Suppression or facilitation of operant behaviour by raclopride dependent on concentration of sucrose reward

Rats responded under continuous reinforcement for 1%, 10% or 95% sucrose pellets, under food deprived or ad libitum access conditions. In both cases responding was highest for 10% sucrose reinforcement, and a small proportion of 95% sucrose was not consumed. Ad libitum food access reduced response rates for all sucrose concentrations. Responding for 10% and 95% sucrose pellets followed a parallel time-course; and accumulation of 95% sucrose pellets was immediate and nonprogressive. Extinction following availability of 95% sucrose pellets caused an increase in response rate, but removal of 10% sucrose led only to a decline in responding. Under both food deprived and non-deprived conditions, the dopamine D-2 antagonist raclopride dose-dependently decreased responding for 1% or 10% sucrose, but increased responding for, and consumption of, 95% sucrose reward. After eight sessions of responding under extinction conditions, the presentation of reward-associated cues increased response rate early in the session. Raclopride had no effect during this period, but decreased responding later in the session. We consider the implications of these results for theories of neuroleptic drug action.