Kynurenine pathway dysregulation in attention-deficit hyperactivity disorder: Impact on N-methyl-D-aspartate receptors and phasic dopamine maturation

Table 1 Differential effects of kynurenine metabolites on neuroreceptor sites

Metabolite	Target receptor	Action	Resulting effect in ADHD
KYNA	NMDA (glycine site)	Competitive antagonist	Decreased in ADHD; loss of neuroprotection
KYNA	Alpha-7 nicotinic acetylcholine receptor	Antagonist	Decreased in ADHD; increased glutamate release
Quinolinic acid	NMDA (glutamate site)	Potent agonist	Increased relative to KYNA; excitotoxicity
3-hydroxykynurenine	Mitochondrial complex II	Inhibitor	Induces oxidative stress and reactive oxygen species production

ADHD: Attention-deficit/hyperactivity disorder; KYNA: Kynurenic acid; NMDA: N-methyl-D-aspartate.

Table 2 Comparative mechanisms of attention-deficit/hyperactivity disorder pharmacotherapy

Feature	Methylphenidate	KMO inhibitors
Primary target	Dopamine/norepinephrine transporters	KMO enzyme
Mechanism	Increases synaptic catecholamines	Blocks neurotoxic branch; raises KYNA
Circuit impact	Direct enhancement of reward signaling	Reduction of metabolic “noise” and excitotoxicity
Side effect profile	Potential appetite/sleep disruption	Targeted metabolic shift (lower systemic risk)
Role in kynurenine pathway	Indirect stabilizer (increases KYNA)	Direct pathway modulator

KMO: Kynurenine 3-monooxygenase; KYNA: Kynurenic acid.