Exposing rats to a predator impairs spatial working memory in the radial arm water maze

This series of studies investigated the effects of predator exposure on working memory in rats trained on the radial arm water maze (RAWM). The RAWM is a modified Morris water maze that contains four or six swim paths (arms) radiating out of an open central area, with a hidden platform located at the end of one of the arms. The hidden platform was located in the same arm on each trial within a day and was in a different arm across days. Each day rats learned the location of the hidden platform during acquisition trials, and then the rats were removed from the maze for a 30-min delay period. During the delay period, the rats were placed either in their home cage (nonstress condition) or in close proximity to a cat (stress condition). At the end of the delay period, the rats were run on a retention trial, which tested their ability to remember which arm contained the platform that day. The first experiment confirmed that the RAWM is a hippocampal-dependent task. Rats with hippocampal damage were impaired at learning the location of the hidden platform in the easiest RAWM under control (non-stress) conditions. The next three experiments showed that stress had no effect on memory in the easiest RAWM, but stress did impair memory in more difficult versions of the RAWM. These findings indicate that the capacity for stress to impair memory is influenced not only by the brain memory system involved in solving the task (hippocampal versus nonhippocampal), but also by the difficulty of the task. This work should help to resolve some of the confusion in the literature regarding the heterogeneous effects of stress on hippocampal-dependent learning and memory.     

Low-dose nicotine facilitates spatial memory in ApoE-knockout mice in the radial arm maze

Here, we investigated the effects of nicotine on spatial memory in ApoE-knockout (ApoE-KO) and wild-type (WT) mice in a radial arm maze. Training occurred on three consecutive days and the test was performed on day 4, with one trial per day. Then on day 4, animals were administered nicotine (0.1, 0.25, 0.5, and 1.0 mg/kg) or the antagonist of nicotinic receptors (nAChRs) mecamylamine (MEC 2 mg/kg) alone or together with 0.1 mg/kg nicotine. The number of errors in the first eight choices was recorded. The results were that 0.1 mg/kg nicotine decreased errors in ApoE-KO mice, while 0.1 and 0.25 mg/kg nicotine reduced errors in WT mice, indicating that lower doses of nicotine elicit a memory improvement. In contrast, 1.0 mg/kg nicotine increased errors in WT mice, but not in ApoE-KO mice. MEC alone had no noticeable effect on errors in either strain of mice. However, co-administration of 0.1 mg/kg nicotine and MEC increased errors and reduced the effects of nicotine in WT mice, but not in ApoE-KO mice. Our study found a biphasic effect of nicotine in WT mice: it improves spatial memory at lower doses and impairs it at a higher dose. In ApoE-KO mice, nicotine improves memory at a low dose and has no effect at a higher dose, suggesting that the ApoE deficiency may influence the efficacy of nicotine. Moreover, a reversal of nicotinic effects with MEC was seen in WT mice, indicating the likelihood of the involvement of nAChRs in the spatial-memory response to nicotine.     

Deprivation of endogenous brain-derived neurotrophic factor results in impairment of spatial learning and memory in adult rats

Brain-derived neurotrophic factor (BDNF) is abundantly expressed in the hippocampus and cerebral cortex and is involved in synaptic plasticity and long-term potentiation (LTP). The present study was under taken to investigate whether endogenous BDNF was required for spatial learning and memory in a rat model. Antibodies to BDNF (anti-BDNF, n=7) or control immunoglobulin G (control, n=6) were delivered into the rat brain continuously for 7 days with an osmotic pump. The rats were then subjected to a battery of behavioral tests. The results show that the average escape latencies in the BDNF antibody treated group were dramatically longer than those of the control (F=13.3, p<0.001). The rats treated with control IgG swam for a significantly longer distance in the P quadrant (where the escape plane had been placed) compared with the other three quadrants (p<0.05). In contrast, anti-BDNF-treated rats swam an equivalent distance in all four quadrants. The average percentage of swimming distance in the P quadrant by anti-BDNF-treated rats was much less than that by control IgG treated rats (p<0.001). These results suggest that endogenous BDNF is required for spatial learning and memory in adult rats.     

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.     

Effects of intraventricular infusion of the N-methyl-D-aspartate (NMDA) receptor antagonist AP5 on spatial memory of rats in a radial arm maze.

Rats were trained to asymptotic performance in an 8-arm radial maze. They then received chronic intraventricular infusion of either artificial CSF or the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5- phosphonopentanoic acid (AP5), at a concentration (30 mM) that has been shown previously to prevent the induction of long-term potentiation in the dentate gyrus of the hippocampus in vivo. Subsequently the rats received another 9 trials in the maze in a quasi-random order, 3 uninterrupted trials, and another 6 trials each with mid-trial delays of 5, 20 or 60 min during which the animals were placed in their home cage. The mean number of errors for the AP5 rats did not differ significantly from that of the controls in the uninterrupted trials throughout the experiment, nor did it differ from that of the controls in any of the 3 delayed trials when these were first introduced. However, the control animals performed better at the longer delays when these were introduced for the second time, whilst there was no such improvement (but rather a deterioration) for the AP5 animals. The impairment of performance in the AP5 rats during the second block of delayed trials was significant, and independent of the length of the delay. These results show that NMDA receptor blockade does not impair working memory in the radial maze per se, but that it does prevent an improvement of working memory persistence with further training.     

Involvement of brain-derived neurotrophic factor in cannabinoid receptor-dependent protection against excitotoxicity

Cannabinoid type 1 (CB1) receptors play a central role in the protection against excitotoxicity induced by treatment of mice with kainic acid (KA). As inactivation of CB1 receptor function in mice blocks KA-induced increase of brain-derived neurotrophic factor (BDNF) mRNA levels in hippocampus, the notion was put forward that BDNF might be a mediator, at least in part, of CB1 receptor-dependent neuroprotection [Marsicano et al. (2003) Science, 302, 84-88]. To assess this signalling cascade in more detail, organotypic hippocampal slice cultures were used, as this in vitro system conserves morphological and functional properties of the hippocampus. Here, we show that both genetic ablation of CB1 receptors and pharmacological blockade with the specific CB1 receptor antagonist SR141716A increased the susceptibility of the in vitro cultures to KA-induced excitotoxicity, leading to extensive neuronal death. Next, we found that the application of SR141716A to hippocampal cultures from wild-type mice abolished the KA-induced increase in BDNF protein levels. Therefore, we tried to rescue these organotypic cultures from neuronal death by exogenously applied BDNF. Indeed, BDNF was sufficient to prevent KA-induced neuronal death after blockade of CB1 receptor signalling. In conclusion, our results strongly suggest that BDNF is a key mediator in CB1 receptor-dependent protection against excitotoxicity, and further underline the physiological importance of the endogenous cannabinoid system in neuroprotection.     

Spatial memory and choice behavior in the radial arm maze after fornix transection

In the radial arm maze task, it is well established that performance of rats with hippocampal damage is severely impaired on the place version, which relies heavily, if not exclusively, on spatial information. However, very little is known about the effects of hippocampal damage on actual choice behavior. To address this issue, sham-operated (SH) and fornix-transected (FX) rats were trained and tested on the place task in the eight-arm radial maze. The following measures were recorded: the frequency of re-entry errors, the number of choices separating repeated visits to the same arm, the latency to arm re-entry, the distribution and targets of microchoices defined as orientations toward an arm or entries in the proximal portion of an arm [Brown, M.F., 1992. Does a cognitive map guide choices in the radial arm maze? J. Exp. Psychol., Anim. Behav. Processes 18, 56-66]. These measures were used as indexes of performance, within-trial retroactive intrusion, memory trace decay and choice behavior, respectively. As generally observed in the literature, the frequency of errors was higher in rats of the FX group than in rats of the SH group; the impairment persisted even after the training criterion was reached. The analysis of latency to arm re-entry and of the number of choices separating re-entries suggested that this impairment was the result of faster memory decay rather than retroactive interference. Both FX and SH groups exhibited a systematic pattern of microchoices, but the frequency of microchoices was higher in FX lesioned rats than in SH controls. Moreover, in lesioned animals, relatively fewer of the initial microchoices were directed toward the baited arms during training-to-criterion. Some of the results provide support to the working memory theory [Olton, D.S., Becker, J.T., Handelmann, G.E., 1979. Hippocampus, space, and memory. Behav. Brain Sci. 2, 313-365], whereas others look more consistent with the cognitive map view [O'Keefe, J., Nadel, L., 1978. The Hippocampus as a Cognitive Map. Clarendon Press, Oxford]. The discussion suggests that both theories and a distinction between prospective and retrospective memory may be required to account for the function of the hippocampal formation in memory.     

Effects of metrifonate on radial arm maze acquisition in middle-aged rats

The efficacy of metrifonate, a well-tolerated cholinesterase (ChE) inhibitor, in attenuating the normal aging- and corticosterone-induced impairments of radial maze performance of rats was compared. Middle-aged Fischer 344 rats were screened for their spatial orientation performance in the Morris water escape task. Good and bad performers were selected: good performers (N=22) were treated with subcutaneous sustained-release corticosterone pellets, resulting in hippocampal cell damage and impaired spatial orientation in the radial maze; age-induced bad performers (N=20) were tested without additional pharmacological intervention. Metrifonate (MFT), administered daily during radial maze testing, 30 min before training, at a dose of 15 mg/kg p.o., facilitated the acquisition of the task in age-impaired rats, but not in corticosterone-impaired rats.     

Chronic restraint stress enhances radial arm maze performance in female rats

Effects of chronic restraint stress (21 and 28 days) on physiological and behavioral parameters in female rats were examined. Total (bound and free) and free corticosterone (CORT) levels were measured at different time points during the stress period. Higher total CORT levels were observed in stressed rats during the stress period but returned to baseline at 15 days post-stress. Additionally, free CORT levels decreased across the stress period. Estrous cyclicity was monitored daily in all animals. Stress had no apparent effects on estrous cyclicity, in rats with either normal length or elongated estrous cycles, but stressed females gained less weight than controls. Following the stress period, subjects were tested for open field activity and radial arm maze (RAM) performance. Females stressed for 21 days showed enhanced spatial memory performance on the RAM. A longer period of restraint, 28 days, also led to less weight gain by stressed subjects and unaltered estrous cycle lengths, but was not associated with enhanced RAM performance. Further analysis indicated that RAM performance was influenced by specific estrous cycle day, particularly during proestrus. Following 21 days of restraint stress all animals in proestrus, regardless of treatment, showed impaired acquisition. After 28 days, stressed females in proestrus performed better than proestrus controls. These results are discussed in relation to previously reported effects of stress in male rats.     

Cerebral ischemia combined with beta-amyloid impairs spatial memory in the eight-arm radial maze task in rats

beta-Amyloid (Abeta), a major component of senile plaques in Alzheimer's disease, has been implicated in neuronal cell death, a characteristic feature of this condition. In our previous experiments using primary cultures of hippocampal neurons, Abeta treatment induced neuronal cell death, displaying morphological characteristics of apoptosis that was significantly enhanced by hypoxia. Based on these results, we developed a simple in vivo rat model of Alzheimer's disease using cerebral ischemia, instead of hypoxia, combined with continuous intracerebroventricular administration of Abeta. The combination of cerebral ischemia and Abeta administration, but not either treatment alone, significantly impaired spatial memory in an eight-arm radial maze. A microdialysis study showed that spontaneous release of acetylcholine (ACh) from the dorsal hippocampus had a tendency to decrease in response to Abeta treatment alone or the combination of ischemia and Abeta. High K(+)-evoked increase in ACh release had a tendency to be inhibited by either ischemia or Abeta treatment alone and was significantly inhibited by the combination of both. Moreover, combination of ischemia and Abeta induced apoptosis of pyramidal neurons in the CA1 region of the hippocampus. Donepezil, a drug currently in clinical use for Alzheimer's disease, improved the impairment of spatial memory induced by cerebral ischemia combined with Abeta. These findings suggest that ischemia is an important factor facilitating the symptoms of Alzheimer's disease, and this model may be useful for developing new drugs for the treatment of Alzheimer's disease.     

Lesions of perirhinal cortex produce spatial memory deficits in the radial maze

Rats with bilateral electrolytic lesions of perirhinal cortex (PRC) or sham control (SHAM) lesions were tested in spatial reference and working memory tasks in the radial arm maze. In experiment 1, one arm of the maze was baited and always located in a fixed position relative to the extra-maze environment. PRC lesioned animals made a significantly greater number of errors than did SHAM animals during initial training in this reference memory task and exhibited a delay-dependent impairment on trial 5 in a series when a delay period of 5, 60, 120, or 240 s was inserted between trials 4 and 5. In experiment 2, when a second group of the animals was tested on the standard radial arm maze working memory task, the performance of the PRC group was markedly impaired relative to controls. These data demonstrate that electrolytic PRC lesions result in a deficit in both spatial reference and spatial working memory tasks. These effects are interpreted as being consistent with the idea that PRC plays an important role in episodic memory processes. These processes may include the storage of information, which is required for the performance of spatial tasks. Hippocampus 1998;8:114–121. © 1998 Wiley-Liss, Inc.     

Intradentate colchicine disrupts the acquisition and performance of a working memory task in the radial arm maze

Rats were given bilateral injections of colchicine into the dorsal and ventral hippocampus to study the role of the dentate gyrus granule cells in the acquisition and performance of a spatial, working memory task in the radial arm maze. Three weeks after intradentate injections, rats were trained in a task in which all eight arms were baited prior to each daily trail. For up to 20 days of training, colchicine-treated rats were significantly impaired in the performance of the task. In another study, rats received 20 days of training and then were given intradentate colchicine. Three weeks later, the performance of the colchicine-treated rats was impaired for up to 20 days of testing. A third experiment tested the ability of colchicine-treated rats to learn a task in which the same four arms of the maze were baited, while the remaining arms were never baited. Colchicine-treated rats were significantly impaired in their ability to perform this version of the task. Histological verification indicated that colchicine resulted in a relatively select loss of granule cells, while sparing pyramidal cells in the hippocampus. These data suggest that the hippocampus plays an integral role in the performance of the place tasks used in these experiments.     

Water maze versus radial maze: differential performance of rats in a spatial delayed match-to-position task and response to scopolamine.

Studies rarely assess treatment effects across tasks; the present experiments addressed this issue. In Experiments 1 and 2, rats (n=12) were trained and then tested with variable delays on a spatial match-to-position task sequentially in the water and radial mazes (in counterbalanced order). Experiment 1 compared the effect of 0-, 60- and 1440-min delays on performance in both mazes. Rats required fewer (P<0.05) mean (+/-S.E.M.) sessions to reach criterion performance in the water (11.0+/-1.0) versus radial maze (19.3+/-2.2). In test sessions, performance was impaired delay-dependently when scores were averaged across the two tasks (P<0.05) but no significant effect of task or task x delay interaction was found. In the second experiment, the same rats were retrained and tested with 0-, 1-, 3- and 5-min delays in both mazes and testing followed the administration of scopolamine (0, 0.1, 0.4 and 0.8 mg/kg). The mean (+/-S.E.M.) number of acquisition sessions was similar in the radial (6.33+/-0.34) and water maze (6.08+/-0.46). On the sample portion of trials, performance was impaired at the 0.8 mg/kg dose of scopolamine (P<0.02) in the radial maze only. On the recognition portion of trials in the radial maze, the 0.4 and 0.8 mg/kg doses of scopolamine impaired performance whereas in the water maze task the 0.8 mg/kg dose impaired performance. The pattern of results may reflect different natural tendencies of rats to alternate (win-shift) versus not alternate (win-stay) in dry land versus swim tasks.     

Long-term effects of developmental halothane exposure on radial arm maze performance in rats

Chronic exposure of rats to low levels of halothane during development, a treatment which retards synaptogenesis, was found to cause a long-term impairment of choice accuracy in the radial-arm maze. In Expt. 1, the relative importance of dose level and dosing regimen was examined. Dose level seemed the more critical variable for causing impaired choice accuracy. Exposure to 100 parts per million (ppm) of halothane in the air either on an intermittent or continuous schedule from day two of conception until 60 days after birth significantly impaired choice accuracy, whereas exposure to 25 ppm on a continuous schedule did not cause a deficit, even though with this condition the total amount of halothane exposure was about the same as with 100 ppm given intermittently. In Expt. 2, the 100 ppm intermittent exposure regimen was used to examine the relative importance of exposure during early and late developmental periods for producing the cognitive effects of halothane. Groups were divided into those exposed to halothane during gestation and until 30 days after birth (early exposure), those exposed from day 31 until day 90 (late exposure) and those exposed during both early and late periods (combined exposure). Adverse effects on choice accuracy were seen with all 3 types of exposure, but surprisingly, it was the late exposure that caused the most severe effects. These results show that developmental exposure to halothane which impairs synaptogenesis also causes long-lasting cognitive impairment. Halothane exposure can be a useful experimental tool for examining the relationship between synaptic and behavioral development.     

Corticostriatal brain-derived neurotrophic factor dysregulation in adult rats following prenatal stress

Prenatal stress represents a well-established experimental protocol resembling some features of schizophrenia, including deficits in social interactions, disruption of prepulse inhibition and enhanced response to psychomotor stimulants. In order to evaluate molecular changes that could participate in long-lasting effects on brain function, we analysed the effects of prenatal stress on the expression of brain-derived neurotrophic factor (BDNF), an important molecular determinant of synaptic plasticity and cellular homeostasis, in adult male rats under basal conditions as well as in response to a chronic stress. The main finding is that BDNF expression is reduced in the prefrontal cortex and striatum of prenatally stressed rats. Furthermore, when exposed to chronic stress in adulthood, these rats display an altered regulation of BDNF expression in these brain structures, implying that adverse life events during gestation may interfere with the expression and function of this neurotrophin at adulthood in a region-specific manner. The dysregulation of corticostriatal BDNF expression might thus contribute to permanent alterations in brain functions leading to heightened susceptibility to psychiatric disorders.     

Spatial competences in Williams syndrome: a radial arm maze study

This study was aimed at evaluating spatial function in subjects with Williams syndrome by using the radial arm maze task and comparing their spatial abilities with those of mental age-matched control subjects. Two different paradigms were administered: the free-choice version for analyzing the aspects linked mainly to procedural and mnesic components, the forced-choice version for disentangling components linked to spatial working memory from the procedural ones.     

Mecamylamine blocks enhancement of reference memory but not working memory produced by post-training injection of nicotine in rats tested on the radial arm maze

The focus of this study was to analyze whether the psychostimulant nicotine would enhance reference and working memory consolidation in rats tested on the 8-arm radial arm maze. Mecamylamine, a nicotine antagonist, was used to attempt to block the enhancement of memory consolidation. All rats were given one training trial/day for 12 consecutive days, and 4 arms were baited. Rats were separated into five groups: the saline-nicotine group received an intraperitoneal (i.p.) injection of saline immediately after each trial followed 15 min later by an subcutaneous (s.c.) injection of nicotine (0.6 mg/kg free base); the nicotine-delay group received an s.c. injection of nicotine 2 h after each training trial, two groups received an i. p. injection of one of two different doses of mecamylamine (2.5 and 6.0 mg/kg) immediately after each trial, which was followed 15 min later by an s.c. nicotine injection, and a control group received an i.p. injection of saline immediately and 15 min after each training trial. Results showed that the saline-nicotine group made fewer reference and working memory errors than the saline- or nicotine-delay groups, but only the effect of nicotine on reference memory was blocked by the higher dose of mecamylamine. It appears from these results that nicotine's effects on reference and working memory may be mediated through different mechanisms.     

慢性癫痫大鼠脑组织中脑源性神经生长因子及其受体的研究

应用免疫组织化学的方法观察了戊四氮诱导的慢性癫痫大鼠脑组织中脑源性神经生长因子（BDNF）及其受体TrkB免疫反映阳性神经元的变化。结果发现慢性癫痫大鼠海马回、齿状回BDNF及TrkB免疫反应阳性神经元数目明显增多。在本次抽搐后3h、72h、7d、10dBDNF阳性神经元均增高,而TrkB阳性神经元仅在末次抽搐后3h升高。24h后恢复到正常水平,结果表明,BDNF及TrkB与癫痫发病有关。     

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.     

Persistence of working memory of rats in an aversively motivated radial maze task

The effects of various within-trial delays on the performance of rats in an 8-arm radial water maze (RWM) were investigated. Rats (n = 13) were trained to escape from 25 degrees C opaque water onto a submerged bench situated at the far end of each channel of the maze. After 20 s the chosen bench was collapsed and the rat had to return to a central platform, 1 cm above water level. After 15 s the platform was lowered and the animal was forced to choose again. This was repeated until all 8 channels had been visited. After 30 daily trials the mean number of correct choices per trial (first 8 choices) was 7.8. In Experiment 1, delays from 40 min up to 21 h were inserted between choices 4 and 5. The number of revisited channels (errors) in the second half of a trial increased rapidly with the duration of the delay, and was not significantly different from chance at 640 min. In Experiment 2, delays of 2.5, 5, 10 or 20 min were inserted between individual choices. Again, performance deteriorated with the duration of the delay. When the incidence of errorless trials was analyzed, performance was not significantly different from random at 5 min inter-choice delays. Comparison with conventional radial maze studies indicates that spatial working memory in the RWM has a slower, but also exponential decay.