Functional organization of cotransmission systems: lessons from small nervous systems.

Small invertebrate nervous systems allow one to ask a series of questions concerning the functional roles of cotransmitters. This review outlines some of the implications of cotransmission for target selectivity in complex neuropils. We suggest the possibility that a unique constellation of cotransmitters in individual identified modulatory neurons allows a specificity of action even when peptides may act over an extended distance, and when individual modulatory substances may be released from several modulatory neurons.