No 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.

Changes in the concentrations of dopamine (DA) and its major metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were characterized in the pituitary gland throughout the 4-day estrous cycle of the rat. Female rats were sacrificed at 2- to 3-hour intervals throughout each day of the 4-day estrous cycle. Pituitary glands were removed, and the concentrations of DA and DOPAC were determined by high-performance liquid chromatography coupled to electrochemical detection. The concentration of prolactin (PRL) in serum from these same animals was determined by radioimmunoassay. The concentration of DA in the anterior lobe was constant throughout most of the 4-day estrous cycle. Prior to initiation of the proestrous surge of PRL, there were significant (p <0.05) decreases in the concentrations of both DA and DOPAC in the anterior lobe which returned to elevated baseline levels just prior to the termination of the proestrous surge of PRL. The concentrations of DA and DOPAC in the intermediate lobe exhibited a daily rhythm. However, in the intermediate as well as in the anterior lobe, there were significant (p < 0.001) decreases in the concentrations of both DA and DOPAC, coincident with the initiation of the proestrous surge of PRL. Similarly, coincident with the peak of the proestrous surge of PRL, there were significant (p < 0.001) increases in the concentrations of DA and DOPAC in the intermediate lobe, followed by a return to basal levels and resumption of the daily rhythm. The pattern of the concentrations of DA and DOPAC in the neural lobe was also daily in nature, with peaks occurring between 13.00 and 15.00 h each day of the 4-day estrous cycle. These data, taken together: (1) confirm that a decrease of the concentrations of DA and DOPAC occurs in the anterior lobe prior to the proestrous surge of PRL; (2) reveal that DA is released in a daily pattern at intermediate and neural lobes, and (3) suggest an apparent role for DA released to the intermediate lobe in the regulation of the proestrous surge of PRL.

No abstract

This study was undertaken to assess the possibility that endothelin-1 (ET-1) and dopamine (DA) can act in concert to modulate PRL secretion. Enzymatically dispersed anterior pituitary cells obtained from random cycling female rats were perifused with Dulbecco's Modified Eagle's Medium supplemented with 0.2% BSA and 100 microM ascorbic acid. In the absence of dopamine, ET-1 (applied at 20 nM for 60 min) rapidly evoked a small transient elevation of PRL release, followed by a sustained inhibitory phase. Overnight perfusion with 500 nM DA-supplemented medium did not change the basic character of ET-1's effects on PRL secretion. Continuation of DA exposure for 48 h dramatically shifted the responsiveness of the lactotrophs to ET-1; the fast stimulatory response was robustly enhanced, whereas the inhibitory phase was replaced by a modest secondary elevation of basal PRL secretion. The stimulatory effect of ET-1 on PRL secretion after DA pretreatment was blocked by an ETA receptor antagonist, BQ-123. The effect of DA can be mimicked completely by a specific D2 receptor agonist (+/-)-2-(N-phenyl-N-propyl)amino-5-hydroxytetraline hydrochloride, whereas pretreatment with a D1 agonist, SKF-39393, failed to change the responsiveness of lactotrophs to ET-1. Our data indicate that persistent activation of D2 receptors, a condition most closely resembling the in vivo environment of the lactotrophs, uncouples the inhibitory signaling pathway from the ETA receptor while synergistically affecting signal transduction, which mediates the ET-induced stimulation of PRL secretion.