Inhibition of frog sciatic nerve compound action potentials by aroma-oil compounds in a manner dependent on their chemical structures

Abstract

Aroma-oil compounds derived from plants have a variety of clinical effects including local anesthesia. We have previously reported that various aroma-oil compounds reduce the peak amplitudes of fast-conducting and Na+-channel blocker tetrodotoxin-sensitive compound action potentials (CAPs) recorded from the frog sciatic nerve in a manner dependent on their chemical structures. The present study further examined this structure-activity relationship by applying the air-gap method to the frog sciatic nerve. Cyclic alcohols ((+)-borneol, (-)-borneol, - terpineol), chain alcohols ((-)linalool, citronellol, geraniol) and esters (bornyl acetate, geranyl acetate) reduced CAP peak amplitudes with the half-maximal inhibitory concentration (IC50) values of 1.5 mM, 2.3mM, 2.7 mM, 2.0 mM, 0.35mM, 0.53 mM, 0.44 mM and 0.51 mM, respectively. This IC50 value for (-)- linalool was similar to that for (+/-)-linalool (1.7mM), a value as reported previously. On the other hand, hydrocarbon (p-cymene) at a high concentration such as 2 mM reduced CAP amplitude by only 20\%. The efficacy sequence of the aroma-oil compounds was esters. alcohols >= hydrocarbons, and was thus consistent with one reported previously, i.e., phenols. aldehydes. esters > alcohols > ketones > oxides >= hydrocarbons. There was a variation in IC50 value among the alcohols used; chain alcohols were more effective in inhibiting CAPs than cyclic ones. Similar IC50 values of (-)- linalool and (+)linalool and also of (+)- borneol and (-)- borneol indicate no difference in efficacy between the steroisomers in inhibiting CAPs. There was no correlation between IC50 value for CAP inhibition by aroma-oil compound and its octanol-water partition coefficient value. The present study confirmed that aroma-oil compounds inhibit nerve conduction in a manner specific to their chemical structures.