Electrical activity of the hypothalamus effects of intraventricular catecholamines.

The injection of epinephrine into the third ventricle of the rat brain causes a biphasic elevation and depression in the integrated multiple-unit electrical activity of the median eminence. Activity in the arcuate nucleus decreases after the injection of the catecholamines. These changes in the integrated multiple-unit electrical activity may be related to the secretion of hormones by the anterior pituitary gland.

Immortalization of hypothalamic GnRH neurons by genetically targeted tumorigenesis.

By genetically targeting tumorigenesis to specific hypothalamic neurons in transgenic mice using the promoter region of the gonadotropin-releasing hormone (GnRH) gene to express the SV40 T-antigen oncogene, we have produced neuronal tumors and developed clonal, differentiated, neurosecretory cell lines. These cells extend neurites, express the endogenous mouse GnRH mRNA, release GnRH in response to depolarization, have regulatable fast Na+ channels found in neurons, and express neuronal, but not glial, cell markers. These immortalized cells will provide an invaluable model system for study of hypothalamic neurosecretory neurons that regulate reproduction. Significantly, their derivation demonstrates the feasibility of immortalizing differentiated neurons by targeting tumorigenesis in transgenic mice to specific neurons of the CNS.

Dissociation of dopamine from its receptor as a signal in the pleiotropic hypothalamic regulation of prolactin secretion.

We have reviewed the literature, which supports an important role for dopamine withdrawal in the regulation of PRL secretion. Concentrations of dopamine in the hypophyseal portal circulation are sufficient to occupy the majority of dopamine receptors (1) and tonically suppress PRL secretion (20-26). Brief escapes from dopaminergic regulation associated with the secretion of PRL have been observed (37-41). Therefore, dopamine regulates secretion of PRL both by occupancy of, as well as dissociation from, specific D2 dopamine receptors. The rapid off rate from its receptor (2) is consistent with signals transmitted through brief decreases in dopamine concentration. The removal of dopamine for 10 min results in increases in intracellular cAMP and presumably activation of protein kinase A (39, 138) as well as activation of phospholipase C (137, 138) and protein kinase C (136). The removal of dopamine results directly in the release of PRL (37-41). Furthermore, the brief removal of dopamine results in the long-term potentiation of the PRL-releasing action of TRH (38-40). The potentiating action of dopamine withdrawal appears to be mediated by the activation of protein kinase A since pretreatment with VIP, a hormone that signals via protein kinase A, also potentiates the action of TRH (39). TRH stimulates PRL release via Ca2+/protein kinase C (177-184). The potentiating action of dopamine removal is selective for the Ca2+/protein kinase C pathway since dopamine removal does not potentiate the PRL-secreting action of VIP (38, 87, 92). The action of TRH is potentiated up to 30 min after the return of dopamine and the suppression of PRL to basal levels (38). In Fig. 10, dopamine dissociation from its receptor or VIP association to its receptor are shown separated by a broken line to indicate that by the time the potentiation of the action of TRH is tested, either dopamine is again occupying its receptor or VIP is no longer present. Therefore, the effect of protein kinase A activation is remembered by the lactotroph. We hypothesize that the responsiveness of the cell to TRH is potentiated by the phosphorylation of proteins by protein kinase A. Two potential substrates for protein kinase A are voltage-dependent Ca2+ channels and protein phosphatase inhibitors that would prolong the action of protein kinase C. When TRH occupies its receptor, intracellular Ca2+ levels are increased first from intracellular stores and subsequently by extracellular Ca2+ influx (187-189). Intracellular Ca2+ is mobilized by increased levels of IP3(128). Extracellular Ca2+ enters the lactotroph via voltage-dependent Ca2+ channels (189, 190).(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency of intrinsic pulsatile gonadotropin-releasing hormone secretion is regulated by the expression of cyclic nucleotide-gated channels in GT1 cells.

Cultures of endogenous GnRH neurons and the GT1 GnRH neuronal cell line release GnRH in pulses (intrinsic pulsatile release) with an interpulse frequency similar to that seen in castrated animals. In both GT1 cells and transgenic rats, lowering cAMP levels by expression of a phosphodiesterase decreased the frequency of intrinsic GnRH pulsatility. We asked whether the cyclic nucleotide-gated cation (CNG) channels expressed in GT1 cells participated in cAMP modulation of intrinsic GnRH pulsatility. Because expression of the CNGA2 subunit is essential for formation of functional CNG channels, we developed an adenovirus (Ad) vector expressing a short interference RNA (siRNA) to the CNGA2 subunit (Ad-CNG-siRNA) or as an infection control, to the coding region of luciferase (Ad-Luc-siRNA). Infection with the Ad-CNG-siRNA of COS cells transfected with a CNGA2 expression vector significantly inhibited CNGA2 protein levels by 74% by Western blot. Infection of GT1-1 cells with Ad-CNG-siRNA resulted in a 68% decrease in the levels of CNGA2 mRNA, a 44% decrease in protein levels, and a clear decrease in immunostaining with an antibody to CNGA2. Infection of GT1-1 cells with Ad-CNG-siRNA decreased spontaneous Ca2+ oscillations compared with Ad-Luc-siRNA-infected or uninfected cells by 71%. Furthermore infection with Ad-CNG-siRNA resulted in a 2-fold increase in the interpulse interval in GnRH secretion (49.4+/-9.1 min) compared with uninfected cells (25.9+/-2.5 min) or Ad-Luc-siRNA (29.3+/-2.8 min)-infected cells. These data provide the first direct evidence that the CNG channel is a downstream signaling molecule in the regulation of the frequency of intrinsic GnRH pulsatility by cAMP.

Regulation of gonadotropin-releasing hormone neurons by basic fibroblast growth factor.

The development of a functional network of gonadotropin-releasing hormone (GnRH) neurons in the central nervous system requires a series of complex regulatory mechanisms, presumably mediated in part by neurotrophic factors. The difficulty in studying factors regulating the development of GnRH neurons stems from their paucity and scattered distribution in the brain; as a result, little was known about the role of neurotrophic factors in the development of the mature GnRH neuronal network. Recent utilization of immortalized GnRH neuronal cell lines (GT1) has enabled us to identify and study specific neurotrophic factors and their functions in vitro. The potent neurotrophic effect of basic fibroblast growth factor (bFGF) and the presence of a high abundance of receptors for bFGF in GT1 cells have led to the hypothesis that bFGF may be an important regulator of GnRH neuron expansion, survival, migration, and connectivity.

Cell lines of the pituitary gonadotrope lineage derived by targeted oncogenesis in transgenic mice.

Study of the molecular and cellular biology of the gonadotropin hormones would be greatly facilitated by the availability of immortalized anterior pituitary gonadotrope cell lines. We directed expression of the simian virus-40 (SV40) T-antigen (Tag) oncogene to specific cells in the anterior pituitary of transgenic mice using the promoter/enhancer region from the human glycoprotein hormone alpha-subunit gene. Transgenic mice carrying this fusion gene developed anterior pituitary tumors. Clonal cell lines established from these tumors express the endogenous mouse alpha-subunit gene and synthesize and secrete alpha-subunit protein. However, they do not express beta-subunit genes. Alpha-subunit mRNA is induced by GnRH in a dose- and time-dependent manner, but is not regulated by TRH. Thus, we have targeted tumorigenesis in transgenic mice to anterior pituitary cells of the gonadotrope lineage to immortalize this specific endocrine cell while maintaining several highly differentiated functions unique to gonadotropes.

16K human prolactin inhibits vascular endothelial growth factor-induced activation of Ras in capillary endothelial cells.

Signaling pathways mediating the antiangiogenic action of 16K human (h)PRL include inhibition of vascular endothelial growth factor (VEGF)-induced activation of the mitogen-activated protein kinases (MAPK). To determine at which step 16K hPRL acts to inhibit VEGF-induced MAPK activation, we assessed more proximal events in the signaling cascade. 16K hPRL treatment blocked VEGF-induced Raf-1 activation as well as its translocation to the plasma membrane. 16K hPRL indirectly increased cAMP levels; however, the blockade of Raf-1 activation was not dependent on the stimulation of cAMP-dependent protein kinase (PKA), but rather on the inhibition of the GTP-bound Ras. The VEGF-induced tyrosine phosphorylation of the VEGF receptor, Flk-1, and its association with the Shc/Grb2/Ras-GAP (guanosine triphosphatase-activating protein) complex were unaffected by 16K hPRL treatment. In contrast, 16K hPRL prevented the VEGF-induced phosphorylation and dissociation of Sos from Grb2 at 5 min, consistent with inhibition by 16K hPRL of the MEK/MAPK feedback on Sos. The inhibition of Ras activation was paralleled by the increased phosphorylation of 120 kDa proteins comigrating with Ras-GAP. Taken together, these findings show that 16K hPRL inhibits the VEGF-induced Ras activation; this antagonism represents a novel and potentially important mechanism for the control of angiogenesis.

The antiangiogenic factor 16K PRL induces programmed cell death in endothelial cells by caspase activation.

We asked whether the antiangiogenic action of 16K human PRL (hPRL), in addition to blocking mitogen-induced vascular endothelial cell proliferation, involved activation of programmed cell death. Treatment with recombinant 16K hPRL increased DNA fragmentation in cultured bovine brain capillary endothelial (BBE) and human umbilical vein endothelial (HUVE) cells in a time- and dose-dependent fashion, independent of the serum concentration. The activation of apoptosis by 16K hPRL was specific for endothelial cells, and the activity of the peptide could be inhibited by heat denaturation, trypsin digestion, and immunoneutralization, but not by treatment with the endotoxin blocker, polymyxin-B. 16K hPRL-induced apoptosis was correlated with the rapid activation of caspases 1 and 3 and was blocked by pharmacological inhibition of caspase activity. Caspase activation was followed by inactivation of two caspase substrates, poly(ADP-ribose) polymerase (PARP) and the inhibitor of caspase-activated deoxyribonuclease (DNase) (ICAD). Furthermore, 16K hPRL increased the conversion of Bcl-X to its proapoptotic form, suggesting that the Bcl-2 protein family may also be involved in 16K hPRL-induced apoptosis. These findings support the hypothesis that the antiangiogenic action of 16K hPRL includes the activation of programmed cell death of vascular endothelial cells.

Steroidogenic adrenocortical cell lines produced by genetically targeted tumorigenesis in transgenic mice.

Studies of adrenal steroidogenesis have been facilitated by the availability of immortalized mouse adrenocortical Y-1 cells. We sought to make new, alternative mouse steroidogenic cell lines by genetically targeted tumorigenesis. Transgenic mice were constructed expressing both the SV40 T-antigen and a bacterial neomycin-resistance gene under the control of the promoter for the human P450 cholesterol side-chain cleavage (P450scc) gene, which encodes the first and rate-limiting enzyme in steroidogenesis. Two female transgenic mice expressed T-antigen in various nonsteroidogenic tissues but generated tumors only in the adrenals, suggesting adrenal tumor formation was an early event. Ovarian tissues, which, unlike the adrenal, do not make steroids in fetal or early postnatal life, did not develop tumors. Cell lines derived from the adrenal tumors were resistant to the neomycin analog G418. Clonal sublines are stable, growing easily in monolayers with a doubling time of 24-60 h. The cell lines secrete progesterone and 11-deoxycorticosterone, indicating these cells express the P450scc system, 3 beta-hydroxysteroid dehydrogenase, and 21-hydroxylase activity. However the 21-hydroxylase activity was not mediated by P450c21, as the cells lacked P450c21 mRNA. The cells did not secrete any 11-hydroxylated steroids, although they contained P450c11 beta mRNA. Both the secretion of progesterone and the abundance of P450scc mRNA increase in response to 8-bromo-cAMP, but not to ACTH or angiotensin II. In addition to expression of steroidogenic enzyme mRNAs, one cell line also expresses mouse renin-1 mRNA, making these cells useful for studies of the role of adrenal renin in regulating adrenal steroidogenesis. These findings represent an approach in transgenic mice to develop highly differentiated adrenal cell lines.

Bovine pituitary folliculo-stellate cells have beta-adrenergic receptors positively coupled to adenosine 3',5'-cyclic monophosphate production.

The specific beta-adrenergic radioligand [125I]iodocyanopindolol (ICYP) was used to identify and characterize beta-adrenergic receptors in bovine pituitary folliculo-stellate cells (bFSC) grown in culture. Saturation analysis demonstrated the binding of ICYP to bFSC particulate fractions to be of high affinity (apparent Kd = 80 pM) and low capacity (Bmax = 37 fmol/mg protein). The specific beta-adrenergic radioligand [3H] dihydroalprenolol also bound to bFSC particulate preparations with parameters compatible with binding to the beta-adrenergic receptor (Kd = 3.0 nM; Bmax = 52 fmol/mg protein). No specific binding was observed with either the dopamine receptor radioligand [3H]spiperone or the alpha-adrenergic radioligand [3H]dihydro-alpha-ergocryptine. The bFSC beta-adrenergic receptors were further characterized by computer modeling of competition studies with a variety of agonists and antagonists selective for beta-adrenergic subtypes. The pharmacological profiles of ICYP binding obtained from these studies indicated that approximately equal proportions of both beta 1- and beta 2-adrenergic subtypes are expressed in cultured bFSC. Bovine FSC beta-adrenergic receptors are functionally coupled to activation of cAMP. The beta-adrenergic agonists isoproterenol, epinephrine, and norepinephrine provoked a rapid and marked stimulation of intracellular cAMP accumulation. The approximately equipotent effect of epinephrine and norepinephrine indicated that the beta-adrenergic effect on cAMP production is principally mediated via the beta 1-adrenergic receptor. The identification of beta-adrenergic receptors on bFSC positively coupled to adenylate cyclase provides a possible regulatory control pathway for the proposed role of pituitary FSC in the modulation of anterior pituitary hormone secretion.

Inhibition of estrogen-induced anterior pituitary enlargement and arteriogenesis by bromocriptine in Fischer 344 rats.

The interrelationship between dopamine (DA) regulation and changes in the blood supply of the anterior pituitary lobe (AP) in the etiology of estradiol (E2)-induced proliferation of pituitary cells was studied in Fischer 344 rats. Rats were implanted with E2-filled or empty Silastic capsules for 21 days alone or in conjunction with pellets of the potent DA agonist bromocriptine (CB-154). Changes in vascularization of the AP, median eminence DA content, and responsiveness to DA of cultured AP cells were measured. Development of a direct arterial blood supply was assessed by the injection of 15-microns microspheres that can only reach the AP by newly formed arteries (arteriogenesis). APs were enzymatically dispersed and cultured for 3 days before challenges with increasing concentrations of DA for 3 h. DA content was measured by radioenzymatic assay, and serum PRL was determined by RIA. E2 treatment increased the weight of the pituitary gland, serum PRL levels, and the number of microspheres in the AP 4.5-, 173-, and 142-fold, respectively, over control values. Median eminence DA content was decreased 71% by E2 treatment, while the ability of DA to suppress PRL secretion in vitro decreased from a maximum of 70% to 40% with no change in the ED50. Simultaneous treatment with CB-154 dramatically decreased the effect of E2 on arteriogenesis, pituitary weight, serum PRL levels, and median eminence DA content. Blockade of E2-induced AP enlargement by increased dopaminergic stimulation was closely correlated with inhibition of arteriogenesis, which further suggests an important role for vascular changes in lactotroph proliferation.

Human prolactin secreting adenoma cells maintained on extracellular matrix.

In vitro maintenance of human prolactin secreting adenoma cells on a extracellular matrix (ECM) produced by bovine corneal endothelial cells is associated with the adoption of morphological properties that are not expressed when the cells are seeded on plastic. Cells seeded on ECM were firmly attached and highly flattened, whereas cells seeded on plastic formed aggregates which were loosely attached or floated in the medium. Moreover, prolactin production from cells on ECM is frequently stable for several weeks. Both tumor cells and normal cells are more likely to resemble their in vivo counterparts when maintained on extracellular matrix than on plastic, and therefore, cells cultured on ECM may provide a better system for oncogenic and endocrine studies.

A specific, high affinity, saturable binding site for the 16-kilodalton fragment of prolactin on capillary endothelial cells.

A 16-kilodalton N-terminal fragment of PRL (16K PRL) is formed by enzymatic cleavage of intact 23-kilodalton PRL (23K PRL) in the pituitary gland and in target tissues for PRL. 16K PRL inhibits the growth of capillary endothelial cells, while intact PRL was inactive suggesting that 16K PRL acted via a receptor other than the PRL receptor. To analyze whether this inhibitory effect could be mediated through an specific 16K PRL receptor, we characterized the binding of 16K PRL to membrane preparations of bovine brain capillary endothelial (BBE) cells. 16K PRL was generated by the proteolysis of rat 23K PRL with a particulate fraction from rat mammary gland homogenates and purified by gel filtration. The specific binding of [125I]16K PRL to BBE cell membranes was high affinity (Kd = 9.9 nM), saturable (Bmax = 4.8 pmol/mg protein), and reversible. In competition studies for [125I]16K PRL binding, 16K PRL was most potent, while little displacement was observed with high concentrations of 23K PRLs, growth hormones, and basic fibroblast growth factor. Blockade of reformation of disulfide bonds by carbamidomethylation of 16K PRL, a procedure which increases the biological activity of the molecule, increased its binding affinity (Kd = 0.9 nM). Cross-linking experiments identified a 52,000 and a 32,000 mol wt protein as the major 16K PRL binding species. These data demonstrate the presence of specific, high affinity, saturable binding sites for 16K PRL on BBE cell membranes and support biological findings that 16K PRL inhibits capillary endothelial cell proliferation, through a novel, high affinity receptor.

Supersensitivity of anterior pituitary dopamine receptors involved in the inhibition of prolactin secretion following destruction of the medial basal hypothalamus.

The medial basal hypothalamus of ovariectomized rats was destroyed using a modified Halász knife. Large increases in prolactin secretion were observed 1 and 14 days following the lesions. Long- and short-term lesioned animals were anesthetized with chloral hydrate and treated with various doses of apomorphine (0.05, 0.2, 2, 5 mg/kg). Blood samples were obtained before and 10, 30 and 60 minutes after the injection. Both the 0.05 and 0.2 mg/kg doses caused significantly greater and longer-lasting inhibition of prolactin in long-term than in short-term lesioned animals. Since the MBH was totally destroyed this study suggests that anterior pituitary dopamine receptors involved in the inhibition of prolactin secretion become supersensitive in long-term lesioned rats.

Effect of dopamine withdrawal on activation of adenylate cyclase and phospholipase C in enriched lactotrophs.

In order to define the roles of cAMP and inositol phosphates (IPx) in the mechanisms governing dopamine (DA)-regulated PRL release, we have carried out studies with enriched lactotrophs from dispersed anterior pituitaries of estrogen-treated rats. Changes in the intracellular levels of cAMP as well as IPx were determined in response to the acute addition or removal of DA. The withdrawal of DA from the incubation medium in cells cultured with DA (500 nM) for 24 h resulted in a rapid and significant increase in cAMP concentration from 1.29 +/- 0.098 to 3.89 +/- 0.199 pmol/dish. On the other hand, the administration of DA for 10 min to cells cultured without it resulted in a significant decrease in intracellular cAMP (from 3.04 +/- 0.208 to 1.62 +/- 0.057 pmol/dish). Similarly, PRL released into the medium was stimulated (95.1 +/- 9%) or inhibited (46.9 +/- 6%) after DA withdrawal or addition, respectively. Cells incubated 24 h with [3H]inositol and DA (500 nM) responded to DA withdrawal with a significant increase in the concentration of [3H]IPx (5148 +/- 199 vs. 8,376 +/- 164 cpm/dish), whereas acute DA administration had no effect on the level of [3H]IPx. The administration of 8-Br-cAMP (0.5 and 2.5 mM) and forskolin (10 microM) for 10 min to cells tonically cultured in the presence of DA had no effect on the intracellular concentration of [3H]IPx, although they decreased the relative proportion of [3H]IP3 fraction from 22.1% to 11.6%, 13.6%, and 9.6%, respectively. The administration of TRH (100 nM), either under tonic DA inhibition or 10 min after the transient removal of DA inhibition, resulted in a similar stimulation of IPx formation (from 5,625 +/- 155 to 21,830 +/- 100 and 24,870 +/- 80 cpm/dish, respectively). However, the release of PRL induced by TRH was potentiated 6-fold (38.2 +/- 2.17 vs. 227 +/- 41 ng/dish) by the transient removal of DA. These findings support the conclusions that: 1) DA receptors in lactotrophs are negatively coupled to adenylate cyclase as seen during the addition or removal of DA. 2) DA receptors are negatively coupled to phospholipase C; however activation is only seen upon the removal of DA. 3) The effect of DA withdrawal on the formation of IPx is not secondary to the activation of adenylate cyclase. 4) The potentiation of the PRL response to TRH after DA withdrawal does not involve increased production of IPx.

A dopaminergic binding site in the high speed supernatant of steer anterior pituitary homogenates.

The high speed supernatant fraction from homogenates of the steer anterior pituitary was shown to contain a high affinity (Kd = 0.10-0.20 nM), saturable (Bmax - 1.2-3.5 fmol/mg protein), stereoselective binding site for 3H-spiperone (3H-SPIP). The Kd (0.10 nM) calculated from the experimentally determined rate constants k1 and k2 was in excellent agreement with that derived from equilibrium measurements. The rank order of potencies of dopaminergic agents to compete for binding was consistent with known dopamine (DA) receptors. Soluble binding sites represented 3% of the total specific binding found in the anterior pituitary, and several lines of evidence suggested they were not due to enzymatic, mechanical or osmotic displacement of membrane receptors. Further studies are necessary to elucidate the possible significance of these soluble binding sites; however, this observation is intriguing since both DA and membrane bound DA receptors enter the cell.

Basic fibroblast growth factor is a neurotropic factor in GT1 gonadotropin-releasing hormone neuronal cell lines.

Basic fibroblast growth factor (bFGF) plays an important role in development of the central nervous system and is neurotropic for a variety of neurons. In this study, we investigated whether bFGF is neurotropic for GT1 GnRH neuronal cell lines and if these cells express functional FGF receptors (FGFRs). The GT1 cell lines generated by genetically targeted tumorigenesis display highly differentiated properties of GnRH neurons. Addition of 2 and 10 ng/ml bFGF increased neurite outgrowth of GT1-7 cells and resulted in a significant increase of GT1 cell survival in serum-free medium. However, bFGF had no effect on [3H]thymidine incorporation at 24 or 48 h. RNase protection assays using riboprobes specific for murine FGFRs 1-3 showed that GT1 cells express FGFRs 1 and 3 but not 2. Occupancy of FGFRs with 10 ng/ml bFGF stimulated the sustained tyrosine phosphorylation of both the 42- and 44-kilodalton mitogen-activated protein kinases (MAPKs) for up to 6 h as shown by Western blot analysis. In addition, phosphorylation of the MAPKs was associated with enzyme activation as shown by an in-gel MAPK assay. GT1-1 and GT1-7 cells also express messenger RNA for bFGF, although the level of bioactive bFGF synthesized by GT1 cells appears suboptimal because GT1 cells can further respond to exogenously added bFGF. Thus, we have demonstrated that bFGF is a neurotropic factor in GT1 GnRh neuronal cell lines, raising the possibility that bFGF may play a role in the neurobiology of GnRH neurons.

Relation between luteinizing hormone and prolactin pulses in ovariectomized rats with or without dopamine inhibition.

GnRH and GnRH-associated peptide (GAP) have been shown to be cosecreted as spontaneous pulses in hypophysial portal blood. In addition, GAP has been proposed as a physiological inhibitor of PRL secretion. The present investigation was performed to determine whether GAP might play a role in the moment to moment regulation of PRL release in the ovariectomized rat. We anticipated that an inverse relation might exist between PRL and LH pulses if GAP is a physiological regulator of PRL and is coreleased with GnRH. Serial blood samples were collected at 6-min intervals over 4 h from ovariectomized rats bearing chronic jugular catheters and were analyzed for plasma concentrations of PRL and LH by RIA. Release patterns were assessed using a pulse detection algorithm. Some animals were pretreated 30 min before blood sampling with domperidone (a dopamine antagonist that does not cross the blood-brain barrier) to unmask PRL inhibitory responses to GAP that might not otherwise be observable in the presence of normal dopamine inhibition. PRL secretory patterns were pulsatile but highly irregular, in contrast to the regular rhythmic patterns of circulating LH. Domperidone treatment significantly increased the number of PRL pulses. PRL pulse amplitudes, and mean PRL concentrations compared to those in vehicle-injected controls. LH pulses after domperidone administration were more frequent, resulting in slightly higher mean LH concentrations. In both vehicle- and domperidone-injected rats, 60-80% of PRL pulses were concordant with LH pulses (concordance defined as PRL and LH peaks occurring within one sample of each other). Assuming that GAP is cosecreted with GnRH, these data fail to support an acute physiological role for GAP during undisturbed PRL release in the ovariectomized rat because the expected relation between PRL and LH pulses in the event of such a role was not observed. To test a role for GAP more directly, domperidone-treated rats were injected with a rabbit anti-GAP serum during serial blood collection. No increase in PRL release was elicited by this treatment, and pulsatile PRL and LH secretion were unaffected compared to those in control animals injected with hyperimmune serum. To determine whether GnRH is the PRL-releasing secretagogue responsible for concordant PRL and LH pulses, some rats were pretreated 4 h before blood sampling with a potent GnRH antagonist, followed by domperidone 30 min before sampling. Treatment with GnRH antagonist virtually abolished LH pulses, but had no effect on PRL pulses.(ABSTRACT TRUNCATED AT 400 WORDS)

Absence of high affinity dopamine receptor in GH3 cells: a prolactin-secreting clone resistant to the inhibitory action of dopamine.

Dopamine (DA) and DA agonists bind with high affinity to anterior pituitary receptors which mediate the inhibition of PRL release. Spiperone (SPIP), a DA antagonist, has also been successfully used to characterize pituitary DA receptors with a dissociation constant (Kd) of less than 1 nM. We studied the binding of SPIP to GH3D6 cells which secrete only PRL and GH. This clone was derived from a radiation-induced tumor of the rat anterior pituitary. Equilibrium binding of [3H]SPIP to living GH3 cells showed no high affinity receptors, but a low affinity (Kd = 0.83 microM) and saturable (0.06 fmol/cell) population of sites was observed. In addition, saturable binding with a similar affinity (Kd = 0.57 microM) was noted in broken GH3 cells. The interaction was completely reversible and temperature dependent. The concentration of various ligands required to compete for half of the [3H]SPIP binding to whole cells were: chlorpromazine, 0.17 microM; haloperidol, 0.68 microM; pimozide, 0.77 microM; d-butaclamol, 1.16 microM; 1-butaclamol, 1.30 microM; SPIP, 1.49 microM; bromergocryptine, 4.98 microM; apomorphine, 13.9 microM; and DA, 100 microM. The absence of a high affinity site in GH3 cells is consistent with the decreased effectiveness of various agonists and antagonists on PRL secretion. It is possible that the low affinity interactions observed in GH3 cells are normally present in the anterior pituitary and brain and do not simply represent an alteration of receptor affinity.

Increased responsiveness of the dopamine-mediated inhibition of prolactin synthesis after destruction of the medial basal hypothalamus.

Long term destruction of the tuberoinfundibular dopaminergic neurons results in increased responsiveness of the anterior pituitary to the suppression of PRL release by dopamine. In the present study we tested whether long term destruction of these neurons by medial basal hypothalamic lesions would also increase the potency of dopamine in suppressing the release and apparent synthesis of PRL. Anterior pituitaries from ovariectomized control rats and ovariectomized rats lesioned for 2 weeks were incubated for 3 h in medium 199 containing [3H]leucine in the presence or absence of various concentrations of dopamine. Labeled PRL was separated by polyacrylamide gel electrophoresis and quantitated by liquid scintillation spectroscopy. Concentrations of dopamine ranging from 10(-8)-10(-6) M caused a dose-dependent suppression of labeled newly synthesized PRL released into the medium from anterior pituitaries of medial basal hypothalamus-lesioned animals. With pituitaries from the nonlesioned rats, newly synthesized PRL release was progressively inhibited by 10(-7) and 10(-6) M dopamine, while 10(-8) M dopamine actually significantly stimulated PRL release. Total labeled PRL (that released and that remaining in the gland) was equally suppressed by all three concentrations of dopamine in pituitaries from lesioned animals, but only a minor effect was observed at the highest concentration of dopamine with the control pituitaries. Therefore, the potency of dopamine to suppress the release and apparent synthesis (total labeled PRL) of newly synthesized PRL from pituitaries of long term lesioned animals was increased.