Differential effects of weekly and daily exposure to the hot plate on the rat's behavior.

Animals were assigned to two groups: weekly stimulated (WS, n = 30) and daily stimulated (DS, n = 30). Three hot plate tests (55.0 +/- 0.5 degree C, 45 s exposure time) were carried out for each rat. Behavior was videotaped and analyzed by a 14-pattern ethogram and a software package. A cluster analysis revealed that naive rats mainly displayed: i) exploratory patterns (walk-sniff, immobile-sniff), ii) primary noxious-evoked elements (forepaw licking, hindpaw licking, stamping), iii) escape responses (learning posture, jumping), and iv) hindleg withdrawal, an independent noxious-evoked element. The main behavioral changes over time were: i) exploratory behavior was habituated in both groups, ii) sensitization of primary noxious-evoked elements was observed only in the WS group, iii) escape elements were enhanced in both groups, although to less extent in the DS group, and iv) hindleg withdrawal was enhanced in both groups. Furthermore, in the WS group, the nociceptive threshold was significantly decreased, and cluster analysis indicated reliable changes over time. Results suggest that a sensitization phenomenon came about when rats were tested weekly, but was minimized by using the daily testing schedule. This study indicated that an ethological analysis is useful to categorize the rat's patterns in the hot plate test, as well as to follow the gradual changes in responses when repeated measures are used.