Hyper-excitability and epilepsy generated by chronic early-life stress

Early-life stress impairs recognition memory and perturbs the functional maturation of prefrontal-hippocampal-perirhinal networks

Early life exposure to stressful situations impairs cognitive performance of adults and contributes to the etiology of several psychiatric disorders. Most of affected cognitive abilities rely on coupling by synchrony within complex neuronal networks, including prefrontal cortex (PFC), hippocampus (HP), and perirhinal cortex (PRH). Yet it remains poorly understood how early life stress (ELS) induces dysfunction within these networks during the course of development. Here we used intermittent maternal separation during the first 2 postnatal weeks to mimic ELS and monitored the recognition memory and functional coupling within prefrontal-hippocampal-perirhinal circuits in juvenile rats. While maternally-separated female rats showed largely normal behavior, male rats experiencing this form of ELS had poorer location and recency recognition memory. Simultaneous multi-site extracellular recordings of network oscillations and neuronal spiking from PFC, HP, and PRH in vivo revealed corresponding decrease of oscillatory activity in theta and beta frequency bands in the PFC of male but not female rats experiencing maternal separation. This deficit was accompanied by weaker cross-frequency coupling within juvenile prefrontal-hippocampal networks. These results indicate that already at juvenile age ELS mimicked by maternal separation induces sex-specific deficits in recognition memory that might have as underlying mechanism a disturbed communication between PFC and HP.

Diagnosing and treating depression in epilepsy

At least one third of patients with active epilepsy suffer from significant impairment of their emotional well-being. A targeted examination for possible depression (irrespective of any social, financial or personal burdens) can identify patients who may benefit from medical attention and therapeutic support. Reliable screening instruments such as the Neurological Disorders Depression Inventory for Epilepsy (NDDI-E) are suitable for the timely identification of patients needing help. Neurologists should be capable of managing mild to moderate comorbid depression but referral to mental health specialists is mandatory in severe and difficult-to-treat depression, or if the patient is acutely suicidal. In terms of the therapeutic approach, it is essential first to optimize seizure control and minimize unwanted antiepileptic drug-related side effects. Psychotherapy for depression in epilepsy (including online self-treatment programs) is underutilized although it has proven effective in ten well-controlled trials. In contrast, the effectiveness of antidepressant drugs for depression in epilepsy is unknown. However, if modern antidepressants are used (e.g. SSRI, SNRI, NaSSA), concerns about an aggravation of seizures and or problematic interactions with antiepileptic drugs seem unwarranted. Epilepsy-related stress ("burden of epilepsy") explains depression in many patients but acute and temporary seizure-related states of depression or suicidality have also been reported. Limbic encephalitits may cause isolated mood alteration without any recognizable psychoetiological background indicating a possible role of neuroinflammation. This review will argue that, overall, a bio-psycho-social model best captures the currently available evidence relating to the etiology and treatment of depression as a comorbidity of epilepsy.

Early-Life Stress Paradigm Transiently Alters Maternal Behavior, Dam-Pup Interactions, and Offspring Vocalizations in Mice

Animal models can help elucidate the mechanisms through which early-life stress (ELS) has pathophysiological effects on the developing brain. One model that has been developed for rodents consists of limiting the amount of bedding and nesting material during the first postnatal weeks of pup life. This ELS environment has been shown to induce "abusive" behaviors by rat dams towards pups, while mouse dams have been hypothesized to display "fragmented care". Here, as part of an ongoing study of gene-environment interactions that impact brain development, we analyzed long observation periods of wild-type C57Bl/6J dams caring for wild-type and Met heterozygous pups. Met encodes for the MET receptor tyrosine kinase, which is involved in cortical and hippocampal synaptogenesis. Dams with limited resources from postnatal day (P)2 to P9 preserved regular long on-nest periods, and instead increased the number of discrete dam-pup interactions during regular off-nest periods. Immediately after dams entered the nest during off-nest periods in this ELS environment, pups responded to these qualitatively different interactions with an increased number of ultrasonic vocalizations (USV) and audible vocalizations (AV), communication signals that have been associated with aversive and painful stimuli. After returning to control conditions, nest entry behaviors normalized, and dams did not show altered anxiety-like or contextual fear learning behaviors after pup weaning. Furthermore, female mice that had undergone ELS as pups did not show atypical nest entry behaviors in control conditions in adulthood, suggesting that these specific maternal behaviors are not learned during the ELS period. The results suggest that atypical responses of both mother and pups during exposure to this ELS environment likely contribute to long-term negative outcomes in mice, and that these responses more closely resemble the effects of limited bedding on rat dams and pups than was previously suggested. Discerning how different early-life stressors mediate changes in maternal-pup interactions can help elucidate the mechanisms of ELS on brain development and behavior.

Positive and negative early life experiences differentially modulate long term survival and amyloid protein levels in a mouse model of Alzheimer's disease

Stress has been implicated as a risk factor for the severity and progression of sporadic Alzheimer's disease (AD). Early life experiences determine stress responsivity in later life, and modulate age-dependent cognitive decline. Therefore, we examined whether early life experiences influence AD outcome in a bigenic mouse model which progressively develops combined tau and amyloid pathology (biAT mice).Mice were subjected to either early life stress (ELS) or to 'positive' early handling (EH) postnatally (from day 2 to 9). In biAT mice, ELS significantly compromised long term survival, in contrast to EH which increased life expectancy. In 4 month old mice, ELS-reared biAT mice displayed increased hippocampal Aβ levels, while these levels were reduced in EH-reared biAT mice. No effects of ELS or EH were observed on the brain levels of APP, protein tau, or PSD-95. Dendritic morphology was moderately affected after ELS and EH in the amygdala and medial prefrontal cortex, while object recognition memory and open field performance were not affected. We conclude that despite the strong transgenic background, early life experiences significantly modulate the life expectancy of biAT mice. Parallel changes in hippocampal Aβ levels were evident, without affecting cognition of young adult biAT mice.

The Diathesis-Epilepsy Model: How Past Events Impact the Development of Epilepsy and Comorbidities

In epilepsy, seizures and comorbidities (e.g., cognitive deficits and depression) occur when specific thresholds are crossed. These thresholds depend on the diathesis (or vulnerability) of a given individual. The diathesis is controlled by multiple genetic and environmental factors. Diathesis changes over multiple timescales: on a daily basis, and as part of the development/aging processes, etc. The diathesis-epilepsy model introduced here provides a conceptual framework to understand how past events (e.g., a very stressful event) can directly influence the occurrence of epilepsy and comorbidities later in life. Experimental evidence supports this model, and the existence of biomarkers predictive of a vulnerability state have led to the development of preventive therapeutic strategies. Epigenetic modifications could be a key determinant of diathesis. Their role is discussed.

Pediatric Epileptic Encephalopathies: Pathophysiology and Animal Models

Epileptic encephalopathies are syndromes in which seizures or interictal epileptiform activity contribute to or exacerbate brain function, beyond that caused by the underlying pathology. These severe epilepsies begin early in life, are associated with poor lifelong outcome, and are resistant to most treatments. Therefore, they represent an immense challenge for families and the medical care system. Furthermore, the pathogenic mechanisms underlying the epileptic encephalopathies are poorly understood, hampering attempts to devise novel treatments. This article reviews animal models of the three classic epileptic encephalopathies-West syndrome (infantile spasms), Lennox-Gastaut syndrome, and continuous spike waves during sleep or Landau-Kleffner syndrome-with discussion of how animal models are revealing underlying pathophysiological mechanisms that might be amenable to targeted therapy.

Targets of polyamine dysregulation in major depression and suicide: Activity-dependent feedback, excitability, and neurotransmission

Major depressive disorder (MDD) is a leading cause of disability worldwide characterized by altered neuronal activity in brain regions involved in the control of stress and emotion. Although multiple lines of evidence suggest that altered stress-coping mechanisms underlie the etiology of MDD, the homeostatic control of neuronal excitability in MDD at the molecular level is not well established. In this review, we examine past and current evidence implicating dysregulation of the polyamine system as a central factor in the homeostatic response to stress and the etiology of MDD. We discuss the cellular effects of abnormal metabolism of polyamines in the context of their role in sensing and modulation of neuronal, electrical, and synaptic activity. Finally, we discuss evidence supporting an allostatic model of depression based on a chronic elevation in polyamine levels resulting in self-sustained stress response mechanisms maintained by maladaptive homeostatic mechanisms.

Toward Understanding How Early-Life Stress Reprograms Cognitive and Emotional Brain Networks

Vulnerability to emotional disorders including depression derives from interactions between genes and environment, especially during sensitive developmental periods. Adverse early-life experiences provoke the release and modify the expression of several stress mediators and neurotransmitters within specific brain regions. The interaction of these mediators with developing neurons and neuronal networks may lead to long-lasting structural and functional alterations associated with cognitive and emotional consequences. Although a vast body of work has linked quantitative and qualitative aspects of stress to adolescent and adult outcomes, a number of questions are unclear. What distinguishes 'normal' from pathologic or toxic stress? How are the effects of stress transformed into structural and functional changes in individual neurons and neuronal networks? Which ones are affected? We review these questions in the context of established and emerging studies. We introduce a novel concept regarding the origin of toxic early-life stress, stating that it may derive from specific patterns of environmental signals, especially those derived from the mother or caretaker. Fragmented and unpredictable patterns of maternal care behaviors induce a profound chronic stress. The aberrant patterns and rhythms of early-life sensory input might also directly and adversely influence the maturation of cognitive and emotional brain circuits, in analogy to visual and auditory brain systems. Thus, unpredictable, stress-provoking early-life experiences may influence adolescent cognitive and emotional outcomes by disrupting the maturation of the underlying brain networks. Comprehensive approaches and multiple levels of analysis are required to probe the protean consequences of early-life adversity on the developing brain. These involve integrated human and animal-model studies, and approaches ranging from in vivo imaging to novel neuroanatomical, molecular, epigenomic, and computational methodologies. Because early-life adversity is a powerful determinant of subsequent vulnerabilities to emotional and cognitive pathologies, understanding the underlying processes will have profound implications for the world's current and future children.

Diversity of Reporter Expression Patterns in Transgenic Mouse Lines Targeting Corticotropin-Releasing Hormone-Expressing Neurons

Transgenic mice, including lines targeting corticotropin-releasing factor (CRF or CRH), have been extensively employed to study stress neurobiology. These powerful tools are poised to revolutionize our understanding of the localization and connectivity of CRH-expressing neurons, and the crucial roles of CRH in normal and pathological conditions. Accurate interpretation of studies using cell type-specific transgenic mice vitally depends on congruence between expression of the endogenous peptide and reporter. If reporter expression does not faithfully reproduce native gene expression, then effects of manipulating unintentionally targeted cells may be misattributed. Here, we studied CRH and reporter expression patterns in 3 adult transgenic mice: Crh-IRES-Cre;Ai14 (tdTomato mouse), Crfp3.0CreGFP, and Crh-GFP BAC. We employed the CRH antiserum generated by Vale after validating its specificity using CRH-null mice. We focused the analyses on stress-salient regions, including hypothalamus, amygdala, bed nucleus of the stria terminalis, and hippocampus. Expression patterns of endogenous CRH were consistent among wild-type and transgenic mice. In tdTomato mice, most CRH-expressing neurons coexpressed the reporter, yet the reporter identified a few non-CRH-expressing pyramidal-like cells in hippocampal CA1 and CA3. In Crfp3.0CreGFP mice, coexpression of CRH and the reporter was found in central amygdala and, less commonly, in other evaluated regions. In Crh-GFP BAC mice, the large majority of neurons expressed either CRH or reporter, with little overlap. These data highlight significant diversity in concordant expression of reporter and endogenous CRH among 3 available transgenic mice. These findings should be instrumental in interpreting important scientific findings emerging from the use of these potent neurobiological tools.