Strategies discovery in the active allothetic place avoidance task.

The Active Allothetic Place Avoidance task is an alternative setup to Morris Water Maze that allows studying spatial memory in a dynamic world in the presence of conflicting information. In this task, a rat, freely moving on a rotating circular arena, has to avoid a sector defined within the room frame where shocks are presented. While for Morris Water Maze several studies have identified animal strategies which specifically affect performance, there were no such studies for the Active Allothetic Place Avoidance task. Using standard machine learning methods, we were able to reveal for the first time, to the best of our knowledge, explainable strategies that the animals employ in this task and demonstrate that they can provide a high-level interpretation for performance differences between an animal group treated with silver nanoparticles (AgNPs) and the control group.

Coating-Dependent Neurotoxicity of Silver Nanoparticles-An In Vivo Study on Hippocampal Oxidative Stress and Neurosteroids.

Silver nanoparticles (AgNPs) are one of the most widely used nanomaterials. The level of exposure to nanosilver is constantly raising, and a growing body of research highlights that it is harmful to the health, especially the nervous system, of humans. The potential pathways through which nanosilver affects neurons include the release of silver ions and the associated induction of oxidative stress. To better understand the mechanisms underlying the neurotoxicity of nanosilver, in this study we exposed male Wistar rats to 0.5 mg/kg body weight of AgNPs coated with bovine serum albumin (BSA), polyethylene glycol (PEG), or citrate, or to AgNO3 as a source of silver ions for 28 days and assessed the expression of antioxidant defense markers in the hippocampus of the exposed animals after 1 week of spatial memory training. We also evaluated the influence of AgNPs coating on neurosteroidogenesis in the rat hippocampus. The results showed that AgNPs disrupted the antioxidant system in the hippocampus and induced oxidative stress in a coating-dependent manner, which could potentially be responsible for neurodegeneration and cognitive disorders. The analysis of the influence of AgNPs on neurosteroids also indicated coating-dependent modulation of steroid levels with a significant decrease in the concentrations of progesterone and 17α-progesterone in AgNPs(BSA), AgNPs(PEG), and Ag+ groups. Furthermore, exposure to AgNPs or Ag+ resulted in the downregulation of selected genes involved in antioxidant defense (Cat), neurosteroid synthesis (Star, Hsd3b3, Hsd17b1, and Hsd17b10), and steroid metabolism (Ar, Er1, and Er2). In conclusion, depending on the coating material used for their stabilization, AgNPs induced oxidative stress and modulated the concentrations of steroids as well as the expression of genes involved in steroid synthesis and metabolism.

Combination of dasatinib and quercetin improves cognitive abilities in aged male Wistar rats, alleviates inflammation and changes hippocampal synaptic plasticity and histone H3 methylation profile.

Aging is associated with cognitive decline and accumulation of senescent cells in various tissues and organs. Senolytic agents such as dasatinib and quercetin (D+Q) in combination have been shown to target senescent cells and ameliorate symptoms of aging-related disorders in mouse models. However, the mechanisms by which senolytics improve cognitive impairments have not been fully elucidated particularly in species other than mice. To study the effect of senolytics on aging-related multifactorial cognitive dysfunctions we tested the spatial memory of male Wistar rats in an active allothetic place avoidance task. Here we report that 8 weeks treatment with D+Q alleviated learning deficits and memory impairment observed in aged animals. Furthermore, treatment with D+Q resulted in a reduction of the peripheral inflammation measured by the levels of serum inflammatory mediators (including members of senescent cell secretome) in aged rats. Significant improvements in cognitive abilities observed in aged rats upon treatment with D+Q were associated with changes in the dendritic spine morphology of the apical dendritic tree from the hippocampal CA1 neurons and changes in the level of histone H3 trimethylation at lysine 9 and 27 in the hippocampus. The beneficial effects of D+Q on learning and memory in aged rats were long-lasting and persisted at least 5 weeks after the cessation of the drugs administration. Our results expand and provide new insights to the existing knowledge associated with effects of senolytics on alleviating age-related associated cognitive dysfunctions.

Silver Nanoparticles Impair Cognitive Functions and Modify the Hippocampal Level of Neurotransmitters in a Coating-Dependent Manner.

Due to their potent antibacterial properties, silver nanoparticles (AgNPs) are widely used in industry and medicine. However, they can cross the brain-blood barrier, posing a risk to the brain and its functions. In our previous study, we demonstrated that oral administration of bovine serum albumin (BSA)-coated AgNPs caused an impairment in spatial memory in a dose-independent manner. In this study, we evaluated the effects of AgNPs coating material on cognition, spatial memory functioning, and neurotransmitter levels in rat hippocampus. AgNPs coated with BSA (AgNPs(BSA)), polyethylene glycol (AgNPs(PEG)), or citrate (AgNPs(Cit)) or silver ions (Ag+) were orally administered at a dose of 0.5 mg/kg b.w. to male Wistar rats for a period of 28 days, while the control (Ctrl) rats received 0.2 mL of water. The acquisition and maintenance of spatial memory related to place avoidance were assessed using the active allothetic place avoidance task, in which rats from AgNPs(BSA), AgNPs(PEG), and Ag+ groups performed worse than the Ctrl rats. In the retrieval test assessing long-term memory, only rats from AgNPs(Cit) and Ctrl groups showed memory maintenance. The analysis of neurotransmitter levels indicated that the ratio between serotonin and dopamine concentration was disturbed in the AgNPs(BSA) rats. Furthermore, treatment with AgNPs or Ag+ resulted in the induction of peripheral inflammation, which was reflected by the alterations in the levels of serum inflammatory mediators. In conclusion, depending on the coating material used for their stabilization, AgNPs induced changes in memory functioning and concentration of neurotransmitters.

Spatial working memory in rats: Crucial role of the hippocampus in the allothetic place avoidance alternation task demanding stimuli segregation.

Working memory is a construct that contains goal maintenance, interference control and memory capacity domains. Spatial working memory in presence of conflicting stimuli requires segregation and maintenance of the relevant information about a goal over a short period of time. Besides the prefrontal cortex, the hippocampus is an anatomical substrate for the working memory. We hypothesized that in a highly challenging task, where spatial stimuli are in a conflict and only some of them describe the goal location, the spatial working memory will be strongly dependant on the hippocampus. To verify this, we used an allothetic place avoidance alternation task (APAAT). Performance of this task demands a small number of entries and a long maximum time avoided between consecutive entries to the shock sector. These parameters reflected both domains of working memory. The experiment was conducted on hippocampal lesioned (HIPP n = 12) and sham-operated (CTRL n = 8) rats trained in four APAAT days, each consisting of four 5-minute stages: habituation, stage1 (st1) and stage2 (st2) of memory training, a 5-minute break followed by a retrieval test. The position of the shock sector was changed each day. The HIPP rats were impaired on both stages of memory training, whereas CTRL rats presented significant memory improvement on stage2. In HIPP rats the cognitive skill learning measured as shock per entrance ratio was compromised. Hippocampal lesions did not impair locomotor activity. In summary, even slight bilateral damage to the hippocampus is blocking working memory formation in a difficult task.

Cellular Senescence in Brain Aging.

Aging of the brain can manifest itself as a memory and cognitive decline, which has been shown to frequently coincide with changes in the structural plasticity of dendritic spines. Decreased number and maturity of spines in aged animals and humans, together with changes in synaptic transmission, may reflect aberrant neuronal plasticity directly associated with impaired brain functions. In extreme, a neurodegenerative disease, which completely devastates the basic functions of the brain, may develop. While cellular senescence in peripheral tissues has recently been linked to aging and a number of aging-related disorders, its involvement in brain aging is just beginning to be explored. However, accumulated evidence suggests that cell senescence may play a role in the aging of the brain, as it has been documented in other organs. Senescent cells stop dividing and shift their activity to strengthen the secretory function, which leads to the acquisition of the so called senescence-associated secretory phenotype (SASP). Senescent cells have also other characteristics, such as altered morphology and proteostasis, decreased propensity to undergo apoptosis, autophagy impairment, accumulation of lipid droplets, increased activity of senescence-associated-ß-galactosidase (SA-ß-gal), and epigenetic alterations, including DNA methylation, chromatin remodeling, and histone post-translational modifications that, in consequence, result in altered gene expression. Proliferation-competent glial cells can undergo senescence both in vitro and in vivo, and they likely participate in neuroinflammation, which is characteristic for the aging brain. However, apart from proliferation-competent glial cells, the brain consists of post-mitotic neurons. Interestingly, it has emerged recently, that non-proliferating neuronal cells present in the brain or cultivated in vitro can also have some hallmarks, including SASP, typical for senescent cells that ceased to divide. It has been documented that so called senolytics, which by definition, eliminate senescent cells, can improve cognitive ability in mice models. In this review, we ask questions about the role of senescent brain cells in brain plasticity and cognitive functions impairments and how senolytics can improve them. We will discuss whether neuronal plasticity, defined as morphological and functional changes at the level of neurons and dendritic spines, can be the hallmark of neuronal senescence susceptible to the effects of senolytics.

A therapeutic dose of memantine improves the performance of rats in an active place avoidance task under the continuous dissociation of distal room and proximal arena cues.

Memory is related to the function of N-methyl-D-aspartate (NMDA) receptors. Depending on the dose, NMDA receptor antagonists (such as memantine or MK-801) can impair memory and/or cognitive as well as procedural functions, while they also can prevent the long-term toxic effects of over-excitation of these receptors in pathophysiological processes. There is an unresolved question of whether memantine at low doses could exert an acute pro-cognitive activity. A therapeutic dose of memantine was found to improve short-term spatial memory tested in the alternation version of active place avoidance in a Carousel Maze, whereas no data are available on long-term memory in various versions of place avoidance. In an effort to reconcile this issue, rats were administered memantine (5 mg/kg) 30 min before a training session and trained in two different versions of place avoidance. A control group received saline injections. In an active place avoidance task (hereby referred to as Room+Arena-), this place was fixed to distal room cues, whereas cues from the arena were misleading. Performance thus demanded the on-going segregation of information that engages cognitive coordination. Following the Room+Arena- training, rats were trained in another place avoidance task (hereby referred to as Arena+), which requires focusing on substratal and idiothetic cues from the arena. In this version, a to-be-avoided sector rotated along with the arena in darkness that hid the extramaze cues. The rats given memantine avoided better than the control rats in the Room+Arena- task. In the Arena+ task, both groups had problems with acquiring the task. Subsequently, memantine was withdrawn and both groups relearned Room+Arena- avoidance with a new sector position. In this task, no effect of groups was seen. In conclusion, memantine at a therapeutic dose improved performance in a task that required the segregation of spatial stimuli into coherent subsets.

Spatial memory formation differentially affects c-Fos expression in retrosplenial areas during place avoidance training in rats.

The retrosplenial cortex is involved in spatial memory function, but the contribution of its individual areas is not well known. To elucidate the involvement of retrosplenial cortical areas 29c and 30 in spatial memory, we analyzed the expression of c-Fos in these areas in the experimental group of rats that were trained in a spatial place avoidance task, i.e. to avoid shocks presented in an unmarked sector of a stable arena under light conditions. Control rats were trained in the same context as the experimental rats either without (Control-noUS) or with shocks (Control-US) that were delivered in a random, noncontingent manner for three days. On the first day of place avoidance learning, the experimental group exhibited c-Fos induction in area 29c, similar to both control groups. In area 30, similarly high levels of c-Fos expression were observed in the experimental and Control-US groups. On the third day of training, when the experimental group efficiently avoided c-Fos expression in areas 29c and 30 was lower compared with the first day of training. In area 29c c-Fos level was also lower in the experimental than in comparison to the Control-US group. In area 30, c-Fos expression in the experimental group was lower than in both control groups. In conclusion, areas 29c and 30 appear to be activated during spatial memory acquisition on the first day of training, whereas area 30 seems suppressed during long-term memory functioning on the third day of training when rats effectively avoid.

MK-801 and memantine act differently on short-term memory tested with different time-intervals in the Morris water maze test.

N-methyl-d-aspartate receptors (NMDARs) play a crucial role in spatial memory formation. In neuropharmacological studies their functioning strongly depends on testing conditions and the dosage of NMDAR antagonists. The aim of this study was to assess the immediate effects of NMDAR block by (+)MK-801 or memantine on short-term allothetic memory. Memory was tested in a working memory version of the Morris water maze test. In our version of the test, rats underwent one day of training with 8 trials, and then three experimental days when rats were injected intraperitoneally with low- 5 (MeL), high - 20 (MeH) mg/kg memantine, 0.1mg/kg MK-801 or 1ml/kg saline (SAL) 30min before testing, for three consecutive days. On each experimental day there was just one acquisition and one test trial, with an inter-trial interval of 5 or 15min. During training the hidden platform was relocated after each trial and during the experiment after each day. The follow-up effect was assessed on day 9. Intact rats improved their spatial memory across the one training day. With a 5min interval MeH rats had longer latency then all rats during retrieval. With a 15min interval the MeH rats presented worse working memory measured as retrieval minus acquisition trial for path than SAL and MeL and for latency than MeL rats. MK-801 rats had longer latency than SAL during retrieval. Thus, the high dose of memantine, contrary to low dose of MK-801 disrupts short-term memory independent on the time interval between acquisition and retrieval. This shows that short-term memory tested in a working memory version of water maze is sensitive to several parameters: i.e., NMDA receptor antagonist type, dosage and the time interval between learning and testing.

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.

Low-dose memantine-induced working memory improvement in the allothetic place avoidance alternation task (APAAT) in young adult male rats.

N-methyl-D-aspartate receptors (NMDAR) are involved in neuronal plasticity. To assess their role simultaneously in spatial working memory and non-cognitive learning, we used NMDAR antagonists and the Allothetic Place Avoidance Alternation Task (APAAT). In this test rats should avoid entering a place where shocks were presented on a rotating arena which requires cognitive coordination for the segregation of stimuli. The experiment took place 30 min after intraperitoneal injection of memantine (5, 10, 20 mg/kg b.w.: MemL, MemM, MemH, respectively) and (+)MK-801 (0.1, 0.2, 0.3 mg/kg b.w.: MK-801L, MK-801M, MK-801H, respectively). Rats from the control group were intact or injected with saline (0.2 ml/kg). Over three consecutive days the rats underwent habituation, two avoidance training intervals with shocks, and a retrieval test. The shock sector was alternated daily. The after-effects of the agents were tested on Day 21. Rats treated with low dose memantine presented a longer maximum time avoided and fewer entrances than the MemH, MK-801M, MK-801H and Control rats. The shocks per entrances ratio, used as an index of cognitive skill learning, showed skill improvement after D1, except for rats treated by high doses of the agents. The activity levels, indicated by the distance walked, were higher for the groups treated with high doses of the agents. On D21 the MK801H rats performed the memory task better than the MemH rats, whereas the rats' activity depended on condition, not on the group factor. These results suggest that in naïve rats mild NMDAR blockade by low-dose memantine improves working memory related to a highly challenging task.

Cumulative benefits of frontal transcranial direct current stimulation on visuospatial working memory training and skill learning in rats.

Transcranial direct current stimulation (tDCS) of the prefrontal cortex, which non-invasively alters cortical activity, has been established to affect executive functions in humans. We hypothesized that changes in excitability by tDCS, found to improve cognitive functions dependent on moderate prefrontal cortex activity, would operate similarly in animals as in humans. To verify this we performed experiments using a rat behavioral model of visuospatial working memory and skill learning paired with tDCS of the frontal cortex. The effect of anodal/cathodal tDCS was examined in three sessions using the allothetic place avoidance alternation task (APAAT) and later re-examined without stimulation. Stimulation had no measurable short term effect on on-going place avoidance learning. However, in the follow-up session on day 21 the rats previously treated with cathodal tDCS showed significantly more efficient place avoidance and skill retention in comparison to the controls. This demonstrates a long-term benefit of diminished excitability by frontal tDCS when paired with training on working memory and skill learning in a novel task. The presented behavioral model provides a tool to evaluate the underlying mechanisms of how tDCS modulates neural network function to support successful behavior.

Divergent effects of age on performance in spatial associative learning and real idiothetic memory in humans.

This study focuses on age-related differences concerning two kinds of spatial memory assessed by: (1) Paired Associates Learning (PAL) test from the CANTAB and (2) a test of Real Idiothetic Memory (RIM) using real-life settings. Despite a clear age-related drop in PAL that is reported in existing studies, age-related differences in idiothetic navigation still remain unclear. In our study we tested 80 healthy volunteers classified according to their age into two groups, i.e. young (aged from 20 to 29 years of life; n=40; 20M/20F) and elderly (from 64 to 77 years; n=40; 20M/20F) healthy volunteers. They were asked in the PAL test to remember the spatial location of visual patterns presented on a computer screen, and in the RIM test to walk on the arena in darkness in order to find a cue place and then to return to the start/exit point. A white noise was switched on at entering the cue place and switched off at leaving this place. Elderly subjects indicated poorer performance than their younger counterparts on the PAL test, as evidenced by all tested outcome measures. In contrast, for the RIM test no clear age effect was evidenced. In both tests no gender effect was observed. A dissociation in age-related changes for these two tests indicates that visuo-spatial associative learning and idiothetic navigation may have different cognitive control which is probably rooted in an interplay of different brain structures.

A spatial paradigm, the allothetic place avoidance alternation task, for testing visuospatial working memory and skill learning in rats.

We present a paradigm for assessing visuospatial working memory and skill learning in a rodent model, based on the place avoidance test. In our allothetic place avoidance alternation task (APAAT) the paradigm is comprised of minimal training sessions, tests various aspects of learning and memory and provides a rich set of parameters. A single working memory session consists of four conditions: habituation (no shock), two place avoidance training intervals (shock activated) and a retrieval test (shock inactivated). The location of the shock sector is alternated for each training day which initially requires extinction of previous representations and further working memory to achieve effective place avoidance across sessions. Visuospatial skill memory was evaluated by the shock/entrance ratio by tracking locomotor activity which is essential to execute a place avoidance strategy. For each day rats learned to avoid a new place with shock, as shown by a decreased number of entrances, and an increased time to the first entrance and maximum avoidance time. Skill learning improved according to the decreased number of shocks per entrance across conditions. These results indicate that complex cognitive functions are captured by this behavioral method. This APAAT paradigm expands and complements existing tools for studying hippocampal-prefrontal dependent functions to support development of treatment interventions.

Retrosplenial cortex lesion affected segregation of spatial information in place avoidance task in the rat.

Retrosplenial cortex (RSC) together with the hippocampus is a component of the spatial memory circuit. To elucidate the role of the RSC in spatial memory formation in the immediate presence of both relevant and irrelevant spatial stimuli, we used a new place avoidance task, in which rats learn to avoid shock in an unmarked place. In the present study, we manipulated the relevance of distal "Room" stimuli and local "Arena" stimuli for place avoidance. Rats with ibotenate lesions of RSC, control sham lesions (Csl) and intact control rats (Cint) initially learned the (Room&Arena)+ task variant in which both Room and Arena stimuli are relevant for defining the shock sector. Afterwards, different subsets of rats from each group were trained in the following task variants: (i) Room+Arena-, in which the arena continuously rotated so that Room stimuli were relevant and Arena stimuli were irrelevant for avoiding shock; (ii) Arena+, in which the arena and shock sector rotated in a dark room so that Arena stimuli were relevant and Room stimuli were irrelevant for avoiding shock; (iii) Room+, in which the arena was covered in shallow water so that only Room stimuli were relevant for avoiding the shock sector whether the arena was stationary or rotating. We found that damage of RSC impaired the Room+Arena- variant that required relevant and irrelevant stimuli to be segregated. Importantly, the same lesions spared task variants that did not require segregation. Our results suggest an involvement of retrosplenial cortex in the segregation of spatial information.

Cognitive flexibility but not cognitive coordination is affected in rats with toxic liver failure.

Hepatic encephalopathy (HE), a consequence of liver damage, is associated with cognitive deficits. In this study, behavioral activity, non-associative learning, associative memory, cognitive coordination and flexibility were investigated in rats with subclinical HE evoked by thioacetamide treatment. Non-associative learning was studied in the open field (OF) set up in 12 HE and 8 saline-injected control rats (C). Memory was examined in spatial place avoidance tasks in 10 HE and 10 C rats. The Room+ Arena- task involved the selection of distal room stimuli from irrelevant arena stimuli (i.e. intramaze cues and/or self-motion information), which engages processes of cognitive coordination. Following the Room+ Arena- training, cognitive flexibility of rats was tested in the Arena+ place avoidance condition, which demands the previously ignored stimuli from arena. In the OF test HE and control rats behaved similar. They displayed high activity in the first block of each session and this pattern was stable. In both groups of rats darkness enhanced locomotor activity in comparison to light only in the first block. The HE and C rats avoided the to-be-avoided place in the Room+ Arena- task, whereas only HE rats were affected in the Arena+ task. In conclusion, these results demonstrate cognitive inflexibility in HE rats. We suggest that (1) the behavioral changes in the TAA model are typical of subclinical HE and (2) test for cognitive flexibility may be modified towards a routine use in patients with subclinical HE.

Selection for body weight induces differences in exploratory behavior and learning in mice.

Mice that were selected for over 108 generations for body weight at the postnatal (PN) day 21 were examined in the open field (OF) test and in the Lashley maze (LM) for their exploratory behavior and spatial learning. Light (L), heavy (C) and control (K) lines of mice in three age groups: PN-21, PN-56 and PN-90, were tested once in the OF and three times in the LM. During the session in the OF the L mice displayed a steady increase of behavioral activity (sum of locomotion and rearing, climbing, sniffing, and grooming acts), whereas mice C and K habituated in the last stage. During entire session in the OF activity of the L mice was lower than that of the C and K mice. The L mice displayed high defecation/urination scores. In the learning task the L mice performed worse than the C and K mice. In conclusion, behavior of the L line was different from that of the two other lines: they showed higher anxiety and poorer spatial learning.

Beyond memory, navigation, and inhibition: behavioral evidence for hippocampus-dependent cognitive coordination in the rat.

Injecting tetrodotoxin (TTX) into one hippocampus impaired avoidance of a place defined by distal cues while rats were on a slowly rotating arena. The impairment could be explained by a deficit in memory, navigation, or behavioral inhibition. Here, we show that the TTX injection abolished the ability of rats to organize place-avoidance behavior specifically when distal room and local arena cues were continuously dissociated. The results provide evidence that injecting TTX into one hippocampus specifically impaired the coordination of representations that support organized behavior because of the following: (1) rats normally coordinate separate room and arena avoidance memories; (2) the TTX injection spared spatial, relational, and representational memory, navigation, and behavioral inhibition; and (3) the TTX-induced impairment of place avoidance depended on the need to coordinate representations of local and distal stimuli.

Behavior of the gray short-tailed opossum (Monodelphis domestica) in the open field and in response to a new object, in comparison with the rat.

We compared the behavior of the gray short-tailed opossums (Monodelphis domestica) and Long-Evans rats during repeated exposures to the open-field (OF) test. Animals were videotaped for 10 min on four consecutive days. A new object was placed in the center of the field on the third day and it was present there again on the fourth day. The rate of locomotor activity in the opossum was always higher than that in the rat. On the first exposure to the open field, both species showed strong thigmotaxy. On the second day, opossums shifted a significant part of their activity to the internal and central parts of the field, while thigmotaxy dominated in the rats' behavior till the end of the experiment. The frequency and time of exploration of a new object placed on the central square was higher in the opossums than in rats. They also showed higher frequency of rearings and lower defecation scores, while the time of grooming was similar to the rats'. These results, that are consistent with those of our earlier experiments in the elevated plus maze (EPM), show that in response to novelty Monodelphis opossums change their behavior from defensive to exploratory faster than rats and then explore it more intensely. These differences may be either a result of different ecologies or evolution of the two species.