The cell that lets go

This system does something almost nobody knows about. It helps you let go of old lessons. And letting go turns out to be half of learning.

In 2002, researchers put mice in a pool of milky water, with a platform hidden just under the surface. You're the mouse. Find the platform.

Swim where you point. The shapes on the walls are your landmarks.

Found it. Remember where it was.

Again, from a different side. Use the landmarks.

Faster. You remembered. That's learning.

Now the platform moves to the other side of the pool. Find it again.

That's where it used to be. It's gone now.

In the real experiment, a mouse that can't find it is shown the way. Here it is.

You let go of the old spot and found the new one. Normal mice do that too.

Now watch two mice swim after the move. The blue trail is a normal mouse. The amber trail is a mouse born without CB1.

Both learned the first spot just as fast. But the one without CB1 keeps going back to where the platform used to be. Again and again, even after it has been shown the new one.

The same year, another lab taught mice to fear a sound. Then they played it again and again, with nothing bad after it. Play the tone.

Each time nothing bad happens, the normal mouse freezes a little less. It's letting go of the fear.

Without CB1, the fear stays.

Now give a normal mouse the blocker, rimonabant, before the tones.

Its fear stays too. The lock has to be working at the moment of letting go.

Now people. In 2011, volunteers wore a tight cuff on one arm and squeezed a spring for as long as they could stand the pain. For two days, they got a painkiller first, and lasted longer.

The next day, they got plain strawberry milk, and were told it was the painkiller. They still lasted longer. Their brains had learned to expect relief.

For another group, the milk also hid rimonabant, the CB1 blocker. On its own, the blocker does nothing to pain. But after an ordinary painkiller, the learned relief was gone.

When the painkiller had been morphine, the blocker changed nothing. That kind of relief runs on the body's opioid system instead.

Letting go of a place. Letting go of a fear. Relief learned from an ordinary painkiller. Each one needed CB1.