Neurotensin receptor expression in the rat forebrain and midbrain: a combined analysis by in situ hybridization and receptor autoradiography

Opposing actions of co-released GABA and neurotensin on the activity of preoptic neurons and on body temperature

Neurotensin (Nts) is a neuropeptide acting as a neuromodulator in the brain. Pharmacological studies have identified Nts as a potent hypothermic agent. The medial preoptic area, a region that plays an important role in the control of thermoregulation, contains a high density of neurotensinergic neurons and Nts receptors. The conditions in which neurotensinergic neurons play a role in thermoregulation are not known. In this study, optogenetic stimulation of preoptic Nts neurons induced a small hyperthermia. In vitro, optogenetic stimulation of preoptic Nts neurons resulted in synaptic release of GABA and net inhibition of the preoptic pituitary adenylate cyclase-activating polypeptide (Adcyap1) neurons firing activity. GABA-A receptor antagonist or genetic deletion of Slc32a1 (VGAT) in Nts neurons unmasked also an excitatory effect that was blocked by a Nts receptor 1 antagonist. Stimulation of preoptic Nts neurons lacking Slc32a1 resulted in excitation of Adcyap1 neurons and hypothermia. Mice lacking Slc32a1 expression in Nts neurons presented changes in the fever response and in the responses to heat or cold exposure as well as an altered circadian rhythm of body temperature. Chemogenetic activation of all Nts neurons in the brain induced a 4-5°C hypothermia, which could be blocked by Nts receptor antagonists in the preoptic area. Chemogenetic activation of preoptic neurotensinergic projections resulted in robust excitation of preoptic Adcyap1 neurons. Taken together, our data demonstrate that endogenously released Nts can induce potent hypothermia and that excitation of preoptic Adcyap1 neurons is the cellular mechanism that triggers this response.

Opposing actions of co-released GABA and neurotensin on the activity of preoptic neurons and on body temperature

Neurotensin (Nts) is a neuropeptide acting as a neuromodulator in the brain. Pharmacological studies have identified Nts as a potent hypothermic agent. The medial preoptic area, a region that plays an important role in the control of thermoregulation, contains a high density of neurotensinergic neurons and Nts receptors. The conditions in which neurotensinergic neurons play a role in thermoregulation are not known. In this study optogenetic stimulation of preoptic Nts neurons induced a small hyperthermia. In vitro, optogenetic stimulation of preoptic Nts neurons resulted in synaptic release of GABA and net inhibition of the preoptic pituitary adenylate cyclase-activating polypeptide (PACAP) neurons firing activity. GABA-A receptor antagonist or genetic deletion of VGAT in Nts neurons unmasked also an excitatory effect that was blocked by a Nts receptor 1 antagonist. Stimulation of preoptic Nts neurons lacking VGAT resulted in excitation of PACAP neurons and hypothermia. Mice lacking VGAT expression in Nts neurons presented changes in the fever response and in the responses to heat or cold exposure as well as an altered circadian rhythm of body temperature. Chemogenetic activation of all Nts neurons in the brain induced a 4-5 °C hypothermia, which could be blocked by Nts receptor antagonists in the preoptic area. Chemogenetic activation of preoptic neurotensinergic projections resulted in robust excitation of preoptic PACAP neurons. Taken together our data demonstrate that endogenously released Nts can induce potent hypothermia and that excitation of preoptic PACAP neurons is the cellular mechanism that triggers this response.

Visualization of real-time receptor endocytosis in dopamine neurons enabled by NTSR1-Venus knock-in mice

Dopamine (DA) neurons are primarily concentrated in substantia nigra (SN) and ventral tegmental area (VTA). A subset of these neurons expresses the neurotensin receptor NTSR1 and its putative ligand neurotensin (Nts). NTSR1, a G protein-coupled receptor (GPCR), which classically activates Gαq/calcium signaling, is a potential route for modulating DA activity. Drug development efforts have been hampered by the receptor’s complex pharmacology and a lack of understanding about its endogenous location and signaling responses. Therefore, we have generated NTSR1-Venus knock-in (KI) mice to study NTSR1 receptors in their physiological context. In primary hippocampal neurons, we show that these animals express functional receptors that respond to agonists by increasing intracellular calcium release and trafficking to endosomes. Moreover, systemic agonist administration attenuates locomotion in KIs as it does in control animals. Mapping receptor protein expression at regional and cellular levels, located NTSR1-Venus on the soma and dendrites of dopaminergic SN/VTA neurons. Direct monitoring of receptor endocytosis, as a proxy for activation, enabled profiling of NTSR1 agonists in neurons, as well as acute SN/VTA containing brain slices. Taken together, NTSR1-Venus animals express traceable receptors that will improve understanding of NTSR1 and DA activities and more broadly how GPCRs act in vivo.

Neurotensin receptor 1 deletion decreases methamphetamine self-administration and the associated reduction in dopamine cell firing

We previously reported that a non-selective pharmacological blockade of neurotensin receptors in the ventral tegmental area (VTA) decreases methamphetamine (METH) self-administration in mice. Here, we explored the consequences of genetic deletion of neurotensin receptor 1 (NtsR1) on METH self-administration and VTA dopamine neuron firing activity. We implanted mice with an indwelling jugular catheter and trained them to nose-poke for intravenous infusions of METH. Mice with NtsR1 deletion (KO) acquired self-administration similar to wildtype (WT) and heterozygous (HET) littermates. However, in NtsR1 KO and HET mice, METH intake and motivated METH seeking decreased when the response requirement was increased to a fixed ratio 3 and when mice were tested on a progressive ratio protocol. After completion of METH self-administration, single cell in vivo extracellular recordings of dopamine firing activity in the VTA were obtained in anesthetized mice. Non-bursting dopamine neurons from KO mice fired at slower rates than those from WT mice, supporting an excitatory role for NtsR1 on VTA dopamine neuronal activity. In WT mice, a history of METH self-administration decreased dopamine cell firing frequency compared with cells from drug-naïve controls. NtsR1 KO and HET mice did not exhibit this decline in dopamine cell firing activity after METH experience. We also observed an increase in population activity following METH self-administration that was strongest in the WT group. Our results suggest a role for NtsR1 in METH-seeking behavior and indicate that ablation of NtsR1 prevents the detrimental effects of prolonged METH self-administration on VTA dopamine cell firing frequency.

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

Direct dopamine terminal regulation by local striatal microcircuitry

Regulation of axonal dopamine release by local microcircuitry is at the hub of several biological processes that govern the timing and magnitude of signaling events in reward-related brain regions. An important characteristic of dopamine release from axon terminals in the striatum is that it is rapidly modulated by local regulatory mechanisms. These processes can occur via homosynaptic mechanisms-such as presynaptic dopamine autoreceptors and dopamine transporters - as well heterosynaptic mechanisms such as retrograde signaling from postsynaptic cholinergic and dynorphin systems, among others. Additionally, modulation of dopamine release via diffusible messengers, such as nitric oxide and hydrogen peroxide, allows for various metabolic factors to quickly and efficiently regulate dopamine release and subsequent signaling. Here we review how these mechanisms work in concert to influence the timing and magnitude of striatal dopamine signaling, independent of action potential activity at the level of dopaminergic cell bodies in the midbrain, thereby providing a parallel pathway by which dopamine can be modulated. Understanding the complexities of local regulation of dopamine signaling is required for building comprehensive frameworks of how activity throughout the dopamine system is integrated to drive signaling and control behavior.

Neurotensin induces hypothermia by activating both neuronal neurotensin receptor 1 and astrocytic neurotensin receptor 2 in the median preoptic nucleus

Neurotensin (NTS) is a neuropeptide acting as a neuromodulator in the brain and is a very potent hypothermic agent. However, the cellular mechanisms of actions are not fully understood. Here we report that NTS increases the firing rate of preoptic GABAergic neurons by activating both neurotensin receptor 1 (NTSR1) and neurotensin receptor 2 (NTSR2), expressed by neurons and astrocytes, respectively. Downstream of NTSR1 the neuropeptide activated an inward current, calcium release from intracellular stores and, postsynaptically, increased frequency and amplitude of inhibitory synaptic events. NTSR2 activation in astrocytes resulted in increased excitatory input in preoptic GABAergic neurons, an effect which was dependent upon the activation of P2X4 receptors. We also found that neuromedin N acted as a selective agonist at the NTSR1. Surprisingly, activation of both NTSR1 and NTSR2 in the median preoptic nucleus was required for activating a full hypothermic response.

Regional and Cellular Mapping of Sortilin Immunoreactivity in Adult Human Brain

Sortilin is a member of the vacuolar protein sorting 10 protein (VPS10P) domain receptor family, which carries out signal transduction and protein transport in cells. Sortilin serves as the third, G-protein uncoupled, receptor of neurotensin that can modulate various brain functions. More recent data indicate an involvement of sortilin in mood disorders, dementia and Alzheimer-type neuropathology. However, data regarding the normal pattern of regional and cellular expression of sortilin in the human brain are not available to date. Using postmortem adult human brains free of neuropathology, the current study determined sortilin immunoreactivity (IR) across the entire brain. Sortilin IR was broadly present in the cerebrum and subcortical structures, localizing to neurons in the somatodendritic compartment, but not to glial cells. In the cerebrum, sortilin IR exhibited differential regional and laminar patterns, with pyramidal, multipolar and polymorphic neurons in cortical layers II–VI, hippocampal formation and amygdaloid complex more distinctly labeled relative to GABAergic interneurons. In the striatum and thalamus, numerous small-to-medium sized neurons showed light IR, with a small group of large sized neurons heavily labeled. In the midbrain and brainstem, sortilin IR was distinct in neurons at the relay centers of descending and ascending neuroanatomical pathways. Dopaminergic neurons in the substantia nigra, cholinergic neurons in the basal nuclei of Meynert and noradrenergic neurons in the locus coeruleus co-expressed strong sortilin IR in double immunofluorescence. In comparison, sortilin IR was weak in the olfactory bulb and cerebellar cortex, with the mitral and Purkinje cells barely visualized. A quantitative analysis was carried out in the lateral, basolateral, and basomedial nuclei of the amygdaloid complex, as well as cortical layers II–VI, which established a positive correlation between the somal size and the intensity of sortilin IR among labeled neurons. Together, the present study demonstrates a predominantly neuronal expression of sortilin in the human brain with substantial regional and cell-type variability. The enriched expression of sortilin in pyramidal, dopaminergic, noradrenergic and cholinergic neurons suggests that this protein may be particularly required for signal transduction, protein trafficking and metabolic homeostasis in populations of relatively large-sized projective neurons.

The role of neurotensin in vulnerability for self-injurious behaviour: studies in a rodent model

BACKGROUND

Self-injurious behaviour is a debilitating characteristic that is commonly expressed in people with autism and other neurodevelopmental disorders, but the neurobiological basis of this maladaptive behaviour is not understood. Abnormal dopaminergic and glutamatergic neurotransmission has been implicated, especially in relation to basal ganglia and mesocorticolimbic circuits. As neurotensin is an important modulator of dopamine and glutamate in these circuits, we investigated its potential role in vulnerability for self-injury, using the pemoline model in rats.

METHODS

Male Long-Evans rats were injected once daily with the psychostimulant pemoline or peanut oil vehicle on each of five consecutive days. Self-injury was quantified by measuring the area of injuries for each rat on each day of the experiment. Each brain was harvested on the sixth day, and the striatum and ventral tegmentum were dissected. Neurotensin-like immunoreactivity was quantified by radioimmunoassay from the dissected brain regions of some of the rats. Membrane and intracellular neurotensin receptor NTS₁ were assayed from the striata of the remaining pemoline-treated or vehicle-treated rats by Western blot. In an additional experiment, male Long-Evans rats were treated with daily injections of vehicle or pemoline, and the NTS₁ neurotensin receptor agonist PD149163 or the NTS₁ receptor antagonist SR48692 (or respective vehicle solutions) was co-administered twice daily throughout the pemoline treatment regimen. The areas of injured tissue were measured, and the duration of self-injurious oral contact was quantified by video-recorded time samples throughout each day.

RESULTS

Striatal neurotensin immunoreactivity was found to be significantly higher in pemoline-treated than in vehicle-treated rats. Moreover, both membrane-bound and intracellular levels of NTS₁ receptor were significantly higher in the striata of pemoline-treated rats than in the striata of the vehicle-treated controls. When the NTS₁ receptor agonist PD149163 was co-administered during the pemoline treatment regimen, it prolonged the daily durations of self-injurious oral contact and increased the severity of the injuries in the self-injurious rats. Conversely, co-administration of the NTS₁ receptor antagonist SR48692 diminished the daily durations of self-injurious oral contact and decreased the severity of the injuries.

CONCLUSIONS

The elevation of striatal neurotensin immunoreactivity during pemoline treatment, coupled with the effects of the NTS₁ agonist and antagonist, suggests that neurotensin transmission in the striatum may be an important modulator of self-injurious behaviour in the pemoline model. Overall, the convergence of the behavioural and biochemical findings suggests that neurotensin signalling could be an important target for pharmacotherapeutic interventions for self-injurious behaviour.

Antagonism of Neurotensin Receptors in the Ventral Tegmental Area Decreases Methamphetamine Self-Administration and Methamphetamine Seeking in Mice

Background

Neurotensin is a peptide that modulates central dopamine neurotransmission and dopamine-related behaviors. Methamphetamine self-administration increases neurotensin levels in the ventral tegmental area, but the consequences for self-administration behavior have not been described. Here we test the hypothesis that antagonizing neurotensin receptors in the ventral tegmental area attenuates the acquisition of methamphetamine self-administration and methamphetamine intake.

Methods

We implanted mice with an indwelling catheter in the right jugular vein and bilateral cannulae directed at the ventral tegmental area. Mice were then trained to nose-poke for i.v. infusions of methamphetamine (0.1 mg/kg/infusion) on a fixed ratio 3 schedule.

Results

Mice receiving microinfusions of the neurotensin NTS1/NTS2 receptor antagonist SR142948A in the ventral tegmental area (10 ng/side) prior to the first 5 days of methamphetamine self-administration required more sessions to reach acquisition criteria. Methamphetamine intake was decreased in SR142948A-treated mice both during training and later during maintenance of self-administration. Drug seeking during extinction, cue-induced reinstatement, and progressive ratio schedules was also reduced in the SR142948A group. The effects of SR142948A were not related to changes in basal locomotor activity or methamphetamine psychomotor properties. In both SR142948A- and saline-treated mice, a strong positive correlation between methamphetamine intake and enhanced locomotor activity was observed.

Conclusion

Our results suggest that neurotensin input in the ventral tegmental area during initial methamphetamine exposure contributes to the acquisition of methamphetamine self-administration and modulates later intake and methamphetamine-seeking behavior in mice. Furthermore, our results highlight the role of endogenous neurotensin in the ventral tegmental area in the reinforcing efficacy of methamphetamine, independent of its psychomotor effects.

Diverse actions of the modulatory peptide neurotensin on central synaptic transmission

Neurotensin (NT) is a 13 amino acid neuropeptide that is expressed throughout the central nervous system and is implicated in the etiology of multiple diseases and disorders. Many primary investigations of NT-induced modulation of neuronal excitability at the level of the synapse have been conducted, but they have not been summarized in review form in nearly 30 years. Therefore, the goal of this review is to discuss the many actions of NT on neuronal excitability across brain regions as well as NT circuit architecture. In the basal ganglia as well as other brain nuclei, NT can act through diverse intracellular signaling cascades to enhance or depress neuronal activity by modulating activity of ion channels, ionotropic and metabotropic neurotransmitter receptors, and presynaptic release of neurotransmitters. Further, NT can produce indirect effects by evoking endocannabinoid release, and recently has itself been identified as a putative retrograde messenger. In the basal ganglia, the diverse actions and circuit architecture of NT signaling allow for input-specific control of reward-related behaviors.

Identification of Neurotensin Receptor Expressing Cells in the Ventral Tegmental Area across the Lifespan

Neurotensin (Nts) promotes activation of dopamine (DA) neurons in the ventral tegmental area (VTA) via incompletely understood mechanisms. Nts can signal via the G protein-coupled Nts receptors 1 and 2 (NtsR1 and NtsR2), but the lack of methods to detect NtsR1- and NtsR2-expressing cells has limited mechanistic understanding of Nts action. To overcome this challenge, we generated dual recombinase mice that express FlpO-dependent Cre recombinase in NtsR1 or NtsR2 cells. This strategy permitted temporal control over recombination, such that we could identify NtsR1- or NtsR2-expressing cells and determine whether their distributions differed between the developing and adult brain. Using this system, we found that NtsR1 is transiently expressed in nearly all DA neurons and in many non-DA neurons in the VTA during development. However, NtsR1 expression is more restricted within the adult brain, where only two thirds of VTA DA neurons expressed NtsR1. By contrast, NtsR2 expression remains constant throughout lifespan, but it is predominantly expressed within glia. Anterograde tract tracing revealed that NtsR1 is expressed by mesolimbic, not mesocortical DA neurons, suggesting that VTA NtsR1 neurons may represent a functionally unique subset of VTA DA neurons. Collectively, this work reveals a cellular mechanism by which Nts can directly engage NtsR1-expressing DA neurons to modify DA signaling. Going forward, the dual recombinase strategy developed here will be useful to selectively modulate NtsR1- and NtsR2-expressing cells and to parse their contributions to Nts-mediated behaviors.

Neurotensin enhances glutamatergic EPSCs in VTA neurons by acting on different neurotensin receptors

Neurotensin (NT) is an endogenous neuropeptide that modulates dopamine and glutamate neurotransmission in several limbic regions innervated by neurons located in the ventral tegmental area (VTA). While several studies showed that NT exerted a direct modulation on VTA dopamine neurons less is known about its role in the modulation of glutamatergic neurotransmission in this region. The present study was aimed at characterising the effects of NT on glutamate-mediated responses in different populations of VTA neurons. Using whole cell patch clamp recording technique in horizontal rat brain slices, we measured the amplitude of glutamatergic excitatory post-synaptic currents (EPSCs) evoked by electrical stimulation of VTA afferents before and after application of different concentrations of NT1-13 or its C-terminal fragment, NT8-13. Neurons were classified as either Ih(+) or Ih(-) based on the presence or absence of a hyperpolarisation activated cationic current (Ih). We found that NT1-13 and NT8-13 produced comparable concentration dependent increase in the amplitude of EPSCs in both Ih(+) and Ih(-) neurons. In Ih(+) neurons, the enhancement effect of NT8-13 was blocked by both antagonists, while in Ih(-) neurons it was blocked by the NTS1/NTS2 antagonist, SR142948A, but not the preferred NTS1 antagonist, SR48692. In as much as Ih(-) neurons are non-dopaminergic neurons and Ih(+) neurons represent both dopamine and non-dopamine neurons, we can conclude that NT enhances glutamatergic mediated responses in dopamine, and in a subset of non-dopamine, neurons by acting respectively on NTS1 and an NT receptor other than NTS1.

An autoradiographic study of neurotensin receptors in the human hypothalamus

The aim of the present investigation was to determine a detailed mapping of neurotensin (NT) in the human hypothalamus, the brain region involved in neuroendocrine control. For this, we investigated the presence and the distribution of neurotensin binding sites in the human hypothalamus, using an in vitro quantitative autoradiography technique and the selective radioligand monoiodo-Tyr3-neurotensin (2000Ci/mM). This study was performed on nine adult human postmortem hypothalami. We first determined the biochemical kinetics of the binding and found that binding affinity constants were of high affinity and do not differ significantly between all cases investigated. Our analysis of the autoradiographic distribution shows that NT binding sites are widely distributed throughout the rostrocaudal extent of the hypothalamus. However, the distribution of NT binding sites is not homogenous and regional variations exist. In general, the highest densities are mainly present in the anterior hypothalamic level, particularly in the preoptic region and the anterior boarding limit (i.e. the diagonal band of Broca). Important NT binding site densities are also present at the mediobasal hypothalamic level, particularly in the paraventricular, parafornical and dorsomedial nuclei. At the posterior level, relatively moderate densities could be observed in the mammillary complex subdivisions, apart from the supramammillary nucleus and the posterior hypothalamic area. In conclusion, the present study demonstrates the occurrence of high concentrations of NT binding sites in various structures in many regions in the human adult hypothalamus, involved in the control of neuroendocrine and/or neurovegetative functions.

Feelings about food: the ventral tegmental area in food reward and emotional eating

Overconsumption of high caloric food plays an important role in the etiology of obesity. Several factors drive such hedonic feeding. High caloric food is often palatable. In addition, when an individual is sated, stress and food-related cues can serve as potent feeding triggers. A better understanding of the neurobiological underpinnings of food palatability and environmentally triggered overconsumption would aid the development of new treatment strategies. In the current review we address the pivotal role of the mesolimbic dopamine reward system in the drive towards high caloric palatable food and its relation to stress- and cue-induced feeding. We also discuss how this system may be affected by both established and potential anti-obesity drug targets.

Selection for stress-induced analgesia affects the mouse hippocampal transcriptome

Stress responsiveness, including pain sensitivity and stress-induced analgesia (SIA), depends on genotype and, partially, is mediated by hippocampus. The present study examined differences in constitutive gene expression in hippocampus in lines of mice bred for high (HA) and low (LA) swim SIA. Between the lines, we found 1.5-fold or greater differences in expression of 205 genes in the hippocampus in nonstressed animals. The identity of these genes indicates that selective breeding for swim SIA affected many aspects of hippocampal neurons physiology, including metabolism, structural changes, and cellular signaling. Genes involved in calcium signaling pathway, including Slc8a1, Slc8a2, Prkcc, and Ptk2b, were upregulated in LA mice. In HA mice, robust upregulation of genes coding some transcription factors (Klf5) or receptors for neurotensin (Ntsr2) and GABA (Gabard) suggests the genetic basis for a novel mechanism of the non-opioid type of SIA in HA animals. Additional groups of differentially expressed genes represented functional networks involved in carbohydrate metabolism, gene expression regulation, and molecular transport. Our data indicate that selection for a single and very specific stress response trait, swim SIA, alters hippocampal gene expression. The results suggest that individual stress responsiveness may be associated with characteristics of the constitutive hippocampal transcriptome.

The regulatory role of neurotensin on the hypothalamic-anterior pituitary axons: emphasis on the control of thyroid-related functions

Neurotensin (NT) is a 13 amino acid neurohormone and/or neuromodulator, located in the synaptic vesicles and released from the neuronal terminals in a calcium-dependent manner. This peptide is present among mammalian and nonmammalian species, mainly in the central nervous system and the gastrointestinal tract. Due to its neuroendocrine activity, NT has been related to the pathophysiology of a series of disorders, such as schizophrenia, drug-abuse, Parkinson's disease, cancer, stroke, eating disorders and other neurodegenerative conditions. Moreover, NT participates in the physiology of pain-induction, central blood pressure control and inflammation. NT also plays an important interactive role in all components of the hypothalamic-anterior pituitary circuit, which is mediated by an endocrine, paracrine or/and autocrine manner, towards most of the anatomical regions that define this circuit. A considerable amount of data implicates NT in thyroid-related regulation through this circuit, the exact mechanisms of which should be further investigated for the potential development of more targeted approaches towards the treatment of thyroid-related endocrine diseases. The aim of this study was to provide an up-to-date review of the literature concerning the regulatory role of NT on the hypothalamic-anterior pituitary axons, with an emphasis on the control of thyroid-related functions.

Stimulation by neurotensin of dopamine and 5-hydroxytryptamine (5-HT) release from rat prefrontal cortex: possible role of NTR1 receptors in neuropsychiatric disorders

The modulation of cortical dopaminergic and serotonergic neurotransmissions by neurotensin (NT) was studied by measuring the release of dopamine (DA) and 5-hydroxytryptamine (5-HT) from the prefrontal cortex (PFC) of freely moving rats. The samples were collected via transversal microdialysis. Dopamine and 5-HT levels in the dialysate were measured using high-performance liquid chromatography (HPLC) with an electrochemical detector. Local administration of neurotensin (1microM or 0.1microM) in the PFC via the dialysis probe produced significant, long-lasting, and concentration-dependent increase in the extracellular release of DA and 5-HT. The increase produced by 1microM neurotensin reached a maximum of about 210% for DA and 340% for 5-HT. A high-affinity selective neurotensin receptor (NTR1) antagonist {2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3yl)carbonylamino tricyclo (3.3.1.1.(3.7)) decan-2-carboxylic acid} (SR 48692), perfused locally at a concentration of 0.1microM and 0.5microM in the PFC antagonized the effects of 1microM neurotensin. Our in vivo neurochemical results indicate, for the first time, that neurotensin is able to regulate cortical dopaminergic and serotonergic neuronal activity in freely moving rats. These effects are possibly mediated by interactions of neurotensin with neurons releasing DA or 5-HT, projecting to the PFC from the ventrotegmental area (VTA) and from the dorsal raphe nuclei (DRN), respectively. The potentiating effects of neurotensin on DA and 5-HT release in the PFC are regulated by NTR1 receptors, probably located on dopaminergic and serotonergic nerve terminals or axons.

Neurotensin receptors as modulators of glutamatergic transmission

Functional studies have provided evidence supporting the concept that the tridecapeptide neurotensin (NT) acts in the central nervous system as a classical neurotransmitter and/or as an important modulator of neuronal signalling. The role of NT in the regulation of the striatal amino acidergic transmission, mainly by antagonising D2 receptor function, will be analysed. In addition, in different rat brain regions, including the basal ganglia, the contribution of NT receptors in modulating and reinforcing glutamate signalling will be shown including the involvement of interactions between NT and NMDA receptors. Since the enhancement of glutamate transmission and in particular the excessive activation of NMDA receptors, has been postulated to be an important factor in the induction of glutamate-mediated neuronal damage, the involvement of NT in the glutamate-induced neurodegenerative effects will be discussed. Moving from these observations and in order to further investigate this issue, results from preliminary behavioural, functional and biochemical experiments will be presented on the putative neuroprotective effect obtained by the blockade of NT receptor 1 (NTS1) via the systemic administration of the selective NTS1 antagonist SR48692 in an in vivo animal model of Parkinson's disease [unilateral nigral 6-hydroxydopamine (6-OHDA) induced lesion of the nigrostriatal pathway].

Blockade of neurotensin receptors during amphetamine discontinuation indicates individual variability

Psychostimulant-induced locomotor sensitization has been related to changes within the mesolimbic dopamine system and has been suggested to be useful to study mechanisms underlying drug craving. Neurotensin is a neuropeptide co-localized with dopamine in the mesolimbic system. The response to novelty has been suggested to be a predictor of enhanced vulnerability to behavioral sensitization. The effects of repeated treatment with the neurotensin antagonist SR48692 after amphetamine discontinuation were investigated in mice previously classified as high responders (HRs) or low responders (LRs) to novelty. Mice were repeatedly treated with 2.0mg/kg amphetamine, every other day for 11 days. During the first 7 days after amphetamine discontinuation, the animals received a daily injection of saline or 0.3mg/kg SR48692. On the eighth day after amphetamine discontinuation all subjects received a 2.0mg/kg amphetamine challenge injection. Then, mice were tested for an open field behavior and after 90min, were sacrificed for Fos expression quantification in the nucleus accumbens. Both HRs and LRs expressed amphetamine-induced sensitized locomotor activation and increased expression of Fos protein. Treatment with SR48692 prevented behavioral sensitization and Fos protein expression enhancement in LRs but not in HRs mice. These data suggest that neurotensin plays a role in individual variability to amphetamine-induced sensitization.

Mesolimbic dopamine and cortico-accumbens glutamate afferents as major targets for the regulation of the ventral striato-pallidal GABA pathways by neurotensin peptides

The tridecapeptide neurotensin (NT) acts in the mammalian brain as a primary neurotransmitter or neuromodulator of classical neurotransmitters. Morphological and functional in vitro and in vivo studies have demonstrated the existence of close interactions between NT and dopamine both in limbic and in striatal brain regions. Additionally, biochemical and neurochemical evidence indicates that in these brain regions NT plays also a crucial role in the regulation of the aminoacidergic signalling. It is suggested that in the nucleus accumbens the regulation of prejunctional dopaminergic transmission induced by NT may be primarily due to indirect mechanism(s) involving mediation via the aminoacidergic neuronal systems with increased glutamate release followed by increased GABA release in the nucleus accumbens rather than a direct action of the peptide on accumbens dopaminergic terminals. The neurochemical profile of action of NT in the control of the pattern of dopamine, glutamate and GABA release in the nucleus accumbens differs to a substantial degree from that shown by the peptide in the dorsal striatum. The neuromodulatory NT mechanisms in the regulation of the ventral striato-pallidal GABA pathways are discussed and their relevance for schizophrenia is underlined.

Neurotensin agonists as an alternative to antipsychotics

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

Brain neurotensin, psychostimulants, and stress--emphasis on neuroanatomical substrates

Neurotensin (NT) is a peptide that is widely distributed throughout the brain. NT is involved in locomotion, reward, stress and pain modulation, and in the pathophysiology of drug addiction and depression. In its first part this review brings together relevant literature about the neuroanatomy of NT and its receptors. The second part focuses on functional-anatomical interactions between NT, the mesotelencephalic dopamine system and structures targeted by dopaminergic projections. Finally, recent data about the actions of NT in processes underlying behavioral sensitization to psychostimulant drugs and the involvement of NT in the regulation of the hypothalamo-pituitary-adrenal gland axis are considered.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

Neurotensin antagonist acutely and robustly attenuates locomotion that accompanies stimulation of a neurotensin-containing pathway from rostrobasal forebrain to the ventral tegmental area

Neurotensin exerts complex effects on the mesolimbic dopamine system that alter motivation and contribute to neuroadaptations associated with psychostimulant drug administration. Activation of abundant neurotensin receptors in the ventral tegmental area (VTA) enhances dopamine neuron activity and associated release of dopamine in the nucleus accumbens (Acb) and cortex. In view of recent anatomical studies demonstrating that 70% of all neurotensin-containing neurons projecting to the VTA occupy the lateral preoptic area-rostral lateral hypothalamus (LPH) and lateral part of the medial preoptic area (MPOA), the present study examined functionality in the LPH-MPOA neurotensinergic pathway in the rat. Disinhibition (resulting ultimately in stimulation-like effects) of LPH-MPOA neurons with microinjected bicuculline (50 or 100 ng in 0.25 microL) produced locomotor activation that was considerably attenuated by systemic administration of the neurotensin antagonist SR 142948 A (0.03 and 0.1 mg/kg). In contrast, locomotion elicited in this manner was completely blocked by SR 142948 A infused directly into the VTA (5.0 and 15.0 ng in 0.25 microL). Baseline locomotion was unaffected by systemic or intra-VTA administration of SR 142948 A and LPH-MPOA-elicited locomotion was unaffected by infusion of SR 142948 A into the substantia nigra pars compacta and sites rostral and dorsal to the VTA. Locomotion was not elicited by infusions of bicuculline into the lateral hypothalamus at sites caudal to the LPH-MPOA, where neurotensin neurons projecting to the VTA are fewer. The results demonstrate the capacity of a neurotensin-containing pathway from LPH-MPOA to VTA to modulate locomotion. This pathway may be important in linking hippocampal and mesolimbic mechanisms in normal behaviour and drug addiction.

Effects of excitotoxic lesions of the medial prefrontal cortex on density of high affinity [125I-Tyr3]neurotensin binding sites within the ventral midbrain and striatum

The present study was aimed at determining the extent to which excitotoxic lesions of the medial prefrontal cortex reduce neurotensin receptors within the striatum, the nucleus accumbens, the ventral tegmental area and the substantia nigra. The medial prefrontal cortex was unilaterally lesioned with ibotenic acid and 10 days later brain sections were processed for neurotensin receptor autoradiographic analysis using 0.1 nM [(125)I-Tyr3]neurotensin with, or without, levocabastine. Analysis revealed at least two sites, one levocabastine-insensitive neurotensin NT(1) and one levocabastine-sensitive neurotensin NT(2)-like. The proportion of the latter site was high within the caudal striatum, the nucleus accumbens and the medial prefrontal cortex. Lesions produced a 60% to 80% reduction in neurotensin NT(1) within the ipsilateral medial prefrontal cortex, but no change in the sub-cortical nuclei. An increase in neurotensin NT(2)-like receptors was found in ipsilateral dorso-caudal caudate. These results show that a significant amount of neurotensin NT(1) receptors are located on neurons within the medial prefrontal cortex but not on their efferent terminals.

Neurotensin receptor activation sensitizes to the locomotor stimulant effect of cocaine: A role for NMDA receptors

This study was aimed at determining whether repeated activation of neurotensin receptors sensitizes to cocaine-induced locomotor activity and whether this effect can be prevented by blockade of N-methyl-d-aspartate receptors. Independent groups of male rats were injected on four occasions, every other day (training phase), with vehicle or one of two doses (4 and 8 mg/kg) of the NMDA antagonist CPP [(+/-)-3-(2-carboxypiperazine-4-yl)-propanephosphonic)] followed by an intracerebroventricular injection of 18 nmol/10 microl of d-Tyr[(11)]neurotensin, or its vehicle. Ambulatory, non-ambulatory and vertical movements were measured for 2 h on every test day. One week after the last day of the training phase, locomotor responses to a single injection of cocaine (7.5 mg/kg, ip) were measured in all rats; a second cocaine challenge test was performed 3 weeks post-training. Results show that during the training phase d-Tyr[(11)]neurotensin produced an initial suppression of all locomotor responses followed by an augmentation of ambulatory and non-ambulatory activity compared to controls, effects that were only slightly altered by CPP. Cocaine produced higher ambulatory and non-ambulatory activity in animals pre-exposed to neurotensin than in the vehicle pre-exposed animals, a sensitization effect that was not prevented by CPP at 1 week post-training but that was blocked at 3 weeks at the high dose. When given alone, the low dose of CPP produced an effect very similar to that of neurotensin on cocaine sensitization. These results further confirm that neurotensin plays a role in sensitization to psychostimulant drugs and suggests that NMDA receptors are involved in the long-term effect of exposure to neurotensin.

Glucocorticoids down-regulate lipopolysaccharide-induced de novo production of neurotensin mRNA in the rat hypothalamic, paraventricular, corticotrophin-releasing hormone neurons

OBJECTIVE

Intraperitoneal injection of the endotoxin lipopolysaccharide (LPS) produces inflammation accompanied by activation of the immune system and the secretion of cytokines. Cytokines stimulate the hypothalamo-pituitary-adrenal (HPA) axis to release the anti-inflammatory corticosterone which controls its own production by acting on the HPA axis. Upstream in the HPA axis are neuroendocrine corticotrophin-releasing hormone (CRH) neurons located in the paraventricular nucleus (PVN), whose multipeptidergic phenotype changes during inflammation: while CRH mRNA is up-regulated in these conditions, neurotensin (NT) mRNA expression is induced de novo. The negative feedback control of glucocorticoids on CRH production is well documented; however, their action on NT production in the PVN of the hypothalamus is poorly documented. The aim of this study was to determine if glucocorticoids modulate the de novo production of NT during inflammation.

METHODS

Using quantitative in situ hybridization histochemistry, we examined whether the absence (adrenalectomy) or excess (corticosterone implants) of glucocorticoids modulate de novo production of NT mRNA in the PVN during inflammation induced by LPS treatment.

RESULTS

A relatively low dose of LPS (50 microg/kg) that is not efficient to induce NT mRNA production in the PVN becomes efficient after adrenalectomy. Moreover, corticosterone excess reduces LPS-induced production of NT mRNA in the PVN.

CONCLUSION

Glucocorticoids exert a negative control on NT mRNA production in the PVN of the hypothalamus, and this effect requires that NT mRNA production be triggered, such as during inflammation.

Neurotensin activates GABAergic interneurons in the prefrontal cortex

Converging data suggest a dysfunction of prefrontal cortical GABAergic interneurons in schizophrenia. Morphological and physiological studies indicate that cortical GABA cells are modulated by a variety of afferents. The peptide transmitter neurotensin may be one such modulator of interneurons. In the rat prefrontal cortex (PFC), neurotensin is exclusively localized to dopamine axons and has been suggested to be decreased in schizophrenia. However, the effects of neurotensin on cortical interneurons are poorly understood. We used in vivo microdialysis in freely moving rats to assess whether neurotensin regulates PFC GABAergic interneurons. Intra-PFC administration of neurotensin concentration-dependently increased extracellular GABA levels; this effect was impulse dependent, being blocked by treatment with tetrodotoxin. The ability of neurotensin to increase GABA levels in the PFC was also blocked by pretreatment with 2-[1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazole-3-yl)carbonylamino]tricyclo(3.3.1.1 [EC] .3.7)decan-2-carboxylic acid (SR48692), a high-affinity neurotensin receptor 1 (NTR1) antagonist. This finding is consistent with our observation that NTR1 was localized to GABAergic interneurons in the PFC, particularly parvalbumin-containing interneurons. Because neurotensin is exclusively localized to dopamine axons in the PFC, we also determined whether neurotensin plays a role in the ability of dopamine agonists to increase extracellular GABA levels. We found that D2 agonist-elicited increases in PFC GABA levels were blocked by pretreatment with SR48692, consistent with data indicating that D2 autoreceptor agonists increase neurotensin release from dopamine-neurotensin axons in the PFC. These findings suggest that neurotensin plays an important role in regulating prefrontal cortical interneurons and that it may be useful to consider neurotensin agonists as an adjunct in the treatment of schizophrenia.

The neurotensin agonist PD149163 increases Fos expression in the prefrontal cortex of the rat

Dopaminergic axons innervating the prefrontal cortex (PFC) target both pyramidal cells and GABAergic interneurons. Many of these dopamine (DA) axons in the rat coexpress the peptide neurotransmitter neurotensin. Previous electrophysiological data have suggested that neurotensin activates GABAergic interneurons in the PFC. Activation of D2-like DA receptors increases extracellular GABA levels in the PFC, as opposed to the striatum, where D2 receptor activation inhibits GABAergic neurons. Because activation of presynaptic D2 release-modulating autoreceptors in the PFC suppresses DA release but increases release of the cotransmitter neurotensin, D2 agonists may enhance the activity of GABAergic interneurons via release of neurotensin. In order to determine if neurotensin can activate GABAergic interneurons, we treated rats with the peptide neurotensin agonist, PD149163, and examined Fos expression in PFC neurons. Systemic administration of PD149163 increased overall Fos expression in the PFC, but not in the dorsal striatum. PD149163 induced Fos in PFC interneurons, as defined by the presence of calcium-binding proteins, and in pyramidal cells. Pretreatment with the high-affinity neurotensin antagonist, SR48692, blocked neurotensin agonist-induced Fos expression. These data suggest that neurotensin activates interneurons in the PFC of the rat.

Blockade of neurotensin receptors affects differently hypo-locomotion and catalepsy induced by haloperidol in mice

Antipsychotic drug treatment increases neurotensin (NT) neurotransmission, and the exogenous administration of NT produces antipsychotic-like effects in rodents. In order to investigate whether "endogenous" NT may act as a natural occurring antipsychotic or may mediate antipsychotic drug activity, the effects of the selective NT receptor antagonists SR 48692 and SR 142948A were analyzed in different behavioural tests of locomotor activity using vehicle, amphetamine, or haloperidol in mice. SR 48692 (0.1-1 mg/kg, i.p.) and SR 142948A (0.03-0.1 mg/kg, i.p.) failed to affect mouse spontaneous locomotor activity and amphetamine-induced (2.5 mg/kg, i.p.) hyper-locomotion. However, SR 48692 (0.1 and 0.3 mg/kg, i.p.) and SR 142948A (0.03 and 0.05 mg/kg, i.p.) significantly alleviated the reduction of locomotor activity elicited by haloperidol (0.01 and 0.04 mg/kg, s.c.) in vehicle- or amphetamine-treated mice. Finally, SR 48692 (0.3 mg/kg, i.p.) and SR 142948A (0.05 and 0.1 mg/kg, i.p.) increased mouse catalepsy produced by haloperidol (0.3 mg/kg, s.c.). The present results indicate that while endogenous NT is not involved in the modulation of either mouse spontaneous locomotor activity or amphetamine-induced hyper-locomotion, it might act by enhancing the therapeutic effects of haloperidol and by attenuating the extrapyramidal side effects elicited by this antipsychotic.

Excitotoxic lesions of the prefrontal cortex attenuate the potentiation of amphetamine-induced locomotion by repeated neurotensin receptor activation

This study was aimed at determining the role of prefrontal cortex neurons in the development of the potentiation of amphetamine-induced locomotor activity by repeated central injections of D-Tyr[11]neurotensin. Excitotoxic lesions of the prefrontal cortex were made by injecting bilaterally at three anterior-posterior placements 2 microg/microl of ibotenic acid. Ten days after surgery, locomotor responses to an intracerebroventricular injection of 0.18 or 18 nmol/10 microl of D-Tyr[11]neurotensin, or vehicle-saline, were measured in different groups of lesioned and sham rats. Ambulatory, non-ambulatory and vertical movements were measured for 2 h in activity cages starting immediately after the injection. This training phase was repeated on four occasions, every second day. One week after the last day of the training phase (day 14), locomotor responses to a single injection of amphetamine (0.75 mg/kg, IP) were measured in all rats. Results show that D-Tyr[11]neurotensin produced in sham animals a dose-dependent initial suppression of locomotor activity followed by an augmentation. The latter behavioral effect tended to be smaller in the lesioned rats, but not statistically different than in sham, suggesting that prefrontal cortex neurons do not play a major role in the stimulant effect of neurotensin on locomotor activity. However, sham rats pre-exposed to the high dose of D-Tyr[11]neurotensin showed stronger non-ambulatory and vertical movements than saline pre-exposed rats when tested with amphetamine; this sensitization effect was not observed in lesioned rats. The present results show that prefrontal cortex neurons are part of the neural circuitry involved in the development of amphetamine sensitization by repeated activation of central neurotensin receptors.

Neurotensin depolarizes globus pallidus neurons in rats via neurotensin type-1 receptor

The globus pallidus is a major component in the indirect pathway of the basal ganglia. There is evidence that neurotensin receptors exist in this nucleus. To determine the electrophysiological effects of neurotensin on pallidal neurons, whole-cell patch-clamp recordings were performed in the acutely prepared brain slices. Under current-clamp recordings, neurotensin at 1 microM depolarized pallidal neurons. Voltage-clamp recordings also showed an inward current induced by neurotensin. The depolarizing effect of neurotensin could be mimicked by the C-terminal fragment, neurotensin (8-13), but not by the N-terminal fragment, neurotensin (1-8). Both SR 142948A, a non-selective neurotensin receptor type-1 and type-2 antagonist, and SR 48692, a selective type-1 receptor antagonist, blocked the depolarizing effect of neurotensin, and which themselves had no effect on membrane potential. Thus, neurotensin type-1 receptors appear to mediate the effect of neurotensin. The depolarization evoked by neurotensin persisted in the presence of tetrodotoxin, ionotropic and metabotropic glutamate and GABA receptor antagonists, indicating that neurotensin excited the pallidal neurons by activating the receptor expressed on the neurons recorded. Current-voltage relationship revealed that both the suppression of a potassium conductance and the activation of a cationic conductance are involved in the neurotensin-induced depolarization. Based on the action of neurotensin in the globus pallidus we hypothesize that alterations of the striatopallidal neurotensin system contribute to symptoms of basal ganglia motor disorders.

Immunohistochemical distribution of NTS2 neurotensin receptors in the rat central nervous system

In the present study, we localized the levocabastine-sensitive neurotensin receptor (NTS2) protein in adult rat brain by using an N-terminally-directed antibody. NTS2-like immunoreactivity was broadly distributed throughout the rat brain. At the cellular level, the reaction product was exclusively associated with neurons and predominantly, although not exclusively, with their dendritic arbors. No NTS2 signal was observed over astrocytes, as confirmed by dual confocal microscopic immunofluorescence studies using the astrocytic marker S100beta. High densities of NTS2-like immunoreactive nerve cell bodies and/or processes were detected in many regions documented to receive a dense neurotensinergic innervation, such as the olfactory bulb, bed nucleus of the stria terminalis, magnocellular preoptic nucleus, amygdaloid complex, anterodorsal thalamic nucleus, substantia nigra, ventral tegmental area, and several brainstem nuclei. Most conspicuous among the latter were structures implicated in the descending control of nociceptive inputs (e.g., the periaqueductal gray, dorsal raphe, gigantocellular reticular nucleus, pars alpha, lateral paragigantocellular, and raphe magnus), in keeping with the postulated role of NTS2 receptors in the mediation of neurotensin's supraspinal antinociceptive actions. However, the distribution of NTS2-like immunoreactivity largely exceeded that of neurotensin terminal fields, and some of the highest concentrations of the receptor were found in areas devoid of neurotensinergic inputs such as the cerebral cortex, the hippocampus, and the cerebellum, suggesting that neurotensin may not be the exclusive endogenous ligand for this receptor subtype.

Pharmacology and Functional Properties of NTS2 Neurotensin Receptors in Cerebellar Granule Cells

The binding and signaling properties of neuronal NTS2 neurotensin (NT) receptors were examined in cultured rat cerebellar granule cells. As shown by reverse transcription-PCR, receptor autoradiography, and confocal microscopic localization of fluorescent NT, these cells selectively express the NTS2 receptor subtype. Accordingly, a single apparent class of (125)I-NT-binding sites, with an affinity of 3.1 nm, was detected in cerebellar granule cell cultures. This binding was competed for with high affinity (IC(50) = 5.7 nm) by the NTS2 ligand levocabastine and with low affinity (IC(50) = 203 nm) by the NTS1 antagonist SR48692. Hypertonic acid stripping of surface-bound ligand and hyperosmolar sucrose treatment revealed that 64% of specifically bound (125)I-NT was internalized at equilibrium via a clathrin-dependent pathway. In cells loaded with the Ca(2+)-sensitive fluorescent dye Fluo4, SR48692, but neither NT nor levocabastine, triggered a marked increase in cytosolic [Ca(2+)](i). By contrast, both NT and levocabastine, but not SR48692, induced a sustained (>60 min) activation of the mitogen-activated protein kinases, p42/p44, indicating functional coupling of NTS2 receptors. Complementary experiments carried out on synaptosomes from adult rat cerebellum demonstrated the presence of presynaptic NTS2 receptors. However, in contrast to perikaryal NTS2 sites, these presynaptic receptors did not internalize in response to NT stimulation. Taken together, the present results demonstrate that NTS2 receptors are present both presynaptically and postsynaptically in central neurons and that NT and levocabastine act as agonists on these receptors.

Receptor-mediated internalization of [3H]-neurotensin in synaptosomal preparations from rat neostriatum

Following its binding to somatodendritic receptors, the neuropeptide neurotensin (NT) internalizes via a clathrin-mediated process. In the present study, we investigated whether NT also internalizes presynaptically using synaptosomes from rat neostriatum, a region in which NT1 receptors are virtually all presynaptic. Binding of [(3)H]-NT to striatal synaptosomes in the presence of levocabastine to block NT2 receptors is specific, saturable, and has NT1 binding properties. A significant fraction of the bound radioactivity is resistant to hypertonic acid wash indicating that it is internalized. Internalization of [(3)H]-NT, like that of [(125)I]-transferrin, is blocked by sucrose and low temperature, consistent with endocytosis occurring via a clathrin-dependent pathway. However, contrary to what was reported at the somatodendritic level, neither [(3)H]-NT nor [(125)I]-transferrin internalization in synaptosomes is sensitive to the endocytosis inhibitor phenylarsine oxide. Moreover, treatment of synaptosomes with monensin, which prevents internalized receptors from recycling to the plasma membrane, reduces [(3)H]-NT binding and internalization, suggesting that presynaptic NT1 receptors, in contrast to somatodendritic ones, are recycled back to the plasma membrane. Taken together, these results suggest that NT internalizes in nerve terminals via an endocytic pathway that is related to, but is mechanistically distinct from that responsible for NT internalization in nerve cell bodies.

Neurons of origin of the neurotensinergic plexus enmeshing the ventral tegmental area in rat: retrograde labeling and in situ hybridization combined

The morphological and physiological substrates that underlie the mutual regulatory interactions of neurotensin and dopamine in the rat mesotelencephalic projections and related structures remain to be fully described. A salient candidate for neurotensinergic effects on the mesotelencephalic dopamine projection is the dense plexus of neurotensin immunoreactive axons that enmeshes the ventral tegmental area and substantia nigra, but the locations of the neurons that give rise to this plexus have not been identified and its systemic context remains obscure. To address this, Fluoro-Gold and the cholera toxin beta subunit, retrogradely transported axonal tracers, were injected into the ventral tegmental area of rats and the brains were processed to demonstrate neurons that contained both retrograde tracer immunoreactivity and a probe against neurotensin/neuromedin N messenger RNA. Substantial numbers of double-labeled neurons were observed in the rostral part of the lateral septum, and in a region centered on the shared boundaries of the bed nucleus of stria terminalis, ventromedial ventral pallidum, diagonal band of Broca, lateral preoptic area and rostral lateral hypothalamus. A few double-labeled neurons were also observed in the dorsal raphe nucleus and adjacent periaqueductal gray. Despite the administration of haloperidol and D-amphetamine to elicit and enhance neurotensin/neuromedin N messenger RNA expression in striatum, including the nucleus accumbens and olfactory tubercle, no double-labeled neurons were observed there. These results identify a novel brain substrate for control of midbrain dopamine levels, which affect reward mechanisms and motivation.

High-affinity neurotensin receptors in the rat nucleus accumbens: subcellular targeting and relation to endogenous ligand

Neurotensin is present in selective mesolimbic dopaminergic projections to the nucleus accumbens (NAc) shell but also is synthesized locally in this region and in the motor-associated NAc core. We examined the electron microscopic immunolabeling of the high-affinity neurotensin receptor (NTR) and neurotensin in these subdivisions of rat NAc to determine the sites for receptor activation and potential regional differences in distribution. Throughout the NAc, NTR immunoreactivity was localized discretely within both neurons and glia. NTR-labeled neuronal profiles were mainly axons and axon terminals with diverse synaptic structures, which resembled dopaminergic and glutamatergic afferents, as well as collaterals of inhibitory projection neurons. These terminals had a significantly higher numerical density in the NAc core than in the shell but were prevalent in both regions, suggesting involvement in both motor and limbic functions. In each region, neurotensin was detected in a few NTR-immunoreactive axon terminals and in terminals that formed symmetric, inhibitory type synapses with NTR-labeled somata and dendrites. The NTR labeling, however, was not seen within these synapses and, instead, was localized to segments of dendritic and glial plasma membranes often near excitatory type synapses. Neuronal NTR immunoreactivity also was associated with cytoplasmic tubulovesicles and nuclear membranes. Our results suggests that, in the NAc shell and core, NTR is targeted mainly to presynaptic sites, playing a role in the regulated secretion and/or retrograde signaling in diverse, neurotransmitter-specific neurons. The findings also support a volume mode of neurotensin actions, specifically affecting excitatory transmission through activation of not only axonal but also dendritic and glial NTR.

Effects of neurotensin on discharge rates of rat suprachiasmatic nucleus neurons in vitro

The neuropeptide neurotensin and two classes of its receptors, the neurotensin receptor-1 and 2, are present in the suprachiasmatic nucleus of the mammalian hypothalamus. The suprachiasmatic nucleus houses the mammalian central circadian pacemaker, but the effects of neurotensin on cellular activity in this circadian pacemaker are unknown. In this study, we examined the effects of neurotensin on the spontaneous discharge rate of rat SCN cells in an in vitro slice preparation. Neurotensin (1-10 microM) increased cell firing rate in approximately 50% of cells tested, while approximately 10% of suprachiasmatic cells tested showed a decrease in firing rate in response to neurotensin. These effects of neurotensin were not altered by the GABA receptor antagonist bicuculline (20 microM) or the glutamate receptor antagonists, D-aminophosphopentanoic acid (50 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). The neurotensin receptor selective antagonists SR48692 and SR142948a (10 microM) failed to antagonise neurotensin responses in the majority of cells examined. Compounds that function as agonists selective for the neurotensin-receptor subtypes 1 and 2, JMV-510 and JMV-431 respectively, elicited neurotensin-like responses in approximately 90% of cells tested. Six out of seven cells tested responded to both JMV-510 and JMV-431. Neuropeptide Y (100nM) treatment of suprachiasmatic nucleus slices was found to elicit profound suppression of neuronal firing rate. Co-application of neurotensin with neuropeptide Y significantly (P<0.05) reduced the duration of the response, as compared to that elicited with neuropeptide Y alone. Together, these results demonstrate for the first time the actions of neurotensin in the suprachiasmatic nucleus and raise the possibility that this neuropeptide may play a role in modulating circadian pacemaker function.

Evidence for a role of endogenous neurotensin in the initiation of amphetamine sensitization

This study was aimed at testing the hypothesis that endogenous neurotensin plays a role in the initiation of sensitization to the locomotor activating effect of amphetamine. During an initial training phase, different groups of male rats were injected on four occasions (every second day: Days 1, 3, 5 and 7) with one of three doses (40, 80 or 160 microg/kg, ip) of the neurotensin antagonist, SR-48692, or its vehicle, followed 30 min later by amphetamine (1.5 mg/kg, ip), or saline. Ambulatory, non-ambulatory, and vertical movements were measured for 2 h in photocell cages immediately following the second injection. One week after the training phase, sensitivity to amphetamine (0.75 mg/kg, ip) was tested in all the rats (sensitization test). The results show that SR-48692, when given alone, produced levels of locomotor activity that were not statistically different from control. At the low dose, it potentiated amphetamine-induced ambulatory and non-ambulatory movements, an effect observed on Day 7 but not on Day 1. On the day of the sensitization test, rats pre-exposed to amphetamine alone displayed stronger ambulatory and non-ambulatory movements than vehicle pre-exposed rats, a sensitization effect that was attenuated and prevented by SR-48692 at 80 and 160 microg/kg, respectively. The present results demonstrate that activation of neurotensin receptors by endogenous neurotensin is required for the initiation of amphetamine sensitization. They provide additional evidence that an increase in central neurotensinergic neurotransmission may lead to a lasting increased sensitivity to psychostimulant drugs.

Distribution of the neurotensin receptor NTS1 in the rat CNS studied using an amino-terminal directed antibody

The distribution of neurotensin receptor 1 immunoreactivity in the rat brain was studied using an antibody against the amino-terminal of the receptor expressed as a fusion protein with glutathione-S transferase. Affinity purified antibodies detected the fusion protein and the complete neurotensin receptor sequence expressed in Escherichia coli. The immunostaining was abolished by preabsorption with the amino-terminal fusion protein. Immunoreactive neurotensin receptor 1 immunoreactivity was detected on cell bodies and their processes in a number of CNS regions. In agreement with previous binding studies neurotensin receptor 1 immunoreactivity was particularly localised in cell bodies in the basal forebrain, nucleus basalis and substantia nigra. At the electron microscope level immunoreactivity was found both in axonal bouton and dendrites and spines in the basal forebrain indicating that neurotensin may act both pre- and post-synaptically. There were several regions such as the substantia gelatinosa, ventral caudate-putamen and the lateral reticular nucleus where the neurotensin receptor 1 positive cells had not previously been reported, indicating that distribution of this receptor is widespread.

Pharmacological, molecular and functional characterization of glial neurotensin receptors

The pharmacological properties, molecular identity and physiopathological regulation of neurotensin receptors expressed by central astrocytes were investigated in primary glial cultures and sections from the adult rat brain. Binding experiments carried out on astrocytes in culture revealed the presence of a single apparent class of neurotensin binding sites. These sites bound [125]neurotensin with an affinity (6 nM) comparable to that of the recently cloned NT2 low-affinity receptor expressed in transfected cells. The glial receptor was sensitive to the antihistamine, levocabastine, but less so than the NT2 site expressed in heterologous expression systems, suggesting the presence of an additional site or a differential coupling of the NT2 receptor in glia. Reverse transcription-polymerase chain reaction experiments demonstrated that both NT2 and NT3 neurotensin receptor sub-types were in fact expressed by cortical glial cells in culture. Confocal microscopic visualization of specifically bound fluorescent neurotensin indicated that this expression concerned only a sub-population of astrocytes in culture, in conformity with earlier reports of a heterogeneous expression of neuropeptides and their receptors by glial cells. To further investigate the functionality of NT2 receptors expressed in astrocytes, dual immunohistochemical labeling of glial fibrillary acidic protein and in situ hybridization of NT2 messenger RNA was performed on sections of normal and lesioned rat brain. In sections from normal brain, only a small subset of immunolabeled astrocytes hybridized NT2 messenger RNA. By contrast, in sections of stab-wounded rat brains, there was a marked increase in the number of NT2-hybridizing astrocytes in the surround of the lesion. Furthermore, NT2 expression within immunopositive reactive astrocytes was significantly enhanced as compared to immunolabeled glial cells in the brain of control animals. These results indicate that NT2 receptor expression is up-regulated during astrocytic reaction, suggesting that NT2 receptors may play a role in regulating glial response to injury.

Stimulation of the ventral tegmental area enhances the effect of vasopressin on blood pressure in conscious rats

The mesolimbic dopamine system projects to a large number of forebrain areas and plays an important role in the regulation of locomotor activity, cognition and reward. We previously found evidence for a functional interaction between the mesolimbic dopamine system and circulating vasopressin and the present study was performed to test the hypothesis that mesolimbic dopamine stimulation modulates the cardiovascular effects of vasopressin. Sprague-Dawley rats were stereotaxically implanted with a guide cannula into the region of origin of the mesolimbic system, the ventral tegmental area, and instrumented with catheters into the abdominal aorta and jugular vein. One week later, separate groups of conscious rats were injected intravenously with 1, 3 or 10 ng kg(-1) of arginine-vasopressin or other vasopressor drugs before and after intra-ventral tegmental area injection of 10 nmol of neurotensin. Intra-ventral tegmental area injections of neurotensin had no significant effect on mean arterial pressure and heart rate but significantly potentiated the pressor response to intravenous administration of vasopressin when compared to saline-injections. However, the vasopressin-induced bradycardia was unaffected. Intravenous pretreatment with raclopride blocked the ability of neurotensin, injected into the ventral tegmental area, to potentiate the vasopressin-induced pressor response. Intra ventral tegmental area injections of neurotensin had no effect on the pressor response and bradycardia induced by intravenous angiotensin II or methoxamine. In conclusion, these results suggest that the mesolimbic dopamine system, in addition to its well-known role in the regulation of behaviour, modulates cardiovascular control by potentiating the effects of vasopressin on mean arterial pressure. British Journal of Pharmacology (2000) 129, 29 - 36

Neurotensin depolarizes cholinergic and a subset of non-cholinergic septal/diagonal band neurons by stimulating neurotensin-1 receptors

Identified cholinergic and a subtype of non-cholinergic, fast-firing neurons were recorded intracellularly in vitro from slices of guinea-pig brain. Recorded neurons were within the boundaries of the medial septum and vertical limb of the diagonal band of the forebrain. The effects of superfused neurotensin and neurotensin receptor antagonists were measured under single-electrode current clamp. Neurotensin consistently caused a dose-dependent, slow depolarization of cholinergic neurons that was accompanied by an increase in membrane resistance and a block of the long-duration (1-10 s) post-spike afterhyperpolarization when present. Neurotensin also blocked a shorter duration, slow afterhyperpolarization, but only in a minority of cholinergic neurons. When present, inhibition of the slow afterhyperpolarization changed the spike pattern from single spikes to short bursts. Inhibition of post-spike afterhyperpolarizations by neurotensin reversed more slowly than did other effects of neurotensin. Tetrodotoxin did not prevent the depolarizing effect of neurotensin. The non-selective neurotensin receptor antagonist, SR142948A, blocked the depolarizing effect of neurotensin but the low-affinity receptor antagonist, levocabastine, did not. A subgroup of noncholinergic, fast-firing neurons (23%) was also depolarized by neurotensin, an effect antagonized by SR142948A but not levocabastine. Neurotensin did not effect post-spike voltage transients or change the firing pattern of non-cholinergic neurons. These data suggest that neurotensin causes a slow depolarization and increased excitability of cholinergic and some noncholinergic neurons in an area of the brain that projects to the hippocampus. Neurotensin type 1 receptors appear to mediate these effects. Neurotensin may modulate hippocampal-dependent learning and memory processes through its effects on septohippocampal neurons.

Sequences required for induction of neurotensin receptor gene expression during neuronal differentiation of N1E-115 neuroblastoma cells

The promoter region of the mouse high affinity neurotensin receptor (Ntr-1) gene was characterized, and sequences required for expression in neuroblastoma cell lines that express high affinity NT-binding sites were characterized. Me(2)SO-induced neuronal differentiation of N1E-115 neuroblastoma cells increased both the expression of the endogenous Ntr-1 gene and reporter genes driven by NTR-1 promoter sequences by 3-4-fold. Deletion analysis revealed that an 83-base pair promoter region containing the transcriptional start site is required for Me(2)SO activation. Detailed mutational analysis of this region revealed that a CACCC box and the central region of a large GC-rich palindrome are the crucial cis-regulatory elements required for Me(2)SO induction. The CACCC box is bound by at least one factor that is induced upon Me(2)SO treatment of N1E-115 cells. The Me(2)SO effect was found to be both selective and cell type-restricted. Basal expression in the neuroblastoma cell lines required a distinct set of sequences, including an Sp1-like sequence, and a sequence resembling an NGFI-A-binding site; however, a more distal 5' sequence was found to repress basal activity in N1E-115 cells. These results provide evidence that Ntr-1 gene regulation involves both positive and negative regulatory elements located in the 5'-flanking region and that Ntr-1 gene activation involves the coordinate activation or induction of several factors, including a CACCC box binding complex.