Fimbria-fornix lesions disrupt the dead reckoning (homing) component of exploratory behavior in mice

Exogenous monosodium glutamate exacerbates lipopolysaccharide-induced neurobehavioral deficits, oxidative damage, neuroinflammation, and cholinergic dysfunction in rat brain

Extensive experimental evidence points to neuroinflammation and oxidative stress as major pathogenic events that initiate and drive the neurodegenerative process. Monosodium glutamate (MSG) is a widely used food additive in processed foods known for its umami taste-enhancing properties. However, concerns about its potential adverse effects on the brain have been raised. Thus, the present study investigated the impact of MSG on lipopolysaccharide (LPS)-induced neurotoxicity in rat brains. Wistar rats weighing between 180 g and 200 g were randomly allocated into four groups: control (received distilled water), MSG (received 1.5 g/kg/day), LPS (received 250 µg/kg/day), and LPS + MSG (received LPS, 250 µg/kg, and MSG, 1.5 g/kg). LPS was administered intraperitoneally for 7 days while MSG was administered orally for 14 days. Our results showed that MSG exacerbated LPS-induced impairment in locomotor and exploratory activities in rats. Similarly, MSG exacerbated LPS-induced oxidative stress as evidenced by increased levels of malondialdehyde (MDA) with a concomitant decrease in levels of superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and glutathione-s-transferase (GST) in the brain tissue. In addition, MSG potentiated LPS-induced neuroinflammation, as indicated by increased levels of pro-inflammatory cytokines such as interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) as well as myeloperoxidase (MPO) and nitric oxide (NO) in the brain. Moreover, MSG aggravated LPS-induced cholinergic dysfunction, as demonstrated by increased activity of acetylcholinesterase (AChE) in the brain. Further, we found a large number of degenerative neurons widespread in hippocampal CA1, CA3 regions, cerebellum, and cortex according to H&E staining. Taken together, our findings suggest that MSG aggravates LPS-induced neurobehavioral deficits, oxidative stress, neuroinflammation, cholinergic dysfunction, and neurodegeneration in rat brains.

Grid cells: the missing link in understanding Parkinson's disease?

The mechanisms underlying Parkinson's disease (PD) are complex and not fully understood, and the box-and-arrow model among other current models present significant challenges. This paper explores the potential role of the allocentric brain and especially its grid cells in several PD motor symptoms, including bradykinesia, kinesia paradoxa, freezing of gait, the bottleneck phenomenon, and their dependency on cueing. It is argued that central hubs, like the locus coeruleus and the pedunculopontine nucleus, often narrowly interpreted in the context of PD, play an equally important role in governing the allocentric brain as the basal ganglia. Consequently, the motor and secondary motor (e.g., spatially related) symptoms of PD linked with dopamine depletion may be more closely tied to erroneous computation by grid cells than to the basal ganglia alone. Because grid cells and their associated central hubs introduce both spatial and temporal information to the brain influencing velocity perception they may cause bradykinesia or hyperkinesia as well. In summary, PD motor symptoms may primarily be an allocentric disturbance resulting from virtual faulty computation by grid cells revealed by dopamine depletion in PD.

Neurobehavioral and biochemical responses to artemisinin-based drug and aflatoxin B1 co-exposure in rats

Populations in malaria endemic areas are frequently exposed to mycotoxin-contaminated diets. The possible toxicological outcome of co-exposure to dietary aflatoxin B₁ (AFB₁) and artemisinin-based combination therapy warrants investigation to ascertain amplification or attenuation of cellular injury. Here, we investigated the neurobehavioral and biochemical responses associated with co-exposure to anti-malarial drug coartem, an artemether-lumefantrine combination (5 mg/kg body weight, twice a day and 3 days per week) and AFB₁ (35 and 70 µg/kg body weight) in rats. Motor deficits, locomotor incompetence, and anxiogenic-like behavior induced by low AFB₁ dose were significantly (p < 0.05) assuaged by coartem but failed to rescue these behavioral abnormalities in high AFB₁-dosed group. Coartem administration did not alter exploratory deficits typified by reduced track plot densities and greater heat map intensity in high AFB₁-dosed animals. Furthermore, the reduction in cerebral and cerebellar acetylcholinesterase activity, anti-oxidant enzyme activities, and glutathione and thiol levels were markedly assuaged by coartem administration in low AFB₁ group but not in high AFB₁-dosed animals. The significant attenuation of cerebral and cerebellar oxidative stress indices namely reactive oxygen and nitrogen species, xanthine oxidase activity, and lipid peroxidation by coartem administration was evident in low AFB₁ group but not high AFB₁ dose. Although coartem administration abated nitric oxide level, activities of myeloperoxidase, caspase-9, and caspase-3 in animals exposed to both doses of AFB₁, these indices were significantly higher than the control. Coartem administration ameliorated histopathological and mophometrical changes due to low AFB₁ exposure but not in high AFB₁ exposure. In conclusion, contrary to AFB₁ alone, behavioral and biochemical responses were not altered in animals singly exposed to coartem. Co-exposure to coartem and AFB₁ elicited no additional risk but partially lessened neurotoxicity associated with AFB₁ exposure.

Age-related changes in the organization of spontaneously occurring behaviors

Age-related changes in spatial and temporal processing have been documented across a range of species. Rodent studies typically investigate differences in performance between adult and senescent animals; however, progressive loss of neurons in the hippocampus and cortex has been observed to occur as early as after adolescence. Therefore, the current study evaluated the effects of age in three- and ten-month-old female rats on the organization of movement in open field and food protection behaviors, two tasks that have previously dissociated hippocampal and cortical pathology. Age-related differences were observed in general measures of locomotion, spatial orientation, and attentional processing. The results of the current study are consistent with age-related changes in the processing of spatial information and motivation that occur earlier in life than previously anticipated. These observations establish a foundation for future studies evaluating interventions that influence these age-related differences in performance.

Sexually dimorphic organization of open field behavior following moderate prenatal alcohol exposure

BACKGROUND

Prenatal alcohol exposure (PAE) can produce deficits in a wide range of cognitive functions but is especially detrimental to behaviors requiring accurate spatial information processing. In open field environments, spatial behavior is organized such that animals establish "home bases" marked by long stops focused around one location. Progressions away from the home base are circuitous and slow, while progressions directed toward the home base are non-circuitous and fast. The impact of PAE on the organization of open field behavior has not been experimentally investigated.

METHODS

In the present study, adult female and male rats with moderate PAE or saccharin exposure locomoted a circular high walled open field for 30 minutes under lighted conditions.

RESULTS

The findings indicate that PAE and sex influence the organization of open field behavior. Consistent with previous literature, PAE rats exhibited greater locomotion in the open field. Novel findings from the current study indicate that PAE and sex also impact open field measures specific to spatial orientation. While all rats established a home base on the periphery of the open field, PAE rats, particularly males, exhibited significantly less clustered home base stopping with smaller changes in heading between stops. PAE also impaired progression measures specific to distance estimation, while sex alone impacted progression measures specific to direction estimation.

CONCLUSIONS

These findings support the conclusion that adult male rats have an increased susceptibility to the effects of PAE on the organization of open field behavior.

Differential organization of open field behavior in mice following acute or chronic simulated GCR exposure

Astronauts undertaking deep space travel will receive chronic exposure to the mixed spectrum of particles that comprise Galactic Cosmic Radiation (GCR). Exposure to the different charged particles of varied fluence and energy that characterize GCR may impact neural systems that support performance on mission critical tasks. Indeed, growing evidence derived from years of terrestrial-based simulations of the space radiation environment using rodents has indicated that a variety of exposure scenarios can result in significant and long-lasting decrements to CNS functionality. Many of the behavioral tasks used to quantify radiation effects on the CNS depend on neural systems that support maintaining spatial orientation and organization of rodent open field behavior. The current study examined the effects of acute or chronic exposure to simulated GCR on the organization of open field behavior under conditions with varied access to environmental cues in male and female C57BL/6 J mice. In general, groups exhibited similar organization of open field behavior under dark and light conditions. Two exceptions were noted: the acute exposure group exhibited significantly slower and more circuitous homeward progressions relative to the chronic group under light conditions. These results demonstrate the potential of open field behavior organization to discriminate between the effects of select GCR exposure paradigms.

Neuroprotective role of gallic acid in aflatoxin B1 -induced behavioral abnormalities in rats

The neurotoxic impact of dietary exposure to aflatoxin B₁ (AFB₁ ) is documented in experimental and epidemiological studies. Gallic acid (GA) is a triphenolic phytochemical with potent anticancer, anti-inflammatory, and antioxidant activities. There is a knowledge gap on the influence of GA on AFB₁ -induced neurotoxicity. This study probed the influence of GA on neurobehavioral and biochemical abnormalities in rats orally treated with AFB₁ per se (75 µg/kg body weight) or administered together with GA (20 and 40 mg/kg) for 28 uninterrupted days. Behavioral endpoints obtained with video-tracking software demonstrated significant (p < .05) abatement of AFB₁ -induced anxiogenic-like behaviors (increased freezing, urination, and fecal bolus discharge), motor and locomotor inadequacies, namely increased negative geotaxis and diminished grip strength, absolute turn angle, total time mobile, body rotation, maximum speed, and total distance traveled by GA. The improvement of exploratory behavior in animals that received both AFB₁ and GA was confirmed by track plots and heat maps appraisal. Abatement of AFB₁ -induced decreases in acetylcholinesterase activity, antioxidant status and glutathione level by GA was accompanied by a marked reduction in oxidative stress markers in the cerebellum and cerebrum of rats. Additionally, GA treatment abrogated AFB₁ -mediated decrease in interleukin-10 and elevation of inflammatory indices, namely tumor necrosis factor-α, myeloperoxidase activity, interleukin-1β, and nitric oxide. Further, GA treatment curtailed caspase-3 activation and histological injuries in the cerebral and cerebellar tissues. In conclusion, abatement of AFB₁ -induced neurobehavioral abnormalities by GA involves anti-inflammatory, antioxidant, and antiapoptotic mechanisms in rats.

Complex Economic Behavior Patterns Are Constructed from Finite, Genetically Controlled Modules of Behavior

Complex ethological behaviors could be constructed from finite modules that are reproducible functional units of behavior. Here, we test this idea for foraging and develop methods to dissect rich behavior patterns in mice. We uncover discrete modules of foraging behavior reproducible across different strains and ages, as well as nonmodular behavioral sequences. Modules differ in terms of form, expression frequency, and expression timing and are expressed in a probabilistically determined order. Modules shape economic patterns of feeding, exposure, activity, and perseveration responses. The modular architecture of foraging changes developmentally, and different developmental, genetic, and parental effects are found to shape the expression of specific modules. Dissecting modules from complex patterns is powerful for phenotype analysis. We discover that both parental alleles of the imprinted Prader-Willi syndrome gene Magel2 are functional in mice but regulate different modules. Our study found that complex economic patterns are built from finite, genetically controlled modules.

The principles and mechanisms involved in constructing complex behavior patterns are not well defined. Stacher Hörndli et al. find that complex foraging patterns in mice are constructed from finite modules, defined as significantly reproducible behavioral sequences. Modules are expressed in a probabilistically defined order to construct complex patterns and controlled by genetic mechanisms.

Kolaviron via anti-inflammatory and redox regulatory mechanisms abates multi-walled carbon nanotubes-induced neurobehavioral deficits in rats

Exposure to multi-walled carbon nanotubes (MWCNTs) reportedly elicits neurotoxic effects. Kolaviron is a phytochemical with several pharmacological effects namely anti-oxidant, anti-inflammatory, and anti-genotoxic activities. The present study evaluated the neuroprotective mechanism of kolaviron in rats intraperitoneally injected with MWCNTs alone at 1 mg/kg body weight or orally co-administered with kolaviron at 50 and 100 mg/kg body weight for 15 consecutive days. Following exposure, neurobehavioral analysis using video-tracking software during trial in a novel environment indicated that co-administration of both doses of kolaviron significantly (p < 0.05) enhanced the locomotor, motor, and exploratory activities namely total distance traveled, maximum speed, total time mobile, mobile episode, path efficiency, body rotation, absolute turn angle, and negative geotaxis when compared with rats exposed to MWCNTs alone. Further, kolaviron markedly abated the decrease in the acetylcholinesterase activity and antioxidant defense system as well as the increase in oxidative stress and inflammatory biomarkers induced by MWCNT exposure in the cerebrum, cerebellum, and mid-brain of rats. The amelioration of MWCNT-induced neuronal degeneration in the brain structures by kolaviron was verified by histological and morphometrical analyses. Taken together, kolaviron abated MWCNT-induced neurotoxicity via anti-inflammatory and redox regulatory mechanisms.

Neurobehavioural and biochemical responses associated with exposure to binary waterborne mixtures of zinc and nickel in rats

Environmental and occupational exposure to metal mixtures due to various geogenic and anthropogenic activities poses a health threat to exposed organisms. The outcome of systemic interactions of metals is a topical area of research because it may cause either synergistic or antagonistic effect. The present study investigated the impact of co-exposure to environmentally relevant concentrations of waterborne nickel (75 and 150 μg NiCl ₂ L^-1) and zinc (100 and 200 μg ZnCl₂ L^-1) mixtures on neurobehavioural performance of rats. Locomotor, motor and exploratory activities were evaluated using video-tracking software during trial in a novel arena and thereafter, biochemical and histological analyses were performed using the cerebrum, cerebellum and liver. Results indicated that zinc significantly (p < 0.05) abated the nickel-induced locomotor and motor deficits as well as improved the exploratory activity of exposed rats as verified by track plots and heat map analyses. Moreover, zinc mitigated nickel-mediated decrease in acetylcholinesterase activity, elevation in biomarkers of liver damage, levels of reactive oxygen and nitrogen species as well as lipid peroxidation in the exposed rats when compared with control. Additionally, nickel mediated decrease in antioxidant enzyme activities as well as the increase in tumour necrosis factor alpha, interleukin-1 beta and caspase-3 activity were markedly abrogated in the cerebrum, cerebellum and liver of rats co-exposed to nickel and zinc. Histological and histomorphometrical analyses evinced that zinc abated nickel-mediated neurohepatic degeneration as well as quantitative reduction in the widest diameter of the Purkinje cells and the densities of viable granule cell layer of dentate gyrus, pyramidal neurones of cornu ammonis 3 and cortical neurons in the exposed rats. Taken together, zinc abrogated nickel-induced neurohepatic damage via suppression of oxido-inflammatory stress and caspase-3 activation in rats.

Behavioral and Neural Subsystems of Rodent Exploration

Animals occupy territories in which resources such as food and shelter are often distributed unevenly. While studies of exploratory behavior have typically involved the laboratory rodent as an experimental subject, questions regarding what constitutes exploration have dominated. A recent line of research has utilized a descriptive approach to the study of rodent exploration, which has revealed that this behavior is organized into movement subsystems that can be readily quantified. The movements include home base behavior, which serves as a central point of attraction from which rats and mice organize exploratory trips into the remaining environment. In this review, we describe some of the features of this organized behavior pattern as well as its modulation by sensory cues and previous experience. We conclude the review by summarizing research investigating the neurobiological bases of exploration, which we hope will stimulate renewed interest and research on the neural systems mediating rodent exploratory behavior.

Antisense oligonucleotide therapy rescues disruptions in organization of exploratory movements associated with Usher syndrome type 1C in mice

Usher syndrome, Type 1C (USH1C) is an autosomal recessive inherited disorder in which a mutation in the gene encoding harmonin is associated with multi-sensory deficits (i.e., auditory, vestibular, and visual). USH1C (Usher) mice, engineered with a human USH1C mutation, exhibit these multi-sensory deficits by circling behavior and lack of response to sound. Administration of an antisense oligonucleotide (ASO) therapeutic that corrects expression of the mutated USH1C gene, has been shown to increase harmonin levels, reduce circling behavior, and improve vestibular and auditory function. The current study evaluates the organization of exploratory movements to assess spatial organization in Usher mice and determine the efficacy of ASO therapy in attenuating any such deficits. Usher and heterozygous mice received the therapeutic ASO, ASO-29, or a control, non-specific ASO treatment at postnatal day five. Organization of exploratory movements was assessed under dark and light conditions at two and six-months of age. Disruptions in exploratory movement organization observed in control-treated Usher mice were consistent with impaired use of self-movement and environmental cues. In general, ASO-29 treatment rescued organization of exploratory movements at two and six-month testing points. These observations are consistent with ASO-29 rescuing processing of multiple sources of information and demonstrate the potential of ASO therapies to ameliorate topographical disorientation associated with other genetic disorders.

Otolith dysfunction alters exploratory movement in mice

The organization of rodent exploratory behavior appears to depend on self-movement cue processing. As of yet, however, no studies have directly examined the vestibular system's contribution to the organization of exploratory movement. The current study sequentially segmented open field behavior into progressions and stops in order to characterize differences in movement organization between control and otoconia-deficient tilted mice under conditions with and without access to visual cues. Under completely dark conditions, tilted mice exhibited similar distance traveled and stop times overall, but had significantly more circuitous progressions, larger changes in heading between progressions, and less stable clustering of home bases, relative to control mice. In light conditions, control and tilted mice were similar on all measures except for the change in heading between progressions. This pattern of results is consistent with otoconia-deficient tilted mice using visual cues to compensate for impaired self-movement cue processing. This work provides the first empirical evidence that signals from the otolithic organs mediate the organization of exploratory behavior, based on a novel assessment of spatial orientation.

Quercetin Improves Neurobehavioral Performance Through Restoration of Brain Antioxidant Status and Acetylcholinesterase Activity in Manganese-Treated Rats

The present study investigated the neuroprotective mechanism of quercetin by assessing the biochemical and behavioral characteristics in rats sub-chronically treated with manganese alone at 15 mg/kg body weight or orally co-treated with quercetin at 10 and 20 mg/kg body weight for 45 consecutive days. Locomotor behavior was monitored using video-tracking software during a 10-min trial in a novel environment whereas the brain regions namely the hypothalamus, cerebrum and cerebellum of the rats were processed for biochemical analyses. Results indicated that co-treatment with quercetin significantly (p < 0.05) prevented manganese-induced locomotor and motor deficits specifically the decrease in total distance travelled, total body rotation, maximum speed, absolute turn angle as well as the increase in time of immobility and grooming. The improvement in the neurobehavioral performance of manganese-treated rats following quercetin co-treatment was confirmed by track and occupancy plot analyses. Moreover, quercetin assuaged manganese-induced decrease in antioxidant enzymes activities and the increase in acetylcholinesterase activity, hydrogen peroxide generation and lipid peroxidation levels in the hypothalamus, cerebrum and cerebellum of the rats. Taken together, quercetin mechanisms of ameliorating manganese-induced neurotoxicity is associated with restoration of acetylcholinesterase activity, augmentation of redox status and inhibition of lipid peroxidation in brain of rats.

Place cells on a maze encode routes rather than destinations

Hippocampal place cells fire at different rates when a rodent runs through a given location on its way to different destinations. However, it is unclear whether such firing represents the animal’s intended destination or the execution of a specific trajectory. To distinguish between these possibilities, Lister Hooded rats (n = 8) were trained to navigate from a start box to three goal locations via four partially overlapping routes. Two of these led to the same goal location. Of the cells that fired on these two routes, 95.8% showed route-dependent firing (firing on only one route), whereas only two cells (4.2%) showed goal-dependent firing (firing similarly on both routes). In addition, route-dependent place cells over-represented the less discriminable routes, and place cells in general over-represented the start location. These results indicate that place cell firing on overlapping routes reflects the animal’s route, not its goals, and that this firing may aid spatial discrimination.

DOI:http://dx.doi.org/10.7554/eLife.15986.001

How does the brain represent the outside world? One way of answering this question is to study the brains of rats, because the basic plan of a rodent’s brain is similar to that of other mammals, such as humans. For example, the brains of rodents and humans both contain a structure called the hippocampus, which plays important roles in navigation and spatial memory. Cells within the hippocampus called place cells support these processes by firing electrical impulses whenever the animal occupies a specific location.

When a rat runs along a corridor in a maze, its place cells often fire as it approaches a choice point. A given place cell will typically fire before the rat chooses a path leading towards one particular location, but not before choices that lead to other locations. The firing that occurs prior to the choice point is termed “prospective firing”. However, it is not known whether the prospective firing of place cells represents the rat’s final destination, or the specific route the animal takes to get there.

To address this question, Grieves et al. designed a maze in which two different paths from a starting corridor led to the same goal location. If place cells represent the goal location, they should fire whichever route the rat chooses. However, if they represent the specific path the rat takes to the goal, they should fire on one or the other route, but not both.

Grieves et al. found that almost all place cells with prospective activity in the starting corridor fired on a single route, as opposed to firing on both routes to the common goal. This suggests that the prospective firing in the hippocampus reflects the route the animal will take, rather than its intended destination. A future challenge will be to understand how the way the hippocampus codes routes interacts with brain circuits that code for intended goals, and how the activity of these circuits influences the animal’s ability to navigate.

DOI:http://dx.doi.org/10.7554/eLife.15986.002

Physiological functions of the small GTPase Arf6 in the nervous system

The small GTPase ADP-ribosylation factor 6 (Arf6) plays important roles in membrane dynamics-based neuronal cell events such as neurite outgrowth and spine formation. However, physiological functions of Arf6 in the nervous system at whole animal level have not yet been explored. We have recently generated conditional knockout mice lacking Arf6 in neurons or oligodendrocytes of central nervous system (CNS) or both cell lineages, and analyzed them. We found that ablation of Arf6 gene from neurons, but not from oligodendrocytes, caused the defect in axon myelination at the fimbria of hippocampus (Fim) and corpus callosum (CC). We also found that migration of oligodendrocyte precursor cells (OPCs) from the subventricular zone to the Fim and CC in mice lacking Arf6 in neurons was impaired. Finally, it was found that secretion of fibroblast growth factor-2 (FGF-2), a guidance factor for OPC migration, from hippocampi lacking Arf6 was impaired. Collectively, these findings demonstrate that Arf6 in neurons of the CNS plays an important role in OPC migration by regulating secretion of FGF-2 from neurons, thereby contributing to the axon myelination. Here, we discuss our current understanding of physiological functions of Arf6 in the nervous system.

Effects of laboratory housing on exploratory behaviour, novelty discrimination and spatial reference memory in a subterranean, solitary rodent, the Cape mole-rat (Georychus capensis)

A large number of laboratory and field based studies are being carried out on mole-rats, both in our research group and others. Several studies have highlighted the development of adverse behaviours in laboratory animals and have emphasised the importance of enrichment for captive animals. Hence we were interested in evaluating how laboratory housing would affect behavioural performance in mole-rats. We investigated exploratory behaviour, the ability to discriminate between novel and familiar environments and reference memory in the solitary Cape mole-rat (Georychuscapensis). Our data showed that both wild and captive animals readily explore open spaces and tunnels. Wild animals were however more active than their captive counterparts. In the Y maze two trial discrimination task, wild animals failed to discriminate between novel and familiar environments, while laboratory housed mole-rats showed preferential spatial discrimination in terms of the length of time spent in the novel arm. The performance of the laboratory and wild animals were similar when tested for reference memory in the Y maze, both groups showed a significant improvement compared to the first day, from the 3rd day onwards. Wild animals made more mistakes whereas laboratory animals were slower in completing the task. The difference in performance between wild and laboratory animals in the Y-maze may be as a result of the lower activity of the laboratory animals. Laboratory maintained Cape mole-rats show classic behaviours resulting from a lack of stimulation such as reduced activity and increased aggression. However, they do display an improved novelty discrimination compared to the wild animals. Slower locomotion rate of the laboratory animals may increase the integration time of stimuli, hence result in a more thorough inspection of the surroundings. Unlike the captive animals, wild animals show flexibility in their responses to unpredictable events, which is an important requirement under natural living conditions.

Lesions of the basal forebrain cholinergic system in mice disrupt idiothetic navigation

Loss of integrity of the basal forebrain cholinergic neurons is a consistent feature of Alzheimer's disease, and measurement of basal forebrain degeneration by magnetic resonance imaging is emerging as a sensitive diagnostic marker for prodromal disease. It is also known that Alzheimer's disease patients perform poorly on both real space and computerized cued (allothetic) or uncued (idiothetic) recall navigation tasks. Although the hippocampus is required for allothetic navigation, lesions of this region only mildly affect idiothetic navigation. Here we tested the hypothesis that the cholinergic medial septo-hippocampal circuit is important for idiothetic navigation. Basal forebrain cholinergic neurons were selectively lesioned in mice using the toxin saporin conjugated to a basal forebrain cholinergic neuronal marker, the p75 neurotrophin receptor. Control animals were able to learn and remember spatial information when tested on a modified version of the passive place avoidance test where all extramaze cues were removed, and animals had to rely on idiothetic signals. However, the exploratory behaviour of mice with cholinergic basal forebrain lesions was highly disorganized during this test. By contrast, the lesioned animals performed no differently from controls in tasks involving contextual fear conditioning and spatial working memory (Y maze), and displayed no deficits in potentially confounding behaviours such as motor performance, anxiety, or disturbed sleep/wake cycles. These data suggest that the basal forebrain cholinergic system plays a specific role in idiothetic navigation, a modality that is impaired early in Alzheimer's disease.

Lesions of the basal forebrain cholinergic system in mice disrupt idiothetic navigation

Loss of integrity of the basal forebrain cholinergic neurons is a consistent feature of Alzheimer's disease, and measurement of basal forebrain degeneration by magnetic resonance imaging is emerging as a sensitive diagnostic marker for prodromal disease. It is also known that Alzheimer's disease patients perform poorly on both real space and computerized cued (allothetic) or uncued (idiothetic) recall navigation tasks. Although the hippocampus is required for allothetic navigation, lesions of this region only mildly affect idiothetic navigation. Here we tested the hypothesis that the cholinergic medial septo-hippocampal circuit is important for idiothetic navigation. Basal forebrain cholinergic neurons were selectively lesioned in mice using the toxin saporin conjugated to a basal forebrain cholinergic neuronal marker, the p75 neurotrophin receptor. Control animals were able to learn and remember spatial information when tested on a modified version of the passive place avoidance test where all extramaze cues were removed, and animals had to rely on idiothetic signals. However, the exploratory behaviour of mice with cholinergic basal forebrain lesions was highly disorganized during this test. By contrast, the lesioned animals performed no differently from controls in tasks involving contextual fear conditioning and spatial working memory (Y maze), and displayed no deficits in potentially confounding behaviours such as motor performance, anxiety, or disturbed sleep/wake cycles. These data suggest that the basal forebrain cholinergic system plays a specific role in idiothetic navigation, a modality that is impaired early in Alzheimer's disease.

Integrating cortico-limbic-basal ganglia architectures for learning model-based and model-free navigation strategies

Behavior in spatial navigation is often organized into map-based (place-driven) vs. map-free (cue-driven) strategies; behavior in operant conditioning research is often organized into goal-directed vs. habitual strategies. Here we attempt to unify the two. We review one powerful theory for distinct forms of learning during instrumental conditioning, namely model-based (maintaining a representation of the world) and model-free (reacting to immediate stimuli) learning algorithms. We extend these lines of argument to propose an alternative taxonomy for spatial navigation, showing how various previously identified strategies can be distinguished as “model-based” or “model-free” depending on the usage of information and not on the type of information (e.g., cue vs. place). We argue that identifying “model-free” learning with dorsolateral striatum and “model-based” learning with dorsomedial striatum could reconcile numerous conflicting results in the spatial navigation literature. From this perspective, we further propose that the ventral striatum plays key roles in the model-building process. We propose that the core of the ventral striatum is positioned to learn the probability of action selection for every transition between states of the world. We further review suggestions that the ventral striatal core and shell are positioned to act as “critics” contributing to the computation of a reward prediction error for model-free and model-based systems, respectively.

Knots: attractive places with high path tortuosity in mouse open field exploration

When introduced into a novel environment, mammals establish in it a preferred place marked by the highest number of visits and highest cumulative time spent in it. Examination of exploratory behavior in reference to this “home base” highlights important features of its organization. It might therefore be fruitful to search for other types of marked places in mouse exploratory behavior and examine their influence on overall behavior.

Examination of path curvatures of mice exploring a large empty arena revealed the presence of circumscribed locales marked by the performance of tortuous paths full of twists and turns. We term these places knots, and the behavior performed in them—knot-scribbling. There is typically no more than one knot per session; it has distinct boundaries and it is maintained both within and across sessions. Knots are mostly situated in the place of introduction into the arena, here away from walls. Knots are not characterized by the features of a home base, except for a high speed during inbound and a low speed during outbound paths. The establishment of knots is enhanced by injecting the mouse with saline and placing it in an exposed portion of the arena, suggesting that stress and the arousal associated with it consolidate a long-term contingency between a particular locale and knot-scribbling.

In an environment devoid of proximal cues mice mark a locale associated with arousal by twisting and turning in it. This creates a self-generated, often centrally located landmark. The tortuosity of the path traced during the behavior implies almost concurrent multiple views of the environment. Knot-scribbling could therefore function as a way to obtain an overview of the entire environment, allowing re-calibration of the mouse's locale map and compass directions. The rich vestibular input generated by scribbling could improve the interpretation of the visual scene.

Exploration is a central component of human and animal behavior that has been studied in rodents for almost a century. It is presently one of the main models for studying the interface between behavior, genetics, drugs, and the brain. Until recently the exploration of an open field by rodents has been considered to be largely stochastic. Lately, this behavior is being gradually deciphered, revealing reference places called home bases, from which the animals perform roundtrips into the environment, tracing well-trodden paths whose features contribute to our understanding of navigation, locational memory, cognition-, and emotion-related behavior. Using advanced computational tools we discover so-called knots, preferred places visited sporadically by mice. Mice perform in these places twists and turns. The measurement of speed on the way in and out of knots reveals that they are attractive for the mice. Knot formation is enhanced by stress, suggesting that stress-related arousal assigns these locales with a special significance that is reinstated by subsequent visits to them. The twists and turns could provide the mouse with multiple views that turn knots into navigational landmarks as well as with rich vestibular input that might improve the perception and subsequent interpretation of the visual input.

The ventral basal ganglia, a selection mechanism at the crossroads of space, strategy, and reward

The basal ganglia are often conceptualised as three parallel domains that include all the constituent nuclei. The 'ventral domain' appears to be critical for learning flexible behaviours for exploration and foraging, as it is the recipient of converging inputs from amygdala, hippocampal formation and prefrontal cortex, putatively centres for stimulus evaluation, spatial navigation, and planning/contingency, respectively. However, compared to work on the dorsal domains, the rich potential for quantitative theories and models of the ventral domain remains largely untapped, and the purpose of this review is to provide the stimulus for this work. We systematically review the ventral domain's structures and internal organisation, and propose a functional architecture as the basis for computational models. Using a full schematic of the structure of inputs to the ventral striatum (nucleus accumbens core and shell), we argue for the existence of many identifiable processing channels on the basis of unique combinations of afferent inputs. We then identify the potential information represented in these channels by reconciling a broad range of studies from the hippocampal, amygdala and prefrontal cortex literatures with known properties of the ventral striatum from lesion, pharmacological, and electrophysiological studies. Dopamine's key role in learning is reviewed within the three current major computational frameworks; we also show that the shell-based basal ganglia sub-circuits are well placed to generate the phasic burst and dip responses of dopaminergic neurons. We detail dopamine's modulation of ventral basal ganglia's inputs by its actions on pre-synaptic terminals and post-synaptic membranes in the striatum, arguing that the complexity of these effects hint at computational roles for dopamine beyond current ideas. The ventral basal ganglia are revealed as a constellation of multiple functional systems for the learning and selection of flexible behaviours and of behavioural strategies, sharing the common operations of selection-by-disinhibition and of dopaminergic modulation.

Head direction cell activity in mice: robust directional signal depends on intact otolith organs

The head direction (HD) cell signal is a representation of an animal's perceived directional heading with respect to its environment. This signal appears to originate in the vestibular system, which includes the semicircular canals and otolith organs. Preliminary studies indicate the semicircular canals provide a necessary component of the HD signal, but involvement of otolithic information in the HD signal has not been tested. The present study was designed to determine the otolithic contribution to the HD signal, as well as to compare HD cell activity of mice with that of rats. HD cell activity in the anterodorsal thalamus was assessed in wild-type C57BL/6J and otoconia-deficient tilted mice during locomotion within a cylinder containing a prominent visual landmark. HD cell firing properties in C57BL/6J mice were generally similar to those in rats. However, in C57BL/6J mice, landmark rotation failed to demonstrate dominant control of the HD signal in 36% of the sessions. In darkness, directional firing became unstable during 42% of the sessions, but landmark control was not associated with HD signal stability in darkness. HD cells were identified in tilted mice, but directional firing properties were not as robust as those of C57BL/6J mice. Most HD cells in tilted mice were controlled by landmark rotation but showed substantial signal degradation across trials. These results support current models that suggest otolithic information is involved in the perception of directional heading. Furthermore, compared with rats, the HD signal in mice appears to be less reliably anchored to prominent environmental cues.

Technical integration of hippocampus, Basal Ganglia and physical models for spatial navigation

Computational neuroscience is increasingly moving beyond modeling individual neurons or neural systems to consider the integration of multiple models, often constructed by different research groups. We report on our preliminary technical integration of recent hippocampal formation, basal ganglia and physical environment models, together with visualisation tools, as a case study in the use of Python across the modelling tool-chain. We do not present new modeling results here. The architecture incorporates leaky-integrator and rate-coded neurons, a 3D environment with collision detection and tactile sensors, 3D graphics and 2D plots. We found Python to be a flexible platform, offering a significant reduction in development time, without a corresponding significant increase in execution time. We illustrate this by implementing a part of the model in various alternative languages and coding styles, and comparing their execution times. For very large-scale system integration, communication with other languages and parallel execution may be required, which we demonstrate using the BRAHMS framework's Python bindings.

Exploration and navigation in the blind mole rat (Spalax ehrenbergi): global calibration as a primer of spatial representation

The aim of this study was to uncover the process of initial spatial mapping of the environment. For this, blind mole rats (Spalax ehrenbergi), were tested in an unfamiliar square arena, in order to reveal how they construct a spatial representation. The mole rats first displayed a build-up phase, in which they gradually formed a path along the perimeter while travelling slowly, frequently pausing and repeating previously travelled segments of the path. This behaviour was followed by a free-travel phase, in which the mole rats appeared to locomote smoothly along the perimeter and through the centre of the arena while travelling faster with fewer stops or repetitions of path segments. Familiarity with the environment was reflected in local shortcuts at the arena corners and global shortcuts (crosscuts) through the arena centre. We suggest that scanning the perimeter throughout the build-up phase constitute a process of calibration, i.e. forming an initial representation of the size and perhaps the shape of the environment--a sort of basic global map. We further suggest that this calibration is later used for navigation, as indicated by the emergence of global crosscuts in the subsequent phase. Further investigation of the build-up phase, e.g. by manipulating environment size, might provide additional insight into the course of establishment of global environment representation (mapping).

Motor activity (exploration) and formation of home bases in mice (C57BL/6) influenced by visual and tactile cues: Modification of movement distribution, distance, location, and speed

The motor activity of mice in tests of "exploration" is organized. Mice establish home bases, operationally defined as places where they spend long periods of time, near physical objects and nesting material from which they make excursions. This organization raises the question of the extent to which mouse motoric activity is modulated by innate predispositions versus environmental influences. Here the influence of contextual cues (visual and tactile) on the motor activity of C57BL/6 mice was examined: (1) on an open field that had no walls, a partial wall, or a complete wall, (2) in the presence of distinct visual cues, room cues, or in the absence of visual cues (infrared light), and (3) in the presence of configurations of visual and tactile cues. Mice were generally less active in the presence of salient cues and formed home bases near those cues. In addition, movement speed, path distribution, and the number and length of stops were modulated by contextual cues. With repeated tests, mice favored tactile cues over visual cues as their home base locations. Although responses to cues were robust over test days, conditioning to context was generally weak. That the exploratory behavior of mice is affected by experience and context provides insights into performance variability and may prove useful in investigating the genetic and neural influences on mouse behavior.

'Looping'-an exploration mechanism in a dark open field

The behavior of Tristram's jird (a species of gerbil) in an illuminated open field resembled that of other rodents, comprising round trips to a home base and alternating between periods of progression (locomoting) and of stopping. In this study, we compared the characteristics of exploration in a dark arena with exploration by the same individuals in a lit arena. In the dark arena, stopping episodes were brief and fewer, suggesting almost continuous locomotion by the rodents. The clear distinction between progression and stopping that had characterized locomotion in an illuminated arena, thus diminished in the dark. There was also no apparent home base in the dark and traveling consisted in moving in a circular path, closing a loop to a recently traveled place that varied from one loop to the next. Locomotion in the dark may thus be regarded as a set of loops (round trips) to a continuously shifting home base, whereas with lights on the round trips converge to a home base using visible environmental landmarks. We suggest that a similar looping mechanism may be applicable to the behavior of hippocampal rats displaying hyperactivity and diversified locomotion, reminiscent of that seen in jirds in a dark arena.

Exploratory behavior of F2 crosses of mouse lines selected for different brain weight: a multivariate analysis

Principal component analysis of behavioural measures together with body and brain weight of hybrid F2 mice crosses between two lines selected for large (LB) and small (SB) brain weight yielded eight-factor solution explaining 75.1% of total variance. Two of eight factors had sufficient loading on brain weight and several behavioural measures. The factor analysis showed that, among F2 hybrids, mice with larger brain weight were characterised, in open-field test, by higher scores of locomotion in the periphery of arena and of rearing, as well as less frequent grooming and freezing than mice with smaller brain weight. F2 hybrids with larger brain weight moved faster and displayed stereotyped behaviour in the cross-maze test more frequently. In general, this diversity is in accord with the behaviour differences between parent LB and SB lines. The results show that, in mice fear-anxiety and stereotypic behaviours, which are known to interfere with normal exploration and learning of the environment, are causally connected with brain weight.