Which of the following does not act as a neurontransmitter ?

Options

A. Acetylcholine
B. Glutamic acid
C. Epinephrine
D. Tyrosine

Answer

To determine which of the following substances does not act as a neurotransmitter, we will analyze each option provided: acetylcholine, glutamic acid, epinephrine, and tyrosine.

### Step-by-Step Solution:

1. **Understanding Neurotransmitters**:
- Neurotransmitters are chemical substances released by neurons to transmit signals across a synapse to target cells, which can be other neurons, muscle cells, or gland cells.

2. **Analyzing Acetylcholine**:
- Acetylcholine is a well-known neurotransmitter. It is released by motor neurons and plays a crucial role in activating muscles. Therefore, acetylcholine **does** act as a neurotransmitter.

3. **Analyzing Glutamic Acid**:
- Glutamic acid is an amino acid that functions as a potent neurotransmitter, specifically known as an excitatory neurotransmitter. It helps in transmitting signals between nerve cells. Thus, glutamic acid **does** act as a neurotransmitter.

4. **Analyzing Epinephrine**:
- Epinephrine, also known as adrenaline, is both a hormone and a neurotransmitter. It is released by the adrenal glands and is involved in the body's fight-or-flight response. Therefore, epinephrine **does** act as a neurotransmitter.

5. **Analyzing Tyrosine**:
- Tyrosine is an aromatic amino acid. While it is a precursor for neurotransmitters like dopamine, norepinephrine, and epinephrine, tyrosine itself does not function as a neurotransmitter. Therefore, tyrosine **does not** act as a neurotransmitter.

6. **Conclusion**:
- Based on the analysis, the substance that does not act as a neurotransmitter is **tyrosine**.

### Final Answer:
The correct answer is **tyrosine** (Option D).