The first identified neurotransmitter was

• A. dopamine.
• B. acetylcholine.
• C. serotonin.
• D. melatonin.

Answer: (B)

Verified step by step guidance

1. Understand the context: Neurotransmitters are chemical messengers that transmit signals across a chemical synapse, such as a neuromuscular junction, from one neuron to another 'target' neuron, muscle cell, or gland cell.
2. Identify the key neurotransmitters mentioned: dopamine, acetylcholine, serotonin, and melatonin.
3. Research the history of neurotransmitter discovery: The first neurotransmitter to be identified was acetylcholine, discovered by Otto Loewi in 1921.
4. Differentiate between the neurotransmitters: Acetylcholine is involved in muscle activation and is a major neurotransmitter in the autonomic nervous system. Dopamine is involved in reward and pleasure systems. Serotonin affects mood and social behavior. Melatonin regulates sleep-wake cycles.
5. Conclude by recognizing that acetylcholine was the first neurotransmitter to be identified, making it the correct answer to the problem.