Tests and refinements of a general structure-activity model for avian repellents.

We tested the robustness of a structure-activity model for avian trigeminal chemoirritants. Fourteen benzoates and acetophenones were tested using European starlingsSturnus vulgaris as a bioassay. In general, the previously proposed model was a reasonable predictor of repellency (i.e., irritant potency). We found that the presence of a phenyl ring was critical to repellency. Basicity of the molecule is the next most critical feature influencing repellency. The presence of an acidic function within the electron-withdrawing functionality seriously detracts from repellency. The presence or absence of an electron-withdrawing or -donating group may potentiate repellent effects, but its presence is not critical, so long as the phenyl ring is electron rich. Our data suggest that there is ano-aminoacetophenone/methyl anthranilate trigeminal chemoreceptor in birds analogous to the mammalian capsaicin receptor. Both receptors contain a benzene site. However, birds seem to lack the associated thiol/hydrogen-bonding site present in mammals which is needed to activate the benzene site. Rather, birds may possess an associated exposed charged site that in turn may interact with the stimulus to activate the benzene site. These differences may explain the differential sensitivity of birds and mammals to aromatic irritants.