Jedynak_Mice with ablated adult brain - Infoscience

Journal of Psychiatric Research 56 (2014) 106e111
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Mice with ablated adult brain neurogenesis are not impaired in
antidepressant response to chronic fluoxetine
Paulina Jedynak a, Tomasz Kos b, Carmen Sandi c, Leszek Kaczmarek a,
Robert K. Filipkowski a, d, e, *
Laboratory of Neurobiology, Nencki Institute of Experimental Biology, Polish Academy of Sciences, 02-093 Warsaw, Poland
Department of Behavioral Neuroscience & Drug Development, Institute of Pharmacology, Polish Academy of Sciences, 31-343 Krakow, Poland
Laboratory of Behavioral Genetics, Brain Mind Institute, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
Department of Biological Psychology, University of Finance and Management in Warsaw, 01-030 Warsaw, Poland
Behavior and Metabolism Research Laboratory, Mossakowski Medical Research Centre, Polish Academy of Sciences, 02-106 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 January 2014
Received in revised form
29 April 2014
Accepted 13 May 2014
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.
© 2014 Elsevier Ltd. All rights reserved.
Adult neurogenesis
Dentate gyrus
Depression-like behavior
1. 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 WarnerSchmidt 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
* Corresponding author. Department of Biological Psychology, University of
Finance and Management in Warsaw, 55, Pawia St., 01-030 Warsaw, Poland.
Tel.: þ48 668403448; fax: þ48 225365412.
fi[email protected],
rfi[email protected]
(R.K. Filipkowski).
0022-3956/© 2014 Elsevier Ltd. All rights reserved.
is supported by the fact that recovery from depression requires
several weeks e 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 depressionlike 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
P. Jedynak et al. / Journal of Psychiatric Research 56 (2014) 106e111
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, 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 neurogenesisdepression studies, e.g., in the original Santarelli et al. (2003)
2.3. Unpredictable chronic mild stress (UCMS) and chronic
fluoxetine treatment
2. Materials and methods
The mice (cD2 KO, n ¼ 29; WT, n ¼ 29) were divided into three
age-matched groups (non-stressed, n ¼ 10; stressed-vehicle,
n ¼ 10; stressed-fluoxetine, n ¼ 9). UCMS protocol was described
before (Bisaz et al., 2011; Bisaz and Sandi, 2012) and used with
some modifications. Non-stressed and stressed groups were
housed in separated and closed housing racks located in the same
room during the duration of the stress procedure. As the experiment started, non-stressed groups were left undisturbed. Stressed
groups were exposed to UCMS procedure which consisted of
different kinds of stressors: cage tilting, damp sawdust, housing in
an empty cage, pairing with another stressed animal, cold room,
water or food deprivation, inversion of the light/dark cycle, lights
on for a short period of time during the dark phase and switching
cages amongst stressed animals. One or two of these stressors were
applied daily at different times and following a semi-random
schedule. Starting from the beginning of the 4th week of UCMS,
stressed mice were given either 10 mg/kg/day (comp. Santarelli
et al., 2003; Bessa et al., 2009a) fluoxetine hydrochloride (Sigma,
PL), dissolved in water and prepared freshly before use, or water.
The treatment lasted for 3 weeks and was provided via oral gavage
(Fine Science Tools Inc., USA). The drug concentration was adjusted
weekly from the average body weight of mice to achieve the
desired doses.
2.1. General experimental design
2.4. Forced swim test (FST)
A cohort of animals (cD2 KO, n ¼ 30; WT, n ¼ 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 experimental
procedures is shown in Fig. 1.
In order to eliminate the effects of acute fluoxetine injections,
FST was performed 3 days after the last delivery of the drug. The
test was conducted as described by Porsolt et al. (1977a, 1977b) by
placing mice into a glass cylinder (25 cm height, 10 cm diameter)
containing 1250 ml of water maintained at 24e25 C. The test
lasted for 6 min. Starting from the 3rd minute of the test, mice were
rated for immobility defined as the absence of active, escapeoriented behaviors such as swimming, jumping, climbing, or diving. Two animals were tested simultaneously.
2.2. Animals
2.5. Tail suspension test (TST)
Cyclin D2 mutant mice (Kowalczyk et al., 2004) were backcrossed into C57BL/6 background over 10 generations and kept as
cyclin D2 heterozygotes (þ/). Their homozygous progeny, cD2 KO
(/) and WT (þ/þ) littermates, were used. If not stated otherwise,
the animals were kept under a natural light/dark cycle in Plexiglas
cages with water and food provided ad libitum. To ensure proper
care and use of laboratory animals, the national rules according to
the Animal Protection Act, ensured by the permission from the First
Warsaw Local Ethics Committee for Animal Experimentation, were
strictly followed. The animals were males, 2e4 months old at the
beginning of the experiments, with their age carefully matched
between WT and cD2 KO mice. Experimenters were always
unaware of the genotype of the mice.
TST procedure was previously described (Steru et al., 1985). Mice
were suspended by the tail temporarily attached to a metal bar
using adhesive scotch tape. Total duration of immobility was
counted during a 6 min test. Three animals were tested simultaneously. One WT animal climbing its tail was excluded from the
2.6. Open field
The test was performed 24 h after the termination of UCMS and
fluoxetine treatment, it was done as described previously (Jedynak
et al., 2012). The apparatus was a wooden floor (50 cm 50 cm)
surrounded by 34 cm high walls. Gray walls and floor as well as
Fig. 1. Schematic plan of experimental procedures and timetable. Mice were subjected to unpredictable chronic mild stress (UCMS) for 6 weeks. Fluoxetine or water was given
during the 4the6th week of UCMS. After 3 days interruption, depression-like behavior was evaluated using forced swim test (FST), then the animals were injected with BrdU and
perfused for immunocytochemistry. Control open field (OF) was performed 24 h after the last fluoxetine treatment.
P. Jedynak et al. / Journal of Psychiatric Research 56 (2014) 106e111
normal room illumination were used to prevent suppression of
activity and exploration behaviors by the anxiety response. Animals' behavior was monitored by a video camera placed above the
center of the apparatus. Mice were put individually in one corner of
the open field facing the wall and were allowed to explore freely for
5 min. Four open fields were employed to test four animals
simultaneously. The floor of the apparatus was cleaned with 5%
ethanol after each session. Data were analyzed using EthoVision 8.0
system (Noldus Information Technology, The Netherlands), and the
total distance moved was acquired.
2.7. Perfusion and sectioning
To assess changes in cell proliferation in DG after chronic stress
and chronic fluoxetine treatment, 24 h after completion of behavioral testing, mice were injected once with bromodeoxyuridine
(150 mg/kg, i.p., Sigma, prepared in PBS, pH 7.4). Two hours after
injection, mice were anesthetized and killed by cardiac perfusion
(4% paraformaldehyde). Brains were then post-fixed overnight in
4% paraformaldehyde at 4 C then transferred into a 30% sucrose/
0.1% sodium azide cryoprotectant until the brains sank. Brains were
embedded in tissue freezing medium, and sectioned using a cryostat to cut 50 mm coronal sections. Six to eight sections from each
brain were analyzed, 1.28 to 2.75 mm from the bregma.
2.8. Bromodeoxyuridine immunohistochemistry
BrdU detection was performed as described elsewhere
(Kowalczyk et al., 2004), with modifications. Free-floating sections
were rinsed with PBS, then incubated in 2 N HCl for 30 min at 37 C
and neutralized by immersing in 0.1 M sodium tetra-borate buffer
for 10 min. Sections were rinsed again in PBS and PBS-TX containing 0.01% Triton x-100 and then blocked with 10% normal
donkey serum (NDS) for 1 h. Primary antibody (sheep anti-BrdU in
1% NDS, 1:500, Abcam, UK) was applied with overnight incubation.
The following day, sections were washed of primary antibody, and
incubated for 1 h with Alexa 488 secondary antibody (donkey antisheep in PBS, 1:500, Abcam, UK). Sections were mounted with
Fluoromount-G containing DAPI nuclear marker (Southern
Biotech., USA) and coverslipped. The mean numbers of BrdUþ cells
per section in SGZ of DG were calculated, using 40 magnification
of an Olympus fluorescent microscope.
2.9. Data analysis
Statistical analyses were performed with GraphPad Prism 5.00
(GraphPad Software, USA). Data are reported as mean þ SEM (if
distributed normally) or as median with inter-quartile range (IQR),
minimum and maximum values (when not distributed normally).
When possible, data were submitted to ANOVA or a nonparametrical ManneWhitney test was used (indicated in the text).
3. Results
3.1. 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) ¼ 24.0) and genotype
(p < 0.01, F(1,39) ¼ 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) ¼ 6.9, p < 0.05). We further confirmed this
observation by testing immobility in FST and tail suspension test
Fig. 2. Behavioral and cellular response to chronic stress and fluoxetine treatment in
cD2 KO and WT mice. In FST (A), behavioral effects of chronic stress (StresseVeh) were
manifested by increased immobility time in both cD2 KO and WT animals. Chronic
fluoxetine treatment (StresseFlu) abolished this difference independently of the genotype as both cD2 KO and WT animals administered with fluoxetine showed
immobility time similar to non-stressed mice. In WT mice, chronic stress caused significant decrease in the number of BrdUþ cells in the SGZ of the hippocampal formation (B, C). The effect was prevented by chronic fluoxetine treatment. There was no
effect on neurogenesis in cD2 KO animals in any conditions. The mean number of new
cells per slice (B) is shown along with representative parts of DG granule cell layer
(GCL) with the SGZ and GCL areas indicated (C). Data shown represent the
means þ SEM (A) or medians with inter-quartile range, minimum and maximum
values (B). *p < 0.05; ***p < 0.001; scale bar, 100 mm.
(TST) using separate groups of mice (not shown). In both tests, nonstressed, naïve cD2 KO mice presented reduced immobility when
compared to WT mice. The decrease in immobility duration [%] was
observed in FST (cD2 KO mice, 35.5 ± 6.4, n ¼ 9; WT mice, 53.7 ± 3.1,
n ¼ 11; F(1,19) ¼ 3.5, p < 0.05, one-way ANOVA) and TST (cD2 KO
P. Jedynak et al. / Journal of Psychiatric Research 56 (2014) 106e111
mice, 25.7 ± 1.6, n ¼ 6; WT mice, 37.3 ± 3.9, n ¼ 6, F(1,11) ¼ 7.5,
p < 0.05, one-way ANOVA).
3.2. Mutant mice show depression-like behaviors after stress and
respond to fluoxetine treatment
The effect of stress was also confirmed as UCMS resulted in an
increase of immobility time [%] in both cD2 KO (57.0 ± 2.7, p < 0.001,
F(1,19) ¼ 17.5) as well as WT mice (63.0 ± 2.5, p < 0.05, F(1,19) ¼ 7.1,
one-way ANOVA). Interestingly, cD2 KO and WT animals responded
also equally well to chronic fluoxetine administration. Two-way
ANOVA revealed effect of fluoxetine treatment (p < 0.01,
F(1,37) ¼ 12.0) but no effect of genotype. Both groups of fluoxetinetreated mice showed reduced immobility time [%] compared with
vehicle-treated animals (cD2 KO mice, 43.6 ± 5.6, p < 0.05,
F(1,17) ¼ 4.9; WT mice, 47.6 ± 5.2, p < 0.05, F(1,17) ¼ 7.5; one-way
Overall influence of fluoxetine treatment on animals activity
was examined in the open field. One-way ANOVA analysis of the
total distance moved reveal no effect of treatment in any group
(cD2 KO mice, F(2,28) ¼ 0.2, p ¼ 0.8; WT mice, F(2,28) ¼ 1.35,
p ¼ 0.3). Importantly, chronic fluoxetine did not increase the total
distance [m] in cD2 KO mice (26.5 ± 2,0) when compared to
vehicle-treated group (28.1 ± 1.9) and non-treated mice
(28.5 ± 2.0). Fluoxetine treatment had a similar effect also in WT
control animals (23.6 ± 1.7; 26.3 ± 2.1; 23.5 ± 1.4; respectively).
3.3. Neurogenic effect of chronic fluoxetine administration observed
only in control animals
Detailed stereological study of adult neurogenesis in naïve cD2
KO mice in comparison to WT littermates was described elsewhere
(Jaholkowski et al., 2009). In the present study, we confirmed that
non-stressed cD2 KO mice showed almost complete lack of
BrdUþ cells in SGZ of DG (0.0; 0.0e0.2, IQR) compared to WT mice
(13.9; 13.2e15.5, IQR; p < 0.001, ManneWhitney, U ¼ 0; Fig. 2BC).
Chronic stress decreased numbers of BrdUþ cells in WT animals
(11.0; 0.2e12.6, IQR), p < 0.05, ManneWhitney, U ¼ 18) when
compared to non-stressed ones, and had no effect in cD2 KO mice
(0.1; 0.0e0.3, IQR; p > 0.05, ManneWhitney). Chronic fluoxetine
treatment caused significant increase of the number of proliferating cells in SGZ of WT mice when compared to vehicle-treated
WT animals (15.2; 9.6e31.0, IQR, p < 0.05, ManneWhitney,
U ¼ 17). There was no neurogenic effect of chronic fluoxetine
administration in cD2 KO animals (0.3; 0.0e0.5, IQR; p > 0.05,
4. 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 circumstances, the
behavioral effect of fluoxetine, and that the increase in adult neurogenesis observed in WT animals is rather an epiphenomenon not
necessary for the drug's direct effect.
There is limited direct evidence supporting reduced hippocampal neurogenesis in the pathophysiology of depression.
Although reduced neurogenesis and depression-like phenotypes
arise in tandem following various chronic stressors (e.g., Surget
et al., 2008; Bessa et al., 2009a), a requirement for reduced
neurogenesis in the development of a depression-like phenotype
has apparently been demonstrated only by Conboy et al. (2011). In
our hands, however, cD2 KO mice, devoid of adult brain neurogenesis, did not show increased immobility in FST and TST tests. On
the contrary, the immobility was reduced in these mice. We have
observed before cD2 KO animals to be more active and explorative
(Jedynak et al., 2012; Ben Abdallah et al., 2013). Please note, we
were still able to increase immobility in mutant mice by UCMS and
again reduce it through chronic fluoxetine treatment. The latter
effect was apparently not a false positive result of an overall increase in mobility since chronic fluoxetine had no effect on the total
distance moved in the open field.
Numerous observations suggest the role of adult brain neurogenesis in the treatment of depression-like behaviors (see Introduction). Importantly, in cD2 KO mice, we did not observe any
increase in cell proliferation in the SGZ following chronic fluoxetine
administration. It can be explained by the lack of cells targeted by
fluoxetine or their proliferation impairment, as it is not clear
whether the disruption of cyclin D2 gene causes developmental
ablation of amplifying neural progenitors or their inability for divisions. The fact that suppression of depression-like symptoms
occurred in both WT and cD2 KO mice, independently of the
presence or lack of adult hippocampal neurogenesis points out a
neurogenesis-independent mechanism behind chronic fluoxetine
action. As concluded before by Bessa et al. (2009a), although temporal coincidence in the occurrence of impaired neurogenesis and
depression-like symptoms and their reversal by ADs treatment are
evident, a direct causeeeffect relationship between neurogenesis
and the alleviation of signs of depression-like behavior is missing. It
was proposed that ADs, irrespectively of their mechanisms of action, trigger neuronal remodeling and synaptic plasticity and the
increase in adult neurogenesis is rather an epiphenomenon (Bessa
et al., 2009a). This notion is supported by recent finding that increase of adult hippocampal neurogenesis causes neither an anxiolytic nor antidepressant behavioral effect (Udo et al., 2008; Sahay
et al., 2011). There is also accumulating evidence that other brain
regions including amygdala, nucleus accumbens, or cingulate cortex are also involved in AD-like activity. It is possible as well that
adult neurogenesis outside of the hippocampus may play a role in
the effects of fluoxetine (Kokoeva et al., 2007). Nevertheless, our
study supports a hippocampal neurogenesis-independent mechanism of fluoxetine-mediated behavioral effect.
It was suggested that fluoxetine acts independently of neurogenesis especially when tested by FST and TST, while other tests,
e.g., novelty-suppressed feeding (NSF) and novelty-induced hypophagia (NIH), are neurogenesis-dependent. However, this quality of
FST and TST, when compared to other set of tests, was demonstrated only by one group, following rarely-used chronic corticosterone treatment (David et al., 2009; Mendez-David et al., 2014).
Moreover, the treatment had no basal effect as corticosterone injections did not increase the immobility levels measured by FST
(David et al., 2009, Fig. 1D) and TST (Mendez-David et al., 2014;
Fig. 3P) in control mice, nor were acute vs. chronic effects of
fluoxetine-delivery investigated. Furthermore, it is known that
glucocorticoids affects food intake (e.g., Solano and Jacobson, 1999)
which is a crucial element of most of the postulated neurogenesisdependent tests, i.e., NSF, NIH. Also, using parallel FST and NSF
protocols, the same group showed that behavioral effects of chronic
fluoxetine in BALB/cJ mice do not require adult neurogenesis
(Holick et al., 2008). Finally, Bessa et al. (2009a) demonstrated that
reduction of adult neurogenesis in rats did not change the effectiveness of several ADs in both FST and sucrose preference test,
while their efficacy was blocked in NSF. This observation added to
the notion that NSF shows anxiolytic rather than antidepressant
properties of investigated drugs (discussed in Bessa et al., 2009a;
P. Jedynak et al. / Journal of Psychiatric Research 56 (2014) 106e111
Bessa et al., 2009b; David et al., 2009). Finally, the prevailing
perception in the literature, suggests FST to be a proper test to show
neurogenesis-dependence of antidepressant efficacy. Several
groups described X-irradiation to block antidepressant effects using
FST (e.g., Zhu et al., 2010; Garza et al., 2012) while correlative observations linking antidepressant treatment effects in FST and adult
hippocampal neurogenesis have been reported numerous times
(Liu et al., 2008; Silva et al., 2008; Schmidt and Duman, 2010;
Wainwright et al., 2011; Wang et al., 2011; Jiang et al., 2012; Lin
and Wang, 2014; Lu et al., 2014).
Our experiment was designed to investigate solely the results of
chronic fluoxetine treatment, since antidepressant effects of the
drug can be observed following acute or short term use, e.g., when
administered 30 min before FST testing in mice (Holick et al., 2008).
Therefore, 3 days interval between the last fluoxetine delivery and
FST testing was introduced (Fig. 1) while even 1 day interval is
frequently used and considered sufficient to eliminate short-term
fluoxetine effects in rodents (e.g., Castro et al., 2010). On the other
hand, fluoxetine-induced adult-born neurons require weeks to
become fully functional (Sahay and Hen, 2007), therefore we used 3
weeks plus 3 days between the introduction of the and FST testing.
Three weeks of fluoxetine treatment of mice were shown before to
have behavioral effects apparently dependent on adult brain neurogenesis (Santarelli et al., 2003; Surget et al., 2008; David et al.,
2009). It was also shown that FST model provides a valid assessment of chronic, and not acute, fluoxetine action in mice (Holick
et al., 2008).
Finally, we realize that our model and, hence, our findings have
limitations. Mutant cD2 KO mice are constitutive knockout animals
with the phenotype not limited to adult brain neurogenesis but also
including reduction in size of several brain structures. There is also
possibility of compensatory mechanisms to operate in these mice
(discussed in Jaholkowski et al., 2009), however, the model is
recognized as valuable and adequate tool in the field (for review see
Frankland, 2013). The results obtained with the use of cD2 KO mice
suggested the lack of relevance of adult brain neurogenesis for
learning and memory in general (Jaholkowski et al., 2009) e a view
which is now widely accepted (e.g., Sahay et al., 2011) e while
suggested and/or confirmed the role of this process in particular
aspects of learning (Ben Abdallah et al., 2013), smell detection
(Jaholkowski et al., 2009), species-typical behaviors (Jedynak et al.,
2012), and alcohol consumption (Jaholkowski et al., 2011). Most of
transgenic models with manipulated adult hippocampal neurogenesis, in contrast to our model, do not achieve complete ablation
of newborn neurons. In case of depression etiology and treatment
studies, partial suppression of adult neurogenesis can be considered as an advantage, as such model has the potential to imitate the
rate of neurogenesis during aging in humans, which is reduced only
partially (Balu and Lucki, 2009). On the other hand, the complete
reduction of neurogenesis may prevent compensative effects from
residual newborn neurons and potential intra- and inter-group
variability. In conclusion, our studies do not support the role of
adult neurogenesis in pharmacological treatment of depression
with fluoxetine as well as put in question a strategy to search for
novel therapeutic approaches aiming at cell-cycle stimulation and
induction of adult neurogenesis (comp. Patricio et al., 2013).
Conflict of interest
All authors declare that they have no conflicts of interest.
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.
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