Final answer:
The substance that is not a neurotransmitter among the listed options is a. glucagon, which is actually a hormone. Glucagon acts on the liver to raise blood glucose levels, whereas the other options are indeed neurotransmitters involved in various neural signaling processes.
Step-by-step explanation:
The correct answer to the question, Which of the following is not a neurotransmitter? is option a. glucagon. Glucagon is a hormone that is involved in regulating blood glucose levels by stimulating the liver to convert glycogen to glucose, which is then released into the bloodstream. The other options, b. glutamine, c. acetylcholine, d. dopamine, and e. serotonin, are all neurotransmitters. Neurotransmitters are chemical messengers that transmit signals across a chemical synapse, such as those between neurons or between neurons and muscles.
For example, acetylcholine is the neurotransmitter released at the neuromuscular junction, dopamine is involved in neurological processes including mood and reward, and serotonin plays a key role in mood regulation and other functions. Glutamine is considered an amino acid and although it is closely related to glutamate, which is a neurotransmitter, glutamine itself is not classified as one. Instead, it serves as a precursor for the synthesis of glutamate.