Actions of neurotensin and (Gln4)-neurotensin on isolated tissues

Hungry for relief: Potential for neurotensin to address comorbid obesity and pain

Chronic pain and obesity frequently occur together. An ideal therapy would alleviate pain without weight gain, and most optimally, could promote weight loss. The neuropeptide neurotensin (Nts) has been separately implicated in reducing weight and pain but could it be a common actionable target for both pain and obesity? Here we review the current knowledge of Nts signaling via its receptors in modulating body weight and pain processing. Evaluating the mechanism by which Nts impacts ingestive behavior, body weight, and analgesia has potential to identify common physiologic mechanisms underlying weight and pain comorbidities, and whether Nts may be common actionable targets for both.

Histamine-enhanced contractile responses of gastric smooth muscle via interstitial cells of Cajal in the Syrian hamster

BACKGROUND

Gastric motility is controlled by the autonomic and enteric nervous systems and by interstitial cells of Cajal (ICCs). Although histamine is known to be released from enterochromaffin-like cells in the gastric mucosa, its regulatory roles in gastric motility are still controversial. Therefore, we investigated the functional roles of histamine in gastric motility.

METHODS

Stomach preparations from hamsters were used because the stomach of hamsters can be easily separated into the forestomach and the glandular stomach. A whole preparation of the stomach was mounted in a Magnus tube, and mechanical responses were recorded using a force transducer.

KEY RESULTS

Exogenous application of histamine had little effect on contractile activity of the glandular stomach. In contrast, the monoamine evoked regular, periodic contractions in the forestomach. An H1 receptor agonist reproduced the contractile responses and an H1 receptor antagonist blocked histamine-evoked contractions. Atropine and tetrodotoxin did not affect the histamine-evoked contractions. Pretreatment with drugs that inhibit the activity of ICCs abolished the effects of histamine.

CONCLUSION & INFERENCES

The findings suggest that histamine regulates gastric motility by acting on ICCs via H1 receptors in the hamster. The remarkable ability of histamine to induce rhythmic contractions would be useful for treatment of gastric dysmotility.

Importance of the enteric nervous system in the control of the migrating motility complex

The migrating motility complex (MMC), a cyclical phenomenon, represents rudimentary motility pattern in the gastrointestinal tract. The MMC is observed mostly in the stomach and gut of man and numerous animal species. It contains three or four phases, while its phase III is the most characteristic. The mechanisms controlling the pattern are unclear in part, although the neural control of the MMC seems crucial. The main goal of this article was to discuss the importance of intrinsic innervation of the gastrointestinal tract in MMC initiation, migration, and cessation to emphasize that various MMC-controlling mechanisms act through the enteric nervous system. Two main neural regions, central and peripheral, are able to initiate the MMC. However, central regulation of the MMC may require cooperation with the enteric nervous system. When central mechanisms are not active, the MMC can be initiated peripherally in any region of the small bowel. The enteric nervous system affects the MMC in response to the luminal stimuli which can contribute to the initiation and cessation of the cycle, and it may evoke irregular phasic contractions within the pattern. The hormonal regulators released from the endocrine cells may exert a modulatory effect upon the MMC mostly through the enteric nervous system. Their central action could also be considered. It can be concluded that the enteric nervous system is involved in the great majority of the MMC-controlling mechanisms.

Mechanisms of regulation of neurotensin receptors

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.

Membrane potential and current responses to neurotensin in the longitudinal muscle of the rectum of the fowl

1. The effects of neurotensin (NT) on membrane potential and membrane current of the longitudinal smooth muscle of chicken rectum were investigated by intracellular recording and whole-cell voltage clamp. 2. NT (3 nM-1.2 microM), when applied via the bathing medium, produced a concentration-dependent membrane depolarization with an EC50 of 18 +/- 2 nM (n = 7) which was accompanied by an increase in the membrane conductance. The effect was biphasic: an initial, rapid depolarization reached a peak within 2-3 min and then declined to a lower but still elevated level which was sustained until washout. 3. Excitatory junction potentials (e.j.ps), which were non-adrenergic non-cholinergic (NANC) in nature, were decreased in amplitude and total duration in the presence of NT (0.6 microM). The depression of the e.j.p. was due mainly to the reduction of the membrane resistance. 4. When NT was applied locally by means of pressure ejection from a micropipette containing NT, some cells responded with a membrane depolarization and some failed to respond, whereas e.j.ps could invariably be elicited from all of them. 5. In single muscle cells enzymatically isolated from the muscle and dialyzed under voltage clamp at -50 mV with a CsCl-rich solution, NT (5 or 10 microM) produced an inward current. NT-induced inward currents were obtained with inclusion of 10 mM EGTA in the pipette solution and their reversal potential was around 0 mV. In cells dialyzed under voltage clamp at 0 mV with a KCl-rich solution, NT (5 microM) produced a brief outward current followed by abolition of spontaneous transient outward currents.6. The present results suggest that the membrane depolarization, which may arise from activation of non-selective cation channels, and release of calcium from internal stores produced by neurotensin are responsible for its contractile activity in the longitudinal smooth muscle of chicken rectum. Further, the depolarizing effect may provide support for the involvement of NT in the NANC transmission in this preparation.

Neurotensin: dual effect on the motor activity of rat duodenum

The effects of neurotensin on mechanical activity of rat duodenum were investigated using an isometric-isovolumic preparation. Neurotensin (1 pM to 10 nM) induced a concentration-dependent, tetrodotoxin (TTX)-insensitive fall in both endoluminal pressure and isometric tension. At higher concentrations of neurotensin (1 nM to 1 microM) the relaxation was followed by a concentration-dependent TTX-insensitive contraction, detected only by an increase in endoluminal pressure. Different concentrations of neurotensin were required to desensitize the relaxant and the contractile actions of the neuropeptide. The relaxation was antagonized by apamin, while the contractile response was blocked by nifedipine. Neurotensin, when tested separately on longitudinal and circular muscular strips, caused relaxation of the longitudinal strips. Circular strips showed contractions in response to neurotensin, following an inhibitory phase, if the strips were spontaneously or pharmacologically activated. The results suggest the presence of two sets of neurotensin receptors with a differential localization between the two muscular layers in rat duodenum.

Neurotensin-related peptides inhibit spontaneous longitudinal contractions of porcine distal jejunum

The tridecapeptide neurotensin (NT) and its C-terminal homologs, including xenopsin (XP) and neuromedin N (NM-N), reduced the amplitude of spontaneous contractions in longitudinal smooth muscle strips from the porcine distal jejunum in vitro. The rank order of potency (IC50 in nM) was XP (0.1) greater than NT (0.9) approximately avian XP (1.0) greater than NM-N (1.6), which could not be explained on the basis of differential peptide degradation. Tachyphylaxis and cross-tachyphylaxis were observed after repeated NT and XP addition to muscle strips. The action of NT was mimicked by norepinephrine (NE), but not by opioid peptides, somatostatin, or vasoactive intestinal peptide. NE was nearly 100-fold less potent than NT and did not produce a state of tachyphylaxis to NT. The effects of NT and NE were unaltered by the neuronal conduction blocker tetrodotoxin (70 nM). However, the actions of NE, unlike those of NT, were reduced by the alpha-adrenoceptor blocker phentolamine (70 nM), the K(+)-channel blocker apamin (7 nM) and the Ca2(+)-channel blocker verapamil (0.7 microM). These results suggest that NT and related peptides, through a nonadrenergic mechanism, interact with smooth muscle receptors to modulate jejunoileal motor function in the pig.

Biological potency of neurotensin metabolites in vivo: importance of alcohol 'fixation' of blood

Neurotensin (NT), a 13-amino acid peptide, is released from the ileum following a meal. It is metabolized principally by the kidney and in the circulation to N-terminal fragments and apparently rapidly degraded C-terminal fragments. The present study was designed to compare the biological activity (plasma pancreatic polypeptide response) and the clearance kinetics of NT(1-13), the N-terminal fragment NT(1-11) and the C-terminal fragment NT(8-13). To measure accurately the circulating concentrations of short-lived NT fragments in the circulation, a method was devised of collecting blood directly into alcohol ('alcohol fixation'). The alcohol fixation procedure prevented the post-collection losses of the C-terminal fragment NT(8-13) and established that, based on blood levels achieved, NT(8-13) had 70% of the pancreatic polypeptide stimulating potency of NT(1-13). Nevertheless, the metabolic clearance rate of NT(8-13) was about sevenfold higher than the intact molecule, NT(1-13), suggesting that circulating C-terminal fragments have a minor physiological role. When the N-terminal fragment NT(1-11) was infused, there was no sustained effect on pancreatic polypeptide secretion although it was cleared at a rate similar to that of NT(1-13). It is concluded that the use of alcohol fixation prevents post-collection losses of NT fragments and enables true biological potencies of short-lived fragments to be assessed. The biological activity of NT resides in the C-terminus which, once split from the protective N-terminus, is rapidly degraded in the circulation. The remaining intact but inactive N-terminus is relatively stable.

Neurotensin and prostaglandin interactions in smooth muscle of the guinea pig stomach

Neurotensin, xenopsin and neuromedin N had a biphasic effect (initial small relaxation followed by a sustained contraction) on the motility of longitudinal muscle strips from the guinea pig gastric corpus. Kinetensin was without effect. Tetrodotoxin, adrenaline, histamine, 5-hydroxytryptamine, opioid peptides, angiotensin II and antagonists of acetylcholine, and desensitization of the strips to bradykinin did not modify the action of neurotensin. The contraction phase was inhibited in a concentration-dependent manner by indomethacin and acetylsalicylate, demonstrating that this activity of neurotensin was dependent on prostaglandin synthesis. The preparation responded to exogenous prostaglandins E1, E2, F1 alpha and F2 alpha with concentration-dependent contractions. The relaxation phase was abolished by verapamil and apamin, indicating the presence of inhibitory neurotensin receptors on smooth muscle cells that are linked to ionic channels.

Effects of guinea pig neurotensin ([Ser7]neurotensin) on gastrointestinal smooth muscle

Guinea pig neurotensin differs from other mammalian neurotensin by the substitution of proline by serine at position 7. [Ser7]neurotensin produced a concentration-dependent contraction of the guinea pig oesophagus and rat fundus, a biphasic effect (initial relaxation followed by more sustained contraction) on the guinea pig stomach corpus, duodenum and longitudinal muscle of the ileum and a relaxation of the guinea pig colon and rat ileum. The potencies (EC50) and % maximum contraction compared with the maximum effect produced by either histamine or acetylcholine of [Ser7]neurotensin and neurotensin were not significantly different in any tissue studied.

Effects of neurotensin on electrical and mechanical properties of smooth muscles in longitudinal and circular layers of the guinea-pig ileum

Effects of neurotensin (NT) on the electrical and mechanical activities of longitudinal and circular muscles of the guinea-pig ileum were investigated using the microelectrode and isometric tension recording methods. In longitudinal muscles, the resting membrane potential was not affected by NT (0.1-30 nmol/l), but NT did provoke the contraction when applied in concentrations over 1 nmol/l. TTX (0.1 mumol/l) neither modified the resting membrane potential nor the contraction evoked by NT, under condition of pretreatment with alpha- and beta-adrenoceptor blockers. In circular muscles, NT (over 0.1 nmol/l) consistently hyperpolarized the membrane and increased the ionic conductance. The hyperpolarization appeared with a transient hyperpolarization, which gradually declined with a long time course. Using apamin and various concentrations of Ca, the NT-induced hyperpolarization was classified into two subtypes; fast and slow. The former was composed of maximum hyperpolarization due to activations of the Ca independent K channel, and the latter was composed of late hyperpolarization, due to activations of the Ca dependent K channel. During the NT-induced hyperpolarization in circular muscles, the amplitude of non-adrenergic, non-cholinergic inhibitory junction potential (i.j.p.) evoked by field stimulation was reduced. This reduction induced by 0.5 nmol/l NT was mainly due to hyperpolarization of the membrane, and that observed in a high concentration of NT (3 nmol/l) was directly involved in ionic mechanisms contributing to the generation of i.j.p. In circular muscles, NT (over 3 nmol/l) did relax the tissue pre-contracted with 17.8 mmol/l K, but NT (below 30 nmol/l) did not relax the tissue pre-contracted by 39.6 mmol/l K.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of neurotensin on the smooth muscle of the chicken crop

1. The effect of neurotensin was studied in the isolated smooth muscle of the chicken crop. 2. This peptide induced concentration-dependent (10(-10)-10(-8) mol/l) contractions of muscle strips. 3. The response to neurotensin was not modified after incubation of the preparation with atropine, indomethacin, naloxone and tetrodotoxin. The neurotensin-exerted contractions are therefore probably myogenic. 4. These properties of neurotensin indicate a possible role for this peptide in the control of crop motility.

Influence of histamine and prostaglandin on desensitization to neurotensin in rat blood pressure

Triphasic depressor-pressor-depressor blood pressure responses to neurotensin (NT: 1.67 micrograms/kg i.v.) in anesthetized rats were not elicited when the second dose of NT was administered 20 min after the first injection. Pretreatment of animals with histamine markedly reduced the depressor response to NT, and vice versa. The triphasic blood pressure pattern remained unaffected with acetylcholine and serotonin treatment, and hypotensive effects of acetylcholine and serotonin were not modified by NT. Attenuation of depressor response induced by the second injection of NT was antagonized by pretreatment with prostaglandin synthesis inhibitors such as indomethacin, mefenamic acid and acetylsalicylic acid. These results suggest that histamine and prostaglandins play a role in the development of desensitization to NT in rat blood pressure.

Characterization of neurotensin binding to rat gastric smooth muscle receptor sites

The binding of monoiodo 125I-Trp11-neurotensin to purified rat gastric fundus smooth muscle plasma membranes was characterized. Specific binding of ligand in subcellular fractions from rat fundus smooth muscle showed a distribution that paralleled that of several plasma membrane marker enzymes. 125I-Trp11-neurotensin binding to smooth muscle plasma membranes at 25 degrees C was maximal at 30 min, reversible and saturable. Scatchard analysis of equilibrium data indicated the existence of two classes of binding sites with dissociation constants (Kd) of 56 pmol and 1.92 nM, and corresponding binding capacities (Bmax) of 6.6 fmol/mg and 11.4 fmol/mg of membrane protein. Analogues and fragments of neurotensin competed for 125I-Trp11-neurotensin binding with a rank order of potency similar to that previously reported for their contracting effect in rat fundus strips. Na+ decreased in a concentration dependent manner the binding of labelled ligand to the high affinity site. At 100 mM, Na+ induced a 6-fold increase in the IC50 of neurotensin for inhibition of 125I-Trp11-neurotensin binding. At this concentration of Na+, the IC50 for neurotensin was 1 nM, a value close to the Kd of the low affinity site.

Isolation of a proform of porcine secretin by ion-exchange and reversed-phase high-performance liquid chromatography

A polypeptide with secretin-like bioactivity has been isolated from upper small intestinal porcine tissue by ion-exchange and reversed-phase high-performance liquid chromatography (HPLC). The purification was followed by determination of biological activity. Its elution position in the ion-exchange HPLC indicated that it was less basic than secretin. Amino acid analysis showed that it contained an additional glycine residue as compared to secretin. Digestions by trypsin and subtilisin established that the polypeptide was a variant form of secretin in which the previously known secretin is extended C-terminally by a glycine residue.

[Neurotensin--what is known about its role as a hormone in the gastrointestinal tract?]

Neurotensin is a tridecapeptide originally isolated and characterized from bovine hypothalamus and later, in identical form, from bovine and human intestine. In the rat about 85% of immunoreactive neurotensin is found in the gut and about 10% in the brain. When an antibody specific for the amino terminal region of neurotensin was used the highest concentrations were found in the mucosa of the ileum, while an antibody specific for the biologically active region, the carboxyl terminus, also detected large amounts in the mucosa of the upper gastrointestinal tract. After a meal neurotensin - as measured by carboxyl terminal antibodies - rises after 5 min, a time in which the chymus has not yet reached the ileum, the main source of whole neurotensin. It is therefore possible that the carboxyl terminal molecules of neurotensin, found in the upper gastrointestinal tract, play an important physiological role. In plasma, neurotensin is rapidly degraded into smaller amino terminal and therefore biologically inactive molecules. Increases of carboxyl terminal neurotensin have been found in plasma in only a very few studies. The nature of this immunoreactive material has not yet been established. Therefore, the physiological role of neurotensin as a circulating hormone is unknown. Potential actions of neurotensin include thermoregulation, regulation of hormone release from brain (pituitary hormones) and gut (glucagon, insulin, somatostatin, pancreatic polypeptide), increase of vascular permeability, vasodilatation, inhibition of gastric acid secretion, stimulation of pancreatic secretion and changes of gut motility from the fasting to the fed type.

Evidence that (Glu4)-neurotensin is the naturally occurring neurotensin in plasma

Neurotensin (NT) is a tridecapeptide (less than Glu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu-OH). It has been suggested that the naturally occurring neurotensin in (Gln4)-NT and that it can be hydrolysed to (Glu4)-NT during extraction. The aim of the present study was to determine whether the neurotensin found in human and rat plasma after fat ingestion is (Gln4)-NT or (Glu4)-NT. Two neurotensin antisera are described that have high avidity for (Glu4)-NT but low avidity for (Gln4)-NT. (Gln4)-NT could be separated from (Glu4)-NT by ion-exchange chromatography. The plasma samples were analyzed without prior extraction to avoid hydrolysis of the naturally occurring peptide. The results indicate that neurotensin in unextracted human and rat plasma is present as (Glu4)-NT, as originally proposed by Carraway and Leeman [4].

Localization of neurotensin binding sites in rat kidney

[3H]Neurotensin ([3H]NT) appears to bind specifically to a single class of sites in slide-mounted rat kidney sections (KD = 8.3 nM; Bmax = 31.6 fmol/mg tissue). Bound [3H]NT can be displaced by nonradioactive NT and a series of its fragments and analogues with relative potencies that correlate well (r = 0.91; p less than 0.005) to their potencies in the rat stomach strip bioassay. These results suggest that NT receptors are similar in both systems. However, they are probably slightly different from those present in the guinea pig atria (r = 0.78; p less than 0.1). We visualized these sites by using the tritium-sensitive LKB film technique analysed by computerized densitometry. [3H]NT binding sites are highly concentrated in the renal cortex while low levels are observed in the renal medulla. The possible physiological and/or pathophysiological significances of the presence of [3H]NT binding sites in the kidney are discussed.

Neurotensin inhibition of gastric exocrine secretions in the cat

The gastric exocrine inhibitory activities of neurotensin were characterized in conscious cats prepared with gastric fistulae. Neurotensin was a potent inhibitor of pentagastrin-stimulated pepsin secretion (ID50, approx. 0.3 mumol . kg-1 . h-1) but was approximately 60 times less potent against acid secretion. Neurotensin did not significantly reduce submaximal histamine-stimulated acid or pepsin secretions. the total 2 h acid and pepsin outputs in response to insulin-hypoglycaemia were not reduced by neurotensin, although the peak 15-min outputs were reduced. The reduction in peak secretion was possibly related to neurotensin antagonism of the ability of insulin to lower blood glucose concentrations. Neurotensin alone was not hyperglycaemic when given as an intravenous infusion. Two C-terminal fragments of neurotensin, the dodecapeptide and nonapeptide, inhibited pentagastrin-stimulated pepsin secretion, but were less potent than neurotensin. The observations with the C-terminal fragments indicate that the major determinants of gastric exocrine inhibitory activity of neurotensin reside in its C-terminal; this agrees with observations on other biological activities of neurotensin. The reduced potency of the dodecapeptide indicates the importance of the N-terminal pyroglutamyl residue for full gastric exocrine inhibitory activity.

A multihormonal tumor of the pancreas producing neurotensin

In a pancreatic adenoma approximately 78.7% of the endocrine cells reacted specifically with antisera to neurotensin, 17.5% to gastrin, 2.8% to pancreatic polypeptide, and 1% to glucagon. The electron microscope revealed that the majority of the endocrine cells were N-cells--morphologically similar to the ileal N-cells which are known to represent the neurotensin-producing cells. Neurotensin was extracted from the tumor and identified by Sephadex, ion-exchange, and high-pressure liquid chromatography. Gastrin, pancreatic polypeptide, and glucagon cells were also identified by the electron microscope; the peptides were extracted and demonstrated by chromatography. The serum concentrations of these hormones were elevated. After total gastrectomy which was necessary because of Zollinger-Ellison syndrome, a jejunoesophageal alkaline reflux, reaching the upper esophagus appeared. As intravenous infusion of synthetic neurotensin in rats caused an increase of luminal enteric pressure, it is suggested that severe jejunoesophageal reflux after gastrectomy may be a clinical feature of a neurotensinoma.

Neurotensin is a potent inhibitor of guinea-pig colon contractile activity

The investigation concerned the effects of neurotensin (NT) on the mechanical activity of segments of guinea-pig proximal colon. The peptide, in concentrations ranging from 0.1 nM to 100 nM, induced a quick relaxation of the preparations which was followed upon washout by a rebound contraction. Tetrodotoxin had no effect on either the NT-induced relaxation or the rebound contraction. Blocking agents of alpha- and beta-adrenoceptors were also without effect. The bee venom toxin apamin inhibited and even converted to contractions the NT-induced relaxations in the guinea-pig colon. Concentration-response curves for the inhibitory effect of NT, adrenaline and ATP indicated that NT (IC50 0.32 nM) was 100 times more potent than adrenaline (IC50 nM) and 50 000 times more potent than ATP (IC50 16 microM). The data show that NT exerts a potent myogenic relaxing effect on the longitudinal smooth muscle of the guinea-pig proximal colon. The properties of this effect suggest the presence of high affinity NT receptors on colonic smooth muscle cells and this points to a possible role for NT as a modulator of guinea-pig proximal colon motility.

Effects of neurotensin and neurotensin analogues on the migrating myoelectrical complexes in the small intestine of rats

The purpose of the present experiments was to study the effect of neurotensin and neurotensin analogues on the migrating myoelectrical complexes in the small intestine of rats. Four bipolar electrodes were implanted into the muscular wall of the small intestine. The electrodes were placed 5, 15, 25 and 35 cm distal to the pylorus. 7-10 days after the operation the animals were fasted for 48 h with free access to water. Some experiments were performed on conscious rats and in others the rats were anesthetized with pentobarbital, 30 mg/kg. I.v. infusion of either neurotensin (NT) or (Gln4)-neurotensin at doses of 1.8, 3.6 and 7.1 pmol X kg-1 X min-1 abolished the migrating myoelectric complexes, which were replaced by increased spiking activity along the whole length of the small intestine from which activity was recorded. The changes in myoelectrical activity were observed within 2-4 min after commencement of the infusion. The activity returned to control levels within 5-15 min after the end of the infusion period. The neurotensin sequences NT 9-13, NT 8-13, NT 4 -13, NT 1-9 and (Gln4)-NT 1-11 did not induce any changes in the electrical activity in the small intestine. The effects of NT and (Gln4)-neurotensin on the myoelectrical activity in the small intestine were indistinguishable. The changes induced by NT or (Gln4)-NT resemble those found after the ingestion of food. The present data indicate that the intact NT sequence, rather than smaller NT fragments, is necessary to induce changes in myoelectrical activity in the small intestine.

Plasma concentration of neurotensin-like immunoreactivity (NTLI) and lower esophageal sphincter (LES) pressure in man following infusion of (Gln4)-neurotensin

(Gln4)-neurotensin was infused i.v. for 5 to 70 min at 3 different infusion rates (6, 12 and 18 pmol X kg-1 X min-1, respectively) in 19 male volunteers, aged 26-47. The plasma concentration of neurotensin-like immunoreactivity (NTLI), the lower esophageal sphincter (LES) pressure, blood pressure, heart rate. ECG and blood glucose concentration were measured. The volunteers did not report any subjective effects during the infusion. Following infusion periods of 30 min or more the volunteers often reported bowel movements starting 5 min or more after cessation of the infusion. Neither blood pressure nor heart rate changed significantly. No changes were noted in the continuous ECG or in the blood glucose concentration. Apparent steady state levels of about 300 pM NTLI were reached at about 40 min during infusion of 12 pmol X kg-1 X min-1 (Gln4)-neurotensin. In all volunteers the LES pressure was significantly reduced within 5 min of starting the infusion. In 6 volunteers 12 pmol X kg-1 X min-1 (Gln4)-neurotensin was infused i.v. for 5 min. The LES pressure decreased significantly (P less than 0.01) from 13.7 +/- 1.3 mmHg to 5.3 +/- 0.8 mmHg. The decrease in the LES pressure occurred at plasma NTLI concentrations of approximately 50 pM, i.e. at levels below those obtained in man after a meal or the ingestion of fat. The present data further support the hypothesis that in man plasma neurotensin, or a neurotensin metabolite is an endocrine hormone involved in the postprandial regulation of the motor functions of the gastrointestinal tract.

Neurotensin-induced contractions in venous smooth muscle

Synthetic neurotensin was shown to be a powerful contractant of the isolated rat portal vein. Concentration-dependent increases in basal tension and frequency of spontaneous contractions were obtained in response to single dose (10-1000 nM, final bath concentrations). The responses reached maxima within 2 min and differed in this respect from comparable responses to angiotensin II, substance P and bradykinin, which were maximal after 2-4 min. The "apparent affinity" for neurotensin (pD2 = 7.5 +/- 0.3, S.D., n = 10) was about 10 times less than for angiotensin II while higher than for both the other peptides and for norepinephrine. The "intrinsic activity" was 0.75 (+/- 0.13, S.D., n = 6) relative to angiotensin II during the first 2 min of the response. A pronounced degree of tachyphylaxis was observed; repetition of the neurotensin dose at the lower range of concentrations (1-10 nM) within 2-4 min was completely ineffective. The results suggest that there is a specific neurotensin receptor in the tissue and that neurotensin-induced contractions may play a part in redistributing the blood flow in the gastrointestinal region following release of this peptide from the ileum into the portal circulation.

Distribution patterns of neurotensin-like immunoreactive cells in the gastro-intestinal tract of higher vertebrates

The endocrine system of the gastro-intestinal tract of selected species representing the five higher vertebrate classes was investigated with reference to occurrence and distribution of neurotensin-like immunoreactive cells. Using antibodies against C-terminal and N-terminal fragments of neurotensin and against the C-terminal sequence of xenopsin it was demonstrated that the intestine of all species studied contains endocrine, neurotensin-like immunoreactive cells. However, large differences in localization and frequency of these neurotensin-like immunoreactive cells were found. Except for a teleostean fish, neurotensin-like immunoreactive cells in the gastro-intestinal tract were more frequent in non-mamalian vertebrates than in mammals. In contrast to mammals, where the highest density of neurotensin-like immunoreactive cells was present in the ileal mucosa, in the non-mammalian vertebrates studied the corresponding cells were most abundant in the pyloric-duodenal junction. The exact mapping of neurotensin-like immunoreactive cells is presented throughout the entire gastro-intestinal tract of six species (Rattus, Coturnix, Lacerta, Rana, Xenopus, Carassius) including a quantitative evaluation of sequential serial sections.

The stimulatory effects of neurotensin and related peptides in rat stomach strips and guinea-pig atria

1 The stimulatory effects of neurotensin (NT) and several NT fragments were evaluated in two pharmacological preparations: rat stomach strips and isolated spontaneously beating atria of guinea-pigs.2 In rat stomach strips, NT elicited a dose-dependent contractile effect in concentrations varying between 1.3 x 10(-9) and 5.4 x 10(-7) M.3 The contractile effect of NT (1.3 and 5.4 x 10(-8) M) in this tissue was not modified by atropine (3.4 x 10(-7) M), methysergide (2.0 x 10(-6) M), a mixture of cimetidine (8.0 x 10(-6) M) and diphenhydramine (7.8 x 10(-6) M), indomethacin (1.4 x 10(-5) M), 8-Leu-angiotensin II (1.0 x 10(-6) M), glucagon (2.0 x 10(-6) M) or somatostatin (3.0 x 10(-7) M).4 Rat stomach strips desensitized by bradykinin (6.1 x 10(-6) M) or substance P (7.4 x 10(-6) M) maintained their sensitivities to NT (1.3 and 5.4 x 10(-8) M).5 In guinea-pig atria, NT produced a dose-dependent positive inotropic action in concentrations varying between 5.4 x 10(-10) and 2.7 x 10(-7) M.6 The inotropic effect of NT (2.7 x 10(-9) M) was not influenced by methysergide (2.8 x 10(-6) M), atropine (3.4 x 10(-7) M), practolol (1.5 x 10(-5) M), 8-Leu-angiotensin II (1.0 x 10(-6) M), or indomethacin (1.4 x 10(-5) M), but it was reduced by 37% by cimetidine (4.0 x 10(-5) and 2.0 x 10(-4) M). A combination of cimetidine (4.0 x 10(-5) M) and diphenhydramine (3.9 x 10(-6) M) did not produce a greater inhibition of NT than cimetidine alone.7 Atria desensitized by bradykinin (6.1 x 10(-6) M) or glucagon (2.0 x 10(-6) M) maintained their sensitivities to NT (2.7 x 10(-9) M). Substance P was inactive both as an agonist or antagonist of NT.8 These results suggest the existence of specific NT receptors in rat stomach strips and guinea-pig atria.9 The data derived from our structure-activity study suggest that the minimum structure required for the full stimulation of NT receptors in these two preparations is H-Arg(9)-Pro(10)-Tyr(11)-Ile(12)-Leu(13)-OH. The sequence PyroGlu(1)-Leu(2)-Tyr(3)-Glu(4)-Asn(5)-Lys(6)-Pro(7)-Arg(8)- and the amino acids Ile(12) and Leu(13) appear to contribute mainly to the affinity or binding of NT to its receptor. The chemical groups responsible for the full activation (intrinsic activity) of NT receptors seem to be located in the sequence -Arg(9)-Pro(10)-Tyr(11).

Immunochemical characterization of neurotensin-like peptides in chicken

Using radioimmunoassay and 3 region specific antisera toward bovine neurotensin (NT), the NT-like peptides in chicken have been shown to differ from NT but to strongly resemble its COOH-terminal region. Three substances were identified, one of which resembled NT biologically and appeared to share 7 or 8 of its COOH-terminal residues. The two other peptides were smaller than NT but seemed to possess 4-6 residue homologies with it. Tissue distribution studies indicated that the chicken pancreas and thymus had unusually high levels of this material (greater than 200 fold than in rat) and that the 3 substances were distributed differently in tissues. Chromatographic studies showed that the peptides obtained form brain, intestine, thymus, and pancreas were similar. These results, demonstrating evolutionary conservation of the COOH-terminal region of NT, are in keeping with the known importance of this region for biological activity. These finding also suggest the exstence of an NT-family of peptides serving multiple biological roles.

Regioal distribution of immunoreactive neurotension in monkey brain

The regional distribution of immunoreactive neurotensin (IRNT) in 66 different areas of the monkey brain and spinal cord was studied by radioimmunoassay using specific rabbit antisera to synthetic neurotensin. Each area was found to contain IRNT in uneven concentrations. Highest concentrations were present in the hypothalamic areas and the interpeduncular nucleus, values reaching around 80 pg IRNT/mg wet tissue weight. The habenula, subthalamus and preoptic area were also rich in IRNT. A relatively selective distribution of IRNT was found in the thalamic regions. Low values were obtained in the cerebral and cerebellar cortex, striatum, substantia nigra and white matter areas. Among the fiber bundle studied, the olfactory tract contained a significant amount of IRNT. The present finding of a selective distribution of IRNT suggests a specific neuronal action of neurotensin in primate central nervous system.

Activity of VIP, somatostatin and other peptides in the mouse vas deferens assay

Non-opiate peptides such as vasoactive intestinal peptide (VIP) and somatostatin were tested for their effects on electrically induced contractions of the vas deferens. VIP(ED50 = 2.7 X 10(-8) M) and to a lesser extent somatostatin (ED50 = 5.2 X 10(-8) M) were found to be in the same general range of activity as enkephalin and the endorphins in this system. Human pancreatic polypeptide (HPP) exerted a biphasic effect, inhibiting the contractions at high concentrations but enhancing them at lower concentrations. A number of other natural occurring brain peptides were ineffective at concentrations of 1 X 10(-6) M. Several somatostatin analogues were tested and their activity on the vas deferens was found to more closely parallel their potency to inhibit the release of gastric acid than of growth hormone. In contrast to the brain opiates, however, the inhibitory effects of VIP, somatostatin and its analogues, and HPP were not reversed by the opiate antagonist naloxone. The results suggest that the vas deferens can be readily used for evaluation of analogues of VIP, somatostatin, and other peptides.

Effects of neurotensin on isolated intestinal smooth muscles

The effects of neurotensin were investigated in intestinal smooth muscle preparations. Neurotensin relaxed the rat ileum, and contracted the guinea-pig ileum and taenia. Neurotensin induced a biphasic response (relaxation followed by contraction) in the contracted guinea-pig ileum. In all systems, half-maximal effects were obtained with 4--5 nM neurotensin and maximal responses with 30--60 nM; tachyphylaxis occurred with higher concentrations. Tetrodotoxin did not affect the responses to neurotensin in the rat ileum and the guinea-pig taenia. Tetrodotoxin abolished the contraction or the contraction phase (but not the relaxation phase) of the biphasic response induced by neurotensin in the guinea-pig ileum. Atropine partially inhibited the Neurotensin-induced contraction in the guinea-pig ileum. These results suggest that neurotensin acts on intestinal smooth muscle both directly (relaxation of the rat and guinea-pig ileum, and contraction of the guinea-pig taenia) and through a nerve-mediated, partly cholinergic, process (contraction of the guinea-pig ileum).