Modulation of calcium current, intracellular calcium levels and cell survival by glucose deprivation and growth factors in hippocampal neurons

Basic fibroblast growth factor (bFGF) and nerve growth factor (NGF) can protect CNS neurons against ischemic/excitotoxic insults, but the mechanism of action is unknown. Imaging of the calcium indicator dye fura-3 and whole-cell patch clamp recordings of calcium currents were used to examine the mechanisms whereby hypoglycemia damages and growth factors protect cultured rat hippocampal neurons. When cultures were deprived of glucose, massive neuronal death occured 16-24 h following the onset of hypoglycemia. Early hypoglycemia-induced changes included calcium current inhibition and a reduction in intracellular free calcium levels ([Ca²⁺]_i) without morphological signs of neuronal damage. Later changes included a large elevation of [Ca²⁺]_i which was causally involved in neuronal damage. NGF and bFGF prevented or reduced both early and later responses to hypoglycemia. The growth factors increased calcium (barium) current and [Ca²⁺]_i to normal limits during the early stages of hypoglycemia and prevented the later elevation in [Ca²⁺]_i and neuronal damage. Nifedipine, but not omega-conotoxin, blocked calcium currents. The increased calcium current caused by the growth factors was apparently not sufficient to protect neurons against hypoglycemic damage since K⁺ depolarization during the early stages of hypoglycemia did not prevent and, in fact exacerbated, the subsequent neuronal damage. In addition, exposure of neurons to K⁺, NGF or bFGF only during the first 1 h of hypoglycemia did not protect against hypoglycemic damage. Taken together, the data suggest that neurons initially respond to hypoglycemia with a reduction in calcium currents which may provide a means to maintain [Ca²⁺]_i within a concentration range conducive to cell survival. Prolonged energy deprivation eventually results in a failure of calcium extrusion systems, glutamate receptor activation and a loss of neuronal calcium homeostasis. Taken together, the data indicate that the mechanism of growth factor protection against energydeprivation involves of the late prevention rise in [Ca²⁺]_i.