Characterization of neurotensin binding sites in intact and solubilized bovine brain membranes

Impact and mechanisms of action of neurotensin on cardiac contractility in the rat left ventricle

Using immunoassay measurements, neurotensin was identified in rat ventricular tissue and in coronary effluent samples. Exogenous neurotensin evoked contractile responses in isolated ventricular preparations, which were equivalent in magnitude to those of norepinephrine and histamine, but greater than those for serotonin and angiotensin II. EC(50) values revealed neurotensin to be as potent as serotonin, but more potent than norepinephrine, histamine and angiotensin II. Structure-activity studies indicated that the contractile effects are attributed to the C-terminal portion of neurotensin. Neurotensin-induced responses were decreased by SR 48692, a specific neurotensin receptor antagonist. Neurotensin elicited an increase in coronary effluent norepinephrine concentrations, and a strong relationship between the magnitude of neurotensin-induced contractile effects and increments in myocardial norepinephrine release were noted. Neurotensin-induced contractile responses were abolished by beta-adrenoceptor antagonists, but not by histamine, serotonin or angiotensin II receptor antagonists. In conclusion, neurotensin increases ventricular contractility through stimulation of myocardial norepinephrine release.

Function and stability of human transcobalamin II: role of intramolecular disulfide bonds C98-C291 and C147-C187

The current studies have investigated the role of three disulfide bonds of human transcobalamin II (TC II), a plasma transporter of cobalamin (Cbl; vitamin B12), in its function and stability. When translated in vitro in the presence or absence of microsomal vesicles, TC II constructs with a single substitution, C3S or C249S, demonstrated synthesis of a stable functional protein. However, TC II synthesized in the presence of microsomal vesicles using constructs with a single (C98S, C147S, C187S, C291S), double (C3/147/S, C98/147/S) or triple (C3/98/147/S) substitution was unstable. In the absence of microsomal vesicles, the percentage of binding to Cbl-Sepharose matrix by TC II expressed by constructs C3S, C3/147/S, C98/147/S, or C3/98/147/S was 100, 49, 52, and 35%, respectively. Upon their reductive alkylation, the binding of TC II expressed by these constructs was reduced to approximately 25-30%. TC II constructs C3S or C249S, when expressed in TC II-deficient fibroblasts, produced a stable functional protein, but those expressed by constructs C147S, C187S, C291S, C3/147/S, C98/147/S, or C3/98/147/S were rapidly degraded. The intracellular degradation of TC II expressed by these constructs was inhibited by lactacystin or MG-132 but not by the lysosomal degradation inhibitors ammonium chloride or chloroquine. These studies suggest that optimal binding of Cbl by human TC II is supported by disulfide bonds C98-C291 and C147-C187 and that their disruption results in loss of Cbl binding and their rapid degradation by the proteasomal machinery.

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.

Stimulation of cell-surface urokinase-type plasminogen activator activity and cell migration in vascular endothelial cells by a novel hexapeptide analogue of neurotensin

To investigate if neurotensin (NT) could induce activation of urokinase-type plasminogen activator (uPA) in vascular endothelial cells, we utilized the acetyl-NT (8-13) analogue, TJN-950, in which the C-terminal leucine is reduced to leucinol. TJN-950 inhibited the binding of 125I-NT to membranes of newborn rat brains and of COS-7 cells transfected with rat NT receptor cDNA, but at 10(4) higher doses than NT (8-13). However, TJN-950 was as effective as NT in inducing the fibrinolytic activity in bovine vascular aortic and human umbilical vein endothelial cells, and enhanced the migration of vascular endothelial cells. Moreover, administration of TJN-950 induced neovascularization in the rat cornea in vivo. TJN-950 had no effect on expression of uPA, plasminogen activator inhibitor-1 or uPA receptor mRNA. The binding of 125I-TJN-950 to cell membranes was blocked by unlabeled uPA and TJN-950, but not the amino-terminal or 12-32 fragment of uPA. TJN-950 may enhance uPA activity in vascular endothelial cells by interacting with the uPA receptor, resulting in induction of angiogenesis.

Detection of a 65 kDa ras binding protein in rat and sheep brain cytosol using a chemical cross linking agent

The ability of a ras protein to associate with proteins present in rat brain cytosol in vitro was investigated using chemical cross-linking agents and the 125I-labelled v-H-ras protein. Two iodinated protein complexes with apparent molecular weights of 40 and 85 kDa were observed when a mixture of rat brain cytosol and [125I] ras was treated with the cross-linking agent disuccinimidyl suberate and subjected to SDS-PAGE. Formation of the [125I] Formation of the[125I] 85 kDa complex was enhanced by a high concentration of EDTA while generation of the 40 kDa species was abolished by this treatment. Formation of the [125I] 85 kDa complex was inhibited by unlabelled ras protein, GTP, GTP gamma S, and GDP but not by ATP gamma S and GMP. Chromatography of the cross-linked brain cytosol[125I] ras mixture on DEAE cellulose partially resolved the [125I] 85 kDa complex from the [125I] ras protein. The [125I] 85 kDa complex (formed using ethyleneglycolbis (succinimidylsuccinate) as the cross-linking agent) could be immunoprecipitated using a rabbit anti-ras polyclonal antibody. Treatment of the immunoprecipitate with hydroxylamine to cleave the cross-link yielded [125]I-labelled ras. A substantial enrichment of the proportion of the [125I] 85 kDa complex in the cross-linked extract was achieved by preparative SDS-PAGE. It is concluded that the in vitro chemical cross-linking approach employed here has detected two ras binding proteins in rat brain cytosol: a 65 kDa heat-sensitive and a 20 kDa heat-stable protein.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Chronic ethanol administration downregulates neurotensin receptors in long- and short-sleep mice

Neurotensin (NT) has been shown to differentially alter many of the physiologic responses to ethanol administration in long-sleep (LS) and short-sleep (SS) mice, which were selectively bred for differences in hypnotic sensitivity to ethanol. These mice have been shown to differ in NT receptor densities in cortical and mesolimbic brain regions and it has been suggested that ethanol actions may be mediated, in part, by neurotensinergic processes. The present study was conducted to further examine this hypothesis by determining the effects of acute and chronic ethanol administration on NT receptor systems in these mice. Scatchard analysis of [3H]NT binding in brain membranes from mice chronically treated with ethanol yielded a one-site model, whereas binding in membranes from control mice were best described by a two-site model. Values for binding capacity (Bmax) were significantly reduced in several brain regions, and binding site density for total, levocabastine-sensitive, and levocabastine-insensitive binding sites were also reduced. The maximum effect was seen after 2 weeks of chronic ethanol consumption. Three weeks after withdrawal from ethanol, Kd and Bmax had returned to control values. Similarly, binding density in all regions for total, levocabastine-sensitive, and levocabastine-insensitive sites had returned to control values within 2 weeks. NT receptor characteristics measured 2 h post-3.0 g/kg ethanol revealed that ethanol caused a rapid downregulation of both subtypes of NT receptors. The finding that both acute and chronic ethanol significantly downregulate the neurotensin receptor systems further supports the hypothesis that ethanol's actions may be mediated in part by neurotensinergic systems.

Cloning and expression of a complementary DNA encoding a high affinity human neurotensin receptor

A human neurotensin receptor (hNTR) cDNA was cloned from the colonic adenocarcinoma cell line HT29. The cloned cDNA encodes a putative peptide of 418 amino acids with 7 transmembrane domains. The amino acid sequence of the hNTR is 84% identical to the rat NTR [Neuron, 4 (1990) 847-854]. Transfection of this cDNA into COS cells results in the expression of receptors with pharmacological properties similar to those found with HT29 cells. Northern blot analysis using the hNTR cDNA probe indicated a single transcript of 4 kb in the brain, the small intestine and blood mononuclear cells.

Different postnatal development of cells expressing mRNA encoding neurotensin receptor

In situ hybridization histochemistry revealed three different ontogenetic patterns of localized expression of the high-affinity type of neurotensin receptor mRNA in the developing rat brain: one comprises sites which showed transient expression of neurotensin receptor mRNA during the first postnatal week, the expression greatly decreasing thereafter (type I); another comprises sites at which there is a gradual increase in neurotensin receptor mRNA after birth, as there is in cell number and intensity, with advancing age, followed by a plateau (type II); the third comprises sites at which there is much expression of neurotensin receptor mRNA already at birth, and a slight decrease thereafter (type III). The cerebral cortex, except retrosplenial and entorhinal cortices, and the anterior dorsal thalamic nucleus exhibit the type I pattern, while the horizontal and vertical limbs of the diagonal band of Broca, magnocellular preoptic nucleus, substantia innominata, ventral part of the suprachiasmatic nucleus, medial habenular nucleus, ventral tegmental area and substantia nigra pars compacta exhibit the type II pattern. The tenia tecta, retrosplenial and entorhinal cortices exhibit the type III pattern. One of the most striking findings in this study was that the entire neocortex and most of the limbic cortex exhibit the type I pattern, i.e. neurotensin receptor mRNA is expressed transiently long before a neuronal network is established there. This suggests that neurotensin plays an important role in cortical development, other than its reported transmitter-like role in the adult.

Regional characterization of brain neurotensin receptor subtypes in LS and SS mice

Neurotensin (NT) receptor subtypes were investigated in nine brain regions from long sleep (LS) and short sleep (SS) mice that were selectively bred for differences in sensitivity to ethanol. Differences in NT receptor densities may mediate, in part, genetically selected differences in ethanol sensitivity between the two lines of mice. The use of [3H] NT at concentrations from 0.02 to 20 nM yielded biphasic binding isotherms as revealed by Scatchard analysis. Membranes from LS ventral midbrain yielded dissociation constants (KD values) of 0.34 and 3.85 nM for the high (NTH) and low (NTL) affinity components, respectively. SS membranes displayed similar KD values, however the maximum number of binding sites (Bmax) for both receptor subtypes were significantly greater in SS than in LS membranes (46.7 vs. 71.5 fmol/mg protein for NTH and 170.2 vs. 208.2 fmol/mg protein for NTL). Using levocabastine, and H1 antagonist with selectivity for NTL, characterization of NTH and NTL binding in nine brain regions was performed. In general, membranes from each brain region of SS mice had higher densities than LS for both receptor subtypes. Significant differences for the total density of receptors and NTL were found in entorhinal cortex, nucleus accumbens, hippocampus, and ventral midbrain. The only region to differ in NTH was the ventral midbrain. Competition experiments using various NT fragments to compete for NTH binding showed the C-terminal amino acids to be essential for binding. The order of potency was NT1-13 = NT8-13 greater than Neuromedin N greater than NT1-8 = NT1-11.

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.

Purification and characterization of neurotensin receptor from rat brain with special reference to comparison between newborn and adult age rats

Scatchard analysis of saturation curves was performed to compared newborn and adult rat neurotensin receptor using [3H] neurotensin as a tracer. The membrane fraction of newborn rat cerebral cortex has a single population of neurotensin receptor (Kd = 0.13 nM, Bmax = 710 fmol/mg protein), whereas adults have two distinct neurotensin binding sites (high affinity site, Kd1 = 0.13 nM; low affinity site, Kd2 = 20 nM). High affinity neurotensin receptor, solubilized with digitonin, was purified from newborn rat cortex by affinity chromatography. An overall purification of 14,000-fold was achieved. The binding of [3H] neurotensin to the purified receptor is saturable and specific, with a Kd of 0.45 nM. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in the presence of 2-mercaptoethanol revealed purified material of a single major band of Mr = 55,000.

Neurotensin and neuromedin N elevate the cytosolic calcium concentration via transiently appearing neurotensin binding sites in cultured rat cortex cells

Through assessment of the changes in the intracellular free-calcium concentration ([Ca2+]i), which was measured using the calcium sensitive dye, fura-2, the character of the neurotensin (NT) binding sites which appeared transiently during the early ontogenetic stage in the rat cerebral cortex was analyzed in primary cultures of cerebral cortex cells from neonatal rats. NT (1-1000 nM) elevated [Ca2+]i of the cells even when extracellular calcium was chelated with 1 mM ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). These findings suggest that the transiently appearing NT-binding sites in the cortex are receptors for NT and that some of the NT-induced increase in [Ca2+]i is due to mobilization from the intracellular calcium store. Further application of NT after 10 min washing caused an increase in [Ca2+]i again. This is in contrast to the findings for cortical slices from adult rats and mRNA-injected oocytes; desensitization due to NT was of long duration and further application of NT failed to activate the neurons which had responded the first time to NT. These facts suggest that the character of the NT-binding sites in the cerebral cortex differs between neonatal and adult rats. In addition, we showed that neuromedin N had a similar property to NT as to mobilization of [Ca2+]i and acted only on NT-responsive cells, suggesting the interaction between NT and neuromedin N at the postsynaptic level via the same receptor.

Agonist-selective protection of the opioid receptor-coupled G proteins from inactivation by 5'-p-fluorosulphonylbenzoyl guanosine

The guanine nucleotide analogue, 5'-p-fluorosulphonylbenzoyl guanosine (FSBG), can react covalently with GTP-binding proteins (G proteins). In rat brain membranes, FSBG causes a time-dependent loss of beta,gamma-imido[8-3H]guanosine 5'-triphosphate binding sites. Using 1 mM FSBG, the guanyl nucleotide modulation of opioid agonist binding is abolished, whereas the guanyl nucleotide sensitivity of neurotensin binding is retained. The action of FSBG can be prevented by the presence of opioid agonists, but not the antagonist naloxone. Iodoacetamide treatment of membranes in the presence of agonist, but not antagonist, can attenuate the action of FSBG in blocking guanyl nucleotide modulation of opioid agonist binding. These results suggest that FSBG covalently modifies essential thiol groups, whose exposure to the reagent is modified by agonist occupancy of the receptor, on a species of G protein linked to opioid receptors, but not on a species of G protein linked to neurotensin receptors. Thus, FSBG may have selectivity for the forms of Gi or Go, proteins associated with opioid receptors.

Neurotensin binding sites in porcine jejunum: biochemical characterization and intramural localization

Neurotensin is present in high concentrations in the mammalian gut, especially in enteroendocrine cells of the mucosa. Exogenous neurotensin has been shown to alter ion transport by the mucosa and contractile activity of intestinal smooth muscle. The purpose of this study was to determine the distribution of neurotensin binding sites within the intestinal wall. Initially, biochemical characteristics of [125I]neurotensin binding sites were determined within two preparations of the distal porcine jejunum: (1) the mucosa and submucosa, and (2) the circular and longitudinal muscle with their intramural plexuses. Ligand binding data for the preparation including the mucosa and submucosa indicated that [125I]neurotensin bound specifically to two sites having apparent equilibrium dissociation constants of approximately 0.046 and 0.37 nM. A binding site with a dissociation constant of approximately 0.38 nM was confirmed for the preparation of muscle and associated intramural plexuses. Xenopsin and neurotensin were equipotent to neurotensin in competing for these binding sites; neuromedin N was approximately 40 times less potent in the preparation of mucosa and submucosa. Receptor autoradiography was used to determine the distribution of [125I]neurotensin binding sites within the wall of the jejunum. Autoradiograms of [125I]neurotensin bound to cross sections of the proximal and distal jejunum showed that the highest densities of silver grains were associated with the internal submucosal ganglia, external submucosal plexus and myenteric ganglia. A moderate density of silver grains was associated with the circular muscle. The localization of neurotensin binding sites to submucosal ganglia is consistent with observations that neurotensin effects on active anion secretion by the mucosa are blocked by tetrodotoxin. Immunohistochemical localization of neurotensin in the porcine jejunum demonstrated a limited population of neurotensin immunoreactive cells within the mucosal epithelium. It is possible that neurotensin released from these cells in the mucosa as well as neurotensin-related peptides released from enteric neurons may be the endogenous ligands for the binding sites visualized in this study.

Opioid receptors in magnesium-digitonin-solubilized rat brain membranes are tightly coupled to a pertussis toxin-sensitive guanine nucleotide-binding protein

Opioid receptors solubilized in Mg2+-digitonin (2%, wt/vol) from Mg2+-pretreated rat brain membranes maintain, in addition to high-affinity opioid agonist binding, the modulation by guanine nucleotides. One of the modes of expression of the latter property is an attenuation of agonist binding by guanine nucleotides in the presence of Na+. To investigate the molecular basis of this modulation and to identify the G protein(s) involved, the soluble receptors were [32P]ADP-ribosylated by means of Bordetella pertussis toxin and subjected to molecular size exclusion chromatography. In addition, soluble extracts were chromatographed on lectin and hydrophobic affinity columns. The binding of 35S- and 3H-labelled analogues of GTP was also monitored in the species separated. The oligomeric G protein-coupled opioid receptors and the guanine nucleotide/pertussis toxin-sensitive species showed similar chromatographic properties in all three systems. This indicates that the biochemically functional G protein-opioid receptor complex formed in Mg2+-pretreated membranes in the absence of an agonist is stable in digitonin solution and to chromatographic separation. Further analysis showed that the guanine nucleotide modulation of opioid receptors is via the pertussis toxin substrates with Mr of 41,000 and 39,000, which are identified as Gi and Go alpha subunits, respectively.