Tumor-evoked sensitization of C nociceptors: a role for endothelin

Abstract

Primary and metastatic cancers that effect bone are frequently associated with pain. Sensitization of primary afferent C nociceptors innervating tissue near the tumor likely contributes to the chronic pain and hyperalgesia accompanying this condition. This study focused on the role of the endogenous peptide endothelin-1 (ET-1) as a potential peripheral algogen implicated in the process of cancer pain. Electrophysiological response properties, including ongoing activity and responses evoked by heat stimuli, of C nociceptors were recorded in vivo from the tibial nerve in anesthetized control mice and mice exhibiting mechanical hyperalgesia following implantation of fibrosarcoma cells into and around the calcaneus bone. ET-1 (100 microM) injected into the receptive fields of C nociceptors innervating the plantar surface of the hind paw evoked an increase in ongoing activity in both control and tumor-bearing mice. Moreover, the selective ETA receptor antagonist, BQ-123 (3 mM), attenuated tumor-evoked ongoing activity in tumor-bearing mice. Whereas ET-1 produced sensitization of C nociceptors to heat stimuli in control mice, C nociceptors in tumor-bearing mice were sensitized to heat, and their responses were not further increased by ET-1. Importantly, administration of BQ-123 attenuated tumor-evoked sensitization of C nociceptors to heat. We conclude that ET-1 at the tumor site contributes to tumor-evoked excitation and sensitization of C nociceptors through an ETA receptor mediated mechanism.

FIG. 1.

Responses of C nociceptors to injection of endothelin-1 (ET-1) in nontumor-bearing (control) and tumor-bearing (cancer) mice. A and D: ongoing activities of C nociceptors in a control mouse and in a tumor-bearing mouse before (baseline) and after injections of vehicle [10 μl of phosphate-buffered saline (PBS)] and ET-1 (100 μM in 10 μl) into its receptive field (RF). B and E: 5 overlapping traces of the constant latency of electrically evoked excitation are illustrated for each C nociceptor. Open arrow, stimulus artifact; solid arrow, action potentials of each C nociceptor. C and F: mean ± SE discharge rate (imp/s) of ongoing activity for C nociceptors in control and tumor-bearing mice at baseline, after injection of vehicle, and after injection of ET-1 into their RFs. *, P < 0.05; **, P < 0.001 for the ongoing activity after injection of ET-1 compared with baseline levels and after injection of vehicle.

FIG. 2.

The effect of the ET_A receptor antagonist BQ-123 on ongoing activity of C nociceptors in tumor-bearing (cancer) mice. A: ongoing activity of a single C nociceptor before (baseline) and after injection of BQ-123 (3 mM in 10 μl). B: 5 overlapping traces of the constant latency of electrically evoked excitation of this C nociceptor are illustrated. Open arrow, stimulus artifact; solid arrow, action potential of this single C nociceptor. C: mean ± SE discharge rate of C nociceptors in tumor-bearing mice before and after injection of BQ-123. *, significant difference from baseline (P < 0.001).

FIG. 3.

Mean ± SE heat-response thresholds of C nociceptors in nontumor-bearing (control) and in tumor-bearing (cancer) mice before (baseline) and after injection of vehicle (10 μl of PBS) or ET-1 (100 μM in 10 μl). C nociceptors in tumor-bearing mice exhibited lower heat-response thresholds than those in control mice at baseline and after injection of vehicle. Injection of ET-1 decreased the heat-response thresholds of C nociceptors in control mice to a level that was similar to heat-response thresholds of C nociceptors in tumor-bearing mice. *, significant difference from baseline (P < 0.001); #, significant differences from control mice (P < 0.001).

FIG. 4.

Effects of ET-1 and the ET_A receptor antagonist BQ-123 on responses of C nociceptors to heat in control and in tumor-bearing (cancer) mice. A: mean ± SE stimulus-response functions for C nociceptors in control mice. B: mean ± SE cumulative number of impulses evoked by all heat stimuli before (baseline) and after injection of vehicle (10 μl of PBS) or ET-1 (100 μM in 10 μl) into the RFs of C nociceptors in control mice. C: mean ± SE stimulus-response functions for C nociceptors in tumor-bearing mice. D: mean ± SE cumulative number of impulse before (baseline) and after injection of vehicle, ET-1, or BQ-123 (3 mM in 10 μl) into the RFs of C nociceptors in tumor-bearing mice. For A–C, *, significant difference from baseline and vehicle (P < 0.05). For D, *, significant difference from baseline, vehicle, and ET-1 (P < 0.005).