Morphological reorganization after repeated

Journal of Chemical Neuroanatomy 38 (2009) 266–272

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Morphological reorganization after repeated corticosterone administration in the hippocampus, nucleus accumbens and amygdala in the rat Julio Ce´sar Morales-Medina a,b, Fremioht Sanchez d, Gonzalo Flores d, Yvan Dumont b, Re´mi Quirion b,c,* a

Dept. of Neurology & Neurosurgery, McGill University, Montreál, QC, Canada H4H 1R3 Douglas Mental Health University Institute, McGill University, Montreál, QC, Canada H4H 1R3 c Dept. of Psychiatry, McGill University, Montreál, QC, Canada H4H 1R3 d Laboratorio de Neuropsiquiatrıá, Instituto de Fisiologıá, Universidad Autońoma de Puebla, 14 Sur 6301, Puebla 72570, Mexico b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 March 2009 Received in revised form 17 May 2009 Accepted 27 May 2009 Available online 6 June 2009

Elevated levels of corticosteroids and stress play key roles in the pathophysiology of affective disorders. Corticosterone (CORT)-treated rats have emerged as a pharmacological model of depression-like behaviors. Previous studies have shown that CORT administration induces neuronal atrophy in the CA3 subfield of the hippocampus and laminae II/III of the prefrontal cortex. However, little attention has been given to other limbic structures such as the amygdala and the nucleus accumbens (NAcc). We investigated here whether 3 weeks of CORT administration in rats causes dendritic remodeling and spine density reorganization in the basolateral amygdala and pyramidal neurons of the CA1 subfield of the hippocampus as well as in spiny medium neurons of NAcc. Quantitative morphological analysis revealed retracted neuronal arborizations and modified configuration of length depending on branch order in medium spiny neurons of the NAcc of CORT-treated animals. Moreover, distal dendritic sections of the NAcc showed massive reductions in the number of spines caused by the CORT treatment. This treatment also induced a reduction in total dendritic length specific to fourth and sixth branch orders of pyramidal CA1 hippocampal neurons. These neurons also showed decreased branching and diminished number of spines. Finally, pyramidal neurons of the basolateral amygdala were apparently not significantly affected by the CORT treatment. Taken together, these data show for the first time neuronal morphological alterations in the NAcc in the CORT model of depression-like behaviors. Our results also add further information about the morphological reorganization occurring in CORT-sensitive regions of the limbic system. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.

Keywords: Animal model Corticosterone Golgi-Cox stain Dendritic morphology Depression Stress

1. Introduction The hypersecretion of corticosteroids (cortisol in humans and corticosterone in rodents) and stress have been associated with various pathological conditions including depression, drug addiction, impaired memory, body weight alterations, hypertension and steroid-induced diabetes in humans (McEwen, 1998; Erickson et al., 2003). Recently, long term CORT administration in rodents has been proposed as an animal model of stress-induced depression-like behaviors (Gregus et al., 2005). Animals repeatedly treated with CORT display a depressive-like state (Kalynchuk et al., 2004; Gregus et al., 2005; Johnson et al., 2006), enhanced fear (Corodimas et al., 1994), decreased hippocampal neurogenesis

* Corresponding author at: Douglas Mental Health University Institute, McGill University, 6875 La Salle Boulevard, Montreál, QC, H4H 1R3 Canada. Tel.: +1 514 761 6131x2934; fax: +1 514 888 4060. E-mail address: [email protected] (R. Quirion).

(Fuchs and Gould, 2000; Pham et al., 2003), downregulated glucocorticoid receptors (Vyas et al., 2002), decreased sexual behavior and reduced body weight in rats (Barr et al., 2000; Kalynchuk et al., 2004). Moreover, CORT treatment affects both the volume (Cerqueira et al., 2005) and rearranges neuronal architecture in layers II/III of the prefrontal cortex (PFC) (Wellman, 2001; Seib and Wellman, 2003) and redistributes dendritic arborization in hippocampal CA3 subfield neurons (Watanabe et al., 1992; Magarinos et al., 1998). Similar neuronal rearrangements are observed in various models of physical or social stress in rats and primates (Magarinos and McEwen, 1995; Magarinos et al., 1996) and depression in humans (Stockmeier et al., 2004). Rather surprisingly, suppression of corticosteroids caused by adrenalectomy also produces dendritic and neuronal atrophy (Cerqueira et al., 2007). However, the major advantage of repeated CORT administration over stress models is the reduced variability since physical stress models cause individual differences in the regulation of hypothalamus–pituitary–adrenal axis as a result of adverse conditions as shown in chronic physical restrain (Gregus

0891-0618/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jchemneu.2009.05.009

J.C. Morales-Medina et al. / Journal of Chemical Neuroanatomy 38 (2009) 266–272

et al., 2005) and chronic unpredictable stress (Vyas et al., 2002). Moreover, repeated CORT administration accurately mimics the elevated levels of cortisol observed following psychosocial stress in tree shrews (Magarinos et al., 1996). Accordingly, the repeated CORT administration model represents a suitable approach to explore possible neuronal rearrangements in stress-related depression-like behaviors. The hippocampus and the amygdala are interconnected with the PFC and send major excitatory projections to the nucleus accumbens (NAcc) (French and Totterdell, 2003). The hippocampus and PFC are known to regulate both corticosteroid release (Rozeboom et al., 2007) and to mediate emotional behaviors (Cerqueira et al., 2005). However, other limbic areas are also involved in conditions associated with high levels of corticosteroids. For example, the basolateral amygdala (BLA) has emerged as a region relevant to the control of non-hippocampal emotional processes (Vyas et al., 2002). The NAcc is also known to play a role in the control of the stress cascade by altering the dopamine (DA) transporter (Sarnyai et al., 1998) and increasing DA metabolism (Abercrombie et al., 1989). High levels of corticosteroids produce diverse effects on mood and neurological processes (Erickson et al., 2003). Accordingly, we hypothesize that these effects may induce neuronal morphological rearrangements not limited to pyramidal neurons of the CA3 subfield of the hippocampus and layers II/III of the PFC. We hypothesize that since CA1 pyramidal neurons receive direct inputs from CA3 pyramidal neurons, some neuronal rearrangement occurs after the chronic CORT treatment in these neurons as well. The aim of the present study was thus to assess whether 3 weeks of CORT treatment causes a remodeling of neuronal and dendritic morphology in pyramidal neurons of BLA and CA1 subfield of the hippocampus as well as spiny medium neurons of the NAcc in adult rats. 2. Methods 2.1. Animals Male Sprague–Dawley rats (Charles River Canada, Montreál, QC, Canada) weighing 140–150 g at the beginning of the treatment were housed two per cage and maintained on a 12 h light/dark cycle schedule with ad libitum access to food (Purina Lab Chow) and water. All procedures were approved by the McGill animal care committee and followed guidelines of the Canadian council on animal care. 2.2. Corticosterone treatment The protocol for CORT administration used as described in earlier studies (Kalynchuk et al., 2004; Gregus et al., 2005). Rats were handled for 1 min for 7 consecutive days prior to CORT treatment. Animals were randomly assigned to either subcutaneous injection of corticosterone 21-acetate (40 mg/ml/kg) dissolved in 0.9% saline solution/1% polyoxyethylene sorbitan monooleate or Tween80; Sigma–Aldrich, Oakville, ON, Canada) or controls (saline solution/1% Tween80), for 21 consecutive days between 0900 and 1100 h. Ten animals were used per group. 2.3. Golgi-Cox stain method On day 22 of treatment, rats were anesthetized with sodium pentobarbital (60 mg/kg, ip) and perfused intracardially with 0.9% saline solution. Brains were removed and stained by the modified Golgi-Cox method (Gibb and Kolb, 1998; Flores et al., 2005; Alquicer et al., 2008). Brains were stored for 14 days in Golgi-Cox solution, followed by 3 days in 30% sucrose solution. Coronal sections of 200 mm thickness of regions to be studied were obtained using a vibrotome (Camden Instrument, MA752, Leicester, UK). Sections were then treated with ammonium hydroxide for 30 min, followed by 30 min in Kodak Film Fixer (Eastman Kodak Company, Rochester, USA) and finally rinsed with distilled water, dehydrated, and mounted with resinous medium. Golgi-impregnated medium spiny neurons from NAcc (shell) (Plates 10–15), pyramidal neurons of the CA1 subfield of the hippocampus (Plates 37–42), and the BLA (Plates 25–30) were studied according to the atlas of Paxinos and Watson (1986). The criteria used to select neurons for reconstruction was as described earlier by our group (Flores et al., 2005; Alquicer et al., 2008). A blind observer identified the three-dimensional dendritic tree of five neurons in each hemisphere (10 neurons per animal), 10 animals per group and reconstructed each neuron in a two-dimensional plane using a camera Lucida at a magnification of 250 (DM 2000 Microscope, Leica Microsystems, Wetzlar, Germany). Dendritic tracing was

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quantified by Sholl analysis (Sholl, 1953; Flores et al., 2005). A transparent grid with concentric rings, equivalent to 10 mm apart, was placed over the dendritic drawing and the number of ring intersections was estimated. In addition, the total dendritic length was calculated by multiplying the total number of intersections of each ring per 10 mm. Another estimate of dendritic arborization is the total number of dendritic branches (branching, indicated by bifurcation), which were counted at each order away from the cell body or dendritic shaft. Finally, the density of dendritic spines was estimated by drawing at least 10 mm long segments from the terminal tips at a 1000 resolution. 2.4. Statistical analysis Data from arborization, total dendritic length, length per branch order and spine densities were analyzed by two-way ANOVA, followed by Bonferroni test for post hoc comparisons, with CORT administration and either regions or orders as independent factors. P < 0.05 was considered significant.

3. Results The arborization, dendritic length for each branching order, total dendritic length, and spine density of NAcc, CA1 subfield of the hippocampus and BLA neurons were measured using a GolgiCox stain as described earlier (Vyas et al., 2002; Flores et al., 2005; Alquicer et al., 2008). Representative control neurons from each region are illustrated in Fig. 1A (NAcc), Fig. 2A (CA1 hippocampus), and Fig. 3A (BLA). Dendritic distal sections of control neurons of the aforementioned regions are shown in Figs. 1B, 2B and 3B. These results indicate that as compared to controls, repeated CORT treatment induces severe reorganization of dendritic morphology (Fig. 1C) and spine density of NAcc medium spiny neurons (Fig. 1D) as illustrated by camera lucida reconstructions. Morphological analysis following CORT treatment revealed reduced branching of medium spiny neurons of the NAcc (two-way ANOVA, corticosterone administration: F1,494 = 145, P < 0.001; intersection per shell: F25,494 = 534, P < 0.001; interaction between corticosterone administration and intersection per shell: F25,494 = 3.7, P < 0.001) (Fig. 1E) with no alteration in total dendritic length (Fig. 1F). Length per branch order analysis shows an increase in dendritic length close to the first and second order of the soma, whereas a decrease in length was observed in the fifth order in CORTtreated versus control animals at the level of NAcc (two-way ANOVA, corticosterone administration: F1,108 = 5.69, P < 0.05; branch order: F5,108 = 96, P < 0.001; interaction between corticosterone treatment and branch order F5,108 = 12.5, P < 0.001) (Fig. 1G). In addition, CORT administration resulted in decreased spine density when compared to control animals (two-way ANOVA, corticosterone administration: F1,54 = 92, P < 0.001) (Fig. 1H). CORT treatment (Fig. 2C) has an effect on the neuroplasticity of CA1 hippocampal pyramidal neurons as illustrated by camera lucida drawings compared to vehicle (control, Fig. 2D). Repeated CORT injection caused debranching in the medial part of the dendritic tree (two-way ANOVA, corticosterone administration: F1,494 = 145, P < 0.001; intersection per shell: F25,494 = 534, P < 0.001; interaction between corticosterone administration and intersection per shell: F25,494 = 3.7, P < 0.001) (Fig. 2E) with a decrease in total dendritic length relative to hippocampal CA1 pyramidal neurons of controls (corticosterone administration: F1,54 = 4.26, P = 0.04; regions: F2,54 = 14.3, P < 0.001, CA1 hippocampus, P < 0.05) (Fig. 2F). Length per branch order analysis also shows specific decreases in length at the fourth and sixth branch order in CORT-treated animals relative to controls (two-way ANOVA, corticosterone administration: F1,144 = 10.7, P < 0.01; branch order: F7,144 = 123, P < 0.001; interaction between corticosterone administration and branch order F7,144 = 1.3, P = 0.25) (Fig. 2G). Further, hippocampal CA1 pyramidal neurons of CORTtreated animals demonstrated a significant decrease in spine density as compared to neurons in control rats (two-way ANOVA, corticosterone administration: F1,54 = 92, P < 0.001 and regions F2,54 = 3.29, P = 0.04, CA1 hippocampus, P < 0.05) (Fig. 2H).

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Fig. 1. Reorganization of dendritic and neuronal arborization in the nucleus accumbens (Nacc). Representative photomicrograph of a Golgi-Cox stained medium spiny neuron of NAcc (A) from control animal. Representative photomicrograph of Golgi-Cox stained distal dendrite of NAcc (B) from control rat. Camera lucida reconstructions of dendritic arbors of medium spiny neurons of the NAcc and distal section of dendritic spines of animals treated with vehicle (C) relative to CORT (D). Sholl analysis (E). Intersections of basal dendrites indicate increase close to the soma (20 mm) and reductions distal to the soma (100–130 mm) in NAcc neurons of CORT-treated animals relative to controls: total dendritic length (F), length of branch order (G). Branch length was augmented in first and second order and dramatically diminished in fifth order: spine density (H). The number of spines per segment was decreased in distal dendritic segment of NAcc. Data are mean SEM with n = 10 rats per group, 8–10 neurons per dendritic segment were drawn for each animal. *P < 0.05, **P < 0.01, ***P < 0.001.

CORT treatment (Fig. 3C) has a limited effect on the neuronal plasticity of BLA pyramidal neurons as observed in camera lucida illustrations relative to vehicle-treated animals (Fig. 3D). Analysis of the effect of long term administration of CORT on pyramidal neurons of BLA: dendritic arborization (two-way ANOVA, corticosterone administration: F1,532 = 3.3, P = 0.69; intersection per shell: F27,532 = 164.2, P < 0.001; interaction between corticosterone administration and intersection per shell: F27,532 = 0.78, P = 0.78) (Fig. 3E), total dendritic length (corticosterone administration: F1,54 = 4.26, P = 0.04; regions: F2,54 = 14.3, P < 0.001, BLA, P > 0.05) (Fig. 3F), branch orders (two-way ANOVA, corticosterone administration: F1,126 = 2.6, P = 0.10; branch order: F6,126 = 75.3, P < 0.001; interaction between corticosterone administration and branch order F6,126 = 2.3, P = 0.06) (Fig. 3G), and spine density (two-way ANOVA, corticosterone administration: F1,54 = 92, P < 0.001) and regions F2,54 = 3.29, P = 0.04, BLA, P > 0.05) (Fig. 3H) were not affected by CORT treatment in pyramidal neurons of BLA. 4. Discussion Elevated CORT levels have been linked to depression, drug addiction and other mental illnesses (McEwen, 1998; Erickson et al., 2003). However, the neurobiology of CORT in disease states is not fully understood. In this study, we characterized CORT-induced changes in some of the brain regions known to be associated with depressive behaviors. In the present study, dramatic reductions in spine density and neuronal atrophy were found in NAcc neurons in CORT-treated rats. A decrease in spine density and branching order

was also clearly seen in pyramidal neurons of the CA1 subfield of the hippocampus. However, in BLA pyramidal neurons, a trend for a reduction in spine density with no dendritic remodeling was observed (failed to reach statistical significance). Taken together, these results suggest that long term increase in CORT levels induce differential morphological changes in key regions known to be associated with depressive behaviors. 4.1. Golgi-Cox staining Morphological changes are known to be correlated with alterations in neurotransmission (Fiala et al., 2002; Pittenger and Duman, 2008), for example a dendritic surface receives more than 95% of synapses of a given neuron (Kolb et al., 1998). Therefore dendritic arborization is correlated to the number of synapses and in addition, adult cortical neurons receive approximately 15,000 synaptic inputs (Huttenlocher, 1984). The Golgi-Cox staining has been used broadly to distinguish differential aspects of dendritic morphology in individual neurons (Gibb and Kolb, 1998) which helps to make inferences about connectivity from dendritic structure. Finally, the Golgi-Cox procedure stains about 1% of cells in brain tissue (Kolb et al., 1998). However, given the random nature of these events, we consider that neurons studied here are representative of the total population. 4.2. Dendritic morphology of NAcc medium spiny neurons To the best of our knowledge, we show for the first time in this study, the existence of morphological alterations in the NAcc, an


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Fig. 2. Morphological analyses of dendritic organization and spine density in CA1 subfield of the hippocampus. Representative photomicrograph of a Golgi-Cox stained pyramidal neuron of CA1 hippocampus (A) from control animal. Representative photomicrograph of Golgi-Cox stained distal dendrites of hippocampus (B) from control rats. Representative camera lucida drawing of pyramidal neurons of CA1 hippocampal region and distal section of dendritic spines from animals treated with either vehicle (C) or CORT (D). Sholl analysis (E). Basal dendritic material of hippocampal neurons is greater in controls compared to CORT-treated animals. Total dendritic length was decreased in CORT-treated versus control animals in hippocampal pyramidal neurons (F), (P < 0.05) and length of branch order (G). CORT treatment induces reductions in length in fourth and sixth order in the CA1 subfield of the hippocampus: spine density (H). The number of spines per segment was decreased in distal dendritic segment of CA1 subfield of the hippocampus in CORT-treated animals compared to controls (P < 0.001). Data are mean SEM with n = 10 rats per group, 8–10 neurons per dendritic segment were drawn for each animal. *P < 0.05, **P < 0.01, ***P < 0.001.

‘‘hedonic’’ nucleus (French and Totterdell, 2003). Neuronal atrophy was restricted to distal segments of the arborization. Neuronal remodeling likely occurs because fifth order branching receives dopamine afferents from the ventral tegmental area as well as excitatory amino acid afferents from the PFC, hippocampus and amygdala (Groenewegen et al., 1999). On the other hand, medium spiny NAcc neurons show hypertrophy in regions close to the soma which may occur to compensate inputs lost in distal segments. These regions receive projections from various thalamic neurons and tonic cholinergic interneurons (Tepper and Bolam, 2004). We also observed dramatic decrease in spine number in the distal segments of dendritic spines of NAcc neurons in CORT-treated animals. Reduced spine density is likely to be relevant in regard to altered inputs from CA1 and PFC pyramidal neurons, since both regions display disrupted morphology in this model (Wellman, 2001). Recently, Bennur et al. (2007) found decreased spinogenesis in medium spiny neurons of medial amygdala after chronic stress treatment. This may suggest that the same population of neurons responded in a similar way to stress-related insults despite the brain region. Additionally, recent evidence suggests that administration of glucocorticoids during pregnancy reduces the number and volume of NAcc neurons in adult offsprings with altered emotionality and increased propensity to drug abuse (Leao et al., 2007). Moreover, in support of role for the NAcc in emotions, antidepressants have been shown to affect NAcc neurons. For example, lithium treatment increases DA D2 receptor mRNA levels in the core and shell of the NAcc (Dziedzicka-Wasylewska and Wedzony, 1996).

4.3. Dendritic morphology of CA1 hippocampal pyramidal neurons The effects of CORT on morphological features of hippocampal CA1 neurons are rather controversial. Here we report decrease in the number of spines, arborization and total dendritic length in CA1 hippocampal neurons following a long term CORT treatment. However, Magarinos and McEwen (1995) reported no alteration in the dendritic morphology of CA1 pyramidal neurons following physical stress while acute physical stress was shown to enhance spine density in the CA1 hippocampal region (Shors et al., 2001). Moreover, in support of current results either a single or 7 days of CORT administration resulted in decreased spine density in CA1 pyramidal neurons (Hajszan et al., 2008). Conversely, chronic physical unpredictable stress induces a reduction in apical dendritic length in the presence of high but not low levels of CORT (Alfarez et al., 2008). These apparent discrepancies could be mostly due to methodological differences. For example, physical stress models have generated differential results based on individual differences in coping with stress (Kalynchuk et al., 2004). It is also well established that CA3 hippocampal neurons are more severely affected by CORT treatment or stress than their CA1 counterparts (Magarinos et al., 1996, 1998). Interestingly, a single CA3 neuron can send projections to apical and basal dendrites of a CA1 pyramidal neuron (Hajszan et al., 2008), making up to 20,000 synapses (Shors et al., 2001). A decrease in spine density can be produced by loss of axons that make synaptic connections with these spines (Fiala et al., 2002). Accordingly, disrupted morphology of CA3 neurons could, at least partly, leads to morphological

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Fig. 3. Analysis of pyramidal neurons of basolateral amygdala (BLA). Representative photomicrograph of a Golgi-Cox stained pyramidal neuron of BLA (A) from control animal. Representative photomicrograph of Golgi-Cox stained distal dendrite of BLA (B). Schematic camera lucida illustration of the pyramidal neurons of BLA distal section of dendritic spines after 21 days of CORT treatment in vehicle-treated animals (C) compared to CORT (D). Sholl analysis (E), total dendritic length (F), length of branch order (G) and spine density (H). CORT treatment has no effect on neuronal morphology of the BLA. Data are mean SEM with n = 10 rats per group, 8–10 neurons per dendritic segment were drawn for each animal.

alterations of CA1 neurons. Another contributor to hippocampalinduced atrophy may be mediated by glutamatergic transmission. This is based on the fact that extracellular glutamate levels are augmented in both CA3 hippocampal region after physical stress (Lowy et al., 1993) and CA1 hippocampal area after corticosterone administration (Venero and Borrell, 1999). Moreover, CGP43487, a NMDA antagonist, prevents hippocampal rearrangement after physical stress (Magarinos and McEwen, 1995). On the other hand, low levels of maternal care resulted in dendritic atrophy of pyramidal neurons in the CA1 subfield (Champagne et al., 2008). Maternally separated animals have been shown to have altered cognitive, emotional and neuroendocrine response to stress including decreased levels of glucocorticoid receptors in the hippocampus and PFC (Meaney, 2001). Taken together, these results suggest that CORT treatment and maternal separation can alter CA1 hippocampal morphology and neurochemistry leading to various behavioral deficits (Shors et al., 2001). 4.4. Dendritic morphology of BLA pyramidal neurons The BLA connects with the dorsal hippocampal CA1 subfield (Kogan and Richter-Levin, 2008) and PFC (Wellman et al., 2007). Since these regions are involved in emotional processes, BLA is largely known for having a pivotal role in emotionality. The PFC and CA1 hippocampal region are affected negatively by both stress and corticosterone treatment (Magarinos and McEwen, 1995; Shors et al., 2001; Wellman, 2001; Seib and Wellman, 2003). In the present study, we observed no significant

alterations in spine density and morphology of pyramidal BLA neurons. Our present data are in agreement with Mitra and Sapolsky (2008) that neuronal anatomy is unaltered after 10 days of CORT treatment in the pyramidal neurons of BLA. Surprisingly, acute CORT treatment results in hypertrophy of amygdaloid neurons (Mitra and Sapolsky, 2008). Moreover, restrained physical stress for 10 days showed a hypertrophy of the dendritic arborization of pyramidal neurons and enhanced spinogenesis of the BLA (Vyas et al., 2002; Mitra et al., 2005), while unpredictable physical stress failed to produce any neuronal rearrangement (Vyas et al., 2002). In addition, 21 days of chronic restrained physical stress resulted in both augmented distribution of dendritic arborization and enhanced spinogenesis in BLA pyramidal neurons in rats (Vyas et al., 2006) and mice (Bennur et al., 2007), while acute stress-induced hypertrophy in the BLA (Mitra et al., 2005). Furthermore, serotonin transporter knockout mice, an animal model of depression-related behaviors, show no morphological rearrangement in pyramidal BLA neurons (Wellman et al., 2007). However, these animals show an increased spine density specifically at the level of the fourth order (Wellman et al., 2007). Contrary to previous observations in CA3 hippocampal and PFC pyramidal neurons where either physical or CORT treatment induced neuronal debranching (Magarinos and McEwen, 1995; Magarinos et al., 1996, 1998; Wellman, 2001), CORT and physical stress produced different effects on the neuronal morphology of BLA. In support of this hypothesis, physical stress failed to induce depression-like behaviors in the forced swim test while both CORT


treatment and uncontrolled and unpredictable physical stress were successful (Vyas et al., 2002; Gregus et al., 2005). In addition, acute CORT treatment induces anxiety-like behavior in rats (Mitra and Sapolsky, 2008), similar to a single exposure to a cat (Adamec et al., 1999). In contrast to depressionrelated behaviors (Kalynchuk et al., 2004), long lasting CORT treatment failed to produce any anxiety-like behaviors (Mitra and Sapolsky, 2008). Taken together, these differences raise the possibility that both the source of stress and the length of time of exposure affect BLA differentially. Therefore, subtle differences may occur in the BLA in different animal models of depression-like behaviors. 4.5. Summary Elevated levels of CORT contribute differentially to neuronal atrophy in areas playing key role in the regulation of the stress response. Neuronal and dendritic reorganization are likely to be involved in sensitization-induced behavioral effects in various models of depressive states. Acknowledgements This study was supported by grants from Canadian Institutes of Health Research (RQ) and VIEP-BUAP (GF). JCMM is a PhD student with fellowship from CONACyT-Mexico. Thanks to Mira Thakur for editing the English-language text. References Abercrombie, E.D., Keefe, K.A., DiFrischia, D.S., Zigmond, M.J., 1989. Differential effect of stress on in vivo dopamine release in striatum, nucleus accumbens, and medial frontal cortex. J. Neurochem. 52, 1655–1658. Adamec, R.E., Burton, P., Shallow, T., Budgell, J., 1999. Unilateral block of NMDA receptors in the amygdala prevents predator stress-induced lasting increases in anxiety-like behavior and unconditioned startle—effective hemisphere depends on the behavior. Physiol. Behav. 65, 739–751. Alfarez, D.N., Karst, H., Velzing, E.H., Joels, M., Krugers, H.J., 2008. Opposite effects of glucocorticoid receptor activation on hippocampal CA1 dendritic complexity in chronically stressed and handled animals. Hippocampus 18, 20–28. Alquicer, G., Morales-Medina, J.C., Quirion, R., Flores, G., 2008. Postweaning social isolation enhances morphological changes in the neonatal ventral hippocampal lesion rat model of psychosis. J. Chem. Neuroanat. 35, 179–187. Barr, A.M., Brotto, L.A., Phillips, A.G., 2000. Chronic corticosterone enhances the rewarding effect of hypothalamic self-stimulation in rats. Brain Res. 875, 196– 201. Bennur, S., Shankaranarayana Rao, B.S., Pawlak, R., Strickland, S., McEwen, B.S., Chattarji, S., 2007. Stress-induced spine loss in the medial amygdala is mediated by tissue-plasminogen activator. Neuroscience 144, 8–16. Cerqueira, J.J., Pego, J.M., Taipa, R., Bessa, J.M., Almeida, O.F., Sousa, N., 2005. Morphological correlates of corticosteroid-induced changes in prefrontal cortex-dependent behaviors. J. Neurosci. 25, 7792–7800. Cerqueira, J.J., Taipa, R., Uylings, H.B., Almeida, O.F., Sousa, N., 2007. Specific configuration of dendritic degeneration in pyramidal neurons of the medial prefrontal cortex induced by differing corticosteroid regimens. Cereb. Cortex 17, 1998–2006. Champagne, D.L., Bagot, R.C., van Hasselt, F., Ramakers, G., Meaney, M.J., de Kloet, E.R., Joels, M., Krugers, H., 2008. Maternal care and hippocampal plasticity: evidence for experience-dependent structural plasticity, altered synaptic functioning, and differential responsiveness to glucocorticoids and stress. J. Neurosci. 28, 6037–6045. Corodimas, K.P., LeDoux, J.E., Gold, P.W., Schulkin, J., 1994. Corticosterone potentiation of conditioned fear in rats. Ann. N.Y. Acad. Sci. 746, 392–393. Dziedzicka-Wasylewska, M., Wedzony, K., 1996. The effect of prolonged administration of lithium on the level of dopamine D2 receptor mRNA in the rat striatum and nucleus accumbens. Acta Neurobiol. Exp. (Wars.) 56, 29–34. Erickson, K., Drevets, W., Schulkin, J., 2003. Glucocorticoid regulation of diverse cognitive functions in normal and pathological emotional states. Neurosci. Biobehav. Rev. 27, 233–246. Fiala, J.C., Spacek, J., Harris, K.M., 2002. Dendritic spine pathology: cause or consequence of neurological disorders? Brain Res. Brain Res. Rev. 39, 29–54. Flores, G., Alquicer, G., Silva-Gomez, A.B., Zaldivar, G., Stewart, J., Quirion, R., Srivastava, L.K., 2005. Alterations in dendritic morphology of prefrontal cortical

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