Adaptor protein-3 produces synaptic vesicles that release phasic dopamine.

The burst firing of midbrain dopamine neurons releases a phasic dopamine signal that mediates reinforcement learning. At many synapses, however, high firing rates deplete synaptic vesicles (SVs), resulting in synaptic depression that limits release. What accounts for the increased release of dopamine by stimulation at high frequency? We find that adaptor protein-3 (AP-3) and its coat protein VPS41 promote axonal dopamine release by targeting vesicular monoamine transporter VMAT2 to the axon rather than dendrites. AP-3 and VPS41 also produce SVs that respond preferentially to high-frequency stimulation, independent of their role in axonal polarity. In addition, conditional inactivation of VPS41 in dopamine neurons impairs reinforcement learning, and this involves a defect in the frequency dependence of release rather than the amount of dopamine released. Thus, AP-3 and VPS41 promote the axonal polarity of dopamine release but enable learning by producing a distinct population of SVs tuned specifically to high firing frequency that confers the phasic release of dopamine.

Gait Abnormalities and Aberrant D2 Receptor Expression and Signaling in Mice Carrying the Human Pathogenic Mutation DRD2I212F.

A dopamine D2 receptor mutation was recently identified in a family with a novel hyperkinetic movement disorder. That allelic variant D2-I212F is a constitutively active and G protein-biased receptor. We now describe mice engineered using CRISPR-Cas9-mediated gene editing technology to carry the D2-I212F variant. Drd2I212F mice exhibited gait abnormalities resembling those in other mouse models of chorea and/or dystonia and had striatal D2 receptor expression that was decreased approximately 30% per Drd2I212F allele. Electrically evoked inhibitory postsynaptic conductances in midbrain dopamine neurons and striatum from Drd2I212F mice, caused by G protein activation of potassium channels, exhibited slow kinetics (e.g., approximately four- to sixfold slower decay) compared with Drd2 +/+ mice. Current decay initiated by photolytic release of the D2 antagonist sulpiride from CyHQ-sulpiride was also ∼fourfold slower in midbrain slices from Drd2I212F mice than Drd2 +/+ mice. Furthermore, in contrast to Drd2 +/+ mice, in which dopamine is several-fold more potent at neurons in the nucleus accumbens than in the dorsal striatum, reflecting activation of Gα o versus Gα i, dopamine had similar potencies in those two brain regions of Drd2I212F mice. Repeated cocaine treatment, which decreases dopamine potency in the nucleus accumbens of Drd2 +/+ mice, had no effect on dopamine potency in Drd2 I212F mice. The results demonstrate the pathogenicity of the D2-I212F mutation and the utility of this mouse model for investigating the role of pathogenic DRD2 variants in early-onset hyperkinetic movement disorders. SIGNIFICANCE STATEMENT: The first dopamine receptor mutation to cause a movement disorder, D2-I212F, was recently identified. The mutation makes receptor activation of G protein-mediated signaling more efficient. To confirm the pathogenesis of D2-I212F, this study reports that mice carrying this mutation have gait abnormalities consistent with the clinical phenotype. The mutation also profoundly alters D2 receptor expression and function in vivo. This mouse model will be useful for further characterization of the mutant receptor and for evaluation of potential therapeutic drugs.

Properties and modulation of excitatory inputs to the locus coeruleus.

Excitatory inputs drive burst firing of locus coeruleus (LC) noradrenaline (NA) neurons in response to a variety of stimuli. Though a small number of glutamatergic LC afferents have been investigated, the overall landscape of these excitatory inputs is largely unknown. The current study used an optogenetic approach to isolate three glutamatergic afferents: the prefrontal cortex (PFC), lateral hypothalamus (LH) and periaqueductal grey (PAG). AAV5-DIO-ChR2 was injected into each region in male and female CaMKII-Cre mice and the properties of excitatory inputs on LC-NA cells were measured. Notably we found differences among these inputs. First, the pattern of axonal innervation differed between inputs such that LH afferents were concentrated in the posterior portion of the LC-NA somatic region while PFC afferents were denser in the medial dendritic region. Second, basal intrinsic properties varied for afferents, with LH inputs having the highest connectivity and the largest amplitude excitatory postsynaptic currents while PAG inputs had the lowest initial release probability. Third, while orexin and oxytocin had minimal effects on any input, dynorphin strongly inhibited excitatory inputs originating from the LH and PAG, and corticotrophin releasing factor (CRF) selectively inhibited inputs from the PAG. Overall, these results demonstrate that individual afferents to the LC have differing properties, which may contribute to the modularity of the LC and its ability to mediate various behavioural outcomes. KEY POINTS: Excitatory inputs to the locus coeruleus (LC) are important for driving noradrenaline neuron activity and downstream behaviours in response to salient stimuli, but little is known about the functional properties of different glutamate inputs that innervate these neurons We used a virus-mediated optogenetic approach to compare glutamate afferents from the prefrontal cortex (PFC), the lateral hypothalamus (LH) and the periaqueductal grey (PAG). While PFC was predicted to make synaptic inputs, we found that the LH and PAG also drove robust excitatory events in LC noradrenaline neurons. The strength, kinetics, and short-term plasticity of each input differed as did the extent of neuromodulation by both dynorphin and corticotrophin releasing factor. Thus each input displayed a unique set of basal properties and modulation by peptides. This characterization is an important step in deciphering the heterogeneity of the LC.

The Evolving Understanding of Dopamine Neurons in the Substantia Nigra and Ventral Tegmental Area.

In recent years, the population of neurons in the ventral tegmental area (VTA) and substantia nigra (SN) has been examined at multiple levels. The results indicate that the projections, neurochemistry, and receptor and ion channel expression in this cell population vary widely. This review centers on the intrinsic properties and synaptic regulation that control the activity of dopamine neurons. Although all dopamine neurons fire action potentials in a pacemaker pattern in the absence of synaptic input, the intrinsic properties that underlie this activity differ considerably. Likewise, the transition into a burst/pause pattern results from combinations of intrinsic ion conductances, inhibitory and excitatory synaptic inputs that differ among this cell population. Finally, synaptic plasticity is a key regulator of the rate and pattern of activity in different groups of dopamine neurons. Through these fundamental properties, the activity of dopamine neurons is regulated and underlies the wide-ranging functions that have been attributed to dopamine.

Neurotensin Induces Presynaptic Depression of D2 Dopamine Autoreceptor-Mediated Neurotransmission in Midbrain Dopaminergic Neurons.

Increased dopaminergic signaling is a hallmark of severe mesencephalic pathologies such as schizophrenia and psychostimulant abuse. Activity of midbrain dopaminergic neurons is under strict control of inhibitory D2 autoreceptors. Application of the modulatory peptide neurotensin (NT) to midbrain dopaminergic neurons transiently increases activity by decreasing D2 dopamine autoreceptor function, yet little is known about the mechanisms that underlie long-lasting effects. Here, we performed patch-clamp electrophysiology and fast-scan cyclic voltammetry in mouse brain slices to determine the effects of NT on dopamine autoreceptor-mediated neurotransmission. Application of the active peptide fragment NT8-13 produced synaptic depression that exhibited short- and long-term components. Sustained depression of D2 autoreceptor signaling required activation of the type 2 NT receptor and the protein phosphatase calcineurin. NT application increased paired-pulse ratios and decreased extracellular levels of somatodendritic dopamine, consistent with a decrease in presynaptic dopamine release. Surprisingly, we observed that electrically induced long-term depression of dopaminergic neurotransmission that we reported previously was also dependent on type 2 NT receptors and calcineurin. Because electrically induced depression, but not NT-induced depression, was blocked by postsynaptic calcium chelation, our findings suggest that endogenous NT may act through a local circuit to decrease presynaptic dopamine release. The current research provides a mechanism through which augmented NT release can produce a long-lasting increase in membrane excitability of midbrain dopamine neurons. SIGNIFICANCE STATEMENT: Whereas plasticity of glutamate synapses in the brain has been studied extensively, demonstrations of plasticity at dopaminergic synapses have been more elusive. By quantifying inhibitory neurotransmission between midbrain dopaminergic neurons in brain slices from mice we have discovered that the modulatory peptide neurotensin can induce a persistent synaptic depression by decreasing dopamine release. This depression of inhibitory synaptic input would be expected to increase excitability of dopaminergic neurons. Induction of the plasticity can be pharmacologically blocked by antagonists of either the protein phosphatase calcineurin or neurotensin receptors, and persists surprisingly long after a brief exposure to the peptide. Since neurotensin-dopamine interactions have been implicated in hyperdopaminergic pathologies, these findings describe a synaptic mechanism that could contribute to addiction and/or schizophrenia.

Mechanisms of 5-HT1A receptor-mediated transmission in dorsal raphe serotonin neurons.

KEY POINTS: In the dorsal raphe nucleus, it is known that serotonin release activates metabotropic 5-HT1A autoreceptors located on serotonin neurons that leads to an inhibition of firing through the activation of G-protein-coupled inwardly rectifying potassium channels. We found that in mouse brain slices evoked serotonin release produced a 5-HT1A receptor-mediated inhibitory postsynaptic current (IPSC) that resulted in only a transient pause in firing. While spillover activation of receptors contributed to evoked IPSCs, serotonin reuptake transporters prevented pooling of serotonin in the extrasynaptic space from activating 5-HT1A -IPSCs. As a result, the decay of 5-HT1A -IPSCs was independent of the intensity of stimulation or the probability of transmitter release. These results indicate that evoked serotonin transmission in the dorsal raphe nucleus mediated by metabotropic 5-HT1A autoreceptors may occur via point-to-point synapses rather than by paracrine mechanisms. ABSTRACT: In the dorsal raphe nucleus (DRN), feedback activation by Gαi/o -coupled 5-HT1A autoreceptors reduces the excitability of serotoninergic neurons, which decreases serotonin release both locally within the DRN and in projection regions. Serotonin transmission within the DRN is thought to occur via transmitter spillover and paracrine activation of extrasynaptic receptors. Here, we tested the volume transmission hypothesis in mouse DRN brain slices by recording 5-HT1A receptor-mediated inhibitory postsynaptic currents (5-HT1A -IPSCs) generated by the activation of G-protein-coupled inwardly rectifying potassium channels (GIRKs). We found that in the DRN of ePET1-EYFP mice, which selectively express enhanced yellow fluorescent protein in serontonergic neurons, the local release of serotonin generated 5-HT1A -IPSCs in serotonin neurons that rose and fell within a second. The transient activation of 5-HT1A autoreceptors resulted in brief pauses in neuron firing that did not alter the overall firing rate. The duration of 5-HT1A -IPSCs was primarily shaped by receptor deactivation due to clearance via serotonin reuptake transporters. Slowing diffusion with dextran prolonged the rise and reduced the amplitude the IPSCs and the effects were potentiated when uptake was inhibited. By examining the decay kinetics of IPSCs, we found that while spillover may allow for the activation of extrasynaptic receptors, efficient uptake by serotonin reuptake transporters (SERTs) prevented the pooling of serotonin from prolonging the duration of transmission when multiple inputs were active. Together the results suggest that the activation of 5-HT1A receptors in the DRN results from the local release of serotonin rather than the extended diffusion throughout the extracellular space.

The timing of dopamine- and noradrenaline-mediated transmission reflects underlying differences in the extent of spillover and pooling.

Metabotropic transmission typically occurs through the spillover activation of extrasynaptic receptors. This study examined the mechanisms underlying somatodendritic dopamine and noradrenaline transmission and found that the extent of spillover and pooling varied dramatically between these two transmitters. In the mouse ventral tegmental area, the time course of D2-receptor-mediated IPSCs (D2-IPSCs) was consistent between cells and was unaffected by altering stimulation intensity, probability of release, or the extent of diffusion. Blocking dopamine reuptake with cocaine extended the time course of D2-IPSCs and suggested that transporters strongly limited spillover. As a result, individual release sites contributed independently to the duration of D2-IPSCs. In contrast, increasing the release of noradrenaline in the rat locus ceruleus prolonged the duration of α2-receptor-mediated IPSCs even when reuptake was intact. Spillover and subsequent pooling of noradrenaline activated distal α2-receptors, which prolonged the duration of α2-IPSCs when multiple release sites were activated synchronously. By using the rapid application of agonists onto large macropatches, we determined the concentration profile of agonists underlying the two IPSCs. Incorporating the results into a model simulating extracellular diffusion predicted that the functional range of noradrenaline diffusion was nearly fivefold greater in the locus ceruleus than dopamine in the midbrain. This study demonstrates that catecholamine synapses differentially regulate the extent of spillover and pooling to control the timing of local inhibition and suggests diversity in the roles of uptake and diffusion in governing metabotropic transmission.

Salience signaling and stimulus scaling of ventral tegmental area glutamate neuron subtypes.

Ventral tegmental area (VTA) glutamatergic neurons participate in reward, aversion, drug-seeking, and stress. Subsets of VTA VGluT2+ neurons are capable of co-transmitting glutamate and GABA (VGluT2+VGaT+ neurons), transmitting glutamate without GABA (VGluT2+VGaT- neurons), or co-transmitting glutamate and dopamine (VGluT2+TH+ neurons), but whether these molecularly distinct subpopulations show behavior-related differences is not wholly understood. We identified that neuronal activity of each VGluT2+ subpopulation is sensitive to reward value but signaled this in different ways. The phasic maximum activity of VGluT2+VGaT+ neurons increased with sucrose concentration, whereas VGluT2+VGaT- neurons increased maximum and sustained activity with sucrose concentration, and VGluT2+TH+ neurons increased sustained but not maximum activity with sucrose concentration. Additionally, VGluT2+ subpopulations signaled consummatory preferences in different ways. VGluT2+VGaT- neurons and VGluT2+TH+ neurons showed a signaling preference for a behaviorally-preferred fat reward over sucrose, but in temporally-distinct ways. In contrast, VGluT2+VGaT+ neurons uniquely signaled a less behaviorally-preferred sucrose reward compared with fat. Further experiments suggested that VGluT2+VGaT+ consummatory reward-related activity was related to sweetness, partially modulated by hunger state, and not dependent on caloric content or behavioral preference. All VGluT2+ subtypes increased neuronal activity following aversive stimuli but VGluT2+VGaT+ neurons uniquely scaled their magnitude and sustained activity with footshock intensity. Optogenetic activation of VGluT2+VGaT+ neurons during low intensity footshock enhanced fear-related behavior without inducing place preference or aversion. We interpret these data such that VTA glutamatergic subpopulations signal different elements of rewarding and aversive experiences and highlight the unique role of VTA VGluT2+VGaT+ neurons in enhancing the salience of behavioral experiences.

A chemogenetic approach for dopamine imaging with tunable sensitivity.

Genetically-encoded dopamine (DA) sensors enable high-resolution imaging of DA release, but their ability to detect a wide range of extracellular DA levels, especially tonic versus phasic DA release, is limited by their intrinsic affinity. Here we show that a human-selective dopamine receptor positive allosteric modulator (PAM) can be used to boost sensor affinity on-demand. The PAM enhances DA detection sensitivity across experimental preparations (in vitro, ex vivo and in vivo) via one-photon or two-photon imaging. In vivo photometry-based detection of optogenetically-evoked DA release revealed that DETQ administration produces a stable 31 minutes window of potentiation without effects on animal behavior. The use of the PAM revealed region-specific and metabolic state-dependent differences in tonic DA levels and enhanced single-trial detection of behavior-evoked phasic DA release in cortex and striatum. Our chemogenetic strategy can potently and flexibly tune DA imaging sensitivity and reveal multi-modal (tonic/phasic) DA signaling across preparations and imaging approaches.

Species differences in somatodendritic dopamine transmission determine D2-autoreceptor-mediated inhibition of ventral tegmental area neuron firing.

The somatodendritic release of dopamine within the ventral tegmental area (VTA) and substantia nigra pars compacta activates inhibitory postsynaptic D2-receptors on dopaminergic neurons. The proposed mechanisms that regulate this form of transmission differ between electrochemical studies using rats and guinea pigs and electrophysiological studies using mice. This study examines the release and resulting dopamine D2-autoreceptor-mediated IPSCs (D2-IPSCs) in the VTA of mouse, rat, and guinea pig. Robust D2-IPSCs were observed in all recordings from neurons in slices taken from mouse, whereas D2-IPSCs in rat and guinea pig were observed less frequently and were significantly smaller in amplitude. In slices taken from guinea pig, dopamine release was more persistent under conditions of reduced extracellular calcium. The decline in the concentration of dopamine was also prolonged and not as sensitive to inhibition of reuptake by cocaine. This resulted in an increased duration of D2-IPSCs in the guinea pig. Therefore, unlike the mouse or the rat, the time course of dopamine in the extracellular space of the guinea pig determined the duration the D2-IPSC. Functionally, differences in D2-IPSCs resulted in inhibition of dopamine neuron firing only in slices from mouse. The results suggest that the mechanisms and functional consequences of somatodendritic dopamine transmission in the VTA vary among species. This highlights the complexity that underlies dopamine-dependent transmission in one brain area. Differences in somatodendritic transmission would be expected in vivo to affect the downstream activity of the mesocorticolimbic dopamine system and subsequent terminal release.

Alterations in neurotransmitter co-release in Parkinson's disease.

Parkinson's disease is a neurological disorder characterized by degeneration of midbrain dopamine neurons, which results in numerous adaptations in basal ganglia circuits. Research over the past twenty-five years has identified that midbrain dopamine neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) co-release multiple other transmitters including glutamate and GABA, in addition to their canonical transmitter, dopamine. This review summarizes previous work characterizing neurotransmitter co-release from dopamine neurons, work examining potential changes in co-release dynamics that result in animal models of Parkinson's disease, and future opportunities for determining how dysfunction in co-release may contribute to circuit dysfunction in Parkinson's disease.

Adaptor Protein-3 Produces Synaptic Vesicles that Release Phasic Dopamine.

The burst firing of midbrain dopamine neurons releases a phasic dopamine signal that mediates reinforcement learning. At many synapses, however, high firing rates deplete synaptic vesicles (SVs), resulting in synaptic depression that limits release. What accounts for the increased release of dopamine by stimulation at high frequency? We find that adaptor protein-3 (AP-3) and its coat protein VPS41 promote axonal dopamine release by targeting vesicular monoamine transporter VMAT2 to the axon rather than dendrites. AP-3 and VPS41 also produce SVs that respond preferentially to high frequency stimulation, independent of their role in axonal polarity. In addition, conditional inactivation of VPS41 in dopamine neurons impairs reinforcement learning, and this involves a defect in the frequency dependence of release rather than the amount of dopamine released. Thus, AP-3 and VPS41 promote the axonal polarity of dopamine release but enable learning by producing a novel population of SVs tuned specifically to high firing frequency that confers the phasic release of dopamine.

miRNA-mediated control of gephyrin synthesis drives sustained inhibitory synaptic plasticity.

Activity-dependent protein synthesis is crucial for many long-lasting forms of synaptic plasticity. However, our understanding of the translational mechanisms controlling inhibitory synapses is limited. One distinct form of inhibitory long-term potentiation (iLTP) enhances postsynaptic clusters of GABAARs and the primary inhibitory scaffold, gephyrin, to promote sustained synaptic strengthening. While we previously found that persistent iLTP requires mRNA translation, the precise mechanisms controlling gephyrin translation during this process remain unknown. Here, we identify miR153 as a novel regulator of Gphn mRNA translation which controls gephyrin protein levels and synaptic clustering, ultimately impacting GABAergic synaptic structure and function. We find that iLTP induction downregulates miR153, reversing its translational suppression of Gphn mRNA and allowing for increased de novo gephyrin protein synthesis and synaptic clustering during iLTP. Finally, we find that reduced miR153 expression during iLTP is driven by an excitation-transcription coupling pathway involving calcineurin, NFAT and HDACs, which also controls the miRNA-dependent upregulation of GABAARs. Overall, this work delineates a miRNA-dependent post-transcriptional mechanism that controls the expression of the key synaptic scaffold, gephyrin, and may converge with parallel miRNA pathways to coordinate gene upregulation to maintain inhibitory synaptic plasticity.

Loss of Mecp2 in substantia nigra dopamine neurons compromises the nigrostriatal pathway.

Mutations in the methyl-CpG-binding protein 2 (MeCP2) result in Rett syndrome (RTT), an X-linked disorder that disrupts neurodevelopment. Girls with RTT exhibit motor deficits similar to those in Parkinson's disease, suggesting defects in the nigrostriatal pathway. This study examined age-dependent changes in dopamine neurons of the substantia nigra (SN) from wild-type, presymptomatic, and symptomatic Mecp2(+/-) mice. Mecp2(+) neurons in the SN in Mecp2(+/-) mice were indistinguishable in morphology, resting conductance, and dopamine current density from neurons in wild-type mice. However, the capacitance, total dendritic length, and resting conductance of Mecp2(-) neurons were less than those of Mecp2(+) neurons as early as 4 weeks after birth, before overt symptoms. These differences were maintained throughout life. In symptomatic Mecp2(+/-) mice, the current induced by activation of D(2) dopamine autoreceptors was significantly less in Mecp2(-) neurons than in Mecp2(+) neurons, although D(2) receptor density was unaltered in Mecp2(+/-) mice. Electrochemical measurements revealed that significantly less dopamine was released after stimulation of striatum in adult Mecp2(+/-) mice compared to wild type. The decrease in size and function of Mecp2(-) neurons observed in adult Mecp2(+/-) mice was recapitulated in dopamine neurons from symptomatic Mecp2(-/y) males. These results show that mutation in Mecp2 results in cell-autonomous defects in the SN early in life and throughout adulthood. Ultimately, dysfunction in terminal dopamine release and D(2) autoreceptor-dependent currents in dopamine neurons from symptomatic females support the idea that decreased dopamine transmission due to heterogeneous Mecp2 expression contributes to the parkinsonian features of RTT in Mecp2(+/-) mice.

Divergent properties and independent regulation of striatal dopamine and GABA co-transmission.

Substantia nigra pars compacta (SNc) dopamine neurons play a key role in regulating the activity of striatal circuits within the basal ganglia. In addition to dopamine, these neurons release several other transmitters, including the major inhibitory neurotransmitter γ-aminobutyric acid (GABA). Both dopamine and GABA are loaded into SNc synaptic vesicles by the vesicular monoamine transporter 2 (VMAT2), and co-release of GABA provides strong inhibition to the striatum by directly inhibiting striatal medium spiny projection neurons (MSNs) through activation of GABAA receptors. Here, we found that despite both dopamine and GABA being co-packaged by VMAT2, the properties of transmission, including Ca2+ sensitivity, release probability, and requirement of active zone scaffolding proteins, differ between the two transmitters. Moreover, the extent by which presynaptic neuromodulators inhibit co-transmission also varied. Differences in modulation and the mechanisms controlling release allow for independent regulation of dopamine and GABA signals despite both being loaded via similar mechanisms.

Control of extracellular dopamine at dendrite and axon terminals.

Midbrain dopamine neurons release dopamine from both axons and dendrites. The mechanism underlying release at these different sites has been proposed to differ. This study used electrochemical and electrophysiological methods to compare the time course and calcium dependence of somatodendritic dopamine release in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) to that of axonal dopamine release in the dorsal striatum. The amount of dopamine released in the striatum was approximately 20-fold greater than in cell body regions of the VTA or SNc. However, the calcium dependence and time to peak of the dopamine transients were similar. These results illustrate an unexpected overall similarity in the mechanisms of dopamine release in the striatum and cell body regions. To examine how diffusion regulates the time course of dopamine following release, dextran was added to the extracellular solution to slow diffusion. In the VTA, dextran slowed the rate of rise and fall of the extracellular dopamine transient as measured by fast-scan cyclic voltammetry yet did not alter the kinetics of the dopamine-dependent IPSC. Dextran failed to significantly alter the time course of the rise and fall of the dopamine transient in the striatum, suggesting a more influential role for reuptake in the striatum. The conclusion is that the time course of dopamine within the extracellular space of the VTA is dependent on both diffusion and reuptake, whereas the activation of D(2) receptors on dopamine neurons is primarily limited by reuptake.

Cocaine shifts dopamine D2 receptor sensitivity to gate conditioned behaviors.

Cocaine addiction is a chronic, relapsing disorder characterized by maladaptation in the brain mesolimbic and nigrostriatal dopamine system. Although changes in the properties of D2-receptor-expressing medium spiny neurons (D2-MSNs) and connected striatal circuits following cocaine treatment are known, the contributions of altered D2-receptor (D2R) function in mediating the rewarding properties of cocaine remain unclear. Here, we describe how a 7-day exposure to cocaine alters dopamine signaling by selectively reducing the sensitivity, but not the expression, of nucleus accumbens D2-MSN D2Rs via an alteration in the relative expression and coupling of G protein subunits. This cocaine-induced reduction of D2R sensitivity facilitated the development of the rewarding effects of cocaine as blocking the reduction in G protein expression was sufficient to prevent cocaine-induced behavioral adaptations. These findings identify an initial maladaptive change in sensitivity by which mesolimbic dopamine signals are encoded by D2Rs following cocaine exposure.

Loss of nigral excitation of cholinergic interneurons contributes to parkinsonian motor impairments.

Progressive loss of dopamine inputs in Parkinson's disease leads to imbalances in coordinated signaling of dopamine and acetylcholine (ACh) in the striatum, which is thought to contribute to parkinsonian motor symptoms. As reciprocal interactions between dopamine inputs and cholinergic interneurons (ChIs) control striatal dopamine and ACh transmission, we examined how partial dopamine depletion in an early-stage mouse model for Parkinson's disease alters nigral regulation of cholinergic activity. We found region-specific alterations in how remaining dopamine inputs regulate cholinergic excitability that differ between the dorsomedial (DMS) and dorsolateral (DLS) striatum. Specifically, we found that dopamine depletion downregulates metabotropic glutamate receptors (mGluR1) on DLS ChIs at synapses where dopamine inputs co-release glutamate, abolishing the ability of dopamine inputs to drive burst firing. This loss underlies parkinsonian motor impairments, as viral rescue of mGluR1 signaling in DLS ChIs was sufficient to restore circuit function and attenuate motor deficits in early-stage parkinsonian mice.

Extinction blunts paraventricular thalamic contributions to heroin relapse.

Here, we use optogenetics and chemogenetics to investigate the contribution of the paraventricular thalamus (PVT) to nucleus accumbens (NAc) pathway in aversion and heroin relapse in two different heroin self-administration models in rats. In one model, rats undergo forced abstinence in the home cage prior to relapse testing, and in the other, they undergo extinction training, a procedure that is likened to cognitive behavioral therapy. We find that the PVT→NAc pathway is both sufficient and necessary to drive aversion and heroin seeking after abstinence, but not extinction. The ability of extinction to reduce this pathway's contribution to heroin relapse is accompanied by a loss of synaptic plasticity in PVT inputs onto a specific subset of NAc neurons. Thus, extinction may exert therapeutic reductions in opioid seeking by altering synaptic plasticity within the PVT→NAc pathway, resulting in reduced aversion during opioid withdrawal as well as reduced relapse propensity.

The time course of dopamine transmission in the ventral tegmental area.

Synaptic transmission mediated by G-protein coupled receptors (GPCR) is not generally thought to be point-to-point. To determine the extent over which dopamine signals in the midbrain, the present study examined the concentration and time course of dopamine that underlies a D(2)-receptor IPSC (D(2)-IPSC) in the ventral tegmental area. Extracellular dopamine was measured electrochemically while simultaneously recording D(2)-IPSCs. The presence of dopamine was brief relative to the IPSC, suggesting that G-protein dependent potassium channel activation determined the IPSC time course. The activation kinetics of D(2) receptor-dependent potassium current was studied using outside-out patch recordings with rapid application of dopamine. Dopamine applied at a minimum concentration of 10 mum for a maximum of 100 ms mimicked the IPSC. Higher concentrations applied for as little as 5 ms did not change the kinetics of the current. The results indicate that both the intrinsic kinetics of G-protein coupled receptor signaling and a rapidly rising high concentration of dopamine determine the time course of the IPSC. Thus, dopamine transmission in the midbrain is more localized then previously proposed.