Allosteric modulation of the [3H]quinuclidinyl benzylate binding to M-cholinoreceptors in rat cerebral cortex by adrenotropic agents

Abstract

The allosteric effects of adrenotropic drugs and the membranotropic agent cocaine on the kinetics of [3H]quinuclidinyl benzylate ([3H]QNB) binding to muscarine cholinoceptors of synaptosomal membranes of rat cerebral cortex were studied. In control, the best results were obtained for the model that assumes the existence of two receptor pools (with high and low affinity) with calculated parameters of the activity (K d), amount (B max), and mono- to dimer receptors ratio (n). For the high-affinity receptors these parameters were K d1 = 0.18 ± 0.08 nM, B m1 = 221.2 ± 56.7 fmol/mg protein, n 1 = 2, and for low-affinity receptors, K d2 = 1.33 ± 0.11 nM, B m2 = 748.7 ± 53.3 fmol/mg protein, n 2 = 2. Allosteric modulation of the activity of specific neurotransmitter receptors can be accomplished by changing the receptor affinity and amount as well as the proportion of mono- and dimer receptors. Under control conditions, muscarine receptors exist as dimers. In the presence of α-adrenoreceptor agonist noradrenaline and β-adrenoreceptor antagonist propranolol, only one pool of the dimer muscarine receptors remains. The number of binding sites for noradrenaline and propranolol decreases approximately by 40% and 20%, respectively. The agonist of α2-adrenoreceptors clonidine, the antagonist of α2-adrenoreceptors yohimbine, and a membranotropic agent cocaine change the ligand binding so that only one receptor pool remains but some of the dimer receptors become monomeric (1 < n < 2). The amount of binding sites reduces by 20%, 25%, and 45% for clonidine, yohimbine, and cocaine, respectively.