Dopaminergic Terminals in the Nucleus Accumbens But Not the Dorsal Striatum Corelease Glutamate

Abstract

Coincident signaling by dopamine and glutamate is thought to be crucial for a variety of motivated behaviors. Previous work has suggested that some midbrain dopamine neurons are themselves capable of glutamate corelease, but this phenomenon remains poorly understood. Here, we expressed the light-activated cation channel Channelrhodopsin-2 (ChR2) in genetically defined midbrain dopamine neurons to stimulate exocytosis specifically from dopaminergic terminals in both the nucleus accumbens (NAc) shell and dorsal striatum of brain slices from adult mice. Optical activation resulted in robust glutamate-mediated EPSCs in all medium spiny neurons examined in the NAc shell. In contrast, optically evoked glutamatergic currents were nearly undetectable in the dorsal striatum. Further, we used a conditional knock-out mouse lacking vesicular glutamate transporter 2 (VGLUT2) specifically in dopamine neurons to determine whether VGLUT2 is required for the exocytotic release of glutamate from dopamine neurons. Our data show that conditional knock-out completely abolished all optically evoked glutamate release. These results provide definitive physiological evidence for VGLUT2-mediated glutamate release by mature dopamine neurons projecting to the NAc shell, but not to the dorsal striatum. Thus, the unique ability of NAc-projecting dopamine neurons to synchronously activate both dopamine and glutamate receptors may have crucial implications for the ability to respond to motivationally significant stimuli.

Full text

Brief Communications
Dopaminergic Terminals in the Nucleus Accumbens But Not
the Dorsal Striatum Corelease Glutamate
Garret D. Stuber,
1,2
* Thomas S. Hnasko,
3
* Jonathan P. Britt,
1,2
Robert H. Edwards,
2,3
and Antonello Bonci
1,2
1
Ernest Gallo Clinic and Research Center, University of California, San Francisco, Emeryville, California 94608, and Departments of
2
Neurology,
and
3
Physiology, University of California, San Francisco, San Francisco, California 94143
Coincident signaling by dopamine and glutamate is thought to be crucial for a variety of motivated behaviors. Previous work has
suggested that some midbrain dopamine neurons are themselves capable of glutamate corelease, but this phenomenon remains
poorly understood. Here, we expressed the light-activated cation channel Channelrhodopsin-2 (ChR2) in genetically defined
midbrain dopamine neurons to stimulate exocytosis specifically from dopaminergic terminals in both the nucleus accumbens
(NAc) shell and dorsal striatum of brain slices from adult mice. Optical activation resulted in robust glutamate-mediated EPSCs in
all medium spiny neurons examined in the NAc shell. In contrast, optically evoked glutamatergic currents were nearly undetect-
able in the dorsal striatum. Further, we used a conditional knock-out mouse lacking vesicular glutamate transporter 2 (VGLUT2)
specifically in dopamine neurons to determine whether VGLUT2 is required for the exocytotic release of glutamate from dopamine
neurons. Our data show that conditional knock-out completely abolished all optically evoked glutamate release. These results
provide definitive physiological evidence for VGLUT2-mediated glutamate release by mature dopamine neurons projecting to the
NAc shell, but not to the dorsal striatum. Thus, the unique ability of NAc-projecting dopamine neurons to synchronously activate
both dopamine and glutamate receptors may have crucial implications for the ability to respond to motivationally significant
stimuli.
Introduction
The role that a neuron plays within a circuit depends to a large extent
on the neurotransmitter(s) released. Midbrain dopamine neurons
project to a variety of forebrain targets, including the dorsal striatum
and nucleus accumbens, where the release of dopamine is thought to
contribute to the generation of motor function and motivated be-
haviors (Wise, 2004). As a variety of adaptive behaviors are ascribed
to specific striatal subregions, it is possible that the striatal microcir-
cuits that underlie these specific behavioral processes have distinct
properties, such as differences in afferent or efferent connectivity.
Accumulating evidence suggests that a subset of midbrain do-
pamine neurons may also corelease the excitatory neurotrans-
mitter glutamate both in vitro (Sulzer et al., 1998; Joyce and
Rayport, 2000; Dal Bo et al., 2004) and in vivo (Chuhma et al.,
2004; Lavin et al., 2005; Hnasko et al., 2010). However, this pos-
sibility has generated controversy due to difficulty stimulating
selectively dopamine neurons or their terminals.
Vesicular glutamate transporters package glutamate into syn-
aptic vesicles and are both necessary and sufficient for the exocy-
totic release of glutamate from neurons (Reimer and Edwards,
2004; Takamori, 2006). In culture, dopamine neurons immuno-
stain for the vesicular glutamate transporter VGLUT2 (Dal Bo et
al., 2004). However the histochemical evidence for VGLUT2 ex-
pression by dopamine neurons in vivo remains less clear. One
study found substantial colocalization of mRNAs encoding the
dopaminergic marker tyrosine hydroxylase (TH) and VGLUT2
in the ventral midbrain, with a higher incidence of colocalization
in medial and caudal regions of the A10 dopamine cell group
(Kawano et al., 2006). A second report detected colocalization only
very rarely, but medial aspects were not examined (Yamaguchi et al.,
2007). Using single-cell RT-PCR and immuno-electron microscopy,
Trudeau and colleagues detected substantial colocalization of
VGLUT2 transcript in dopamine neurons (Mendez et al., 2008) and
VGLUT2 protein in TH⫹terminals of the nucleus accumbens (Des-
carries et al., 2008; Bérubé-Carriére et al., 2009). However, the level
of colocalization declined significantly across development (Mendez
et al., 2008; Be´rube´ -Carrie`re et al., 2009), suggesting that glutamate
corelease may similarly downregulate in adulthood.
To determine whether dopamine neurons release synapti-
cally relevant amounts of glutamate in the adult brain, a
method to selectively stimulate exocytotic release from dopa-
mine terminals is required. We therefore used a multifaceted
strategy, combining optogenetics and conditional gene dis-
ruption to selectively stimulate dopaminergic terminals while
recording postsynaptic currents from medium spiny neurons
(MSNs) in either the ventral or dorsal striatum of control or
conditional knock-out mice lacking VGLUT2 specifically in
dopamine neurons.
Received April 6, 2010; revised May 5, 2010; accepted May 11, 2010.
Thisworkwas supportedby ABMRFand National Alliancefor Researchon SchizophreniaandDepression (G.D.S.),
the A. P. Giannini Foundation (T.S.H.), and the State of California for Medical Research on Alcohol and Substance
Abuse through the University of California, San Francisco (UCSF) (A.B.). We thank Kurt Thorn and the Nikon Imaging
Center at UCSF for assistance and Karl Deisseroth for the AAV-DIO-ChR2-mcherry construct.
*G.D.S. and T.S.H. contributed equally to this work.
Correspondenceshould be addressedto Dr.Antonello Bonci,Department ofNeurology andErnest GalloClinic and
Research Center, University of California, San Francisco, Emeryville, CA 94608. E-mail: antonello.bonci@ucsf.edu.
DOI:10.1523/JNEUROSCI.1754-10.2010
Copyright © 2010 the authors 0270-6474/10/308229-05$15.00/0
The Journal of Neuroscience, June 16, 2010 •30(24):8229 – 8233 • 8229

Materials and Methods
Experimental subjects. Age-matched, sex-matched
adult male and female mice (⬃25 g) were used
as subjects. Control mice carried one copy of
cre recombinase driven by dopamine trans-
porter regulatory elements (Zhuang et al.,
2005) and one conditional VGLUT2 allele
(Hnasko et al., 2010) (Slc6a3
⫹/cre
; Slc17a6
⫹/lox
).
Conditional VGLUT2 knock-out mice were
identical but carried two conditional VGLUT2
alleles (Slc6a3
⫹/cre
; Slc17a6
lox/lox
). Mice were
group housed in a colony maintained with a
standard 12 h light/dark cycle and given food
and water ad libitum. Experiments were con-
ducted in accordance with the Guide for the
Care and Use of Laboratory Animals, as adopted
by the National Institutes of Health, and with
approval of the University of California, San
Francisco Institutional Animal Care and Use
Committee.
Stereotaxic recombinant adeno-associated vi-
rus injection. Methods were adapted from (Tsai
et al., 2009). Briefly, mice were anesthetized
with ketamine/xylazine, placed in a stereotaxic
frame (Kopf), the skull leveled, small holes
drilled, and 1
␮
l (0.25
␮
l/min) of AAV5-EF1
␣
-
DIO-ChR2-mcherry (⬃3⫻10
12
genomes/ml)
was injected using a Hamilton syringe bilater-
ally into the VTA (coordinates in mm relative
to bregma: ⫺3.25 anteroposterior, ⫺4.50 dor-
soventral, and ⫾0.5 mediolateral). Mice were
allowed to recover for at least 3 weeks before
further procedures.
Immunohistochemistry. Mice were perfused
with cold PBS followed by 4% paraformalde-
hyde; their brains were removed, postfixed,
cryoprotected in 30% sucrose, and frozen in
superchilled isopentane; and 35
␮
m sections
were cut on a cryostat and floated in PBS. Sec-
tions were rinsed with PBS containing 0.2%
Triton X-100, blocked 1 h with 4% normal
donkey serum, and incubated overnight at 4°C
in sheep anti-TH (Pel-Freze) and rabbit anti-
DsRed (Clontech), both at 1:2000 dilution in
blocking solution. Sections were rinsed, incubated 2 h with Cy2-
conjugated anti-sheep and DyLite549-conjugated anti-rabbit secondar-
ies (1:500, Jackson ImmunoResearch), rinsed again, mounted on slides,
dehydrated through alcohol/xylenes, and coverslipped with DPX.
Images were collected using a Nikon Eclipse Ti-E motorized inverted
microscope equipped with epifluorescence and a Photometrics Coolsnap
HQ2 camera or a Nikon FN1 upright C1si spectral confocal microscope.
For quantification of TH/ChR2-mcherry
⫹
neurons in the VTA, confocal
images were taken using a 60⫻objective. ChR2-mcherry
⫹
neurons were
identified and then scored for TH content.
Brain slice preparation. Mice were anesthetized and rapidly decapi-
tated. Dissected brains were transferred to ice-cold artificial CSF (ACSF)
containing the following (in mM): 75 sucrose, 87 NaCl, 2.5 KCl, 7 MgCl
2
,
0.5 CaCl
2
, 25 NaHCO
3
, 1.25 NaH
2
PO
4
, and 1 ascorbic acid (saturated
with 95% O
2
and 5% CO
2
). Coronal sections of the striatum (200
␮
m)
were cut with a vibratome (VT1200, Leica). Slices were incubated for at
least 30 min in a holding chamber containing the following (in mM): 126
NaCl, 2.5 KCl, 1.2 MgCl
2
, 2.5 CaCl
2
, 26 NaHCO
3
, 1.2 NaH
2
PO
4
,11
glucose, and 1 ascorbic acid (saturated with 95% O
2
and 5% CO
2
). Dur-
ing recording, slices were superfused (2 ml/min) with this same ACSF at
⬃32°C but with picrotoxin (100
␮
M, to block GABA
A
receptor-mediated
synaptic currents) and without ascorbic acid.
Patch-clamp electrophysiology. Whole-cell voltage-clamp recordings
from MSNs located in the nucleus accumbens (NAc) shell and dorsal
striatum (DS) were obtained under visual control on a differential inter-
ference contrast, upright microscope with infrared illumination. Record-
ings were obtained using 3– 6 M⍀resistance pipettes backfilled with
internal solution containing the following (in mM): 120 CsCH
3
SO
3
,20
HEPES, 0.4 EGTA, 2.8 NaCl, 5 N(CH
2
CH
3
)
4
Cl, 2.5 Mg-ATP, and 0.25
Mg-GTP, pH 7.3. Currents were measured using either an Axopatch 1D
or 200A amplifier (2 kHz low-pass Bessel filter) with a DigiData 1440
interface (5 kHz digitization) and pClamp software (Molecular Devices).
MSNs, identified by their morphology and hyperpolarized resting mem-
brane potential, were voltage clamped at ⫺70 mV. An optical fiber (200
␮
m core diameter, 0.2 numerical aperture) coupled to a diode-pumped
solid-state 473 nm laser was placed 200
␮
m from the site of recording,
and was used to deliver optical stimulation at a frequency of 0.1 Hz.
Stimulus intensity ranged from 1 to 30 mW with a pulse duration of 5 ms.
Series resistance (5–20 M⍀) was monitored online witha5mVhyper-
polarizing step (50 ms) given after each stimulus.
To quantify EPSC amplitudes, six sweeps were collected from each cell
at each light intensity (0, 1, 2, 5, and 10 mW for input/output experiment,
30 mW for maximal stimulation and pharmacology experiments).
Sweeps were then averaged offline to determine EPSC amplitude at a
particular intensity. Light stimulus intensities were presented in a ran-
dom order and counterbalanced across recordings. For AMPA receptor
(AMPAR) antagonism experiments, 10
␮
M6,7-dinitroquinoxaline-2,3-
dione (DNQX) was used, and for dopamine (DA) receptor antagonism
experiments, 2
␮
MR-(⫹)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-
Figure 1. Expression of ChR2-mcherry in midbrain dopamine neurons and their projections to the striatum. A, Immunostaining
for ChR2-mcherry (red) and TH (green) in the VTA/SN ⬃30 d after injection of AAV-DIO-ChR2-mcherry in a mouse expressing cre
recombinase under the control of the DAT promoter. B, Confocal microscopy shows that nearly all ChR2-mcherry-expressing
neurons in the VTA colabel for TH. C, Staining as described above in a coronal plane including both DS and NAc shell. D, Confocal
images of dopaminergic fibers in the NAc shell indicate that essentially all ChR2-mcherry-labeled puncta also contain TH.
8230 •J. Neurosci., June 16, 2010 •30(24):8229 – 8233 Stuber et al. •Optogenetic Glutamate Corelease from Dopamine Terminals

tetrahydro-1H-3-benzazepine (SCH23390) and
2
␮
Mraclopride were used to block D
1
and D
2
receptors, respectively. Experimenters were blind
to the animal’s genotype during data acquisition.
Fast-scan cyclic voltammetry. Methods were
adapted from previous work (Stuber et al.,
2008). T-650 carbon fiber microelectrodes
(100 –200
␮
m in length) were used for detec-
tion of dopamine. Electrodes were placed in
the NAc shell or dorsal striatum of Slc6a3
⫹/cre
;
Slc17a6
⫹/lox
mice. The potential applied to the
electrode was ramped from ⫺0.4Vto⫹1.3 V
to ⫺0.4 V versus a Ag/AgCl reference every 100
ms at a rate of 400 V/s, and resulting electro-
chemical data were acquired using custom-
written software in LabVIEW. Electrochemical
data were low-pass filtered at 1 kHz offline.
Background-subtracted cyclic voltammograms
were generated immediately after optical stimu-
lation of the slice, and were characteristic of do-
pamine (peak oxidation potential 600 –700 mV).
Current resulting from the oxidation of dopa-
mine (monitored at the peak oxidation potential
determined from cyclic voltammograms) were
then converted into concentration changes using
a calibration factor of 10 nA/
␮
MDA.
Results
An adeno-associated virus encoding a
conditional allele of the light-activated
cation channel channelrhodopsin-2 (ChR2)
activated by cre-mediated recombination
(AAV5-DIO-ChR2-mcherry) (Tsai et al.,
2009) was bilaterally injected into the ven-
tral tegmental area (VTA) of adult mice
(2– 4 months old) expressing cre recom-
binase under the control of dopamine
transporter (DAT) regulatory sequences
(Zhuang et al., 2005). Four weeks after in-
jection, immunohistochemistry revealed
robust, selective expression of ChR2-mch-
erry in VTA dopamine neurons (Fig. 1 A,B).
Indeed, 99.5% of ChR2-mcherry
⫹
neurons
identified in the VTA (n⫽409 cells) were
also labeled for the catecholamine biosyn-
thetic enzyme TH. Furthermore, essen-
tially all ChR2-mcherry-labeled fibers and
puncta in striatal terminals costained for
TH (Fig. 1C,D).
We then prepared coronal sections
through striatal regions from injected
mice and light-evoked stimulation was
used to depolarize and stimulate neuro-
transmitter release from the dopaminer-
gic terminals selectively expressing ChR2.
Whole-cell voltage-clamp recordings from
visually identified MSNs revealed that op-
tical stimulation of dopaminergic termi-
nals in the NAc shell resulted in EPSCs
⬎10 pA in 24/24 neurons, which in-
creased in amplitude as a function of light
stimulus intensity (Fig. 2A,B). Light-
evoked EPSCs in the NAc shell were
blocked by AMPAR antagonism (Fig.
3A,C). The rapid response to light sug-
Figure 2. Dopaminergic terminals in the nucleus accumbens shell but not dorsal striatum corelease glutamate. A, Example
EPSCsrecordedin theNAcshell at⫺70 mV followinga5msbluelight pulseatmaximal intensity.B,Light-evoked EPSCamplitudes
recorded in the NAc shell were significantly greater in DAT:VGLUT2 heterozygous mice than in conditional knock-outs (n⫽9 –24
cells per group; 2-way ANOVA for the interaction of genotype and light intensity: F
(4,128)
⫽2.56, p⬍0.05). C, Example EPSCs
recorded in the DS at ⫺70 mV followinga5msblue light pulse at maximal intensity. D, Light-evoked EPSC amplitudes recorded
in the DS were not significantly different in DAT:VGLUT2 heterozygous mice versus conditional knock-outs as a function of light
intensity (2-way ANOVA for the interaction of genotype and light intensity: F
(4,87)
⫽0.31, p⫽0.91). EPSC amplitudes (indepen-
dent of light intensity) were significantly different across genotypes (F
(1,87)
⫽12.80, p⫽0.006).
Figure 3. EPSCs evoked by dopamine terminal stimulation in the nucleus accumbens shell are blocked by AMPAR antagonism
but unaffected by D
1
R/D
2
R antagonism. A, Example EPSCs recorded at ⫺70 mV in the NAc shell from DAT:VGLUT2 heterozygous
mice before and after bath application of DNQX. B, Example EPSCs recorded at ⫺70 mV in the NAc shell from DAT:VGLUT2
heterozygous mice before and after bath application of SCH23390/raclopride. C, Light-evoked EPSCs from dopamine terminals
were significantly reduced by DNQX bath application (n⫽5 cells; t
(4)
⫽2.61; p⬍0.05) but not by SCH23390/raclopride (n⫽9
cells; t
(8)
⫽0.40; p⫽0.70). All data indicate mean ⫾SEM.
Stuber et al. •Optogenetic Glutamate Corelease from Dopamine Terminals J. Neurosci., June 16, 2010 •30(24):8229 – 8233 • 8231

gests a monosynaptic event, but to ensure
that the glutamate-mediated currents did
not reflect the postsynaptic action of do-
pamine, we applied D
1
- and D
2
-type do-
pamine receptor antagonists to the bath.
The addition of SCH23390 and raclopride
did not significantly affect light-evoked
EPSCs (Fig. 3 B,C), indicating that they do
not require the light-evoked release of do-
pamine. Rather, dopamine neurons them-
selves appear to release glutamate.
The ability of dopamine neurons to re-
lease glutamate implies that they express a
vesicular glutamate transporter. To deter-
mine whether VGLUT2 expression by do-
pamine neurons is required for their
release of glutamate, we performed the
same experiments in conditional knock-
out (cKO) mice (Slc6a3
⫹/cre
; Slc17a6
lox/lox
)
where cre recombinase drives both selec-
tive excision in dopamine neurons of the
gene encoding VGLUT2 (Hnasko et al.,
2010) and, following virus injection, the
selective expression of ChR2 in dopamine
neurons. In contrast to control mice, light
never evoked EPSCs in cKO mice (Fig.
2A,B), demonstrating that VGLUT2 is re-
quired for glutamate packaging and re-
lease from dopaminergic terminals.
Although previous results have sug-
gested that mesolimbic dopamine neu-
rons release glutamate (Chuhma et al.,
2004; Hnasko et al., 2010), it is unknown
whether dopamine neurons that project
to the DS also have the capacity to release
glutamate. Therefore, we also examined
brain slices through the DS. Optical stim-
ulation of dopaminergic terminals in the
DS resulted in ⬃10-fold smaller mean glutamate-mediated cur-
rents than in the NAc shell of control mice, with no detectable
release in the DS from mice lacking VGLUT2 in dopamine neu-
rons (Fig. 2C,D). Importantly, light-evoked dopamine release as
measured by fast-scan cyclic voltammetry was detected in both
the DS (Fig. 4A,C) and NAc shell (Fig. 4B,D) of virally injected
mice. Within the striatum, glutamate corelease is thus relatively
restricted to dopamine terminals in the ventral striatum.
Discussion
Our results provide direct physiological evidence that mature
dopamine neurons projecting to the NAc shell corelease gluta-
mate. Previous studies have suggested that mesolimbic dopamine
neurons release glutamate (Chuhma et al., 2004, 2009; Lavin et
al., 2005; Hnasko et al., 2010), but electrical stimulation nonspe-
cifically activates all VTA neurons, making it difficult to exclude
glutamate release from nondopaminergic neurons. Indeed, there
exist VGLUT2-positive neurons in the VTA that do not colabel
for TH (Kawano et al., 2006; Yamaguchi et al., 2007; Nair-
Roberts et al., 2008). Although some of these neurons appear to
project locally (Dobi et al., 2010), it is unknown whether they
may also project to striatal and/or limbic brain regions. Our abil-
ity to drive ChR2 expression selectively in dopamine neurons of
the VTA and thus to selectively activate dopamine terminals in
striatal regions with light eliminates the possibility of inadver-
tently activating these nondopaminergic VTA glutamate neu-
rons. Essentially all ChR2-mcherry-expressing neurons in the
midbrain and puncta in the striatum were also labeled for TH
by confocal microscopy (Fig. 1B,D). Additionally, no ChR2-
mcherry-expressing neurons were seen in brain regions known to
send glutamatergic projections to the NAc shell, such as cortex,
thalamus, hippocampus, amygdala, or lateral hypothalamus.
Surprisingly, we did observe small clusters of ChR2-expressing
neurons in both the lateral habenula and in the ventral premam-
millary nucleus (data not shown), but these neurons are not
known to project to the NAc shell.
Due to technical limitations, earlier electrophysiological stud-
ies of glutamate corelease in the NAc shell have reported relatively
small AMPAR currents limited to brain slices made from young
(⬃3 weeks old) animals (Chuhma et al., 2004; Hnasko et al.,
2010). Given that VGLUT2 expression by dopamine neurons
appears to upregulate in culture (Dal Bo et al., 2004; Mendez et
al., 2008) and downregulate over development in vivo (Mendez et
al., 2008; Be´rube´-Carrie`re et al., 2009), it has remained unclear
whether mature dopamine neurons also corelease glutamate.
However, we now find that VGLUT2 in dopamine neurons is
required for glutamate release from dopaminergic terminals in
the NAc shell of adult mice, demonstrating that VGLUT2-
dependent glutamate corelease persists into adulthood. Although
some evidence suggests that only a small fraction of dopamine
Figure 4. Optical stimulation of dopamine terminals in dorsal striatum and nucleus accumbens shell results in dopamine
release. A, Example trace of dopamine release resulting from optical stimulation of dopamine terminals in the DS (1 pulse, 5 ms
pulse duration). Inset shows background-subtracted cyclic voltammograms taken immediately after optical stimulation. The color
plot below shows current measured as the electrode across the entire scanned potential range. B, Example trace of dopamine
release resulting from optical stimulation of dopamine terminals in the NAc shell. C, Average dopamine release (solid line; dashed
lines indicate ⫾SEM) in the dorsal striatum following one-pulse optical stimulation (n⫽7 recording sites). D, Average dopamine
release in the nucleus accumbens shell following one-pulse optical stimulation (n⫽5 recording sites).
8232 •J. Neurosci., June 16, 2010 •30(24):8229 – 8233 Stuber et al. •Optogenetic Glutamate Corelease from Dopamine Terminals

neurons express VGLUT2 in adult rodents (Yamaguchi et al.,
2007), we see ubiquitous light-evoked glutamatergic currents in
MSNs of the NAc shell. Therefore, it is likely that a significant
number of dopamine neurons in the adult VTA express VGLUT2
at levels too low to detect by conventional means, or alternatively,
a small percentage of VGLUT2-expressing dopamine neurons
may send axon collaterals to essentially all MSNs in the NAc shell.
Although we were able to observe substantial ChR2-mcherry
expression and light-evoked dopamine release in the DS, gluta-
mate currents were much smaller and less frequent in the DS
relative to the NAc shell. These results are consistent with a pre-
vious report (Kawano et al., 2006) showing the absence of
VGLUT2 in the substantia nigra pars compacta compared to the
VTA. More recently, we reported that cKO mice lacking
VGLUT2 selectively in dopamine neurons show a presynaptic
reduction in dopamine storage and release that is restricted to the
ventral striatum (Hnasko et al., 2010). Together, these findings
strongly suggest that within the striatum, VGLUT2 expression
and glutamate corelease is restricted to mesolimbic projections,
which may also extend to other limbic structures (Lavin et al.,
2005).
In conclusion, our findings provide unequivocal evidence that
a subset of dopaminergic terminals release glutamate capable of
postsynaptic signaling. Mice lacking VGLUT2 in dopamine neu-
rons show deficits in psychostimulant-induced locomotion
(Hnasko et al., 2010; Walle´n-Mackenzie et al., 2010), apparently
due to a presynaptic role for glutamate in vesicular monoamine
filling (Amilhon et al., 2010; Hnasko et al., 2010). Although we
still understand little about the role for glutamate released by
monoamine neurons as an independent signal, rapid, synaptic
signaling mediated by glutamate may provide the temporal spec-
ificity required for reward prediction and/or incentive salience
(Lapish et al., 2006, 2007).
References
Amilhon B, Lepicard E, Renoir T, Mongeau R, Popa D, Poirel O, Miot S, Gras
C, Gardier AM, Gallego J, Hamon M, Lanfumey L, Gasnier B, Giros B, El
Mestikawy S (2010) VGLUT3 (vesicular glutamate transporter type 3)
contribution to the regulation of serotonergic transmission and anxiety.
J Neurosci 30:2198–2210.
Be´ rube´-Carrie` re N, Riad M, Dal Bo G, Le´vesque D, Trudeau LE, Descarries L
(2009) The dual dopamine-glutamate phenotype of growing mesence-
phalic neurons regresses in mature rat brain. J Comp Neurol 517:
873–891.
Chuhma N, Zhang H, Masson J, Zhuang X, Sulzer D, Hen R, Rayport S
(2004) Dopamine neurons mediate a fast excitatory signal via their glu-
tamatergic synapses. J Neurosci 24:972–981.
Chuhma N, Choi WY, Mingote S, Rayport S (2009) Dopamine neuron glu-
tamate cotransmission: frequency-dependent modulation in the
mesoventromedial projection. Neuroscience 164:1068–1083.
Dal Bo G, St-Gelais F, Danik M, Williams S, Cotton M, Trudeau LE (2004)
Dopamine neurons in culture express VGLUT2 explaining their capacity
to release glutamate at synapses in addition to dopamine. J Neurochem
88:1398–1405.
Descarries L, Be´rube´-Carrie`re N, Riad M, Bo GD, Mendez JA, Trudeau LE
(2008) Glutamate in dopamine neurons: synaptic versus diffuse trans-
mission. Brain Res Rev 58:290–302.
Dobi A, Margolis EB, Wang HL, Harvey BK, Morales M (2010) Glutama-
tergic and nonglutamatergic neurons of the ventral tegmental area estab-
lish local synaptic contacts with dopaminergic and nondopaminergic
neurons. J Neurosci 30:218–229.
Hnasko TS, Chuhma N, Zhang H, Goh GY, Sulzer D, Palmiter RD, Rayport S,
Edwards RH (2010) Vesicular glutamate transport promotes dopamine
storage and glutamate corelease in vivo. Neuron 65:643–656.
Joyce MP, Rayport S (2000) Mesoaccumbens dopamine neuron synapses
reconstructed in vitro are glutamatergic. Neuroscience 99:445–456.
Kawano M, Kawasaki A, Sakata-Haga H, Fukui Y, Kawano H, Nogami H,
Hisano S (2006) Particular subpopulations of midbrain and hypotha-
lamic dopamine neurons express vesicular glutamate transporter 2 in the
rat brain. J Comp Neurol 498:581–592.
Lapish CC, Seamans JK, Chandler LJ (2006) Glutamate-dopamine cotrans-
mission and reward processing in addiction. Alcohol Clin Exp Res
30:1451–1465.
Lapish CC, Kroener S, Durstewitz D, Lavin A, Seamans JK (2007) The abil-
ity of the mesocortical dopamine system to operate in distinct temporal
modes. Psychopharmacology (Berl) 191:609–625.
Lavin A, Nogueira L, Lapish CC, Wightman RM, Phillips PE, Seamans JK
(2005) Mesocortical dopamine neurons operate in distinct temporal do-
mains using multimodal signaling. J Neurosci 25:5013–5023.
Mendez JA, Bourque MJ, Dal Bo G, Bourdeau ML, Danik M, Williams S,
Lacaille JC, Trudeau LE (2008) Developmental and target-dependent
regulation of vesicular glutamate transporter expression by dopamine
neurons. J Neurosci 28:6309–6318.
Nair-Roberts RG, Chatelain-Badie SD, Benson E, White-Cooper H, Bolam
JP, Ungless MA (2008) Stereological estimates of dopaminergic,
GABAergic and glutamatergic neurons in the ventral tegmental area, sub-
stantia nigra and retrorubral field in the rat. Neuroscience 152:1024–
1031.
Reimer RJ, Edwards RH (2004) Organic anion transport is the primary
function of the SLC17/type I phosphate transporter family. Pflugers Arch
447:629–635.
Stuber GD, Klanker M, de Ridder B, Bowers MS, Joosten RN, Feenstra MG,
Bonci A (2008) Reward-predictive cues enhance excitatory synaptic
strength onto midbrain dopamine neurons. Science 321:1690–1692.
Sulzer D, Joyce MP, Lin L, Geldwert D, Haber SN, Hattori T, Rayport S
(1998) Dopamine neurons make glutamatergic synapses in vitro. J Neu-
rosci 18:4588–4602.
Takamori S (2006) VGLUTs: ‘exciting’ times for glutamatergic research?
Neurosci Res 55:343–351.
Tsai HC, Zhang F, Adamantidis A, Stuber GD, Bonci A, de Lecea L, Deisseroth
K (2009) Phasic firing in dopaminergic neurons is sufficient for behav-
ioral conditioning. Science 324:1080–1084.
Walle´n-Mackenzie A, Wootz H, Englund H (2010) Genetic inactivation of
the vesicular glutamate transporter 2 (VGLUT2) in the mouse: what have
we learnt about functional glutamatergic neurotransmission? Ups J Med
Sci 115:11–20.
Wise RA (2004) Dopamine, learning and motivation. Nat Rev Neurosci
5:483–494.
Yamaguchi T, Sheen W, Morales M (2007) Glutamatergic neurons are
present in the rat ventral tegmental area. Eur J Neurosci 25:106–118.
Zhuang X, Masson J, Gingrich JA, Rayport S, Hen R (2005) Targeted gene
expression in dopamine and serotonin neurons of the mouse brain.
J Neurosci Methods 143:27–32.
Stuber et al. •Optogenetic Glutamate Corelease from Dopamine Terminals J. Neurosci., June 16, 2010 •30(24):8229 – 8233 • 8233