Induction of TRPV1 desensitization by a biased receptor agonist

Resolvin E1 Inhibits Substance P-Induced Potentiation of TRPV1 in Primary Sensory Neurons

The neuropeptide substance P (SP) is expressed in primary sensory neurons and is commonly regarded as a “pain” neurotransmitter. Upon peripheral inflammation, SP activates the neurokinin-1 (NK-1) receptor and potentiates activity of transient receptor potential vanilloid subtype 1 (TRPV1), which is coexpressed by nociceptive neurons. Therefore, SP functions as an important neurotransmitter involved in the hypersensitization of inflammatory pain. Resolvin E1 (RvE1), derived from omega-3 polyunsaturated fatty acids, inhibits TRPV1 activity via activation of the chemerin 23 receptor (ChemR23)—an RvE1 receptor located in dorsal root ganglion neurons—and therefore exerts an inhibitory effect on inflammatory pain. We demonstrate here that RvE1 regulates the SP-induced potentiation of TRPV1 via G-protein coupled receptor (GPCR) on peripheral nociceptive neurons. SP-induced potentiation of TRPV1 inhibited by RvE1 was blocked by the Gαi-coupled GPCR inhibitor pertussis toxin and the G-protein inhibitor GDPβ-S. These results indicate that a low concentration of RvE1 strongly inhibits the potentiation of TRPV1, induced by the SP-mediated activation of NK-1, via a GPCR signaling pathway activated by ChemR23 in nociceptive neurons. RvE1 might represent a new therapeutic target for the treatment of inflammatory pain as a prospective endogenous inhibitor that strongly inhibits TRPV1 activity associated with peripheral inflammation.

Phenylarsine oxide as a redox modulator of transient receptor potential vanilloid type 1 channel function

Transient receptor potential vanilloid type 1 (TRPV1) channels are capable of detecting and integrating noxious stimuli and play an important role in nociceptor activation and sensitization. It has been demonstrated that oxidizing agents are capable of positively modulating (sensitizing) the TRPV1 channel. The present study investigates the ability of the thiol-oxidizing agent phenylarsine oxide (PAO) to modulate TRPV1 currents under voltage-clamp conditions. We assessed the ability of PAO to modulate both proton- and capsaicin-activated currents mediated by recombinant human TRPV1 channels as well as native rat and human TRPV1 channels in dorsal root ganglion (DRG) neurons. Experiments with other oxidizing and reducing agents having various membrane-permeating properties supported the intracellular oxidizing mechanism of PAO modulation. The PAO modulation of proton-activated currents was consistent across the cell types studied, with an increase in current across the proton concentrations studied. PAO modulation of the capsaicin-activated current in hTRPV1/Chinese hamster ovary cells consisted of potentiation of the current elicited with low capsaicin concentrations and inhibition of the current at higher concentrations. This same effect was seen with these recombinant cells in calcium imaging experiments and with native TRPV1 channels in rat DRG neurons. Contrary to this, currents in human DRG neurons were potentiated at all capsaicin concentrations tested after PAO treatment. These results could indicate important differences in the reduction-oxidation modulation of human TRPV1 channels in a native cellular environment.

From opto- to radio-genetics: A switch in the wavelength

The use of ion channels to control defined events in defined cell types at defined times in the context of living tissue or whole organism represent one of the major advance of the last decade, and optogenetics (i.e the combination of genetic and optical methods) obviously played a key role in this achievement.¹ Although the existence of light-activated ion channels (i.e ospin channels) has been known since 1971,² it took about 35 y before the concept of an ion channel used for bioengineering control of cell or tissue activity becomes reality.³ From that moment forward, rhodopsine channels^4,5 (i.e blue light-gated non-specific Na⁺ channels that depolarize cells thus increasing cell excitability) or halorhodopsin channels⁶ (i.e yellow light-gated Cl^- channels that hyperpolarize cells thus decreasing cell excitability) have been extensively used to turn neurons on and off in response to diverse colors of light, with an extremely high temporal precision (i.e milliseconds range). Although optogenetics has been originally established in neuroscience, it addresses now to non-neuronal systems, including cardiac, smooth and skeletal muscles, glial cells or even embryonic stem cells.^7-9 However, although light stimulation allows control of cell excitability with a high spatio-temporal specificity, light waves present the disadvantage to not penetrate deep tissue, and implanted devices are required for in vivo light stimulation. In contrast to visible light-waves, radio-waves (i.e longer wavelength and lower frequency) can penetrate deep tissues with minimal energy absorption.