Abstract

Physiological studies suggest that the formamidine pesticides, chlordimeform (CDM) and N-demethylchlordimeform (DCDM), may affect octopaminergic neurotransmission. To elucidate the possible mechanism of this interaction, the present study examined in detail the biochemical effects of these compounds on a very active octopamine-sensitive adenylate cyclase present in washed particulate preparations of the firefly light organ. DCDM was a partial agonist (70% of octopamine V_max) and was 6-fold more potent (K_a = 2.2 x 10^-6 M) than octopamine (K_a = 1.4 x 10^-5 M) in activating enzyme activity. At high concentrations (IC₅₀ = 3 x 10^-4 M), DCDM caused a 30% inhibition of octopamine stimulation. Activation by DCDM was reversible, nonadditive to that caused by octopamine, and could be competitively inhibited by several receptor antagonists, including cyproheptadine (K_i = 2 x 10^-6 M), clozapine (K_i = 4 x 10^-6 M), fluphenazine (K_i = 6 x 10^-6 M), phentolamine (K_i = 1.8 x 10^-5 M), and propranolol (K_i = 4.7 x 10^-5 M). These inhibitory constants correlated well with those for inhibiting octopamine stimulation. The agonist activity of DCDM was specific for tissue containing an octopamine-activated adenylate cyclase; enzyme activity in the rat caudate nucleus (activated by dopamine) and in the heart and liver (activated by isoproterenol) was little affected by DCDM. In contrast to DCDM, CDM was a weak octopamine agonist (K_a = 3 x 10^-5 M; 9% of octopamine V_max) in the light organ. At higher concentrations (IC₅₀ = 3 to 10 x 10^-4 M), CDM was an octopamine antagonist, causing nearly complete inhibition of octopamine stimulation at 1 to 3 x 10^-3 M. This inhibitory effect of CDM was reversible, pH-dependent, and noncompetitive with octopamine. It was also nonselective, since CDM inhibited dopamine-sensitive, beta-adrenergic-sensitive, and non-hormone-dependent adenylate cyclases in mammalian brain and liver. The above results indicate that, at low concentrations, DCDM can bind specifically and reversibly to octopamine receptors with a resultant activation of adenylate cyclase. CDM, on the other hand, has little direct effect on the octopamine receptor; its octopamimetic actions in vivo are a probable result of its conversion to DCDM. At high, nonpharmacological doses, CDM exerts noncompetitive and non-receptor-specific inhibitory effects on adenylate cyclase. These data have relevance to understanding the structural requirements necessary for interaction with octopamine-sensitive adenylate cyclase.