Compulsive-like effects of quinpirole on drinking behavior in rats are inhibited by substituting ethanol for water

We have previously reported that in rats given the choice between operant and free access to water (contrafreeloading: CFL), repeated administrations of quinpirole, a D2/D3 dopamine receptor agonist, shifted the animals towards the operant access and inhibited water intake. The purpose of the present study was to investigate the influence of substituting different concentrations of ethanol (2, 4, 6%) for water on the effects of repeated daily administrations of vehicle or quinpirole (0.5 mg/kg i.p.) in rats that for 6 days were given access to the fluid according to an FR3 schedule of reinforcement and for the following 9 days were given the choice between operant and free access to the fluid. On the first day quinpirole completely suppressed operant behavior, which however progressively increased in the subsequent sessions, approaching control levels by day 6. Ethanol presentation did not alter these effects of quinpirole. When the resource was also freely available, quinpirole produced the expected shift from free to operant access to water (CFL). Substituting ethanol for water resulted in a concentration-related reduction of the over-responding and, consequently, of CFL induced by quinpirole. In vehicle-injected subjects ethanol did not affect responding and only marginally reduced fluid intake. Thus, ethanol appears to prevent perseveration in performing needless instrumental behavior induced by repeated activation of D2/D3 receptors.