The effects of chronic glucocorticoid exposure on dendritic length, synapse numbers and glial volume in animal models: Implications for hippocampal volume reductions in depression

“Although glucocorticoids are normally elevated as a neuroendocrine response to acute threats to maintain homeostasis [1, 2], they are chronically elevated in a number of neuropsychiatric disorders (e.”

“The rodent hippocampus has a high density of both GR and MR, whereas in primates the hippocampus has a high density of MR [9, 10].”

“The rodent hippocampus has a high density of both GR and MR, whereas in primates the hippocampus has a high density of MR [9, 10].”

“Although cell loss in the hippocampus had been considered as a major factor contributing to the volume reduction [65], Czeh and Lucassen [66] have concluded that factors other than cell loss, such as alterations in dendritic, axonal, synaptic and glial processes are likely to explain the volume differences in the popu…”

“Sheline [36] and her colleagues were the first to report smaller volume in both right and left hippocampi of women with a history of major depression.”

“A number of other studies report unilateral volume reductions [42-45].”

“One study of PTSD utilizing twins to explore the relationship between PTSD and hippocampal volume concluded that a smaller volume can predispose an individual to PTSD [61].”

“However, subsequent metaanalyses of studies that used MRI to assess the volume of the hippocampus in patients with major depression showed that differences in scanning resolution, age at onset of depression, severity of depression at the time of MRI scanning, and gender cannot account for the discrepancy [54-56].”

“On the contrary, factors such as the duration of depression and number of episodes or days of untreated depression are inversely correlated with hippocampal volume [37, 41, 57-59].”

“A number of studies have reported dendritic atrophy after exposure to stress [23, 110, 111] or corticosterone administration [22, 24, 112].”

“On the contrary, factors such as the duration of depression and number of episodes or days of untreated depression are inversely correlated with hippocampal volume [37, 41, 57-59].”

“The magnitude of loss is fairly large, but corresponds to the magnitude of dendritic atrophy reported (25-40%) [22, 24] for the dose used.”

“The elevations occur during a critical period of development characterized by a “hypo-responsive” hypothalamic-pituitary-adrenal axis which serves to protect the developing brain from the catabolic actions of glucocorticoid elevations [78].”

“On the contrary, factors such as the duration of depression and number of episodes or days of untreated depression are inversely correlated with hippocampal volume [37, 41, 57-59].”

“Like the hippocampus, the frontal lobe dysfunction has been implicated in depression [29-31] and the frontal lobe, like the hippocampus undergoes structural modification in response to glucocorticoids and stress [32-35].”

“The duration of depression and the number of depressive episodes contribute to an 8%-10% reduction of hippocampal volume in patients with major depressive syndrome compared to controls [56, 60].”

“Dendritic alterations also have been reported after corticosterone treatment of the same or longer duration [22, 81] and after social or unpredictable stress in rats and tree shrews [24, 82].”

“[4] reported a negative correlation between the hippocampal volume and plasma cortisol levels in this clinical population; higher levels of cortisol were associated with lower volumetric measurements of the hippocampus.”

“corticosterone treatment (40mg/kg) or unpredictable stress caused a significant decrease in the total number of mossy fiber-CA3 synapses as well as a reduction in the volume of the mossy fiber terminals and surface area of their plasmalemma [24].”

“The hippocampus, a structure important for learning and memory [11], mediates the regulation of glucocorticoid release through its indirect action on the Hypothalamic-Pituitary-Adrenal axis activity [12, 13].”

“Like the hippocampus, the frontal lobe dysfunction has been implicated in depression [29-31] and the frontal lobe, like the hippocampus undergoes structural modification in response to glucocorticoids and stress [32-35].”

“However, subsequent metaanalyses of studies that used MRI to assess the volume of the hippocampus in patients with major depression showed that differences in scanning resolution, age at onset of depression, severity of depression at the time of MRI scanning, and gender cannot account for the discrepancy [54-56].”

“Hippocampal histological examination showed no loss of pyramidal cells in major-depressed patients [67, 69, 70].”

“Although previous investigators have also reported the appearance of pyknotic cells in primate CA3 after exposure to glucocorticoids [106-109], our qualitative observations in tissue handled to avoid artifacts with appearances similar to pyknoses yielded no indicators of dying cells [72].”

“The finding that glucocorticoid receptors are also expressed in the brain [8] raised the possibility that elevated GCs may affect brain structure and function.”

“, depression, Cushing's syndrome) [3, 4] as well as during chronic stress [5, 6].”

“Like the hippocampus, the frontal lobe dysfunction has been implicated in depression [29-31] and the frontal lobe, like the hippocampus undergoes structural modification in response to glucocorticoids and stress [32-35].”

“Restraint stress, which would elevate glucocorticoids, increased the density of vesicles in the mossy fiber terminals and caused a relocalization of vesicles near the active zones [89] suggesting that more vesicles are available for glutamate release.”

“In animals, regulation of GFAP, the major intermediate filament protein in mature astrocytes [94], has received the most attention.”

“In the one study that controlled for cell size [71], cell loss was not reported.”

“Subsequent investigators found a correlation between plasma adrenocorticoids and and astrocyte reactivity [18] and that controlling plastma adrenoscorticoids over age reduced cell loss [17].”

“suggested that reductions in hippocampal volume are usually seen in studies that used higher MRI resolution and tended to include subjects with early-onset unipolar depression, in contrast to late onset or bipolar forms of the disorder [53].”

“However, a significant number of studies failed to report smaller volume in patients with depression [48-52]”

“The hippocampus, a structure important for learning and memory [11], mediates the regulation of glucocorticoid release through its indirect action on the Hypothalamic-Pituitary-Adrenal axis activity [12, 13].”

“The finding that tumor removal and the consequent reductions in plasma cortisol corresponded to increases in hippocampal volume [25] perhaps provides the best support obtained from human subjects for a direct causal relationship between glucocorticoid elevations and hippocampal volume.”

“It has been proposed that repeated exposure to elevated glucocorticoids, producing dendritic retraction, could lead to greater vulnerability to cell loss from metabolic challenge or mechanical trauma [80].”

“A number of other studies report unilateral volume reductions [42-45].”

“Similarly, in the original report of cell loss, the animals were older [19].”

“A number of other studies report unilateral volume reductions [42-45].”

“However, a significant number of studies failed to report smaller volume in patients with depression [48-52]”

“Further work is needed to understand whether the plasma elevations serve as an appropriate representation of those seen in Cushing's disease and major depressive disorder [85, 86].”

“Like the hippocampus, the frontal lobe dysfunction has been implicated in depression [29-31] and the frontal lobe, like the hippocampus undergoes structural modification in response to glucocorticoids and stress [32-35].”

“It is well established that a large proportion of people suffering from depression are characterized by hyperactivity of the Hypothalamic-Pituitary-Adrenal axis [27], resulting in plasma cortisol elevations [3, 28].”

“The hippocampus, a structure important for learning and memory [11], mediates the regulation of glucocorticoid release through its indirect action on the Hypothalamic-Pituitary-Adrenal axis activity [12, 13].”

“It is well established that a large proportion of people suffering from depression are characterized by hyperactivity of the Hypothalamic-Pituitary-Adrenal axis [27], resulting in plasma cortisol elevations [3, 28].”

“Like the hippocampus, the frontal lobe dysfunction has been implicated in depression [29-31] and the frontal lobe, like the hippocampus undergoes structural modification in response to glucocorticoids and stress [32-35].”

“The latter observations are in agreement with the failure to detect signs of apoptosis in CA3 post-mortem analysis of subjects with major depressive disorder [68].”

“Over the course of the three years, patients with incomplete remission and relapses had larger volume declines in the hippocampus, thereby supporting an active process of decline [63].”

“These findings have been confirmed in drug free depressed patients [46, 47].”

“The mechanism of reduced cell number is likely to be through a suppression of neurogenesis [79].”

“Hippocampal histological examination showed no loss of pyramidal cells in major-depressed patients [67, 69, 70].”

“Like the hippocampus, the frontal lobe dysfunction has been implicated in depression [29-31] and the frontal lobe, like the hippocampus undergoes structural modification in response to glucocorticoids and stress [32-35].”

“Similarly, CA3 neuronal loss was also reported after the same duration of corticosterone treatment [21] or one month of stress [20].”

“On the contrary, factors such as the duration of depression and number of episodes or days of untreated depression are inversely correlated with hippocampal volume [37, 41, 57-59].”

“However, a significant number of studies failed to report smaller volume in patients with depression [48-52]”

“[88] found that 21 days of restraint stress caused a loss of total synapse number in the stratum lucidum.”

“Further raising concern that volume may precede rather than follow depression is a finding that variance in hippocampal volume in children is as great as in adulthood, and greater than the difference in volume reported between controls and patients with depression [62].”

“Since then, many investigators have reported bilateral reductions in depressed subjects [37-41].”

“Dendritic alterations also have been reported after corticosterone treatment of the same or longer duration [22, 81] and after social or unpredictable stress in rats and tree shrews [24, 82].”

“On the contrary, factors such as the duration of depression and number of episodes or days of untreated depression are inversely correlated with hippocampal volume [37, 41, 57-59].”

“, depression, Cushing's syndrome) [3, 4] as well as during chronic stress [5, 6].”

“Consistent with the above findings, chronic cortisol administration (1 year) to nonhuman primates (macaques) [76] or exposure of tree shrews to psychosocial stress [77] failed to cause neuronal loss in the CA3 or CA1 hippocampal fields.”

“However, a significant number of studies failed to report smaller volume in patients with depression [48-52]”

“As expected, there are reports of cognitive deficits that correspond to the deficits expected from impaired hippocampal function [4, 26]”

“So far we have seen that glucocorticoids can cause a volume reduction [73] and dendritic atrophy [87] but these measures are not direct measures of synaptic contacts.”

“Since then, many investigators have reported bilateral reductions in depressed subjects [37-41].”

“The regulation of GFAP by glucocorticoids is further supported by increases in GFAP levels after adrenalectomy, an effect that is reversed by corticosterone replacement [96-98].”

“Altogether, cell culture studies support the possibility that changes in GFAP reported in post-mortem samples from patients with depression could have been decreased by glucocorticoids, but the data from animal models suggest that corticosterone produces no effect on glial volume or produces tendencies toward glial pr…”

“A number of other studies report unilateral volume reductions [42-45].”

“Glial cells express both MR and GR [90, 91], although at lower levels than in neurons [92].”

“The hippocampus is not the sole structure providing negative feedback; additional structures have been involved in the regulation of the HypothalamicPituitary-Adrenal axis activity [14, 15], the high expression glucocorticoid receptors in the hippocampus may render this brain structure a target of glucocorticoid eleva…”

“These findings have been confirmed in drug free depressed patients [46, 47].”

“[83] report that the cell body layer in DG, CA3 and CA1 is 17%, 22% and 14% of the overall volume, respectively.”

“4% of volume in the dentate gyrus [104].”

“Consistent with the above findings, chronic cortisol administration (1 year) to nonhuman primates (macaques) [76] or exposure of tree shrews to psychosocial stress [77] failed to cause neuronal loss in the CA3 or CA1 hippocampal fields.”

“The regulation of GFAP by glucocorticoids is further supported by increases in GFAP levels after adrenalectomy, an effect that is reversed by corticosterone replacement [96-98].”

“However, a significant number of studies failed to report smaller volume in patients with depression [48-52]”

“The glial volume fraction in these areas is far lower than in the CA1 region, where glial processes make up ~15% of the volume [105], which is closer to reports from the cerebellar molecular layer (~18%) [102].”

“For example, in aging rats, astrocytic volume fraction increases in the dentate gyrus whereas thickness of the molecular layer does not change [93].”

“Similarly, CA3 neuronal loss was also reported after the same duration of corticosterone treatment [21] or one month of stress [20].”

“In addition, short or long exposure of cerebral cortex astrocyte cultures to corticosterone increased GFAP gene expression [99, 100].”

“Hippocampal histological examination showed no loss of pyramidal cells in major-depressed patients [67, 69, 70].”

“Further work is needed to understand whether the plasma elevations serve as an appropriate representation of those seen in Cushing's disease and major depressive disorder [85, 86].”

“The hippocampus is not the sole structure providing negative feedback; additional structures have been involved in the regulation of the HypothalamicPituitary-Adrenal axis activity [14, 15], the high expression glucocorticoid receptors in the hippocampus may render this brain structure a target of glucocorticoid eleva…”

“The dose used significantly elevates plasma corticosterone [84], and can have a dramatic effect on organ weights [74].”

“Corticosterone administration (40 mg/kg) for at least 90 days in rats can decrease hippocampal volume [73], but does not decrease volume after 30 [75] or 60 days [74].”

“, depression, Cushing's syndrome) [3, 4] as well as during chronic stress [5, 6].”

“In addition, short or long exposure of cerebral cortex astrocyte cultures to corticosterone increased GFAP gene expression [99, 100].”

“The regulation of GFAP by glucocorticoids is further supported by increases in GFAP levels after adrenalectomy, an effect that is reversed by corticosterone replacement [96-98].”

“The experience of negative emotions may be like pouring hydrochloric acid on the brain’s memory cells [422].”

“Sustained glucocorticoid exposure leads to dendritic atrophy in hippocampal subfields and decreases neuronal cell survival by evoking a decline in BDNF expression in hippocampal and cortical regions (31, 32).”

“…hypothesis discussed above, because the degree of ippocampal shrinkage is directly proportional to the number and uration of prior depressive episodes, and to the total duration of vioral Reviews 37 (2013) 2331–2371 2337 illness (Sheline et al., 2003; Colla et al., 2007; Tata and Anderson, 2010).”

“28 Recently, McAuley et al 29 showed, from an in silico model, a potential 30% to 40% decrease in hippocampal activity with long-term elevated levels of cortisol.”

“The role of a sustained increased level of glucocorticoids involved in stress-related excitotoxicity has been well documented in relation to hippocampal and ACC GMV atrophy (Arborelius et al., 1999; Tata and Anderson, 2010), but differences in the plasticity of these structures compared with the amygdala in the contex…”

“consequent synaptic and morphological changes in these structures.(69,80,81) Some of these changes appear to be reversible.”

“…HPA axis), glucocorticoids, which typically provide a negative feedback loop, are pathologically overexpressed in depression (reviewed by Sapolsky, 2000; Tata and Anderson, 2010) and have been linked to neuronal death and hippocampal atrophy (reviewed by Lupien et al., 2007; Fales et al., 2008).”

“With change in cellular morphology or cell death, either regional or whole brain atrophy results, depending on the severity and type of injury (Pitkanen et al., 1998; Bramlett and Dietrich, 2007; Lifshitz et al., 2007; Tata and Anderson, 2010).”

“In a review of animal studies, both behaviorally-induced stress and mimicry of the stress response via corticosterone treatment significantly altered hippocampal structure and function [324].”

“, hippocampal CA3 spines remodel in response to postnatal stressor or corticosteroid exposure (Tata and Anderson, 2010).”

“…the activity of glutamatergic N-methyl-D-aspartate receptor (Weiland et al., 1997), and cause synaptic and morphological alterations in the hippocampus (Conrad, 2008; McLaughlin et al., 2009; Tata & Anderson, 2010), which in turn may contribute to greater susceptibility of older adults to anxiety.”

“Whereas initial studies reported that chronic exposure to glucocorticoid could result in neuronal loss of the hippocampus [43], subsequent studies failed to consistently replicate the deleterious effects of glucocorticoid on the hippocampus [44,45].”

“It is well known that early stress can cause hippocampal damages characterized by synaptic loss and atrophy (McEwen, 1999; Tata and Anderson, 2010), and these changes are accompanied by reduced hippocampal 5-HT concentration (Hayashi et al., 1998).”

“However, a recent study found that chronically elevated CORT levels did not reduce cell number but caused a pronounced loss of synapses, suggesting that volume measures can substantially underestimate the effects of CORT on hippocampal structure and importantly on function (Tata et al., 2006; Tata and Anderson, 2010).”

“In animal models, the effects of persistent glucocorticoids on glutamate neurones are increased glutaminergic excitability with astrocytic decline, reduced astroglial plasticity, and reduced dendritic connectivity in hippocampal and frontal cortex regions [72,73].”

“Extensive research has exhibited that chronic stress or prolonged exposure to glucocorticoids negatively affects the rodent hippocampal neuronal morphology, neural proliferation, and volume (Tata and Anderson, 2010; Wilson et al., 2015), medial prefrontal cortex (mPFC) volume and function (Lo Iacono and Carola, 2017;…”

“Our results are theoretically consistent with preclinical and clinical models of MDD that posit that stress-induced increases in glucocorticoid levels shrink dendrites and reduce the number of spines in the hippocampus, resulting in atrophy (McEwen et al., 2015; Tata & Anderson, 2010).”

“Recent research has changed the perception of neuroglia from being no more than silent supportive cells of neurons to being dynamic partners participating in brain metabolism and communication between neurons [15, 72–74].”

“Repeated exposure to elevated glucocorticoid hormone levels have been shown to reduce axonal length and synapse number in the hippocampus in laboratory animals (Sheline, 2000; Tata and Anderson, 2010), and hypothalamic-pituitary-adrenal (HPA) axis dysregulation and hypercortisolemia are established findings in some pa…”

“Moreover, there is evidence that glial processes make up a larger proportion of the tissue after exposure to prolonged psychosocial stress and glucocorticoid exposure (Czéh et al., 2006; Tata et al., 2006; Tata and Anderson, 2010).”

“Chronic elevation of glucocorticoids such as corticosterone in rodents leads to a decrease in volume of the hippocampus and prefrontal cortex, due to a loss of neuropil as a consequence of a loss of synapses and, in some cases, whole dendrites (Tata et al., 2006; Tata and Anderson, 2010; Zhu et al., 2007; McEwen, 2005…”

“Indeed, volume changes in the hippocampus, mPFC, or amygdala are found both in patients with depression and in animal models of depression [51,52].”

“(Tata and Anderson, 2010; Wilson et al., 2015), mPFC volume and function (Yang et al.”

“Other findings have indicated that changes in glial cell density and the complexity of dendritic arbors might also account for volumetric changes in depression (Tata and Anderson, 2010; Gittins and Harrison, 2011).”

“Though GR and MR are enriched in this sub-region (Ozawa, 2005), the result seems to simulate a condition of stress that no obvious cell loss occurs in hippocampus (Tata and Anderson, 2010), but rather dendritic retraction in the CA3 or reduced neurogenesis in the DG is expected (Joels, 2008).”

“…the consensus based on unbiased stereological histology data appears to be that the reduced hippocampal volumes are not due to a reduced number of cells, but are rather caused by changes in the neuropil of the affected regions (Müller et al., 2001; Czéh and Lucassen, 2007; Tata and Anderson, 2010).”

“The morphological changes in the various hippocampal regions have been related to changes in spatial learning after chronic stress (see Conrad, 2006; Tata and Anderson, 2010).”

“Serotonin and GABA [55], as well as brain derived neurotrophic factor (BDNF) [56], have been implicated in stress-induced remodeling.”

“Imbalanced glucocorticoids induced by an abnormal HPA axis cause neurotoxicity, neuron apoptosis, neuron decrease, loss of synapses, dendritic length decrease, and hippocampal volume reduction among other effects, and are related to depression (Sapolsky, 2000; Saylam et al., 2006; Sheline et al., 1999; Tata and Anders…”

“…changes are related to volume measures, Tata and co-workers carried out a study of the effects of long-term glucocorticoids on hippocampal CA3 volume, and identified altered dendritic and glial processes as well as altered numbers and sizes of synapses (Tata et al. 2006; Tata and Anderson 2010).”

“In animal models of stress effects on hippocampal morphology, the loss of hippocampal volume arises from losses in the neuropil involving losses of dendrites and synapses [39] suggesting a substrate for loss of hippocampal volume in humans.”

“Accordingly, it has been shown that patients and animal models of this disorder show changes in the volume of certain cerebral regions, such as the hippocampus, the medial prefrontal cortex or the amygdala, which are related to the reorganization of neuronal structure and may affect their connectivity [3,4].”

“Fewer dendritic branches and spines are hypothesized to underlie the reduced volume and activity seen in brain regions involved in MDD, such as within the PFC and hippocampus, as previously reviewed [63, 64].”

“Glucocorticoid exposure has also been shown to cause reductions similar to those found following PS such as a reduction in hippocampal volume, dendritic length and number of dendritic branch points as well as synapse number (Tata and Anderson, 2009).”

“…1997; Filipovic et al., 2013; Hillerer et al., 2013; MacPherson et al., 2005; Magarinos et al., 2011; Stein-Behrens et al., 1992; for review see, Tata and Anderson, 2010; see also, Tombaugh et al., 1992; Tynan et al., 2013; Tynan et al., 2010; Woolley et al., 1990b), and impairments in…”

“In the context of hippocampal volume decreases an increased packing density due to decreased neuropil volume has been proposed (Tata and Anderson, 2010).”

“In this structure, which is involved in the inhibition of the HPA system and in encoding spatial information in contextual memory, elevated corticosteroid levels induce loss of complexity and decreased length of apical dendritic branches in the CA1 and CA3 areas (Dranovsky & Leonardo, 2012; McEwen et al., 2012).”

“The effects of chronic or repeated stress in humans and in animal models are mediated primarily by the prolonged elevations of glucocorticoids (cortisol in humans, corticosterone in rodents) that are the endpoint of activation of the HPA axis [19,20].”

“Chronic exposure to stress induces permanent synaptic and dendritic alterations [34] [35], increases hippocampal glutamate levels [36] [37] and decreases AHN [9].”

“Changes in hippocampal morphology and loss of neurons are the contributors to hippocampal volume changes [9].”

“Contrary to the initial belief that neuron loss predominantly contributes to hippocampal volume reduction, recent studies suggest that dendritic and synaptic alterations are more likely to be responsible for hippocampal shrinkage [20].”

“This association is partially mediated by elevated levels of glucocorticoids, reflecting an end-point of HPA-axis activation (Brown, 2009; Tata and Anderson, 2010).”

“The data from human and animal studies have shown that higher HPA activity contributes to major depressive disorders (MDD) (37, 38).”

“The brain is rich in glucocorticoid and mineralocorticoid receptors, both activated by cortisol (28), and neurotoxic effects of chronic hypercortisolism have been demonstrated in animal models (29).”

“In patients with MDD, with more frequent exposure to stress, the glial cells of the Hb may be damaged by excessive neuroinflammation, oxidative stress, and excitatory toxicity, leading to a decrease in neuropils and reduced brain volume [49].”

“In animal models, the effects of persistent glucocorticoids on glutamate neurones are: increased glutaminergic excitability with astrocytic decline, reduced astroglial plasticity and reduced dendritic connectivity in hippocampal and frontal cortex regions (44, 45).”

“Such individuals would be expected to have experienced multiple depressive episodes over a longer duration, which may correlate with greater exposure to elevated glucocorticoid levels (40) thought to contribute to hippocampal volume reductions (41).”

“Within dendritic regions, which make up 78–86% of gray matter tissue volume in the hippocampus (Sousa et al., 1998; Tata et al., 2002; Tata and Anderson, 2010), the layers with small dendritic processes and higher synaptic density have higher metabolic demand than areas with large dendrites and smaller synaptic densit…”

“Hippocampal neuron cells, especially in the area of CA1, express high level of GRs compared to the expression in other regions of the brain and are the principal target sites for glucocorticoid-associated neuronal cell death (Tata and Anderson, 2010; Wang et al., 2013).”

“The durable increases in glucocorticoids (corticosterone in rodents, cortisol in humans) which are the endpoint of HPA axis activation mainly mediate the influence of repeated or chronic stress in animal models as well as in humans [6,7].”

“Stress-related structural reorganization of hippocampal neurons Hippocampal CA3 neurons are highly sensitive to stressor exposure (Tata and Anderson, 2010).”

“For example, elevated levels of GCs have been shown to cause atrophy of dendritic processes in the CA3 region, loss of hippocampal neurons, and increased neuronal damage in pathological conditions such as seizures and stroke (Patil et al., 2007; Tata and Anderson, 2010).”