Apomorpine inhibits the prolactin but not the TSH response to thyrotropin releasing hormone.

Pretreatment of normal subjects with apomorphine, a dopamine receptor agonist, resulted in significant impairment of the subsequent prolactin (PRL) response to thyrotropin releasing hormone (TRH). The mean maximal increment of PRL was 27.9+/-2.4 ng/ml after TRH alone, and 11.9+/-3.0 ng/ml (P less than 0.001) after apomorphine plus TRH. In contrast, the.thyrotropin (TSH) response to TRH was unaffected by apomorphine (10.5+/-2.9 vs. 9.5+/-1.8 muU/ml, P greater than 0.5). These results demonstrate that dopaminergic effects are capable of inhibiting PRL responses to TRH, probably via a direct effect on the lactotrope cell. They also suggest that dopaminergic influences are not important in the regulation of TSH secretion.     

Basal serum prolactin levels and prolactin responses to constant infusions of thyrotropin releasing hormone in healthy aging men

We measured serum prolactin (PRL) levels by RIA before and during a 240-min constant infusion of TRH (0.4 microgram/min iv) in three similarly sized groups of healthy aging men 30 to 49, 50 to 69, and 70 to 96 years. Basal data were evaluated by analysis of variance with Duncan's multiple range test and regression analysis. Mean basal serum PRL level was elevated (p less than .05) in the oldest group, attributable to PRL elevations (between 20 and 40 ng/ml) in 4 men over 75 years. Serum PRL levels decreased (p less than .001) from -30 min to 0 min before TRH infusion in all groups, but there was no age-dependent difference (p greater than .3) in the magnitude of the reduction. Repeated measures analysis of variance showed increased serum PRL levels (p less than .001) during TRH infusion in all age groups, and an age-dependent increase (p less than .05) in magnitude of peak PRL response. This significant difference was between the two oldest age groups early in the infusion. Chi-square analysis revealed an increased (p less than .05) frequency of early (less than 120 min) peak responses in the oldest age group. The present data suggest that basal and TRH-stimulated PRL secretion may be augmented in some healthy older men.     

Effect of mammalian thyrotropin releasing hormone on prolactin secretion by bullfrog adenohypophyses in vitro.

The effects of the mammalian thyrotropin-releasing hormone (TRH) on the secretion of prolactin by bullfrog adenohypophysis in vitro were investigated in short-term incubation and 24-hr organ culture experiments. Prolactin in the medium or in the incubated tissue was measured by either polyacrylamide disc gel electrophoresis and densitometry or by a homologous radioimmunoassay. The TRH was consistently effective in promoting prolactin release in vitro in concentrations of 10 ng/ml to 10 μg/ml. The tripeptide also caused an increase in the tissue prolactin content over a wide range of concentrations. These results indicate that TRH may function as a prolactin-releasing factor in the bullfrog.     

Effect of gestational mastectomy on postpartum gonadotropin releasing hormone and thyrotropin releasing hormone-induced luteinizing hormone and prolactin response in first lactation Holstein cattle

First lactation Holstein cows were divided into two treatment groups to evaluate thyrotropin releasing hormone (TRH, 0.25 microgram/kg body weight) and gonadotropin releasing hormone (GnRH; 200 micrograms) induced secretion of prolactin (PRL) and luteinizing hormone (LH) on days 7 and 16 postpartum. Disregarding treatment, LH response was greater (p less than 0.01) on day 16 than day 7 postpartum (7.5 +/- 0.3 ng/ml on day 7 vs 10.2 +/- 0.3 ng/ml serum on day 16). Mastectomized cattle had similar time for initiation of LH increase, but peak concentrations were achieved later. Peak PRL concentrations were reached 12 to 15 min after injection and returned to baseline within 2.5 h in both groups. However, intact cows had higher (p less than 0.01) mean serum PRL than the mastectomized cows for 1 h following injection. Peak PRL concentration was 83.3 +/- 17.6 ng/ml for mastectomized cows vs 128.0 +/- 24.7 ng/ml for intact cows. It appears that udder removal allows for greater pituitary responsiveness to GnRH but diminishes PRL response to TRH suggesting the mammary gland differentially affects pituitary secretion of LH and PRL.     

Lack of dissociation of prolactin responses to thyrotropin releasing hormone and metoclopramide in chronic alcoholic men

Prolactin, growth hormone and thyroid stimulating hormone responses to thyrotropin releasing hormone (TRH) and metoclopramide were determined in 12 alcoholic men with biopsy proven liver disease and were compared to those of 8 age matched normal controls. All subjects were challenged with TRH (400 micrograms) and metoclopramide (10 mg) given as an intravenous bolus each on a separate day. Alcoholics had increased basal prolactin and growth hormone levels compared to controls. Alcoholics had a brisk and statistically significant (p less than 0.01) response for each of the 3 hormones studied in response to TRH. In contrast to the alcoholics, the controls did not demonstrate a growth hormone response to TRH. Moreover, the TSH response to TRH of the alcoholics was exaggerated (p less than 0.05) compared to that of the controls. In response to metoclopramide, alcoholics had a brisk prolactin response, failed to demonstrate a TSH response, and had a decline in growth hormone when compared to controls. These results for alcoholics with liver disease differ from those reported for individuals with renal failure while those for the controls are similar to previously reported normal responses. These data suggest that liver disease and renal disease must differ in terms of their patterns of hypothalamic-pituitary neuroregulation as documented by their differing pituitary hormone responses to TRH and metoclopramide.     

Ontogeny of the subcellular compartmentalization of thyrotropin releasing hormone and luteinizing hormone releasing hormone in the rat hypothalamus.

The 900 x g supernatant fluid prepared from hypothalamic homogenates from male and female rats (ranging in age from -1 to 120 days) was fractionated by means of continuous sucrose density gradient centrifugation. Thyrotropin releasing hormone (TRH) and LH releasing hormone (LHRH) in the gradient fractions were quantified by radioimmunoassay. In adult hypothalamic homogenates, TRH and LHRH were associated with two populations of particles distinguishable by their sedimentation properties. Each peptide was in turn distributed in two subpopulations of parties differing in size but similar in density. The distribution of each peptide within its subpopulations of particles was found to be a function of age. In hypothalami of 22-day-old fetuses, TRH was associated almost entirely with the subpopulation of small particles. However, in the neonates, an age-dependent increase in the fractional amount of the TRH confined to the subpopulation of large particles was observed. By the 7th day of age, the peptide was equally distributed in the two subpopulations. The buoyant density of the 1-day-old neonatal particles and that of the adult small and large particles containing TRH was similar. The ontogeny of the subcellular compartmentalization of LHRH differed appreciably from that of TRH. LHRH was barely detectable in hypothalami of 22-day-old neonates. Nevertheless, at this age, the peptide was confined primarily to the subpopulation of large particles, and a similar compartmentalization was noted in hypothalami of 5- and 7-day-old neonates. However, in hypothalami of 14-day-old males and 21-day-old females, association of LHRH with the subpopulation of small particles was evident. It is concluded that 1) the nature of the hypothalamic subcellular compartmentalization of TRH and LHRH is age dependent, 2) the compartmentalization of each peptide in neonatal hypothalami differs from that in the adults, and 3) the development of the mature profile of subcellular compartmentalization of TRH and LHRH proceeds asynchronously.     

Modulation of the prolactin response to thyrotropin releasing hormone by ovarian steroids in ovariectomized turkeys (Meleagris gallopavo)

The effect of thyrotropin releasing hormone (TRH) administered intramuscularly (im) on serum levels of prolactin (Prl) in ovariectomized (ovx) adult turkeys before and following the onset of photostimulation, before and during daily administration (im) of progesterone (P; 0.1, 0.4, or 1.0 mg/kg), estradiol benzoate (EB; 0.01, 0.1, or 0.2 mg/kg), or their combination (1.0 mg/kg EB + 0.1 mg/kg P) were studied. Ovariectomy reduced Prl levels in the serum of photostimulated turkeys, and blunted the Prl response to TRH administration. Progesterone treatment had no effect on basal serum Prl levels but the Prl response to TRH was higher in P-treated turkeys than in non-treated ovx turkeys. Basal serum Prl levels were higher (P less than 0.05) in the EB-treated ovx turkeys than in the untreated birds. The Prl response to TRH in ovx EB-treated turkeys was greatly increased (P less than 0.05). Progesterone treatment of EB-primed ovx turkey did not alter the basal levels of serum Prl or the Prl response to TRH administration. These results suggest that ovarian steroids may be responsible for the increased Prl secretion in the female turkey associated with laying.     

Response to thyrotropin releasing hormone: an objective criterion for the adequacy of thyrotropin suppression therapy.

Most serum thyrotropin (TSH) assays do not adequately discriminate between normal values and absent TSH. We therefore evaluated the TSH response to thyrotropin releasing hormone (TRH) as a criterion for the adequacy of TSH suppression therapy. Twenty-six outpatients with various thyroid disorders (cancer, 10; nodules, 9; miscellaneous, 4; hypothyroidism after 131I therapy for Graves' disease, 3) were studied. Using the frequent sampling technique (samples every 20 min) in two normal volunteers and one untreated patient who was TRH-responsive, we first confirmed the observation that TSH secretion occurred episodically throughout the 24-h period. In contrast, serum TSH was undetectable (less than 0.6 micronU/ml) throughout the 24-h period in 5 patients on TSH suppression therapy who were TRH-unresponsive and one who had a minimal response to TRH. Thus, TRH-unresponsive patients did not secrete measurable amounts of TSH throughout the 24-h period. To suppress TSH secretion, all patients were treated with L-thyroxine (T4) at doses which resulted in undetectable TSH values in random plasma samples. TRH tests were carried out only when random TSH concentrations were less than 0.6 micronU/ml. Seven of the twenty-six patients (27%) including two with thyroid cancer were TRH-responsive indicating a potential for TSH secretion. In these seven, the T4 dose was adjusted until they were TRH-unresponsive. The mean change in T4 dose of these 7 patients was 20+/-10 (SD) microng/day and this resulted in a mean increase of 1.5+/-1.1 microng/dl for T4 and 20+/-20 ng/dl for T3. For all patients, the mean T4 dose required for TSH suppression was 172+/-53 microng/day or 2.6+/-0.8 microng per day per kg body weight. Twenty-three of 26 patients required between 100-200 microng/day and the remaining 3, 250-300 microng/day. The T4 dose required to suppress TSH resulted in normal serum concentrations of T4. 9.1+/-2.0 MICRONG/DL, AND T3, 136.7+/-33.6 NG/DL. These T4 doses did not produce a rapid heart rate, either awake or asleep, arrhythmias, or electrocardiographic abnormalties as assessed by 24-h Holter monitor tracings in 11 patients. Our results thus show that the T4 dose which results in an unresponsive TRH test ensures that serum TSH will remain undetectable (less than 0.6 micronU/ml) throughout the 24-h period. An unresponsive TRH test, therefore, appears to be a very useful and reliable index of TSH suppression.     

Distribution of thyrotropin releasing hormone receptor in rats: an immunohistochemical study

Thyrotropin releasing hormone (TRH) receptor was identified immunohistochemically in the rat tissues using anti-peptide antiserum. Anti-TRH receptor antiserum was raised in New Zealand white rabbits immunized with a conjugate of synthetic TRH-receptor peptide (15-28) to bovine serum albumin. Immunohistochemical analysis was performed by the avidin-biotin complex method. TRH-receptor immunoreactivity was visualized in the central nervous system and anterior pituitary thus supporting previous investigations of TRH receptor distribution using in vitro autoradiographic ligand binding. Significant stain was detected in neural perikarya, axons and dendrites as well as in many cells of the retina, adrenal medulla, stomach mucosa, Auerbach's nervous branch and Mysner's nervous branch of the stomach, small intestine and colon. When using antiserum preincubated with synthetic TRH-receptor peptide (15-28) or rat anterior pituitary homogenate which contains TRH-receptor peptide, no significant stain of the anterior pituitary cells or neurons in the hypothalamus was detected. These findings suggest that TRH-receptor is widely distributed and that this method is valuable in studying the distribution of TRH-receptor in rats.     

Hypothalamic secretion of thyrotropin releasing hormone declines in aging rats

Young and aged male rats were used in experiments to investigate a possible decline in hypothalamic secretion of thyrotropin releasing hormone (TRH) to the anterior pituitary of aging mammals. We observed a 66% decrease in basal TRH release by incubated rat hypothalami with aging. Thyroid hormone-responsive hepatic alpha-glycerophosphate dehydrogenase (GPD) and malic enzyme (ME) levels in aged rats did not differ from 5-month-old controls in spite of a significant fall in serum thyroxine (T4) levels with aging. Other results suggest that these particular indicators of thyroidal status should not change in the aging rat because serum T3 is maintained in the normal range. Serum thyrotropin (TSH) levels, which normally rise when serum T4 levels decline, did not change with aging. These data suggest that gradual loss of the essential TRH stimulation of TSH release with aging may be compensated for by a decline in T4 inhibition of TSH release at the pituitary.     

Annexin 5 inhibits thyrotropin releasing hormone (TRH) stimulated prolactin release in the primary culture of rat anterior pituitary cells

Annexin 5, a novel calcium-phospholipid binding protein, is thought to be involved in hormone secretion by the anterior pituitary gland. Gonadotropin releasing hormone stimulates annexin 5 synthesis, which, in turn, enhances gonadotoropin secretion. On the other hand, annexin 5 was shown to inhibit prolactin release in vitro. To understand the nature of the opposing effects of annexin 5 on these two major pituitary hormones, the present study examines the inhibitory effect of annexin 5 on prolactin release in relation to thyrotropin stimulating hormone (TRH) using primary cultures of anterior pituitary cells of adult female rats. While recombinant rat annexin 5 was found to have little effect on basal prolactin release, it significantly inhibited TRH-stimulated prolactin release. Addition of specific anti-annexin 5 serum to the culture increased basal prolactin release in a concentration dependent manner, and no further increase in prolactin release was observed following application of TRH in the presence of anti-annexin 5. The enhanced basal prolactin release induced by anti-annexin 5 was reversed by the simultaneous administration of indomethacin, an inhibitor of cyclooxygenase. These results demonstrate that endogenous pituitary annexin 5 exerts an inhibitory effect on prolactin release and suggest that this is attained by suppression of eicosanoid synthesis in vitro.     

Changes of pituitary thyrotropin releasing hormone (TRH) receptor level and prolactin response to TRH during the rat estrous cycle.

The plasma PRL and TSH responses to TRH injected iv at different stages of the estrous cycle in normal rats under Surital anesthesia were maximal during the afternoon of proestrus and morning of estrus and lowest on diestrus I. As calculated from the areas under the plasma response curves, a 10-fold difference was found between the maximal and minimal PRL responses while a 2-fold difference was measured for TSH. The plasma PRL and TSH responses to TRH showed a correlation with the binding of [3H]TRH to anterior pituitary gland, a 3-fold difference being observed between the minimal binding measured on the morning of diestrus II and the maximal value found on the evening of proestrus. Contrary to findings with LHRH and LH, repeated injections of a small dose (10 ng) of TRH in the afternoon of proestrus abolished PRL and TSH responses to subsequent injection of the neurohormone.     

Pituitary-thyroid hormone economy in healthy aging men: basal indices of thyroid function and thyrotropin responses to constant infusions of thyrotropin releasing hormone

In order to assess the effects of aging, as distinct from those of thyroid disease or extrathyroidal illness, on certain indices of thyroid function, we studied 74 healthy, ambulatory men recruited from the Baltimore Longitudinal Study on Aging. We determined basal serum values of T4, T3, rT3, thyroxine-binding globulin (TBG), and T3 resin uptake (T3RU) and calculated the free T4 index (FT4I = T4 X T3RU/100), free T3 index (FT3I = T3 X T3RU/100), and T4/TBG ratio for each subject. We used an ultrasensitive RIA to measure variations in basal concentrations of TSH within the normal range. We then infused TRH at a constant rate (0.4 microgram/min iv) for 240 min into 63 of the same men; serum samples, collected at 15-min intervals during the infusion, were analyzed for TSH by routine RIA. Subjects were divided into 3 groups according to age; A (n = 26, mean age = 39.4), B (n = 23, mean age = 60.0), and C (n = 25, mean age = 79.6). Analysis of variance with Duncan's multiple range test and regression analysis were used to evaluate data. There was no significant (P greater than 0.05) variation with age of basal serum values of T4, TBG, or T3RU. Comparison of groups A and C showed significant decreases of mean values of serum T3 (-11%, P less than 0.05), FT3I (-13%, P = 0.02), FT4I (-11%, P less than 0.01), and T4/TBG ratio (-12%, P less than 0.01) and an increase in serum TSH (+38%, P less than 0.05). For these variables, the mean values for group B were intermediate between, but not significantly different from, those of A and C. Regression analysis showed significant correlations of age with T3, FT3I, FT4I, T4/TBG, and TSH at P levels similar to those obtained by Duncan's test. No elderly individual exhibited a baseline elevation of TSH (greater than 7 microU/ml) or depression of T4 (less than 5 micrograms/dl), suggesting that primary hypothyroidism was not present in our old group. The basal TSH concentration did not correlate significantly with any index of thyroid function except with FT3I in group C (r = -0.43, P less than 0.05). In all age groups the TSH responses to TRH exhibited a biphasic pattern with early and late peaks.(ABSTRACT TRUNCATED AT 400 WORDS)