Antagonism by SR 48692 of mechanical responses to neurotensin in rat intestine

Neurotensin receptor-1 antagonist SR48692 modulation of high-fat diet induced reproductive impairment in male mice

Neurotensin (NTS), a tridecapeptide of the gastrointestinal tract, has been implicated in the facilitation of lipid absorption on ingestion of a high-fat diet (HFD) especially via NTS receptors, NTSR1, NTSR2, and NTSR3, to cause lipid metabolic dysregulation and imbalance of the oxidant-antioxidant system. Oxidative stress induced a negative impact on reproductive function, affecting the reproductive organ and related reproductive hormones. The present study elucidated the efficacy of NTSR1 antagonist SR48692 in the modulation of HFD-induced reproductive impairment in male mice. Swiss albino mice (male, 23 ± 2 g) were maintained (6/group) for eight weeks; Group-I chow diet (CD), Group-II HFD, Group-III (HFD+SR48692L), Group-IV (HFD+SR48692H), Group-V (CD+SR48692L) and Group-VI (CD+SR48692H). SR48692 low (100 µg/kg b.w./SR48692L) and high-dose (400 µg/kg b.w./SR48692H) were given intraperitoneally for the last four weeks. Treatment with low-dose (SR48692L) to HFD-fed mice showed some efficacy in mitigating lipid dysregulation, oxidative stress, and reproductive impairment as evidenced by decreased triglycerides, total cholesterol, low-density lipoprotein cholesterol, leptin, and increased high-density lipoprotein cholesterol, increased antioxidant defense enzymes, reduction of histopathological scores in testis and increase in plasma level of LH, FSH and testosterone compared to that of HFD, but not up to CD. With the high-dose of antagonist (SR48692H) showed more adverse effects even from that of HFD. Treatment of both doses of SR48692 to CD-fed mice these effects become more extended. Less effectiveness of NTSR1 antagonist with the doses tried (low and high) in normalizing the lipid dysregulation and reproductive impairments might be due to the persistence of NTSR2/NTSR3-mediated lipid absorption.

Neurotensin and xenin stimulates pancreatic exocrine secretion through the peripheral cholinergic nerves in conscious sheep

The present study aimed to clarify the effects of neurotensin and xenin on pancreatic exocrine secretion in conscious sheep and their mechanism of actions. The animals were equipped with two silastic cannulae in the common bile duct to separately collect pancreatic fluid and bile, and a silastic cannula in the proximal duodenum to continuously return the mixed fluids. NT and xenin were intravenously injected at range of 0.01-3.0 nmol/kg during the phase I of duodenal migrating motor complex. A single intravenous NT injection significantly and dose-dependently increased pancreatic fluid, protein, and bicarbonate outputs. The effect of NT at 1 nmol/kg was completely inhibited by a background intravenous infusion of atropine methyl nitrate at a dose of 10 nmol/kg/min, however, the effect was not altered by a prior injection of the neurotensin receptor subtype (NTR)-1 antagonist SR 48692 at 60 nmol/kg. Moreover, a single intravenous xenin-25 injection significantly and dose-dependently increased pancreatic fluid and protein output, whereas the effect of xenin-25 did not clearly show dose-dependence. The prior SR 48692 injection at 30 nmol/kg did not significantly alter the effects of xenin-25 at 0.3 nmol/kg, while the atropine infusion significantly inhibited the increase in fluid secretion. Under the atropine infusion, xenin-25 at 0.3 nmol/kg did not increase protein and bicarbonate outputs, whereas the inhibitory effect of the atropine was not significant compared to that of the single injection of xenin-25. A single intravenous injection of NTR-2 agonist levocabastine at 0.1-3 nmol/kg did not alter pancreatic exocrine secretion. These results suggest that both NT and xenin-25 effectively stimulates pancreatic exocrine secretion through the peripheral cholinergic system in sheep and that NTR-2 is not involved in the regulation of pancreatic exocrine secretion, however, we did not precisely determine the role of NTR-1 in the actions of both the peptides on pancreatic exocrine secretion.

Pharmacodynamic and pharmacokinetic profiles of a neurotensin receptor type 2 (NTS2) analgesic macrocyclic analog

The current opioid crisis highlights the urgent need to develop safe and effective pain medications. Thus, neurotensin (NT) compounds represent a promising approach, as the antinociceptive effects of NT are mediated by activation of the two G protein-coupled receptor subtypes (i.e., NTS1 and NTS2) and produce potent opioid-independent analgesia. Here, we describe the synthesis and pharmacodynamic and pharmacokinetic properties of the first constrained NTS2 macrocyclic NT(8-13) analog. The Tyr¹¹ residue of NT(8-13) was replaced with a Trp residue to achieve NTS2 selectivity, and a rationally designed side-chain to side-chain macrocyclization reaction was applied between Lys⁸ and Trp¹¹ to constrain the peptide in an active binding conformation and limit its recognition by proteolytic enzymes. The resulting macrocyclic peptide, CR-01-64, exhibited high-affinity for NTS2 (K_i 7.0 nM), with a more than 125-fold selectivity over NTS1, as well as an improved plasma stability profile (t_1/2 > 24 h) compared with NT (t_1/2 ~ 2 min). Following intrathecal administration, CR-01-64 exerted dose-dependent and long-lasting analgesic effects in acute (ED₅₀ = 4.6 µg/kg) and tonic (ED₅₀ = 7.1 µg/kg) pain models as well as strong mechanical anti-allodynic effects in the CFA-induced chronic inflammatory pain model. Of particular importance, this constrained NTS2 analog exerted potent nonopioid antinociceptive effects and potentiated opioid-induced analgesia when combined with morphine. At high doses, CR-01-64 did not cause hypothermia or ileum relaxation, although it did induce mild and short-term hypotension, all of which are physiological effects associated with NTS1 activation. Overall, these results demonstrate the strong therapeutic potential of NTS2-selective analogs for the management of pain.

A (99m) Tc-tricine-HYNIC-labeled peptide targeting the neurotensin receptor for single-photon imaging in malignant tumors

In this study, a new neurotensin (NT) analog was labeled with (99m) Tc via HYNIC chelator and tricine as coligand and investigated further. An NT (7-13) analog was prepared, and labeling with (99m) Tc was performed. The internalization rate and biodistribution of radiopeptide were studied in HT-29 cells and nude mice bearing tumor, respectively. Radiolabeling with (99m) Tc was performed at high specific activities (54 MBq/nmol) with an acceptable labeling yield (>95%). In vitro cell line studies showed a specific internalization uptake up to 13.23 ± 0.45% during 4 h which was blocked in the presence of excess cold peptide to 0.83 ± 0.15%. In biodistribution studies, uptake was observed in NT receptor-positive organs so that after 1 h the uptakes in mouse intestine and tumor were 1.23 ± 0.16% ID/g and 1.12 ± 0.11% ID/g, respectively. In animals co-injected with excess cold peptide, reduction uptake in tumor and intestines were 73% (1.10% vs. 0.29% ID/g at 4 h) and 61% (1.22% vs. 0.47% ID/g at 4 h) respectively. Predominant renal excretion pathway with a highest accumulation of activity in bladder was observed for this radiopeptide. This radiolabeled peptide could be a candidate for detection of NT positive tumors.

Intrathecal neurotensin is hypotensive, sympathoinhibitory and enhances the baroreflex in anaesthetized rat

BACKGROUND AND PURPOSE

The neuromodulatory effects of the gut-neuropeptide neurotensin on sympathetic vasomotor tone, central respiratory drive and adaptive reflexes in the spinal cord, are not known.

EXPERIMENTAL APPROACH

Neurotensin (0.5 µM-3 mM) was administered into the intrathecal (i.t.) space at the sixth thoracic spinal cord segment in urethane-anaesthetized, paralysed, vagotomized male Sprague-Dawley rats. Pulsatile arterial pressure, splanchnic sympathetic nerve activity (sSNA), phrenic nerve activity, ECG and end-tidal CO(2) were recorded.

KEY RESULTS

Neurotensin caused a dose-related hypotension, sympathoinhibition and bradycardia. The maximum effects were observed at 3000 µM, where the decreases in mean arterial pressure (MAP), heart rate (HR) and sSNA reached -25 mmHg, -26 beats min(-1) and -26% from baseline, respectively. The sympathetic baroreflex was enhanced. Changes in central respiratory drive were characterized by a fall in the amplitude of the phrenic nerve activity. Finally, administration of SR 142948A (5 mM), a potent, selective antagonist at neurotensin receptors, caused a potent hypotension (-35 mmHg), bradycardia (-54 beats min(-1) ) and sympathoinhibition (-44%). A reduction in the amplitude and frequency of the phrenic nerve activity was also observed.

CONCLUSIONS AND IMPLICATIONS

The data demonstrate that neurotensin plays an important role in the regulation of spinal cardiovascular function, affecting both tone and adaptive reflexes.

Double-stabilized neurotensin analogues as potential radiopharmaceuticals for NTR-positive tumors

INTRODUCTION

Overexpression of neurotensin (NT) receptors in exocrine pancreatic cancer and other neuroendocrine cancers make them interesting targets for tumor imaging and therapy. Modifications at the cleavage bonds 8-9 and 11-12 led to the synthesis of NT-XII, NT-XIII and NT-XVIII, three new stabilized analogues. (NalphaHis)Ac was coupled to the N-terminus for labeling with [(99m)Tc]-tricarbonyl.

METHODS

Stability was tested in vitro in human plasma and HT-29 cells. Binding to NT1 receptors and internalization/efflux were analyzed in intact HT-29 cells. Biodistribution studies were performed in nude mice bearing HT-29 xenografts.

RESULTS

All analogues were very stable in human plasma, with half-lives of 20-21 days. Degradation in HT-29 cells was more rapid (t(1/2) of 6.5, 5 and 2.5 h for NT-XII, NT-XIII and NT-XVIII, respectively). They also showed high affinity and specificity for NT1 receptors. Bound activity was rapidly internalized at 37 degrees C. The pattern of externalization was different. NT-XII was released more slowly than NT-XIII and NT-XVIII (half of the activity still inside the cells after 24 h). Bigger differences were found in the biodistribution studies. NT-XII showed the highest tumor uptake as well as the best tumor to nontumor ratios.

CONCLUSION

The modifications introduced in NT(8-13) increased plasma stability, maintaining unaffected the in vitro binding properties. The best biodistribution corresponded to NT-XII, which shows to be a good candidate for NT1 receptors overexpressing tumors. First clinical trials are ongoing.

Neurotensin-induced Cl(-) current in guinea-pig dorsal root ganglion cells

In guinea-pig dorsal root ganglion cells held under voltage-clamp at -80 mV, neurotensin elicited an inward current (I(NT)) whose amplitude increased with increasing neurotensin concentration (40-4000 nM). The effect was blocked by a nonpeptide neurotensin antagonist. I(NT) occurred in the absence of the extracellular Na(+), but not in the absence of the intracellular Cl(-), and it was outward directed by reversing the driving force for Cl(-). I(NT), like the gamma-amino-butyric acid (GABA)-induced Cl(-) current (I(GABA)), remained little changed after virtual elimination of cytosolic free-ionized Ca(2+) or after treatment with a Ca(2+)-activated Cl(-) channel blocker, but, in contrast to I(GABA) it was resistant to the I(GABA) blocker picrotoxin, slower in time course and more easily desensitized when repeatedly elicited. I(NT) and I(GABA) were additive to each other. AG-protein inhibitor markedly reduced I(NT), and a G-protein activator produced an inward current during which no current could be elicited by neurotensin. These results show that neurotensin exerts an effect to activate Ca(2+)-insensitive Cl(-) channels distinct from those activated by GABA in guinea-pig dorsal root ganglion cells, and the effect may arise through a G-protein-dependent mechanism.

In vitro functional evidence of different neurotensin-receptors modulating the motor response of human colonic muscle strips

1. The newly developed non-peptide neurotensin (NT)-receptor antagonists SR 48692 and SR 142948 were used to challenge NT responses of human colonic circular smooth muscle strips in vitro. The presence of NT1 and NT2 receptor transcripts in this tissue was tested by reverse transcriptase polymerase chain reaction (RT - PCR) analysis. 2. NT potently and dose-dependently contracted muscle strips, with significant regional differences in potency and efficacy between the transverse and distal colon: EC50, 3.6 and 7.5 nM; the maximal effect was 70 and 55% of 0.1 mM carbachol. Colonic responses to NT in both segments were virtually the same in the presence of atropine (1 microm), levocabastine (10 microM) or tetrodotoxin (1 microM). 3. SR 142948 (10 nM - 1 microM) competitively antagonized NT responses in the transverse and distal colon with similar affinities: pA2 values 8.71 and 8.45, slopes 0.98 and 0.99. SR 48692 (10 nM - 10 microM) antagonized the NT response competitively in the distal colon (pA2 6.55, slope 0.79) and non-competitively in the transverse colon (pA2 8.0, slope 0.51). Neither compound had any agonist effect. 4. The fact that the specific antagonists prevented NT-evoked atropine- and tetrodotoxin-insensitive mechanical responses of colonic muscle strips is highly consistent with the presence in these tissues of non-neuronal NT receptors, whose heterogeneity in the transverse segment is supported by the non-competitive antagonism of SR 48692. The finding of NT1 receptor transcript in both transverse and distal colon suggests its identity with the lower affinity site disclosed functionally by SR 48692 in these segments.

Characterization of neurotensin receptors in intestinal smooth muscle using a nonpeptide antagonist

Neurotensin reduced substance P-induced tension in ileal muscle strips and the relaxant effect was inhibited by a nonpeptide antagonist, SR 48692, 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3-yl)car bonylamino]tricyclo(3.3.1.1.(3.7)decan-2-carboxylic acid with a half-maximal concentration (IC50) of 7.4+/-2.1 nM (n = 9) and a dissociation constant (Kb) of 0.9+/-0.2 nM. Neurotensin produced a contractile response in ileal muscle strips pretreated with apamin (50 nM) and in isolated chick rectums and both contractile effects were inhibited by SR 48692 with IC50 of 31.6+/-9.5 nM and Kh of 3.2+/-0.9 nM (n = 6) and with IC50 of 28.9+/-6.9 nM and Kb of 5.4+/-1.0 nM (n = 7), respectively. The Kb values for the contractile effects were not significantly different from each other, but significantly different from that for the relaxant effect, suggesting that both types of effect are mediated via distinct subtypes of neurotensin receptor in the intestinal smooth muscles. Contractile responses and excitatory junction potentials evoked by electrical stimulation of nonadrenergic, noncholinergic (NANC) nerves in isolated chick rectums were not inhibited by SR 48692 (up to 3.3 microM). This does not provide functional evidence for the idea that neurotensin acts as an unidentified excitatory neurotransmitter of NANC nerves in the avian rectum.

Pretreatment with SR48692 has different effects on central neurotensin-induced gastric mucosal defense and inhibition of gastric acid secretion in rats

Neurotensin is a tridecapeptide present in the brain and gastrointestinal tract. Administration of neurotensin into the brain results in responses in the gastrointestinal tract, suggesting a role for neurotensin in the interrelationships that comprise the brain-gut axis. Intracerebroventricular (i.c.v.) administration of neurotensin protects the gastric mucosa against injury caused by cold water restraint (CWR) and also inhibits gastrin-stimulated gastric acid secretion. The hypothesis tested was that these two actions of neurotensin are mediated via its high-affinity receptor. Rats were given neurotensin (60 microgram, i.c.v.) prior to CWR or pylorus ligation after pretreatment with SR48692, a nonpeptide antagonist of the high-affinity neurotensin receptor (0.25 or 2.5 microgram, i.c.v., or 10, 100, or 500 microgram kg-1, i.p.). Neurotensin reduced cold water restraint (CWR)-induced gastric mucosal injury and inhibited gastrin-stimulated acid secretion. Pretreatment with SR48692 (2.5 microgram, i.c.v., or 100 microgram kg-1, i.p.) prior to CWR blocked neurotensin's protection of the gastric mucosa against injury. In contrast, pretreatment with 2.5 microgram SR48692, i.c.v., did not block neurotensin-induced inhibition of acid secretion, whereas 500 microgram kg-1, i.p., partially blocked the inhibition. SR48692 (2.5 microgram, i.c.v.) inhibited acid secretion, suggesting that SR48692 has agonist activity in this system. These results suggest that central neurotensin protects the gastric mucosa against CWR-induced injury via its high-affinity receptor. The receptor that mediates central neurotensin-induced inhibition of gastric acid secretion does not appear to be the high-affinity receptor since the neurotensin receptor antagonist SR48692, when given i.c.v., had agonist activity, inhibiting stimulated acid secretion. High-affinity neurotensin receptors in the periphery appear to play a role in inhibition of stimulated gastric acid secretion.

Potentiation by neurotensin of carbachol-induced tension development in beta-escin-skinned smooth muscle of guinea-pig ileum

Effect of neurotensin (NT) on carbachol(CCh)-induced tension development due to Ca2+ release from intracellular stores was investigated in beta-escin-skinned smooth muscle of guinea-pig ileum. NT (10 nM) increased the tension development in response to CCh. NT also increased the tension response to caffeine, another store-Ca2+ releaser. NT did not exert such an effect in pertusis toxin (PTX)-treated preparations. Treatment with isoprenaline to elevate endogenous cyclic AMP levels or with dibutyryl cyclic AMP did not affect the effect of NT. A nonpeptide NT antagonist, SR 48692, failed to block the effect of NT. NT shifted the pCa-tension relationship in the lower direction of Ca2+ concentration. NT was incapable of releasing Ca2+ from intracellular stores. The results suggest that NT may cause an increase in Ca2+ sensitivity of contractile elements to potentiate the CCh-induced tension development due to release of stored Ca2+ and that the effect is mediated by SR 48692-insensitive NT receptors linked to a PTX-sensitive G protein which works with no relation to a change in cytosolic cyclic AMP levels.

Mode and mechanism of neurotensin action in rat proximal colon

This study examined the mechanism of action of neurotensin on intraluminal pressure in rat proximal colon. The direct and indirect contractile response to neurotensin (100 nM) was abolished in Ca(2+)-free solution, and was antagonized by nifedipine (1-5-10 nM) and potentiated by Bay K 8644 (methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)- pyridine-5-carboxylate) (10-100-1000 nM). Neurotensin, in the presence of nifedipine (10 nM) and atropine (1 microM), induced a tetrodotoxin-insensitive inhibitory effect, which was antagonized by SR 48692 (2[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxy-phenyl)pyrazol-3-yl) carbonyl amino]tricyclo (3.3.1.1.(3.7)) decan-2-carboxylic acid) (300 nM) or apamin (0.1 microM). The results demonstrate that the neurotensin response is dependent on the influx of Ca2+ via L-type channels and results from summation of excitatory and inhibitory effects.