Fluctuations in endogenous kynurenic acid control hippocampal glutamate and memory

Abstract

Kynurenic acid (KYNA), an astrocyte-derived metabolite, antagonizes the α7 nicotinic acetylcholine receptor (α7nAChR) and, possibly, the glycine co-agonist site of the NMDA receptor at endogenous brain concentrations. As both receptors are involved in cognitive processes, KYNA elevations may aggravate, whereas reductions may improve, cognitive functions. We tested this hypothesis in rats by examining the effects of acute up- or downregulation of endogenous KYNA on extracellular glutamate in the hippocampus and on performance in the Morris water maze (MWM). Applied directly by reverse dialysis, KYNA (30-300 nM) reduced, whereas the specific kynurenine aminotransferase-II inhibitor (S)-4-(ethylsulfonyl)benzoylalanine (ESBA; 0.3-3 mM) raised, extracellular glutamate levels in the hippocampus. Co-application of KYNA (100 nM) with ESBA (1 mM) prevented the ESBA-induced glutamate increase. Comparable effects on hippocampal glutamate levels were seen after intra-cerebroventricular (i.c.v.) application of the KYNA precursor kynurenine (1 mM, 10 μl) or ESBA (10 mM, 10 μl), respectively. In separate animals, i.c.v. treatment with kynurenine impaired, whereas i.c.v. ESBA improved, performance in the MWM. I.c.v. co-application of KYNA (10 μM) eliminated the pro-cognitive effects of ESBA. Collectively, these studies show that KYNA serves as an endogenous modulator of extracellular glutamate in the hippocampus and regulates hippocampus-related cognitive function. Our results suggest that pharmacological interventions leading to acute reductions in hippocampal KYNA constitute an effective strategy for cognitive improvement. This approach might be especially useful in the treatment of cognitive deficits in neurological and psychiatric diseases that are associated with increased brain KYNA levels.

Figure 1

Dose-dependent effect of KYNA on extracellular levels of glutamate in the hippocampus. After 2 h of baseline collection, KYNA was applied intra-hippocampally for 2 h by reverse dialysis (bar). Data are expressed as a percentage of baseline (see Results for absolute value) and are the mean±SEM (n=5 per group). ^# and ^*P<0.05 vs baseline for 100 and 300 nM KYNA, respectively (two-way ANOVA with Bonferroni's post-hoc test).

Figure 2

Dose-dependent effects of ESBA on extracellular KYNA and glutamate levels in the hippocampus. After 2 h of baseline collection, ESBA was applied intra-hippocampally for 2 h by reverse dialysis (bar). KYNA (a) and glutamate (b) were measured in the same dialysate. Data are expressed as a percentage of baseline values and are the mean±SEM (n=4 per group). ^# and ^*P<0.05 vs baseline for 1 and 3 mM ESBA, respectively (two-way ANOVA with Bonferroni's post-hoc test). (c) After 2 h of baseline collection, ESBA (1 mM) and KYNA (100 nM) were co-infused into the hippocampus for 2 h (bar). Data are expressed as a percentage of baseline values and are the mean±SEM (n=5). Perfusion with ESBA+KYNA did not affect extracellular glutamate levels (P>0.05; two-way ANOVA with Bonferroni's post-hoc test). Absolute baseline values of KYNA and glutamate are given under Results.

Figure 3

Acute, unilateral i.c.v. infusion of kynurenine or ESBA affects neurochemistry in the contralateral hippocampus. After 2 h of baseline collection, test compounds were applied i.c.v. (a) kynurenine (1 mM); (b) ESBA (10 mM); (c) ESBA (10 mM)+KYNA (10 μM)) (arrows) and the microdialysate was collected from the contralateral hippocampus. KYNA and glutamate were measured in the same dialysate where applicable. Data are expressed as a percentage of baseline (see Results for absolute values) and are the mean±SEM (n=4 for kynurenine; n=5 for ESBA; n=5 for ESBA+KYNA). ^# and ^*P<0.05 vs the respective baseline (two-way ANOVA with Bonferroni's post-hoc test).

Figure 4

Acute i.c.v. infusion of kynurenine impairs spatial memory. (a) Vehicle (control) or kynurenine (1 mM) was infused i.c.v. 90 min prior to MWM testing each day. See text for experimental details. The data are the mean±SEM (n=12 per group). ^*P<0.05 vs control (post-hoc Student's t-test). (b) Number of platform crossings during the probe trial (video tracking analysis). The data are the mean±SEM (n=12 per group). ^*P<0.05 vs control (Student's t-test). (c) Representative swim path traces for the probe trial. The former platform location is indicated by small black circles.

Figure 5

Acute i.c.v. infusion of ESBA improves spatial memory. (a) Vehicle (control) or ESBA (10 mM) were infused i.c.v. 90 min prior to MWM testing each day. See text for experimental details. The data are the mean±SEM (n=12 per group). ^*P<0.05 vs control (post-hoc Student's t-test). (b) Number of platform crossings during the probe trial (video tracking analysis). The data are the mean±SEM (n=12 per group). ^*P<0.05 vs control (Student's t-test). (c) Representative swim path traces for the probe trial. The former platform location is indicated by small black circles.

Figure 6

Acute i.c.v. co-infusion of KYNA prevents the ESBA-induced cognitive enhancement. (a) Vehicle (control), ESBA (10 mM), or ESBA (10 mM)+KYNA (10 μM) was infused i.c.v. 90 min prior to MWM testing each day. See text for experimental details. The data are the mean±SEM (n=12 for control; n=12 for ESBA; n=11 for ESBA+KYNA). ^*P<0.05 (ESBA+KYNA vs ESBA alone; post-hoc Student's t-test). (b) Number of platform crossings during the probe trial (video tracking analysis). The data are the mean±SEM (n=12 for control; n=12 for ESBA; n=11 for ESBA+KYNA). ^**P<0.01, n.s.: not significant (P>0.05) (Student's t-test). (c) Representative swim path traces for the probe trial. The former platform location is indicated by small black circles.