Depression is a real disease, as you surely know; not just a "feeling the blahs." Depression is expeted to be secnd only to heart disease as the world's major cause of disease burden.
Of course, our anticipation for astro-markers includes Mercury and Saturn and Neptune... but there is lot more. --I suggest you read my overviews of depression in The Astrological Timing of Critical Illness.
Midpoint pictures figure strongly in the vulnerability to depression: Neptune=Mercury/Saturn is the archetype. --Then we have pictures that often connote environmental happenings that pull from us deep, possibly depressed reaction: Saturn=Neptune/Pluto, which stongly suggests "the presence of grief, weakness, torment"; Pluto=Saturn/Ascendant, "violent upset, deep anguish, being put down by others"; Ascendant=Saturn/Pluto, "feeling a loss of identity." --And of course, "=" connotes high-tension aspect, bsed on the fourth harmonic.
Remember the "mind set" is so very often portrayed by the 3rd House and its rulership condition.
Hope this helps you into a fascinating study