Interaction of formamidine pesticides with insect neural octopamine receptors: Effects on radioligand binding and cyclic AMP production

The formamidines chlordimeform, demethylchlordimeform (DCDM), amitraz and BTS-2727l were tested for their respective abilities to inhibit ³H]mianserin binding in membrane preparations of cockroach brain and, in the same tissue, to stimulate octopamine-sensitive receptors coupled to adenylate cyclase. Analysis of ³H]mianserin binding indicated negative cooperativity between two binding sites with respective K_d and B_max values of 3.82 mM and 0.886 pmol (mg protein)^?1 for a high-affinity site and 218 nM and 13.56pmol (mg protein)^?1 for a low-affinity site. DCDM, BTS-27271 and amitraz inhibit ³H]mianserin binding with IC₅₀ values similar to those obtained with octopamine and the μ-adrenergic antagonist, phentolamine, whereas chlordimeform is a poor competitor for the binding sites. Similarly DCDM, BTS-27271 and amitraz elevate cyclic AMP production in brain membrane preparations in a dose-dependent manner with K_a values of 0.32 uM, 1.5 uM and 3.4 uM respectively, whereas chlordimeform was again without effect. The formamidine-mediated responses were fully additive with the evaluation of cyclic AMP obtained using the maximal concentration of dopamine but not with octopamine-mediated increases; thus the formamidine effects appear to be expressed through partial agonism of octopamine receptors that are coupled to adenylate cyclase.