Long-term deficits in water maze spatial conditional alternation performance following retrohippocampal lesions in rats

The effects of large bilateral retrohippocampal lesions on long-term performance of conditional spatial alternation, incorporating a strong working memory component, were examined using a T-maze task motivated by swim-escape. The lesions, which included entorhinal cortex, subiculum, pre- and parasubiculum and invaded the molecular layer of the dentate gyrus, completely eliminated the previously acquired conditional alternation learning, and performance failed to recover with 40 days of testing. These findings support the contention that retrohippocampal structures are an important and necessary component of the neural circuitry mediating working memory.     

Endovascular perforation subarachnoid hemorrhage fails to cause Morris water maze deficits in the mouse

Cognitive dysfunction is the primary driver of poor long-term outcome in aneurysmal subarachnoid hemorrhage (SAH) survivors; modeling such deficits preclinically is thus key for mechanistic and translational investigation. Although rat SAH causes long-term deficits in learning and memory, it remains unknown whether similar deficits are seen in the mouse, a species particularly amenable to powerful, targeted genetic manipulation. We thus subjected mice to endovascular perforation SAH and assessed long-term cognitive outcome via the Morris water maze (MWM), the most commonly used metric for rodent neurocognition. No significant differences in MWM performance (by either of two protocols) were seen in SAH versus sham mice. Moreover, SAH caused negligible hippocampal CA1 injury. These results undercut the potential of commonly used methods (of SAH induction and assessment of long-term neurocognitive outcome) for use in targeted molecular studies of SAH-induced cognitive deficits in the mouse.     

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.     

Spatial discrimination deficits by excitotoxic lesions in the Morris water escape task

The effects of the cholinesterase inhibitors (ChEI) metrifonate and donepezil were assessed on spatial performance of rats with bilateral lesions of the entorhinal cortex (EC), which is thought to model early changes in the brains of patients suffering from Alzheimer's disease. In the present study, we found that spatial discrimination deficits in rats, induced by bilateral ibotenic acid (IBO) lesions of the EC region can partially be antagonised by treatment with the cholinesterase inhibitors metrifonate (30 mg kg(-1)) and donepezil (0.3 and 3 mg kg(-1)). Performance was improved in the spatial discrimination task compared with that of the EC-lesioned control group. It is concluded that the rat with bilateral EC lesions is a suitable deficit model for the assessment of effects of putative Alzheimer therapeutics.     

Brain lesions and delayed water maze learning deficits after intracerebroventricular spermine

The effects of spermine on the acquisition and retention of spatial learning in the Morris water maze were studied. Spermine 25 and 125 nmol i.c.v. did not alter the ability of rats to find a hidden platform in the water maze when administered before training over 5 days. However, the inhibitory effect of the benzodiazepine, diazepam (3 mg/kg i.p., 30 min prior to training), on path length to target was markedly potentiated by the higher dose of spermine, consistent with spermine acting as a functional antagonist at the NMDA receptor. This drug combination did not affect performance on visible platform trials. Administration of doses of 125 and 250 nmol (but not 62.5 nmol) of spermine i.c.v. in the week prior to training (daily for 5 days) dose-dependently inhibited subsequent learning of a platform position in the absence of drug. These higher doses of spermine produced neuronal loss and increased

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binding indicating microglial activation predominantly in the hippocampus and to a lesser extent in the striatum, septum, thalamus and amygdala. Spermine 125 nmol i.c.v. (daily for 7 days) also abolished retention of a previously learned platform position when administered in an interval between training and retention testing. The inhibitory effects of spermine 125 nmol i.c.v. (daily for 7 days) on subsequent spatial learning were not antagonised by concomitant administration of 30 nmol dizocilpine. These results demonstrate that spermine produces a delayed neurotoxic effect in particular neuronal populations in the brain that selectively impair spatial learning and recall.     

Allopregnanolone inhibits learning in the Morris water maze

The progesterone metabolite allopregnanolone (3alpha-OH-5alpha-pregnane-20-one) inhibits neural functions, enhancing the GABA induced GABA(A) receptor activation. This effect is benzodiazepine like and benzodiazepines are known to impair memory. Acute effects of allopregnanolone on the hippocampus dependent spatial learning in the Morris water maze have not been studied. Adult male Wistar rats where injected (i.v.) with allopregnanolone (2 mg/kg), or vehicle, daily for 11 days. At 8 or 20 min after each injection, studies of place navigation were performed in the Morris water maze. Allopregnanolone concentrations in plasma and in nine different brain areas where analyzed by radioimmunoassay. The latency to find the platform was increased 8 min after the allopregnanolone injection, while normal learning was seen after 20 min. Swim speed did not differ between groups. A higher number of rats were swimming close to the pool wall (thigmotaxis) in the 8 min allopregnanolone group compared to the other groups. Allopregnanolone concentrations in the brain tissue at 8 min were 1.5 to 2.5 times higher then at 20 min after the allopregnanolone injections. After vehicle injections the brain concentrations of allopregnanolone were at control levels. Plasma concentrations of allopregnanolone followed the same pattern as in the brain, with the exception of an increase 8 min after vehicle injections. The natural progesterone metabolite allopregnanolone can inhibit learning in the Morris water maze, an effect not caused by motor impairment. The learning impairment might be due to a combination of changed swimming behavior and difficulties in navigation.     

Impaired acquisition of the Morris water maze following global ischemic damage in the gerbil.

Five minutes of global ischemia in the Mongolian gerbil impaired acquisition of a Morris water maze task when testing began 72 h after surgery. In spite of extensive damage to CA1 pyramidal cells, ischemic animals eventually learned to locate a submerged platform and performed normally on a subsequent retention test. Animals that were allowed a more protracted recovery period (21 days) acquired the task as readily as control gerbils. These results suggest that undamaged structures within and external to the hippocampal formation allow spatial learning to proceed at a somewhat reduced rate.     

Memantine affects cognitive flexibility in the Morris water maze

Alzheimer's disease (AD) is multi-factorial mental disorder characterized by a copious array of congruent features cumulating in disrupted memory and dysthymia. Though the mechanism remains elusive, the highly unspecific pharmaceutical, memantine, provides modest benefits for patients with moderate-to-severe AD. A greater understanding of how memantine affects cognitive function promises to facilitate the search for better therapeutics. We therefore examined cognitive flexibility of mice following 5 and 10 mg/kg memantine administration using a platform re-location water maze. Strikingly, subjects receiving memantine demonstrated memory impairment relative to controls when re-trained off drug, revealing a novel and unusual disruption of cognitive flexibility.     

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 fornix transection and combined fornix transection, mammillary body lesions and hippocampal ablations on object-pair association memory in the rhesus monkey

Cynomolgus monkeys were tested on an associative memory task in which they had to remember object-object pairings. Eight animals (4 unoperated monkeys and 4 monkeys with lesions to components of the 'hippocampal-mammillary' circuit) were trained initially on a conditional object-pair association task that was similar to simple discrimination learning, but which involved presentation of object-pairs instead of single objects. Object-pairs were made up from 4 different objects, each with an identical copy. Animals had to learn which pairings of the 4 objects were rewarded (e.g. AB or CD) and which pairings were not (e.g. AC or BD). Objects appeared as a member of both rewarded and non-rewarded pairs. After several stages of training all monkeys reached a high level of performance making greater than 90% correct choices. In a performance test the monkeys had to discriminate between 3 single objects depending on their past associations with the other objects. Animals had to choose the two objects which in initial training were rewarded as an object pair. Monkeys with the 'hippocampal-mammillary' circuit lesions learned the initial object-pair discriminations at the same rate as control monkeys. In the performance test, however, monkeys with the 'hippocampal-mammillary' circuit lesions performed at chance levels, whereas the control monkeys performed significantly better. The results demonstrate a dissociation between the 'habit' and 'mediational' memory systems for ostensibly the same object-pair associations. They also indicate an important contribution of the 'hippocampal-mammillary' circuit in nonspatial association memory.     

Automatic recognition of explorative strategies in the Morris water maze

Notwithstanding the development of reliable tracking systems, the quantification methodology of the Morris water maze (MWM) has witnessed an operational mismatch between the indexes used to quantify MWM performance and the cognitive concepts derived from these indexes. Indeed, escape latency is the main, and often unique, performance measure used for the quantification of behavior. Aim of the present work was to overcome this limitation by presenting a methodology that allows for automatic categorical pattern recognition of the behavioral strategies performed in the MWM. By selecting few a priori and user-defined behavioral categories, many quantitative variables and regions of interest (ROIs), we used discriminant analysis (DA) to obtain 97.9% of correct automatic recognition of categories. The developed discriminant model (DM) also allowed to predict category membership of newly recorded swim paths with the same statistical efficacy (96%), and to identify the variables that better discriminate between adjacent categories. The combination of DA with a tracking system, a selection of many variables, different ROIs and qualitative categorization, reduces the gap between the measurement process and the categories used to describe a given behavior, and offers a methodology to computationally reproduce the human categorization of behaviors in the MWM.     

Disorientation combined with bilateral parietal cortex lesions causes path integration deficits in the water maze

The navigational abilities of rats were examined using the water maze after disorientation induced by rotation and/or swimming in darkness. Control and light-disoriented groups performed similarly, whereas the dark group and the dark-disoriented groups were initially much slower but improved to control levels. After receiving bilateral parietal lesions, multiple start position tests showed that both rotation groups were severely impaired in finding the hidden platform. The effects of disorientation induced by darkness and by rotation are therefore separable.     

Dense amnesia in the monkey after transection of fornix, amygdala and anterior temporal stem

The traditional explanation of dense amnesia after medial temporal lesions is that the amnesia is caused by damage to the hippocampus and related structures. An alternative view is that dense amnesia after medial temporal lesions is caused by the interruption of afferents to the temporal cortex from the basal forebrain. These afferents travel to the temporal cortex through three pathways, namely the anterior temporal stem, the amygdala and the fornix-fimbria, and all these three pathways are damaged in dense medial temporal amnesia. In four experiments using different memory tasks, we tested the effects on memory of sectioning some or all of these three pathways in macaque monkeys. In a test of scene-specific memory for objects, which is analogous in some ways to human episodic memory, section of fornix alone, or section of amygdala and anterior temporal stem sparing the fornix, each produced a significant but mild impairment. When fornix section was added to the section of anterior temporal stem and amygdala in this task, however, a very severe impairment resulted. In an object recognition memory task (delayed matching-to-sample) a severe impairment was seen after section of anterior temporal stem and amygdala alone, with or without the addition of fornix section; this impairment was significantly more severe than that which was seen in the same task after amygdalectomy leaving the temporal stem intact, with or without fornix section. Animals with combined section of anterior temporal stem, amygdala and fornix were also impaired in object-reward association learning. However, the retention of pre-operatively acquired object-reward associations was at a high level. These results show that the pattern of impairments after section of anterior temporal stem, amygdala and fornix in the monkey, leaving hippocampus intact, resembles human dense amnesia and is different from the effects of hippocampal lesions in the monkey.     

Differential effects of fornix and caudate nucleus lesions on two radial maze tasks: evidence for multiple memory systems

The present experiments were designed to examine the hypothesis that the mammalian brain contains anatomically distinct memory systems. Rats with bilateral lesions of caudate nucleus or fimbria-fornix and a control group were tested postoperatively on 1 of 2 versions of the radial maze task. In a standard win-shift version, each of the 8 arms of the maze was baited once, and the number of errors (revisits) in the first 8 choices of each trial was recorded. Fimbria-fornix rats were impaired in choice accuracy, while caudate animals were unimpaired relative to controls. Different groups of rats with similar lesions were tested on a newly developed win-stay version of the radial maze, in which the location of 4 randomly selected baited arms was signaled by a light at the entrance to each arm, and which required rats to revisit arms in which reinforcement had been previously acquired within a trial. Rats with fimbria-fornix lesions were superior to controls in choice accuracy on the win-stay radial maze task, while caudate animals were impaired relative to controls. The results demonstrate a double dissociation of the mnemonic functions of the hippocampus and caudate nucleus. Some implications of the presence of 2 memory systems in the mammalian brain are discussed.     

On-demand platform improves accuracy of the Morris water maze procedure

In order to prevent chance finding of the hidden target in the Morris water tank task, the rigid underwater platform is replaced with a collapsible platform, resting at the bottom of the pool. A computerized videosystem tracks the rat's movement across the pool and raises the platform when the animal has stayed in the target area for a predetermined time. Acquisition of the task with the collapsible platform proceeds at a similar rate as with the rigid platform when the criterion conditions are easy (target distance 15 cm, target time 2.5 s), but gradually deteriorates when the target time increases to 10.0 s. Successful solution of the modified task requires accurate localization of the target under open loop conditions and is thus well suited for investigation of the fine structure of the cognitive maps and of their changes induced by lesions or drugs.     

Methylphenidate improves cue navigation in the Morris water maze in rats

Despite its still rapidly increasing use in the treatment of the attention deficit/hyperactivity disorder, the effects of methylphenidate on behavior and learning are not yet fully understood. We have used the Morris water maze to study effects of methylphenidate (1 mg/kg) on target-oriented behavior and visuospatial learning in young rats. Although the relatively low dose of 1 mg/kg methylphenidate changed the behavior in the Morris water maze when the goal was visible, reducing times to reach the visible platform, learning of the position of the hidden platform, was not influenced, nor was 'relearning' after shifting the position of the platform. These results indicate that methylphenidate can influence goal-oriented behavior at doses that do not change visuospatial learning.     

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.     

Hippocampal kindled seizures impair spatial cognition in the Morris water maze

We investigated the effects of hippocampally kindled seizures on spatial performance of rats in the Morris water maze (MWM). Seizures were elicited with stimulation of field CA1 of dorsal hippocampus 25-45 min prior to daily testing in the water maze. One group of rats was naive to the MWM (acquisition groups), while another group received pretraining in the MWM (retention groups). These groups were further subdivided into rats that experienced non-convulsive seizures prior to daily testing and rats that experienced fully generalized convulsive seizures prior to daily testing. We found that CA1 seizures significantly disrupted water maze performance during both acquisition and retention, and the effects were similar when either non-convulsive or fully generalized convulsive seizures were evoked. Our findings are consistent with previous reports suggesting that epileptiform activity in the hippocampus acutely impairs performance in tasks sensitive to spatial learning and memory deficits and suggest that both new learning and demonstration of an established place response are susceptible to such disruption.