What could moving a needle across a piece of vinyl and shining a laser over a mouse brain have in common? They are both techniques that allow us to play back memory of a specific event. The concept of stamping the present moment— instruments, sound waves, matter, protons, and electrons—into a concrete piece of matter is so out-of-this world; I may never truly grasp the concept. For me, it’s just as daunting as chasing consciousness, the white rabbit of neuroscience.
When I ask people about recording music, I get some friendly answers: “the waves are then transferred to the grooves” or “to the digital components.” These explanations do nothing except tell me that a moment in time can be stamped onto a tangible piece of plastic and made available for instant playback whenever I’m feeling funky. With a memory, the same thing occurs in the brain. Pieces of the present environment are stamped into specific cells for later recall. This knowledge is a small part of a bigger question: how is memory formed and stored in the brain?
In two papers, one from last year, and one published July 26th in Science, Susumu Tonegawa and his colleagues at MIT figured out how to insert a memory into the brain and play it back when they felt funky. They did this in mice with a behavioral learning paradigm called fear conditioning and a technique known as optogenetics. Fear conditioning is basically just a way to create a measurable memory. Scientists pair a specific context with a threatening event, such as a footshock, to elicit a behavioral response (freezing). The result is an association between the context and footshock, or a contextual fear memory in the mouse. Scientists can test this memory by placing the animal into the same or a different context and looking for freezing (which would indicate memory of the footshock). With optogenetics as a compass, scientists have chartered a small bit of contextual memory territory in the brain, and Tonegawa et al. sought out to explore distinct contextual fear memories in specific populations of neurons in the dentate gyrus (DG) of the hippocampus.
To do it, they inserted a super-fancy virus into the part of the DG associated with a context. Then they fear conditioned the mice and shone a laser into the brain to activate the virus which turned on the cells. This combo stamped that “fear” memory into those specific cells, and salut! They had successfully recorded their contextual fear memory. Tonegawa’s virus was like a logic puzzle. It only worked if certain components of it were on or off at certain times. It let them really find that perfect song. They could then play it back with the laser like the needle on the groove of a record. If the mice froze, they knew they were on the right track. Yea, I know. It pretty much sounds like “the waves are then transferred to the grooves.” – by JoAnna Klein