<qa>  What is the cholinergic system responsible for?Several paths of evidence converge in implicating a role for the cholinergic system in the pathophysiology of affective illness. In both unipolar depressed and euthymic bipolar subjects, cholinomimetic drugs i.e., muscarinic agonists, acetylcholinesterase inhibitors exacerbate depressive signs and symptoms such as dysphoria, psychomotor retardation, impairment of attention and memory, hypothalamic pituitary adrenal axis hyperactivity and sleep EEG abnormalities. In healthy subjects, the acetylcholinesterase inhibitor physostigmine elicits a range of depressive symptoms including dysphoria, anergia, psychomotor slowing, emotional lability, sleep disturbances, memory and concentration impairment, and with higher doses, tearfulness and depression. These effects have been shown to reflect stimulation of muscarinic receptors. Cholinomimetics also exacerbate behavioral despair in putative animal models of depression. Conversely, the anticholinergic agent biperidine improved symptoms of depression in a placebo controlled study. Moreover, muscarinic cholinomimetics and a choline rich nutrient, lecithin phosphatidylcholine exert antimanic effects in bipolar subjects. 