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

Abstract

The effects of chronic ovarian steroid treatment on the secretory activity of individual lactotropes and the mechanisms modulating their responsiveness to dopamine (DA) were studied. Female rats were ovariectomized (OVX) and implanted with Silastic capsules containing progesterone (P₄), 17β), 17β-estradiol (E₂) or both E₂ (E₂+P₄). Ten days after surgery, anterior pituitaries were enzymatically dispersed and the reverse hemolytic plaque assay (RHPA) was performed to assess the release of prolactin (PRL) from individual lactotropes. RHPA was combined with immunocytochemistry (ICC) for PRL, G_αs or G_iα3/G_αo proteins. E₂ treatment alone or in combination with P₄ increased the percentage of immunoreactive lactotropes among anterior pituitary cells. Incidence of active (plaque-forming) lactotropes, however, was increased both in P₄-, and E₂-treated rats and E₂+P₄ treatment increased it even further. While P₄ treatment did not affect the frequency distribution of lactotropes, both E₂ and E₂+P₄ treatments increased the large plaque-forming lactotrope population. This increase was reflected by the significantly greater mean plaque areas of lactotropes from E₂- and E₂+P₄-treated rats compared to OVX or P₄-treated animals. The responsiveness of lactotropes to DA from P₄-treated rats did not differ from that of OVX rats: thus challenge with 1 µM DA inhibited the release of PRL, while 100 pM DA had no effect. E₂ and E₂+P₄ treatments, however, profoundly changed the lactotrope’s responsiveness: challenge with 1 µM DA had no effect and 100 pM DA resulted in moderate stimulation of PRL release in E₂+P₄ rats. Double-label ICC revealed that ovarian steroid treatments did not affect the expression of G_αs in lactotropes. The incidence of G_iα3/G_αo-immunoreactive lactotropes, however, decreased after E₂ treatment, alone or in combination with P₄. Although expression of G_αs was similar in all plaque-forming cells regardless of plaque size, lactotropes expressing G_iα3/G_αo were more likely to form small plaques in all treatment groups. These data suggest: (1) ovarian steroid treatment recruits quiescent lactotropes to release PRL; (2) E₂ treatment alone or in combination with P₄ increases the amount of PRL rleased by individual lactotropes; (3) E₂-induced alterations in the frequency distribution and lactotrope responsiveness to DA may be due in part to a decreased expression of G_iα3/G_αo.

Estrogen inhibits D2S receptor-regulated Gi3 and Gs protein interactions to stimulate prolactin production and cell proliferation in lactotropic cells

“…We have recently shown that the pituitary levels of specific groups of G proteins (Gi3 and Gs) are highly responsive to E 2 , whereas Gq11 and other G proteins (Gi1 and Gi2) are insensitive or very moderately responsive to E 2 (Chaturvedi & Sarkar 2008). E 2 stimulatory effect on Gs and inhibitory effect on Gi3 expression have also been demonstrated in lactotropes (Livingstone et al 1998). However, the contributions of each D2 receptor isoform in regulation of basal and E 2 modulation of G proteins have not been determined.…”

Trajectories of brain development: point of vulnerability or window of opportunity?

“…Gonadal hormones influence behavior directly for reproduction [1] or indirectly via their interactions with neurotransmitter systems [22,24,152]. Gonadal hormones modulate neurotransmitter effectiveness via membranebound estrogen receptors [152,155,214], which exert their effects non-genomically to alter G protein activity [52,145,231]. It is at this stage that the functional expression of an earlier insult emerges as hormonal changes influence the pharmacological sensitivity in such a way so as to unmask any deficit.…”

Genesis of Prolactinomas: Studies Using Estrogen-Treated Animals

“…The steroid decreases the potency of dopamine to inhibit PRL release by uncoupling the D2 receptors with Gαi/αo. The uncoupling of the D2 receptor might be the result of the phosphorylation of Gαi/αo [112] and downregulation of Gαi/αo protein synthesis [113]. Estrogen also regulates the expression of a splicing factor thus favoring the production of the D2L [114].…”