Divergent Whole Brain Projections from the Ventral Midbrain in Macaques

To understand the connectome of the axonal arborizations of dopaminergic midbrain neurons, we investigated the anterograde spread of highly sensitive viral tracers injected into the ventral tegmental area (VTA) and adjacent areas in 3 macaques. In 2 monkeys, injections were centered on the lateral VTA with some spread into the substantia nigra, while in one animal the injection targeted the medial VTA with partial spread into the ventro-medial thalamus. Double-labeling with antibodies against transduced fluorescent proteins (FPs) and tyrosine hydroxylase indicated that substantial portions of transduced midbrain neurons were dopaminergic. Interestingly, cortical terminals were found either homogeneously in molecular layer I, or more heterogeneously, sometimes forming patches, in the deeper laminae II-VI. In the animals with injections in lateral VTA, terminals were most dense in somatomotor cortex and the striatum. In contrast, when the medial VTA was transduced, dense terminals were found in dorsal prefrontal and temporal cortices, while projections to striatum were sparse. In all monkeys, orbitofrontal and occipito-parietal cortex received strong and weak innervation, respectively. Thus, the dopaminergic ventral midbrain sends heterogeneous projections throughout the brain. Furthermore, our results suggest the existence of subgroups in meso-dopaminergic neurons depending on their location in the primate ventral midbrain.

Single-hole, ruptured parenchymal arteriovenous fistula of the mesencephalon: not known vascular malformation of the brain or a posthemorrhagic entity?

The subtypes of brain arteriovenous malformations, with direct, single-hole fistulas without co-existing nidus are not described as existing entities inside the brain parenchyma but on the pial surface. True parenchymal arteriovenous malformations present with nidal structure, even if they are small, whereas surface lesions may present a direct fistulous configuration. In this case of midbrain haemorrhage a direct arteriovenous fistula was detected at the level of the red nucleus between a paramedian midbrain perforator artery and a paramedian parenchymal vein, with pseudo-aneurysm formation at the fistulous connection, without signs of adjacent nidus structure. The hypothesis whether a pre-existing arteriovenous fistula ruptured or a spontaneous haemorrhage has caused the fistulous connection is discussed.

Increased levels of midbrain immune-related transcripts in schizophrenia and in murine offspring after maternal immune activation

The pathophysiology of dopamine dysregulation in schizophrenia involves alterations at the ventral midbrain level. Given that inflammatory mediators such as cytokines influence the functional properties of midbrain dopamine neurons, midbrain inflammation may play a role in schizophrenia by contributing to presynaptic dopamine abnormalities. Thus, we quantified inflammatory markers in dopaminergic areas of the midbrain of people with schizophrenia and matched controls. We also measured these markers in midbrain of mice exposed to maternal immune activation (MIA) during pregnancy, an established risk factor for schizophrenia and other psychiatric disorders. We found diagnostic increases in SERPINA3, TNFα, IL1β, IL6, and IL6ST transcripts in schizophrenia compared with controls (p < 0.02-0.001). The diagnostic differences in these immune markers were accounted for by a subgroup of schizophrenia cases (~ 45%, 13/28) showing high immune status. Consistent with the human cohort, we identified increased expression of immune markers in the midbrain of adult MIA offspring (SERPINA3, TNFα, and IL1β mRNAs, all p ≤ 0.01), which was driven by a subset of MIA offspring (~ 40%, 13/32) with high immune status. There were no diagnostic (human cohort) or group-wise (mouse cohort) differences in cellular markers indexing the density and/or morphology of microglia or astrocytes, but an increase in the transcription of microglial and astrocytic markers in schizophrenia cases and MIA offspring with high inflammation. These data demonstrate that immune-related changes in schizophrenia extend to dopaminergic areas of the midbrain and exist in the absence of changes in microglial cell number, but with putative evidence of microglial and astrocytic activation in the high immune subgroup. MIA may be one of the contributing factors underlying persistent neuroimmune changes in the midbrain of people with schizophrenia.

Controllable fusion of human brain organoids using acoustofluidics

The fusion of human organoids holds promising potential in modeling physiological and pathological processes of tissue genesis and organogenesis. However, current fused organoid models face challenges of high heterogeneity and variable reproducibility, which may stem from the random fusion of heterogeneous organoids. Thus, we developed a simple and versatile acoustofluidic method to improve the standardization of fused organoid models via a controllable spatial arrangement of organoids. By regulating dynamic acoustic fields within a hexagonal acoustofluidic device, we can rotate, transport, and fuse one organoid with another in a contact-free, label-free, and minimal-impact manner. As a proof-of-concept to model the development of the human midbrain-to-forebrain mesocortical pathway, we acoustically fused human forebrain organoids (hFOs) and human midbrain organoids (hMOs) with the controllable alignment of neuroepithelial buds. We found that post-assembly, hMO can successfully project tyrosine hydroxylase neurons towards hFO, accompanied by an increase of firing rates and synchrony of excitatory neurons. Moreover, we found that our controllable fusion method can regulate neuron projection (e.g., range, length, and density), projection maturation (e.g., higher firing rate and synchrony), and neural progenitor cell (NPC) division in the assembloids via the initial spatial control. Thus, our acoustofluidic method may serve as a label-free, contact-free, and highly biocompatible tool to effectively assemble organoids and facilitate the standardization and robustness of organoid-based disease models and tissue engineering.

Neurodevelopmental defects and neurodegenerative phenotypes in human brain organoids carrying Parkinson's disease-linked DNAJC6 mutations

Loss-of-function mutations of DNAJC6, encoding HSP40 auxilin, have recently been identified in patients with early-onset Parkinson's disease (PD). To study the roles of DNAJC6 in PD pathogenesis, we used human embryonic stem cells with CRISPR-Cas9-mediated gene editing. Here, we show that DNAJC6 mutations cause key PD pathologic features, i.e., midbrain-type dopamine (mDA) neuron degeneration, pathologic α-synuclein aggregation, increase of intrinsic neuronal firing frequency, and mitochondrial and lysosomal dysfunctions in human midbrain-like organoids (hMLOs). In addition, neurodevelopmental defects were also manifested in hMLOs carrying the mutations. Transcriptomic analyses followed by experimental validation revealed that defects in DNAJC6-mediated endocytosis impair the WNT-LMX1A signal during the mDA neuron development. Furthermore, reduced LMX1A expression during development caused the generation of vulnerable mDA neurons with the pathologic manifestations. These results suggest that the human model of DNAJC6-PD recapitulates disease phenotypes and reveals mechanisms underlying disease pathology, providing a platform for assessing therapeutic interventions.

Delayed Hemiparkinsonism Associated with Kernohan's Notch in a Patient with a Ruptured Arteriovenous Malformation

A 24-year-old female patient was admitted for a right frontal intracranial hematoma with an uncal herniation due to a ruptured arteriovenous malformation and therefore underwent emergency surgery. Neuroimaging revealed left-sided midbrain notching against the tentorium, indicating Kernohan's notch phenomenon. She denied experiencing any short-term neurological deficits but right-sided delayed hemiparkinsonism developed 18 months later. Dopamine transporter tracer uptake was severely reduced in the left striatum, suggesting nigrostriatal degeneration secondary to Kernohan's notch. Uncal herniations are potentially fatal, but surgery can save the patient's life and improve the functional outcomes. Clinicians should therefore be aware of delayed hemiparkinsonism as a rare complication of Kernohan's notch phenomenon.

Distributional Reinforcement Learning in the Brain

Learning about rewards and punishments is critical for survival. Classical studies have demonstrated an impressive correspondence between the firing of dopamine neurons in the mammalian midbrain and the reward prediction errors of reinforcement learning algorithms, which express the difference between actual reward and predicted mean reward. However, it may be advantageous to learn not only the mean but also the complete distribution of potential rewards. Recent advances in machine learning have revealed a biologically plausible set of algorithms for reconstructing this reward distribution from experience. Here, we review the mathematical foundations of these algorithms as well as initial evidence for their neurobiological implementation. We conclude by highlighting outstanding questions regarding the circuit computation and behavioral readout of these distributional codes.

Differentiation of ciliated human midbrain-derived LUHMES neurons

Many human cell types are ciliated, including neural progenitors and differentiated neurons. Ciliopathies are characterized by defective cilia and comprise various disease states, including brain phenotypes, where the underlying biological pathways are largely unknown. Our understanding of neuronal cilia is rudimentary, and an easy-to-maintain, ciliated human neuronal cell model is absent. The Lund human mesencephalic (LUHMES) cell line is a ciliated neuronal cell line derived from human fetal mesencephalon. LUHMES cells can easily be maintained and differentiated into mature, functional neurons within one week. They have a single primary cilium as proliferating progenitor cells and as postmitotic, differentiating neurons. These developmental stages are completely separable within one day of culture condition change. The sonic hedgehog (SHH) signaling pathway is active in differentiating LUHMES neurons. RNA-sequencing timecourse analyses reveal molecular pathways and gene-regulatory networks critical for ciliogenesis and axon outgrowth at the interface between progenitor cell proliferation, polarization and neuronal differentiation. Gene expression dynamics of cultured LUHMES neurons faithfully mimic the corresponding in vivo dynamics of human fetal midbrain. In LUHMES cells, neuronal cilia biology can be investigated from proliferation through differentiation to mature neurons.

DSCAM regulates delamination of neurons in the developing midbrain

For normal neurogenesis and circuit formation, delamination of differentiating neurons from the proliferative zone must be precisely controlled; however, the regulatory mechanisms underlying cell attachment are poorly understood. Here, we show that Down syndrome cell adhesion molecule (DSCAM) controls neuronal delamination by local suppression of the RapGEF2-Rap1-N-cadherin cascade at the apical endfeet in the dorsal midbrain. Dscam transcripts were expressed in differentiating neurons, and DSCAM protein accumulated at the distal part of the apical endfeet. Cre-loxP-based neuronal labeling revealed that Dscam knockdown impaired endfeet detachment from ventricles. DSCAM associated with RapGEF2 to inactivate Rap1, whose activity is required for membrane localization of N-cadherin. Correspondingly, Dscam knockdown increased N-cadherin localization and ventricular attachment area at the endfeet. Furthermore, excessive endfeet attachment by Dscam knockdown was restored by co-knockdown of RapGEF2 or N-cadherin Our findings shed light on the molecular mechanism that regulates a critical step in early neuronal development.

Comparison of operation of spinal locomotor networks activated by supraspinal commands and by epidural stimulation of the spinal cord in cats

KEY POINTS

Epidural electrical stimulation (ES) of the spinal cord restores/improves locomotion in patients. ES-evoked locomotor movements differ to some extent from the normal ones. Operation of the locomotor network during ES is unknown. We compared the activity of individual spinal neurons during locomotion initiated by signals from the brainstem and by ES. We demonstrated that the spinal network generating locomotion under each of the two conditions is formed by the same neurons. A part of this network operates similarly under the two conditions, suggesting that it is essential for generation of locomotion under both conditions. Another part of this network operates differently under the two conditions, suggesting that it is responsible for differences in the movement kinematics observed under the two conditions.

ABSTRACT

Locomotion is a vital motor function for both animals and humans. Epidural electrical stimulation (ES) of the spinal cord is used to restore/improve locomotor movements in patients. However, operation of locomotor networks during ES has never been studied. Here we compared the activity of individual spinal neurons recorded in decerebrate cats of either sex during locomotion initiated by supraspinal commands (caused by stimulation of the mesencephalic locomotor region, MLR) and by ES. We found that under both conditions, the same neurons had modulation of their activity related to the locomotor rhythm, suggesting that the network generating locomotion under the two conditions is formed by the same neurons. About 40% of these neurons had stable modulation (i.e. small dispersion of their activity phase in sequential cycles), as well as a similar phase and shape of activity burst in MLR- and ES-evoked locomotor cycles. We suggest that these neurons form a part of the locomotor network that operates similarly under the two conditions, and are critical for generation of locomotion. About 23% of the modulated neurons had stable modulation only during MLR-evoked locomotion. We suggest that these neurons are responsible for some differences in kinematics of MLR- and ES-evoked locomotor movements. Finally, 25% of the modulated neurons had unstable modulation during both MLR- and ES-evoked locomotion. One can assume that these neurons contribute to maintenance of the excitability level of locomotor networks necessary for generation of stepping, or belong to postural networks, activated simultaneously with locomotor networks by both MLR stimulation and ES.

Developing a behavioral model of Restless Legs Syndrome utilizing mice with natural variances in ventral midbrain iron

BACKGROUND

The primary symptoms of Restless Legs Syndrome (RLS) are circadian-dependent, leading to increased activity or decreased rest, especially at night. The primary pathology in RLS is brain iron insufficiency despite normal systemic iron stores. Natural variances in brain and peripheral iron concentrations across recombinant inbred (RI) murine strains provide a biological model of RLS. The question is whether these RI mice strains show a behavioral analog to circadian-dependent clinical phenotype of RLS.

METHODS

The home cage activity of eight female RI strains was measured over a 72-h period. The ratio of the average activity in the last 2 h of the active period relative to that in the total 12-h active period (late active period activity ratio, LAPAR) was the primary outcome variable. The relation of average LAPAR scores to measures of ventral midbrain (VMB) iron was evaluated across strains in this study.

RESULTS

RI strain 40 (LAPAR = 1.28) and RI strain 21 (LAPAR = 1.02) were the only strains to show an increased activity in the last part of the active period. ANOVA showed the increased activity was significantly greater during the last 2 h compared to the preceding 10 h of the active phase only for the RI strain 40. Average LAPAR across the eight strains did not significantly correlate with the VMB iron content (r = -0.27, p < 0.10) but did correlate with changes in VMB iron with iron deficiency (r = 0.71, p < 0.05) and diurnal change in VMB iron (r = 0.65, p < 0.05).

CONCLUSION

The female RI strain 40 mice exhibited a distinct end-of-active-period behavior analogous to circadian-dependent clinical phenotype of RLS.

Neural fluctuation cues for simultaneous notched-noise masking and profile-analysis tasks: Insights from model midbrain responses

Results of simultaneous notched-noise masking are commonly interpreted as reflecting the bandwidth of underlying auditory filters. This interpretation assumes that listeners detect a tone added to notched-noise based on an increase in energy at the output of an auditory filter. Previous work challenged this assumption by showing that randomly and independently varying (roving) the levels of each stimulus interval does not substantially worsen listener thresholds [Lentz, Richards, and Matiasek (1999). J. Acoust. Soc. Am. 106, 2779-2792]. Lentz et al. further challenged this assumption by showing that filter bandwidths based on notched-noise results were different from those based on a profile-analysis task [Green (1983). Am. Psychol. 38, 133-142; (1988). (Oxford University Press, New York)], although these estimates were later reconciled by emphasizing spectral peaks of the profile-analysis stimulus [Lentz (2006). J. Acoust. Soc. Am. 120, 945-956]. Here, a single physiological model is shown to account for performance in fixed- and roving-level notched-noise tasks and the Lentz et al. profile-analysis task. This model depends on peripheral neural fluctuation cues that are transformed into the average rates of model inferior colliculus neurons. Neural fluctuations are influenced by peripheral filters, synaptic adaptation, cochlear amplification, and saturation of inner hair cells, an element not included in previous theories of envelope-based cues for these tasks. Results suggest reevaluation of the interpretation of performance in these paradigms.

Classification of Midbrain Dopamine Neurons Using Single-Cell Gene Expression Profiling Approaches

Dysfunctional dopamine (DA) signaling has been associated with a broad spectrum of neuropsychiatric disorders, prompting investigations into how midbrain DA neuron heterogeneity may underpin this variety of behavioral symptoms. Emerging literature indeed points to functional heterogeneity even within anatomically defined DA clusters. Recognizing the need for a systematic classification scheme, several groups have used single-cell profiling to catalog DA neurons based on their gene expression profiles. We aim here not only to synthesize points of congruence but also to highlight key differences between the molecular classification schemes derived from these studies. In doing so, we hope to provide a common framework that will facilitate investigations into the functions of DA neuron subtypes in the healthy and diseased brain.

Distribution of Midbrain Cholinergic Axons in the Thalamus

Cholinergic transmission is essential for adaptive behavior and has been suggested to play a central role in the modulation of brain states by means of the modulation of thalamic neurons. Midbrain cholinergic neurons from the pedunculopontine nucleus (PPN) and the laterodorsal tegmental nucleus (LDT) provide dense innervation of the thalamus, but a detailed connectivity mapping is missing. Using conditional tracing of midbrain cholinergic axons in the rat, together with a detailed segmentation of thalamic structures, we show that projections arising in PPN and LDT are topographically organized along the entire extent of the thalamus. PPN cholinergic neurons preferentially innervate thalamic relay structures, whereas LDT cholinergic neurons preferentially target thalamic limbic nuclei. Moreover, both PPN and LDT provide a dense innervation of the intralaminar thalamic nuclei. Notably, we observe a differential synaptic density that functionally dissociates between PPN and LDT innervation. Our results show that midbrain cholinergic neurons innervate virtually all thalamic structures and this innervation is functionally segregated.

Neuroprotection of dopamine neurons by xenon against low-level excitotoxic insults is not reproduced by other noble gases

Using midbrain cultures, we previously demonstrated that the noble gas xenon is robustly protective for dopamine (DA) neurons exposed to L-trans-pyrrolidine-2,4-dicarboxylate (PDC), an inhibitor of glutamate uptake used to generate sustained, low-level excitotoxic insults. DA cell rescue was observed in conditions where the control atmosphere for cell culture was substituted with a gas mix, comprising the same amount of oxygen (20%) and carbon dioxide (5%) but 75% of xenon instead of nitrogen. In the present study, we first aimed to determine whether DA cell rescue against PDC remains detectable when concentrations of xenon are progressively reduced in the cell culture atmosphere. Besides, we also sought to compare the effect of xenon to that of other noble gases, including helium, neon and krypton. Our results show that the protective effect of xenon for DA neurons was concentration-dependent with an IC50 estimated at about 44%. We also established that none of the other noble gases tested in this study protected DA neurons from PDC-mediated insults. Xenon's effectiveness was most probably due to its unique capacity to block NMDA glutamate receptors. Besides, mathematical modeling of gas diffusion in the culture medium revealed that the concentration reached by xenon at the cell layer level is the highest of all noble gases when neurodegeneration is underway. Altogether, our data suggest that xenon may be of potential therapeutic value in Parkinson disease, a chronic neurodegenerative condition where DA neurons appear vulnerable to slow excitotoxicity.

Functional MRI Investigation of Audiovisual Interactions in Auditory Midbrain

The brain integrates information from different sensory modalities to form a representation of the environment and facilitate behavioral responses. The auditory midbrain or inferior colliculus (IC) is a pivotal station in the auditory system, integrating ascending and descending information from various auditory sources and cortical systems. The present study investigated the modulation of auditory responses in the IC by visual stimuli of different frequencies and intensities in rats using functional MRI (fMRI). Low-frequency (1 Hz) high-intensity visual stimulus suppressed IC auditory responses. However, high-frequency (10 Hz) or low-intensity visual stimuli did not alter the IC auditory responses. This finding demonstrates that cross-modal processing occurs in the IC in a manner that depends on the stimulus. Furthermore, only low-frequency high-intensity visual stimulus elicited responses in non-visual cortical regions, suggesting that the above cross-modal modulation effect may arise from top-down cortical feedback. These fMRI results provide insight to guide future studies of cross-modal processing in sensory pathways.

Ring of Power: A Band of Peptidergic Midbrain Neurons that Binds Motivation

A recent Cell paper identifies a novel population of neurons within the ventral tegmental area producing the endogenous opioid nociceptin that regulates dopamine neuron firing and acts uniquely to gate motivation in reward seeking. These results highlight neuropeptidergic signaling as a critical component of functional heterogeneity in the midbrain.

[The roles of habenula and related neural circuits in neuropsychiatric diseases]

The habenula is a small and bilateral nucleus above dorsal thalamus, which contains several different types of neurons. The habenula has extensive connections with the forebrain, septum and monoaminergic nuclei in the midbrain and brainstem. Habenula is known as an 'anti-reward' nucleus, which can be activated by aversive stimulus and negative reward prediction errors. Accumulating researchs have implicated that the habenula is involved in several behaviors crucial to survival. Meanwhile, the roles of the habenula in neuropsychiatric diseases have received increasing attention. This review summaries the studies regarding the roles of habenula and the related circuits in neuropathic pain, depression, drug addiction and schizophrenia, and discusses the possibility to use the habenula as a treatment target.

The fMRI Data of Thompson et al. (2006) Do Not Constrain How the Human Midbrain Represents Interaural Time Delay

This commentary provides an alternate interpretation of the fMRI data that were presented in a communication to the journal Nature Neuroscience (Thompson et al., Nat. Neurosci. 9: 1096-1098, 2006 ). The authors argued that their observations demonstrated that traditional models of binaural hearing which incorporate "internal delays," such as the coincidence-counting mechanism proposed by Jeffress and quantified by Colburn, are invalid, and that a new model for human interaural time delay processing must be developed. We argue that the fMRI data presented do not strongly favor either the refutation or the retention of the traditional models, although they may be useful in constraining the physiological sites of various processing stages. The conclusions of Thompson et al. are based on the locations of maximal activity in the midbrain in response to selected binaural signals. These locations are inconsistent with well-known perceptual attributes of the stimuli under consideration, as is noted by the authors, which suggests that further processing is involved in forming the percept of subjective lateral position.

Social Behavior: How the Brain Thinks like a Mom

Becoming a parent changes our choices and actions. Identifying the underlying neural circuits is necessary to understand the transformation of an animal's behavior post-parenthood. Multiple nodes of the 'parenting circuit' have now been identified to reveal the workings of a single brain region key to the orchestration of parent-specific behaviors.

[Preoperative and postoperative ophthalmic symptoms in patients with space-occupying lesions of the midbrain and pineal region]

The most common clinical manifestations of space-occupying lesions of the midbrain and pineal region are oculomotor and pupil disorders and ophthalmoscopic signs of intracranial hypertension.

PURPOSE

To identify patterns of neuro-ophthalmic symptoms before and after surgical treatment in patients with space-occupying lesions of the midbrain and pineal region.

MATERIAL AND METHODS

We analyzed neurological symptoms in 231 patients with space-occupying lesions of the midbrain and pineal region before and after surgical treatment. Malignant tumors were detected in 121 patients; benign tumors were present in 73 patients; 37 patients were diagnosed with pineal gland cysts. Patients with suspicion of germinoma underwent a tumor biopsy only; the other patients underwent tumor resection.

RESULTS AND DISCUSSION

Before surgery, oculomotor and pupil disorders were detected in more than half of the (67%) patients; ophthalmoscopic signs of intracranial hypertension were present in 38% of the patients. Neuro-ophthalmic symptoms significantly more often occurred in patients with malignant tumors. Midbrain symptoms were significantly more pronounced in germ cell tumors than in other malignant neoplasms. In the early postoperative period after tumor resection, deterioration of oculomotor and pupillary functions occurred in 46% of cases; there were no changes in 51% of cases; improvement occurred in 3% of cases. After tumor biopsy, symptoms in all patients with germinomas remained at the preoperative level. Developed symptoms partially regressed in the long-term period, and finally, only 29% of patients had deterioration of oculomotor and pupillary functions compared to the preoperative level.

Transplantation of Human Fetal Tissue from Spontaneous Abortions to a Rodent Model of Parkinson's Disease

The use of human fetal tissue from elective abortions for cell transplantation therapies has been the subject of considerable controversy. Because of concerns regarding the use of tissue from elective abortions, tissue from spontaneous abortions has been suggested as an alternate donor source. In the present study we have evaluated human fetal tissue from spontaneous abortions to assess its viability, growth potential, and functional expression. Viable cells (Grade I) from a donor (7 wk postconception) were transplanted as a suspension into the striatum of rats with unilateral 6-OHDA lesions of the nigrostriatal pathway. A second group of animals received solid grafts of tissue from a Grade I donor 14 wk postconception. Tissue from Grade II and III specimens were not sufficiently viable for transplantation. Locomotor responses were monitored over a period of 15 wk and revealed an amelioration of rotational asymmetry by animals that received tissue from the 7 wk donor. Animals receiving tissue from the 14 wk donor showed no functional improvement. We found numerous graft-derived tyrosine hydroxylase (TH) immunopositive neurons contained within the transplantation site, and a rich plexus of TH-immunopositive fibers extending into the striatum of animals receiving tissue from the 7 wk donor. Animals receiving tissue from the 14 wk donor exhibited tissue necrosis at the transplant site and were devoid of TH-immunopositive neurons. These results suggest that human fetal ventral mesencephalic cells from spontaneous abortions can survive and develop after transplantation, and rectify locomotor deficits associated with experimental parkinsonism if the donor tissue is of the appropriate gestational age at the time of implantation. Our study further suggests, however, that the availability of tissue from spontaneous abortions of sufficient viability is quite limited and may thus restrict its potential use in cell transplantation therapies for Parkinson's disease.

Tyrosine hydroxylase gene expression is facilitated by alcohol followed by the degradation of the protein by ubiquitin proteasome system

OBJECTIVES

Alcohol intake induces brief periods of euphoria; however, its continuous consumption can lead the development of alcohol tolerance. The euphoria, an intense feeling of wellbeing, is deeply associated with dopamine. Dopamine biosynthesis is strictly regulated by tyrosine hydroxylase (TH), a rate-limiting enzyme of dopamine. The aim of this study was to examine the transient or chronic effects of ethanol treatment on TH protein level in vitro.

METHODS

Cultured primary mesencephalic neurons were prepared and exposed to 100 mM ethanol for 48 hours or 168 hours. TH and cAMP-responsive element (CRE)-mediated transcriptional activity was measured by reporter gene assay using pTH9.0kb-Luc and pCRE-Luc reporter plasmid. TH protein expression and TH phosphorylation was analyzed by Western blot analysis. Dopamine content was measured by high-performance liquid chromatography (HPLC).

RESULTS

Ethanol treatment for 48 hours facilitates TH transcriptional activity and TH protein expression in a cAMP-dependent protein kinase A (PKA) and MAPK/Erk kinase (MEK)-dependent manner in cultured mesencephalic neurons. Ethanol also facilitated TH phosphorylation, which resulted in the elevation of dopamine content. On the other hand, treatment with ethanol for 168 hours did not show significant elevation of TH gene expression and dopamine biosynthesis. Intriguingly, simultaneous treatment with MG-132, a 26S proteasomal inhibitor, recovered the ethanol-induced increase of TH protein expression and dopamine biosynthesis.

CONCLUSION

Transient ethanol-treatment facilitates TH gene expression and its phosphorylation in a PKA- and MEK-dependent manner to elevate dopamine biosynthesis, whereas continuous exposure to ethanol abolishes its potent effects on the dopaminergic function to reduce dopamine content. This reduction seems to originate from the decrease of TH protein level by degradation of the protein. Our current data may contribute to the better understanding of alcohol tolerance associated with degradation of TH protein to reduce total-TH level and dopamine biosynthesis.