depressed vs downcast

depressed is not a noun while downcast is a noun.

Word	Noun	Adjective	Verb	Adverb
depressed	No	Yes	No	No
downcast	Yes	Yes	No	No

As adjectives, depressed and downcast are synonyms defined as:

Other synonyms of depressed include blue, dispirited, down, down in the mouth, downhearted, gloomy, grim, low-spirited, low.

depressed (noun)	downcast (noun)
	a ventilation shaft through which air enters a mine

depressed (adjective)	downcast (adjective)
filled with melancholy and despondency	filled with melancholy and despondency
lower than previously	directed downward
flattened downward as if pressed from above or flattened along the dorsal and ventral surfaces