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Mice with global deletion of one brain-derived neurotrophic factor (BDNF) allele or with forebrain-restricted deletion of both alleles show elevated aggression, but this phenotype is accompanied by other behavioral changes, including increases in anxiety and deficits in cognition. Here we performed behavioral characterization of conditional BDNF knockout mice generated using a Cre recombinase driver line, KA1-Cre, which expresses Cre in few areas of brain: highly at hippocampal area CA3 and moderately in dentate gyrus, cerebellum and facial nerve nucleus. The mutant animals exhibited elevated conspecific aggression and social dominance, but did not show changes in anxiety-like behaviors assessed using the elevated plus maze and open field test. There were no changes in depression-like behaviors tested in the forced swim test, but small increase in immobility in the tail suspension test. In cognitive tasks, mutants showed normal social recognition and normal spatial and fear memory, but exhibited a deficit in object recognition. Thus, this knockout can serve as a robust model for BDNF-dependent aggression and object recognition deficiency.
Here, we made an attempt to dissociate BDNF-dependent phenotypes by generating mice with a more restricted BDNF knockout, which involves hippocampal area CA3, one of the few brain regions with high expression of BDNF. The BDNF knockout mice were generated using a kainate receptor promoter driven-line of cre (KA1-Cre) for the first time. This line exhibits high levels of Cre expression in the hippocampal area CA3, moderate expression in the dentate gyrus and facial nuclei and low expression in the anterodorsal thalamus, cerebellar granule cell layers and vestibular nuclei (Nakashiba et al. 2008). Surprisingly, despite significant loss of hippocampal BDNF, mutant mice did not display major deficits in several hippocampus-dependent tasks, except the object recognition task, but exhibited high levels of aggression and thus may serve as a robust animal model of BDNF-dependent aggressive behaviors and object recognition deficit.
- Top of page
- Materials and methods
- Supporting Information
This study characterizes a line of region-restricted BDNF knockout mice with a Cre driver line KA1-Cre, that exhibit no major behavioral changes except elevated aggression, a mild deficit in object recognition and higher immobility in the tail suspension, but not in the forced swim test for depression-like behaviors.
As BDNF acts in different brain areas, manipulation of BDNF expression in the brain has been found to alter several behaviors. Heterozygous BDNF knockout mice showed increased aggression (Lyons et al. 1999), but no changes in depression or anxiety-like behaviors (MacQueen et al. 2001). Forebrain-restricted knockout mice with EMX-Cre line showed similar phenotypes: increased aggression, no changes in depression or anxiety-like behaviors and learning deficits (Gorski et al. 2003). Another forebrain-restricted BDNF knockout mouse with CaMKII promoter-driven Cre line exhibited hyperactivity, aggression and elevated anxiety-like behaviors (Rios et al. 2001). In those mutants, it is difficult to dissociate aggression from other phenotypes, because of limited spatial and temporal restriction of the mutation. Recently published animal models, in which BDNF expression is altered in a more restricted manner, allow more targeted analysis of BDNF functions. For example, animals that received local delivery of BDNF protein or Cre virus for BDNF deletion, showed more limited behavioral changes. Knockdown of BDNF in the nucleus accumbens prevented development of social aversion (Berton et al. 2006). BDNF infusion into the dentate gyrus had an antidepressant effect (Shirayama et al. 2002), whereas BDNF knockout from dentate gyrus or CA1 by adeno associated virus Cre attenuated antidepressant actions of desipramine and citalopram (Adachi et al. 2008). Likewise, using inducible knockout of BDNF showed that early developmental BDNF deletion results in more pronounced cognitive deficits (Monteggia et al. 2004), and analysis of BDNF Val66Met mutants showed deficit in the extinction, but not in acquisition or retention of the aversive memories (Yu et al. 2009). Recent work by Sakata et al. (2010) showed that the elimination of promoter IV-driven BDNF transcription increases depression-like behaviors and reduces locomotion, but does not cause anxiety-like phenotype, which is another example of dissociation between BDNF-dependent behaviors achieved by selective alteration of BDNF expression.
In contrast to several mouse models with alterations in BDNF expression, our BDNF knockout had normal locomotion, several forms of memory or anxiety-like behaviors, all of which are known to be modulated by BDNF. In the depression-like behaviors, our KO mice did not show changes in the forced swim test, but had 25% higher immobility in the tail suspension test, which was also observed in mice-lacking BDNF promoter IV (Sakata et al. 2010). Mice with the forebrain-restricted BDNF knockouts showed even greater immobility in the same test, but surprisingly low immobility in the forced swim test, possibly resulting from hyperactivity (Chan et al. 2006).
The elevated aggression in the mutant animals was evident from two observations: first, KO mice attacked and injured their home cage partners and second, singly housed KO mice exhibited shorter latency and more frequent attacks against an intruder. In addition, KO mice became dominant over WT home cage partners before the onset of the injurious attacks.
Certain memory deficits may lead to aggression. For example, loss of social memory, which requires hippocampus (Broadbent et al. 2004; Maaswinkel et al. 1996; van Wimersma Greidanus & Maigret 1996), can compromise the recognition between cage partners and cause confrontation. At the same time, loss of contextual memory, which also depends on hippocampus (Anagnostaras et al. 2001), may result in a perception of a familiar environment as more novel and thus hostile (Nelson & Trainor 2007). However, it was surprising that KO mice, which lack BDNF gene in CA3 pyramidal cells, a major BDNF source in the hippocampus (Conner et al. 1997), did not show cognitive deficit when tested in social recognition, contextual fear conditioning or spatial version of the Morris water maze. Thus, the elevated aggression does not appear to be secondary to other deficits.
The only cognitive phenotype in KO mice was a deficit in object recognition, which is a hippocampus-dependent task (Mansuy et al. 1998; Myhrer 1988a,b). This finding, together with the observations of pattern completion deficiency in CA3-restricted N-methyl-D–aspartate receptor knockout mice (Nakazawa et al. 2002), suggests that interfering with the function of CA3 results in rather selective cognitive deficits.
It is intriguing that despite this deficit, KO mice displayed normal social recognition of ovariectomized females. As social recognition in rodents utilizes olfaction as a key factor (Matochik 1988) and involves distinct circuitries from object recognition (Caffe et al. 1987), this segregation might be because of differences of the modalities involved in each task and smaller contribution of the hippocampus in processing olfactory information.
Besides hippocampus, KA1-Cre line exhibits moderate Cre activity in the cerebellum and facial nerve nuclei (Nakashiba et al. 2008), which have not been implicated in aggression. Yet, our study does not exclude a possibility that Cre activity observed in the thalamus, brain stem or peripheral organs, even though it's diffuse and affecting small number of cells (Gene_Expression_Database_(Gxd)), may influence aggressive behaviors by deleting BDNF gene.
Several studies implicated hippocampus in aggression. In cats, electrical stimulation of the ventral hippocampus increases aggressive response to the electrical stimulation of hypothalamus (Siegel & Flynn 1968). In mice, density of mossy fibers negatively correlates with aggression (Guillot et al. 1994). Moreover, hippocampus projects to the lateral septum, which suppress aggression, and to the medial hypothalamus, which enhances aggression (Siegel 2005). Evidence of hippocampal involvement in aggression in rodents and cats has been recently complemented by a study in humans showing that hippocampal activity positively correlates with anger ruminations following verbal insults (Denson et al. 2009). As deletion of BDNF gene in our KO mice is so prominent in the hippocampal CA3 pyramidal cells, the decrease in hippocampal BDNF could be one of the direct causes for the aggression. Finally, BDNF KO mice with relatively restricted behavioral phenotypes can be used as a tool to search for compounds with selective behavioral effects.