Postnatal Fluoxetine Treatment Alters Perineuronal Net Formation and Maintenance in the Hippocampus

1-Methyltryptophan treatment ameliorates high-fat diet-induced depression in mice through reversing changes in perineuronal nets

Depression and obesity are prevalent disorders with significant public health implications. In this study, we used a high-fat diet (HFD)-induced obese mouse model to investigate the mechanism underlying HFD-induced depression-like behaviors. HFD-induced obese mice exhibited depression-like behaviors and a reduction in hippocampus volume, which were reversed by treatment with an indoleamine 2,3-dioxygenase (IDO) inhibitor 1-methyltryptophan (1-MT). Interestingly, no changes in IDO levels were observed post-1-MT treatment, suggesting that other mechanisms may be involved in the anti-depressive effect of 1-MT. We further conducted RNA sequencing analysis to clarify the potential underlying mechanism of the anti-depressive effect of 1-MT in HFD-induced depressive mice and found a significant enrichment of shared differential genes in the extracellular matrix (ECM) organization pathway between the 1-MT-treated and untreated HFD-induced depressive mice. Therefore, we hypothesized that changes in ECM play a crucial role in the anti-depressive effect of 1-MT. To this end, we investigated perineuronal nets (PNNs), which are ECM assemblies that preferentially ensheath parvalbumin (PV)-positive interneurons and are involved in many abnormalities. We found that HFD is associated with excessive accumulation of PV-positive neurons and upregulation of PNNs, affecting synaptic transmission in PV-positive neurons and leading to glutamate-gamma-aminobutyric acid imbalances in the hippocampus. The 1-MT effectively reversed these changes, highlighting a PNN-related mechanism by which 1-MT exerts its anti-depressive effect.

Linking epileptic phenotypes and neural extracellular matrix remodeling signatures in mouse models of epilepsy

Epilepsies are multifaceted neurological disorders characterized by abnormal brain activity, e.g. caused by imbalanced synaptic excitation and inhibition. The neural extracellular matrix (ECM) is dynamically modulated by physiological and pathophysiological activity and critically involved in controlling the brain's excitability. We used different epilepsy models, i.e. mice lacking the presynaptic scaffolding protein Bassoon at excitatory, inhibitory or all synapse types as genetic models for rapidly generalizing early-onset epilepsy, and intra-hippocampal kainate injection, a model for acquired temporal lobe epilepsy, to study the relationship between epileptic seizures and ECM composition. Electroencephalogram recordings revealed Bassoon deletion at excitatory or inhibitory synapses having diverse effects on epilepsy-related phenotypes. While constitutive Bsn mutants and to a lesser extent GABAergic neuron-specific knockouts (BsnDlx5/6cKO) displayed severe epilepsy with more and stronger seizures than kainate-injected animals, mutants lacking Bassoon solely in excitatory forebrain neurons (BsnEmx1cKO) showed only mild impairments. By semiquantitative immunoblotting and immunohistochemistry we show model-specific patterns of neural ECM remodeling, and we also demonstrate significant upregulation of the ECM receptor CD44 in null and BsnDlx5/6cKO mutants. ECM-associated WFA-binding chondroitin sulfates were strongly augmented in seizure models. Strikingly, Brevican, Neurocan, Aggrecan and link proteins Hapln1 and Hapln4 levels reliably predicted seizure properties across models, suggesting a link between ECM state and epileptic phenotype.

Perinatal serotonergic manipulation shapes anhedonic and cognitive behaviors in a sex- and age-dependent manner: Identification of related biological functions at central and peripheral level

Poor knowledge about psychiatric disorders often results in similar diagnoses for patients with different symptoms, thus limiting the effectiveness of the available medications. As suggested by several lines of evidence, to improve these shortcomings, it is essential to identify biomarkers associated with specific symptoms and to stratify patients into more homogeneous populations taking a further step toward personalized medicine. Here, we aimed to associate specific behavioral phenotypes with specific molecular alterations by employing an animal model based on the pharmacological manipulation of the serotonergic system, which mimics a condition of vulnerability to develop psychiatric disorders. In particular, we treated female and male rats with fluoxetine (FLX 15 mg/kg dissolved in drinking water) during prenatal or early postnatal life, and we evaluated different pathological-like phenotypes (cognitive deficit, anhedonia, and anxiety) by exposing the rats to a battery of behavioral tests during adolescence and adulthood. In addition, we carried out molecular analyses on specific brain areas and in the blood. Our results showed that perinatal FLX administration determined age- and sex-dependent effects, with males being more sensitive to prenatal manipulation and manifesting anhedonic-like behavior and females to early postnatal exposure, exhibiting cognitive deficits and a less anxious phenotype. Furthermore, we identified, peripherally and centrally, biological functions altered by perinatal serotonin modulation regardless of the timing of exposure and sex, and other pathways specific for the pathological-like phenotypes. The results presented here provide new insights into potential biomarkers associated with specific behavioral phenotypes that may be useful for broadening knowledge about psychiatric conditions.

Perineuronal Nets: Subtle Structures with Large Implications

Perineuronal nets (PNNs) are specialized structures of the extracellular matrix that surround the soma and proximal dendrites of certain neurons in the central nervous system, particularly parvalbumin-expressing interneurons. Their appearance overlaps the maturation of neuronal circuits and the closure of critical periods in different regions of the brain, setting their connectivity and abruptly reducing their plasticity. As a consequence, the digestion of PNNs, as well as the removal or manipulation of their components, leads to a boost in this plasticity and can play a key role in the functional recovery from different insults and in the etiopathology of certain neurologic and psychiatric disorders. Here we review the structure, composition, and distribution of PNNs and their variation throughout the evolutive scale. We also discuss methodological approaches to study these structures. The function of PNNs during neurodevelopment and adulthood is discussed, as well as the influence of intrinsic and extrinsic factors on these specialized regions of the extracellular matrix. Finally, we review current data on alterations in PNNs described in diseases of the central nervous system (CNS), focusing on psychiatric disorders. Together, all the data available point to the PNNs as a promising target to understand the physiology and pathologic conditions of the CNS.

MPFC PV+ interneurons are involved in the antidepressant effects of running exercise but not fluoxetine therapy

Depression is a complex psychiatric disorder. Previous studies have shown that running exercise reverses depression-like behavior faster and more effectively than fluoxetine therapy. GABAergic interneurons, including the PV⁺ interneuron subtype, in the medial prefrontal cortex (MPFC) are involved in pathological changes of depression. It was unknown whether running exercise and fluoxetine therapy reverse depression-like behavior via GABAergic interneurons or the PV⁺ interneurons subtype in MPFC. To address this issue, we subjected mice with chronic unpredictable stress (CUS) to a 4-week running exercise or fluoxetine therapy. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that running exercise enriched GABAergic synaptic pathways in the MPFC of CUS-exposed mice. However, the number of PV⁺ interneurons but not the total number of GABAergic interneurons in the MPFC of mice exposed to CUS reversed by running exercise, not fluoxetine therapy. Running exercise increased the relative gene expression levels of the PV gene in the MPFC of CUS-exposed mice without altering other subtypes of GABAergic interneurons. Moreover, running exercise and fluoxetine therapy both significantly improved the length, area and volume of dendrites and the spine morphology of PV⁺ interneurons in the MPFC of mice exposed to CUS. However, running exercise but not fluoxetine therapy improved the dendritic complexity level of PV⁺ interneurons in the MPFC of mice exposed to CUS. In summary, the number and dendritic complexity level of PV⁺ interneurons may be important therapeutic targets for the mechanism by which running exercise reverses depression-like behavior faster and more effectively than fluoxetine therapy.

Enriched environmental exposure reduces the onset of action of the serotonin norepinephrin reuptake inhibitor venlafaxine through its effect on parvalbumin interneurons plasticity in mice

Mood disorders are associated with hypothalamic-pituitary-adrenal axis overactivity resulting from a decreased inhibitory feedback exerted by the hippocampus on this brain structure. Growing evidence suggests that antidepressants would regulate hippocampal excitatory/inhibitory balance to restore an effective inhibition on this stress axis. While these pharmacological compounds produce beneficial clinical effects, they also have limitations including their long delay of action. Interestingly, non-pharmacological strategies such as environmental enrichment improve therapeutic outcome in depressed patients as in animal models of depression. However, whether exposure to enriched environment also reduces the delay of action of antidepressants remains unknown. We investigated this issue using the corticosterone-induced mouse model of depression, submitted to antidepressant treatment by venlafaxine, alone or in combination with enriched housing. We found that the anxio-depressive phenotype of male mice was improved after only two weeks of venlafaxine treatment when combined with enriched housing, which is six weeks earlier than mice treated with venlafaxine but housed in standard conditions. Furthermore, venlafaxine combined with exposure to enriched environment is associated with a reduction in the number of parvalbumin-positive neurons surrounded by perineuronal nets (PNN) in the mouse hippocampus. We then showed that the presence of PNN in depressed mice prevented their behavioral recovery, while pharmacological degradation of hippocampal PNN accelerated the antidepressant action of venlafaxine. Altogether, our data support the idea that non-pharmacological strategies can shorten the onset of action of antidepressants and further identifies PV interneurons as relevant actors of this effect.

Matrix disequilibrium in Alzheimer's disease and conditions that increase Alzheimer's disease risk

Alzheimer's Disease (AD) and related dementias are a leading cause of death globally and are predicted to increase in prevalence. Despite this expected increase in the prevalence of AD, we have yet to elucidate the causality of the neurodegeneration observed in AD and we lack effective therapeutics to combat the progressive neuronal loss. Throughout the past 30 years, several non-mutually exclusive hypotheses have arisen to explain the causative pathologies in AD: amyloid cascade, hyper-phosphorylated tau accumulation, cholinergic loss, chronic neuroinflammation, oxidative stress, and mitochondrial and cerebrovascular dysfunction. Published studies in this field have also focused on changes in neuronal extracellular matrix (ECM), which is critical to synaptic formation, function, and stability. Two of the greatest non-modifiable risk factors for development of AD (aside from autosomal dominant familial AD gene mutations) are aging and APOE status, and two of the greatest modifiable risk factors for AD and related dementias are untreated major depressive disorder (MDD) and obesity. Indeed, the risk of developing AD doubles for every 5 years after ≥ 65, and the APOE4 allele increases AD risk with the greatest risk in homozygous APOE4 carriers. In this review, we will describe mechanisms by which excess ECM accumulation may contribute to AD pathology and discuss pathological ECM alterations that occur in AD as well as conditions that increase the AD risk. We will discuss the relationship of AD risk factors to chronic central nervous system and peripheral inflammation and detail ECM changes that may follow. In addition, we will discuss recent data our lab has obtained on ECM components and effectors in APOE4/4 and APOE3/3 expressing murine brain lysates, as well as human cerebrospinal fluid (CSF) samples from APOE3 and APOE4 expressing AD individuals. We will describe the principal molecules that function in ECM turnover as well as abnormalities in these molecular systems that have been observed in AD. Finally, we will communicate therapeutic interventions that have the potential to modulate ECM deposition and turnover in vivo.

Influence of Chronic Electroconvulsive Seizures on Plasticity-Associated Gene Expression and Perineuronal Nets Within the Hippocampi of Young Adult and Middle-Aged Sprague-Dawley Rats

BACKGROUND

Electroconvulsive seizure therapy is often used in both treatment-resistant and geriatric depression. However, preclinical studies identifying targets of chronic electroconvulsive seizure (ECS) are predominantly focused on animal models in young adulthood. Given that putative transcriptional, neurogenic, and neuroplastic mechanisms implicated in the behavioral effects of chronic ECS themselves exhibit age-dependent modulation, it remains unknown whether the molecular and cellular targets of chronic ECS vary with age.

METHODS

We subjected young adult (2-3 months) and middle-aged (12-13 months), male Sprague Dawley rats to sham or chronic ECS and assessed for despair-like behavior, hippocampal gene expression, hippocampal neurogenesis, and neuroplastic changes in the extracellular matrix, reelin, and perineuronal net numbers.

RESULTS

Chronic ECS reduced despair-like behavior at both ages, accompanied by overlapping and unique changes in activity-dependent and trophic factor gene expression. Although chronic ECS had a similar impact on quiescent neural progenitor numbers at both ages, the eventual increase in hippocampal progenitor proliferation was substantially higher in young adulthood. We noted a decline in reelin⁺ cell numbers following chronic ECS only in young adulthood. In contrast, an age-invariant, robust dissolution of perineuronal net numbers that encapsulate parvalbumin⁺ neurons in the hippocampus were observed following chronic ECS.

CONCLUSION

Our findings indicate that age is a key variable in determining the nature of chronic ECS-evoked molecular and cellular changes in the hippocampus. This raises the intriguing possibility that chronic ECS may recruit distinct, as well as overlapping, mechanisms to drive antidepressant-like behavioral changes in an age-dependent manner.

The tetrapartite synapse in neuropsychiatric disorders: Matrix metalloproteinases (MMPs) as promising targets for treatment and rational drug design

Inflammation and an exacerbated immune response are widely accepted contributing mechanisms to the genesis and progression of major neuropsychiatric disorders. However, despite the impressive advances in understanding the neurobiology of these disorders, there is still no approved drug directly linked to the regulation of inflammation or brain immune responses. Importantly, matrix metalloproteinases (MMPs) comprise a group of structurally related endopeptidases primarily involved in remodeling extracellular matrix (ECM). In the central nervous system (CNS), these proteases control synaptic plasticity and strength, patency of the blood-brain barrier, and glia-neuron interactions through cleaved and non-cleaved mediators. Several pieces of evidence have pointed to a complex scenario of MMPs dysregulation triggered by neuroinflammation. Furthermore, major psychiatric disorders' affective symptoms and neurocognitive abnormalities are related to MMPs-mediated ECM changes and neuroglia activation. In the past decade, research efforts have been directed to broad-spectrum MMPs inhibitors with frustrating clinical results. However, in the light of recent advances in combinatorial chemistry and drug design technologies, specific and CNS-oriented MMPs modulators have been proposed as a new frontier of therapy for regulating ECM properties in the CNS. Therefore, here we aim to discuss the state of the art of MMPs and ECM abnormalities in major neuropsychiatric disorders, namely depression, bipolar disorder, and schizophrenia, the possible neuro-immune interactions involved in this complex scenario of MMPs dysregulation and propose these endopeptidases as promising targets for rational drug design.

Neuronal pericellular baskets: neurotransmitter convergence and regulation of network excitability

A pericellular basket is a presynaptic configuration of numerous axonal boutons outlining a target neuron soma and its proximal dendrites. Recent studies show neurochemical diversity of pericellular baskets and suggest that neurotransmitter usage together with the dense, soma-proximal boutons may permit strong input effects on different timescales. Here we review the development, distribution, neurochemical phenotypes, and possible functions of pericellular baskets. As an example, we highlight pericellular baskets formed by projections of certain Pet1/Fev neurons of the serotonergic raphe nuclei. We propose that pericellular baskets represent convergence sites of competition or facilitation between neurotransmitter systems on downstream circuitry, especially in limbic brain regions, where pericellular baskets are widespread. Study of these baskets may enhance our understanding of monoamine regulation of memory, social behavior, and brain oscillations.

Parvalbumin interneuron alterations in stress-related mood disorders: A systematic review

Stress-related psychiatric disorders including depression involve complex cellular and molecular changes in the brain, and GABAergic signaling dysfunction is increasingly implicated in the etiology of mood disorders. Parvalbumin (PV)-expressing neurons are fast-spiking interneurons that, among other roles, coordinate synchronous neuronal firing. Mounting evidence suggests that the PV neuron phenotype is altered by stress and in mood disorders. In this systematic review, we assessed PV interneuron alterations in psychiatric disorders as reported in human postmortem brain studies and animal models of environmental stress. This review aims to 1) comprehensively catalog evidence of PV cell function in mood disorders (humans) and stress models of mood disorders (animals); 2) analyze the strength of evidence of PV interneuron alterations in various brain regions in humans and rodents; 3) determine whether the modulating effect of antidepressant treatment, physical exercise, and environmental enrichment on stress in animals associates with particular effects on PV function; and 4) use this information to guide future research avenues. Its principal findings, derived mainly from rodent studies, are that stress-related changes in PV cells are only reported in a minority of studies, that positive findings are region-, age-, sex-, and stress recency-dependent, and that antidepressants protect from stress-induced apparent PV cell loss. These observations do not currently translate well to humans, although the postmortem literature on the topic remains limited.

Serotonin-related rodent models of early-life exposure relevant for neurodevelopmental vulnerability to psychiatric disorders

Mental disorders including depression and anxiety are continuously rising their prevalence across the globe. Early-life experience of individuals emerges as a main risk factor contributing to the developmental vulnerability to psychiatric disorders. That is, perturbing environmental conditions during neurodevelopmental stages can have detrimental effects on adult mood and emotional responses. However, the possible maladaptive neural mechanisms contributing to such psychopathological phenomenon still remain poorly understood. In this review, we explore preclinical rodent models of developmental vulnerability to psychiatric disorders, focusing on the impact of early-life environmental perturbations on behavioral aspects relevant to stress-related and psychiatric disorders. We limit our analysis to well-established models in which alterations in the serotonin (5-HT) system appear to have a crucial role in the pathophysiological mechanisms. We analyze long-term behavioral outcomes produced by early-life exposures to stress and psychotropic drugs such as the selective 5-HT reuptake inhibitor (SSRI) antidepressants or the anticonvulsant valproic acid (VPA). We perform a comparative analysis, identifying differences and commonalities in the behavioral effects produced in these models. Furthermore, this review discusses recent advances on neurodevelopmental substrates engaged in these behavioral effects, emphasizing the possible existence of maladaptive mechanisms that could be shared by the different models.