Elsevier

Journal of Psychiatric Research

Volume 56, September 2014, Pages 106-111
Journal of Psychiatric Research

Mice with ablated adult brain neurogenesis are not impaired in antidepressant response to chronic fluoxetine

https://doi.org/10.1016/j.jpsychires.2014.05.009Get rights and content

Highlights

  • Cyclin D2 KO mice have markedly reduced hippocampal neurogenesis.

  • These mice do not show depressive symptoms, such as increased immobility.

  • They show stressed-evoked increase of immobility in forced swim test.

  • This increase is reduced by chronic fluoxetine without any changes in neurogenesis.

  • Efficiency of antidepressant fluoxetine does not depend on adult brain neurogenesis.

Abstract

The neurogenesis hypothesis of major depression has two main facets. One states that the illness results from decreased neurogenesis while the other claims that the very functioning of antidepressants depends on increased neurogenesis. In order to verify the latter, we have used cyclin D2 knockout mice (cD2 KO mice), known to have virtually no adult brain neurogenesis, and we demonstrate that these mice successfully respond to chronic fluoxetine. After unpredictable chronic mild stress, mutant mice showed depression-like behavior in forced swim test, which was eliminated with chronic fluoxetine treatment, despite its lack of impact on adult hippocampal neurogenesis in cD2 KO mice. Our results suggest that new neurons are not indispensable for the action of antidepressants such as fluoxetine. Using forced swim test and tail suspension test, we also did not observe depression-like behavior in control cD2 KO mice, which argues against the link between decreased adult brain neurogenesis and major depression.

Introduction

Major depression is a common mental disorder, however, its pathophysiology remains poorly understood. Stress, the main cause of depression, most severely affects hippocampal formation (McEwen et al., 2012), while hippocampal subgranular zone (SGZ) of the dentate gyrus (DG) is a region where new neurons originate throughout mammalian adulthood. Moreover, stress leads to significant decrease of adult neurogenesis (for review see Warner-Schmidt and Duman, 2006), implicating a role of this decrease in the onset of depression-like symptoms, however, this notion remains controversial (for reviews see Sahay and Hen, 2007, Balu and Lucki, 2009, Eisch and Petrik, 2012).

Hippocampal neurogenesis has also been proposed as a crucial process involved in the therapeutic efficacy of chronic antidepressants (ADs) treatment (e.g., Santarelli et al., 2003). This hypothesis is supported by the fact that recovery from depression requires several weeks – the timescale that overlaps with the time-course of AD-stimulated neurogenesis and is also paralleled with the time needed for differentiation and incorporation of newborn neurons into existing neuronal hippocampal networks (Sahay and Hen, 2007, Balu and Lucki, 2009). Moreover, majority of the antidepressant approaches elevate neurogenesis by increasing proliferation rate and/or by enhancing newborn cells survival (for review see Samuels and Hen, 2011). Indeed, it was shown that animals with blocked adult neurogenesis do not recover from depression-like behavior when chronically administered with ADs (Santarelli et al., 2003, Surget et al., 2008, David et al., 2009, Onksen et al., 2011, Perera et al., 2011). However, recent studies suggest both neurogenesis-dependent and independent mechanisms underlying ADs action, as more studies show none or only partial effect of reducing neurogenesis on restoration of behavioral homeostasis by ADs (Meshi et al., 2006, David et al., 2007, Holick et al., 2008, Surget et al., 2008, Bessa et al., 2009a, David et al., 2009, Singer et al., 2009, Nollet et al., 2012).

Herein, we have employed cyclin D2 knockout (cD2 KO) mice showing lack of adult brain neurogenesis. We showed before that mice with mutated cyclin D2 gene display largely impaired proliferation of neuronal precursors in SGZ (Kowalczyk et al., 2004, Jaholkowski et al., 2009, Jedynak et al., 2012). Notably, cyclin D2 mutant mice show deficits in some hippocampal-dependent behaviors (Jedynak et al., 2012), but not in learning in general (Jaholkowski et al., 2009, Jedynak et al., 2012, Urbach et al., 2013), while selected cognitive functions are impaired (Ben Abdallah et al., 2013, Garthe et al., 2014).

In the present study, we set out to test the reaction of cD2 KO mice to chronic fluoxetine administration following chronic stress. We have chosen unpredictable chronic mild stress (UCMS) as known to cause ADs-reversible behavioral changes in rodents that parallel symptoms of major depression (Willner, 2005). This paradigm was also used in the original study suggesting a causal link between ADs efficacy and adult neurogenesis (Santarelli et al., 2003). Also, we have chosen forced swim test (FST) as one of the most widely used test of ADs action (Petit-Demouliere et al., 2005, Krishnan and Nestler, 2011), which results in reduced immobility following their acute (Porsolt et al., 1977a, Porsolt et al., 1977b) and chronic delivery (e.g., Detke et al., 1997, Dulawa et al., 2004, Holick et al., 2008, Lin and Wang, 2014) as well as in reversal of increased immobility following UCMS (Willner, 2005). Finally, we have used fluoxetine as most commonly used AD in adult neurogenesis-depression studies, e.g., in the original Santarelli et al. (2003) publication.

Section snippets

General experimental design

A cohort of animals (cD2 KO, n = 30; WT, n = 30) was subjected to unpredictable chronic mild stress (UCMS) and chronically administered with fluoxetine. When stress and fluoxetine administration ended, all animals were tested behaviorally in the forced swim test (FST) and cell proliferation was assessed in DG using bromodeoxyuridine (BrdU) injection and immunocytochemistry. There was also an open field control test performed 24 h after the last fluoxetine administration. The overall design of

Mutant cD2 KO mice did not show increased immobility

Basal depression-like behavior, behavioral response to UCMS as well as chronic fluoxetine treatment were assessed in FST (Fig. 2A). Two-way ANOVA of no stress and stress-vehicle groups revealed effect of both the stress (p < 0.001, F(1,39) = 24.0) and genotype (p < 0.01, F(1,39) = 9.3). The latter effect was confirmed by one-way ANOVA: cD2 KO mice showed decreased time [%] spent immobile compared to WT animals (cD2 KO mice, 38.6 ± 3.4; WT mice, 51.5 ± 3.5; F(1,19) = 6.9, p < 0.05). We further

Discussion

Despite numerous publications, the exact function of new neurons in the etiology and treatment of mood disorders remains elusive. In the present study, we used cD2 KO mice with ablated adult neurogenesis to investigate the role of this process in ADs action. We have found that fluoxetine treatment was efficient in the FST independently of the presence of newborn cells in the SGZ of mice. These data suggest that neurogenesis-independent mechanisms underlie, at least in some specific

Conflict of interest

All authors declare that they have no conflicts of interest.

Contributors

LK, RKF, and PJ designed the study. TK and CS introduced FST and UCMS techniques, respectively, into the laboratory. PJ performed the experiments and undertook the statistical analysis. PJ and RKF wrote the manuscript and prepared the figures. All authors contributed to and have approved the final manuscript.

Role of funding sources

This work was supported by the European Union structural funds Innovative Economy Operational Program, project no. POIG.01.01.02-00-109/09 and by statutory funds of the University of Finance and Management in Warsaw. PJ was supported by FEBS Short-Term Collaborative Experimental Scholarship for Central & Eastern Europe. The funding sources had no role in the study design; collection, analysis or interpretation of data; writing of the report; or decision to submit the paper for publication.

Acknowledgments

None.

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