Receptor site analysis using neurosensory responses of the boll weevil to analogs of the cyclohexylideneethanol of its aggregation pheromone

The active sites of II neurons (Dickens, 1990c) in both sexes of the boll weevil, Anthonomus grandis Boh., were investigated using structure—activity relationship studies. Hectrophysiological responses of antennal olfactory receptors in male and female boll weevils were recorded in response to components II and III of its aggregation pheromone and various functional, geometric, and 2-fluoroalkenyl analogs. Dose—response curves for electroantennogram responses of both sexes to both the 2-fluoro analog of II (E-II-F) and II (Z-II) were essentially identical. Correspondingly, dose—response curves for the geometric isomer of II (E-II), and its 2-fluoro analog (Z-II-F), had similar shapes but responses to E-II were significantly greater at several doses. Responses of single neurons associated with type I sensilla to 1 μg stimulus loads of various analogs of II showed significant differences in the numbers of impulses elicited by each during a 500 ms stimulation period: Z-II > E-II-F > III-α-F, III > Z-II-F, E-II, IV. These results demonstrated the importance of the cis geometry between the dimethyl group at the 3-position on the six-carbon ring, and the functional moiety at the 1-position. The limited activity of E-II and Z-II-F on II neurons indicates these compounds may be stimulating other receptor types, since EAGs to these compounds were large. Dose—response curves for an II neuron were similar for Z-D and E-II-F, as were curves for III and III-α-F, indicating that the fluorine atom at the 2-position had little effect on receptor binding. Our data support a previous hypothesis that II neurons have separate receptors sites for pheromone components II and III.