Neurotensin increases extracellular striatal dopamine levels in vivo

The Role of Intra-Amygdaloid Neurotensin and Dopamine Interaction in Spatial Learning and Memory

Neurotransmitter and neuromodulator neurotensin (NT) has been proved to facilitate spatial and passive avoidance learning after microinjected into the rat central nucleus of amygdala (CeA). These previous studies of our laboratory also revealed that neurotensin-1 receptor (NTS1) is involved in the mentioned actions of NT. Extensive literature confirms the interaction between neurotensinergic and dopaminergic systems, and our research group also suppose that the mesolimbic dopaminergic system (MLDS) is involved in the spatial learning and memory-facilitating effect of NT in the CeA. In the present work, NT and dopamine (DA) interaction has been examined in the Morris water maze and passive avoidance tests. Rats received 100 ng NT, 5 µg dopamine D2 receptor antagonist sulpiride in itself, sulpiride as a pretreatment before NT or vehicle solution into the CeA. NT microinjection significantly decreased target-finding latency in the Morris water maze test and significantly increased entrance latency in the passive avoidance test, as was expected based on our previous findings. The DA D2 receptor antagonist pretreatment was able to inhibit both effects of NT. The results confirm the facilitatory effect of NT on spatial learning and memory and let us conclude that these actions can be exerted via the DA D2 receptors.

HIV-Associated Apathy/Depression and Neurocognitive Impairments Reflect Persistent Dopamine Deficits

Individuals living with human immunodeficiency virus type 1 (HIV-1) are often plagued by debilitating neurocognitive impairments and affective alterations;the pathophysiology underlying these deficits likely includes dopaminergic system dysfunction. The present review utilized four interrelated aims to critically examine the evidence for dopaminergic alterations following HIV-1 viral protein exposure. First, basal dopamine (DA) values are dependent upon both brain region andexperimental approach (i.e., high-performance liquid chromatography, microdialysis or fast-scan cyclic voltammetry). Second, neurochemical measurements overwhelmingly support decreased DA concentrations following chronic HIV-1 viral protein exposure. Neurocognitive impairments, including alterations in pre-attentive processes and attention, as well as apathetic behaviors, provide an additional line of evidence for dopaminergic deficits in HIV-1. Third, to date, there is no compelling evidence that combination antiretroviral therapy (cART), the primary treatment regimen for HIV-1 seropositive individuals, has any direct pharmacological action on the dopaminergic system. Fourth, the infection of microglia by HIV-1 viral proteins may mechanistically underlie the dopamine deficit observed following chronic HIV-1 viral protein exposure. An inclusive and critical evaluation of the literature, therefore, supports the fundamental conclusion that long-term HIV-1 viral protein exposure leads to a decreased dopaminergic state, which continues to persist despite the advent of cART. Thus, effective treatment of HIV-1-associated apathy/depression and neurocognitive impairments must focus on strategies for rectifying decreases in dopamine function.

Gut-brain peptides in corticostriatal-limbic circuitry and alcohol use disorders

Peptides synthesized in endocrine cells in the gastrointestinal tract and neurons are traditionally considered regulators of metabolism, energy intake, and appetite. However, recent work has demonstrated that many of these peptides act on corticostriatal-limbic circuitry and, in turn, regulate addictive behaviors. Given that alcohol is a source of energy and an addictive substance, it is not surprising that increasing evidence supports a role for gut-brain peptides specifically in alcohol use disorders (AUD). In this review, we discuss the effects of several gut-brain peptides on alcohol-related behaviors and the potential mechanisms by which these gut-brain peptides may interfere with alcohol-induced changes in corticostriatal-limbic circuitry. This review provides a summary of current knowledge on gut-brain peptides focusing on five peptides: neurotensin, glucagon-like peptide 1, ghrelin, substance P, and neuropeptide Y. Our review will be helpful to develop novel therapeutic targets for AUD.

Intra-accumbens shell injections of SR48692 enhanced cocaine self-administration intake in rats exposed to an environmentally-elicited reinstatement paradigm

Neurotensin (NT) is a neuropeptide involved in cocaine reward, and in learning and memory processes related to drug use within the mesolimbic dopamine (DA) system. Studies have demonstrated that NT receptor antagonists have potential as pharmacotherapeutical tools for cocaine abuse. Therefore, it is important to understand the molecular profile of NT within mesolimbic neurons and the behavioral effects of NT receptor inhibitors on environmentally-elicited cocaine seeking behavior. To address this issue, male Sprague Dawley rats were trained to self-administer cocaine and to discriminate between environmental cues signaling cocaine vs. saline availability. Then, following extinction, these cues were used to induce reinstatement of cocaine seeking behavior. A differential expression profile was observed throughout the experiment. Particularly, a significant increase of NT levels was observed within the nucleus accumbens (NAc) shell subregion during the acquisition phase of training. To further examine the implications of this increase, separate groups of animals received intra NAc shell injections of one of three doses (25, 50, 100 nM) of the NT1 receptor antagonist SR48692 after reaching stable self-administration. Animals were injected prior to placement in the operant conditioning chambers for four consecutive sessions. An increase in lever pressing was observed following antagonist treatment, whereas no major changes in locomotor activity were observed. We propose that the observed increase in lever pressing may be a compensatory response to a decrease in reinforcement, possibly due to decreased DA release, as previous studies show that chronic SR48692 decreases basal DA release in the NAc shell.

Brain levels of neuropeptides in human chronic methamphetamine users

Animal data show that neuropeptide systems in the dopamine-rich brain areas of the striatum (caudate, putamen, and nucleus accumbens) are influenced by exposure to psychostimulants, suggesting that neuropeptides are involved in mediating aspects of behavioral responses to drugs of abuse. To establish in an exploratory study whether levels of neuropeptides are altered in brain of human methamphetamine users, we measured tissue concentrations of dynorphin, metenkephalin, neuropeptide Y, neurotensin, and substance P in autopsied brains of 16 chronic methamphetamine users and 17 matched control subjects. As expected, levels of most neuropeptides were enriched in dopamine-linked brain regions such as the nucleus accumbens and striatum of normal human brain. In contrast to animal findings of increased neuropeptide levels following short-term methamphetamine exposure, striatal neuropeptide concentrations were either normal or moderately decreased in the methamphetamine users. In other examined dopamine-poor cortical and subcortical brain areas, neuropeptide levels were generally either normal or variably reduced. Although the neuropeptide differences might be explained by methamphetamine-induced damage to neuropeptide-containing neurons, our human data are consistent with the possibility that, at least in the human striatum, long-term methamphetamine exposure leads to an adaptive process that is distinct from that which increases neuropeptide levels after acute methamphetamine exposure.

Neurotensin reduces glutamatergic transmission in the dorsolateral striatum via retrograde endocannabinoid signaling

Neurotensin is a peptide that has been suggested to mimic the actions of antipsychotics, but little is known about how it affects synaptic transmission in the striatum, the major input nucleus of the basal ganglia. In this study we measured the effects of neurotensin on EPSCs from medium spiny projection neurons in the sensorimotor striatum, a region implicated in habit formation and control of motor sequences. We found that bath-applied neurotensin reduced glutamate release from presynaptic terminals, and that this effect required retrograde endocannabinoid signaling, as it was prevented by the CB1 cannabinoid receptor antagonist AM251. Neurotensin-mediated inhibition of striatal EPSCs was also blocked by antagonists of D2-like dopamine receptors and group I metabotropic glutamate receptors, as well as by intracellular calcium chelation and phospholipase C inhibition. These results suggest that neurotensin can indirectly engage an endocannabinoid-mediated negative feedback signal to control glutamatergic input to the basal ganglia.

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.

Amphetamine-elicited striatal Fos expression is attenuated in neurotensin null mutant mice

Neurotensin (NT) has been suggested to interact with dopamine systems in different forebrain sites to exert both antipsychotic- and psychostimulant-like effects. We previously found that genetic or pharmacological manipulations that disrupt endogenous NT signaling attenuate antipsychotic drug-induced Fos expression in the dorsolateral and central striatum but not other striatal regions. To assess the role of NT in psychostimulant responses, we examined the ability of d-amphetamine (AMP) to induce Fos in wild-type and NT null mutant mice. AMP-elicited Fos expression was significantly attenuated in the medial striatum of NT null mutant mice, but was unaffected in other striatal territories. Similar results were obtained in rats and mice pretreated with the high affinity neurotensin receptor (NTR1) antagonist SR 48692. The effect of the NTR1 antagonist was particularly apparent in the striatal patch (striosome) compartment, as defined by mu-opioid receptor immunoreactivity. These data suggest that NT is required for the full activation by AMP of medial striatal neurons.

Differential neurotensin responses to low and high doses of methamphetamine in the terminal regions of striatal efferents

Neurotensin is a neuropeptide associated with basal ganglia dopaminergic neurons. Because levels of neurotensin in striatal tissue are differentially affected by low or high doses of methamphetamine, we employed microdialysis to assess the dose-dependent effects of methamphetamine on neurotensin release from the terminals of striatonigral and striatopallidal neurons. A low (0.5 mg/kg), but not high (10 mg/kg), dose of methamphetamine significantly increased nigral extracellular levels of neurotensin. The low-dose effect on extracellular nigral neurotensin levels was blocked by pretreatment with either a dopamine D1 or D2 receptor antagonist. In the globus pallidus, only half of the animals demonstrated increased neurotensin release after the low dose of methamphetamine. These findings suggest that low and high doses of methamphetamine differentially affect the release of neurotensin from the terminals of striatonigral neurons and that both dopamine D1 and D2 receptor activation contributes to the low-dose methamphetamine effects in the substantia nigra.

The effects of systemic NT69L, a neurotensin agonist, on baseline and drug-disrupted prepulse inhibition

Centrally administered neurotensin (NT) produces behavioral and biochemical effects that are very similar to the effects of antipsychotic drugs. Therefore, there is much interest in the potential use of NT agonists as antipsychotic drugs. We have previously reported that PD149163, a NT(8-13) analogue, produced effects on prepulse inhibition (PPI) of startle after systemic administration that were suggestive of an atypical antipsychotic-like drug profile. To determine if these effects are shared by other peripherally administered NT agonists, we tested the effects of NT69L, a recently developed NT agonist that penetrates the CNS, on drug-induced PPI deficits. In the first experiment, rats received subcutaneous (s.c.) injections of NT69L (vehicle, 0.08, 0.25, and 1.0mg/kg) followed 30min later by subcutaneous saline or D-amphetamine (2.0mg/kg). In the second experiment, NT69L injections were followed by saline or the non-competitive NMDA antagonist dizocilpine (0.1mg/kg). Both D-amphetamine and dizocilpine significantly decreased PPI as expected. In the first experiment, NT69L significantly increased PPI levels at baseline and after D-amphetamine. In the second experiment, NT69L attenuated PPI deficits produced by dizocilpine, without increasing baseline PPI. In addition, NT69L had no effect on startle magnitude. The effects of NT69L in these studies were similar in some ways to the effects of PD149163 and were also consistent with the preclinical effects of atypical antipsychotic drugs. These data provide further support for the notion that NT agonists may have use as novel antipsychotic drugs. Furthermore, the ability of NT69L and PD149163 to attenuate dizocilpine-disrupted PPI, an antipsychotic drug effect not mediated by dopamine, suggests that NT agonists may produce some of their antipsychotic-like effects by modulating neurotransmitter systems other than dopamine, such as serotonin, noradrenaline or glutamate.

Enhanced neurotensin neurotransmission is involved in the clinically relevant behavioral effects of antipsychotic drugs: evidence from animal models of sensorimotor gating

To date, none of the available antipsychotic drugs are curative, all have significant side-effect potential, and a receptor-binding profile predictive of superior therapeutic ability has not been determined. It has become increasingly clear that schizophrenia does not result from the dysfunction of a single neurotransmitter system, but rather from an imbalance between several interacting systems. Targeting neuropeptide neuromodulator systems that concertedly regulate all affected neurotransmitter systems could be a promising novel therapeutic approach for schizophrenia. A considerable database is concordant with the hypothesis that antipsychotic drugs act, at least in part, by increasing the synthesis and release of the neuropeptide neurotensin (NT). In this report, we demonstrate that NT neurotransmission is critically involved in the behavioral effects of antipsychotic drugs in two models of antipsychotic drug activity: disrupted prepulse inhibition of the acoustic startle response (PPI) and the latent inhibition (LI) paradigm. Blockade of NT neurotransmission using the NT receptor antagonist 2-[[5-(2,6-dimethoxyphenyl)-1-(4-(N-(3-dimethylaminopropyl)-N-methylcarbamoyl)-2-isopropylphenyl)-1H- pyrazole-3-carbonyl]-amino]-adamantane-2-carboxylic acid, hydrochloride (SR 142948A) prevented the normal acquisition of LI and haloperidol-induced enhancement of LI. In addition, SR 142948A blocked the PPI-restoring effects of haloperidol and the atypical antipsychotic drug quetiapine in isolation-reared animals deficient in PPI. We also provide evidence of deficient NT neurotransmission as well as a left-shifted antipsychotic drug dose-response curve in isolation-reared rats. These novel findings, together with previous observations, suggest that neurotensin receptor agonists may represent a novel class of antipsychotic drugs.

The effects of glutamate receptor agonists on neurotensin release using in vivo microdialysis

In the present study, extracellular concentrations of neurotensin were measured from the striatum, nucleus accumbens and the medial prefrontal cortex in the awake, freely moving rat. Using a highly sensitive solid phase radioimmunoassay, basal concentrations of neurotensin were 2-5 pg/sample. In each region, glutamate receptor agonists, N-methyl-D-aspartate (NMDA) and kainic acid, increased neurotensin release 2-3-fold. Preincubation with the Na(+) channel blocker tetrodotoxin abolished the glutamate receptor agonist-induced increases except in the striatum following kainic acid infusion. These findings indicate that activation of glutamate receptors may indirectly stimulate neurotensin release.

Chronic, but not acute, dosing of antipsychotic drugs alters neurotensin binding in rat brain regions

The present study compared high affinity neurotensin (NT) binding in rat brain following acute or chronic treatment with the classical antipsychotic, haloperidol, and the newer antipsychotic drugs, clozapine and zotepine. Drugs were given orally, as an acute treatment (1 dose) or chronically (21 day dosing) and binding to the NT high affinity receptor was examined in three brain regions; striatum, nucleus accumbens/olfactory tubercle and frontal cortex. Acute dosing with either vehicle, haloperidol, clozapine or zotepine produced no significant changes in NT binding from controls (naïve rats). Chronic (21 day) dosing resulted in an increase in the K:(D:) and B(max) of high affinity receptors in the striatum following haloperidol, but not clozapine, zotepine or vehicles. In contrast, the newer antipsychotics, clozapine and zotepine but not haloperidol or vehicles, significantly altered NT binding in the nucleus accumbens/olfactory tubercle by decreasing the K:(D:) and B(max). Further differentiation between the two newer antipsychotic drugs occurred in the frontal cortex. Clozapine had no significant effect on NT binding, whereas zotepine significantly reduced the K:(D:) of the high affinity receptor with no alteration in B(max). The antipsychotic drugs tested did not interact directly with the NT high affinity receptor. Therefore, they must be acting indirectly via an alternative receptor mechanism to alter NT high affinity binding. In accordance with previously reported NT/dopamine receptor interactions, this would suggest cross-talk between these systems. Overall, these data demonstrate that chronic, but not acute, administration of antipsychotic drugs alters NT binding in the rat brain. In addition, anatomical differences in NT binding arise according to the antipsychotic drug under test. This may be predictive of drug side-effect profile, antipsychotic efficacy or atypicality.

Does neurotensin mediate the effects of antipsychotic drugs?

The possibility that the neuropeptide neurotensin (NT) may function as an endogenous antipsychotic compound was first hypothesized almost two decades ago. Since that time, considerable effort has been directed towards determining whether NT neurons mediate the effects of antipsychotic drugs (APDs). The anatomic, biochemical, behavioral, and clinical relevance of this hypothesis is reviewed. Although the majority of the available evidence is indirect, the availability of several NT receptor (NTR) antagonists have now made possible the direct examination of the involvement of the NT system in the mechanism of action of APDs. Preliminary studies in our laboratory demonstrate the ability of a selective NTR antagonist to block the effects of APDs in two models of sensory motor gating deficits characteristic of schizophrenia. These data, taken together with a compelling series of studies demonstrating that increases of NT/neuromedin N mRNA expression and NT content in the nucleus accumbens and striatum after chronic administration of APDs are predictive of clinical efficacy and extrapyramidal side effects, respectively, provide direct preclinical evidence for a role of the NT system in the clinical efficacy of APDs. Although effects of selective NTR antagonists in normal volunteers or schizophrenic patients have not been studied, and nonpeptidergic NTR agonists have not yet been identified, these cumulative results provide the groundwork for the use of NT-ergic compounds in the treatment of schizophrenia.

Neurotensin, N-acetyl-aspartylglutamate and beta-endorphin modulate [3H]dopamine release from guinea pig nucleus accumbens, prefrontal cortex and caudate-putamen

Dopaminergic hyperactivity in nucleus accumbens and dopaminergic hypoactivity in prefrontal cortex are thought to underlie positive and negative symptoms of schizophrenia, respectively. The caudate putamen is the neuroanatomical substrate for extrapyramidal side effects resulting from chronic antipsychotic treatment. We sought to identify potential endogenous regulators of dopamine release that might produce differential effects in these brain areas. We tested neurotensin, N-acetyl-aspartyl-glutamate and beta-endorphin for potential regulation of [3H]dopamine release in these regions of guinea pig brain. All three peptides stimulated dopamine release, above basal activity, at all concentrations tested in the three regions. Neurotensin significantly enhanced and N-acetyl-aspartyl-glutamate had no significant effect on N-methyl-D-aspartate-stimulated release from all three regions. In contrast, beta-endorphin significantly inhibited N-methyl-D-aspartate-stimulated release in nucleus accumbens and caudate putamen. These results suggest that these neuropeptides may regulate endogenous dopamine release and therefore may be potential therapeutic targets for antipsychotic drug development.

Regulation of neurotensin receptor mRNA expression by the receptor antagonist SR 48692 in the rat midbrain dopaminergic neurons

In this study, we demonstrated that the tyrosine hydroxylase-like immuno-reactive (possibly dopaminergic) neurons express neurotensin receptor mRNA in the rat substantia nigra and in the ventral tegmental area. Additionally, 2 weeks treatment with the neurotensin receptor antagonist SR 48692 increased mRNA levels in the substantia nigra. These data suggest that neurotensin receptor expression in the perikarya and in the terminal regions of dopaminergic neurons is regulated by its endogenous agonist in vivo.

SR 48692-sensitive neurotensin receptors modulate acetylcholine release in the rat striatum

The effects of stimulation and blockade of neurotensin receptors on striatal acetylcholine release were examined in anaesthetized rats using microdialysis. Local perfusion with neurotensin (100 nM) did not influence the release of acetylcholine. Application of neurotensin (100 nM) 30 min after haloperidol (125 micrograms/kg, i.p.) increased acetylcholine levels to 188% compared to 120% when haloperidol was administered alone. SR 48692 (3-100 micrograms/kg, i.p.) dose-dependently reduced the stimulatory effect of neurotensin in the presence of haloperidol. Comparable antagonism was observed with SR 48527, a chemically-related compound with high affinity for neurotensin receptors, but not with SR 49711, its low-affinity antipode. These results indicate that high affinity neurotensin receptors regulate acetylcholine release, when D2-dopaminergic inhibitory input is suppressed.

Heterogeneity of melanized neurons expressing neurotensin receptor messenger RNA in the substantia nigra and the nucleus paranigralis of control and Parkinson's disease brain

We have recently cloned the neurotensin receptor from human substantia nigra. Using in situ hybridization techniques, with an 35S-labeled antisense RNA probe complementary to this receptor complementary DNA, we studied the expression of the human neurotensin receptor in the brain from control and Parkinson's disease subjects. We also performed an analogous study with rat brain. Neurotensin receptor messenger RNA was present in high levels in melanized neurons of the substantia nigra pars compacta and the nucleus paranigralis (the ventral tegmental area for rat brain). Background levels of signals for neurotensin receptor messenger RNA were detected in the nucleus ruber, the colliculus inferior and the striatal subdivisions (the nucleus caudatus, the putamen and the nucleus accumbens) of both human and rat brain. All these areas, except the nucleus ruber and the collicus inferior, contain very high to high levels of neurotensin receptor binding sites. Additionally, Parkinson's disease brains had markedly fewer melanized (possibly dopaminergic) neurons, as expected, and correspondingly very low or background levels of messenger RNA for neurotensin receptor. We have also demonstrated heterogeneity among the melanized cells expressing messenger RNA encoding the neurotensin receptor in the substantia nigra and the nucleus paranigralis of human brain. The neurons in the nucleus paranigralis had lower melanin pigmentation and higher expression of neurotensin receptor messenger RNA. In general, the expression of the messenger RNA within the highly and evenly melanized neurons was lower than that found in low or unevenly pigmented neurons. The neurons in the nucleus paranigralis had lower melanin pigmentation and higher expression of neurotensin receptor messenger RNA. The low pigmented neurons in the ventral tier of the substantia nigra had relatively high expression. On the other hand, highly and evenly melanized neurons in these regions of the brain had low expression of neurotensin receptor messenger RNA. Together with the previous binding data, it is suggested that not only in rat brain, but also in human brain, melanized (possibly dopaminergic) neurons in the substantia nigra and the nucleus paranigralis (ventral tegmental area of rat brain) synthesize neurotensin receptors and express them in their perikarya and the terminal regions. Additionally, the heterogeneity of the melanized neurons in human brain may play a role in the normal function of dopaminergic systems and probably in the etiology of some neurological and psychiatric disorders.

Astrocyte-dependent and -independent phases of the development and survival of rat embryonic day 14 mesencephalic, dopaminergic neurons in culture

A primary neuronal culture was prepared from the ventral mesencephalon, centered on the A8, A9 and A10 dopaminergic nuclei of the embryonic day 14 rat, and studied from 12 h to 28 days. At 12 h after plating, and before cell death ensued, 95% of the cells stained positive for neuron specific enolase; 20% for tyrosine hydroxylase; 5% for vimentin and < 0.1% for glial fibrillary acidic protein. In the presence of the mitotic inhibitor cytosine arabinoside (2.0 microM), neuronal growth and survival were surprisingly normal up to the ninth day in culture, but deteriorated rapidly thereafter. In the absence of a mitotic inhibitor, and in the presence of proliferating but non-confluent glia, the tyrosine hydroxylase positive neurons that survived to the 10th day, had retracted neurites and a rounded soma, suggesting an inhibition of cell development. Those tyrosine hydroxylase positive neurons that survived this adverse phase of development tended to produce elaborate neuritic profiles after the 11th day, coincident with confluence of the astrocyte monolayer at the 12th day. By the 21st day in culture, and persisting up to the 28th day, 60% (61 +/- 10, n = 20) of the surviving neurons stained positive for tyrosine hydroxylase. When plated on an established, ventral mesencephalic monolayer of astrocytes, at the seventh day in culture, neuritic growth and branching of the tyrosine hydroxylase positive neurons were greater, compared with similar neurons grown on poly-D-lysine, and the signs of arrested development (retraction of neurites and rounded soma) seen at the 10th day after plating on poly-D-lysine, were not observed. We conclude that in the primary culture studied, and under the experimental conditions used, the survival of dopaminergic neurons was independent of glia during the first nine days, and critically dependent on glia thereafter. The resurgence of growth of dopaminergic neurons after 10 days in vitro, and their subsequent selective survival in culture, suggest that confluent type-1 astrocytes produce factors that act selectively on the dopaminergic neuronal phenotype. The successful identification of these dopaminergic-specific, neurotrophic factors could lead to an increased understanding of the etiology of Parkinson's disease, and suggest new directions for therapeutic intervention.

Mesencephalic microinjections of neurotensin-(1-13) and its C-terminal fragment, neurotensin-(8-13), potentiate brain stimulation reward

Using the curve shift method, we assessed the effects of ventromedial mesencephalic tegmental (VMT) microinjections of an equimolar concentration of neurotensin-(1-13) (NT-(1-13)) and of its C-terminal fragment, neurotensin-(8-13) (NT-(8-13)), on operant responding for rewarding electrical stimulation of the caudal mesencephalic central gray. The effects of NT-(1-13) and NT-(8-13) on brain stimulation reward (BSR) were also compared to those of systemically administered quinpirole (0.1 and 0.2 mg/kg, s.c.), a direct dopamine agonist, and GBR-12909 (10 and 20 mg/kg, i.p.), a selective dopamine uptake blocker. At the concentration injected, NT-(8-13) was as effective as NT-(1-13) at facilitating BSR, producing significant leftward shifts of the function relating the rate of responding to the stimulation frequency (R/F function); neither form of the peptide was effective when injected in regions dorsal to the VMT. Similarly, GBR-12909 produced a parallel leftward shift of the R/F function, but, unlike NT-(1-13), also significantly increased the asymptotic rates of responding. In contrast, the high dose of quinpirole produced non-parallel leftward shifts of the R/F function and suppressed the asymptote. The similarity between the effects of neurotensin and GBR-12909 on one hand, and the differences between those of neurotensin and quinpirole on the other, suggest that activation VMT neurotensin receptors potentiate BSR by enhancing increases in dopamine neurotransmission that are contingent upon operant responding or rewarding brain stimulation, or both.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotensin modulates differently potassium, veratridine and 4-aminopyridine-evoked release of dopamine in rat striatal slices

We have studied the effects of neurotensin (NT) on the release of [3H]dopamine ([3H]DA) evoked by terminal depolarization with either K+, veratridine or 4-aminopyridine (4-AP). NT (1-1000 nM) induced a net potentiation (up to 170%) of the K+ (25 mM)-evoked release of [3H]DA. The capacity of NT to potentiate the effect of K+ ions decreased as the K+ concentration rose from 25 to 50 mM and totally disappeared at this high K+ concentration. NT (100 nM; 1,000 nM) had no significant effect on the veratridine (1.5; 5 microM) or 4-AP (20 microM) -evoked release of [3H]DA. The relevance of these experimental models of DA release to physiological transmitter release remains to be established. Those data highlight the complexity of the modulation of evoked neurotransmitter release by pharmacological agents.