4-Bromoacetamidoprocaine: An affinity ligand for brain muscarinic and nicotinic cholinergic receptors

Abstract

This study describes the synthesis, receptor binding characteristics, and some behavioral effects of p-bromoacetamidoprocaine (BAP), a new affinity ligand for brain muscarinic and nicotinic cholinergic receptors. The reversible binding of [³H]QNB to rat brain membranes was inhibited in a concentration dependent and saturable manner by both procaine and BAP, with Ki values of 4×10⁻⁶ and 3×10⁻⁷ M, respectively, and complete inhibition at 1×10⁻⁵ M. Both procaine and BAP, although at much concentrations, inhibited the binding of [³H]methylcarbamylcholine in a concentration dependent manner, with Ki values of 5×10⁻⁵ and 1×10⁻⁵ M, respectively, and complete inhibition for both at 1×10⁻³ M. Plots of the % irreversible inhibition of [³H]QNB, [³H]nicotine, and [³H]MCC vs [BAP] yielded Ki values of 7×10⁻⁸, 1×10⁻⁴, and 6×10⁻⁵ M, respectively. In behavioral studies BAP was able to antagonize the QNB-induced hyperactivity in mice; however, BAP did not appear to alter nicotine-induced seizure activity or other behavioral effects in mice. A plot of the time course of inhibition by BAP for [³H]QNB binding revealed that the inhibition was almost complete within 10 min exposure at 37°. The findings indicate that BAP is a useful affinity ligand for examining the biochemical and functional characteristics of brain cholinergic receptors, particularly the muscarinic which has an affinity near the nM concentration range.