Lipopolysaccharide and opioids activate distinct populations of Mytilus edulis immunocytes.

Studies in Mytilus edulis have indicated that immunoregulatory activities comparable to those in vertebrates also exist in invertebrates. Mytilus immunocytes resemble cells of the vertebrate monocyte/macrophage lineage and are activated by similar substances. We searched for differential effects of opioids on these cells in comparison with those of lipopolysaccharide (LPS), in order to determine if different subpopulations of immunoactive hemocytes are involved. We showed that Mytilus immunocytes respond to LPS in a fashion similar to that in vertebrate granulocytes by flattening, and increasing in cellular perimeter and mobility, that LPS administered in vivo results in a lowering of the number of free hemocytes that can be obtained from the animal, and that distinct immunoactive cell populations seem to exist since apparently different subsets of cells react when exposed to LPS or opioids and the opioid antagonist naloxone.