Reduced acetylcholine receptor density, morphological remodeling, and butyrylcholinesterase activity can sustain muscle function in acetylcholinesterase knockout mice

Nerve-evoked contractions were studied in vitro in phrenic nerve-hemidiaphragm preparations from strain 129X1 acetylcholinesterase knockout (AChE-/-) mice and their wild-type littermates (AChE+/+). The AChE-/- mice fail to express AChE but have normal levels of butyrylcholinesterase (BChE) and can survive into adulthood. Twitch tensions elicited in diaphragms of AChE-/- mice by single supramaximal stimuli had larger amplitudes and slower rise and decay times than did those in wild-type animals. In AChE-/- preparations, repetitive stimulation at frequencies of 20 and 50 Hz and at 200 and 400 Hz produced decremental muscle tensions; however, stimulation at 70 and 100 Hz resulted in little or no loss of tension during trains. Muscles from AChE+/+ mice maintained tension at all frequencies examined but exhibited tetanic fade after exposure to the selective AChE inhibitor 1,5-bis(4-allyldimethyl-ammoniumphenyl)pentane-3-one (BW 284C51). The ability of diaphragm muscles from AChE-/- mice to maintain tension at 70 and 100 Hz suggests a partial compensation for impairment of acetylcholine (ACh) hydrolysis. Three mechanisms--including a reliance on BChE activity for termination of ACh action, downregulation of nicotinic acetylcholine receptors (nAChRs), and morphological remodeling of the endplate region--were identified. Studies of neuromuscular transmission in this model system provide an excellent opportunity to evaluate the role of AChE without complications arising from use of inhibitors.