Neuronal hypertrophy dampens neuronal intrinsic excitability and stress responsiveness during chronic stress

Repeated exposure to multiple concurrent stressors alters visual processing in the adult posterior parietal cortex

Chronic stress is well known to erode cognitive functions. Yet, our understanding of how repeated stress exposure impacts one of the fundamental bases of cognition: sensory processing, remains limited. The posterior parietal cortex (PPC) is a high order visual region, known for its role in visually guided decision making, multimodal integration, attention, and working memory. Here, we used functional measures to determine how repeated exposure to multiple concurrent stressors (RMS) affects sensory processing in the PPC in adult male mice. A longitudinal experimental design, repeatedly surveying the same population of neurons using in vivo two-photon imaging, revealed that RMS disrupts the balanced turnover of visually responsive cells in layer 2/3 of the PPC. Across the population, RMS-induced changes in visual responsiveness followed a bimodal distribution suggesting idiosyncratic stress effects. In cells that maintained their responsiveness across recording sessions, we found that stress reduced visual response magnitudes and feature selectivity. While we did not observe stress-induced elimination of excitatory synapses, noise correlation statistics indicated that RMS altered visual input to the neuronal population. The impact of RMS was restricted to visually evoked responses and was not evident in neuronal activity associated with locomotion onset. Together, our results indicate that despite no apparent synaptic reorganization, stress exposure in adulthood can disrupt sensory processing in the PPC, with the effects showing remarkable individual variation.

The effects of early-life and intergenerational stress on the brain

Stress experienced during ontogeny can have profound effects on the adult phenotype. However, stress can also be experienced intergenerationally, where an offspring's phenotype can be moulded by stress experienced by the parents. Although early-life and intergenerational stress can alter anatomy, physiology, and behaviour, nothing is known about how these stress contexts interact to affect the neural phenotype. Here, we examined how early-life and intergenerational stress affect the brain in eastern fence lizards (Sceloporus undulatus). Some lizard populations co-occur with predatory fire ants, and stress from fire ant attacks exerts intergenerational physiological and behavioural changes in lizards. However, it is unclear if intergenerational stress, or the interaction between intergenerational and early-life stress, modulates the brain. To test this, we captured gravid females from fire ant invaded and uninvaded populations, and subjected offspring to three early-life stress treatments: (1) fire ant attack, (2) corticosterone, or (3) a control. Corticosterone and fire ant attack decreased some aspects of the neural phenotype while population of origin and the interaction of early-life stress and population had no effects on the brain. These results suggest that early-life stressors may better predict adult brain variation than intergenerational stress in this species.

Peripheral nerve injury elicits microstructural and neurochemical changes in the striatum and substantia nigra of a DYT-TOR1A mouse model with dystonia-like movements

The relationship between genotype and phenotype in DYT-TOR1A dystonia as well as the associated motor circuit alterations are still insufficiently understood. DYT-TOR1A dystonia has a remarkably reduced penetrance of 20-30%, which has led to the second-hit hypothesis emphasizing an important role of extragenetic factors in the symptomatogenesis of TOR1A mutation carriers. To analyze whether recovery from a peripheral nerve injury can trigger a dystonic phenotype in asymptomatic hΔGAG3 mice, which overexpress human mutated torsinA, a sciatic nerve crush was applied. An observer-based scoring system as well as an unbiased deep-learning based characterization of the phenotype showed that recovery from a sciatic nerve crush leads to significantly more dystonia-like movements in hΔGAG3 animals compared to wildtype control animals, which persisted over the entire monitored period of 12 weeks. In the basal ganglia, the analysis of medium spiny neurons revealed a significantly reduced number of dendrites, dendrite length and number of spines in the naïve and nerve-crushed hΔGAG3 mice compared to both wildtype control groups indicative of an endophenotypical trait. The volume of striatal calretinin+ interneurons showed alterations in hΔGAG3 mice compared to the wt groups. Nerve-injury related changes were found for striatal ChAT+, parvalbumin+ and nNOS+ interneurons in both genotypes. The dopaminergic neurons of the substantia nigra remained unchanged in number across all groups, however, the cell volume was significantly increased in nerve-crushed hΔGAG3 mice compared to naïve hΔGAG3 mice and wildtype littermates. Moreover, in vivo microdialysis showed an increase of dopamine and its metabolites in the striatum comparing nerve-crushed hΔGAG3 mice to all other groups. The induction of a dystonia-like phenotype in genetically predisposed DYT-TOR1A mice highlights the importance of extragenetic factors in the symptomatogenesis of DYT-TOR1A dystonia. Our experimental approach allowed us to dissect microstructural and neurochemical abnormalities in the basal ganglia, which either reflected a genetic predisposition or endophenotype in DYT-TOR1A mice or a correlate of the induced dystonic phenotype. In particular, neurochemical and morphological changes of the nigrostriatal dopaminergic system were correlated with symptomatogenesis.

Windows into stress: a glimpse at emerging roles for CRHPVN neurons

The corticotropin-releasing hormone cells in the paraventricular nucleus of the hypothalamus (CRH^PVN) control the slow endocrine response to stress. The synapses on these cells are exquisitely sensitive to acute stress, leveraging local signals to leave a lasting imprint on this system. Additionally, recent work indicates that these cells also play key roles in the control of distinct stress and survival behaviors. Here we review these observations and provide a perspective on the role of CRH^PVN neurons as integrative and malleable hubs for behavioral, physiological, and endocrine responses to stress.

KMT2E Haploinsufficiency Manifests Autism-Like Behaviors and Amygdala Neuronal Development Dysfunction in Mice

Autism spectrum disorders (ASD) are highly heterogeneous neurodevelopmental disorders characterized by impaired social interaction skills. Whole exome sequencing has identified loss-of-function mutations in lysine methyltransferase 2E (KMT2E, also named MLL5) in ASD patients and it is listed as an ASD high-risk gene in humans. However, experimental evidence of KMT2E in association with ASD-like manifestations or neuronal function is still missing. Relying on KMT2E^+/- mice, through animal behavior analyses, positron emission tomography (PET) imaging, and neuronal morphological analyses, we explored the role of KMT2E haploinsufficiency in ASD-like symptoms. Behavioral results revealed that KMT2E haploinsufficiency was sufficient to produce social deficit, accompanied by anxiety in mice. Whole-brain ¹⁸F-FDG-PET analysis identified that relative amygdala glycometabolism was selectively decreased in KMT2E^+/- mice compared to wild-type mice. The numbers and soma sizes of amygdala neurons in KMT2E^+/- mice were prominently increased. Additionally, KMT2E mRNA levels in human amygdala were significantly decreased after birth during brain development. Our findings support a causative role of KMT2E in ASD development and suggest that amygdala neuronal development abnormality is likely a major underlying mechanism.

Urocortin-3 neurons in the perifornical area are critical mediators of chronic stress on female infant-directed behavior

Infant avoidance and aggression are promoted by activation of the Urocortin-3 expressing neurons of the perifornical area of hypothalamus (PeFA^Ucn3) in male and female mice. PeFA^Ucn3 neurons have been implicated in stress, and stress is known to reduce maternal behavior. We asked how chronic restraint stress (CRS) affects infant-directed behavior in virgin and lactating females and what role PeFA^Ucn3 neurons play in this process. Here we show that infant-directed behavior increases activity in the PeFA^Ucn3 neurons in virgin and lactating females. Chemogenetic inhibition of PeFA^Ucn3 neurons facilitates pup retrieval in virgin females. CRS reduces pup retrieval in virgin females and increases activity of PeFA^Ucn3 neurons, while CRS does not affect maternal behavior in lactating females. Inhibition of PeFA^Ucn3 neurons blocks stress-induced deficits in pup-directed behavior in virgin females. Together, these data illustrate the critical role for PeFA^Ucn3 neuronal activity in mediating the impact of chronic stress on female infant-directed behavior.

Restraint Stress and Repeated Corticosterone Administration Differentially Affect Neuronal Excitability, Synaptic Transmission and 5-HT7 Receptor Reactivity in the Dorsal Raphe Nucleus of Young Adult Male Rats

Exogenous corticosterone administration reduces GABAergic transmission and impairs its 5-HT₇ receptor-dependent modulation in the rat dorsal raphe nucleus (DRN), but it is largely unknown how neuronal functions of the DRN are affected by repeated physical and psychological stress. This study compared the effects of repeated restraint stress and corticosterone injections on DRN neuronal excitability, spontaneous synaptic transmission, and its 5-HT₇ receptor-dependent modulation. Male Wistar rats received corticosterone injections for 7 or 14 days or were restrained for 10 min twice daily for 3 days. Repeated restraint stress and repeated corticosterone administration evoked similar changes in performance in the forced swim test. They increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) recorded from DRN neurons. In contrast to the treatment with corticosterone, restraint stress-induced changes in sEPSC kinetics and decreased intrinsic excitability of DRN neurons did not modify inhibitory transmission. Repeated injections of the 5-HT₇ receptor antagonist SB 269970 ameliorated the effects of restraint on excitability and sEPSC frequency but did not restore the altered kinetics of sEPSCs. Thus, repeated restraint stress and repeated corticosterone administration differ in consequences for the intrinsic excitability of DRN projection neurons and their excitatory and inhibitory synaptic inputs. Effects of repeated restraint stress on DRN neurons can be partially abrogated by blocking the 5-HT₇ receptor.

Alterations in the intrinsic discharge activity of CA1 pyramidal neurons associated with possible changes in the NADPH diaphorase activity in a rat model of autism induced by prenatal exposure to valproic acid

Autism spectrum disorder is a neurodevelopmental disorder characterized by sensory abnormalities, social skills impairment and cognitive deficits. Although recent evidence indicated that induction of autism-like behavior in animal models causes abnormal neuronal excitability, the impact of autism on neuronal properties is still an important issue. Thus, new findings at the cellular level may shed light on the pathophysiology of autism and may help to find effective treatment strategies. Here, we investigated the behavioral, electrophysiological and histochemical impacts of prenatal exposure to valproic acid (VPA) in rats. Findings revealed that VPA exposure caused a significant increase in the hot plate response latency. The novel object recognition ability was also impaired in VPA-exposed rats. Along with these behavioral alterations, neurons from VPA-exposed animals exhibited altered excitability features in response to depolarizing current injections relative to control neurons. In the VPA-exposed group, these changes consisted of a significant increase in the amplitude, evoked firing frequency and the steady-state standard deviation of spike timing of action potentials (APs). Moreover, the half-width, the AHP amplitude and the decay time constant of APs were significantly decreased in this group. These changes in the evoked electrophysiological properties were accompanied by intrinsic hyperexcitability and lower spike-frequency adaptation and also a significant increase in the number of NADPH-diaphorase stained neurons in the hippocampal CA1 area of the VPA-exposed rats. Taken together, findings demonstrate that abnormal nociception and recognition memory is associated with alterations in the neuronal responsiveness and nitrergic system in a rat model of autism-like.

State-dependent activity dynamics of hypothalamic stress effector neurons

The stress response necessitates an immediate boost in vital physiological functions from their homeostatic operation to an elevated emergency response. However, the neural mechanisms underlying this state-dependent change remain largely unknown. Using a combination of in vivo and ex vivo electrophysiology with computational modeling, we report that corticotropin releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN), the effector neurons of hormonal stress response, rapidly transition between distinct activity states through recurrent inhibition. Specifically, in vivo optrode recording shows that under non-stress conditions, CRH_PVN neurons often fire with rhythmic brief bursts (RB), which, somewhat counterintuitively, constrains firing rate due to long (~2 s) interburst intervals. Stressful stimuli rapidly switch RB to continuous single spiking (SS), permitting a large increase in firing rate. A spiking network model shows that recurrent inhibition can control this activity-state switch, and more broadly the gain of spiking responses to excitatory inputs. In biological CRH_PVN neurons ex vivo, the injection of whole-cell currents derived from our computational model recreates the in vivo-like switch between RB and SS, providing direct evidence that physiologically relevant network inputs enable state-dependent computation in single neurons. Together, we present a novel mechanism for state-dependent activity dynamics in CRH_PVN neurons.

Adolescent Stress Confers Resilience to Traumatic Stress Later in Life: Role of the Prefrontal Cortex

Background

Adolescent brains are sensitive to stressors. However, under certain circumstances, developmental stress can promote an adaptive phenotype, allowing individuals to cope better with adverse situations in adulthood, thereby contributing to resilience.

Methods

Sprague Dawley rats (50 males, 48 females) were subjected to adolescent chronic variable stress (adol CVS) for 2 weeks at postnatal day 45. At postnatal day 85, a group was subjected to single prolonged stress (SPS). After a week, animals were evaluated in an auditory-cued fear conditioning paradigm, and neuronal recruitment during reinstatement was assessed by Fos expression. Patch clamp electrophysiology (17-35 cells/group) was performed in male rats to examine physiological changes associated with resilience.

Results

Adol CVS blocked fear potentiation evoked by SPS. We observed that SPS impaired extinction (males) and enhanced reinstatement (both sexes) of the conditioned freezing response. Prior adol CVS prevented both effects. SPS effects were associated with a reduction of infralimbic (IL) cortex neuronal recruitment after reinstatement in males and increased engagement of the central amygdala in females, both also prevented by adol CVS, suggesting different neurocircuits involved in generating resilience between sexes. We explored the mechanism behind reduced IL recruitment in males by studying the intrinsic excitability of IL pyramidal neurons. SPS reduced excitability of IL neurons, and prior adol CVS prevented this effect.

Conclusions

Our data indicate that adolescent stress can impart resilience to the effects of traumatic stress on neuroplasticity and behavior. Our data provide a mechanistic link behind developmental stress-induced behavioral resilience and prefrontal (IL) cortical excitability in males.

Physical restraint mouse models to assess immune responses under stress with or without habituation

Physical confinement, or restraint, is a psychological stressor used in rodent studies. A single restraint episode elevates blood corticosterone levels, a hallmark of stress responses. Repeated restraint results in habituation (or desensitization), whereas chronic exposure to unpredictable stressors fails to induce habituation. Here, we provide our protocols and guidelines in using three mouse restraint models, namely prolonged restraint stress, repeated restraint stress, and chronic variable stress, to examine immunological homeostasis/competence, or lack thereof, under stress with or without habituation.

For complete information on the generation and use of these protocols, please refer to Rudak et al. (2021).

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Three physical restraint mouse models to study the impact of long-term stress on immunity

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A model of prolonged restraint stress altering immune homeostasis/competence

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A model of repeated daily restraint stress resulting in habituation in animals

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An optimized protocol for chronic variable stress circumventing habituation

Anxiety and ultrastructural consequences of chronic mild stress in rats

Detrimental consequences following exposure to severe stress, either acute or chronic are well recognized. Chronic mild stress (CMS) is also a leading cause of emotional distress and neuropsychiatric conditions such as anxiety disorders. However, the neurobiological substrates of the latter, particularly at the ultrastructural levels have not been adequately investigated. In this study, adult male Wistar rats were subjected to 4 h daily mild restraint for 20 days and their behavior in open field and elevated plus maze (EPM) were evaluated 24 h after the last restraint. Anxiety-like behavior was evident in CMS exposed rats by increases in rearing and grooming in the open field and the avoidance of open arms in the EPM. Concomitant ultrastructural alterations such as chromatolysis, agglutination of synaptic vesicles or mitochondrial damage were also observed in the central nucleus of amygdala (CNA), an area intimately involved in emotional and fear response, in CMS exposed rats. These results while confirming detrimental consequences of CMS, also suggest that ultrastructural alterations in CNA may be a basis for CMS-induced anxiety.

Early life stress induces anxiety-like behavior during adulthood through dysregulation of neuronal plasticity in the basolateral amygdala

AIMS

Early life stress (ELS) increases the risk of psychiatric diseases such as anxiety disorders and depression in later life. Hyperactivation of the basolateral amygdala (BLA) neurons plays a pivotal role in the pathogenesis of stress-related diseases. However, the functional roles of BLA neurons in ELS-induced anxiety disorders are not completely understood.

MAIN METHODS

Mice were subjected to maternal separation (MS) during postnatal days 3 to 21 to mimic ELS. Anxiety-like behavior was tested by open field test (OFT), elevated plus maze (EPM), and novelty suppressed feeding (NSF). Then, c-fos expression, a proxy for neuronal activity, was evaluated by immunofluorescence. Finally, synaptic transmission and intrinsic excitability were measured by whole-cell patch-clamp recordings.

KEY FINDINGS

MS significantly increased anxiety-like behavior in adulthood, as indicated by less time spent in the center area of the OFT, less time spent in and fewer entries to the open arms of the EPM, and increased latency to feed in NSF. Mechanistically, MS increased the expression of c-fos in BLA. MS enhanced the excitatory, but not inhibitory, synaptic transmission onto BLA projection neurons (PNs), which was caused by enhanced presynaptic glutamate release. Moreover, MS also markedly increased the intrinsic neuronal excitability of BLA PNs, probably due to the reduced medium afterhyperpolarization (mAHP) in BLA PNs.

SIGNIFICANCE

Our results suggest that the changes of neuronal activity and synaptic transmission in the BLA PNs may play a crucial role in ELS-induced anxiety-like behavior, and these findings provide new insights into the pathological mechanisms of stress-related anxiety disorders.

Plasticity of intrinsic excitability across the estrous cycle in hypothalamic CRH neurons

Stress responses are highly plastic and vary across physiological states. The female estrous cycle is associated with a number of physiological changes including changes in stress responses, however, the mechanisms driving these changes are poorly understood. Corticotropin-releasing hormone (CRH) neurons are the primary neural population controlling the hypothalamic-pituitary-adrenal (HPA) axis and stress-evoked corticosterone secretion. Here we show that CRH neuron intrinsic excitability is regulated over the estrous cycle with a peak in proestrus and a nadir in estrus. Fast inactivating voltage-gated potassium channel (I_A) currents showed the opposite relationship, with current density being lowest in proestrus compared to other cycle stages. Blocking I_A currents equalized excitability across cycle stages revealing a role for I_A in mediating plasticity in stress circuit function over the female estrous cycle.