Downregulation of nicotinic and muscarinic receptor function in rats after subchronic exposure to diazinon

Diazinon (DZN) is an organophosphate insecticide which exerts its effect through the inhibition of acetylcholinesterase enzyme (AChE). In this work, we studied the development of tolerance to subchronic p.o. administration of DZN in rats, under both in vivo and in vitro conditions. A group of 20 rats (2 groups, n = 10) was administered p.o. the 1/10 of established LD(50) DZN (namely 55.87 mg/kg bw) for 28 days. On the 14th and 28th day of study with isolated diaphragm and ileum, we examined the downregulation of nicotinic and muscarinic receptor function through Electrical Field Stimulation (EFS). Maximum contractility of the diaphragm was recorded on the 14th day of the study (25% higher compared to the non-treated rats), while on the 28th day the contractions almost did not differ from the values found in non-treated rats. EFS of isolated ileum on the 14th day of study caused significantly higher contractions compared to the non-treated rats, but after 28 days, ileum contractions decreased approximately to the level of contractions in non-treated rats. On the 14th study day, we also recorded increased amplitude of spontaneous ileum contractions, compared to non-treated rats. The application of increasing ACh concentrations caused dose-dependent ileum contractions, without statistically significant differences of median effective concentration (EC(50)) values in non-treated and treated rats. Tolerance to subchronic DZN administration develops due to various adaptation mechanisms, including the most important one—downregulation of nicotinic and muscarinic receptor function.