Dissociation of spatial reference memory, spatial working memory, and hippocampal mossy fiber distribution in two rat strains differing in emotionality

Impact of Hyperbaric Oxygen Therapy on Cognitive Functions: a Systematic Review

Hyperbaric oxygen therapy (HBOT) is a modality of treatment in which patients inhale 100% oxygen inside a hyperbaric chamber pressurised to greater than 1 atmosphere. The aim of this review is to discuss neuropsychological findings in various neurological disorders treated with HBOT and to open new perspectives for therapeutic improvement. A literature search was conducted in the MEDLINE (via PubMed) database from the inception up 10 May 2020. Eligibility criteria included original articles published in English. Case studies were excluded. Full-text articles were obtained from the selected studies and were reviewed on the following inclusion criteria (1) performed cognitive processes assessment (2) performed HBOT with described protocol. Two neuropsychologists independently reviewed titles, abstracts, full texts and extracted data. The initial search retrieved 1024 articles, and a total of 42 studies were finally included after applying inclusion and exclusion criteria. The search yielded controversial results with regard to the efficiency of HBOT in various neurological conditions with cognitive disturbance outcome. To the best of our knowledge this is the first state-of-the art, systematic review in the field. More objective and precise neuropsychological assessment methods are needed to exact evaluation of the efficacy of HBOT for neuropsychological deficits. Future studies should widen the assessment of HBOT effects on different cognitive domains because most of the existing studies have focussed on a single process. Finally, there is a need for further longitudinal studies.

Modeling heritability of temperamental differences, stress reactivity, and risk for anxiety and depression: Relevance to research domain criteria (RDoC)

Animal models provide important tools to study biological and environmental factors that shape brain function and behavior. These models can be effectively leveraged by drawing on concepts from the National Institute of Mental Health Research Domain Criteria (RDoC) Initiative, which aims to delineate molecular pathways and neural circuits that underpin behavioral anomalies that transcend psychiatric conditions. To study factors that contribute to individual differences in emotionality and stress reactivity, our laboratory utilized Sprague-Dawley rats that were selectively bred for differences in novelty exploration. Selective breeding for low versus high locomotor response to novelty produced rat lines that differ in behavioral domains relevant to anxiety and depression, particularly the RDoC Negative Valence domains, including acute threat, potential threat, and loss. Bred Low Novelty Responder (LR) rats, relative to their High Responder (HR) counterparts, display high levels of behavioral inhibition, conditioned and unconditioned fear, avoidance, passive stress coping, anhedonia, and psychomotor retardation. The HR/LR traits are heritable, emerge in the first weeks of life, and appear to be driven by alterations in the developing amygdala and hippocampus. Epigenomic and transcriptomic profiling in the developing and adult HR/LR brain suggest that DNA methylation and microRNAs, as well as differences in monoaminergic transmission (dopamine and serotonin in particular), contribute to their distinct behavioral phenotypes. This work exemplifies ways that animal models such as the HR/LR rats can be effectively used to study neural and molecular factors driving emotional behavior, which may pave the way toward improved understanding the neurobiological mechanisms involved in emotional disorders.

Coordinated prefrontal-hippocampal activity and navigation strategy-related prefrontal firing during spatial memory formation

Learning the location of relevant places in the environment is crucial for survival. Such capacity is supported by a distributed network comprising the prefrontal cortex and hippocampus, yet it is not fully understood how these structures cooperate during spatial reference memory formation. Hence, we examined neural activity in the prefrontal-hippocampal circuit in mice during acquisition of spatial reference memory. We found that interregional oscillatory coupling increased with learning, specifically in the slow-gamma frequency (20 to 40 Hz) band during spatial navigation. In addition, mice used both spatial and nonspatial strategies to navigate and solve the task, yet prefrontal neuronal spiking and oscillatory phase coupling were selectively enhanced in the spatial navigation strategy. Lastly, a representation of the behavioral goal emerged in prefrontal spiking patterns exclusively in the spatial navigation strategy. These results suggest that reference memory formation is supported by enhanced cortical connectivity and evolving prefrontal spiking representations of behavioral goals.

Working Memory in the Prefrontal Cortex

The prefrontal cortex participates in a variety of higher cognitive functions. The concept of working memory is now widely used to understand prefrontal functions. Neurophysiological studies have revealed that stimulus-selective delay-period activity is a neural correlate of the mechanism for temporarily maintaining information in working memory processes. The central executive, which is the master component of Baddeley's working memory model and is thought to be a function of the prefrontal cortex, controls the performance of other components by allocating a limited capacity of memory resource to each component based on its demand. Recent neurophysiological studies have attempted to reveal how prefrontal neurons achieve the functions of the central executive. For example, the neural mechanisms of memory control have been examined using the interference effect in a dual-task paradigm. It has been shown that this interference effect is caused by the competitive and overloaded recruitment of overlapping neural populations in the prefrontal cortex by two concurrent tasks and that the information-processing capacity of a single neuron is limited to a fixed level, can be flexibly allocated or reallocated between two concurrent tasks based on their needs, and enhances behavioral performance when its allocation to one task is increased. Further, a metamemory task requiring spatial information has been used to understand the neural mechanism for monitoring its own operations, and it has been shown that monitoring the quality of spatial information represented by prefrontal activity is an important factor in the subject's choice and that the strength of spatially selective delay-period activity reflects confidence in decision-making. Although further studies are needed to elucidate how the prefrontal cortex controls memory resource and supervises other systems, some important mechanisms related to the central executive have been identified.

Comparison of spatial learning in the partially baited radial-arm maze task between commonly used rat strains: Wistar, Spargue-Dawley, Long-Evans, and outcrossed Wistar/Sprague-Dawley

Strain-related differences in animals' cognitive ability affect the outcomes of experiments and may be responsible for discrepant results obtained by different research groups. Therefore, behavioral phenotyping of laboratory animals belonging to different strains is important. The aim of the present study was to compare the variation in allothetic visuospatial learning in most commonly used laboratory rat strains: inbred Wistar (W) and Sprague-Dawley (SD), outcrossed Wistar/Sprague-Dawley (W/SD), and outbred Long Evans (LE) rats. All rats were trained to the arbitrary performance criterion of 83 % correct responses in the partially baited 12-arm radial maze allowing for simultaneous evaluation of both working and reference memory. In the present study, testing albino versus pigmented and inbred versus outcrossed rats revealed significant strain-dependent differences with the inbred SD rats manifesting lower performance on all learning measures compared to other strains. On the other hand, the outcrossed W/SD rats showed a lower frequency of reference memory errors and faster rate of task acquisition compared to both LE and W rats, with W rats showing a lower frequency of working memory errors compared to other strains. In conclusion, albinism apparently did not reduce the animals' performance in the allothetic visuospatial learning task, while outcrossing improved the spatial learning. A differential effect of strain on the contribution of each error type to the animals' overall performance was observed. The strain-dependent differences were more pronounced between subpopulations of learning-deficient individuals ("poor" learners), and generally the reference memory errors contributed more to the final behavioral output than did the working memory errors.

Transferrin Receptor 2 Dependent Alterations of Brain Iron Metabolism Affect Anxiety Circuits in the Mouse

The Transferrin Receptor 2 (Tfr2) modulates systemic iron metabolism through the regulation of iron regulator Hepcidin (Hepc) and Tfr2 inactivation causes systemic iron overload. Based on data demonstrating Tfr2 expression in brain, we analysed Tfr2-KO mice in order to examine the molecular, histological and behavioural consequences of Tfr2 silencing in this tissue. Tfr2 abrogation caused an accumulation of iron in specific districts in the nervous tissue that was not accompanied by a brain Hepc response. Moreover, Tfr2-KO mice presented a selective overactivation of neurons in the limbic circuit and the emergence of an anxious-like behaviour. Furthermore, microglial cells showed a particular sensitivity to iron perturbation. We conclude that Tfr2 is a key regulator of brain iron homeostasis and propose a role for Tfr2 alpha in the regulation of anxiety circuits.

Hyperbaric Oxygen Therapy Alleviates Carbon Monoxide Poisoning-Induced Delayed Memory Impairment by Preserving Brain-Derived Neurotrophic Factor-Dependent Hippocampal Neurogenesis

OBJECTIVE

To test the hypothesis that hyperbaric oxygen therapy ameliorates delayed cognitive impairment after acute carbon monoxide poisoning by promoting neurogenesis through upregulating the brain-derived neurotrophic factor in the hippocampus.

DESIGN

Laboratory animal experiments.

SETTING

University/Medical center research laboratory.

SUBJECTS

Adult, male Sprague-Dawley rats.

INTERVENTIONS

Rats were divided into five groups: (1) non-carbon monoxide-treated control, (2) acute carbon monoxide poisoning, (3) acute carbon monoxide poisoning followed by 7-day hyperbaric oxygen treatment, (4) carbon monoxide + hyperbaric oxygen with additional intracerebroventricular infusion of Fc fragment of tyrosine kinase receptor B protein (TrkB-Fc) chimera, and (5) acute carbon monoxide poisoning followed by intracerebroventricular infusion of brain-derived neurotrophic factor. Acute carbon monoxide poisoning was achieved by exposing the rats to carbon monoxide at 2,500 ppm for 40 minutes, followed by 3,000 ppm for 20 minutes. Hyperbaric oxygen therapy (at 2.5 atmospheres absolute with 100% oxygen for 60 min) was conducted during the first 7 days after carbon monoxide poisoning. Recombinant human TrkB-Fc chimera or brain-derived neurotrophic factor was infused into the lateral ventricle via the implanted osmotic minipump. For labeling of mitotic cells in the hippocampus, bromodeoxyuridine was injected into the peritoneal cavity. Distribution of bromodeoxyuridine and two additional adult neurogenesis markers, Ki-67 and doublecortin, in the hippocampus was evaluated by immunohistochemistry or immunofluorescence staining. Tissue level of brain-derived neurotrophic factor was assessed by enzyme-linked immunosorbent assay. Cognitive behavior was evaluated by the use of eight-arm radial maze.

MEASUREMENTS AND MAIN RESULTS

Acute carbon monoxide poisoning significantly suppressed adult hippocampal neurogenesis evident by the reduction in number of bromodeoxyuridine-positive, Ki-67⁺, and doublecortin⁺ cells in the subgranular zone of the dentate gyrus. This suppression of adult neurogenesis by the carbon monoxide poisoning was appreciably alleviated by early treatment of hyperbaric oxygen. The hyperbaric oxygen treatment also promoted a sustained increase in hippocampal brain-derived neurotrophic factor level. Blockade of hippocampal brain-derived neurotrophic factor signaling with intracerebroventricular infusion of recombinant human TrkB-Fc chimera significantly blunted the protection by the hyperbaric oxygen on hippocampal neurogenesis; whereas intracerebroventricular infusion of brain-derived neurotrophic factor mimicked the action of hyperbaric oxygen and preserved hippocampal neurogenesis after acute carbon monoxide poisoning. Furthermore, acute carbon monoxide poisoning resulted in a delayed impairment of cognitive function. The hyperbaric oxygen treatment notably restored the cognitive impairment in a brain-derived neurotrophic factor-dependent manner.

CONCLUSIONS

The early hyperbaric oxygen treatment may alleviate delayed memory impairment after acute carbon monoxide poisoning by preserving adult neurogenesis via an increase in hippocampal brain-derived neurotrophic factor content.

Early Life in a Barren Environment Adversely Affects Spatial Cognition in Laying Hens (Gallus gallus domesticus)

Spatial cognition in vertebrates is adversely affected by a lack of environmental complexity during early life. However, to our knowledge, no previous studies have tested the effect of early exposure to varying degrees of environmental complexity on specific components of spatial cognition in chickens. There are two main rearing systems for laying hens in the EU: aviaries and cages. These two systems differ from one another in environmental complexity. The aim of the present study was to test the hypothesis that rearing in a barren cage environment relative to a complex aviary environment causes long-lasting deficits in the ability to perform spatial tasks. For this purpose, 24 white Dekalb laying hens, half of which had been reared in an aviary system and the other half in a conventional cage system, were tested in a holeboard task. Birds from both treatment groups learnt the task; however, the cage-reared hens required more time to locate rewards and had poorer levels of working memory. The latter finding supports the hypothesis that rearing in a barren environment causes long-term impairment of short-term memory in chickens.

Hippocampal Y2 receptor-mediated mossy fiber plasticity is implicated in nicotine abstinence-related social anxiety-like behavior in an outbred rat model of the novelty-seeking phenotype

Experimentally naïve outbred rats display varying rates of locomotor reactivity in response to the mild stress of a novel environment. Namely, some display high rates (HR) whereas some display low rates (LR) of locomotor reactivity. Previous reports from our laboratory show that HRs, but not LRs, develop locomotor sensitization to a low dose nicotine challenge and exhibit increased social anxiety-like behavior following chronic intermittent nicotine training. Moreover, the hippocampus, specifically hippocampal Y2 receptor (Y2R)-mediated neuropeptide Y signaling is implicated in these nicotine-induced behavioral effects observed in HRs. The present study examines the structural substrates of the expression of locomotor sensitization to a low dose nicotine challenge and associated social anxiety-like behavior following chronic intermittent nicotine exposure during adolescence in the LRHR hippocampi. Our data showed that the expression of locomotor sensitization to the low dose nicotine challenge and the increase in social anxiety-like behavior were accompanied by an increase in mossy fiber terminal field size, as well as an increase in spinophilin mRNA levels in the hippocampus in nicotine pre-trained HRs compared to saline pre-trained controls. Furthermore, a novel, selective Y2R antagonist administered systemically during 1 wk of abstinence reversed the behavioral, molecular and neuromorphological effects observed in nicotine-exposed HRs. These results suggest that nicotine-induced neuroplasticity within the hippocampus may regulate abstinence-related negative affect in HRs, and implicate hippocampal Y2R in vulnerability to the behavioral and neuroplastic effects of nicotine in the novelty-seeking phenotype.

The role of fear in one-trial passive avoidance learning in Japanese quail chicks genetically selected for long or short duration of the tonic immobility reaction

Emotional arousal has been shown to affect learning in mammals, but little is known about the relationship between fear and learning in birds. In order to investigate this relationship, the learning abilities of Japanese quail chicks from lines that have been divergently selected for high or low levels of underlying fearfulness, as measured by the duration of tonic immobility behaviour, were compared. Day-old chicks from both lines were trained in a one-trial passive avoidance task. In this task, young chicks spontaneously peck at a small, visually conspicuous bead. If the bead has been coated with a gustatory aversant, the chicks learn in a single trial not to peck a similar, uncoated bead upon subsequent presentation. Significantly more chicks of the low fear line pecked the training bead compared to those of the high fear line. However, 2 h later, chicks of both lines trained on a methyl anthranilate-coated bead showed similar avoidance of the test bead. Therefore, although fear affected performance during training, it did not appear to directly affect memory formation in this task.

Spatial memory impairments in amnestic mild cognitive impairment in a virtual radial arm maze

OBJECTIVE

This study aims to apply the virtual radial arm maze (VRAM) task to find spatial working memory and reference memory impairments in patients of amnestic mild cognitive impairment (aMCI) and Alzheimer's disease (AD). Spatial memory functions between aMCI converters and nonconverters are also compared using VRAM results.

METHODS

We assessed the spatial memory in 20 normal controls, 20 aMCI, and 20 mild AD subjects using VRAM. The Mini-Mental State Examination, Clinical Dementia Rating scale, and other neuropsychological tests were given to the subjects in conjunction with the VRAM test. Scores in working memory errors and reference memory errors were compared among the three groups using repeated measures analysis of variance. In addition, aMCI patients were followed-up after 5 years and surveyed for AD conversion rate.

RESULTS

In AD patients, both spatial working and reference memory were impaired. However, in aMCI subjects, only spatial reference memory was impaired. Significant spatial reference memory impairment was found in the aMCI converter group when compared to the nonconverter group.

CONCLUSION

Spatial working memory is less impaired in aMCI while reference memory is similarly damaged in AD. In aMCI patients, more severe spatial reference memory deficit is a neuropsychological marker for AD conversion. VRAM may be well utilized in humans to assess spatial memory in normal aging, in aMCI, and in AD.

Structural synaptic plasticity in the hippocampus induced by spatial experience and its implications in information processing

INTRODUCTION

Long-lasting memory formation requires that groups of neurons processing new information develop the ability to reproduce the patterns of neural activity acquired by experience.

DEVELOPMENT

Changes in synaptic efficiency let neurons organise to form ensembles that repeat certain activity patterns again and again. Among other changes in synaptic plasticity, structural modifications tend to be long-lasting which suggests that they underlie long-term memory. There is a large body of evidence supporting that experience promotes changes in the synaptic structure, particularly in the hippocampus.

CONCLUSION

Structural changes to the hippocampus may be functionally implicated in stabilising acquired memories and encoding new information.

Long-term cognitive impairments induced by chronic cannabinoid exposure during adolescence in rats: a strain comparison

RATIONALE

During cerebral development, adolescence is a critical phase in which the endocannabinoid system plays an important role in regulating various neurotransmitters. Moreover, evidence from both human and animal studies suggests that chronic cannabinoid exposure during this vulnerable period can induce persistent brain and behavioural alterations.

OBJECTIVES

The aim of this study was to compare the long-term cognitive consequences of chronic adolescence cannabinoid exposure between Lister Hooded rats and Wistar rats.

METHODS

Rats of both strains were injected daily throughout their adolescent or adult periods with vehicle or with incremental doses of the synthetic cannabinoid CB1 receptor agonist CP55,940 (CP). Short-term and spatial working memories were assessed using the object recognition and object location, tasks respectively. For both tasks, the effect of a 30- or 120-min delay between the learning and the testing phase was investigated.

RESULTS

In the object recognition task, adolescent CP exposure impaired short-term memory after both delays in both strains. In contrast, in the object location task, adolescent CP exposure impaired spatial working memory in the Wistar rats after a 30-min delay, whereas the Lister Hooded rats exhibited a similar effect only after a 120-min delay. In these tests, no long-term deleterious effects were found following adult CP exposure in either strain.

CONCLUSIONS

Our results confirm that adolescence is a critical period for the deleterious effects of cannabinoids on cognition and that these deleterious effects on spatial working memory are more strain-dependent than the effects observed on short-term memory.

Progesterone and low-dose vitamin D hormone treatment enhances sparing of memory following traumatic brain injury

Progesterone (PROG) has been shown to protect the brain from traumatic injury and is now in Phase III clinical trials. Our work shows that PROG's beneficial effects can be reduced in vitamin D hormone (VDH)-deficient subjects. VDH can modulate neuronal apoptosis, trophic factors, inflammation, oxidative stress, excitotoxicity, and myelin and axon repair. We investigated whether VDH combined with PROG could improve behavioral outcomes more than PROG alone in VDH-sufficient rats given bilateral contusions of the medial frontal cortex. PROG and different doses of VDH (1 μg/kg, VDH1; 2.5 μg/kg, VDH2; 5 μg/kg, VDH3) were injected intraperitoneally 1 h post-injury. Eight additional doses of PROG were given subcutaneously over 8 days with tapering over the last 2 days. Neurobehavioral tests, necrotic cavity, neuronal death and activation of astrocytes were evaluated 21 days post-injury. We found that PROG and PROG + VDH preserve spatial memory processing. VDH1 + PROG improved performance in acquisition more effectively than PROG alone, indicating that the low VDH dose is optimal for combination therapy. There were no significant differences in necrotic cavity size among the groups. The density of positive staining for reactive astrocytes (glial fibrillary acidic protein (GFAP)) increased and the cell bodies and processes of GFAP-positive cells were enlarged in the PROG + VDH1 group. Our data indicate that the combination of PROG and VDH is more effective than PROG alone in preserving spatial and reference memory, and that PROG plus low-dose VDH can activateGFAP reactions up to 21 days after injury. This effect may be one of the mechanisms underlying PROG's neuroprotective effects in combination with VDH.

Developmental underpinnings of differences in rodent novelty-seeking and emotional reactivity

Innate differences in human temperament strongly influence how individuals cope with stress and also predispose towards specific types of psychopathology. The present study examines the developing brain in an animal model of temperamental differences to examine how altered neurodevelopment may engender differences in emotional reactivity that are stable throughout the animal's life. We utilize selectively-bred High Responder (bHR) and Low Responder (bLR) rats that exhibit dramatic emotional behavior differences, with bHRs exhibiting exaggerated novelty-exploration, aggression, impulsivity and drug self-administration, and bLRs showing marked behavioral inhibition and exaggerated anxiety-like and depressive-like behavior. Using Affymetrix microarrays, we assessed bLR and bHR gene expression in the developing brain on postnatal days (P)7, 14 and 21, focusing on the hippocampus and nucleus accumbens, two regions related to emotionality and known to differ in adult bLR and bHR rats. We found dramatic gene expression differences between bLR and bHR in the P7 and P14 hippocampus, with minimal differences in the nucleus accumbens. Some of the most profound differences involved genes critical for neurodevelopment and synaptogenesis. Stereological studies evaluated hippocampal structure in developing bHR and bLR pups, revealing enhanced hippocampal volume and cell proliferation in bLR animals. Finally, behavioral studies showed that the characteristic bHR and bLR behavioral phenotypes emerge very early in life, with exploratory differences apparent at P16 and anxiety differences present by P25. Together these data point to specific brain regions and critical periods when the bHR and bLR phenotypes begin to diverge, which may eventually allow us to test possible therapeutic interventions to normalize extreme phenotypes (e.g. the anxiety-prone nature of bLRs or drug addiction proclivity of bHRs).

Strain-dependent variations in spatial learning and in hippocampal synaptic plasticity in the dentate gyrus of freely behaving rats

Hippocampal synaptic plasticity is believed to comprise the cellular basis for spatial learning. Strain-dependent differences in synaptic plasticity in the CA1 region have been reported. However, it is not known whether these differences extend to other synapses within the trisynaptic circuit, although there is evidence for morphological variations within that path. We investigated whether Wistar and Hooded Lister (HL) rat strains express differences in synaptic plasticity in the dentate gyrus in vivo. We also explored whether they exhibit differences in the ability to engage in spatial learning in an eight-arm radial maze. Basal synaptic transmission was stable over a 24-h period in both rat strains, and the input–output relationship of both strains was not significantly different. Paired-pulse analysis revealed significantly less paired-pulse facilitation in the HL strain when pulses were given 40–100 ms apart. Low frequency stimulation at 1 Hz evoked long-term depression (>24 h) in Wistar and short-term depression (<2 h) in HL rats; 200 Hz stimulation induced long-term potentiation (>24 h) in Wistar, and a transient, significantly smaller potentiation (<1 h) in HL rats, suggesting that HL rats have higher thresholds for expression of persistent synaptic plasticity. Training for 10 days in an eight-arm radial maze revealed that HL rats master the working memory task faster than Wistar rats, although both strains show an equivalent performance by the end of the trial period. HL rats also perform more efficiently in a double working and reference memory task. On the other hand, Wistar rats show better reference memory performance on the final (8–10) days of training. Wistar rats were less active and more anxious than HL rats. These data suggest that strain-dependent variations in hippocampal synaptic plasticity occur in different hippocampal synapses. A clear correlation with differences in spatial learning is not evident however.

Assessment of spatial memory in mice

Improvements in health care have greatly increased life span in the United States. The focus is now shifting from physical well-being to improvement in mental well-being or maintenance of cognitive function in old age. It is known that elderly people suffer from cognitive impairment, even without neurodegeneration, as a part of 'normal aging'. This 'age-associated memory impairment' (AAMI), can have a devastating impact on the social and economic life of an individual as well as the society. Scientists have been experimenting to find methods to prevent the memory loss associated with aging. The major factor involved in these experiments is the use of animal models to assess hippocampal-based spatial memory. This review describes the different types of memory including hippocampal-based memory that is vulnerable to aging. A detailed overview of various behavioral paradigms used to assess spatial memory including the T-maze, radial maze, Morris water maze, Barnes maze and others is presented. The review also describes the molecular basis of memory in hippocampus called as 'long-term potentiation'. The advantages and limitations of the behavioral models in assessing memory and the link to the long-term potentiation are discussed. This review should assist investigators in choosing suitable methods to assess spatial memory in mice.

Nicotinamide prevents the long-term effects of perinatal asphyxia on apoptosis, non-spatial working memory and anxiety in rats

There is no established treatment for the long-term effects produced by perinatal asphyxia. Thus, we investigated the neuroprotection provided by nicotinamide against the effects elicited by perinatal asphyxia on hippocampus and behaviour observed at 30-90 days of age. Asphyxia was induced by immersing foetuses-containing uterine horns, removed from ready-to-deliver rats into a water bath at 37 degrees C for 20 min. Caesarean-delivered siblings were used as controls. Saline or nicotinamide (0.8 mmol/kg, i.p.) was administered to control and asphyxia-exposed animals 24, 48, and 72 h after birth. The animals were examined for morphological changes in hippocampus, focusing on delayed cell death and mossy fibre sprouting, and behaviour, focusing on cognitive behaviour and anxiety. At the age of 30-45 days, asphyxia-exposed rats displayed (1) increased apoptosis, assessed in whole hippocampus by nuclear Hoechst staining, and (2) increased mossy fibre sprouting, restricted to the stratum oriens of dorsal hippocampus, assessed by Timm's staining. Rats from the same cohorts displayed (3) deficits in non-spatial working memory, assessed by a novel object recognition task, and (4) increased anxiety, assessed by an elevated plus-maze test when examined at the age of 90 days. Nicotinamide prevented the effects elicited by perinatal asphyxia on apoptosis, working memory, and anxiety.

Intrauterine environment-genome interaction and children's development (4): Brain-behavior phenotypying of genetically-engineered mice using a comprehensive behavioral test battery on research of neuropsychiatric disorders

Despite massive research efforts, the exact pathogenesis and pathophysiology of psychiatric disorders, such as schizophrenia and bipolar disorder, remain largely unknown. Animal models can serve as essential tools for investigating the etiology and treatment of such disorders. Some mutant mouse strains were found to exhibit behavioral abnormalities reminiscent of human psychiatric disorders. Here we outline our unique approach of extrapolating findings in mice to humans, and present studies on alpha-CaMKII heterozygous knockout (alpha-CaMKII+/-) mice as examples. Alpha-CaMKII+/- mice have profoundly dysregulated behavior and impaired neuronal development in the dentate gyrus (DG). The behavioral abnormalities include a severe working memory deficit and an exaggerated infradian rhythm, which are similar to symptoms seen in schizophrenia, bipolar mood disorder and other psychiatric disorders. By conducting a series of experiments, we discovered that almost all the neurons in the mutant DG were very similar to the immature DG neurons of normal rodents. In other words, alpha-CaMKII+/- mice have an "immature DG". We proposed that an "immature DG" in adulthood might induce alterations in behavior and serve as a promising candidate endophenotype of schizophrenia and other human psychiatric disorders. The impact of a large-scale mouse phenotyping on studies of psychiatric disorders and the potential utility of an "animal-model-array" of psychiatric disorders for the development of suitable therapeutic agents is also discussed.

Spatial memory in rats after 25 hours

We investigated the time course of spatial-memory decay in rats using an eight-arm radial maze. It is well established that performance remains high with retention intervals as long as 4 hr, but declines to chance with a 24-hr retention interval (e.g., Beatty & Shavalia, 1980b). It is possible that 24 hr reflects a genuine retention limitation of rat spatial memory. Alternatively, it may be possible to identify factors that might support memory performance even after very long delays. The current experiment was conducted to test the above two hypotheses. We evaluated performance using two intertrial intervals (24 and 48 hr) and two retention intervals (1 and 25 hr). Increasing the intertrial interval produced an approximately constant increase in performance for both retention intervals. This improvement is consistent with a trial-spacing effect (i.e., the superiority of spaced over massed trials). Rat spatial memory apparently lasts at least 25 hr.

Evaluation of spatial memory of C57BL/6J and CD1 mice in the Barnes maze, the Multiple T-maze and in the Morris water maze

Evaluation of spatial learning and memory is mainly carried out using the Morris water maze as a single paradigm. We intended to test whether mice in the Barnes maze and Multiple T-maze would lead to comparable results and to test two individual mouse strains with different anxiety levels. C57BL/6J and CD1 male mice were used in the experiments. During the acquisition phase, learning was measured using parameters latency, path length, errors in the BM and correct decisions in MTM. Mice were trained for 4 days and probe trials were performed on days 5 and 12. Latencies reduction over the training period indicated that both strains learned all tasks. During retention phase at days 5 and 12 C57BL/6J performed the Barnes maze and Multiple T-maze task better than CD1 mice while CD1 performed better than C57BL/6J in the Morris water maze. In the BM at day 12, C57BL/6J kept the level of visits to target observed at day 5 whereas CD1 performed worse. Strain- and task-dependent differences were observed using the three mazes. Therefore, fair evaluation of spatial memory demands application of (at least) two different test systems, a water- and a land maze. Different anxiety-related behaviour as well as stress-responses in the strains used may help to interpret the findings reported and again may propose the use of at least two mouse strains when robust evaluation of spatial memory is considered.

Impaired long-term memory retention and working memory in sdy mutant mice with a deletion in Dtnbp1, a susceptibility gene for schizophrenia

BACKGROUND

Schizophrenia is a complex genetic disorder caused by multiple genetic and environmental factors. The dystrobrevin-binding protein 1 (DTNBP1: dysbindin-1) gene is a major susceptibility gene for schizophrenia. Genetic variations in DTNBP1 are associated with cognitive functions, general cognitive ability and memory function, and clinical features of patients with schizophrenia including negative symptoms and cognitive decline. Since reduced expression of dysbindin-1 has been observed in postmortem brains of patients with schizophrenia, the sandy (sdy) mouse, which has a deletion in the Dtnbp1 gene and expresses no dysbindin-1 protein, could be an animal model of schizophrenia. To address this issue, we have carried out a comprehensive behavioral analysis of the sdy mouse in this study.

RESULTS

In a rotarod test, sdy mice did not exhibit motor learning whilst the wild type mice did. In a Barnes circular maze test both sdy mice and wild type mice learned to selectively locate the escape hole during the course of the training period and in the probe trial conducted 24 hours after last training. However, sdy mice did not locate the correct hole in the retention probe tests 7 days after the last training trial, whereas wild type mice did, indicating impaired long-term memory retention. A T-maze forced alternation task, a task of working memory, revealed no effect of training in sdy mice despite the obvious effect of training in wild type mice, suggesting a working memory deficit.

CONCLUSION

Sdy mouse showed impaired long-term memory retention and working memory. Since genetic variation in DTNBP1 is associated with both schizophrenia and memory function, and memory function is compromised in patients with schizophrenia, the sdy mouse may represent a useful animal model to investigate the mechanisms of memory dysfunction in the disorder.

Differential effects of stress on adult hippocampal cell proliferation in low and high aggressive mice

Male wild house mice selected for a long (LAL) or a short (SAL) latency to attack a male intruder generally show opposing behavioural coping responses to environmental challenges. LAL mice, unlike SAL mice, adapt to novel challenges with a highly reactive hypothalamic-pituitary-adrenal axis and show an enhanced expression of markers for hippocampal plasticity. The present study aimed to test the hypothesis that these features of the more reactive LAL mice are reflected in parameters of hippocampal cell proliferation. The data show that basal cell proliferation in the subgranular zone (SGZ) of the dentate gyrus, assessed by the endogenous proliferation marker Ki-67, is lower in LAL than in SAL mice. Furthermore, application of bromodeoxyuridine (BrdU) over 3 days showed an almost two-fold lower cell proliferation rate in the SGZ in LAL versus SAL mice. Exposure to forced swimming resulted, 24 h later, in a significant reduction in BrdU + cell numbers in LAL mice, whereas cell proliferation was unaffected by this stressor in SAL mice. Plasma corticosterone and dentate gyrus glucocorticoid receptor levels were higher in LAL than in SAL mice. However, no differences between the SAL and LAL lines were found for hippocampal NMDA receptor binding. In conclusion, the data suggest a relationship between coping responses and hippocampal cell proliferation, in which corticosterone may be one of the determinants of line differences in cell proliferation responses to environmental challenges.

Reduced reactivity to novelty, impaired social behavior, and enhanced basal synaptic excitatory activity in perforant path projections to the dentate gyrus in young adult mice deficient in the neural cell adhesion molecule CHL1

The neural cell adhesion molecule CHL1 is implicated in neural development in the mouse and has been related to psychiatric disorders in humans. Here we report that mice constitutively deficient for CHL1 display reduced reactivity to environmental stimuli and reduced expression of social behaviors, whereas cognitive, motor and olfactory functions are normal. Basal synaptic transmission and plasticity in seven major excitatory connections in the hippocampus were analyzed to test whether dysfunctions in this brain region, which controls complex behaviors, correlate with the behavioral alterations of CHL1 deficient mice. We found that basal synaptic transmission in lateral and medial perforant path projections to the dentate gyrus is elevated in CHL1-deficient mice. Taking in consideration the function of these synapses in processing information from cortical areas, we hypothesize that constitutive ablation of CHL1 leads to reduced capability to react to external stimuli due to dysfunctions in the dentate gyrus.

Learning-induced axonal remodeling: Evolutionary divergence and conservation of two components of the mossy fiber system within Rodentia

Damage to the hippocampal formation results in profound impairments in spatial navigation in rats and mice leading to the widely accepted assumption that the hippocampal cellular and molecular memory mechanisms of both genera are conserved. Recently our group has shown in two rat strains that hippocampal-dependent training in the water maze specifically induces robust 'sprouting' of granule cell suprapyramidal mossy fiber axon terminal fields. Here we sought to investigate whether the pronounced remodeling of adult hippocampal circuitry observed in the rat is also present in the mouse motivated by the thought that subsequent studies using genetically-engineered mice could then be implemented to explore the molecular mechanisms underlying training-dependent axonal growth in adult rodents. However, in contrast to Wistar rats, no changes in the Timm's-stained area of mossy fiber terminal fields (MFTFs) were observed in C57BL/6J or 129Sv/EmsJ inbred wild-type mice after water maze training. Neither extending the duration of training nor scaling down the size of the apparatus was able to induce sprouting in mouse mossy fiber pathways. Though there may be similarities in the ultimate output of the hippocampus of rats and mice as inferred from lesion studies, the current results, as well as differences in learning and memory characteristics between the two genera, suggest that the way in which the component circuitry functions is likely to be different; a not too surprising conclusion given the substantial evolutionary distance between them (>20 million years). The present findings afford an opportunity for uncovering linkages between evolutionarily significant alterations in hippocampal circuitry in relation to genera-specific information storage requirements.

Enhancing effects of chronic lithium on memory in the rat

BACKGROUND

In spite of recent enrichment of neurochemical and behavioural data establishing a neuroprotective role for lithium, its primary effects on cognitive functioning remain ambiguous. This study examines chronic lithium effects on spatial working memory and long-term retention.

METHODS

In three discrete experiments, rats subjected to 30 daily intraperitoneal injections (2mmol/kg) of lithium (lithium groups: serum lithium=0.5+/-0.4mEq/l, 12h post-injection) or saline (controls) were trained in 0-s delay T-maze alternation and then tested in 30-, 45- and 60-s delay alternation (Experiments 1, 2, 3, respectively). Animals from Experiment 1 were further tested in one-trial step-through passive avoidance under mild shock parameters (0.5mA, 1s). Retention was assessed 6h later. Daily lithium or saline injections continued throughout behavioural testing.

RESULTS

Lithium animals were indistinguishable from controls during 0-delay alternation baseline (Experiments 1-3, accuracy>88%) but showed significantly higher accuracy than controls at 30- and 45-s delays (93% versus 85% and 92% versus 82%, Experiments 1 and 2, respectively). At 60-s delay (Experiment 3) this beneficial effect of lithium was no longer apparent (lithium and control accuracy=78%). In Experiment 4, the shock used did not support 6-h passive avoidance retention in controls, whereas lithium animals showed significant step-through latency increases.

CONCLUSIONS

Chronic lithium enhanced spatial working memory and promoted long-term retention of a weak aversive contingency. The results suggest that lithium may have potential as a cognitive enhancer.

Learning spatial orientation tasks in the radial-maze and structural variation in the hippocampus in inbred mice

In the present paper we review a series of experiments showing that heritable variations in the size of the hippocampal intra- and infrapyramidal mossy fiber (IIPMF) terminal fields correlate with performance in spatial, but not non-spatial radial-maze tasks. Experimental manipulation of the size of this projection by means of early postnatal hyperthyroidism produces the effects predicted from the correlations obtained with inbred mouse strains. Although the physiological mechanisms behind these correlations are unknown as yet, several lines of evidence indicate that these correlations are causal.

The metabotropic glutamate receptor, mGluR5, is a key determinant of good and bad spatial learning performance and hippocampal synaptic plasticity

Hippocampal synaptic plasticity is expressed to very different extents in distinct rat strains in vivo. This may correlate with differences in learning ability. We investigated whether the metabotropic glutamate receptor mGluR5 contributes to differences in long-term potentiation (LTP) and learning in freely moving hooded Lister (HL) and Wistar rats. High-frequency tetanization (HFT) generated robust CA1 LTP in Wistar rats (> 24 h) and incremental potentiation in HL rats. The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP; 1.8 microg), applied intracerebrally, impaired LTP from approximately 60 min onwards in Wistar and from 24 h in HL rats. HFT generated LTP in the dentate gyrus (DG) of Wistar rats (> 24 h), which was blocked by MPEP, and MPEP-resistant short-term depression in HL rats. Training for 10 days in an eight-arm radial maze revealed no working memory differences, but better reference memory performance in Wistar compared with HL rats. Daily application of MPEP (1.8 microg) impaired working and reference memory in Wistar rats. In HL rats, working memory was impaired but reference memory was unaffected. Western blot analysis revealed lower expression of mGluR5 in HL compared with Wistar rats. MGluR1 expression was equivalent. These data reveal striking mGluR5-dependent differences in spatial learning in different rat strains, which correlate to synaptic plasticity and mGluR5 expression levels.

The relationship between dominance rank and spatial ability among male meadow voles (Microtus pennsylvanicus)

Males of many mammalian species exhibit contest competition and scramble competition for mates, but the relationship between these 2 forms of competition remains poorly understood. The authors measured dominance rank and spatial ability as traits likely to be selected by contest and scramble competition, respectively, among male meadow voles (Microtus pennsylvanicus). The spatial ability of males was assessed using water maze tests, and dominance rank was determined using paired trials in a neutral arena. Dominant males had better spatial-learning ability and tended to have quicker learning speed but did not have better spatial memory than less aggressive subordinates. Therefore, the authors found no evidence that contest and scramble competition have favored alternative reproductive phenotypes among male meadow voles.

Agonistic Onset Marks Emotional Changes and Dispersal Propensity in Wild House Mouse Males (Mus domesticus)

The authors investigated implications of agonistic onset for anxiety and dispersive motivation in maturing wild house mouse males (Mus domesticus). Laboratory-kept fraternal pairs either developed agonistic dominance or stayed amicable during their first 2 months of life, when the authors assessed open-field behavior and dispersal propensity. State anxiety was lower in amicable than agonistic males and higher in subordinate than dominant ones. During subsequent dispersal trials, 1 dominant and 1 amicable male from 2 fraternal pairs were concomitantly introduced into seminatural enclosures containing 3 females. One male invariably became territorial. The defeated males, if previously dominant, dispersed at significantly higher rates than if previously amicable. The authors conclude that agonistic onset during development represents an adaptive behavioral switch from a submissive-philopatric to agonistic-dispersive coping strategy.

Role of a neuronal small non-messenger RNA: behavioural alterations in BC1 RNA-deleted mice

BC1 RNA is a small non-messenger RNA common in dendritic microdomains of neurons in rodents. In order to investigate its possible role in learning and behaviour, we compared controls and knockout mice from three independent founder lines established from separate embryonic stem cells. Mutant mice were healthy with normal brain morphology and appeared to have no neurological deficits. A series of tests for exploration and spatial memory was carried out in three different laboratories. The tests were chosen as to ensure that different aspects of spatial memory and exploration could be separated and that possible effects of confounding variables could be minimised. Exploration was studied in a barrier test, in an open-field test, and in an elevated plus-maze test. Spatial memory was investigated in a Barnes maze and in a Morris water maze (memory for a single location), in a multiple T-maze and in a complex alley maze (route learning), and in a radial maze (working memory). In addition to these laboratory tasks, exploratory behaviour and spatial memory were assessed under semi-naturalistic conditions in a large outdoor pen. The combined results indicate that BC1 RNA-deficient animals show behavioural changes best interpreted in terms of reduced exploration and increased anxiety. In contrast, spatial memory was not affected. In the outdoor pen, the survival rates of BC1-depleted mice were lower than in controls. Thus, we conclude that the neuron-specific non-messenger BC1 RNA contributes to the aptive modulation of behaviour.

Tetrachlorodibenzo-p-dioxin exposure alters radial arm maze performance and hippocampal morphology in female AhR +/- mice

Perinatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been reported to alter spatial learning in rats tested on a radial arm maze (RAM). TCDD is believed to exert most of its effects through binding to the aryl hydrocarbon receptor (AhR). To determine whether the AhR mediates TCDD-induced alterations in spatial learning, we tested male and female AhR-knockout (AhR-/-), heterozygous (AhR+/-) and wild-type (AhR+/+) mice on the RAM. AhR+/- male and female mice were time mated, and treated dams were dosed with 5 microg TCDD/kg body weight on day 13 of gestation. When offspring reached adulthood, male and female AhR+/+, AhR+/- and AhR-/- mice from TCDD-exposed and unexposed litters were tested on the eight-arm RAM. After testing, we examined hippocampal morphology as visualized by the Timm's silver sulfide stain. TCDD-exposed female AhR+/- mice made more errors than their respective controls on the RAM and exhibited a decrease in the size of the intra- and infrapyramidal mossy fiber (IIP-MF) field of the hippocampus. None of the other TCDD-exposed groups differed from their respective control groups with regard to maze performance or hippocampal morphology. The reduction of IIP-MF field indicates a possible morphological basis for the learning deficit that was observed in the female AhR+/- mice. It is hypothesized that the effect of TCDD exposure is AhR dependent and that TCDD may alter GABAergic activity in the hippocampus of female mice during development.

Hippocampal mossy fibre terminal field size is differentially affected in a rat model of risk-taking behaviour

Individual differences in novelty-induced exploratory activity identify rats which can serve as a model of human sensation-seeking, risk-taking behaviour. Experimentally naïve rats, when exposed to mild stress of a novel environment, exhibit variability in their exploratory activity. Some rats display high rates of locomotor reactivity to novelty (high responders (HR)), and others display low rates (low responders (LR)). The LRHR phenotype is a reliable predictor of drug-taking behaviour and is linked to differences in hippocampal glucocorticoid receptor mRNA expression. In this study, we investigated whether the LRHR phenotype is associated with differences in the quantitative morphology of the hippocampal field CA3, dentate gyrus molecular layer, granule cell layer and mossy fibres. LRs and HRs showed no significant differences in the volumes of CA3 and dentate molecular layer volume or the number of dentate granule cells. However, LRs had a significantly larger suprapyramidal mossy fibre terminal field volume when compared to HRs. The infrapyramidal mossy fibres did not differ between phenotypes. Also, we found a LRHR phenotype-independent significant negative correlation between molecular layer volume per granule cell and the total number of granule cells. These findings implicate the SP-MF in vulnerability for risk-taking behaviour, and we propose that LR and HR hippocampi may differ in the way novelty information is processed.

Sialyltransferase ST8Sia-II Assembles a Subset of Polysialic Acid That Directs Hippocampal Axonal Targeting and Promotes Fear Behavior

Polysialic acid (PSA) is a post-translational protein modification that is widely expressed among neural cell types during development. Found predominantly on the neural cell adhesion molecule (NCAM), PSA becomes restricted to regions of neurogenesis and neuroplasticity in the adult. In the mammalian genome, two polysialyltransferases termed ST8Sia-II and ST8Sia-IV have been hypothesized to be responsible for the production of PSA in vivo. Approaches to discover PSA function have involved the application of endoneuraminidase-N to remove PSA and genetic manipulations in the mouse to deplete either NCAM or ST8Sia-IV. Here we report the production and characterization of mice deficient in the ST8Sia-II polysialyltransferase. We observed alterations in brain PSA expression unlike those observed in mice lacking ST8Sia-IV. This included a PSA deficit in regions of neurogenesis but without changes in the frequency of mitotic neural progenitor cells. In further contrast with ST8Sia-IV deficiency, loss of ST8Sia-II did not impair hippocampal synaptic plasticity but instead resulted in the misguidance of infrapyramidal mossy fibers and the formation of ectopic synapses in the hippocampus. Consistent with studies of animal models bearing these morphological changes, ST8Sia-II-deficient mice exhibited higher exploratory drive and reduced behavioral responses to Pavlovian fear conditioning. PSA produced by the ST8Sia-II polysialyltransferase modifies memory and behavior processes that are distinct from the neural roles reported for ST8Sia-IV. This genetic partitioning of PSA formation engenders discrete neurological processes and reveals that this post-translational modification forms the predominant basis for the multiple functions attributed to the NCAM glycoprotein.

Exploration, emotionality, and hippocampal mossy fibers in nonaggressive AB/Gat and congenic highly aggressive mice

AB/Gat mice and congenic mice bred for high aggressiveness (CS/ag) were tested for exploratory behavior in novel situations and anxiety-related behavior, using an open-field test and the elevated plus-maze test. Subsequently, the size of hippocampal mossy fiber terminal fields was evaluated. Considerably higher exploratory activity was found in nonaggressive mice, whereas aggressive mice exhibited more anxiety-related behavior. Larger intra- and infrapyramidal mossy fiber terminal fields (IIP-MF) and a larger hilus were found in the highly aggressive strain. Within the nonaggressive AB/Gat strain, larger IIP-MF were correlated with higher exploratory behavior and lower anxiety in the plus-maze test. Within the aggressive strain, no individual correlations between hippocampal morphometry and behavior were found. The results corroborate the "ecotype hypothesis," which suggests that mice of subpopulations with highly aggressive males tend to display reduced exploratory behavior. The findings support the view that genetic factors involved in aggressive behavior also affect hippocampal connectivity. However, our results do not support the hypothesis that a higher level of aggressiveness is necessarily related to smaller IIP-MF.

The effects of novelty and behavioral stereotype on the development of amnesia in mice

The aim of the present study was to analyze the significance of the interaction between the basic behavioral strategy, the extinction of the novelty of information, and the efficacy of amnesia-inducing influences. Using a combination of training to passive avoidance with holding the animal in the unsafe sector of the apparatus, comparative analysis was performed of the reproduction of a memory trace in aggressive and submissive mice of line C57BL/6J with and without six sessions of familiarization with the apparatus. These experiments showed that preliminary habituation prevented the development of amnesia in submissive but not aggressive individuals. The cause of these differences in the effects of preexposure on the development of amnesia involves the selectivity of the process of extinction of information novelty characteristic for the behavioral stereotype.

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.

Hippocampal mossy fibers and swimming navigation learning in two vole species occupying different habitats

We showed previously for mice that size differences of the infrapyramidal hippocampal mossy fiber projection (IIP-MF) correlate with spatial learning abilities. In order to clarify the role of the IIP-MF in a natural environment, we studied the bank vole (Clethrionomys glareolus), adapted to a wide range of different habitats, and the root vole (Microtus oeconomus), living in homogenous grassland habitats with small home ranges. Morphometry on Timm-stained horizontal brain sections of six C. glareolus and six M. oeconomus revealed that the size of the entire mossy fiber projection was 42% larger in C. glareolus than M. oeconomus. C. glareolus had also an IIP-MF projection about 230% larger than that of the root vole. A sample of captured animals was then transferred to the laboratory (C. glareolus, n = 23; M. oeconomus, n = 15) and underwent testing for swimming navigation according to a standardized protocol used to assess water maze learning in about 2,000 normal and transgenic mice. Both species learned faster than laboratory mice. Overall escape times showed no differences, but path length was significantly reduced in C. glareolus, which also showed superior performance in a variety of scores assessing spatial search patterns. On the other hand, M. oeconomus showed faster swimming speed, and strong thigmotaxis combined with circular swimming. M. oeconomus also scored at chance levels during the probe trial, about as poorly as mutant knockout mice considered to be deficient in spatial memory. These differences probably reflect differential styles of water maze learning rather than spatial memory deficits: C. glareolus appears to be superior in inhibiting behavior interfering with proper spatial search behavior, while M. oeconomus succeeds in escaping by using rapid circular swimming. We assume that size variations of the IIP-MF correspond to a mechanism stabilizing hippocampal processing during spatial learning or complex activities. This corresponds to the ecological lifestyle of the two species and is in line with previous observations on the role of the IIP-MF.

Previous training in the water maze: differential effects in NMRI and C57BL mice

It has been shown that acquisition rates in the water maze vary across strains of mice, although the differential effects of previous experience in this spatial task have been scarcely evaluated. The aim of the present study was to evaluate the effects of training in the water maze at an early age (2 months) in two strains of mice (NMRI and C57BL) using a longitudinal study. Mice with or without previous training were tested when they were 6 months, and retested when 10 months old. The results showed that trained NMRI mice performed better than all the other groups, both at test and retest, indicating that previous training had more beneficial effects in NMRI than in C57BL mice. These results demonstrate that the effects of an early training in the water maze may be influenced by the characteristics of the strain of mice. It could have implications in longitudinal studies evaluating effects of pharmacological or behavioral manipulations.