Analgesic effect of capsaicin pharmaceutical acupuncture on formalin-induced pain in rats

Abstract

Acupuncture combined with pharmaceutical agents has been used to treat diseases in humans and animals. Capsaicin, the active element in hot chili peppers, has selective actions on unmyelinated C-fibres and thinly myelinated A primary sensory neurons. Capsaicin induces depolarization of nerve cell by intracellular accumulation of cation. However, high concentration of intracellular calcium ion is known as voltage sensitive cation channel’s blocker for long time. As a result, continuous intracellular accumulation of cations induce desensitization of nociceptive neuron. This implies capsaicin may be used to relieve pain as one of pharmaceutical agents which can be combined with acupuncture. Manganese-enhanced magnetic resonance imaging (MEMRI) is based upon neuronal activity-dependent manganese (Mn) uptake which can provide useful information about functions of the nervous system. However, no systematic studies regarding processing of pain using MEMRI have not been conducted. The present study was conducted to determine whether capsaicin acupuncture as one of pharmaceutical acupuncture can relieve pain effectively, using formalin test, MEMRI, and c-Fos immunohistochemistry in rats. Male Sprague-Dawley rats (230 ~ 250 g) were used in this study. For MEMRI investigation, MnCl2 solution (50 ㎕) was injected into subarachnoid space. After then, capsaicin was injected by concentration (0, 0.1, 1, and 2%) at the Zusanli (ST36) acupoint of the left leg. Formalin solution (5 %, 50 ㎕) was injected into the plantar side of left hind paw and pain-related behavior (flinching) was immediately observed for 1 hr. After behavioral test, MEMRI was performed in a Biospec 4.7 T MRI system. For analyzing MnCl2 distribution, a set of noncontiguous T1-weighted (T1W) images were acquired. Lastly, immunohistochemistry was performed to observe c-fos expression.

Capsaicin injected at ST36 acupoint inhibited formalin-induced pain behaviors. In formalin test, flinching behavior of high concentration capsaicin-injected rats was significantly reduced compared to 0% capsaicin-injected group. MEMRI results showed difference of intensity induced by manganese ion between formalin only treated group and capsaicin treated group. In addition, c-Fos expression level was effectively decreased in capsaicin-treated group compared to formalin only treated group. In conclusion, this experiment using formalin test, MEMRI and c-Fos immunohistochemistry support that capsaicin can reduce formalin-induced pain. It suggests that capsaicin acupuncture as one of pharmaceutical acupunctures may be effective in relieving pain.