EFFECTS OF OESTROGENS ON PROLACTIN AND THYROTROPHIN RESPONSES TO TRH IN WOMEN DURING THE MENSTRUAL CYCLE AND UNDER ORAL CONTRACEPTIVE TREATMENT

The effects of physiological and pharmacological variations of oestrogens on prolactin and thyrotrophin (TSH) secretion have been studied during the menstrual cycle and under oral contraceptive treatment. Ten women were tested for prolactin and TSH responses to 200 μg TRH in the early follicular (days 4-6), periovulatory (days 14-15) and luteal phases (days 22-26) of the same menstrual cycle. Circulating plasma prolactin levels did not significantly vary in three phases, but TSH basal levels were lower in the luteal than in the follicular and periovulatory periods. The prolactin response to TRH was significantly enhanced in the periovulatory phase, while the TSH response was slightly decreased. Seven women on sequential contraceptives exhibited increased basal and TRH-induced prolactin secretion during the oestrogen treatment, with an unaltered TSH secretion throughout therapy. Treatment with combined contraceptives did not alter either basal or TRH-induced prolactin secretion in eight women, but basal TSH secretion and its response to TRH were both reduced. These data show that oestrogens may produce different regulatory effects on prolactin and TSH secretion, particularly in the pituitary sensitivity to TRH stimulation. Physiological variations of oestrogen secretion such as those observed during the menstrual cycle can likewise modify prolactin levels. These results could provide some support for a regulatory role for prolactin in the menstrual cycle.     

Serum thyrotrophin, prolactin and growth hormone, response to TRH during oestrogen treatment.

The serum levels of thyrotrophin (TSH), prolactin (PRL) and growth hormone (GH) and the response of these hormones to 500 mug thyrotrophin-releasing hormone (TRH) iv were studied in menstruating women, in post-menopausal women before and after 2 mg oestradiol valerate for 5 consecutive days, and in men on long term oestrogen treatment. Oestrogen treatment had no effect on basal serum TSH levels, which were within the normal range in all groups. The TSH response to TRH was not different in menstruating and post-menopausal women and was not changed in the latter group after oestrogen treatment. In men treated chronically with oestrogens, the TSH response to TRH was similar to that found in normal male subjects.     

THE TSH, FSH AND PROLACTIN RESPONSES TO CONTINUOUS INFUSIONS OF TRH AND THE EFFECTS OF OESTROGEN ADMINISTRATION IN NORMAL MALES

Four normal males received a constant infusion of 0.9% NaCl for 1 hr followed immediately by 500 μg of TRH infused over the same period. A rise in serum TSH was observed in all subjects while in three there was also a significant FSH response. The prolactin response, unlike that of TSH, was markedly pulsatile indicating that different mechanisms exist for the release of these two hormones from the pituitary after TRH. Circulating levels of LH were unaffected. Ethinyl oestradiol, 30 μg daily for 3 days, was administered to two of the subjects and the infusions were repeated. Both basal FSH and LH levels were depressed, as was the FSH response to the infusion of TRH. By contrast, however, the TSH response to thyrotrophin releasing hormone was enhanced after oestrogen. In one subject the basal prolactin levels were significantly higher while in both there was an augmented prolactin response to TRH, the pulsatile pattern of release being maintained.     

Somatostatin partially impedes the stimulatory effects of thyrotrophin-releasing hormone and dibutyryl cyclic AMP on prolactin release: prolactin release through multiple routes.

Patterns of prolactin release were examined using stimulating and inhibiting agents. Primary cultured pituitary cells primed with oestrogens were used for perifusion experiments. TRH (100 nmol/l) increased the peak prolactin concentration to 360% of the basal concentration, while TRH, under inhibition by 1 nmol somatostatin/l, raised the peak prolactin concentration to 185% of the basal levels. When the somatostatin concentration was increased to 10, 100 and 1000 nmol/l, TRH still stimulated prolactin release to 128%, 121% and 140% respectively, indicating that concentrations of somatostatin of 10 nmol/l or higher did not further suppress the stimulatory effect of TRH. TRH (1 mumol/l) stimulated prolactin release under the influence of 0 (control), 1, 10, 100 and 1000 nmol dopamine/l (plus 0.1 mmol ascorbic acid/l) to 394, 394, 241, 73 and 68% of the basal concentration respectively, showing that the dopamine concentrations and peak prolactin concentrations induced by TRH have an inverse linear relationship in the range 1-100 nmol dopamine/l. The stimulatory effect of dibutyryl cyclic AMP (dbcAMP) on prolactin release was also tested. The relationship between dbcAMP and somatostatin was similar to that between TRH and somatostatin. When adenohypophyses of male rats were used for perifusion experiments, somatostatin (100 nmol/l) did not inhibit basal prolactin release from the fresh male pituitary in contrast with the primary cultured pituitary cells, but dopamine (1 mumol/l) effectively inhibited prolactin release. In conclusion, (1) oestrogen converts the somatostatin-insensitive route into a somatostatin-sensitive route for basal prolactin release, (2) TRH-induced prolactin release passes through both somatostatin-sensitive and -insensitive routes, (3) dopamine blocks both somatostatin-sensitive and -insensitive routes and (4) cAMP activates both somatostatin-sensitive and -insensitive routes.     

Anterolateral hypothalamic deafferentation inhibits histamine-induced prolactin secretion and potentiates TRH-induced thyrotropin secretion in male rats

We studied the effect of histamine on serum prolactin and thyrotropin (TSH) levels in male rats with anterolateral hypothalamic deafferentation of hypothalamic connections or anterolateral cut (ALC). The success of ALC was confirmed by immunohistochemistry of somatostatin (SRIF) in the medial basal hypothalamus. ALC did not affect basal prolactin or TSH levels. Thyrotropin-releasing hormone (TRH, 200 ng/rat, i.p.) did not affect prolactin secretion either in sham-operated or ALC rats. In sham-operated rats intracerebroventricularly administered histamine increased significantly prolactin levels. Hypothalamic deafferentation abolished the effect of histamine on prolactin levels. TRH increased significantly serum TSH levels both in sham-operated controls and ALC rats. In the latter, however, the TSH-secretory response to TRH was significantly (p less than 0.05) larger compared to the controls. Intracerebroventricularly infused histamine (2 micrograms/rat) did not change the TRH-induced TSH secretion in either group of rats. These results show that (1) the effect of histamine on prolactin secretion is mediated through nerve tracts which are destroyed by ALC, and (2) cutting of afferent TRH (through sensitization) and SRIF fibers (through lacking inhibition) entering medial basal hypothalamus may both contribute to the enhanced TSH response to exogenous TRH.     

Studies of TRH-induced prolactin secretion and its inhibition by dopamine, using ovine pituitary cells

Prolactin secretion from ovine pituitary cell cultures was stimulated by thyrotropin-releasing hormone (TRH) (10(-10)-10(-7) M) with a half-maximal effect at approximately 2.5 X 10(-9) M. A maximally effective concentration of TRH produced a peak secretory response, 5-10-fold stimulation over basal release, within 15 min. Dopamine (10(-10)-10(-7) M) but not somatostatin caused a dose-related inhibition of TRH (10(-8) M) stimulated prolactin release. Both dopamine (10(-7) M) and somatostatin (10(-7) M) inhibited basal secretion from the cells. TRH did not significantly increase pituitary cell cyclic AMP levels under any of the conditions tested. Stimulation of prolactin secretion by TRH was not prevented when Ca2+ was omitted from the incubation medium. Dopamine inhibited secretion induced by TRH under low Ca2+ conditions. Our results are consistent with a hypothesis that TRH may stimulate prolactin secretion via release of intracellular Ca2+ rather than increased cellular Ca2+ uptake, and imply that dopamine inhibition involves a lowering of intracellular Ca2+ levels.     

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.     

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.     

ALTERATIONS IN THYROID HORMONE METABOLISM DURING CHEMOTHERAPY IN PATIENTS WITH TESTICULAR CARCINOMA

In a prospective study, the effects of chemotherapy on thyroid function in patients with non-seminoma testicular carcinoma were evaluated. Thirty-one patients were studied; in sixteen immunoassayable HCG was present, but altered thyroid function could not be established. In fifteen patients an exaggerated TSH response to TRH was observed. In these patients, although T3 and T4N values were normal, basal TSH levels were higher compared to patients with a normal TSH response, probably due to preceding lymphangiography.During chemotherapy, T4N, T3 and rT3 levels rose significantly, but basal TSH levels and the TSH response to TRH decreased. In contrast, the prolactin responses to TRH increased. The observed changes in thyroid function during chemotherapy appear to result from delayed thyroid hormone clearance, probably caused by an effect of chemotherapy on deiodinating enzyme activity. This would result, in an increase in T4N and rT3 levels and a fall in TSH levels and in the TSH response to TRH. Furthermore, after therapy the raised T4N and lowered TSH levels remained, whilst the FT3 level did not change either during or after therapy, suggesting an unaltered hypothalamic/pituitary axis.     

THE EFFECT OF ENDOGENOUS AND EXOGENOUS GONADOTROPHIN-RELEASING HORMONE ON THE PROLACTIN RESPONSE TO TRH

The prolactin response to TRH in a group of patients with Kallmann's syndrome was found to be significantly lower compared to a group of hypergonadotrophic hypogonadal patients. Since levels of testicular products are comparably low in both groups, we hypothesize that high endogenous LHRH production might be associated with an increased prolactin response to TRH. In support of this, we were, indeed, able to establish a positive correlation between the magnitude of the prolactin response to TRH and basal and LHRH-stimulated LH/FSH levels (the latter serving as an index of endogenous LHRH production) in: (1) eugonadal men, (2) men with Kallmann's syndrome, (3) oestrogen-treated agonadal men, (4) men with severely impaired spermatogenesis and, (5) agonadal men. A direct relation between LHRH and the prolactin response to TRH was demonstrated in a group of eugonadal men, the prolactin response to TRH being greater after prolonged LHRH pretreatment. We speculate that an increase of endogenous or exogenous LHRH might be associated with decreased hypothalamic dopamine secretion which could directly increase prolactin synthesis. Indirectly, decreased dopamine secretion could augment the potency of TRH in releasing prolactin.     

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

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

THE EFFECT OF THYROTROPHIN-RELEASING HORMONE ON PLASMA PROLACTIN AND THYROTROPHIN LEVELS IN PRIMARY HYPOTHYROIDISM

Plasma prolactin and thyrotrophin (TSH) were measured by radioimmunoassay before, at 20 min and 60 min after the intravenous administration of 200 μg thyrotrophin-releasing hormone (TRH) in thirty-two patients with untreated primary hypothyroidism and in sixteen normal volunteers. Whereas basal plasma TSH was markedly elevated in all the patients with hypothyroidism, a slight, but significant increase (P<0.05) in basal plasma prolactin in primary hypothyroidism could only be demonstrated by matching for age, sex and circulating gonadotrophin levels, ten patients with hypothyroidism with ten normal volunteers. There was, however, no significant difference between the two groups, matched or unmatched, in the plasma prolactin levels, in contrast to the plasma TSH levels, following TRH administration. No apparent relationship was found between basal prolactin and follicle-stimulating hormone (FSH), luteinizing hormone (LH) or TSH. Assuming the release of prolactin by TRH to be of physiological significance, the results suggest that TRH secretion by the hypothalamus may be increased in untreated hypothyroidism and that low levels of circulating thyroid hormone increase the sensitivity of the pituitary thyrotrophs, but not the prolactin secreting cells, to TRH. Markedly elevated plasma prolactin levels associated with galactorrhoea were not seen in primary hypothyroidism in the absence of the puerperium or oestrogen therapy.     

Evidence for autonomous secretion of prolactin in some alcoholic men with cirrhosis and gynecomastia

Prolactin responses to provocative thyrotropin releasing hormone (TRH) stimulation were evaluated in 20 cirrhotic men with gynecomastia. Fifteen of these cirrhotic men had normal responses with a minimum doubling of the prolactin concentration above basal in response to TRH. Five had abnormal (autonomous) responses in that they failed to double their basal level or had a paradoxical decrease from basal in response to TRH. Moreover, these same five men failed to have a sleep-related increase in plasma prolactin. Three of them also failed to respond to chlorpromazine stimulation. Such abnormal responses are generally associated with the presence of a prolactin secreting pituitary tumor. Basal plasma levels of prolactin were measured in all 20 men studied. The five men who failed to respond to TRH had significantly greater basal prolactin concentrations (80.5 ± 18.7 ng/ml) than did the 15 men who responded normally (33.7 ± 4.3 ng/ml) (p < 0.01), although all 20 had increased prolactin levels relative to that of controls (10.8 ± 0.9 ng/ml) (both p < 0.01).     

The effect of 2-bromo-alpha-ergocryptine and 2-chloro-alpha-methylergoline-8beta-acetonitrile (lergotrile mesylate) on the prolactin secretion of the ewe.

The effects of two ergot alkaloids, 2-bromo-alpha-ergocryptine methane-sulphonate (CB154, Sandoz) and lergotrile mesylate (LM, E. Lilly & Co.), on the basal secretion of prolactin and on the prolactin response to TRH or the milking stimulus was investigated in the sheep. Both CB154 (c. 0.5 mg/kg) and LM (c. 0.75 mg/kg) markedly reduced basal levels of prolactin and inhibited the TRH-induced prolactin release in ewes in the mid-luteal phase of the oestrous cycle. Both compounds also suppressed basal levels of prolactin in lactating ewes and inhibited the prolactin response to the milking stimulus in lactating ewes.     

Study of prolactin levels in the ferret

A radioimmunoassay for canine prolactin has been used to measure prolactin in the ferret. Serial dilutions of extracts of ferret pituitary glands and of ferret plasma yielded curves that were parallel with the canine prolactin standard curve. The sensitivity, accuracy, reproducibility and precision of the assay were within acceptable limits. Plasma prolactin levels increased after the administration of thyrotrophin releasing hormone (TRH) or chlorpromazine, but not after giving luteinizing hormone releasing hormone. Female ferrets, which were anoestrous, oestrous or spayed, and male ferrets had similar basal prolactin levels when sampled under sodium pentobarbitone anaesthesia. These basal levels were higher than in conscious males and the latter also showed a lesser response to TRH. Hypophysectomy significantly reduced basal prolactin levels in female ferrets by 2 h postoperatively and abolished the response to TRH.     

Danazol and prolactin status in patients with endometriosis

A group of 55 women with endometriosis was studied before and during danazol therapy. An unexpectedly high proportion (36%) had a raised serum prolactin level before treatment which was reduced after 50 days of danazol (before treatment 783 +/- 333 mU/l; on danazol 243 +/- 113 mU/l, P less than 0.001). In contrast patients with normal serum prolactin levels showed no significant drop on danazol therapy. In all patients serum oestradiol was significantly reduced during treatment (before treatment 449 +/- 188 pmol/l; on danazol 207 +/- 117 pmol/l, P less than 0.001). In one patient with hyperprolactinaemia danazol reduced both basal and stimulated prolactin levels, whereas in 5 women with normal prolactin levels we could detect no gross alteration in metoclopramide or TRH stimulated prolactin levels associated with danazol therapy. The possibility that normalisation of raised prolactin levels may be secondary to reduced oestrogens and that patients with endometriosis have an increased sensitivity to oestrogen-induced prolactin secretion is discussed.     

THE DYNAMICS OF PROLACTIN SECRETION DURING THE PUERPERIUM IN WOMEN

This study deals with serum prolactin concentrations during various conditions in the early puerperium in an attempt to investigate some characteristics of the mechanisms responsible for the regulation of the lactotrophes. In nursing, in non-nursing, non-medicated and in non-nursing, bromocriptine-treated women prolactin and 17β-oestradiol were measured during the early puerperium. In the first and the third group this was repeated during and after challenge with oestradiol-benzoate. The pituitary responsiveness to TRH was also determined in these two groups, challenged and unchallenged with oestradiol-benzoate. Nursing women had higher prolactin levels than the non-nursing groups, while bromocriptine decreased prolactin to very low levels. Non-nursing non-medicated women had prolactin values between those of nursing and those of bromocriptine-treated mothers. The already elevated prolactin levels in nursing women were not influenced by chronic oestradiol administration. In non-nursing puerperal women treated with bromocriptine, exogenous oestradiol caused a significant rise in plasma prolactin. The prolactin response to TRH in nursing women was clearly reduced in comparison with the normal menstrual cycle. In the bromocriptine-treated group the basal concentration of prolactin and its response to TRH stimulation was similar to normal non-pregnant women. In nursing and in non-nursing women treated with bromocriptine prolactin responses to TRH were increased after oestradiol challenge.     

Failure of 3,3'-diiodothyronine administration to alter TSH and prolactin responses to TRH stimulation.

In order to determine whether elevations in serum 3,3'-diiodothyronine (3,3'T2) concentrations influence the hypothalamic-pituitary--thyroid axis, thyrotropin (TSH) and prolactin responses to thyrotropin-releasing hormone (TRH) were assessed in five patients both prior to and during 3,3'T2 administration. Mean (+/- SE) peak TSH responses to TRH were 168 +/- 64 microU/ml during 3,3'T2 administration and 168 +/- 65 muU/ml during 3,3'T2 administration. Mean basal and peak prolactin concentrations after TRH were 6 +/- 3 ng/ml and 54 +/- 26 ng/ml, whereas during 3,3'T2 administration the basal and peak prolactin levels were 6 +/- 2 ng/ml and 55 +/- 28 ng/ml, respectively. Hypothyroid rats administered triiodothyronine (10 migrogram b.i.d.) for 5 days had a mean TSH response to TRH stimulation of 0.051 +/- 0.003 mU/ml, whereas rats to whom saline or 3,3'T2 (50 microgram b.i.d.) had been given for the same time interval had mean TRH-induced TSH responses of 1.127 +/- 0.179 mU/ml and 1.324 +/- 0.286 mU/ml, respectively. None of the TSH or prolactin responses to TRH, in either human or rat studies, were apparently altered by 3,3'T2. These observations suggest that elevation of serum 3,3'T2 levels are not associated with alterations in the hypothalamic--pituitary--thyroid axis in the experimental systems employed.     

Pituitary-thyroid function in spironolactone treated hypertensive women.

Four weeks high dose spironolactone treatment (Aldactone Searle, 100 mg q. i. d.) significantly enhanced the TSH (delta max. 8.5 +/- 4.1 vs. 4.6 +/- 3.1 microunits/ml, P less than 0.05) and T3 (delta max. 32 +/- 27 vs. 11 +/- 16 ng/100 ml, P less than 0.05) responses to an intravenous TRH/LH-RH bolus injection in 6 eumenorrhoeic euthyroid hypertensive women, without affecting basal serum TSH, T3 or T4 levels or the basal and stimulated LH, FSH and prolactin values (P greater than 0.10). The mean serum testosterone, 17-hydroxyprogesterone and oestradiol levels were also similar before and during therapy. Spironolactone, possibly by virtue of its antiandrogenic action, may exert its enhancing effect on pituitary-thyroid function by modulating the levels of receptors for TRH in the thyrotrophs or by altering the T3 receptor in the pituitary permitting a greater response to TRH.     

Mechanisms of release of prolactin from fowl anterior pituitary glands incubated in vitro: effects of calcium and cyclic adenosine monophosphate

Fowl anterior pituitary glands were bisected and each half was pretreated in either Medium 199 or medium containing EGTA to deplete endogenous calcium (Ca2+) stores, after which they were incubated in Medium 199, or Ca2+-free medium, containing prolactin release-stimulating agents and verapamil, a Ca2+ channel blocker. High K+ concentrations, hypothalamic extract, synthetic thyrotrophin-releasing hormone (TRH) and dibutyryl cyclic AMP (dbcAMP) all stimulated release of prolactin from control (non EGTA-treated) hemianterior pituitary glands. The effects of TRH and dbcAMP were not additive, but the response to submaximal concentrations of TRH was augmented by theophylline, a phosphodiesterase inhibitor. Reduction of Ca2+ availability with EGTA or verapamil reduced basal release of prolactin, prevented the prolactin-stimulating effects of high K+ concentrations and TRH, and markedly attenuated responses to hypothalamic extract and dbcAMP, EGTA being more effective than verapamil. Increasing the Ca2+ concentration of the medium did not augment basal or stimulated release of prolactin. These results suggest that both Ca2+ and cyclic AMP may act as intracellular mediators in the release of prolactin. Both basal and stimulated release of prolactin depend upon the presence of Ca2+. Although influx from the medium may be the major source of Ca2+, endogenous stores of Ca2+, perhaps mobilized by dbcAMP, may be able to maintain some release of prolactin. The prolactin-stimulating effects of TRH may be mediated by cyclic AMP.