Differential Spatial Patterns of Fos Induction Following Generalized Clonic and Generalized Tonic Seizures

The expression of generalized clonic and generalized tonic seizures has been suggested to result from the activation of different and independent neuronal circuits. Using the induction of the c-fosprotein (Fos) as a marker of neuronal activity, we identified brain structures that are differentially associated with the expression of electroconvulsive shock-induced generalized clonic and generalized tonic seizures. Expression of either seizure phenotype resulted in a similar bilaterally symmetrical increase in Fos immunoreactivity in many forebrain structures, including the bed nucleus of the stria terminalis, hippocampal dentate gyrus, amygdala, and piriform cortex, compared to controls. However, following tonic hindlimb extension (THE), the degree of labeling in specific thalamic, hypothalamic, and brain stem areas was significantly greater than that of either controls or animals exhibiting clonic seizures. While a greater number of neurons in the hypothalamus (e.g., ventromedial nucleus), subparafascicular thalamic nucleus, peripeduncular area, deep medial superior colliculus, dorsal and lateral central gray, and paralemniscal nuclei were robustly labeled following THE, noticeably fewer cells were immunoreactive following face and forelimb clonic seizure behaviors. These differences were also found to be independent of the stimulus magnitude. In animals stimulated with the same current intensity but expressing either of the two seizure phenotypes, the pattern of Fos induction was consistent with the seizure phenotype expressed. These results demonstrate that specific subsets of neurons are differentially activated following the expression of different generalized seizure behaviors and that activity in discrete mesencephalic and diencephalic structures is more frequently associated with the expression of generalized tonic seizures than with the expression of generalized clonic seizures.     

Prevention of Kainic Acid-induced Limbic Seizures and Fos Expression by the GABA-A Receptor Agonist Muscimol

Fos oncoprotein expression has been shown to be a sensitive marker for sequential neuronal activation in response to a specific stimulus. The present study investigated the effect of the γ-aminobutyric acid (GABA)-A receptor agonist muscimol on kainic acid (KA)-induced limbic seizures and Fos expression in the rat forebrain. One hour after KA injection, a substantial Fos expression was observed in the hippocampal dentate gyrus, whereas only a low level of Fos induction was seen in CA1–3 fields. Six hours post-injection a prominent increase of Fos expression occurred in most forebrain structures, including the whole hippocampus. Following 0.5 mg/kg muscimol treatment a remarkable decrease of Fos expression occurred but only in the caudate putamen and core of the accumbens nucleus. Treatment with 1 mg/kg muscimol led to further significant decreases of Fos expression in CA1–3 pyramidal neurons and the disappearance of Fos induction in the cerebral cortex above the rhinal fissure, reticular thalamic nucleus, claustrum, fundus striati, ventral pallidum, septal nucleus, lateral habenular nucleus, and lateral amygdaloid nucleus. When 2 mg/kg muscimol was injected, animals exhibited 'absence seizures' instead of limbic seizures, and Fos expression in the hippocampus was effectively blocked. These results suggest that a reduction of GABAergic inhibition plays a crucial role not only in limbic seizure genesis in the dentate gyrus, but also in the seizure spread mechanism in many brain structures, among which the hippocampal CA1–3 fields are most markedly involved, less marked in the cerebral cortex and some other structures, and least marked in the caudate putamen and core of the accumbens nucleus.     

Hippocampus of the seizure-sensitive gerbil is a specific site for anatomical changes in the GABAergic system

The brains of seizure-sensitive (SS) and seizure-resistant (SR) gerbils were studied with an immunocytochemical method to localize glutamic acid decarboxylase (GAD) to determine whether a defect existed in the inhibitory GABAergic system similar to that which has been reported in animal models of focal epilepsy in which GABAergic cell bodies and terminals are decreased in number. A major difference between the two strains of gerbils was found in the number of GABAergic neurons in the hippocampal formation. Specifically, a paradoxical increase occurred in the number of glutamate decarboxylase GAD-immunoreactive neurons: there were approximately 65% more GABAergic cells within the dentate gyrus and the CA3 region of the hippocampus in the SS gerbils. Furthermore, the density of GAD-immunoreactive puncta, the light microscopic correlates of synaptic boutons, was greater in the SS animals. Other histological methods were used to determine if the difference between SS and SR gerbils was specific for the GABAergic system. Nissl-stained preparations showed that the number of granule cells in the dentate gyrus was 20% greater in SS gerbils than in SR gerbils. An examination of some hippocampal afferents, efferents, and intrinsic connections with acetylcholinesterase histochemistry and the Timm's stain for heavy metals demonstrated no differences between the two strains. In addition, Golgi-stained preparations of the dentate gyrus indicated that the morphology of basket cells did not differ between the two strains nor between the gerbil and the rat. Several brain regions in addition to the hippocampus were studied to determine whether or not the increased number of GAD-immunoreactive neurons was specific for the hippocampal formation. These regions included the substantia nigra, motor cortex, and nucleus reticularis thalami and were selected because they contain large populations of GABAergic neurons and have been implicated in seizure activity. No differences between the two strains were detected in any of these regions. Therefore, a major morphological difference between the brains of SS and SR gerbils exists in the hippocampal formation of SS gerbils in which an increase occurs in the number of GABAergic neurons and granule cells. If these additional inhibitory neurons act mainly to inhibit other inhibitory neurons, the net effect would be increased disinhibition of the principal excitatory neurons of the hippocampal formation. This could lead to seizure activity within the hippocampal formation and at distant sites through multiple synaptic connections.     

Induction of immediate-early gene proteins in dentate granule cells and somatostatin interneurons after hippocampal seizures.

The expression of the protein products of the immediate-early genes c-fos, Fos B, Fos-related proteins (FRAs), c-jun, jun B, jun D and krox-24 was investigated in the rat hippocampus at various times after electrically-induced hippocampal seizures. Hippocampal seizures induced all the immediate-early gene proteins in dentate granule cells with differing time-courses. In addition, Krox-24, Fos and Jun D were also induced in somatostatin-containing interneurons throughout the hippocampus and also in a small percentage of parvalbumin-containing interneurons. Thus, hippocampal seizures induce waves of immediate-early gene protein expression in dentate granule cells and a selective expression of krox-24, Fos and Jun D in hippocampal somatostatin interneurons. These results suggest that biochemical and/or morphological changes occurring in dentate granule cells and somatostatin interneurons after seizures may be regulated by immediate-early gene expression, and that these immediate-early gene proteins may be involved in seizure development in the nervous system.     

Response of seizure-induced newborn neurons in the dentate gyrus of adult rats to second episode of seizures

In this study we examined the unknown issue of whether seizure-induced newborn hippocampal neurons in freely moving adult rats are able to respond to pathophysiological stimuli in the same way as their neighboring neurons do. Three days after pentylenetrazol (PTZ)-induced generalized seizures, rats received 5-bromodeoxyuridine (BrdU) injections to label dividing cells, followed 4 weeks later by the second PTZ injection to induce second episode of generalized seizures. We observed that the first episode of PTZ-induced seizures resulted in a significant increase in the number of newborn neurons in the adult hippocampal dentate gyrus. In comparison with vehicle-injected control rats that exhibited no Fos immunoreactivity and mild glutamic acid decarboxylase 67 (GAD67) expression in the dentate granule cells, rats killed 2-6 h following the second PTZ injection showed intensive Fos and GAD67 expression in virtually all granule cells with or without BrdU double-labeling. These findings provide important evidence indicating that seizure-induced newborn neurons in freely moving adult rats are able to respond to pathophysiological stimuli in the same way as neighboring neurons do.     

Testing the disinhibition hypothesis of epileptogenesis in vivo and during spontaneous seizures.

The "disinhibition" hypothesis contends that (1) seizures begin when granule cells in the dentate gyrus of the dorsal hippocampus are disinhibited and (2) disinhibition occurs because GABAergic interneurons are excessively inhibited by other GABAergic interneurons. We tested the disinhibition hypothesis using the experimental model that inspired it-naturally epileptic Mongolian gerbils. To determine whether there is an excess of GABAergic interneurons in the dentate gyrus of epileptic gerbils, as had been reported previously, GABA immunocytochemistry, in situ hybridization of GAD67 mRNA, and the optical fractionator method were used. There were no significant differences in the numbers of GABAergic interneurons. To determine whether granule cells in epileptic gerbils were disinhibited during the interictal period, IPSPs were recorded in vivo with hippocampal circuits intact in urethane-anesthetized gerbils. The reversal potentials and conductances of IPSPs in granule cells in epileptic versus control gerbils were similar. To determine whether the level of inhibitory control in the dentate gyrus transiently decreases before seizure onset, field potential responses to paired-pulse perforant path stimulation were obtained from the dorsal hippocampus while epileptic gerbils experienced spontaneous seizures. Evidence of reduced inhibition was found after, but not before, seizure onset, indicating that seizures are not triggered by disinhibition in this region. However, seizure-induced depression of inhibition may amplify and promote the spread of seizure activity to other brain regions. These findings do not support the disinhibition hypothesis and suggest that in this model of epilepsy seizures initiate by another mechanism or at a different site.     

On the role of seizure activity in the hippocampal damage produced by trimethyltin

Trimethyltin (TMT) causes a pattern of hippocampal damage in rats that is similar to that caused by convulsant chemicals or seen in the brains of some human epileptics. Therefore, we investigated the possible role that TMT-induced seizure activity might play in the hippocampal damage produced by this organotin. The morphologic effects of systemically administered TMT were compared to those of kainic acid given by the same route. Unlike kainate, TMT produced seizures in only a subset of treated animals and with a latency of days rather than minutes. Evaluation of morphology during the acute seizure period revealed that TMT-induced seizures were associated with a variable pattern of granule and pyramidal cell necrosis and acute dendritic swelling in the two associational/commissural hippocampal pathways, one from CA3 to CA1-CA3 and the other from the hilus to the proximal dendrites of dentate granule cells. The TMT-induced damage contrasted sharply with the acute pattern of kainate-induced damage that consisted of acute dendritic swellings in the distal granule cell dendrites, hilus and mossy fiber region. TMT-treated rats that did not exhibit seizures in the one week after injection exhibited minimal pathology during this period. These results suggest that at least part of the damage to granule and pyramidal cells produced by TMT is mediated by the seizure activity produced by this compound. Although the resulting lesions to the CA1-CA3 pyramidal cells may appear similar in both TMT- and kainate-treated rats long after injection, evaluation of acute pathology during the active seizure phase indicates that these compounds induce seizure activity in different hippocampal pathways and cause different patterns of irreversible neuronal damage as a result.     

Resistance of immature hippocampus to morphologic and physiologic alterations following status epilepticus or kindling.

Seizures in adult rats result in long-term deficits in learning and memory, as well as an enhanced susceptibility to further seizures. In contrast, fewer lasting changes have been found following seizures in rats younger than 20 days old. This age-dependency could be due to differing amounts of hippocampal neuronal damage produced by seizures at different ages. To determine if there is an early developmental resistance to seizure-induced hippocampal damage, we compared the effects of kainic acid (KA)-induced status epilepticus and amygdala kindling on hippocampal dentate gyrus anatomy and electrophysiology, in immature (16 day old) and adult rats. In adult rats, KA status epilepticus resulted in numerous silver-stained degenerating dentate hilar neurons, pyramidal cells in fields CA1 and CA3, and marked numerical reductions in CA3c pyramidal neuron counts (-57%) in separate rats. Two weeks following the last kindled seizure, some, but significantly less, CA3c pyramidal cell loss was observed (-26%). Both KA status epilepticus and kindling in duced mossy-fiber sprouting, as evidenced by ectopic Timm staining in supragranular layers of the dentate gyrus. In hippocampal slices from adult rats, paired-pulse stimulation of perforant path axons revealed a persistent enhancement of dentate granule-cell inhibition following KA status epilepticus or kindling. While seizures induced by KA or kindling in 16-day-old rats were typically more severe than in adults, the immature hippocampus exhibited markedly less KA-induced cell loss (-22%), no kindling-induced loss, no detectable synaptic rearrangement, and no change in dentate inhibition. These results demonstrate that, in immature rats, neither severe KA-induced seizures nor repeated kindled seizures produce the kind of hippocampal damage and changes associated with even less severe seizures in adults. The lesser magnitude of seizure-induced hippocampal alterations in immature rats may explain their greater resistance to long-term effects of seizures on neuronal function, as well as future seizure susceptibility. Conversely, hippocampal neuron loss and altered synaptic physiology in adults may contribute to increased sensitivity to epileptogenic stimuli, spontaneous seizures, and behavioral deficits.     

Differential and long-lasting alterations of high-voltage activated calcium currents in CA1 and dentate granule neurons after status epilepticus

The effects on high-voltage activated (HVA) calcium currents were examined in hippocampal CA1 cells and dentate gyrus (DG) granule neurons, 2 days (short-term; ST) and 2-3 months (long-term; LT) after electrically induced, limbic electrographic and behavioural seizures in rats. Whole-cell voltage-clamp recordings in dissociated CA1 neurons of LT rats showed a decrease in the sustained HVA calcium current amplitude and a faster inactivation of the current both in rats that had experienced a status epilepticus (post-SE rats) and those in which the stimulation did not lead to SE (non-SE rats). In CA1 neurons of LT-SE rats this resulted in a reduced Ca2+ entry through the HVA channels. Perforated-patch voltage-clamp recordings in dissociated DG granule neurons of LT-SE rats showed an increased sustained HVA current amplitude compared to controls and non-SE rats, leading to an increased Ca2+ entry via HVA calcium channels. Two days after SE, we observed an increased Ca2+ entry for a defined depolarization, although the change in HVA current amplitude and inactivation rate did not reach significance. We also observed a decrease in calbindin-D28k staining in DG post-SE neurons, but this change was not associated with a change in HVA current inactivation. The opposite changes in neuronal Ca2+ entry through HVA channels in CA1 vs. DG cells depended strongly on whether rats had experienced SE and later spontaneous seizure activity. These changes are likely to contribute to regionally different effects on local network excitability.     

Long-term genetic fate mapping of adult generated neurons in a mouse temporal lobe epilepsy model

In the epileptic brain, seizures can increase hippocampal neurogenesis, while opposingly seizure-associated brain pathology has been shown to detrimentally affect neurogenesis. The long-term impact of recurrent seizures on the number of new neurons as well as their relative contribution to the granule cell layer remains an open question. Therefore we analyzed neuron addition based on genetic fate mapping in a chronic model of epilepsy comparing non-kindled animals and kindled animals having at least one generalized seizure with and without further seizures. The number of all new granule cells added to the dentate gyrus following the onset of kindling was significantly increased (7.0-8.9 fold) in kindled groups. The hyperexcitable kindled state and a prior seizure history proved to be sufficient to cause a pronounced long-term net effect on neuron addition. An ongoing continuous occurrence of seizures did not further increase the number of new granule cells in the long-term. In contrast, a correlation was found between the cumulative duration of seizures and neuron addition following a kindled state. In addition, the overall number of seizures influenced the relative portion of new cells among all granule cells. Non-kindled animals showed 1.6% of new granule cells among all granular cells by the end of the experiment. This portion reached 5.7% in the animals which experienced either 10 or 22 seizures. A percentage of 8.4% new cells were determined in the group receiving 46 seizures which is a significant increase in comparison to the control group. In conclusion, permanent genetic fate mapping analysis demonstrated that recurrent seizures result in a lasting change in the makeup of the granule cell layer with alterations in the relative contribution of newborn neurons to the granule cell network. Interestingly, the formation of a hyperexcitable kindled network even without recent seizure activity can result in pronounced long-term alterations in the absolute number of new granule cells. However, seizure density also seems to play a critical role with more frequent seizures resulting in increased fractions of new neurons.     

Hippocampal granule cell activity and c-Fos expression during spontaneous seizures in awake, chronically epileptic, pilocarpine-treated rats: implications for hippocampal epileptogenesis

The process of postinjury hippocampal epileptogenesis may involve gradually developing dentate granule cell hyperexcitability caused by neuron loss and synaptic reorganization. We tested this hypothesis by repeatedly assessing granule cell excitability after pilocarpine-induced status epilepticus (SE) and monitoring granule cell behavior during 235 spontaneous seizures in awake, chronically implanted rats. During the first week post-SE, granule cells exhibited diminished paired-pulse suppression and decreased seizure discharge thresholds in response to afferent stimulation. Spontaneous seizures often began during the first week after SE, recruited granule cell discharges that followed behavioral seizure onsets, and evoked c-Fos expression in all hippocampal neurons. Paired-pulse suppression and epileptiform discharge thresholds increased gradually after SE, eventually becoming abnormally elevated. In the chronic epileptic state, interictal granule cell hyperinhibition extended to the ictal state; granule cells did not discharge synchronously before any of 191 chronic seizures. Instead, granule cells generated only low-frequency voltage fluctuations (presumed "field excitatory postsynaptic potentials") during 89% of chronic seizures. Granule cell epileptiform discharges were recruited during 11% of spontaneous seizures, but these occurred only at the end of each behavioral seizure. Hippocampal c-Fos after chronic seizures was expressed primarily by inhibitory interneurons. Thus, granule cells became progressively less excitable, rather than hyperexcitable, as mossy fiber sprouting progressed and did not initiate the spontaneous behavioral seizures. These findings raise doubts about dentate granule cells as a source of spontaneous seizures in rats subjected to prolonged SE and suggest that dentate gyrus neuron loss and mossy fiber sprouting are not primary epileptogenic mechanisms in this animal model.     

On the role of somatostatin in seizure control: clues from the hippocampus

The role of the hippocampal somatostatin (somatotropin release-inhibiting factor, SRIF) system in the control of partial complex seizures is discussed in this review. The SRIF system plays a role in the inhibitory modulation of hippocampal circuitries under normal conditions: 1) SRIF neurons in the dentate gyrus are part of a negative feedback circuit modulating the firing rate of granule cells; 2) SRIF released in CA3 interacts both with presynaptic receptors located on associational/commissural terminals and with postsynaptic receptors located on pyramidal cell dendrites, reducing excitability of pyramidal neurons; 3) in CA1, SRIF exerts a feedback inhibition and reduces the excitatory drive on pyramidal neurons. Significant changes in the hippocampal SRIF system have been documented in experimental models of temporal lobe epilepsy (TLE), in particular in the kindling and in the kainate models. SRIF biosynthesis and release are increased in the kindled hippocampus, especially in the dentate gyrus. This hyper-function may be instrumental to control the latent hyperexcitability of the kindled brain, preventing excessive discharge of the principal neurons and the occurrence of spontaneous seizures. In contrast, the hippocampal SRIF system undergoes damage in the dentate gyrus following kainate-induced status epilepticus. Although surviving SRIF neurons appear to hyperfunction, the loss of hilar SRIF interneurons may compromise inhibitory mechanisms in the dentate gyrus, facilitating the occurrence of spontaneous seizures. In keeping with these data, pharmacological activation of SRIF1 (sst2) receptors, i.e. of the prominent receptor subtype on granule cells, exerts antiseizure effects. Taken together, the data presented suggest that the hippocampal SRIF system plays a role in the control of partial complex seizures and, therefore, that it may be proposed as a therapeutic target for TLE.     

The functional organization of the hippocampal dentate gyrus and its relevance to the pathogenesis of temporal lobe epilepsy

Temporal lobe seizures are frequently associated with a characteristic pattern of hippocampal pathology (hippocampal sclerosis), as well as pathology in other temporal lobe structures. Despite more than a century of study, the relationship between pathology and epileptogenesis remains unclear. Endfolium sclerosis, which is characterized by the loss of dentate hilar neurons that are presumed to govern dentate granule cell excitability, is evident whenever hippocampal sclerosis exists and is the only temporal lobe pathology in some patients. Because prolonged seizures or head trauma produce endfolium sclerosis and granule cell hyperexictability in experimental animals, hilar neuron loss may be the common pathological denominator and primary network defect underlying development of a hippocampal seizure “focus.” Physiological studies suggest that vulnerable hilar mossy cells normally excite neurons that mediate granule cell inhibition. Recent anatomical studies indicate that the axons of mossy cells project longitudinally, out of the lamellar plane in which their cell bodies lie. If mossy cells in one lamella excite inhibitory neurons in surrounding lamellae, neocortical excitation of one segment of the granule cell layer may produce lateral inhibition and limit neocortical excitation to the targeted lamella. In patients who have had status epilepticus, prolonged febrile seizures, head trauma, or encephalitis, loss of dentate mossy cells may deafferent inhibitory neurons, render them “dormant,” and thereby disinhibit an encephalitis, loss of dentate mossy cells may deafferent inhibitory neurons, render them “dormant,” and thereby disinhibit an enlarged expanse of the granule cell layer. The selective loss of neurons that normally govern lateral inhibition in the dentate gyrus may cause functional delamination of the granule cell layer and result in synchronous, multilamellar discharges in response to cortical stimuli. Repetitive seizures may ultimately produce the full pattern of hippocampal and mesial temporal sclerosis by destroying cells within the seizure circuit that were not injured irreversibly by the initial insult. Thus, hippocampal pathology may be both the cause and effect of seizures that originate in the temporal lobe.     

Roles of astrocytes and microglia in seizure‐induced aberrant neurogenesis in the hippocampus of adult rats

Recent evidence showed that epileptic seizures increase hippocampal neurogenesis in the adult rat, but prolonged seizures result in the aberrant hippocampal neurogenesis that often leads to a recurrent excitatory circuitry and thus contributes to epileptogenesis. However, the mechanism underlying the aberrant neurogenesis after prolonged seizures remains largely unclear. In this study, we examined the role of activated astrocytes and microglia in the aberrant hippocampal neurogenesis induced by status epilepticus. Using a lithium‐pilocarpine model to mimic human temporal lobe epilepsy, we found that status epilepticus induced a prominent activation of astrocytes and microglia in the dentate gyrus 3, 7, 14, and 20 days after the initial seizures. Then, we injected fluorocitrate stereotaxicly into the dentate hilus to inhibit astrocytic metabolism and found that fluorocitrate failed to prevent the seizure‐induced formation of ectopic hilar basal dendrites but instead promoted the degeneration of dentate granule cells after seizures. In contrast, a selective inhibitor of microglia activation, minocycline, inhibited the aberrant migration of newborn neurons at 14 days after status epilepticus. Furthermore, with stereotaxic injection of lipopolysaccharide into the intact dentate hilus to activate local microglia, we found that lipopolysaccharide promoted the development of ectopic hilar basal dendrites in the hippocampus. These results indicate that the activated microglia in the epileptic hilus may guide the aberrant migration of newborn neurons and that minocycline could be a potential drug to impede seizure‐induced aberrant migration of newborn neurons. © 2009 Wiley‐Liss, Inc.     

Concentration- and cell type-specific effects of calbindin D28k on vulnerability of hippocampal neurons to seizure-induced injury

The calcium-binding protein calbindin D28k (CB) is expressed in limited subpopulations of neurons in the brain. In the hippocampus, CB is expressed in all dentate granule cells and a subpopulation of CA1 pyramidal neurons, but is absent from CA3 neurons. This pattern of CB expression is inversely correlated with neuronal vulnerability to seizure-induced damage suggesting the possibility that expression of CB confers resistance to excitotoxicity. While data from cell culture studies support an excitoprotective role for calbindin, it is not known whether CB is a key determinant of neuronal vulnerability in vivo. We therefore examined the pattern of damage to hippocampal neurons following intrahippocampal injection of the seizure-inducing excitotoxin kainate in CB homozygous (CB-/-) and CB heterozygous (CB+/-) knockout mice in comparison with wild-type mice (CB+/+). Whereas the extent of damage to CA1 neurons was similar in CB-/- and CB+/+ mice, damage to CA1 neurons was significantly reduced in CB+/- mice. Dentate granule neurons were not damaged following kainate-induced seizures in CB+/+, CB+/- or CB-/- mice. These findings suggest that CB can modify vulnerability of hippocampal CA1 neurons to seizure-induced injury, and that either CB is not a critical determinant of resistance of dentate granule neurons, or compensatory changes occur and lack of CB is not the only difference between CB-/- and CB+/+ mice.     

Activation of somatostatin-synthesizing neurons in the hippocampal formation through kindling-induced seizures

The present study was designed to determine if and to what extent somatostatin (SST) synthesizing neurons of the hippocampal formation are activated during seizures, elicited through kindling of the perforant pathway. Tissue was used and analyzed from animals which had experienced a single afterdischarge, or a stage 3 or stage 5 seizure. The protein expression of the oncogene c-fos in activated, depolarizing neurons was utilized to identify seizure-activated SST-synthesizing neurons. Combined immunocytochemical and in situ hybridization methods were used to identify these double-labeled, Fos protein, and SST mRNA-containing neurons. The results were quantified and compared across seizure stages. The resulting data demonstrate that at every stage of seizure development, a majority of SST-synthesizing neurons is activated, but that these activated SST mRNA-containing neurons represent only a minority of all seizure-activated, Fos-expressing neurons in the hippocampal formation. The data further reveal a numerical hierarchy in which the majority of double-labeled neurons is present in the hilus of the dentate, followed by the stratum oriens of CA1. It is concluded that SST-synthesizing neurons represent an integral component of the kindling activated neuronal network and, since the SST synthesizing neurons represent the minority of all seizure-activated neurons in the hippocampal formation, that this neuronal network is likely to be of considerable neurochemical complexity. & 1995 Wiley-Liss, Inc.     

Seizures induce proliferation and dispersion of doublecortin-positive hippocampal progenitor cells

One neuropathological hallmark of temporal lobe epilepsy is granule cell dispersion, a widening of the hippocampal granule cell layer (GCL) with abnormally positioned excitatory neurons. The finding that seizure activity also induces adult hippocampal neurogenesis was taken largely as indicative of a regenerative attempt, not as part of the pathology. The aim of our study was to characterize a potential relationship between granule cell dispersion and seizure-induced neurogenesis. Kainic acid (KA)-induced seizures in mice led to increased cell proliferation and new neurons persisted for months after the seizures. We show that the proliferative stimulus did not affect nestin-expressing early precursor cells that primarily respond to physiologic mitogenic stimuli, but stimulated the division of late type-3 progenitor cells, which express doublecortin (DCX), a protein associated with cell migration. This delayed proliferation presumably interfered with migration, leading to a significant dispersion of DCX-positive progenitors and early postmitotic neurons within the dentate gyrus granule cell layer. We propose that initial seizures induce ectopic precursor cell proliferation resulting in the dispersion of immature neurons within the adult granule cell layer. Thus, seizure-generated neurons might contribute to the disease process of epilepsy.     

GDNF family ligands and receptors are differentially regulated after brain insults in the rat

Expression of mRNAs for glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN) and their receptors was studied in adult rat brain using in situ hybridization after 40 kindling-evoked, rapidly recurring seizures or 10 min of global forebrain ischaemia. Following seizures, GDNF and NTN mRNAs were elevated in dentate granule cells, and c-Ret mRNA in hilar neurons and non-pyramidal cells in CA1 and CA3 regions. GFRalpha-1 mRNA levels showed more widespread increases in the dentate granule cell layer and hilus, CA1 and CA3 pyramidal layers, basolateral amygdala and parietal cortex. The expression of GFRalpha-2 mRNA increased in the piriform cortex and decreased in the CA1 region and basolateral amygdala. Forebrain ischaemia induced elevated expression of GDNF mRNA in dentate granule cells, GFRalpha-1 mRNA in the dentate granule cell layer, hilus and CA3 pyramidal layer, and GFRalpha-2 mRNA in the parietal cortex. The gene expression patterns observed here suggest that GDNF and NTN may act as target-derived factors, but also in an autocrine or paracrine manner. GFRalpha-1 can be coexpressed with GFRalpha-2 and c-Ret mRNAs in the same hippocampal or thalamic neurons, but other neurons contain GFRalpha-1 alone or together with c-Ret mRNA. The gene expression changes for the ligands, and the receptor components are region-, cell- and insult-specific, and occur independently of each other, mainly within 24 h after seizures or ischaemia. This dynamic regulation of GDNF and NTN circuits primarily at the receptor level may be important for the effectiveness of neuroprotective responses but could also trigger plastic changes, e.g. those underlying the development of epileptic syndromes.