Calcium-Binding Proteins Protect GABAergic Neurons of the Hippocampus from Hypoxia and Ischemia in vitro

Disturbances in cerebral blood flow cause hypoxic and ischemic processes that lead to damaging and death of neurons. Some populations of GABAergic neurons are characterized by greater sensitivity to oxygen-glucose deprivation. Massive damage and death of the cells (more than 80%) take place in hippocampal cultures during long oxygen-glucose deprivation (40 min). Astrocytes and GABAergic neurons are destroyed first, which in turn leads to the neuroglial network disturbances accompanied by massive death of glutamatergic neurons. In the present work we investigated a protective role of calcium-binding proteins (CaBPs) in the population of GABAergic neurons under hypoxic-like and ischemic-like conditions. The preconditioning was evaluated by suppression of the NMDAR activity after short-term episodes of hypoxia. The posthypoxic hyperexcitability was estimated by the appearance of synchronous spontaneous calcium impulses (s[Ca²⁺]_i) at the reoxigenation stage. The cells damaged during hypoxia and ischemia were detected by the presence of the irreversible increase of [Ca²⁺]_i. The type of neurons and presence of CaBPs (parvalbumin (PV), calbindin (CB), calretinin (CR)) were determined by immunohistochemistry after registration of the [Ca²⁺]_i dynamics. We have shown that any calcium-binding protein in GABAergic neurons can play the role of an endogenous neuroprotector, which prevents calcium overload and subsequent death even without preconditioning. GABAergic neurons containing any CaBP are characterized by lower magnitudes of the calcium responses to the NMDA application. These neurons are not preconditioned by repeated short-term episodes of hypoxia. It was shown that GABAergic neurons containing CR are characterized by the absence of irreversible calcium increases and survive during oxygen-glucose deprivation. However, the presence of PV or CB can lead to the appearance of lag phases with different durations. These two CaBPs reduce the rate of calcium increase and possibly in that way prevent the death of GABAergic neurons under the ischemia-like conditions.