Residual granule cells can maintain susceptibility of CA3 pyramidal cells to kainate-induced epileptiform discharges

Cocaine- and amphetamine-regulated transcript peptide (CART) is a selective marker of rat granule cells and of human mossy cells in the hippocampal dentate gyrus

Cocaine- and amphetamine-regulated transcript (CART) peptide immunocytochemistry was used to reveal cellular localization in the dentate gyrus and in Ammon's horn of the rat and human hippocampal formations. In the rat dentate gyrus, only granule cells were labeled, whereas in humans, only mossy cells of the hilar region expressed CART peptide immunoreactivity. In the rat, CART-positive granule cells were located at the molecular layer border of the granule cell layer and had no features that would distinguish them from other granule cells. The mossy fiber bundle was labeled in the hilus as well as along the entire CA3 area of Ammon's horn. In the human, CART-immunoreactive mossy cells displayed the characteristic thorny excrescences both on their somata and their main dendrites. Axon collaterals of mossy cells could be seen in the hilus and the main axons formed a dense band in the inner molecular layer of the dentate gyrus, suggesting that mossy cells are the principal source of the associational pathway. Granule cells of the dentate gyrus and pyramidal neurons of the human hippocampal formation were devoid of CART peptide immunoreactivity. A few labeled non-pyramidal cells and a large group of strongly immunostained axons of unknown origin were present in all layers of CA1-3. Granule cells are the main excitatory cell population of the dentate gyrus while mossy cells are in a key position in controlling activity of granule cells. The specific location of CART peptide in the dentate granule cells of rodents and in the mossy cells of the human hippocampus may indicate involvement of neuronal circuitry of the dentate gyrus in the memory-related effects of cocaine and amphetamine. Independently of its functional role, CART peptide can be used as a specific marker of human mossy cells and of the dentate associational pathway. The sensitivity of CART peptide to postmortem autolysis may restrict the use of this marker in surgically removed hippocampi or in human brains removed and fixed shortly after death.

Analysis of Interleukin-1β-induced Cell Signaling Activation in Rat Hippocampus following Exposure to Gamma Irradiation

Among the many reported effects of irradiation in cells is activation of the stress-activated protein kinase, c-Jun N-terminal kinase (JNK), which has been shown to result in apoptotic cell death. The trigger that leads to JNK activation has not been identified, although, in rat hippocampus at least, irradiation-induced apoptosis has been coupled with increased accumulation of reactive oxygen species (ROS). Significantly, irradiation-induced changes in hippocampus are abrogated by treatment of rats with the polyunsaturated fatty acid, eicosapentaenoic acid (EPA). A close coupling between ROS accumulation and concentration of the pro-inflammatory cytokine, interleukin-1 beta (IL-1 beta) in hippocampus has been reported, and the evidence suggests that IL-1 beta may be responsible for the enhanced ROS production. Here we set out to assess the possibility that whole body gamma-irradiation increases IL-1 beta concentration in hippocampus and to investigate the consequences of such a change. We present evidence that reveals that the irradiation-induced increase in IL-1 beta concentration in hippocampus is accompanied by increased expression of IL-1 type I receptor and IL-1 accessory protein and increased activation of IL-1 receptor-activated kinase. These changes, which were coupled with increased activation of JNK and evidence of apoptotic cell death, were absent in hippocampus of rats that received EPA treatment. Significantly, EPA treatment enhanced hippocampal IL-10 concentration that was inversely correlated with IL-1 beta concentration. The data are consistent with the idea that EPA exerts anti-inflammatory and neuroprotective effects in the central nervous system.

Neuroprotective effect of eicosapentaenoic acid in hippocampus of rats exposed to gamma-irradiation

Exposure to irradiation leads to detrimental changes in several cell types. In this study we assessed the changes induced in hippocampus by exposure of rats to whole body irradiation; the findings reveal that irradiation leads to apoptotic cell death in hippocampus, and as a consequence, long term potentiation in perforant path-granule cell synapses is markedly impaired. The evidence is consistent with the view that irradiation induced an increase in reactive oxygen species and that this leads to stimulation of the stress-activated protein kinase, JNK, and activation of the transcription factor, c-Jun. Consequent upon activation of JNK, a cascade of cell signaling events was stimulated that ultimately resulted in apoptosis, as suggested by parallel increases in cytochrome c translocation, caspase-3 activation, poly(ADP-ribose) polymerase cleavage, and terminal dUTP nick-end labeling staining. Treatment of rats with eicosapentaenoic acid inhibited the irradiation-induced increase in reactive oxygen species production and the subsequent cellular signaling events, suggesting that oxidative stress triggered apoptotic cell death in the hippocampus of rats exposed to irradiation. Significantly, when the compromise in cell viability induced by irradiation was prevented by eicosapentaenoic acid, long term potentiation was sustained in a manner similar to that in the sham-treated control group.

Rapid activation of microglial cells by hypoxia, kainic acid, and potassium ions in slice preparations of the rat hippocampus

Microglial activation induced by hypoxia, kainic acid and elevated potassium concentration, all of which alter neuronal function, was studied in hippocampal slices. The activation of microglia was detected by immunostaining with a monoclonal antibody (OX-42) raised against a type 3 complement receptor (CD11b). During activation the phenotype of microglia changes and the intensity of staining of individual cells increases. Oxygen deprivation depressed the focal responses of CA1 neurons to stratum radiatum volleys. Microglial activation was time dependent. Ten minute hypoxia caused mild activation, and after 20 min, a strong microglial reaction could be observed. Although neuronal function returned during reoxygenation, the morphological signs of microglial activation remained. Epileptiform activity of hippocampal neurons, followed by depression, was induced by application of 0.5 mM kainic acid, in a time and dose dependent manner. Washing out kainic acid did not alter microglial reaction. Elevated concentrations of potassium ions induced microglial changes similar to those induced by hypoxia and kainic acid. It is therefore suggested that an elevated extracellular potassium ion concentration may be the common factor in microglial activation observed in these experiments since this is raised both in hypoxia and under the effect of excitotoxins.

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.