The vanilloid receptor TRPV1 is tonically activated in vivo and involved in body temperature regulation

Abstract

The vanilloid receptor TRPV1 (transient receptor potential vanilloid 1) is a cation channel that serves as a polymodal detector of pain-producing stimuli such as capsaicin, protons (pH <5.7), and heat. TRPV1 antagonists block pain behaviors in rodent models of inflammatory, neuropathic, and cancer pain, suggesting their utility as analgesics. Here, we report that TRPV1 antagonists representing various chemotypes cause an increase in body temperature (hyperthermia), identifying a potential issue for their clinical development. Peripheral restriction of antagonists did not eliminate hyperthermia, suggesting that the site of action is predominantly outside of the blood-brain barrier. Antagonists that are ineffective against proton activation also caused hyperthermia, indicating that blocking capsaicin and heat activation of TRPV1 is sufficient to produce hyperthermia. All TRPV1 antagonists evaluated here caused hyperthermia, suggesting that TRPV1 is tonically activated in vivo and that TRPV1 antagonism and hyperthermia are not separable. TRPV1 antagonists caused hyperthermia in multiple species (rats, dogs, and monkeys), demonstrating that TRPV1 function in thermoregulation is conserved from rodents to primates. Together, these results indicate that tonic TRPV1 activation regulates body temperature.

Figure 1.

AMG9810 blocks capsaicin-induced hypothermia and causes hyperthermia by itself in rats. a, Body temperatures of rats recorded from implanted radiotelemetry probes. Rats were first administered with either vehicle or 30 mg/kg AMG9810 intraperitoneally and 30 min later were challenged with capsaicin (20 μg/20 μl). Note that the capsaicin-induced decrease in body temperature of rats at 60 min was greater in the vehicle/capsaicin group (red) compared with the AMG9810/capsaicin group (green) (t = 4.63; p < 0.001). AMG9810 caused a significant increase in rat body temperature at 20 and 30 min (F _(2,14) = 20.98; F _(2,14) = 26.60; p < 0.001). b, Body temperature of rats administered with either vehicle or 30 mg/kg AMG9810 intraperitoneally. A significant increase in body temperature was seen 20 and 30 min after AMG9810 administration (t = 4.20 and 3.85; p < 0.01). Error bars indicate SEM.

Figure 2.

Both brain-penetrant (AMG8163) and peripherally restricted (AMG1629 and AMG3731) TRPV1 antagonists cause hyperthermia in rats. a–c, Body temperatures of rats administered with vehicle or 3 mg/kg AMG8163(a), 3 mg/kg AMG1629 (b), and 10 mg/kg AMG3731 (c). All TRPV1 antagonists were given orally to rats, and body temperature was monitored for 120 min after administration. Compared with vehicle treatment, all four antagonists caused significant increase in body temperature (p < 0.01) at each time point or the average temperature from 0 to 120 min. d, Time course of body temperature in rats administered with vehicle or 0.1, 1, or 10 mg/kg AMG8163. During hours 2–11, ANOVA at each time point was significant (p < 0.01), and Dunnett's multiple comparison to vehicle was significant at each time point for 1 and 10 mg/kg doses (p < 0.01). During hours 16–21, ANOVA at each time point was nonsignificant (p > 0.05). Error bars indicate SEM.

Figure 3.

JYL1421 blocks capsaicin-induced flinch behavior in rats but does not induce body temperature changes. a, JYL1421 blocks capsaicin-induced flinch in rats. Different doses (3–30 mg/kg) of JYL1421 were administered orally to rats 60 min before capsaicin challenge. The number (No.) of flinches in the first minute was counted and plotted against the dose of JYL1421. A 100% inhibition of capsaicin-induced flinch was observed at 30 mg/kg JYL1421 (F _(3,27) = 10.68; n = 8; p < 0.05 only for 30 mg/kg). Plasma concentrations of JYL1421 at 10 and 30 mg/kg were 2546 ± 600 and 14182 ± 3953 ng/ml, respectively. v/c, vehicle/capsaicin; D/R, dose response. b, Body temperature of vehicle (veh) or different doses (range, 3–100 mg/kg) of JYL1421-administered (orally) rats. No significant change in body temperature was observed at all doses tested (p > 0.05 for all Dunnett's multiple comparisons at each time point). Plasma concentrations of JYL1421 at 10, 30, and 100 mg/kg were 2725 ± 3329, 3876 ± 1448, 7867 ± 3570 ng/ml, respectively. c, Capsaicin (0.5 μm)-induced or heat (45°C)-induced ⁴⁵Ca²⁺ uptake into CHO cells stably expressing rat TRPV1 in the absence (100%) or presence of different concentrations of JYL1421. Note that JYL1421 fully blocks capsaicin (red) and partially blocks heat (blue) activation. d, Low pH (proton)-induced ⁴⁵Ca²⁺ uptake into CHO cells expressing rat TRPV1 in the absence (100%) or presence of different concentrations of JYL1421. JYL1421 shows partial inhibition of pH 5.5-induced ⁴⁵Ca²⁺ uptake at the highest concentration tested (4 μm). Note the potentiation of pH 5-induced ⁴⁵Ca²⁺ uptake above 100% by JYL1421 at concentrations >30 nm. max, Maximum. Error bars indicate SEM.

Figure 4.

JYL1421 blocks capsaicin, heat, and proton activation of dog and cynomolgus monkey TRPV1 and causes hyperthermia in both species. a, Capsaicin (0.5 μm)-induced or heat (45°C)-induced ⁴⁵Ca²⁺ uptake into HEK293 cells transiently expressing dog TRPV1 in the absence (100%) or presence of different concentrations of JYL1421. Note that JYL1421 fully blocks capsaicin (red) and partially blocks heat (blue) activation. b, Capsaicin (0.5 μm)-induced or heat (45°C)-induced ⁴⁵Ca²⁺ uptake into CHO cells stably expressing cynomolgus monkey TRPV1 in the absence (100%) or presence of different concentrations of JYL1421. Note that JYL1421 fully blocks capsaicin (red) and heat (blue) activation. c, Low pH (proton)-induced ⁴⁵Ca²⁺ uptake into HEK293 cells transiently expressing dog TRPV1 in the absence (100%) or presence of different concentrations of JYL1421. JYL1421 shows partial inhibition of pH 5.5-induced ⁴⁵Ca²⁺ uptake. d, Low pH (proton)-induced ⁴⁵Ca²⁺ uptake into CHO cells expressing cynomolgus monkey TRPV1 in the absence (100%) or presence of different concentrations of JYL1421. JYL1421 shows full block of pH 5.5 activation (green) and partially blocks pH 5 activation (brown). e, Time course of body temperature of dogs administered with either vehicle or 30 mg/kg JYL1421. JYL1421 caused a significant increase in body temperature compared with vehicle treatment from 1 to 5 h after administration (t = 3.01–8.94; p < 0.05). f, Time course of body temperature of cynomolgus monkeys administered with either vehicle or 100 mg/kg JYL1421. JYL1421 caused a significant increase in body temperature compared vehicle treatment from 1 to 17 h after administration (t = 2.98–8.66; p < 0.05). max, Maximum; Veh, vehicle. Error bars indicate SEM.