Precise changes in the connectivity between neurons caused by an imbalance of gut bacteria may help explain depressive symptoms in bipolar disorder, a new animal study suggests.

Researchers from Zhejiang University in China used fecal transplants to transfer gut bacteria from people with bipolar disorder into mice. The volunteers had all been diagnosed as being in a depressive phase of bipolar within the last 24 hours.

Through a combination of brain imaging, genetic sequencing, and behavioral tests, the scientists determined that the mice began to exhibit signs of depression – moving less and showing less interest in treats, for example.

What’s more, crucial measures of brain connectivity weakened. In the medial prefrontal cortex (mPFC) part of the brain, where a lot of decision making and emotional regulation happens, there were fewer connections (synapses) between cells, and the brain’s reward center was effectively cut off.