Preferred Name

Amanda Powell

Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Date of Graduation


Document Type


Degree Name

Master of Arts (MA)


Department of Graduate Psychology


Melanie Shoup-Knox

Corey Cleland

S. Jeanne Horst


The gut-brain axis is a bidirectional pathway that acts as a connection between the gut and the brain. Bacterial changes in the gut alter this pathway, affecting organism’s health, cognition, and behavior. Commensal bacteria in the gut can reduce inflammation and increase longevity while pathogenic bacteria can have opposite effects. Reduced commensal gut bacteria can result in an increase in stress activation, depression, and anxiety in both human and animal models. Increases in commensal bacteria and decreases in pathobiontic bacteria can decrease hypothalamic pituitary adrenal (HPA) system activation, anxious behavior, and depressive behavior. Alternatively, increases in pathobionts can lead to decreases in neurogenesis, synaptogenesis, and synaptic plasticity, and can be observed behaviorally through deficits in hippocampally-dependent tasks. Therefore, changes to gut microflora diversity due to diet, age, antibiotic use, or probiotic use can alter the functioning of the individual. Antibiotics reduce both harmful and commensal gut microflora, which can lead to reduced hippocampal neurogenesis, impaired spatial memory performance, and increased depressive-like behavior. Administration of antibiotics can have long-term consequences following the cessation of antibiotic treatment. Probiotics may rescue these effects by reinstating commensal gut bacteria. As people age, a decrease in gastrointestinal tract functioning occurs, altering the gut microbiome and resulting in decreased diversity of commensal bacteria and an increased diversity of harmful bacteria. These effects are associated with reduced cognitive performance. Antibiotic use also contributes to the loss of gut bacteria diversity, and may augment cognitive-related deficits in older adults. The current study examined the effects of antibiotic and probiotic administration on spatial memory performance and depression across two age groups in Long-Evans rats. Spatial memory performance was assessed via object location task and depressive behavior was assessed via forced swim test. Baseline spatial memory performance and depressive behavior were compared to the same behaviors following antibiotic treatment and then again following probiotic treatment. Antibiotic consumption predicted forced swim test performance; however, no differences were observed between treatment groups and age. Probiotic consumption did not predict behavioral performance. Although not expected, results suggested that a possible dosage effect exists for the amount of treatment to yield an effect.



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