<qa>  What is the mAChR receptor system?Despite the data implicating the mAChR receptor system in mood disorders, no direct in vivo investigations of the central mAChR have been performed in depressed subjects. A novel PET radioligand, 18FFP-TZTP was recently developed by Eckelman as a selective agonist of M2 receptors. Because the M2 receptor functions predominately as a presynaptic release-controlling autoreceptor, decreased distribution volume V of this receptor could conceivably give rise to increased postsynaptic muscarinic receptor sensitivity. 