**4. Effects of synthetic glucocorticoids on astroglia**

GRs and MRs are expressed not only by neurons, but also by glial cells or *neuroglia* (Bohn et al. 1991; Vielkind et al. 1990). Glial cells are non-neuronal cells that preserve neural homeostasis, form myelin, and provide support and protection for neurons. In fact, corticoids exert several effects into the brain by targeting glial cells that, in turn, modify the cerebral functioning (Figure 3). Astrocytes, collectively known as *astroglia*, are the most abundant glial cells and play multiple roles into the brain, such as: Neurotransmitter reuptake and release, modulation of synaptic transmission, nervous system repair, hormonal signalling, vascular tone regulation, preservation of blood–brain barrier and, in some cases, astrocytes may function as neural stem cells (Gonzalez-Perez & Alvarez-Buylla 2011; Kettenmann & Ransom 2005).

Astroglia expresses the intermediate filament glial fibrilliary acidic protein (GFAP), which is used as cellular marker of these cells (Ihrie & Alvarez-Buylla 2008). Interestingly, astrocytes contain high number of GRs and MRs; consequently, astrocyte function and their GFAP expression are highly susceptible to glucocorticoids (Lambert et al. 2000; Rozovsky et al. 1995). Some of the effects of glucocorticoids on gene and protein expression in astrocytes are summarized in the table 3.


Table 3. Effects of glucocorticoids on protein and gene expression in astrocytes.

*In vitro* administration of dexamethasone, corticosterone or aldosterone inhibits astrocyte proliferation in a dose-dependent manner (Crossin et al. 1997). This effect seems to be mediated by neural cell adhesion molecules, which inhibit activation of mitogen-activated protein (MAP) kinase (Krushel et al. 1998). The corticoid-induced inhibition of cell

GRs and MRs are expressed not only by neurons, but also by glial cells or *neuroglia* (Bohn et al. 1991; Vielkind et al. 1990). Glial cells are non-neuronal cells that preserve neural homeostasis, form myelin, and provide support and protection for neurons. In fact, corticoids exert several effects into the brain by targeting glial cells that, in turn, modify the cerebral functioning (Figure 3). Astrocytes, collectively known as *astroglia*, are the most abundant glial cells and play multiple roles into the brain, such as: Neurotransmitter reuptake and release, modulation of synaptic transmission, nervous system repair, hormonal signalling, vascular tone regulation, preservation of blood–brain barrier and, in some cases, astrocytes may function as neural stem cells (Gonzalez-Perez & Alvarez-Buylla

Astroglia expresses the intermediate filament glial fibrilliary acidic protein (GFAP), which is used as cellular marker of these cells (Ihrie & Alvarez-Buylla 2008). Interestingly, astrocytes contain high number of GRs and MRs; consequently, astrocyte function and their GFAP expression are highly susceptible to glucocorticoids (Lambert et al. 2000; Rozovsky et al. 1995). Some of the effects of glucocorticoids on gene and protein expression in astrocytes are

**effect** 

Cell differentiation Upregulation (Nichols et al. 2005)

**Reference** 

1991; Ramos-Remus et al. 2002)

Zschocke et al. 2005)

Vardimon et al. 1999)

2006)

1995)

Upregulation (O'Callaghan et al.

Upregulation (Reagan et al. 2004;

Upregulation (Hansson 1989;

Upregulation (Niu et al. 1997)

Upregulation (Van den Hove et al.

Upregulation (McLeod & Bolton

Downregulation (Niu et al. 1997)

Downregulation (Avola et al. 2004)

**Protein / gene Function Glucocorticoid** 

Intermediate filament protein

Neurotransmitter recycling

recycling

Neurotrophic protein

neurotrophic protein

protein

Neurotrophic protein

filament

Table 3. Effects of glucocorticoids on protein and gene expression in astrocytes.

*In vitro* administration of dexamethasone, corticosterone or aldosterone inhibits astrocyte proliferation in a dose-dependent manner (Crossin et al. 1997). This effect seems to be mediated by neural cell adhesion molecules, which inhibit activation of mitogen-activated protein (MAP) kinase (Krushel et al. 1998). The corticoid-induced inhibition of cell

**4. Effects of synthetic glucocorticoids on astroglia** 

2011; Kettenmann & Ransom 2005).

summarized in the table 3.

Glial fibrilliary acidic protein (GFAP)

Glial glutamate transporter (GLT-1)

Basic fibroblast growth factor (bFGF)

N-myc downstreamregulated gene (Ndrg2)

Nerve growth factor (NGF)

Glutamine synthetase Neurotransmitter

S100β Ca2+-binding

Lipocortin-1 Anti-inflammatory

Vimentin Intermediate

proliferation and growth retardation may also be enhanced by a concomitant reduction in the production of insulin-like growth factor 1 (IGF-1) in parenchymal astrocytes (Adamo et al. 1988). On the other hand, the overexpression of GFAP and chondroitin sulfate proteoglycans in reactive astrocytes has been related to a deficient neuronal repair and less neurite outgrowth. Methylprednisolone can revert these adverse effects by downregulating astrocyte activation (Liu et al. 2008) and reducing the number of GFAP-expressing astroglia (Sabolek et al. 2006). Further studies indicate that dexamethasone can modify hippocampal neuron development and survival by decreasing the mRNA levels of nerve growth factor (NGF) (Niu et al. 1997) and GFAP in hippocampus and neocortex (Aleong et al. 2003). In contrast, other reports using corticosterone (Bridges et al. 2008), prednisone (Ramos-Remus et al. 2002) and deflazacort (Gonzalez-Castaneda et al. 2007) reported an increase in the number and cytoplasmic processes of hippocampal and cortical astrocytes. The reason for these discrepancies is not well-known, but they appear to be mediated by dose- and regiondependent phenomena (Gonzalez-Perez et al. 2001).

Glucocorticoids not only modify the function of glial cells in adult stages, but also during prenatal development. Prenatal betamethasone administration delays both astrocyte and capillary tight junction maturation (Huang et al. 2001a), as well as the myelination in the corpus callosum (Huang et al. 2001b). Remarkably, glucocorticoid effects are not limited to the modulation of cell morphology or molecular expression in neuroglia. Instead, they are key modulators of glycogen metabolism and neurotransmitter transporter homeostasis as demonstrated in several experimental models. For instance, cortical astrocytes exposed to dexamethasone show a reduction of noradrenaline-induced glycogen synthesis (Allaman et al. 2004). Prednisolone, betamethasone and dexamethasone inhibit the transporter uptake of monoamines producing effects on physiological and behavioral processes (Hill et al. 2010). The glial glutamate transporter-1 (GLT-1) is also affected by synthetic glucocorticoids. In cortical astrocytes, dexamethasone provokes a marked increase in the GLT-1 transcription and GLT-1 protein levels (Zschocke et al. 2005). GABAergic neurons are also affected by synthetic glucocorticoids that impair their rhythmic firing, which may lead to cognitive deficit (Hu et al. 2010). Taken together, this evidence indicates that synthetic glucocorticoids exert a strong modulation on neural cells by modifying protein expression, neurite growth, cell proliferation, neurotransmitter uptake, neuronal firing, vasculature function and neuronal degeneration.

#### **4.1 Implications in autoimmune neurological disorders. Pathological features observed in patients upon GCs administration**

One major application of synthetic glucocorticoids is the treatment of acute and chronic neuroinflammatory disorders, such as multiple sclerosis, autoimmune encephalomyelitis, immune rejection, Parkinson's disease, retinal degeneration and others. Increasing evidence indicates that the therapeutic efficacy of glucocorticoids against autoimmune disorders may rely not only on their well-known anti-inflammatory effects, but also on their properties of neuro-gliomodulation (Gonzalez-Perez et al. 2007). For instance, methylprednisolone has a synergistic effect with Nogo-66 receptor protein, which promotes functional recovery and axonal growth in a model of spinal cord contusion (Ji et al. 2005). Methylprednisolone also mediates anti-apoptotic effects on oligodendrocytes by activating STAT5 proteins, which up-regulate a splicing isoform of the bcl-x gene (Xu et al. 2009). On the other hand, prednisone contributes to attenuate experimental autoimmune encephalomyelitis by preventing the reduction of brain-derived neurotrophic factor (BDNF) and NGF mRNA

Synthetic Glucocorticoids Modulate Function of Neural

**5. Conclusion** 

deterioration.

**7. References** 

**6. Acknowledgments** 

(179-81).

33).

Stroke (NIH R01 NS070021-01).

psychiatric and neurological conditions throughout life.

Cells: Implications in Autoimmune Neurological Disorders 331

Taken together, this evidence indicates that synthetic glucocorticoid may have detrimental effects on glial and neuronal integrity. Therefore, some authors have proposed that uncontrolled use of glucocorticoids may predispose to the development of a range of

Synthetic glucocorticoids are a valuable therapeutic strategy against neuroinflammation and autoimmune disorders with neurological involvement. In fact, anti-inflammatory strategies receive growing attention for their potential to prevent pathological deterioration in multiple sclerosis (the most prevalent chronic autoimmune disease of the central nervous system), Parkinson's disease, autoimmune encephalomyelitis and other severe neurological disorders. Nevertheless, the uncontrolled use of glucocorticoids must be avoided because of their deleterious potential on cognition, neuronal survival and apoptosis induction. Yet, in those clinical situations where glucocorticoid use is necessary, a continuous neuropsychological assessment is strongly recommended to detect a possible neurological

This work was supported in part by grants from the Consejo Nacional de Ciencia y Tecnologia's (CB-2008-101476) and the National Institute of Neurological Disorders and

Adamo, M., Werner, H., Farnsworth, W., et al. (1988). Dexamethasone reduces steady state

Allaman, I., Pellerin, L., & Magistretti, P.J. (2004). Glucocorticoids modulate

Amylon, M.D., Perrine, S.P., & Glader, B.E. (1986). Prednisone stimulation of erythropoiesis

Arguelles, S., Herrera, A.J., Carreno-Muller, E., et al. (2010). Degeneration of dopaminergic

Avola, R., Di Tullio, M.A., Fisichella, A., et al. (2004). Glial fibrillary acidic protein and

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expression into the brain (Chen et al. 2009). Promising results have also been obtained with dexamethasone and fluocinolone in several studies, i.e. dexamethasone reduces astroglial reactivity to implanted neuroprosthetic devices in rat cortex (Spataro et al. 2005), whereas intravitreous administration of fluocinolone attenuates retinal degeneration (Glybina et al. 2010). Further evidence suggests that dexamethasone produces immunosuppressive effects on the astrocyte response to interleukin-1-beta stimulation (Pousset et al. 1999) and counteracts blood–brain barrier failure by decreasing transendothelial permeability (Cucullo et al. 2004).

Despite synthetic glucocorticoids have demonstrated an adequate safety profile, increasing clinical experience and experimental studies indicate that corticoids are able to promote cognitive dysfunction, anxiety, cerebral atrophy, depression and steroid psychosis. One of the first studies that associated the glucocorticoid delivery with mood disorders in humans was reported in prednisone-treated asthmatic children (Bender et al. 1991). However, adults are also affected by corticoids as demonstrated in healthy volunteers that, after receiving a high-dose prednisone or dexamethasone, showed mood changes and memory impairment (Keenan et al. 1996; Schmidt et al. 1999; Wolkowitz 1994). Cerebral atrophy was reported after a long-term treatment with glucocorticoids in patients with no previous history of central nervous system affection (Bentson et al. 1978; Hara et al. 1981). Other immunologic disorders, such as systemic corticosteroid hypersensitivity (de Sousa et al. 2010; Rachid et al. 2011), toxic epidermal necrolysis (Navarro Llanos et al. 1996) or urticaria-angioedema (Gomez et al. 2002), have also been associated with the administration of glucocorticoids.

Under specific circumstances synthetic corticoids may impair or even potentiate the progress of neurological disorders as reported in experimental models of Alzheimer's disease, hypoxia or prenatal glucocorticoid delivery. This fact appears to be particularly important in neurodegenerative disorders related to oxysterol production such as Alzheimer's disease and multiple sclerosis. Oxysterols are oxidized forms of cholesterol that provokes oligodendrocyte apoptosis. Dexamethasone exacerbates the apoptotic effects of oxysterols on oligodendrocytes, resulting in secondary necrosis (Trousson et al. 2009). Cerebral vasculature is also altered by exposure to dexamethasone that may deteriorate hippocampal functions (Neigh et al. 2010). In hypoxia models, dexamethasone increases the expression of Bnip3, a pro-apoptotic Bcl-2 family, which impairs hypoxic tissue damage (Sandau & Handa 2007).

Neuronal function and survival are also affected by synthetic corticoids. Dexamethasone increases oxidative stress and expression of monoamine oxidase A and B, resulting in a higher loss of dopaminergic neurons (Arguelles et al. 2010). Oral administration of prednisone or deflazacort promotes neuronal degeneration of pyramidal neurons in CA1 and CA3 hippocampal regions (Gonzalez-Castaneda et al. 2007; Gonzalez-Perez et al. 2007; Ramos-Remus et al. 2002). Dexamethasone also decreases the number of neurons in the striatum (dorsomedial caudate-putamen) and hippocampus (dentate gyrus, CA1 and CA3 subfields), which may account for some of the cognitive deficits seen following administration of glucocorticoids to healthy volunteers (Haynes et al. 2001). Glucocorticoids also target the developing brain as reported in children exposed to synthetic glucocorticoids *in uterus,* who showed a reduction in fetal and, in some cases, newborn and infant HPA axis activity (Tegethoff et al. 2009). Other studies indicate that prenatal dexa- or betamethasone exposure also affects postnatal cognitive functions (Hauser et al. 2007; Hauser et al. 2006), reduces the survival of cholinergic neurons (Emgard et al. 2007), and produces permanent changes in the cytoarchitecture within midbrain dopamine nuclei (McArthur et al. 2005). Taken together, this evidence indicates that synthetic glucocorticoid may have detrimental effects on glial and neuronal integrity. Therefore, some authors have proposed that uncontrolled use of glucocorticoids may predispose to the development of a range of psychiatric and neurological conditions throughout life.
