Dentate Granule Cell Neurogenesis Research Articles

Beneficial effects of enriched environment following status epilepticus in immature rats.

There is increasing evidence that enriching the environment can improve cognitive and motor deficits following a variety of brain injuries. Whether environmental enrichment can improve cognitive impairment following status epilepticus (SE) is not known. To determine whether the environment in which animals are raised influences cognitive function in normal rats and rats subjected to SE. Rats (n = 100) underwent lithium-pilocarpine-induced SE at postnatal (P) day 20 and were then placed in either an enriched environment consisting of a large play area with toys, climbing objects, and music, or in standard vivarium cages for 30 days. Control rats (n = 32) were handled similarly to the SE rats but received saline injections instead of lithium-pilocarpine. Rats were then tested in the water maze, a measure of visual-spatial memory. A subset of the rats were killed during exposure to the enriched or nonenriched environment and the brains examined for dentate granule cell neurogenesis using bromodeoxyuridine (BrdU) and phosphorylated cyclic AMP response element binding protein (pCREB) immunostaining, a brain transcription factor important in long-term memory. Both control and SE rats exposed to the enriched environment performed significantly better than the nonenriched group in the water maze. There was a significant increase in neurogenesis and pCREB immunostaining in the dentate gyrus in both control and SE animals exposed to the enriched environment compared to the nonenriched groups. Environmental enrichment resulted in no change in SE-induced histologic damage. Exposure to an enriched environment in weanling rats significantly improves visual-spatial learning. Even following SE, an enriched environment enhances cognitive function. An increase in neurogenesis and activation of transcription factors may contribute to this enhanced visual-spatial memory.

Seizure-induced neurogenesis: are more new neurons good for an adult brain?

The idea that neural stem cells may play a role in the pathophysiology or potential treatment of specific epilepsy syndromes is relatively new. This notion relates directly to advances in the field of stem cell biology over the past decade, which have confirmed prior theories that both neural stem cells and neurogenesis, the birth of new neurons, persist in specific regions of the adult mammalian brain. The physiological role of persistent neurogenesis is not known, although recent work implicates this process in specific learning and memory tasks. Knowledge of the normal neurogenic pathways in the mature brain has led to recent studies of neurogenesis in rodent models of acute seizures or epileptogenesis. Most of these studies have examined neurogenesis in the adult rodent dentate gyrus, and current evidence indicates that single brief or prolonged seizures, as well as repeated kindled seizures, increase dentate granule cell (DGC) neurogenesis. The models studied to date include pilocarpine and kainic acid models of temporal lobe epilepsy, limbic kindling, and intermittent perforant path stimulation. Recent work also suggests that pilocarpine-induced status epilepticus increases rostral forebrain subventricular zone (SVZ) neurogenesis and caudal SVZ gliogenesis. Several lines of evidence implicate newly generated neurons in structural and functional network abnormalities in the epileptic hippocampal formation of adult rodents. These abnormalities include aberrant mossy fiber reorganization, persistence of immature DGC structure (e.g. basal dendrites), and the abnormal migration of newborn neurons to ectopic sites in the dentate gyrus. Taken together, these findings suggest a pro-epileptogenic role of seizure- or injury-induced neurogenesis in the epileptic hippocampal formation. However, the induction of forebrain SVZ neurogenesis and directed migration to injury after seizures and other brain insults underscores the potential therapeutic use of neural stem cells as a source for neuronal replacement after injury.

Differential regulation of basic helix–loop–helix mRNAs in the dentate gyrus following status epilepticus

In various chemoconvulsant models of human temporal lobe epilepsy, the induction of epileptogenesis by a prolonged period of continuous seizure activity is accompanied by significant changes in hippocampal structure. These changes include an increase in neurogenesis within the proliferative subgranular zone (SGZ) of the dentate gyrus and induction of mossy fiber sprouting in mature dentate granule cells. As dentate granule cell neurogenesis and axon outgrowth are also hallmarks of hippocampal development, we hypothesized that molecules involved in normal development may also play a role in similar changes associated with epileptogenesis. To begin to test this hypothesis, we have analyzed the expression patterns of multiple members of the basic helix–loop–helix (bHLH) family of transcription factors in both normal and epileptic adult rats. bHLH protein expression has been found recently in dentate granule cells at specific developmental stages, and analysis of developmental models suggests specific neural differentiation functions for these molecules. We show that mRNA expression of all seven bHLH family members examined in this study, as well as the divergent homeobox protein Prox1, is present in the adult. Patterns of expression varied considerably between family members, ranging from the limited expression of Mash1 in the neurogenic SGZ of the dentate gyrus to the scattered, widespread profile of Hes5 throughout the dentate gyrus and the hippocampus proper. Moreover, these varied profiles of expression were differentially regulated following status epilepticus, with some increasing (Mash1, Id2), some falling (Hes5, Prox1), and others remaining mostly unchanged (NeuroD/BETA2, NeuroD2/NDRF, Id3, Rath2/Nex1). While the function of these molecules in the adult brain remains to be characterized, our findings support the idea that molecules controlling cell-fate decisions in the developing dentate gyrus are also operative during seizure-induced neurogenesis and plasticity.

Enhancement of progenitor cell division in the dentate gyrus triggered by initial limbic seizures in rat models of epilepsy.

Mitogenic effects of seizures on granule cell progenitors in the dentate gyrus were studied in two rat models of epilepsy. We investigated which stage of epileptogenesis is critical for eliciting progenitor cell division and whether seizure-induced neuronal degeneration is responsible for the enhancement of progenitor cell division. Seizures were induced by either kainic acid (KA) administration or electrical kindling. Neurogenesis of dentate granule cells was evaluated using the bromodeoxyuridine (BrdU) labeling method, and neuronal degeneration was assessed by in situ DNA fragmentation analysis. After injection of KA, the number of BrdU-positive granule cells began to increase at day 3 after the treatment, peaked at day 5, and returned to baseline at day 10. By day 13, the values were lower than control. After kindling, the number of BrdU-positive cells began to increase after five consecutive experiences of stage I seizures. The increase occurred from day 1 to day 3 after the last electrical stimulation, but returned to baseline by day 7. After generalized seizures were well established, repeated stimulation did not facilitate division of granule cell progenitors. DNA fragmentation was noted in pyramidal neurons in the CA1, CA3, and hilus regions at 18 h after KA injection, but not in the kindling model. These observations indicate that a mechanism in epileptogenesis boosts dentate progenitor cell division, but progenitor cells may become unreactive to prolonged generalized seizures. Pyramidal neuronal degeneration is not necessary for triggering the upregulation. It is suggested that newly born granule cells may play a role in the network reorganization that occurs during epileptogenesis.

Inhibition of Dentate Granule Cell Neurogenesis with Brain Irradiation Does Not Prevent Seizure-Induced Mossy Fiber Synaptic Reorganization in the Rat

Aberrant reorganization of dentate granule cell axons, the mossy fibers, occurs in human temporal lobe epilepsy and rodent epilepsy models. Whether this plasticity results from the remodeling of preexisting mossy fibers or instead reflects an abnormality of developing dentate granule cells is unknown. Because these neurons continue to be generated in the adult rodent and their production increases after seizures, mossy fibers that arise from either developing or mature granule cells are potential substrates for this network plasticity. Therefore, to determine whether seizure-induced, mossy fiber synaptic reorganization arises from either developing or mature granule cell populations, we used low-dose, whole-brain x-irradiation to eliminate proliferating dentate granule cell progenitors in adult rats. A single dose of 5 Gy irradiation blocked cell proliferation and eliminated putative progenitor cells in the dentate subgranular proliferative zone. Irradiation 1 d before pilocarpine-induced status epilepticus significantly attenuated dentate granule cell neurogenesis after seizures. Two irradiations, 1 d before and 4 d after status epilepticus, essentially abolished dentate granule cell neurogenesis but failed to prevent mossy fiber reorganization in the dentate molecular layer. These results indicate that dentate granule cell neurogenesis in the mature hippocampal formation is vulnerable to the effects of low-dose ionizing irradiation. Furthermore, the development of aberrant mossy fiber remodeling in the absence of neurogenesis suggests that mature dentate granule cells contribute substantially to seizure-induced network reorganization.

Environmental enrichment inhibits spontaneous apoptosis, prevents seizures and is neuroprotective.

The mammalian brain has a high degree of plasticity, with dentate granule cell neurogenesis and glial proliferation stimulated by an enriched environment combining both complex inanimate and social stimulation. Moreover, rodents exposed to an enriched environment both before and after a cerebral insult show improved cognitive performance. One of the most robust associations of environmental enrichment is improved learning and memory in the Morris water maze, a spatial task that mainly involves the hippocampus. Furthermore, clinical evidence showing an association between higher educational attainment and reduced risk of Alzheimer and Parkinson-related dementia indicates that a stimulating environment has positive effects on cerebral health that may provide some resilience to cerebral insults. Here we show that in addition to its effects on neurogenesis, an enriched environment reduces spontaneous apoptotic cell death in the rat hippocampus by 45%. Moreover, these environmental conditions protect against kainate-induced seizures and excitotoxic injury. The enriched environment induces expression of glial-derived neurotrophic factor and brain-derived neurotrophic factor and increases phosphorylation of the transcription factor cyclic-AMP response element binding protein, indicating that the influence of the environment on spontaneous apoptosis and cerebral resistance to insults may be mediated through transcription factor activation and induction of growth factor expression.

Increased dentate granule cell neurogenesis following amygdala kindling in the adult rat

Structural neuronal network plasticity is associated with epileptogenesis during limbic kindling, but the full extent of network changes is not well understood. We investigated whether dentate granule cell (DGC) neurogenesis, which continues into adulthood in the rodent, is altered in the amygdala kindling model of epileptogenesis. Adult rats were stimulated to either 4–6, 9–10 or 19–20 class 4/5 (generalized) kindled seizures. 5-Bromo-2′-deoxyuridine labeling showed that cell proliferation increased in the dentate gyrus only in animals that experienced nine or more class 4/5 kindled seizures. Immunocytochemistry for neuronal markers revealed that many of the newly generated cells differentiated into DGCs in the inner aspect of the DGC layer. The lack of increased DGC neurogenesis after fewer kindled seizures or at early timepoints following kindling suggests that this process is not involved in kindling development. Instead, newly generated DGCs may be important for maintenance of the kindled state or the increased susceptibility to spontaneous recurrent seizures.

Dentate Granule Cell Neurogenesis Is Increased by Seizures and Contributes to Aberrant Network Reorganization in the Adult Rat Hippocampus

The dentate granule cell layer of the rodent hippocampal formation has the distinctive property of ongoing neurogenesis that continues throughout adult life. In both human temporal lobe epilepsy and rodent models of limbic epilepsy, this same neuronal population undergoes extensive remodeling, including reorganization of mossy fibers, dispersion of the granule cell layer, and the appearance of granule cells in ectopic locations within the dentate gyrus. The mechanistic basis of these abnormalities, as well as their potential relationship to dentate granule cell neurogenesis, is unknown. We used a systemic chemoconvulsant model of temporal lobe epilepsy and bromodeoxyuridine (BrdU) labeling to investigate the effects of prolonged seizures on dentate granule cell neurogenesis in adult rats, and to examine the contribution of newly differentiated dentate granule cells to the network changes seen in this model. Pilocarpine-induced status epilepticus caused a dramatic and prolonged increase in cell proliferation in the dentate subgranular proliferative zone (SGZ), an area known to contain neuronal precursor cells. Colocalization of BrdU-immunolabeled cells with the neuron-specific markers turned on after division, 64 kDa, class III beta-tubulin, or microtubule-associated protein-2 showed that the vast majority of these mitotically active cells differentiated into neurons in the granule cell layer. Newly generated dentate granule cells also appeared in ectopic locations in the hilus and inner molecular layer of the dentate gyrus. Furthermore, developing granule cells projected axons aberrantly to both the CA3 pyramidal cell region and the dentate inner molecular layer. Induction of hippocampal seizure activity by perforant path stimulation resulted in an increase in SGZ mitotic activity similar to that seen with pilocarpine administration. These observations indicate that prolonged seizure discharges stimulate dentate granule cell neurogenesis, and that hippocampal network plasticity associated with epileptogenesis may arise from aberrant connections formed by newly born dentate granule cells.

An in vivo study of the ontogeny of long-term potentiation (LTP) in the CA1 region and in the dentate gyrus of the rat hippocampal formation

The influences of stimulus intensity, intratrain frequency, and number of trains were studied for their effects on the development of long-term potentiation (LTP) in the hippocampal formation. LTP was analyzed with full input-output curves for population spike (PS) amplitudes (PS-LTP) calculated from responses elicited in the CA1 region and in the dentate gyrus by monosynaptic activation. A standardized protocol employing a sequence of stimuli was devised to systematically compare LTP in the dentate gyrus to that in the CA1 region in rats of various ages ranging from postnatal day (PN) 6 to adults (PN 60). In adult animals, the degrees of LTP were comparable in the dentate gyrus and CA1 region for the 3 stimulus strengths studied (intensity just subthreshold for PS, intensity giving 1 4 maximal PS, and intensity giving 1 2 maximal PS). LTP developed at different rates in the two regions, reaching adult values about two weeks after birth in CA1 but about 3 weeks after birth in the dentate gyrus. We postulate that differences in the rate of development in CA1 and in the dentate gyrus are related to the later neurogenesis of dentate granule cells and also possibly to a later functional maturation of N- methyl- d-aspartate (NMDA) receptor-channel complexes on these cells.