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. Author manuscript; available in PMC: 2011 Apr 29.
Published in final edited form as: Prog Brain Res. 2002;135:365–375. doi: 10.1016/S0079-6123(02)35033-7

Is neuronal death required for seizure-induced epileptogenesis in the immature brain?

Tallie Z Baram 1,*, Mariam Eghbal-Ahmadi 1, Roland A Bender 1
PMCID: PMC3084550  NIHMSID: NIHMS290164  PMID: 12143355

Abstract

Do seizures cause neuronal death? At least in the immature hippocampus, this may not be the critical question for determining the mechanisms of epileptogenesis. Neuronal injury and death have clearly been shown to occur in most epilepsy models in the mature brain, and are widely considered a prerequisite to seizure-induced epilepsy. In contrast, little neuronal death occurs after even a severe and prolonged seizure prior to the third postnatal week. However, seizures early in life, for example prolonged experimental febrile seizures, can profoundly and permanently change the hippocampal circuit in a pro-epileptogenic direction. These seizure-induced alterations of limbic excitability may require transient structural injury, but are mainly due to functional changes in expression of gene coding for specific receptors and channels, leading to altered functional properties of hippocampal neurons. Thus, in some pro-epileptogenic models in the developing brain, neither the death of neurons nor death-induced abnormalities of surviving neurons may underlie the formation of an epileptic circuit. Rather, findings in the experimental prolonged febrile seizure model suggest that persistent functional alterations of gene expression (‘neuroplasticity’) in diverse hippocampal neuronal populations may promote pro-epileptogenic processes induced by these seizures. These findings also suggest that during development, relatively short, intense bursts of neuronal activity may disrupt ‘normal’ programmed maturational processes to result in permanent, selective alterations of gene expression, with profound functional consequences. Therefore, determining the cascade of changes in the programmed expression of pertinent genes, including their temporal and cell-specific spatial profiles, may provide important information for understanding the process of transformation of an evolving, maturing hippocampal network into one which is hyperexcitable.

Introduction

The issue of neuronal Injury and death caused by seizures has haunted investigators in the field for several decades (Margerison and Corsellis, 1966; Babb and Brown, 1987; Armstrong, 1993). The striking cell loss in specific hippocampal regions and layers found in humans with temporal lobe epilepsy has suggested that seizures may cause epilepsy by killing neurons, leading to reduced inhibition or to the development of excitatory synapses and circuits (e.g., Sloviter, 1991; Dudek et al., 1994; Scharfman, 1999).

Indeed, a strong body of evidence using models of seizure-induced epilepsy has documented that prolonged limbic seizures (status epilepticus) as well as kindling (Cavazos et al. , 1994) result in hippocampal cell loss in a pattern reminiscent of that found in human temporal lobe epilepsy (e.g., Lothman and Collins, 1981 ; Nadler, 1981; Ben-Ari, 1985; Sperk et al., 1985; Obenaus et al., 1993; Buckmaster and Dudek, 1997). This cell loss has been found to be progressive (Sutula, 1991; Kalviainen et al., 1998), and associated with — in fact, probably required for (Schauwecker et al., 2000) — synaptic reorganization and increased excitability in the involved circuits (Dudek et al., 1994; Scharfman et al., 2000).

In analogy to the usefulness of mature animal models for elucidating the role of seizure-induced neuronal death in the epileptogenic process in human adults, limbic seizures have been induced during the first 2 weeks of life in the rat, using kainic acid and other convulsants (e.g., Albala et al., 1984; Nitecka et al., 1984; Holmes and Thompson, 1988; Veliskova et al., 1988; Holmes et al., 1998; McCabe et al., 2001). These experiments have attempted to provide clues about the immediate and long-term consequences of early-life seizures in the human, and in particular, about the relationship of seizures, such as those induced by neonatal asphyxia (Jensen et al., 1991), fever (Shinnar, 1998) or other triggers (Baram and Hatalski , 1998), to subsequent epilepsy. Specifically, whether seizures early in life cause cell death, and whether this death is required, sufficient, or even related, to subsequent epilepsy has been a topic of intense investigation (for discussions see Holmes and Ben-Ari, 1998; Sperber et al., 1999; Dube et al., 2000; Kubova et al., 2001; Bender and Baram, 2002).

Because seizures provoked by fever are the most common human developmental seizures which are associated with subsequent temporal lobe epilepsy and hippocampal cell loss, we developed and characterized a model of experimental prolonged febrile seizures in the immature rat. Here we discuss the issues of the occurrence of cell death after these seizures, of other structural alterations in the ‘wiring’ of the hippocampal network, and of the functional changes induced by these seizures, including enhanced susceptibility to further seizures. We consider the molecular and cellular processes that may underlie these changes. These findings indicate important differences between mechanisms of seizure-induced epileptogenicity in mature and still developing hippocampus, and suggest that the interaction of a bout of intense neuronal activity with programmed maturation of the hippocampal network may lead to long-lasting neuroplastic alterations which promote a hyperexcitable state without the requirement for neuronal death.

The rationale for studying prolonged febrile seizures and temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most prevalent type of human focal epilepsy, yet the processes leading to spontaneous seizures involving the hippocampus have not been fully determined. Specifically, the relationship of childhood febrile seizures to adult TLE has remained a focus of intense controversy (for brief recent reviews see Shinnar, 1998; Sloviter and Pedley, 1998; Lewis, 1999; Dube et al., 2000). Epidemiological evidence from prospective studies has convincingly shown that most febrile seizures carry a benign outcome: they do not lead to development of subsequent unprovoked seizures (epilepsy). However, retrospective analyses of adults with TLE have demonstrated a high prevalence (30% to > 60%) of a history of prolonged (longer than 10-15 min) febrile seizures during early childhood, suggesting an etiological role for these seizures in the development of TLE (e.g., Cendes et al., 1993; French et al., 1993). Specifically, neuronal damage induced by febrile seizures has been suggested as a mechanism for the development of mesial temporal sclerosis, the pathological hallmark of TLE (Bruton, 1988; Armstrong, 1993). In addition, recent imaging studies in children with prolonged febrile seizures have shown acute hippocampal swelling in some, indicating acute neuronal injury (VanLandingham et al., 1998). However, these data should not be taken to indicate a causal relationship, and alternative mechanisms may exist for the correlation of prolonged febrile seizures and TLE. For example, pre-existing (genetic or acquired, functional or structural) neuronal abnormalities may provide a predisposition to the occurrence of prolonged febrile seizures or for subsequent neuronal injury and TLE.

Given the high prevalence of febrile seizures (1 in 20-30 children), understanding their consequences for the developing brain is critical, because even a relatively small contribution by these seizures to the development of epilepsy will result in large numbers of affected individuals. However, febrile seizures cannot be induced, and the critical question of the causal relationship of prolonged febrile seizures and epilepsy cannot be studied in children. Therefore, an immature rat model for these seizures has been created. Because ‘complex’, typically prolonged, febrile seizures are those that have been associated with subsequent limbic epilepsy (TLE), the model aimed to reproduce this subtype of human febrile seizures as closely as possible. Seizures in the model are provoked by generating brain temperatures seen physiologically in ill infants, and seizure duration is regulated to reproduce relatively prolonged febrile seizures (~20 min), those considered complex by the International League Against Epilepsy, and associated retrospectively with the development of TLE. Several features of the model, which render it suitable for study of the mechanisms and consequences of these prolonged febrile seizures, have been characterized. These include an age-specificity, temperatures required for seizure induction, lack of mortality or acute morbidity and pharmacological profile, which are similar to those of human febrile seizures (Knudsen, 1996; Baram et al., 1997; Dube et al., 2000). In addition, the neuronal circuits involved in the seizures have been mapped (Chen et al., 1999; Dube et al., 2000; Hatalski et al., 2000).

Note that throughout this chapter, reference is made to the developmental stage when febrile seizures may be ‘generated or provoked’. This, in the animal model, implies postnatal days 9–14 (Baram et al., 1997), and the seizures are induced on days 10–11. It is fully recognized that direct correlations of rat brain development to that in the human are imprecise. However, in general, evidence based on rates of neuronal birth and myelination (Dobbing and Sands, 1973, 1979), and saltatory growth stages (Gottlieb et al., 1977) suggest that the 5–7-days-old rat may be ‘equivalent’ to the human newborn. Rat brain development at 10–15 postnatal days, the age of maximal susceptibility to triggered seizures (including those induced by hyperthermia; Hjeresen and Diaz, 1988; Jensen et al., 1991; Baram and Hatalski, 1998) may thus best correspond to the stage of brain development during human infancy (Gottlieb et al., 1977).

This model of complex (prolonged) febrile seizures has been used to address the fundamental question of whether these seizures promote epilepsy by causing death of vulnerable hippocampal neuronal populations leading to mesial temporal sclerosis, neuroanatomical matrix of TLE. It should be noted that this specific pattern of neuronal loss is found in the majority of individuals with TLE (Armstrong, 1993; Kuzniecky et al., 1997), but whether neuronal death promotes epilepsy, or whether recurrent seizures result in neuronal death is unclear.

Prolonged experimental febrile seizures result in permanent increase of hippocampal excitability, and in susceptibility to the development of further limbic seizures

Whether prolonged febrile seizures in the immature rat model lead to the development of spontaneous seizures was investigated using both in vivo and in vitro approaches (Dube et al., 2000). After induction of such seizures, animals were allowed to mature (three months), then underwent extensive hippocampal-EEG and behavioral monitoring. Both EEGs and behavioral measures failed to demonstrate spontaneous seizures. However, these animals showed a fourfold increase in susceptibility to kainic acid, an activator of the AMPA-type glutamate receptors. In essence, a dose that failed to provoke seizures in most adult rats which had not experienced prolonged febrile seizures in ‘infancy’ (normothermic controls and those who were subjected to hyperthermia but in which seizures had been blocked) led to severe seizures in all adult animals who had prolonged experimental febrile seizures early in life. This increased susceptibility to limbic convulsants was confirmed in vitro: spontaneous epileptiform discharges were not observed in hippocampal–entorhinal cortex slices derived from either control or experimental groups. However, Schaffer collateral stimulation induced prolonged, self-sustaining, status-epilepticus-like discharges exclusively in slices from experimental rats. These data indicate that experimental prolonged febrile seizures do not cause spontaneous limbic seizures during adulthood. However, they reduce thresholds to chemical convulsants in vivo and to electrical stimulation in vitro, indicating persistent enhancement of hippocampal excitability that may facilitate the development of epilepsy.

The mechanisms by which prolonged febrile seizures enhance excitability do not involve cell death

As mentioned earlier, a large body of literature involving adult animal models supports the notion that provoked seizures may lead to spontaneous seizures (epilepsy) by killing vulnerable neurons in hippocampus and amygdala, altering the balance of excitation/inhibition in the limbic network (Sloviter, 1994). Indeed, experimental models using pilocarpine (Mello et al., 1993; Obenaus et al., 1993) or kainic acid (Pollard et al., 1994; Buckmaster and Dudek, 1997) document that neuronal loss induced by these seizures precedes the emergence of spontaneous seizures. Therefore, the hypothesis was tested that experimental prolonged febrile seizures increased hippocampal excitability by causing the loss of vulnerable neuronal populations.

Using molecular methods for visualizing neuronal death, no excess of in situ end-labeled cells was found 1, 4, 8.5, 20 or 48 h after seizures lasting 20 or 60 min (Toth et al., 1998). In addition, stereological counts in amygdala (Toth et al., 1998) or in specific hippocampal sub-populations that are vulnerable to seizure-induced death in other models, failed to reveal loss of mossy cells or other seizure-sensitive neuronal populations (Bender and Baram, 1999, 2002). Thus, it was concluded that experimental prolonged febrile seizures do not lead to cell loss. This is in accord with data from other single prolonged developmental seizures such as those induced by hypoxia (Jensen and Baram, 2000) or by chemical provocation, during the second postnatal week (Sperber et al., 1992), and indicates that the mechanisms by which prolonged febrile seizures increase hippocampal excitability may not involve hippocampal cell death. In addition, the findings support the notion that these mechanisms may involve interaction with concurrent maturational processes in the developing hippocampus.

Neurogenesis is not altered by prolonged experimental febrile seizures

Death of hippocampal CA3 pyramidal cells or of hilar neurons deprives the excitatory granule cells of their targets and leads to abnormal growth (‘sprouting’) of their axons, the mossy fibers. The excitatory synaptic connections formed by these axons are widely considered to contribute to the mechanism of enhanced hippocampal excitability and epileptogenesis (e.g., Pollard et al., 1994; Buckmaster and Dudek, 1997). A second trigger for abnormal granule cell excitatory innervation is seizure-induced alteration of their postnatal proliferation rate and total numbers (Parent et al., 1997; Parent and Lowenstein, 2002, this volume), as shown for several experimental seizure models in adult animals. However, no evidence for altered granule cell proliferation rate and only modest ‘sprouting’ of the granule cell axons was found after prolonged febrile seizures in the immature rat model (Baram et al., 2000; Bender et al., 2000). These findings reinforce the notion that these seizures affect the hippocampal network via processes which are distinct from those implicated in adult epileptogenesis, and which must interact with — and perhaps disrupt — developmental events coinciding with these seizures (Jensen and Baram, 2000).

Prolonged ‘febrile’ seizures injure specific populations of hippocampal neurons

The data discussed above demonstrated a dichotomy between the functional and neuroanatomical changes induced by experimental prolonged febrile seizures. Namely, enhanced susceptibility to further seizures was observed long-term, but this was achieved without any evidence for acute or chronic cell death. Therefore, we evaluated the possibility that experimental prolonged febrile seizures induced neuronal injury that was sufficient to permanently alter the properties of these neurons, without leading to their death. A method considered sensitive to changes in cytoskeletal elements of neurons was chosen (Gallyas et al., 1990; Toth et al., 1998) to visualize the potential effects of hyperthermic seizures on neuronal structure. The overall approach involved a comparison of three experimental groups: normothermic and hyperthermic controls and animals subjected to prolonged experimental febrile seizures. For analysis of neuronal injury using the Gallyas ‘dark’-neuron silver stain, animals were sacrificed 24 h, 1 week or 2 weeks after seizure induction. For cell counting, animals (n = 12, four per experimental group) were sacrificed 4 weeks following the hyperthermic seizures.

As shown in Fig. 1, significant and prolonged alterations in the physicochemical properties of neurons in the pyramidal layer of the hippocampal CA1 and all the CA3 subfields were found. Specifically, starting within 24 h of seizures and persisting for at least 2 weeks, numerous pyramidal cells a well as less abundant hilar neurons exhibited pronounced avidity to silver stain (argyrophilia). The distribution of these argyrophilic neurons indicated the pattern of neuronal vulnerability to febrile seizures in this model, and shared significant similarities with the pattern of injury found with other limbic seizure types (and in human temporal lobe epilepsy). In the hippocampus, major involvement of CA3 and CA1 pyramidal cell layers and relative sparing of the granule cell layer and subiculum were consistent with vulnerability patterns in adult models of kainic acid-(Nadler et al., 1978; Sperk et al., 1983; Ben-Ari, 1985; Pollard et al., 1994) and pilocarpine-induced status epilepticus (Clifford et al., 1987; Mello et al., 1993; Liu et al., 1994).

Fig. 1.

Fig. 1

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Injury to hippocampal neurons after a single 20-min episode of intense neuronal activity induced by hyperthermia (hyperthermic seizure, an experimental model of prolonged febrile seizures). Sections obtained from immature rats killed 24 h after a seizure. Silver-stained neurons (Gallyas’ dark-neuron method; Gallyas et al., 1992) are evident in the CA3c pyramidal cell layer in this high-magnification photomicrograph. The distribution of the argyrophilic neurons involved also CA3a and b, CA1, some hilar interneurons, and discrete nuclei in amygdala and perirhinal cortex (Toth et al., 1998). Such neurons were not observed in animals subjected to hyperthermia alone, i.e., when the seizures were blocked. s.p. and s.r. are strata pyramidale and radiatum, respectively.

However, unlike the chronic outcome of these seizures in adult rats, no evidence of neuronal cell loss was evident at any time after the hyperthermic seizures, and cell counts revealed no evidence of loss in specific vulnerable hippocampal cell populations (see above). Thus, these data, revealing striking but transient alterations of neuronal integrity in regions known to be affected by other limbic seizure paradigms, might provide a mechanism to reconcile conflicting reports regarding the effects of developmental limbic seizures on neuronal survival. Specifically, our findings suggest that similar neuronal populations share vulnerability to limbic seizures in both the immature and mature hippocampus, but immature neurons may undergo injury followed by recovery, whereas mature neurons progress from injury to death (Chang and Baram, 1994; Owens et al., 1997).

Do argyrophilia, acid-fuchsin staining or in situ end-labeling (ISEL) of neurons indicate their death?

The data presented above demonstrate that the physicochemical properties of neurons may change dramatically, permitting increased avidity to silver stains (Gallyas et al., 1990; Van den Pol and Gallyas, 1990; Toth et al., 1998) without these changes being followed by neuronal degeneration. The relative nature of silver uptake by different classes of intact and injured neurons and subcellular organelles has been discussed (Gallyas et al., 1990). Indeed, avidity to silver staining can be induced by subjecting the brain to postmortem trauma, indicating that this process is independent from the process of cell death (Gallyas et al., 1992). This fact, together with earlier studies (Chang and Baram, 1994), raised the question as to the interpretation of neuroanatomical methods used to demonstrate ‘cell death’. Put differently, acquisition of the avidity to silver in a variety of methods may not necessarily mean neuronal death. The changes or injury which renders a cell argyrophilic may be reversible and not lead to cell loss (death). In analogy, the acid-fuchsin method, described originally for hypoglycemic cell death, may merely imply increased avidity to this dye, without ‘fatal’ injury to the cell (see discussion in Chang and Baram, 1994, and by Nehlig and Pereira de Vasconcelos, 1996; Motte et al., 1998; Pineau et al., 1999). Somewhat more controversial, the interpretation of TUNEL and ISEL, methods which rely on labeling of end-terminals of DNA, has come under question. Whereas breaking of DNA into pieces of roughly equal length (‘laddering’) has been described in the process of apoptotic cell death, this form of death may occur without overt DNA fragmentation. In addition, injury-induced repair may also yield enhanced numbers of DNA ‘ends’ with enzymatic labeling. Finally, in the immature brain, normal cell death occurs, and these dying cells may also be labeled. Because of the potential ambiguities of each of the methods described here regarding its specificity for cell death, a recent trend (Sankar et al., 1998; Toth et al., 1998; Kubova et al., 2001) has been the adoption of several methods in combination. In addition, several time points have often been analyzed in recent studies of seizure-induced neuronal death in the immature brain. Thus, Toth et al., 1998 (see above) found reversible silver staining with absence of in situ end-labeling or of neuronal dropout, and concluded that the prolonged hyperthermic seizures did not kill hippocampal neurons. Sankar et al. ( 1998) used several methods, including Fluoro-jade, to assess pilocarpine-induced cell death. In an elegant recent study, Kubova et al. (2001) resorted to electron microscopy to determine categorically that seizures killed specific populations in discrete thalamic nuclei.

Changes in gene expression follow prolonged experimental febrile seizures, and may alter neuronal function to promote hippocampal excitability

The paragraphs above suggest that experimental prolonged febrile seizures induce pro-epileptogenic changes in the immature hippocampal network, and that neuronal death may not be required for this process. What then, might the responsible mechanisms be?

In addition to the absence of cell death, cell birth rate (and cell fate) has been found not to change after these seizures. Thus, recent research has focused on potential sequential changes in the expression of genes which may impact excitability in the developing hippocampal circuit. Because experimental (and clinical) febrile seizures occur uniquely in a distinct phase of development, emphasis has been placed on molecules that influence hippocampal excitability during this age.

We first tested the hypothesis that excitatory drive to the hippocampal circuit is enhanced persistently via long-term upregulation of the expression of the excitatory neuropeptide, corticotropin-releasing hormone (CRH), which is normally expressed in hippocampal interneurons (for review see Yan et al., 1998; Y. Chen et al., 2001). CRH immunoreactive interneurons are more numerous in the principal cell layers of the immature rat, compared with the adult, peaking during the second and third postnatal weeks (Y. Chen et al., 2001), and the pro-excitatory actions of the peptide on hippocampal neurons are also maximal during this developmental age (for review see Baram and Hatalski, 1998). Indeed, within hour of the seizures, robust increase in CRH-mRNA levels in the principal cell layers of the hippocampal formation was evident (Hatalski et al., 2000; Fig. 2). However, these changes persisted only for 24–48 h and were no longer evident at 1 week or 1 month after the seizures (Eghbal-Ahmadi, 2000), whereas the enhanced excitability in the limbic circuit persisted (Dube et al., 2000). It should be noted that the enhanced expression of CRH was not due simply to the stressful effects of the seizures, since it did not occur with other stressors such as cold (Hatalski et al., 2000). In addition, this enhanced gene expression required neuronal activity, since it was abolished when the seizures (but not the hyperthermia) were eliminated via pre-treatment with the short-acting barbiturate pentobarbital. Finally, lack of persistent changes in hippocampal CRH expression was not due to the inability of this transcript to undergo long-term upregulation, since protracted enhancement (up to 12 months) of CRH-mRNA levels in the hippocampus was induced in the immature rat by other means (Brunson et al., 2001).

Fig. 2.

Fig. 2

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Time-course of CRH expression in the ammon's horn pyramidal layer (CA1 and CA3) as well as in the granule cell layer of the hippocampal formation after prolonged experimental febrile seizures. CRH expression in these layers is confined to basket and chandelier-type interneurons (Yan et al., 1998; Y. Chen et al., 2001).

The permanent functional changes in the hippocampal network following prolonged experimental febrile seizures led to a search for persistent alteration in the expression of other candidate genes. One such candidate emerged from the in vitro electrophysiological studies (K. Chen et al., 2001; Thon et al., 2002). Whereas enhanced excitability of the hippocampal circuit was evident from both in vivo and in vitro data (Dube et al., 2000, and see above), single-cell electrophysiology (Chen et al., 1999) suggested that experimental prolonged febrile seizures led to significantly increased GABA release onto CA1 pyramidal cells. This change indicated enhancement of inhibitory drive (Walker and Kullmann, 1999), and failed to explain the augmented excitability in the hippocampal network after these seizures. However, more recent electrophysiology data (K. Chen et al., 2001) resolved this conundrum, demonstrating a remarkable and persistent change in an ion channel, which can convert the potentiated inhibition into excitation. Specifically, a channel activated by hyperpolarization and leading to depolarization by permitting entry of Na+ ions was altered, in a manner that rendered it more highly activated under physiological conditions. This channel (hyperpolarization-activated, cyclic AMP-regulated mixed-cation channel, HCN), producing the hyperpolarization activated (IH) current, is composed of several, recently characterized subunit isoforms (Santoro et al., 1998, 2000; Siegelbaum, 2000).

Because the functional properties of the HCN channels are governed also by the subunit or isoform make up of the channel (Santoro et al., 1998, 2000; Siegelbaum, 2000), current experiments are aimed at determining whether prolonged experimental febrile seizures alter the relative expression of these subunit isoforms. The existence of these molecules in developing hippocampus has been demonstrated (Bender et al., 2001). Furthermore, the expression of each of the three hippocampal-expressed isoforms is highly age-dependent, demonstrating a distinct spatio-temporal expression profile in both interneuronal populations and pyramidal cells (Bender et al., 2001). Therefore, current experiments are testing the hypothesis that prolonged experimental febrile seizures may disrupt the normal evolution of maturational changes in HCN subunit expression, thus leading to permanent alteration in the expression of these subunits, and ultimately, to altered channel function.

Conclusion

Experimental prolonged febrile seizures lead to enhanced excitability in the limbic circuit. However, this is not accompanied by neuronal death. Transient neuronal injury is induced by these seizures, manifested as enhanced avidity to silver stains. This structural alteration is associated with both transient and persistent functional changes that may derive from disruption of programmed, sequential and orderly expression of genes critical to normal hippocampal maturation. Thus, a single experimental febrile seizure may modify gene expression and resultant neuronal function persistently, leading to altered properties of the hippocampal network long-term.

References

  1. Albala BJ, Moshe SL, Okada R. Kainic-acid-induced seizures: a developmental study. Brain Res. 1984;315:139–148. doi: 10.1016/0165-3806(84)90085-3. [DOI] [PubMed] [Google Scholar]
  2. Armstrong DD. The neuropathology of temporal lobe epilepsy. J. Neuropathal. Exp. Neural. 1993;52:433–443. doi: 10.1097/00005072-199309000-00001. [DOI] [PubMed] [Google Scholar]
  3. Babb TL, Brown BJ. Pathological findings in epilepsy. In: Engel J, editor. Surgical Treatment of the Epilepsies. Raven; New York, NY: 1987. pp. 511–552. [Google Scholar]
  4. Baram TZ, Hatalski CG. Neuropeptide-mediated excitability: a key triggering mechanism for seizure generation in the developing brain. Trends Neurosci. 1998;21:471–476. doi: 10.1016/s0166-2236(98)01275-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Baram TZ, Gerth A, Schultz L. Febrile seizures: an appropriate-aged model suitable for long-term studies. Brain Res. Dev. Brain Res. 1997;98:265–270. doi: 10.1016/s0165-3806(96)00190-3. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Baram TZ, Mina E, Bender RA. Neurogenesi of dentate gyrus granule cells is not altered by prolonged febrile seizures in the immature rat. Epilepsia. 2000;41(Suppl. 7):13. [Google Scholar]
  7. Ben-Ari Y. Limbic seizure and brain damage produced by kainic acid: mechanisms and relevance to human temporal lobe epilepsy. Neuroscience. 1985;14:375–403. doi: 10.1016/0306-4522(85)90299-4. [DOI] [PubMed] [Google Scholar]
  8. Bender RA, Baram TZ. Effects of febrile seizures in the immature rat model on interneuronal parvalbumin (PV) expression in the dentate gyrus. Epilepsia. 1999;40(Suppl. 7):120. [Google Scholar]
  9. Bender RA, Baram TZ. Do prolonged febrile seizures injure hippocampal neurons? Insights from animal models. In: Baram TZ, Shinnar S, editors. Febrile Seizures. Academic Press; San Diego, CA: 2002. pp. 127–137. [Google Scholar]
  10. Bender RA, Dube C, Baram TZ. Mossy fiber sprouting into the dentate gyrus inner molecular layer follows prolonged febrile seizures in the immature rat model. Epilepsia. 2000;41(Suppl. 7):76. [Google Scholar]
  11. Bender RA, Brewster A, Santoro B, Ludwig A, Hofmann F, Biel M, Baram TZ. Differential and age-dependent expression of hyperpolarization-activated, cyclic nucleotide-gated cation channel isoforms 1–4 suggest evolving roles in the developing rat hippocampus. Neuroscience. 2001;106:689–698. doi: 10.1016/s0306-4522(01)00314-1. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Brunson KL, Eghbal-Ahmadi M, Bender R, Chen Y, Baram TZ. Long-term, progressive hippocampal cell loss and dysfunction induced by early-life administration of corticotropin-releasing hormone reproduce the effects of early-life stress. Proc. Natl. Acad. Sci. USA. 2001;98:8856–8861. doi: 10.1073/pnas.151224898. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Bruton CJ. The Neuropathology of Temporal Lobe Epilepsy. Oxford University Press; New York: 1988. Maudsley Monographs, No. 31. [Google Scholar]
  14. Buckmaster PS, Dudek FE. Neuron loss, granule cell axon reorganization and functional changes in the dentate gyrus of epileptic kainate-treated rats. J. Comp. Neurol. 1997;385:385–404. [PubMed] [Google Scholar]
  15. Cavazos JE, Das I, Sutula TP. Neuronal loss induced in limbic pathways by kindling: evidence for induction of hippocampal sclerosis by repeated brief seizures. J. Neurosci. 1994;14:3106–3121. doi: 10.1523/JNEUROSCI.14-05-03106.1994. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Cendes E, Andermann F, Gloor P, Lopes-Cendes I, Ander-mann E, Melanson D, Jones-Gotman M, Robitaille Y, Evans A, Peters T. Atrophy of mesial structures in patients with temporal lobe epilepsy: cause or consequence of repeated seizures?. Ann. Neurol. 1993;34:795–801. doi: 10.1002/ana.410340607. [DOI] [PubMed] [Google Scholar]
  17. Chang D, Baram TZ. Status epilepticus results in reversible neuronal injury in infant rat hippocampus: novel use of a marker. Brain Res. Dev. Brain Res. 1994;77:133–136. doi: 10.1016/0165-3806(94)90220-8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Chen K, Baram TZ, Soltesz I. Febrile seizures in the developing brain result in persistent modification of neuronal excitability in limbic circuits [see comments]. Nat. Med. 1999;5:888–894. doi: 10.1038/11330. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Chen K, Aradi I, Thon N, Eghbal-Ahmadi M, Baram TZ, Soltesz I. Persistently modified H-channels after complex febrile seizures convert the seizure-induced enhancement of inhibition to hyperexcitability. Nat. Med. 2001;7:1–7. doi: 10.1038/85480. [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Chen Y, Bender RA, Frotscher M, Baram TZ. Novel and transient populations of corticotropin-releasing hormone-expressing neurons in developing hippocampus suggest unique functional roles: a quantitative spatiotemporal analysis. J. Neurosci. 2001;21:7171–7181. doi: 10.1523/JNEUROSCI.21-18-07171.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Clifford DB, Olney JW, Maniotis A, Collins RC, Zorumski CF. The functional anatomy and pathology of lithium–pilocarpine and high-dose pilocarpine seizures. Neuroscience. 1987;23:953–968. doi: 10.1016/0306-4522(87)90171-0. [DOI] [PubMed] [Google Scholar]
  22. Dobbing J, Sands J. Quantitative growth and development of human brain. Arch. Dis. Child. 1973;48:757–767. doi: 10.1136/adc.48.10.757. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Dobbing J, Sands J. Comparative aspects of the brain growth spurt. Early Hum. Dev. 1979;3:79–83. doi: 10.1016/0378-3782(79)90022-7. [DOI] [PubMed] [Google Scholar]
  24. Dube C, Chen K, Eghbal-Ahmadi M, Brunson K, Soltesz I, Baram TZ. Prolonged febrile seizures in immature rat model enhance hippocampal excitability long-term. Ann. Neurol. 2000;47:336–344. [PMC free article] [PubMed] [Google Scholar]
  25. Dudek FE, Obenaus A, Schweitzer JS, Wuarin J-P. Functional significance of hippocampal plasticity in epileptic brain: electrophysiological changes of the dentate granule cells associated with mossy fiber sprouting. Hippocampus. 1994;4:259–265. doi: 10.1002/hipo.450040306. [DOI] [PubMed] [Google Scholar]
  26. Eghbal-Ahmadi M. Thesis. University of California; 2000. Potential role of corticotropin releasing hormone (CRH) in age-specific developmental seizures. [Google Scholar]
  27. French JA, Williamson PD, Thadani VM, Darcey TM, Mattson RH, Spencer SS, Spencer DD. Characteristics of medial temporal lobe epilepsy. I. Results of history and physical examination. Ann. Neurol. 1993;34:774–780. doi: 10.1002/ana.410340604. [DOI] [PubMed] [Google Scholar]
  28. Gallyas F, Guldner FH, Zoltay G, Wolff JR. Golgi-like demonstration of ‘dark’ neurons with an argyrophil III method for experimental neuropathology. Acta Neuropathol. 1990;79:620–628. doi: 10.1007/BF00294239. [DOI] [PubMed] [Google Scholar]
  29. Gallyas F, Zoltay G, Horvath Z. Light microscopic response of neuronal somata, dendrites and axons to postmortem concussive head injury. Acta Neuropathol. 1992;83:499–503. doi: 10.1007/BF00310026. [DOI] [PubMed] [Google Scholar]
  30. Gottlieb A, Keydar I, Epstein HT. Rodent brain growth stages: an analytical review. Biol. Neonate. 1977;32:166–176. doi: 10.1159/000241012. [DOI] [PubMed] [Google Scholar]
  31. Hatalski CG, Brunson KL, Tantayanubutr B, Chen Y, Baram TZ. Neuronal activity and stress differentially regulate hippocampal and hypothalamic corticotropin-releasing hormone expression in the immature rat. Neuroscience. 2000;101:571–580. doi: 10.1016/s0306-4522(00)00386-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Hjeresen DL, Diaz J. Ontogeny of susceptibility to experimental febrile seizures in rats. Dev. Psychobiol. 1988;21:261–275. doi: 10.1002/dev.420210307. [DOI] [PubMed] [Google Scholar]
  33. Holmes GL, Ben-Ari Y. Seizures in the developing brain: perhaps not so benign after all. Neuron. 1998;21:1231–1234. doi: 10.1016/s0896-6273(00)80642-x. [DOI] [PubMed] [Google Scholar]
  34. Holmes GL, Thompson JL. Effects of kainic acid on seizure susceptibility in the developing brain. Bran Res. 1988;467:51–59. doi: 10.1016/0165-3806(88)90066-1. [DOI] [PubMed] [Google Scholar]
  35. Holmes GL, Gairsa JL, Chevassus Au Louis N, Ben Ari Y. Consequences of neonatal seizures in the rat: morphological and behavioral effects. Ann. Neurol. 1998;44:845–857. doi: 10.1002/ana.410440602. [DOI] [PubMed] [Google Scholar]
  36. Jensen FE, Baram TZ. Developmental seizures induced by common early-li fe insults: short- and long-term effects on seizure susceptibility. Ment. Retard. Dev. Disabil. Res. Rev. 2000;6:253–257. doi: 10.1002/1098-2779(2000)6:4<253::AID-MRDD4>3.0.CO;2-P. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Jensen FE, Applegate CD, Holtzman D, Belin TR, Burchfiel JL. Epileptogenic effect of hypoxia in the immature rodent brain. Ann. Neurol. 1991;29:629–637. doi: 10.1002/ana.410290610. [DOI] [PubMed] [Google Scholar]
  38. Kalviainen R, Salmenpera T, Partanen K, Vainio P, Riekkinen P, Pitkanen A. Recurrent seizures may cause hippocampal damage in temporal lobe epilepsy. Neurology. 1998;50:1377–1382. doi: 10.1212/wnl.50.5.1377. [DOI] [PubMed] [Google Scholar]
  39. Knudsen FU. Febrile seizures — treatment and outcome. Brain Dev. 1996;18:438–449. doi: 10.1016/s0387-7604(96)00059-9. [DOI] [PubMed] [Google Scholar]
  40. Kubova H, Druga R, Lukasiuk K, Suchomelova L, Haugvicova R, Jirmanova I, Pitkanen A. Status epilepticus causes necrotic damage in the mediodorsal nucleus of the thalamus in immature rats. J. Neurosci. 2001;21:3593–3599. doi: 10.1523/JNEUROSCI.21-10-03593.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Kuzniecky RI, Bilir C, Gilliam F, Faught E, Palmer C, Morawetz R, Jackson G. Multimodality MRI in mesial temporal sclerosis: relative sensitivity and specificity. Neurology. 1997;49:774–778. doi: 10.1212/wnl.49.3.774. [DOI] [PubMed] [Google Scholar]
  42. Lewis DV. Febrile convulsions and mesial temporal sclerosis. Curr. Opin. Neurol. 1999;12:197–201. doi: 10.1097/00019052-199904000-00011. [DOI] [PubMed] [Google Scholar]
  43. Liu Z, Nagao T, Desjardin GC, Gloor P, Avoli M. Quantitative evaluation of neuronal loss in the dorsal hippocampus in rats with long-term pilocarpine seizures. Epilepsy Res. 1994;17:237–247. doi: 10.1016/0920-1211(94)90054-x. [DOI] [PubMed] [Google Scholar]
  44. Lothman EW, Collins RC. Kainic acid induced limbic seizures: metabolic, behavioral, electroencephalographic and neuropathological correlates. Brain Res. 1981;218:299–318. doi: 10.1016/0006-8993(81)91308-1. [DOI] [PubMed] [Google Scholar]
  45. Margerison JH, Corsellis JAN. Epilepsy and the temporal lobes: a clinical electroencephalographic and neuropathological study of the brain in epilepsy, with particular reference to the temporal lobes. Brain. 1966;399:1246–1283. doi: 10.1093/brain/89.3.499. [DOI] [PubMed] [Google Scholar]
  46. McCabe BK, Silveira DC, Cilio MR, Cha BH, Liu X, Sogawa Y, Holmes GL. Reduced neurogenesis following neonatal seizures. J. Neurosci. 2001;21:2094–2103. doi: 10.1523/JNEUROSCI.21-06-02094.2001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Mello LE, Cavalheiro EA, Tan AM, Kupfer WR, Pretorius JK, Babb TL, Finch DM. Circuit mechanism of seizures in the pilocarpine model of chronic epilepsy: cell loss and mossy fiber sprouting. Epilepsia. 1993;34:985–995. doi: 10.1111/j.1528-1157.1993.tb02123.x. [DOI] [PubMed] [Google Scholar]
  48. Motte J, Fernandes MJ, Baram TZ, Nehlig A. Spatial and temporal evolution of neuronal activation, stress and injury in lithium–pilocarpine seizures in adult rats. Brain Res. 1998;793:61–72. doi: 10.1016/s0006-8993(98)00135-8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Nadler JV. Kainic acid as a tool for the study of temporal lobe epilepsy. Life Sci. 1981;29:2031–2041. doi: 10.1016/0024-3205(81)90659-7. [DOI] [PubMed] [Google Scholar]
  50. Nadler JV, Perry BW, Cotman CW. Intraventricular kainic acid preferentially destroys hippocampal pyramidal cells. Nature. 1978;271:676–677. doi: 10.1038/271676a0. [DOI] [PubMed] [Google Scholar]
  51. Nehlig A, Pereira de Vasconcelos A. The model of pentylenetetrazol-induced status epilepticus in the immature rat: short- and long-term effects. Epilepsy Res. 1996;26:93–103. doi: 10.1016/s0920-1211(96)00045-9. [DOI] [PubMed] [Google Scholar]
  52. Nitecka L, Tremblay C, Charton G, Bouillot JP, Berger ML, Ben-Ari Y. Maturation of kainic acid seizure-brain damage syndrome in the rat, II. Histopathological sequelae. Neuroscience. 1984;13:1073–1094. doi: 10.1016/0306-4522(84)90289-6. [DOI] [PubMed] [Google Scholar]
  53. Obenaus A, Esclapez M, Houser CR. Loss of glutamate decarboxylase mRNA-containing neurons in the rat dentate gyrus following pilocarpine-induced seizures. J. Neurophysiol. 1993;13:4470–4485. doi: 10.1523/JNEUROSCI.13-10-04470.1993. [DOI] [PMC free article] [PubMed] [Google Scholar]
  54. Owens JJ, Robbins CA, Wenzel HJ, Schwartzkroin PA. Acute and chronic effects of hypoxia on the developing hippocampus. Ann. Neurol. 1997;41:187–199. doi: 10.1002/ana.410410210. [DOI] [PubMed] [Google Scholar]
  55. Parent JM, Lowenstein DH. Seizure-induced neu-rogenesis: are more new neurons good for an adult brain? In: Sutula T, Pitkänen A, editors. Do Seizures Damage the Brain. Progress in Brain Research, Vol. 135. Elsevier; Amsterdam: 2002. pp. 121–132. [DOI] [PubMed] [Google Scholar]
  56. Parent JM, Yu TW, Leibowitz RT, Geschwind DH, Sloviter RS, Lowenstein DH. Dentate granule cell neurogenesis is increased by seizures and contributes to aberrant network reorganization in the adult rat hippocampus. J. Neurosci. 1997;17:3727–3738. doi: 10.1523/JNEUROSCI.17-10-03727.1997. [DOI] [PMC free article] [PubMed] [Google Scholar]
  57. Pineau N, Charriaut-Marlangue C, Motte J, Nehlig A. Pentylenetetrazol seizures induce cell suffering but not death in the immature rat brain. Brain Res. Dev. Brain Res. 1999;112:139–144. doi: 10.1016/s0165-3806(98)00158-8. [DOI] [PubMed] [Google Scholar]
  58. Pollard H, Charriaut-Marlangue C, Cantagrel S, Represa A, Robain D, Moreau J, Ben-Ari Y. Kainate-induced apoptotic cell death in hippocampal neurons. Neuro-science. 1994;63:7–18. doi: 10.1016/0306-4522(94)90003-5. [DOI] [PubMed] [Google Scholar]
  59. Sankar R, Shin DH, Liu H, Mazarati A, Pereira de Vasconcelos A, Wasterlain C. Patterns of status epilepticus-induced neuronal injury during development and long-term consequences. J. Neurosci. 1998;18:8382–8393. doi: 10.1523/JNEUROSCI.18-20-08382.1998. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Santoro B, Liu DT, Yao H, Bartsch D, Kandel ER, Siegel-baum SA, Tibbs GR. Identification of a gene encoding a hyperpolarization-activated pacemaker channel of brain. Cell. 1998;93:717–729. doi: 10.1016/s0092-8674(00)81434-8. [DOI] [PubMed] [Google Scholar]
  61. Santoro B, Chen S, Lüthi A, Pavlidis P, Shumyatsky GP, Tibbs GR, Siegelbaum SA. Molecular and functional heterogeneity of hyperpolarization-activated pacemaker channels in the mouse CNS. J. Neurosci. 2000;20:5264–5275. doi: 10.1523/JNEUROSCI.20-14-05264.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
  62. Scharfman HE. The role of nonprincipal cells in dentate gyrus excitability and its relevance to animal models of epilepsy and temporal lobe epilepsy. In: Delgado-Esqueta AV, Wilson WA, Olson RA, Porter RJ, editors. Basic Mechanisms of the Epilepsies: Molecular and Cellular Approaches. 3rd ed. Lippincott–Raven; New York, NY: 1999. pp. 805–820. [PubMed] [Google Scholar]
  63. Scharfman HE, Goodman JH, Sollas AL. Granule-like neurons at the Hilar/CA3 border after status epilepticus and their synchrony with area CA3 pyramidal cells: functional implications of seizure-induced neurogenesis. J. Neurosci. 2000;20:6144–6158. doi: 10.1523/JNEUROSCI.20-16-06144.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
  64. Schauwecker PE, Ramirez JJ, Steward O. Genetic dissection of the signals that induce synaptic reorganization. Exp. Neurol. 2000;161:139–152. doi: 10.1006/exnr.1999.7251. [DOI] [PubMed] [Google Scholar]
  65. Siegelbaum SA. Presynaptic facilitation by hyperpolarization-activated pacemaker channels. Nat. Neurosci. 2000;3:101–102. doi: 10.1038/72038. [DOI] [PubMed] [Google Scholar]
  66. Shinnar S. Prolonged febrile seizures and mesial temporal sclerosis. Ann. Neurol. 1998;43:411–412. doi: 10.1002/ana.410430402. [DOI] [PubMed] [Google Scholar]
  67. Sloviter RS. Permanently altered hippocampal structure, excitability, and inhibition after experimental status epilepticus in the rat: the ‘dormant basket cell’ hypothesis and its possible relevance to temporal lobe epilepsy. Hippocampus. 1991;1:41–66. doi: 10.1002/hipo.450010106. [DOI] [PubMed] [Google Scholar]
  68. Sloviter RS. The functional organization of the hippocampal dentate gyrus and its relevance to the pathogenesis of temporal lobe epilepsy. Ann. Neurol. 1994;35:640–654. doi: 10.1002/ana.410350604. [DOI] [PubMed] [Google Scholar]
  69. Sloviter RS, Pedley TA. Subtle hippocampal malformation: importance in febrile seizures and development of epilepsy. Neurology. 1998;50:846–849. doi: 10.1212/wnl.50.4.846. [DOI] [PubMed] [Google Scholar]
  70. Sperber EF, Stanton PK, Haas K, Ackerman RF, Moshe SL. Developmental differences in the neurobiology of epileptic brain damage. In: Engel J Jr., Waster-lain C, Cavalheiro EA, Heinemann U, Avanzini G, editors. Molecular Neurobiology of Epilepsy. Elsevier; Amsterdam: 1992. pp. 67–81. [PubMed] [Google Scholar]
  71. Sperber EF, Germano IM, Friedman LK, Velísková J, Romero MT. The resiliency of the immature brain to seizure-induced damage. In: Nehlig A, Motte J, Moshé SL, Plouin P, editors. Childhood Epilepsies and Brain Development. John Libbey; London: 1999. pp. 255–262. [Google Scholar]
  72. Sperk G, Lassmann H, Baran H, Kish SJ, Seitelberger F, Hornykiewicz O. Kainic acid induced seizures: neurochemical and histopathological changes. Neuroscience. 1983;10:1301–1315. doi: 10.1016/0306-4522(83)90113-6. [DOI] [PubMed] [Google Scholar]
  73. Sperk G, Lassman H, Baran H, Seitelberger F, Hornykiewicz O. Kainic acid-induced seizures: Dose relationship of behavioural, neurochemical and histopathological changes. Brain Res. 1985;338:289–295. doi: 10.1016/0006-8993(85)90159-3. [DOI] [PubMed] [Google Scholar]
  74. Sutula TP. Reactive changes in epilepsy: cell death and axon sprouting induced by kindling. Epilepsy Res. 1991;10:62–70. doi: 10.1016/0920-1211(91)90096-x. [DOI] [PubMed] [Google Scholar]
  75. Thon N, Chen K, Aradi I, Soltesz I. Physiology of limbic hyperexcitability after experimental complex febrile seizures: interactions of seizure-induced alterations at multiple levels of neuronal organization. In: Baram TZ, Shinnar S, editors. Febrile Seizures. Academic Press; San Diego, CA: 2002. pp. 202–213. [Google Scholar]
  76. Toth Z, Van XX, Haftoglou S, Ribak CE, Baram TZ. Seizure-induced neuronal injury: vulnerability to febrile seizures in an immature rat model. J. Neurosci. 1998;18:4285–4294. doi: 10.1523/JNEUROSCI.18-11-04285.1998. [DOI] [PMC free article] [PubMed] [Google Scholar]
  77. Van den Pol AN, Gallyas F. Trauma-induced Golgi-like staining of neurons: a new approach to neuronal organization and response to injury. J. Comp. Neurol. 1990;296:654–673. doi: 10.1002/cne.902960410. [DOI] [PubMed] [Google Scholar]
  78. VanLandingham KE, Heinz ER, Cavazos JE, Lewis DV. Magnetic resonance imaging evidence of hippocampal injury after prolonged focal febrile convulsions. Ann. Neurol. 1998;43:413–426. doi: 10.1002/ana.410430403. [DOI] [PubMed] [Google Scholar]
  79. Veliskova J, Velisek L, Mares P. Epileptic phenomena produced by kainic acid in laboratory rats during ontogenesis. Physiologia Bohemoslovaca. 1988;37:395–405. [PubMed] [Google Scholar]
  80. Walker MC, Kullmann DM. Febrile convulsions: a ‘benign’ condition? Nat. Med. 1999;5:871–872. doi: 10.1038/11308. [DOI] [PubMed] [Google Scholar]
  81. Yan XX, Toth Z, Schultz L, Ribak CE, Baram TZ. Corticotropin-releasing hormone (CRH)-containing neurons in the immature rat hippocampal formation: light and electron microscopic features and colocalization with glutamate decarboxylase and parvalbumin. Hippocampus. 1998;8:231–243. doi: 10.1002/(SICI)1098-1063(1998)8:3<231::AID-HIPO6>3.0.CO;2-M. [DOI] [PMC free article] [PubMed] [Google Scholar]

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