 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. 