Evidence for intraocular synthesis of kynurenic acid, a putative endogenous neuroprotectant

Kynurenic acid (KYNA), an excitatory amino acid antagonist preferentially active at glycine binding site of the NMDA receptor, has been previously identified in the brain. This study was designed to examine its presence in the rabbit vitreous humor. Mean (+/- SD) level of KYNA in the vitreous was 22.3 +/- 3.9 pmol/ml as determined by HPLC. Intravitreal administration of 10 mmol aminooxyacetic acid (AOAA), a KYNA synthesis inhibitor, diminished its production by 9.6% after 2 h, 47.8% after 24 h and 21.5% after 48 h. It can be concluded that AOAA decreases the intravitreal concentration of KYNA, providing evidence of its intraocular origin.