A feature-specific prediction error model explains dopaminergic heterogeneity.

Substantia nigra pars compacta (SNc) dopamine neurons and their targets are involved in addiction and cue-induced relapse. However, afferents onto SNc dopamine neurons themselves appear insensitive to drugs of abuse, such as cocaine, when afferents are collectively stimulated electrically. This contrasts with ventral tegmental area (VTA) dopamine neurons, whose glutamate afferents react robustly to cocaine. We used an optogenetic strategy to isolate identified SNc inputs and determine whether cocaine sensitivity in the mouse SNc circuit is conferred at the level of three glutamate afferents: dorsal raphé nucleus (DR), pedunculopontine nucleus (PPN), and subthalamic nucleus (STN). We found that excitatory afferents to SNc dopamine neurons are sensitive to cocaine in an afferent-specific manner. A single exposure to cocaine in vivo led to PPN-innervated synapses reducing the AMPA-to-NMDA receptor-mediated current ratio. In contrast to work in the VTA, this was due to increased NMDA receptor function with no change in AMPA receptor function. STN synapses showed a decrease in calcium-permeable AMPA receptors after cocaine, but no change in the AMPA-to-NMDA ratio. Cocaine also increased the release probability at DR-innervated and STN-innervated synapses, quantified by decreases in paired-pulse ratios. However, release probability at PPN-innervated synapses remained unaffected. By examining identified inputs, our results demonstrate a functional distribution among excitatory SNc afferent nuclei in response to cocaine, and suggest a compelling architecture for differentiation and separate parsing of inputs within the nigrostriatal system.SIGNIFICANCE STATEMENT Prior studies have established that substantia nigra pars compacta (SNc) dopamine neurons are a key node in the circuitry that drives addiction and relapse, yet cocaine apparently has no effect on electrically stimulated excitatory inputs. Our study is the first to demonstrate the functional impact of a drug of abuse on synaptic mechanisms of identified afferents to the SNc. Optogenetic dissection of inputs originating from dorsal raphé, pedunculopontine, and subthalamic nuclei were tested for synaptic modifications following in vivo cocaine exposure. Our results demonstrate that cocaine differentially induces modifications to SNc synapses depending on input origin. This presents implications for understanding dopamine processing of motivated behavior; most critically, it indicates that dopamine neurons selectively modulate signal reception processed by afferent nuclei.

This study examined the effect of the p.o. administration of the putative atypical antipsychotic drug Y-931 (8-fluoro-12-(4-methylpiperazin-1-yl)-6H-[1]benzothieno[2,3b][1,5] benzodiazepine maleate) on the activity of spontaneously active dopamine (DA) neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNC) in anesthetized male Sprague-Dawley rats. This was accomplished using in vivo electrophysiology. The acute p.o. administration of Y-931 did not significantly alter the number of spontaneously active SNC DA neurons compared to vehicle-treated animals. A single p.o. administration of 3 and 10 mg/kg of Y-931 significantly increased and decreased, respectively, the number of spontaneously active VTA DA neurons compared to vehicle-treated animals. The acute administration of 3 mg/kg of Y-931 significantly altered the firing pattern parameters for all spontaneously active SNC DA. The 3 and 10 mg/kg doses of Y-931 significantly increased the degree of bursting and irregular activity of spontaneously active VTA and SNC DA neurons firing in a bursting pattern. The repeated p.o. administration (21 days) of 1, 3, or 10 mg/kg of Y-931 significantly decreased the number of spontaneously active VTA DA neurons but had no significant effect on SNC DA neurons compared to vehicle-treated animals. The repeated administration of Y-931 did not significantly alter the firing pattern of all spontaneously active SNC or VTA DA neurons. Our findings indicate that the acute and chronic administration of Y-931 significantly alters the activity of midbrain DA neurons in rats and the electrophysiological profile of chronic Y-931 resembles that of atypical antipsychotic agents.

How does the activity of midbrain dopamine (DA) neurons reinforce actions? A prominent hypothesis is that the activity of ventral tegmental area (VTA) DA neurons instructs representations of predicted reward, or value, in downstream neurons1. To directly test this model, we performed comprehensive striatal recordings in mice engaged in a trial-and-error probabilistic learning task where they continuously adapted their choices to obtain a reward of optogenetic stimulation of VTA DA neurons (paired with an auditory cue). We then assessed neural representations of action values (estimated from a behavioral model), revealing for the first time that VTA DA stimulation is sufficient to generate downstream neural correlates of action value. Surprisingly, these value correlates were strongest in the intermediate caudoputamen (CP) and weakest in the nucleus accumbens (NAc), despite NAc being the major projection target of VTA DA neurons2,3. This was true not only for the value of each choice, but also for state value (reward expectation) and relative value (the decision variable). However, value representations were differentially organized within the intermediate CP, with ventromedial domains (which receive inputs from orbitofrontal cortex) preferentially encoding state value and dorsolateral domains (which receive inputs from motor cortex) preferentially encoding relative value. A difference in learning rate for the value computation between NAc and CP did not explain the relatively weak value correlates in NAc. Instead, we found that VTA DA stimulation was sufficient to produce learned neural responses to the stimulation-paired auditory cue throughout the striatum, including in the NAc, and that animals work for this cue rather than for VTA DA stimulation itself. Overall, this suggests that VTA DA neurons support trial-and-error learning indirectly, by making stimuli valuable (“conditioned reinforcers”), which in turn support the generation of action value representations in the CP.

Role of ventral tegmental area dopamine neurons in fear memory retention in post-traumatic stress disorder model rats

Connexin-36 (Cx36) gap junctions (GJs) appear to be involved in the synchronization of GABA interneurons in many brain areas. We have previously identified a population of Cx36-connected ventral tegmental area (VTA) GABA neurons that may regulate mesolimbic dopamine (DA) neurotransmission, a system implicated in reward from both natural behaviors and drugs of abuse. The aim of this study was to determine the effect mefloquine (MFQ) has on midbrain DA and GABA neuron inhibition, and the role Cx36 GJs play in regulating midbrain VTA DA neuron activity in mice. In brain slices from adolescent wild-type (WT) mice the Cx36-selective GJ blocker mefloquine (MFQ, 25 μM) increased VTA DA neuron sIPSC frequency sixfold, and mIPSC frequency threefold. However, in Cx36 KO mice, MFQ only increased sIPSC and mIPSC frequency threefold. The nonselective GJ blocker carbenoxolone (CBX, 100 μM) increased DA neuron sIPSC frequency twofold in WT mice, did not affect Cx36 KO mouse sIPSCs, and did not affect mIPSCs in WT or Cx36 KO mice. Interestingly, MFQ had no effect on VTA GABA neuron sIPSC frequency. We also examined MFQ effects on VTA DA neuron firing rate and current-evoked spiking in WT and Cx36 KO mice, and found that MFQ decreased WT DA neuron firing rate and current-evoked spiking, but did not alter these measures in Cx36 KO mice. Taken together these findings suggest that blocking Cx36 GJs increases VTA DA neuron inhibition, and that GJs play in key role in regulating inhibition of VTA DA neurons. Synapse, 2011. © 2011 Wiley-Liss, Inc.

Long-term morphine use is associated with serious side effects, such as morphine-induced hyperalgesia and analgesic tolerance. Previous investigations have documented the association between dopamine (DA) neurons in the ventral tegmental area (VTA) and pain. However, whether VTA DA neurons are implicated in morphine-induced hyperalgesia and analgesic tolerance remains elusive. Initially, we observed behavioural effects of lidocaine administration into VTA or ablation of VTA DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance. Subsequently, c-Fos expression in nucleus accumbens (NAc) shell-projecting and medial prefrontal cortex (mPFC)-projecting VTA DA neurons after chronic morphine treatment was respectively investigated. Afterwards, the effects of chemogenetic manipulation of NAc shell-projecting or mPFC-projecting DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance were observed. Additionally, effects of chemogenetic manipulation of VTA GABA neurons on c-Fos expression in VTA DA neurons were investigated. Lidocaine injection into VTA relieved established hyperalgesia and anti-nociceptive tolerance whereas ablation of VTA DA neurons prevented the development of morphine-induced hyperalgesia and anti-nociceptive tolerance. Chronic morphine treatment increased c-Fos expression in NAc shell-projecting DA neurons, rather than in mPFC-projecting DA neurons. Chemogenetic manipulation of NAc shell-projecting DA neurons had influence on morphine-induced hyperalgesia and tolerance. However, chemogenetic manipulation of mPFC-projecting DA neurons had no significant effects on morphine-induced hyperalgesia and anti-nociceptive tolerance. Chemogenetic manipulation of VTA GABA neurons affected the c-Fos expression in VTA DA neurons. These findings revealed the involvement of NAc shell-projecting VTA DA neurons in morphine-induced hyperalgesia and anti-nociceptive tolerance, and may shed new light on the clinical management of morphine-induced hyperalgesia and analgesic tolerance. This study demonstrated that NAc shell-projecting DA neurons rather than mPFC-projecting DA neurons in the VTA were implicated in morphine-induced hyperalgesia and anti-nociceptive tolerance. Our findings may pave the way for the discovery of novel therapies for morphine-induced hyperalgesia and analgesic tolerance.

Stimulation of the prefrontal cortex (PFC) has been shown to have an excitatory influence on dopamine (DA) neurons. We report here that, under nonstimulated conditions, the activity of DA neurons in the ventral tegmental area (VTA) also covaries, on a subsecond timescale, with the activity of PFC cells. Thus, in 67% of VTA DA neurons recorded in chloral hydrate-anesthetized rats, the firing of the cell displayed a slow oscillation (SO) that was highly coherent with the activity of PFC neurons. The SO was suppressed by transections immediately caudal to the PFC or by intra-PFC infusion of tetrodotoxin, suggesting that it depends on inputs derived from the PFC. Unexpectedly, the SO in most VTA DA neurons was reversed in phase relative to PFC cell activity, suggesting that at least part of PFC information is transferred to DA neurons indirectly through inhibitory relay neurons. These results, together with those reported previously, suggest that the PFC can act through multiple pathways to exert both excitatory and inhibitory influences on DA neurons. The observed functional coupling between DA and PFC neurons further suggests that these pathways not only allow a bidirectional control of DA neurons by the PFC, but also enable action potential-dependent DA release to be coordinated, on a subsecond timescale, with glutamate release from PFC terminals. Further understanding of this coordinated activity may provide important new insights into brain functions and disorders thought to involve both VTA DA and PFC neurons.

BackgroundDisruptions in dopamine (DA) signaling are central to the pathophysiology of several major psychiatric disorders, including schizophrenia. Thus, discovery of novel therapeutic approaches that normalize DA signaling is a major focus of research. One pathway that is critical for DA regulation originates from the ventral subiculum (vSub) of the hippocampus and controls ventral tegmental area (VTA) DA neuron activity. A potent regulator of hippocampal function is the medial septum (MS); a sub-region of the basal forebrain that widely innervates the hippocampus, including the vSub, via cholinergic, GABAergic, and glutamatergic projections, drives hippocampal theta rhythms, and affects goal-directed learning and memory. Despite this, it has never been determined if the MS is an afferent regulator of the midbrain DA system, and therefore may be a novel therapeutic treatment target for DA-related disorders.MethodsEffects of MS activation (NMDA, 0.75 µg/ 0.2 µL) were examined in intact and methylazoxymethanol acetate- treated (MAM) male Sprague-Dawley rats using anesthetized single unit DA recordings in the VTA and substantia nigra pars compacta (SNc) and locomotor behavior in an open field following systemic amphetamine (0.75 mg/kg).ResultsMS activation produced a prolonged 71% increase in the number of spontaneously active DA cells in the VTA, and an opposing 40% decrease in the number of active DA cells in the SNc, compared to vehicle infusions. These effects were mediated by the vSub and ventral pallidum as local infusion of TTX (1 µM /0.5 µL) and bicuculline (1 ng/ 0.5 µL), respectively, reversed DA population activity changes in both regions. MS activation also decreased the locomotor response to amphetamine (49% reduction in distance traveled during peak ambulation compared to vehicle). MS activation-induced changes in both DA population activity and amphetamine-induced hyperlocomotion were selectively mediated by different neurotransmitter populations from MS to vSub as infusion of scopolamine (8 µg/1.0 µL) into the vSub selectively prevented DA population activity changes in the VTA. In contrast, infusion of bicuculline (12.5 ng/0.5 µL) selectively prevented DA population activity changes in the SNc and the decrease in amphetamine-induced hyperlocomotion. In MAM rats, MS activation produced opposite effects on DA population activity versus controls as it decreased VTA DA activity by 51% and increased SNc DA activity by 47%. This was accompanied by a similar reversal in amphetamine-induced hyperlocomotion, with MS activation increasing locomotion in MAM animals (113% increase in distance traveled during peak ambulation compared to control animals). The reversal in behavioral output is likely due to disrupted GABAergic projections from MS to vSub as bicuculline infusion into vSub prevented the increase in locomotor behavior.DiscussionThese data indicate that the MS differentially regulates both VTA and SNc DA neuron activity and behavioral output, but via distinct pathways, and that this regulation is disrupted in a well-validated animal model of schizophrenia. Therefore, this suggests that in normal animals the MS might activate the VTA to increase information processing while delaying action via SNc inhibition. In contrast, the inhibition of VTA and activation of SNc in MAM rats might promote a rapid response without adequate processing of information.

RGS Proteins as Critical Regulators of Motor Function and Their Implications in Parkinson’s Disease

BackgroundThe initiation of behavioral sensitization to cocaine and other psychomotor stimulants is thought to reflect N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic plasticity in the mesolimbic dopamine (DA) circuitry. The importance of drug induced NMDAR mediated adaptations in ventral tegmental area (VTA) DA neurons, and its association with drug seeking behaviors, has recently been evaluated in Cre-loxp mice lacking functional NMDARs in DA neurons expressing Cre recombinase under the control of the endogenous dopamine transporter gene (NR1DATCre mice).Methodology and Principal FindingsUsing an additional NR1DATCre mouse transgenic model, we demonstrate that while the selective inactivation of NMDARs in DA neurons eliminates the induction of molecular changes leading to synaptic strengthening, behavioral measures such as cocaine induced locomotor sensitization and conditioned place preference remain intact in NR1DATCre mice. Since VTA DA neurons projecting to the prefrontal cortex and amygdala express little or no detectable levels of the dopamine transporter, it has been speculated that NMDA receptors in DA neurons projecting to these brain areas may have been spared in NR1DATCre mice. Here we demonstrate that the NMDA receptor gene is ablated in the majority of VTA DA neurons, including those exhibiting undetectable DAT expression levels in our NR1DATCre transgenic model, and that application of an NMDAR antagonist within the VTA of NR1DATCre animals still blocks sensitization to cocaine.Conclusions/SignificanceThese results eliminate the possibility of NMDAR mediated neuroplasticity in the different DA neuronal subpopulations in our NR1DATCre mouse model and therefore suggest that NMDARs on non-DA neurons within the VTA must play a major role in cocaine-related addictive behavior.

The circuitry mediating top-down control of dopamine (DA) neurons in the ventral tegmental area (VTA) is exceedingly complex. Characterizing these networks will be critical to our understanding of fundamental behaviors, such as motivation and reward processing, as well as several disease states. Previous work suggests that the medial prefrontal cortex (mPFC) exerts a profound influence on VTA DA neuron firing. Recently, our group reported that inhibition of the infralimbic subdivision of the medial prefrontal cortex (ilPFC) increases the proportion of VTA DA neurons that are spontaneously active (i.e., "population activity") and that this effect depends on activity in the ventral subiculum of the hippocampus (vSub). However, there is no direct projection from the mPFC to the vSub. Anatomical evidence suggests that communication between the two structures is mediated by the nucleus reuniens of the midline thalamus (RE). Here, we used in vivo electrophysiological and behavioral approaches in rats to explore the role of the RE in the circuitry governing VTA DA neuron firing. We show that pharmacological stimulation of the RE enhances VTA DA neuron population activity and amphetamine-induced hyperlocomotion, a behavioral indicator of an over-responsive DA system. Furthermore, the effect of RE stimulation on population activity is prevented if vSub is also inhibited. Finally, pharmacological inhibition of ilPFC enhances VTA DA neuron population activity, but this effect does not occur if RE is also inhibited. These findings suggest that disruption of ilPFC-RE-vSub communication could lead to a dysregulated, hyperdopaminergic state, and may play a role in psychiatric disorders. Dopamine (DA) neurons in the ventral tegmental area (VTA) are involved in a variety of fundamental brain functions. To understand the neurobiological basis for these functions it is essential to identify regions controlling DA neuron activity. The medial prefrontal cortex (mPFC) is emerging as a key regulator of DA neuron activity, but the circuitry by which it exerts its influence remains poorly described. Here, we show that the nucleus reuniens of the midline thalamus gates mPFC control of VTA DA neuron firing by the hippocampus. These data identify a unique role for this corticothalamic-hippocampal circuit, and suggest that dysfunction in these regions likely influences the pathophysiology of psychiatric disorders.

Ventral tegmental area (VTA) dopamine (DA) neurons play a pivotal role in processing reward-related information and are involved in drug addiction and mental illness in humans. Information is conveyed to the VTA in large part by glutamatergic afferents that arise in various brain nuclei, including the pedunculopontine nucleus (PPN). Using a unique rat brain slice preparation, we found that PPN stimulation activates afferents targeting GluR2-containing AMPA receptors (AMPAR) on VTA DA neurons, and these afferents did not exhibit long-term depression (LTD). In contrast, activation of glutamate afferents onto the same DA neurons via stimulation within the VTA evoked EPSCs mediated by GluR2-lacking AMPARs that demonstrated LTD or EPSCs mediated by GluR2-containing AMPA receptors that did not express LTD. Twenty four hours after single cocaine injections to rats, GluR2-lacking AMPARs were increased at both PPN and local VTA projections, and this permitted LTD expression in both pathways. Single injections with the main psychoactive ingredient of marijuana, delta-9-tetrahydrocannabinol (Δ9-THC), increased GluR2-lacking AMPA receptors and permitted LTD in only the PPN pathway, and these effects were prevented by in vivo pretreatment with the cannabinoid CB1 receptor antagonist AM251. These results demonstrate that cocaine more globally increases GluR2-lacking AMPA receptors at all glutamate synapses on VTA dopamine neurons, whereas Δ9-THC selectively increased GluR2-lacking AMPA receptors at subcortical PPN synapses. This suggests that different abused drugs may exert influence over distinct sets of glutamatergic afferents to VTA DA neurons which may be associated with different reinforcing or addictive properties of these drugs.

Parkinson’s disease (PD) is characterized by the death of substantia nigra pars compacta (SNc) dopamine (DA) neurons, but the pathophysiological mechanisms that precede and drive their death remain unknown. The activity of DA neurons is likely altered in PD, but we understand little about if or how chronic changes in activity may contribute to degeneration. To address this question, we developed a chemogenetic (DREADD) mouse model to chronically increase DA neuron activity and confirmed this increase using ex vivo electrophysiology. Chronic hyperactivation of DA neurons resulted in prolonged increases in locomotor activity during the light cycle and decreases during the dark cycle, consistent with chronic changes in DA release and circadian disturbances. We also observed early, preferential degeneration of SNc projections, recapitulating the PD hallmarks of selective vulnerability of SNc axons and the comparative resilience of ventral tegmental area axons. This was followed by the eventual loss of midbrain DA neurons. Continuous DREADD activation resulted in a sustained increase in baseline calcium levels, supporting a role for increased calcium in the neurodegeneration process. Finally, spatial transcriptomics from DREADD mice examining midbrain DA neurons and striatal targets, and cross-validation with human patient samples, provided insights into potential mechanisms of hyperactivity-induced toxicity and PD. Our results thus reveal the preferential vulnerability of SNc DA neurons to increased neural activity and support a potential role for increased neural activity in driving degeneration in PD.

Systemic administration of cocaine is thought to decrease the firing rates of ventral tegmental area (VTA) dopamine (DA) neurons. However, this view is based on categorizations of recorded neurons as DA neurons using preselected electrophysiological characteristics lacking neurochemical confirmation. Without applying cellular preselection, we recorded the impulse activity of VTA neurons in response to cocaine administration in anesthetized adult rats. The phenotype of recorded neurons was determined by their juxtacellular labeling and immunohistochemical detection of tyrosine hydroxylase (TH), a DA marker. We found that intravenous cocaine altered firing rates in the majority of recorded VTA neurons. Within the cocaine-responsive neurons, half of the population was excited and the other half was inhibited. Both populations had similar discharge rates and firing regularities, and most neurons did not exhibit changes in burst firing. Inhibited neurons were more abundant in the posterior VTA, whereas excited neurons were distributed evenly throughout the VTA. Cocaine-excited neurons were more likely to be excited by footshock. Within the subpopulation of TH-positive neurons, 36% were excited by cocaine and 64% were inhibited. Within the subpopulation of TH-negative neurons, 44% were excited and 28% were inhibited. Contrary to the prevailing view that all DA neurons are inhibited by cocaine, we found a subset of confirmed VTA DA neurons that is excited by systemic administration of cocaine. We provide evidence indicating that DA neurons are heterogeneous in their response to cocaine and that VTA non-DA neurons play an active role in processing systemic cocaine.

Early life aversive experiences can trigger persistent deficits in neuronal signaling within the mesolimbic pathway, most notably in the dopamine (DA) neurons of the ventral tegmental area (VTA). The identity of such cellular mechanisms currently appears as an important issue. To address this concern, we investigated whether early life maternal deprivation (MD) would affect the electrical activity of VTA DA neurons, via in vivo extracellular single-unit recording. Male Wistar rats were deprived of their dams for 3h per day from postnatal days (PND) 1-14. Thereafter, the adult animals (PND 70-80) were tested for the discharge activity of putative VTA DA neurons. The VTA DA neurons displayed a decrease in firing rate and an increase in the variability of baseline discharge activity in deprived animals. MD also caused a decrease in burst firing of VTA DA neurons compared to control subjects. In summary, early life MD induces a hypoactive VTA DA system, which may contribute to lifespan psychopathologies.