Neurotensin reduces the affinity of dopamine D2 receptors in membranes from post mortem human caudate-putamen

Continuous versus extended antipsychotic dosing in schizophrenia: Less is more

Antipsychotic drugs temper psychotic symptoms by interacting with dopamine D2 receptors to reduce dopamine neurotransmission. Currently, the standard of care involves antipsychotic treatment protocols that achieve steady-state levels of medication. Maintaining patients on continuous treatment is thought to be necessary to keep them stabilised. However, continuous antipsychotic exposure increases the risk of adverse effects over time. These effects include metabolic and cardiovascular disorders, extrapyramidal complications, and dopamine receptor supersensitivity, the latter of which could potentially promote both treatment tolerance and psychosis relapse. In the present review, we describe evidence showing that continuous exposure to antipsychotic drugs can not only worsen long-term outcome, but-past acute phase treatment-it is also unnecessary to effectively manage schizophrenia symptoms. We also describe evidence that regular but extended dosing, allowing predictable periods of lower antipsychotic levels/D2 occupancy, is both safe and effective in patients, and it greatly reduces drug exposure overall. Studies in laboratory animals show that compared to continuous antipsychotic exposure, regular but extended dosing actually has superior antipsychotic-like efficacy, and it also substantially reduces the likelihood of both motor side effects and dopamine receptor supersensitivity. We propose that regular, but extended dosing should be considered in the long-term treatment of people with schizophrenia, because the available evidence suggests it can be just as effective as continuous treatment, while decreasing overall drug exposure and potentially reducing harmful side effects.

Selective agonists for dopamine/neurotensin receptor heterodimers

The neuromodulatory peptide neurotensin has been described to functionally interact with dopaminergic pathways of the human brain. We employed radioligand binding studies to investigate the physical interaction between co-expressed dopamine D(2L) or D₃ and neurotensin NTS₁ or NTS₂ receptors. Substantial cross-inhibitory effects of both receptor subtypes NTS(1) and NTS₂ on the agonist binding of D(2L) or D₃ were detected in the presence of neurotensin. To identify ligand-specific modulation and subtype-dependent differences, the novel dopamine receptor agonists 5 and 6 bearing the 7-OH-DPAT pharmacophore were synthesized. Exceptional ligand specificity was observed for D₃-NTS₂ co-expression, which gave a 20-fold decrease in affinity for biphenylcarboxamide 5 in the presence of neurotensin. Comparing the binding properties of dopaminergic compounds in the presence of neurotensin, dopamine receptor subtype-selective profiles of the cross-inhibitory effect of neurotensin were observed.

Cross-receptor interactions between dopamine D2L and neurotensin NTS1 receptors modulate binding affinities of dopaminergics

Dopaminergic systems have been described to functionally interact with the neuromodulatory peptide neurotensin. Employing fluorescence detected coimmunoprecipitation and radioligand binding experiments, we herein demonstrate that coexpression of dopamine D(2L) receptor and the neurotensin receptor subtype NTS(1) leads to physical interaction and the formation of heteromers in transfected human embryonic kidney 293 cells. In this in vitro system, a trans-inhibitory effect on the agonist binding affinity of D(2) was observed in presence of neurotensin. To correlate between the functional properties of dopaminergic agents and the magnitude of neurotensin-induced modulation of D(2L) binding affinities in cells coexpressing D(2L) and NTS(1), a structurally diverse set of dopamine receptor agonists, partial agonists, and antagonists was tested. Ligand specific profiles indicating substantial bias between ligand efficacy and transmodulation were discovered, suggesting a heteromerization-based functional selectivity. In the presence of neurotensin, the novel D(2) agonist FAUC 326 displayed a 34-fold decrease of binding affinity in cells coexpressing D(2L) and NTS(1).

Measuring endogenous 5-HT release by emission tomography: promises and pitfalls

Molecular in vivo neuroimaging techniques can be used to measure regional changes in endogenous neurotransmitters, evoked by challenges that alter synaptic neurotransmitter concentration. This technique has most successfully been applied to the study of endogenous dopamine release using positron emission tomography, but has not yet been adequately extended to other neurotransmitter systems. This review focuses on how the technique has been applied to the study of the 5-hydroxytryptamine (5-HT) system. The principles behind visualising fluctuations in neurotransmitters are introduced, with reference to the dopaminergic system. Studies that aim to image acute, endogenous 5-HT release or depletion at 5-HT receptor targets are summarised, with particular attention to studies in humans. Radiotracers targeting the 5-HT(1A), 5-HT(2A), and 5-HT(4) receptors and the serotonin reuptake transporter have been explored for their sensitivity to 5-HT fluctuations, but with mixed outcomes; tracers for these targets cannot reliably image endogenous 5-HT in humans. Shortcomings in our basic knowledge of the mechanisms underlying changes in binding potential are addressed, and suggestions are made as to how the selection of targets, radiotracers, challenge paradigms, and experimental design might be optimised to improve our chances of successfully imaging endogenous neurotransmitters in the future.

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

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

Nigral neurotensin receptor regulation of nigral glutamate and nigroventral thalamic GABA transmission: a dual-probe microdialysis study in intact conscious rat brain

Dual-probe microdialysis in the awake rat was employed to investigate the effects of intranigral perfusion with the tridecapeptide neurotensin on local dialysate glutamate and GABA levels in the substantia nigra pars reticulata and on dialysate GABA levels in the ventral thalamus. Intranigral neurotensin (10-300nM, 60min) dose-dependently increased (+29+/-3% and +46+/-3% vs basal for the 100 and 300nM concentrations, respectively) local dialysate glutamate levels, while the highest 300nM concentration of the peptide exerted a long-lasting and prolonged reduction in both local and ventral thalamic (-20+/-4% and -22+/-2%, respectively) GABA levels. Intranigral perfusion with the inactive neurotensin fragment neurotensin(1-7) (10-300nM, 60min) was without effect. Furthermore, the non-peptide neurotensin receptor antagonist SR 48692 (0.2mg/kg) and tetrodotoxin (1microM) fully counteracted the intranigral neurotensin (300nM)-induced increase in local glutamate. SR 48692 (0.2mg/kg) also counteracted the decreases in nigral and ventral thalamic GABA release induced by the peptide. In addition, intranigral perfusion with the dopamine D(2) receptor antagonist raclopride (1microM) fully antagonized the neurotensin (300nM)-induced decreases in nigral and ventral thalamic GABA levels. The ability of nigral neurotensin receptor activation to differently influence glutamate and GABA levels, whereby it increases nigral glutamate and decreases both nigral and ventral thalamic GABA levels, suggests the involvement of neurotensin receptor in the regulation of basal ganglia output at the level of the nigra.

Thalamostriatal projection neurons in birds utilize LANT6 and neurotensin: a light and electron microscopic double-labeling study

Based on its location, connectivity and neurotransmitter content, the dorsal thalamic zone in birds appears to be homologous to the intralaminar, midline, and mediodorsal nuclear complex in the thalamus of mammals. We investigated the neuroactive substances used by thalamostriatal projection neurons of the dorsal thalamic zone in the pigeon. Single-labeling experiments showed that many neurons in the dorsal thalamic zone are immunoreactive for neurotensin and the neurotensin-related hexapeptide, (Lys8,Asn9)NT(8-13) (LANT6). Double-labeling experiments, using the retrograde fluorescent tracer, FluoroGold, combined with fluorescence immunocytochemistry for either LANT6 or neurotensin, showed that neurotensin- and LANT6-containing neurons in the dorsal thalamic zone project to the striatum of the basal ganglia. Immunofluorescence double-labeling experiments showed that neurotensin and LANT6 are often (possibly always) co-expressed in neurons in the dorsal thalamic zone. Electron microscopic immunohistochemical double-labeling showed that LANT6 terminals in the striatum make asymmetric contacts with heads of spines labeled for substance P and heads of spines not labeled for substance P, suggesting that these terminals synapse with both substance P-containing and non-substance P-containing medium spiny striatal projection neurons. These findings indicate that LANT6 and neurotensin may be utilized as neurotransmitters in thalamostriatal projections in birds and raise the possibility that this may also be the case in other amniotes.

Blockade of neurotensin-induced motor activity by inhibition of protein kinase

The administration of neurotensin into the ventral tegmental area stimulates dopamine neurons and locomotor activity. Furthermore, when neurotensin is microinjected daily into the ventral tegmental area the motor stimulant response increases. The role of protein kinases in the motor stimulant effect of neurotensin was evaluated by coadministration of the protein kinase inhibitors H8 and H7 into the ventral tegmental area with neurotensin. It was found that the acute motor stimulant effect of neurotensin was abolished in a dose-dependent fashion by H8 coadministration. Neurotensin-induced activity was also blocked by H7. However, acute motor stimulation following microinjection of the mu opioid, Tyr-d-Ala-Gly-MePhe-Gly(ol) or the potassium channel antagonist apamin into the ventral tegmental area was not affected by coadministration with H8. The behavioral sensitization produced by daily neurotensin microinjection into the ventral tegmental area was also prevented by the coadministration of H8. These data indicate that the motor stimulation produced by acute and repeated neurotensin microinjection into the ventral tegmental area is dependent upon activation of protein kinase(s). Furthermore, Tyr-d-Ala-Gly-MePhe-Gly(ol) and apamine elicit locomotion independently of protein kinase(s).

Receptor-receptor interactions as an integrative mechanism in nerve cells

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

Neuromedin N is a potent modulator of dopamine D2 receptor agonist binding in rat neostriatal membranes

In the concentration range of 1-10 nM, neuromedin N produced a significant concentration-related increase in the Kd values of [3H]L-(-)-N-propylnorapomorphine binding sites in rat neostriatal membranes with a peak action at 10 nM (36% increase versus the control group mean value). The Bmax values were not affected by neuromedin N. Neurotensin at 10 nM induced an increase in the Kd values, which was not affected by a threshold concentration of neuromedin N (0.1 nM). In view of the higher potency of neuromedin N versus neurotensin to modulate neostriatal D2 receptors in contrast to the higher potency of neurotensin versus neuromedin N to bind to the cloned neurotensin receptors, it seems possible that the neuromedin N activated neostriatal neurotensin receptors controlling the D2 receptors represent a distinct subtype of neurotensin receptors.

Intramembrane receptor-receptor interactions: integration of signal transduction pathways in the nervous system

During recent years a large number of observations have been made indicating that neuropeptides and other transmitters in various brain areas can regulate the affinity of monoamine receptors via the activation of their own receptors. These "receptor--receptor interactions" can either take place at the plasma membrane level or use intracytoplasmatic loops. This review is mainly focused on the evidence for hetero-regulation of dopamine (DA) D2 receptors in the basal ganglia. The existence of such receptor--receptor interactions increases the plasticity of transmission and opens up the possibility of developing new drugs which indirectly modulate receptor recognition and decoding processes. This would avoid the use of direct receptor agonists or antagonists which induce major side effects such as tolerance and abstinence. Disturbances in the receptor--receptor interactions, including DA D2 receptors, may be involved in the development of neurological and mental diseases such as schizophrenia.

Intramembrane interactions between neurotensin receptors and dopamine D2 receptors as a major mechanism for the neuroleptic-like action of neurotensin

Evidence has been presented that behavioral actions of NT, inducing its neuroleptic-like action, can be explained on the basis of NT-D2 intramembrane receptor-receptor interactions in the basal ganglia, unrelated to the coexistence phenomenon, leading to reduced affinity and transduction of the D2 agonist binding site. By reducing selectively D2 receptor transduction at the pre- and postsynaptic level, the NT receptor appears capable of switching the DA synapses towards a D1 receptor-mediated transduction, illustrating how receptor-receptor interactions can increase the functional plasticity of central synapses (FIG. 12).

Neurotensin and cholecystokinin octapeptide control synergistically dopamine release and dopamine D2 receptor affinity in rat neostriatum

Combined perfusion of the neostriatum with 1 nM of cholecystokinin octapeptide (CCK-8) and 0.01, 0.1 or 1 nM of neurotensin was done in the halothane-anesthetized rat after systemic apomorphine treatment (0.05 mg/kg, s.c.). Neurotensin (1 nM) plus CCK-8 (1 nM) effectively counteracted the apomorphine-induced inhibition of neostriatal perfusate levels of dopamine (DA). With a constant concentration of CCK-8 (1 nM), the apomorphine-induced inhibition of DA release was counteracted dose relatedly by neurotensin in concentrations of 0.01, 0.1 and 1 nM. The results of binding experiments demonstrated that threshold concentrations of CCK-8 and neurotensin significantly increased the KD values of the high-affinity D2 receptors without significant alterations in the low-affinity D2 receptors or in the proportion of D2 receptors in the high-affinity state. Thus, neurotensin and CCK receptors may regulate synergistically, via intramembrane interactions with the D2 receptors, the binding characteristics and the signal transduction of D2 autoreceptors in the neostriatum. The combined presence of very low concentrations of CCK-8 and neurotensin in the extracellular fluid may be sufficient to regulate D2 receptor transduction, underlining the important role of these peptide receptor interactions with the D2 receptors.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Brain neuropeptides: changes by treatment with the convulsants pentylenetetrazole and bicuculline

1. The effects of chemically induced convulsions, clinically similar to those elicited by electroconvulsive treatment (ECT), on brain regional distribution of neuropeptide Y-, neurokinin A-, substance P- and neurotensin-like immunoreactivities were studied in the rat. 2. Pentylenetetrazole (PTZ) and bicuculline (BIC) were used to induce grand mal seizures. Rats were divided into three groups receiving one of the following treatments: Saline, PTZ (45 mg/kg) or BIC (1.5 mg/kg). 3. After sacrifice by focused microwave irradiation, brains were dissected, peptides extracted and measured by specific radioimmunoassays. 4. Repeated grand mal convulsions induced by PTZ, in similarity to ECT, markedly increased NPY-LI concentrations in frontal cortex and hippocampus. In contrast to ECT, no changes in NKA- or SP-LI levels were seen. NT-LI was lowered in striatum. 5. Bicuculline effects were more circumscribed: some animals developed grand mal and died while convulsing (peptides not measured), others did not develop generalized seizures and were sacrificed after the fourth treatment. 6. The results demonstrate a similar effect of PTZ and ECT on regional NPY-LI concentrations and raise the possibility that grand mal, regardless of etiology, is necessary for effects on peptides.

Coactivation of dopamine D1 and D2 receptors increases the affinity of cholecystokinin-8 receptors in membranes from post-mortem human caudate-putamen

The effects of dopamine in vitro were investigated on the binding sites for cholecystokinin-8 (sulphated, CCK-8) and neurotensin in membrane preparations of the caudate-putamen and nucleus accumbens of post-mortem human brains. Dopamine reduced the IC50 value of competition curves with CCK-8 for [125I]CCK-8 binding in membranes from the caudate-putamen, but not the nucleus accumbens, with a maximal decrease of -25 +/- 9% at 300 nM of dopamine. This decrease could be antagonized by 100 nM of SCH 23390 or 100 nM of raclopride. Kinetic analysis of [125I]CCK-8 binding showed a decrease in the first order dissociation rate constant and in the kinetic Kd (-22 +/- 6% and -24 +/- 6%, respectively) at 300 nM of dopamine, without any significant effect on the apparent or actual association rate constant. Competition curves with neurotensin versus [125I]neurotensin were not affected by dopamine (10-1000 nM) in membranes from the caudate-putamen or the nucleus accumbens. These results suggest that dopamine, by synergistic stimulation of both D1 and D2 receptors, selectively increases the affinity of CCK-8 receptors in the human caudate-putamen, by a selective inhibition of ligand dissociation. This increase may reflect a positive feed-back mechanism, further enhancing the modulatory effects of CCK-8 on dopamine neurotransmission.

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.

Neurotensin modulates K(+)-stimulated dopamine release from the caudate-putamen but not the nucleus accumbens of mice with differential sensitivity to ethanol

Slices of caudate-putamen (CP) and nucleus accumbens (NA) prepared from Long-Sleep (LS) and Short-Sleep (SS) mice were used to determine the effects of neurotensin (NT) and ethanol on K(+)-stimulated 3H-dopamine (3H-DA) release and to test the hypothesis that ethanol acts, in part, via NT receptor-mediated processes. Slices prepared from either LS or SS CP or NA did not differ in submaximal (25 mM) K(+)-stimulated 3H-DA release but 60 mM K+ induced significantly greater 3H-DA release from LS CP slices compared with SS CP slices. NT had no effect on unstimulated 3H-DA overflow but enhanced 25 mM K(+)-stimulated 3H-DA release from slices of the CP of both lines of mice. Augmentation of DA release by NT from caudate slices was concentration dependent and tetrodotoxin (TTX) insensitive, implicating a role of presynaptic neurotensin receptors located on nigrostriatal DA neurones. In contrast, NT did not enhance K(+)-stimulated 3H-DA release from NA slices from either line of mice. The absence of an NT effect in NA slices was not due to a rapid desensitization of NT receptors but the data were consistent with the absence of presynaptic NT receptors on dopaminergic terminals in the NA. Between-line differences were observed in the effect of ethanol on NT enhancement of 25 mM K(+)-stimulated 3H-DA release from CP slices. Ethanol (100 mM) applied concomitantly with NT blocked the NT enhancement of 3H-DA release from CP slices of LS but not SS mice.(ABSTRACT TRUNCATED AT 250 WORDS)

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 decreases the affinity of dopamine D2 agonist binding by a G protein-independent mechanism

To examine whether GTP-binding proteins (G proteins) mediate the ability of neurotensin to lower the affinity of dopamine D2 agonist binding, the modulation by neurotensin in vitro of N-[3H]propylnorapomorphine [( 3H]-NPA) binding was investigated following pretreatment with pertussis toxin and N-ethylmaleimide in rat neostriatal membranes. Preincubation with N-ethylmaleimide (100 microM) markedly inhibited pertussis toxin-induced back-ADP ribosylation of three proteins with apparent molecular masses of 41, 40, and 39 kDa, respectively. This inhibition was prevented by adding dithiothreitol (250 microM) during the preincubation. N-Ethylmaleimide increased the KD (180 +/- 30%) and decreased the Bmax (-31 +/- 9%) of [3H]NPA binding sites but did not affect the binding properties of the selective D2 antagonist [3H]raclopride. N-Ethylmaleimide pretreatment did not affect the neurotensin (3 nM)-induced increase in the KD of [3H]NPA binding sites. Pertussin toxin treatment in vivo and in vitro was similarly ineffective. In conclusion, the present study indicates that neurotensin modulation of D2 agonist binding in neostriatal membranes is not mediated by G proteins.

The electrophysiological actions of neurotensin in the central nervous system

The endogenous neuropeptide, neurotensin (NT) alters the firing frequencies of certain neurons in the central nervous system (CNS). This is one of the findings that support the hypothesis that NT is a neurotransmitter substance. The direct application of NT on CNS neurons causes predominantly excitatory effects. These effects occur in a dose-related fashion via a calcium-dependent postsynaptic mechanism. The C-terminal hexapeptide fragment, NT 8-13 exerts similar electrophysiological effects to NT, while the N-terminal octapeptide fragment, NT 1-8 is devoid of such activity. NT produces a significant increase in the firing rates of individual neurons in the substantia nigra (SN), ventral tegmental area (VTA), medial prefrontal cortex (MPF), hypothalamus, and periaqueductal grey (PAG). This excitation occurs with a rapid onset and is readily reversible after cessation of NT application. In contrast, NT has no effect or weak inhibitory effects on the firing rates of neurons in the locus coeruleus (LC) and cerebellum. These electrophysiological actions of NT appear to be unique and not shared by other neurotransmitter and neuropeptide receptor antagonists and agonists that have been studied via direct co-application. NT attenuates dopamine (DA)-induced inhibition associated with direct application onto neurons in the SN and VTA both in vivo and in vitro. Intracellular recordings suggest that direct application of higher concentrations of NT appears to produce 'depolarization block' on individual neurons in the SN, VTA, MPF, and hypothalamus. The electrophysiological consequences of NT application not only show similarities to clinically efficacious antipsychotic medications, but also demonstrate the ability of NT to modulate the activity of dopamine (DA) neurons at the cellular level via specific NT binding sites. These findings further underscore the possibility that NT may play a pre-eminent role in the pathogenesis of, and psychopharmacological management of neurological and psychiatric disorders purportedly related to perturbation of CNS DA systems including schizophrenia.

Intraventricular injection of neurotensin reduces dopamine D2 agonist binding in rat forebrain and intermediate lobe of the pituitary gland. Relationship to serum hormone levels and nerve terminal coexistence

In order to investigate neurotensin-dopamine receptor interactions in vivo, the effects of intraventricular injection of neurotensin were analyzed on S(-)[N-propyl-3H(N)]propylnorapomorphine [( 3H]NPA) binding in cryostat sections of the forebrain, hypothalamus and pituitary gland, and on serum levels of prolactin, luteinizing hormone and corticosterone in the male rat. The relationship of modulation of [3H]NPA binding with neurotensin-dopamine coexistence in nerve terminals was analyzed by investigating coexistence of neurotensin and tyrosine hydroxylase (TH) immunoreactive nerve terminals in various brain areas, using a double immunohistofluorescence procedure. Intraventricular injections of neurotensin (0.03-3 nmol, 30 min) reduced dose-dependently specific [3H]NPA binding (0.25 nM) in the caudate-putamen (-38 +/- 4%), nucleus accumbens (-42 +/- 5%), tuberculum olfactorium (-52 +/- 7%) and in the intermediate lobe of the pituitary gland (-17 +/- 2%). Coexistence of neurotensin and TH was demonstrated in nerve terminals in the prefrontal, cingulate, piriform and entorhinal cortex and in the cortical and deep nuclei of the amygdaloid cortex. It was not possible to demonstrate coexistence in the caudate-putamen, nucleus accumbens, tuberculum olfactorium and median eminence, in view of the high density of dopamine nerve terminals present in relation to the few visualized neurotensin terminals. Nor could coexistence be demonstrated in the few remaining TH-positive nerve terminals following unilateral 6-hydroxydopamine lesions (8 micrograms per 4 microliters; one week) in spite of increased numbers of neurotensin-containing cell bodies and terminals in the ipsilateral dorsomedial caudate. Neurotensin injection markedly decreased serum prolactin levels and increased serum corticosterone levels by about 60%, whereas serum levels of luteinizing hormone were unaffected. The present study indicates that central dopamine D2 receptors may be regulated by neurotensin in vivo and that the neurotensin involved most likely is released from nerve terminals not containing dopamine, since fibers showing coexistence were only found in prefrontal and limbic cortical areas.