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