Choice for response alternatives differing in reinforcement frequency in dopamine D₂ receptor mutant and Swiss-Webster mice

Rationale: A previous study showed that dopamine (DA) D₂ receptors (D₂Rs) are involved in the reinforcing effectiveness of food, but the specific involvement of DA D₂Rs in choice among food reinforcers remains unclear. Objectives: The current study used genetic and pharmacological approaches to assess the role of D₂Rs in choice among food-reinforcement frequencies using the generalized matching law (GML), which specifies that logged response and time allocation ratios vary linearly with logged reinforcer ratios. Methods: Congenic D₂R knockout (KO) and wild-type (WT) mice were exposed to concurrent variable-interval schedules of reinforcement with scheduled relative-reinforcement rates from 4:1 to 1:4. Effects of the D₂R antagonist (-)-eticlopride (0.1-1.0 mg/kg) were assessed in Swiss-Webster mice. Results: Response and time allocation ratios were related to obtained reinforcement ratios as predicted by the GML. GML fits accounted for ≥92 % of the variance in allocation ratios and did not differ in D₂R KO and WT mice. Similarly, there were no significant effects of (-)-eticlopride dose on GML fits, despite effects on overall response rates. Conclusions: The current results demonstrate that neither deletion nor acute blockade of D₂Rs affects choice among response alternatives varying in food-reinforcement frequencies. Because previously published results suggest a role of D₂Rs in choice between response alternatives differing in reinforcer magnitude and delay or magnitude and probability, the current findings suggest that D₂Rs play a role in choice only among certain parameters of reinforcement. Furthermore, these findings suggest parameters of reinforcement may only be fungible in a complex manner.