Functional implications of seizure-induced neurogenesis

Abstract

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?

Figure 1

Schematic diagram of changes in neurogenesis of hippocampal granule cells after seizures. Top left: A low level of neurogenesis is normal in the adult dentate gyrus of the rat. A diagram of a transverse section of the rat hippocampus is shown. Neurogenesis normally occurs in a layer positioned immediately below the granule cell layer which is called the subgranular zone (SGZ). MOL = molecular layer, GCL = granule cell layer. Center: Increased neurogenesis occurs after seizures. This is thought to arise from the SGZ primarily. Lower right: After seizures induced by pilocarpine, some of the newly born cells appear to migrate from the SGZ incorrectly, entering the molecular layer and hilus (arrows).

Figure 2

Ectopic hilar granule cells in the dentate gyrus after kainic acid-induced status epilepticus. A) A section from a saline-injected rat demonstrates calbindin-immunoreactivity predominantly in the cell bodies, dendrites and axons of dentate gyrus granule cells. PCL = pyramidal cell layer; HIL = hilus; GCL= granule cell layer. Calibration = 200 μm. B) A section from a kainic acid-injected rat, over 1 month after status epilepticus. Numerous calbindin-immunoreactive neurons surround the border of the hilus and pyramidal cell layer (arrows). Same calibration as A. Used with permission, ref. .

Figure 3

Morphology and electrophysiology of ectopic hilar granule cells born after seizures. A) The morphology of a hilar granule cell from a pilocarpine-treated rat (arrowhead) is shown after intracellular injection of Neurobiotin. For comparison, two granule cells in the granule cell layer (arrow) were also injected. G = granule cell layer; H = hilus; P = pyramidal cell layer. Calibration = 100 μm. B) Physiology of hilar granule cells. Responses to direct current injection (rectangular current pulses, as shown at the bottom) are superimposed for a granule cell located in the granule cell layer (left, “granule cell”) and a granule-like neuron located in the hilus (right, “hilar cell”) of the same slice. This slice was from a pilocarpine-treated rat which had status epilepticus and recurrent seizures. A diagram of the location of these cells is shown at top right. G = granule cell layer, H = hilus, P = pyramidal cell layer. Arrowheads mark spontaneous synaptic potentials. Calibration = 20 mV, 50 msec. C) Comparison of pyramidal cell and ectopic granule cell activity. Simultaneous extracellular recordings from the pyramidal cell layer of CA3b (bottom) and intracellular recordings are shown for a hilar granule cell (left) and a pyramidal cell (right). This slice was from a pilocarpine-treated rat that had status epilepticus and recurrent seizures. A diagram of the location of the cells (dots) and extracellular recording (x) is shown at top right. The arrows point to the population spike of the spontaneous burst discharge recorded extracellularly. Calibration same as B. Used with permission, ref. .

Figure 4

Ectopic hilar granule cells are born after status epilepticus. A-B: Following injection of BrdU after status epilepticus, tissue sections were double-labeled for BrdU and either NeuN (A) or calbindin (B). The results showed numerous double-labeled neurons (arrows) at the border of the hilus and pyramidal cell layer. Calibration = 30 μm (A) and 60 μm (B). PCL = end of pyramidal cell layer. From ref. with permission: C-D. In a different animal, double-labeling with BrdU and neuropeptide Y (C) or BrdU and parvalbumin (D) demonstrated no double-labeled cells, but numerous hilar neurons that were immunoreactive for neuropeptide Y or parvalbumin (arrows). Calibration (in A) = 60 μm (C) and 120 μm (D).

Figure 5

Ectopic granule cells in the hilus are active during a spontaneous seizure. Immunoreactivity for c-fos and calbindin in a pilocarpine-treated animal that had spontaneous seizures and was sacrificed 3 hours after a spontaneous seizure was observed. A-B) There are two double-labeled neurons in the hilus (arrow), enlarged in B. Calibration = 100 μm (A); 25 μm (B). C) Double-labeled hilar neurons in another animal are shown. Calibration (in A) = 50 μm. Used with permission, ref. .

Figure 6

Schematic illustration of the development of widespread seizure activity from initial synchronized burst discharges among CA3 pyramidal cells and ectopic hilar granule cells. 1. CA3 burst. Initially, activity that is synchronized in area CA3 neurons develops after status epilepticus. The reasons may include increased recurrent excitatory activity due to loss of interneurons, as well as sprouting among residual pyramidal cells. 2. CA3 to hilus. Based on empirical findings in pilocarpine-treated rats, synchronized burst discharges develop among pyramidal cells, newly-born hilar ectopic granule cells (EGCs) and hilar mossy cells (MCs) several months after status epilepticus. This is likely to be due to the normal projection of pyramidal cells to hilar neurons, and the mossy fiber axon that develops in EGCs. 3. hilus to GC. Because EGCs have mossy fiber collaterals that contribute to the sprouted fiber plexus in the inner molecular layer, and MCs also project there, the bursts can potentially lead to activation of granule cells located in the granule cell layer. This would be limited by whatever interneurons are innervated by EGCs and MCs. 4. GC to GC. GC activation may be amplified because they are interconnected by sprouted fibers. However, this also would be limited by the extent that sprouted fibers activate GABAergic neurons. 5. GCs to CA3 to CA1. Strong excitatory activity that develops in the granule cell layer could potentially exit the hippocampus and trigger a limbic seizure by the trisynaptic circuit.

Figure 7

Effects of ectopic hilar granule cells on normal signal processing in the trisynaptic circuit. It is proposed that the development of ectopic hilar granule cells could contribute to cognitive deficits in epileptics by interfering with the normal trisynaptic circuit. A) The trisynaptic circuit is schematized as perforant path axons innervating distal granule cell dendrites (1), granule cell mossy fibers innervating proximal CA3 pyramidal cell dendrites (2), and CA3 pyramidal cell projections to apical dendrites of CA1 pyramidal cells (3). The accumulation of hilar ectopic granule cells with axons that innervate pyramidal cells, hilar neurons, and the inner molecular layer, suggests a multitude of complex pathways that would potentially interfere with normal transmission along the trisynaptic pathway, as shown diagrammatically in B.

Figure 8

Interaction of newly-born granule cells and the host environment. A-B) A schematic illustrates the potential interactions between developing granule cells born after seizures and the adjacent adult (host) neurons. After seizures, new cells are born in the subgranular zone, and at a similar time, there is altered expression of various proteins in the surrounding adult granule cells and adult hilar neurons. The fact that these two phenomena occur at a similar time may be no coincidence, because the proteins may enhance growth, survival, and proliferation of newly-born cells. Examples include BDNF, zinc, neuropeptide Y and somatostatin. BDNF and zinc are normally present in adult granule cells, and BDNF expression increases in granule cells after seizures. NPY and somatostatin are present in a subset of dentate gyrus GABAergic neurons. After seizures, some of these cells increase their expression of NPY, and many somatostatin-containing neurons die. BDNF, NPY, and zinc appear to have a positive influence and somatostatin a negative influence on processes relating to cell proliferation, so the increase in BDNF, NPY and decrease in somatostatin may foster phenomena associated with neurogenesis. Interestingly, these same substances also have effects on adult synaptic transmission, making their overall influence on the epileptic dentate gyrus network difficult to predict. For further discussion, see text.