Monoiodo-[Trp11]neurotensin, a highly radioactive ligand of neurotensin receptors. Preparation, biological activity, and binding properties to rat brain synaptic membranes

Regulation of neurotensin-containing neurons in the rat striatum and substantia nigra. Effects of unilateral nigral lesion with 6-hydroxydopamine on neurotensin content and its binding site density

The effect of unilateral lesion of the rat substantia nigra with 6-hydroxydopamine (6-OHDA) was investigated on the endogenous contents of neurotensin (NT) and its binding site densities in the striatum and substantia nigra. Tyrosine hydroxylase (T-OH) activity, gamma-aminobutyric acid (GABA) content, binding site densities of dihydrotetrabenazine (TBZOH), a marker of dopaminergic synaptic vesicles, and of iodosulpiride, a ligand for dopamine D2 receptors, were also determined. Fourteen days following nigral lesions, these markers were analyzed quantitative autoradiography for NT, TBZOH and iodosulpiride binding site densities. Unilateral nigral lesion with 6-OHDA provoked only ipsilateral modifications in dopamine markers. T-OH activity and TBZOH binding site densities significantly decreased in both the ipsilateral striatum and substantia nigra. Iodosulpiride binding sites decreased in the substantia nigra and increased in the striatum on the ipsilateral side. In contrast to these unilateral changes observed for dopamine markers, dramatic increased in NT contents were found in both the ipsi- and contralateral striata. No change was found in nigral NT levels on either side. On the other hand, NT binding sites decreased in the ipsilateral striatum and substantia nigra, which reflected the destruction of dopaminergic elements in these regions. The present results strongly suggest a dopaminergic control of striatal NT systems and demonstrate that a unilateral loss of this control may lead to strong bilateral alterations in NT levels.

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.

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.

Neurotensin receptors on circular smooth muscle of canine small intestine: role of disulfide bridges

We have studied the effect of sulfhydryl agents on the binding of 125I-tyr3-neurotensin to the purified plasma membranes from circular smooth muscle and on the in vitro response of circular muscle strips of canine small intestine to neurotensin. Dithiothreitol (DTT) enhanced the binding by about 80%. Cysteine (a reductant) also enhanced the binding while cystine (an oxidant) reduced the binding to the similar extent. DTT stimulated the tissue in the organ bath and abolished the stimulatory response to low concentrations of neurotensin. The stimulatory response to acetylcholine was not altered by DTT. The implications of the role of disulfide bridges in the neurotensin response is discussed.

Neurotensin elevates cytosolic calcium in small cell lung cancer cells

The ability of neurotensin (NT) to elevate cytosolic Ca2+ in small cell lung cancer (SCLC) cells was investigated using the fluorescent Ca2+ indicator Fura 2-AM. Using SCLC cell line NCI-H345, NT elevated cytosolic Ca2+ levels in a concentration-dependent manner. Using a 10 nM dose, NT and C-terminal fragments such as NT(8-13) but not N-terminal fragments such as NT(1-8) elevated the cytosolic Ca2+ levels. Because EGTA (5 mM) did not affect the NT response, NT may cause release of Ca2+ from intracellular stores. These data indicate that SCLC NT receptors may use Ca2+ as a second messenger.

Autoradiographic localization of mu-opioid and neurotensin receptors within the mesolimbic dopamine system

In vitro autoradiographic techniques were coupled with selective chemical lesions of the A10 dopamine cells and intrinsic perikarya of the A10 region to delineate anatomical localization of the mu-opioid receptors, labeled with 125I-Tyr-D-Ala-NMePhe-Gly-OH (125I-DAGO), and neurotensin receptors, labeled with 125I-[Tyr3]neurotensin within the mesolimbic dopamine system. Unilateral lesions of dopamine perikarya produced by 6-hydroxydopamine (6-OHDA), administered in the ventral mesencephalon, produced a unilateral loss of specific neurotensin binding (65%), but did not affect mu-opioid receptor density or distribution. Unilateral lesions of intrinsic perikarya by quinolinic acid (250 nmol) injected into the A10 dopamine region produced a significant reduction in mu-opioid receptors (50%), as well as a concomitant reduction in neurotensin receptors. Unilateral 6-OHDA- or quinolinic-induced lesions in the ventral mesencephalon failed to cause significant reduction in mu-opioid receptors in the caudate putamen or limbic forebrain. In contrast, mesencephalic lesions produced significant reductions (50%) in neurotensin binding in the caudate putamen and lateral nucleus accumbens. However, neurotensin binding within the medial nucleus accumbens and adjacent limbic nuclei were unaffected by these treatments. These results are consistent with the pharmacological effects of mu-opioids and neurotensin, and suggest an indirect modulation of the mesolimbic dopamine neurons by mu-opioid agonists and the endogenous opioid peptides.

Comparison of neurotensin levels, receptors and actions in LS/Ibg and SS/Ibg mice

Neurotensin (NT), injected centrally, markedly enhances sensitivity to ethanol-induced anesthesia in SS but not in LS mice (4). Since LS and SS mice were bred selectively for differential sensitivity to ethanol, these findings suggest that neurotensinergic neuronal processes mediate some of ethanol's actions and that LS and SS mice might differ genetically in neurotensinergic systems. Indeed, in biochemical studies it was shown that LS and SS mice differ in NT-like immunoreactivity in specific brain regions, i.e., hypothalamus, and in NT receptor densities (Bmax) in frontal cortex and striatum. In other experiments LS and SS mice differed in behavioral responses to centrally administered NT. Intracerebroventricular (ICV) administration of NT produced dose-dependent changes in motor activity, hypothermia, and analgesia in both LS and SS mice. SS mice appeared to be more sensitive than LS to NT-induced analgesia but not hypothermia. Neurotensin increased or decreased locomotor activity in both SS and LS mice following intraventral tegmental area or ICV administration, respectively. The results indicate that LS and SS mice, which were selectively bred for differences in ethanol sensitivity, differ genetically in NT concentrations, receptor densities in specific brain regions, and in some receptor-mediated behavioral responses to NT.

Characteristics and displaceability of neurotensin binding sites in the rat cerebral cortex and corpus striatum

1. Neurotensin binding to synaptosomes isolated from the rat cerebral cortex and corpus striatum reached saturation after 4 min of incubation. 2. The rates of binding were found to be higher in the corpus striatum than in the cerebral cortex. 3. Concentration study revealed the presence of two different and independent classes of neurotensin binding sites with a negative cooperative interaction within each class of binding sites. 4. At saturation levels, the low affinity binding component was found to have a flat regional difference, while the high affinity binding component showed regional differences in terms of maximal binding capacity, suggesting a higher number of high affinity binding sites in the striatum than in the cortex. 5. In the competition related experiments, dopamine was found to displace around 70% of neurotensin binding sites in the corpus striatum and the cerebral cortex. 5. Substance P displaced around 50% of [3H]neurotensin in the cerebral cortex. Whereas, 50% and 70% displacement were observed in the striatum at high and low affinity binding sites, respectively. 6. These results show the overlap in the binding sites of neurotensin, dopamine and substance P.

Heterogeneity of [3H]neurotensin bindings: studies with dynorphin, L-156,903 and levocabastine

The binding of [3H]neurotensin (NT) to membranes from rat forebrain was complex, exhibiting 'high' affinity (Kd approximately 0.5 nM) and 'low' affinity (Kd approximately 5.0 nM) binding components. Dynorphin A(1-13) (DYN A(1-13] and L-156,903 (N-oxo-3-(10H-phenothiazine-10-yl)propyl-1- arginyl-1-prolyl-1-phenylalanine) potently inhibited [3H]NT binding to brain with shallow biphasic competition curves. Saturation binding studies conducted in the presence or absence of DYN A(1-13) or L-156,903 indicated that these compounds, like levocabastine, exhibited substantial selectivity for 'low' affinity NT site. Structure-activity studies indicated rigid structural requirements for the NT binding activity of DYN A(1-13) and L-156,903. In contrast to the results using brain tissue, DYN A(1-13), L-156,903 and levocabastine were very weak or inactive to inhibit [3H]NT binding to rat uterus. These studies further characterize the heterogeneity of [3H]NT binding in vitro and demonstrate clear tissue differences in binding within a given species.

Brain neurotensin receptors in mice bred for differences in sensitivity to ethanol

The hypothesis that some of ethanol's acute effects are mediated via neurotensinergic systems was investigated by characterizing neurotensin (NT) receptors in mice (LS and SS) selectively bred for differences in sensitivity to ethanol. [3H]Neurotensin binding in brain membranes from both mouse lines was specific, saturable, reversible, and linear with protein concentrations. Subcellular localization studies showed specific NT binding to be concentrated in the microsomal/synaptosomal fractions. Scatchard analyses of [3H]NT binding indicated similar KD values for membranes from various brain regions of LS and SS mice. However, Bmax values in frontal cortex, cerebellum, and striatum were greater in SS than in LS mice. In competitive binding studies IC50 values were lower for NT8-13 than for NT1-13, and IC50 values for NT1-8, NT1-11, D-Trp11-NT, and D-Tyr11-NT were greater than 1000 nM. Association and dissociation rate constants for [3H]NT and resulting KD values (0.8 nM) were similar for LS and SS brain membranes. Ethanol, in vitro, had no effect on NT binding characteristics, but as expected various cations markedly increased KD values.

The ontogeny of brain neurotensin receptors studied by autoradiography

Using in vitro receptor autoradiographic techniques we have analysed the pre- and postnatal development of neurotensin receptors in the rat brain. Receptors were labeled with [3H] or [125I]neurotensin in mounted tissue sections from animals of ages gestational day 14 until the postnatal day 21 as well as young adult animals. Very low densities of neurotensin receptors were visualized on gestational days 14 and 15. Between gestational days 16 and 18 a marked increase in the density of neurotensin receptors was seen in the developing neocortex. Densities in other brain areas, particularly the midbrain and brainstem were much lower than cortical densities. The density of neurotensin receptors in the cortex increased through the last part of the gestation and early postnatal life until it peaked at the end of the first postnatal week. After that, neurotensin receptor binding decreased dramatically reaching lower densities seen in the adult animal at the end of the third postnatal week. Development of neurotensin receptors in other brain areas followed very different time patterns. Neurotensin receptors in the midbrain were seen first at gestational day 18 and increased slowly with development to reach adult levels at about the second week of postnatal life. Neurotensin receptors in the hippocampal formation demonstrated postnatal development; they were detected at postnatal day 5 and showed a developmental peak around the second week. These patterns were seen with both 3H- and 125I-labeled neurotensin, thus excluding possible differential quenching artifacts. These clear differential regional ontogenetic patterns for neurotensin receptors are the main findings of these experiments. The very high densities present in the cortex even in fetal stages suggest that neurotensin could play a role in the development of the brain.

Different subcellular localization of neurotensin-receptor and neurotensin-acceptor sites in the rat brain dopaminergic system

The subcellular localization of neurotensin-receptor sites (NT2 sites) and neurotensin-acceptor sites (NT1 sites) was studied in rat caudate-putamen by isopycnic centrifugation in sucrose density gradients. [3H]Neurotensin binding to NT2 sites occurred as a major peak at higher sucrose densities, colocalized with [3H]dopamine uptake, and as a small peak at a lower density; whereas binding to NT1 sites occurred as a single large peak at an intermediate density. 6-Hydroxydopamine lesions of the median forebrain bundle resulted in a total loss of NT2 sites in the caudate-putamen but did not affect NT2 sites in the nucleus accumbens and the olfactory tubercle. NT1 sites were not affected. Kainic acid injections into the rat caudate-putamen led to a partial decrease of NT1 sites in this region 5 days later. After a few weeks they returned to normal. Therefore NT2 sites are probably associated with presynaptic nigrostriatal dopaminergic terminals in the caudate-putamen but not in the nucleus accumbens and the olfactory tubercle. A possible association of NT1 sites with glial cells is suggested.

Neurotensin binds with high affinity to small cell lung cancer cells

Several small cell lung cancer cell lines bound 125I-neurotensin with high affinity. Radiolabeled neurotensin bound with high affinity (Kd = 4 nM) to a single class of sites (2300/cell) using cell line NCI-H209. Binding was time dependent and reversible. Pharmacology studies indicated that the C-terminal of neurotensin was important for the high affinity binding activity. Because high concentrations of neurotensin and its receptors are associated with small cell lung cancer, neurotensin may function as a regulatory peptide in this disease.

Two populations of neurotensin binding sites in murine brain: discrimination by the antihistamine levocabastine reveals markedly different radioautographic distribution

Monoiodo-[125I-Tyr3]neurotensin (NT) bound to a high affinity, low capacity binding component and a lower affinity, high capacity component in rat brain synaptic membranes. The antihistamine H1 agent levocabastine, which bears no structural relationship to NT, selectively and totally inhibited NT binding to its low affinity binding sites. The IC50 for levocabastine was 7 nM. Lowering the temperature of the binding assay from 25 to 4 degrees C markedly reduced the affinity of the high affinity NT binding site but did not affect the ability of levocabastine to discriminate between high and low affinity NT binding sites in rat brain membranes and tissue sections. Radioautographic studies of [125I-Tyr3]NT binding to rat brain tissue sections in the absence and presence of levocabastine revealed markedly different regional distributions of the two NT binding components. The levocabastine-sensitive NT binding site was present in membranes from rat and mouse brain but absent from rabbit brain membranes and from human brain tissue sections. It was also absent from mouse neuroblastoma N1E115 and human colonic adenocarcinoma HT29 cell membranes, two cell lines which have previously been shown to possess NT receptors functionally coupled to intracellular second messenger-generating systems. These findings are discussed in the light of the known properties of the high and low affinity NT binding sites in rat brain.

Distribution of neurotensin binding sites in rat brain: a light microscopic radioautographic study using monoiodo [125I]Tyr3-neurotensin

The topographic distribution of specifically labeled neurotensin binding sites was examined by light microscopic radioautography in rat brain sections incubated with monoiodo [125I]Tyr3-neurotensin. Preliminary experiments indicated that under the present experimental conditions [125I]neurotensin specifically binds to a single apparent population of sites with a dissociation constant of 7.7 +/- 0.3 nM, and that fixation of the labeled sections with glutaraldehyde ensures regionally proportional retention of more than 70% of bound [125I]neurotensin molecules. High concentrations of [125I]neurotensin binding sites were detected in the olfactory bulb and tubercle, parts of the neocortex, the lateral septum, the diagonal band of Broca, the caudate putamen, the amygdala, the dentate gyrus, the anterior dorsal nucleus of the thalamus, the suprachiasmatic nucleus of the hypothalamus, the medial habenula, the zona incerta, the substantia nigra and the ventral tegmental area. In certain areas, such as in the diagonal band of Broca, the substantia innominata, the nucleus basalis and the pars compacta of the substantia nigra, discrete accumulations of silver grains were apparent over neuronal perikarya and their proximal dendrites. In most areas, however, the label appeared more or less uniformly distributed over nerve cell bodies and surrounding neuropil. In several instances, the labeling conformed with the distribution of cell bodies of origin and terminal aborizations of specific projection systems, suggesting that neurotensin receptors might be distributed both proximally and distally on the plasma membrane of certain neurons. Such putative "neurotensinoceptive" projection systems might involve part of the mesostriatal, mesocortical and mesolimbic dopamine systems, as well as the raphe-prosencephalic serotonin system and the habenulo-interpeduncular and basal forebrain-cortical cholinergic systems. Finally, areas of dense [125I]neurotensin labeling often corresponded to zones previously shown to exhibit intense acetylcholinesterase staining, suggesting the existence of a possible link between the expression of neurotensin binding sites and that of acetylcholinesterase in certain neuronal populations.

Neurotensin receptors on the rat liver plasma membranes

Neurotensin (NT) is now classified as a brain-gut peptide in the central nervous system and gastrointestinal tract. In the present study, we characterized the NT receptors on the rat liver plasma membranes. The specific binding of [3H]NT was time dependent, reversible, and saturable. Scatchard analysis of the specific binding data yielded two classes of binding sites, a high affinity site and a low affinity site. The average maximum number of binding sites (Bmax) amounted to 13.3 +/- 1.1 fmol/mg protein at high affinity site and 122.3 +/- 21.5 fmol/mg protein at low affinity site, respectively. The dissociation constant (Kd) had values of 0.39 +/- 0.01 nM at high affinity site and 8.1 +/- 1.1 nM at low affinity site, respectively. The amount of specifically bound [3H]NT was significantly reduced in the presence of mono and divalent cations, EDTA, EGTA and a peptidase inhibitor bacitracin, NT1-13 competed with [3H]NT for its binding site with an IC50 of 0.19 nM at high affinity site (0.2 nM concentration of [3H]NT) and 0.7 nM at low affinity site (4.0 nM concentration of [3H]NT). Xenopsin, a NT analogue separated from the skin of Xenopus laevis, was equipotent (IC50 0.75 nM) with NT1-13 at 4.0 nM concentration of [3H]NT. C-terminal sequence of NT contains the structure necessary for interaction with NT binding sites whereas N-terminal sequence had no binding activity. Since NT has a hyperglysemic and a hypercholesterolemic effects in rats, these NT receptors on the rat liver plasma membranes may be involved in the hyperglycemia and/or hypercholesteroremia induced by NT.

Different postnatal ontogeny of two [3H]neurotensin binding sites in rat brain

Two distinct neurotensin binding sites have been identified in rat brain: the NT1-acceptor site (levocabastine-sensitive) and the NT2-receptor site. In rat forebrain, NT2-receptors were present at birth, revealed a maximal level (13.8 fmol/mg tissue) on day 10 of postnatal life but a much lower plateau (3.0 fmol/mg tissue) in adult rats. NT1-acceptors were not detected before day 10 and became maximal at day 30. It is suggested that the loss of NT2-receptor sites during the postnatal development may be the expression of the regression of a transient redundancy of neuronal connections.

Neurotensin receptors in canine intestinal smooth muscle: preparation of plasma membranes and characterization of (Tyr3-125I)-labelled neurotensin binding

To study the binding of (Tyr3-125I)-labelled neurotensin to intestinal muscle, plasma membranes have been purified from dog intestinal circular smooth muscle. Purification was done by differential centrifugation followed by separation on a sucrose gradient. Electron microscopic study revealed that the dissected circular muscles used as the source of membranes were free of myenteric plexus and that the plasma membrane fraction obtained was free of any mitochondria or synaptosomes. The fraction used was obtained at the interface of 14%-33% sucrose density on the gradient and was 25-times enriched in the plasma membrane marker enzyme 5'-nucleotidase activity as compared to post-nuclear supernatant. This fraction contained negligible activity of mitochondrial membrane marker enzyme cytochrome c oxidase and low activity of a putative endoplasmic reticulum marker enzyme NADPH-cytochrome-c reductase. This membrane fraction contained a high density of neurotensin binding sites. This binding was studied by kinetic and by saturation approaches. Analysis of data from saturation binding studies by the computer programs (EBDA and LIGAND) suggested the presence of a two-site model (Kd1 = 0.118 nM, Kd2 = 3.18 nM, Bmax1 = 9.73 fmol/mg and Bmax2 = 129.8 fmol/mg). A part of specifically bound neurotensin was rapidly dissociated. No cooperativity between the two receptor types could be detected. A kinetic analysis of binding gave the Kd value equal to 0.107 nM. Carboxy terminal amino acid residues 8-13 were found to be essential for the binding activity and replacement of Tyr11 by tryptophan reduced the affinity of the peptide by 10 times in displacement studies. Binding was modulated by sodium ions and a guanine nucleotide Gpp[NH]p. MgCl2, CaCl2 and KCl were also found to reduce the specific binding. Evidence was found of a high specific binding to another membrane fraction poor in plasma membranes and rich in synaptosomes. We concluded that plasma membrane of canine intestinal circular muscle contains neurotensin receptors with recognition properties distinct from those obtained in previous studies of neurotensin binding sites in murine tissues. Another neurotensin binding site may be present on neuronal membranes.

Neurotensin selectively alters ethanol-induced anesthesia in LS/Ibg and SS/Ibg lines of mice

Neurotensin (NT) differentially altered ethanol-induced anesthesia as measured by duration of loss of righting response or by blood ethanol levels producing loss of righting response in mice (LS and SS) which were selectively bred for differences in response to ethanol. At doses of 5-500 ng i.c.v., NT increased ethanol sensitivity in SS mice, but not in LS mice, as measured by blood ethanol concentrations at loss of righting response. At higher doses, 0.5-10 micrograms i.c.v., NT enhanced the sensitivity of both SS and LS mice to ethanol-induced anesthesia. The hypothermic effect of ethanol determined at loss of righting response was not altered in either LS or SS mice at low doses of NT, but at higher doses NT enhanced ethanol-induced hypothermia in both lines of mice. The altered anesthetic sensitivity was specific for ethanol in that NT did not alter pentobarbital-induced sleep time in either LS or SS mice and halothane anesthesia was altered slightly only in LS mice. NT analogues, N-acetyl-NT8-13, and [D-Trp11]-NT but not NT1-8 enhanced the anesthetic action of ethanol in SS mice. Bombesin, cholecystokinin sulfate, substance P, [D-Trp8, D-Cys14]-somatostatin and corticotropin releasing hormone (CRF) were not effective in enhancing ethanol-induced anesthesia in LS or SS mice. CRF appeared to decrease ethanol sensitivity in LS but not in SS mice. Beta-Endorphin (beta-END) markedly increased the ethanol sensitivity of SS and to a lesser extent of LS mice at relatively high doses, e.g. 0.5-1.0 micrograms i.c.v. The results of the present study indicate that differences in brain sensitivity of LS and SS mice to ethanol may be mediated by genetic differences in NT systems. Likewise, NT, and probably beta-endorphin, may interact with other neurochemical processes that are involved in the mechanism of ethanol-induced anesthesia and that differ genetically in LS and SS mice.

Identification of neurotensin receptors associated with calcium channels and prolactin release in rat pituitary

Neurotensin (NT) is now reasonably well established as a neurotransmitter or neuromodulator candidate in the CNS. In the present study, we characterized the NT receptors in dispersed cells from the anterior lobe of rat pituitary and investigated the involvement of both cyclic AMP and calcium in the release of prolactin (PRL) induced by NT receptor stimulation. The [3H]NT binding to membranes from anterior pituitary dispersed cells was found saturable and stereospecific. Scatchard analysis of the data gave a straight line indicating a Bmax value of 121 +/- 11 fmol/mg protein and a KD value of 1.4 +/- 0.2 nM. The calculated IC50 values for [3H]NT binding were 5.8 nM for NT, 7.8 nM for L-Phe-NT, and 3,000 nM for the pharmacologically inactive form D-Phe-NT. NT, up to a concentration of 1 microM, did not affect the cyclic AMP generating system in homogenates of anterior pituitary from male or lactating female rats. The same pattern of results was obtained for cyclic AMP formation in intact cells. NT and its analogs stereospecifically enhanced the influx of calcium into dispersed cells from rat anterior pituitary. The effect was time- and dose-dependent. It appeared to be associated with neurotransmitter-operated calcium channels since: preincubation of the cells with tetrodotoxin did not affect the increase in calcium influx induced by NT; concentrations of verapamil that counteract the influx of calcium induced by potassium lacked the capacity to modify the influx of calcium induced by NT; and NT lost its capacity to release PRL in the absence of extracellular calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

High-affinity receptor sites and rapid proteolytic inactivation of neurotensin in primary cultured neurons

The present article describes the interaction of neurotensin with specific receptors in pure primary cultured neurons and the mechanisms by which this peptide is inactivated by these cells. Neurotensin binding sites are not detectable in nondifferentiated neurons and appear during maturation. The binding at 37 degrees C of [monoiodo-Tyr3]neurotensin to monolayers of neurons 96 h after plating is saturable and characterized by a dissociation constant of 300 pM and a maximal binding capacity of 178 fmol/mg of protein. The binding parameters as well as the specificity of these receptors toward neurotensin analogues reveal close similarities between the binding sites present in primary cultured neurons and those described in other membrane preparations or cells. Neurotensin is rapidly degraded by primary cultured neurons. The sites of primary inactivating cleavages are the Pro7-Arg8, Arg8-Arg9, and Pro10-Tyr11 bonds. Proline endopeptidase is totally responsible for the cleavage at the Pro7-Arg8 bond and contributes to the hydrolysis mainly at the Pro10-Tyr11 site. However, the latter breakdown is also generated by a neurotensin-degrading neutral metallopeptidase. The cleavage at the Arg8-Arg9 bond is due to a peptidase that can be specifically inhibited by N-[1(R,S)-carboxy-2-phenylethyl]-alanyl-alanyl-phenylalanyl-p- aminobenzoate. The secondary processing occurring on neurotensin degradation products are: a bestatin-sensitive aminopeptidasic conversion of neurotensin11-13 to free Tyr11, and a rapid cleavage of neurotensin8-13 by proline endopeptidase. A model for the inactivation of neurotensin in primary cultured neurons is proposed and compared to that previously described for purified rat brain synaptic membranes.

Photoaffinity labeling of neurotensin binding sites on rat brain sections

The photoaffinity labeling of neurotensin (NT) binding sites was carried out on rat midbrain sections using a monoiodo analogue of NT (125I-azidobenzoyl [Trp11] NT; 125IAB-NT). Autoradiographic data showed that the 125IAB-NT binding site localization was quite similar to that obtained with 125I-NT, with high densities in both substantia nigra and ventral tegmental area. Covalent specific binding was only observed when sections were irradiated with UV after the incubation, followed by various histological treatments necessary for light and electron microscopy.

Evidence for a displaceable non-specific [3H]neurotensin binding site in rat brain

Levocabastine is a potent antihistamine drug, structurally unrelated to neurotensin. In rat and mouse brain but not in other animal species, it inhibited 60% of the [3H]neurotensin binding displaced by unlabelled neurotensin or neurotensin(8-13). The levocabastine-sensitive site or "site 1" displayed high affinity properties for levocabastine (IC50 = 25 nM) and was highly sterospecific (IC50-value higher than 10 microM for one of the isomers). Binding to the "site 1" in rat brain corresponded to the [3H]neurotensin binding displaceable by 1 microM levocabastine, whereas binding to the "site 2" corresponded to the binding displaced by 1 microM neurotensin when the "site 1" was occluded by 1 microM levocabastine. Both "site 1" and "site 2" appeared to be saturable. Scatchard plots obtained in rat bulbus olfactorius allowed to calculate a KD-values of 7.1 nM and a Bmax-values of 37.2 fmol/mg original tissue for "site 1", while "site 2" displayed a KD-value of 0.7 nM and a Bmax-value of 16.3 fmol/mg original tissue. The regional distributions of both sites showed marked differences. The "site 1" was homogeneously distributed throughout all rat brain areas, whereas the amount of "site 2" binding was markedly different in separate brain areas: bulbus olfactorius and substantia nigra had the highest amounts (8.9 and 7.8 fmol/mg tissue) while cerebellum had the lowest (0.4 fmol/mg tissue). In spite of its high affinity and stereospecificity, "site 1" has to be considered as an acceptor or recognition site for [3H]neurotensin because of its species-link, low saturability and homogeneous distribution in all rat brain areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuromedin N: high affinity interaction with brain neurotensin receptors and rapid inactivation by brain synaptic peptidases

Neuromedin N (NN) is a novel neurotensin (NT)-like hexapeptide recently isolated from porcine spinal cord. NN competitively inhibited the binding of monoiodinated [Trp11]NT to rat brain synaptic membranes with a 19-fold lower potency than NT. In the presence of 1 mM 1,10-phenanthroline or 10 microM bestatin, the potency of NN relative to NT was increased about 5-fold. NN was readily degraded by rat brain synaptic membranes, and NN-(2-6) was the major degradation product. NN-(2-6) did not bind to NT receptors at concentrations up to 1 microM whether or not peptidase inhibitors were present in the binding assay. The rate of degradation by synaptic membranes was nearly 2.5 times higher for NN than for NT. NN degradation by membranes was totally prevented by 1,10-phenanthroline and markedly inhibited by bestatin. The presence of NN in the central nervous system, its high potency to interact with brain NT receptors and its rapid inactivation by brain synaptic peptidases make it a potential neurotransmitter candidate acting at the NT receptor.

Binding of [3H]neurotensin in human brain: properties and distribution

The binding of [3H]neurotensin to membranes from human brain at 0 degrees C was specific, saturable, and reversible. In the frontal cortex, the equilibrium dissociation constant (KD) for [3H]neurotensin determined from the ratio of rate constants (k-1/k1), saturation isotherms, and inhibition binding experiments was 0.80, 2.0, and 2.0 nM, respectively, and the maximum number of binding sites (Bmax) from the saturation isotherms and the competitive binding experiments was 2.4 and 2.2 pmol/g of tissue, respectively. Hill coefficients for binding were equal to 1, indicating the presence of single, noncooperative binding sites. Inhibition of specific binding of [3H]-neurotensin by several analogs of neurotensin showed that [Gln4]neurotensin and neurotensin(8-13) had the highest affinities for these binding sites in human frontal cortex, with each analog being approximately 13-fold more potent than neurotensin. In addition, these data showed that the carboxy-terminal portion of neurotensin played an important part in the binding of this neuropeptide in human brain, a result described for other species. Regional distribution of binding sites was different from that reported for animal brains. Of the 33 different regions investigated, the uncus and substantia nigra showed the highest specific binding of [3H]neurotensin, whereas such areas as the pineal body, medulla, and corpus callosum had few binding sites.

Neurotensin and its analogs--correlation of specific binding with stimulation of cyclic GMP formation in neuroblastoma clone N1E-115

The receptors which mediate neurotensin-stimulated intracellular cyclic GMP formation in murine neuroblastoma clone N1E-115 [J. A. Gilbert and E. Richelson, Eur. J. Pharmac. 99, 245 (1984)] were further characterized. The binding of [3H]neurotensin to intact N1E-115 cells at 0 degree displayed specificity, saturability, reversibility, and tissue linearity. A single class of neurotensin receptors was demonstrated with an apparent KD of 9-11 nM and a Bmax of 180-250 fmoles/10(6) cells, determined by the type of serum employed in the cellular culture medium. A number of neurotensin analogs and fragments were compared for their ability to inhibit [3H]neurotensin binding and stimulate intracellular cyclic GMP formation with intact N1E-115 cells. A direct correlation was found to exist between the KD and EC50 for each peptide. The carboxyl-terminal portion of neurotensin proved to be responsible for the binding and biochemical activities of this peptide with clone N1E-115. Neurotensin(8-13) was, in fact, fifty times more potent than native neurotensin in stimulating intracellular cyclic GMP formation and had an 18-fold higher affinity for the neurotensin receptor on this neuronal cell type.

Covalent labeling of neurotensin receptors in rat gastric fundus plasma membranes

Neurotensin receptors from plasma membranes of rat gastric fundus smooth muscle were specifically and covalently labeled either by using the photoreactive analogue 125I-labeled azidobenzoyl (Trp11)-neurotensin or by cross-linking (monoiodo-Tyr3)neurotensin to the membrane preparation by means of disuccinimidyl suberate. Analysis of plasma membranes by sodium dodecylsulfate-polyacrylamide gel electrophoresis and autoradiography revealed that the same protein band with an apparent molecular weight of 110,000 was specifically labeled by both methods. This band consisted of a single chain protein since its apparent size was found to be the same with or without reduction of membrane samples before electrophoresis. Only neurotensin and its biologically active analogues were able to protect plasma membranes against specific labeling of the protein band of molecular weight 110,000. Comparison of these results with those obtained from rat brain synaptic membranes shows that although rat central and peripheral neurotensin receptors exhibit similar specificities towards a series of neurotensin analogues, their subunit structures are different.

Characterization of functional receptors for vasoactive intestinal peptide in bovine cerebral arteries

This study reports the characterization of receptors for vasoactive intestinal peptide (VIP) on membranes prepared from bovine cerebral arteries. By use of HPLC we prepared two purified monoiodinated VIP radioligands with nearly equivalent cerebral vasorelaxant potency as native VIP, [Tyr(125I)10 )VIP and [Tyr(125I)22]VIP. The former resulted in a higher proportion of specific binding to arterial membranes than the latter and was therefore thought to be the superior radioligand for receptor characterization. The binding of [Tyr(125I)10]VIP to cerebral arterial membranes was saturable, specific, reversible, and dependent on time and temperature. Scatchard analysis suggested the presence of a high- and a low-affinity binding site with KD values of 0.2 and 11 nM and receptor concentrations of 79 and 737 fmol/mg of protein, respectively. The dose-response curves for binding to the VIP receptor by the VIP-homologous peptides PHI, PHM, and rat growth hormone-releasing factor (GRF) were very similar to their dose-response curves for relaxation of cerebral arteries. The order of potency was VIP greater than PHM greater than PHI greater than rat GRF. It is suggested that the characteristics of the vascular VIP binding sites and the close correlation between the binding and vasorelaxant properties of VIP and its related peptides argue for the vascular binding sites being functional receptors for VIP.

Strategies in neuropeptide receptor binding research

Strategies and general approaches used in neuropeptide receptor binding assays are described. Special attention is given to the nature of the ligand, its physical and chemical stability and the demonstration of an appropriate ligand selectivity pattern. Examples are given to illustrate critical aspects of neuropeptide receptor binding assays. Strong correlation between binding and bioassay data is also stressed.

Regulation of cyclic GMP levels by neurotensin in neuroblastoma clone N1E115

The binding of 125I-labeled [monoiodo-Tyr3]neurotensin to intact neuroblastoma N1E115 cells and the effect of neurotensin on the intracellular concentration of cyclic nucleotides were studied at 37 degrees C and under physiological conditions of pH and ionic strength. The radiolabeled neurotensin analogue bound specifically to differentiated cells with a dissociation constant of 0.75 nM and a maximal binding capacity of 45 fmol/10(6) cells. Incubation of neuroblastoma cells with neurotensin in the presence of calcium ions resulted in a transient increase of 10 fold over basal level of the intracellular cyclic GMP concentration. Half-maximal stimulation was obtained with 2 nM neurotensin. Under identical conditions the cyclic AMP concentration only decreased by 20-30%. These results suggest that cyclic GMP is a second messenger of neurotensin in neuroblastoma clone N1E115.

Loss of high affinity neurotensin receptors in substantia nigra from parkinsonian subjects

Monoiodo [125I-Tyr3]-Neurotensin binding was studied in post mortem substantia nigra from 17 control and 15 parkinsonian subjects. Binding to individual homogenates was decreased by 58%, 49% and 26% at 0.36, 1.4, 5.5 M(-9) concentration of ligand, respectively. Saturation analysis using pooled substantia nigra demonstrated an almost complete loss of the high affinity component of the neurotensin receptor complex, yielding a 24% loss of the total binding capacity, with no alteration of the low affinity component. Similarly an important loss of binding was observed in monoiodo[125I-Tyr3]-Neurotensin autoradiograms of two substantia nigra from parkinsonian subjects. These results support the hypothesis of neurotensin receptors occurring on dopamine cell bodies and/or dendrites in human substantia nigra. Role of neurotensin may be of importance in the regulation of dopamine pathway involved in parkinsonism.

Characterization of neurotensin binding to rat gastric smooth muscle receptor sites

The binding of monoiodo 125I-Trp11-neurotensin to purified rat gastric fundus smooth muscle plasma membranes was characterized. Specific binding of ligand in subcellular fractions from rat fundus smooth muscle showed a distribution that paralleled that of several plasma membrane marker enzymes. 125I-Trp11-neurotensin binding to smooth muscle plasma membranes at 25 degrees C was maximal at 30 min, reversible and saturable. Scatchard analysis of equilibrium data indicated the existence of two classes of binding sites with dissociation constants (Kd) of 56 pmol and 1.92 nM, and corresponding binding capacities (Bmax) of 6.6 fmol/mg and 11.4 fmol/mg of membrane protein. Analogues and fragments of neurotensin competed for 125I-Trp11-neurotensin binding with a rank order of potency similar to that previously reported for their contracting effect in rat fundus strips. Na+ decreased in a concentration dependent manner the binding of labelled ligand to the high affinity site. At 100 mM, Na+ induced a 6-fold increase in the IC50 of neurotensin for inhibition of 125I-Trp11-neurotensin binding. At this concentration of Na+, the IC50 for neurotensin was 1 nM, a value close to the Kd of the low affinity site.

Specific binding of tritiated neurotensin to rat brain membranes: characterization and regional distribution

The characteristics of [3H]neurotensin binding were studied using membranes prepared from the rat brain. Binding of [3H]neurotensin was found to be specific, saturable and reversible. Under the conditions of the assay non-specific binding represented less than 20% of the total binding at a radioligand concentration of 2 nM. The specific [3H]neurotensin binding increased linearly with protein concentration and was dependent on the pH and the temperature of the incubation medium. At 25 degrees C equilibrium was reached rapidly and the association kinetics appeared to be monophasic. The dissociation was not monophasic and it could be resolved into two distinct components. Scatchard analysis of the saturation data indicated a single population of binding sites with a density of 432 fmol/mg protein and an equilibrium dissociation constant of 2.85 nM. A Hill transformation of the competitive inhibition of specific [3H]neurotensin binding by increasing concentrations of unlabelled peptide yielded a slope not significantly different from unity. Neurotensin 1-13 and various neurotensin analogues were tested for their ability to compete with [3H]neurotensin for its binding site. Neurotensin 1-13, neurotensin 8-13 and the amphibian skin peptide xenopsin were equipotent and strongly inhibited the specific binding of [3H]neurotensin. Neurotensin 9-13 and the chicken intestinal peptide Lys8,Asn9-neurotensin 8-13 were weakly active, whereas neurotensin 10-13 and the amino-terminal fragments neurotensin 1-6, neurotensin 1-8 and neurotensin 1-11 were inactive. Physiological concentrations of sodium chloride inhibited specific [3H]neurotensin binding, whereas divalent cations and guanyl nucleotides did not produce a significant change in either the equilibrium dissociation constant or the total number of binding sites. There was no apparent correlation between the content of neurotensin-like immunoreactivity in the rat brain and the density of [3H]neurotensin binding sites in the various brain regions. The highest density of binding sites was found in the hypothalamus and the frontal cortex, intermediate levels in striatum, thalamus, midbrain, hippocampus and olfactory bulb, and low levels in cerebellum and pons-medulla oblongata. In general, there was less variation between different brain regions in the number of [3H]neurotensin binding sites than in the content of neurotensin-like immunoreactivity. The characteristics of this binding assay are consistent with [3H]neurotensin binding to a physiological receptor.

Chronic neuroleptic treatment enhances neurotensin receptor binding in human and rat substantia nigra

Antischizophrenic neuroleptic drugs interact with brain dopamine systems in producing both therapeutic and unwanted side effects. Short-term administration of neuroleptics may produce a parkinsonian, hypokinetic syndrome mimicking symptoms of dopamine depletion, and presumably caused by dopamine receptor blockade. Paradoxically, some 20% of patients treated chronically with these agents develop tardive dyskinesia. The adventitious movements typical of this condition mimic symptoms of dopamine excess despite continuing receptor blockade. Dopamine receptor supersensitivity does develop in such conditions, but its extent does not correlate well with the presence or absence of tardive dyskinesia. Exploration of neuroleptic-induced alterations in other dopamine-associated systems may thus provide insight into these processes. Dopamine-containing cells of the substantia nigra, prominently implicated in the motor side effects of neuroleptics, possess dense concentrations of receptors for the putative peptide neurotransmitter, neurotensin. Here we report that these receptors are substantially increased in both rats and humans after chronic treatment with neuroleptic drugs, and discuss possible implications of this finding for our understanding of neuroleptic actions.

Preparation of neurotensin selectively iodinated on the tyrosine 3 residue. Biological activity and binding properties on mammalian neurotensin receptors

[Monoiodo- Tyr3 ]neurotensin, a neurotensin analogue that contains a single iodine atom on the side chain of Tyr 3 was prepared and purified. This analogue can be labeled at any specific radioactivity between 0 and 2000 Ci/mmol; its binding and biological properties on rat and guinea pig neurotensin receptors are identical to those of the parent peptide. These properties make [monoiodo- Tyr3 ]neurotensin the best suitable radioligand for detection and characterization of neurotensin receptors in various tissues and species.

Characterization and visualization of neurotensin binding to receptor sites in human brain

The binding of monoiodo [125I-Tyr3]-neurotensin to human brain was characterized and visualized using radioreceptorassay and autoradiographic techniques. Specific binding to homogenates of human substantia nigra at 25 degrees C was maximal at 20 min, reversible and saturable. Scatchard analysis of equilibrium data indicated the existence of two populations of binding sites with Kd values of 0.26 nM and 4.3 nM. Corresponding binding capacities were 26 and 89 fmol/mg of protein. Neurotensin analogs inhibited the binding of iodinated neurotensin with relative potencies that demonstrated the crucial role of the C-terminal hexapeptide portion of neurotensin for binding to its receptors. Autoradiography of human substantia nigra sections incubated with iodinated neurotensin revealed high levels of specific binding in the nucleus paranigralis and substantia nigra, pars compacta, and low levels in the substantia nigra, pars reticulata.

High-affinity neurotensin binding sites in differentiated neuroblastoma N1E115 cells

This paper describes the interaction of neurotensin with mouse neuroblastoma N1E115 cells. Neurotensin binding sites are undetectable in nondifferentiated neuroblastoma cells. They appear during cell differentiation in the presence of a low serum concentration and dimethyl sulfoxide, and reach a maximal level after 50-60 h of incubation under these conditions. The binding of monoiodo[Trp11]neurotensin to homogenates of differentiated N1E115 cells is specific, saturable, and reversible. The interaction is characterized by a dissociation constant of 150 pM and a maximal binding capacity of 9 fmol/mg of protein at 0 degrees C, pH 7.5. These binding parameters, as well as the specificity toward a series of neurotensin analogues, are similar for neurotensin receptors in N1E115 cells and for the high-affinity binding sites that had been previously characterized in rat brain synaptic membranes by means of the same radiolabeled ligand. The presence of high-affinity binding sites for neurotensin in the neuroblastoma N1E115 provides a useful model to study the cellular responses that are generated by the association of neurotensin to its receptor in electrically excitable cells.

[Radioactive analogs of neurotensin. I. Solid phase synthesis and biological characterization of Trp 11-neurotensin, a precursor of an iodized ligand]

In order to generate highly labelled neurotensin analogues, synthesis has been performed of two types of precursors, one for iodination and one for tritiation. Iodination of native neurotensin occurs on both tyrosines in position 3 and 11 and thus affects greatly its binding capacities. In this article, synthesis and chemical characterization of [Trp11]-neurotensin are described which can be iodinated without loss of activity. Synthesis was by solid phase procedure on an experimental support, Pab-resin, alpha-(4-chloromethylphenylacetamido)-benzyl copoly (styrene 1 per cent divinylbenzene). After esterification of Boc-Leu by its cesium salt on the Pab-resin, each amino acid was incorporated by a double coupling with dicyclohexylcarbodiimide on a Beckman 990 synthesizer. The trifunctionnal amino acids were protected as follows : Tyr as the 2,6-dichlorobenzyl ether, Glu as benzyl ester, Lys by the benzyloxycarbonyl group, Arg by the tosyl group, and Trp by the formyl group. Boc-Asn was incorporated by the HOBt procedure. The cleavage of peptide-resin bond and the removal of lateral chain protecting groups was realized by hydrofluoric acid with 10 per cent anisol for 1 h at 0 degrees C. The peptide obtained was then treated by NH4HCO3 1 M, pH 9, for 24 h for the removal of tryptophan formyl protecting group. Purification of the crude peptide on Bio-Gel P2 followed by ion exchange chromatography on carboxymethylcellulose (CM 52) and a final desalting on Bio-Gel P2 proved very efficient in removing several shorter contaminants.(ABSTRACT TRUNCATED AT 250 WORDS)

[Radioactive analogs of neurotensin. II. Preparation of tritiated neurotensin from a synthetic multi-unsaturated derivative]

In this second paper on the synthesis of neurotensin analogues as precursors for radiolabelling, solid phase synthesis of two polyunsaturated peptides, [Dah6, delta Pro7,10]-neurotensin and acetyl-[delta Pro10]-neurotensin-(8-13), are described. The first one contains one triple bond and two double bonds susceptible to tritiation in the same molecule, the second one contains one double bond in the shortest sequence having neurotensin activity. The C-terminal residue, Boc-Leu, was esterified on the chloromethyl-resin by its cesium salt. For the other amino acids a double coupling was carried out, the first one with dicyclohexylcarbodimide and the second one with the amino acid hydroxybenzotriazole ester. Acylation of the second amino acid, on the resin, presented some difficulties to achieve completeness and several acetylations and benzoylations had to be performed in order to block the last 4 per cent of free amines. It seems that these difficulties are related to some batches of chloromethyl-resin. Incorporation of both acetylenic lysine, N alpha-Boc-N epsilon-Z-L-2,6-diamino-4-hexynoic acid, whose synthesis is described, and N alpha-Boc-L-3,4-dehydroproline was without problems in this synthesis. After cleavage by hydrofluoric acid the crude peptides were purified by gel filtration on Bio-Gel P2 and ion exchange chromatography on carboxymethylcellulose (CM 52). [Dah6, delta Pro7,10]-neurotensin so obtained (51 per cent compared to starting Boc-Leu-resin) was in homogeneous form as characterized by amino acid analysis, thin layer chromatography in different systems and high performance liquid chromatography. The hydrogenation or tritiation product was identical with native neurotensin. Unsaturated derivative and neurotensin obtained after catalytic hydrogenation were as active as native neurotensin in inhibition of 125I-[Trp11]-neurotensin binding to rat brain synaptic membranes and in guinea pig ileum contractility test. Substitution of proline and lysine by their dehydro-derivatives did not affect the biological properties of neurotensin. The tritiated neurotensin (160-180 Ci/mmol) should be a good agent for biological characterization of neurotensin receptors and for investigation of the peptide metabolism.