Decrease in hypothalamic epinephrine concentration and other neurochemical changes produced by quinpirole, a dopamine agonist, in rats.

Quinpirole, (4 aR-trans)-4, 4a, 5, 6, 7, 8, 8a, 9-octahydro-5-propyl-1 H-pyrazolo [3, 4-g]quinoline, is a dopamine agonist selective for the D2 subtype of dopamine receptors. In rats, quinpirole at doses of 0.3 mg/kg i.p. and higher decreased hypothalamic epinephrine concentrations. The doses required for this effect are only slightly higher than the minimum doses that decreased the concentration of dopamine metabolites in cerebral hemispheres. At higher doses of quinpirole (2-3 mg/kg i.p.), dopamine concentration was increased and norepinephrine concentration was decreased in hypothalamus, and MHPG sulfate (the norepinephrine metabolite) concentration was increased in brain stem and in hypothalamus. All of these neurochemical effects of quinpirole were blocked by pretreatment with spiperone, a dopamine antagonist. The effects were not produced by SKF 38393, a selective D1 agonist, or by the dextrorotatory enantiomer of quinpirole, which lacks D2 agonist activity. The data support the interpretation that quinpirole, by activating D2 receptors, results in a decrease in dopamine metabolites, a decrease in hypothalamic epinephrine concentration, and an increased conversion of norepinephrine to MHPG sulfate in rat brain probably through enhanced norepinephrine release.