Enhanced evoked responses after early adversity and repeated platform exposure: the neurobiology of vulnerability?

Vasopressin deficiency diminishes acute and long-term consequences of maternal deprivation in male rat pups

Early life events have special importance in the development as postnatal environmental alterations may permanently affect the lifetime vulnerability to diseases. For the interpretation of the long-term consequences it is important to understand the immediate effects. As the role of vasopressin in hypothalamic-pituitary-adrenal axis regulation as well as in affective disorders seem to be important we addressed the question whether the congenital lack of vasopressin will modify the stress reactivity of the pups and will influence the later consequences of single 24h maternal deprivation (MD) on both stress-reactivity and stress-related behavioral changes. Vasopressin-producing (di/+) and deficient (di/di) Brattleboro rat were used. In 10-day-old pups MD induced a remarkable corticosterone rise in both genotypes without adrenocorticotropin (ACTH) increase in di/di rats. Studying the later consequences at around weaning (25-35-day-old rats) we found somatic and hormonal alterations (body weight reduction, dysregulation of the stress axis) which were not that obvious in di/di rats. The more anxious state of MD rats was not detectable in di/di rats both at weaning and in adulthood (7-12-week-old). The lack of vasopressin abolished all chronic stress and anxiety-like tendencies both at weaning and in adulthood probably as a consequence of reduced ACTH rise immediately after MD in pups. This finding suggests that postnatal stress-induced ACTH rise may have long-term developmental consequences.

Have studies of the developmental regulation of behavioral phenotypes revealed the mechanisms of gene-environment interactions?

This review addresses the recent convergence of our long-standing knowledge of the regulation of behavioral phenotypes by developmental experience with recent advances in our understanding of mechanisms regulating gene expression. This review supports a particular perspective on the developmental regulation of behavioral phenotypes: That the role of common developmental experiences (e.g. maternal interactions, peer interactions, exposure to a complex environment, etc.) is to fit individuals to the circumstances of their lives within bounds determined by long-standing (evolutionary) mechanisms that have shaped responses to critical and fundamental types of experience via those aspects of gene structure that regulate gene expression. The phenotype of a given species is not absolute for a given genotype but rather variable within bounds that is determined by mechanisms regulated by experience (e.g. epigenetic mechanisms). This phenotypic variation is not necessarily random, or evenly distributed along a continuum of description or measurement, but often highly disjointed, producing distinct, even opposing, phenotypes. The potentiality for these varying phenotypes is itself the product of evolution, the potential for alternative phenotypes itself conveying evolutionary advantage. Examples of such phenotypic variation, resulting from environmental or experiential influences, have a long history of study in neurobiology, and a number of these will be discussed in this review: neurodevelopmental experiences that produce phenotypic variation in visual perception, cognitive function, and emotional behavior. Although other examples will be discussed, particular emphasis will be made on the role of social behavior on neurodevelopment and phenotypic determination. It will be argued that an important purpose of some aspects of social behavior is regulation of neurobehavioral phenotypes by experience via genetic regulatory mechanisms.

Unraveling the time domains of corticosteroid hormone influences on brain activity: rapid, slow, and chronic modes

Brain cells are continuously exposed to corticosteroid hormones, although the levels vary (e.g., after stress). Corticosteroids alter neural activity via two receptor types, mineralocorticoid (MR) and glucocorticoid receptors (GR). These receptors regulate gene transcription but also, as we now know, act nongenomically. Via nongenomic pathways, MRs enhance and GRs suppress neural activity. In the hypothalamus, inhibitory GR effects contribute to negative feedback regulation of the stress axis. Nongenomic MR actions are also important extrahypothalamically and help organisms to immediately select an appropriate response strategy. Via genomic mechanisms, corticosteroid actions in the basolateral amygdala and ventral-most part of the cornu ammonis 1 hippocampal area are generally excitatory, providing an extended window for encoding of emotional aspects of a stressful event. GRs in hippocampal and prefrontal pyramidal cells increase surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and strengthen glutamatergic signaling through pathways partly overlapping with those involved in long-term potentiation. This raises the threshold for subsequent induction of synaptic potentiation and promotes long-term depression. Synapses activated during stress are thus presumably strengthened but protected against excitatory inputs reaching the cells later. This restores higher cognitive control and promotes, for example, consolidation of stress-related contextual information. When an organism experiences stress early in life or repeatedly in adulthood, the ability to induce synaptic potentiation is strongly reduced and the likelihood to induce depression enhanced, even under rest. Treatment with antiglucocorticoids can ameliorate cellular effects after chronic stress and thus provide an interesting lead for treatment of stress-related disorders.

Electrophysiological insights into the enduring effects of early life stress on the brain

Increasing evidence links exposure to stress early in life to long-term alterations in brain function, which in turn have been linked to a range of psychiatric and neurological disorders in humans. Electrophysiological approaches to studying these causal pathways have been relatively underexploited. Effects of early life stress on neuronal electrophysiological properties offer a set of potential mechanisms for these susceptibilities, notably in the case of epilepsy. Thus, we review experimental evidence for altered cellular and circuit electrophysiology resulting from exposure to early life stress. Much of this work focuses on limbic long-term potentiation, but other studies address alterations in electrophysiological properties of ion channels, neurotransmitter systems, and the autonomic nervous system. We discuss mechanisms which may mediate these effects, including influences of early life stress on key components of brain synaptic transmission, particularly glutamate, GABA and 5-HT receptors, and influences on neuroplasticity (primarily neurogenesis and synaptic density) and on neuronal network activity. The existing literature, although small, provides strong evidence that early life stress induces enduring, often robust effects on a range of electrophysiological properties, suggesting further study of enduring effects of early life stress employing electrophysiological methods and concepts will be productive in illuminating disease pathophysiology.

Early-life stress and antidepressant treatment involve synaptic signaling and Erk kinases in a gene-environment model of depression

Stress has been shown to interact with genetic vulnerability in pathogenesis of psychiatric disorders. Here we investigated the outcome of interaction between genetic vulnerability and early-life stress, by employing a rodent model that combines an inherited trait of vulnerability in Flinders Sensitive Line (FSL) rats, with early-life stress (maternal separation). Basal differences in synaptic signaling between FSL rats and their controls were studied, as well as the consequences of early-life stress in adulthood, and their response to chronic antidepressant treatment (escitalopram). FSL rats showed basal differences in the activation of synapsin I and Erk1/2, as well as in alpha CaM kinase II/syntaxin-1 and alpha CaM kinase II/NMDA-receptor interactions in purified hippocampal synaptosomes. In addition, FSL rats displayed a blunted response of Erk-MAP kinases and other differences in the outcome of early-life stress in adulthood. Escitalopram treatment restored some but not all alterations observed in FSL rats after early-life stress. The marked alterations found in key regulators of presynaptic release/neurotransmission in the basal FSL rats, and as a result of early-life stress, suggest synaptic dysfunction. These results show that early gene-environment interaction may cause life-long synaptic changes affecting the course of depressive-like behavior and response to drugs.

Early life stress as an influence on limbic epilepsy: an hypothesis whose time has come?

The pathogenesis of mesial temporal lobe epilepsy (MTLE), the most prevalent form of refractory focal epilepsy in adults, is thought to begin in early life, even though seizures may not commence until adolescence or adulthood. Amongst the range of early life factors implicated in MTLE causation (febrile seizures, traumatic brain injury, etc.), stress may be one important contributor. Early life stress is an a priori agent deserving study because of the large amount of neuroscientific data showing enduring effects on structure and function in hippocampus and amygdala, the key structures involved in MTLE. An emerging body of evidence directly tests hypotheses concerning early life stress and limbic epilepsy: early life stressors, such as maternal separation, have been shown to aggravate epileptogenesis in both status epilepticus and kindling models of limbic epilepsy. In addition to elucidating its influence on limbic epileptogenesis itself, the study of early life stress has the potential to shed light on the psychiatric disorder that accompanies MTLE. For many years, psychiatric comorbidity was viewed as an effect of epilepsy, mediated psychologically and/or neurobiologically. An alternative – or complementary – perspective is that of shared causation. Early life stress, implicated in the pathogenesis of several psychiatric disorders, may be one such causal factor. This paper aims to critically review the body of experimental evidence linking early life stress and epilepsy; to discuss the direct studies examining early life stress effects in current models of limbic seizures/epilepsy; and to suggest priorities for future research.

Remodelling by early-life stress of NMDA receptor-dependent synaptic plasticity in a gene-environment rat model of depression

An animal model of depression combining genetic vulnerability and early-life stress (ELS) was prepared by submitting the Flinders Sensitive Line (FSL) rats to a standard paradigm of maternal separation. We analysed hippocampal synaptic transmission and plasticity in vivo and ionotropic receptors for glutamate in FSL rats, in their controls Flinders Resistant Line (FRL) rats, and in both lines subjected to ELS. A strong inhibition of long-term potentiation (LTP) and lower synaptic expression of NR1 subunit of the NMDA receptor were found in FSL rats. Remarkably, ELS induced a remodelling of synaptic plasticity only in FSL rats, reducing inhibition of LTP; this was accompanied by marked increase of synaptic NR1 subunit and GluR2/3 subunits of AMPA receptors. Chronic treatment with escitalopram inhibited LTP in FRL rats, but this effect was attenuated by prior ELS. The present results suggest that early gene-environment interactions cause lifelong synaptic changes affecting functional and molecular aspects of plasticity, partly reversed by antidepressant treatments.

Stress load during childhood affects psychopathology in psychiatric patients

BACKGROUND

Childhood stress and trauma have been related to adult psychopathology in different psychiatric disorders. The present study aimed at verifying this relationship for stressful experiences during developmental periods by screening stress load across life in adult psychiatric inpatients with different diagnoses compared to healthy subjects. In addition, a relationship between the amount of adverse experiences and the severity of pathology, which has been described as a 'building block' effect in posttraumatic stress disorder (PTSD), was explored for non-traumatic events in psychiatric disorders other than PTSD.

METHODS

96 patients with diagnoses of Major Depressive Disorder (MDD), schizophrenia, drug addiction, or personality disorders (PD) and 31 subjects without psychiatric diagnosis were screened for adverse experiences in childhood (before the age of six years), before onset of puberty, and in adulthood using the Early Trauma Inventory and the Posttraumatic Stress Diagnostic Scale. Effects of stress load on psychopathology were examined for affective symptoms, PTSD, and severity of illness by regression analyses and comparison of subgroups with high and low stress load.

RESULTS

High stress load in childhood and before puberty, but not in adulthood, was related to negative affect in all participants. In patients, high stress load was related to depressive and posttraumatic symptoms, severity of disorder, and the diagnoses of MDD and PD.

CONCLUSION

Results support the hypothesis of stress-sensitive periods during development, which may interact with genetic and other vulnerability factors in their influence on the progress of psychiatric disorders. A 'dose' effect of stress load on the severity of psychopathology is not restricted to the relationship between traumata and PTSD.

Hypothalamic-pituitary-gut axis dysregulation in irritable bowel syndrome: plasma cytokines as a potential biomarker?

BACKGROUND & AIMS

Irritable bowel syndrome (IBS) is a functional disorder with an etiology that has been linked to both psychological stress and infection. The primary aim of this study was to examine the hypothalamic-pituitary-adrenal axis in patients with IBS and to relate such response to plasma cytokine profiles.

METHODS

A total of 151 subjects, 76 patients and 75 controls, were recruited. The patients with IBS were diagnosed according to Rome II criteria. Forty-nine patients and 48 matched controls had cytokine levels measured, and a subset of 21 patients and 21 controls also underwent a corticotropin-releasing hormone (CRH) stimulation test with plasma levels of adrenocorticotropic hormone (ACTH) and cortisol measured. The remaining 27 patients and 27 controls underwent a dexamethasone (1 mg) challenge.

RESULTS

Cortisol and the proinflammatory cytokines interleukin (IL)-6 (together with its soluble receptor) and IL-8 were elevated in all IBS subgroups (diarrhea predominant, constipated, and alternators), although the elevation was most marked in the constipated subgroup. There was no alteration in the anti-inflammatory cytokine IL-10. Following CRH infusion, an exaggerated release of both ACTH and cortisol was observed in patients with IBS. There was a significant correlation between the ACTH response (deltaACTH) and the IL-6 levels. A similar relationship existed between the deltaACTH/deltacortisol ratio and the IL-6 levels. Dexamethasone suppression of cortisol was similar in patients and controls.

CONCLUSIONS

IBS is characterized by an overactivation of the hypothalamic-pituitary-adrenal axis and a proinflammatory cytokine increase.