Multiple binding sites for phencyclidine on the nicotinic acetylcholine receptor from Torpedo ocellata electric organ

Abstract

Binding of [ 3H]-phencyclidine ([ 3H]-PCP) to acetylcholine-receptor enriched membrane from Torpedo ocellata electric organ was studied over a ligand concentration range of 1 to 200 μM. The results indicate that [ 3H]-PCP is boud to two classes of sites: high affinity (Kd = 6–9 μM) and low affinity (Kd = 85 μM) binding sites. In the absence of cholinergic drugs the ratio of high affinity [ 3H]-PCP binding sites to 125I-α-bungarotoxin (α-Bgt) binding sites is 0.37, and that of low affinity [ 3H]-PCP binding sites to 125I-α-Bgt is 1.06. Low affinity [ 3H]-PCP binding can be completely inhibited by α-bungarotoxin (α-Bgt), carbamylcholine and d-tubocurarine. This inhibition, together with the one to one stoichiometry with 125I-α-Bgt, suggests that the sites to which [ 3H]-PCP binds with low affinity are the acetylcholine (AcCho) binding sites. In the presence of 1 μM α-Bgt which blocks binding of [ 3H]-PCP to the AcCho binding sites, the ratio of high affinity [ 3H]-PCP sites to 125I-α-Bgt sites is 0.5, indicating the existence of one high affinity PCP site per receptor molecule, The toxin, however, decreases the apparent affinity of [ 3H]-PCP towards the AcCho receptor as well as the potency of tetracaine or dibucaine in inhibiting [ 3H]-PCP binding to that receptor. In the latter case the effect involves changes from a biphasic to a simple inhibition curve. The results suggest that non-competitive blockers to the AcCho receptors may affect their own sites as well, and that they do this also by binding to the AcCho binding sites. This is also inferred from the accelerated dissociation of [ 3H]-PCP from its high affinity binding sites by unlabeled PCP in the concentration range of 10 −3 to 10 −4 M, at which the drug occupies AcCho binding sites as well.