Probiotic Lactobacillus Plantarum 299v decreases kynurenine concentration and improves cognitive functions in patients with major depression: A double-blind, randomized, placebo controlled study

•There was an improvement in cognitive functions in group of depressed patients receiving L. Plantarum 299v (LP299v) compared to the placebo group.•There was a significant decrease in kynurenine concentration in the LP299v group compared to the placebo group.•There was a significant increase in 3-hydroxykynurenine : kynurenine ratio in the LP299v group compared with the placebo group.•Decreased kynurenine concentration could contribute to the improvement of cognitive functions in the LP299v group compared to the placebo group. Interactions between the digestive system and the brain functions have become in recent years an important field of psychiatric research. These multidirectional interactions take place in the so called microbiota-gut-brain axis and emerging scientific data indicate to the significant role of microbiota in the modulation of the central nervous system (CNS) including affective and cognitive functions. An assessment of psychobiotic and immunomodulatory effects of probiotic bacteria Lactobacillus Plantarum 299v (LP299v) by measuring affective, cognitive functions and biochemical parameters in patients with MDD undergoing treatment with selective serotonin reuptake inhibitors (SSRI). Seventy nine patients with MDD were randomized and allocated to a double-blind, placebo-controlled trial. Participants received either a SSRI with the probiotic LP299v (n = 40) for a period of 8 weeks or a SSRI with the placebo of the probiotic (n = 39) for the same period. The severity of psychiatric symptoms was assessed using Hamilton Depression Rating Scale (HAM-D 17), Symptom Checklist (SCL-90) and Perceived Stress Scale (PSS-10). Cognitive functions were assessed using the Attention and Perceptivity Test (APT), Stroop Test parts A and B, Ruff Figural Fluency Test (RFFT), Trail Making Test (TMT) Parts A and B and the California Verbal Learning Test (CVLT). Biochemical parameters such as tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA), 3-hydroxykynurenine (3HKYN), anthranilic acid (AA), 3-hydroxy anthranilic acid (3HAA), tumor necrosis factor-alpha (TNF-α), interleukin 6 (IL-6), interleukin 1-beta (IL-1b) and cortisol plasma concentrations were measured. Sixty participants finished the study and were analyzed: 30 participants in the LP299v group and 30 participants in the placebo group. There was an improvement in APT and in CVLT total recall of trials 1–5 in the LP299v group compared with the placebo between baseline and after 8 weeks of interv