Researchers from the University of Houston in Texas believe they are a step closer to understanding how memories form, which could ultimately provide better treatments to improve memory in people of all ages.
The findings, published in Current Biology, promise a more complete understanding of how memories form, both at the molecular level and through neural circuit activity.
For people with dementia, memory loss can have a profound effect on their daily lives. Struggling to remember things they have recently learned, asking for the same information every few minutes or forgetting important events can compound feelings of confusion and loss of control.
Gregg Roman, associate professor of biology and biochemistry at the university, explains:
"Memory is essential to our daily function and is also central to our sense of self. To a large degree, we are the sum of our experiences. When memories can no longer be retrieved or we have difficulty in forming new memories, the effects are frequently tragic. In the future, our work will enable us to have a better understanding of how human memories form."
For the research, Roman and his postdoctoral assistant Shixing Zhang looked at the brains of fruit flies from the genus Drosophila.
Roman explains their choice:
"Drosophila represents the Goldilocks principle of neural research, with sufficient behavioral complexity, while maintaining a huge advantage in neural simplicity. The complex behaviors allow us to examine many behavioral processes like learning, attention, aggression and addiction-like behaviors, while the simplicity allows us to dissect the crucial neural activities down to single cells."
While fruit fly brain structure is much simpler with far fewer neurons, researchers say the mushroom body is comparable to the perirhinal cortex in humans, which serves the same function of sensory integration and learning. The team says the simplicity of the brain structure allows them to gain insights into how memories are acquired, stored and retrieved.
Inside the fly's brain, the scientists were able to identify the cells associated with olfactory learning and memory. Olfactory learning is an example of classical conditioning, which Pavlov demonstrated with his experiments on dogs.
Gamma lobe neuron responses
For this study, the flies were trained to associate a weak electric shock with a particular odor. After exposure to the shock, the flies avoided that smell.
Roman explains further:
"We found that these particular nerve cells - the gamma lobe neurons of the mushroom bodies in the insect brain - are activated by odors. Training the flies to associate an odor with an electric shock changed how these cells responded to odors by developing a modification in gamma lobe neuron activity, known as a memory trace."
"Interestingly," he adds, "we found that training caused the gamma lobe neurons to be more weakly activated by odors that were not paired with an electric shock, while the odors paired with electric shock maintained a strong activation of these neurons. Thus, the gamma lobe neurons responded more strongly to the trained odor than to the untrained odor."
The team found that a specific protein - the heterotrimeric G(o) protein - naturally inhibits gamma lobe neurons. Removing the activity of this protein only within the gamma lobe neurons resulted in a loss of the memory trace and, thus, poor learning, Roman says.
The researchers claim that inhibiting the release of neurotransmitters from these neurons through the actions of the G(o) protein is key to forming the memory trace and associated memories.