Calibrated forceps: a sensitive and reliable tool for pain and analgesia studies.

UNLABELLED: Devices designed for mechanical pain threshold studies are often difficult to implement. The purpose of this study was to investigate a simple tool based on calibrated forceps to induce quantifiable mechanical stimulation in the rat on a linear scale. The most suitable protocol was tested by determining the effects of 3 repetitive measurements on both hind paws, respectively, during long-term (9 days), mid-term (1 day), and short-term (2 hours). Only threshold increase related to weight gain over long-term was observed, suggesting that moderate rat training can be used. The capacity of the device to reveal hyperalgesia was tested in a model of carrageenan-induced inflammation in the hind paw. The hyperalgesia was maximal 6 hours after carrageenan injection and progressively decreased. Similar, although more variable, responses were observed with von Frey filaments. Morphine-induced analgesia resulted in a dose-dependent increase of paw threshold. Tolerance to morphine administrated on a once daily schedule (10 mg/kg) during 5 days was revealed by a significant decrease in analgesia by day 3. Taken together, these results demonstrated accuracy of this device for easy, fast, and reproducible measure of mechanical pain threshold on rat limbs. Moreover, it allows the performance of rat testing with minimal constraint, which reduces data variability. PERSPECTIVE: The calibrated forceps is an easy to use device well-suited to rapidly test mechanical pain threshold with accuracy. It is well-designed for preclinical behavioral screening of noxious or analgesic properties of molecules.