CHARACTERIZATION OF VARIABLES DEFINING HINDPAW WITHDRAWAL LATENCY EVOKED BY RADIANT THERMAL STIMULI

We have examined the stability and sources of variation within the noc iceptive model of rat hind paw withdrawal from an under-glass radiant stimulus (Hargreaves et al., 1988) using a system where stimulus inten sity and floor temperature can be controlled and reproducibly changed. The current study demonstrates that: (i) increased stimulus intensity with a fixed surface temperature is associated with a monotonic decre ase in mean response latency and its variance; (ii) for a fixed stimul us intensity, the mean paw withdrawal latency:and variance increased a s the glass floor temperature is lowered from 30 degrees C to room tem perature (25 degrees C). Using subcutaneously-implanted thermocouples and a 30 degrees C glass surface, the subcutaneous paw temperature obs erved at an interval corresponding to the time at which the animal dis played a paw withdrawal did not differ across multiple heating rates ( 41-42.5 degrees C). This finding is in agreement with human studies of pain thresholds and C-fiber activity. These studies emphasize the imp ortance of maintaining a fixed surface temperature to reduce experimen tal variability and the utility of this apparatus across multiple stim ulus intensities to define agonist efficacy. (C) 1997 Elsevier Science B.V.