Elsevier

Neuropharmacology

Volume 58, Issue 8, June 2010, Pages 1199-1205
Neuropharmacology

Hyperactivity of the dopaminergic system in NTS1 and NTS2 null mice

https://doi.org/10.1016/j.neuropharm.2010.02.015Get rights and content

Abstract

Neurotensin (NT) is a tridecapeptide that acts as a neuromodulator in the central nervous system mainly through two NT receptors, NTS1 and NTS2. The functional–anatomical interactions between NT, the mesotelencephalic dopamine system, and structures targeted by dopaminergic projections have been studied. The present study was conducted to determine the effects of NT receptor subtypes on dopaminergic function with the use of mice lacking either NTS1 (NTS1−/−) or NTS2 (NTS2−/−). Basal and amphetamine-stimulated locomotor activity was determined. In vivo microdialysis in freely moving mice, coupled with HPLC-ECD, was used to detect basal and d-amphetamine-stimulated striatal extracellular dopamine levels. In vitro radioligand binding and synaptosomal uptake assays for the dopamine transporters were conducted to test for the expression and function of the striatal pre-synaptic dopamine transporter. NTS1−/− and NTS2−/− mice had higher baseline locomotor activity and higher basal extracellular dopamine levels in striatum. NTS1−/− mice showed higher locomotor activity and exaggerated dopamine release in response to d-amphetamine. Both NTS1−/− and NTS2−/− mice exhibited lower dopamine D1 receptor mRNA expression in the striatum relative to wild type mice. Dopamine transporter binding and dopamine reuptake in striatum were not altered. Therefore, lack of either NTS1 or NTS2 alters the dopaminergic system. The possibility that the dysregulation of dopamine transmission might stem from a deficiency in glutamate neurotransmission is discussed. The data strengthen the hypothesis that NT receptors are involved in the pathogenesis of schizophrenia and provide a potential model for the biochemical changes of the disease.

Introduction

The interaction between neurotensin (NT) and dopamine (DA) has been implicated in the pathogenesis and treatment of schizophrenia (Nemeroff et al., 1983b). The involvement of the dopaminergic system in the behavioral effects of schizophrenia has emerged from the observations that drugs used to treat schizophrenia act through blocking DA receptors (Carlsson and Lindqvist, 1963) and that DA agonists such as d-amphetamine, provoke certain psychotic reactions in normal humans and exacerbate some psychotic symptoms in schizophrenics (Gainetdinov et al., 2001).

NT is a neuropeptide that is co-localized with, and is expressed throughout the brain within the nigrostriatal and mesolimbic DA system (Jennes et al., 1982, Leeman and Carraway, 1982) modulating DA neurotransmission (Fuxe et al., 1992b, Kitabgi et al., 1989, Lambert et al., 1995). Additionally, NT and NT agonists exhibit neuroleptic-like properties in DA-mediated animal models of psychosis, such as amphetamine-induced locomotor activity, apomorphine-induced climbing, and drug-induced disruption of prepulse inhibition (PPI) (Boules et al., 2001, Cusack et al., 2000, Ervin et al., 1981, Feifel et al., 1999, Ford and Marsden, 1990, Jolicoeur et al., 1983, Jolicoeur et al., 1993, Kalivas et al., 1983, Kalivas et al., 1984, Nemeroff et al., 1983a, Sarhan et al., 1997, Shilling et al., 2003).Hence, NT has been labeled the endogenous neuroleptic (Nemeroff, 1980).

NT mediates its effects through its receptors that were first identified by radioligand binding techniques. The two, most widely studied receptors are: the high affinity, levocabastine-insensitive NT receptor subtype 1 (NTS1) (Tanaka et al., 1990, Vita et al., 1993) and the low affinity, levocabastine-sensitive subtype 2 receptor (NTS2) (Chalon et al., 1996, Mazella et al., 1996). NTS1 is localized both pre- and post-synaptically with DA D2 receptors (Delle Donne et al., 2004) in the striatum, ventral midbrain, and nucleus accumbens. Its activation stimulates Ca++ release (Beauregard et al., 1992) and modulates DA transmission (Gully et al., 1993, Leonetti et al., 2004). Additionally, the lack of NT causes altered responses to antipsychotic drugs in mice (Dobner et al., 2001). NTS2 is widely distributed throughout the brain (Sarret et al., 2003) and has been implicated in spinal analgesia (Sarret et al., 2005).

Several studies have shown behavioral differences that are relevant to schizophrenia between wild type and NT or NT receptor knockout mice. Kinkead et al. (2005) reported that NT knockout mice had reduced PPI and were not sensitive to the PPI-disrupting effects of amphetamine as compared to wild type mice, a result suggesting the importance of endogenous NT in the effects of amphetamine on PPI. Additionally, administration of NT receptor agonists reversed apomorphine-induced climbing in wild type, but not in NTS1−/− mice (Mechanic et al., 2009), and administration of PD149163, an NT receptor agonist, significantly facilitated PPI and decreased the acoustic startle response in wild type but not in NTS1−/− mice (Feifel et al., 2010).

Uncovering the roles NT receptors play in the brain has been approached with the use NT receptor agonists and antagonists, as well as mice lacking NT or its receptors (NTS1 or NTS2). In the present study, we used mice lacking either NTS1 or NTS2 to determine further the role of NT receptors on DA neurotransmission. Our results suggest that both NT receptor subtypes are necessary for normal dopaminergic function.

Section snippets

Materials and methods

All animal protocols were approved by the Mayo Foundation Institutional Animal Use and Care Committee in accordance with the NIH Guide for Care and Use of Laboratory Animals. All animals were kept in an environmentally controlled room with 12 h light/dark cycle and free access to water and food.

NTS1−/− and NTS2−/− mice

NTS1−/− and NTS2−/− knockout mice were viable at birth with no obvious physical abnormalities.

Discussion

NTS1−/− mice exhibited an increased baseline locomotor activity in a novel environment and increased sensitivity to d-amphetamine-induced hyperactivity as compared to wild type mice. This increase in baseline activity in the null mice is contradictory to results reported by Remaury et al. (2002). The discrepancy in these locomotor activity results may be due to differences in the experimental designs. In the present study, the mice were acclimated to the activity chamber for 2 h and the

Acknowledgments

This work was funded by NIMH grant # MH71241 and Mayo Foundation for Medical Education and Research.

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    Yanqi Liang and Mona Boules contributed equally to this study.

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