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Psychopharmacology (Berl). Author manuscript; available in PMC 2020 June 24.
Published in final edited form as:
Psychopharmacology (Berl). 2020 June ; 237(6): 1723–1735. doi:10.1007/s00213-020-05495-y.
Activation of alpha7 nicotinic and NMDA receptors is necessary
for performance in a working memory task
David Phenis1,2, Sarah A. Vunck1, Valentina Valentini3, Hugo Arias4, Robert Schwarcz5,
John P. Bruno1,2
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1Department
of Psychology, The Ohio State University, Columbus, OH, USA
2Department
of Neuroscience, The Ohio State University, Columbus, OH, USA
3Department
of Biomedical Sciences, University of Cagliari, Cagliari, Italy
4Department
of Pharmacology and Physiology, College of Osteopathic Medicine, Oklahoma State
University Center for Health Sciences, Tahlequah, OK, USA
5Maryland
Psychiatric Research Center, University of Maryland School of Medicine, Baltimore,
MD, USA
Abstract
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Rationale—Working memory deficits are present in schizophrenia (SZ) but remain insufficiently
resolved by medications. Similar cognitive dysfunctions can be produced acutely in animals by
elevating brain levels of kynurenic acid (KYNA). KYNA’s effects may reflect interference with
the function of both the a7 nicotinic acetylcholine receptor (α7nAChR) and the glycineB site of
the NMDA receptor.
Objectives—The aim of the present study was to examine, using pharmacological tools, the
respective roles of these two receptor sites on performance in a delayed non-match-to-position
working memory (WM) task (DNMTP).
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Methods—DNMTP consisted of 120 trials/session (5, 10, and 15 s delays). Rats received two
doses (25 or 100 mg/kg, i.p.) of L-kynurenine (KYN; bioprecursor of KYNA) or L-4chlorokynurenine (4-Cl-KYN; bioprecursor of the selective glycineB site antagonist 7-Clkynurenic acid). Attenuation of KYN- or 4-Cl-KYN-induced deficits was assessed by coadministration of galantamine (GAL, 3 mg/kg) or PAM-2 (1 mg/kg), two positive modulators of
a7nAChR function. Reversal of 4-Cl-KYN- induced deficits was examined using D-cycloserine
(DCS; 30 mg/kg), a partial agonist at the glycineB site.
Results—Both KYN and 4-Cl-KYN administration produced dose-related deficits in DNMTP
accuracy that were more severe at the longer delays. In KYN-treated rats, these deficits were
John P. Bruno Bruno.1@osu.eduz.
Compliance with ethical standards
Animal care and experimentation was performed in accordance with protocols approved by The Ohio State University Institutional
Laboratory Animal Care and Use Committee and was consistent with the NIH Guide for the Care and Use of Laboratory Animals.
Publisher′s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Phenis et al.
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reversed to control levels by GAL or PAM-2 but not by DCS. In contrast, DCS eliminated
performance deficits in 4-Cl-KYN-treated animals.
Conclusions—These experiments reveal that both a7nAChR and NMDAR activity are necessary
for normal WM accuracy. They provide substantive new support for the therapeutic potential of
positive modulators at these two receptor sites in SZ and other major brain diseases.
Keywords
Delayed non-matching to position task; Kynurenic acid; Working memory; Galantamine; PAM-2;
D-cycloserine; 4-Cl-kynurenic acid
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The construct of working memory (WM) occupies a critical position in the series of
operations that support normal executive function or cognitive control and has been shown
to be impaired in patients with schizophrenia (SZ; Elvevag and Goldberg 2000; GoldmanRakic 1999; Park and Holzman 1992), major depressive disorder (MDD; Mohn and Rund
2016), and Alzheimer’s disease (AD; Castel et al. 2009). Cognitive deficits, including
impairments in attention (Luck and Gold 2008; Nuechterlein et al. 2009), WM (AbiDargham et al. 2002; Perlstein et al. 2001), and cognitive control (Everett et al. 2001; Thoma
et al. 2007), are in fact core symptoms of SZ, as they often present prior to the initial clinical
episode and are also observed in first-degree relatives who do not display the respective
phenotype (Snitz et al. 2006). Unfortunately, while the severity of these deficits is predictive
of the patient’s functional outcome (Green 1996; Green et al. 2004), these symptoms are not
effectively relieved by current medications or behavioral interventions (Bosia et al. 2017;
Nielsen et al. 2015). Thus, additional information on the neurochemical events involved in
these cognitive limitations, particularly when obtained from validated animal models and by
using cognitive tasks that reliably measure the relevant behavioral constructs, is highly
desirable and may lead to innovative treatment strategies in SZ as well as other disorders.
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Impaired executive function in SZ includes dysregulations of multiple brain circuits and
neurotransmitter systems. Of special relevance in this regard, dysfunctions in attention, WM,
and cognitive flexibility critically involve impairments of cholinergic (Berman et al. 2007)
and glutamatergic (Merritt et al. 2013; Paz et al. 2008; Thoma and Daum 2013)transmission
in the prefrontal cortex (PFC), an area involved in the mediation/regulation of higher order
cognition. Cholinergic abnormalities include reduced expression of the alpha 7 nicotinic
acetylcholine receptor (α7nAChR) in the dorsolateral PFC (Mathew et al. 2007), as well as
dysfunction of genes encoding the α7nAChR (Araud et al. 2011; Stefansson et al. 2008;
Zammit et al. 2007). With regard to glutamatergic transmission, a considerable number of
imaging and other studies in patients indicate hypofunctional NMDAR signaling within the
PFC (Bickel and Javitt 2009; Poels et al. 2014), though AMPA/kainate or metabotropic
glutamate receptors also appear to play distinct roles in the dysregulation of glutamatergic
signaling (Javitt 2012; Howes et al. 2015 for review).
An additional pathophysiological feature reported in patients with SZ and MDD that may
contribute significantly to both the cholinergic and glutamatergic dysfunctions discussed
above is a marked elevation in brain levels of the tryptophan metabolite kynurenic acid
(KYNA) (Erhardt et al. 2001; Miller et al. 2006; Sathyasaikumar et al. 2011; Schwarcz et al.
Psychopharmacology (Berl). Author manuscript; available in PMC 2020 June 24.
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2001). KYNA is derived from its immediate bioprecursor L-kynurenine (KYN), a pivotal
product of tryptophan metabolism, and is synthesized in astrocytes (Guillemin et al. 2001).
Upon release, nanomolar concentrations of KYNA negatively modulate α7nAChR function
in vivo (Alexander et al. 2013; Pocivavsek etal. 2012), though the underlying mechanism(s)
may be indirect and remain to be clarified (Albuquerque and Schwarcz 2013; Flores-Barrera
et al. 2017; Hilmas et al. 2001; Lopes et al. 2007; Stone 2019). In turn, antagonism of
presynaptic α7nAChRs decreases the release and tone of several major neurotransmitters,
including ACh (Zmarowski et al. 2009), glutamate (Beggiato et al. 2013; KonradssonGeuken et al. 2010; Potter et al. 2010; Wu et al. 2010), GABA (Beggiato et al. 2014), and
dopamine (Livingstone et al. 2010). At higher (low micromolar) concentrations, KYNA
competitively antagonizes the glycine co-agonist site of the NMDAR (the “glycineB”
receptor; Birch et al. 1988; Kessler et al. 1989; Stone and Darlington 2013). Not
surprisingly, therefore, acute elevations of brain KYNA in adult rodents result in
impairments that mimic several of the cognitive dysfunctions seen in SZ, including deficits
in sensorimotor gating (Erhardt et al. 2004), WM (Chess et al. 2007), contextual learning
(Chess et al. 2009; Pocivavsek et al. 2011), and cognitive flexibility (Alexander et al. 2012).
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The aims of the present study were twofold. First, we examined the impact of acute
elevations in brain KYNA on performance in a well-studied operant WM task by
administering KYN systemically to adult rats, using the delayed nonmatching to position
task (DNMTP) developed by Dunnett (1985) as an outcome measure. We then assessed the
respective roles of the α7nAChR and the glycineB receptor by pharmacological means. To
this end, we compared the cognition-impairing effects of KYN with those of 4-Clkynurenine (4-Cl-KYN), the bioprecursor of the selective glycineB receptor antagonist 7-Clkynurenic acid (7-Cl-KYNA; Kemp et al. 1988; Hilmas et al. 2001) and used positive
allosteric modulators (PAMs) of α7 nAChRs, including galantamine (GAL; Lopes et al.
2007; Wadenberg et al. 2017) and PAM-2 (3-furan-2-yl-N-p-tolyl-acrylamide) (Arias et al.
2011, 2016; Potasiewicz et al. 2015, 2017; Targowska-Duda et al. 2016), and the partial
glycineB receptor agonist D-cycloserine (DCS; Goff 2012) as tools to reverse performance
deficits.
Methods
Animals
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Adult male Wistar rats (Charles River Laboratory, Wilmington, MA) weighing 225–250 g at
the start of training were singly housed under a 12-h light/dark cycle (lights on 06:00–18:00
h). Food was available ad libitum, but water intake was progressively restricted over the
course of 1 week (water deprived successively 2, 4, 6, 12, 16, 20, and 22 h per day). Access
was then maintained at 2 h/day throughout the duration of the experiment (including the test
session). During this week, animals were also habituated to being handled, and weighed.
Animal care and experimentation was performed in accordance with protocols approved by
The Ohio State University Institutional Laboratory Animal Care and Use Committee and
was consistent with the NIH Guide for the Care and Use of Laboratory Animals.
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Drugs and treatment groups
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As the optimal doses, injection routes, and time intervals used for testing the central effects
of KYN and 4-Cl-KYN are well- established, the brain levels of newly produced KYNA and
7-Cl- KYNA were not measured in the present study. Specifically, the doses of KYN used
were based on previous work showing the detrimental effects of an acute administration of
this KYNA precursor on cognition (Alexander et al. 2012; Chess et al. 2007), and the doses
of 4-Cl-KYN were guided by the affinity of 7-Cl-KYNA to the NMDA/glycineB receptor
(IC50: 560 nM; Kemp et al. 1988). Thus, our i.p. administration of KYN rapidly raises the
concentration of extracellular KYNA (Alexander et al. 2012), and an i.p. injection of 4-ClKYN leads to the presence of extracellular 7-Cl-KYNA (Wu et al. 1997) in the brain. Both
these effects are dose-dependent, resulting in peak concentrations of ~ 40 nM KYNA
(Alexander et al. 2012; Bortz et al. 2017) and ~ 50 nM 7-Cl-KYNA (Linderholm et al. 2010;
Wu et al. 2000), respectively, approximately 2 h after the application of 100 mg/kg of the
precursor.
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Four groups were used in these experiments and are identified in Table 1 according to the
between- and within-group treatment differences. Three of the four groups were tested four
times in the task; the other (PAM-2 group) was tested three times. Table 1 also illustrates the
dosing schemes used. The first three groups tested the acute effects of KYN (Sai Advantium,
Hyderabad, India) on DNMTP performance as well as the effects of three putative cognition
enhancers (GAL, Sigma St. Louis, MO, USA; PAM-2, synthesized as in Arias et al., (2011);
DCS, Bioexpress, Kaysville, UT, USA) on affecting KYN-induced deficits. The fourth
group was used to determine if administration of 4-Cl-KYN (VistaGen Therapeutics Inc.,
Burlingame, CA, USA) impairs working memory and whether the co-agonist DCS
influences performance deficits in this paradigm. All drugs were dissolved in 0.9% saline
and administered (i.p.) in a volume of 1 ml/kg.
Testing the ability of GAL and PAM-2 to reverse deficits was guided by the assumption that
the doses used restricted these compounds to their abilities to selectively affect α7nAChR
function (Arias et al. 2011; Choueiry et al. 2019; Kita et al. 2013). The GAL dose was based
on recovery from deficits in an attentional set shifting task induced by acute administration
of KYN (Alexander et al. 2012), and the dose of PAM-2 was selected based on its ability to
effect promnesic and pro-cognitive activity through the potentiation of α7 nAChR function
(Potasiewicz et al. 2015; Targowska-Duda et al. 2016; Potasiewicz et al. 2017). The dose of
DCS was chosen based on rodent studies using a range of informative memory tasks
(Gabriele and Packard 2007; Leslie et al. 2012; Shaw et al. 2009; Vurbic et al. 2011).
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Training and testing procedure
All rats were trained in a DNMTP using a procedure that was modified from the original
protocol. A standard rat operant system (10 chambers; Med Associates, St Albans, VT,
USA) was used for training and testing. Each chamber was fitted with two fixed levers
positioned on either side of a central water dispenser. Two white stimulus lights were located
above each lever. Reinforcement was provided by water dispensed from a central reward
port after the appropriate lever response. A house light was positioned in the top center of
the back panel, opposite the levers and water dispenser. The training procedure involved 7
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phases: lever pressing, sample phase, choice phase (subdivided into 5, 10, and 15 s delays
sequentially added), a baseline performance acquisition phase testing that basal accuracy had
returned prior to drug challenges, and, finally, a WM testing phase during which drugs were
administered. Details regarding the criterion necessary for advancement to the next phase are
outlined below.
Habituation to operant chambers and lever pressing
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On day one of training, rats were placed in the operant chambers for 100 trials or a 60-min
duration (whichever came first) and trained with a continuous FR1 reinforcement procedure.
Each trial began when the house light was illuminated and levers were extended. If the rat
pressed either lever, a droplet of water (0.02 ml) was dispensed at the water port. The
development of a side bias was minimized by making each lever inactive after five
consecutive presses, reactivating it only after the opposite lever was pressed. Criterion for
advancement to sample phase training was met when both the left and right levers were
pressed a minimum of 50 times within 1 h.
Sample phase
During sample phase training, a signal light illumination was added as a condition for
reinforcement. Presentation of the left and right signals was balanced overall. Pressing the
lever signaled at the start of sample phase (i.e., match) resulted in a water reward (0.02 ml),
ending illumination of the signal light and the trial. In later stages of task training, for
example when choice phase is introduced, the position of the signal light during the sample
phase would determine the correct choice (i.e., non-matched) after the delay.
Delayed non-matching to position training and testing
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Subjects were trained with the addition of a DNMTP rule where a delay (5 s) was added
after the sample phase response. Both signal lights were then illuminated (choice phase). If
the rat pressed the lever opposite to the sample (i.e., non-matching position), a correct
response was recorded, both lights extinguished, and the rat was rewarded with a water
droplet (0.05 ml). If the rat pressed the same lever as the sample (i.e., matching), an
incorrect response was recorded, and the animal was not rewarded. The sample phase
continued to be rewarded with 0.02 ml of water, while the choice phase was rewarded with
0.05 ml of water. The sample phase was continually reinforced to ensure that the animal reoriented itself toward the reward dispenser, thus reducing the development of mediating
strategies (Dudchenko 2004; Dunnett 1985). The larger water reward for the choice phase
was provided to motivate completion of the full trial. Each trial was followed by a fixed
inter-trial interval (ITI) of 10 s during which the house light was not illuminated; each
session consisted of 120 trials. Advancement criterion for the 5 s delay length required >
70% correct responses and completion of at least 75% of the 120 trials for one session.
Once the advancement criterion was met, delay conditions (first 10 s, then 15 s) were added.
Delays were randomly presented and interspersed throughout the session with
approximately equal frequency. The final criteria for all three combined delays were: (a) >
80% overall correct responses, (b) > 80% correct in both 5 and 10 s delays, (c) > 75%
correct in 15 s delay, and (d) > 75% of trials completed. Once these criteria were met on
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three consecutive days, drug testing days were initiated, and rats were required to meet final
criteria for two consecutive training days before each successive drug test. This ensured that
animals maintained baseline performance, and that a minimum of 48 h passed between each
test. Approximately every 2 weeks animals were given at least a 24-h period of ad libitum
access to water to help maintain body fluid balance and renal health.
Omissions
During each trial, the rat had 3 s to respond to the sample lever. If the lever was not pressed
during this period, the signal and house lights were extinguished, and the event was recorded
(“sample omission”). This was followed by an ITI of 5 s with the house light off. Failure to
respond to the choice phase presentation after responding to a sample lever (“delay
omission”) was recorded separately for each delay (5,10, and 15 s).
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Statistical analysis
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Drug effects were evaluated using two-way repeated measures ANOVAs. Both delay and
drug conditions were within- subject factors, with performance determined at each delay
level and drug dose (n = 7–8 rats/group). The first set of ANOVAs was designed to isolate
the dose- and delay-dependent effects of KYN and 4-Cl-KYN. Following significant main
and interaction effects, a second set of ANOVAs was used to compare the effects of putative
cognition enhancers with the drug effects (including the vehicle/vehicle condition). Multiple
post hoc comparisons were conducted in order to compare correct trial percentage
(accuracy) at three different levels of delay and three different drug doses against vehicle
performance. In order to minimize the probability of Type II errors, comparisons were made
using a Bonferroni-corrected t test for dependent measures (Abdi 2007). Omission data were
compiled and analyzed separately for selected treatment groups using drug dose as the
independent variable (dose = 4 levels). The four groups were combined for acquisition
(means ± SEM). All comparisons were made using commercial software (Prism 5.0c,
GraphPad Software, Inc., 2004). P < 0.05 was considered statistically significant.
Results
Acquisition of DNMTP
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Rats in all treatment groups and conditions readily acquired each stage of the DNMTP task
as follows (expressed in days; mean ± SEM of all groups): Lever Press (1.2 ±0.1), Sample
Phase (2.7 ±0.2), Choice Phases (5 s, 8.2 ±0.5; 10 s, 8.7 ± 0.7; 15 s, 15.7 ±2.3), and
Stabilization of Baseline (13.7 ± 0.6). Total training time from lever press to initiation of
testing averaged 50.6±3.0 days. One rat was excluded from the KYN + GAL condition after
losing weight below our removal criterion of 15% loss from baseline.
Task performance
Acute KYN administration and the effect of galantamine—Initial testing revealed
that acute administration of KYN produced significant overall reductions in performance
accuracy on the DNMTP task when compared to the vehicle/vehicle condition (Fig. 1). As
outlined in the data analysis portion of the “Methods” section, the dose- and delay-impairing
effects of KYN were first calculated in the absence of the GAL treatment condition. The
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negative effects of KYN on performance became more pronounced as the dose of KYN
increased from 25 to 100 mg/kg (main effect of dose: F2, 36 = 54.40, P < 0.0001). There
was also an overall main effect of delay (F2,18 = 85.75, P < 0.0001). Thus, both vehicle- and
KYN-treated rats exhibited poorer task performance as the delay length between sample and
choice phases increased. Finally, we observed a greater sensitivity to the disrupting effects of
10 and 15 s delays in KYN-treated than in vehicle-treated animals (interaction effect: F4, 36
= 5.57, P = 0.001). The 5 s (P = 0.002) and 10 s (P = 0.0003) delays were affected only by
100 mg/kg KYN, whereas the longest delay was affected by both 25 mg/kg (P = 0.0036) and
100 mg/kg KYN (P <0.0001).
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Figure 1 also illustrates that co-administration of GAL (3 mg/kg) reversed the impairing
effects of KYN on performance in the DNMTP task to vehicle control levels at all delays.
As described in the data analysis section, a second set of ANOVAs was calculated to reveal
the pro-cognitive effects of GAL treatment. With the GAL treatment condition included, the
previously described main and interaction effects were preserved (delay: F2,18 = 65.21, P
<0.0001; dose: F3,54 = 52.72, P <0.0001; interaction: F6,54 = 5.29, P =0.0002). Post hoc
analyses revealed that GAL was most effective when tested under conditions that led to the
greatest deficits, i.e., at the higher dose of KYN (100 mg/kg) and at the longest delay (15 s)
(P < 0.0001 compared to KYN alone, P = 0.68 compared to vehicle).
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Acute KYN administration and the effect of PAM-2—A separate group of KYNtreated animals was used to test the effectiveness of PAM-2, a highly selective α7nAChR
PAM (Arias et al. 2011; Arias et al. 2016). To relate the results to those shown in Fig. 1, only
rats receiving 100 mg/kg KYN or vehicle were compared. An initial analysis excluding the
PAM-2 condition revealed main effects of KYN dose (F1,21 = 19.50; P = 0.0002), delay
(F2,21 = 8.26; P = 0.002), and a trend toward an interaction between dose and delay (F2,21
= 2.91; P = 0.07). An additional analysis including the PAM-2 condition revealed main
effects of group (F2,42= 13.97; P <0.0001), delay (F2,21 = 9.41; P = 0.001), and a trend
toward an interaction between group and delay (F4,42 = 2.25; P = 0.07). As shown in Fig. 2,
the KYN- induced deficit at the 15-s delay interval was completely abolished by the
administration of PAM-2 (1 mg/kg, P = 0.001), reaching an accuracy level that was
indistinguishable from vehicle-treated controls (P = 0.99).
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KYN-induced deficits: no reversal by the NMDA/glycineB agonist Dcycloserine with higher cognitive load—The next experiment was designed to assess
whether the impairing effects of KYN on task performance included, in addition to
α7nAChR-based mechanisms, antagonism of NMDA-mediated transmission. To this end,
we determined the ability of DCS, an NMDA/glycineB receptor agonist that does not bind to
α7nAChRs (Hood et al. 1989), to counteract the impairing effects of KYN (see Fig. 3).
We first confirmed that administration of KYN resulted in significant effects of dose (F2,36
= 38.06; P < 0.0001), delay (F2,18 = 49.84; P <0.0001), and an interaction between the two
(F4,36 = 3.34; P = 0.02). Including the DCS condition, we again found significant effects of
dose (F3,54 = 22.56; P< 0.0001), delay (F2,18 = 118.6; P < 0.0001), and an interaction
between the two (F6,54 = 3.34; P =0.007). However, unlike the case with GAL and PAM-2,
administration of DCS (30 mg/kg) did not counteract the deficits produced by KYN (100
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mg/kg; P = 0.99; Fig. 3) at the longest delay length (15 s). There was, however, a DCSmediated recovery of KYN-induced deficits (100 mg/kg; P =0.0046) at the medium delay
length (10 s).
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Acute blockade of NMDA/glycineB receptors: reversal of 4-Cl-KYN-induced
deficits—Previous studies have shown that administration of NMDA receptor antagonists
causes deficits in WM (Didriksen et al. 2007; Galizio et al. 2012). Here we determined
whether 4-Cl-KYN administration, resulting in the formation of the specific NMDA/
glycineB receptor antagonist 7-Cl-KYNA (Wu et al. 1997), would be sufficient to produce
deficits in WM. Acute administration of 4-Cl-KYN caused significant reductions in
accuracy in the DNMTP task compared to vehicle treatment (main effect of dose: F2,36 =
26.5, P <0.0001; Fig. 4), and the magnitude of the reduction increased with the length of the
delay (main effect of delay: F2,18 = 84.02, P < 0.0001). Notably, the lower dose of 4-ClKYN (25 mg/kg) affected only the intermediate and longest delays, whereas 100 mg/kg
reduced performance even at the shortest delay. For 4-Cl-KYN, there was no interaction
between delay and dose (F4,36= 1.142, P =0.35).
Regardless of delay length, co-administration of DCS (30 mg/kg) with 4-Cl-KYN (100
mg/kg) restored performance to levels that did not differ from vehicle-treated controls (all P
values >0.05; Fig. 4). With the DCS treatment condition included, the previously described
main effects were maintained (delay: F2,18 = 42.16, P <0.0001; dose: F3,54 = 21.17, P <
0.0001) while there was an interaction effect that trended toward statistical significance
(F6,54 = 2.116, P = 0.066).
Trials completed and task omissions
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The performance data were compiled from completed trials only, i.e., any trial that the
animal did not complete (by either not responding to the sample or choice phase of a trial)
was not incorporated into accuracy data. This prevented trials that were not completed (i.e.,
omissions) from influencing the accuracy results. We did, however, analyze the omission
data as a means of determining whether drug affects could reflect declines in motivational
state (i.e., satiety) or sensorimotor capacity that were then interpreted as a specific cognitive
effect on WM.
Acute KYN administration and the effect of galantamine on task omissions
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Initial testing revealed that acute administration of KYN produced significant increases in
the number of omissions when compared to the vehicle/vehicle condition (main effect of
dose: F2,18 = 59.40 P < 0.0001; Fig. 5). This effect was seen after administration of 100
mg/kg (P <0.0001) but not 25 mg/kg (P = 0.26) KYN. The increase in omissions occurred
due to reduced sample phase responding (sample omissions 39.86 ±2.34 vs. delay omissions
4.57 ±1.42). Additional analysis revealed that KYN significantly increased omissions,
relative to the vehicle condition, during the sample phase (F1,6 = 90.86, P <0.001) but not
the choice phase (F1,9 = 2.82, P = 0.12).
Notably, the restoration of performance accuracy by GAL in KYN-treated animals was not
accompanied by a reduction in the number of omissions. Co-administration of KYN and
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GAL was indistinguishable from the high dose KYN condition (P = 0.99) and remained
higher than the number of omissions compared to the vehicle/vehicle condition (F3,24 =
50.67, P <0.0001).
Acute 4-Cl-KYN administration and the effect of D-cycloserine on task
omissions—Acute administration of 4-Cl-KYN did not significantly change the number of
omissions when compared to the vehicle/vehicle condition (main effect of dose: F2,18 =
0.66 P = 0.52; Fig. 6). The restoration of performance accuracy by DCS also did not alter the
number of omissions compared to the vehicle/vehicle condition (F3,24 = 0.54, P <0.66). Coadministration of 4-Cl-KYN and DCS condition was indistinguishable from all other dose
conditions (P = 0.99).
Discussion
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The present study led to several novel and significant observations. First, an acute single
injection of KYN (the immediate bioprecursor of KYNA) reduced accuracy of adult rats in
the DNMTP WM task in both a dose- and delay-dependent fashion. Second, coadministration of GAL, an acetylcholinesterase inhibitor with positive allosteric modulatory
(PAM) activity on nAChRs (Lopes et al. 2007), or PAM-2, a highly selective PAM of
α7nAChRs (Arias et al. 2011, 2016; Targowska-Duda et al. 2016), eliminated KYN-induced
impairments and restored task performance to normal levels. Third, performance deficits in
KYN-treated rats were not reversed by addition of the glycineB/NMDA receptor agonist
DCS. Finally, acute de novo synthesis of the selective glycineB/NMDA antagonist 7-ClKYNA (via systemic injection of its precursor 4-Cl-KYN) also produced deficits in
performance accuracy which were similar in magnitude to those seen following KYN
administration. These detrimental effects of 4-Cl-KYN were prevented by co-administration
of DCS, a compound that was ineffective in alleviating deficits induced by KYN at the long
delay (i.e., high cognitive load). Collectively, these observations indicate that both α7nACh
and NMDA receptor function are required for normal WM. Moreover, they support the
conclusion that the cognitive impairments seen following acute elevations of KYNA,
through administration of up to 100 mg/kg of its precursor KYN (Alexander et al. 2012;
Chess et al. 2009; Pocivavsek et al. 2011), are mediated by reductions in α7nACh function
and not by NMDA receptors—a mechanism that may become operative, however, following
greater and more prolonged disruptions to the kynurenine pathway (Forrest et al. 2015). The
discussion that follows focuses on the mechanisms underlying the performance deficits
produced by KYN and 4-Cl-KYN.
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In our experiments, the magnitude of the impairments was dose- and delay-dependent—a
profile consistent with a deficit in the construct of WM (Goldman-Rakic 1994). As expected,
the greatest deficits were observed at the higher dose and at a longer delay interval (i.e., 15
s). Use of putative cognition enhancers such as GAL, PAM-2, and DCS as pharmacological
probes indicated that the impairments in WM induced by KYN, as well as those seen after
the administration of 4-Cl-KYN, were caused by identifiable and readily dissociable
neurochemical mechanisms. Specifically, though they cannot distinguish between direct and
indirect inhibition of the receptor (Albuquerque and Schwarcz 2013; Stone 2019), our
results support a critical role of α7nAChRs in the deleterious effects of KYN described in
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the present study. Thus, elimination of the KYN-induced WM deficits by GAL and PAM-2
indicates that the WM impairment was triggered by KYNA’s action as an effective
inhibitory modulator of α7nAChR function in vivo. This interpretation is in line with the
fact that α7nAChR inhibition also plays a central role in other cognitive impairments which
are seen in rodents after a systemic injection of 100 mg/kg KYN (Alexander et al. 2012;
Pocivavsek et al. 2011) and, more generally, and that both GAL and PAM-2 produce a
plethora of pro-cognitive and promnesic effects in rodents through their action on
α7nAChRs (Ludwig et al. 2010; Potasiewicz et al. 2015, 2017; Targowska-Duda et al. 2016;
Wadenberg et al. 2017). These effects were antagonized by methyllycaconitine (a selective
α7nAChR antagonist) and showed synergistic efficacy when co-administered with α7
nAChR-selective agonists, supporting the notion that α7* nAChR potentiation is involved in
these processes. Of special note in this context, though GAL acts as a cholinesterase
inhibitor at higher concentrations (Bickel et al. 1991), the drug’s ability to neutralize the
neurochemical consequences of increased KYNA levels (Beggiato et al. 2013, 2014; Wu et
al. 2010), as well as several other biochemical and electrophysiological effects of GAL (Kita
et al. 2013; Kroker et al. 2013; Schilström et al. 2007), are not duplicated by the
cholinesterase inhibitor donepezil, another established cognition enhancer. This evidence
supports the view that the effect elicited by GAL is mediated by its PAM activity on
endogenous α7* nAChRs. Although our results support an important role of α7* nAChRs in
WM, there is ample experimental evidence indicating that both α7nACh and NMDA
receptors interact in the modulation of circuits devoted to WM, including those involving the
hippocampus and PFC. For example, during early development, α7* nAChRs are needed for
glutamatergic synapse formation (Lozada et al. 2012), whereas α7nAChR gene deletion
decreases synaptic NMDAR levels and glutamate-related synaptic formation in cortical
neurons with concomitant cognitive deficits (Lin et al. 2014). On the other hand, α7nAChR
activation upregulates NMDARs (Li et al. 2013) and induces long-term potentiation (LTP)
(Mann and Greenfield 2003) in hippocampal neurons, and these effects, important for
learning and memory, are abolished by NMDAR blockers (Mann and Greenfield 2003) or
when the structural interaction between α7nAChRs and NMDARs is disrupted (Li et al.
2013).
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In marked contrast to GAL and PAM-2, DCS, a pro-cognitive partial agonist at the glycineB
site of the NMDA receptor (Gabriele and Packard 2007; Goff 2012; Leslie et al. 2012),
failed to counter the most detrimental effect of KYN on WM in our study. This provides
further support for the argument that NMDARs are not a primary target of KYNA in
mediating its adverse effects on cognition. This is of relevance since NMDAR blockade is
clearly linked to a number of cognitive dysfunctions including WM impairment (GonzalezBurgos and Lewis 2012; Neill et al. 2010; Rowland et al. 2005), and as higher
concentrations of KYNA inhibit the obligatory glycineB site of the NMDAR in a
competitive manner (Birch et al. 1988; Kessler et al. 1989). Indeed, DCS readily prevented
the WM dysfunction induced by 4-Cl-KYN, the bioprecursor of the selective glycineB
receptor antagonist 7-Cl-KYNA, in the present study.
Admittedly, the systemic route of administration of the precursors KYN and 4-Cl-KYN in
the present study makes it challenging to define the precise target region(s) which accounted
for the observed DNMTP deficits. However, we assume a central role of the PFC since the
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Phenis et al.
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integrity of prefrontal cholinergic and glutamatergic transmission is critical for several
components of cognitive control, including attention (Benn and Robinson 2014; Parikh and
Sarter 2008; St Peters et al. 2011) and WM (Aultman and Moghaddam 2001; Furey 2000).
Interestingly, acute administration of KYN not only raises extracellular KYNA levels but
also causes a secondary, GAL-sensitive decrease in extracellular glutamate in the PFC
(Alexander et al. 2012; Wu et al. 2010). In line with earlier studies demonstrating the
presence of functional α7nAChRs on glutamatergic nerve terminals in the PFC (Dickinson
et al. 2008; Marchi etal. 2002; Udakis etal. 2016), these datathere- fore suggest an attractive
mechanism, linking elevated prefrontal KYNA levels indirectly to glutamate hypofunction
and WM deficits. Taken together, the results described here provide significant support for
the hypothesis that KYNA malfunction in the brain is causally involved in the
pathophysiology of major psychiatric diseases (cf. Introduction).
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Finally, while the accuracy data were calculated on the basis of completed trials, the patterns
depicted in Figs. 5 and 6 suggest that the impairments seen in KYN- and 4-Cl-KYN- treated
animals did not simply reflect drug-induced reductions in motivation or sensorimotor
capacity. Moreover, the procognitive effects of GAL and DCS were clearly dissociable from
their inability to affect numbers of omissions. This dissociation of the omission effects of
KYN from performance is consistent with other findings showing increases in errors of
radial arm maze accuracy in the absence of changes in locomotor behavior or latency to
consume reward (Chess et al. 2007).
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Taken together, the data described here add substantive knowledge regarding the impact of
KYNA, a well-established, astrocyte-derived neuromodulator (Schwarcz and Stone 2017),
on cholinergic and glutamatergic processes involved in cognitive functions. Using
informative pharmacological tools, our experiments provide new conceptual insights through
proof-of-principle studies. From a therapeutic perspective, the results provide further
justification for accelerated development of cognition-enhancing agents based on the
positive modulation of both α7nACh and NMDA receptors (Koola 2018).
Acknowledgements
This work was supported by USPHS grants RO1 MH083729 (JB & RS) and P50 MH103222 (RS; Conte Center for
Translational Research).
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Fig. 1.
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Effects of acute kynurenine (KYN) administration, with and without galantamine (GAL), on
working memory performance in the delay non-match to position (DNMTP) task. Data
represent the percentage of correctly completed trials (mean ± SEM), in a single group of
rats (n = 7), during four drug sessions (order randomized: vehicle, 25 mg/kg KYN, 100
mg/kg KYN, 100 mg/kg KYN + 3 mg/kg GAL). Choice accuracy is depicted for all three
delay intervals (5, 10, 15 s). Acute administration of KYN resulted in a delay-dependent
reduction in accuracy that also interacted with drug dose. The low dose of KYN (25 mg/kg)
reduced choice accuracy relative to vehicle controls during 15s delay trials (but not during
the two shorter delays). The high dose of KYN (100 mg/kg) reduced choice accuracy during
all three delays. Co-administration of GAL (3 mg/kg) fully prevented the performance
deficit caused by KYN (100 mg/kg) during all three delays. Significance is denoted as P <
0.01 (**), P < 0.001 (***), and P <0.0001 (****)
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Fig.2.
Effects of acute kynurenine (KYN) administration, with and without PAM-2, on working
memory performance in the delay non-match to position (DNMTP) task. Data represent the
percentage of correctly completed trials (mean ± SEM), in a single group of rats (n = 8),
during three drug sessions (order randomized: vehicle, 100 mg/kg KYN, 100 mg/kg KYN +
1 mg/kg PAM-2). Choice accuracy is depicted for all three delay intervals (5, 10, 15 s).
Acute administration of KYN resulted in a reduction in accuracy that interacts with delay
length. KYN (100 mg/kg) reduced choice accuracy during the 10 and 15s delay trials but not
during 5 s delay trials. Co-administration of PAM-2 (1 mg/kg) prevented the performance
deficit caused by KYN (100 mg/kg) during the 10 and 15 s delay trials. Significance is
denoted as P < 0.05 (*), P <0.01 (**), and P <0.001 (***)
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Fig.3.
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Effects of acute kynurenine (KYN) administration, with and without D-cycloserine (DCS),
on working memory performance in the delay non-match to position (DNMTP) task. Data
represent the percentage of correctly completed trials (mean ± SEM), in a single group of
rats (n = 7), during four drug sessions (order randomized: vehicle, 25 mg/kg KYN, 100
mg/kg KYN, 100 mg/kg KYN + 30 mg/kg DCS). Choice accuracy is depicted for all three
delay intervals (5, 10, 15 s). Acute administration of KYN resulted in a delay-dependent
reduction in accuracy that also interacted with drug dose. Compared to vehicle, the low dose
of KYN (25 mg/kg) resulted in a deficit during 15 s delay trials but not during shorter delay
trials. The high dose of KYN (100 mg/kg) reduced choice accuracy during the 10 and 15 s
delay trials but not during 5 s delay trials. Co-administration of DCS (30 mg/kg) prevented
the performance deficit caused by KYN (100 mg/kg) during the 10 s delay trials but did not
significantly restore performance for 15 s delay trials. Significance is denoted as P <0.01
(**), P <0.001 (***) and P <0.0001 (****)
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Fig. 4.
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Effects of acute 4-chlorokynurenine (4-Cl-KYN) administration, with and without Dcycloserine (DCS), on working memory performance in the delay non-match to position
(DNMTP) task. Data represent the percentage of correctly completed trials (mean ± SEM),
in a single group of rats (n = 7), during four drug sessions (order randomized: vehicle, 25
mg/kg 4-Cl-KYN, 100 mg/kg 4-Cl-KYN, 100 mg/kg 4-Cl-KYN + 30 mg/kg DCS). Choice
accuracy is depicted for all three delay intervals (5, 10, 15 s). Acute administration of 4-ClKYN resulted in a delay- dependent reduction in accuracy that did not show a dose × delay
interaction. The low dose of 4-Cl-KYN (25 mg/kg) resulted in a deficit during 10 and the
15s delay trials but not during 5 s delay trials. The high dose of 4-Cl-KYN (100 mg/kg)
reduced choice accuracy during all three delays. Co-administration of DCS (30 mg/kg)
prevented the performance deficit caused by 4-Cl-KYN (100 mg/kg) during the 10 and 15 s
delay trials but did not significantly restore performance for 5 s delay trials. Significance is
denoted as P <0.05 (*), P <0.01 (**), P <0.001 (***), and P <0.0001 (****)
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Fig. 5.
Effects of acute kynurenine (KYN) administration, with and without galantamine (GAL), on
task omissions in the delay non-match to position (DNMTP) task. Data represent the number
of omissions (mean ± SEM), in a single group of rats (n = 7), during four drug sessions
(order randomized: vehicle, 25 mg/kg KYN, 100 mg/kg KYN, 100 mg/kg KYN + 3 mg/kg
GAL). Acute administration of KYN resulted in a dose-dependent increase in the number of
omissions. The high dose (100 mg/kg) but not the low dose (25 mg/kg) of KYN significantly
increased omissions compared to vehicle. Treatment with KYN (100 mg/kg) induced an
increase in omissions. Co-administration of GAL (3 mg/kg) did not return the number of
omissions to control levels despite the fact that accuracy values were normalized (cf. Fig. 1).
Significance is denoted as P < 0.0001 (****)
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Fig. 6.
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Effects of acute 4-chlorokynurenine (4-Cl-KYN) administration, with and without Dcycloserine (DCS), on task omissions in the delay non-match to position (DNMTP) task.
Data represent the number of omissions (mean ± SEM), in a single group of rats (n = 7),
during four drug sessions (order randomized: vehicle, 25 mg/kg 4-Cl-KYN, 100 mg/kg 4Cl-KYN, 100 mg/kg 4-Cl-KYN+ 30 mg/kg DCS). Acute administration of 4-Cl-KYN did
not significantly increase the number of omissions at either the high dose (100 mg/kg) or the
low dose (25 mg/kg). Co-administration of 4-Cl-KYN and DCS also had no significant
effect on the number of omissions despite the fact that DCS returned choice accuracy to
control levels (cf. Fig. 4)
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Table 1
Treatment group
Within subject injections
Kynurenine + galantamine
Vehicle + vehicle
25 mg/kg KYN + vehicle
100 mg/kg KYN + vehicle
100 mg/kg KYN + 3 mg/kg GAL
Kynurenine + PAM-2
Vehicle + vehicle
n/a
100 mg/kg KYN + vehicle
100 mg/kg KYN + 1 mg/kg PAM-2
Psychopharmacology (Berl). Author manuscript; available in PMC 2020 June 24.
Kynurenine + D-cycloserine
Vehicle + vehicle
25 mg/kg KYN + vehicle
100 mg/kg KYN + vehicle
100 mg/kg KYN + 30 mg/kg DCS
4-Chlorokynurenine + D-cycloserine
Vehicle + vehicle
25 mg/kg 4-Cl-KYN + vehicle
100 mg/kg 4-Cl-KYN + vehicle
100 mg/kg 4-Cl-KYN + 30 mg/kg DCS
Phenis et al.
Summary of treatment groups and within subject injections
Four separate treatment groups of rats are represented in these experiments. The first column lists the drug used, as a between subject factor, to impair task performance (KYN kynurenine, 4-Cl-KYN 4chlorokynurenine). The next four columns represent the individual injections given to all animals within each treatment group prior to each DNMTP drug session to reinstate performance (vehicle, GAL
galantamine, PAM-2 3-furan-2-yl-N-p-tolyl-acrylamide, DCS D-cycloserine). The order of injections was randomized with a Latin Square to prevent order effects
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