Thank goodness for wikipedia! L-Theanine increases GABA (which basically has the opposite effect on the brain as glutamate), serotonin, and dopamine levels in the brain (link
). Other things that increase GABA levels include alcohol and benzodiazapines (e.g. valium), which is probably the explanation for why tea has calming effects. Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed as anti-depressants, and essentially increase serotonin levels. Dopamine is another neurotransmitter associated with mood. Another drug which increases dopamine levels is cocaine, which may explain Salsero sometimes. (bwahahaha
So, ironically, while it could be considered a "depressant" (as alcohol and benzodiazapines are), it may have some anti-depressant (in the clinical sense) properties. That said, I don't know enough about the compound to say for certain whether or not it acts like an anti-depressant, or whether the amount normally consumed in tea is enough to have such an effect on the brain.
Of course, the usual disclaimer applies; be sure to check with your doctor before using tea as a medication.