Rapid effects of hyperprolactinemia on basal prolactin secretion and dopamine turnover in the medial and lateral median eminence.

Subcutaneous injections of ovine PRL (oPRL; 4 mg/kg) were used to study the negative feedback of PRL on its own secretion in the adult male rat. A single injection of oPRL significantly suppressed the endogenous secretion of rat PRL within 3-4 h, an effect that persisted until the oPRL was substantially cleared from the circulation some 4-6 h later. In rats injected every 8 h, rat PRL levels were suppressed for 48 h, while LH titers increased significantly at some time points, and FSH levels varied in the same direction as LH. LHRH concentrations in 10 brain structures containing cell bodies, axons, and terminal boutons were not affected by 48 h of oPRL treatment. Dopamine turnover in both medial and lateral aspects of the median eminence increased as early as 2 h after the first oPRL injection and remained elevated after 10 and 26 h of oPRL exposure. The results are consistent with the hypothesis that the tuberoinfundibular dopamine neurons mediate the negative feedback action of PRL on its own secretion. Further, under these experimental conditions, neurons projecting to both medial and lateral aspects of the median eminence are equally sensitive to elevated PRL levels.