Which neurotransmitters are known to be peptides?

The correct answer refers to the classification of neurotransmitters, particularly those that are peptides. Peptide neurotransmitters are chains of amino acids that function as signaling molecules in the nervous system. Prominent examples of peptide neurotransmitters include endorphins, enkephalins, and substance P, which play crucial roles in pain regulation, mood, and stress response.

Serotonin and dopamine are classified as monoamines, which are different from peptide neurotransmitters. They are derived from single amino acids and are pivotal in mood regulation, reward pathways, and numerous other physiological functions. Therefore, while they significantly influence neurotransmission, they do not fit the category of peptides.

The other pairs of neurotransmitters listed also consist of monoamine neurotransmitters or are not classified as peptides. Norepinephrine and epinephrine, for example, are catecholamines derived from the same precursor, tyrosine, while acetylcholine and histamine, although important in many neurological and physiological functions, do not fall into the peptide category either. Thus, the correct identification of peptide neurotransmitters is essential to understand how different signaling mechanisms operate within the nervous system.