**Core Concept**
These substances are neurotransmitters, which play a crucial role in transmitting signals across synapses in the nervous system. They can act as either excitatory or inhibitory neurotransmitters, influencing various physiological processes, including mood regulation, pain perception, and vasodilation.

**Why the Correct Answer is Right**
These neurotransmitters function by binding to specific receptors on the surface of target cells, triggering a cascade of intracellular signaling events. The binding of neurotransmitters to their receptors can activate or inhibit the activity of downstream effectors, such as ion channels or enzymes, which ultimately lead to the desired physiological response. For example, the binding of acetylcholine to nicotinic receptors can trigger muscle contraction, while the binding of somatostatin to its receptors can inhibit the release of other neurotransmitters.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incomplete and does not provide a clear description of the function of the substances listed.
**Option B:** This option is too vague and does not accurately describe the specific role of neurotransmitters.
**Option C:** This option is incorrect because while some of the substances listed may have hormone-like functions, they are primarily classified as neurotransmitters.

**Clinical Pearl / High-Yield Fact**
Neurotransmitters play a crucial role in maintaining homeostasis and regulating various physiological processes. An imbalance or dysfunction of neurotransmitters has been implicated in various neurological and psychiatric disorders, such as depression, anxiety, and Parkinson's disease.

**Correct Answer: C. Neurotransmitters.**