Functional implications of seizure-induced neurogenesis

The neurobiological doctrine governing the concept of neurogenesis has undergone a revolution in the past few years. What was once considered dubious is now well accepted: new neurons are born in the adult brain. Science fiction is quickly becoming a reality as scientists discover ways to convert skin, bone, or blood cells into neurons. In the epilepsy arena, widespread interest has developed because of the evidence that neurogenesis increases after seizures, trauma, and other insults or injuries that alter seizure susceptibility. This review discusses some of the initial studies in this field, and their often surprising functional implications. The emphasis will be on the granule cells of hippocampus, because they are perhaps more relevant to epilepsy than other areas in which neurogenesis occurs throughout life, the olfactory bulb and subventricular zone. In particular, the following questions will be addressed: 1. Do granule cells that are born in the adult brain become functional, and what are the limits of their function? Do they behave homogeneously? Results from our own laboratory have focused on cells that become established outside the normal boundaries of the granule cell layer, forming a group of "ectopic" granule cells in the hilar region. 2. Is increased neurogenesis beneficial, or might it actually exacerbate seizures? Evidence is presented that supports the hypothesis that new granule cells may not necessarily act to ameliorate seizures, and might even contribute to them. Furthermore, cognitive deficits following seizures might in part be due to new circuits that develop between new cells and the host brain. 3. How do the new cells interact with the host brain? Several changes occur in the dentate gyrus after seizures, and increased neurogenesis is only one of many. What is the interdependence of this multitude of changes, if any? 4. Is neurogenesis increased after seizures in man? Research suggests that the data from human epileptics are actually inconsistent with the studies in animal models of epilepsy, because there is little evidence of increased neurogenesis in epileptic tissue resected from intractable epileptics. Yet neurogenesis has been shown to occur in humans throughout adult life. What might be the reasons for these seemingly disparate results?

Morphological transformations of cells immunopositive for GFAP, TrkA or p75 in the CA1 hippocampal area following transient global ischemia in the rat. A quantitative study

Transient global ischemia induces intensive neuronal degeneration in the hippocampal CA1 pyramidal layer, accompanied by reactive transformation of glial cells. Previously, we have shown using the double immunostaining method that the NGF receptors (NGFR) p75 and TrkA are expressed mainly on subpopulations of GFAP+ astrocytes, and this expression increases progressively after ischemia. In the presented study, we analyzed quantitatively the morphological transformations of cells immunopositive for GFAP or NGF receptors in the stratum radiatum of the CA1 hippocampal area in different survival periods after ischemia, evoked by 10-min cardiac arrest in adult rats. In control brains, NGF receptors were expressed only on small cells with poorly ramified processes. After ischemia, the NGFR+ cells increased in size and morphological complexity (measured using fractal analysis). However, even 2 weeks after ischemia these cells did not reach the size and value of the fractal dimension typical of the largest GFAP+ astrocytes. Moreover, the reaction of NGFR+ cells was significantly delayed in comparison with the total astrocyte population. The obtained results suggest that NGF receptors are expressed mainly by immature astrocytes and ischemia induces the maturation of these cells.

Life-long stability of neurons: a century of research on neurogenesis, neuronal death and neuron quantification in adult CNS

In this chapter we provide an extensive review of 100 years of research on the stability of neurons in the mammalian brain, with special emphasis on humans. Although Cajal formulated the Neuronal Doctrine, he was wrong in his beliefs that adult neurogenesis did not occur and adult neurons are dying throughout life. These two beliefs became accepted "common knowledge" and have shaped much of neuroscience research and provided much of the basis for clinical treatment of age-related brain diseases. In this review, we consider adult neurogenesis from a historical and evolutionary perspective. It is concluded, that while adult neurogenesis is a factor in the dynamics of the dentate gyrus and olfactory bulb, it is probably not a major factor during the life-span in most brain areas. Likewise, the acceptance of neuronal death as an explanation for normal age-related senility is challenged with evidence collected over the last fifty years. Much of the problem in changing this common belief of dying neurons was the inadequacies of neuronal counting methods. In this review we discuss in detail implications of recent improvements in neuronal quantification. We conclude: First, age-related neuronal atrophy is the major factor in functional deterioration of existing neurons and could be slowed down, or even reversed by various pharmacological interventions. Second, in most cases neuronal degeneration during aging is a pathology that in principle may be avoided. Third, loss of myelin and of the white matter is more frequent and important than the limited neuronal death in normal aging.

Effects of cortical ischemia and postischemic environmental enrichment on hippocampal cell genesis and differentiation in the adult rat

The study aimed to elucidate the effects of cortical ischemia and postischemic environmental enrichment on hippocampal cell genesis. A cortical infarct was induced by a permanent ligation of the middle cerebral artery distal to the striatal branches in 6-month-old spontaneously hypertensive rats. Bromodeoxyuridine (BrdU) was administered as 7 consecutive daily injections starting 24 hours after surgery and animals were housed in standard or enriched environment. Four weeks after completed BrdU administration, BrdU incorporation and its co-localization with the neuronal markers NeuN and calbindin D28k, and the astrocytic marker glial fibrillary acidic protein in the granular cell layer and subgranular zone of the hippocampal dentate gyrus were determined with immunohistochemistry and were quantified stereologically. Compared with sham-operated rats, rats with cortical infarcts had a five-to sixfold ipsilateral increase in BrdU-labeled cells. About 80% of the new cells were neurons. Differential postischemic housing did not influence significantly the total number of surviving BrdU-labeled cells or newborn neurons. However, postischemic environmental enrichment increased the ipsilateral generation of astrocytes normalizing the astrocyte-to-neuron ratio, which was significantly reduced in rats housed in standard environment postischemically.