I know that physiological changes occur in the brain parenchyma following cannulation and probe insertion that could affect quality of both neurotransmitter or metabolite recovery or delivery. The maximum amount of time I have seen in the literature was 30 days in the pineal gland in a microdialysis study. Accessory question: would you suggest I run any control tests during the microdialysis experiments to ensure microdialysis is optimally performed? Obviously, aside from measuring the concentration of molecule of interest and preferably without having to sacrifice any rats in the midst of the experiment.
In the literature you can find a time of 31 days period after chronic cannula insertion (Lecca et al, 2006 a, b; 2007 a Psychopharmacology; Bassareo et al 2015b, Behav Brain Res) and a maximum period of 48 days (Lecca et al., 2007 Psychopharmacology).
In general, when a rat is implanted with a microcannula or electrode will produce a glial reaction within hours or days, the best solution would be to use a holder device that allows the cannula replacement each study. If you want details on this issue you can review our publications.
Depending on how well the implant is fixed on the skull (screws, how dry the skull was during surgery) you may have the implant stable for over a month. But of course, you are right, astrogliosis and the resulting glial scar is an issue. However, how this will affect neurotransmitter level is probably more dependent on how often you insert the probe into the cannula as this will extend beyond the tip and will disrupt additional ´fresh´ (or less fresh after multiple insertions) tissue. In practice I therefore kept the probe overnight in the cannula before measurements and kept at least a week of recovery in between measurements. Finally, I would not recommend over 4 measurements. You may check effects of repeated insertions on neurotransmitter levels by comparing the baseline throughout multiple measurement-days.
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