Effect of resiniferatoxin on the noxious heat threshold temperature in the rat: a novel heat allodynia model sensitive to analgesics

Abstract

1. An increasing-temperature hot plate (ITHP) was introduced to measure the noxious heat threshold (45.3+/-0.3 degrees C) of unrestrained rats, which was reproducible upon repeated determinations at intervals of 5 or 30 min or 1 day. 2. Morphine, diclofenac and paracetamol caused an elevation of the noxious heat threshold following i.p. pretreatment, the minimum effective doses being 3, 10 and 200 mg kg(-1), respectively. 3. Unilateral intraplantar injection of the VR1 receptor agonist resiniferatoxin (RTX, 0.048 nmol) induced a profound drop of heat threshold to the innocuous range with a maximal effect (8-10 degrees C drop) 5 min after RTX administration. This heat allodynia was inhibited by pretreatment with morphine, diclofenac and paracetamol, the minimum effective doses being 1, 1 and 100 mg kg(-1) i.p., respectively. 4. The long-term sensory desensitizing effect of RTX was examined by bilateral intraplantar injection (0.048 nmol per paw) which produced, after an initial threshold drop, an elevation (up to 2.9+/-0.5 degrees C) of heat threshold lasting for 5 days. 5. The VR1 receptor antagonist iodo-resiniferatoxin (I-RTX, 0.05 nmol intraplantarly) inhibited by 51% the heat threshold-lowering effect of intraplantar RTX but not alpha,beta-methylene-ATP (0.3 micromol per paw). I-RTX (0.1 or 1 nmol per paw) failed to alter the heat threshold either acutely (5-60 min) or on the long-term (5 days). The heat threshold of VR1 receptor knockout mice was not different from that of wild-type animals (45.6+/-0.5 vs 45.2+/-0.4 degrees C). 6. In conclusion, the RTX-induced drop of heat threshold measured by the ITHP is a novel heat allodynia model exhibiting a high sensitivity to analgesics.

Figure 1

Reproducibility of the noxious heat threshold temperature upon repeated measurements in three different groups of untreated rats. Heat threshold of either paw (black columns) refers to the lower value of the left and right paws threshold, which represents the heat threshold of the animal. The heating rate was 12°C min⁻¹ in the first group (upper panel) and 6°C min⁻¹ in the second and third groups. Data are means with s.e.m. of 12 animals in each group. Statistical analysis revealed no significant differences between the thresholds of the same paw or that of the animal (labelled as either paw) measured at various time points (ANOVA followed by Newman–Keuls test) and between the threshold of the left and right paws measured at any time point (Student's t-test for unpaired samples).

Figure 2

Effect of morphine, diclofenac or paracetamol pretreatment (i.p., 30 min) on the noxious heat threshold. Increase of threshold refers to the difference between the postdrug and the control thresholds. Data are means with s.e.m. of 6-12 animals. As all solvents failed to significantly alter the heat threshold, the postdrug threshold was compared to the control using Student's t-test for paired samples. Asterisks indicate statistically significant (P<0.05) drug effects.

Figure 3

Effect of unilateral intraplantar injection of RTX (0.048 nmol per paw) on the heat threshold. Data are means with s.e.m. of 12 animals. Asterisks indicate values significantly different from the control threshold of the treated paw (P<0.05, calculated with ANOVA followed by Newman–Keuls test). The solvent of RTX (100 μl 0.03% ethanol in saline) failed to alter the heat threshold and only the paw-licking reaction was observed in the vehicle-treated animals (data not shown).

Figure 4

Effect of diclofenac pretreatment (1 mg kg⁻¹ i.p., 25 min before RTX injection) as compared to its solvent on the RTX-induced drop of heat threshold. Data are means with s.e.m. of thresholds of the RTX-treated paw (n=12 in both groups). The effect of diclofenac was significant (P<0.05, Student's t-test for unpaired samples) comparing the threshold drop at 5 min (as shown) or the sum of threshold drops at 5, 10, 15 and 20 min in the solvent- versus drug-treated animals.

Figure 5

Dose–response relation for the inhibitory effect of morphine, diclofenac or paracetamol pretreatment (i.p., 25 min before RTX injection) on the RTX-induced drop of heat threshold. Percentage inhibition of threshold drop was calculated with the formula (Drop_solv−Drop_drug)/Drop_solv × 100, where Drop_soiv and Drop_drug refer to the average of threshold drops measured in the solvent- and drug-treated animals, respectively (n=10–12 in each group). Solid lines show percentage inhibition values calculated on the basis of threshold drop 5 min after RTX injection, whereas dashed lines indicate those determined with the sum of threshold drops at 5, 10, 15 and 20 min. Asterisks indicate statistically significant (P<0.05) drug effects as calculated by comparisons to their solvents with Student's t-test for unpaired samples. The asterisks refer to points of both lines in case of each drug.

Figure 6

Long-term effect of bilateral intraplantar injection of RTX (0.048 nmol per paw) on the noxious heat threshold. Data are means with s.e.m. of 12 animals. Asterisks indicate values significantly different from control (P<0.05, as calculated with ANOVA followed by Newman–Keuls test). The solvent of RTX (100 μl 0.03% ethanol in saline) applied also bilaterally failed to evoke any significant changes of threshold (data not shown).

Figure 7

Effect of I-RTX (0.05 nmol per paw) on the RTX (0.048 nmol per paw)- induced and α-β-methylene-ATP (meATP, 0.3 μmol per paw)-induced drop of heat threshold. Shown are heat threshold values measured 5 min after RTX or meATP injection in the solvent- and I-RTX-pretreated animals (upper panel) as well as the sum of threshold drops (at 5, 10, 15 and 20 min for RTX; 5 and 10 min for meATP) during the allodynic/hyperalgesic response induced by RTX or meATP (lower panel). Data are means with s.e.m. of 8 animals. Asterisks indicate values significantly different compared to solvent pretreatment (P<0.05, as calculated with Student's t-test for unpaired samples).