Cardioactive neuropeptides in gastropods.

At least five neuropeptides that are active on an isolated snail heart can be recovered from extracts of gastropod nervous tissue. These peptides have been divided into three classes. The class of the lowest molecular weight, termed the small cardioactive peptides (SCPs), is made up of two peptides. SCPs have been found in all gastropods studied and appear to be involved in the control of the gut. They have been localized by microdissection and bioassay to several identified central neurons that send their axons out to innervate the gut. These neurons act centrally to enhance the motor output of the ganglia responsible for the control of feeding, and peripherally to modulate gut activity. In one pair of these neurons, the classical transmitter acetylcholine coexists with an SCP. The next larger peptide class (medium cardioactive peptide), found only in Aplysia, shares both its mode of cardiac activity and tissue distribution with the SCPs. As yet, there is no evidence that either of these peptide classes acts as a physiological modulator of cardiac activity. The class of the highest molecular weight (large cardioactive peptide [LCPs]) is made up of two peptides and is found only in Helix. The LCPs are circulating neurohormones involved in the regulation of heart, gut, and neuromuscular activity. Their primary release site is a neurohemal region in the auricle. The significance of these findings is discussed in light of recent advances in the study of mammalian neuropeptides.