Scopolamine impairs learning performance of rats in a 14-unit T-maze

Apigenin Ameliorates Scopolamine-Induced Cognitive Dysfunction and Neuronal Damage in Mice

We investigated the protective effect and mechanisms of apigenin against cognitive impairments in a scopolamine-injected mouse model. Our results showed that intraperitoneal (i.p.) injection of scopolamine leads to learning and memory dysfunction, whereas the administration of apigenin (synthetic compound, 100 and 200 mg/kg/day) improved cognitive ability, which was confirmed by behavioral tests such as the T-maze test, novel objective recognition test, and Morris water maze test in mice. In addition, scopolamine-induced lipid peroxidation in the brain was attenuated by administration of apigenin. To further evaluate the protective mechanisms of apigenin on cognitive and memory function, Western blot analysis was carried out. Administration of apigenin decreased the B-cell lymphoma 2-associated X/B-cell lymphoma 2 (Bax/Bcl-2) ratio and suppressed caspase-3 and poly ADP ribose polymerase cleavage. Furthermore, apigenin down-regulated the β-site amyloid precursor protein-cleaving enzyme, along with presenilin 1 (PS1) and PS2 protein levels. Apigenin-administered mice showed lower protein levels of a receptor for advanced glycation end-products, whereas insulin-degrading enzyme, brain-derived neurotrophic factor (BDNF), and tropomyosin receptor kinase B (TrkB) expression were promoted by treatment with apigenin. Therefore, this study demonstrated that apigenin is an active substance that can improve cognitive and memory functions by regulating apoptosis, amyloidogenesis, and BDNF/TrkB signaling pathways.

Stereotypy and spontaneous alternation in deer mice and its response to anti-adenosinergic intervention

Repetitive behavioral phenotypes are a trait of several neuropsychiatric disorders, including obsessive-compulsive disorder (OCD). Such behaviors are typified by complex interactions between cognitive and neurobiological processes which most likely contribute to the suboptimal treatment responses often observed. To this end, exploration of the adenosinergic system may be useful, since adenosine-receptor modulation has previously shown promise to restore control over voluntary behavior and improve cognition in patients presenting with motor repetition. Here, we employed the deer mouse (Peromyscus maniculatus bairdii) model of compulsive-like behavioral persistence, seeking to investigate possible associations between stereotypic motor behavior and cognitive flexibility as measured in the T-maze continuous alternation task (T-CAT). The effect of istradefylline, a selective adenosine A_2A receptor antagonist at two doses (10 and 20 mg kg^-1  day^-1 ) on the expression of stereotypy and T-CAT performance in high (H) and non-(N) stereotypical animals, was investigated in comparison to a control intervention (six groups; n = 8 or 9 per group). No correlation between H behavior and T-CAT performance was found. However, H but not N animals presented with istradefylline-sensitive spontaneous alternation and stereotypy, in that istradefylline at both doses significantly improved the spontaneous alternation scores and attenuated the stereotypical expression of H animals. Thus, evidence is presented that anti-adenosinergic drug action improves repetitive behavior and spontaneous alternation in stereotypical deer mice, putatively pointing to a shared psychobiological construct underlying naturalistic stereotypy and alterations in cognitive flexibility in deer mice.

Central muscarinic cholinergic involvement in serial pattern learning: Atropine impairs acquisition and retention in a serial multiple choice (SMC) task in rats

Atropine sulfate is a muscarinic cholinergic antagonist which impairs acquisition and retention performance on a variety of cognitive tasks. The present study examined the effects of atropine on acquisition and retention of a highly-structured serial pattern in a serial multiple choice (SMC) task. Rats were given daily intraperitoneal injections of either saline or atropine sulfate (50mg/kg) and trained in an octagonal operant chamber equipped with a lever on each wall. They learned to press the levers in a particular order (the serial pattern) for brain-stimulation reward in a discrete-trial procedure with correction. The two groups learned a pattern composed of eight 3-element chunks ending with a violation element: 123-234-345-456-567-678-781-818 where the digits represent the clock-wise positions of levers in the chamber, dashes indicate 3-s pauses, and other intertrial intervals were 1s. Central muscarinic cholinergic blockade by atropine caused profound impairments during acquisition, specifically in the encoding of chunk-boundary elements (the first element of chunks) and the violation element of the pattern, but had a significant but negligible effect on the encoding of within-chunk elements relative to saline-injected rats. These effects persisted when atropine was removed, and similar impairments were also observed in retention performance. The results indicate that intact central muscarinic cholinergic systems are necessary for learning and producing appropriate responses at places in sequences where pattern structure changes. The results also provide further evidence that multiple cognitive systems are recruited to learn and perform within-chunk, chunk-boundary, and violation elements of a serial pattern.

Neuroprotection provided by dietary restriction in rats is further enhanced by reducing glucocortocoids

Glucocorticoids (GC)--corticosterone (CORT) in rodents and cortisol in primates--are stress-induced hormones secreted by adrenal glands that interact with the hypothalamic pituitary axis. High levels of cortisol in humans are observed in neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), as well as in diabetes, post-traumatic stress syndrome, and major depression. Experimental models of diabetes in rats and mice have demonstrated that reduction of CORT reduces learning and memory deficits and attenuates loss of neuronal viability and plasticity. In contrast to the negative associations of elevated GC levels, CORT is moderately elevated in dietary restriction (DR) paradigms which are associated with many healthy anti-aging effects including neuroprotection. We demonstrate here in rats that ablating CORT by adrenalectomy (ADX) with replenishment to relatively low levels (30% below that of controls) prior to the onset of a DR regimen (ADX-DR) followed by central administration of the neurotoxin, kainic acid (KA), significantly attenuates learning deficits in a 14-unit T-maze task. The performance of the ADX-DR KA group did not differ from a control group (CON) that did not receive KA and was fed ad libitum (AL). By contrast, the sham-operated DR (SHAM-DR KA) group, SHAM-AL KA group, and ADX-AL KA group demonstrated poorer learning behavior in this task compared to the CON group. Stereological analysis revealed equivalent DR-induced neuroprotection in the SH-DR KA and ADX-DR KA groups, as measured by cell loss in the CA2/CA3 region of the hippocampus, while substantial cell loss was observed in SH-AL and ADX-AL rats. A separate set of experiments was conducted with similar dietary and surgical treatment conditions but without KA administration to examine markers of neurotrophic activity, brain-derived neurotrophic factor (BDNF), transcriptions factors (pCREB), and chaperone proteins (HSP-70). Under these conditions, we noted elevations in both BDNF and pCREB in ADX DR rats compared to the other groups; whereas, HSP-70, was equivalently elevated in ADX-DR and SH-DR groups and was higher than observed in both SH-AL and ADX-AL groups. These results support findings that DR protects hippocampal neurons against KA-induced cellular insult. However, this neuroprotective effect was further enhanced in rats with a lower-than control level of CORT resulting from ADX and maintained by exogenous CORT supplementation. Our results then suggest that DR-induced physiological elevation of GC may have negative functional consequences to DR-induced beneficial effects. These negative effects, however, can be compensated by other DR-produced cellular and molecular protective mechanisms.

Age-associated learning and memory deficits in two mouse versions of the Stone T-maze

We have previously reported that a modified Stone T-maze (STM), using escape from water as motivation, was effective in evaluating learning and memory ability in young C57/BL6 mice. Here we report on the effectiveness and sensitivity of the STM in the assessment of age-related learning and memory deficits in mice using either escape from foot shock or water as the motivational manipulations. C57BL/6Nia mice 7-, 12-, 20- and 24-months old received 15 massed trials in the escape from foot shock motivated STM while C57BL/6Nia mice 5-, 12-, and 25-months old were tested in the escape from water STM. Analysis of errors, the main performance variable, revealed similar results in both versions of the task with younger mice making fewer errors. Notably, mice of all ages in the water-motivated version moved quickly through the maze, while all ages of mice in the shock-motivated version tended to wait for shock to be initiated to move forward. Overall, both versions of the STM appear to be sensitive to age-related changes in learning and memory and provide an alternative to other testing paradigms such as the Morris water maze which are susceptible to performance confounds which can lead to uninterpretable results.

Development of a water-escape motivated version of the Stone T-maze for mice

Mice provide a highly valuable resource for investigating learning and memory processes; however, many of the established tasks for evaluating learning and memory were developed for rats. Behaviors of mice in these tasks appear to be driven by different motivational factors, and as a result, they often do not perform reliably on tasks involving rewards traditionally used for rats. Because of difficulties in measuring learning and memory in mice as well as the need to have a task that can reliably measure these behavioral processes, we have developed a mouse version of the Stone T-maze utilizing what appears to be the primary motivation of mice, escape to a safe location. Specifically, we have constructed a task that requires the mouse to wade through water to reach a dark and dry goal box. To escape this aversive environment, the Stone T-maze requires learning the correct sequence of 13 left and right turns to reach the goal box. Through a series of experiments examining a variety of protocols, it was found that mice will reliably perform this task. This task can be used to assess learning and memory without the potential performance confounds that can affect performance of mice in other tasks. We believe this task offers a valuable new tool for evaluating learning and memory in mice not previously available to researchers.

Striatal lesions interfere with acquisition of a complex maze task in rats

The 14-unit T-maze has proven to be a valuable tool for investigating age-associated memory impairment (AAMI). While another task widely used to evaluate AAMI, the water maze, is primarily used to evaluate allocentric hippocampal-dependent spatial memory, the 14-unit T-maze can assess egocentric procedural memory. Although several brain structures, e.g. hippocampus, parietal cortex, have been implicated in acquisition and retention performance in the 14-unit T-maze, there has been no evaluation of the involvement of the striatum, a brain region implicated in procedural learning and memory. The current study revealed that excitotoxic lesions of the medial or lateral striatum significantly impaired acquisition, as measured by errors and latency, on this task without disruption of motor function. These results indicate that the 14-unit T-maze most likely is requires a large egocentric procedural learning component, and previously observed AAMI may involve age-related dysfunction of the striatum.

Combined administration of subthreshold doses of the nitric oxide inhibitor, nitro-L-arginine, and muscarinic receptor antagonist, scopolamine, impairs complex maze learning in rats

Traditionally, research into the neurobiological mechanisms of age-related memory impairments has focused on single neurotransmitter systems. As normal and abnormal age-related declines in memory function probably involve alterations in more than one system, a more effective approach for elucidating underlying neurobiological changes and resulting impairments may be to evaluate the roles of multiple systems simultaneously. This study evaluated the interaction of the cholinergic and nitric oxide systems in rats on acquisition in the 14-unit T-maze. This task requires learning a series of turns to avoid foot shock, and most likely reflects procedural learning. Administration of scopolamine (0.1 mg/kg) or N-nitro-L-arginine methyl ester (30 mg/kg) alone did not impair acquisition, whereas administration of the same doses in combination increased both the latency to complete the maze and number of errors committed. These data suggest that manipulation of learning and memory processes with multiple compounds potentially offers a clinically relevant paradigm for investigating cognitive function in normal and abnormal aging.

A blueberry-enriched diet provides cellular protection against oxidative stress and reduces a kainate-induced learning impairment in rats

Young male Fischer-344 rats were fed a diet containing 2% blueberry (BB) extract or control diet for at least 8 weeks and then received bilateral hippocampal injections of kainic acid (KA 200 ng/0.5 microl) or phosphate buffered saline (PBS). One week later rats were trained in one-way active footshock avoidance in a straight runway followed the next day by training in a footshock motivated 14-unit T-maze with documented sensitivity to hippocampal glutamatergic manipulations. Based on analyses of several performance variables, KA-treated rats exhibited clearly impaired learning performance; however, the BB diet significantly reduced this impairment. Supporting the behavioral findings, stereological assessment of CA1 pyramidal neurons documented greater neuronal loss in KA-treated controls compared to KA-treated rats on the BB diet. In an in vitro experiment, FaO cells grown in medium supplemented with serum from BB-fed rats had enhanced viability after exposure to hydrogen peroxide. These findings suggest that BB supplementation may protect against neurodegeneration and cognitive impairment mediated by excitotoxicity and oxidative stress.

Preclinical investigation of the topical administration of phenserine: transdermal flux, cholinesterase inhibition, and cognitive efficacy

Phenserine (PS) was designed as a selective acetylcholinesterase (AChE) inhibitor, with a tartrate form (PST) for oral administration in mild to moderate Alzheimer's disease (AD). Recent phase 3 trials of PST in Europe indicate that any clinically relevant activity of PST may be limited by its duration of action. Like many oral drugs, bioavailability and plasma concentrations of PST are regulated by hepatic and gastrointestinal first-pass effects. To minimize the kinetic limitations of first-pass metabolism, transdermal formulations of PS and PST (ointment/patch) were developed and characterized in vitro and in vivo. Initial in vitro kinetic characterization of PS or PST formulations used a diffusion cell chamber and skin samples isolated from hairless mice. Liquid paraffin and fatty alcohol/propylene glycol (FAPG) were found to be suitable vehicles for ointment formulation. Addition of a penetration enhancer, 1-[2-(decylthio)ethyl]-azacyclopentane-2-one (HPE-101), improved stratum corneum permeability. Application of the optimal formulation of PS/HPE-101/FAPG to the shaved back of rats resulted in significantly lowered plasma and brain AChE activities and improved cognitive performance in animals with scopolamine-induced cognitive impairment. These results suggest that the transdermal application of AChE inhibitors may represent an effective therapeutic strategy for AD. Particular benefits over oral therapies might include avoiding first-pass metabolic effects and improved dosing compliance.

Sildenafil citrate attenuates a complex maze impairment induced by intracerebroventricular infusion of the NOS inhibitor Nomega-nitro-L-arginine methyl ester

In a previous study, our laboratory reported that sildenafil citrate, a cyclic nucleotide phosphodiesterase type 5 inhibitor, reversed a learning impairment in rats induced by systemic inhibition of nitric oxide synthase (60 mg/kg, i.p., Nomega-nitro-L-arginine methyl ester; L-NAME). To limit the peripheral effects of L-NAME and further localize the site of action of sildenafil, L-NAME (48 microg, i.c.v.) was infused bilaterally into the lateral cerebral ventricles 30 min prior to maze training. Saline or sildenafil citrate (1.5 or 3.0 mg/kg, i.p.) was administered systemically 15 min before training. Drug injections occurred 24 h after pretraining rats to avoid foot shock on a one-way active avoidance straight runway. Following drug treatment, the rats received 15 training trials on a 14-unit T-maze task that requires learning a complex sequence of turns to avoid mild foot shock. This complex maze paradigm is sensitive to aging and blockade of cholinergic, N-methyl-D-aspartate and nitric oxide signaling systems. Behavioral measures of performance included deviations from the correct pathway (errors), runtime from start to goal (latency), shock frequency and shock duration. Statistical analysis revealed that central infusion of L-NAME impaired maze performance and that sildenafil (3.0 mg/kg) significantly attenuated the impairment. These results suggest that sildenafil citrate may serve as a cognitive enhancer by modulating central nitric oxide/cGMP signal transduction following N-methyl-D-aspartate receptor activation. This pathway has been implicated in age-related cognitive decline and may be a useful target for pharmacological intervention of neurodegenerative disease.

Phosphodiesterase inhibition by sildenafil citrate attenuates a maze learning impairment in rats induced by nitric oxide synthase inhibition

RATIONALE

The nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signal transduction pathway has been implicated in some forms of learning and memory. Recent findings suggest that inhibition of phosphodiesterase (PDE) enzymes that degrade cGMP may have memory-enhancing effects.

OBJECTIVES

We examined whether treatment with sildenafil citrate, a PDE type 5 inhibitor, would attenuate a learning impairment induced by inhibition of NO synthase [60 mg/kg N(omega)-nitro-L-arginine methyl ester (L-NAME), i.p.].

METHODS

Rats were pretrained in a one-way active avoidance of foot shock in a straight runway and, on the next day, received 15 training trials in a 14-unit T-maze, a task that has been shown to be sensitive to aging and impairment of central NO signaling systems. Combined treatments of L-NAME or saline and sildenafil (1.0, 1.5, 3.0, or 4.5 mg/kg, i.p.) or vehicle were given 30 and 15 min before training, respectively. Behavioral measures of performance included entries into incorrect maze sections (errors), run time from start to goal (latency), shock frequency, and shock duration.

RESULTS

Statistical analysis revealed that L-NAME impaired maze performance and that sildenafil (1.5 mg/kg) significantly attenuated this impairment. Control experiments revealed that administration of L-NAME alone did not significantly increase latencies in a one-way active avoidance test and that different doses of sildenafil alone did not significantly alter complex maze performance.

CONCLUSIONS

The results indicate that sildenafil may improve learning by modulating NO-cGMP signal transduction, a pathway implicated in age-related cognitive decline and neurodegenerative disease.

Investigations of the cholinergic deficit hypothesis in the hippocampus of the aged rat brain with physostigmine and scopolamine

Using histochemically demonstrated acetylcholinesterase activity and (14)C-2-deoxyglucose uptake as the respective indices, a study was set up to determine whether cerebral (hippocampal) metabolism was stimulated by a cholinergic agonist and/or inhibited by a cholinergic antagonist. For this 36 12-month-old (adult) and 48 27-month-old (aged) Fischer 344 rats were given intraperitoneal injections of physostigmine 0.05, 0.1 or 0.2 mg/kg or scopolamine 0.01, 0.03 or 0.1 mg/kg for 5 days. In the aged rats there was a slight increase in acetylcholinesterase activity after physostigmine but no convincing evidence of enhanced (14)C-2-deoxyglucose uptake. In neither age group was glucose uptake significantly reduced by scopolamine; it was in fact increased, as was - slightly but significantly - acetylcholinesterase activity. Findings for acetylcholinesterase activity and (14)C-2-deoxyglucose uptake in aged Fischer 344 rats thus do not provide firm corroboration of physostigmine-induced stimulation of mental performance found in behavioural studies, while scopolamine did not adversely affect the hippocampal variables studied. It is concluded that cholinergic agents such as physostigmine and scopolamine have only a marginal effect on the functional and metabolic deficits associated with cerebral aging.

APP23 mice as a model of Alzheimer's disease: an example of a transgenic approach to modeling a CNS disorder

Animal models are considered essential in research ensuing elucidation of human disease processes and subsequently, testing of potential therapeutic strategies. This is especially true for neurodegenerative disorders, in which the first steps in pathogenesis are often not accessible in human patients. Alzheimer's disease is vastly becoming a major medical and socioeconomic problem in our aging society. Valid animal models for this uniquely human condition should exhibit histopathological, biochemical, cognitive, and behavioral alterations observed in Alzheimer's disease patients. Major progress has been made since the understanding of the genetic basis of Alzheimer's disease and the development and improvement of transgenic mouse models. All present Alzheimer's disease models developed are partial but nevertheless essential in further unraveling the nature and spatial and temporal development of the complex molecular pathology underlying this condition. One of the more recent transgenic attempts to model Alzheimer's disease is the APP23 transgenic mouse. This article describes the development and assessment of this human amyloid precursor protein overexpression model. We summarize histopathological and biochemical, cognitive and behavioral observations made in heterozygous APP23 mice, thereby emphasizing the model's contribution to clarification of neurodegenerative disease mechanisms. In addition, the first therapeutic interventions in the APP23 model are included.

Residential maze as a task for testing rats' maze learning ability: effects of hippocampal lesions and cholinergic receptor antagonists

We investigated whether rats can learn mazes by a procedure in which rats were left in the maze (residential maze) in groups for 1 h a day. Water and food locations, which served as the start and goal boxes respectively in the test trial, were at the opposite ends of the maze. On the test trial conducted everyday before the residence period, animals put in the start box showed a significant decrease of the error response into the blind alleys and running time to reach the goal box. Systemic administration of scopolamine (0.25, 0.5 mg/kg), a muscarinic receptor antagonist, dose-dependently increased the number of errors, but mecamylamine, a nicotinic receptor antagonist, had no effect. Bilateral hippocampal lesions retarded both the acquisition and retention of this maze learning. The results suggest that this residential maze procedure is useful for testing maze learning ability in rats.

Effects of predictable and unpredictable intermittent noise on spatial learning in rats

The effects of predictable (periodic) and unpredictable (aperiodic) intermittent noise of moderate intensity (68 dB) on the learning of a complex T-maze by genetically defined rats were investigated. In Experiment 1, three groups (n=8) of rats learned a multiple T-maze, one group under control conditions, one group with predictable intermittent noise and one group with unpredictable intermittent noise. Results showed a profound effect of noise on learning and behavioural scores. Noise-exposed animals made less errors, finished their trials sooner and explored less. There was no difference between predictable and unpredictable noise. Further tests, during which formerly noise-exposed groups learned a new route under control conditions (Experiment 2) or the former controls learned a new route with noise (Experiment 3), suggest that the effects of noise on learning were caused by an effect of noise on memory formation and/or retrieval, rather than by long-term shifts in behavioural strategies.

Passive avoidance and complex maze learning in the senescence accelerated mouse (SAM): age and strain comparisons of SAM P8 and R1

Two strains of the senescence accelerated mouse, P8 and R1,were tested in footshock-motivated passive avoidance (PA; P8, 3-21 months; R1, 3-24 months) and 14-unit T-maze (P8 and R1, 9, and 15 months) tasks. For PA, entry to a dark chamber from a lighted chamber was followed by a brief shock. Latency to enter the dark chamber 24 hours later served as a measure of retention. Two days of active avoidance training in a straight runway preceded 2 days (8 trials/day) of testing in the 14-unit T-maze. For PA retention, older P8 mice entered the dark chamber more quickly than older R1 mice, whereas no differences were observed between young P8 or R1 mice. In the 14-unit T-maze, age-related learning performance deficits were reflected in higher error scores for older mice. P8 mice were actually superior learners; that is, they had lower error scores compared with those of age-matched R1 counterparts. Although PA learning results were in agreement with other reports, results obtained in the 14-unit T-maze were not consistent with previous reports of learning impairments in the P8 senescence accelerated mouse.

Nootropic-like effect of ashwagandha (Withania somnifera L.) in mice

Ashwagandha (Withania somnifera L.) root extract (50, 100 and 200 mg/kg; orally) improved retention of a passive avoidance task in a step-down paradigm in mice. Ashwagandha (50, 100 and 200 mg/kg; orally) also reversed the scopolamine (0.3 mg/kg)-induced disruption of acquisition and retention and attenuated the amnesia produced by acute treatment with electroconvulsive shock (ECS), immediately after training. Chronic treatment with ECS, for 6 successive days at 24 h intervals, disrupted memory consolidation on day 7. Daily administration of ashwagandha for 6 days significantly improved memory consolidation in mice receiving chronic ECS treatment. Ashwagandha, administered on day 7, also attenuated the disruption of memory consolidation produced by chronic treatment with ECS. On the elevated plus-maze, ashwagandha reversed the scopolamine (0.3 mg/kg)-induced delay in transfer latency on day 1. On the basis of these findings, it is suggested that ashwagandha exhibits a nootropic-like effect in naive and amnesic mice.

Involvement of dopamine D(2) receptors in complex maze learning and acetylcholine release in ventral hippocampus of rats

In the current study we focus on the involvement of dopamine D(2) receptors in the ventral hippocampus in memory performance and acetylcholine release. Using the aversively motivated 14-unit T-maze (Stone maze) the injection of raclopride, a D(2) receptor antagonist, into the ventral hippocampus (8 microg/kg) was found to impair memory performance. Co-injection of quinpirole, a D(2) receptor agonist (8 microg/kg), overcame the impairment in performance. Microdialysis study revealed that quinpirole infusion (10-500 microM) into the ventral hippocampus stimulated acetylcholine release in a dose-dependent manner, and systemic injection of quinpirole (0.5 mg/kg, i.p.) also stimulated acetylcholine release in the ventral hippocampus. Infusion of eticlopride, another D(2) receptor antagonist, into the ventral hippocampus suppressed acetylcholine release in the hippocampus induced by systemic injection of quinpirole. Taken together, we suggest that D(2) receptors in the ventral hippocampus are involved in memory performance, possibly through the regulation of acetylcholine.

Influence of cholinergic system on motor learning during aging in mice

Three, 12- and 20-month-old C57Bl6 mice, reared in standard conditions or in an enriched environment, were administered subcutaneously either scopolamine hydrobromide (SIGMA), 0.6 and 1.2 mg kg(-1), or physiological saline 15 min before testing their motor skills (muscular strength, dynamic equilibrium and motor coordination) and motor learning abilities (number of trials needed to reach a learning criterion on a rotorod rotating at 27 revolutions per min). The results demonstrated a lack of correlation between motor skill scores and between motor skill and motor learning scores, suggesting that the rotorod training procedure measures motor learning and not motor skills or is insensitive to changes in motor skills. They also demonstrated that motor skills decreased with age but were insensitive to environmental rearing and to scopolamine. In contrast, the learning scores, which also decreased with age, were very sensitive to scopolamine, particularly in the oldest mice. These results are discussed according to the role of cholinergic system in motor learning during aging.

Differential participation of the NBM in the acquisition and retrieval of conditioned taste aversion and Morris water maze

Deficits in both learning and memory after lesions of the cholinergic basal forebrain, in particular the nucleus basalis magnocellularis (NBM), have been widely reported. However, the participation of the cholinergic system in either acquisition or retrieval of memory process is still unclear. In this study, we tested the possibility that excitotoxic lesions of the NBM affect either acquisition or retrieval of two tasks. In the first experiment, animals were trained for two conditioned taste aversion tasks using different flavors, saccharine and saline. The acquisition of the first task was before NBM lesions (to test retrieval) and the acquisition of the second task was after the lesions (to test acquisition). Accordingly, in the first part of the second experiment, animals were trained in the Morris water maze (MWM), lesioned and finally tested. In the final part of this experiment, another set of animals was lesioned, then trained in the MWM and finally tested. All animals were able to retrieve conditioned taste aversion (CTA) and MWM when learned before NBM lesions; however, lesions disrupted the acquisition of CTA and MWM. The results suggest that the NBM and cholinergic system may play an important role in acquisition but not during retrieval of aversive memories.

The effects of some K(+) channel blockers on scopolamine- or electroconvulsive shock-induced amnesia in mice

The effects of three K(+) channel blockers, 4-aminopyridine, 3, 4-diaminopyridine and apamin, on scopolamine- or electroconvulsive shock-induced amnesia were investigated in mice by using a one-trial step-down passive avoidance system. Scopolamine and electroconvulsive shock reduced the retention latency of passive avoidance, which indicated the amnestic effect of these treatments. 4-Aminopyridine, 3,4-diaminopyridine and apamin injected immediately after the acquisition trial, reversed the amnestic effect of scopolamine or electroconvulsive shock in a dose-dependent manner. None of the drugs or electroconvulsive shock treatment affected the rotarod or activity cage performance of the mice. These results indicate that K(+) channel blockers may improve cognitive deficits when memory is impaired by a drug or any other manipulation.

Hemodynamic changes during aging associated with cerebral blood flow and impaired cognitive function

This study investigates the age associated changes in hemorheological properties and cerebral blood flow. Partial correlations indicate that part of the age-dependent decrease in flow velocities can be attributed to a hemorheological decrement resulting in part from enhanced oxidative stress in the aged. A possible link with Alzheimer's pathology is suggested by the augmented hemorheological impairment resulting from in vitro incubation of red cells with amyloids. These results suggest that in aging, oxidative stress as well as amyloids may influence the fluid properties of blood, resulting in a potential decrement in blood flow and oxygen delivery to the brain. Animal intervention studies further demonstrate that altered hemorheological properties of blood can actually influence cognitive function. The relationships shown to exist between hemorheology, blood flow, amyloids, oxidative stress, and cognitive function suggest that these factors may be one of the mechanisms operating in the complex etiology of Alzheimer's disease.

Amnesic effects of the anticholinergic drugs, trihexyphenidyl and biperiden: differences in binding properties to the brain muscarinic receptor

An amnesic effect of anticholinergic drugs was previously described from several behavioral studies. We examined this effect induced by trihexyphenidyl and biperiden, clinically used in the parkinsonism and schizophrenic patients, by using passive avoidance tasks. Both of these drugs (0.1-10 mg/kg, s.c.) showed dose-dependent amnesic effects in the acquisition and retrieval phases. However, the effect induced by trihexyphenidyl was transient, whereas that of biperiden was long-lasting. To clarify the reason for the different duration of the amnesic activity, binding to the muscarinic receptor was examined. In the Scatchard analysis, trihexyphenidyl competed with [(3)H]quinuclidinyl benzilate ([(3)H]QNB) on the muscarinic receptor (showed increased K(d) and unchanged B(max) value), while biperiden decreased [(3)H]QNB binding (B(max) value) significantly. Furthermore, in an exchange assay for receptor inactivation, trihexyphenidyl binding to muscarinic receptors was exchanged by [(3)H]QNB completely, but biperiden decreased the exchangeable binding of [(3)H]QNB in a dose dependent manner (0.1-100 nM). These results suggested that the binding of trihexyphenidyl and biperiden to muscarinic receptor might be completely reversible and partially irreversible, respectively, whereas the K(i) values of these two drugs were similar. In conclusion, this difference in binding property may explain the difference in the time-course of the amnesic effect induced by trihexyphenidyl and biperiden.

Maze learning impairment is associated with stress hemopoiesis induced by chronic treatment of aged rats with human recombinant erythropoietin

Mean cell volume (MCV) of erythrocytes has been reported to increase with age in humans, and to be negatively correlated with memory performance in humans and rats. We evaluated hematological changes in 21-mo old male Fischer 344 rats undergoing a 3-mo twice weekly subcutaneous injection of human recombinant erythropoietin (EPO). A baseline hematocrit (HCT) was obtained initially and repeated at monthly intervals to determine the effectiveness of EPO treatment. At 24-mo of age and after 3 mo EPO treatment, the rats were tested for their ability to learn a 14-unit T maze. Following maze testing, blood was drawn for hematologic analyses, including HCT, MCV, maximum swollen cell volume (MCVS), mean cell transit time (MCTT), and the membrane shear modulus of elasticity (G), the latter a derived measure of the relative elasticity of the red cell membrane. After 1 mo EPO treatment, HCT significantly increased compared to saline-injected controls. After 2 mo treatment, HCT began to decline but remained elevated above baseline levels even after 3 mo treatment. After 3 mo EPO treatment, MCV was significantly lower in EPO-treated rats compared to controls. These changes imply altered hemopoiesis to produce cells which undergo shrinkage associated with accelerated cellular aging. The lower MCV would have predicted a shorter MCTT which instead was unchanged. This observation suggested the presence of an additional factor contributing to the MCTT. The G, which measures the membrane contribution to deformability, very significantly increased with EPO treatment. This finding indicates an increased contribution of membrane properties to the MCTT after EPO treatment, which cancels the expected decrease in MCTT for smaller cells. After 3 mo of EPO treatment, aged rats exhibited significantly impaired maze learning compared to controls. A relationship between, changes in erythrocyte membrane properties and impaired function was indicated by a significant correlation (r=0.67, p <0.04) between G and errors in the 14-unit T-maze. These findings suggest that stress-induced erythropoiesis produces accelerated aging in the red blood cell population that may have functional implications (i.e., impaired learning ability).

Age-associated memory impairment. Assessing the role of nitric oxide

Several neurotransmitter systems have been investigated to assess hypothesized mechanisms underlying the decline in recent memory abilities in normal aging and in Alzheimer's disease. Examining the performance of F344 rats in a 14-unit T-maze (Stone maze), we have focused on the muscarinic cholinergic (mACh) and the N-methyl-D-aspartate (NMDA) glutamate (Glu) systems and their interactions. Maze learning is impaired by antagonists to mACh or NMDA receptors. We have also shown that stimulation of mACh receptors can overcome a maze learning deficit induced by NMDA blockade, and stimulation of the NMDA receptor can overcome a similar blockade of mACh receptors. No consistent evidence in rats has been produced from our laboratory to reveal significant age-related declines in mACh or NMDA receptor binding in the hippocampus (HC), a brain region that is greatly involved in processing of recent memory. Thus, we have directed attention to the possibility of a common signal transduction pathway, the nitric oxide (NO) system. Activated by calcium influx through the NMDA receptor, NO is hypothesized to be a retrograde messenger that enhances presynaptic Glu release. Maze learning can be impaired by inhibiting the synthetic enzyme for NO, nitric oxide synthase (NOS), or enhanced by stimulating NO release. However, we have found no age-related loss of NOS-containing HC neurons or fibers in rats. Additionally, other laboratories have reported no evidence of an age-related loss of HC NOS activity. In a microdialysis study we have found preliminary evidence of reduced NO production following NMDA stimulation. We are currently working to identify the parameters of this phenomenon as well as testing various strategies for safely stimulating the NO system to improve memory function in aged rats.

The effects of piracetam on morphine-induced amnesia and analgesia: The possible contribution of central opiatergic mechanisms on the antiamnestic effect of piracetam

The involvement of opiatergic mechanisms on the antiamnestic effects of piracetam was investigated in mice. First, the effects of piracetam and naloxone on the amnesia induced by scopolamine, electroconvulsive shock and morphine were evaluated by using elevated plus maze apparatus. Second, the effects of electroconvulsive shock and piracetam on the antinociceptive action of morphine were tested by means of radiant heat tail-flick experiment. Piracetam and naloxone reversed the drug- or electrically-induced amnestic effects. On the other hand, electroconvulsive shock treatment enhanced the antinociceptive effect of morphine while piracetam decreased the same activity. These results suggest an important role of the opiatergic system on the learning and memory process as well as on the antiamnestic effect of piracetam.

Phenserine, a novel acetylcholinesterase inhibitor, attenuates impaired learning of rats in a 14-unit T-maze induced by blockade of the N-methyl-D-aspartate receptor

The present study evaluated the interaction of the glutamatergic and acetylcholinergic systems in memory formation, with an overall emphasis on developing multi-system approaches for treating age-related cognitive decline and Alzheimer' s disease. Specifically, we used a 14-unit T-maze to investigate whether phenserine (PHEN), a long-acting acetylcholinesterase inhibitor, could overcome a learning deficit in rats induced by the NMDA receptor antagonist, 3-(+/-) 2-carboxypiperzin-4-yl) propyl phosphonic acid (CPP). Prior to drug treatment, 3-month-old male Fischer-344 rats were trained to criterion (13 of 15 shock avoidances) in a straight runway. Twenty-four hours later, rats were given i.p. injections of saline (SAL), CPP (9 mg/kg) + SAL or CPP + PHEN (0.25, 0.5 or 0.75 mg/kg) and received 15 massed training trials in a 14-unit T-maze. CPP significantly increased the number of errors made in the maze relative to controls, and phenserine significantly reduced the number of errors made relative to rats receiving CPP only, with the lowest dose being the most effective. These results provide further support of phenserine's potent, cognitive-enhancing properties, and suggest that combined modulation of glutamatergic and acetylcholinergic systems may be of potential benefit in developing new pharmacotherapies for Alzheimer's disease and age-related cognitive decline.

Learning in a 14-unit T-maze is impaired in rats following systemic treatment with N omega-nitro-L-arginine

We examined whether inhibition of nitric oxide synthase (NO synthase) impairs learning in male Fischer-344 rats (9 mo) in a shock-motivated 14-unit T-maze. Rats were pretrained in one-way active avoidance of foot shock to a criterion of 13/15 avoidances in a straight runway. The next day, rats received intraperitoneal (i.p.) injections of 0.9% NaCl as controls or Nomega-nitro-L-arginine (N-Arg: 3.0. 4.5, or 6.0 mg/kg) to inhibit NO synthase 30 min before maze training. During 15 trials, rats were required to negotiate each of 5 segments within 10 s to avoid footshock. Performance variables included errors (deviations from the correct pathway), runtime from start to goal, shock frequency and duration. N-Arg treatment impaired performance on all variables in a dose-dependent manner. Specifically, only the 6 mg/kg N-Arg dose significantly increased errors compared to controls over the last 10 trials but not the first 5 trials. Controls and rats treated with 3 or 4.5 mg/kg N-Arg were retested in the maze 7-10 days following training, with half receiving N-Arg (6 mg/kg i.p.) 30 min in advance. In this retention test, maze performance was not significantly affected; thus, these results indicated that NO synthase inhibition primarily impaired acquisition without impacting upon noncognitive aspects of performance. This conclusion was further reinforced by the demonstration that 6 mg/kg N-Arg did not significantly affect sensorimotor performance in a rotarod task. When rats were treated with sodium nitroprusside, an NO donor, at 1 min, but not 30 min, prior to training, the N-Arg induced impairment (6 or 8 mg/kg i.p.) in maze learning was significantly attenuated.

An assessment of behavioral aging in the Mongolian gerbil

Male Mongolian gerbils (Meriones unguiculatus) 14-54 months old (n = 77) were evaluated in a battery of psychomotor (open field, locomotor, and runwheel activity, rotorod performance) and learning (one-way active avoidance in a straight runway and in 14-unit T-maze performance) tests. Body weight and seizure activity were also monitored. According to linear regression analysis, runwheel activity decreased with age; and the number of errors in the 14-unit T-maze increased as a function of age (ps < 0.05). None of the other behavioral measures or body weight were significantly correlated with age. This gerbil strain (Tumblebrook Farms; West Brookfield, MA) tended to be very prone to seizures with 64% of the gerbils experiencing at least one seizure while being tested. Seizures tended to occur when the gerbil was exposed to a novel situation (e.g., initial weighing, placement on the rotorod). An age-related decline in some aspects of psychomotor and learning performance was observed, suggesting the gerbil as an additional mammalian model of aging. The high incidence of seizure activity presented a complicating and confounding variable to the interpretation of the results of the behavioral tests used in the present study. Interventions to control seizure activity (e.g., systematic, controlled breeding; adaptation to apparati) in this model will likely increase its viability as a mammalian model of aging.

Methylcobalamin attenuates the hypoxia/hypoglycemia- or glutamate-induced reduction in hippocampal fiber spikes in vitro

The effects of methylcobalamin, a vitamin B12 analogue, on the hypoxia/hypoglycemia- or glutamate-induced reduction in hippocampal CA1 presynaptic fiber spikes elicited by Schaffer collateral stimulation in rat brain slices were evaluated. Hippocampal slices were exposed to 15 min of hypoxia/hypoglycemia, and then these slices were returned to oxygenated and glucose-containing buffer for 3 h. Hypoxia/hypoglycemia reduced CA1 presynaptic potentials in vitro. Treatment with 10 microM methylcobalamin attenuated the impairment of CA1 presynaptic potentials induced by hypoxia/hypoglycemia or glutamate application (10 mM). Daily injection of methylcobalamin (0.5 mg/kg i.p./day) for 3 days in vivo also attenuated the hypoxia/hypoglycemia- or glutamate-induced reduction in presynaptic potentials in hippocampal slices. Pretreatment with cyanocobalamin at 10 microM failed to attenuate the impairment of CA1 presynaptic potentials. However, daily injection of cyanocobalamin (0.5 mg/kg i.p./day) for 3 days caused a protective action against the hypoxia/hypoglycemia- or glutamate-induced functional deficit. Furthermore, co-treatment of L-arginine (100 microM), a substrate for nitric oxide synthase, with methylcobalamin in vitro reversed the methylcobalamin-induced functional recovery. The present results demonstrate that methylcobalamin application in vivo or in vitro leads to functional recovery from hypoxia/hypoglycemia- or glutamate-induced impairment of CA1 presynaptic potentials. Neuroprotection was obtained by in vivo application of cyanocobalamin, but not by its in vitro application. It is reported that in vivo injected cyanocobalamin converted to methylcobalamin in the hepatic cells. Therefore, the results suggest that a transmethylation reaction in the hippocampal regions may be involved in the methylcobalamin-induced functional recovery from ischemic impairment.

Relationship of hematological variables to learning performance in aged Fischer-344 rats

The relationship between hematological variables and the ability to perform behaviorally in two learning tests was evaluated in male F344 rats aged 22-24 months. Rats were screened for ability to meet criterion for learning one-way active avoidance in a straight runway task. Rats failing to meet criterion were given no further testing and were assigned to Group 1 (G1). Rats meeting criterion were tested in a 14-unit T-maze (2 days, 10 trials/day). Failure to negotiate the T-maze within 600 s on any three trials resulted in assignment to Group 2 (G2) with no further testing. Rats successfully completing both tasks constituted Group 3 (G3). Trunk blood was collected following behavioral testing and was assayed to determine red blood cell count (RBC), hematocrit (HCT), hemoglobin (HGB), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), white blood cell count (WBC), bands (BND), polymorphs (POLY), lymphocytes (LYM), monocytes (MON), and eosinophils (EOS). The combined G1/G2 group had significantly lower RBC, HCT, HGB, and EOS but significantly higher MCV and MCH than G3 rats. Correlation analysis revealed a positive relationship of group membership (i.e., learning test completion) to RBC, HCT, HGB, and EOS, but a negative correlation of group membership to MCH. No significant correlation emerged between any hematological characteristic and performance in either behavioral task. These results suggest that a simple blood test to determine HCT may be a useful screen for removal of moribund rats from aging studies attempting to control for effects of health on behavioral performance in rodent models.

Spermidine potentiates dizocilpine-induced impairment of learning performance by rats in a 14-unit T-maze

The NMDA receptor, a ligand-gated ion channel complex, has been reported to be involved in memory processes. Learning is impaired following administration of dizocilpine, a non-competitive antagonist of the NMDA receptor. Polyamines, such as spermine and spermidine, interact with the NMDA receptor to enhance binding of dizocilpine, which blocks the ion channel. The present study assessed action of polyamines as modulators of learning via NMDA receptor activation. Dizocilpine (0.05 mg/kg) was given i.p. before maze learning, at a dose that produced a slight, nonsignificant impairment of maze learning. Pretreatment with 80 mg/kg but not 15 or 40 mg/kg spermidine (i.p.) before dizocilpine impaired maze learning compared to saline controls. Administration of 80 mg/kg spermidine without dizocilpine did not impair maze learning. The results are consistent with the view that systemic injection of a polyamine can modulate learning processes involving the NMDA receptor.

Thrombosis of parietal, but not striate, cortex impairs acquisition of a 14-unit T-maze in the rat

We evaluated parietal cortex (PCTX) involvement in learning a shock-motivated, 14-unit T-maize in young F344 rats. In Experiment 1, each rat was placed into a stereotaxic instrument, and the scalp was retracted before either the photosensitive dye, rose bengal, or saline (CON) was provided via a jugular catheter. After 60 s, halogen lamps were used to illuminate the cranium bilaterally over PCTX for 40 min to activate the dye and generate a thrombus in dye-injected rats. Active avoidance pretraining in a straight runway occurred 12 days after surgery. Each rat was provided acquisition (ACQ) trials in the 14-unit T-maze 24-h later. The PCTX rats were impaired relative to CON in errors, run time, alternation errors, and shock frequency measures in the 14-unit T-maze, but not in shock duration. The thrombosis extended to areas comprising anteromedial and posterior PCTX, and also into the striate cortex (SCTX). In Experiment 2, rats were divided into four groups: a PCTX group that received bilateral illumination; PCTX+SCTX and SCTX groups that were illuminated at midline; and a CON group that received the surgery and either dye or illumination. Only PCTX differed from CON, with maze performance similar to Experiment 1. The thrombosis for the PCTX group in Experiment 2 was confined to anteromedial and posterior PCTX. Thus, PCTX, but not SCTX, was implicated in ACQ of this maze task, suggesting that PCTX might be involved in the robust age-related impairments in ACQ previously observed.

Scopolamine induces recovery of shuttle box avoidance behavior after frontal cortex ablation

The learning and reversal of shuttle box active avoidance behavior in animals with a bilateral frontal cortex ablation was investigated during and after scopolamine or pilocarpine treatment. Scopolamine facilitated the performance of the avoidance task in normal animals and in those with frontal cortex lesions and also increased the number of intertrial responses, while pilocarpine increased the deleterious effects of the lesions. Furthermore, in the absence of scopolamine, the animals previously treated with the drug showed that its beneficial effects persisted while the number of intertrial responses were no longer increased. The results indicate that the beneficial effects of scopolamine treatment on active avoidance behavior are independent from the effects observed on intertrial activity since only the former are observed after drug withdrawal. Therefore, scopolamine treatment seems to induce a long lasting recovery process in frontal cortex ablated animals.

New pharmacological strategies for cognitive enhancement using a rat model of age-related memory impairment

We have developed the Stone maze paradigm for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Evidence produced thus far clearly implicates both the cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results have been very inconsistent regarding the cognitive enhancing abilities of cholinomimetics for use in Alzheimer's disease, new classes of cholinesterase inhibitors may offer greater therapeutic efficacy. The use of glycine and polyamine agonists appears to be a viable strategy for positive modulation of the NMDA receptor. In addition, an approach that combines stimulation both of cholinergic and glutamatergic systems may have greater potential than agonism of either separately. Manipulation of signal transduction events might also have potential for cognitive enhancement. The influx of Ca2+ through the NMDA receptor stimulates production of NO via the action of NOS. By using NARG to block NOS activity, we have demonstrated in rats that NO production appears to influence learning in the Stone maze. We are currently exploring the age-related changes in NOS activity in specific brain regions of rats to determine if loss in the NO generating system is related to age-related memory impairment observed in the Stone maze. In addition, we are exploring pharmacological strategies for inducing NO production; however, because of the potential neurotoxicity for NO overstimulation, this strategy will present some obstacles. The identification of NO as a simple molecule serving vital physiological functions but representing potential for neurotoxicity presents an important unifying area for neurobiological investigations searching for mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease.

Behavioral assessment of aging in male Fischer 344 and brown Norway rat strains and their F1 hybrid

Male Fischer-344 (F344) and Brown Norway (BN) rats 7-, 13-, and 24-month-old and their F344 x BN hybrid (F1) 7-, 13-, 24- and 31-month-old were tested in a behavioral battery (15-min and 24-h locomotor activity, inclined screen, rod suspension, rotorod, shock-motivated learning in a straight runway and 14-unit T maze). Necropsy was performed 3 days later and the results rated for pathology (i.e., severity of lesions observed). Age-related performance declines were observed in all behavioral tests except 15-min locomotor activity. Strain effects were observed in 15-min (BN more active than F344 and F1) and 24-h locomotor activity test (F344 more active than BN and F1 strains); rotorod performance (F344 fell more than BN and F1); and in all measures [errors (E), runtime (RT)], shock frequency (SF), and duration (SD)] in the 14-unit T maze (F344 worse than BN, BN worse than F1). T maze performance of 31-month-old F1 rats was deficient in RT, SD, and SF but E performance was equivalent to that of 7-month-old F1 rats. In a second experiment, only 7- and 31-month-old F1 rats were tested in the 14-unit T maze and the results obtained in Experiment 1 were replicated. Gross necropsy revealed age and strain effects in the number of lesions observed and the mean ratings of pathology. The 24-month-old F344 rats exhibited the greatest number of lesions and had the highest ratings (generally observed as chronic nephrosis and enlarged spleens characteristic of mononuclear cell leukemia). BN rats exhibited a high incidence of hydronephrosis at all age levels. While experiencing less obvious pathology, F1 rats experienced a significant number of lesions in the 31-month-old group. Pathology ratings correlated with behavioral performance but only for a few tests (e.g., SD and RT in 14 unit T maze in 24-month-old F344). Thus, behavioral performance declined with age and the battery of tests differentiated between the strains tested (in general, F344 worse than BN; BN worse than F1). The correlation of pathology ratings at gross necropsy with behavior did not appear to be systematic, suggesting that morbidity was not responsible for the age-related performance declines. However, more extensive evaluation of the relationship of age-related changes in health status to behavior with larger samples of rats is suggested.

Rodent models of memory dysfunction in Alzheimer's disease and normal aging: moving beyond the cholinergic hypothesis

The Stone maze paradigm has been developed for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Results from several studies have demonstrated the involvement of both cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results of clinical studies on the cognitive enhancing abilities of cholinomimetics for treatment of memory impairment in Alzheimer's disease have been inconsistent, new classes of cholinesterase inhibitors offer greater potential for therapeutic efficacy. The physostigimine derivative, phenserine, appears to have marked efficacy for improving learning performance of aged rats or of young rats treated with scopolamine in the Stone maze. Declines in markers of glutamatergic neurotransmission in Alzheimer's disease and in normal aging suggest that pharmacological manipulation of this system might also prove beneficial for cognitive enhancement. Treatment with glycine and/or polyamine agonists is suggested as a strategy for activating the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. In addition, the use of combined pharmacological activation of cholinergic and glutamatergic systems is suggested. Manipulation of signal transduction events should also be considered as a strategy for cognitive enhancement. The influx of Ca2+ through the channel formed by the NMDA receptor stimulates the production of the oxyradical, nitric oxide (NO*), via the action of nitric oxide synthase (NOS). Compounds that inhibit NOS activity impair acquisition in the Stone maze, suggesting an involvement of NO*. Thus, strategies for inducing NO* production to enhance cognitive performance may be beneficial. Because of the potential neurotoxicity for NO*, this strategy is not straightforward. Although many new directions beyond the cholinergic hypothesis can be suggested, each has its potential benefits which must be weighed against its risks. Nonetheless, an important unifying area for neurobiological research examining mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease, might emerge from the identification of NO* as a simple molecule serving vital physiological functions but representing potential for neurotoxicity.

Impaired acquisition in a 14-unit T-maze following medial septal lesions in rats is correlated with lesion size and hippocampal acetylcholinesterase staining

Septohippocampal cholinergic system involvement in acquisition of an aversively motivated 14-unit T-maze was evaluated in 4-month-old male Fischer-344 rats. Each rat was assigned to one of two groups that received either a bilateral electrolytic lesion to the medial septal area (MSA) or a sham operation. One week after surgery, each rat began pretraining in one-way active avoidance (footshock = 0.8 mA) consisting of 10 trials per day on each of 3 consecutive days. Criterion for successful completion of pretraining was 8/10 avoidances on the third day. On the day following completion of pretraining, each rat received 10 trials in a shock-motivated 14-unit T-maze. The performance requirement was to move through each of five maze segments within 10 s to avoid footshock (0.8 mA). A second 10-trial session was provided 24 h later. Performance measures included errors, alternation errors, runtime, shock frequency, and duration. Following maze training, each rat was sacrificed, and formalin-fixed brains were frozen for histology, which included procedures for thionin Nissl and acetylcholinesterase (AChE) staining. MSA-lesioned rats were observed to be significantly impaired on all measures of maze performance compared to sham-operated controls. Densitometric analysis of hippocampal AChE staining revealed a 30% reduction in relative AChE staining of MSA-lesioned rats compared to sham-operated controls. Lesion size was observed to be highly positively correlated with maze errors. A negative correlation of mean error score with density of AChE staining was observed for MSA-lesioned rats, but not for sham-operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenserine: a physostigmine derivative that is a long-acting inhibitor of cholinesterase and demonstrates a wide dose range for attenuating a scopolamine-induced learning impairment of rats in a 14-unit T-maze

Phenserine ((-)-N-phenylcarbamoyl eseroline), a carbamate analog of physostigmine (Phy), is a long-acting inhibitor of cholinesterase. We have assessed the potential clinical value of phenserine for cholinomimetic therapy of cognitive impairments associated with aging and Alzheimer's disease by evaluating its duration of in vivo activity against rat plasma acetylcholinesterase (AChE) and its effect on attenuating a scopolamine-induced impairment in learning performance of young rats in a shock-motivated 14-unit T-maze. Phenserine achieved maximum AChE inhibition of 73.5% at 5 min and maintained a high and relatively constant inhibition for more than 8 h. For analysis of effects on learning performance, 69, 3-month-old male Fischer-344 rats were pretrained in a straight runway to avoid electric footshock. On the following day, each animal received 15 trials in the 14-unit T-maze. Sixty minutes prior to the maze training, each rat received the first IP injection of either vehicle (Tween 80, ethanol and 0.9% NaCl) or phenserine at 1.5, 3.0, 4.0, 5.0, 7.5, or 10.0 mg/kg. Then 30 min prior to the training, each animal received a second IP injection of either 0.9% NaCl or scopolamine hydrochloride (0.75 mg/kg; SCOP). Compared to the vehicle-SCOP group, all but the 7.5 mg/kg dose of phenserine significantly ameliorated error performance, runtime, shock frequency and shock duration in SCOP-treated rats at the final block of three trials. Appearing to have a long effect and a wide therapeutic window, phenserine deserves further study as a cognitive enhancer.

The long-acting cholinesterase inhibitor heptyl-physostigmine attenuates the scopolamine-induced learning impairment of rats in a 14-unit T-maze

Heptyl-physostigmine (heptyl-Phy), a new carbamate derivative of physostigmine (Phy), has been assessed for potential clinical value by evaluating its in vitro activity against human erythrocyte acetylcholinesterase (AChE) and plasma butyrylcholinesterase (BChE), its duration of in vivo activity against rat plasma AChE, and its effects on attenuating a scopolamine-induced impairment in learning performance of young rats in a 14-unit T-maze. Heptyl-Phy demonstrated potent cholinesterase inhibition, with activity similar to that of Phy against AChE, IC50 values 21.7 +/- 2.0 nM and 27.9 +/- 2.4 nM, respectively, and significantly greater than that of Phy against BChE, IC50 values 5.0 +/- 0.1 nM and 16.0 +/- 2.9 nM, respectively. Heptyl-Phy achieved maximum AChE inhibition of 92.5% at 60 min and maintained a high and relatively constant inhibition for more than 8 h. For analysis of effects on learning performance, heptyl-Phy at 1.0, 1.5, 2.0 or 3.0 mg/kg, or vehicle was administered i.p. to 52 3-month-old male Fischer-344 rats 60 min prior to maze training. Thirty minutes prior to training, each animal received either 0.9% NaCl or scopolamine hydrochloride (0.75 mg/kg). Only a 2.0 mg/kg dose of heptyl-Phy significantly reduced the number of errors in scopolamine-treated rats. The other doses did not improve any aspect of maze performance. Although the therapeutic window of heptyl-Phy did not appear wide enough for clinical use, the longer duration of action of heptyl-Phy would appear beneficial.

Reduced density of NMDA receptors and increased sensitivity to dizocilpine-induced learning impairment in aged rats

About 20 min prior to training in a shock-motivated 14-unit T-maze, young (3-4 months) and aged (24-25 months) male Fischer-344 rats were given s.c. injections of either saline or dizocilpine (MK-801, 0.02 or 0.04 mg/kg), a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. The aged rats showed a dose-dependent impairment in maze performance. Deficiencies were manifested as increases in errors, in runtime from start to goal, and in the number and duration of shocks received. In contrast, young rats exhibited no detrimental effects of dizocilpine on maze performance. Analysis of [3H]glutamate binding in these rats revealed a marked age-related decline in NMDA receptor binding in hippocampus. A significant correlation was observed between errors in the maze and hippocampal [3H]-glutamate binding, but the correlation was positive, i.e., rats that made the most errors had the highest level of NMDA receptor binding. Thus, compared to young rats, aged rats were more sensitive to the behavioral effects of NMDA receptor antagonism and they showed a hippocampal loss of [3H]glutamate in binding, which may be related to the increased sensitivity to dizocilpine. The positive correlation between poor maze performance and NMDA receptor binding suggests that the behaviors assessed involve complex interactions between NMDA receptors and other neuronal systems in the hippocampus.

Delayed matching-to-sample performance by rats in a new avoidance-motivated maze: response to scopolamine and fimbria-fornix lesions

A new avoidance-motivated detour-maze in which memory for an immediately preceding sample event could be assessed was evaluated by testing seven 6-month-old male F-344 rats with a delayed matching-to-sample (DMTS) paradigm. Rats first received extensive pretraining with this paradigm over several months and after a minimum of 1,390 choice-trials demonstrated great proficiency in this maze. Studies were then conducted to establish cholinergic and hippocampal involvement in the DMTS task by using drug manipulations (scopolamine and physostigmine), and after lesions to the fimbria-fornix (FF) pathway. A high dose (1.0 mg/kg) of scopolamine but not a low dose (0.3 mg/kg) significantly interfered with choice accuracy as measured by errors and trials to criterion; physostigmine (0.01 and 0.03 mg/kg) had no significant effect; and fimbria lesions significantly disrupted both choice accuracy and runtime performance. Disruption was most pronounced on difficult problems (different paths to the goal). After lesions only, considerable within-trial perseverative errors occurred during the early postlesion weeks on four difficult problems from among the 18 tested. Results were discussed in terms of (a) specificity of this disruption, (b) indications of proactive interference effects, and (c) the movement-related excitation component of maze learning. The present results accord with earlier findings of disruption by scopolamine and FF lesions in a 14-unit T-maze, both mazes having similar performance requirements of shock avoidance and multiple 90-degree turns along the paths to the goal. The present results affirm that this new detour maze provides a viable approach for assessing cognitive performance in a within-subject design and thereby offers new possibilities for testing various aspects of cognitive processing, particularly for aged rodent models, in a complex aversive situation.

Behavioral microanalysis of spatial delayed alternation performance: rehearsal through overt behavior, and effects of scopolamine and chlordiazepoxide

Rats were trained in an operant spatial delayed alternation task utilizing retention intervals from 2 to 32 s. In addition to response accuracy, operations of the levers during the retention intervals were recorded and analyzed. Animals were tested following the administration of the muscarinic antagonists scopolamine hydrobromide and methylbromide, and the benzodiazepine receptor agonist chlordiazepoxide. In vehicle-treated animals, the relative number of correct responses and correct rehearsal operations (operation of the forthcoming correct lever during retention intervals) varied with the length of the retention intervals, and these measures were correlated. The response rate for rehearsal operations increased with the length of the retention intervals. It is speculated that the delay-dependent increase in response rate reflects an effect of delayed reward that was also associated with a delay-dependent increase in the tendency to alternate between levers. The effects of delay on the accuracy of rehearsal operations may have contributed to the delay-dependent correct responding. Scopolamine hydrobromide (0.01, 0.03, 0.1, 0.3 mg/kg) and methylbromide (0.1, 0.3 mg/kg) impaired correct responding, but did not seem to interfere with the relative number of correct rehearsal operations. As only the presentation of the panel light indicated trial onset, it is speculated that the cholinergic receptor blockade resulted in an increase in the probability of a repositioning response that was triggered by light onset. Chlordiazepoxide (1, 3, 5, 10 mg/kg) did not affect behavioral performance. These results suggest that in tasks that allow the development of rehearsal operations, delay-dependent response accuracy does not represent a sufficient condition for conclusions on task demands on memory.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic blockade and response timing in rats

The effects of central cholinergic blockade on the temporal regulation of behaviour were studied with a two-level DRL schedule. Five-month-old male Wistar rats had to press lever A and then wait for a minimum of 5 s before pressing lever B to obtain the reinforcer (sweetened milk). After a stable baseline performance, subjects were injected in random order with the general cholinergic blocker, scopolamine, 0.15 and 0.5 mg/kg, the peripheral cholinergic blocker, methylscopolamine, 0.15 and 0.5 mg/kg, and a combination of the cholinesterase inhibitor, physostigmine, 0.2 mg/kg, and scopolamine, 0.5 mg/kg. Each drug treatment was separated by 2 days of saline treatment. Results showed that scopolamine at 0.5 mg/kg significantly impaired the temporal regulation of the A-B response sequence: the median A-B inter-response time (IRT) was shortened and the coefficient of variation of the A-B IRT distribution was increased, thus revealing a loss in the sensitivity to time. This disruption of accurate timing behaviour lowered efficiency. The drug changed neither the duration of the B-A interval nor the A-B response rate, but significantly increased the rate of the superfluous B-B sequences. Methylscopolamine was without effects and physostigmine totally or partially reversed all the scopolamine effects. These results suggest that scopolamine at 0.5 mg/kg specifically affected the mechanism(s) underlying response timing, and that the effects were not secondary to changes in activity or motivation. They partly corroborate data obtained in other procedures and support the idea that the central cholinergic system is involved in the temporal regulation of behaviour.

NMDA receptor channel antagonism by dizocilpine (MK-801) impairs performance of rats in aversively motivated complex maze tasks

To determine the involvement of the N-methyl-D-aspartate (NMDA) receptor in shock-motivated complex maze performance, the drug dizocilpine (DIZO; a.k.a. MK-801) was administered a) to naive, 3-month-old male F-344 rats prior to acquisition (AQ) in the 14-unit T-maze (Experiment 1), and b) to well-trained 11-month-old male F-344 rats prior to testing in a delayed-matching-to-sample (DMTS) task in the detour maze (Experiment 2). For Experiment 1, rats first were pretrained in a straight runway on one-way active avoidance (13/15 correct avoidances) for a maximum of 30 trials. On the following day, either DIZO 0.025 (n = 8), 0.05 (n = 8), 0.1 (n = 8), mg/kg, or saline (SAL; n = 15) was administered subcutaneously (SC) 20 min prior to 15 AQ trials in the shock-motivated 14-unit T-maze. The highest dose disrupted all measures of maze performance including errors, alternation errors, runtime, shock duration and frequency, but also produced marked motor ataxia. The 0.05-mg/kg group displayed significant impairment in AQ of this task but only on the cognitive measures, errors and alternation errors, and the 0.025-mg/kg group was impaired on the alternation measure only. One week later, the 15 SAL rats were divided into 2 groups and tested on retention with either SAL or 0.05 mg/kg DIZO. No effects on maze performance were observed. For Experiment 2, after receiving extensive pretraining in the shock-motivated detour maze, 7 rats were exposed to a novel sequence of 4 problems (P) during each of 7 daily sessions. Performance was evaluated 20 min after SC injection of either DIZO--0.025, 0.05, 0.125 mg/kg, or SAL. The 0.125-mg/kg dose caused extreme motor ataxia which precluded testing during that session. The 0.05-mg/kg but not the 0.025-mg/kg dose significantly disrupted performance on both error and trials to criterion measures. Both problem and interaction effects were significant. Disruption was most evident on two specific problems, those involving a side change from the first to second detour. Also, rats had more difficulty switching sides from problem to problem (few errors on P-1 and most on P-4), suggesting proactive interference effects. In sum, DIZO was observed to significantly disrupt performance in both mazes in a dose-related manner similar to effects observed in previous studies following administration of the anticholinergic drug scopolamine. For the 14-unit T-maze, the present results simulate age-related deficits previously found in acquisition of that task.

Acetyl-1-carnitine. 2: Effects on learning and memory performance of aged rats in simple and complex mazes

Acetyl-1-carnitine (AC) was administered via drinking water for six months to one group (OLD-AC) of male F-344 rats beginning at 16 months of age, while another group (OLD-CON) of rats was given water only during that period. The rats were maintained on this treatment throughout behavioral testing, which began at 22 months of age. Performance of the OLD-AC and OLD-CON rats was compared to that of young control (YG-CON) rats on the following set of tasks: spontaneous alternation in the arms of a T-maze, two-choice simultaneous discrimination in the stem of a T-maze, rewarded alternation in the arms of a T-maze, spatial discrimination and reversal on a circular platform, spatial working memory in the radial 8-arm maze, long-term memory in the 14-unit T-maze, and for preference of the light or dark chamber of a two-compartment box. AC improved the long-term memory performance in the split-stem T-maze and on the circular platform but had no discernable effects on performance of aged rats in the other tasks. Possible reasons for the selectivity of this agent's action on behavior are suggested.

WEB 1881 FU ameliorates impairment of working memory induced by scopolamine and cerebral ischemia in the three-panel runway task

Using a repeated acquisition procedure in a 3-panel runway apparatus, the effect of WEB 1881 FU on impairment of working memory produced either by scopolamine or by cerebral ischemia was investigated in rats and compared with those of aniracetam and Ca hopantenate. Intraperitoneal injection of scopolamine at 0.56 mg/kg significantly increased the number of errors (pushes made on the two incorrect panels of the three panel-gates located at each choice point). WEB 1881 FU at 10-32 mg/kg, p.o., caused a dose-related reduction in the increase of errors expected in the scopolamine-treated rats. Aniracetam at 10-100 mg/kg, p.o., or Ca hopantenate at 100 and 560 mg/kg, p.o., also significantly diminished the increase in errors induced by 0.56 mg/kg of scopolamine. Cerebral ischemia for 5 min significantly increased errors in the 3-panel runway task. WEB 1881 FU at 32 and 56 mg/kg, administered p.o. immediately after blood flow recirculation and again 1 hr before the runway test, conducted 24 hr after ischemia, significantly reduced the increase in errors expected to occur after 5 min of ischemia. Aniracetam at 32 and 100 mg/kg, p.o., similarly diminished the increase in errors in ischemic rats. These findings suggest that WEB 1881 FU has a beneficial effect on memory that has been impaired by scopolamine or by cerebral ischemia.