Presynaptic regulation of the electrically evoked release of endogenous dopamine from the isolated neurointermediate lobe or isolated neural lobe of the rat pituitary gland in vitro.

Isolated neurointermediate lobes (NILs) or isolated neural lobes (NLs) of the rat pituitary gland were incubated in Krebs-HEPES solution which contained pargyline and the dopamine uptake inhibitor GBR 12921. The release of endogenous dopamine was determined by HPLC with electrochemical detection. Electrical stimulation of the pituitary stalk induced a frequency-dependent release of dopamine. The release of dopamine from the combined NIL evoked by stimulation at 15 Hz was increased by 130% in the presence of the dopamine D2 receptor antagonist, (-)-sulpiride; the (+)-enantiomer of sulpiride had virtually no effect. When the stimulation frequency was 3 Hz (-)-sulpiride caused an increase in dopamine release by 230%. A similar increase was observed in the presence of domperidone, another dopamine D2 receptor antagonist. The dopamine receptor agonists, apomorphine and quinpirole, had no significant effects on the evoked release of dopamine indicating that under the present incubation conditions endogenous dopamine may have been maximally activating the autoinhibition. However, in the presence of 1 mumol/l (-)-sulpiride, apomorphine as well as quinpirole reduced the evoked release of dopamine in a concentration-dependent manner. The dopamine D1 receptor selective antagonist, SCH 23390, had no effect on the evoked release of dopamine at a concentration of 1 mumol/l. Only at a concentration of 10 mumol/l did SCH 23390 cause a small increase in dopamine release; this effect was, however, abolished in the presence of 1 mumol/l (-)-sulpiride.(ABSTRACT TRUNCATED AT 250 WORDS)