Cobalt mapping of the nervous system: evidence that cobalt can cross a neuronal membrane.

Exposing intact nerves of Aplysia to exogenous CoCl2 was sufficient to fill neurons by axonal iontophoresis. There were no significant differences between filling intact nerves and nerves whose ends had been cut. Nonneuronal elements were not filled appreciably. These results show that Co2+ can cross intact axonal membranes and suggest that in tracing neuronal pathways Co2+ may not remain confined to neurons whose interiors are directly exposed to it.