Similarities and differences between the brain networks underlying allocentric and egocentric spatial learning in rat revealed by cytochrome oxidase histochemistry

Learning and metabolic brain differences between juvenile male and female rats in the execution of different training regimes of a spatial memory task

Spatial memory is responsible for encoding spatial information to form a path, storing this mental representation, and evaluating and recovering spatial configurations to find a target location in the environment. It is mainly supported by the hippocampus and its interaction with other structures, such as the prefrontal cortex, and emerges in rodents around postnatal day (PND) 20. Sex differences in spatial tasks have been found in adults, with a supposedly better performance in males. However, few studies have examined sex differences in orientation throughout postnatal development. This study aimed to analyse the performance of juvenile (PND 23) male (n=18) and female (n=21) Wistar rats in a spatial reference memory task in the Morris water maze (MWM) with two different training regimes in the acquisition phase, and their subjacent metabolic brain activity. Based on sex, subjects were assigned to two different groups: one that performed four learning trials per day (n=9 males and n=8 females) and the other that was submitted to two trials per day (n=9 males and n=13 females). After the behavioural protocols, metabolic activity was evaluated using cytochrome c oxidase histochemistry. Results showed no metabolic brain or behavioural differences in the four-trial protocol performance, in which both sexes reached the learning criterion on the fourth day. By contrast, the two-trial protocol revealed an advantage for females, who reached the learning criterion on day four, whereas males needed more training and succeeded on day six. The female group showed lower metabolic activity than the male group in the cingulate and prelimbic cortex. These results suggest a faster consolidation process in the female group than the male group. Further research is needed to understand sex differences in spatial memory at early stages.

Neurobehavioral basis of Maier 3-table and other matching-to-place tasks

The Maier 3-table task comprises three phases conducted each day. During the exploration phase, rats explore the entire apparatus. During the information phase, the rats are placed on one of the three tables where food is found. During the test phase, the animals are placed at the starting point on one of the two remaining tables and must enter the goal table where they previously ate. The acquisition of the Maier 3-table task was slowed down after lesions of the septum, fornix, hippocampus, medial prefrontal cortex, or posterior parietal cortex. Because of its time-consuming nature, the Maier 3-table task has more recently been superseded by appetitive matching-to-place in Y- or T-mazes or the circular water maze, because experimenters skip over the exploration phase. Nevertheless, like the Maier 3-table task, the acquisition of the Y- or T-maze matching-to-place task was retarded after lesions of the medial septum or medial prefrontal cortex, more particularly its prelimbic-infralimbic part. Like the previous task, the water-maze version is sensitive to lesions of the medial septum or retrosplenial cortex. Despite methodological differences between the three procedures, these results indicate common neurobiological bases of matching-to-place learning.

Transformer maze for the evaluation of the learning and memory in rodents

Background

Currently, different types of mazes are used to assess spatial learning and memory of rodents. The typical disadvantage is the inability to separate and exclude coincidences of the result of random choice with the correct one. The other problem is the impossibility of knowing whether the animal is guided by particular cues of the environment, or a map.

New method

Our novel transformer maze can be used to test learning and memory of rodents and their navigation. It is a multiple T-maze with passages in the interior walls. Its modular design allows to quickly change routes. The task can include external signals; for example, the colors of the interior walls, or it can be used without any cues.

Results

We compared Wistar and dopamine transporter heterozygous (DAT-HET) rats’ behavior in this novel paradigm using the black color of the wall as a cue. Entering a cul-de-sac compartment was considered an error. While Wistar rats learned the rule abruptly with the total number of errors rapidly decreasing, DAT-HET rats’ errors decreased gradually. We suppose that this reflects different strategies: insightful learning behavior is typical for Wistar rats, and trial-and-error learning is typical for DAT-HET rats.

Comparison with existing methods

The diversity of the chains of choices gives us confidence that trained animals do not make a choice randomly and are guided precisely by the cues. Moreover, we propose to use the same arena for a task with route-based navigation without any cues, and for a task with a visible and invisible feeder to study the path integration navigation within one box.

Conclusions

We suggest that the transformer maze could be a valuable tool for behavioral and pharmacological research to study learning, memory and navigation mechanisms.

Recovering Spatial Information through Reactivation: Brain Oxidative Metabolism Involvement in Males and Females

Memory involves a complex network system of interconnected brain areas in which labile trace memories are transformed into enduring ones and reorganized in a time-dependant manner. Although it has been observed that remote memories are less prone to destabilizing, they can become fragile and lead to behavioural decline. We explored the behavioural outcomes of male and female rats in response to the reactivation of a previously acquired allocentric spatial reference memory, under conditions in which animals have shown a retrieval decay. In addition, we assessed their brain metabolic activity through cytochrome c oxidase (CCO) histochemistry. Our results show that a spatial memory amnesia-like behaviour with a time interval of 45 days can be recovered after re-exposure to the environmental configuration with the reinforced contingencies. Moreover, we observed that, following reactivation, male rats reveal a decrease in metabolic activity in septal nuclei and thalamic structures, whereas female rats add a metabolic reduction in the hippocampus, amygdala, mPFC, and retrosplenial, parietal and rhinal cortices, suggesting that they efficiently employ these brain areas when reactivation a memory that has suffered a decay with time. Finally, although male and female rats perform the behavioural task equally, we found sex differences at the brain metabolism level, revealing the differential contribution of brain limbic system energy demands by sex, even when their performance is similar. In conclusion, our work provides behavioural and brain data about remote spatial retrieval and memory reactivation processes.

Environmental Enrichment Results in Both Brain Connectivity Efficiency and Selective Improvement in Different Behavioral Tasks

Exposure to environmental enrichment (EE) has been a useful model for studying the effects of experience on brain plasticity, but to date, few is known about the impact of this condition on the brain functional networks that probably underlies the multiple behavioral improvements. Hence, we assessed the effect of an EE protocol in adult Wistar rats on the performance in several behavioral tasks testing different domains (Open field (OP): locomotor activity; Elevated-zero maze (EZM): anxiety-related behaviors; 5-choice serial reaction time task (5-CSRTT): attentional processes; 4-arm radial water maze (4-RAWM): spatial memory) in order to check its effectiveness in a wide range of functions. After this, we analyzed the functional brain connectivity underlying each experimental condition through cytochrome C oxidase (COx) histochemistry. Our EE protocol reduced both locomotor activity in the OP and anxiety-related behaviors in the EZM. On the other hand, enriched rats showed more accuracy in the 4-RAWM, whereas 5-CSRTT performance was not significantly ameliorated by EE condition. In relation to COx functional connectivity, we found that EE reduced the number of strong positive correlations both in basal and training conditions, suggesting a modulating effect on specific brain connections. Our results suggest that EE seems to have a selective effect on specific brain regions, such as prefrontal cortex and hippocampus, leading to a more efficient brain connectivity.

Lack of anticipatory behavior in Gpr88 knockout mice showed by automatized home cage phenotyping

Mouse models are widely used to understand genetic bases of behavior. Traditional testing typically requires multiple experimental settings, captures only snapshots of behavior and involves human intervention. The recent development of automated home cage monitoring offers an alternative method to study mouse behavior in their familiar and social environment, and over weeks. Here, we used the IntelliCage system to test this approach for mouse phenotyping, and studied mice lacking Gpr88 that have been extensively studied using standard testing. We monitored mouse behavior over 22 days in 4 different phases. In the free adaptation phase, Gpr88 ^-/- mice showed delayed habituation to the home cage, and increased frequency of same corner returns behavior in their alternation pattern. In the following nose-poke adaptation phase, non-habituation continued, however, mutant mice acquired nose-poke conditioning similar to controls. In the place learning and reversal phase, Gpr88^-/- mice developed preference for the water/sucrose corner with some delay, but did not differ from controls for reversal. Finally, in a fixed schedule-drinking phase, control animals showed higher activity during the hour preceding water accessibility, and reduced activity after access to water was terminated. Mutant mice did not show this behavior, showing lack of anticipatory behavior. Our data therefore confirm hyperactivity, non-habituation and altered exploratory behaviors that were reported previously. Learning deficits described in other settings were barely detectable, and a novel phenotype was discovered. Home cage monitoring therefore extends previous findings and shows yet another facet of GPR88 function that deserves further investigation.

Training-induced elevations in extracellular lactate in hippocampus and striatum: Dissociations by cognitive strategy and type of reward

Recent evidence suggests that astrocytes convert glucose to lactate, which is released from the astrocytes and supports learning and memory. This report takes a multiple memory perspective to test the role of astrocytes in cognition using real-time lactate measurements during learning and memory. Extracellular lactate levels in the hippocampus or striatum were determined with lactate biosensors while rats were learning place (hippocampus-sensitive) or response (striatum-sensitive) versions of T-mazes. In the first experiment, rats were trained on the place and response tasks to locate a food reward. Extracellular lactate levels in the hippocampus increased beyond those of feeding controls during place training but not during response training. However, striatal lactate levels did not increase beyond those of controls when rats were trained on either the place or the response version of the maze. Because food ingestion itself increased blood glucose and brain lactate levels, the contribution of feeding may have confounded the brain lactate measures. Therefore, we conducted a second similar experiment using water as the reward. A very different pattern of lactate responses to training emerged when water was used as the task reward. First, provision of water itself did not result in large increases in either brain or blood lactate levels. Moreover, extracellular lactate levels increased in the striatum during response but not place learning, whereas extracellular lactate levels in the hippocampus did not differ across tasks. The findings from the two experiments suggest that the relative engagement of the hippocampus and striatum dissociates not only by task but also by reward type. The divergent lactate responses of the hippocampus and striatum in place and response tasks under different reward conditions may reflect ethological constraints tied to foraging for food and water.

Deletion of the serotonin receptor type 7 disrupts the acquisition of allocentric but not egocentric navigation strategies in mice

Spatial navigation is achieved through both egocentric (body-centered) and allocentric (externally-centered) strategies but decline with age, especially allocentric strategies. A better understanding of the neurobiological mechanisms underlying these strategies would allow the development of new treatments to mitigate this deterioration. Among them, the modulation of 5-HT₇ receptor (5-HT₇R) may constitute a potential strategy. Indeed, this receptor is known to play a role in spatial navigation, however its precise role in egocentric and allocentric strategies remains unclear. Here, we first examined the effect of 5-HT₇ genetic invalidation (knock-out (KO) mice) in two versions of a water cross-maze task in which only egocentric or allocentric strategies were efficient to solve the task. Our results demonstrated that KO mice are able to learn an allocentric strategy. However, contrary to wild-type mice (WT mice), the acquisition rate was slower compared to the task requiring the acquisition of an egocentric strategy. Mice were then trained in a third version of the water maze, allowing the use of both egocentric and allocentric strategies. When facing conflicting spatial information, both KO and WT mice preferentially used an egocentric strategy. However, only WT mice displayed a greater latency to achieve the task. This suggests that WT mice are able to learn both information in parallel, but not KO mice (i.e. only learning an egocentric strategy). Altogether, these results provide evidence for the essential role of the 5HT₇R in the acquisition of an allocentric strategy and in the ability to learn concomitantly both strategies.

Main target of minimal hepatic encephalopathy: Morphophysiological, inflammatory and metabolic view

Although often not considered clinically relevant and, therefore, not diagnosed or treated, minimal hepatic encephalopathy (MHE) has been shown to affect daily functioning, quality of life, driving and overall mortality. To discover early impairments involved in MHE, we studied one of its precipitating factors, portal hypertension. Rats were trained on a stimulus-response task using the Morris water maze. Two groups of animals were used: a SHAM (sham-operated) group (n= 13) and a portal hypertension (PH) group (n= 13). The triple portal vein ligation method was used to create an animal model of an early developmental phase of HE. Brain metabolic activity was studied with cytochrome c-oxidase histochemistry (C.O.). Neuronal nuclear volume was assessed by nucleator probe; the number of glial fibrillary acidic protein-immunoreactive astrocytes (GFAP-IR) and proinflammatory mediators was measured. The results revealed that the PH group was not able to reach the behavioural criterion, in contrast to the SHAM group. The metabolic brain consumption revealed decreased C.O. activity in the ventral striatum. The PH group showed lower density of GFAP-IR and an increase in the tumour necrotic factor-α (TNF-α). The PH group showed decreased neuronal nuclear volume in the dorsal striatum. On the contrary, increased neuronal nuclear volume was found in the ventral striatum. For the first time, a relationship has been established between inflammation, astrocytic and neural damage, and brain metabolic impairment in a model of MHE. Disruption of the striatum and related structures was highlighted as the main target in early stages of HE. Finally, a simple task was presented to assess the subtle impairments found in the clinic, which could provide fresh insights into the development of new tools for the assessment of MHE.

Spatial memory deficit across aging: current insights of the role of 5-HT7 receptors

Elderly persons often face biological, psychological or social changes over time that may cause discomfort or morbidity. While some cognitive domains remain stable over time, others undergo a decline. Spatial navigation is a complex cognitive function essential for independence, safety and quality of life. While egocentric (body-centered) navigation is quite preserved during aging, allocentric (externally-centered) navigation-based on a cognitive map using distant landmarks-declines with age. Recent preclinical studies showed that serotonergic 5-HT7 receptors are localized in brain regions associated with allocentric spatial navigation processing. Behavioral assessments with pharmacological or genetic tools have confirmed the role of 5-HT7 receptors in allocentric navigation. Moreover, few data suggested a selective age-related decrease in the expression of 5-HT7 receptors in pivotal brain structures implicated in allocentric navigation such as the hippocampal CA3 region. We aim to provide a short overview of the potential role of 5-HT7 receptors in spatial navigation, and to argue for their interests as therapeutic targets against age-related cognitive decline.

Brain network function during shifts in learning strategies in portal hypertension animals

Patients with minimal hepatic encephalopathy exhibit early impairments in their ability to shift attentional set. We employed a task-switching protocol to evaluate brain network changes. Strategy switching requires the modification of both the relevant stimulus dimension and the required memory system. Rats were trained in an allocentric (A) and a cue-guided (C) task using a four-arm maze. To examine priming, we changed the order in which the tasks were presented. Five groups of animals were used: a SHAM (sham-operated) A-C group (n=10), a SHAM C-A group (n=8), a PH (portal hypertension) A-C group (n=8), PH C-A group (n=8), and a naïve group (n=10). The triple portal vein ligation method was used to create an animal model of the early evolutive phase of PH. The animals were tested in the four-arm radial water maze in a single 10-trial session each day for six days (three days for the allocentric task and three days for the cue-guided task). The metabolic activities of the brains were studied with cytochrome oxidase histochemistry, and brain network changes were assessed with principal component analysis. The behavioural results revealed significant increases in the numbers of correct choices across training days in all groups studied, and facilitation of the acquisition of the second task was present in the C-A groups. Moreover, different brain network activities were found; in the experimental groups, the performance of A-C switch involved the prefrontal cortex, and the key structures involved in the C-A switch in the other groups were the dentate gyrus of the dorsal hippocampus and the basolateral and central amygdala. These networks have a common nucleus of structures (i.e., the parietal cortex and the dorsal and ventral striatum), whereas other structures were specifically involved in each type of strategy, suggesting that these regions are part of both circuits and may interact with one another during learning.

Age-dependent effects of environmental enrichment on brain networks and spatial memory in Wistar rats

We assessed the effect of 3h of environmental enrichment (EE) exposure per day started at different ages (3 and 18months old) on the performance in a spatial memory task and on brain regions involved in the spatial learning (SPL) process using the principal component analysis (PCA). The animals were tested in the four-arm radial water maze (4-RAWM) for 4days, with six daily trials. We used cytochrome c oxidase (COx) histochemistry to determine the brain oxidative metabolic changes related to age, SPL and EE. Behavioural results showed that the enriched groups, regardless of their age, achieved better performance in the spatial task. Interestingly, in the case of the distance travelled in the 4-RAWM, the effect of the EE was dependent on the age, so the young enriched group travelled a shorter distance compared to the aged enriched group. Respect to COx histochemistry results, we found that different brain mechanisms are triggered in aged rats to solve the spatial task, compared to young rats. PCA revealed the same brain functional network in both age groups, but the contribution of the brain regions involved in this network was slightly different depending on the age of the rats. Thus, in the aged group, brain regions involved in anxiety-like behaviour, such as the amygdala or the bed nucleus of the stria terminalis had more relevance; whereas in the young enriched group the frontal and the hippocampal subregions had more contribution.