It’s the first solid proof that long-term exposure to a toxin can cause this disease, says Abraham Lieberman, medical director of the National Parkinson Foundation in Miami, Florida. “It’s a very provocative study, and you can’t just throw it away.” But he says the study needs to be repeated in primates.

The pesticide the researchers investigated is called rotenone and is extracted from the derris plant. It is widely used as an insecticide in gardens and to kill pests on pets. It is also a favourite of organic farmers and gardeners.

Michelle Burton is a pesticides advisor for the Soil Association, which promotes organic agriculture. She says: “Organic farmers can’t use rotenone routinely – they have to apply and have a good reason. But in the light of this research, we will be looking into this product, and perhaps look into phasing it out.”

Nature or nurture

Studies of twins suggest that genetic factors are to blame for Parkinson’s in people under 50 years of age. But this isn’t true for late-onset Parkinson’s, the most common form of the disease. This affects one per cent of people over 65.

So researchers have searched for environmental risk factors. Earlier this year, a study of 1000 people found that those who often used pesticides at home had a 70 per cent increased risk of getting Parkinson’s. Another study done earlier this year revealed that California counties with the most pesticide use also had the most deaths from the disease.

But these studies couldn’t show that the pesticide actually caused the disease. So Tim Greenamyre of Emory University in Atlanta, Georgia, and his colleagues gave rats low levels of a pesticide called rotenone over several weeks.

The rats gradually lost their dopamine neurons, and developed tremors and cellular protein deposits just like those seen in people with Parkinson’s. Greenamyre’s work will be published in December in Nature Neuroscience.

Partial proof

“It clearly shows that environmental factors can be sufficient to produce all the features of Parkinson’s disease,” says Greenamyre. He doesn’t think this is proof that pesticides cause Parkinson’s in humans.

But he believes the study suggests that we need to look again at the way we assess the safety of pesticides. “There are a large number of pesticides that act by the same mechanism of action,” he says.

Lieberman fears that the study will create “a tremendous flurry of worried people” wanting to know if pesticides have doomed them to Parkinson’s. He also thinks that lawyers will use the study to seek damages on behalf of Parkinson’s patients exposed to pesticides.

American scientists have shown for the first time that monkeys strive to inject themselves with the active chemical in marijuana in the same way that they seek out cocaine or morphine. The finding has created a firestorm of controversy over the implications for people who smoke the drug.

Last week, a survey revealed that one in ten people in England and Wales has used cannabis in the past year. The report by the European Monitoring Centre for Drugs and Drug Addiction also found that 45 million Europeans are regular users.

Serious habit

Therapists and psychiatrists who treat people for marijuana dependency have hailed the study as a breakthrough. They think it will make people take the abuse potential of cannabis a lot more seriously.

“A lot of people think it’s not addictive,” says Ron Kadden of the University of Connecticut Health Center. “They’ve been told by treatment professionals and their friends that they couldn’t really be addicted to marijuana.”

Kadden says he recently found plenty of takers when he advertised a treatment program designed specifically for cannabis dependence.

But campaigners for the legalisation of dope have called the study “pseudoscience”, designed to fulfil a political agenda.

“We take umbrage with the government’s seemingly never-ending penchant to prove that cannabis is harmful enough to justify the 70-year prohibition,” says Allen St. Pierre of the National Organization for the Reform of Marijuana Laws in Washington, DC.

Tough test

For years, scientists have been unable to get animals to self-administer THC, the active chemical in marijuana and cannabis. Some critics of laws banning marijuana have used this fact to argue that the drug can’t be addictive.

Steven Goldberg and his colleagues at the National Institute on Drug Abuse in Baltimore, Maryland gave squirrel monkeys small injections of THC every time they pressed a lever.

The monkeys quickly learned to self-administer the drug, giving themselves up to 30 injections in a one-hour session. Goldberg then gave the animals a drug that blocks the brain’s cannabinoid receptors. This eliminated the lever pressing.

Perfect puff

“We’re interested in looking at potential compounds that might be used as medications for drug abuse,” says Goldberg. “Now that we have a model of how THC supports drug behaviour we can go in and try to intervene.”

Goldberg thinks he has a good model for human marijuana use, because the monkeys received the same amount of THC in each injection as a person would get from one puff of a point.

When the team took the monkeys’ body weight into account, they found the THC dose was five times lower than scientists have tried in the past.

Source: Nature Neuroscience (vol 3, p 1073)