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The regulation of intracellular calcium in cultured hippocampal neurons and the influence of calcium buffers Abdel-Hamid, Khaled Mohammed


Excessive activation of the major neurotransmitter glutamate receptors has been blamed as a causative mechanism in a large number of neuro-pathological situations via a process known as excitotoxicity. Excessive influx of calcium (Ca²+, and the consequent loss of Ca²+ homeostasis in neurons subject to this form of excessive activation have been suggested to play a central role in the triggering of a series of events ending in neuronal death. Neuronal populations differ in their vulnerability to excitotoxicity and this difference has, controversially, been attributed to the variability in the calcium-load handling capacity of neurons. The latter may possibly reflect the level of expression of the high-capacity calcium-binding proteins such as calbindin-D28K (CaBP). This work aims at studying the potential influence of CaBP expression in cultured hippocampal neurons on agonist- and depolarization-induced Ca²+ responses, and on the vulnerability of these neurons to excitotoxic stimuli. Manipulation of neuronal Ca²+ buffering capacity could also be achieved by loading neurons with BAPTA-like Ca²+ buffers. We tested the hypothesis that enhancing neuronal Ca²+ buffering capacity, either by the expression of CaBP or by loading neurons with BAPTA-like buffers, will render them more resistant to glutamate-induced neuronal death in vitro. We found that neurons expressing CaBP or loaded with an artificial Ca²+ buffer, contrary to our hypothesis, were more vulnerable to excitotoxicity. The mechanism(s) involved in the enhancement of neuronal loss under conditions of increases Ca²+ buffering capacity are likely to include an increase in the net influx of Ca²+, secondary to the impairment ofCa²+-mediated inhibition of Ca²+ influx, and the prolongation of the recovery phase at the end of the excitotoxic stimulus. Such a prolongation may be a result of the increased Ca²+ influx and competition between the recovery mechanisms on one side, and the large number of high affinity Ca²+ binding sites of the buffer on the other. In neurons that express CaBP, an added vulnerability factor may be the consistently higher peak Ca²+ responses to glutamate receptor agonists and to depolarization observed in these neurons when compared to responses in neurons lacking CaBP. The biological effects of enhanced Ca²+ buffering capacity, particularly using fast and mobile buffers, would be subject to the cell-specific nature of the spatial and temporal pattern of the Ca²+ signal. Thus, the influence of Ca²+ buffering on neuronal vulnerability may be difficult to predict on a theoretical basis particularly in view of our results indicating the large impact of many factors in the culture environment on the vital properties of neurons in vitro. [Scientific formulae used in this abstract could not be reproduced.]

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