Neurotransmitter co-transmission: synaptic architectures, functional logic, and emerging tools

Neurotransmitter co-transmission has become recognized as a fundamental organizing principle of neural communication, challenging the traditional view that individual neurons operate through a single transmitter system. Current evidence demonstrates that many neurons utilize multiple transmitters via distinct synaptic architectures, such as co-packaging within the same vesicle, release from separate vesicle pools within the same terminal, and segregation of transmitters across different boutons or neuronal processes. These organizational modes are not simply structural variants; they impose distinct rules for release, target engagement, and short-term dynamics, thereby shaping circuit function in specific ways. Across neural systems, several common principles have emerged: co-transmission expands signaling across multiple timescales, enhances target specificity, and allows transmitter balance to shift according to firing patterns and circuit state. A major conceptual and technical challenge in the field is that no single method can definitively establish the release mechanism. Consequently, recent advances have relied on integrating molecular profiling, electrophysiology, high-resolution anatomy, optogenetics, and genetically encoded neurotransmitter sensors. Collectively, these approaches are beginning to clarify how multi-transmitter neurons are organized and how their signaling is regulated. Future progress will likely depend on multimodal strategies that connect synaptic architecture to release dynamics, circuit computation, and behavior in vivo. In this context, co-transmission should be viewed not as an exception to canonical neurotransmission but as a versatile mechanism that enhances the flexibility, precision, and context-dependence of neural circuit output.