The effect of dopamine on thyrotropin-releasing hormone-induced prolactin secretion in vitro

The direct effects on PRL release of acute changes in dopamine (DA) and TRH concentrations were measured in an in vitro perifusion system. Hemisected anterior pituitaries of lactating rats were perifused with medium that received a coinfusion of DA at 20 ng/ml. These tissues released PRL at 35% of the release rate of controls in the absence of DA. Interruption of the DA coinfusion for 9 min caused a 2-fold increase in PRL release, which was resuppressed when the DA treatment was resumed. During continuous Da exposure, TRH administration (10 ng/ml for 12 min) induced a gradual but slight increase in PRL release. However, when this TRH treatment was administered immediately after the end of the DA interruption, it evoked an immediate 2-fold increase in PRL release to 4 times the initial release rate in the presence of DA. This pronounced effect of TRH after the brief DA interruption was also observed when an 18 min interval was imposed between the two manipulations. During continuous coinfusion of DA at 100 ng/ml, TRH was totally ineffective in eliciting PRL release. However, even after this DA treatment had been interrupted briefly and an increase in PRL release had been evoked, TRH still was not an effective stimulus for PRL release. T was imposed between the two manipulations. During continuous coinfusion of DA at 100 ng/ml, TRH was totally ineffective in eliciting PRL release. However, even after this DA treatment had been interrupted briefly and an increase in PRL release had been evoked, TRH still was not an effective stimulus for PRL release. T was imposed between the two manipulations. During continuous coinfusion of DA at 100 ng/ml, TRH was totally ineffective in eliciting PRL release. However, even after this DA treatment had been interrupted briefly and an increase in PRL release had been evoked, TRH still was not an effective stimulus for PRL release. These data indicate that DA not only can serve as a PRL-inhibiting factor for tonic release of PRL but also may determine by its presence or brief absence, and concentration whether acute release occurs in the presence of a PRL-releasing factor. The direct effect of DA on PRL release and its interference with the action of a PRL-releasing factor appear to be independent of each other.