Interleukin-1β Immunoreactivity and Microglia Are Enhanced in the Rat Hippocampus by Focal Kainate Application: Functional Evidence for Enhancement of Electrographic Seizures

Using immunocytochemistry and ELISA, we investigated the production of interleukin (IL)-1β in the rat hippocampus after focal application of kainic acid inducing electroencephalographic (EEG) seizures and CA3 neuronal cell loss. Next, we studied whether EEG seizures per se induced IL-1β and microglia changes in the hippocampus using bicuculline as a nonexcitotoxic convulsant agent. Finally, to address the functional role of this cytokine, we measured the effect of human recombinant (hr)IL-1β on seizure activity as one marker of the response to kainate. Three and 24 hr after unilateral intrahippocampal application of 0.19 nmol of kainate, IL-1β immunoreactivity was enhanced in glia in the injected and the contralateral hippocampi. At 24 hr, IL-1β concentration increased by 16-fold ( p < 0.01) in the injected hippocampus. Reactive microglia was enhanced with a pattern similar to IL-1β immunoreactivity. Intrahippocampal application of 0.77 nmol of bicuculline methiodide, which induces EEG seizures but not cell loss, enhanced IL-1β immunoreactivity and microglia, although to a less extent and for a shorter time compared with kainate. One nanogram of (hr)IL-1β intrahippocampally injected 10 min before kainate enhanced by 226% the time spent in seizures ( p < 0.01). This effect was blocked by coinjection of 1 μg (hr)IL-1β receptor antagonist or 0.1 ng of 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate, selective antagonists of IL-1β and NMDA receptors, respectively. Thus, convulsant and/or excitotoxic stimuli increase the production of IL-1β in microglia-like cells in the hippocampus. In addition, exogenous application of IL-1β prolongs kainate-induced hippocampal EEG seizures by enhancing glutamatergic neurotransmission.