Cotransmitters

Cotransmitters are chemical messengers released from a presynaptic neuron alongside the primary transmitter, contributing to synaptic signaling in a coordinated way. The term encompasses a broad range of molecules, including small-molecule transmitters that accompany another transmitter, as well as neuropeptides that are released from distinct vesicle populations. Cotransmission describes the phenomenon in which a neuron emits more than one transmitter and these signaling molecules interact to shape the postsynaptic response.

Release and mechanisms

Many neurons contain two vesicle systems: small clear vesicles carrying classical fast transmitters, and large dense-core vesicles containing neuropeptides or other modulators. Release of these vesicles can be frequency- and Ca2+-dependent, with high-frequency or sustained activity favoring the release of neuropeptides. A presynaptic neuron may therefore release a fast transmitter to produce an immediate effect and one or more cotransmitters to modulate the response over longer timescales. Cotransmitters can act on distinct receptors, produce depolarization or hyperpolarization, influence ion channels, and engage second messenger cascades. They can also modulate the release probability of the primary transmitter via presynaptic receptors, adding another layer of control to synaptic communication.

Examples and significance

Cotransmission is well documented in both peripheral and central nervous systems. In the sympathetic system, norepinephrine is often co-released with ATP and neuropeptide Y, contributing to rapid signaling and longer-term modulation. In other circuits, acetylcholine or dopamine may be released together with neuropeptides or purines, expanding the range of physiological effects. Understanding cotransmitters illuminates how neurons diversify signaling, influence plasticity, and participate in complex processes such as pain, mood, arousal, and autonomic regulation.

Notes

Cotransmitters are an active area of research, with ongoing debates about the exact roles of specific molecules in different circuits and how release dynamics shape function.