Interactions between allosteric modulators and 4-DAMP and other antagonists at muscarinic receptors: Potential significance of the distance between the N and carboxyl C atoms in the molecules of antagonists

Allosteric enhancement of the affinity of muscarinic receptors for their li gands offers a new way to influence cholinergic. neurotransmission. The str ucture of the allosteric binding domain(s) and the features of agonists, an tagonists and modulators which determine the occurrence of either positive or negative cooperativity require clarification. We tested interactions bet ween allosteric, modulators alcuronium, strychnine and brucine and eight an tagonists at muscarinic receptors expressed in CHO cells. In experiments wi th unlabeled antagonists, all three modulators enhanced the affinity for 4- diphenylacetoxy-N-dimethylpiperidinium (4-DAMP) at the M-2 receptors, and s trychnine did so also at the M-4 receptors. Positive interactions were also observed between alcuronium and L-hyoscyamine (M-2) and scopolamine (M-2), between strychnine and butylscopolamine (M-4), L-hyoscyamine M-2 and M-4) and scopolamine (M-4), and between brucine and scopolamine (M-2). Positive effects of alcuronium, strychnine and brucine on the affinity of the M-2 re ceptors for 4-DAMP have been confirmed by direct measurements of the bindin g of [H-3]-4-DAMP. A comparison of molecular models of several antagonists which are esters revealed that antagonists in which the distance between th e N and the carboxyl C atoms corresponds to five chemical bonds are more li kely to display positive cooperativity with alcuronium at the M-2 receptors than the antagonists in which the N-carboxyl C distance corresponds to fou r chemical bonds.