Hippocampus lesions induced deficits in social and spatial recognition in Octodon degus

Social odor discrimination and its enhancement by associative learning in the hippocampal CA2 region

Although the hippocampus is crucial for social memory, how social sensory information is combined with contextual information to form episodic social memories remains unknown. Here, we investigated the mechanisms for social sensory information processing using two-photon calcium imaging from hippocampal CA2 pyramidal neurons (PNs)-which are crucial for social memory-in awake head-fixed mice exposed to social and non-social odors. We found that CA2 PNs represent social odors of individual conspecifics and that these representations are refined during associative social odor-reward learning to enhance the discrimination of rewarded compared with unrewarded odors. Moreover, the structure of the CA2 PN population activity enables CA2 to generalize along categories of rewarded versus unrewarded and social versus non-social odor stimuli. Finally, we found that CA2 is important for learning social but not non-social odor-reward associations. These properties of CA2 odor representations provide a likely substrate for the encoding of episodic social memory.

Age-Dependent Behavioral and Synaptic Dysfunction Impairment Are Improved with Long-Term Andrographolide Administration in Long-Lived Female Degus (Octodon degus)

In Octodon degus, the aging process is not equivalent between sexes and worsens for females. To determine the beginning of detrimental features in females and the ways in which to improve them, we compared adult females (36 months old) and aged females (72 months old) treated with Andrographolide (ANDRO), the primary ingredient in Andrographis paniculata. Our behavioral data demonstrated that age does not affect recognition memory and preference for novel experiences, but ANDRO increases these at both ages. Sociability was also not affected by age; however, social recognition and long-term memory were lower in the aged females than adults but were restored with ANDRO. The synaptic physiology data from brain slices showed that adults have more basal synaptic efficiency than aged degus; however, ANDRO reduced basal activity in adults, while it increased long-term potentiation (LTP). Instead, ANDRO increased the basal synaptic activity and LTP in aged females. Age-dependent changes were also observed in synaptic proteins, where aged females have higher synaptotagmin (SYT) and lower postsynaptic density protein-95 (PSD95) levels than adults. ANDRO increased the N-methyl D-aspartate receptor subtype 2B (NR2B) at both ages and the PSD95 and Homer1 only in the aged. Thus, females exposed to long-term ANDRO administration show improved complex behaviors related to age-detrimental effects, modulating mechanisms of synaptic transmission, and proteins.

The prelimbic cortex but not the anterior cingulate cortex plays an important role in social recognition and social investigation in mice

The prefrontal cortex (PFC) has been implicated in social cognitive functions and emotional behaviors in rodents. Each subregion (prelimbic cortex, PL; infralimbic cortex; and anterior cingulate cortex, ACC) of the PFC appears to play a different role in social and emotional behaviors. However, previous investigations have produced inconsistent data, and few previous studies directly compared the roles of the PFC subregions using the same experimental paradigm. Accordingly, in the present study, we examined the role of the PL and the ACC in short-term social recognition, social investigation, and anxiety-related behaviors in C57BL/6J mice. We subjected mice with a lesioned PL or ACC, as well as those in a sham control group, to tests of social recognition and social novelty where juvenile and adult male mice were used as social stimuli. In the social recognition test, the PL-lesioned mice exhibited habituation but not dishabituation regardless of whether they encountered juvenile or adult mice. In a subsequent social novelty test, they spent less time engaged in social investigation compared with the control mice when adult mice were used as social stimuli. These results suggest that PL lesions impaired both social recognition and social investigation. In contrast, ACC-lesioned mice did not exhibit impaired short-term social recognition or social investigation regardless of the social stimulus. Furthermore, PL lesions and ACC lesions did not affect anxiety-related behavior in the open field test or light-dark transition test. Our findings demonstrate that the PL but not the ACC plays an important role in social recognition and social investigation.

Grid cell remapping under three-dimensional object and social landmarks detected by implantable microelectrode arrays for the medial entorhinal cortex

Grid cells with stable hexagonal firing patterns in the medial entorhinal cortex (MEC) carry the vital function of serving as a metric for the surrounding environment. Whether this mechanism processes only spatial information or involves nonspatial information remains elusive. Here, we fabricated an MEC-shaped microelectrode array (MEA) to detect the variation in neural spikes and local field potentials of the MEC when rats forage in a square enclosure with a planar, three-dimensional object and social landmarks in sequence. The results showed that grid cells exhibited rate remapping under social conditions in which spike firing fields closer to the social landmark had a higher firing rate. Furthermore, global remapping showed that hexagonal firing patterns were rotated and scaled when the planar landmark was replaced with object and social landmarks. In addition, when grid cells were activated, the local field potentials were dominated by the theta band (5-8 Hz), and spike phase locking was observed at troughs of theta oscillations. Our results suggest the pattern separation mechanism of grid cells in which the spatial firing structure and firing rate respond to spatial and social information, respectively, which may provide new insights into how the brain creates a cognitive map.

Prefrontal cortex is necessary for long-term social recognition memory in mice

The prefrontal cortex (PFC) plays critical roles in social cognition and emotional regulation in humans and rodents; however, its involvement in social recognition memory in mice remains unclear. Here, we examined the roles of the PFC in short-term and long-term social recognition memory, social motivation, and anxiety-related behavior in C57BL/6J male mice. Sham control and PFC-lesioned mice underwent four different behavioral tests. In the social recognition test, composed of three daily trials over 3 consecutive days, the control mice spent less time investigating the juvenile stimulus mouse both within each day and across days. By contrast, while social investigation behavior in PFC-lesioned mice decreased across the three trials within each day, it did not decrease over the 3-day testing period. These results indicate that the PFC has an important role in long-term, but not short-term, social recognition memory. The control and PFC-lesioned mice exhibited similar social motivation in the three-chamber test - both groups preferred the juvenile mouse to the empty cylinder and did not prefer the adult mouse. In addition, the PFC lesion had no impact on anxiety-related behavior or general activity in the light-dark transition test or the open field test. Our findings demonstrate that the PFC is essential for long-term social recognition memory and that it plays a critical role in higher-order social cognition.

Sociability, but not spatial memory, is correlated with regional brain volume variation in the striped mouse Rhabdomys spp

Local environmental conditions associated with different geographic areas may elicit variations in behavioural responses in animals, leading to concomitant differences in functional brain region volumes. We investigated the behavioural correlates of hippocampus and amygdala volumes in three sister taxa of the murid rodent genus Rhabdomys, occurring in different environments. We used a Barnes maze to test spatial memory, dyadic encounters to test social behaviour, and histological brain sections to calculate hippocampus and amygdala volumes. Arid-occurring R. pumilio made fewer errors and had shorter latencies in locating the escape tunnel compared to moist grassland-occurring R. d. dilectus and R. d. chakae in two probe trials, 48 and 96 h after the last learning trial. R. pumilio was more amicable than the R. dilectus subspecies in intra-specific dyadic encounters. R. pumilio had larger hippocampus and amygdala volumes than the other species. Smaller amygdala volumes were correlated with longer latencies in females for probe trial 1, but males showed similar latencies regardless of taxon. Higher amicability scores were correlated with larger amygdala volumes in all taxa. Higher amicability scores were correlated with larger hippocampus volumes in R. pumilio and R. d. chakae but smaller hippocampus volumes in R. d. dilectus. Correlative relationships between spatial memory and amygdala volume appeared 48 h, but not 96 h, after the last learning trial. Local environmental conditions may influence spatial navigation, but social correlates drive regional brain size within cryptic striped mouse taxa.

A Multivariate Assessment of Age-Related Cognitive Impairment in Octodon degus

Aging is a progressive functional decline characterized by a gradual deterioration in physiological function and behavior. The most important age-related change in cognitive function is decline in cognitive performance (i.e., the processing or transformation of information to make decisions that includes speed of processing, working memory, and learning). The purpose of this study is to outline the changes in age-related cognitive performance (i.e., short-term recognition memory and long-term learning and memory) in long-lived Octodon degus. The strong similarity between degus and humans in social, metabolic, biochemical, and cognitive aspects makes it a unique animal model for exploring the mechanisms underlying the behavioral and cognitive deficits related to natural aging. In this study, we examined young adult female degus (12- and 24-months-old) and aged female degus (38-, 56-, and 75-months-old) that were exposed to a battery of cognitive-behavioral tests. Multivariate analyses of data from the Social Interaction test or Novel Object/Local Recognition (to measure short-term recognition memory), and the Barnes maze test (to measure long-term learning and memory) revealed a consistent pattern. Young animals formed a separate group of aged degus for both short- and long-term memories. The association between the first component of the principal component analysis (PCA) from short-term memory with the first component of the PCA from long-term memory showed a significant negative correlation. This suggests age-dependent differences in both memories, with the aged degus having higher values of long-term memory ability but poor short-term recognition memory, whereas in the young degus an opposite pattern was found. Approximately 5% of the young and 80% of the aged degus showed an impaired short-term recognition memory; whereas for long-term memory about 32% of the young degus and 57% of the aged degus showed decreased performance on the Barnes maze test. Throughout this study, we outlined age-dependent cognitive performance decline during natural aging in degus. Moreover, we also demonstrated that the use of a multivariate approach let us explore and visualize complex behavioral variables, and identified specific behavioral patterns that allowed us to make powerful conclusions that will facilitate further the study on the biology of aging. In addition, this study could help predict the onset of the aging process based on behavioral performance.

"Live together, die alone": The effect of re-socialization on behavioural performance and social-affective brain-related proteins after a long-term chronic social isolation stress

Loneliness affects group-living mammals triggering a cascade of stress-dependent physiological disorders. Indeed, social isolation stress is a major risk factor for several neuropsychiatric disorders including anxiety and depression. Furthermore, social isolation has a negative impact on health and fitness. However, the neurobiological consequences of long-term chronic social isolation stress (LTCSIS) manifested during the adulthood of affected individuals are not fully understood. Our study assessed the impact of LTCSIS and social buffering (re-socialization) on the behavioural performance and social-affective brain-related proteins in diurnal, social, and long-lived Octodon degus (degus). Thereby, anxiety-like and social behaviour, and social recognition memory were assessed in male and female animals subjected to a variety of stress-inducing treatments applied from post-natal and post-weaning until their adulthood. Additionally, we evaluated the relationship among LTCSIS, Oxytocin levels (OXT), and OXT-Ca2+-signalling proteins in the hypothalamus, the hippocampus, and the prefrontal cortex. Our findings suggest that LTCSIS induces anxiety like-behaviour and impairs social novelty preference whereas sociability is unaffected. On the other hand, re-socialization can revert both isolation-induced anxiety and social memory impairment. However, OXT and its signalling remained reduced in the abovementioned brain areas, suggesting that the observed changes in OXT-Ca2+ pathway proteins were permanent in male and female degus. Based on these findings, we conclude degus experience social stress differently, suggesting the existence of sex-related mechanisms to cope with specific adaptive challenges.

Widespread Doublecortin Expression in the Cerebral Cortex of the Octodon degus

It has been demonstrated that in adulthood rodents show newly born neurons in the subgranular layer (SGL) of the dentate gyrus (DG), and in the subventricular zone (SVZ). The neurons generated in the SVZ migrate through the rostral migratory stream (RMS) to the olfactory bulb. One of the markers of newly generated neurons is doublecortin (DCX). The degu similarly shows significant numbers of DCX-labeled neurons in the SGL, SVZ, and RMS. Further, most of the nuclei of these DCX-expressing neurons are also labeled by proliferating nuclear antigen (PCNA) and Ki67. Finally, whereas in rats and mice DCX-labeled neurons are predominantly present in the SGL and SVZ, with only a few DCX neurons present in piriform cortex, the degu also shows significant numbers of DCX expressing neurons in areas outside of SVZ, DG, and PC. Many areas of neocortex in degu demonstrate DCX-labeled neurons in layer II, and most of these neurons are found in the limbic cortices. The DCX-labeled cells do not stain with NeuN, indicating they are immature neurons.

Western Diet Consumption During Development: Setting the Stage for Neurocognitive Dysfunction

The dietary pattern in industrialized countries has changed substantially over the past century due to technological advances in agriculture, food processing, storage, marketing, and distribution practices. The availability of highly palatable, calorically dense foods that are shelf-stable has facilitated a food environment where overconsumption of foods that have a high percentage of calories derived from fat (particularly saturated fat) and sugar is extremely common in modern Westernized societies. In addition to being a predictor of obesity and metabolic dysfunction, consumption of a Western diet (WD) is related to poorer cognitive performance across the lifespan. In particular, WD consumption during critical early life stages of development has negative consequences on various cognitive abilities later in adulthood. This review highlights rodent model research identifying dietary, metabolic, and neurobiological mechanisms linking consumption of a WD during early life periods of development (gestation, lactation, juvenile and adolescence) with behavioral impairments in multiple cognitive domains, including anxiety-like behavior, learning and memory function, reward-motivated behavior, and social behavior. The literature supports a model in which early life WD consumption leads to long-lasting neurocognitive impairments that are largely dissociable from WD effects on obesity and metabolic dysfunction.

Social spatial cognition: social distance dynamics as an identifier of social interactions

We suggest that socio-spatial behavior, which is an interaction between social and spatial cognition, can be viewed as a set of excursions that originate and end in close proximity to another individual(s). We present an extension of earlier studies that perceived spatial behavior in individual animals as a series of excursions originating from a particular location. We measured here the momentary distance between two individuals (social distance) to differentiate among eight possible types of social excursion originating in a state of proximity between excursion-participants. The defined excursion types are based on whether or not the excursion initiator also concludes the excursion, whether or not the excursion starts and ends at the same location, and the dynamics of the distance between excursion participants. We validated this approach to socio-spatial behavior as a set of excursions using it to analyze the behavior of the two sexes in rodents, of normal vs. stereotyped rats, as well as of different rodent species. Each of these groups displays a prevalent excursion type that reflects a distinct social dynamics. Our approach offers a useful and comprehensive tool for studying socio-spatial cognition, and can also be applied to distinguish among different social situations in rodents and other animals.

Female degus show high sociality but no preference for familiar peers

Group-living animals vary in social behavior across multiple dimensions, including in the selectivity of social interactions with familiar versus unfamiliar peers. Standardized behavioral tests can be used to tease apart different dimensions of behavior. These serve a dual function-on one hand, helping to isolate behavioral factors that may support collective behavior in natural habitats, and, on another, providing a basis for comparative approaches to understanding physiological mechanisms of behavior. Degus (Octodon degus) are South American caviomorph rodents that nest and forage in groups with relatively low genetic relatedness. Flexibility in group membership is likely supported by gregariousness toward strangers, but the relative preference for strangers compared with familiar individuals has not been systematically tested. We assessed the specificity of social preferences in female degus using a same-sex partner preference test. Degus huddled extensively with both familiar and unfamiliar peers, with no average preference for one over the other. Detailed analysis of social interactions demonstrated an effect of familiarity on social investigation and aggressive behaviors, indicating that degus distinguished between familiar and unfamiliar conspecifics, even though it did not impact huddling. This behavioral profile is thus far unique to degus; in similar tests, meadow and prairie voles exhibit strong partner preferences for known peers, while mice exhibit low social huddling and spend relatively less time in social chambers. Understanding how group-living species differ in specific aspects of social behavior such as familiarity/novelty preference and propensity for social contact will offer a foundation to interpret differences in neural systems supporting sociality.

Auditory-induced visual illusions in rodents measured by spontaneous behavioural response

When two brief sounds are presented with a short flash of light, we often perceive that the flash blinks twice. This phenomenon, called the “sound-induced flash illusion”, has been investigated as an example of how humans finely integrate multisensory information, more specifically, the temporal content of perception. However, it is unclear whether nonhuman animals experience the illusion. Therefore, we investigated whether the Mongolian gerbil, a rodent with relatively good eyesight, experiences this illusion. The novel object recognition (NOR) paradigm was used to evaluate the gerbil’s natural (i.e., untrained) capacity for multimodal integration. A light-emitting diode embedded within an object presented time-varying visual stimuli (different flashing patterns). The animals were first familiarised with repetitive single flashes. Then, various sound stimuli were introduced during test trials. An increase in exploration suggested that the animals perceived a flashing pattern differently only when the contradicting sound (double beeps) was presented simultaneously with a single flash. This result shows that the gerbil may experience the sound-induced flash illusion and indicates for the first time that rodents may have the capacity to integrate temporal content of perception in a sophisticated manner as do humans.

New Insights into the Spontaneous Human Alzheimer's Disease-Like Model Octodon degus: Unraveling Amyloid-β Peptide Aggregation and Age-Related Amyloid Pathology

Alzheimer's disease (AD) is the most common cause of dementia worldwide. Despite advances in our understanding of the molecular milieu driving AD pathophysiology, no effective therapy is currently available. Moreover, various clinical trials have continued to fail, suggesting that our approach to AD must be revised. Accordingly, the development and validation of new models are highly desirable. Over the last decade, we have been working with Octodon degus (degu), a Chilean rodent, which spontaneously develops AD-like neuropathology, including increased amyloid-β (Aβ) aggregates, tau hyperphosphorylation, and postsynaptic dysfunction. However, for proper validation of degu as an AD model, the aggregation properties of its Aβ peptide must be analyzed. Thus, in this study, we examined the capacity of the degu Aβ peptide to aggregate in vitro. Then, we analyzed the age-dependent variation in soluble Aβ levels in the hippocampus and cortex of third- to fifth-generation captive-born degu. We also assessed the appearance and spatial distribution of amyloid plaques in O. degus and compared them with the plaques in two AD transgenic mouse models. In agreement with our previous studies, degu Aβ was able to aggregate, forming fibrillar species in vitro. Furthermore, amyloid plaques appeared in the anterior brain structures of O. degus at approximately 32 months of age and in the whole brain at 56 months, along with concomitant increases in Aβ levels and the Aβ42/Aβ40 ratio, indicating that O. degus spontaneously develops AD-like pathology earlier than other spontaneous models. Based on these results, we can confirm that O. degus constitutes a valuable model to improve AD research.

Loss of Function of Phosphodiesterase 11A4 Shows that Recent and Remote Long-Term Memories Can Be Uncoupled

Systems consolidation is a process by which memories initially require the hippocampus for recent long-term memory (LTM) but then become increasingly independent of the hippocampus and more dependent on the cortex for remote LTM. Here, we study the role of phosphodiesterase 11A4 (PDE11A4) in systems consolidation. PDE11A4, which degrades cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), is preferentially expressed in neurons of CA1, the subiculum, and the adjacently connected amygdalohippocampal region. In male and female mice, deletion of PDE11A enhances remote LTM for social odor recognition and social transmission of food preference (STFP) despite eliminating or silencing recent LTM for those same social events. Measurement of a surrogate marker of neuronal activation (i.e., Arc mRNA) suggests the recent LTM deficits observed in Pde11 knockout mice correspond with decreased activation of ventral CA1 relative to wild-type littermates. In contrast, the enhanced remote LTM observed in Pde11a knockout mice corresponds with increased activation and altered functional connectivity of anterior cingulate cortex, frontal association cortex, parasubiculum, and the superficial layer of medial entorhinal cortex. The apparent increased neural activation observed in prefrontal cortex of Pde11a knockout mice during remote LTM retrieval may be related to an upregulation of the N-methyl-D-aspartate receptor subunits NR1 and NR2A. Viral restoration of PDE11A4 to vCA1 alone is sufficient to rescue both the LTM phenotypes and upregulation of NR1 exhibited by Pde11a knockout mice. Together, our findings suggest remote LTM can be decoupled from recent LTM, which may have relevance for cognitive deficits associated with aging, temporal lobe epilepsy, or transient global amnesia.

Neonatal Congenital Central Hypoventilation Syndrome: Why We Should not Sleep on it. Literature Review of Forty-two Neonatal Onset Cases

Congenital Central Hypoventilation Syndrome (CCHS), also referred with the expression "Ondine's Curse", is a rare genetic life-long disease resulting from the mutation of PHOX2B gene on chromosome 4p12.3. CCHS represents an autonomic nervous system disorder; its more fearsome manifestation is central hypoventilation, due to a deficient response of chemoreceptors to hypercapnia and hypoxia. Several associated symptoms can occur, such as pupillary anomalies, arrhythmias, reduced heart rate variability, esophageal dysmotility, and structural comorbidities (Hirschsprung's Disease or neural crest tumours). CCHS typical onset is during the neonatal period, but cases of delayed diagnosis have been reported; moreover, both sporadic or familial cases can occur. In preterm newborns, asphyxia and typical prematurity-related findings may overlap CCHS clinical manifestations and make it harder to formulate a correct diagnosis. The early recognition of CCHS allows appropriate management, useful to reduce immediate and long- term consequences.

Oxytocin and vasopressin in the rodent hippocampus

The role of the hippocampus in social memory and behavior is under intense investigation. Oxytocin (Oxt) and vasopressin (Avp) are two neuropeptides with many central actions related to social cognition. Oxt- and Avp-expressing fibers are abundant in the hippocampus and receptors for both peptides are seen throughout the different subfields, suggesting that Oxt and Avp modulate hippocampal-dependent processes. In this review, we first focus on the anatomical sources of Oxt and Avp input to the hippocampus and consider the distribution of their corresponding receptors in different hippocampal subfields and neuronal populations. We next discuss the behavioral outcomes related to social memory seen with perturbation of hippocampal Oxt and Avp signaling. Finally, we review Oxt and Avp modulatory mechanisms in the hippocampus that may underlie the behavioral roles for both peptides.

Toxoplasma gondii alters NMDAR signaling and induces signs of Alzheimer's disease in wild-type, C57BL/6 mice

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disease associated with cognitive decline and complete loss of basic functions. The ubiquitous apicomplexan parasite Toxoplasma gondii (T. gondii) infects up to one third of the world's population and is implicated in AD.

METHODS

We infected C57BL/6 wild-type male and female mice with 10 T. gondii ME49 cysts and assessed whether infection led to behavioral and anatomical effects using immunohistochemistry, immunofluorescence, Western blotting, cell culture assays, as well as an array of mouse behavior tests.

RESULTS

We show that T. gondii infection induced two major hallmarks of AD in the brains of C57BL/6 male and female mice: beta-amyloid (Aβ) immunoreactivity and hyperphosphorylated Tau. Infected mice showed significant neuronal death, loss of N-methyl-D-aspartate receptor (NMDAR) expression, and loss of olfactory sensory neurons. T. gondii infection also caused anxiety-like behavior, altered recognition of social novelty, altered spatial memory, and reduced olfactory sensitivity. This last finding was exclusive to male mice, as infected females showed intact olfactory sensitivity.

CONCLUSIONS

These results demonstrate that T. gondii can induce advanced signs of AD in wild-type mice and that it may induce AD in some individuals with underlying health problems.

Automated dissection of permanent effects of hippocampal or prefrontal lesions on performance at spatial, working memory and circadian timing tasks of C57BL/6 mice in IntelliCage

To evaluate permanent effects of hippocampal and prefrontal cortex lesion on spatial tasks, lesioned and sham-operated female C57BL/6 mice were exposed to a series of conditioning schemes in IntelliCages housing 8–10 transponder-tagged mice from each treatment group. Sequential testing started at 51–172 days after bilateral lesions and lasted for 154 and 218 days in two batches of mice, respectively. Spontaneous undisturbed behavioral patterns clearly separated the three groups, hippocampals being characterized by more erratic hyperactivity, and strongly impaired circadian synchronization ability. Hippocampal lesions led to deficits in spatial passive avoidance, as well as in spatial reference and working memory tasks. Impairment was minimal in rewarded preference/reversal schemes, but prominent if behavioral responses required precise circadian timing or included punishment of wrong spatial choices. No differences between sham-operated and prefrontally lesioned subjects in conditioning success were discernible. These results corroborate the view that hippocampal dysfunction spares simple spatial learning tasks but impairs the ability to cope with conflicting task-inherent spatial, temporal or emotional cues. Methodologically, the results show that automated testing and data analysis of socially kept mice is a powerful, efficient and animal-friendly tool for dissecting complex features and behavioral profiles of hippocampal dysfunction characterizing many transgenic or pharmacological mouse models.

Aging Impairs Hippocampal- Dependent Recognition Memory and LTP and Prevents the Associated RyR Up-regulation

Recognition memory comprises recollection judgment and familiarity, two different processes that engage the hippocampus and the perirhinal cortex, respectively. Previous studies have shown that aged rodents display defective recognition memory and alterations in hippocampal synaptic plasticity. We report here that young rats efficiently performed at short-term (5 min) and long-term (24 h) hippocampus-associated object-location tasks and perirhinal cortex-related novel-object recognition tasks. In contrast, aged rats successfully performed the object-location and the novel-object recognition tasks only at short-term. In addition, aged rats displayed defective long-term potentiation (LTP) and enhanced long-term depression (LTD). Successful long-term performance of object-location but not of novel-object recognition tasks increased the protein levels of ryanodine receptor types-2/3 (RyR2/RyR3) and of IP₃R1 in young rat hippocampus. Likewise, sustained LTP induction (1 h) significantly increased RyR2, RyR3 and IP₃R1 protein levels in hippocampal slices from young rats. In contrast, LTD induction (1 h) did not modify the levels of these three proteins. Naïve (untrained) aged rats displayed higher RyR2/RyR3 hippocampal protein levels but similar IP₃R1 protein content relative to young rats; these levels did not change following exposure to either memory recognition task or after LTP or LTD induction. The perirhinal cortex from young or aged rats did not display changes in the protein contents of RyR2, RyR3, and IP₃R1 after exposure at long-term (24 h) to the object-location or the novel-object recognition tasks. Naïve aged rats displayed higher RyR2 channel oxidation levels in the hippocampus compared to naïve young rats. The RyR2/RyR3 up-regulation and the increased RyR2 oxidation levels exhibited by aged rat hippocampus are likely to generate anomalous calcium signals, which may contribute to the well-known impairments in hippocampal LTP and spatial memory that take place during aging.

Identification of Cerebral Metal Ion Imbalance in the Brain of Aging Octodon degus

The accumulation of redox-active transition metals in the brain and metal dyshomeostasis are thought to be associated with the etiology and pathogenesis of several neurodegenerative diseases, and Alzheimer’s disease (AD) in particular. As well, distinct biometal imaging and role of metal uptake transporters are central to understanding AD pathogenesis and aging but remain elusive, due inappropriate detection methods. We therefore hypothesized that Octodon degus develop neuropathological abnormalities in the distribution of redox active biometals, and this effect may be due to alterations in the expression of lysosomal protein, major Fe/Cu transporters, and selected Zn transporters (ZnTs and ZIPs). Herein, we report the distribution profile of biometals in the aged brain of the endemic Chilean rodent O. degus—a natural model to investigate the role of metals on the onset and progression of AD. Using laser ablation inductively coupled plasma mass spectrometry, our quantitative images of biometals (Fe, Ca, Zn, Cu, and Al) appear significantly elevated in the aged O. degus and show an age-dependent rise. The metals Fe, Ca, Zn, and Cu were specifically enriched in the cortex and hippocampus, which are the regions where amyloid plaques, tau phosphorylation and glial alterations are most commonly reported, whilst Al was enriched in the hippocampus alone. Using whole brain extracts, age-related deregulation of metal trafficking pathways was also observed in O. degus. More specifically, we observed impaired lysosomal function, demonstrated by increased cathepsin D protein expression. An age-related reduction in the expression of subunit B2 of V-ATPase, and significant increases in amyloid beta peptide 42 (Aβ42), and the metal transporter ATP13a2 were also observed. Although the protein expression levels of the zinc transporters, ZnT (1,3,4,6, and 7), and ZIP7,8 and ZIP14 increased in the brain of aged O. degus, ZnT10, decreased. Although no significant age-related change was observed for the major iron/copper regulator IRP2, we did find a significant increase in the expression of DMT1, a major transporter of divalent metal species, 5′-aminolevulinate synthase 2 (ALAS2), and the proto-oncogene, FOS. Collectively, our data indicate that transition metals may be enriched with age in the brains of O. degus, and metal dyshomeostasis in specific brain regions is age-related.

Genotypic differences in intruder-evoked immediate early gene activation in male, but not female, vasopressin 1b receptor knockout mice

Background

The neuropeptide arginine vasopressin (Avp) modulates social behaviors via its two centrally expressed receptors, the Avp 1a receptor and the Avp 1b receptor (Avpr1b). Recent work suggests that, at least in mice, Avp signaling through Avpr1b within the CA2 region of the hippocampus is critical for normal aggressive behaviors and social recognition memory. However, this brain area is just one part of a larger neural circuit that is likely to be impacted in Avpr1b knockout (−/−) mice. To identify other brain areas that are affected by altered Avpr1b signaling, genotypic differences in immediate early gene activation, i.e. c-FOS and early growth response factor 1 (EGR-1), were quantified using immunocytochemistry following a single exposure to an intruder.

Results

In females, no genotypic differences in intruder-evoked c-FOS or EGR-1 immunoreactivity were observed in any of the brain areas measured. In males, while there were no intruder-evoked genotypic differences in c-FOS immunoreactivity, genotypic differences were observed in EGR-1 immunoreactivity within the ventral bed nucleus of the stria terminalis and the anterior hypothalamus; with Avpr1b −/− males having less EGR-1 immunoreactivity in these regions than controls.

Conclusions

These data are the first to identify specific brain areas that may be a part of a neural circuit that includes Avpr1b-expressing cells in the CA2 region of the hippocampus. It is thought that this circuit, when working properly, plays a role in how an animal evaluates its social context.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-016-0310-7) contains supplementary material, which is available to authorized users.

Sex-dependent changes in neuronal morphology and psychosocial behaviors after pediatric brain injury

Chronic social behavior problems after pediatric traumatic brain injury (TBI) significantly contribute to poor quality of life for survivors. Using a well-characterized mouse model of early childhood TBI, we have previously demonstrated that young brain-injured mice develop social deficits by adulthood. As biological sex may influence both normal and aberrant social development, we here evaluated potential sex differences in post-TBI psychosocial deficits by comparing the behavior of male and female mice at adulthood (8 weeks post-injury). Secondly, we hypothesized that pediatric TBI would influence neuronal morphology identified by Golgi-Cox staining in the hippocampus and prefrontal cortex, regions involved in social cognition and behavior, before the onset of social problems (3 weeks post-injury). Morphological analysis of pyramidal neurons in the ipsilateral prefrontal cortex and granule cells of the hippocampal dentate gyrus revealed a reduction in dendritic complexity after pediatric TBI. This was most apparent in TBI males, whereas neurons from females were less affected. At adulthood, consistent with previous studies, TBI males showed deficits in sociability and social recognition. TBI females also showed a reduction in sociability, but intact social recognition and increased sociosexual avoidance. Together, these findings indicate that sex is a determinant of regional neuroplasticity and social outcomes after pediatric TBI. Reduced neuronal complexity in the prefrontal cortex and hippocampus, several weeks after injury in male mice, appears to precede the subsequent emergence of social deficits. Sex-specific alterations in the social brain network are thus implicated as an underlying mechanism of social dysfunction after pediatric TBI.

Dynamic microglial modulation of spatial learning and social behavior

Microglia are active players in inflammation, but also have important supporting roles in CNS maintenance and function, including modulation of neuronal activity. We previously observed an increase in the frequency of excitatory postsynaptic current in organotypic brain slices after depletion of microglia using clodronate. Here, we describe that local hippocampal depletion of microglia by clodronate alters performance in tests of spatial memory and sociability. Global depletion of microglia by high-dose oral administration of a Csf1R inhibitor transiently altered spatial memory but produced no change in sociability behavior. Microglia depletion and behavior effects were both reversible, consistent with a dynamic role for microglia in the regulation of such behaviors.

Neonatal inflammatory pain and systemic inflammatory responses as possible environmental factors in the development of autism spectrum disorder of juvenile rats

Background

Autism spectrum disorder (ASD) affects many children and juveniles. The pathogenesis of ASD is not well understood. Environmental factors may play important roles in the development of ASD. We examined a possible relationship of inflammatory pain in neonates and the development of ASD in juveniles.

Methods

Acute inflammation pain was induced by 5 % formalin (5 μl/day) subcutaneous injection into two hindpaws of postnatal day 3 to 5 (P3–P5) rat pups. Western blot, immunohistochemical, and behavioral examinations were performed at different time points after the insult.

Results

Formalin injection caused acute and chronic inflammatory responses including transient local edema, increased levels of inflammatory cytokines, TNF-α, and IL-1β in the blood as well as in the brain, and increased microglia in the brain. One day after the pain insult, there was significant cell death in the cortex and hippocampus. Two weeks later, although the hindpaw local reaction subsided, impaired axonal growth and demyelization were seen in the brain of P21 juvenile rats. The number of bromodeoxyuridine (BrdU) and doublecortin (DCX) double-positive cells in the hippocampal dentate gyrus of P21 rats was significantly lower than that in controls, indicating reduced neurogenesis. In the P21 rat’s brain of the formalin group, the expression of autism-related gene neurexin 1 (NRXN1), fragile X mental retardation 1 (FMR1), and oxytocin was significantly downregulated, consistent with the gene alteration in ASD. Juvenile rats in the formalin group showed hyperalgesia, repetitive behaviors, abnormal locomotion, sleep disorder, and distinct deficits in social memory and social activities. These alterations in neuroinflammatory reactions, gene expression, and behaviors were more evident in male than in female rats. Importantly, an anti-inflammation treatment using indomethacin (10 mg/kg, i.p.) at the time of formalin injections suppressed inflammatory responses and neuronal cell death and prevented alterations in ASD-related genes and the development of abnormal behaviors.

Conclusions

These novel observations indicate that severe inflammatory pain in neonates and persistent inflammatory reactions may predispose premature infants to development delays and psychiatric disorders including ASD. The prevention of pain stimuli and prompt treatments of inflammation during development appear vitally important in disrupting possible evolution of ASD syndromes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0575-x) contains supplementary material, which is available to authorized users.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Recognizing Others: Rodent's Social Memories

We provide in this chapter a brief overview of the present knowledge about social memory in laboratory rodents with a focus on mice and rats. We discuss in the first part the relevance of the processing of olfactory cues for social recognition in these animals and present information about the brain areas involved in the generation of a long-term social memory including cellular mechanisms thought to underlie memory consolidation. In the second part, we suggest that sensory modalities beyond olfaction may also be important in contributing to the long-term social memory trace including audition and taction (and vision). The exposure to stimuli activating the auditory system and taction is able to produce interference phenomena at defined time points during the consolidation of social memory. This ability of such-nonsocial-stimuli may provide a new approach to dissect the brain processes underlying the generation of the social memory trace in further studies.

Anisomycin administered in the olfactory bulb and dorsal hippocampus impaired social recognition memory consolidation in different time-points

To identify an individual as familiar, rodents form a specific type of memory named social recognition memory. The olfactory bulb (OB) is an important structure for social recognition memory, while the hippocampus recruitment is still controversial. The present study was designed to elucidate the OB and the dorsal hippocampus contribution to the consolidation of social memory. For that purpose, we tested the effect of anisomycin (ANI), which one of the effects is the inhibition of protein synthesis, on the consolidation of social recognition memory. Swiss adult mice with cannulae implanted into the CA1 region of the dorsal hippocampus or into the OB were exposed to a juvenile during 5 min (training session; TR), and once again 1.5 h or 24 h later to test social short-term memory (S-STM) or social long-term memory (S-LTM), respectively. To study S-LTM consolidation, mice received intra-OB or intra-CA1 infusion of saline or ANI immediately, 3, 6 or 18 h after TR. ANI impaired S-LTM consolidation in the OB, when administered immediately or 6h after TR. In the dorsal hippocampus, ANI was amnesic only if administered 3 h after TR. Furthermore, the infusion of ANI in either OB or CA1, immediately after training, did not affect S-STM. Moreover, ANI administered into the OB did not alter the animal's performance in the buried food-finding task. Altogether, our results suggest the consolidation of S-LTM requires both OB and hippocampus participation, although in different time points. This study may help shedding light on the specific roles of the OB and dorsal hippocampus in social recognition memory.

Lesions to the CA2 region of the hippocampus impair social memory in mice

The function of the CA2 region of the hippocampus is poorly understood. Although the CA1 and CA3 regions have been extensively studied, for years the CA2 region has primarily been viewed as a linking area between the two. However, the CA2 region is known to have distinct neurochemical and structural features that are different from the other parts of the hippocampus and in recent years it has been suggested that the CA2 region may play a role in the formation and/or recall of olfactory-based memories needed for normal social behavior. Although this hypothesis has been supported by hippocampal lesion studies that have included the CA2 region, no studies have attempted to specifically lesion the CA2 region of the hippocampus in mice to determine the effects on social recognition memory and olfaction. To fill this knowledge gap, we sought to perform excitotoxic N-methyl-D-aspartate lesions of the CA2 region and determine the effects on social recognition memory. We predicted that lesions of the CA2 region would impair social recognition memory. We then went on to test olfaction in CA2-lesioned mice, as social memory requires a functional olfactory system. Consistent with our prediction, we found that CA2-lesioned animals had impaired social recognition. These findings are significant because they confirmed that the CA2 region of the hippocampus is a part of the neural circuitry that regulates social recognition memory, which may have implications for our understanding of the neural regulation of social behavior across species.

Impaired hippocampal neuroligin-2 function by chronic stress or synthetic peptide treatment is linked to social deficits and increased aggression

Neuroligins (NLGNs) are cell adhesion molecules that are important for proper synaptic formation and functioning, and are critical regulators of the balance between neural excitation/inhibition (E/I). Mutations in NLGNs have been linked to psychiatric disorders in humans involving social dysfunction and are related to similar abnormalities in animal models. Chronic stress increases the likelihood for affective disorders and has been shown to induce changes in neural structure and function in different brain regions, with the hippocampus being highly vulnerable to stress. Previous studies have shown evidence of chronic stress-induced changes in the neural E/I balance in the hippocampus. Therefore, we hypothesized that chronic restraint stress would lead to reduced hippocampal NLGN-2 levels, in association with alterations in social behavior. We found that rats submitted to chronic restraint stress in adulthood display reduced sociability and increased aggression. This occurs along with a reduction of NLGN-2, but not NLGN-1 expression (as shown with western blot, immunohistochemistry, and electron microscopy analyses), throughout the hippocampus and detectable in different layers of the CA1, CA3, and DG subfields. Furthermore, using synthetic peptides that comprise sequences in either NLGN-1 (neurolide-1) or NLGN-2 (neurolide-2) involved in the interaction with their presynaptic partner neurexin (NRXN)-1, intra-hippocampal administration of neurolide-2 led also to reduced sociability and increased aggression. These results highlight hippocampal NLGN-2 as a key molecular substrate regulating social behaviors and underscore NLGNs as promising targets for the development of novel drugs for the treatment of dysfunctional social behaviors.

Acute and chronic ethanol intake: effects on spatial and non-spatial memory in rats

Abusive alcohol consumption produces neuronal damage and biochemical alterations in the mammal brain followed by cognitive disturbances. In this work rats receiving chronic and acute alcohol intake were evaluated in a spontaneous delayed non-matching to sample/position test. Chronic alcohol-treated rats had free access to an aqueous ethanol solution as the only available liquid source from the postnatal day 21 to the end of experiment (postnatal day 90). Acute alcoholic animals received an injection of 2 g/kg ethanol solution once per week. Subjects were evaluated in two tests (object recognition and spatial recognition) based on the spontaneous delayed non-matching to sample or to position paradigm using delays of 1 min, 15 min and 60 min. Results showed that chronic and acute alcohol intake impairs the rats' performance in both tests. Moreover, chronic alcohol-treated rats were more altered than acute treated animals in both tasks. Our results support the idea that chronic and acute alcohol administration during postnatal development caused widespread brain damage resulting in behavioral disturbances and learning disabilities.

The vasopressin 1b receptor and the neural regulation of social behavior

To date, much of the work in rodents implicating vasopressin (Avp) in the regulation of social behavior has focused on its action via the Avp 1a receptor (Avpr1a). However, there is mounting evidence that the Avp 1b receptor (Avpr1b) also plays a significant role in Avp's modulation of social behavior. The Avpr1b is heavily expressed on the anterior pituitary cortiocotrophs where it acts as an important modulator of the endocrine stress response. In the brain, the Avpr1b is prominent in the CA2 region of the hippocampus, but can also be found in areas such as the paraventricular nucleus of the hypothalamus and the olfactory bulb. Studies that have employed genetic knockouts or pharmacological manipulation of the Avpr1b point to the importance of central Avpr1b in the modulation of social behavior. However, there continues to be a knowledge gap in our understanding of where in the brain this is occurring, as well as how and if the central actions of Avp acting via the Avpr1b interact with the stress axis. In this review we focus on the genetic and pharmacological studies that have implicated the Avpr1b in the neural regulation of social behaviors, including social forms of aggressive behavior, social memory, and social motivation. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Weak and nondiscriminative responses to conspecifics in the rat hippocampus

Little is known about how hippocampal neurons in rodents respond to and represent conspecifics. To address this question, we let rats interact while quantifying hippocampal neuronal activation patterns with extracellular recordings and immediate-early gene (c-Fos) expression. A total of 319 single putative pyramidal neurons was recorded in dorsal hippocampus. In sessions with multiple stimulus rats, no cell responded differentially to individual rats (N = 267 cells). We did find, however, that the presence of other rats induced a significant enhancement or suppression of firing in a fraction of neurons (n = 22 of 319; 7%). As expected, a large fraction of neurons (n = 170; 53%) had place fields. There was no evidence for place-independent responses to rats. Rather, the modulations were linked to the spatial responses. While neurons did not discriminate between individual rats, they did discriminate between rats and inanimate objects. Surprisingly, neuronal responses were more strongly modulated by objects than by rats, even though subjects spent more time near their conspecifics. Consistent with the low fraction of rat-modulated cells, social encounters did not induce c-Fos expression in the hippocampus, while there was a social interaction-specific expression in the basolateral amygdala. In both interacting and non-interacting rats, the fraction of c-Fos-expressing cells in the hippocampus was very low. Our investigation of social coding in the rat hippocampus, along with other recent work, showed that social responses were rare and lacked individual specificity, altogether speaking against a role of rodent dorsal hippocampus in social memory.