THE EFFECTS OF DRUGS ON THE INCORPORATION OF A CONFORMATIONALLY-SENSITIVE, HYDROPHOBIC PROBE INTO THE ION-CHANNEL OF THE NICOTINIC ACETYLCHOLINE-RECEPTOR

The pattern of incorporation of the hydrophobic photolabel thyl)-3-(m- [I-125]iodophenyl)diazirine([I-125]TID) into the nicotinic acetylcholi ne receptor (AChR) is a sensitive measure of AChR conformation (restin g state or desensitized). We determined the ability of tetracaine, dib ucaine, procaine, lidocaine, chlorpromazine or phencyclidine to inhibi t [I-125]TID photolabeling of the AChR as a function of drug concentra tion, both as a measure of the ability of these drugs to desensitize t he AChR, and to characterize the [I-125]TID binding site. To localize the site(s) of drug action, experiments were performed in the absence and presence of saturating concentrations of alpha-bungarotoxin (BgTx) , to block drug binding to the agonist binding site. On the basis of t he concentration dependence of their effects, which was not altered by the presence of BgTx, tetracaine and dibucaine appeared to block [I-1 25]TID incorporation competitively, suggesting that the high-affinity [I-125]TID binding site is the non-competitive blocker binding site pr esumed to exist in the interior of the AChR ion channel. Procaine, chl orpromazine, lidocaine and phencyclidine blocked [I-125]TID incorporat ion at lower concentrations in the absence of BgTx than in its presenc e, suggesting that these drugs block incorporation by inducing desensi tization when bound to their high-affinity non-competitive blocker bin ding sites and that BgTx countered the drug effect by allosterically s tabilizing the resting state.