Repeated ()-ketamine administration ameliorates the spatial working memory impairment in mice with chronic pain: role of the gut microbiota-brain axis.

Chronic pain is commonly linked with diminished working memory. This study explores the impact of the anesthetic ()-ketamine on spatial working memory in a chronic constriction injury (CCI) mouse model, focusing on gut microbiome. We found that multiple doses of ()-ketamine, unlike a single dose, counteracted the reduced spontaneous alteration percentage (%SA) in the Y-maze spatial working memory test, without affecting mechanical or thermal pain sensitivity. Additionally, repeated ()-ketamine treatments improved the abnormal composition of the gut microbiome (β-diversity), as indicated by fecal 16S rRNA analysis, and increased levels of butyrate, a key gut - brain axis mediator. Protein analysis showed that these treatments also corrected the upregulated histone deacetylase 2 (HDAC2) and downregulated brain-derived neurotrophic factor (BDNF) in the hippocampi of CCI mice. Remarkably, fecal microbiota transplantation from mice treated repeatedly with ()-ketamine to CCI mice restored %SA and hippocampal BDNF levels in CCI mice. Butyrate supplementation alone also improved %SA, BDNF, and HDAC2 levels in CCI mice. Furthermore, the TrkB receptor antagonist ANA-12 negated the beneficial effects of repeated ()-ketamine on spatial working memory impairment in CCI mice. These results indicate that repeated ()-ketamine administration ameliorates spatial working memory impairment in CCI mice, mediated by a gut microbiota - brain axis, primarily through the enhancement of hippocampal BDNF - TrkB signaling by butyrate.