D-2 selective receptor antagonists suppress sucrose sham feeding in the rat

Dopamine receptor blockade by pimozide has previously been shown to reduce the intake of pellets and of a sweet solution by intact, food-deprived rats and to reduce the sham intake of sucrose solutions by food-deprived rats with an open chronic gastric fistula. To determine if this effect was due to the blockade of the D-2 subclass of dopamine receptors, we investigated the effects of the selective D-2 receptor antagonists sultopride and (-)-sulpiride as well as the effects of the preferential D-2 receptor antagonists, haloperidol and pimozide, on sucrose sham feeding. All four neuroleptics administered intraperitoneally decreased the sham intake of a 10% sucrose solution significantly. The relative inhibitory potencies of these drugs for decreasing the sham intake of 10% sucrose were similar to their relative potencies for binding striatal D-2 receptors in vitro, namely HALOPERIDOL greater than or equal to PIMOZIDE much greater than SULTOPRIDE greater than or equal to (-)-SULPIRIDE. Additional evidence for the specificity of these effects was that the less active isomer (+)-sulpiride did not inhibit sham feeding of sucrose and that a dose of pimozide and of haloperidol which decreased sham feeding of sucrose after food deprivation did not decrease sham drinking of water after water deprivation. Taken together, these data support the hypothesis that dopaminergic activation of a central D-2 receptor mechanism is necessary for the positive reinforcing effect of sucrose that maintains sham feeding.