Hippocampal N-methyl-d-aspartatereceptor-mediated encoding and retrieval processes in spatial working memory: Delay-interposed radial maze performance in rats

Effects of chronic forced-swim stress on behavioral properties in rats with neonatal repeated MK-801 treatment

The two-hit hypothesis has been used to explain the onset mechanism of schizophrenia. It assumes that predisposition to schizophrenia is originally attributed to vulnerability in the brain which stems from genetic or early developmental factors, and that onset is triggered by exposure to later detrimental factors such as stress in adolescence or adulthood. Based on this hypothesis, the present study examined whether rats that had received neonatal repeated treatment with an N-methyl-d-aspartate (NMDA) receptor antagonist (MK-801), an animal model of schizophrenia, were vulnerable to chronic stress. Rats were treated with MK-801 (0.2mg/kg) or saline twice daily on postnatal days 7-20, and animals in the stress subgroups were subjected to 20days (5days/week×4weeks) of forced-swim stress in adulthood. Following this, behavioral tests (prepulse inhibition, spontaneous alternation, open-field, and forced-swim tests) were carried out. The results indicate that neonatal repeated MK-801 treatment in rats inhibits an increase in immobility in the forced-swim test after they have experienced chronic forced-swim stress. This suggests that rats that have undergone chronic neonatal repeated NMDA receptor blockade could have a reduced ability to habituate or adapt to a stressful situation, and supports the hypothesis that these rats are sensitive or vulnerable to stress.

Differential requirements of hippocampal de novo protein and mRNA synthesis in two long-term spatial memory tests: Spontaneous place recognition and delay-interposed radial maze performance in rats

Hippocampal de novo mRNA and protein synthesis has been suggested to be critical for long-term spatial memory. However, its requirement in each memory process (i.e. encoding, consolidation and retrieval) and the differences in the roles of de novo mRNA and protein synthesis in different situations where spatial memory is tested have not been thoroughly investigated. To address these questions, we examined the effects of hippocampal administration of the protein synthesis inhibitors, anisomycin (ANI) and emetine (EME), as well as that of an mRNA synthesis inhibitor, 5,6-dichlorobenzimidazole 1-β-D-ribofuranoside (DRB), on rat performance in two long-term spatial memory tests. In a spontaneous place recognition test with a 6 h delay, ANI, administered either before or immediately after the sample phase, but not before the test phase, eliminated the exploratory preference for the object in a novel place. This amnesic effect was replicated by both EME and DRB. In a 6 h delay-interposed radial maze task, however, administering ANI before the first-half and before the second-half, but not immediately or 2 h after the first-half, impaired performance in the second-half. This disruptive effect of ANI was successfully replicated by EME. However, DRB administered before the first-half performance did not impair the second-half performance, while it did impair it if injected before the second-half. None of these drugs caused amnesic effects during the short (5 min)/non-delayed conditions in either tests. These results suggest that 1) hippocampal protein synthesis is required for the consolidation of spatial memory, while mRNA synthesis is not necessarily required, and 2) hippocampal mRNA and protein synthesis requirement for spatial memory retrieval depends on the types of memory tested, probably because their demands are different.

MK-801 and memantine act differently on short-term memory tested with different time-intervals in the Morris water maze test

N-methyl-d-aspartate receptors (NMDARs) play a crucial role in spatial memory formation. In neuropharmacological studies their functioning strongly depends on testing conditions and the dosage of NMDAR antagonists. The aim of this study was to assess the immediate effects of NMDAR block by (+)MK-801 or memantine on short-term allothetic memory. Memory was tested in a working memory version of the Morris water maze test. In our version of the test, rats underwent one day of training with 8 trials, and then three experimental days when rats were injected intraperitoneally with low- 5 (MeL), high - 20 (MeH) mg/kg memantine, 0.1mg/kg MK-801 or 1ml/kg saline (SAL) 30min before testing, for three consecutive days. On each experimental day there was just one acquisition and one test trial, with an inter-trial interval of 5 or 15min. During training the hidden platform was relocated after each trial and during the experiment after each day. The follow-up effect was assessed on day 9. Intact rats improved their spatial memory across the one training day. With a 5min interval MeH rats had longer latency then all rats during retrieval. With a 15min interval the MeH rats presented worse working memory measured as retrieval minus acquisition trial for path than SAL and MeL and for latency than MeL rats. MK-801 rats had longer latency than SAL during retrieval. Thus, the high dose of memantine, contrary to low dose of MK-801 disrupts short-term memory independent on the time interval between acquisition and retrieval. This shows that short-term memory tested in a working memory version of water maze is sensitive to several parameters: i.e., NMDA receptor antagonist type, dosage and the time interval between learning and testing.

The effect of AMPA receptor blockade on spatial information acquisition, consolidation and expression in juvenile rats

Improvement on spatial tasks in rats is observed during a late, postnatal developmental period (post-natal day (PND) 18 - PND 20). The developmental emergence of this spatial function occurs in conjunction with hippocampal connectivity changes and enhanced hippocampal-AMPA receptor-mediated synaptic responses. The current work investigated the effect of AMPAr blockade on the emergence and long-term storage of spatial information in juvenile rats and associated neural activity patterns in the dorsal hippocampus CA1 region. Male, Long Evans rats between the ages of PND 18 and PND 20 were systemically (i.p.) administered the AMPAr antagonist, NBQX, (0, 5 or 10mg/kg) every day prior to hidden platform water maze training (PND 18, 19 and 20), every day immediately post-training or immediately before the probe test (PND 41). NBQX administration prior to training prolonged latencies, pathlength and increased thigmotaxis during the acquisition phase. Administration of NBQX immediately posttraining had no effect on the day-to-day performance. When given a probe test 3weeks later, the saline group across all conditions spent more time in the target quadrant. Rats treated with pretraining 5mg NBQX dose showed a preference for the target quadrant while the posttraining and pretesting 5mg NBQX doses impaired the target quadrant preference. Groups injected with 10mg of NBQX pretraining, posttraining or pretesting did not show a preference for the target quadrant. c-Fos labeling in the CA1 reflected these differences in probe performance in that groups showing greater than chance dwell time in the target quadrant showed more c-Fos labeling in the CA1 region than groups that did not show a target quadrant preference. These findings provide support for the critical role of AMPA receptor-mediated function in the organization and long-term storage of spatial memories acquired during the juvenile period.

Hippocampus NMDA receptors selectively mediate latent extinction of place learning

Extinction of maze learning may be achieved with or without the animal performing the previously acquired response. In typical "response extinction," animals are given the opportunity to make the previously acquired approach response toward the goal location of the maze without reinforcement. In "latent extinction," animals are not given the opportunity to make the previously acquired response and instead are confined to the previous goal location without reinforcement. Previous evidence indicates that the effectiveness of these protocols may depend on the type of memory being extinguished. Thus, one aim of the present study was to further examine the effectiveness of response and latent extinction protocols across dorsolateral striatum (DLS)-dependent response learning and hippocampus-dependent place learning tasks. In addition, previous neural inactivation experiments indicate a selective role for the hippocampus in latent extinction, but have not investigated the precise neurotransmitter mechanisms involved. Thus, the present study also examined whether latent extinction of place learning might depend on NMDA receptor activity in the hippocampus. In experiment 1, adult male Long-Evans rats were trained in a response learning task in a water plus-maze, in which animals were reinforced to make a consistent body-turn response to reach an invisible escape platform. Results indicated that response extinction, but not latent extinction, was effective at extinguishing memory in the response learning task. In experiment 2, rats were trained in a place learning task, in which animals were reinforced to approach a consistent spatial location containing the hidden escape platform. In experiment 2, animals also received intra-hippocampal infusions of the NMDA receptor antagonist 2-amino-5-phosphopentanoic acid (AP5; 5.0 or 7.5 ug/0.5 µg) or saline vehicle immediately before response or latent extinction training. Results indicated that both extinction protocols were effective at extinguishing memory in the place learning task. In addition, intra-hippocampal AP5 (7.5 µg) impaired latent extinction, but not response extinction, suggesting that hippocampal NMDA receptors are selectively involved in latent extinction. © 2016 Wiley Periodicals, Inc.

Involvement of hippocampal NMDA receptors in retrieval of spontaneous object recognition memory in rats

The involvement of hippocampal N-methyl-d-aspartate (NMDA) receptors in the retrieval process of spontaneous object recognition memory was investigated. The spontaneous object recognition test consisted of three phases. In the sample phase, rats were exposed to two identical objects several (2-5) times in the arena. After the sample phase, various lengths of delay intervals (24h-6 weeks) were inserted (delay phase). In the test phase in which both the familiar and the novel objects were placed in the arena, rats' novel object exploration behavior under the hippocampal treatment of NMDA receptor antagonist, AP5, or vehicle was observed. With 5 exposure sessions in the sample phase (experiment 1), AP5 treatment in the test phase significantly decreased discrimination ratio when the delay was 3 weeks but not when it was one week. On the other hand, with 2 exposure sessions in the sample phase (experiment 2) in which even vehicle-injected control animals could not discriminate the novel object from the familiar one with a 3 week delay, AP5 treatment significantly decreased discrimination ratio when the delay was one week, but not when it was 24h. Additional experiment (experiment 3) showed that the hippocampal treatment of an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, NBQX, decreased discrimination ratio with all delay intervals tested (24h-3 weeks). Results suggest that hippocampal NMDA receptors play an important role in the retrieval of spontaneous object recognition memory especially when the memory trace weakens.

Frontal cortex and hippocampus neurotransmitter receptor complex level parallels spatial memory performance in the radial arm maze

Several neurotransmitter receptors have been proposed to be involved in memory formation. However, information on receptor complexes (RCs) in the radial arm maze (RAM) is missing. It was therefore the aim of this study to determine major neurotransmitter RCs levels that are modulated by RAM training because receptors are known to work in homo-or heteromeric assemblies. Immediate early gene Arc expression was determined by immunohistochemistry to show if prefrontal cortices (PFC) and hippocampi were activated following RAM training as these regions are known to be mainly implicated in spatial memory. Twelve rats per group, trained and untrained in the twelve arm RAM were used, frontal cortices and hippocampi were taken, RCs in membrane protein were quantified by blue-native PAGE immunoblotting. RCs components were characterised by co-immunoprecipitation followed by mass spectrometrical analysis and by the use of the proximity ligation assay. Arc expression was significantly higher in PFC of trained as compared to untrained rats whereas it was comparable in hippocampi. Frontal cortical levels of RCs containing AMPA receptors GluA1, GluA2, NMDA receptors GluN1 and GluN2A, dopamine receptor D1, acetylcholine nicotinic receptor alpha 7 (nAChR-α7) and hippocampal levels of RCs containing D1, GluN1, GluN2B and nAChR-α7 were increased in the trained group; phosphorylated dopamine transporter levels were decreased in the trained group. D1 and GluN1 receptors were shown to be in the same complex. Taken together, distinct RCs were paralleling performance in the RAM which is relevant for interpretation of previous and design of future work on RCs in memory studies.

Low-dose memantine-induced working memory improvement in the allothetic place avoidance alternation task (APAAT) in young adult male rats

N-methyl-D-aspartate receptors (NMDAR) are involved in neuronal plasticity. To assess their role simultaneously in spatial working memory and non-cognitive learning, we used NMDAR antagonists and the Allothetic Place Avoidance Alternation Task (APAAT). In this test rats should avoid entering a place where shocks were presented on a rotating arena which requires cognitive coordination for the segregation of stimuli. The experiment took place 30 min after intraperitoneal injection of memantine (5, 10, 20 mg/kg b.w.: MemL, MemM, MemH, respectively) and (+)MK-801 (0.1, 0.2, 0.3 mg/kg b.w.: MK-801L, MK-801M, MK-801H, respectively). Rats from the control group were intact or injected with saline (0.2 ml/kg). Over three consecutive days the rats underwent habituation, two avoidance training intervals with shocks, and a retrieval test. The shock sector was alternated daily. The after-effects of the agents were tested on Day 21. Rats treated with low dose memantine presented a longer maximum time avoided and fewer entrances than the MemH, MK-801M, MK-801H and Control rats. The shocks per entrances ratio, used as an index of cognitive skill learning, showed skill improvement after D1, except for rats treated by high doses of the agents. The activity levels, indicated by the distance walked, were higher for the groups treated with high doses of the agents. On D21 the MK801H rats performed the memory task better than the MemH rats, whereas the rats' activity depended on condition, not on the group factor. These results suggest that in naïve rats mild NMDAR blockade by low-dose memantine improves working memory related to a highly challenging task.

Role of a hippocampal SRC-family kinase-mediated glutamatergic mechanism in drug context-induced cocaine seeking

Glutamatergic neurotransmission in the dorsal hippocampus (DH) is necessary for drug context-induced reinstatement of cocaine-seeking behavior in an animal model of drug relapse. Furthermore, in vitro studies suggest that the Src family of tyrosine kinases critically regulates glutamatergic cellular functions within the DH. Thus, Src-family kinases in the DH may similarly control contextual cocaine-seeking behavior. To test this hypothesis, rats were trained to lever press for un-signaled cocaine infusions in a distinct context followed by extinction training in a different context. Cocaine-seeking behavior (non-reinforced active lever pressing) was then assessed in the previously cocaine-paired and extinction contexts after AP5 (N-methyl-D-aspartate glutamate (NMDA) receptor (NMDAR) antagonist; 0.25 or 2.5 μg/0.5 μl/hemisphere), PP2 (Src-family kinase inhibitor; 6.25 or 62.5 ng/0.5 μl/hemisphere), Ro25-6981 (NR2B subunit-containing NMDAR antagonist; 0.2 or 2 μg/0.5 μl/hemisphere), or vehicle administration into the DH. Administration of AP5, PP2, or Ro25-6981 into the DH dose-dependently impaired drug context-induced reinstatement of cocaine-seeking behavior relative to vehicle, without altering instrumental behavior in the extinction context or food-reinforced instrumental responding and general motor activity in control experiments. Cocaine-seeking behavior during the first 20 min of the test session in the cocaine-paired context was associated with an increase in NR2B subunit activation, and intra-DH PP2 pretreatment disrupted this relationship. Together, these findings suggest that Src-family kinase activation, NMDAR stimulation, and likely Src-family kinase-mediated NR2B subunit-containing NMDAR activation in the DH are necessary for incentive motivational and/or memory processes that promote contextual cocaine-seeking behavior.

Effect of exercise training on long-term potentiation and NMDA receptor channels in rats with cerebral infarction

The aim of the present study was to investigate the effects of exercise training on the characteristics of long-term potentiation (LTP) and N-methyl-D aspartate (NMDA) receptor channels in the hippocampal CA3 neurons of rats with cerebral infarction. Wistar rats were randomly allocated into the model without any training and rehabilitation with exercise training. A model of cerebral infarction was established by middle cerebral artery occlusion. Using chronically embedded electrodes combined with an electrophysiological method, the population spike (PS) amplitude and latency, as well as changes in the NMDA single channel current in the hippocampal neurons were determined prior to and following Y-maze discrimination learning 60 times in the two groups. The formation of learning-dependent LTP and synaptic efficacy in the hippocampal CA3 area after exercise training in the rehabilitation group was significantly faster compared with that in the model group without any training (P<0.05). The incubation period of the PS in the CA3 area of the rats in the rehabilitation group was significantly shorter compared with that in the model group. The PS amplitude in the rehabilitation group was significantly higher compared with that in the model group. Furthermore, the opening probability of the NMDA receptor channel in the rehabilitation group was significantly higher compared with that in the model group. In conclusion, exercise training improved the opening conductance level, time and probability of NMDA receptor channels and accelerated the formation of learning-dependent LTP in the contralateral hippocampal CA3 area.

Robust training attenuates TBI-induced deficits in reference and working memory on the radial 8-arm maze

Globally, it is estimated that nearly 10 million people sustain severe brain injuries leading to hospitalization and/or death every year. Amongst survivors, traumatic brain injury (TBI) results in a wide variety of physical, emotional and cognitive deficits. The most common cognitive deficit associated with TBI is memory loss, involving impairments in spatial reference and working memory. However, the majority of research thus far has characterized the deficits associated with TBI on either reference or working memory systems separately, without investigating how they interact within a single task. Thus, we examined the effects of TBI on short-term working and long-term reference memory using the radial 8-arm maze (RAM) with a sequence of four baited and four unbaited arms. Subjects were given 10 daily trials for 6 days followed by a memory retrieval test 2 weeks after training. Multiple training trials not only provide robust training, but also test the subjects' ability to frequently update short-term memory while learning the reference rules of the task. Our results show that TBI significantly impaired short-term working memory function on previously acquired spatial information but has little effect on long-term reference memory. Additionally, TBI significantly increased working memory errors during acquisition and reference memory errors during retention testing 2 weeks later. With a longer recovery period after TBI, the robust RAM training mitigated the reference memory deficit in retention but not the short-term working memory deficit during acquisition. These results identify the resiliency and vulnerabilities of short-term working and long-term reference memory to TBI in the context of robust training. The data highlight the role of cognitive training and other behavioral remediation strategies implicated in attenuating deficits associated with TBI.

Can rats control previously acquired spatial information? Evidence of "directed forgetting" phenomenon in delay-interposed radial maze behavior

"Directed forgetting" is a method and a phenomenon used to investigate whether organisms can control memory with instructions to forget, and to remember. We examined directed forgetting phenomenon in rats, using a modified radial maze task. In the delay-interposed radial maze task, a 120-min delay followed four correct choices, during which rats were placed in either a white or black box, that signaled whether a second-half performance will be required (remember (R)-trial), or not (forget (F)-trial) after the delay. R- and F-trials were repeated 5 times each, followed by a "probe test," in which the second-half performance was conducted, although the F-cue was presented during the delay period. Rats made more errors in the probe test than in the R-trial. These results demonstrate the "directed forgetting" phenomenon in a radial maze task in rats, and suggest that rats can indeed control previously acquired spatial information, even situations with a long retention interval interposed. Although the mechanism by which presentation of the "forget-cue" influences rats' information processing remains unclear, several brain regions, including the medial prefrontal cortex and the hippocampal CA3 area, were shown to be more activated in the "remember"-cued trial than in the "forget"-cued trial, suggesting that these areas are potentially related to the "directed forgetting" phenomenon of spatial information demonstrated in the study.

MK-801 increases the baseline level of anxiety in mice introduced to a spatial memory task without prior habituation

C57BL/6J mice were introduced to a nine arm radial maze without prior habituation and trained in the acquisition of a working memory task in 16 sessions, one session per day. In this maze mice need to climb onto an upward inclined bridge in order to reach and cross onto an arm. They received in each session an i.p. injection of MK-801 (0.1 mg/kg) 30 min before training or immediately after training. MK-801 pre-treated mice made significantly more entries onto the bridges, fewer entries onto the arms and took significantly longer time to make a first arm visit compared to saline and MK-801 post-treated mice during the first 3 session blocks (4 sessions per block). These results indicate that MK-801 induced anxiety which was extended throughout the first 3 session blocks. MK-801 pre-treated mice made also significantly more errors and required more sessions to reach the criterion compared to saline and MK-801 post-treated mice. Administration of MK-801 after training did not affect the acquisition of the task. The present results indicate that MK-801 pre-treatment impaired the acquisition of a spatial task and this can be accounted for by its effect on the baseline level of anxiety which was elevated. The introduction of mice to the acquisition of the task without prior habituation demonstrates that a drug treatment can affect learning and memory by increasing and/or prolonging anxiety. Such effect may be confounded with learning and memory performance and not detected with pre-habituation training procedures, particularly when the number of sessions is determined a-priori.

Behavioural studies with a newly developed neuroprotective KYNA-amide

The neuroactive properties and neuroprotective potential of endogenous L: -kynurenine, kynurenic acid (KYNA) and its derivatives are well established. KYNA acts as an antagonist on the obligatory co-agonist glycine site, and has long been at the focus of neuroprotective trials. Unfortunately, KYNA is barely able to cross the blood-brain barrier. Accordingly, the development and synthesis of KYNA analogs which can readily cross the BBB have been at the focus of research interest with the aim of neuroprotection. Earlier we reported a new KYNA-amide crosses the BBB and proved neuroprotective in several experiments. In the present study, we investigated the locomotor activity, working memory performance, and also the long-lasting, consolidated reference memory of animals treated intraperitoneally (i.p.) with the novel analog. The effects of the novel analog on the spatial orientation and learning ability of rats were assessed in the Morris water maze (MWM) paradigm. The effects on locomotor activity of mice was assessed in the open field (OF) paradigm, and those on the spatial orientation and learning ability of mice were investigated in the radial arm maze (RAM) paradigm. It emerged that there is a dose of this KYNA-amide which is neuroprotective, but does not worsen the cognitive function of the brain. This result is significant in that a putative neuroprotectant without adverse cognitive side-effects is of great benefit.

Effects of histamine H(1) receptor antagonists on delayed spatial win-shift task in rats

We investigated the effects of histamine H(1) receptor antagonists in 3 different stages of spatial working memory in comparison with scopolamine. The effects of drugs were measured using an eight-arm radial maze with a delayed spatial win-shift task in rats. Drugs were injected 10 min before the training phase (timing 1), immediately after the training phase (timing 2), or 10 min before the test phase (timing 3). Intrahippocampal injection of pyrilamine and diphenhydramine resulted in significant effects on the number of correct choices and across-phase errors in all 3 timings. Scopolamine impaired the number of correct choices and across-phase errors in timing 1 and 3; however, no significant effect was observed in timing 2. These results indicate that pyrilamine and diphenhydramine impaired encoding, retention and retrieval processes of spatial working memory, whereas scopolamine did not impair the retention process.

NMDA receptor involvement in spatial delayed alternation in developing rats

Two experiments examined the effect of the noncompetitive NMDA receptor antagonist, dizocilpine maleate (MK-801), on spatial working memory during development. Rats were trained on spatial delayed alternation (SDA) in a T-maze after ip administration of 0.06 mg/kg MK-801, 0.1 mg/kg MK-801, or saline on postnatal days (P) P23 and P33 (Experiment 1), or following bilateral intrahippocampal administration of 2.5 or 5.0 microg per side MK-801 or saline on P26 (Experiment 2). In Experiment 1, MK-801 dose-dependently impaired SDA learning at both ages. Because the same doses of systemic MK-801 have no effect on T-maze position discrimination learning, impairment of SDA by MK-801 likely reflects disruption of spatial working memory. Both doses of MK-801 abolished acquisition of SDA performance in Experiment 2. Disruption of hippocampal plasticity may account for the effects produced by systemic MK-801 administration. These results confirm and extend earlier lesion studies by implicating plasticity of hippocampal neurons in the ontogeny of spatial delayed alternation.

Altered mnemonic functions and resistance to N-METHYL-d-Aspartate receptor antagonism by forebrain conditional knockout of glycine transporter 1

Converging evidence from pharmacological and molecular studies has led to the suggestion that inhibition of glycine transporter 1 (GlyT1) constitutes an effective means to boost N-methyl-d-aspartate receptor (NMDAR) activity by increasing the extra-cellular concentration of glycine in the vicinity of glutamatergic synapses. However, the precise extent and limitation of this approach to alter cognitive function, and therefore its potential as a treatment strategy against psychiatric conditions marked by cognitive impairments, remain to be fully examined. Here, we generated mutant mice lacking GlyT1 in the entire forebrain including neurons and glia. This conditional knockout system allows a more precise examination of GlyT1 downregulation in the brain on behavior and cognition. The mutation was highly effective in attenuating the motor-stimulating effect of acute NMDAR blockade by phencyclidine, although no appreciable elevation in NMDAR-mediated excitatory postsynaptic currents (EPSC) was observed in the hippocampus. Enhanced cognitive performance was observed in spatial working memory and object recognition memory while spatial reference memory and associative learning remained unaltered. These findings provide further credence for the potential cognitive enhancing effects of brain GlyT1 inhibition. At the same time, they indicated potential phenotypic differences when compared with other constitutive and conditional GlyT1 knockout lines, and highlighted the possibility of a functional divergence between the neuronal and glia subpopulations of GlyT1 in the regulation of learning and memory processes. The relevance of this distinction to the design of future GlyT1 blockers as therapeutic tools in the treatment of cognitive disorders remains to be further investigated.

Dissociation of the effects of MTEP [3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]piperidine] on conditioned reinstatement and reinforcement: comparison between cocaine and a conventional reinforcer

To advance understanding of the potential of metabotropic glutamate receptor (mGluR) 5 as treatment targets for cocaine addiction, the effects of MTEP [3-[(2-methyl-1,3-thiazol-4-yl) ethynyl]piperidine] (a selective mGluR5 antagonist) on conditioned reinstatement of cocaine seeking were examined. To test whether modification of conditioned reinstatement by MTEP is selective for drug-directed behavior or reflects general actions on motivated behavior, effects of MTEP on reinstatement induced by a stimulus conditioned to palatable conventional reward, sweetened condensed milk (SCM), were also evaluated. Previous data suggest that mGluR manipulations preferentially interfere with conditioned reinstatement compared with cocaine self-administration. Therefore, the effects of MTEP on cocaine self-administration were compared with MTEP's effects on SCM-reinforced behavior using the same cocaine doses and SCM concentrations employed for establishing conditioned reinstatement. Male Wistar rats were trained to associate a discriminative stimulus (S(D)) with response-contingent availability of cocaine or SCM and subjected to reinstatement tests after extinction of cocaine or SCM-reinforced behavior. MTEP (0.3-10 mg/kg i.p.) dose-dependently attenuated the response-reinstating effects of both the cocaine S(D) and SCM S(D). MTEP also decreased cocaine self-administration without a clear graded dose-response profile and did not modify SCM-reinforced responding. The findings implicate mGluR5-regulated glutamate transmission in appetitive behavior controlled by reward-related stimuli but without selectivity for cocaine seeking. However, the data suggest a differential role for mGluR5 in the acute reinforcing effects of cocaine versus conventional reward. These observations identify mGluR5 as potential treatment targets for cocaine relapse prevention, although the profile of action of mGluR5 antagonists remains to be more closely examined for potential anhedonic effects.

The effects of clonidine on discrete-trial delayed spatial alternation in two rat models of memory loss

Spatial memory impairments observed in Alzheimer's disease and schizophrenia have been attributed to many factors, including glutamate hypofunction and reduced hippocampal volume. Clonidine, a non-specific alpha(2) adrenergic receptor agonist, improves spatial memory in animals treated with the N-methyl-D-aspartate (NMDA) receptor antagonist, phencyclidine; however, its effects on memory deficits produced by other NMDA antagonists or hippocampal damage have not been fully characterized. The purpose of this study was to determine if clonidine could alleviate memory deficits produced by the NMDA antagonist, MK-801, or by excitotoxic hippocampal damage. In the first phase of the study, male rats were pretreated with clonidine (0.01 or 0.05 mg/kg) or saline, and treated with MK-801 (0.1 mg/kg) or saline prior to discrete-trial delayed alternation or radial-arm maze testing. MK-801 impaired delayed alternation performance and increased the number of arm revisits in the radial-arm maze. Clonidine pretreatment significantly alleviated these drug-induced deficits. In the second phase of the study, excitotoxic damage was produced in the dorsal hippocampus with NMDA. Hippocampal damage produced a significant impairment in the delayed alternation task, yet pretreatment with clonidine did not alleviate this damage-induced deficit. Taken together, the data indicate that clonidine alleviates memory impairments produced by glutamate hypofunction, but not by hippocampal damage. This caveat may be important in designing treatments for memory disorders not linked to a single pathophysiological mechanism.

Aniracetam reversed learning and memory deficits following prenatal ethanol exposure by modulating functions of synaptic AMPA receptors

Specific pharmacological treatments are currently not available to address problems resulting from fetal ethanol exposure, described as Fetal Alcohol Syndrome or Fetal Alcohol Spectrum Disorders (FASD). The present study evaluated the therapeutic effects of aniracetam against cognitive deficits in a well-characterized and sensitive FASD Sprague-Dawley rat model. Ethanol, administered orally at a moderate dose (4 g/kg/24 h; 38% v/v) during the entire course of pregnancy, caused severe cognitive deficits in offspring. Furthermore, both progeny genders were affected by a spectrum of behavioral abnormalities, such as a delay in the development of the righting reflex, poor novelty seeking behavior, and high anxiety levels in female rats. Cognitive disabilities, monitored in adult rats by a two-way active avoidance task, correlated well with a significant reduction of AMPA (alpha-amino-3 hydro-5 methyl-isoxazole propionic acid) receptor-mediated miniature excitatory postsynaptic responses (mEPSCs) in the hippocampus. Administration of aniracetam for 10 days (post-natal days (PND) 18-27), at a dose of 50 mg/kg reversed cognitive deficits in both rat genders, indicated by a significant increase in the number of avoidances and the number of 'good learners'. After the termination of the nootropic treatment, a significant increase in both amplitude and frequency of AMPA receptor-mediated mEPSCs in hippocampal CA-1 pyramidal cells was observed. Significant anxiolytic effects on PND 40 also preceded acquisition improvements in the avoidance task. This study provides evidence for the therapeutic potential of aniracetam in reversing cognitive deficits associated with FASD through positive post-natal modulation of AMPA receptors.

Repeated treatment with N-methyl-d-aspartate antagonists in neonatal, but not adult, rats causes long-term deficits of radial-arm maze learning

Brain glutamatergic system is involved in synaptic plasticity as a base for learning and neural development. This study investigated the effects of neonatal and adult chronic antagonism of N-methyl-d-aspartate (NMDA) receptors, a subtype of glutamate receptors, on learning and/or memory. Rats were trained in the radial-maze learning, which is known as a measure of spatial working memory capacities, in adulthood after neonatal or adult repeated treatment of MK-801 (dizocilpine; 5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine), a non-competitive antagonist, or neonatal repeated treatment of CGS 19755 (cis-4-phosphonomethyl-2-piperadine carboxilic acid), a competitive antagonist. Neonatal repeated treatment of MK-801 or CGS 19755 markedly impaired the radial-arm maze learning. In addition, the treatment altered activities differently in the radial-maze and in the open-field. On the other hand, adult repeated treatment with MK-801 affected neither the radial-maze learning nor activities. Results suggest that chronic blockade of NMDA receptors in a neonatal stage may produce long-lasting deteriorative effects on spatial working memory in adulthood.

Hippocampal activity, but not plasticity, is required for early consolidation of fear conditioning with a short trace interval

The dorsal hippocampus is required for explicit cue fear conditioning only when a temporal gap is inserted between conditioned stimulus (CS) termination and unconditioned stimulus (US) onset (trace fear conditioning). To examine the role of the dorsal hippocampus in associating temporally discontiguous stimuli and to minimize the potential contribution of contextual cues, fear conditioning was conducted using a relatively short (3-s) trace interval. Inactivation of the dorsal hippocampus using the AMPA receptor antagonist NBQX (3 microg/hemisphere) or the GABA(A) agonist muscimol (5 microg/hemisphere) disrupted trace fear conditioning when conducted immediately following training. Trace conditioning was not disrupted significantly when NBQX was infused either before or 2 h after training. Similarly, NBQX infusions were not effective when the CS and US overlapped (delay conditioning). Moreover, trace conditioning was not impaired by intrahippocampal infusion of either the NMDA receptor antagonist AP5 (5 microg/hemisphere) or the L-type voltage-gated calcium channel (VGCC) blocker diltiazem (20 or 40 microg/hemisphere). These data suggest that the involvement of the dorsal hippocampus in short trace interval fear conditioning is largely restricted to the early period of memory consolidation, during which time it mediates the storage of long-term memory in other brain regions.

Involvement of hippocampal metabotropic glutamate receptors in radial maze performance

We investigated the role of hippocampal metabotropic glutamate receptors in spatial learning and memory, using an eight-arm radial maze task. (S)-4-Carboxyphenylglycine, a group I metabotropic glutamate receptor antagonist, or trans-(+/-)-1-amino-(1S,3R)-cyclopentanedicarboxylic acid, a broad-spectrum metabotropic glutamate receptor agonist, was administered into the dorsal hippocampus after rats had acquired the task. Both of these agents significantly impaired radial maze performance, suggesting a functional importance of hippocampal metabotropic glutamate receptors in spatial working memory.

Androgens’ performance-enhancing effects in the inhibitory avoidance and water maze tasks may involve actions at intracellular androgen receptors in the dorsal hippocampus

Androgens can have performance-enhancing effects in some cognitive tasks, but the mechanism of these effects has not been established. Experiments examined whether androgens' actions to bind to intracellular androgen receptors (ARs) in the hippocampus are necessary to enhance cognitive performance in the inhibitory avoidance and water maze tasks. If androgens' binding at ARs are essential, then blocking them through intrahippocampal administration of flutamide, an AR receptor antagonist, should attenuate androgens' performance-enhancing effects in the inhibitory avoidance and water maze tasks. In Experiments 1 and 2, flutamide was administered through intrahippocampal inserts to intact male rats immediately pre- and post-training in the inhibitory avoidance and water maze tasks. Both pre- and post-training administration of flutamide to the dorsal hippocampus, but not missed sites, produced significantly poorer performance in the inhibitory avoidance and water maze tasks, without influencing control measures such as flinch/jump threshold or swim speed. In Experiment 3, flutamide administration to the hippocampus was delayed two hours following training in the inhibitory avoidance and water maze tasks. There was no significant effect of delayed administration of flutamide on performance in either of these tasks. Together, these findings suggest that blocking ARs in the dorsal hippocampus with flutamide administration immediately pre- or post-training can produce decrements in cognitive performance, which implies that androgens' performance-enhancing effects may occur, in part, through binding at intracellular androgen receptors in the dorsal hippocampus.

Hypofunction of the dorsal hippocampal NMDA receptors impairs retrieval of memory to partially presented foreground context in a single-trial fear conditioning in rats

In the present study, the effects of N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonopentanoic acid (AP5) bilaterally infused into the dorsal hippocampus (2.0 microl /5 microg), on the retrieval of fear memory to partial and whole foreground cues were evaluated by using a step-through passive avoidance and Pavlovian fear conditioning. In the both conditioning tasks, following a 30-s preshock exposure period to the shock-associated context, rats received a single shock in a foreground manner for fear memory exhibition by freezing. Rats with AP5 infusion 5 min before the retrieval tests showed profound freezing deficits either immediately or 48 h after the shock in the testing section of the passive avoidance chamber where foreground cues was partially presented. In the Pavlovian conditioning chamber where fear conditioning was tested in the whole of the context that was explicitly paired with the shock, AP5 rats in all infusion schedules exhibited robust freezing responses. These results showed that hypofunction of the hippocampal NMDA receptors impaired the retrieval of fear memory to partial, and not whole, foreground cues. This suggests that NMDA receptors of the hippocampus are involved in the formation of background context representations about foreground events when there is a deficit in perceiving certain sensory properties of the foreground retrieval cues.

Opposing effects of AMPA and 5-HT1A receptor blockade on passive avoidance and object recognition performance: correlation with AMPA receptor subunit expression in rat hippocampus

It has been suggested that antagonists at serotonin 5-HT1A receptors may exert a procognitive effect by facilitating glutamatergic neurotransmission. Here we further explored this issue by looking for the ability of a 5-HT1A antagonist to prevent the learning deficit induced by AMPA receptor blockade in two behavioural procedures in rats, and for concomitant molecular changes presumably involved in memory formation in the hippocampus. Pretraining administration of the competitive AMPA receptor antagonist, NBQX, produced a dose-related retention impairment in a passive avoidance task 24h later, and also impaired retention in a novel object recognition test when an intertrial interval of 3h was selected. Pretreatment with the selective 5-HT1A receptor antagonist, WAY-100635, prevented the learning deficit induced by NBQX in the two behavioural procedures. In biochemical studies performed on rat hippocampus after the retention tests, we found that learning increased the membrane levels of AMPA receptor GluR1 and GluR2/3 subunits, as well as the phosphorylated forms of GluR1, effects that were abolished by NBQX administration before the training session. Pretreatment with WAY-100635 counteracted the NBQX effects and restored the initial learning-specific increase in Ca2+/calmodulin-dependent protein kinase II (CaMKII) function and the later increase in GluR2/3 and phosphorylated GluR1 surface expression. Moreover, administration of WAY-100635 before object recognition training improved recognition memory 24h later and potentiated the learning-associated increase in AMPA receptor subunits. The results support the proposed utility of 5-HT1A antagonists in the treatment of cognitive disorders.

Effect of δ-aminolevulinic acid treatment on N-methyl-d-aspartate receptor at different ages in the rat brain

We report here the effects of the chronic treatment with the oxidant agent delta-aminolevulinic acid (ALA) on the N-methyl-D-aspartate (NMDA) receptors in 4-, 12- and 24-month-old male Wistar rats. ALA was administered daily for 15 days (40 mg/kg i.p). The study was performed by membrane homogenate binding and autoradiography, using tritiated 5-methyl-10, 11-dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate ([3H]MK-801). [3H]MK-801 binding was significantly decreased in most areas studied (cortex and hippocampus) at all ages in treated rats with respect to their controls. Furthermore, Western blot assays were performed using antibodies against the NMDA receptor NR2A subunit, which is widely distributed in the brain, mainly in cortex and hippocampus. In cortex but not in hippocampus, the ALA treatment induced significant decreases in the amounts of NR2A subunit in 12- and 24-month-old animals. We conclude that chronic treatment with ALA is able to induce NMDA receptor decreases in an age-independent way and that NR2A subunit seems to be involved in these decreases in cerebral cortex, but not in the other structures studied.