Yes, aspartame is the methyl ester of phenylalanine and aspartic acid,
two amino acids. According to Richard Wurtman of MIT, you can get
increases in brain levels of phenylalanine if low-protein foods such
as soft drinks are consumed on an empty stomach.

The brain requires vitamins B-6 and C to convert phenylalanine into
the natural stimulant noradrenaline and another important brain chemical,
dopamine (which is also, among other things, a stimulant and euphoric)

Thus even if you are taking no supplemental phenylalanine, you can
still get beneficial effects on mood, etc., by taking C and B-6
because your brain will become more efficient at converting the
phenylalanine that is already present in your diet.

Speaking of diets, phenylalanine also stimulates the release of
cholecystokinin, which is the body's own appetite-suppressant,
used to tell the brain when the stomach has had enough. So phenylalanine
acts indirectly as an appetite-suppressant.

Note that all of the effects of phenylalanine are greatly enhanced if
it is taken either on an empty stomach or with low-protein foods,
because various proteins compete with phenylalanine for transport
across the blood-brain barrier.

Dosage: 500mg to 1g phenylalanine, plus 1g C, 30 to 50mg B-6.

WARNING: Do NOT use phenylalanine: 
        *If you are using MAO inhibitors, unless they are the extremely 
                selective types such as deprenyl (selegiline); 
        *If you have a cardiac arrythmia; 
        *If you have hypertension; 
        *If you have phenylketonuria; 
        *If you have a psychosis; 
        *If you have a pigmented malignant melanoma-type cancer; 
        *If you have a violent temper. 

If you are taking other stimulants such as phenylpropanolamine, ephedrine,
or even, potentially, caffiene or theophylline, you should monitor your
blood pressure carefully because there is a possiblity of blood pressure
rise and other problems characteristic of adrenaline-like drug oversoda.