Interactions of neurotensin with dopamine-containing neurons in the central nervous system

The reversal of amphetamine-induced locomotor activation by a selective neurotensin-1 receptor agonist does not exhibit tolerance

RATIONALE

Neurotensin-1 (NT1) receptor agonists have been proposed as putative antipsychotic drugs. Recently, brain-penetrating NT analogs produced by stability-enhancing modification of the smallest NT fragment, NT(8-13), have demonstrated antipsychotic-like efficacy after acute systemic injection in several preclinical animal tests predictive for antipsychotic efficacy. However, the evidence regarding the persistence versus tolerance of these effects after repeated administration is ambiguous. Previous studies have used compounds that nonselectively activated both NT1 and NT2 receptors or used continuous slow, central infusion of doses rather than daily acute administration, both factors which may have contributed to the ambiguity in the literature regarding the emergence of tolerance.

OBJECTIVES

To determine if tolerance develops to the antipsychotic-like effects of NT1 receptor agonists, we investigated the effects of subchronic daily systemic administration of PD149163, a brain-penetrating NT analog with selectivity for the NT1 receptor, on amphetamine-induced locomotor activation, a classic preclinical test of antipsychotic efficacy.

MATERIALS AND METHODS

Sprague-Dawley rats were pretreated with eight consecutive daily subcutaneous (SC) injections of PD149163 or saline. On the ninth day, rats received a pair of SC injections consisting of PD149163 or saline, followed by amphetamine (0.5 mg/kg) or saline. Locomotor activity was then measured in photobeam-equipped cages.

RESULTS

The results indicated that repeated daily administration of PD149163 was able to antagonize amphetamine's locomotor-activating effect comparable to that of the first dose, despite that repeated administration of PD149163 produced an increase in baseline locomotor activity not seen after the first dose.

CONCLUSIONS

The results do not support the development of tolerance for the acute antipsychotic-like effect of NT1 agonists and thus lend support to the contention that NT1 agonists are viable candidates as putative novel antipsychotic drugs.

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

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

Determination of extracellular release of neurotensin in discrete rat brain regions utilizing in vivo microdialysis/electrospray mass spectrometry

In vivo microdialysis was used together with structure-specific high sensitivity nano-flow capillary liquid chromatography/micro-electrospray mass spectrometry to quantify and compare extracellular neurotensin from discrete regions of the rat brain. Microdialysis probes were implanted in the hypothalamus or globus pallidus/ventral pallidum in unanesthetized freely moving animals. Utilizing this specific methodology, recovered basal levels of neurotensin were detectable in hypothalamus and globus pallidus/ventral pallidum. The basal level of neurotensin in these regions were slightly higher in hypothalamus (101+/-11 amol/10 microl, n=6) compared to those in the globus pallidus/ventral pallidum region (74+/-12 amol/10 microl, n=8) in samples collected for 30 min at a flow-rate of 0.4 microl/min 150-180 min after the microdialysis probe implantation. After a pulse of 1.0 microl of 100 mM KCl-containing artificial cerebrospinal fluid during the next 30-min sampling period (180-210 min), the recovered neurotensin increased in hypothalamus and globus pallidus/ventral pallidum by 544% (548+/-90 amol/10 microl) and 674% (499+/-99 amol/10 microl), respectively. The basal levels of endogenously released neurotensin in the hypothalamus and globus pallidus/ventral pallidum were lower in the present study compared to those previously reported in the rat brain using in vivo microdialysis and radioimmunoassays. Our data demonstrate the effectiveness of combining in vivo microdialysis and structure-specific micro-electrospray mass spectrometry for the quantitation of basal and stimulated in vivo levels of endogenous neurotensin (NT) in different brain areas.

Dde-I restriction endonuclease fragmentation: a novel method of generating cDNA probes for in situ hybridization in brain

We present a novel procedure for detection of low- and high-abundance messenger RNAs in the brain by in situ hybridization histochemistry, by using fragmented double-stranded cDNA as molecular probes. The procedure involves digesting the cDNA of interest with the restriction endonuclease from Desulfocibrio desulfuricans (Dde I digestion), followed by random primed labeling, which generates a family of high specific activity cDNA fragments. This procedure is a rapid, straightforward, and reproducible method of obtaining sensitive probes for in situ hybridization and is generally applicable to the analysis of the expression of a large number of genes. Here we report the use of this procedure to prepare probes for the detection of synapsin I, p150Glued, neurotensin, c-fos, and c-jun mRNAs in brain, using both isotopic and non-isotopic labeling methods. Because this procedure does not require complex recombinant DNA manipulations or oligonucleotide design, it should prove useful to the non-molecular biologist examining the expression of genes in the central nervous 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.

Neuropharmacological profile of non-peptide neurotensin antagonists

Neurotensin, an endogenous peptide widely distributed throughout the brain, fulfils neurotransmitter criteria. When administered centrally, neurotensin induces various effects and modulates the activity of the mesolimbic dopamine system. It antagonizes the behavioural action of dopamine in a manner similar, but not identical, to antipsychotic drugs. Neurotensin is even considered to be an endogenous neuroleptic. In fact, microinjection of neurotensin elicits different effects depending on both the dose and the cerebral structures into which the injection is made. Our work on the development of orally-active neurotensin antagonists has led to the identification of SR 48692, the first non-peptide antagonist of the neurotensin receptor, and some analogues. This small molecule reveals a surprising neuropharmacological profile. It antagonizes turning behaviour induced in mice and rats (after striatal or ventral tegmental area administration of neurotensin, respectively), hypolocomotion induced by intracerebroventricular injection of neurotensin in rats, and reverses the inhibitory effect of neurotensin (nucleus accumbens injection) on amphetamine-induced hyperlocomotion in rats. However, SR 48692 cannot reverse either dopamine release in the nucleus accumbens evoked by neurotensin injection in ventral tegmental area, or hypothermia and analgesia induced by intracerebroventricular injection of neurotensin. As direct and indirect dopamine agonists have been reported to promote neurotensin release in the cortex, behavioural studies were performed using injection of apomorphine. In these experiments, SR 48692 inhibited only turning and yawning. It did not antagonize other apomorphine-dependent effects such as climbing, hypothermia, hypo- or hyperlocomotion, penile erection and stereotypies. All together, these data raise the question of the existence of neurotensin receptor subtypes and confirm that the nature of neurotensin and dopamine interactions depends on the brain structures considered.

Autoradiographic and electrophysiological evidence for the existence of neurotensin receptors on cultured astrocytes

By means of autoradiography we have studied the cellular localization of binding sites for [3H]neurotensin and its nonpeptide receptor antagonist [3H]SR-48692 in explant cultures of rat neocortex, striatum, brain stem and spinal cord. Binding sites for the peptide and its antagonist were observed on a great number of astrocytes in all CNS regions studied. Simultaneous staining of the cultures with a monoclonal antibody against glial fibrillary acidic protein has shown that the labelled cells in the outgrowth zone of the cultures were glial fibrillary acidic protein-positive and could therefore be identified as astrocytes. In addition to astrocytes, many neurons and outgrowing nerve fibres were labelled by the radioligands. Binding of [3H]neurotensin and [3H]SR-48692 (10(-8)M) to neurons and glial cells was markedly reduced or inhibited by the unlabelled compounds at high concentration (10(-6)M), suggesting "specific" binding of the radioligands. Electrophysiological studies have shown that addition of neurotensin to the bathing solution caused a hyperpolarization of the majority of astrocytes tested. There was a dose-response relationship between the magnitude of the hyperpolarization and the concentration of the peptide (10(-10)-10(-7)M); 10(-10)M being the threshold concentration. The specificity of the action of neurotensin was confirmed by the selective nonpeptide neurotensin receptor antagonist SR-48692 which reversibly blocked or markedly reduced the hyperpolarization by the peptide on all astrocytes tested. Our electrophysiological findings together with our autoradiographic data provide strong evidence for the presence of specific and functional neurotensin receptors on astrocytes.

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)

Pharmacokinetics of (Me)Arg-Lys-Pro-Trp-tert-Leu-Leu-OEt and (Me)Arg-Lys-Pro-Trp-tert-Leu-Leu administered intravenously to beagle dogs

The pharmacokinetics of two analogues (NT-1 and NT-2) of the smallest active fragment of neurotensin were investigated in beagle dogs after intravenous (iv) administration at the doses of 0.1 and 0.5 mg/kg for NT-1 and of 0.25 and 0.5 mg/kg for NT-2. After iv administration of these two drugs, the plasma levels decreased with time with a biexponential pattern, and linear kinetic behavior was observed. The pharmacokinetic parameters (mean +/- standard error of mean) after iv administration of NT-1 at 0.1 mg/kg were as follows: the half-life of the distribution phase (t1/2 alpha) was 0.29 +/- 0.11 h, the half-life of the terminal phase (t1/2 beta) was 1.99 +/- 0.41 h, total plasma clearance (CL) was 33.2 +/- 6.3 mL/h/kg, steady-state volume of distribution (Vdss) was 57.1 +/- 2.8 mL/kg, and mean residence time (MRT) was 2.20 +/- 0.55 h. The t1/2 alpha, t1/2 beta, CL, Vdss, and MRT values after iv administration of NT-2 at 0.25 mg/kg were 0.11 +/- 0.02 h, 0.58 +/- 0.04 h, 257.3 +/- 17.2 mL/h/kg, 180.3 +/- 4.6 mL/kg, and 0.71 +/- 0.04 h, respectively. After iv administration of NT-1, NT-2 was detected in the plasma. It was confirmed from this result that NT-1 was metabolically hydrolyzed to NT-2 in the body of beagle dog.

Autoradiographic distribution of [3H]neurotensin receptors in the brains of superoxide dismutase transgenic mice

Superoxide dismutase (SOD) plays an important role in the protection of cells against the deleterious effects of free radicals by dismutating the toxic superoxide anion radical. Although oxygen-based radicals have been implicated in the process of aging and in neurodegenerative disorders such as Parkinson's disease, the contribution of these free radicals to the pathology of these entities has yet to be clarified. It is also not certain that increased levels of free radical scavenging enzymes would attenuate the molecular and cellular processes that lead to these pathological states. In order to assess the contribution of increased SOD gene dosage to the pathogenesis of Down's syndrome, transgenic mice have been constructed that overexpress the human CuZnSOD. We are also using this model to evaluate the role of free radicals in age-associated changes in brain neurotransmitters and their receptors. In the present study, transgenic mice and their nontransgenic littermates, aged 6 weeks and 21 months, were used in an autoradiographic receptor study of the distribution of brain neurotensin receptors. At 6 weeks of age, there were no significant differences between the two groups of mice in most brain regions. In addition, [3H]NT binding sites showed parallel age-related decreases in the majority of the areas examined in both groups. However, significant age-related decreases in the septum, the diagonal band of Broca, and in some subdivisions of the caudate-putamen were observed only in SOD-Tg mice. In contrast, significant age-related decreases in the core area of the nucleus accumbens and the dorsal aspect of the dentate gyrus of the hippocampus were seen only in non-Tg mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotensin increases extracellular striatal dopamine levels in vivo

This study employed intracranial microdialysis to assess the effects of neurotensin (NT) infusion on extracellular dopamine (DA) and DA metabolite concentrations in the rat striatum and nucleus accumbens, and the effects of NT on alterations in extracellular DA levels induced by cocaine and the DA D-2 receptor agonist, quinpirole. Direct NT infusion (.10, 1.0, 10.0 microM) did not significantly affect extracellular DA in the nucleus accumbens, but did produce a significant increase in the DA metabolite homovanillic acid (HVA). In contrast, direct NT infusion produced an increase in striatal DA levels, without altering DA metabolites. Neurotensin infusion (.10 microM) into the striatum significantly attenuated the peak DA increase induced by an intraperitoneal (IP) injection of a low dose (10.0 mg/kg) but not a high dose (30.0 mg/kg) of cocaine. Neurotensin infusion (.10 microM) did not affect the decrease in DA and its metabolites induced by an IP injection of a low dose of quinpirole (.03 mg/kg), but did alter the decrease in HVA induced by a high dose of quinpirole (.10 mg/kg). These results suggest that NT differentially affects in vivo DA release in the striatum and nucleus accumbens, and further strengthens the assertion that NT is an important modulator of dopaminergic function.

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

Anatomical, biochemical and behavioral data suggest functional interactions between dopamine and neurotensin in regions of the brain receiving a co-existent and/or distinct innervation by these two transmitters. We therefore measured and compared the effects of iontophoretically applied dopamine and neurotensin in the prefrontal and anterior cingulate cortex (co-existent innervation) vs the nucleus accumbens and neostriatum (distinct innervation) of urethane-anesthetized rats. In every region, the firing rate of most spontaneously active neurons was depressed by dopamine. Neurotensin had no effect on the same cells, except for a few nucleus accumbens units which were inhibited by the peptide. When dopamine and neurotensin were concomitantly applied, the magnitude of maximal inhibitions induced by dopamine was modified in the majority of neurons tested. A significant decrease in dopamine inhibition was observed in 100% of anterior cingulate, 74% of prefrontal cortex and 48% of accumbens units. On the contrary, in neostriatum, dopamine inhibition was significantly increased in 60% of the units tested. In every region, the remaining neurons showed less than 30% changes in dopamine responsiveness, and were therefore considered unaffected by neurotensin. In the anterior cingulate cortex, inhibitions, respectively, induced by the dopamine D1 agonist, SKF 38393, and the D2 agonist, LY 171555, were also decreased by simultaneous application of neurotensin. Together with currently available data on the cellular localization of neurotensin receptors in rat brain, these results suggest that the modulation of dopamine inhibition by neurotensin may have opposite effects depending on whether the neurotensin receptors are located postsynaptically on target neurons (antagonistic effects) or presynaptically on dopamine terminals (potentiating effects).

Muscarinic antagonists attenuate neurotensin-stimulated accumbens and striatal dopamine metabolism

The effect of scopolamine and atropine upon the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by central injection of neurotensin was examined in the nucleus accumbens and the striatum of anaesthetized rats using in vivo differential pulse voltammetry with carbon fibre electrodes. Scopolamine (1 and 3 mg/kg, i.p.) and atropine (20 micrograms, i.c.v.) did not alter the 3,4-dihydroxyphenylacetic acid level in the nucleus accumbens or the striatum, measured for 60 min after administration. Neurotensin (10 micrograms, i.c.v.) increased the 3,4-dihydroxyphenylacetic acid peak height in both regions. Pretreatment with scopolamine (1 mg/kg) 15 min before neurotensin injection blocked the increase in extracellular 3,4-dihydroxyphenylacetic acid in the striatum but not in the nucleus accumbens whilst scopolamine (3 mg/kg) partially attenuated the effect of neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid in the striatum. Atropine partially attenuated the effect produced by neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid induced by the peptide in the striatum. However, the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by haloperidol (1 mg/kg, s.c.) was not altered by scopolamine (1 mg/kg) or atropine. Also, the increase in dopamine metabolism in the nucleus accumbens and the striatum after centrally injected haloperidol (10 micrograms, i.c.v.) was not altered by atropine (20 micrograms, i.c.v.). Together, the results demonstrate a functional interaction between muscarinic antagonists and neurotensin on in vivo dopamine metabolism in the nucleus accumbens and the striatum but with a greater effect in the latter region.

Characterization of neurotensin binding sites on rat mesencephalic cells in primary culture

Many studies have reported the presence of high amounts of neurotensin (NT) binding sites in the mesencephalon of adult rat, and their possible role in mediating the effects of the peptide on the activity of mesencephalic dopaminergic neurons. In the present study, we demonstrate the presence of NT sites in primary cultures of embryonic rat mesencephalic cells. On these cells, a single class of high affinity 125I-NT binding sites was observed. The value of the apparent affinity constant (0.3 nM) did not show any significant change throughout time, from 3 to 14 days in culture. The number of sites, however, increased until day 11 and decreased thereafter. Acetylneurotensin (8-13), NT and neuromedin N were potent competitors of 125I-NT binding, while NT (1-10), NT (1-11) and levocabastine were uneffective. These results indicate that the sites detected in the mesencephalic cultures share common binding properties with the high-affinity NT sites already described in adult rat brain. The neuronal localization of the NT sites was suggested by their presence in neuron-enriched serum-free cultures and their absence in glial cultures. Autoradiographic studies confirmed the cellular localization of NT sites and indicated that, under our experimental conditions, cells labeled by 125I-NT represented 0.14% of the initially plated cell number. Taken together, these results show that the development of mesencephalic neurons in primary culture is associated with an increased expression of NT binding sites. Since such cultures have been shown previously to contain functional dopaminergic neurons, we suggest that they could provide a good model to investigate the modulation of the activity of these neurons by NT.

Neurotensin effects on calcium/calmodulin-dependent protein phosphorylation in rat neostriatal slices

Neurotensin (NT) is an endogenous brain tridecapeptide for which high affinity binding sites exist in the central nervous system. We have investigated the effects of NT incubation with rat neostriatal slices on calcium/calmodulin (Ca/CaM)-dependent protein phosphorylation. Slices were incubated with NT (5 or 50 nM) for 3, 10, 16 or 30 min followed by in vitro phosphorylation, electrophoresis and autoradiography. NT significantly altered the phosphorylation of a 62 kDa protein which is likely the beta subunit of the Ca/CaM dependent protein kinase. These changes may reflect the ability of NT to influence calcium mediated signal transduction.

Use of amfonelic acid to discriminate between classical and atypical neuroleptics and neurotensin: an in vivo voltammetric study

Previous ex vivo studies have shown that the non-amphetamine stimulant amfonelic acid potentiates the increase in DOPAC induced by classical but not by atypical neuroleptics. In the present study, we have demonstrated that this neurochemical model can be used to discriminate typical from atypical neuroleptics in vivo using differential pulse voltammetry with carbon fibre electrodes. The study also compared the effect of intracerebroventricular (i.c.v.) administration of neurotensin, on extracellular striatal DOPAC following amfonelic acid, with the effects of both classical and atypical neuroleptics. Saline or amfonelic acid (2.5 mg/kg s.c.) were administered; followed 5 min later by the classical neuroleptics haloperidol, perphenazine, or the atypical neuroleptics clozapine, thioridazine, or by neurotensin. After drug administration extracellular striatal DOPAC was recorded every 5 min for 90 min. Amfonelic acid did not alter basal striatal DOPAC but potentiated the increase in DOPAC induced by haloperidol (1.0 and 0.05 mg/kg s.c.) and perphenazine (10 mg/kg s.c.). Both clozapine (30 mg/kg i.p.) and thioridazine (20 mg/kg s.c.) increased extracellular DOPAC, but pretreatment with amfonelic acid prevented the increase in DOPAC produced by both drugs. Neurotensin (10 micrograms, i.c.v.), in a similar manner to the atypical neuroleptics, increased extracellular DOPAC in the striatum and the effect was prevented by amfonelic acid. The present study demonstrates that pretreatment with amfonelic acid is a valuable tool to discriminate between classical and atypical neuroleptics in vivo. The results also indicate that neurotensin in the presence of amfonelic acid has a profile similar to the atypical neuroleptics.

Neurotensin regulation of endogenous acetylcholine release from rat striatal slices is independent of dopaminergic tone

The effects of neurotensin (NT) alone or in combination with the dopamine antagonist sulpiride were tested on the release of endogenous acetylcholine (ACh) from striatal slices. NT enhanced potassium (25 mM)-evoked ACh release from striatal slices in a dose-dependent manner. This effect was tetrodotoxin-insensitive, suggesting an action directly on cholinergic elements. The dopamine antagonist sulpiride (5 x 10(-5) M) significantly increased (63%) potassium-evoked ACh release from striatal slices; potassium-evoked ACh release was further increased (90%) in the presence of NT (10(-5) M) and sulpiride (5 x 10(-5) M). The second set of experiments tested the effects of 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra on NT-induced increases of potassium-evoked ACh release. These lesions did not alter the NT regulation of potassium-evoked ACh release from striatal slices, but did significantly increase spontaneous (33%) and potassium-evoked (40%) ACh release from striatal slices. Striatal choline acetyltransferase activity was not affected by 6-OHDA lesions. In addition, following 6-OHDA lesions, sulpiride was ineffective in altering ACh release from striatal slices. Furthermore, evoked ACh release in the presence of the combination of NT and sulpiride was not different from that in the presence of NT alone. These results suggest that in the rat striatum, NT regulates cholinergic interneuron activity by interacting with NT receptors associated with cholinergic elements. Moreover, the NT modulation of cholinergic activity is independent of either an interaction of NT with D2 dopamine receptors or the sustained release of dopamine.

Neurotensin causes a greater increase in the metabolism of dopamine in the accumbens than in the striatum in vivo

Differential pulse voltammetry with carbon fibre electrodes was used to study the effect of central administration of neurotensin on the extracellular level of 3,4-dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens and the striatum in anaesthetised rats. Intracerebroventricular injection of neurotensin (10 micrograms) increased the peak height for DOPAC 20 min after administration in the nucleus accumbens but only after 40 min in the striatum. The maximum increase was similar in both regions, with 30% and 27% above the pre-injection basal level, respectively. Neurotensin (1 micrograms) however increased the extracellular level of DOPAC in the nucleus accumbens alone. Neurotensin (0.1, 1.0 and 3.0 micrograms/0.5 microliter), injected into the ventral tegmental area, induced a potent and long-lasting elevation of the peak height for DOPAC in the nucleus accumbens, while the same doses in the substantia nigra produced effects on the metabolism of dopamine in the striatum of smaller amplitude and shorter duration. The maximum effect of each dose was about 2.5 times greater in the mesolimbic, compared to the nigrostriatal system. Amphetamine (2 mg/kg, s.c.) decreased the extracellular level of DOPAC with a similar magnitude, both in the nucleus accumbens (52%) and the striatum (47%). Intracerebroventricular administration of neurotensin (1 micrograms), 5 min after amphetamine, did not alter the effect of amphetamine on the extracellular level of DOPAC either in the nucleus accumbens or the striatum. However, neurotensin (10 micrograms) partially reversed the effect of amphetamine in the nucleus accumbens and had a similar but smaller and delayed effect in the striatum. The results from the present study, together with previous neurobehavioural studies, suggest that neurotensin has a relatively selective action on the mesolimbic dopaminergic system in the rat.

Structure-activity studies of neurotensin on muscular rigidity and tremors induced by 6-hydroxydopamine lesions in the posterolateral hypothalamus of the rat

It has previously been reported that intracerebroventricular administration of neurotensin (30 micrograms) reduced muscular rigidity and tremors, induced by a neurochemical lesion with 6-hydroxydopamine in the posterolateral hypothalamus of rats. In the present study, the effects of two fragments (NT1-10 and NT8-13) and two analogues ([D-Tyr11]-NT and [Ala11]-NT) of neurotensin on the grasping time (index of muscle rigidity) and tremors in 6-hydroxydopamine-lesioned rats are reported. Intracerebroventricular administration with 120 micrograms of NT1-10 and [Ala11]-NT had no effect on the muscle rigidity and tremors induced by the neurochemical lesion. The administration of NT8-13 60 micrograms) significantly attenuated both behavioural responses. The analogue [D-Tyr11]-NT produced a much greater attenuation of the muscle rigidity and tremors. The dose of 1.8 micrograms of [D-Tyr11]-NT significantly reduced the grasping time, while the number of tremors was attenuated with the threshold dose of 0.9 micrograms. Together, these results suggest that the effects of neurotensin on muscle rigidity and tremors, induced by pretreatment with 6-hydroxydopamine injected into the posterolateral hypothalamus, were not caused by non-specific effects but largely depended on the carboxy terminal of the peptide. The tyrosine residue in position 11 of the molecule plays a critical role in the action of neurotensin, as shown with the high potency and duration of action of the analogue [D-Tyr11]-NT. As previously suggested, the greater effect with [D-Tyr11]-NT may be due to greater resistance of the analogue to enzymatic degradation because of the incorporation of the D-Tyr amino acid, in position 11 of neurotensin.