Wash-resistantly bound xanomeline inhibits acetylcholine release by persistent activation of presynaptic M₂ and M₄ muscarinic receptors in rat brain

We studied the effects of 3-[3-hexyloxy-1,2,5-thiadiazo-4-yl]-1,2,5,6- tetrahydro-1-methylpyridine (xanomeline) wash-resistant binding on presynaptic muscarinic regulation of electrically evoked [³H]acetylcholine (ACh) release from rat brain slices. In both cortical and striatal tissues that possess M₂ and M₄ autoreceptors, respectively, immediate application of 10 μM xanomeline had no effect on evoked [³H]ACh release or its inhibition by 10 μM carbachol. In contrast, preincubation with 1, 10, or 100 μM xanomeline for 15 min decreased evoked release of ACh measured after 53 min of washing in xanomeline-free medium in a concentration-dependent manner. The maximal inhibitory effect equaled the immediate effect of the muscarinic full agonist carbachol, and it was completely (at 1 and 10 μM xanomeline) or partially (at 100 μM xanomeline) blocked by 1 μM N-methylscopolamine. Neither presence of N-methylscopolamine during 100 μM xanomeline treatment nor previous irreversible inactivation of the classical receptor binding site using propylbenzylcholine mustard in cortical slices prevented the inhibitory effect of wash-resistantly bound xanomeline. Treatment of cortical slices with xanomeline slightly decreased the number of muscarinic binding sites, and it markedly decreased affinity for N-methylscopolamine. When applied as in acetylcholine release experiments, xanomeline did not impair presynaptic α₂-adrenoceptor-mediated regulation of noradrenaline release. The functional studies in brain tissue reported in this work demonstrate that xanomeline can function as a wash-resistant agonist of native presynaptic muscarinic M₂ and M ₄ receptors with both competitive and allosteric components of action.