Aberrant hippocampal neurogenesis after limbic kindling: Relationship to BDNF and hippocampal-dependent memory
Introduction
Neurogenesis, the birth of new neurons, was long thought to be exclusive to the prenatal brain. However, a plethora of findings has revealed that in the mammalian brain, neurogenesis occurs throughout adulthood [1], [2] and is regulated by a variety of physiological, pharmacological, or pathological stimuli [3], [4], [5], [6]. One of the most studied neurogenic brain regions is the dentate gyrus of the hippocampus, where newborn neurons of the subgranular zone (SGZ) migrate to the granule cell layer (GCL) and integrate into local hippocampal circuitry. Adult generated neurons in the hippocampus are functionally important, with numerous reports indicating that these new neurons play crucial roles in learning and spatial memory [7], [8], [9], [10], associative fear learning [11], [12], and anxiety-related behaviors [13].
Although the integration of adult generated neurons into hippocampal circuitry is generally considered to be beneficial for cognition and emotion, pathological states such as temporal lobe epilepsy (TLE) seem to promote an aberrant form of neurogenesis that could contribute to the cognitive impairment and emotional dysfunction often seen in patients with this disorder [14], [15]. For example, seizures are known to increase hippocampal cell proliferation and survival, and some of these newborn neurons migrate ectopically into the hilus and develop basal dendrites [14], [16], [17], [18], [19]. Many other newborn neurons do migrate normally into the GCL, but they mature more rapidly than normal. However, colocalization of adult generated neurons with the behaviorally relevant immediate early gene c-Fos [20] has indicated that mature adult generated neurons born under conditions of kindling may not be functionally active during hippocampal-dependent memory tests [14]. One issue that remains unresolved is whether the effect of seizures on adult hippocampal neurogenesis depends on the brain site where the seizures originate or whether epileptic activity anywhere in the brain can promote aberrant neurogenesis. This is an important question because there is evidence that the behavioral comorbidities associated with TLE vary depending on the location of the epileptic focus. This is seen quite clearly in the preclinical kindling model of TLE, where kindling of the basolateral amygdala (BLA) or dorsal hippocampus (dHip) impairs hippocampal-dependent fear conditioning [14], [21], [22] and spatial learning in the Morris water maze and radial arm maze [23], [24] and enhances unconditioned fear behaviors in the open-field test and elevated plus maze [25], [26], [27]. In contrast, kindling of nonlimbic brain sites, such as the caudate nucleus (CN), has minimal effects on any of these behavioral and cognitive comorbidities [22], [28]. As adult hippocampal neurogenesis is involved in anxiety-related behaviors and spatial learning and memory, these behavioral dissociations raise the possibility that aberrant neurogenesis may be present following seizures that originate in limbic, but not nonlimbic, regions of the brain.
To test this idea, we examined the effect of different site kindling on aberrant adult hippocampal neurogenesis and cognition. We also investigated the effects of kindling on brain-derived neurotrophic factor (BDNF) expression across the hippocampus. Brain-derived neurotrophic factor is a modulator of many physiological functions, including synaptogenesis, neurogenesis, and activity-dependent plasticity [29], [30], [31]. Brain-derived neurotrophic factor belongs to the neurotrophin family of growth factors and is synthesized as a precursor protein (pre–proBDNF) that is cleaved into proBDNF and mature BDNF (mBDNF), which in turn activates TrkB receptors. Previous studies have shown that seizures significantly increase BDNF expression, regardless of the method of seizure induction [32], [33], [34], [35], [36]. Brain-derived neurotrophic factor is known to increase hippocampal neurogenesis [31], [37], promote hilar-CA3 ectopic granule cells [38], influence the morphological features of dendritic spines [39], [40], and stimulate axonal growth and the development of hilar basal dendrites in dentate granule cells [41]. Elevated levels of BDNF following seizures could therefore stimulate aberrant forms of neurogenesis. We expected to see kindling-induced changes in hippocampal BDNF expression that parallel changes in hippocampal neurogenesis and cognition. That is, if limbic kindling promotes aberrant neurogenesis and impairs hippocampal-dependent cognition, it should also enhance hippocampal BDNF expression. Further, if nonlimbic kindling does not influence hippocampal neurogenesis or cognition, it should have no effect on BDNF levels.
Section snippets
Animals
We used 88 male Long-Evans rats (n = 40 for postmortem analyses; n = 48 for behavioral analyses) that were purchased from Charles River (QC, Canada). The rats weighed 200–250 g at the time of arrival from the breeder. Rats were individually housed in rectangular polypropylene cages containing standard laboratory bedding with free access to food and water. The colony room was maintained at a temperature of 20 ± 1 °C on a 12:12 h light–dark cycle (lights on at 8 am). All experimental procedures were in
Kindling progression
Kindling progressed normally in all rats. The number of stimulations required to elicit the first class 5 convulsion was 11.8 ± 1.2 for the BLA-kindled rats and 48.1 ± 3.3 for the dHip-kindled rats. The number of stimulations required to elicit three consecutive class 5 convulsions was 17.3 ± 2.4 for the BLA-kindled rats and 53.4 ± 3.2 for the dHip-kindled rats. Finally, the total number of class 5 convulsions was 76.2 ± 4.0 for the BLA-kindled rats and 43.9 ± 3.2 for the dHip-kindled rats. In contrast to
Discussion
The results of this experiment reveal site-specific effects of kindled seizures on adult hippocampal neurogenesis, hippocampal BDNF protein levels, and hippocampal-dependent learning and memory. We noted several distinct findings. First, our cell counts revealed that the BLA- and dHip-kindled rats had significantly more BrdU-ir cells than did the control or CN-kindled rats. Interestingly, the BLA- and dHip-kindled rats also had more BrdU-ir ectopic granule neurons located in the hilus-CA3
Conclusions
Our results indicate that limbic seizures dramatically increase adult hippocampal neurogenesis and BDNF protein levels whereas nonlimbic seizures do not. Accelerated hippocampal neurogenesis and dendritic complexity in limbic-kindled rats paralleled elevated levels of hippocampal BDNF protein and coincided with significant impairments on a hippocampal-dependent memory task. Collectively, these results suggest that limbic kindling facilitates adult hippocampal neurogenesis and promotes aberrant
Acknowledgments
This work was supported by a grant from the Natural Sciences and Engineering Research Council (NSERC, #222912) to LEK. LEK holds a CIHR-Canada Research Chair in Behavioral Neuroscience. JJB was supported by an NSERC Canada Graduate Scholarship and a Savoy Foundation Studentship.
Conflict of interest
The authors declare no conflicts of interest.
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