Neurotensin increases tyrosine hydroxylase messenger RNA-positive neurons in substantia nigra after retrograde axonal transport

Neurotensin in reward processes

Neurotensin (NTS) is a neuropeptide neurotransmitter expressed in the central and peripheral nervous systems. Many studies over the years have revealed a number of roles for this neuropeptide in body temperature regulation, feeding, analgesia, ethanol sensitivity, psychosis, substance use, and pain. This review provides a general survey of the role of neurotensin with a focus on modalities that we believe to be particularly relevant to the study of reward. We focus on NTS signaling in the ventral tegmental area, nucleus accumbens, lateral hypothalamus, bed nucleus of the stria terminalis, and central amygdala. Studies on the role of NTS outside of the ventral tegmental area are still in their relative infancy, yet they reveal a complex role for neurotensinergic signaling in reward-related behaviors that merits further study. This article is part of the special issue on 'Neuropeptides'.

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.

Where and what is the paralaminar nucleus? A review on a unique and frequently overlooked area of the primate amygdala

The primate amygdala is composed of multiple subnuclei that play distinct roles in amygdala function. While some nuclei have been areas of focused investigation, others remain virtually unknown. One of the more obscure regions of the amygdala is the paralaminar nucleus (PL). The PL in humans and non-human primates is relatively expanded compared to lower species. Long considered to be part of the basal nucleus, the PL has several interesting features that make it unique. These features include a dense concentration of small cells, high concentrations of receptors for corticotropin releasing hormone and benzodiazepines, and dense innervation of serotonergic fibers. More recently, high concentrations of immature-appearing cells have been noted in the primate PL, suggesting special mechanisms of neural plasticity. Following a brief overview of amygdala structure and function, this review will provide an introduction to the history, embryology, anatomical connectivity, immunohistochemical and cytoarchitectural properties of the PL. Our conclusion is that the PL is a unique subregion of the amygdala that may yield important clues about the normal growth and function of the amygdala, particularly in higher species.

NT69L blocks ethanol-induced increase of dopamine and glutamate levels in striatum of mouse

Recent study shows that NT69L, an analog of neurotensin (NT) (8-13), reduces ethanol consumption and preference in mice through modulation of neurotensin receptor subtype one. The current study showed that NT69L significantly decreased ethanol-induced increase of dopamine and glutamate levels in striatum of mouse. These data suggest that NT69L prevents ethanol consumption through the modulation of both dopaminergic and glutamatergic systems implicated in ethanol addiction. NT agonists may provide novel treatment for alcohol addiction.

Hyperactivity of the dopaminergic system in NTS1 and NTS2 null mice

Neurotensin (NT) is a tridecapeptide that acts as a neuromodulator in the central nervous system mainly through two NT receptors, NTS1 and NTS2. The functional-anatomical interactions between NT, the mesotelencephalic dopamine system, and structures targeted by dopaminergic projections have been studied. The present study was conducted to determine the effects of NT receptor subtypes on dopaminergic function with the use of mice lacking either NTS1 (NTS1(-/-)) or NTS2 (NTS2(-/-)). Basal and amphetamine-stimulated locomotor activity was determined. In vivo microdialysis in freely moving mice, coupled with HPLC-ECD, was used to detect basal and d-amphetamine-stimulated striatal extracellular dopamine levels. In vitro radioligand binding and synaptosomal uptake assays for the dopamine transporters were conducted to test for the expression and function of the striatal pre-synaptic dopamine transporter. NTS1(-/-) and NTS2(-/-) mice had higher baseline locomotor activity and higher basal extracellular dopamine levels in striatum. NTS1(-/-) mice showed higher locomotor activity and exaggerated dopamine release in response to d-amphetamine. Both NTS1(-/-) and NTS2(-/-) mice exhibited lower dopamine D(1) receptor mRNA expression in the striatum relative to wild type mice. Dopamine transporter binding and dopamine reuptake in striatum were not altered. Therefore, lack of either NTS1 or NTS2 alters the dopaminergic system. The possibility that the dysregulation of dopamine transmission might stem from a deficiency in glutamate neurotransmission is discussed. The data strengthen the hypothesis that NT receptors are involved in the pathogenesis of schizophrenia and provide a potential model for the biochemical changes of the disease.

Anatomical localization of leucine-rich repeat kinase 2 in mouse brain

Mutations in leucine-rich repeat kinase 2 (LRRK2) have recently been identified in autosomal dominant late-onset Parkinson's disease. Expression of LRRK2 has previously been reported in brain; however, no precise anatomical information is yet available. We have performed in situ hybridization and quantitative reverse transcription polymerase chain reaction to map LRRK2 mRNA expression in mouse brain. We find LRRK2 is highly expressed in the striatum, cortex and olfactory tubercle; however, little or no expression is found in the substantia nigra, where dopaminergic neurons preferentially degenerate in Parkinson's disease. These findings suggest that LRRK2 mRNA is expressed in dopamine-receptive areas rather than in the dopamine-synthesizing neurons. Consistent with a role LRRK2 in Parkinson's disease, dysfunction of leucine-rich repeat kinase 2 protein in dopamine-innervated areas may to lead to altered dopaminergic neurotransmission and degeneration of the nigro-striatal pathway.

Neurotensin agonists: possible drugs for treatment of psychostimulant abuse

Although many neuropeptides have been implicated in the pathophysiology of psychostimulant abuse, the tridecapeptide neurotensin holds a prominent position in this field due to the compelling literature on this peptide and psychostimulants. These data strongly support the hypothesis that a neurotensin agonist will be clinically useful to treat the abuse of psychostimulants, including nicotine. This paper reviews the evidence for a role for neurotensin in stimulant abuse and for a neurotensin agonist for its treatment.

Transcriptional regulation of the tyrosine hydroxylase gene by neurotensin in human neuroblastoma CHP212 cells

The human neuroblastoma cell line CHP212 was found to express functional high affinity neurotensin (NTS-1) receptor subtype. Based on the functional interactions between neurotensin and dopamine transmission, we have used this cell line to investigate the short- and long-term modulation of tyrosine hydroxylase gene expression by the stable neurotensin agonist JMV 449. After exposure of the cells to 1 microM JMV 449 for 5 or 72 h, tyrosine hydroxylase protein and mRNA levels were significantly increased as detected by western blot analysis and quantitative RT-PCR, respectively. Transfection of CHP212 cells with a plasmid containing the luciferase reporter gene under the control of a limited proximal region of the cloned tyrosine hydroxylase promoter, revealed that the effect of JMV 449 results from an increase in the transcriptional activity of the TH gene. These results indicate that modulation of tyrosine hydroxylase gene expression may constitute one of the mechanisms involved in the control of dopamine transmission by neurotensin. Such neurotensin-mediated changes in tyrosine hydroxylase expression may also participate in multiple adaptation processes within the central nervous system to environmental conditions where neurotensin is released such as stress and food intake.

Involvement of the neurotensin receptor subtype NTR3 in the growth effect of neurotensin on cancer cell lines

The expression of the 3 currently known neurotensin receptors was studied in human cancer cells of prostatic, colonic or pancreatic origin by means of RT-PCR analysis and binding experiments. All the cells selected for this work have been shown to exhibit a growth response to neurotensin. We found that the 7 transmembrane domain, levocabastine insensitive receptor (NTR1) is expressed in most but not all of the cells studied whereas the 7 transmembrane domain, levocabastine sensitive receptor (NTR2) is present in none of these cells. The 100 kDa-type I neurotensin receptor (NTR3) is expressed in all the cells assayed. Moreover, we demonstrated that neurotensin can stimulate the growth of CHO cells stably transfected with the NTR3. Taken together, our results strongly suggest that the NTR3 subtype could be involved in the growth response of human cancer cells to neurotensin.

Ligand-induced internalization of neurotensin in transfected COS-7 cells: differential intracellular trafficking of ligand and receptor

The neuropeptide neurotensin (NT) is known to be internalized in a receptor-mediated fashion into its target cells. To gain insight into the mechanisms underlying this process, we monitored in parallel the migration of the NT1 neurotensin receptor subtype and a fluorescent analog of NT (fluo-NT) in COS-7 cells transfected with a tagged NT1 construct. Fluo-NT internalization was prevented by hypertonic sucrose, potassium depletion and cytosol acidification, demonstrating that it proceeded via clathrin-coated pits. Within 0-30 minutes, fluo-NT accumulated together with its receptor in Acridine Orange-positive, acidic organelles. These organelles concentrated transferrin and immunostained positively for rab 5A, therefore they were early endosomes. After 30-45 minutes, the ligand and its receptor no longer colocalized. Fluo-NT was first found in rab 7-positive late endosomes and later in a nonacidic juxtanuclear compartment identified as the Trans-Golgi Network (TGN) by virtue of its staining for syntaxin 6. This juxtanuclear compartment also stained positively for rab 7 and for the TGN/pericentriolar recycling endosome marker rab 11, suggesting that the ligand could have been recruited to the TGN from either late or recycling endosomes. By that time, internalized receptors were detected in Lamp-1-immunoreactive lysosomes. These results demonstrate that neurotensin/NT1 receptor complexes follow a recycling cycle that is unique among the G protein-coupled receptors studied to date, and provide the first evidence for the targeting of a nonendogenous protein from endosomes to the TGN.

Alteration of neurotensin receptors in MPTP-treated mice

We examined the sequential changes in neurotensin receptors in the striatum and substantia nigra of mouse brains lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) by receptor autoradiography, in comparison with the alterations in dopamine uptake sites. The mice received four intraperitoneal injections of MPTP (10 mg/kg) at 1-h intervals and then the brains were analyzed at 6 h and 1, 3, 7, and 21 days after the treatments. [3H]Neurotensin and [3H]mazindol were used to label neurotensin receptors and dopamine uptake sites, respectively. [3H]Neurotensin binding was significantly decreased in the striatum from 6 h to 21 days after MPTP treatment. In the substantia nigra, pars reticulata also showed a significant decrease in [3H]neurotensin binding from 3 to 21 days post-MPTP treatment. However, no significant change in [3H]neurotensin binding was observed in the pars compacta even after 21 days. On the other hand, [3H]mazindol binding was markedly decreased in the striatum and substantia nigra from 6 h to 21 days after MPTP treatment. These results indicate that neurotoxin MPTP can produce a severe decrease in neurotensin receptors and dopamine uptake sites in the striatum and substantia nigra of mice. Thus, our findings provide evidence that the dysfunction in neurotensin receptors may be involved in the degenerative processes causing Parkinson's disease.

Correlative ultrastructural distribution of neurotensin receptor proteins and binding sites in the rat substantia nigra

Neurotensin (NT) produces various stimulatory effects on dopaminergic neurons of the rat substantia nigra. To gain insight into the subcellular substrate for these effects, we compared by electron microscopy the distribution of immunoreactive high-affinity NT receptor proteins (NTRH) with that of high-affinity 125I-NT binding sites in this region of rat brain. Quantitative analysis showed a predominant association of immunogold and radioautographic labels with somata and dendrites of presumptive dopaminergic neurons, and a more modest localization in myelinated and unmyelinated axons and astrocytic leaflets. The distributions of immunoreactive NTRH and 125I-NT binding sites along somatodendritic plasma membranes were highly correlated and homogeneous, suggesting that membrane-targeted NTRH proteins were functional and predominantly extrasynaptic. Abundant immunocytochemically and radioautographically labeled receptors were also detected inside perikarya and dendrites. Within perikarya, these were found in comparable proportions over membranes of smooth endoplasmic reticulum and Golgi apparatus, suggesting that newly synthesized receptor proteins already possess the molecular and conformational properties required for effective ligand binding. By contrast, dendrites showed a proportionally higher concentration of immunolabeled than radiolabeled intracellular receptors. A fraction of these immunoreactive receptors were found in endosomes, suggesting that they had undergone ligand-induced internalization and were under a molecular conformation and/or in a physical location that precluded their recognition by and/or access to exogenous ligand. Our results provide the first evidence that electron microscopic immunocytochemistry of the NT receptor identifies sites for both the binding and trafficking of NT in the substantia nigra.

Activation of receptor gene transcription is required to maintain cell sensitization after agonist exposure. Study on neurotensin receptor

Neurotensin (NT) acts through specific G protein-coupled receptors to induce effects in the central nervous system and periphery. In this study we have shown that in the human neuroblastoma cell line CHP 212, an NT agonist, JMV 449, induced high affinity neurotensin receptor (NTR) gene activation. 125I-NT binding of cells challenged with JMV 449 rapidly decreased then reappeared and subsequently stabilized at 50% of the control values after 48 h of agonist exposure. These receptors, which reappeared at the cell surface, are as active as those found in control cells as demonstrated by Ca2+ mobilization. Furthermore, the tyrosine hydroxylase (TH) gene, a known NT target gene, remained activated after prolonged NT agonist exposure in this cell line. In the murine neuroblastoma cell line, N1E-115, NT did not stimulate NTR gene activation but induced NTR mRNA destabilization after long term agonist exposure. In this cell line, NT binding dropped to 15% of control values and remained at this value after agonist treatment. The TH expression, which was originally activated upon NT agonist exposure, decreased to control values after prolonged agonist exposure. These observations combined with the data obtained from a complementary study with HT-29 cells (Souazé, F., Rostène, W., and Forgez, P. (1997) J. Biol. Chem. 272, 10087-10094) revealed the crucial role of agonist-induced receptor gene transcription in the maintenance of cell sensitivity. A model for G protein-coupled receptor regulation induced by prolong and intense agonist stimulation is proposed.

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.

Desensitization of the neurokinin-1 receptor (NK1-R) in neurons: effects of substance P on the distribution of NK1-R, Galphaq/11, G-protein receptor kinase-2/3, and beta-arrestin-1/2

Observations in reconstituted systems and transfected cells indicate that G-protein receptor kinases (GRKs) and beta-arrestins mediate desensitization and endocytosis of G-protein-coupled receptors. Little is known about receptor regulation in neurons. Therefore, we examined the effects of the neurotransmitter substance P (SP) on desensitization of the neurokinin-1 receptor (NK1-R) and on the subcellular distribution of NK1-R, Galphaq/11, GRK-2 and -3, and beta-arrestin-1 and -2 in cultured myenteric neurons. NK1-R was coexpressed with immunoreactive Galphaq/11, GRK-2 and -3, and beta-arrestin-1 and -2 in a subpopulation of neurons. SP caused 1) rapid NK1-R-mediated increase in [Ca2+]i, which was transient and desensitized to repeated stimulation; 2) internalization of the NK1-R into early endosomes containing SP; and 3) rapid and transient redistribution of beta-arrestin-1 and -2 from the cytosol to the plasma membrane, followed by a striking redistribution of beta-arrestin-1 and -2 to endosomes containing the NK1-R and SP. In SP-treated neurons Galphaq/11 remained at the plasma membrane, and GRK-2 and -3 remained in centrally located and superficial vesicles. Thus, SP induces desensitization and endocytosis of the NK1-R in neurons that may be mediated by GRK-2 and -3 and beta-arrestin-1 and -2. This regulation will determine whether NK1-R-expressing neurons participate in functionally important reflexes.

G protein-coupled receptors in gastrointestinal physiology. II. Regulation of neuropeptide receptors in enteric neurons

Neuropeptides exert their diverse biological effects by interacting with G protein-coupled receptors (GPCRs). In this review we address the question, What regulates the ability of a target cell, in particular a neuron, to respond to a neuropeptide? Available evidence from studies of many GPCRs in reconstituted systems and transfected cell lines indicates that much of this regulation occurs at the level of the receptor and serves to alter the capacity of the receptor to bind ligands with high affinity and to couple to heterotrimeric G proteins. Although some of the knowledge gained from these studies is applicable to the regulation of neuropeptide receptors on neurons, at present there are far more questions than answers.

Endocytosis and recycling of neurokinin 1 receptors in enteric neurons

Neurotransmission depends on the availability of transmitter and on the presence of functional, high-affinity receptors at the plasma membrane that are capable of binding ligand. The pathway, mechanism and function of endocytosis and recycling of the substance P or neurokinin 1 receptor in enteric neurons were studied using fluorescent substance P, receptor antibodies and confocal microscopy. In both the soma and neurites, substance P induced rapid, clathrin-mediated internalization of the neurokinin 1 receptor into early endosomes, which also contained the transferrin receptor. After 4-8 h, there was a return in surface neurokinin 1 receptor immunoreactivity in the soma, which was not prevented by cycloheximide, and was thus independent of new protein synthesis. This return was prevented by acidotropic agents, therefore required endosomal acidification. This suggests that the neurokinin 1 receptor recycles in the soma. In contrast, in neurites, substance P and the neurokinin 1 receptor remained in endosomes and recycling was not detected. Neurons of the myenteric plexus were heavily innervated by substance P-containing nerve fibers, and K(+)-stimulated release of endogenous substance P from cultured neurons induced internalization of the neurokinin 1-receptor. Therefore, endogenous substance P may induce endocytosis of the neurokinin 1 receptor. In the soma, endocytosis and recycling correlated with loss and recovery of functional binding sites for substance P. suggesting that this process contributes to the regulation of peptidergic neurotransmission. Thus, ligand-induced endocytosis of the neurokinin 1 receptor in myenteric neurons is associated with a loss of surface receptors and functional binding sites. Since release of endogenous substance P induces neurokinin 1 receptor internalization, and neurokinin 1 receptor neurons are innervated by substance P-containing fibers, endocytosis of neuropeptide receptors may regulate neurotransmission.

Detection of naturally expressed receptors for gastrin-releasing peptide and tachykinins using cyanine 3-labelled neuropeptides

Peptides labelled with the fluorophore cyanine 3 were used to study naturally expressed neuropeptide receptors by confocal microscopy in continuous cell lines, primary cultures, and unfixed tissue. Swiss 3T3 fibroblasts bound cyanine 3-gastrin-releasing peptide at 4 degrees C, and internalized the peptide after 10 min at 37 degrees C. Internalization was specific, since it was blocked by incubation with unlabelled peptide. Primary cultures of myenteric neurons of the guinea pig incubated with cyanine 3-substance P at 4 degrees C had specific surface labelling. After 30 s at 37 degrees C, the peptide was internalized into vesicles in both the soma and neurites. Direct observation of live neurons showed movement of fluorescent vesicles to a perinuclear region after 30 min. Endocytosis was associated with a loss of surface binding sites. Unfixed whole mounts of guinea pig and rat ileum were incubated with cyanine 3-neurokinin A at 4 degrees C. After 5 min at 37 degrees C, Cy3-neurokinin A was specifically internalized in neurons and smooth muscle cells. After 30 min, a perinuclear labelling occurred in some cells. Labelling in rat neurons was diminished by the NK3-R antagonist SR142801. Thus, cyanine 3-neuropeptides are valuable tools to study expression and endocytosis of naturally expressed receptors.

Axonal and dendritic transport of internalized neurotensin in rat mesostriatal dopaminergic neurons

Previous studies have demonstrated that neurotensin is internalized and retrogradely transported in neurons of the substantia nigra following its intracerebral injection in the neostriatum. The aim of the present study was to compare the intracellular distribution of retrogradely transported material with that observed following internalization of the peptide at the somatodendritic level and to confirm that the internalization was confined to dopamine neurons. To document somatodendritic internalization, slices (350 microns) from the rat ventral midbrain were incubated in vitro with 20 mM fluoresceinylated neurotensin, a fluorescent derivative of neurotensin, and immunostained 5-60 min later for tyrosine hydroxylase. To document retrograde transport, rats were injected with the same compound into the neostriatum and the brains processed for tyrosine hydroxylase immunohistochemistry 4.5 and 8 h later. Confocal laser microscopic examination of superfused slices revealed that fluoresceinylated neurotensin was internalized at the level of the perikarya and processes of neurons in the substantia nigra, ventral tegmental area and interfascicular nucleus. At short time intervals, the label was detected in the form of small, intensely fluorescent particles distributed within the cytoplasm of both perikarya and dendrites. At longer time intervals, these fluorescent particles were larger, less numerous and confined to the perikarya where they eventually clustered against the nucleus. Following intrastriatal injection of fluoresceinylated neurotensin, retrogradely labeled cells were apparent throughout the substantia nigra, pars compacta, as well as in the lateral part of the ventral tegmental area. Here again, the label took the form of small fluorescent particles, comparable in size, shape and distribution to those detected following superfusion of midbrain slices. In both labeling conditions, fluoresceinylated neurotensin was almost exclusively confined to tyrosine hydroxylase-immunoreactive cells. These results indicate that neurotensin is internalized throughout the terminal and dendritic arborization of mesostriatal dopamine cells and that the internalized peptide is transported centripetally from both locations to the soma of the cells. The clustering of fluorescent particles in the perinuclear region of the cells further suggests that the internalized process may play a role in the long term transcellular signalling.

Neuroprotective effects of colchicine in the gerbil model of cerebral ischaemia

The tropolonic alkaloid colchicine significantly reduces the behavioural, electroencephalographic and histological damage seen after a 6-min occlusion of the two common carotid arteries of the Mongolian gerbil if the compound is administered at 2 or 4 mg/kg i.p. immediately upon reperfusion. A 45% increase in high-frequency ECoG activity and significant reduction of 80% in the hypermotility of the gerbils, with 63% less faults in a passive avoidance paradigm, were observed in conjunction with considerable protection of the hippocampus, after a single dose of 4 mg/kg colchicine. No adverse effects of colchicine treatment on animal movement and body weight were observable. Colchicine's possible mode of action, via inhibition of cellular transport systems, is discussed.

Blockade of neurotensin receptors by the antagonist SR 48692 partially prevents retrograde axonal transport of neurotensin in rat nigrostriatal system

The effect of SR 48692, a potent and selective non-peptide antagonist of the neurotensin receptor, was investigated on the retrograde axonal transport of neurotensin in the rat nigrostriatal dopamine pathway. When rats were injected in the striatum with (3-[125I]iodotyrosyl3)neurotensin, a substantial accumulation of radioactivity appeared in the ipsilateral substantia nigra 1.5 h after injection, and highest levels (336 +/- 23 dpm/mg of protein) were observed 2.5-3.5 h after the injection. The phenomenon required a pretreatment of the animals with thiorphan (30 micrograms) an inhibitor of endopeptidase. The amount of radioactivity accumulated (3.5 h) was found to be reduced (25%) by local (100 nM) or peripheral administration of SR 48692 (5, 10, 20 mg/kg, i.p.; 25%, 40%, 40%, respectively). Our results indicate that blockade of neurotensin receptors by a selective non-peptide receptor antagonist affects the retrograde axonal transport of the tridecapeptide, and further suggest the notion that this process involves neurotensin receptors.

Retrograde axonal transport of neurotensin in rat nigrostriatal dopaminergic neurons. Modulation during ageing and possible physiological role

Biochemical and anatomical data are reported which demonstrate for the first time the existence of a retrograde axonal transport process for a neuropeptide, neurotensin, in rat brain. Neurotensin receptors are mainly located in the striatum on nerve terminals of the nigrostriatal dopaminergic pathway. Thus, the association of specific neurotensin receptors on a well defined pathway provides an excellent model to investigate the existence of such a process. Two hours after the intrastriatal injection of iodinated neurotensin, radioactivity started to accumulate in the ipsilateral substantia nigra. The levels were maximal during the fourth hour. The appearance of this labelling was prevented by injection of a large excess of unlabelled neurotensin or of neurotensin 8-13, an active neurotensin fragment, but not by neurotensin 1-8 which had no affinity for neurotensin receptors. These results suggest that the appearance of radioactivity in the ipsilateral substantia nigra was dependent on the initial binding of this peptide to its receptors in the striatum. HPLC studies demonstrated that the radioactivity found in the substantia nigra corresponded to intact neurotensin and to degradation products of this peptide. Moreover, it has been shown that this retrograde transport was microtubule-dependent and occurred in dopaminergic nigrostriatal neurons. Light and electron microscopic data confirmed and extended the present results. Four and a half hours after intrastriatal injection of iodinated neurotensin, silver grains were mainly detected in dopaminergic perikarya of the substantia nigra pars compacta. The vast majority were associated with neuronal elements and their localization within cell bodies suggests that retrogradely transported neurotensin may be processed along a variety of intracellular pathways including those mediating recycling in the rough endoplasmic reticulum and degradation in lysosomes. However, the presence of silver grains over the nucleus, as well as the increase in tyrosine-hydroxylase mRNA expression in the ipsilateral substantia nigra 4 hr after intrastriatal injection of neurotensin support the concept that neurotensin alone, or associated with its receptor, might be involved in the regulation of gene expression. Finally, we have demonstrated that in old rats the quantity of retrogradely transported neurotensin was significantly decreased as compared to that observed in young adult rats. This retrograde axonal transport of a neuropeptide may represent, as already suggested for growth factors, an important dynamic process conveying information from nerve terminals to the cell body.

From receptor internalization to nuclear translocation. New targets for long-term pharmacology

Receptors involved in intercellular communication at the cell surface share the capacity to desensitize through molecular and cellular mechanisms. Cellular desensitization is a rapid and dynamic process whereby membrane receptors internalize in response to an excess of agonists. The internalized receptors may recycle rapidly or undergo down-regulation when following a degradative pathway. However, receptor internalization does not necessarily mean degradation; it also represents the initial step of a retrograde signalling system whereby an "interiorized" message, the ligand-receptor complex, can be transported in contrast to second messengers, along axons or in the cytoplasm leading to long-term effects in the nucleus. Such "third messengers" have to undergo nuclear translocation to serve as transcriptional regulators in the control of gene expression. The "third messengers" are thus cytoplasmic proteins, including the receptor itself, which may be associated with internalized vesicles and released by mechanisms which have not yet been elucidated. They represent already good targets for the development of new drugs, and multi-targeting and synergistic approaches are likely to increase their usefulness.

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.

Growth factors in Parkinson's disease

The etiology of Parkinson's disease, one of the most frequent neurodegenerative disorders in human, is unknown. New hopes concerning satisfactory therapies include transplants of autologous adrenal medullary chromaffin tissue, fetal mesencephalic dopaminergic neurons, and local application of growth factors with a neurotrophic capacity. A large body of evidence supports the notion that neurons require trophic support not only during a limited period of ontogenesis, but during their whole lifespan. Relevant molecules promote survival, transmitter synthesis and other differentiated properties, and become crucially important when a neuron is metabolically or toxically impaired. Several molecules, most of which occur in the striatum and the substantia nigra, have been identified that protect lesioned dopaminergic nigrostriatal neurons in culture or in animal models of Parkinson's disease. These include members of the neurotrophin, fibroblast growth factor, and insulin-like growth factor families as well as epidermal growth factor/transforming growth factor alpha, interleukins and ciliary neurotrophic factor. Whether their effects are merely pharmacological, or reflect a physiological role in the nigrostriatal system, is unclear as yet. This article reviews experiments that document the trophic effects of these factors on dopaminergic neurons and discusses their possible physiological and therapeutic relevance.

Immunohistochemical evidence for a neurotensin striatonigral pathway in the rat brain

The distribution and origin of neurotensin-like immunoreactivity in the substantia nigra pars reticulata of the rat have been analysed using immunohistochemistry combined with different drug treatments and lesioning techniques. In normal rats, a distinct but weakly fluorescent network of neurotensin-immunoreactive fibers was found in the central part of the substantia nigra pars reticulata. When the animals were treated with reserpine, which suppresses dopamine transmission, a similar pattern of immunoreactivity was found, though the intensity of staining was slightly enhanced. However, when rats were treated with methamphetamine, a potent dopamine releaser, the intensity of immunoreactivity was dramatically increased. In particular, densely packed neurotensin-immunoreactive fibers were found at the dorsal border and at the ventral periphery of the substantia nigra pars reticulata. This pattern of immunoreactivity was found to be similar to that displayed by dynorphin. In the nucleus caudatus, several neurotensin-immunoreactive cell bodies were seen after reserpine treatment. Morphologically similar perikarya were observed in methamphetamine-treated rats, but they were less numerous, whereas no cell bodies were detectable in untreated animals. When a unilateral mechanical transection or an ibotenic acid injection was performed in the striatum, the patterns of neurotensin as well as dynorphin and substance P immunoreactivities in the substantia nigra pars reticulata were strongly affected. Both types of lesion caused a marked, parallel depletion of all three immunoreactive substances on the side ipsilateral to the lesion, where a restricted area was virtually devoid of immunoreactive elements. Thus the present study provides evidence for the existence of a unilateral neurotensin striatonigral pathway, terminating in the pars reticulata. The origin of the neurotensin fibers in the pars compacta has not been established but does not appear to be the caudate nucleus. These results together with evidence from the literature suggest that methamphetamine induced a massive release of dopamine from nigral dendrites acting on presynaptic D1 dopamine receptors located on neurotensinergic terminals leading to a marked increase in neurotensin-like immunoreactivity in the pars reticulata.

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.