The effect of gastrin-releasing peptide on growth hormone secretion in the rat

Bombesin-like peptides stimulate growth hormone secretion mediated by the gastrin-releasing peptide receptor in cattle

This study was designed to determine the effects of bombesin-like peptides (BLPs) on the secretion of growth hormone (GH) and to characterize the receptor subtypes mediating these effects in cattle. Four experiments were conducted: (1) six steers were randomly assigned to receive intravenous (IV) bolus injections of 0, 0.2, 1.0, 12.5 and 50.0 μg/kg neuromedin C (NMC); (2) seven pre-weaned calves were IV injected with 1.0 μg/kg NMC; (3) six steers were IV injected with 2.5μg/kg bovine gastrin-releasing peptide (GRP), 1.0 μg/kg NMC combined with 20.0 μg/kg [d-Lys(3)]-GHRP-6 (an antagonist for the GH secretagogue receptor type 1a [GHS-R1a]), 1.0 μg/kg NMC combined with 20.0 μg/kg N-acetyl-GRP(20-26)-OCH(2)CH(3) (N-GRP-EE, an antagonist for the GRP receptor), 20.0 μg/kg N-GRP-EE alone, 1.0 μg/kg neuromedin B (NMB); and (4) four rats were IV injected 1.0 μg/kg NMC. A serial blood sample was collected before and after injection. Plasma GH levels dose-dependently increased at 5 min after NMC injection and the minimal effective dose was 1.0 μg/kg. Plasma GH level was elevated by GRP, but not by NMB. The NMC-induced elevation of GH was completely blocked by N-GRP-EE. The administration of NMC elevated GH level in pre-weaned calves but not in rats. Ghrelin level was unaffected by any treatments; and [d-Lys(3)]-GHRP-6 did not block the NMC-induced elevation of GH. The results indicate BLP-induced elevation of GH levels is mediated by the GRP receptor but not through a ghrelin/GHS-R1a pathway in cattle.

Bombesin regulation of adrenocorticotropin release from ovine anterior pituitary cells

Mammalian members of the bombesin-like peptide family (gastrin releasing peptides; GRP) have been localized in the ovine median eminence and in hypophysial-portal blood, suggesting a role in the regulation of anterior pituitary function. In this study we have shown that although bombesin cannot stimulate ACTH secretion alone, it potentiates release by ovine CRF, an effect blocked by the GRP receptor antagonist D-Tyr6bombesin (6-13) propylamide. Bombesin did not potentiate AVP-stimulated ACTH release; instead release was attenuated when bombesin was given at a 10-fold or greater molar excess over AVP, with no interaction seen at lower concentrations. We conclude that ovine corticotrophs express bombesin receptors, and that GRP may act in concert with other hypothalamic releasing factors to regulate ACTH secretion.

Role of dopamine in the inhibitory control of growth hormone and prolactin release by gastrin-releasing peptide

The ability of gastrin-releasing peptide to inhibit the release of growth hormone and prolactin by a hypothalamic mechanism has been previously reported. To determine whether or not these effects involved the hypothalamic dopaminergic neurons, ovariectomized female rats were pretreated with the dopamine receptor blocker, pimozide (0.6 mg/kg, SC), or the diluent 35 min prior to administration of gastrin-releasing peptide (2 micrograms in 2 microliters 0.9% NaCl) into the third ventricle. The elevation in plasma growth hormone in response to growth hormone-releasing factor (GRF; 1 microgram/kg, i.v.) was blocked by gastrin-releasing peptide as previously reported following pretreatment with the diluent, but occurred following the injection of pimozide, reversing the inhibitory action of gastrin-releasing peptide. Some control animals that were injected with the pimozide vehicle exhibited elevated plasma prolactin levels compared to levels in uninjected controls. Gastrin-releasing peptide significantly lowered plasma prolactin concentrations in this group of animals. As expected, plasma prolactin levels were elevated following pimozide treatment due to the removal of inhibitory dopaminergic control. Prolactin concentrations were unresponsive to gastrin-releasing peptide treatment in this paradigm and remained elevated throughout the 40-min duration of sampling. We interpret these data to indicate that gastrin-releasing peptide exerts its inhibitory actions on the release of growth hormone and prolactin through a dopaminergic mechanism. Gastrin-releasing peptide may elicit the release of dopamine from tuberoinfundibular neurons, which a) stimulates release of somatostatin, thus inhibiting the release of growth hormone, and b) reaches the pituitary directly via the hypophysial portal vessels to inhibit the release of prolactin from the lactotropes.

Characterization and distribution of bombesin binding sites in the goldfish hypothalamic feeding center and pituitary

Bombesin (BBS)/gastrin-releasing peptide (GRP) binding sites were characterized and their distribution examined in the goldfish brain and pituitary by radioligand binding and autoradiography. Binding of 125I-[Tyr4]-BBS-14 to tissue sections was found to be saturable, reversible, time-dependent and displaceable by BBS/GRP-like peptides. Analysis of saturable equilibrium binding revealed a one-site model fit with a Kd of 0.665 +/- 0.267 nM. This binding site displayed high affinity for members of the BBS subfamily of peptides, including GRP10 (Ki; 0.292 +/- 0.038 nM) and GRP27 (Ki; 2.034 +/- 1.597 nM), but showed no affinity for the BBS8-14 fragment. While an approximate 100-fold lower binding affinity was displayed by the binding site for neuromedin B (Ki; 6.15 +/- 28.2 nM), litorin was highly effective in displacing radiolabeled BBS binding (Ki; 1.469 +/- 0.427 nM). The localization of saturable and high affinity BBS/GRP binding sites in specific areas of the goldfish brain and pituitary generally revealed a similar anatomical distribution to BBS/GRP-like immunoreactive material reported previously by our laboratory. Quantitative densitometric analysis of radiolabeled BBS binding to brain nuclei and the pituitary revealed a moderate concentration of BBS/GRP binding sites in the hypothalamic feeding area, including the nucleus diffusus libi inferioris, nucleus recessus lateralis, nucleus lateral tuberis, and nucleus anterior tuberis. Other brain nuclei known to influence the brain feeding center which contained a high density of BBS/GRP binding sites included nuclei of the dorsal and ventro-medial telencephalon, the preoptic hypothalamus, and the optic tectum. High densities of BBS/GRP binding sites were also localized in the dorsal cerebellum, and nucleus habenularis. In the pituitary, BBS/GRP binding sites were present in high concentration in the neurointermediate lobe, with a relatively lower density localized in the pars distalis. The present study further supports a role for BBS/GRP-like peptides in the regulation of feeding behavior and anterior pituitary hormone secretion in teleosts.

Effects of BIM26226, a potent and specific bombesin receptor antagonist, on amylase release and binding of bombesin-like peptides to AR4-2J cells

Using AR4-2J rat pancreatic carcinoma cells, the effects of a novel bombesin (BN) receptor antagonist [D-F5Phe6, D-Ala11]BN(6-13)OMe (BIM26226) on BN- or GRP-stimulated amylase release and binding of radio-labeled bombesin-like peptides to these cells were examined and compared to [D-Phe6,Leu13 psi(CH2NH)Leu14]BN(6-14) (Psi Bn(6-14)), one of the most potent BN receptor antagonists presently known. BN and GRP both stimulated amylase release with EC50 values in the nanomolar range. Both antagonists were devoid of agonist activity when tested alone. BIM26226 was most potent, antagonizing BN- or GRP-stimulated amylase release with IC50 values in the nanomolar range, whereas Psi Bn(6-14) was approximately ten times less potent. With 125I-[Tyr15]GRP bound to these cells, the binding affinities were BIM26226 > GRP > Psi Bn(6-14) >> neuromedin B. BIM 22626 was not able to inhibit binding of radio-labeled CCK-33, gastrin-17 or VIP. These results suggest that BIM26226 is one of the most potent and specific bombesin receptor antagonists in vitro and seems to be a useful tool to define the physiologic role of GRP in vivo.

Determination of the structural requirements for the inhibitory action of gastrin-releasing peptide on growth hormone release

We have previously determined that gastrin-releasing peptide (GRP) suppresses plasma growth hormone (GH) levels and blocks the response to growth hormone releasing factor (GRF) following its injection into the third ventricle (3V) of conscious male rats. To determine the portion of the peptide required for this action, fragments of the peptide were injected at various doses to determine their effect on plasma growth hormone and the response to a test dose of GRF (1 microgram/kg IV). The GRP 1-16 lowered plasma GH and blocked the response to GRF following an intraventricular injection of 2 micrograms but not 0.2 microgram of the peptide. Carboxy-terminal fragments were more effective. AcGRP20-27 was partially effective at a 100 times lower dose of 2.0 ng, a result similar to that obtained with the related peptide, bombesin. Even the C-terminal heptapeptide AcGRP-27 was effective at a dose of 2 ng. The results indicate that the C-terminal heptapeptide is the minimal requirement for intraventricularly injected GRP to suppress plasma GH levels and block the response to GRF.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Bombesin acts to suppress feeding behavior and alter serum growth hormone in goldfish

The acute effects of a single injection of bombesin (BBS) on feeding behavior and serum growth hormone (GH) levels in goldfish were examined. When injected intraperitoneally (IP), BBS (0.5-100 ng/g) caused a dose-dependent decrease in food intake within 30 and 45 min of administration; maximal suppression was achieved at 50 ng/g BBS and was accompanied by an elevation in serum GH levels. Associated with IP injection of BBS was a pronounced spitting out behavior in which food pellets were taken into the oral cavity but immediately expelled. When injected into goldfish deprived of food for 72 h, 50 ng/g BBS was still potent in suppressing feeding behavior and increasing serum GH. Additionally, IP injection of BBS (10 or 100 ng/g) into groups of fish caused a significant increase in circulating serum GH levels at 1.5 h postinjection. Finally, when injected into the third brain ventricle (ICV), 60 ng/g BBS also caused a suppression in food intake and a concomitant increase in serum GH. Groups of fish injected ICV with 5 or 50 ng/g BBS also exhibited a graded increase in serum GH levels at 45 min postinjection. Overall, these data are the first to demonstrate in any lower vertebrate that a neuropeptide acts to suppress food intake and cause concomitant alterations in circulating serum GH levels, following either peripheral or central administration.

High potency of a new bombesin antagonist (RC-3095) in inhibiting serum gastrin levels; comparison of different routes of administration

This study was performed to evaluate the efficacy and duration of action of a new bombesin antagonist D-Tpi6,Leu13 psi (CH2NH)Leu14-bombesin (6-14) (RC-3095), given by different routes of administration, in suppressing gastrin releasing-peptide (GRP(14-27))-stimulated gastrin release in rats. First, we showed that GRP(14-27) itself was highly active when administered by different routes. GRP(14-27), given to rats at a dose of 25 micrograms/100 g b.w. significantly increased serum gastrin levels 3 and 6 min after intravenous and for more than 30 min after subcutaneous administration or pulmonary inhalation. RC-3095 was then injected subcutaneously, intravenously and also delivered by pulmonary inhalation at a dose of 10 micrograms/100 g b.w. in each case to seven male rats 2, 30, 60 or 120 min prior to i.v. administration of 5 micrograms GRP(14-27). RC-3095 administered 2 min prior to GRP(14-27) decreased the gastrin response to GRP(14-27), measured as area under the curve, by 81% in the intravenously injected group and 64% in the pulmonary inhalation group in the first 6 min. When GRP(14-27), was given 30 min after administration of RC-3095, the gastrin response was decreased by 52% in the subcutaneous group, 49% in the pulmonary inhalation group and 11% in the intravenous group during the first 6 min. RC-3095 delivered subcutaneously or by pulmonary inhalation 1 h before GRP(14-27) was also able to significantly inhibit gastrin release. Analysis of the data revealed that the bioavailability of RC-3095 given by the pulmonary inhalation route was about 69% of the s.c. route.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocrine effects of new bombesin/gastrin-releasing peptide antagonists in rats

Four new and specific pseudononapeptide bombesin/gastrin-releasing peptide (GRP) receptor antagonists, containing the D-forms of Trp or Trp analogue (Tpi) at position 6, were studied for their effects on the endocrine pancreas and GRP-(14-27)-induced gastrin release in pentobarbital-anesthetized rats. One of the analogues, D-Tpi6,Leu13-psi (CH2NH)Leu14-bombesin-(6-14) (RC-3095), was injected into the lateral brain ventricle just preceding intracerebroventricular administration of GRP-(14-27) to evaluate its antagonistic effect on GRP-induced serum growth hormone (GH) suppression. Analogues RC-3095, D-Trp6,Leu13-psi (CH2NH)Leu14-bombesin-(6-14) (RC-3125), and D-Trp6,Leu13-psi (CH2NH)Phe14-bombesin-(6-14) (RC-3420), but not D-Tpi6,Leu13-psi (CH2NH)Phe14-bombesin-(6-14) (RC-3105), significantly (P < 0.01) inhibited GRP-(14-27)-stimulated serum gastrin secretion. Analogues RC-3095, RC-3420, and RC-3105, but not RC-3125, demonstrated significant (P < 0.05) antagonistic activities on GRP-(14-27)-stimulated plasma glucagon secretion. Intracerebroventricular injection of RC-3095 (10 micrograms) immediately before GRP-(14-27) (1 microgram) completely prevented the GRP-(14-27)-induced serum GH suppression. These results indicate that 1) marked differences exist in the ability of these analogues to antagonize GRP-(14-27)-induced gastrin or glucagon release, suggesting the existence of different bombesin/GRP receptor subtypes, and 2) the central effect of bombesin/GRP on GH release from the pituitary is probably mediated through specific bombesin/GRP receptors.

Gastrin releasing peptide (GRP) is present in a GRP(1-27) form in anterior pituitary cells of the guinea pig

Immunohistochemical and chromatographic studies were performed on the guinea pig anterior pituitary gland with an antiserum recognizing an epitope within the gastrin releasing peptide (GRP) carboxyterminal amino acid sequence Val-Gly-His-Leu-Met-NH2. Within the anterior pituitary gland GRP-like immunoreactive cells were identified. The GRP-like immunoreactive cells were distributed heterogenously in the gland, predominantly located in ventral aspects of the anterior pituitary. Intracellularly, the immunoreactivity elements were identified as granula-like structures in the cytoplasma. To further characterize the peptide displaying GRP-like immunoreactivity within the pituitary cells, the GRP-like substances were analyzed by radioimmunoassay and gel filtration chromatography. Using this analytical approach it was determined that the guinea pig pituitary extract contained a peptide with characteristics similar to that of authentic porcine GRP(1-27). Only trace amounts of smaller C-terminal fragments were identified. These results indicate, in contrast to findings in other tissues, the GRP(1-27) is not further degraded into smaller peptide fragments.

Distribution of two distinct messenger ribonucleic acids encoding gastrin-releasing peptide in rat brain

There are two distinct mRNAs that encode the precursor to gastrin-releasing peptide (GRP) in rat brain. These two messages arise from separate transcription initiation sites located approximately 400 base pairs apart, which are presumably regulated by separate promoters. In the present study, we mapped the distribution of neurons containing GRP mRNAs by in situ hybridization using cRNA and synthetic DNA probes specific for the 1.5 kb GRP transcript and probes complementary to both the 1.5 kb and 1.1 kb transcripts. The distribution of neurons expressing GRP mRNA appears to be wider than that previously observed by immunohistochemical studies, suggesting an important functional role for this neuropeptide in a number of brain regions. We detected the 1.5 kb transcript only in cingulate cortex, Ammon's horn of the hippocampus and in subiculum. In contrast, the probe which hybridized to both GRP mRNAs labeled a broad range of brain areas, including those containing the 1.5 kb mRNA. These data suggest that the 1.5 kb mRNA encoding rat GRP is expressed only in specific parts of the limbic system, whereas the expression of the 1.1 kb GRP message is more widespread.

Evidence for a physiological role of hypothalamic gastrin-releasing peptide to suppress growth hormone and prolactin release in the rat

Gastrin-releasing peptide (GRP) is localized to hypothalamic neurons and is a potent inhibitor of basal and growth hormone (GH)-releasing factor-induced GH secretion in the rat. It also acts similarly to inhibit opiate- and stress-induced prolactin (PRL) release. To determine the physiological significance of the peptide in the control of the release of these two hormones, a highly specific antiserum against GRP was injected into the third brain ventricle to immunoneutralize hypothalamic GRP. The injection of the antiserum initially did not alter levels of the hormones; however, both PRL and GH levels in the plasma began to increase within 3 and 3.5 hr, respectively. They were still significantly elevated 24 hr after the injection. There was no change in the plasma levels of either hormone in animals injected intraventricularly with a similar volume of normal rabbit serum (NRS). Mean plasma GH levels 24 hr after antiserum injection were more than twice those of the NRS-injected controls, whereas the PRL concentrations were 14-fold higher in the antiserum injected as compared to the control NRS-injected animals. A second similar injection of antiserum 24 hr after the first administration resulted in a slight and transient further increase in both GH and PRL levels so that they were both significantly (P less than 0.001) higher than those of the animals given a second injection of NRS. The anti-GRP antiserum was highly specific for GRP by radioimmunoassay procedures and this antiserum produced positive immunostaining of GRP neuronal perikarya and terminals within discrete hypothalamic nuclei. Beaded fibers and terminals were observed in the suprachiasmatic nucleus (SCN) and the area lateral and dorsal to the SCN in the region of the periventricular nucleus (PeVN). GRP-positive perikarya were observed in the parvocellular neurons of the paraventricular nucleus. In addition, GRP-positive cell bodies were observed in the PeVN in close proximity to the third ventricle. Furthermore, the median eminence displayed no immunostaining for GRP, and all traces of positive staining were abolished by preabsorption of the antiserum with GRP-27 (30 micrograms/ml), confirming the specificity of the antiserum. The combined results with immunoneutralization of GRP and the immunostaining of GRP neuronal elements in the hypothalamus support the physiological role of this peptide in the inhibitory control of both GH and PRL release.

The role of brain peptides in neuroimmunomodulation

Since neuroimmunomodulation is brought about in part, at least, by secretion of pituitary hormones involved in stress and immune responses, we review briefly the hypothalamic control of the release of ACTH, growth hormone, and prolactin. The release of ACTH is controlled particularly by corticotropin-releasing factor (CRF), but vasopressin has intrinsic releasing activity and potentiates the action of CRF at both hypothalamic and pituitary levels. Oxytocin may even potentiate the action of CRF, but has little, if any, ACTH-releasing activity by itself. In addition, epinephrine may augment responses to the CRFs. In contrast, growth hormone is under dual control by growth-hormone-releasing factor (GRF) and somatostatin, and prolactin is under multifactorial control by a series of inhibitors and stimulators. Dopamine is accepted as a physiological prolactin-inhibiting factor (PIF), but probably GABA and possibly acetylcholine as well are PIFs. There is good evidence for a peptide PIF as well. There are a number of prolactin-releasing factors (PRFs) which include oxytocin, vasoactive intestinal polypeptide, PHI and TRH. Several other peptides can also release prolactin, including angiotensin II. In response to stress there is a complex interaction of peptides intrahypothalamically. CRF augments its own release by an ultra short-loop positive feedback, and there is negative ultra short-loop feedback of GRF and somatostatin. Vasopressin appears to augment CRF release as well as to act directly on the pituitary, and there are complex interactions of various peptides to influence prolactin and GH release.

Delta sleep-inducing peptide (DSIP) stimulates growth hormone (GH) release in the rat by hypothalamic and pituitary actions

To evaluate possible effects of delta sleep-inducing peptide on GH release, the peptide was micro-injected into conscious animals with third ventricular cannulae and blood samples were drawn from indwelling external jugular vein cannulae. Ovariectomized animals were used in order to eliminate gonadal steroid feedback. In the initial experiment, intraventricular injection of 5 micrograms of the peptide induced an elevation of GH which became significant by 30 min and persisted for the 120 min duration of the experiment after the injection. Diluent-injected animals showed a slight initial drop in GH and then no increase. The increase in plasma GH induced by the peptide was dose-related with a minimal effective dose of 0.1 microgram and a linear log-dose increase to a dose of 10 micrograms. This effect is presumably mediated hypothalamically via a dopaminergic mechanism since it could be blocked by pre-treatment of the animals with pimozide, a dopamine receptor blocker. Dispersed overnight, cultured pituitary cells from ovariectomized rats exhibited a dose-related increase in GH release in static incubations with DSIP. A response occurred with the lowest dose tested (10(-12) M) which increased to a maximum at 10(-10) M DSIP. The responses then declined at higher doses such that they were no longer significant at doses of 10(-7) and 10(-5) M. The increase even at the most effective dose was approximately 50% above the basal values. The results are consistent with the hypothesis that DSIP may be involved in GH release via a dopaminergic mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distribution of gastrin-releasing peptide- and somatostatin-like immunoreactivity in the rabbit hypothalamus

Regional distribution of gastrin-releasing peptide- (GRP) and somatostatin (SRIF)-like immunoreactivity in the discrete nuclei of the hypothalamus was examined in the rabbit according to Palkovits' microdissection method. GRP-like immunoreactivity (LI) was detected abundantly in the hypothalamus as compared with the cerebral cortex when measured by radioimmunoassay using the antiserum recognizing the C-terminal portion of synthetic porcine GRP. On gel-filtration chromatography of the hypothalamic extracts, two major peaks of GRP-LI were eluted; the peak with larger molecular size corresponded to synthetic porcine GRP1-27 and the smaller size to porcine GRP14-27. A concentration of GRP-LI was highest in the infundibular nuclei (IFN) as well as the ventromedial nuclei (VMN), and next high in the paraventricular nuclei (PVN), suprachiasmatic nuclei (SCN) and periventricular nuclei (PEV). The content of GRP-LI in the median eminence was not so much when compared with them. On the other hand, SRIF was localized in the highest concentration in the ME, followed by the VMN and IFN, as well as the PEV. The findings indicate that porcine GRP-LI exists in the hypothalamus of rabbits with characteristic regional distribution. Concurrent localization of GRP-LI and SRIF in some parts of the hypothalamus may suggest the interaction of both peptides in these areas under various physiological and pathological status.

Growth hormone neurosecretory dysfunction

The basis for understanding clinical disorders in the neuroregulation of GH secretion is derived from the complexity of the CNS-hypothalamic-pituitary axis. Studies in animals and humans demonstrate an anatomic, physiological and pharmacological evidence for neurosecretory control over GH secretion including neurohormones (GRH, somatostatin), neurotransmitters (dopaminergic, adrenergic, cholinergic, serotonergic, histaminergic, GABAergic), and neuropeptides (gut hormones, opioids, CRH, TRH, etc). The observation of a defect in the neuroregulatory control of GH secretion in CNS-irradiated humans and animals led to the hypothesis of a disorder in neurosecretion, GHND, as a cause for short stature. We speculate that in this heterogeneous group of children a disruption in the neurotransmitter-neurohormonal functional pathway could modify secretion ultimately expressed as poor growth velocity and short stature.