Binding of N-[propionyl- 3H]propionylated α-bungarotoxin and l-[benzilic-4,4'- 3H] quinuclidinyl benzilate to CNS extracts of the cockroach Periplaneta Americana

Abstract

1. The nerve cord of the cockroach ( Periplaneta americana) contains distinct saturable components of specific binding for the ligands N-[propionyl- 3H]propionylated α-bungarotoxin and l-[benzilic-4,4'- 3H]quinuclidinyl benzilate. 2. N-[Propionyl- 3H]propionylated α-bungarotoxin bound reversibly to homogenates with a K d of 4.8 nM and B maxof 910 fmol mg −1. The association rate constant (1.9 × 10 5 M −1s −1) and dissociation rate constant (1.2 × 10 −4s −1) yielded a K d of 0.6 nM. 3. Nicotinic ligands were found to displace toxin binding most effectively. 4. The binding sites characterized in this way showed many similarities with the properties of the vertebrate neuronal α-bungarotoxin binding site. 5. For a range of cholinergic ligands, inhibition constants calculated from toxin binding studies closely corresponded to their effectiveness in blocking the depolarizing response to acetylcholine recorded by electrophysiological methods from an identified cockroach motoneurone. 6. The N-[propionyl- 3H]propionylated α-bungarotoxin binding component therefore appears to be a constituent of a functional CNS acetylcholine receptor. 7. Binding of l-[benzilic-4-4'- 3H]quinuclidinyl benzilate was reversible with a K dof 8 nM and B maxof 138 fmol mg −1, determined from equilibrium binding experiments. The K d calculated from the association rate constant (2.4 × 10 5M −1 s −1) and dissociation rate constant (1.3 × 10 −4s −1) was 1.9nM. 8. Muscarinic ligands were the most potent inhibitors of quinuclidinyl benzilate binding. The characteristics of this binding site resembled those of vertebrate CNS muscarinic cholinergic receptors. 9. In contrast with vertebrate CNS, the nerve cord of Periplaneta americana contains more (∼ × 7) α-bungarotoxin binding sites than quinuclidinyl benzilate binding sites.