Studies on rats have made a connection between LTD in the hippocampus and memory. In one study, rats were exposed to a novel environment, and homosynaptic plasticity (LTD) in CA1 was observed. After the rats were brought back to their initial environment, LTD activity was lost. It was found that if the rats were exposed to novelty, the electrical stimulation required to depress synaptic transmission was of lower frequency than without novelty. When the rat was put in a novel environment, acetylcholine was released in the hippocampus from the medial septum fiber, resulting in LTD in CA1. Therefore, it has been concluded that acetylcholine facilitates LTD in CA1.