Effect of long-term GHRH and somatostatin administration on GH release and body weight in prepubertal female rats

In order to find a chronic GHRH administration capable of stimulating growth rate without depleting pituitary GH content, prepubertal female rats were subcutaneously (sc) treated with GHRH (1-29)-NH2 and somatostatin (SS). In experiment 1, the rats received sc injections of GHRH and cyclic natural SS for 19 days. In the second study, female rats were continuously treated during 21 days with GHRH, using a slow release pellet, alone or combined with one daily injection of long acting SS (octreotide). In experiment 1, body weight was significantly increased when GHRH was administered at the highest daily dosage (1200 microg/day), accompanied by an slight increment in pituitary GH content. Hypothalamic SS concentrations decreased when GHRH or SS were administered alone whereas the combined treatment with both peptides did not modify this parameter, which suggests the existence of a balance between the chronic actions of both peptides on hypothalamus. In experiment 2, the continuous infusion of GHRH increased plasma GH levels and tended to enhance pituitary GH content. Nevertheless, GHRH effect was not effective enough to increase body weight. By adding one daily injection of SS both GHRH effects on the pituitary gland were abolished. Our study indicates that female rats retain responsiveness to chronic GHRH and SS treatments at both pituitary and hypothalamic levels.     

Possible role of prolactin in growth regulation

To study the possible role of prolactin on growth regulation pituitary grafted rats of different ages and their sham-operated controls have been used. After the transplant operation of one pituitary gland from a litter-mate donor on day 5 or on day 30 of life, a marked prolactin increase has been observed. This increase has been immediate in 30 day-old rats and delayed in 5 day-old rats in which the elevation starts being significant on day 20 for females and on day 25 for male rats. Pituitary grafting on day 5 of life, with an adult donor gland, resulted in an immediate and marked increase of prolactin values in both sexes. Litter-mate donor pituitary grafting, on day 5 of life, resulted in an increase in body weight that could be directly correlated with the increase in prolactin levels. Adult pituitary grafting resulted in an increased body weight in females with no effects being detected in males. In 30 day-old grafted male and female rats, marked body weight increases were seen, over the whole studied period, together with an increase in nose-tail length (1 cm in female and 1.5 cm in males longer than the control animals). All these changes do not seem not be related to GH modifications in pituitary grafted rats, since GH changes were very slight with a final tendency to lower values in female rats but not in males. All these data could suggest that prolactin might exert a direct effect on growth both in male and female rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Naltrexone partially inhibits the estrogen-induced increase in prolactin secretion and anterior pituitary weight

The effects of naltrexone, a specific opiate antagonist, on stimulation by estradiol benzoate (EB) of prolactin (PRL) release and anterior pituitary (AP) weight, were studied in gonadectomized female and male Sprague-Dawley rats. One week after castration, rats were injected for 10 days once daily with 2 μg EB alone, or together with twice daily injections of 2 mg naltrexone/kg body weight (BW). Blood was collected for radioimmunoassay of PRL by orbital sinus puncture on days 0 and 6, and by decapitation on day 11, at which time the AP was quickly removed, weighed and assayed for PRL.Serum PRL concentrations and AP weights were significantly increased by EB administration. These effects of EB were partially but significantly inhibited by naltrexone. These results suggest that endegenous opiates may be involved in the estrogen-induced rise in serum PRL and increase in pituatary weight.     

Pancreatic polypeptides affect luteinizing and growth hormone secretion in rats

We have examined the effects of third cerebroventricular (3V) injections of avian and bovine pancreatic polypeptide (APP and BPP) and the C-terminal hexapeptide amide of human PP (CHPP) on the secretion of anterior pituitary hormones in conscious ovariectomized rats. Injection of APP (2.0 micrograms; 472 pmoles) or BPP (5.0 micrograms; 1191 pmoles) decreased plasma levels of luteinizing hormone (LH) when compared to pre-injection levels in these animals or to saline-injected controls. The lower dose of BPP (0.5 micrograms; 119 pmoles) decreased plasma LH versus pre-injection levels and control animals, however, these effects diminished at later times. Plasma growth hormone (GH) also decreased following 3V injections of APP (2.0 micrograms) or BPP (5.0 micrograms). The lower dose of BPP (0.5 microgram) initially inhibited GH release, however, this effect was rapidly reversed and GH levels were significantly greater than those in controls at 60 and 120 min. Injections of BPP or APP did not alter prolactin (PRL) or thyroid stimulating hormone (TSH) secretion. Administration of 2.0 micrograms and 0.2 microgram of CHPP (2488 and 249 pmoles) produced no significant effects on plasma LH, GH, PRL or TSH. APP and BPP had no consistent effects on hormone secretion from dispersed anterior pituitary cells. The results indicate that APP and BPP exert potent central effects which inhibit LH and GH release from the pituitary gland.     

Effects of thyroidectomy and thyroxine replacement on the release of luteinizing hormone and gonadotropin-releasing hormone in vitro

The effects of thyroidectomy and thyroxine (T4) replacement on the release of luteinizing hormone (LH) and gonadotropin-releasing hormone (GnRH) in ovariectomized (Ovx) rats were studied. Immediately after ovariectomy, rats were thyroidectomized (Tx) or sham-Tx. The Ovx-Tx rats were injected subcutaneously with either saline or T4 (2 micrograms/100 g body weight) daily for 30 days before sacrifice. Sham-Tx rats were treated with saline only. Twenty hours after the last injection, the blood sample was obtained by decapitation. The excised anterior pituitary gland (AP) was bisected and incubated in vitro with or without 0.1, 0.5, 2.5, and 50 ng GnRH at 37 degrees C for 4 h. The mediobasal hypothalamus (MBH) was bisected and incubated with or without the AP of Ovx donor rat in vitro. Concentrations of LH and GnRH in the medium and that of LH in the serum were measured by radioimmunoassay. LH in the serum of Tx rats was higher than that in the serum of sham-Tx and Tx-T4 rats. Thyroidectomy resulted in an increase of LH release by Ovx rat AP, stimulated with or without 0.1 and 50 ng GnRH, as well as in an increase of immunoreactive GnRH release from MBH of Ovx rats in vitro. After a 4-hour incubation with donor APs, the LH in the medium containing MBH obtained from Tx rats was significantly higher than that obtained from sham-Tx and Tx-T4 rats. LH concentrations, in both sera and media, as well as GnRH concentration in the media of euthyroid and T4-replaced Tx groups were nonsignificantly different. These results suggest that T4 is inhibitory to the basal and GnRH-stimulated LH release as well as to the release of GnRH in the absence of ovarian hormones.     

Localization of fertility factor SP22 to specific cell types within the anterior pituitary gland

Sperm protein 22 (SP22) was recently identified in the anterior pituitary gland (AP) of male Golden Syrian hamsters using ion trap mass spectrometry. SP22 has been implicated in apoptosis, androgen receptor function, fertility, and ontogeny of early-onset Parkinson's disease. However, the role of SP22 in the pituitary has not been investigated. We cloned the cDNA for full-length SP22 from AP and posterior lobe (posterior pituitary and intermediate lobe) of the pituitary gland in adult male rats and Golden Syrian hamsters, confirming the presence of SP22 mRNA in the AP and posterior lobe. Because gonadal steroids are important regulators of AP function, and SP22 is associated with androgen receptor function, we used Western blots to compare SP22 in the AP of intact and orchidectomized male rats given placebo or a low or high dose of testosterone. SP22 did not differ with treatment, indicating that AP SP22 concentration was not regulated by testosterone. To localize SP22 to specific cells of the AP, mirror-image paraffin sections were labeled against SP22 and either luteinizing hormone (LH)beta, thyroid-stimulating hormone (TSH)beta, prolactin, adrenocorticotropic hormone (ACTH), or growth hormone (GH) using peroxidase-conjugated secondary antibody. Additional sections were colabeled with SP22 and one of the AP hormones using fluorescent secondary antibodies. SP22 colocalized in somatotropes and thyrotropes in rat and hamster. We identified SP22 in a small percentage of corticotropes, gonadotropes, and lactotropes. This is the first report that SP22 mRNA is present specifically in the AP, and SP22 is localized primarily in somatotropes and thyrotropes. SP22 may help regulate AP function and be particularly important for the control of GH and TSH secretion.     

The effect of cyclosporine administration on growth hormone release and serum concentrations of insulin-like growth factor-I in male rats.

The aim of this work was to study the effect of cyclosporine on the somatotropic axis. Accordingly, growth hormone (GH) secretion, circulating insulin-like growth factor I (IGF-I) and IGF binding proteins (IGFBPs) in response to cyclosporin A (CsA) treatment were examined in adult male Wistar rats. Cyclosporine administration (5, 10 or 20 mg/Kg daily) over 8 days did not modify the body weight, but it did decrease serum concentration of corticosterone and increased serum IGF-I and GH levels. Rats treated with 5 and 10 mg/Kg of cyclosporine had similar levels of serum IGFBPs to control rats, but there was an increase in circulating IGFBP-3 and IGFPB-1,2 in the group treated with 20 mg/Kg of CsA. The increase in circulating GH correlates with a decrease in pituitary GH content in CsA treated rats, with no modification in hypothalamic somatostatin content, suggesting an increase in pituitary GH release. In order to test this hypothesis, anterior pituitary cell cultures were exposed to different CsA concentrations during a 4 h incubation period. Cyclosporine increased GH secretion in cultured pituitary cells (p<0.05). These data suggest that cyclosporine increases circulating IGF-I and GH by stimulating pituitary GH release.     

Growth hormone and prolactin secretion in hypophysial stalk-transected pigs as affected by growth hormone and prolactin-releasing and inhibiting factors.

Control of growth hormone (GH) and prolactin (PRL) release was investigated in hypophysial stalk-transected (HST) and stalk-intact pigs by determining the effects of analogs of GH-releasing factors (GHRF), somatostatin (SRIF), arginine, thyrotropin-releasing hormone, alpha-methyl-rho-tyrosine, and haloperidol. HST and control gilts were challenged with intravenous injections of human pancreatic GHRF(1-40)OH, thyrotropin-releasing hormone, and analogs of rat hypothalamic GHRF. HST animals remained acutely responsive to GHRF by releasing 2-fold greater quantities of GH than seen in controls. This occurred in spite of a 38% reduction in pituitary gland weight and a 32 and 55% decrease in GH concentration and total content. During SRIF infusion, GH remained at similar basal concentrations in HST and control gilts, but increased immediately after stopping SRIF infusion only in the controls. Releasable pituitary GH appears to accumulate during SRIF infusion. GHRF given during SRIF infusion caused a 2-fold greater release of GH than seen in animals receiving only GHRF. Arginine increased (P less than 0.05) GH release in controls, but not in HST gilts, which suggests that it acts through the central nervous system. Basal PRL concentrations were greater (P less than 0.05) in HST gilts than in control gilts. TRH acutely elevated circulating PRL (P less than 0.001) in HST gilts, suggesting that it acts directly on the pituitary gland. Haloperidol, a dopamine receptor antagonist, increased circulating PRL in controls but not in HST animals. alpha-Methyl-rho-tyrosine did not consistently increase circulating PRL, however, suggesting that it did not sufficiently alter turnover rate of the tyrosine hydroxylase pool. The results indicate that the isolated pituitary after HST remains acutely responsive to hypothalamic releasing and inhibiting factors for both GH and PRL release in the pig.     

Stimulation of growth hormone release in dwarf and normal chickens by thyrotrophin releasing hormone (TRH) or human pancreatic growth hormone releasing factor (hpGRF)

The effect of thyrotrophin releasing hormone (TRH) or human pancreatic growth hormone releasing factor (hpGRF) on growth hormone (GH) release was studied in both dwarf and normal Rhode Island Red chickens with a similar genotype except for a sex-linked dw gene. Both TRH (10 micrograms/kg) and hpGRF (20 micrograms/kg) injections stimulated plasma GH release within 15 min in young and adult chickens. The increase in GH release was higher in young cockerels than that in adult chickens. The age-related decline in the response to TRH stimulation was observed in both strains, while hpGRF was a still potent GH-releaser in adult chickens. The maximal and long acting response was observed in young dwarf chickens, suggesting differences in GH pools releasable by TRH and GRF in the anterior pituitary gland. The pituitary gland was stimulated directly by perifusion with hpGRF (1 microgram/ml and 10 micrograms/ml) or TRH (1 microgram/ml). Repeated perifusion of GRF at 40 min intervals blunted further increase in GH release, but successive perifusion with TRH stimulated GH release. The results suggest the possibility that desensitization to the effects of hpGRF occurs in vitro and that the extent of response depends on the number of receptors for hpGRF or TRH and/or the amount of GH stored in the pituitary gland.     

Effect of glucagon on growth hormone secretion in rats

Gentled rats injected subcutaneously with glucagon (20 microgram/100 g body weight) showed a significant decrease in plasma growth hormone (GH) at 15 min after glucagon injection. A subcutaneous injection of 50% glucose did not cause the early suppression as shown at 15 min after glucagon injection, but at 30 min after glucose injection a tendency to decrease in plasma GH was observed. In urethane anesthetized rats, a subcutaneous administration of glucagon (1 microgram or 10 microgram/100 g body weight) failed to elicit an increase in plasma GH. In vitro incubation of anterior pituitary fragments with glucagon failed to decrease the release of GH, suggesting that glucagon does not act directly on the anterior pituitary.     

Increased synthesis of prolactin and growth hormone during incubation in the pituitary of broody Nagoya hens

Synthesis and release rates of prolactin and growth hormone (GH) in the anterior pituitary of laying and incubating broody chickens (Nagoya breed) were determined by a disc electrophoretic technique after in vitro incubation of anterior pituitaries with a labeled amino acid. Prolactin synthesis and release were two-fold higher in incubating than in laying hens, resulting in twofold increase in the concentration of prolactin in the gland. GH synthesis was three-fold higher in incubating than in laying hens but GH release was not affected by the incubation. GH concentration in the pituitary gland also increased in incubating hens. It is suggested that these changes in hormone synthesis, release, and concentrations are related to nesting behaviour and nutritional condition of incubating hens.     

Feedback regulation of growth hormone synthesis and secretion in fish and the emerging concept of intrapituitary feedback loop

Growth hormone (GH) is known to play a key role in the regulation of body growth and metabolism. Similar to mammals, GH secretion in fish is under the control of hypothalamic factors. Besides, signals generated within the pituitary and/or from peripheral tissues/organs can also exert a feedback control on GH release by effects acting on both the hypothalamus and/or anterior pituitary. Among these feedback signals, the functional role of IGF is well conserved from fish to mammals. In contrast, the effects of steroids and thyroid hormones are more variable and appear to be species-specific. Recently, a novel intrapituitary feedback loop regulating GH release and GH gene expression has been identified in fish. This feedback loop has three functional components: (i) LH induction of GH release from somatotrophs, (ii) amplification of GH secretion by GH autoregulation in somatotrophs, and (iii) GH feedback inhibition of LH release from neighboring gonadotrophs. In this article, the mechanisms for feedback control of GH synthesis and secretion are reviewed and functional implications of this local feedback loop are discussed. This intrapituitary feedback loop may represent a new facet of pituitary research with potential applications in aquaculture and clinical studies.     

Rescue from Dwarfism by Thyroid Function Compensation in rdw Rats.

The rdw rat was initially reported as having hereditary dwarfism caused by pituitary dysfunction. Subsequent studies on the rdw rat, however, have demonstrated that the primary cause of rdw dwarfism is present in the thyroid gland but not in the pituitary gland. The primary cause of rdw rat disorders is a missense mutation of the thyroglobulin (Tg) gene by a one-point mutation. In the present study, we attempted to rescue the dwarfism of the rdw rats using a diet supplemented with thyroid powder (T-powder) and a thyroid graft (T-graft). The infants of the rdw rat were successfully raised to a mature stage body weight, accompanied by elevation of serum growth hormone (GH) and prolactin (PRL), by the T-powder. Furthermore, the T-graft successfully increased the body weight with fertility. The serum GH and PRL levels in the T-graft rdw rat significantly increased. The serum thyroid-stimulating hormone (TSH) levels in the T-graft rdw rat were significantly decreased but were significantly higher than those in the control rat. The GH and PRL mRNA expression in the rdw rat with the T-graft was virtually the same as that of the control, but the TSH beta mRNA differed from that of the control rats. Thus, the dwarfism in the rdw rat is rescued by thyroid function compensation, such as that afforded by T-powder and T-graft.     

Prolactin and growth hormone synthesis: effects of perphenazine, alpha-methyltyrosine and estrogen in different thyroid states.

The effects of thyroid hormones on prolactin (PRL) and growth hormone (GH) synthesis by the rat anterior pituitary gland were assessed in vitro. A marked reduction (84-87%) in the rate of H3-leucine incorporation into GH was evident 2-4 weeks after thyroidectomy, while incorporation into PRL was 52-71% less than that measured in glands from intact rats. A single injection of T4 (200 mug/kg) administered to thyroidectomized (THX) rats 48 hr before sacrifice significantly increased incorporation into both pituitary hormones, although the stimulation of GH synthesis was much more dramatic. Perphenazine, alpha-methyltyrosine and estrogen enhanced the rate of PRL synthesis in intact rats. Thyroid ablation did not affect the response to perphenazine, but significantly increased the response to alpha-methyltyrosine and estrogen. On the other hand, administration of T4 to THX rats receiving perphenazine, alpha-methyltyrosine or estrogen diminished the stimulatory influence of these treatments on PRL synthesis. Perphenazine, alpha-methyltyrosine and estrogen had no effect on the rate of GH synthesis in THX rats, nor did they alter the ability of T4 to restore GH synthesis in these animals. These results indicate that GH synthesis in the rat is dependent upon thyroid hormones and support the concept that these hormones exert their stimulatory effect directly on pituitary somatotrophs. Pituitary lactotrophs, however, appear to retain much of their capacity to synthesize PRL under conditions of thyroid deficiency. The changes in pituitary PRL levels and synthesis rate induced by thyroid ablation might reflect differences in the number rather than the activity of these cells.     

Studies of anterior pituitary-grafted rats: II. Normal growth hormone secretion

Of the various animal models used to study chronic hyperprolactinemia, the otherwise intact rat implanted with extra anterior pituitary glands (AP) under the kidney capsule is assumed to be normal except for excess circulating prolactin (PRL). Since the ectopic glands contain numerous somatotropes in addition to abundant and active lactotropes, it was important to assess growth hormone (GH) secretion as well in this model of hyperprolactinemia. The structural and functional similarities of PRL and GH are such that it is necessary to demonstrate that metabolic abnormalities noted in AP-implanted rats are due to hyperprolactinemia and not to altered GH secretion. AP-implanted female rats have significantly higher resting serum PRL concentrations when compared to sham-operated control rats, but baseline serum GH levels are similar in normal and pituitary-grafted rats. Suppression of GH by insulin and clonidine is comparable in AP-implanted and control rats. The intrasellar pituitary GH concentration is also similar (ca. 20 μg/mg wet weight) in hyperprolactinemic and normal rats. We conclude that GH secretion is normal in the non-hypophysectomized AP-implanted rat, in contrast to the hypophysectomized AP-implanted rat model which has been reported to have diminished GH secretion. Despite the presence of recognizable somatotropes, the ectopic anterior pituitary does not appear to secrete significant amounts of GH, making the intact rat bearing multiple pituitary grafts an excellent model of chronic hyperprolactinemia.     

Pituitary response to growth hormone-releasing factor in rats with functional or anatomical lesions of the central nervous system that inhibit endogenous growth hormone secretion

The pituitary growth hormone (GH) response to the growth hormone-releasing factor, hpGRF-44, was evaluated in male rats with various lesions of the central nervous system. These included an electrical lesion of the ventromedial hypothalamus, a chemical lesion of the arcuate nucleus induced by neonatal treatment with monosodium glutamate, a functional lesion of catecholamine synthesis with alpha-methyl-p-tyrosine or a functional lesion of catecholamine storage with reserpine. The first three lesions appear to partially inhibit normal somatostatin secretion since in every instance hpGRF-44 administration induced a significant increase in plasma GH concentrations. In contrast, reserpine blocked the GH response to hpGRF-44, presumably by stimulating somatostatin secretion. The pituitary GH response to hpGRF-44 in the above described models was enhanced by pretreatment of the rats with antibodies against somatostatin. The pituitary GH response to repeated injections of hpGRF-44 was also evaluated in rats with an anatomical lesion of the arcuate nucleus or a functional lesion of catecholamine synthesis. The maximum GH response did not vary over time to the repeated injections of hpGRF-44 in rats with lesions of the arcuate nucleus; however, interruption of catecholamine synthesis resulted in a significant decrease in the GH response to hpGRF-44 over time.     

Prostaglandin E2 (PGE2)-induced growth hormone (GH) release: Effect of intrahypothalamic and intrapituitary implants

Overiectomized rats were unilaterally implanted with a 23-gauge stainless steel cannula in different hypothalamic areas or in the pituitary gland and subsequently were treated with estrogen (sc, 10 μg estradiol benzoate, Eb). Two days after the estrogen injection, an inner cannula containing PGE₂ or PHF_2α at its tip was inserted into the cannula. Other animals were implanted with empty inner cannula. Plasma GH concentrations were measured by RIA in blood samples drawn from the jugular vein while the animals were lightly etherized before (−2) and at 20, 40, 60 and 120 min following the implantation. Plasma GH levels in control animals bearing an empty cannula in the body of the arcuate nucleus-median eminence region (BARH-ME) were signifantly depressed by the ether stress. The implantation of PGF_2α in this area was completely ineffective in preventing ether stress-induced decline in plasma GH. By contrast, PGE₂ implanted in BARH-ME or the post-chiasmatic region of the hypothalamus (HARH-ME) elevated plasma GH 20 min following its implantation and partially prevented the subsequent decrease in GH levels induced by ether stress. PGE₂ implants located in several other hypothalamic areas failed to induce GH release or to prevent the decline in GH levels induced by ether stress. However, PGE₂ implanted in the pituitary gland elicited a marked increase in plasma GH at 20 min and completely prevented the subsequent ether stress-induced decline in GH levels.The results suggest that PGE₂ can act at both hypothalamic (ARH-ME) and pituitary levels to stimulate GH release. At the hypothalamus, PGE₂ may inhibit GH-inhibiting factor (GIF) release or induce release of GH releasing factor (GHRF).     

Influence of maternal dexamethasone treatment on morphometric characteristics of pituitary GH cells and body weight in near-term rat fetuses

Growth hormone (GH) and glucocorticoids have a powerful influence on controlling fetal growth, differentiation and maturation of numerous tissues. In the present study, the effect of maternal dexamethasone (Dx) treatment on GH cells and body weight in 19- and 21-day-old rat fetuses was investigated using immunocytochemical and morphometric methods. Pregnant female rats received daily injections of 1.0-0.5-0.5 mg Dx/kg b.w. on days 16-18 of pregnancy (experimental group), while the control group received an equal volume of saline. Dx treatment of pregnant rats enhanced immunostaining intensity and significantly increased (p<0.05) GH nuclear and cell volume, as well as volume density and number of GH cells per square millimeter in 19-day-old fetuses compared to the controls. In 21-day-old fetuses after maternal Dx administration, immunoreactivity, volume density and number of GH cells remained significantly increased (p<0.05). Dx treatment of pregnant rats resulted in marked body weight reduction of 21-day-old but not 19 days old fetuses in comparison with the corresponding controls. The presented results demonstrate that maternal Dx application has pronounced effect on morphometric parameters of GH cells of 19- and 21-day-old fetuses. Also, in near-term rat fetuses body weight was largely independent of pituitary GH cell activity.