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Scope Probiotics exert immunomodulatory effects and may influence tryptophan metabolism in the host. Deficiency of nutrients related to C1 metabolism might stimulate inflammation by enhancing the kynurenine pathway. This study used Sprague Dawley rats to investigate whether a methyl‐deficient diet (MDD) may influence tryptophan/kynurenine pathways and cytokines and whether probiotics can mitigate these effects. Methods and Results Rats are fed a control or MDD diet. Animals on the MDD diet received vehicle, probiotics (L. helveticus R0052 and B. longum R0175), choline, or probiotics + choline for 10 weeks (n = 10 per group). Concentrations of plasma kynurenine metabolites and the methylation and inflammatory markers in plasma and liver are measured. Results MDD animals (vs controls) show upregulation of plasma kynurenine, kynurenic acid, xanthurenic acid, 3‐hydroxyxanthranilic acid, quinolinic acid, nicotinic acid, and nicotinamide (all p < 0.05). In the MDD rats, the probiotics (vs vehicle) cause lower anthranilic acid and a trend towards lower kynurenic acid and picolinic acid. Compared to probiotics alone, probiotics + choline is associated with a reduced enrichment of the bacterial strains in cecum. The interventions have no effect on inflammatory markers. Conclusions Probiotics counterbalance the effect of MDD diet and downregulate downstream metabolites of the kynurenine pathway.

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Glutamatergic signaling via N-methyl-D-aspartate receptors (NMDARs) is important for physiological functioning, but can also induce cell death via excitotoxic mechanisms in many neuropathological diseases, such as stroke. Altering the cellular response to excitotoxic insults by modulating the downstream effects of NMDAR activation represents a promising therapeutic approach. For example, α7 nicotinic acetylcholine receptors (α7 nAChRs) signaling has been shown to be able to change NMDA-induced neurotoxicity in some models. However, both neuroprotective and neurotoxic effects have been reported. In this study, we examined the effect of co-activation of α7 nAChRs on NMDA-mediated cell death in rat organotypic hippocampal slice cultures (OHSCs). Our results show that α7 nAChR stimulation did not significantly influence NMDA-induced excitotoxic cell damage as measured by propidium iodide uptake. However, treatment of OHSCs with the α7 nAChR agonist choline alone induced an increase in the propidium iodide signal. Both the α7 nAChR antagonist methyllycaconitine (MLA) and the NMDAR antagonist (RS)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) were able to block this effect in the dentate gyrus and hippocampal subfield CA3.