Hmm.. depressants too have a rebound effect, but as you're saying, it mostly consists of excitation which - depending on tolerance, intensity and other factors - can be anything from pleasurable to absolutely hellish. I think we know pretty much about the monoamines, so there is little surprise, but when viewing all things together it becomes more complicated.
Dissociatives for example have both components. Physically they are more of a stimulant or neutral but not a depressant, but on the mind they can be quite sedating. I remember becoming very sleepy on ketamine. And they are interesting because they're quite unique in that both the acute effects as well as the after effects can be pleasant - that afterglow thingy. It seems that by blocking the NMDAR's, more glutamate gets released, probably through auto-receptors. This should imply a depressant come-down, but there is at least a 1:1 chance to catch a stimulating come-down. My thoughts here are about that one transmitter always inhibits the other above a certain threshold, so when glutamate levels fall, you might get a rise of dopamine.
Also I'm not sure if the monoamine depletion explains it all, it looks like that when you're only using a psychostimulant, but there's no real explanation why adding a NMDA antagonist (memantine can be enough) seems to - at least temporarily - solve the depletion, making the stimulants more sustainable. Oxidative stress plays a role as well, when I used emoxypine along with e.g. isopropylphenidate, there seemed to be much less 'depletion' happening. Smoother comedown, no rebound. And more sustainable. Granted, if dopamine oxidation is a major factor for it's degradation, then this would explain it.
Maybe some tweakers can help out with experiences about staying up forever?
Somehow I have the theory that with enough knowledge and access to the right substances, one would be able to avoid comedowns nearly completely soon.. at least within the biological limits (which are quite elastic, but there are definite limits).
