Environmental enrichment reverses reduction in glucocorticoid receptor expression in the hippocampus of and improves behavioral responses of anxiety in early malnourished rats

A bibliometric analysis of studies on environmental enrichment spanning 1967-2024: patterns and trends over the years

Environmental Enrichment (EE) has received considerable attention for its potential to enhance cognitive and neurobiological outcomes in animal models. This bibliometric analysis offers a comprehensive evaluation of the EE research spanning from 1967 to 2024, utilizing data extracted from Scopus and analyzed through R and VOSviewer. The volume of publications, citation patterns, and collaborations were systematically reviewed, highlighting important contributions and emerging trends within the field of animal research. Core concepts of EE research are mapped, revealing key themes such as neuroplasticity, cognitive function, and behavioral outcomes. A significant increase in EE research is demonstrated, particularly after the year 2000, reflecting growing scientific and public interest in EE paradigms. This analysis provides insights into the global contributions and collaborative networks that have shaped EE studies over time. The role of EE in advancing the understanding of neurobiological, neurodevelopmental, and neurodegenerative processes is underscored. Influential contributors, leading countries, and high-impact journals in the field of EE are identified, offering a valuable resource for researchers seeking to understand or extend the current knowledge base. The strategic selection of keywords and rigorous data curation methods ensure that the findings accurately reflect the most impactful aspects of EE research in animals. This study serves as an essential reference for future explorations and applications of EE across disciplines. By providing a clear and structured overview of the field, this paper aims to serve as a foundation for ongoing and future research initiatives, encouraging more robust investigations and applications of EE to enhance cognitive and neurological health globally.

Exploring the role of environmental enrichment and early life adversity on emotional development

Early life adversity has been linked with a higher probability of developing behavioral impairments and environmental manipulation is a strategy that may reduce the negative effects of exposure to adversity in early life. Here, we focused on exploring the influence of environmental enrichment (EE) as a protective factor in the context of early life adversity. We hypothesized that 24 hours of maternal deprivation (MD), in the second week of life, could induce anxiety-like behavior alterations and that exposure to EE could induce resilience to these behaviors due to alterations in the serotonergic system. Male Wistar rats were exposed to MD, on postnatal days 11 and 13, and to EE, after weaning. In adulthood, we performed a series of behavioral tests for fear, anxiety, and locomotor activity. We also measured the levels of serotonin in the amygdala and dorsal raphe nucleus. Our results revealed that MD does not impact fear behavior or the levels of serotonin, while EE decreases locomotor activity in a novel environment and enhances exploration in the predator odor test. EE also decreases serotonin in the amygdala and increases its turnover rate levels. Our findings provide insights into the critical timeframe during which stress exposure impacts the development and confirm that exposure to EE has an independent and protective effect for anxiety-like behaviors later in life.

Environmental enrichment enhances the antidepressant effect of ketamine and ameliorates spatial memory deficits in adult rats

Ketamine is a rapid-acting antidepressant associated with various cognitive side effects. To mitigate these side effects while enhancing efficacy, it can be co-administered with other antidepressants. In our study, we adopted a similar strategy by combining ketamine with environmental enrichment, a potent sensory-motor paradigm, in adult male Wistar rats. We divided the animals into four groups based on a combination of housing conditions and ketamine versus vehicle injections. The groups included those housed in standard cages or an enriched environment for 50 days, which encompassed a 13-day-long behavioral testing period. Each group received either two doses of ketamine (20 mg/kg, IP) or saline as a vehicle. We tested the animals in the novel object recognition test (NORT), forced swim test (FST), open field test (OFT), elevated plus maze (EPM), and Morris water maze (MWM), which was followed by ex vivo c-Fos immunohistochemistry. We observed that combining environmental enrichment with ketamine led to a synergistic antidepressant effect. Environmental enrichment also ameliorated the spatial memory deficits caused by ketamine in the MWM. There was enhanced neuronal activity in the habenula of the enrichment only group following the probe trial of the MWM. In contrast, no differential activity was observed in enriched animals that received ketamine injections. The present study showed how environmental enrichment can enhance the antidepressant properties of ketamine while reducing some of its side effects, highlighting the potential of combining pharmacological and sensory-motor manipulations in the treatment of mood disorders.

Increased Hippocampal Glucocorticoid Receptor Expression and Reduced Anxiety-Like Behavior Following Tuina in a Rat Model With Allergic Airway Inflammation

OBJECTIVE

This study aimed to explore the influence mechanism of Tuina on anxiety-like behavior in immature rats with allergic airway inflammation (AAI).

METHODS

A total of 27 Sprague-Dawley male rats (aged ∼5 weeks) were divided randomly into control, AAI, and AAI with Tuina groups (9 rats per group). The anxiety-like behavior was assessed by an open field test and elevated plus-maze test. Allergic airway inflammation was assessed based on the pathological score of the lung, plasma ovalbumin-specific immunoglobulin E, interleukin 4, interleukin 5, and tumor necrosis factor-alpha levels. Glucocorticoid receptor (GR) messenger RNA and protein expression in the hippocampus and lung were detected by polymerase chain reaction and immunohistochemistry, respectively. Meanwhile, corticotropin-releasing hormone (CRH) messenger RNA in the hypothalamus, the plasma levels of adrenocorticotropic hormone and corticosterone were also determined respectively by polymerase chain reaction and enzyme-linked immunosorbent assay for hypothalamic-pituitary-adrenal axis (HPA) function.

RESULTS

The AAI group had obvious anxiety-like behavior and hyperactive HPA axis, along with decreased GR expression in the hippocampus and lung. Following Tuina, AAI and the anxiety-like behavior were efficiently reduced, and the hyperactivity of HPA axis was efficiently inhibited, along with enhanced GR expression in the hippocampus and lung.

CONCLUSION

Glucocorticoid receptor expression in the hippocampus and lung was enhanced, and anxiety-like behavior was reduced following Tuina in rats with AAI.

Behavioral consequences of postnatal undernutrition and enriched environment during later life

In rat models, large litter groups during suckling are used in the study of undernutrition. Large litter sizes are known to promote alterations in memory processes and anxiety-like behavior. Nevertheless, the effect of large litter size on sexual behavior and the reproductive system is still unknown. Environmental enrichment has been reported to (EE) enhance behavior and to correct some of the alterations produced by postnatal undernutrition. We used the Elevated Plus Maze (EPN), Morris Water Maze (MWM), Object Recognition test (OR) and several parameters of sexual behavior to determine the effect of large litter size on rats exposed to enriched and non-enriched environments. Newborn Wistar rats of both sexes were assigned to be suckled under lactation conditions, in litters of 8 pups or 16 pups. The large litter size (16 pups) caused a reduction in weight gain during the lactation period. On PND 45, four experimental groups were established for both sexes: Well-nourished Non-enriched (WN); Well-nourished Enriched (WE); undernourished Non-enriched (UN); Undernourished Enriched (UE). On PND 90, the UN males spent more time in the open arms on EPM. On PND 100, the UE females increased the latency to find the platform in training days (D1-4) in MWM. On probe day (D5) the UE males spent more time in the target quadrants in MWM. On PND 110, irrespective of EE the large litter size had increased the exploration time in both groups (UN) and (UE) in OR test. On PND 120, the performance of sexual behavior was more evident by effect of EE irrespective of the litter size. In conclusion, the large litter size showed no effects on sexual behavior, in contrast, EE has a sharp influence on sexual behavior. Conversely, memory processes and anxiety-like behavior are altered by large litter size.

Handling has an anxiolytic effect that is not affected by the inhibition of the protein kinase C pathway in adult prenatal undernourished male rat offspring

Environmental enrichment (EE) after birth has been reported as an intervention improving the anxiety-like behavior and cognitive deficit due to maternal restraint, foot-shock, or social stress during pregnancy. However, it remains unclear whether EE after birth could benefit the early prenatal undernourished offspring. In this study, we examined the effect of daily handling as a simple EE intervention on the aberrant behavior of prenatally undernourished rats. The male rat offspring exhibited anxiety-like behavior at 9 weeks of age due to maternal food restriction in early pregnancy. Our study shows that the daily handling after weaning has an anxiolytic effect in the prenatally undernourished offspring without affecting the behavior of prenatally well-nourished offspring. Conversely, the concentrations of dopamine, serotonin, norepinephrine, and their metabolites were not altered in the prefrontal cortex by prenatal undernutrition or daily handling after weaning. We investigated whether the anxiolytic effect of daily handling was mediated by the protein kinase C (PKC) pathway using the PKC inhibitor, chelerythrine. The anxiolytic effect of the handling was not canceled by chelerythrine injection in prenatally undernourished offspring, whereas chelerythrine induced an anxiety-like behavior in control rats. Our results suggest that maternal undernutrition in early pregnancy induces an anxiety-like behavior accompanied with a PKC pathway-hyporesponsiveness; however, daily handling ameliorates the anxiety-like behavior through a PKC-independent pathway.

Feed restriction since lactation has reduced anxiety in adult Wistar rats

ABSTRACT Objective To evaluate the effects of macronutrients (protein, carbohydrate and/or lipid) in the diet of young adult (72 days) and adult (182 days) Wistar rats treated ad libitum and with 30% restriction from birth on anxiety in the elevated plus-maze. Methods We used 238 rats treated from birth, composing the groups: Control, Protein, Carbohydrate, Lipid, Carbohydrate and Lipid, Control Restriction, Protein Restriction, Carbohydrate Restriction, Lipid Restriction and Carbohydrate and Lipid Restriction. The animals were weighed at the beginning and at the end of the experiment and tested in the elevated plus-maze. Data were submitted to analysis of variance, followed by the Newman-Keuls Test (p<0.05). Results Among the animals treated ad libitum, the Control, Carbohydrate plus Lipid and Lipid gained more weight than the Carbohydrate and Protein; ad libitum animals gained more weight than those on restriction; among the restrictions, Carbohydrate Restriction rats were the ones that gained less weight. Diet-restricted animals exhibited reduced first-entry latency, greater percentage of entries and time spent, frequency of open arm extremity visits, head dipping (protected and unprotected), and length of stay in the central area of the elevated plus-maze. The animals with 182 days presented greater latency for first entry, reduced frequency of false entries and visits to the ends of the open arms and protected head dipping. Conclusion Food restricted animals, regardless of the macronutrient present in the diet, were less anxious and/or increased their impulsivity and those at 182 days were more anxious and/or with reduced impulsivity.

Early exposure to environmental enrichment modulates the effects of prenatal ethanol exposure upon opioid gene expression and adolescent ethanol intake

Prenatal ethanol exposure (PEE) promotes ethanol consumption in the adolescent offspring accompanied by the transcriptional regulation of kappa opioid receptor (KOR) system genes. This study analysed if environmental enrichment (EE, from gestational day 20 to postnatal day 26) exerts protective effects upon PEE-modulation of gene expression, ethanol intake and anxiety responses. Pregnant rats were exposed to PEE (0.0 or 2.0 g/kg ethanol, gestational days 17-20) and subsequently the dam and offspring were reared under EE or standard conditions. PEE upregulated KOR mRNA levels in amygdala (AMY) and prodynorphin (PDYN) mRNA levels in ventral tegmental area (VTA) with the latter effect associated with lower DNA methylation at the gene promoter. These effects were normalized by exposure to EE. PEE modulated BDNF mRNA levels in VTA and Nucleus accumbens (AcbN), and EE mitigated the changes in AcbN. EE induced a protective effect on ethanol intake and preference, an effect more noticeable in males than in females, and in prenatal vehicle-treated (PV) than in PEE rats. The male offspring drank significantly less ethanol than the female offspring. The latter result suggests that the protective effect of EE on ethanol drinking may only emerge at lower levels of drinking. In the dams, PEE induced an upregulation of PDYN and KOR in AcbN. PDYN gene expression was normalized by exposure to EE. These results suggest that EE is a promising treatment to inhibit the effects of PEE. The results confirm that PEE effects are mediated by alterations in the transcriptional regulation of KOR system genes.

Basic fibroblast growth factor increased glucocorticoid receptors in cortical neurons through MAP kinase pathway

Prolonged and intense stress chronically increases blood concentration of glucocorticoids, which in turn causes downregulation of glucocorticoid receptor (GR) in the central nervous system (CNS). This process has been suggested to be involved in the pathogenesis of major depressive disorder (MDD). Here, we found that basic fibroblast growth factor (bFGF) increased the expression of GR in the rat cerebral cortex and cultured cortical neurons and restored the reduced GR expression caused by glucocorticoid exposure. Among intracellular signaling pathways stimulated by bFGF, extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway was responsible for the upregulation of GR. The bFGF-induced GR was functional as a transcription factor to enhance transcription of a target gene. Because high stress augments bFGF levels in the brain, it is likely that bFGF plays a compensating role for reduced GR expression after stress and thus should be studied as a therapeutic target for the treatment of MDD.

Nutritional status and social behavior in preschool children: the mediating effects of neurocognitive functioning

Early malnutritional status has been associated with reduced cognitive ability in childhood. However, there are almost no studies on the effect of malnutrition on positive social behavior, and no tests of possible mediating mechanisms. This study tests the hypothesis that poor nutritional status is associated with impaired social functioning in childhood, and that neurocognitive ability mediates this relationship. We assessed 1553 male and female 3-year-olds from a birth cohort on measures of malnutrition, social behavior and verbal and spatial neurocognitive functions. Children with indicators of malnutrition showed impaired social behavior (p < .0001) as compared with children in the control group with adequate nutritional status. These associations even persisted after controlling for social adversity and parental education. Findings were not moderated by gender or ethnicity, and there was no interaction effect with parental education. A dose-response relationship was observed between degree of malnutrition and degree of social behavior, with increased malnutrition associated with more impaired social behavior. Neurocognitive ability was found to mediate the nutrition-social behavior relationship. The mediation effect of neurocognitive functioning suggests that poor nutrition negatively impacts brain areas that play important roles in developing positive social behavior. Findings suggest that reducing poor nutrition, alternatively promoting good nutrition, may help promote positive social behavior in early childhood during a critical period for social and neurocognitive development, with implications for improving positive health in adulthood.

Tactile stimulation partially prevents neurodevelopmental changes in visual tract caused by early iron deficiency

Iron deficiency has a critical impact on maturational mechanisms of the brain and the damage related to neuroanatomical parameters is not satisfactorily reversed after iron replacement. However, emerging evidence suggest that enriched early experience may offer great therapeutic efficacy in cases of nutritional disorders postnatally, since the brain is remarkably responsive to its interaction with the environment. Given the fact that tactile stimulation (TS) treatment has been previously shown to be an effective therapeutic approach and with potential application to humans, here we ask whether exposure to TS treatment, from postnatal day (P) 1 to P32 for 3min/day, could also be employed to prevent neuroanatomical changes in the optic nerve of rats maintained on an iron-deficient diet during brain development. We found that iron deficiency changed astrocyte, oligodendrocyte, damaged fiber, and myelinated fiber density, however, TS reversed the iron-deficiency-induced alteration in oligodendrocyte, damaged fiber and myelinated fiber density, but failed to reverse astrocyte density. Our results suggest that early iron deficiency may act by disrupting the timing of key steps in visual system development thereby modifying the normal progression of optic nerve maturation. However, optic nerve development is sensitive to enriching experiences, and in the current study we show that this sensitivity can be used to prevent damage from postnatal iron deficiency during the critical period.

Short-term environmental enrichment is sufficient to counter stress-induced anxiety and associated structural and molecular plasticity in basolateral amygdala

Moderate levels of anxiety enable individual animals to cope with stressors through avoidance, and could be an adaptive trait. However, repeated stress exacerbates anxiety to pathologically high levels. Dendritic remodeling in the basolateral amygdala is proposed to mediate potentiation of anxiety after stress. Similarly, modulation of brain-derived neurotrophic factor is thought to be important for the behavioral effects of stress. In the present study, we investigate if relatively short periods of environmental enrichment in adulthood can confer resilience against stress-induced anxiety and concomitant changes in neuronal arborisation and brain derived neurotrophic factor within basolateral amygdala. Two weeks of environmental enrichment countermanded the propensity of increased anxiety following chronic immobilization stress. Environmental enrichment concurrently reduced dendritic branching and spine density of projection neurons of the basolateral amygdala. Moreover, stress increased abundance of BDNF mRNA in the basolateral amygdala in agreement with the dendritic hypertrophy post-stress and role of BDNF in promoting dendritic arborisation. In contrast, environmental enrichment prevented stress-induced rise in the BDNF mRNA abundance. Gain in body weights and adrenal weights remained unaffected by exposure to environmental enrichment. These observations suggest that a short period of environmental enrichment can provide resilience against maladaptive effects of stress on hormonal, neuronal and molecular mediators of anxiogenesis.