Neurotrophic receptor TrkB activation as an orchestrator of neuronal plasticity

Structural brain plasticity is an essential process to adjust maladapted networks, but it dramatically declines after closure of the critical periods during early postnatal life. Growing evidence suggests, however, that certain interventions, such as environmental enrichment and antidepressant treatment, can reinstate a network plasticity that is similar to that observed during the critical periods. Chronic treatment with the antidepressant fluoxetine, for example, can reopen visual cortex plasticity when combined with monocular deprivation. Further, it promotes the erasure of previously acquired fear memory when combined with extinction training. Fluoxetine can bind to and activate the neurotrophic TrkB receptor and can therefore regulate the downstream pathway to induce synaptic plasticity. Considering that TrkB receptors are expressed in essentially all neurons, the question to be answered is through which neuronal subpopulation are the plasticity effects regulated within these two circuitries. Visual cortex plasticity is tightly regulated by the inhibitory Parvalbumin (PV)-specific GABAergic network, which highly expresses TrkB receptors. During the critical periods TrkB’s ligand BrainDerived Neurotrophic Factor (BDNF) promotes the maturation of PV interneurons, thereby stimulating a precocious onset of critical periods. Hence, our first aim was to understand TrkB actions specifically in PV interneurons and their subsequent effects on visual cortex plasticity during adulthood. We used optically activated TrkB (optoTrkB) expressed only in PV interneurons of the visual cortex and found that optoTrkB activation by light combined with monocular deprivation is sufficient to induce ocular dominance plasticity. Strikingly, optoTrkB activation rapidly induces LTP in layer II/III of the visual cortex after theta burst stimulation (TBS). This potentiation in excitatory transmission is mediated by rapid decreases in the intrinsic excitability of PV regulated by reduced expressions of Kv3.1 and Kv3.2 mRNA. In addition, optoTrkB activation promotes the removal of perineuronal nets (PNNs) and shifts the PV and PNN networks into a plastic, immature configuration. Conversely, deleting TrkB from PV interneurons and using chronic fluoxetine treatment to pharmacologically induce plasticity prevented the effects of fluoxetine treatment. Our second aim was to identify the effects of optoTrkB activation expressed specifically in pyramidal neurons of the ventral hippocampus on the fear circuitry. We therefore directed the expression of optoTrkB to pyramidal neurons of the ventral hippocampus. During fear extinction optoTrkB was activated with light, and spontaneous recovery and fear renewal were tested one and three (remote memory) weeks after extinction training. We found that optoTrkB activation during extinction training promoted the erasure of remote fear memory. This effect was accompanied by increased LTP expression after brief TBS stimulation. Finally, fluoxetine and methylmercury (MeHg) are a common intervention and stressor, respectively, in our society, and exposure to either during pregnancy is known to impact brain development and functioning. An altered critical period can result in impairments that are retained into adulthood. Our aim was to understand how perinatal exposure to fluoxetine or MeHg affects the development of PV and PNNs, two well-established markers for the time course of critical periods, in the hippocampus and basolateral amygdala. We found that upon closure of the normal critical periods (P24) the number of PV and PNNs, and PV cell intensity increase. Perinatal fluoxetine treatment resulted in reduced expression of PNNs throughout critical periods, indicating a delayed closure. In contrast, perinatal MeHg exposure impaired the development of PV interneurons and PV expression at the onset of critical periods (P17), which were, however, restored upon critical period closure (P24), suggesting a delayed onset. Our results provide new evidence that TrkB activation in PV interneurons rapidly orchestrates cortical networks by reducing the intrinsic excitability of PV cells regulated by decreased expression of Kv3.1 and Kv3.2 channels, subsequently promoting excitatory transmission. In contrast, TrkB activation in pyramidal neurons of the ventral hippocampus also potentiates excitatory transmission. These opposite findings demonstrate that TrkB employs different mechanisms to increase the excitability of the neuronal network to induce plasticity. We propose that TrkB is a promising therapeutic target for the treatment of neuropsychiatric diseases that benefit from high plasticity modes. We further shed light on the effects of fluoxetine and MeHg exposure during pregnancy on the time course of the critical periods, which can help in developing better guidelines for the use and consumption of both during pregnancy.Aivojen rakenteellinen muovautuvuus on keskeinen prosessi hermoverkkojen hienosäädössä erityisesti silloin kun signalointi on jotenkin häiriintynyt. Tämän mekanismin aktiivisuus kuitenkin laskee huomattavasti pian syntymän jälkeen kriittisten periodien sulkeutuessa. Kasvava määrä todisteita viittaa siihen että jotkin interventiot, kuten elinympäristön rikastuttaminen tai masennuslääkehoito voivat palauttaa hermoverkkojen muovautumiskyvyn kriittisen periodin kaltaiselle tasolle. Esimerkiksi pitkäaikainen hoito masennuslääke fluoksetiinilla voi palauttaa muovautumiskyvyn näköaivokuorella, kun se yhdistetään silmän sulkemiseen. Tämän lisäksi fluoksetiinihoidon on todettu edistävän pelkomuiston häviämistä kun se yhdistetään häviämistä edistävään harjoitukseen. Fluoksetiini sitoutuu neurotrofiinireseptori TrkB:hen ja aktivoi sen ja siten säätelee synaptista muovatuvuutta lisääviä signalointireittejä. Vaikka TrkB-reseptori ilmenee laajasti hermostossa, on vielä epäselvää mikä hermosolupopulaatio välittää signaloinnin vaikutukset. Parvalbumiinia ilmentävät ja GABA-välittäjäainetta käyttävät välihermosolut muodostavat hermoverkon, joka säätelee tarkoin näköaivokuoren muovatuvuutta. Nämä solut ilmentävät myös suuria määriä TrkB- reseptoria. Kriittisen periodin aikana TrkB:n ligandi aivoperäinen hermokasvutekijä BDNF edistää PV- välittäjähermosolujen kypsymistä joka taas aikaistaa kriittisten periodien sulkeutumista. Ensimmäinen tavoitteemme oli siis ymmärtää TrkB:n toimintoja erityisesti PV-välihermosoluissa ja tämän vaikutuksia aikuisten hiirten näköaivokuoressa. Hyödynsimme tutkimuksissa valoaktivoituvaa TrkB-reseptoria (optoTrkB), jota ilmennettiin vain näköaivokuoren PV-välittäjähermosoluissa ja huomasimme, että optoTrkB:n aktivointi valolla yhdistettynä silmän sulkemiseen oli riittävä aikaansaamaan silmän dominanssimuutoksen. OptoTrkB:n aktivointi indusoi häkellyttävällä tavalla sähköstimulaation aikaansaamaa hermosolujen pitkäkestoista herkistymistä (LTP) näköaivokuoren kerroksella II/III. Tämä hermoverkkoja kiihdyttävän viestinnän aktivoituminen johtuu parvalbumiinisolujen aktivoitumiskynnyksen nopeasta laskusta, jota säätelee kaliumkanavien vähentynyt ilmentyminen. Tämän lisäksi optoTrkB:n aktivointi edistää perineuronaalisten verkkojen (PNN) hajoamista ja kääntää PV:n ja PNN:n viestintäverkostot muuntumiskykyiseen tilaan. TrkB-reseptorin poistaminen PV-välihermosoluista taas estää fluoksetiinihoidon farmakologiset vaikutukset hermoverkkojen muovautumiskykyyn. Toinen tavoitteemme oli tutkia optoTrkB-aktivaation vaikutuksia ventraalisen hippokampuksen pyramidisoluissa hermoverkossa, joka välittää pelkosignaaleja. OptoTrkB aktivoitiin valolla samalla, kun pelkoreaktiota hälvennettiin, minkä jälkeen spontaania toipumista ja pelon uusiutumista seurattiin yhden ja kolmen viikon päästä pelonhälventämisharjoituksista. Ilmeni, että optoTrkB:n aktivointi yhdessä pelon hälventämisharjoituksen kanssa edistää pelkomuiston häviämistä. Tämän vaikutuksen lisäksi LTP:n ilmentymisen todettiin kohonneen lyhyen sähköstimulaation jälkeen. Fluoksetiini ja metyylielohopea (MeHg) vaikuttavat kumpikin eri lailla todistetusti aivojen kehitykseen ja toimintaan kun niille altistutaan raskauden aikana. Muuntunut kriittinen periodi voi johtaa vielä aikuisuudessakin ilmeneviin heikentyneisiin toimintoihin. Tavoitteenamme oli ymmärtää, miten syntymänaikainen altistuminen fluoksetiinille tai metyylielohopealle vaikuttaa PV:n ja PNN:n kehitykseen, sillä molemmat ovat vakiintuneita kriittisen periodin ajoitukseen liittyviä merkkiaineita hippokampuksessa ja mantelitumakkeessa. Selvisi, että normaalin kriittisen periodin sulkeutuessa (P24) PV-solujen intensiteetti kasvoi kuten myös PV:n ja PNN:n määrä. Syntymänaikainen fluoksetiinikäsittely johti vähentyneeseen PNN:n ilmentymiseen läpi koko kriittisen periodin, mikä viittaa periodin viivästyneeseen sulkeutumiseen. Metyylielohopealle altistuminen taas heikensi PV-välittäjähermosolujen kehitystä ja PV:n ilmentymistä kriittisen periodin alussa (P17), mikä kuitenkin palautui normaalitasolle periodin sulkeutuessa, viitaten viivästyneeseen kriittisen periodin alkuun. Tuloksemme osoittavat, että TrkB-reseptorin aktivaatio PV-välittäjähermosoluissa nopeasti orkestroi aivokuoren hermoverkkoja madaltamalla PV-solujen stimulointikynnystä kaliumkanavien vähentyneen ilmentymisen välityksellä, mikä taas edistää kiihdyttävää viestinvälitystä. Toisaalta TrkB-reseptorin aktivointi ventraalisen hippokampuksen pyramiidisoluissa vahvistaa niin ikään kiihdyttävää viestinvälitystä. Nämä TrkB:n vastakkaiset kiihdyttävissä ja hiljentävissä hermosoluissa osoittavat, että TrkB käyttää eri soluissa eri mekanismeja jotka kuitenkin molemmat madaltavat hermoverkkojen stimulointikynnystä ja siten edistävät muovautuvuutta. Esitämme TrkB-reseptorin olevan lupaava hoitokohde sellaisten neuropsykiatristen sairauksien hoidossa, joita lisääntynyt muovautuvuus parantaa. Tuloksemme auttavat myös ymmärtämään raskaudenaikaisen fluoksetiinille ja metyylielohopealle altistumisen vaikutuksia kriittisten periodien ajoittumiseen. Tätä voitaisiin käyttää hyväksi kun kehitetään näiden aineiden raskaudenaikaista käytöä ja altistumista koskevia säännöksiä ja ohjeita

Chondroitinase and Antidepressants Promote Plasticity by Releasing TRKB from Dephosphorylating Control of PTP sigma in Parvalbumin Neurons

Perineuronal nets (PNNs) are an extracellular matrix structure rich in chondroitin sulfate proteoglycans (CSPGs), which preferentially encase parvalbumin-containing (PV+) interneurons. PNNs restrict cortical network plasticity but the molecular mechanisms involved are unclear. We found that reactivation of ocular dominance plasticity in the adult visual cortex induced by chondroitinase ABC (chABC)-mediated PNN removal requires intact signaling by the neurotrophin receptor TRKB in PV+ neurons. Additionally, we demonstrate that chABC increases TRKB phosphorylation (pTRKB), while PNN component aggrecan attenuates brain-derived neurotrophic factor (BDNF)-induced pTRKB in cortical neurons in culture. We further found that protein tyrosine phosphatase sigma (PTP sigma, PTPRS), receptor for CSPGs, interacts with TRKB and restricts TRKB phosphorylation. PTP sigma deletion increases phosphorylation of TRKB in vitro and in vivo in male and female mice, and juvenile-like plasticity is retained in the visual cortex of adult PTP sigma-deficient mice (PTP sigma(+/-)). The antidepressant drug fluoxetine, which is known to promote TRKB phosphorylation and reopen critical period-like plasticity in the adult brain, disrupts the interaction between TRKB and PTP sigma by binding to the transmembrane domain of TRKB. We propose that both chABC and fluoxetine reopen critical period-like plasticity in the adult visual cortex by promoting TRKB signaling in PV+ neurons through inhibition of TRKB dephosphorylation by the PTP sigma-CSPG complex.Peer reviewe

Pharmacological and optical activation of TrkB in Parvalbumin interneurons regulate intrinsic states to orchestrate cortical plasticity

Elevated states of brain plasticity typical for critical periods of early postnatal life can be reinstated in the adult brain through interventions, such as antidepressant treatment and environmental enrichment, and induced plasticity may be critical for the antidepressant action. Parvalbumin-positive (PV) interneurons regulate the closure of developmental critical periods and can alternate between high and low plasticity states in response to experience in adulthood. We now show that PV plasticity states and cortical networks are regulated through the activation of TrkB neurotrophin receptors. Visual cortical plasticity induced by fluoxetine, a widely prescribed selective serotonin reuptake inhibitor (SSRI) antidepressant, was lost in mice with reduced expression of TrkB in PV interneurons. Conversely, optogenetic gain-of-function studies revealed that activation of an optically activatable TrkB (optoTrkB) specifically in PV interneurons switches adult cortical networks into a state of elevated plasticity within minutes by decreasing the intrinsic excitability of PV interneurons, recapitulating the effects of fluoxetine. TrkB activation shifted cortical networks towards a low PV configuration, promoting oscillatory synchrony, increased excitatory-inhibitory balance, and ocular dominance plasticity. OptoTrkB activation promotes the phosphorylation of Kv3.1 channels and reduces the expression of Kv3.2 mRNA providing a mechanism for the lower excitability. In addition, decreased expression and puncta of Synaptotagmin2 (Syt2), a presynaptic marker of PV interneurons involved in Ca2+-dependent neurotransmitter release, suggests lower inputs onto pyramidal neurons suppressing feed-forward inhibition. Together, the results provide mechanistic insights into how TrkB activation in PV interneurons orchestrates the activity of cortical networks and mediating antidepressant responses in the adult brain.Peer reviewe

Polygenic risk score for schizophrenia was not associated with glycemic level (HbA1c) in patients with non-affective psychosis: Genetic Risk and Outcome of Psychosis (GROUP) cohort study

Introduction: Type 2 diabetes (T2D) is a common comorbidity in patients with schizophrenia (SCZ). The underlying pathophysiologic mechanisms are yet to be fully elucidated, although it can be argued that shared genes, environmental factors or their interaction effect are involved. This study investigated the association between polygenic risk score of SCZ (PRSSCZ) and glycated haemoglobin (HbA1c) while adjusting for polygenic risk score of T2D (PRST2D), and clinical and demographic covariables. Methods: Genotype, clinical and demographic data of 1129 patients with non-affective psychosis were extracted from Genetic Risk and Outcome of Psychosis (GROUP) cohort study. The glycated haemoglobin (HbA1c) was the outcome. PRS was calculated using standard methods. Univariable and multivariable linear regression analyses were applied to estimate associations. Additionally, sensitivity analysis based on multiple imputation was done. After correction for multiple testing, a two-sided p-value ≤.003 was considered to discover evidence for an association. Results: Of 1129 patients, 75.8% were male with median age of 29 years. The mean (standard deviation) HbA1c level was 35.1 (5.9) mmol/mol. There was no evidence for an association between high HbA1c level and increased PRSSCZ (adjusted regression coefficient (aβ) = 0.69, standard error (SE) = 0.77, p-value =.37). On the other hand, there was evidence for an association between high HbA1c level and increased PRST2D (aβ = 0.93, SE = 0.32, p-value =.004), body mass index (aβ = 0.20, SE = 0.08, p-value =.01), diastolic blood pressure (aβ = 0.08, SE = 0.04, p-value =.03), late age of first psychosis onset (aβ = 0.19, SE = 0.05, p-value =.0004) and male gender (aβ = 1.58, SE = 0.81, p-value =.05). After multiple testing correction, there was evidence for an association between high HbA1c level and late age of first psychosis onset. Evidence for interaction effect between PRSscz and antipsychotics was not observed. The multiple imputation-based sensitivity analysis provided consistent results with complete case analysis. Conclusions: Glycemic dysregulation in patients with SCZ was not associated with PRSSCZ. This suggests that the mechanisms of hyperglycemia or diabetes are at least partly independent from genetic predisposition to SCZ. Our findings show that the change in HbA1c level can be caused by at least in part due to PRST2D, late age of illness onset, male gender, and increased body mass index and diastolic blood pressure

Genetic copy number variants, cognition and psychosis: a meta-analysis and a family study

The burden of large and rare copy number genetic variants (CNVs) as well as certain specific CNVs increase the risk of developing schizophrenia. Several cognitive measures are purported schizophrenia endophenotypes and may represent an intermediate point between genetics and the illness. This paper investigates the influence of CNVs on cognition. We conducted a systematic review and meta-analysis of the literature exploring the effect of CNV burden on general intelligence. We included ten primary studies with a total of 18,847 participants and found no evidence of association. In a new psychosis family study, we investigated the effects of CNVs on specific cognitive abilities. We examined the burden of large and rare CNVs (>200 kb, <1% MAF) as well as known schizophrenia-associated CNVs in patients with psychotic disorders, their unaffected relatives and controls (N = 3428) from the Psychosis Endophenotypes International Consortium (PEIC). The carriers of specific schizophrenia-associated CNVs showed poorer performance than non-carriers in immediate (P = 0.0036) and delayed (P = 0.0115) verbal recall. We found suggestive evidence that carriers of schizophrenia-associated CNVs had poorer block design performance (P = 0.0307). We do not find any association between CNV burden and cognition. Our findings show that the known high-risk CNVs are not only associated with schizophrenia and other neurodevelopmental disorders, but are also a contributing factor to impairment in cognitive domains such as memory and perceptual reasoning, and act as intermediate biomarkers of disease risk.This work was supported by the Medical Research Council (G0901310) and the Wellcome Trust (grants 085475/B/08/Z, 085475/Z/08/Z). This study was supported by the NIHR Biomedical Research Centre at University College London Hospitals NHS Foundation Trust and University College London and by the NIHR Biomedical Research Centre for Mental Health at the South London and Maudsley NHS Foundation Trust at King’s College London. Further support to EB: Mental Health Research UK’s John Grace QC award, BMA Margaret Temple grants 2016 and 2006, MRC—Korean Health Industry Development Institute Partnering Award (MC_PC_16014), MRC New Investigator Award and a MRC Centenary Award (G0901310), National Institute of Health Research UK post-doctoral fellowship, the Psychiatry Research Trust, the Schizophrenia Research Fund, the Brain and Behaviour Research foundation’s NARSAD Young Investigator Awards 2005, 2008, Wellcome Trust Research Training Fellowship, the NIHR Biomedical Research Centre at UCLH, and the NIHR Biomedical Research Centre for Mental Health at the South London and Maudsley NHS Foundation Trust and Institute of Psychiatry King’s College London. Further support to co-authors: The Brain and Behaviour Research foundation’s (NARSAD’s) Young Investigator Award (Grant 22604, awarded to CI). The BMA Margaret Temple grant 2016 to JT. A 2014 European Research Council Marie Curie award to A Díez-Revuelta. HI has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 747429. A Medical Research Council doctoral studentship to JH-S, IA-Z and AB. A Mental Health Research UK studentship to RM. VB is supported by a Wellcome Trust Seed Award in Science (200589/Z/16/Z). FWO Senior Clinical Fellowship to RvW. The infrastructure for the GROUP consortium is funded through the Geestkracht programme of the Dutch Health Research Council (ZON-MW, grant number 10-000-1001), and matching funds from participating pharmaceutical companies (Lundbeck, AstraZeneca, Eli Lilly, Janssen Cilag) and universities and mental health care organisations (Amsterdam: Academic Psychiatric Centre of the Academic Medical Centre and the mental health institutions: GGZ Ingeest, Arkin, Dijk en Duin, GGZ Rivierduinen, Erasmus Medical Centre, GGZ Noord Holland Noord. Groningen: University Medical Centre Groningen and the mental health institutions: Lentis, GGZ Friesland, GGZ Drenthe, Dimence, Mediant, GGNet Warnsveld, Yulius Dordrecht and Parnassia psycho-medical centre The Hague. Maastricht: Maastricht University Medical Centre and the mental health institutions: GGZ Eindhoven en De Kempen, GGZ Breburg, GGZ Oost-Brabant, Vincent van Gogh voor Geestelijke Gezondheid, Mondriaan, Virenze riagg, Zuyderland GGZ, MET ggz, Universitair Centrum Sint-Jozef Kortenberg, CAPRI University of Antwerp, PC Ziekeren Sint-Truiden, PZ Sancta Maria Sint-Truiden, GGZ Overpelt, OPZ Rekem. Utrecht: University Medical Centre Utrecht and the mental health institutions Altrecht, GGZ Centraal and Delta). The Santander cohort was supported by Instituto de Salud Carlos III (PI020499, PI050427, PI060507), SENY Fundació (CI 2005-0308007), Fundacion Ramón Areces and Fundacion Marqués de Valdecilla (API07/011, API10/13). We thank Valdecilla Biobank for providing the biological PAFIP samples and associated data included in this study and for its help in the technical execution of this work; we also thank IDIVAL Neuroimaging Unit for its help in the acquisition and processing of imaging PAFIP data

Cognitive functioning throughout adulthood and illness stages in individuals with psychotic disorders and their unaffected siblings.

Important questions remain about the profile of cognitive impairment in psychotic disorders across adulthood and illness stages. The age-associated profile of familial impairments also remains unclear, as well as the effect of factors, such as symptoms, functioning, and medication. Using cross-sectional data from the EU-GEI and GROUP studies, comprising 8455 participants aged 18 to 65, we examined cognitive functioning across adulthood in patients with psychotic disorders (n = 2883), and their unaffected siblings (n = 2271), compared to controls (n = 3301). An abbreviated WAIS-III measured verbal knowledge, working memory, visuospatial processing, processing speed, and IQ. Patients showed medium to large deficits across all functions (ES range = -0.45 to -0.73, p < 0.001), while siblings showed small deficits on IQ, verbal knowledge, and working memory (ES = -0.14 to -0.33, p < 0.001). Magnitude of impairment was not associated with participant age, such that the size of impairment in older and younger patients did not significantly differ. However, first-episode patients performed worse than prodromal patients (ES range = -0.88 to -0.60, p < 0.001). Adjusting for cannabis use, symptom severity, and global functioning attenuated impairments in siblings, while deficits in patients remained statistically significant, albeit reduced by half (ES range = -0.13 to -0.38, p < 0.01). Antipsychotic medication also accounted for around half of the impairment in patients (ES range = -0.21 to -0.43, p < 0.01). Deficits in verbal knowledge, and working memory may specifically index familial, i.e., shared genetic and/or shared environmental, liability for psychotic disorders. Nevertheless, potentially modifiable illness-related factors account for a significant portion of the cognitive impairment in psychotic disorders.The European Community’s Seventh Framework Programme under grant agreement No. HEALTH-F2-2010-241909 (EU-GEI)

Targeted Sequencing of 10,198 Samples Confirms Abnormalities in Neuronal Activity and Implicates Voltage-Gated Sodium Channels in Schizophrenia Pathogenesis

Background Sequencing studies have pointed to the involvement in schizophrenia of rare coding variants in neuronally expressed genes, including activity-regulated cytoskeleton-associated protein (ARC) and N-methyl-D-aspartate receptor (NMDAR) complexes; however, larger samples are required to reveal novel genes and specific biological mechanisms. Methods We sequenced 187 genes, selected for prior evidence of association with schizophrenia, in a new dataset of 5207 cases and 4991 controls. Included among these genes were members of ARC and NMDAR postsynaptic protein complexes, as well as voltag

iPlasticity : Induced juvenile-like plasticity in the adult brain as a mechanism of antidepressants

The network hypothesis of depression proposes that mood disorders reflect problems in information processing within particular neural networks. Antidepressants (AD), including selective serotonin reuptake inhibitors (SSRI), function by gradually improving information processing within these networks. AD have been shown to induce a state of juvenile-like plasticity comparable to that observed during developmental critical periods: Such critical-period-like plasticity allows brain networks to better adapt to extrinsic and intrinsic signals. We have coined this drug-induced state of juvenile-like plasticity iPlasticity.' A combination of iPlasticity induced by chronic SSRI treatment together with training, rehabilitation, or psychotherapy improves symptoms of neuropsychiatric disorders and issues underlying the developmentally or genetically malfunctioning networks. We have proposed that iPlasticity might be a critical component of AD action. We have demonstrated that iPlasticity occurs in the visual cortex, fear erasure network, extinction of aggression caused by social isolation, and spatial reversal memory in rodent models. Chronic SSRI treatment is known to promote neurogenesis and to cause dematuration of granule cells in the dentate gyrus and of interneurons, especially parvalbumin interneurons enwrapped by perineuronal nets in the prefrontal cortex, visual cortex, and amygdala. Brain-derived neurotrophic factor (BDNF), via its receptor tropomyosin kinase receptor B, is involved in the processes of synaptic plasticity, including neurogenesis, neuronal differentiation, weight of synapses, and gene regulation of synaptic formation. BDNF can be activated by both chronic SSRI treatment and neuronal activity. Accordingly, the BDNF/tropomyosin kinase receptor B pathway is critical for iPlasticity, but further analyses will be needed to provide mechanical insight into the processes of iPlasticity.Peer reviewe

Kainate Receptor Auxiliary Subunit NETO2-Related Cued Fear Conditioning Impairments Associate with Defects in Amygdala Development and Excitability

NETO2 is an auxiliary subunit for kainate-type glutamate receptors that mediate normal cued fear expression and extinction. Since the amygdala is critical for these functions, we asked whether Neto2−/− mice have compromised amygdala function. We measured the abundance of molecular markers of neuronal maturation and plasticity, parvalbumin-positive (PV+), perineuronal net-positive (PNN+), and double positive (PV+PNN+) cells in the Neto2−/− amygdala. We found that Neto2−/− adult, but not postnatal day (P)23, mice had 7.5% reduction in the fraction of PV+PNN+ cells within the total PNN+ population, and 23.1% reduction in PV staining intensity compared with Neto2+/+ mice, suggesting that PV interneurons in the adult Neto2−/− amygdala remain in an immature state. An immature PV inhibitory network would be predicted to lead to stronger amygdalar excitation. In the amygdala of adult Neto2−/− mice, we identified increased glutamatergic and reduced GABAergic transmission using whole-cell patch-clamp recordings. This was accompanied by increased spine density of thin dendrites in the basal amygdala (BA) compared with Neto2+/+ mice, indicating stronger glutamatergic synapses. Moreover, after fear acquisition Neto2−/− mice had a higher number of c-Fos-positive cells than Neto2+/+ mice in the lateral amygdala (LA), BA, and central amygdala (CE). Altogether, our findings indicate that Neto2 is involved in the maturation of the amygdala PV interneuron network. Our data suggest that this defect, together with other processes influencing amygdala principal neurons, contribute to increased amygdalar excitability, higher fear expression, and delayed extinction in cued fear conditioning, phenotypes that are common in fear-related disorders, including the posttraumatic stress disorder (PTSD).Peer reviewe

Activation of TrkB in Parvalbumin interneurons is required for the promotion of reversal learning in spatial and fear memory by antidepressants

Critical period-like plasticity (iPlasticity) can be reinstated in the adult brain by several interventions, including drugs and optogenetic modifications. We have demonstrated that a combination of iPlasticity with optimal training improves behaviors related to neuropsychiatric disorders. In this context, the activation of TrkB, a receptor for BDNF, in Parvalbumin-positive (PV+) interneurons has a pivotal role in cortical network changes. However, it is unknown if the activation of TrkB in PV+ interneurons is important for other plasticity-related behaviors, especially for learning and memory. Here, using mice with heterozygous conditional TrkB deletion in PV+ interneurons (PV-TrkB hCKO) in IntelliCage and fear erasure paradigms, we show that chronic treatment with fluoxetine, a widely prescribed antidepressant drug that is known to promote the activation of TrkB, enhances behavioral flexibility in spatial and fear memory, largely depending on the expression of the TrkB receptor in PV+ interneurons. In addition, hippocampal long-term potentiation was enhanced by chronic treatment with fluoxetine in wild-type mice, but not in PV-TrkB hCKO mice. Transcriptomic analysis of PV+ interneurons after fluoxetine treatment indicated intrinsic changes in synaptic formation and downregulation of enzymes involved in perineuronal net formation. Consistently, immunohistochemistry has shown that the fluoxetine treatment alters PV expression and reduces PNNs in PV+ interneurons, and here we show that TrkB expression in PV+ interneurons is required for these effects. Together, our results provide molecular and network mechanisms for the induction of critical period-like plasticity in adulthood.Peer reviewe