This study shows how activated microglial (killer cells), which are believed to destroy dopamine neurons, can be be greatly attenuated by dextromethorphan and another even more effective drug. Though the research was done for drug abuse, it implies that DXM could be used for PD. I don't know what an equivalent human DXM dose would be but this study seems to back Dr. Hongs research done at the NIH.
Ashley

http://www.nida.nih.gov/NIDA_notes/N...N6/Evokes.html
Drs. Kuhn and Thomas injected mice with either MK-801 or DXM and then methamphetamine (5 mg/kg of body weight) 15 minutes later, repeating this sequence four times at 2-hour intervals. A control group of mice received the same regimen, but with saline substituted for methamphetamine. Forty-eight hours after the last injection, the researchers assayed the brains of the mice for Cox-2 and TNF-α, the indicators of microglia activation, and for striatal dopamine levels, a widely used index of damage to dopamine neurons. Dr. Kuhn says, "We found that both DXM and MK-801 significantly reduced the markers of striatal microglial activation associated with methamphetamine exposure and protected against dopamine nerve terminal damage in the striatum. The close association between the ability of MK-801 and DXM to significantly lower both microglial activation and neuronal damage suggests a causal link between the two. It looks as though the damage associated with methamphetamine abuse is the result of microglial action."