Negative regulation by dexamethasone of the potentiation of neuromedin C-induced growth hormone and prolactin release by estradiol in anterior pituitary cell aggregates

Stimulation of growth hormone release by 5-hydroxytryptamine (5-HT) in cultured rat anterior pituitary cell aggregates: evidence for mediation by 5-HT2B, 5-HT7, 5-HT1B, and ketanserin-sensitive receptors

5-Hydroxytryptamine (5-HT) promotes the release of GH by a hypothalamic site of action. The present study explores a putative pituitary action in a perifused rat anterior pituitary aggregate cell culture system. In aggregates cultured with 1 nM estradiol for expression of the 5-HT4, -5, and -6 receptor (R), 5-HT promptly stimulated GH secretion with a dose dependency between 1 and 10 nM. The effect of 5-HT was partially blocked by methiothepin and methysergide; by SB-206553, a 5-HTR2B/C antagonist; SB-269970, a 5-HTR7/5A antagonist; and SB-224289, a 5-HTR1B antagonist. The GH response was fully blocked by combined administration of SB-206553+SB-269970 and SB-206553+ketanserin but not by SB-206553+spiperone. Culturing the aggregates without estradiol diminished the magnitude of the GH response to 5-HT as well as the impact of 5-HTR7/5 blockade on the response. Basal GH release was stimulated by the 5-HTR2 agonists 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, m-chlorophenyl piperazine, and alpha-methyl 5-HT; 5-carboxytryptamine (agonist at 5-HTR1, -5, and -7); tryptamine (preferential 5-HTR7 agonist); and the selective 5-HTR1B agonist CP93129 but not the 5-HTR1A agonists 7-(dipropylamino)tetralin-1-ol-8-hydroxy-2-(di-n-propylamino)tetralin and the 5-HTR1B/D agonist sumatriptan. The selective 5-HTR2B agonist BW 723C86 stimulated GH release, and the selective 5-HTR2B antagonist SB-204741 attenuated the GH response to 5-HT. The present data suggest that 5-HT may release GH through a pituitary site of action, and that the 5-HTR2B, 5-HTR7 and 5-HTR1B mediate this response, with possibly an inhibitory component of the 5-HTR1D. The relative contribution of these receptors may be modulated by estrogen.

Signal transduction mechanisms mediating rapid, nongenomic effects of cortisol on prolactin release

While the mechanisms governing genomically mediated glucocorticoid actions are becoming increasingly understood, relatively little is known with regard to the cell signaling pathways that transduce rapid glucocorticoid actions. Studies of the cultured tilapia rostral pars distalis (RPD), a naturally segregated region of the fish pituitary gland that contains a 95-99% pure population of prolactin (PRL) cells and is easily dissected and maintained in a completely defined, serum-free media, indicate that physiological concentrations of cortisol rapidly inhibit PRL release. The attenuative action of cortisol on PRL release occurs within 10-20 min, is insensitive to the protein synthesis inhibitor, cycloheximide, and mimicked by its membrane impermeable analog, cortisol-21 hemisuccinate-conjugated bovine serum albumin (BSA). Cortisol and somatostatin, a peptide known to work through membrane receptors to inhibit PRL release, rapidly and reversibly reduces intracellular free Ca(2+) (Ca(i)(2+)), and inhibits 45Ca(2+) influx and BAYK-8644 induced PRL release. Preliminary investigations show cortisol, but not somatostatin, suppresses phospholipase C (PLC) activity in PRL cell membrane preparations. In addition, cortisol and somatostatin reduce intracellular cAMP and membrane adenylyl cyclase activity. These findings indicate that the acute inhibitory effects of cortisol on PRL release occur through a nongenomic mechanism involving interactions with the plasma membrane and inhibition of both the Ca(2+) and cAMP signal transduction pathways. Cortisol may reduce Ca(i)(2+) by inhibiting influx through L-type voltage-gated channels and possibly release through a PLC/inositol triphosphate sensitive intracellular Ca(2+) pool. In addition, it is also likely the steroid inhibits adenylyl cyclase activity in events leading to reduced cAMP production and the subsequent release of PRL.

Steroid effects on secretion from subsets of lactotrophs: role of folliculo-stellate cells and annexin 1

Prolactin secretion is controlled by the hypothalamus, and by circulating steroids; oestrogens stimulate, but glucocorticoids inhibit prolactin release. Lactotrophs express intracellular receptors for oestrogens, but apparently not glucocorticoids. Therefore, a genomic effect of oestrogens could be direct, but that of glucocorticoids appears to be indirect. Lactotrophs are not a homogeneous cell population: some have large irregular dense-cored vesicles, others have small round vesicles, but the functional significance of this inhomogeneity is far from clear. Oestradiol and testosterone can stimulate rapid release of prolactin selectively from type II lactotrophs characterised by small round vesicles. Progesterone and other steroids do not exert this effect, which results from a non-genomic action of oestradiol and testosterone. Glucocorticoid inhibition of secretagogue-induced prolactin secretion is mimicked by annexin 1 (lipocortin 1), a protein induced by glucocorticoids in the pituitary and many other tissues, and can be blocked by annexin 1 immunoneutralisation and antisense. Glucocorticoid inhibition of ACTH and growth hormone secretion also involves annexin 1. Pituitary annexin 1 is located in folliculo-stellate cells; these express glucocorticoid receptors, and glucocorticoids induce annexin-1 synthesis. Annexin 1 is externalised from folliculo-stellate cells in response to glucocorticoids, despite the fact that it lacks a secretory signal sequence and is not packaged in vesicles. Inhibition of annexin 1 externalisation by glyburide suggests involvement of an ABC (ATP-binding cassette) transporter in externalisation. Both oestradiol and glucocorticoids therefore influence the secretion of prolactin by novel direct and indirect mechanisms, in addition to their much better understood effects on transcription via classical intracellular steroid receptors.

Effect of adrenalectomy and dexamethasone treatment on prolactin secretion of lactating rats

The contribution of corticosteroids to the control of prolactin secretion in lactating rats was investigated. The prolactin response to domperidone (20 microg/kg b.w., i.v.), a dopamine receptor antagonist and to domperidone plus formalin stress was tested in adrenalectomized and/or dexamethasone-treated continuously nursing rats. Animals were adrenalectomized on the 3rd day of lactation and tested on the 7th day of lactation. Dexamethasone was injected s.c. 24 h before testing (400 microg/kg b.w.) and on the day of testing (200 microg/kg b.w.). Domperidone caused a significant rise in plasma prolactin levels. The prolactin response to domperidone was twice as high in solely adrenalectomized dams and in solely dexamethasone-treated rats compared to controls. In adrenalectomized animals treated with dexamethasone, the prolactin response to domperidone was like in controls. Formalin injection to either adrenalectomized plus domperidone-treated animals or to animals injected with dexamethasone plus domperidone, resulted in a statistically significant depletion of plasma prolactin. In controls and in adrenalectomized animals receiving dexamethasone and domperidone, the prolactin response to formalin was very similar, i.e., plasma prolactin levels did not change after the administration of formalin. The present findings suggest that in lactating rats, corticosteroids are involved in the prolactin response to domperidone and to formalin stress.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Annexin 1 (lipocortin 1) mediates the glucocorticoid inhibition of cyclic adenosine 3',5'-monophosphate-stimulated prolactin secretion

Our previous studies have identified a role for annexin 1 (also called lipocortin 1) in the regulatory actions of glucocorticoids (GCs) on the release of PRL from the rat anterior pituitary gland. In the present study we used antisense and immunoneutralization strategies to extend this work. Exposure of rat anterior pituitary tissue to corticosterone (1 nM) or dexamethasone (100 nM) in vitro induced 1) de novo annexin 1 synthesis and 2) translocation of the protein from intracellular to pericellular sites. Both responses were prevented by the inclusion in the medium of an annexin 1 antisense oligodeoxynucleotide (ODN; 50 nM), but not by the corresponding sense and scrambled ODN sequences. Unlike the GCs, 17beta-estradiol, testosterone, and aldosterone (1 nM) had no effect on either the synthesis or the cellular disposition of annexin 1; moreover, none of the steroids or ODNs tested influenced the expression of annexin 5, a protein closely related to annexin 1. The increases in PRL release induced in vitro by drugs that signal via cAMP/protein kinase A [vasoactive intestinal polypeptide (10 nM), forskolin (100 microM), 8-bromo-cAMP (0.1 microM)] or phospholipase C (TRH, 10 nM) were attenuated by preincubation of the pituitary tissue with either corticosterone (1 nM) or dexamethasone (100 nM). The inhibitory actions of the steroids on the secretory responses to vasoactive intestinal polypeptide, forskolin, and 8-bromo-cAMP were specifically quenched by inclusion in the medium of the annexin 1 antisense ODN (50 nM) or a neutralizing antiannexin 1 monoclonal antibody (antiannexin 1 mAb, diluted 1:15,000). By contrast, the ability of the GCs to suppress the TRH-induced increase in PRL release was unaffected by both the annexin 1 antisense ODN and the antiannexin 1 mAb. In vivo, interleukin-1beta (10 ng, intracerebroventricularly) produced a significant increase in the serum PRL concentration (P < 0.01), which was prevented by pretreatment of the rats with corticosterone (100 microg/100 g BW, sc). The inhibitory actions of the steroid were specifically abrogated by peripheral administration of an antiannexin 1 antiserum (200 microl, sc); by contrast, when the antiserum was given centrally (3 microl, intracerebroventricularly), it was without effect. These results support our premise that annexin contributes to the regulatory actions of GCs on PRL secretion and suggest that it acts at point distal to the formation of cAMP.

Bombesin regulation of adrenocorticotropin release from ovine anterior pituitary cells

Mammalian members of the bombesin-like peptide family (gastrin releasing peptides; GRP) have been localized in the ovine median eminence and in hypophysial-portal blood, suggesting a role in the regulation of anterior pituitary function. In this study we have shown that although bombesin cannot stimulate ACTH secretion alone, it potentiates release by ovine CRF, an effect blocked by the GRP receptor antagonist D-Tyr6bombesin (6-13) propylamide. Bombesin did not potentiate AVP-stimulated ACTH release; instead release was attenuated when bombesin was given at a 10-fold or greater molar excess over AVP, with no interaction seen at lower concentrations. We conclude that ovine corticotrophs express bombesin receptors, and that GRP may act in concert with other hypothalamic releasing factors to regulate ACTH secretion.

Characterization of a rat anterior pituitary cell bioassay

We have described the protocols and characterization of a pituicyte culture, which became established as a reliable and reproducible bioassay for the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The bioassay was used to measure the bioactivity of factors that inhibit and stimulate gonadotrophin secretion. The protocol that was used involved the culling of female Wistar rats (200 to 250 g weight), at random stages of their cycle, and dispersal of their pituicytes in a concentration of 0.4 x 10(6) cells.ml-1.well-1 in serum-free medium (Dulbecco's modified Eagle's medium/Ham's F12 mixture, supplemented with insulin and transferrin) in Falcon 3047 24-well culture plates. After 24 h of pre-culture, the medium was changed and the cells cultured for a further 48 h. The supernatant was removed and assayed for basal secretion of FSH and LH. The cells were then stimulated with 10(-8) M GnRH for 4 h and the supernatant assayed for gonadotrophin-releasing hormone (GnRH)-stimulated FSH and LH secretion. All samples were assayed as pairs of duplicates (i.e. quadruplicate samples) which were randomly added to the plates to minimize plate effects. Random number tables were used to achieve this randomization.

Autoradiographic demonstration of 125I-Tyr4-bombesin binding sites on rat anterior pituitary cells

Because several direct effects of bombesin related peptides on pituitary hormone release have been demonstrated, we chose to study the presence of bombesin binding sites in the adult male rat anterior pituitary in aggregate cell culture by autoradiographic localization of 125I-tyrosine4-bombesin (125I-Tyr4-BBN) binding and immunocytochemical localization of the anterior pituitary hormones. When aggregates were cultured in medium without hormonal supplements, the number of cells with detectable 125I-Tyr4-BBN binding was below 1%. In cell aggregates cultured in the presence of 1 nM estradiol (E2) 125I-Tyr4-BBN binding was detected on 5.4 +/- 0.8% of the cells after redispersion and on 5.8 +/- 1.1% of the cells in sections of paraffin embedded aggregates. The binding cell types were mainly lactotrophs and somatotrophs. The binding of 125I-Tyr4-BBN (3 or 5 nM) was specific because it was inhibited by the addition of an excess of unlabelled Tyr4-BBN or the bombesin receptor antagonist L686,095. In aggregates cultured in the presence of 1 nM E2 and 4 nM dex, the percentage of cells with detectable 125I-Tyr4-BBN binding was significantly lower than that in aggregates cultured in the presence of 1 nM E2 alone. Binding on somatotrophes almost completely disappeared. The present data show that specific 125I-Tyr4-BBN binding sites are present on anterior pituitary cells, are detectable mainly on a small subpopulation of lactotrophs and somatotrophs, and are affected by hormonal conditions.(ABSTRACT TRUNCATED AT 250 WORDS)