If I remember correctly 5-htp is a precursor of TRP
Nope, the opposite. Part of TRP in the brain is converted to 5-HTP through 5-hydroxytryptophan to serotonin. Since I didn't notice an effect from supplemental 5-HTP (except for countering insomnia) or from SSRIs, I assume that pathway is working okay and gets adequate TRP. Most (90%) TRP in the brain gets converted into kynurenines. Kynurenic acid is neuroprotective. Some of the others are neurotoxic.
I just came across this paper about quinolinic acid: www.hindawi.com/journals/omcl/2013/104024/ I'm too brainfogged today to make full sense of it, but it's full of findings that seem to relate to ME/CFS. "QUIN can potentiate its own toxicity and that of other excitotoxins, like NMDA and glutamate, producing progressive mitochondrial dysfunction" "Recently, Schuck and coworkers [54] have shown that QUIN inhibits the 14CO2 production and increased glucose uptake in cerebral cortex homogenates of young rats indicating that this kynurenine stimulated the transport and/or utilization of this substrate by the brain. QUIN also inhibits around 35% succinate dehydrogenase (SDH), an enzyme involved in the citric acid cycle and in the respiratory chain."
Those who have their own theories about ME/CFS would likely find supporting evidence in this paper. Not that this is proof of anything, but QUIN (and other kynurenines) certainly seem like they could play a role in ME/CFS. Lots of possibilities for feedback loops to get locked into. There probably isn't much ME/CFS research into kynurenines, since it requires CSF samples rather than blood, which makes it more difficult/expensive. I still think ME/CFS is primarily a cerebral disorder, and that the common physical effects are secondary effects which are leading researchers down the wrong path.
"On the other hand, the administration of a TRP-free diet to rats for 15 days resulted in a doubling of QUIN concentrations in the cortex." Dang! Maybe that's why my attempt at a TRP-free diet didn't reduce my symptom severity.
I also came across: www.ncbi.nlm.nih.gov/pubmed/9597672 which concluded that: 'Selegiline has a small but significant therapeutic effect in CFS which appears independent of an antidepressant effect.' in case anyone here is desperate for even a small theraputic effect.