Effects of anticholinergic drugs on rabbit efferent phrenic discharges

In conscious, vagotomized, curarized, and artificially-ventilated rabbits, the efferent phrenic discharges were recorded. When scopolamine, atropine, pirenzepine or AF-DX 116 (11-2[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro- 6H[2,3-6] [1,4]benzodiazepine-6-one) was injected into the cerebello-medullary cistern, the frequency and voltage of phrenic discharges were decreased (P less than 0.05) by scopolamine (0.5 mg.kg-1) and pirenzepine (0.5 mg.kg-1), but were increased (P less than 0.01) by atropine (0.05 mg.kg-1) and AF-DX 116 (0.1 mg.kg-1). It is probable that scopolamine inhibits the respiratory center by blocking the M1 cholinergic receptors while atropine excites the respiratory center blocking the M2 cholinergic receptors.