The Brain Metabolite Kynurenic Acid Inhibits alpha 7 Nicotinic Receptor Activity and Increases Non- alpha 7 Nicotinic Receptor Expression: Physiopathological Implications

The tryptophan metabolite kynurenic acid (KYNA) has long been recognized as an NMDA receptor antagonist. Here, interactions between KYNA and the nicotinic system in the brain were investigated using the patch-clamp technique and HPLC. In the electrophysiological studies, agonists were delivered via a U-shaped tube, and KYNA was applied in admixture with agonists and via the background perfusion. Exposure ( greater than or equal to 4 min) of cultured hippocampal neurons to KYNA ( greater than or equal to 100 nM) inhibited activation of somatodendritic alpha 7 nAChRs; the IC sub(50) for KYNA was similar to 7 mu M. The inhibition of alpha 7 nAChRs was noncompetitive with respect to the agonist and voltage independent. The slow onset of this effect could not be accounted for by an intracellular action because KYNA (1 mM) in the pipette solution had no effect on alpha 7 nAChR activity. KYNA also blocked the activity of preterminal/presynaptic alpha 7 nAChRs in hippocampal neurons in cultures and in slices. NMDA receptors were less sensitive than alpha 7 nAChRs to KYNA. The IC sub(50) values for KYNA-induced blockade of NMDA receptors in the absence and presence of glycine (10 mu M) were similar to 15 and 235 mu M, respectively. Prolonged (3 d) exposure of cultured hippocampal neurons to KYNA increased their nicotinic sensitivity, apparently by enhancing alpha 4 beta 2 nAChR expression. Furthermore, as determined by HPLC with fluorescence detection, repeated systemic treatment of rats with nicotine caused a transient reduction followed by an increase in brain KYNA levels. These results demonstrate that nAChRs are targets for KYNA and suggest a functionally significant cross talk between the nicotinic cholinergic system and the kynurenine pathway in the brain.