Research reportDifferential alteration of NMDA receptor subunits in the gerbil dentate gyrus and subiculum following seizure
Introduction
Epilepsy is a chronic condition characterized by the presence of spontaneous episodes of neuronal discharges. Many reports have suggested that this may, at least in part, result from abnormal glutamatergic synaptic transmission, which may involve modulation of the glutamate receptors properties [16], [24], [34], [35].
The N-methyl-d-aspartate (NMDA) receptors, one of the ionotropic glutamate receptors, modulate glutamate postsynaptic neurotransmission by generating long-lasting Ca2+ channel openings for longer depolarization. The NMDA receptors are composed of two classes of subunits: one subunit of NR1 that has at least eight splice variants and four subunits of NR2(A–D). It is known that the NR1 gene expresses a functional receptor with a weak response when activated by glutamate and glycine, while none of the NR2 subunits are functional when expressed alone [7], [26], [27]. However, physiologically functional NMDA receptors, which exist as heteromeric complexes containing NR1 combined with one or more NR2 subunits, produce NMDA-induced currents up to 100-fold larger than a homomeric NR1 channel [7], [26], [27].
It has been firmly established that NMDA receptors in particular play an essential role in the epileptic phenomena. This was as a result of the observation that NMDA receptor activation induces a pattern of burst firing in neurons that is reminiscent of a paroxysmal depolarization shift in conjunction with the demonstration that NMDA receptor-selective antagonists have powerful anticonvulsant activity in a variety of animal in vivo and in vitro models [2], [6], [31].
Although the contribution of NMDA receptor to epileptiform activity has been investigated in various models, there is a great deal of discrepancy. In the hippocampus, increased NMDA receptor activity after seizure has been reported in both the dentate gyrus and CA3 regions [16], [23], [24]. No lasting changes in the expression of NR1 mRNA have been demonstrated [4], [5], [13], [32]. Furthermore, acute effects, including increases [9], [10], [17], [21] and decreases [5] and no changes [13] have been reported for NR2A and NR2B in the hippocampus. Thus, it has not been clearly established whether NMDA receptor expression in the hippocampal complex altered in the experimental epilepsy model.
Mongolian gerbils provide an excellent model for the studying genetic epilepsy because this animal offers genetic control, breeding proclivity and ease of behavioral testing [1], [30]. The gerbils exhibit spontaneous motor seizures in response to a variety of stimuli. The seizures exhibited by the gerbils are consistent over time. Therefore, it is possible to correlate the seizure intensity records with the morphological observations [11], [12], [30]. Thus, to clarify the question described above, a chronological and comparative analysis of NMDA receptor expression was made in both seizure resistant (SR) and seizure sensitive (SS) gerbils.
Section snippets
Experimental animals
This study utilized the progeny of Mongolian gerbils (Meriones unguiculatus) obtained from Experimental Animal Center (Hallym University, Chunchon, South Korea). The animals were provided with a commercial diet and water ad libitum under controlled temperature, humidity and lighting conditions (22±2°C, 55±5% and a 12:12 light/dark cycle with lights). Procedures involving animals and their care were conducted in conformity with the institutional guidelines that are in compliance with
Results
In the present study, the NR immunoreactivities remained unaffected in either the hippocampal proper or the entorhinal cortex following the onset of seizure (data not shown). The NR immunoreactivity changes were restricted to the dentate gyrus and the subiculum. Thus alterations in the NR immunoreactivities in these regions are described.
Discussion
Rapid excitatory signal transduction in the central nervous system is largely mediated through glutamate receptors [25], [28]. Glutamatergic impulses from the entorhinal cortex constitute the major excitatory input to the hippocampus and a shift in glutamate-mediated excitability may be involved in the pathogenesis of epileptic discharges. Moreover, the NMDA receptor may be responsible for the seizure-induced selective excitotoxic cell death of certain hippocampal neuronal populations since
Acknowledgements
The authors would like to thank Mr. Suek Han and Kyung Jin Lee for their technical help during this study. This work was supported by Korea Research Foundation Grant (KRF-2000-042-G00030).
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2005, NeuropharmacologyCitation Excerpt :On the other hand, Ribak and Khan (1987) reported the termination of seizure activity in seizure prone gerbils with bilateral lesions of the perforant path. These observations led us to speculate upon the possible existence of enhanced feedback/feedforward glutamatergic excitation to the dentate gyrus of gerbils, although no differences in the N-methyl-d-aspartate receptor (NMDA) system were found between seizure prone and normal gerbils (Suh et al., 2001). Glutamate has to be loaded into synaptic vesicles by proton-dependent transporters before its exocytotic release (Ozkan and Ueda, 1998; Reimer et al., 1998; Erickson and Varoqui, 2000; Gasnier, 2000).
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2005, Brain ResearchCitation Excerpt :Bonferroni's test was used for post-hoc comparisons. P values below either 0.01 or 0.05 were considered statistically significant [15–19,41]. In the present study, there was no difference of morphological and electrophysiological data between sham-operated animals and only hypoxia induced animals.
Elevated voltage-gated Ca <sup>2+</sup> channel immunoreactivities in the hippocampus of seizure-prone gerbil
2004, Brain ResearchCitation Excerpt :Moreover, N-methyl-d-aspartate (NMDA) receptor may be responsible for the seizure-induced selective excitotoxic cell death of certain hippocampal neuronal populations, since NMDA receptor antagonists provide protection against such damage [4,5]. However, in our previous study [35], no differences were observed in the NMDA receptor systems of seizure-sensitive (SS) gerbils and seizure-resistant (SR) gerbils. For this reason, NMDA receptor expression may not initiate seizure activity in SS gerbils, although it could be that other glutamate receptor (e.g., kainate-, AMPA-, or metabotropic glutamate receptor) might be involved in seizure activity in gerbils.