Vulnerability to chronic stress in male rats: Additive effect of low positive affectivity and high hedonic response...

Stress contributes to the development of psychiatric disorders. We have previously shown that rats with low inherent positive affectivity, assessed by 50-kHz ultrasonic vocalizations (USV), are more vulnerable to stress, and that free-fed rats with persistently lower consumption of sucrose are less sensitive. Hence we compared the association of these traits with the effect of chronic stress within a single experiment. Male Wistar rats were classified as of high (HC) or low (LC) positive affectivity based on their average 50-kHz USV response, and as young adults the rats were further divided into high and low sucrose-consuming (HSuc and LSuc, respectively). Four groups comprising twenty rats each were formed, and half of the animals submitted to chronic variable stress (CVS) for 5 weeks. CVS was followed by behavioural tests and ex vivo biochemical analyses. In elevated plus-maze, CVS increased the anxiety-related measures most prominently in the LC-HSuc rats. Stress reduced amphetamine-induced 50-kHz USVs statistically significantly only in LC-HSuc rats. Serum glucose and adrenal weight, as well as levels of monoamines and their metabolites in the frontal cortex, striatum, nucleus accumbens, amygdala and hypothalamus, were altered by chronic stress mostly in a phenotype-dependent manner. CVS also increased 5-HT_2 C-receptor gene expression in the striatum of LC-HSuc rats. Conclusively, previous separate findings that individual differences in positive affectivity and sucrose intake (hedonic response) can contribute to stress vulnerability were confirmed. Importantly, the highest vulnerability to stress was found if low positive affectivity and high sucrose consumption coincided.