Opioids are small-sized peptide hormones of the endorphin or enkephalin type present both in the brain and the intestinal tract (proenkephalin A also in adrenal cortex) of all vertebrates. The presence of enkephalins in invertebrates was unequivocally demonstrated in a mollusc and a crab and there is also convincing evidence for an endorphin and enkephalins as well as an high-affinity enkephalin binding site in Schistosoma mansoni. This species also produces opiate-like substances of the morphine and codein type. Both opioids and opiates have predominantly analgesic and immunomodulatory functions, besides their local activity as paracrine factors in the intestinal tract.
Parasitic infections of mammals are often associated with changes in the endogenous opioid system, but it cannot be decided at the moment whether opioids or opiates from the parasite also contribute to the effects. Infection of the gastrointestinal tract of mice, independently of whether the protozoan Eimeria vermiformis or the nematode Nippostrongylus brasiliensis is used, lead to an increase in antinociceptive responses. These responses were blocked by administration of opioid receptor antagonists. In addition to this significant analgesic activity, the levels of infected animals were also reduced and were restored to normal values by application of opioid antagonists. But not all behavioural changes of parasitized mice are mediated via the opioid system: avoidance of predators is less pronounced in parasitized mice but in this case GABA receptors are involved.