Bombesin acts in the brain to decrease gastric vagal efferent discharge in rats

Preproghrelin-derived peptide, obestatin, fails to influence food intake in lean or obese rodents

OBJECTIVES

Obestatin has been initially characterized as a new peptide derived from the ghrelin precursor, which suppresses food intake and inhibits the orexigenic and prokinetic actions of ghrelin when injected peripherally or centrally in lean mice. However, reproducing these data remains controversial. Reasons for the disparity may be the use of different doses, routes, and animal models. We aimed to investigate the effects of peripheral and intracisternal (IC) injection of obestatin on feeding, gastric motility, and blood glucose in rats as well as in diet-induced obese (DIO) mice.

RESEARCH METHODS AND PROCEDURES

Food intake and gastric emptying of a semi-liquid caloric meal were measured after intraperitoneal (IP) injection of obestatin in rats and DIO mice. Gastric phasic motility and blood glucose were monitored in urethane-anesthetized rats after IC or intravenous (IV) injection of obestatin.

RESULTS

Obestatin injected intraperitoneally at doses ranging from 0.1 to 3 mg/kg influenced neither acute food intake nor gastric emptying in rats. Obestatin injected intravenously at 0.3 or 3 mg/kg and IC at 7.5 or 30 microg/rat modified neither fasted gastric phasic motility nor blood glucose levels, while ghrelin (30 microg/kg, IV) increased and vagotomy suppressed gastric motility, and an oligosomatostatin analog (3 microg/rat, IC) decreased blood glucose. Obestatin, injected intraperitoneally (0.3 mg/kg) in DIO mice, did not alter feeding response to a fast, while urocortin 1 (10 microg/kg, IP) induced a 73.3% inhibition at 2 hours.

DISCUSSION

Our data demonstrate that peripheral administration of obestatin did not modify food intake in rats or obese mice or gastric motor function in rats.

The CRF(1) receptor antagonist, NBI-35965, abolished the activation of locus coeruleus neurons induced by colorectal distension and intracisternal CRF in rats

Corticotropin-releasing factor (CRF) receptors have been reported to play a role in tonic colorectal distension (CRD)-induced activation of locus coeruleus (LC) neurons. We examined the influence of repeated phasic CRDs and intracisternal (ic) CRF on the spontaneous discharge rate of LC neurons in chloral hydrate-anesthetized rats and the role of CRF receptors using the nonselective CRF(1)/CRF(2) antagonist, astressin, and the water-soluble CRF(1) receptor antagonist, NBI-35965. Two consecutive phasic CRDs (43.7 +/- 1.1 mm Hg, 30 s each) at a 10-min interval increased LC activity to 184.9 +/- 15% and 171.9 +/- 12.2%, respectively. There was no difference in magnitude, onset (within 1 s), and duration (5-7 min) of the LC responses between the 1st and 2nd CRDs. CRF (300 ng/rat, ic) injected 10 min after the 2nd CRD increased LC activity to 191.1 +/- 11.2%. Astressin (3 mug, ic) completely blocked the 2nd CRD- and ic CRF-induced LC activation. Neither ic vehicle nor astressin influenced basal LC neuronal activity. NBI-35965 (10 mg/kg, iv) prevented the 2nd CRD- and ic CRF-induced LC neuronal activation, while at 5 mg significantly reduced the LC response to the 2nd CRD by 80%, but did not block that of ic CRF injected 30 min later. These findings indicate a primary role of brain CRF interacting with CRF(1) receptors in mediating the activation of LC neurons in response to a phasic CRD within the nociceptive range (>40 mm Hg). This activation may have relevance to irritable bowel syndrome characterized by lower pain threshold to CRD and hypervigilance to colonic input.

GRP mediates an inhibitory response of gut-related vagal motor neurons to PVN stimulation

We previously characterized neurons in the dorsal motor nucleus of the vagus (DMNV) that were modulated by electrical stimulation of the PVN and by gastrointestinal distention. Bombesin has been identified in a subset of PVN neurons projecting to the DMNV. It is currently unknown whether this neurotransmitter is involved in descending communication from PVN to DMNV neurons. In this study we determined whether the specific bombesin antagonist, N-acetyl-GRP(20-26), influenced (1) the basal firing rate of DMNV neurons and (2) the response to electrical current stimulation of the PVN. Our results indicate that N-acetyl-GRP(20-26), significantly attenuated the inhibitory response of DMNV neurons to PVN stimulation. These results provide a possible mechanism by which bombesin regulates gastrointestinal function, body temperature homeostasis, and feeding behaviors.

Microinjection of bombesin into the ventrolateral reticular formation inhibits peripherally stimulated gastric acid secretion through spinal pathways in rats

Bombesin injected into the cisterna magna potently inhibits gastric acid secretion stimulated by intravenous infusion of pentagastrin. Sites in the medulla oblongata where bombesin acts to suppress gastric acid secretion were investigated in urethane-anesthetized rats with gastric cannula. Bombesin or vehicle was injected into the medullary parenchyma or intracisternally (i.c.) 60 min after the start of an intravenous pentagastrin infusion; gastric acid secretion was monitored every 10 min for 20 min before and 150 min after the start of pentagastrin. Bombesin (0.2, 0.6 or 6.2 pmol) microinjected into the ventrolateral reticular formation (VLRF) inhibited dose-dependently the net acid response to pentagastrin by 40.8+/-11.1, 75.4+/-12.8 and 96.7+/-19.4%, respectively, at the 40-50 min period after microinjection compared with the vehicle group. Bombesin action in the VLRF was long lasting (96% inhibition still observed at 90 min after 6.2 pmol), and completely abolished by cervical spinal cord transection at the C6 level. By contrast, bombesin injected i.c. at 0.2 or 0.6 pmol had no effect while at 6.2 pmol, there was a 79.0+/-3.9% peak inhibition of pentagastrin-stimulated acid secretion. Bombesin (6.2 pmol) injected into the dorsal motor nucleus reduced the acid response to pentagastrin by 29%. The parvicellular and gigantocellular reticular nuclei were not responsive to bombesin. These results indicate that bombesin acts in the VLRF to inhibit pentagastrin-stimulated gastric acid secretion through spinal pathways, suggesting a potential role of medullary VLRF area in the sympathetic control of gastric acid secretion.

Bombesin and the brain-gut axis

Bombesin is the first peptide shown to act in the brain to influence gastric function and the most potent peptide to inhibit acid secretion when injected into the cerebrospinal fluid (CSF) in rats and dogs. Bombesin responsive sites include specific hypothalamic nuclei (paraventricular nucleus, preoptic area and anterior hypothalamus), the dorsal vagal complex as well as spinal sites at T9-T10. The antisecretory effect of central bombesin encompasses a variety of endocrine/paracrine (gastrin, histamine) or neuronal stimulants. Bombesin into the CSF induces an integrated gastric response (increase in bicarbonate, and mucus, inhibition of acid, pepsin, vagally mediated contractions) enhancing the resistance of the mucosa to injury through autonomic pathways. The physiological significance of central action of bombesin on gastric function is still to be unraveled.

Synthesis of nitric oxide in the dorsal motor nucleus of the vagus mediates the inhibition of gastric acid secretion by central bombesin

1. Central administration of bombesin inhibits gastric acid production independently of the centrally or peripherally-acting stimuli employed. This study evaluates the role and location of the cerebral nitric oxide (NO) implicated in the inhibitory effect of central bombesin on in vivo rat gastric acid secretion, as induced by distension with 15 cm H2O, insulin (0.75 u.i. kg-1 i.p.) TRH (1.2 microg kg-1, i.c.) or pentagastrin (100 microg kg-1, i.p.). 2. The acid-inhibitory effect of i.c. bombesin (40 ng kg-1) was prevented by prior administration of L-NAME (80 microg kg-1) in the dorsal motor nucleus of the vagus (DMN). This dose of L-NAME when administered into the nucleus of the tractus solitarious (NTS) did not influence the effects of bombesin. Administration of L-arginine (400 microg kg-1) into the DMN restored the acid-inhibitory effect of i.c. bombesin in animals treated with L-NAME. 3. Microinjection of bombesin (12 ng kg-1) into the paraventricular nucleus of the hypothalamus (PvN) inhibits acid secretion stimulated by pentagastrin. This inhibitory effect was prevented by a previous injection of L-NAME (80 microg kg-1) into the DMN. 4. The release of NO in the DMN following i.c. administration of bombesin was confirmed by in vivo electrochemical detection. 5. Administration by microdialysis in the DMN of the NO-donor SNAP (25 mM in 1.5 microl min-1) into the DMN inhibits pentagastrin-stimulated gastric acid secretion. 6. The present study suggests that nNOS-containing neurons in the DMN have an inhibitory role in the control of gastric acid responses.

Intracisternal sauvagine is more potent than corticotropin-releasing factor to decrease gastric vagal efferent activity in rats

Consecutive intracisternal (ic) injections of corticotropin-releasing factor (CRF) (21, 63, and 126 pmol, ic) or sauvagine (2.1, 6.3, and 21 pmol, ic) decreased gastric vagal efferent multiunit discharge (GVED) to 82%, 75% and 69% and 71%, 40% and 21%, respectively, from preinjection basal levels (taken as 100%). The inhibitory action was dose related (magnitude and duration of the response, 7-45 min). The CRF antagonist, [D-Phe12,Nle21,38,Calpha-MeLeu37]-rCRF12-4 1 (6.25 nmol, ic) increased GVED by 43.5+/-4.3% and blocked the decrease in GVED induced by CRF (21 pmol, ic) for >90 min with a complete recovery after 3 h. Vehicles (injected intracisternally) had no effect. These data indicate that: 1) CRF injected intracisternally decreases GVED through the activation of CRF receptors and sauvagine is more potent than CRF to inhibit GVED; and 2) endogenous CRF exerts an inhibitory tone on basal GVED in urethane-anesthetized rats undergoing surgery.

Ultrastructure of bombesin-like immunoreactive nerve terminals in the nucleus of the solitary tract and the dorsal motor nucleus

Bombesin (gastrin-releasing peptide 14-27) inhibits gastric function and feeding when microinjected into the nucleus of the solitary tract (NTS)/dorsal motor nucleus of the vagus (DMV) complex. We performed a preembedding immunoelectron microscopic study in rats to describe the bombesin containing nerve terminals and to characterize their postsynaptic structures. 228 bombesin-L1 nerve terminals which made synaptic contacts in the NTS/DMV complex were studied. Labeling was heaviest over dense core vesicles and lighter over small clear vesicles. The dense core vesicles were typically located along the plasmalemma away from the synaptic face, a finding that is typical of neuropeptide containing nerve terminals. The postsynaptic structures were most often medium sized dendrites (56%) and small sized dendrites (27%), with similar percentages in the NTS and DMV. In the DMV, synapses on cell bodies (8%) were more frequent than in the NTS (1%). In the NTS, synapses on dendritic spines (10%) were more frequent than in the DMV (4%). Only a single axo-axonal contact was identified. These findings add to the increasing body of evidence that bombesin is a neurotransmitter/neuromodulator in the NTS/DMV complex. Bombesin rarely makes presynaptic (axo-axonal) contacts that might inhibit the release of excitatory neurotransmitters, but rather makes postsynaptic contacts potentially effecting vagal motoneurons.

Intracisternal TRH and RX 77368 potently activate gastric vagal efferent discharge in rats

The influence of intracisternal (ic) TRH and the stable TRH analog, RX 77368, on gastric vagal efferent discharge (GVED) was investigated in urethane-anesthetized rats. Consecutive IC injections of TRH (3, 30, and 300 ng) at 60 min intervals stimulated dose dependently multi-unit GVED with a peak increase of 90 +/- 21%, 127 +/- 18% and 145 +/- 16% respectively. In two separate studies, IC injection of RX 77368 at 1.5 or 15 ng stimulated multi-unit GVED by 142 +/- 24% and 244 +/- 95% respectively. Saline injection IC had no effect on GVED. RX 77368 (1.5 ng, ic) action was long lasting (84 +/- 13 min) compared with TRH (3 ng: 44 +/- 7 min). Single-unit analysis also showed that 13 of 13 units responded to ic RX 77368 (1.5 ng) by an increase in activity. These data indicate that low doses of TRH injected ic stimulate vagal efferent outflow to the rat stomach and that RX 77368 action is more potent than TRH.

Distribution of bombesin-like immunoreactivity in the nucleus of the solitary tract and dorsal motor nucleus of the rat and human: colocalization with tyrosine hydroxylase

Bombesin is a peptide neurotransmitter/neuromodulator with important autonomic and behavioral effects that are mediated, at least in part, by bombesin-containing neurons and nerve terminals in the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (DMV). The distribution of bombesin-like immunoreactive nerve terminals/fibers and cell bodies in relation to a viscerotopically relevant subnuclear map of this region was studied by using an immunoperoxidase technique. In the rat, bombesin fiber/terminal staining was heavy in an area that included the medial subnucleus of the NTS and the DMV over their full rostral-caudal extent. Distinctly void of staining were the gelatinous, central, and rostral commissural subnuclei and the periventricular area of the NTS, regions to which gastric, esophageal, cecal, and colonic primary afferents preferentially project. The caudal commissural and dorsal subnuclei had light bombesin fiber/terminal staining, as did the intermediate, interstitial, ventral, and ventrolateral subnuclei. With colchicine pretreatment, numerous cell bodies were stained in the medial and dorsal subnuclei, with fewer neurons in the caudal commissural, intermediate, interstitial, ventral, and ventrolateral subnuclei. Bombesin-like immunoreactive neurons were found in numerous other areas of the brain, including the ventrolateral medulla, the parabrachial nucleus, and the medial geniculate body. In the human NTS/DMV complex, the distribution of bombesin fiber/terminal staining was very similar to the rat. In addition, occasional bombesin-like immunoreactive neurons were labeled in a number of subnuclei, with clusters of neurons labeled in the dorsal and ventrolateral subnuclei. Double immunofluorescence studies in rat demonstrated that bombesin colocalizes with tyrosine hydroxylase in neurons in the dorsal subnucleus of the NTS. Bombesin does not colocalize with tyrosine hydroxylase in any other location in the brain. In conclusion, the distribution of bombesin in the NTS adheres to a viscerotopically relevant map. This is the anatomical substrate for the effects of bombesin on gastrointestinal function and satiety and its likely role in concluding a meal. The anatomic similarities between human and rat suggest that bombesin has similar functions in the visceral neuraxis of these two species. Bombesin coexists with catecholamines in neurons in the dorsal subnucleus, which likely mediate, in part, the cardiovascular effects of bombesin.

Centrifugal gastric vagal afferent unit activities: another source of gastric "efferent" control

Our previous studies indicated that in rats about 10% of ventral gastric vagal efferent discharges do not originate from supracervical neural elements. To determine the origin of these efferent activities, an in vitro subdiaphragmatic vagus nerve-esophagus preparation was used. Action potentials with the same amplitude and waveform, and behaving 'all or none' characteristic are considered to be recorded from a nerve fiber and defined as an unit activity. Because these centrifugal unit activities were recorded from the proximal cut end of the ventral gastric vagal strands, they are ostensibly considered to be efferent activities. However, about 50% of unit action potential samples (21 out of 40) behave like unit activities recorded from mechanoreceptive afferent fibers. They have spot-like or diffuse mechanoreceptive fields on the subdiaphragmatic esophagus. When these receptive fields were stimulated the sensory nerve terminals in the fields generate afferent unit action potentials. These afferent potentials not only propagate orthodromically to the central nerve system, but also can be transmitted centrifugally to the gastric branches of the same vagal afferent neuron. Together with the efferent discharges of gastric vagal motor neurons, these centrifugal sensory potentials can be intercepted from the proximal cut end of gastric vagal nerve strands at gastroesophageal junction. Three types of mechanoresponsive centrifugal afferent unit activities were observed: rapidly adapting (n = 8), with or without after-discharge; slowly adapting (n = 8), with or without after-discharge, and initial high frequency followed by a plateau, with long-lasting after-discharge (n = 5). Of the tested units (n = 24), 25% were either activated or inhibited by esophageal inflation and 23% (n = 22) by esophageal deflation. It is evident that not all centrifugal unit action potentials recorded from the proximal cut end of gastric vagal nerve strands are generated from the vagal motor neurons, the recorded centrifugal unit activities may contain antidromic unit action potentials generated from the esophageal collateral branches of the gastric vagal afferent nerve fibers. These results suggest that gastric vagal afferent neurons possess collateral branches innervating the esophagus, activation of esophageal terminals may exert an effect on the gastric terminals via collateral reflex, analogous to the 'axon reflex' mechanism.

Local and commissural neuropeptide-containing projections of the nucleus of the solitary tract to the dorsal vagal complex in the pigeon

The neuropeptide content of neurons of the nucleus of the solitary tract (NTS), which have local and commissural projections to the dorsal motor nucleus of the vagus (DMNX) and to NTS, were demonstrated in the pigeon (Columba livia) by using a combined fluorescein-bead retrograde-transport-immunofluorescence technique. The specific peptides studied were bombesin, cholecystokinin, enkephalin, galanin, neuropeptide Y, neurotensin, and substance P. Perikarya immunoreactive for bombesin were located in medial tier subnuclei of NTS and the caudal NTS. Most galanin- and substance P-immunoreactive cells were found in subnucleus medialis ventralis. Cells immunoreactive for neuropeptide Y were found in the medial tier of NTS and in the lateral tier, especially in subnucleus lateralis dorsalis intermedius. The majority of enkephalin- and neurotensin-immunoreactive cells were found centrally in subnuclei medialis dorsalis and medialis intermedius. Cells immunoreactive for cholecystokinin were located in subnuclei lateralis dorsalis pars anterior, medialis superficialis, and the caudal NTS. Based on the presence of retrogradely labeled cells, numerous neurons of the medial tier of NTS, but extremely few lateral tier NTS neurons, had projections to the ipsilateral and contralateral DMNX and NTS. The number of retrogradely labeled NTS cells was always greater ipsilaterally than contralaterally. The percentages of peptide-immunoreactive NTS cells that projected to the ipsilateral and contralateral DMNX were in the ranges of 29-61% and 10-48%, respectively. The percentages of peptide-immunoreactive NTS cells that projected to the contralateral NTS ranged from 13 to 60%. Peptide-immunoreactive NTS cells that have local and commissural projections to DMNX and NTS may act as interneurons in vagovagal reflex pathways and in the integration of visceral sensory and forebrain input to NTS and DMNX.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.