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Next, a look at why some brain scientists think marijuana could be helping people like Sgt. Ryan Begin. NPR's Jon Hamilton reports on new research showing how marijuana affects brain circuits involved in PTSD.

JON HAMILTON, BYLINE: When a typical person encounters something scary, the brain's fear system goes into overdrive. The heart pounds, muscles tighten. Then, once the danger is past, everything goes back to normal.

Kerry Ressler, of Emory University, says that's not what happens in the brain of someone with PTSD.

KERRY RESSLER: One way of thinking about PTSD is an overactivation of the fear system that can't be inhibited, can't be normally modulated.

HAMILTON: Ressler says researchers suspected that marijuana might help people with PTSD by quieting an overactive fear system. But it wasn't clear how. Then about a decade ago, scientists in Germany found an explanation while studying fear in some unusual mice. Ressler says these mice had lost the ability to respond to brain chemicals called cannabinoids.

RESSLER: And they showed in those animals that they were more anxious, and they had difficulty recovering from fear.

HAMILTON: That suggests the brain uses cannabinoids to modulate the fear system.

There are two common sources of cannabinoids. One is the brain itself. The other is Cannabis sativa, the marijuana plant. So Ressler says researchers began treating traumatized mice with the active ingredient in marijuana, THC.

RESSLER: A number of studies looked pharmacologically at the process and basically said, if we give a mouse an equivalent of marijuana, what will happen? And they look, for the most part, initially less anxious, more calm - you know, many of the things that you might imagine.

HAMILTON: Unfortunately, Ressler says, the drug's effect on fear doesn't seem to last.

RESSLER: There's very explicit data now that if you give animals a THC-like compound and they have it for a couple of days in a row, you know, it's basically in their system. It doesn't work to inhibit fear anymore.

HAMILTON: Ressler says prolonged exposure to THC seems to make brain cells less sensitive to the chemical. Andrew Holmes, at the National Institute on Alcohol Abuse and Alcoholism, says there's another reason marijuana isn't ideal for treating PTSD - side effects.

ANDREW HOLMES: You may, indeed, get a reduction in anxiety. But you're also going to get all of these unwanted effects: loss of short-term memory, increases in feeding might be something that's not something you want.

HAMILTON: So for several years now, Holmes and other scientists have been testing drugs that appear to work like marijuana, but with fewer drawbacks. Holmes says some of the drugs amplify the effect of the brain's own cannabinoids, which are called endocannabinoids.

HOLMES: What's encouraging about the effects of these endocannabinoid-acting drugs is that they may allow for long-term reductions in anxiety - in other words, weeks, if not months.

HAMILTON: Holmes says the drugs work well in mice. Tests in people are just beginning. In the meantime, researchers are learning more about how the active ingredient in marijuana affects the fear system in people. At least one team has had success giving a single dose of THC during a type of therapy often used on people with PTSD. The therapy is designed to induce something called fear extinction, which is when the brain stops reacting to something that previously triggered a fearful response.

Holmes says THC seems to make the results last longer in both mice and people.

HOLMES: Giving human volunteers THC or a synthetic relative to THC during a fear extinction test produced long-lasting reductions in anxiety very similar to what we were seeing in our animal models.

HAMILTON: So THC may be most useful when combined with other therapy.

Kerry Ressler, from Emory, says that as the evidence piles up, it may become unrealistic to expect people with PTSD to wait for something better than marijuana and THC.

RESSLER: I'm a pragmatist and I think if there are medications, including drugs like marijuana, that can be used in the right way, there's an opportunity there, potentially.

HAMILTON: An opportunity that some veterans are already exploring on their own.

Jon Hamilton, NPR News.

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