| Abstract: | The structure-activity relationships between five amides previously isolated from the root extract of Piper guineense Schum and Thonn were studied. The methyl-terminated amides, pellitorine and kalecide, were found to be more polar and caused higher knockdown, but showed less lethal action against insects than their methylenedioxyphenyl-terminated counterparts, pipercide and guineensine. Guineensinamide showed weak activity and this was correlated with higher unsaturation and the presence of a pyrrolidyl group in the molecule. There was an association between the polarity of the amides and the recovery of the insects from their effects, much more recovery being observed with the methylterminated than with the methylenedioxyphenyl-terminated amides. Electrophysiological investigations of the effects of pellitorine and pipercide on miniature excitatory postsynaptic potentials (MEPSPs) from the locust retractor unguis nerve-muscle preparation showed that the amides caused a reduction in both the frequency and amplitude of MEPSPs, pipercide being between 100 and 1000 times more effective than pellitorine. An action of the amides on presynaptic transmission is suggested. The effects of pipercide and pellitorine on the isolated nerve-muscle preparation were correlated with the symptoms of poisoning at the whole animal level. |
