Neuropeptide function: the invertebrate contribution.

The following is a list of generalizations that arise from considering the present state of knowledge concerning the functions of invertebrate peptides. Some of these clearly also apply to vertebrates. Invertebrate peptides can be classified into structurally related groups. Structural similarity of peptides may represent true evolutionary homology by selection acting on an original gene. Alternatively, independent evolution of similar genes may have occurred because certain amino acid sequences represent optimal solutions to complex functional problems. Invertebrate neuropeptides have multiple functions. Thus, proctolin is a cardioactive peptide, a skeletal neuromuscular transmitter, a hindgut neuropeptide, a peptide of CNS interneurons (Keshishian & O'Shea 1984) and may have humoral roles. Invertebrate peptides act through a variety of molecular mechanisms. Generalizations about the mechanism cannot yet be made. Thus, proctolin's action on crustacean skeletal muscle is not associated with stimulation of cyclic-AMP or protein phosphorylation, but the action of SCPB on molluscan skeletal muscle involves elevation of cAMP. Invertebrate peptide inactivation can be caused by proteolysis that can also function to enhance peptide bioactivity. Proctolin is made virtually biologically inactive by any proteolysis, but alpha-BCP bioactivity is enhanced by two steps of carboxy peptidase digestion before being functionally inactivated. Protease action on released peptides is not necessarily a "simple" form of transmitter inactivation. Protease action also involves functional processing whereby the temporal and spatial parameters of a peptide's action may be shaped. Invertebrate neuropeptides are frequently co-localized with other neuroeffectors. Peptides may be co-localized and released with other active peptides as in the bag-cell example, or may be coactive with more conventional transmitters, as in the Ds motoneuron example. In such circumstances there is no reason to view either transmitter as primary or secondary. Invertebrate neuropeptides are widely involved in the control of muscle contraction. These effects may be locally and directly mediated as in the Ds motoneuron example or may be humoral. The peptide may act directly on the muscle contractile system or function to modulate the muscles' response to other motor input. Muscle contraction may be induced by a neuropeptide without depolarization of the muscle cells, for example see proctolin. Invertebrate neuropeptides are frequently involved in the control of oscillatory functions. In several examples peptides activate rhythmic myogenic contractions of cardiac and skeletal muscle.(ABSTRACT TRUNCATED AT 400 WORDS)