Thyrotropin-releasing hormone stimulates growth hormone release from the anterior pituitary of hypothyroid rats in vitro

We have examined the interaction of thyroid hormone and TRH on GH release from rat pituitary monolayer cultures and perifused rat pituitary fragments. TRH (10(-9) and 10(-8)M) consistently stimulated the release of TSH and PRL, but not GH, in pituitary cell cultures of euthyroid male rats. Basal and TRH-stimulated TSH secretion were significantly increased in cells from thyroidectomized rats cultured in medium supplemented with hypothyroid serum, and a dose-related stimulation of GH release by 10(-9)-10(-8) M TRH was observed. The minimum duration of hypothyroidism required to demonstrate the onset of this GH stimulatory effect of TRH was 4 weeks, a period significantly longer than that required to cause intracellular GH depletion, decreased basal secretion of GH, elevated serum TSH, or increased basal secretion of TSH by cultured cells. In vivo T4 replacement of hypothyroid rats (20 micrograms/kg, ip, daily for 4 days) restored serum TSH, intracellular GH, and basal secretion of GH and TSH to normal levels, but suppressed only slightly the stimulatory effect of TRH on GH release. The GH response to TRH was maintained for up to 10 days of T4 replacement. In vitro addition of T3 (10(-6) M) during the 4-day primary culture period significantly stimulated basal GH release, but did not affect the GH response to TRH. A GH stimulatory effect of TRH was also demonstrated in cultured adenohypophyseal cells from rats rendered hypothyroid by oral administration of methimazole for 6 weeks. TRH stimulated GH secretion in perifused [3H]leucine-prelabeled anterior pituitary fragments from euthyroid rats. A 15-min pulse of 10(-8) M TRH stimulated the release of both immunoprecipitable [3H]rat GH and [3H]rat PRL. The GH release response was markedly enhanced in pituitary fragments from hypothyroid rats, and this enhanced response was significantly suppressed by T4 replacement for 4 days. The PRL response to TRH was enhanced to a lesser extent by thyroidectomy and was not affected by T4 replacement. These data suggest the existence of TRH receptors on somatotrophs which are suppressed by normal amounts of thyroid hormones and may provide an explanation for the TRH-stimulated GH secretion observed clinically in primary hypothyroidism.     

Influence of starvation on hormonal control of hypophyseal secretion in rats

The reduction of hypophyseal hormone secretion during starvation is not completely understood. A previous study showed that the concomitant reduction of plasma TSH and T3 may be related to an increased sensitivity of the thyrotrope cell to T3. This suggests that regulation of hypophyseal secretion by peripheral hormones may be altered in starved rats. As GH and PRL secretion are under the control of thyroid and steroid hormones, the aim of the present study was to investigate the modification of feed-back control by T3 or E2 on hypophyseal secretion during starvation. For this purpose, pituitary GH, PRL and TSH contents and their plasma responses to TRH injection were measured in euthyroid, thyroidectomized (Tx), T3-supplemented Tx and E2-treated male Wistar rats before and after a 3-day starvation. TRH (0.25 micrograms/100 g) was injected iv through a chronically-implanted catheter. Our results show that GH content and GH plasma response to TRH are dramatically increased in T3-treated Tx starved rats, suggesting that starvation also increases the effectiveness of T3 influence on somatotrope cell secretion. By contrast, effects of T3 on PRL secretion remain unchanged during starvation. Furthermore, starvation in E2-treated rats is associated with a marked rise in the PRL and GH responsiveness to TRH without any significant change of hormonal pituitary content. This suggests that, in starved rats, E2 increases the effects of TRH on lactotrope and somatotrope secretion. No significant effect on TSH secretion could be demonstrated. Thus, starvation seems to act differentially on the feed-back mechanisms controlling the hormonal secretion of the three adenohypophyseal target cells to TRH.     

The paraventricular nucleus of the hypothalamus has a major role in thyroid hormone feedback regulation of thyrotropin synthesis and secretion

The role of the hypothalamic paraventricular nucleus (PVN) in thyroid hormone regulation of TSH synthesis during hypothyroidism was studied in adult male rats that were normal (n = 10), had primary hypothyroidism with sham lesions in the hypothalamus (n = 17), and had primary hypothyroidism with PVN lesions (n = 14). Two and 4 weeks after initiation of treatment, plasma levels of thyroid hormones (TSH, corticosterone and PRL) and pituitary content of TSH beta and alpha-subunit mRNA were measured. TRH mRNA levels in the PVN were determined by in situ hybridization histochemistry. At 2 weeks, despite a decrease in plasma free T4 in both hypothyroid groups, plasma TSH levels increased, but to a lesser degree, in the hypothyroid PVN lesioned compared to hypothyroid sham-lesioned group (7.8 +/- 1.3 vs. 20.5 +/- 1.1 ng/dl; P less than 0.05). Similarly, at 4 weeks, the hypothyroid PVN-lesioned group demonstrated a blunted TSH response compared to the hypothyroid sham-lesioned group (6.8 +/- 0.7 vs. 24.0 +/- 1.3 ng/dl; P less than 0.05). Plasma corticosterone and PRL did not significantly differ between sham-lesioned and PVN-lesioned groups. TSH beta mRNA levels markedly increased in hypothyroid sham-lesioned rats compared to those in euthyroid controls at 2 weeks (476 +/- 21% vs. 100 +/- 39%; P less than 0.05) and 4 weeks (1680 +/- 270% vs. 100 +/- 35%; P less than 0.05). In contrast, TSH beta mRNA levels did not increase with hypothyroidism in the PVN-lesioned group compared to those in euthyroid controls at 2 weeks (140 +/- 16%, P = NS) and only partially increased at 4 weeks (507 +/- 135; P less than 0.05). alpha mRNA levels at 4 weeks markedly increased in hypothyroid sham-lesioned rats compared to those in euthyroid controls (1121 +/- 226% vs. 100 +/- 48%; P less than 0.05), but did not increase in the hypothyroid PVN-lesioned rats (61 +/- 15%; P = NS). TRH mRNA in the PVN increased in the hypothyroid sham-lesioned rats compared to those in euthyroid controls (16.6 +/- 1.3 vs. 4.8 +/- 1.2 arbitrary densitometric units; P less than 0.05), and TRH mRNA was not detectable in the PVN of hypothyroid-lesioned rats at 2 weeks. In summary, lesions in rat PVN prevented the full increase in plasma TSH, pituitary TSH beta mRNA, and alpha mRNA levels in response to hypothyroidism. Thus, factors in the PVN are important in thyroid hormone feedback regulation of both TSH synthesis and secretion.     

Prolactin and TSH response to TRH and metoclopramide before and after l-thyroxine therapy in subclinical hypothyroidism

The effects of the dopamine (DA) receptor antagonist metoclopramide on the plasma thyroid stimulating hormone (TSH) and prolactin (PRL) levels were studied in 8 patients with subclinical hypothyroidism (defined as absence of clinical signs of hypothyroidism with normal thyroid hormone levels, normal or slightly increased basal plasma TSH levels and increased and long-lasting TSH response to TRH) before and after l-thyroxine replacement therapy. Metoclopramide induced a significant (p less than 0.01) TSH release in the subclinical hypothyroid patients. Two weeks after l-thyroxine replacement therapy (50 micrograms/day), the TSH response to metoclopramide was completely blunted in subclinical hypothyroidism. In these patients a significant (p less than 0.01) inhibition of TSH response to intravenous thyrotropin-releasing hormone (TRH) was also observed after treatment with thyroid hormone. In analogy to the TSH behavior, plasma PRL secretion in response to metoclopramide and TRH administration was significantly (p less than 0.05) inhibited in the subclinical hypothyroid patients after l-thyroxine replacement therapy.     

Small dose of domperidone potentiated the effects of TRH and serotonin on prolactin release in ovariectomized, estrogen-treated rats

A transient antagonism of the dopaminergic action by using a dopamine antagonist, domperidone, plus a dopamine agonist, CB154, has been shown to potentiate the effect of thyrotropin-releasing hormone (TRH) on prolactin (PRL) secretion. In order to test whether the serotonin (5-HT)-induced PRL secretion can also be enhanced in a similar way, we used 5-HT instead of TRH in our first experiment. We found that the dose of domperidone used (10 micrograms/rat) seems to be excessive since it induced a marked and substantial increase in PRL release and the use of CB154 further masked the action of 5-HT. We used a smaller dose of domperidone (1 microgram/rat) without the CB154 and found that it induced a moderate amount of PRL release which lasted for over 1 h. Given TRH (1 microgram/rat) or 5-HT (0.3 mg/rat) 1 h later resulted in a significant increase in plasma PRL which was much higher than that induced by TRH or 5-HT alone. The potentiating effect of domperidone was even more significant for the 5-HT- than the TRH-stimulated PRL secretion. Pretreatment with 5-HT or vasoactive intestinal polypeptide (10 micrograms/rat) were without any effect in potentiating the action of TRH. In conclusion, antagonizing the dopamine action appears to enhance the stimulatory effect of TRH and 5-HT on PRL secretion.     

The effect of transient dopamine antagonism on thyrotropin-releasing hormone-induced prolactin release in ovariectomized rats treated with estradiol and/or progesterone

PRL release was studied in ovariectomized (OVX) rats pretreated with estradiol benzoate (EB), progesterone (P), or a combination of both steroids using a protocol that was selected to mimic ovarian steroid changes that have been observed during the female rat 4-day estrous cycle and early pregnancy. On the morning of the experiment, the animals received injections of either the dopamine (DA) antagonist domperidone (0.01 mg/rat iv) or vehicle (acetic acid in saline). Five minutes later, all animals received injections of the DA agonist 2-bromo-alpha-ergocryptine (CB-154; 0.5 mg/rat, iv) followed 60 min later by the administration of TRH (1.0 microgram/rat, iv). Plasma obtained from blood samples taken during the experiment was assayed for PRL by RIA. In OVX or P-treated OVX rats, a transient blockade of DA by domperidone did not alter the sensitivity of the pituitary to TRH administration, as measured by an increase in plasma PRL. However, such an effect of DA blockade was induced by 2 days of EB treatment and was maintained and amplified by P administration after EB injections. We conclude that enhancement of the PRL-releasing effect of TRH by DA antagonism, a mechanism we previously observed in female rats during midlactation, proestrus, estrus, and metestrus using the present drug protocol, can be induced by estrogen and maintained by P. Further, our data suggest that the previously observed loss of this secretory mechanism on the morning of diestrus may be due to the decrease in plasma P that takes place between metestrus and diestrus.     

Pyridostigmine and metoclopramide do not restore the TSH response to TRH inhibited by L-thyroxine treatment in children with goiter

To define the role of somatostatin and dopamine in TSH suppression induced by L-thyroxine, 16 children (12 F, 4 M) on suppressive doses of L-thyroxine (3-4 microg/kg/day) for endemic goiter were studied. Firstly a conventional TRH test was performed in all subjects, in order to evaluate TSH, PRL and GH (basal study). A week later a second TRH test was carried out; one hour before the test, however, group A (9 patients) was given 60 mg pyridostigmine bromide po (pyridostigmine study) and group B (7 patients) 10 mg metoclopramide po (metoclopramide study). In the basal study, TSH was suppressed in both groups and levels did not increase following TRH administration, while PRL increased significantly and GH levels remained stable. In the pyridostigmine study, TSH levels did not increase following TRH administration, while PRL and GH levels were both significantly raised. In the metoclopramide study, TSH and GH levels were not raised following TRH administration, while a significantly greater increase of PRL was observed. In conclusion, suppressive doses of L-thyroxine inhibit the TSH response to TRH, while they do not seem to affect GH and PRL secretion. Somatostatin and/or dopamine do not seem to play a significant role in the L-thyroxine-induced TSH suppression.     

Effect of transient dopamine antagonism on thyrotropin-releasing hormone-induced prolactin secretion in the serotonin-blocked, estrogen-treated ovariectomized rats

Recent studies showed that a brief interruption of dopamine (DA) action markedly increased the thyrotropin-releasing hormone (TRH)-stimulated prolactin (PRL) release. It is thus of interest to delineate whether the estrogen-induced afternoon PRL surge involves the same mechanism. Long-term ovariectomized rats pretreated with polyestradiol phosphate (PEP, 0.1 mg/rat s.c.) for 6 days were used in this study. They also received either p-chlorophenylalanine (PCPA, 250 mg/kg i.p.) or ketanserin (Ket, 10 mg/kg i.p.), two serotonergic drugs known to inhibit the estrogen-induced afternoon PRL surge. Then the animals were either treated with a DA antagonist, domperidone (Domp, 0.01 mg/rat i.v.), or vehicle at 16.00 h on the sampling day. Ten minutes later, the ones receiving Domp were injected with a DA agonist, 2-bromo-alpha-ergocryptine (CB154, 0.5 mg/rat i.v.), followed 50 min later by the administration of TRH (1 microgram/rat i.v.). Plasma samples taken through indwelling intraatrial catheters were assayed for PRL by radioimmunoassay. The estrogen-induced afternoon PRL surges were completely blocked in both PCPA- and Ket-treated animals. A significant PRL surge with similar amplitude, however, was induced by either Domp or TRH, although pretreatment with Domp did not cause any potentiating effect on the action of TRH. On the other hand, Domp induced only a small rise of PRL secretion and TRH was totally ineffective in rats untreated with PEP. It is concluded that both DA antagonism and TRH stimulation can induce significant PRL release in the afternoon of estrogen-treated, serotonin-blocked rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that thyrotropin-releasing hormone is not a major prolactin-releasing factor during suckling in the rat

The purpose of this study was to investigate whether TRH could be an important PRL-releasing factor during suckling in the rat. Plasma PRL, TSH, beta-endorphin-like immunoreactivity, and GH responses in serial blood samples from unanesthetized suckled rats were determined. The resulting hormonal profile was compared with that obtained when TRH (500 ng/kg BW, iv) was injected at the onset of suckling. Suckling evoked a rise in plasma levels of PRL, beta-endorphin-like immunoreactivity, and GH, but not in TSH. In contrast, exogenous TRH caused a 9-fold increase in plasma TSH levels during suckling without further increasing the PRL response. Since plasma PRL responses are reportedly enhanced by previous suckling, we also determined plasma PRL and TSH levels when TRH (25 ng/rat, iv) was given 30 min after a brief suckling episode. TRH caused a 2.5-fold increase in plasma TSH, but did not significantly increase plasma PRL levels. Since suckling increases plasma PRL without increasing plasma TSH, and TRH increases TSH but not PRL levels, we conclude that TRH is not a major PRL-releasing factor during suckling.     

Effect of thyroid status and paraventricular area lesions on the release of thyrotropin-releasing hormone and catecholamines into hypophysial portal blood

TRH is a potent stimulator of pituitary TSH release, but its function in the physiological regulation of thyroid activity is still controversial. The purpose of the present study was to investigate TRH and catecholamine secretion into hypophysial portal blood of hypothyroid and hyperthyroid rats, and in rats bearing paraventricular area lesions. Male rats were made hypothyroid with methimazole (0.05% in drinking water) or hyperthyroid by daily injections with T4 (10 micrograms/100 g BW). Untreated male rats served as euthyroid controls. On day 8 of treatment they were anesthetized to collect peripheral and hypophysial stalk blood. In euthyroid, hypothyroid and hyperthyroid rats plasma T3 was 1.21 +/- 0.04, 0.60 +/- 0.04, and 7.54 +/- 0.33 nmol/liter, plasma T4 50 +/- 3, 16 +/- 2, and 609 +/- 74 nmol/liter, and plasma TSH 1.58 +/- 0.29, 8.79 +/- 1.30, and 0.44 +/- 0.03 ng RP-2/ml, respectively. Compared with controls, hyperthyroidism reduced hypothalamic TRH release (0.8 +/- 0.1 vs. 1.5 +/- 0.2 ng/h) but was without effect on catecholamine release. Hypothyroidism did not alter TRH release, but the release of dopamine increased 2-fold and that of noradrenaline decreased by 20%. Hypothalamic TRH content was not affected by the thyroid status, but dopamine content in the hypothalamus decreased by 25% in hypothyroid rats. Twelve days after placement of bilateral electrolytic lesions in the paraventricular area plasma thyroid hormones and TSH levels were lower than in control rats (T3: 0.82 +/- 0.05 vs. 1.49 +/- 0.07 nmol/liter; T4: 32 +/- 4 vs. 66 +/- 3 nmol/liter; TSH: 1.08 +/- 0.17 vs. 3.31 +/- 0.82 ng/ml). TRH release in stalk blood in rats with lesions was 15% of that of controls, whereas dopamine and adrenaline release had increased by 50% and 40%, respectively. These results suggest that part of the feedback action of thyroid hormones is exerted at the level of the hypothalamus. Furthermore, TRH seems an important drive for normal TSH secretion by the anterior pituitary gland, and thyroid hormones seem to affect the hypothalamic release of catecholamines.     

Modulation of pituitary thyrotropin releasing hormone receptor levels by estrogens and thyroid hormones.

The effect of estradiol and thyroid hormone treatment on pituitary TRH binding and TSH and PRL responses to the neurohormone was studied. A significant increase in the number of pituitary TRH binding sites was observed between 2 and 4 days after daily administration of estradiol benzoate with a plateau at 300% of control being reached at 7 days. Plasma PRL levels showed a similar early pattern of response. In animals rendered hypothyroid by a 2-month treatment with propylthiouracil or 1 month after surgical thyroidectomy, the level of pituitary TRH receptors was increased approximately 2-fold, this elevation being completely reversed by treatment with thyroid hormone. Estradiol-17beta administered with L-thyroxine partially reversed the inhibitory effect of thyroid hormone on TRH receptor levels in hypothyroid animals. The antagonism between estrogens and thyroid hormone is also apparent on the TSH response to TRH since estrogen administration can reverse the marked inhibition by thyroxine of the TSH response to TRH either partially or completely in intact and hypothyroid animals, respectively. The PRL response to TRH is 55 and 40% inhibited in hypothyroid and intact rats, respectively, by thyroid hormone when combined with estrogen treatment. The present data clearly show that estrogens and thyroid hormones can affect TSH and PRL secretion, the effect of estrogens being predominantly on PRL secretion while thyroid hormone affects mainly TSH. The close correlation observed between the level of TRH receptors and PRL and TSH responses to TRH suggests that estrogens and, to a lesser extent, thyroid hormones, exert their action by modulation of the level of receptors for the neurohormone in both thyrotrophs and mammotrophs.     

Induction of hypothyroidism and hypoprolactinemia by growth hormone producing rat pituitary tumors.

The GH3 rat pituitary tumor cell line which secretes both growth hormone (GH) and prolactin (PRL) stopped releasing PRL when transplanted to animals; furthermore, it suppressed PRL production by the hosts' pituitary glands. When the same tumor was transferred back to cell culture, PRL production resumed. The PRL to GH ratio in cell culture medium and cells ranged from 5 to 1 while in the tumor and serum of the host animals it averaged 0.09 and 0.001, respectively. To investigate further this phenomenon, female rats were transplanted with GH3 tumors (T) and compared to intact normal (N) and to thyroidectomized (Tx) rats. T animals were larger and had splanchnomegaly but smaller pituitaries and thyroids. Serum PRL concentrations in the basal state were decreased, as were levels of triiodothyronine (T3), thyroxine (T4), and free T4 index. Despite reduced serum thyroid hormone concentrations, and in contrast to Tx animals, the serum thyrotropin (TSH) level in T rats was not elevated and they did not show a supranormal TSH response to thyrotropin-releasing hormone (TRH) administration. The PRL response to TRH in T animals was completely abolished while all N and Tx animals responded by a significant increase in serum PRL. Serum corticosteroids and estrogens were normal in T rats. Pituitary content of PRL was decreased and that of TSH increased in T rats. Tx animals, however, had a reduced pituitary content of PRL, TSH, and GH. When GH3 cells were grown in cell culture media containing serum from T animals, there was a reduction of PRL content in cells and released in the medium. Addition of T3 to the T serum did not alter its suppressive effect on PRL nor did rat GH added to N serum alter PRL production and release in vitro. In a preliminary experiment, rats injected ip with 50 mug hGH in two divided doses for eighteen days, suppressed serum T4 and T3 concentrations; pituitary content of TSH was significantly increased and that of PRL slightly decreased. Injection with 250 mug oPRL or saline, on the same schedule and for the same length of time, had no significant effect on the levels of serum thyroid hormones. Thus, GH, but also possibly other substance(s) secreted by GH3 tumors in vivo a) suppress the production of tumor and pituitary PRL; b) suppress the release of TSH, causing mild hypothyroidism; c) inhibit the PRL and TSH responses to TRH; and d) decrease the production of PRL in tissue culture. Although no simple and unifying theory could explain these findings, an hypothesis implicating somatomedin is presented.     

The effects of transient dopamine antagonism on thyrotropin-releasing hormone-induced prolactin release in pseudopregnant rats

The effectiveness of TRH in releasing PRL after transient dopamine (DA) blockade was investigated in female rats between days 3 and 11 of pseudopregnancy (PSP). At 0930 h on the morning of the experiment, each animal was injected with the DA antagonist domperidone (0.01 mg/rat, iv) or vehicle (acetic acid in saline); 5 min later, the DA agonist 2-bromo-alpha-ergocryptine maleate (CB-154; 0.5 mg/rat, iv) was administered. Sixty minutes later, TRH (1.0 micrograms/rat, iv) was administered. Blood samples were withdrawn via indwelling catheters before, 5, 20, 40, and 70 min after domperidone or vehicle administration, and 5 and 10 min after TRH administration. On day 3 of PSP, TRH-induced PRL release was significantly enhanced by the domperidone-CB154 treatment compared to that in vehicle-treated control rats. By day 9 of PSP, the effectiveness of TRH in stimulating PRL release after domperidone treatment was decreased by 50% compared to that on day 3 of PSP. This reduction in PRL response to TRH was not due to decreased progesterone levels, as no difference was observed in plasma progesterone between days 3 and 9. Rats that were given domperidone on day 11 of PSP did not exhibit a significant increase in sensitivity to TRH; however, the effectiveness of TRH was enhanced by domperidone on day 11 of PSP in animals that were hysterectomized on day 2 of PSP. Since DA receptor blockage increased the sensitivity to a putative PRL-releasing factor (TRH) and this mechanism was eliminated around the time that the PRL surges of PSP disappear, we suggest that this pituitary mechanism is a critical component of the PRL release mechanism during the surges of PSP. Further, the observed loss of the mechanism between days 9 and 11 of PSP may be due to the direct influence at the anterior pituitary of a uterine PRL inhibitory factor which has been recently described.     

Regulation of thyrotropin-releasing hormone receptors and responses by L-triiodothyronine in dispersed rat pituitary cell cultures

The effects of physiological concentrations of L-T3 (T3) were examined in dispersed cell cultures of pituitaries obtained from 10- to 12-day-old rats. T3 inhibited TSH secretion by 50% and blunted the TSH response to TRH. The PRL response to TRH was also inhibited by T3, and GH secretion was increased 2-fold. These responses were half-maximal at 0.1 nM added T3 in medium supplemented with 10% hypothyroid calf serum, corresponding to a free T3 concentration of 5 pM. In the presence or absence of added T3, TRH effects were half-maximal at 0.5-3 nM, and T3 suppression was not overcome by high concentrations of TRH (up to 1 microM). Maximal inhibition of TSH responses to TRH occurred when cultures were preincubated with thyroid hormone for 24 h; a significant effect was observed after 8 h. The specific binding of [3H]TRH to dispersed rat pituitary cells was decreased 55-70% by T3 in a dose-dependent manner. Inhibition of TSH secretion by T3 was reversible within 24 h, and the fraction of thyrotrophs in the cultures (0.22) was not altered by T3 over the course of the experiments. The results demonstrate that physiological concentrations of T3 regulate TSH and PRL responses to TRH and control TRH receptor levels by a direct action on normal rat pituitary cells.     

Effects of in vivo bolus versus continuous TRH administration on TSH secretion, biosynthesis, and glycosylation in normal and hypothyroid rats

The effects of in vivo TRH administered either as bolus or continuous doses on TSH secretion, synthesis, and glycosylation were studied in normal and hypothyroid rats. Nine-week-old normal or 3-week postthyroidectomy rats were administered bolus doses of saline or TRH (0.5 mg/kg) twice daily or continuous saline or TRH (1 mg/kg/day) via an osmotic pump. After 5 days, pituitaries were removed and incubated with [35S]methionine (MET) and [3H]glucosamine (GLCN), with or without 10(-8) M TRH, for 6 and 24 h. Samples were precipitated with anti-TSH beta sera and then analyzed by gel electrophoresis. In normal rats, plasma TSH, T4 and T3 increased with continuous in vivo TRH but not with bolus TRH; in hypothyroid rats, plasma TSH, T4 and T3 were not altered by continuous or bolus doses of TRH. Additionally, in normal rats, continuous in vivo TRH increased incorporation of MET in secreted TSH (477 vs. 212 X 10(3) dpm/mg DNA; P less than 0.05) and intrapituitary TSH (5035 vs. 2124 X 10(3) dpm/mg DNA; P less than 0.05), and GLCN in secreted TSH (148 vs. 50 dpm/mg DNA; P less than 0.05) and intrapituitary TSH (2344 vs. 744 X 10(3) dpm/mg DNA; P less than 0.05). In contrast, in hypothyroid animals, continuous in vivo TRH did not alter MET or GLCN incorporation in TSH. Bolus TRH did not alter secreted or intrapituitary MET or GLCN incorporation into TSH in the normal rat. However, bolus TRH in the intrapituitary MET or GLCN incorporation into TSH in the normal rat.(ABSTRACT TRUNCATED AT 250 WORDS)     

Histidyl-proline-diketopiperazine (cyclo-his-pro) inhibits the suckling- and domperidone-induced transformation of prolactin in the lactating rat

The histidyl-proline-diketopiperazine (cyclo-his-pro) metabolite of TRH inhibited the transformation of prereleasable PRL within the anterior pituitary and as a consequence, the normal rise in plasma PRL levels when injected before suckling. The dose (400 ng) was administered five times at 1-min intervals. PRL suppression was similar to that resulting from dopamine injected at the same rate. Combined injections of cyclo-his-pro and dopamine, each at the 400 ng/min for 5 min rate, provoked a greater inhibition of PRL than did either alone. Neither cyclo-his-pro nor dopamine inhibited the suckling-induced release of PRL in rats whose PRL had previously been transformed by a short (10-min) period of suckling. The stimulatory effects of iv administered domperidone (0.005 or 0.01 mg/rat) and haloperidol (0.2 mg/rat) upon PRL transformation and release in the lactating rat were substantially reduced by cyclo-his-pro in a dose-related fashion over a range of doses from 200-800 ng/min for 5 min. These results demonstrate that cyclo-his-pro inhibits PRL secretion in the lactating rat after suckling, primarily through inhibition of the transformation phase, and that the inhibitory mechanism may involve an interaction with dopamine.     

Hypothalamic regulation of pulsatile thyrotopin secretion.

To determine the mechanism underlying pulsatile TSH secretion, 24-h serum TSH levels were measured in three groups of five healthy volunteers by sampling blood every 10 min. The influence of an 8-h infusion of dopamine (200 mg), somatostatin (500 micrograms), or nifedipine (5 mg) on the pulsatile release of TSH was tested using a cross-over design. The amount of TSH released per pulse was significantly lowered by these drugs, resulting in significantly decreased mean basal TSH serum levels. However, pulses of TSH were still detectable at all times. The TSH response to TRH (200 micrograms) tested in separate experiments was significantly lowered after 3 h of nifedipine infusion compared to the saline control value. Nifedipine treatment did not alter basal, pulsatile, or TRH-stimulated PRL secretion. The persistence of TSH pulses under dopamine and somatostatin treatment and the blunted TSH responses to nifedipine infusion support the hypothesis that pulsatile TSH secretion is under the control of hypothalamic TRH. The 24-h TSH secretion pattern achieved under stimulation with exogenous TRH in two patients with hypothalamic destruction through surgical removal of a craniopharyngioma provided further circumstantial evidence for this assumption. No TSH pulses and low basal TSH secretion were observed under basal conditions (1700-2400 h), whereas subsequent repetitive TRH challenge (25 micrograms/2 h to 50 micrograms/1 h) led to a pulsatile release of TSH with fusion of TSH pulses, resulting in a TSH secretion pattern strikingly similar to the circadian variation. These data suggest that pulsatile and circadian TSH secretions are predominantly controlled by TRH.     

Serum thyrotropin and prolactin in the syndrome of generalized resistance to thyroid hormone: responses to thyrotropin-releasing hormone stimulation and short term triiodothyronine suppression

Serum TSH and PRL levels and their response to TRH were measured in 11 patients with generalized resistance to thyroid hormone (GRTH), 6 euthyroid subjects, and 6 patients with primary hypothyroidism. TSH and PRL levels and their response to TRH were also measured after the consecutive administration of 50, 100, and 200 micrograms T3 daily, each for a period of 3 days. Using a sensitive TSH assay, all GRTH patients had TSH values that were elevated or within the normal range. On the basis of a normal or elevated TSH level, GRTH patients were classified as GRTH-N1 TSH (5 patients) or GRTH-Hi TSH (6 patients), respectively. Only GRTH patients with previous thyroid ablative therapy had basal TSH values greater than 20 mU/L. TSH responses, in terms of percent increment above baseline, were appropriate for the basal TSH level in all subjects. No GRTH patient had an elevated basal PRL level. PRL responses to TRH were significantly increased only in the hypothyroid controls compared to values in all other groups. On 50 micrograms T3, 7 of 12 (58%) nonresistant (euthyroid and hypothyroid) and 1 of 11 (9%) resistant subjects had a greater than 75% suppression of the TSH response to TRH. On the same T3 dose, 2 of 12 (17%) nonresistant and 4 of 11 (36%) resistant subjects had a greater than 50% suppression of the PRL response to TRH. On 200 micrograms T3, all subjects, except for 1 with GRTH, had a greater than 75% suppression of the TSH response to TRH. On the same T3 dose, while 11 of 12 (92%) nonresistant subjects had a greater than 50% reduction of the PRL response to TRH, only 3 of 10 (30%) resistant patients showed this degree of suppression (P less than 0.005). Without previous ablative therapy, serum TSH in patients with GRTH is usually normal or mildly elevated. The TSH response to TRH is proportional to the basal TSH level and is suppressed by exogenous T3. However, on 200 micrograms T3 basal TSH was not detectable (less than 0.1 mU/L) in all euthyroid subjects, but it was measurable in three of four GRTH patients with normal TSH levels before T3 treatment. PRL levels in GRTH are normal even when TSH is elevated. The PRL response to TRH is not increased in GRTH. In all subjects, exogenous T3 suppresses the PRL response to TRH to a lesser degree than the TSH response, but this difference is much greater in patients with GRTH.     

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.     

Hypothalamo-hypophysial-thyroid axis in streptozotocin-induced diabetes.

Diabetes mellitus is frequently associated with reduced levels of TSH, PRL, GH, and gonadotropins. In this study we have wanted to determine whether chemically induced diabetes mellitus is associated with a decreased hypothalamic release of TRH. Male rats were made diabetic with streptozotocin (STZ; 65 mg/kg), whereas controls received vehicle. After 2 weeks, STZ diabetic rats had 25% lower body weights, 3.5-fold higher blood glucose, and 40-60% lower plasma TSH, T3, and T4 levels than controls. The plasma T4 dialyzable fraction had increased 2.5-fold in STZ diabetic rats, and the plasma free T4 concentration was similar to that in controls. Thus, treatment with STZ results in decreased plasma TSH and T4 levels, but does not reduce free T4 concentrations. The content of TRH in hypothalami of 2-week STZ diabetic rats was similar to that in controls, but in vitro these hypothalami released less TRH than those of control rats. In 2-week STZ diabetic rats, TRH in hypophysial stalk blood was 30% lower than that in control rats. The in vitro TRH secretion from hypothalami of untreated rats was dependent on the glucose concentrations in the incubation medium; increasing the glucose concentration from 10 to 30 mM did not alter TRH secretion, but basal TRH release increased in the absence of glucose. In conclusion, STZ-induced diabetes in the rat is associated with reduced hypothalamic secretion of TRH, which, in turn, may be responsible for the reduced plasma TSH and thyroid hormone levels. Furthermore, it is suggested that the inhibitory effect of STZ-induced diabetes on TRH secretion is probably not due to hyperglycemia.