A 25-year-old woman is admitted to the hospital because of a 6-week history of double vision and difficulty to talk after prolonged speaking. Her husband reports fluctuating droopy eyelids in the morning and evening. An immunogenic assay detects the presence of circulating auto andibodies against the certain receptors at the neuromuscular junction. Affected binding of which of the neurotransmitters is the most likely cause of the patient's symptoms?