Acute elevations of brain kynurenic acid impair cognitive flexibility: normalization by the alpha7 positive modulator galantamine

Abstract

Rationale: Cognitive deficits represent a core symptom cluster in schizophrenia (SZ) that is predictive of outcome but not effectively treated by current antipsychotics. Thus, there is a need for validated animal models for testing potential pro-cognitive drugs.

Objective: As kynurenic acid levels are increased in prefrontal cortex (PFC) of individuals with SZ, we acutely increased brain levels of this astrocyte-derived, negative modulator of alpha7 nicotinic acetylcholine receptors (α7nAChRs) by administration of its bioprecursor kynurenine and measured the effects on extracellular kynurenic acid and glutamate levels in PFC and also performance in a set-shifting task.

Results: Injections of kynurenine (100 mg/kg, i.p.) increased extracellular kynurenic acid (1,500%) and decreased glutamate levels (30%) in PFC. Kynurenine also produced selective deficits in set-shifting. Saline- and kynurenine-treated rats similarly acquired the compound discrimination and intra-dimensional shift (saline, 7.0 and 6.3 trials, respectively; kynurenine, 8.0 and 6.7). Both groups required more trials to acquire the initial reversal (saline, 15.3; kynurenine, 22.2). Only kynurenine-treated rats were impaired in acquiring the extra-dimensional shift (saline, 8.2; kynurenine, 21.3). These deficits were normalized by administering the α7nAChR positive allosteric modulator galantamine (3.0 mg/kg, i.p) prior to kynurenine, as trials were comparable between galantamine + kynurenine (7.8) and controls (8.2). Bilateral local perfusion of the PFC with galantamine (5.0 μM) also attenuated kynurenine-induced deficits.

Conclusions: These results validate the use of animals with elevated brain kynurenic acid levels in SZ research and support studies of drugs that normalize brain kynurenic acid levels and/or positively modulate α7nAChRs as pro-cognitive treatments for SZ.

Figure 1

(Top Panel) Effects of systemic kynurenine administration (100 mg/kg, i.p.) on extracellular levels of kynurenic acid (KYNA) and glutamate in the PFC. The two analytes were determined in the same microdialysate samples (n = 5 rats). Kynurenine led to a marked increase in KYNA levels in the first post-injection collection, reaching a maximum 2 h later, and approaching baseline levels by the end of the microdialysis session. Glutamate levels exhibited a mirror image profile, decreasing in the first collection interval, reaching a nadir 90 min later, and returning to baseline levels 2 ½ h after injection. (Bottom Panel) Effects of a combined systemic administration of kynurenine (100 mg/kg, i.p.) and galantamine (3 mg/kg, i.p.) on extracellular levels of KYNA or glutamate in the PFC. The two analytes were again determined in the same microdialysate samples (n = 5 rats). KYNA levels exhibited the same pattern of elevation seen following kynurenine alone (top panel). However, pretreatment with galantamine eliminated the kynurenine-induced reduction in prefrontal glutamate levels. a,b = significantly different from levels during the last baseline (collection 4), just prior to drug injection (P's < 0.05). See text for absolute baseline concentrations.

Figure 2

Mean trials to criterion (± S.E.M.) for drug treatment groups in various stages of the attentional set-shifting task. Animals from each treatment group were tested in each of the stages of the task in the order indicated. All groups of rats readily acquired the single (SD) and compound (CD) discriminations. As expected, each group required more trials to learn the initial reversal (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. Kynurenine-treated rats exhibited marked deficits in the ability to make an extra-dimensional shift (ED), and this deficit was normalized following pretreatment with galantamine. (n = 5–6 rats/drug group). ^a = significantly different, within treatment group, from the trials to criterion for the CD stage; ^b = significantly different, within treatment group, from the trials to criterion for the ID stage (all P's < 0.05).

Figure 3

Representative photomicrograph showing bilateral placements of guide cannula for microdialysis probes that were used for the local perfusion of aCSF or galantamine into the medial PFC (Experiment 3). The left hemisphere placement depicts the tract of a cannula (arrow) implanted at a 20° rostral angle, while the right hemisphere placement depicts the tract implanted at a 20° caudal angle (arrow). The left and right guide cannulae terminated approximately 200 μm posterior and anterior, respectively, to this section.

Figure 4

Mean trials to criterion (± S.E.M.) for drug treatment groups in various stages of the attentional set-shifting task. The four drug conditions received an intra-PFC infusion of either aCSF or galantamine (5 μM) for 1 h prior to the onset of testing and, 15 min after the start of the perfusion, an i.p. injection of either saline or kynurenine (100 mg/kg). The results were strikingly similar to those seen in Figure 2. Neither local perfusion of aCSF nor the delivery of galantamine to saline-treated controls affected the results seen in Experiment 2 (Figure 2). All groups of rats readily acquired the single (SD) and compound (CD) discriminations but required more trials to learn the initial reversal (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. Finally, kynurenine-treated rats exhibited marked deficits in the ability to make an extra-dimensional shift (ED), and this deficit was normalized following the intra-PFC perfusion of galantamine. (n = 5 rats/drug group). ^a = significantly different, within treatment group, from the trials to criterion for the CD stage; ^b = significantly different, within treatment group, from the trials to criterion for the ID stage (all P's < 0.05).