Inhibition of rapid heat responses in nociceptive primary sensory neurons of rats by vanilloid receptor antagonists

Pharmacological differences between the human and rat vanilloid receptor 1 (VR1)

Vanilloid receptors (VR1) were cloned from human and rat dorsal root ganglion libraries and expressed in Xenopus oocytes or Chinese Hamster Ovary (CHO) cells. Both rat and human VR1 formed ligand gated channels that were activated by capsaicin with similar EC(50) values. Capsaicin had a lower potency on both channels, when measured electrophysiologically in oocytes compared to CHO cells (oocytes: rat=1.90+/-0.20 microM; human=1.90+/-0.30 microM: CHO cells: rat=0.20+/-0.06 microM; human=0.19+/-0.08 microM). In CHO cell lines co-expressing either rat or human VR1 and the calcium sensitive, luminescent protein, aequorin, the EC(50) values for capsaicin-induced responses were similar in both cell lines (rat=0.35+/-0.06 microM, human=0.53+/-0.03 microM). The threshold for activation by acidic solutions was lower for human VR1 channels than that for rat VR1 (EC(50) pH 5.49+/-0.04 and pH 5.78+/-0.09, respectively). The threshold for heat activation was identical (42 degrees C) for rat and human VR1. PPAHV was an agonist at rat VR1 (EC(50) between 3 and 10 microM) but was virtually inactive at the human VR1 (EC(50)>10 microM). Capsazepine and ruthenium red were both more potent at blocking the capsaicin response of human VR1 than rat VR1. Capsazepine blocked the human but not the rat VR1 response to low pH. Capsazepine was also more effective at inhibiting the noxious heat response of human than of rat VR1.

Comparison of currents activated by noxious heat in rat and chicken primary sensory neurons

The vanilloid receptor 1 (VR1) gene is responsible for both capsaicin-, and low threshold (LT) noxious heat-sensitivity in mammalian primary sensory neurons. Although, birds lack capsaicin-sensitivity they express LT noxious heat-sensitivity. Here, we compared LT noxious heat-activated whole-cell currents produced by rat and chicken cultured dorsal root ganglion neurons in order to find out the similarities and differences in the LT noxious heat transduction mechanisms between the two species. No significant differences between rat and chicken neurons were found in the mean cell diameter of the LT noxious heat-sensitive cells (20.4+/-0.8 microm, n=19 and 20.6+/-0.6 microm, n=11, respectively) and the average threshold (45.7+/-0.5 degrees C, n=19 and 46.1+/-0.7 degrees C, n=11, respectively) and peak amplitude (-2.9+/-0.6 nA, n=19 and -2.1+/-0.6 nA, n=11, respectively) of the heat-evoked responses. The current-voltage curves of the responses both in rat and chicken cells reversed at the same range (-19.5+/-3.8 mV, n=4 and -15.5+/-1. 2 mV, n=3, respectively) and showed strong outward rectification at negative membrane potentials. While all LT noxious heat-sensitive rat cells responded to capsaicin, none of the chicken neurons produced detectable response to it. These findings suggest that a VR1 homologue which lacks to sequence for capsaicin-sensitivity is possibly the LT noxious heat transducer in chicken.

Inactivation and tachyphylaxis of heat-evoked inward currents in nociceptive primary sensory neurones of rats

Membrane currents evoked by repeated noxious heat stimuli (43-47 degrees C) of 3 s duration were investigated in acutely dissociated dorsal root ganglion (DRG) neurones of adult rats. The heat stimuli generated by a fast solution exchanger had a rise time of 114 +/- 6 ms and a fall time of 146 +/- 13 ms. When heat stimuli were applied to heat-sensitive small (< or = 32.5 microm) DRG neurones, an inward membrane current (I(heat)) with a mean peak of 2430 +/- 550 pA was observed (n = 19). This current started to activate and deactivate with no significant latency with respect to the heat stimulus. The peak of I(heat) was reached with a rise time of 625 +/- 115 ms. When the heat stimulus was switched off I(heat) deactivated with a fall time of 263 +/- 17 ms. During constant heat stimulation I(heat) decreased with time constants of 4-5 s (inactivation). At the end of a 3 s heat stimulus the peak current was reduced by 44 +/- 5 % (n = 19). Current-voltage curves revealed outward rectifying properties of I(heat) and a reversal potential of -6.3 +/- 2.2 mV (n = 6). Inactivation was observed at all membrane potentials investigated (-80 to 60 mV); however, inactivation was more pronounced for inward currents (37 +/- 5 %) than for outward currents (23 +/- 6 %, P < 0.05). When neurones were investigated with repeated heat stimuli (3 to 5 times) of the same temperature, the peak current relative to the first I(heat) declined by 48 +/- 6 % at the 3rd stimulus (n = 19) and by 54 +/- 18 % at the 5th stimulus (n = 4; tachyphylaxis). In the absence of extracellular Ca2+ (buffered with 10 mM EGTA) inactivation (by 53 +/- 6 %) and tachyphylaxis (by 42 +/- 7 % across three stimuli) were still observed (n = 8). The same was true when intracellular Ca2+ was buffered by 10 mM BAPTA (inactivation by 49 +/- 4 %, tachyphylaxis by 52 +/- 7 % across three stimuli; n = 13). Thus, inactivation and tachyphylaxis were mainly independent of intra- and extracellular Ca2+. These results indicate that inactivation and tachyphylaxis of heat-evoked inward currents can be observed in vitro, similar to adaptation and suppression of action potential discharges elicited by comparably fast heat stimuli in vivo. Whereas the voltage dependence of I(heat) resembles that of capsaicin-induced membrane currents (I(Caps)), the independence of inactivation and tachyphylaxis of I(heat) from calcium is in clear contrast to I(Caps). A similar difference in calcium dependence of inactivation has been reported between heat-evoked and capsaicin-induced currents through the cloned capsaicin receptor channel VR1. Thus, the properties of I(heat) and of VR1 largely account for the adaptation and suppression of heat-evoked nociceptor discharges.

Low-threshold heat response antagonized by capsazepine in chick sensory neurons, which are capsaicin-insensitive

The heat-transducing receptor VR1 cloned from rat sensory neurons can be activated by both noxious heat and capsaicin. As the response of sensory neurons to capsaicin is species dependent, it is conceivable that the responses to noxious heat and to capsaicin are transduced by distinct receptors across different species. Therefore, we investigated responses to noxious heat from a capsaicin-insensitive (chick) and a capsaicin-sensitive (rat) species. In chick, whole-cell patch-clamp experiments in isolated dorsal root ganglion neurons revealed two populations of neurons with different thresholds to noxious heat, activated at approximately 43 degrees C and approximately 53 degrees C. In cobalt uptake experiments, the proportion of neurons showing a heat-induced response increased with increasing heat stimuli. Application of capsaicin (1-10 microM) did not result in inward currents or cobalt uptake. Rat neurons yielded comparable results in heat experiments, but were capsaicin-sensitive. Although chick neurons are insensitive to capsaicin, the competitive capsaicin antagonist capsazepine (1-10 microM) was effective in blocking heat-induced responses, verified by patch-clamp and cobalt uptake methods. The noncompetitive capsaicin antagonist ruthenium red (10 microM) reduced to almost nil the proportion of heat-responsive neurons identified with the cobalt uptake method. These findings suggest that chick DRG neurons express a low-threshold heat-transducing receptor with a pharmacological profile distinct from the low-threshold heat receptor VR1 cloned from rat DRG neurons. The data support the idea that there might be heat receptor subtypes with differences in the capsaicin binding site.

Impaired nociception and pain sensation in mice lacking the capsaicin receptor

The capsaicin (vanilloid) receptor VR1 is a cation channel expressed by primary sensory neurons of the "pain" pathway. Heterologously expressed VR1 can be activated by vanilloid compounds, protons, or heat (>43 degrees C), but whether this channel contributes to chemical or thermal sensitivity in vivo is not known. Here, we demonstrate that sensory neurons from mice lacking VR1 are severely deficient in their responses to each of these noxious stimuli. VR1-/- mice showed normal responses to noxious mechanical stimuli but exhibited no vanilloid-evoked pain behavior, were impaired in the detection of painful heat, and showed little thermal hypersensitivity in the setting of inflammation. Thus, VR1 is essential for selective modalities of pain sensation and for tissue injury-induced thermal hyperalgesia.