Brain circuits

Learning to avoid negative experiences requires an interplay of two distinct brain circuits, one to interpret and drive learning, and the other, unexpectedly; to dial in the strength of that memory, a new fruit fly study shows. “This kind of gain control is probably present in many levels of the nervous system; in many organisms,” says Scripps Research neuroscientist Seth Tomchik, Ph.D., lead author of the study.

Therefore the human brain is comprising of a staggering number of neurons about 86 billion; according to recent studies. Given that complexity; so neuroscientists working to understand how learning and memory work begin with simpler model organisms. Humans and flies share a degree of fundamental biology; but including reliance on the neurotransmitter dopamine in learning. The fruit fly brain contains eight groups of neurons that produce dopamine.

Dopamine-producing groups

Three of them can found in what’s known as the fly brain’s “mushroom body.” Humans don’t have an exact analogous brain section, but other brain regions perform similar functions. But in Drosophila melanogaster, aka the fruit fly, the mushroom body is an area highly responsive to odors. Past fly brain studies have shown that one of the dopamine-producing groups projecting into the mushroom; the body handles desire-inducing memories connected to odors.

To find out the role of the third group, referred to as PPL2; research associate and first author Tamara Boto, Ph.D., trained the flies with an experiment that involved exposing them to fruit-like odors while simultaneously giving them a mild electric shock. Their conditioned response could visualized under a microscope by adding a green fluorescent protein that releases light upon reacting to calcium.

The neurons communicate

Calcium ions are released when neurons communicate. Stimulating the PPL2 neurons during the odor experiments changed the brightness of the fluorescence when presented with the odor, an indication that the structures involved in learning and memory had altered the degree of response. “When they activated this PPL2 set of neurons, it would actually modulate the strength of that memory,” Tomchik says.

“So they see there are dopaminergic neurons that encode the aversive stimulus itself; then there is this additional set that can turn the volume up or down on that memory. Studying the brain circuitry underlying experience; learning and memory in model organisms can offer insights into our own, more complex brains; insights that could help us understand issues like addiction, PTSD, depression, and neurodevelopmental disorders.

“They researcher want to understand more about what their fundamental function; what types of stimuli activate them under what conditions,” he says. “Translating the learned information into behavioral execution, through the neurons in between, that’s where I expect a lot of discoveries in the next few years are going to be.”