Pharmacological characterization and autoradiographic localization of dopamine receptors in the human adrenal cortex

Psychotropic Drug Effects on Steroid Stress Hormone Release and Possible Mechanisms Involved

There is no doubt that chronic stress accompanied by adrenocortical stress hormone release affects the development and treatment outcome of several mental disorders. Less attention has been paid to the effects of psychotropic drugs on adrenocortical steroids, particularly in clinical studies. This review focuses on the knowledge related to the possible modulation of cortisol and aldosterone secretion under non-stress and stress conditions by antipsychotic drugs, which are being used in the treatment of several psychotic and affective disorders. The molecular mechanisms by which antipsychotic drugs may influence steroid stress hormones include the modulation of central and/or adrenocortical dopamine and serotonin receptors, modulation of inflammatory cytokines, influence on regulatory mechanisms in the central part of the hypothalamic-pituitary axis, inhibition of corticotropin-releasing hormone gene promoters, influencing glucocorticoid receptor-mediated gene transcription, indirect effects via prolactin release, alteration of signaling pathways of glucocorticoid and mineralocorticoid actions. Clinical studies performed in healthy subjects, patients with psychosis, and patients with bipolar disorder suggest that single and repeated antipsychotic treatments either reduce cortisol concentrations or do not affect its secretion. A single and potentially long-term treatment with dopamine receptor antagonists, including antipsychotics, has a stimulatory action on aldosterone release.

Significance of dopamine D1 receptor signalling for steroidogenic differentiation of human induced pluripotent stem cells

Human induced pluripotent stem cells (hiPSCs) are expected to be both a revolutionary cell source for regenerative medicine and a powerful tool to investigate the molecular mechanisms underlying human cell development in vitro. In the present study, we tried to elucidate the steroidogenic differentiation processes using hiPSC-derived intermediate mesoderm (IM) that is known to be the origin of the human adrenal cortex and gonads. We first performed chemical screening to identify small molecules that induce steroidogenic differentiation of IM cells expressing Odd-skipped related 1 (OSR1), an early IM marker. We identified cabergoline as an inducer of 3β-hydroxysteroid dehydrogenase, an essential enzyme for adrenogonadal steroidogenesis. Although cabergoline is a potent dopamine D₂ receptor agonist, additional experiments showed that cabergoline exerted effects as a low-affinity agonist of D₁ receptors by increasing intracellular cyclic AMP. Further analysis of OSR1⁺ cells transfected with steroidogenic factor-1/adrenal 4 binding protein revealed that D₁ receptor agonist upregulated expression of various steroidogenic enzymes and increased secretion of steroid hormones synergistically with adrenocorticotropic hormone. These results suggest the importance of dopamine D₁ receptor signalling in steroidogenic differentiation, which contributes to effective induction of steroidogenic cells from hiPSCs.

NEFM (Neurofilament Medium) Polypeptide, a Marker for Zona Glomerulosa Cells in Human Adrenal, Inhibits D1R (Dopamine D1 Receptor)–Mediated Secretion of Aldosterone

Heterogeneity among aldosterone-producing adenomas (APAs) has been highlighted by the discovery of somatic mutations. KCNJ5 mutations predominate in large zona fasciculata (ZF)–like APAs; mutations in CACNA1D , ATP1A1, ATP2B3 , and CTNNB1 are more likely to be found in small zona glomerulosa (ZG)–like APAs. Microarray comparison of KCNJ5 mutant versus wild-type APAs revealed significant differences in transcriptomes. NEFM , encoding a neurofilament subunit which is a D1R (dopamine D1 receptor)–interacting protein, was 4-fold upregulated in ZG-like versus ZF-like APAs and 14-fold more highly expressed in normal ZG versus ZF. Immunohistochemistry confirmed selective expression of NEFM (neurofilament medium) polypeptide in ZG and in ZG-like APAs. Silencing NEFM in adrenocortical H295R cells increased basal aldosterone secretion and cell proliferation; silencing also amplified aldosterone stimulation by the D1R agonist, fenoldopam, and inhibition by the D1R antagonist, SCH23390. NEFM coimmunoprecipitated with D1R, and its expression was stimulated by fenoldopam. Immunohistochemistry for D1R was mainly intracellular in ZG-like APAs but membranous in ZF-like APAs. Aldosterone secretion in response to fenoldopam in primary cells from ZF-like APAs was higher than in cells from ZG-like APAs. Transfection of mutant KCNJ5 caused a large reduction in NEFM expression in H295R cells. We conclude that NEFM is a negative regulator of aldosterone production and cell proliferation, in part by facilitating D1R internalization from the plasma membrane. Downregulation of NEFM in ZF-like APAs may contribute to a D1R/D2R imbalance underlying variable pharmacological responses to dopaminergic drugs among patients with APAs. Finally, taken together, our data point to the possibility that ZF-like APAs are in fact ZG in origin.

Somatostatin and dopamine receptors as targets for medical treatment of Cushing's Syndrome

Somatostatin (SS) and dopamine (DA) receptors are widely expressed in neuroendocrine tumours that cause Cushing's Syndrome (CS). Increasing knowledge of specific subtype expression within these tumours and the ability to target these receptor subtypes with high-affinity compounds, has driven the search for new SS- or DA-based medical therapies for the various forms of CS. In Cushing's disease, corticotroph adenomas mainly express dopamine receptor subtype 2 (D(2)) and somatostatin receptor subtype 5 (sst(5)), whereas sst(2) is expressed at lower levels. Activation of these receptors can inhibit ACTH-release in primary cultured corticotroph adenomas and compounds that target either sst(5) (pasireotide, or SOM230) or D(2) (cabergoline) have shown significant efficacy in subsets of patients in recent clinical studies. Combination therapy, either by administration of both types of compounds separately or by treatment with novel somatostatin-dopamine chimeric molecules (e.g. BIM-23A760), appears to be a promising approach in this respect. In selected cases of Ectopic ACTH-producing Syndrome (EAS), the sst(2)-preferring compound octreotide is able to reduce cortisol levels effectively. A recent study showed that D(2) receptors are also significantly expressed in the majority of EAS and that cabergoline may decrease cortisol levels in subsets of these patients. In both normal adrenal tissue as well as in adrenal adenomas and carcinomas that cause CS, sst and DA receptor expression has been demonstrated. Although selected cases of adrenal CS may benefit from sst or DA-targeted treatment, its total contribution to the treatment of these patients is likely to be low as surgery is effective in most cases.

Novel insights in dopamine receptor physiology

The dopaminergic system has a pivotal role in the central nervous system but also plays important roles in the periphery, mainly in the endocrine system. Dopamine exerts its functions via five different receptors, named D(1)-D(5), belonging to the category of G protein coupled membrane receptors. Dopamine receptors are heterogeneously expressed in different cells, tissues and organs, where they stimulate or inhibit different functions, including neurotransmission and hormone synthesis and secretion. In particular, the dopamineric system has a pivotal role in the physiological regulation of the hypothalamus-pituitary-adrenal axis. Recent data have demonstrated the expression and function of dopamine receptors not only in endocrine organs but also in endocrine tumors, mainly those belonging to the hypothalamus-pituitary-adrenal axis, and also in the so-called 'neuroendocrine' tumors. These data confirm the important role of the dopaminergic system in this endocrine axis, as well as in the neuroendocrine system. This review summarizes the main structural and functional characteristics of dopamine receptors, emphasizing the most recent novelties, and focused on the physiological and pathological regulation of the hypothalamus-pituitary-adrenal axis by the dopaminergic system. In addition, the recent findings on the relationship between dopamine receptors and neuroendocrine tumors are summarized.

Dopamine Antagonist Alters Serum Cortisol and Prolactin Secretion in Lactating Holstein Cows

The role of dopamine in regulating glucocorticoid and prolactin secretion was investigated in lactating Holstein cows by characterizing serum cortisol and prolactin responses to fluphenazine, a dopamine receptor antagonist. Twelve anovulatory cows received an intravenous bolus injection of either saline (n = 6) or 0.3 mg of fluphenazine/kg of body weight (n = 6) in wk 2 postpartum. Blood samples were collected every 30 min for 4 h before and 4 h after saline or fluphenazine injection. Serum progesterone concentration was 0.13 +/- 0.1 ng/mL and did not differ between groups. No difference in serum cortisol concentrations was detected between groups before treatments. Fluphenazine increased serum cortisol concentrations within 30 min after fluphenazine administration (>30 ng/mL) and concentrations remained elevated throughout the sampling period. Cortisol remained unchanged in saline-treated cows (<10 ng/mL). Prolactin concentrations also increased after fluphenazine administration (103.1 +/- 3.1 ng/mL), but were unaffected by saline (18 +/- 3.1 ng/mL). Prolactin concentrations remained elevated throughout the sampling period in fluphenazine-treated cows. Our results indicated that a dopamine antagonist increased cortisol, suggesting that endogenous dopamine, at least in part, regulates cortisol and prolactin secretion. These effects are regulated through dopamine receptors in anovulatory lactating dairy cows during the early postpartum period.

The actions of dopamine on the airways

BACKGROUND

Dopamine exerts inhibitory and excitatory effects on different systems. In the lungs, dopamine modulates respiratory functions through carotid bodies and modulates pulmonary blood vessel tone, alveolar liquids, and bronchial exchange, and possibly participates in the regulation of airways diameter. It has been reported that dopamine has no acute effect on human airways in normal subjects or those with asthma background. However, inhaled or infused dopamine decreased histamine-induced bronchoconstriction in both normal and asthmatic subjects. We have examined the possible modulating effect of dopamine on bronchial diameter by administering inhaled dopamine and the DA2 dopaminergic blocker metoclopramide to subjects with various degrees of bronchial tone.

METHODS

We examined 56 volunteers. Arterial blood pressure and heart rate were determined in every subject. By means of spirometry, we measured forced vital capacity, forced expiratory volume in the first second, maximal forced expiratory flow, and forced expiratory flow at 50% of vital capacity, before and after each treatment. By inhalation with a nebulizer, we administered dopamine (0.5 microg/kg/min) to 10 healthy subjects, 10 subjects with asthma without acute bronchospasm, and 16 subjects with acute asthma attack; intravenous metoclopramide (7 microg/kg/min) was administered to 10 healthy subjects and 10 subjects with asthma without acute bronchospasm. For ethical reasons, metoclopramide was not used in subjects with acute asthma attack.

STATISTICS

non-parametric Wilcoxon tests for paired samples, ANOVA tests, and Bonferroni multiple comparison tests were performed.

RESULTS

Inhaled dopamine increased forced expiratory volume in the first second, forced vital capacity, maximal forced expiratory flow, and forced expiratory flow at 50% of vital capacity in the acute asthma attack group, but there were no modifications in the healthy group or in the asthma without acute bronchospasm group. Metoclopramide did not induce changes in respiratory parameters in healthy individuals or in those with asthma without acute bronchospasm.

CONCLUSIONS

Inhaled dopamine was able to induce bronchodilatation when the bronchial tone was already increased by acute asthma attack, but it did not modify the resting bronchial tone in normals or in asthmatics without acute bronchospasm. DA2 blockade with metoclopramide did not modify resting bronchial tone either. We suggest that dopamine exerts a modulatory effect on bronchial tone of human airways depending on the degree of existing basal tone.

Inhibitory effects of bromocriptine on corticosterone secretion in male rats

Bromocriptine, a dopamine D2 receptor agonist, is widely used for treating prolactinoma, Parkinson's disease and galactorrhea. However, the influence of bromocriptine on the endocrine system, especially adrenal function, is not clear. The present study was aimed to investigate the effects of bromocriptine on corticosterone production in rats. Male rats were treated or not treated by bromocriptine (5 mg/kg, s.c.) twice per day for 2 days before decapitation. The adrenal zona fasciculata-reticularis cells were prepared and incubated with adrenocorticotropic hormone (ACTH), forskolin (an adenylyl cyclase activator), 8-bromo-adenosine 3':5' cyclic monophosphate (8-Br-cAMP, a membrane-permeable analogue of cAMP), and steroidogenic precursors including 25-OH-cholesterol and pregnenolone. The concentrations of prolactin, corticosterone and pregnenolone in the plasma and/or medium were measured by radioimmunoassay (RIA). The protein expression of cytochrome P450 side-chain cleavage (P450scc) enzyme and steroidogenic acute regulatory protein (StAR) was analyzed by Western blotting. Administration of bromocriptine in vivo resulted in a decrease in the levels of plasma prolactin and corticosterone. Basal--and ACTH--as well as forskolin-stimulated corticosterone secretion by zona fasciculata-reticularis cells was also lower in bromocriptine-treated rats than in control animals. The decreased production of corticosterone in zona fasciculata-reticularis cells could be reversed by administration of 8-Br-cAMP. The corticosterone and pregnenolone release induced by 25-OH-cholesterol in zona fasciculata-reticularis cells was reduced by administration of bromocriptine. The protein expression of both StAR protein and P450scc in zona fasciculata-reticularis cells was inhibited in the bromocriptine-treated group. Administration of bromocriptine in vitro reduced the release of corticosterone stimulated by ACTH and forskolin in rat zona fasciculata-reticularis cells. These results suggested that bromocriptine caused adrenal dysfunction through inhibition of ACTH action and of the activity of adenylyl cyclase, and impaired the early steps of corticosterone biosynthesis.

Polymorphism in exon 6 of the dopamine D(2) receptor gene (DRD2) is associated with elevated blood pressure and personality disorders in men

A deficient dopamine D(2) receptor (DA2) formation or action may contribute to hypertension via an increase of catecholamine release. In addition, Axis II personality disorders that appears odd or eccentric (cluster A) is associated with a low density of DA2. This study sought to examine if a NcoI restriction fragment length polymorphism (C to T transition) in exon 6 of the dopamine D(2) receptor gene (DRD2) was associated with these characteristics. The genotypes (CC, CT and TT) were compared in anthropometric, endocrine, metabolic and haemodynamic variables as well as estimates of personality disorders in 284 randomly selected 51-year-old men. Homozygotes for the C allele constituted 49% of the men and homozygotes for the T allele 9%, while heterozygotes were 41%. The TT genotype was associated with elevated systolic and diastolic blood pressure, independent of obesity and endocrine abnormalities, including the hypothalamic-pituitary-adrenal axis regulation. Moreover, the TT genotype was significantly more frequent among subjects with grade 1 (mild) hypertension (>140/90 mm Hg) compared to normotensive subjects (<130/85 mm Hg). The polymorphism in exon 6 of the DRD2 was also significantly associated with cluster A personality disorders. These results suggest that a polymorphism in exon 6 of the DRD2examined with the restriction enzyme NcoI is associated with an elevated blood pressure, independent of obesity. Paranoid or schizoid personality disorders is also associated with a polymorphism of the DRD2, which might be associated with a previously demonstrated low density of this receptor.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Regulation of plasma aldosterone levels by metoclopramide: a reappraisal of its mechanism from dopaminergic antagonism to serotonergic agonism

It has been thought, since the late 1970s, that dopamine exerts a tonic suppression of plasma aldosterone levels in human subjects. This action, however, had not been established directly using dopamine and dopamine mimetic drugs, which do not, in fact, affect the aldosterone levels. Rather, the conclusion was arrived at indirectly, based on the increase in aldosterone levels seen with dopamine receptor blockers; metoclopramide in particular, considered at the time of its discovery in the 1960s to be a new generation dopamine antagonist. However, metoclopramide is not a pure drug and in fact, shows intermediate affinity at certain serotonin receptor subtypes. Studies have been recently carried out in human subjects on the role of serotonergic transmission in mediating the metoclopramide as an aldosterone secretagogue effect. Here we briefly review this work and attempt to reassess the action of metoclopramide as an aldosterone secretagogue, from dopamine D2 antagonism to serotonin 5-HT4 partial agonism.

Innervation of the adrenal cortex, its physiological relevance, with primary focus on the noradrenergic transmission

The current knowledge of the catecholaminergic innervation of the mammalian adrenal cortex is summarized, and macro- and microscopic neuromorphology, including the central nervous system connections of the adrenal cortex, is briefly discussed. Morphological and functional data on the catecholaminergic (i.e., noradrenergic) innervation of the adrenal cortex are reviewed. Experimental data suggest that in addition to the regulation of adrenal blood flow, the noradrenergic innervation has a primary influence on zona glomerulosa cells possibly via beta 1 adrenergic and dopaminergic receptors (DA2 subtype via inhibiting T-type Ca2+ channels) It is concluded that the local, modulatory effect of noradrenergic nerve fibres, terminating in the close vicinity of the zona glomerulosa cells, on the systemic renin-angiotensin-aldosterone and other peptide cascade may be influenced by neuropeptides, particularly neuropeptide Y and vasoactive intestinal peptide.

Localization of dopamine D1A receptor protein and messenger ribonucleic acid in rat adrenal cortex

Pharmacological, physiological, and autoradiographic studies have suggested the presence of dopamine receptors in the adrenal gland. Dopaminergic ligands have been shown to modulate adrenocortical aldosterone biosynthesis and secretion as well as adrenomedullary catecholamine production and release. Using a combination of light microscopic immunochemistry and in situ amplification and hybridization, the present study sought to determine the site-specific expression of the recently cloned D1A receptor subtype in rat adrenal gland. Light microscopic immunohistochemistry was conducted using polyclonal antisera raised to the putative rat D1A receptor. Immunoreactive product was detected using an avidin-biotin immunoperoxidase method. D1A receptor messenger RNA (mRNA) was detected using a transcription-based isothermal in situ amplification and hybridization approach using receptor-specific mRNA oligonucleotide probes. The amplified product was localized using an alkaline phosphatase 4-nitro blue tetrazolium chloride/5-bromo-4-chloro-3-indolyl-phosphate technique. This combined experimental approach, using both receptor subtype-selective antibodies and oligonucleotide probes, allows for the site-specific localization of the D1A receptor subtype, which would otherwise not be possible with the pharmacological methods currently available. The D1A receptor protein and mRNA were expressed solely in the zona glomerulosa of the rat adrenal gland, with no signal evident in any of the other cortical layers or in the medulla. Such a distribution raises the possibility that the D1A receptor subtype could modulate, at least in part, some of the known effects of dopamine on aldosterone secretion.

Genomic structure and tissue distribution of the mouse dopamine D4 receptor

The mouse dopamine D4 receptor gene was isolated from a genomic DNA library by plaque hybridization. The D4 receptor gene encoded an open reading frame consisting of 387 amino acids, interrupted by three introns. Distribution of the D4 receptor mRNA in brain regions and peripheral tissues of mice was examined by reverse transcription-polymerase chain reaction (RT-PCR). There was a remarkable expression of the receptor mRNA in various brain regions, showing the highest expression level in the cerebellum. Also, in the peripheral tissues a high level expression of the D4 mRNA was detected in the eye, adrenal gland and testes. We observed several differences in tissue distribution of the D4 mRNA in mice from that reported in other mammalian species.