Behavioural sensitization to a dopamine agonist is associated with reversal of stress-induced anhedonia

Chronic exposure to very mild unpredictable stress (CMS) has previously been found to reduce the consumption of palatable sweet solutions and to impair place preference conditioning; evidence has been presented that these effects may reflect a dysfunction of the mesolimbic dopamine system. In the present study, rats were subjected to CMS for a total of 9 weeks. CMS reduced the consumption of a 1% sucrose solution. During weeks 6 and 7, animals received quinpirole (0-400 micrograms/kg) twice weekly. Both CMS-treated animals and controls showed sensitization to the locomotor stimulant effects of quinpirole. Subsequently, a sustained recovery of sucrose drinking was observed in quinpirole-treated stressed animals. During week 8, all animals received a single pair of place preference conditioning trials, in which quinpirole (200 micrograms/kg) was administered in a distinctive environment, and vehicle in a different environment. Non-stressed animals showed an increase in preference for the environment associated with quinpirole, as did stressed animals that had been sensitized to quinpirole, this effect was absent in untreated stressed animals. Finally, in week 9, acute administration of raclopride (150 micrograms/kg) was found to reverse the recovery of sucrose drinking in quinpirole-treated stressed animals, suggesting that these effects are mediated by an increase in dopamine function.