Serotonergic mechanism of the relieving effect of bee venom acupuncture on oxaliplatin-induced neuropathic cold allodynia in rats

Abstract

Background: Oxaliplatin, an important chemotherapy drug for advanced colorectal cancer, often induces peripheral neuropathy, especially cold allodynia. Our previous study showed that bee venom acupuncture (BVA), which has been traditionally used in Korea to treat various pain symptoms, potently relieves oxaliplatin-induced cold allodynia in rats. However, the mechanism for this anti-allodynic effect of BVA remains poorly understood. We investigated whether and how the central serotonergic system, a well-known pathway for acupuncture analgesia, mediates the relieving effect of BVA on cold allodynia in oxaliplatin-injected rats.

Methods: The behavioral signs of cold allodynia in Sprague-Dawley (SD) rats were induced by a single injection of oxaliplatin (6 mg/kg, i.p.). Before and after BVA treatment, the cold allodynia signs were evaluated by immersing the rat's tail into cold water (4°C) and measuring the withdrawal latency. For BVA treatment, a diluted BV (0.25 mg/kg) was subcutaneously administered into Yaoyangguan (GV3) acupoint, which is located between the spinous processes of the fourth and the fifth lumbar vertebra. Serotonin was depleted by a daily injection of DL-p-chlorophenylalanine (PCPA, 150 mg/kg, i.p.) for 3 days. The amount of serotonin in the spinal cord was measured by ELISA. Serotonergic receptor antagonists were administered intraperitoneally or intrathecally before BVA treatment.

Results: The serotonin levels in the spinal cord were significantly increased by BVA treatment and such increase was significantly reduced by PCPA. This PCPA pretreatment abolished the relieving effect of BVA on oxaliplatin-induced cold allodynia. Either of methysergide (mixed 5-HT1/5-HT2 receptor antagonist, 1 mg/kg, i.p.) or MDL-72222 (5-HT3 receptor antagonist, 1 mg/kg, i.p) blocked the anti-allodynic effect of BVA. Further, an intrathecal injection of MDL-72222 (12 μg) completely blocked the BVA-induced anti-allodynic action, whereas NAN-190 (5-HT1A receptor antagonist, 15 μg, i.t.) or ketanserin (5-HT2A receptor antagonist, 30 μg, i.t.) did not.

Conclusions: These results suggest that BVA treatment alleviates oxaliplatin-induced acute cold allodynia in rats via activation of the serotonergic system, especially spinal 5-HT3 receptors. Thus, our findings may provide a clinically useful evidence for the application of BVA as an alternative therapeutic option for the management of peripheral neuropathy, a dose-limiting side effect that occurs after an administration of oxaliplatin.

Figure 1

Effects of PCPA on BVA-induced increases in 5-HT and TWL in oxaliplatin-injected rats. (a) Levels of 5-HT in the spinal cord measured with ELISA. Vehicle + N/S, Vehicle + BV, and PCPA + BV (n = 5/group). Data are presented as mean ± SEM. *p < 0.05, **p < 0.01, by one-way ANOVA followed by Dunnet’s test. (b) PCPA, but not vehicle pretreatment (n = 8/group), blocked the significant increase in TWL by BVA at GV3 acupoint. Data are presented as mean ± SEM. ***p < 0.001, ns = no significant, by paired t-test.

Figure 2

Effects of systemic injection of 5-HT antagonists on BVA-induced anti-allodynic action. The behavioral tests for cold allodynia were performed before an intraperitoneal injection of antagonists and after BVA treatment in the four groups of animals: (a) N/S + BV (left, n = 7) and DMSO + BV (right, n = 4), (b) methysergide + BV, and (c) MDL-72222+ BV (n = 7/group). Data are presented as mean ± SEM. **p < 0.01, ***p < 0.001, ns = no significant, by paired t-test.

Figure 3

Effects of intrathecal injection of 5-HT antagonists on BVA-induced anti-allodynic action. The behavioral tests for cold allodynia were performed before an intrathecal injection of antagonists and after BVA treatment in the five groups of animals: (a) N/S + BV (left, n = 7) and DMSO + BV (right, n = 3), (b) NAN-190 + BV, (c) ketanserin + BV, and (d) MDL-72222 + BV (n = 8/group). Data are presented as mean ± SEM (bar graph) and individual traces in TWL change. *p < 0.05, **p < 0.01, ns = no significant, by paired t-test.