Regional effects of neurotensin on the electrically stimulated release of [3H]dopamine and [14C]acetylcholine in the rat nucleus accumbens

Neurotensin receptor 1-biased ligand attenuates neurotensin-mediated excitation of ventral tegmental area dopamine neurons and dopamine release in the nucleus accumbens

Strong expression of the G protein-coupled receptor (GPCR) neurotensin receptor 1 (NTR1) in ventral tegmental area (VTA) dopamine (DA) neurons and terminals makes it an attractive target to modulate DA neuron activity and normalize DA-related pathologies. Recent studies have identified a novel class of NTR1 ligand that shows promising effects in preclinical models of addiction. A lead molecule, SBI-0654553 (SBI-553), can act as a positive allosteric modulator of NTR1 β-arrestin recruitment while simultaneously antagonizing NTR1 Gq protein signaling. Using cell-attached recordings from mouse VTA DA neurons we discovered that, unlike neurotensin (NT), SBI-553 did not independently increase spontaneous firing. Instead, SBI-553 blocked the NT-mediated increase in firing. SBI-553 also antagonized the effects of NT on dopamine D2 auto-receptor signaling, potentially through its inhibitory effects on G-protein signaling. We also measured DA release directly, using fast-scan cyclic voltammetry in the nucleus accumbens and observed antagonist effects of SBI-553 on an NT-induced increase in DA release. Further, in vivo administration of SBI-553 did not notably change basal or cocaine-evoked DA release measured in NAc using fiber photometry. Overall, these results indicate that SBI-553 blunts NT's effects on spontaneous DA neuron firing, D2 auto-receptor function, and DA release, without independently affecting these measures. In the presence of NT, SBI-553 has an inhibitory effect on mesolimbic DA activity, which could contribute to its efficacy in animal models of psychostimulant use.

Neurotensinergic Excitation of Dentate Gyrus Granule Cells via Gαq-Coupled Inhibition of TASK-3 Channels

Neurotensin (NT) is a 13-amino acid peptide and serves as a neuromodulator in the brain. Whereas NT has been implicated in learning and memory, the underlying cellular and molecular mechanisms are ill-defined. Because the dentate gyrus receives profound innervation of fibers containing NT and expresses high density of NT receptors, we examined the effects of NT on the excitability of dentate gyrus granule cells (GCs). Our results showed that NT concentration dependently increased action potential (AP) firing frequency of the GCs by the activation of NTS1 receptors resulting in the depolarization of the GCs. NT-induced enhancement of AP firing frequency was not caused indirectly by releasing glutamate, GABA, acetylcholine, or dopamine, but due to the inhibition of TASK-3 K(+) channels. NT-mediated excitation of the GCs was G protein dependent, but independent of phospholipase C, intracellular Ca(2+) release, and protein kinase C. Immunoprecipitation experiment demonstrates that the activation of NTS1 receptors induced the association of Gαq/11 and TASK-3 channels suggesting a direct coupling of Gαq/11 to TASK-3 channels. Endogenously released NT facilitated the excitability of the GCs contributing to the induction of long-term potentiation at the perforant path-GC synapses. Our results provide a cellular mechanism that helps to explain the roles of NT in learning and memory.

Activation of neurotensin receptor 1 facilitates neuronal excitability and spatial learning and memory in the entorhinal cortex: beneficial actions in an Alzheimer's disease model

Neurotensin (NT) is a tridecapeptide distributed in the CNS, including the entorhinal cortex (EC), a structure that is crucial for learning and memory and undergoes the earliest pathological alterations in Alzheimer's disease (AD). Whereas NT has been implicated in modulating cognition, the cellular and molecular mechanisms by which NT modifies cognitive processes and the potential therapeutic roles of NT in AD have not been determined. Here we examined the effects of NT on neuronal excitability and spatial learning in the EC, which expresses high density of NT receptors. Brief application of NT induced persistent increases in action potential firing frequency, which could last for at least 1 h. NT-induced facilitation of neuronal excitability was mediated by downregulation of TREK-2 K(+) channels and required the functions of NTS1, phospholipase C, and protein kinase C. Microinjection of NT or NTS1 agonist, PD149163, into the EC increased spatial learning as assessed by the Barnes Maze Test. Activation of NTS1 receptors also induced persistent increases in action potential firing frequency and significantly improved the memory status in APP/PS1 mice, an animal model of AD. Our study identifies a cellular substrate underlying learning and memory and suggests that NTS1 agonists may exert beneficial actions in an animal model of AD.

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.

High-affinity neurotensin receptors in the rat nucleus accumbens: subcellular targeting and relation to endogenous ligand

Neurotensin is present in selective mesolimbic dopaminergic projections to the nucleus accumbens (NAc) shell but also is synthesized locally in this region and in the motor-associated NAc core. We examined the electron microscopic immunolabeling of the high-affinity neurotensin receptor (NTR) and neurotensin in these subdivisions of rat NAc to determine the sites for receptor activation and potential regional differences in distribution. Throughout the NAc, NTR immunoreactivity was localized discretely within both neurons and glia. NTR-labeled neuronal profiles were mainly axons and axon terminals with diverse synaptic structures, which resembled dopaminergic and glutamatergic afferents, as well as collaterals of inhibitory projection neurons. These terminals had a significantly higher numerical density in the NAc core than in the shell but were prevalent in both regions, suggesting involvement in both motor and limbic functions. In each region, neurotensin was detected in a few NTR-immunoreactive axon terminals and in terminals that formed symmetric, inhibitory type synapses with NTR-labeled somata and dendrites. The NTR labeling, however, was not seen within these synapses and, instead, was localized to segments of dendritic and glial plasma membranes often near excitatory type synapses. Neuronal NTR immunoreactivity also was associated with cytoplasmic tubulovesicles and nuclear membranes. Our results suggests that, in the NAc shell and core, NTR is targeted mainly to presynaptic sites, playing a role in the regulated secretion and/or retrograde signaling in diverse, neurotransmitter-specific neurons. The findings also support a volume mode of neurotensin actions, specifically affecting excitatory transmission through activation of not only axonal but also dendritic and glial NTR.

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.

Neurotensin modulation of acetylcholine and GABA release from the rat hippocampus: an in vivo microdialysis study

The effects of neurotensin (NT) on the release of acetylcholine (ACh), aspartate (Asp), glutamate (Glu) and gamma-aminobutyric acid (GABA) from the hippocampus of freely moving rats were studied by transversal microdialysis. ACh was detected by High Performance Liquid Chromatography (HPLC) with electrochemical detection while GABA, glutamate and aspartate were measured using HPLC with fluorometric detection. Neurotensin (0.2 and 0.5 microM) administered locally through the microdialysis probe to the hippocampus produced a long-lasting and concentration-dependent increase in the basal extracellular levels of GABA and ACh but not of glutamate and aspartate. The increase in the extracellular levels of GABA and ACh produced by 0.5 microM neurotensin in the hippocampus reached a maximum of about 310% for GABA and 250% for ACh. This stimulant effect of NT was antagonized by the NT receptor antagonist SR 48692 (100 microg/kg, i.p.). Local infusion of tetrodotoxin (1 microM) decreased the basal release of ACh, GABA, Asp, Glu and prevented the 0.2 microM NT-induced increase in GABA and ACh release. The effect of NT on the release of ACh was blocked by the GABA(A) receptor antagonist bicuculline (2-10 microM). Our findings indicate for the first time that neurotensin plays a neuromodulatory role in the regulation of GABAergic and cholinergic neuronal activity in the hippocampus of awake and freely moving rats. The potentiating effects of neurotensin on GABA and ACh release in the hippocampus are probably mediated by (i) NT receptors located on GABAergic cell bodies and (ii) through GABA(A) receptors located on cholinergic nerve terminals.

Involvement of potentially distinct neurotensin receptors in neurotensin-induced stimulation of striatal [3H]dopamine release evoked by KCl versus electrical depolarization

We intended to determine whether the effect of neurotensin (NT) on K+ and electrically evoked [3H]dopamine (DA) release from rat and guinea-pig striatal slices involved different mechanisms and/or receptors. In the two species, NT and three NT agonists were found to exhibit different relative potencies to enhance K+- and electrically-evoked [3H]DA release. NT(1-13) increased [3H]DA release with EC50 values in the nanomolar range and Emax values in the range of 100% of control. NT(8-13) and Eisai hexapeptide were both as active as NT(1-13) under K+ depolarization, but did not exceed 40% of the NT(1-13) effect under electrical depolarization. In rats, when [3H]DA release was stimulated with two successive K+ depolarizations, in the presence of NT(1-13), the NT effect during the second exposure to K+ was drastically decreased, suggesting that the NT receptor was desensitized. The desensitization process was essentially observed on Emax values, EC50 values being weakly affected. Similar results were obtained in guinea pig. In contrast, with two electrical depolarizations or with two different depolarizations (K+ followed by electrical), the NT effect during the second depolarization was not significantly affected. Concerning NT antagonists, SR 48692 antagonized with IC50 values in the nanomolar range the NT(1-13) stimulated K+-evoked [3H]DA release but did not affect, up to 10(-6) M, the NT(1-13) enhancement of electrically stimulated [3H]DA release. On the contrary, SR 142948A antagonized the NT(1-13) effect on K+- and electrically-evoked [3H]DA release. In conclusion, these results suggest the possible existence of potentially distinct neurotensin receptors differentially involved in the control exerted by NT on DA release under KCl vs electrical depolarization.

Neurochemical and behavioural effects of neurotensin vs [D-Tyr11]neurotensin on mesolimbic dopaminergic function

Microinjection of neurotensin(1-13) or neurotensin(8-13) into the ventral tegmental area (VTA) of anaesthetized rats produced dose-dependent (1-100 pg) dopamine release in the nucleus accumbens as measured by differential pulse amperometry (DPA). Higher doses (100 pg-10 ng) of [D-Tyr11]neurotensin were required to produce an identical effect. In addition, the 3 peptides enhanced the K(+)-evoked [3H]DA release from nucleus accumbens slices. The stimulatory actions produced by 10(-8) M neurotensin(1-13) and neurotensin(8-13) were respectively of 96% and 72% while the effect of [D-Tyr11]neurotensin was only of 79% at 10(-6) M. Unilateral application of the 3 peptides in the VTA of cannulated rats produced contralateral circling. [D-Tyr11]neurotensin was effective in a dose-dependent manner, between 40 and 320 ng. Similar effects were observed with 80 ng of neurotensin(1-13) and neurotensin(8-13) in presence of the protease inhibitor thiorphan. In view of the higher potency of neurotensin(1-13) and neurotensin(8-13) versus [D-Tyr11]neurotensin to stimulate DA release both in vivo and in vitro and the higher efficacy of [D-Tyr11]neurotensin to induce circling, this study further strengthens the concept of neurotensin receptor heterogeneity.

In vitro and in vivo evidence of neurotensin release from preganglionic axon terminals in the stellate ganglion of the cat

We have previously shown that the neurotensin (NT) store in preganglionic axon terminals of the cat stellate ganglion (SG) is reversibly depleted by prolonged preganglionic stimulation. The present study addresses the questions of whether the preganglionic axon terminals release NT in response to depolarizing stimuli in vitro and whether in vivo NT is released by the tonic firing of the sympathetic preganglionic neurons. Slices of the SG of the anaesthetized cat, maintained in oxygenated Ringer solution, released NT. The efflux increased when the K concentration was increased from 5 to 25 or 45 mM or when veratridine was added to the medium. In Ca-free medium, efflux was suppressed. The effect of veratridine was blocked by tetrodotoxin (TTX). In awake, freely moving cats, in which TTX was applied for 4 days to the preganglionic input of the right SG, the NT content of this ganglion doubled by comparison with the left SG. Since NT accumulates proximal to a ligature on the preganglionic input of the SG, the increased NT content is likely to result from suppression of action potential-dependent release while influx into the terminals persists. This result suggests that the steady state of the NT store in sympathetic preganglionic terminals is the result of a steady influx from the soma balanced by action potential-dependent loss, presumably release.