A role for mGluRs and Ca2+ in the inhibitory effect of tumour necrosis factor-α on long-term potentiation

Derval Cumiskey & John J. O’Connor. UCD School of Biomolecular and Biomedical Science, Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland.

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 laboratories that the inhibition of long-term potentiation (LTP) in the dentate gyrus region of the rat hippocampus, by tumour 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 (Butler et al., 2004). Many of the factors involved in the early modulation of LTP by TNF-α have yet to be elucidated. We have therefore investigated the effects of the mGluR antagonist MPEP on the effect of TNF-α on LTP in the rat dentate gyrus in vitro.

Recordings of field excitatory postsynaptic potentials (EPSPs) were made from the medial perforant path using standard methods. Results are expressed as mean±sem. Students t-test was used to compare data. When TNF-α (5.5ng/ml) was applied to the hippocampal slice 20 min pre-high frequency stimulus (HFS) LTP was significantly impaired (TNF-α /LTP 103 ± 9%, versus control LTP 167 ± 7%, n=4-9, 1 h post-tetanus, P< 0.001) as has previously been published (Butler et al., 2002). Perfusion of the mGluR5 specific antagonist MPEP (5μM) for 40 min prior to application of TNF-α reversed the inhibitory effect of TNF-α on LTP (141 ± 4% compared to TNF- a alone at 1 h, n=4). To investigate this further we isolated the NMDA mediated EPSP by using the AMPA antagonist NBQX (2μM). We found that TNF-α caused a significant reduction in the NMDA EPSP (50 ± 6% versus control 90 ± 2% at 2 h post drug application, P< 0.001, n=4). To investigate if TNF-α inhibition of LTP had a pre-synaptic component involving increased glutamate release, paired pulse stimuli were carried out. TNF-α had no effect on paired pulse depression, (control PPD at 50ms ISI was 87 ± 3%, n=4. At 2 h post TNF-α application, PPD was 85 ± 4%, n=4). To investigate a role for intra cellular ca2+ in the TNF- a effect, a low concentration of ryanodine (500nM-1μM), which had no significant effect on its own, was perfused for 20 min prior to TNF-α . Ryanodine reversed the inhibitory effect of TNF-α on LTP (174 ± 12% versus 103 ± 9% at 1 h post HFS, n=4, P<0.05).

This work shows for the first time a role for mGluRs, NMDARs and intracellular calcium in the TNF-α mediated inhibition of LTP. These studies will further help our understanding of the mechanisms of action of TNF-α on synaptic plasticity.

Butler, M.P., Moynagh, P.N., O’Connor, J.J. (2002) J Neurosci Meth, 119(2), 185-190.
Butler M.P., O’ Connor J.J., Moynagh P.N. (2004) Neurosci. 124(2), 319-26.

The Higher Education Authority of Ireland supported this work