Mice lacking TrkB in parvalbumin-positive cells exhibit sexually dimorphic behavioral phenotypes

Parvalbumin interneurons in human ventromedial prefrontal cortex: a comprehensive post-mortem study of myelination and perineuronal nets in neurotypical individuals and depressed suicides with and without a history of child abuse

Cortical parvalbumin interneurons (PV+) are major regulators of excitatory/inhibitory information processing, and their maturation is associated with the opening of developmental critical periods (CP). Recent studies reveal that cortical PV+ axons are myelinated, and that myelination along with perineuronal net (PNN) maturation around PV+ cells is associated with the closures of CP. Although PV+ interneurons are susceptible to early-life stress, their relationship between their myelination and PNN coverage remains unexplored. This study compared the fine features of PV+ interneurons in well-characterized human post-mortem ventromedial prefrontal cortex samples (n = 31) from depressed suicides with or without a history of child abuse (CA) and matched controls. In healthy controls, 81% of all sampled PV+ interneurons displayed a myelinated axon, while a subset (66%) of these cells also displayed a PNN, proposing a relationship between both attributes. Intriguingly, a 3-fold increase in the proportion of unmyelinated PV+ interneurons with a PNN was observed in CA victims, along with greater PV-immunofluorescence intensity in myelinated PV+ cells with a PNN. This study, which is the first to provide normative data on myelination and PNNs around PV+ interneurons in human neocortex, sheds further light on the cellular and molecular consequences of early-life adversity on cortical PV+ interneurons.

Sex differences in socioemotional behavior and changes in ventral hippocampal transcription across aging in C57Bl/6J mice

Socioemotional health is positively correlated with improved cognitive and physical aging. Despite known sex differences in socioemotional behaviors and the trajectory of aging, the interactive effects between sex and aging on socioemotional outcomes are poorly understood. We performed the most comprehensive assessment of sex differences in socioemotional behaviors in C57Bl/6J mice across aging to date. Compared to males, females exhibited decreased anxiety-like behavior and social preference but increased social recognition. With age, anxiety-like behavior, cued threat memory generalization, and social preference increased in both sexes. To investigate potential neural mechanisms underlying these behavioral changes, we analyzed transcriptional neuropathology markers in the ventral hippocampus and found age-related changes in genes related to activated microglia, angiogenesis, and cytokines. Sex differences emerged in the timing, direction, and magnitude of these changes, independent of reproductive senescence in aged females. Interestingly, female-specific upregulation of autophagy-related genes correlated with age-related behavioral changes selectively in females. These novel findings reveal critical sex differences in trajectories of ventral hippocampal aging that may contribute to sex- and age-related differences in socioemotional outcomes.

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.

[Research for sex differences in fear extinction]

Fear memory has an important role in survival, but excess fear memory could be cause to fear-related disorders such as post traumatic disorder (PTSD). People who undergo a fearful event do not always develop PTSD, suggesting that there are vulnerable factors to develop the disorder. It is believed that sex, in particular female, is one of the factors as it is known that the prevalence rate of PTSD is higher in women than in men. However, the biological mechanisms underlying the sex differences in their prevalence rate remain unclear. Because previous studies reported that patients with fear-related disorders show an impairment of fear extinction, investigation of sex differences in fear extinction may be useful for understanding the greater vulnerability of women to fear-related disorders. Although number of papers investigating sex differences in fear extinction, these reports have increased recently. This review introduces fear extinction, sex differences in fear extinction is low, and the molecular mechanisms of sex differences in fear extinction. In the molecular mechanisms, we focused brain-derived neurotrophic factor (BDNF)-tropomyosin-related kinase B (TrkB) receptor signal which has been investigated by independent research groups.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

A brain-specific pgc1α fusion transcript affects gene expression and behavioural outcomes in mice

This study shows that loss of a brain-specific fusion isoform of PGC1a leads to up-regulation of genes and motor impairments in mice, suggesting functional differences between PGC1 isoforms in the brain.

PGC1α is a transcriptional coactivator in peripheral tissues, but its function in the brain remains poorly understood. Various brain-specific Pgc1α isoforms have been reported in mice and humans, including two fusion transcripts (FTs) with non-coding repetitive sequences, but their function is unknown. The FTs initiate at a simple sequence repeat locus ∼570 Kb upstream from the reference promoter; one also includes a portion of a short interspersed nuclear element (SINE). Using publicly available genomics data, here we show that the SINE FT is the predominant form of Pgc1α in neurons. Furthermore, mutation of the SINE in mice leads to altered behavioural phenotypes and significant up-regulation of genes in the female, but not male, cerebellum. Surprisingly, these genes are largely involved in neurotransmission, having poor association with the classical mitochondrial or antioxidant programs. These data expand our knowledge on the role of Pgc1α in neuronal physiology and suggest that different isoforms may have distinct functions. They also highlight the need for further studies before modulating levels of Pgc1α in the brain for therapeutic purposes.

Deletion of TrkB in parvalbumin interneurons alters cortical neural dynamics

Signaling by neurotrophins such as the brain‐derived neurotrophic factor (BDNF) is known to modulate development of interneurons, but the circuit effects of this modulation remain unclear. Here, we examined the impact of deleting TrkB, a BDNF receptor, in parvalbumin‐expressing (PV) interneurons on the balance of excitation and inhibition (E‐I) in cortical circuits. In the mouse olfactory cortex, TrkB deletion impairs multiple aspects of PV neuronal function including synaptic excitation, intrinsic excitability, and the innervation pattern of principal neurons. Impaired PV cell function resulted in aberrant spiking patterns in principal neurons in response to stimulation of sensory inputs. Surprisingly, dampened PV neuronal function leads to a paradoxical decrease in overall excitability in cortical circuits. Our study demonstrates that, by modulating PV circuit plasticity and development, TrkB plays a critical role in shaping the evoked pattern of activity in a cortical network.

Effects of repeated anodal transcranial direct current stimulation on auditory fear extinction in C57BL/6J mice

BACKGROUND

Trauma-based psychotherapy is a first line treatment for post-traumatic stress disorder (PTSD) but not all patients achieve long-term remission. Transcranial direct current stimulation (tDCS) received considerable attention as a neuromodulation method that may improve trauma-based psychotherapy.

OBJECTIVE

We explored the effects of repeated anodal tDCS over the prefrontal cortex (PFC) on fear extinction in mice as a preclinical model for trauma-based psychotherapy.

METHODS

We performed auditory fear conditioning with moderate or high shock intensity on C57BL6/J mice. Next, mice received anodal tDCS (0.2 mA, 20 min) or sham stimulation over the PFC twice daily for five consecutive days. Extinction training was performed by repeatedly exposing mice to the auditory cue the day after the last stimulation session. Early and late retention of extinction were evaluated one day and three weeks after extinction training respectively.

RESULTS

We observed no significant effect of tDCS on the acquisition or retention of fear extinction in mice subjected to fear conditioning with moderate intensity. However, when the intensity of fear conditioning was high, tDCS significantly lowered freezing during the acquisition of extinction, regardless of the extinction protocol. Moreover, when tDCS was combined with a strong extinction protocol, we also observed a significant improvement of early extinction recall. Finally, we found that tDCS reduced generalized fear induced by contextual cues when the intensity of conditioning is high and extinction training limited.

CONCLUSIONS

Our data provide a rationale to further explore anodal tDCS over the PFC as potential support for trauma-based psychotherapy for PTSD.

Neurobiology of BDNF in fear memory, sensitivity to stress, and stress-related disorders

Brain-derived neurotrophic factor (BDNF) is widely accepted for its involvement in resilience and antidepressant drug action, is a common genetic locus of risk for mental illnesses, and remains one of the most prominently studied molecules within psychiatry. Stress, which arguably remains the "lowest common denominator" risk factor for several mental illnesses, targets BDNF in disease-implicated brain regions and circuits. Altered stress-related responses have also been observed in animal models of BDNF deficiency in vivo, and BDNF is a common downstream intermediary for environmental factors that potentiate anxiety- and depressive-like behavior. However, BDNF's broad functionality has manifested a heterogeneous literature; likely reflecting that BDNF plays a hitherto under-recognized multifactorial role as both a regulator and target of stress hormone signaling within the brain. The role of BDNF in vulnerability to stress and stress-related disorders, such as posttraumatic stress disorder (PTSD), is a prominent example where inconsistent effects have emerged across numerous models, labs, and disciplines. In the current review we provide a contemporary update on the neurobiology of BDNF including new data from the behavioral neuroscience and neuropsychiatry literature on fear memory consolidation and extinction, stress, and PTSD. First we present an overview of recent advances in knowledge on the role of BDNF within the fear circuitry, as well as address mounting evidence whereby stress hormones interact with endogenous BDNF-TrkB signaling to alter brain homeostasis. Glucocorticoid signaling also acutely recruits BDNF to enhance the expression of fear memory. We then include observations that the functional common BDNF Val66Met polymorphism modulates stress susceptibility as well as stress-related and stress-inducible neuropsychiatric endophenotypes in both man and mouse. We conclude by proposing a BDNF stress-sensitivity hypothesis, which posits that disruption of endogenous BDNF activity by common factors (such as the BDNF Val66Met variant) potentiates sensitivity to stress and, by extension, vulnerability to stress-inducible illnesses. Thus, BDNF may induce plasticity to deleteriously promote the encoding of fear and trauma but, conversely, also enable adaptive plasticity during extinction learning to suppress PTSD-like fear responses. Ergo regulators of BDNF availability, such as the Val66Met polymorphism, may orchestrate sensitivity to stress, trauma, and risk of stress-induced disorders such as PTSD. Given an increasing interest in personalized psychiatry and clinically complex cases, this model provides a framework from which to experimentally disentangle the causal actions of BDNF in stress responses, which likely interact to potentiate, produce, and impair treatment of, stress-related psychiatric disorders.

Brain-Derived Neurotrophic Factor Val66Met polymorphism interacts with adolescent stress to alter hippocampal interneuron density and dendritic morphology in mice

Brain-derived neurotrophic factor (BDNF) plays essential roles in GABAergic interneuron development. The common BDNF val66met polymorphism, leads to decreased activity-dependent release of BDNF. The current study used a humanized mouse model of the BDNF val66met polymorphism to determine how reduced activity-dependent release of BDNF, both on its own, and in combination with chronic adolescent stress hormone, impact hippocampal GABAergic interneuron cell density and dendrite morphology. Male and female Val/Val and Met/Met mice were exposed to corticosterone (CORT) or placebo in their drinking water from weeks 6-8, before brains were perfuse-fixed at 15 weeks. Cell density and dendrite morphology of immunofluorescent labelled inhibitory interneurons; somatostatin, parvalbumin and calretinin in the CA1, and 3 and dentate gyrus (DG) across the dorsal (DHP) and ventral hippocampus (VHP) were assessed by confocal z-stack imaging, and IMARIS dendritic mapping software. Mice with the Met/Met genotype showed significantly lower somatostatin cell density compared to Val/Val controls in the DHP, and altered somatostatin interneuron dendrite morphology including branch depth, and spine density. Parvalbumin-positive interneurons were unchanged between genotype groups, however BDNF val66met genotype influenced the dendritic volume, branch level and spine density of parvalbumin interneurons differentially across hippocampal subregions. Contrary to this, no such effects were observed for calretinin-positive interneurons. Adolescent exposure to CORT treatment also significantly altered somatostatin and parvalbumin dendrite branch level and the combined effect of Met/Met genotype and CORT treatment significantly reduced somatostatin and parvalbumin dendrite spine density. In sum, the BDNF^Val66Met polymorphism significantly alters somatostatin and parvalbumin-positive interneuron cell development and dendrite morphology. Additionally, we also report a compounding effect of the Met/Met genotype and chronic adolescent CORT treatment on dendrite spine density, indicating that adolescence is a sensitive period of risk for Val66Met polymorphism carriers.

Sex-dependent opposite effects of a tropomyosin-related kinase B receptor (TrkB) agonist 7,8-dihydroxyflavone on cued fear extinction in mice

Tropomyosin-related kinase B receptor (TrkB) is one of the new candidate receptors for drugs targeting psychiatric and neurodegenerative disorders. Recently, 7,8-dihydroxyflavone (7,8-DHF) has been identified as a selective TrkB agonist that crosses the blood-brain barrier after oral or intraperitoneal administration, and it enhances cued fear extinction in male rodents. However, its effects on females remain unclear. Preclinical research including both sexes is important for the development of treatment, particularly, for stress-related disorders such as post-traumatic stress disorder because such disorders are more prevalent in women. Therefore, we investigated the effects of 7,8-DHF on cued and contextual fear extinction in both male and female mice. Here we demonstrated that the administration of 7,8-DHF before each extinction session attenuated cued fear extinction in females; conversely, it enhanced cued fear extinction in males. However, administration of 7,8-DHF immediately after each extinction session did not affect cued fear extinction in either sex. Moreover, in contextual fear extinction, administration of 7,8-DHF before each extinction session did not affect fear extinction in either sex. Thus, 7,8-DHF showed sex-dependent opposite effects on cued fear extinction in mice when administered before but not immediately after each extinction session. Our results could contribute to the development of pharmacotherapy involving 7,8-DHF, particularly for stress-related disorders.

Loss of TrkB Signaling in Parvalbumin-Expressing Basket Cells Results in Network Activity Disruption and Abnormal Behavior

The GABAergic system is regulated by the brain-derived neurotrophic factor (BDNF)/Tropomyosin-related kinase B (TrkB) pathway, but the cell-intrinsic role of TrkB signaling in parvalbumin cortical interneuron development and function is unclear. We performed conditional ablation of the TrkB receptor in parvalbumin-expressing (PV) interneurons to study whether postnatal loss of TrkB in parvalbumin cells affects their survival, connectivity, spontaneous and evoked neuronal activity and behavior. Using in vivo recordings of local field potentials, we found reduced gamma oscillations in the sensory cortex of PVcre+; TrkBF/F conditional knockout mice (TrkB cKO), along with increased firing of putative excitatory neurons. There was a significant downregulation in parvalbumin neuron number in cerebral and cerebellar cortices of TrkB cKO mice. In addition, inhibitory synaptic connections between basket cells and pyramidal neurons were profoundly reduced in the neocortex of TrkB cKO mice and there was a loss of cortical volume. TrkB cKO mice also showed profound hyperactivity, stereotypies, motor deficits and learning/memory defects. Our findings demonstrate that the targeting and/or synapse formation of PV-expressing basket cells with principal excitatory neurons require TrkB signaling in parvalbumin cells. Disruption of this signaling has major consequences for parvalbumin interneuron connectivity, network dynamics, cognitive and motor behavior.

Beneficial effects of sound exposure on auditory cortex development in a mouse model of Fragile X Syndrome

BACKGROUND

Fragile X syndrome (FXS) is the most common genetic cause of autism and intellectual disability. Fragile X mental retardation gene (Fmr1) knock-out (KO) mice display core deficits of FXS, including abnormally increased sound-evoked responses, and show a delayed development of parvalbumin (PV) cells. Here, we present the surprising result that sound exposure during early development reduces correlates of auditory hypersensitivity in Fmr1 KO mice.

METHODS

Fmr1 KO and wild-type (WT) mice were raised in a sound-attenuated environment (AE) or sound-exposed (SE) to 14 kHz tones (5 Hz repetition rate) from P9 until P21. At P21-P23, event-related potentials (ERPs), dendritic spine density, PV expression and phosphorylation of tropomyosin receptor kinase B (TrkB) were analyzed in the auditory cortex of AE and SE mice.

RESULTS

Enhanced N1 amplitude of ERPs, impaired PV cell development, and increased spine density in layers (L) 2/3 and L5/6 excitatory neurons were observed in AE Fmr1 KO compared to WT mice. In contrast, developmental sound exposure normalized ERP N1 amplitude, density of PV cells and dendritic spines in SE Fmr1 KO mice. Finally, TrkB phosphorylation was reduced in AE Fmr1 KO, but was enhanced in SE Fmr1 KO mice, suggesting that BDNF-TrkB signaling may be regulated by sound exposure to influence PV cell development.

CONCLUSIONS

Our results demonstrate that sound exposure, but not attenuation, during early developmental window restores molecular, cellular and functional properties in the auditory cortex of Fmr1 KO mice, and suggest this approach as a potential treatment for sensory phenotypes in FXS.

The impact of silencing feed-forward parvalbumin-expressing inhibitory interneurons in the cortico-thalamocortical network on seizure generation and behaviour

Feed-forward inhibition (FFI) is an essential mechanism within the brain, to regulate neuronal firing and prevent runaway excitation. In the cortico-thalamocortical (CTC) network, fast spiking parvalbumin-expressing (PV+) inhibitory interneurons regulate the firing of pyramidal cells in the cortex and relay neurons in the thalamus. PV+ interneuron dysfunction has been implicated in several neurological disorders, including epilepsy. Previously, we demonstrated that loss of excitatory AMPA-receptors, specifically at synapses on PV+ interneurons in CTC feedforward microcircuits, occurs in the stargazer mouse model of absence epilepsy. These mice present with absence seizures characterized by spike and wave discharges (SWDs) on electroencephalogram (EEG) and concomitant behavioural arrest, similar to childhood absence epilepsy. The aim of the current study was to investigate the impact of loss of FFI within the CTC on absence seizure generation and behaviour using new Designer Receptor Exclusively Activated by Designer Drug (DREADD) technology. We crossed PV-Cre mice with Cre-dependent hM4Di DREADD strains of mice, which allowed Cre-recombinase-mediated restricted expression of inhibitory G_i-DREADDs in PV+ interneurons. We then tested the impact of global and focal (within the CTC network) silencing of PV+ interneurons. CNO mediated silencing of all PV+ interneurons by intraperitoneal injection caused the impairment of motor control, decreased locomotion and increased anxiety in a dose-dependent manner. Such silencing generated pathological oscillations similar to absence-like seizures. Focal silencing of PV+ interneurons within cortical or thalamic feedforward microcircuits, induced SWD-like oscillations and associated behavioural arrest. Epileptiform activity on EEG appeared significantly sooner after focal injection compared to peripheral injection of CNO. However, the mean duration of each oscillatory burst and spike frequency was similar, irrespective of mode of CNO delivery. No significant changes were observed in vehicle-treated or non-DREADD wild-type control animals. These data suggest that dysfunctional feed-forward inhibition in CTC microcircuits may be an important target for future therapy strategies for some patients with absence seizures. Additionally, silencing of PV+ interneurons in other brain regions may contribute to anxiety related neurological and psychiatric disorders.

Delayed injury of hippocampal interneurons after neonatal hypoxia-ischemia and therapeutic hypothermia in a murine model

Delayed hippocampal injury and memory impairments follow neonatal hypoxia-ischemia (HI) despite the use of therapeutic hypothermia (TH). Death of hippocampal pyramidal cells occurs acutely after HI, but characterization of delayed cell death and injury of interneurons (INs) is unknown. We hypothesize that injury of INs after HI is: (i) asynchronous to that of pyramidal cells, (ii) independent of injury severity, and (iii) unresponsive to TH. HI was induced in C57BL6 mice at p10 with unilateral right carotid ligation and 45 min of hypoxia (FiO₂  = 0.08). Mice were randomized to normothermia (36 °C, NT) or TH (31 °C) for 4 hr after HI and anesthesia-exposed shams were use as controls. Brains were studied at 24 hr (p11) or 8 days (p18) after HI. Vglut1, GAD65/67, PSD95, parvalbumin (PV) and calbindin-1 (Calb1) were measured. Cell death was assessed using cresyl violet staining and TUNEL assay. Hippocampal atrophy and astroglyosis at p18 were used to assess injury severity and to correlate with number of PV + INs. VGlut1 level decreased by 30% at 24 hr after HI, while GAD65/67 level decreased by ∼50% in forebrain 8 days after HI, a decrease localized in CA1 and CA3. PSD95 levels decreased in forebrain by 65% at 24 hr after HI and remained low 8 days after HI. PV + INs increased in numbers (per mm² ) and branching between p11 and p18 in sham mice but not in NT and TH mice, resulting in 21-52% fewer PV + INs in injured mice at p18. Calb1 protein and mRNA were also reduced in HI injured mice at p18. At p18, somatodendritic attrition of INs was evident in all injured mice without evidence of cell death. Neither hippocampal atrophy nor astroglyosis correlated with the number of PV + INs at p18. Thus, HI exposure has long lasting effects in the hippocampus impairing the development of the GABAergic system with only partial protection by TH independent of the degree of hippocampal injury. © 2018 Wiley Periodicals, Inc.

Sex differences in fear extinction

Despite the exponential increase in fear research during the last years, few studies have included female subjects in their design. The need to include females arises from the knowledge gap of mechanistic processes underlying the behavioral and neural differences observed in fear extinction. Moreover, the exact contribution of sex and hormones in relation to learning and behavior is still largely unknown. Insights from this field could be beneficial as fear-related disorders are twice as prevalent in women compared to men. Here, we review an up-to-date summary of animal and human studies in adulthood that report sex differences in fear extinction from a structural and functional approach. Furthermore, we describe how these factors could contribute to the observed sex differences in fear extinction during normal and pathological conditions.

Sexual dimorphism in vestibular function and dysfunction

It has been recognized for some time that females appear to be overrepresented in the incidence of many vestibular disorders, and recent epidemiological studies further support this idea. While it is possible that this is due to a reporting bias, another possibility is that there are actual differences in the incidence of vestibular dysfunction between males and females. If this is true, it could be due to a sexual dimorphism in vestibular function and therefore dysfunction, possibly related to the hormonal differences between females and males, although the higher incidence of vestibular dysfunction in females appears to last long after menopause. Many other neurochemical differences exist between males and females, however, that could be implicated in sexual dimorphism. This review critically explores the possibility of sexual dimorphism in vestibular function and dysfunction, and the implications it may have for the treatment of vestibular disorders.

Prazosin during fear conditioning facilitates subsequent extinction in male C57Bl/6N mice

RATIONALE

Recovery from a traumatic experience requires extinction of cue-based fear responses, a process that is impaired in post-traumatic stress disorder. While studies suggest a link between fear behavioral flexibility and noradrenaline signaling, the role of specific receptors and brain regions in these effects is unclear.

OBJECTIVES

Here, we examine the role of prazosin, an α1-adrenergic receptor (α1-AR) antagonist, in auditory fear conditioning and extinction.

METHODS

C57Bl/6N mice were subjected to auditory fear conditioning and extinction in combination with systemic (0.1-2 mg/kg) or local microinjections (3 or 6 mM) of the α1-AR antagonist prazosin into the prelimbic division of medial prefrontal cortex or basolateral amygdala. Conditioned fear and anxiety-like behaviors were compared with vehicle-injected control animals.

RESULTS

Mice that received systemic prazosin prior to fear conditioning exhibited similar initial levels of cue-elicited freezing compared to vehicle controls on the following day. However, at all doses tested, fear that was acquired during prazosin treatment was more readily extinguished, whereas anxiety-like behavior on the day of extinction was unaffected. A similar pattern of results was observed when prazosin was microinjected into the basolateral amygdala but not the prelimbic cortex. In contrast to pre-conditioning injections, prazosin administration prior to extinction had no effect on freezing.

CONCLUSIONS

Our results indicate that α1-AR activity during aversive conditioning is dispensable for memory acquisition but renders conditioned fear more impervious to extinction. This suggests that behavioral flexibility is constrained by noradrenaline at the time of initial learning via activation of a specific AR isoform.

Cortistatin-expressing interneurons require TrkB signaling to suppress neural hyper-excitability

Signaling of brain-derived neurotrophic factor (BDNF) via tropomyosin receptor kinase B (TrkB) plays a critical role in the maturation of cortical inhibition and controls expression of inhibitory interneuron markers, including the neuropeptide cortistatin (CST). CST is expressed exclusively in a subset of cortical and hippocampal GABAergic interneurons, where it has anticonvulsant effects and controls sleep slow-wave activity (SWA). We hypothesized that CST-expressing interneurons play a critical role in regulating excitatory/inhibitory balance, and that BDNF, signaling through TrkB receptors on CST-expressing interneurons, is required for this function. Ablation of CST-expressing cells caused generalized seizures and premature death during early postnatal development, demonstrating a critical role for these cells in providing inhibition. Mice in which TrkB was selectively deleted from CST-expressing interneurons were hyperactive, slept less and developed spontaneous seizures. Frequencies of spontaneous excitatory post-synaptic currents (sEPSCs) on CST-expressing interneurons were attenuated in these mice. These data suggest that BDNF, signaling through TrkB receptors on CST-expressing cells, promotes excitatory drive onto these cells. Loss of excitatory drive onto CST-expressing cells that lack TrkB receptors may contribute to observed hyperexcitability and epileptogenesis.

Postnatal TrkB ablation in corticolimbic interneurons induces social dominance in male mice

The tight balance between synaptic excitation and inhibition (E/I) within neocortical circuits in the mammalian brain is important for complex behavior. Many loss-of-function studies have demonstrated that brain-derived neurotrophic factor (BDNF) and its cognate receptor tropomyosin receptor kinase B (TrkB) are essential for the development of inhibitory GABAergic neurons. However, behavioral consequences of impaired BDNF/TrkB signaling in GABAergic neurons remain unclear, largely due to confounding motor function deficits observed in previous animal models. In this study, we generated conditional knockout mice (TrkB cKO) in which TrkB was ablated from a majority of corticolimbic GABAergic interneurons postnatally. These mice showed intact motor coordination and movement, but exhibited enhanced dominance over other mice in a group-housed setting. In addition, immature fast-spiking GABAergic neurons of TrkB cKO mice resulted in an E/I imbalance in layer 5 microcircuits within the medial prefrontal cortex (mPFC), a key region regulating social dominance. Restoring the E/I imbalance via optogenetic modulation in the mPFC of TrkB cKO mice normalized their social dominance behavior. Taken together, our results provide strong evidence for a role of BDNF/TrkB signaling in inhibitory synaptic modulation and social dominance behavior in mice.

Sex differences in brain-derived neurotrophic factor signaling: Functions and implications

Brain-derived neurotrophic factor (BDNF) regulates diverse processes such as neuronal survival, differentiation, and plasticity. Accumulating evidence suggests that molecular events that direct sexual differentiation of the brain interact with BDNF signaling pathways. This Mini-Review first examines potential hormonal and epigenetic mechanisms through which sex influences BDNF signaling. We then examine how sex-specific regulation of BDNF signaling supports the development and function of sexually dimorphic neural circuits that underlie male-specific genital reflexes in rats and song production in birds. Finally, we discuss the implications of sex differences in BDNF signaling for gender-biased presentation of neurological and psychiatric diseases such as Alzheimer's disease. Although this Mini-Review focuses on BDNF, we try to convey the general message that sex influences brain functions in complex ways and underscore the requirement for and challenge of expanding research on sex differences in neuroscience. © 2016 Wiley Periodicals, Inc.

Sexual dimorphic expression of TrkB, TrkB-T1, and BDNF in the medial preoptic area of the Syrian hamster

Neurotrophins regulate many aspects of neuronal function and activity. Specifically, the binding of Brain-derived neurotrophic factor (BDNF) to Tyrosine receptor kinase-B (TrkB) or its truncated version, TrkB-T1, can cause growth and differentiation or dominant inhibition of receptor signaling, respectively. There is evidence that these neurotropic effects on nervous tissue, in both the central and peripheral nervous system, behave differently between the sexes. This study used western blots to examine the expression of these neurotrophins in the medial preoptic area (MPOA), a sexually dimorphic region of the hamster brain that controls male sex behavior. We report that TrkB-FL and BDNF show greater expression in male MPOA tissue, when compared to female. On the contrary, TrkB-T1 is expressed in greater abundance in the female MPOA. Our results indicate a clear sexual dimorphism of neurotrophins in the MPOA of the Syrian hamster. Furthermore, the greater expression of TrkB-FL and BDNF in the male MPOA suggests that these neurotrophins could be promoting synaptic growth to facilitate male-typical copulation. In contrast, the greater TrkB-T1 expression in the female MPOA suggests a possible inhibition of synaptic growth, and may contribute to the lack of male-typical copulation. Altogether, our data suggests that neurotrophins may play a larger role sexual differentiation than previously thought.

Changes in the Prefrontal Glutamatergic and Parvalbumin Systems of Mice Exposed to Unpredictable Chronic Stress

The prefrontal cortex (PFC) is highly sensitive to the effects of stress, a known risk factor of mood disorders including anxiety and depression. Abnormalities in PFC functioning have been well described in humans displaying stress-induced depressive symptoms, and hypoactivity of the PFC is now recognized to be a key feature of the depressed brain. However, little is known about the causes and mechanisms leading to this altered prefrontal functional activity in the context of stress-related mood disorders. We previously showed that unpredictable chronic mild stress (UCMS) in mice increases prefrontal expression of parvalbumin (PV), an activity-dependent calcium-binding albumin protein expressed in a specific subtype of GABAergic neurons, highlighting a potential mechanism through which chronic stress leads to hypofunction of the PFC. In this study, we aimed to investigate the mechanisms by which chronic stress alters the prefrontal GABA system. We hypothesized that chronic stress-induced enhancement of glutamatergic transmission in the PFC is a crucial contributing factor to changes within the prefrontal GABAergic and, specifically, PV system. BALB/c male and female mice were exposed to daily handling (control) or 2 or 4 weeks of UCMS. Female mice displayed a more severe altered phenotype than males, as shown by increased anxiety- and depressive-like behaviors and deficits in PFC-dependent cognitive abilities, particularly after exposure to 2 weeks of UCMS. This behavioral phenotype was paralleled by a large increase in prefrontal PV messenger RNA (mRNA) and number of PV-expressing neurons, supporting our previous findings. We further showed that the expression of pre- and postsynaptic markers of glutamatergic transmission (VGlut1 presynaptic terminals and pERK1/2, respectively) onto PV neurons was increased by 2 weeks of UCMS in a sex-specific manner; this was associated with sex-specific changes in the mRNA expression of the NR2B subunit of the NMDA receptor. These findings provide evidence of increased glutamatergic transmission onto prefrontal PV neurons, particularly in female mice, which could potentially contribute to their increased PV expression and the extent of their behavioral impairment following UCMS. Finally, our analysis of activity of subcortical regions sending glutamatergic afferents to the PFC reveals that glutamatergic neurons from the basolateral amygdala might be specifically involved in UCMS-induced changes in prefrontal glutamatergic transmission.

Multimodal and Site-Specific Plasticity of Amygdala Parvalbumin Interneurons after Fear Learning

Stimulus processing in fear conditioning is constrained by parvalbumin interneurons (PV-INs) through inhibition of principal excitatory neurons. However, the contributions of PV-IN microcircuits to input gating and long-term plasticity in the fear system remain unknown. Here we interrogate synaptic connections between afferent pathways, PV-INs, and principal excitatory neurons in the basolateral amygdala. We find that subnuclei of this region are populated two functionally distinct PV-IN networks. PV-INs in the lateral (LA), but not the basal (BA), amygdala possess complex dendritic arborizations, receive potent excitatory drive, and mediate feedforward inhibition onto principal neurons. After fear conditioning, PV-INs exhibit nucleus- and target-selective plasticity, resulting in persistent reduction of their excitatory input and inhibitory output in LA but not BA. These data reveal previously overlooked specializations of amygdala PV-INs and indicate specific circuit mechanisms for inhibitory plasticity during the encoding of associative fear memories.

BDNF Signaling Promotes Vestibular Compensation by Increasing Neurogenesis and Remodeling the Expression of Potassium-Chloride Cotransporter KCC2 and GABAA Receptor in the Vestibular Nuclei

Reactive cell proliferation occurs rapidly in the cat vestibular nuclei (VN) after unilateral vestibular neurectomy (UVN) and has been reported to facilitate the recovery of posturo-locomotor functions. Interestingly, whereas animals experience impairments for several weeks, extraordinary plasticity mechanisms take place in the local microenvironment of the VN: newborn cells survive and acquire different phenotypes, such as microglia, astrocytes, or GABAergic neurons, whereas animals eventually recover completely from their lesion-induced deficits. Because brain-derived neurotrophic factor (BDNF) can modulate vestibular functional recovery and neurogenesis in mammals, in this study, we examined the effect of BDNF chronic intracerebroventricular infusion versus K252a (a Trk receptor antagonist) in our UVN model. Results showed that long-term intracerebroventricular infusion of BDNF accelerated the restoration of vestibular functions and significantly increased UVN-induced neurogenesis, whereas K252a blocked that effect and drastically delayed and prevented the complete restoration of vestibular functions. Further, because the level of excitability in the deafferented VN is correlated with behavioral recovery, we examined the state of neuronal excitability using two specific markers: the cation-chloride cotransporter KCC2 (which determines the hyperpolarizing action of GABA) and GABAA receptors. We report for the first time that, during an early time window after UVN, significant BDNF-dependent remodeling of excitability markers occurs in the brainstem. These data suggest that GABA acquires a transient depolarizing action during recovery from UVN, which potentiates the observed reactive neurogenesis and accelerates vestibular functional recovery. These findings suggest that BDNF and/or KCC2 could represent novel treatment strategies for vestibular pathologies.

SIGNIFICANCE STATEMENT

In this study, we report for the first time that brain-derived neurotrophic factor potentiates vestibular neurogenesis and significantly accelerates functional recovery after unilateral vestibular injury. We also show that specific markers of excitability, the potassium-chloride cotransporter KCC2 and GABAA receptors, undergo remarkable fluctuations within vestibular nuclei (VN), strongly suggesting that GABA acquires a transient depolarizing action in the VN during the recovery period. This novel plasticity mechanism could explain in part how the system returns to electrophysiological homeostasis between the deafferented and intact VN, considered in the literature to be a key parameter of vestibular compensation. In this context, our results open new perspectives for the development of therapeutic approaches to alleviate the vestibular symptoms and favor vestibular function recovery.

Activity-dependent signaling: influence on plasticity in circuits controlling fear-related behavior

Fear regulation is impaired in anxiety and trauma-related disorders. Patients experience heightened fear expression and reduced ability to extinguish fear memories. Because fear regulation is abnormal in these disorders and extinction recapitulates current treatment strategies, understanding the underlying mechanisms is vital for developing new treatments. This is critical because although extinction-based exposure therapy is a mainstay of treatment, relapse is common. We examine recent findings describing changes in network activity and functional connectivity within limbic circuits during fear regulation, and explore how activity-dependent signaling contributes to the neural activity patterns that control fear and anxiety. We review the role of the prototypical activity-dependent molecule, brain-derived neurotrophic factor (BDNF), whose signaling has been critically linked to regulation of fear behavior.

Increased stereotypy in conditional Cxcr4 knockout mice

Chemokines play important roles in the central nervous system, including mediating neuroinflammation and guiding the intracortical migration of interneurons during development. Alteration in parvalbumin-positive interneurons is a key neuropathological hallmark of multiple mental conditions. We recently reported a significant reduction in the expression of CXCL12 in olfactory neurons from sporadic cases with schizophrenia compared with matched controls, suggesting a role for CXCR4/CXCL12 signaling in mental conditions. Thus, we depleted the chemokine receptor Cxcr4 from mice using the parvalbumin-2A-Cre line. The conditional knockout mice exhibited a unique behavioral phenotype involving increased stereotypy. Stereotypy is observed in many psychiatric conditions, including schizophrenia, autism, and dementia. Thus, the Cxcr4 conditional knockout mice may serve as a model for this symptomatic feature.

Ablation of intact hypothalamic and/or hindbrain TrkB signaling leads to perturbations in energy balance

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB), play a paramount role in the central regulation of energy balance. Despite the substantial body of genetic evidence implicating BDNF- or TrkB-deficiency in human obesity, the critical brain region(s) contributing to the endogenous role of BDNF/TrkB signaling in metabolic control remain unknown.

METHODS

We assessed the importance of intact hypothalamic or hindbrain TrkB signaling in central regulation of energy balance by generating Nkx2.1-Ntrk2-/- and Phox2b-Ntrk2+/- mice, respectively, and comparing metabolic parameters (body weight, adiposity, food intake, energy expenditure and glucose homeostasis) under high-fat diet or chow fed conditions.

RESULTS

Our data show that when fed a high-fat diet, male and female Nkx2.1-Ntrk2-/- mice have significantly increased body weight and adiposity that is likely driven by reduced locomotor activity and core body temperature. When maintained on a chow diet, female Nkx2.1-Ntrk2-/- mice exhibit an increased body weight and adiposity phenotype more robust than in males, which is accompanied by hyperphagia that precedes the onset of a body weight difference. In addition, under both diet conditions, Nkx2.1-Ntrk2-/- mice show increased blood glucose, serum insulin and leptin levels. Mice with complete hindbrain TrkB-deficiency (Phox2b-Ntrk2-/-) are perinatal lethal, potentially indicating a vital role for TrkB in visceral motor neurons that control cardiovascular, respiratory, and digestive functions during development. Phox2b-Ntrk2+/- heterozygous mice are similar in body weight, adiposity and glucose homeostasis parameters compared to wild type littermate controls when maintained on a high-fat or chow diet. Interestingly, despite the absence of a body weight difference, Phox2b-Ntrk2+/- heterozygous mice exhibit pronounced hyperphagia.

CONCLUSION

Taken together, our findings suggest that the hypothalamus is a key brain region involved in endogenous BDNF/TrkB signaling and central metabolic control and that endogenous hindbrain TrkB likely plays a role in modulating food intake and survival of mice. Our findings also show that female mice lacking TrkB in the hypothalamus have a more robust metabolic phenotype.

Nmf9 Encodes a Highly Conserved Protein Important to Neurological Function in Mice and Flies

Many protein-coding genes identified by genome sequencing remain without functional annotation or biological context. Here we define a novel protein-coding gene, Nmf9, based on a forward genetic screen for neurological function. ENU-induced and genome-edited null mutations in mice produce deficits in vestibular function, fear learning and circadian behavior, which correlated with Nmf9 expression in inner ear, amygdala, and suprachiasmatic nuclei. Homologous genes from unicellular organisms and invertebrate animals predict interactions with small GTPases, but the corresponding domains are absent in mammalian Nmf9. Intriguingly, homozygotes for null mutations in the Drosophila homolog, CG45058, show profound locomotor defects and premature death, while heterozygotes show striking effects on sleep and activity phenotypes. These results link a novel gene orthology group to discrete neurological functions, and show conserved requirement across wide phylogenetic distance and domain level structural changes.

Genome sequencing projects have identified large numbers of genes that encode proteins of unknown function. Many of these genes show strong evolutionary conservation, predicting important and well-conserved functions. A fraction of these show strong conservation of core domains but dynamic changes in other domains, predicting both conserved and lineage-dependent functions. Here we identify neurological functions associated with one such gene identified by a forward genetic screen in mice. We use recently developed genome editing tools both to confirm the mouse studies and to test comparative functions in a model insect, the fruit fly Drosophila melanogaster. Each of these species has a single homolog of this gene family, but differ by inclusion of a ras-association (RA) domain present in most invertebrate species but missing in mammals. Null mutations in both mice and flies produce neurological phenotypes, but while the mouse phenotypes are comparatively mild (vestibular deficits, mild tremor, hyperactivity, mild circadian phenotypes and abnormal fear learning–but normal viability and breeding), null flies rarely survive to adulthood and surviving flies have severe locomotor deficits. Interestingly, heterozygous flies have significant sleep-related phenotypes. Together, our results provide a detailed first look at comparative function for a gene lineage with an unusual evolutionary history.