http://www.sciencenews.org/articles/20051015/fob7.asp

High Times for Brain Growth: Marijuana-like drug multiplies neurons

Christen Brownlee

In the stoner stereotype, pot smokers and dying brain cells go hand
in hand. However, new research suggests the situation may be more
uplifting than that. A drug that functions as concentrated marijuana
does may spur neurogenesis, the process by which the brain gives
birth to new nerve cells.

Previous research had suggested that neurogenesis happens only in
select locations in the brain, such as the hippocampus, a region
involved in learning and memory. Some studies have shown that this
process is inhibited by most illicit drugs, such as cocaine, heroin,
and methamphetamine. However, says Xia Zhang of the University of
Saskatchewan in Saskatoon, marijuana's effect on neurogenesis has not
been clear.

He and his colleagues started investigating this mystery by searching
cell surfaces in live, cultured slices of rat hippocampus for
receptors that respond to marijuana and a few other similar drugs,
called cannabinoids. They reasoned that if marijuana affected
neurogenesis in the hippocampus, then cells in that area must have a
way to recognize the drug. Sure enough, 95 percent of hippocampus
cells responsible for neurogenesis showed evidence of cannabinoid 1
(CB1) receptors, one of two receptors that respond to cannabinoid
drugs.

Next, Zhang's team incubated samples of rat hippocampus with a
solution containing HU210, a drug that stimulates CB1 receptors with
a strength 100 times greater than that of pot. Other rat-hippocampus
cells were incubated with the same solution minus the drug or with
AM281, a drug that blocks CB1 receptors. After 2 days, the
researchers found a significant increase in the number of new brain
cells in the samples incubated with HU210, but no significant
increase of such cells in the other samples.

Finally, the researchers injected adult rats with various doses of
HU210. A single high-dose injection seemed to make no significant
difference in the number of new nerve cells. However, animals
injected with high daily doses of the drug over the course of 2 weeks
had about 30 percent more newborn nerve cells than did rats given
AM281 or a solution without either drug.

Animals given the 2-week course of HU210 also showed less anxiety and
depressionlike behavior than did rats not given the drug. When the
researchers irradiated the rats' hippocampi with X rays, which kill
off new neurons, animals given HU210 responded to these tests much as
did animals that didn't receive the drug. These results suggest that,
while these new neurons probably don't increase intelligence, they
could be responsible for antianxiety and antidepressive effects, says
Zhang.

He and his team report their findings in the November Journal of
Clinical Investigation.

While data suggesting that cannabinoid drugs can accelerate
neurogenesis are "interesting and potentially promising," it's too
early to tell whether high doses of marijuana over long periods have
a similar effect on depression and anxiety in people, says Ron Duman,
a neuroscientist at Yale University. "There is very little clinical
evidence demonstrating that cannabinoid administration produces an
antidepressant response," he says.