Determinants of phencyclidine potency on the nicotinic acetylcholine receptors from muscle and electric organ

1. Phencyclidine (PCP) is an inhibitor of the nicotinic acetylcholine recep tor (AChR) with characteristics of an open-channel blocker. The location of PCP binding site on the AChR molecule is unknown. 2. PCP inhibits the AChR from electric organ with a higher potency than mus cle AChR. To find the molecular basis of this difference, we expressed the two native and six hybrid receptors, and two receptors containing mutated m ouse gamma subunits in Xenopus laevis oocytes. The inhibition of ACh-induce d current in these receptors by PCP was studied using whole-cell voltage-cl amp. All hybrid receptors generated robust ACh-induced currents, while inco mplete receptors (gamma-less or delta-less) did not. 3. PCP potency was higher on hybrids containing Torpedo beta and gamma subu nits regardless of the alpha and delta subunit origin. A mouse gamma subuni t containing the asparagine 6' to the serine mutation in the M2 segment con ferred a high sensitivity to PCP. 4. These results support the conclusion that the amino acid residues at the position 6' of the M2 segments contribute to the PCP potency difference be tween Torpedo and mouse receptors. 5. Another noncompetitive inhibitor of the AChR, the cembranoid eupalmerin acetate (EUAC), also inhibited the electric organ receptor with a somewhat higher potency than muscle AChR. However, the IC50 values for EUAC inhibiti on of hybrid receptors did not follow the pattern observed for PCP. Therefo re, these two inhibitors interact differently with the AChR molecule.