The lithium–pilocarpine model of status epilepticus (SE) was used to study the type and distribution of seizure-induced neuronal injury in the rat and its consequences during development. Cell death was evaluated in hematoxylin- and eosin-stained sections and by electron microscopy. Damage to the CA1 neurons was maximal in the 2- and 3-week-old pups and decreased as a function of age. On the other hand, damage to the hilar and CA3 neurons was minimal in the 2-week-old rat pups but reached an adult-like pattern in the 3-week-old animals, and damage to amygdalar neurons increased progressively with age. The 3-week-old animals also demonstrated vulnerability of the dentate granule cells. To evaluate neuronal apoptosis, we used terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) stain, confocal fluorescence microscopy of ethidium bromide-stained sections, electron microscopy, and DNA electrophoresis. Neurons displaying all of those features of apoptotic death in response to SE were seen in the CA1 region of the 2-week-old pups and in the hilar border of the dentate granule cells of the 3-week-old animals. Some (3/11) of the animals that underwent SE at 2 weeks of age and most of the animals that underwent SE at 3 or 4 weeks of age (8/11 and 6/8, respectively) developed spontaneous seizures later in life; the latter showed SE-induced synaptic reorganization as demonstrated by Timm methodology. These results provide strong evidence for the vulnerability of the immature brain to seizure-induced damage, which bears features of both necrotic and apoptotic death and contributes to synaptic reorganization and the development of chronic epilepsy.
- status epilepticus
- mossy fiber sprouting
In adult animals, seizures cause selective neuronal death as a result of excessive neuronal activation, even in the absence of systemic complications (Meldrum et al., 1973;Nevander et al., 1985). Status epilepticus (SE) can produce neuronal damage by excessive activation of both NMDA and non-NMDA ionotropic glutamate receptors, and both NMDA and non-NMDA receptor antagonists have demonstrated neuroprotective properties in various seizure paradigms (Fujikawa, 1994, 1995; Penix and Wasterlain, 1994; Penix et al., 1996). Different neuronal populations in the hippocampus demonstrate selectively apoptotic or necrotic features of cell death in response to perforant path stimulation, a model of focal SE (Sloviter et al., 1996). Administration of kainic acid (KA) by intra-amygdaloid (Pollard et al., 1994), subcutaneous (Morrison et al., 1996), or intraperitoneal (Filipkowski et al., 1994) route results in SE and apoptosis of select populations of neurons. In the intra-amygdaloid model (Pollard et al., 1994), coadministration of diazepam prevented the seizures and apoptosis of neurons in the CA3 region, but not in the amygdala, suggesting that the apoptotic hippocampal neurons reflect seizure-induced excitotoxic damage.
In the developing brain, the effect of seizures on neuronal survival and brain growth have been controversial (Camfield, 1997; Wasterlain, 1997). Excitotoxic neuronal death is influenced by brain maturation (Portera-Cailliau et al., 1997a,b). Because apoptosis plays an important role in early brain development it is possible that the immature brain displays a special vulnerability to programmed cell death in response to seizures. Animal models suggest the existence in the young of both unique protective mechanisms as well as enhanced vulnerability to specific types of experimental SE (Sankar et al., 1995). Rat pups as young as 2 weeks demonstrate seizure-induced elevation in serum neuron-specific enolase accompanied by histological evidence of damage as a result of SE (Sankar et al., 1997b). Our understanding of seizure-induced brain damage in immature animals remains patchy because available reports are based on a limited sampling of models and ages (Albala et al., 1984; Okada et al., 1984;Sperber et al., 1991, 1992; Thompson and Wasterlain, 1997; Thompson et al., 1998), or they have examined specific aspects such as EEGs (Hirsch et al., 1992) or behavior (Liu et al., 1994) without a detailed study of the histopathology.
The cholinergic agent pilocarpine (PC) produces limbic and generalized SE in rodents accompanied by widespread brain damage in mature rats (Turski et al., 1983). The use of PC-induced seizures as a model for studying SE and SE-induced brain damage and epileptogenesis has been well described (Cavalheiro, 1995; Cavalheiro et al., 1996). Furthermore, the susceptibility of rats to PC-induced seizures appeared to be age-dependent (Cavalheiro et al., 1987). Clifford and collaborators (1987) have shown that pretreatment with lithium (Li) potentiates the epileptogenic action of PC (Honchar et al., 1983; Jope et al., 1986), reduces mortality, and avoids many of the peripheral cholinomimetic side effects of PC.
We report here the pattern of apoptosis and necrosis resulting from SE induced by the administration of lithium and pilocarpine in both developing and adult rat brains. Both morphological and biochemical methods have been used to detect the pattern of seizure-induced brain damage at different developmental stages. We also report the development of spontaneous seizures and seizure-associated synaptic plasticity in the mossy fiber terminals as visualized by Timm histochemistry.
Some of the results have appeared previously in abstract form (Sankar et al., 1997a).
MATERIALS AND METHODS
Animals. Wistar rats (Simonsen Labs, Gilroy, CA) of either sex were used in this study. The Committees on Animal Research at the University of California, Los Angeles, and the Sepulveda Veterans Affairs Medical Center approved all protocols. Rat pups of 2, 3, and 4 weeks of age and mature rats (12–16 weeks of age) were given 3 mEq/kg lithium chloride (Sigma, St. Louis, MO) intraperitoneally on the day before the induction of SE. Seizures progressing to SE were induced by subcutaneous injection of 60 mg/kg pilocarpine hydrochloride (Sigma). Control animals were given an equal volume of saline subcutaneously. Rats were observed for behavioral evidence of seizures. Only rats displaying behavioral manifestations of seizures described previously (Jope et al., 1986; Hirsch et al., 1992) were used. The selected animals underwent blood gas monitoring 5 min before the administration of PC, and 2, 4, and 6 hr after. We have previously recorded electrographic seizures from both the hippocampus and parietal cortex of selected animals of each age and have previously published selected epochs (Kornblum et al., 1997). A detailed ontogenetic study of the EEG during lithium–pilocarpine (LiPC)-induced SE has been published by Hirsch et al. (1992).
Histology. The animals were subjected to methoxyflurane (Mallinckrodt, Mundelein, IL) anesthesia 24 hr after the pilocarpine treatment. Subsequently, they underwent transcardiac perfusion-fixation with PBS followed by 4% phosphate-buffered paraformaldehyde. Brains were left in situ for 2 hr and then put in 4% paraformaldehyde overnight. Each brain was then dehydrated in graded ethanol solutions, cleared with d-limonene (Fisher, Santa Clara, CA), embedded in paraffin, sectioned to 8 μm thickness, deparaffinized and hydrated, and stained with modified hematoxylin and eosin. Slides were examined under fluorescent light so that cells that sustained injury and became acidophilic (eosinophilic) could be identified by their bright eosin fluorescence as described in detail earlier (Sankar et al., 1997b). Bilateral sections of the dorsal hippocampus approximating the region −3.8 mm posterior to bregma (Paxinos and Watson, 1982) were used for cell counts of subfields CA1, CA3, and hilus. A few animals were maintained for 6 months after SE and then killed for examination of Nissl-stained sections. Tissue was cut into 30 μm sections and stained with cresyl violet.
Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining. Paraffin-embedded sections were labeled with a commercially available in situ labeling kit (Oncor, Gaithersburg, MD). Briefly, sections were deparaffinized ind-limonene (Fisher), incubated with proteinase K (Oncor) followed by terminal deoxynucleotidyl transferase and biotinylated-16-dUTP. The reaction product was visualized with diaminobenzidine (Sigma) and lightly counterstained with 1% methyl green (Sigma).
Ethidium bromide staining. Some animals were perfused as described above, and the brains were removed, cryoprotected in 30% sucrose, and then cut in 30 μm frozen sections. Sections were soaked in d-limonene (Fisher), hydrated to water, stained in ethidium bromide (EtBr) (0.1 μg/ml) for 3 min, rinsed in water, and then dehydrated before mounting. Slides were viewed under fluorescent light using a rhodamine filter.
Optical sections were also obtained with an Olympus IX-70 microscope equipped with an epifluorescence adapter and a 100× oil immersion objective lens. Images were recorded with the CELLscan System (Scanalytics, Billerica, MA) equipped with a SenSys CCD camera system (Photometrics, Tucson, AZ), a piezoelectric z-axis focus device, and a computer-controlled excitation light shutter. The light haze contributed by fluorescently labeled structures located above and below the plane of optimal focus was mathematically reassigned to its proper places of origin (Exhaustive Photon Reassignment software, Scanalytics) after accurate characterization of the blurring function of the optical system. The blurring function of the optical system (the point-spread function) was characterized by imaging a through-focus series of optical sections of a 0.2-μm-diameter fluorescent bead (Molecular Probes, Eugene OR) using the same optical conditions as those used to obtain the specimen image. The patented Exhaustive Photon Reassignment (EPR) process successively iterates estimates of the true signal and intensity distribution until such time as the estimate, when blurred by the point-spread function, yields the image as detected by the camera. This can be expressed as follows: I =O * PSF, where I = image of the object as detected by the camera, O = the true object as it exists on the microscope slide, * = convolution,PSF = point-spread function image stack as measured from the optical setup. During restoration, EPR uses the PSF image data to refocus light and haze in the raw specimen image.
DNA extraction and agarose gel electrophoresis. Samples of hippocampi (8–10) were dissected from fresh brains of rats (24 hr after SE), immediately frozen in dry ice, and stored at −70°C until use. DNA was purified using the technique of Iwasa et al. (1996). Hippocampi were incubated overnight in digestion buffer [100 mm NaCl, 10 mm Tris-Cl, pH 8.0, 25 mm EDTA, pH 8.0, 0.5% SDS, 0.1 mg/ml proteinase K, 0.1 mg/ml RNase A (Sigma)]. DNA was extracted using equal proportions of the lysate and phenol, chloroform, and isoamyl alcohol (Life Technologies, Gaithersburg, MD). The aqueous layer was incubated at 37°C with additional RNase A (0.1 mg/ml) for 60–90 min. Phenol extraction was repeated and DNA was precipitated with 2.5 vol of ethanol and 1/10 vol of sodium acetate and placed in dry ice for 30 min. Precipitated DNA was spun at 11,000 rpm (14,500 ×g) for 15 min and washed three times with 50 ml cold 70% ethanol. The supernatant was then decanted, and the pellet was air-dried for 15 min. Pellet was resuspended in 200–500 μl of 10 mm Tris-Cl, pH 8.5, in 55°C shaking water bath overnight. Spectrophotometry revealed A 260/280 ratios of 1.6–2.1, indicating relatively pure DNA in concentrations of ∼1–4 μg/μl. DNA (20 μg per lane) was run on 2% agarose gels containing 0.5% EtBr at 4 V/cm gel length. The gel was viewed under UV transillumination (302 nm) and photographed with a Polaroid camera.
Electron microscopy. Some animals were perfused with PBS (20 ml for 2-week-old animals, 30 ml for 3-week-old animals) followed by 3% glutaraldehyde (50 ml for 2-week-old animals, 60 ml for 3-week-old animals) (Electron Microscopy Sciences, Ft. Washington, PA) and 1% paraformaldehyde (Fisher) in 0.1 m phosphate buffer (PB), pH 7.4. The brain was removed and post-fixed for 1 d and then cut on a vibratome into 100 μm sections. The sections were washed with PB, stained for 1 hr in 2% OsO4 (Electron Microscopy Sciences), and stained en block in 2% aqueous uranyl acetate (Mallinckrodt) for 30 min. The sections were then dehydrated in ascending graded solutions of alcohol for 10 min each and then placed in 1:1 100% EtOH/propylene oxide for 5 min followed by two incubations in propylene oxide for 5 min. The sections were then incubated in 1:1 and 1:3 propylene oxide/Durcupan (Fluka, Buchs, Switzerland) for 1 hr each followed by Durcupan overnight. The sections were placed between strips of Aclar plastic and cured for 24 hr at 60°C, after which they were cut on an ultramicrotome. They were placed on single hole grids, covered by butvar, stained with 6% uranyl acetate and lead citrate, and examined with a Philips Electron Microscope 201C.
Timm histochemistry. Rats that had been subjected to LiPC-induced SE 4 months earlier and control rats were anesthetized with methoxyflurane (Mallinckrodt) and perfused transcardially with an aqueous solution of 250 ml of 0.1% (w/v) sodium sulfide, followed by 250 ml of 4% paraformaldehyde solution. The brains were removed and left in a 30% (w/v) sucrose solution in the fixative overnight at 4°C. Coronal 30 μm frozen sections were developed in the dark for 30 min in a 6:3:1 mixture of gum arabic (20%, w/v), hydroquinone (5.6%, w/v), citric acid–sodium citrate buffer with 1.5 ml of a silver nitrate solution (17%) (Danscher, 1981).
Monitoring for spontaneous seizures. Animals that underwent LiPC-induced SE were selected for monitoring when they began to demonstrate behavioral seizures 3 months or longer after the initial experiments. Animals were anesthetized with ketamine (60 mg/kg) and xylazine (15 mg/kg). A bipolar stimulating electrode was implanted into the angular bundle of the perforant path (4.5 mm left of and 0.5 mm posterior to lambda), and a bipolar recording electrode was implanted into the granule cell layer of the ipsilateral dentate gyrus (2.2 mm left of and 3.5 mm anterior to lambda). The depth of both electrodes was optimized by finding the population spike of maximal amplitude, evoked from the dentate gyrus by stimulation of the perforant path. After recovery from surgery for 1 week, the animals were monitored for 24 hr a day with a video camera and continuous EEG using the Monitor 81 program from Stellate Software. The software was configured to detect and save seizures and spikes. The EEGs were analyzed off-line using the same software.
The 60 mg/kg dose of pilocarpine used in this study produced cortical and subcortical SE in all the ages studied (EEG data not shown). The latency to SE was 10–20 min for all ages. The 2-week-old pups developed head bobbing followed by intermittent forelimb clonus and hyperextension of the tail (which persisted for most of the duration of the SE) and of the hindlimbs intermittently. The 3- and 4-week-old rats showed forelimb clonus leading to frequent kindled-like seizures with rearing and falling, and occasional running seizures with vocalization, which sometimes culminated in tonic extension and death. The adults exhibited forelimb clonus and kindled-like seizures with frequent periods of behavioral arrest during which chewing movements and twitching of the face and/or vibrissae were often observed. All the 2-week-old rat pups survived the lithium–pilocarpine SE, whereas approximately one-third of the 3- and 4-week-old animals and 50% of the adult animals died. Arterial blood gases were sampled in the 2-week-old group (data not shown), and none of the animals experienced any hypoxemia during SE.
In all four groups, extensive hippocampal and extra-hippocampal neuronal death followed SE. However, both the nature and the distribution of this neuronal death were highly age-specific.
Neuronal damage in the CA1 subfield of the hippocampus (Fig.1 A–D) as ascertained by the presence of “ischemic cell change” (Brown and Brierley, 1968) with a pyknotic nucleus and an eosinophilic cytoplasm was maximal in the 2- and 3-week-old rat pups (in which 36 ± 5% and 34 ± 10% of CA1 neurons, respectively, had this appearance) and decreased progressively with age to 25 ± 8% in the 4-week-old animals and 9 ± 3% (p < 0.05) in the adults (Table 1).
The neuronal damage in the CA3 region of the 2-week-old rats was barely discernible at 1%. The damage at 3, 4, and 9–12 weeks (20 ± 9, 18 ± 7, and 14 ± 3%, respectively) did not differ significantly (Table 1). Two-week-old rat pups were resistant to SE-induced hilar damage. The extent of damage in the 3-week-old rat pups (31 ± 2%) was comparable to that in the 4-week-old (27 ± 2%) and adult rats (28 ± 2%) (Table 1, Fig.2 A–D). Damage to the dentate granule cells is also shown in Figures2 A–D. The 3-week-old rat pups demonstrated a special vulnerability to SE-induced damage in this cell population (33 ± 7%) that is different from that seen in younger or older animals (5 ± 0.5, 7 ± 2, and 5 ± 1% in 2-, 4-, and 9–12-week-old animals, respectively) (Table 1, Fig.2 A–D). The cell injury demonstrated by eosin fluorescence at 24 hr after SE was confirmed as permanent loss of cells by cresyl violet-stained sections in selected animals 6 months after the initial SE (Fig.3 A–F). We have reported previously on the age-related SE-induced damage to extrahippocampal structures (Sankar et al., 1997b). The neocortex, caudate, and septum showed damage scores similar to adults by 3 weeks of age, whereas the damage to amygdala showed a trend toward progressively increasing damage with age. Damage to the thalamus was maximal in the 2- and 3-week-old rats and decreased with age (Sankar et al., 1997b).
Type of neuronal death
Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) stain-positive neurons were seen in large numbers only in the 2- and 3-week-old pups after SE. The distribution of these neurons was age-dependent, with the 2-week-olds demonstrating such injury predominantly in the CA1 region (Fig.4 A,C), the subiculum, and the thalamus, with lesser labeling of the inner dentate granule cells. In the 3-week-old animals, only the inner layer of the dentate granule cells (Fig. 4 B,D) and a few thalamic neurons were TUNEL-positive. Fluorescence microscopy of EtBr-stained sections (Fig. 5 A,B) revealed fragmented nuclei in the same areas that were TUNEL-stained. In the 3-week-old animals, the neuronal injury delineated by the TUNEL and EtBr methods (Figs. 4 B,D, 5 B) seen in the inner layer of the granule cells was different from that seen in the outer granular layer neurons visualized by their eosinophilic cytoplasm (Fig. 2 B). Two types of damage also appear to coexist in the CA1 subfield of 2-week-old pups subjected to SE.
Electron micrographs of CA1 in 2-week-old pups (Fig.6 A) and of the inner granule cell region and hilus (Fig. 6 B) of the 3-week-olds clearly demonstrated the presence of distinctive nuclear and cytoplasmic changes suggestive of necrosis in some neurons and of apoptosis in others. Twenty-four hours after SE, many CA1 neurons displayed chromatin condensation coexisting with intact cytoplasmic and nuclear membranes and with relatively intact cytoplasmic organelles. In that same CA1 region, however, swollen neurons with ruptured membranes, extensive cytoplasmic damage, and all the hallmarks of necrosis were also seen. In the 3-week-old dentate gyrus, inner granule cells with chromatin margination and nearly intact cytoplasms were seen in close proximity to clearly necrotic hilar neurons.
The DNA extracted from the hippocampi of rats of various ages subjected to LiPC SE and of their age-matched controls was studied by agarose gel electrophoresis (Fig. 7). Laddering was not evident in the lanes in which DNA from control rats was placed. Discrete bands corresponding to fragments of DNA with a periodicity approximating 180 base pairs, suggesting oligonucleosomal breaks, were seen best in the hippocampal DNA from the 2- and 3-week-old rats that underwent SE. A less prominent degree of laddering was also seen in the lanes containing DNA from the hippocampi and neocortices (the latter not shown) of 4-week-old and adult rats that underwent SE, although we did not see a significant number of TUNEL-positive neurons, or apoptotic bodies, at the two time points (24 and 72 hr) studied in those animals.
Three of the 11 rats subjected to SE at 2 weeks of age that were observed for at least 4 months developed spontaneous seizures that resembled Racine’s (1972) stages 3–5 in kindled animals and seizures triggered by handling. The latter were characterized by a few myoclonic jerks sometimes leading to a stage 3–5 seizure. Of the 11 rats subjected to SE at 3 weeks of age, eight (and six out of eight that experienced SE at 4 weeks) displayed spontaneous seizures after a latent period of <3 months, shorter than that seen in the animals subjected to SE at 2 weeks of age. Recordings of spontaneous seizures from a rat that underwent LiPC SE at 2 weeks is shown in Figure8 A, whereas Figure8 B shows multiple spontaneous seizures recorded from a rat that experienced LiPC SE at 3 weeks of age.
Mossy fiber synaptic reorganization
The degree of staining seen in the rats that developed spontaneous seizures after undergoing SE at 2 weeks of age (Fig.8 E) was not distinguished from that of controls (Fig.8 C). The pattern of Timm staining of the inner molecular layer seen in a rat with spontaneous seizures that had undergone SE at 3 (Fig. 8 F) and 4 weeks of age is similar to that seen in adults.
We have used different methods known to be sensitive to distinctive mechanisms that may lead to neuronal damage as a result of the recurrent excitation caused by SE during development. Previous studies by others (Albala et al., 1984; Sperber et al., 1991, 1992) have focused on specific aspects of hippocampal response to seizures such as hilar cell loss and mossy fiber sprouting and have remarked on the relative resiliency of the immature brain to seizures. Our results show enhanced vulnerability to CA1 neurons during early development at a stage when hilar injury and mossy fiber synaptic reorganization are not prominent. The damage seen at this age appears to involve both necrosis, as evidenced by the eosinophilic cells, and apoptosis as demonstrated by various techniques that included TUNEL staining, visualization of fragmented nuclei under fluorescence microscopy of EtBr-stained sections, and electron microscopy. We have also presented DNA electrophoresis data compatible with the process of apoptosis.
Franck and Schwartzkroin (1984) had found preferential loss of CA1 neurons in immature rabbits subjected to SE induced by systemic KA and had observed the similarity of this lesion to that induced by ischemia and/or hypoxia. It was subsequently shown in hippocampal slices obtained from immature rabbits that the rise in extracellular potassium after repetitive stimulation was regulated less effectively in CA1 compared with CA3 (Haglund and Schwartzkroin, 1990). More recently, such vulnerability of the CA1 neurons to LiPC SE has been demonstrated in immature rabbits in the absence of hypoxia (Thompson and Wasterlain, 1997). In the rat, Hamon and Heinemann (1988) had shown that during the second postnatal week, the apical dendrites of pyramidal neurons in area CA1 become more sensitive to NMDA, which is expressed by big influxes of calcium at this level. Harris and Teyler (1984) showed that long-term potentiation in area CA1 of the rat was maximal at 15 d. Excitability comparable to that seen in fully mature animals could be demonstrated in the CA1 region after postnatal day 14, whereas the inhibitory processes did not reach an adult stage of maturation until several weeks later (Michelson and Lothman, 1989). Our observation of preferential CA1 damage at 2 weeks of age is consistent with the observations of the development of synaptic inhibition lagging in CA1 compared with area CA3 (Swann et al., 1989) or the dentate gyrus (Bekenstein and Lothman, 1991). Target kainate receptor development at the mossy fiber terminals and amygdala reached maturation (Berger et al., 1984) only toward the end of the third week such that the hilus and CA3 are resistant to SE-induced damage in the 2-week-old rat.
In the CA1 region of the 2-week-old rat, both acidophilic neurons and TUNEL-positive neurons are seen in close proximity to each other. Our electron micrographs confirm the presence of nuclear profiles suggesting necrosis in the same region as those suggesting apoptosis. It has been shown that both apoptosis and necrosis can result from qualitatively similar types of excitotoxic stimulus, with the intensity of NMDA receptor activation deciding the process (Bonfoco et al., 1995). The neuronal damage seen maximally in the CA1 of our 2 week-old rats is attributable to the seizures, because hypoxia was not encountered in these animals. Furthermore, hypoxia at PaO2 of 20 mmHg for 20 min without ischemia did not result in cell loss or even morphological evidence of injury as evidenced by the lack of expression of heat-shock protein in mature rats (Pearigen et al., 1996).
Damage to the hilus and CA3 in this model of SE appears to be at least as severe in the 3-week-old rat pups as in mature rats, but 2-week-old pups show little or no damage. The development of vulnerability to seizure-induced damage in the hilus and CA3 may be attributed to the maturation of the mossy fiber terminals. This was suggested by Nitecka et al. (1984) in a detailed study of SE-induced damage in the KA model that documented progressive damage to limbic structures from postnatal day 18. Previous studies demonstrating resistance to SE-induced hilar and CA3 damage in the developing brain have compared mature rats only to 2-week-old pups (Albala et al., 1984; Sperber et al., 1991, 1992) in the kainic acid model or have used groupings such as 15- to 21-d-old rats in the PC model of SE (Cavalheiro et al., 1987).
Recent studies in immature animals have suggested that certain methods such as acid fuchsin staining (Chang and Baram, 1994) and silver degeneration methods (Toth et al., 1998) may delineate reversible neuronal injury rather than serve as markers of dying neurons. We have demonstrated that the pattern of neuronal damage discernible by eosin fluorescence at 24 hr is reflected as permanent cell drop-out in cresyl violet-stained sections obtained from surviving rats several months after SE.
We saw evidence of damage to the granule cells of the dentate gyrus mainly in the 21-d-old rat pups. It is striking that strongly eosinophilic neurons are seen mainly in the outermost layers of the granule cells, whereas scattered TUNEL-positive neurons are seen along the hilar border. The distribution of TUNEL-positive neurons in our experiments is reminiscent of the results reported by Goodman et al. (1993) in ischemic rat pups, and by Bengzon et al. (1997) after kindling or KA administration in adult rats. Apoptosis of granule cells has also been shown after repetitive perforant path stimulation (Sloviter et al., 1996) and after adrenalectomy (Sloviter et al., 1993;Hu et al., 1997), in mature rats. Under these circumstances, apoptosis does not appear to be restricted to the granule cells along the hilar border. In hypoxic–ischemic rat pups, the distribution of apoptosis was limited to the inner granule cells that did not express the calcium-binding protein calbindin D28k, suggesting a mechanism for this age-dependent vulnerability, which might also apply to our pups.
Weiss et al. (1996) reported a significantly different pattern of DNA fragmentation in the hippocampus of rats subjected to KA-induced SE. It is likely that the pattern of DNA fragmentation detected by a particular technique is influenced by the timing after the excitotoxic insult as well as the variations in the technique. Didier et al. (1996)have suggested that the in situ nick translation method using DNA polymerase 1 is very sensitive and labels single-strand breaks and that such damage may be reversible, whereas the nick end labeling with terminal deoxynucleotidyl transferase identifies double-strand breaks.
Charriaut-Marlangue and Ben-Ari (1995) cautioned on the use of the TUNEL stain as the sole criterion for the demonstration of apoptosis and suggested that methods such as Hoechst 33258 staining be used to supplement the data. We have used EtBr staining in such a manner. It is of interest that significant TUNEL labeling was not seen in our experiments in regions where extensive eosinophilic neurons are encountered in animals older than the 3 week-old pups. This is consistent with the results of Fujikawa et al. (1997) who did not see significant evidence for apoptosis in mature rats subjected to LiPC SE. On the other hand, “DNA laddering” on agarose gel is widely presented as evidence of apoptosis based on the suggestion that necrosis would lead to a “smear” DNA pattern (Walker et al., 1994). Our results show that laddering may be seen when there is extensive necrosis, although cells matching the morphological criteria for apoptosis may be extremely rare. Similar observations have been made byFujikawa et al. (1997). These results are consistent with recent reports that examined the consequences of excitotoxic exposure of cultured murine cortical neurons (Gwag et al., 1997; Sohn et al., 1998). In these studies, early cell swelling and early damage to the plasma membrane were observed when the integrity of the nuclear membrane was preserved along with DNA ladders. Hence, we would like to sound a cautionary note on the use of agarose gel electrophoresis of extracted DNA as a sole or major criterion for apoptosis.
Rat pups that were subjected to SE at 2 weeks of age developed spontaneous seizures after a longer latency than those that underwent SE at P21, and in much smaller (3/11 vs 8/11, respectively) numbers. Even the rats that developed spontaneous seizures after SE at 2 weeks of age did not show evidence of sprouting as seen by Timm stain. A recent report had suggested (Longo and Mello, 1997) that spontaneous seizures could develop after SE in the absence of mossy fiber sprouting. This conclusion was based on experiments in which sprouting had been blocked by the administration of cycloheximide, a protein synthesis inhibitor, without abolishing the development of spontaneous seizures. Our results do not distinguish whether sprouting and synaptic reorganization of the mossy fiber pathway is a cause or result of repeated seizures.
In conclusion, we have shown that the pattern of seizure-induced cell death is related to development. Apoptosis and necrosis can result from seizures in an age-dependent distribution, and even young animals can develop spontaneous seizures as a consequence of early seizures. The demonstration of seizure-related apoptosis in the immature brain may have implications for the development of neuroprotective agents tailored for use in the young.
R.S. was supported by Clinical Investigator Development Award NS01792 from the National Institute of Neurological Disorders and Stroke (NINDS), National Institutes of Health. C.G.W. was supported by Grant NS13515 from NINDS, National Institutes of Health, and by the Research Service of the Veterans Health Administration. We thank Roger Baldwin, Rosie Lezama, Steve Shinmei, and Don Santos for technical assistance.
Correspondence should be addressed to Dr. Raman Sankar, Pediatric Neurology (22-474 Marion Davies Children’s Center), Box 951752, UCLA School of Medicine, Los Angeles, CA 90095-1752.