Ask a neuroscientist which superpower they’d most like to have, and they’ll probably choose the ability to see into someone’s head and literally observe their thoughts. Well, thanks to a “tissue transparency technique” called CLARITY, researchers are now able to convert mouse brains into see-through thought blobs – like snow-globes of consciousness. In doing so, they’ve managed to disentangle some long-standing mysteries regarding how pleasure and pain are encoded in the brain.
Publishing their findings in the journal Cell, the study authors explain how a brain region called the medial prefrontal cortex (mPFC) has previously been shown to become activated in response to both pleasant and aversive experiences. However, scientists have been scratching their heads for years about whether signals from these two types of stimuli are transmitted by the same or different types of brain cells.
To investigate, they genetically engineered mice using a technology called ArcTRAP, which causes their neurons to become labeled with a fluorescent stain when activated. At the same time, they used CLARITY to remove the fatty tissue from the animals’ brains, making them see-through. As such, the team was able to observe the full workings of their subjects’ brains when encoding a range of different experiences.
They then repeatedly gave some of the mice cocaine, while the others received an electric shock. Peering into the crystal-ball-like rodent brains, the study authors observed how the two types of stimuli activated separate neuronal pathways, involving different types of brain cells that communicated with the various other brain regions via separate input and output routes.
In addition, the researchers discovered a key difference in the mechanism by which pleasurable and fearsome experiences provoke their respective neuronal responses. Aversive stimuli, for instance, generate a lasting spike in brain activity by upregulating the expression of a gene called NPAS4, leading to the formation of fearsome memories. Pleasant stimuli, however, do not have this effect.
Based on these findings, the study authors hope that it may one day be able to develop new treatments for depression and other mental health disorders. Talking to New Scientist, co-researcher Karl Deisseroth of Stanford University explained that “now we know the signals for fear and pleasure can be transmitted by different axonal projections, new targeted treatments might be envisioned.”
