Impact of Environmental Enrichment on Perineuronal Nets in the Prefrontal Cortex following Early and Late Abstinence from Sucrose Self-Administration in Rats

From molecules to behavior: Implications for perineuronal net remodeling in learning and memory

Perineuronal nets (PNNs) are condensed extracellular matrix (ECM) structures found throughout the central nervous system that regulate plasticity. They consist of a heterogeneous mix of ECM components that form lattice-like structures enwrapping the cell body and proximal dendrites of particular neurons. During development, accumulating research has shown that the closure of various critical periods of plasticity is strongly linked to experience-driven PNN formation and maturation. PNNs provide an interface for synaptic contacts within the holes of the structure, generally promoting synaptic stabilization and restricting the formation of new synaptic connections in the adult brain. In this way, they impact both synaptic structure and function, ultimately influencing higher cognitive processes. PNNs are highly plastic structures, changing their composition and distribution throughout life and in response to various experiences and memory disorders, thus serving as a substrate for experience- and disease-dependent cognitive function. In this review, we delve into the proposed mechanisms by which PNNs shape plasticity and memory function, highlighting the potential impact of their structural components, overall architecture, and dynamic remodeling on functional outcomes in health and disease.

Degradation of perineuronal nets in the medial prefrontal cortex promotes extinction and reduces reinstatement of methamphetamine-induced conditioned place preference in female mice

The high rate of relapse to compulsive methamphetamine (MA)-taking and seeking behaviors after abstinence constitutes a major obstacle to the treatment of MA addiction. Perineuronal nets (PNNs), essential components of the extracellular matrix, play a critical role in synaptic function, learning, and memory. Abnormalities in PNNs have been closely linked to a series of neurological diseases, such as addiction. However, the exact role of PNNs in MA-induced related behaviors remains elusive. Here, we established a MA-induced conditioned place preference (CPP) paradigm in female mice and found that the number and average optical density of PNNs increased significantly in the medial prefrontal cortex (mPFC) of mice during the acquisition, extinction, and reinstatement stages of CPP. Notably, the removal of PNNs in the mPFC via chondroitinase ABC (ChABC) before extinction training not only facilitated the extinction of MA-induced CPP and attenuated the relapse of extinguished MA preference but also significantly reduced the activation of c-Fos in the mPFC. Similarly, the ablation of PNNs in the mPFC before reinstatement markedly lessened the reinstatement of MA-induced CPP, which was accompanied by the decreased expression of c-Fos in the mPFC. Collectively, our results provide more evidence for the implication of degradation of PNNs in facilitating extinction and preventing relapse of MA-induced CPP, which indicate that targeting PNNs may be an effective therapeutic option for MA-induced CPP memories.

Effects of adolescent intermittent ethanol exposure on cortical perineuronal net and parvalbumin expression in adulthood mediate behavioral inflexibility

BACKGROUND

Alcohol is commonly consumed by adolescents in a binge-like pattern, which can lead to long-lasting cognitive deficits, including reduced behavioral flexibility. We and others have determined that adolescent intermittent ethanol (AIE) exposure leads to increased number of perineuronal net (PNN) numbers in brain regions that are important for behavioral flexibility. However, whether altered neurochemistry stemming from AIE exposure plays a significant role in reduced behavioral flexibility is unknown.

METHODS

We measured the number and size of parvalbumin expressing (PV+) interneurons and associated PNNs within the orbitofrontal cortex (OFC), prelimbic cortex (PrL), infralimbic cortex (IL), and anterior insular cortex (AIC) of female and male rats following AIE or control exposure and subsequent training on an attentional set-shift task (ASST). We then ran analyses to determine whether AIE-induced changes in PV and PNN measures statistically mediated the AIE-induced behavioral deficit in reversal learning.

RESULTS

We demonstrate that AIE exposure impaired behavioral flexibility on reversal two of the ASST (i.e., recalling the initial learned associations), and led to smaller PV+ cells and increased PNN numbers in the AIC. Interestingly, PNN size and number were not altered in the PrL or IL following AIE exposure, in contrast to prior reports. Mediation analyses suggest that AIE alters behavioral flexibility, at least in part through changes in PV and PNN fluorescent measures in the AIC.

CONCLUSIONS

This study reveals a significant link between AIE exposure, neural alterations, and diminished behavioral flexibility in rats, and highlights a potential novel mechanism comprising changes in PV and PNN measures within the AIC. Future studies should explore the impact of PNN degradation within the AIC on behavioral flexibility.

Virtual Reality Environmental Enrichment Effects on Craving for Cigarette in Smokers

Background: Preclinical studies suggested the exposure to environmental enrichment (EE) as an intervention able to prevent or reduce nicotine-taking and nicotine-seeking behaviors. Virtual reality (VR) may help to test the effects of EE in smokers in a reproducible and feasible manner. Materials and Methods: In the present study, 31 smokers (14 women) were divided into two groups: (1) exposure to a virtual EE (VR-EE) and (2) exposure to a virtual neutral environment (VR-NoEE). Cigarette craving was assessed as basal and evoked, at different timepoints during the session. Behavior activity during VR exposure, mood, and subjective measures were also collected. Results: EE exposure in VR significantly reduced craving scores from basal timepoint. This was not observed in the VR-NoEE group, which significantly increased craving compared with values at neutral scenario. When both groups were exposed to smoking-related VR scenario, the VR-EE group showed an increased craving compared with previous timepoint up to score values not different from those in the VR-NoEE group. A significant positive correlation between basal craving scores and interactive behavior with virtual smoking cues was observed in the VR-NoEE but not in the VR-EE group. Conclusion: These findings suggest that virtual EE might have an inhibitory effect in smokers on basal, but not on evoked cigarette craving. Noteworthily, the interactive activity correlation to craving scores in the VR-NoEE participants was not observed in the VR-EE group, adding further evidence that the enrichment simulation was nonetheless able to modify behavior in the smoking-related scenario.

Perineuronal Net Microscopy: From Brain Pathology to Artificial Intelligence

Perineuronal nets (PNN) are a special highly structured type of extracellular matrix encapsulating synapses on large populations of CNS neurons. PNN undergo structural changes in schizophrenia, epilepsy, Alzheimer's disease, stroke, post-traumatic conditions, and some other brain disorders. The functional role of the PNN microstructure in brain pathologies has remained largely unstudied until recently. Here, we review recent research implicating PNN microstructural changes in schizophrenia and other disorders. We further concentrate on high-resolution studies of the PNN mesh units surrounding synaptic boutons to elucidate fine structural details behind the mutual functional regulation between the ECM and the synaptic terminal. We also review some updates regarding PNN as a potential pharmacological target. Artificial intelligence (AI)-based methods are now arriving as a new tool that may have the potential to grasp the brain's complexity through a wide range of organization levels-from synaptic molecular events to large scale tissue rearrangements and the whole-brain connectome function. This scope matches exactly the complex role of PNN in brain physiology and pathology processes, and the first AI-assisted PNN microscopy studies have been reported. To that end, we report here on a machine learning-assisted tool for PNN mesh contour tracing.

Timing of methamphetamine exposure during adolescence differentially influences parvalbumin and perineuronal net immunoreactivity in the medial prefrontal cortex of female, but not male, rats

INTRODUCTION

Adolescence involves significant reorganization within the medial prefrontal cortex (mPFC), including modifications to inhibitory neurotransmission that may be mediated through parvalbumin (PV) interneurons and their surrounding perineuronal nets (PNNs). These developmental changes, which can result in increased PV neuron activity in adulthood, may be disrupted by drug use resulting in lasting changes in mPFC function and behavior. Methamphetamine (METH), which is a readily available drug used by some adolescents, increases PV neuron activity and could influence the activity-dependent maturational process of these neurons.

METHODS

In the present study, we used male and female Sprague Dawley rats to test the hypothesis that METH exposure influences PV and PNN expression in a sex- and age-specific manner. Rats were injected daily with saline or 3.0 mg/kg METH from early adolescence (EA; 30-38 days old), late adolescence (LA; 40-48 days old), or young adulthood (60-68 days old). One day following exposure, effects of METH on PV cell and PNN expression were assessed using immunofluorescent labeling within the mPFC.

RESULTS

METH exposure did not alter male PV neurons or PNNs. Females exposed in early adolescence or adulthood had more PV expressing neurons while those exposed in later adolescence had fewer, suggesting distinct windows of vulnerability to changes induced by METH exposure. In addition, females exposed to METH had more PNNs and more intense PV neuron staining, further suggesting that METH exposure in adolescence uniquely influences development of inhibitory circuits in the female mPFC.

CONCLUSIONS

This study indicates that the timing of METH exposure, even within adolescence, influences its neural effects in females.

Perineuronal nets are associated with decision making under conditions of uncertainty in female but not male mice

Biological sex influences decision-making processes in significant ways, differentiating the responses animals choose when faced with a range of stimuli. The neurobiological underpinnings that dictate sex differences in decision-making tasks remains an important open question, yet single-sex studies of males form most studies in behavioural neuroscience. Here we used female and male BALB/c mice on two spatial learning and memory tasks and examined the expression of perineuronal nets (PNNs) and parvalbumin interneurons (PV) in regions correlated with spatial memory. Mice underwent the aversive active place avoidance (APA) task or the appetitive trial-unique nonmatching-to-location (TUNL) touchscreen task. Mice in the APA cohort learnt to avoid the foot-shock and no differences were observed on key measures of the task nor in the number and intensity of PNNs and PV. On the delay but not separation manipulation in the TUNL task, females received more incorrect trials and less correct trials compared to males. Furthermore, females in this cohort exhibited higher intensity PNNs and PV cells in the agranular and granular retrosplenial cortex, compared to males. These data show that female and male mice perform similarly on spatial learning tasks. However, sex differences in neural circuitry may underly differences in making decisions under conditions of uncertainty on an appetitive task. These data emphasise the importance of using mice of both sexes in studies of decision-making neuroscience.

Timing of methamphetamine exposure during adolescence differentially influences parvalbumin and perineuronal net immunoreactivity in the medial prefrontal cortex of female, but not male, rats

Adolescence involves significant reorganization within the medial prefrontal cortex (mPFC), including modifications to inhibitory neurotransmission mediated through parvalbumin (PV) interneurons and their surrounding perineuronal nets (PNNs). These developmental changes, which result in increased PV neuron activity in adulthood, may be disrupted by drug use resulting in lasting changes in mPFC function and behavior. Methamphetamine (METH), which is a readily available drug used by some adolescents, increases PV neuron activity and could influence the activity-dependent maturational process of these neurons. In the present study, we used male and female Sprague Dawley rats to test the hypothesis that METH exposure influences PV and PNN expression in a sex- and age-specific manner. Rats were injected daily with saline or 3.0 mg/kg METH from early adolescence (EA; 30-38 days old), late adolescence (LA; 40-48 days old), or young adulthood (60-68 days old). One day following exposure, effects of METH on PV cell and PNN expression were assessed using immunofluorescent labeling within the mPFC. METH exposure did not alter male PV neurons or PNNs. Females exposed in early adolescence or adulthood had more PV expressing neurons while those exposed in later adolescence had fewer, suggesting distinct windows of vulnerability to changes induced by METH exposure. In addition, females exposed to METH had more PNNs and more intense PV neuron staining, further suggesting that METH exposure in adolescence uniquely influences development of inhibitory circuits in the female mPFC. This study indicates that the timing of METH exposure, even within adolescence, influences its neural effects in females.

Dynamic regulation of the extracellular matrix in reward memory processes: a question of time

Substance use disorders are a global health problem with increasing prevalence resulting in significant socioeconomic burden and increased mortality. Converging lines of evidence point to a critical role of brain extracellular matrix (ECM) molecules in the pathophysiology of substance use disorders. An increasing number of preclinical studies highlight the ECM as a promising target for development of novel cessation pharmacotherapies. The brain ECM is dynamically regulated during learning and memory processes, thus the time course of ECM alterations in substance use disorders is a critical factor that may impact interpretation of the current studies and development of pharmacological therapies. This review highlights the evidence for the involvement of ECM molecules in reward learning, including drug reward and natural reward such as food, as well as evidence regarding the pathophysiological state of the brain's ECM in substance use disorders and metabolic disorders. We focus on the information regarding time-course and substance specific changes in ECM molecules and how this information can be leveraged for the development of therapeutic strategies.

Net gain and loss: influence of natural rewards and drugs of abuse on perineuronal nets

Overindulgence, excessive consumption, and a pattern of compulsive use of natural rewards, such as certain foods or drugs of abuse, may result in the development of obesity or substance use disorder, respectively. Natural rewards and drugs of abuse can trigger similar changes in the neurobiological substrates that drive food- and drug-seeking behaviors. This review examines the impact natural rewards and drugs of abuse have on perineuronal nets (PNNs). PNNs are specialized extracellular matrix structures that ensheathe certain neurons during development over the critical period to provide synaptic stabilization and a protective microenvironment for the cells they surround. This review also analyzes how natural rewards and drugs of abuse impact the density and maturation of PNNs within reward-associated circuitry of the brain, which may contribute to maladaptive food- and drug-seeking behaviors. Finally, we evaluate the relatively few studies that have degraded PNNs to perturb reward-seeking behaviors. Taken together, this review sheds light on the complex way PNNs are regulated by natural rewards and drugs and highlights a need for future studies to delineate the molecular mechanisms that underlie the modification and maintenance of PNNs following exposure to rewarding stimuli.

The differential effects of brief environmental enrichment following social isolation in rats

Environmental enrichment (EE) in rodents is associated with a wide range of physiological, affective, and cognitive benefits. A seemingly opposite housing condition, social isolation (SI), is used as a rodent model of stress, negatively affecting several neurobiological mechanisms and hampering cognitive performance. Experimental designs that involve switching between these housing conditions produced mixed results. We evaluated different behavioral and cognitive effects of brief EE following long-term, SI-induced stress. We revealed the influence of enrichment after 30 days of isolation on behavioral despair, anxiety-like behavior, and spatial working memory in adult male Wistar rats and found a substantial anxiolytic effect in the experimental (SI to EE) group. Interestingly, rats exposed to EE also showed increased behavioral despair compared with the control (continuous SI) group. There was no difference in spatial working memory performance at the end of a 5-day water Y-maze (WYM) test. However, the SI to EE animals displayed better memory performance in the first 2 days of the WYM, indicating faster learning. In line with this difference, we recorded significantly more c-Fos-immunopositive (c-Fos+) cells in the retrosplenial and perirhinal cortices of the SI to EE animals. The lateral and basolateral nuclei of the amygdala showed no such difference. These results suggest that brief enrichment following isolation stress leads to differential results in affective and cognitive systems.

Structural and Functional Deviations of the Hippocampus in Schizophrenia and Schizophrenia Animal Models

Schizophrenia is a grave neuropsychiatric disease which frequently onsets between the end of adolescence and the beginning of adulthood. It is characterized by a variety of neuropsychiatric abnormalities which are categorized into positive, negative and cognitive symptoms. Most therapeutical strategies address the positive symptoms by antagonizing D2-dopamine-receptors (DR). However, negative and cognitive symptoms persist and highly impair the life quality of patients due to their disabling effects. Interestingly, hippocampal deviations are a hallmark of schizophrenia and can be observed in early as well as advanced phases of the disease progression. These alterations are commonly accompanied by a rise in neuronal activity. Therefore, hippocampal formation plays an important role in the manifestation of schizophrenia. Furthermore, studies with animal models revealed a link between environmental risk factors and morphological as well as electrophysiological abnormalities in the hippocampus. Here, we review recent findings on structural and functional hippocampal abnormalities in schizophrenic patients and in schizophrenia animal models, and we give an overview on current experimental approaches that especially target the hippocampus. A better understanding of hippocampal aberrations in schizophrenia might clarify their impact on the manifestation and on the outcome of this severe disease.

The gut microbiota of environmentally enriched mice regulates visual cortical plasticity

Exposing animals to an enriched environment (EE) has dramatic effects on brain structure, function, and plasticity. The poorly known "EE-derived signals'' mediating the EE effects are thought to be generated within the central nervous system. Here, we shift the focus to the body periphery, revealing that gut microbiota signals are crucial for EE-driven plasticity. Developmental analysis reveals striking differences in intestinal bacteria composition between EE and standard rearing (ST) mice, as well as enhanced levels of short-chain fatty acids (SCFA) in EE mice. Depleting the microbiota of EE mice with antibiotics strongly decreases SCFA and prevents activation of adult ocular dominance plasticity, spine dynamics, and microglia rearrangement. SCFA treatment in ST mice mimics EE induction of ocular dominance plasticity and microglial remodeling. Remarkably, transferring the microbiota of EE mice to ST recipients activates adult ocular dominance plasticity. Thus, experience-dependent changes in gut microbiota regulate brain plasticity.

Brief Environmental Enrichment exposure enhances contextual-induced sucrose-seeking with and without memory reactivation in rats

Chronic Environmental Enrichment (EE) has been shown to prevent the relapse to addictive behaviours, such as drug-taking and -seeking. Recently, acute EE was shown to reduce cue-induced sucrose-seeking, but its effects on contextual (Cx)-induced sucrose-seeking is still unknown. Here we report the effects of brief EE exposure on Cx-induced sucrose-seeking with and without prior Cx-memory reactivation. Adult male Sprague-Dawley rats were trained to sucrose self-administration associated to a specific conditioning Cx (CxA), followed by a 7-day extinction in a different Cx (CxB). Afterwards, rats were exposed for 22 h to EE, and 1 h later to either i) Cx-induced sucrose-seeking (1 h, renewal without Cx-memory reactivation), ii) or two different Cx-memory reactivations: short (2-min) and long (15-min) CxA-retrieval session (Cx-Ret). In Cx-Ret experiments, CxA-induced sucrose-seeking test (1 h) was done after a subsequent 3-day extinction phase. The assessment of molecular markers of memory reactivation/reconsolidation, Zif-268 and rpS6P, was performed 2 h after Cx-Ret. Brief EE exposure enhanced Cx-induced sucrose-seeking without and with short but not long Cx-retrieval. Moreover, EE impaired discriminative responding at test prior to long, whereas improved it with or without short Cx-retrieval. Different changes in Zif-268 and rpS6P expression induced by short vs. long Cx-Ret were correlated to behavioural data, suggesting the occurrence of different memory processes affected by EE. Our data show that brief EE exposure may differently affect subsequent appetitive relapse depending on the modality of re-exposure to conditioned context. This finding suggests caution and further studies to understand the proper conditions for the use of EE against appetitive and addiction disorders.

The interaction between Environmental Enrichment and fluoxetine in inhibiting sucrose-seeking renewal in mice depend on social living condition

RATIONALE

Several single or combined therapeutic approaches have been developed to treat addiction, however with partial efficacy in preventing relapse. Recently, the living environment has been suggested as a critical intervening factor determining the treatment outcomes. Despite accumulating evidence confirming a role of living conditions in the vulnerability to addictive behaviours, their impact on single or integrative therapeutic strategies preventing relapse is yet to be identified.

OBJECTIVES

Here, we explore the possible interaction between brief Environmental Enrichment (EE) exposure and acute fluoxetine administration in inhibiting sucrose-seeking behaviours, and whether this effect could be affected by living environment.

METHODS

Social and isolated adult male C57BL/6 mice were trained to sucrose self-administration associated to a specific conditioning context (CxA), followed by a 7-day extinction in a different context (CxB). Afterwards, mice were exposed for 22 h to EE and then injected with fluoxetine (10 mg/kg, i.p.) 1 h before a CxA-induced sucrose-seeking test.

RESULTS

Brief EE exposure and acute fluoxetine administration alone inhibited context-induced sucrose-seeking in both housing conditions; however, they exhibited additive properties only in social condition.

CONCLUSIONS

Our data show that social environment may influence the EE/fluoxetine interaction in inhibiting relapse to sucrose. These findings suggest that setting up proper living conditions to boost the efficacy of therapeutic approaches may represent a fundamental strategy to treat addiction disorders.

Functional consequences of postnatal interventions in a mouse model of Fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS) is a leading genetic cause of autism and intellectual disability with cortical hyperexcitability and sensory hypersensitivity attributed to loss and hypofunction of inhibitory parvalbumin-expressing (PV) cells. Our studies provide novel insights into the role of excitatory neurons in abnormal development of PV cells during a postnatal period of inhibitory circuit refinement.

METHODS

To achieve Fragile X mental retardation gene (Fmr1) deletion and re-expression in excitatory neurons during the postnatal day (P)14-P21 period, we generated Cre^CaMKIIa/Fmr1^Flox/y (cOFF) and Cre^CaMKIIa/Fmr1^FloxNeo/y (cON) mice, respectively. Cortical phenotypes were evaluated in adult mice using biochemical, cellular, clinically relevant electroencephalogram (EEG) and behavioral tests.

RESULTS

We found that similar to global Fmr1 KO mice, the density of PV-expressing cells, their activation, and sound-evoked gamma synchronization were impaired in cOFF mice, but the phenotypes were improved in cON mice. cOFF mice also showed enhanced cortical gelatinase activity and baseline EEG gamma power, which were reduced in cON mice. In addition, TrkB phosphorylation and PV levels were lower in cOFF mice, which also showed increased locomotor activity and anxiety-like behaviors. Remarkably, when FMRP levels were restored in only excitatory neurons during the P14-P21 period, TrkB phosphorylation and mouse behaviors were also improved.

CONCLUSIONS

These results indicate that postnatal deletion or re-expression of FMRP in excitatory neurons is sufficient to elicit or ameliorate structural and functional cortical deficits, and abnormal behaviors in mice, informing future studies about appropriate treatment windows and providing fundamental insights into the cellular mechanisms of cortical circuit dysfunction in FXS.

Factors modulating the incubation of drug and non-drug craving and their clinical implications

It was suggested in 1986 that cue-induced cocaine craving increases progressively during early abstinence and remains high during extended periods of time. Clinical evidence now supports this hypothesis and that this increase is not specific to cocaine but rather generalize across several drugs of abuse. Investigators have identified an analogous incubation phenomenon in rodents, in which time-dependent increases in cue-induced drug seeking are observed after abstinence from intravenous drug or palatable food self-administration. Incubation of craving is susceptible to variation in magnitude as a function of biological and/or the environmental circumstances surrounding the individual. During the last decade, the neurobiological correlates of the modulatory role of biological (sex, age, genetic factors) and environmental factors (environmental enrichment and physical exercise, sleep architecture, acute and chronic stress, abstinence reinforcement procedures) on incubation of drug craving has been investigated. In this review, we summarized the behavioral procedures adopted, the key underlying neurobiological correlates and clinical implications of these studies.

Timely N-Acetyl-Cysteine and Environmental Enrichment Rescue Oxidative Stress-Induced Parvalbumin Interneuron Impairments via MMP9/RAGE Pathway: A Translational Approach for Early Intervention in Psychosis

Research in schizophrenia (SZ) emphasizes the need for new therapeutic approaches based on antioxidant/anti-inflammatory compounds and psycho-social therapy. A hallmark of SZ is a dysfunction of parvalbumin-expressing fast-spiking interneurons (PVI), which are essential for neuronal synchrony during sensory/cognitive processing. Oxidative stress and inflammation during early brain development, as observed in SZ, affect PVI maturation. We compared the efficacy of N-acetyl-cysteine (NAC) and/or environmental enrichment (EE) provided during juvenile and/or adolescent periods in rescuing PVI impairments induced by an additional oxidative insult during childhood in a transgenic mouse model with gluthation deficit (Gclm KO), relevant for SZ. We tested whether this rescue was promoted by the inhibition of MMP9/RAGE mechanism, both in the mouse model and in early psychosis (EP) patients, enrolled in a double-blind, randomized, placebo-controlled clinical trial of NAC supplementation for 6 months. We show that a sequential combination of NAC+EE applied after an early-life oxidative insult recovers integrity and function of PVI network in adult Gclm KO, via the inhibition of MMP9/RAGE. Six-month NAC treatment in EP patients reduces plasma sRAGE in association with increased prefrontal GABA, improvement of cognition and clinical symptoms, suggesting similar neuroprotective mechanisms. The sequential combination of NAC+EE reverses long-lasting effects of an early oxidative insult on PVI/perineuronal net (PNN) through the inhibition of MMP9/RAGE mechanism. In analogy, patients vulnerable to early-life insults could benefit from a combined pharmacological and psycho-social therapy.

Environmental Enrichment Induces Meningeal Niche Remodeling through TrkB-Mediated Signaling

Neural precursors (NPs) present in the hippocampus can be modulated by several neurogenic stimuli, including environmental enrichment (EE) acting through BDNF-TrkB signaling. We have recently identified NPs in meninges; however, the meningeal niche response to pro-neurogenic stimuli has never been investigated. To this aim, we analyzed the effects of EE exposure on NP distribution in mouse brain meninges. Following neurogenic stimuli, although we did not detect modification of the meningeal cell number and proliferation, we observed an increased number of neural precursors in the meninges. A lineage tracing experiment suggested that EE-induced β3-Tubulin⁺ immature neuronal cells present in the meninges originated, at least in part, from GLAST⁺ radial glia cells. To investigate the molecular mechanism responsible for meningeal reaction to EE exposure, we studied the BDNF-TrkB interaction. Treatment with ANA-12, a TrkB non-competitive inhibitor, abolished the EE-induced meningeal niche changes. Overall, these data showed, for the first time, that EE exposure induced meningeal niche remodeling through TrkB-mediated signaling. Fluoxetine treatment further confirmed the meningeal niche response, suggesting it may also respond to other pharmacological neurogenic stimuli. A better understanding of the neurogenic stimuli modulation for meninges may be useful to improve the effectiveness of neurodegenerative and neuropsychiatric treatments.

Structural and Functional Modulation of Perineuronal Nets: In Search of Important Players with Highlight on Tenascins

The extracellular matrix (ECM) of the brain plays a crucial role in providing optimal conditions for neuronal function. Interactions between neurons and a specialized form of ECM, perineuronal nets (PNN), are considered a key mechanism for the regulation of brain plasticity. Such an assembly of interconnected structural and regulatory molecules has a prominent role in the control of synaptic plasticity. In this review, we discuss novel ways of studying the interplay between PNN and its regulatory components, particularly tenascins, in the processes of synaptic plasticity, mechanotransduction, and neurogenesis. Since enhanced neuronal activity promotes PNN degradation, it is possible to study PNN remodeling as a dynamical change in the expression and organization of its constituents that is reflected in its ultrastructure. The discovery of these subtle modifications is enabled by the development of super-resolution microscopy and advanced methods of image analysis.

Impact of Perineuronal Nets on Electrophysiology of Parvalbumin Interneurons, Principal Neurons, and Brain Oscillations: A Review

Perineuronal nets (PNNs) are specialized extracellular matrix structures that surround specific neurons in the brain and spinal cord, appear during critical periods of development, and restrict plasticity during adulthood. Removal of PNNs can reinstate juvenile-like plasticity or, in cases of PNN removal during early developmental stages, PNN removal extends the critical plasticity period. PNNs surround mainly parvalbumin (PV)-containing, fast-spiking GABAergic interneurons in several brain regions. These inhibitory interneurons profoundly inhibit the network of surrounding neurons via their elaborate contacts with local pyramidal neurons, and they are key contributors to gamma oscillations generated across several brain regions. Among other functions, these gamma oscillations regulate plasticity associated with learning, decision making, attention, cognitive flexibility, and working memory. The detailed mechanisms by which PNN removal increases plasticity are only beginning to be understood. Here, we review the impact of PNN removal on several electrophysiological features of their underlying PV interneurons and nearby pyramidal neurons, including changes in intrinsic and synaptic membrane properties, brain oscillations, and how these changes may alter the integration of memory-related information. Additionally, we review how PNN removal affects plasticity-associated phenomena such as long-term potentiation (LTP), long-term depression (LTD), and paired-pulse ratio (PPR). The results are discussed in the context of the role of PV interneurons in circuit function and how PNN removal alters this function.

Acute, but not longer-term, exposure to environmental enrichment attenuates Pavlovian cue-evoked conditioned approach and Fos expression in the prefrontal cortex in mice

Exposure to environmental enrichment can modify the impact of motivationally relevant stimuli. For instance, previous studies in rats have found that even a brief, acute (~1 day), but not chronic, exposure to environmentally enriched (EE) housing attenuates instrumental lever pressing for sucrose-associated cues in a conditioned reinforcement setup. Moreover, acute EE reduces corticoaccumbens activity, as measured by decreases in expression of the neuronal activity marker "Fos." Currently, it is not known whether acute EE also reduces sucrose seeking and corticoaccumbens activity elicited by non-contingent or "forced" exposure to sucrose cues, which more closely resembles cue exposure encountered in daily life. We therefore measured the effects of acute/intermittent (1 day or 6 day of EE prior to test day) versus chronic (EE throughout conditioning lasting until test day) EE on the ability of a Pavlovian sucrose cue to elicit sucrose seeking (conditioned approach) and Fos expression in the medial prefrontal cortex (mPFC), orbitofrontal cortex (OFC), and nucleus accumbens (NAc) in mice. One day, but not 6 day or chronic EE , reduced sucrose seeking and Fos in the deep layers of the dorsal mPFC. By contrast, 1 day, 6 day, and chronic EE all reduced Fos in the shallow layers of the OFC. None of the EE manipulations modulated NAc Fos expression. We reveal how EE reduces behavioral reactivity to sucrose cues by reducing activity in select prefrontal cortical brain areas. Our work further demonstrates the robustness of EE in its ability to modulate various forms of reward-seeking across species.

Diurnal changes in perineuronal nets and parvalbumin neurons in the rat medial prefrontal cortex

Perineuronal nets (PNNs) surrounding fast-spiking, parvalbumin (PV) interneurons provide excitatory:inhibitory balance, which is impaired in several disorders associated with altered diurnal rhythms, yet few studies have examined diurnal rhythms of PNNs or PV cells. We measured the intensity and number of PV cells and PNNs labeled with Wisteria floribunda agglutinin (WFA) and also the oxidative stress marker 8-oxo-deoxyguanosine (8-oxo-dG) in rat prelimbic medial prefrontal cortex (mPFC) at Zeitgeber times (ZT) ZT0 (lights-on, inactive phase), ZT6 (mid-inactive phase), ZT12 (lights-off, active phase), and ZT18 (mid-active phase). Relative to ZT0, the intensities of PNN and PV labeling were increased in the dark (active) phase compared with the light (inactive) phase. The intensity of 8-oxo-dG was decreased from ZT0 at all times (ZT6,12,18). We also measured GAD 65/67 and vGLUT1 puncta apposed to PV cells with and without PNNs. There were more excitatory puncta on PV cells with PNNs at ZT18 vs. ZT6, but no changes in PV cells without PNNs and no changes in inhibitory puncta. Whole-cell slice recordings in fast-spiking (PV) cells with PNNs showed an increased ratio of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor:N-methyl-D-aspartate receptor (AMPA: NMDA) at ZT18 vs. ZT6. The number of PV cells and PV/PNN cells containing orthodenticle homeobox 2 (OTX2), which maintains PNNs, showed a strong trend toward an increase from ZT6 to ZT18. Diurnal fluctuations in PNNs and PV cells are expected to alter cortical excitatory:inhibitory balance and provide new insights into treatments for diseases impacted by disturbances in sleep and circadian rhythms.

Effects of Social Isolation on Perineuronal Nets in the Amygdala Following a Reward Omission Task in Female Rats

Negative urgency is a facet of impulsivity associated with negative affect and risky behavior that may involve the amygdala. The current study determined if social isolation during development alters negative urgency and c-Fos activity in the basolateral amygdala (BLA). Female Sprague-Dawley rats were raised in an isolated condition (IC), a standard social condition (SC), or an enriched condition (EC) and then were tested for locomotor activity, novelty place preference, and negative urgency using a reward omission task. Following performance on the reward omission task, the brains were analyzed for c-Fos expression in Ca2+/calmodulin kinase II (CaMKII) and calbindin (CB) neurons, as well as in parvalbumin (PV) neurons associated with perineuronal nets (PNNs) in BLA. IC rats exhibited enhanced locomotion compared with both SC and EC rats, as well as enhanced novelty place preference compared with EC rats; only IC rats showed increased responding following omission of an expected reward (negative urgency). Following completion of the reward omission task, IC rats also displayed increased percent of c-Fos neurons in BLA associated with CaMKII, CB, and PV neurons compared with SC and EC rats. In IC rats, c-Fos activation in BLA occurred following the omission of an expected reward. Finally, IC rats displayed reduced PNN intensity associated with PV neurons compared with EC rats, but the percent of these neurons co-expressing c-Fos was greater in IC rats; SC rats were intermediate between IC and EC rats. Negative urgency was observed in IC rats, but not SC or EC rats. While multiple mechanisms are likely involved, this behavioral effect was associated with an isolation-induced increase in activity of excitatory neurons in BLA, as well as decreased PNN intensity surrounding GABAergic neurons in the same region.

Dynamics of perineuronal nets over amphibian metamorphosis

Extracellular matrix materials known as perineuronal nets (PNNs) have been shown to have remarkable consequences for the maturation of neural circuits and stabilization of behavior. It has been proposed that, due to the possibly long-lived biochemical nature of their components, PNNs may be an important substrate by which long-term memories are stored in the central nervous system. However, little empirical evidence exists that shows that PNNs are themselves stable once established. Thus, the question of their temporal dynamics remains unresolved. We leverage the dramatic morphological and behavioral transformations that occur during amphibian metamorphosis to show that PNNs can be highly dynamic in nature. We used established lectin histochemistry to show that PNNs undergo drastic reconstruction during the metamorphic transition. Pre-metamorphic tadpoles have abundant lectin-labeled pericellular material, which we interpret to be PNNs, surrounding neurons throughout the central nervous system. During the metamorphic transition, these structures degrade, and begin to reform in the months following metamorphosis. We show that PNN sizes and staining intensity further change over metamorphosis, suggesting compositional rearrangement. We found PNNs in brain regions with putative homology to regions in mammals with known PNN function, and in other shared regions where PNN function is unknown. Our results suggest that PNNs are susceptible to remodeling by endogenous mechanisms during development. Interpreting the roles of PNNs in circuit maturation and stability requires understanding their temporal relationship with the neurons and synapses they surround. Our work provides further impetus to investigate this relationship in tandem with developmental and behavioral studies.

Comparative analysis of the modulation of perineuronal nets in the prefrontal cortex of rats during protracted withdrawal from cocaine, heroin and sucrose self-administration

Relapse into drug use is a significant problem for people recovering from addiction. The ability that conditioned cues have to reinstate and reinvigorate drug-seeking is potentiated over time (incubation of seeking), posing an additional difficulty for maintaining abstinence. While the prefrontal cortex has been involved in the incubation phenomenon and the extracellular matrix, perineuronal nets (PNNs) in particular, may play a vital role in brain plasticity associated to drug relapse, there are no comparative analyses between different drug classes and natural reinforcers. Here, we compare the effects of early (1 day) and protracted (30 days) withdrawal from to cocaine, heroin and sucrose self-administration on the total density and density per intensity range of PNNs of different territories of the prefrontal cortex of male Lewis rats. Our results show that cocaine self-administration increases the density of PNNs in the dorsal prelimbic, infralimbic and ventral orbitofrontal cortices, while protracted withdrawal reversesthis effect in the dorsal prelimbic cortex. Also, heroin self-administration increases the density of PNNs in the infralimbic cortex and ventral orbitofrontal cortices, but this effect is lost after 30 days of withdrawal in the infralimbic cortex. Finally, the self-administration of sucrose-sweetened water or the protracted withdrawal from this powerful reinforcer does not affect any of the PNN parameters analysed. Our results show that two different drugs of abuse (but not a natural reward) with specific pharmacological and physiological actions, differentially modulate PNNs in specific areas of the rodent prefrontal cortex with potential implications for the incubation of seeking phenomenon.

Sex and region-specific effects of high fat diet on PNNs in obesity susceptible rats

Perineuronal nets (PNNs) are specialized extracellular matrix structures that primarily surround fast-spiking parvalbumin (PV)-containing interneurons within the PFC. They regulate PV neuron function and plasticity to maintain cortical excitatory/inhibitory balance. For example, reductions in PNN intensity are associated with reduced local inhibition and enhanced pyramidal neuron firing. We previously found that exposure to dietary high fat reduced PNN intensity within the PFC of male Sprague-Dawley (SD) rats. However, how high fat affects PNNs in the PFC of females or in obesity-vulnerable vs. -resistant models is unknown. Therefore, we gave male and female SD, selectively bred obesity-prone (OP), and obesity-resistant rats (OR) free access to standard lab chow or 60% high fat for 21 days. We then measured the number of PNN positive cells and PNN intensity (determined by Wisteria floribunda agglutinin [WFA] staining) as well as the number of PV positive neurons using immunohistochemistry. We found sex and region-specific effects of dietary high fat on PNN intensity, in the absence of robust changes in cell number. Effects were comparable in SD and OP but differed in OR rats. Specifically, high fat reduced PNN intensities in male SD and OP rats but increased PNN intensities in female SD and OP rats. In contrast, effects in ORs were opposite, with males showing increases in PNN intensity and females showing a reduction in intensity. Finally, these effects were also region specific, with diet-induced reductions in PNN intensity found in the prelimbic PFC (PL-PFC) and ventral medial orbital frontal cortex (vmOFC) of SD and OP males in the absence of changes in the infralimbic PFC (IL-PFC), and increases in PNN intensity in the IL-PFC of SD and OP females in the absence of changes in other regions. These results are discussed in light of roles PNNs may play in influencing PFC neuronal activity and the differential role of these sub-regions in food-seeking and motivation.

Environmental enrichment reduces food seeking and taking in rats: A review

Environmental enrichment (EE) for rodents is generally defined as providing subjects with an environment enhanced with access to conspecifics, novel and tactile stimuli, and in many preparations, more space. EE exposure, in particular as an "intervention" in adult rodents, decreases food and drug seeking and taking. This review focuses on the reduction of sucrose seeking and taking in rats assessed in operant-based procedures. The operant-based model provides a means to evaluate addiction-related behaviors. Findings using the model might translate to clinically-relevant addiction behaviors directed towards both drugs and food. Both overnight (acute) and one month (chronic) EE effects on behavior are described, including a recent evaluation of the persistence of EE effects following its removal. EE effects on neurobiology related to sucrose seeking using the model are outlined, with a special emphasis on meso-cortico-limbic terminals. Overall, our working hypothesis for how EE reduces sucrose seeking and taking is that EE alters processing of incentive valence. This may also be accompanied by changes in learning and affect. Anti-seeking and anti-taking effects of EE have translational implications for the prevention and treatment of both drug addiction and food-focused behaviors ("food addiction").

Sleep disruption elevates oxidative stress in parvalbumin-positive cells of the rat cerebral cortex

We used a novel automated sleep disruption (SD) apparatus to determine the impact of SD on sleep and molecular markers of oxidative stress in parvalbumin (PV) neurons in the rat prefrontal cortex (PFC). Rats were subjected to two 6 hr SD sessions from zeitgeber time (ZT) 0 to ZT6, one by the gentle handling method and the other by an automated agitator running the length of the rat's home cage floor (a novel SD method). The same rats were later subjected to a 12 hr SD session from ZT0 to ZT12. Sleep was disrupted with both methods, although rats slept less during gentle handling than during the automated condition. Immediately after both SD sessions, rats displayed compensatory sleep characterized by elevated slow-wave activity. We measured in the prelimbic prefrontal cortex (prelimbic PFC; 6 and 12 hr SD) and orbital frontal cortex (12 hr SD) the intensity of the oxidative stress marker, 8-oxo-2'-deoxyguanosine (8-oxo-dG) as well as the staining intensity of PV and the PV cell-associated perineuronal net marker, Wisteria floribunda agglutinin (WFA). In the prelimbic PFC, 6 hr SD increased the intensity of 8-oxo-dG, PV, and WFA. After 12 hr SD, the intensity of 8-oxo-dG was elevated in all neurons. PV intensity was elevated only in neurons colabeled with 8-oxo-dG or WFA, and no changes were found in WFA intensity. We conclude that in association with SD-induced sleep drive, PV neurons in the prelimbic PFC exhibit oxidative stress.

Incubation of food craving in rats: A review

Incubation of food craving is an abstinence-dependent increase in responding for reward-paired cues. Incubation of craving was first reported for rats responding for cocaine-paired cues, and later generalized to several drugs of abuse and for food. Incubation of drug and food craving has been reported in clinical studies as well. Incubation of food craving by rats has been reported for standard chow as well as for high fat and sucrose reinforcers. Parametric and other evaluations of the incubation of food craving reveal manipulations that reduce incubation, including environmental enrichment and pharmacological manipulation of dopamine, glutamate, and endogenous opiates. Several brain regions are likely involved in the effect, including mesolimbic terminals and the central nucleus of the amygdala. Further study of the incubation of food craving could facilitate development of treatments for cravings that precede relapse characteristic of drug and food addictions.

The Neuroscience of Drug Reward and Addiction

Drug consumption is driven by a drug's pharmacological effects, which are experienced as rewarding, and is influenced by genetic, developmental, and psychosocial factors that mediate drug accessibility, norms, and social support systems or lack thereof. The reinforcing effects of drugs mostly depend on dopamine signaling in the nucleus accumbens, and chronic drug exposure triggers glutamatergic-mediated neuroadaptations in dopamine striato-thalamo-cortical (predominantly in prefrontal cortical regions including orbitofrontal cortex and anterior cingulate cortex) and limbic pathways (amygdala and hippocampus) that, in vulnerable individuals, can result in addiction. In parallel, changes in the extended amygdala result in negative emotional states that perpetuate drug taking as an attempt to temporarily alleviate them. Counterintuitively, in the addicted person, the actual drug consumption is associated with an attenuated dopamine increase in brain reward regions, which might contribute to drug-taking behavior to compensate for the difference between the magnitude of the expected reward triggered by the conditioning to drug cues and the actual experience of it. Combined, these effects result in an enhanced motivation to "seek the drug" (energized by dopamine increases triggered by drug cues) and an impaired prefrontal top-down self-regulation that favors compulsive drug-taking against the backdrop of negative emotionality and an enhanced interoceptive awareness of "drug hunger." Treatment interventions intended to reverse these neuroadaptations show promise as therapeutic approaches for addiction.

Reduced perineuronal net expression in Fmr1 KO mice auditory cortex and amygdala is linked to impaired fear-associated memory

Fragile X Syndrome (FXS) is a leading cause of heritable intellectual disability and autism. Humans with FXS show anxiety, sensory hypersensitivity and impaired learning. The mechanisms of learning impairments can be studied in the mouse model of FXS, the Fmr1 KO mouse, using tone-associated fear memory paradigms. Our previous study reported impaired development of parvalbumin (PV) positive interneurons and perineuronal nets (PNN) in the auditory cortex of Fmr1 KO mice. A recent study suggested PNN dynamics in the auditory cortex following tone-shock association is necessary for fear expression. Together these data suggest that abnormal PNN regulation may underlie tone-fear association learning deficits in Fmr1 KO mice. We tested this hypothesis by quantifying PV and PNN expression in the amygdala, hippocampus and auditory cortex of Fmr1 KO mice following fear conditioning. We found impaired tone-associated memory formation in Fmr1 KO mice. This was paralleled by impaired learning-associated regulation of PNNs in the superficial layers of auditory cortex in Fmr1 KO mice. PV cell density decreased in the auditory cortex in response to fear conditioning in both WT and Fmr1 KO mice. Learning-induced increase of PV expression in the CA3 hippocampus was only observed in WT mice. We also found reduced PNN density in the amygdala and auditory cortex of Fmr1 KO mice in all conditions, as well as reduced PNN intensity in CA2 hippocampus. There was a positive correlation between tone-associated memory and PNN density in the amygdala and auditory cortex, consistent with a tone-association deficit. Altogether our studies suggest a link between impaired PV and PNN regulation within specific regions of the fear conditioning circuit and impaired tone memory formation in Fmr1 KO mice.

Examining persistence of acute environmental enrichment-induced anti-sucrose craving effects in rats

A single, overnight (acute) environmental enrichment (EE; a large environment with conspecifics and novel objects) experience robustly decreases sucrose consumption (taking) and responsiveness to sucrose-paired cues (seeking) in rats. Persisting effects of acute EE on sucrose seeking and taking have not yet been identified. In the present study, rats were trained to self-administer a 10% sucrose solution paired with a compound tone + light stimulus for 10 days in 2-h sessions. We then examined the persistence of acute EE effects at reducing sucrose seeking and taking in a 12-h test immediately following acute EE (Exp. 1), or for 7 days with daily 1-h tests immediately following acute EE, or after a 24-h delay (Exp. 2). We also examined the persistence of acute EE effects on sucrose taking in rats responding on a PR schedule in 7 daily sessions following acute EE (Exp. 3). We found that acute EE was effective at reducing responding for both sucrose and a sucrose-paired cue, persisting throughout the 12-h test (Exp. 1). A reduction in sucrose seeking persisted for 24 h and a reduction in sucrose taking persisted for 72 h following acute EE plus a 24-h delay prior to testing (Exp. 2). Decreased PR responding for sucrose was observed following acute EE; this reduction persisted for 48 h (Exp. 3). These findings indicate that acute exposure to EE has persisting effects at reducing sucrose seeking and taking in rats. Acute EE may have translational value as a non-pharmacological intervention to curb sucrose craving.

Perineuronal Nets, Inhibitory Interneurons, and Anxiety-Related Ventral Hippocampal Neuronal Oscillations Are Altered by Early Life Adversity

BACKGROUND

In humans, accumulated adverse experiences during childhood increase the risk of anxiety disorders and attention-deficit/hyperactivity disorder. In rodents, the ventral hippocampus (vHIP) is associated with anxiety regulation, and lesions in this region alter both anxiety-like behavior and activity levels. Neuronal oscillations in the vHIP of the theta frequency range (4-12 Hz) have been implicated in anxious states and derive in part from the activity of inhibitory interneurons in the hippocampus, some of which are enwrapped with perineuronal nets (PNNs), extracellular matrix structures known to regulate plasticity. We sought to investigate the associations among early life stress-induced anxiety and hyperactivity with vHIP neuronal oscillations, inhibitory interneurons, and PNNs in mice.

METHODS

We used repeated maternal separation with early weaning (MSEW) to model accumulated early life adversity in mouse offspring and studied the underlying cellular and electrophysiological changes in the vHIP that are associated with excessive anxiety and hyperactivity.

RESULTS

We found increased anxiety-like behavior and activity levels in MSEW adult males, along with increased theta power and enhanced theta-gamma coupling in the vHIP. MSEW mice showed reduced intensity of parvalbumin as well as increased PNN intensity around parvalbumin-positive interneurons in the vHIP. We further observed that MSEW increased orthodenticle homeobox protein 2, a transcription factor promoting PNN development, in the choroid plexus, where it is produced, as well as in parvalbumin-positive interneurons, where it is sequestered.

CONCLUSIONS

These findings raise the possibility of causal links among parvalbumin-positive interneurons, PNNs, orthodenticle homeobox protein 2, and MSEW-induced anxiety and hyperactivity.

Sucrose Abstinence and Environmental Enrichment Effects on Mesocorticolimbic DARPP32 in Rats

Dopamine- and cAMP-regulated neuronal phosphoprotein 32 kDa (DARPP32) is a signaling molecule that could serve as a molecular switch, promoting or restraining sucrose seeking. We measured DARPP32 and pThr34 DARPP32 in the brains of male Long-Evans rats with a history of sucrose self-administration followed by 1 or 30 days of abstinence and exposure to either overnight (acute) or one month (chronic) environmental enrichment (EE). Brains were extracted following a 1 h cue reactivity test or no exposure to the test environment. Micropunches (prelimbic, infralimbic, and anterior cingulate areas of the medial prefrontal cortex, orbitofrontal cortex, dorsal striatum, nucleus accumbens, and ventral tegmental area) were then processed using Western blot. Abstinence increased, while EE decreased, sucrose seeking. DARPP32 and pThr34 DARPP32 levels were affected by testing, abstinence, and/or EE in most regions. Especially salient results were observed in the nucleus accumbens core, a region associated with relapse behaviors. Both acute and chronic EE reduced DARPP32 in the nucleus accumbens core and acute EE increased the ratio of phosphorylated to total DARPP32. Degree of DARPP32 phosphorylation negatively correlated with sucrose seeking. These findings demonstrate a potential role for DARPP32 in mediating the “anti-craving” effect of EE.

Erasure of striatal chondroitin sulfate proteoglycan-associated extracellular matrix rescues aging-dependent decline of motor learning

Cognitive decline is a feature of aging. Accumulating evidence suggests that the brain extracellular matrix (ECM) is involved in the process of aging-dependent cognitive impairment and neurodegeneration by regulating synaptic neurotransmission and affecting neuroplasticity. Age-related changes in brain structure and cognition are not uniform across the whole brain. Being one of the most vulnerable brain regions to aging-dependent alterations, striatum is integral to several central nervous system functions, such as motor, cognition, and affective control. However, the striatal ECM is largely understudied. We first describe 2 major types of chondroitin sulfate proteoglycan (CSPG)-associated ECM in striatum: perineuronal nets and diffusive ECM. Both types of ECM accumulate in an aging-dependent manner. The accumulation of CSPG-associated ECM correlates with aging-dependent decline in striatum-related cognitive functions, including motor learning and working memory. Enzymatic depletion of CSPG-associated ECM in aged mice via chondroitinase ABC significantly improves motor learning, suggesting that changes in neural ECM CSPGs regulate striatal plasticity. Our study provides a greater understanding of the role of neural ECM underlying striatal plasticity, which is an important precursor to design appropriate therapeutic strategies for normal and pathologic aging.

Complex housing causes a robust increase in dendritic complexity and spine density of medial prefrontal cortical neurons

Prelimbic cortex and infralimbic cortex, parts of the ventromedial prefrontal cortex, are critical brain regions for generating a flexible behavioral response to changing environmental contingencies. This includes the role of these brain structures in the extinction of learned fear, decision making and retrieval of remote memories. Dendritic structure of medial prefrontal cortex neurons retains significant structural plasticity in adulthood. This has been mainly demonstrated as dendritic atrophy and loss of dendritic spines due to chronic stress. It remains unknown if housing condition of the animals itself can cause opposing changes in the dendritic organization. In that backdrop, here we report that short-term increase in complexity of the housing causes a robust increase in complexity of dendritic architecture of prelimbic and infralimbic neurons. This is reflected in the dendritic expansion of prelimbic neurons and increase in spine density of prelimbic and infralimbic neurons. These results suggest that non-invasive changes in the housing environment can be harnessed to study brain reserves for the flexible and species-typical behaviors.

Consumption of a High-Fat Diet Alters Perineuronal Nets in the Prefrontal Cortex

A key factor in the development of obesity is the overconsumption of fatty foods, which, in addition to facilitating weight gain, alters neuronal structures within brain reward circuitry. Our previous work demonstrates that sustained consumption of a high-fat diet (HFD) attenuates spine density in the prefrontal cortex (PFC). Whether HFD promotes structural adaptation among inhibitory cells of the PFC is presently unknown. One structure of interest is the perineuronal net (PNN), a specialized extracellular matrix surrounding, primarily, parvalbumin-containing GABAergic interneurons. PNNs contribute to synaptic stabilization, protect against oxidative stress, regulate the ionic microenvironment within cells, and modulate regional excitatory output. To examine diet-induced changes in PNNs, we maintained rats on one of three dietary conditions for 21 days: ad libitum chow, ad libitum 60% high fat (HF-AL), or limited-access calorically matched high fat (HF-CM), which produced no significant change in weight gain or adiposity with respect to chow controls. The PNN “number” and intensity were then quantified in the prelimbic (PL-PFC), infralimbic (IL-PFC), and ventral orbitofrontal cortex (OFC) using Wisteria floribunda agglutinin (WFA). Our results demonstrated that fat exposure, independent of weight gain, induced a robust decrease in the PNN intensity in the PL-PFC and OFC and a decrease in the PNN number in the OFC.

Novel approaches to alcohol rehabilitation: Modification of stress-responsive brain regions through environmental enrichment

Relapse remains the most prominent hurdle to successful rehabilitation from alcoholism. The neural mechanisms underlying relapse are complex, but our understanding of the brain regions involved, the anatomical circuitry and the modulation of specific nuclei in the context of stress and cue-induced relapse have improved significantly in recent years. In particular, stress is now recognised as a significant trigger for relapse, adding to the well-established impact of chronic stress to escalate alcohol consumption. It is therefore unsurprising that the stress-responsive regions of the brain have also been implicated in alcohol relapse, such as the nucleus accumbens, amygdala and the hypothalamus. Environmental enrichment is a robust experimental paradigm which provides a non-pharmacological tool to alter stress response and, separately, alcohol-seeking behaviour and symptoms of withdrawal. In this review, we examine and consolidate the preclinical evidence that alcohol seeking behaviour and stress-induced relapse are modulated by environmental enrichment, and these are primarily mediated by modification of neural activity within the key nodes of the addiction circuitry. Finally, we discuss the limited clinical evidence that stress-reducing approaches such as mindfulness could potentially serve as adjunctive therapy in the treatment of alcoholism. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".

Distinct roles for hyaluronan in neural stem cell niches and perineuronal nets

Adult neurogenesis in mammals is a tightly regulated process where neural stem cells (NSCs), especially in the subgranular zone (SGZ) of the hippocampal dentate gyrus, proliferate and differentiate into new neurons that form new circuits or integrate into old circuits involved in episodic memory, pattern discrimination, and emotional responses. Recent evidence suggests that changes in the hyaluronan (HA)-based extracellular matrix of the SGZ may regulate neurogenesis by controlling NSC proliferation and early steps in neuronal differentiation. These studies raise the intriguing possibility that perturbations in this matrix, including HA accumulation with aging, could impact adult neurogenesis and cognitive functions, and that alterations to this matrix could be beneficial following insults to the central nervous system that impact hippocampal functions.

Perineuronal nets in subcortical auditory nuclei of four rodent species with differing hearing ranges

Perineuronal nets (PNs) are aggregates of extracellular matrix molecules that surround some neurons in the brain. While PNs occur widely across many cortical areas, subcortical PNs are especially associated with motor and auditory systems. The auditory system has recently been suggested as an ideal model system for studying PNs and their functions. However, descriptions of PNs in subcortical auditory areas vary, and it is unclear whether the variation reflects species differences or differences in staining techniques. Here, we used two staining techniques (one lectin stain and one antibody stain) to examine PN distribution in the subcortical auditory system of four different species: guinea pigs (Cavia porcellus), mice (Mus musculus, CBA/CaJ strain), Long-Evans rats (Rattus norvegicus), and naked mole-rats (Heterocephalus glaber). We found that some auditory nuclei exhibit dramatic differences in PN distribution among species while other nuclei have consistent PN distributions. We also found that PNs exhibit molecular heterogeneity, and can stain with either marker individually or with both. PNs within a given nucleus can be heterogeneous or homogenous in their staining patterns. We compared PN staining across the frequency axes of tonotopically organized nuclei and among species with different hearing ranges. PNs were distributed non-uniformly across some nuclei, but only rarely did this appear related to the tonotopic axis. PNs were prominent in all four species; we found no systematic relationship between the hearing range and the number, staining patterns or distribution of PNs in the auditory nuclei.

Releasing Addiction Memories Trapped in Perineuronal Nets

Drug addiction can be conceptualized at a basic level as maladaptive learning and memory. Addictive substances elicit changes in brain circuitry involved in reward, cognition, and emotional state, leading to the formation and persistence of strong drug-associated memories that lead to craving and relapse. Recently, perineuronal nets (PNNs), extracellular matrix (ECM) structures surrounding neurons, have emerged as regulators of learning, memory, and addiction behaviors. PNNs do not merely provide structural support to neurons but are dynamically remodeled in an experience-dependent manner by metalloproteinases. They function in various brain regions through constituent proteins such as brevican that are implicated in neural plasticity. Understanding the function of PNN components in memory processes may lead to new therapeutic approaches to treating addiction.

A lack of GluN2A-containing NMDA receptors confers a vulnerability to redox dysregulation: Consequences on parvalbumin interneurons, and their perineuronal nets

The GluN2A subunit of NMDA receptors (NMDARs) plays a critical role during postnatal brain development as its expression increases while Glun2B expression decreases. Mutations and polymorphisms in GRIN2A gene, coding for GluN2A, are linked to developmental brain disorders such as mental retardation, epilepsy, schizophrenia. Published data suggest that GluN2A is involved in maturation and phenotypic maintenance of parvalbumin interneurons (PVIs), and these interneurons suffer from a deficient glutamatergic neurotransmission via GluN2A-containing NMDARs in schizophrenia. In the present study, we find that although PVIs and their associated perineuronal nets (PNNs) appear normal in anterior cingulate cortex of late adolescent/young adult GRIN2A KO mice, a lack of GluN2A delays PNN maturation. GRIN2A KO mice display a susceptibility to redox dysregulation as sub-threshold oxidative stress and subtle alterations in antioxidant systems are observed in their prefrontal cortex. Consequently, an oxidative insult applied during early postnatal development increases oxidative stress, decreases the number of parvalbumin-immunoreactive cells, and weakens the PNNs in KO but not WT mice. These effects are long-lasting, but preventable by the antioxidant, N-acetylcysteine. The persisting oxidative stress, deficit in PVIs and PNNs, and reduced local high-frequency neuronal synchrony in anterior cingulate of late adolescent/young adult KO mice, which have been challenged by an early-life oxidative insult, is accompanied with microglia activation. Altogether, these indicate that a lack of GluN2A-containing NMDARs alters the fine control of redox status, leading to a delayed maturation of PNNs, and conferring vulnerability for long-term oxidative stress, microglial activation, and PVI network dysfunction.

Disruption of perineuronal nets increases the frequency of sharp wave ripple events

Hippocampal sharp wave ripples (SWRs) represent irregularly occurring synchronous neuronal population events that are observed during phases of rest and slow wave sleep. SWR activity that follows learning involves sequential replay of training-associated neuronal assemblies and is critical for systems level memory consolidation. SWRs are initiated by CA2 or CA3 pyramidal cells (PCs) and require initial excitation of CA1 PCs as well as participation of parvalbumin (PV) expressing fast spiking (FS) inhibitory interneurons. These interneurons are relatively unique in that they represent the major neuronal cell type known to be surrounded by perineuronal nets (PNNs), lattice like structures composed of a hyaluronin backbone that surround the cell soma and proximal dendrites. Though the function of the PNN is not completely understood, previous studies suggest it may serve to localize glutamatergic input to synaptic contacts and thus influence the activity of ensheathed cells. Noting that FS PV interneurons impact the activity of PCs thought to initiate SWRs, and that their activity is critical to ripple expression, we examine the effects of PNN integrity on SWR activity in the hippocampus. Extracellular recordings from the stratum radiatum of horizontal murine hippocampal hemisections demonstrate SWRs that occur spontaneously in CA1. As compared with vehicle, pre-treatment (120 min) of paired hemislices with hyaluronidase, which cleaves the hyaluronin backbone of the PNN, decreases PNN integrity and increases SWR frequency. Pre-treatment with chondroitinase, which cleaves PNN side chains, also increases SWR frequency. Together, these data contribute to an emerging appreciation of extracellular matrix as a regulator of neuronal plasticity and suggest that one function of mature perineuronal nets could be to modulate the frequency of SWR events.

Region-specific impairments in parvalbumin interneurons in social isolation-reared mice

Many neuropsychiatric disorders show localized dysfunction in specific cortical regions. The mechanisms underlying such region-specific vulnerabilities are unknown. Post-mortem analyses have demonstrated a selective reduction in the expression of parvalbumin (PV) in GABAergic interneurons in the frontal rather than the sensory cortex of patients with neuropsychiatric disorders such as schizophrenia, autism spectrum disorders, and bipolar disorders. PV neurons are surrounded by perineuronal nets (PNNs), and are protected from oxidative stress. Previous studies have shown that the characteristics of PNNs are brain region-specific. Therefore, we hypothesized that PV neurons and PNNs may be targeted in region-specific lesions in the brain. Oxidative stress was induced in mice by rearing them in socially isolated conditions. We systemically examined the distribution of PV neurons and PNNs in the brains of these mice as well as a control group. Our results show that the regions frequently affected in neuropsychiatric disorders show significantly lower PV expression and a lower percentage of PV neurons surrounded by PNNs in the brains of socially isolated mice. These results indicate that PV neurons and PNNs exhibit region-specific vulnerabilities. Our findings may be useful for elucidating the mechanisms underlying region-specific disruption of the brain in neuropsychiatric disorders.