Cerebral ischemic preconditioning induces lasting effects on CA1 neuronal survival, prevents memory impairments but not ischemia-induced hyperactivity

In ischemic preconditioning, prior exposure to a short 3-min global ischemia provides substantial protection against the deleterious effects of a subsequent prolonged ischemic insult in rats. The objective of the present study was to determine if the neuronal protection induced by ischemic preconditioning influence functional recovery following a 6-min ischemic insult in rats. Animals received either sham-operation, a 3-min ischemia, a preconditioning 3-min global ischemia followed 3 days later by a 6-min global ischemia or a single 6-min global ischemia. Open field habituation, memory performance in the 8-arm radial maze and object recognition were assessed at different intervals following ischemia. Our findings revealed that preconditioning reversed ischemia-induced spatial memory deficits in the 8-arm radial maze, as suggested by significant reduction of working memory errors in preconditioned as compared to ischemic animals. Preconditioning also attenuated ischemia-induced object recognition deficits at short-term intervals. Nonetheless, preconditioning failed to alter ischemia-induced hyperactivity as demonstrated by enhanced behavioral activity in the open field in both preconditioned and ischemic animals compared to 3-min ischemic and sham-operated rats. CA1 cell counts revealed significant neuronal sparing in preconditioned animals that was observed 6-month following reperfusion. Together, these findings suggest that neuronal survival in preconditioned rats is associated with significant improvements of hippocampal-dependent memory functions and, further support that ischemia-induced hyperactivity may not solely depend on selective neuronal damage to hippocampal neurons.     

Food restriction attenuates ischemia-induced spatial learning and memory deficits despite extensive CA1 ischemic injury

The purpose of the present study was to examine whether short-term food restriction (40% less food over a 3-month period) can attenuate ischemia-induced CA1 neuronal degeneration, and whether this attenuation translated into improved recovery of functional impairments following global ischemia. There was a significant loss of pyramidal CA1 neurons in ischemic compared to sham-operated rats but no difference between the ad lib and food-restricted ischemic animals. Although the diet did not influence neuronal damage in ischemic animals, the performance of food-restricted ischemic rats in spatial task such as the radial arm maze was significantly better than that of ad lib fed ischemic rats. Food-restricted ischemic rats made equivalent numbers of working memory errors as sham-operated animals and took the same time to complete a standard 8-arm radial arm maze task. They also displayed higher activity level in the open field compared to ad libitum fed ischemic rats, and spent considerably more time in the open arms of the elevated plus maze compared to the other groups, suggesting decreased anxiety in these ischemic rats. The relative sparing of spatial memory performance in food-restricted ischemic animals suggests that food restriction facilitates functional recovery.     

Facilitative effects of an adenosine A1/A2 receptor blockade on spatial memory performance of rats: selective enhancement of reference memory retention during the light period

The present experiment was designed to examine the role of adenosine in spatial working and reference memory in rats using an 8-arm radial maze task which requires the integrity of the hippocampal formation. We investigated the effects of the unselective adenosine A1/A2 receptor antagonist theophylline on acquisition and retention of spatial working and reference memory. As there is evidence that brain extracellular adenosine levels vary significantly during the light-dark cycle, we tested the effects of theophylline both during the light and the dark period. Acquisition of the task was investigated for 10 consecutive days after rats received daily injections of vehicle or theophylline (15 mg/kg, intraperitoneally). Retention was tested in two nondrug sessions 7 and 14 days after completion of acquisition. The results demonstrate that in saline-treated control rats acquisition and retention of reference memory and, to a lesser extent, working memory was superior in the dark period. The results further revealed that daily administration of theophylline interacted with days to selectively enhance reference memory acquisition in the light, but not in the dark, period. In addition, reference memory retention was significantly enhanced in those rats who learned the task under theophylline treatment during the light period. Overall, the results show that in saline-treated control rats the effectiveness of acquisition and retention of spatial information in a radial maze strongly depends on the time of day. The higher levels of maze performance in the dark period might be related to a better functioning of involved brain systems in the active period of the rat. Furthermore, theophylline-induced blockade of adenosine A1/A2 receptors in the light, but not in the dark, period selectively enhanced reference memory acquisition and retention. Variations of brain extracellular adenosine levels during the light-dark cycle might account for the restriction of reference memory enhancing effects of theophylline to the light period.     

Improved learning and memory in aged rats with chronic administration of the nicotinic receptor agonist GTS-21

The ability of two synthetic nicotine receptor ligands, GTS-21 and DMAB, to chronically enhance the cognitive function of aged rats was evaluated in three diverse tasks and compared to the cognition-enhancing effects of nicotine administration. 15 min prior to daily behavioral testing, aged 22–24 month old rats received an i.p. injection of nicotine (0.2 mg/kg), GTS-21 (1 mg/kg), DMAB (2 mg/kg), or saline vehicle and were tested in either one-way active avoidance pole jumping, Lashley III maze, or a 17-arm radial maze. GTS-21 pretreatment was as effective as nicotine for enhancing the acquisition of aged rats in both one-way active avoidance and Lashley III maze training. In 17-arm radial maze testing, GTS-21 improved both general learning and reference (long-term) memory to the same extent as nicotine. Although DMAB pretreatment enhanced reference memory in 17-arm radial maze testing to the same extent as nicotine, it did not affect general learning in this complex task and did not exert any cognition-enhancing effects in Lashley III maze training. These results indicate that GTS-21 has cognition-enhancing abilities in aged rats that are comparable to those of nicotine in a variety of behavioral tasks. Since GTS-21 acts preferentially on brain nicotinic receptors and is less toxic than nicotine, these results further indicate that GTS-21 may have substantive therapeutic value in the treatment of age-associated memory impairment (AAMI) and/or Alzheimer's disease.     

Memory impairments following basal forebrain lesions

The functional contribution of the nucleus basalis magnocellularis (NBM) and medial septal area (MSA) to memory was evaluated in 4 behavioral tasks. The tasks were postoperative acquisition of a win-stay spatial discrimination in a T-maze, a win-shift spatial discrimination on a radial arm maze, active avoidance in a shuttle box, and passive avoidance in a shuttle box. Bilateral lesions were made by injecting ibotenic acid (IBO) into the NBM or MSA. Control rats received operations in which no neurotoxin was injected. When compared to controls, rats with lesions in either the NBM or MSA had significantly impaired choice accuracy in the T-maze and radial maze tasks, took significantly fewer trials to reach criterion in the acquisition, but not the retention of an active avoidance task, and significantly more trials to reach criterion in the passive avoidance task. The results show that equivalent behavioral changes are obtained from lesions in the NBM and MSA in tasks that vary in their type of motivation, reinforcement, response-reinforcement contingency, and response. These behavioral changes suggest that the NBM and MSA may both be involved in memory.     

Preoperative training modifies radial maze performance in rats with ischemic hippocampal injury

Rats exposed to 30 minutes of four-vessel occlusion reliably develop severe bilateral CA1 hippocampal injury; under certain conditions of radial maze training, such rats perform the reference memory component as well as controls yet perform the working memory component worse than controls. Reference memory is thought to depend on invariable and working memory on variable spatial information. We assessed the effect of training before ischemia. In Experiment 1, rats trained for 36 trials on 12-arm radial mazes before ischemia demonstrated a persistent impairment on the working memory task but eventually performed the reference memory task comparable to controls. Ischemic rats made more working memory errors as the number of choices increased. This pattern of working memory errors was similar to that in controls except, as expected, ischemic rats made many more errors. In Experiment 2, training for 80 trials before ischemia in rats decreased the severity of both the working and the reference memory impairment. Ischemia did not affect motor behavior in either experiment. These results characterize the working memory deficit in ischemic rats and demonstrate the importance of experimental factors, particularly in the design of treatment strategies to reduce functional impairments caused by ischemia.     

The role of the hippocampal-nucleus accumbens pathway in radial-arm maze performance

The role of the glutamatergic hippocampal-nucleus accumbens pathway in relaying hippocampal information via the nucleus accumbens to the motor system was investigated behaviorally using the radial-arm maze paradigm in rats. Bilateral injections of kynurenic acid, a glutamate antagonist, into the nucleus accumbens increased the latency to initiate movement during performance of an 8-arm radial maze with all arms baited and with 4 arms baited. Injections of kynurenic acid did not change the number of visits to previously visited arms (i.e. working memory errors) on both versions of the 8-arm radial maze. However, on the 8-arm radial maze with 4 arms baited, injections increased the number of visits to unbaited arms (i.e. reference memory errors). Similar injections were made in rats with ibotenic acid lesions of the prefrontal cortex in order to eliminate the glutamatergic prefrontal cortex-nucleus accumbens pathway so as to investigate the glutamatergic hippocampal-nucleus accumbens pathway. These rats displayed similar deficits on the radial-arm maze as non-lesioned rats (i.e. enhanced latency to initiate movement and reference memory errors). These findings suggest that the glutamatergic hippocampal-accumbens pathway plays a role in radial-arm maze performance by transferring information required for performing a radial-arm maze to the motor system.     

Forebrain ischemia induces selective behavioral impairments associated with hippocampal injury in rats

Two groups of rats were tested on a variety of motor and cognitive tasks after either 10 minutes of two-vessel occlusion forebrain ischemia (n = 8) or sham operative procedures (n = 6). Histological injury was absent in the sham-operated group. In the ischemic group, hippocampal injury was restricted to field CA1, while damage in the neocortex and caudoputamen was sparse. Motor tests performed on postoperative days 18 and 28 revealed no significant differences between the ischemic and sham-operated groups. Retention performance of a radial maze discrimination task was impaired, with a significant but transient increase in both working and reference memory errors. Passive avoidance acquisition and retention were not significantly affected, although conclusions concerning the utility of this task must be reserved because of variability in the behavior of the sham-operated rats. Morris maze spatial navigation (place learning) and open-field activity were insensitive to treatment group. These functional results are consistent with the observed histological injury and what is known about hippocampal injury and behavior, and they provide further guidance for the development of neurological assays appropriate for discriminating outcome from forebrain ischemia in rats.     

Post insult enriched housing improves the 8-arm radial maze performance but not the Morris water maze task in ventral subicular lesioned rats

The present study attempted to evaluate the efficacy of enriched housing conditions in promoting behavioral recovery following subicular lesion. Rats with bilateral ibotenic acid lesions of ventral subiculum were exposed to either enriched housing conditions or standard housing for 6 h daily for 10 days. The performance of the lesioned rats reared in standard housing conditions was severely impaired in both 8-arm radial maze and water maze tasks. Lesioned rats exposed to enriched housing showed behavioral recovery in the 8-arm radial maze but not in the water maze. Our study suggests the possibility that recovery may be based on the functional demand of spatial tasks and may require more appropriate environmental stimulation.     

Aniracetam improves radial maze performance in rats.

The memory enhancing effect of the pyrrolidinone derivative aniracetam was investigated in rats trained in a delayed-response task in an 8-arm radial maze. Oral administration of aniracetam (100, 200, 400, or 800 mg kg-1) 16 h and again 1 h prior to a first trial of exposure to a given configuration of 4 baited arms resulted in a significant improvement in performance during a second trial in the maze given 3 h later in which there was access to all 8 arms but only the other 4 arms were baited. The pattern of baited arms was varied daily. The performance enhancement was greatest for the highest doses. These results extend the demonstration of the cognition enhancing effects of aniracetam to a spatial memory task in rats.     

Deficits in spatial-memory tasks following lesions of septal dopaminergic terminals in the rat

The behavioral effects of 6-hydroxydopamine, injected bilaterally into the lateral septum, were investigated in two tests of spatial memory (radial 8-arm and T-maze). Three different experiments were conducted in the radial maze. In experiment I, rats were permitted to learn the task with food reinforcement in all arms of the maze. In experiment II, retention of the spatial information (working memory) learned in experiment I was tested by interposing various time intervals between choice 4 and 5 of each trial. In experiment III, reference and working memory were simultaneously assessed by only reinforcing 4 choices in the radial maze. Performances were compared in spaced versus massed trials. In the T-maze, the rats were first tested for learning a spatial discrimination between the two arms of the maze, and subsequently for reversal of the previously learned response. The results showed that the rats with lesions were impaired in all experiments. This impairment was particularly marked in some aspects of the procedures used: (1) in the search for the last 4 pellets in experiment I, (2) in the first presentations of various intervals interposed between choices 4 and 5, (3) in the search for food in the baited arms when the trials were massed in experiment III and (4) in the reversal of previously learned spatial discrimination in the T-maze. These behavioral deficits in the rats with septal dopaminergic lesions were interpreted as an increased susceptibility to interference. The lesions were shown to have selectively depleted dopamine concentrations in the septum without damaging noradrenergic terminals or cholinergic cell bodies. It was concluded that dopaminergic neurons could have a modulatory influence on memory processes.     

Persistent behavioral impairments and neuroinflammation following global ischemia in the rat

Cognitive deficits associated with cardiac arrest have been well documented; however, the corresponding deficits in animal models of global ischemia have not been comprehensively assessed, particularly after long‐term, clinically relevant survival times. We exposed male Sprague–Dawley rats to 10 min of bilateral carotid artery occlusion + systemic hypotension (40–45 mmHg) or sham surgery, and used histopathological assessments for short‐term survival animals (16 days) and both behavioral and histopathological assessments for long‐term survival animals (270 days). Analyses revealed significant long‐term deficits in ischemic animals’ learning, memory (T‐maze, radial arm maze), working memory (radial arm maze), and reference memory (Morris water maze, radial arm maze) abilities that were not associated with a general cognitive decline. Histological results showed significant increases in glial fibrillary acidic protein, neuron glia 2, OX‐42 and ED‐1 staining, as well as significant decreases in microtubule‐associated protein 2 staining and cornu ammonis area 1 (CA1) cell counts 16 days post‐ischemia. The pattern at 270 days was similar, but notably there was a persistent elevation of ED‐1 staining, suggesting recent cell death as well as significant atrophy of CA1. Whereas previous work has primarily reported transient changes in behavior after global ischemia, this study describes disturbances in several different functional domains following CA1 cell loss at clinically relevant survival times. Moreover, the histopathological outcome is suggestive of a spontaneous repopulation of CA1, but this was not sufficient to offset the behavioral impairments arising from the ischemic insult.     

Post-training injection of the acetylcholine M2 receptor antagonist AF-DX 116 improves memory

The present study examined the effect of systemic post-training administration of the acetylcholine muscarinic M2 receptor antagonist AF-DX 116 on the acquisition of two 8-arm radial maze tasks. On a win-stay visual discrimination task, a light cue signalled the location of food in 4 randomly selected maze arms, and rats were required to visit each of the 4 lit arms twice within a trial. Rats were given one trial per day and injected immediately post-training on day 5. AF-DX 116 (0.5 and 1.0 mg/kg) significantly improved win-stay acquisition relative to vehicle-injected controls. On a win-shift task, rats were allowed to visit 4 randomly selected maze arms, followed by a delay period. After the delay, rats were returned to the maze for a retention test in which only those 4 arms not visited prior to the delay contained food. On the test (i.e. drug) trial, rats were removed from the maze after the first 4 choices and injected with AF-DX 116 or vehicle. The retention test was given following an 18 h delay. AF-DX 116 (2.0 mg/kg) significantly improved retention relative to vehicle controls. When the injections were given 2 h post-training, no effect on retention was observed in either task. The results demonstrate that post-training injection of the selective M2 receptor antagonist AF-DX 116 improves memory in a time-dependent manner. The findings may have implications for the cholinergic pharmacotherapy of Alzheimer's disease.     

Isolation stress during the third postnatal week alters radial arm maze performance and corticosterone levels in adulthood

Stressful experiences during development cause long-lasting changes in neuroendocrine systems as well as lasting changes in behavior. The present study examines the long-term consequences of daily periods of social isolation during the third postnatal week on radial arm maze performance in adulthood. Male rat pups were either isolated for 6 h per day between postnatal days 15-21 or remained in the home cage. This manipulation caused a significant increase in plasma corticosterone during the isolation period. As adults, these animals were tested on a 12-arm radial arm maze. Rats that experienced social isolation during development made more working memory errors during initial acquisition but reached an asymptotic level of performance comparable to controls. The pattern of reference memory errors across testing was comparable to the pattern of working memory errors, though the difference between isolated and control animals was not significant. Blood samples taken in adulthood revealed that social isolation during development results in an long-term elevation in plasma corticosterone levels. These findings indicate that isolation stress during the third week of life leads to lasting impairments in cognition and HPA axis activity and suggest a potential alteration in hippocampal function.     

Effects of electrolytic lesions of the medial prefrontal cortex or its subfields on 4-arm baited, 8-arm radial maze, two-way active avoidance and conditioned fear tasks in the rat

The present study tested the effects of electrolytic lesions in two mPFC subregions, the dorsal anterior cingulate area (dACA) and prelimbic cortex, as well as the effects of a larger medial prefrontal cortex (mPFC) lesion which included both subregions, on 4-arm baited, 4-arm unbaited, 8-arm radial maze task and its reversal (Experiments 1 and 4), two-way active avoidance (Experiments 2 and 5) and conditioned emotional response (Experiments 3 and 6). Rats with large or small lesions of the mPFC learned the location of the 4 baited arms in the training and reversal stages of the radial maze task similarly to sham rats, indicating that these lesions did not affect animals' capacity to process and remember spatial information. dACA and mPFC lesions produced a transient deficit in the acquisition of the radial maze task, suggestive of an involvement of these regions in mnemonic processes. However, in view of the normal performance of these groups by the end of training and during reversal, this deficit is better interpreted as stemming from a difficulty to learn the memory-based strategy used to solve the task. Only mPFC lesion led to better avoidance performance at the beginning of training and tended to increase response during the presentation of a stimulus previously paired with shock, compared to sham rats. Both effects can be taken as an indication of reduced emotionality following mPFC lesion. The results are discussed in relation to known behavioral functions of the mPFC and the suggested functional specialization within this region.     

Failure of chronic physostigmine to ameliorate working memory deficits after medial septal lesions

To assess the potential usefulness of chronic acetylcholinesterase inhibition in the treatment of learning/memory disorders arising from central cholinergic deficient states, physostigmine was administered chronically to rats with medial septal lesions and the retention of a spatial/working memory task investigated. Three dose levels of physostigmine (0.025, 0.05, 0.075 mg/kg) were administered three times per day following medial septal lesions. Retention of a standard radial 8-arm maze task was assessed. Although the lesions transiently disrupted task performance, physostigmine therapy did not improve either daily performance or total recovery time. Our results suggest that chronic acetylcholinesterase inhibition is not effective in ameliorating the working memory deficits that occur after medial septal lesions.     

Male mice exhibit better spatial working and reference memory than females in a water-escape radial arm maze task

The present study examined sex differences in spatial working and reference memory in C57BL/6 mice. Males and females were tested in a version of the spatial 8-arm radial arm maze in which the motivating stimulus was escape from water. To test spatial working memory, four arms were baited with submerged escape platforms, each of which was removed after it was found. Four arms that never contained platforms assessed spatial reference memory. In addition to determining the number of working memory and reference memory errors made in each session, working memory errors made in each trial were analyzed to examine performance as the number of arms to be remembered (i.e. the working memory load) increased. Males committed significantly fewer working memory and reference memory errors than females throughout testing. Within a session, males committed fewer working memory errors than females as the working memory load increased. These sex differences were particularly evident during task acquisition. The data indicate that male C57BL/6 mice learn both the working and reference memory components of a water-escape motivated radial arm maze task better than female mice.     

Septo-hippocampal and nBM-cortical cholinergic neurones exhibit differential time-courses of activation as a function of both type and duration of spatial memory testing in mice

We previously showed that the initial acquisition session of a spatial discrimination (mixed reference/working memory) test in an 8-arm radial maze induced differential activations in the ascending cholinergic septo-hippocampal and nBM-cortical pathways in mice. This data showed that the duration of post-test cholinergic activation was longer in the nBM-cortical pathway than in the septo-hippocampal projection. Moreover, the post-test durations but not the immediate post-test amplitudes of activation in each pathway decreased progressively as a function of repeated daily acquisition sessions. In the present study we have thus tested the hypotheses that the time-courses of post-test cholinergic activation in the septo-hippocampal and nBM-cortical pathways may vary both as a function of the type of memory used (working vs. reference) and according to the duration of repeated daily testing. Cholinergic activity in vivo in the hippocampus or frontal cortex of mice was quantified using measures of sodium-dependent high-affinity choline uptake at two different times (30 s and 15 min) following specific spatial working or reference memory testing in an 8-arm radial maze. The memory tests were administered daily over a 13-day period to attain high levels of performance in each type of task. In comparison to control groups both types of memory testing induced significant post-test cholinergic activations in each brain region on Day 15. However, cholinergic activity remained elevated in frontal cortex at 15 min post-test following reference memory testing, whereas significantly shorter durations of cortical and hippocampal cholinergic activation were observed following working memory testing using short (1 min) retention intervals. The possible significance of these differential modifications to the time-course of the post-test activations in these cholinergic pathways in working and reference memory processes and the putative transsynaptic mechanisms involved are discussed.     

The disruption of spatial cognition and changes in brain amino acid, monoamine and acetylcholine in rats with transient cerebral ischemia

We investigated the disruption of spatial cognition due to transient forebrain ischemia using an 8-arm radial arm maze task in rats. Five or 10 min of ischemia did not affect the task acquisition. When rats established spatial cognition by daily training of the task, 10 min of ischemia significantly decreased the number of correct choices and increased the errors in the task when performed 24 h after reperfusion. These changes, however, returned to the normal level after about 4 days of daily training. Glutamic acid (Glu) and acetylcholine (ACh) release from the dorsal hippocampus (DH) was observed to transiently increase during ischemia. However, neither the content of noradrenaline (NA) nor the release of NA in the DH changed during ischemia. The NA and ACh release from the DH, however, gradually decreased during reperfusion, and the decrease became significant at 24 h after reperfusion. The NA content of the frontal cortex (FO and the DH increased 7 days after reperfusion. These results suggest that the disruption of spatial cognition induced by 10 min of ischemia may be attributed to a greater degree to the dysfunction of the hippocampal ACh and NA, and cortical NA systems, rather than to the development of neuronal cell death in these areas.     

Seizures in the developing brain cause adverse long-term effects on spatial learning and anxiety

PURPOSE: Seizures in the developing brain cause less macroscopic structural damage than do seizures in adulthood, but accumulating evidence shows that seizures early in life can be associated with persistent behavioral and cognitive impairments. We previously showed that long-term spatial memory in the eight-arm radial-arm maze was impaired in rats that experienced a single episode of kainic acid (KA)-induced status epilepticus during early development (postnatal days (P) 1-14). Here we extend those findings by using a set of behavioral paradigms that are sensitive to additional aspects of learning and behavior. METHODS: On P1, P7, P14, or P24, rats underwent status epilepticus induced by intraperitoneal injections of age-specific doses of KA. In adulthood (P90-P100), the behavioral performance of these rats was compared with that of control rats that did not receive KA. A modified version of the radial-arm maze was used to assess short-term spatial memory; the Morris water maze was used to evaluate long-term spatial memory and retrieval; and the elevated plus maze was used to determine anxiety. RESULTS: Compared with controls, rats with KA seizures at each tested age had impaired short-term spatial memory in the radial-arm maze (longer latency to criterion and more reference errors), deficient long-term spatial learning and retrieval in the water maze (longer escape latencies and memory for platform location), and a greater degree of anxiety in the elevated plus maze (greater time spent in open arms). CONCLUSIONS: These findings provide additional support for the concept that seizures early in life may be followed by life-long impairment of certain cognitive and behavioral functions. These results may have clinical implications, favoring early and aggressive control of seizures during development.