In the typical von Frey assay, a filament is applied to the plantar surface of the mouse hindpaw. Sometimes that is all that is said in the methods section. There are number of important variables here though that need to be consistent and well-described: site of application and duration of application.
Different areas of the hindpaw are more or less sensitive to mechanical stimulation. But for a usual mechanical sensibility testing, where on the mouse's paw do you apply the filament?
- Near the toes, ahead of the foot pads
- In the anterior-middle area, in the skin between the footpads?
- Posterior to the footpads
Certain models, such as the SNI model, denervate the skin and leave only certain territories available for testing, such as sural territory in this case.
And how about duration of application? This is something people don't often talk about, but apparently it can make a difference in the withdrawal threshold, with longer durations leading to lower thresholds. So it's important to be consistent here.
So what do you do?