Biphasic NF-κB activation in the excitotoxic hippocampus

Excitotoxic stimulation of NMDA receptors results in the activation of a variety of cellular responses. The inducible transcription factor NF-κB is known to be involved in excitotoxic responses by neurons. Here, we show that NF-κB activation occurs in a biphasic manner in hippocampal slices following a 20-min N-methyl-d-aspartate (NMDA) exposure. The biphasic activation profile consists of an early, rapid phase at 0.5–1 h post-insult, and a delayed phase evident 10–24 h post-insult. Endogenous inhibitors of NF-κB, IκBs, were examined for their involvement in the biphasic activation. IκBβ exhibited marked degradation in response to the excitotoxity, while changes in the levels of IκBα and p105 isoforms were not detected. The initial decline in IκBβ occurred in as little as 30 min post-NMDA exposure, coinciding with early NF-κB activity. A second, more gradual phase of IκBβ degradation was also evident, possibly giving rise to the delayed activation of the transcription factor. While both phases of NF-κB activation were disrupted by the NMDA receptor antagonist AP5, they were distinct with regard to the composition of activated complexes and their responsiveness to altered culture conditions. The two phases of NF-κB activity also were associated with distinct gene regulation events. Up-regulation of bcl-2 message occurred early after the excitotoxic insult and remained up-regulated for an extended period. In contrast, bax message initially remained unchanged after the insult, but then exhibited an increase 24 h later, corresponding with the second phase of the NF-κB response. These results indicate that distinct phases of NF-κB activation are generated in the excitotoxic hippocampus, and that the phases may be involved in opposing cellular responses.