Interleukin-18 mediated inhibition of LTP in the rat dentate gyrus is attenuated in the presence of mGluR antagonists

Pro-inflammatory cytokines are known to be elevated in several neuropathological states that are associated with learning and memory impairments. We have previously demonstrated the inhibition of long-term potentiation (LTP), a recognised model for memory, in the dentate gyrus region of the rat hippocampus, by interleukin-18. We have also previously shown that the inhibitory effect of TNF-α on LTP can be attenuated by inhibitors of metabotropic glutamate receptors (mGluRs). We therefore went on to investigate the effects of the mGluR antagonists MPEP and MTPG on the effect of IL-18 on LTP in the rat dentate gyrus in vitro. Recordings of field excitatory post-synaptic potentials (EPSPs) were made from the medial perforant path of rat hippocampal slices. IL-18 (100 ng/ml) applied for 20 min before-HFS had no significant effect on baseline EPSPs but significantly impaired LTP (IL-18 LTP 116 ± 9%, versus control LTP 163 ± 6% 1 h post-tetanus, P < 0.001, n = 5). Perfusion of the mGluR5 specific antagonist MPEP (5 μM) for 40 min prior to application of IL-18 had no significant effect on baseline EPSPs but significantly attenuated the inhibitory effect of IL-18 on LTP at 30 min but not 1 h (177 ± 2% and 138 ± 8%, respectively, compared to controls; n = 5). Perfusion of the group II mGluR antagonist MTPG (50 μM) for 40 min prior to application of IL-18 had no significant effect on baseline EPSPs but was found to significantly reverse the inhibitory effect of IL-18 on LTP at 1 h (164 ± 6% compared to IL-18 alone, n = 5). This study provides novel evidence of the involvement of mGluRs in the IL-18 mediated inhibition of LTP.