I reallly wish that I had a pharmacologically acceptable answer for the reason why Amantadine works so well in conjunction with Sinemet. Some call it a "Weak agonist" with few side effects. However, as apharm chemist, i see it molecularly as "a ball of fat (hydrocarbon) attached to one valence (bonding "hand")of ammonia". Not "much of a drug" when one first loooks at its structure, but nevertheless, a drug that has incredible antiviral abilities, along with it's most useful to us PWP's properties as being a "synergistic" compound when taken with sinemet.
Let me tell you my about my 10 years use of amantadine every day, at 200mg/day. At first, before my neuro wanted to give me the "sinemet test", he put me on amantadine. I woke up the next day and bounded down the ctairs and into the shower and was at work within 15 minutes like I had been for a decade before PD signs showed up. My wife commented on how "changed " i was, because it helped "striaghten out my left leaning gait, and my general mood. This all slipped away after 3 months. Then he added sinemet, and through agonist after agonist trial, the ony thing that continues to "keep me on my feet" Is 800mg of dopa plus carbidopa = sinemet, plus the amantadine. IF i try to do without it, it is like my whole response to sinemet "Goes wild". That's all i can say about it. It is an observation, and reveals no ryme nor reason about how or why amantadine oes what it does for ME. And the operative word is me, as we PWP are all different.