Comparison of NADPH diaphorase histochemistry, somatostatin immunohistochemistry, and silver impregnation in detecting structural and functional impairment in experimental status epilepticus

Ictal onset sites and γ-aminobutyric acidergic neuron loss in epileptic pilocarpine-treated rats

OBJECTIVE

The present study tested whether ictal onset sites are regions of more severe interneuron loss in epileptic pilocarpine-treated rats, a model of human temporal lobe epilepsy.

METHODS

Local field potential recordings were evaluated to identify ictal onset sites. Electrode sites were visualized in Nissl-stained sections. Adjacent sections were processed with proximity ligation in situ hybridization for glutamic acid decarboxylase 2 (Gad2). Gad2 neuron profile numbers at ictal onset sites were compared to contralateral regions. Other sections were processed with immunocytochemistry for reelin or nitric oxide synthase (NOS), which labeled major subtypes of granule cell layer-associated interneurons. Stereology was used to estimate numbers of reelin and NOS granule cell layer-associated interneurons per hippocampus.

RESULTS

Ictal onset sites varied between and within rats but were mostly in the ventral hippocampus and were frequently bilateral. There was no conclusive evidence of more severe Gad2 neuron profile loss at sites of earliest seizure activity compared to contralateral regions. Numbers of granule cell layer-associated NOS neurons were reduced in the ventral hippocampus.

SIGNIFICANCE

In epileptic pilocarpine-treated rats, ictal onset sites were mostly in the ventral hippocampus, where there was loss of granule cell layer-associated NOS interneurons. These findings suggest the hypothesis that loss of granule cell layer-associated NOS interneurons in the ventral hippocampus is a mechanism of temporal lobe epilepsy.

Seizure frequency correlates with loss of dentate gyrus GABAergic neurons in a mouse model of temporal lobe epilepsy

Epilepsy occurs in one of 26 people. Temporal lobe epilepsy is common and can be difficult to treat effectively. It can develop after brain injuries that damage the hippocampus. Multiple pathophysiological mechanisms involving the hippocampal dentate gyrus have been proposed. This study evaluated a mouse model of temporal lobe epilepsy to test which pathological changes in the dentate gyrus correlate with seizure frequency and help prioritize potential mechanisms for further study. FVB mice (n = 127) that had experienced status epilepticus after systemic treatment with pilocarpine 31-61 days earlier were video-monitored for spontaneous, convulsive seizures 9 hr/day every day for 24-36 days. Over 4,060 seizures were observed. Seizure frequency ranged from an average of one every 3.6 days to one every 2.1 hr. Hippocampal sections were processed for Nissl stain, Prox1-immunocytochemistry, GluR2-immunocytochemistry, Timm stain, glial fibrillary acidic protein-immunocytochemistry, glutamic acid decarboxylase in situ hybridization, and parvalbumin-immunocytochemistry. Stereological methods were used to measure hilar ectopic granule cells, mossy cells, mossy fiber sprouting, astrogliosis, and GABAergic interneurons. Seizure frequency was not significantly correlated with the generation of hilar ectopic granule cells, the number of mossy cells, the extent of mossy fiber sprouting, the extent of astrogliosis, or the number of GABAergic interneurons in the molecular layer or hilus. Seizure frequency significantly correlated with the loss of GABAergic interneurons in or adjacent to the granule cell layer, but not with the loss of parvalbumin-positive interneurons. These findings prioritize the loss of granule cell layer interneurons for further testing as a potential cause of temporal lobe epilepsy.

Glutamate and GABA imbalance promotes neuronal apoptosis in hippocampus after stress

Background

People who experience traumatic events have an increased risk of post-traumatic stress disorder (PTSD). However, PTSD-related pathological changes in the hippocampus and prefrontal cortex remain poorly understood.

Material/Methods

We investigated the effect of a PTSD-like animal model induced by severe stress. The experimental rats received 20 inescapable electric foot shocks in an enclosed box for a total of 6 times in 3 days. The physiological state (body weight and plasma corticosterone concentrations), emotion, cognitive behavior, brain morphology, apoptosis, and balance of gamma-aminobutyric acid (GABA) and glutamate in the hippocampus and prefrontal cortex were observed. Cell damages were examined with histological staining (HE, Nissl, and silver impregnation), while apoptosis was analyzed with flow cytometry using an Annexin V and propidium iodide (PI) binding and terminal deoxynucleotidyl transferase mediated-dUTP nick end labeling (TUNEL) method.

Results

In comparison with the sham litter-mates, the stressed rats showed decreased body weight, inhibition of hypothalamic-pituitary-adrenal (HPA) axis activation, increase in freezing response to trauma reminder, hypoactivity and anxiety-like behaviors in elevated plus maze and open field test, poor learning in Morris water maze, and shortened latency in hot-plate test. There were significant damages in the hippocampus but not in the prefrontal cortex. Imbalance between glutamate and GABA was more evident in the hippocampus than in the prefrontal cortex.

Conclusions

These results suggest that neuronal apoptosis in the hippocampus after severe traumatic stress is related to the imbalance between glutamate and GABA. Such modifications may resemble the profound changes observed in PTSD patients.

NOS-positive local circuit neurons are exclusively axo-dendritic cells both in the neo- and archi-cortex of the rat brain

Neuronal nitric oxide synthase (nNOS)-containing neurons and axon terminals were examined in the rat somatosensory and temporal neocortex, in the CA3/a-c areas of Ammon's horn and in the hippocampal dentate gyrus. In these areas, only nonpyramidal neurons were labeled with the antibody against nNOS. Previous observations suggested that all nNOS-positive nonpyramidal cells are GABAergic local circuit neurons, which form exclusively symmetric synapses. In agreement with this, nNOS-positive axon terminals in the hippocampal formation formed symmetric synapses exclusively with dendritic shafts. In the neocortex, in contrast, in addition to the nNOS-positive axon terminals that formed synapses with unlabeled spiny and aspiny dendrites and with nNOS-positive aspiny dendrites, a small proportion of the nNOS-positive axon terminals formed symmetric synapses with dendritic spines. These results suggest that nNOS-positive local circuit neurons form a distinct group of axo-dendritic cells displaying slightly different domain specificity in the archi- and neocortex. However, nNOS-positive cells show no target selectivity, because they innervate principal cells and local circuit neurons. Afferents to the NOS-positive cells display neither domain nor target selectivity, because small unlabeled terminals formed synapses with both the soma or dendrites of nNOS-positive neurons and an adjacent unlabeled dendrite or spine in both the hippocampal formation and in neocortex.

Distribution of NADPH-diaphorase and expression of nNOS, N-methyl-D-aspartate receptor (NMDAR1) and non-NMDA glutamate receptor (GlutR2) genes in the neurons of the hippocampus after domoic acid-induced lesions in adult rats

Neuronal degeneration followed by detection of nitric oxide (NO)-producing neurons of the hippocampus was investigated at 4 h, 16 h, 24 h, 2 days, 5 days, and 14 days after administration of domoic acid (DA), in the present study. Histopathological analysis (Nissl staining) displayed dark-stained degenerating neurons in the hippocampus at 24 h to 14 days after DA administration, with degeneration most severe at 5-14 days. NADPH-d-positive neurons were observed in different subfields of the hippocampus in control rats and DA treated rats at 4-24 h. Complete loss of NADPH-d-positive neurons in the CA1 and CA3 subfields and also in the hilus of dentate gyrus (DG) was observed at 5 days and 14 days after the administration of DA. In contrast, at 4-24 h, neuronal nitric oxide synthase (nNOS)-immunoreactive cells were absent from the hippocampal subfields in control and DA-treated animals but were observed at 5 days and 14 days after DA administration. N-methyl-D-aspartate receptor (NMDAR1) immunoreactivity was increased in the hippocampal neurons at 5 days after DA administration and double immunofluorescence demonstrated its coexpression with induced nNOS expression. No significant change could be observed in the immunoreactivity of non-NMDA receptor (GlutR2) as compared with the controls, while occasional immunoreactive neurons were colocalized with induced nNOS expression. Reverse transcription-polymerase chain reaction analysis showed the upregulated expression of nNOS and downregulated expression of NMDAR1 at 5 days after the administration of DA. Although nNOS mRNA expression was rapidly induced at 5 days after DA administration, in situ hybridization analysis revealed complete loss of nNOS mRNA expression in the region of neuronal degeneration in the hippocampus at 24 h and 5 days after DA administration. The present study has shown that NADPH-d and nNOS express differentially in the neurons of the hippocampus in DA-induced neurotoxicity. It is speculated that induction of nNOS and glutamate receptor genes in the neurons of the hippocampus in response to DA-induced neurotoxicity could have contributed to the neuronal degeneration.

The spiny rat Proechimys guyannensis as model of resistance to epilepsy: chemical characterization of hippocampal cell populations and pilocarpine-induced changes

At variance with pilocarpine-induced epilepsy in the laboratory rat, pilocarpine administration to the tropical rodent Proechimys guyannensis (casiragua) elicited an acute seizure that did not develop in long-lasting status epilepticus and was not followed by spontaneous seizures up to 30 days, when the hippocampus was investigated in treated and control animals. Nissl staining revealed in Proechimys a highly developed hippocampus, with thick hippocampal commissures and continuity of the rostral dentate gyri at the midline. Immunohistochemistry was used to study calbindin, parvalbumin, calretinin, GABA, glutamic acid decarboxylase, and nitric oxide synthase expression. The latter was also investigated with NADPH-diaphorase histochemistry. Cell counts and densitometric evaluation with image analysis were performed. Differences, such as low calbindin immunoreactivity confined to some pyramidal cells, were found in the normal Proechimys hippocampus compared to the laboratory rat. In pilocarpine-treated casiraguas, stereological cell counts in Nissl-stained sections did not reveal significant neuronal loss in hippocampal subfields, where the examined markers exhibited instead striking changes. Calbindin was induced in pyramidal and granule cells and interneuron subsets. The number of parvalbumin- or nitric oxide synthase-containing interneurons and their staining intensity were significantly increased. Glutamic acid decarboxylase(67)-immunoreactive interneurons increased markedly in the hilus and decreased in the CA1 pyramidal layer. The number and staining intensity of calretinin-immunoreactive pyramidal cells and interneurons were significantly reduced. These findings provide the first description of the Proechimys hippocampus and reveal marked long-term variations in protein expression after an epileptic insult, which could reflect adaptive changes in functional hippocampal circuits implicated in resistance to limbic epilepsy.

Effect of neonatal dentate gyrus lesion on allothetic and idiothetic navigation in rats

Goal-directed navigation is believed to be the combined product of idiothetic and allothetic orientation. Although both navigation systems require the hippocampal formation, it is probable that different circuits implement them. Examination of Long-Evans rats with dentate gyrus lesions induced by neonatal X-ray irradiation may show the dissociation of these two components of navigation. Two recently developed place avoidance tasks on a rotating circular arena were used to test this hypothesis. In the first test, the position of the punished area is stable in the room frame but is permanently changing on the surface of the arena. This task requires the rat to use allothetic orientation and to disregard idiothetic orientation. In the second test, the prohibited area is fixed in the coordinate system of the arena and the experiment is conducted in complete darkness, forcing the rat to rely exclusively on idiothesis supported by substratal cues. The results suggest that the dentate gyrus lesion interferes less with idiothetic orientation than with allothetic orientation. In addition, an attempt was made to control the number of developing granule cells by exact timing of a single high dose of perinatal irradiation, and to measure the ensuing behavioral deficits. Rats irradiated at 6, 18, or 24 h after birth were tested as adults in the Morris water maze. Irradiated animals showed significant, but highly variable, learning deficit, but histological examination indicated that the granule cell loss did not correlate with the degree of behavioral impairment.

Differential changes in neuropeptide Y and nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in rat hippocampus after kainic acid-induced seizure

Changes in neuropeptide Y (NPY) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-positive neurons in the hippocampus were investigated 5, 10 and 20 days after kainic acid (KA) administration using a double labeling method. The numbers of NADPH-d-positive-only and NPY/NADPH-d-positive neurons decreased in the CA1/2-CA3 regions of the hippocampus, 5, 10 and 20 days after KA administration, however, the number of NPY-positive-only neurons increased in the same regions 5 and 10 days after KA administration. In the dentate gyrus (DG) region of the hippocampus, the numbers of NPY-positive-only, NADPH-d-positive-only and NPY/NADPH-d-positive neurons increased 5 days after KA administration, and 20 days after KA administration, the number of NADPH-d-positive-only neurons decreased to levels similar to or lower than the level of the controls. However, the numbers of NPY/NADPH-d-positive and NPY-positive-only neurons in the DG region 20 days after KA administration remained at control levels. These results indicate that, NADPH-d-positive-only neurons are vulnerable to, and NPY-positive-only neurons are resistant to KA-induced seizures in the whole hippocampus, but that NPY/NADPH-d-positive neurons have different sensitivities in subregions of the hippocampus to KA-induced seizures. In addition, the present findings provide the first statistical and morphological evidence, which demonstrates that NPY-positive-only, NADPH-d-positive-only and NPY/NADPH-d-positive neurons in the hippocampus have different sensitivities to KA-induced seizures.

Nitric oxide synthase immunoreactivity in the rat hippocampus after status epilepticus induced by perforant pathway stimulation

Nitric oxide has recently been implicated in mediation of neuronal excitotoxicity and damage. This study aimed at elucidating the changes in the expression of neuronal isoform of nitric oxide synthase (nNOS) in the hippocampus after status epilepticus induced by perforant pathway stimulation. nNOS-immunoreactivity (nNOS-ir) and neuronal damage, assessed by silver staining, were evaluated separately in different hippocampal subfields 2 weeks after induction of status epilepticus. Perforant pathway stimulation resulted in an increase in the number of nNOS-immunoreactive neurons in the stratum radiatum of the CA1 and CA3 subfields of the hippocampus proper, and the hilus of the dentate gyrus. The morphology and distribution of the nNOS-ir neurons resembled that of interneurons. No correlation of the number of nNOS-ir neurons to the neuronal damage score was observed. Our results suggest that status epilepticus provokes a de novo expression of nNOS protein, and the nNOS expressing neurons may be selectively resistant to epileptic brain injury.

Changes in NADPH-diaphorase positivity induced by status epilepticus in allocortical structures of the immature rat brain

The distribution and time course of changes of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) positivity were studied in immature rats (12 and 25 days old) surviving motor status epilepticus (SE) induced by a high dose of pilocarpine. Motor SE characterized by continuous convulsions was interrupted after 2 h by an injection of clonazepam (0.5 mg/kg or 1 mg/kg in 12- and 25-day-old rats, respectively) in order to reduce mortality. Correlation between electroencephalographic and behavioral seizure activity was confirmed using animals with electrodes implanted bilaterally in the hippocampus and sensorimotor cortex. Brains were examined 2, 6, 13, and 21 days after motor SE using NADPH-diaphorase histochemistry. Two types of changes were found in both age groups: (a) decrease of NADPH-d positivity occurred in both neuropil and cell bodies in piriform, periamygdalar, and entorhinal cortices; and (b) NADPH-d positivity was induced in the cell bodies in the hippocampal fields CA1/2, CA3, and dentate gyrus. These changes were more intense in animals surviving SE at postnatal day 25 than in younger age group, and they peaked 2 days after SE. The changes observed after SE disappeared quickly in 12-day-old rat pups, where only moderate changes could be observed in piriform, periamygdalar, and entorhinal cortices 6 days after SE, whereas the changes in the histochemical positivity persisted in older animals even 21 days after SE.

Hippocampal damage after injection of kainic acid into the rat entorhinal cortex

Several experimental models of epilepsy have used kainic acid in animals to induce seizures and neuropathological changes which mimic those observed in human temporal lobe epilepsy. These models differ in the location and manner in which kainic acid is applied. In the present study, we characterized the seizure activity and neuropathological changes that occur in awake rats after kainic acid (25 ng/250 nl) is injected into the entorhinal cortex of freely moving rats. In 91% of the animals, this induced generalized motor seizures. Moreover, all of the animals survived status epilepticus. Animals were perfused two weeks after the injection for neuropathological examination. Silver-impregnation revealed that kainic acid caused pyramidal cell damage which was most severe in the CA1 subfield and to a lesser degree in the CA3c area. A loss of NADPH diaphorase-containing neurons in the hilus and the CA1 area was also consistently seen and, in most cases, a population of somatostatin-immunoreactive neurons was diminished. Our findings show that a minute amount of kainic acid delivered directly to the entorhinal cortex on unanesthetized animals reliably produces generalized seizures as well as a consistent pattern of cell damage in the hippocampus. Therefore, this model may be suitable for investigating the mechanisms underlying temporal lobe epilepsy, and may prove useful in assessing different treatment strategies for preventing seizure-induced structural damage.

Neocortex in the hippocampus: an anatomical and functional study of CA1 heterotopias after prenatal treatment with methylazoxymethanol in rats

Migration disorders cause neurons to differentiate in an abnormal heterotopic position. Although significant insights have been gained into the etiology of these disorders, very little is known about the anatomy of heterotopias. We have studied heterotopic masses arising in the hippocampal CA1 region after prenatal treatment with methylazoxymethanol (MAM) in rats. Heterotopic cells were phenotypically similar to neocortical supragranular neurons and exhibited the same temporal profile of migration and neurogenesis. However, they did not express molecules characteristic of CA1 neurons such as the limbic-associated membrane protein. Horseradish peroxidase injections in heterotopia demonstrated labeled fibers not only in the neocortex and white matter but also in the CA1 stratum radiatum and stratum lacunosum. To study the pathophysiological consequences of this connectivity, we compared the effects of neocortical and limbic seizures on the expression of Fos protein and on cell death in MAM animals. After metrazol-induced seizures, Fos-positive cells were present in CA1 heterotopias, the only hippocampal region to be activated with the neocortex. By contrast, kainic acid-induced seizures caused a prominent delayed cell death in limbic regions and in CA1 heterotopias. Together, these results suggest that neocortical heterotopias in the CA1 region are integrated in both the hippocampal and neocortical circuitry.