptor Potential Vanilloid 1 (TRPV1) channel is a polymodal protein with functions widely linked to the generation of pain. Several agonists of exogenous and endogenous nature have been described for this ion channel. Nonetheless, detailed mechanisms and description of binding sites have been resolved only for a few endogenous agonists. This review focuses on summarizing discoveries made in this particular field of study and highlighting the fact that studying the molecular details of activation of the channel by different agonists can shed light on biophysical traits that had not been previously demonstrated.