Early developmental elevations of brain kynurenic acid impair cognitive flexibility in adults: reversal with galantamine

Abstract

Levels of kynurenic acid (KYNA), an endogenous α7 nicotinic acetylcholine receptor (α7nAChR) antagonist, are elevated in the brain of patients with schizophrenia (SZ) and might contribute to the pathophysiology and cognitive deficits seen in the disorder. As developmental vulnerabilities contribute to the etiology of SZ, we determined, in rats, the effects of perinatal increases in KYNA on brain chemistry and cognitive flexibility. KYNA's bioprecursor l-kynurenine (100mg/day) was fed to dams from gestational day 15 to postnatal day 21 (PD21). Offspring were then given regular chow until adulthood. Control rats received unadulterated mash. Brain tissue levels of KYNA were measured at PD2 and PD21, and extracellular levels of KYNA and glutamate were determined by microdialysis in the prefrontal cortex in adulthood (PD56-80). In other adult rats, the effects of perinatal l-kynurenine administration on cognitive flexibility were assessed using an attentional set-shifting task. l-Kynurenine treatment raised forebrain KYNA levels ∼3-fold at PD2 and ∼2.5-fold at PD21. At PD56-80, extracellular prefrontal KYNA levels were moderately but significantly elevated (+12%), whereas extracellular glutamate levels were not different from controls. Set-shifting was selectively impaired by perinatal exposure to l-kynurenine, as treated rats acquired the discrimination and intra-dimensional shift at the same rate as controls, yet exhibited marked deficits in the initial reversal and extra-dimensional shift. Acute administration of the α7nAChR-positive modulator galantamine (3.0mg/kg, i.p.) restored performance to control levels. These results validate early developmental exposure to l-kynurenine as a novel, naturalistic animal model for studying cognitive deficits in SZ.

Figure 1

Effects of pre- and postnatal exposure to kynurenine on forebrain tissue levels of KYNA at PD2. Dams were exposed to kynurenine (100 mg/day) as a component of a wet chow mash from GD15 to weaning on PD21 (devKYN). Control animals were fed a standard wet mash without kynurenine (devCTL). Male progeny (n = 6 per group) were euthanized at PD2. KYNA levels are expressed as the mean ± S.E.M. ^a = P < 0.05 vs. devCTL.

Figure 2

Effects of pre- and postnatal exposure to kynurenine on KYNA levels in the prefrontal cortex at PD21. Dams were exposed to kynurenine (100 mg/day) as a component of a wet chow mash from GD15 to weaning on PD21 (devKYN). Control animals were fed a standard wet mash without kynurenine (devCTL). Male progeny (n = 7 per group) were euthanized at PD21. KYNA levels are expressed as the mean ± S.E.M. ^a = P < 0.05 vs. devCTL.

Figure 3

Effects of pre- and postnatal exposure to kynurenine on basal extracellular levels (mean ± S.E.M.) of KYNA (top) and glutamate (bottom) in the prefrontal cortex in adult rats (≥PD56-80). Dams were exposed to kynurenine (100 mg/day) as a component of a wet chow mash from GD15 to weaning on PD21. Control animals were fed a standard wet mash without kynurenine. All rats were fed standard laboratory chow pellets from the time of weaning until the microdialysis studies were conducted between PD56 and PD80. See text for absolute basal KYNA and glutamate levels in devCTL (n=6) and devKYN animals (n = 7) of the levels seen in devCTL rats (n = 6). a = P < 0.05 vs. devCTL.

Figure 4

Mean trials to criterion (± S.E.M.) for adult rats (PD56-80) in various stages of the attentional set-shifting task. Animals were derived from two treatment conditions: devCTL + SAL (saline; n = 6) and devKYN + SAL (n = 8). Saline was administered as a vehicle solution for subsequent groups that received galantamine as a potential cognition-enhancer. Both groups of rats readily acquired the single (SD) and compound (CD) discriminations but required more trials to learn the initial reversal (REV1). The devKYN + SAL group required more trials than the devCTL + SAL group to acquire REV1. Each group demonstrated comparable abilities to form an attentional set, as evidenced by the rapid acquisition of an intra-dimensional shift (ID) to a novel stimulus. DevKYN rats exhibited a marked deficit in the ability to make an extra-dimensional shift (ED). ^a = significantly different, within treatment group, from the trials to criterion for the CD stage; ^b = significantly different, within the REV1 stage, from the devCTL + SAL group;^c = significantly different, within treatment group, from the trials to criterion for the ID stage; ^d = significantly different, within the ED stage, from the devCTL + SAL group (all P values < 0.05).

Figure 5

Mean trials to criterion (± S.E.M.) for adult rats (PD56-80) in various stages of the attentional set-shifting task. The pro-cognitve effects of galantamine (GAL) were assessed in two treatment groups: devCTL+ GAL (n = 6) and devKYN + GAL (n = 8). GAL (3 mg/kg, i.p.) was injected 45 min prior to the onset of the task. Both groups of rats readily acquired the single (SD) and compound (CD) discriminations but required more trials to learn the initial reversal (REV1). The devKYN + SAL group required more trials than the devCTL + SAL group to acquire REV1 (Fig. 4). This difference was eliminated following the administration of GAL (Fig. 5). ^a = significantly different, within treatment group, from the respective trials to criterion for the CD stage; ^b = significantly different, within treatment group, from the trials to criterion for the ID stage (all P values < 0.05).

Figure 6

Mean errors to criterion (± S.E.M.) for adult rats (PD56-80) in select stages of the attentional set-shifting task. The focus was on the REV1 and ED stages as these were the stages that revealed significant group differences in Figure 4. The error analysis paralleled the effects shown in Figure 4; namely that the number of errors made in the run to criterion was significantly higher in the devKYN + SAL group, during REV1 and ED, than any of the remaining 3 groups. ^a = devKYN + SAL different from any other group in that particular stage (both P values < 0.05).