OBJECTIVE: To assess whether some of the alterations in energy homeostasis present in thyroid function disorders and GH deficiency could be mediated by ghrelin. DESIGN: To assess the influence of thyroid status on ghrelin, adult male Sprague-Dawley rats were treated with vehicle (euthyroid), amino-triazole (hypothyroid) or l-thyroxine (hyperthyroid). The influence of GH on ghrelin was assessed in wild-type (control) and GH-deficient (dwarf) Lewis rats. Evaluation of gastric ghrelin mRNA expression in the stomach was carried out by Northern blot. Circulating levels of ghrelin were measured by radioimmunoassay. RESULTS: Hypothyroidism resulted in an increase in gastric ghrelin mRNA levels (euthyroid: 100+/-3.2% vs hypothyroid: 127.3+/-6.5%; P<0.01), being decreased in hyperthyroid rats (70+/-5.4%; P<0.01). In keeping with these results, circulating plasma ghrelin levels were increased in hypothyroid (euthyroid: 124+/-11 pg/ml vs hypothyroid: 262+/-39 pg/ml; P<0.01) and decreased in hyperthyroid rats (75+/-6 pg/ml; P<0.01). Using an experimental model of GH deficiency, namely the dwarf rat, we found a decrease in gastric ghrelin mRNA levels (controls: 100+/-6% vs dwarf: 66+/-5.5%; P<0.01) and circulating plasma ghrelin levels (controls: 124+/-12 pg/ml vs dwarf: 81+/-7 pg/ml; P<0.01). CONCLUSION: This study provides the first evidence that ghrelin gene expression is influenced by thyroid hormones and GH status and provides further evidence that ghrelin may play an important role in the alteration of energy homeostasis and body weight present in these pathophysiological states.
JE Caminos, LM Seoane, SA Tovar, FF Casanueva and C Dieguez
E Carro, LM Seoane, R Senaris, FF Casanueva and C Dieguez
BACKGROUND: Leptin has recently been shown to have a stimulatory effect on basal GH secretion. However, the mechanisms by which leptin exert this effect are not yet clear. GHRH and GH-releasing peptide (GHRP)-6 are the two most potent GH secretagogues described to date. OBJECTIVE: To determine if leptin could also enhance in vivo GH responses to a maximal dose of GHRH. DESIGN: Leptin (10microg i.c.v.) or vehicle was administered at random before GHRH (10microg/kg i,v.) or GHRP-6 (50microg/kg i.v.), to freely-moving rats with food available ad libitum and to (48h) food-deprived rats. METHODS: Leptin and GH concentrations were measured by radioimmunoassay. Comparison between the different groups was assessed by the Mann-Whitney test. RESULTS: In comparison with fed rats, food-deprived rats showed a marked decrease in GH responses to GHRH as assessed by the area under the curve (5492+/-190ng/ml in fed rats and 1940+/-128ng/ml in fasted rats; P<0.05) and GHRP-6 (3695+/-450 in fed rats and 1432+/-229 in fasted rats; P<0.05). In comparison with its effects in vehicle-treated rats, leptin administered to food-deprived rats markedly increased GH responses to both GHRH (6625+/-613ng/ml; P<0.05) and GHRP-6 (5862+/-441ng/ml; P<0.05). CONCLUSIONS: These data suggest that the blunted GH response to GHRH and GHRP-6 in food-deprived rats is a functional and reversible state, and that the decreased leptin concentrations could be the primary defect responsible for the altered GH secretion in food-deprived rats.
LM Seoane, SA Tovar, D Perez, F Mallo, M Lopez, R Senaris, FF Casanueva and C Dieguez
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.
LM Seoane, S Tovar, R Baldelli, E Arvat, E Ghigo, FF Casanueva and C Dieguez
Ghrelin is a growth hormone-releasing acylated peptide from stomach. The purified peptide consist of 28 amino acids in which the serine 3 residue is n-octanoylated. Ghrelin has been reported to increase in vitro GH secretion as well as in vivo plasma GH levels in pentobarbital anaesthetized rats. The aim of this work was to characterize the stimulatory effect of Ghrelin on in vivo GH secretion in freely-moving rats. Furthermore, we compare the effect of Ghrelin with GHRH. In addition to vehicle, we administered different doses of Ghrelin (3 nmol/Kg, 12 nmol/Kg and 60 nmol/Kg); GHRH (3 nmol/Kg and 12 nmol/kg). Plasma GH levels were measured in blood samples taken at 5, 10, 15, 20, 30 and 45 min after their administration as an i.v. bolus at 0 min. Administration of Ghrelin led to an increase in plasma GH levels at all time-points tested (5, 10, 15, 20 and 30 min, P<0.01; and 45 min, P<0.05) in comparison to control untreated rats. A maximal stimulatory effect on plasma GH was observed following administration of 12 nmol/Kg of Ghrelin, the effect being similar to the one obtained with 60 nmol/Kg in terms of both AUC and mean peak GH levels. At the dose of 3 nmol/Kg GHRH and Ghrelin exhibited a similar stimulatory effect in term of both, AUC and mean peak GH levels. However following administration of a dose of 12 nmol/Kg, the effect of Ghrelin was much greater than the same dose of GHRH in terms of both AUC and mean peak GH levels. In summary, this study provides the first evidences that Ghrelin exert a marked stimulatory effect in plasma GH levels in freely-moving rats and provides further evidences that Ghrelin may play an important role in the physiological control of GH secretion.
L Pinilla, LM Seoane, L Gonzalez, E Carro, E Aguilar, FF Casanueva and C Dieguez
The aim of this study was to investigate the regulation of serum leptin levels by gender and gonadal steroid milieu. Thus, we measured serum leptin levels by radioimmunoassay in (a) intact male and female rats, (b) female rats at different stages of the estrous cycle and (c) ovariectomized or orchidectomized rats. Gonadectomized groups were or were not implanted with silastic capsules (10 or 30 mm in length, 1.519mm internal diameter; 3.06 mm external diameter) containing estradiol or testosterone and decapitated two weeks later. We found (i) intact female rats weighing 50 g, 250 g and 300 g exhibited higher serum leptin concentrations than intact male rats of similar body weight; (ii) leptin concentrations were not affected by the phase of the estrous cycle; (iii) two weeks after gonadectomy serum leptin concentrations increased in both male (from 4.47+/-1.87 to 8.76+/-1.24 ng/ml) and female (from 1.97+/-0.46 to 5.29+/-0.51 ng/ml) rats. The ovariectomy-induced increase in serum leptin levels was not dependent, at least completely, on changes in body weight since it could be observed when comparisons were made between ovariectomized rats and intact rats in estrus matched for body weight. In contrast the effect of orchidectomy on serum leptin levels appears to be dependent on changes in body weight since it was no longer observed when comparisons were made with a group of intact male rats matched for body weight. In conclusion, these results suggest that serum leptin concentrations are controlled by gonadal function either directly or as a consequence of changes in body weight.