The GLP-1 analog, liraglutide prevents the increase of proinflammatory mediators in the hippocampus of male rat pups submitted to maternal perinatal food restriction.

BACKGROUND: Perinatal maternal malnutrition is related to altered growth of tissues and organs. The nervous system development is very sensitive to environmental insults, being the hippocampus a vulnerable structure, in which altered number of neurons and granular cells has been observed. Moreover, glial cells are also affected, and increased expression of proinflammatory mediators has been observed. We studied the effect of Glucagon-like peptide-1 receptor (GLP-1R) agonists, liraglutide, which have very potent metabolic and neuroprotective effects, in order to ameliorate/prevent the glial alterations present in the hippocampus of the pups from mothers with food restriction during pregnancy and lactation (maternal perinatal food restriction-MPFR). METHODS: Pregnant Sprague-Dawley rats were randomly assigned to 50% food restriction (FR; n = 12) or ad libitum controls (CT, n = 12) groups at day of pregnancy 12 (GD12). From GD14 to parturition, pregnant FR and CT rats were treated with liraglutide (100 µg/kg) or vehicle. At postnatal day 21 and before weaning, 48 males and 45 females (CT and MPFR) were sacrificed. mRNA expression levels of interleukin-1ß (IL1ß), interleukin-6 (IL-6), nuclear factor-κß, major histocompatibility complex-II (MHCII), interleukin 10 (IL10), arginase 1 (Arg1), and transforming growth factor (TGFß) were assessed in the hippocampus by quantitative real-time polymerase chain reaction. Iba1 and GFAP-immunoreactivity were assessed by immunocytochemistry. RESULTS: The mRNA expression IL1ß, IL6, NF-κB, and MHCII increased in the hippocampus of male but not in female pups from MPFR. In addition, there was an increase in the percentage of GFAP and Iba1-immupositive cells in the dentate gyrus compared to controls, indicating an inflammatory response in the brain. On the other hand, liraglutide treatment prevented the neuroinflammatory process, promoting the production of anti-inflammatory molecules such as IL10, TGFß, and arginase 1, and decreasing the number and reactivity of microglial cells and astrocytes in the hippocampus of male pups. CONCLUSION: Therefore, the GLP-1 analog, liraglutide, emerges as neuroprotective drug that minimizes the harmful effects of maternal food restriction, decreasing neuroinflammation in the hippocampus in a very early stage.

Liraglutide Enhances the Activity of the ACE-2/Ang(1-7)/Mas Receptor Pathway in Lungs of Male Pups from Food-Restricted Mothers and Prevents the Reduction of SP-A.

In utero growth restriction and being born small for gestational age are risk factors for respiratory morbidity. IUGR (in utero growth retardation) is associated to overall reduction in lung weight, surfactant content and activity, impaired maturation of the alveolar type II cells, and decreased alveolar formation. The renin-angiotensin system (RAS) may be a key target underlying pathophysiological lung alterations. GLP-1 and agonists of its receptor modulate the expression levels of different components of RAS and also are very important for lung maturation and the production of surfactant proteins. The aim of this study was to elucidate the effects of IUGR induced by perinatal food restriction of the mother in the lung function of pups at early stages of life (PD21) and to determine if liraglutide had any effect during gestational period. Sprague-Dawley pregnant rats were randomly assigned to 50% food restriction (MPFR) or ad libitum control (CT) groups at day of pregnancy 12 (GD12). From GD14 to parturition, pregnant MPFR and CT rats were treated with liraglutide or vehicle. At postnatal day 21 and before weaning, 20 CT and 20 FR male pups were sacrificed and lungs were analyzed by RT-PCR. Liraglutide restored surfactant protein A (SP-A) mRNA expression in pup lungs from food-restricted mothers. Surfactant protein B (SP-B) mRNA expression is not affected by neither IUGR nor liraglutide treatment. Moreover, liraglutide modulated different elements of RAS, increasing angiotensin-converting enzyme 2 (ACE2) and MasR mRNA expression only in pups from food-restricted mothers (MPFR), despite food restriction had not any direct effect at this early stage. Liraglutide also increased endothelial nitric oxide synthase (eNOS) expression in MPFR lungs, reflecting the activation of MasR by angiotensin 1-7. In conclusion, liraglutide prevented the alteration in lung function induced by IUGR and promoted the positive effects of ACE2-Ang(1-7)-MasR in restoring lung function.

Prepuberal evolution of plasma leptin levels in gilts of thrifty genotype (Iberian pig) and lean commercial crosses (Large White × Landrace).

Current study aimed to determine possible differences in plasma leptin levels during the prepuberal period and their relationship with the onset of puberty in gilts of obese thrifty genotype (Iberian breed) and lean genotype (Large White × Landrace commercial crosses) reared under similar conditions. Plasma leptin concentration increased linearly during the 7 weeks prior to the day of puberty attainment in both genotypes (P<0.005, r=0.707 for LW × L and P<0.0005, r=0.874 for Iberian gilts). However, leptin levels in the Iberian gilts was higher from the first sample of the experimental period, with females having 16 weeks-old (2.7±0.3 vs 1.7±0.2 ng/ml in LW × L; P<0.001), to the onset of puberty (8.5±0.7 vs 2.8±0.3 ng/ml in LW × L; P<0.005). Thus, the current study reinforces previous data on changes in around puberty and evidences, for the first time, profound differences in prepuberal plasma leptin levels between gilts of obese (Iberian) and lean genotypes (LW×L).

Plasma leptin, ghrelin and indexes of glucose and lipid metabolism in relation to the appearance of post-weaning oestrus in Mediterranean obese sows (Iberian pig).

Iberian pig is the most abundant Mediterranean swine. The lack of knowledge of the reproductive physiology of Mediterranean genotypes, with predisposition to obesity, led us to evaluate the influence of body condition and metabolic status at weaning on the resumption of follicular growth and the appearance of post-weaning oestrus. Females failing to display post-weaning oestrus showed a high decrease in backfat mass during lactation; backfat depth at weaning was therefore lower than in sows becoming in oestrus. Females not bearing oestrus behaviour showed lower plasma leptin levels and higher ghrelin concentrations at weaning. Moreover, these sows evidenced dyslipidemic profile (increased triglyceridemia and cholesterolemia) and mobilization of fat reserves. Hence, changes in metabolic regulation of Iberian pigs may originate large effects on the resumption of ovulatory activity after weaning.

Glucogenic supply increases ovulation rate by modifying follicle recruitment and subsequent development of preovulatory follicles without effects on ghrelin secretion.

This study determined the effects of short-term energy inputs on ghrelin secretion and possible links with changes in the follicle population or the ovulation rate. Oestrous cycle was synchronized in 16 Manchega sheep using progestagen sponges and cloprostenol. Half of the animals were treated from days 0 to 4 by the oral administration, twice daily, of 200 ml of a glucogenic mixture containing 70% of glycerol, 20% of 1,2-propanediol and 10% of water; the control group received 200 ml water. The mean (+/-S.E.M.) plasma glucose increased immediately after the first administration (3.9+/-0.3 vs 3.0+/-0.1 mmol/l in control group, P<0.05), remaining statistically different during the treatment. However, plasma ghrelin levels were similar in both groups. On the other hand, the results indicated that short-term energy inputs modify ovulation rate (1.9+/-0.1 vs 1.3+/-0.2 in control group, P<0.05) by increasing the number of follicles able to be selected to ovulate during the period of treatment (>or=4 mm in size; 5.9+/-0.6 vs 4.3+/-0.4 at day 2, P<0.05). After sponge withdrawal, the number of these follicles decreased throughout follicular phase (5.8+/-0.8 to 1.5+/-0.4, P<0.0005) while the number of large follicles increased (>or=6 mm in size; 0.8+/-0.4 to 2.0+/-0.3, P<0.05); this would indicate an active growth of preovulatory follicles that were not found in the control group. Thus, the increases of ovulation rate by high-energy inputs would be caused by an enhancement in the developmental competence of preovulatory follicles.

Ghrelin improves growth hormone responses to growth hormone-releasing hormone in a streptozotocin-diabetic model of delayed onset.

GH secretion is markedly altered in diabetes mellitus (DM) in both rats and humans, albeit in opposite directions. In the rat, diabetes suppresses pulsatile GH secretion, especially high amplitude pulses, and decreases GH responses to secretagogue, depending inversely on severity of metabolic alteration. In the present study, we wanted to address the GH responses to GHRH and low doses of ghrelin in a streptozotocin (STZ) model of diabetes characterized by the delayed onset of the metabolic alterations. We have shown that the administration of high doses of STZ (90 mg/kg in 0.01 M solution of chloride-sodium, ip) to five-day-old pups (n5-STZ) can induce the appearance of a characteristic diabetic syndrome in adult age, the diabetic triad, with elevated plasma glucose levels: polyuria, polydipsia, hyperphagia, and reduced body weight gain. At the age of 3 months, in these n5-STZ male and female rats the GH responses to GHRH (1 microg/kg) and GHRH combined with ghrelin (1+3 microg/kg) had diminished both in punctual times and in the area under the curve (AUC). However, the combined administration of GHRH and ghrelin, being the more potent stimulus, elicited a synergistic GH response. Thus, male and female rats with delayed onset diabetes displayed an altered GH response to GHRH, although the combined administration of GHRH and ghrelin was able to restore the GH secretion with a synergistic effect.

Orexin A suppresses in vivo GH secretion.

BACKGROUND/AIMS: Orexins (OXs) are a newly described family of hypothalamic neuropeptides. Based on the distribution of OX neurons and their receptors in the brain, it has been postulated that they could play a role in the regulation of neuroendocrine function. GH secretion is markedly influenced by nutritional status and body weight. To investigate the role OX-A plays in the neuroregulation of GH secretion we have studied its effect on spontaneous GH secretion as well as GH responses to GHRH and ghrelin in freely moving rats. Finally, we also assessed the effect of OX-A on in vitro GH secretion. METHODS: We administered OX-A (10 microg, i.c.v.) or vehicle (10 microl, i.c.v.) to freely moving rats. Spontaneous GH secretion was assessed over 6 h with blood samples taken every 15 min. RESULTS: Administration of OX-A led to a decrease in spontaneous GH secretion in comparison with vehicle-treated rats, as assessed by mean GH levels (means+/-s.e.m. 4.2+/-1.7 ng/ml vs 9.4+/-2.2 ng/ml; P<0.05), mean GH amplitude (3.6+/-0.5 ng/ml vs 20.8+/-5.6 ng/ml; P<0.01) and area under the curve (848+/-379 ng/ml per 4 h vs 1957+/-458 ng/ml per 4 h; P<0.05). In contrast, OX-A failed to modify in vivo GH responses to GHRH (10 microg/kg, i.v.) although it markedly blunted GH responses to ghrelin (40 microg/kg, i.v.) (mean peak GH levels: 331+/-71 ng/ml, vehicle, vs 43+/-11 ng/ml in OX-A-treated rats; P<0.01). Finally, OX-A infusion (10(-7), 10(-8) or 10(-9) M) failed to modify in vitro basal GH secretion or GH responses to GHRH, ghrelin and KCl. CONCLUSIONS: These data indicate that OX-A plays an inhibitory role in GH secretion and may act as a bridge among the regulatory signals that are involved in the control of growth, nutritional status and sleep regulation.

Regulation of hypothalamic somatostatin and growth hormone releasing hormone mRNA levels by inhibin.

Although it is well established that inhibin plays a major role in the regulation of the hypothalamic-pituitary-gonadal axis, its influence in the regulation of other neuroendocrine functions is still poorly understood. Recent results indicate that inhibin suppresses plasma GH levels, but its site of action is yet unknown. Therefore, in the present work we investigated the effects of inhibin on somatostatin and growth hormone releasing hormone (GHRH) mRNA levels in the hypothalamus by 'in situ' hybridization. We found that inhibin administration (4, 12 and 24 h, i.c.v.) led to an increase in somatostatin mRNA levels in the periventricular nucleus, and to a decrease in GHRH mRNA content in the arcuate nucleus of the hypothalamus. These findings indicate that inhibin regulates the hypothalamic levels of somatostatin and GHRH mRNA.

Inhibin suppresses in vivo growth hormone secretion.

Although the effects of inhibin on gonadotropin synthesis and secretion have been extensively studied, the role of inhibin in the neuroregulation of in vivo growth hormone (GH) secretion still remains to be elucidated. In the present work, we investigated the effects of inhibin on spontaneous GH secretion in three different groups of conscious adult male rats: intact, gonadectomized, and dihydrotestosterone (DHT)-treated gonadectomized animals. We found that inhibin administration (100 microg/kg, i.v.) led to a marked suppression in spontaneous GH secretion in all the groups studied. This significant decrease was assessed by the area under the curve in intact (311.1 +/- 163.3 vs. 3,882.1 +/- 1,084.6 ng/ml/6 h, p < 0. 01), gonadectomized (416.6 +/- 120.9 vs. 2,078.5 +/- 298.4 ng/ml/6 h, p < 0.01) and gondadectomized rats treated with DHT (755.0 +/- 102. 3 vs. 4,539.3 +/- 1,670.6 ng/ml/6 h, p < 0.01). Furthermore, intravenous inhibin significantly reduced in vivo GH responses to GHRH (10 microg/kg, i.v.) in both intact and gonadectomized rats. These findings suggest a role of inhibin on in vivo GH secretion in the male rat.

Growth hormone (GH) response to GH-releasing peptide-6 in type 1 diabetic patients with exaggerated GH-releasing hormone-stimulated GH secretion.

In type 1 diabetes mellitus (DM 1), high GH basal levels and exaggerated GH responses to several stimuli, including GHRH, have been described. GH-releasing peptide-6 (GHRP-6) is a synthetic hexapeptide that specifically stimulates GH release, both in vitro and in vivo. The aim of this study was to evaluate the effects of GHRP-6 alone or in combination with GHRH on GH secretion in DM 1. Six type 1 diabetic males and six age-, sex-, and body mass index-matched control volunteers were studied. Each subject received GHRH (100 microg iv), GHRP-6 (90 microg iv), and GHRH plus GHRP-6 on three separate days. GH peak values were higher in DM 1 patients than in control volunteers, after GHRH (52.2+/-9.8 vs. 19.3+/-6.0 microg/L; P = 0.016), GHRP-6 (66.2+/-9.6 vs. 39.9+/-6.3 microg/L; P = 0.05), and GHRH plus GHRP-6 (81.8+/-4.4 vs. 53.7+/-8.2 microg/L; P = 0.01). An additive GH response to combined administration of these two peptides was observed in diabetic patients. Serum insulin-like growth factor (IGF)-1 levels were diminished in DM 1, with respect to normal subjects (145.2+/-21.5 vs. 269.7+/-42.0 microg/L; P = 0.01), whereas IGF-binding protein-3 levels were not significantly different between DM-1 and controls. In summary, GHRP-6 is a potent stimulus for GH secretion in DM 1. The combined administration of GHRP-6 plus GHRH constitutes the most powerful stimulus for GH secretion in DM 1. These patients exhibit a greater GH secretory capacity than normal subjects, probably caused by a diminished tone in the IGF-1 sustained negative feedback control exerted upon somatotroph responsiveness.

Influence of endogenous cholinergic tone and growth hormone-releasing peptide-6 on exercise induced growth hormone release.

OBJECTIVE: The neuroendocrine mechanisms of exercise-induced GH release remain incompletely understood. In this study we have investigated the influence of endogenous cholinergic tone and growth hormone-releasing peptide-6 (GHRP-6) on exercise induced GH release. DESIGN: Analysis of responses of serum GH to administration of pyridostigmine (PD, 120 mg, orally) or GHRP-6 (100 micrograms, i.v.), to exercise alone, and to the combinations of PD plus exercise or GHRP-6 plus exercise. An indirect estimation of the secretory pattern of GH was calculated by the deconvolution technique. Exercise was performed on a bicycle ergometer with a 20-minute workload near the individual lactate threshold of 4 mmol/l. The five tests were performed in random order after an overnight fast, and at least 3 days apart. SUBJECTS: Eleven healthy, non-obese male subjects (age 23.9 +/- 0.3 years, body mass index 23 +/- 0.7 kg/m2, VO2 max: 52.4 +/- 2.0 ml/min/kg body weight; mean +/- SEM) participated in this study. MEASUREMENTS: Serial blood samples from an indwelling catheter were taken before, during and after exercise for analysis of GH (IRMA), lactate (YSI 2300) and haematocrit (micromethod). RESULTS: Irrespective of the tests, peak values of GH were found between the 18th and 26th minute. The secretory pattern showed differences between the tests. Exercise alone induced relatively short lasting peaks of medium amplitude, whereas PD induced long lasting peaks with low amplitudes. PD plus exercise showed additive effects on the amplitude of GH peaks. GHRP-6 induced long lasting peaks with high amplitude, and GHRP-6 plus exercise also had additive effects on the amplitude. CONCLUSIONS: The increase in frequency and amplitude of GH peaks, which occurred during the GHRP-6 plus exercise, indicates that exercise-induced GH release is not mediated through an increment in the release of an endogenous GHRP-6-like ligand, favouring the possibility that exercise-induced GH release is mediated through an increase in endogenous GHRH release.

Retinoic acid inhibits in vivo thyroid-stimulating hormone secretion.

Retinoids are needed for normal growth and development. Retinoic acid (RA), an active metabolite of vitamin A, acts through nuclear receptors that belongs to the superfamily which also includes the T3 receptors and 1-25-dihydroxyvitamin D receptor. In order to assess whether RA is a regulator of in vivo thyroid-stimulating hormone (TSH) secretion, we studied the effect of RA administration on spontaneous basal TSH secretion and TSH responses to TRH in either euthyroid or hypothyroid rats. We found that rats treated with RA showed a decrease in spontaneous basal TSH levels and TSH responses to TRH. Similarly, RA administration to hypothyroid rats led to a decrease on TSH responses to TRH. Our data suggests that RA plays an important inhibitory role on in vivo secretion and this effect is unrelated to the thyroid status of the animals.

Role of glucocorticoids in the neuroregulation of growth hormone secretion.

Elevated glucocorticoid (GC) levels produce a marked impairment in somatic growth in both rodents and primates. In addition, GC play an important role in the regulation of growth hormone (GH) synthesis and secretion. Blunted GH response to stimulation tests in conditions of chronic exposure to excessive cortisol secretion or administration are well documented. In contrast, acute administration of GC to normal human subjects induces a transient increase in plasma GH levels. This dual action of GC on GH secretion is probably due to the fact that they act at different loci; i.e. in the regulation of GH transcription and GHRH and somatostatin receptors at the pituitary level as well as GHRH, somatostatin and GH receptor gene expression at the hypothalamic level.

Growth hormone releasing hexapeptide-6 (GHRP-6) test in the diagnosis of GH-deficiency.

Pituitary GH reserve can be assessed by substances that act directly at the somatotroph, such as GHRH, or by a variety of metabolic and neuropharmacological tests acting at the hypothalamic level, such as hypoglycemia, clonidine or L-Dopa. In order to evaluate GHRP-6 as a test of pituitary GH reserve, we studied GH responses of i.v. administered GHRP-6 in a group of short-statured children, as well as in a group of adults diagnosed with growth hormone deficiency (GHD) by conventional GH testing. Although we found that the GH response to GHRP-6 was lower in patients with GHD than in normal children, on an individual basis a considerable degree of overlap was observed between the two groups. In contrast, we found an almost complete blockade of GH response to either GHRP-6 or GHRH plus GHRP-6 in patients with pituitary stalk transection, suggesting that this could be a cost-effective test for the diagnosis of this condition. A similar finding was also obtained in GH response to the combined administration of GHRH plus GHRP-6 in patients with GHD of adult onset; this test may well prove valuable in the diagnosis of this clinical entity.

Basic and acidic fibroblast growth factor increase prolactin mRNA in a dose-dependent and specific manner in GH3 cells.

In the present report we have studied the effects of acidic and basic molecular forms of the fibroblast growth factor (aFGF, bFGF) on prolactin (PRL) mRNA production and PRL secretion in GH3 cells, a rat pituitary cell line, and their interactions with 17 beta-estradiol (beta E2). To meet this purpose we measured mRNA levels in the cells by both Northern blot and dot blot hybridization analysis, and rPRL immunoreactivity in the culture medium by specific RIA. We observed a marked increase in PRL mRNA levels following 24 h incubation with both basic and acidic FGF. This effect was dose-dependent, with maximal responses ranging between 300 and 600% above the control values. bFGF appeared to be much more potent than aFGF (10-50 times), considering the ED50 of the dose-response curves. Prior incubation with beta E2 (10(-8) M) produced an enhancement in the responses to low doses of bFGF and aFGF, but not to high doses, as revealed by dot-hybridization analysis. Northern blot analysis showed also that both aFGF and bFGF, may have a partially additive effect with beta E2, upon the mature form (1 kb) of rPRL mRNA in GH3 cells. Considering that bFGF is present at high levels in the pituitary, our results suggest that FGF could be a physiological regulatory factor for prolactin production and secretion.

Plasma growth hormone response to growth hormone-releasing hexapeptide (GH-RP-6) in children with short stature.

Eighteen children with short stature were evaluated for growth hormone (GH) reserve after pharmacological tests and a single iv injection of GH-RP-6. These children were divided into two groups: 10 were diagnosed as having idiopathic GH deficiency by classical stimulation tests (group A) and the remaining 8 (group B) were considered growth-retarded children with normal GH secretion, following conventional stimulation, but reduced endogenous GH secretion. The results were compared with a group of 12 normal children. As a group, patients in group A showed a lower GH response to GH-RP-6, while patients in group B had a similar response as normal controls. However, on an individual basis, a considerable degree of overlapping in responses among the three groups was evident. These data indicate that, on an individual basis, GH-RP-6 testing is not of diagnostic value in children suspected of having idiopathic GH deficiency.

Metabolic clearance rate of biosynthetic growth hormone after endogenous growth hormone suppression with a somatostatin analogue in chronic renal failure patients and control subjects.

OBJECTIVE: Several disturbances in the regulation of growth hormone secretion have been reported in chronic renal failure. The general assumption is that an altered hormonal clearance is at the basis of such GH alterations. Nevertheless, details of GH elimination kinetics in uraemia are not available. To clarify the role played by the kidney in its catabolism, GH elimination kinetics were studied in uraemic and control subjects after suppression of endogenous secretion of GH. DESIGN: In all subjects an analogue of somatostatin (octreotide 100 micrograms i.v.) was administered as a bolus before GH (-60 minutes). Sixty minutes later (0 min) biosynthetic GH (0.5 IU = 200 micrograms) was administered intravenously as a bolus. PATIENTS: Six chronic renal failure patients before dialysis and six matched normal volunteers. MEASUREMENTS: Plasma GH levels were measured by an immunoradiometric assay. RESULTS: In both groups, the GH elimination curve fitted a bi-exponential model. The calculated plasma volume and GH concentration at 0 minutes were similar in both groups, while uraemic patients presented a reduced distribution volume. In all parameters measuring GH elimination, chronic renal failure patients showed an impaired clearance. In fact, the area under the curve (mU/l/150 min) was 912.8 +/- 170.6 for controls and 3524.8 +/- 642.8 for chronic renal failure patients (P < 0.005). The GH half-life was 13.8 +/- 1.6 and 26.4 +/- 2.9 minutes for control and uraemic subjects respectively (P < 0.05), and the metabolic clearance rate MCR (ml/min/m2) was 265.3 +/- 50.6 for controls and 79.9 +/- 16.4 for uraemic patients (P < 0.05). The GH mean residence time (minutes) (MRT) calculated was 12.0 +/- 0.5 for controls and 31.8 +/- 4.6 for chronic renal failure patients (P < 0.05). CONCLUSIONS: Contrary to previous estimates, GH elimination kinetics follows a bi-exponential model and in normal subjects the GH half-life of the second phase is 13.8 +/- 1.6 minutes. Uraemic patients have impaired clearance of GH, suggesting that the kidney plays a role in GH disposal. However, the degree of impairment does not fully explain the alterations in GH secretion previously described in chronic renal failure.

Regulation of growth hormone secretion by the growth hormone releasing hexapeptide (GHRP-6).

Growth hormone (GH) secretion is regulated by a complex system of central and peripheral signals. Recently, a new GH-releasing hexapeptide (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2) called GHRP-6 which specifically releases GH has been studied. In the present work the mechanism of action of GHRP-6 has been addressed in experimental animal models as well as in obese subjects. GHRP-6 releases GH independently of the hypothalamic factors GHRH and somatostatin and is a powerful GH releaser in obesity.

Regulation of His-dTrp-Ala-Trp-dPhe-Lys-NH2 (GHRP-6)-induced GH secretion in the rat.

His-dTrp-Ala-Trp-dPhe,Lys-NH2(GHRP-6) is a synthetic compound that releases GH in a dose-response and specific manner in several species and that may well be related to an endogenous compound of similar structure. The aim of this study was to investigate the in vivo GH responses to GHRP-6 in pentobarbital anesthetized rats. Specifically and in order to avoid the influence of endogenous GHRH and somatostatin secretion we studied the GH responses to GHRP-6 in animals with surgical ablation of the hypothalamus, confirmed by histological assessment, as well as in hypophysectomyzed-transplanted rats bearing two hypophyses under the renal capsule. Since it has been previously reported that rats pretreated with GHRH (10 micrograms/kg i.p. every 12 h for 15 days) rather than saline-treated rats have greater GH responses to acutely administered GHRH, we compared the self-potentiating effect of chronic GH pretreatment with GHRP-6 (10 micrograms/kg i.p. every 12 h). Furthermore we also studied the influence of estrogens, glucocorticoids, free fatty acids (FFA) and bombesin on somatotroph responsiveness to GHRP-6 in intact rats. We found a greater GH response to GHRP-6 in rats that underwent a surgical ablation of the hypothalamus 36 h prior to the test than in sham-operated rats. A direct stimulatory effect of GHRP-6 on in vivo GH secretion was demonstrated by a clear GH response to GHRP-6 in hypophysectomyzed-transplanted rats. In addition, we found a similar response whether the animals were pretreated with GHRH or GHRP-6 over the previous 2 weeks. Finally, we found that both estrogen- and testosterone-treated rats have greater GH responses to GHRP-6 than untreated rats. On the other hand, chronic dexamethasone administration, acute elevation of circulating FFA levels and bombesin administration markedly inhibited GH responses to GHRP-6. In contrast to the effects exerted on GH responses to GHRP-6 estrogen administration led to a decrease in GH responses to GHRH while dexamethasone did not affect the GH responses to GHRH, highlighting a differential regulation of these hormones on somatotroph responsiveness to these peptides.

Effect of neurotensin on growth hormone release in vivo.

In order to investigate the mechanisms involved in the in vivo Growth Hormone (GH) response to Neurotensin (NT) we assessed the influence of estrogen status as well as the effect of passive immunization with antisomatostatin and anti-Growth Hormone-Releasing Hormone (GHRH) on NT-induced GH secretion in pentobarbital anesthetized rats. We found that, contrary to GH responses to GHRH, estrogen-treated rats (one single injection of 200 micrograms s.c. of estradiol valerate, 3 days before the experiment), exhibited markedly increased GH responses to different doses of NT (7.5, 15 and 30 micrograms/kg, i.v.). The stimulatory effect of NT (30 micrograms/kg) on estrogen-treated rats was similar in rats that received normal rabbit serum or passively immunized with antisomatostatin or anti-GHRH serum. In conclusion, estrogens play a facilitatory role on NT-induced GH release in the rat, which is exerted through a mechanism independent of hypothalamic GHRH or somatostatin release.