Native opioid-like peptides in Squilla mantis ganglia

To detect the presence of a mammalian-like enkephalin precursor in suboesophageal ganglia of Squilla mantis, an arthropod shown to be sensitive in vivo to opiates [8], protein acid extracts were fractionated by gel filtration into three large pools: A(Mr greater than 65,000), B(10,000 less than Mr less than 65,000) and C(Mr less than 10,000). Only the low molecular weight pool, pool C, showed opioid-like activity when assayed by displacing labeled D-Ala2-D-Leu5-enkephalin from rat brain membranes. After trypsin and carboxypeptidase B proteolysis, pool A remained inactive, while pool B turned out to be active and was shown to inhibit the twitch response of electrically stimulated guinea-pig ileum. After HPLC fractionation of proteolyzed pool B, most of the opioid-like activity was found to be associated with a fraction showing an elution volume different from that of opioid peptide standards. Furthermore, no fraction showed immunoreactivity with anti-Met-enkephalin antibodies. The results suggest that native opioid-like peptides are present in Squilla mantis and are most likely released from higher molecular weight precursor(s).