Met-enkephaline-like immunoreactivity in a cephalopod neurohemal organ.

Using a highly specific antibody we found Met-enkephalin-like immunoreactivity in large granules of numerous distinct cells that are embedded in a layer of secretory terminals inside the octopus vena cava. Application of antibodies against fragments of the mammalian pro-opiocortin precursor -- alpha-MSH, ACTH, beta-endorphin and 16k-fragment -- and against growth hormone did not produce immunostaining in the octopus enkephalin cells. The vena cava neuropil may represent a favourable system for the examination of the physiological role of the opioid peptide enkephalin.