Chapter 7. Neurotransmitters & Neuromodulators
A medical student is doing a research project in a neurophysiology laboratory that is studying the neuromuscular junction. As a first experiment, he tested the effects of three different drugs on the potential recorded from the skeletal muscle during stimulation of the nerve to the skeletal muscle. Drug A enhanced the response, drug B blocked the response, and drug C did not alter the potential in the skeletal muscle. Drugs A, B, and C might be the following types of drugs, respectively.
A. An acetylcholinesterase inhibitor, a muscarinic receptor antagonist, and a nicotinic receptor agonist
B. A nicotinic receptor agonist, a GPCR antagonist, and a muscarinic receptor agonist
C. A glycine receptor antagonist, a GABA receptor agonist, and a glutamate receptor agonist
D. A glutamate receptor agonist, a glutamate receptor antagonist, and a GABA reuptake inhibitor
E. An acetylcholinesterase inhibitor, a nicotinic receptor antagonist, and a muscarinic receptor antagonist
The correct answer is E. The motor neuron releases acetylcholine that binds to a nicotinic receptor on the skeletal muscle end plate to increase N+ and K+ conductance to generate an end plate potential followed by an action potential in the skeletal muscle fiber and eventual muscle contraction. The duration of action of acetylcholine is very short but can be prolonged by the administration of an acetylcholinesterase inhibitor to prevent the hydrolysis of acetylcholine to choline and acetate. A nicotinic receptor antagonist will prevent the binding of the acetylcholine released by nerve stimulation to the nicotinic receptor, thus preventing a response in the skeletal muscle. There are no muscarinic receptors in the neuromuscular junction, thus a muscarinic receptor agonist will not alter the response to electrical stimulation. This also rules out option A as the best answer; this option is also incorrect because administration of a nicotinic receptor agonist would activate the muscle. Option B can be ruled out because the receptors at the neuromuscular junction are ionotropic receptors not GPCR. Options C and D are ruled out because the neuromuscular junction does not contain excitatory or inhibitory amino acid receptors.
A medical student is studying transmission through autonomic ganglia. She studied the effects of two different drugs on the activity of a postganglionic neuron. Drug A induced an EPSP in the postganglionic neuron, and drug B blocked the EPSP produced by electrical stimulation of a preganglionic nerve. Drugs A and B might be the following drugs, respectively.
C. Strychnine and atenolol