Can I apply for a patent for the treatment of depression?

You can't. According to Article 25 of the Patent Law, no patent right shall be granted for the following items: (1) Scientific discovery; (2) rules and methods of intellectual activities; (3) Methods of diagnosis and treatment of diseases; (4) Species of animals and plants; (5) substances obtained by nuclear transformation; (six) the design of the pattern, color or the combination of the two.