Tumor necrosis factor-α signaling maintains the ability of cortical synapses to express synaptic scaling

Abstract

Glial tumor necrosis factor-α (TNFα) is essential for scaling up of synapses during prolonged activity blockade, but whether TNFα is an instructive or permissive signal is not known. Here we show in rat cortical neurons that the effects of TNFα and activity blockade are not additive; whereas TNFα increased AMPA quantal amplitude at control synapses, TNFα reduced quantal amplitude at prescaled synapses, demonstrating state-dependent effects of TNFα signaling on the scaling process. Whereas synaptic scaling during prolonged activity blockade [24 h tetrodotoxin (TTX)] was prevented by blocking TNFα signaling, early scaling (6 h TTX) was not, unless TNFα signaling was first blocked for 24 h. Moreover, when synapses were prescaled, prolonged (24 h) but not brief (6 h) blockade of TNFα signaling reversed scaling. Finally, prolonged block of TNFα signaling modified the synaptic localization of several scaffold proteins, suggesting that maintenance of postsynaptic density composition is TNFα dependent. Together, these data suggest that TNFα is not an instructive signal for scaling but rather is critical for maintaining synapses in a plastic state in which synaptic scaling can be expressed.

Figures

Figure 1.

The effects of TTX and TNFα on mEPSC amplitude are not additive. A, D, Schematic of experimental protocol for coapplication of TTX and TNFα. B, E, left, Examples of mEPSC recordings for the indicated conditions; right, average mEPSC waveforms for the same conditions. C, F, Average mEPSC amplitude ± SEM for indicated conditions. *p < 0.05, **p < 0.01. G, Analysis of change in distribution of mEPSC amplitude following TTX (left), TNFα (middle,), or TTX + TNFα (right) treatment. Black line is unity line (baseline plotted against itself), symbols are experimental condition (experimental plotted against baseline), and dashed line is best linear fit for the experimental condition.

Figure 2.

Prolonged but not brief blockade of TNFα signaling prevents synaptic scaling. A, B, Average mEPSC amplitude for the indicated conditions. C, Schematic of experimental conditions for data shown in D. D, Average mEPCS amplitude for the various conditions. *p < 0.05, **p < 0.01.

Figure 3.

Prolonged blockade of TNFα signaling has subtle effects on the composition of the postsynaptic density. A, Example images showing colocalization between SAP102 and GluR2 (left) or PSD95 and VGlut (right) in control and after 24 h of sTNFR treatment. B, Summary of changes in length density after 24 h of sTNFR treatment. C, Summary of changes in puncta fluorescence intensity after 24 h of sTNFR treatment. D, Summary of the changes in colocalization rates for SAP102 (left), TARP (middle), and PSD95 (right). *p < 0.05, **p < 0.01.