The Impact of NMDA Receptor Blockade on Human Working Memory-Related Prefrontal Function and Connectivity

Preclinical research suggests that N-methyl-D-aspartate glutamate receptors (NMDA-Rs) have a crucial role in working memory (WM). In this study, we investigated the role of NMDA-Rs in the brain activation and connectivity that subserve WM. Because of its importance in WM, the lateral prefrontal cortex, particularly the dorsolateral prefrontal cortex and its connections, were the focus of analyses. Healthy participants (n=22) participated in a single functional magnetic resonance imaging session. They received saline and then the NMDA-R antagonist ketamine while performing a spatial WM task. Time-course analysis was used to compare lateral prefrontal activation during saline and ketamine administration. Seed-based functional connectivity analysis was used to compare dorsolateral prefrontal connectivity during the two conditions and global-based connectivity was used to test for laterality in these effects. Ketamine reduced accuracy on the spatial WM task and brain activation during the encoding and early maintenance (EEM) period of task trials. Decrements in task-related activation during EEM were related to performance deficits. Ketamine reduced connectivity in the DPFC network bilaterally, and region-specific reductions in connectivity were related to performance. These results support the hypothesis that NMDA-Rs are critical for WM. The knowledge gained may be helpful in understanding disorders that might involve glutamatergic deficits such as schizophrenia and developing better treatments.     

Neurobehavioral,testicular and erectile impairments of chronic ketamine administration: Pathogenesis and ameliorating effect of N-acetyl cysteine

Ketamine, a dissociative anesthetic, recently has spread as a recreational drug. Its abuse lead to neurobehavioral disturbance in addition to toxic effects on other body organs. To evaluate the toxic effects of chronic administration of low ketamine doses on the memory, testicles, and erection, explore its pathophysiology through oxidative stress mechanism and examine the ameliorating effect of N-acetyl cysteine (NAC). A total of 40 male albino rats were assigned to control, vehicle, ketamine only I.P. (10 mg/kg), and ketamine (10 mg/kg) + NAC (150 mg/kg) groups. Assessment of memory affection and erectile function by Passive Avoidance, Novel Object Recognition, and copulatory tests were performed. Estimation of malondialdehyde (MDA), catalase (CAT), and total antioxidant capacity (TAC) in serum and prefrontal & hippocampal homogenate, and luteinizing hormone (LH), testosterone in serum were done. Prefrontal cortex, hippocampus, and testes were collected for histopathology. Chronic ketamine administration induced significant memory deficits (P < 0.05), reduced erectile function (P < 0.05), severe hypospermatogenesis, increased MDA, reduced CAT, TAC levels in serum, and tissue homogenate (P < 0.05) and reduction of LH, and testosterone (P < 0.05). Treatment with NAC resulted in significant improvement of memory function, improved erectile function, and decrease in oxidative injury in both serum and tissue homogenates. Testosterone and LH levels exhibited significant difference between treatment groups and controls (P < 0.05). NAC reduced the deleterious histopathological changes. These data suggest that long-term ketamine affects short and long memory, induces erectile and testicular dysfunction through oxidative stress. Co-administration with NAC ameliorates these toxic effects.     

Ketamine impairs multiple cognitive domains in rhesus monkeys

Available evidence suggests that recreational use and abuse of the dissociative anaesthetic ketamine is increasing. Characterization of the cognitive risks of ketamine exposure contributes substantially to understanding this growing public health threat. Although prior human studies demonstrate that ketamine impairs a range of cognitive skills, investigation in nonhuman models permits more precise exploration of neurochemical mechanisms which may underlie detrimental behavioral effects. Adult male rhesus monkeys (N=7) were trained on a neuropsychological battery including tests of memory (delayed match-to-sample, DMS; self-ordered spatial search, SOSS), reaction time (RT), reinforcer efficacy and sustained attention (progressive ratio, PR) and fine motor coordination (bimanual motor skill, BMS). Battery performance was then serially challenged with acute doses of ketamine (0.3, 1.0, 1.78 mg/kg IM). Ketamine impaired DMS and SOSS in a dose x difficulty dependent manner with the most difficult task conditions disrupted at the 1.0 and 1.78 mg/kg doses. Thus, both visual recognition memory and working memory indices were affected. Ketamine also slowed RT and BMS performance and interfered with PR performance at the 1.78 mg/kg dose. Overall the present findings confirm that ketamine interferes with multiple aspects of cognition at subanesthetic doses in monkeys.     

Effects of ketamine on nociception and gastrointestinal motility in mice are unaffected by naloxone

The present study investigates the effects of ketamine on nociception towards chemical and thermic stimuli and on gastrointestinal transit in mice. The reversibility of these effects by the opioid antagonist naloxone (10 mg/kg) was also assessed. Ketamine promoted dose-related analgesia in both the acetic acid-induced writhing and hot plate tests. Analgesia was not influenced by pretreatment with naloxone. Contrasting the constipation induced by opioids, ketamine enhanced gastrointestinal transit in a dose-dependent manner and this was not modified by naloxone. These results suggest that although ketamine can elicit analgesia, it does not activate opioid mechanisms in subanesthetic doses.     

Effects of repeated treatment of phencyclidine on cognition and gene expression in C57BL/6 mice

A number of studies indicate that glutamatergic N-methyl-D-aspartate (NMDA) neurotransmission is disturbed in schizophrenia partly based on the findings that NMDA receptor antagonists such as phencyclidine (PCP) can reproduce a schizophrenia-like syndrome in both humans and rodents. This study investigated whether repeated administration of low doses of PCP can induce cognitive dysfunctions in mice at doses which produce no sensorimotor disturbances. In addition, the effects on cognition were related to the expression of two genes, Arc and spinophilin, which have been related to neuronal plasticity and learning. Adult male C57Bl/6J mice received daily s.c. doses of PCP (0.5-2.0 mg/kg) or saline for 7 d. Testing was performed 24 h after the last day of treatment. Only the 2.0 mg/kg PCP dose produced a consistent impairment in spatial learning and working memory performed in the water-maze task without any apparent sensorimotor deficits. Importantly, the 2.0 mg/kg PCP dose produced no impairment in a non-spatial learning paradigm in the water-maze task. PCP treatment altered Arc mRNA levels in the hippocampus and retrosplenial agranular cortex while leaving the striatum and prefrontal cortex unaffected. The mRNA expression of spinophilin was down-regulated in striatum by repeated PCP treatment. These results demonstrate that repeated treatment with low doses of PCP in mice can produce specific cognitive deficits which are associated with alterations in gene expression in brain regions that appear to play a role in the pathophysiology of schizophrenia. These results suggest that the low-dose PCP model may have significant potential in characterizing the behavioural and molecular mechanisms underlying cognitive deficits seen in schizophrenia patients.     

Positron emission tomographic studies of brain dopamine and serotonin transporters in abstinent (±)3,4-methylenedioxymethamphetamine (“ecstasy”) users: relationship to cognitive performance

BACKGROUND: (+/-)3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") is a recreational drug and brain serotonin (5-HT) neurotoxin. Under certain conditions, MDMA can also damage brain dopamine (DA) neurons, at least in rodents. Human MDMA users have been found to have reduced brain 5-HT transporter (SERT) density and cognitive deficits, although it is not known whether these are related. This study sought to determine whether MDMA users who take closely spaced sequential doses, which engender high plasma MDMA concentrations, develop DA transporter (DAT) deficits, in addition to SERT deficits, and whether there is a relationship between transporter binding and cognitive performance. MATERIALS AND METHODS: Sixteen abstinent MDMA users with a history of using sequential MDMA doses (two or more doses over a 3- to 12-h period) and 16 age-, gender-, and education-matched controls participated. Subjects underwent positron emission tomography with the DAT and SERT radioligands, [(11)C]WIN 35,428 and [(11)C]DASB, respectively. Subjects also underwent formal neuropsychiatric testing. RESULTS: MDMA users had reductions in SERT binding in multiple brain regions but no reductions in striatal DAT binding. Memory performance in the aggregate subject population was correlated with SERT binding in the dorsolateral prefrontal cortex, orbitofrontal cortex, and parietal cortex, brain regions implicated in memory function. Prior exposure to MDMA significantly diminished the strength of this relationship. CONCLUSIONS: Use of sequential MDMA doses is associated with lasting decreases in brain SERT, but not DAT. Memory performance is associated with SERT binding in brain regions involved in memory function. Prior MDMA exposure appears to disrupt this relationship. These data are the first to directly relate memory performance to brain SERT density.     

Hyperphosphorylated tau in the brains of mice and monkeys with long-term administration of ketamine

Ketamine, a non-competitive antagonist at the glutamatergic N-methyl-d-aspartate (NMDA) receptor, might impair memory function of the brain. Loss of memory is also a characteristic of aging and Alzheimer's disease. Hyperphosphorylation of tau is an early event in the aging process and Alzheimer's disease. Therefore, we aimed to find out whether long-term ketmaine administration is related to hyperphosphorylation of tau or not in the brains of mice and monkeys. Results showed that after 6 months’ administration of ketamine, in the prefrontal and entorhinal cortical sections of mouse and monkey brains, there were significant increases of positive sites for the hyperphosphorylated tau protein as compared to the control animals receiving no ketamine administration. Furthermore, about 15% of hyperphosphorylated tau positive cells were also positively labeled by terminal dUTP nick end labeling (TUNEL) indicating there might be a relationship between hyperphosphorylation of tau and apoptosis. Therefore, the long-term ketamine toxicity might involve neurodegenerative process similar to that of aging and/or Alzheimer's disease.     

Acute effects of ketamine on memory systems and psychotic symptoms in healthy volunteers.

N-methyl-D-aspartate (NMDA) receptor antagonists have been demonstrated to induce schizophrenia-like symptoms and cognitive impairment in humans. The NMDA receptor has been strongly implicated in memory, but research to date on the effects of NMDA antagonists has examined only some aspects of human memory functions. This study used a double-blind, placebo-controlled, independent groups design with 54 healthy volunteers to examine the effects of infusions of two doses (0.4, 0.8 mg/kg) of the NMDA antagonist ketamine upon the five human memory systems, aspects of executive functioning and schizophrenia-like and dissociative symptoms. Ketamine produced a dose-dependent impairment to episodic and working memory and a slowing of semantic processing. Ketamine also impaired recognition memory and procedural learning. Attention, perceptual priming and executive functioning were not affected following the drug. In addition, ketamine induced schizophrenia-like and dissociative symptoms, which were not correlated with the cognitive measures. These data suggest that, in humans, ketamine produces a selective pattern of impairments to working, episodic, and procedural memory but not to perceptual priming, attention or aspects of executive functioning.     

Effects of MK-801 and ketamine on short-term memory deficits in passive avoidance step-down task paradigm in mice.

The phenomenon of long-term potentiation (LTP) formation in hippocampal and neocortical brain areas has been suggested as a mechanism for learning and memory where NMDA-receptors play a significant role. Various agonists have been proposed to facilitate LTP and thereby learning and memory. Competitive and non-competitive antagonists of NMDA-receptors block LTP formation and produce attentional or acquisition deficit in animals. A series of experiments were carried out with noncompetitive NMDA antagonists, MK-801 (10-100 micrograms/kg) and ketamine (1-10 mg/kg), in passive avoidance step-down task paradigm in mice. MK-801 showed complete disruption of acquisition at higher doses, while very low doses showed improvement in retention. MK-801 showed additive or potentiating influence on scopolamine-induced deficits. The results of the interaction of NMDA antagonists with scopolamine provide a basis for the speculation that cholinergic- and NMDA-antagonism may play a hand in hand role in short-term memory disturbances in passive avoidance step-down task paradigm in mice.     

Selective cognitive impairments associated with NMDA receptor blockade in humans.

Hypofunction of the N-methyl-D-aspartate receptor (NMDAR) may be involved in the pathophysiology of schizophrenia. NMDAR antagonists like ketamine induce schizophrenia-like features in humans. In rodent studies, NMDAR antagonism impairs learning by disrupting long-term potentiation (LTP) in the hippocampus. This study investigated the effects of ketamine on spatial learning (acquisition) vs retrieval in a virtual Morris water task in humans. Verbal fluency, working memory, and learning and memory of verbal information were also assessed. Healthy human subjects participated in this double-blinded, placebo-controlled study. On two separate occasions, ketamine/placebo was administered and cognitive tasks were assessed in association with behavioral ratings. Ketamine impaired learning of spatial and verbal information but retrieval of information learned prior to drug administration was preserved. Schizophrenia-like symptoms were significantly related to spatial and verbal learning performance. Ketamine did not significantly impair attention, verbal fluency, or verbal working memory task performance. Spatial working memory was slightly impaired. In conclusion, these results provide evidence for ketamine's differential impairment of verbal and spatial learning vs retrieval. By using the Morris water task, which is hippocampal-dependent, this study helps bridge the gap between nonhuman animal and human NMDAR antagonism research. Impaired cognition is a core feature of schizophrenia. A better understanding of NMDA antagonism, its physiological and cognitive consequences, may provide improved models of psychosis and cognitive therapeutics.     

Acute and chronic effects of ketamine upon human memory: a review

Introduction Ketamine is attracting increasing research interest not only because of its powerful amnestic effects but also as a putative model of schizophrenia and as a substance with an expanding following of recreational users.Objective This article reviews the existing literature on the effects of acute ketamine on the memory of healthy volunteers and of repeated doses of ketamine in recreational users.Current trends Although there have been relatively few, often methodologically diverse, studies to date of the mnemonic effects of ketamine, there is an emerging consensus that an acute dose of the drug impairs the manipulation of information in working memory and produces decrements in the encoding of information into episodic memory. Preliminary evidence suggests that ketamine may differ from other classic amnestic drugs in impairing aspects of semantic memory. Acute-on-chronic effects in ketamine users generally mimic the pattern seen in controlled studies with healthy volunteers. However, chronic ketamine use may be associated with a more specific pattern of memory decrements and with episodic memory impairment, which might not abate following cessation of use.Future trends An important aim of future research should be to detail the specificity of ketamine’s amnestic effects on both a neuropharmacological and a cognitive level.     

Neonatal ketamine exposure results in changes in biochemical substrates of neuronal growth and synaptogenesis, and alters adult behavior irreversibly

Ketamine, an anaesthetic agent used in newborns and toddlers, has been shown to induce neurodegeneration and alter adult behavior in mice, when administered during the neonatal period. Mammals have a marked period of rapid brain growth and development (BGS), which is postnatal in mice and rats, spanning the first 3-4 weeks of life and reaching its peak around postnatal day 10. CaMKII and GAP-43 play important roles during the BGS in mammals. In the present study, 10 days old mice were exposed to 5-25 mg ketamine/kg bw and 24 h later brains were analyzed for calcium/calmodulin-dependent protein kinase II (CaMKII) and growth associated protein-43 (GAP-43) and at an age of 2 and 4 months the animals were tested for spontaneous behavior. The protein analysis showed that CaMKII increased significantly in hippocampus, but not in cortex, in animals 24h after exposure to ketamine. GAP-43 showed a significant increase in hippocampus, but a significant decrease in cortex for the highest ketamine dose. When looking at the adult behavior it was clear that neonatal ketamine exposure affected spontaneous behavior and habituation in a dose-response-related manner and that these behavioral disturbances were not transient but still persisted 2 months later. Taken together, this shows that ketamine affects important proteins involved in normal maturation of the brain and induce functional deficits in the adult individual, which further strengthen our findings concerning ketamine as a developmental neurotoxicological agent.     

The effects of sub-anaesthetic doses of ketamine on memory, cognitive performance and subjective experience in healthy volunteers

The cognitive and subjective effects of sub-anaesthetic doses of ketamine on healthy volunteers were examined. Twelve healthy volunteers received 25 mg ketamine, 10 mg ketamine and saline placebo, i.m. in a double-blind, Latin square design. A cognitive, perceptual and self-report test battery was administered over 45 min. The order of tests was rotated to control for timing effects. Ketamine (25 mg) significantly affected verbal learning and memory, parallel visual search, some measures of psychomotor performance, measures of arousal, subjective mood ratings and visual perception. Measures of attention and frontal lobe functioning were relatively unaffected. Thus, low doses of ketamine had selective, dose-related effects on memory, perceptual and psychomotor functions. The disruption of memory and perceptual processes may help to explain the unique subjective state induced by ketamine.     

Neuronal development of the hyperdopaminergic animal model

Dopamine transporter knockout (DAT KO) mice exhibited hyperdopaminergic tone in the nucleus accumbens and striatum, whereas they showed normal levels of extracellular dopamine in the prefrontal cortex. DAT KO mice showed numerous behavioral alterations that can be linked to abnormal dopaminergic function, including hyperlocomotion, deficits of prepulse inhibition (1PI) and impairment of working memory. PPI deficits were also shown in schizophrenic patients and hyperlocomotion was observed in AD/HD patients; therefore DAT KO mice had face validity for these psychiatric disorders. Impairment of neuronal development such as brain volume loss and decrease in spine density was reported especially in the prefrontal cortex of schizophrenia and AD/HD patients. We therefore investigated the neuronal development of DAT KO mice. Our results indicated that DAT KO mice had deficits of neuronal development in the prefrontal cortex similar to schizophrenia and AD/HD patients at least in part. These findings suggest that DAT KO mice are one of the useful models to investigate the impairment of neuronal development observed in psychiatric disorders including schizophrenia and AD/HD.     

Ketamine impairs response inhibition and is positively reinforcing in healthy volunteers: a dose–response study

Rationale Ketamine is an N-methyl-d-aspartate (NMDA) receptor antagonist that has medical indications but is also used as a recreational drug. Previous research has found persisting cognitive and psychotogenic effects of ketamine in chronic abusers of this drug 3 days after an acute dose.Objective The present study aimed to investigate the effects of ketamine on two processes related to drug abuse, response inhibition and reinforcement, and to examine whether an acute dose of ketamine produced residual cognitive effects in healthy volunteers.Methods Fifty-four healthy volunteers were given an 80-min infusion of one of two doses (0.4, 0.8 mg kg^–1) of ketamine or placebo. Subjects completed a battery of tests at three time points: pre-infusion, during the infusion and 3 days later at follow-up. The battery consisted of tests of episodic and semantic memory, schizophrenic-like and dissociative symptoms, response inhibition and measures of subjective effects, including mood, bodily symptoms and enjoyment of and desire for the drug.Results Ketamine acutely impaired response inhibition and had related biphasic effects on the subjective reinforcing effects of the drug. Ketamine also acutely impaired episodic but not semantic memory and increased schizophrenic-like and dissociative symptoms. No residual cognitive effects were observed 3 days following an acute dose.Conclusions The lack of residual effects in healthy volunteers on day 3 indicates that impairments found on day 3 in ketamine abusers are chronic effects. The abuse of ketamine may be related to its capacity both to reinforce and to decrease response inhibition.     

Sub-Anesthetic Ketamine Modulates Intrinsic BOLD Connectivity Within the Hippocampal-Prefrontal Circuit in the Rat

Dysfunctional connectivity within the hippocampal-prefrontal circuit (HC-PFC) is associated with schizophrenia, major depression, and neurodegenerative disorders, and both the hippocampus and prefrontal cortex have dense populations of N-methyl-D-aspartate (NMDA) receptors. Ketamine, a potent NMDA receptor antagonist, is of substantial current interest as a mechanistic model of glutamatergic dysfunction in animal and human studies, a psychotomimetic agent and a rapidly acting antidepressant. In this study, we sought to understand the modulatory effect of acute ketamine administration on functional connectivity in the HC-PFC system of the rat brain using resting-state fMRI. Sprague–Dawley rats in four parallel groups (N=9 per group) received either saline or one of three behaviorally relevant, sub-anesthetic doses of S-ketamine (5, 10, and 25 mg/kg, s.c.), and connectivity changes 15- and 30-min post-injection were studied. The strongest effects were dose- and exposure-dependent increases in functional connectivity within the prefrontal cortex and in anterior–posterior connections between the posterior hippocampus and retrosplenial cortex, and prefrontal regions. The increased prefrontal connectivity is consistent with ketamine-induced increases in HC-PFC electroencephalographic gamma band power, possibly reflecting a psychotomimetic aspect of ketamine''s effect, and is contrary to the data from chronic schizophrenic patients suggesting that ketamine effect does not necessarily parallel the disease pattern but might rather reflect a hyperglutamatergic state. These findings may help to clarify the brain systems underlying different dose-dependent behavioral profiles of ketamine in the rat.     

Impairments in water maze learning of aged rats that received dextromethorphan repeatedly during adolescent period

Rationale Dextromethorphan (DM), an over-the-counter cough suppressant, has been recently used as a drug of abuse by teenage groups in some countries, such as the United States, Canada, and Korea. We previously showed that repeated administration of DM, a noncompetitive antagonist of N-methyl-d-aspartate (NMDA) receptors, impairs spatial learning performance in adolescent rats. Objectives In the present study, long-term adverse effects of repetitive DM use at adolescence were examined in rats. Methods Male and female Sprague–Dawley rat pups received either intraperitoneal DM (40 mg/kg) or saline daily during postnatal days 28–37, and were then subjected to the Morris water maze task at the age of 18 months. Expression levels of NMDAR1, functional subunit of NMDA receptors, in the prefrontal cortex and the hippocampus were examined by Western blot analysis. Changes in plasma corticosterone levels responding to stress were determined by radioimmunoassay. Results DM-experienced male rats exhibited deficits in the probe trial, and female rats in the initial learning and the reversal training, in water maze performance. Expression levels of NMDAR1 in the brain regions were significantly increased in DM-experienced rats, compared to control rats. Stress-induced increases in plasma corticosterone levels were blunted both in male and female DM rats. Conclusions The results suggest that repeated administration of DM at high doses during adolescent period may induce permanent deficits in cognitive function and that increased expression of NMDAR1 in the prefrontal cortex and the hippocampus may take a role in DM-induced memory deficits.     

Effects of chronic ketamine use on frontal and medial temporal cognition

Background

Recreational ketamine use has been on the rise worldwide. Previous studies have demonstrated that it disrupts various memory systems, but few studies have examined how it affects learning and frontal functioning. The present study investigates the effects of repeated ketamine self-administration on frontal fluency, attention, learning, and memory along the verbal/nonverbal axis.

Methods

Twenty-five ketamine users and 30 healthy controls took a battery of neuropsychological tests. Frontal fluency was measured by the Verbal Fluency Test for semantic organization ability and the Figural Fluency Test for nonverbal executive functioning. Learning and memory were measured with the Chinese Auditory-Verbal Learning Test for acquisition and retention abilities of verbal information, as well as with the Continuous Visual Memory Test for nonverbal information. Participants also took several tests tapping subdomains of attention. To test for the potential effects of other drug use, 10 polydrug controls were included for comparison with the ketamine users and healthy controls.

Results

Ketamine users had impaired verbal fluency, cognitive processing speed, and verbal learning. Verbal learning impairment was strongly correlated with estimated lifetime ketamine use. Ketamine users showed no impairments in figural fluency, sustained attention, selective attention, visual learning, or verbal/nonverbal memory. However, heavier lifetime ketamine use was significantly correlated with deficits in verbal memory (both immediate recall and delayed recall) and visual recognition memory. Deficits in cognitive processing speed and verbal learning persisted even after polydrug controls were included in the control group, but their inclusion did make the impairment in verbal fluency barely reach statistical significance.

Conclusions

This study suggests that repeated ketamine use causes differential impairment to multiple domains of frontal and medial temporal functioning, possibly specific to verbal information processing.     

Impaired spatial memory after ketamine administration in chronic low doses

Ketamine is a noncompetitive antagonist of the NMDA-receptors, used as a dissociative anesthetic, presently included in the category of the psychoactive substances known as "club drugs". Ketamine administration was associated with impaired working memory and increased psychopathological symptoms, but there is a lack of information regarding the effects of chronic sub-anesthetic doses. Adult Wistar rats were administered ketamine, 5 and 10 mg/kg twice daily, subcutaneously for 14 days. One week later, rats were tested in an object recognition/object location task and in the open field arena. There was altered performance in both the object recognition/location and in the open field tests by the group chronically exposed to the lower dose of ketamine. These animals displayed a decreased discrimination index (p<0.05) in the object recognition task, were unable to recognize the displacement of a familiar object and displayed decreased activity across open filed sessions. Importantly, these alterations were not observed in animals administered a higher dose of ketamine. Collectively, these results consistently show that chronic administration of ketamine in sub-anesthetic doses may lead to decreased habituation and inability to update spatial representations.