Acidosis enhances toxicity induced by kainate and zinc exposure in aged cultured astrocytes

Abstract

A key feature of cerebral ischemia, one of the leading causes of death associated with ageing, is excessive accumulation of glutamate in the synaptic cleft. In some forms of cerebral ischemia, like transient global ischemia, high levels or synaptic glutamate are complemented by a concomitant increase in extracellular Zn²⁺ as result of the release of the cation that is present in the pre-synaptic vesicles of glutamatergic neurons. Interestingly, while neurons are very sensitive to the toxicity triggered by exposure to either glutamate or Zn²⁺, astrocytes show less vulnerability to these toxins. We examined the vulnerability of cortical type 1 astrocytes to a combined exposure to the AMPA/kainate receptor agonist kainate and Zn²⁺. Astrocytes exposed to 1 mM kainate for 1 h did not exhibit any degeneration in the following 24 h, and addition of 50 μM Zn²⁺ to the kainate exposure failed to produce any further glial loss. Another hallmark of cerebral ischemia is parechymal acidosis and therefore, we tested the susceptibility of our cultured astrocytes to a kainate/Zn²⁺ exposure performed under acidotic conditions. We found that the combination of 1 h exposure to 1 mM kainate + 50 μM Zn²⁺ at pH 6.2 produced a strong increase in intracellular free Zn²⁺ ([Zn²⁺]_i), and extensive glial injury. Comparing [Zn²⁺]_i rises triggered by kainate/Zn²⁺ exposure at pH 7.4 or pH 6.2 we found that acidosis promotes increased toxic [Zn²⁺]_i levels as a result of a lethal combination of both enhanced Zn²⁺ influx through Zn²⁺ permeable AMPA/kainate channels and impaired intracellular buffering of the cation.