The paper Relative nutritional deficiencies associated with centrally acting monoamines postulates a new aproach for the treatment of many mental illnesses.

Different in the way that it's underlying mechanism is based on a new understanding of monoamine transporters which simply wasn't there before.

Specifically it is based around the organic cation transporter types.

Basically said when there is no absolute nutritional deficiencie e.g. not enough proteins and cofactors and there is an relative nutritional deficiencie just adding the amino acid which is needed or it's neurotransmitters precursors like 5-htp, l-dopa, tyrosine didn't satisfy.

For parkinson we see that as soon as the l-dopa treatment starts, conditions worsen. This is why the L-dopa is kind of a last resort.

With depression it's the same, 5-htp instantly pushes the available Serotonin in the brain but doesn't really improve depressive symptoms and in fact also worsens the symptoms with time.

The same goes for SSRI's which have a totally different mechanism but with time also worsen symptoms in the same way.

What Marty Hinz, Alvin Stein and Thomas Uncini postulated is a relative simple method for producing a neurotransmitter equilibrium in the brain .

After all, the lack of this equilibrium of centrally acting monoamines is the actual scientific explanation for most of the actual mental diseases.

There are other papers which show the research done:
The discrediting of the monoamine hypothesis
5-HTP efficacy and contraindications
The dual-gate lumen model of renal monoamine transport
Monoamine depletion by reuptake inhibitors
Amino acid management of Parkinson’s disease: a case study
Treatment of attention deficit hyperactivity disorder with monoamine amino acid precursors and organic cation transporter assay interpretation