[3H]neurotensin(8-13) binds in human brain to the same sites as does [3H]neurotensin but with higher affinity

Development of a Fluorescent Ligand for the Intracellular Allosteric Binding Site of the Neurotensin Receptor 1

The membrane protein family of G protein-coupled receptors (GPCRs) represents a major class of drug targets. Over the last years, the presence of additional intracellular binding sites besides the canonical orthosteric binding pocket has been demonstrated for an increasing number of GPCRs. Allosteric modulators harnessing these pockets may represent valuable alternatives when targeting the orthosteric pocket is not successful for drug development. Starting from SBI-553, a recently discovered intracellular allosteric modulator for neurotensin receptor subtype 1 (NTSR1), we developed the fluorescent molecular probe 14. Compound 14 binds to NTSR1 with an affinity of 0.68 μM in the presence of the agonist NT(8-13). NanoBRET-based ligand binding assays with 14 were established to derive the affinity and structure-activity relationships for allosteric NTSR1 modulators in a direct and nonisotopic manner, thereby facilitating the search for and optimization of novel allosteric NTSR1 ligands. As a consequence of cooperativity between the ligands binding to the allosteric and orthosteric pocket, compound 14 can also be used to investigate orthosteric NTSR1 agonists and antagonists. Moreover, employing 14 as a probe in a drug library screening, we identified novel chemotypes as binders for the intracellular allosteric SBI-553 binding pocket of NTSR1 with single-digit micromolar affinity. These hits may serve as interesting starting points for the development of novel intracellular allosteric ligands for NTSR1 as a highly interesting yet unexploited drug target in the fields of pain and addiction disorder therapy.

Improved Pharmacokinetic and Pharmacodynamic Profile of Deuterium-Reinforced Tricyclic Antidepressants Doxepin, Dosulepin, and Clomipramine in Animal Models

BACKGROUND AND OBJECTIVES

Doxepin, dosulepin, and clomipramine are tricyclic antidepressants (TCAs) that act as serotonin and noradrenaline reuptake inhibitors. The metabolites formed by N-dealkylation of these tricyclic antidepressants contribute to overall poor pharmacokinetics and efficacy. Deuteration of the methyl groups at metabolically active sites has been reported to be a useful strategy for developing more selective and potent antidepressants. This isotopic deuteration can lead to better bioavailability and overall effectiveness. The objective is to study the effect of site-selective deuteration of TCAs on their pharmacokinetic and pharmacodynamic profile by comparison with their nondeuterated counterparts.

METHODS

In the current study, the pharmacokinetic profile and antidepressant behavior of deuterated TCAs were evaluated using the forced swim test (FST) and tail suspension test (TST), using male Wistar rats and male Swiss albino mice, respectively; additionally, a synaptosomal reuptake study was carried out.

RESULTS

Compared with the nondeuterated parent drugs, deuterated forms showed improved efficacy in the behavior paradigm, indicating improved pharmacological activity. The pharmacokinetic parameters indicated increased maximum concentration in the plasma (Cmax), elimination half-life (t1/2), and area under the concentration-time curve (AUC)  in deuterated compounds. This can have a positive clinical impact on antidepressant treatment. Synaptosomal reuptake studies indicated marked inhibition of the reuptake mechanism of serotonin (5-HT) and norepinephrine.

CONCLUSIONS

Deuterated TCAs can prove to be potentially better molecules in the treatment of neuropsychiatric disorders as compared with nondeuterated compounds. In addition, we have demonstrated a concept that metabolically active, site-selective deuteration can be beneficial for improving the pharmacokinetic and pharmacodynamic profiles of TCAs. A further toxicological study of these compounds is needed to validate their future clinical use.

Stabilization of pre-existing neurotensin receptor conformational states by β-arrestin-1 and the biased allosteric modulator ML314

The neurotensin receptor 1 (NTS1) is a G protein-coupled receptor (GPCR) with promise as a drug target for the treatment of pain, schizophrenia, obesity, addiction, and various cancers. A detailed picture of the NTS1 structural landscape has been established by X-ray crystallography and cryo-EM and yet, the molecular determinants for why a receptor couples to G protein versus arrestin transducers remain poorly defined. We used 13CεH3-methionine NMR spectroscopy to show that binding of phosphatidylinositol-4,5-bisphosphate (PIP2) to the receptor's intracellular surface allosterically tunes the timescale of motions at the orthosteric pocket and conserved activation motifs - without dramatically altering the structural ensemble. β-arrestin-1 further remodels the receptor ensemble by reducing conformational exchange kinetics for a subset of resonances, whereas G protein coupling has little to no effect on exchange rates. A β-arrestin biased allosteric modulator transforms the NTS1:G protein complex into a concatenation of substates, without triggering transducer dissociation, suggesting that it may function by stabilizing signaling incompetent G protein conformations such as the non-canonical state. Together, our work demonstrates the importance of kinetic information to a complete picture of the GPCR activation landscape.

Facile solid phase peptide synthesis with a Re-lysine conjugate generated via a one-pot procedure

We have synthesized a Re(CO)3-modified lysine via a one-pot Schiff base formation reaction that can be used in the solid phase peptide synthesis. To demonstrate its potential use, we have attached it to a neurotensin fragment and observed uptake into human umbilical vascular endothelial cells.

The discovery of indole full agonists of the neurotensin receptor 1 (NTSR1)

Neurotensin (NT) is an endogenous tridecapeptide found in the central nervous system (CNS) and in peripheral tissues. Neurotensin exerts a wide range of physiological effects and it has been found to play a critical role in a number of human diseases, such as schizophrenia, Parkinson's disease and drug addiction. The discovery of small-molecule non-peptide neurotensin receptor (NTSR) modulators would represent an important breakthrough as such compounds could be used as pharmacological tools, to further decipher the cellular functions of neurotensin, and potentially as therapeutic agents to treat human disease. Herein, we report the identification of non-peptide low-micromolar neurotensin receptor 1 (NTSR1) full agonists, discovered through structural optimization of the known NTSR1 partial agonist 1. In vitro cellular screenings, based on an intracellular Ca(2+) mobilization assay, revealed our best hit molecule 8 (SR-12062) to have an EC50 of 2 μM at NTSR1 with full agonist behaviour (Emax=100%), showing a higher efficacy and ∼90-fold potency improvement compared to parent compound 1 (EC50=178 μM; Emax=17%).

Glycosylated neurotensin analogues exhibit sub-picomolar anticonvulsant potency in a pharmacoresistant model of epilepsy

Neurotensin (NT) is an endogenous neuropeptide involved in a variety of central and peripheral neuromodulatory effects. Herein we show the effects of site-specific glycosylation on the in vitro and in vivo pharmacological properties of this neuropeptide. NT analogues containing O-linked disaccharides (beta-melibiose and alpha-TF antigen) or beta-lactose units linked by a PEG(3) spacer were designed and chemically synthesized using Fmoc chemistry. For the latter analogue, Fmoc-Glu-(beta-Lac-PEG(3)-amide) was prepared. Our results indicate that the addition of the disaccharides does not negatively affect the sub-nanomolar affinity or the low-nanomolar agonist potency for the neurotensin receptor subtype 1 (NTS1). Interestingly, three glycosylated analogues exhibited sub-picomolar potency in the 6 Hz limbic seizure mouse model of pharmacoresistant epilepsy following intracerebroventricular administration. Our results suggest for the first time that chemically modified NT analogues may lead to novel antiepileptic therapies.

The neurotensin-1 receptor agonist PD149163 blocks fear-potentiated startle

Preliminary evidence suggests that the neuropeptide, neurotensin (NT) may regulate fear/anxiety circuits. We investigated the effects of PD149163, a NT1 receptor agonist, on fear-potentiated startle (FPS). Sprague Dawley rats were trained to associate a white light with a mild foot shock. In one experiment, animals were treated with either subcutaneous vehicle or PD149163 (0.01, 0.1 or 1.0 mg/kg) 24 h after training. Twenty minutes later their acoustic startle response in the presence or absence of the white light was tested. In a second experiment, saline and 1.0 mg/kg PD149163 were tested using a separate group of rats. In the first experiment, PD149163 produced a non-significant decrease in baseline acoustic startle at all three doses. As expected, saline-treated rats exhibited significant FPS. An ANOVA of percentage FPS revealed no significant effect of treatment group overall but the high dose group did not display FPS strongly suggesting an FPS effect at this dose. This finding was confirmed in the second experiment where the high dose of PD149163 reduced percent FPS relative to saline (P < 0.05). These data suggest that systemically administered NT1 agonists modulate the neural circuitry that regulates fear and anxiety to produce dose-dependent anxiolytic-like effects on FPS.

Neurotensin agonists as an alternative to antipsychotics

Neurotensin (NT) is a 13 amino acid neuropeptide that is found in the central nervous system and in the gastrointestinal tract. In brain, this peptide is prominently associated anatomically with dopaminergic, as well as other neurotransmitter systems. Based on animal studies, already decades old, researchers have hypothesised that NT receptor agonists will have antipsychotic properties in patients. However, to date no one has obtained a non-peptide NT receptor agonist. Therefore, there has been great interest in obtaining peptide analogues of NT, that, unlike NT resist degradation by peptidases and cross the blood-brain barrier, yet have the pharmacological characteristics of native NT, for therapeutic use in the treatment of schizophrenia, as well as other neuropsychiatric diseases such as Parkinson's disease and addiction to psychostimulants. In this review, we present the rationale for development of NT receptor agonists for treatment of certain central nervous system diseases, as well as a review of those peptide agonists that are in early stages of development.

Reversal of sensorimotor gating deficits in Brattleboro rats by acute administration of clozapine and a neurotensin agonist, but not haloperidol: a potential predictive model for novel antipsychotic effects

Prepulse inhibition (PPI) of acoustic startle is decreased in unmedicated schizophrenia patients and similar deficits can be induced in rats through pharmacological, environmental, or neuroanatomical manipulations. Recently, we reported that Brattleboro (BB) rats, a Long Evans (LE) strain with a single gene mutation, have inherent deficits in PPI homologous to those observed in schizophrenia patients. We also reported that PPI deficits in BB rats could be reversed by chronic but not acute administration of 0.5 mg/kg haloperidol. No other dose or drug was tested in that experiment. In this study, we tested the effects of acute subcutaneous administration of several doses of haloperidol as well as the second-generation antipsychotic, clozapine, and the putative novel antipsychotic, PD149163, a neurotensin mimetic that crosses the blood-brain barrier. Consistent with our previous report, BB rats exhibited PPI deficits compared to LE rats and none of the doses of haloperidol produced a significant effect on this PPI deficit. In contrast, 10 and 15 mg/kg of clozapine and all the doses of PD149163 tested reversed the PPI deficits in BB rats. In addition, haloperidol, but not clozapine or PD149163 produced significant catalepsy in BB rats, supporting the notion that PD149163 has a profile consistent with atypical antipsychotics and providing support for the predictive validity of the PPI results. These results further strengthen the notion that the BB rat is a useful predictive model of antipsychotic efficacy and suggest that this model may differentiate between antipsychotics belonging to different therapeutic categories, for example, first- and second-generation agents.

The effects of systemic NT69L, a neurotensin agonist, on baseline and drug-disrupted prepulse inhibition

Centrally administered neurotensin (NT) produces behavioral and biochemical effects that are very similar to the effects of antipsychotic drugs. Therefore, there is much interest in the potential use of NT agonists as antipsychotic drugs. We have previously reported that PD149163, a NT(8-13) analogue, produced effects on prepulse inhibition (PPI) of startle after systemic administration that were suggestive of an atypical antipsychotic-like drug profile. To determine if these effects are shared by other peripherally administered NT agonists, we tested the effects of NT69L, a recently developed NT agonist that penetrates the CNS, on drug-induced PPI deficits. In the first experiment, rats received subcutaneous (s.c.) injections of NT69L (vehicle, 0.08, 0.25, and 1.0mg/kg) followed 30min later by subcutaneous saline or D-amphetamine (2.0mg/kg). In the second experiment, NT69L injections were followed by saline or the non-competitive NMDA antagonist dizocilpine (0.1mg/kg). Both D-amphetamine and dizocilpine significantly decreased PPI as expected. In the first experiment, NT69L significantly increased PPI levels at baseline and after D-amphetamine. In the second experiment, NT69L attenuated PPI deficits produced by dizocilpine, without increasing baseline PPI. In addition, NT69L had no effect on startle magnitude. The effects of NT69L in these studies were similar in some ways to the effects of PD149163 and were also consistent with the preclinical effects of atypical antipsychotic drugs. These data provide further support for the notion that NT agonists may have use as novel antipsychotic drugs. Furthermore, the ability of NT69L and PD149163 to attenuate dizocilpine-disrupted PPI, an antipsychotic drug effect not mediated by dopamine, suggests that NT agonists may produce some of their antipsychotic-like effects by modulating neurotransmitter systems other than dopamine, such as serotonin, noradrenaline or glutamate.

Synthesis and biological studies of novel neurotensin(8-13) mimetics

Novel neurotensin (NT) (8-13) (Arg(8)-Arg(9)-Pro(10)-Tyr(11)-Ile(12)-Leu(13)) mimetics 3, 4 were designed by adopting all intrinsic functional groups of the native neurotensin(8-13) and using a substituted indole as a template to mimic the pharmacophore of NT(8-13). Biological studies at subtype 1 of the NT receptor showed that 3 has a 55 and 580 nM binding affinity at rat and human neurotensin receptors, respectively. As a comparison, compounds 5 and 6 were also synthesized. The binding difference between 3, 4 and 5, 6 argues the importance of the carboxylic group in achieving higher potency NT(8-13) mimetics.

Neurotensin: peptide for the next millennium

Neurotensin is an endogenous tridecapeptide neurotransmitter (pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Try-Ile-Leu-OH) that was discovered by Carraway and Leeman in bovine hypothalami in the early 1970s. Since then this peptide has been the subject of a multitude of articles detailing discoveries related to its activity, receptors, localization, synthesis, and interactions with other systems. This review article does not intend to summarize again all the history of this fascinating peptide and its receptors, since this has been done quite well by others. The reader will be directed to these other reviews, where appropriate. Instead, this review attempts to provide a summary of current knowledge about neurotensin, why it is an important peptide to study, and where the field is heading. Special emphasis is placed on the behavioral studies, particularly with reference to agonists, antagonists, and antisense studies, as well as, the interaction of neurotensin with other neurotransmitters.

Highly potent neurotensin analog that causes hypothermia and antinociception

The tridecapeptide neurotensin has long been proposed as an endogenous neuroleptic. However, for neurotensin [or neurotensin(8-13) [NT(8-13)], the active fragment] to cause its effects, it must be administered centrally. Here, we report on an analog of NT(8-13), (N-methyl-Arg),Lys,Pro,L-neo-Trp,tert-Leu,Leu (NT69L), which contains a novel amino acid, L-neo5 degrees C (rectal), with a significant effect persisting for over 7 h. NT69L also caused a rapid (within 15 min) and persistent (for over 5 h) antinociceptive effect, as determined by the hot plate test. NT69L was overall the most potent and longest lasting neurotensin analog that has been reported. These studies provide the background for further testing of a stable, potent and long lasting neurotensin analog as a potential neuroleptic.

In vitro binding and CNS effects of novel neurotensin agonists that cross the blood-brain barrier

Animal studies with neurotensin (NT) directly injected into brain suggest that it has pharmacological properties similar to those of antipsychotic drugs. Here, we present radioligand binding data for some novel hexapeptide analogs of NT(8-13) at the molecularly cloned rat and human neurotensin receptors (NTR-1), along with behavioral and physiological effects of several of these peptides after intraperitoneal (i.p.) administration in rats. One unique analog, NT66L, which had high affinity (0.85 nM) for the molecularly cloned rat neurotensin receptor (NTR-1), caused a drop in body temperature and antinociception at doses as low as 0.1 mg/kg after i.p. injection. At 30 min post-injection, the ED50 for NT66L-induced hypothermia (rectal temperature) and antinociception (hot plate test) was 0.5 and 0.07 mg/kg, respectively. At a dose of 1 mg/kg i.p., NT66L caused 100% of the maximum possible effect for antinociception for up to 2 h after administration. At this dose body temperature lowering was greater than -2.5 degrees C from 20 to 120 min after i.p. administration. These results in animals suggest that NT66L has agonist properties at NTR-1 in vivo after extracranial administration and provide support for its further study in behavioral tests predictive of neuroleptic activity.

Evidence for additional neurotensin receptor subtypes: neurotensin analogs that distinguish between neurotensin-mediated hypothermia and antinociception

Neurotensin (NT), a tridecapeptide, is a neurotransmitter that elicits potent effects including hypothermia and antinociception in mice and rats. To date, there are two types of the neurotensin receptor (NTR) that have been molecularly cloned from the rat. However, several lines of evidence suggest the presence of additional NTR subtypes. We have identified a NT analog of the NT(8-13) fragment, NT27, that selectively causes only the hypothermic response in vivo, when microinjected into the periaqueductal gray (PAG) of rats. A dose of 18 nmol of NT or NT27 caused a body temperature lowering of 1.8 and 1.2 degrees C, respectively. This same dose of NT or NT27 yielded a hotplate maximum physiological effect of 75% and 25%, respectively. Interestingly, despite its high KD (620 nM) at the cloned NTR-1, NT27-I (the iodinated form of NT27) exerted a potent hypothermic effect even at a very low dose (0.6 nmol). Equally intriguing, was that NT24, a sterioisomer of NT27, with a much higher affinity (KD=0. 5 nM) at NTR-1, did not selectively induce hypothermia in mice, but did selectively induce hypothermia in rats.

In vivo studies with low doses of levocabastine and diphenhydramine, but not pyrilamine, antagonize neurotensin-mediated antinociception

The present study describes in vivo experiments in the rat addressing the role of levocabastine, and two other specific histamine H1 antagonists, diphenhydramine and pyrilamine, at neurotensin (NT)-mediated hypothermia and antinociception (hotplate). Levocabastine given i.p. or microinjected directly into the periaqueductal gray (PAG) did not cause antinociception or hypothermia. This indicates that despite the results with the recently-cloned levocabastine-sensitive NT receptors (NTR) in the rat (NTR-2) and mouse (NTRL), levocabastine by itself does not mediate either hypothermia or antinociception at NT receptors. However, pretreatment with 5 or 50 microg/kg of levocabastine or 5 microg/kg diphenhydramine all caused over a three-fold reduction in NT-mediated antinociception. Higher doses (500 or 5000 microg/kg) of levocabastine did not cause any antagonism of NT-mediated antinociception. All three antihistamines did not affect NT-mediated hypothermia. In addition, histamine H1 pathways are not involved in NT-mediated antinociception, as pretreatment with the much more potent histamine H1 antagonist pyrilamine did not affect antinociception mediated by NT. Therefore, these data may suggest the presence of yet unidentified NTR subtypes responsible for NT-mediated hypothermia and antinociception.

Neurotensin receptors in the human amygdaloid complex. Topographical and quantitative autoradiographic study

The distribution of high affinity 125I-neurotensin (NT) binding sites were investigated in the amygdaloid complex of adult humans by means of dry film and emulsion autoradiography. Autoradiograms were analysed quantitatively using [125I] standards and an image analyser system, and data obtained were converted to nCi of ligand bound per mg tissue. High densities of 125I-NT binding sites were found in the following amygdaloid structures the dorsal part of the accessory basal nucleus, the medial part of the cortical nucleus, the lateral subdivision of the central nucleus, the paralaminar nucleus, the amygdalohippocampal transition area and the rostral portions of the anterior amygdaloid area. The ventral part of the accessory basal nucleus, the intercalated cell groups and the remaining parts of the anterior amygdaloid area showed moderate density of NT binding sites, while the medial, basal and lateral amygdaloid nuclei, the lateral part of the cortical nucleus, the medial subdivision of the central nucleus, as well as the corticoamygdaloid transition area exhibited low densities of 125I-NT binding sites. At microscopic level, silver grains appeared more or less evenly distributed over both neuronal perikarya and the surrounding neuropil. In comparison to NT-immunoreactivity, NT receptors showed mismatching distribution throughout most parts of the amygdala, with the exception of the lateral subdivision of the central nucleus, where NT-immunoreactive perikarya and nerve fibers as well as 125I-NT binding sites were found in high density.

Proposed ligand binding site of the transmembrane receptor for neurotensin(8-13)

We report here the first proposed ligand binding site of the transmembrane receptor for neurotensin(8-13) in human and rat, the corresponding bound conformation of the peptide ligand, and site-directed mutagenesis studies that support the binding site model. These three-dimensional structures were generated by using a heuristic approach in conjunction with experimental data. The proposed neurotensin(8-13) binding site is primarily composed of eight residues (i.e., Phe326, Ile329, Trp334, Phe337, Tyr339, Phe341, Tyr342, and Tyr344 in the human receptor; Phe331, Ile334, Trp339, Phe342, Phe344, Phe346, Tyr347, and Tyr349 in the rat receptor) located in the third extracellular loop. The seven aromatic residues form an aromatic pocket on the extracellular surface of the neurotensin receptor to accommodate its ligands apparently by cation-pi, pi-pi, and hydrogen bonding interactions. The neurotensin(8-13) ligand adopts a compact conformation at the proposed binding site. In the bound conformation of neurotensin(8-13), the backbone of Arg9-Pro10-Tyr11-Ile12 forms the proline type I turn, and the hydroxy group of Tyr11 interacts with the two guanidinium groups of Arg8 and Arg9. These guanidinium groups are curled toward the hydroxy group so that they interact electrostatically with the hydroxy group, and that the guanidinium group of Arg9 forms an intra-hydrogen bond with the hydroxy group. The proposed three-dimensional structure may not only provide a basis for rationalizing mutations of the neurotensin receptor gene but also offer insights into understanding the binding of many neurotensin analogs, biological functions of the neurotensin receptors, and structural elements for species specificity of the neurotensin receptors, and may expedite developing nonpeptidic neurotensin mimetics for the potential treatment of the neuropsychiatric diseases.

Rational design of novel neurotensin mimetics: discovery of a pharmacologically unprecedented agent exhibiting concentration-dependent dual effects as antagonist and full agonist

We report the rational design of novel neurotensin mimetics through use of the Multiple Template Approach. This approach is based on our notion that a flexible peptide can be replaced by a partially flexible molecule, identified through testing a comparatively small number of molecules possessing a different intrinsic availability of conformations of the native peptide. The Multiple Template Approach has culminated in the discovery of a pharmacologically unprecedented agent, which behaves as a neurotensin antagonist at low concentration and as a full neurotensin agonist at high concentration.

The pharmacology of neurotensin analogues on neurones in the rat substantia nigra, pars compacta in vitro

The effects of neurotensin and neurotensin analogues on dopamine neurones were studied in an in vitro slice preparation of rat substantia nigra, pars compacta using extracellular and intracellular recording techniques. Neurotensin had an EC50 of 13 nM in these experiments. This study showed that the C-terminal 5 amino acids of neurorotensin in neurotensin-(9-13) retained agonist activity on substantia nigra neurones. The naturally occurring neurotensin analogues neuromedin N and avian neurotensin were also active whilst kinetensin was inactive. Kinetensin differs from the C-terminal neurotensin 5-amino acids by two amino acids. The difference between the activity of neurotensin and the inactivity of kinetensin was investigated using synthetic peptides which contained single amino acid substitutions. These results show that position 12 of neurotensin is important for agonist activity in the substantia nigra.

Co-occurrence of gamma-aminobutyric acid, parvalbumin and the neurotensin-related neuropeptide LANT6 in pallidal, nigral and striatal neurons in pigeons and monkeys

Immunohistochemical double-labeling techniques were used to examine the co-localization of the neurotransmitter gamma-aminobutyric acid (GABA), the calcium-binding protein parvalbumin and the neurotensin-related hexapeptide LANT6 in neurons of the striatum and its target areas in pigeons and monkeys. The studies revealed the existence of a population of striatal interneurons apparently containing all three of these substances in both monkeys and pigeons. The results also revealed that GABA and LANT6 were co-localized in numerous pallidal and nigral reticulata neurons that also contained parvalbumin in both species. Examination of diverse other cell groups in avian forebrain and midbrain revealed that parvalbumin and LANT6 were typically co-localized to GABAergic neurons. In light of the presence of pallidal, reticulata and striatal neurons containing these three substances in two widely divergent amniote groups such as pigeons and monkeys, it seems likely that: (1) comparable neuronal populations are present in other avian and mammalian species; and (2) these neuronal populations play a fundamental role in basal ganglia functions that requires these three substances.

Mesencephalic microinjections of neurotensin-(1-13) and its C-terminal fragment, neurotensin-(8-13), potentiate brain stimulation reward

Using the curve shift method, we assessed the effects of ventromedial mesencephalic tegmental (VMT) microinjections of an equimolar concentration of neurotensin-(1-13) (NT-(1-13)) and of its C-terminal fragment, neurotensin-(8-13) (NT-(8-13)), on operant responding for rewarding electrical stimulation of the caudal mesencephalic central gray. The effects of NT-(1-13) and NT-(8-13) on brain stimulation reward (BSR) were also compared to those of systemically administered quinpirole (0.1 and 0.2 mg/kg, s.c.), a direct dopamine agonist, and GBR-12909 (10 and 20 mg/kg, i.p.), a selective dopamine uptake blocker. At the concentration injected, NT-(8-13) was as effective as NT-(1-13) at facilitating BSR, producing significant leftward shifts of the function relating the rate of responding to the stimulation frequency (R/F function); neither form of the peptide was effective when injected in regions dorsal to the VMT. Similarly, GBR-12909 produced a parallel leftward shift of the R/F function, but, unlike NT-(1-13), also significantly increased the asymptotic rates of responding. In contrast, the high dose of quinpirole produced non-parallel leftward shifts of the R/F function and suppressed the asymptote. The similarity between the effects of neurotensin and GBR-12909 on one hand, and the differences between those of neurotensin and quinpirole on the other, suggest that activation VMT neurotensin receptors potentiate BSR by enhancing increases in dopamine neurotransmission that are contingent upon operant responding or rewarding brain stimulation, or both.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase C activation by neurotensin and neuromedin N in Chinese hamster ovary cells expressing the rat neurotensin receptor

The rat neurotensin receptor cDNA sequence was transfected in Chinese hamster ovary cells and cellular clones which stably express the corresponding protein were isolated and characterized. The Scatchard analysis of the specific binding of [3H]neurotensin indicated a Kd value of 0.45 +/- 0.08 nM and a Bmax value of 3.27 +/- 0.29 pmol/mg of protein. Displacement experiments using peptidic analogs of neurotensin and levocabastine confirmed that the transfected receptor exhibits the binding properties of the neurotensin receptor characterized in the rat brain. Neurotensin stimulated the phosphoinositides hydrolysis in a time- and concentration-dependent manner and this effect was mimicked by neurotensin(8-13) and by neuromedin N. The stimulation of phosphoinositides hydrolysis was not inhibited by pertussis toxin. These results indicate that the transfected cells actively express the rat neurotensin receptor which is functionally coupled to phospholipase C through a pertussis toxin-insensitive GTP-binding protein, and that neuromedin N is able to induce the phosphoinositides turnover by interaction with the neurotensin receptor.

Effects of neurotensin on midbrain dopamine neurons: are they mediated by formation of a neurotensin-dopamine complex?

The effects of neurotensin on midbrain dopamine neuron activity were studied in brain slices using single-unit recording techniques. At low concentrations (0.2-10 nM), neurotensin attenuated dopamine-induced inhibition without a significant effect on the basal firing rate. At higher concentrations (greater than 10 nM), however, it consistently caused an increase in cell activity. At even higher concentrations (greater than 100 nM), a sudden cessation of cell activity preceded by an increase in firing rate was observed. Whether this effect of neurotensin was due to depolarization inactivation or to a toxic effect of the peptide at high concentrations remains to be determined. To determine whether the effects of neurotensin were mediated by formation of a neurotensin-dopamine complex, several neurotensin analogues were studied. Neurotensin (8-13), which binds to both neurotensin receptors and dopamine, mimicked the effects of native neurotensin. Neuromedin N, which competes with neurotensin for the same receptor but does not bind to dopamine, also mimicked the effects. However, neurotensin (1-11), which forms a complex with dopamine but is inactive in competing for neurotensin receptors, was ineffective. In addition, the excitatory effect of neurotensin was not attenuated in the presence of dopamine receptor blockade by sulpiride. These results suggest that formation of a neurotensin-dopamine complex may not account for the action of neurotensin on dopamine cells. When combined with the fact that there is a high density of neurotensin receptors on dopamine cells, our results support the suggestion that the observed effects of neurotensin on dopamine neurons are most likely mediated by an activation of neurotensin receptors.

Neurotensin high affinity binding sites and endopeptidase 24.11 are present respectively in the meningothelial and in the fibroblastic components of human meningiomas

The presence of neurotensin receptors and endopeptidase 24.11 (E-24.11) in 16 human meningioma specimens, obtained at surgery, was assessed by measuring the binding of 125I-[tyrosyl3]neurotensin(1-13) (125I-NT) and the inhibitor 3H-N(2RS)-3-hydroxyaminocarbonyl-2-benzyl-1-oxopropyl)glycine (3H-HACBO-Gly), for the receptor and enzyme, respectively. E-24.11 activity was also measured. Autoradiography, on the 16 meningiomas, showed that specific 125I-NT labeling (nonspecific labeling was assessed in the presence of excess NT) was exclusively located in the meningothelial regions. In contrast, specific 3H-HACBO-Gly labeling (nonspecific labeling was assessed in the presence of an excess of the E-24.11 inhibitor thiorphan) was exclusively found in fibroblastic regions. No specific labeling of either ligand was found on collagen or blood vessels. In vitro binding assays were performed on membranes of 10 of the 16 meningiomas. In the 4 meningiomas rich in meningothelial cells, 125I-NT specifically bound to one population of sites with Bmax ranging from 57 to 405 fmol/mg protein and Kd around 0.3 nM. These sites share common properties with the brain NT receptor, since the carboxy terminal acetyl NT(8-13) fragment bound to the same sites but with a higher affinity. The carboxy terminal analogue of NT, neuromedin N, also bound to the same sites with a 10-fold lower affinity and the sites were bradykinin and levocabastine insensitive. In the 4 meningiomas rich in fibroblastic cells, 3H-HACBO-Gly specifically bound to one population of sites with Bmax ranging from 251 to 739 fmol/mg protein and Kd around 2.8 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Canine neurotensin, neurotensin6-13 and neuromedin N: primary structures and receptor activity

Canine neurotensin (NT) and neuromedin N (NMN) were isolated from extracts of ileal mucosa using radioimmunoassay for detection. The structures determined were consistent with those predicted by earlier cDNA work. The molar ratio of NT to NMN was ca. 7, suggesting that the NT/NMN precursor, which contains one copy of each peptide, undergoes complex posttranslational processing or that other NT-precursors lacking NMN exist. In addition to NT, small quantities of NT6-13 and NT2-13 were obtained. Native and synthetic preparations of these peptides were indistinguishable in a radioreceptor assay employing rat brain membranes and 125I-labeled NT; NT6-13 was ca. 8-times more potent than NT and NMN was about one-sixth as potent as NT. NT6-13 was also ca. 10 times more potent than NT in inhibiting spontaneous contractile activity in longitudinally-oriented smooth muscle strips of porcine jejunum. Preparations of intestinal N-cells as well as N-cell vesicles also appeared to contain NT2-13 and NT6-13; however, it is not yet clear whether these peptides are utilized physiologically or simply represent metabolites of NT. These results suggest that further work on the processing of NT precursor and on biologic abilities of partial sequences of NT could be fruitful.

Neurotensin reduces the affinity of dopamine D2 receptors in membranes from post mortem human caudate-putamen

The effects of neurotensin in vitro were investigated on the binding characteristics of N-[3H]propylnorapomorphine [( 3H]NPA) binding sites in crude membrane preparations from post mortem human caudate-putamen, obtained within 10 h after death from 3 patients without reported neurological or psychiatric diseases. Neurotensin (0.1-30 nM) was found to increase the KD value of [3H]NPA binding sites with a maximal increase of about 50% (420 +/- 45 pM) at 3 nM of neurotensin, without significantly affecting the Bmax value (123 +/- 20 pmol/g protein). These results indicate the existence of an intramembrane regulation by neurotensin receptors of D2 receptors in the human caudate-putamen.

Neurotensin(8-13): comparison of novel analogs for stimulation of cyclic GMP formation in neuroblastoma clone N1E-115 and receptor binding to human brain and intact N1E-115 cells

Neurotensin(8-13), the carboxyl-terminal portion of neurotensin, is 4-50 times more potent than native neurotensin in binding to intact neuroblastoma N1E-115 cells and human brain tissue and in stimulation of intracellular cyclic GMP production and inositol phospholipid hydrolysis in clone N1E-115 (Gilbert JA and Richelson E, Eur J Pharmacol 99: 245-246, 1984; Gilbert JA et al., Biochem Pharmacol 35: 391-397, 1986; Kanba KS et al., J Neurochem 46: 946-952, 1986; and Kanba KS and Richelson E, Biochem Pharmacol 36: 869-874, 1987). A series of novel analogs of neurotensin (8-13) was synthesized, and a structure-activity study was done comparing the abilities of these peptides to stimulate intracellular cyclic GMP production in intact neuroblastoma clone N1E-115 and to inhibit the binding of [3H]neurotensin to these cells and to membranal preparations from human brain. A direct correlation was found for each analog between its EC50 for biochemical activity and its KD for binding ability in studies with clone N1E-115. Furthermore, a strong correlation existed for each peptide between its KD for binding to neurotensin receptors on these cells and its KD for binding to neurotensin receptors in human brain tissue. In this study, the residues that were important to the biochemical and binding activities of neurotensin (8-13) proved to be identical to the amino acids that are necessary for the functional integrity of native neurotensin (Gilbert JA et al., Biochem Pharmacol 35: 391-397, 1986.

Neurotensin receptors in the human spinal cord: a quantitative autoradiographic study

The anatomical localization of neurotensin receptors in the human spinal cord was examined in 12 cases aged 4-68 years using quantitative autoradiographic methods following the incubation of fresh, unfixed cryostat sections with 4 nM [3H]neurotensin. Characterization of the pharmacological specificity of the [3H]neurotensin binding sites in the human spinal cord from displacement studies with neurotensin and various neurotensin fragments indicated that, whereas 1.0 microM neurotensin and the carboxy-terminal fragment neurotensin almost completely displaced [3H]neurotensin binding (4 nM), the amino-terminal fragments neurotensin and neurotensin1-11 were weak inhibitors. This requirement for the carboxy-terminal fragment neurotensin is consistent with [3H]neurotensin binding to specific neurotensin receptors in the human spinal cord. In all cases the autoradiograms demonstrated that neurotensin receptors were distributed in a similar fashion in the gray matter of the cervical, thoracic, lumbar, sacral and coccygeal regions of the human spinal cord. At all 21 spinal levels examined, the highest density of neurotensin receptors was localized in lamina II of the dorsal horn. Within lamina II the receptors were especially concentrated in the deeper inner segment (IIi) where they formed a dense band lying immediately dorsal to lamina III. The density of receptors in this inner region of lamina II (23.5 fmol/mg) was almost double that in the outer segment of lamina II (12.2 fmol/mg), which showed the next highest density of receptors, and more than three times that in the adjacent lamina I (6.9 fmol/mg) and lamina III (7.1 fmol/mg). A moderate density of receptors was present in the intermediomedial (8.0 fmol/mg) and intermediolateral (8.0 fmol/mg) nuclei of lamina VII, and in lamina IX (4.4 fmol/mg). The density of labelling in the remaining laminae of the spinal cord was very low. These results indicate that neurotensin receptors are mainly localized in somatic and visceral sensory and motor regions of the human spinal cord and suggest that neurotensin may play a role in modulating sensory-motor functions in the human spinal cord.