Ontogenesis of hypothalamic control of adenohypophyseal secretions in the human foetus (author's transl)]

The endocrine glands of the human foetus are active early in gestation, and various foetal and placental hormonal contributions are essential for growth and sexual differentiation. 1. The anterior pituitary gland has the ability to synthesize, store and secrete hormones early in gestation. The patterns of change in plasma concentrations of hGH (Fig. 1), ACTH, LH and FSH (Fig. 2) during gestation indicate that secretion is at a maximum at mid-gestation, followed by a progressive decrease towards term. The high levels at mid-gestation can be interpreted as due simultaneously to a high secretion rate, low peripheral catabolism and absence of feedback mechanism. In contrast, the secretions of PRL (Fig. 1) and TSH are moderate at mid-gestation and only increase in the last trimester of gestation. 2. Effective control by the central nervous system (CNS) of the pituitary secretions is still immature at mid-gestation. The presence in the foetal hypothalamus of releasing factors such as LRF (Fig. 5) and TRF, and of somatostatin (Fig. 6), a growth hormone release inhibiting factor (GIF), has been established. TRF and GIF, but not LRF, are also present in the cerebral cortex. It has been postulated that, early in life, relatively autonomous and unrestrained secretion of hypothalamic hypophysiotropic releasing factors occurs, and that, later in development, there is a maturation of inhibitory or restraining influences mediated via the CNS (feedback mechanisms) that modulates the secretion of the foetal adenohypophyseal hormones (Fig. 3 and 4). 3. Observations made with anencephalic newborn confirm that a functional hypothalamus is necessary during foetal life for the secretion of each of the hormones of the anterior pituitary gland with the exception of PRL, the secretion of which is normal in anencephaly. Although somatostatin probably participates in the regulation of hGH during foetal life, it appears evident from the anencephaly data that this regulation can only be fully understood by postulating the existence of a growth hormone releasing factor (GRF).     

Targeted ablation of pituitary gonadotropes in transgenic mice.

LH, FSH, and TSH are heterodimeric glycoprotein hormones composed of a common alpha-subunit and unique beta-subunits. The alpha-subunit is produced in two distinct specialized cell types of the pituitary gland: gonadotropes, which synthesize LH and FSH, and thyrotropes, which synthesize TSH. We have demonstrated that 313 base pairs of the bovine-alpha subunit promoter direct expression of diphtheria toxin A chain specifically to the gonadotropes in transgenic mice. Animals carrying this transgene generally exhibit reproductive failure and lack of gonadal differentiation, consistent with gonadotrope ablation. Lack of gonadotrope activity was verified by RIA and immunohistochemical staining for LH. The phenotype of these transgenic mice is nearly identical to mice homozygous for the spontaneous mutation, hpg, which is due to a deletion in the gene encoding GnRH. Thyrotrope function was judged normal based on overall growth of the animals, appearance of their thyroids, T4 levels measured by RIA, and immunohistochemical staining for TSH. The ablation of gonadotropes but not thyrotropes suggests that separate cis-acting elements are necessary for expression of the alpha-subunit gene in these two cell types. Pituitary content of ACTH and GH was apparently normal, while PRL synthesis and storage were reduced. Thus, in a pituitary almost completely devoid of gonadotropes, most other pituitary functions were normal. This suggests that most pituitary cells are able to differentiate independently of terminal gonadotrope differentiation and can function in the absence of paracrine signaling provided by gonadotropes.     

Galactorrhea in subclinical hypothyroidism

Galactorrhea was found in 5 patients with subclinical hypothyroidism. The galactorrhea consisted of the discharge of a few drops of milk only under pressure. Serum T4 was in the lower level of the normal range, but serum T3 was normal (T4: 6.3 +/- 1.2 micrograms/dl, T3: 113 +/- 7 ng/dl). Basal serum TSH and PRL were slightly increased only in 2 and 1 cases, respectively. The PRL responses to TRH stimulation were exaggerated in all cases, although the basal levels were normal. An enlarged pituitary gland was observed in 1 patient by means of CT scanning. All patients were treated by T4 replacement. In serial TRH tests during the T4 replacement therapy, the PRL response was still increased even when the TSH response was normalized. Galactorrhea disappeared when the patients were treated with an increased dose of T4 (150-200 micrograms/day). Recurrence of galactorrhea was not observed even though replacement dose of T4 was later decreased to 100 micrograms/day in 4 cases. In patients with galactorrhea of unknown origin, subclinical hypothyroidism should not be ruled out even when their serum T4, T3, TSH and PRL are in the normal range. The TRH stimulation test is necessary to detect an exaggerated PRL response, as the cause of the galactorrhea. To differentiate this from pituitary microadenoma, observation of the effects of T4 replacement therapy on galactorrhea is essential.     

Effects of tartrate thyrotropin-releasing hormone treatment on serum thyrotropin, free triiodothyronine, free thyroxine and prolactin levels in patients with spinocerebellar degeneration.

The aim of the present study was to investigate the pituitary-thyroid axis function during the long-term (30 days) intramuscular administration of 4 mg/day of thyrotropin-releasing hormone tartrate (TRH-T) in 15 patients with spinocerebellar degeneration. The study was performed as follows: (1) acute 4 mg TRH-T test with hourly prolactin (PRL) and thyroid-stimulating hormone (TSH) level evaluations for 6 h; (2) placebo; and (3) 4 mg/day of TRH-T administration for 30 days with TSH, PRL, and free T3 and T4 (FT3 and FT4) levels evaluated on days 1, 15 and 30. Hormone determination was performed just before and 1 h after placebo or TRH-T administration. The acute administration of TRH-T caused a sustained rise of TSH which lasted until the 6th hour and of PRL which declined after 1 h (p < 0.01). During placebo administration, no change of TSH, PRL, FT3 or FT4 was observed. On the 1st day of treatment, 1 h after the TRH-T injection, a significant increase of both TSH and PRL levels occurred (p < 0.01). As compared to the 1st day, a significant decrease of the TSH (p < 0.01) levels occurred on the 15th and 30th days before TRH-T: the TSH response to TRH-T administration was present although less than on the 1st day (p < 0.01). Moreover, throughout the whole period of treatment, no difference was recorded for PRL levels before or after TRH-T administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of L-tryptophan and restraint stress on hypothalmic and brain serotonin turnover, and pituitary TSH and prolactin release in rats.

The dose response and time course effects of L-tryptophan and restraint stress on the metabolism of serotonin and release of thyroid stimulating hormone (TSH) and prolactin (PRL) were tested in male rats. Both treatments increased serotonin turnover in the hypothalamus (H) and remaining brain tissue minus the cerebellum (brain) as determined by enhanced accumulation of serotonin following monoamine oxidase (MAO) inhibition. L-tryptophan but not restraint stress elevated levels of tryptophan in the cerebellum. Both L-tryptophan and restraint stress inhibited TSH release and stimulated PRL release. These findings indicate that enhanced rates of serotonin turnover produced by L-tryptophan and physical restraint are associated with inhibition of TSH and stimulation of PRL release from the anterior pituitary.     

Alteration of stimulated TSH and prolactin response in children treated with growth hormone.

The plasma TSH and prolactin responses to thyrotropin releasing hormone (TRH) were measured in 5 children with isolated growth hormone deficiency prior to, during and after the administration of human growth hormone (hGH). TSH and prolactin secretory patterns were not uniformly concordant. TSH responses to TRH infusion were suppressed in 4 subjects after 5 days or 1 month of hGH administration despite normal serum thyroxin concentrations. Prolactin responses were suppressed in all 5 subjects after 5 days of hGH administration. After 8 months of hGH therapy both TSH and prolactin responses returned toward pre-hGH values. Our finding that suppression of the TRH-induced TSH and prolactin secretory responses are reversible during hGH administration supports the concept of altered neuroregulation in this form of hypothalamic disorder.     

Episodic thyrotropin (TSH) and prolactin (PRL) secretion during aging in postmenopausal women.

While aging is known to decrease episodic thyrotropin (TSH) secretion in men, no detailed information is available as to age-related alterations in the TSH and prolactin (PRL) release patterns in postmenopausal women (PMW). Accordingly, we compared the TSH and prolactin (PRL) secretory profiles of 6 euthyroid younger PMW (mean age: 53.0 years) with those of 7 euthyroid older PMW (mean age: 80.4 years). In all PMW, blood samples were obtained at 10 minute intervals for 10 hours for serial determinations of TSH and PRL by RIA. While thyroxine (T4) serum concentrations were not different in younger from older PMW, triiodothyronine (T3) levels markedly (p less than 0.05) decreased in older PMW. In both younger and older PMW, TSH and PRL were secreted episodically (by Cluster pulse algorithm), with considerable inter-individual variabilities in either study group. TSH and PRL pulse attributes (interpulse intervals, frequencies, amplitudes) were comparable in younger and older PMW, although a tendency of mean TSH to increase (p = 0.18) was noted for older PMW. Mean TSH and PRL serum concentrations were positively (r = 0.94, p less than 0.01) correlated in older, whereas not in younger PMW. These observations demonstrate that the pulse characteristics of episodic TSH and PRL secretion are preserved in PMW even of old age. However, in view of markedly decreased circulating T3 concentrations and of no substantial change in the TSH pulsatile secretion in older PMW, the negative feedback on the hypothalamic-pituitary unit may be impaired in elderly women.     

Effect of sulpiride on serum growth hormone and prolactin concentrations following L-DOPA administration in man.

The effect of chronic administration of sulpiride on serum human growth hormone (hGH), prolactin and thyroid stimulating hormone (TSH) was examined in 6 normal subjects. Sulpiride was given orally at a dose of 300 mg (t.i.d.) for 30 days. Sulpiride raised serum prolactin levels in all subjects examined. In addition, sulpiride suppressed hGH release induced by L-dopa, although the basal hGH level was not changed. Sulpiride treatment appeared to antagonize partially the inhibitory effect of L-dopa on prolactin release. Following thyrotropin-releasing hormone (TRH) injection, the percent increment in prolactin levels from the baseline in sulpiride-treated subjects was less than in controls without sulpiride. In contrast, both the basal and TRH-stimulated TSH levels were not influenced by sulpiride. These observations suggest that sulpiride suppresses L-dopa-induced hGH release and stimulates prolactin release, presumably by acting against the dopaminergic mechanism either on the hypothalamus or on the pituitary. The decreased prolactin response to TRH after sulpiride treatment may indicate a diminished reserve capacity in pituitary prolactin release.     

Differential in vivo activities of bovine growth hormone analogues

In rodents, bovine (b) growth hormone (GH) binds only to GH receptors, while human (h) GH binds to both GH and PRL receptors. The phenotypic consequences of expression of bGH and hGH in transgenic mice are different and, in some cases, opposite. In the present study, site-directed in vitro mutagenesis of the bGH gene was used systematically to eliminate its differences from hGH at one, two, three or four sites suspected of conferring lactogenic activity: D11, H18, S57 and T60, respectively (corresponding to sites 12, 19, 57 and 60 of the bGH molecule). The resulting bGH analogues were expressed in cell lines and in transgenic mice. All of the seven bGH analogues produced retained their ability to bind to GH receptors and exhibited somatogenic activity in vitro and in vivo. However, none of them were able to bind to PRL receptors or to elicit detectable lactogenic response in vitro. Transgenic animals expressing any of the generated analogues were characterized by gigantism and splanchnomegaly. The effects of expression of each of the double, triple or quadruple mutants on the seminal vesicle weight resembled the effects of wild-type hGH and differed from the effects of expression of wild-type bGH. There were differences between the effects of the expression of different bGH analogues on plasma PRL levels and on the PRL response to pharmacological blockade of catecholamine synthesis. Plasma LH levels in ovariectomized females were suppressed by several of the analogues tested, an effect not seen in animals expressing wild-type bGH or hGH. Dopamine turnover in the median eminence of male mice was also altered in animals expressing different bGH analogues but not in those expressing wild-type bGH or hGH. In ovariectomized females, the effects of different bGH analogs on the turnover of dopamine and norepine phrine in the median eminence included changes resembling those detected in animals expressing hGH, as well as alterations differing from the effects of bot h bGH and hGH.The results indicate that biological actions of these bGH analogues cannot be characterized simply in terms of enhanced or reduced somatogenic or lactogenic activity and raise a possibility that different sites, domains or features of the tri-dimensional structure of GH are involved in its actions on different cellular targets     

Further evidence for a hypothalamic site of action of atrial natriuretic factor: inhibition of prolactin secretion in the conscious rat

The presence of atrial natriuretic factor (ANF) in the hypothalamus and pituitary gland suggests a possible neuroendocrine action of the peptide. Because ANF has been shown to alter the activity of hypothalamic neurons and to interact with brain dopamine systems, we examined the possibility that it might be involved in the hypothalamic control of prolactin (PRL) and thyroid-stimulating hormone (TSH) secretion. Neither basal not stimulated release of PRL or TSH from cultured dispersed anterior pituitary cells was altered by doses of ANF ranging from 10(-11) to 10(-6) M. Similarly, the in vitro inhibition of PRL release by dopamine was not affected by the presence of ANF (10(-7) M). Plasma levels of PRL and TSH in conscious male rats infused for 30 min with 0.01 or 0.1 microgram ANF-kg-1.min-1 did not differ significantly from those present in saline infused controls. Third-cerebroventricular injection of saline (2 microL) or saline plus ANF (0.02, 0.1, 1.0, or 2.0 nmol) did not significantly alter TSH secretion; however, injection of the two highest doses of ANF resulted in significant inhibition of PRL release. Levels of PRL remained significantly reduced for 90 min after injection of 2 nmol ANF. The results indicate that ANF can act centrally to alter the release of neural factors responsible for the hypothalamic control of lactotroph function.     

Autocrine stimulation of Nb2 cell proliferation by secreted, but not intracellular, prolactin

We have introduced expression constructs for mouse PRL (mPRL) or a nonsecreted form of mPRL into the PRL-responsive Nb2 rat lymphoma cell line. Cell lines resulting from transfection of Nb2 cells with the wild type mPRL construct synthesize and secrete mPRL. These cells are able to grow independently of added lactogens, and conditioned media and cell extracts from these cultures stimulate the growth of Nb2 cells. In contrast, cells synthesizing the nonsecreted mPRL do not proliferate in the absence of added lactogenic hormones, and conditioned media from these cell cultures do not have PRL-like activity in the Nb2 cell growth assay. PRL protein is detected in these nonsecreting cell lines; however, extracts from these lines are generally unable to stimulate Nb2 cell proliferation. These results indicate that cells can respond in an autocrine fashion to PRL, but that an intracellular form of PRL is unable to activate Nb2 cell growth.     

Evidence that the thyroid-stimulating hormone (TSH) receptor transmembrane domain influences kinetics of TSH binding to the receptor ectodomain

Thyroid-stimulating hormone (TSH)-induced reduction in ligand binding affinity (negative cooperativity) requires TSH receptor (TSHR) homodimerization, the latter involving primarily the transmembrane domain (TMD) but with the extracellular domain (ECD) also contributing to this association. To test the role of the TMD in negative cooperativity, we studied the TSHR ECD tethered to the cell surface by a glycosylphosphatidylinositol (GPI) anchor that multimerizes despite the absence of the TMD. Using the infinite ligand dilution approach, we confirmed that TSH increased the rate of dissociation (k(off)) of prebound (125)I-TSH from CHO cells expressing the TSH holoreceptor. Such negative cooperativity did not occur with TSHR ECD-GPI-expressing cells. However, even in the absence of added TSH, (125)I-TSH dissociated much more rapidly from the TSHR ECD-GPI than from the TSH holoreceptor. This phenomenon, suggesting a lower TSH affinity for the former, was surprising because both the TSHR ECD and TSH holoreceptor contain the entire TSH-binding site, and the TSH binding affinities for both receptor forms should, theoretically, be identical. In ligand competition studies, we observed that the TSH binding affinity for the TSHR ECD-GPI was significantly lower than that for the TSH holoreceptor. Further evidence for a difference in ligand binding kinetics for the TSH holoreceptor and TSHR ECD-GPI was obtained upon comparison of the TSH K(d) values for these two receptor forms at 4 °C versus room temperature. Our data provide the first evidence that the wild-type TSHR TMD influences ligand binding affinity for the ECD, possibly by altering the conformation of the closely associated hinge region that contributes to the TSH-binding site.     

The Circadian Rhythms of Thyrotrophin and Prolactin Secretion

As with other anterior pituitary hormones, the secretion of both thyrotrophin (TSH) and prolactin (PRL) displays a circadian variation with different patterns for each hormone. In recent years there has been a substantial increase in the understanding of the neuroregulation oof TSH and PRL. However the primary events involved in the generation of their circadian rhythms remains unclear. Regulatory pathways comprise two major groups: central factors, where the control is exerted by the central nervous system via the hypothalamus and peripheral factors, which include all extra CNS mechanisms. The first group is represented mainly by neuropeptides and neurotransmitters controlling TSH and PRL release, whereas the second one comprises both physical phenomena such as variations in plasma volume or postural changes and hormonal influences arising from target glands such as the adrenal, the thyroid and the gonads.     

The involvement of cyclic-3',5'-AMP in the release of hormones from the anterior pituitary in vitro.

DBcAMP significantly increased the release of GH but not of LH, FSH, TSH, or PRL, except in the presence of hypothalamic extract when it augmented the release of LH, FSH, and GH, reversed the inhibition of PRL, but did not further influence TSH release. Theophylline increased release of GH and PRL while inducing increased tissue content of cAMP without consistently increasing the release of TSH, LH, or FSH. Hypothalamic extractor K+-stimulated hormone rel-ase was consistently and significantly potentiated by theophylline. Neither hypothalamic extract, increased [K+], or synthetic TRH and LRH were able to raise tissue content of cAMP while producing their expected effects on hormone release. Cholera enterotoxin produced a highly significant increase in tissue content of the cyclic nucleotide but increased the release of GH only, and not that of LH, FSH, TSH, or PRL. DBcAMP was able to lower the threshold concentration of K+ required to stimulate release of GH, LH, and FSH and also to augment K+-stimulated release to the higher levels induced by the hypothalamic releasing hormones. It did not augment K+-induced release of TSH.     

The pituitary-thyroid axis in patients with pituitary disorders

The pituitary-thyroid axis of 12 patients, exposed to transsphenoidal pituitary microsurgery because of nonfunctioning adenomas (6), prolactinomas (3) and craniopharyngioma (1), or to major pituitary injury (1 apoplexy, 1 accidental injury), was controlled more than 6 months following the incidents. The patients did not receive thyroid replacement therapy and were evaluated by measurement of the serum concentration of thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), T3-resin uptake test and thyrotropin (TSH, IRMA method) before and after 200 micrograms thyrotropin releasing hormone (TRH) iv. The examination also included measurement of prolactin (PRL) and cortisol (C) in serum. Apart from 1 patient with pituitary apoplexy all had normal basal TSH levels and 9 showed a significant TSH response to TRH. Compared to 40 normal control subjects the 12 patients had significantly decreased levels of T4, T3 and rT3 (expressed in free indices), while the TSH levels showed no change. Five of the patients, studied before and following surgery, had all decreased and subnormal FT4I (free T4 index) after surgery, but unchanged FT3I and TSH. The levels of FT4I were positively correlated to both those of FT3I and FrT3I, but not to TSH. The TSH and thyroid hormone values showed no relationship to the levels of PRL or C of the patients exposed to surgery. It is concluded that the risk of hypothyroidism in patients exposed to pituitary microsurgery is not appearing from the TSH response to TRH, but from the thyroid hormone levels.     

Differential Effect of Sodium and Magnesium on the Pituitary of Goldfish Adapted to Calcium-Free Environments

Adaptation to deionized water (DW) affects several cell types in the goldfish. The pars intermedia PAS-positive cells are highly stimulated. Their low response or the absence of changes in goldfish kept in 1/3 Ca-free seawater (SW) and in Ca-free SW-adapted eels, respectively, suggest that sodium and/or magnesium are interfering. To test this hypothesis, young goldfish were adapted to DW supplemented or not with sodium (50 and 140 mM) for 8 and 16 days or with magnesium (16.5 and 50 mM) for 16 and 30 days. Cytological and morphometric studies of the pituitary showed that prolactin (PRL) cell activity was reduced by sodium. Thyrotropic (TSH) cells were stimulated. The activity of melanocyte-stimulating (MSH) cells increased in DW + Na+. Stimulation of the pars intermedia PAS+ cells in DW was partly inhibited by adding sodium; the cellular and nuclear areas increased only moderately, the endoplasmic reticulum (ER) was not conspicuous and mitotic activity disappeared. In DW + Mg²⁺ the activity of PRL, TSH and MSH cells tended to be lower after a long-term adaptation. The response of the PAS+ cells was as high as that noted in DW; complete degranulation, enlargement of the ER and important mitotic activity. Differential responses to Na+ and Mg²⁺ are not due to pH differences in the solutions. External sodium is able then to reduce the response of the PAS+ cells in a Ca-free environment, while magnesium is not inhibitory. Other cell types are also affected by high levels of Na+ and Mg²⁺.     

Inhibitory effects of cysteamine on neuroendocrine function

The action of cysteamine on anterior pituitary hormone secretion was studied in vivo using conscious, freely moving male rats and in vitro using anterior pituitary cells in monolayer culture. Administration of 500 micrograms cysteamine into the lateral cerebral ventricles of normal rats caused the complete inhibition of pulsatile GH secretion for a minimum of 6 h. This treatment also significantly decreased plasma concentrations of LH for at least 6 h in orchiectomized rat, TSH in short-term (0.5 month) thyroidectomized rats, and PRL in long-term (6 months) thyroidectomized rats. The in vivo stimulation of GH, LH, TSH and PRL with their respective releasing hormones 60 min after administration of cysteamine was not different from the response observed in rats pretreated with saline except for PRL where cysteamine pretreatment significantly inhibited the expected PRL increase. In vitro, 1 mM cysteamine decreased basal and TRH stimulated PRL release while not affecting basal or stimulated GH, LH, TSH and ACTH secretion. These data demonstrate the dramatic and wide-ranging effects of cysteamine on anterior pituitary hormone secretion. This action appears to be mediated through hypothalamic pathways for GH, LH and TSH and through a pituitary pathway for PRL.     

Clinical use of metoclopramide test in the diagnosis of women with hyperprolactinemia

14 women with elevated prolactin (PRL) serum levels (greater than 25 ng/ml) were given 2.5 mg of metoclopramide, by bolus intravenous injection, to evaluate its diagnosic potential as a stimulus for PRL release. Following metoclopramide injection there was a prompt increase in serum PRL in normal subjects and in patients with moderate PRL elevations associated with galactorrhea-oligomenorrhea. The women with amenorrhea-galactorrhea regardless of the presence of absence of a pituitary tumor, showed a blunted response. Metoclopramide failed to induce TSH secretion in all cases. In conclusion: the use of the metoclopramide test provides no additional clinical information to that furnished by the basal serum PRL concentration for the hyperprolactinemic patient.     

Effect of triiodothyronine administration on the plasma TSH and prolactin responses to TRH in patients with hypothalamic-pituitary insufficiency.

A study was carried out in 10 patients with multiple pituitary hormone deficiencies to determine the response of thyroid-stimulating hormone (TSH) and prolactin (PRL) to thyrotropin-releasing hormone (TRH) and their suppressibility by treatment with triiodothyronine (T3) given at a dose of 60 microgram/day for 1 week. In 3 patients the basal tsh values were normal and in 7 patients, 2 of whom had not received regular thyroid replacement therapy, they were elevated. The response of TSH to TRH was normal in 6 patients and exaggerated in 4 (of these, 1 patient had not received previous substitution therapy and 2 had received only irregular treatment). The basal and stimulated levels of TSH were markedly suppressed by the treatment with T3. The basal PRL levels were normal in 7 and slightly elevated in 3 patients. The response of PRL to TRH stimulation was exaggerated in 2, normal in 6 and absent in 2 patients. The basal PRL levels were not suppressible by T3 treatment but in 4 patients this treatment reduced the PRL response to TRH stimulation. From these findings the following conclusions are drawn: (1) T3 suppresses TSH at the pituitary level, and (2) the hyperreactivity of TSH to TRH and the low set point of suppressibility are probably due to a lack of TRH in the type of patients studied.     

Thyroid stimulating hormone and prolactin secretion: reduced sensitivity to TRH-stimulated prolactin release after midpregnancy in rats

Prolactin (PRL) and thyroid stimulating hormone (TSH) plasma concentrations were measured during the latter part of the dark period in early and mid-late pregnancy in the rat. On Days 4-5 and 7-8 of pregnancy, plasma PRL concentrations surged between 22:00 and 06:00 hr and TSH values increased between 22:00 and 02:00 hr. While the TSH pattern was maintained during the second-half of pregnancy, surges in PRL release ceased and PRL levels remained at less than 10 ng/ml. The effects of thyrotropin releasing hormone (TRH) administration on PRL and TSH secretion were then measured to determine whether the second-half of pregnancy is associated with a decrease in sensitivity to an agent that can stimulate PRL release. Injection (iv) of cannulated pregnant rats with a low dosage (20 ng) of TRH stimulated a twofold increase in plasma TSH during both early (Days 5-9) and later (Days 14-18) pregnancy but did not change plasma PRL levels. Treatment with a high dosage (2 micrograms) of TRH induced a sixfold rise in plasma TSH during both phases of gestation. The higher dose of TRH also stimulated elevations in plasma PRL during early and mid-late pregnancy; however, both the absolute increase in the amount of PRL in plasma and the percentage increase over baseline levels were greater from Days 5-9 than from Days 14-16 of gestation. These data indicate that the neuroendocrine sensitivity to factors that stimulate PRL secretion changes as pregnancy progresses, and suggest that nocturnal secretion of PRL and TSH during pregnancy may be regulated, in part, by a common trophic factor.