In Vitro and in Vivo Cholinesterase Inhibition in Lacertides by Phosphonate- and Phosphorothioate-Type Organophosphates

The acute toxicity of two organophosphorus (OP) insecticides (trichlorphon and parathion) was assessed in lizards by in vitro and in vivo experiments. Serum and brain cholinesterase (ChE) activities were measured in lizards treated orally with the OPs. Inhibitory effects of trichlorphon were measured 24 h after exposure and those of parathion were determined according to a treatment/sampling schedule designed to investigate the toxic effects at different times of day, since parathion undergoes bioactivation before it inhibits brain ChE. A batch of lizards (batch A) was treated at 10 and sampled 6 h later; a second batch was exposed to parathion at 4 and sampled 24 h later (batch B); a third was treated at 4 and ChE measured 6 h later (batch C). In vitro assays showed that brain ChE activity was more sensitive to paraoxon (IC50 = 2.2 · 10-6 M) and dichlorvos (IC50 = 3.2 · 10-5 M) than their parent compounds. Treatment with trichlorphon caused a significant dose-related inhibition of brain and serum ChE activities at doses of 50 and 100 mg/kg. Inhibition of ChEs by parathion depended more on administration time than dose. Significant dose-dependent inhibition of serum ChE was found in lizards of batches A and B. Brain ChE activity was depressed at all treatment doses (> 50% inhibition) in batch A and at the highest dose in batch B (70%). Afternoon treatment and sampling caused dose-related inhibition of serum ChE but no inhibition of brain ChE activity, irrespective of parathion dose. Attempts to correlate the two ChE activities showed a linear relationship in lizards treated with trichlorphon and a nonlinear relationship in lizards treated with parathion. This may be due to bioactivation of parathion to its oxon analogue paraoxon, which inhibits brain ChE activity, and to the buffer effect of serum ChE, which lessens the impact of the organophosphate on the central nervous system. These results show (i) that reptiles have a high resistance to organophosphorus insecticides compared to other vertebrates (e.g., birds and mammals); (ii) that in lacertides the toxic effects of phosphorothioate-type organophosphates may depend strongly on the time of day that exposure occurs; (iii) that a relationship exist between brain and serum ChEs in lizards treated with OP insecticides, suggesting that serum ChE could be used as a surrogate biomarker of effect.