Corticosteroid actions on the expression of kainate receptor subunit mRNAs in rat hippocampus

Structural plasticity of the adult brain: how animal models help us understand brain changes in depression and systemic disorders related to depression

The brain interprets experiences and translates them into behavioral and physiological responses. Stressful events are those which are threatening or, at the very least, unexpected and surprising, and the physiological and behavioral responses are intended to promote adaptation via a process called “allostasis. ” Chemical mediators of allostasis include cortisol and adrenalin from the adrenal glands, other hormones, and neurotransmitters, the parasympathetic and sympathetic nervous systems, and cytokines and chemokines from the immune system. Two brain structures, the amygdala and hippocampus, play key roles in interpreting what is stressful and determining appropriate responses. The hippocampus, a key structure for memories of events and contexts, expresses receptors that enable it to respond to glucocorticoid hormones in the blood, it undergoes atrophy in a number of psychiatric disorders; it also responds to stressors with changes in excitability, decreased dendritic branching, and reduction in number of neurons in the dentate gyrus. The amygdala, which is important for “emotional memories, ” becomes hyperactive in posttraumatic stress disorder and depressive illness, in animal models of stress, there is evidence for growth and hypertrophy of nerve cells in the amygdala. Changes in the brain after acute and chronic stressors mirror the pattern seen in the metabolic, cardiovascular, and immune systems, that is, short-term adaptation (allostasis) followed by long-term damage (allostatic load), eg, atherosclerosis, fat deposition obesity, bone demineralization, and impaired immune function. Allostatic load of this kind is seen in major depressive illness and may also be expressed in other chronic anxiety and mood disorders.

Recent findings on neuroprotection against excitotoxicity in the hippocampus of female rats

Newborn mammals are totally dependent on maternal milk and care for survival. The mother's brain undergoes different behavioural, physiological and emotional adaptations that make the mother more likely to satisfy the demands of the offspring. Recent reports from our group show that, compared to nulliparous rats, lactation diminishes cell damage induced by excitotoxicity in the dorsal hippocampus of the dam after systemic or i.c. administration of kainic acid (KA) and the resulting motor seizures. Elevated levels of prolactin (PRL), oxytocin, progesterone and glucocorticoids are characteristics of lactation, and the pronounced fluctuation of these hormones occurring in this phase may play a role protecting the hippocampus. Indeed, PRL administration to ovariectomised rats significantly diminishes the deleterious effects of KA in the dorsal hippocampus and reduces the progression of KA-induced seizures. Thus, lactation is a natural model for neuroprotection because it effectively prevents acute and chronic cell damage of the hippocampus induced by excitotoxicity.

Regulation of kainate receptor subunit mRNA by stress and corticosteroids in the rat hippocampus

Kainate receptors are a class of ionotropic glutamate receptors that have a role in the modulation of glutamate release and synaptic plasticity in the hippocampal formation. Previous studies have implicated corticosteroids in the regulation of these receptors and recent clinical work has shown that polymorphisms in kainate receptor subunit genes are associated with susceptibility to major depression and response to anti-depressant treatment. In the present study we sought to examine the effects of chronic stress and corticosteroid treatments upon the expression of the mRNA of kainate receptor subunits GluR5-7 and KA1-2. Our results show that, after 7 days, adrenalectomy results in increased expression of hippocampal KA1, GluR6 and GluR7 mRNAs, an effect which is reversed by treatment with corticosterone in the case of KA1 and GluR7 and by aldosterone treatment in the case of GluR6. 21 days of chronic restraint stress (CRS) elevated the expression of the KA1 subunit, but had no effect on the expression of the other subunits. Similarly, 21 days of treatment with a moderate dose of corticosterone also increased KA1 mRNA in the dentate gyrus, whereas a high corticosterone dose has no effect. Our results suggest an interaction between hippocampal kainate receptor composition and the hypothalamic-pituitary-adrenal (HPA) axis and show a selective chronic stress induced modulation of the KA1 subunit in the dentate gyrus and CA3 that has implications for stress-induced adaptive structural plasticity.

Neuroprotective effects of lactation against kainic acid treatment in the dorsal hippocampus of the rat

Marked hippocampal changes in response to excitatory amino acid agonists occur during pregnancy (e.g. decreased frequency in spontaneous recurrent seizures in rats with KA lesions of the hippocampus) and lactation (e.g. reduced c-Fos expression in response to N-methyl-d,l-aspartic acid but not to kainic acid). In this study, the possibility that lactation protects against the excitotoxic damage induced by KA in hippocampal areas was explored. We compared cell damage induced 24 h after a single systemic administration of KA (5 or 7.5 mg/kg bw) in regions CA1, CA3, and CA4 of the dorsal hippocampus of rats in the final week of lactation to that in diestrus phase. To determine cellular damage in a rostro-caudal segment of the dorsal hippocampus, we used NISSL and Fluorojade staining, immunohistochemistry for active caspase-3 and TUNEL, and we observed that the KA treatment provoked a significant loss of neurons in diestrus rats, principally in the pyramidal cells of CA1 region. In contrast, in lactating rats, pyramidal neurons from CA1, CA3, and CA4 in the dorsal hippocampus were significantly protected against KA-induced neuronal damage, indicating that lactation may be a natural model of neuroprotection.

Ethanol modifies the effect of handling stress on gene expression: problems in the analysis of two-way gene expression studies in mouse brain

Studies analysing the effects of acute treatments on animal behaviour and brain biochemistry frequently use pairwise comparisons between sham-treated and -untreated animals. In this study, we analyse expression of tPA, Grik2, Smarca2 and the transcription factor, Sp1, in mouse cerebellum following acute ethanol treatment. Expression is compared to saline-injected and -untreated control animals. We demonstrate that acute i.p. injection of saline may alter gene expression in a gene-specific manner and that ethanol may modify the effects of sham treatment on gene expression, as well as inducing specific effects independent of any handling related stress. In addition to demonstrating the complexity of gene expression in response to physical and environmental stress, this work raises questions on the interpretation and validity of studies relying on pairwise comparisons.

Glucocorticoids, depression, and mood disorders: structural remodeling in the brain

The hippocampal formation expresses high levels of adrenal steroid receptors and is a malleable brain structure that is important for certain types of learning and memory. It is also vulnerable to the effects of stress and trauma. The amygdala is an important target of stress and mediates physiological and behavioral responses associated with fear and strong emotions. The prefrontal cortex plays an important role in working memory and executive function and is also involved in extinction of learning. All 3 regions are targets of stress hormones, and stress is known to precipitate and exacerbate mood disorders. In long-term depressive illness, the hippocampus and prefrontal cortex undergo atrophy, whereas the amygdala is hyperactive in anxiety and mood disorders and may undergo a biphasic change in structure--increasing in size in acute depression and shrinking on long-term depression. In animal models of acute and chronic stress, neurons in the hippocampus and prefrontal cortex respond to repeated stress by showing atrophy that leads to memory impairment, whereas neurons in amygdala show a growth response that leads to increased anxiety and aggression. Yet, these are not necessarily "damaged" and may be treatable with the right medications. The mechanisms that distinguish between protection and damage of brain cells from stress are discussed in this context.

Provocation of kainic acid receptor mRNA changes in the rat paraventricular nucleus by insulin-induced hypoglycaemia

Hypoglycaemia induced by insulin injection is a powerful stimulus to the hypothalamic-pituitary-adrenal (HPA) axis and drives the secretion of corticotropin-releasing hormone and vasopressin from the neurones in the paraventricular nucleus (PVN), as well as the downstream hormones, adrenocorticotropic hormone and corticosterone. In some brain regions, hypoglycaemia also provokes increases in extracellular fluid concentrations of glutamate. Regulation of glutamatergic mechanisms could be involved in the control of the HPA axis during hypoglycaemic stress and one potential site of regulation might be at the receptors for glutamate, which are expressed in the PVN. Insulin (2.0 IU/kg, i.p.) or saline was administered to adult male Sprague-Dawley rats and the animals were sacrificed 30 min, 180 min and 24 h after injection. The amount of several kainic acid-preferring glutamate receptor mRNAs (i.e. KA2, GluR5 and GluR6) were assessed in the PVN by in situ hybridisation histochemistry. Injection of insulin induced a rapid fall in plasma glucose concentrations, which was mirrored by an increase in plasma corticosterone concentrations. KA2 and GluR5 mRNAs are highly expressed within the rat PVN, and responded to hypoglycaemia with robust increases in expression that endured beyond the period of hypoglycaemia itself. However, GluR6 mRNA is expressed in the areas adjacent to the PVN and hypoglycaemic stress failed to alter expression of this mRNA. These experiments suggest that kainic acid-preferring glutamate receptors are responsive to changes in plasma glucose concentrations and may participate in the activation of the PVN neurones during hypoglycaemic stress.

Synergistic effects of corticosterone and kainic acid on neurite outgrowth in axotomized dorsal root ganglion

Corticosterone is the main adrenal glucocorticoids induced by stress in rats. Therapeutic use of high concentration of synthetic glucocorticoids in clinical treatment of spinal cord injury suggests that pharmacological action of glucocorticoids might be beneficial for nerve repair. In this article we cultured axotomized rat dorsal root ganglion neurons to investigate the effects of corticosterone and a glutamate receptor agonist kainic acid on neurite outgrowth. Our results revealed a synergistic effect of corticosterone and kainic acid in promoting neurite outgrowth when applied as early as one and two days in vitro, but not effective at three and four days in vitro. In addition, applied corticosterone and kainic acid were neurotoxic at three and four days in vitro but not at one and two days in vitro. The minimal concentrations of corticosterone and kainic acid to be effective were 10 microM and 1 mM, respectively. The neurotrophic effect of corticosterone and kainic acid was attenuated by the receptor tyrosine kinase A (TrkA) inhibitor AG-879. Western blot analysis and immunocytochemical studies revealed an increase of expressions of both TrkA and growth-associated protein GAP-43 in dorsal root ganglion neurons with combined treatment of corticosterone and kainic acid. Immunocytochemistry showed that corticosterone+kainic acid increase nerve growth factor immunoreactivity in dorsal root ganglion neurites and enhance GAP-43 immunointensity in dorsal root ganglion neurons. These results suggest that the neurotrophic effect of glucocorticoids on axonal regeneration might require facilitation of excitatory stimulation at an early stage of nerve injury, and nerve growth factor may mediate a growth signaling to accomplish the effect.

Effect of adrenalectomy on membrane properties and synaptic potentials in rat dentate granule cells

Adrenalectomy is known to accelerate both neurogenesis and cell death of granule cells located in the suprapyramidal blade of the rat dentate gyrus. Three days after adrenalectomy, some granule cells have already died by apoptosis while newly formed cells are not yet incorporated in the cell layer, resulting in a temporary loss of granule cells. Concomitantly, the field response to stimulation of perforant path afferents is reduced. While the temporary cell loss is likely to attenuate synaptic field responses, adrenalectomy-induced changes in properties of the surviving cells may also contribute to the reduction in field response amplitude. To address this possibility, we here investigated the membrane properties and synaptic responses of dentate granule cells, 3 days after adrenalectomy. We found that passive and most of the active membrane properties of granule cells in adrenalectomized rats were not significantly different from the cell properties in sham-operated controls. However, intracellularly recorded synaptic responses from surviving granule cells were markedly reduced after adrenalectomy. The N-methyl-D-aspartate (NMDA)- and the non-NMDA receptor-mediated components were reduced to a similar extent, suggesting that the attenuation of synaptic transmission after adrenalectomy could be partly of presynaptic origin. The data indicate that the earlier observed attenuation of synaptic field responses after adrenalectomy may be partly due to a diminished glutamatergic input to the dentate gyrus and not exclusively to a loss of granule cells participating in the synaptic circuit.

Metyrapone, an inhibitor of corticosterone synthesis, blocks the kainic acid-induced expression of HSP 70

It is shown in the present study that metyrapone (100 mg/kg), an inhibitor of corticosterone synthesis, given twice, 30 min before and 6 h after kainic acid (10 mg/kg) administration, blocks the kainic acid-evoked induction of heat shock proteins 72 kDa (HSP 70). Specifically, it was observed that metyrapone completely prevented kainic acid-induced appearance of HSP 70 in the rat amygdala, habenula, parietal cortex, and significantly decreased the number of HSP 70-positive neurons in the CA1, CA3, and CA4 subregions of hippocampus. The reduction in HSP 70 induction was paralleled by a complete prevention of the kainic acid-induced rise in the circulating corticosterone level by metyrapone; however, in applied doses metyrapone evoked slight enhancement of blood corticosterone. Despite the fact that metyrapone blocked/attenuated the kainic acid-evoked induction of HSP 70, its administration did not affect the behavioral effects of kainic acid, regarded as "limbic status epilepticus." It is concluded that the blockade of corticosterone synthesis might have neuroprotective effects in the pathological states associated with the overstimulation of glutamatergic receptors.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.

Chronic stress alters synaptic terminal structure in hippocampus

Repeated psychosocial or restraint stress causes atrophy of apical dendrites in CA3 pyramidal neurons of the hippocampus, accompanied by specific cognitive deficits in spatial learning and memory. Excitatory amino acids mediate this atrophy together with adrenal steroids and the neurotransmitter serotonin. Because the mossy fibers from dentate granule neurons provide a major excitatory input to the CA3 proximal apical dendrites, we measured ultrastructural parameters associated with the mossy fiber-CA3 synapses in control and 21-day restraint-stressed rats in an effort to find additional morphological consequences of stress that could help elucidate the underlying anatomical as well as cellular and molecular mechanisms. Although mossy fiber terminals of control rats were packed with small, clear synaptic vesicles, terminals from stressed animals showed a marked rearrangement of vesicles, with more densely packed clusters localized in the vicinity of active zones. Moreover, compared with controls, restraint stress increased the area of the mossy fiber terminal occupied by mitochondrial profiles and consequently, a larger, localized energy-generating capacity. A single stress session did not produce these changes either immediately after or the next day following the restraint session. These findings provide a morphological marker of the effects of chronic stress on the hippocampus that points to possible underlying neuroanatomical as well as cellular and molecular mechanisms for the ability of repeated stress to cause structural changes within the hippocampus.