Neonatal low-protein diet changes deiodinase activities and pituitary TSH response to TRH in adult rats.

Protein malnutrition during neonatal programs for a lower body weight and hyperthyroidism in the adult offspring were analyzed. Liver deiodinase is increased in such animals, contributing to the high serum triiodothyronine (T3) levels. The level of deiodinase activities in other tissues is unknown. We analyzed the effect of maternal protein restriction during lactation on thyroid, skeletal muscle, and pituitary deiodinase activities in the adult offspring. For pituitary evaluation, we studied the in vitro, thyrotropin-releasing hormone (TRH)-stimulated thyroid-stimulating hormone (TSH) secretion. Lactating Wistar rats and their pups were divided into a control (C) group, fed a normal diet (23% protein), and a protein-restricted (PR) group, fed a diet containing 8% protein. At weaning, pups in both groups were fed a normal diet until 180 days old. The pituitary gland was incubated before and after TRH stimulation, and released TSH was measured by radioimmunoassay. Deiodinase activities (D1 and D2) were determined by release of (125)I from [(125)I]reverse triiodothyronine (rT3). Maternal protein malnutrition during lactation programs the adult offspring for lower muscle D2 (-43%, P<0.05) and higher muscle D1 (+83%, P<0.05) activities without changes in thyroidal deiodinase activities, higher pituitary D2 activity (1.5 times, P<0.05), and lower TSH response to in vitro TRH (-56%, P<0.05). The evaluations showed that the lower in vivo TSH detected in adult PR hyperthyroid offspring, programmed by neonatal undernutrition, may be caused by an increment of pituitary deiodination. As described for liver, higher skeletal muscle D1 activity suggests a hyperthyroid status. Our data broaden the knowledge about the adaptive changes to malnutrition during lactation and reinforce the concept of neonatal programming of the thyroid function.

Peptide YY (PYY)3-36 modulates thyrotropin secretion in rats.

Peptide YY (PYY)3-36 is a gut-derived hormone, with a proposed role in central mediation of postprandial satiety signals, as well as in long-term energy balance. In addition, recently, the ability of the hormone to regulate gonadotropin secretion, acting at pituitary and at hypothalamus has been reported. Here, we examined PYY3-36 effects on thyrotropin (TSH) secretion, both in vitro and in vivo. PYY3-36-incubated rat pituitary glands showed a dose-dependent decrease in TSH release, with 44 and 62% reduction at 10(-8) and 10(-6) M (P < 0.05 and P < 0.001 respectively), and no alteration in TSH response to thyrotropin-releasing hormone. In vivo, PYY3-36 i.p. single injection in the doses of 3 or 30 cg/kg body weight, administered to rats fed ad libitum, was not able to change serum TSH after 15 or 30 min. However, in fasted rats, PYY3-36 at both doses elicited a significant rise (approximately twofold increase, P < 0.05) in serum TSH observed 15 min after the hormone injection. PYY3-36 treatment did not modify significantly serum T4, T3, or leptin. Therefore, in the present paper, we have demonstrated that the gut hormone PYY3-36 acts directly on the pituitary gland to inhibit TSH release, and in the fasting situation, in vivo, when serum PYY3-36 is reduced, the activity of thyroid axis is reduced as well. In such a situation, systemically injected PYY3-36 was able to acutely activate the thyrotrope axis, suggesting a new role for PYY3-36 as a regulator of the hypothalamic-pituitary-thyroid axis.

Thyroid hormones modulate the endocrine and autocrine/paracrine actions of leptin on thyrotropin secretion.

We investigated the influence of hypo- and hyperthyroidism on the ability of leptin to modulate TSH secretion. Two hours after receiving leptin (8 mug leptin/100 g BW; s.c.), hyperthyroid rats (10 mug thyroxine (T4)/100 g body weight (BW) for 5 days) showed a 1.7-fold increase in serum TSH (P<0.05); in hypothyroid rats, leptin had no effect. Hemi-pituitaries of hyperthyroid rats incubated with 10(-9) and 10(-7)M leptin showed reductions in TSH release of 40 and 50% respectively (P<0.05); incubation with 1:2000 and 1:500 dilutions of antiserum against leptin resulted in 3- and 4-fold higher TSH release (P<0.05 and P<0.001 respectively). However, in hypothyroid pituitaries leptin or the antiserum had no effect. The results suggest that the in vivo and in vitro responsiveness of TSH to leptin is abolished in hypothyroidism and is preserved in short-term hyperthyroidism, in comparison to previous reports in euthyroidism. In addition, the inhibitory action of pituitary leptin is enhanced in hyperthyroid glands, which may suggest a role for locally produced leptin in the suppression of TSH release associated with hyperthyroidism.

Sex steroids modulate rat anterior pituitary and liver iodothyronine deiodinase activities.

In this study, we investigated the sex hormone regulation of 5'-iodothyronine deiodinase activity, which is responsible for enzymatic conversion of thyroxine into the bioactive form, triiodothyronine. Pituitary homogenates and liver microsomes from: 1) ovariectomized rats injected with 17-beta-estradiol benzoate and/or progesterone (0.7 and 250 microg/100 g body weight, respectively, subcutaneously, over 10 days); 2) male castrated rats treated or not with 0.4 mg/100 g body weight testosterone propionate, intramuscular, over 7 days, were assayed for type 1 and type 2 deiodinase activity in the pituitary. Enzyme activities were measured by release of (125)I from deiodination of (125)I reverse triiodothyronine under varying assay conditions. Estrogen stimulated anterior pituitary and liver type 1 deiodinase activity in ovariectomized rats (45 and 30 %, p < 0.05). Progesterone inhibited the liver enzyme (40 %, p < 0.05), and had no effect on the pituitary, but in both tissues, blocked estrogen stimulatory effect on type 1 deiodinase. In males, testosterone normalized the reduced liver type 1 deiodinase of castrated rats. However, in the pituitary, castration increased (50 %) type 1 deiodinase independent of testosterone treatment, suggesting the existence of a inhibitory testicular regulator of pituitary type 1 enzyme. Treatments did not alter pituitary type 2 deiodinase activity. In conclusion, gonads and sex steroids differentially modulate type 1 deiodinase activity in rat pituitary and liver.

An unliganded thyroid hormone receptor causes severe neurological dysfunction.

Congenital hypothyroidism and the thyroid hormone (T(3)) resistance syndrome are associated with severe central nervous system (CNS) dysfunction. Because thyroid hormones are thought to act principally by binding to their nuclear receptors (TRs), it is unexplained why TR knock-out animals are reported to have normal CNS structure and function. To investigate this discrepancy further, a T(3) binding mutation was introduced into the mouse TR-beta locus by homologous recombination. Because of this T(3) binding defect, the mutant TR constitutively interacts with corepressor proteins and mimics the hypothyroid state, regardless of the circulating thyroid hormone concentrations. Severe abnormalities in cerebellar development and function and abnormal hippocampal gene expression and learning were found. These findings demonstrate the specific and deleterious action of unliganded TR in the brain and suggest the importance of corepressors bound to TR in the pathogenesis of hypothyroidism.

The somatostatin analogue octreotide modulates Iodothyronine deiodinase activity and pituitary neuromedin B.

Somatostatin inhibits growth hormone and thyrotropin (TSH) secretion. It also enhances the inhibitory effect of thyroid hormone (TH) on TSH by poorly understood mechanisms. We investigated the acute effect of the long-acting somatostatin analogue, octreotide (OCT), on anterior pituitary type 1 (D1) and 2 (D2) deiodinase activity, on liver D1, and on pituitary content of neuromedin B (NB), an autocrine inhibitor of TSH secretion, which is positively regulated by thyroid hormones. Euthyroid or hypothyroid rats were sacrificed at different times after a single subcutaneous injection of OCT (1 microg/kg body weight [BW]). D1 and D2 activities were measured by the release of 125I from 125I reverse triiodothyronine (rT3) under different assay conditions. NB, TSH, T3, and thyroxine (T4) were quantitated by radioimmunoassay (RIA). In euthyroid rats, liver and pituitary D1 activities were decreased (50%) 6 hours after OCT injection; pituitary D2 and NB remained unchanged. In hypothyroid rats, OCT increased near to the level of normal rats both pituitary D1 activity (but not liver) and NB content, at 24 hours and at 6 and 24 hours, respectively (p < 0.05). Pituitary D2, greatly increased by hypothyroidism, showed a small (25%) but significant reduction at 3 hours, persisting at 24 hours (p < 0.01), although it remained higher than that of euthyroid control. Serum thyroid hormones were not affected by OCT injection. The results show that octreotide acutely regulates pituitary deiodinases and NB content, both representing mechanisms that potentially can contribute to somatostatin and octreotide actions on pituitary growth hormone (GH) and TSH secretion and to modulate these cells sensitivity to thyroid hormone action.

Effect of medroxyprogesterone acetate on thyrotropin secretion in adult and old female rats.

Steroid hormones have been implicated in the modulation of TSH secretion; however, there are few and controversial data regarding the effect of progesterone (Pg) on TSH secretion. Medroxyprogesterone acetate (MPA) is a synthetic alpha-hydroxyprogesterone analog that has been extensively employed in therapeutics for its Pg-like actions, but that also has some glucocorticoid and androgen activity. Both hormones have been shown to interfere with TSH secretion. The objective of the present study was to investigate the effects of MPA or Pg administration to ovariectomized (OVX) rats on in vivo and in vitro TSH release and pituitary TSH content. The treatment of adult OVX rats with MPA (0. 25 mg/100 g body weight, sc, daily for 9 days) induced a significant (P<0.05) increase in the pituitary TSH content, which was not observed when the same treatment was used with a 10 times higher MPA dose or with Pg doses similar to those of MPA. Serum TSH was similar for all groups. MPA administered to OVX rats at the lower dose also had a stimulatory effect on the in vitro basal and TRH-induced TSH release. The in vitro basal and TRH-stimulated TSH release was not significantly affected by Pg treatment. Conversely, MPA had no effect on old OVX rats. However, in these old rats, ovariectomy alone significantly reduced (P<0.05) basal and TRH-stimulated TSH release in vitro, as well as pituitary TSH content. The results suggest that in adult, but not in old OVX rats, MPA but not Pg has a stimulatory effect on TSH stores and on the response to TRH in vitro.

Effect of medroxyprogesterone acetate on thyrotropin secretion in adult and old female rats

Steroid hormones have been implicated in the modulation of TSH secretion; however, there are few and controversial data regarding the effect of progesterone (Pg) on TSH secretion. Medroxyprogesterone acetate (MPA) is a synthetic alpha-hydroxyprogesterone analog that has been extensively employed in therapeutics for its Pg-like actions, but that also has some glucocorticoid and androgen activity. Both hormones have been shown to interfere with TSH secretion. The objective of the present study was to investigate the effects of MPA or Pg administration to ovariectomized (OVX) rats on in vivo and in vitro TSH release and pituitary TSH content. The treatment of adult OVX rats with MPA (0.25 mg/100 g body weight, sc, daily for 9 days) induced a significant (P<0.05) increase in the pituitary TSH content, which was not observed when the same treatment was used with a 10 times higher MPA dose or with Pg doses similar to those of MPA. Serum TSH was similar for all groups. MPA administered to OVX rats at the lower dose also had a stimulatory effect on the in vitro basal and TRH-induced TSH release. The in vitro basal and TRH-stimulated TSH release was not significantly affected by Pg treatment. Conversely, MPA had no effect on old OVX rats. However, in these old rats, ovariectomy alone significantly reduced (P<0.05) basal and TRH-stimulated TSH release in vitro, as well as pituitary TSH content. The results suggest that in adult, but not in old OVX rats, MPA but not Pg has a stimulatory effect on TSH stores and on the response to TRH in vitro

Low-protein diet changes thyroid function in lactating rats.

Lactating rats were fed with free access to an 8% protein-restricted diet (PR); the control group was fed a 23% protein diet (C). An energy-restricted (pair-fed) group was given the same food as the animals in the control group, but the amounts of food consumed by both PF and PR were about the same. The body weight and serum albumin concentration of PR and PF dams were significantly (P < 0. 05) lower than that of the controls. The PR group had a significant increase in serum-free triiodothyronine (FT3) concentration, 24-hr mammary gland and milk radioiodine (I131) uptake (67%, 278%, and 200%, respectively) as compared with the controls. On the other hand, those animals had a significantly lower serum-free thyroxine (FT4) concentration and 2- and 24-hr thyroid I131 uptake (67%, 64%, and 74%, respectively). Protein malnutrition during lactation did not alter thyroid or liver 5'-deiodinase activity significantly. However, PF dams had a significantly lower (25%) thyroid 5'-deiodinase activity. These data suggest that protein-restricted lactating dams had an adaptive change in the thyroid function, which could be important to increase the transference of iodine or triiodothyronine through the milk to their pups and prevent sequelae of neonatal hypothyroidism.

Effect of testosterone propionate treatment on thyrotropin secretion of young and old rats in vitro.

The aim of this study was to evaluate the influence of androgens on TSH secretion during aging in Dutch rats. Male young (2 months) and old (16-21 months) rats were castrated (Cx) or sham-operated (C) and received testosterone propionate (TP--4 mg/Kg B.W., i.m., 7 days) or vehicle. Female adult (3 months) and old (12 and 17 months) intact rats received TP or corn oil in the same dose. The rats were decapitated, trunk blood was collected and anterior pituitaries were dissected out for in vitro incubation. In Cx young male rats, only TSH pituitary content showed lower levels than in their controls. Cx TP-treated rats showed higher serum TSH and in vitro basal and TRH-induced TSH secretion, but TP only partially reversed the decrease in pituitary TSH promoted by castration. The old male rats showed lower basal in vitro TSH secretion and pituitary TSH content. In Cx old male rats, serum and basal in vitro TSH concentrations were higher than those of old controls and TP treatment further increased basal in vitro TSH secretion, as well as, stimulated TRH-induced TSH secretion. Interestingly, TP had no effect on intact young or old male rats. However, in intact old female rats, TP stimulated in vitro TSH secretion but, as observed in the intact male, TP had no effect on adult female rats. These results suggest a stimulatory role of testosterone on TSH secretion of young and old male rats. Thereafter, it seems that the testes of old rats secrete some testicular factor that inhibits TSH secretion. However, in male rats with normal testosterone levels TP treatment did not increase further TSH secretion, but in old female rats it had a stimulatory effect.

Effects of estradiol benzoate on 5'-iodothyronine deiodinase activities in female rat anterior pituitary gland, liver and thyroid gland

There is little information on the possible effects of estrogen on the activity of 5'-deiodinase (5'-ID), an enzyme responsible for the generation of T3, the biologically active thyroid hormone. In the present study, anterior pituitary sonicates or hepatic and thyroid microsomes from ovariectomized (OVX) rats treated or not with estradiol benzoate (EB, 0.7 or 14 mug/100 g body weight, sc, for 10 days) were assayed for type I 5'-ID (5'-ID-I) and type II 5'-ID (5'-ID-II, only in pituitary) activities. The 5'-ID activity was evaluated by the release of (125)I from deiodinated (125)I rT3, using specific assay conditions for type I or type II. Serum TSH and free T3 and free T4 were measured by radioimmunoassay. OVX alone induced a reduction in pituitary 5'-ID-I (control = 723.7 + 67.9 vs OVX = 413.9 + 26.9; P<0.05), while the EB-treated OVX group showed activity similar to that of the normal group. Thyroid 5'-ID-I showed the same pattern of changes, but these changes were not statistically significant. Pituitary and hepatic 5'-ID-II did not show major alterations. The treatment with the higher EB dose (14 mug), contrary to the results obtained with the lower dose, had no effect on the reduced pituitary 5'-ID-I of OVX rats. However, it induced an imporatnt increment of 5'-ID-I in the thyroid gland (0.8 times higher than that of the normal group: control = 131.9 + 23.7 vs OVX + EB 14 mug = 248.0 + 31.2; P<0.05), which is associated with increased serum TSH (0.6-fold vs OVX, P<0.05) but normal serum free T3 and free T4. The data suggest that estrogen is a physiological stimulator of anterior pituitary 5'-ID-I and a potent stimulator of the thyroid enzyme when employed at high doses.

Dose-dependent effects of 17-beta-estradiol on pituitary thyrotropin content and secretion in vitro

We studied the basal and thyrotropin-releasing hormone (TRH) (50 nM) induced thyrotropin (TSH) release in isolated hemipituitaries of ovariectomized rats treated with near-physiological or high doses of 17-Beta-estradiol benzoate (EB; sc, daily for 10 days) or with vehicle (untreated control rats, OVX). One group was sham-operated (normal control). The anterior pituitary glands were incubated in Krebs-Ringer bicarbonate medium, pH 7.4 at 37 C in an atmosphere of 95 percent O2/5 percent CO2. Medium and pituitary TSH was measured by specific RIA (NIDDK-RP-3). Ovariectomy induced a decrease (P<0.05) in basal TSH release (normal control = 44.1 + 7.2; OVX = 14.7 + 3.0 ng/ml) and tendend to reduce TRH-stimulated TSH release (normal control = 33.0 + 8.1; OVX = 16.6 + 2.4 ng/ml). The lowest dose of EB (0.7 mug/100 g body weight) did not reverse this alteration, but markedly increased the pituitary TSH content (0.6 + 0.06 mug/hemipituitary; P<0.05) above that of OVX (0.4 + 0.03 mug/hemipituitary) and normal rats (0.46 + 0.03 mug/hemipituitary). The intermediate EB dose (1.4 mug/100 g body weight) induced a nonsignificant tendency to a higher TSH response to TRH compared to OVX and a lower response compared to normal rats. Conversely, in the rats treated with the highest dose (14 mug/100 g body weight), serum 17-Beta-estradiol was 17 times higher than normal, and the basal and TRH-stimulated TSH release, as well as the pituitary TSH content, was significantly (P<0.05) reduced compared to normal rats and tended to be even lower than the values observed for the vehicle-treated OVX group, suggesting an inhibitory effect of hyperestrogenism. In conclusion, while reinforcing the concept of a positive physiological regulatory role of estradiol on the tSH response to TRH and on the pituitary stores of the hormone, the present results suggest an inhibitory effect of high levels of estrogen on these responses.

Pituitary neuromedin B content in experimental fasting and diabetes mellitus and correlation with thyrotropin secretion.

Fasting and diabetes mellitus in the rat model have been associated with abnormalities of thyrotropin (TSH) secretion. Neuromedin B is a bombesin-like peptide highly concentrated in the pituitary gland that has been shown to have inhibitory action on TSH secretion, acting as an autocrine/paracrine factor. Here, we aimed to determine if the pituitary content of neuromedin B would change in fasted rats (1, 2, 3, and 4 days of food deprivation) and streptozotocin (55 mg/kg body weight)-diabetic rats. The total pituitary content of neuromedin B was decreased in fasted rats, except at 2 days of fasting, as was the total protein content in the gland; however, the concentration of the peptide (femtomoles per milligram protein) did not significantly change until the fourth day of food deprivation, when an abrupt decrease in total protein happened and therefore neuromedin B concentration increased. In rats after 20 days of diabetes induction, pituitary neuromedin B increased. Serum thyroxine (T4) and triiodothyronine (T3) decreased in both disorders, whereas serum TSH was normal or decreased in 4-day fasted rats. Therefore, the caloric deprivation of diabetes and fasting changed the pituitary neuromedin B content and concentration, by mechanisms that remain to be elucidated. Since neuromedin B has been shown to act as a local inhibitor of TSH release, the results raise the possibility that increased neuromedin B concentration might be involved in the altered TSH secretion of diabetes mellitus and fasting.

Effects of estradiol benzoate on 5'-iodothyronine deiodinase activities in female rat anterior pituitary gland, liver and thyroid gland.

There is little information on the possible effects of estrogen on the activity of 5'-deiodinase (5'-ID), an enzyme responsible for the generation of T3, the biologically active thyroid hormone. In the present study, anterior pituitary sonicates or hepatic and thyroid microsomes from ovariectomized (OVX) rats treated or not with estradiol benzoate (EB, 0.7 or 14 micrograms/100 g body weight, s.c., for 10 days) were assayed for type I 5'-ID (5'-ID-I) and type II 5'-ID (5'-ID-II, only in pituitary) activities. The 5'-ID activity was evaluated by the release of 125I from deiodinated 125I rT3, using specific assay conditions for type I or type II. Serum TSH and free T3 and free T4 were measured by radioimmunoassay. OVX alone induced a reduction in pituitary 5'-ID-I (control = 723.7 +/- 67.9 vs OVX = 413.9 +/- 26.9; P < 0.05), while the EB-treated OVX group showed activity similar to that of the normal group. Thyroid 5'-ID-I showed the same pattern of changes, but these changes were not statistically significant. Pituitary and hepatic 5'-ID-II did not show major alterations. The treatment with the higher EB dose (14 micrograms), contrary to the results obtained with the lower dose, had no effect on the reduced pituitary 5'-ID-I of OVX rats. However, it induced an important increment of 5'-ID-I in the thyroid gland (0.8 times higher than that of the normal group: control = 131.9 +/- 23.7 vs OVX + EB 14 micrograms = 248.0 +/- 31.2; P < 0.05), which is associated with increased serum TSH (0.6-fold vs OVX, P < 0.05) but normal serum free T3 and free T4. The data suggest that estrogen is a physiological stimulator of anterior pituitary 5'-ID-I and a potent stimulator of the thyroid enzyme when employed at high doses.

Dose-dependent effects of 17-beta-estradiol on pituitary thyrotropin content and secretion in vitro.

We studied the basal and thyrotropin-releasing hormone (TRH) (50 nM) induced thyrotropin (TSH) release in isolated hemipituitaries of ovariectomized rats treated with near-physiological or high doses of 17-beta-estradiol benzoate (EB; sc, daily for 10 days) or with vehicle (untreated control rats, OVX). One group was sham-operated (normal control). The anterior pituitary glands were incubated in Krebs-Ringer bicarbonate medium, pH 7.4, at 37 degrees C in an atmosphere of 95% O2/5% CO2. Medium and pituitary TSH was measured by specific RIA (NIDDK-RP-3). Ovariectomy induced a decrease (P < 0.05) in basal TSH release (normal control = 44.1 +/- 7.2; OVX = 14.7 +/- 3.0 ng/ml) and tended to reduce TRH-stimulated TSH release (normal control = 33.0 +/- 8.1; OVX = 16.6 +/- 2.4 ng/ml). The lowest dose of EB (0.7 microgram/100 g body weight) did not reverse this alteration, but markedly increased the pituitary TSH content (0.6 +/- 0.06 microgram/hemipituitary; P < 0.05) above that of OVX (0.4 +/- 0.03 microgram/hemipituitary) and normal rats (0.46 +/- 0.03 microgram/hemipituitary). The intermediate EB dose (1.4 micrograms/100 g body weight) induced a nonsignificant tendency to a higher TSH response to TRH compared to OVX and a lower response compared to normal rats. Conversely, in the rats treated with the highest dose (14 micrograms/100 g body weight), serum 17-beta-estradiol was 17 times higher than normal, and the basal and TRH-stimulated TSH release, as well as the pituitary TSH content, was significantly (P < 0.05) reduced compared to normal rats and tended to be even lower than the values observed for the vehicle-treated OVX group, suggesting an inhibitory effect of hyperestrogenism. In conclusion, while reinforcing the concept of a positive physiological regulatory role of estradiol on the TSH response to TRH and on the pituitary stores of the hormone, the present results suggest an inhibitory effect of high levels of estrogen on these responses.

Acute effect of thyroxine on pituitary neuromedin B content of hypothyroid rats and its correlation with TSH secretion.

Neuromedin B (NB) is a bombesin-like peptide that has been recently characterized as a physiological paracrine/autocrine inhibitor of thyrotropin (TSH) secretion. We report here the time course of the effect of thyroxine (T4) administration to hypothyroid rats on the anterior pituitary content of NB. Dutch-Miranda male rats weighing 250-300 g received 0.03% methimazole in the drinking water for 3 weeks. T4 (0.8 microgram/100 g body weight, sc) was given 1/2, 1, 3 or 6 h before sacrifice. One group received saline rather than T4 (hypothyroid control). The groups contained 6 to 8 animals each. NB, extracted from tissue by boiling in acetic acid, was measured by radioimmunoassay, using a highly specific antiserum. Pituitary NB content was significantly increased 4-fold, as early as 1/2 h after T4 injection, while serum TSH level was similar to that of the hypothyroid control group. The peak response to T4 was at 3 h after injection, when NB content was increased 8-fold (hypothyroid: 45 +/- 8; 1/2 h, 223 +/- 15; 1 h, 203 +/- 48; 3 h, 383 +/- 31; 6 h, 224 +/- 30 fmol/mg protein) and serum TSH decreased to the level of normal rats (0.93-1.5 ng/ml) generally observed in our laboratory (hypothyroid: 31 +/- 3; 1/2 h, 26 +/- 3; 1 h, 31 +/- 2; 3 h, 1.3 +/- 0.1; 6 h, 3.7 +/- 0.6 ng/ml). These data suggest that NB synthesis is rapidly induced by thyroxine and this might represent a new regulatory path involved in the acute inhibitory effect of thyroid hormones on TSH secretion.

Acute effect of thyroxine on pituitary neuromedin B content of hypothyroid rats and its correlation with TSH secretion

Neuromedin B (NB) is a bombesin-like peptide that has been recently characterized as a physiological paracrine/autocrine inhibitor of thyrotropin (TSH) secretion. We report here the time course of the effect of thyroxine (T4) administration to hypothyroid rats on the anterior pituitary content of NB. Dutch-Miranda male rats weighing 250-300 g received 0.03 percent methimazole in the drinking water for 3 weeks. T4(0.8µg/100 g body weigh, sc) was given 1/2, 1,2 or 6 h before sacrifice. One group recived saline rather than T4 (hypothyroid control). The groups contained 6 to 8 animals each. NB, extracted from tissue by boiling in acetic acid, was measured by radioimmunoassay, using a highly specific anti-serum. Pituitary NB content was significantly increased 4-fold, as ealry 1/2 h after T4 injection, while serum TSH level was similar to that of the hypophyroid control group. The peak response to T4 was at 4 h after injection, when NB content was increased 8-fold (hypothyroid: 45 ñ 8; 1/2h, 223 ñ 15; 1h, 203 ñ 48; 3h, 383 ñ 31; 6h, 224 ñ 30 fmol/mg protein) and serum TSH decreased to the level of normal rats (0.93-1.5 ng/ml) generally observed in our laboratory (jypothyroid: 31 ñ 3; 1/2h, 26 ñ 3; 1h, 31 ñ 2; 3h, 1.3 ñ 0.1; 6h, 3.7 ñ 0.6 ng/ml). These data suggest that NB synthesis is rapidly induced by thyroxine and this might represent a new regulatory pathy involved in the acute inhibitory effect of thyroid hormones on TSH secretion

Paradoxical effect of neuromedin B and thyroxin on thyrotropin secretion from isolated hyperthyroid pituitaries.

Neuromedin B (NB) is a bombesin-like peptide that we recently characterized as a physiological autocrine inhibitor of thyrotropin (TSH) secretion. We now report the effect of NB, thyroxin (T4) and NB + thyroxin on basal and THR (50 nM)-stimulated TSH release from isolated hemipituitaries of hyperthyroid rats. To induce hyperthyroidism, 20 rats were treated with 0.03% methimazole for one month and then received T4, 4 micrograms/100 g body weight, sc, daily for 7 days. Each experimental group consisted of 7 to 9 hemipituitaries. TSH was measured using a rat TSH kit provided by NIDDK. Basal TSH release was paradoxically increased in the presence of 0.1 microM T4 or 0.1 microM NB and even two times higher in the presence of both (Control: 30.0 +/- 4.2 ng/ml; T4: 58.6 +/- 5.6 ng/ml; NB: 53.4 +/- 6.1 ng/ml; T4 + NB: 90.4 +/- 8.5 ng/ml). The percent increment above basal TSH levels after TRH was higher only in the presence of NB (Control: 44.5 +/- 8.2%, NB: 105.3 +/- 18.8%; P < 0.05). Altered responsiveness in hyperthyroidism and direct modification of the intracellular metabolism of T4 are mechanisms that could explain this paradoxical effect.

Paradoxical effect of neuromedin B and thyroxin on thyrotropin secretion from isolated hyperthyroid pituitaries

Neuromedin B (NB) is a bombesin-like peptide that we recently characterized as a physiological autocrine inhibitor of thyrotropin (TSH) secretion. We now report the effect of NB, thyroxin (T4) and NB + thyroxin on basal and THR (50 nM)-stimulated TSH release from isolated hemipituitaries of hyperthyroid rats. To induce hyperthyroidism, 20 rats were treated with 0.03 per cent methimazole for one month and then received T4, 4 micrograms/100 g body weight, sc, daily for 7 days. Each experimental group consisted of 7 to 9 hemipituitaries. TSH was measured using a rat TSH kit provided by NIDDK. Basal TSH release was paradoxically increased in the presence of 0.1 microM T4 or 0.1 microM NB and even two times higher in the presence of both (Control: 30.0 +/- 4.2 ng/ml; T4: 58.6 +/- 5.6 ng/ml; NB: 53.4 +/- 6.1 ng/ml; T4 + NB: 90.4 +/- 8.5 ng/ml). The percent increment above basal TSH levels after TRH was higher only in the presence of NB (Control: 44.5 +/- 8.2 per cent , NB: 105.3 +/- 18.8 per cent ; P < 0.05). Altered responsiveness in hyperthyroidism and direct modification of the intracellular metabolism of T4 are mechanisms that could explain this paradoxical effect