Intracellular staining of neurons with nickel-lysine

A new method of intracellularly staining neurons is described. Nickel-lysine (NL) can be used for both intracellular injection by pressure or iontophoresis and retrograde labelling (axonal backfilling). Once introduced into neurons, NL is reacted with dithiooximide dissolved in dimethyl sulfoxide (DMSO) to produce a blue-black precipitate. Small diameter processes are easily detected. For pressure injections, mixing NL with carboxyfluorescein provides a simple way to gauge how much dye has been injected, in that the latter is readily visible when illuminated with blue light. NL appears to move within neurons by axonal transport. Staining over long distances can be obtained in 12-24 h. NL does not appear to cross electrotonic synapses and remains confined to the neurons into which it has been injected. NL staining is simple, flexible and inexpensive. It has the additional advantage that it is compatible with other staining techniques.