2-Photon Microscopy Shows Learning Involves Various Areas of Brain

To explore how learning and memory-building take place in the brain, scientists at Johns Hopkins University School of Medicine used a laser-assisted imaging tool to monitor and measure levels of AMPAR molecules, which help send messages between neurons, in mouse brains. Their experiments, they said, add to evidence that motor-based learning can occur in multiple areas of the brain, even in areas not typically associated with motor control.

The scientists injected DNA-encoding AMPARs carrying a fluorescent tag into the brains of mice and used an electrical pulse to force the neurons to absorb the AMPAR DNA. Using in vivo two-photon microscopy, they measured the amount of fluorescence coming from the tagged AMPARs. Higher amounts of fluorescence indicated increased AMPAR activity and messaging between neurons, a sign that learning and memory-building was taking place.

AMPA receptors in green on neurons in magenta at one time-point in a live mouse. Courtesy of Richard Roth and Richard Huganir.

While the mice were learning how to reach for a food pellet with their paws (a task they normally complete using their mouth), the scientists observed approximately a 20% increase in AMPAR activity in the motor cortex, a part of the brain that controls and moves muscles. The experiments showed the same increase in AMPAR activity levels in the visual cortex. “This made sense because vision is very important for motor control,” researcher Richard Roth said. “So, we did the same experiment again, but with the lights switched off.”

Although the mice learned how to grab the food, there was a smaller increase (10%) in AMPAR activity in the visual cortex when infrared light (a wavelength not visible to mice) was used to measure it. “We believe the mice brains are using different sets of sensory cues in the dark to learn the motor task, including touch and smell, enabling these other senses to take over,” Roth said.

The research team repeated the experiments using specialized light-activated modulators to shut down neurons in either the motor or visual cortex. They found that optogenetic inhibition of visual cortex activity impaired task performance; if the mice were trained to get the pellet with the room lights on, they could not complete the task if their visual cortex was shut down. However, mice initially trained to grab the pellet in the dark could still complete the task, even if their visual cortex was shut down.

“We’ve traditionally thought that motor-based learning happens solely in the motor part of the brain, but our studies and others now show that it’s not as specific as we had thought. There is more of a brain-wide effect in learning,” Roth said.

“Scientists should be looking at the entire brain to understand specific types of learning,” professor Richard Huganir said. “Different parts of the brain contribute to learning in different ways, and studying brain cell receptors can help us decipher how this works.”

The research, which was published in Neuron (www.doi.org/10.1016/j.neuron.2019.12.005), could support efforts to develop treatments for learning-based and neurocognitive disorders.

Researchers at Johns Hopkins University School of Medicine have successfully used a laser-assisted imaging tool to “see” what happens in brain cells of mice learning to reach out and grab a pellet of food. Their experiments, they say, add to evidence that such motor-based learning can occur in multiple areas of the brain, even ones not typically associated with motor control. Courtesy of Richard Roth and Richard Huganir.