An experimental drug for Parkinson's disease has been shown to trigger new nerve growth in the brain, the first time any treatment has reversed the brain damage caused by the condition.

Neuroscientists at Frenchay hospital in Bristol made the discovery when they examined the brain of a patient who took part in a trial of the drug GDNF four years ago. All five patients on the trial showed dramatic improvements.

Seth Love, a consultant neuropathologist at the hospital, examined the brain of a 62- year-old patient who had been on the trial but recently died of a heart attack. He found that nerve fibres in a region of the brain called the putamen had regrown. The loss of these fibres, and the chemical dopamine they produce, leads to Parkinson's disease.

Tests showed that the drug improved patients' control of their movements by between 50% and 80%. Also, they have experienced no deterioration since treatment stopped six months ago.

"This is the first time that any treatment at all has been shown to reverse the disease process. All the other drugs have just treated the symptoms," said Professor Love, whose study is published in the journal Nature Medicine.

But the finding does not mean a new treatment for Parkinson's disease is on the horizon. Amgen, the US company that owns GDNF, withdrew the drug last year amid concerns over its safety and efficacy.

Helen Garner of the Parkinson's Disease Society said the trial was encouraging. "This only based on one person though, and there are 120,000 people with Parkinson's disease in Britain."