Regulation of GABAergic Inputs to CA1 Pyramidal Neurons by Nicotinic Receptors and Kynurenic Acid

Impaired alpha 7 nicotinic acetylcholine receptor (nAChR) function and GABAergic transmission in the hippocampus and elevated brain levels of kynurenic acid (KYNA), an astrocyte-derived metabolite of the kynurenine pathway, are key features of schizophrenia. KYNA acts as a noncompetitive antagonist with respect to agonists at both alpha 7 nAChRs and N-methyl-D-aspartate receptors. Here, we tested the hypothesis that in hippocampal slices tonically active alpha 7 nAChRs control GABAergic transmission to CA1 pyramidal neurons and are sensitive to inhibition by rising levels of KYNA. The alpha 7 nAChR-selective antagonist alpha-bungarotoxin (alpha-BGT; 100 nM) and methyllycaconitine (MLA; 10 nM), an antagonist at alpha 7 and other nAChRs, reduced by 51.3 +/- 1.3 and 65.2 +/- 1.5%, respectively, the frequency of GABAergic postsynaptic currents (PSCs) recorded from CA1 pyramidal neurons. MLA had no effect on miniature GABAergic PSCs. Thus, GABAergic synaptic activity in CA1 pyramidal neurons is maintained, in part, by tonically active alpha 7 nAChRs located on the preterminal region of axons and/or the somatodendritic region of interneurons that synapse onto the neurons under study. L-Kynurenine (20 or 200 mu M) or KYNA (20-200 mu M) suppressed concentration-dependently the frequency of GABAergic PSCs; the inhibitory effect of 20 mu M L-kynurenine had an onset time of approximately 35 min and could not be detected in the presence of 100 nM alpha-BGT. These results suggest that KYNA levels generated from 20 mu M kynurenine inhibit tonically active alpha 7 nAChR-dependent GABAergic transmission to the pyramidal neurons. Disruption of nAChR-dependent GABAergic transmission by mildly elevated levels of KYNA can be an important determinant of the cognitive deficits presented by patients with schizophrenia.