Measuring hedonic status using SP test

To assess the hedonic status in rats that were ‘insensitive’ and ‘sensitive’ to negative/positive feedback, the animals were subjected to SP tests (Fig. 1c). A decreased preference for the sweet sucrose solution has been previously postulated to be an indicator of anhedonia in rodents (Papp et al. 1991; Willner et al. 1992), and the applied test has been described elsewhere (Rygula et al. 2013). In brief, during the SP test, the rats were separated into single cages and were offered a voluntary choice between two bottles for 1 h, where one bottle contained a 0.8% (w/v) sucrose solution and the other bottle contained tap water. To prevent potential effects of side preference in drinking, the position of the bottles was switched after 0.5 h. The consumption of water and sucrose solution was measured by weighing the bottles. The preference for sucrose was calculated from the amount of sucrose solution consumed and is expressed as a percentage of the total amount of liquid that was consumed.

Experimental schedule. Initially, (a) using a series of PRL tests, we classified individual animals as sensitive or insensitive to negative feedback and sensitive or insensitive to positive feedback. The rats classified in this way were subsequently tested in a battery of behavioural tests aimed at measuring the motivational, hedonic and affective (anxiety) correlates of depressive symptoms in rodents (b, c, d and e). Appetitive motivation (b) was assessed in the PRSR paradigm, hedonic status (c) was evaluated in the SP test, locomotor and exploratory activities (d) were measured in the OF test, and finally, the level of anxiety (e) was measured in the LDB test