| Kynurenic acid (KYNA) is an endogenous metabolite that acts at multiple neuronal receptor types, including alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). Millimolar KYNA concentrations competitively antagonize AMPARs; however, how micromolar KYNA concentrations affect AMPARs has not been determined. We used AMPAR-expressing oocytes and human embryonic kidney (HEK) cells to study the effects of micromolar concentrations of KYNA on AMPARs. We discovered that KYNA has dual effects on AMPARs, acting as a positive modulator at 10 muM and as a competitive antagonist at concentrations of 1 mM or greater. Though the overall effect of KYNA on AMPARs is primarily determined by KYNA concentration, additional factors may play a role. These factors include L-glutamate concentration, with lower L-glutamate concentrations favoring KYNA's competitive antagonist effect, and AMPAR splice variant, with flop isoforms more sensitive to positive modulation and flip isoforms more sensitive to competitive antagonism by KYNA. Using mutant AMPARs, we found that KYNA exerts its positive modulatory effect through binding at, or near, the serine 497 site of the GluR2 subunit. Using outside-out patches from AMPAR-expressing HEK cells, we found that 20 muM KYNA does not alter the time constant of desensitization.; In the rat hippocampal slice preparation, we observed that high micromolar to millimolar concentrations of KYNA exert an antagonistic effect on excitatory synaptic transmission; however, we did not observe positive modulation of synaptic transmission by lower KYNA concentrations. The lack of evidence for a dual effect of KYNA in the slice preparation, as compared to the oocytes preparation, may be due to the presence of endogenous KYNA in the slice preparation. Therefore, we lowered KYNA levels, using aminooxyacetic acid (AOAA); however, effects of AOAA on other compounds, such as gamma-aminobutyric acid, made these results difficult to interpret.; Finally, we tested the effect of synaptic activity on low extracellular calcium-induced antagonism of excitatory synaptic transmission in the rat hippocampal slice. We observed a concentration-specific effect, where extracellular calcium concentrations between 0.78 and 0.845 mM lead to use-dependent antagonism and lower concentrations antagonized synaptic transmission independently of stimulation frequency. |
