The effect of alkaloids on sugar receptors and the feeding behaviour of the blowfly

ABSTRACT. The deterrent effect of alkaloids on feeding by the black blowfly Phormia regina Meigen (Diptera: Calliphoridae) was tested by determining tarsal thresholds for mixtures of sucrose and alkaloids. The following alkaloids were used: atropine sulphate, berberine sulphate, quinine mono-hydrochloride, caffeine, yohimbine sulphonic ester, pilocarpine hydrochloride, coniine hydrochloride and codeine. The same alkaloids were tested electrophysiologically on tarsal chemoreceptors (D hairs). Both behaviourally and electrophysiologically alkaloids reduced response to sucrose. Deterrence and peripheral inhibition could be blocked by increasing the concentration of sucrose. Application of kinetic analyses to the electrophysiological data ruled out competitive, non-competitive, and uncompetitive inhibition at receptor sites. There is no correlation of thresholds with available data on lipid solubility or octanol/water partition coefficients. The diverse pharmacological properties of alkaloids suggest that there is no uniform limiting mechanism for this multiform array of compounds.