Major depressive disorder

A clinically assessed syndrome involving persistent depressed mood and/or anhedonia, with associated cognitive, motivational, sleep and bodily symptoms and functional impairment. Similar symptom profiles need not have one biological cause.

A family of proposals relating depression to reduced or dysregulated monoamine signalling. A simple universal deficiency account is stronger than the claim that a monoaminergic manipulation can alter symptoms. Drug-target engagement, therapeutic mediation and original disease cause are separate causal claims.

An experimental manipulation intended to reduce the availability of the serotonin precursor tryptophan. Effects in a previously treated remitted sample test dependence of that state on precursor availability; they do not directly measure a universal baseline serotonin deficit.

Reduced interest or pleasure. Anticipation, experienced pleasure, learning from outcomes and willingness to exert effort are distinguishable components, so one reward task does not measure them all.