The concerted activity of many neuropeptides has been implicated in th
e neurohormonal control of specific behaviors and various physiologica
l functions in some invertebrate model systems. What are the functiona
l consequences of this neuropeptide multiplicity? The distinct actions
of closely related neuropeptides have been detected in molluscs and i
nsects; however, recent work provides examples of systems in which som
e of the multiple isoforms may be functionally redundant, Groups of fu
nctionally distinct neuropeptides encoded by the same gene can be expr
essed in different neurons by alternative gene splicing or cell-specif
ic post-translational processing; therefore, as shown recently, they c
an be targeted for release as 'cocktails' to act on specific sets of m
uscles or neurons. One prominent role of neuropeptides is to modulate
the activity of rhythm-generating circuits, as exemplified by recent r
esearch on mollusc neural networks, the crab stomatogastric ganglion,
and fly circadian pacemakers.