Involvement of dopamine D2 receptor in the diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in female rats

Background

An endogenous dopaminergic (DA) tone acting on D₃ receptors has been shown to inhibit tuberoinfundibular (TI) DA neuron activity and stimulate prolactin (PRL) surge in the afternoon of estrogen-primed ovariectomized (OVX+E₂) rats. Whether D₂ receptor (D₂R) is also involved in the regulation of TIDA and PRL rhythms was determined in this study.

Results

Intracerebroventricular (icv) injection of PHNO, a D₂R agonist, in the morning inhibited TIDA and midbrain DA neurons’ activities, and stimulated PRL secretion. The effects of PHNO were significantly reversed by co-administration of raclopride, a D₂R antagonist. A single injection of raclopride at 1200 h significantly reversed the lowered TIDA neuron activity and the increased serum PRL level at 1500 h. Dopamine D₂R mRNA expression in medial basal hypothalamus (MBH) exhibited a diurnal rhythm, i.e., low in the morning and high in the afternoon, which was opposite to that of TIDA neuron activity. The D₂R rhythm was abolished in OVX+E₂ rats kept under constant lighting but not in OVX rats with regular lighting exposures. Pretreatment with an antisense oligodeoxynucleotides (AODN, 10 μg/3 μl/day, icv) against D₂R mRNA for 2 days significantly reduced D₂R mRNAs in central DA neurons, and reversed both lowered TIDA neuron activity and increased serum PRL level in the afternoon on day 3. A diurnal rhythm of D₂R mRNA expression was also observed in midbrain DA neurons and the rhythm was significantly knocked down by the AODN pretreatment.

Conclusions

We conclude that a diurnal change of D₂R mRNA expression in MBH may underlie the diurnal rhythms of TIDA neuron activity and PRL secretion in OVX+E₂ rats.     

Evidence for increased endogenous dopaminergic inhibition of aldosterone secretion in prolactinoma

Aldosterone responsiveness to consecutive i.v. injections of metoclopramide 1 mg, 2.5 mg and 10 mg was studied in 8 patients with prolactinoma and normally preserved adrenal function and in 14 healthy volunteers. In the patients, aldosterone response to metoclopramide 1 mg was blunted. After metoclopramide 10 mg, aldosterone rose to the same levels in patients and volunteers. In the patients, however, percentage rise of aldosterone was enhanced, since the appropriate base line concentration of aldosterone was decreased. Thus, there is evidence for increased endogenous dopaminergic inhibition of aldosterone secretion in prolactinoma.     

Evidence for dopaminergic modulation of pancreatic polypeptide secretion in man

This study examines the role of dopaminergic mechanisms in the regulation of human pancreatic polypeptide (hPP) secretion in 11 normal male volunteers. Administration of domperidone (20 mg iv), an extracerebral inhibitor of dopamine receptors, resulted in a hPP rise (p<0.05) within 10 min and a peak response (p<0.01) at 15 min after drug administration. Administration of the dopaminergic agonist, bromocriptine, 2.5 mg tid for 4 days eliminated hPP responses to isometric handgrip exercise in these 11 volunteers. These results suggest that dopaminergic mechanisms may exert a tonic inhibitory effect on hPP secretion in normal subjects.     

Involvement of opioid receptor subtypes in rat feeding behavior

The short-acting opiate antagonist naloxone decreases food intake in three models of ingestive behavior: free feeding, food-deprivation induced feeding and deoxyglucose-induced feeding. Twenty-four hours after administration, the long-acting, mu1 selective antagonist naloxonazine inhibits food intake to the same extent as naloxone in freely feeding and food-deprived rats, but not in animals treated with 2-deoxyglucose. These results indicate that 1) opiates modulate feeding through multiple opioid receptor mechanisms, one of which is the mu subtype, and 2) the feeding observed in various experimental paradigms are modulated by different receptor subtypes. Furthermore, these results illustrate the usefulness of naloxone in defining a behavior as opioid but point out its limitations in discriminating between opioid receptor subtypes.     

Involvement of hypothalamic dopamine in the regulation of prolactin secretion

The neuroendocrine control of prolactin (PRL) secretion is known to be a multifactorial process, but dopamine (DA) secreted by the tuberoinfundibular dopaminergic (TIDA) neurons of the hypothalamus is believed to exert a predominant inhibitory control on the secretion of PRL. The secretory activity of the TIDA neurons, including the rate of biosynthesis of DA and the rate of release of the neurohormone into hypophysial portal blood, can be readily evaluated in the rat. In most conditions in which an altered secretion of PRL has been documented, an altered secretory activity of the TIDA neurons has been found. When an acute reduction in the secretion of DA is observed, an increased secretion of PRL is associated, with an inverse relationship between DA and PRL concentrations in hypophysial portal and systemic blood, respectively. However, the secretion of PRL can be regulated by PRL itself through stimulation of the secretory activity of the TIDA neurons, and consequently hyperprolactinemia can be observed concomitantly with a sustained high secretion of DA, as seen after treatment with estrogen. The short loop feedback of PRL secretion seems to be impaired in the aging rat, since a sustained reduced hypothalamic secretion of DA is observed in spite of long-term hyperprolactinemia.     

Effects of fasting on aldosterone secretion in ovariectomized rats

The present study was designed to assess the effect of fasting on aldosterone secretion in ovariectomized (Ovx) rats. Ovx rats were divided into fed (allowed access to food ad libitum) and fasted (deprived of food for 24 hours) groups. The trunk blood of fed and fasted rats was collected after decapitation. In the in vitro study, adrenal zona glomerulosa (ZG) cells from fed or fasted rats were incubated with angiotensin II (Ang II, 10(-6) M), adrenocorticotropic hormone (ACTH, 10(-9) M), or forskolin (an activator of adenylyl cyclase, 10(-6) M) at 37 degrees C for 30 min. The levels of aldosterone in medium and plasma extracts were measured by radioimmunoassay. Results showed that the levels of plasma aldosterone in fasted rats were lower than those in fed rats. There were no significant differences in basal and Ang II-stimulated aldosterone secretion between fed and fasted groups. The increment of aldosterone induced by ACTH in fasted group was significantly less than that in fed group. Administration of forskolin led to a significant increase in aldosterone secretion in both fed and fasted groups. Fasted group had a decreased aldosterone secretion in response to forskolin as compared with fed group. In summary, these results suggest that fasting decreases aldosterone secretion in Ovx rats through a mechanism in part involving a reduction of aldosterone production in response to ACTH, a decreased activity of adenylyl cyclase, and/or an inhibition of post-cAMP pathway in ZG cells.     

Involvement of prostaglandin E receptor EP3 subtype in duodenal bicarbonate secretion in rats

We investigated the involvement of prostaglandin E (PGE) receptor subtype EP3 in the regulatory mechanism of duodenal HCO₃⁻ secretion in rats. A proximal duodenal loop or a chambered stomach was perfused with saline, and HCO₃⁻ secretion was measured using a pH-stat method and by adding 2 mM HCl. Mucosal acidification was achieved through 10 min of exposure to 10 mM HCl in the duodenum or 100 mM HCl in the stomach. Various EP agonists or the EP4 antagonist were given i.v., while the EP1 or EP3 antagonist was given s.c. or i.d., respectively. Sulprostone (EP1/EP3 agonists) stimulated duodenal HCO₃⁻ secretion in a dose-dependent manner, and this response was inhibited by AE5-599 (EP3 antagonist) but not AE3-208 (EP4 antagonist). AE1-329 (EP4 agonist) also increased duodenal HCO₃⁻ secretion, and this action was inhibited by AE3-208 but not AE5-599. The response to PGE₂ or acidification in the duodenum was partially attenuated by AE5-599 or AE3-208 alone but completely abolished by the combined administration. Duodenal damage caused by mucosal perfusion with 150 mM HCl for 4 h was worsened by pretreatment with AE5-599 and AE3-208 as well as indomethacin and further aggravated by co-administration of these antagonists. Neither the EP3 nor EP4 antagonist had any effect on the gastric response induced by PGE₂ or acidification. These results clearly demonstrate the involvement of EP3 receptors, in addition to EP4 receptors, in the regulation of duodenal HCO₃⁻ secretion as well as the maintenance of the mucosal integrity of the duodenum against acid injury.     

Effects of estradiol on aldosterone secretion in ovariectomized rats

The effects and action mechanisms of estradiol on aldosterone secretion in female rats were studied. Replacement of estradiol benzoate (EB) increased the levels of plasma estradiol and aldosterone in ovariectomized (Ovx) rats. The aldosterone release from zona glomerulosa (ZG) cells was higher in EB-treated rats than in oil-treated animals. EB treatment potentiated the responses of aldosterone release to adrenocorticotropic hormone (ACTH), forskolin (FSK), and 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP). Administration of EB in vivo did not alter cAMP production in response to ACTH or FSK. Although angiotensin II (Ang II) increased aldosterone secretion by rat ZG cells, the stimulatory effect of Ang II on the release of aldosterone was not altered by EB treatment. The conversions of [3H]-deoxycorticosterone to [3H]-corticosterone and [3H]-corticosterone to [3H]-aldosterone in EB-treated groups were greater than those in the oil-treated group. These results suggest that estradiol increases aldosterone secretion in part through the mechanisms involving the activation of the post-cAMP pathway, 11beta-hydroxylase and aldosterone synthase activity.     

Role of endothelin receptor subtypes in volume-stimulated ANF secretion

The role of endothelin (ET) receptors was tested in volume-stimulated atrial natriuretic factor (ANF) secretion in conscious rats. Mean ANF responses to slow infusions (3 x 3.3 ml/8 min) were dose dependently reduced (P < 0.05) by bosentan (nonselective ET-receptor antagonist) from 64.1 +/- 18.1 (SE) pg/ml (control) to 52.6 +/- 16.1 (0.033 mg bosentan/rat), 16.1 +/- 7.6 (0. 33 mg/rat), and 11.6 +/- 6.5 pg/ml (3.3 mg/rat). The ET-A-receptor antagonist BQ-123 (1 mg/rat) had no effect relative to DMSO controls, whereas the putative ET-B antagonist IRL-1038 (0.1 mg/rat) abolished the response. In a second protocol, BQ-123 (>/=0.5 mg/rat) nonsignificantly reduced the peak ANF response (106.1 +/- 23.0 pg/ml) to 74.0 +/- 20.5 pg/ml for slow infusions (3.5 ml/8.5 min) but reduced the peak response (425.3 +/- 58.1 pg/ml) for fast infusions (6.6 ml/1 min) by 49.9% (P < 0.001) and for 340 pmoles ET-1 (328.8 +/- 69.5 pg/ml) by 83.5% (P < 0.0001). BQ-123 abolished the ET-1-induced increase in arterial pressure (21.8 +/- 5.2 mmHg at 1 min). Changes in central venous pressure were similar for DMSO and BQ-123 (slow: 0.91 and 1.14 mmHg; fast: 4.50 and 4.13 mmHg). The results suggest 1) ET-B receptors mainly mediate the ANF secretion to slow volume expansions of <1.6%/min; and 2) ET-A receptors mainly mediate the ANF response to acute volume overloads.     

Prostaglandin E receptor subtypes involved in stimulation of gastroduodenal bicarbonate secretion in rats and mice.

We investigated prostaglandin E (EP) receptor subtypes responsible for the HCO3- stimulatory action of prostaglandin E2 (PGE2) in the gastroduodental mucosa, by examining the effects of various prostanoids with subtype specific EP receptor agonists in rats and those of PGE2 in knockout mice lacking EP1 or EP3 receptors. In rats, gastric HCO3- secretion was stimulated by i.v. administration of PGE2, 17-phenyl PGE2 the selective EP1 agonist as well as sulprostone the EP1 and EP3 agonist, but was not affected by other EP agonists such as butaprost the selective EP2 agonist, ONO-NT-012 the selective EP3 agonist or 11-deoxy PGE1 the EP3 and EP4 agonist. In contrast, the HCO3- secretion in rat duodenums was stimulated by PGE2, sulprostone, ONO-NT-012 as well as 11-deoxy PGE1 but not affected by either 17-phenyl PGE2 or butaprost. The HCO stimulatory effect of sulprostone in the stomach was significantly inhibited by ONO-AE-829, the selective EP1 antagonist. On the other hand, PGE2 applied topically to the mucosa for 10 min caused a dose-dependent increase of HCO3- secretion in both the stomach and duodenum of wild-type mice. The HCO3- stimulatory action of PGE2 in the stomach was also observed dose-dependently in knockout mice lacking EP3-receptors but was absent in EP1-receptor knockout mice, while the stimulatory effect in the duodenum was observed in EP1-receptor knockout mice, similar to wild-type animals, but not in knockout mice lacking EP3-receptors. These results indicate that PGE2 stimulates HCO3- secretion via different EP receptor subtypes in the stomach and duodenum; the former is mediated by EP1-receptors, while the latter mediated by EP3-receptors.     

Endogenous dopaminergic inhibition of aldosterone and prolactin secretion is apparently not increased in primary aldosteronism

To examine the previous suggestion that the endogenous dopaminergic activity would be increased in patients with primary aldosteronism, dose-response curves of aldosterone and prolactin stimulation by the dopamine antagonist metoclopramide were established in a pilot study by injecting metoclopramide 1, 2.5, and 10 mg i.v. consecutively at hourly intervals to 6 patients with primary aldosteronism and 14 healthy volunteers. All three metoclopramide doses induced clear-cut rises in aldosterone levels both in patients with primary aldosteronism and healthy controls. Basal aldosterone concentration was higher in range in the patients but the dose-response curves were nearly parallel one with the other. Prolactin responsiveness was also very similar. Thus, the present findings do not support the hypothesis of an increase in endogenous dopaminergic activity in primary aldosteronism.     

G-protein-coupled receptors: the new dopamine receptor subtypes.

The family of genes encoding G-protein-coupled dopamine receptors continues to grow with the recent cloning of a fifth member. The availability of these clones has revolutionized the dopamine receptor field. Expression of individual dopamine receptors is permitting the detailed analysis of their pharmacology and coupling to second messenger systems, while probes based on the receptors' nucleotide sequences are being used to gain new insights into their tissue distribution and genetics.     

Repeated stressful experiences differently affect brain dopamine receptor subtypes

The binding of tritiated spiperone (D2 antagonist) and tritiated SCH 23390 (D1 antagonist), in vivo, was investigated in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of mice submitted to ten daily restraint stress sessions. Mice sacrificed 24 hr after the last stressful experience presented a 64% decrease of D2 receptor density (Bmax) but no changes in D1 receptor density in the NAS. In the CP a much smaller (11%) reduction of D2 receptor density was accompanied by a 10% increase of D1 receptors. These results show that the two types of dopamine (DA) receptors adapt in different or even opposite ways to environmental pressure, leading to imbalance between them.     

Dopaminergic modulation of aldosterone secretion: effect of sodium balance and postural changes

The influence of an increased endogenous production of angiotensin II and of sodium homeostasis upon the response of plasma aldosterone to metoclopramide administration has been investigated in 5 normal volunteers. Our results show that the increase of plasma aldosterone after metoclopramide administration is independent of angiotensin II, ACTH and potassium, and that it increases even further due to the endogenous production of angiotensin II induced by postural changes. The state of sodium balance seems to influence the response of plasma aldosterone to metoclopramide administration as it occurs with other stimuli of aldosterone secretion.     

Age and the effect of adrenocorticotropic hormone on aldosterone secretion in rats

Age-related changes in functional activity of the glomerular zone were studied in the adrenal cortex of Wistar rats. It was shown that secretion of the basic mineralocorticoid hormone aldosterone was decreased with age. The glomerular zone of old rats showed the reduced sensitivity to the stimulant action of corticotropin, marked by the pronounced reaction to less doses of the hormone administered. At the same time the reactivity and the range of shifts in the course of ACTH dosage build-up decrease with aging.     

A patient with a prolactinoma associated with an aldosterone producing adrenal adenoma: differences in dopaminergic regulation of PRL and aldosterone secretion.

A patient with a rare combination of prolactinoma and aldosterone producing adrenal adenoma (APA) was reported in relation to studies concerning dopaminergic regulation of PRL and aldosterone secretion. The patient is a 38-year-old female with plasma PRL and aldosterone concentrations (PAC) of 563 ng/ml and 54 ng/dl, respectively. A bolus of 10 mg of metoclopramide significantly increased plasma PRL in 6 normal subjects and in 4 patients with APA, whereas the responses were blunted in 7 patients with prolactinoma and in our patient. The response of aldosterone to metoclopramide was less than that of PRL, but similar in all studied subjects, indicating that the dopaminergic inhibition of aldosterone secretion is less than that of PRL in normal subjects and did not change in patients with APA or prolactinoma. Oral administration of 2.5 mg of bromocriptine suppressed plasma PRL significantly in all the subjects studied, but did not produce any consistent changes in PAC. Discrepancies in the response of PRL and aldosterone to metoclopramide and to bromocriptine suggest a difference in the dopaminergic regulation of PRL and aldosterone secretion in both normal subjects and patients with prolactinoma and APA. It is unlikely that reduced dopaminergic inhibition is the basis for hypersecretion of PRL and aldosterone in our patient.     

Simulated microgravity impairs aldosterone secretion in rats: possible involvement of adrenomedullin

The prolonged exposure to microgravity (MG) or simulated MG (SMG) has been reported to cause hypotension, mainly due to reduced vascular contractility, and dysregulation of fluid and electrolyte balance. However, the mechanism(s) involved in these MG- or SMG-induced effects is not yet completely elucidated. Hence, we investigated in the rat the effect of prolonged (15 day) SMG, in the form of hindlimb unweighting, on the renin-angiotensin-aldosterone system (RAAS), as well as on atrial natriuretic peptide (ANP) and adrenomedullin (ADM), two hypotensive peptides that play a major role in the regulation of RAAS activity by inhibiting adrenal aldosterone secretion. SMG caused a mild hypotension in rats, associated with the blockade of body weight gain. Plasma aldosterone concentration and basal and agonist-stimulated in vitro aldosterone secretion from adrenal slices were decreased, and plasma renin activity was moderately increased. Neither Na(+) and K(+) serum concentrations nor ACTH and corticosterone blood levels were significantly affected. Plasma ANP concentration did not display significant alterations, while ADM blood concentration underwent a marked rise. The administration of the ADM-receptor antagonist ADM-(22-52) during the last 3 days of hindlimb unweighting reversed the SMG-induced hypotension and hypoaldosteronism. Collectively, these findings allow us to suggest that prolonged SMG impairs RAAS activity in rats, through a mechanism probably involving upregulation of the ADM system. Both hypoaldosteronism and increased ADM secretion may contribute to the development of hypotension during prolonged exposure to SMG.     

Inhibitors of aldosterone secretion

Aldosterone secretion may be inhibited by potassium depletion, inhibitors of the renin-angiotensin system, dopamine and atrial natriuretic factor. The latter appears to be an important physiological regulator of aldosterone secretion. ANF inhibits basal, ACTH, Angiotensin II and potassium-stimulated aldosterone production in vitro by a direct action on the adrenal gland. In vivo data also support a direct inhibitions of aldosterone. The stimulation of aldosterone secretion by infusions of Angiotensin II and potassium is inhibited by simultaneous infusions of ANF. Infusions of ANF lower the basal aldosterone secretion in man. The mechanism by which ANF inhibits aldosterone is not known. No unifying first step has been identified to explain ANF's ability to inhibit all stimuli. In vivo, part of the lowering of aldosterone levels may be due to inhibition of renin secretion. This effect of ANF upon renin is inconsistent and appears to depend upon the experimental conditions.