Abstract
Long-term potentiation (LTP) is regulated in part by metaplasticity, the activity-dependent alterations in neural state that coordinate the direction, amplitude, and persistence of future synaptic plasticity. Previously, we documented a heterodendritic metaplasticity effect whereby high-frequency priming stimulation in stratum oriens (SO) of hippocampal CA1 suppressed subsequent LTP in the stratum radiatum (SR). The cytokine tumor necrosis factor (TNF) mediated this heterodendritic metaplasticity in wild-type rodents and in a mouse model of Alzheimer’s disease. Here, we investigated whether LTP at other afferent synapses to CA1 pyramidal cells were similarly affected by priming stimulation. We found that priming stimulation in SO inhibited LTP only in SR and not in a second independent pathway in SO, nor in stratum lacunosum moleculare (SLM). Synapses in SR were also more sensitive than SO or SLM to the LTP-inhibiting effects of pharmacological TNF priming. Neither form of priming was sex-specific, while the metaplasticity effects were absent in TNFR1 knock-out mice. Our findings demonstrate an unexpected pathway specificity for the heterodendritic metaplasticity in CA1. That Schaffer collateral/commissural synapses in SR are particularly susceptible to such metaplasticity may reflect an important control of information processing in this pathway in addition to its sensitivity to neuroinflammation under disease conditions.
Highlights
Long-term potentiation (LTP) and long-term depression (LTD) are widely accepted candidates as neural mechanisms underlying memory formation
We reported that tumour necrosis factor (TNF) but not IL-1β mediates the long-range metaplastic inhibition of LTP in CA1 of rat hippocampus, while pharmacological priming by TNF but not IL-1β protein causes a similar inhibition of L TP8
Priming in stratum oriens (SO) (Figs. 1c, 2e,g) had no effect on LTP at synapses in stratum lacunosum moleculare (SLM) (Student’s t-test; p = 0.39; Fig. 2f,h). These pathway differences occurred in a different pattern to what was observed for LTP in the SO priming pathway across the three experiments (one-way ANOVA F(2, 15) = 4.750, p = 0.025)
Summary
Long-term potentiation (LTP) and long-term depression (LTD) are widely accepted candidates as neural mechanisms underlying memory formation. In a unique form of metaplasticity in area CA1 of the rodent hippocampus, prior electrical activity (priming activity) in either stratum oriens (SO) or stratum radiatum (SR) inhibits subsequent LTP and promotes LTD at a separate set of synapses in SR. Aβ causes inhibition of LTP in SR but not in SO17,18 This raises the question whether SR synapses are more sensitive to TNF-mediated metaplasticity effects than SO synapses. Contains the pyramidal cell apical tuft dendrites, is more like SO or SR in its sensitivity to TNF-mediated inhibition of LTP To address these questions, we compared the relative sensitivity of SO, SR and SLM synapses to electrical and TNF priming. Electrical and TNF priming effects in SR occurred in both females and males and required TNFR1 activation
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