Stress and GABA receptors

Repeated administration of imipramine modifies GABAergic transmission in rat frontal cortex

Alterations in the functions of brain gamma-aminobutyric acid (GABA) inhibitory system and a distortion in the balance between excitatory and inhibitory synaptic transmission have been hypothesized to be possible causes of mood disorders. Experimental evidence points to modifications of GABAergic transmission as a result of prolonged treatment with antidepressant drugs, however, the influence of the tricyclic antidepressant imipramine on inhibitory synaptic transmission in the rat cerebral cortex has not yet been investigated. Therefore, in the present study the effects of single and repeated administration of imipramine were evaluated ex vivo in slices of the rat frontal cortex using electrophysiological approach. In slices prepared 2 days after the last drug administration from animals receiving imipramine for 14 days (dose 10 mg/kg p.o., twice daily) the mean frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from layer II/III pyramidal neurons was decreased, while the mean amplitude of sIPSCs was increased. These effects were absent in slices obtained from rats which received imipramine once. Application of N,N′-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN 082), a selective mGluR7 allosteric agonist, to the slice incubation medium resulted in a decrease in the mean frequency of sIPSCs in preparations obtained from repeated imipramine-treated animals, in contrast to slices originating from control rats where no AMN 082-induced effects were observed. Repeated imipramine treatment reduced protein density levels of the three tested GABA_A receptor subunits: α₁, β₂ and γ₂. These data indicate that repeated treatment of normal rats with imipramine results in a modification of the release mechanism of GABA from presynaptic terminals and its modulation by mGluR7 receptors as well as in an alteration in GABA_A receptor subunit protein levels in the rat cerebral cortex.

Role of IL-6 in the etiology of hyperexcitable neuropsychiatric conditions: experimental evidence and therapeutic implications

Many neuropsychiatric conditions are primed or triggered by different types of stressors. The mechanisms through which stress induces neuropsychiatric disease are complex and incompletely understood. A ‘double hit’ hypothesis of neuropsychiatric disease postulates that stress induces maladaptive behavior in two phases separated by a dormant period. Recent research shows that the pleiotropic cytokine IL-6 is released centrally and peripherally following physical and psychological stress. In this article, we analyze evidence from clinics and animal models suggesting that stress-induced elevation in the levels of IL-6 may play a key role in the etiology of a heterogeneous family of hyperexcitable central conditions including epilepsy, schizophrenic psychoses, anxiety and disorders of the autistic spectrum. The cellular mechanism leading to hyperexcitable conditions might be a decrease in inhibitory/excitatory synaptic balance in either or both temporal phases of the conditions. Following these observations, we discuss how they may have important implications for optimal prophylactic and therapeutic pharmacological treatment.

Exposure to a maternal n-3 fatty acid-deficient diet during brain development provokes excessive hypothalamic-pituitary-adrenal axis responses to stress and behavioral indices of depression and anxiety in male rat offspring later in life

Brain docosahexaenoic acid (DHA, 22:6n-3) accumulates rapidly during brain development and is essential for normal neurological function. The aim of this study was to evaluate whether brain development was the critical period in which DHA deficiency leads to dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis in response to stress later in life. Rats were exposed to an n-3 fatty acid-deficient diet or the same diet supplemented with fish oil as an n-3 fatty acid-adequate diet either throughout the preweaning period from embryo to weaning at 3 weeks old or during the postweaning period from 3 to 10 weeks old. Exposure to the n-3 fatty acid-deficient diet during the preweaning period resulted, at weaning, in a significant decrease in hypothalamic DHA levels and a reduced male offspring body weight. DHA deficiency during the preweaning period significantly increased and prolonged restraint stress-induced changes in colonic temperature and serum corticosterone levels, caused a significant increase in GABA(A) antagonist-induced heart rate changes and enhanced depressive-like behavior in the forced swimming test and anxiety-like behavior in the plus-maze test in later life. These effects were not seen in male rats fed the n-3 fatty acid-deficient diet during the postweaning period. These results suggest that brain development is the critical period in which DHA deficiency leads to excessive HPA responses to stress and elevated behavioral indices of depression and anxiety in adulthood. We propose that these effects of hypothalamic DHA deficiency during brain development may involve a GABA(A) receptor-mediated mechanism.

Angiotensin II AT1 receptor blocker candesartan prevents the fast up-regulation of cerebrocortical benzodiazepine-1 receptors induced by acute inflammatory and restraint stress

Centrally acting Angiotensin II AT(1) receptor blockers (ARBs) protect from stress-induced disorders and decrease anxiety in a model of inflammatory stress, the systemic injection of bacterial endotoxin lipopolysaccharide (LPS). In order to better understand the anxiolytic effect of ARBs, we treated rats with LPS (50 μg/kg) with or without 3 days of pretreatment with the ARB candesartan (1mg/kg/day), and studied cortical benzodiazepine (BZ) and corticotrophin-releasing factor (CRF) receptors. We compared the cortical BZ and CRF receptors expression pattern induced by LPS with that produced in restraint stress. Inflammation stress produced a generalized increase in cortical BZ(1) receptors and reduced mRNA expression of the GABA(A) receptor γ(2) subunit in cingulate cortex; changes were prevented by candesartan pretreatment. Moreover, restraint stress produced similar increases in cortical BZ(1) receptor binding, and candesartan prevented these changes. Treatment with candesartan alone increased cortical BZ(1) binding, and decreased γ(2) subunit mRNA expression in the cingulate cortex. Conversely, we did not find changes in CRF(1) receptor expression in any of the cortical areas studied, either after inflammation or restraint stress. Cortical CRF(2) receptor binding was undetectable, but CRF(2) mRNA expression was decreased by inflammation stress, a change prevented by candesartan. We conclude that stress promotes rapid and widespread changes in cortical BZ(1) receptor expression; and that the stress-induced BZ(1) receptor expression is under the control of AT(1) receptor activity. The results suggest that the anti-anxiety effect of ARBs may be associated with their capacity to regulate stress-induced alterations in cortical BZ(1) receptors.

The stress-induced cytokine interleukin-6 decreases the inhibition/excitation ratio in the rat temporal cortex via trans-signaling

BACKGROUND

Although it is known that stress elevates the levels of pro-inflammatory cytokines and promotes hyper-excitable central conditions, a causal relationship between these two factors has not yet been identified. Recent studies suggest that increases in interleukin 6 (IL-6) levels are specifically associated with stress. We hypothesized that IL-6 acutely and directly induces cortical hyper-excitability by altering the balance between synaptic excitation and inhibition.

METHODS

We used patch-clamp to determine the effects of exogenous or endogenous IL-6 on electrically evoked postsynaptic currents on a cortical rat slice preparation. We used control subjects or animals systemically injected with lipopolysaccharide or subjected to electrical foot-shock as rat models of stress.

RESULTS

In control animals, IL-6 did not affect excitatory postsynaptic currents but selectively and reversibly reduced the amplitude of inhibitory postsynaptic currents with a postsynaptic effect. The IL-6-induced inhibitory postsynaptic currents decrease was inhibited by drugs interfering with receptor trafficking and/or internalization, including wortmannin, Brefeldin A, 2-Br-hexadecanoic acid, or dynamin peptide inhibitor. In both animal models, stress-induced decrease in synaptic inhibition/excitation ratio was prevented by prior intra-ventricular injection of an analog of the endogenous IL-6 trans-signaling blocker gp130.

CONCLUSIONS

Our results suggest that stress-induced IL-6 shifts the balance between synaptic inhibition and excitation in favor of the latter, possibly by decreasing the density of functional γ-aminobutyric acid A receptors, accelerating their removal and/or decreasing their insertion rate from/to the plasma membrane. We speculate that this mechanism could contribute to stress-induced detrimental long-term increases in central excitability present in a variety of neurological and psychiatric conditions.

Discovering imaging endophenotypes for major depression

Psychiatry research lacks an in-depth understanding of mood disorders phenotypes, leading to limited success of genetics studies of major depressive disorder (MDD). The dramatic progress in safe and affordable magnetic resonance-based imaging methods has the potential to identify subtle abnormalities of neural structures, connectivity and function in mood disordered subjects. This review paper presents strategies to improve the phenotypic definition of MDD by proposing imaging endophenotypes derived from magnetic resonance spectroscopy measures, such as cortical gamma-amino butyric acid (GABA) and glutamate/glutamine concentrations, and from measures of resting-state activity and functional connectivity. The proposed endophenotypes are discussed regarding specificity, mood state-independence, heritability, familiarity, clinical relevance and possible associations with candidate genes. By improving phenotypic definitions, the discovery of new imaging endophenotypes will increase the power of candidate gene and genome-wide associations studies. It will also help to develop and evaluate novel therapeutic treatments and enable clinicians to apply individually tailored therapeutic approaches. Finally, improvements of the phenotypic definition of MDD based on neuroimaging measures will contribute to a new classification system of mood disorders based on etiology and pathophysiology.

The GABAergic deficit hypothesis of major depressive disorder

Increasing evidence points to an association between major depressive disorders (MDDs) and diverse types of GABAergic deficits. In this review, we summarize clinical and preclinical evidence supporting a central and causal role of GABAergic deficits in the etiology of depressive disorders. Studies of depressed patients indicate that MDDs are accompanied by reduced brain concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and by alterations in the subunit composition of the principal receptors (GABA(A) receptors) mediating GABAergic inhibition. In addition, there is abundant evidence that suggests that GABA has a prominent role in the brain control of stress, the most important vulnerability factor in mood disorders. Furthermore, preclinical evidence suggests that currently used antidepressant drugs (ADs) designed to alter monoaminergic transmission and nonpharmacological therapies may ultimately act to counteract GABAergic deficits. In particular, GABAergic transmission has an important role in the control of hippocampal neurogenesis and neural maturation, which are now established as cellular substrates of most if not all antidepressant therapies. Finally, comparatively modest deficits in GABAergic transmission in GABA(A) receptor-deficient mice are sufficient to cause behavioral, cognitive, neuroanatomical and neuroendocrine phenotypes, as well as AD response characteristics expected of an animal model of MDD. The GABAergic hypothesis of MDD suggests that alterations in GABAergic transmission represent fundamentally important aspects of the etiological sequelae of MDDs that are reversed by monoaminergic AD action.

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.

The reciprocal regulation of stress hormones and GABA(A) receptors

Stress-derived steroid hormones regulate the expression and function of GABA(A) receptors (GABA(A)Rs). Changes in GABA(A)R subunit expression have been demonstrated under conditions of altered steroid hormone levels, such as stress, as well as following exogenous steroid hormone administration. In addition to the effects of stress-derived steroid hormones on GABA(A)R subunit expression, stress hormones can also be metabolized to neuroactive derivatives which can alter the function of GABA(A)Rs. Neurosteroids allosterically modulate GABA(A)Rs at concentrations comparable to those during stress. In addition to the actions of stress-derived steroid hormones on GABA(A)Rs, GABA(A)Rs reciprocally regulate the production of stress hormones. The stress response is mediated by the hypothalamic-pituitary-adrenal (HPA) axis, the activity of which is governed by corticotropin releasing hormone (CRH) neurons. The activity of CRH neurons is largely controlled by robust GABAergic inhibition. Recently, it has been demonstrated that CRH neurons are regulated by neurosteroid-sensitive, GABA(A)R δ subunit-containing receptors representing a novel feedback mechanism onto the HPA axis. Further, it has been demonstrated that neurosteroidogenesis and neurosteroid actions on GABA(A)R δ subunit-containing receptors on CRH neurons are necessary to mount the physiological response to stress. Here we review the literature describing the effects of steroid hormones on GABA(A)Rs as well as the importance of GABA(A)Rs in regulating the production of steroid hormones. This review incorporates what we currently know about changes in GABA(A)Rs following stress and the role in HPA axis regulation.

Chronic variable stress alters inflammatory and cholinergic parameters in hippocampus of rats

In the present study we investigated the effect of chronic variable stress (CVS) on some parameters of the immune system, including levels of cytokines [interleukin 1β (IL-1 β), interleukin 6 (IL-6), tumor necrosis factor α (TNF- α)] and chemokine CCL2 (MCP-1) in the hippocampus of rats. Acetylcholinesterase activity was also evaluated. Sixty-day old Wistar rats were submitted to different mild stressors for 40 days. After the last stress section, the cytokines and MCP-1 were determined by immunoassay and acetylcholinesterase activity by colorimetric method. Results showed that chronic stress significantly increased the levels of IL-1β, IL-6 and TNF-α, but did not alter the levels of MCP-1. In addition, acetylcholinesterase activity was increased in the hippocampus of rats subjected to CVS. These findings suggest that inflammation and cholinergic dysfunction may be, at least in part, important contributors to the neurological dysfunction observed in some depressed patients.

Quantitative localisation of synaptic and extrasynaptic GABAA receptor subunits on hippocampal pyramidal cells by freeze-fracture replica immunolabelling

Hippocampal CA1 pyramidal cells, which receive γ-aminobutyric acid (GABA)ergic input from at least 18 types of presynaptic neuron, express 14 subunits of the pentameric GABA(A) receptor. The relative contribution of any subunit to synaptic and extrasynaptic receptors influences the dynamics of GABA and drug actions. Synaptic receptors mediate phasic GABA-evoked conductance and extrasynaptic receptors contribute to a tonic conductance. We used freeze-fracture replica-immunogold labelling, a sensitive quantitative immunocytochemical method, to detect synaptic and extrasynaptic pools of the alpha1, alpha2 and beta3 subunits. Antibodies to the cytoplasmic loop of the subunits showed immunogold particles concentrated on distinct clusters of intramembrane particles (IMPs) on the cytoplasmic face of the plasma membrane on the somata, dendrites and axon initial segments, with an abrupt decrease in labelling at the edge of the IMP cluster. Neuroligin-2, a GABAergic synapse-specific adhesion molecule, co-labels all beta3 subunit-rich IMP clusters, therefore we considered them synapses. Double-labelling for two subunits showed that virtually all somatic synapses contain the alpha1, alpha2 and beta3 subunits. The extrasynaptic plasma membrane of the somata, dendrites and dendritic spines showed low-density immunolabelling. Synaptic labelling densities on somata for the alpha1, alpha2 and beta3 subunits were 78-132, 94 and 79 times higher than on the extrasynaptic membranes, respectively. As GABAergic synapses occupy 0.72% of the soma surface, the fraction of synaptic labelling was 33-48 (alpha1), 40 (alpha2) and 36 (beta3)% of the total somatic surface immunolabelling. Assuming similar antibody access to all receptors, about 60% of these subunits are in extrasynaptic receptors.