Induction of thyroxine and triiodothyronine release by thyrotrophin-releasing hormone in the hen

Effect of genetic selection on growth parameters and tonic immobility in Leghorn pullets

Four genetic stocks of Leghorn pullets were used to evaluate the effects of genetic selection on growth and fearfulness behavior. Three of the stocks were the Ottawa randombred control stocks from 1950 (CS5), 1959 (CS7), and 1972 (CS10). The fourth stock was a 1993 commercial laying stock (CCS) whose ancestors were involved in the formation of the randombred control stocks. Pullets were reared in a brood and grow poultry house with flat deck cages. Each stock was comprised of 840 birds with 21 replicates per strain. Body weight and feed consumption were monitored biweekly. At 16 wk of age, a 20-hen sample from each strain was analyzed for BW, body composition, and tonic immobility. There were significant (P < 0.05) differences among the stocks for BW of 1,403; 1,333; 1,332; and 1,428 g for the CS5, CS7, CS10, and CCS stocks, respectively. Furthermore, significant differences occurred with regard to feed consumption, livability, and frame size. There were no differences among the stocks in tonic immobility. Measurement of circulating corticosterone levels were shown to be significantly (P < 0.05) higher in the CCS stock (7.64 ng/mL) than for both the CS5 (4.50 ng/mL) and CS7 (4.61 ng/mL) stocks, whereas the CS10 stock was intermediate with 6.45 ng/mL. Genetic selection has affected growth parameters, although there appears to be no change in fearfulness behavior but an increase in corticosterone levels in stocks from later years.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Birds and Its Role in Bird Development and Reproduction

This article reviews thyroid function and its hypothalamic-pituitary-thyroid (HPT) axis control in birds with emphasis on the similarities and differences in thyroid function compared to mammals and other vertebrate classes. Thyroid hormones are important in metabolism and the thermogenesis required for homeothermy in birds, as in mammals, the other homeothermic class of vertebrates. Thyroid hormones play important roles in development and growth in birds, as is the case for all vertebrate classes. The developmental effects of thyroid hormones in birds are presented in the context of differences in precocial and altricial patterns of development and growth with emphasis on oviparous development. The sections on thyroid hormone actions include discussion of effects on the development of a number of tissue types as well as on seasonal organismal processes and interactions of the thyroid axis with reproduction. The current picture of how environmental chemicals may disrupt avian thyroid function is relatively limited and is presented in the context of the assessment endpoints that have been used to date. These endpoints are categorized as thyroid and HPT axis endpoints versus target organ endpoints. The final section discusses two recommended assay protocols, the avian two-generation toxicity assay and the avian one-generation assay, and whether these protocols can evaluate thyroid disruption in birds.

Thyroid hormone deiodination in birds

Because the avian thyroid gland secretes almost exclusively thyroxine (T4), the availability of receptor-active 3,3',5-triiodothyronine (T3) has to be regulated in the extrathyroidal tissues, essentially by deiodination. Like mammals and most other vertebrates, birds possess three types of iodothyronine deiodinases (D1, D2, and D3) that closely resemble their mammalian counterparts, as shown by biochemical characterization studies in several avian species and by cDNA cloning of the three enzymes in chicken. The tissue distribution of these deiodinases has been studied in detail in chicken at the level of activity and mRNA expression. More recently specific antibodies were used to study cellular localization at the protein level. The abundance and distribution of the different deiodinases shows substantial variation during embryonic development and postnatal life. Deiodination in birds is subject to regulation by hormones from several endocrine axes, including thyroid hormones, growth hormone and glucocorticoids. In addition, deiodination is also influenced by external parameters, such as nutrition, temperature, light and also a number of environmental pollutants. The balance between the outer and inner ring deiodination resulting from the impact of all these factors ultimately controls T3 availability.

Pituitary and extrapituitary action sites of the novel nonpeptidyl growth hormone (GH) secretagogue L-692,429 in the chicken

Chickens were used as a model to further analyze the efficacy and specificity of L-692,429, a novel nonpeptidyl mimic of growth hormone (GH)-releasing peptide-6 (GHRP-6), which is a specific GH-releasing secretagogue in mammals. Actions at the level of the pituitary and the hypothalamus were studied. Pituitaries isolated from 1-day-old (C1) chicks responded in a dose-dependent manner to L-692,429 (ED50 = 10 nM). Using equimolar concentrations of thyrotropin-releasing hormone (TRH), human GH-releasing hormone (hGHRH1-29), and L-692,429 (10 nM), L-692,429 had 20-25% the in vitro potency of the two endogenous releasing factors. There was an additive effect between hGHRH1-29 (10 nM) and L-692,429 (10 or 100 nM) on GH release from C1 pituitaries but no such additive effect was observed when pituitaries were exposed to both TRH (10 nM) and L-692,429 (100 nM). An acute challenge with 50 microg L-692,429 resulted in increased plasma GH levels within 5 min, which remained elevated for up to 15 min (C1 chickens). This increase in GH was accompanied by a drop in hypothalamic TRH content by 5 min. Hypothalamic somatostatin (SRIH) content did not change. Plasma corticosterone concentrations were increased following L-692,429 treatment, whereas plasma alpha-subunit, T4, and T3 levels were unchanged. To confirm the role of the decreased hypothalamic TRH concentrations in the GH-releasing activity of L-692,429 in the chicken, chickens (C1) were pretreated with normal rabbit serum (NRS) or a TRH antiserum (1/50) 1 h prior to the L-692,429 challenge. Both groups showed an increase in circulating GH but the increase was within 5 min inhibited by the TRH antiserum pretreatment, whereas no differences were noted in plasma corticosterone levels. It is concluded that in the chicken the GH secretagogue L-692,429 has a dual action site: (1) directly at the level of the pituitary and (2) centrally through an increase in hypothalamic TRH release.

Serum pattern of thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) in fed and fasted cocks following TRH stimulation

Food deprivation for 27 and 75h in cocks decreased serum levels of T3 maximally by 61.1% and increased the level of rT3 by 51.4% and that of T4 by 22.4%. Injection of a single dose of TRH (10 micrograms/kg b.w.) increased serum levels of all 3 iodothyronines in both fed and food-deprived animals. In fasted, TRH-treated birds the peak of serum rT3 was 80.5 and 73.3% above control levels in 27- and 75-hr food-deprived cocks, respectively (fed birds: 14.6 and 9.7%); the relevant data for T3 were: 78.3 and 40.4% (fed birds: 95.7 and 127.5%); for T4: 30.0 and 27.0% (fed birds: 9.2 and 6.1%). The greater relative increment of serum rT3 than T3 in food-deprived and TRH-treated cocks was supported by a fall of the serum T3/rT3 ratio to 78.7 and 27.9% in fed cocks and 27- and 75-hr fasted animals, respectively. We conclude, that food deprivation modifies the response of the hypophysis-thyroid axis to exogenous TRH in the sense that during fasting the production of rT3 is enhanced relative to T3.

Evidence for the role of thyroxine as a hormone in the physiology of a lizard

In vivo effects of thyroxine (T4) and triiodothyronine (T3) were studied in male lizards. T4 or T3 (0.5 to 2 nmol) was administered per day over 10 days to surgically thyroidectomized male lizards (35 +/- 2.0 g) and the responses were measured in terms of scale shedding, whole body oxygen consumption (OC), and testicular weight. T3 was more effective in stimulating OC as compared to T4. T4 accelerated scale shedding to a greater extent as compared to T3. T4 was more effective than T3 in restoring the decline in the gonadal weight of thyroidectomized animals. Effect of inhibition of peripheral conversion of T4----T3 by iopanoic acid (IOP) was studied on the above response parameters. IOP at both dose levels inhibited extrathyroidal conversion of T4----T3. The T4-stimulated increase in OC of IOP-treated animals was suppressed, clearly indicating that the T4 effect could be attributed to its conversion to T3. But in these same animals, IOP failed to inhibit T4-stimulated scale shedding and gonadal weight. As the proportion of T4 converted to T3 decreased as a result of IOP treatment, the effectiveness of T4 increased in terms of scale shedding and restoration of gonadal weight. From these studies it appears that all effects of T4 in lizards need not necessarily be mediated via conversion to T3.

Thyroid regulation of body temperature in anaesthetized chickens

1. Anaesthesia caused marked decreases in the plasma concentrations of triiodothyronine (T3) and thyroxine (T4) and in the body temperature of young fowl. 2. Exogenous T4 or a thyroid hormone secretagogue (somatostatin antiserum), increased endogenous T3 and T4 concentrations and body temperature in conscious birds and prevented the body temperature decline in anaesthetized fowl. 3. These results provide further evidence for a role of T3 and T4 in temperature regulation in birds, particularly during anaesthesia.

Thyroid function in sex-linked and autosomal dwarf chickens

Thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) elevated plasma thyroxine (T4) and triiodothyronine (T3) concentrations in a (euthyroid) control line of chickens and in an autosomal dwarf strain. These agents were ineffective in sex-linked dwarf (SLD) chickens. Similarly, while somatostatin (SRIF) lowered plasma T4 and T3 concentrations in autosomal dwarf (ADW) chickens and controls, it had no inhibitory effect in the SLD strain. These results suggest that an impairment in the hypothalamus-pituitary-thyroid axis is at least partly responsible for hypothyroidism in the SLD strain.

The effect of severe starvation and captivity stress on plasma thyroxine and triiodothyronine concentrations in an antarctic bird (emperor penguin)

The effect of confinement and severe starvation on the plasma thyroxine (T4) and triiodothyronine (T3) concentrations was determined in emperor penguins (Aptenodytes forsteri). During their annual cycle, emperor penguins fast freely for periods of up to 4 months and may thus represent a unique subject to study endocrine adaptations to fasting. Plasma T4 concentrations progressively decreased following capture and confinement of naturally fasting penguins, and within 15-20 days stabilized at levels three times lower than in free-living penguins. A transient fourfold increase in plasma T3 concentration developed within the day following confinement in parallel with a rise in daily body mass loss. Both plasma T3 concentration and mass loss subsided to normal levels within 15 days. The decrease in plasma T4 concentration is in accordance with the well-known inhibitory effect of stress on thyroid function in birds and mammals, whereas the transient increase in plasma T3 concentration seems related to enhancement of energy expenditure as a consequence of restlessness. Starvation severe enough to exhaust fat stores and to activate protein catabolism induced a 6- and 5 to 10-fold fall in plasma T4 and T3, respectively. This is in marked contrast with maintenance of plasma thyroid levels during long-term natural fasting associated with protein sparing (R. Groscolas and J. Leloup (1986) Gen. Comp. Endocrinol. 63, 264-274). Surprisingly, there was a final reincrease in plasma T4 concentration in very lean penguins. These results suggest that the effect of starvation on plasma thyroid hormones seems to depend on how much protein catabolism is activated and demonstrate the acute sensitivity of thyroid hormone balance to stress in penguins.

Effect of fatty acid administration on plasma thyroid hormones in the domestic fowl

In birds a severe stress is associated with a reduction in concentrations of plasma thyroxine. Studies in man and the rat have demonstrated that severe illness is associated with an increase in serum concentrations of free fatty acids, notably oleic acid, and that they are associated with a reduction in concentrations of serum thyroxine (T4) and/or triiodothyronine (T3). Since stress is associated with increased fatty acids in birds, we have, in the present study, examined the role of oleic acid and another polyunsaturated fatty acid, arachidonic acid, on thyroid function tests (plasma thyroxine, triiodothyronine, and T3 resin uptake (RT3U) index) and on the thyroidal response to exogenous thyroid-stimulating hormone (TSH) in the domestic fowl. In the first study we observed that the iv administration of arachidonic (10 mg/kg) or oleic acid (15 mg/kg) to groups of 10-week-old cockerels (six per group) was associated with a significant reduction in concentrations of plasma T4, whereas there was little change in saline-injected controls. However, administration of fatty acids to chickens was not associated with a significant change in RT3U index or in the levels of plasma T3. In the second study, groups of animals (n = 6) were injected with bovine TSH (0.5 IU/kg, im) or saline 2.5 hr after the fatty acid injection and blood samples were obtained at -2.5 to 24 hr after the TSH injection. A similar progressive increase in serum T4 was observed for the three groups studied whereas there was little change in the concentrations of plasma T3.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between insulin-like growth factor-1, growth hormone, thyroid hormones and insulin in chickens selected for growth

The concentrations of circulating insulin-like growth factor I, growth hormone, insulin and thyroid hormones were measured in broilers selected for an increase in growth, broilers in which selection pressure was relaxed and in White Leghorns. Growth hormone levels increased in all lines between 3 and 4 weeks of age followed by a decline to adult levels. The lines with the slowest rate of growth had the highest growth hormone concentrations. Insulin-like growth factor I concentrations increased significantly in all three lines of birds during the 10 weeks of study and was significantly correlated with the increase in body weight. There were no consistent differences in plasma IGF-1 levels between the lines. Thyroxine levels increased consistently throughout the study but the levels of triiodothyronine decreased between 5 and 6 weeks of age in all lines. There were no consistent changes in plasma insulin levels. The highest rate of growth in these animals is accompanied by an increase in growth hormone concentration followed by an increase in plasma IGF-1. However, despite differences in plasma growth hormone, plasma concentrations of IGF-1 are not different between lines and are not related to between line differences in growth rate.

Thyrotrophin-releasing hormone induces growth hormone secretion in adult hypothyroid fowl

While thyrotrophin-releasing hormone (TRH) stimulated growth hormone (GH) secretion in adult anesthetized cockerels, the GH response was blocked in anesthetized birds pretreated with thyroxine (T4) or triiodothyronine (T3). Moreover, whereas GH secretion in conscious adult birds was poorly responsive to TRH stimulation, conscious birds made hypothyroid by goitrogen pretreatment (with propylthiouracil, methimazole, or thiourea) were responsive to TRH challenge. Basal circulating GH concentrations in the goitrogen-pretreated birds were also higher than in the vehicle-injected controls. Surgical thyroidectomy similarly increased the basal GH concentration in adult birds and promoted TRH-induced GH secretion. These results demonstrate inhibitory effects of the thyroid hormones on basal and stimulated GH secretion in adult domestic fowl and suggest that GH release in adults is partly under tonic thyroidal inhibition.

Thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of the domestic mallard

The distribution of immunoreactive thyrotropin-releasing hormone (TRH) in the central nervous system of the domestic mallard was studied by means of the peroxidase-antiperoxidase technique. After colchicine pretreatment, the highest number of TRH-immunoreactive perikarya was found in the parvocellular subdivision of the paraventricular nucleus and in the preoptic region; a smaller number of immunostained perikarya was observed in the lateral hypothalamic area and in the posterior medical hypothalamic nucleus. TRH-immunoreactive nerve fibers were detected throughout the hypothalamus, forming a dense network in the periventricular area, paraventricular nucleus, preoptic-suprachiasmatic region, and baso-lateral hypothalamic area. TRH-containing nerve fibers and terminals occurred in the organon vasculosum of the lamina terminalis and in the external zone of the median eminence in juxtaposition with hypophyseal protal vessels. Scattered fibers were also seen in the internal zone of the median eminence and in the rostral portion of the neural lobe. Numerous TRH-immunoreactive fibers were detected in extrahypothalamic brain regions: the highest number of immunoreactive nerve fibers was found in the lateral septum, nucleus accumbens, olfactory tubercle, and parolfactory lobe. Moderate numbers of fibers were located in the basal forebrain, dorsomedial thalamic nuclei, hippocampus, interpeduncular nucleus, and the central gray of the mesencephalon. The present findings suggest that TRH may be involved in hypophysiotropic regulatory mechanisms and, in addition, may also act as neuromodulator or neurotransmitter in other regions of the avian brain.

The influence of methimazole on the thyrotrophic and peripheral activity of thyrotrophin and thyrotrophin-releasing hormone in the chick embryo and growing chicken

Plasma concentrations of thyroxine (T4) and triiodothyronine (T3) were profoundly depressed both in chick embryos and growing chickens after methimazole (MMI) treatment. There was no response of T4 and T3 levels to TRH or TSH injections in the MMI group, either in embryos or growing chickens. Peroxidase activity measured in the thyroid gland was significantly higher in embryos and growing chickens treated with MMI. However, neither TRH nor TSH affected this activity 2 hr after injection in either control or the MMI-treated group. Hepatic 5'-monodeiodinase activity was significantly stimulated in the MMI-treated groups of embryos and growing chickens but only when additional sulphydryl groups (DTT) were provided. In embryos, monodeiodination activity 2 hr after TSH injection was not significantly different from control values for either DTT-stimulated or unstimulated conditions within the control and MMI-infused groups. However, in both control and MMI-treated embryos monodeiodination activity significantly increased 2 hr after TRH injection. In the growing chickens, monodeiodination activity 2 hr after TRH or TSH injection was not significantly different from control values in either stimulated or unstimulated conditions of each group.

Influence of orally administered thyrotropin-releasing hormone on plasma growth hormone, thyroid hormones, growth, feed efficiency, and organ weights of broiler chickens

Thyrotropin-releasing hormone (TRH) was administered continuously or intermittently in the drinking water of male broiler chickens from 2 to 21 days of age. Intermittent administration of TRH was accomplished by giving birds access to the solution for 2 hr, then removing it for 2 hr, with six repetitions of this procedure each 24 hr. The TRH concentration was such that the birds each would ingest 10 to 20 micrograms of the material during each 2-hr period that it was available. Plasma growth hormone (GH) levels were elevated within 30 min after an episode of TRH administration at 2, 7, and 14 days of age but not at 21 days. Continuous administration of TRH had no effect on GH in plasma at any age. Thyroxine concentrations in plasma were increased within 30 min of first exposure to TRH at 2 days of age, but they were unaffected by intermittent episodes of TRH at 7, 14, and 21 days of age. Triiodothyronine concentrations in plasma were unaffected by TRH at any age. Even though intermittent TRH administration elicited significant elevation in plasma GH for at least 14 days of the experiment, it had no effect on body weight, feed consumption, or weight of the gastrocnemius muscle, the pectoralis major muscle, liver, tibia, or abdominal fat pad.

Opiate participation in thyroid hormone regulation in domestic fowl

Administration of morphine sulfate (at 5 and 50 mg/kg) to immature domestic fowl lowered the circulating thyroxine (T4) concentration in a dose- and time-dependent manner. The inhibitory effect of morphine on T4 secretion was mediated via opiate receptors, since it was completely blocked in the presence of naloxone (50 mg/kg). The possibility that T4 secretion may be under tonic opioid inhibition was suggested by a stimulatory effect of naloxone on plasma T4 concentrations. Morphine sulfate (50 mg/kg) enhanced plasma triiodothyronine concentrations, an effect that was completely suppressed in the presence of naloxone. These results demonstrate differential effects of opiates on T4 and T3 concentrations and suggest that opiates tonically inhibit T4 release and accelerate its rate of peripheral monodeiodination.

Calcium participation in thyroid function in fowl (Gallus domesticus)

The effects of the calcium antagonists ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraace tic acid (EGTA), cobalt chloride (CoCl2), and magnesium chloride (MgCl2) on the concentrations of plasma thyroxine (T4) and triiodothyronine (T3) and on the basal and stimulated release of T4 from incubated thyroid glands have been determined in the domestic fowl. Plasma T4 levels were consistently reduced 2 hr after the administration of each calcium antagonist, although only EGTA and CoCl2 lowered the concentration of plasma total calcium. Concentrations of plasma T3 were increased following MgCl2 treatment but reduced after CoCl2 administration. The basal release of T4 by incubated thyroid glands was reduced following in vivo EGTA and CoCl2 treatment, but increased after MgCl2 injection. The addition of bovine thyroid stimulating hormone (TSH, 200 mU) to the incubation consistently stimulated in vitro T4 release, although the magnitude of the stimulation was increased following in vivo EGTA or CoCl2 treatment and reduced after MgCl2 administration. These results demonstrate the involvement of calcium dependent mechanisms in the control of T4 release in fowl.

Somatostatin inhibits thyroid function in fowl

Plasma concentrations of thyroxine (T4) and triiodothyronine (T3) were marginally (less than 25%) lowered 10 and 60 min, respectively, following somatostatin (SRIF) administration (at doses of 10-30 micrograms/kg) in immature domestic fowl. When plasma iodothyronine levels were elevated by the administration of anti-SRIF (1.0 ml/kg) 10 min prior to SRIF challenge, the inhibitory effect of SRIF on T4 and T3 concentrations was greatly augmented. The relative reduction in the T3 concentration was greater than the decline in the T4 level, resulting in a decline in the T3:T4 ratio. These data indicate that SRIF inhibits thyroid function in fowl, possibly by direct effects on the thyroid gland and by effects on the peripheral conversion of T4 to T3.

Plasma prolactin, thyroxine, triiodothyronine, testosterone, and luteinizing hormone during a photoinduced reproductive cycle in mallard drakes

The temporal relationships between plasma concentrations of prolactin, thyroxine (T4) and triiodothyronine (T3) were determined in a group of six wild mallard drakes during the development and maintenance of long-day refractoriness after transfer from 6 h light: 18 h darkness (6L:18D) to 20L:4D for 24 weeks. As shown by changes in the plasma concentrations of luteinizing hormone (LH) and testosterone, the birds came into breeding condition and then became long-day refractory within 5 weeks of photostimulation. Long-day refractoriness was maintained for the remainder of the study. Plasma prolactin began to increase immediately after photostimulation, although not as fast as the increases in plasma LH and testosterone. The concentration of plasma T4 also increased after photostimulation but, as shown by decreased plasma LH and testosterone levels, only after the birds had become long-day refractory. The development of long-day refractoriness was thus directly correlated with an increased plasma prolactin and not with a change in plasma concentration of T4. Plasma T3 decreased after photostimulation but returned to prestimulation values as the birds became long-day refractory and remained stable for the remainder of the study. Concentrations of plasma T4 and prolactin returned to baseline values after about 15 weeks photostimulation showing that the long-term maintenance of long-day refractoriness is not directly related to continuously high plasma concentrations of either hormone.

The effects of glucagon and insulin on plasma thyroid hormone levels in fed and fasted domestic fowls

Glucagon injection (50 micrograms kg-1) produced a biphasic response in plasma thyroxine (T4) level in both fed and fasted chickens. An initial inhibition was followed by an increase to levels above control value. Glucagon reduced plasma triidothyronine (T3) possibly as a consequence of inhibition of peripheral monodeiodination. This inhibition persisted in fasted animals despite a glucagon induced hyperglycaemia. Insulin injection (4 IU kg-1) decreased plasma T4 concomitant with a profound hypoglycaemia. These effects were more pronounced in fasted birds. Insulin induced hypoglycaemia was associated with decreased plasma T3 probably as a consequence of reduced thyroidal T4 secretion and reduced peripheral monodeiodination. Glucagon and insulin may play direct or indirect roles in the regulation of thyroid hormone secretion and metabolism in the domestic fowl.

The relative potencies of L-T4 and L-T3 on the pituitary-thyroid axis in the Japanese quail

We have studied indirectly the relative contribution of the two thyroid hormones T4 and T3 to the feedback regulation of thyrotropin (TSH) secretion in quails, by means of the TRH-test. Adult euthyroid quails received a single injection of thyrotropin-releasing hormone (TRH) (1 or 10 micrograms/100 g body wt) and were killed different times after. Peak level of T4 occurred at 40 min and further served as the TRH-test response. The administration of two different doses of T4 or T3 (0.5 or 2 micrograms/100 g body wt) increased plasma T4 or T3 levels by 3- to 12-fold, respectively, according to the dose. The dose of 0.5 micrograms T4 slightly impaired the TRH-test response, while 2 micrograms rapidly blunted this response. Changes in plasma T3 levels had no deep repercussion on the TRH-test response. It is concluded that the pituitary of the quail is more sensitive to changes in plasma T4 levels than to changes in plasma T3.

Growth hormone secretion in anaesthetized fowl. 1. Refractoriness to repeated stimulation by human pancreatic growth hormone-releasing factor (hpGRF) or thyrotrophin releasing hormone (TRH)

The intravenous (iv) administration of thyrotrophin releasing hormone (TRH, at 1.0 or 10.0 micrograms/kg) or human pancreatic growth hormone-releasing factor (hpGRF(1-44)NH2, at 10 micrograms/kg) markedly increased the growth hormone (GH) concentration in the plasma of immature or adult cockerels anaesthetized by sodium pentabarbitone (30 mg/kg, iv). A second injection of either TRH or hpGRF failed to increase the GH concentration in immature chicks when administered 15, 30, or 60 min after the first injection. However, significant GH responses to TRH or hpGRF were observed when the interval between injections was either 120 or 240 min. The magnitude of the GH responses to TRH were, however, diminished by 83.3 and 26.7% when given 120 or 240 min after the initial TRH injection, and the responses to hpGRF were similarly reduced by 68.3 and 33.6%. A similar period of GH refractoriness to TRH or hpGRF stimulation was also observed in adult birds, although the recovery of GH responsiveness occurred earlier. While a second injection of hpGRF was ineffective in increasing the plasma GH concentration if given within 30 min of the first, it was fully effective when the interval between injections was greater than 60 min. In response to a second injection of TRH, the GH concentration was elevated when the interval between injections was greater than 120 min, after which the magnitude of the response evoked was greater than that induced by the initial injection. Growth hormone secretion secretion in immature and adult fowl is therefore refractory to repeated provocative stimuli, although the mechanism involved is unknown.

The effect of a single injection of thyrotrophin on serum concentrations of thyroxine, triiodothyronine, and reverse triiodothyronine in the immature chicken (Gallus domesticus)

Immature domestic fowl were given a single injection of 0.1 or 0.25 IU bovine thyrotrophin (TSH)/kg body wt by the intramuscular route and serum concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), and 3,3',5'-reverse T3 (rT3) were measured during the following 24 hr. The increase in T4 was markedly greater than that of T3. Serum rT3 concentration increased after an injection of 0.25 IU TSH/kg but the maximal concentration (75 pg/ml) was very low.

Hypothalamo-adenohypophyseal-thyroid interrelationships in the developing chick embryo. VII. Immunocytochemical demonstration of thyrotrophin-releasing hormone

Thyrotrophin-releasing hormone (TRH) was demonstrated immunocytochemically in the infundibulum of the chick embryo as early as Day 4.5 of incubation. From Days 4.5 through 19.5 of embryonic development there is a gradual increase within the developing hypothalamus in the number of TRH-positive perikarya as well as the amount of immunoreactive-TRH (IR-TRH) per cell. There are no abrupt changes in either parameter during the critical time period (Days 10.5-13.5 of incubation) in the maturation of the pituitary-thyroid axis. Thus, although TRH is probably not directly responsible for the dramatic increase in the number of thyrotrophin-producing cells which occurs in the pars distalis of 10.5- to 11.5-day-old embryos (R. C. Thommes, J. B. Martens, W. E. Hopkins, J. Caliendo, M. J. Sorrentino, and J. E. Woods (1983). Gen. Comp. Endocrinol. 51, 434-443) the marked change in the activity of the pituitary-thyroid unit at this time may well reflect the response of these newly differentiated thyrothrophs to low levels of plasma TRH. This hypothesis is supported by the observations that between Days 10.5 and 11.5 the hypothalamic-adenohypophyseal-thyroid (HAT) axis is first responsive to cold (R. C. Thommes, J. B. Martens, J. B. Hopkins, D. A. Griesbach, D. J. Williams, M. J. Sorrentino, P. Wernke, and J. E. Woods. In "Proceedings, Ninth International Symposium on Comparative Endocrinology Hong Kong, 7-11 December 1981" (B. Lofts, ed.). Hong Kong Univ. Press, Hong Kong, in press) and also that the pituitary-thyroid unit exhibits a marked increase in its sensitivity to exogenous TRH (R. C. Thommes, D. J. Williams, and J. E. Woods (1984). Gen. Comp. Endocrinol. 55, 275-279).

Hormonal and environmental interactions on thyroid function in the chick embryo and posthatching chicken

Recent research has focused on developmental changes in thyroxine (T4) metabolism as well as hormonal and environmental interactions upon peripheral monodeiodination. Toward the end of incubation and the time of air space membrane perforation, concentrations of 3,3', 5'-triiodothyronine T3 increase more rapidly than T4, while reverse T3 (rT3) decreases. Administration of glucocorticoids or prolactin (hormones known to increase at the end of incubation) 2 to 3 days prior to air chamber perforation can induce this change in T4 metabolism. Attention has been focused on two major environmental factors that influence T4 concentrations in posthatch chicks. T4 disappearance rate and peripheral T4 conversion are adjusted during thermal acclimation. More T3 will be generated in cold-exposed chickens, and more rT3 will be produced at higher ambient temperatures. Peripheral T4 metabolism acts as an independent regulation pathway but accompanies changes in hypothalamo-thyroid activity. Changes in thyroid hormone levels following ambient temperature changes, therefore, are the result of a balance between central and peripheral control mechanisms, depending on age of the animal and degree and duration of the temperature change. In the hen, feeding also plays an important role in the regulation of the absolute levels and daily rhythms of plasma thyroid hormones. Fasting results in an increase in circulating T4 levels, but a decrease in T3 levels, owing to a lower 5'deiodinase activity, and also abolishes the circadian circulating T3 rhythm.

Effect of short exposure to cold on plasma thyroxine in Coturnix quail: role of the infundibular complex and its neural afferents

Exposure of control quail to low ambient temperature (4 degrees) for a short duration (15 min) led to a rapid increase in plasma thyroxine (T4) levels. A peak appeared 40 min after the cold began and was followed by a progressive and slow decline. T4 levels were elevated in birds maintained for up to 3 hr at 4 degrees. Restraint stress could be accompanied by a slight and late decrease in thyroxine concentration, indicating that glucocorticoids could partly inhibit thyroid function. Both cold and restraint stresses elicited sustained adrenocortical activation. No thyroid response to cold appeared after complete or partial neural deafferentation of the hypothalamus, indicating that cold signals were conveyed to the thyrotropic centers from peripheral and/or deep thermoreceptors through neural posterior-lateral afferents to the hypothalamus. No thalamic relay appeared to be necessary since normal thyroidal stimulation was observed after thalamic-hypothalamic disconnection. Such a response persisted in hemispherectomized quail.

Concentrations of triiodothyronine, growth hormone, and luteinizing hormone in the plasma of thyroidectomised fowl (Gallus domesticus)

Surgical thyroidectomy increased (P less than 0.05) the basal concentrations of growth hormone (GH) and luteinizing hormone (LH) in the plasma of 10- to 12-week-old domestic fowl. The administration of thyrotrophin releasing hormone (TRH) (100 micrograms, sc) increased (P less than 0.01) the GH concentration in both intact and thyroidectomised birds. The magnitude of the TRH-induced increase in GH level was greater (P less than 0.01) in thyroidectomised birds than in intact controls. Although TRH had no effect on LH secretion in the controls, it induced a small (P less than 0.05) rise in the plasma LH level in thyroidectomised birds. In both the intact and thyroidectomised birds the LH concentration was enhanced (P less than 0.05) following the administration of LH-releasing hormone (LH-RH) (20 micrograms, sc). The increase in the LH level by LH-RH in the thyroidectomised birds was greater (P less than 0.001) than that in the intact controls. Plasma GH concentrations were unaffected by LH-RH treatment. These results suggest that thyroid hormones inhibit the secretion of LH and GH in birds. In thyroidectomised birds low levels of immunoreactive triiodothyronine (T3)-like material were measurable in the circulation, despite the absence of regenerated thyroid tissue. The administration of TRH (100 micrograms, sc) did not enhance the plasma level of this material in thyroidectomised birds, whereas plasma T3 concentrations were enhanced in intact birds following TRH treatment. These results suggest that the T3 immunoreactive substance in thyroidectomised birds is extrathyroidal in origin.

Comparison of plasma thyroxine levels following short exposure to cold and TRH administration in intact and pituitary autografted quail (Coturnix coturnix japonica)

The thyrotropic functional abilities of ectopically transplanted anterior pituitaries were tested by subjecting quail bearing their adenohypophysis in juxtarenal position either to a short cold exposure or to an intravenous injection of TRH. Thyroxine was determined in plasma samples collected from 20 to 120 min after treatment. Intact birds exhibited increasing T4 levels up to a peak at 40 min, then decreasing slowly within 2 hr after either cold or TRH stimulation. Autografted birds exhibited significant although lower and delayed increase of plasma thyroxine following both stimuli.

Effect of blocking T4-monodeiodination on hatching in chickens

Chick embryos were injected with iopanoic acid (IOP) on either day 17 or 18 of incubation, and radioimmunoassays of triiodothyronine (T3) and thyroxine (T4) in serum and thyroid glands were performed from day 19 on through pipping and hatching and 1 day after hatching. The IOP was able to block T4 to T3 conversion in chick embryos from day 19 of incubation. Blocking T4 conversion did not delay hatching significantly, nor did it affect embryonic mortality significantly. Yolk sac retraction was not affected at hatching. A rise in serum reverse T3 (rT3) and T4 was observed after IOP administration. The rise in T4 could not be explained solely by a decreased T4 conversion. The results indicated that peripheral monodeiodination occurs in the late chick embryo.

In vivo thyroxine release in day-old cockerels in response to acute stimulation by mammalian and avian pituitary hormones

Thyroxine (T4) level in the blood of newly hatched cockerels were measured at different times after the injection of bovine thyrotropin. A linear response to increasing doses of mammalian thyrotropin was seen when a two-injection protocol was used with a fifteen-hour interval between injections. Blood T4 levels peaked 5 hr after the second injection and declined thereafter. This response was shown to be highly specific for thyrotropin from both mammalian and avian sources; gonadotropins, prolactin, and growth hormone had negligible activity, although the last two hormones were able to synergise with thyrotropin under certain circumstances to augment the response. Experiments conducted at different times of day indicated that diurnal fluctuations in the response of the thyroid to exogenous thyrotropins may exist. As a result, animals were injected and bled at the same time of day in all subsequent experiments. Under these circumstances, in vivo thyroxine release in cockerels appears to be precise, simple, and sensitive bioassay for thyrotropin. This bioassay can be used to demonstrate that thyrotropin purified from ostrich pituitaries is distinct from gonadotropin and active in an avian species.