Dopamine in dendrites of substantia nigra neurons: suggestions for a role in dendritic terminals

Altered parabrachial nucleus nociceptive processing may underlie central pain in Parkinson's disease

The presence of central neuropathic pain in Parkinson's disease suggests that the brain circuits that allow us to process pain could be dysfunctional in the disorder. However, there is to date no clear pathophysiological mechanism to explain these symptoms. In this work, we present evidence that the dysfunction of the subthalamic nucleus and/or substantia nigra pars reticulata may impact nociceptive processing in the parabrachial nucleus (PBN), a low level primary nociceptive structure in the brainstem, and induce a cellular and molecular neuro-adaptation in this structure. In rat models of Parkinson's disease with a partial dopaminergic lesion in the substantia nigra compacta, we found that the substantia nigra reticulata showed enhanced nociceptive responses. Such responses were less impacted in the subthalamic nucleus. A total dopaminergic lesion produced an increase in the nociceptive responses as well as an increase of the firing rate in both structures. In the PBN, inhibited nociceptive responses and increased expression of GABA_A receptors were found following a total dopaminergic lesion. However, neuro-adaptations at the level of dendritic spine density and post-synaptic density were found in both dopaminergic lesion groups. These results suggest that the molecular changes within the PBN following a larger dopaminergic lesion, such as increased GABA_A expression, is a key mechanism to produce nociceptive processing impairment, whilst other changes may protect function after smaller dopaminergic lesions. We also propose that these neuro-adaptations follow increased inhibitory tone from the substantia nigra pars reticulata and may represent the mechanism generating central neuropathic pain in Parkinson's disease.

Molecular and Epigenetic Aspects of Opioid Receptors in Drug Addiction and Pain Management in Sport

Opioids are substances derived from opium (natural opioids). In its raw state, opium is a gummy latex extracted from Papaver somniferum. The use of opioids and their negative health consequences among people who use drugs have been studied. Today, opioids are still the most commonly used and effective analgesic treatments for severe pain, but their use and abuse causes detrimental side effects for health, including addiction, thus impacting the user's quality of life and causing overdose. The mesocorticolimbic dopaminergic circuitry represents the brain circuit mediating both natural rewards and the rewarding aspects of nearly all drugs of abuse, including opioids. Hence, understanding how opioids affect the function of dopaminergic circuitry may be useful for better knowledge of the process and to develop effective therapeutic strategies in addiction. The aim of this review was to summarize the main features of opioids and opioid receptors and focus on the molecular and upcoming epigenetic mechanisms leading to opioid addiction. Since synthetic opioids can be effective for pain management, their ability to induce addiction in athletes, with the risk of incurring doping, is also discussed.

Dendritic involvement in inhibition and disinhibition of vulnerable dopaminergic neurons in healthy and pathological conditions

Dopaminergic neurons in the substantia nigra pars compacta (SNc) differentially degenerate in Parkinson's Disease, with the ventral region degenerating more severely than the dorsal region. Compared with the dorsal neurons, the ventral neurons in the SNc have distinct dendritic morphology, electrophysiological characteristics, and circuit connections with the basal ganglia. These characteristics shape information processing in the ventral SNc and structure the balance of inhibition and disinhibition in the striatonigral circuitry. In this paper, I review foundational studies and recent work comparing the circuitry of the ventral and dorsal SNc neurons and discuss how loss of the ventral neurons early in Parkinson's Disease could affect the overall balance of inhibition and disinhibition of dopamine signals.

Brain region-specific microglial and astrocytic activation in response to systemic lipopolysaccharides exposure

Microglia cells are the macrophage population within the central nervous system, which acts as the first line of the immune defense. These cells present a high level of heterogeneity among different brain regions regarding morphology, cell density, transcriptomes, and expression of different inflammatory mediators. This region-specific heterogeneity may lead to different neuroinflammatory responses, influencing the regional involvement in several neurodegenerative diseases. In this study, we aimed to evaluate microglial response in 16 brain regions. We compared different aspects of the microglial response, such as the extension of their morphological changes, sensitivity, and ability to convert an acute inflammatory response to a chronic one. Then, we investigated the synaptic alterations followed by acute and chronic inflammation in substantia nigra. Moreover, we estimated the effect of partial ablation of fractalkine CX3C receptor 1 (CX3CR1) on microglial response. In the end, we briefly investigated astrocytic heterogeneity and activation. To evaluate microglial response in different brain regions and under the same stimulus, we induced a systemic inflammatory reaction through a single intraperitoneal (i.p.) injection of lipopolysaccharides (LPS). We performed our study using C57BL6 and CX3CR1^+/GFP mice to investigate microglial response in different regions and the impact of CX3CR1 partial ablation. We conducted a topographic study quantifying microglia alterations in 16 brain regions through immunohistochemical examination and computational image analysis. Assessing Iba1-immunopositive profiles and the density of the microglia cells, we have observed significant differences in region-specific responses of microglia populations in all parameters considered. Our results underline the peculiar microglial inflammation in the substantia nigra pars reticulata (SNpr). Here and in concomitance with the acute inflammatory response, we observed a transient decrease of dopaminergic dendrites and an alteration of the striato-nigral projections. Additionally, we found a significant decrease in microglia response and the absence of chronic inflammation in CX3CR1^+/GFP mice compared to the wild-type ones, suggesting the CX3C axis as a possible pharmacological target against neuroinflammation induced by an increase of systemic tumor necrosis factor-alpha (TNFα) or/and LPS. Finally, we investigated astrocytic heterogeneity in this model. We observed different distribution and morphology of GFAP-positive astrocytes, a heterogeneous response under inflammatory conditions, and a decrease in their activation in CX3CR1 partially ablated mice compared with C57BL6 mice. Altogether, our data confirm that microglia and astrocytes heterogeneity lead to a region-specific inflammatory response in presence of a systemic TNFα or/and LPS treatment.

Visualizing synaptic dopamine efflux with a 2D composite nanofilm

Chemical neurotransmission constitutes one of the fundamental modalities of communication between neurons. Monitoring release of these chemicals has traditionally been difficult to carry out at spatial and temporal scales relevant to neuron function. To understand chemical neurotransmission more fully, we need to improve the spatial and temporal resolutions of measurements for neurotransmitter release. To address this, we engineered a chemi-sensitive, two-dimensional composite nanofilm that facilitates visualization of the release and diffusion of the neurochemical dopamine with synaptic resolution, quantal sensitivity, and simultaneously from hundreds of release sites. Using this technology, we were able to monitor the spatiotemporal dynamics of dopamine release in dendritic processes, a poorly understood phenomenon. We found that dopamine release is broadcast from a subset of dendritic processes as hotspots that have a mean spatial spread of ≈ 3.2 µm (full width at half maximum [FWHM]) and are observed with a mean spatial frequency of one hotspot per ≈ 7.5 µm of dendritic length. Major dendrites of dopamine neurons and fine dendritic processes, as well as dendritic arbors and dendrites with no apparent varicose morphology participated in dopamine release. Remarkably, these release hotspots co-localized with Bassoon, suggesting that Bassoon may contribute to organizing active zones in dendrites, similar to its role in axon terminals.

To form the vast and complex network necessary for an organism to sense and react to the world, neurons must connect at highly specialized junctions. Individual cells communicate at these ‘synapses’ by releasing chemical signals (or neurotransmitters) such as dopamine, a molecule involved in learning and motivation.

Despite the central role that synapses play in the brain, it remains challenging to measure exactly where neurotransmitters are released and how far they travel from their release site. Currently, most tools available to scientists only allow bulk measurements of neurotransmitter release.

To tackle this limitation, Bulumulla et al. developed a new way to measure neurotransmitter release from neurons, harnessing a technique which uses fluorescent nanosensors that glow brighter when exposed to dopamine. These sensors form a very thin film upon which neurons can grow; when the cells release dopamine, the sensors ‘light up’ as they encounter the molecule. Dubbed DopaFilm, the technology reveals exactly where the neurotransmitter comes from and how it spreads between cells in real time. In particular, the approach showed that dopamine emerges from 'hot spots' at specific sites in cells; it also helped Bulumulla et al. study how dopamine is released from subcellular compartments that have previously not been well characterized.

Improving the sensors so that the film could detect other neurotransmitters besides dopamine would broaden the use of this approach. In the future, combining this technology with other types of imaging should enable studies of individual synapses with intricate detail.

The potential convergence of NLRP3 inflammasome, potassium, and dopamine mechanisms in Parkinson's disease

The pathology of Parkinson's disease (PD) is characterized by α-synuclein aggregation, microglia-mediated neuroinflammation, and dopaminergic neurodegeneration in the substantia nigra with collateral striatal dopamine signaling deficiency. Microglial NLRP3 inflammasome activation has been linked independently to each of these facets of PD pathology. The voltage-gated potassium channel Kv1.3, upregulated in microglia by α-synuclein and facilitating potassium efflux, has also been identified as a modulator of neuroinflammation and neurodegeneration in models of PD. Evidence increasingly suggests that microglial Kv1.3 is mechanistically coupled with NLRP3 inflammasome activation, which is contingent on potassium efflux. Potassium conductance also influences dopamine release from midbrain dopaminergic neurons. Dopamine, in turn, has been shown to inhibit NLRP3 inflammasome activation in microglia. In this review, we provide a literature framework for a hypothesis in which Kv1.3 activity-induced NLRP3 inflammasome activation, evoked by stimuli such as α-synuclein, could lead to microglia utilizing dopamine from adjacent dopaminergic neurons to counteract this process and fend off an activated state. If this is the case, a sufficient dopamine supply would ensure that microglia remain under control, but as dopamine is gradually siphoned from the neurons by microglial demand, NLRP3 inflammasome activation and Kv1.3 activity would progressively intensify to promote each of the three major facets of PD pathology: α-synuclein aggregation, microglia-mediated neuroinflammation, and dopaminergic neurodegeneration. Risk factors overlapping to varying degrees to render brain regions susceptible to such a mechanism would include a high density of microglia, an initially sufficient supply of dopamine, and poor insulation of the dopaminergic neurons by myelin.

Dopamine signaling modulates microglial NLRP3 inflammasome activation: implications for Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is characterized by the loss of nigral dopaminergic neurons leading to impaired striatal dopamine signaling, α-synuclein- (α-syn-) rich inclusions, and neuroinflammation. Degenerating neurons are surrounded by activated microglia with increased secretion of interleukin-1β (IL-1β), driven largely by the NLRP3 inflammasome. A critical role for microglial NLRP3 inflammasome activation in the progression of both dopaminergic neurodegeneration and α-syn pathology has been demonstrated in parkinsonism mouse models. Fibrillar α-syn activates this inflammasome in mouse and human macrophages, and we have shown previously that the same holds true for primary human microglia. Dopamine blocks microglial NLRP3 inflammasome activation in the MPTP model, but its effects in this framework, highly relevant to PD, remain unexplored in primary human microglia and in other in vivo parkinsonism models.

METHODS

Biochemical techniques including quantification of IL-1β secretion and confocal microscopy were employed to gain insight into dopamine signaling-mediated inhibition of the NLRP3 inflammasome mechanism in primary human microglia and the SYN120 transgenic mouse model. Dopamine and related metabolites were applied to human microglia together with various inflammasome activating stimuli. The involvement of the receptors through which these catecholamines were predicted to act were assessed with agonists in both species.

RESULTS

We show in primary human microglia that dopamine, L-DOPA, and high extracellular K⁺, but not norepinephrine and epinephrine, block canonical, non-canonical, and α-syn-mediated NLRP3 inflammasome-driven IL-1β secretion. This suggests that dopamine acts as an inflammasome inhibitor in human microglia. Accordingly, we provide evidence that dopamine exerts its inhibitory effect through dopamine receptor D1 and D2 (DRD1 and DRD2) signaling. We also show that aged mice transgenic for human C-terminally truncated (1-120) α-syn (SYN120 tg mice) display increased NLRP3 inflammasome activation in comparison to WT mice that is diminished upon DRD1 agonism.

CONCLUSIONS

Dopamine inhibits canonical, non-canonical, and α-syn-mediated activation of the NLRP3 inflammasome in primary human microglia, as does high extracellular K⁺. We suggest that dopamine serves as an endogenous repressor of the K⁺ efflux-dependent microglial NLRP3 inflammasome activation that contributes to dopaminergic neurodegeneration in PD, and that this reciprocation may account for the specific vulnerability of these neurons to disease pathology.

A dopaminergic mechanism of antipsychotic drug efficacy, failure, and failure reversal: the role of the dopamine transporter

Antipsychotic drugs are effective interventions in schizophrenia. However, the efficacy of these agents often decreases over time, which leads to treatment failure and symptom recurrence. We report that antipsychotic efficacy in rat models declines in concert with extracellular striatal dopamine levels rather than insufficient dopamine D2 receptor occupancy. Antipsychotic efficacy was associated with a suppression of dopamine transporter activity, which was reversed during failure. Antipsychotic failure coincided with reduced dopamine neuron firing, which was not observed during antipsychotic efficacy. Synaptic field responses in dopamine target areas declined during antipsychotic efficacy and showed potentiation during failure. Antipsychotics blocked synaptic vesicle release during efficacy but enhanced this release during failure. We found that the pharmacological inhibition of the dopamine transporter rescued antipsychotic drug treatment outcomes, supporting the hypothesis that the dopamine transporter is a main target of antipsychotic drugs and predicting that dopamine transporter blockers may be an adjunct treatment to reverse antipsychotic treatment failure.

Blockade of the dopaminergic neurotransmission with AMPT and reserpine induces a differential expression of genes of the dopaminergic phenotype in substantia nigra

Dopaminergic neurons have the ability to release Dopamine from their axons as well as from their soma and dendrites. This somatodendritically-released Dopamine induces an autoinhibition of Dopaminergic neurons mediated by D2 autoreceptors, and the stimulation of neighbor GABAergic neurons mediated by D1 receptors (D1r). Here, our results suggest that the somatodendritic release of Dopamine in the substantia nigra (SN) may stimulate GABAergic neurons that project their axons into the hippocampus. Using semiquantitative multiplex RT-PCR we show that chronic blockade of the Dopaminergic neurotransmission with both AMPT and reserpine specifically decreases the expression levels of D1r, remarkably this may be the result of an antagonistic effect between AMPT and reserpine, as they induced the expression of a different set of genes when treated by separate. Furthermore, using anterograde and retrograde tracing techniques, we found that the GABAergic neurons that express D1r also project their axons in to the CA1 region of the hippocampus. Finally, we also found that the same treatment that decreases the expression levels of D1r in SN, also induces an impairment in the performance in an appetitive learning task that requires the coding of reward as well as navigational skills. Overall, our findings show the presence of a GABAergic interconnection between the SNr and the hippocampus mediated by D1r.

Proliferation of Inhibitory Input to the Substantia Nigra in Experimental Parkinsonism

The substantia nigra pars reticulata (SNr) is one of the output nuclei of the basal ganglia (BG) and plays a vital role in movement execution. Death of dopaminergic neurons in the neighboring nucleus, the substantia nigra pars compacta (SNc), leads to Parkinson's disease. The ensuing dopamine depletion affects all BG nuclei. However, the long-term effects of dopamine depletion on BG output are less characterized. In this in vitro study, we applied electrophysiological and immunohistochemical techniques to investigate the long-term effects of dopamine depletion on GABAergic transmission to the SNr. The findings showed a reduction in firing rate and regularity in SNr neurons after unilateral dopamine depletion with 6-OHDA, which we associate with homeostatic mechanisms. The strength of the GABAergic synapses between the globus pallidus (GP) and the SNr increased but not their short-term dynamics. Consistent with this observation, there was an increase in the frequency and amplitude of spontaneous inhibitory synaptic events to SNr neurons. Immunohistochemistry revealed an increase in the density of vGAT-labeled puncta in dopamine depleted animals. Overall, these results may suggest that synaptic proliferation can explain how dopamine depletion augments GABAergic transmission in the SNr.

Alpha-melanocyte stimulating hormone increases the activity of melanocortin-3 receptor-expressing neurons in the ventral tegmental area

KEY POINTS

Alpha-melanocyte stimulating hormone (α-MSH) is an anorexigenic peptide. Injection of the α-MSH analog MTII into the ventral tegmental area (VTA) decreases food and sucrose intake and food reward. Melanocortin-3 receptors (MC3R) are highly expressed in the VTA, suggesting that the effects of intra-VTA α-MSH may be mediated by α-MSH changing the activity of MC3R-expressing VTA neurons. α-MSH increased the firing rate of MC3R VTA neurons in acute brain slices from mice, although it did not affect the firing rate of non-MC3R VTA neurons. The α-MSH induced increase in MC3R neuron firing rate is probably activity-dependent, and was independent of fast synaptic transmission and intracellular Ca²⁺ levels. These results help us to better understand how α-MSH acts in the VTA to affect feeding and other dopamine-dependent behaviours.

ABSTRACT

The mesocorticolimbic dopamine system, the brain's reward system, regulates multiple behaviours, including food intake and food reward. There is substantial evidence that the melanocortin system of the hypothalamus, an important neural circuit controlling feeding and body weight, interacts with the mesocorticolimbic dopamine system to affect feeding, food reward and body weight. For example, melanocortin-3 receptors (MC3Rs) are expressed in the ventral tegmental area (VTA) and our laboratory previously showed that intra-VTA injection of the MC3R agonist, MTII, decreases home-cage food intake and operant responding for sucrose pellets. However, the cellular mechanisms underlying the effects of intra-VTA alpha-melanocyte stimulating hormone (α-MSH) on feeding and food reward are unknown. To determine how α-MSH acts in the VTA to affect feeding, we performed electrophysiological recordings in acute brain slices from mice expressing enhanced yellow fluorescent protein in MC3R neurons to test how α-MSH affects the activity of VTA MC3R neurons. α-MSH significantly increased the firing rate of VTA MC3R neurons without altering the activity of non-MC3R expressing VTA neurons. In addition, the α-MSH-induced increase in MC3R neuron activity was independent of fast synaptic transmission and intracellular Ca²⁺ levels. Finally, we show that the effect of α-MSH on MC3R neuron firing rate is probably activity-dependent. Overall, these studies provide an important advancement in the understanding of how α-MSH acts in the VTA to affect feeding and food reward.

Extrasynaptic Neurotransmission Mediated by Exocytosis and Diffusive Release of Transmitter Substances

This review article deals with the mechanisms of extrasynaptic release of transmitter substances, namely the release from the soma, axon and dendrites in the absence of postsynaptic counterparts. Extrasynaptic release occurs by exocytosis or diffusion. Spillover from the synaptic cleft also contributes to extrasynaptic neurotransmission. Here, we first describe two well-known examples of exocytosis from the neuronal soma, which may release copious amounts of transmitter for up to hundreds of seconds after electrical stimulation. The mechanisms for somatic exocytosis of the low molecular weight transmitter serotonin, and the peptides oxytocin and vasopressin have been studied in detail. Serotonin release from leech neurons and oxytocin and vasopressin from rodent neurons have a common multi-step mechanism, which is completely different from that for exocytosis from presynaptic endings. Most transmitters and peptides released extrasynaptically seem to follow this same mechanism. Extrasynaptic exocytosis may occur onto glial cells, which act as intermediaries for long-term and long-distance transmission. The second part of this review article focuses on the release upon synthesis of the representative diffusible molecules nitric oxide (NO) and endocannabinoids. Diffusible molecules are synthesized “on demand” from postsynaptic terminals in response to electrical activity and intracellular calcium elevations. Their effects include the retrograde modulation of presynaptic electrical activity and transmitter release. Extrasynaptic neurotransmission is well exemplified in the retina. Light-evoked extrasynaptic communication sets the gain for visual responses and integrates the activity of neurons, glia and blood vessels. Understanding how extrasynaptic communication changes the function of hard-wired circuits has become fundamental to understand the function of the nervous system.

Mesencephalic dopamine neurons interfacing the shell of nucleus accumbens and the dorsolateral striatum in the rat

Parallel corticostriatonigral circuits have been proposed that separately process motor, cognitive, and emotional‐motivational information. Functional integration requires that interactions exist between neurons participating in these circuits. This makes it imperative to study the complex anatomical substrate underlying corticostriatonigral circuits. It has previously been proposed that dopaminergic neurons in the ventral mesencephalon may play a role in this circuit interaction. Therefore, we studied in rats convergence of basal ganglia circuits by depositing an anterograde neuroanatomical tracer into the ventral striatum together with a retrograde fluorescent tracer ipsilaterally in the dorsolateral striatum. In the mesencephalon, using confocal microscopy, we looked for possible appositions of anterogradely labeled fibers and retrogradely labeled neurons, “enhancing” the latter via intracellular injection of Lucifer Yellow. Tyrosine hydroxylase (TH) immunofluorescence served to identify dopaminergic neurons. In neurophysiological experiments, we combined orthodromic stimulation in the medial ventral striatum with recording from ventral mesencephalic neurons characterized by antidromic stimulation from the dorsal striatum. We observed terminal fields of anterogradely labeled fibers that overlap populations of retrogradely labeled nigrostriatal cell bodies in the substantia nigra pars compacta and lateral ventral tegmental area (VTA), with numerous close appositions between boutons of anterogradely labeled fibers and nigrostriatal, TH‐immunopositive neurons. Neurophysiological stimulation in the medial ventral striatum caused inhibition of dopaminergic nigrostriatal neurons projecting to the ventrolateral striatal territory. Responding nigrostriatal neurons were located in the medial substantia nigra and adjacent VTA. Our results strongly suggest a functional link between ventromedial, emotional‐motivational striatum, and the sensorimotor dorsal striatum via dopaminergic nigrostriatal neurons.

Desensitized D2 autoreceptors are resistant to trafficking

Dendritic release of dopamine activates dopamine D2 autoreceptors, which are inhibitory G protein-coupled receptors (GPCRs), to decrease the excitability of dopamine neurons. This study used tagged D2 receptors to identify the localization and distribution of these receptors in living midbrain dopamine neurons. GFP-tagged D2 receptors were found to be unevenly clustered on the soma and dendrites of dopamine neurons within the substantia nigra pars compacta (SNc). Physiological signaling and desensitization of the tagged receptors were not different from wild type receptors. Unexpectedly, upon desensitization the tagged D2 receptors were not internalized. When tagged D2 receptors were expressed in locus coeruleus neurons, a desensitizing protocol induced significant internalization. Likewise, when tagged µ-opioid receptors were expressed in dopamine neurons they too were internalized. The distribution and lack of agonist-induced internalization of D2 receptors on dopamine neurons indicate a purposefully regulated localization of these receptors.

Transplantation of Mesencephalic Cell Suspensions from Wild-Type and Heterozygous Weaver Mice into the Denervated Striatum: Assessing the Role of Graft-Derived Dopaminergic Dendrites in the Recovery of Function

The Weaver (wv) mutation leads to a loss of mesencephalic dopamine cells and nigrostriatal dopamine axons in homozygosity (wv/wv) and to a deficiency of nigral dopaminergic dendrites without a concomitant loss of dopamine cell somata or axons in heterozygosity (wv/+). Previous studies have shown that grafts of foetal dopamine cells from wild-type (+/+) donors can survive when implanted into the wv/wv striatum, supply both an axonal and a dendritic innervation to the host, establish synaptic connections with host striatal neurons, and bring about a functional recovery evidenced by rotational asymmetry tests. The aims of the present study were to examine whether wv/+ dopamine cells maintain a “dendrite-poor” phenotype after transplantation to the denervated striatum, and to compare their functional effects with those of wild-type (+/+) grafts in reversing amphetamine-induced turning behaviour. To that end, +/+ and wv/+ ventral mesencephalic tissue (dissected out from E10-E12 foetal mice and made into a cell suspension by enzymatic and mechanical dissociation) was stereotactically grafted into the right striatum of either wv/wv hosts or +/+ hosts subjected in advance to 6-OHDA lesions of the right substantia nigra. Viability and morphology of grafted neurons were assessed by tyrosine hydroxylase immunocytochemistry on serial sections of the host forebrains. Dopamine cell bodies survived in comparable numbers in the grafts regardless of donor genotype; however, grafts of either genotype contained fewer dopaminergic cells when they were hosted in the wv/wv striatum as compared to the striatum of +/+ mice with 6-OHDA lesions. Despite the survival of cell somata, the dendritic arborisation of wv/+ cells was strikingly poorer than that of +/+ cells in grafts placed into both host types, most likely reflecting their in situ phenotypic abnormality. Recipient wv/wv mice with +/+ and wv/+ grafts exhibited 88% and 83% left rotations, respectively; 6-OHDA hosts with +/+ and wv/+ grafts showed 178% and 165% reversals of asymmetry, respectively. The differences between the effects of +/+ and wv/+ grafts were not statistically significant. We conclude that (i) wv/+ and +/+ dopamine cell somata survive in comparable numbers after intrastriatal grafting; (ii) grafted wv/+ dopamine cells express an anatomical phenotype consistent with that seen in the wv/+ substantia nigra in situ; and (iii) the axonal innervation supplied by wv/+ grafts to the denervated striatum induces a functional recovery comparable to that brought about by +/+ cells, which in addition supply a substantial dendritic innervation to the host; (iv) the wv/wv host environment may be associated with smaller numbers of graft dopamine neurons compared to the environment of +/+ mouse hosts with 6-OHDA lesions.

Intranigral Grafts of Fetal Ventral Mesencephalic Tissue in Adult 6-Hydroxydopamine-Lesioned Rats can Induce Behavioral Recovery

Intrastriatal grafts of fetal ventral mesencephalon in rats with unilateral 6-hydroxydopamine lesions can reduce and even reverse rotational behavior in response to direct and indirect dopamine agonists. These grafts can ameliorate deficits on simple spontaneous behaviors, but do not improve complex behaviors that require the skilled integration of the use of both paws. We report here that rats with grafts into the DA-depleted substantia nigra, that receive cyclosporine A, can experience recovery on spontaneous behaviors that mimic those observed in Parkinson's disease. Specific cyclosporine A treatment conditions can differentially affect whether intranigral grafts normalize paw use during initiation or termination of a movement sequence. These findings may have important implications for the treatment of Parkinson's disease.

Dendritic Release of Neurotransmitters

Release of neuroactive substances by exocytosis from dendrites is surprisingly widespread and is not confined to a particular class of transmitters: it occurs in multiple brain regions, and includes a range of neuropeptides, classical neurotransmitters, and signaling molecules, such as nitric oxide, carbon monoxide, ATP, and arachidonic acid. This review is focused on hypothalamic neuroendocrine cells that release vasopressin and oxytocin and midbrain neurons that release dopamine. For these two model systems, the stimuli, mechanisms, and physiological functions of dendritic release have been explored in greater detail than is yet available for other neurons and neuroactive substances. © 2017 American Physiological Society. Compr Physiol 7:235-252, 2017.

Dopamine Autoreceptor Regulation of a Hypothalamic Dopaminergic Network

How autoreceptors contribute to maintaining a stable output of rhythmically active neuronal circuits is poorly understood. Here, we examine this issue in a dopamine population, spontaneously oscillating hypothalamic rat (TIDA) neurons, that underlie neuroendocrine control of reproduction and neuroleptic side effects. Activation of dopamine receptors of the type 2 family (D2Rs) at the cell-body level slowed TIDA oscillations through two mechanisms. First, they prolonged the depolarizing phase through a combination of presynaptic increases in inhibition and postsynaptic hyperpolarization. Second, they extended the discharge phase through presynaptic attenuation of calcium currents and decreased synaptic inhibition. Dopamine reuptake blockade similarly reconfigured the oscillation, indicating that ambient somatodendritic transmitter concentration determines electrical behavior. In the absence of D2R feedback, however, discharge was abolished by depolarization block. These results indicate the existence of an ultra-short feedback loop whereby neuroendocrine dopamine neurons tune network behavior to echoes of their own activity, reflected in ambient somatodendritic dopamine, and also suggest a mechanism for antipsychotic side effects.

Somatodendritic dopamine release: recent mechanistic insights

Dopamine (DA) is a key transmitter in motor, reward and cogitative pathways, with DA dysfunction implicated in disorders including Parkinson's disease and addiction. Located in midbrain, DA neurons of the substantia nigra pars compacta project via the medial forebrain bundle to the dorsal striatum (caudate putamen), and DA neurons in the adjacent ventral tegmental area project to the ventral striatum (nucleus accumbens) and prefrontal cortex. In addition to classical vesicular release from axons, midbrain DA neurons exhibit DA release from their cell bodies and dendrites. Somatodendritic DA release leads to activation of D2 DA autoreceptors on DA neurons that inhibit their firing via G-protein-coupled inwardly rectifying K(+) channels. This helps determine patterns of DA signalling at distant axonal release sites. Somatodendritically released DA also acts via volume transmission to extrasynaptic receptors that modulate local transmitter release and neuronal activity in the midbrain. Thus, somatodendritic release is a pivotal intrinsic feature of DA neurons that must be well defined in order to fully understand the physiology and pathophysiology of DA pathways. Here, we review recent mechanistic aspects of somatodendritic DA release, with particular emphasis on the Ca(2+) dependence of release and the potential role of exocytotic proteins.

Neurotensin inhibits both dopamine- and GABA-mediated inhibition of ventral tegmental area dopamine neurons

Dopamine is an essential neurotransmitter that plays an important role in a number of different physiological processes and disorders. There is substantial evidence that the neuropeptide neurotensin interacts with the mesolimbic dopamine system and can regulate dopamine neuron activity. In these studies we have used whole cell patch-clamp electrophysiology in brain slices from mice to examine how neurotensin regulates dopamine neuron activity by examining the effect of neurotensin on the inhibitory postsynaptic current generated by somatodendritic dopamine release (D2R IPSC) in ventral tegmental area (VTA) dopamine neurons. Neurotensin inhibited the D2R IPSC and activated an inward current in VTA dopamine neurons that appeared to be at least partially mediated by activation of a transient receptor potential C-type channel. Neither the inward current nor the inhibition of the D2R IPSC was affected by blocking PKC or calcium release from intracellular stores, and the inhibition of the D2R IPSC was greater with neurotensin compared with activation of other Gq-coupled receptors. Interestingly, the effects of neurotensin were not specific to D2R signaling as neurotensin also inhibited GABAB inhibitory postsynaptic currents in VTA dopamine neurons. Finally, the effects of neurotensin were significantly larger when intracellular Ca(2+) was strongly buffered, suggesting that reduced intracellular calcium facilitates these effects. Overall these results suggest that neurotensin may inhibit the D2R and GABAB IPSCs downstream of receptor activation, potentially through regulation of G protein-coupled inwardly rectifying potassium channels. These studies provide an important advance in our understanding of dopamine neuron activity and how it is controlled by neurotensin.

Neuroendocrine-autonomic integration in the paraventricular nucleus: novel roles for dendritically released neuropeptides

Communication between pairs of neurones in the central nervous system typically involves classical 'hard-wired' synaptic transmission, characterised by high temporal and spatial precision. Over the last two decades, however, knowledge regarding the repertoire of communication modalities used in the brain has notably expanded to include less conventional forms, characterised by a diffuse and less temporally precise transfer of information. These forms are best suited to mediate communication among entire neuronal populations, now recognised to be a fundamental process in the brain for the generation of complex behaviours. In response to an osmotic stressor, the hypothalamic paraventricular nucleus (PVN) generates a multimodal homeostatic response that involves orchestrated neuroendocrine (i.e. systemic release of vasopressin) and autonomic (i.e. sympathetic outflow to the kidneys) components. The precise mechanisms that underlie interpopulation cross-talk between these two distinct neuronal populations, however, remain largely unknown. The present review summarises and discusses a series of recent studies that have identified the dendritic release of neuropeptides as a novel interpopulation signalling modality in the PVN. A current working model is described in which it is proposed that the activity-dependent dendritic release of vasopressin from neurosecretory neurones in the PVN acts in a diffusible manner to increase the activity of distant presympathetic neurones, resulting in an integrated sympathoexcitatory population response, particularly within the context of a hyperosmotic challenge. The cellular mechanism underlying this novel form of intercellular communication, as well as its physiological and pathophysiological implications, is discussed.

Neuroprotection by 6-(methylsulfinyl)hexyl isothiocyanate in a 6-hydroxydopamine mouse model of Parkinson׳s disease

A number of pathogenic factors have been implicated in the progression of Parkinson׳s disease (PD), including oxidative stress, mitochondrial dysfunction, inflammation, excitotoxicity, and signals mediating apoptosis cascade. 6-(methylsulfinyl)hexyl isothiocyanate (6-MSITC) is a major component in wasabi, a very popular spice in Japan and a member of the Brassica family of vegetables. This study was designed to investigate the neuroprotective effects of 6-MSITC in a PD mouse model. Mice were treated with 6-MSITC (5mg/kg twice a week) for four weeks after the unilateral intrastriatal injection of 6-hydroxydopamine (6-OHDA). On the 28th day, 6-OHDA-injected mice showed behavioral impairments, a significant decrease in tyrosine hydroxylase (TH) and an increase in apoptosis. In addition, lesioned mice showed reduced glutathione levels and glutathione-S-transferase and glutathione reductase activities. Notably, 6-MSITC demonstrated neuroprotective effects in our experimental model strongly related to the preservation of functional nigral dopaminergic neurons, which contributed to the reduction of motor dysfunction induced by 6-OHDA. Furthermore, this study provides evidence that the beneficial effects of 6-MSITC could be attributed to the decrease of apoptotic cell death and to the activation of glutathione-dependent antioxidant systems. These findings may render 6-MSITC as a promising molecule for further pharmacological studies on the investigation for disease-modifying treatment in PD.

Preparation of primary cultured dopaminergic neurons from mouse brain

Dopaminergic neurons are involved in a variety of normal brain functions; degenerations of these neurons cause diseases in human. Investigation of how dopaminergic neurons respond to extracellular signals and molecular mechanisms regulating dopaminergic neuron survival and death often requires reliable, reproducible, and high-quality primary cultures of dopaminergic neurons. Here, we described methods to dissect and culture these neurons from embryonic mesencephalon of mouse brain. We utilize coverslips made from Aclar film to maximize the number of surviving dopaminergic neuron in the culture and immunocytochemistry of tyrosine hydroxylase (TH) to identify dopaminergic neuron.

Dendritic trafficking for neuronal growth and plasticity

Among the largest cells in the body, neurons possess an immense surface area and intricate geometry that poses many unique cell biological challenges. This morphological complexity is critical for neural circuit formation and enables neurons to compartmentalize cell-cell communication and local intracellular signalling to a degree that surpasses other cell types. The adaptive plastic properties of neurons, synapses and circuits have been classically studied by measurement of electrophysiological properties, ionic conductances and excitability. Over the last 15 years, the field of synaptic and neural electrophysiology has collided with neuronal cell biology to produce a more integrated understanding of how these remarkable highly differentiated cells utilize common eukaryotic cellular machinery to decode, integrate and propagate signals in the nervous system. The present article gives a very brief and personal overview of the organelles and trafficking machinery of neuronal dendrites and their role in dendritic and synaptic plasticity.

Food restriction increases glutamate receptor-mediated burst firing of dopamine neurons

Restriction of food intake increases the acquisition of drug abuse behavior and enhances the reinforcing efficacy of those drugs. However, the neurophysiological mechanisms responsible for the interactions between feeding state and drug use are largely unknown. Here we show that chronic mild food restriction increases the burst firing of dopamine neurons in the substantia nigra. Dopamine neurons from food-restricted mice exhibited increased burst firing in vivo, an effect that was enhanced by an injection of the psychomotor stimulant cocaine (10 mg/kg, i.p.). Food restriction also enhanced aspartic acid-induced burst firing of dopamine neurons in an ex vivo brain slice preparation, consistent with an adaptation occurring in the somatodendritic compartment and independent of a circuit mechanism. Enhanced burst firing persisted after 10 d of free feeding following chronic food restriction but was not observed following a single overnight fast. Whole-cell patch-clamp recordings indicated that food restriction also increased electrically evoked AMPAR/NMDAR ratios and increased D2 autoreceptor-mediated desensitization in dopamine neurons. These results identify dopamine neurons in the substantia nigra as a convergence point for the interactions between feeding state and drugs of abuse. Furthermore, increased glutamate transmission combined with decreased autoreceptor inhibition could work in concert to enhance drug efficacy in response to food restriction.

Neurotransmitter-mediated control of neurogenesis in the adult vertebrate brain

It was long thought that no new neurons are added to the adult brain. Similarly, neurotransmitter signaling was primarily associated with communication between differentiated neurons. Both of these ideas have been challenged, and a crosstalk between neurogenesis and neurotransmitter signaling is beginning to emerge. In this Review, we discuss neurotransmitter signaling as it functions at the intersection of stem cell research and regenerative medicine, exploring how it may regulate the formation of new functional neurons and outlining interactions with other signaling pathways. We consider evolutionary and cross-species comparative aspects, and integrate available results in the context of normal physiological versus pathological conditions. We also discuss the potential role of neurotransmitters in brain size regulation and implications for cell replacement therapies.

Modulation of ventral tegmental area dopamine receptors inhibit nicotine-induced anxiogenic-like behavior in the central amygdala

Nicotine, the major addictive substance in tobacco, increases the activity of the central amygdala (CeA). Amygdala is directly implicated in anxiety modulation and sends projections to the vicinity of the midbrain dopamine neurons, including the ventral tegmental area (VTA) which is a key area that controls nicotine dependence processes. In this study, the role of dopamine D(1) and D(2)/(3) receptors of the VTA on anxiogenic-like behavior induced with intra-CeA injection of nicotine has been investigated. Male Wistar rats with cannula aimed to the left CeA and the left VTA were submitted to the elevated plus-maze (EPM). The nicotine injection (1 μg/rat) into the CeA decreased the percentage of open arm time and open arm entries, but not locomotor activity, indicating an anxiogenic-like response. Intra-VTA injection of a dopamine D1 receptor antagonist, SCH23390 (0.25 μg/rat), and a dopamine D2/3 receptor antagonist, sulpiride (0.7 μg/rat), inhibited the anxiogenic-like response caused by intra-CeA injection of nicotine. These results suggest that the relationship between the VTA and the CeA may be involved in nicotine-induced anxiogenic-like behavior via dopamine D(1) and D(2)/(3) receptors. An understanding of these cellular processes will be crucial for the development of new intervention to combat nicotine effect.

Investigations into Potential Extrasynaptic Communication between the Dopaminergic and Nitrergic Systems

Nitric oxide is unconstrained by cell membranes and can therefore act along a broad distance as a volume transmitter. Spillover of nitric oxide between neurons may have a major impact on central nervous system diseases and particularly on neurodegeneration. There is evidence whereby communication between nitrergic and dopaminergic systems plays an essential role in the control of the nigrostriatal pathway. However, there is sparse information for either the coexistence or overlap of nitric oxide and dopaminergic structures. The dual localization of immunoreactivity for nitric oxide synthase (NOS) and tyrosine hydroxylase, enzymes responsible for the synthesis of nitric oxide and dopamine, respectively, was examined in neurons of the nigrostriatal pathway in the rat brain by means of a double-immunohistochemical method and confocal laser scanning microscopy, acquired at the resolution limit. After perfusional fixation, the brains were cut and double-immunostained. A proximity analysis of tyrosine hydroxylase and NOS structures was done using binary masks generated from the respective maximum projections, using confocal laser microscopy. Unrevealed regions were determined somatodendritic positive for both NOS and tyrosine hydroxylase, within an image limit resolution at 2 μm-wide margin. The described interconnected localization of nNOS(+) and TH(+) containing neuronal fibers and cells bodies in the nigrostriatal pathway propose a close anatomical link between the two neurotransmitters.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Insulin in the ventral tegmental area reduces hedonic feeding and suppresses dopamine concentration via increased reuptake

Mesolimbic dopamine (DA) signaling has been implicated in the incentive, reinforcing and motivational aspects of food intake. Insulin receptors are expressed on dopaminergic neurons of the ventral tegmental area (VTA), and insulin may act in the VTA to suppress feeding. However, the neural mechanisms underlying insulin effects in the VTA are poorly understood. Here, we measured the effects of insulin on evoked DA concentration in the VTA using fast-scan cyclic voltammetry. Insulin concentration-dependently reduced evoked somatodendritic DA in the VTA, requiring activation of phosphoinositol 3-kinase and mTOR signaling. Insulin depression of somatodendritic DA was abolished in the presence of a selective DA transporter (DAT) inhibitor, GBR 12909, as well as in VTA slices of DAT knockout mice, suggesting that insulin upregulated the number or function of DAT to reduce DA concentration. Finally, insulin administered to the VTA depressed sated feeding of sweetened high-fat food. Taken together, these results indicate that insulin depresses DA concentration in the VTA via increased reuptake of DA through DAT. Insulin-mediated decrease of DA in the VTA may suppress salience of food once satiety is reached.

Physiology of quantal norepinephrine release from somatodendritic sites of neurons in locus coeruleus

Norepinephrine (NE) released from the nerve terminal of locus coeruleus (LC) neurons contributes to about 70% of the total extracellular NE in primates brain. In addition, LC neurons also release NE from somatodendritic sites. Quantal NE release from soma of LC neurons has the characteristics of long latency, nerve activity-dependency, and autoinhibition by α₂-adrenergic autoreceptor. The distinct kinetics of stimulus-secretion coupling in somata is regulated by action potential patterns. The physiological significance of soma and dendritic release is to produce negative-feedback and to down-regulate neuronal hyperactivity, which consequently inhibit NE release from axon terminal of LC projecting to many brain areas. Recent discoveries about the LC somatodendritic release may provide new insights into the pathogenesis of clinic disease involving LC-NE system dysfunction, and may help developing remedy targeted to the LC area.

Dopamine release in the basal ganglia

Dopamine (DA) is a key transmitter in the basal ganglia, yet DA transmission does not conform to several aspects of the classic synaptic doctrine. Axonal DA release occurs through vesicular exocytosis and is action potential- and Ca²⁺-dependent. However, in addition to axonal release, DA neurons in midbrain exhibit somatodendritic release by an incompletely understood, but apparently exocytotic, mechanism. Even in striatum, axonal release sites are controversial, with evidence for DA varicosities that lack postsynaptic specialization, and largely extrasynaptic DA receptors and transporters. Moreover, DA release is often assumed to reflect a global response to a population of activities in midbrain DA neurons, whether tonic or phasic, with precise timing and specificity of action governed by other basal ganglia circuits. This view has been reinforced by anatomical evidence showing dense axonal DA arbors throughout striatum, and a lattice network formed by DA axons and glutamatergic input from cortex and thalamus. Nonetheless, localized DA transients are seen in vivo using voltammetric methods with high spatial and temporal resolution. Mechanistic studies using similar methods in vitro have revealed local regulation of DA release by other transmitters and modulators, as well as by proteins known to be disrupted in Parkinson's disease and other movement disorders. Notably, the actions of most other striatal transmitters on DA release also do not conform to the synaptic doctrine, with the absence of direct synaptic contacts for glutamate, GABA, and acetylcholine (ACh) on striatal DA axons. Overall, the findings reviewed here indicate that DA signaling in the basal ganglia is sculpted by cooperation between the timing and pattern of DA input and those of local regulatory factors.

Intrinsic and integrative properties of substantia nigra pars reticulata neurons

The GABA projection neurons of the substantia nigra pars reticulata (SNr) are output neurons for the basal ganglia and thus critical for movement control. Their most striking neurophysiological feature is sustained, spontaneous high frequency spike firing. A fundamental question is: what are the key ion channels supporting the remarkable firing capability in these neurons? Recent studies indicate that these neurons express tonically active type 3 transient receptor potential (TRPC3) channels that conduct a Na-dependent inward current even at hyperpolarized membrane potentials. When the membrane potential reaches -60 mV, a voltage-gated persistent sodium current (I(NaP)) starts to activate, further depolarizing the membrane potential. At or slightly below -50 mV, the large transient voltage-activated sodium current (I(NaT)) starts to activate and eventually triggers the rapid rising phase of action potentials. SNr GABA neurons have a higher density of I(NaT), contributing to the faster rise and larger amplitude of action potentials, compared with the slow-spiking dopamine neurons. I(NaT) also recovers from inactivation more quickly in SNr GABA neurons than in nigral dopamine neurons. In SNr GABA neurons, the rising phase of the action potential triggers the activation of high-threshold, inactivation-resistant Kv3-like channels that can rapidly repolarize the membrane. These intrinsic ion channels provide SNr GABA neurons with the ability to fire spontaneous and sustained high frequency spikes. Additionally, robust GABA inputs from direct pathway medium spiny neurons in the striatum and GABA neurons in the globus pallidus may inhibit and silence SNr GABA neurons, whereas glutamate synaptic input from the subthalamic nucleus may induce burst firing in SNr GABA neurons. Thus, afferent GABA and glutamate synaptic inputs sculpt the tonic high frequency firing of SNr GABA neurons and the consequent inhibition of their targets into an integrated motor control signal that is further fine-tuned by neuromodulators including dopamine, serotonin, endocannabinoids, and H₂O₂.

Chronic valproate treatment blocks D2-like receptor-mediated brain signaling via arachidonic acid in rats

BACKGROUND AND OBJECTIVE

Hyperdopaminergic signaling and an upregulated brain arachidonic acid (AA) cascade may contribute to bipolar disorder (BD). Lithium and carbamazepine, FDA-approved for the treatment of BD, attenuate brain dopaminergic D(2)-like (D(2), D(3), and D(4)) receptor signaling involving AA when given chronically to awake rats. We hypothesized that valproate (VPA), with mood-stabilizing properties, would also reduce D(2)-like-mediated signaling via AA.

METHODS

An acute dose of quinpirole (1 mg/kg) or saline was administered to unanesthetized rats that had been treated for 30 days with a therapeutically relevant dose of VPA (200 mg/kg/day) or vehicle. Regional brain AA incorporation coefficients, k*, and incorporation rates, J(in), markers of AA signaling and metabolism, were measured by quantitative autoradiography after intravenous [1-(14)C]AA infusion. Whole brain concentrations of prostaglandin (PG)E(2) and thromboxane (TX)B(2) also were measured.

RESULTS

Quinpirole compared to saline significantly increased k* in 40 of 83 brain regions, and increased brain concentrations of PGE(2) in chronic vehicle-treated rats. VPA treatment by itself reduced concentrations of plasma unesterified AA and whole brain PGE(2) and TXB(2), and blocked the quinpirole-induced increments in k* and PGE(2).

CONCLUSION

These results further provide evidence that mood stabilizers downregulate brain dopaminergic D(2)-like receptor signaling involving AA.

Extrastriatal D2-like receptors modulate basal ganglia pathways in normal and Parkinsonian monkeys

According to traditional models of the basal ganglia-thalamocortical network of connections, dopamine exerts D2-like receptor (D2LR)-mediated effects through actions on striatal neurons that give rise to the "indirect" pathway, secondarily affecting the activity in the internal and external pallidal segments (GPi and GPe, respectively) and the substantia nigra pars reticulata (SNr). However, accumulating evidence from the rodent literature suggests that D2LR activation also directly influences synaptic transmission in these nuclei. To further examine this issue in primates, we combined in vivo electrophysiological recordings and local intracerebral microinjections of drugs with electron microscopic immunocytochemistry to study D2LR-mediated modulation of neuronal activities in GPe, GPi, and SNr of normal and MPTP-treated (parkinsonian) monkeys. D2LR activation with quinpirole increased firing in most GPe neurons, likely due to a reduction of striatopallidal GABAergic inputs. In contrast, local application of quinpirole reduced firing in GPi and SNr, possibly through D2LR-mediated effects on glutamatergic inputs. Injections of the D2LR antagonist sulpiride resulted in effects opposite to those of quinpirole in GPe and GPi. D2 receptor immunoreactivity was most prevalent in putative striatal-like GABAergic terminals and unmyelinated axons in GPe, GPi, and SNr, but a significant proportion of immunoreactive boutons also displayed ultrastructural features of glutamatergic terminals. Postsynaptic labeling was minimal in all nuclei. The D2LR-mediated effects and pattern of distribution of D2 receptor immunoreactivity were maintained in the parkinsonian state. Thus, in addition to their preferential effects on indirect pathway striatal neurons, extrastriatal D2LR activation in GPi and SNr also influences direct pathway elements in the primate basal ganglia under normal and parkinsonian conditions.

Somatodendritic dopamine release requires synaptotagmin 4 and 7 and the participation of voltage-gated calcium channels

Somatodendritic (STD) dopamine (DA) release is a key mechanism for the autoregulatory control of DA release in the brain. However, its molecular mechanism remains undetermined. We tested the hypothesis that differential expression of synaptotagmin (Syt) isoforms explains some of the differential properties of terminal and STD DA release. Down-regulation of the dendritically expressed Syt4 and Syt7 severely reduced STD DA release, whereas terminal release required Syt1. Moreover, we found that although mobilization of intracellular Ca(2+) stores is inefficient, Ca(2+) influx through N- and P/Q-type voltage-gated channels is critical to trigger STD DA release. Our findings provide an explanation for the differential Ca(2+) requirement of terminal and STD DA release. In addition, we propose that not all sources of intracellular Ca(2+) are equally efficient to trigger this release mechanism. Our findings have implications for a better understanding of a fundamental cell biological process mediating transcellular signaling in a system critical for diseases such as Parkinson disease.

Differential calcium dependence of axonal versus somatodendritic dopamine release, with characteristics of both in the ventral tegmental area

Midbrain dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) exhibit somatodendritic release of DA. Previous studies indicate a difference between the Ca²⁺ dependence of somatodendritic DA release in the SNc and that of axonal DA release in dorsal striatum. Here, we evaluated the Ca²⁺ dependence of DA release in the VTA and nucleus accumbens (NAc) shell for comparison with that in the SNc and dorsal striatum. Release of DA was elicited by single-pulse stimulation in guinea-pig brain slices and monitored with subsecond resolution using carbon-fiber microelectrodes and fast-scan cyclic voltammetry. In dorsal striatum and NAc, DA release was not detectable at extracellular Ca²⁺ concentrations ([Ca²⁺]_o) below 1 mM; however, a progressive increase in evoked extracellular DA concentration ([DA]_o) was seen with [Ca²⁺]_o ≥ 1.5 mM. By contrast, in SNc and VTA, robust increases in [DA]_o could be elicited in 0.25 mM [Ca²⁺]_o that were ∼60% of those seen in 1.5 mM [Ca²⁺]_o. In SNc, a plateau in single-pulse evoked [DA]_o was seen at [Ca²⁺]_o ≥ 1.5 mM, mirroring the release plateau reported previously for pulse-train stimulation in SNc. In VTA, however, evoked [DA]_o increased progressively throughout the range of [Ca²⁺]_o tested (up to 3.0 mM). These functional data are consistent with the microanatomy of the VTA, which includes DA axon collaterals as well as DA somata and dendrites. Differences between axonal and somatodendritic release data were quantified using Hill analysis, which showed that the Ca²⁺ dependence of axonal DA release is low affinity with high Ca²⁺ cooperativity, whereas somatodendritic release is high affinity with low cooperativity. Moreover, this analysis revealed the dual nature of DA release in the VTA, with both somatodendritic and axonal contributions.

Mechanisms and function of dendritic exocytosis

Dendritic exocytosis is required for a broad array of neuronal functions including retrograde signaling, neurotransmitter release, synaptic plasticity, and establishment of neuronal morphology. While the details of synaptic vesicle exocytosis from presynaptic terminals have been intensely studied for decades, the mechanisms of dendritic exocytosis are only now emerging. Here we review the molecules and mechanisms of dendritic exocytosis and discuss how exocytosis from dendrites influences neuronal function and circuit plasticity.

Drug-driven AMPA receptor redistribution mimicked by selective dopamine neuron stimulation

Background

Addictive drugs have in common that they cause surges in dopamine (DA) concentration in the mesolimbic reward system and elicit synaptic plasticity in DA neurons of the ventral tegmental area (VTA). Cocaine for example drives insertion of GluA2-lacking AMPA receptors (AMPARs) at glutamatergic synapes in DA neurons. However it remains elusive which molecular target of cocaine drives such AMPAR redistribution and whether other addictive drugs (morphine and nicotine) cause similar changes through their effects on the mesolimbic DA system.

Methodology / Principal Findings

We used in vitro electrophysiological techniques in wild-type and transgenic mice to observe the modulation of excitatory inputs onto DA neurons by addictive drugs. To observe AMPAR redistribution, post-embedding immunohistochemistry for GluA2 AMPAR subunit was combined with electron microscopy. We also used a double-floxed AAV virus expressing channelrhodopsin together with a DAT Cre mouse line to selectively express ChR2 in VTA DA neurons. We find that in mice where the effect of cocaine on the dopamine transporter (DAT) is specifically blocked, AMPAR redistribution was absent following administration of the drug. Furthermore, addictive drugs known to increase dopamine levels cause a similar AMPAR redistribution. Finally, activating DA VTA neurons optogenetically is sufficient to drive insertion of GluA2-lacking AMPARs, mimicking the changes observed after a single injection of morphine, nicotine or cocaine.

Conclusions / Significance

We propose the mesolimbic dopamine system as a point of convergence at which addictive drugs can alter neural circuits. We also show that direct activation of DA neurons is sufficient to drive AMPAR redistribution, which may be a mechanism associated with early steps of non-substance related addictions.

Extrastriatal dopaminergic circuits of the Basal Ganglia

The basal ganglia are comprised of the striatum, the external and internal segment of the globus pallidus (GPe and GPi, respectively), the subthalamic nucleus (STN), and the substantia nigra pars compacta and reticulata (SNc and SNr, respectively). Dopamine has long been identified as an important modulator of basal ganglia function in the striatum, and disturbances of striatal dopaminergic transmission have been implicated in diseases such as Parkinson's disease (PD), addiction and attention deficit hyperactivity disorder. However, recent evidence suggests that dopamine may also modulate basal ganglia function at sites outside of the striatum, and that changes in dopaminergic transmission at these sites may contribute to the symptoms of PD and other neuropsychiatric disorders. This review summarizes the current knowledge of the anatomy, functional effects and behavioral consequences of the dopaminergic innervation to the GPe, GPi, STN, and SNr. Further insights into the dopaminergic modulation of basal ganglia function at extrastriatal sites may provide us with opportunities to develop new and more specific strategies for treating disorders of basal ganglia dysfunction.

A transient receptor potential channel regulates basal ganglia output

The substantia nigra pars reticulata (SNr) is a key basal ganglia output nucleus critical for movement control. A hallmark of the SNr gamma-aminobutyric acid (GABA)-containing projection neurons is their depolarized membrane potential, accompanied by rapid spontaneous spikes. Parkinsonian movement disorders are often associated with abnormalities in SNr GABA neuron firing intensity and/or pattern. A fundamental question is the molecular identity of the ion channels that drive these neurons to a depolarized membrane potential. Recent data show that SNr GABA projection neurons selectively express type 3 canonical transient receptor potential (TRPC3) channels. Such channels are tonically active and mediate an inward, Na(+)-dependent current, leading to a substantial depolarization and ensuring appropriate firing intensity and pattern in SNr GABA projection neurons. Equally important, TRPC3 channels in SNr GABA neurons are up-regulated by dopamine (DA) released from neighboring nigral DA neuron dendrites. Co-activation of D1 and D5 DA receptors leads to a TRPC3 channel-mediated inward current and increased firing in SNr GABA neurons, whereas D1-like receptor blockade reduces SNr GABA neuron firing frequency and increases their firing irregularity. TRPC3 channels serve as the effector channels mediating an ultra-short SNc-->SNr DA pathway that regulates the firing intensity and pattern of the basal ganglia output neurons.

Ultrastructural localization and function of dopamine D1-like receptors in the substantia nigra pars reticulata and the internal segment of the globus pallidus of parkinsonian monkeys

The motor symptoms of Parkinson's disease (PD) are commonly attributed to striatal dopamine loss, but reduced dopamine innervation of basal ganglia output nuclei, the internal globus pallidus (GPi) and the substantia nigra pars reticulata (SNr) may also contribute to symptoms and signs of PD. Both structures express dopamine D1 and D5 receptors under normal conditions, and we have recently demonstrated that their local activation reduces neuronal discharge rates and enhances bursts and oscillatory activity in both nuclei of normal monkeys [M.A. Kliem et al. (2007)J. Neurophysiol., 89, 1489-1500]. Here, we determined the ultrastructural localization and function of D1-like receptors in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys. In both normal and MPTP-treated monkeys, most of the D1 and D5 receptor immunoreactivity was associated with unmyelinated axons, but we also found significant postsynaptic D5 receptor immunostaining in dendrites of GPi and SNr neurons. A significant proportion of axonal D1 immunostaining was bound to the plasma membrane in both normal and MPTP-treated monkeys. Local microinjections of the D1/D5 receptor agonist SKF82958 significantly reduced discharge rates in GPi and SNr neurons, while they increased burst firing and oscillatory activity in the 3-15-Hz band in SNr, but not in GPi, of parkinsonian monkeys. Together with our recent findings from normal monkeys, these data provide evidence that functional D1/D5 receptors are expressed in GPi and SNr in both normal and parkinsonian states, and that their activation by endogenous dopamine (under normal conditions) or dopamine receptor agonists (in parkinsonism) may regulate basal ganglia outflow.

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.