Which one of the following transmitter is not a protein/ glycopeptide/polypeptide?
**Core Concept**
The question is testing the student's knowledge of neurotransmitters and their classification. Neurotransmitters are chemical messengers released by neurons to transmit signals to other neurons or to muscles or glands. They can be classified into different categories based on their chemical structure and function.
**Why the Correct Answer is Right**
Neurotransmitters can be broadly classified into two categories: amino acid-based neurotransmitters and non-amino acid-based neurotransmitters. The amino acid-based neurotransmitters include glutamate, aspartate, glycine, GABA, and dopamine, which are all polypeptides. The non-amino acid-based neurotransmitters include acetylcholine, which is a small molecule. Acetylcholine is synthesized from acetyl-CoA and choline through the action of the enzyme choline acetyltransferase. It acts on muscarinic and nicotinic receptors to produce its effects.
**Why Each Wrong Option is Incorrect**
**Option A:** Glutamate is an amino acid-based neurotransmitter and is a polypeptide. It acts on NMDA, AMPA, and kainate receptors to produce its excitatory effects.
**Option B:** GABA (gamma-aminobutyric acid) is also an amino acid-based neurotransmitter and is a polypeptide. It acts on GABA receptors to produce its inhibitory effects.
**Option C:** Dopamine is another amino acid-based neurotransmitter and is a polypeptide. It acts on D1, D2, D3, D4, and D5 receptors to produce its effects.
**Clinical Pearl / High-Yield Fact**
It's worth noting that the classification of neurotransmitters into amino acid-based and non-amino acid-based categories is not always clear-cut, and some neurotransmitters may be classified differently depending on the source.
**Correct Answer: D. Acetylcholine is a small molecule, not a protein/glycopeptide/polypeptide.**