Opposite effects of neurotensin on dopamine inhibition in different regions of the rat brain: An iontophoretic study

Neurotensin and Its Involvement in Female Hormone-Sensitive Cancers

Neurotensin (NT) is a peptide involved in digestion, neuromodulation, and cancer progression. NT and its receptors (NTR1 and SORT1 mainly) have been widely studied in oncology. Data show that NT expression is under the control of sex steroid hormones, in particular estradiol. We focused on its involvement in three main female hormone-sensitive cancers, breast, ovarian, and endometrial cancer, in a narrative review. NT, NTR1, and SORT1 are mostly expressed in these three cancers, and their involvement in oncologic processes such as proliferation and invasion seems to match, as does their impact on prognosis for most. The development of NT receptor-targeted therapies, including theranostics and radioligand treatments, presents a promising avenue for personalized cancer treatment.

Ventral Midbrain NTS1 Receptors Mediate Conditioned Reward Induced by the Neurotensin Analog, D-Tyr[11]neurotensin

The present study was aimed at characterizing the mechanisms by which neurotensin (NT) is acting within the ventral midbrain to induce a psychostimulant-like effect. In a first experiment, we determine which subtype(s) of NT receptors is/are involved in the reward-inducing effect of ventral midbrain microinjection of NT using the conditioned place-preference (CPP) paradigm. In a second study, we used in vitro patch clamp recording technique to characterize the NT receptor subtype(s) involved in the modulation of glutamatergic neurotransmission (excitatory post-synaptic current, EPSC) in ventral tegmental neurons that expressed (Ih+), or do not express (Ih-), a hyperpolarization-activated cationic current. Behavioral studies were performed with adult male Long-Evans rats while electrophysiological recordings were obtained from brain slices of rat pups aged between 14 and 21 days. Results show that bilateral ventral midbrain microinjections of 1.5 and 3 nmol of D-Tyr[¹¹]NT induced a CPP that was respectively attenuated or blocked by co-injection with 1.2 nmol of the NTS1/NTS2 antagonist, SR142948, and the preferred NTS1 antagonist, SR48692. In electrophysiological experiments, D-Tyr[¹¹]NT (0.01-0.5 μM) attenuated glutamatergic EPSC in Ih+ but enhanced it in Ih- neurons. The attenuation effect (Ih+ neurons) was blocked by SR142948 (0.1 μM) while the enhancement effect (Ih- neurons) was blocked by both antagonists (0.1 μM). These findings suggest that (i) NT is acting on ventral midbrain NTS1 receptors to induce a rewarding effect and (ii) that this psychostimulant-like effect could be due to a direct action of NT on dopamine neurons and/or an enhancement of glutamatergic inputs to non-dopamine (Ih-) neurons.

Schizophrenia as a disorder of too little dopamine: implications for symptoms and treatment

Antipsychotics represent the first effective therapy for schizophrenia, with their benefits linked to dopamine D2 blockade. Schizophrenia was soon identified as a hyperdopaminergic disorder, and antipsychotics proved to be reasonably effective in controlling positive symptoms. However, over the years, schizophrenia has been reconceptualized more broadly, now defined as a heterogeneous disorder with multiple symptom domains. Negative and cognitive features, not particularly responsive to antipsychotic therapy, have taken on increased importance--current thinking suggests that these domains predate the onset of positive symptoms and are more closely tied to functional outcome. That they are better understood in the context of decreased dopamine activity suggests that schizophrenia may fundamentally represent a hypodopaminergic disorder. This shift in thinking has important theoretical implications from the standpoint of etiology and pathophysiology, but also clinically in terms of treatment and drug development.

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.

Hyperactivity of the dopaminergic system in NTS1 and NTS2 null mice

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

Clozapine administration modifies neurotensin effect on synaptosomal membrane Na+, K+ -ATPase activity

Na+, K+-ATPase is inhibited by neurotensin, an effect which involves the peptide high affinity receptor (NTS1). Neurotensin effect on cerebral cortex synaptosomal membrane Na+, K+-ATPase activity of rats injected i.p. with antipsychotic clozapine was studied. Whereas 3.5 x 10(-6) M neurotensin decreased 44% Na+, K+-ATPase activity in the controls, the peptide failed to modify enzyme activity 30 min after a single 3.0, 10.0 and 30.0 mg/kg clozapine dose. Neurotensin decreased Na+, K+-ATPase activity 40 or 20% 18 h after 3.0 or 5.6 mg/kg clozapine administration, respectively, and lacked inhibitory effect 18 h after 17.8 and 30.0 mg/kg clozapine doses. Results indicated that the clozapine treatment differentially modifies the further effect of neurotensin on synaptosomal membrane Na+, K+-ATPase activity according to time and dose conditions employed. Taken into account that clozapine blocks the dopaminergic D2 receptor, findings obtained favor the view of an interplay among neurotensinergic receptor, dopaminergic D2 receptor and Na+, K+-ATPase at synaptic membranes.

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.

Neurotensin modulation of spontaneous EPSCs in the nucleus accumbens of Lewis and Fischer 344 rats

Fischer 344 (F344) and Lewis (LEW) rats are inbred strains that are differentially sensitive to drugs of abuse and that respond differently to the endogenous neuropeptide neurotensin (NT). To understand the mechanisms involved we used whole cell patch clamp recording technique to study the effects of an equimolar concentration of NT and its active analog, d-Tyr[11]neurotensin (d-NT), on the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in nucleus accumbens medium spiny (MS) neurons in brain slices. NT and d-NT produced an increase in the amplitude but not in the frequency of sEPSCs in all neurons tested in both F344 and LEW rats. In LEW rats, NT and d-NT produced an increase in sEPSCs of the same magnitude. In contrast, in F344 rats, d-NT produced an increase in sEPSCs that was 2.4 times larger than that of NT. Moreover, the effect of d-NT in F344 rats was also significantly larger than that measured in LEW rats whereas NT produced an effect of the same magnitude in both strains. These results demonstrate that MS neurons in F344 rats are more responsive to the activation of NT receptors sensitive to d-NT than LEW animals. This finding parallels previous behavioral data and provides additional evidence that the NT circuitry differs in the two strains, in a brain region known to play a key role in the rewarding effects of drugs of abuse.

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.

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.

Activation of central neurotensin receptors reinstates cocaine seeking in the rat: modulation by a D1/D5, but not D2/D3, receptor antagonist

RATIONALE

Neurotensin (NT) has been implicated in some of the behavioral effects of psychostimulants. Thus, there is reason to think that NT may play a role in the reinstatement of cocaine seeking, and that it may do so via an interaction with dopamine (DA).

OBJECTIVES

To assess (1) whether NT and an NT analog, D-TYR[11]NT, induce reinstatement of cocaine seeking; (2) whether the effects of NT receptor activation on reinstatement can be modulated by D1/D5 or D2/D3 antagonists; (3) the specificity of the effects of NT receptor activation on the reinstatement of cocaine seeking.

METHODS

In Experiment 1, rats were initially trained to self-administer cocaine. Following a subsequent period of extinction training, they were tested for the reinstatement of cocaine seeking by NT or D-TYR[11]NT (15, 30 microg i.c.v.). In Experiment 2, rats were pretreated with the D1/D5 antagonist, SCH 23390 (0.05, 0.10 mg/kg i.p.) or the D2/D3 antagonist, raclopride (0.25, 0.50 mg/kg i.p.), prior to testing for reinstatement by D-TYR[11]NT (15 microg i.c.v.). In Experiment 3, rats that had been trained to self-administer sucrose pellets were tested for the reinstatement of sucrose seeking by D-TYR[11]NT (15, 30 microg i.c.v.).

RESULTS

(1) Both NT and D-TYR[11]NT produced robust reinstatement of cocaine seeking; (2) the effect of the analog was attenuated by pretreatment with the D1/D5, but not D2/D3, receptor antagonist; (3) the analog did not induce the reinstatement of sucrose seeking.

CONCLUSIONS

The findings suggest that an interaction between NT and DA may contribute to the neurobiology of reinstatement in animals with a history of cocaine self-administration.

The inhibitory effect of neurotensin on synaptosomal membrane Na+, K+-ATPase is altered by antipsychotic administration

Synaptosomal membrane Na+, K+-ATPase is inhibited by neurotensin, an effect which involves its high affinity receptor (NTS1) [Lopez Ordieres MG, Rodriguez de Lores Arnaiz, G. Peptides 2000; 21:571-576.]. Herein, the effect of neurotensin on synaptosomal membrane Na+, K+-ATPase of rats 18 h after i.p. administration of antipsychotic haloperidol (2 mg/kg) or clozapine (10 mg/kg) was studied. Basal enzyme activity after these treatments did not differ from that in vehicle-treated rats. It was observed that 3.5 x 10(-6) M neurotensin reduced roughly 40% cerebral cortex Na+, K+-ATPase from vehicle-injected rats, produced no effect on the enzyme from rats injected with haloperidol but enhanced 26% that from rats injected with clozapine. The peptide decreased 40% striatal Na+, K+-ATPase from vehicle-injected rats or from rats injected with clozapine, whereas it failed to alter this enzyme activity from rats injected with haloperidol. Haloperidol and clozapine (1 x 10(-6) M) added in vitro failed to alter Na+, K+-ATPase activity in cerebral cortex synaptosomal membranes. Results obtained after antipsychotic administration may well offer an alternative explanation for the particular side effects recorded in therapeutics by typical (haloperidol) versus atypical (clozapine) antipsychotic drugs.

Endogenous neurotensin attenuates dopamine-dependent locomotion and stereotypy

The neuropeptide neurotensin (NT) is highly sensitive to changes in dopaminergic signaling in the striatum, and is thought to modulate dopamine-mediated behaviors. To explore the interaction of NT with the dopamine system, we utilized mice with a targeted deletion of dopamine synthesis specifically in dopaminergic neurons. Dopamine levels in dopamine-deficient (DD) mice are less than 1% of control mice, and they require daily administration of the dopamine precursor L-dihydroxyphenylalanine (L-DOPA) for survival. DD mice are supersensitive to the effects of dopamine, becoming hyperactive relative to control mice in the presence of L-DOPA. We show that 24 h after L-DOPA treatment, when DD mice are in a "dopamine-depleted" state, Nt mRNA levels in the striatum of DD mice are similar to those in control mice. Administration of L-DOPA or L-DOPA plus the L-amino acid decarboxylase inhibitor, carbidopa, (C/L-DOPA) induced Nt expression in the striatum of DD mice. The dopamine D1 receptor antagonist, SCH23390, blocked C/L-DOPA-induced Nt. To test the hypothesis that this striatal Nt expression modulated dopamine-mediated behavior in DD mice, we administered SR 48692, an antagonist of the high affinity NT receptor, together with L-DOPA or C/L-DOPA. L-DOPA-induced hyperlocomotion and C/L-DOPA-induced stereotypy were potentiated by peripheral administration of SR 48692. Furthermore, intrastriatal microinjections of SR 48692 augmented L-DOPA-induced hyperlocomotion. These results demonstrate a dynamic regulation of striatal Nt expression by dopamine via D1 receptors in DD mice, and point to a physiological role for endogenous striatal NT in counteracting motor behaviors induced by an overactive dopamine system.

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.

Endogenous neurotensin down-regulates dopamine efflux in the nucleus accumbens as revealed by SR-142948A, a selective neurotensin receptor antagonist

SR-142948A belongs to the second generation of potent, selective, non-peptide antagonists of neurotensin receptors. It was used to investigate the role of endogenous neurotensin in the regulation of dopamine efflux in the nucleus accumbens and striatum of anaesthetized and pargyline-treated rats. All the data were obtained using in vivo electrochemistry. Electrically evoked (20 Hz, 10 s) dopamine efflux was monitored by differential pulse amperometry, whereas variations in basal (tonic) dopamine efflux were monitored by differential normal pulse voltammetry. Like the first-generation compound SR-48692, SR-142948A did not affect the tonic and evoked dopamine efflux, but dose-dependently enhanced haloperidol (50 microg/kg, i.p.) induced facilitation of the electrically evoked dopamine release in the nucleus accumbens. In contrast to SR-48692, SR-142948A dose-dependently potentiated haloperidol (50 microg/kg, i.p.) induced increase in the basal dopamine level in the nucleus accumbens. This potentiating effect did not appear in the striatum. When dopaminergic and/or neurotensinergic transmissions were modified by a higher dose of haloperidol (0.5 mg/kg, i.p.), apomorphine, amphetamine or nomifensine, SR-142948A pre-treatment affected only the effect of apomorphine on the basal dopamine level in the nucleus accumbens. These results strengthen the hypothesis that endogenous neurotensin could exert a negative control on mesolimbic dopamine efflux.

Neurotensin attenuates the quinpirole-induced inhibition of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area

In the present study we describe the excitatory effects of the bioactive peptide neurotensin on the electrical activity of dopamine neurons (simultaneously recorded) in the substantia nigra pars compacta and the ventral tegmental area. The neurotensin fragment (8-13) induced comparable increases in firing rate of the substantia nigra and ventral tegmental area dopamine neurons (EC50 values 30 and 45 nM, respectively). The neurotensin receptor antagonist SR142948A antagonized the excitatory effects of neurotensin fragment (8-13) (pA2 values 8.4 and 8.2, respectively). Furthermore, it was found that a low concentration of neurotensin fragment (8-13) (1 nM) attenuated the inhibition of the firing rate by the selective dopamine D2 receptor agonist quinpirole in both neuron types (e.g., the effect of 0.01 microM quinpirole was reduced by approximately 60% in the presence of 1 nM neurotensin fragment [8-13]). Antagonism of this neurotensin fragment (8-13) effect by SR142948A confirms that neurotensin receptors can reduce the effect of dopamine D2 receptors at the single-cell level. These results are discussed in the light of possible roles for neurotensin in neurological disorders such as Parkinson's disease and schizophrenia.

Neuropeptide Y, neurokinin A and neurotensin in brain regions of Fawn Hooded "depressed", Wistar, and Sprague Dawley rats. Effects of electroconvulsive stimuli

1. Concentrations of neuropeptide Y (NPY)-, neurokinin A (NKA)- and neurotensin (NT)-like immunoreactivity (-LI) were measured in brain tissues of Fawn Hooded (FH) (a model of depression), Wistar (W) (control for depression) and Sprague Dawley (SD) rats (control for strain) with the aim to explore possible associations between neuropeptides and models of depression. 2. In addition, peptides were determined after six electroconvulsive stimuli (ECS) or six sham ECS ("baseline") in order to investigate ECS mechanisms of action. 3. Baseline NPY-LI concentrations were markedly lower in the hippocampus of the "depressed" FH compared to the W and SD animals. 4. Baseline NKA-LI concentrations were higher in the occipital cortex and NT-LI concentrations in the occipital cortex, frontal cortex, and hypothalamus of the FH and W compared to the SD rats. 5. ECS increased NPY-LI in the hippocampus, frontal cortex and occipital cortex of all three strains. In the hippocampus, the increase was significantly larger in the FH compared to the W and SD rats. ECS also increased NKA-LI in the hippocampus. 6. In contrast, ECS decreased NT-LI in the occipital cortex of the FH and W animals. 7. The results indicate that NPY may play a role in depression and that changes in NPY and NKA probably constitute one of the mechanisms of ECT action. More speculatively, NT may also be involved in depression.

Activation of neurotensin receptors in the prefrontal cortex stimulates midbrain dopamine cell firing

The effects of medial prefrontal cortex microinjections of 3 nmol/0.5 microl of neurotensin-(1-13), the inactive fragment neurotensin-(1-8), or vehicle on the firing rate of midbrain dopamine neurons were studied in anesthetized rats. Twelve of 19 cells tested with neurotensin-(1-13) showed an average 20-25% increase in firing rate between 10 and 20 min after the injection. This effect was not mimicked by neurotensin-(1-8) (9 cells), nor by a control injection (10 cells) suggesting that it is mediated by high-affinity neurotensin receptors. These results suggest that activation of neurotensin receptors in the medial prefrontal cortex can modulate neural activity of a subpopulation of midbrain dopamine neurons.

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.

Neurotransmitter interactions in the stomatogastric system of the spiny lobster: one peptide alters the response of a central pattern generator to a second peptide

Two of the peptides found in the stomatogastric nervous system of the spiny lobster, Panulirus interruptus, interacted to modulate the activity of the cardiac sac motor pattern. In the isolated stomatogastric ganglion, red-pigment-concentrating hormone (RPCH), but not proctolin, activated the bursting activity in the inferior ventricular (IV) neurons that drives the cardiac sac pattern. The cardiac sac pattern normally ceased within 15 min after the end of RPCH superfusion. However, when proctolin was applied within a few minutes of that time, it was likewise able to induce cardiac sac activity. Similarly, proctolin applied together with subthreshold RPCH induced cardiac sac bursting. The amplitude of the excitatory postsynaptic potentials from the IV neurons to the cardiac sac dilator neuron CD2 (1 of the 2 major motor neurons in the cardiac sac system) was potentiated in the presence of both proctolin and RPCH. The potentiation in RPCH was much greater than in proctolin alone. However, the potentiation in proctolin after RPCH was equivalent to that recorded in RPCH alone. Although we do not yet understand the mechanisms for these interactions of the two modulators, this study provides an example of one factor that can determine the "state" of the system that is critical in determining the effect of a modulator that is "state dependent," and it provides evidence for yet another level of flexibility in the motor output of this system.

Effects of D-amphetamine and phencyclidine on behavior and extracellular concentrations of neurotensin and dopamine in the ventral striatum and the medial prefrontal cortex of the rat

The effects of systemically administered phencyclidine (PCP; 2.5 mg/kg, s.c.) and D-amphetamine (1.5 mg/kg, s.c.) on the extracellular concentrations of neurotensin-like immunoreactivity (NT-LI) and dopamine (DA) in the ventral striatum (vSTR) and the medial prefrontal cortex (mPFC) were studied in freely moving rats using microdialysis. In separate animals, the effects of PCP and D-amphetamine on open field activity were also analyzed. PCP, but not D-amphetamine, caused a significant increase (156% over baseline) of NT-LI levels in the vSTR which was relatively short lasting, i.e., of less than 2 h duration. In contrast, both drugs significantly increased NT-LI concentrations in the mPFC by almost 100% during the same period. PCP and D-amphetamine also significantly increased extracellular levels of DA in the vSTR by 83 and 364%, respectively. However, the peak effect of PCP on DA appeared later than that of D-amphetamine, i.e., at 150 and 60 min, respectively, after drug administration. Also in the mPFC, both PCP and D-amphetamine significantly increased DA concentrations by 98 and 284%, respectively. Generally, effects on DA levels of both PCP and D-amphetamine were, in contrast to their effects on NT-LI levels, clearly more long-lasting, i.e., of 3-4 h duration. Behaviorally, D-amphetamine produced a more pronounced, general activation than PCP, with a faster onset of activation, i.e. within 30 vs 90 min after administration. However, both drugs produced long-lasting effects on the spatial organization of behavioral activity, which lasted for 3-4 h. In conclusion, the more pronounced behavioral stimulation by D-amphetamine (1.5 mg/kg, s.c.) vs PCP (2.5 mg/kg, s.c.) in the rat may largely be explained by its more potent DA-releasing effect in the brain. Initial behavioral suppression by PCP, e.g., of rearing, as well as its rather poor locomotor stimulant action in general, might relate to release of NT in the vSTR. The long-lasting, behavioral disorganization by both PCP and D-amphetamine may, however, be related to increased release of DA rather than NT in the mesolimbocortical areas.

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.

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.

SR 48692, a non-peptide neurotensin receptor antagonist differentially affects neurotensin-induced behaviour and changes in dopaminergic transmission

Unilateral microinjection of neurotensin in the ventral tegmental area of the rat (2.5 micrograms/0.5 microliter) produced behavioural excitation illustrated by contralateral circling. Given orally, SR 48692, a selective and potent non-peptide neurotensin receptor antagonist, significantly reduced these rotations with a triphasic dose-effect relationship. Inhibition occurred at 0.12 mg/kg; further increases in dose up to 2.5 mg/kg produced no significant antagonism, then at doses > or = 5 mg/kg, a second phase of antagonism was observed. Bilateral injection of neurotensin (0.5 microgram each side) into the nucleus accumbens antagonized the increase in locomotor activity following intraperitoneal injection of amphetamine. Given orally, SR 48692 reduced dose-dependently (0.1-1 mg/kg) these intra-accumbens neurotensin effects. Using high pressure liquid chromatography with electrochemical detection, we showed that microgram amounts of neurotensin injected into the ventral tegmental area increased dihydroxyphenylacetate/dopamine ratios in the nucleus accumbens. Using in vivo voltammetry techniques, we found that the injection of nanogram and picogram amounts of neurotensin in the ventral tegmental area stimulated dopamine efflux in the nucleus accumbens. None of these biochemical changes were affected by SR 48692 (0.1-10 mg/kg). These results indicate complex interactions between neurotensin and the mesolimbic dopamine system. More particularly, the differential ability of SR 48692 to affect neurotensin-evoked behavioural versus biochemical changes supports the concept of neurotensin receptor heterogeneity.

Neurotensin injected into the nucleus accumbens blocks the psychostimulant effects of cocaine but does not attenuate cocaine self-administration in the rat

The neuropeptide neurotensin (NT) has been shown to modulate mesolimbic dopaminergic activity. Neurotensin injected into the VTA produces motor stimulation and release of dopamine in the nucleus accumbens. In contrast, when neurotensin is administered into the nucleus accumbens, it produces neuroleptic-like effects such as attenuation of the locomotor activity elicited by psychostimulants. In the present study, the hypothesis that neurotensin injected into the nucleus accumbens might modulate the psychostimulant and reinforcing actions of cocaine was tested. In experiment one, rats were trained to self-administer cocaine intravenously on an FR5 schedule of reinforcement. Following the establishment of baseline responding, rats were implanted with bilateral cannulae in the nucleus accumbens. One week later, rats were injected into the nucleus accumbens with various doses of neurotensin (4.2, 8.4 and 16.7 micrograms, total doses bilaterally) immediately prior to the self-administration session. No significant effects were found with any of the doses of neurotensin tested on the self-administration of cocaine. However, in experiment 2, neurotensin at doses of 4.2 and 16.7 micrograms injected into the nucleus accumbens significantly reduced the locomotor activation induced by an acute injection of cocaine (15 mg/kg i.p.) and a dose of 16.7 micrograms attenuated the locomotor activation induced by amphetamine (0.75 mg/kg i.p.). Thus, neurotensin in the nucleus accumbens appears to specifically modulate the acute locomotor activating properties of cocaine but not cocaine self-administration. Different mechanisms by which NT interacts with dopamine in the nucleus accumbens may provide a means of selectively altering psychostimulant motor actions without affecting psychostimulant reinforcement.

Effect of neurotensin and immunneutralization with anti-neurotensin-serum on dopaminergic-cholinergic interaction in the striatum

The effect of neurotensin (NT) on the release of acetylcholine (ACh) and dopamine (DA) from striatal slices of the rat brain was studied. Neurotensin, 1-150 nM, was able to release ACh from cholinergic interneurons of the striatum. Like the response to electrical stimulation, the ACh-releasing effect of NT was completely inhibited by tetrodotoxin indicating that neuronal firing is involved in its effect. Immunneutralization reduced the stimulation-evoked release of ACh, an effect that was much marked when the inhibitory dopaminergic input was suspended by sulpiride-selective antagonists of D2 receptors. Sulpiride, 0.1 mM, induced a 2-fold increase in the NT- and electrically-induced release of ACh. A quantitatively similar increase was also observed after degeneration of the nigrostriatal DA pathway with 6-hydroxydopamine (6-OHDA) (2 x 250 micrograms/animal, i.c.v.). However, the D2 receptor agonist quinpirole, 0.01 mM, significantly reduced the NT-induced release of ACh by 77%. Neurotensin enhanced the stimulation-evoked release of [3H]DA. These findings indicate that, using field stimulation when dopaminergic, cholinergic and NT-containing neurons are stimulated in concert, NT is capable of releasing both ACh and DA in the striatum, but its effect on ACh release is masked unless the D2 receptor-mediated tonic inhibitory effect of DA released from the nigro-striatal pathway is attenuated. Thus, in Parkinson's disease where the dopaminergic input is impaired, NT may be involved in producing cholinergic dominance.