Inhibition of thyrotropin and prolactin secretions in primary hypothyroidism by 2-Br-alpha-ergocryptine

Hypotensive action of bromocriptine in the DOCA-salt hypertensive rat: contribution of spinal dopamine receptors

To assess the role of spinal dopamine receptors in mediation of hypotension induced by systemic administration of the dopamine D2 receptor agonist, bromocriptine, conscious deoxycorticosterone acetate (DOCA)-salt hypertensive rats were pretreated with either intravenous (i.v.; 500 micrograms/kg) or intrathecal (i.t.; 40 micrograms/rat at T9-T10) domperidone, a selective dopamine D2 receptor antagonist that does not cross the blood-brain barrier. In DOCA-salt hypertensive rats, i.v. administration of a sub-maximal dose of bromocriptine (150 micrograms/kg) induced a significant decrease in mean aortic pressure (MAP) which was greater and longer lasting than that in uninephrectomized control rats. Intravenous or i.t. pretreatment with domperidone reduced partially, but significantly, the hypotensive effect of bromocriptine (reduction of about 57% and 45% of the maximal effect, respectively). The remaining responses observed during the 60 min postinjection period were still statistically significant as compared with vehicle injection. In contrast, the bromocriptine-induced hypotension was fully abolished by i.v. pretreatment with metoclopramide (300 micrograms/kg), a dopamine D2 receptor antagonist that crosses the blood-brain barrier, or by combined pretreatment with i.v. and i.t. domperidone. These results suggest that, in DOCA-salt hypertensive rats, the hypotension induced by i.v. bromocriptine is mediated partly through a peripheral D2 dopaminergic mechanism and partly through stimulation of spinal dopamine D2 receptors, has been demonstrated in conscious normotensive rats.

Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold

It seems clear from the studies reviewed here that there is adequate evidence to support the concept of a biphasic response of the thyroid gland to cold as first postulated by Moll et al. (1972). The initial response to acute exposure to cold begins at the level of the hypothalamus as a result of either neural stimuli from skin and other areas and/or blood of somewhat lower than normal temperature reaching the hypothalamus (Andersson et al., 1963). As a result, the secretion of norepinephrine and/or dopamine may increase, and serotonin and/or somatostatin may decrease. The net result of these is an increase in the release of TRH from the hypothalamus. This, in turn, stimulates the cascade for the release of TSH from the anterior pituitary gland and thyroid hormone from the thyroid gland. Moll et al. (1972) postulated the lack of a feedback limb in this acute phase, and, indeed, this may be the case. It is possible, however, that certain hormones, such as somatostatin, norepinephrine, T3, and T4 could act in the capacity of feedback inhibitors. Additional experiments will be required to assess this possibility. The transitional link between the acute (less than 1 day) and chronic (greater than 1 day) phases of the response of the thyroid gland to cold could be T4 itself. An increase in the concentration of T4 in plasma has been reported to increase peripheral deiodination of T4 to T3 by kidneys and liver of rats. There are no studies at present to indicate that hepatic conjugation can be increased by elevation of plasma levels of T4 and T3. If it can, these responses would provide adequate reasons as to why peripheral metabolism of thyroid hormones increases during chronic exposure to cold. The time-course for these changes to occur needs to be studied in greater detail to establish the sequence of events following acute exposure to cold. The latter may also increase urinary excretion of T4 and T3 in man, but not the rat. This suggests that another aspect of exposure to cold needing additional study is measurement of the binding affinities of T4 and T3 for their transport proteins during exposure to cold as compared to affinities prior to exposure to cold. If binding affinities are reduced, the amount of free hormones would increase and, consequently the likelihood of being excreted into urine and conjugated by the liver would also increase.(ABSTRACT TRUNCATED AT 400 WORDS)

Prolactin and thyrotropin in serum during electroconvulsive therapy in patients with major depressive illness

Thirty-three patients with major depressive illness received electroconvulsive therapy (ECT), and serum prolactin (PRL) and thyrotropin (TSH) levels were measured 30 min before and 1, 5, 15, 30, and 60 min after the treatment. There was a threefold increase in PRL levels with a maximum after 15 min. The TSH plasma levels rose significantly with a maximum at 30 min after ECT. No change in PRL and TSH concentrations was seen in control experiments when the patients received anaesthesia only. In 15 patients the hormone levels were studied both during the first and sixth (last) ECT. The PRL and TSH levels were significantly higher following the first as compared to the sixth ECT. Patients on phenothiazines had higher PRL and lower TSH levels than those on other drugs or without medication, but there was no significant difference in the mean increment by ECT. Dopamine depresses PRL and TSH secretion. The diminished hormone release after a series of ECT may be explained by ECT-induced increase of postsynaptic dopamine receptor function leading to inhibition of PRL and TSH release from the pituitary gland.

Nomifensine decreases the thyroid-stimulating-hormone response to thyrotropin-releasing-hormone in normal subjects

The Thyroid-stimulating-hormone (TSH) secretion has been studied in 12 normal euthyroid subjects (4 males, 8 females) after nomifensine (NOM) administration (200 mg po). NOM is a drug which activates dopaminergic neurotransmission at the Central Nervous System level. Blood samples were drawn every h for 4 h after NOM or placebo, respectively. At the 4th hour thyrotropin-releasing-hormone (TRH) was administered in bolus (200 micrograms iv), in both studies, and additional samples were collected for 90 min. The results show a moderate suppression (NS) of TSH and a clear-cut reduction in the secretory response to TRH after NOM administration (secretory area: TRH after placebo 723 +/- 132, TRH after NOM 400 +/- 83; p less than 0.01). The data appear to confirm that dopaminergic neurotransmission exerts an inhibitory role upon TSH secretion. The mechanism by which NOM induced dopaminergic activation leads to the suppression of TSH release after TRH has not yet been elucidated. An interference in TSH storage and/or in the post receptor mechanisms involved in TRH action might be hypothesized.

Dopaminergic modulation of TSH and its subunits: in vivo and in vitro studies

We have studied the effects of dopamine on the secretion of TSH and its subunits in vivo and in vitro. Four normal controls, seven patients with primary hypothyroidism, two patients with peripheral resistance to thyroid hormone (PRTH), and two patients with alpha-secreting pituitary tumours underwent a 3-h dopamine infusion (4 micrograms/kg/min). Serial blood samples were drawn for TSH, PRL, alpha, and TSH-beta subunit. In normal subjects, TSH fell from 2.1 +/- 0.9 (+/- SE) to 0.7 +/- 0.1 microU/ml (P less than 0.05), and alpha declined from 1.5 +/- 0.4 to 1.0 +/- 0.1 ng/ml (P less than 0.01). TSH-beta was at or slightly above the detection limits of the assay before and after dopamine. In hypothyroidism, basal serum TSH was 81 +/- 14 microU/ml. With dopamine, TSH fell to 35 +/- 8 microU/ml (P less than 0.001), while alpha decreased from 3.2 +/- 0.4 to 2.0 +/- 0.3 ng/ml (P less than 0.01). Serum TSH-beta also declined from 0.97 +/- 0.06 to 0.57 +/- 0.05 ng/ml (P less than 0.001). A similar fall in TSH and alpha was seen in the two patients with PRTH. In normals and hypothyroid patients, the percentage change in alpha concentration was significantly less than that observed for intact TSH. This is due presumably to the contribution of the gonadotrophs to the circulating alpha pool. TSH and TSH-beta were undetectable in the two pituitary tumour patients, and alpha declined only slightly in each patient after dopamine. The in vitro effects of dopamine were studied using cultured bovine anterior pituitary cells. Dopamine (10(-4)-10(-8) mol/l) did not change basal TSH, alpha, or TSH-beta release. However, dopamine at all doses significantly blunted TRH (10(-7) mol/l)-stimulated TSH and TSH-beta release, and blunted TRH-mediated alpha release at the two highest dopamine doses. These data suggest that dopamine modulates both TSH and TSH subunit secretion. These effects may be exerted directly at the level of the thyrotroph.

Cardiovascular dopamine receptors: physiological, pharmacological and therapeutic implications

Dopamine receptor activation can lead to pronounced changes in cardiovascular function. The myriad of effects produced by dopamine receptor agonists results from the activation of dopamine receptors located at different anatomical sites in the cardiovascular system. Further basic research is required to better characterize these dopamine receptors so as to allow the development of more specific dopamine receptor agonists. Endogenous dopamine may be involved in the physiological control of fluid and electrolyte balance and continuing research efforts in this area should provide for a better understanding of the role of cardiovascular dopamine receptors in the maintenance of overall circulatory homeostasis. Cardiovascular dopamine receptor stimulation represents an important and promising approach for the development of novel therapeutic agents for the treatment of hypertension, angina pectoris, congestive heart failure and acute renal failure.

Pharmacological interference with in vitro tests of thyroid function

Numerous drugs may cause changes in the serum concentrations of T4 and of T3. If such alterations are not recognized an incorrect diagnosis may result. In moderate degrees of hypo- and hyperthyroidism thyroid hormone levels may be spuriously normal, or the influence of pharmacological substances may lead to false diagnosis of thyroid disease in euthyroid patients. Since prediction of such alterations remains uncertain, it may be necessary to perform additional investigations when a potential artefact is recognized. On the other hand many pharmacological agents, especially those which interact with neurotransmitters, may influence TSH secretion, too. The TRH-test may show an increase or decreased TSH response, although complete suppression is only rarely seen during high-dose glucocorticoid treatment when low TRH doses are applied. Because of TRH-test gives such wide separation between different clinical states false interpretations are generally less likely than with drug-induced changes in T4 and T3 values.

Abnormal thyroid stimulating hormone following pituitary surgery

Biologically inactive thyroid stimulating hormone (TSH) has been reported in hypothyroid patients. We report the first case of immunoreactive, but abnormal TSH in a euthyroid patient following hypophysectomy for a prolactin secreting pituitary adenoma. Indirect evidence indicates that this abnormal TSH was biologically inactive. The TSH was characterized by gel chromatography and has a molecular weight of approximately 23,000 daltons.

The endocrine profile of bromocriptine: its application in endocrine diseases

Bromocriptine, a potent agonist at Dz receptors, was developed as a therapeutic agent for inhibiting prolactin (PRL) secretion in patients with hyperprolactinemia. Besides, its PRL-suppressive effect and a short-lasting growth hormone (GH)-releasing effect in normal volunteers, bromocriptine has no other endocrine effects in healthy subjects. On the other hand, bromocriptine lowers GH secretion in acromegalic patients and ACTH secretion in some patients with Cushing's disease or Nelson's syndrome. The paper reviews the endocrine actions of bromocriptine in man, in normal and pathological conditions, the bromocriptine's mechanism of action and its clinical applications in endocrinology.

Altered dopaminergic regulation of thyrotrophin release in patients with prolactinomas: comparison with other tests of hypothalamic-pituitary function

This study was carried out to test the hypothesis that sustained hyperprolactinaemia in patients with prolactinomas stimulates hypothalamic dopaminergic activity via a short loop positive feedback effect of prolactin (PRL). The intensity of dopamine (DA) effects on the pituitary around the adenoma was evaluated by measuring thyroid stimulating hormone (TSH) responses to intravenous injection of domperidone (10 mg) a new DA receptor blocking drug that does not penetrate the blood-brain barrier. TSH responses have been compared with those of PRL to the same agent. Eight females with prolactinomas showed greater TSH release after domperidone than nine normal females (sum of TSH increments over 20 min 17.5 +/- 1.7 v. 8.9 +/- 1.5 mu/l, P less than 0.001) whilst PRL release was reduced (sum of PRL increments over 120 min 5.9 +/- 2.4 v. 21.8 +/- 3.8 mu/l x 10(-3), P less than 0.01). Amongst nineteen hyperprolactinaemic females with apparently normal pituitary fossae (plain skull X-ray), ten showed an exaggerated TSH response (delta TSH, 4.2 +/- 0.6 mu/l, range 2.5-9.0 mu/1) and reduced PRL response to domperidone, comparable with established tumor cases. In the remaining nine normal fossa hyperprolactinaemic females, the TSH and PRL responses to dopaminergic were similar to normal females. These results support the initial hypothesis and indicate the coexistence of a defect in the dopaminergic inhibition of PRL release and increased dopaminergic inhibition of TSH release in patients with prolactinomas. The presence of an exaggerated TSH response to DA antagonism in a euthyroid, radiologically normal (plain skull X-ray), hyperprolactinaemic patient is compatible with the presence of an autonomously-functioning, PRL secreting, pituitary microadenoma and the TSH changes seen in these patients after DA antagonist administration can be readily detected by sensitive TSH radioimmunoassay.

Bromocriptine suppresses the thyrotrophin response to thyrotrophin releasing hormone during human pregnancy

Thyrotrophin (TSH) responses to 200 microgram of intravenous thyrotrophin releasing hormone (TRH) were measured in fifteen healthy women in normal early pregnancy before and at the end of a bromocriptine treatment of 5.0-7.5 mg daily for 1-2 weeks. Bromocriptine did not change the basal levels of TSH, triiodothyronine (T3) and thyroxine (T4) during pregnancy. Before the start of bromocriptine, TRH caused a significant TSH elevation from 12.8 +/- 0.5 muu/ml (mean +/- SE) to 21.2 +/- 1.9 muu/ml after 20 min. During bromocriptine intake, TRH caused a TSH elevation from 11.9 +/- 0.4 muu/ml to only 15.5 +/- 1.1 muu/ml which is significantly less (P less than 0.001) than before bromocriptine. Similarly, the mean maximal TSH increment of 8.4 +/- 1.5 muu/ml before bromocriptine was greater (P less than 0.001) than that of 3.8 +/- 60 muu/ml during bromocriptine intake. When women were retested with TRH before and during bromocriptine after legal abortion, bromocriptine did not change the basal levels of TSH, T3 and T4 or the TSH response to TRH. Therefore, the TSH inhibition caused by bromocriptine is specifically related to the pregnancy itself, but the mechanism for this inhibition remains unknown.

Evidence for increased dopaminergic inhibition of secretion of thyroid-stimulating hormone in hyperprolactinemic patients with pituitary microadenoma

The administration of a dopamine-receptor antagonist, metoclopramide (10 mg, intravenous bolus), elicited a significant increase in the circulating concentration of thyroid-stimulating hormone (TSH) which was substantially greater for hyperprolactinemic patients with pituitary microadenoma than for normal women during the early follicular phase of the cycle. This study provides evidence of an inhibitory role of hypothalamic dopamine in the regulation of secretion of TSH and suggests an increased dopaminergic inhibition of secretion of TSH in these patients with prolactin-secreting tumors.

Inhibition of prolactin secretion by nomifensine in man

Serum prolactin, TSH and GH levels were measured in thirty healthy volunteers in fasting conditions and following oral administration of 200 mg nomifensine. In addition, the effect of the drug on serum prolactin and TSH response to synthetic TRH was studied in twenty normal subjects. Inhibition of endogenous catecholamine reuptake by nomifensine significantly inhibited prolactin release whereas it had no effect on TSH and GH levels. Nomifensine administration had no appreciable effect on the TRH-induced release of TSH and prolactin.

Chronic dopamine receptor stimulation using bromocriptine: failure to modify thyroid function

Administration of 2.5 mg bromocriptine (Parlodel), a dopamine agonist, on two occasions to six normal volunteers did not alter the plasma TSH response to an i.v. injection of 100 micrograms TRH, but significantly (P less than 0.01) blunted it after 200 micrograms. Chronic bromocriptine treatment (7.5--50 mg/day) of fifteen acromegalic subjects failed to influence basal plasma TSH or the response pattern to 200 micrograms TRH. The thyroxine Binding Index (TBI) and the levels of T3 and T4 were not modified by treatment. These results indicate that chronic dopaminergic therapy with bromocriptine does not alter thyroid function.

Bromocriptine suppression of TRH-stimulated prolactin and thyrotrophin release and accompanying inhibition of bromocriptine induced growth hormone release by TRH in normal man

Six normal fasting males received on four separate occasions in random order (1) a placebo tablet followed 60 min later by 200 microgram of TRH intravenously (2) bromocriptine 2.5 mg orally followed by TRH intravenously (3) bromocriptine 2.5 mg orally followed by a placebo injection and (4) placebo tablet followed by placebo injection. Plasma prolactin and TSH responses to TRH were decreased following bromocriptine pretreatment. The rise of plasma growth hormone after bromocriptine was inhibited by TRH. The rise in plasma FSH seen after TRH injection was not influenced by bromocriptine pretreatment. Circulating LH and insulin concentrations were unaffected by any drug administration. These results suggest a dopaminergic influence on prolactin and TSH release in normal men, an inhibitory effect of TRH on bromocriptine stimulated growth hormone secretion, and no dopaminergic modulation of basal insulin secretion.

Dopamine is a physiological regulator of thyrotrophin (TSH) secretion in normal man

Using a sensitive and precise radioimmunoassay for human TSH we have demonstrated significant elevations in serum TSH levels in euthyroid volunteers following administration of the dopamine receptor blocking drug metoclopramide when compared with placebo. The degree of TSH response is significantly greater in females than in males and is sustained over a 3-hour period after a single oral 10 mg dose of metoclopramide. The degree of TSH release after metoclopramide is inversely related to the basal TSH level suggesting that dopamine is a determinant of low daytime TSH levels and is thus implicated in the circadian rhythm of TSH secretion. Pretreatment with 10 mg of metoclopramide orally, one hour before TRH administration leads to significant enhancement of the TSH response to TRH. Our findings provide further evidence for the physiological inhibitory role of dopamine in the contol of TSH secretion in normal man. The possible mode of action of dopamine and the clinical implications of this neuroregulatory pathway are discussed.

Effect of apomorphine and piribedil on the secretion of thyrotropin and prolactin in patients with primary hypothyroidism

The administration of apomorphine and piribedil, two dopaminergic agents, significantly reduced thyrotropin (TSH) and prolactin levels in six female patients with primary hypothyroidism. These data provide further evidence for an inhibitory role of dopaminergic stimulation on TSH secretion.

Failure of oral bromocriptine to affect hypergastrinemia in two patients with the Zollinger-Ellison syndrome

Bromocriptine was administered to 2 subjects with gastrin-secreting tumors of the pancreas. The absorption of the drug was confirmed by a rise in growth hormone levels but no change in the elevated serum gastrin levels were observed. Bromocriptine does not appear to affect gastrin hypersecretion in the way that it influences the hypersecretion of pituitary hormones.

Bromocriptine suppression of the thyrotrophin response to thyrotrophin releasing hormone

A single oral dose of 5 mg of bromocriptine significantly lowered the TSH response to 200 microgram TRH intravenously in eight healthy men compared with control experiments in the same subjects. This finding may be relevant in chronic bromocriptine therapy.

Metergoline inhibition of thyrotrophin and prolactin secretions in primary hypothyroidism

The effect of acute oral administration of metergoline on serum thyrotrophin and prolactin levels in six patients with primary hypothyroidism was studied. Metergoline 4 mg by mouth caused a significant decrease in the concentration of serum thyrotrophin and prolactin in all subjects. There was no consistent change in serum thyroxine and triiodothyronine concentrations during the experiment. These findings suggest that metergoline inhibits prolactin and thyrotrophin secretion by a direct action on the hypothalamus or pituitary gland.

Inhibition of thyrotropin and prolactin secretion by dopamine in man

The effect of dopamine on thyrotropin (TSH) and prolactin (PRL) levels was studied in 5 normal subjects, 7 women with galactorrhea, 9 acromegalics and 4 patients with primary hypothyroidism. Dopamine infused at the rate of 280 micrograms/min produced significant decrease in plasma TSH and PRL levels in all four groups, though a lower fall in TSH was noted in acromegalics. A similar reduction in PRL was also noted after 28 micrograms/min dopamine. Phentolamine infusion (0.5 mg/min) had no effect on PRL response to dopamine. These results indicate that a dopaminergic stimulation led to an inhibition of TSH and PRL secretion. Since the high polarity of dopamine impedes its passage through the blood-brain barrier, its site of action should be outside this barrier, probably in the pituitary.

Evidence for dopaminergic control of thyrotrophin secretion in man

After administration of the dopamine-receptor-blocking drug, metoclopramide (10 mg orally), there is significant release of thyrotrophin (T.S.H.) in hypothyroid patients which is not evident in euthyroid subjects. This is not due to spontaneous fluctuation in basal T.S.H. levels, and it indicates inhibitory dopaminergic control of T.S.H. release in man. The lack of significant T.S.H. release in euthyroid subjects may be due to the inhibitory effects of normal circulating levels of T3 and T4 on T.S.H. release. The T.S.H. response in hypothyroidism is significantly correlated with both T3 and T4 levels, suggesting suppression of this inhibitory pathway in increasingly severe hypothyroidism.

Bromocryptine therapy in cases of amenorrhea-galactorrhea

Of 28 patients presenting with amenorrhea-galactorrhea, pituitary tumors were confirmed in eight. Six patients had occult hypothyroidism and the rest had an endocrine profile suggestive of pituitary tumor or of an idiopathic etiology. Treatment with bromocryptine resulted in suppression of the inappropriate lactation and restoration of regular menstrual function. In five cases, however, the galactorrhea was only diminished and in four of these cases, normal ovarian function did not return. Of the 19 patients that were seeking fertility and continued the medication for at least 20 days, nine pregnancies resulted. A similar response to bromocryptine was observed regardless of the underlying cause of the amenorrhea-galactorrhea.