Dopamine D₂ receptors and ingestive behavior: brainstem mediates inhibition of intraoral intake and accumbens mediates aversive taste behavior in male rats

Abstract.

Rationale: One of the factors that terminate the ingestion of an intraorally infused solution of sucrose may be an increase in the perceived aversiveness of its taste. Objectives: We tested the hypothesis that dopamine D₂, as opposed to D₁, receptors in the brainstem or nucleus accumbens inhibit intraoral intake by enhancing the aversiveness of the taste of the infused solution. Methods: Male rats were infused intraorally with a 2 M sucrose solution (1 ml/min) and intake and the display of gapes and chin rubs, i.e. taste-related aversive behavior, was measured. Gapes and chin rubs were also measured in rats during and 40 s after brief intraoral infusion (1 ml/min during 20 s) of a 0.3 mM solution of quinine HCl. The full D₁ receptor agonist dihydrexidine (0.1–3.0 mg/kg) and antagonist SCH-23390 (0.03–0.1 mg/kg), the D₂ receptor agonist quinpirole (0.3 mg/kg) and antagonist raclopride (1.7 mg/kg) were injected IP. Quinpirole (14–55 µg) and raclopride (5 µg) were also infused into the fourth brain ventricle. In addition, quinpirole (2 or 10 µg) was infused into the shell region of the nucleus accumbens. Results: IP dihydrexidine and quinpirole inhibited the intraoral intake of sucrose and pretreatment with raclopride, but (in the case of dihydrexidine) not SCH-23390, attenuated this effect. Injection of quinpirole into the fourth ventricle produced raclopride-reversible inhibition of intraoral intake but did not stimulate the display of gapes and chin rubs. Infusion of quinpirole into the shell region of the nucleus accumbens had the opposite effects. The intake of sucrose was suppressed by the addition of quinine HCl but this suppression was unaffected by dopamine agonist or antagonist treatment. Conclusions: It is suggested that brainstem dopamine D₂ receptors mediate suppression of consummatory ingestive behavior and that D₂ receptors in the shell region of the nucleus accumbens mediate the display of gapes and chin rubs, but that neither of these D₂ receptor populations mediate the hedonic evaluation of taste.