The sympathetic nervous system plays a crucial role in preparing the body for “fight or flight” responses. Preganglionic neurons in this system release neurotransmitters to communicate with postganglionic neurons. Knowing which neurotransmitters are secreted by these neurons helps in understanding both normal physiology and pathological conditions.

Neurotransmitters Released by Sympathetic Preganglionic Neurons

Sympathetic preganglionic neurons primarily release acetylcholine as their neurotransmitter at the synapse with postganglionic neurons. Additionally, they can release some neuropeptides like neurotensin, enkephalins, and substance P as co-transmitters that modulate neural signaling.

• Neurotensin is known to be released as a neuromodulator or co-transmitter in some sympathetic preganglionic neurons.

• Enkephalins (opioid peptides) can be found in sympathetic neurons as co-transmitters that modulate neurotransmission.

• Substance P is another neuropeptide that can be released, contributing to pain transmission and autonomic functions.

Neurotransmitters NOT Released by Sympathetic Preganglionic Neurons

Serotonin (5-HT), on the other hand, is not released by sympathetic preganglionic neurons. Serotonin primarily functions as a neurotransmitter in different regions of the brain and in the central nervous system, where it regulates mood, appetite, and other functions. It is not involved in synaptic transmission between sympathetic pre- and postganglionic neurons.

Summary

To answer the question directly:

(3) Serotonin is NOT released by sympathetic preganglionic neurons.