Glutamatergic neurons are present in the rat ventral tegmental area

Corticotropin-releasing factor binding protein within the ventral tegmental area is expressed in a subset of dopaminergic neurons

Corticotropin-releasing factor (CRF) and related peptides play a role in mediating neuronal effects of stress. These peptides mediate stress responses by their interactions with the CRF receptors and the CRF-binding protein (CRF-BP). Because the CRF-BP is implicated in neurotransmission within the ventral tegmental area (VTA), we investigated whether the CRF-BP is expressed in VTA neurons. By in situ hybridization, we detected cellular expression of CRF-BP mRNA in the VTA; no such expression was seen in neighboring substantia nigra pars compacta (SNC) or substantia nigra pars reticulata. By double in situ hybridization, we determined that VTA neurons with CRF-BP mRNA coexpressed transcripts encoding either tyrosine hydroxylase [TH; a marker for dopamine (DA) neurons] or glutamic acid decarboxylase [GAD; synthesizing enzyme of gamma-aminobutyric acid (GABA)]. Neurons with CRF-BP mRNA represented 25% of the total population of TH-expressing neurons and 28% of the total population of GAD-expressing neurons, indicating that discrete subpopulations of dopaminergic and GABAergic neurons are present in the VTA. Within the total population of neurons containing CRF-BP mRNA, 70% coexpressed TH mRNA and only 27% coexpressed GAD mRNA. As far as we are aware, we provide the first anatomical evidence that a molecule, CRF-BP, is encoded by DAergic neurons of the VTA but not by those of the SNC. We propose, based on the observation that the majority of VTA neurons expressing CRF-BP mRNA are DAergic, that in the VTA interactions of CRF-BP with CRF, or with CRF-related peptides, are likely to be mediated predominantly by DAergic neurons.

Developmental and target-dependent regulation of vesicular glutamate transporter expression by dopamine neurons

Mesencephalic dopamine (DA) neurons have been suggested to use glutamate as a cotransmitter. Here, we suggest a mechanism for this form of cotransmission by showing that a subset of DA neurons both in vitro and in vivo expresses vesicular glutamate transporter 2 (VGluT2). Expression of VGluT2 decreases with age. Moreover, when DA neurons are grown in isolation using a microculture system, there is a marked upregulation of VGluT2 expression. We provide evidence that expression of this transporter is normally repressed through a contact-dependent interaction with GABA and other DA neurons, thus providing a partial explanation for the highly restricted expression of VGluT2 in DA neurons in vivo. Our results demonstrate that the neurotransmitter phenotype of DA neurons is both developmentally and dynamically regulated. These findings may have implications for a better understanding of the fast synaptic action of DA neurons as well as basal ganglia circuitry.

Ventral tegmental glutamate: a role in stress-, cue-, and cocaine-induced reinstatement of cocaine-seeking

Ventral tegmental dopamine neurons are activated by primary rewards and, when such rewards are predictable' by reward-predicting stimuli. Glutamatergic input to the ventral tegmental area contributes to this activation: in animals trained to self-administer cocaine, cocaine-predictive cues trigger ventral tegmental glutamate release and dopaminergic activation. Mild footshock stress similarly causes glutamate release and dopaminergic activation in cocaine-trained but not cocaine-naïve animals. The ability of cocaine-predictive and stress-associated cues to activate the dopamine system and to trigger cocaine craving appears to be related to changes in the ability of glutamate to activate dopaminergic neurons, changes known to be caused by experience with stress or with drugs of abuse.

Sensory effects of intravenous cocaine on dopamine and non-dopamine ventral tegmental area neurons

Intravenous (iv) cocaine mimics salient somato-sensory stimuli in their ability to induce rapid physiological effects, which appear to involve its action on peripherally located neural elements and fast neural transmission via somato-sensory pathways. To further clarify this mechanism, single-unit recording with fine glass electrodes was used in awake rats to examine responses of ventral tegmental area (VTA) neurons, both presumed dopamine (DA) and non-DA, to iv cocaine and tail-press, a typical somato-sensory stimulus. To exclude the contribution of DA mechanisms to the observed neuronal responses to sensory stimuli and cocaine, recordings were conducted during full DA receptor blockade (SCH23390+eticloptide). Iv cocaine (0.25 mg/kg delivered over 10 s) induced significant excitations of approximately 63% of long-spike (presumed DA) and approximately 70% of short-spike (presumed non-DA) VTA neurons. In both subgroups, neuronal excitations occurred with short latencies (4-8 s), peaked at 10-20 s (30-40% increase over baseline) and disappeared at 30-40 s after the injection onset. Most long-(67%) and short-spike (89%) VTA neurons also showed phasic responses to tail-press (5-s). All responsive long-spike cells were excited by tail-press; excitations were very rapid (peak at 1 s) and strong (100% rate increase over baseline) but brief (2-3 s). In contrast, both excitations (60%) and inhibitions (29%) were seen in short-spike cells. These responses were also rapid and transient, but excitations of short-spike units were more prolonged and sustained (10-15 s) than in long-spike cells. These data suggest that in awake animals iv cocaine, like somato-sensory stimuli, rapidly and transiently excites VTA neurons of different subtypes. Therefore, along with direct action on specific brain substrates, central effects of cocaine may occur, via an indirect mechanism, involving peripheral neural elements, visceral sensory nerves and rapid neural transmission. Via this mechanism, cocaine, like somato-sensory stimuli, can rapidly activate DA neurons and induce phasic DA release, creating the conditions for DA accumulation by a later occurring and prolonged direct inhibiting action on DA uptake. By providing a rapid neural signal and triggering transient neural activation, such a peripherally driven action might play a crucial role in the sensory effects of cocaine, thus contributing to learning and development of drug-taking behavior.

Mesoprefrontal dopamine neurons distinguish themselves

By distinguishing groups of dopamine neurons that differ in their projection patterns and intrinsic properties, Lammel and colleagues report in this issue of Neuron that mesocorticolimbic dopamine neurons of the ventral tegmental area (VTA) form a distinct subclass of dopamine cells.

Novel neurons in ventral tegmental area fire selectively during the active phase of the diurnal cycle

The ventral tegmental area (VTA) contains dopamine (DA) and gamma-aminobutyric acid (GABA) neurons involved in motivation and behavioral state. These phenomena are also influenced by circadian factors. The goal of our studies was to examine the impulse activity of neurochemically identified VTA neurons during dark (active) vs light (rest) phases of the circadian cycle. Using extracellular single-unit recordings with juxtacellular labeling in anesthetized rats, we found multiple neuronal subpopulations including 'novel neurons' that selectively fired during the dark phase. These novel neurons were electrophysiologically categorized into two groups, 'novel wide-spike' and 'novel thin-spike' neurons. Characterization of novel wide-spike neurons found they were consistently non-dopaminergic and non-GABAergic [tyrosine hydroxylase (TH)(-), glutamic acid decarboxylase (GAD)(-)]. However, they were inhibited by the D2 agonist quinpirole, an effect that could be reversed by the D2 antagonist eticlopride. Physiologically, they were fast firing (mean = 18.9 +/- 1.2 spikes/s), low bursting neurons (median = 6.2 +/- 3.0% of spikes in bursts) with spike durations > or = 2.0 ms, but slightly shorter than TH(+) neurons. They were also consistently non-responsive to footpad stimulation. The novel thin-spike neurons were neurochemically heterogeneous, and were located more ventrally than thin-spike neurons found during the light phase. These findings reveal previously unknown populations of VTA neurons whose activities are sensitive to diurnal phase, and whose functions may be in the temporal regulation of arousal and motivational processes.

Trace amine-associated receptor 1 modulates dopaminergic activity

The recent identification of the trace amine-associated receptor (TAAR)1 provides an opportunity to dissociate the effects of trace amines on the dopamine transporter from receptor-mediated effects. To separate both effects on a physiological level, a Taar1 knockout mouse line was generated. Taar1 knockout mice display increased sensitivity to amphetamine as revealed by enhanced amphetamine-triggered increases in locomotor activity and augmented striatal release of dopamine compared with wild-type animals. Under baseline conditions, locomotion and extracellular striatal dopamine levels were similar between Taar1 knockout and wild-type mice. Electrophysiological recordings revealed an elevated spontaneous firing rate of dopaminergic neurons in the ventral tegmental area of Taar1 knock-out mice. The endogenous TAAR1 agonist p-tyramine specifically decreased the spike frequency of these neurons in wild-type but not in Taar1 knockout mice, consistent with the prominent expression of Taar1 in the ventral tegmental area. Taken together, the data reveal TAAR1 as regulator of dopaminergic neurotransmission.

The vesicular glutamate transporter VGLUT3 synergizes striatal acetylcholine tone

Three subtypes of vesicular transporters accumulate glutamate into synaptic vesicles to promote its vesicular release. One of the subtypes, VGLUT3, is expressed in neurons, including cholinergic striatal interneurons, that are known to release other classical transmitters. Here we showed that disruption of the Slc17a8 gene (also known as Vglut3) caused an unexpected hypocholinergic striatal phenotype. Vglut3(-/-) mice were more responsive to cocaine and less prone to haloperidol-induced catalepsy than wild-type littermates, and acetylcholine release was decreased in striatum slices lacking VGLUT3. These phenotypes were associated with a colocalization of VGLUT3 and the vesicular acetylcholine transporter (VAChT) in striatal synaptic vesicles and the loss of a synergistic effect of glutamate on vesicular acetylcholine uptake. We propose that this vesicular synergy between two transmitters is the result of the unbalanced bioenergetics of VAChT, which requires anion co-entry for continuing vesicular filling. Our study reveals a previously unknown effect of glutamate on cholinergic synapses with potential functional and pharmacological implications.

Stereological estimates of dopaminergic, GABAergic and glutamatergic neurons in the ventral tegmental area, substantia nigra and retrorubral field in the rat

Midbrain dopamine neurons in the ventral tegmental area, substantia nigra and retrorubral field play key roles in reward processing, learning and memory, and movement. Within these midbrain regions and admixed with the dopamine neurons, are also substantial populations of GABAergic neurons that regulate dopamine neuron activity and have projection targets similar to those of dopamine neurons. Additionally, there is a small group of putative glutamatergic neurons within the ventral tegmental area whose function remains unclear. Although dopamine neurons have been intensively studied and quantified, there is little quantitative information regarding the GABAergic and glutamatergic neurons. We therefore used unbiased stereological methods to estimate the number of dopaminergic, GABAergic and glutamatergic cells in these regions in the rat. Neurons were identified using a combination of immunohistochemistry (tyrosine hydroxylase) and in situ hybridization (glutamic acid decarboxylase mRNA and vesicular glutamate transporter 2 mRNA). In substantia nigra pars compacta 29% of cells were glutamic acid decarboxylase mRNA-positive, 58% in the retrorubral field and 35% in the ventral tegmental area. There were further differences in the relative sizes of the GABAergic populations in subnuclei of the ventral tegmental area. Thus, glutamic acid decarboxylase mRNA-positive neurons represented 12% of cells in the interfascicular nucleus, 30% in the parabrachial nucleus, and 45% in the parainterfascicular nucleus. Vesicular glutamate transporter 2 mRNA-positive neurons were present in the ventral tegmental area, but not substantia nigra or retrorubral field. They were mainly confined to the rostro-medial region of the ventral tegmental area, and represented approximately 2-3% of the total neurons counted ( approximately 1600 cells). These results demonstrate that GABAergic and glutamatergic neurons represent large proportions of the neurons in what are traditionally considered as dopamine nuclei and that there are considerable heterogeneities in the proportions of cell types in the different dopaminergic midbrain regions.

Dopamine alters AMPA receptor synaptic expression and subunit composition in dopamine neurons of the ventral tegmental area cultured with prefrontal cortex neurons

Excitatory synapses onto dopamine (DA) neurons of the ventral tegmental area (VTA) represent a critical site of psychostimulant-induced synaptic plasticity. This plasticity involves alterations in synaptic strength through AMPA receptor (AMPAR) redistribution. Here, we report an in vitro model for studying regulation of AMPAR trafficking in DA neurons under control conditions and after elevation of DA levels, mimicking cocaine exposure. We used cocultures containing rat VTA neurons and prefrontal cortex (PFC) neurons from enhanced cyan fluorescent protein-expressing mice. In VTA-PFC cocultures, D1 receptor activation (10 min) increased synaptic and nonsynaptic glutamate receptor subunit 1 (GluR1) and GluR2 surface expression on DA neurons. NMDA or AMPA receptor antagonists blocked this effect, and it was not observed in pure VTA cultures, suggesting that DA agonists acted on D1 receptors on PFC neurons, altering their excitatory transmission onto VTA DA neurons and, thus, influencing AMPARs. To mimic the longer elevation in extracellular DA levels produced by systemic cocaine, cocultures were incubated with DA for 1 h. Synaptic GluR1 was increased 24 h later, reminiscent of the increased AMPA/NMDA ratio at excitatory synapses onto VTA DA neurons 24 h after cocaine injection (Ungless et al., 2001). In contrast, GluR2 was unchanged. Analysis of colocalization of surface GluR1-3 labeling suggested that control DA neurons express a substantial number of GluR1/2, GluR2/3, and homomeric GluR1 receptors and that the increase in surface AMPARs 24 h after DA exposure may in part reflect increased GluR1/3-containing receptors. These results help define the cellular basis for plasticity underlying the development of behavioral sensitization.

Functional implications of glutamatergic projections to the ventral tegmental area

Glutamatergic afferents of the ventral tegmental area (VTA) play an important role in the functioning of the VTA and are involved in the pathophysiology of drug addiction. It has recently been demonstrated that the VTA is densely innervated by glutamatergic axons and that glutamatergic neurons projecting to the VTA are situated in almost all structures that project there. While the projection from the prefrontal cortex is essentially entirely glutamatergic, subcortical glutamatergic neurons innervating the VTA intermingle with non-glutamatergic, most likely GABAergic and/or peptidergic VTA-projecting neurons. The first part of this review focuses on the origins and putative functional implications of various glutamatergic projections to the VTA. In the second part we consider how different neuropeptides via different mechanisms modulate glutamatergic actions in the VTA. We conclude by developing a model of how the glutamatergic afferents might together contribute to the functions of the VTA.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Activation of afferents to the ventral tegmental area in response to acute amphetamine: a double-labelling study

The ventral tegmental area (VTA), primary source of the mesocorticolimbic dopaminergic system, is regarded as a critical site for initiation of behavioural sensitization to psychostimulants. The present study was undertaken to identify the neural pathways converging on the VTA that are potentially implicated in this process. Rats were sensitized by a single exposure to amphetamine (5 mg/kg, s.c.). The distribution of VTA-projecting neurons activated by amphetamine was examined by combining retrograde transport of the cholera toxin beta subunit (CTb), injected into the VTA, with immunodetection of Fos. The quantitative analysis of CTb-Fos double labelling demonstrates that amphetamine induced a rapid activation of Fos in a large number of brain areas projecting to the VTA. More than half of the CTb-Fos double-labelled neurons were located in the prefrontal cortex, lateral preoptic area-lateral hypothalamus, pontomesencephalic tegmentum, dorsal raphe nucleus, ventral pallidum and nucleus accumbens. In addition, scattered CTb-Fos double-labelled cells were observed in many other VTA afferent structures, such as claustrum, lateral septum, diagonal band-magnocellular preoptic nucleus, deep mesencephalic nucleus, oral part of pontine reticular nucleus and dorsomedial tegmental area. This suggests that systemic amphetamine activates a wide population of neurons projecting to the VTA that may be important for the modulation of neurobehavioural plasticity produced by this psychostimulant.

Orexin axons in the rat ventral tegmental area synapse infrequently onto dopamine and gamma-aminobutyric acid neurons

Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.

Ventral tegmental area neurons in learned appetitive behavior and positive reinforcement

Ventral tegmental area (VTA) neuron firing precedes behaviors elicited by reward-predictive sensory cues and scales with the magnitude and unpredictability of received rewards. These patterns are consistent with roles in the performance of learned appetitive behaviors and in positive reinforcement, respectively. The VTA includes subpopulations of neurons with different afferent connections, neurotransmitter content, and projection targets. Because the VTA and substantia nigra pars compacta are the sole sources of striatal and limbic forebrain dopamine, measurements of dopamine release and manipulations of dopamine function have provided critical evidence supporting a VTA contribution to these functions. However, the VTA also sends GABAergic and glutamatergic projections to the nucleus accumbens and prefrontal cortex. Furthermore, VTA-mediated but dopamine-independent positive reinforcement has been demonstrated. Consequently, identifying the neurotransmitter content and projection target of VTA neurons recorded in vivo will be critical for determining their contribution to learned appetitive behaviors.