This 2015 New York fruit fly study found:

“Forgetting is regulated by multiple neural pathways that impinge upon a memory center.

Forgetting over time and the acute forgetting of conflicting memory during reversal learning rely on separable neural circuits.

Inactivating these neurons inhibits memory decay without altering learning, whereas activating them promotes forgetting. These neurons [include] a cluster of dopaminergic neurons and a pair of glutamatergic neurons.

Although activity of these neurons is required for memory decay over time, they are not required for acute forgetting during reversal learning. Our results thus not only establish the presence of multiple neural pathways for forgetting in Drosophila but also suggest the existence of diverse circuit mechanisms of forgetting in different contexts.”

Here’s a 3D view of the glutamatergic neurons:

http://movie-usa.glencoesoftware.com/video/10.1073/pnas.1512792112/video-2

http://www.pnas.org/content/112/48/E6663.full “Dissecting neural pathways for forgetting in Drosophila olfactory aversive memory”