By Pallab Ghosh BBC science correspondent

One patient has learned to laugh again and has regained his sense of smell.

The doctors say it is too soon to tell whether the treatment is long lasting or could be used on all sufferers of Parkinson's.

The treatment involves putting a drug called GDNF into a mechanical pump, which feeds it to the most damaged parts of the brain.

"We can deliver a drug very precisely to areas in the brain in the concentrations that we need to cause recovery and we can control that very precisely," said Stephen Gill, consultant neurosurgeon at Frenchay Hospital.

He said he and his colleagues were very surprised at the effects of the treatment.

Rapid progress

"We thought that this drug would take some months or years to be effective [but] we found that really within a month or two patients were noticing significant changes."

Patients with Parkinson's disease lose their sense of smell and taste but this recovered very early, he said.

Roger Nelson, who took part in the Bristol trial, told the BBC what the treatment had meant for him.

"Being able to talk better, walk better, smile and also laugh again.

"My wife passed a slightly risque comment just after I got home from hospital and I burst out laughing, which I hadn't been able to do for several years," he said.

Further research

Nik Patel, consultant neurosurgeon at Frenchay Hospital, counsels caution.

"I think we're a way from a possible cure yet and we have many hurdles to cross and many theories to prove but that remains a possibility.

"We have to prove that this drug is continually effective, safe and does reverse the disease, and in time we may see the results on a Pet (Positron Emission Tomography) scan confirming these cells have re-grown," he told the BBC.

The new treatment has worked well on five people - but that is too few to know for sure whether it is an effective therapy. But the results so far are promising and it has given Roger a new lease of life.