A previous study from this laboratory showed that dopamine (DA) D2 receptors (D2Rs) are involved in the reinforcing effectiveness of different magnitudes of food reinforcement. We attempted to assess the further influence of D2Rs in choice among food-reinforcement frequencies rather than magnitudes. We used the generalized matching law (GML) to assess the role of food reinforcement frequency. The GML specifies that the ratio of responses (or the times allocated to various responses) will match the ratios of reinforcement frequencies obtained from the responses. 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. Acute elimination of D2Rs was assessed using the dopamine antagonist (-)-eticlopride. 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. The results demonstrate that neither deletion of D2Rs, nor their acute elimination by pharmacological antagonism, affects choice among response alternatives varying in food-reinforcement frequencies. Because our previous results indicate a role of D2Rs in choice between response alternatives differing in reinforcer magnitude, the current findings suggest that D2Rs play a role in choice only among certain parameters of reinforcement. Further, these findings suggest that different parameters of reinforcement (magnitude, rate) may only be fungible in a complex manner.
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