Evoked and Ongoing Pain-Like Behaviours in a Rat Model of Paclitaxel-Induced Peripheral Neuropathy

Abstract

Paclitaxel-induced neuropathic pain is a major dose-limiting side effect of paclitaxel therapy. This study characterises a variety of rat behavioural responses induced by intermittent administration of clinically formulated paclitaxel. 2 mg/kg paclitaxel or equivalent vehicle was administered intraperitoneally on days 0, 2, 4, and 6 to adult male Sprague-Dawley rats. Evoked pain-like behaviours were assessed with von Frey filaments, acetone, or radiant heat application to plantar hind paws to ascertain mechanical, cold, or heat sensitivity, respectively. Motor coordination was evaluated using an accelerating RotaRod apparatus. Ongoing pain-like behaviour was assessed via spontaneous burrowing and nocturnal wheel running. Mechanical and cold hypersensitivity developed after a delayed onset, peaked approximately on day 28, and persisted for several months. Heat sensitivity and motor coordination were unaltered in paclitaxel-treated rats. Spontaneous burrowing behaviour and nocturnal wheel running were significantly impaired on day 28, but not on day 7, indicating ongoing pain-like behaviour, rather than acute drug toxicity. This study comprehensively characterises a rat model of paclitaxel-induced peripheral neuropathy, providing the first evidence for ongoing pain-like behaviour, which occurs in parallel with maximal mechanical/cold hypersensitivity. We hope that this new data improve the face validity of rat models to better reflect patient-reported pain symptoms, aiding translation of new treatments to the clinic.

Figure 1

Time course of weight gain in rats following paclitaxel or vehicle administration. Graph shows mean ± SEM of rats' weight (g) from before paclitaxel/vehicle administration (BL) up to day 174. Arrows indicate four injections of 2 mg/kg paclitaxel or equivalent volume of vehicle on days 0, 2, 4, and 6 (n=18 per group).

Figure 2

Time course of paclitaxel-induced mechanical hypersensitivity. Graphs show the mean ± SEM of the number of withdrawal responses to (a) von Frey 4 g, (b) von Frey 8 g, and (c) von Frey 15 g, respectively. Arrows indicate four injections of 2 mg/kg paclitaxel or equivalent volume of vehicle on days 0, 2, 4, and 6. BL = baseline before paclitaxel/vehicle administration. Res = resolution of mechanical hypersensitivity, which occurred between days 174 and 219. ^∗p < 0.05, two-tailed unpaired t-tests with Bonferroni correction (n=15–21 per group).

Figure 3

Time course of paclitaxel-induced cold hypersensitivity. Graph shows the median ± interquartile range of the hind paw response to acetone application catergorised by the cold score. BL = baseline before paclitaxel/vehicle administration. Arrows indicate four injections of 2 mg/kg paclitaxel or equivalent volume of vehicle on days 0, 2, 4, and 6. ^∗p < 0.05, the Friedman test with Dunn's post hoc test comparing the baseline scores in each group (n=12 per group). As this analysis is nonparametric, the error bars appear larger than expected from parametric analysis.

Figure 4

Effect of paclitaxel administration on heat hypersensitivity. Graph shows the mean ± SEM of the hind paw withdrawal latency to a radiant heat stimulus. BL = baseline before paclitaxel/vehicle administration. Arrows indicate four injections of 2 mg/kg paclitaxel or equivalent volume of vehicle on days 0, 2, 4, and 6 (n=6 per group).

Figure 5

Effect of paclitaxel administration on motor coordination. Graph shows the mean ± SEM of the latency to fall from an accelerating RotaRod apparatus. BL = baseline before paclitaxel/vehicle administration. Arrows indicate four injections of 2 mg/kg paclitaxel or equivalent volume of vehicle on days 0, 2, 4, and 6 (n=6 per group).

Figure 6

Effect of paclitaxel administration on spontaneous burrowing behaviour. Graphs show the mean ± SEM of (a) gravel displaced from burrowing tubes and (b) latency to start burrowing by individual rats before paclitaxel/vehicle administration (BL) and then on day 7 and day 28 following the initial injection of 2 mg/kg paclitaxel or vehicle. ^∗p < 0.05, one-way repeated measures ANOVA with Dunnett's post hoc test comparing the baseline burrowing capacity (n=6 per group).

Figure 7

Effect of paclitaxel administration on overnight spontaneous wheel-running behaviour. Graphs show the mean ± SEM of the (a) time spent on the wheel, (b) total distance travelled, (c) maximal acceleration generated, and (d) number of times the wheel was accessed by individual rats overnight (7.10 pm–6.30 am) before paclitaxel/vehicle administration (BL) and then on day 7, day 28, and day 35 following the initial injection of 2 mg/kg paclitaxel or vehicle. Arrows indicate four injections of 2 mg/kg paclitaxel or equivalent volume of vehicle on days 0, 2, 4, and 6. ^∗p < 0.05, two-way repeated measures ANCOVA with Bonferroni pairwise analysis (n=10 vehicle rats and n=9 paclitaxel rats).