INHIBITION OF ACH RELEASE AT AN APLYSIA SYNAPSE BY NEUROTOXIC PHOSPHOLIPASES A(2) - SPECIFIC RECEPTORS AND MECHANISMS OF ACTION

1. Monochain (OS2) and multichain (taipoxin) neurotoxic phospholipases A(2) (PLA(2)), purified from taipan snake venom, both inhibited ACh r elease at a concentration of 20 nM (90% inhibition in 2 h) at an ident ified synapse from buccal ganglion of Aplysia, californica. 2. The Na current tvas unchanged upon application of either OS2 or taipoxin. Co nversely, presynaptic K+ currents (I-A and I-K) were increased by taip oxin but not by OS2. In addition, OS2 induced a significant decrease o f the presynaptic Ca2+ current (30%) while taipoxin increased this lat ter current by 20-30%.3. Bee venom PLA(2), another monochain neurotoxi c PLA(2), also inhibited ACh release while non-toxic enzymatically act ive PLA(2)s like OS1 (also purified from taipan snake venom) or porcin e pancreatic PLA(2) elicited a much weaker inhibition of ACh release, suggesting a specific action of neurotoxic PLA(2)s versus non-toxic PL A(2)s on ACh release. 4. Using iodinated OS2, specific high affinity b inding sites with molecular masses of 140 and 18 kDa have been identif ied on Aplysia ganglia. The maximal binding capacities were 55 and 300 -400 fmol (mg protein)(-1) for membrane preparations from whole and bu ccal ganglia, respectively These binding sites are of high affinity fo r neurotoxic PLA(2)s (K-d values, 100-800 pM) and of very low affinity for non-toxic PLA(2)s (K-d values in the micromolar range), thus indi cating that these binding sites are presumably involved in the blockad e of ACh release by neurotoxic PLA(2)s.