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Department of Pharmacology and Therapeutics, University College Cork, Cork, Ireland; and the Department of Medicine, University of Virginia Health Sciences Center (R.M.C.), Charlottesville, Virginia 22908
Address all correspondence and requests for reprints to: Dr. Damian P. OConnell, Department of Pharmacology and Therapeutics, Clinical Investigation Wing, Cork University Hospital, Cork, Ireland E-mail: damian{at}ucc.ie
| Abstract |
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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.
| Introduction |
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The presence of peripheral dopaminergic receptors has been described by many research groups, including our own, in such tissues as the kidney (5), the gastrointestinal tract (6), and the heart (7, 8). An extensive literature suggests the presence of DA receptors in the adrenal gland [reviewed by Missale et al. (9)], where it has been proposed that DA regulates adrenocortical aldosterone secretion (9) and adrenomedullary catecholamine release (10). With pharmacological and receptor-ligand binding methods using [3H]ADNT and [3H]SCH 23390 as D1-like receptor ligands, [3H]spiperone and [3H](-)sulpiride as D2-like receptor ligands, and [3H]DPAT and [3H]clozapine as D3- and D4-like receptor ligands, respectively, dopaminergic receptors have been identified in bovine, rat, and human adrenal glands (11, 12, 13). These studies are reviewed in detail by Missale (9). Functional studies have demonstrated that aldosterone secretion can be inhibited by dopaminergic agonists such as bromocriptine (14), whereas DA antagonists such as metoclopramide can stimulate secretion when used in in vivo studies in humans and experimental animals (15). Recent studies have indicated that adrenocortical DA is derived from the circulating precursor L-DOPA, rather than from free or conjugated plasma DA or by de novo synthesis from tyrosine (16). In this manner, the formation and action of DA may be analogous to that observed in the kidney, in which DA is synthesized from filtered L-DOPA, stored, and released to act on adjacent DA receptors localized in the renal vasculature, glomeruli, and tubule system (17). Based on the potential similarities of a paracrine/autocrine model of DA function in both the renal and adrenal cortex and given our recent demonstration that the kidney expresses the D1A receptor (5), we sought to determine whether the rat adrenal cortex also contains D1A-specific messenger RNA (mRNA) and receptor protein. As peripheral tissues, such as the kidney and adrenal gland, express receptor-specific mRNA in substantially lower quantities than are demonstrable in the brain, we employed a novel transcription-based amplification system to amplify in situ a unique portion of the rat D1A complementary DNA (cDNA), which was subsequently detected by nonradioactive in situ hybridization. The present experiments are, to our knowledge, the first to localize simultaneously a specific DA receptor protein and mRNA in the adrenal gland according to their recent molecular classification. We do not know of any other studies that have used both immunochemistry and in situ mRNA histochemistry to define the nature of adrenal DA receptors. These studies confirm previous work, using ligand-binding techniques, that has suggested the presence of a DA1-like receptor in the adrenal cortex (11) and support functional studies that have implicated a DA1-like receptor in the control of aldosterone secretion (18).
| Materials and Methods |
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Light microscopic immunohistochemistry was conducted with
affinity-purified polyclonal antisera that had been raised against the
synthetic peptide sequences:
166KPTWPLDGNFTSLEDTEDDNC186,241CQTTAGNGNPVE252
and 299GSEETQPFC307 corresponding to epitopes
located on the third intracellular and second and third extracellular
loops, respectively, of the putative rat D1A receptor amino
acid sequence (Fig. 1A
). The chosen peptide sequences
have no significant homology to other G protein-coupled receptors in
the rat as verified by search of the Swiss Protein Database. The
specificity of the antisera was verified by attenuation of the
enzyme-linked immunosorbent assay response after preincubation of the
antiserum with its immunization peptide and by the ability of the
antiserum to recognize the native receptor expressed in a murine
fibroblast (LTK-) cell line, stably transfected with the
full-length cDNA of the rat D1A receptor (5). An
immunoperoxidase immunohistochemical protocol that we have previously
described (5) was employed thereafter, in which the adrenal sections
were incubated with one of the different D1A antipeptide
antisera as described above. Adrenal sections underwent two washes in
Tris saline (0.05 M) and saponin (SAP; 0.001%; Sigma
Chemical Co., Dorset, UK) for 5 min each. This was followed by a
blocking step with 1% H2O2 in methanol for 45
min. The sections were washed in 1% normal goat serum (NGS; Sigma
Chemical Co.) in TS-SAP before another blocking step, this time with
3% NGS in TS-SAP for 1 h. All sections were washed again, and the
primary antibody was prepared. A number of concentrations of the
D1A antibodies were used by dilution in TS buffer, and
aliquots were added to the appropriate sections. As a control, similar
dilutions of the preimmune serum were prepared and added to sections.
All slides were incubated in a humid chamber at 4 C overnight. The
following day, a number of wash steps with 1% NGS in TS-SAP were
carried out before the addition of the biotinylated secondary antibody,
which was left incubating for 45 min. The ABC avidin biotin solution
(Vectastain ABC reagent, Vector Laboratories, Peterborough, UK) was
made up and added to the sections, which then underwent a further
45-min incubation period. Diaminobenzidine (Sigma Chemical Co.) was
prepared (0.1%) and applied to the sections. Finally, the adrenal
sections were lightly counterstained with hematoxylin, dehydrated by
passing through a graded series of ethanol dilutions, delipidated with
xylene, and coverslipped in DPX (BDH Laboratory Supplies, Poole,
UK).
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In situ hybridization was conducted using 24-mer HPLC-purified antisense and sense probes complimentary to a D1A mRNA sequence lying within the amplified product coding for N-terminal amino acid residues. The oligonucleotides were labeled by tailing the 3'-end with digoxigenin-11-dUTP using terminal transferase (DIG Oligonucleotide 3' end-labeling kit, Boehringer Mannheim, East Sussex, UK). Prehybridization of both amplified and nonamplified tissue sections was carried out in a humid chamber at room temperature for 1 h. Thereafter, the hybridization solution, containing either the antisense or sense probe, was applied to the appropriate sections and incubated overnight at 37 C in the OmniSlide In Situ system. After hybridization, the tissue sections were washed in decreasing concentrations of SSC buffer, including a stringency wash of 0.5 x SSC at 37 C for 30 min. After the posthybridization washes, the sections were immersed in a series of different buffers, and tissue-bound alkaline phosphatase activity was detected by incubating the tissue sections in 4-nitro blue tetrazolium chloride/5-bromo-4-chloro-3-indolyl-phosphate (NBT/BCIP) color solution (Sigma Chemical Co.) for 224 h. The enzymatic reaction was stopped by rinsing the sections in PBS. Finally, the sections were counterstained with 0.2% neutral red and coverslipped as already described. In addition to the sense control, other slides had either no labeled oligonucleotide or no antidigoxigenin secondary antibody added, both of which served to control for nonspecific hybridization signal and background. All of these treatments resulted in the loss of the hybridization signal. Tissue areas for immuno- and in situ histochemistry were examined using a Leica DMR Photomicroscope at various magnifications (x1001000). Photographs were made using color film (Fujichrome 100 ASA color film).
| Results |
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Figure 2
illustrates the results obtained for rat
adrenal sections processed for light immunohistochemistry using the
avidin-biotin peroxidase reaction to identify the D1A
receptor sites. Positive staining is represented by the brown
diaminobenzidine substrate reaction. Figure 2A
indicates the presence
of positive receptor protein in rat adrenal cortex. The staining
pattern is confined to the zona glomerulosa. Figure 2B
illustrates a
higher magnification of zona glomerulosa staining. Figure 2C
shows a
high power magnification of positively stained glomerulosa cells.
Figure 2D
illustrates the preimmune serum control of a consecutive
section at a similar magnification as Fig. 2A
, showing absence of the
signal. The specificity of the D1A receptor antiserum was
verified by demonstration of immunostaining in rat brain sections known
to contain D1A receptor protein. Figure 4A
illustrates
positive staining that was obtained in the rat brain cortex in an area
rich in dopaminergic neurons expressing this DA receptor subtype.
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Sections processed for nonradioactive in situ hybridization
for D1A mRNA revealed positive signal in the zona
glomerulosa region of the adrenal cortex, (Fig. 3A
).
Those cells labeled with D1A antibodies by
immunocytochemistry in consecutive sections also were shown to contain
D1A mRNA (Fig. 3B
). In addition, hybridization signal was
absent from the other adrenocortical cell layers as well as from the
medulla. Consecutive adrenal sections probed with sense oligonucleotide
(Fig. 3C
) or the absence of antidigoxigenin secondary antibody failed
to reveal any hybridization signal. The specificity of the
D1A mRNA hybridization in the adrenal sections was again
verified by demonstration of positive hybridization signal in rat brain
sections shown previously to express D1A receptor protein.
Figure 4B
illustrates the positive staining that was
obtained in the rat brain neocortex in a region known to contain
D1A receptor mRNA (4).
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| Discussion |
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The experimental approach adopted in this study is effective in defining the site-specific localization of the D1A receptor in the rat adrenal gland. Both immunohistochemistry and in situ amplification allow for the specific detection of a receptor subtype. Receptor protein and mRNA, respectively, are targeted by both antibodies and oligonucleotide probes directed exclusively to the receptor. Our laboratories have successfully employed this experimental approach to localize the D1A receptor in a number of peripheral tissues, including rat and human kidney, rat colon, and rat myocardium (5, 6, 7, 8). In addition, the use of receptor-specific antibodies in the localization of DA receptors has been successfully employed by other groups to define regional and cellular distribution patterns of such receptors (19, 20). In contrast, techniques such as autoradiography and radioligand binding can only identify the general receptor grouping, e.g. D1-like, because the ligands cannot target the DA receptor subtypes (D1A and D1B, specifically). Such general experimental approaches have previously identified the presence of DA1 and DA2 receptors in both the adrenal cortex and medulla (10, 11, 12, 13).
The zona glomerulosa has been shown to be the exclusive site within the adrenal gland for the synthesis and secretion of aldosterone (21). Aldosterone secretion can be influenced by circulating plasma potassium, ACTH, and angiotensin II levels, all of which play a stimulatory role (22). There is substantial evidence to suggest that DA also plays a role in the modulation of aldosterone secretion (9, 15, 22). Evidence that dopaminergic mechanisms might be involved in the control of aldosterone secretion came originally from in vivo studies in humans and animals using metoclopramide, a competitive DA antagonist in both the central nervous system and the periphery (23, 24). It is now well documented that administration of metoclopramide results in an increase in the plasma aldosterone concentration independent of changes in PRA and circulating sodium, potassium, and ACTH concentrations (9, 15). In addition, experimental evidence suggests that alterations in angiotensin-induced adrenocortical responsiveness with different sodium balance states may be related to dopaminergic modulation (25). The DA agonist, bromocriptine, has been shown to inhibit the aldosterone responses to acute diuresis and to the infusion of angiotensin II or ACTH (14, 26), but has no effect on basal aldosterone secretion (27). Ibopamine, an orally active DA agonist, inhibits aldosterone secretion and blunts the metoclopramide-induced release of this hormone (28).
The dopaminergic modulation of aldosterone secretion is due to a direct interaction of DA or dopaminergic ligands with the zona glomerulosa aldosterone-secreting mechanism (29). Moreover, this modulation is transduced by specific dopaminergic receptor activation. The transductional capability of DA receptors in rat adrenal glomerulosa homogenates has been analyzed with the measurement of cAMP formation in response to selective agonists (30). The results obtained suggested the presence of DA1 and DA2 receptors with opposing roles in the formation of cAMP in the adrenal glomerulosa. In addition, in vitro binding studies have demonstrated saturable, reversible, and stereospecific DA1 and DA2 binding sites in the adrenal cortex (11, 12, 13). Several studies have provided compelling evidence for a role for DA2 receptors in the inhibitory action of DA on aldosterone secretion (9). DA may act by decreasing the glomerulosa intracellular calcium concentration, as DA reduces angiotensin-stimulated calcium influx. It appears that the effect of angiotensin II on aldosterone secretion is calcium dependent, involving mobilization of both extracellular and intracellular calcium induced by direct receptor activation of phospholipase C (18). Studies also have shown that DA blocks voltage-dependent T-type calcium channels in cultured rat adrenal zona glomerulosa cells in a sustainable and reversible manner (31). Experiments on isolated rat zona glomerulosa cells have demonstrated that DA blocks angiotensin II-induced calcium influx and decreases inositol phosphate production (18, 32). Further studies in primary cultures of rat adrenal glomerulosa cells have demonstrated that DA can stimulate aldosterone secretion in conjunction with enhanced cAMP production and to have an additive effect on angiotensin-stimulated aldosterone secretion. These effects were DA1 receptor specific, in that they were completely suppressed by a specific DA1 antagonist and were unaffected by a ß-adrenergic antagonist (18). Further, this DA1 effect appeared to be calcium dependent, in that it could be blocked by calcium channel antagonists. These latter studies would suggest a more complex effect of DA on aldosterone secretion than previously suspected, with potential interactions between DA1 and DA2 receptor subtypes in an manner analogous to that observed for the effect of DA on the arachidonic cascade (33).
The results of the present experiments raise the possibility that the effects of DA may be mediated at least in part by the newly cloned D1A receptor. Support for this hypothesis comes from investigation of the signal transduction pathways of the human D1A receptor, which has demonstrated that this receptor can induce multiple cell-specific signals, including elevation of cAMP, cAMP-dependent activation and potentiation of the opening of voltage-dependent calcium channels, and a phosphatidyl inositol-linked mobilization of intracellular calcium (34, 35).
In summary, we have shown that the D1A receptor subtype is present in the zona glomerulosa of the rat adrenal cortex. The receptor protein was localized by light microscopic immunohistochemistry. The mRNA for the D1A receptor subtype was localized to the same region by means of the novel transcription-based amplification system, followed by in situ hybridization. We speculate that the D1A receptor subtype may modulate the previously described dopaminergic effects on aldosterone secretion from the adrenal cortex.
| Footnotes |
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Received June 14, 1996.
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