Evidence for dopaminergic control of thyrotrophin secretion in man

Causal link between thyroid function and schizophrenia: a two-sample Mendelian randomization study

Schizophrenia is a chronic psychiatric disorder with inconsistent behavioral and cognitive abnormalities with profound effects on the individual and the society. Individuals with schizophrenia have altered thyroid function, but results from observational studies are conflicting. To date, it remains unclear whether and in which direction there is a causal relationship between thyroid function and schizophrenia. To investigate causal paths, a bidirectional two-sample Mendelian randomization (MR) study was conducted using summary statistics from genome-wide association studies including up to 330,132 Europeans. Thyroid function was described by the normal-range thyroid-stimulating hormone (TSH) and free thyroxine levels as well as an increased and decreased TSH status. The iterative radial inverse-variance weighted approach with modified second order weights was used as the main method. Based on a discovery and replication sample for schizophrenia, pooled effect estimates were derived using a fixed-effect meta-analysis. Robustness of results was assessed using both a range of pleiotropy robust methods and a network analysis that clustered genetic instruments potentially responsible for horizontal pleiotropy. Genetic liability for hypothyroidism was inversely associated with schizophrenia ([Formula: see text]; 95% CI: (-0.10; -0.02); [Formula: see text]). No notable associations were observed between other thyroid parameters and schizophrenia. Furthermore, no associations could be detected in the reverse direction. Our results suggest that an elevated level of TSH reduce the risk for schizophrenia. The role of thyroid function and the hypothalamic-pituitary-thyroid axis in the development of schizophrenia should be subject of further research.

Risperidone-associated hyperprolactinemia

OBJECTIVE

To compare the prolactogenic effects of risperidone, clozapine, and typical antipsychotic agents in an outpatient community-based psychiatric population.

METHODS

Prolactin and thyroid-stimulating hormone (TSH) concentrations were measured in 68 outpatients with schizophrenia who were receiving antipsychotic medications and were recruited from a community mental health clinic.

RESULTS

The percentage of women with increased prolactin concentrations was significantly greater in the risperidone group (100%, 12 of 12 patients) than in the clozapine group (25%, 1 of 4) (P = 0.0071) but not in comparison with the typical antipsychotic agent group (83%, 5 of 6) (P = 0.333). The percentage of men with increased prolactin concentrations was significantly greater in the risperidone group (94%, 17 of 18) than in the clozapine group (18%, 3 of 17) (P<0.0001) and in comparison with the typical antipsychotic agent group (27%, 3 of 11) (P = 0.0003). The mean prolactin concentration (all ng/mL +/- standard deviation) was also significantly higher in patients taking risperidone (women, 125.0 +/- 56.6; men, 37.3 +/- 23.9) than clozapine (women, 22.0 +/- 25.9; men, 13.3 +/- 22.4) (female patients, P = 0.0004; male patients, P<0.0001) or typical antipsychotic agents (women, 69.0 +/- 59.8; men, 13. 3 +/- 9.1) (female patients, P = 0.036; male patients, P = 0.0003). In the risperidone group, gender affected prolactin level, with women having higher concentrations than men, but the duration of therapy did not. In this group, prolactin was inversely dependent on age. No difference was noted in TSH concentrations between medication groups.

CONCLUSION

Risperidone is a potent inducer of hyperprolactinemia in outpatients with schizophrenia in a community population. The higher and more frequently increased prolactin concentrations caused by risperidone could adversely affect patient health and compliance.

Reduced thyroid-stimulating hormone response to thyrotropin-releasing hormone in autistic boys

The thyroid-stimulating hormone (TSH) and prolactin responses to thyrotropin-releasing hormone (TRH) were compared among four groups of boys--41 autistic, 12 mentally retarded (MR), 12 with minimal brain dysfunction (MBD) and five controls. The autistic boys were divided into two groups: DQ(IQ) greater than or equal to 80 and DQ(IQ) less than 80. Mean TSH basal and peak levels were significantly lower in both autistic groups than in the MR, MBD and control groups. Mean TSH peak value minus basal value (p-b) was significantly lower in both autistic groups than in the control group. Mean prolactin levels and p-b value did not differ among groups. It is suggested that there may be enhanced dopaminergic and/or reduced serotonergic activity in the central nervous system of autistic children, together with hypothalamic dysfunction.

Bromocriptine reduces rat thyrotropin beta-subunit mRNA stability

We have examined the effect of orally administered bromocriptine on TSH beta-subunit messenger (m)RNA in the anterior pituitary glands of Sprague-Dawley rats using in situ and dot-blot hybridization histochemistry. Quantitative in situ hybridization of pituitary sections demonstrated a 60% reduction in TSH beta-subunit mRNA probe binding from rats fed a diet containing bromocriptine 10 mg/kg/day. This was confirmed by dot-blot analysis of nuclear and cytoplasmic pituitary extracts from the same tissue. Hybridization of cytoplasmic extracts of pituitary cells cultured under actinomycin D-induced transcription arrest showed that part of the effect of bromocriptine appeared to be mediated through a change in TSH beta-subunit mRNA stability and implies that the acute influence of dopamine on TSH metabolism may be transduced by control of TSH beta-subunit mRNA catabolism. This suggests a mechanism by which cells with relatively stable tissue specific mRNAs appear to respond rapidly to hormonal effects at the transcriptional level.

Effect of fenoldopam, a dopamine D-1 receptor agonist, on pituitary, gonadal and thyroid hormone secretion

The influence of fenoldopam, a dopamine (DA) D-1 receptor agonist, on basal and GnRH/TRH stimulated PRL, GH, LH, TSH, testosterone and thyroid hormone secretion was studied in nine normal men. All men received 4-h infusions of either 0.9% saline or fenoldopam at an infusion rate of 0.5 microgram/kg min, 12-16 ml/h, adjusted according to weight. After 3 h of infusion, 50 micrograms GnRH and 100 micrograms TRH was given i.v. Blood samples were collected every 15 min from 1 h before to 1 h after the infusion for a total of 6 h for measurements of PRL, LH, FSH, GH, TSH, testosterone, T4 and T3. The median PRL concentration increased significantly (P less than 0.01) to 128%, range 87-287, of preinfusion levels, compared to the decline during control infusion (85%, 78-114). Basal TSH levels declined significantly to 71% (60-91) during fenoldopam compared with 82% (65-115) during control infusion (P less than 0.05). Basal LH, FSH, GH and thyroid hormones were similar during fenoldopam and control infusions (P greater than 0.05). The LH response to GnRH/TRH was significantly (P less than 0.02) increased by fenoldopam infusion. Basal and stimulated testosterone concentration was lower during fenoldopam (P less than 0.01) infusion compared with control. Other hormones were similar after GnRH/TRH stimulation during fenoldopam and saline infusions. These results suggest that DA D-1 receptors are involved in the modulation of pituitary hormone secretion. We suggest that the effect of fenoldopam on PRL and TSH is mainly at the hypothalamic level. Regarding the effect on LH concentrations, an additional direct effect of fenoldopam on testosterone regulation can not be excluded.

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)

The circadian rhythms of thyrotrophin and prolactin secretion

As with other anterior pituitary hormones, the secretion of both thyrotrophin (TSH) and prolactin (PRL) displays a circadian variation with different patterns for each hormone. In recent years there has been a substantial increase in the understanding of the neuroregulation of TSH and PRL. However the primary events involved in the generation of their circadian rhythms remains unclear. Regulatory pathways comprise two major groups: central factors, where the control is exerted by the central nervous system via the hypothalamus and peripheral factors, which include all extra CNS mechanisms. The first group is represented mainly by neuropeptides and neurotransmitters controlling TSH and PRL release, whereas the second one comprises both physical phenomena such as variations in plasma volume or postural changes and hormonal influences arising from target glands such as the adrenal, the thyroid and the gonads.

The preoperative and postoperative investigation of TSH and prolactin release in the management of patients with hyperprolactinaemia due to prolactinomas and nonfunctional pituitary tumours: relationship to adenoma size at surgery

We report here our results of the pre- and post-operative assessment of prolactin and TSH status in 41 hyperprolactinaemic patients who underwent pituitary surgery over a 5 year period. Preoperatively in patients with prolactinomas (n = 33) the TSH response to domperidone decreased with increasing adenoma size. When the data are expressed on a group mean basis the exaggerated TSH response to domperidone in preoperative prolactinoma patients was reduced significantly in patients rendered normoprolactinaemic by surgery but persisted in those who remained hyperprolactinaemic. Similarly the reduced preoperative PRL responses to domperidone and TRH were significantly increased by successful surgery. In contrast patients with stalk-compression hyperprolactinaemia (n = 6) due to larger lesions which were not prolactinomas all showed reduced or absent TSH responses to domperidone. The PRL responses to domperidone and TRH were reduced or absent both in patients with prolactinomas and in those with stalk-compression hyperprolactinaemia. All patients with stalk-compression hyperprolactinaemia showed a delayed pattern of TSH response to TRH with 60 min values being greater than 20 min ones. In contrast a normal pattern of TSH response to TRH was observed in all patients with hyperprolactinaemia due to prolactinomas. Postoperatively TSH and PRL responses were largely unchanged in patients with stalk-compression hyperprolactinaemia regardless of whether normoprolactinaemia was restored by surgery. In conclusion a reduced or absent PRL response to TRH or domperidone is not diagnostic of the presence of a prolactinoma since it occurs in hyperprolactinaemic patients with prolactinomas or stalk-compression. In contrast, the TSH response to acute dopamine antagonism is exaggerated in most patients with small prolactinomas but not in those with stalk-compression hyperprolactinaemia and we have found this to be helpful diagnostically since the presence of an exaggerated TSH response to dopamine antagonism is evidence against the presence of stalk-compression hyperprolactinaemia. The observation of a delayed TSH response to TRH in a hyperprolactinaemic patient should alert the clinician to the possibility of stalk-compression hyperprolactinaemia due to a large lesion which may not be a prolactinoma.

Effect of low-dose dopamine infusion on basal and stimulated TSH and prolactin concentrations in man

Dopamine (DA) infused at pharmacological doses in man inhibits thyrotrophin (TSH) secretion, although the physiological significance of this observation is unclear. The effect of low-dose DA infusion (0.1 microgram/kg/min) on TSH and prolactin (PRL) concentrations during stimulation with thyrotrophin releasing hormone (TRH) in normal male subjects is reported. Six subjects were given intravenous DA or placebo infusions for 165 min on separate days. A bolus of TRH (7.5 micrograms) was given at + 90 min, followed by infusion of the tripeptide (750 ng/min) for 45 min during both DA and placebo studies. In all subjects TRH administration caused a small rise in TSH which was partially inhibited by DA (peak 5.73 +/- 0.85 mU/l vs 4.58 +/- 1.09, P less than 0.05). PRL response to TRH was almost totally inhibited by DA (620 +/- 164 mU/l vs 234 +/- 96, P less than 0.05); integrated TSH and PRL responses to TRH were similarly inhibited by DA. Circulating plasma DA concentration during infusion of the catecholamine was 3.46 +/- 1.00 ng/ml, which is within the range reported in pituitary stalk plasma of other species. These data support the hypothesis that DA is a physiological modulator of TSH secretion in normal man. Major differences in the time course of TSH and PRL responses to TRH, and in the suppressive effect of DA on these responses suggest that there are fundamental differences in stimulus-secretion coupling for TRH and the lactotroph and thyrotroph.

Prolactin and TSH responses to both domperidone and TRH in normal and hyperprolactinaemic women after dopamine synthesis blockade

A study of the effect of alpha-methyl-1-tryosine (metyrosine) blockade (2 g/d for 2 d) of dopamine (DA) synthesis on the PRL and TSH response to domperidone (DOM) and TRH in normal women and subjects with pathological hyperprolactinaemia is reported. In the normal subjects, there was a marked increase in basal PRL (51.7 +/- 11.1 vs 5.7 +/- 1.0 ng/ml) and the PRL and TSH responses to DOM were abolished. The PRL response to TRH was also reduced. In the hyperprolactinaemic subjects, metyrosine had no effect on basal PRL nor on the virtually non-existent PRL response to DOM, whereas it abolished the exaggerated TSH response. The conclusion is drawn that the response of both PRL and TSH to DA receptor blockers is really dependent upon DA inhibitory tone. A fall in this tone can also be postulated as responsible for the hyporesponsiveness of PRL to DOM frequently observed in pathological hyperprolactinaemia. In addition, the fact that metyrosine also abolished the exaggerated TSH response to DOM shows that the latter is totally dependent on enhanced DA inhibition of the thyrotrophs. Lastly, the reduced PRL response to TRH after metyrosine indicates that DA partly determines the ability of the lactotrophs to respond to TRH.

The effect of lisuride hydrogen maleate on anterior pituitary hormones, oestradiol and cortisol in normal and hyperprolactinaemic women

The effects of single oral doses of 0.1 mg lisuride hydrogen maleate (lisuride), a semisynthetic ergot derivative given at 09.00 h, on the serum levels of prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), thyroid-stimulating hormone (TSH), oestradiol and cortisol were studied in six normally cycling women and six patients with hyperprolactinaemic amenorrhoea. The same subjects received placebo and the results were compared with those of lisuride. Within 1 h after oral administration of lisuride, PRL levels decreased significantly (P less than 0.05) and reached the lowest level at 4 h after ingestion and stayed low until 8 h in both normal women and hyperprolactinaemic patients. LH levels slightly but not significantly decreased after lisuride administration in hyperprolactinaemic patients. Serum cortisol levels increased at 13.00 h, probably due to the effect of meal. No significant differences between the lisuride group and placebo group were observed in GH, TSH, FSH and oestradiol levels. These results suggest that lisuride acts as a long-acting prolactin suppressor and that it has little effects on the secretion of other pituitary hormones. Therefore, this drug could be useful for treatment of hyperprolactinaemic patients.

Altered responses of prolactin, luteinizing hormone and follicle stimulating hormone secretion to thyrotrophin releasing hormone/gonadotrophin releasing hormone stimulation in cyclical mastalgia

A generalized abnormality of hypothalamopituitary function was found in 17 patients with cyclical pronounced mastalgia compared with 11 controls by using a combined thyrotrophin releasing hormone and gonadotrophin releasing hormone test. The release of prolactin, luteinizing hormone and follicle stimulating hormone was significantly greater in cyclical mastalgia patients than in controls. Basal thyrotrophin, T3 and T4 levels were within the normal range in both groups indicating normal thyroid status in benign breast disease. The single measurement of oestrogen and progesterone in the luteal phase was not abnormal. These data demonstrate an alteration in lactotroph and gonadotroph function in patients with cyclical mastalgia. It is unknown at present whether this represents an appropriate cellular response to altered central or peripheral signals. There is no evidence to suggest, however, that the anterior pituitary cell types are abnormal per se.

Estrogenic treatment does not modify the TSH and PRL responses to domperidone and TRH in patients with tumoral hyperprolactinemia

To verify if the low estrogen regimen could condition the TSH hyperresponsivity and PRL hyporesponsivity to antidopaminergic drugs seen by us and others in patients bearing prolactinoma, the effect of ethynilestradiol treatment (50 micrograms/day/14 days) on TSH and PRL responses to domperidone in 6 women with tumoral hyperprolactinemia and hypoestrogenemia were studied. Estrogenic treatment was unable to modify the TSH and PRL responsiveness either to domperidone and TRH. These data do not support the hypothesis that hypoestrogenemia could cause the peculiar TSH and PRL pattern in response to antidopaminergic drugs, in patients bearing prolactinoma. Also the TRH releasable pool of TSH and PRL in these patients seems to be unaffected by estrogenic treatment.

Recovery of serum TSH and thyroid hormones after 3'isopropyl-3,5-diiodo-L-thyronine (DIIP) treatment: absence of inhibiting effect of bromocriptine on TSH secretion and evidence for autoregulation of serum T3 levels

Bromocriptine, because of its dopaminergic activity, could possibly inhibit TSH secretion. This hypothesis was tested in 7 normal male volunteers. Thyroid function was suppressed with the very potent thyromimetic analogue, 3'isopropyl-3,5-diiodo-L-thyronine (DIIP). We wanted to observe, after stopping this treatment, whether the return of serum T3, T4, rT3 and TSH values was influenced by concomitant bromocriptine treatment (5 mg/day). For comparison, the DIIP study was also performed without bromocriptine treatment. In both cases, serum T4 and serum T3, which had decreased by, respectively, 34% +/- 5% and 17% +/- 5%, returned over the same time interval to their initial values. Serum TSH and TRH-mediated TSH secretion were also unaffected by the bromocriptine treatment. DIIP does not interfere in the serum determination of T3, T4 and rT3. During suppression with DIIP serum T3 fell less than serum T4 and returned more rapidly to its initial concentrations. It is concluded that dopaminergic inhibition by bromocriptine, at this dose, is insufficient to alter normal thyroid function. In addition, during suppression and shortly thereafter there is a tendency for the serum T3 levels to be maintained.

Inhibition by phospholipid liposomes of the prolactin and cortisol response to insulin hypoglycemia in man

This study was designed to further investigate the purported dopaminergic activity of phospholipid liposomes (PL) prepared from bovine cerebral extracts, and to obtain further indications about their pituitary or suprapituitary site of action. In eight normal subjects, we have studied the effects of PL administration (250 mg as IV bolus plus additional 250 mg infused IV over a 60-min period), compared to placebo, on the prolactin (PRL), cortisol and growth hormone (GH) response to an insulin tolerance test (ITT). In eight additional subjects, the effects of PL on the PRL and TSH response to TRH were evaluated. PL medication blunted the PRL and cortisol response in the ITT: significant differences, with respect to placebo administration, were observed between mean peak PRL values (51.9 +/- 13.63 SEM vs 83.4 +/- 26.35 ng/ml, P less than 0.05) and mean cortisol values at 120 min time (20.9 +/- 0.67 vs 26.7 +/- 2.46 micrograms/dl, P less than 0.05). In contrast, PL administration did not modify the ITT-related GH rise or the PRL and TSH release in response to TRH. These findings favour the view that PL are endowed with intrinsic biological activity which is dopamine-mediated, and point to the hypothalamus as their primary site of action.

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.

Domperidone. A review of its pharmacological activity, pharmacokinetics and therapeutic efficacy in the symptomatic treatment of chronic dyspepsia and as an antiemetic

Domperidone is a dopamine antagonist that does not readily enter the central nervous system. Given parenterally or orally it increases gastric emptying of liquids and increases lower oesophageal sphincter pressure in healthy subjects. The antiemetic and pharmacodynamic profile of domperidone is similar to that of metoclopramide, although domperidone has a lower propensity to cause extrapyramidal side effects. Domperidone effectively alleviates symptoms of chronic postprandial dyspepsia and nausea and vomiting due to a wide variety of underlying causes and in some studies has been superior to metoclopramide. Vomiting associated with the administration of moderately emetic cytotoxic drugs is controlled in the majority of patients. Alleviation of the dose-limiting peripheral side effects (nausea and vomiting) of the anti-Parkinsonian drugs bromocriptine and levodopa, enables a higher optimum dose, with consequent improvement in Parkinsonian symptoms. Domperidone does not aggravate the extrapyramidal side effects of neuroleptic drugs. Control of cytotoxic-induced, and postprandial nausea and vomiting in children has been achieved with domperidone without evidence of extrapyramidal side effects. Indeed, side effects have seldom occurred with therapeutic doses of domperidone.

Reciprocal pattern of the TSH and PRL responses to dopamine receptor blockade in women with physiological or pathological hyperprolactinaemia

The TSH and PRL responses to administration of the two dopamine (DA) receptor antagonists sulpiride and domperidone, were studied in fifteen normoprolactinaemic subjects, twenty-two post-partum women and sixteen subjects with presumptive evidence of (six subjects) or surgically confirmed (ten subjects) prolactinomas. Sulpiride (100 mg i.m.) or domperidone (10 mg i.v.) elicited a slight increase in basal TSH levels in both normoprolactinaemic and post-partum women, but induced a clear-cut TSH rise in subjects with a tumour. Conversely, sulpiride and domperidone strikingly stimulated PRL secretion in normoprolactinaemic and post-partum women, but only slightly enhanced base-line PRL levels in women with prolactinomas. The reason for the reciprocal pattern of TSH and PRL responsiveness to DA receptor blockade of post-partum women and subjects with prolactinomas is presently obscure. Since baseline PRL levels in the two hyperprolactinaemic states overlapped widely, it would appear that hyperprolactinaemia per se is not responsible for such behaviour.

Domperidone. A review of its pharmacological activity, pharmacokinetics and therapeutic efficacy in the symptomatic treatment of chronic dyspepsia and as an antiemetic

Domperidone is a dopamine antagonist that does not readily enter the central nervous system. Given parenterally or orally it increases gastric emptying of liquids and increases lower oesophageal sphincter pressure in healthy subjects. The antiemetic and pharmacodynamic profile of domperidone is similar to that of metoclopramide, although domperidone has a lower propensity to cause extrapyramidal side effects. Domperidone effectively alleviates symptoms of chronic postprandial dyspepsia and nausea and vomiting due to a wide variety of underlying causes and in some studies has been superior to metoclopramide. Vomiting associated with the administration of moderately emetic cytotoxic drugs is controlled in the majority of patients. Alleviation of the dose-limiting peripheral side effects (nausea and vomiting) of the anti-Parkinsonian drugs bromocriptine and levodopa, enables a higher optimum dose, with consequent improvement in Parkinsonian symptoms. Domperidone does not aggravate the extrapyramidal side effects of neuroleptic drugs. Control of cytotoxic-induced, and postprandial nausea and vomiting in children has been achieved with domperidone without evidence of extrapyramidal side effects. Indeed, side effects have seldom occurred with therapeutic doses of domperidone.

Impaired dopaminergic control of thyroid stimulating hormone secretion in chronic renal failure

After administration of intravenous metoclopramide, a dopaminergic receptor blocking agent, no rise in thyroid stimulating hormone (TSH) could be found in patients with chronic renal failure, in contrast to non-uraemic controls. Basal TSH values were normal in the uraemic patients but the TSH response to thyrotrophin-releasing hormone (TRH) was significantly reduced. These results suggest that a discrete abnormality in the hypothalamo-pituitary axis exists in uraemia which may in part be due to interference with central dopaminergic control by a uraemic toxin.

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.

Prolactin response to exercise, metoclopramide and other provacative agents in children

Metoclopramide (MCP), a derivative of procainamide was compared with exercise, arginine, insulin and thyrotropin releasing hormone (TRH) as a prolactin (PRL) releaser in children. The peak response of plasma PRL after oral administration of MCP was greater than that after strenuous exercise and after i.v. administration of pharmacodynamic agents. Normal PRL and TSH responses were observed after TRH administration in all subjects. Variable PRL responses were seen after exercise and after i.v. administration of arginine and insulin, despite significant growth hormone (GH) release following the administration of these agents. MCP produced no increase in plasma TSH. Metoclopramide may be useful for dynamic testing of PRL release in children. It can be taken orally and is free of side-effects.

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.

In vivo and in vitro studies on the effect of metoclopramide on aldosterone secretion

Metoclopramide, a dopamine antagonist drug, elevated plasma aldosterone and prolactin levels without significantly affecting plasma renin activity, ACTH or potassium. Studies with isolated perfused rat zona glomerulosa cells showed that metoclopramide could directly stimulate aldosterone release and that this action was blocked by dopamine. These results suggest that dopamine may play an important inhibitory role in the control of aldosterone secretion.

Catecholaminergic interactions in the regulation of thyrotropin (TSH) secretion in man

The TSH responses to the catecholamine depleting agent monoiodotyrosine (1 g po) and the dopamine receptor blocking drug metoclopramide (10 mg po) have been compared in euthyroid and hypothyroid subjects. Both drugs lead to a prompt and significant rise in TSH levels (peak values at 60-75 min) which is very much greater in hypothyroid than euthyroid subjects. The findings lend further support to the inhibitory role of dopamine in the control of TSH release in man, since TSH release follows either dopamine depletion with monoiodotyrosine or dopamine receptor blockade with metoclopramide.

TSH and prolactin stimulation by the decarboxylase inhibitor benserazide in primary hypothyroidism

200 mg of the 4-aromatic amino acid decarboxylase inhibitor benserazide were administered orally to five patients with primary hypothyroidism. Benserazide induced a significant increase in prolactin and TSH plasma concentration in all subjects. Since the drug blocks the conversion of DOPA to dopamine, these hormonal changes may be due to a reduction in circulating and/or median eminence levels of dopamine.

Domperidone: a novel agent for the investigation of anterior pituitary function and control in man

Administration of the novel agent domperidone (10 mg iv), which combines the properties of specific dopamine receptor blockade and inability to cross the blood-brain barrier, leads to acute and significant TSH and PRL release in man. This suggests that the in vivo site of action of endogenous dopamine in the inhibitory control of these two hormones is either the anterior pituitary or median eminence, since these tissues lie outside the blood-brain barrier. This class of drug is of potential value both clinically and experimentally in the investigation of anterior pituitary control mechanisms.

Effects of sulpiride isomers on the control of anterior pituitary secretion in normal man

The effects of (+)- and (-)-sulpiride on anterior pituitary hormone secretion (LH, FSH, PL, hGH, TSH) have been studied in five normal men. Both (+)- and (-)-sulpiride increase PL and TSH secretion and decrease LH and FSH secretion, whereas they do not seem to affect hGH secretion. These results are discussed in view of the different action of (+)- and (-)-sulpiride on DA receptors in the forebrain with respect to DA receptors involved in the control of anterior pituitary secretion.

The effect of lisuride hydrogen maleate, an ergot derivative on anterior pituitary hormone secretion in man

The effects of single oral doses of 0.2 mg of lisuride hydrogen maleate, a semisynthetic ergot derivative, on serum levels of prolactin (PRL), growth hormone (GH), thyroid stimulating hormone (TSH), luteinizing hormone (LH), follicle stimulating hormone (FSH), cortisol and blood glucose were studied in six normal males. Lisuride effectively inhibited basal PRL secretion as well as the PRL response to TRH given 3 h later. In addition, the drug raised basal GH levels and decreased basal and TRH stimulated TSH secretion. No significant differences between lisuride and control were observed in basal LH and FSH, LHRH stimulated gonadotrophins or in cortisol. Drowsiness was noted by all subjects, one became nauseated and another vomited, 60 and 90 min respectively after administration of lisuride. No changes were seen in pulse rate and blood pressure. The endocrine effects of lisuride were attenuated by the prior administration of the dopamine antagonist metoclopramide. These results suggest that lisuride acts as a long-acting dopamine agonist and that therefore this drug could be of therapeutic use in hyperprolactinaemic states and acromegaly.

Thyroid hormones and TSH, prolactin and LH responses to repeated TRH and LRH injections in depressed patients

Twelve patients with unipolar depressive disorders received 600 microgram of synthetic TRH or LRH, in a random order, for 3 days each. Placebo injections were given as two 3-day courses prior to and between the active treatments. Serum TSH, prolactin (Prl) and LH were measured by radioimmunoassays prior to the experiment as well as immediately before and 20 min after each injection. Serum T4 and T3 were determined by radioimmunoassays before the treatments and 24 h after the first two TRH injections. Serum T4 level in depressed patients did not differ from controls. Serum T3 level in depressed patients was significantly below, and the reverse T3 level was slightly above the normal mean. However, the latter difference was not statistically significant due to the large variation of rT3 levels among the depressed patients. The TSH responses did not differ from that of controls after the first injection but the responses after the second injection were lower than in a control study. The Prl responses to TRH were sub-normal in seven out of eight patients. Blunted LH responses to LRH were seen in three patients.

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.

Increased thyrotrophin secretion induced by sulpiride in man

100 mg i.m. sulpiride (a dopamine-receptor-blocking drug) led to a significant rise in plasma TSH in normal womem, in female patients with galactorrhoea, and, to a much more marked degree, in male and female patients with primary hypothyroidism. In the hypothyroid patients, there was a significant positive correlation between basal TSH and its maximum increment after sulpiride. The drug proved to be an even more potent stimulator of PRL, at least in subjects with normal blood PRL. Normal males, on the other hand, displayed no significant changes in TSH after sulpiride. Continuous administration (150 mg/day per os for 15 days) also resulted in enhancement of TSH in normal women. These results suggest that TSH release is controlled by a dopaminergic mechanism in man. The more accentuated TSH response in hypothyroid patients may perhaps be attributable to the absence of negative-feedback on the part of thyroid hormones.

Hypothalamo-pituitary-thyroid function in anorexia nervosa: influence of weight gain

The functional state of the hypothalamo-pituitary-thyroid axis was assessed in 14 women and girls with anorexia nervosa when at low body weight and again in 12 cases after they had gained weight. Mean serum thyroxine concentrations were low before and after weight gain. Mean serum triiodothyronine (T3) concentrations were substantially reduced at low weight and doubled after weight gain, the absolute values being linearly correlated with body weight expressed as a percentage of the ideal. Concentrations of reverse T3 were greatly increased in some patients initially and fell with weight gain. Basal concentrations of thyroid-stimulating hormone (TSH) were unchanged after weight gain but the TSH response to thyrotrophin-releasing hormone was significantly augmented; delayed patterns of response were found in seven out of 12 patients tested before and three out of 12 patients tested after weight gain. Changes in the hypothalamo-pituitary-thyroid axis are common in anorexia nervosa and probably represent both peripheral and central adaptations to the altered nutritional state.

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.

Monoamines and sexual behaviour in non-human primates

The experiments described were concerned with elucidating the involvement of monoamines in the sexual behaviour of rhesus monkeys. The measurement of perturbations in the levels and turnover of the acid metabolites of serotonin and dopamine in cisternal cerebrospinal fluid, occurring in response to hormonal manipulations which alter proceptivity and receptivity, has not clarified the ways in which hormones and amines interact. However, the use of psychoactive drugs which alter the activity of monamine neurons in the brain has demonstrated that sexual activity can be profoundly influenced by such procedures. Thus, depletion of serotonin (5-hydroxytryptamine) in the brain using p-chlorophenylalanine reversed the decrease in proceptivity that followed adrenalectomy. Conversely, inhibition of serotonin uptake using chlorimipramine reduced proceptivity and receptivity to very low levels. Both findings point to the apparently important role of serotoninergic neurons in the control of sexual behaviour. Low doses of the dopamine agonist alpha-bromocriptine enhanced the proceptivity of female rhesus monkeys but high doses had no such effect. This behavioural change was not related to the concomitant suppression of serum prolactin and may have been related to presynaptic actions of the drug in a manner postulated to explain similar behavioural effects in rats. The beta-blocker oxprenolol had the interesting property of improving the sexual performance of male rhesus monkeys, a behavioural effect consistent with an anxiolytic action, although endocrine (prolactin, cortisol) measures did not provide support for such a view. Defining the nature of the interaction between hormones and monoamine-containing neurons in the brain in behavioural contexts is shown to depend largely on the application of precise neuroanatomical and neuropharmacological techniques. However, the use of systemic treatment with psychoactive drugs used widely in clinical practice, in carefully controlled, behavioural and and endocrine experiments, is likely to provide invaluable information on where and how to investigate the neural mechanisms involved.