Anterograde Tracing - Techniques

Techniques

Several methods exist to trace projections originating from the soma towards their target areas. These techniques initially relied upon the direct physical injection of various visualizable tracer molecules (e.g. Green fluorescent protein, lipophylic dyes or radioactively tagged amino acids) into the brain. These molecules are absorbed locally by the soma (cell body) of various neurons and transported to the axon terminals, or they are absorbed by axons and transported to the soma of the neuron. Other tracer molecules allow for the visualization of large networks of axonal projections extending from the neurons exposed to the tracer.

The aforementioned tracers such as GFP and DiI are technically not anterograde tracers: they do not selectively travel in a certain direction and are not actively transported by the cell. Furthermore, they do not cross the synaptic cleft, and therefore only label the neurons that were directly at the site of tracer application. These dyes are mostly used as a filler to mark cells that have been recorded using electrophysiology: this allows the cells that have been recorded to also be morphologically reconstructed in 3D.

The anterograde tracers that can cross the synaptic cleft and label multiple neurons within a pathway can be divided in two categories: genetic and molecular tracers.