Calcitonin inhibits basal and thyrotropin-releasing hormone-induced release of prolactin from anterior pituitary cells: evidence for a selective action exerted proximal to secretagogue-induced increases in cytosolic Ca2+

Salmon calcitonin (sCT)-like peptide is present in the central nervous system and pituitary gland of the rat, and this peptide inhibits basal and TRH-stimulated PRL release from cultured rat anterior pituitary (AP) cells. The present studies were designed to examine further the inhibitory actions of sCT on basal and TRH-stimulated PRL release and investigated 1) the temporal dynamics of the responses, 2) the effects of sCT on PRL release induced by other secretogogues, and particularly those acting via elevations of cytosolic Ca2+, and 3) the selectivity of sCT action on basal and stimulated AP hormone release. The inhibition of basal PRL release by sCT (0.1-10 nM) was dose-dependent and was characterized by a rapid onset with a gradual recovery to normal rates of release after the period of sCT inhibition. The inhibitory effect of sCT on basal PRL release was reversed by treatment with either the Ca2+ ionophore A23187 or with the phorbol ester, phorbol myristate acetate (PMA). sCT infusion did not affect the basal release of GH, TSH, FSH, or LH by perifused AP cells. When administered in short pulses, TRH, at concentrations from 1-100 nM, elicited a dose-dependent increase in PRL release. When coadministered with short 10 nM TRH, sCT (1-100 nM) inhibited TRH-induced PRL release in a dose-dependent manner, with a maximal inhibition of 78% at a concentration of 10 nM, and an ED50 concentration of approximately 3 nM. During longer (30 min) pulses of TRH (100 nM), PRL release increased sharply over 4-fold within 2 min, followed within 12 min by a rapid decline to a level 1.5-2-fold higher than basal, and this level was maintained for the remainder of the stimulation period. sCT pretreatment inhibited the overall PRL response to TRH. In contrast to its inhibition of TRH-induced PRL release, sCT failed to prevent the stimulation of PRL release by either ionophore A23187, PMA, vasoactive intestinal peptide, or forskolin. In addition, sCT failed to block TRH-induced TSH release or GnRH-induced LH release.(ABSTRACT TRUNCATED AT 400 WORDS)     

Maternal modulation of growth hormone secretion in the neonatal rat. I. Involvement of milk factors

The present experiments investigated the time course of maternal modulation of GH secretion and examined the possible role of milk in the regulation of GH secretion in neonatal rats. Serum GH concentrations in neonatal rats were high at birth and declined over time postpartum. Separation of rat pups from their mothers decreased, while a subsequent period of suckling increased, serum GH levels in rat pups on postpartum days 1-14, but not on day 20. The water-soluble fraction (infranatant) of rat milk contained immunoreactive (ir) rat GH-releasing hormone (rGHRH)-like material (725.06 +/- 81.29 pg/ml), ir-somatostatin-like activity (1.64 +/- 0.2 ng/ml), and irGH (4.79 +/- 0.73 ng/ml). The concentrations of these hormones tended to decrease with time postpartum and were positively correlated with each other (r = 0.70; P less than or equal to 0.0001). IrPRL was also present in the infranatant (148.44 +/- 14.55 ng/ml), but levels were not correlated with the other hormones detected. Milk infranatant stimulated GH secretion from perifused neonatal rat pituitary glands in vitro. Milk infranatant also stimulated GH secretion in vivo when administered sc or intragastrically to 2- or 8-day-old rat pups. The GH-releasing effect was not due to gastric distension or nonspecific nutritive components, as neither 0.9% saline nor nutrients (5% glucose and 10% BSA) increased serum GH levels. The presence of high concentrations of irGHRH in rat milk infranatant and the strong correlation between the irGHRH concentrations of milk samples and the in vitro GH response (r = 0.71; P less than or equal to 0.005) suggested that this peptide is a major candidate for producing the in vitro and in vivo GH-stimulating activity in rat milk. However, the minimally effective concentration of synthetic rGHRH required to stimulate GH release in the superfusion system was between 1-10 nM, which exceeds milk irGHRH levels by 100- to 1000-fold. Moreover, in vivo administration of synthetic rGHRH (sc or intragastrically) was unable to increase serum GH concentrations in 2-day-old pups, although a large dose (100 ng/g) of human GHRH sc was effective. These findings indicate that rat milk may be an important maternal factor that modulates GH secretion and, consequently, growth during the neonatal period. Rat milk has GH-releasing activity both in vivo and in vitro in neonatal rats, but the GH-releasing activities of milk are probably only minimally due to its rGHRH content.     

Does calcitonin modulate anterior pituitary hormone secretion?

In a group of 5 healthy subjects salmon CT (sCT) infusion was unable to induce significant variations on basal secretory levels of LH, FSH, PRL and TSH. In a second group of 5 normal subjects, GnRH and TRH tests were performed both during sCT and saline infusion; a clear inhibition of TSH-stimulated levels and of PRL area was documented, while gonadotropin secretion was not significantly affected by sCT infusion. These results suggest that CT effect might be attributed to a change in intracellular calcium of pituitary cells; however the different behavior between TRH-and GnRH-stimulated hormones might be due to a different hormonal release mechanism. Furthermore the widespread recognition of CT-like immunoreactivity in adenohypophysis and in portions of the central nervous system suggests that CT may be a neurotransmitter or paracrine regulator.     

Hyperprolactinemia after neonatal prolactin (PRL) deficiency in rats: evidence for altered anterior pituitary regulation of PRL secretion

Previous findings from this laboratory suggest a role for milk-borne PRL in the development of the inhibitory neuroendocrine controls over PRL secretion. Thus, rats that consumed milk deficient in PRL on days 2-5 postpartum show reduced concentrations and turnover of DA in the median eminence and elevated serum levels of PRL at 30-35 days of age. The present experiments were undertaken to investigate whether these consequences of neonatal PRL deficiency persist beyond puberty, and whether alterations in pituitary responsiveness to hypothalamic hormones may be involved. Lactating rats received sc injections of either saline or the dopamine (DA) agonist bromocriptine (125 micrograms/rat.day) on each of days 2-5 postpartum, a treatment that reduces the amount of PRL in milk without abolishing lactation. Blood samples were obtained from male and female offspring at various postnatal ages, and PRL concentrations were determined by RIA. Serum PRL concentrations in offspring from both groups were low until after weaning, but the female offspring of bromocriptine-treated mothers showed significantly elevated serum PRL between days 30 and 90 postpartum. Male offspring of bromocriptine-treated mothers also had transiently increased serum PRL levels, which returned to control levels by day 40. The turnover rate of DA in the median eminence, calculated from the rate of decline after synthesis inhibition, was reduced on day 35 in neonatally PRL-deficient offspring, as shown previously. However, no differences in DA turnover between the two groups were apparent on day 60, indicating a recovery of normal dopaminergic activity. Anterior pituitary cells of 100-day-old control and neonatally PRL-deficient animals were dispersed, cultured for 3 days, and then exposed to either TRH, to stimulate PRL release, or to the DA agonist bromocriptine, which inhibits PRL release. Pituitary cells of neonatally PRL-deficient offspring were almost completely unresponsive to bromocriptine with regard to suppression of PRL release and cytoplasmic PRL mRNA levels. In contrast, pituitary cells of neonatal PRL-deficient offspring were somewhat more responsive to TRH in stimulating PRL release and increasing the levels of PRL mRNA. These results suggest that a brief period of PRL deficiency during the neonatal period may result in long-lasting alterations in control of PRL secretion. The resultant hyperprolactinemia may be initiated by a reduction in the release of DA from the hyothalamus, perhaps reflecting a role for milk-derived PRL in the functional development of this neurosecretory system, and maintained in part by a reduction in pituitary responsiveness to DA.     

Calcitonin-like peptide containing gonadotrophs are juxtaposed to cup-shaped lactotrophs

Calcitonin (CT) is known to inhibit basal and TRH-stimulated prolactin release in cultured anterior pituitary cells in vitro and pituitary CT-like peptide (pit-CT) is synthesized and released by isolated anterior pituitary cells. However, the specific cell type containing pit-CT has not been identified. To determine this, double label immunohistochemistry was performed on pituitary sections from male rats using antisera for specific marker peptides of gonadotrophs, thyrotrophs, lactotrophs, somatotrophs, corticotrophs, and folliculo-stellate cells. CT was only colocalized with gonadotroph-specific markers and the distribution of pit-CT immunoreactive (IR) cells followed the patterns of gonadotroph distribution in male and female rats. Double and triple label immunohistochemistry using antiserum for CT, FSH, and PRL showed an apposition of calcitonin-like peptide containing gonadotrophs to cup-shaped lactotrophs. To examine whether pit-CT IR was altered, similarly to gonadotrophs, with known changes in PRL serum levels, studies were extended to ovariectomized, pregnant, and lactating rats. The area covered by pit-CT immunoreactivity and the tissue content of pit-CT significantly differed between physiological states and the pit-CT level was inversely related to the known PRL status. Pit-CT containing gonadotrophs were in all cases apposed to cup-shaped lactotrophs. These results provide histological support for previous studies proposing that pit-CT serves as a paracrine inhibitor of PRL release.     

Stimulation of prolactin cell differentiation in vitro by a milk-borne peptide

We have previously reported that the normal expression of PRL-secreting cells in neonatal rats requires a maternal signal specific to the first few days of lactation. These results raised the possibility that a milk-borne factor(s) ingested by the neonate and absorbed into the circulation might induce the ontogenic appearance of PRL cells. The purpose of the present study was to determine whether milk from this period could directly stimulate the differentiation of PRL secretors in culture. Monodispersed anterior pituitary cells from 1-day-old pups were cultured for 6 days with aqueous extracts of milk from early (days 2, 3, and 4) and late (days 15 and 16) lactation and then subjected to reverse hemolytic plaque assays for PRL and GH release. We found that the addition of milk extracts (10 mg/ml) from either early or late lactation stimulated the differentiation of PRL secretors (to 6.1 +/- 1.0% and 2.4 +/- 0.7% of all pituitary cells, respectively; mean +/- SE; n = 3) above that in control cultures without milk (0.2 +/- 0.2%). Thus, early milk was more than twice as effective as late milk in this regard (P less than 0.05). This effect appeared to be specific to PRL cell differentiation, since the relative abundance of GH secretors was not different between cells treated with either early or late milk (29.3 +/- 4.8% and 33.7 +/- 3.9%, respectively). On the other hand, late milk was more than twice as effective as early milk at increasing the capacity of GH secretors to release hormone (P less than 0.05). Preliminary characterization by gel filtration chromatography and proteolytic hydrolysis indicates that the bioactivity that differentiates PRL secretors is a small peptide(s) of 2000-6000 daltons. Taken together, our results demonstrate that a milk-borne peptide(s) is capable of specifically stimulating the differentiation of PRL-secreting cells in vitro, and that this bioactivity is more prevalent in milk from early lactation.     

Correlation of rat pituitary prolactin messenger ribonucleic acid and hormone content with serum levels during the estrogen-induced surge

The model of the serum PRL surge generated in the ovariectomized rat after estradiol benzoate (EB) treatment was used to study the relationship between serum and pituitary PRL levels and pituitary PRL mRNA levels. Adult ovariectomized rats were injected sc with 7 micrograms EB or vehicle at noon on day 0. Three days later (day 3), the rats were decapitated every 4 h over a 24-h period (0800 h on day 3 to 0400 h on day 4) for determination of serum and pituitary PRL and GH levels by RIA. In addition, PRL and GH mRNA content was determined using dot blot hybridization with cDNAs. Administration of EB resulted in a significant rise in serum PRL levels at 1200, 1600, and 2000 h on day 3 compared to control values. At other times, serum PRL levels in the EB group were the same as control values. EB treatment also elicited a marked increase in pituitary PRL content at all time periods examined except during (1600 and 2000 h) and after the PRL surge (2400 h on day 3) when there was a significant reduction in stored pituitary PRL. The pituitary PRL mRNA content in the EB-treated group was significantly elevated (4- to 6-fold) over control levels throughout the study. Furthermore, PRL mRNA levels in EB-treated rats were significantly higher at 2000 and 2400 h on day 3 than at other time periods. In contrast to its effects on PRL, EB treatment had a slight inhibitory effect on pituitary GH content at 2000 and 2400 h on day 3 compared to control values; otherwise, this steroid had no effect on serum GH levels and pituitary GH mRNA content. Interestingly, serum GH levels and pituitary GH mRNA content in both treatment and control groups fluctuated in a pattern consistent with circadian rhythms, with peak values occurring during the lights-on hours. These data show that estrogen has a stimulatory effect on pituitary content of PRL and its corresponding mRNA in the rat 3 days after injection. These elevated PRL mRNa levels may be necessary for the occurrence of PRL surges. Furthermore, the facts that serum PRL levels were elevated only at certain times (1200-2000 h on day 3) while PRL mRNA content was increased at all times in the EB-treated rats suggest a differential regulation between PRL release and biosynthesis.     

Calcitonin decreases thyrotropin-releasing hormone-stimulated prolactin release through a mechanism that involves inhibition of inositol phosphate production

Calcitonin is present in both the hypothalamus and pituitary of the rat, and normal rat anterior pituitary cells express calcitonin receptors. Calcitonin has been reported to inhibit or to stimulate PRL release from rat anterior pituitary cells. We have investigated the effects of salmon calcitonin on basal and stimulated PRL release from rat anterior pituitary cells and have studied the effects of this peptide on the intracellular biochemical pathways involved in PRL release. Salmon calcitonin had no significant effect on basal PRL release, but inhibited (P less than 0.01) TRH-stimulated PRL release without affecting PRL release promoted by angiotensin II, neurotensin, phorbol myristate acetate (a protein kinase C activator), or maitotoxin (a calcium channel activator). Salmon calcitonin had no effect on the increase in PRL release and intracellular cAMP concentration after exposure of pituitary cells to vasoactive intestinal peptide or forskolin. Salmon calcitonin significantly decreased (P less than 0.01) the TRH-stimulated rise in inositol phosphates without affecting the angiotensin II-stimulated increase in inositol phosphates. Similarly, salmon calcitonin decreased the TRH-stimulated increase in cytosolic calcium and arachidonate liberation by pituitary cells. We conclude that salmon calcitonin selectively decreases TRH-stimulated PRL release by a mechanism that involves a decrease in inositol phosphate production, as well as a subsequent reduction in cytosolic calcium levels and in arachidonate liberation.     

Posterior pituitary lobectomy abolishes the suckling-induced rise in prolactin (PRL): evidence for a PRL-releasing factor in the posterior pituitary

The aim of this study was to determine the role of the posterior pituitary in the regulation of PRL release during suckling. Lactating rats were subjected to posterior pituitary lobectomy (LOBEX) or sham surgery (SHAM) and separation from pups in the evening; experimental manipulations and blood collection were performed the next morning. In the first experiment rats were divided into three groups: SHAM, LOBEX, and LOBEX treated with a vasopressin analog, 1-desamino-8-D-arginine vasopressin and oxytocin. Plasma PRL levels in SHAM rats increased 20- to 25-fold upon introduction of pups and remained elevated for the duration of suckling. In contrast, basal plasma PRL levels in LOBEX rats were 3- to 4-fold higher than in SHAM but suckling failed to induce a further increase. Treatment of LOBEX rats with 1-desamino-8-D-arginine vasopressin and oxytocin reduced water consumption and allowed for milk ejection and milk intake by the pups but did not restore the suckling-induced rise in PRL. The second experiment tested the functional integrity of the hypothalamic dopamine (DA) and serotonergic systems after LOBEX and the ability of LOBEX-lactating rats to respond to PRL-releasing stimuli other than suckling. Injections of alpha-methyl-para tyrosine, an inhibitor of tyrosine hydroxylase, and 5-hydroxytryptophan, a precursor of serotonin, caused 20- to 30-fold rises in plasma PRL levels in both LOBEX and SHAM rats. Exposure to ether elicited a 3- to 4-fold rise in PRL which was higher in magnitude and of longer duration in LOBEX than in SHAM rats. Conclusions: Removal of the posterior pituitary from lactating rats results in an increase in basal PRL levels and a complete abolishment of the suckling-induced rise. Vasopressin and oxytocin restore water balance and milk ejection in the LOBEX rat but fail to affect PRL secretion. The LOBEX-lactating rat is not refractory to PRL-releasing stimuli other than suckling and its hypothalamic DA and serotonergic systems are functionally intact. In addition to DA, the posterior pituitary appears to contain a PRL-releasing factor(s) which mediates the suckling-induced rise in PRL.     

Calcitonin inhibition of prolactin secretion in lactating rats: mechanism of action

The effects of intracerebroventricular (10 ng/rat) or intravenous (10 or 40 microg/15 min/rat) administration of salmon calcitonin (sCT) on the prolactin (PRL) response to suckling and the activity of tyrosine hydroxylase (TH) were examined in lactating rats. Plasma concentration of PRL increased dramatically in control rats after the onset of the suckling stimulus, while administration of sCT resulted in inhibition of PRL response to suckling. The action of sCT was much more effective with intracerebroventricular administration, which totally blocked PRL release, compared to intravenous administration. The intracerebroventricular administration of sCT increased TH activity of tuberoinfundibular dopamine neuron (TIDA) in the stalk-median eminence, as measured by DOPA accumulation, while completely suppressing the PRL response to suckling. Injection of alpha-methyl-p-tyrosine (alpha-MT; 50 mg/kg), an inhibitor of TH and thus dopamine synthesis, increased PRL levels, and suckling caused a further increase in plasma concentrations of PRL. Injection of sCT (intracerebroventricularly) did not inhibit the PRL response to suckling in the presence of a depletion of dopamine. These results suggest that sCT inhibition of PRL secretion in lactating rats is mediated mainly by TIDA neurons without involvement of other neuroendocrine mechanisms.     

Inhibitory effects of centrally administered /ASU1-7/eel calcitonin on basal and stimulated prolactin release in rats

We investigated the effects of /ASU1-7/eel calcitonin (ASU1-7eelCT) on basal and stimulated prolactin (PRL) release in male rats. /ASU1-7/eelCT was administered intracerebroventricularly (icv) into freely moving rats with indwelling catheters. The administration of /ASU1-7/eelCT (2.5 micrograms/rat, icv) significantly inhibited basal PRL secretion. When PRL secretion was stimulated by exposing rats to restraint stress, /ASU1-7/eelCT (250 ng; 800 ng; 2.5 micrograms/rat, icv) dose-relatedly inhibited the PRL surges at 10 min after stress. The same doses of icv /ASU1-7/eelCT were effective in inhibiting morphine (6 mg/kg, intracarotid, ia-induced PRL release. No effect on stress-induced PRL secretion was observed when the peptide was administered intracarotid at the dose of 10 micrograms/rat. These results demonstrate that /ASU1-7/eelCT, as we previously observed with salmon calcitonin (sCT), has central inhibitory activity on PRL secretion, probably through enhancement of hypothalamic inhibitory pathways involved in the control of PRL.     

Calcitonin inhibits thyrotropin-releasing hormone-induced increases in cytosolic Ca2+ in isolated rat anterior pituitary cells

Calcitonin (CT) and related peptides, such as CT gene-related peptide and salmon CT (sCT)-like peptide, are present in the rat nervous system and the pituitary gland, and sCT markedly inhibits basal and TRH-stimulated PRL release from anterior pituitary (AP) cells. Because TRH-induced PRL release is known to involve increases in cytosolic free Ca2+ derived from both extracellular and intracellular sources, the objective of the present study was to test whether sCT interferes with this effect. Secretogogue-induced elevations of cytosolic free Ca2+ ([Ca2+]i) in acutely dispersed AP cells were monitored using the fluorescent Ca2+ indicator Indo-1 AM and flow cytometry. AP cells were enzymatically dispersed to single cell suspensions and loaded with 20 microM Indo-1 AM for 30 min. Indo-1-loaded AP cells were scanned at a rate of approximately 500 cells/sec for 200-300 sec in a flow cytometer, and the ratio of fluorescence due to Ca2+ bound to Indo-1 to free Indo-1 (Indo-1 ratio), which is an index of [Ca2+]i, was determined for each cell. Under basal conditions, AP cells showed stable Indo-1 ratios during the scans, and 100% of the cells responded to the Ca2+ ionophore ionomycin with increases in the Indo-1 ratio. Approximately 25-30% of the AP cells responded to a 1 microM pulse of TRH with marked increases in the Indo-1 ratio, indicative of increases in [Ca2+]i, with the response consisting of two phases, an initial rapid rise that was unaffected by the presence of EGTA in the extracellular environment, followed by a decrease to a sustained secondary phase that was completely eliminated by EGTA. In a normal extracellular Ca2+ environment, pretreatment with 100 nM sCT almost totally inhibited the response to 1 microM TRH. In EGTA-pretreated AP cells, the initial EGTA-insensitive phase of the TRH-induced [Ca2+]i increase was also abolished by prior exposure to sCT. These results suggest that sCT inhibits TRH-stimulated PRL release in AP cells by attenuating the TRH-induced increase in [Ca2+]i, an effect that probably occurs as a consequence of inhibition of the stimulatory effect of TRH on the Ca2+/phospholipid messenger system.     

Preferential increase in pituitary prolactin versus vasoactive intestinal peptide as a function of estradiol benzoate dose in the ovariectomized rat

Vasoactive intestinal peptide (VIP) is synthesized in various tissues, including the anterior pituitary gland, where it may stimulate the release of PRL. Because estrogen plays a central role in the regulation of PRL, it becomes important to determine the effects of this steroid on both pituitary VIP and PRL. To study this, pituitary VIP and PRL and plasma PRL were assayed in ovariectomized rats after treatment with estradiol benzoate (EB; 0.007, 0.07, 0.7, 7 or 70 microgram/rat). Pituitary and plasma TSH were also determined as well as VIP content in the medial basal hypothalamus, suprachiasmatic region, cerebral cortex, and jejunum. Oil-treated rats served as controls. Injection of 0.7 or 7 microgram EB resulted in a significant increase in pituitary PRL without changing plasma PRL levels or pituitary VIP content compared to values in the control group. Only treatment with 70 microgram EB produced a significant increase in both pituitary VIP and PRL as well as in plasma PRL compared to control values. EB treatment at any of the doses used had no significant effect on pituitary and plasma TSH or VIP content in any of the other tissues examined. These data show that pituitary PRL and VIP are differentially regulated in response to estrogen. The increases in pituitary VIP and basal plasma PRL after treatment with the highest dose of EB suggest that pituitary VIP may be involved in the development of estrogen-induced hyperprolactinemia. These data also show that the regulations of pituitary VIP and TSH are independent of each other in the estrogen-treated rat.     

Thyrotropin-releasing hormone stimulates growth hormone release from the anterior pituitary of hypothyroid rats in vitro

We have examined the interaction of thyroid hormone and TRH on GH release from rat pituitary monolayer cultures and perifused rat pituitary fragments. TRH (10(-9) and 10(-8)M) consistently stimulated the release of TSH and PRL, but not GH, in pituitary cell cultures of euthyroid male rats. Basal and TRH-stimulated TSH secretion were significantly increased in cells from thyroidectomized rats cultured in medium supplemented with hypothyroid serum, and a dose-related stimulation of GH release by 10(-9)-10(-8) M TRH was observed. The minimum duration of hypothyroidism required to demonstrate the onset of this GH stimulatory effect of TRH was 4 weeks, a period significantly longer than that required to cause intracellular GH depletion, decreased basal secretion of GH, elevated serum TSH, or increased basal secretion of TSH by cultured cells. In vivo T4 replacement of hypothyroid rats (20 micrograms/kg, ip, daily for 4 days) restored serum TSH, intracellular GH, and basal secretion of GH and TSH to normal levels, but suppressed only slightly the stimulatory effect of TRH on GH release. The GH response to TRH was maintained for up to 10 days of T4 replacement. In vitro addition of T3 (10(-6) M) during the 4-day primary culture period significantly stimulated basal GH release, but did not affect the GH response to TRH. A GH stimulatory effect of TRH was also demonstrated in cultured adenohypophyseal cells from rats rendered hypothyroid by oral administration of methimazole for 6 weeks. TRH stimulated GH secretion in perifused [3H]leucine-prelabeled anterior pituitary fragments from euthyroid rats. A 15-min pulse of 10(-8) M TRH stimulated the release of both immunoprecipitable [3H]rat GH and [3H]rat PRL. The GH release response was markedly enhanced in pituitary fragments from hypothyroid rats, and this enhanced response was significantly suppressed by T4 replacement for 4 days. The PRL response to TRH was enhanced to a lesser extent by thyroidectomy and was not affected by T4 replacement. These data suggest the existence of TRH receptors on somatotrophs which are suppressed by normal amounts of thyroid hormones and may provide an explanation for the TRH-stimulated GH secretion observed clinically in primary hypothyroidism.     

Estriol affects prolactin and LH secretion in rats

The effects of estriol on serum prolactin (PRL) and LH levels, on the pituitary response to TRH and LHRH and on the synthesis and release of PRL from the anterior pituitary gland were investigated in female rats. The increase of serum PRL levels after estradiol administration was found to be associated with an increase of glutamic acid decarboxylase (GAD) and GABA-transaminase (GABA-T) in the hypothalamus. Thus, a study was carried out on the effects of estradiol and estriol on PRL secretion and on GAD, GABA-T and gamma-amino butyric acid (GABA) in the hypothalamus and the anterior pituitary. Under basal and TRH-stimulated conditions, estriol increased serum PRL levels, decreased basal serum LH levels, and increased the response to LHRH, in terms of LH release. Estradiol and estriol increased the synthesis and release of 3H-PRL from hemipituitary glands in incubations of pretreated animals. Both estrogens induced hyperprolactinemia, concomitantly with an increase of hypothalamic GAD and GABA-T activity. Estriol increased hypothalamic GABA concentration, but did not modify GABA concentration in the pituitary glands. Our results show that estriol, at relatively high doses, seems to be active in increasing PRL synthesis and release and in decreasing serum LH levels; it can also modify pituitary response to TRH and LHRH stimulation.     

Prolactin inhibition at the end of lactation programs for a central hypothyroidism in adult rat

Malnutrition during lactation is associated with hypoprolactinemia and failure in milk production. Adult rats whose mothers were malnourished presented higher body weight and serum tri-iodothyronine (T(3)). Maternal hypoprolactinemia at the end of lactation caused higher body weight in adult life, suggesting an association between maternal prolactin (PRL) level and programming of the offspring's adult body weight. Here, we studied the consequences of the maternal PRL inhibition at the end of lactation by bromocriptine (BRO) injection, a dopaminergic agonist, upon serum TSH and thyroid hormones, thyroid iodide uptake, liver mitochondrial alpha-glycerophosphate dehydrogenase (mGPD), liver and pituitary de-iodinase activities (D1 and/or D2), and in vitro post-TRH TSH release in the adult offspring. Wistar lactating rats were divided into BRO - injected with 1 mg/twice a day, daily for the last 3 days of lactation, and C - control, saline-injected with the same frequency. At 180 days of age, the offspring were injected with (125)I i.p. and after 2 h, they were killed. Adult animals whose mothers were treated with BRO at the end of lactation presented lower serum TSH (-51%), T(3) (-23%), and thyroxine (-21%), lower thyroid (125)I uptake (-41%), liver mGPD (-55%), and pituitary D2 (-51%) activities, without changes in the in vitro post-TRH TSH release. We show that maternal PRL suppression at the end of lactation programs a hypometabolic state in adulthood, in part due to a thyroid hypofunction, caused by a central hypothyroidism, probably due to decreased TRH secretion. We suggest that PRL during lactation can regulate the hypothalamus-pituitary-thyroid axis and programs its function.     

Neonatal diethylstilbestrol treatment: response of prolactin to dopamine or estradiol in adult mice

The effects of neonatal diethylstilbestrol (DES) treatment on pituitary PRL response to dopamine (DA) inhibition or estrogen stimulation were studied in C3H/MTV+ mice at 10 weeks, 5 months and 8 months of age using synthesis and release of [3H]PRL by the pituitary as endpoints. In the DA study, anterior pituitaries from ovariectomized or intact, neonatally hormone-treated mice were incubated for 6 h in the presence or absence of DA. In the estrogen study, serum and anterior pituitaries were obtained from ovariectomized neonatally hormone-treated mice injected with oil or 17 beta-estradiol for 7 days before death. Neonatal treatment of mice with 2.5 micrograms/day DES altered basal [3H]PRL synthesis and release as well as circulating PRL concentrations. The results of analysis of variance indicated a significant interaction between the effects of neonatal hormone treatment and DA treatment on [3H]PRL release. Neonatal DES treatment did not alter the response of pituitary PRL to estrogen treatment at the ages examined.     

Hyperprolactinaemia in the spontaneously hypertensive rat.

A hypothalamic role in the aetiology of hypertension in the spontaneously hypertensive rat (SHR) has been suggested by prior observations. In an attempt to determine whether the central control of prolactin (PRL) release is altered in the SHR we have compared the PRL response to immobilization stress, thyrotrophin releasing hormone (TRH), haloperidol, and L-DOPA in the SHR and in normotensive Wistar control rats. Carotid artery catheters were inserted 48 h prior to the PRL response studies and the catheters were maintained patent with heparinized saline. Timed blood samples were obtained in SHR and control rats weighing 180-225 g. The SHR demonstrated elevated basal serum levels of PRL and greater PRL responses to stress. However, administration of L-DOPA resulted in a similar suppression of serum PRL in the SHR and in the normotensive controls. These findings suggest alteration in the central control of PRL release in the SHR. Observations of elevated basal PRL, exaggerated PRL in response to L-DOPA in SHR are consistent with normal pituitary responsiveness to dopamine suppression of PRL release, but defective hypothalamic metabolism of dopamine. Alterations in central dopamine control mechanisms in the SHR may play a role in the pathogenesis of essential hypertension in these animals.     

Nonpuerperal lactation and normal prolactin regulation.

Prolactin secretion was evaluated in 11 consecutive patients referred with nonpuerperal lactation who did not have clinical evidence of pituitary tumors. Six patients had normal fasting prolactin (PRL) levels, 13.8 plus or minus 1.8 ng per ml (Group I), and the other 5 women had elevated basal serum PRL concentrations, 182 plus or minus 72 ng per ml (Group II). All Group II patients had amenorrhea; however, 5 of 6 Group I patients had menstrual periods. The 24-h mean serum PRL concentrations were 12.8 plus or minus 1.2 (SEM), 13.3 plus or minus 0.7, and 165 plus or minus 62 ng per ml for the controls and Group I and II, respectively. A pattern of intermittent PRL discharge during the day characterized each group; however, a normal sleep related increase in serum PRL concentration was absent in the Group II patients. Chlorpromazine produced greater than two-fold increases in serum PRL concentrations in the controls and Group I patients; however, this response was absent in Group II. L-Dopa produced appropriate suppression of serum PRL concentrations in the normals and both patient groups. Normal serum growth hormone, thyroxine, and plasma cortisol characterized each group. Serum estrogen and/or LH and FSH were decreased in 3 of 5 Group II patients; however, the serum concentrations of these hormones were normal in 5 of 6 Group I patients. Short term L-dopa therapy was effective in suppressing lactation in 3 of 5 Group II patients, and it also decreased lactation in the 4 treated Group I patients without significantly altering 24-h mean serum PRL concentrations in the latter group. Conclusion: hypothalamic-pituitary dysfunction was present in certain patients with nonpuerperal lactation who had elevated 24-h mean PRL concentrations and no PRL release following chlorpromazine and sleep. Frequently, however, nonpuerperal lactation is associated with normal prolactin secretion.