While I can’t give you a specific answer, I can tell you where to look.
There have been a large number of studies using radioisotopes as nutrients for nerve tissue (often in vivo) or initial growth substrates for neural tissue cultures.
The solid component of neuron is almost entirely carbon, nitrogen, oxygen and hydrogen, and each of these has a commonly used, relatively harmless medical research isotope with a long enough half life (and other useful properties allowing them to be distinguished by e.g. NMR imaging) that we can observe atomic turnover directly. Such studies were common in biochemistry through the 1950s/60s
Once we had the outline of cell processes, radiolabelling experiments were still important, but they were more specialized. e.g. manufacturing radio-glucose with a C-14 atom instead of C-12 at a specific site to determine the diffusion, uptake and metabolism of the sugar, the fate of each specific atom in the molecule, etc.
While radioisotopes have slightly different reaction rates than the most common “stable” isotope, due to their slightly higher (usually) mass, they still gave us a reasonable estimate, which could be refined later by comparing the rate of each individual (enzyme catalyzed) reaction with both isotopes.
Damn you! Epiphany told me she wasn’t seeing anyone else. Does she ever mention me?
Maybe. Is your name Eureka?
Nope, she always refered to me by my nickname, “I’mcoming”. 
I just had an epistaxis looking up all those other words at WebMD.
Damn you, QADGOP.
I’ve said this before; I’m saying it again; and it most likely will come up again, but…
It must be awful lonesome on the Isle of Man.
Babysitting computers is lonely work.