Actions of neurotensin: a review of the electrophysiological studies

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.

Exogenous neurotensin modulates sperm function in Japanese Black cattle

Recently, the conception rates after artificial insemination have been pointed out to decline continuously. To overcome this problem, the control of frozen and thawed sperm quality is required. However, the mechanism of bovine sperm functional regulation is still largely unknown. In mammals, the ejaculated sperm are capable of showing fertilizing ability during migration in the female reproductive organs. It is well known that these female organs secrete several factors contributing to sperm capacitation. We previously reported that neurotensin (NT) secreted from the oviduct and cumulus cells enhanced sperm capacitation and acrosome reaction in mice. In this study, we confirmed the expression of the NT receptor (NTR1) in the bovine sperm neck region and the secretion of NT in the bovine uterus and oviduct. The similar expression patterns of NT and NTR1 suggests a conserved mechanism of sperm functional regulation between mouse and cattle. Thus, we examined the effects of exogenous NT on the bovine sperm functions. First, we showed that NT induced sperm protein tyrosine phosphorylation in a dose-dependent manner, suggesting that NT enhances sperm capacitation. Second, we showed that NT induced acrosome reactions of capacitated sperm in a dose-dependent manner, suggesting that NT facilitates acrosome reaction. Finally, we used a computer-aided sperm analysis system to show that NT did not have a great effect on sperm motility. These results suggest that NT acts as a facilitator of sperm capacitation and acrosome reaction in the female reproductive tracts in cattle, highlighting the importance of NT-mediated signaling to regulate sperm functions.

The role of endogenous neurotensin in psychostimulant-induced disruption of prepulse inhibition and locomotion

The neuropeptide neurotensin (NT) is closely associated with dopaminergic and glutamatergic systems in the rat brain. Central injection of NT into the nucleus accumbens (NAcc) or peripheral administration of NT receptor agonists, reduces many of the behavioral effects of psychostimulants. However, the role of endogenous NT in the behavioral effects of psychostimulants (e.g. DA agonists and NMDA receptor antagonists) remains unclear. Using a NTR antagonist, SR142948A, the current studies were designed to examine the role of endogenous NT in DA receptor agonist- and NMDA receptor antagonist-induced disruption of prepulse inhibition of the acoustic startle response (PPI), locomotor hyperactivity and brain-region specific c-fos mRNA expression. Adult male rats received a single i.p. injection of SR142948A or vehicle followed by D-amphetamine, apomorphine or dizocilpine challenge. SR142948A had no effect on baseline PPI, but dose-dependently attenuated d-amphetamine- and dizocilpine-induced PPI disruption and enhanced apomorphine-induced PPI disruption. SR142948A did not significantly affect either baseline locomotor activity or stimulant-induced hyperlocomotion. Systemic SR142948A administration prevented c-fos mRNA induction in mesolimbic terminal fields (prefrontal cortex, lateral septum, NAcc, ventral subiculum) induced by all three psychostimulants implicating the VTA as the site for NT modulation of stimulant-induced PPI disruption. Further characterization of the NT system may be valuable to find clinical useful compounds for schizophrenia and drug addiction.

Neurotensin triggers dopamine D2 receptor desensitization through a protein kinase C and beta-arrestin1-dependent mechanism

The peptide neurotensin (NT) is known to exert a potent excitatory effect on the dopaminergic system by inhibiting D2 dopamine (DA) receptor (D2R) function. This regulation is dependent on activation of PKC, a well known effector of the type 1 NT receptor (NTR1). Because PKC phosphorylation of the D2R has recently been shown to induce its internalization, we hypothesized that NT acts to reduce D2R function through heterologous desensitization of the D2R. In the present study, we first used HEK-293 cells to demonstrate that NT induces PKC-dependent D2R internalization. Furthermore, internalization displayed faster kinetics in cells expressing the D2R short isoform, known to act as an autoreceptor in DA neurons, than in cells expressing the long isoform, known to act as a postsynaptic D2R. In patch clamp experiments on cultured DA neurons, overexpression of a mutant D2S lacking three key PKC phosphorylation sites abrogated the ability of NT to reduce D2R-mediated cell firing inhibition. Short interfering RNA-mediated inhibition of β-arrestin1 and dynamin2, proteins important for receptor desensitization, reduced agonist-induced desensitization of D2R function, but only the inhibition of β-arrestin1 reduced the effect of NT on D2R function. Taken together, our data suggest that NT acutely regulates D2 autoreceptor function and DA neuron excitability through PKC-mediated phosphorylation of the D2R, leading to heterologous receptor desensitization.

Bidirectional regulation of dopamine D2 and neurotensin NTS1 receptors in dopamine neurons

Several lines of evidence suggest a close association between dopamine (DA) and neurotensin (NT) systems in the CNS. Indeed, in the rodent brain, abundant NT-containing fibres are found in DA-rich areas such as the ventral tegmental area and substantia nigra. Moreover, it has been shown in vivo that NT, acting through its high-affinity receptor (NTS1), reduces the physiological and behavioural effects of DA D2 receptor (D2R) activation, a critical autoreceptor feedback system regulating DA neurotransmission. However, the mechanism of this interaction is still elusive. The aim of our study was thus to reproduce in vitro the interaction between D2R and NTS1, and then to characterize the mechanisms implicated. We used a primary culture model of DA neurons prepared from transgenic mice expressing green fluorescent protein under the control of the tyrosine hydroxylase promoter. In these cultures, DA neurons endogenously express both D2R and NTS1. Using electrophysiological recordings, we show that activation of D2R directly inhibits the firing rate of DA neurons. In addition, we find that NT, acting through a NTS1-like receptor, is able to reduce D2R autoreceptor function independently of its ability to enhance DA neuron firing, and that this interaction occurs through a protein kinase C- and Ca(2+)-dependent mechanism. Furthermore, prior activation of D2R reduces the ability of NTS1 to induce intracellular Ca(2+) mobilization. Our findings provide evidence for bidirectional interaction between D2R and NTS1 in DA neurons, a regulatory mechanism that could play a key role in the control of the activity of these neurons.

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.

Virally mediated increased neurotensin 1 receptor in the nucleus accumbens decreases behavioral effects of mesolimbic system activation

Dopamine receptor agonist and NMDA receptor antagonist activation of the mesolimbic dopamine system increases locomotion and disrupts prepulse inhibition of the acoustic startle response (PPI), paradigms frequently used to study both the pharmacology of antipsychotic drugs and drugs of abuse. In rats, virally mediated overexpression of the neurotensin 1 (NT1) receptor in the nucleus accumbens antagonized d-amphetamine- and dizocilpine-induced PPI disruption, hyperlocomotion, and D-amphetamine-induced rearing. The NT receptor antagonist SR 142948A [2-[[5-(2,6-dimethoxyphenyl)-1-(4-N-(3-dimethylaminopropyl)-N-methylcarbamoyl)-2-isopropylphenyl)-1H-pyrazole-3-carbonyl]amino] adamantane-2-carboxylic acid, hydrochloride] blocked inhibition of dizocilpine-induced hyperlocomotion mediated by overexpression of the NT1 receptor. Together, these results suggest that increased nucleus accumbens NT neurotransmission, via the NT1 receptor, can decrease the effects of activation of the mesolimbic dopamine system and disruption of the glutamatergic input from limbic cortices, resembling the action of the atypical antipsychotic drug clozapine. In contrast to clozapine, virally mediated overexpression of the NT1 receptor in the nucleus accumbens had prolonged protective effects (up to 4 weeks after viral injection) without perturbing baseline PPI and locomotor behaviors. These data further confirm the NT1 receptor as the receptor mediating the antistimulant- and antipsychotic-like properties of NT and provide rationale for the development of NT1 receptor agonists as novel antipsychotic drugs. In addition, the NT1 receptor vector might be a valuable tool for understanding the mechanism of action of antipsychotic drugs and drugs of abuse and may have potential therapeutic applications.

The electrophysiological effects of neurotensin on spontaneously active neurons in the nucleus accumbens: an in vivo study

Considerable evidence obtained from neuroanatomical and neurochemical studies suggests an interaction between the endogenous tridecapeptide neurotensin (NT) and central nervous system dopamine (DA) neurons. Centrally administered NT blocks many of the actions of synaptic DA in limbic brain areas; the specific mechanism and receptors involved remain under investigation. The electrophysiological effects of NT were studied using extracellular recording techniques and iontophoretic application in 243 spontaneously active neurons in the nucleus accumbens (NAc), with a positive/negative waveform. NT was directly applied to 208 neurons in a pulsatile fashion by iontophoresis (21+/-1 nA). NT had no effect on the firing rate of 120 neurons ((0.31+/-0.72)%), decreased the firing rate in 51 neurons ((-27.87+/-1.52)%), and increased the firing rates of 37 neurons ((33.38+/-2.6)%). One hundred ninety nine (81.9%) of the neurons studied were sensitive to iontophoretically applied DA (>15% decrease in firing rate). The effects of continuous NT application on DA-induced inhibitions were studied in 169 neurons. NT attenuated neuronal responses to directly applied DA by (49.95+/-4.52)%, with antagonism in the "core" subregion (n=96) of (33.41+/-7.75)% when compared with antagonism in the "shell" subregion (n=71) of (61.39+/-5.2)%. The effects of NT on DA were consistent and independent of the effects of NT alone. These data provide further evidence that NT functions as a true neuromodulator in the NAc, exerting minimal direct effects, but blocking the actions of DA.

Neurotensin effect on Na+, K+-ATPase is CNS area- and membrane-dependent and involves high affinity NT1 receptor

We have previously shown that peptide neurotensin inhibits cerebral cortex synaptosomal membrane Na+, K+-ATPase, an effect fully prevented by blockade of neurotensin NT1 receptor by antagonist SR 48692. The work was extended to analyze neurotensin effect on Na+, K+-ATPase activity present in other synaptosomal membranes and in CNS myelin and mitochondrial fractions. Results indicated that, besides inhibiting cerebral cortex synaptosomal membrane Na+, K+-ATPase, neurotensin likewise decreased enzyme activity in homologous striatal membranes as well as in a commercial preparation obtained from porcine cerebral cortex. However, the peptide failed to alter either Na+, K+-ATPase activity in cerebellar synaptosomal and myelin membranes or ATPase activity in mitochondrial preparations. Whenever an effect was recorded with the peptide, it was blocked by antagonist SR 48692, indicating the involvement of the high affinity neurotensin receptor (NT1), as well as supporting the contention that, through inhibition of ion transport at synaptic membrane level, neurotensin plays a regulatory role in neurotransmission.

Circuits and systems in stress. I. Preclinical studies

This paper reviews the preclinical literature related to the effects of stress on neurobiological and neuroendocrine systems. Preclinical studies of stress provide a comprehensive model for understanding neurobiological alterations in post-traumatic stress disorder (PTSD). The pathophysiology of stress reflects long-standing changes in biological stress response systems and in systems involved in stress responsivity, learning, and memory. The neural circuitry involved includes systems mediating hypothalamic-pituitary-adrenal (HPA) axis, norepinephrine (locus coeruleus), and benzodiazepine, serotonergic, dopaminergic, neuropeptide, and central amino acid systems. These systems interact with brain structures involved in memory, including hippocampus, amygdala, and prefrontal cortex. Stress responses are of vital importance in living organisms; however excessive and/or repeated stress can lead to long-lasting alterations in these circuits and systems involved in stress responsiveness. Intensity and duration of the stressor, and timing of the stressor in life, have strong impact in this respect.

The role of neurotensin in the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs

It has become increasingly clear that schizophrenia does not result from the dysfunction of a single neurotransmitter system, but rather pathologic alterations of several interacting systems. Targeting of neuropeptide neuromodulator systems, capable of concomitantly regulating several transmitter systems, represents a promising approach for the development of increasingly effective and side effect-free antipsychotic drugs. Neurotensin (NT) is a neuropeptide implicated in the pathophysiology of schizophrenia that specifically modulates neurotransmitter systems previously demonstrated to be dysregulated in this disorder. Clinical studies in which cerebrospinal fluid (CSF) NT concentrations have been measured revealed a subset of schizophrenic patients with decreased CSF NT concentrations that are restored by effective antipsychotic drug treatment. Considerable evidence also exists concordant with the involvement of NT systems in the mechanism of action of antipsychotic drugs. The behavioral and biochemical effects of centrally administered NT remarkably resemble those of systemically administered antipsychotic drugs, and antipsychotic drugs increase NT neurotransmission. This concatenation of findings led to the hypothesis that NT functions as an endogenous antipsychotic. Moreover, typical and atypical antipsychotic drugs differentially alter NT neurotransmission in nigrostriatal and mesolimbic dopamine (DA) terminal regions, and these effects are predictive of side effect liability and efficacy, respectively. This review summarizes the evidence in support of a role for the NT system in both the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs.

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.

Neurotensin inhibits neuronal Na+,K+-ATPase activity through high affinity peptide receptor

Neurotensin is a peptide present in mammalian CNS and peripheral tissues, which may play a major role in neurotransmission or neuromodulation, subserving diverse physiological functions. We studied the effect of added neurotensin on ATPase activities in synaptosomal membranes isolated from rat cerebral cortex. Neurotensin at 3 x 10(-8)-3 x 10(-6) M concentration decreased 20-44% Na+,K+-ATPase activity but failed to modify Mg2+-ATPase activity; lower neurotensin concentrations (3 x 10(-14)-3 x 10(-10) M) had no effect on enzyme activities. This inhibitory effect was abolished by neurotensin heating, by enzyme preincubation with neurotensin during periods exceeding 10 min, or by adding 1 x 10(-6) M SR 48692, a high affinity neurotensin receptor antagonist. Levocabastine, which blocks low affinity neurotensin receptor, failed to alter enzyme inhibition by the peptide. It is suggested that the sodium pump may be a target for neurotensin effects at neuronal level involving the participation of high affinity neurotensin receptor.

Agonist induced conformation alteration of neurotensin receptor and the mechanism behind Na+ inhibition of 125I-NT binding

In the absence of Na+, 125I-Neurotensin (125I-NT) binding to the Neurotensin receptor (NTR) produces a stable noncovalent 125I-NT-NTR complex whose dissociation rate is extremely low even after the addition of 1 microM NT, 100 microM SR48692 (antagonist), 100 microM GPPNHP or 100 mM NaCl. Lowering the medium pH to 4.5 enhances the process (approximately 70% in 10 minutes). Labeling by photoactivatable 125I-Tyr3-Azo4-NT identifies a approximately 50 KD Mr band along with several other minor components. Interestingly, the labeling intensity is drastically reduced when binding is performed in the presence of Na+ or GPPNHP. However, a minor reduction is noticed when Na+ or GPPNHP is added to the medium after binding. The binding kinetics indicates that Na+ lowers the rate of 125I-NT association by acting as a noncompetitive inhibitor. On the contrary, Na+ favors the interaction of antagonist, SR48692 by lowering the value of Ki. GTPgamma35S binding to membranes in the presence of 30 mM NaCl suggests that Na+ inhibition of 125I-NT binding is due to the uncoupling of NTR associated G protein(s). In order to explain the entire phenomenon, a two-step, binding model has been proposed. In Step-1, interaction between NT and NTR produces a transient complex, which attains a stable state in the absence of NaCl via step-2, thereby altering the native NTR conformation. The presence of Na+ prevents step-2 by dissociating the transition complex.

Neurontensin studies in alcohol naive, preferring and non-preferring rats

Neurotensin is a tridecapeptide, present in the central nervous system and the gastrointestinal tract in man and animals. Previous studies in mice selectively bred for differences in hypnotic sensitivity to ethanol have provided data to suggest that neurotensinergic systems may mediate differences in ethanol's actions in these animals. The present study sought to determine if brain neurotensin levels differed between two lines of rats which have been selectively bred for alcohol preferring or non-preferring behaviors. In addition, electroencephalographic and event-related potential responses to intracerebroventricular saline and neurotensin (10 or 30 microg) were evaluated between the rat lines. Similar to human subjects at high genetic risk for alcoholism, preferring rats were found to have more electroencephalographic fast frequency activity and lowered amplitude of the P3 component of the event-related potential in cortical sites under the saline condition. Overall, electrophysiological response to neurotensin, in the two rats lines, was substantially similar to what has been reported previously in outbred Wistar rats, and consisted of dose-related decreases in overall electroencephalographic spectral power concomitant with increases in amplitude and decreases in the latency of the N1 component of the event-related potential. However, differences in neurotensin responses between the preferring and non-preferring rat lines were also found. The differences in electroencephalographic high-frequency activity and in P3 amplitude seen between the rat lines under control conditions were eliminated by administration of neurotensin. In addition, preferring rats appeared to be more sensitive to neurotensin-induced increases in N1 amplitude. Brain neurotensin concentrations were also found to differ between the lines. Significantly lower concentrations of neurotensin were found in the frontal cortex of preferring rats when compared to non-preferring rats or outbred Wistars. Taken together, these studies suggest that differences in the regulation of neurotensin neurons may contribute to the expression of behavioral preference for ethanol consumption in selective rat lines. Additionally, drugs targeting the neurotensinergic system may plausibly be of utility in the treatment of alcoholism.

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

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

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.

Complementarity of radioautographic and immunohistochemical techniques for localizing neuroreceptors at the light and electron microscopy level

To assess relationships between neuropeptide-binding sites and receptor proteins in rat brain, the distribution of radioautographically labeled somatostatin and neurotensin-binding sites was compared to that of immunolabeled sst2A and NTRH receptor subtypes, respectively. By light microscopy, immunoreactive sst2A receptors were either confined to neuronal perikarya and dendrites or diffusely distributed in tissue. By electron microscopy, areas expressing somatodendritic sst2A receptors displayed only low proportions of membrane-associated, as compared to intracellular, receptors. Conversely, regions displaying diffuse sst2A labeling exhibited higher proportions of membrane-associated than intracellular receptors. Furthermore, the former showed only low levels of radioautographically labeled somatostatin-binding sites whereas the latter contained high densities of somatostatin-binding suggesting that membrane-associated receptors are preferentially recognized by the radioligand. In the case of NTRH receptors, there was a close correspondence between the light microscopic distribution of NTRH immunoreactivity and that of labeled neurotensin-binding sites. Within the substantia nigra, the bulk of immuno- and autoradiographically labeled receptors were associated with the cell bodies and dendrites of presumptive DA neurons. By electron microscopy, both markers were detected inside as well as on the surface of labeled neurons. At the level of the plasma membrane, their distribution was highly correlated and characterized by a lack of enrichment at the level of synaptic junctions and by a homogeneous distribution along the remaining neuronal surface, in conformity with the hypothesis of an extra-synaptic action of this neuropeptide. Inside labeled dendrites, there was a proportionally higher content of immunoreactive than radiolabeled receptors. Some of the immunolabeled receptors not recognized by the radioligand were found in endosome-like organelles suggesting that, as in the case of sst2A receptors, they may have undergone endocytosis subsequent to binding to the endogenous peptide.

Involvement of cortical neurotensin in the regulation of rat meso-cortico-limbic dopamine neurons: evidence from changes in the number of spontaneously active A10 cells after neurotensin receptor blockade

In order to further assess the role of endogenous neurotensin on midbrain dopaminergic neuronal function, the effects of the selective neurotensin receptor antagonists SR 48692 and SR 48527 were investigated on the number of spontaneously active A9 and A10 dopaminergic neurons in rats. Single intraperitoneal administration of SR 48692 (0.1-3 mg/kg) dose-dependently increased the number of active A10, but not A9 cells. SR 48527 (1 mg/kg) had a similar profile, but not SR 49711, its low affinity R-enantiomer, indicating that the effects observed were mediated through neurotensin receptor blockade. Five-week treatment with SR 48692 (3 mg/kg/day) produced a significant decrease of the number of active A10, but not A9 cells, which was reversed by apomorphine, suggesting that these cells were under depolarization block. Single co-administration of inactive doses of SR 48692 (0.1 mg/kg) and haloperidol (0.0625 mg/kg) significantly increased the number of active A10 cells. Conversely, co-administered active doses of SR 48692 or SR 48527 and haloperidol (1 and 0.25 mg/kg, respectively) induced an apomorphine-sensitive decrease of the number of A10 active cells. Finally, SR 48692 (10 mg/kg) modified neither accumbal nor cortical basal DA release. Local micro-injection of SR 48692 (10[-11]-10[-9] M), but not that of SR 49711 (10[-9] M), into the prefrontal cortex, increased the number of active A10 cells in a concentration-dependent manner. These results suggest that neurotensin receptor blockade counteracts a tonic inhibitory regulation by endogenous neurotensin of mesolimbic dopaminergic function and indicate that the prefrontal cortex is critically involved in this regulation.

Neurotensin and neuroendocrine regulation

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

Cellular distribution of neurotensin receptors in rat brain: immunohistochemical study using an antipeptide antibody against the cloned high affinity receptor

Receptors for the neuropeptide, neurotensin, were localized by immunohistochemistry in the rat brain by using an antibody raised against a sequence of the third intracellular loop of the cloned high affinity receptor. Selective receptor immunostaining was observed throughout the brain and brainstem. This immunostaining was totally prevented by preadsorbing the antibody with the immunogenic peptide. The regional distribution of the immunoreactivity conformed for the most part to that of [3H]- or [125I]-neurotensin binding sites previously identified by autoradiography. Thus, the highest levels of immunostaining were observed in the islands of Calleja, diagonal band of Broca, magnocellular preoptic nucleus, pre- and parasubiculum, suprachiasmatic nucleus, anterodorsal nucleus of the thalamus, substantia nigra, ventral tegmental area, pontine nuclei and dorsal motor nucleus of the vagus, all of which had previously been documented to contain high densities of neurotensin binding sites. There were, however, a number of regions reportedly endowed with neurotensin binding sites, including the central amygdaloid nucleus, periaqueductal gray, outer layer of the superior colliculus and dorsal tegmental nucleus, which showed no or divergent patterns of immunostaining, suggesting that they might be expressing a molecularly distinct form of the receptor. At the cellular level, neurotensin receptor immunoreactivity was predominantly associated with perikarya and dendrites in some regions (e.g., in the basal forebrain, ventral midbrain, pons and rostral medulla) and with axons and axon terminals in others (e.g., in the lateral septum, bed nucleus of the stria terminalis, neostriatum, paraventricular nucleus of the thalamus and nucleus of the solitary tract). These data indicate that neurotensin may act both post- and presynaptically in the central nervous system and confirm that some of its effects are exerted on projection neurons. There were also areas, such as the cerebral cortex, nucleus accumbens and para- and periventricular nucleus of the hypothalamus, which contained both immunoreactive perikarya/dendrites and axon terminals, consistent with either a joint association of the receptor with afferent and efferent elements or its presence on interneurons. Taken together, these results also suggest that the neurotensin high affinity receptor protein is associated with a neuronal population that is more extensive than originally surmised from in situ hybridization studies.

Effects of neurotensin on visual neurons in the superficial laminae of the hamster's superior colliculus

Autoradiography with 125I-neurotensin in normal and enucleated hamsters was used to define the distribution of receptors for this peptide in the superficial layers of the superior colliculus (SC). Neurotensin binding sites were densely distributed in the stratum griseum superficiale (SGS), and results from the enucleated animals indicated that they were not located on retinal axons. The effects of neurotensin on individual superficial layer cells were tested in single-unit recording experiments. Neurotensin was delivered via micropressure ejection during visual stimulation (n = 75 cells), or during electrical stimulation of either the optic chiasm (OX; n = 47 cells) or visual cortex (CTX; n = 29 cells). In comparison with control values, application of neurotensin decreased visual responses of all SC cells tested to 54.1 +/- 34.9% (mean +/- standard deviation; range of decrement 7.5 to 100%; nine cells showed no effect or an increase in visual activity, which for four of these was > or = 30%). Neurotensin application also reduced responses to electrical stimulation of either OX or CTX, respectively, to 65.8 +/- 36.5% of control values (range of decrement 2.6 to 97.4%; 12 neurons showed a weak increment < or = 30%) and 68.0 +/- 38.5% (range of decrement 3.3 to 100%; five cells showed no effect or an increment, in one case > or = 30%). Of the 25 neurons tested with both OX and CTX stimulation, the correlation of evoked response suppression by neurotensin was highly significant (r = 0.70; P < 0.001). This suggests that the suppressive effects of neurotensin were common to both pathways. To test whether the inhibitory effects of neurotensin were presynaptic or postsynaptic, Mg2+ ions were ejected iontophoretically to abolish synaptic responses, and the neurons (n = 16) were activated by iontophoresis of glutamate and then tested with neurotensin. Neurotensin reduced the glutamate-evoked responses to an average 59.3 +/- 37.9% of control values (range 2.3 to 92.5%; one cell showed an increment > 30%). This result suggests that the site of action of neurotensin is most likely postsynaptic.

Low doses of neurotensin in the preoptic area produce hyperthermia. Comparison with other brain sites and with neurotensin-induced analgesia

High amounts of neurotensin (NT) are found in the preoptic area of the hypothalamus, an area known to be involved in the regulation of body temperature. It is generally believed that NT is a peptide that produces hypothermia, and several sites in the brain have been proposed to mediate NT-induced hypothermia, including the preoptic area. However, the doses of NT used in these experiments were always very high (microgram order) whereas, according to Goedert, the total brain content of NT in the rat does not exceed 10 ng. We therefore reinvestigated the effects of microinjections of NT in the brain, using high (5 micrograms) and low (50 and 5 ng) doses, into the preoptic area and other brain sites (cerebral ventricles, posterior hypothalamus, and nucleus accumbens), and we also studied, as a comparison, the effects of high and low doses of NT on pain sensitivity in the same sites. The results show that the preoptic area has unique properties in the regulation of body temperature: low doses of NT in the preoptic area produce a hyperthermic response, whereas high doses produce hypothermia. In comparison, NT produces hypothermia in the posterior hypothalamus whatever the dose, and NT has analgesic effects in the preoptic area only at high doses. Besides, NT has no thermic effect, but does have an analgesic effect, in the nucleus accumbens. The selectivity of the actions of high doses of NT, as well as the mechanism of action of NT (possibly an endogenous neuroleptic), are discussed.

Effects of neurotensin on neurons in the rat central amygdaloid nucleus in vitro

The effects of neurotensin (NT) on neurons in the central amygdaloid nucleus (ACe) were investigated in rat brain slice preparations by adding the peptide to the perfusing medium. Of 115 ACe neurons, 69 cells (60%) showed excitatory responses and 10 cells (9%) showed inhibitory responses to application of NT. The excitatory response to NT was observed in a dose-dependent manner and the threshold concentration was approximately 3 x 10(-9) M. The excitatory effects of NT persisted under blockade of synaptic transmission. The NT fragment neurotensin 8-13 and the NT analogue neuromedin N showed effects similar to those of NT, whereas the NT fragment neurotensin 1-8 had no effect on ACe neurons. Of 43 neurons in the septal nucleus, 8 cells (19%) and 3 cells (7%) showed excitatory and inhibitory responses, respectively, to NT. The results suggest that NT exerts a potent excitatory effect on ACe neurons through a direct action on specific receptors, in which NT may play a role in amygdala-relevant functions.

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

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

Effect of muscimol on haloperidol-induced alteration of neurotensin gene expression in the striatum and nucleus accumbens in the rat

Acute neuroleptic administration increases the expression of neurotensin/neuromedin (NT/N) gene in rat dorsolateral striatum and shell sector of the nucleus accumbens. The purpose of this study was to examine modulation of neuroleptic induction of NT/N and the proto-oncogene c-fos expression by the GABAA agonist muscimol. Adult male Sprague-Dawley rats were treated with saline, haloperidol (1 mg/kg); muscimol (3.2 mg/kg); or haloperidol (1 mg/kg) plus muscimol (3.2 mg/kg). Animals were sacrificed 1 h after drug administration. Expression of NT/N and c-fos mRNA was examined by in situ hybridization using 35S-antisense probes. Muscimol alone had no measurable effect on basal levels of NT/N or c-fos mRNA in either the dorsolateral striatum or the nucleus accumbens. However, co-administration of muscimol with haloperidol reduced haloperidol-induced increases in NT/N as well as c-fos mRNA in the dorsolateral striatum. In contrast, NT/N mRNA expression in accumbal shell induced by haloperidol was not modulated by co-administration of muscimol. These data suggest that GABAA receptors may be involved in regulation of NT/N gene expression in the DLSt, but not in the nucleus accumbens.

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.

Effects of neurotensin on EEG and event-related potentials in the rat

Neurotensin has neuromodulatory actions on multiple brain functions including motor, sensory and limbic processes. However, little is known about how neurotensin affects general arousal and/or attention states. The present study evaluated the effects of neurotensin on spontaneous brain activity as well as auditory evoked responses using electrophysiological measures. Electroencephalographic and event-related potential recordings were obtained in awake animals following intracerebroventricular administration of neurotensin (1.0, 10.0 and 30.0 micrograms). Twenty rats were implanted with recording electrodes in the frontal cortex, dorsal hippocampus, amygdala and nucleus accumbens. Neurotensin was found to produce a dose-related effect on behavior and electrophysiological measures. Lower doses (10 micrograms) produced no obvious behavioral changes, but significantly reduced EEG power in the lower frequency ranges (2-6 Hz) in the frontal cortex, the anterior amygdaloid complex and the nucleus accumbens. At higher doses (30 micrograms), rats appeared behaviorally inactivated, and EEG power was reduced in all structures in both the lower frequency ranges (2-6 Hz) and the higher frequency ranges (8-32 Hz). Auditory processing, as assessed by event-related potentials, was affected most significantly in amygdala and dorsal hippocampus. In the amygdala, the amplitude of the P3 component of the auditory event-related potential was increased significantly by doses of 10.0 and 30.0 micrograms. In the dorsal hippocampus, the amplitude and the area of the N1 component was increased dose dependently and significance was reached at the 30 micrograms dose. These electrophysiological findings indicate that neurotensin does not reduce the arousal level of the animals and in fact may enhance neurosensory processing in limbic areas through increased arousal and/or enhanced stimulus evaluation.

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

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

Facilitation of GABA release by neurotensin is associated with a reduction of dopamine release in rat nucleus accumbens

The main aim of the present study was to investigate the effects of local perfusion with the tridecapeptide neurotensin on extracellular GABA and dopamine levels in the nucleus accumbens of the halothane-anaesthetized rat, using in vivo microdialysis. In an initial set of characterization studies we examined the Na+ dependence of neurotransmitter release by local perfusion with ouabain, veratridine and tetrodotoxin. Local perfusion with the Na+ ATPase inhibitor ouabain (10 microM) or the Na+ channel agonist veratridine (20 microM) perfused into the nucleus accumbens increased both extracellular GABA and dopamine levels. The Na+ channel antagonist tetrodotoxin (1 microM) consistently decreased (24% of basal) dopamine levels, while even at 10 microM it did not affect GABA. However, tetrodotoxin (10 microM) abolished the veratridine-induced increase in both GABA and dopamine, demonstrating that Na(+)-dependent neuronal activity is involved in this release mechanism. In a second set of experiments a hypothesis for a functional link between neurotensin, dopamine and GABA in the medial nucleus accumbens was tested. Towards this aim, the effects of local perfusion with a high 1 microM concentration of neurotensin into the nucleus accumbens increased both GABA (210% of basal value) and dopamine (145% of basal) release. However, a low (10 nM) concentration of neurotensin again increased GABA release (160% of basal), but decreased that of dopamine (75% of basal value). Furthermore, the local perfusion with the GABAA receptor antagonist bicuculline abolished the neurotensin (10 nM) induced inhibition of dopamine release without affecting the increase in GABA release. These findings suggest that neurotensin modulates both GABA and dopamine neurotransmission in the nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)

The non-peptide neurotensin receptor antagonist SR48692 is not a potent antagonist of neurotensin(8-13) responses of rat substantia nigra neurones in vitro

The pharmacology of the neurotensin response of dopamine-sensitive neurons in the substantia nigra compacta was examined by testing the effect of a novel neurotensin receptor antagonist, SR48692, on the response to neurotensin(8-13). Extracellular recordings of action potentials from neurons were made from coronal rat brain slices maintained in vitro. SR48692 was unable to antagonise the actions of neurotensin(8-13) until microM concentrations were applied. The equilibrium constant for SR48692 was 4.9 microM compared with low nM Ki values reported in binding experiments. The data suggests that the pharmacology of the neurotensin response in the rat substantia nigra may be different to that predicted from radioligand-binding studies in cell lines and brain homogenates.

Up-regulation of neurotensin mRNA in the rat striatum after acute methamphetamine treatment

The effect of acute subcutaneous administration of methamphetamine on the expression of neurotensin mRNA was investigated in the adult rat striatum. At different time points (2, 6 and 24 h) following drug administration rats were killed, and mRNA levels were quantified both on films and emulsion-dipped tissue sections from two striatal levels. Two hours after methamphetamine injection, a dramatic increase in neurotensin mRNA levels was detected in different areas of the striatum at both rostral and caudal levels. Numerous positive cells were observed in the dorsomedial, dorsolateral and ventrolateral parts of the striatum. This up-regulation reflected an increase both in the number of cells expressing neurotensin mRNA and in the mean mRNA levels. This increase was still present after 6 h and was similar to the 2 h treated group at the rostral level of the striatum, but lower at the caudal one. Twenty-four hours after methamphetamine injection, neurotensin mRNA levels were back to control values, or in some areas even below. A strong increase in neurotensin mRNA-expressing cells was also seen in the olfactory tubercle, and the time-course was similar to the one observed in the striatum. In a second set of experiments, the effect of methamphetamine was evaluated on adjacent striatal sections hybridized with probes directed against neurotensin and substance P mRNAs, respectively. Two hours after drug administration, a significant increase in the levels of both peptide mRNAs was observed (+190% for neurotensin, +80% for substance P). These results demonstrate that methamphetamine is able to induce a dramatic, rapid and transient increase in striatal neurotensin mRNA levels, which may partly account for the elevation in neurotensin peptide levels observed in the striatonigral pathway after methamphetamine. The different anatomical localization of neurotensin mRNA-expressing cells observed after haloperidol and methamphetamine treatments, as well as the fact that the D1 receptor antagonist SCH-23390 is able to counteract the effect of methamphetamine but not that of haloperidol on neurotensin mRNA expression, suggests that there are at least two different subpopulations of neurotensin cells in the striatum. One population is regulated via D1 receptors and projects to the substantia nigra pars reticulata. The second is sensitive to D2 receptor stimulation and may project to the globus pallidus and/or may represent interneurons.

The pharmacology of neurotensin analogues on neurones in the rat substantia nigra, pars compacta in vitro

The effects of neurotensin and neurotensin analogues on dopamine neurones were studied in an in vitro slice preparation of rat substantia nigra, pars compacta using extracellular and intracellular recording techniques. Neurotensin had an EC50 of 13 nM in these experiments. This study showed that the C-terminal 5 amino acids of neurorotensin in neurotensin-(9-13) retained agonist activity on substantia nigra neurones. The naturally occurring neurotensin analogues neuromedin N and avian neurotensin were also active whilst kinetensin was inactive. Kinetensin differs from the C-terminal neurotensin 5-amino acids by two amino acids. The difference between the activity of neurotensin and the inactivity of kinetensin was investigated using synthetic peptides which contained single amino acid substitutions. These results show that position 12 of neurotensin is important for agonist activity in the substantia nigra.

Receptor-receptor interactions as an integrative mechanism in nerve cells

Several lines of evidence indicate that interactions among transmission lines can take place at the level of the cell membrane via interactions among macromolecules, integral or associated to the cell membrane, involved in signal recognition and transduction. The present view will focus on this last subject, i.e., on the interactions between receptors for chemical signals at the level of the neuronal membrane (receptor-receptor interaction). By receptor-receptor interaction we mean that a neurotransmitter or modulator, by binding to its receptor, modifies the characteristics of the receptor for another transmitter or modulator. Four types of interactions among transmission lines may be considered, but mainly intramembrane receptor-receptor interactions have been dealt with in this article, exemplified by the heteroregulation of D2 receptors via neuropeptide receptors and A2 receptors. The role of receptor-receptor interactions in the integration of signals is discussed, especially in terms of filtration of incoming signals, of integration of coincident signals, and of neuronal plasticity.