Effect of D2 dopamine agonists on tuberoinfundibular dopamine neurons

The effects of piribedil and the selective D2 dopamine agonists, quinpirole and quinelorane, on the synthesis and metabolism of dopamine, within tuberoinfundibular neurons, were studied. The synthesis and metabolism of dopamine within these hypothalamic neurons were assessed by measuring the accumulation of DOPA after inhibition of DOPA decarboxylase and the concentration of DOPAC in the median eminence. Quinpirole (0.1-2.5 mg/kg, i.p.) produced a dose-related increase in accumulation of DOPA and concentrations of DOPAC in the median eminence. The increased accumulation of DOPA after administration of quinpirole was evident for at least 4 hr. The accumulation of DOPA in the median eminence also was enhanced after the administration of quinelorane (0.025 mg/kg, i.p.) and piribedil (50 mg/kg, i.p.). The stimulatory effect of quinpirole on accumulation of DOPA in the median eminence was antagonized by haloperidol (1 mg/kg, i.p.) but not by SCH 23390 (0.5 mg/kg, i.p.). Although D2 agonists have been shown to acutely suppress the synthesis and metabolism of dopamine in nigrostriatal and mesocorticolimbic dopamine neurons, it is apparent that activation of D2 receptors enhanced the synthesis and metabolism of dopamine within tuberoinfundibular neurons in the hypothalamus.