Don't knock the ganja, i believe the green stuff may actually a viable chemical path for bipolar disorder. One of my closest friends took his own life July 23rd, and the simple reason is that there is no reliable treatment for severe depression in bipolar patients because the SSRIs can induce rapid cycling, intense manias by the nature of their function increase and sustain arousal. I don't have statistics, and i don't know much about the cannabinoid pathway except that with judicious use it might indeed offer an antidepressant option that is more compatible with the bipolar patient.

Regarding the above post: The only difference between amphetamine and dopamine is a pair of hydroxyl groups on the 4th and 5th carbons of the phenyl group. Which gives amphetamine the ability to dock with dopamine receptors thus bypassing the Pre-synaptic neuron.  
From random reading of articles I've gathered that one of the more immediate consequences of increased serotonin release is to facilitate the firing of the post-synaptic neuron. Additionally I wanna say that serotonin is modulated more by the extremity of a reward as opposed to the presence or expected presence of a reward as you see in some dopaminergic regions like the Striatum.  
T