PARIS -- Experimental compounds used on monkeys helped curb the involuntary jerks and spasms that are a common side-effect of L-dopa, the mainstream drug for treating Parkinson's disease, AFP quoted doctors as reporting.

Parkinson's, a tragic degenerative disease of the nervous system, occurs when there is a loss of cells in a part of the brain that produces dopamine, a neurotransmitter that communicates with other brain cells which regulate motor functions.

The conventional medical response is to raise dopamine levels in the brain by using a drug, L-dopa. The drug binds to cells that have the D3 receptor.

L-dopa initially reduces the tremor, slow movement and muscular rigidity associated with Parkinson's.

But in most patients, a jerky and writhing side effect, called dyskInesia, sets in after a number of years, possibly as a result of the cells' excess exposure to dopamine.

Dyskinesia is resistant to medicines, which means the only way to tackle it is by a "drug holiday" in which the patient stops taking L-dopa for a while, or takes a reduced dose, but this is a short-lived solution because the symptoms of Parkinson's then return.

Pierre Sokoloff and colleagues at the Center Paul Broca, which is part of France's National Institute for Medical Research, INSERM, believe they have uncovered a highly promising path for tackling dyskinesia.

They tried out three compounds that, like dopamine attach to the D3 receptor and act rather like valves, opening up or reducing the amount of L-dopa that is admitted into the cell.

The three molecules, called BP897, ST198 and Nafadotride, were tested on five macaque monkeys whose dopamine cells had been destroyed and which were displaying dyskinesia after a prolonged treatment with L-dopa.

When used in combination, these so-called "partial agonist" drugs were able to eliminate dyskinesia yet not lose the beneficial effects of L-dopa.

The experiment -- while encouraging -- has a long way to go before it can be tested on humans.

Parkinson's affects millions of people. Sufferers include Pope John Paul II, Muhammad Ali and the screen actor Michael J. Fox.