They looked closely at brain cell receptors to which cannabinoids bind, allowing their effects to be felt.
They also studied structures called microglia, which activate the brain's immune response.
Microglia collect near the plaque deposits associated with Alzheimer's disease and, when active, cause inflammation.
The researchers found that the presence of amyloid protein in the rats' brains activated immune cells.
However, rats that also received the cannabinoid showed no sign of microglia activation.
Using cell cultures, the researchers confirmed that cannabinoids counteracted the activation of microglia and thus reduced inflammation.