<qa>  What is a neurotransmitter that binds to certain brain receptors calledThe cholinergic system involves the regulation of neurotransmitters and the brain receptors to which they bind. Evidence suggests that the cholinergic system may play a role in the development of depression. Acetylcholine is a neurotransmitter that binds to certain brain receptors called muscarinic cholinergic receptors. Cholinomimetic drugs drugs that stimulate the cholinergic system often exacerbate depressive symptoms in people with mood disorders and in healthy individuals. This increase in depressive symptoms may be caused by stimulation of muscarinic acetylcholine receptors mAChRs, but further study is needed to confirm this. This study will use positron emission tomography PET and magnetic resonance imaging MRI to study the function of mAChRs in individuals with depression. 