Additive interaction of intrathecal ginsenosides and neostigmine in the rat formalin test

BACKGROUND: The authors evaluated the effect of intrathecal mixture of ginsenosides with neostigmine on formalin-induced nociception and made further clear the role of the spinal muscarinic (M) receptors on the activity of ginsenosides. METHODS: A catheter was located in the intrathecal space of male Sprague-Dawley rats. Pain was evoked by injection of formalin solution (5%, 50 µl) to the hindpaw. Isobolographic analysis was done to characterize drug interaction between ginsenosides and neostigmine. The antagonism of ginsenosides-mediated antinociception was determined with M1 receptor antagonist (pirenzepine), M2 receptor antagonist (methoctramine), M3 receptor antagonist (4-DAMP), M4 receptor antagonist (tropicamide). The expression of muscarinic receptor subtypes was examined with RT-PCR. RESULTS: Intrathecal ginsenosides and neostigmine produced an antinociceptive effect during phase 1 and phase 2 in the formalin test. Isobolographic analysis revealed an additive interaction between ginsenosides and neostigmine in both phases. Intrathecal pirenzepine, methoctramine, 4-DAMP, and tropicamide reversed the antinociception of ginsenosides in both phases. M1-M4 receptors mRNA detected in spinal cord of naïve rats and the injection of formalin decreased the expression of M1 receptor mRNA, but it had no effect on the expression of other three muscarinic receptors mRNA. Intrathecal ginsenosides little affected the expression of all of muscarinic receptors mRNA in formalin-injected rats. CONCLUSIONS: Intrathecal ginsenosides additively interacted with neostigmine in the formalin test. Furthermore, M1-M4 receptors exist in the spinal cord, all of which contribute to the antinocieption of intrathecal ginsenosides.