Evidence for a role for the group I metabotropic glutamate receptor in the inhibitory effect of tumor necrosis factor-α on long-term potentiation

Pro-inflammatory cytokines are known to be elevated in several neuropathological states that are associated with learning and memory. We have previously demonstrated in our laboratory that the inhibition of long-term potentiation (LTP) in the dentate gyrus region of the rat hippocampus, by tumor necrosis factor (TNF)-α, represents a biphasic response, an early phase dependent on p38 mitogen activated protein kinase (MAPK) activation and a later phase, possible dependent on protein synthesis. Many of the factors involved in the early modulation of LTP by TNF-α have yet to be elucidated. This study investigated if metabotropic glutamate receptors (mGluRs) are functionally linked to the inhibitory effect of TNF-α on LTP in the rat dentate gyrus in vitro. We report that the impairment of early-LTP by TNF-α is significantly attenuated by prior application of the group I/II mGluR antagonist MCPG and more specifically the mGluR5 antagonist MPEP. Since TNF-α is now known to cause transient increases in intracellular Ca2+ levels from ryanodine-sensitive stores, we explored the possibility that disruption of intracellular Ca2+ homeostasis could be involved. Ryanodine was found to significantly reverse the inhibition of LTP by TNF-α. From these studies we propose that the TNF-α inhibition of LTP is dependent upon the activation of TNFR1 and mGlu5-receptors. Importantly this study provides the first proof of the involvement of ryanodine-sensitive intracellular Ca2+ stores in TNF-α mediated inhibition of LTP.