Microdialysis A technique used to study the biochemical activity in the extracellular space. Micro : refers to the extremely small scale. Dialysis: the movement of chemicals across a permeable membrane. Microdialysis allows us to monitor chemical changes in a behaving animal

Most commonly, a concentric microdialysis probe is used. Usually the fluid flows in the opposite direction to the arrows in the diagram, but it doesn’t really matter as long as the fluid flows by the membrane.

A microdialysis probe is similar in size to a bipolar stimulating electrode. It can therefore be implanted stereotaxically for both Acute and Chronic experiments. The fluid circulating through the dialysis probe is artificial cerebrospinal fluid (ACSF). ACSF is very similar to the CSF in the extracellular space so a negligible amount of ACSF or CSF will cross the membrane. There is thus very little tissue damage or trauma. Soluble molecules will cross the membrane down their concentration gradient. Molecules can cross in either direction. A dialysis probe can be used to deliver chemicals to the brain. If a higher concentration of the chemical of interest is in the probe, it will tend to cross the membrane and enter the extracellular space. This application is often called Retrodialysis.

Calibration The recovery capabilities of probes vary. Before a microdialysis probe can be used, it must be calibrated. This is done in vitro by testing the probe with a known concentration of the molecule of interest in ACSF. Recovery = conc(outflow) conc(ACSF)

The recovery in brain will not be as efficient as the recovery in vitro. Once the probe has been calibrated it can be used. Before samples are collected, ACSF is run through the probe for approx. 20 minutes to allow the probe to equilibrate.

Factors influencing the dialysis rate Surface area of dialysis membrane. Magnitude of concentration gradient. Rate of flow through the probe. Type of dialysis membrane. Different membranes have different pore sizes The size of the molecule.

Applications Temporal characteristics of a drug crossing the BBB. Release of neurotransmitters during behaviour. Activation of brain areas by various agonists (Retrodialysis) Activation of one brain area with one probe. Recording neurotransmitter release from another area with a second probe. Mixed use? Using a probe to both deliver and sample.

Images from J.P.J.Pinel, Biopsychology 4 th ed © 2000 pages 123 & 365 Release of DA in the nucleus accumbens being measured during sexual behaviour (left) & in response to intracranial self-stimulation (right).

Pitfalls Although microdialysis provides the sample, you still need a suitable method to analyze the sample. Some methods require only microlitres of solution while others might require millilitres. Temporal resolution will be limited. If it takes 10 minutes to collect a sample, that is the temporal resolution. Some areas of interest are too small for the probe.