Our previous studies have demonstrated that administration of quinpirole (LY171555), a potent and highly selective dopamine (DA) D2 receptor agonist, to conscious Sprague-Dawley rats produces increases in arterial pressure through the activation of sympathetic outflow and vasopressinergic activity. To test the hypotheses that quinpirole inhibits in vivo release of DA from central dopaminergic neurons by activation of DA receptors in the central nervous system (CNS) and that this mechanism may be altered in the desoxycorticosterone acetate (DOCA)/NaCl model of hypertension, we examined the effects of quinpirole on stores of DA and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in brain regions of 4-week DOCA/NaCl-hypertensive rats and their normotensive controls. Levels of DA and DOPAC were measured in brain regions by HPLC 15 min after the i.v. administration of quinpirole (1 mg/kg). Quinpirole resulted in a significant increase in DA stores and decrease in DOPAC stores in most brain regions examined in both DOCA/NaCl-hypertensive rats and normotensive controls, presumably by inhibiting DA release through a presynaptic mechanism. In the vehicle-treated groups, DA stores in the anterior hypothalamus and DOPAC stores in the nucleus accumbens were lower in DOCA/NaCl-hypertensive rats than in H2O controls. Following quinpirole administration, DA stores in the anterior hypothalamus increased significantly in DOCA/NaCl-treated rats but not in H2O controls and DOPAC stores in the nucleus accumbens decreased significantly in H2O control rats but not in DOCA/NaCl-treated rats. These observations provide further evidence for the presence of inhibitory DA D2 receptors which modulate the activity of dopaminergic neurons in the CNS. This regulatory mechanism appears to be altered in the DOCA/NaCl model of hypertension. The increased responsiveness to quinpirole in the anterior hypothalamus and nucleus accumbens of DOCA/NaCl-treated rats could represent upregulation of central DA D2 receptors as a consequence of reduced central dopaminergic activity, as previously found in this model of hypertension. © 1987.
