http://www.pdf.org/en/parkinson_research_highlights_10

In a new analysis of current Parkinson’s research, Robert E. Burke, M.D., and his colleagues at Columbia University, with funding from a PDF center grant, found that only about 30 percent of a person’s dopamine neurons have died by the time that individual is diagnosed with the disease. These new findings contrast with a widely-cited number from previous studies that 50 percent or more of these cells have been lost at time of diagnosis.

There's a paper, published in 1991, by Pakkenberg et al. [1].

"Using an unbiased stereological technique, the total numbers of pigmented and non-pigmented neurons were estimated in the substantia nigra of seven patients with Parkinson's disease and seven control patients. Compared with the controls, in which the average total number of pigmented neurons was 550 000, the number of neurons was reduced by 66% in the patients."

"Others have reported the reduction of neuron numbers to be approximately 80% in the SN in patients with Parkinson's disease."

They were having the same sorts of debate 20 years ago!

Note:
1. the absolute number: 550,000.
2. numbers are at the time of death.

[1] "The absolute number of nerve cells in substantia
nigra in normal subjects and in patients with
Parkinson's disease estimated with an unbiased
stereological method"
B Pakkenberg, A M0ller, H J G Gundersen, A Mouritzen Dam, H Pakkenberg
Journal of Neurology, Neurosurgery, and Psychiatry 1991;54'30-33
http://www.ncbi.nlm.nih.gov/pmc/arti...00499-0040.pdf

John

Well, if I think about it, it would be weird to see the first symptoms after 70-80 % of neurons have died. Why ? This means majority of your brain would have way more neurons than necessary. Why would evolution create a brain with so many excess neurons ? That doesn't make any sense. You could say ... to have some reserve in case you lose some. But usually normal people don't lose this many neurons. And you can't say evolution would take into account old age, because we never lived this long.

Also ... I assume people have a similar degeneration of neurons during their lifetimes. At some moment PD gets triggered and this process gets speeded up. If at the time of first symptoms you would only have 20 % of neurons, it means that the person must have lost 80 % of them in a very short timespan ... It would mean that in an even shorter time period, you must lose all your remaining dopamine neurons. And this is not the case.

i'd want to read the actual article.
you can live on 1 kidney, the remaining one works a lot harder.
could be the same with dopamine neurons, the remaining ones work harder, it's a feedback loop i assume so the remaining neurons likely produce more dopamine.

percentages are less important it would appear than the actual number of cells, 80% of a small number is a small number.

regardless, i don't see this information keeping researchers from working on treatments that try to rescue the existing cells, namely gdnf and the research ceregene is doing plus others. and lots of research on identifying pd as early as possible so neuroprotective agents/activitities can be tested.

Definitely ... I think this is what happens. But I doubt 20 % of neurons can do the work of 100 % of neurons.

I tend to disagree on this one .

Of course ... this doesn't change the way people will continue doing research.

your're assuming 100% of the neurons are always active at the same rate.