You may get responses to your request that are criticizing, but I would like to say - Congratulations to you and your daughter for the open communication that you have! It's wonderful that she can talk with you openly and that she can ask for help.
My ex-husband, who I've known for 25 years has suffered his whole life with chronic depression and more recently is being treated for bi-polor disorder. He also has addiction problems; these commonly go hand-in-hand with depression/pi-polar. There are interesting articles being published now about how depression is "inherited". Not genetic, but learned from your family members.
I don't see treatment of these conditions to be only a drugs/no drugs debate (that some will turn this into). Help for your daughter could involve counseling for behavior modification and to learn techniques to help her manage her emotions, and a regular exercise program (read "Spark: the effects of exercise on the brain" for the amazing help that exercise can bring), along with the possibility of drug treatment. Medications can be helpful, when carefully monitored until the ones that work best for the individual are discovered.
I would not tell family, friends, or school anything that they do not need to know until you discover what works best for your daughter. There may not be any need to disclose anything, or only to select people.
Good luck to you both in working through this, and again, you are so far ahead in that you can talk openly about this.