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Increased cell proliferation in the adult mouse hippocampus following chronic administration of group II metabotropic glutamate receptor antagonist, MGS0039
BBRC Biochemical and Biophysical Research Communications 315 (2004) 493–496 www.elsevier.com/locate/ybbrc
Increased cell proliferation in the adult mouse hippocampus following chronic administration of group II metabotropic glutamate receptor antagonist, MGS0039 Takao Yoshimizu* and Shigeyuki Chaki Psychiatric Diseases and Pain Research, Medicinal Pharmacology Laboratory, Medicinal Research Laboratories, Taisho Pharmaceutical Co., Ltd., 1-403 Yoshino-cho, Kita-ku, Saitama, Saitama 331-9530, Japan Received 26 December 2003
Abstract We have previously reported that MGS0039, a novel antagonist of group II metabotropic glutamate receptors (mGluRs), exerts antidepressant-like effects in experimental animal models. Recent studies suggest that the behavioral effects of chronic antidepressant treatment are mediated by the stimulation of neurogenesis in the hippocampus. In the present study, we examined the effects of MGS0039 on cell proliferation in the adult mouse hippocampus. MGS0039 (5 or 10 mg/kg) or fluvoxamine was administered chronically to male ICR mice over a period of 14 days. Multiple bromodeoxyuridine (BrdU) administrations were performed after the last drug injection to label dividing cells. Immunohistochemical analyses after BrdU injections revealed that chronic MGS0039 treatment enhanced BrdU-positive cells in the dentate gyrus (62% increase) in the same manner as chronic fluvoxamine treatment. This is the first in vivo study to demonstrate an increase in cell proliferation following a blockade of group II mGluRs. These findings raise the possibility that MGS0039 may exert antidepressant-like effects by modulating cell proliferation in the hippocampus. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Group II metabotropic glutamate receptor antagonist; MGS0039; Antidepressant; Cell proliferation; Neurogenesis; Hippocampus; Mice
Metabotropic glutamate receptors (mGluRs), a family of G-protein-coupled receptors, are divided into three groups based on sequence homologies, their ability to activate specific second messenger systems, and specific sensitivities for agonists [1]. Group II mGluRs such as mGluR2 and mGluR3 (mGluR2/3) share significant sequence homologies and are similarly distributed in many brain regions. Various pharmacological and histochemical studies have implicated their role in the control of emotional state. Recently, we reported that (1R, 2R, 3R, 5R, 6R)-2-amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039), a novel mGluR2/3 antagonist, exhibits antidepressant-like effects in rats subjected to forced swimming tests and mice subjected to tail suspension tests [2]. Atrophy of hippocampal neurons has been observed in patients with major and prolonged depression [3]. The *
Corresponding author. Fax: +81-48-652-7254. E-mail address: [email protected] (T. Yoshimizu). 0006-291X/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2004.01.073
hippocampus is one of only a few brain regions where production of neurons occurs throughout the lifetime of animals, including humans [4]. Hippocampal neurogenesis can be influenced by several environmental factors and stimuli. For instance, neurogenesis has been shown to decline in animal models following periods of chronic stress [5]. Furthermore, recent studies have shown that the administration of different classes of antidepressant drugs or electroconvulsive seizure (ECS) promotes adult neurogenesis in the hippocampus [6,7]. The present study sought to determine whether MGS0039, a potent and selective antagonist of group II mGluRs, might influence cell proliferation in the dentate gyrus of adult mice.
Materials and methods Experimental animals. Male ICR mice, 8–10 weeks old (Charles River, Japan), were used for this study. All mice were housed in groups of 10 per cage, under standard conditions; room temperature
494
T. Yoshimizu, S. Chaki / Biochemical and Biophysical Research Communications 315 (2004) 493–496
(23 3 °C); light–dark cycle (light phase 7:00–19:00). The mice were given free access to food and water except during the experiment. This study was conducted with the approval of Taisho Pharmaceutical Co., Ltd. Animal Care Committee. Chemicals and drug administration. For chronic dosing, MGS0039 (5 or 10 mg/kg) or fluvoxamine (30 mg/kg) was administered intraperitoneally to mice for 14 days. MGS0039 or fluvoxamine was dissolved in 1/15 M phosphate buffer (pH 8.0). To evaluate the effects of the compounds on cell proliferation, the thymidine analog bromodeoxyuridine (BrdU) was used as a marker for dividing cells. BrdU (Sigma, St. Louis, MO; 3 75 mg/kg every 2 h, i.p.) was administered 24 h after the last injection of MGS0039, fluvoxamine, or vehicle. Immunohistochemistry. All mice were killed 24 h after BrdU injection via intracardial perfusion with 4% paraformaldehyde under anesthetization with sodium pentobarbital (100 mg/kg, i.p.). After perfusion, all brains were post-fixed overnight in the identical fixative at 4 °C, followed by sequential incubation with 10%, 20%, and 30% sucrose, after which they were frozen and stored at )80 °C. Thirty micrometer-thick coronal brain sections were cut on a cryostat and mounted on slides. Every fifth section was collected between stereotaxic coordinates Bregma )1.2 to )3.0. Sections were first denatured in 50% formamide/2 saline sodium citrate (SSC) at 65 °C for 2 h and in 2 N HCl at 37 °C for 30 min. They were then incubated in 0.1 M boric acid (pH 8.5) for 10 min followed by incubation for 30 min in 0.3% H2 O2 /methanol to eliminate endogenous peroxidases. After blocking in phosphate-buffered saline (PBS) containing 1.5% normal horse serum and 0.05% Tween 20 (blocking buffer), cells were incubated with mouse monoclonal antibodies against BrdU (1:1000; Sigma) in a blocking buffer. Sections were then incubated for 1 h with secondary
antibodies (biotinylated horse anti-mouse IgG; Vector Laboratories, Burlingame, CA) followed by amplification with an avidin–biotin complex (ABC elite; Vector Laboratories), and cells were visualized with 0.05% of 3,30 -diaminobenzidine (Wako Pure Chemical Industries, Japan). Data quantification of BrdU-labeling and statistical analysis. All BrdU-labeled cells in the dentate gyrus (granule cell layer) and hilus were counted in each section. This count was performed bilaterally in 12 spanning across the rostro-caudal coordinates of the hippocampus of each mouse (Bregma )1.2 to )3.0). To distinguish single cells within clusters, all counts were performed at 400 magnification under a light microscope (ZEISS Axioplan2), omitting cells in the outermost focal plane. The total number of BrdU-labeled cells per section was determined and multiplied by 5 to obtain the total number of cells per dentate gyrus. Data were analyzed by one-way ANOVA and are presented as the mean value of BrdU-positive cells per section SEM. Significant differences between groups were determined by Dunnett’s test (p < 0:05).
Results and discussion BrdU-labeled nuclei of proliferating cells were frequently observed in clusters, localized in the subgranular zone of the hippocampus in all treated mice (Fig. 1). In chronic fluvoxamine-treated animals, the total number of BrdU-positive cells in the dentate gyrus and hilus was
Fig. 1. The number of BrdU-positive cells in the dentate gyrus increases following chronic MGS0039 administration. ICR mice received injections of BrdU (75 mg/kg) 24 h after the last fluvoxamine or MGS0039 treatment. Their coronal brain sections were pretreated for DNA denaturation and stained with antibodies against BrdU. Shown are representative photomicrographs from vehicle (A), 30 mg/kg fluvoxamine (B), 5 mg/kg (C), or 10 mg/kg (D) MGS0039.
T. Yoshimizu, S. Chaki / Biochemical and Biophysical Research Communications 315 (2004) 493–496
Fig. 2. Chronic MGS0039 administration resulted in cell proliferation in the adult mouse hippocampus. Mice received BrdU injections as described in Fig. 1. The results are expressed as means SEM number of BrdU-positive cells in the dentate gyrus (n ¼ 6 or 7). *p < 0:05 indicates a significant difference from the vehicle control.
2019 59 (mean SEM) compared with 1499 75 in the vehicle group, which corresponds to a significant increase (35%) (p < 0:001). Interestingly, chronic administration of MGS0039 at 5 and 10 mg/kg promoted progenitor cell proliferation in the dentate gyrus in the same way that chronic fluvoxamine treatment did. The 5 mg/kg dose of MGS0039 produced a significant increase in the number of BrdU-positive cells (2430 66, 62% increase; p < 0:0001). Compared to the lower dose, MGS0039 given at 10 mg/kg increased (2114 114, 41% increase; p < 0:0001) at the same level as chronic fluvoxamine treatment (Fig. 2). In the present study, we investigated the effects of MGS0039 on cell proliferation in the adult mouse hippocampus. As has been observed with various other antidepressants, including ECS, chronic MGS0039 treatment at a pharmacologically effective dose increased cell proliferation in the dentate gyrus (62% increase). It was reported that chronic administration of fluoxetine, a selective serotonin reuptake inhibitor, increases the number of BrdU-positive cells by about 60% in the dentate gyrus of mice, and approximately 70% of the cells differentiate into neurons [8]. Furthermore, Chen et al. [9] reported that the mood-stabilizing agent lithium has a similar effect on cell proliferation in the adult mouse hippocampus (25% increase), and that 66% of these cells express a neuronal marker protein [9]. Although we did not continue to monitor the fate of proliferating cells in this study after MGS0039 administration, most of the dividing cells may differentiate into mature neurons, as in the case of several antidepressant drug treatments. The mechanisms by which MGS0039 promotes cell proliferation in the dentate gyrus are unclear. With
495
regard to the relationship between the antidepressantlike effects and cell proliferation in the hippocampus, many interesting studies focusing on brain-derived neurotrophic factor (BDNF) have been reported. In the central nervous system, including the hippocampus, BDNF plays an important role in long-term survival, in the differentiation of neurons, and in synaptic development. Heterozygous BDNF knockout (BDNF +/)) mice, which have reduced BDNF protein in the hippocampus, exhibit decreased proliferation and survival of BrdU-labeled cells in the dentate gyrus, suggesting that BDNF is an important molecule in the regulation of cell proliferation, differentiation, and survival in the adult mouse hippocampus [10]. BDNF conditional deletion mice are hyperaggressive and hyperphagic, and have increased levels of anxiety [11]. Interestingly, chronic administration of antidepressant drugs or ECS completely blocked the down-regulation of BDNF and its receptor, trkB, in the hippocampus in response to restraint stress [12]. Based on the above evidences, BDNF is thought to be a critical molecule in exerting antidepressant-like activity [13]. Recent studies suggest that the activation of AMPA (a-amino-3-hydroxy-5-methyl4-isoxazolepropionate) receptors, ionotropic glutamate receptors, plays an important role in BDNF production and neurogenesis in the adult hippocampus. AMPA potentiator upregulates BDNF expression [14] and increases progenitor cell proliferation in the adult rat dentate gyrus [15]. Furthermore, AMPA potentiators exhibited antidepressant-like effects in forced swim and tail suspension tests [16], as did MGS0039. Group II mGluRs inhibit neurotransmitter release as autoreceptors located on glutamatergic terminals or as presynaptic heteroreceptors. Treatment with the mGluR2/3 antagonist in vivo produced a dose-dependent increase in extracellular glutamate [17]. Therefore, it is the intriguing hypothesis that the moderate elevation of glutamate level by MGS0039 may cause increased BDNF production and cell proliferation in the hippocampus via the activation of postsynaptic AMPA receptors. Indeed, in a preliminary study, we observed that the antidepressant-like effect of MGS0039 in an experimental animal model was attenuated by an AMPA receptor antagonist, NBQX, suggesting that AMPA receptor stimulation is involved in the antidepressant-like effects of MGS0039 (S. Chaki, in preparation). In this paper, we have shown that MGS0039, a group II mGluRs antagonist, can promote cell proliferation in the adult mouse hippocampus. Recent reports indicate that the behavioral effects of chronic antidepressants are mediated by the stimulation of neurogenesis in the hippocampus [8]. In light of studies by Santarelli et al. showing that hippocampal neurogenesis is required to exert antidepressant-like effects, the increase in cell proliferation in the hippocampus may be at least partially implicated in the pharmacological action of
496
T. Yoshimizu, S. Chaki / Biochemical and Biophysical Research Communications 315 (2004) 493–496
MGS0039. As above, the blockage of group II mGluRs may be an effective way of treating subjects suffering from clinical depression.
[9]
[10]
References [1] P.J. Conn, J.P. Pin, Pharmacology and functions of metabotropic glutamate receptors, Annu. Rev. Pharmacol. Toxicol. 37 (1997) 205–237. [2] S. Chaki, R. Yoshikawa, S. Hirota, T. Shimazaki, M. Maeda, N. Kawashima, T. Yoshimizu, A. Yasuhara, K. Sakagami, S. Okuyama, S. Nakanishi, A. Nakazato, MGS0039: a potent and selective group II metabotropic glutamate receptor antagonist with antidepressant-like activity, Neuropharmacology (in press). [3] J.D. Bremner, M. Narayan, E.R. Anderson, L.H. Staib, H.L. Miller, D.S. Charney, Hippocampal volume reduction in major depression, Am. J. Psychiatry 157 (2000) 115–118. [4] P.S. Eriksson, E. Perfilieva, T. Bjork-Eriksson, A.M. Alborn, C. Nordborg, D.A. Peterson, F.H. Gage, Neurogenesis in the adult human hippocampus, Nat. Med. 4 (1998) 1313–1317. [5] E. Gould, B.S. McEwen, P. Tanapat, L.A. Galea, E. Fuchs, Neurogenesis in the dentate gyrus of the adult tree shrew is regulated by psychosocial stress and NMDA receptor activation, J. Neurosci. 17 (1997) 2492–2498. [6] R.S. Duman, J. Malberg, S. Nakagawa, Regulation of adult neurogenesis by psychotropic drugs and stress, J. Pharmacol. Exp. Ther. 299 (2001) 401–407. [7] J.E. Malberg, A.J. Eisch, E.J. Nestler, R.S. Duman, Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus, J. Neurosci. 20 (2000) 9104–9110. [8] L. Santarelli, M. Saxe, C. Gross, A. Surget, F. Battaglia, S. Dulawa, N. Weisstaub, J. Lee, R. Duman, O. Arancio, C. Belzung, R. Hen, Requirement of hippocampal neurogenesis for
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[14]
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[16]
[17]
the behavioral effects of antidepressants, Science 301 (2003) 805– 809. G. Chen, G. Rajkowska, F. Du, N. Seraji-Bozorgzad, H.K. Manji, Enhancement of hippocampal neurogenesis by lithium, J. Neurochem. 75 (2000) 1729–1734. J. Lee, W. Duan, M.P. Mattson, Evidence that brain-derived neurotrophic factor is required for basal neurogenesis and mediates, in part, the enhancement of neurogenesis by dietary restriction in the hippocampus of adult mice, J. Neurochem. 82 (2002) 1367–1375. M. Rios, G. Fan, C. Fekete, J. Kelly, B. Bates, R. Kuehn, R.M. Lechan, R. Jaenisch, Conditional deletion of brain-derived neurotrophic factor in the postnatal brain leads to obesity and hyperactivity, Mol. Endocrinol. 15 (2001) 1748–1757. M. Nibuya, S. Morinobu, R.S. Duman, Regulation of BDNF and trkB mRNA in rat brain by chronic electroconvulsive seizure and antidepressant drug treatments, J. Neurosci. 15 (1995) 7539–7547. Y. Shirayama, A.C. Chen, S. Nakagawa, D.S. Russell, R.S. Duman, Brain-derived neurotrophic factor produces antidepressant effects in behavioral models of depression, J. Neurosci. 22 (2002) 3251–3261. M. Mackowiak, M.J. O’Neill, C.A. Hicks, D. Bleakman, P. Skolnick, An AMPA receptor potentiator modulates hippocampal expression of BDNF: an in vivo study, Neuropharmacology 43 (2002) 1–10. F. Bai, M. Bergeron, D.L. Nelson, Chronic AMPA receptor potentiator (LY451646) treatment increases cell proliferation in adult rat hippocampus, Neuropharmacology 44 (2003) 1013–1021. X. Li, J.P. Tizzano, K. Griffey, M. Clay, T. Lindstrom, P. Skolnick, Antidepressant-like actions of an AMPA receptor potentiator (LY392098), Neuropharmacology 40 (2001) 1028– 1033. Z.X. Xi, S. Ramamoorthy, D.A. Baker, H. Shen, D.J. Samuvel, P.W. Kalivas, Modulation of group II metabotropic glutamate receptor signaling by chronic cocaine, J. Pharmacol. Exp. Ther. 303 (2002) 608–615.
Increased cell proliferation in the adult mouse hippocampus following chronic administration of group II metabotropic glutamate receptor antagonist, MGS0039 Takao Yoshimizu* and Shigeyuki Chaki Psychiatric Diseases and Pain Research, Medicinal Pharmacology Laboratory, Medicinal Research Laboratories, Taisho Pharmaceutical Co., Ltd., 1-403 Yoshino-cho, Kita-ku, Saitama, Saitama 331-9530, Japan Received 26 December 2003
Abstract We have previously reported that MGS0039, a novel antagonist of group II metabotropic glutamate receptors (mGluRs), exerts antidepressant-like effects in experimental animal models. Recent studies suggest that the behavioral effects of chronic antidepressant treatment are mediated by the stimulation of neurogenesis in the hippocampus. In the present study, we examined the effects of MGS0039 on cell proliferation in the adult mouse hippocampus. MGS0039 (5 or 10 mg/kg) or fluvoxamine was administered chronically to male ICR mice over a period of 14 days. Multiple bromodeoxyuridine (BrdU) administrations were performed after the last drug injection to label dividing cells. Immunohistochemical analyses after BrdU injections revealed that chronic MGS0039 treatment enhanced BrdU-positive cells in the dentate gyrus (62% increase) in the same manner as chronic fluvoxamine treatment. This is the first in vivo study to demonstrate an increase in cell proliferation following a blockade of group II mGluRs. These findings raise the possibility that MGS0039 may exert antidepressant-like effects by modulating cell proliferation in the hippocampus. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Group II metabotropic glutamate receptor antagonist; MGS0039; Antidepressant; Cell proliferation; Neurogenesis; Hippocampus; Mice
Metabotropic glutamate receptors (mGluRs), a family of G-protein-coupled receptors, are divided into three groups based on sequence homologies, their ability to activate specific second messenger systems, and specific sensitivities for agonists [1]. Group II mGluRs such as mGluR2 and mGluR3 (mGluR2/3) share significant sequence homologies and are similarly distributed in many brain regions. Various pharmacological and histochemical studies have implicated their role in the control of emotional state. Recently, we reported that (1R, 2R, 3R, 5R, 6R)-2-amino-3-(3,4-dichlorobenzyloxy)-6-fluorobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (MGS0039), a novel mGluR2/3 antagonist, exhibits antidepressant-like effects in rats subjected to forced swimming tests and mice subjected to tail suspension tests [2]. Atrophy of hippocampal neurons has been observed in patients with major and prolonged depression [3]. The *
Corresponding author. Fax: +81-48-652-7254. E-mail address: [email protected] (T. Yoshimizu). 0006-291X/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2004.01.073
hippocampus is one of only a few brain regions where production of neurons occurs throughout the lifetime of animals, including humans [4]. Hippocampal neurogenesis can be influenced by several environmental factors and stimuli. For instance, neurogenesis has been shown to decline in animal models following periods of chronic stress [5]. Furthermore, recent studies have shown that the administration of different classes of antidepressant drugs or electroconvulsive seizure (ECS) promotes adult neurogenesis in the hippocampus [6,7]. The present study sought to determine whether MGS0039, a potent and selective antagonist of group II mGluRs, might influence cell proliferation in the dentate gyrus of adult mice.
Materials and methods Experimental animals. Male ICR mice, 8–10 weeks old (Charles River, Japan), were used for this study. All mice were housed in groups of 10 per cage, under standard conditions; room temperature
494
T. Yoshimizu, S. Chaki / Biochemical and Biophysical Research Communications 315 (2004) 493–496
(23 3 °C); light–dark cycle (light phase 7:00–19:00). The mice were given free access to food and water except during the experiment. This study was conducted with the approval of Taisho Pharmaceutical Co., Ltd. Animal Care Committee. Chemicals and drug administration. For chronic dosing, MGS0039 (5 or 10 mg/kg) or fluvoxamine (30 mg/kg) was administered intraperitoneally to mice for 14 days. MGS0039 or fluvoxamine was dissolved in 1/15 M phosphate buffer (pH 8.0). To evaluate the effects of the compounds on cell proliferation, the thymidine analog bromodeoxyuridine (BrdU) was used as a marker for dividing cells. BrdU (Sigma, St. Louis, MO; 3 75 mg/kg every 2 h, i.p.) was administered 24 h after the last injection of MGS0039, fluvoxamine, or vehicle. Immunohistochemistry. All mice were killed 24 h after BrdU injection via intracardial perfusion with 4% paraformaldehyde under anesthetization with sodium pentobarbital (100 mg/kg, i.p.). After perfusion, all brains were post-fixed overnight in the identical fixative at 4 °C, followed by sequential incubation with 10%, 20%, and 30% sucrose, after which they were frozen and stored at )80 °C. Thirty micrometer-thick coronal brain sections were cut on a cryostat and mounted on slides. Every fifth section was collected between stereotaxic coordinates Bregma )1.2 to )3.0. Sections were first denatured in 50% formamide/2 saline sodium citrate (SSC) at 65 °C for 2 h and in 2 N HCl at 37 °C for 30 min. They were then incubated in 0.1 M boric acid (pH 8.5) for 10 min followed by incubation for 30 min in 0.3% H2 O2 /methanol to eliminate endogenous peroxidases. After blocking in phosphate-buffered saline (PBS) containing 1.5% normal horse serum and 0.05% Tween 20 (blocking buffer), cells were incubated with mouse monoclonal antibodies against BrdU (1:1000; Sigma) in a blocking buffer. Sections were then incubated for 1 h with secondary
antibodies (biotinylated horse anti-mouse IgG; Vector Laboratories, Burlingame, CA) followed by amplification with an avidin–biotin complex (ABC elite; Vector Laboratories), and cells were visualized with 0.05% of 3,30 -diaminobenzidine (Wako Pure Chemical Industries, Japan). Data quantification of BrdU-labeling and statistical analysis. All BrdU-labeled cells in the dentate gyrus (granule cell layer) and hilus were counted in each section. This count was performed bilaterally in 12 spanning across the rostro-caudal coordinates of the hippocampus of each mouse (Bregma )1.2 to )3.0). To distinguish single cells within clusters, all counts were performed at 400 magnification under a light microscope (ZEISS Axioplan2), omitting cells in the outermost focal plane. The total number of BrdU-labeled cells per section was determined and multiplied by 5 to obtain the total number of cells per dentate gyrus. Data were analyzed by one-way ANOVA and are presented as the mean value of BrdU-positive cells per section SEM. Significant differences between groups were determined by Dunnett’s test (p < 0:05).
Results and discussion BrdU-labeled nuclei of proliferating cells were frequently observed in clusters, localized in the subgranular zone of the hippocampus in all treated mice (Fig. 1). In chronic fluvoxamine-treated animals, the total number of BrdU-positive cells in the dentate gyrus and hilus was
Fig. 1. The number of BrdU-positive cells in the dentate gyrus increases following chronic MGS0039 administration. ICR mice received injections of BrdU (75 mg/kg) 24 h after the last fluvoxamine or MGS0039 treatment. Their coronal brain sections were pretreated for DNA denaturation and stained with antibodies against BrdU. Shown are representative photomicrographs from vehicle (A), 30 mg/kg fluvoxamine (B), 5 mg/kg (C), or 10 mg/kg (D) MGS0039.
T. Yoshimizu, S. Chaki / Biochemical and Biophysical Research Communications 315 (2004) 493–496
Fig. 2. Chronic MGS0039 administration resulted in cell proliferation in the adult mouse hippocampus. Mice received BrdU injections as described in Fig. 1. The results are expressed as means SEM number of BrdU-positive cells in the dentate gyrus (n ¼ 6 or 7). *p < 0:05 indicates a significant difference from the vehicle control.
2019 59 (mean SEM) compared with 1499 75 in the vehicle group, which corresponds to a significant increase (35%) (p < 0:001). Interestingly, chronic administration of MGS0039 at 5 and 10 mg/kg promoted progenitor cell proliferation in the dentate gyrus in the same way that chronic fluvoxamine treatment did. The 5 mg/kg dose of MGS0039 produced a significant increase in the number of BrdU-positive cells (2430 66, 62% increase; p < 0:0001). Compared to the lower dose, MGS0039 given at 10 mg/kg increased (2114 114, 41% increase; p < 0:0001) at the same level as chronic fluvoxamine treatment (Fig. 2). In the present study, we investigated the effects of MGS0039 on cell proliferation in the adult mouse hippocampus. As has been observed with various other antidepressants, including ECS, chronic MGS0039 treatment at a pharmacologically effective dose increased cell proliferation in the dentate gyrus (62% increase). It was reported that chronic administration of fluoxetine, a selective serotonin reuptake inhibitor, increases the number of BrdU-positive cells by about 60% in the dentate gyrus of mice, and approximately 70% of the cells differentiate into neurons [8]. Furthermore, Chen et al. [9] reported that the mood-stabilizing agent lithium has a similar effect on cell proliferation in the adult mouse hippocampus (25% increase), and that 66% of these cells express a neuronal marker protein [9]. Although we did not continue to monitor the fate of proliferating cells in this study after MGS0039 administration, most of the dividing cells may differentiate into mature neurons, as in the case of several antidepressant drug treatments. The mechanisms by which MGS0039 promotes cell proliferation in the dentate gyrus are unclear. With
495
regard to the relationship between the antidepressantlike effects and cell proliferation in the hippocampus, many interesting studies focusing on brain-derived neurotrophic factor (BDNF) have been reported. In the central nervous system, including the hippocampus, BDNF plays an important role in long-term survival, in the differentiation of neurons, and in synaptic development. Heterozygous BDNF knockout (BDNF +/)) mice, which have reduced BDNF protein in the hippocampus, exhibit decreased proliferation and survival of BrdU-labeled cells in the dentate gyrus, suggesting that BDNF is an important molecule in the regulation of cell proliferation, differentiation, and survival in the adult mouse hippocampus [10]. BDNF conditional deletion mice are hyperaggressive and hyperphagic, and have increased levels of anxiety [11]. Interestingly, chronic administration of antidepressant drugs or ECS completely blocked the down-regulation of BDNF and its receptor, trkB, in the hippocampus in response to restraint stress [12]. Based on the above evidences, BDNF is thought to be a critical molecule in exerting antidepressant-like activity [13]. Recent studies suggest that the activation of AMPA (a-amino-3-hydroxy-5-methyl4-isoxazolepropionate) receptors, ionotropic glutamate receptors, plays an important role in BDNF production and neurogenesis in the adult hippocampus. AMPA potentiator upregulates BDNF expression [14] and increases progenitor cell proliferation in the adult rat dentate gyrus [15]. Furthermore, AMPA potentiators exhibited antidepressant-like effects in forced swim and tail suspension tests [16], as did MGS0039. Group II mGluRs inhibit neurotransmitter release as autoreceptors located on glutamatergic terminals or as presynaptic heteroreceptors. Treatment with the mGluR2/3 antagonist in vivo produced a dose-dependent increase in extracellular glutamate [17]. Therefore, it is the intriguing hypothesis that the moderate elevation of glutamate level by MGS0039 may cause increased BDNF production and cell proliferation in the hippocampus via the activation of postsynaptic AMPA receptors. Indeed, in a preliminary study, we observed that the antidepressant-like effect of MGS0039 in an experimental animal model was attenuated by an AMPA receptor antagonist, NBQX, suggesting that AMPA receptor stimulation is involved in the antidepressant-like effects of MGS0039 (S. Chaki, in preparation). In this paper, we have shown that MGS0039, a group II mGluRs antagonist, can promote cell proliferation in the adult mouse hippocampus. Recent reports indicate that the behavioral effects of chronic antidepressants are mediated by the stimulation of neurogenesis in the hippocampus [8]. In light of studies by Santarelli et al. showing that hippocampal neurogenesis is required to exert antidepressant-like effects, the increase in cell proliferation in the hippocampus may be at least partially implicated in the pharmacological action of
496
T. Yoshimizu, S. Chaki / Biochemical and Biophysical Research Communications 315 (2004) 493–496
MGS0039. As above, the blockage of group II mGluRs may be an effective way of treating subjects suffering from clinical depression.
[9]
[10]
References [1] P.J. Conn, J.P. Pin, Pharmacology and functions of metabotropic glutamate receptors, Annu. Rev. Pharmacol. Toxicol. 37 (1997) 205–237. [2] S. Chaki, R. Yoshikawa, S. Hirota, T. Shimazaki, M. Maeda, N. Kawashima, T. Yoshimizu, A. Yasuhara, K. Sakagami, S. Okuyama, S. Nakanishi, A. Nakazato, MGS0039: a potent and selective group II metabotropic glutamate receptor antagonist with antidepressant-like activity, Neuropharmacology (in press). [3] J.D. Bremner, M. Narayan, E.R. Anderson, L.H. Staib, H.L. Miller, D.S. Charney, Hippocampal volume reduction in major depression, Am. J. Psychiatry 157 (2000) 115–118. [4] P.S. Eriksson, E. Perfilieva, T. Bjork-Eriksson, A.M. Alborn, C. Nordborg, D.A. Peterson, F.H. Gage, Neurogenesis in the adult human hippocampus, Nat. Med. 4 (1998) 1313–1317. [5] E. Gould, B.S. McEwen, P. Tanapat, L.A. Galea, E. Fuchs, Neurogenesis in the dentate gyrus of the adult tree shrew is regulated by psychosocial stress and NMDA receptor activation, J. Neurosci. 17 (1997) 2492–2498. [6] R.S. Duman, J. Malberg, S. Nakagawa, Regulation of adult neurogenesis by psychotropic drugs and stress, J. Pharmacol. Exp. Ther. 299 (2001) 401–407. [7] J.E. Malberg, A.J. Eisch, E.J. Nestler, R.S. Duman, Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus, J. Neurosci. 20 (2000) 9104–9110. [8] L. Santarelli, M. Saxe, C. Gross, A. Surget, F. Battaglia, S. Dulawa, N. Weisstaub, J. Lee, R. Duman, O. Arancio, C. Belzung, R. Hen, Requirement of hippocampal neurogenesis for
[11]
[12]
[13]
[14]
[15]
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