DIFFERENTIAL-EFFECTS OF WEEKLY AND DAILY EXPOSURE TO THE HOT PLATE ONTHE RATS BEHAVIOR

Animals were assigned to two groups: weekly stimulated (WS, n = 30) an d daily stimulated (DS, n = 30). Three hot plate tests (55.0 +/- 0.5 d egrees C, 45 s exposure time) were carried out for each rat. Behavior was videotaped and analyzed by a 14-pattern ethogram and a software pa ckage. A cluster analysis revealed that naive rats mainly displayed: i ) exploratory patterns (walk-sniff, immobile-sniff), ii) primary noxio us-evoked elements (forepaw licking, hindpaw licking, stamping), iii) escape responses (leaning posture, jumping), and iv) hindleg withdrawa l, 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 onl y in the WS group, iii) escape elements were enhanced in both groups, although to less extent in the DS group, and iv) hindleg withdrawal wa s enhanced in both groups. Furthermore, in the WS group, the nocicepti ve threshold was significantly decreased, and cluster analysis indicat ed reliable changes over time. Results suggest that a sensitization ph enomenon came about when rats were tested weekly, but was minimized by using the daily testing schedule. This study indicated that an etholo gical analysis is useful to categorise the rat's patterns in the hot p late test, as well as to follow the gradual changes in responses when repeated measures are used.