Lack of autoreceptor mediated regulation of the spontaneous dopamine turnover in the isolated neurointermediate lobe of the rat pituitary gland in vitro.

Isolated neurointermediate lobes of the rat pituitary gland were incubated in Krebs-HEPES solution and the spontaneous outflow of endogenous dopamine and its metabolites (DOPAC, HVA and MOPET) was determined by HPLC with electrochemical detection. The spontaneous outflow of dopamine metabolites (about 1500 fmol/10 min) largely exceeded that of dopamine (about 60 fmol/10 min). Apomorphine concentration-dependently (IC50, 205 nmol/l) reduced the spontaneous outflow of the dopamine metabolites. The effect of apomorphine developed slowly and was progressive over an observation period of 70 min. After 1 h of exposure to a maximall effective concentration of apomorphine (10 mumol/l), the outflow of metabolites was inhibited by 43%. The effect of apomorphine was not affected by the dopamine D2 receptor antagonist (-)-sulpiride nor by the dopamine D1 receptor antagonist SCH 23390. Neither quinpirole nor fenoldopam significantly affected the spontaneous outflow of dopamine metabolites. It was previously shown that the high rate of spontaneous outflow of dopamine metabolites from the dopaminergic nerves in the neurointermediate lobe reflects largely the immediate catabolism of newly synthesized dopamine. This high rate of spontaneous dopamine synthesis in the neurointermediate lobe is not controlled by dopamine autoreceptors. Apomorphine appears to inhibit the spontaneous dopamine turnover by an inhibition not mediated by dopamine receptors.