- Email: [email protected]
S.15.05 Genetic epidemiology in addiction research
S210
Convergent functional genomics in addiction research
S.15.03 From the gene to behaviour to more genes: the mu-opioid receptor as an example in addiction research B. Kieffer1 ° , K. Befort1 , C. Goeldner2 , J. Becker1 , J. Le Merrer1 . 1 Institut de G´ en´etique et de Biologie Mol´eculaire et Cellulaire, CNRS/INSERM/ULP, Illkirch Cedex, France; 2 Hoffman-La-Roche, Neuroscience, Basel, Switzerland Addiction is a relapsing brain disorder. Chronic exposure to drugs of abuse causes neural adaptations, which persist long after drug effects and acute withdrawal have dissipated. Prolonged abstinence from drugs of abuse involves dysphoria, high stress responsiveness and craving, however the neurobiology of drug abstinence is poorly understood. Here we present a set of studies, centered on the mu-opioid receptor, that have led us to discover a molecular signature of protracted abstinence. The mu-opioid receptor is essential to mediate drug reward, including reward produced by non-opioid drugs [1]. As recreational drug use gradually evolves towards drug abuse, mu-opioid receptors are activated continually, a phenomenon which likely represents a first line event in the development of addiction [1,2]. Using a genome-wide approach and mu-opioid receptor knockout mice, we identified a set of one hundred genes, whose expression is specifically regulated upon excessive mu-opioid receptor activation in the extended amygdala [3]. We then examined these mu-receptor dependent genes either immediately after chronic morphine, nicotine, D9-tetrahydrocannabinol or alcohol, or following four-week abstinence. At this time point, low sociability anddespair behavior have developed in a mouse model of opiate abstinence that we have recently developed. We found that gene regulations strikingly converge in the abstinent groups, and reveal a novel gene network previously unreported in addiction research. Regardless the drug, therefore, a specific regulation pattern emerges in the abstinent brain. This pattern may represent a novel unitary mechanism in addictive disorders, and provides novel gene target opportunities for addiction treatment.
Interestingly, all major addictive drugs trigger synaptic strengthening of excitatory synapses on dopamine neurons. Deletion of receptors necessary for such strengthening, selectively in the dopaminergic cells using the Cre/loxP system, leaves cocaine reward intact but affects the persistence of drug seeking. Thus, deletion of GluR1 blocks extinction and deletion of NMDA receptors blocks reinstatement of conditioned place preference to cocaine [1]. Further, reinstatement of self-administration is attenuated in mice in which NMDA receptors in dopaminergic cells were deleted in adulthood [2]. It was also shown that cocaineinduced synaptic plasticity in the dopaminergic neurons is critical for subsequent plasticity events in the nucleus accumbens. In this nucleus, metabotropic glutamate receptors are critical for important forms of synaptic plasticity. We used RNAi to knock down the metabotropic glutamate receptor 5 selectively in dopamine D1 receptor expressing neurons. Interestingly, these mice also showed attenuated cue-induced reinstatement of cocaine seeking [3]. Together with studies using tetracycline-induced expression systems and gene deletions by stereotactic injection of viral vectors, these data show that molecular changes in glutamatergic synapses in the mesolimbic dopaminergic system are critical underpinnings of relapse behavior. References [1] Engblom D, Bilbao A, Sanchis-Segura C, Dahan L, Perreau-Lenz S, Balland B, Rodriguez Parkitna J, Lujan R, Halbout B, Mameli M, Parlato R, Sprengel R, L¨uscher C, Sch¨utz G, Spanagel R. 2008 Glutamate receptors on dopaminergic neurons control the persistence of drug-seeking. Neuron, 59, 497–508. [2] Mameli M, Halbout B, Creton C, Engblom D, Rodriguez Parkitna J, Spanagel R, Luscher C. 2009 Cocaine-evoked synaptic plasticity: persistence in the VTA triggers adaptations in the NAc. Nat Neurosci, 12, 1036−41. [3] Novak M, Halbout B, O’Connor E, Rodriguez J, Su T, Chai M, Crombag H, Bilbao A, Spanagel R, Stephens DN, Sch¨utz G, Engblom D. 2010 Incentive learning underlying cocaine relapse requires mGluR5 receptors located on dopamine D1 receptor-expressing neurons. J Neurosci, 30, 11973−82.
S.15.05 Genetic epidemiology in addiction research References [1] Contet, C.S., Kieffer, B.L. and Befort, K., 2004. Mu opioid receptor: a gateway to drug addiction. Curr. Op. Neurobiol. 14, 1−9. [2] Befort, K, Ghate, A., Becker, J., Filliol, D., Darcq, E., Muller, J., Lardenois, A., Poch, O., Thibault, C., Dembele, D., Matifas, A. and Kieffer, B.L., 2008. Mu opioid receptor activation induces transcriptional plasticity in the extended amygdala. Eur. J. Neurosci. 27, 2973– 2984. [3] Goeldner, C., Lutz, P.E., Darcq, E., Halter, T., Clesse, D, Ouagazzal, A.M. and Kieffer, B.L., 2011. Impaired emotional behavior and serotoninergic function during protracted abstinence from chronic morphine. Biol Psychiatry 69, 236−44.
S.15.04 Reverse genetics in addiction research D. Engblom1 ° . 1 Link¨oping University, Department of Clinical and Experimental Medicine, Link¨oping, Sweden Repeated intake of addictive drugs induces aberrant reward-related learning processes that are critical factors behind the development and persistence of addiction. These learning processes are most likely underpinned by drug-induced molecular changes resulting in synaptic plasticity in different components of the mesolimbic dopaminergic system. We and others have used reverse genetics in the mouse to identify the role of such plastic changes.
M. Munaf`o1 ° . 1 University of Bristol, Department of Experimental Psychology, Bristol, United Kingdom It is well known from twin, family and adoption studies that addictive behaviours are under a degree of genetic influence, which is reflected in moderate to high heritability coefficients for these phenotypes. In the last twenty years, considerable effort has been invested in attempting to determine the molecular genetic basis of this influence, with mixed success. Historically, genetic studies of risk of substance use and addiction have included twin, family and adoption studies, and more recently candidate gene and linkage studies. These have yielded only modest advances in understanding. However, recent research, in particular using genomewide association, gene × environment and intermediate phenotype methods, has begun to identify genetic variants which appear to robustly associate with addiction phenotypes [1]. Genetic variants which influence the metabolism of specific substances, such as alcohol and nicotine, appear to influence the intake of those substances. A number of genes which encode drug targets also appear to influence drug-taking behaviour, such as the alpha-5 nicotinic acetylcholine receptor subunit, which has been shown to associate with heaviness of smoking. However, the proportion of phenotypic variance explained by these variants
S.16. Stress, neuroplasticity, cognition and psychopathology remains small. Potential reasons for this include imprecision in phenotype assessment. References [1] Ho MK, Goldman D, Heinz A, Kaprio J, Kreek MJ, Li MD, Munaf`o MR, Tyndale RF. 2010. Breaking barriers in the genomics and pharmacogenetics of drug addiction. Clin Pharmacol Ther; 88: 779–791.
S.16. Stress, neuroplasticity, cognition and psychopathology S.16.01 Structural plasticity in relation to stress and depression (models) P.J. Lucassen1 ° . 1 University of Amsterdam, SILS Center for Neuroscience, Amsterdam, The Netherlands Stress during development increases the risk for psychopathologies in adult life [1]. In rodents, stress during the first two weeks of life can have a lasting impact on the hypothalamo– pituitary–adrenal (HPA) axis and on structural plasticity [1,2]. Recent studies suggest that mild early life stress may program the hippocampus such that it is optimally adapted to stress later in life. Here we tested if ‘adaptive programming’ also holds under severely adverse early life conditions, i.e. 24 h maternal deprivation (MD). In young-adult male rats subjected to MD, adult hippocampal neurogenesis was reduced. Also, mature dentate granule cells showed alterations in the proximal part of the dendritic tree. Lasting structural changes due to MD were paralleled by impaired water maze acquisition, but did not affect long-term potentiation in the dentate gyrus. Importantly, in the presence of high stress levels of corticosterone, long term potentiation in the dentate gyrus of MD animals was facilitated. In addition to this, contextual learning in a high-stress environment was enhanced in MD rats [3]. These morphological, electrophysiological and behavioral observations show that even a severely adverse early life environment does not evolve into overall impaired hippocampal functionality later in life. Rather, adversity early in life can prepare the organism to perform optimally under emotional stress conditions in adulthood [3]. References [1] Lucassen PJ er al. Regulation of adult neurogenesis by stress, sleep disruption, exercise and inflammation: Implications for depression and antidepressant action. Eur Neuropsychopharmacol. 2010, 20(1), 1−17. [2] PJ Lucassen, MW Stumpel, Q Wang, E Aronica. Decreased numbers of progenitor cells but no response to antidepressant drugs in the hippocampus of elderly depressed patients. Neuropharmacology 2010, 58, 940–949. [3] Oomen CA et al. Severe early life stress hampers spatial learning and neurogenesis, but improves hippocampal synaptic plasticity and emotional learning under high-stress conditions in adulthood. J Neurosci. 2010, 30(19), 6635−45. Disclosure statement: Support by NWO, ISAO, EU, Corcept Inc, HersenStichting Nederland.
S.16.02 Stress effects on mood and sociability − cell adhesion molecules as molecular targets C. Sandi1 ° , M. Fantin1 , J. Grosse1 . 1 Ecole Polytechnique Federale de Lausanne (EPFL), Brain Mind Institute, Lausanne, Switzerland Chronic stress is a vulnerability factor for the development of a number of neuropsychiatric disorders, frequently characterized
S211
by both cognitive dysfunction and anti-social behaviors. Previous work showed that chronic stress evokes profound structural and molecular rearrangements in several brain regions. In the hippocampus, stress was found to induce profound structural remodelling of excitatory axo-spinous synaptic connectivity and a decrease in CA1 spine density, as well as atrophy of dendritic branching in CA3 pyramidal cells. Our previous work implicated the cell adhesion molecules NCAM and L1 in the effects of stress on hippocampal remodeling and cognitive deficits [1−3]. Our recent work has focused in the potential involvement of synapse specific cell adhesion molecules of the Nectin (Nectin-1 and Nectin-3) and Neuroligin (Neuroligin-1, Neuroligin-2 and Neuroligin-3) families in the social and cognitive abnormalities induced by chronic restraint stress in the rat. The impact of chronic stress on the expression levels of these molecules in different hippocampal areas correlated with alterations in social motivation and social cognition. By means of AAV-mediated Nectin overexpression in the hippocampus, the effects of stress in social interactions were counteracted without affecting general anxiety-like and exploratory behaviors or corticosterone responses to stress. These results strongly implicate these synapse specific molecules in stress-induced alterations in mood and sociability highlighting both Nectins and Neuroligins as potential targets for the development of novel treatments for stress-induced neuropsychiatric disorders. Furthermore, they emphasize a key role of the social domain in the mediation of stress-induced alterations in mood disorders. References [1] Sandi, C., 2004 Stress, cognitive impairment and cell adhesion molecules. Nat Rev Neurosci 5, 917–930. [2] Bisaz, R., Schachner, M., Sandi, C. 2011 Causal evidence for the involvement of the neural cell adhesion molecule, NCAM, in chronic stress-induced cognitive impairments. Hippocampus 21, 56−71. [3] Conboy, L., Tanrikut, C., Zoladz, P.R., Campbell, A.M., Park, C.R., Gabriel, C., Mocaer, E., Sandi, C., Diamond, D.M. 2009 The antidepressant agomelatine blocks the adverse effects of stress on memory and enables spatial learning to rapidly increase neural cell adhesion molecule (NCAM) expression in the hippocampus of rats. Int J Neuropsychopharmacol 12, 329–341.
S.16.03 Hippocampal volume decrease in depression: what can we learn from animal models? B. Cz´eh1 ° . 1 Max Planck Institute of Psychiatry, Molecular Neurobiology, Munich, Germany Structural imaging studies demonstrate smaller hippocampal volumes in depressed patients [1], but the exact underlying cellular mechanisms are still unclear [2]. The traditional glucocorticoid vulnerability hypothesis suggested neurotoxic effects. Since chronic stress paradigms in animal experiments can replicate the hippocampal shrinkage, thus these models are valuable to study the cellular changes. The prime candidates are neuronal death, suppressed adult neurogenesis and dendritic atrophy, all due to the elevated levels of glucocorticoids. However, histopathological studies failed to confirm massive neuronal loss in the hippocampi of depressed patients, or in animals subjected to chronic stress. Stress inhibits adult neurogenesis, this is well documented in animal experiments, but convincing clinical evidence on decreased neurogenesis in depressed individuals is still lacking. Notably, adult neurogenesis is a very rare event in adult humans, thus it is unlikely that it could significantly contribute to the hippocampal shrinkage. Dendritic atrophy is another well documented event in chronically stressed animals, and recent data suggest similar
Convergent functional genomics in addiction research
S.15.03 From the gene to behaviour to more genes: the mu-opioid receptor as an example in addiction research B. Kieffer1 ° , K. Befort1 , C. Goeldner2 , J. Becker1 , J. Le Merrer1 . 1 Institut de G´ en´etique et de Biologie Mol´eculaire et Cellulaire, CNRS/INSERM/ULP, Illkirch Cedex, France; 2 Hoffman-La-Roche, Neuroscience, Basel, Switzerland Addiction is a relapsing brain disorder. Chronic exposure to drugs of abuse causes neural adaptations, which persist long after drug effects and acute withdrawal have dissipated. Prolonged abstinence from drugs of abuse involves dysphoria, high stress responsiveness and craving, however the neurobiology of drug abstinence is poorly understood. Here we present a set of studies, centered on the mu-opioid receptor, that have led us to discover a molecular signature of protracted abstinence. The mu-opioid receptor is essential to mediate drug reward, including reward produced by non-opioid drugs [1]. As recreational drug use gradually evolves towards drug abuse, mu-opioid receptors are activated continually, a phenomenon which likely represents a first line event in the development of addiction [1,2]. Using a genome-wide approach and mu-opioid receptor knockout mice, we identified a set of one hundred genes, whose expression is specifically regulated upon excessive mu-opioid receptor activation in the extended amygdala [3]. We then examined these mu-receptor dependent genes either immediately after chronic morphine, nicotine, D9-tetrahydrocannabinol or alcohol, or following four-week abstinence. At this time point, low sociability anddespair behavior have developed in a mouse model of opiate abstinence that we have recently developed. We found that gene regulations strikingly converge in the abstinent groups, and reveal a novel gene network previously unreported in addiction research. Regardless the drug, therefore, a specific regulation pattern emerges in the abstinent brain. This pattern may represent a novel unitary mechanism in addictive disorders, and provides novel gene target opportunities for addiction treatment.
Interestingly, all major addictive drugs trigger synaptic strengthening of excitatory synapses on dopamine neurons. Deletion of receptors necessary for such strengthening, selectively in the dopaminergic cells using the Cre/loxP system, leaves cocaine reward intact but affects the persistence of drug seeking. Thus, deletion of GluR1 blocks extinction and deletion of NMDA receptors blocks reinstatement of conditioned place preference to cocaine [1]. Further, reinstatement of self-administration is attenuated in mice in which NMDA receptors in dopaminergic cells were deleted in adulthood [2]. It was also shown that cocaineinduced synaptic plasticity in the dopaminergic neurons is critical for subsequent plasticity events in the nucleus accumbens. In this nucleus, metabotropic glutamate receptors are critical for important forms of synaptic plasticity. We used RNAi to knock down the metabotropic glutamate receptor 5 selectively in dopamine D1 receptor expressing neurons. Interestingly, these mice also showed attenuated cue-induced reinstatement of cocaine seeking [3]. Together with studies using tetracycline-induced expression systems and gene deletions by stereotactic injection of viral vectors, these data show that molecular changes in glutamatergic synapses in the mesolimbic dopaminergic system are critical underpinnings of relapse behavior. References [1] Engblom D, Bilbao A, Sanchis-Segura C, Dahan L, Perreau-Lenz S, Balland B, Rodriguez Parkitna J, Lujan R, Halbout B, Mameli M, Parlato R, Sprengel R, L¨uscher C, Sch¨utz G, Spanagel R. 2008 Glutamate receptors on dopaminergic neurons control the persistence of drug-seeking. Neuron, 59, 497–508. [2] Mameli M, Halbout B, Creton C, Engblom D, Rodriguez Parkitna J, Spanagel R, Luscher C. 2009 Cocaine-evoked synaptic plasticity: persistence in the VTA triggers adaptations in the NAc. Nat Neurosci, 12, 1036−41. [3] Novak M, Halbout B, O’Connor E, Rodriguez J, Su T, Chai M, Crombag H, Bilbao A, Spanagel R, Stephens DN, Sch¨utz G, Engblom D. 2010 Incentive learning underlying cocaine relapse requires mGluR5 receptors located on dopamine D1 receptor-expressing neurons. J Neurosci, 30, 11973−82.
S.15.05 Genetic epidemiology in addiction research References [1] Contet, C.S., Kieffer, B.L. and Befort, K., 2004. Mu opioid receptor: a gateway to drug addiction. Curr. Op. Neurobiol. 14, 1−9. [2] Befort, K, Ghate, A., Becker, J., Filliol, D., Darcq, E., Muller, J., Lardenois, A., Poch, O., Thibault, C., Dembele, D., Matifas, A. and Kieffer, B.L., 2008. Mu opioid receptor activation induces transcriptional plasticity in the extended amygdala. Eur. J. Neurosci. 27, 2973– 2984. [3] Goeldner, C., Lutz, P.E., Darcq, E., Halter, T., Clesse, D, Ouagazzal, A.M. and Kieffer, B.L., 2011. Impaired emotional behavior and serotoninergic function during protracted abstinence from chronic morphine. Biol Psychiatry 69, 236−44.
S.15.04 Reverse genetics in addiction research D. Engblom1 ° . 1 Link¨oping University, Department of Clinical and Experimental Medicine, Link¨oping, Sweden Repeated intake of addictive drugs induces aberrant reward-related learning processes that are critical factors behind the development and persistence of addiction. These learning processes are most likely underpinned by drug-induced molecular changes resulting in synaptic plasticity in different components of the mesolimbic dopaminergic system. We and others have used reverse genetics in the mouse to identify the role of such plastic changes.
M. Munaf`o1 ° . 1 University of Bristol, Department of Experimental Psychology, Bristol, United Kingdom It is well known from twin, family and adoption studies that addictive behaviours are under a degree of genetic influence, which is reflected in moderate to high heritability coefficients for these phenotypes. In the last twenty years, considerable effort has been invested in attempting to determine the molecular genetic basis of this influence, with mixed success. Historically, genetic studies of risk of substance use and addiction have included twin, family and adoption studies, and more recently candidate gene and linkage studies. These have yielded only modest advances in understanding. However, recent research, in particular using genomewide association, gene × environment and intermediate phenotype methods, has begun to identify genetic variants which appear to robustly associate with addiction phenotypes [1]. Genetic variants which influence the metabolism of specific substances, such as alcohol and nicotine, appear to influence the intake of those substances. A number of genes which encode drug targets also appear to influence drug-taking behaviour, such as the alpha-5 nicotinic acetylcholine receptor subunit, which has been shown to associate with heaviness of smoking. However, the proportion of phenotypic variance explained by these variants
S.16. Stress, neuroplasticity, cognition and psychopathology remains small. Potential reasons for this include imprecision in phenotype assessment. References [1] Ho MK, Goldman D, Heinz A, Kaprio J, Kreek MJ, Li MD, Munaf`o MR, Tyndale RF. 2010. Breaking barriers in the genomics and pharmacogenetics of drug addiction. Clin Pharmacol Ther; 88: 779–791.
S.16. Stress, neuroplasticity, cognition and psychopathology S.16.01 Structural plasticity in relation to stress and depression (models) P.J. Lucassen1 ° . 1 University of Amsterdam, SILS Center for Neuroscience, Amsterdam, The Netherlands Stress during development increases the risk for psychopathologies in adult life [1]. In rodents, stress during the first two weeks of life can have a lasting impact on the hypothalamo– pituitary–adrenal (HPA) axis and on structural plasticity [1,2]. Recent studies suggest that mild early life stress may program the hippocampus such that it is optimally adapted to stress later in life. Here we tested if ‘adaptive programming’ also holds under severely adverse early life conditions, i.e. 24 h maternal deprivation (MD). In young-adult male rats subjected to MD, adult hippocampal neurogenesis was reduced. Also, mature dentate granule cells showed alterations in the proximal part of the dendritic tree. Lasting structural changes due to MD were paralleled by impaired water maze acquisition, but did not affect long-term potentiation in the dentate gyrus. Importantly, in the presence of high stress levels of corticosterone, long term potentiation in the dentate gyrus of MD animals was facilitated. In addition to this, contextual learning in a high-stress environment was enhanced in MD rats [3]. These morphological, electrophysiological and behavioral observations show that even a severely adverse early life environment does not evolve into overall impaired hippocampal functionality later in life. Rather, adversity early in life can prepare the organism to perform optimally under emotional stress conditions in adulthood [3]. References [1] Lucassen PJ er al. Regulation of adult neurogenesis by stress, sleep disruption, exercise and inflammation: Implications for depression and antidepressant action. Eur Neuropsychopharmacol. 2010, 20(1), 1−17. [2] PJ Lucassen, MW Stumpel, Q Wang, E Aronica. Decreased numbers of progenitor cells but no response to antidepressant drugs in the hippocampus of elderly depressed patients. Neuropharmacology 2010, 58, 940–949. [3] Oomen CA et al. Severe early life stress hampers spatial learning and neurogenesis, but improves hippocampal synaptic plasticity and emotional learning under high-stress conditions in adulthood. J Neurosci. 2010, 30(19), 6635−45. Disclosure statement: Support by NWO, ISAO, EU, Corcept Inc, HersenStichting Nederland.
S.16.02 Stress effects on mood and sociability − cell adhesion molecules as molecular targets C. Sandi1 ° , M. Fantin1 , J. Grosse1 . 1 Ecole Polytechnique Federale de Lausanne (EPFL), Brain Mind Institute, Lausanne, Switzerland Chronic stress is a vulnerability factor for the development of a number of neuropsychiatric disorders, frequently characterized
S211
by both cognitive dysfunction and anti-social behaviors. Previous work showed that chronic stress evokes profound structural and molecular rearrangements in several brain regions. In the hippocampus, stress was found to induce profound structural remodelling of excitatory axo-spinous synaptic connectivity and a decrease in CA1 spine density, as well as atrophy of dendritic branching in CA3 pyramidal cells. Our previous work implicated the cell adhesion molecules NCAM and L1 in the effects of stress on hippocampal remodeling and cognitive deficits [1−3]. Our recent work has focused in the potential involvement of synapse specific cell adhesion molecules of the Nectin (Nectin-1 and Nectin-3) and Neuroligin (Neuroligin-1, Neuroligin-2 and Neuroligin-3) families in the social and cognitive abnormalities induced by chronic restraint stress in the rat. The impact of chronic stress on the expression levels of these molecules in different hippocampal areas correlated with alterations in social motivation and social cognition. By means of AAV-mediated Nectin overexpression in the hippocampus, the effects of stress in social interactions were counteracted without affecting general anxiety-like and exploratory behaviors or corticosterone responses to stress. These results strongly implicate these synapse specific molecules in stress-induced alterations in mood and sociability highlighting both Nectins and Neuroligins as potential targets for the development of novel treatments for stress-induced neuropsychiatric disorders. Furthermore, they emphasize a key role of the social domain in the mediation of stress-induced alterations in mood disorders. References [1] Sandi, C., 2004 Stress, cognitive impairment and cell adhesion molecules. Nat Rev Neurosci 5, 917–930. [2] Bisaz, R., Schachner, M., Sandi, C. 2011 Causal evidence for the involvement of the neural cell adhesion molecule, NCAM, in chronic stress-induced cognitive impairments. Hippocampus 21, 56−71. [3] Conboy, L., Tanrikut, C., Zoladz, P.R., Campbell, A.M., Park, C.R., Gabriel, C., Mocaer, E., Sandi, C., Diamond, D.M. 2009 The antidepressant agomelatine blocks the adverse effects of stress on memory and enables spatial learning to rapidly increase neural cell adhesion molecule (NCAM) expression in the hippocampus of rats. Int J Neuropsychopharmacol 12, 329–341.
S.16.03 Hippocampal volume decrease in depression: what can we learn from animal models? B. Cz´eh1 ° . 1 Max Planck Institute of Psychiatry, Molecular Neurobiology, Munich, Germany Structural imaging studies demonstrate smaller hippocampal volumes in depressed patients [1], but the exact underlying cellular mechanisms are still unclear [2]. The traditional glucocorticoid vulnerability hypothesis suggested neurotoxic effects. Since chronic stress paradigms in animal experiments can replicate the hippocampal shrinkage, thus these models are valuable to study the cellular changes. The prime candidates are neuronal death, suppressed adult neurogenesis and dendritic atrophy, all due to the elevated levels of glucocorticoids. However, histopathological studies failed to confirm massive neuronal loss in the hippocampi of depressed patients, or in animals subjected to chronic stress. Stress inhibits adult neurogenesis, this is well documented in animal experiments, but convincing clinical evidence on decreased neurogenesis in depressed individuals is still lacking. Notably, adult neurogenesis is a very rare event in adult humans, thus it is unlikely that it could significantly contribute to the hippocampal shrinkage. Dendritic atrophy is another well documented event in chronically stressed animals, and recent data suggest similar