Which one of the following acts solely as an inhibitory neurotransmitter?

Options

A. Nor-epinephrine
B. Gamma`(lambda)` amino butyric acid
C. Acetylcholine
D. Dopamine

Answer

To determine which neurotransmitter acts solely as an inhibitory neurotransmitter, we can analyze the options provided:

1. **Norepinephrine**: This neurotransmitter is primarily known for its role as an excitatory neurotransmitter. It is involved in the fight-or-flight response and increases alertness and arousal. Therefore, it does not act solely as an inhibitory neurotransmitter.

2. **Gamma-Aminobutyric Acid (GABA)**: GABA is well-known for its inhibitory effects in the central nervous system. It decreases the action potential of neurons, meaning it reduces the likelihood of a neuron firing. This makes GABA the correct answer as it acts solely as an inhibitory neurotransmitter.

3. **Acetylcholine**: This neurotransmitter can have both excitatory and inhibitory effects depending on the type of receptors it binds to. It is involved in muscle movement and can also play a role in inhibiting certain functions in the heart. Thus, it does not act solely as an inhibitory neurotransmitter.

4. **Dopamine**: Dopamine is primarily an excitatory neurotransmitter that plays a key role in reward, motivation, and motor control. It does not act solely as an inhibitory neurotransmitter.

Based on this analysis, the correct answer is **Gamma-Aminobutyric Acid (GABA)**, as it is the only neurotransmitter listed that acts solely as an inhibitory neurotransmitter.

### Step-by-Step Solution:
1. **Identify the neurotransmitters listed in the options.**
2. **Analyze the function of each neurotransmitter:**
- Norepinephrine: Excitatory
- GABA: Inhibitory
- Acetylcholine: Both excitatory and inhibitory
- Dopamine: Excitatory
3. **Determine which neurotransmitter acts solely as an inhibitory neurotransmitter.**
4. **Conclude that GABA is the correct answer.**