Opiate agonists activate feeding in Limax: comparison of in vivo and in vitro effects

The neural control system for feeding in the terrestrial mollusc Limax maximus is modulated by at least two major families of peptides. Sequence homology between one of the peptides known to modulate Limax feeding and some members of the opioid peptide family suggested that opioid peptides might also modulate Limax feeding. Experiments with the mu agonist morphine and the kappa agonist U50,488H showed that the probability of feeding, but not meal size, was increased by morphine injection into intact animals, whereas the length of feeding motor program responses elicited from the isolated lip-brain preparation of Limax was augmented by U50,488H. The behavioral effect of morphine was blocked by naltrexone injection, whereas the physiological effect of U50,488H was blocked by naloxone. Factors that influence the behavioral and electrophysiological effects of opioids on mollusc feeding are discussed.