Transferring the blues: depression-associated gut microbiota induces neurobehavioural changes in the rat
J.R. Kelly1,2 °, Y. Borre1, S. El Aidy1,4, J. Deane3, E. Patterson3, P.J. Kennedy1, S. Beers1, K. Scott1,
G. Moloney1, L. Scott2, P. Ross3, C. Stanton3, G. Clarke1,2, J.F. Cryan1,5, T.G. Dinan1,2.
1APC Microbiome Institute, University College Cork, Cork Ireland;
2University College Cork, Department of Psychiatry and Neurobehavioural Science, Cork,
Ireland;
3Teagasc Food Research Centre, Moorepark, Fermoy, Ireland;
4Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen,
Groningen, The Netherlands;
5University College Cork, Department of Anatomy and Neuroscience, Cork, Ireland

Introduction: The gut microbiota is a complex metabolic ecosystem, which interacts with the host via neuroimmune, neuroendocrine and neural pathways. These pathways are integral components of the brain-gut-microbiota axis and pre-clinical evidence suggests that the microbiota can recruit this bidirectional communication system to modulate brain development, function and behaviour [1,2].
Although it is well acknowledged that the pathophysiology of depression involves neuroimmune-endocrine dysregulation [3,4], the extent to which changes in the gut microbiota composition and function mediate dysregulation of these pathways in depression is currently unknown.

Objectives: To determine the composition of the gut microbiota and its relationship to immune activity (plasma cytokines), hypothalamic-pituitary-adrenal axis (HPAaxis) function and tryptophan metabolism in depressed patients. Furthermore, to determine the behavioural and physiological effects of a fecal microbiota transplantation from depressed patients and health controls to a
microbiota-deficient antibiotic rat model.

Methods: Thirty-four patients with DSM IV major depression and 33 healthy subjects matched for gender, age and ethnicity were recruited. Plasma C-reactive protein (CRP) and a panel of cytokines were measured ELISA.
Salivary cortisol levels were determined by ELISA. Plasma tryptophan and kynurenine were determined HPLC.
Plasma Lipopolysaccharide binding protein (LBP) was determined by ELISA. Fecal samples were collected for 16sRNA metagenomic sequencing. A fecal microbiota transplant was prepared from a sub group of depressed patients and controls and transferred by oral gavage to a microbiota-deficient antibiotic rat model.

Results: We demonstrate that depression is associated with altered gut microbiota composition, richness and phylogenetic diversity. Moreover, we show that fecalmicrobiota transplantation from depressed patients to microbiota-deficient rats can induce the development of
behavioural and physiological features of depression in the recipient animals. This includes anhedonia, indicated by reduced sucrose preference (p = 0.022) and anxiety like
behaviours, indicated by a decrease in visits to the open arms in the elevated plus maze (p = 0.029) and a decrease in time spent in the open field (p = 0.013). Physiologically, rats that received the depressed fecal microbiota had increased plasma kynurenine levels (p = 0.029) and an
increased kynurenine/tryptophan ratio (p = 0.008).

Conclusion: Our results confirm that depression is associated with a distinct microbial signature which is capable of inducing alterations in behaviour and physiology when transferred to microbiota-deficient animals. This suggests that the gut microbiota may play a causal role in the development of core behavioural and neurobiological features of depression and may provide a tractable target in the treatment and prevention of depression.

Reference(s)
[1] Cryan, J.F. & Dinan, T.G., Mind-altering microorganisms: the impact of the gut microbiota on
brain and behaviour, 2012. Nat Rev Neuroscience., 13(22968153): p. 701–712.
[2] Dinan, T.G. & Cryan, J.F., Melancholic microbes: a link between gut microbiota and depression?. 2013. Neurogastroenterology and Motility, 2013. 25(9): p.713−9.
[3] Dowlati, Y., Herrmann, N., Swardfager, W., Liu, H., Sham, L., Reim, E.K., & Lanctot, K. L., 2010. A meta-analysis of cytokines in major depression. Biological Psychiatry. 67(5): p. 446−57.
[4] Lupien, S.J., McEwen, B.S., Gunnar, M.R., & Heim, C., 2009. Effects of stress throughout the
lifespan on the brain, behaviour and cognition. Nat Rev Neuroscience. 10(6): p. 434−45.
Disclosure statement: The authors are funded by Science Foundation Ireland (SFI), through the Irish Governments National Development Plan in the form of a centre grant (Alimentary Pharmabiotic Centre Grant Number SFI/12/RC/2273). The Alimentary Pharmabiotic Centre has conducted studies in collaboration with several companies including GSK, Pfizer, Cremo, Suntory, Wyeth and Mead Johnson.
TGD has until recently been on the Board of Alimentary Health.
Funding is also provided by the Health Research Board (HRB) through Health
Research Awards (HRA_POR/2012/32 JFC, TGD and HRA-POR-2-14-647: GC, TGD).
GC is supported by a NARSAD Young Investigator Grant from the Brain and Behaviour Research Foundation (Grant Number 20771).