Brainstem functional connectivity integrates microbiome-gut-brain signals in healthy older adults: a cross-sectional evaluation and a randomised controlled probiotic trial

Background: Despite increasing interest in the gut-brain axis, the role of the autonomic nervous system and the brainstem in human gut-brain signalling remain largely unknown. Objective: To characterise resting state functional connectivity (FC) of the brainstem in healthy older adults and to examine its association with a variety of factors relevant to ageing and the gut-brain axis, and to assess if brainstem FC is altered by probiotic intervention. Design: Randomised, double-blinded, placebo-controlled trial in 60-80-year-olds (55 women/28 men) with six-weeks Lacticaseibacillus rhamnosus HN001 intervention in a non-encapsulated and microencapsulated formulation. Brainstem-optimised functional magnetic resonance imaging was performed before and after intervention. Results: At baseline, age correlated positively with FC of locus coeruleus (LC) and dorsal raphe nucleus (DRN) with pre-/postcentral gyrus, as well as of periaqueductal gray (PAG) with temporal/supramarginal gyrus and planum temporale. FC of PAG with precuneus cortex and cingulate gyrus correlated with cognitive status and mood. Perceived stress correlated with FC between nucleus tractus solitarius and occipital cortex, angular/temporal gyrus. FC of DRN differed between men and women. All p<0.0063. Following probiotic intervention, a significant time*group interaction and main time effect were observed for FC of LC and ventral tegmental area. FC of both with occipital pole/cortex and cerebellum, and of LC with pre-/postcentral gyrus, superior parietal lobule, insular/opercular cortex, putamen, and planum polare showed within-group changes (non-encapsulated), no between-group differences were observed (all p<0.0056). Conclusion: Brainstem-to-cortex FC at rest correlates with several phenotypic characteristics implicated in healthy ageing and may be modifiable by probiotic supplementation.