Which of the following is an example of a neurotransmitter peptide?

Dynorphin is indeed classified as a neuropeptide, which is a specific type of neurotransmitter peptide. Neuropeptides are larger than traditional neurotransmitters and consist of chains of amino acids. They play critical roles in modulating a wide range of biological functions, including pain regulation, stress response, and emotional processing. Dynorphin, in particular, is known for its role in pain perception and its involvement in the body's response to stress.

In contrast, acetylcholine, GABA (gamma-aminobutyric acid), and dopamine are classified as classical neurotransmitters. Acetylcholine is essential for muscle activation and cognitive functions, GABA serves primarily as an inhibitory neurotransmitter in the central nervous system, and dopamine is involved in reward, motivation, and several neurological functions. These neurotransmitters typically function by acting on specific receptors to transmit signals across synapses, but they do not fall within the peptide category. This distinction is crucial in understanding the different types of signaling molecules in the nervous system and their varying mechanisms of action.