What are the techniques used to introduce neuronal tracer into the tissue?

The three main techniques used to introduce neuronal tracers into the tissue are pressure injection, iontophoretic injection, and insertion of crystalline dyes. 

1. Pressure injection: Pressure injection is the most common technique used. In this technique, the tracer is injected directly into the cell in liquid form. 
2. Iontophoretic injection: Also known as iontophoresis, this technique involves using an iontophoresis unit to apply a current to a 1-5% concentration of tracer solution within an electrode. The tracer molecules pick up a charge and are driven into the cell by the force of the electric field. 
3. Insertion of dye crystals: This is a highly efficient labeling technique but can only be performed when the tissue is openly accessible. It is the preferred technique for focal applications of carbocyanine dyes such as DiI and DiAsp and is only