Behavioral effects of corpus callosum transection and environmental enrichment in adult rats

Transection of the ventral hippocampal commissure impairs spatial reference but not contextual or spatial working memory

Plasticity is a neural phenomenon in which experience induces long-lasting changes to neuronal circuits and is at the center of most neurobiological theories of learning and memory. However, too much plasticity is maladaptive and must be balanced with substrate stability. Area CA3 of the hippocampus provides such a balance via hemispheric lateralization, with the left hemisphere dominant in providing plasticity and the right specialized for stability. Left and right CA3 project bilaterally to CA1; however, it is not known whether this downstream merging of lateralized plasticity and stability is functional. We hypothesized that interhemispheric convergence of input from these pathways is essential for integrating spatial memory stored in the left CA3 with navigational working memory facilitated by the right CA3. To test this, we severed interhemispheric connections between the left and right hippocampi in mice and assessed learning and memory. Despite damage to this major hippocampal fiber tract, hippocampus-dependent navigational working memory and short- and long-term memory were both spared. However, tasks that required the integration of information retrieved from memory with ongoing navigational working memory and navigation were impaired. We propose that one function of interhemispheric communication in the mouse hippocampus is to integrate lateralized processing of plastic and stable circuits to facilitate memory-guided spatial navigation.

Histamine H2 receptor negatively regulates oligodendrocyte differentiation in neonatal hypoxic-ischemic white matter injury

Neonatal hypoxic-ischemic encephalopathy (HIE) with the pathological characteristic of white matter injury often leads to lifelong cognitive and neurobehavioral dysfunction, but relevant therapies to promote remyelination are still unavailable. We found that histamine H₂ receptor (H₂R) negatively regulated the oligodendrocyte differentiation rate without affecting the oligodendrocytes at the oligodendrocyte precursor cell stage or mature stage following oxygen-glucose deprivation in vitro. Notably, selective deletion of the H₂R gene (Hrh2) in differentiating oligodendrocytes (Hrh2^fl/fl;CNPase-Cre) improved their differentiation, remyelination, and functional recovery following neonatal hypoxia-ischemia in mice. The regulation of oligodendrocyte differentiation by H₂R is mediated by binding with Axin2, which leads to up-regulation of the Wnt/β-catenin signaling pathway. Furthermore, H₂R antagonists also promoted oligodendrocyte differentiation and remyelination and the recovery of cognition and motor functions following neonatal hypoxia-ischemia. Thus, histamine H₂R in oligodendrocytes could serve as a novel and effective therapeutic target for the retard of oligodendrocyte differentiation and remyelination following neonatal hypoxia-ischemia. The H₂R antagonists may have potential therapeutic value for neonatal HIE.

The impact of environmental and social factors on learning abilities: a meta-analysis

Since the 1950s, researchers have examined how differences in the social and asocial environment affect learning in rats, mice, and, more recently, a variety of other species. Despite this large body of research, little has been done to synthesize these findings and to examine if social and asocial environmental factors have consistent effects on cognitive abilities, and if so, what aspects of these factors have greater or lesser impact. Here, we conducted a systematic review and meta-analysis examining how different external environmental features, including the social environment, impact learning (both speed of acquisition and performance). Using 531 mean-differences from 176 published articles across 27 species (with studies on rats and mice being most prominent) we conducted phylogenetically corrected mixed-effects models that reveal: (i) an average absolute effect size |d| = 0.55 and directional effect size d = 0.34; (ii) interventions manipulating the asocial environment result in larger effects than social interventions alone; and (iii) the length of the intervention is a significant predictor of effect size, with longer interventions resulting in larger effects. Additionally, much of the variation in effect size remained unexplained, possibly suggesting that species differ widely in how they are affected by environmental interventions due to varying ecological and evolutionary histories. Overall our results suggest that social and asocial environmental factors do significantly affect learning, but these effects are highly variable and perhaps not always as predicted. Most notably, the type (social or asocial) and length of interventions are important in determining the strength of the effect.

Extinction of Contextual Fear with Timed Exposure to Enriched Environment: A Differential Effect

BACKGROUND

Extinction of fear memory depends on the environmental and emotional cues. Furthermore, consolidation of extinction is also dependent on the environmental exposure. But, the relationship of the time of the exposure to a variety of environmental cues is not well known. The important region involved in facilitation of extinction of fear memory is through diversion of the flow of information leaving the lateral nucleus of amygdala.

PURPOSE

The study aimed to address a question to explain how these brain regions react to environmental stimulation during the retention and extinction of fear memory.

METHODS

An enriched environment (EE) is assumed to mediate extinction of fear memory, we examined the apparent discrepancy between the effects of defensive response, the freezing behavior induced by Pavlovian classical fear conditioning by subjecting them to variance in the timing to EE. The different timing of EE exposure was 10 days of EE either before fear conditioning and/or after extinction training to the rats. The local field potentials was recorded from CA1 hippocampus, lateral nucleus of amygdala and infralimbic region of medial prefrontal cortex (mPFC) during the fear learning and extinction from the control rats and rats exposed to EE before and after fear conditioning.

RESULTS

Exposure to EE before the fear conditioning and after extinction training was more effective in the extinction fear memory. In addition, we also found switching from exploratory locomotion to freezing during retention of contextual fear memory which was associated with decreased theta power and reduced synchronized theta oscillations in CA1-hippocampus, lateral nucleus of amygdala, and infralimbic region of mPFC.

CONCLUSION

Thus, we propose that the timing of exposure to EE play a key role in the extinction of fear memory.

Interleukin-1β impedes oligodendrocyte progenitor cell recruitment and white matter repair following chronic cerebral hypoperfusion

Subcortical ischemic vascular dementia (SIVD) caused by chronic cerebral hypoperfusion exhibits progressive white matter and cognitive impairments. However, its pathogenetic mechanisms are poorly understood. We investigated the role of interleukin-1β (IL-1β) and its receptor IL-1 receptor type 1 (IL-1R1) in an experimental SIVD model generated via right unilateral common carotid arteries occlusion (rUCCAO) in mice. We found that IL-1β expression was elevated in the corpus callosum at the early stages after rUCCAO. IL-1 receptor antagonist (IL-1Ra), when delivered at an early stage, as well as IL-1R1 knockout, rescued the downregulation of myelin basic protein (MBP) and improved remyelination at the later stage after rUCCAO. Our data suggest that the recruitment of OPCs, but not the proliferation or differentiation of OPCs, is the only compromised step of remyelination following chronic cerebral ischemia. IL-1Ra treatment and IL-1R1 knockout had no effect on the oligodendrocyte progenitor cell (OPC) proliferation, but did promote the recruitment of newly generated OPCs to the corpus callosum, which can be reversed by compensatory expression of IL-1R1 in the SVZ of IL-1R1 knockout mice. Further, we found that recruited OPCs contribute to oligodendrocyte regeneration and functional recovery. In transwell assays, IL-1β inhibited OPC migration through IL-1R1. Moreover, KdPT which can enter the brain to block IL-1R1 also showed comparable protection when intraperitoneally delivered. Our results suggest that IL-1β during the early stages following chronic cerebral hypoperfusion impedes OPC recruitment via IL-1R1, which inhibits white matter repair and functional recovery. IL-1R1 inhibitors may have potential uses in the treatment of SIVD.

Temporal dynamics of L5 dendrites in medial prefrontal cortex regulate integration versus coincidence detection of afferent inputs

Distinct brain regions are highly interconnected via long-range projections. How this inter-regional communication occurs depends not only upon which subsets of postsynaptic neurons receive input, but also, and equally importantly, upon what cellular subcompartments the projections target. Neocortical pyramidal neurons receive input onto their apical dendrites. However, physiological characterization of these inputs thus far has been exclusively somatocentric, leaving how the dendrites respond to spatial and temporal patterns of input unexplored. Here we used a combination of optogenetics with multisite electrode recordings to simultaneously measure dendritic and somatic responses to afferent fiber activation in two different populations of layer 5 (L5) pyramidal neurons in the rat medial prefrontal cortex (mPFC). We found that commissural inputs evoked monosynaptic responses in both intratelencephalic (IT) and pyramidal tract (PT) dendrites, whereas monosynaptic hippocampal input primarily targeted IT, but not PT, dendrites. To understand the role of dendritic integration in the processing of long-range inputs, we used dynamic clamp to simulate synaptic currents in the dendrites. IT dendrites functioned as temporal integrators that were particularly responsive to dendritic inputs within the gamma frequency range (40-140 Hz). In contrast, PT dendrites acted as coincidence detectors by responding to spatially distributed signals within a narrow time window. Thus, the PFC extracts information from different brain regions through the combination of selective dendritic targeting and the distinct dendritic physiological properties of L5 pyramidal dendrites.

Protective effect of carnosine on subcortical ischemic vascular dementia in mice

AIMS

Recently, we found carnosine protects against N-Methyl-D-Aspartate (NMDA) induced excitotoxicity through a histaminergic pathway. The aim of this study was to determine whether the carnosine-histidine-histamine pathway also played a protective role in subcortical ischemic vascular dementia (SIVD).

METHODS

Adult male mice (C57BL/6 strain) were subjected to right unilateral common carotid arteries occlusion (rUCCAO) and treated with carnosine or histidine. Object recognition test, passive avoidance task, Morris water maze, and immunohistochemical analyses were performed after rUCCAO.

RESULTS

We found that carnosine (200, 500 mg/kg) ameliorated white matter lesion and cognitive impairment evaluated by object recognition test, passive avoidance task, and Morris water maze test after rUCCAO in both wide-type mice and histidine decarboxylase knockout mice, which are lack of endogenous histamine. However, administration of histidine did not show the same effect. The myelin basic protein in the corpus callosum decreased obviously at day 37 after rUCCAO, which was largely reversed by carnosine (200, 500 mg/kg). Carnosine (200, 500 mg/kg) suppressed the activation of microglia and astrocyte as attenuating the elevation of glial fibrillary acidic protein (GFAP) and Iba-1 fluorescent intensity. Moreover, carnosine (200, 500 mg/kg) significantly attenuated the increase in reactive oxygen species generation after rUCCAO.

CONCLUSION

These data suggest that the neuroprotective effect of carnosine on rUCCAO in mice is not dependent on the histaminergic pathway, but may be due to a suppression of reactive oxygen species generation, glia activation, and myelin degeneration.

White matter abnormalities and animal models examining a putative role of altered white matter in schizophrenia

Schizophrenia is a severe mental disorder affecting about 1% of the population worldwide. Although the dopamine (DA) hypothesis is still keeping a dominant position in schizophrenia research, new advances have been emerging in recent years, which suggest the implication of white matter abnormalities in schizophrenia. In this paper, we will briefly review some of recent human studies showing white matter abnormalities in schizophrenic brains and altered oligodendrocyte-(OL-) and myelin-related genes in patients with schizophrenia and will consider abnormal behaviors reported in patients with white matter diseases. Following these, we will selectively introduce some animal models examining a putative role of white matter abnormalities in schizophrenia. The emphasis will be put on the cuprizone (CPZ) model. CPZ-fed mice show demyelination and OLs loss, display schizophrenia-related behaviors, and have higher DA levels in the prefrontal cortex. These features suggest that the CPZ model is a novel animal model of schizophrenia.

A single mild fluid percussion injury induces short-term behavioral and neuropathological changes in the Long-Evans rat: support for an animal model of concussion

Brain concussion is a serious public health concern and is associated with short-term cognitive impairments and behavioral disturbances that typically occur in the absence of significant brain damage. The current study addresses the need to better understand the effects of a mild lateral fluid percussion injury on rat behavior and neuropathology in an animal model of concussion. Male Long-Evans rats received either a single mild fluid percussion injury or a sham-injury, and either a short (24h) or long (4 weeks) post-injury recovery period. After recovery, rats underwent a detailed behavioral analysis consisting of tests for rodent anxiety, cognition, social behavior, sensorimotor function, and depression-like behavior. After testing all rats were sacrificed and brains were examined immunohistochemically with markers for microglia/macrophage activation, reactive astrocytosis, and axonal injury. Injured rats (mean injury force: 1.20 ±.03 atm) displayed significant short-term cognitive impairments in the water maze and significantly more anxiolytic-like behavior in the elevated-plus maze compared to sham controls. Neuropathological analysis of the brains of injured rats showed an acute increase in reactive astrogliosis and activated microglia in cortex and evidence of axonal injury in the corpus callosum. There were no significant long-term effects on any behavioral or neuropathological measure 4 weeks after injury. These short-term behavioral and neuropathological changes are consistent with findings in human patients suffering a brain concussion, and provide further evidence for the use of a single mild lateral fluid percussion injury to study concussion in the rat.

Consequences of different housing conditions on brain morphology in laying hens

The aim of this study was to analyze the impact of physical and social stress on the avian forebrain morphology. Therefore, we used laying hens kept in different housing systems from puberty (approximately 16 weeks old) until the age of 48 weeks: battery cages, small littered ground pen, and free range system. Cell body sizes and catecholaminergic and serotonergic innervation patterns were investigated in brain areas expected to be sensitive to differences in environmental stimulation: hippocampal substructures and the nidopallium caudolaterale (NCL), a functional analogue of the prefrontal cortex. Our analysis shows both structures differing in the affected morphological parameters. Compared to battery cage hens, hens in the free range system developed larger cells in the dorsomedial hippocampus. Only these animals exhibited an asymmetry in the tyrosine hydroxylase density with more fibres in the left dorsomedial hippocampus. We assume that the higher spatial complexity of the free range system is the driving force of these changes. In contrast, in the NCL the housing systems affected only the serotonergic innervation pattern with highest fibre densities in free range hens. Moreover hens of the free range system displayed the worst plumage condition, which most likely is caused by feather pecking causing an altered serotonergic innervation pattern. Considering the remarkable differences between the three housing conditions, their effects on hippocampal structures and the NCL were surprisingly mild. This observation suggests that the adult brain of laying hens displays limited sensitivity to differences in social and physical environment induced post-puberty, which warrants further studies.

Behavioral deficits in the cuprizone-induced murine model of demyelination/remyelination

The neurotoxicant cuprizone has been used extensively to create a mouse model of demyelination. However, the effects on behavior of cuprizone treatment have not been previously reported. We have analyzed the behavioral changes of mice given a diet containing 0.2% cuprizone for 6 weeks followed by 6 weeks of recovery. Behavior was assessed using a range of tests: the functional observation battery, the open-field test and the rota-rod test. Concurrent with the start of demyelination, at 3 and 4 weeks of 0.2% cuprizone treatment, the animals exhibited an increase in central nervous system activity and an inhibited anxiogenic response to the novelty challenge test. At 5 weeks of treatment (the period of maximal demyelination) equilibrium was altered and sensorimotor reactivity was also affected. Further, rota-rod analysis demonstrated that the treated group had poorer motor co-ordination than control animals. This effect was not reversed 6 weeks after cuprizone withdrawal. The animals in the recovery period also exhibited difficulties in the rota-rod progressive learning task. Our results indicate that behavioral deficits follow the course of demyelination-remyelination induced by administration of 0.2% cuprizone, and that some of the changes persist even after 6 weeks on normal diet.