Effects of SR 48692 on neurotensin-induced calcium-activated chloride currents in the Xenopus oocyte expression system: agonist-like activity on the levocabastine-sensitive neurotensin receptor and absence of antagonist effect on the levocabastine insensitive neurotensin receptor

Activation of ventral tegmental area neurotensin Receptor-1 neurons promotes weight loss

Dopamine (DA) neurons in the ventral tegmental area (VTA) modulate physical activity and feeding behaviors that are disrupted in obesity. Yet, the heterogeneity of VTA DA neurons has hindered determination of which ones might be leveraged to support weight loss. We hypothesized that increased activity in the subset of VTA DA neurons expressing neurotensin receptor-1 (NtsR1) might promote weight loss behaviors. To test this, we used Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to activate VTA NtsR1 neurons in normal weight and diet-induced obese mice. Acute activation of VTA NtsR1 neurons (24hr) significantly decreased body weight in normal weight and obese mice by reducing food intake and increasing physical activity. Moreover, daily activation of VTA NtsR1 neurons in obese mice sustained weight loss over 7 days. Activating VTA NtsR1 neurons also suppressed how much mice worked to obtain sucrose rewards, even when there was high motivation to consume. However, VTA NtsR1 neural activation was not reinforcing, nor did it invoke liabilities associated with whole-body NtsR1 agonism such as anxiety, vasodepressor response or hypothermia. Activating VTA NtsR1 neurons therefore promotes dual behaviors that support weight loss without causing adverse effects, and is worth further exploration for managing obesity.

Neurotensins and their therapeutic potential: research field study

The natural tridecapeptide neurotensin has been emerged as a promising therapeutic scaffold for the treatment of neurological diseases and cancer. In this work, we aimed to identify the top 100 most cited original research papers as well as recent key studies related to neurotensins. The Web of Science Core Collection database was searched and the retrieved research articles were analyzed by using the VOSviewer software. The most cited original articles were published between 1973 and 2013. The top-cited article was by Carraway and Leeman reporting the discovery of neurotensin in 1973. The highly cited terms were associated with hypotension and angiotensin-converting-enzyme. The conducted analysis reveals the therapeutic potentials of neurotensin, and further impactful research toward its clinical development is warrantied.

Identification of Neurotensin Receptor Expressing Cells in the Ventral Tegmental Area across the Lifespan

Neurotensin (Nts) promotes activation of dopamine (DA) neurons in the ventral tegmental area (VTA) via incompletely understood mechanisms. Nts can signal via the G protein-coupled Nts receptors 1 and 2 (NtsR1 and NtsR2), but the lack of methods to detect NtsR1- and NtsR2-expressing cells has limited mechanistic understanding of Nts action. To overcome this challenge, we generated dual recombinase mice that express FlpO-dependent Cre recombinase in NtsR1 or NtsR2 cells. This strategy permitted temporal control over recombination, such that we could identify NtsR1- or NtsR2-expressing cells and determine whether their distributions differed between the developing and adult brain. Using this system, we found that NtsR1 is transiently expressed in nearly all DA neurons and in many non-DA neurons in the VTA during development. However, NtsR1 expression is more restricted within the adult brain, where only two thirds of VTA DA neurons expressed NtsR1. By contrast, NtsR2 expression remains constant throughout lifespan, but it is predominantly expressed within glia. Anterograde tract tracing revealed that NtsR1 is expressed by mesolimbic, not mesocortical DA neurons, suggesting that VTA NtsR1 neurons may represent a functionally unique subset of VTA DA neurons. Collectively, this work reveals a cellular mechanism by which Nts can directly engage NtsR1-expressing DA neurons to modify DA signaling. Going forward, the dual recombinase strategy developed here will be useful to selectively modulate NtsR1- and NtsR2-expressing cells and to parse their contributions to Nts-mediated behaviors.

Neurotensin: A role in substance use disorder?

Neurotensin is a tridecapeptide originally identified in extracts of bovine hypothalamus. This peptide has a close anatomical and functional relationship with the mesocorticolimbic and nigrostriatal dopamine system. Neural circuits containing neurotensin were originally proposed to play a role in the mechanism of action of antipsychotic agents. Additionally, neurotensin-containing pathways were demonstrated to mediate some of the rewarding and/or sensitizing properties of drugs of abuse.This review attempts to contribute to the understanding of the role of neurotensin and its receptors in drug abuse. In particular, we will summarize the potential relevance of neurotensin, its related compounds and neurotensin receptors in substance use disorders, with a focus on the preclinical research.

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.

Neurotensin and its high affinity receptor 1 as a potential pharmacological target in cancer therapy

Cancer is a worldwide health problem. Personalized treatment represents a future advancement for cancer treatment, in part due to the development of targeted therapeutic drugs. These molecules are expected to be more effective than current treatments and less harmful to normal cells. The discovery and validation of new targets are the foundation and the source of these new therapies. The neurotensinergic system has been shown to enhance cancer progression in various cancers such as pancreatic, prostate, lung, breast, and colon cancer. It also triggers multiple oncogenic signaling pathways, such as the PKC/ERK and AKT pathways. In this review, we discuss the contribution of the neurotensinergic system to cancer progression, as well as the regulation and mechanisms of the system in order to highlight its potential as a therapeutic target, and its prospect for its use as a treatment in certain cancers.

Anticonvulsant neuropeptides as drug leads for neurological diseases

Anticonvulsant neuropeptides are best known for their ability to suppress seizures and modulate pain pathways. Galanin, neuropeptide Y, somatostatin, neurotensin, dynorphin, among others, have been validated as potential first-in-class anti-epileptic or/and analgesic compounds in animal models of epilepsy and pain, but their therapeutic potential extends to other neurological indications, including neurodegenerative and psychatric disorders. Disease-modifying properties of neuropeptides make them even more attractive templates for developing new-generation neurotherapeutics. Arguably, efforts to transform this class of neuropeptides into drugs have been limited compared to those for other bioactive peptides. Key challenges in developing neuropeptide-based anticonvulsants are: to engineer optimal receptor-subtype selectivity, to improve metabolic stability and to enhance their bioavailability, including penetration across the blood–brain barrier (BBB). Here, we summarize advances toward developing systemically active and CNS-penetrant neuropeptide analogs. Two main objectives of this review are: (1) to provide an overview of structural and pharmacological properties for selected anticonvulsant neuropeptides and their analogs and (2) to encourage broader efforts to convert these endogenous natural products into drug leads for pain, epilepsy and other neurological diseases.

Cancer, chemistry, and the cell: molecules that interact with the neurotensin receptors

The literature covering neurotensin (NT) and its signalling pathways, receptors, and biological profile is complicated by the fact that the discovery of three NT receptor subtypes has come to light only in recent years. Moreover, a lot of this literature explores NT in the context of the central nervous system and behavioral studies. However, there is now good evidence that the up-regulation of NT is intimately involved in cancer development and progression. This Review aims to summarize the isolation, cloning, localization, and binding properties of the accepted receptor subtypes (NTR1, NTR2, and NTR3) and the molecules known to bind at these receptors. The growing role these targets are playing in cancer research is also discussed. We hope this Review will provide a useful overview and a one-stop resource for new researchers engaged in this field at the chemistry-biology interface.

Molecular evolution of neuropeptide receptors with regard to maintaining high affinity to their authentic ligands

Recently, we cloned many of the bullfrog neuropeptide G protein-coupled receptors (GPCRs), including receptors for vasotocin (VT), mesotocin, gonadotropin-releasing hormone (GnRH), neurotensin, apelin, and metastin. Bullfrog GPCRs usually have high affinity for bullfrog ligands but relatively low affinity for mammalian ligands. Reciprocally, synthetic agonists and antagonists developed based upon mammalian ligands display lower affinity at bullfrog receptors. Studies using chimeric or domain-swapped receptors indicate that the motifs responsible for differential ligand selectivity usually reside within transmembrane domain 6 (TMD6)-extracellular loop 3 (ECL3)-transmembrane domain 7 (TMD7). Triple mutation of mammalian V1aR (Phe(6.51) to Tyr, Ile(6.53) to Thr, and Pro(7.33) to Thr) increases VT affinity but greatly reduces arginine vasopressin affinity. This binding profile is similar to that of bullfrog VT1R. Changing just three amino acids in the bullfrog GnRH receptor-1 (i.e. Ser-Gln-Ser in the ECL3) to those found in the type-I mammalian GnRH receptor (i.e. Ser-Glu-Pro) reverses GnRH selectivity. In conclusion, specific receptor motifs that govern ligand selectivity can be determined by comparative molecular analyses of GPCRs and their ligands. Such analysis provides clues for understanding how GPCRs maintain high affinity to their authentic ligands.

Heterogeneity of ghrelin/growth hormone secretagogue receptors. Toward the understanding of the molecular identity of novel ghrelin/GHS receptors

Ghrelin is a gastric polypeptide displaying strong GH-releasing activity by activation of the type 1a GH secretagogue receptor (GHS-R1a) located in the hypothalamus-pituitary axis. GHS-R1a is a G-protein-coupled receptor that, upon the binding of ghrelin or synthetic peptidyl and non-peptidyl ghrelin-mimetic agents known as GHS, preferentially couples to G(q), ultimately leading to increased intracellular calcium content. Beside the potent GH-releasing action, ghrelin and GHS influence food intake, gut motility, sleep, memory and behavior, glucose and lipid metabolism, cardiovascular performances, cell proliferation, immunological responses and reproduction. A growing body of evidence suggests that the cloned GHS-R1a alone cannot be the responsible for all these effects. The cloned GHS-R1b splice variant is apparently non-ghrelin/GHS-responsive, despite demonstration of expression in neoplastic tissues responsive to ghrelin not expressing GHS-R1a; GHS-R1a homologues sensitive to ghrelin are capable of interaction with GHS-R1b, forming heterodimeric species. Furthermore, GHS-R1a-deficient mice do not show evident abnormalities in growth and diet-induced obesity, suggesting the involvement of another receptor. Additional evidence of the existence of another receptor is that ghrelin and GHS do not always share the same biological activities and activate a variety of intracellular signalling systems besides G(q). The biological actions on the heart, adipose tissue, pancreas, cancer cells and brain shared by ghrelin and the non-acylated form of ghrelin (des-octanoyl ghrelin), which does not bind GHS-R1a, represent the best evidence for the existence of a still unknown, functionally active binding site for this family of molecules. Finally, located in the heart and blood vessels is the scavenger receptor CD36, involved in the endocytosis of the pro-atherogenic oxidized low-density lipoproteins, which is a pharmacologically and structurally distinct receptor for peptidyl GHS and not for ghrelin. This review highlights the most recently discovered features of GHS-R1a and the emerging evidence for a novel group of receptors that are not of the GHS1a type; these appear involved in the transduction of the multiple levels of information provided by GHS and ghrelin.

NTS2 modulates the intracellular distribution and trafficking of NTS1 via heterodimerization

Neurotensin (NT) receptors NTS1 and NTS2 are known to display considerable distributional overlap in mammalian central nervous system (CNS). Using co-immunoprecipitation approaches, we demonstrated here that NTS1 forms constitutive heterodimers with NTS2 in transfected COS-7 cells. We also showed that co-expression of NTS2 with NTS1 markedly decreases the cell surface density of NTS1 without affecting ERK1/2 MAPK activity or NT-induced NTS1 internalization. However, radioligand-binding studies indicated that upon prolonged NT stimulation, cell surface NTS1 receptors are more resistant to down-regulation in cells co-expressing NTS1 and NTS2 than in cells expressing NTS1 alone. Taken together, these data suggest that NTS1/NTS2 heterodimerization affects the intracellular distribution and trafficking of NTS1 by making it more similar to that of NTS2 as witnessed in cells expressing NTS2 alone. NTS1/NTS2 heterodimerization might therefore represent an additional mechanism in the regulation of NT-triggered responses mediated by NTS1 and NTS2 receptors.

Interactions between neurotensin receptors and G proteins

Three neurotensin (NT) receptors have been cloned to date, two of which, NTS1 and NTS2, belong to the family of seven transmembrane domain receptors coupled to G proteins (GPCRs). NTS1 and NTS2 may activate multiple signal transduction pathways, involving several G proteins. However, whereas NT acts as an agonist towards all NTS1-mediated pathways, this peptide may exert either agonist or antagonist activities, depending on the NTS2-mediated pathway in question. Studies on these receptors reinforce the concept of independence between multiple signals potentially mediated through a single GPCR, generating a wide diversity of functional responses depending on the host cell and the ligand.

Functional roles of the NTS2 and NTS3 receptors

Neurotensin exerts its actions in the central nervous system and the periphery through three identified receptors. Two of them, the NTS2 and NTS3, display unusual properties either because of their complex signal transduction mechanisms (NTS2) or because of their structural composition as a non-G-protein-coupled receptor (NTS3). Here, we review the transduction mechanisms, cellular trafficking, and potential physiological roles of these two unconventional receptors.

Bioactive analogs of neurotensin: focus on CNS effects

Neurotensin (NT) is a 13-amino acid neuropeptide found in the central nervous system and in the gastrointestinal tract. It is closely associated anatomically with dopaminergic and other neurotransmitter systems, and evidence supports a role for NT agonists in the treatment of various neuropsychiatric disorders. However, NT is readily degraded by peptidases, so there is much interest in the development of stable NT agonists, that can be injected systemically, cross the blood-brain barrier (BBB), yet retains the pharmacological characteristics of native NT for therapeutic use in the treatment of diseases such as schizophrenia, Parkinson's disease and addiction.

Neurotensin activation of the NTR1 on spinally-projecting serotonergic neurons in the rostral ventromedial medulla is antinociceptive

Microinjection of neurotensin (NT) in the rostral ventromedial medulla (RVM) produces dose-dependent antinociception. The NTR1 (Neurotensin Receptor Subtype 1) may mediate part of this response, however definitive evidence is lacking, and the spinal mediators of NTR1-induced antinociception are unknown. In the present study, we used immunohistochemical techniques to show that the NTR1, but not the NTR2 is expressed by spinally projecting serotonergic neurons of the RVM. We also show that microinjection of NT or the NTR1-selective agonist PD149163 in the RVM both produce dose-dependent antinociception in the tail-flick test that is blocked by the NTR1-selective antagonist SR48692. The antinociception produced by NT or PD149163 is also blocked by intrathecal administration of the non-selective serotonergic receptor antagonist methysergide. The results of these experiments provide anatomical and behavioral evidence that activation of NTR1-expressing spinally projecting neurons in the RVM produces antinociception through release of serotonin in the spinal dorsal horn. These results support the conclusion that the NTR1 plays an important role in the central modulation of nociception.

Identification and Functional Characterization of a 5-Transmembrane Domain Variant Isoform of the NTS2 Neurotensin Receptor in Rat Central Nervous System

The present study demonstrated that alternative splicing of the rat nts2 receptor gene generates a 5-transmembrane domain variant isoform (vNTS2) that is co-expressed with the full-length NTS2 receptor throughout the brain and spinal cord, as evidenced by reverse transcription-PCR. The vNTS2 polypeptide is 281 amino acids in length, which is 135 amino acids shorter than the full-length isoform. Immunohistochemical and radioligand binding studies revealed that the HA-tagged recombinant vNTS2 receptor is poorly targeted to plasma membranes in transfected COS-7 cells. Binding studies also showed that the truncated receptor displayed a 5000-fold lower affinity for neurotensin (NT) than its full-length counterpart (IC(50) of 10 mum and 2 nm, respectively). Yet NT binding induced efficient internalization of receptor-ligand complexes in vNTS2-transfected cells. Furthermore, it produced a rapid (<5 min) activation of the mitogen-activated protein kinases (ERK1/2) pathway, indicating functional coupling of the variant receptor. This activation is sustained (>1 h) and is also produced by the NTS2 agonist levocabastine. Western blotting experiments suggested that vNTS2 is not expressed in monomeric form in the rat central nervous system. However, it does appear to form a variety of multimeric complexes, including homodimers and heterodimers, with the full-length NTS2. Indeed, co-immunoprecipitation studies in dually transfected cells demonstrated that the two receptor isoforms can form stable associations. Taken together, the present results indicated that the rat vNTS2 is a functional receptor that may play a role in NT signaling in mammalian central nervous system.

Low-affinity neurotensin receptor (NTS2) signaling: internalization-dependent activation of extracellular signal-regulated kinases 1/2

The role and signaling properties of the low-affinity neurotensin receptor (NTS2) are still controversial. In particular, it is unclear whether neurotensin acts as an agonist, inverse agonist, or antagonist at this site. In view of the growing evidence for a role of NTS2 in antinociception, the elucidation of the pharmacological and coupling properties of this receptor is particularly critical. In the present study, we demonstrate that in Chinese hamster ovary (CHO) cells expressing the rat NTS2 receptor, neurotensin (NT), levocabastine, neuromedin N, and the high-affinity NT receptor antagonist SR48692 [2-[[1-(-7-chloroquinolin-4-yl)-5-(2,6-dimethoxyphenyl)-1H-pyrazole-3-carbonyl]amino]adamantane-2-carboxylic acid] all bind to and activate the NTS2 receptor. This activation is followed by ligand-induced internalization of receptor-ligand complexes, as evidenced by confocal microscopy using a fluorescent NT analog. All compounds tested produced a rapid and sustained activation of extracellular signal-regulated kinases 1/2 (ERK1/2) but were without specific effect on Ca(2+) mobilization. The agonist-induced activation of ERK1/2 was completely abolished by preincubation of the cells with the endocytosis inhibitors phenylarsine oxide and monodansylcadaverine as well as overexpression of a dominant-negative mutant of dynamin 1 (DynK44A), indicating that receptor internalization was required for ERK1/2 activation. NTS2-induced activation of ERK1/2 was not species-specific, because the same agonistic effects of NT and analogs were observed in CHO cells transfected with the human NTS2 receptor. In conclusion, this study demonstrates that NTS2 is a bona fide NT receptor and that activation of this receptor by NT or NT analogs results in an internalization-dependent activation of the ERK1/2 signaling cascade.

Neurotensin stimulates both calcium mobilization from inositol trisphosphate-sensitive intracellular stores and calcium influx through membrane channels in frog pituitary melanotrophs

Neurotensin (NT) is a potent stimulator of electrical and secretory activities in frog pituitary melanotrophs. The aim of the present study was to characterize the transduction pathways associated with activation of NT receptors in frog melanotrophs. Application of synthetic frog NT (fNT) increased the cytosolic calcium concentration ([Ca2+]c) and stimulated the formation of inositol trisphosphate (IP3). The phospholipase C inhibitor U-73122 blocked the electrophysiological and secretory effects of fNT. Intracellular application of the IP3 receptor antagonist heparin abolished fNT-induced electrical activity. Suppression of Ca2+ in the incubation medium markedly reduced the effect of NT on [Ca2+]c, firing rate, and alpha-melanocyte-stimulating hormone (alphaMSH) secretion. Similarly, the inhibitor of IP3-induced Ca2+ release and store-operated Ca2+ channels, 2-Aminoethoxydiphenylborane, and the nonselective Ca2+ channel blockers GdCl3 and NiCl2, attenuated the [Ca2+]c increase and the electrical and secretory responses evoked by fNT. Coapplication of the L- and N-type Ca2+ channel blockers nifedipine and omega-CgTx GVIA reduced the effects of fNT on action potential discharge, [Ca2+]c increase, and alphaMSH release. The protein kinase C (PKC) inhibitors, PKC-(19-31) and chelerythrine, reduced the electrophysiological and secretory responses induced by iterative applications of fNT. Collectively, these results demonstrate that, in frog melanotrophs, NT stimulates the phospholipase C/PKC pathway and increases [Ca2+]c. Both Ca2+ release from intracellular stores and Ca2+ influx through L- and N-type Ca2+ channels are involved in fNT-induced alphaMSH secretion. In addition, the present data indicate that PKC plays a crucial role in maintenance of the responsiveness of melanotrophs to NT.

Pharmacology and Functional Properties of NTS2 Neurotensin Receptors in Cerebellar Granule Cells

The binding and signaling properties of neuronal NTS2 neurotensin (NT) receptors were examined in cultured rat cerebellar granule cells. As shown by reverse transcription-PCR, receptor autoradiography, and confocal microscopic localization of fluorescent NT, these cells selectively express the NTS2 receptor subtype. Accordingly, a single apparent class of (125)I-NT-binding sites, with an affinity of 3.1 nm, was detected in cerebellar granule cell cultures. This binding was competed for with high affinity (IC(50) = 5.7 nm) by the NTS2 ligand levocabastine and with low affinity (IC(50) = 203 nm) by the NTS1 antagonist SR48692. Hypertonic acid stripping of surface-bound ligand and hyperosmolar sucrose treatment revealed that 64% of specifically bound (125)I-NT was internalized at equilibrium via a clathrin-dependent pathway. In cells loaded with the Ca(2+)-sensitive fluorescent dye Fluo4, SR48692, but neither NT nor levocabastine, triggered a marked increase in cytosolic [Ca(2+)](i). By contrast, both NT and levocabastine, but not SR48692, induced a sustained (>60 min) activation of the mitogen-activated protein kinases, p42/p44, indicating functional coupling of NTS2 receptors. Complementary experiments carried out on synaptosomes from adult rat cerebellum demonstrated the presence of presynaptic NTS2 receptors. However, in contrast to perikaryal NTS2 sites, these presynaptic receptors did not internalize in response to NT stimulation. Taken together, the present results demonstrate that NTS2 receptors are present both presynaptically and postsynaptically in central neurons and that NT and levocabastine act as agonists on these receptors.

The effects of deleting the mouse neurotensin receptor NTR1 on central and peripheral responses to neurotensin

Mice deficient in the neurotensin (NT)-1 receptor (NTR1) were developed to characterize the NT receptor subtypes that mediate various in vivo responses to NT. F2 generation (C57BL6/Sv129J) NTR1 knockout (-/-) mice were viable, and showed normal growth and overt behavior. The -/- mice lacked detectable NTR1 radioligand binding in brain, whereas NTR2 receptor binding density appeared normal compared with wild-type (+/+) mice. The gene deletion also resulted in the loss of NTR1 expression as determined by reverse transcription-polymerase chain reaction and in situ hybridization. Intracerebroventricular injection of NT (1 microg) to +/+ mice caused a robust hypothermic response (5-6 degrees C) and a significant increase in hot-plate latency. These effects were absent in the -/- mice. Similar results were obtained with i.p. injections of the brain-penetrant NT analog NMe-Arg-Lys-Pro-Trp-Tle-Leu (NT-2, 1 mg/kg i.p.). NT-2 administration also impaired rotarod performance in wild-type mice, but had no effect on motor coordination in knockout mice. In vitro, NT and NT-2 at 30 nM caused predominantly contraction and relaxation in isolated distal colon and proximal ileum, respectively, from +/+ mice, but no responses were observed with tissues from -/- mice. A similar loss of the contractile effects of NT was observed in the isolated stomach fundus from the knockout mice. In vivo, NT-2 administration reduced colonic propulsion substantially in wild-type mice. In contrast, NT-2 had no effect in NTR1 null mice, whereas the hypomotility effect of clonidine was intact. These data indicate that NTR1 mediates several of the central and peripheral effects of NT.

Agonism, inverse agonism, and neutral antagonism at the constitutively active human neurotensin receptor 2

Two G protein-coupled neurotensin (NT) receptors, termed NTR1 and NTR2, have been identified so far. In contrast to the NTR1, which has been extensively studied, little is known about the pharmacological and biological properties of the NTR2. In the course of characterizing NT analogs that exhibited binding selectivity for the NTR2, we discovered that this receptor constitutively activated inositol phosphate (IP) production. Here, we report on the constitutive activity of the human NTR2 (hNTR2) transfected in COS cells and on compounds that exhibit agonism, inverse agonism, and neutral antagonism at this receptor. IP levels increased linearly with time, whereas they remained constant in mock-transfected cells. Furthermore, IP production was proportional to the amount of hNTR2 present at the cell membrane. SR 48692, a nonpeptide antagonist of the NTR1, stimulated IP production, whereas levocabastine, a nonpeptide histamine H1 antagonist that binds the NTR2 but not the NTR1, behaved as a weak partial inverse agonist. NT analogs modified at position 11 of the NT molecule, in particular by the introduction of bulky aromatic D amino acids, exhibited binding selectivity at the hNTR2 and also behaved as partial inverse agonists, reversing constitutive IP production up to 50%. Finally, NT barely affected constitutive IP production but antagonized the effects of both agonist and inverse agonist compounds, thus behaving as a neutral antagonist. The unique pharmacological profile of the hNTR2 is discussed in the light of its sequence similarity with the NTR1 and the known binding site topology of NT and SR 48692 in the NTR1.

Effects of neurotensin on discharge rates of rat suprachiasmatic nucleus neurons in vitro

The neuropeptide neurotensin and two classes of its receptors, the neurotensin receptor-1 and 2, are present in the suprachiasmatic nucleus of the mammalian hypothalamus. The suprachiasmatic nucleus houses the mammalian central circadian pacemaker, but the effects of neurotensin on cellular activity in this circadian pacemaker are unknown. In this study, we examined the effects of neurotensin on the spontaneous discharge rate of rat SCN cells in an in vitro slice preparation. Neurotensin (1-10 microM) increased cell firing rate in approximately 50% of cells tested, while approximately 10% of suprachiasmatic cells tested showed a decrease in firing rate in response to neurotensin. These effects of neurotensin were not altered by the GABA receptor antagonist bicuculline (20 microM) or the glutamate receptor antagonists, D-aminophosphopentanoic acid (50 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). The neurotensin receptor selective antagonists SR48692 and SR142948a (10 microM) failed to antagonise neurotensin responses in the majority of cells examined. Compounds that function as agonists selective for the neurotensin-receptor subtypes 1 and 2, JMV-510 and JMV-431 respectively, elicited neurotensin-like responses in approximately 90% of cells tested. Six out of seven cells tested responded to both JMV-510 and JMV-431. Neuropeptide Y (100nM) treatment of suprachiasmatic nucleus slices was found to elicit profound suppression of neuronal firing rate. Co-application of neurotensin with neuropeptide Y significantly (P<0.05) reduced the duration of the response, as compared to that elicited with neuropeptide Y alone. Together, these results demonstrate for the first time the actions of neurotensin in the suprachiasmatic nucleus and raise the possibility that this neuropeptide may play a role in modulating circadian pacemaker function.

Sortilin/neurotensin receptor-3: a new tool to investigate neurotensin signaling and cellular trafficking?

The identification of gp95sortilin, a sorting protein, as being the 100 kDa neurotensin (NT) receptor, a non-G-protein coupled receptor, constitutes a new and interesting but intriguing step in the neuropeptide signaling as well as in cellular trafficking. The isolation of the same protein by three different experimental approaches sum up the complexity for researchers involved in the functional significance of the so-called sortilin/neurotensin receptor 3 (NTR3). This review will concentrate on the putative physiological and cellular roles of sortilin/NTR3 as most results so far have proposed hypothetical conclusions rather than concrete evidence.

Pharmacological, molecular and functional characterization of glial neurotensin receptors

The pharmacological properties, molecular identity and physiopathological regulation of neurotensin receptors expressed by central astrocytes were investigated in primary glial cultures and sections from the adult rat brain. Binding experiments carried out on astrocytes in culture revealed the presence of a single apparent class of neurotensin binding sites. These sites bound [125]neurotensin with an affinity (6 nM) comparable to that of the recently cloned NT2 low-affinity receptor expressed in transfected cells. The glial receptor was sensitive to the antihistamine, levocabastine, but less so than the NT2 site expressed in heterologous expression systems, suggesting the presence of an additional site or a differential coupling of the NT2 receptor in glia. Reverse transcription-polymerase chain reaction experiments demonstrated that both NT2 and NT3 neurotensin receptor sub-types were in fact expressed by cortical glial cells in culture. Confocal microscopic visualization of specifically bound fluorescent neurotensin indicated that this expression concerned only a sub-population of astrocytes in culture, in conformity with earlier reports of a heterogeneous expression of neuropeptides and their receptors by glial cells. To further investigate the functionality of NT2 receptors expressed in astrocytes, dual immunohistochemical labeling of glial fibrillary acidic protein and in situ hybridization of NT2 messenger RNA was performed on sections of normal and lesioned rat brain. In sections from normal brain, only a small subset of immunolabeled astrocytes hybridized NT2 messenger RNA. By contrast, in sections of stab-wounded rat brains, there was a marked increase in the number of NT2-hybridizing astrocytes in the surround of the lesion. Furthermore, NT2 expression within immunopositive reactive astrocytes was significantly enhanced as compared to immunolabeled glial cells in the brain of control animals. These results indicate that NT2 receptor expression is up-regulated during astrocytic reaction, suggesting that NT2 receptors may play a role in regulating glial response to injury.

Sequences required for induction of neurotensin receptor gene expression during neuronal differentiation of N1E-115 neuroblastoma cells

The promoter region of the mouse high affinity neurotensin receptor (Ntr-1) gene was characterized, and sequences required for expression in neuroblastoma cell lines that express high affinity NT-binding sites were characterized. Me(2)SO-induced neuronal differentiation of N1E-115 neuroblastoma cells increased both the expression of the endogenous Ntr-1 gene and reporter genes driven by NTR-1 promoter sequences by 3-4-fold. Deletion analysis revealed that an 83-base pair promoter region containing the transcriptional start site is required for Me(2)SO activation. Detailed mutational analysis of this region revealed that a CACCC box and the central region of a large GC-rich palindrome are the crucial cis-regulatory elements required for Me(2)SO induction. The CACCC box is bound by at least one factor that is induced upon Me(2)SO treatment of N1E-115 cells. The Me(2)SO effect was found to be both selective and cell type-restricted. Basal expression in the neuroblastoma cell lines required a distinct set of sequences, including an Sp1-like sequence, and a sequence resembling an NGFI-A-binding site; however, a more distal 5' sequence was found to repress basal activity in N1E-115 cells. These results provide evidence that Ntr-1 gene regulation involves both positive and negative regulatory elements located in the 5'-flanking region and that Ntr-1 gene activation involves the coordinate activation or induction of several factors, including a CACCC box binding complex.

Neurotensin and neurotensin receptors

Neurotensin is a brain and gastrointestinal peptide that fulfils many central and peripheral functions through its interaction with specific receptors. Three subtypes of neurotensin receptors have been cloned. Two of them belong to the family of G protein-coupled receptors, whereas the third one is an entirely new type of neuropeptide receptor and is identical to gp95/sortilin, a 100 kDa-protein with a single transmembrane domain. In this review, the present knowledge regarding the molecular and pharmacological properties of the three cloned neurotensin receptors is summarized and the relationship between these receptors and the known pharmacological effects of neurotensin is discussed.

Identification of the receptor subtype involved in the analgesic effect of neurotensin

The neuropeptide neurotensin (NT) elicits hypothermic and naloxone-insensitive analgesic responses after brain injection. Recent pharmacological evidence obtained with NT agonists and antagonists suggests that these effects are mediated by a receptor distinct from the initially cloned high-affinity NT receptor (NTR1). The recent cloning of a second NT receptor (NTR2) prompted us to evaluate its role in NT-induced analgesia. Intracerebroventricular injections in mice of two different antisense oligodeoxynucleotides from the NTR2 markedly decreased NTR2 mRNA and protein and reduced NT-induced analgesia. This effect was specific, because NTR1 levels were unaffected, and sense or scramble oligodeoxynucleotides had no effect. Structure-activity studies revealed a close correlation between the analgesic potency of NT analogs and their affinity for the NTR2 and disclosed potent and selective agonists of this receptor. These data confirm that NTR1 is involved in the NT-elicited turning behavior and demonstrate that the NTR2 mediates NT-induced analgesia.

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.

Neurotensin is an antagonist of the human neurotensin NT2 receptor expressed in Chinese hamster ovary cells

The human levocabastine-sensitive neurotensin NT2 receptor was cloned from a cortex cDNA library and stably expressed in Chinese hamster ovary (CHO) cells in order to study its binding and signalling characteristics. The receptor binds neurotensin as well as several other ligands already described for neurotensin NT1 receptor. It also binds levocabastine, a histamine H1 receptor antagonist that is not recognised by neurotensin NT1 receptor. Neurotensin binding to recombinant neurotensin NT2 receptor expressed in CHO cells does not elicit a biological response as determined by second messenger measurements. Levocabastine, and the peptides neuromedin N and xenin were also ineffective on neurotensin NT2 receptor activation. Experiments with the neurotensin NT1 receptor antagonists SR48692 and SR142948A, resulted in the unanticipated discovery that both molecules are potent agonists on neurotensin NT2 receptor. Both compounds, following binding to neurotensin NT2 receptor, enhance inositol phosphates (IP) formation with a subsequent [Ca2+]i mobilisation; induce arachidonic acid release; and stimulate mitogen-activated protein kinase (MAPK) activity. Interestingly, these activities are antagonised by neurotensin and levocabastine in a concentration-dependent manner. These activities suggest that the human neurotensin NT2 receptor may be of physiological importance and that a natural agonist for the receptor may exist.