Environmental enrichment improves long-term memory impairment and aberrant synaptic plasticity by BDNF/TrkB signaling in nerve-injured mice

Maintaining a Dynamic Brain: A Review of Empirical Findings Describing the Roles of Exercise, Learning, and Environmental Enrichment in Neuroplasticity from 2017-2023

Brain plasticity, also termed neuroplasticity, refers to the brain's life-long ability to reorganize itself in response to various changes in the environment, experiences, and learning. The brain is a dynamic organ capable of responding to stimulating or depriving environments, activities, and circumstances from changes in gene expression, release of neurotransmitters and neurotrophic factors, to cellular reorganization and reprogrammed functional connectivity. The rate of neuroplastic alteration varies across the lifespan, creating further challenges for understanding and manipulating these processes to benefit motor control, learning, memory, and neural remodeling after injury. Neuroplasticity-related research spans several decades, and hundreds of reviews have been written and published since its inception. Here we present an overview of the empirical papers published between 2017 and 2023 that address the unique effects of exercise, plasticity-stimulating activities, and the depriving effect of social isolation on brain plasticity and behavior.

Brain-Derived Neurotrophic Factor, Nociception, and Pain

This article examines the involvement of the brain-derived neurotrophic factor (BDNF) in the control of nociception and pain. BDNF, a neurotrophin known for its essential role in neuronal survival and plasticity, has garnered significant attention for its potential implications as a modulator of synaptic transmission. This comprehensive review aims to provide insights into the multifaceted interactions between BDNF and pain pathways, encompassing both physiological and pathological pain conditions. I delve into the molecular mechanisms underlying BDNF's involvement in pain processing and discuss potential therapeutic applications of BDNF and its mimetics in managing pain. Furthermore, I highlight recent advancements and challenges in translating BDNF-related research into clinical practice.

Chronic Pain-Related Cognitive Deficits: Preclinical Insights into Molecular, Cellular, and Circuit Mechanisms

Cognitive impairment is a common comorbidity of chronic pain, significantly disrupting patients' quality of life. Despite this comorbidity being clinically recognized, the underlying neuropathological mechanisms remain unclear. Recent preclinical studies have focused on the fundamental mechanisms underlying the coexistence of chronic pain and cognitive decline. Pain chronification is accompanied by structural and functional changes in the neural substrate of cognition. Based on the developments in electrophysiology and optogenetics/chemogenetics, we summarized the relevant neural circuits involved in pain-induced cognitive impairment, as well as changes in connectivity and function in brain regions. We then present the cellular and molecular alternations related to pain-induced cognitive impairment in preclinical studies, mainly including modifications in neuronal excitability and structure, synaptic plasticity, glial cells and cytokines, neurotransmitters and other neurochemicals, and the gut-brain axis. Finally, we also discussed the potential treatment strategies and future research directions.

Alleviating Neonatal Intensive Care Unit Stress: A Chinese Medicine Approach in Neonatal Rats

Background

Premature infants are exposed to numerous stressors in neonatal intensive care unit (NICU) during a crucial period for brain development; this period exerts long-term influences on cognitive and behavioral development.

Aims

To evaluate the effect of NICU-related stress on neonatal rat pups and explore the effect of Chinese medicine treatment (CMT).

Methods

Sixty male rat pups were randomly assigned to three groups: the control group, the NICU group (NICU-related stress), and the CMT group (NICU-related stress plus CMT). All stressors and interventions were administered from 0 to 7 days after birth. Body weight, serum corticosterone levels, and behavior in the open field (OF) test, elevated plus maze (EPM) test, sucrose preference test, and Morris water maze (MWM) test were recorded, and blood samples were collected at five different time points (T0, T1, T2, T3, and T4).

Results

The body weights of rats in the CMT and control groups were heavier than those in the NICU group in both early life and adulthood (P < 0.05). Serum corticosterone levels significantly differed with time (except T0 vs. T1 and T3 vs. T4) but did not significantly differ among the three groups (F = 0.441, P = 0.894). Regardless of age, spatial memory and anxiety-like and depression-like behavior did not differ among the three groups.

Conclusion

NICU-related stress exerted a long-term effect on rat growth and development but did not affect spatial memory, anxiety-like behavior, depression-like behavior, or serum corticosterone levels. CMT alleviated the impact of NICU-related stress on rats and promoted the growth and development of neonatal rats.

Repeated (S)-ketamine administration ameliorates the spatial working memory impairment in mice with chronic pain: role of the gut microbiota-brain axis

Chronic pain is commonly linked with diminished working memory. This study explores the impact of the anesthetic (S)-ketamine on spatial working memory in a chronic constriction injury (CCI) mouse model, focusing on gut microbiome. We found that multiple doses of (S)-ketamine, unlike a single dose, counteracted the reduced spontaneous alteration percentage (%SA) in the Y-maze spatial working memory test, without affecting mechanical or thermal pain sensitivity. Additionally, repeated (S)-ketamine treatments improved the abnormal composition of the gut microbiome (β-diversity), as indicated by fecal 16S rRNA analysis, and increased levels of butyrate, a key gut - brain axis mediator. Protein analysis showed that these treatments also corrected the upregulated histone deacetylase 2 (HDAC2) and downregulated brain-derived neurotrophic factor (BDNF) in the hippocampi of CCI mice. Remarkably, fecal microbiota transplantation from mice treated repeatedly with (S)-ketamine to CCI mice restored %SA and hippocampal BDNF levels in CCI mice. Butyrate supplementation alone also improved %SA, BDNF, and HDAC2 levels in CCI mice. Furthermore, the TrkB receptor antagonist ANA-12 negated the beneficial effects of repeated (S)-ketamine on spatial working memory impairment in CCI mice. These results indicate that repeated (S)-ketamine administration ameliorates spatial working memory impairment in CCI mice, mediated by a gut microbiota - brain axis, primarily through the enhancement of hippocampal BDNF - TrkB signaling by butyrate.

Emphasizing roles of BDNF promoters and inducers in Alzheimer's disease for improving impaired cognition and memory

Brain-derived neurotrophic factor (BDNF) is a crucial neurotrophic factor adding to neurons' development and endurance. The amount of BDNF present in the brain determines susceptibility to various neurodegenerative diseases. In Alzheimer's disease (AD), often it is seen that low levels of BDNF are present, which primarily contributes to cognition deficit by regulating long-term potentiation (LTP) and synaptic plasticity. Molecular mechanisms underlying the synthesis, storage and release of BDNF are widely studied. New molecules are found, which contribute to the signal transduction pathway. Two important receptors of BDNF are TrkB and p75NTR. When BDNF binds to the TrkB receptor, it activates three main signalling pathways-phospholipase C, MAPK/ERK, PI3/AKT. BDNF holds an imperative part in LTP and dendritic development, which are essential for memory formation. BDNF supports synaptic integrity by influencing LTP and LTD. This action is conducted by modulating the glutamate receptors; AMPA and NMDA. This review paper discusses the aforesaid points along with inducers of BDNF. Drugs and herbals promote neuroprotection by increasing the hippocampus' BDNF level in various disease-induced animal models for neurodegeneration. Advancement in finding pertinent molecules contributing to the BDNF signalling pathway has been discussed, along with the areas that require further research and study.

Enriched environment rescues neonatal pain induced cognitive deficits and the impaired hippocampal synaptic plasticity later in life

Although extensive and untreated pain that occurs during a critical developmental window may impair cognition later in life, environmental interventions at early might promote. However, the underlying mechanism is poorly understood. Our current study utilized a rat model of "repetitive needle pricks" from the day of birth (P0) to postnatal day 7 (P7) to mimic the painful experience of preterm neonates in the Neonatal intensive care unit (NICU). Enriched environment (EE) during development period (from P15 to P70) was implemented as a nonpharmacological intervention approach. Electrophysiological recording, behavioral tests and biochemical analysis were performed after the end of EE (between P71 and P80). Results showed neonatal repetitive pain resulted in a reduction in mechanical withdrawal thresholds by the von Frey test in P70 (P < 0.001). Furthermore, neonatal repetitive pain impaired spatial learning and memory (P < 0.05) and even led to dysfunction in fear memory (P < 0.01). In contrast, EE rescued neonatal pain induced cognitive deficits and normalized hippocampal long-term potentiation in rats exposed to neonatal pain (P < 0.05). The beneficial effect of EE might be the improvements in hippocampal synaptic plasticity via up-regulating neurotrophic factors and N-methyl-D-aspartate (NMDA) receptors in the hippocampus. Our findings provide evidence that early environmental intervention might be a safe strategy to overcome neurodevelopmental abnormalities in preterm infants who experienced multiple procedural painful events during the early critical period. This article is protected by copyright. All rights reserved.

Neurotrophin Signaling Impairment by Viral Infections in the Central Nervous System

Neurotrophins, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3), NT-4, and NT-5, are proteins involved in several important functions of the central nervous system. The activation of the signaling pathways of these neurotrophins, or even by their immature form, pro-neurotrophins, starts with their recognition by cellular receptors, such as tropomyosin receptor kinase (Trk) and 75 kD NT receptors (p75NTR). The Trk receptor is considered to have a high affinity for attachment to specific neurotrophins, while the p75NTR receptor has less affinity for attachment with neurotrophins. The correct functioning of these signaling pathways contributes to proper brain development, neuronal survival, and synaptic plasticity. Unbalanced levels of neurotrophins and pro-neurotrophins have been associated with neurological disorders, illustrating the importance of these molecules in the central nervous system. Furthermore, reports have indicated that viruses can alter the normal levels of neurotrophins by interfering with their signaling pathways. This work discusses the importance of neurotrophins in the central nervous system, their signaling pathways, and how viruses can affect them.

Environmental Enrichment Sex-dependently Rescues Memory Impairment in FABP5 KO Mice Not Mediated by Brain-Derived Neurotrophic Factor

Fatty acid-binding proteins (FABPs) are intracellular carriers of bioactive lipids and play a role in the trafficking of endocannabinoids as well as polyunsaturated fatty acids. Mice lacking the FABP5 gene have memory impairments. Environmental enrichment is a potent manipulation known to rescue or improve memory performance. The extent to which the memory impairments in FABP5 knockout (KO) mice can be rescued or improved through environmental conditions remains to be understood. To address this, we raised wild type (WT) and FABP5 KO mice in either socially isolated or environmental enrichment conditions during adolescence. Once in adulthood, mice were tested for Novel Object Recognition (NOR), T-maze, and Morris Water Maze (MWM) to evaluate memory performance. Mice were then euthanized to assess hippocampal brain-dervied neurotrophic factor (BDNF) concentrations. MWM results showed that male FABP5 KO mice performed worse compared to WT counterparts. Male and female mice raised in an enriched environment improved performance regardless of genotype. Results on the NOR test showed that male FABP5 KO mice displayed lower object recognition compared to WT counterparts across both environments. No differences of genotype or environment were seen in female mice. T maze findings showed that impaired performance in socially isolated FABP5 KO mice. Adolescent environmental enrichment rescued this deficit in male, but not female, FABP5 KO mice. Lastly, environmental enrichment increased hippocampal BDNF levels in male WT mice only. Our results corroborate the previously observed role of the FABP5 gene on memory performance and identify an important interaction with the environment during adolescence.

Aerobic exercise as a promising nonpharmacological therapy for the treatment of substance use disorders

Despite the prevalence and public health impact of substance use disorders (SUDs), effective long-term treatments remain elusive. Aerobic exercise is a promising, nonpharmacological treatment currently under investigation as a strategy for preventing drug relapse. Aerobic exercise could be incorporated into the comprehensive treatment regimens for people with substance abuse disorders. Preclinical studies of SUD with animal models have shown that aerobic exercise diminishes drug-seeking behavior, which leads to relapse, in both male and female rats. Nevertheless, little is known regarding the effects of substance abuse-induced cellular and physiological adaptations believed to be responsible for drug-seeking behavior. Accordingly, the overall goal of this review is to provide a summary and an assessment of findings to date, highlighting evidence of the molecular and neurological effects of exercise on adaptations associated with SUD.

Enriched environment remedies cognitive dysfunctions and synaptic plasticity through NMDAR-Ca2+-Activin A circuit in chronic cerebral hypoperfusion rats

Chronic cerebral ischemia (CCI) is one of the critical factors in the occurrence and development of vascular cognitive impairment (VCI). Apoptosis of nerve cells and changes in synaptic activity after CCI are the key factors to induce VCI. Synaptic stimulation up-regulates intraneuronal Ca2+ level through N-methyl-D-aspartic acid receptor (NMDAR) via induction of the activity-regulated inhibitor of death (AID) expression to produce active-dependent neuroprotection. Moreover, the regulation of synaptic plasticity could improve cognition and learning ability. Activin A (ActA), an exocrine protein of AID, can promote NMDAR phosphorylation and participate in the regulation of synaptic plasticity. We previously found that exogenous ActA can improve the cognitive function of rats with chronic cerebral ischemia and enhance the oxygenated glucose deprivation of intracellular Ca2+ level. In addition to NMDAR, the Wnt pathway is critical in the positive regulation of LTP through activation or inhibition. It plays an essential role in synaptic transmission and activity-dependent synaptic plasticity. The enriched environment can increase ActA expression during CCI injury. We speculated that the NMDAR-Ca2+-ActA signal pathway has a loop-acting mode, and the environmental enrichment could improve chronic cerebral ischemia cognitive impairment via NMDAR-Ca2+-ActA, Wnt/β-catenin pathway is involved in this process. For the hypothesis verification, this study intends to establish chronic cerebral hypoperfusion (CCH) rat model, explore the improvement effect of enriched environment on VCI, detect the changes in plasticity of synaptic morphology and investigate the regulatory mechanism NMDAR-Ca2+-ActA-Wnt/β-catenin signaling loop, providing a therapeutic method for the treatment of CCH.

Inhibition of monoamine oxidase attenuates social defeat-induced memory impairment in goldfish, (Carassius auratus): A possible involvement of synaptic proteins and BDNF

Social defeat (SD) has been implicated in different modulatory effects of physiology and behaviour including learning and memory. We designed an experiment to test the functional role of monoamine oxidase (MAO) in regulation of synaptic transmission, synaptic plasticity and memory in goldfish Carassius auratus. To test this, individuals were divided into three groups: (i) control; (ii) social defeat (SD) group (individuals were subjected to social defeat for 10 min by Pseudotropheus demasoni) and (iii) SD + MAO inhibitor pre-treated group. All experimental groups were subjected to spatial learning and then memory. Our results suggest that SD affects a spatial learning and memory, whereas SD exerts no influence on MAOI pre-treated group. In addition, we noted that the expression of monoamine oxidase-A (MAO-A) was up-regulated and level of serotonin (5-hydroxytryptamine; 5-HT), expression of serotonin transporter (SERT), synaptophysin (SYP), synaptotagmin -1 (SYT-1), N-methyl-D-asparate (NMDA) receptors subunits (NR2A and NR2B), postsynaptic density-95 (PSD-95) and brain-derived neurotrophic factor (BDNF) were reduced by SD, while MAOIs pretreatment protects the effect of SD. Taken together, our results suggest that MAO is an essential component in the serotonergic system that finely tunes the level of 5-HT, which further regulates the molecules involving in synaptic transmission, synaptic plasticity and memory.

Entacapone promotes hippocampal neurogenesis in mice

Entacapone, a catechol-O-methyltransferase inhibitor, can strengthen the therapeutic effects of levodopa on the treatment of Parkinson’s disease. However, few studies are reported on whether entacapone can affect hippocampal neurogenesis in mice. To investigate the effects of entacapone, a modulator of dopamine, on proliferating cells and immature neurons in the mouse hippocampal dentate gyrus, 60 mice (7 weeks old) were randomly divided into a vehicle-treated group and the groups treated with 10, 50, or 200 mg/kg entacapone. The results showed that 50 and 200 mg/kg entacapone increased the exploration time for novel object recognition. Immunohistochemical staining results revealed that after entacapone treatment, the numbers of Ki67-positive proliferating cells, doublecortin-positive immature neurons, and phosphorylated cAMP response element-binding protein (pCREB)-positive cells were significantly increased. Western blot analysis results revealed that treatment with tyrosine kinase receptor B (TrkB) receptor antagonist significantly decreased the exploration time for novel object recognition and inhibited the expression of phosphorylated TrkB and brain-derived neurotrophic factor (BDNF). Entacapone treatment antagonized the effects of TrkB receptor antagonist. These results suggest that entacapone treatment promoted hippocampal neurogenesis and improved memory function through activating the BDNF-TrkB-pCREB pathway. This study was approved by the Institutional Animal Care and Use Committee of Seoul National University (approval No. SNU-130730-1) on February 24, 2014.

Emotional and cognitive impairments in the peripheral nerve chronic constriction injury model (CCI) of neuropathic pain: A systematic review

BACKGROUND AND OBJECTIVE

Emotional and cognitive impairments are common comorbidities of chronic neuropathic pain that significantly impact the quality of life of patients. While the nociceptive components of the peripheral nerve chronic constriction injury (CCI) animal model have been extensively analyzed, data related to the development of mood and cognitive disorders, and especially its impact on female rats remains fragmented. We systematically reviewed the literature analyzing the methods used to induce and evaluate the development of emotional- and cognitive-like impairments and sex-specific differences in the CCI model.

DATABASES AND DATA TREATMENT

We searched PubMed, Google Scholar and Web of Science from inception to September 30th, 2019, and a total of 44 papers were considered eligible for inclusion. We included animal studies assessing nociception, locomotion, anxious-like, depressive-like and cognitive behaviours after the CCI induction.

RESULTS

The overall quality of the studies was considered moderate to high. Overall, the induction of CCI leads to the development of emotional impairments, namely anxiety- and depressive-like behaviours, as well as cognitive impairments. With the majority of the studies using male subjects, the lack of evidence on female animals prevents the evaluation of sex-specific differences.

CONCLUSIONS

This review supports the development of an anxiodepressive-like phenotype, associated with cognitive impairments, in CCI-induced animals. These results support the use of this animal model for the study of the mechanisms underlying these comorbidities, as well as a screening tool for the development/repurposing of drugs that tackle both the neuropathy-induced nociceptive and emotional impairments, such as tricyclic antidepressants. Importantly, our review also highlights the need for studies performed in female rodents as these are almost non-existent.

Repetitive Pain in Neonatal Male Rats Impairs Hippocampus-Dependent Fear Memory Later in Life

Preterm infants in neonatal intensive care units are inevitably subjected to numerous painful procedures. However, little is known about the consequences of early pain experience on fear memory formation later in life. We hypothesized that exposure to repetitive pain in early life triggered hippocampal synaptic plasticity and resulted in memory deficiency in prepubertal and adult rats. From the day of birth (P0) to postnatal day 7 (P7), neonatal male rat pups were randomly assigned to either needle pricks or tactile touches repetitively every 6 h. Trace fear conditioning was performed on rats on P24-P26 and P87-P89. On P24 and P87, rats were sacrificed for molecular and electrophysiological studies. On P24-26 and P87-89, rats that experienced neonatal needle treatment showed a significant reduction in freezing time in the contextual fear conditioning (P < 0.05) and trace fear conditioning tests (P < 0.05). Moreover, repetitive neonatal procedural pain caused a significant decrease in the magnitude of hippocampal long-term potentiation induced by high-frequency stimulation. Furthermore, rats that experienced neonatal needle treatment demonstrated sustained downregulation of NR1, NR2A, NR2B, and GluR1 expression in the hippocampus. Therefore, neonatal pain is related to deficits in hippocampus-related fear memory later in life and might be caused by impairments in hippocampal synaptic plasticity.

Environmental enrichment improves pain sensitivity, depression-like phenotype, and memory deficit in mice with neuropathic pain: role of NPAS4

Patients suffering from neuropathic pain have a higher incidence of depression and cognitive decline. Although environment enrichment (EE) may be effective in the treatment of neuropathic pain, the precise mechanisms underlying its actions remain determined. The aim of the study was to examine the molecular mechanisms underlying the EE's beneficial effects in mice with neuropathic pain. EE attenuated the pain threshold reduction, depression-like phenotype, and memory deficit in mice after chronic constriction injury (CCI). Furthermore, EE attenuated decreased neurogenesis and increased inflammation in the hippocampus of mice with neuropathic pain after CCI. Moreover, the suppression of adult hippocampal neurogenesis by temozolomide antagonized the beneficial effects of EE on depression-like phenotype and cognitive deficit in the mice with neuropathic pain. In addition, lipopolysaccharide-induced increase in tumor necrosis factor-α (TNF-α) in the hippocampus antagonized the beneficial effects of EE for these behavioral abnormalities in mice with neuropathic pain. Knock-down of NPAS4 (neuronal PAS domain protein 4) in the hippocampus by lentivirus targeting NPAS4 blocked these beneficial effects of EE in the mice with neuropathic pain. These all findings suggest that hippocampal NPAS4 plays a key role in the beneficial effects of EE on the pain sensitivity, depression-like phenotype, and memory deficit in mice with neuropathic pain. Therefore, it is likely that NPAS4 would be a new therapeutic target for perceptional, affective, and cognitive dimensions in patients with chronic pain.