Figure 3. Local pharmacological methods of circuit manipulation: microinjection and microdialysis.

Microinjection and microdialysis are used to manipulate neural activity near the cannula or probe tip by delivering drugs or other substances. In addition, microdialysis can be used to collect samples of substances from the brain for analysis. (A) Drugs and other receptor agonists or antagonists can be delivered to a small volume (typically <1 mm³) of brain tissue using microinjection. (B) Microdialysis is used to collect samples of drugs, neurotransmitters, or other small molecules from the brain to measure concentrations or other analyses. When the perfusate is isosmotic with the surrounding brain tissue, there is no net fluid delivery with microdialysis, in contrast with microinjection. (C) Microdialysis is also used to deliver drugs (often referred to as “reverse microdialysis”). In microdialysis and reverse microdialysis, diffusion of substances into or out of the probe tip depends on the pore size of the semi-permeable membrane.