'This is very unique to cannabis'

What was the experiment, in brief?

We treated old mice with a low dose of the active ingredient in cannabis. Learning and memory functions improved dramatically in these animals, and they performed as well as mice that were much, much younger.

How did you react when you saw for the first time that old mice had developed younger brains?

Complete disbelief, basically.

How was the experiment conducted?

We implanted so-called osmotic mini-pumps—tiny pumps that we put under the skin of these mice—and they constantly released the drug that we wanted to administer. And then, after four weeks, we stopped the delivery, waited for a week, and then afterwards, there were several tests on learning and memory.

How do you test a mouse's memory?

A very simple test, for example, is that mouse needs to learn how to find a hidden platform. And mice learn this typically within a few days—actually, they're quite good.

So after the old mice receive THC you tested them again….

And what we observed was quite staggering. They remembered where the platform was just as well as a young animal—and also, their working memory was absolutely indistinguishable. Then we tested the animals in many different paradigms that also test memory function, and we basically couldn't distinguish [between] the old mice and young mice. These [old mice] were between 12 and 18 months, which is very old for a mouse.

Is there any other substance, chemical compound—anything out there—that you've seen administered to mice that has this kind of effect?

No, not that I know of. This is very unique to cannabis.

Does this mean I can walk outside, smoke a joint and improve my memory?

No, you're too young. Cannabis is a drug that has opposite effect on young animals as opposed to old animals. It worsens memory in young mice. It also worsens memory in young humans. Don't smoke dope.

When young people smoke marijuana, what is happening at the chemical level to worsen cognitive function or to worsen memory?

We don't know. We know it's temporary. If you stop smoking pot your memory functions will improve. But while you're smoking pot, you'll perform worse.

Did you have a hunch that you might find these results?

We had previously observed that the endocannabinoid tone declines as we age, so the levels of endogenous substances decrease in older individuals. And this means they're present inside our bodies to begin with. So what the plant compound does is just mimic our endogenous compounds. What we were attempting to do is to normalize the level, or bring it back to the level of young individuals. 

Andreas Zimmer is a professor at the Institute of Molecular Psychiatry at the University of Bonn. Together with colleagues at The Hebrew University of Jerusalem, he conducted the brain research on mice and co-authored the paper published in Nature Medicine entitled, "A chronic low dose of Δ9-tetrahydrocannabinol (THC) restores cognitive function in old mice."