Inhibition of acetylcholine-mediated effects by borneol

We previously reported that the aqueous extract from a medicinal plant Dryobalanops aromatica specifically inhibits the nicotinic acetylcholine receptor (nAChR) (Oh et al. Pharmacol Res 2000;42(6):559-64). Here, the effect of borneol, the main constituent of D. aromatica, on nAChR activity was investigated in bovine adrenal chromaffin cells. Borneol inhibited a nAChR agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP)-induced calcium increase with a half maximal inhibitory concentration (IC₅₀) of 56±9μM. In contrast, borneol did not affect the calcium increases induced by high K⁺, veratridine, and bradykinin. The sodium increase induced by DMPP was also inhibited by borneol with similar potency (49±12μM), suggesting that the activity of nAChRs is inhibited by borneol. Borneol inhibited DMPP-induced secretion of [³H]norepinephrine with an IC₅₀ of 70±12μM. Carbon-fiber amperometry also confirmed the inhibition of DMPP-induced exocytosis by borneol in single chromaffin cells. [³H]nicotine binding, however, was not affected by borneol. The inhibitory effect by borneol is more potent than the effect by lidocaine, a commonly used local anesthetic. The data suggest that borneol specifically inhibits the nAChR-mediated effects in a noncompetitive way.