Interleukin 10 Restores Lipopolysaccharide-Induced Alterations in Synaptic Plasticity Probed by Repetitive Magnetic Stimulation

Abstract

Systemic inflammation is associated with alterations in complex brain functions such as learning and memory. However, diagnostic approaches to functionally assess and quantify inflammation-associated alterations in synaptic plasticity are not well-established. In previous work, we demonstrated that bacterial lipopolysaccharide (LPS)-induced systemic inflammation alters the ability of hippocampal neurons to express synaptic plasticity, i.e., the long-term potentiation (LTP) of excitatory neurotransmission. Here, we tested whether synaptic plasticity induced by repetitive magnetic stimulation (rMS), a non-invasive brain stimulation technique used in clinical practice, is affected by LPS-induced inflammation. Specifically, we explored brain tissue cultures to learn more about the direct effects of LPS on neural tissue, and we tested for the plasticity-restoring effects of the anti-inflammatory cytokine interleukin 10 (IL10). As shown previously, 10 Hz repetitive magnetic stimulation (rMS) of organotypic entorhino-hippocampal tissue cultures induced a robust increase in excitatory neurotransmission onto CA1 pyramidal neurons. Furthermore, LPS-treated tissue cultures did not express rMS-induced synaptic plasticity. Live-cell microscopy in tissue cultures prepared from a novel transgenic reporter mouse line [C57BL/6-Tg(TNFa-eGFP)] confirms that ex vivo LPS administration triggers microglial tumor necrosis factor alpha (TNFα) expression, which is ameliorated in the presence of IL10. Consistent with this observation, IL10 hampers the LPS-induced increase in TNFα, IL6, IL1β, and IFNγ and restores the ability of neurons to express rMS-induced synaptic plasticity in the presence of LPS. These findings establish organotypic tissue cultures as a suitable model for studying inflammation-induced alterations in synaptic plasticity, thus providing a biological basis for the diagnostic use of transcranial magnetic stimulation in the context of brain inflammation.

Microglia mediate synaptic plasticity induced by 10 Hz repetitive transcranial magnetic stimulation

“…In our recent work we demonstrated concentration-dependent effects of TNFα on synaptic plasticity, where low concentrations of TNFα promoted plasticity induction, whereas high concentrations impeded the ability of neurons to express synaptic plasticity (Maggio and Vlachos, 2018). Consistent with these findings we were able to show that a pathological activation of microglia, as seen under conditions of bacterial infection and sepsis (Strehl et al, 2014), blocks the ability of neurons to express synaptic plasticity induced by 10 Hz repetitive magnetic stimulation (rMS; (Lenz et al, 2020)). We therefore theorized that the "physiological" (i.e., plasticity-inducing) effects of rMS depend on microglia.…”

“…Finally, we prepared tissue cultures from a transgenic TNFα reporter mouse line, which expresses the enhanced green fluorescent protein under the control of the TNF-promoter (C57BL/6-Tg(TNFa-eGFP)). Previously, we used this approach to demonstrate a pathological activation of microglia in the presence of bacterial lipopolysaccharides, which occludes the ability of neurons to express synaptic plasticity (Lenz et al, 2020). Live-cell microscopy revealed no obvious changes in eGFP fluorescence following 10 Hz rMS (Figure 9D), which is in line with a production and release of plasticity-promoting low concentrations of TNFα.…”

“…The plasticity modulating effects of secreted microglial factors are well-established (Cao et al, 2014;Chai et al, 2019;Habbas et al, 2015;Heir and Stellwagen, 2020;Lenz et al, 2020;Maggio and Vlachos, 2018;Santello et al, 2011;Stellwagen and Malenka, 2006;Tancredi et al, 2000). Specifically, microglial TNFα, Il6 and Il1b have been linked to the ability of neurons to express synaptic plasticity (Lenz et al, 2020;Rizzo et al, 2018;Stellwagen and Malenka, 2006;Tancredi et al, 2000). In this context, we were recently able to demonstrate that low concentrations of TNFα promote the ability of neurons to express synaptic plasticity (Maggio and Vlachos, 2018).…”

“…Preparation of tissue cultures. Entorhino-hippocampal tissue cultures were prepared at postnatal day 3-5 from C57BL/6J, HexB-tdTom (Masuda et al, 2020), and C57BL/6-Tg(TNFa-eGFP) (Lenz et al, 2020) mice of either sex as previously described (Lenz et al, 2015).…”

“…In these experiments propidium iodide (PI) stainings were used to assess viability and cell death in our preparations (Figure 3; c.f., (Lenz et al, 2020)). PI is a cell-membrane impermeant fluorescent molecule that binds to DNA.…”