Naltrexone, dopamine receptor agonists and antagonists, and food intake in rats: 1. Food deprivation

Different forms of food intake are reduced by both agonists and antagonists of dopamine D₁ and D₂ receptors as well as general opioid antagonists. The present study evaluated whether deprivation (24 h)-induced food intake was altered following systemic administration of either the D₁ agonist, SKF-38393, the D₁ antagonist, SCH-23390, the D₂ agonist, quinpirole, or the D₂ antagonist, haloperidol, alone or in combination with the general opioid antagonist, naltrexone. Both SKF-38393 (5-10 mg/kg) and SCH-23390 (100-200 μg/kg) significantly and dose dependently reduced deprivation-induced intake. Whereas quinpirole (0.5-1 mg/kg) failed to alter deprivation-induced intake, haloperidol increased deprivation-induced intake at low (50 μg) doses and decreased intake at higher (100-500 μg/kg) doses. Naltrexone (2.5-10 mg/kg) significantly inhibited deprivation-induced intake. When naltrexone was paired with behaviorally ineffective doses of either SCH-23390 (2.5-100 μg/kg), quinpirole (0.01-1 mg/kg), or haloperidol (50 μg/kg), the degree of reduction of deprivation-induced intake was significantly greater than that produced by naltrexone alone. Pairing naltrexone with SKF-38393 produced reductions of deprivation-induced intake comparable to that of naltrexone alone.