Differential alterations of cholinergic muscarinic receptors during chronic and acute tolerance to organophosphorus insecticides

Male mice treated for 2 weeks with the anticholinesterase insecticide disulfoton (O,O-diethylS-2-(ethylthio)-ethyl] phosphorodithioate; 10 mg per kg per day) became tolerant to the hypothermic and antinociceptive effects of disulfoton itself and of oxotremorine, a muscarinic cholinergic agonist. Homogenates of brain and ileum from tolerant animals exhibited reduced binding of the specific muscarinic antagonist ³H]quinuclidinyl benzilate (³H]QNB). In forebrains of tolerant animals, the number of receptors (B_max) was decreased 40% with no change in the affinity constant. Acetylcholinesterase (AChE) activity was 15% of control. Forty-eight hours after a single injection of disulfoton (10 mg/kg) mice were more resistant than their controls to the hypothermic and antinociceptive effects of a second administration of the same insecticide and of oxotremorine. Tolerance was not present 96 hr after a single administration of disulfoton. A single injection of disulfoton produced 74, 65 and 27% inhibition of AChE activity after 4, 48 and 96 hr respectively. Four hours after a second injection at 48 or 96 hr, 73 or 72% inhibition was found. ³H]QNB binding of animals treated with a single injection of disulfoton and of controls did not differ at either time point. An increase in the K_i for inhibition of ³H]QNB binding by unlabeled oxotremorine was observed in forebrain from mice killed 48 hr after a single injection of disulfoton, indicating a decreased affinity of the muscarinic receptor for agonists. Binding of ³H]oxotremorine-M was decreased significantly 48 hr after a single injection of disulfoton and after chronic treatment. It is suggested that a differential down-regulation of muscarinic receptors occurs in acute and chronic tolerance, involving agonist and antagonist binding sites and depending on duration of exposure.