Chronic mild stress has no effect on responding by rats for sucrose under a progressive ratio schedule

Exposure of rats to chronic unpredictable mild stress (CMS) has been shown to produce a syndrome in which a wide range of consummatory behaviors are attenuated, resembling a state of anhedonia, which may be reversed by treatment with antidepressant drugs. The aim of the present study was to determine whether CMS would also affect a rat's motivation to respond for a sucrose solution, as assessed by its performance under a progressive ratio (PR) schedule of reinforcement. Control studies demonstrated that break points in nonstressed rats were sensitive to both the concentration of sucrose solution used, as well as the period of food and water deprivation used prior to testing. Exposure of rats to CMS had no effect upon break points when responding under a progressive ratio schedule for either a 1% or 7% sucrose solution, although subjects did display the typical reduction in consumption of a freely consumed 1% sucrose solution. These results are not readily understood within the theoretical framework of the CMS model of anhedonia and imply instead that both the neural and psychological correlates of motivation may be less susceptible to modulation by the effects of CMS than the free consumption of sweet solutions.