Slugs and snails and opiate tales: opioids and feeding behavior in invertebrates.

There is accumulating evidence that opioid systems are involved in the regulation of fundamental behavioral and physiological processes in invertebrates. Feeding is a basic physiological function that is essential for maintaining homeostasis. Results of studies examining the feeding responses of molluscs and arthropods treated with various opiate agonists and antagonists indicate that delta, kappa, mu, and possibly sigma opioid systems differentially and selectively mediate the components of their natural feeding behavior. Moreover, it appears that at an early evolutionary stage the mu and kappa systems have developed to selectively affect the components of feeding behavior associated with the acquisition and ingestion of food. In addition, evidence suggests that neuropeptides that have been proposed as possible endogenous antagonists of opioid-mediated feeding in mammals may also be involved in the control of feeding in invertebrates. This indicates that there may be an interplay of opioid agonists and antagonists in the regulation of feeding and satiation in invertebrates analogous to that proposed for vertebrates. Moreover, these findings indicate that opioid influences on feeding have been conserved through evolution.