Prolactin release from subpopulations of rat lactotrophs in the presence and absence of dopamine

Abstract

In this study we examined the effects of dopamine (DA) and its withdrawal on in vitro prolactin (PRL) release from subpopulations of lactotrophs from two regions of the anterior pituitary obtained from untreated ovariectomized (OVX) rats or OVX rats treated with estrogen, progesterone or a combination of the two. Anterior pituitaries were cut horizontally into an inner (dorsal) zone and an outer (ventral) zone. Each of these regions was enzymatically dispersed and the resulting cells were otherwise untreated (unseparated) or centrifuged through a discontinuous Percoll gradient to separate the cells into two subpopulations (light and heavy cells). Each of these types of cells was perifused for 1 h with culture medium containing 1 microM DA followed by medium devoid of DA for 1 h. Prolactin released into the perifusion medium, collected as 5-min fractions, was measured by radioimmunoassay and normalized to the number of lactotrophs in the cellular pools as determined by immunocytochemistry. In the presence of DA, PRL release from unseparated cells of the outer zone was significantly increased by estradiol treatment compared with the release from similar cells from OVX rats. (Differences were considered significant where P < 0.05.) However, no effect of estradiol treatment was observed with unseparated cells of the inner zone or light or heavy cells from either zone. Progesterone had no effect on any cell type when administered alone. However, when progesterone was given following estradiol, PRL release from unseparated cells of the inner zone was increased significantly compared with similar cells from the other steroid-treated groups. Similar significant increases were observed with light and heavy cells of the outer zone, but there was no effect of the combined steroid treatment on light or heavy cells from the inner zone. When DA was withdrawn, prolactin release was significantly increased from all cells except unseparated cells of the outer zone of OVX rat pituitaries. However, when the cells of the outer zone from OVX rats were separated into light and heavy cells, they responded to the withdrawal of DA with significant and equivalent increases in prolactin release. Light cells of the inner zone of pituitaries from OVX rats were more responsive to DA withdrawal than were heavy cells. Estradiol increased the response to the withdrawal of DA by light and heavy cells of the outer zone and heavy cells of the inner zone. Progesterone significantly reversed these effects of estradiol on separated cells. These results suggest that lactotrophs in two regions of rat pituitaries respond differently to dopamine and to its withdrawal, that subpopulations of lactotrophs within these regions also respond differently and that steroids modulate these responses.

Ovarian Steroids Modulate Responsiveness to Dopamine and Expression of G-Proteins in Lactotropes

“…These results suggest that ovarian steroids modulate the mechanisms which mediate DA-induced inhibition and stimulation of the release of PRL. When comparing our data to previous reports on steroid modulation of the lactotropes responsiveness to DA [31], it should be noted that RHPA detects the sum of the changes in PRL release over 3 h. Therefore the dynamics of PRL release cannot be determined with such a fine time resolution as is possible with perifusion. On the other hand, RHPA combined with ICC allows us to study the expression of regulatory proteins and secretory characteristics of the same cell.…”

Estrogen Regulates the Serum Level of Phosphorylated Prolactin in Mice

“…Metoclopramide is a potent dopamine antagonist that enters the central nervous system. It acts on the dopamine 2 receptor and induces an increase of PRL levels in serum [15,16]. We examined whether the levels of PPRL in the pituitary gland and serum were changed by inhibiting dopamine action using its antagonist.…”