Oxiracetam and D-pyroglutamic acid antagonize a disruption of passive avoidance behaviour induced by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate

The uncompetitive NMDA receptor antagonists ketamine and memantine preferentially increase the choice for a small, immediate reward in low-impulsive rats

RATIONALE

Impulsive behavior is categorically differentiated between impulsive action, the inability to withhold from acting out a response, and impulsive choice, the greater preference for an immediate and smaller reward over a delayed but more advantageous reward. While the effects of N-methyl-D-aspartic acid (NMDA) receptor antagonists on impulsive action have been extensively characterized, there are very few and conflicting reports on the effects of this class of drugs on impulsive choice.

OBJECTIVES

Using a modified adjusting delay task, we investigated the effects of uncompetitive and competitive blockade of NMDA receptors on impulsive choice.

METHODS

Male Wistar rats were trained in a modified adjusting delay task, which involved repeated choice between a low reinforcing solution delivered immediately and a highly reinforcing solution delivered after a variable delay. Rats were then administered either the NMDA receptor uncompetitive antagonists ketamine or memantine, or the competitive antagonists D-AP-5 or CGS 19755.

RESULTS

Ketamine treatment dose-dependently increased impulsive choice, and this effect was selective for low-impulsive but not high-impulsive rats. Similarly, memantine treatment dose-dependently increased impulsive choice with a preferential effect for low-impulsive rats. While D-AP-5 treatment did not affect impulsive choice, CGS 19755 increased impulsivity, however, at the same doses at which it caused a marked response inhibition.

CONCLUSIONS

NMDA receptor uncompetitive, but not competitive, antagonists significantly increased impulsive choice, preferentially in low-impulsive rats. These findings demonstrate that the effects of NMDA receptor blockade on impulsive choice are not generalizable and depend on the specific mechanism of action of the antagonist used.

Mild cognitive impairment: animal models

Mild cognitive impairment (MCI) is an aspect of cognitive aging that is considered to be a transitional state between normal aging and the dementia into which it may convert. Appropriate animal models are necessary in order to understand the pathogenic mechanisms of MCI and develop drugs for its treatment. In this review, we identify the features that should characterize an animal model of MCI, namely old age, subtle memory impairment, mild neuropathological changes, and changes in the cholinergic system, and the age at which these features can be detected in laboratory animals. These features should occur in aging animals with normal motor activity and feeding behavior. The animal models may be middle-aged rats and mice, rats with brain ischemia, transgenic mice overexpressing amyloid precursor protein and presenilin 1 (tested at an early stage), or aging monkeys. Memory deficits can be detected by selecting appropriately difficult behavioral tasks, and the deficits can be associated with neuropathological alterations. The reviewed literature demonstrates that, under certain conditions, these animal species can be considered to be MCI models, and that cognitive impairment in these models responds to drug treatment.

Oxiracetam pre- but not post-treatment prevented social recognition deficits produced with trimethyltin in rats

The social recognition paradigm was used to investigate the effect of trimethyltin (TMT) in adult male rats. Consequently, the effect of chronic oxiracetam (OXI) treatment in TMT impaired animals was evaluated. In all experiments, a behavioural testing was performed 3 weeks after TMT administration. Experiment 1: A single TMT oral dose, 5 and 7.5 but not 2.5mg/kg, impaired the natural ability of the adults to recognize a juvenile conspecific that they encountered 30 min before. The dose of 5mg/kg TMT was chosen to be used in subsequent experiments. Experiment 2: Chronic OXI pre-+post-treatment, daily 3 or 30 mg/kg sc for 7 days before and 7 days after the insult, protected the adults against recognition deficit produced by TMT. Experiment 3: OXI pre- but not post-treatment (always 3 and 30 mg/kg) had beneficial effects on the social recognition. The findings suggest that social recognition ability of adult male rats pre-treated sufficiently long with OXI is resistant to the neurotoxicity effect of TMT.

Oxiracetam prevented the scopolamine but not the diazepam induced memory deficits in mice

In mice, the elevated plus-maze paradigm was used to investigate the effect of scopolamine hydrobromide and diazepam and their interaction with oxiracetam on the retrieval of spatial memory trace. This paradigm measures (using the transfer latency) an animal's capacity to escape from the open arm to the enclosed one. The retention session followed 24 h after the acquisition one. Experiment 1: Scopolamine (0.25 and 0.5 mg/kg) and diazepam (0.5 and 1.0 mg/kg) given 30 min before the retention session significantly prolonged the transfer latency as compared with the saline treated mice and those given the lowest dose of scopolamine (0.125 mg/kg) and diazepam (0.25 mg/kg). Experiment 2: Oxiracetam administered at doses of 3, 10 and 30 mg/kg immediately after the acquisition session prevented the scopolamine induced prolongation of the transfer latency. Thus, oxiracetam forestalled the impairment of retrieval of memory trace: the animals were able to remember the spatial configuration of the plus-maze. On the contrary, oxiracetam was not effective in the diazepam treated mice. We suggest that beneficial effect of oxiracetam might be confounded or blocked by the anxiolytic effect of diazepam.

Oxiracetam prevents the MK-801 induced amnesia for the elevated plus-maze in mice

We investigated the effect of the nootropic substance oxiracetam on the impairment of memory induced in mice by the non-competitive NMDA antagonist MK-801. Memory capacities of animals having different experience were evaluated using the elevated plus-maze test. Oxiracetam was injected immediately after the acquisition session(s), MK-801 was given 30 min before the retention session which followed 24 h after the acquisition session(s). In slightly experienced animals (Section 3.1), oxiracetam (3 and 30 mg/kg, s.c.) prevented MK-801 (0.15 mg/kg, i.p.) induced memory deficits characterized by a prolongation of the transfer latency. In well-trained animals (Section 3.2), oxiracetam (30 mg/kg, s.c.) attenuated MK-801 (0.15,0. 25 and 0.4 mg/kg, i.p.) induced amnesia for a spatial orientation in the elevated plus-maze. These results show that oxiracetam interacted with the glutamatergic NMDA receptor system and forestalled the impairment of retrieval of long-term memory. The results also justify the usage of the elevated plus-maze method in the evaluation of potential anti-amnesic or nootropic drugs.

Putative cognition enhancers reverse kynurenic acid antagonism at hippocampal NMDA receptors

Oxiracetam, aniracetam and D-cycloserine, three putative cognition enhancers, were examined in a functional assay for NMDA receptors. Rat hippocampal slices or synaptosomes were labeled with [3H]noradrenaline and exposed to NMDA or glutamate in superfusion. NMDA (100 microM) elicited a remarkable rise (about 500%) in the release of [3H]noradrenaline from slices. The effect of NMDA was antagonized by the glutamate receptor blocker, kynurenic acid. The antagonism by 100 microM kynurenate was reduced by submicromolar concentrations of oxiracetam and totally reversed by 1 microM of the drug. The concentration-antagonism curve for kynurenic acid was shifted to the right in the presence of 0.2 or 1 microM oxiracetam. Aniracetam and D-cycloserine, as well as glycine and D-serine, behaved similarly to oxiracetam: all compounds, tested at 1 microM, reversed the antagonism by 100 microM kynurenate of the NMDA-evoked [3H]noradrenaline release. In superfused hippocampal synaptosomes, 100 microM NMDA or glutamic acid elicited the release of [3H]noradrenaline; the evoked release was enhanced by glycine, but not by oxiracetam. In this preparation 1 microM glycine or 1 microM oxiracetam prevented the antagonism by kynurenate of the NMDA- or the glutamate-evoked [3H]noradrenaline release. As kynurenic acid is an endogenous glutamate receptor antagonist whose brain levels are known to increase in conditions associated to cognitive deficits, it is proposed that the putative cognition enhancers tested may act in vivo by relieving the antagonism produced by excessive endogenous kynurenate.

Traumatic brain injury enhances the amnesic effect of an NMDA antagonist in rats

The authors have examined the effect of experimental traumatic brain injury on the amnesia produced by the N-methyl-D-aspartate (NMDA) antagonist MK-801. Rats were either subjected to a moderate level of fluid-percussion injury or prepared for injury but not injured ("sham injury"). Nine days following injury or sham injury, the rats were injected either with saline (sham/saline group, nine rats; injured/saline group, nine rats) or with 0.1 mg/kg of MK-801 (sham/MK-801 group, nine rats; injured/MK-801 group, eight rats) 30 minutes before being trained on a passive-avoidance task. Twenty-four hours later, the rats were tested for retention of the passive-avoidance task. Results revealed that the low dose of MK-801 did not significantly affect retention of the passive-avoidance task in the sham-injured group. In injured animals, administration of MK-801 produced a profound amnesia in contrast to the sham-injured animals treated with MK-801 and the injured animals treated with saline. To further investigate this enhanced sensitivity to the amnesic effects of MK-801 exhibited by the injured animals, nine injured and eight sham-injured rats were injected with 0.3 mg/kg of MK-801 15 minutes before injury. Results indicated that the animals treated with MK-801 before injury did not significantly differ from the sham-injured animals in retention of the passive-avoidance task. In addition, test results in the animals treated with MK-801 before injury and reinjected with MK-801 before passive-avoidance testing did not differ from those in untreated injured animals reinjected with saline before passive-avoidance testing. These findings indicate that MK-801 treatment before injury prevented the enhanced sensitivity to MK-801-induced amnesia that follows traumatic brain injury.

Piracetam and other structurally related nootropics

Nearly three decades have now passed since the discovery of the piracetam-like nootropics, compounds which exhibit cognition-enhancing properties, but for which no commonly accepted mechanism of action has been established. This review covers clinical, pharmacokinetic, biochemical and behavioural results presented in the literature from 1965 through 1992 (407 references) of piracetam, oxiracetam, pramiracetam, etiracetam, nefiracetam, aniracetam and rolziracetam and their structural analogues. The piracetam-like nootropics are capable of achieving reversal of amnesia induced by, e.g., scopolamine, electroconvulsive shock and hypoxia. Protection against barbiturate intoxication is observed and some benefit in clinical studies with patients suffering from mild to moderate degrees of dementia has been demonstrated. No affinity for the alpha 1-, alpha 2-, beta-, muscarinic, 5-hydroxytryptamine-, dopamine, adenosine-A1-, mu-opiate, gamma-aminobutyric acid (GABA) (except for nefiracetam (GABAA)), benzodiazepine and glutamate receptors has been found. The racetams possess a very low toxicity and lack serious side effects. Increased turnover of different neurotransmitters has been observed as well as other biochemical findings, e.g., inhibition of enzymes such as prolylendopeptidase. So far, no generally accepted mechanism of action has, however, emerged. We believe that the effect of the racetams is due to a potentiation of already present neurotransmission and that much evidence points in the direction of a modulated ion flux by, e.g., potentiated calcium influx through non-L-type voltage-dependent calcium channels, potentiated sodium influx through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor gated channels or voltage-dependent channels or decreases in potassium efflux. Effects on carrier mediated ion transport are also possible.

Relationships between arousal and cognition-enhancing effects of oxiracetam

Cognition-enhancing effects of no-otropic drugs are currently ascribed to an increase in arousal level. In order to test this hypothesis, we studied the effects of three doses of oxiracetam (25, 50 and 100 mg/kg IP) on a radial maze task and on slow wave sleep (SWS) latency in a familiar environment. The 25- and 100-mg/kg doses, but not the 50-mg/kg, significantly improved performance in the memory task. On the other hand, SWS latency was significantly increased by 50 and 100 mg/kg, with the effect of the 25-mg/kg dose going in the same direction but only approaching significance. These results give only a partial support to the "arousal factor hypothesis." Other factors are probably involved in the promnesic effects of oxiracetam.

Memory disorders: novel treatments, clinical perspective

The aging of the society is accompanied by a strong pressure to develop drugs to treat age-related memory disorders. This paper examines the discrepancy between the results of preclinical tests, which have identified a large number of putative cognition enhancers, and the limited clinical efficacy of most of them. The predictivity of the preclinical tests is discussed, and the criteria for evaluating the therapeutic efficacy of the cognition enhancers are examined. An analysis is made of the novel treatments presently available or under investigation. In light of the results of the trials with tacrine a warning is given not to expect striking clinical improvements by only attempting to restore signal transmission, disrupted by aging and dementia, without modifying the underlying morphological alterations.

In search of the mechanism of action of the nootropics: new insights and potential clinical implications

The positive action of nootropics on the memory has up to now primarily been discussed in the context of effects on energy metabolism and cholinergic or glutaminergic neurotransmission. Recent findings have shown that the memory-enhancing effect is steroid-sensitive. Since corticosteroids are potent modulators of gene transcription, it appears possible that the nootropics may exert a modulatory action on protein synthesis. This assumption is supported on the one hand by the fact that the nootropics improve the memory even if they are administered several hours after the learning trial, and on the other hand by the observation that their memory-enhancing effect does not become detectable until 16-24 hours after the treatment and learning trial. Provided the memory-enhancing effect in animal experiments and the therapeutic effect in patients come about by way of the same mode of action, the fact that high levels of corticosteroids suppress the effects of the nootropics could also have clinical implications: in the light of the observation that the majority of Alzheimer patients have elevated steroid levels it could explain why there is always only a small proportion of patients in clinical trials that respond to treatment with nootropics.

The pharmacology of the nootropics; new insights and new questions

Up to now, the memory-enhancing effect of the nootropics has chiefly been investigated in the context of effects on energy metabolism and on cholinergic and glutamatergic neurotransmission. Recent studies have also shown that the effect on memory is steroid-sensitive. The present review article summarizes the available results and discusses them in the context of a new hypothesis on the mechanism of action and with respect to clinical implications.

Oxiracetam prevents haloperidol-induced passive avoidance impairment in mice

The nootropic drug oxiracetam (50 mg/kg) prevented passive avoidance impairment induced by posttraining administration of haloperidol (0.25 and 0.5 mg/kg). Conversely, oxiracetam did not antagonize either locomotor depression or suppression of active avoidance responses induced by the dopamine receptor blocking agent. The results indicate that prevention of haloperidol-induced retention impairment, by oxiracetam, may be due to a not yet defined protective action, common to other nootropic agents, on different types of experimental amnesias, rather than to a specific interaction with dopaminergic mechanisms.

Effects of intra-amygdala injections of NMDA receptor antagonists on acquisition and retention of inhibitory avoidance

These experiments examined the effects of intra-amygdala injections of NMDA receptor antagonists on the acquisition and retention of inhibitory avoidance. In Expt. I, rats received bilateral intra-amygdala injections of the NMDA antagonists D,L-AP5 (1-10 micrograms), D-AP5 (0.03-1 micrograms), CPP (0.125 or 0.375 microgram), or MK-801 (0.2 or 0.5 microgram) prior to training in a continuous multiple-trial inhibitory avoidance (CMIA) task. Acquisition of the task was not significantly affected by any of the drug injections. In contrast, all three competitive antagonists, D,L-AP5, D-AP5 and CPP, produced dose-dependent impairment of 48 h retention performance. Although the MK-801 injections did not significantly impair retention performance, the retention scores of the 0.5 microgram MK-801 group were bimodally distributed, indicating retention impairment in a subgroup of the animals given that dose. Intra-amygdala injections of 3 or 10 micrograms D,L-AP5 did not affect footshock sensitivity (Expt. II) or locomotor activity (Expt. III) and their retention-impairing effects were not due to induction of state dependency (Expt. IV). The retention-impairing effects of intra-amygdala injections of NMDA antagonists were not due to diffusion of the drugs dorsally: injections of 1 microgram D-AP5 into the striatal area directly above the amygdala impaired acquisition but not retention performance (Expt. V). The retention-impairing effects of 1 microgram D-AP5 or 0.5 microgram MK-801 were attenuated by giving additional training to the animals shortly after receiving intra-amygdala injections (Expt. VI). The implications of these findings for hypotheses concerning amygdala function in learning and memory are discussed.

Excitatory amino acids and memory: evidence from research on Alzheimer's disease and behavioral pharmacology

The excitatory amino acid transmitter (EAA) system is believed to play a crucial role in a variety of physiological processes related to neuronal plasticity. Substantial neurophysiological evidence suggests that, under normal physiological conditions. EAAs may be involved in mechanisms underlying learning and memory. However, overactivity of this system produces excitotoxic damage to neurons which is believed to be an etiological factor in various neurological conditions, such as epilepsy, and stroke-induced impairments. The fact that EAAs have been implicated in both, normal cognitive function and in degenerative neurological conditions suggests that they may contribute to the etiology of Alzheimer's disease (AD), because AD is characterized by memory deficits and specific histopathological signs of neuronal damage. This paper summarizes information regarding 1) the involvement of EAAs in Alzheimer's disease and 2) results from psychopharmacological studies of EAAs in laboratory animal models of learning. Investigations of the pathophysiology of AD indicate that glutamatergic deficits are associated with this syndrome. However, there is controversy concerning the nature of this defect. As a result, it is unclear whether it is a consequence of excitotoxic changes produced by glutamatergic overactivity or result from a decrease in glutamatergic function. Evidence from behavioral studies is consistent with the conclusion that EAAs may be involved in the acquisition of conditioned responses. However, in parallel with the clinical findings, learning impairments have been produced by treatments which either increase or decrease activity within this transmitter system. Therefore, although present results suggest that the EAAs play a role in cognition and in clinical syndromes in which such function is compromised, the specific nature of that role needs to be elucidated by future research.

Induction of cognitive impairment by scopolamine and noncholinergic agents in rhesus monkeys

In primates, treatment with scopolamine impairs performance of a spatial delayed response task in a way which mimics deficits seen spontaneously in aged primates and demented patients. Despite their efficacy in reversing scopolamine induced disruption, the effects of cholinergic agonists on cognition in aged primates and dements are unimpressive, suggesting that other neurotransmitter systems are also involved in this type of deficit. We have induced a scopolamine-like impairment of spatial delayed response performance in rhesus monkeys using phencyclidine (0.1-0.2 mg/kg i.m.), lorazepam (0.4-0.6 mg/kg s.c.) or tetrahydrocannabinol (1-4 mg/kg p.o.), but not amphetamine (0.1-0.4 mg/kg i.m.), yohimbine (0.1-1.0 mg/kg i.m.) or morphine (2-4 mg/kg i.m.). Our findings suggest that disruption of specific neurotransmitter systems other than acetylcholine may contribute importantly to cognitive decline in aging and dementia.