Bulk application of fluorescent dyes into an area of interest allows us to visualize, by means of confocal microscopy, the gross morphology of particular neuronal pathways involved in a functional system. By injecting two dyes possessing distinct optical characteristics into different brain areas, we can uncover putative connections between the axon terminals originating from the various regions. The immunostaining technique, on its side, offers the opportunity to stain neuron populations based on the transmitter they utilize (for example GABA and serotonin) or according to the presence of certain molecules, such as synapsin, which is present in all presynaptic terminals.