Targeting VGLUT2 in Mature Dopamine Neurons Decreases Mesoaccumbal Glutamatergic Transmission and Identifies a Role for Glutamate Co-release in Synaptic Plasticity by Increasing Baseline AMPA/NMDA Ratio.

Maria Papathanou,Åsa Wallén-Mackenzie,Hanna Pettersson,Meaghan Creed,Zisis Bimpisidis,Gilad Silberberg,Sylvie Dumas,Camilla Bellone,Matthijs C Dorst,Christian Lüscher

doi:10.3389/fncir.2018.00064

Abstract

Expression of the Vglut2/Slc17a6 gene encoding the Vesicular glutamate transporter 2 (VGLUT2) in midbrain dopamine (DA) neurons enables these neurons to co-release glutamate in the nucleus accumbens (NAc), a feature of putative importance to drug addiction. For example, it has been shown that conditional deletion of Vglut2 gene expression within developing DA neurons in mice causes altered locomotor sensitization to addictive drugs, such as amphetamine and cocaine, in adulthood. Alterations in DA neurotransmission in the mesoaccumbal pathway has been proposed to contribute to these behavioral alterations but the underlying molecular mechanism remains largely elusive. Repeated exposure to cocaine is known to cause lasting adaptations of excitatory synaptic transmission onto medium spiny neurons (MSNs) in the NAc, but the putative contribution of VGLUT2-mediated glutamate co-release from the mesoaccumbal projection has never been investigated. In this study, we implemented a tamoxifen-inducible Cre-LoxP strategy to selectively probe VGLUT2 in mature DA neurons of adult mice. Optogenetics-coupled patch clamp analysis in the NAc demonstrated a significant reduction of glutamatergic neurotransmission, whilst behavioral analysis revealed a normal locomotor sensitization to amphetamine and cocaine. When investigating if the reduced level of glutamate co-release from DA neurons caused a detectable post-synaptic effect on MSNs, patch clamp analysis identified an enhanced baseline AMPA/NMDA ratio in DA receptor subtype 1 (DRD1)-expressing accumbal MSNs which occluded the effect of cocaine on synaptic transmission. We conclude that VGLUT2 in mature DA neurons actively contributes to glutamatergic neurotransmission in the NAc, a finding which for the first time highlights VGLUT2-mediated glutamate co-release in the complex mechanisms of synaptic plasticity in drug addiction.

Highlights

Drug addiction is a multifaceted neuropsychiatric disease characterized by a neurobiological interplay between the reinforcing effect of supraphysiological dopamine (DA) levels upon initial drug intake and the lasting alterations in glutamatergic synaptic strength upon repeated drug consumption (Volkow and Morales, 2015; Lüscher, 2016)
While DA neurons in the midbrain are confined within the ventral tegmental area (VTA)/substantia nigra pars compacta (SNc) area, Vglut2 gene expression was sparse within this area compared to its expression throughout the other areas of the midbrain, for example the red nucleus (RN) located immediately dorsal of the VTA
Tyrosine hydroxylase (Th) mRNA was used to illustrate the outline of the subareas of the VTA and the SNc (Figures 1C,D ) which was subsequently superimposed on the images of Vglut2 mRNA (Figures 1A,B )

Summary

Introduction

Drug addiction is a multifaceted neuropsychiatric disease characterized by a neurobiological interplay between the reinforcing effect of supraphysiological dopamine (DA) levels upon initial drug intake and the lasting alterations in glutamatergic synaptic strength upon repeated drug consumption (Volkow and Morales, 2015; Lüscher, 2016). In addition to forebrainderived glutamatergic transmission within the NAc, a group of VTA DA neurons possess the capacity for glutamate release within this area. Based on their unique capacity to co-release glutamate and DA, these VTA neurons, often referred to as ‘‘dual-signaling,’’ ‘‘bi-lingual,’’ ‘‘combinatorial’’ or ‘‘co-releasing’’ neurons (El Mestikawy et al, 2011; Trudeau et al, 2014; Pupe and Wallén-Mackenzie, 2015; Morales and Margolis, 2017), might be of particular interest to drug addiction, but their role in VTA circuitry and behavior remains to be fully clarified. Vglut mRNA has been visualized by in situ hybridization throughout several developing brain regions at embryonal day (E) 12.5 in the mouse, and co-localizes already at this stage with TH immunoreactivity within midbrain DA neurons (Birgner et al, 2010; Nordenankar et al, 2014)

Methods

Results

Conclusion