Assessment of a cholinergic contribution to chlordiazepoxide-induced deficits of place learning in the Morris water maze

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.

NOGO-66 receptor deficient mice show slow acquisition of spatial memory task performance

The Nogo-66 receptor (NgR1) is part of a co-receptor complex on neurons that transmits a signal for inhibition of neurite outgrowth. In addition, NgR1 function has also been related to other disorders such as schizophrenia and Alzheimer's disease. Here, we studied the effect of life-long deletion of NgR1 (ngr(-/-)) in tests for cognition and positive symptoms of schizophrenia. In the water maze, ngr(-/-) mice learned to locate the hidden platform as well as wild type mice, although with slower acquisition. Deletion of NgR1 did not affect amphetamine- or phencyclidine (PCP)-induced hyperactivity, two models of positive symptoms of schizophrenia. Taken together, ngr(-/-) animals show slower acquisition of a spatial learning and memory task.

Flumazenil administration attenuates cognitive impairment in immature rats after controlled cortical impact

Evidence suggests that the gamma-aminobutyric acid (GABA)ergic system may be involved in cognitive dysfunction following traumatic brain injury (TBI). We investigated the effect of flumazenil treatment, a benzodiazepine antagonist approved by the U.S. Food and Drug Administration, on learning and memory in the immature rat following experimental brain injury. Post-natal day 17 rats were injured using controlled cortical impact. Systemic treatment with flumazenil at 1, 5, and 10 mg/kg was initiated on post-injury day 1 and administered for 13 days via daily intraperitoneal injections. Morris water maze (MWM) testing was used to measure latency to find a submerged platform and the results from experimental and control animals were compared. We demonstrated a significant dose-dependent improvement in MWM performance in drug-treated animals. This is the first study demonstrating the efficacy of flumazenil in reducing post-TBI cognitive deficits and we propose that these effects may be related to modulation of the GABA(A) receptor.

Intact learning and memory in rats following treatment with the dual orexin receptor antagonist almorexant

RATIONALE

Orexins play a key role in the maintenance of alertness and are implicated in the modulation of diverse physiological processes, including cognitive function. Almorexant, a dual orexin receptor antagonist, transiently and reversibly blocks the action of orexin peptides at both OX(1) and OX(2) receptors and increases time spent in rapid eye movement (REM) and non-REM sleep.

OBJECTIVES

We explored the direct effects on learning and memory of single and repeated administration of almorexant in rats.

METHODS

Following administration of high doses of almorexant (300 mg/kg, p.o.), scopolamine (0.8 mg/kg, i.p.), combination almorexant-scopolamine, or vehicle alone, rats were trained on a Morris water maze spatial navigation task, or on a passive avoidance task.

RESULTS

Rats treated with almorexant learned the spatial navigation task with similar efficacy as vehicle-treated animals. After 4 days, almorexant-but not vehicle-treated rats had established spatial memory; after 8 days, spatial memory had been established in both vehicle-and almorexant-treated rats. Scopolamine-treated rats failed to learn the spatial task. Both vehicle-and almorexant-but not scopolamine-treated rats demonstrated passive avoidance learning. Almorexant did not ameliorate scopolamine-induced impairment of learning in either task.

CONCLUSIONS

Rats treated with almorexant are fully capable of spatial and avoidance learning.

Bacopa monniera exerts antiamnesic effect on diazepam-induced anterograde amnesia in mice

RATIONALE

As Benzodiazepines are known to produce amnesia by involvement of the GABAergic system, we examined Bacopa monniera, an herb known for memory enhancement for reversal of memory deficits caused by diazepam.

OBJECTIVES

The objective of the study was to study the effect of standardized extract of B. monniera on diazepam-induced amnesia in mice using Morris water maze.

MATERIALS AND METHODS

We used the rota rod test as a screening measure for muscle incoordination followed by the Morris water maze scale to evaluate the effect of B. monniera on amnesia. The index of acquisition and retrieval was recorded with varying doses of Bacopa.

RESULTS

The results revealed antiamnesic effects of B. monniera (120 mg kg(-1) oral) on diazepam (1.75 mg kg(-1) intraperitoneal)-induced amnesia. The degree of reversal by Bacopa was significant as it progressively reduced escape latency time when mice treated with diazepam were subjected to acquisition trials.

CONCLUSIONS

The antiamnesic effects of Bacopa suggest likely a gamma-aminobutyric acid-benzodiazepine pathway possibly affecting long-term potentiation.

Effects of scopolamine on morphine-induced conditioned place preference in mice

It is well known that the cholinergic system plays a crucial role in learning and memory. Psychopharmacological studies in humans and animals have shown that a systemic cholinergic blockade may induce deficits in learning and memory. Accumulated studies have indicated that learning and memory play an important role in drug addition. In the present study, in order to get a further understanding about the functions of the cholinergic system in drug-related learning and memory, we examined the effects of scopolamine (0.5, 1.0 and 2.0 mg/kg) on morphine-induced conditioned place preference (CPP). Two kinds of morphine exposure durations (4 days and 12 days) were used. The main finding was that all doses of scopolamine enhanced the extinction of morphine-induced CPP in mice treated with morphine for 12 days. However, in mice treated with morphine for 4 days, all doses of scopolamine did not inhibit morphine-induced CPP. The highest dose (2.0 mg/kg) of scopolamine even significantly delayed the extinction of morphine-induced CPP. Our results suggest that the effects of a systemic cholinergic blockade on morphine-induced CPP depend on the morphine exposure time.

BetaCCM but not physostigmine enhancement of memory retrieval depends on emotional processes in mice

RATIONALE

The effects of methyl beta-carboline-3-carboxylate (betaCCM, an inverse agonists of GABA/benzodiazepine receptors) or physostigmine (a cholinesterase inhibitor) on retrieval processes and relationships with anxiety have been only marginally studied.

OBJECTIVE

This study investigates in mice the effects of acute betaCCM or physostigmine injections on retrieval of previously acquired discriminations involving distinct contextual cues (serial contextual discrimination; SCD) in a four-hole-board. Animals submitted to SCD were also evaluated for emotional reactivity in an elevated-plus maze.

METHODS

Mice were injected before the learning session began with a saline solution. Twenty-four hours later, mice were replaced on the context of the initial acquisition and a single dose of saline or betaCCM (0.5 or 1.5 mg/kg) or physostigmine (0.05 and 1.0 mg/kg) was injected 20 min before testing.

RESULTS

The highest dose of either betaCCM or physostigmine improved performance of the first discrimination in the SCD task. The higher dose of betaCCM produced anxiety-like reactivity in the plus maze, and scores of "anxiety" were significantly correlated with memory scores; in contrast, memory performance of physostigmine-treated subjects were totally independent of emotional reactivity.

CONCLUSION

These results show that, as opposed to physostigmine, betaCCM acts on retrieval processes specifically through its emotional component.

Role of platelet activating factor in triazolobenzodiazepines-induced retrograde amnesia

Benzodiazepine (diazepam), triazolobenzodiazepines (brotizolam, triazolam) and platelet activating factor (PAF) antagonist (BN 52021) are administered to mice before acquisition and retrieval trials conducted using Morris water maze. Benzodiazepine has produced only anterograde amnesia and it has not produced retrograde amnesia. Triazolobenzodiazepines have produced both anterograde and retrograde amnesia. PAF antagonist (BN 52021) has only produced retrograde amnesia and it has not produced anterograde amnesia. The anterograde amnesia produced by benzodiazepine and triazolobenzodiazepines, has been prevented by benzodiazepine receptor antagonist (flumazenil). It suggests that benzodiazepine- and triazolobenzodiazepines-induced anterograde amnesia may be mediated through benzodiazepine receptors. On the other hand, retrograde amnesia produced by PAF antagonist (BN 52021) and triazolobenzodiazepines has been attenuated by PAF and PAF acetyl hydrolase inhibitors such as cigarette smoke extract (CSE) and phenylmethanesulfonylflouride. It suggests that triazolobenzodiazepine-induced retrograde amnesia may be mediated through blockade of PAF receptors.

Influence of benzodiazepine binding site ligands on fear-conditioned contextual memory

Eight compounds that bind to the benzodiazepine binding site on the gamma-amino butyric acid(A) (GABA(A)) receptor were assessed for their influence on contextual memory, an aspect of memory affected in various cognitive disorders including Alzheimer's disease. Using a Pavlovian fear-conditioning paradigm, each ligand was evaluated in C57Bl/6 mice in regards to its direct affect on contextual memory and whether the ligand could attenuate scopolamine-induced contextual memory impairment. Of the eight ligands tested, one impaired contextual memory (agonist), six attenuated scopolamine-induced contextual memory impairment (inverse agonists), and one antagonized the ability of an inverse agonist to attenuate scopolamine-induced contextual memory impairment. Hence, further demonstrating the bi-directional influence benzodiazepine binding site ligands are able to exert on memory modulation. This study serves as an initial starting point in the development of pharmacological tools to be used in deciphering how GABA(A) receptors influence contextual memory.

Effects of glucose on scopolamine-induced learning deficits in rats performing the Morris water maze task

In order to assess the effects of glucose on drug-induced spatial learning deficits, three experiments were conducted using the Morris water maze. Scopolamine and glucose were injected ip at various stages of training. Rats of Wistar strain served as subjects. In Experiment 1, scopolamine (0.4 mg/kg) and 10, 100, or 500 mg/kg of glucose were administered every day from the start of training, and the effect on acquisition was evaluated. In Experiment 2, scopolamine and 100 or 500 mg/kg of glucose were administered after 6 days of training, and the effect on performance was assessed. In Experiment 3, scopolamine and 500 mg/kg of glucose were injected after 2 days of training, and the effect on the following trial was tested. In all experiments, scopolamine impaired acquisition/performance of the task. Glucose at 500 mg/kg showed a significant enhancing effect on acquisition regardless of scopolamine injection only when injected daily from the start of training (Experiment 1). Glucose injected after the performance has reached asymptote (Experiment 2) did not affect performance, and glucose in the middle of training showed a slight but insignificant enhancing effect (Experiment 3). These results may suggest that the effect of glucose changes as a function of the degree of learning of the spatial learning task. The possibility of task specificity of the glucose effect was also discussed in relation to the cholinergic systems and local cerebral glucose utilization.

The allocentric place discrimination task is selectively and highly dependent on the central muscarinic system in rats

The allocentric place discrimination task (APDT) is useful in evaluating working memory separately from and simultaneously with motivation, motor and sensory ability. Muscarinic acetylcholine receptor antagonist scopolamine has been shown to selectively impair the accuracy of APDT without changing swimming speed, distance, and still time. For further evaluation of other neurotransmitters' roles in the APDT, pharmacological manipulations were performed. Neither diazepam 3.0 mg/kg, mecamylamine 10 mg/kg, haloperidol 0.5 mg/kg, nor 8-OH DPAT 1.0 mg/kg affected accuracy of place discrimination. Two kinds of responses were observed following the administration of MK-801 0.3 mg/kg: the accuracy of rats for longer swimming distance tended to decrease, and the accuracy of rats for normal swimming distance did not change. Therefore, NM-801 did not seem to affect the working memory selectively. In addition, neither flumazenil 10 mg/kg, ondansetron 0.3 mg/kg nor R(-)-alpha-metylhistamine 10 mg/kg attenuated the scopolamine-induced deficits. These results suggest that the central muscarinic receptors are selectively and highly important in the APDT.

Simultaneous evaluation of spatial working memory and motivation by the allocentric place discrimination task in the water maze in rats

In order to evaluate learning and memory deficits separately from and simultaneously with motivational, motor and sensory impairments in identical animals, we developed the allocentric place discrimination task test using a water maze in rats. For this assessment task, two similar, visible platforms, one was fixed and the other was floating, were simultaneously present in a pool, and the working memory of the allocentric place discrimination task was evaluated. After training, the task accuracy was high about 85% correct and animals were used repeatedly. The accuracy decreased significantly when the pool was surrounded with a black curtain. Muscarinic receptor antagonist scopolamine 0.5 mg/kg selectively impaired the accuracy. Muscle relaxant dantrolene 10 mg/kg selectively decreased swimming speed. Under low motivational condition (warm water), still time increased and swimming speed decreased, but the accuracy was not affected. Similar to warm water, opioid receptor agonist morphine 15 mg/kg increased still time and decreased swimming speed. These results suggest that the allocentric place discrimination task is useful in evaluating spatial working memory ability independently of and concurrently with also visual, motor ability and motivation in identical animals.

Physostigmine's impact on brief shock-induced hypoalgesia parallels its effect on memory

Past research indicates that the anticholinergic drug scopolamine disrupts memory and environmentally induced hypoalgesia in rats. The present study examined the impact of the centrally active cholinesterase inhibitor physostigmine, which enhances memory and central cholinergic activity, on brief shock-induced hypoalgesia on the tail-flick test using Sprague-Dawley rats. It is reported that physostigmine (0.1 mg/kg) potentiates the magnitude of this hypoalgesia. Contrary to past research, our results showed that omission of baseline testing did not eliminate hypoalgesia or its potentiation by physostigmine. Similar to its effects on memory, physostigmine (0.04, 0.1, and 0.25 mg/kg) has a nonmonotonic impact on brief shock-induced hypoalgesia; low doses potentiated hypoalgesia (0.1 mg/kg), whereas a high dose (0.25 mg/kg) disrupted it. These results provide further evidence that the cholinergic system indirectly affects pain reactivity by modulating the memory of the aversive event.

Learning impairment in transgenic mice with central overexpression of corticotropin-releasing factor

The present studies were designed to test the learning and memory capacities of transgenic mice with central overexpression of corticotropin-releasing factor in a forced alternation water T-maze task and in the Morris water maze. In T-maze testing, littermate control mice reached a criterion of 70% correct responses after five days of trials, while the performance of transgenic subjects was still random after the same training. In Morris maze testing, control subjects reached the submerged platform significantly faster (F(1.48) = 4.51, P < 0.05) after three days of trials, while the performance of transgenic mice was unimproved over the same period. The deficit in Morris maze performance in transgenic mice was reversed when the platform was visible above the surface of the water. Pre-test administration of the benzodiazepine anxiolytic, chlordiazepoxide (10 mg/kg), before acquisition training also produced a significant (F(4.40) = 16.61, P < 0.001) and persistent improvement in Morris maze performance in transgenic mice when compared to vehicle-treated transgenic litter mates. Finally, there was no evidence of hippocampal cell loss in transgenic brains. The results suggest that corticotropin-releasing factor-overexpressing mice exhibit a profound learning deficit without sensory or motor-related impairments, and that memory plasticity can be restored by anxiolytic pre-treatment. Thus, constitutive overabundance of brain corticotropin-releasing factor may produce hyperemotionality that interferes with learned behaviors. Stress-related disorders characterized by co-morbid deficits in learning/memory may benefit from pharmacological normalization of brain corticotropin-releasing factor systems.

Possible confounding influence of strain, age and gender on cognitive performance in rats

There are substantial differences in the performance of various rat strains in tasks of learning, memory and attention. Strain, age and sex differences are not consistent over procedures: poor performance in one paradigm does not predict poor performance in a different paradigm. Some strain differences are not readily apparent until a direct comparison is made between one or more strains. Moreover, large differences in nominally the same strain but obtained from different suppliers have been observed in behavioural, pharmacological and physiological parameters and can have important consequences for interpretation of drug effects. Longevity, and the effects of ageing can differ dramatically from one strain to another; drug effects can alter radically with increasing age and show strain (and individual) differences in their action. Sex can further complicate interpretation of results. Thus, non-cognitive factors may exert a major effect on results in cognitive testing, and strain-dependent effects may account for many conflicting results in the literature concerning mnemonic performance. Strain differences in particular must be identified and used to help identify fundamental effects on memory, rather than continue to be ignored and allowed to obscure interpretation of drug effects on cognitive processes.

Differential effect of antipsychotics on place navigation of rats in the Morris water maze. A comparative study between novel and reference antipsychotics

A group of novel neuroleptics (e.g. olanzapine, seroquel, sertindole and ziprasidone) and already marketed compounds (e.g. clozapine, haloperidol and risperidone) were tested for acute effect on spatial learning and memory in Morris' water maze task. Young rats were trained for 4 consecutive days (three trials/day) to find a platform situated beneath the water surface. Two compounds, sertindole and seroquel, were without effect on spatial performance, whereas clozapine impaired performance on the first 2 test days but showed no effect compared to the controls on the last 2 test days. Ziprasidone and olanzapine markedly impaired spatial memory without affecting motor function (measured by the swimming speed). Risperidone and haloperidol also impaired performance but in addition both compounds significantly lowered the swimming speed. The present study indicates that several of the compounds impair spatial learning in Morris water maze. This might be of clinical importance in the treatment of schizophrenics, as many of these patients already show severe cognitive deficits. Therefore, certain antipsychotics could worsen the preexisting memory deficits in schizophrenic patients and this aspect should be considered before antipsychotic treatment.

Cholinergic receptor agonists inhibit pirenzepine-induced dysfunction of spontaneous alternation performance in the mouse

1. The present study was designed to examine the effects of intracerebroventricular injection of several cholinergic drugs on the impairment of spontaneous alternation performance induced by the M1-selective muscarinic receptor antagonist pirenzepine. 2. Pirenzepine (3 and 10 micrograms) significantly reduced spontaneous alteration performance related to working memory without producing any marked increase in total arm entries, which are considered to reflect locomotor activity. 3. Physostigmine (3.47 micrograms), a cholinesterase inhibitor, and McN-A-343 (20 micrograms), and M1-selective muscarinic receptor agonist, significantly improved the pirenzepine (3 micrograms)-induced impairment of spontaneous alternation performance, although oxotremorine (0.68 microgram), a nonselective muscarinic receptor agonist, showed a tendency to reverse the pirenzepine (3 micrograms)-induced impairment. 4. These findings suggest that the blockade of muscarinic M1 but not M2 receptors results in the impairment of spontaneous alternation performance associated with working memory.

The attenuation of suppression of motility by triazolam in the conditioned fear stress task is exacerbated by ethanol in mice

We investigated whether triazolam attenuated the suppression of motility in the conditioned fear stress task in mice and whether ethanol modified the effects of triazolam. When mice were placed 24 hours later (retention test) in the same environment in which they had previously been exposed to an electric foot shock (training), they exhibited a marked suppression of motility (conditioned fear stress). Triazolam (0.01-0.1 mg/kg, s.c.), administered before training, attenuated the suppression of motility in the conditioned fear stress task in a dose-dependent manner, without affecting the sensitivity to an electric foot shock. The doses of triazolam that attenuated the suppression of motility were much lower that those of chlordiazepoxide (5-10 mg/kg, s.c.). Neither drug, administered before the retention test, attenuated the suppression of motility in the conditioned fear stress task. These results suggest that both benzodiazepines may inhibit the process of acquisition, but not the process of recall, of memory. Ethanol (1 g/kg, p.o.), which, by itself, did not affect either the suppression of motility or the sensitivity to an electric foot shock, exacerbated the attenuation of the suppression of motility in the conditioned fear stress task induced by both triazolam (0.01 mg/kg) and chlordiazepoxide (5 mg/kg). These results suggest that ethanol exacerbates the effects of benzodiazepines.

Involvement of cholinergic systems in the deficit of place learning in Morris water maze task induced by baclofen in rats

Effects of oxotremorine on the deficit of place learning in the Morris water maze task induced by baclofen and scopolamine were examined to determine the involvement of brain cholinergic systems in the deficit of learning induced by baclofen. Rats were given 4 training trials per day with the submerged platform at a fixed location in the maze for 4 days. On day 4, rats were required to swim in the pool without the platform after the 4th training trial (probe test). Baclofen as well as scopolamine dose-dependently increased the escape latency in the training trials. In the probe test, baclofen as well as scopolamine dose-dependently reduced the duration in the quadrant where the platform had been originally located. Increased latency in the training trials and reduced duration in the probe test induced by scopolamine were dose-dependently attenuated by oxotremorine. Increased latency and reduced duration in the baclofen-treated rats were improved by oxotremorine as well as 2-hydroxysaclofen. Baclofen but not scopolamine induced motor incoordination in the rotarod test. Oxotremorine failed to improve motor incoordination induced by baclofen. These results suggest that cholinergic systems may be involved in the deficit of place learning induced by baclofen, and that the ameliorative effects of oxotremorine may not be due to improvement of motor incoordination.

Muscimol induces state-dependent learning in Morris water maze task in rats

Effects of muscimol on the place learning in Morris water maze task were investigated in rats. Rats were given 4 training trials per day with the submerged platform at a fixed location in the maze for 4 days. On day 4, rats were required to swim in the pool without the platform after 4 training trials (probe test). Compared to the saline-treated rats, the rats treated with muscimol on day 1-4 showed no modifications of place learning in the training trials and the probe test. However, in the rats treated with muscimol on day 1-3 and treated with saline on day 4, there was increased latency to reach the platform and reduced duration in the quadrant where the platform had been located on day 4. The increased latency in the training trials and reduced duration in the probe test on day 4 was blocked by bicuculline, when bicuculline and muscimol were co-administered on day 1-3, and saline was injected on day 4. Moreover, in the rats treated with muscimol on day 1-3, co-administration of bicuculline and muscimol on day 4 blocked place learning: increased latency in the training trials and reduced duration in the probe test was observed. These results suggest that muscimol induces state-dependent learning (SDL) in Morris water maze task, and that muscimol-induced SDL is mediated by GABAA receptors.

Neuromodulation and cortical function: modeling the physiological basis of behavior

Neuromodulators including acetylcholine, norepinephrine, serotonin, dopamine and a range of peptides alter the processing characteristics of cortical networks through effects on excitatory and inhibitory synaptic transmission, on the adaptation of cortical pyramidal cells, on membrane potential, on the rate of synaptic modification, and on other cortical parameters. Computational models of self-organization and associative memory function in cortical structures such as the hippocampus, piriform cortex and neocortex provide a theoretical framework in which the role of these neuromodulatory effects can be analyzed. Neuromodulators such as acetylcholine and norepinephrine appear to enhance the influence of synapses from afferent fibers arising outside the cortex relative to the synapses of intrinsic and association fibers arising from other cortical pyramidal cells. This provides a continuum between a predominant influence of external stimulation to a predominant influence of internal recall (extrinsic vs. intrinsic). Modulatory influence along this continuum may underlie effects described in terms of learning and memory, signal to noise ratio, and attention.

Effects of hormonal treatment and history on scopolamine inhibition of lordosis

The muscarinic receptor blocker, scopolamine, inhibits the display of lordosis behavior in female rats but its effectiveness depends on hormonal conditions. In these experiments, systemic administration of scopolamine (0.031-4 mg/kg) inhibited lordosis in ovariectomized rats brought into receptivity by treatment with a low dose of estradiol benzoate (EB, 0.25 micrograms for 3 days) with progesterone (P, 500 micrograms for 1 day), or a high dose of EB (25 micrograms for 3 days) with and without P. However, the effectiveness of scopolamine was reduced at the high dose of EB and with the addition of P. Furthermore, scopolamine failed to inhibit lordosis in females treated on a second week with the high dose of EB with or without P, unless an interval of at least 3 weeks separated the two sets of steroid treatments. The reduced effectiveness of scopolamine cannot be explained by peripheral mechanisms because its inhibitory effect on lordosis also was reduced following repeated hormonal exposure even when scopolamine was infused directly into the lateral ventricles.

Ferritin-dependent inactivation of microsomal glucose-6-phosphatase

Glucose-6-phosphatase (G6Pase) is a microsomal enzyme which is very sensitive to inactivation by lipid peroxidation. Experiments were carried out to evaluate whether ferritin, which is the major storage form of iron within cells, could catalyze inactivation of G6Pase and to determine the mechanism responsible for this effect of ferritin. Incubation of microsomes with NADPH in the absence of ferritin led to decreased activity of G6Pase. Ferritin stimulated this inactivation of G6Pase in a time- and concentration-dependent manner. Ferritin did not stimulate G6Pase inactivation when NADH replaced NADPH as the microsomal reductant. Superoxide dismutase but not catalase or DMSO prevented the ferritin-stimulated inactivation of G6Pase suggesting a role for superoxide, but not H2O2 or hydroxyl radical, in the overall mechanism. Trolox, at concentrations which prevent lipid peroxidation, also prevented the ferritin-catalyzed inactivation of G6Pase. Inhibition of G6Pase by ferritin was further enhanced in the presence of ATP but was inhibited in the presence of EDTA or desferrioxamine; ferric-ATP stimulates, whereas ferric-EDTA inhibits microsomal lipid peroxidation. The redox cycling agent paraquat increased the ability of ferritin to inactivate G6Pase by a reaction prevented by superoxide dismutase, trolox, EDTA, and desferrioxamine, but not by catalase or DMSO. Ferritin stimulated microsomal light emission, a reaction reflecting lipid peroxidation, with time and concentration dependence, and sensitivity to scavengers (trolox, superoxide dismutase), iron chelators and paraquat, identical to the inactivation of G6Pase. These results indicate that one possible toxicological consequence of ferritin-catalyzed lipid peroxidation is inhibition of microsomal enzymes such as G6Pase.

Characterization of the effects of scopolamine on the habituation of exploratory activity: differential effects of oxotremorine and physostigmine

1. Habituation was regarded as a difference between exploratory activity measured first (session 1) and that measured second (session 2) in a novel environment. 2. Scopolamine (1.0 mg/kg) significantly increased the horizontal activity in sessions 1 and 2 when administered prior to session 1, resulting in the impairment of habituation. 3. Haloperidol (0.2 mg/kg) inhibited scopolamine-induced hypermotility in session 1, but it did not inhibit the scopolamine-induced impairment of habituation in session 2. 4. The direct cholinergic agonist oxotremorine (0.03 mg/kg), unlike the cholinesterase inhibitor physostigmine, significantly inhibited the scopolamine-induced impairment of habituation in the horizontal and vertical activities. 5. These results suggest that the direct stimulation of cholinergic receptors is more effective for scopolamine-induced amnesia than the indirect stimulation of cholinergic receptors by cholinesterase inhibitors in the habituation task.

Effects of intracranial infusions of chlordiazepoxide on spatial learning in the Morris water maze. II. Neuropharmacological specificity

In the preceding paper it was found that infusions of chlordiazepoxide (CDP) into the medial septal region, but not several other regions possessing a high density of benzodiazepine receptors, impaired spatial learning, but not cue learning or swim speed, in the Morris water maze. The present investigation sought to further characterize the neuropharmacological profile of this effect. Initially, it was reconfirmed that systemically administered CDP impaired spatial learning, but not cue learning or swim speed, in the water maze. Additionally, it was found that systemically administered scopolamine, a muscarinic antagonist, impaired both spatial and cue learning, but not swim speed, confirming the detrimental effects of cholinergic hypofunction on maze learning. In new rats, a dose-response assessment revealed that 60 and 30 nmol, but not 10 nmol, CDP infused into the medial septum impaired spatial learning, but not cue learning or swim speed. On the following day, rats from each dose group, now undrugged, acquired a reversed platform location at control levels, suggesting that the previously observed impairment was not due to a neurotoxic effect. Additionally, it was found that systemically administered flumazenil (10 mg/kg) blocked the spatial learning deficit produced by the 60 nmol dose of CDP infused into the medial septum. However, intraseptal infusions of flumazenil (10, 20, or 30 nmol) failed to attenuate the spatial learning deficit produced by systemically administered CDP. Finally, systemically administered tetrahydroaminoacridine (1 or 3 mg/kg), an acetylcholinesterase inhibitor, failed to attenuate the spatial learning deficit produced by intraseptal CDP (60 nmol). Together these results implicate benzodiazepine receptors in the medial septum in the amnesic actions of CDP but suggest that additional sites also mediate this action. The present results fail to support the idea that the spatial learning deficit produced by intraseptal infusions of CDP is due to a suppression of septo-hippocampal cholinergic activity and it is proposed that CDP impairs spatial learning by exacerbating hippocampal inhibition by inhibiting septo-hippocampal GABAergic projection neurons.

Scopolamine does not differentially affect Morris maze performance in adult rats exposed prenatally to alcohol

Rats exposed prenatally to alcohol have shown deficits in spatial learning in radial-arm and Morris mazes. Prenatal exposure to alcohol in rats has also been shown to alter central nervous system (CNS) cholinergic function. Since cholinergic dysfunction disrupts spatial learning in normal rats, the present experiment assessed the role of putative prenatal alcohol-induced cholinergic dysfunction in spatial learning in rats. Pregnant rats were fed alcohol via liquid diet from gestation day 6 to 20. Control dams were pair-fed liquid diet without alcohol or fed ad lib lab chow and water. Group housed adult male and female offspring (postnatal days 110 to 135) were given scopolamine-HCl (0, 0.5, or 1.0 mg/kg/day) and tested in a Morris maze, with four trials per day for four days. A 15-s probe trial preceded testing on days 2-4. On day 5, the rats were given four trials to learn a new platform location. Scopolamine produced dose-dependent increases in latency to find the platform for all groups. There were no significant differences among prenatal treatment groups in scopolamine-induced shifts in performance. The results did not support the hypothesis that prenatal alcohol-induced CNS cholinergic dysfunction is related to spatial learning performance in these rats.

The neuropharmacological and neurochemical basis of place learning in the Morris water maze

The Morris water maze (MWM) offers several advantages over other methods of studying the neurochemical basis of learning and memory, particularly with respect to its ability to dissociate deficits in memory formation from deficits in sensory, motor, motivational and retrieval processes. The contributions of nearly all of the major neurotransmitter systems have been investigated and consistent patterns have emerged. Normal function in glutamatergic and cholinergic systems is necessary for spatial learning, as blockade of NMDA receptors and cholinergic hypofunction prevents spatial learning but does not impair recall. Peptides such as adrenal and sex hormones and somatostatin may also be necessary for spatial learning. In contrast, activity in either GABAergic or opioidergic systems impairs spatial learning, though by quite different means. GABAergic activity prevents memory function, whereas opioidergic activity reduces motivation. Normal monoaminergic activity is necessary for normal performance in the MWM, but not for spatial learning per se. However, noradrenergic and serotonergic systems may enhance cholinergic-mediated mnemonic processes. Further research into the relative contributions of different receptor subtypes as well as interactions between neurochemical systems should provide significant advances in our understanding of the neural basis of learning and memory in mammals.