BDNF regulates the maturation of inhibition and the critical period of plasticity in mouse visual cortex

Development of visual response selectivity in cortical GABAergic interneurons

The visual cortex of carnivores and primates displays a high degree of modular network organization characterized by local clustering and structured long-range correlations of activity and functional properties. Excitatory networks display modular organization before the onset of sensory experience, but the developmental timeline for modular networks of GABAergic interneurons remains under-explored. Using in vivo calcium imaging of the ferret visual cortex, we find evidence that before visual experience, interneurons display weak orientation tuning and widespread, correlated activity in response to visual stimuli. Robust modular organization and orientation tuning are evident with as little as one week of visual experience. Furthermore, we find that the maturation of orientation tuning requires visual experience, while the reduction in widespread, correlated network activity does not. Thus, the maturation of inhibitory cortical networks occurs in a delayed, parallel process relative to excitatory neurons.

Excitatory neurons in the carnivore and primate visual cortex display orientation selectivity arranged in a modular fashion before the onset of visual experience, but the developmental timeline for visual response selectivity of inhibitory neurons is unknown. Using in vivo calcium imaging in ferret visual cortex, the authors show that experience-dependent emergence of orientation selectivity develops in a modular fashion for inhibitory interneurons over the first week of visual experience.

Fingolimod increases parvalbumin-positive neurons in adult mice

In recent years, it has been shown that central nervous system agents, such as antidepressants and antiepileptic drugs, reopen a critical period in mature animals. Fingolimod, which is used for the treatment of multiple sclerosis, also restores neuroplasticity. In this study, we investigated the effects of parvalbumin (PV)-positive neurons and perineuronal nets (PNN) on fingolimod administration with respect to neuroplasticity. Fingolimod was chronically administered intraperitoneally to mature mice. PV-positive neurons and PNN in the hippocampus, prefrontal cortex, and somatosensory cortex were analyzed. An increase in PV-positive neurons was observed in the hippocampus, prefrontal cortex, and somatosensory cortex of the fingolimod-treated mice. An increase in Wisteria floribunda agglutinin-positive PNN was confirmed in mice treated with fingolimod in the somatosensory cortex only. Fingolimod increased the density of PV-positive neurons in the brains of mature mice. The results indicate that fingolimod may change the critical period in mature animals.

Origin, Development, and Synaptogenesis of Cortical Interneurons

The mammalian cerebral cortex represents one of the most recent and astonishing inventions of nature, responsible of a large diversity of functions that range from sensory processing to high-order cognitive abilities, such as logical reasoning or language. Decades of dedicated study have contributed to our current understanding of this structure, both at structural and functional levels. A key feature of the neocortex is its outstanding richness in cell diversity, composed by multiple types of long-range projecting neurons and locally connecting interneurons. In this review, we will describe the great diversity of interneurons that constitute local neocortical circuits and summarize the mechanisms underlying their development and their assembly into functional networks.

Nurr1 Is Not an Essential Regulator of BDNF in Mouse Cortical Neurons

Nurr1 and brain-derived neurotrophic factor (BDNF) play major roles in cognition. Nurr1 regulates BDNF in midbrain dopaminergic neurons and cerebellar granule cells. Nurr1 and BDNF are also highly expressed in the cerebral cortex, a brain area important in cognition. Due to Nurr1 and BDNF tissue specificity, the regulatory effect of Nurr1 on BDNF in different brain areas cannot be generalized. The relationship between Nurr1 and BDNF in the cortex has not been investigated previously. Therefore, we examined Nurr1-mediated BDNF regulation in cortical neurons in activity-dependent and activity-independent states. Mouse primary cortical neurons were treated with the Nurr1 agonist, amodiaquine (AQ). Membrane depolarization was induced by KCl or veratridine and reversed by nimodipine. AQ and membrane depolarization significantly increased Nurr1 (p < 0.001) and BDNF (pAQ < 0.001, pKCl < 0.01) as assessed by real-time qRT-PCR. However, Nurr1 knockdown did not affect BDNF gene expression in resting or depolarized neurons. Accordingly, the positive correlation between Nurr1 and BDNF expression in AQ and membrane depolarization experiments does not imply co-regulation because Nurr1 knockdown did not affect BDNF gene expression in resting or depolarized cortical neurons. Therefore, in contrast to midbrain dopaminergic neurons and cerebellar granule cells, Nurr1 does not regulate BDNF in cortical neurons.

Postnatal development of the microstructure of cortical GABAergic synapses and perineuronal nets requires sensory input

The brain synaptic circuitry is formed as a result of pre-defined genetic programs and sensory experience during postnatal development. Perineuronal nets ensheath synaptic boutons and control several crucial features of the synapse physiology. Formation of the perineuronal net microstructure during the brain development remains largely unstudied. Here we provide a detailed quantitative description of the 3-dimensional geometry of the synapse and the surrounding perineuronal net in the mouse somatosensory cortex layer IV. We compare the morphology of the synapse+perineuronal net complex in the adult brain formed under normal conditions or in the whisker shaving model of somatosensory deprivation. We demonstrate that the sensory deprivation causes flattening of the 3D PNN mesh geometry and reduction of the VGAT-positive cluster volume in presynaptic boutons. These results reveal a mechanism of the sensory input-dependent synapse morphogenesis during the brain development.

Brain Proteome and Behavioural Analysis in Wild Type, BDNF+/- and BDNF-/- Adult Zebrafish (Danio rerio) Exposed to Two Different Temperatures

Experimental evidence suggests that environmental stress conditions can alter the expression of BDNF and that the expression of this neurotrophin influences behavioural responses in mammalian models. It has been recently demonstrated that exposure to 34 °C for 21 days alters the brain proteome and behaviour in zebrafish. The aim of this work was to investigate the role of BDNF in the nervous system of adult zebrafish under control and heat treatment conditions. For this purpose, zebrafish from three different genotypes (wild type, heterozygous BDNF^+/- and knock out BDNF^-/-) were kept for 21 days at 26 °C or 34 °C and then euthanized for brain molecular analyses or subjected to behavioural tests (Y-maze test, novel tank test, light and dark test, social preference test, mirror biting test) for assessing behavioural aspects such as boldness, anxiety, social preference, aggressive behaviour, interest for the novel environment and exploration. qRT-PCR analysis showed the reduction of gene expression of BDNF and its receptors after heat treatment in wild type zebrafish. Moreover, proteomic analysis and behavioural tests showed genotype- and temperature-dependent effects on brain proteome and behavioural responding. Overall, the absent expression of BDNF in KO alters (1) the brain proteome by reducing the expression of proteins involved in synapse functioning and neurotransmitter-mediated transduction; (2) the behaviour, which can be interpreted as bolder and less anxious and (3) the cellular and behavioural response to thermal treatment.

Parvalbumin-Positive Interneurons Regulate Cortical Sensory Plasticity in Adulthood and Development Through Shared Mechanisms

Parvalbumin-positive neurons are the largest class of GABAergic, inhibitory neurons in the central nervous system. In the cortex, these fast-spiking cells provide feedforward and feedback synaptic inhibition onto a diverse set of cell types, including pyramidal cells, other inhibitory interneurons, and themselves. Cortical inhibitory networks broadly, and cortical parvalbumin-expressing interneurons (cPVins) specifically, are crucial for regulating sensory plasticity during both development and adulthood. Here we review the functional properties of cPVins that enable plasticity in the cortex of adult mammals and the influence of cPVins on sensory activity at four spatiotemporal scales. First, cPVins regulate developmental critical periods and adult plasticity through molecular and structural interactions with the extracellular matrix. Second, they activate in precise sequence following feedforward excitation to enforce strict temporal limits in response to the presentation of sensory stimuli. Third, they implement gain control to normalize sensory inputs and compress the dynamic range of output. Fourth, they synchronize broad network activity patterns in response to behavioral events and state changes. Much of the evidence for the contribution of cPVins to plasticity comes from classic models that rely on sensory deprivation methods to probe experience-dependent changes in the brain. We support investigating naturally occurring, adaptive cortical plasticity to study cPVin circuits in an ethologically relevant framework, and discuss recent insights from our work on maternal experience-induced auditory cortical plasticity.

Non-Cell-Autonomous Factors Implicated in Parvalbumin Interneuron Maturation and Critical Periods

From birth to adolescence, the brain adapts to its environmental stimuli through structural and functional remodeling of neural circuits during critical periods of heightened plasticity. They occur across modalities for proper sensory, motor, linguistic, and cognitive development. If they are disrupted by early-life adverse experiences or genetic deficiencies, lasting consequences include behavioral changes, physiological and cognitive deficits, or psychiatric illness. Critical period timing is orchestrated not only by appropriate neural activity but also by a multitude of signals that participate in the maturation of fast-spiking parvalbumin interneurons and the consolidation of neural circuits. In this review, we describe the various signaling factors that initiate critical period onset, such as BDNF, SPARCL1, or OTX2, which originate either from local neurons or glial cells or from extracortical sources such as the choroid plexus. Critical period closure is established by signals that modulate extracellular matrix and myelination, while timing and plasticity can also be influenced by circadian rhythms and by hormones and corticosteroids that affect brain oxidative stress levels or immune response. Molecular outcomes include lasting epigenetic changes which themselves can be considered signals that shape downstream cross-modal critical periods. Comprehensive knowledge of how these signals and signaling factors interplay to influence neural mechanisms will help provide an inclusive perspective on the effects of early adversity and developmental defects that permanently change perception and behavior.

Critical Periods in Vision Revisited

For four decades, investigations of the biological basis of critical periods in the developing mammalian visual cortex were dominated by study of the consequences of altered early visual experience in cats and nonhuman primates. The neural deficits thus revealed also provided insight into the origin and neural basis of human amblyopia that in turn motivated additional studies of humans with abnormal early visual input. Recent human studies point to deficits arising from alterations in all visual cortical areas and even in nonvisual cortical regions. As the new human data accumulated in parallel with a near-complete shift toward the use of rodent animal models for the study of neural mechanisms, it is now essential to review the human data and the earlier animal data obtained from cats and monkeys to infer general conclusions and to optimize future choice of the most appropriate animal model. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Body Weight-Supported Treadmill Training Ameliorates Motoneuronal Hyperexcitability by Increasing GAD-65/67 and KCC2 Expression via TrkB Signaling in Rats with Incomplete Spinal Cord Injury

Spasticity is a typical consequence after spinal cord injury (SCI). The critical reasons are reducing the synthesis of Gamma-Aminobutyric Acid (GABA), glycine and potassium chloride co-transporter 2 (KCC2) inside the distal spinal cord. The current work aimed to test whether exercise training could increase the expression of glutamic acid decarboxylase 65/67 (GAD-65/67, the key enzymes in GABA synthesis) and KCC2 in the distal spinal cord via tropomyosin-related kinase B (TrkB) signaling. The experimental rats were randomly assigned to the following five groups: Sham, SCI/phosphate-buffered saline (PBS), SCI-treadmill training (TT)/PBS, SCI/TrkB-IgG, and SCI-TT/TrkB-IgG. After that, the model of T10 contusion SCI was used, then TrkB-IgG was used to prevent TrkB activity at 7 days post-SCI. Body weight-supported treadmill training started on the 8^th day post-SCI for four weeks. The Hmax/Mmax ratio and the rate-dependent depression of H-reflex were used to assess the excitability of spinal motoneuronal networks. Western blotting and Immunohistochemistry techniques were utilized for measuring the expression of GAD-65, GAD-67, and KCC2. The findings revealed that exercise training could reduce motoneuronal excitability and boost GAD-65, GAD-67, and KCC2 production in the distal region of the spinal cord after SCI. The effects of exercise training were decreased after the TrkB signaling was inhibited. The present exploration demonstrated that exercise training increases GAD-65, GAD-67, and KCC2 expression in the spinal cord via TrkB signaling and that this method could also improve rats with motoneuronal hyperexcitability and spasticity induced by incomplete SCI.

Positive Aspects of Oxidative Stress at Different Levels of the Human Body: A Review

Oxidative stress is the subject of numerous studies, most of them focusing on the negative effects exerted at both molecular and cellular levels, ignoring the possible benefits of free radicals. More and more people admit to having heard of the term "oxidative stress", but few of them understand the meaning of it. We summarized and analyzed the published literature data in order to emphasize the importance and adaptation mechanisms of basal oxidative stress. This review aims to provide an overview of the mechanisms underlying the positive effects of oxidative stress, highlighting these effects, as well as the risks for the population consuming higher doses than the recommended daily intake of antioxidants. The biological dose-response curve in oxidative stress is unpredictable as reactive species are clearly responsible for cellular degradation, whereas antioxidant therapies can alleviate senescence by maintaining redox balance; nevertheless, excessive doses of the latter can modify the redox balance of the cell, leading to a negative outcome. It can be stated that the presence of oxidative status or oxidative stress is a physiological condition with well-defined roles, yet these have been insufficiently researched and explored. The involvement of reactive oxygen species in the pathophysiology of some associated diseases is well-known and the involvement of antioxidant therapies in the processes of senescence, apoptosis, autophagy, and the maintenance of cellular homeostasis cannot be denied. All data in this review support the idea that oxidative stress is an undesirable phenomenon in high and long-term concentrations, but regular exposure is consistent with the hormetic theory.

Enzymatic Degradation of Cortical Perineuronal Nets Reverses GABAergic Interneuron Maturation

Perineuronal nets (PNNs) are specialised extracellular matrix structures which preferentially enwrap fast-spiking (FS) parvalbumin interneurons and have diverse roles in the cortex. PNN maturation coincides with closure of the critical period of cortical plasticity. We have previously demonstrated that BDNF accelerates interneuron development in a c-Jun-NH₂-terminal kinase (JNK)–dependent manner, which may involve upstream thousand-and-one amino acid kinase 2 (TAOK2). Chondroitinase-ABC (ChABC) enzymatic digestion of PNNs reportedly reactivates ‘juvenile-like’ plasticity in the adult CNS. However, the mechanisms involved are unclear. We show that ChABC produces an immature molecular phenotype in cultured cortical neurons, corresponding to the phenotype prior to critical period closure. ChABC produced different patterns of PNN-related, GABAergic and immediate early (IE) gene expression than well-characterised modulators of mature plasticity and network activity (GABA_A-R antagonist, bicuculline, and sodium-channel blocker, tetrodotoxin (TTX)). ChABC downregulated JNK activity, while this was upregulated by bicuculline. Bicuculline, but not ChABC, upregulated Bdnf expression and ERK activity. Furthermore, we found that BDNF upregulation of semaphorin-3A and IE genes was TAOK mediated. Our data suggest that ChABC heightens structural flexibility and network disinhibition, potentially contributing to ‘juvenile-like’ plasticity. The molecular phenotype appears to be distinct from heightened mature synaptic plasticity and could relate to JNK signalling. Finally, we highlight that BDNF regulation of plasticity and PNNs involves TAOK signalling.

Manipulating the Level of Sensorimotor Stimulation during LI-rTMS Can Improve Visual Circuit Reorganisation in Adult Ephrin-A2A5-/- Mice

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive brain stimulation technique that has the potential to treat a variety of neurologic and psychiatric disorders. The extent of rTMS-induced neuroplasticity may be dependent on a subject's brain state at the time of stimulation. Chronic low intensity rTMS (LI-rTMS) has previously been shown to induce beneficial structural and functional reorganisation within the abnormal visual circuits of ephrin-A2A5^-/- mice in ambient lighting. Here, we administered chronic LI-rTMS in adult ephrin-A2A5^-/- mice either in a dark environment or concurrently with voluntary locomotion. One day after the last stimulation session, optokinetic responses were assessed and fluorescent tracers were injected to map corticotectal and geniculocortical projections. We found that LI-rTMS in either treatment condition refined the geniculocortical map. Corticotectal projections were improved in locomotion+LI-rTMS subjects, but not in dark + LI-rTMS and sham groups. Visuomotor behaviour was not improved in any condition. Our results suggest that the beneficial reorganisation of abnormal visual circuits by rTMS can be significantly influenced by simultaneous, ambient visual input and is enhanced by concomitant physical exercise. Furthermore, the observed pathway-specific effects suggest that regional molecular changes and/or the relative proximity of terminals to the induced electric fields influence the outcomes of LI-rTMS on abnormal circuitry.

Retinal Plasticity

Brain plasticity is a well-established concept designating the ability of central nervous system (CNS) neurons to rearrange as a result of learning, when adapting to changeable environmental conditions or else while reacting to injurious factors. As a part of the CNS, the retina has been repeatedly probed for its possible ability to respond plastically to a variably altered environment or to pathological insults. However, numerous studies support the conclusion that the retina, outside the developmental stage, is endowed with only limited plasticity, exhibiting, instead, a remarkable ability to maintain a stable architectural and functional organization. Reviewed here are representative examples of hippocampal and cortical paradigms of plasticity and of retinal structural rearrangements found in organization and circuitry following altered developmental conditions or occurrence of genetic diseases leading to neuronal degeneration. The variable rate of plastic changes found in mammalian retinal neurons in different circumstances is discussed, focusing on structural plasticity. The likely adaptive value of maintaining a low level of plasticity in an organ subserving a sensory modality that is dominant for the human species and that requires elevated fidelity is discussed.

Defining neuroplasticity

Neuroplasticity, i.e., the modifiability of the brain, is different in development and adulthood. The first includes changes in: (i) neurogenesis and control of neuron number; (ii) neuronal migration; (iii) differentiation of the somato-dendritic and axonal phenotypes; (iv) formation of connections; (v) cytoarchitectonic differentiation. These changes are often interrelated and can lead to: (vi) system-wide modifications of brain structure as well as to (vii) acquisition of specific functions such as ocular dominance or language. Myelination appears to be plastic both in development and adulthood, at least, in rodents. Adult neuroplasticity is limited, and is mainly expressed as changes in the strength of excitatory and inhibitory synapses while the attempts to regenerate connections have met with limited success. The outcomes of neuroplasticity are not necessarily adaptive, but can also be the cause of neurological and psychiatric pathologies.

Multisensory Integration: Is Medial Prefrontal Cortex Signaling Relevant for the Treatment of Higher-Order Visual Dysfunctions?

In the mammalian brain, information processing in sensory modalities and global mechanisms of multisensory integration facilitate perception. Emerging experimental evidence suggests that the contribution of multisensory integration to sensory perception is far more complex than previously expected. Here we revise how associative areas such as the prefrontal cortex, which receive and integrate inputs from diverse sensory modalities, can affect information processing in unisensory systems via processes of down-stream signaling. We focus our attention on the influence of the medial prefrontal cortex on the processing of information in the visual system and whether this phenomenon can be clinically used to treat higher-order visual dysfunctions. We propose that non-invasive and multisensory stimulation strategies such as environmental enrichment and/or attention-related tasks could be of clinical relevance to fight cerebral visual impairment.

Dysregulation of GABAergic Signaling in Neurodevelomental Disorders: Targeting Cation-Chloride Co-transporters to Re-establish a Proper E/I Balance

The construction of the brain relies on a series of well-defined genetically and experience- or activity -dependent mechanisms which allow to adapt to the external environment. Disruption of these processes leads to neurological and psychiatric disorders, which in many cases are manifest already early in postnatal life. GABA, the main inhibitory neurotransmitter in the adult brain is one of the major players in the early assembly and formation of neuronal circuits. In the prenatal and immediate postnatal period GABA, acting on GABA_A receptors, depolarizes and excites targeted cells via an outwardly directed flux of chloride. In this way it activates NMDA receptors and voltage-dependent calcium channels contributing, through intracellular calcium rise, to shape neuronal activity and to establish, through the formation of new synapses and elimination of others, adult neuronal circuits. The direction of GABA_A-mediated neurotransmission (depolarizing or hyperpolarizing) depends on the intracellular levels of chloride [Cl⁻]_i, which in turn are maintained by the activity of the cation-chloride importer and exporter KCC2 and NKCC1, respectively. Thus, the premature hyperpolarizing action of GABA or its persistent depolarizing effect beyond the postnatal period, leads to behavioral deficits associated with morphological alterations and an excitatory (E)/inhibitory (I) imbalance in selective brain areas. The aim of this review is to summarize recent data concerning the functional role of GABAergic transmission in building up and refining neuronal circuits early in development and its dysfunction in neurodevelopmental disorders such as Autism Spectrum Disorders (ASDs), schizophrenia and epilepsy. In particular, we focus on novel information concerning the mechanisms by which alterations in cation-chloride co-transporters (CCC) generate behavioral and cognitive impairment in these diseases. We discuss also the possibility to re-establish a proper GABA_A-mediated neurotransmission and excitatory (E)/inhibitory (I) balance within selective brain areas acting on CCC.

Child abuse associates with increased recruitment of perineuronal nets in the ventromedial prefrontal cortex: a possible implication of oligodendrocyte progenitor cells

Child abuse (CA) is a strong predictor of psychopathologies and suicide, altering normal trajectories of brain development in areas closely linked to emotional responses such as the prefrontal cortex (PFC). Yet, the cellular underpinnings of these enduring effects are unclear. Childhood and adolescence are marked by the protracted formation of perineuronal nets (PNNs), which orchestrate the closure of developmental windows of cortical plasticity by regulating the functional integration of parvalbumin interneurons into neuronal circuits. Using well-characterized post-mortem brain samples, we show that a history of CA is specifically associated with increased densities and morphological complexity of WFL-labeled PNNs in the ventromedial PFC (BA11/12), possibly suggesting increased recruitment and maturation of PNNs. Through single-nucleus sequencing and fluorescent in situ hybridization, we found that the expression of canonical components of PNNs is enriched in oligodendrocyte progenitor cells (OPCs), and that they are upregulated in CA victims. These correlational findings suggest that early-life adversity may lead to persistent patterns of maladaptive behaviors by reducing the neuroplasticity of cortical circuits through the enhancement of developmental OPC-mediated PNN formation.

Intrauterine Growth Restriction Disrupts the Postnatal Critical Period of Synaptic Plasticity in the Mouse Dorsal Hippocampus in a Model of Hypertensive Disease of Pregnancy

INTRODUCTION

Intrauterine growth restriction (IUGR) from hypertensive disease of pregnancy complicates up to 10% of all pregnancies. Significant hippocampal-dependent cognitive and memory impairments as well as neuropsychiatric disorders have been linked to IUGR. Because disturbance of the hippocampal critical period (CPd) of synaptic plasticity leads to impairments similar to those described in IUGR human offspring, we hypothesized that IUGR would perturb the CPd of synaptic plasticity in the mouse hippocampus in our model.

METHODS

IUGR was produced by a micro-osmotic pump infusion of the potent vasoconstrictor U-46619, a thromboxane A2-agonist, at embryonic day 12.5 in C57BL/6J mouse dams to precipitate hypertensive disease of pregnancy and IUGR. Sham-operated mice acted as controls. At P10, P18, and P40, we assessed astrogliosis using GFAP-IHC. In dorsal CA1 and CA3 subfields, we assessed the immunoreactivities (IR) (IF-IHC) to (i) parvalbumin (PV) and glutamate decarboxylase (GAD) 65/67, involved in CPd onset; (ii) PSA-NCAM that antagonizes CPd onset; (iii) NPTX2, necessary for excitatory synapse formation and engagement of CPd; and (iv) MBP and WFA, staining perineural nets (PNNs), marking CPd closure. ImageJ/Fiji and IMARIS were used for image processing and SPSS v24 for statistical analysis.

RESULTS

Although PV+ interneuron numbers and IR intensity were unchanged, development of GAD65/67+ synaptic boutons was accelerated at P18 IUGR mice and inversely correlated with decreased expression of PSA-NCAM in the CA of P18 IUGR mice at P18. NPTX2+ puncta and total volume were persistently decreased in the CA3 pyramidal and radiatum layers of IUGR mice from P18 to P40. At P40, axonal myelination (MBP+) in CA3 of IUGR mice was decreased and correlated with NPTX2 deficits. Lastly, the volume and integrity of the PNNs in the dorsal CA was disrupted in IUGR mice at P40.

DISCUSSION/CONCLUSION

IUGR disrupts the molecular and structural initiation, consolidation, and closure of the CPd of synaptic plasticity in the mouse hippocampus in our model, which may explain the learning and memory deficits observed in juvenile IUGR mice and the cognitive disorders seen in human IUGR offspring. The mechanistic links warrant further investigation, to identify therapeutic targets to prevent neurodevelopmental deficits in patients affected by IUGR.

Sensitive period-regulating genetic pathways and exposure to adversity shape risk for depression

Animal and human studies have documented the existence of developmental windows (or sensitive periods) when experience can have lasting effects on brain structure or function, behavior, and disease. Although sensitive periods for depression likely arise through a complex interplay of genes and experience, this possibility has not yet been explored in humans. We examined the effect of genetic pathways regulating sensitive periods, alone and in interaction with common childhood adversities, on depression risk. Guided by a translational approach, we: (1) performed association analyses of three gene sets (60 genes) shown in animal studies to regulate sensitive periods using summary data from a genome-wide association study of depression (n = 807,553); (2) evaluated the developmental expression patterns of these genes using data from BrainSpan (n = 31), a transcriptional atlas of postmortem brain samples; and (3) tested gene-by-development interplay (dGxE) by analyzing the combined effect of common variants in sensitive period genes and time-varying exposure to two types of childhood adversity within a population-based birth cohort (n = 6254). The gene set regulating sensitive period opening associated with increased depression risk. Notably, 6 of the 15 genes in this set showed developmentally regulated gene-level expression. We also identified a statistical interaction between caregiver physical or emotional abuse during ages 1-5 years and genetic risk for depression conferred by the opening genes. Genes involved in regulating sensitive periods are differentially expressed across the life course and may be implicated in depression vulnerability. Our findings about gene-by-development interplay motivate further research in large, more diverse samples to further unravel the complexity of depression etiology through a sensitive period lens.

Systemic LPS-induced microglial activation results in increased GABAergic tone: A mechanism of protection against neuroinflammation in the medial prefrontal cortex in mice

Neuroinflammation with excess microglial activation and synaptic dysfunction are early symptoms of most neurological diseases. However, how microglia-associated neuroinflammation regulates synaptic activity remains obscure. We report here that acute neuroinflammation induced by intraperitoneal injection of lipopolysaccharide (LPS) results in cell-type-specific increases in inhibitory postsynaptic currents in the glutamatergic, but not the GABAergic, neurons of medial prefrontal cortex (mPFC), coinciding with excessive microglial activation. LPS causes upregulation in levels of GABA_AR subunits, glutamine synthetase and vesicular GABA transporter, and downregulation in brain-derived neurotrophic factor (BDNF) and its receptor, pTrkB. Blockage of microglial activation by minocycline ameliorates LPS-induced abnormal expression of GABA signaling-related proteins and activity of synaptic and network. Moreover, minocycline prevents the mice from LPS-induced aberrant behavior, such as a reduction in total distance and time spent in the centre in the open field test; decreases in entries into the open arm of elevated-plus maze and in consumption of sucrose; increased immobility in the tail suspension test. Furthermore, upregulation of GABA signaling by tiagabine also prevents LPS-induced microglial activation and aberrant behavior. This study illustrates a mode of bidirectional constitutive signaling between the neural and immune compartments of the brain, and suggests that the mPFC is an important area for brain-immune system communication. Moreover, the present study highlights GABAergic signaling as a key therapeutic target for mitigating neuroinflammation-induced abnormal synaptic activity in the mPFC, together with the associated behavioral abnormalities.

Heterosynaptic Plasticity and the Experience-Dependent Refinement of Developing Neuronal Circuits

Neurons remodel the structure and strength of their synapses during critical periods of development in order to optimize both perception and cognition. Many of these developmental synaptic changes are thought to occur through synapse-specific homosynaptic forms of experience-dependent plasticity. However, homosynaptic plasticity can also induce or contribute to the plasticity of neighboring synapses through heterosynaptic interactions. Decades of research in vitro have uncovered many of the molecular mechanisms of heterosynaptic plasticity that mediate local compensation for homosynaptic plasticity, facilitation of further bouts of plasticity in nearby synapses, and cooperative induction of plasticity by neighboring synapses acting in concert. These discoveries greatly benefited from new tools and technologies that permitted single synapse imaging and manipulation of structure, function, and protein dynamics in living neurons. With the recent advent and application of similar tools for in vivo research, it is now feasible to explore how heterosynaptic plasticity contribute to critical periods and the development of neuronal circuits. In this review, we will first define the forms heterosynaptic plasticity can take and describe our current understanding of their molecular mechanisms. Then, we will outline how heterosynaptic plasticity may lead to meaningful refinement of neuronal responses and observations that suggest such mechanisms are indeed at work in vivo. Finally, we will use a well-studied model of cortical plasticity—ocular dominance plasticity during a critical period of visual cortex development—to highlight the molecular overlap between heterosynaptic and developmental forms of plasticity, and suggest potential avenues of future research.

Inhibition of Cdk5 in PV Neurons Reactivates Experience-Dependent Plasticity in Adult Visual Cortex

Cyclin-dependent kinase 5 (Cdk5) has been shown to play a critical role in brain development, learning, memory and neural processing in general. Cdk5 is widely distributed in many neuron types in the central nervous system, while its cell-specific role is largely unknown. Our previous study showed that Cdk5 inhibition restored ocular dominance (OD) plasticity in adulthood. In this study, we specifically knocked down Cdk5 in different types of neurons in the visual cortex and examined OD plasticity by optical imaging of intrinsic signals. Downregulation of Cdk5 in parvalbumin-expressing (PV) inhibitory neurons, but not other neurons, reactivated adult mouse visual cortical plasticity. Cdk5 knockdown in PV neurons reduced the evoked firing rate, which was accompanied by an increment in the threshold current for the generation of a single action potential (AP) and hyperpolarization of the resting membrane potential. Moreover, chemogenetic activation of PV neurons in the visual cortex can attenuate the restoration of OD plasticity by Cdk5 inhibition. Taken together, our results suggest that Cdk5 in PV interneurons may play a role in modulating the excitation and inhibition balance to control the plasticity of the visual cortex.

BDNF polymorphism and inter hemispheric balance of motor cortex excitability: a preliminary study

Preclinical studies have demonstrated that Brain-Derived Neurotrophic Factor (BDNF) plays a crucial role in the homeostatic regulation of cortical excitability and excitation/inhibition balance. Using transcranial magnetic stimulation (TMS) techniques we investigated whether BDNF polymorphism could influence cortical excitability of the left and right primary motor cortex in healthy humans. Twenty-nine participants were recruited and genotyped for the presence of the BDNF Val66Met polymorphism, namely homozygous for the valine allele (Val/Val), heterozygotes (Val/Met), and homozygous for the methionine allele (Met/Met). Blinded to the latter, we evaluated inhibitory and facilitatory circuits of the left (LH) and right motor cortex (RH) by measuring resting (RMT) and active motor threshold (AMT), short interval intracortical inhibition (SICI) and intracortical facilitation (ICF). For each neurophysiological metric we also considered the inter-hemispheric balance expressed by the Laterality Index (LI). Val/Val participants (n= 21) exhibited an overall higher excitability of the LH compared to the RH, as probed by lower motor thresholds, lower SICI and higher ICF. Val/Val participants displayed positive LI, especially for AMT and ICF (all p< 0.05), indicating higher LH excitability and more pronounced inter-hemispheric excitability imbalance as compared to Met carriers. Our preliminary results suggest that BDNF Val66Met polymorphism might influence interhemispheric balance of motor cortex excitability.

Ibudilast attenuates cocaine self-administration and prime- and cue-induced reinstatement of cocaine seeking in rats

Ibudilast is a non-selective phosphodiesterase (PDE) inhibitor and glial cell modulator which has shown great promise for the treatment of drug and alcohol use disorders in recent clinical studies. However, it is unknown whether and how ibudilast affects cocaine seeking behavior. Here we show that systemic administration of ibudilast dose-dependently reduced cocaine self-administration under fixed- and progressive-ratio reinforcement schedules in rats and shifted cocaine dose-response curves downward. In addition, ibudilast decreased cocaine prime- and cue-induced reinstatement of cocaine seeking. These results indicate that ibudilast was effective in reducing the reinforcing effects of cocaine and relapse to cocaine seeking. Chronic cocaine exposure induces cAMP-related neuroadaptations in the reward circuitry of the brain. To investigate potential mechanisms for ibudilast-induced attenuation of cocaine self-administration, we recorded from ventral tegmental area (VTA) dopamine neurons in ex vivo midbrain slices prepared from rats that had undergone saline and cocaine self-administration. We found cocaine self-administration led to a decrease in inhibitory postsynaptic currents (IPSCs), an increase in the AMPAR/NMDAR ratio, and an increase in the excitation to inhibition (E/I) ratio. Ibudilast pretreatments enhanced GABAergic inhibition and did not further change cocaine-induced potentiation of excitation, leading to normalization of the E/I ratio. Restoration of the balance between excitation and inhibition in VTA dopamine neurons may contribute to the attenuation of cocaine self-administration by ibudilast.

REST/NRSF drives homeostatic plasticity of inhibitory synapses in a target-dependent fashion

The repressor-element 1-silencing transcription/neuron-restrictive silencer factor (REST/NRSF) controls hundreds of neuron-specific genes. We showed that REST/NRSF downregulates glutamatergic transmission in response to hyperactivity, thus contributing to neuronal homeostasis. However, whether GABAergic transmission is also implicated in the homeostatic action of REST/NRSF is unknown. Here, we show that hyperactivity-induced REST/NRSF activation, triggers a homeostatic rearrangement of GABAergic inhibition, with increased frequency of miniature inhibitory postsynaptic currents (IPSCs) and amplitude of evoked IPSCs in mouse cultured hippocampal neurons. Notably, this effect is limited to inhibitory-onto-excitatory neuron synapses, whose density increases at somatic level and decreases in dendritic regions, demonstrating a complex target- and area-selectivity. The upscaling of perisomatic inhibition was occluded by TrkB receptor inhibition and resulted from a coordinated and sequential activation of the Npas4 and Bdnf gene programs. On the opposite, the downscaling of dendritic inhibition was REST-dependent, but BDNF-independent. The findings highlight the central role of REST/NRSF in the complex transcriptional responses aimed at rescuing physiological levels of network activity in front of the ever-changing environment.

Age- and sex-specific effects of stress on parvalbumin interneurons in preclinical models: Relevance to sex differences in clinical neuropsychiatric and neurodevelopmental disorders

Stress is a major risk factor for neurodevelopmental and neuropsychiatric disorders, with the capacity to impact susceptibility to disease as well as long-term neurobiological and behavioral outcomes. Parvalbumin (PV) interneurons, the most prominent subtype of GABAergic interneurons in the cortex, are uniquely responsive to stress due to their protracted development throughout the highly plastic neonatal period and into puberty and adolescence. Additionally, PV + interneurons appear to respond to stress in a sex-specific manner. This review aims to discuss existing preclinical studies that support our overall hypothesis that the sex-and age-specific impacts of stress on PV + interneurons contribute to differences in individual vulnerability to stress across the lifespan, particularly in regard to sex differences in the diagnostic rate of neurodevelopmental and neuropsychiatric diseases in clinical populations. We also emphasize the importance of studying sex as a biological variable to fully understand the mechanistic and behavioral differences between males and females in models of neuropsychiatric disease.

Hippocampal chloride transporter KCC2 contributes to excitatory GABA dysregulation in the developmental rat model of schizophrenia

Recent studies have revealed an altered expression of NKCC1 and KCC2 in prefrontal cortex (PFC) and hippocampus of schizophrenic patients. Despite extensive considerations, the alteration of NKCC1 and KCC2 co-transporters at different stages of development has not been fully studied. Therefore, we evaluated the expression of these transporters in PFC and hippocampus at time points of four, eight, and twelve weeks in post-weaning social isolation rearing rat model. For this purpose, 23-25 days-old rats were classified into social- or isolation-reared groups. The levels of NKCC1 and KCC2 mRNA expression were evaluated at hippocampus or PFC regions at the time-points of four, eight, and twelve weeks following housing. Post-weaning isolation rearing decreased the hippocampal KCC2 mRNA expression level, but does not affect the NKCC1 mRNA expression. However, no significant difference was observed in the PFC mRNA levels of NKCC1 and KCC2 in the isolation-reared group compared to the socially-reared group during the course of modeling. Further, we assessed the therapeutic effect of selective NKCC1 inhibitor bumetanide (10 mg/kg), on improvement of prepulse inhibition (PPI) test on twelve weeks isolation-reared rats. Intraperitoneal administration of bumetanide (10 mg/kg) did not exert beneficial effects on PPI deficit. Our findings show that isolation rearing reduces hippocampal KCC2 expression level and may underlie hippocampal GABA excitatory. In addition, 10 mg/kg bumetanide is not effective in improving the reduced PPI of twelve weeks isolation-reared rats. Collectively, our findings show that hippocampal chloride transporter KCC2 contributes to excitatory GABA dysregulation in the developmental rat model of schizophrenia.

Vitamin A Deficiency Exacerbates Gut Microbiota Dysbiosis and Cognitive Deficits in Amyloid Precursor Protein/Presenilin 1 Transgenic Mice

Vitamin A deficiency (VAD) plays an essential role in the pathogenesis of Alzheimer’s disease (AD). However, the specific mechanism by which VAD aggravates cognitive impairment is still unknown. At the intersection of microbiology and neuroscience, the gut-brain axis is undoubtedly contributing to the formation and function of neurological systems, but most of the previous studies have ignored the influence of gut microbiota on the cognitive function in VAD. Therefore, we assessed the effect of VAD on AD pathology and the decline of cognitive function in AD model mice and determined the role played by the intestinal microbiota in the process. Twenty 8-week-old male C57BL/6J amyloid precursor protein/presenilin 1 (APP/PS1) transgenic mice were randomly assigned to either a vitamin A normal (VAN) or VAD diet for 45 weeks. Our results show that VAD aggravated the behavioral learning and memory deficits, reduced the retinol concentration in the liver and the serum, decreased the transcription of vitamin A (VA)-related receptors and VA-related enzymes in the cortex, increased amyloid-β peptides (Aβ40 and Aβ42) in the brain and gut, upregulate the translation of beta-site APP-cleaving enzyme 1 (BACE1) and phosphorylated Tau in the cortex, and downregulate the expression of brain-derived neurotrophic factor (BDNF) and γ-aminobutyric acid (GABA) receptors in the cortex. In addition, VAD altered the composition and functionality of the fecal microbiota as exemplified by a decreased abundance of Lactobacillus and significantly different α- and β-diversity. Of note, the functional metagenomic prediction (PICRUSt analysis) indicated that GABAergic synapse and retinol metabolism decreased remarkably after VAD intervention, which was in line with the decreased expression of GABA receptors and the decreased liver and serum retinol. In summary, the present study provided valuable facts that VAD exacerbated the morphological, histopathological, molecular biological, microbiological, and behavioral impairment in the APP/PS1 transgenic mice, and the intestinal microbiota may play a key mediator role in this mechanism.

Association between the improvement in depressive symptoms and serum BDNF levels in drug-naive first episode patients with schizophrenia: A longitudinal follow-up

Depressive symptoms are frequent clinical manifestations in patients with schizophrenia. Decreased brain-derived neurotrophic factor (BDNF) has been shown to be involved in the development of depressive symptoms. However, the detailed molecular mechanism of BDNF in the depressive symptoms of schizophrenia patients remains to be fully elucidated. This study aimed to investigate the role of BDNF in the depressive symptoms in drug-naïve first-episode (DNFE) patients with schizophrenia and whether BDNF levels were associated with the improvement of depressive symptoms after olanzapine treatment. 50 DNFE schizophrenia patients and 55 healthy controls were recruited, and their serum BDNF levels were compared. All patients were treated with olanzapine monotherapy for 12 weeks, and 45 patients completed the trial. The serum BDNF levels and depressive symptoms were measured again at follow-up. We found that DNFE patients had lower BDNF levels, compared to controls. Last observation carried forward (LOCF) analysis was used for patients who dropped out after the second month, and 50 patients were included in the statistical analysis with LOCF. After 12 weeks of treatment with olanzapine, BDNF levels were significantly increased and depressive symptoms were significantly decreased. Correlation analysis showed that the change of BDNF levels after treatment was correlated with the change of HAMD total score from baseline. Further regression analysis showed that the change in BDNF levels was an independent predictor for the improvement in depressive symptoms, after controlling age, BMI change and the decrease of PANSS total score. However, the baseline BDNF levels were not associated with an improvement in depressive symptoms in patients. Our findings reveal that olanzapine treatment can increase BDNF levels and improve depressive symptoms in schizophrenia patients. Moreover, the changes in serum BDNF levels were related to the improvement in depressive symptoms.

Molecular impacts of childhood abuse on the human brain

Childhood abuse (CA) is a prevalent global health concern, increasing the risk of negative mental health outcomes later in life. In the literature, CA is commonly defined as physical, sexual, and emotional abuse, as well as neglect. Several mental disorders have been associated with CA, including depression, bipolar disorder, schizophrenia, and post-traumatic stress disorder, along with an increased risk of suicide. It is thought that traumatic life events occurring during childhood and adolescence may have a significant impact on essential brain functions, which may persist throughout adulthood. The interaction between the brain and the external environment can be mediated by epigenetic alterations in gene expression, and there is a growing body of evidence to show that such changes occur as a function of CA. Disruptions in the HPA axis, myelination, plasticity, and signaling have been identified in individuals with a history of CA. Understanding the molecular impact of CA on the brain is essential for the development of treatment and prevention measures. In this review, we will summarize studies that highlight the molecular changes associated with CA in the human brain, along with supporting evidence from peripheral studies and animal models. We will also discuss some of the limitations surrounding the study of CA and propose extracellular vesicles as a promising future approach in the field.

Vision is required for the formation of binocular neurons prior to the classical critical period

Depth perception emerges from the development of binocular neurons in primary visual cortex. Vision is required for these neurons to acquire their mature responses to visual stimuli. The prevailing view is that vision does not influence binocular circuitry until the onset of the critical period, about a week after eye opening, and that plasticity of visual responses is triggered by increased inhibition. Here, we show that vision is required to form binocular neurons and to improve binocular tuning and matching from eye opening until critical period closure. Enhancing inhibition does not accelerate this process. Vision soon after eye opening improves the tuning properties of binocular neurons by strengthening and sharpening ipsilateral eye cortical responses. This progressively changes the population of neurons in the binocular pool, and this plasticity is sensitive to interocular differences prior to critical period onset. Thus, vision establishes binocular circuitry and guides binocular plasticity from eye opening.

Activity-based anorexia animal model: a review of the main neurobiological findings

BACKGROUND

The genesis of anorexia nervosa (AN), a severe eating disorder with a pervasive effect on many brain functions such as attention, emotions, reward processing, cognition and motor control, has not yet been understood. Since our current knowledge of the genetic aspects of AN is limited, we are left with a large and diversified number of biological, psychological and environmental risk factors, called into question as potential triggers of this chronic condition with a high relapse rate. One of the most valid and used animal models for AN is the activity-based anorexia (ABA), which recapitulates important features of the human condition. This model is generated from naïve rodents by a self-motivated caloric restriction, where a fixed schedule food delivery induces spontaneous increased physical activity.

AIM

In this review, we sought to provide a summary of the experimental research conducted using the ABA model in the pursuit of potential neurobiological mechanism(s) underlying AN.

METHOD

The experimental work presented here includes evidence for neuroanatomical and neurophysiological changes in several brain regions as well as for the dysregulation of specific neurochemical synaptic and neurohormonal pathways.

RESULTS

The most likely hypothesis for the mechanism behind the development of the ABA phenotype relates to an imbalance of the neural circuitry that mediates reward processing. Evidence collected here suggests that ABA animals show a large set of alterations, involving regions whose functions extend way beyond the control of reward mechanisms and eating habits. Hence, we cannot exclude a primary role of these alterations from a mechanistic theory of ABA induction.

CONCLUSIONS

These findings are not sufficient to solve such a major enigma in neuroscience, still they could be used to design ad hoc further experimental investigation. The prospect is that, since treatment of AN is still challenging, the ABA model could be more effectively used to shed light on the complex AN neurobiological framework, thus supporting the future development of therapeutic strategies but also the identification of biomarkers and diagnostic tools. Anorexia Nervosa (AN) is a severe eating disorder with a dramatic effect on many functions of our brain, such as attention, emotions, cognition and motion control. Since our current knowledge of the genetic aspects behind the development of AN is still limited, many biological, psychological and environmental factors must be taken into account as potential triggers of this condition. One of the most valid animal models for studying AN is the activity-based anorexia (ABA). In this model, rodents spontaneously limit food intake and start performing increased physical activity on a running wheel, a result of the imposition of a fixed time schedule for food delivery. In this review, we provide a detailed summary of the experimental research conducted using the ABA model, which includes extended evidence for changes in the anatomy and function of the brain of ABA rodents. The hope is that such integrated view will support the design of future experiments that will shed light on the complex brain mechanisms behind AN. Such advanced knowledge is crucial to find new, effective strategies for both the early diagnosis of AN and for its treatment.

NGF Eye Administration Recovers the TrkB and Glutamate/GABA Marker Deficit in the Adult Visual Cortex Following Optic Nerve Crush

Eye-drop recombinant human nerve growth factor (ed-rhNGF) has proved to recover the retina and optic nerve damage in animal models, including the unilateral optic nerve crush (ONC), and to improve visual acuity in humans. These data, associated with evidence that ed-rhNGF stimulates the brain derived neurotrophic factor (BDNF) in retina and cortex, suggests that NGF might exert retino-fugal effects by affecting BDNF and its receptor TrkB. To address these questions, their expression and relationship with the GABAergic and glutamatergic transmission markers, GAD65 and GAD67, vesicular inhibitory amino acid transporter (VGAT), and vesicular glutamate transporters 1 and 2 (VGLUT-1 and VGLUT-2) were investigated in adult ONC rats contralateral and ipsilateral visual cortex (VCx). Ed-rhNGF recovers the ONC-induced alteration of GABAergic and glutamatergic markers in contralateral VCx, induces an upregulation of TrkB, which is positively correlated with BDNF precursor (proBDNF) decrease in both VCx sides, and strongly enhances TrkB+ cell soma and neuronal endings surrounded by GAD65 immuno-reactive afferents. These findings contribute to enlarging the knowledge on the mechanism of actions and cellular targets of exogenously administrated NGF, and suggest that ed-rhNGF might act by potentiating the activity-dependent TrkB expression in GAD+ cells in VCx following retina damage and/or ONC.

The critical period: neurochemical and synaptic mechanisms shared by the visual cortex and the brain stem respiratory system

The landmark studies of Wiesel and Hubel in the 1960's initiated a surge of investigations into the critical period of visual cortical development, when abnormal visual experience can alter cortical structures and functions. Most studies focused on the visual cortex, with relatively little attention to subcortical structures. The goal of the present review is to elucidate neurochemical and synaptic mechanisms common to the critical periods of the visual cortex and the brain stem respiratory system in the normal rat. In both regions, the critical period is a time of (i) heightened inhibition; (ii) reduced expression of brain-derived neurotrophic factor (BDNF); and (iii) synaptic imbalance, with heightened inhibition and suppressed excitation. The last two mechanisms are contrary to the conventional premise. Synaptic imbalance renders developing neurons more vulnerable to external stressors. However, the critical period is necessary to enable each system to strengthen its circuitry, adapt to its environment, and transition from immaturity to maturity, when a state of relative synaptic balance is attained. Failure to achieve such a balance leads to neurological disorders.

Development of parvalbumin neurons and perineuronal nets in the visual cortex of normal and dark-exposed cats

During development, the visual system maintains a high capacity for modification by expressing characteristics permissive for plasticity, enabling neural circuits to be refined by visual experience to achieve their mature form. This period is followed by the emergence of characteristics that stabilize the brain to consolidate for lifetime connections that were informed by experience. Attenuation of plasticity potential is thought to derive from an accumulation of plasticity-inhibiting characteristics that appear at ages beyond the peak of plasticity. Perineuronal nets (PNNs) are molecular aggregations that primarily surround fast-spiking inhibitory neurons called parvalbumin (PV) cells, which exhibit properties congruent with a plasticity inhibitor. In this study, we examined the development of PNNs and PV cells in the primary visual cortex of a highly visual mammal, and assessed the impact that 10 days of darkness had on both characteristics. Here, we show that labeling for PV expression emerges earlier and reaches adult levels sooner than PNNs. We also demonstrate that darkness, a condition known to enhance plasticity, significantly reduces the density of PNNs and the size of PV cell somata but does not alter the number of PV cells in the visual cortex. The darkness-induced reduction of PV cell size occurred irrespective of whether neurons were surrounded by a PNN, suggesting that PNNs have a restricted capacity to inhibit plasticity. Finally, we show that PV cells surrounded by a PNN were significantly larger than those without one, supporting the view that PNNs may mediate trophic support to the cells they surround.

Time-delimited signaling of MET receptor tyrosine kinase regulates cortical circuit development and critical period plasticity

Normal development of cortical circuits, including experience-dependent cortical maturation and plasticity, requires precise temporal regulation of gene expression and molecular signaling. Such regulation, and the concomitant impact on plasticity and critical periods, is hypothesized to be disrupted in neurodevelopmental disorders. A protein that may serve such a function is the MET receptor tyrosine kinase, which is tightly regulated developmentally in rodents and primates, and exhibits reduced cortical expression in autism spectrum disorder and Rett Syndrome. We found that the peak of MET expression in developing mouse cortex coincides with the heightened period of synaptogenesis, but is precipitously downregulated prior to extensive synapse pruning and certain peak periods of cortical plasticity. These results reflect a potential on-off regulatory synaptic mechanism for specific glutamatergic cortical circuits in which MET is enriched. In order to address the functional significance of the 'off' component of the proposed mechanism, we created a controllable transgenic mouse line that sustains cortical MET signaling. Continued MET expression in cortical excitatory neurons disrupted synaptic protein profiles, altered neuronal morphology, and impaired visual cortex circuit maturation and connectivity. Remarkably, sustained MET signaling eliminates monocular deprivation-induced ocular dominance plasticity during the normal cortical critical period; while ablating MET signaling leads to early closure of critical period plasticity. The results demonstrate a novel mechanism in which temporal regulation of a pleiotropic signaling protein underlies cortical circuit maturation and timing of cortical critical period, features that may be disrupted in neurodevelopmental disorders.

The role of early visual experience in the development of spatial-frequency preference in the primary visual cortex

KEY POINTS

The mature functioning of the primary visual cortex depends on postnatal visual experience, while the orientation/direction preference is established just after eye-opening, independently of visual experience. In this study, we find that visual experience is required for the normal development of spatial-frequency (SF) preference in mouse primary visual cortex. We show that age- and experience-dependent shifts in optimal SFs towards higher frequencies occurred similarly in excitatory neurons and parvalbumin-positive interneurons. We also show that some excitatory and parvalbumin-positive neurons preferentially responded to visual stimuli consisting of very high SFs and posterior directions, and that the preference was established at earlier developmental stages than the SF preference in the standard frequency range. These results suggest that early visual experience is required for the development of SF representation and shed light on the experience-dependent developmental mechanisms underlying visual cortical functions.

ABSTRACT

Early visual experience is crucial for the maturation of visual cortical functions. It has been demonstrated that the orientation and direction preferences in individual neurons of the primary visual cortex are well established immediately after eye-opening. The postnatal development of spatial frequency (SF) tuning and its dependence on visual experience, however, has not been thoroughly quantified. In this study, macroscopic imaging with flavoprotein autofluorescence revealed that the optimal SFs shift towards higher frequency values during normal development in mouse primary visual cortex. This developmental shift was impaired by binocular deprivation during the sensitive period, postnatal 3 weeks (PW3) to PW6. Furthermore, two-photon Ca²⁺ imaging revealed that the developmental shift of the optimal SFs, depending on visual experience, concurrently occurs in excitatory neurons and parvalbumin-positive inhibitory interneurons (PV neurons). In addition, some excitatory and PV neurons exhibited a preference for visual stimuli consisting of particularly high SFs and posterior directions at relatively early developmental stages; this preference was not affected by binocular deprivation. Thus, there may be two distinct developmental mechanisms for the establishment of SF preference depending on the frequency values. After PW3, SF tuning for neurons tuned to standard frequency ranges was sharper in excitatory neurons and slightly broader in PV neurons, leading to considerably attenuated SF tuning in PV neurons compared to excitatory neurons by PW5. Our findings suggest that early visual experience is far more important than orientation/direction selectivity for the development of the neural representation of the diverse SFs.

Single Exposure to the Cathinones MDPV and α-PVP Alters Molecular Markers of Neuroplasticity in the Adult Mouse Brain

Synthetic cathinones have gained popularity among young drug users and are widely used in the clandestine market. While the cathinone-induced behavioral profile has been extensively investigated, information on their neuroplastic effects is still rather fragmentary. Accordingly, we have exposed male mice to a single injection of MDPV and α-PVP and sacrificed the animals at different time points (i.e., 30 min, 2 h, and 24 h) to have a rapid readout of the effect of these psychostimulants on neuroplasticity in the frontal lobe and hippocampus, two reward-related brain regions. We found that a single, low dose of MDPV or α-PVP is sufficient to alter the expression of neuroplastic markers in the adult mouse brain. In particular, we found increased expression of the transcription factor Npas4, increased ratio between the vesicular GABA transporter and the vesicular glutamate transporter together with changes in the expression of the neurotrophin Bdnf, confirming the widespread impact of these cathinones on brain plasticity. To sum up, exposure to low dose of cathinones can impair cortical and hippocampal homeostasis, suggesting that abuse of these cathinones at much higher doses, as it occurs in humans, could have an even more profound impact on neuroplasticity.

Exercise training modulates glutamic acid decarboxylase-65/67 expression through TrkB signaling to ameliorate neuropathic pain in rats with spinal cord injury

Neuropathic pain is one of the most frequently stated complications after spinal cord injury. In post-spinal cord injury, the decrease of gamma aminobutyric acid synthesis within the distal spinal cord is one of the main causes of neuropathic pain. The predominant research question of this study was whether exercise training may promote the expression of glutamic acid decarboxylase-65 and glutamic acid decarboxylase-67, which are key enzymes of gamma aminobutyric acid synthesis, within the distal spinal cord through tropomyosin-related kinase B signaling, as its synthesis assists to relieve neuropathic pain after spinal cord injury. Animal experiment was conducted, and all rats were allocated into five groups: Sham group, SCI/PBS group, SCI-TT/PBS group, SCI/tropomyosin-related kinase B-IgG group, and SCI-TT/tropomyosin-related kinase B-IgG group, and then T10 contusion SCI model was performed as well as the tropomyosin-related kinase B-IgG was used to block the tropomyosin-related kinase B activation. Mechanical withdrawal thresholds and thermal withdrawal latencies were used for assessing pain-related behaviors. Western blot analysis was used to detect the expression of brain-derived neurotrophic factor, tropomyosin-related kinase B, CREB, p-REB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord. Immunohistochemistry was used to analyze the distribution of CREB, p-CREB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord dorsal horn. The results showed that exercise training could significantly mitigate the mechanical allodynia and thermal hyperalgesia in post-spinal cord injury and increase the synthesis of brain-derived neurotrophic factor, tropomyosin-related kinase B, CREB, p-CREB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord. After the tropomyosin-related kinase B signaling was blocked, the analgesic effect of exercise training was inhibited, and in the SCI-TT/tropomyosin-related kinase B-IgG group, the synthesis of CREB, p-CREB, glutamic acid decarboxylase-65, and glutamic acid decarboxylase-67 within the distal spinal cord were also significantly reduced compared with the SCI-TT/PBS group. This study shows that exercise training may increase the glutamic acid decarboxylase-65 and glutamic acid decarboxylase-67 expression within the spinal cord dorsal horn through the tropomyosin-related kinase B signaling, and this mechanism may play a vital role in relieving the neuropathic pain of rats caused by incomplete SCI.

Implications of Extended Inhibitory Neuron Development

A prolonged developmental timeline for GABA (γ-aminobutyric acid)-expressing inhibitory neurons (GABAergic interneurons) is an amplified trait in larger, gyrencephalic animals. In several species, the generation, migration, and maturation of interneurons take place over several months, in some cases persisting after birth. The late integration of GABAergic interneurons occurs in a region-specific pattern, especially during the early postnatal period. These changes can contribute to the formation of functional connectivity and plasticity, especially in the cortical regions responsible for higher cognitive tasks. In this review, we discuss GABAergic interneuron development in the late gestational and postnatal forebrain. We propose the protracted development of interneurons at each stage (neurogenesis, neuronal migration, and network integration), as a mechanism for increased complexity and cognitive flexibility in larger, gyrencephalic brains. This developmental feature of interneurons also provides an avenue for environmental influences to shape neural circuit formation.

Kv3.1 channels regulate the rate of critical period plasticity

Experience-dependent plasticity within visual cortex is controlled by postnatal maturation of inhibitory circuits, which are both morphologically diverse and precisely connected. Gene-targeted disruption of the voltage-dependent potassium channel K_v3.1 broadens action potentials and reduces net inhibitory function of parvalbumin (PV)-positive GABA subtypes within the neocortex. In mice lacking K_v3.1, the rate of input loss from an eye deprived of vision was slowed two-fold, despite otherwise normal critical period timecourse and receptive field properties. Rapid ocular dominance plasticity was restored by local or systemic enhancement of GABAergic transmission with acute benzodiazepine infusion. Diazepam instead exacerbated a global suppression of slow-wave oscillations during sleep described previously in these mutant mice, which therefore did not account for the rescued plasticity. Rapid ocular dominance shifts closely reflected K_v3.1 gene dosage that prevented prolonged spike discharge of their target pyramidal cells in vivo or the spike amplitude decrement of fast-spiking cells during bouts of high-frequency firing in vitro. Late postnatal expression of this unique channel in fast-spiking interneurons thus subtly regulates the speed of critical period plasticity with implications for mental illnesses.

Adult trkB Signaling in Parvalbumin Interneurons is Essential to Prefrontal Network Dynamics

Inhibitory interneurons expressing parvalbumin (PV) are central to cortical network dynamics, generation of γ oscillations, and cognition. Dysfunction of PV interneurons disrupts cortical information processing and cognitive behavior. Brain-derived neurotrophic factor (BDNF)/tyrosine receptor kinase B (trkB) signaling regulates the maturation of cortical PV interneurons but is also implicated in their adult multidimensional functions. Using a novel viral strategy for cell-type-specific and spatially restricted expression of a dominant-negative trkB (trkB.DN), we show that BDNF/trkB signaling is essential to the integrity and maintenance of prefrontal PV interneurons in adult male and female mice. Reduced BDNF/trkB signaling in PV interneurons in the medial prefrontal cortex (mPFC) resulted in deficient PV inhibition and increased baseline local field potential (LFP) activity in a broad frequency band. The altered network activity was particularly pronounced during increased activation of the prefrontal network and was associated with changed dynamics of local excitatory neurons, as well as decreased modulation of the LFP, abnormalities that appeared to generalize across stimuli and brain states. In addition, our findings link reduced BDNF/trkB signaling in prefrontal PV interneurons to increased aggression. Together our investigations demonstrate that BDNF/trkB signaling in PV interneurons in the adult mPFC is essential to local network dynamics and cognitive behavior. Our data provide direct support for the suggested association between decreased trkB signaling, deficient PV inhibition, and altered prefrontal circuitry.SIGNIFICANCE STATEMENT Brain-derived neurotrophic factor (BDNF)/tyrosine receptor kinase B (trkB) signaling promotes the maturation of inhibitory parvalbumin (PV) interneurons, neurons central to local cortical dynamics, γ rhythms, and cognition. Here, we used a novel viral approach for reduced BDNF/trkB signaling in PV interneurons in the medial prefrontal cortex (mPFC) to establish the role of BDNF/trkB signaling in adult prefrontal network activities. Reduced BDNF/trkB signaling caused pronounced morphologic alterations, reduced PV inhibition, and deficient prefrontal network dynamics. The altered network activity appeared to manifest across stimuli and brain states and was associated with aberrant local field potential (LFP) activities and increased aggression. The results demonstrate that adult BDNF/trkB signaling is essential to PV inhibition and prefrontal circuit function and directly links BDNF/trkB signaling to network integrity in the adult brain.

Deletion of BDNF in Pax2 Lineage-Derived Interneuron Precursors in the Hindbrain Hampers the Proportion of Excitation/Inhibition, Learning, and Behavior

Numerous studies indicate that deficits in the proper integration or migration of specific GABAergic precursor cells from the subpallium to the cortex can lead to severe cognitive dysfunctions and neurodevelopmental pathogenesis linked to intellectual disabilities. A different set of GABAergic precursors cells that express Pax2 migrate to hindbrain regions, targeting, for example auditory or somatosensory brainstem regions. We demonstrate that the absence of BDNF in Pax2-lineage descendants of Bdnf^Pax2KOs causes severe cognitive disabilities. In Bdnf^Pax2KOs, a normal number of parvalbumin-positive interneurons (PV-INs) was found in the auditory cortex (AC) and hippocampal regions, which went hand in hand with reduced PV-labeling in neuropil domains and elevated activity-regulated cytoskeleton-associated protein (Arc/Arg3.1; here: Arc) levels in pyramidal neurons in these same regions. This immaturity in the inhibitory/excitatory balance of the AC and hippocampus was accompanied by elevated LTP, reduced (sound-induced) LTP/LTD adjustment, impaired learning, elevated anxiety, and deficits in social behavior, overall representing an autistic-like phenotype. Reduced tonic inhibitory strength and elevated spontaneous firing rates in dorsal cochlear nucleus (DCN) brainstem neurons in otherwise nearly normal hearing Bdnf^Pax2KOs suggests that diminished fine-grained auditory-specific brainstem activity has hampered activity-driven integration of inhibitory networks of the AC in functional (hippocampal) circuits. This leads to an inability to scale hippocampal post-synapses during LTP/LTD plasticity. BDNF in Pax2-lineage descendants in lower brain regions should thus be considered as a novel candidate for contributing to the development of brain disorders, including autism.

Synaptic Connectivity and Cortical Maturation Are Promoted by the ω-3 Fatty Acid Docosahexaenoic Acid

Brain development is likely impacted by micronutrients. This is supported by the effects of the ω-3 fatty acid docosahexaenoic acid (DHA) during early neuronal differentiation, when it increases neurite growth. Aiming to delineate DHA roles in postnatal stages, we selected the visual cortex due to its stereotypic maturation. Immunohistochemistry showed that young mice that received dietary DHA from birth exhibited more abundant presynaptic and postsynaptic specializations. DHA also increased density and size of synapses in a dose-dependent manner in cultured neurons. In addition, dendritic arbors of neurons treated with DHA were more complex. In agreement with improved connectivity, DHA enhanced physiological parameters of network maturation in vitro, including bursting strength and oscillatory behavior. Aiming to analyze functional maturation of the cortex, we performed in vivo electrophysiological recordings from awake mice to measure responses to patterned visual inputs. Dietary DHA robustly promoted the developmental increase in visual acuity, without altering light sensitivity. The visual acuity of DHA-supplemented animals continued to improve even after their cortex had matured and DHA abolished the acuity plateau. Our findings show that the ω-3 fatty acid DHA promotes synaptic connectivity and cortical processing. These results provide evidence that micronutrients can support the maturation of neuronal networks.

Accelerated Hyper-Maturation of Parvalbumin Circuits in the Absence of MeCP2

Methyl-CpG-binding protein 2 (MeCP2) mutations are the primary cause of Rett syndrome, a severe neurodevelopmental disorder. Cortical parvalbumin GABAergic interneurons (PV) make exuberant somatic connections onto pyramidal cells in the visual cortex of Mecp2-deficient mice, which contributes to silencing neuronal cortical circuits. This phenotype can be rescued independently of Mecp2 by environmental, pharmacological, and genetic manipulation. It remains unknown how Mecp2 mutation can result in abnormal inhibitory circuit refinement. In the present manuscript, we examined the development of GABAergic circuits in the primary visual cortex of Mecp2-deficient mice. We identified that PV circuits were the only GABAergic interneurons to be upregulated, while other interneurons were downregulated. Acceleration of PV cell maturation was accompanied by increased PV cells engulfment by perineuronal nets (PNNs) and by an increase of PV cellular and PNN structural complexity. Interestingly, selective deletion of Mecp2 from PV cells was sufficient to drive increased structure complexity of PNN. Moreover, the accelerated PV and PNN maturation was recapitulated in organotypic cultures. Our results identify a specific timeline of disruption of GABAergic circuits in the absence of Mecp2, indicating a possible cell-autonomous role of MeCP2 in the formation of PV cellular arbors and PNN structures in the visual cortex.

Spontaneous Afferent Activity Carves Olfactory Circuits

Electrical activity has a key role in shaping neuronal circuits during development. In most sensory modalities, early in development, internally generated spontaneous activity sculpts the initial layout of neuronal wiring. With the maturation of the sense organs, the system relies more on sensory-evoked electrical activity. Stimuli-driven neuronal discharge is required for the transformation of immature circuits in the specific patterns of neuronal connectivity that subserve normal brain function. The olfactory system (OS) differs from this organizational plan. Despite the important role of odorant receptors (ORs) in shaping olfactory topography, odor-evoked activity does not have a prominent role in refining neuronal wiring. On the contrary, afferent spontaneous discharge is required to achieve and maintain the specific diagram of connectivity that defines the topography of the olfactory bulb (OB). Here, we provide an overview of the development of olfactory topography, with a focus on the role of afferent spontaneous discharge in the formation and maintenance of the specific synaptic contacts that result in the topographic organization of the OB.

An Extracellular Perspective on CNS Maturation: Perineuronal Nets and the Control of Plasticity

During restricted time windows of postnatal life, called critical periods, neural circuits are highly plastic and are shaped by environmental stimuli. In several mammalian brain areas, from the cerebral cortex to the hippocampus and amygdala, the closure of the critical period is dependent on the formation of perineuronal nets. Perineuronal nets are a condensed form of an extracellular matrix, which surrounds the soma and proximal dendrites of subsets of neurons, enwrapping synaptic terminals. Experimentally disrupting perineuronal nets in adult animals induces the reactivation of critical period plasticity, pointing to a role of the perineuronal net as a molecular brake on plasticity as the critical period closes. Interestingly, in the adult brain, the expression of perineuronal nets is remarkably dynamic, changing its plasticity-associated conditions, including memory processes. In this review, we aimed to address how perineuronal nets contribute to the maturation of brain circuits and the regulation of adult brain plasticity and memory processes in physiological and pathological conditions.