Structure of the rat's behaviour in the hot plate test

The objectives of the study were: (i) to elaborate an ethogram of the rat's behaviour during the hot plate test, (ii) to analyse the rat's responses to the nociceptive heat stimulus by using ethological techniques, and (iii) to discern the suitable pattern or patterns to evaluate the rat's reaction to the noxious stimulus. Male Wistar rats (n = 30) were employed, and one hot plate test was carried out for each rat. The platform (Socrel DS37) was maintained at 55.0 +/- 0.5 degrees C, and the exposure time was 45 s. Behaviour was videotaped and analysed by an ethogram and a software package. Every pattern parameter (frequency, duration and latency for the first occurrence) was quantified. Cluster and Markovian analyses were also employed, whereby a dendrogram and a Markov chain were obtained, respectively. The ethogram was composed of twelve patterns: immobile-sniff, walk-sniff, rearing, forepaw-licking, hindpaw-licking, face-washing, body-cleaning, stamping, hindleg-withdrawal, leaning posture, jumping off, and freezing. Cluster analysis indicated that the most frequent patterns could be classified into three categories: sniffing responses (immobile-sniff, walk-sniff), primary noxious-evoked patterns (forepaw-licking, hindpaw-licking, stamping), and escape noxious-evoked responses (leaning posture, jumping off). Hindleg-withdrawal would be an independent noxious-evoked pattern, and freezing an alerting behaviour. The ethological information could be useful to better elucidate effects on rats' behaviour of drug treatment or other phenomena during the hot plate test.