The Effects of Alistipes-Produced GABA on the Murine Gut-Brain Serotonergic System and Major Depressive Disorder: A Research Protocol

Abstract

Introduction: Inspired by the link established between gut dysbiosis and Major Depressive Disorder (MDD), this research protocol aims to clarify the role of Alitsipes bacteria in the context of this disorder. Gut dysbiosis has been implicated in MDD, but the specific role of the Alistipes genus remains unclear. Alistipes, being an underrepresented constituent of gut microbiome-related literature, were chosen as a potential link to MDD via the hypothesized pathway of Alistipes-released GABA, subsequent production of serotonin from enterochromaffin (EC) cells, and eventual vagus nerve stimulation.

Methods: Four groups of 60 male adult gnotobiotic mice (strain C57BL/6) will be inoculated with different levels of Alistipes bacteria (none, low, baseline, high). Experiment 1 aims to understand the localization and quantity of Alistipes-derived GABA using PET/MRI. Subsequently, either bicuculline (a GABA_A receptor antagonist), CGP56433A (a GABA_B receptor antagonist), both in conjunction, or neither will be administered to understand how serotonin output varies based on GABAergic activity; PET/MRI will be used again for serotonin visualization. Lastly, an in-vivo whole-cell patch clamp experiment will help quantitatively assess vagus nerve stimulation resulting from serotonin secretion.

Results: We hypothesize that GABA in the GI tract will be localized at EC cells, and that inhibiting GABA_B receptors will produce the highest serotonin output and subsequently the greatest vagus nerve stimulation.

Discussion: Current literature postulates that higher GABA_B activity correlates with worse MDD symptoms, supporting the therapeutic potential of antagonizing this receptor’s activity. The serotonin output-related excitatory GABA_A receptor responses are well known, calling for the exploration of the optimal combination of Alistipes concentration to support GABA_A activity and proper dosage to block GABA_B receptors.

Conclusion: This study may have clinical implications for MDD therapies seeing that Selective Serotonin Reuptake Inhibitor treatments are ineffective for many patients; approaching this problem by drawing clearer conclusions about how GABA in the gut affects serotonergic activity in the brain and other nervous system components may help devise more novel treatments. If the proposed pathway holds true, a probiotic to help regulate the Alistipes level in the gut may serve as a potential therapeutic direction to take for such treatments.