Journal of Chemical Neuroanatomy 73 (2016) 33–42
Contents lists available at ScienceDirect
Journal of Chemical Neuroanatomy
journal homepage: www.elsevier.com/locate/jchemneu
Multiplexed neurochemical signaling by neurons of the ventral
tegmental area
David J. Barker, David H. Root, Shiliang Zhang, Marisela Morales*
Neuronal Networks Section, Integrative Neuroscience Research Branch, National Institute on Drug Abuse, 251 Bayview Blvd Suite 200, Baltimore, MD 21224,
United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 26 June 2015
Received in revised form 31 December 2015
Accepted 31 December 2015
Available online 4 January 2016
The ventral tegmental area (VTA) is an evolutionarily conserved structure that has roles in rewardseeking, safety-seeking, learning, motivation, and neuropsychiatric disorders such as addiction and
depression. The involvement of the VTA in these various behaviors and disorders is paralleled by its
diverse signaling mechanisms. Here we review recent advances in our understanding of neuronal
diversity in the VTA with a focus on cell phenotypes that participate in ‘multiplexed’ neurotransmission
involving distinct signaling mechanisms. First, we describe the cellular diversity within the VTA,
including neurons capable of transmitting dopamine, glutamate or GABA as well as neurons capable of
multiplexing combinations of these neurotransmitters. Next, we describe the complex synaptic
architecture used by VTA neurons in order to accommodate the transmission of multiple transmitters.
We specifically cover recent findings showing that VTA multiplexed neurotransmission may be mediated
by either the segregation of dopamine and glutamate into distinct microdomains within a single axon or
by the integration of glutamate and GABA into a single axon terminal. In addition, we discuss our current
understanding of the functional role that these multiplexed signaling pathways have in the lateral
habenula and the nucleus accumbens. Finally, we consider the putative roles of VTA multiplexed
neurotransmission in synaptic plasticity and discuss how changes in VTA multiplexed neurons may relate
to various psychopathologies including drug addiction and depression.
Published by Elsevier B.V.
Keywords:
Reward
Addiction
Depression
Aversion
Co-transmission
Dopamine
Glutamate
GABA
1. Introduction
Midbrain dopamine neurons (DA) are most often associated
with reward processing of both natural rewards (e.g., food, water,
etc.) and drugs of abuse (Schultz, 2002; Wise, 2004; Sulzer, 2011).
Over fifty years of intense research has led to the proposal that
neurons belonging to the ventral tegmental area (VTA), which
includes but is not limited to DA neurons, are paramount to reward
processing. Many hypotheses have been put forward regarding the
specific function of VTA DA neurons in reward processing, such as
decision making (Salamone and Correa, 2002a, 2002b; Saddoris
et al., 2015), flexible approach behaviors (Nicola, 2010), incentive
salience (Berridge and Robinton, 1998; Berridge, 2007), and
learning or the facilitation of memory formation (Adcock et al.,
2006; Steinberg et al., 2013). However, several studies have also
shown that VTA DA neurons are involved in the processing of
aversive outcomes (Laviolette et al., 2002; Young, 2004; Pezze and
Feldon, 2004; Brischoux et al., 2009; Lammel et al., 2012; Twining
* Corresponding author.
E-mail address: mmorales@intra.nida.nih.gov (M. Morales).
http://dx.doi.org/10.1016/j.jchemneu.2015.12.016
0891-0618/ Published by Elsevier B.V.
et al., 2014; Hennigan et al., 2015), fear (Abraham et al., 2014),
aggression (Yu et al., 2014a, 2014b), depression (Tidey and Miczek,
1996; Tye et al., 2013), and drug withdrawal (Grieder et al., 2014).
Other hypotheses have proposed that VTA DA neurons play a more
general role in processes such as associative learning (Brown et al.,
2012), arousal (Horvitz, 2000), or general motivational salience
and cognition (Bromberg-Martin et al., 2010).
The functional diversity associated with the VTA may be
mediated, in part, by different VTA subpopulations of neurons. A
particular advancement that may subserve the functional diversity
of the VTA is the recent discovery of neurons that are capable of
signaling using one or more neurotransmitters. In the present
review, we cover recent literature on the diversity of VTA neuronal
phenotypes as they relate to ‘multiplexed neurotransmission’.
We refer the reader to recent comprehensive reviews detailing
VTA cellular composition, VTA efferent and afferents, and VTA
functions (Oades and Halliday, 1987; Fields et al., 2007; Ikemoto,
2007; Nair-Roberts et al., 2008; Morales and Pickel, 2012;
Trudeauet al., 2013; Morales and Root 2014; Pignatelli and Bonci,
2015; Saunders et al., 2015b; Lüthi and Lüscher, 2014). Moreover,
the present review does not cover co-transmission of
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D.J. Barker et al. / Journal of Chemical Neuroanatomy 73 (2016) 33–42
neurotransmitters and neuropeptides, which has long been known
and recently reviewed (Morales and Pickel, 2012). Here, we use the
phrase “multiplexed neurotransmission” to describe neurons that
are capable of signaling using two or more neurotransmitters. In
many circuits, our understanding of the specific mechanisms by
which neurons utilize multiple neurotransmitters is limited. Thus,
we have chosen the term multiplexed neurotransmission to
encompass known and unknown mechanisms of co-release and
co-transmission (e.g., Nusbaum et al., 2001; El Mestikawy et al.,
2011), while also allowing for the possibility of independent
release of individual neurotransmitters either in time or space.
2. Cellular diversity in the ventral tegmental area
Following the discovery of DA as a chemical neurotransmitter in
the brain (Montagu, 1957), the DAergic neurons in the “ventral
tegmental area of Tsai” (Nauta, 1958) were identified by
formaldehyde histofluorescence (Carlsson et al., 1962). These
neurons, along with other catecholaminergic and serotonergic
neurons throughout the brain were shown to comprise twelve
discrete cell groups (labeled as A1–A12 groups; Dahlström and
Fuxe, 1964). One feature of the A10 group, in particular, is the
heterogeneous morphology among its neurons. Based on cytoarchitecture, the A10 region has been divided into two lateral nuclei
[the Parabrachial Pigmented Nucleus (PBP) and Paranigral Nucleus
(PN)], and three midline nuclei [the Rostral Linear Nucleus of the
Raphe (RLi), Interfasicular Nucleus (IF), and Caudal Linear Nucleus
(CLi)]. Traditionally, the VTA has been considered to include just
the lateral nuclei (PBP, PN) (Swanson, 1982), however, modern
conceptions of VTA function have often included the midline nuclei
(RLi, IF, CLi) as subnuclei of the VTA (Ikemoto, 2007; Nair-Roberts
et al., 2008; Morales and Root, 2014). Thus, in this review, we use
the term VTA to define the midbrain A10 structure containing
lateral (PBP, PN) and midline nuclei (RLi, IF, CLi). The cellular
heterogeneity within the VTA subnuclei, together with findings
showing that a single A10 neuron rarely innervates multiple
structures (Swanson, 1982; Takada and Hattori, 1987; Lammel
et al., 2008; Hosp et al., 2015), suggests that the VTA utilizes highly
specific projections from different sets of neurons.
Dopamine neurons, defined by the expression of tyrosine
hydroxylase (TH) protein (Fig. 1), are interspersed throughout all
VTA nuclei, but are most prevalent in the lateral PBP and PN
(Swanson, 1982; Ikemoto, 2007; Li et al., 2013). In addition to the
co-expression of TH and aromatic decarboxylase (AADC), the
majority of rat lateral PBP and lateral PN neurons co-express the
dopamine transporter (DAT), D2 receptor (D2R), and vesicular
monoamine transporter 2 (VMAT2) mRNA (Li et al., 2013). More
medially within the rat PBP and PN, as well as within the RLi, CLi,
and IF, subsets of TH-expressing neurons either express or lack
different combinations of DAT, VMAT2, or D2 receptor (Li et al.,
2013; reviewed in Morales and Root, 2014). Our understanding of
diversity among DAergic neurons in other species than the rat is
less understood. However, recent studies have shown that, while
all VTA neurons in the rat VTA expressing TH mRNA co-express the
TH protein, some mouse VTA neurons expressing TH mRNA lack TH
protein (Yamaguchi et al., 2015). In addition, ventrally to the VTA
within the interpeduncular nucleus, there is in the mouse, but not
in the rat, a subpopulation of neurons expressing TH mRNA, but
lacking TH protein (Yamaguchi et al., 2015; Lammel et al., 2015). So
far, detailed molecular characterizations of VTA neurons in
nonhuman primates or humans has not been reported.
Rat TH-expressing neurons within the lateral PBP and lateral PN
have also been electrophysiologically characterized (so-called
‘primary’ neurons) based on their long-duration action potentials
and hyperpolarization-activated cation currents (Grace and Onn,
1989). However, recent findings have shown that not all VTA THexpressing neurons share these electrophysiological criteria
(Margolis et al., 2006). In addition, although lack of direct
electrophysiological responses to the m-opioid receptor agonist
DAMGO has been proposed as a property shared by VTA DAergic
neurons (Johnson and North, 1992), the VTA has a subpopulation of
TH-expressing neurons that are directly excited or inhibited by
DAMGO (Margolis et al., 2014). So far, it seems that hyperpolarization-activated cation currents, spike duration, inhibition by D2R
agonist and other electrophysiological properties are unreliable
predictors for the identification of all VTA DAergic neurons
(Margolis et al., 2006), further supporting the heterogeneity of
VTA DAergic neurons.
Fig. 1. Neurons in the ventral tegmental area (VTA) are capable of multiplexed neurotransmission. Detection of tyrosine hydroxylase (TH) immunoreactivity within the VTA,
(low magnification, left panel). VTA combined immunohistochemistry and in situ hybridization showing at high magnification (right panel) neurons expressing TH (dopamine
neurons; green cells), glutamic acid decarboxylase mRNA (GABA neurons; GAD 65/67; purple cells), vesicular glutamate transporter 2 mRNA (glutamate neurons; VGluT2;
green or white grain aggregates) or combinations of these cell markers.
Abbreviations: Left: RLi, Rostral Linear Nucleus, IF, Interfasicular Nucleus, PBP, Parabrachial Pigmented Nucleus, PN, Paranigral Nucleus, SNc, Substantia Nigra Pars Compacta,
fr, fasciculus retroflexus, mp, Mammillary Peduncle, Right: TH, tyrosine hydroxylase, GAD, glutamic acid decarboxylase, VGluT2, vesicular glutamate transporter 2.
D.J. Barker et al. / Journal of Chemical Neuroanatomy 73 (2016) 33–42
Along with DAergic neurons, g-aminobutyric-acid (GABA)
neurons are also present in the VTA (Nagai et al., 1983; Kosaka
et al., 1987). These GABAergic neurons are relatively less prevalent
than the DAergic neurons, and are identified by their expression of
glutamic acid decarboxylase (GAD) 65 or 67 mRNA, isoforms of the
enzyme involved in the synthesis of GABA. GABAergic VTA neurons
are also identified by their expression of vesicular GABA
transporter (VGaT) mRNA. Electrophysiologically, putative VTA
GABAergic ‘secondary’ neurons have been characterized based on
the observation that these cells are hyperpolarized by m-opioid
agonists (Johnson and North, 1992). As with VTA DAergic neurons,
VTA GABAergic neurons are pharmacologically and electrophysiologically heterogeneous. For example, approximately 60% of VTA
GABAergic neurons are inhibited by the m-opioid receptor agonist
DAMGO, but all seem to be unaffected by the GABAB agonist
baclofen (Margolis et al., 2012). GABAergic neurons in the VTA are
known to establish local inhibitory connections on DAergic
neurons (Johnson and North, 1992; Omelchenko and Sesack,
2009), but have also been shown to project outside of the VTA to
the ventral striatum (nAcc; Van Bockstaele and Pickel, 1995) basal
forebrain (Taylor et al., 2014), the prefrontal cortex (Steffensen
et al., 1998; Carr and Sesack, 2000), the lateral habenula (LHb;
Stamatakis et al., 2013; Root et al., 2014a; Taylor et al., 2014;
Lammel et al., 2015), lateral hypothalamus, preoptic area, and
amygdala, as well as to structures in the thalamus, midbrain, pons
and medulla (Taylor et al., 2014). GABAergic neurons are scattered
throughout the A10 region, and although a detailed subregional
mapping of these neurons has not been yet reported, a dense group
of GABAergic neurons has been identified in an area ventro-caudal
to the VTA, referred as the ‘tail of the VTA’ (tVTA; Kaufling et al.,
2009) or the rostromedial tegmental area (RMTg; Jhou, 2005; Jhou
et al., 2009a, 2009b; Geisler et al., 2008; Lavezzi and Zahm, 2011).
The GABAergic neurons of the tVTA/RMTg provide a major
35
inhibitory control to VTA DAergic neurons (Kaufling et al., 2010;
Matsui and Williams, 2011).
In addition to VTA DAergic and GABAergic neurons, early
electrophysiological studies of the midbrain suggested the
possibility of glutamatergic signaling by some VTA neurons
(Wilson et al., 1982; Mercuri et al., 1985; Sulzer et al., 1998; Joyce
and Rapport, 2000; Chuhma et al., 2004; Ungless et al., 2004;
Lavin et al., 2005; Chuhma et al., 2009). Anatomical identification
of glutamatergic neurons has recently become possible due to
the cloning of three distinct vesicular glutamate transporters
(VGluT1, VGluT2, and VGluT3; Bellocchio et al., 1998; Bai et al.,
2001; Fremeau et al., 2001, 2002; Fujiyama et al., 2001; Hayashi
et al., 2001; Herzog et al., 2001; Takamori et al., 2000; Varoqui
et al., 2002; Gras et al., 2002). By in situ hybridization, it has
been demonstrated that some neurons within the VTA (Kawano
et al., 2006; Yamaguchi et al., 2007; 2011), substantia nigra and
retrorubral field (Yamaguchi et al., 2013) express VGluT2 mRNA,
but not VGluT1 or VGluT3. The VTA-VGluT2 neurons are present
in all A10 nuclei, but are particularly prevalent within midline
nuclei (Yamaguchi et al., 2007, 2011). In fact, glutamatergic
neurons outnumber the DAergic neurons in the rostral and medial
portions of the VTA (Yamaguchi et al., 2007, 2011). Thus, these
neurons represent a major subpopulation in certain parts of the
VTA. Similar to VTA GABAergic neurons, VGluT2-glutamatergic
neurons in the VTA establish local and extrinsic synapses (Dobi
et al., 2010; Yamaguchi et al., 2011; Zhang et al., 2015; Wang et al.,
2015). Specifically, glutamatergic VTA neurons establish local
asymmetric synapses with both DAergic and non-DAergic
neurons (Dobi et al., 2010; Wang et al., 2015). Additionally,
glutamatergic VTA neurons project to other regions of the brain
including the LHb (Root et al., 2014a, 2014b), nAcc (Zhang et al.,
2015), amygdala, basal forebrain, and prefrontal cortex (Hnasko
et al., 2012; Taylor et al., 2014).
Fig. 2. Ultrastructural immunolabeling reveals unpredicted mechanisms of neurotransmission within the mesohabenular and mesoaccumbal pathways. (a) Lateral habenula
micrograph (left panel) showing a single mesohabenular axon terminal containing VGluT2 (scattered dark material detected by immunoperoxidase labeling) and VGaT (gold
particles detected by immunogold; blue arrowheads). This single axon terminal forms both an asymmetric synapse (green arrow) and symmetric synapses (blue arrows) with
a common postsynaptic dendrite (De). Postsynaptic to a single axon terminal (middle panel), GluR1 receptors (green arrowhead) are found adjacent to asymmetric synapses
(green arrows), while GABAA receptors (blue arrowhead) are found adjacent to symmetric synapses (blue arrow). (b) Nucleus Accumbens micrograph showing a
messoaccumbal axon containing both VMAT2 (scattered dark material) and VGluT2 (gold particles). VMAT2 and VGluT2 are segregated within the same axon. Note that the
VGluT2 microdomain corresponds to an axon terminal establishing an asymmetric synapse (arrow) with a postsynaptic dendritic spine (sp). All scale bars represent 200 nm.
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D.J. Barker et al. / Journal of Chemical Neuroanatomy 73 (2016) 33–42
Moreover, increasing evidence indicates that subpopulations of
VTA neurons are capable of releasing DA and GABA, or DA and
glutamate (Kosaka et al., 1987; Sulzer et al., 1998; Rayport, 2001;
Dal Bo et al., 2004; Trudeau, 2004; Seutin, 2005; Lapish et al., 2006;
Yamaguchi et al., 2007, 2011; Hnasko et al., 2010; Tritsch et al.,
2012; Li et al., 2013; Mingote et al., 2015). Recent work from our
laboratory has shown that there is a subset of VTA neurons capable
of co-releasing DA and glutamate or glutamate and GABA (Zhang
et al., 2015; Root et al., 2014a).
Multiplexed neurotransmission by some VTA neurons and
associated circuits is a property shared by other brain structures.
For instance, glutamate and GABA co-neurotransmission has been
reported in epilepsy models within mossy fiber terminals
(Gutiérrez et al., 2003; Gutiérrez, 2003, 2005; Trudeau and
Gutiérrez, 2007; Münster-Wandowski et al., 2013), developing
medial trapezoid body terminals from the lateral superior olive
(Gillespie et al., 2005; Noh et al., 2010), entopeduncular nucleus
projection to the LHb (Shabel et al., 2014), and cortex (Fattorini
et al., 2015). In addition, there is evidence for GABA and DA cotransmission in by substantia nigra pars compacta neurons as well
as retinal amacrine neurons (Tritsch et al., 2012; Hirasawa et al.,
2012). Other forms of neurotransmission include GABA and
histamine by hypothalamic neurons (Yu et al., 2015), glutamate
and acetylcholine co-transmission in striatal interneurons or
medial habenula neurons (Gras et al., 2008; Ren et al., 2011; Higley
et al., 2011; Nelson et al., 2014), and GABA and acetylcholine cotransmission in corticopetal globus pallidus neurons (Saunders
et al., 2015a).
Based on the discovery that some VTA neurons exhibit multiple
vesicular transporters, we have applied ultrastructural and
electrophysiological approaches to determine the possible cellular
mechanisms by which multiple neurotransmitters are released at
the synaptic level. In the process of answering this question, we
have revealed complex ultrastructural architectures suggesting
that glutamate and GABA neuronal signaling by VTA neurons can
be integrated into a single complex terminal with spatially distinct
synaptic release sites for glutamate or GABA (Fig. 1) (Root et al.,
2014a). We have also revealed that DA and glutamate neuronal
signaling by VTA neurons can be segregated to distinct microdomains within the same axon (Fig. 2), allowing for the spatially
distinct release of DA or glutamate (Zhang et al., 2015).
3. Multiplexed signaling by VTA-GluT2 neurons
Following the discovery of VTA-VGluT2 neurons, further
characterization of these neurons demonstrated that they are
very diverse in their molecular composition, signaling properties
and neuronal connectivity. Whereas many VTA-VGluT2 neurons
lack both DAergic and GABAergic markers, there are subpopulations of VTA-VGluT2 neurons that co-express molecules responsible for the synthesis or vesicular transport of either DA or GABA
(Li et al., 2013; Root et al., 2014a). Although the distinct targets
for VTA-VGluT2 neurons remains to be determined, emerging
evidence suggests preferential target sites for specific subsets of
VTA-VGluT2 neurons. For instance, by a combination of retrograde
tract tracing and in situ hybridization, it has been demonstrated
that VTA VGluT2(+)/GAD(+) neurons provide the major mesohabenular input to the LHb (Root et al., 2014a), and by contrast, VTA
VGluT2(+)/TH(+) neurons largely target the nAcc shell (Yamaguchi
et al., 2011). While the molecular characterization of mesohabenular and mesoaccumbens neurons provides support for multiplexed signaling by some VTA neurons, this characterization does
not provide information on the cellular mechanisms by which
these neurons release more than one neurotransmitter. As
discussed below, recent findings obtained by a combination of
cell-type specific anterograde tract tracing and immuno-electron
microscopy have demonstrated that the VTA-VGluT2 neurons
establish unique synaptic architectures for multiplexed signaling
in both the LHb (Root et al., 2014a) and the nAcc (Zhang et al.,
2015).
3.1. Multiplexed signaling by mesohabenular VGluT2 neurons
Recently available viral vectors and transgenic mice have
facilitated the elucidation of the cellular mechanisms by which
some VTA neurons use two distinct signaling molecules. To
determine the synaptic ultrastructural features of mesohabenular
axons we have taken advantage of the cell-specific viral tagging of
VTA-VGluT2 neurons through the Cre-dependent expression of
mCherry tethered to channelrhodopsin (ChR2) under the regulation of the VGluT2 promoter in VGluT2:Cre mice. By applying cellspecific tagging we estimated that more than 70% of mesohabenular axon terminals within the LHb co-express VGluT2 and VGaT
(Root et al., 2014a). Moreover, we estimated that within the LHb
both VGluT2 and VGaT are present in half of the total population of
axon terminals, some of which derive from brain structures others
than the VTA (e.g., from the basal ganglia; Shabel et al., 2014).
While the presence of VGluT2 and VGaT within the same axon
terminal has been established by immuno-electron microscopy, it
remains to be determined whether each vesicular transporter is
integrated into the membrane of distinct vesicles or in the same
vesicular membrane. However, ultrastructural findings of the
synaptic composition of individual VGluT2(+)/VGaT(+) axon terminals show that the plasma membrane of single VGluT2(+)/VGaT(+)
axon terminals participates in the formation of both asymmetric
and symmetric synapses, suggesting that glutamate-signaling is
segregated to the asymmetric synapse and GABA-signaling to the
symmetric (Fig. 2).
In addition to the suggestion that asymmetric synapses
participate in excitatory neurotransmission (Peters and Palay,
1996), GluR1-containing AMPA receptors are located in the
membrane postsynaptic to the mesohabenular asymmetric
synapses, but not to the symmetric synapses (Root et al.,
2014a). In contrast, consistent with the suggestion that symmetric
synapses participate in inhibitory neurotransmission (Peters and
Palay, 1996), GABAA receptors are located postsynaptically to the
mesohabenular symmetric synapses, but not to the asymmetric
synapses (Root et al., 2014a). The selective postsynaptic distribution of GluR1 to VGluT2(+)/VGaT(+) mesohabenular terminals
making asymmetric synapses and GABAA to those making
symmetric synapses indicates that VGluT2(+)/VGaT(+) terminals
release glutamate at the asymmetric synapse, and GABA at the
symmetric synapses (Root et al., 2014a). Besides the formation of
asymmetric and symmetric synapses by individual VGluT2(+)/
VGaT(+) axon terminals, both synapses may target separate
postsynaptic dendritic spines or dendritic shafts or share a
common postsynaptic dendrite. These ultrastructural findings
underlie the multiplexed signaling and potential neuroplastic
capacity endowed by dual VGluT2(+)/VGaT(+) axon terminals from
the VTA to the LHb.
3.2. Multiplexed Signaling by Mesoaccumbens VGluT2-DA neurons
Pioneering electrophysiological in vitro studies demonstrated
that dopamine neurons in primary culture have the capability to
release glutamate, which lead to the hypothesis that midbrain
neurons co-transmit DA and glutamate (Sulzer et al., 1998; Joyce
and Rayport 2000; Bourque and Trudeau, 2000). Since then,
anatomical studies demonstrated that subsets of TH-positive
neurons co-express VGluT2 mRNA throughout the brain (Stornetta
et al., 2002; Kawano et al., 2006), including some TH-neurons
within the midline nuclei of the VTA in rats (Yamaguchi et al., 2007,
D.J. Barker et al. / Journal of Chemical Neuroanatomy 73 (2016) 33–42
2011) and mice (Yamaguchi et al., 2015). Moreover, findings from
optogenetic electrophysiological ex vivo recordings have shown
that the VGluT2(+)/TH(+) mesoaccumbens neurons use glutamate as
signaling molecule (Stuber et al., 2010; Tecuapetla et al., 2010;
Zhang et al., 2015; Mingote et al., 2015), and recent in vitro
voltammetry measurements have shown that VGluT2-TH coexpressing neurons that project to nAcc release DA (Zhang et al.,
2015).
So far two opposing hypotheses have been proposed to
mediate the dual glutamate and DA signaling by VGluT2(+)/TH(+)
mesoaccumbens neurons. One of them proposes that glutamate
and DA coexist (and are co-released) from the same pool of
vesicles (Hnasko et al., 2010; Hnasko and Edwards, 2012). This
hypothesis has been based on the co-immunoprecipitation of
VMAT2 and VGluT2 from nAcc preparations (Hnasko et al., 2010).
In clear contrast, a recent study has shown lack of VMAT2 and
VGluT2 co-immunoprecipitation when ultrastructurally confirmed pure nAcc synaptic vesicles were used (Zhang et al.,
2015). These recent findings have led to the hypothesis that dual
VGluT2(+)/TH(+) mesoaccumbens neurons contain independent
pools of vesicles for the accumulation of either DA or glutamate.
Moreover, immuno-electron microscopy findings from intact
brain tissue have shown that TH and VGluT2 do not coexist in the
same axon terminal in the nAcc of either adult rats (BérubéCarrière et al., 2009; Moss et al., 2011) or mice of any age (BérubéCarrière et al., 2012). These immuno-electron microscopy
findings are consistent with nAcc structural studies published
over the last 40 years showing that axonal compartments
engaged in excitatory signaling do not overlap with axonal
compartments engaged in DA signaling (reviewed in Morales and
Pickel, 2012). The lack of overlap between DA-vesicles and
glutamate-vesicles may result from their segregation into two
different sets of axons or segregation into micro-domains within
the same axon. Although the possibility of vesicular segregation
to different axons has not been discarded, recent immunoelectron microscopy findings indicate that VGluT2-vesicles from
VGluT2(+)/TH(+) neurons are located in axon terminals that
establish asymmetric synapses, and that axonal segments
adjacent to these VGluT2- axonal terminals contain TH, VMAT2
and DAT (Zhang et al., 2015). The segregation between glutamatevesicles and DA-vesicles within the same axon appears to be
highly regulated, as in vivo overexpression of VMAT2, in the rat,
does not disrupt the segregation between these two different
types of vesicles. Moreover, the vesicular segregation by VGluT2(+)/
TH(+) mesoaccumbens neurons is maintained in the nAcc of
transgenic mice expressing ChR2 (following their viral mediated
expression in VTA neurons under the regulation of either the THpromoter or VGluT2-promoter; Zhang et al., 2015).
In summary, the characterization of mesoaccumbens and
mesohabenular ultrastructural features together with the characterization of their electrophysiological and chemical properties
have provided evidence for multiplexed signaling by VTAVGluT2 neurons. These findings have demonstrated that dual
rodent mesoaccumbens VGluT2(+)/TH(+) neurons have adjacent
cellular compartments that participate in independent glutamatesignaling and DA-signaling (Zhang et al., 2015). In contrast, the
dual rodent mesohabenular VGluT2(+)/GABA(+) neurons concentrate both glutamate-vesicles and GABA-vesicles within a single
axon terminal that establishes both excitatory and inhibitory
synapses (Root et al., 2014a). Future studies are necessary to
determine the molecular and signaling mechanisms involved in
the sorting and retention of VGluT2-vesicles, GABA-vesicles (VGaT)
and DA-vesicles (VMAT2) to specific microdomains within the
same axon. Additional studies are also necessary to determine the
extent to which the multiplexed signaling by VTA neurons is
affected in brain disorders, such as addiction and depression.
37
4. Functional diversity by VTA neurons
The functional diversity of VTA neurons has been constantly
updated (Unlgess et al., 2004; Stamatakis et al., 2013; Root et al.,
2014b; Mejias-Aponte et al., 2015; Eddine et al., 2015; Kotecki et al.,
2015; Beier et al., 2015). As detailed above, the multiplexed
neurotransmission of the VTA-VGluT2 neurons is an emerging
factor involved in the complexity of VTA function. Based on
observations that different combinations of neurotransmitters are
multiplexed throughout the brain (Trudeau, 2004; Gillespie et al.,
2005; Zhou et al., 2005; Gras et al., 2008; Noh et al., 2010; Higley
et al., 2011; Tritsch et al., 2012; Hnasko and Edwards, 2012;
Münster-Wandowski et al., 2013; Nelson et al., 2014; Root et al.,
2014a; Shabel et al., 2014; Qi et al., 2014; Zhang et al., 2015;
Fattorini et al., 2015; Saunders et al., 2015a, 2015b), we suggest that
multiplexed neurotransmission conveys distinct messages
depending on the neurotransmitter content of each circuit,
momentary singular or multiplexed signaling, and perhaps even
the time scale of neurotransmitter function. Furthermore, we
speculate that changes in the influence of one or more of the
multiplexed neurotransmitters, by way of either presynaptic of
postsynaptic changes, may result from and result in observable
changes in behavior. Recent advances in the functional diversity
within the VTA neurons targeting the LHb or nAcc will be
presented in the following paragraphs.
4.1. Functional diversity by VGluT2 mesohabenular neurons
An example of the circuit specific nature of multiplexed
neurotransmission is found in the LHb. By combination of
optogenetics and electrophysiology, we have shown that activation
of the mesohabenular pathway evokes release of GABA and
glutamate, and that the co-transmitted GABA is capable of
shunting the co-transmitted glutamate-mediated currents (Root
et al., 2014a). Therefore, the simultaneous release of glutamate and
GABA may be a mechanism by which the glutamatergic excitation
within the LHb is autoregulated by the co-transmitted GABA. In
vivo recordings of LHb neurons following ChR2 activation of
mesohabenular fibers have shown that this activation results in
GABA-induced decreases in the firing rates of most recorded LHb
neurons, and in glutamate-induced increases in firing rates in
fewer neurons. In addition, secondary firing patterns are often
observed in which initial increases in firing rates are followed by
decreased firing rates or initial decreases in firing rates are
followed by increased firing rates. The in vivo recordings of LHb
neurons suggest that stimulation on mesohabenular fibers induces
predominantly GABAergic neurotransmission. Nevertheless, the
observed secondary firing patterns suggest that signaling might
also occur over multiple time-scales or that the contribution of
each neurotransmitter might be shifted in response to specific
stimuli. For instance, rat depression models reduce GABA signaling
from the multiplexed glutamate-GABA inputs to LHb from
entopeduncular neurons (Shabel et al., 2014). In addition, findings
from combinations of optogenetics and behavioral analysis have
shown that mesohabenular stimulation of fibers from different
pools of VTA neurons, including multiplexed signaling neurons,
promotes different behaviors. For instance, a LHb GABA receptormediated reward is evoked by mesohabenular stimulation of fibers
expressing ChR2 under the TH-promoter (Stamatakis et al., 2013),
likely to include activation of fibers from VGluT2(+)/GAD(+)/TH(+),
VGluT2(+)/TH(+)/GAD( ), and VGluT2( )/GAD(+)/TH(+) mesohabenular neurons. However, a mild reward is evoked by mesohabenular
stimulation of fibers expressing ChR2 under the GAD2-promoter
(Lammel et al., 2015), likely to include activation of fiber from
VGluT2(+)/GAD(+)/TH(+), VGluT2(+)/GAD(+)/TH( ), VGluT2( )/GAD(+)/
TH( ), and VGluT2( )/GAD(+)/TH(+) mesohabenular neurons. In
38
D.J. Barker et al. / Journal of Chemical Neuroanatomy 73 (2016) 33–42
contrast, a LHb glutamate receptor-mediated conditioned place
aversion is evoked by mesohabenular stimulation of fibers
expressing ChR2 under the VGluT2-promoter (Root et al., 2014b;
Lammel et al., 2015), likely to include activation of fibers
fromVGluT2(+)/GAD(+)/TH( ),
VGluT2(+)/GAD(+)/TH(+),
and
VGluT2(+)/GAD( ) neurons. These behavioral findings underlie
the need for targeted intersectional approaches to dissect the
behavioral contributions of each mesohabenular neuronal phenotype.
Multiplexed neurotransmission may affect neuronal regulation
over multiple time scales, for instance “prolonged slow-actions” by
monoamines (i.e., serotonin or dopamine) and “fast short actions”
provided by the concomitant release of glutamate or GABA. This
multiple time scale neurotransmission, by neurons endowed with
the capacity for multiplexed signaling, may be found in a single DAglutamate mesoaccumbens axon establishing segregated postsynaptic targets for DA- or glutamate-signaling (Zhang et al., 2015).
Although the extent to which these mesoaccumbens DA-glutamate
fibers participate in the neurobiology of drugs of abuse remains to
be determined, we speculate that these axons may participate in
the regulation of neuronal activity in cocaine self-administration.
Specifically, electrophysiological recordings have shown that nAcc
neurons exhibit rapid phasic firing patterns to related cues and
actions to obtain the drug (Peoples et al., 1998; Ghitza et al., 2003,
2004, 2006; Fabbricatore et al., 2009, 2010; Coffey et al., 2015). The
nAcc neurons also exhibit slow-phasic and tonic changes in firing
rate that correlate with the pharmacological effects of cocaine, and
do not correlate with the rapid phasic firing patterns (Fabbricatore
et al., 2010). Furthermore, slow phasic pharmacologic and rapid
phasic behavioral firing patterns are similarly processed in
downstream accumbal targets (ventral pallidum and lateral
preoptic area; Root et al., 2012, 2013; Barker et al., 2014). These
dissociable fast and slow signaling patterns in the accumbens are
consistent with findings suggesting that glutamate and dopamine
each have specific roles in addiction-associated behaviors (Birgner
et al., 2010; Alsiö et al., 2011).
4.2. Functional diversity by TH mesohabenular neurons
Phenotypic characterizations of VTA-TH neurons have revealed
the heterogeneous expression of several transcripts, some of which
may be expressed transiently during development or may be
induced in the adult brain in response to insults (e.g., drugs, stress,
illness). In addition, some of these transcripts may not be
translated into detectable protein levels under normal conditions,
instead, this translation may depend on VTA circuit activity or be
induced as a result of various brain insults (e.g., Bayer and Pickel,
1990, 1991; García-Pérez et al., 2014). For instance, we have
identified a subset of VTA neurons, in wild type mice, that express
TH mRNA, but lack detectable levels of TH-protein in cell bodies,
dendrites and axons. Some of these neurons send projections to the
LHb (Yamaguchi et al., 2015). In agreement with these findings,
revealing TH-mRNA(+)/TH-protein( ) mesohabenular neurons in
wild type mice, viral-induced expression of reporter genes (i.e.,
green-fluorescent-protein under the regulation of the TH-promoter) within the VTA of TH:cre mice has shown expression of
fluorescent fibers without detectable TH-protein in the LHb
(Stamatakis et al., 2013; Lammel et al., 2015; Stuber et al., 2015).
These findings underlie the need to better characterize the VTA
cellular composition in wild type mice, and reveal that expression
of reporter genes in the mouse under the control of the THpromoter does not guarantee the selective manipulation or
mapping of DA projections.
In contrast to the mouse TH-mRNA(+)/TH-protein( ) mesohabenular neurons, subsets of rat mesohabenular neurons contain
detectable levels of TH-protein in the cell bodies, dendrites and
axons (Root et al., 2015). However, these rat TH-protein(+)
mesohabenular neurons rarely co-express VMAT2-mRNA in their
cell bodies or VMAT2-protein in their axon terminals in LHb (Root
et al., 2015). The lack of VMAT2 within mesohabenular neurons has
also been documented in the mouse (Stamatakis et al., 2013;
Lammel et al., 2015). Overall, these findings provide crucial
information when considering the functional properties of multineurotransmitter neurons, as they demonstrate that specific
neuronal subsets have the capacity to synthesize DA but lack
the capability to package DA into synaptic vesicles for traditional
vesicular release. These finding are intriguing because DA has been
detected in LHb homogenates (Phillipson and Pycock, 1982; Root
et al., 2015), D2 receptors have been found in a subset of LHb
neurons (Aizawa et al., 2012), and exogenous DA evokes currents in
LHb neurons, currents that are eliminated by D2 or D4-receptor
antagonists (Jhou et al., 2013; Good et al., 2013; Root et al., 2015).
However, recordings of LHb neurons from rats treated with toxins
for either the elimination of VTA-TH neurons or noradrenergic
fibers have demonstrated that noradrenergic habenular afferents
specifically activate D4-receptors in the LHb neurons and that VTA
TH-expressing neurons are not necessary for this effect (Root et al.,
2015). Thus, it seems that the LHb effects on DA-receptors
previously ascribed to DA release from mesohabenular fibers may
be instead mediated by noradrenergic fibers.
5. Multiplexed transmission: future directions and
considerations for synaptic plasticity
Our ever-expanding knowledge of multiplexed signaling opens
the door to new predictions about synaptic plasticity. For example,
though activation of the mesohabenular projection results in
glutamate and GABA release, firing patterns of LHb neurons
indicate a predominant GABA-induced decrease in firing rate of
LHb neurons in rodents (Root et al., 2014a). Drugs of abuse,
depression, and stress alter LHb function to favor glutamatergic
excitation and demote GABAergic inhibition (Meshul et al., 1998; Li
et al., 2011; Shabel et al., 2014), suggesting the potential for
mesohabenular plasticity in mediating part of these effects. The
ability of “neurotransmitter-switching” depending on circadian
and seasonal variations has also been documented (Dulcis et al.,
2013; Farajnia et al., 2014), and further investigation is necessary to
determine if these factors influence multiplexed signaling.
The postsynaptic signaling dominance by one neurotransmitter
may also be used to balance signals from other neurotransmitters
and maintain homeostasis. Indeed, it has been shown that the
same neurotransmitter may elicit different responses depending
on the overall extracellular environment (Laviolette et al., 2002;
Twining et al., 2014). For example in the mesohabenular
projection, depending on the membrane potential of the postsynaptic LHb neuron, mesohabenular stimulation results in GABAA
receptor or AMPA-receptor currents (Root et al., 2014b). With this
in mind, we speculate that drugs of abuse, neurodegenerative
diseases, or other circumstances that affect synapses capable of
multiplexed neurotransmission may produce aberrant signaling
and thus affect cognition and behavior. It is likely that the recent
discoveries of unpredicted synaptic arrangements will lead to
novel experimental approaches to have a better understanding of
how neurotransmission shifts from homeostatic conditions, how
certain neurotransmitters become amplified or silenced, and how
multiplexed signals might be simultaneously sent and received.
Because many neurotransmitters have multiple postsynaptic
and presynaptic effects,multiplexed neurotransmission expands
the repertoire of synaptic capabilities of single neurons. In this
regard, electrophysiological evidence indicates that DA neurons
are capable of acting on GABAAreceptors (Tritsch et al., 2012; Kim
et al., 2015; Hoerbelt et al., 2015). Thus, when glutamate and DA are
D.J. Barker et al. / Journal of Chemical Neuroanatomy 73 (2016) 33–42
multiplexed together – as they are in some mesoaccumbal
projections (Zhang et al., 2015) – DA may act to facilitate
glutamatergic signaling, counter glutamatergic signaling, or might
even behave differently depending on the postsynaptic cell (e.g.,
targeting of D1 receptor neurons or D2 receptor neurons). With
this in mind, it is clear that novel technologies and intersectional
genetic strategies will be necessary in order to decipher the unique
contributions of each component of multiplexed signals (Pupe and
Wallén-Mackenzie, 2015).
A better understanding of the presynaptic and postsynaptic
elements will allow better understanding of how these specialized
synapses, described above, manage multiplexed neurotransmission in the presynaptic terminal and are subsequently integrated
by the postsynaptic neuron. For example, these elements may
work together to facilitate spike-timing dependent mechanisms
for plasticity (e.g., Watanabe et al., 2002), as it is known that
neuromodulators can affect the temporal window necessary for
spike timing dependent activation, or that neuromodulators can
cause a switch from long-term potentiation to long term
depression (Caporale and Dan 2008; Bissière et al., 2003). Thus,
the spatiotemporal relationship of segregated DAergic and
glutamatergic signaling in the nAcc may act to enhance the
probability of signal transduction when both transmitters are
released within a short time window. A similar mechanism might
apply to mesohabenular signaling, although the precise mechanism by which glutamate and GABA might work to facilitate or
shunt one another is still unclear. One possibility is that the
integration for either a GABAergic or a glutamate response by the
post-synaptic cell would depend on the timing of other habenular
afferents.
Overall, it is becoming clear that the VTA is far more complex
than was initially realized. Indeed, many studies have reported
heterogeneous responses of specific VTA neurons (Brischoux et al.,
2009; Borgkvist et al., 2011; Margolis et al., 2014; Eddine et al.,
2015; Mrejeru et al., 2015; Mejias-Aponte et al., 2015), and it would
seem likely that this diversity is due (at least in part) to cells that
are capable of multiplexed neurotransmission. With this in mind, it
is clear that that the discovery of compound cell types has widereaching implications for our understanding of VTA circuit
functions and that the use of intersectional strategies will become
increasingly critical. Moreover, multiplexed signaling neurons are
increasingly identified throughout the brain, suggesting that this
unique type of signaling plays important roles in health and
disease.
Acknowledgements
The Intramural Research Program of the National Institute on
Drug Abuse, US National Institutes of Health (IRP/NIDA/NIH)
supported this work.
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