The Science of Cannabis and the Brain [io9.com]

Annalee Newitz — Archaeologists recently found a 2,700-year-old pot stash, so we know humans have been smoking weed for thousands of years. But it was only about 20 years ago that neuroscientists began to understand how it affects our brains.

Scientists have known for a while that the active ingredient in cannabis was a chemical called delta-9-tetrahydrocannabinol, or THC for short. Ingesting or smoking THC has a wide range of effects, from the psychoactive “getting high” to the physiological relief of pain and swelling. It also acts as both a stimulant and depressant. How could one substance do all that?

Meet the cannabinoid receptor

In the 1980s and 90s, researchers identified cannabinoid receptors, long, ropy proteins that weave themselves into the surfaces of our cells and process THC. They also process other chemicals, many of them naturally occurring in our bodies. Once we’d discovered these receptors, we knew exactly where THC was being processed in our bodies and brains, as well as what physical systems it was affecting. Scattered throughout the body, cannabinoid receptors come in two varieties, called CB1 and CB2 – most of your CB1 receptors are in your brain, and are responsible for that “high” feeling when you smoke pot. CB2 receptors, often associated with the immune system, are found all over the body. THC interacts with both, which is why the drug gives you the giggles and also (when interacting with the immune system) reduces swelling and pain.

Cannabinoid receptors evolved in sea squirts about 500 million years ago; humans and many other creatures inherited ours from a distant ancestor we share with these simple sea creatures. THC binds to receptors in animals as well as humans, with similar effects.

Tasty, tasty, tasty

Cannabis notoriously makes people hungry – even cancer patients who had lost all desire to eat. One study showed that cancer patients who thought food smelled and tasted awful suddenly regained an ability to appreciate food odors after ingesting a THC compound. There are CB1 receptors in your hypothalamus, a part of your brain known to regulate appetite, and your body’s own cannabinoids usually send the “I’m hungry” message to them. But when you ingest THC, you artificially boost the amount of cannabinoids sending that message to your hypothalamus, which is why you get the munchies.

Understanding this process has actually led to a new body of research into safe diet drugs that would block those cannabinoid receptors. That way, your hypothalamus wouldn’t receive signals from your body telling it to eat, and would reduce hunger cravings in dieters.

What you’re forgetting

What’s happening in your brain when smoking pot makes you forget what you’re saying in the middle of saying it? According to the book Marijuana and Medicine (National Academies Press):

One of the primary effects of marijuana in humans is disruption of short-term memory. That is consistent with the abundance of CB1 receptors in the hippocampus, the brain region most closely associated with memory. The effects of THC resemble a temporary hippocampal lesion.

That’s right – smoking a joint creates the effect of temporary brain damage.

What happens is that THC shuts down a lot of the normal neuroprocessing that goes on in your hippocampus, slowing down the memory process. So memories while stoned are often jumpy, as if parts are missing. That’s because parts literally are missing: Basically you are saving a lot less information to your memory. It’s not that you’ve quickly forgotten what’s happened. You never remembered it at all.

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