Neurotensin and Alcohol Use Disorders: Towards a Pharmacological Treatment

Harmful alcohol use is responsible for a group of disorders collectively named alcohol use disorders (AUDs), according to the DSM-5 classification. The damage induced by alcohol depends on the amount, time, and consumption patterns (continuous and heavy episodic drinking). It affects individual global well-being and social and familial environments with variable impact. Alcohol addiction manifests with different degrees of organ and mental health detriment for the individual, exhibiting two main traits: compulsive drinking and negative emotional states occurring at withdrawal, frequently causing relapse episodes. Numerous individual and living conditions, including the concomitant use of other psychoactive substances, lie in the complexity of AUD. Ethanol and its metabolites directly impact the tissues and may cause local damage or alter the homeostasis of brain neurotransmission, immunity scaffolding, or cell repair biochemical pathways. Brain modulator and neurotransmitter-assembled neurocircuitries govern reward, reinforcement, social interaction, and consumption of alcohol behaviors in an intertwined manner. Experimental evidence supports the participation of neurotensin (NT) in preclinical models of alcohol addiction. For example, NT neurons in the central nucleus of the amygdala projecting to the parabrachial nucleus strengthen alcohol consumption and preference. In addition, the levels of NT in the frontal cortex were found to be lower in rats bred to prefer alcohol to water in a free alcohol-water choice compared to wild-type animals. NT receptors 1 and 2 seem to be involved in alcohol consumption and alcohol effects in several models of knockout mice. This review aims to present an updated picture of the role of NT systems in alcohol addiction and the possible use of nonpeptide ligands modulating the activity of the NT system, applied to experimental animal models of harmful drinking behavior mimicking alcohol addiction leading to health ruin in humans.

Oncogenic role of neurotensin and neurotensin receptors in various cancers

Neurotensin (NTS) has long been recognized as a neurotransmitter or neuromodulator in the central nervous system and as an endocrine agent in the periphery via actions mediated through neurotensin receptors (NTSRs). Many studies support a role for NTS in the endocrine, autocrine and paracrine growth stimulation of cancer, with oncogenic actions described for NTS in different types of cancers and cancer cell lines at each step of cancer progression, ranging from tumour growth and survival to metastatic spread. The mechanisms underlying the effects of the NTS/NTSR system in cell proliferation, migration and invasion, as well as the anti-apoptotic effects of this system, have been elucidated in different types of cancers, and include mitogen-activated protein kinases, phosphatidylinositol 3-kinase and RhoGTPases. The present mini review summarizes recent findings relating to the oncogenic function of the NTS/NTSR system.

Multiple D2 heteroreceptor complexes: new targets for treatment of schizophrenia

The dopamine (DA) neuron system most relevant for schizophrenia is the meso-limbic-cortical DA system inter alia densely innervating subcortical limbic regions. The field of dopamine D2 receptors and schizophrenia changed markedly with the discovery of many types of D2 heteroreceptor complexes in subcortical limbic areas as well as the dorsal striatum. The results indicate that the D2 is a hub receptor which interacts not only with many other G protein-coupled receptors (GPCRs) including DA isoreceptors but also with ion-channel receptors, receptor tyrosine kinases, scaffolding proteins and DA transporters. Disturbances in several of these D2 heteroreceptor complexes may contribute to the development of schizophrenia through changes in the balance of diverse D2 homo- and heteroreceptor complexes mediating the DA signal, especially to the ventral striato-pallidal γ-aminobutyric acid (GABA) pathway. This will have consequences for the control of this pathway of the glutamate drive to the prefrontal cortex via the mediodorsal thalamic nucleus which can contribute to psychotic processes. Agonist activation of the A2A protomer in the A2A-D2 heteroreceptor complex inhibits D2 Gi/o mediated signaling but increases the D2 β-arrestin2 mediated signaling. Through this allosteric receptor-receptor interaction, the A2A agonist becomes a biased inhibitory modulator of the Gi/o mediated D2 signaling, which may the main mechanism for its atypical antipsychotic properties especially linked to the limbic A2A-D2 heterocomplexes. The DA and glutamate hypotheses of schizophrenia come together in the signal integration in D2-N-methyl-d-aspartate (NMDA) and A2A-D2-metabotropic glutamate receptor 5 (mGlu5) heteroreceptor complexes, especially in the ventral striatum. 5-Hydroxytryptamine 2A (5-HT2A)-D2 heteroreceptor complexes are special targets for atypical antipsychotics with high potency to block their 5-HT2A protomer signaling in view of the potential development of pathological allosteric facilitatory 5-HT2A-D2 interaction increasing D2 protomer signaling. Neurotensin (NTS1)-D2 heterocomplexes also exist in the ventral and dorsal striatum, and likely also in midbrain DA nerve cells as NTS1-D2 autoreceptor complexes where neurotensin produces antipsychotic and propsychotic actions, respectively.

A marine analgesic peptide, Contulakin-G, and neurotensin are distinct agonists for neurotensin receptors: uncovering structural determinants of desensitization properties

Neurotensin receptors have been studied as molecular targets for the treatment of pain, schizophrenia, addiction, or cancer. Neurotensin (NT) and Contulakin-G, a glycopeptide isolated from a predatory cone snail Conus geographus, share a sequence similarity at the C-terminus, which is critical for activation of neurotensin receptors. Both peptides are potent analgesics, although affinity and agonist potency of Contulakin-G toward neurotensin receptors are significantly lower, as compared to those for NT. In this work, we show that the weaker agonist properties of Contulakin-G result in inducing significantly less desensitization of neurotensin receptors and preserving their cell-surface density. Structure-activity relationship (SAR) studies suggested that both glycosylation and charged amino acid residues in Contulakin-G or NT played important roles in desensitizing neurotensin receptors. Computational modeling studies of human neurotensin receptor NTS1 and Contulakin-G confirmed the role of glycosylation in weakening interactions with the receptors. Based on available SAR data, we designed, synthesized, and characterized an analog of Contulakin-G in which the glycosylated amino acid residue, Gal-GalNAc-Thr10, was replaced by memantine-Glu10 residue. This analog exhibited comparable agonist potency and weaker desensitization properties as compared to that of Contulakin-G, while producing analgesia in the animal model of acute pain following systemic administration. We discuss our study in the context of feasibility and safety of developing NT therapeutic agents with improved penetration across the blood-brain barrier. Our work supports engineering peptide-based agonists with diverse abilities to desensitize G-protein coupled receptors and further emphasizes opportunities for conotoxins as novel pharmacological tools and drug candidates.

A site-directed mutagenesis study on the conserved alanine residue in the distal third intracellular loops of cholecystokininB and neurotensin receptors

1. An alanine residue at the C-terminal tail of the third intracellular loop is highly conserved among various Gq protein-coupled receptors including rat cholecystokininB (CCKB) and neurotensin receptors. To investigate the functional significance of the conserved alanine in the activation of Gq proteins and phospholipase C (PLC) by CCKB and neurotensin receptors, the alanine residue was mutated in the present study. Subsequently, the ability of resulting mutant receptors to activate PLC was investigated by measuring the formation of inositol phosphates (IP) in COS-7 cells and recording Ca(2+)-activated chloride currents from Xenopus oocytes. 2. Site-directed mutagenesis was performed to mutate alanine at position 332 of rat CCKB receptor to glutamate. When the (A332E) mutant receptor was expressed in COS-7 cells and Xenopus oocytes, the efficacy and the potency of sulphated cholecystokinin octapeptide (CCK-8) to stimulate polyphosphoinositide hydrolysis in COS-7 cells and evoke calcium-dependent Cl- currents in oocytes were not significantly affected. 3. Alanine residue at position 302 of rat neurotensin receptor was also mutated to glutamate. When expressed in COS-7 cells and Xenopus oocytes, the resulting (A302E) mutant receptor was strongly defective in stimulating phosphatidylinositol turnover in COS-7 cells and evoking Ca(2+)-dependent chloride currents in oocytes. 4. In summary, the present study demonstrates that alanine residue at the C-terminus of third cytoplasmic domain is required for the full activation of Gq proteins and PLC by neurotensin receptors. However, in contrast to other Gq protein-coupled receptors, alanine at the distal third intracellular loop does not play a significant role in CCKB receptor activation of PLC.