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Nitric oxide has been postulated as a retrograde intercellular messenger for long-term potentiation, a form of synaptic plasticity that is associated with learning and memory processes. [8] In the present study we investigated whether the loss or survival of nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase-containing neurons, which are known to synthesize nitric oxide, [19] would be an useful indicator for evaluating the structural and functional state of the rat hippocampus after status epilepticus that is induced by intraperitoneal injection of kainic acid. Besides NADPH diaphorase histochemistry, two other histological parameters were studied: the grade of cell damage evaluated from silver-impregnated sections, and the number of somatostatin-containing neurons in different hippocampal subfields. We found that the number of NADPH diaphorase-containing neurons in the hilus and granule cell layer correlated well with spatial learning and memory performance as assessed by the Morris water-maze test. The extent of cell damage in the CA1 subfield analysed in silver-impregnated sections and the number of hilar somatostatin-containing neurons also significantly correlated with latencies in the water-maze test. Furthermore, linear regression analysis revealed that the number of somatostatin-containing neurons in the hilus explains about 50% of the variation in water-maze learning. These findings emphasize that although general structural preservation is of crucial importance for the function of the hippocampus also interneurons, such as somatostatin- and NADPH diaphorase-containing neurons, may play an important role during the acquisition phase and processing of information in hippocampal circuitry. Therefore, in addition to evaluating general cell damage, analysis of the cell loss that occurs in the interneuron subpopulations will be beneficial in verifying structural and functional deficits of the hippocampus after status epilepticus.

Our preliminary results showed that mossy fibres do not undergo sprouting after global ischaemia in gerbils, although the pattern of hippocampal cell damage resembled that seen in ischaemic and epileptic rats, where mossy fibre sprouting is known to occur. In order to investigate whether the observed differences in the appearance of mossy fibre sprouting are related to the animal model or species used, this study was undertaken to compare the neuropathological changes induced in gerbils by systemic injection of kainate or by occlusion of carotid arteries with the changes induced in rats by injection of kainate. The pattern of pyramidal cell damage was very similar in each group. Mossy fibre sprouting was present in epileptic rats but not in ischaemic or epileptic gerbils. The number of somatostatin-immunoreactive neurons was decreased in the hilus of epileptic rats and ischaemic gerbils, but not in epileptic gerbils. The analysis of calretinin immunoreactivity in the dentate gyrus revealed differences between the rat and gerbil. The most striking difference between these species was that the mossy cells contained calretinin in gerbils but not in rats. Cell counting showed that the calretinin-containing mossy cells had survived both in epileptic and ischaemic gerbils. Therefore, since the mossy cells are known to be highly susceptible to excitotoxic insult in rats and degeneration of these cells is thought to be a key element in the induction of mossy fibre sprouting, we propose that the absence of mossy fibre sprouting in gerbils is related to the survival of the mossy cells.

Neurons containing nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase exhibit high resistance to several excitotoxins. In the neocortex and striatum, however, these neurons are sensitive to kainic acid. Here we report that, 2 weeks after i.p. injection of kainic acid, the number of NADPH diaphorase neurons in the hilus and CA1 subfield was decreased, whereas the cell counts in the other hippocampal areas were to a great extent similar to those for the controls. We propose that the loss of NADPH diaphorase neurons in the hippocampus after systemic injection of kainic acid is associated with the pathophysiological processes involved in the spreading of epileptic seizure activity rather than to the direct neurotoxic effect of the kainic acid per se.

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.

Kainic acid (KA)-induced convulsions are accompanied by histopathological changes that are most prominent in the temporal lobe structures. In the present study, we investigated whether a selective alpha2-adrenoceptor agonist, dexmedetomidine could attenuate KA-induced epileptic convulsions and subsequent neuronal damage in the rat hippocampus. Rats were pretreated 30 min before KA injection (9 mg/kg, i.p.) with dexmedetomidine (3 micrograms/kg, s.c.). The behavior of animals was observed for at least 3 h. Dexmedetomidine suppressed the development (p0.001), generalization (p0.05) and severity (p0.01) of convulsions. In addition, histological analysis revealed that dexmedetomidine-treated animals without convulsions or with only partial convulsions had no neuronal damage in the principal cell layers of the hippocampus. A selective alpha2-antagonist, atipamezole (1 mg/kg, s.c.) potentiated KA-induced convulsions and increased the mortality in status epilepticus. In conclusion, the present study demonstrated that dexmedetomidine, in addition to possessing anticonvulsant properties, has a neuroprotective effect in the KA model of status epilepticus.

We studied the neuroprotective effect of vigabatrin (gamma-vinyl GABA, VGB) in the rat hippocampus after status epilepticus (SE) induced by kainic acid (KA). Rats were treated with VGB (500 or 1000 mg/kg, i.p.) 24 h before KA injection (9 mg/kg, i.p.). The lower dose of VGB had no effect on the generation or severity of convulsions. However, VGB decreased neuronal damage in the CA3a (P < 0.05) and CA1 (P < 0.01) subfields of the hippocampus. The higher dose of VGB attenuated the severity of convulsions (P < 0.05) but had no effect on the development or generalization of convulsions. This finding may have clinical implications in the prevention of neuronal damage induced by drug refractory seizures or SE.

SummaryAn in vivo screen was performed in search of chemicals capable of enhancing neuron formation in the hippocampus of adult mice. Eight of 1000 small molecules tested enhanced neuron formation in the subgranular zone of the dentate gyrus. Among these was an aminopropyl carbazole, designated P7C3, endowed with favorable pharmacological properties. In vivo studies gave evidence that P7C3 exerts its proneurogenic activity by protecting newborn neurons from apoptosis. Mice missing the gene encoding neuronal PAS domain protein 3 (NPAS3) are devoid of hippocampal neurogenesis and display malformation and electrophysiological dysfunction of the dentate gyrus. Prolonged administration of P7C3 to npas3−/− mice corrected these deficits by normalizing levels of apoptosis of newborn hippocampal neurons. Prolonged administration of P7C3 to aged rats also enhanced neurogenesis in the dentate gyrus, impeded neuron death, and preserved cognitive capacity as a function of terminal aging.PaperClip