Intermittent individual housing increases survival of newly proliferated cells

Environmental enrichment: a systematic review on the effect of a changing spatial complexity on hippocampal neurogenesis and plasticity in rodents, with considerations for translation to urban and built environments for humans

Introduction

Hippocampal neurogenesis is critical for improving learning, memory, and spatial navigation. Inhabiting and navigating spatial complexity is key to stimulating adult hippocampal neurogenesis (AHN) in rodents because they share similar hippocampal neuroplasticity characteristics with humans. AHN in humans has recently been found to persist until the tenth decade of life, but it declines with aging and is influenced by environmental enrichment. This systematic review investigated the impact of spatial complexity on neurogenesis and hippocampal plasticity in rodents, and discussed the translatability of these findings to human interventions.

Methods

Comprehensive searches were conducted on three databases in English: PubMed, Web of Science, and Scopus. All literature published until December 2023 was screened and assessed for eligibility. A total of 32 studies with original data were included, and the process is reported in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement and checklist.

Results

The studies evaluated various models of spatial complexity in rodents, including environmental enrichment, changes to in-cage elements, complex layouts, and navigational mazes featuring novelty and intermittent complexity. A regression equation was formulated to synthesize key factors influencing neurogenesis, such as duration, physical activity, frequency of changes, diversity of complexity, age, living space size, and temperature.

Conclusion

Findings underscore the cognitive benefits of spatial complexity interventions and inform future translational research from rodents to humans. Home-cage enrichment and models like the Hamlet complex maze and the Marlau cage offer insight into how architectural design and urban navigational complexity can impact neurogenesis in humans. In-space changing complexity, with and without physical activity, is effective for stimulating neurogenesis. While evidence on intermittent spatial complexity in humans is limited, data from the COVID-19 pandemic lockdowns provide preliminary evidence. Existing equations relating rodent and human ages may allow for the translation of enrichment protocol durations from rodents to humans.

The exercise-glucocorticoid paradox: How exercise is beneficial to cognition, mood, and the brain while increasing glucocorticoid levels

Exercise is known to have beneficial effects on cognition, mood, and the brain. However, exercise also activates the hypothalamic-pituitary-adrenal axis and increases levels of the glucocorticoid cortisol (CORT). CORT, also known as the "stress hormone," is considered a mediator between chronic stress and depression and to link various cognitive deficits. Here, we review the evidence that shows that while both chronic stress and exercise elevate basal CORT levels leading to increased secretion of CORT, the former is detrimental to cognition/memory, mood/stress coping, and brain plasticity, while the latter is beneficial. We propose three preliminary answers to the exercise-CORT paradox. Importantly, the elevated CORT, through glucocorticoid receptors, functions to elevate dopamine in the medial prefrontal cortex under chronic exercise but not chronic stress, and the medial prefrontal dopamine is essential for active coping. Future inquiries may provide further insights to promote our understanding of this paradox.

Social isolation prevents exercise-induced proliferation of hippocampal progenitor cells in female rats

Social isolation negatively affects the behavior and health of laboratory rats. Recently, it has been found that social isolation retards exercise-induced neurogenesis in the hippocampal dentate gyrus (DG) of male rats (Stranahan et al. (2006) Nat Neurosci 9:526-533). Since male and female rats react differently to housing changes and exercise opportunities, we investigated whether social isolation would also suppress the exercise-dependent increase in proliferation of dentate gyrus progenitor cells in females. Accordingly, female rats were housed either alone (isolated) or in groups (social) with (exercise) or without (sedentary) the opportunity to run in an exercise wheel. Proliferating progenitor cells were labeled with bromodeoxyuridine (BrdU). As expected, exercise increased the number of BrdU+ cells in socially housed animals. However, isolation prevented this running-induced increase. Our results expand upon previous findings by showing that the female brain is also susceptible to the suppressive effect of social isolation on exercise-induced neurogenesis.

Housing conditions modulate escitalopram effects on antidepressive-like behaviour and brain neurochemistry

Despite limited understanding of the pathophysiology of depression and the underlying mechanisms mediating antidepressant effects, there are several efficient treatments. The anhedonia symptoms of depression are characterized by decreased motivation and drive and imply possible malfunctioning of the mesolimbic dopamine system, whereas cognitive deficits might reflect decreased plasticity in hippocampus. In female Flinders Sensitive Line (FSL) rats, a model of depression, we compared the effects of three long-term antidepressant treatments: voluntary running, escitalopram and the combination of both on antidepressant-like behaviour in the Porsolt swim test (PST), and on regulation of mRNA for dopamine and neuropeptides in striatal dopamine pathways and brain-derived neurotrophic factor (BDNF) in hippocampus. Escitalopram diet attenuated running behaviour in FSL rats but not in non-depressed controls rats. In the PST the running group had increased climbing activity (noradrenergic/dopaminergic response), whereas the combination of escitalopram and running-wheel access increased swimming (serotonergic response). Running elevated mRNA for dynorphin in caudate putamen and BDNF in hippocampus. The combined treatment down-regulated D1 receptor and enkephalin mRNA in accumbens. Escitalopram alone did not affect behaviour or mRNA levels. We demonstrate a novel behavioural effect of escitalopram, i.e. attenuation of running in 'depressed' rats. The antidepressant-like effect of escitalopram was dependent on the presence of a running wheel, but not actual running indicating that the environment influenced the antidepressant effect of escitalopram. Different patterns of mRNA changes in hippocampus and brain reward pathways and responses in the PST by running and escitalopram suggest that antidepressant-like responses by running and escitalopram are achieved by different mechanisms.

Social isolation increases number of newly proliferated cells in hippocampus in female flinders sensitive line rats

Genetic background influences the responsiveness to stress and plays a crucial role in the pathophysiology of depression. In an animal model of depression, Flinders Sensitive Line rats, and Sprague Dawley controls we analyzed if 7 weeks of social isolation of adult animals affect the number of newly proliferated cells in the dentate gyrus or mRNAs of Neuropeptide Y (NPY), the NPY-Y1 receptor, nociceptin, BDNF, and the serotonin 5HT1A and 5HT2A receptors, which are molecules involved in hippocampal plasticity. Since depressive illness more frequently affects women than men, and females seem to respond differently to stressful experiences than males, female rats were used in this study. Bromodeoxyuridine, which is a thymidin analogue that is incorporated into the DNA of newly formed cells, was administered during 9 days to even out the effects of hormonal fluctuations. Social isolation increased the number of newly proliferated Bromodeoxyuridine-immunoreactive cells in the Flinders Sensitive Line rats, whereas it had no impact on the number of cells in the Sprague Dawley strain. Group housed Sprague Dawley rats had a higher expression of BDNF, NPY, and the serotonin 5HT2A receptor mRNA than "depressed" Flinders Sensitive Line. Social isolation downregulated these molecules in Sprague Dawley but not in Flinders Sensitive Line rats thereby eliminating the differences between the two strains. We demonstrate strain and gender specific responses to stress induced regulation of factors important for hippocampal plasticity.

Functional recovery in rats with chronic spinal cord injuries after exposure to an enriched environment

BACKGROUND/OBJECTIVE

The objective of this study was to determine the effect of environmental enrichment on the sensorimotor function of rats with chronic spinal cord injuries.

DESIGN

Adult Sprague-Dawley rats received a contusive injury of moderate severity at vertebral level T8 using a weight-drop device. Three months after injury, 1 randomized group (n = 16) of rats was placed in an enriched environment, whereas the control group (n = 16) remained housed in standard laboratory cages (2/cage).

METHODS

Animals were placed in an enriched environment for 4 weeks beginning at 3 months after injury. The enriched environment consisted of a large cage (5-6 rats/cage) with access to items such as tubes, ramps, and running wheel, with items changed daily.

MAIN OUTCOME MEASURES

Functional evaluation consisted of the open field Basso, Beattie and Bresnahan (BBB) locomotor test and the tests that form the combined behavioral score (CBS). The CBS includes motor score, toe spread, placing, withdrawal, righting, inclined plane, hot plate, and swim tests. Behavioral testing was repeated 7 times before and after the period of intervention.

RESULTS

The group placed in the enriched environment scored significantly better on the BBB (ANOVA repeated-measures, P < 0.01) test and CBS (ANOVA repeated-measures, P < 0.01).

CONCLUSIONS

Environmental enrichment results in significant functional improvement in animals with spinal cord injury even with a substantial delay in initiating treatment after injury. The features of an enriched environment that may be responsible for the improvement include social interactions, exercise, and novel items in an interesting environment. These findings suggest a continued plasticity of the chronically injured rat spinal cord and a possible therapeutic intervention for people with spinal cord injury.

Neither environmental enrichment nor voluntary wheel running enhances recovery from incomplete spinal cord injury in rats

Environmental enrichment and exercise may be neuroprotective or promote recovery after different forms of CNS injury. Here, we tested the possible effects of moderate environmental enrichment and voluntary exercise on the outcome of incomplete spinal cord injury in rats. We provided rats in standard cages with basic environmental enrichment (carton house, nesting material, tube, gnawing sticks). We also analyzed the effect of increased activity by housing spinal-cord-injured rats in cages with or without access to running wheels. In a third experiment, we looked at the possible effect of pre-injury training. In all experiments, a battery of behavior tests were used. Enriched environment provided before, after or both before and after injury did not alter the outcome on any of these tests. Similarly, despite excessive running after injury, no differences in terms of recovery and behavior were found in the running experiment. Similarly, running prior to injury did not significantly decrease the degree of functional deficit caused by the injury. Since there were no effects of further enrichment, above the possible effects of being socially housed, and since exercise did not improve the outcome, we conclude that these forms of increased activity do not render the animals significantly less sensitive to spinal cord injury and do not cause robust improvement when initiated after injury. While these results pose a limit to how helpful environmental and physical training programs may be in rodent impact injury models, they do not contradict the fact that voluntary and guided training can be effective tools in human spinal cord rehabilitation.