Torpor induces reversible tau hyperphosphorylation and accumulation in mice expressing human tau.

Tau protein hyperphosphorylation and aggregation are key pathological events in neurodegenerative tauopathies such as Alzheimer's disease. Interestingly, seasonal hibernators show extensive tau hyperphosphorylation during torpor, i.e., the hypothermic and hypometabolic state of hibernation, which is completely reversed during arousal. Torpor-associated mechanisms that reverse tau hyperphosphorylation may be of therapeutic relevance, however, it is currently not known to what extent they apply to human tau. Here we addressed this issue using daily torpor in wildtype mice that express mouse tau (mtau) and in mice that lack mtau expression and instead express human tau (htau). AT8, AT100 and Ser396 immunoblotting and immunohistochemistry were used to assess tau (hyper)phosphorylation at clinically relevant phosphorylation sites. We found that torpor robustly and reversibly increases the levels of phosphorylated tau in both mtau and htau mice. Immunohistochemistry revealed four brain areas that show prominent tau phosphorylation: the hippocampus, posterior parietal cortex, piriform cortex and cortical amygdala. Whereas wildtype mice primarily showed increased levels of diffusely organized hyperphosphorylated tau during torpor, htau mice contained clear somato-dendritic accumulations of AT8 reactivity resembling tau pre-tangles as observed in the Alzheimer brain. Interestingly, AT8-positive accumulations disappeared upon arousal, and tau phosphorylation levels at 24 h after arousal were lower than observed at baseline, suggesting a beneficial effect of torpor-arousal cycles on preexisting hyperphosphorylated tau. In conclusion, daily torpor in mice offers a quick and standardized method to study tau phosphorylation, accumulation and clearance in mouse models relevant for neurodegeneration, as well as opportunities to discover new targets for the treatment of human tauopathies.

Fractionated brain X-irradiation profoundly reduces hippocampal immature neuron numbers without affecting spontaneous behavior in mice.

Whole brain radiotherapy (WBRT) is used to improve tumor control in patients with primary brain tumors, or brain metastasis from various primary tumors to improve tumor control. However, WBRT can lead to cognitive decline in patients. We assessed whether fractionated WBRT (fWBRT) affects spontaneous behavior of mice in automated home cages and cognition (spatial memory) using the Barnes maze. Male C57Bl/6j mice received bi-lateral fWBRT at a dosage of 4 Gy/day on 5 consecutive days. In line with previous reports, immunohistochemical analysis of doublecortin positive cells in the dentate gyrus showed a profound reduction in immature neurons 4 weeks after fWBRT. Surprisingly, spontaneous behavior as measured in automated home cages was not affected. Moreover, learning and memory measured with Barnes maze, was also not affected 4-6 weeks after fWBRT. At 10-11 weeks after fWBRT a significant difference in escape latency during the learning phase, but not in the probe test of the Barnes maze was observed. In conclusion, although we confirmed the serious adverse effect of fWBRT on neurogenesis 4 weeks after fWBRT, we did not find similar profound effects on spontaneous behavior in the automated home cage nor on learning abilities as measured by the Barnes maze. The relationship between the neurobiological effects of fWBRT and cognition seems more complex than often assumed and the choice of animal model, cognitive tasks, neurobiological parameters, and experimental set-up might be important factors in these types of experiments.

Dysregulation of synaptic and developmental transcriptomic/proteomic profiles upon depletion of MUNC18-1.

Absence of presynaptic protein MUNC18-1 (gene: Stxbp1) leads to neuronal cell death at an immature stage before synapse formation. Here, we performed transcriptomic and proteomic profiling of immature Stxbp1 knockout (KO) cells to discover which cellular processes depend on MUNC18-1. Hippocampi of Stxbp1 KO mice showed cell-type specific dysregulation of 2123 transcripts primarily related to synaptic transmission and immune response. To further investigate direct, neuron-specific effects of MUNC18-1 depletion, a proteomic screen was performed on murine neuronal cultures at two developmental timepoints prior to onset of neuron degeneration. 399 proteins were differentially expressed, which were primarily involved in synaptic function (especially synaptic vesicle exocytosis) and neuron development. We further show that many of the downregulated proteins upon loss of MUNC18-1 are normally upregulated during this developmental stage. Thus, absence of MUNC18-1 extensively dysregulates the transcriptome and proteome, primarily affecting synaptic and developmental profiles. Lack of synaptic activity is unlikely to underlie these effects, as the changes were observed in immature neurons without functional synapses, and minimal overlap was found to activity-dependent proteins. We hypothesize that presence of MUNC18-1 is essential to advance neuron development, serving as a 'checkpoint' for neurons to initiate cell death in its absence.Significance StatementPresynaptic protein MUNC18-1 is essential for neuronal functioning. Pathogenic variants in its gene, STXBP1, are among the most common found in patients with developmental delay and epilepsy. To discern the pathogenesis in these patients, a thorough understanding of MUNC18-1's function in neurons is required. Here, we show that loss of MUNC18-1 results in extensive dysregulation of synaptic and developmental proteins in immature neurons before synapse formation. Many of the downregulated proteins are normally upregulated during this developmental stage. This indicates that MUNC18-1 is a critical regulator of neuronal development, which could play an important role in the pathogenesis of STXBP1 variant carriers.

Stitching the synapse: Cross-linking mass spectrometry into resolving synaptic protein interactions.

Synaptic transmission is the predominant form of communication in the brain. It requires functionally specialized molecular machineries constituted by thousands of interacting synaptic proteins. Here, we made use of recent advances in cross-linking mass spectrometry (XL-MS) in combination with biochemical and computational approaches to reveal the architecture and assembly of synaptic protein complexes from mouse brain hippocampus and cerebellum. We obtained 11,999 unique lysine-lysine cross-links, comprising connections within and between 2362 proteins. This extensive collection was the basis to identify novel protein partners, to model protein conformational dynamics, and to delineate within and between protein interactions of main synaptic constituents, such as Camk2, the AMPA-type glutamate receptor, and associated proteins. Using XL-MS, we generated a protein interaction resource that we made easily accessible via a web-based platform (http://xlink.cncr.nl) to provide new entries into exploration of all protein interactions identified.

A Potential Role for the STXBP5-AS1 Gene in Adult ADHD Symptoms.

We aimed to detect Attention-deficit/hyperactivity (ADHD) risk-conferring genes in adults. In children, ADHD is characterized by age-inappropriate levels of inattention and/or hyperactivity-impulsivity and may persists into adulthood. Childhood and adulthood ADHD are heritable, and are thought to represent the clinical extreme of a continuous distribution of ADHD symptoms in the general population. We aimed to leverage the power of studies of quantitative ADHD symptoms in adults who were genotyped. Within the SAGA (Study of ADHD trait genetics in adults) consortium, we estimated the single nucleotide polymorphism (SNP)-based heritability of quantitative self-reported ADHD symptoms and carried out a genome-wide association meta-analysis in nine adult population-based and case-only cohorts of adults. A total of n = 14,689 individuals were included. In two of the SAGA cohorts we found a significant SNP-based heritability for self-rated ADHD symptom scores of respectively 15% (n = 3656) and 30% (n = 1841). The top hit of the genome-wide meta-analysis (SNP rs12661753; p-value = 3.02 × 10-7) was present in the long non-coding RNA gene STXBP5-AS1. This association was also observed in a meta-analysis of childhood ADHD symptom scores in eight population-based pediatric cohorts from the Early Genetics and Lifecourse Epidemiology (EAGLE) ADHD consortium (n = 14,776). Genome-wide meta-analysis of the SAGA and EAGLE data (n = 29,465) increased the strength of the association with the SNP rs12661753. In human HEK293 cells, expression of STXBP5-AS1 enhanced the expression of a reporter construct of STXBP5, a gene known to be involved in "SNAP" (Soluble NSF attachment protein) Receptor" (SNARE) complex formation. In mouse strains featuring different levels of impulsivity, transcript levels in the prefrontal cortex of the mouse ortholog Gm28905 strongly correlated negatively with motor impulsivity as measured in the five choice serial reaction time task (r2 = - 0.61; p = 0.004). Our results are consistent with an effect of the STXBP5-AS1 gene on ADHD symptom scores distribution and point to a possible biological mechanism, other than antisense RNA inhibition, involved in ADHD-related impulsivity levels.

Author Correction: Susceptible genes and disease mechanisms identified in frontotemporal dementia and frontotemporal dementia with Amyotrophic Lateral Sclerosis by DNA-methylation and GWAS.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Susceptible genes and disease mechanisms identified in frontotemporal dementia and frontotemporal dementia with Amyotrophic Lateral Sclerosis by DNA-methylation and GWAS.

Frontotemporal dementia (FTD) is a neurodegenerative disorder predominantly affecting the frontal and temporal lobes. Genome-wide association studies (GWAS) on FTD identified only a few risk loci. One of the possible explanations is that FTD is clinically, pathologically, and genetically heterogeneous. An important open question is to what extent epigenetic factors contribute to FTD and whether these factors vary between FTD clinical subgroup. We compared the DNA-methylation levels of FTD cases (n = 128), and of FTD cases with Amyotrophic Lateral Sclerosis (FTD-ALS; n = 7) to those of unaffected controls (n = 193), which resulted in 14 and 224 candidate genes, respectively. Cluster analysis revealed significant class separation of FTD-ALS from controls. We could further specify genes with increased susceptibility for abnormal gene-transcript behavior by jointly analyzing DNA-methylation levels with the presence of mutations in a GWAS FTD-cohort. For FTD-ALS, this resulted in 9 potential candidate genes, whereas for FTD we detected 1 candidate gene (ELP2). Independent validation-sets confirmed the genes DLG1, METTL7A, KIAA1147, IGHMBP2, PCNX, UBTD2, WDR35, and ELP2/SLC39A6 among others. We could furthermore demonstrate that genes harboring mutations and/or displaying differential DNA-methylation, are involved in common pathways, and may therefore be critical for neurodegeneration in both FTD and FTD-ALS.

Temporal profiling of depression vulnerability in a preclinical model of sustained depression.

Major Depression is a prevalent mental disorder that is characterized by negative mood and reduced motivation, and frequently results in social withdrawal and memory-related deficits. Repeated stressors, such as adverse life events, increase the risk for development of the disorder. Consequently, individual variability in stress response greatly weighs on depression-vulnerability and -resilience. Here, we employed the social defeat-induced persistent stress (SDPS) paradigm to identify depression-prone individuals and to examine the temporal development of depression in the months following exposure to brief defeat stress. Male Wistar rats were socially defeated (5 defeat episodes) and single-housed for a prolonged period of time (~24 weeks). We assessed the emergence of a sustained depressive-like state by repeatedly evaluating social motivation (social approach avoidance) and spatial memory (object place recognition) in SDPS rats during the isolation period. Individual variability in the effects of SDPS yielded two extreme subpopulations: an SDPS-prone group that showed gradual affective and cognitive deterioration in terms of social approach and memory retention, and a SDPS-resilient group that did not develop this phenotype. Notably, in SDPS-prone individuals, the affective deficits preceded later cognitive impairments, providing a novel temporal profile of the development of pathology in this preclinical model of sustained depression.

Cognitive flexibility deficits in a mouse model for the absence of full-length dystrophin.

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disorder, caused by mutations in the DMD gene and the resulting lack of dystrophin. The DMD gene has seven promoters, giving rise to multiple full-length and shorter isoforms. Besides the expression of dystrophin in muscles, the majority of dystrophin isoforms is expressed in brain and dystrophinopathy can lead to cognitive deficits, including intellectual impairments and deficits in executive function. In contrast to the muscle pathology, the impact of the lack of dystrophin on the brain is not very well studied. Here, we study the behavioral consequences of a lack of full-length dystrophin isoforms in mdx mice, particularly with regard to domains of executive functions and anxiety. We observed a deficit in cognitive flexibility in mdx mice in the absence of motor dysfunction or general learning impairments using two independent behavioral tests. In addition, increased anxiety was observed, but its expression depended on the context. Overall, these results suggest that the absence of full-length dystrophin in mice has specific behavioral effects that compare well to deficits observed in DMD patients.

Progression and recovery of Parkinsonism in a chronic progressive MPTP-induction model in the marmoset without persistent molecular and cellular damage.

Chronic exposure to low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in marmoset monkeys was used to model the prodromal stage of Parkinson's disease (PD), and to investigate mechanisms underlying disease progression and recovery. Marmosets were subcutaneously injected with MPTP for a period of 12weeks, 0.5mg/kg once per week, and clinical signs of Parkinsonism, motor- and non-motor behaviors were recorded before, during and after exposure. In addition, postmortem immunohistochemistry and proteomics analysis were performed. MPTP-induced parkinsonian clinical symptoms increased in severity during exposure, and recovered after MPTP administration was ended. Postmortem analyses, after the recovery period, revealed no alteration of the number and sizes of tyrosine hydroxylase (TH)-positive dopamine (DA) neurons in the substantia nigra. Also levels of TH in putamen and caudate nucleus were unaltered, no differences were observed in DA, serotonin or nor-adrenalin levels in the caudate nucleus, and proteomics analysis revealed no global changes in protein expression in these brain areas between treatment groups. Our findings indicate that parkinsonian symptoms can occur without detectable damage at the cellular or molecular level. Moreover, we show that parkinsonian symptoms may be reversible when diagnosed and treated early.

Neurobiological changes by cytotoxic agents in mice.

Cognitive deficit is a frequently reported side-effect of adjuvant chemotherapy. A large number of animal studies has been performed to examine the neurobiological mechanisms underlying this phenomenon, however, definite conclusions from these studies are restricted due to differences in experimental set-up. We systematically investigated the effects of 6 cytotoxic agents on various neurobiological parameters. C57Bl/6J mice were treated with cyclophosphamide, docetaxel, doxorubicin, 5-fluorouracil, methotrexate, or topotecan. The animals were sacrificed 3 or 15 weeks after treatment and the effect on neurogenesis, blood vessel density, and neuroinflammation was analyzed using immunohistochemistry. None of the cytostatic agents tested affected neurogenesis (cell survival or cell proliferation). Blood vessel density was increased in the hippocampus and prefrontal cortex 3 weeks after treatment with docetaxel and doxorubicin compared with control animals. A decrease in the number of microglial cells was observed in the prefrontal cortex after treatment with cyclophosphamide, docetaxel, 5-FU, and topotecan compared with control mice. The observed decrease in microglia cells is indicative of inflammation that occurred after treatment. Overall, the magnitude of the effects was relatively modest. Therefore, we conducted a similar study with topotecan in Abcg2;Abcb1a/b knock out and wildtype FVB mice. Animals were sacrificed 3 weeks after treatment and no notable effect was seen in hippocampal cell differentiation (DCX), microglia activation, or blood vessel density. Perhaps the FVB strain is more resistant to the neurotoxic effects of topotecan which makes this not the correct model to study the mechanism of chemotherapy-induced cognitive impairment.

Neurobeachin Regulates Glutamate- and GABA-Receptor Targeting to Synapses via Distinct Pathways.

Neurotransmission and synaptic strength depend on expression of post-synaptic receptors on the cell surface. Post-translational modification of receptors, trafficking to the synapse through the secretory pathway, and subsequent insertion into the synapse involves interaction of the receptor with A-kinase anchor proteins (AKAPs) and scaffolding proteins. Neurobeachin (Nbea), a brain specific AKAP, is required for synaptic surface expression of both glutamate and GABA receptors. Here, we investigated the role of Nbea-dependent targeting of postsynaptic receptors by studying Nbea interaction with synapse-associated protein 102 (SAP102/Dlg3) and protein kinase A subunit II (PKA II). A Nbea mutant lacking the PKA binding domain showed a similar distribution as wild-type Nbea in Nbea null neurons and partially restored GABA receptor surface expression. To understand the relevance of Nbea interaction with SAP102, we analysed SAP102 null mutant mice. Nbea levels were reduced by ~80% in SAP102 null mice, but glutamatergic receptor expression was normal. A single-point mutation in the pleckstrin homology domain of Nbea (E2218R) resulted in loss of binding with SAP102. When expressed in Nbea null neurons, this mutant fully restored GABA receptor surface expression, but not glutamate receptor expression. Our results suggest that the PKA-binding domain is not essential for Nbea's role in receptor targeting and that Nbea targets glutamate and GABA receptors to the synapse via distinct molecular pathways by interacting with specific effector proteins.

Functional characterization of the PCLO p.Ser4814Ala variant associated with major depressive disorder reveals cellular but not behavioral differences.

Genome-wide association studies have suggested a role for a genetic variation in the presynaptic gene PCLO in major depressive disorder (MDD). As with many complex traits, the PCLO variant has a small contribution to the overall heritability and the association does not always replicate. One variant (rs2522833, p.Ser4814Ala) is of particular interest given that it is a common, nonsynonymous exon variant near a calcium-sensing part of PCLO. It has been suggested that the molecular effects of such variations penetrate to a variable extent in the population due to phenotypic and genotypic heterogeneity at the population level. More robust effects may be exposed by studying such variations in isolation, in a more homogeneous context. We tested this idea by modeling PCLO variation in a mouse knock-in model expressing the Pclo(SA)(/)(SA) variant. In the highly homogeneous background of inbred mice, two functional effects of the SA-variation were observed at the cellular level: increased synaptic Piccolo levels, and 30% increased excitatory synaptic transmission in cultured neurons. Other aspects of Piccolo function were unaltered: calcium-dependent phospholipid binding, synapse formation in vitro, and synaptic accumulation of synaptic vesicles. Moreover, anxiety, cognition and depressive-like behavior were normal in Pclo(SA)(/)(SA) mice. We conclude that the PCLO p.Ser4814Ala missense variant produces mild cellular phenotypes, which do not translate into behavioral phenotypes. We propose a model explaining how (subtle) cellular phenotypes do not penetrate to the mouse behavioral level but, due to genetic and phenotypic heterogeneity and non-linearity, can produce association signals in human population studies.

HCN channels are a novel therapeutic target for cognitive dysfunction in Neurofibromatosis type 1.

Cognitive impairments are a major clinical feature of the common neurogenetic disease neurofibromatosis type 1 (NF1). Previous studies have demonstrated that increased neuronal inhibition underlies the learning deficits in NF1, however, the molecular mechanism underlying this cell-type specificity has remained unknown. Here, we identify an interneuron-specific attenuation of hyperpolarization-activated cyclic nucleotide-gated (HCN) current as the cause for increased inhibition in Nf1 mutants. Mechanistically, we demonstrate that HCN1 is a novel NF1-interacting protein for which loss of NF1 results in a concomitant increase of interneuron excitability. Furthermore, the HCN channel agonist lamotrigine rescued the electrophysiological and cognitive deficits in two independent Nf1 mouse models, thereby establishing the importance of HCN channel dysfunction in NF1. Together, our results provide detailed mechanistic insights into the pathophysiology of NF1-associated cognitive defects, and identify a novel target for clinical drug development.

Cognitive impact of cytotoxic agents in mice.

RATIONALE AND OBJECTIVES: Adjuvant chemotherapy is associated with changes in cognition in a subgroup of cancer patients. Chemotherapy is generally given as a combination of cytotoxic agents, which makes it hard to define the agent responsible for these observed changes. Literature on animal experiments has been difficult to interpret due to variance in experimental setup. METHODS: We examined the effects of cytotoxic agents administered separately on various cognitive measures in a standardized animal model. Male C57Bl/6 mice received cyclophosphamide, docetaxel, doxorubicin, 5-fluorouracil, methotrexate, or topotecan. These agents represent different compound classes based on their working mechanism and are frequently prescribed in the clinic. A control group received saline. Behavioral testing started 2 or 15 weeks after treatment and included testing general measures of behavior and cognitive task performance: spontaneous behavior in an automated home cage, open field, novel location recognition (NLR), novel object recognition (NOR), Barnes maze, contextual fear conditioning, and a simple choice reaction time task (SCRTT). RESULTS: Cyclophosphamide, docetaxel, and doxorubicin administration affected spontaneous activity in the automated home cage. All cytotoxic agents affected memory (NLR and/or NOR). Spatial memory measured in the Barnes maze was affected after administration with doxorubicin, 5-fluorouracil, and topotecan. Decreased inhibition in the SCRTT was observed after treatment with cyclophosphamide, docetaxel, and topotecan. CONCLUSIONS: Our data show that, in mice, a single treatment with a cytotoxic agent causes cognitive impairment. Not all cytotoxic agents affected the same cognitive domains, which might be explained by differences in working mechanisms of the various agents.

High-throughput phenotyping of avoidance learning in mice discriminates different genotypes and identifies a novel gene.

Recognizing and avoiding aversive situations are central aspects of mammalian cognition. These abilities are essential for health and survival and are expected to have a prominent genetic basis. We modeled these abilities in eight common mouse inbred strains covering ∼75% of the species' natural variation and in gene-trap mice (>2000 mice), using an unsupervised, automated assay with an instrumented home cage (PhenoTyper) containing a shelter with two entrances. Mice visited this shelter for 20-1200 times/24 h and 71% of all mice developed a significant and often strong preference for one entrance. Subsequently, a mild aversive stimulus (shelter illumination) was automatically delivered when mice used their preferred entrance. Different genotypes developed different coping strategies. Firstly, the number of entries via the preferred entrance decreased in DBA/2J, C57BL/6J and 129S1/SvImJ, indicating that these genotypes associated one specific entrance with the aversive stimulus. Secondly, mice started sleeping outside (C57BL/6J, DBA/2J), indicating they associated the shelter, in general, with the aversive stimulus. Some mice showed no evidence for an association between the entrance and the aversive light, but did show markedly shorter shelter residence times in response to illumination, indicating they did perceive illumination as aversive. Finally, using this assay, we screened 43 different mutants, which yielded a novel gene, specc1/cytospinB. This mutant showed profound and specific delay in avoidance learning. Together, these data suggest that different genotypes express distinct learning and/or memory of associations between shelter entrance and aversive stimuli, and that specc1/cytospinB is involved in this aspect of cognition.

Independent genetic loci for sensorimotor gating and attentional performance in BXD recombinant inbred strains.

A startle reflex in response to an intense acoustic stimulus is inhibited when a barely detectable pulse precedes the startle stimulus by 30-500 ms. It has been theorized that this phenomenon, named prepulse inhibition (PPI) of a startle response, is an automatic early-stage gating process contributing to the ability to focus attention. Deficits in PPI may therefore contribute to deficits in attentional processing. Both deficits are observed in schizophrenia spectrum disorders. Here, we investigated whether there is overlap in genetic control of PPI and attentional processing phenotypes in the panel of BXD recombinant inbred strains of mice. Using an individually titrated prepulse intensity to handle differences in perceived prepulse intensities among strains, we identified a significant quantitative trait locus (QTL) for PPI at the mid-distal end of chromosome 17. A measure of attentional processing in the five-choice serial reaction time task, response variability, mapped to a different locus on proximal-mid chromosome 16. In addition, the estimated genetic and environmental correlations between PPI and several attentional phenotypes were low and not significant. Taken together, the observation of separate genetic loci for PPI and attention and the absence of genetic and environmental correlations indicate that differences in sensorimotor gating do not contribute to differences in attentional performance. Therefore, it is worth pursuing the causative genes residing in both attention and PPI QTL, as these may contribute to separate molecular pathways implicated in neuropsychiatric diseases, such as schizophrenia.

Functional gene group analysis identifies synaptic gene groups as risk factor for schizophrenia.

Schizophrenia is a highly heritable disorder with a polygenic pattern of inheritance and a population prevalence of ~1%. Previous studies have implicated synaptic dysfunction in schizophrenia. We tested the accumulated association of genetic variants in expert-curated synaptic gene groups with schizophrenia in 4673 cases and 4965 healthy controls, using functional gene group analysis. Identifying groups of genes with similar cellular function rather than genes in isolation may have clinical implications for finding additional drug targets. We found that a group of 1026 synaptic genes was significantly associated with the risk of schizophrenia (P=7.6 × 10(-11)) and more strongly associated than 100 randomly drawn, matched control groups of genetic variants (P<0.01). Subsequent analysis of synaptic subgroups suggested that the strongest association signals are derived from three synaptic gene groups: intracellular signal transduction (P=2.0 × 10(-4)), excitability (P=9.0 × 10(-4)) and cell adhesion and trans-synaptic signaling (P=2.4 × 10(-3)). These results are consistent with a role of synaptic dysfunction in schizophrenia and imply that impaired intracellular signal transduction in synapses, synaptic excitability and cell adhesion and trans-synaptic signaling play a role in the pathology of schizophrenia.

Reduction in hippocampal neurogenesis after social defeat is long-lasting and responsive to late antidepressant treatment.

Major depressive disorder is a chronic disabling disease, often triggered and exacerbated by stressors of a social nature. Hippocampal volume reductions have been reported in depressed patients. In support of the neurogenesis theory of depression, in several stress-based animal models of depression, adult hippocampal neurogenesis was reduced and subsequently rescued by parallel antidepressant treatment. Here, we investigated whether repeated social defeat and subsequent individual housing for 3 months induces long-lasting changes in adult hippocampal neurogenesis in rats, and whether these can be normalized by late antidepressant treatment, as would match human depression. Neurogenesis was analysed by stereological quantification of the number of immature doublecortin (DCX)-immunopositive cells, in particular young (class I) and more mature (class II) DCX(+) cells, to distinguish differential effects of stress or drug treatment on these subpopulations. Using this social defeat paradigm, the total DCX(+) cell number was significantly reduced. This was most profound for older (class II) DCX(+) cells with long apical dendrites, whereas younger, class I cells remained unaffected. Treatment with the broad-acting tricyclic antidepressant imipramine, only during the last 3 weeks of the 3-month period after social defeat, completely restored the reduction in neurogenesis by increasing both class I and II DCX(+) cell populations. We conclude that despite the lack of elevated corticosterone plasma levels, neurogenesis is affected in a lasting manner by a decline in a distinct neuronal population of more mature newborn cells. Thus, the neurogenic deficit induced by this social defeat paradigm is long-lasting, but can still be normalized by late imipramine treatment.