Caapi and Syrian Rue contain harmine, harmaline, and caapi contains tetrahydroharmine. These are all substances called RIMA's (reversible inhibitors of monoamine oxidase). Monoamine oxidase inhibitors have a history of being used as anti depressants. MAO-A inhibition reduces the breakdown of serotonin. Serotonin has long been theorized to play a role in depression. Typically, it is thought that low serotonin levels, and low uptake of serotonin, play a role in depression. Also, DMT is a serotonin agonist (A serotonin receptor agonist is a compound that activates serotonin receptors, mimicking the effect of the neurotransmitter, serotonin). Anyway, just saying from a "scientific" point of view, both ayahuasca and dmt play SOME role in the activity of the neurotransmitter serotonin, and low activity of serotonin, or low levels of serotonin, are thought to play a role in depression. Don't take the "science" part too seriously. Usually when I try to write something chemistry like here, someone comes back and says my info is crap. However, at the most basic level, the point is that ayahuasca and dmt positively affect serotonin activity, and serotonin is thought to play a role in depression (general idea).
Anyhoo, rima's tend to induce of a temporary state of well being, and reduced anxiety. The combination of harmalas with dmt seems to put the body in a very.... relaxed state, and put the psyche in a very...... interesting state. Both the mind and body state of ayahuasca are temporary. People do report ayahuasca afterglows, in which they feel very good for days after. But I think the potential therapeutic value ayahuasca is not the temporary state that it puts one in, but the revelations about ones own emotions, and ones own life while under the influece. People seem to develop an ability to "work through" emotions while on ayahuasca. They take their realizations with them in their lives afterwards. The altered psychedelic state allows one to look at their life from a different perspective, a perspective which is very difficult, or impossible to see while one is in a ordinary state, or for one who has never tried psychedelics. It is the shift in perspective in the psychedelic state which is potentially therapeutic, probably not so much the temporary increase in serotonin activity. The psychedelic perspective may reveal mistakes that a person keeps making over and over. The person then takes what they have learned on their trip, and uses it in their life. It can be very therapeutic. Maybe. I don't want to suggest that ayahuasca is therapeutic for everyone. If you want to try it, the option is there. Also, it is generally agreed that the therapeutic value of psychedelics in general lies in the low dose range.