Higher activities of acetylcholinesterase and choline acetyltransferase in jaw-opening than jaw-closing motoneurones in the rat

The activities of acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) in rat masticatory motoneurones were measured. Anterior digastric (jaw-opening) and masseter (jaw-closing) motoneurones were retrogradely labelled with the fluoroscent tracers nuclear yellow and bisbenzimide, respectively. The animals were pretreated with an irreversible AChE inhibitor, diisopropyl fluorophosphate, for the measurement of AChE activities. After transcardial perfusion, serial frozen sections, 20-μm thick, of the brainstem were prepared and processed for AChE histochemical analysis. Sections of 30-μm thickness were also prepared and processed for ChAT immunohistochemical analysis using anti-ChAT antibodies and the peroxidase–antiperoxidase complex. The AChE and ChAT activities in motoneurones identified by their fluorescence were determined by measuring their absorbance in the cytoplasm at 470 and 450 nm, respectively. Each of the enzymatic activities was significantly higher in the anterior digastric than in masseter motoneurones (p<0.001, student t-test).