With only 100,000 neurons, the fruit fly Drosophila Melanogaster confirms that sleep is not only good for skin and mood, but also for memory. American researchers at Florida Campus of The Scripps Research Institute published in the journal Cell that remembering and forgetting are two processes closely related, dependent on the activation or inactivation of a type of neurons that produce dopamine. This discovery reduces the gap between psychology and neuroscience about sleep and memory.
Psychology says that during sleep human brain is not in contact with any external stimulus, and this state of isolation facilitates memory retention. On the other hand, neuroscience argues that not forgetting is an active process, called consolidation, which develops during sleep.
“Many scientists have tried to figure out how we learn and how our memories become stabilized,” said Dr. Davis, the senior author of this article, “But far less attention has been paid to forgetting, which is a fundamental function for the brain and potentially has profound consequences for the development of memory therapeutics. Our current study merges the neuroscience of forgetting, that is, the brain mechanisms that lead to forgetting, and the psychology of forgetting into an integrated picture.”
Human brain contains approximately 86 billion neurons and over than 200 different types of neurons exist, with different sizes, shapes or functions.
Dr. Davis and colleagues found that forgetting is dependent on the activation of dopamine neurons, a subclass of cells of the midbrain that are the main source of dopamine. They control several processes such as voluntary movement, mood, reward, addiction, and stress. The same group demonstrated few years before in Drosophila that dopamine is also required for learning.
By inducing sleep in fruit flies (by either drugs or by genetically stimulating the neural sleep circuit) and training them by exposure to aversive olfactory conditioning, the researchers observed that sleep reduces dopamine neurons activation, thus stabilizing memories. By contrast, increasing arousal stimulates dopamine signaling and accelerates forgetting. That means that dopamine-dependent forgetting is not a constant process, but it is dependent on behavior and on the level of arousal caused by external stimuli.
Dopamine is important for learning and for the retention of memories. Further studies will clarify whether consolidation of memory and forgetting are parallel and independent events, or whether reduced forgetting may favor consolidation during sleep. Whatever the mechanism, these findings could have could have a strong impact on the development of drugs to promote memory.
Reference: Berry JA et al. Sleep Facilitates Memory by Blocking Dopamine Neuron-Mediated Forgetting. Cell. 2015 Jun 18;161(7):1656-67.