Dopamine withdrawal elicits prolonged calcium rise to support prolactin rebound release

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Activation of D2 dopamine receptors inhibits estrogen response element-mediated estrogen receptor transactivation in rat pituitary lactotrophs

Estrogen and dopamine are major opposing regulators of the endocrine functions of pituitary lactotrophs. Dopamine inhibits estrogen-induced changes in the synthesis and secretion of prolactin, and lactotroph proliferation. We studied the mechanism of the inhibitory effects of dopaminergic stimulation on estrogen-induced functional changes of rat lactotrophs in primary culture. The dopaminergic agonist, bromocriptine (BC), suppressed 17β-estradiol-stimulated lactotroph proliferation, prolactin promoter activity, and mRNA expression of some estrogen-responsive genes. In lactotroph-enriched pituitary cells, BC treatment inhibited the estrogen response element (ERE) DNA sequence-mediated estrogen receptor (ER) transcriptional activity. Using a lactotroph-specific ERE transcriptional assay, we found that BC inhibition of the ERE-mediated ER transcriptional activity partly involved D2 dopamine receptor-mediated, pertussis toxin-sensitive G protein-coupled, cAMP/protein kinase A-dependent signaling. BC treatment had no effect on the cellular concentration of ERα or its phosphorylation status at Ser-118. Similar transcriptional inhibition by BC was also found in GH4ZR7 cells, a D2 dopamine receptor-expressing somatomammotrophic cell line. These results suggest that activation of the D2 dopamine receptors inhibits estrogen-dependent lactotroph functions in part via attenuation of ERE-mediated ER transactivation.

Estrogen regulation of the dopamine-activated GIRK channel in pituitary lactotrophs: implications for regulation of prolactin release during the estrous cycle

Prolactin (PRL), synthesized and secreted from lactotrophs of the anterior pituitary gland, is tonically inhibited by hypothalamic dopamine (DA) throughout the female reproductive (estrous) cycle. Our laboratory has shown that DA hyperpolarizes these cells by activating G protein-coupled inwardly rectifying K(+) (GIRK) channels; however, this response is only observed on proestrus. While the cellular mechanisms that allow for functional expression of this unique DA-signaling pathway are unclear, we hypothesized that activation of the DA-GIRK effector pathway is due to the rise in circulating estrogen (E₂) during the preceding day of diestrus. Thus, we examined the effects of E₂ on primary lactotrophs isolated from female rats. Treatment with a physiological concentration of E₂ (40-80 pg/ml, in vivo or in vitro) induced a proestrous phenotype in diestrous lactotrophs. These cells exhibited a DA-induced membrane hyperpolarization, as well as a secretory rebound of PRL following DA withdrawal (characteristic of proestrous cells). Internal dialysis of GTPγS demonstrated that E₂ exposure enabled functional expression of GIRK channels, and this regulation by E₂ did not involve the D₂R. The effect of E₂ was blocked by the receptor antagonist, ICI 182,780, and by the protein synthesis inhibitor, cycloheximide. Single-cell analysis revealed increased mRNA expression of GIRK channel subunits in E₂-treated lactotrophs. While E₂ is known to have multiple actions on the lactotroph, the present findings illuminate a novel action of E₂ in lactotrophs-regulation of the expression of a DA effector, the GIRK channel.

Investigating heterogeneity of intracellular calcium dynamics in anterior pituitary lactotrophs using a combined modelling/experimental approach

Cell responses are commonly heterogeneous, even within a subpopulation. In the present study, we investigate the source of heterogeneity in the Ca(2+) response of anterior pituitary lactotrophs to a Ca(2+) mobilisation agonist, thyrotrophin-releasing hormone. This response is characterised by a sharp increase of cytosolic Ca(2+) concentration as a result of mobilisation of Ca(2+) from intracellular stores, followed by a decrease to an elevated plateau level that results from Ca(2+) influx. We focus on heterogeneity of the evoked Ca(2+) spike under extracellular Ca(2+) free conditions. We introduce a method that uses the information provided by a mathematical model to characterise the source of heterogeneity. This method compares scatter plots of features of the Ca(2+) response obtained experimentally with those made from the mathematical model. The model scatter plots reflect random variation of parameters over different ranges, and matching the experimental and model scatter plots allows us to predict which parameters are most variable. We find that a large degree of variation in Ca(2+) efflux is a likely key contributor to the heterogeneity of Ca(2+) responses to thyrotrophin-releasing hormone in lactotrophs. This technique is applicable to any situation in which the heterogeneous biological response is described by a mathematical model.

Toxicity assessment of pramipexole in juvenile rhesus monkeys

Pramipexole (PPX) is a dopamine agonist approved for the treatment of the signs and symptoms of idiopathic Parkinson's disease as well as restless leg syndrome. The objective of this study was to investigate the toxicity of PPX when administered orally to juvenile rhesus monkeys once daily for 30 weeks, and to assess the reversibility of toxicity during a 12-week recovery. Rhesus monkeys (N=4 males and 4 females/group; 22-24 months of age) were orally treated daily for 30 weeks with 0.0, 0.1, 0.5 or 2.0 mg/kg PPX, and subjects were assessed daily using the NCTR Operant Test Battery (OTB). Clinical chemistry, hematology, ophthalmology and other standard postmortem toxicological evaluations, including histopathology and neuropathology as well as toxicokinetics were performed. The systemic exposure to PPX was higher than that at therapeutic doses in man and AUC(0-24 h)-data increased proportionally to dose. Blood pressure significantly decreased over time in all groups including control. Near the end of treatment, there were statistically significant decreases in heart rate for the 0.5 and 2.0 mg/kg/day groups compared to control. After 4 weeks of dosing, serum prolactin was significantly decreased in all treatment groups compared to control. This decrease remained at the end of treatment in the 0.5 and 2.0 mg/kg/day groups. In summary, administration of PPX at doses of up to 2.0 mg/kg/day for 30 weeks to juvenile rhesus monkeys produced adverse findings which were attributable to its pharmacological properties, including hypoprolactinemia.

Prolactin secretory rhythm of mated rats induced by a single injection of oxytocin

Mating or vaginocervical stimulation [copulatory stimulus (CS)] induces two daily surges of the hormone prolactin (PRL) in rats. This unique secretory pattern of PRL surges is characteristic for the first half of pregnancy and is also present in ovariectomized (OVX) rats. Studies have shown that CS additionally provokes an acute release of the hormone oxytocin (OT). In this study, we tested whether a single injection of OT (iv) is sufficient to initiate the PRL secretion pattern of OVX/CS rats. Furthermore, we measured the 24-h profile of dopamine (DA) content in the anterior lobe of the pituitary gland, because DA is the major inhibitory factor of PRL secretion. The results indicated that a single injection of OT induces a PRL secretory rhythm and a DA release pattern similar to that initiated by CS. Immunocytochemical investigation showed that particular OTergic neurons in the hypothalamus express receptors for PRL, as well as for vasoactive intestinal polypeptide, which indicates an involvement in generating the PRL rhythm and entraining it to the ambient photoperiod. On the basis of this study, we suggest that the PRL-DA inhibitory feedback loop between lactotrophs and DAergic neurons plays a crucial role in generating the oscillatory PRL secretion pattern in CS rats. A timing signal, likely provided by the hypothalamic suprachiasmatic nucleus, entrains the autonomous PRL oscillation to a particular time of day. Mathematical modeling was used to illustrate the proposed network function. The experimental results further suggest an additional feedback mechanism in which certain hypothalamic OTergic neurons are influenced by PRL.

Functional expression of the dopamine-activated K(+) current in lactotrophs during the estrous cycle in female rats: correlation with prolactin secretory responses

It is well established that hypothalamic dopamine (DA) is the major physiologic regulator of prolactin (PRL) secretion, exerting a tonic inhibition throughout most of the estrous cycle. A dramatic drop in the amount of DA perfusing the anterior pituitary occurs in the afternoon of proestrus and is critical for the production of the surge of PRL that occurs at that time. In my laboratory, we have identified and characterized a DA-activated K(+) channel (K(DA)) in lactotrophs derived from proestrous rats that underlies DA-induced membrane hyperpolarization of lactotrophs. We have also demonstrated that this hyperpolarization plays a critical role in both the inhibition of PRL release from proestrous cells and the PRL secretory rebound that occurs following DA withdrawal. We now report that the ability of DA to activate the K(DA) channel and elicit hyperpolarization in primary lactotrophs changes dramatically during the estrous cycle. Lactotrophs isolated from cycling female rats were studied using whole-cell voltage clamp. DA (1 microM) elicited a robust membrane K(+) current in the majority of proestrous lactotrophs (86%; 24.0 +/- 2.9 pA). By contrast, DA activated a considerably smaller membrane current (3.3 pA) in very few lactotrophs isolated from rats on either diestrus or estrus (8 and 0%, respectively). Using a perifusion system to examine temporal patterns of PRL release, we found that following application and withdrawal of DA, proestrous cells produced a robust secretory rebound, but diestrous and estrous cells did not. However, DA inhibited PRL release to the same extent regardless the stage of the cycle from which the cells were derived. These data are consistent with the presence of multiple DA effectors in lactotrophs and demonstrate that their relative importance shifts dramatically with changes in the endocrine status of the animal. We propose that the DA-activated K(+) channel (K(DA)) is a critical effector governing the unique secretory profile of PRL observed in proestrous animals.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Identification of G protein-coupled, inward rectifier potassium channel gene products from the rat anterior pituitary gland

Dopamine (DA) is a physiological regulator of PRL secretion, exerting tonic inhibitory control. DA activates an inward rectifier K(+) (IRK) channel in rat lactotropes, causing membrane hyperpolarization and inhibition of Ca(2+)-dependent action potentials. Both the activation of this effector K(+) channel and the inhibition of PRL release are mediated by D(2)-type receptor activation and pertussis toxin- sensitive G proteins. To study the molecular basis of this physiologically relevant channel, a homology-based PCR approach was employed to identify members of the IRK channel family expressed in the anterior pituitary gland. Nondegenerate primers corresponding to regions specific for IRK channels known to be G protein activated (GIRKs; gene subfamily Kir 3.0) were synthesized and used in the PCR with reverse transcribed female rat anterior pituitary messenger RNA as the template. PCR products of predicted sizes for Kir 3.1, 3.2, and 3.4 were consistently observed by ethidium bromide staining after 16 amplification cycles. The identities of the products were confirmed by subcloning and sequencing. Expression of each of these gene products in anterior pituitary was confirmed by Northern blot analysis. Functional analysis of the GIRK proteins was performed in the heterologous expression system, Xenopus laevis oocytes. Macroscopic K(+) currents were examined in oocytes injected with different combinations of Kir 3.0 complementary RNA (cRNA) and G protein subunit (beta(1)gamma(2)) cRNA. The current-voltage relationships demonstrated strong inward rectification for each individual and pairwise combination of GIRK channel subunits. Oocytes coinjected with any pair of GIRK subunit cRNA exhibited significantly larger inward K(+) currents than oocytes injected with only one GIRK channel subtype. Ligand-dependent activation of only one of the GIRK combinations (GIRK1 and GIRK4) was observed when channel subunits were coexpressed with the D(2) receptor in Xenopus oocytes. Dose-response data fit to a Michaelis-Menten equation gave an apparent K(d) similar to that for DA binding in anterior pituitary tissue. GIRK1 and GIRK4 proteins were coimmunoprecipitated from anterior pituitary lysates, confirming the presence of native GIRK1/GIRK4 oligomers in this tissue. These data indicate that GIRK1 and GIRK4 are excellent candidate subunits for the D(2)-activated, G protein-gated channel in pituitary lactotropes, where they play a critical role in excitation-secretion coupling.

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.

Intracellular mechanisms involved in dopamine-induced actin cytoskeleton organization and maintenance of a round phenotype in cultured rat lactotrope cells

The participation of the actin cytoskeleton in the control of PRL secretion by dopamine (DA) is not yet fully understood. Recently, we demonstrated that DA induces cortical actin assembly and stabilization in anterior pituitary PRL-secreting cells (lactotropes) that can be linked to DA-induced inhibition of PRL secretion. Here we show that DA prevents cell flattening and the formation of cytoplasmic actin cables in cultured rat lactotropes. The effects of DA were reversible, mediated by D2 receptors, exclusive to lactotropes, and independent of other anterior pituitary cells present in the cultures. Because cAMP and Ca2+ mediate DA-induced inhibition of PRL secretion and synthesis, we investigated whether morphological responses to DA were dependent on these second messengers. Either inhibition of protein kinase A activity with the specific inhibitor KT5720 or blockade of Ca2+ channels with nifedipine inhibited cell flattening and induced cytoplasmic actin filament breakdown. Nifedipine was as effective as DA, but KT5720 was less effective than DA. Increased intracellular cAMP levels provoked cell flattening, which was blocked by nifedipine and KT5720, but not by DA. The results suggest that Ca2+-dependent pathways control cell shape in most lactotropes; however, in a subpopulation of lactotropes, cAMP-dependent pathways may also contribute to DA morphological responses. Next, we studied the participation of the Rho family of guanosine triphosphatases, which is known to regulate the dynamics of actin filaments. Inactivation of Rho by C3 exoenzyme induced cytoplasmic actin cable disassembly and lactotrope rounding up. No additive effects were observed among Rho-, cAMP-, and Ca2+-dependent pathways. However, C3-induced morphological responses were blocked by increased cAMP levels, suggesting that Rho-dependent steps are upstream cAMP-dependent steps. DA-induced actin cytoskeleton reorganization in lactotropes may involve modifications in the expression and localization of actin-binding proteins. DA increased expression of the actin anchoring proteins talin and alpha-actinin, but not of vinculin. DA enhanced association of talin to cell membranes. Increased talin-membrane interaction may be implicated in DA-induced maintenance of a round phenotype in lactotrope cells.

The cortical actin cytoskeleton of lactotropes as an intracellular target for the control of prolactin secretion

We investigated the role of cortical actin filaments (F-actin) in the regulation of PRL secretion in cultured normal anterior pituitary cells. F-actin dynamics were evaluated by fluorescence microscopy, and PRL secretion from attached cells was measured by the reverse hemolytic plaque assay. F-actin localized to the periphery of lactotropes. PRL-releasing factors such as TRH, vasoactive intestinal peptide (VIP), and forskolin, or removal of the PRL-inhibiting factor dopamine (DA) from cultures chronically exposed to DA, caused fragmentation, i.e. focal disassembly of cortical F-actin. Basal, VIP-, and DA withdrawal-induced cortical F-actin disassembly were dependent on extracellular Ca2+ whereas TRH- and forskolin-induced disassembly were not. Short-term (5 min) treatment of cells with the F-actin-disrupting agent cytochalasin D (CD) enhanced basal PRL secretion but did not further stimulate TRH- or VIP-induced PRL secretion. The results support the existence of a causal link between F-actin disassembly and increased PRL secretion. On the other hand, exposure of cultures to DA decreased the percentage of cells showing cortical F-actin disassembly within minutes. Longer treatments (2-4 h) caused stabilization of cortical actin filaments as revealed by the protection vis-a-vis the depolymerizing effect of CD. The protective effect was specific for lactotropes and was evident with DA concentrations as low as 50 nM. Chronic exposure of the cells to DA blocked CD- and TRH-evoked actin disassembly and PRL secretion while VIP-induced effects were partially inhibited. Stabilization of F-actin with the marine sponge venom, jasplakinolide, also decreased basal and stimulated PRL secretion. In conclusion, our results suggest that, first, the cortical actin cytoskeleton of lactotropes is an integrator of the multiple factors regulating PRL secretion directly on the lactotrope, and second, the tonic inhibition of PRL secretion is mediated, at least in part, by DA-induced stabilization of cortical F-actin.

Pituitary adenylate cyclase-activating polypeptide potentiation of Ca2+ entry via protein kinase C and A pathways in melanotrophs of the pituitary pars intermedia of rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been reported to stimulate melanotroph secretion, and PACAP-like immunoreactivity and expression of PACAP type I receptor messenger RNA have been identified in the pituitary pars intermedia (PI). The present study showed that PACAP messenger RNA is also expressed in the PI. To examine the mechanism of PACAP action in the PI, cytosolic Ca2+ concentrations ([Ca2+]i) and ionic currents were measured in acutely dissociated rat melanotrophs. In about 40% of the melanotrophs studied, PACAP induced an increase in [Ca2+]i, which was suppressed by extracellular Ca2+ removal; extracellular Na+ replacement; the blocker of L-type Ca2+ channels, nicardipine; or the secreto-inhibitory neurotransmitter, dopamine. The PACAP-induced [Ca2+]i increase was mimicked by activators of protein kinase A (PKA) and protein kinase C (PKC), Sp-diastereomer of cAMP and 1-oleoyl-2-acetyl-sn-glycerol, and was reduced by inhibitors of PKA and PKC, Rp-diastereomer of cAMP and staurosporine. Patch-clamp analysis revealed that PACAP caused inward currents with a reversal potential of -0.8 mV and facilitated voltage-dependent Ba2+ currents. It further revealed that PACAP-induced inward currents were mimicked by 1-oleoyl-2-acetyl-sn-glycerol and inhibited by staurosporine, and that Sp-diastereomer of cAMP facilitated Ba2+ currents. These results suggest that PACAP potentiates Ca2+ entry mechanisms of rat melanotrophs by activation of nonselective cation channels via PKC and facilitation of voltage-dependent Ca2+ channels via PKA.

Endothelin activates large-conductance K+ channels in rat lactotrophs: reversal by long-term exposure to dopamine agonist

Endothelin-1 (ET-1) inhibits PRL secretion from cultured rat lactotrophs. However, ET-1 stimulates PRL secretion after cultured lactotrophs have been exposed for 48 h to dopamine or D2 dopamine agonists. In the present study, we have used cell-attached and inside-out patch recordings to establish an ionic basis for these effects. Bath application of 20 nM ET-1 to untreated lactotrophs evoked a robust and persistent activation of large-conductance K+ channels in cell-attached patches. This effect of ET-1 had a long latency to onset, was maintained for as long as ET-1 was present, and required at least 10 min of washing in control saline before complete recovery was achieved. The stimulatory effect of 20 nM ET-1 on these channels was markedly attenuated in the presence of the selective ET(A) receptor antagonist BQ-610 (200 nM), or after pertussis toxin (200 ng/ml, 16 h) pretreatment. The unitary slope conductance of the ET-1 activated channels in cell attached patches was 165 and 95 pS when the recording electrodes contained 150 and 5.4 mM KCl, respectively. These channels were voltage-sensitive and their activity increased upon patch depolarization. Previously activated channels in cell-attached patches became quiescent immediately upon patch excision into Ca2+-free bath saline. Exposure of the intracellular surface to 0.1 microM Ca2+ restored the activity of these channels similar to the level seen before patch excision. In addition, preincubating the cells with the membrane-permeable Ca2+-chelator BAPTA-AM, or using Ca2+-free solution in the recording pipettes, prevented the activation of these channels by ET-1. The ET-1 activated large-conductance Ca2+-dependent K+ (BK(Ca)) channels were blocked by 20 mM tetraethylammonium but were insensitive to the K+ channel blockers apamin (1 microM), charybdotoxin (200 nM), or iberiotoxin (200 nM). Acute application of 10 microM dopamine and 20 nM ET-1 caused activation of BK(Ca) channels with indistinguishable kinetic properties, although the effect of dopamine occurred with shorter latency. After 48-h exposure to the specific D2 dopamine receptor agonist (+/-)-2-(N-phenyl-N-propyl) amino-5-hydroxytetralin hydrochloride (PPHT, 500 nM), bath application of 20 nM ET-1 resulted in inhibition of spontaneously active BK(Ca) channels. These data suggest that both the stimulatory and inhibitory effects of ET-1 on PRL secretion are mediated, at least in part, by actions on BK(Ca) channels, and that long term exposure to dopamine or D2 agonists alters the signaling pathways from the ET(A) receptor to BK(Ca) channels.