Which of the following is NOT considered a small molecule neurotransmitter?

The correct answer highlights that endorphins are not classified as small molecule neurotransmitters, and this distinction is crucial in understanding the different types of neurotransmitters in the nervous system.

Small molecule neurotransmitters are typically synthesized from dietary precursors and are usually released in response to neuronal activity. They include substances like acetylcholine and biogenic amines, which play a significant role in rapid synaptic transmission.

Endorphins, on the other hand, are classified as neuropeptides. Neuropeptides are larger molecules that are made up of chains of amino acids. They are generally involved in modulating neurotransmission and can have a longer-lasting effect compared to small molecule neurotransmitters. Due to their structural makeup and functional role, endorphins fall outside the category of small molecule neurotransmitters.

Understanding these classifications is essential as it helps delineate the functions and mechanisms of different neurotransmiiters in the brain and body. The other choices presented are indeed examples of small molecule neurotransmitters, reinforcing the distinction that endorphins represent a different class altogether.