Electrophysiological evidence for putative subtypes of neurotensin receptors in guinea-pig mesencephalic dopaminergic neurons

Diverse actions of the modulatory peptide neurotensin on central synaptic transmission

Neurotensin (NT) is a 13 amino acid neuropeptide that is expressed throughout the central nervous system and is implicated in the etiology of multiple diseases and disorders. Many primary investigations of NT-induced modulation of neuronal excitability at the level of the synapse have been conducted, but they have not been summarized in review form in nearly 30 years. Therefore, the goal of this review is to discuss the many actions of NT on neuronal excitability across brain regions as well as NT circuit architecture. In the basal ganglia as well as other brain nuclei, NT can act through diverse intracellular signaling cascades to enhance or depress neuronal activity by modulating activity of ion channels, ionotropic and metabotropic neurotransmitter receptors, and presynaptic release of neurotransmitters. Further, NT can produce indirect effects by evoking endocannabinoid release, and recently has itself been identified as a putative retrograde messenger. In the basal ganglia, the diverse actions and circuit architecture of NT signaling allow for input-specific control of reward-related behaviors.

Neurotensin Induces Presynaptic Depression of D2 Dopamine Autoreceptor-Mediated Neurotransmission in Midbrain Dopaminergic Neurons

Increased dopaminergic signaling is a hallmark of severe mesencephalic pathologies such as schizophrenia and psychostimulant abuse. Activity of midbrain dopaminergic neurons is under strict control of inhibitory D2 autoreceptors. Application of the modulatory peptide neurotensin (NT) to midbrain dopaminergic neurons transiently increases activity by decreasing D2 dopamine autoreceptor function, yet little is known about the mechanisms that underlie long-lasting effects. Here, we performed patch-clamp electrophysiology and fast-scan cyclic voltammetry in mouse brain slices to determine the effects of NT on dopamine autoreceptor-mediated neurotransmission. Application of the active peptide fragment NT8-13 produced synaptic depression that exhibited short- and long-term components. Sustained depression of D2 autoreceptor signaling required activation of the type 2 NT receptor and the protein phosphatase calcineurin. NT application increased paired-pulse ratios and decreased extracellular levels of somatodendritic dopamine, consistent with a decrease in presynaptic dopamine release. Surprisingly, we observed that electrically induced long-term depression of dopaminergic neurotransmission that we reported previously was also dependent on type 2 NT receptors and calcineurin. Because electrically induced depression, but not NT-induced depression, was blocked by postsynaptic calcium chelation, our findings suggest that endogenous NT may act through a local circuit to decrease presynaptic dopamine release. The current research provides a mechanism through which augmented NT release can produce a long-lasting increase in membrane excitability of midbrain dopamine neurons.

SIGNIFICANCE STATEMENT

Whereas plasticity of glutamate synapses in the brain has been studied extensively, demonstrations of plasticity at dopaminergic synapses have been more elusive. By quantifying inhibitory neurotransmission between midbrain dopaminergic neurons in brain slices from mice we have discovered that the modulatory peptide neurotensin can induce a persistent synaptic depression by decreasing dopamine release. This depression of inhibitory synaptic input would be expected to increase excitability of dopaminergic neurons. Induction of the plasticity can be pharmacologically blocked by antagonists of either the protein phosphatase calcineurin or neurotensin receptors, and persists surprisingly long after a brief exposure to the peptide. Since neurotensin-dopamine interactions have been implicated in hyperdopaminergic pathologies, these findings describe a synaptic mechanism that could contribute to addiction and/or schizophrenia.

Neurotensin enhances glutamatergic EPSCs in VTA neurons by acting on different neurotensin receptors

Neurotensin (NT) is an endogenous neuropeptide that modulates dopamine and glutamate neurotransmission in several limbic regions innervated by neurons located in the ventral tegmental area (VTA). While several studies showed that NT exerted a direct modulation on VTA dopamine neurons less is known about its role in the modulation of glutamatergic neurotransmission in this region. The present study was aimed at characterising the effects of NT on glutamate-mediated responses in different populations of VTA neurons. Using whole cell patch clamp recording technique in horizontal rat brain slices, we measured the amplitude of glutamatergic excitatory post-synaptic currents (EPSCs) evoked by electrical stimulation of VTA afferents before and after application of different concentrations of NT1-13 or its C-terminal fragment, NT8-13. Neurons were classified as either Ih(+) or Ih(-) based on the presence or absence of a hyperpolarisation activated cationic current (Ih). We found that NT1-13 and NT8-13 produced comparable concentration dependent increase in the amplitude of EPSCs in both Ih(+) and Ih(-) neurons. In Ih(+) neurons, the enhancement effect of NT8-13 was blocked by both antagonists, while in Ih(-) neurons it was blocked by the NTS1/NTS2 antagonist, SR142948A, but not the preferred NTS1 antagonist, SR48692. In as much as Ih(-) neurons are non-dopaminergic neurons and Ih(+) neurons represent both dopamine and non-dopamine neurons, we can conclude that NT enhances glutamatergic mediated responses in dopamine, and in a subset of non-dopamine, neurons by acting respectively on NTS1 and an NT receptor other than NTS1.

To ingest or rest? Specialized roles of lateral hypothalamic area neurons in coordinating energy balance

Survival depends on an organism’s ability to sense nutrient status and accordingly regulate intake and energy expenditure behaviors. Uncoupling of energy sensing and behavior, however, underlies energy balance disorders such as anorexia or obesity. The hypothalamus regulates energy balance, and in particular the lateral hypothalamic area (LHA) is poised to coordinate peripheral cues of energy status and behaviors that impact weight, such as drinking, locomotor behavior, arousal/sleep and autonomic output. There are several populations of LHA neurons that are defined by their neuropeptide content and contribute to energy balance. LHA neurons that express the neuropeptides melanin-concentrating hormone (MCH) or orexins/hypocretins (OX) are best characterized and these neurons play important roles in regulating ingestion, arousal, locomotor behavior and autonomic function via distinct neuronal circuits. Recently, another population of LHA neurons containing the neuropeptide Neurotensin (Nts) has been implicated in coordinating anorectic stimuli and behavior to regulate hydration and energy balance. Understanding the specific roles of MCH, OX and Nts neurons in harmonizing energy sensing and behavior thus has the potential to inform pharmacological strategies to modify behaviors and treat energy balance disorders.

Repeated effects of the neurotensin receptor agonist PD149163 in three animal tests of antipsychotic activity: assessing for tolerance and cross-tolerance to clozapine

Neurotensin is an endogenous neuropeptide closely associated with the mesolimbic dopaminergic system and shown to possess antipsychotic-like effects. In particular, acute neurotensin receptor activation can inhibit conditioned avoidance response (CAR), attenuate phencyclidine (PCP)-induced prepulse inhibition (PPI) disruptions, and reverse PCP-induced hyperlocomotion. However, few studies have examined the long term effects of repeated neurotensin receptor activation and results are inconsistent. Since clinical administration of antipsychotic therapy often requires a prolonged treatment schedule, here we assessed the effects of repeated activation of neurotensin receptors using an NTS1 receptor selective agonist, PD149163, in 3 behavioral tests of antipsychotic activity. We also investigated whether reactivity to the atypical antipsychotic clozapine was altered following prior PD149163 treatment. Using both normal and prenatally immune activated rats generated through maternal immune activation with polyinosinic:polycytidylic acid, we tested PD149163 in CAR, PCP (1.5mg/kg)-induced PPI disruption, and PCP (3.2mg/kg)-induced hyperlocomotion. For each paradigm, rats were first repeatedly tested with vehicle or PD149163 (1.0, 4.0, 8.0mg/kg, sc) along with vehicle or PCP for PPI and hyperlocomotion tests, then challenged with PD149163 after 2 drug-free days. All rats were then challenged with clozapine (5.0mg/kg, sc). During the repeated test period, PD149163 exhibited antipsychotic-like effects in all three models. On the PD149163 challenge day, prior drug treatment only caused a tolerance effect in CAR. This tolerance in CAR was transferrable to clozapine, as it enhanced clozapine tolerance in the same group of animals. Although no tolerance effect was seen in the PD149163 challenge for the PCP-induced hyperlocomotion test, the clozapine challenge showed increased sensitivity in groups previously exposed to repeated PD149163 treatment. Our findings suggest that repeated exposure to NTS1 receptor agonists can induce a dose-dependent tolerance and cross-tolerance to clozapine to some of its behavioral effects but not others.

The role of neurotensin in positive reinforcement in the rat central nucleus of amygdala

In the central nervous system neurotensin (NT) acts as a neurotransmitter and neuromodulator. It was shown that NT has positive reinforcing effects after its direct microinjection into the ventral tegmental area. The central nucleus of amygdala (CeA), part of the limbic system, plays an important role in learning, memory, regulation of feeding, anxiety and emotional behavior. By means of immunohistochemical and radioimmune methods it was shown that the amygdaloid body is relatively rich in NT immunoreactive elements and NT receptors. The aim of our study was to examine the possible effects of NT on reinforcement and anxiety in the CeA. In conditioned place preference test male Wistar rats were microinjected bilaterally with 100 or 250 ng NT in volume of 0.4 microl or 35 ng neurotensin receptor 1 (NTS1) antagonist SR 48692 alone, or NTS1 antagonist 15 min before 100 ng NT treatment. Hundred or 250 ng NT significantly increased the time rats spent in the treatment quadrant. Prior treatment with the non-peptide NTS1 antagonist blocked the effects of NT. Antagonist itself did not influence the reinforcing effect. In elevated plus maze test we did not find differences among the groups as far as the anxiety index (time spent on the open arms) was concerned. Our results suggest that in the rat ACE NT has positive reinforcing effects. We clarified that NTS1s are involved in this action. It was also shown that NT does not influence anxiety behavior.

Neurotensin receptor mechanisms and its modulation of glutamate transmission in the brain: relevance for neurodegenerative diseases and their treatment

The extracellular accumulation of glutamate and the excessive activation of glutamate receptors, in particular N-methyl-D-aspartate (NMDA) receptors, have been postulated to contribute to the neuronal cell death associated with chronic neurodegenerative disorders such as Parkinson's disease. Findings are reviewed indicating that the tridecaptide neurotensin (NT) via activation of NT receptor subtype 1 (NTS1) promotes and reinforces endogenous glutamate signalling in discrete brain regions. The increase of striatal, nigral and cortical glutamate outflow by NT and the enhancement of NMDA receptor function by a NTS1/NMDA interaction that involves the activation of protein kinase C may favour the depolarization of NTS1 containing neurons and the entry of calcium. These results strengthen the hypothesis that NT may be involved in the amplification of glutamate-induced neurotoxicity in mesencephalic dopamine and cortical neurons. The mechanisms involved may include also antagonistic NTS1/D2 interactions in the cortico-striatal glutamate terminals and in the nigral DA cell bodies and dendrites as well as in the nigro-striatal DA terminals. The possible increase in NT levels in the basal ganglia under pathological conditions leading to the NTS1 enhancement of glutamate signalling may contribute to the neurodegeneration of the nigro-striatal dopaminergic neurons found in Parkinson's disease, especially in view of the high density of NTS1 receptors in these neurons. The use of selective NTS1 antagonists together with conventional drug treatments could provide a novel therapeutic approach for treatment of Parkinson's disease.

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

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

The neurotensin agonist PD149163 increases Fos expression in the prefrontal cortex of the rat

Dopaminergic axons innervating the prefrontal cortex (PFC) target both pyramidal cells and GABAergic interneurons. Many of these dopamine (DA) axons in the rat coexpress the peptide neurotransmitter neurotensin. Previous electrophysiological data have suggested that neurotensin activates GABAergic interneurons in the PFC. Activation of D2-like DA receptors increases extracellular GABA levels in the PFC, as opposed to the striatum, where D2 receptor activation inhibits GABAergic neurons. Because activation of presynaptic D2 release-modulating autoreceptors in the PFC suppresses DA release but increases release of the cotransmitter neurotensin, D2 agonists may enhance the activity of GABAergic interneurons via release of neurotensin. In order to determine if neurotensin can activate GABAergic interneurons, we treated rats with the peptide neurotensin agonist, PD149163, and examined Fos expression in PFC neurons. Systemic administration of PD149163 increased overall Fos expression in the PFC, but not in the dorsal striatum. PD149163 induced Fos in PFC interneurons, as defined by the presence of calcium-binding proteins, and in pyramidal cells. Pretreatment with the high-affinity neurotensin antagonist, SR48692, blocked neurotensin agonist-induced Fos expression. These data suggest that neurotensin activates interneurons in the PFC of the rat.

Role of calcium in neurotensin-evoked enhancement in firing in mesencephalic dopamine neurons

Neurotensin (NT) increases neurotransmission within the mesolimbic dopamine system by enhancing the firing rate of dopaminergic (DAergic) neurons and by acting at the nerve terminal level. The signal transduction pathways involved in these effects have not been characterized, but NT receptors are coupled to the phospholipase C pathway and Ca(2+) mobilization. However, an enhancement of intracellular Ca(2+) concentration ([Ca(2+)](i)) evoked by NT in DAergic neurons has yet to be demonstrated. Furthermore, the hypothesis that the excitatory effects of NT in DAergic neurons are Ca(2+) dependent is currently untested. In whole-cell recording experiments, DAergic neurons in culture were identified by their selective ability to express a cell-specific green fluorescent protein reporter construct. These experiments confirmed that NT increases firing rate in cultured DAergic neurons. This effect was Ca(2+) dependent because it was blocked by intracellular dialysis with BAPTA. Using Ca(2+) imaging, we showed that NT caused a rapid increase in [Ca(2+)](i) in DAergic neurons. Most of the Ca(2+) originated from the extracellular medium. NT-induced excitation and Ca(2+) influx were blocked by SR48692, an antagonist of the type 1 NT receptor. Blocking IP(3) receptors using heparin prevented the excitatory effect of NT. Moreover, Zn(2+) and SKF96365 both blocked the excitatory effect of NT, suggesting that nonselective cationic conductances are involved. Finally, although NT can also induce a rise in [Ca(2+)](i) in astrocytes, we find that NT-evoked excitation of DAergic neurons can occur independently of astrocyte activation.

Neurotensin enhances glutamate excitotoxicity in mesencephalic neurons in primary culture

The tridecapeptide neurotensin has been demonstrated to increase glutamate release in discrete rat brain regions, leading to the hypothesis of a possible involvement of the peptide in neurodegenerative pathologies. The role of neurotensin in modulating glutamate excitotoxicity and the possible neuroprotective action of the neurotensin receptor antagonist SR48692 were investigated in primary cultures of mesencephalic neurons by measuring [(3)H]dopamine uptake and tyrosine hydroxylase immunocytochemistry 24 hr after glutamate treatment. The exposure to glutamate (30 and 100 microM, 10 min) decreased [(3)H]dopamine uptake into mesencephalic neurons. Neurotensin (10 and 100 nM), added before glutamate (30 microM) exposure, significantly enhanced the glutamate-induced reduction of [(3)H]dopamine uptake. In addition, the peptide (10 nM) also significantly enhanced the effect of 100 microM glutamate. The effects of neurotensin were counteracted by the neurotensin receptor antagonist SR48692 (100 nM) and by the protein kinase C inhibitor calphostin C. The exposure to 100 microM, but not 30 microM, glutamate significantly reduced the number of tyrosine hydroxylase-immunoreactive cells, and neurotensin (10 nM) significantly enhanced this effect. SR48692 (100 nM) prevented the neurotensin-induced action. These findings support the view of a possible pathophysiological role of neurotensin in mesencephalic dopamine neuronal function. Furthermore, selective neurotensin antagonists in combination with conventional drug treatments could provide a novel therapeutic approach for the treatment of neurodegenerative disorders, such as Parkinson's disease.

Presynaptic action of neurotensin on cultured ventral tegmental area dopaminergic neurones

Dopamine-containing neurones of the ventral tegmental area express neurotensin receptors which are involved in regulating cell firing and dopamine release. Although indirect evidence suggests that some neurotensin receptors may be localised on the nerve terminals of dopaminergic neurones in the striatum and thus locally regulate dopamine release, a clear demonstration of such a mechanism is lacking and a number of indirect sites of action are possible. We have taken advantage of a simplified preparation in which cultured rat ventral tegmental area dopaminergic neurones establish nerve terminals that co-release glutamate to determine whether neurotensin can act at presynaptic sites. We recorded glutamate-mediated synaptic currents that were generated by dopaminergic nerve terminals as an index of presynaptic function. The neurotensin receptor agonist NT(8-13) caused an inward current and an enhancement of the firing rate of dopaminergic neurones together with an increase in the frequency of spontaneous glutamate receptor-mediated excitatory postsynaptic currents (EPSCs). Incompatible with a direct excitatory action on nerve terminals, NT(8-13) failed to change the amplitude of individual action potential-evoked EPSCs or the frequency of miniature EPSCs recorded in the presence of tetrodotoxin. However, NT(8-13) reduced the ability of terminal D2 dopamine receptors to inhibit action potential-evoked EPSCs in isolated dopaminergic neurones. Taken together, our results suggest that in addition to its well-known somatodendritic excitatory effect leading to an increase in firing rate, neurotensin also acts on nerve terminals. The main effect of neurotensin on nerve terminals is not to produce a direct excitation, but rather to decrease the effectiveness of D2 receptor-mediated presynaptic inhibition.

Neurotensin-induced modulation of dopamine D2 receptors and their function in rat striatum: counteraction by a NTR1-like receptor antagonist

The present study investigated the neurotensin (NT) receptor subtype (NTR) involved in the antagonistic neurotensin modulation of striatal dopamine D2 receptors observed in vitro and in vivo. The NT induced increase of the IC50 values of dopamine (DA) competition for [125I]iodosulpiride binding sites was counteracted by the NTR1-like antagonist SR48692 in rat striatal slices. Intrastriatal perfusion of pergolide induced in the awake rat an inhibition of striatal DA release that was antagonized by NT. This action of NT was counteracted by co-perfusion with the NTR1 like antagonist SR48692. These data indicate that there exists in the striatum at the prejunctional level an intramembrane antagonistic NT receptor/DA D2 receptor-receptor interaction where NTR1 like receptor activation reduces the DA D2 autoreceptor function.

Effects of neurotensin on discharge rates of rat suprachiasmatic nucleus neurons in vitro

The neuropeptide neurotensin and two classes of its receptors, the neurotensin receptor-1 and 2, are present in the suprachiasmatic nucleus of the mammalian hypothalamus. The suprachiasmatic nucleus houses the mammalian central circadian pacemaker, but the effects of neurotensin on cellular activity in this circadian pacemaker are unknown. In this study, we examined the effects of neurotensin on the spontaneous discharge rate of rat SCN cells in an in vitro slice preparation. Neurotensin (1-10 microM) increased cell firing rate in approximately 50% of cells tested, while approximately 10% of suprachiasmatic cells tested showed a decrease in firing rate in response to neurotensin. These effects of neurotensin were not altered by the GABA receptor antagonist bicuculline (20 microM) or the glutamate receptor antagonists, D-aminophosphopentanoic acid (50 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). The neurotensin receptor selective antagonists SR48692 and SR142948a (10 microM) failed to antagonise neurotensin responses in the majority of cells examined. Compounds that function as agonists selective for the neurotensin-receptor subtypes 1 and 2, JMV-510 and JMV-431 respectively, elicited neurotensin-like responses in approximately 90% of cells tested. Six out of seven cells tested responded to both JMV-510 and JMV-431. Neuropeptide Y (100nM) treatment of suprachiasmatic nucleus slices was found to elicit profound suppression of neuronal firing rate. Co-application of neurotensin with neuropeptide Y significantly (P<0.05) reduced the duration of the response, as compared to that elicited with neuropeptide Y alone. Together, these results demonstrate for the first time the actions of neurotensin in the suprachiasmatic nucleus and raise the possibility that this neuropeptide may play a role in modulating circadian pacemaker function.

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 regulates intracellular calcium in ventral tegmental area astrocytes: evidence for the involvement of multiple receptors

Recent evidence suggests that some types of neurotensin receptors may be expressed by astrocytes. In order to explore the function of neurotensin receptors in astrocytes, the effect of a neurotensin receptor agonist, neurotensin(8-13), on intracellular Ca(2+) dynamics in mixed neuronal/glial cultures prepared from rat ventral tegmental area was examined. It was found that neurotensin(8-13) induces a long-lasting rise in intracellular Ca(2+) concentration in a subset of glial fibrilary acidic protein-positive glial cells. This response displays extensive desensitization and appears to implicate both intracellular and extracellular Ca(2+) sources. In the absence of extracellular Ca(2+), neurotensin(8-13) evokes only a short-lasting rise in intracellular Ca(2+). The neurotensin-evoked intracellular Ca(2+) accumulation is blocked by the phospholipase C inhibitor U73122 and by thapsigargin, suggesting that it is initiated by release of Ca(2+) from an inositol triphosphate-dependent store. The Ca(2+)-mobilizing action of neurotensin(8-13) in astrocytes is dependent on at least two receptors, because the response is blocked in part only by SR48692, a type 1 neurotensin receptor antagonist, and is blocked completely by SR142948A, a novel neurotensin receptor antagonist. The finding that the type 2 neurotensin receptor agonist levocabastine fails to mimic or alter the effects of neurotensin(8-13) on intracellular Ca(2+) makes it unlikely that the type 2 neurotensin receptor is involved. In summary, these results show that functional neurotensin receptors are present in cultured ventral tegmental area astrocytes and that their activation induces a highly desensitizing rise in intracellular Ca(2+). The pharmacological profile of this response suggests that a type 1 neurotensin receptor is involved but that another, possibly novel, non-type 2 neurotensin receptor is also implicated. If present in vivo, such signalling could be involved in some of the physiological actions of neurotensin.

Neurotensin increases endogenous glutamate release in rat cortical slices

In the present study, the effects of the tridecapeptide neurotensin [NT(1-13)] and its fragments, NT(1-7) and NT(8-13), on endogenous glutamate release from rat cortical slices, were evaluated. NT(1-13) (100-1000 nM) slightly increased spontaneous glutamate release, while it was ineffective at 1 and 10 nM concentrations. Neither the biologically active NT fragment NT(8-13) nor the inactive one NT(1-7) affected basal glutamate release. NT(1-13) (1-1000 nM) enhanced potassium (35 mM)-evoked glutamate release displaying a bell-shaped concentration response curve. In addition NT(8-13) (10 nM) increased K+-evoked-glutamate release similarly to the parent peptide (10 nM), while the biologically inactive fragment NT(1-7) (10-100 nM) was ineffective. The effects of NT(1-13) and NT(8-13) were fully counteracted by the selective neurotensin receptor antagonist SR48692 (100 nM). These findings suggest that NT plays a role in regulating cortical glutamate transmission.

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.

Neurotensin depolarizes cholinergic and a subset of non-cholinergic septal/diagonal band neurons by stimulating neurotensin-1 receptors

Identified cholinergic and a subtype of non-cholinergic, fast-firing neurons were recorded intracellularly in vitro from slices of guinea-pig brain. Recorded neurons were within the boundaries of the medial septum and vertical limb of the diagonal band of the forebrain. The effects of superfused neurotensin and neurotensin receptor antagonists were measured under single-electrode current clamp. Neurotensin consistently caused a dose-dependent, slow depolarization of cholinergic neurons that was accompanied by an increase in membrane resistance and a block of the long-duration (1-10 s) post-spike afterhyperpolarization when present. Neurotensin also blocked a shorter duration, slow afterhyperpolarization, but only in a minority of cholinergic neurons. When present, inhibition of the slow afterhyperpolarization changed the spike pattern from single spikes to short bursts. Inhibition of post-spike afterhyperpolarizations by neurotensin reversed more slowly than did other effects of neurotensin. Tetrodotoxin did not prevent the depolarizing effect of neurotensin. The non-selective neurotensin receptor antagonist, SR142948A, blocked the depolarizing effect of neurotensin but the low-affinity receptor antagonist, levocabastine, did not. A subgroup of noncholinergic, fast-firing neurons (23%) was also depolarized by neurotensin, an effect antagonized by SR142948A but not levocabastine. Neurotensin did not effect post-spike voltage transients or change the firing pattern of non-cholinergic neurons. These data suggest that neurotensin causes a slow depolarization and increased excitability of cholinergic and some noncholinergic neurons in an area of the brain that projects to the hippocampus. Neurotensin type 1 receptors appear to mediate these effects. Neurotensin may modulate hippocampal-dependent learning and memory processes through its effects on septohippocampal neurons.

Stimulation by neurotensin of (3H)5-hydroxytryptamine (5HT) release from rat frontal cortex slices

The effect of neurotensin (NT) on the K+-evoked (3H)5HT release from brain frontal cortex slices was studied in rats. NT(1-13) and NT(8-13) increased (3H)5HT release with EC50 values in the nanomolar range and Emax values in the range of 100% of control, whereas D-tyr11-NT was inactive. Concerning NT receptor antagonists, SR 48692 and SR 142948A antagonized with IC50 values of 4.8+/-1.8 nM and 4.5+/-1.8 nM respectively, the NT stimulated K+-evoked (3H)5HT release. SR 48527 also antagonized NT induced (3H)5HT release with an IC50 value of 0.95+/-0.06 nM whereas the inactive R(-) enantiomer SR 49711 only inhibited this effect with IC50 value close to 10(-6)M. The 5HT-releasing effect of NT was completely inhibited by tetrodotoxin suggesting that NT receptors involved in the control of 5-HT release are not located on 5-HT terminals. After a first NT (10(-7)M) application, the NT (10(-7)M, 10(-6)M) effect under K+ depolarization was drastically decreased, indicating that the NT receptor could be desensitized. No potentiating effect of NT on K+-evoked (3H)5HT release was observed in striatal and hippocampal slices. These results suggest that, in the rat frontal cortex, NT regulates 5HT release through a high affinity NT receptor not associated with 5HT terminals.