**Core Concept**
Dopamine is a critical neurotransmitter in the substantia nigra, a region of the midbrain involved in movement control and reward processing. The substantia nigra pars compacta is particularly rich in dopamine-producing neurons, which project to the striatum to regulate motor function.

**Why the Correct Answer is Right**
Dopamine acts as a major neurotransmitter in the substantia nigra by facilitating the transmission of signals between neurons. In the substantia nigra pars compacta, dopamine is synthesized from the amino acid tyrosine through the action of the enzyme tyrosine hydroxylase. Dopamine then binds to postsynaptic D1 and D2 receptors in the striatum, modulating motor function and movement. Dysfunction of the dopaminergic system in the substantia nigra is associated with Parkinson's disease, a neurodegenerative disorder characterized by motor symptoms such as tremors and rigidity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the neurotransmitter involved in the substantia nigra. While it may be a neurotransmitter, it is not the major one in this region.
**Option B:** This option is incorrect because it is not a neurotransmitter, but rather a type of cell involved in the transmission of signals.

**Clinical Pearl / High-Yield Fact**
The nigrostriatal pathway, which involves the substantia nigra and striatum, is a critical circuit in the brain involved in movement control. Dysfunction of this pathway is associated with Parkinson's disease, a disorder characterized by motor symptoms such as tremors and rigidity.

**Correct Answer:** C. Dopamine.