Dichlorvos exposure to the Kölliker-fuse nuclei is sufficient but not necessary for OP induced apnea

• Organophosphate (OP) exposure to the Kölliker-fuse nuclei alone is sufficient for central apnea
• OP induced respiratory failure was similar for animals exposed subcutaneously and injection into the Kölliker-fuse nuclei
• OpdA enzyme protection of the KF mitigates organophosphate induced central apnea

Patients exposed to organophosphate (OP) compounds demonstrate a central apnea. The Kölliker-fuse nuclei (KF) are cholinergic nuclei in the brainstem involved in central respiratory control. We hypothesize that exposure of the KF is both necessary and sufficient for OP induced central apnea. We performed an animal study of acute OP exposure. Anesthetized and spontaneously breathing Wistar rats (n = 24) were exposed to a lethal dose of dichlorvos using three experimental models. Experiment 1 (n = 8) involved systemic OP poisoning using subcutaneous (SQ) 2,2-dichlorovinyl dimethyl phosphate (dichlorvos) at 100 mg/kg or 3× LD50. Experiment 2 (n = 8) involved isolated poisoning of the KF using stereotactic microinjections of dichlorvos (625 μg in 50μl) into the KF. Experiment 3 (n = 8) involved systemic OP poisoning with isolated protection of the KF using SQ dichlorvos (100 mg/kg) and stereotactic microinjections of organophosphatase A (OpdA), an enzyme that degrades dichlorvos. Respiratory and cardiovascular parameters were recorded continuously. Animals were followed post exposure for 1 h or until death. There was no difference in respiratory depression between animals with SQ dichlorvos and those with dichlorvos microinjected into the KF. Despite differences in amount of dichlorvos (100 mg/kg vs. 1.8 mg/kg) and method of exposure (SQ vs. CNS microinjection), 10 min following dichlorvos both groups (SQ vs. microinjection respectively) demonstrated a similar percent decrease in respiratory rate (51.5 vs. 72.2), minute ventilation (49.2 vs. 68.8) and volume of expired gas (17.5 vs. 0.0). Animals with OpdA exposure to the KF during systemic OP exposure demonstrated less respiratory depression, compared to SQ dichlorvos alone (p < 0.04). No animals with SQ dichlorvos survived past 25 min post exposure, compared to 50% of animals with OpdA exposure to the KF. In conclusion, exposure of the KF is sufficient but not necessary for OP induced apnea. Protection of the KF during systemic OP exposure mitigates OP induced apnea.