Neurotensin stimulates formation of cyclic GMP in murine neuroblastoma clone N1E-115

NT Agonist Regulates Expression of Nuclear High-affinity Neurotensin Receptors

Neurotensin (NT) exerts multiple functions in the central nervous system and peripheral tissues. Its actions are mainly mediated by a high-affinity G-protein-coupled receptor, the NT-1 receptor. In this study we demonstrated a nuclear NT binding site in different cellular models. We first noted that a large percentage of NT-1 receptor cell body immunoreactivity was located in the nuclear soma and nuclear envelope of rat substantia nigra, a brain area rich in NT-containing axon terminals. The NT-1 receptor was also visualized in purified nuclei from CHO cells stably transfected with NT-1 receptor coupled to the enhanced green fluorescence protein by immunocytochemistry. We observed that both the nuclear envelope and the nuclear soma were labeled, and the labeling intensity significantly increased after NT agonist treatment. These results suggested that NT-1 receptors, present in both the nuclear soma and the nuclear envelope, can be modulated by the ligand. Lastly, [125I]-NT binding experiments performed on isolated nuclei from a human lung cancer cell line endogenously expressing NT-1 receptor and NT, LNM35, revealed the existence of nuclear Gpp(NHp)-sensitive binding sites. These binding sites markedly decreased when cells were chronically treated with an NT-1 receptor antagonist, SR 48692. Taken together, these data suggest that the agonist regulates the expression of nuclear NT-1 receptors.

Elucidating the role of neurotensin in the pathophysiology and management of major mental disorders

Neurotensin (NT) is a neuropeptide that is closely associated with, and is thought to modulate, dopaminergic and other neurotransmitter systems involved in the pathophysiology of various mental disorders. This review outlines data implicating NT in the pathophysiology and management of major mental disorders such as schizophrenia, drug addiction, and autism. The data suggest that NT receptor analogs have the potential to be used as novel therapeutic agents acting through modulation of neurotransmitter systems dys-regulated in these disorders.

Diverse roles of neurotensin agonists in the central nervous system

Neurotensin (NT) is a tridecapeptide that is found in the central nervous system (CNS) and the gastrointestinal tract. NT behaves as a neurotransmitter in the brain and as a hormone in the gut. Additionally, NT acts as a neuromodulator to several neurotransmitter systems including dopaminergic, sertonergic, GABAergic, glutamatergic, and cholinergic systems. Due to its association with such a wide variety of neurotransmitters, NT has been implicated in the pathophysiology of several CNS disorders such as schizophrenia, drug abuse, Parkinson's disease (PD), pain, central control of blood pressure, eating disorders, as well as, cancer and inflammation. The present review will focus on the role that NT and its analogs play in schizophrenia, endocrine function, pain, psychostimulant abuse, and PD.

Role of neuropeptides in appetite regulation and obesity--a review

Obesity represents the most prevalent nutritional problem worldwide which in the long run predisposes to development of diabetes mellitus, hypertension, endometrial carcinoma, osteoarthritis, gall stones and cardiovascular diseases. Despite significant reductions in dietary fat consumption, the prevalence of obesity is on a rise and is taking on pandemic proportions. Obesity develops when energy intake exceeds energy expenditure over time. Recently, a close evolutionary relationship between the peripheral and hypothalamic neuropeptides has become apparent. The hypothalamus being the central feeding organ mediates regulation of short-term and long-term dietary intake via synthesis of various orexigenic and anorectic neuropeptides. The structure and function of many hypothalamic peptides (neuropeptide Y (NPY), melanocortins, agouti-related peptide (AGRP), cocaine and amphetamine regulated transcript (CART), melanin concentrating hormone (MCH), orexins have been characterized in rodent models The peripheral neuropeptides such as cholecystokinin (CCK), ghrelin, peptide YY (PYY3-36), amylin, bombesin regulate important gastrointestinal functions such as motility, secretion, absorption, provide feedback to the central nervous system on availability of nutrients and may play a part in regulating food intake. The pharmacological potential of several endogenous peripheral peptides released prior to, during and/or after feeding are being explored. Long-term regulation is provided by the main circulating hormones leptin and insulin. These systems implicated in hypothalamic appetite regulation provide potential targets for treatment of obesity which could potentially pass into clinical development in the next 5 years. This review summarizes various effects and interrelationship of these central and peripheral neuropeptides in metabolism, obesity and their potential role as targets for treatment of obesity.

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.

In vitro analysis of stable, receptor-selective neurotensin[8-13] analogues

A set of neurotensin[8-13] (NT[8-13]) analogues featuring substitution of non-natural cationic amino acids in the Arg(8) position have been synthesized and tested for binding potencies against the three cloned human NT receptors (hNTR-1, hNTR-2, hNTR-3), functional agonism of the hNTR1 and for rat serum stability. Three distinct classes of peptides have been identified: Class 1 features alkyl-Arg analogues at Arg(8), Class 2 features alpha-azido-cationic amino acids at Arg(8), and Class 3 feature modified Arg(8) and Tyr(11) residues. Most of the peptides maintain or exceed the binding potency of NT[8-13] to hNTR-1. Class 2 analogues exceed the binding potency of NT[8-13] to hNTR-2 with KK19 binding with higher affinity to hNTR-2 than hNTR-1. Peptides with enhanced binding potencies for hNTR-3 were not found. All analogues are functional agonists of the hNTR1 receptor as indicated by phosphoinositide (PI) determination. Serum stability increased with peptide classification where the half-life of Class 1 < Class 2 < Class 3 which are stable to rat serum for > 24 h.

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-mediated activation of MAPK pathways and AP-1 binding in the human pancreatic cancer cell line, MIA PaCa-2

Neurotensin (NT), a gastrointestinal (GI) hormone, binds its receptor (NTR) to stimulate proliferation of normal and neoplastic GI tissues; the molecular mechanisms remain largely undefined. Mitogen-activated protein kinases (MAPKs) are a family of intracellular kinases that transmit mitogenic signals by translocating to the nucleus and activating transcription factors. The purposes of this study were: (1) to identify whether the MAPKs (ERK1/2 and JNK) are activated by NT and (2) to determine the effect of NT on downstream transcription factors using the human pancreatic adenocarcinoma cell line, MIA PaCa-2, which possesses high-affinity NTR. Both ERK and JNK activity were stimulated within 3-6 min by treatment with NT (10 nM); steady-state levels of ERK and JNK protein were unchanged. Moreover, NT treatment resulted in increased AP-1 binding activity as determined by gel shift analysis. Delineating the signal transduction mechanisms regulating the cellular effects of NT will provide important insights into the molecular pathways responsible for NT-mediated effects on both normal and neoplastic cells.

Analysis of binding sites and efficacy of a species-specific peptide at rat and human neurotensin receptors

We have developed a neurotensin analog, L-[3,1'-naphthylalanine11]NT(8-13), NT34, that can distinguish between rat and human neurotensin receptors, and exhibits more than a 100-fold difference in binding affinities and a 60-fold difference in functional coupling to phosphatidylinositol turnover. Using cells transfected with different numbers of the appropriate receptors, we measured the changes in phosphatidylinositol production, and then evaluated the efficiency of receptor-effector coupling based on Furchgott's design. The binding of NT34 at both rat and human neurotensin receptors stably expressed in CHO-K1 cells was to two sites, while the binding of NT was to one site. At the rat receptor the equilibrium dissociation constant (Kd) for NT34 at the high-affinity site was 0.058 nM, while that at the low-affinity site was 3.1 nM. For the human receptor at the high-affinity site, the Kd for NT34 was 18 nM, while that at the low-affinity site was 180 nM. For both species the percentage of receptors representing the high-affinity site was approximately 60-70% with 30-40% at the low-affinity site. We derived agonist dissociation constants (Ka) for NT and NT34, which suggest that for NT34, the low-affinity site is functionally coupled to phosphatidylinositol turnover. Finally, we compared the relative efficacies of both compounds and found that NT34 was about 2-fold and 4-fold more efficacious than NT in stimulating phosphatidylinositol turnover in rat and human NT receptors, respectively.

Characterization of high affinity neurotensin receptor NTR1 in HL-60 cells and its down regulation during granulocytic differentiation

1. We investigated responses to neurotensin in human promyelocytic leukaemia HL-60 cells. 2. Neurotensin increased the cytosolic calcium concentration ([Ca2+]i) in a concentration-dependent manner and also produced inositol 1,4,5-trisphosphate (InsP3). 3. Among the tested neurotensin analogues, neurotensin 8-13, neuromedin-N, and xenopsin also increased [Ca2+]i, whereas neurotensin 1-11 and neurotensin 1-8 did not elicit detectable responses. 4. SR48692, an antagonist of NTR1 neurotensin receptors, blocked the neurotensin-induced [Ca2+]i increase, whereas levocabastine, which is known as an NTR2 neurotensin receptor antagonist, did not attenuate the neurotensin-evoked effect. 5. The expression of NTR1 neurotensin receptors was confirmed by Northern blot analysis and reverse transcriptase-polymerase chain reaction (RT-PCR). 6. During 1.25% dimethylsulfoxide (DMSO)-triggered granulocytic differentiation of HL-60 cells, the neurotensin-induced [Ca2+]i rise became gradually smaller and completely disappeared 4 days after treatment with DMSO. The mRNA level for neurotensin receptors was also decreased after differentiation. 7. The results show that HL-60 cells express NTR1 neurotensin receptors and suggest that granulocytic differentiation involves transcriptional regulation of the receptors resulting in down-regulation of the neurotensin-induced signalling.

Simultaneous activation of two different receptor systems by enkephalin/neurotensin conjugates having spacer chains of various lengths

Bivalent ligands composed of enkephalin and neurotensin were prepared and their action in the receptor-receptor interaction was studied with the neuroblastoma cell, NG108-15. Enkephalin was connected via oligosarcosine spacer chain to the N-terminus of neurotensin(8-13). The bivalent ligand stimulated the cGMP production more than plain neurotensin. The affinity of the bivalent ligands for the neurotensin receptor changed with varying lengths of the spacer chain. When the spacer chain was an octamer or a dodecamer of sarcosine, the receptor affinity of those bivalent ligands was higher than neurotensin(1-13), and significantly decreased in the presence of excess amount of enkephalin. These results suggest that the bivalent ligands bind to opioid receptor and neurotensin receptor simultaneously, leading to receptor-receptor interaction. On the other hand, some bivalent ligands, especially that without a spacer chain seemed to bind to the neurotensin receptor by the help of the enkephalin part interacting with a receptor exosite.

Neurotensin enhances agonist-induced cAMP accumulation in PC3 cells via Ca2+ -dependent adenylyl cyclase(s)

A human prostate cancer cell line (PC3) with abundant neurotensin (NT) receptors was used to demonstrate that NT potentiated 3',5'-cyclic adenosine monophate (cAMP) accumulation in response to a variety of stimuli, including both direct forskolin (F) and indirect (prostaglandin, (PGE2), isoproterenol (ISO) and cholera toxin (CTx)) activators of adenylyl cyclase. Several mechanisms were investigated and our results indicated an effect on the rate of cAMP formation and not on degradation or extrusion. For each stimulus, NT enhanced efficacy without altering EC50. The effect of NT did not involve stimulatory G-protein (Gs)-activation or interference with a tonic inhibitory G-protein (Gi)-mediated inhibition. A similar response was obtained when NT was added with the stimulus or given as a two minute pulse which was removed prior to addition of stimulus. The potentiating activity disappeared with a t1,2 of approximately 15 min. NT transiently elevated cellular [Ca2+]i and its effects on cAMP could be mimicked by [Ca2+]i-elevating agents (uridine triphosphate (UTP), thapsigargin and ionomycin). Buffering cellular [Ca2+]i with 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) inhibited cAMP responses to ISO and F in presence and absence of NT. These data support the idea that NT potentiated cAMP formation in response to a variety of stimuli by facilitating the activation of Ca2+ -dependent adenylyl cyclases.

Mechanisms of regulation of neurotensin receptors

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

Neurotensin and neuroendocrine regulation

More than two decades of research indicate that the peptide neurotensin (NT) and its cognate receptors participate to a remarkable extent in the regulation of mammalian neuroendocrine systems, potentially at multiple levels in a given system. NT-synthesizing neurons appear to exert a direct or indirect stimulatory influence on neurosecretory cells that synthesize gonadotropin-releasing hormone, dopamine (DA), somatostatin, and corticotropin-releasing hormone (CRH). In addition, context-specific synthesis of NT occurs in hypothalamic neurosecretory cells located in the arcuate nucleus and parvocellular paraventricular nucleus, including distinct subsets of cells which release DA, CRH, or growth hormone-releasing hormone into the hypophysial portal circulation. At the level of the anterior pituitary, NT stimulates secretion of prolactin and occurs in subsets of gonadotropes and thyrotropes. Moreover, circulating hormones influence NT synthesis in the hypothalamus and anterior pituitary, raising the possibility that NT mediates certain feedback effects of the hormones on neuroendocrine cells. Gonadal steroids alter NT levels in the preoptic area, arcuate nucleus, and anterior pituitary; adrenal steroids alter NT levels in the hypothalamic periventricular nucleus and arcuate nucleus; and thyroid hormones alter NT levels in the hypothalamus and anterior pituitary. Finally, clarification of the specific neuroendocrine roles subserved by NT should be greatly facilitated by the use of newly developed agonists and antagonists of the peptide.

Activation of mitogen-activated protein kinase couples neurotensin receptor stimulation to induction of the primary response gene Krox-24

Neurotensin (NT) is a neuropeptide that is important in a variety of biological processes such as signal transduction and cell growth. NT effects are mediated by a single class of cell-surface receptors, known as neurotensin receptors (NTRs), which exhibit structural features of the G-protein-coupled receptors superfamily. We investigated NTR signalling properties with Chinese hamster ovary (CHO) cells stably transformed with human NTR (hNTR). First, we showed that NTR stimulation by NT induced the activation of the mitogen-activated protein kinases (MAPKs) in time- and dose-dependent manners. Both p42 and p44 MAPK isoforms were retarded in gel-shift assays, which was consistent with their activation by phosphorylation. In addition we showed that NT caused a prolonged activation of MAPK as measured by in-gel kinase assay. Secondly, we demonstrated that NT induced the expression of the growth-related gene Krox-24 at the protein level, as assessed by Western-blot analysis, and at the transcriptional level, as demonstrated in CHO cells transfected with hNTR and a reporter gene for Krox-24. Activation of MAPK and induction of Krox-24 were both prevented by the NTR antagonist SR 48692, confirming the specific action on NTR. Furthermore we observed coupling of NTR to a mitogenic pathway and Krox-24 induction in the human adenocarcinoma cell line HT29, which naturally expresses NTRs. Considering coupling pathways between NTR stimulation and MAPK activation, we observed a partial inhibition by pertussis toxin (PTX) and a complete blockade by the protein kinase C (PKC) inhibitor GF 109203X. Taken together, these results suggest that (1) stimulation of NTR activates the MAPK pathway by mechanisms involving dual coupling to both PTX-sensitive and PTX-insensitive G-proteins as well as PKC activation, and (2) these effects are associated with the induction of Krox-24, which might be a target of MAPK effector.

Receptor affinity of neurotensin message segment immobilized on liposome

Neurotensin derivatives having a dioctadecyl group were synthesized and immobilized on DMPC liposome to construct a multivalent-ligand system. The derivatives are Ac-Glu[N(C18H37)2]-(Sar-Sar-Pro)n-Arg-Arg-Pro-Tyr-Ile-Leu-OH (D3nNT, n = 0,1,2,3), where a dioctadecyl group was connected to the N-terminal side of neurotensin 8-13 fragment directly or through a hydrophilic and flexible spacer chain of different lengths. The derivatives were spontaneously immobilized on DMPC liposome upon incubation overnight. The receptor affinity of the derivatives increased significantly upon immobilization on liposome. The maximum affinity was obtained by D9NT immobilized on DMPC liposome at the molar ratio of DMPC and D9NT of 200. This affinity is slightly better than the neurotensin 8-13 fragment, the message segment of the derivatives. The fluorescent microscopy using rhodamine-labelled liposome revealed that the multivalent-ligand system binds to specific receptors without dissociation of the derivative from DMPC liposome.

Neurotensin levels and receptors in HAS and LAS rat brains: effects of ethanol

Previous studies of neurotensin (NT) levels and NT receptor densities in specific brain regions of mice selectively bred for differences in sensitivity to ethanol have shown that NTergic processes may mediate some actions of ethanol. In the present study, we have determined the levels of NT and NT receptor densities in specific brain regions of HAS and LAS rats that have been selectively bred for differences in sensitivity to ethanol-induced loss of righting response. Regional differences in NT levels were observed in brains from both HAS and LAS rats and values in hypothalamus, ventral midbrain, and nucleus accumbens from female rats were 25 to 75% higher than levels in corresponding regions from male rats. However, there were no significant line differences in NT-ir levels in corresponding regions from HAS and LAS animals. High-affinity binding (NTH Bmax values), measured by Scatchard analyses, were higher in ventral midbrain from HAS males than from LAS males. NTH receptor densities were higher in HAS males than in HAS females; sex differences were not observed in the LAS line. There were no significant line or sex differences between HAS and LAS in low-affinity (NTL) Bmax values in any brain region. In HAS females, subhypnotic doses of ethanol produced a decrease in NT levels in nucleus accumbens, whereas, hypnotic doses caused an increase in NT levels. Likewise, hypnotic doses elicited increases in NT levels in hypothalamus of female HAS and LAS, but not in ventral midbrain or caudate putamen. These results are consistent with low dose activation of mesolimbic and nigrostriatal dopaminergic neurons in which NT is colocalized with dopamine and with high dose inhibition of these pathways.

Neurotensin induces calcium oscillations in cultured amacrine cells

The peptide, neurotensin, is found in a class of amacrine cells synapsing chiefly with other amacrine cells in the chicken retina (Li & Lam, 1990; Watt et al., 1991). To investigate the possible effects of neurotensin, we have used Ca2+ imaging to measure cytosolic Ca2+ concentrations in cultured chick amacrine cells. Following a delay of about 2 min, neurotensin (300 nM) induced oscillations in Ca2+ concentration that typically had a period of 2 min and peak values of about 300 nM when averaged over the cell body. The phospholipase C inhibitors U-73, 112 and 4'-bromophenacyl bromide terminated oscillations induced by neurotensin but the protein kinase inhibitors H7 and staurosporine did not inhibit oscillations, increasing their frequency instead. In the absence of external Ca2+, neurotensin induced only a single Ca2+ transient, much briefer than when external Ca2+ was present. Together these results suggest that neurotensin activates phospholipase C, thereby producing IP3 that triggers Ca2+ release from an internal store. Although this released Ca2+ contributes to periodic Ca2+ peaks, the majority of cytosolic Ca2+, even in the first peak, comes from Ca2+ influx across the plasmalemma.

Antipsychotics and neuropeptides: the atypical profile of CI-943 and its relationship to neurotensin

CI-943 is a new drug candidate with antipsychotic-like activity in a variety of behavioural tests in rodents and primates, but without any affinity for brain dopamine receptors. CI-943 does not cause dystonia in monkeys, a predictive symptom of extrapyramidal side effects (EPS). Its mechanism of action remains unclear. Neurotensin (NT) concentration in nucleus accumbens and caudate is increased by CI-943; this may be associated with its antipsychotic effect. Indeed various observations suggest that the clinical action of antipsychotic drugs may at least be partially mediated by some neuropeptides. Various actions of neurotensin are reviewed. The hypothesis on the role of neurotensin represents a new strategy in the development of pharmacological tools for the treatment of schizophrenia.

Neuropharmacological profile of non-peptide neurotensin antagonists

Neurotensin, an endogenous peptide widely distributed throughout the brain, fulfils neurotransmitter criteria. When administered centrally, neurotensin induces various effects and modulates the activity of the mesolimbic dopamine system. It antagonizes the behavioural action of dopamine in a manner similar, but not identical, to antipsychotic drugs. Neurotensin is even considered to be an endogenous neuroleptic. In fact, microinjection of neurotensin elicits different effects depending on both the dose and the cerebral structures into which the injection is made. Our work on the development of orally-active neurotensin antagonists has led to the identification of SR 48692, the first non-peptide antagonist of the neurotensin receptor, and some analogues. This small molecule reveals a surprising neuropharmacological profile. It antagonizes turning behaviour induced in mice and rats (after striatal or ventral tegmental area administration of neurotensin, respectively), hypolocomotion induced by intracerebroventricular injection of neurotensin in rats, and reverses the inhibitory effect of neurotensin (nucleus accumbens injection) on amphetamine-induced hyperlocomotion in rats. However, SR 48692 cannot reverse either dopamine release in the nucleus accumbens evoked by neurotensin injection in ventral tegmental area, or hypothermia and analgesia induced by intracerebroventricular injection of neurotensin. As direct and indirect dopamine agonists have been reported to promote neurotensin release in the cortex, behavioural studies were performed using injection of apomorphine. In these experiments, SR 48692 inhibited only turning and yawning. It did not antagonize other apomorphine-dependent effects such as climbing, hypothermia, hypo- or hyperlocomotion, penile erection and stereotypies. All together, these data raise the question of the existence of neurotensin receptor subtypes and confirm that the nature of neurotensin and dopamine interactions depends on the brain structures considered.

Neurotensin receptors: binding properties, transduction pathways, and structure

Neurotensin is a 13-amino acid peptide (pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu) originally isolated from hypothalami (Carraway and Leeman, 1973) and later from intestines (Kitabgi et al., 1976) of bovine. The peptide is present throughout the animal kingdom, suggesting its participation to important processes basic to animal life (Carraway et al., 1982). Neurotensin and its analogue neuromedin-N (Lys-Ile-Pro-Tyr-Ile-Leu) (Minamino et al., 1984) are synthesized by a common precursor in mammalian brain (Kislauskis et al., 1988) and intestine (Dobner et al., 1987). The central and peripheral distribution and effects of neurotensin have been extensively studied. In the brain, neurotensin is exclusively found in nerve cells, fibers, and terminals (Uhl et al., 1979), whereas the majority of peripheral neurotensin is found in the endocrine N-cells located in the intestinal mucosa (Orci et al., 1976; Helmstaedter et al., 1977). Central or peripheral injections of neurotensin produce completely different pharmacological effects (Table I) indicating that the peptide does not cross the blood-brain barrier. Many of the effects of centrally administered neurotensin are similar to those of neuroleptics or can be antagonized by simultaneous administration of TRH (Table I). The recently discovered nonpeptide antagonist SR 48692 (Gully et al., 1993) can inhibit several of the central and peripheral effects of neurotensin (Table I). Like many other neuropeptides, neurotensin is a messenger of intracellular communication working as a neurotransmitter or neuromodulator in the brain (Nemeroff et al., 1982) and as a local hormone in the periphery (Hirsch Fernstrom et al., 1980). Thus, several pharmacological, morphological, and neurochemical data suggest that one of the functions of neurotensin in the brain is to regulate dopamine neurotransmission along the nigrostriatal and mesolimbic pathways (Quirion, 1983; Kitabgi, 1989). On the other hand, the likely role of neurotensin as a parahormone in the gastrointestinal tract has been well documented (Rosell and Rökaeus, 1981; Kitabgi, 1982). Both central and peripheral modes of action of neurotensin imply as a first step the recognition of the peptide by a specific receptor located on the plasma membrane of the target cell. Formation of the neurotensin-receptor complex is then translated inside the cell by a change in the activity of an intracellular enzyme. This paper describes the binding and structural properties of neurotensin receptors as well as the signal transduction pathways that are activated by the peptide in various target tissues and cells.

Pharmacological and biochemical profiles of unique neurotensin 8-13 analogs exhibiting species selectivity, stereoselectivity, and superagonism

Recently, the rat neurotensin receptor and the two human neurotensin receptor clones (differing by one amino acid residue) have been isolated. We present results with 33 newly synthesized neurotensin analogs. We have evaluated their binding potency at the three neurotensin receptor clones by determining equilibrium dissociation constants and coupling to phosphatidylinositol turnover. Our work focused on position 8 and 9 substitutions as well as position 11 of the neurotensin hexamer NT8-13. The results presented include: 1) the development of a compound that is species selective, with a binding potency at the rat receptor that is 20-fold more potent than at the human receptor; 2) the development of a pair of stereoselective compounds with the L-isomer exhibiting 190-700-fold more potency than the D-isomer; and 3) the development of an agonist that has a Kd of 0.3 and 0.2 nM at the human and rat neurotensin receptor, respectively, ranking it as among the most potent tested. Also, we present the first evidence that 1) the effect of pi electrons at position 11 (L-Tyr) are important for binding to the neurotensin receptor, and 2) the length of the side chain on position 9 (L-Arg) changes binding potency.

Agonist properties of a stable hexapeptide analog of neurotensin, N alpha MeArg-Lys-Pro-Trp-tLeu-Leu (NT1)

The major signal transduction pathway for neurotensin (NT) receptors is the G-protein-dependent stimulation of phospholipase C, leading to the mobilization of intracellular free Ca2+ ([Ca2+]i) and the stimulation of cyclic GMP. We investigated the functional actions of an analog of NT(8-13), N alpha MeArg-Lys-Pro-Trp-tLeu-Leu (NT1), and other NT related analogs by quantitative measurement of the cytosolic free Ca2+ concentration in HT-29 (human colonic adenocarcinoma) cells using the Ca(2+)-sensitive dye fura-2/AM and by effects on cyclic GMP levels in rat cerebellar slices. The NT receptor binding affinities for these analogs to HT-29 cell membranes and newborn (10-day-old) mouse brain membranes were also investigated. Data obtained from HT-29 cell and mouse brain membrane preparations showed saturable single high-affinity sites and binding densities (Bmax) of 130.2 and 87.5 fmol/mg protein, respectively. The respective KD values were 0.47 and 0.39 nM, and the Hill coefficients were 0.99 and 0.92. The low-affinity levocabastine-sensitive site was not present (K1 > 10,000) in either membrane preparation. Although the correlation of binding between HT-29 cell membranes and mouse brain membranes was quite significant (r = 0.92), some of the reference agents had lower binding affinities in the HT-29 cell membranes. The metabolically stable compound NT1 plus other NT analogs and related peptides [NT, NT(8-13), xenopsin, neuromedin N, NT(9-13), kinetensin and (D-Trp11)-NT] increased intracellular Ca2+ levels in HT-29 cells, indicating NT receptor agonist properties. The effect of NT1 in mobilizing [Ca2+]i blocked by SR 48692, a non-peptide NT antagonist. Receptor binding affinities of NT analogs to HT-29 cell membranes were positively correlated with potencies for mobilizing intracellular calcium in the same cells. In addition, NT1 increased cyclic GMP levels in rat cerebellar slices, confirming the latter findings of its NT agonist action. These results substantiate the in vitro NT agonist properties of the hexapeptide NT analog NT1.

Human umbilical vein endothelial cells express high affinity neurotensin receptors coupled to intracellular calcium release

The binding of 125I-neurotensin (NT) to human umbilical vein endothelial cell monolayers was studied. At 20 degrees C, 125I-NT bound to a single class of binding sites with a dissociation constant of 0.23 +/- 0.08 nM and a binding site density of 5500 +/- 1300 sites/cell (n = 3). 125I-NT also bound to human aortic endothelial cells with a dissociation constant of 0.6 +/- 0.26 nM and a binding site density of 32000 +/- 1700 sites/cell. Association and dissociation kinetics were of a pseudo-first order and gave association and dissociation rate constant values of 1.6 x 10(6) M-1 s-1 and 3.5 x 10(-4) s-1, respectively. 125I-NT binding was inhibited by NT analogues with a rank order of potency similar to that characterizing brain high affinity NT binding sites (K0.5, nM): NT8-13 (0.11) > NT (0.35) > acetyl-NT8-13 (1.5) > [Phe11]NT (12) > [D-Tyr11]NT (> 1000). 125I-NT binding was also inhibited by the non-peptide NT antagonist SR 48692 (Ki = 16 nM) but was not affected by levocabastine, an inhibitor of low affinity brain NT binding sites. NT had no effect on cGMP levels in endothelial cells but NT and its analogues increased 45Ca2+ efflux from endothelial cells at nanomolar concentrations with a rank order of potency which was identical to that observed in binding experiments. This effect was inhibited by SR 48692 (IC50 = 8 nM). NT was able to increase phosphoinositide turnover in these cells, and this effect was blocked by SR 48692. The correlation between dissociation constants of NT analogues in binding experiments and IC50 values in 45Ca2+ efflux experiments was very high (r = 0.997) with a slope near unity, indicating that 125I-NT binding sites are functional NT receptors coupled to phosphoinositide hydrolysis and Ca2+ release in human umbilical vein endothelial cells.

Differential effects of the nonpeptide neurotensin antagonist, SR 48692, on the pharmacological effects of neurotensin agonists

In in vitro studies, SR 48692, a nonpeptide neurotensin receptor antagonist, inhibited the binding of [3H] or [125I]neurotensin to membrane preparations from 10-day-old mouse brains and from HT-29 cells with Ki values of 3.9 and 8.6 nM, respectively. SR 48692 also antagonized the neurotensin-induced mobilization of intracellular calcium in HT-29 cells, in agreement with previous findings. In rat cerebellar slices SR 48692 blocked the increase in cyclic GMP levels evoked by neurotensin in a dose-dependent manner. In vivo, SR 48692 antagonized the increase in rat brain mesolimbic dopamine turnover induced by the systemically active neurotensin peptide, EI [(N-Me)Arg-Lys-Pro-Trp-tert-Leu-Leu]. No effects on dopamine turnover of either EI or SR 48692 were observed in the striatum. SR 48692 did not antagonize the EI-induced decreases in mouse body temperature and spontaneous locomotor activity (LMA) or the decreases in LMA induced by ICV-administered neurotensin. Although other explanations are possible, these findings support the hypothesis that a subtype of the NT receptor may mediate the locomotor and hypothermic actions of this peptide and that it is different from the NT receptor that is involved in dopamine turnover.

Neurochemical and behavioural effects of neurotensin vs [D-Tyr11]neurotensin on mesolimbic dopaminergic function

Microinjection of neurotensin(1-13) or neurotensin(8-13) into the ventral tegmental area (VTA) of anaesthetized rats produced dose-dependent (1-100 pg) dopamine release in the nucleus accumbens as measured by differential pulse amperometry (DPA). Higher doses (100 pg-10 ng) of [D-Tyr11]neurotensin were required to produce an identical effect. In addition, the 3 peptides enhanced the K(+)-evoked [3H]DA release from nucleus accumbens slices. The stimulatory actions produced by 10(-8) M neurotensin(1-13) and neurotensin(8-13) were respectively of 96% and 72% while the effect of [D-Tyr11]neurotensin was only of 79% at 10(-6) M. Unilateral application of the 3 peptides in the VTA of cannulated rats produced contralateral circling. [D-Tyr11]neurotensin was effective in a dose-dependent manner, between 40 and 320 ng. Similar effects were observed with 80 ng of neurotensin(1-13) and neurotensin(8-13) in presence of the protease inhibitor thiorphan. In view of the higher potency of neurotensin(1-13) and neurotensin(8-13) versus [D-Tyr11]neurotensin to stimulate DA release both in vivo and in vitro and the higher efficacy of [D-Tyr11]neurotensin to induce circling, this study further strengthens the concept of neurotensin receptor heterogeneity.

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.

Receptor mediated internalization of neurotensin in transfected Chinese hamster ovary cells

After association with intact Chinese hamster ovary (CHO) cells expressing the rat neurotensin receptor, tritiated neurotensin was rapidly internalized. Internalization was maximal after 30 min and accounted for about 90% of the total associated ligand. Neurotensin internalization was not observed at 0-4 degrees and was inhibited by an excess of unlabelled neurotensin or by the neurotensin non peptide antagonist, SR 48692. Moreover, the incubation of intact cells for 30 min with 10 nM neurotensin resulted in a significant decrease in the number of the cell surface neurotensin receptors. These results indicate that the endocytosis of membrane bound neurotensin in transfected CHO cells resulted from the internalization of the ligand-receptor complex inside the cell, through an agonist-induced process.

Expression of a rat neurotensin receptor in Escherichia coli

With the goal of obtaining sufficient quantities of seven-helix G-protein-coupled receptors for structural analysis, we have studied the functional expression of a rat neurotensin receptor cDNA in Escherichia coli with and without a signal sequence and as a fusion with the gene coding for maltose-binding protein. The addition of an N-terminal signal peptide resulted in increased expression levels. In vitro translation at a high level revealed that the codon usage of the rat neurotensin receptor cDNA was not critical for overproduction. Expression of neurotensin receptor cDNA fused to the 3' end of the gene encoding maltose-binding protein resulted in a 40-fold increase in neurotensin-binding sites. Binding of [3H]neurotensin to intact bacteria or E. coli membranes was saturable, with a dissociation constant, KD, of 0.23 nM (Bmax. = 450 sites/bacterium or 15 pmol/mg of crude membrane protein). The binding properties of all recombinant receptors presented in this study were similar and corresponded to those of the high-affinity binding sites in rat brain. For immunological detection and future purification of neurotensin receptor, a C-terminal pentahistidine/c-myc tail was introduced. Western-blot analysis revealed the association of neurotensin receptor with E. coli membranes.

Role of intercellular and intracellular communication by nitric oxide in coupling of muscarinic receptors to activation of guanylate cyclase in neuronal cells

Muscarinic receptor-mediated cyclic GMP formation and release of nitric oxide (NO) (or a precursor thereof) were compared in mouse neuroblastoma N1E-115 cells. [3H]Cyclic GMP was assayed in cells prelabeled with [3H]guanine. Release of NO upon the addition of muscarinic agonists to unlabeled neuroblastoma cells (NO donor cells) was quantitated indirectly by its ability to increase the [3H]cyclic GMP level in labeled cells whose muscarinic receptors were inactivated by irreversible alkylation (NO detector cells). Carbachol increased NO release in a concentration-dependent manner, with half-maximal stimulation at 173 microM (compared to 96 microM for direct activation of cyclic GMP formation). The maximal effect of carbachol in stimulating release of NO when measured indirectly was lower than that in elevating [3H]cyclic GMP directly in donor cells. Hemoglobin was more effective in blocking the actions of released NO than in attenuating direct stimulation of [3H]cyclic GMP synthesis. There was a good correlation between the ability of a series of muscarinic agonists to release NO or to activate [3H]cyclic GMP formation directly, and the potency of pirenzepine in inhibiting the two responses. Furthermore, there was a similar magnitude of desensitization of both responses by prolonged receptor activation or stimulation of protein kinase C. NO release was also regulated in relation to the cellular growth phase. A model is proposed in which a fraction of NO generated upon receptor activation does not diffuse extracellularly and stimulates cyclic GMP synthesis within the same cell where it is formed (locally acting NO). The remainder of NO that is extruded extracellularly might travel to neighboring cells (neurotransmitter NO) or might be taken back into the cells of origin (homing NO).

Postnatal ontogeny of the rat brain neurotensin receptor mRNA

Total RNA was purified from rat forebrain at different postnatal ages and analyzed by Northern blot using a specific neurotensin receptor RNA probe. The rat neurotensin receptor mRNA was present in high amount during the first 10 days of life. Thereafter, it rapidly decreased and was undetected after 20 days. [3H]neurotensin binding experiments performed on the same tissues indicated that the total amount of neurotensin receptors increased during the first week and was maximal between day 7 and day 10. This plateau was followed by an important loss (70%) of neurotensin receptors. These results indicate that an important reduction in the genetic expression of the neurotensin receptor after day 10 may probably account for the [3H]neurotensin binding profile observed in rat forebrain during the postnatal ontogeny.

Genetic-based differences in neurotensin levels and receptors in brains of LS x SS mice

Levels of endogenous neurotensin (NT-IR) in the LS x SS RI strains differed by 3.0-, 4.7-, 5.4-, and 6.9-fold in the ventral midbrain (VMB), hypothalamus (HY), nucleus accumbens (NA), and caudate putamen (CP), respectively. Frequency distributions and estimates of the number of genes indicate that differences in NT-IR are polygenically influenced. The NT-IR levels in NA and CP were significantly correlated, but levels in the VMB did not correlate with those in the NA or CP. Specific binding to either low (NTL)- or high (NTH)-affinity receptors as measured in the absence or presence of levocabastine differed significantly in brain regions from among LS X SS mouse strains. Results indicate a polygenic influence mediating the differences in receptor densities and suggest differences in genetic regulation of NTL and NTH receptors.

Further characterization of neurotensin receptor desensitization and down-regulation in clone N1E-115 neuroblastoma cells

Murine neuroblastoma clone N1E-115 cells possess neurotensin receptors that are coupled to polyphosphoinositide hydrolysis and cyclic guanosine 3',5'-monophosphate (cGMP) formation. These responses rapidly desensitize and these receptors rapidly down-regulate nearly completely in about 15 min. Although neurotensin is rapidly degraded by peptidases, in this study we show that at 37 degrees neurotensin (100 nM) in the absence of peptidase inhibitors caused this rapid desensitization and down-regulation (32 +/- 5 and 24 +/- 2% of control, respectively) of neurotensin receptors in N1E-115 cells. In addition, we demonstrated that this desensitization, resensitization, down-regulation and recovery of binding sites were temperature dependent. These data suggest that a certain degree of phospholipid fluidity or activity of some enzymes is required for these processes to occur. After addition of sodium nitroprusside or ionomycin to cells, cGMP increased in desensitized cells to the same degree as in control cells. Additionally, desensitization and down-regulation occurred in the absence of a change in the affinity of neurotensin for the remaining sites. These data suggest that desensitization is not caused by changes in nitric oxide synthesis, guanylyl cyclase activity or receptor affinity, but predominantly by a decrease in receptor number.

Intramembrane interactions between neurotensin receptors and dopamine D2 receptors as a major mechanism for the neuroleptic-like action of neurotensin

Evidence has been presented that behavioral actions of NT, inducing its neuroleptic-like action, can be explained on the basis of NT-D2 intramembrane receptor-receptor interactions in the basal ganglia, unrelated to the coexistence phenomenon, leading to reduced affinity and transduction of the D2 agonist binding site. By reducing selectively D2 receptor transduction at the pre- and postsynaptic level, the NT receptor appears capable of switching the DA synapses towards a D1 receptor-mediated transduction, illustrating how receptor-receptor interactions can increase the functional plasticity of central synapses (FIG. 12).

The rat neurotensin receptor expressed in Chinese hamster ovary cells mediates the release of inositol phosphates

To study second messenger synthesis mediated by the cloned rat neurotensin receptor, we derived a cell line stably expressing this receptor. The cDNA clone of this receptor was subcloned into the pcDNA1neo expression vector. This construct was then used to transfect Chinese hamster ovary (CHO)-K1 cells. Colony clones, selected for resistance to antibiotic G-418 sulfate, were isolated and grown separately. Nineteen individual clones were screened for total [3H]neurotensin binding as an indication of neurotensin receptor expression. The clone (CHO-rNTR-10) showing the highest level of specific [3H]neurotensin binding was characterized further. With intact cells, the equilibrium dissociation constant (KD) for specific [3H]neurotensin binding was 18 nM, and the maximal number of binding sites (Bmax) was 900 fmol/mg of protein or 740 fmol/10(6) cells (approximately 4.4 x 10(5) sites on the cellular surface). Whereas the KD was similar to that found in other cellular systems, for example, the murine neuroblastoma clone N1E-115, the Bmax exceeded previously reported values. Incubation of intact CHO-rNTR-10 cells with neurotensin caused the release of inositol phosphates in a dose-dependent manner (EC50 = 3 nM), results indicating that the expressed transfected receptor was functional. Neurotensin did not inhibit cyclic AMP levels stimulated by forskolin. As with other systems, neurotensin (8-13) was more potent than neurotensin Neurotensin-mediated inositol phosphate release is the first report of second messenger synthesis for this receptor expressed in a transfected cell line. These results suggest that the relation between structure and function of the neurotensin receptor can be readily studied in transfected cell lines.

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.

Neurotensin increases extracellular striatal dopamine levels in vivo

This study employed intracranial microdialysis to assess the effects of neurotensin (NT) infusion on extracellular dopamine (DA) and DA metabolite concentrations in the rat striatum and nucleus accumbens, and the effects of NT on alterations in extracellular DA levels induced by cocaine and the DA D-2 receptor agonist, quinpirole. Direct NT infusion (.10, 1.0, 10.0 microM) did not significantly affect extracellular DA in the nucleus accumbens, but did produce a significant increase in the DA metabolite homovanillic acid (HVA). In contrast, direct NT infusion produced an increase in striatal DA levels, without altering DA metabolites. Neurotensin infusion (.10 microM) into the striatum significantly attenuated the peak DA increase induced by an intraperitoneal (IP) injection of a low dose (10.0 mg/kg) but not a high dose (30.0 mg/kg) of cocaine. Neurotensin infusion (.10 microM) did not affect the decrease in DA and its metabolites induced by an IP injection of a low dose of quinpirole (.03 mg/kg), but did alter the decrease in HVA induced by a high dose of quinpirole (.10 mg/kg). These results suggest that NT differentially affects in vivo DA release in the striatum and nucleus accumbens, and further strengthens the assertion that NT is an important modulator of dopaminergic function.

Agonist-induced changes in [Ca2+]i in N1E-115 cells: differential effects of bradykinin and carbachol

The addition of bradykinin to populations of fura-2 loaded N1E-115 neuroblastoma cells produced an increase in intracellular calcium which rapidly reached a peak and returned to baseline within 60 s. The response was concentration dependent and unaffected by removal of extracellular calcium or addition of the inorganic channel blocker Ni2+. Similar transient responses were seen with histamine and angiotensin II and experiments monitoring manganese entry suggest that agonist responses in this cell line involve mainly release of calcium from intracellular stores. However, unlike bradykinin, the response to carbachol, at all concentrations, failed to return completely to baseline suggesting a small secondary influx component and highlighting possible differences between the mechanisms of calcium elevation by these two agonists.

Block of neurotensin receptor down-regulation by an aminosteroid in N1E-115 cells

We have demonstrated that the aminosteroid, U-73122 (1-(6-([17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino)hexyl)-1H-pyrrole- 2,5-dione) blocks agonist induced down-regulation of neurotensin receptors in murine neuroblastoma clone N1E-115 cells. U-73122 is known to inhibit polyphosphoinositide hydrolysis by affecting the coupling of GTP-binding proteins. Our results may indicate that GTP-binding proteins play a role in the mechanism of down-regulation of the neurotensin receptor.

Neurotensin modulates K(+)-stimulated dopamine release from the caudate-putamen but not the nucleus accumbens of mice with differential sensitivity to ethanol

Slices of caudate-putamen (CP) and nucleus accumbens (NA) prepared from Long-Sleep (LS) and Short-Sleep (SS) mice were used to determine the effects of neurotensin (NT) and ethanol on K(+)-stimulated 3H-dopamine (3H-DA) release and to test the hypothesis that ethanol acts, in part, via NT receptor-mediated processes. Slices prepared from either LS or SS CP or NA did not differ in submaximal (25 mM) K(+)-stimulated 3H-DA release but 60 mM K+ induced significantly greater 3H-DA release from LS CP slices compared with SS CP slices. NT had no effect on unstimulated 3H-DA overflow but enhanced 25 mM K(+)-stimulated 3H-DA release from slices of the CP of both lines of mice. Augmentation of DA release by NT from caudate slices was concentration dependent and tetrodotoxin (TTX) insensitive, implicating a role of presynaptic neurotensin receptors located on nigrostriatal DA neurones. In contrast, NT did not enhance K(+)-stimulated 3H-DA release from NA slices from either line of mice. The absence of an NT effect in NA slices was not due to a rapid desensitization of NT receptors but the data were consistent with the absence of presynaptic NT receptors on dopaminergic terminals in the NA. Between-line differences were observed in the effect of ethanol on NT enhancement of 25 mM K(+)-stimulated 3H-DA release from CP slices. Ethanol (100 mM) applied concomitantly with NT blocked the NT enhancement of 3H-DA release from CP slices of LS but not SS mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-antinociceptive activity of neurotensin and some novel analogues in the periaqueductal gray region of the brainstem

Neurotensin, an endogenous tridecapeptide, produces a potent, naloxone-insensitive antinociceptive response when it is microinjected into the periaqueductal gray region of the rat brainstem. In the present study, the ED50 for neurotensin in inducing antinociception was 1.5 nmol, two times more potent than morphine. We sought to find whether neurotensin's antinociceptive effects were mediated by the same receptor that mediates its other functions. We found that the structure-activity relationship of neurotensin-induced antinociception was different from that required for the stimulation of intracellular cyclic GMP production in neuroblastoma clone N1E-115 and the binding to N1E-115 cells, human brain tissue, or rat periaqueductal gray. These data suggest there exists a subtype of neurotensin receptors in neural tissue that mediates its antinociceptive actions.

Developmental regulation of neurotensin receptor expression and function in murine neuroblastoma clone N1E-115

Murine neuroblastoma cells (clone N1E-115) during their growth from log to late stationary phase, expressed no specific neurotensin binding sites until day 7 in culture. From day 7 to day 20, binding sites increased 6-fold in number/cell and more than 4-fold in sites/mg protein. Neurotensin-mediated cyclic [3H]GMP synthesis was not detected until day 17. For these cells these data show that neurotensin receptor binding and function are regulated with respect to growth cycle and that presence of neurotensin binding sites is not sufficient for receptor function.

Structure and functional expression of the cloned rat neurotensin receptor

A functional cDNA clone for the rat neurotensin receptor was isolated by combining molecular cloning in an RNA expression vector with an electrophysiological assay in Xenopus oocytes. The neurotensin receptor consists of 424 amino acids with seven putative transmembrane domains and belongs to the family of G protein-coupled receptors. The cloned receptor expressed in mammalian cells or in Xenopus oocytes shows a selective and high-affinity binding to neurotensin peptides and undergoes potent desensitization by repeated application of neurotensin. The neurotensin receptor mRNA is expressed in both the brain and the peripheral tissues at different levels. This investigation discloses the molecular nature of the neurotensin receptor, which mediates the diverse neuronal and peripheral actions of neurotensin by effecting the G protein-associated second messenger system.

Low doses of ethanol reduce neurotensin levels in discrete brain regions from LS/Ibg and SS/Ibg mice

Studies were designed to examine the previously proposed hypothesis that some of the pharmacological actions of ethanol are mediated by neurotensinergic processes. Neurotensin-immunoreactivity (NT-ir) was extracted from various brain regions and shown by high performance liquid chromatography to possess the same retention time as authentic bovine NT1-13. The highest levels of NT-ir were observed in the hypothalamus with intermediate levels in the midbrain and striatum and lowest levels in the frontal cortex. Levels of NT-ir were higher in hypothalamus and midbrain from long-sleep (LS) than from short-sleep (SS) mice. Ethanol, in vivo, produced a dose-dependent decrease in NT-ir in several brain regions; low doses, 1.5 to 3.0 g/kg, but not high doses, 4.1 g/kg, of ethanol significantly decreased NT-ir in hypothalamus, midbrain, and striatum of LS and SS mice. Levels of NT-ir in the frontal cortex were not altered by ethanol administration. Ethanol-induced decreases in NT-ir were of rapid onset with a maximum decrease in 5 min after intraperitoneal (i.p.) injection, and they were of long duration with levels remaining depressed for 4 hr. These findings show that subhypnotic, intoxicating doses of ethanol enhance NT release, in vivo, and support the hypothesis that some of ethanol's actions are mediated by neurotensinergic systems.

The effect of unsaturated fatty acids on sodium nitroprusside stimulation of guanylate cyclase in the human astrocytoma clone, D384, and the human neuroblastoma clone, NB1-G

Sodium nitroprusside (SNP) stimulates cGMP formation to a greater extent in 20,000 g supernatant fractions of the human neuroblastoma clones NB1-G and SH-SY5Y than in the human astrocytoma clone D384. This suggests that these cell lines contain the soluble form of guanylate cyclase. Arachidonic, 8,11,14- and 11,14,17-eicosatrienoic acids inhibit SNP (10(-4) M)-stimulated cGMP formation more potently than the C18 unsaturated fatty acids linolenic and linoleic acids in D384 and NB1-G. In contrast the C20 saturated fatty acid, arachidic acid had little effect even at 10(-4) M concentration. In addition arachidonic and 8,11,14-eicosatrienoic acids inhibited basal guanylate cyclase activity, in NB1-G, over the same concentration range as they inhibited SNP-stimulated cGMP formation. No evidence could be obtained for the stimulation of guanylate cyclase by arachidonic acid in either NB1-G or D384. These results provide further support for suggestions that arachidonic acid or its metabolites may be important regulators of cGMP formation in the nervous system.

Cardiovascular effects of neurotensin and some analogues on rats

When administered in vivo into the femoral vein of normotensive rats, neurotensin, neurotensin-(8-13), and [D-Lys8]neurotensin-(8-13) decreased diastolic blood pressure in a dose-dependent manner, without change in heart rate. All three peptides evoked tachyphylaxis and a triphasic depressor-pressor-depressor, response at higher doses. The rank order of potency was neurotensin greater than [D-Lys8]neurotensin-(8-13) greater than neurotensin-(8-13). In organ chamber experiments, both neurotensin and neurotensin-(8-13) at a range of concentrations which induced dose-dependent decreases in blood pressure, did not significantly change tension in rat aorta rings with or without endothelium. In contrast, [D-Lys8]neurotensin-(8-13) induced weak dose-dependent relaxation of both rings with or without endothelium. However, this effect was not obtained at concentrations able to decrease the blood pressure. Indomethacin did not change the vasodilator effect of [D-Lys8]neurotensin-(8-13). There was no correlation between the vasodilator effect of this peptide and its ability to decrease blood pressure. These experiments suggest that the hypotension was not due to a direct vasodilator effect on the smooth muscle. In addition, since the rank order of peptide potency was opposite of those found in previous studies of second messenger synthesis and binding to neural tissue, these data suggest that there is a second receptor for neurotensin or that neurotensin can also bind to a different unknown receptor.

The effects of acute exposure to ethanol on neurotensin and guanine nucleotide-stimulation of phospholipase C activity in intact NIE-115 neuroblastoma cells

Both ethanol and neurotensin produce sedation and hypothermia. When administered in combination the behavioral effects of these two substances are potentiated. In order to better understand the biochemical nature of this interaction, the direct effects of ethanol on neurotensin receptors and an associated signal transduction process were determined in NIE-115 neuroblastoma cells. Ethanol in physiologically relevant concentrations (50mM) significantly reduced neurotensin stimulated [3H]inositol phosphate production while having no effect on the specific binding of [3H]neurotensin. In addition, ethanol up to 200 mM had no effect on GTPYS mediated [3H]inositol phosphate production. The results indicate that acute exposure to ethanol partially disrupts the normal coupling of activated neurotensin receptors to the guanine nucleotide binding protein associated with phospholipase C.

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.