I wonder if this is because they are somehow suppressed rather than out right lost.

Gee, I sure hope so! This is true of me. On my recent Datscan, the result was

"'Period-shaped' scan indicating normal anterior (caudate nuclei) dopaminergic function and greatly decreased dopaminergic putamena function bilaterally. This pattern is consistent with Parkinson's disease."

Greatly decreased dopaminergic function -- and all I have is minimal leg tremor.