Comparison of the behavioural and endocrine response to forced swimming stress in five inbred strains of rats

Some inbred strains of rats showed behavioural differences in the forced swimming test, which is considered a putative animal model of depression. In the present work, the behavioural and physiological responses to forced swimming were studied in male and female rats of five inbred strains of rats: Brown-Norway (BN), Fischer 344 (FIS), Lewis (LEW), Spontaneously Hypertensive (SHR) and Wistar-Kyoto (WKY). Physiological measures were aimed at characterizing emotional reactivity, a very important issue which has usually been approached by studying a single endocrine system, and its relationship to the forced swimming behaviour. The four indices of reactivity to stress used were serum glucose, ACTH, corticosterone and prolactin. No behavioural differences between sexes were observed in the forced swimming test. In addition, BN and WKY rats showed passive behaviour compared with the other three strains, the FIS strain being the most active. Whereas only minor differences were found in the resting levels of the variables studied with regard to either sex or strain, pituitary-adrenal (PA) and glucose responses to 15 min forced swimming differed among sexes and strains. Stress-induced hyperglycaemia was lowest in WKY and highest in SHR, being lower in females than in males. The lowest ACTH and corticosterone responses to forced swimming were observed in LEW and the highest in FIS. Female rats showed a clearly higher PA response to stress in all strains. Prolactin response to stress was very similar between sexes and strains. It might thus be concluded that: (i) there are important inter-strain differences in the forced swimming behaviour, with no differences between sexes; (ii) the various physiological indices of emotional reactivity follow a different trend and no warranted conclusion on differences in emotional reactivity should be based upon a single endocrine system or even only upon physiological measures; (iii) we cannot be sure, therefore, whether or not there are differences in emotionality between the strains studied in spite of well-established inter-strains differences in the forced swimming behaviour.