C. Netti

Evidence for a Central Inhibitory Role of Growth Hormone Secretagogues and Ghrelin on Gastric Acid Secretion in Conscious Rats

We examined the possible central and peripheral effects of synthetic growth hormone secretagogues (GHS), hexarelin (Hexa) and EP 40737 (D-Thr-D-Trp (2-Me)-Ala- Trp-D-Phe-Lys-NH2), and of their endogenous counterpart, ghrelin, on gastric acid secretion. The compounds were administered intracerebroventricularly (i.c.v.) or subcutaneously (s.c.) in conscious male rats and the volume of gastric secretion and gastric acid output were examined 3 h after pylorus ligation (Shay-test). Central Hexa, EP 40737 and ghrelin administration (from 0.1 pmol to 1 nmol/rat, i.c.v.) significantly inhibited gastric acid secretion. The maximum inhibitory effect on gastric acid output was detected at the dose of 10 pmol/rat, i.c.v. for Hexa (–51.3%), of 100 pmol/rat, i.c.v. for EP 40737 (–70%) and of 1 pmol/rat, i.c.v. for ghrelin (–60%). All peptides were less effective at the highest dose used (1 nmol/rat, i.c.v.). Hexa, EP 40737 and ghrelin injected s.c. did not modify gastric acid secretion. The inhibitory action of Hexa on gastric acid secretion seems to involve brain somatostatinergic system since Hexa (10 pmol/rat, i.c.v.) did not inhibit gastric acid secretion in rats pretreated (4 h before) with cysteamine (300 mg/kg, s.c.), a depletor of endogenous somatostatin. These results show that synthetic GHS and ghrelin exert a central long-lasting inhibitory effect on gastric acid secretion in conscious pylorus-ligated rats. The fact that very low doses of ghrelin and GHS inhibit gastric secretion, provide evidence for a tonic inhibitory role of the peptides in the central control of gastric secretory function.

The central GABAergic system and control of food intake under different experimental conditions.

Intracerebroventricular injections of gamma-aminobutyric acid (GABA) and of the GABA-transaminase inhibitor, ethanolamine-O-sulphate (EOS), decreased the food intake of freely-fed (GABA and EOS) and food-deprived rats (EOS). The effect, still evident 24 h after treatment, was not decreased by the GABA receptor-blocker bicuculline. In contrast, intracerebroventricular injections of the GABA receptor-agonist, muscimol, caused an increase in food intake of freely-fed rats that was antagonized by bicuculline. The eating of animals receiving only bicuculline was stimulated in free-feeding and depressed in food-deprived conditions. These opposite results suggest that muscimol binds preferentially to some GABA receptors, probably those within the satiety-controlling areas (i.e. ventromedial hypothalamus), and that bicuculline influences mainly those postsynaptic neurons where GABAergic inputs prevail.These observations and the data from EOS- and GABA-treated rats provide evidence for involvement of GABA neurons in the regulation of feeding behaviour. The balance of the different effects produced in each of these areas by this modulation appears to be a decrease in feeding behaviour.

Protection by amylin of gastric erosions induced by indomethacin or ethanol in rats

1 The effect of amylin on gastric ulcers induced by oral administration of indomethacin (Indo, 20 mg kg−1 at a dosing volume of 5 ml) or ethanol 50% (EtOH, 1 ml/rat) was investigated in conscious rats. 2 Amylin given intracerebroventricularly (0.22, 0.66 and 2.2 μg/rat, i.c.v.) demonstrated a dose‐dependent cytoprotective effect against both Indo and EtOH‐induced ulcers. In contrast, amylin, given subcutaneously at doses effective in inhibiting acid gastric secretion (2.5, 10 and 40 μg kg−1, s.c), did not show any cytoprotective effect. 3 The interaction between amylin and endogenous nitric oxide (NO) in the maintenance of gastric mucosal integrity was investigated by pretreating the rats with a selective inhibitor of NO‐synthesis, NG‐nitro‐L‐arginine methyl ester (l‐NAME, 25 and 70 mg kg−1, s.c). Administration of L‐NAME to rats did not significantly increase the degree of the Indo‐induced ulcer index and was not able to remove the protective effect of amylin on Indo‐induced ulcers, thus excluding a role for endogenous NO in mediating the protective effect of this peptide. 4 To determine whether the cytoprotective effect of amylin was mediated by endogenous prostaglandins, we studied the effect of amylin (2.2 μg/rat, i.c.v.) on EtOH‐induced ulcers in rats pretreated with Indo (10 mg kg−1, s.c.) to inhibit prostanoid biosynthesis; Indo was injected 30 min before amylin and EtOH after a further 30 min. Pretreatment with Indo did not significantly increase the ulcer index induced by EtOH but counteracted the ability of amylin to prevent the ulcer formation. 5 These findings suggest that amylin exerts a gastroprotective activity that is not strictly related to inhibition of acid gastric secretion and can be partly explained through a prostaglandin‐dependent mechanism mediated by receptors for the peptide in the brain. Amylin might be considered as a new brain‐gut peptide.

Ghrelin inhibits inflammatory pain in rats: Involvement of the opioid system

This study examined the effects of intracerebroventricular (i.c.v.), intraperitoneal (i.p.) or intraplantar (i.pl.) administration of ghrelin, the endogenous ligand of the growth hormone secretagogue receptor, in the development of hyperalgesia and edema induced by intraplantar injection of carrageenan in rats. Central ghrelin (4 ng to 4 microg/rat) given 5 min before carrageenan produced a dose-related reversal of carrageenan-induced mechanical hyperalgesia measured by Randall-Selitto test with an ED50 of 81.7 ng/rat. Ghrelin at the dose of 4 microg/rat i.c.v. was also effective in inhibiting edema. When ghrelin (4 microg/rat i.c.v.) was administered 150 min after carrageenan, it failed to modify either hyperalgesia or the paw volume. Given i.p., 30 min before carrageenan, ghrelin (20-160 microg/kg) induced a significant dose-dependent inhibition of hyperalgesia with an ED50 of 77 microg/kg and a slight reduction of edema. Intraplantar ghrelin (40 ng to 12 microg/rat) did not significantly modify both the hyperalgesic and edematous activities of carrageenan. The anti-hyperalgesic and anti-edematous effects of ghrelin (4 microg/rat i.c.v.) were reversed by naloxone (10 microg/rat i.c.v.). Systemic administration of the peripheral selective opioid antagonist, naloxone methiodide (3 mg/kg s.c.), did not antagonize antinociception elicited by i.p. ghrelin. Overall these data indicate that ghrelin exerts an inhibitory role on inflammatory pain through an interaction with the central opioid system.

Central ghrelin gastroprotection involves nitric oxide/prostaglandin cross-talk

Ghrelin, a gut‐brain peptide, is considered a gastroprotective factor in gastric mucosa. We investigated the role of prostaglandins (PG) and the possible interplay between PGs and nitric oxide (NO) in ghrelin gastroprotection against ethanol (EtOH)‐induced gastric lesions.

Intracerebroventricular acute and chronic administration of obestatin minimally affect food intake but not weight gain in the rat.

We studied the effect of the acute central administration of obestatin on food intake and body weight in short-term starved male rats, and those of 28-day continuous intracerebroventricular (icv) infusion of obestatin in free feeding rats. In 16-h starved rats, obestatin induced a trend toward a reduction of food intake that did not reach statistical significance. In fed rats, the icv infusion of obestatin significantly decreased food consumption in the first day of treatment; but the anorexigenic effect of obestatin vanished thereafter. Interestingly, the body weight of rats infused for 28 days with obestatin was superimposable to that of the respective control at all time intervals. In all, our results indicate that the anorexigenic effect of obestatin is of little account and that the peptide does not modify energy metabolism in the long-term administration.

Amylin given by central and peripheral routes inhibits acid gastric secretion

The effect of rat amylin on acid gastric secretion in the pylorus-ligated, unanesthetized rat system (Shay test) was examined. Amylin administered peripherally (2.5, 5, 10, 40, 100, or 160 micrograms/kg, SC) or intracerebroventricularly (1.5, 2.7, or 5 micrograms/rat, ICV) decreased acid gastric secretion in a dose-dependent manner. Central administration of amylin gave a stronger suppression of gastric secretion than peripheral injection. In addition, ICV-injected amylin inhibited insulin-stimulated acid gastric secretion and was effective in suppressing acid gastric secretion in rats depleted of somatostatin by pretreatment with cysteamine. This study suggests that amylin may participate in the central regulation of acid gastric secretion and indicates a possible biological function of amylin as a gastrointestinal peptide.

Calcitonin inhibition of physiological and stimulated prolactin secretion in rats

Abstract The administration of salmon Calcitonin (sCT) intravenously (2.5 or 10 μg/kg) or into the lateral cerebral ventricles (2.5 or 25 ng/rat, i.c.v.) of unanaestized male rats induced clearcut decreases in plasma prolactin(PRL) levels. The i.c.v. injection of one of these doses of sCT (25 ng/rat) into rats with median eminence lesions was completely ineffective, while it induced a dramatic decrease in plasma PRL levels of sham-operated rats. Morphine- and heat stress-stimulated PRL levels were also abolished by sCT injection (250 ng/rat i.c.v.). The sCT-induced decrease in PRL levels was completely overcome by haloperidol, a dopamine-receptor blocker. We conclude that sCT may affect PRL secretion via an hypothalamic system, probably involving dopaminergic neurons. The present results indicate that CT, like many others peptides, may affect PRL secretion, directly or indirectly, even though further research is necessary to determine whether this effect has pharmacological or physiological importance.

Calcitonin gene-related peptide: antinociceptive activity in rats, comparison with calcitonin

The effects of synthetic human calcitonin gene-related peptide (CGRP) on nociceptive response were evaluated in rats by two behavioral tests (tail-flick and hot-plate) and by electrophysiological recording of the firing of thalamic neurons evoked by peripheral noxious mechanical stimuli. CGRP was administered intracerebroventricularly (i.c.v.) and its effects were compared with that of salmon calcitonin (sCT). In the tail-flick test, CGRP (0.25, 2.5 and 5 micrograms/rat) dose-dependently increased response latencies, whereas sCT (0.125, 2.5, 5 and 10 micrograms/rat) did not. Conversely, in the hot-plate test CGRP was effective in enhancing response latencies only at the highest dose of 10 micrograms/rat, while sCT (0.125, 0.25 and 2.5 micrograms/rat) inhibited the hot-plate response dose-dependently. In electrophysiological studies, CGRP (2.5 micrograms/rat, i.c.v.) completely inhibited the evoked neuronal thalamic firing and the same dose of sCT induced only a partial reduction. Furthermore, the antinociceptive effects of CGRP in the tail-flick test and in the electrophysiological studies were not prevented by naloxone. These results demonstrate that central administration of CGRP is effective in inhibiting nociceptive responses and its action like that of sCT does not involve an opioid mechanism. The differences in the antinociceptive profiles of CGRP and sCT suggest that the inhibitory effects of these peptides may involve different neuronal pathways.

Evidence for a Role of the GHS‐R1a Receptors in Ghrelin Inhibition of Gastric Acid Secretion in the Rat

Ghrelin, the endogenous ligand of the GH secretagogue receptor (GHS‐R) has been previously shown to inhibit gastric acid secretion in pylorus‐ligated rats. Two isoforms of GHS‐R have been identified: GHS‐R1a and GHS‐R1b. The present study aimed: (i) to characterise the type of GHS‐R involved in the central gastric inhibitory activity of ghrelin by using des‐octanoyl ghrelin, and synthetic GHS‐R1a agonist (EP1572) and antagonist (D‐Lys3‐GHRP‐6) and (ii) to investigate the relationship between ghrelin and cortistatin (CST) in the control of gastric acid secretion by using the natural neuropeptide CST‐14 and the synthetic octapeptide CST‐8. The specific interactions of all the compounds with GHS‐R1a were determined by comparing their ability to displace labelled ghrelin or somatostatin from its receptors on rat hypothalamic membranes or on rat cardiomyocyte, respectively. Intracerebroventricular administration of 0.01 and 1 nmol/rat des‐octanoyl ghrelin did not affect gastric acid secretion in pylorus‐ligated rats, whereas EP1572 either i.c.v. (0.01–1 nmol/rat) or i.p. (10 and 20 nmol/kg) inhibited acid gastric secretion. Preteatment with D‐Lys3GHRP‐6 (3 nmol/rat, i.c.v.) was able to remove the inhibitory action of ghrelin (0.01 nmol/rat, i.c.v.) on gastric acid volume and acid output, thus indicating that the type 1a GHS‐R likely mediates the gastric inhibitory action of ghrelin. This is supported by binding data showing that D‐Lys3GHRP‐6, but not des‐octanoyl ghrelin, binds to hypothalamic GHS‐R. CST‐14 (1 nmol/rat, i.c.v.) did not affect either basal or ghrelin inhibition of gastric acid secretion. CST‐8 (1 nmol/rat, i.c.v.) was able to counteract the gastric ghrelin response. The observation that CST‐14 binds both GHR‐S and somatostatin receptors, whereas CST‐8 specifically displaces only ghrelin binding, indicates that CST‐8 behaves as a GHS‐R1a antagonist.