The Role of Dopamine in Forgetting Competing Information
Post by Shannon Kelly
The takeaway
Actively forgetting memories that compete with useful memories is important for our ability to remember information that supports our goals. This study found that the neurotransmitter dopamine in the prefrontal cortex plays a critical role in the active forgetting of competing memories.
What's the science?
Unlike the passive decay of memories that are not used over time, active forgetting is a process by which distracting memories are selectively forgotten to support our ability to retrieve useful memories. Active forgetting can happen when we retrieve certain memories and must inhibit competing memories (i.e., retrieval-induced forgetting). Although the neurotransmitter dopamine is known to play an important role in skills that relate to active forgetting, such as the ability to flexibly change behaviors, its direct involvement in retrieval-induced forgetting is not well understood. This week in The Journal of Neuroscience, Gallo and colleagues found that active forgetting in rats was hindered by reducing the brain’s ability to respond to the presence of dopamine and improved by increasing dopamine-related activity in the prefrontal cortex.
How did they do it?
The authors tested the effects of dopamine-related drugs on rats’ memory. They first showed rats pairs of objects to remember, then injected either harmless salt water or a drug that affects dopamine-related activity in the medial prefrontal cortex (mPFC; similar to the dorsolateral prefrontal cortex in humans). In the retrieval condition, rats then were shown pairs of one previously seen object and one new object so that they used their memory to guide their exploration. In the control conditions, rats either were shown pairs of two new objects or rested for the same amount of time. Finally, they showed the rats a pair of one new and one previously seen object and measured how long the rats examined each object. Since rats are known to prefer new objects, a large difference in time spent exploring the new object compared to the old one indicated that the rats remembered the previously seen object. If the rats preferred the new object in both control conditions but showed no or less preference for the new object in the retrieval condition, this indicated that the rats showed retrieval-induced forgetting of the previously seen object. The authors tested the effects of (1) a drug that decreases reactivity to dopamine by blocking dopamine receptors, (2) a drug that inhibits dopamine release into the mPFC from the ventral tegmental area (VTA), and (3) a drug that activates dopamine receptors on retrieval-induced forgetting.
What did they find?
Rats that were injected with either the drug that blocks dopamine receptors or the drug that suppresses the release of dopamine from VTA into mPFC showed no difference in behavior between the retrieval and control conditions, indicating no retrieval-induced forgetting. These findings show that dopamine-related activity in the mPFC is necessary for active forgetting of competing memories. For rats whose dopamine release from VTA was suppressed, retrieval-induced forgetting was improved back to normal levels when given the drug that activates dopamine receptors in mPFC. Finally, rats given only the dopamine-enhancing drug showed better retrieval-induced forgetting compared to control rats during a more difficult version of the memory test. Together, these findings show that active forgetting can be causally improved or hindered by increasing or decreasing dopamine-related activity in mPFC.
What's the impact?
This study showed that the ability to selectively forget distracting information can be directly influenced by affecting the level of dopamine-related activity in the prefrontal cortex of rats. These findings clarify the mechanisms by which experience helps determine which memories are maintained and which are forgotten. Based on findings that active forgetting involves similar brain mechanisms in humans as in rats, these findings suggest that prefrontal dopamine plays a critical role in our ability to adapt our memories based on the demands of our environments.