Stimulatory Effects Of Dopamine Research Articles

The effects of interaction of Dopaminergic and Kisspeptin neural pathways on Ghrelin secretion in rats

Dopamine, kisspeptin and ghrelin are important factors for regulating the reproduction and food intake. Finding the effective central or peripheral factors on ghrelin secretion attracted the attention of most researches. In the present experimental study, the effects of dopamine, kisspeptin and the GPR54 receptor signaling pathway role in the mediating the dopamine effects was determined on ghrelin secretion. Forty Wistar male rats weighing 220-250g in 8 groups received saline, 1nmol kisspeptin, 5, 15 or 45 microgram dopamine hydrochloride, simultaneous injections of 1nmol peptide 234 and kisspeptin, 15microgram dopamine and kisspeptin or peptide234 and 15microgram dopamine via third cerebral ventricle respectively. Blood samples were collected via tail vein. Mean serum ghrelin concentrations were determined by rat ghrelin kit and ELISA method. Kisspeptin significantly decreased mean serum concentration compared to saline group, while 15 or 45 microgram dopamine significantly increased mean serum ghrelin level compared to saline group. Kisspeptin significantly blocked the stimulatory effects of dopamine on ghrelin secretion compared to dopamine group. Dopaminergic and kisspeptin/GPR54 signaling pathways may interact to control the ghrelin secretion at hypothalamic level. Stimulatory effects of dopamine on ghrelin secretions could exert partly via decreasing the activity of hypothalamic kisspeptin neurons.

Low dose of dopamine may stimulate prolactin secretion by increasing fast potassium currents

Dopamine (DA) released from the hypothalamus tonically inhibits pituitary lactotrophs. DA (at micromolar concentration) opens potassium channels, hyperpolarizing the lactotrophs and thus preventing the calcium influx that triggers prolactin hormone release. Surprisingly, at concentrations approximately 1000 lower, DA can stimulate prolactin secretion. Here, we investigated whether an increase in a K+ current could mediate this stimulatory effect. We considered the fast K+ currents flowing through large-conductance BK channels and through A-type channels. We developed a minimal lactotroph model to investigate the effects of these two currents. Both IBK and IA could transform the electrical pattern of activity from spiking to bursting, but through distinct mechanisms. IBK always increased the intracellular Ca2+ concentration, while IA could either increase or decrease it. Thus, the stimulatory effects of DA could be mediated by a fast K+ conductance which converts tonically spiking cells to bursters. In addition, the study illustrates that

Catecholamines increase lung edema clearance in rats with increased left atrial pressure.

During hydrostatic pulmonary edema, active Na(+) transport and alveolar fluid reabsorption are decreased. Dopamine (DA) and isoproterenol (ISO) have been shown to increase active Na(+) transport in rat lungs by upregulating Na(+)-K(+)-ATPase in the alveolar epithelium. We studied the effects of DA and ISO in isolated rat lungs with increased left atrial pressure (Pla = 15 cmH(2)O) compared with control rats with normal Pla (Pla = 0). Alveolar fluid reabsorption decreased from control value of 0.51 +/- 0.02 to 0.27 +/- 0.02 ml/h when Pla was increased to 15 cmH(2)O (P < 0.001). DA and ISO increased the alveolar fluid reabsorption back to control levels. Treatment with the D(1) antagonist SCH-23390 inhibited the stimulatory effects of DA (0.30 +/- 0.02 ml/h), whereas fenoldopam, a specific D(1)-receptor agonist, increased alveolar fluid reabsorption in rats exposed to Pla of 15 cmH(2)O (0.47 +/- 0.04 ml/h). Propranolol, a beta-adrenergic-receptor antagonist, blocked the stimulatory effects of ISO; however, it did not affect alveolar fluid reabsorption in control or DA-treated rats. Amiloride (a Na(+) channel blocker) and ouabain (a Na(+)-K(+)-ATPase inhibitor), either alone or together, inhibited the stimulatory effects of DA. Colchicine, which disrupts the cellular microtubular transport of ion-transporting proteins to the plasma membrane, inhibited the stimulatory effects of DA, whereas the isomer beta-lumicolchicine did not block the stimulatory effects of DA. These data suggest that DA and ISO increase alveolar fluid reabsorption in a model of increased Pla by regulating active Na(+) transport in rat alveolar epithelium. The effects of DA and ISO are mediated by the activation of dopaminergic D(1) receptors and the beta-adrenergic receptors, respectively.

Roles of renal dopamine and kallikrein-kinin systems in antihypertensive mechanisms of exercise in rats.

We have previously shown that both renal dopamine (DA) and kallikrein-kinin systems are activated by exercise in mild hypertensives. We aimed to confirm the effects of exercise on the renal DA system and the stimulatory effects of DA on the renal kallikrein-kinin system in rats. In experiment 1, 12 male Dahl salt-sensitive (DS) rats given a 4% salt diet were divided into two groups. Rats in the exercise group were forced to run at 8 m/min, 60 min/day, 5 days/week for 4 weeks. Daily urinary volume, urinary excretion of sodium, free DA, and kallikrein activity were measured weekly. Renal aromatic-L-amino-acid decarboxylase (AADC) activities were assayed at the end of the experiment. In experiment 2, 15 male Sprague-Dawley (SD) rats were randomly divided into 3 groups, a DA-5 (5 microg of DA/kg/min), a DA-10 (10 microg of DA/kg/min), and a control group. DA or vehicle was administered subcutaneously with an osmotic pump for 2 weeks. Daily urinary volume, urinary excretion of sodium, aldosterone, DA, and kallikrein activity were measured weekly. Plasma renin activity, aldosterone concentration, and renal kallikrein mRNA levels were determined at the end of the experiment. In experiment 1, urinary excretion of free DA and renal AADC activities in the exercise group were significantly higher than those in the non-exercise group at week 4. In experiment 2, renal kallikrein mRNA levels and urinary volume were significantly increased in the DA-10 group compared to the control group, although there were no differences in urinary kallikrein activities. Plasma aldosterone concentration was significantly decreased in the DA-10 group compared to that in the control group despite a lack of differences in plasma renin activities. In conclusion, exercise increased the urinary excretion of free DA, probably through increased renal AADC activity in DS rats. DA amplified renal kallikrein mRNA levels and decreased plasma aldosterone levels, probably through its suppression of aldosterone in the adrenal glands. Activation of the kallikrein-kinin system might be counteracted by post-transcriptional modification of aldosterone. These results suggest that exercise enhances renal dopamine production by activating renal AADC activity, which in turn stimulates the renal kallikrein-kinin system.

Effect of gonadal steroids and gamma-aminobutyric acid on LH release and dopamine expression and activity in the zona incerta in rats.

A dopaminergic system in the zona incerta stimulates LH release and may mediate the positive feedback effects of the gonadal steroids on LH release. In this study the mechanisms by which steroids might increase dopamine activity in the zona incerta were investigated. In addition, experiments were conducted to determine whether the inhibitory effects of gamma-aminobutyric acid (GABA) on LH release in the zona incerta are due to suppression of dopamine activity in this area or conversely whether the stimulatory effects of dopamine on LH release are due to suppression of a tonic inhibitory GABAergic system. Ovariectomized rats were treated s.c. with oil, 5 micrograms oestradiol benzoate or 5 micrograms oestradiol benzoate followed 48 h later by 0.5 mg progesterone, and killed 54 h after the oestradiol benzoate injection. At this time the LH concentrations were suppressed in the oestradiol benzoate group and increased in the group treated with oestradiol benzoate and progesterone. The ratio of tyrosine hydroxylase:beta-actin mRNA in the zona incerta was significantly increased by the oestradiol benzoate treatment, but the addition of progesterone resulted in values similar to those in the control group. At the same time, the progesterone treatment increased tyrosine hydroxylase activity in the zona incerta as indicated by an increase in L-dihydroxyphenylalanine (L-DOPA) accumulation after 100 mg 3-hydroxybenzylhydrazine hydrochloric acid (NSD1015) kg-1 and an increase in dopamine release as indicated by a increase in dihydroxyphenylacetic acid (DOPAC) concentrations (one of the major metabolites of dopamine). Ovariectomized rats treated with oestradiol benzoate plus progesterone were also injected i.p. with 75 mg gamma-acetylenic GABA kg-1 (a GABA transaminase inhibitor) to increase GABA concentrations in the brain. This treatment had no effect on the ratio of tyrosine hydroxylase:beta-actin mRNA but decreased L-DOPA accumulation and DOPAC concentrations in the zona incerta, indicating a post-translational inhibition of dopamine synthesis and release. Treatment of ovariectomized rats with oestradiol benzoate followed by 100 mg L-DOPA i.p. to increase dopamine concentrations in the whole brain had no effect on glutamic acid decarboxylase mRNA expression in the zona incerta, although it increased the glutamic acid decarboxylase:beta-actin mRNA ratio in other hypothalamic areas (that is, the medical preoptic area, ventromedial nucleus and arcuate nucleus). In conclusion, the steroids act to increase dopamine activity in different ways: oestrogen increases tyrosine hydroxylase mRNA expression and progesterone acts after translation to increase tyrosine hydroxylase activity and dopamine release (as indicated by increases in DOPAC concentrations). This latter effect may be due to progesterone removing a tonic GABAergic inhibition from the dopaminergic system.

Stimulation of the dopamine 1 receptor increases lung edema clearance.

We previously reported that lung edema clearance was stimulated by dopamine (DA). The purpose of this study was to determine whether the DA-mediated stimulation of edema clearance occurs via an adrenergic or dopaminergic regulation of alveolar epithelial Na, K-ATPase. When isolated perfused rat lungs were coinstilled with DA and SCH 23390 (a specific D(1) receptor antagonist), there was a dose-dependent attenuation of the stimulatory effects of DA. Coinstillation with S-sulpiride (a specific D(2) receptor antagonist) or propranolol (a beta-adrenergic antagonist) did not alter DA-stimulated clearance. Similarly, the specific dopaminergic D(1) agonist fenoldopam increased lung edema clearance, but quinpirole (a specific dopaminergic D(2) agonist) did not. (125)I-SCH 23982 binding studies suggested that D(1) receptors are expressed on alveolar type II (ATII) cells with an apparent dissociation constant (K(d)) of 4.4 nM and binding maximum (Bmax) 9.8 pmol/mg. Consistent with these results, the D(1) receptor messenger RNA (mRNA) and protein were detected in ATII cells by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. These data demonstrate a novel mechanism involving the activation of dopaminergic D(1) receptors which mediates DA-stimulated edema removal from rat lungs.

Modification of cataleptic responses to dopamine receptor antagonists after withdrawal from chronic cocaine or cocaine plus dopamine antagonist administration

1. 1. In mice pretreated chronically with cocaine (indirect dopamine agonist: 10 mg/Kg S.C. on alternating days for 15 days), haloperidol (dopamine D 2 antagonist: 0.3 mg/kg i.p.) exerted an enhanced cataleptic response, but SCH23390 (dopamine D 1 antagonist: 0.3 mg/kg i.p.) produced an attenuated response at 24 h, which converted to a supernormal response, when it was administered 15–60 days after withdrawal from cocaine. 2. 2. A challenge dose of SCH23390 exhibited enhanced catalepsy when given 15 days, but not at 24 h, after the last pretreatment dose of SCH23390 (0.1–1.0 mg/kg s.c.). In contrast, haloperidol catalepsy was not affected by the SCH23390 pretreatment. 3. 3. However, in animals chronically pretreated with haloperidol (0.1–1.0 mg/kg s.c.), a challenge dose of SCH23390 as well as haloperidol exhibited attenuated cataleptic effects at 24 h and normal cataleptic responses at 15 days after the last dose of the pretreatment regimen. 4. 4. Challenge doses of haloperidol or SCH23390 given to mice 24 h after chronic cocaine pretreatment produced enhanced and attenuated cataleptic responses, respectively; however, these responses were no longer produced when haloperidol or SCH23390 was given to mice pretreated chronically with a combination of cocaine and either haloperidol or SCH23390. 5. 5. The enhanced catalepsy produced by a challenge dose of SCH23390 (15–60 days after chronic cocaine) was further potentiated when it was administered to animals that had been pretreated chronically with a combination of SCH23390 and cocaine, but was antagonized in animals pretreated chronically with haloperidol and cocaine. In contrast, the degree of enhanced cataleptic responses produced by a challenge dose of haloperidol 30–60 days after pretreatment chronically with a combination of cocaine + SCH23390 was similar to that seen after chronic cocaine alone. However, this enhanced response was antagonized in animals that had been pretreated chronically with the combination of cocaine+haloperidol. 6. 6. The results suggest that the coadministration of SCH23390 with cocaine was able to block indirectly dopamine D 2 receptor inhibition (subsensitivity) induced during the early withdrawal period from chronic cocaine, despite the fact that by itself SCH23390 did not have an effect on haloperidol catalepsy. Accordingly, the stimulatory effects of dopamine D 2 receptors by a single administration of cocaine may be mediated mainly by an indirect stimulation of dopamine D 2 receptor function via its D 1 receptor stimulating action. 7. 7. The coadministration of SCH23390 with cocaine rather aggravate the subsensitive effect of dopamine D 1 receptors (increased SCH23390 catalepsy) produced during long-term withdrawal period from chronic cocaine, but did not affect that of the dopamine D 2 receptor. On the other hand, the coadministration of haloperidol with cocaine normalized both D 1 and D 2 receptor subsensitive effect. 8. 8. These result suggest that a single administration of SCH23390 or haloperidol after long-term withdrawal periods from chronic cocaine may not be effective as antipsychotic drugs because of further aggravation of suppressive behaviors. These results also provide evidence that D 2 receptor antagonists may be more effective as antipsychotic drugs than dopamine D 1 receptor antagonist, since the coadministration of haloperidol with cocaine normalized the abnormal behaviors seen during early and long-term withdrawal periods from chronic cocaine.

Stimulation of Na(+)-K(+)-2Cl- cotransport in rat medullary thick ascending limb by dopamine.

Dopamine receptors are present in the medullary thick ascending limb (mTAL) of Henle, but their effect on ion transport in this nephron segment has not been tested. Therefore, we studied the short-term effects of dopamine on Na(+)-K(+)-2Cl- cotransport (assessed by 100 microM bumetanide-sensitive 86Rb uptake) in rat mTAL tubular suspensions. Dopamine (1 microM) stimulated bumetanide-sensitive 86Rb uptake (72.1 +/- 10.6% vs. control, n = 5) by increasing total 86Rb uptake and by decreasing bumetanide-insensitive 86Rb uptake; this effect was concentration dependent. The dopamine-induced stimulation of Na(+)-K(+)-2Cl- cotransport activity was mimicked by calyculin A, a protein phosphatase (PP) inhibitor, and Sp isomer of adenosine 3',5'-cyclic monophosphothioate (Sp-cAMP[S]), a protein kinase A (PKA) agonist, and blocked by Rp isomer of 8-(4-chlorophenylthio)-cAMP[S] (Rp-8-CPT-cAMP[S]), a PKA inhibitor (n = 5). Dopamine did not increase the stimulatory effect of the PP inhibitor. However, the stimulatory effect of the PP inhibitor and PKA agonist was additive and approached the stimulatory effect of dopamine. The stimulatory effects of dopamine, PP inhibitor, and PKA agonist persisted even when intracellular sodium was clamped by 5 microM monensin. When K+ channels were blocked by 1 mM BaCl2, the effects of dopamine and calyculin A on the cotransport were no longer apparent, although the stimulatory effect of the PKA agonist was attenuated. We conclude that dopamine stimulates Na(+)-K(+)-2Cl- cotransport activity. This action is mediated mainly by PKA-dependent phosphorylation/dephosphorylation processes and modulated by dopamine actions on K+ channels.

Effects of dopamine on adenylyl cyclase activity and amylase secretion in rat parotid tissue.

Several previous studies have shown that dopamine causes amylase secretion from rat parotid tissue. However, the mechanism of this dopamine action is still unclear. The present study was designed to characterize dopamine action in rat parotid gland tissue by examining the effects of dopamine on cyclic AMP accumulation, adenylyl cyclase activity, and amylase release. Dopamine significantly enhanced accumulation of cyclic AMP in parotid slices and stimulated adenylyl cyclase activity in parotid membrane preparations. It also significantly stimulated amylase release from parotid slices. The stimulatory effects of dopamine on cyclic AMP accumulation, adenylyl cyclase activity, and amylase release were effectively blocked with propranolol, a beta-adrenergic antagonist, but not by either SCH 23390, a preferential D1 antagonist, or butaclamol, a preferential D2 antagonist. No substantial specific binding sites for D1 receptors were detectable by [3H]SCH 23390 binding in parotid membranes. These results suggest that the stimulatory effect of dopamine on amylase secretion in rat parotid tissue is not mediated through specific D1 dopamine receptors but rather through beta-adrenergic receptors.

Evidence that stimulatory dopamine receptors may be involved in the regulation of prolactin secretion

It is well established that dopamine (DA) effectively inhibits PRL secretion from anterior pituitary mammotropes via D2-DA receptors. Paradoxically, it is reported that the monoamine can actually increase PRL release under appropriate experimental conditions. Although the mechanism underlying this stimulatory effect remains undefined, the ability of D1- and D5-DA receptors to activate adenylyl cyclase raises the possibility that a similar receptor subtype is present in the anterior pituitary and mediates the stimulatory effects of DA on PRL release. The purpose of the present study was to explore this possibility. First, we tested whether D1 and D5 receptors could couple to and stimulate PRL secretion. Subclones of GH4C1 cells (which secrete PRL, but do not express DA receptors) stably expressing human D1 or D5 receptors were treated with DA (10(-16)-10(-6) M), and the medium PRL content was measured by RIA. Subclones transfected with short or long forms of the human D2 receptor were also tested. As expected, DA (10(-6) M) inhibited PRL release from cells expressing either short or long D2 receptors by 41% and 39%, respectively (P < 0.01; n = 4 separate experiments). In contrast, comparable concentrations of DA (10(-(8) and 10(-6) M) increased PRL release from cells expressing D1 or D5 receptors by 76% and 122%, respectively (P < 0.01; n = 4). Thus, both D1 and D5 receptors were fully capable of stimulating PRL release from transfected GH4C1 cells. We next sought to determine whether the gene for at least one of these structurally similar receptors was expressed in rat anterior pituitary tissue. First strand cDNA was synthesized, using a rat D5-specific oligonucleotide primer and reverse transcriptase, from total RNA extracted from the anterior pituitary glands of five lactating female rats. The specifically primed cDNA then served as a template for 35 cycles of polymerase chain reaction amplification in which nested primers specific for the rat D5 receptor were used. Electrophoresis of the DNA resolved a 696-basepair band corresponding to a fragment of the D5 receptor in each of five anterior pituitary samples (verified by digestion with three different restriction endonucleases). Taken together, these results demonstrate that both D1 and D5 receptors are capable of mediating the stimulatory effects of DA on PRL release and that the mRNA for DA D5 receptors is present in rat anterior pituitary glands. Our findings support the view that PRL release in vivo may be modulated via one or more stimulatory DA receptors.

Evidence that stimulatory dopamine receptors may be involved in the regulation of prolactin secretion.

It is well established that dopamine (DA) effectively inhibits PRL secretion from anterior pituitary mammotropes via D2-DA receptors. Paradoxically, it is reported that the monoamine can actually increase PRL release under appropriate experimental conditions. Although the mechanism underlying this stimulatory effect remains undefined, the ability of D1- and D5-DA receptors to activate adenylyl cyclase raises the possibility that a similar receptor subtype is present in the anterior pituitary and mediates the stimulatory effects of DA on PRL release. The purpose of the present study was to explore this possibility. First, we tested whether D1 and D5 receptors could couple to and stimulate PRL secretion. Subclones of GH4C1 cells (which secrete PRL, but do not express DA receptors) stably expressing human D1 or D5 receptors were treated with DA (10(-16)-10(-6) M), and the medium PRL content was measured by RIA. Subclones transfected with short or long forms of the human D2 receptor were also tested. As expected, DA (10(-6) M) inhibited PRL release from cells expressing either short or long D2 receptors by 41% and 39%, respectively (P < 0.01; n = 4 separate experiments). In contrast, comparable concentrations of DA (10(-(8) and 10(-6) M) increased PRL release from cells expressing D1 or D5 receptors by 76% and 122%, respectively (P < 0.01; n = 4). Thus, both D1 and D5 receptors were fully capable of stimulating PRL release from transfected GH4C1 cells. We next sought to determine whether the gene for at least one of these structurally similar receptors was expressed in rat anterior pituitary tissue. First strand cDNA was synthesized, using a rat D5-specific oligonucleotide primer and reverse transcriptase, from total RNA extracted from the anterior pituitary glands of five lactating female rats. The specifically primed cDNA then served as a template for 35 cycles of polymerase chain reaction amplification in which nested primers specific for the rat D5 receptor were used. Electrophoresis of the DNA resolved a 696-basepair band corresponding to a fragment of the D5 receptor in each of five anterior pituitary samples (verified by digestion with three different restriction endonucleases). Taken together, these results demonstrate that both D1 and D5 receptors are capable of mediating the stimulatory effects of DA on PRL release and that the mRNA for DA D5 receptors is present in rat anterior pituitary glands. Our findings support the view that PRL release in vivo may be modulated via one or more stimulatory DA receptors.

Dopamine and nicotine, but not serotonin, modulate the crustacean ventilatory pattern generator.

Dopamine (DA) causes a dose-dependent increase in the frequency of motor neuron bursts [virtual ventilation (fR)] produced by deafferented crab ventilatory pattern generators (CPGv). Domperidone, a D2-specific DA antagonist, by itself reversibly depresses fR and also blocks the stimulatory effects of DA. Serotonin (5HT) has no direct effects on this CPGv. Nicotine also causes dramatic dose-dependent increases in the frequency of motor bursts from the CPGv. The action is triphasic, beginning with an initial reversal of burst pattern typical of reversed-mode ventilation, followed by a 2- to 3-min period of depression and then a long period of elevated burst rate. Acetylcholine chloride (ACh) alone is ineffective, but in the presence of eserine is moderately stimulatory. The inhibitory effects of nicotine are only partially blocked by curare. The excitatory action of nicotine is blocked by prior perfusion of domperidone, but not by SKF-83566.HCl, a D1-specific DA antagonist. SKF-83566 had no effects on the ongoing pattern of firing. These observations support the hypothesis that dopaminergic pathways are involved in the maintenance of the CPGv rhythm and that the acceleratory effects of nicotine may involve release of DA either directly or via stimulation of atypical ACh receptors at intraganglionic sites.

The stimulatory and inhibitory effects of dopamine on prolactin secretion involve different G-proteins.

The reverse hemolytic plaque assay (RHPA) was used in this study to further characterize the mechanism whereby low concentrations of dopamine (DA) stimulate PRL secretion in vitro. Female Sprague-Dawley rats were used as a source of anterior pituitary cells for the RHPA. Pituitary cells were infused into Cunningham chambers along with a suspension of protein-A-coated ovine red blood cells. Excess cells were rinsed from the chambers leaving a monolayer of cells attached to the glass. The cells were then incubated with solutions containing PRL antiserum (1:40) and various concentrations of DA. After 4 h, a solution containing guinea pig complement (1:60) was infused into the chambers. Thirty minutes later, the cells were fixed and plaques (zones of hemolysis) surrounding PRL-producing cells (lactotrophs) were measured and used as an index of the amount of PRL secreted. Control cells that received no DA had a mean plaque area of 8,000 microns 2 and two distinct subpopulations of plaque sizes. This biphasic population of cells consisted of a small and a large plaque producing population. The mean plaque area surrounding lactotrophs was significantly (P less than 0.05) decreased if 1 microM or 10 microM DA was present (4,500 microns 2 and 3,500 microns 2, respectively). These cells which received inhibitory concentrations of DA demonstrated a monophasic distribution of plaque-forming cells. On the other hand, mean plaque area was significantly (P less than 0.05) increased if 0.1 nM or 1 nM DA was presented to the cells (15,000 microns 2 and 14,500 microns 2, respectively). These cells receiving stimulatory doses of DA exhibited a multiphasic distribution of plaque-forming cells. The possibility that the two physiological opposing actions of DA on PRL secretion might be mediated by different GTP binding proteins was also examined using cholera toxin (CTX) and pertussis toxin (PTX). Anterior pituitary cells were pretreated with either CTX (50 micrograms/ml) or PTX (5 micrograms/ml) for 1 h before initiation of the RHPA. In the RHPA, cells received no DA, a stimulatory dose of DA (0.1 nM), or a inhibitory dose of DA (10 microM). The effects of toxin pretreatment on mean plaque area of DA-treated cells was determined. PTX pretreatment significantly attenuated the inhibitory effects of DA while having no effect on the stimulatory effects of DA on PRL secretion. CTX significantly (P less than 0.05) potentiated the stimulatory effects of DA on PRL secretion and had no effect on inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulatory effects of quinpirole hydrochloride D 2-dopamine receptor agonist, at low concentrations on prolactin release in female rats in vitro

Dopamine (DA) has dual actions (inhibitory and stimulatory) in the regulation of prolactin (PRL) release, depending on its concentration. To investigate the stimulatory effects of DA, perifused rat anterior pituitary cells were exposed to the highly-specific DA D 2 receptor agonist, quinpirole hydrochloride (LY). Very low concentrations of LY (10 −12–10 −10M) stimulated PRL release and potentiated thyrotropin-releasing hormone (TRH)-induced PRL release. Higher concentrations of LY did not stimulate. Pretreatment with pertussis toxin (30 ng/ml, 24 h) completely abolished these effects of LY. The D 2 receptor antagonist, metocloprimide, also blocked the potentiation by LY of TRH-induced PRL release. These data indicate that very low concentrations of dopamine stimulate PRL release via an interaction with a D 2 receptor connected to a pertussis toxin-sensitive G protein.

Suckling unmasks the stimulatory effect of dopamine on prolactin release: possible role for alpha-melanocyte-stimulating hormone as a mammotrope responsiveness factor.

Mounting evidence indicates that dopamine (DA) can stimulate as well as inhibit PRL release when given in appropriately low doses. In the present study, we investigated whether the suckling stimulus could influence this response. Pituitary cultures from suckled or nonsuckled rats were exposed to DA (10(-16) - 10(-6) M) during a reverse hemolytic plaque assay for PRL. Pituitary cells from nonsuckled rats exhibited only the inhibitory response to DA; exposure to high-dose DA (10(-6) M) reduced plaque area to 42.3 +/- 7.2% (mean +/- SEM) of control. A low dose of DA (10(-12) M) had no effect on PRL secretion (79.3 +/- 13.3% of control). In striking contrast, a brief suckling stimulus (10 min) rendered the mammotropes responsive to stimulation by low-dose DA (to 152.7 +/- 12.5% of control). Thus, suckling appears to be a requirement for expression of the stimulatory effect of DA in lactators. In a subsequent series of experiments we explored the possibility that a hypophysial factor, released during nursing, might mimic the effects of suckling on mammotrope responsiveness. Accordingly, we tested the effects of alpha-melanocyte-stimulating hormone (10(-7) M) and low-dose DA, alone or in combination, on pituitary cells from nonsuckled rats. Although neither agent alone had a dramatic effect on PRL secretion, concurrent administration of both of these significantly stimulated PRL release to 130.0 +/- 4.2% of control. Taken together, these results demonstrate that suckling renders mammotropes responsive to the stimulatory effects of DA. Moreover, our data indicate that alpha-melanocyte-stimulating hormone could function as a responsiveness factor in this phenomenon.

Pharmacologic Evidence that a D2 Receptor Subtype Mediates Dopaminergic Stimulation of Prolactin Secretion from the Anterior Pituitary Gland

The ability of low concentrations of dopamine (DA) to stimulate the secretion of prolactin (PRL) was examined in perifused or monolayer cultures of anterior pituitary cells. In cultures perifused with media containing 100 nM DA, changing the DA concentration to either 1 or 100 pM caused a significant dose-dependent stimulatory PRL secretory response within 6 min when compared to the PRL secretory response to removal of DA altogether. Picomolar concentrations of DA caused a biphasic PRL secretory response. This response is characterized by an immediate increase in the rate of PRL secretion similar to that seen when the cells were treated with 100 nM thyrotropin-releasing hormone followed by a decrease in the rate of PRL secretion to levels comparable to cells receiving media alone. In a monolayer culture system DA, at concentrations between 10 nM and 1.0 pM, caused significant stimulation of PRL secretion relative to media alone. Maximal stimulation occurred at nanomolar concentrations of DA (approximately 60% greater than control). Although the D2 agonists, bromocriptine and 2-(N-phenethyl-N-propyl)-amino-5-hydroxytetralin hydrochloride (PPHT) caused significant (p less than 0.05) inhibition of PRL secretion at nanomolar concentrations and above, neither had stimulatory activity. The D1 agonists, SKF 38393 and SKF 82958, had no effect on PRL secretion when tested at 0.1 pM to 1 microM. These data suggest that DA not only inhibits PRL secretion in vitro, but also stimulates PRL secretion at relatively low concentrations. Stimulation is mediated by a DA receptor which is neither recognized by D2 nor D1 agonists, suggesting a possible third DA receptor subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine stimulation of cAMP production in cultured opossum kidney cells

Dopamine receptors have been identified in many tissues including the kidney. To establish an in vitro system as a model for dopamine action, we studied the effect of dopamine (DA) receptor agonists and antagonists on adenosine 3',5'-cyclic monophosphate (cAMP) formation in opossum kidney (OK) cells. The stimulation of cAMP production in these cells by dopamine was dose dependent, and markedly higher levels were observed in the presence of dopamine plus a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. Half-maximal stimulation was found with 1.15 +/- 0.22 microM dopamine. A DA1-receptor agonist, SKF 82526J, stimulated cAMP production, whereas a DA2-receptor agonist, Ly 171555, did not. The stimulatory effects of dopamine and SKF 82526J were abolished by a specific DA1-receptor antagonist, Sch 23390 with half-maximal inhibition concentrations of 1.24 +/- 0.18 and 4.0 +/- 0.5 nM, respectively. In contrast, the DA2-receptor antagonist, spiperone, had no inhibitory effect on dopamine- and SKF 82526J-stimulated cAMP production. Beta-Adrenergic antagonists failed to attenuate the stimulatory effects of dopamine and SKF 82526J on cAMP production. In addition, the beta-adrenergic receptor agonist, isoproterenol, did not stimulate cAMP production. These results suggest that the action of dopamine was not mediated through beta-adrenergic receptors. Furthermore, our results clearly demonstrated the existence of DA1-receptors linked to adenylate cyclase in OK cells.

Comparison of the inhibitory effects of neuroleptic drugs on adenylate cyclase in rat tissues stimulated by dopamine, noradrenaline and glucagon

The effect of neuroleptic drugs of various chemical classes on adenylate cyclase from a variety of rat tissues has been investigated. These were the dopamine-stimulated adenylate cyclase in rat striatal homogenates, the noradrenaline-stimulated adenylate cyclase in adipocyte membrane fragments and the glucagon-stimulated adenylate cyclase in liver membranes. The stimulatory effects of dopamine on rat striatal adenylate cyclase were apparent within a minute of the start of incubations and over a wide range of temperatures. Moreover the stimulation could be rapidly inhibited by several classes of neuroleptic agent. The inhibitory effect of neuroleptics of all classes was completely reversed by high concentrations of dopamine and this reversal took place rapidly at 30°. Neuroleptics were also able to inhibit the stimulatory effects of noradrenaline on adipocyte adenylate cyclase at concentrations where the basal enzyme activity was not affected. However the effects of drugs in this system occurred at much higher concentrations than in the dopamine-stimulated system. In addition drug potencies as inhibitors of noradrenaline did not correlate with neuroleptic activity, α and β-flupenthixol for example were equipotent as noradrenaline antagonists. Neuroleptics were able to inhibit the stimulation of liver membrane adenylate cyclase by glucagon. The potencies of drugs as inhibitors in this system agian did not correlate with neuroleptic potencies. Higher concentrations of glucagon were not able to reverse the inhibition produced by neuroleptics. It is concluded that the effects of neuroleptic drugs on dopamine-stimulated adenylate cyclase correlate most closely with their clinical neuroleptic potencies.