ABSTRACT

Antibodies specific to the dopamine D2 receptor have been raised in rabbits using synthetic peptides. The resulting antiserum was sensitive to picogram quantities of peptide as measured by enzyme-linked immunoassay and was shown to have a 33% cross-reactivity with partially purified D2 receptor protein. No detectable cross-reactivity with similarly prepared fungal membranes was observed. D2 receptor preparations from normal rat pituitary cells were used in Western blot analysis. Bands of M _r = 95 000 and 34 000 were detected in these preparations with a third faint band at 120 000. These correspond to the pituitary D2 receptor.

Journal of Endocrinology (1992) 134, 227–233

ABSTRACT

The rat adrenal cortex contains quantities of dopamine that are compatible with its function as a neurotransmitter, suggesting that locally released dopamine may act as a neuroregulator within the gland. This possibility has been tested by comparing the effects of dopamine on aldosterone secretion in the perfused adrenal with the effects of stimuli designed to provoke the release of intraglandular dopamine.

Infusion of dopamine (0·1–100 μmol/l for 10-min periods) into the isolated perfused rat adrenal gland resulted in a transient, dose-related reduction of aldosterone secretion to a minimum of approximately 50% of the basal value at 1 μmol dopamine/l (ratio of experimental to control measurements, R = 0·53 ± 0·06 (s.e.m.); n = 5). In contrast, dopamine (1–100 μmol/l) had no effect on aldosterone production by dispersed zona glomerulosa cell preparations incubated in vitro.

The effects of changes in K⁺ concentration (3·9–52 mmol/l) on aldosterone secretion in the perfused gland and dispersed cell preparations were also compared. A similar bell-shaped dose–response relationship was seen in both preparations between 6 and 32 mmol K⁺/l, with a maximum at 8·4 mmol K⁺/l and a return to control values with 16, 24 or 32 mmol K⁺/l. However, infusion of media with very high K⁺ concentrations (42 or 52 mmol K⁺/l) reduced the secretion of aldosterone by the perfused gland to approximately 50% of the basal value (R = 0·51 ± 0·05, n = 9; R = 0·49± 0·08, n = 9; respectively) but produced no change in aldosterone production by zona glomerulosa cells. Electrical field stimulation (pulse width 1 ms, 1 Hz at 60 V for 5 min) of the perfused gland also resulted in a reduction in aldosterone secretion (R = 0·66 ± 0·66, n = 6). In the presence of 1 μmol haloperidol/l, a dopamine antagonist, no effect on aldosterone secretion was seen under control conditions, but the responses to 1 μmol dopamine/l, 52 mmol K⁺/l and field stimulation were eliminated.

The results are consistent with the view that aldosterone secretion by the perfused adrenal gland is subject to an inhibitory dopaminergic control, which may originate from catecholaminergic neurones within the gland itself.

Journal of Endocrinology (1992) 133, 275–282

ABSTRACT

The chemical structure of dopamine includes an ortho-catechol group which is labile to oxidizing agents. Ascorbic acid, a reducing agent, has in the past been added to the incubation medium in order to protect dopamine against oxidation. However, there has been no thorough examination of the biological effect of ascorbic acid on prolactin release. In this present study we have shown that ascorbic acid has neither a stimulatory nor an inhibitory effect on prolactin release but reduces by approximately two orders of magnitude the concentration of dopamine necessary to inhibit prolactin release from cultured anterior pituitary cells. The strong potentiation effect of ascorbic acid was reproduced using apomorphine. We compared the effect of ascorbic acid and isoascorbic acid on dopamine inhibition of prolactin release. Isoascorbic acid is an epimer of ascorbic acid, having the same reduction–oxidation potential as ascorbic acid, but is less biologically active. Isoascorbic acid was less effective in potentiating the dopaminergic effect than was ascorbic acid, which supports the notion that potentiation by ascorbic acid is not entirely due to its reducing property.

In order to dissociate further the chemical protection of dopamine from the biological potentiation, the inhibitory effects of freshly made and 3-h-old dopamine solutions were compared. Neither one of the two solutions contained any ascorbic acid, yet the two solutions did not show any difference in their ability to inhibit prolactin release during the 3-h incubation period, indicating that no significant amount of dopamine was oxidized. The minimum effective concentration of ascorbic acid necessary to demonstrate potentiation was between 0·001 and 0·01 mmol/l. The potentiation effect was shown after 1, 2, or 3 h of exposure to dopamine, and was evident in both 2- and 6-day-old cultured cells.

The effect of ascorbic acid can either be a pharmacological potentiation or a physiological effect on the primary cultured pituitary cells. However, it is quite clear that ascorbic acid is not a simple anti-oxidant but produces a strong potentiating effect on the dopaminergic inhibition of prolactin release by some other means.

J. Endocr. (1988) 118, 287–294

Instituto Latinoamericano de Fisiología de la Reproducción (ILAFIR), CC10, San Miguel 1663, PBA, Argentina

(Received 1 September 1976)

Ever since the initial observations demonstrating the importance of central monoaminergic mechanisms in the control of ovulation (Sawyer, Markee & Townsend, 1949) evidence has accumulated that gonadotrophin and prolactin secretion and sexual behaviour can be changed by altering monoamine receptor activity in the brain, particularly in the hypothalamus. It has also become apparent that the central effects of sex steroids can be partly explained via changing activity of central monoaminergic neurones. A variety of transport mechanisms for their respective neurotransmitters have been described in peripheral and central neurones and considerable evidence indicates that the removal of the released transmitter from the synaptic cleft constitutes a major mechanism to terminate neurotransmission (Iversen, 1971). High concentrations (10⁻⁴ to 10⁻⁵ mol/l) of oestradiol and progesterone added in vitro have been found to decrease noradrenaline, dopamine

ABSTRACT

The administration of thyrotrophin-releasing hormone (TRH) causes a variety of dopamine-related biological events. To understand the specific role of TRH on rat hypothalamic dopamine neurones, we examined the in-vivo effects of intraventricular (i.c.v.) infusion of TRH on the release and synthesis of prolactin in the rat pituitary gland and on the changes in binding of [³H]MeTRH and dopamine turnover rates in rat hypothalamus. We have also examined the in-vitro effects of TRH on the release of [³H]dopamine from dispersed tuberoinfundibular dopamine neurones.

Female rats were treated with i.c.v. infusions of 1 μmol TRH/l daily for 1, 3 and 7 days using Alzet osmotic pumps. Following 7 days of treatment the serum prolactin concentrations were significantly decreased. A reduction in hypothalamic TRH-binding sites (B_max) was also apparent but the dissociation constant (K _d) was unaffected. Northern blot analysis of total RNA isolated from the pituitary glands of control animals using ³²P-labelled prolactin cDNA as a probe indicated the presence of three species of prolactin gene transcripts of approximately 3·7, 2·0 and 1·0 kb in size, and these were decreased by TRH treatment. We examined the turnover rate of dopamine in the rat hypothalamus when TRH was administered i.c.v. for 7 days. There was a significant increase in 3,4-dihydroxyphenylacetic acid/dopamine ratio with TRH treatment. Moreover, exposure to TRH stimulated [³H]dopamine release from rat tuberoinfundibular neurones in a time- and dose-dependent manner. Dopamine receptor antagonists such as SCH23390 and (−)sulpiride, and other neuropeptides such as vasoactive intestinal peptide and oxytocin did not affect TRH-stimulated [³H]dopamine release.

These data suggest that i.c.v. administration of TRH might decrease both prolactin secretion and accumulation of prolactin gene transcripts in the pituitary by stimulating dopamine release from tuberoinfundibular neurones.

Journal of Endocrinology (1992) 133, 59–66

ABSTRACT

The effect of dopamine (1 μg/kg per min) on corticosteroid response to ACTH (0·1, 1 and 10 ng/kg per min) was compared with that of a placebo in sodium-replete (150 mmol/day) and -deplete (10 mmol/day) normal man. Dopamine had no effect on aldosterone, cortisol or corticosterone responses in either dietary phase, but increased deoxycorticosterone (897·0 ± 126·4 (s.e.m.) vs 590·0 ±84·3 pmol/l, normal Na⁺; 1264·2 ±84·3 vs 764·5 ±84·3 pmol/l, low Na⁺) and deoxycortisol (6·033 ± 0·583 vs 5·048±0·680 nmol/l, normal Na⁺; 5·112 ± 0·600 vs 4·130± 0·367 nmol/l, low Na⁺) levels during ACTH administration (all P <0·01). Deoxycorticosterone and corticosterone responses to ACTH were greater during sodium depletion than repletion (both P <0·01).

Dopamine therefore increased 11-deoxycorticosteroid concentrations during ACTH-stimulated steroidogenesis. This may reflect action of dopamine to increase extra-adrenal formation of 11-deoxycorticosteroids.

J. Endocr. (1986) 109, 339–344

ABSTRACT

The effect of drug-induced hypo- and hyperprolactinaemia on pituitary gonadotrophin releasing hormone receptors (GnRH-R), serum and pituitary gonadotrophins (LH and FSH) and prolactin was investigated in intact adult male and female rats. Hypoprolactinaemia (serum prolactin <20% of control values) resulting from dopamine agonist (bromocriptine) infusion (4 mg/kg per day for 7 days) was accompanied by a 40–50% increase in GnRH-R in both male and female animals, though this was not accompanied by any major change in serum or pituitary LH and FSH. Hyperprolactinaemia (serum prolactin greater than ten times control values) induced by the dopamine receptor antagonist metoclopramide (65 mg/kg per day for 7 days) increased GnRH-R between 35 and 45% in both male and female rats without altering serum gonadotrophins. Domperidone (1 mg twice daily for 14 days) also increased GnRH-R by 50% but only in female rats. Both dopamine antagonists significantly increased pituitary prolactin content. Pituitary FSH increased in female rats treated with both metoclopramide and domperidone.

The stimulatory effects of bromocriptine and metoclopramide on GnRH-R in male rats were prevented by concurrent treatment with a GnRH antiserum, suggesting that the drug effects were mediated through alteration in endogenous GnRH secretion.

Induction of massive (serum prolactin > 2000 μg/l) hyperprolactinaemia in male and female rats with a transplantable prolactin-secreting pituitary tumour did not reduce GnRH-R concentration, although serum gonadotrophins were suppressed and pituitary gonadotrophin content was increased.

These results indicate a dissociation between serum prolactin concentrations and pituitary GnRH receptor content and indicate that dopamine agonist and antagonist agents can influence GnRH-R independently of prolactin, possibly by acting on central dopamine receptors responsible for catecholaminergic regulation of GnRH secretion.

J. Endocr. (1984) 102, 215–223

Abstract

In this study we examined the effects of dopamine (DA) and its withdrawal on in vitro prolactin (PRL) release from subpopulations of lactotrophs from two regions of the anterior pituitary obtained from untreated ovariectomized (OVX) rats or OVX rats treated with estrogen, progesterone or a combination of the two. Anterior pituitaries were cut horizontally into an inner (dorsal) zone and an outer (ventral) zone. Each of these regions was enzymatically dispersed and the resulting cells were otherwise untreated (unseparated) or centrifuged through a discontinous Percoll gradient to separate the cells into two subpopulations (light and heavy cells). Each of these types of cells was perifused for 1 h with culture medium containing 1 μm DA followed by medium devoid of DA for 1 h. Prolactin released into the perifusion medium, collected as 5-min fractions, was measured by radioimmunoassay and normalized to the number of lactotrophs in the cellular pools as determined by immunocytochemistry.

In the presence of DA, PRL release from unseparated cells of the outer zone was significantly increased by estradiol treatment compared with the release from similar cells from OVX rats. (Differences were considered significant where P<0·05.) However, no effect of estradiol treatment was observed with unseparated cells of the inner zone or light or heavy cells from either zone. Progesterone had no effect on any cell type when administered alone. However, when progesterone was given following estradiol, PRL release from unseparated cells of the inner zone was increased significantly compared with similar cells from the other steroid-treated groups. Similar significant increases were observed with light and heavy cells of the outer zone, but there was no effect of the combined steroid treatment on light or heavy cells from the inner zone. When DA was withdrawn, prolactin release was significantly increased from all cells except unseparated cells of the outer zone of OVX rat pituitaries. However, when the cells of the outer zone from OVX rats were separated into light and heavy cells, they responded to the withdrawal of DA with significant and equivalent increases in prolactin release. Light cells of the inner zone of pituitaries from OVX rats were more responsive to DA withdrawal than were heavy cells. Estradiol increased the response to the withdrawal of DA by light and heavy cells of the outer zone and heavy cells of the inner zone. Progesterone significantly reversed these effects of estradiol on separated cells.

These results suggest that lactotrophs in two regions of rat pituitaries respond differently to dopamine and to its withdrawal, that subpopulations of lactotrophs within these regions also respond differently and that steroids modulate these responses.

Journal of Endocrinology (1996) 148, 113–120

The effect of the dopamine antagonist, haloperidol, on arginine-vasopressin (AVP) release induced by angiotensin II was studied in six dehydrated conscious dogs. Angiotensin II (10 ng/kg per min) alone caused a twofold increase (P<0·05) in plasma AVP concentration, a 25 mmHg increase (P<0·01) in mean arterial blood pressure (ABP) and a 70% decrease (P<0·01) in plasma renin activity (PRA). In the presence of haloperidol (3 μg/kg per min), angiotensin II caused similar changes in mean ABP (+25 mmHg; P<0·01) and PRA (−65%, P<0·01), but a small insignificant decrease in plasma AVP (−22%). The AVP response to angiotensin II in the presence of haloperidol was significantly (P<0·05) different from its response to angiotensin II alone. Neither haloperidol alone nor the two vehicles had any effect on plasma AVP or mean ABP but PRA dropped slightly. The results suggest that a dopaminergic mechanism may be involved in angiotensin II-induced AVP release.

Plasma concentrations of prolactin in anoestrous ewes were respectively lowered or raised by the separate infusion of dopamine or thyrotrophin releasing hormone (TRH). Combined treatment with dopamine and TRH lowered the concentration of prolactin in plasma but the values increased markedly after the treatment was stopped and reached a level equivalent to that found in ewes treated with TRH alone. The results are interpreted as evidence that both dopamine and TRH play a regulatory role in determining the secretion of prolactin in the ewe.