Intracranial self-stimulation and sucrose intake differ as hedonic measures following chronic mild stress: interstrain and interindividual differences

The present study was designed to assess the utility of sucrose intake and intracranial self-stimulation (ICSS) as hedonic measures for chronic mild s tress (CMS) induced behavioural deficits. Wistar and PVG hooded rats were e xposed to a variety of mild stressors, e.g. periods of food and/or water de privation, soiled cage, light/dark reversal, confinement to small cages and pairing, during 6-9 weeks. The intake of 1% sucrose solution was significa ntly reduced in stressed PVG hooded rats compared to control animals. The s ucrose intake in stressed Wistar rats remained unaltered, indicating that C MS-induced decreases in sucrose intake are strain dependent. However, sucro se intake has in our experience been shown to be unreliable as the observed decreases following CMS were inconsistent over time. ICSS behaviour was ev aluated from rate/frequency functions by determining the frequency that sup ported 50% of maximal response rate. Neither the Wistar nor the PVG hooded rats showed an overall decrease in ICSS behaviour following CMS. However, t he ICSS measures revealed interindividual differences in both rat strains. In the stress groups a subgroup (14 +/- 2.4%) of rats progressively exhibit ed an attenuated ICSS behaviour. These findings may reflect the interindivi dual variability observed in humans as stress does not invariably lead to d epression. The model may in its present form be used to study the pathophys iology of depressive disorders. However, the utility of the CMS model to st udy antidepressant drug actions has to be questioned. Our results show ther e is a need for rat strains in which there is a greater sensitivity for det ecting stress effects. It emphasises the fact that replication of CMS-induc ed decreases in ICSS behaviour can be as problematic as inducing decreases in sucrose intake. (C) 2000 Elsevier Science B.V. All rights reserved.