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It was assumed for many years that a single neuron would synthesize and release only one neurotransmitter. We now know that it is not true.
New scientific investigations have indicated that some neurons can synthesize, store, and release more than one neurotransmitter. Instead of thinking about a single chemical signal being responsible for transmitting a single message from the presynaptic nerve terminal to the postsynaptic cell, the possibility of multiple signals carrying independent, complementary, or mutually reinforcing messages must be considered. These multiple signals are referred to as cotransmitters. In fact, supposed cotransmitters are sometimes found stored in separate vesicles and sometimes in the same vesicles, an indication, that both types of situations may exist in the brain.