Regulation of norepinephrine transporter abundance by catecholamines and desipramine in vivo

doi:10.1016/S0006-8993(02)02889-5

Brain Research

Volume 946, Issue 2, 16 August 2002, Pages 239-246

https://doi.org/10.1016/S0006-8993(02)02889-5 Get rights and content

Abstract

The norepinephrine transporter (NET) regulates adrenoreceptor signaling by controlling the availability of synaptic norepinephrine (NE), and it is a direct target for some classes of antidepressant drugs. NET levels are normal in dopamine β-hydroxylase knockout (Dbh −/−) mice that lack NE, demonstrating that the NET does not require endogenous NE for appropriate regulation under physiological conditions. In contrast, tyrosine hydroxylase knockout (Th −/−) mice that lack both NE and dopamine (DA) have reduced levels of NET, suggesting that it is down-regulated by a complete absence of catecholamines and not NE per se. Chronic treatment with the NET inhibitor, desipramine (DMI), reduced NET levels in both control and Dbh −/− mice, demonstrating that NE is not required for the regulation of NET by antidepressant drugs. There are some qualitative and quantitative differences in the down-regulation of the NET by catecholamine depletion and DMI treatment, suggesting that different mechanisms may be involved.

Introduction

Norepinephrine (NE) is an abundant neurotransmitter in the peripheral and central nervous systems, where it is involved in many physiological and behavioral processes, including cardiovascular function, metabolism, embryonic development, arousal, seizure susceptibility, maternal behavior, and responses to drugs of abuse [9], [11], [26], [27], [28]. The norepinephrine transporter (NET) is a Na⁺/Cl⁻-dependent transporter expressed by noradrenergic neurons, where it modulates noradrenergic signaling by clearing secreted norepinephrine via selective uptake (e.g. Ref. [1]).

To maintain homeostatic NE signaling, NET levels would be expected to decrease when synaptic NE content is low. The work of Lee et al. [19] supports this, as NET levels decrease when NE is depleted with reserpine. These results suggest that NE levels are sensed by the neuron and NET abundance is then regulated accordingly. However, because reserpine depletes dopamine (DA) and serotonin (5-HT) as well as NE, it is unclear whether the depletion of other monoamines contributes to the observed down-regulation of the NET.

Among the drugs that inhibit the NET and block NE reuptake are antidepressants such as DMI and reboxetine [13]. Blockade of NE reuptake by antidepressant drugs happens on the order of minutes to hours, while alleviation of depressive symptoms requires weeks of chronic drug administration (e.g. Ref. [21]). Therefore, it is thought that adaptive changes that follow chronic reuptake blockade are responsible for the efficacy of antidepressant drugs. One molecular change that follows the time course of clinical efficacy is down-regulation of the NET. For example, chronic, but not acute antidepressant treatment decreases monoamine transporter density in rats [4], [6]. However, it is not known whether the antidepressant-induced increase in extracellular monoamines is required for this effect.

To determine if NE, the primary substrate for the NET, is required for NET regulation under basal conditions or by chronic exposure to antidepressants in vivo, we measured NET levels before and after chronic DMI administration in dopamine β-hydroxylase (DBH) knockout (Dbh −/−) mice. These mice lack NE because DBH is essential for the conversion of DA to NE. Because the NET is capable of transporting DA [14], [22] and Dbh −/− mice have elevated DA levels [30], NET levels were also determined in tyrosine hydroxylase knockout (Th −/−) mice that lack both NE and DA.

Section snippets

Animals

Dbh knockout (Dbh −/−) and Th knockout (Th −/−) mice, maintained on a 129/SvEv and C57BL/6J hybrid background, were developed and generated as described [30], [33] with the following modifications. Th +/− males were bred to Th +/− females. To rescue the embryonic lethality caused by the lack of catecholamines in Th −/− mice, pregnant females were given L-DOPA (50 mg/kg, 1.5 mg/ml) and vitamin C (2.5 mg/ml) both by daily intraperitoneal (i.p.) injection and in the drinking water until the birth

Dbh −/− mice have normal NET levels

NET levels were measured in adult Dbh +/− and Dbh −/− mice by [³H]nisoxetine binding. No differences between genotypes were detected in any of the five brain regions measured (Fig. 1A), demonstrating that NET levels do not change in response to an absence of NE in mice.

Th −/− mice have reduced NET levels

Because DBH converts DA to NE, Dbh −/− mice produce DA from ‘noradrenergic’ terminals by default. Central DA tissue levels are ∼10% higher in Dbh −/− mice, although DA release has not been measured [30]. In addition, the affinity

Overview

Although the regulation of NET levels by NE and antidepressants has been examined both in vitro and in vivo, the mechanisms remain poorly understood. We have used mice with a genetic NE deficiency to show that in vivo, NET levels appear to be modulated by endogenous catecholamines and an antidepressant drug. The absence of all catecholamines reduces NET levels in terminal regions, but a loss of only NE has no effect, suggesting that substrate transport is important for NET regulation. Reduction

Acknowledgements

We thank Sumitomo Pharmaceuticals for the generous donation of l-3,4 dihydroxyphenylserine (DOPS), R. Steiner for use of his anesthesia equipment, N. Rust and N. Miller for maintaining the Dbh mouse colony, M. Hunsley for help with statistical tests, and D. Kim and M. Szczypka for critical reading of this manuscript. This work was supported by the Howard Hughes Medical Institute (D.W. and R.D.P.), the National Alliance for Research on Schizophrenia and Depression and the Department of Veterans

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Research reportRegulation of norepinephrine transporter abundance by catecholamines and desipramine in vivo

Abstract

Introduction

Section snippets

Animals

Dbh −/− mice have normal NET levels

Th −/− mice have reduced NET levels

Overview

Acknowledgements

Brain Res.

Int. J. Dev. Neurosci.

Brain Res.

J. Biol. Chem.

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Brain Res. Mol. Brain Res.

Cell

Cell

Neurotransmitter transporters: recent progress

Annu. Rev. Neurosci.

Acute regulation of norepinephrine transport: II. PKC-modulated surface expression of human norepinephrine transporter proteins

J. Pharmacol. Exp. Ther.

[3H]Desipramine binding to rat brain tissue: binding to both noradrenaline uptake sites and sites not related to noradrenaline neurons

J. Neurochem.

Cocaine and antidepressant-sensitive biogenic amine transporters exist in regulated complexes with protein phosphatase 2A

J. Neurosci.

Effects of chronic antidepressant treatments on serotonin transporter function, density, and mRNA level

J. Neurosci.

Blockade of the noradrenaline carrier increases extracellular dopamine concentrations in the prefrontal cortex: evidence that dopamine is taken up in vivo by noradrenergic terminals

J. Neurochem.

Importance of the noradrenaline-dopamine coupling in the locomotor activating effects of D-amphetamine

J. Neurosci.

Research report
Regulation of norepinephrine transporter abundance by catecholamines and desipramine in vivo

[³H]Desipramine binding to rat brain tissue: binding to both noradrenaline uptake sites and sites not related to noradrenaline neurons