Effect of thyrotropin-releasing hormone and dopamine on the in vitro secretion of prolactin by the bullfrog pituitary gland

Pituitary glands of bullfrogs (Rana catesbeiana) were incubated in medium containing thyrotropin-releasing hormone (TRH) and/or dopamine in order to see their effects on the release and synthesis of prolactin, which was measured by a homologous radioimmunoassay. Prolactin synthesis was measured by monitoring the incorporation of [3H]leucine into prolactin. TRH (0.1-10 ng/ml) stimulated the release of immunoassayable prolactin and newly synthesized [3H]prolactin into the medium in a dose-dependent manner; however, it was ineffective in increasing total prolactin (medium plus pituitary) and the incorporation of [3H]leucine into the total prolactin during the experimental period (20 hr). The TRH-induced elevation of prolactin release was suppressed by the addition of dopamine (5 X 10(-7) M) to the medium.     

Unchanged thyrotropin and prolactin responses to thyrotropin releasing hormone after indomethacin treatment.

Experimental effects of prostaglandin synthetase inhibitors have been considered in the literature as a clue to the possible interactions of prostaglandins with the hypothalamic releasing hormones at the pituitary level. Some results of the administration to man of these drugs are apparently in contrast with the in vivo and in vitro animal data. The present investigation deals with the comparison between the thyrotropin releasing hormone (TRH) effect on prolactin and thyrotropin when the hormone was administered intravenously at doses of 50, 100 and 200 microgram respectively to three groups of six men (aged 22 to 30 years), before and on the sixth day of indomethacin administration (50 mg orally at 6-hour intervals). No significant change in the releasing hormone effect was observed either in the case of prolactin, where TRH caused a consistently similar release of the hormone at every dose employed, or in the case of thyrotropin, where a dose-dependent releasing effect was obtained before and after indomethacin treatment.     

Estradiol-17 beta stimulates basal and thyrotropin releasing hormone induced prolactin secretion by bovine pituitary cells in primary culture.

A series of experiments were conducted which demonstrate that estradiol-17β directly affects bovine pituitary cells in primary culture causing an increase in basal and thyrotropin releasing hormone (TRH)-induced prolactin secretion. Prolactin release by pituitary cells incubated with TRH at concentrations of 0.001, 0.01, 0.1 and 1 ng/ml increased linearly with increasing log concentrations. Exposure of pituitary cells to 5, 50 or 500 ng/ml estradiol for 4 h did not affect basal or TRH-induced prolactin release. However, when the period of exposure to estradiol was prolonged to 6, 12, or 24 h, 0.5, 5 or 50 ng estradiol/ml medium caused pituitary cells to release more prolactin and there was more total prolactin in the system (medium +cell content) than for comparable controls. These increases were linearly related to increasing log concentrations of estradiol used. To determine the chronic effect of estradiol on prolactin secretion, pituitary cells were incubated with estradiol-17β for 11 days during which medium was collected at 24 h intervals beginning on day 3. On day 3, prolactin accumulation in medium of control cultures averaged 2.5 ng/ml, and decreased gradually reaching relatively low levels by day 11 (100 ng/ml). Although prolactin secretion decreased during the culture period, stimulatory effects of estradiol were evident throughout. In addition, these cells still released prolactin in response to TRH (1 ng/ml) on day 11 and magnitude of TRH-induced prolactin release increased with increasing concentrations of estradiol-17β. We conclude that estradiol will increase basal and TRH-induced prolactin release by bovine lactotrophs. These results are consistent with the view that the increase in estradiol that occurs at the end of pregnancy in cattle, may participate in the prolactin surge that occurs at parturition in this species.     

Enhancement by proopiomelanocortin-derived peptides of growth hormone and prolactin secretion by bullfrog pituitary cells.

Corticotrophs in the bullfrog (Rana catesbeiana) are situated mainly in the rostral region of the anterior lobe of the pituitary gland, which receives its blood supply primarily from the portal vessel. On the assumption that the proopiomelanocortin (POMC)-derived peptides released into the pituitary circulation may influence the function of other pituitary cells situated downstream, the effects of three POMC-derived peptides, namely, N-terminal peptide of POMC (NPP), adrenocorticotropic hormone (ACTH), and joining peptide (JP), on the secretion of growth hormone (GH) and prolactin (PRL) by bullfrog dispersed anterior pituitary cells were examined. NPP and ACTH, but not JP, stimulated the release of GH and PRL in a concentration-dependent manner. It was also found that ACTH1-17, but not alpha-melanocyte-stimulating hormone, was effective in enhancing GH and PRL release. A marked difference between the response to NPP and ACTH and the response to thyrotropin-releasing hormone employed as a reference secretagogue in terms of the time required for stimulating the release of GH and PRL was noted. Northern blot analysis of GH and PRL mRNA levels and radioimmunoassay for GH and PRL in the cultured cells revealed that ACTH increases the syntheses of both pituitary hormones as well. The possibility that NPP and ACTH act on neighboring cells to maintain their overall secretory function is discussed.     

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.     

Suppression of prolactin and thyrotropin secretion in the rat by antiserum to thyrotropin-releasing hormone.

Administration of antiserum to synthetic thyrotropin-releasing hormone (TRH) to male and female rats cause a 50% and a 70% suppression in serum levels of prolactin and thyrotropin, respectively, as compared with controls injected with normal rabbit serum. The degree of suppression was similar in diestrous and proestrous female rats and in male rats. These findings support the view that, in addition to its original designation, TRH also has a physiological role in regulating release of pituitary prolactin.     

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.     

Inhibitory effect of prolactin secretion by luteinizing hormone releasing hormone agonist (buserelin)

The LH RH agonist Buserelin was continuously administered through the nose over an extended period to 5 patients with endometriosis at daily doses ranging between 300 micrograms and 1,200 micrograms. The clinical findings and endocrinological dynamics, with emphasis on PRL secretion function, were investigated and the findings below were obtained. The clinical findings showed Buserelin to be effective against endometriosis. The administration of Buserelin markedly inhibited the secretion of LH and FSH under LH-RH loading. The administration of Buserelin markedly inhibited the secretion of PRL and the degree of inhibition appeared to be dependent on the dose and duration of Buserelin administration. No correlation between E2 and PRL was observed during Buserelin administration. The findings suggest that Buserelin inhibits PRL secretion by direct CNS activity, not indirectly through inhibition of E2 production.     

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.     

1,25-Dihydroxyvitamin D3 enhances thyrotropin releasing hormone induced thyrotropin secretion in normal pituitary cells

The findings of specific binding of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] in normal rat pituitary tissue and selective effects of 1,25-(OH)2D3 on gene expression in clonal pituitary tumour cells have suggested that vitamin D may regulate pituitary function. Therefore, the in vitro effect of 1,25-(OH)2D3 on normal pituitary cells was investigated. Primary anterior pituitary cell cultures prepared from female rats were maintained in experimental medium +/- 10(-8) M 1,25-(OH)2D3 for up to 24 h and then incubated with fresh experimental medium containing TRH (10(-10)-10(-8) M) or vehicle for 1 h. Pretreatment with 1,25-(OH)2D3 for 24 h led to increased TSH release at all TRH concentrations tested (P less than 0.0001), a decrease in the half-maximal stimulatory dose of TRH for TSH release from 2 X 10(-9) M to 0.4 X 10(-9) M, a 22% increase in maximal TSH release (P less than 0.01), and an 81% increase in TSH release at 10(-9) M TRH (P less than 0.001). 1 X 10(-9) M 1,25-(OH)2D3 increased TRH (10(-9) M)-induced TSH release by 20% (P less than 0.05) but 10(-7) M and 10(-6) M 25-hydroxyvitamin D3 (25-OH D3) had no effect. The effect of 1,25-(OH)2D3 on TRH (10(-9) M)-induced TSH release was evident within 8 h and was maximal by 16 h. There was no effect on basal TSH release, TSH accumulation in the medium in the preceding 24 h nor on cell-associated TSH. 1,25-(OH)2D3 pretreatment had no effect on TRH-induced PRL secretion, PRL accumulation in the medium nor on cell-associated PRL. We have shown that 1,25-(OH)2D3 acts selectively on the thyrotroph to enhance in vitro responsiveness to physiologically relevant concentrations of TRH. These findings are consistent with the reported autoradiographic localization of [3H]-1,25-(OH)2D3 in the thyrotroph and support a permissive or regulatory role of vitamin D in the normal pituitary gland.     

Regulation of prolactin secretion by dopamine and thyrotropin-releasing hormone in lactating rat adenohypophyses: influence of intracellular age of the hormone

We studied the effect of dopamine (DA) on the rate of processing of PRL after biosynthesis and, with TRH, on the secretion of PRL of different intracellular ages. In these studies anterior pituitary (AP) PRL of lactating rats was pulse labeled either in vivo with [3H]leucine (3 microCi/g BW, injected iv 0.2, 1, 4, 8, 16, or 24 h before removing the AP for incubation) or in vitro with [3H]- or [14C]leucine (5 microCi/ml Krebs-Ringer bicarbonate buffer for 5 min), followed by a chase period of 15-240 min of AP fragments in medium 199. Also, to determine if endogenous DA influenced PRL synthesis, the rate of [14C]-leucine incorporation into PRL was determined in AP fragments from alpha-methyl-p-tyrosine (alpha MpT)-pretreated (200 mg/kg BW) rats. Tissue and medium PRL levels were quantified by polyacrylamide gel electrophoresis densitometric and liquid scintillation techniques. In APs from alpha MpT-treated animals [14C]-leucine incorporation into PRL increased about 30% above control values after the 5-min pulse period, and the release of labeled PRL from alpha MpT-treated APs was about 80% higher than control values after 4 h of incubation. On the other hand, when DA (50 microM) was present in the incubation medium, AP concentrations and release of in vitro synthesized [3H]PRL were significantly decreased (45-55% compared to control values; P less than 0.001). The [3H]PRL concentration, but not that of total, i.e. unlabeled PRL, fell within the AP as the time from in vivo pulse labeling to removal of the pituitary gland increased from 8 to 24 h, thus suggesting that a loss of labeled hormone occurred as it aged within the gland. Also, it was found that biosynthesis and/or processing of PRL were markedly depressed in APs from rats whose pups were removed for 24 h. Under basal conditions, in vitro secretion of [3H]PRLs during the first 30-60 min of incubation consisted primarily of mature [3H]PRL, i.e. those labeled 4 and 8 h previously, whereas newly synthesized (labeled 0.2 and 1 h previously) and old [3H]PRL (labeled 16 and 24 h previously) were secreted at much lower rates. These data confirm previous in vivo and in vitro results on the sequential release of different age PRLs. DA (17 microM) had a significantly greater inhibitory effect on newly synthesized and older stored PRL than on PRL labeled 4-8 h previously. The converse was true with regard to the PRL stimulatory effects of TRH; it provoked greater stimulation of PRL labeled 4-8 h before incubation, thus suggesting an interdependence of the actions of DA or TRH with intracellular age of the hormone.     

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.