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BACKGROUND. Chronic pain is a debilitating illness with currently limited therapy, in part due to difficulties in translating treatments derived from animal models to patients. The transient receptor potential vanilloid 1 (TRPV1) channel is associated with noxious heat detection and inflammatory pain, and reports of adverse effects in human trials have hindered extensive efforts in the clinical development of TRPV1 antagonists as novel pain relievers. METHODS. We examined 2 affected individuals (A1 and A2) carrying a homozygous missense mutation in TRPV1, rendering the channel nonfunctional. Biochemical and functional assays were used to analyze the mutant channel. To identify possible phenotypes of the affected individuals, we performed psychophysical and medical examinations. RESULTS. We demonstrated that diverse TRPV1 activators, acting at different sites of the channel protein, were unable to open the cloned mutant channel. This finding was not a consequence of impairment in the expression, cellular trafficking, or assembly of protein subunits. The affected individuals were insensitive to application of capsaicin to the mouth and skin and did not demonstrate aversive behavior toward capsaicin. Furthermore, quantitative sensory testing of A1 revealed an elevated heat-pain threshold but also, surprisingly, an elevated cold-pain threshold and extensive neurogenic inflammatory, flare, and pain responses following application of the TRPA1 channel activator mustard oil. CONCLUSION. Our study provides direct evidence in humans for pain-related functional changes linked to TRPV1, which is a prime target in the development of pain relievers. FUNDING. Supported by the Israel Science Foundation (368/19); Teva's National Network of Excellence in Neuroscience grant (no. 0394886) and Teva's National Network of Excellence in Neuroscience postdoctoral fellowship., Introduction The ability to sense noxious stimuli is crucial for protecting organisms from tissue damage. The discovery of the capsaicin and heat-activated polymodal ion channel transient receptor potential vanilloid 1 [...]

The article focuses on Bill Pak's pioneering use of the PDA phenotype to screen a network of vision-related genes and associated metabolic pathways, highlighting its significant impact on the understanding of phototransduction in Drosophila. It mentions the approach, utilizing the PDA screen to identify visual mutants based on their prolonged depolarizing afterpotential, revolutionized the field by enabling the isolation.

The light-activated channels of Drosophila photoreceptors transient receptor potential (TRP) and TRP-like (TRPL) show voltage-dependent conductance during illumination. Recent studies implied that mammalian members of the TRP family, which belong to the TRPV and TRPM subfamilies, are intrinsically voltage-gated channels. However, it is unclear whether the Drosophila TRPs, which belong to the TRPC subfamily, share the same voltage-dependent gating mechanism. Exploring the voltage dependence of Drosophila TRPL expressed in S2 cells, we found that the voltage dependence of this channel is not an intrinsic property since it became linear upon removal of divalent cations. We further found that [Ca.sup.2+] blocked TRPL in a voltage-dependent manner by an open channel block mechanism, which determines the frequency of channel openings and constitutes the sole parameter that underlies its voltage dependence. Whole cell recordings from a Drosophila mutant expressing only TRPL indicated that [Ca.sup.2+] block also accounts for the voltage dependence of the native TRPL channels. The open channel block by [Ca.sup.2+] that we characterized is a useful mechanism to improve the signal to noise ratio of the response to intense light when virtually all the large conductance TRPL channels are blocked and only the low conductance TRP channels with lower [Ca.sup.2+] affinity are active.

Drosophila melanogaster photoreceptor cells are capable of detecting single photons. This utmost sensitivity is critically dependent on the maintenance of an exceedingly low, dark, spontaneous activity of photoreceptor cells. However, the underlying mechanisms of this hallmark of phototransduction are not fully understood. An analysis of the Drosophila visual heterotrimeric ([alpha][beta][gamma]) Gq protein revealed that wild-type Drosophila flies have about a twofold excess of G[beta] over G[alpha] subunits of the visual Gq protein. Studies of G[[beta].sub.e] mutants in which the excess of G[beta] was genetically eliminated showed dramatic dark, spontaneous activity of the photoreceptor cells, whereas concurrent genetic reduction of the G[alpha] subunit, which restored the excess of G[beta], abolished this effect. These results indicate that an excess of G[beta] over G[alpha] is a strategy used in vivo for the suppression of spontaneous activity, thereby yielding a high signal to noise ratio, which is characteristic of the photoreceptor light response. This mechanism could be relevant to the regulation of G protein signaling in general.

Minke, Baruch, and Boaz Cook. TRP Channel Proteins and Signal Transduction. Physiol Rev 82: 429-472, 2002; 10.1152/physrev.00001.2002.--TRP channel proteins constitute a large and diverse family of proteins that are expressed [...]

Measurement of the extracellular changes in Ca2+ (delta Ca2+)o in the photoreceptors of Drosophila retina using Ca2+-selective microelectrodes in inactivation-but-no-afterpotential (inaC) and transient receptor potential (trp) mutants reveal that light causes a decrease in extracellular Ca2+ and this light-induced decrease in (Ca2+)o is increased and suppressed in the inaC and trp mutants, respectively. The reduction of (Ca2+)o indicates the light-induced Ca2+ influx into photoreceptors and the inhibition of this Ca2+ influx by trp mutation. The suppression of the increased Ca2+ signal in a inaC;trp double mutant indicates that the trp mutant dominates over the protein kinase C-deficient inaC mutant.

Studies on photoreceptors from dissociated Drosophila ommatidia suggest that defects in inaC halp obtain high Ca2+ levels that are not usually attained under moderate illumination conditions. This causes the Ca2+ dependent inactivation of light-sensitive channels. The trp mutant does not exhibit Ca2+ and voltage dependent light-induced current. Protein kinase C may be essential for feedback mechanism regulating photoreceptor Ca2+ level.

Studies on Drosophila photoreceptor Ca2+-permeable plasma membrane channels reveal that the trp gene-dependent channels may possess low unit conductance. The power spectra of 'rundown current' noise in wild type and trp are similar and amenable to being fitted with Lorentzians. The effective 'mean channel open time' of 1-2 ms and a smaller component at frequencies about 0.2 ms characterize the process.

Calcium-selective microelectrodes were used to test the effect of La3+ or nss mutation on Ca2+ influx in fly photoreceptors. The microelectrodes were applied into the compound eyes of normal blowfly Lucilia and into the retina of the no steady state (nss) mutant of the blowfly to monitor modifications in light-induced extracellular Ca2+ concentration. Both La3+ and nss mutation were found to suppress light-induced Ca2+ influx into the photoreceptor. The inhibitory effect of La3+ and nss mutation were identical to the inhibitory activity of trp mutation in Drosophila.