Reversing the action of noncompetitive inhibitors (MK-801 and cocaine) on a protein (nicotinic acetylcholine receptor)-mediated reaction

Abstract

The nicotinic acetylcholine receptor (nAChR) is one of five structurally related membrane proteins required for communication between approximately 10(12) cells of the mammalian nervous system. The receptor is inhibited by both therapeutic agents and abused drugs. Understanding the mechanism of noncompetitive allosteric inhibitors of the nicotinic acetylcholine receptor is a long-standing and intensely investigated problem. During the past two decades, many attempts have been made to find drugs that prevent cocaine inhibition, including the synthesis of hundreds of cocaine analogues and derivatives, so far without success. The use of newly developed transient kinetic techniques in investigations of the inhibition of the receptor by the anticonvulsant MK-801 [(+)-dizocilpine] and the abused drug cocaine led to an inhibition mechanism not previously proposed. This mechanism indicates the properties of compounds that would prevent allosteric inhibition of the receptor and how to test for such compounds. Here we present the first evidence that small organic compounds (cocaine derivatives) exist that prevent cocaine and MK-801 inhibition of this receptor. These compounds are RTI-4229-70, a previously synthesized cocaine derivative, and based on its structure four newly synthesized cocaine derivatives, RCS-III-143, RCS-III-140A, RCS-III-218, and RCS-III-202A. Because the nAChR desensitizes rapidly, to make the required measurements a cell-flow technique with a time resolution of 10 ms was used to equilibrate BCH(3) cells containing the fetal mouse muscle-type nAChR with carbamoylcholine. The resulting whole-cell current pertaining to the nondesensitized nAChR was determined. Inhibitors and compounds that alleviate inhibition were tested by their effect on the whole-cell current.