Stem cell technology, whether it turns out to be adult, embryonic, corld blood, whataver.. has unfathomable potential.
It is possible that the stem cells could be coaxed into regrowing into Dopamine or GABA producing cells, or any other cell.
The fact that this can be done shows how amazing the human body and our biology is.
Ps 139:14 -
I will praise thee; for I am fearfully and wonderfully made: marvellous are thy works; and that my soul knoweth right well.
One very strong reason for taking this route is, the stem cells are from the patient's body, thus histamine reactions when reintroduced would be avoided.