INTERLEUKIN-3 PREVENTS DELAYED NEURONAL DEATH IN THE HIPPOCAMPAL CA1 FIELD

In the central nervous system, interleukin (IL)-3 has been shown to ex ert a trophic action only on septal cholinergic neurons in vitro and i ll vivo, but a widespread distribution of IL-3 receptor (IL-3R) ill th e brain does not conform to such a selective central action of the lig and. Moreover, the mechanism(s) underlying the neurotrophic action of IL-3 has not been elucidated, although an erythroleukemic cell, line i s known to enter apoptosis after IL-3 starvation possibly due to a rap id decrease in Bcl-2 expression. This in vivo study focused on whether IL-3 rescued noncholinergic hippocampal neurons from lethal ischemic damage by modulating the expression of Bcl-x(L), a Bcl-2 family protei n produced in the mature brain. 7-d IL-3 infusion into the lateral ven tricle of gerbils with transient forebrain ischemia prevented signific antly hippocampal CA1 neuron death and ischemia-induced learning disab ility. TUNEL (terminal deoxynucleotidyltransferase-mediated 2'-deoxyur idine 5'-triphosphate-biotin nick end labeling) staining revealed that IL-3 infusion caused a significant reduction in the number of CA1 neu rons exhibiting DNA fragmentation 7 d after ischemia. The neuroprotect ive action of IL-3 appeal-ed to be mediated by a postischemic transien t upregulation of the IL-3R alpha subunit in the hippocampal CA1 field where IL-3R alpha was barely detectable under normal conditions. In s itu hybridization histochemistry and immunoblot analysis demonstrated that Bcl-x(L) mRNA expression, even though upregulated transiently in CA1 pyramidal neurons after ischemia, did not lead to the production o f Bcl-x(L) protein in ischemic gerbils infused with vehicle. However, IL-3 infusion prevented the decrease in Bcl-x(L) protein expression in the CA1 field of ischemic gerbils. Subsequent in vitro experiments sh owed that IL-3 induced the expression of Bcl-x(L), mRNA and protein in cultured neurons with IL-3R alpha: and attenuated neuronal damage cau sed by a free radical-producing agent FeSO4. These findings suggest th at IL-3 prevents delayed neuronal death in the hippocampal CA1 field t hrough a receptor-mediated expression of Bcl-x(L) protein, which is kn own to facilitate neuron survival. Since IL-3R alpha in the hippocampa l CA1 region, even though upregulated ill response to ischemic insult, is much less intensely expressed than that in the CA3 region tolerant to ischemia, the paucity of IL-3R interacting with the ligand may acc ount for the vulnerability of CA1 neurons to ischemia.