Regional mechanisms within anterior pituitary of lactating rats may regulate prolactin secretion.

Prolactin (PRL) release was compared after incubating the central and peripheral regions of the anterior pituitary of lactating rats, either nonsuckled or suckled in conditioned medium obtained by incubating medium with the same anterior pituitary regions from nonsuckled or suckled rats. To collect conditioned media, anterior pituitary regions were incubated in Earle's medium for 4 h, and conditioned medium was filtered and employed double concentrated. Each anterior pituitary was incubated in conditioned medium for 30 min. PRL released in vitro was determined by polyacrylamide gel electrophoresis. As a control, anterior pituitary regions from lactators were incubated in medium conditioned by male rat anterior pituitary regions, and they showed no changes of PRL release compared with those cultured in Earle's medium. In general, conditioned media from both anterior pituitary regions of nonsuckled and suckled rats inhibited PRL release in peripheral anterior pituitary regions, whereas PRL release was stimulated in central regions of both nonsuckled and suckled rats. A higher number of stimulatory effects was provoked by conditioned media from suckled than from nonsuckled rats, and most of these effects were from conditioned media of the peripheral region of suckled rats. Together, these results suggest the existence within anterior pituitary regions of factors that regulate PRL secretion and that their action depends on the physiologic condition of the animal.