Dopamine Systems in the Forebrain

Understanding How Acute Alcohol Impacts Neural Encoding in the Rodent Brain

Alcohol impacts neural circuitry throughout the brain and has wide-ranging effects on the biophysical properties of neurons in these circuits. Articulating how these wide-ranging effects might eventually result in altered computational properties has the potential to provide a tractable working model of how alcohol alters neural encoding. This chapter reviews what is currently known about how acute alcohol influences neural activity in cortical, hippocampal, and dopaminergic circuits as these have been the primary focus of understanding how alcohol alters neural computation. While other neural systems have been the focus of exhaustive work on this topic, these brain regions are the ones where in vivo neural recordings are available, thus optimally suited to make the link between changes in neural activity and behavior. Rodent models have been key in developing an understanding of how alcohol impacts the function of these circuits, and this chapter therefore focuses on work from mice and rats. While progress has been made, it is critical to understand the challenges and caveats associated with experimental procedures, especially when performed in vivo, which are designed to answer this question and if/how to translate these data to humans. The hypothesis is discussed that alcohol impairs the ability of neural circuits to acquire states of neural activity that are transiently elevated and characterized by increased complexity. It is hypothesized that these changes are distinct from the traditional view of alcohol being a depressant of neural activity in the forebrain.

Immunolabeling-compatible PEGASOS tissue clearing for high-resolution whole mouse brain imaging

Novel brain clearing methods revolutionize imaging by increasing visualization throughout the brain at high resolution. However, combining the standard tool of immunostaining targets of interest with clearing methods has lagged behind. We integrate whole-mount immunostaining with PEGASOS tissue clearing, referred to as iPEGASOS (immunostaining-compatible PEGASOS), to address the challenge of signal quenching during clearing processes. iPEGASOS effectively enhances molecular-genetically targeted fluorescent signals that are otherwise compromised during conventional clearing procedures. Additionally, we demonstrate the utility of iPEGASOS for visualizing neurochemical markers or viral labels to augment visualization that transgenic mouse lines cannot provide. Our study encompasses three distinct applications, each showcasing the versatility and efficacy of this approach. We employ whole-mount immunostaining to enhance molecular signals in transgenic reporter mouse lines to visualize the whole-brain spatial distribution of specific cellular populations. We also significantly improve the visualization of neural circuit connections by enhancing signals from viral tracers injected into the brain. Last, we show immunostaining without genetic markers to selectively label beta-amyloid deposits in a mouse model of Alzheimer's disease, facilitating the comprehensive whole-brain study of pathological features.

The neurobiological effects of senescence on dopaminergic system: A comprehensive review

Over time, the body undergoes a natural, multifactorial, and ongoing process named senescence, which induces changes at the molecular, cellular, and micro-anatomical levels in many body systems. The brain, being a highly complex organ, is particularly affected by this process, potentially impairing its numerous functions. The brain relies on chemical messengers known as neurotransmitters to function properly, with dopamine being one of the most crucial. This catecholamine is responsible for a broad range of critical roles in the central nervous system, including movement, learning, cognition, motivation, emotion, reward, hormonal release, memory consolidation, visual performance, sexual drive, modulation of circadian rhythms, and brain development. In the present review, we thoroughly examine the impact of senescence on the dopaminergic system, with a primary focus on the classic delimitations of the dopaminergic nuclei from A8 to A17. We provide in-depth information about their anatomy and function, particularly addressing how senescence affects each of these nuclei.

The Effect of Methylphenidate Treatment on Olfactory Function in Children and Adolescents With ADHD

OBJECTIVE

This study aimed to research whether there is an olfactory disorder in ADHD, and if so, what is the effect of methylphenidate on this condition.

METHOD

This is a cross-sectional study aiming to evaluate olfactory threshold, identification, discrimination and threshold, discrimination, and identification (TDI) scores in 109 children and adolescents, 33 of whom have ADHD without medication, 29 with ADHD with medication and 47 control groups.

RESULT

In the post hoc tests, the mean odor discrimination test, the mean odor identification test, and the mean TDI scores of the unmedicated ADHD group were significantly lower than those of the other two groups, and that the mean odor threshold test scores of the medicated ADHD group were significantly lower than those of the control and unmedicated groups.

CONCLUSION

Olfactory function could be a useful tool to monitor treatment effects and may be a promising candidate as a biomarker in ADHD.

Increased oligodendrogenesis and myelination in the subventricular zone of aged mice and gray mouse lemurs

The adult rodent subventricular zone (SVZ) generates neural stem cells (NSCs) throughout life that migrate to the olfactory bulbs (OBs) and differentiate into olfactory interneurons. Few SVZ NSCs generate oligodendrocyte precursor cells (OPCs). We investigated how neurogliogenesis is regulated during aging in mice and in a non-human primate (NHP) model, the gray mouse lemur. In both species, neuronal commitment decreased with age, while OPC generation and myelin content unexpectedly increased. In the OBs, more tyrosine hydroxylase interneurons in old mice, but fewer in lemurs, marked a surprising interspecies difference that could relate to our observation of a continuous ventricle in lemurs. In the corpus callosum, aging promoted maturation of OPCs into mature oligodendrocytes in mice but blocked it in lemurs. The present study highlights similarities and dissimilarities between rodents and NHPs, revealing that NHPs are a more relevant model than mice to study the evolution of biomarkers of aging.

Cocaine perturbs mitovesicle biology in the brain

Cocaine, an addictive psychostimulant, has a broad mechanism of action, including the induction of a wide range of alterations in brain metabolism and mitochondrial homeostasis. Our group recently identified a subpopulation of non-microvesicular, non-exosomal extracellular vesicles of mitochondrial origin (mitovesicles) and developed a method to isolate mitovesicles from brain parenchyma. We hypothesised that the generation and secretion of mitovesicles is affected by mitochondrial abnormalities induced by chronic cocaine exposure. Mitovesicles from the brain extracellular space of cocaine-administered mice were enlarged and more numerous when compared to controls, supporting a model in which mitovesicle biogenesis is enhanced in the presence of mitochondrial alterations. This interrelationship was confirmed in vitro. Moreover, cocaine affected mitovesicle protein composition, causing a functional alteration in mitovesicle ATP production capacity. These data suggest that mitovesicles are previously unidentified players in the biology of cocaine addiction and that target therapies to fine-tune brain mitovesicle functionality may be beneficial to mitigate the effects of chronic cocaine exposure.

Signaler's Vasotocin Alters the Relationship between the Responder's Forebrain Catecholamines and Communication Behavior in Lizards (Anolis carolinensis)

Dynamic fluctuations in the distribution of catecholamines across the brain modulate the responsiveness of vertebrates to social stimuli. Previous work demonstrates that green anoles (Anolis carolinensis) increase chemosensory behavior in response to males treated with exogenous arginine vasotocin (AVT), but the neurochemical mechanisms underlying this behavioral shift remains unclear. Since central catecholamine systems, including dopamine, rapidly activate in response to social stimuli, we tested whether exogenous AVT in signalers (stimulus animals) impacts catecholamine concentrations in the forebrain (where olfactory and visual information are integrated and processed) of untreated lizard responders. We also tested whether AVT influences the relationship between forebrain catecholamine concentrations and communication behavior in untreated receivers. We measured global catecholamine (dopamine = DA, epinephrine = Epi, and norepinephrine = NE) concentrations in the forebrain of untreated responders using high-performance liquid chromatography-mass spectrometry following either a 30-min social interaction with a stimulus male or a period of social isolation. Stimulus males were injected with exogenous AVT or vehicle saline (SAL). We found that global DA, but not Epi or NE, concentrations were elevated in lizards responding to SAL-males relative to isolated lizards. Lizards interacting with AVT-males had DA, Epi and NE concentrations that were not significantly different from SAL or isolated groups. For behavior, we found a significant effect of social treatment (AVT vs. SAL) on the relationships between (1) DA concentrations and the motivation to perform a chemical display (latency to tongue flick) and (2) Epi concentrations and time spent displaying mostly green body coloration. We also found a significant negative correlation between DA concentrations and the latency to perform a visual display but found no effect of social treatment on this relationship. These data suggest that catecholamine concentrations in the forebrain of untreated responders are associated with chemical and visual communication in lizards and that signaler AVT alters this relationship for some, but not all, aspects of social communication.

Olfactory Bulb D2/D3 Receptor Availability after Intrastriatal Botulinum Neurotoxin-A Injection in a Unilateral 6-OHDA Rat Model of Parkinson's Disease

Olfactory deficits occur as early non-motor symptoms of idiopathic Parkinson's disease (PD) in humans. The first central relay of the olfactory pathway, the olfactory bulb (OB), depends, among other things, on an intact, functional crosstalk between dopaminergic interneurons and dopamine receptors (D2/D3R). In rats, hemiparkinsonism (hemi-PD) can be induced by unilateral injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle (MFB), disrupting dopaminergic neurons of the substantia nigra pars compacta (SNpc). In a previous study, we showed that subsequent injection of botulinum neurotoxin-A (BoNT-A) into the striatum can reverse most of the pathological motor symptoms and normalize the D2/D3R availability. To determine whether this rat model is suitable to explain olfactory deficits that occur in humans with PD, we examined the availability of D2/D3R by longitudinal [18F]fallypride-PET/CT, the density of tyrosine hydroxylase immunoreactivity in the OB, olfactory performance by an orienting odor identification test adapted for rats, and a connectome analysis. PET/CT and immunohistochemical data remained largely unchanged after 6-OHDA lesion in experimental animals, suggesting that outcomes of the 6-OHDA hemi-PD rat model do not completely explain olfactory deficits in humans. However, after subsequent ipsilateral BoNT-A injection into the striatum, a significant 8.5% increase of the D2/D3R availability in the ipsilateral OB and concomitant improvement of olfactory performance were detectable. Based on tract-tracing meta-analysis, we speculate that this may be due to indirect connections between the striatum and the OB.

Unraveling the Role of Dopaminergic and Calretinin Interneurons in the Olfactory Bulb

The perception and discriminating of odors are sensory activities that are an integral part of our daily life. The first brain region where odors are processed is the olfactory bulb (OB). Among the different cell populations that make up this brain area, interneurons play an essential role in this sensory activity. Moreover, probably because of their activity, they represent an exception compared to other parts of the brain, since OB interneurons are continuously generated in the postnatal and adult period. In this review, we will focus on periglomerular (PG) cells which are a class of interneurons found in the glomerular layer of the OB. These interneurons can be classified into distinct subtypes based on their neurochemical nature, based on the neurotransmitter and calcium-binding proteins expressed by these cells. Dopaminergic (DA) periglomerular cells and calretinin (CR) cells are among the newly generated interneurons and play an important role in the physiology of OB. In the OB, DA cells are involved in the processing of odors and the adaptation of the bulbar network to external conditions. The main role of DA cells in OB appears to be the inhibition of glutamate release from olfactory sensory fibers. Calretinin cells are probably the best morphologically characterized interneurons among PG cells in OB, but little is known about their function except for their inhibitory effect on noisy random excitatory signals arriving at the main neurons. In this review, we will mainly describe the electrophysiological properties related to the excitability profiles of DA and CR cells, with a particular view on the differences that characterize DA mature interneurons from cells in different stages of adult neurogenesis.

The role of dopamine receptors in lymphocytes and their changes in schizophrenia

Dopamine and its 5 receptors, which are grouped into two families (D1-like and D2-like), modulate functions at a systemic level in both the central nervous system and periphery. The central nervous system and the immune system are the main adaptive systems, which participate in a continuous and functional crosstalk to guarantee homeostasis. On binding to its 5 dopamine receptors, dopamine acts as a co-regulator of the immune system, contributing to the interaction of the central nervous system and inflammatory events and as a source of communication between the different immune cells. Dopaminergic perturbations in the central nervous system are observed in several neurological and psychiatric disorders. Schizophrenia is one of the most common mental disorders with a poorly understood pathoaetiology that includes genetic and environmental components that promote alterations in the dopaminergic system. Interestingly, abnormalities in dopamine receptors expression in lymphocytes of schizophrenia patients have been reported, often significantly correlating with the severity of the psychotic illness. Here, we review the current literature regarding the dopaminergic system in human lymphocytes and its alterations in schizophrenia.

1.

The existence of DA in the bloodstream suggests the presence of dopaminergic components that modulate functions at a systemic level; therefore, its effects are not limited to the CNS and the signalling in the neuronal dopaminergic system should be independent from that of the peripheral systems.

2.

The effects by DA-mediated activation of different DRs on immune cells show different sensitivities to DA, but binding profiles of DA on T cells are similar to those in neuronal membranes, suggesting receptors act similarly to those found in neurons.

3.

All DRs are expressed on the LYM membrane. However, more detailed information is required on the expression patterns of DR in immune cells in healthy conditions and in pathologies.

4.

DA has been observed to influence LYM functions acting in a variety of important processes, like cytokine secretion, cell adhesion, chemotaxis, and cytotoxicity.

5.

In human LYM, DA on D1-like receptors decreases oxidative metabolism and apoptosis, activates the selective secretion of IL-10 and TNFα, and facilitates NK cells. In contrast, most of the immunostimulatory DA effects on LYM depend on stimulation of D2-like receptors including activation, proliferation, differentiation, and suppression of NK cells.

6.

To date, an altered expression or signalling of neurotransmitter receptors is observed in immune cells during psychiatric disorders and, consequently, these cells also markedly respond to antipsychotics.

7.

Numerous technologies have been used in search of biomarkers for SCZ. However, after a century of studying SCZ their application in psychiatry remains rare and there are currently no validated biomarkers for the diagnosis and prognosis of patients with SCZ or the prediction of treatment efficacy.

Dopaminergic and serotonergic changes in rabbit fetal brain upon repeated gestational exposure to diesel engine exhaust

Limited studies in humans and in animal models have investigated the neurotoxic risks related to a gestational exposure to diesel exhaust particles (DEP) on the embryonic brain, especially those regarding monoaminergic systems linked to neurocognitive disorders. We previously showed that exposure to DEP alters monoaminergic neurotransmission in fetal olfactory bulbs and modifies tissue morphology along with behavioral consequences at birth in a rabbit model. Given the anatomical and functional connections between olfactory and central brain structures, we further characterized their impacts in brain regions associated with monoaminergic neurotransmission. At gestational day 28 (GD28), fetal rabbit brains were collected from dams exposed by nose-only to either a clean air or filtered DEP for 2 h/day, 5 days/week, from GD3 to GD27. HPLC dosage and histochemical analyses of the main monoaminergic systems, i.e., dopamine (DA), noradrenaline (NA), and serotonin (5-HT) and their metabolites were conducted in microdissected fetal brain regions. DEP exposure increased the level of DA and decreased the dopaminergic metabolites ratios in the prefrontal cortex (PFC), together with sex-specific alterations in the hippocampus (Hp). In addition, HVA level was increased in the temporal cortex (TCx). Serotonin and 5-HIAA levels were decreased in the fetal Hp. However, DEP exposure did not significantly modify NA levels, tyrosine hydroxylase, tryptophan hydroxylase or AChE enzymatic activity in fetal brain. Exposure to DEP during fetal life results in dopaminergic and serotonergic changes in critical brain regions that might lead to detrimental potential short-term neural disturbances as precursors of long-term neurocognitive consequences.

COVID-19 and Parkinson's disease: Defects in neurogenesis as the potential cause of olfactory system impairments and anosmia

Anosmia, a neuropathogenic condition of loss of smell, has been recognized as a key pathogenic hallmark of the current pandemic SARS-CoV-2 infection responsible for COVID-19. While the anosmia resulting from olfactory bulb (OB) pathology is the prominent clinical characteristic of Parkinson's disease (PD), SARS-CoV-2 infection has been predicted as a potential risk factor for developing Parkinsonism-related symptoms in a significant portion of COVID-19 patients and survivors. SARS-CoV-2 infection appears to alter the dopamine system and induce the loss of dopaminergic neurons that have been known to be the cause of PD. However, the underlying biological basis of anosmia and the potential link between COVID-19 and PD remains obscure. Ample experimental studies in rodents suggest that the occurrence of neural stem cell (NSC) mediated neurogenesis in the olfactory epithelium (OE) and OB is important for olfaction. Though the occurrence of neurogenesis in the human forebrain has been a subject of debate, considerable experimental evidence strongly supports the incidence of neurogenesis in the human OB in adulthood. To note, various viral infections and neuropathogenic conditions including PD with olfactory dysfunctions have been characterized by impaired neurogenesis in OB and OE. Therefore, this article describes and examines the recent reports on SARS-CoV-2 mediated OB dysfunctions and defects in the dopaminergic system responsible for PD. Further, the article emphasizes that COVID-19 and PD associated anosmia could result from the regenerative failure in the replenishment of the dopaminergic neurons in OB and olfactory sensory neurons in OE.

Adult Endogenous Dopaminergic Neuroregeneration Against Parkinson's Disease: Ideal Animal Models?

Parkinson's disease (PD) is the second most common neurodegenerative disease. The etiology of PD remains an enigma with no available disease modifying treatment or cure. Pharmacological compensation is the only quality of life improving treatments available. Endogenous dopaminergic neuroregeneration has recently been considered a plausible therapeutic strategy for PD. However, researchers have to first decipher the complexity of adult endogenous neuroregeneration. This raises the need of animal models to understand the underlying molecular basis. Mammalian models with highly conserved genetic homology might aid researchers to identify specific molecular mechanisms. However, the scarcity of adult neuroregeneration potential in mammals obfuscates such investigations. Nowadays, non-mammalian models are gaining popularity due to their explicit ability to neuroregenerate naturally without the need of external enhancements, yet these non-mammals have a much diverse gene homology that critical molecular signals might not be conserved across species. The present review highlights the advantages and disadvantages of both mammalian and non-mammalian animal models that can be essentially used to study the potential of endogenous DpN regeneration against PD.

Illuminating and Sniffing Out the Neuromodulatory Roles of Dopamine in the Retina and Olfactory Bulb

In the central nervous system, dopamine is well-known as the neuromodulator that is involved with regulating reward, addiction, motivation, and fine motor control. Yet, decades of findings are revealing another crucial function of dopamine: modulating sensory systems. Dopamine is endogenous to subsets of neurons in the retina and olfactory bulb (OB), where it sharpens sensory processing of visual and olfactory information. For example, dopamine modulation allows the neural circuity in the retina to transition from processing dim light to daylight and the neural circuity in the OB to regulate odor discrimination and detection. Dopamine accomplishes these tasks through numerous, complex mechanisms in both neural structures. In this review, we provide an overview of the established and emerging research on these mechanisms and describe similarities and differences in dopamine expression and modulation of synaptic transmission in the retinas and OBs of various vertebrate organisms. This includes discussion of dopamine neurons’ morphologies, potential identities, and biophysical properties along with their contributions to circadian rhythms and stimulus-driven synthesis, activation, and release of dopamine. As dysregulation of some of these mechanisms may occur in patients with Parkinson’s disease, these symptoms are also discussed. The exploration and comparison of these two separate dopamine populations shows just how remarkably similar the retina and OB are, even though they are functionally distinct. It also shows that the modulatory properties of dopamine neurons are just as important to vision and olfaction as they are to motor coordination and neuropsychiatric/neurodegenerative conditions, thus, we hope this review encourages further research to elucidate these mechanisms.

The Zebrafish Amygdaloid Complex - Functional Ground Plan, Molecular Delineation, and Everted Topology

In mammals and other tetrapods, a multinuclear forebrain structure, called the amygdala, forms the neuroregulatory core essential for emotion, cognition, and social behavior. Currently, higher circuits of affective behavior in anamniote non-tetrapod vertebrates (“fishes”) are poorly understood, preventing a comprehensive understanding of amygdala evolution. Through molecular characterization and evolutionary-developmental considerations, we delineated the complex amygdala ground plan of zebrafish, whose everted telencephalon has made comparisons to the evaginated forebrains of tetrapods challenging. In this radical paradigm, thirteen telencephalic territories constitute the zebrafish amygdaloid complex and each territory is distinguished by conserved molecular properties and structure-functional relationships with other amygdaloid structures. Central to our paradigm, the study identifies the teleostean amygdaloid nucleus of the lateral olfactory tract (nLOT), an olfactory integrative structure that links dopaminergic telencephalic groups to the amygdala alongside redefining the putative zebrafish olfactory pallium (“Dp”). Molecular characteristics such as the distribution of substance P and the calcium-binding proteins parvalbumin (PV) and calretinin (CR) indicate, that the zebrafish extended centromedial (autonomic and reproductive) amygdala is predominantly located in the GABAergic and isl1-negative territory. Like in tetrapods, medial amygdaloid (MeA) nuclei are defined by the presence of substance P immunoreactive fibers and calretinin-positive neurons, whereas central amygdaloid (CeA) nuclei lack these characteristics. A detailed comparison of lhx5-driven and vGLut2a-driven GFP in transgenic reporter lines revealed ancestral topological relationships between the thalamic eminence (EmT), the medial amygdala (MeA), the nLOT, and the integrative olfactory pallium. Thus, the study explains how the zebrafish amygdala and the complexly everted telencephalon topologically relate to the corresponding structures in mammals indicating that an elaborate amygdala ground plan evolved early in vertebrates, in a common ancestor of teleosts and tetrapods.

The Impact of Mitochondrial Dysfunction on Dopaminergic Neurons in the Olfactory Bulb and Odor Detection

Understanding non-motor symptoms of Parkinson’s disease is important in order to unravel the underlying molecular mechanisms of the disease. Olfactory dysfunction is an early stage, non-motor symptom which occurs in 95% of Parkinson’s disease patients. Mitochondrial dysfunction is a key feature in Parkinson’s disease and importantly contributes to the selective loss of dopaminergic neurons the substantia nigra pars compacta. The olfactory bulb, the first olfactory processing station, also contains dopaminergic neurons, which modulate odor information and thereby enable odor detection as well as odor discrimination. MitoPark mice are a genetic model for Parkinson’s disease with severe mitochondrial dysfunction, reproducing the differential vulnerability of dopaminergic neurons in the midbrain. These animals were used to investigate the impact of mitochondrial dysfunction on olfactory-related behavior and olfactory bulb dopaminergic neuron survival. Odor detection was severely impaired in MitoPark mice. Interestingly, only the small anaxonic dopaminergic subpopulation, which is continuously replenished by neurogenesis, was moderately reduced in number, much less compared with dopaminergic neurons in the midbrain. As a potential compensatory response, an enhanced mobilization of progenitor cells was found in the subventricular zone. These results reveal a high robustness of dopaminergic neurons located in the olfactory bulb towards mitochondrial impairment, in striking contrast to their midbrain counterparts.

Dopaminergic Modulation of Glomerular Circuits in the Mouse Olfactory Bulb

Dopaminergic neurons are located in several brain areas including the olfactory bulb (OB) and involved in many physiological and pathophysiological processes. In the OB, dopamine (DA) is released exclusively by a population of interneurons termed short axon cells (SACs) in the glomerular layer, the initial synaptic integration site of the whole olfactory system. SACs corelease GABA and extend their processes to many glomeruli forming the interglomerular circuit. Two major groups of DA receptors D1-like (D1LRs) and D2-like (D2LRs) types are differentially distributed in the OB, i.e., D1LRs are broadly present except the most superficial olfactory nerve (ON) layer while D2LRs are predominantly confined to the ON and glomerular layers, suggesting that they mediate different physiological functions. In contrast to the well-known D2LR-mediated presynaptic inhibition of ON terminals in the OB, the cellular and circuit targets of the D1LR-mediated DA actions remain unclear even though D1LR activation improves odor detection and discrimination. We recently demonstrated that endogenous DA released from SACs or exogenous DA excites a population of excitatory glomerular neurons termed external tufted cells (ETCs) via D1LRs. But the physiological significance of this D1LR activation is largely unknown. In the present study, we addressed these questions by a systematic examination of exogenous DA actions on synaptic activities and excitabilities in most glomerular neurons and OB output neurons with the following major findings: (1) DA via D1LRs enhances OB output by potentiating the ETC-mediated feedforward excitation to the OB output neurons but suppresses spontaneous excitatory synaptic activities in both types of inhibitory glomerular interneurons periglomerular (PGCs) and SACs; (2) this suppression of excitatory synaptic activities in PGCs and SACs depends on activation of GABA_B receptors; (3) DA via D1LRs augments spontaneous inhibitory synaptic activities in all glomerular neurons and OB output neurons; (4) DA selectively activates SACs via D1LRs. These findings suggest that activation of D1LRs elevates the system’s sensitivity to odor stimuli and provide a mechanistic basis for the functional roles of DA in modulating odor detection and discrimination.

Spiking and Membrane Properties of Rat Olfactory Bulb Dopamine Neurons

The mammalian olfactory bulb (OB) has a vast population of dopamine (DA) neurons, whose function is to increase odor discrimination through mostly inhibitory synaptic mechanisms. However, it is not well understood whether there is more than one neuronal type of OB DA neuron, how these neurons respond to different stimuli, and the ionic mechanisms behind those responses. In this study, we used a transgenic rat line (hTH-GFP) to identify fluorescent OB DA neurons for recording via whole-cell electrophysiology. These neurons were grouped based on their localization in the glomerular layer ("Top" vs. "Bottom") with these largest and smallest neurons grouped by neuronal area ("Large" vs. "Small," in μm²). We found that some membrane properties could be distinguished based on a neuron's area, but not by its glomerular localization. All OB DA neurons produced a single action potential when receiving a sufficiently depolarizing stimulus, while some could also spike multiple times when receiving weaker stimuli, an activity that was more likely in Large than Small neurons. This single spiking activity is likely driven by the Na⁺ current, which showed a sensitivity to inactivation by depolarization and a relatively long time constant for the removal of inactivation. These recordings showed that Small neurons were more sensitive to inactivation of Na⁺ current at membrane potentials of -70 and -60 mV than Large neurons. The hyperpolarization-activated H-current (identified by voltage sags) was more pronounced in Small than Large DA neurons across hyperpolarized membrane potentials. Lastly, to mimic a more physiological stimulus, these neurons received ramp stimuli of various durations and current amplitudes. When stimulated with weaker/shallow ramps, the neurons needed less current to begin and end firing and they produced more action potentials at a slower frequency. These spiking properties were further analyzed between the four groups of neurons, and these analyses support the difference in spiking induced with current step stimuli. Thus, there may be more than one type of OB DA neuron, and these neurons' activities may support a possible role of being high-pass filters in the OB by allowing the transmission of stronger odor signals while inhibiting weaker ones.

A chromosomal connectome for psychiatric and metabolic risk variants in adult dopaminergic neurons

BACKGROUND

Midbrain dopaminergic neurons (MDN) represent 0.0005% of the brain's neuronal population and mediate cognition, food intake, and metabolism. MDN are also posited to underlay the neurobiological dysfunction of schizophrenia (SCZ), a severe neuropsychiatric disorder that is characterized by psychosis as well as multifactorial medical co-morbidities, including metabolic disease, contributing to markedly increased morbidity and mortality. Paradoxically, however, the genetic risk sequences of psychosis and traits associated with metabolic disease, such as body mass, show very limited overlap.

METHODS

We investigated the genomic interaction of SCZ with medical conditions and traits, including body mass index (BMI), by exploring the MDN's "spatial genome," including chromosomal contact landscapes as a critical layer of cell type-specific epigenomic regulation. Low-input Hi-C protocols were applied to 5-10 × 103 dopaminergic and other cell-specific nuclei collected by fluorescence-activated nuclei sorting from the adult human midbrain.

RESULTS

The Hi-C-reconstructed MDN spatial genome revealed 11 "Euclidean hot spots" of clustered chromatin domains harboring risk sequences for SCZ and elevated BMI. Inter- and intra-chromosomal contacts interconnecting SCZ and BMI risk sequences showed massive enrichment for brain-specific expression quantitative trait loci (eQTL), with gene ontologies, regulatory motifs and proteomic interactions related to adipogenesis and lipid regulation, dopaminergic neurogenesis and neuronal connectivity, and reward- and addiction-related pathways.

CONCLUSIONS

We uncovered shared nuclear topographies of cognitive and metabolic risk variants. More broadly, our PsychENCODE sponsored Hi-C study offers a novel genomic approach for the study of psychiatric and medical co-morbidities constrained by limited overlap of their respective genetic risk architectures on the linear genome.

Dopamine-induced calcium signaling in olfactory bulb astrocytes

It is well established that astrocytes respond to the major neurotransmitters glutamate and GABA with cytosolic calcium rises, whereas less is known about the effect of dopamine on astroglial cells. In the present study, we used confocal calcium imaging in mouse brain slices of the olfactory bulb, a brain region with a large population of dopaminergic neurons, to investigate calcium signaling evoked by dopamine in astrocytes. Our results show that application of dopamine leads to a dose-dependent cytosolic calcium rise in astrocytes (EC₅₀ = 76 µM) which is independent of neuronal activity and mainly mediated by PLC/IP₃-dependent internal calcium release. Antagonists of both D₁- and D₂-class dopamine receptors partly reduce the dopaminergic calcium response, indicating that both receptor classes contribute to dopamine-induced calcium transients in olfactory bulb astrocytes.

Comparative Analysis of the Organization of the Catecholaminergic Systems in the Brain of Holostean Fishes (Actinopterygii/Neopterygii)

Living holosteans, comprising 8 species of bowfins and gars, form a small monophyletic group of actinopterygian fishes, which are currently considered as the sister group to the enormously numerous teleosts and have largely been neglected in neuroanatomical studies. We have studied the catecholaminergic (CAergic) systems by means of antibodies against tyrosine hydroxylase (TH) and dopamine (DA) in the brain of representative species of the 3 genera included in the 2 orders of holostean fishes: Amia calva (Amiiformes) and Lepisosteus platyrhincus, Lepisosteus oculatus, and Atractosteus spatula (Lepisosteiformes). Different groups of TH/DA-immunoreactive (ir) cells were observed in the olfactory bulb, subpallium, and preoptic area of the telencephalon. Hypothalamic groups were labeled in the suprachiasmatic nucleus, tuberal (only in A. calva), retrotuberal, and retromamillary areas; specifically, the paraventricular organ showed only DA immunoreactivity. In the diencephalon, TH/DA-ir groups were detected in the prethalamus, posterior tubercle, and pretectum. In the caudal hindbrain, the solitary tract nucleus and area postrema presented TH/DA-ir cell groups, and also the spinal cord and the retina. Only in A. calva, particular CAergic cell groups were observed in the habenula, the mesencephalic tegmentum, and in the locus coeruleus. Following a neuromeric analysis, the comparison of these results with those obtained in other classes of fishes and tetrapods shows many common traits of CAergic systems shared by most vertebrates and in addition highlights unique features of actinopterygian fishes.

The A30P α-synuclein mutation decreases subventricular zone proliferation

Parkinson's disease (PD) is associated with olfactory defects in addition to dopaminergic degeneration. Dopaminergic signalling is necessary for subventricular zone (SVZ) proliferation and olfactory bulb (OB) neurogenesis. Alpha-synuclein (α-syn or Snca) modulates dopaminergic neurotransmission, and SNCA mutations cause familial PD, but how α-syn and its mutations affect adult neurogenesis is unclear. To address this, we studied a bacterial artificial chromosome transgenic mouse expressing the A30P SNCA familial PD point mutation on an Snca-/- background. We confirmed that the SNCA-A30P transgene recapitulates endogenous α-syn expression patterns and levels by immunohistochemical detection of endogenous α-syn in a wild-type mouse and transgenic SNCA-A30P α-syn protein in the forebrain. The number of SVZ stem cells (BrdU+GFAP+) was decreased in SNCA-A30P mice, whereas proliferating (phospho-histone 3+) cells were decreased in Snca-/- and even more so in SNCA-A30P mice. Similarly, SNCA-A30P mice had fewer Mash1+ transit-amplifying SVZ progenitor cells but Snca-/- mice did not. These data suggest the A30P mutation aggravates the effect of Snca loss in the SVZ. Interestingly, calbindin+ and calretinin (CalR)+ periglomerular neurons were decreased in both Snca-/-, and SNCA-A30P mice but tyrosine hydroxylase+ periglomerular OB neurons were only decreased in Snca-/- mice. Cell death decreased in the OB granule layer of Snca-/- and SNCA-A30P mice. In the same region, CalR+ numbers increased in Snca-/- and SNCA-A30P mice. Thus, α-syn loss and human A30P SNCA decrease SVZ proliferation, cell death in the OB and differentially alter interneuron numbers. Similar disruptions in human neurogenesis may contribute to the olfactory deficits, which are observed in PD.

Differentiation of human mesenchymal stem cells (MSC) to dopaminergic neurons: A comparison between Wharton's Jelly and olfactory mucosa as sources of MSCs

The generation of dopaminergic neurons from stem cells is a potential therapeutic approach to treat neurodegenerative disorders, such as Parkinson's disease. The current study aims to investigate the potential of two different types of mesenchymal stem cells derived from human Wharton's jelly and nasal cavity for differentiation into dopaminergic neurons. The differentiation capacities of both cell types were evaluated using real-time PCR, immunocytochemistry, flow cytometry and HPLC. Wharton's jelly-derived mesenchymal stem cells (WJ-MSCs) are noted for their capability to differentiate into mesodermal and non-mesodermal cells, including neurons. However, it was demonstrated that having the same neuroectodermal origin as the nervous system, the olfactory ectomesenchymal stem cells (OE-MSCs) expressed the neural marker MAP2 as well as dopaminergic markers such as tyrosine hydroxylase (TH), dopamine transporter (DAT) and PITX3 to a greater extent than the WJ-MSCs both at the level of mRNA and protein. Furthermore, quantitative flow cytometric evaluation of these markers at 12 days post-induction supported the above-mentioned results. Finally, the assessment of the functionality of differentiated cells and their ability to synthesize dopamine measured by HPLC revealed that the OE-MSC-derived dopaminergic cells released almost the same amount of dopamine as that secreted by WJ-MSC-derived cells. Thus it showed the difference in their functionality to be negligible. Overall, it may be concluded that higher proliferation and differentiation capacity of OE-MSCs, along with their easier harvestability and autologous transplantability compared with WJ-MSCs, makes them a better cell source for stem cell therapy of neurodegenerative disorders which are caused by degeneration of dopaminergic neurons.

Genetic labeling reveals temporal and spatial expression pattern of D2 dopamine receptor in rat forebrain

The D2 dopamine receptor (Drd2) is implicated in several brain disorders such as schizophrenia, Parkinson’s disease, and drug addiction. Drd2 is also the primary target of both antipsychotics and Parkinson’s disease medications. Although the expression pattern of Drd2 is relatively well known in mouse brain, the temporal and spatial distribution of Drd2 is lesser clear in rat brain due to the lack of Drd2 reporter rat lines. Here, we used CRISPR/Cas9 techniques to generate two knockin rat lines: Drd2::Cre and Rosa26::loxp-stop-loxp-tdTomato. By crossing these two lines, we produced Drd2 reporter rats expressing the fluorescence protein tdTomato under the control of the endogenous Drd2 promoter. Using fluorescence imaging and unbiased stereology, we revealed the cellular expression pattern of Drd2 in adult and postnatal rat forebrain. Strikingly, the Drd2 expression pattern differs between Drd2 reporter rats and Drd2 reporter mice generated by BAC transgene in prefrontal cortex and hippocampus. These results provide fundamental information needed for the study of Drd2 function in rat forebrain. The Drd2::Cre rats generated here may represent a useful tool to study the function of neuronal populations expressing Drd2.

The pleiotropic transcriptional regulator COUP-TFI plays multiple roles in neural development and disease

Transcription factors are expressed in a dynamic fashion both in time and space during brain development, and exert their roles by activating a cascade of multiple target genes. This implies that understanding the precise function of a transcription factor becomes a challenging task. In this review, we will focus on COUP-TFI (or NR2F1), a nuclear receptor belonging to the superfamily of the steroid/thyroid hormone receptors, and considered to be one of the major transcriptional regulators orchestrating cortical arealization, cell-type specification and maturation. Recent data have unraveled the multi-faceted functions of COUP-TFI in the development of several mouse brain structures, including the neocortex, hippocampus and ganglionic eminences. Despite NR2F1 mutations and deletions in humans have been linked to a complex neurodevelopmental disease mainly associated to optic atrophy and intellectual disability, its role during the formation of the retina and optic nerve remains unclear. In light of its major influence in cortical development, we predict that its haploinsufficiency might be the cause of other cognitive diseases, not identified so far. Mouse models offer a unique opportunity of dissecting COUP-TFI function in different regions during brain assembly; hence, the importance of comparing and discussing common points linking mouse models to human patients' symptoms.

Effects of Pinealectomy and Gonadectomy on Olfactory Bulb Dopaminergic Neurons in Rats

Background:

Olfactory disorder is an early manifestation of Parkinson's disease (PD), likely to be associated with abnormalities of the dopaminergic neurons in the olfactory bulb (OB); however, the causes of olfactory disorder in PD are not entirely clear. Some studies showed that melatonin (MT) and androgens (mainly testosterone, T) might participate in the pathogenesis of PD. The research aimed to investigate effects of MT or T deficiency on OB dopaminergic neurons in rats.

Methods:

One hundred and twenty normal male Wistar rats were randomly divided into the control, sham operation pinealectomy (PX), sham operation gonadectomy (GDX), PX, GDX, and PX + GDX groups. After 60 days, glial cell hyperplasia and neuronal apoptosis were examined with hematoxylin and eosin and the TUNEL method; the expression levels of tyrosine hydroxylase (TH), Bax, and Bcl-2 were measured using immunohistochemistry (IH) by the streptavidin peroxidase conjugated method. Comparison among multiple sets used analysis of variance and LSD method or Kruskal-Wallis test and Nemenyi method.

Results:

There were no significant differences between the sham operation groups and the control group; thus, they were merged into Group A. There was no significant glial cell hyperplasia (P > 0.05) or change in shape in any of the groups after PX or GDX. The number of apoptotic cells in Groups A (1.41 ± 0.56), PX (12.31 ± 4.68), GDX (20.52 ± 5.13), and PX + GDX (30.23 ± 5.25) successively significantly increased (P < 0.05). The number of TH (+) cells in Groups A (42.62 ± 5.63), PX (37.31 ± 4.32), GDX (31.07 ± 4.21), and PX + GDX (25.22 ± 3.66) was successively significantly decreased (P < 0.05). The gray value of TH (+) cells and fibers in Groups A (98.51 ± 10.36), PX (108.96 ± 13.01), GDX (119.02 ± 12.98), and PX + GDX (128.99 ± 13.39) was successively significantly increased (P < 0.05). The results of Bax staining were as follows: Group A+, Group PX++, Group GDX++, and Group PX+ GDX+++, the results of Bcl-2 in all groups were +.

Conclusions:

PX or GDX could lead to OB neurotoxicity in the following groups of rats in the following order: PX < GDX < PX + GDX. PX or GDX increased the ratio of Bax/Bcl-2. The effect of PX and GDX was equal, but both were less than that of PX + GDX. Neurotoxicity as a result of PX or GDX was not related to inflammation.

The Stem Cell Marker Lgr5 Defines a Subset of Postmitotic Neurons in the Olfactory Bulb

Lgr5, leucine-rich repeat-containing G-protein coupled receptor 5, is a bona fide biomarker for stem cells in multiple tissues. Lgr5 is also expressed in the brain, but the identities and properties of these Lgr5⁺ cells are still elusive. Using an Lgr5-EGFP reporter mouse line, we found that, from early development to adulthood, Lgr5 is highly expressed in the olfactory bulb (OB), an area with ongoing neurogenesis. Immunostaining with stem cell, glial, and neuronal markers reveals that Lgr5 does not label stem cells in the OB but instead labels a heterogeneous population of neurons with preference in certain subtypes. Patch-clamp recordings in OB slices reveal that Lgr5-EGFP⁺ cells fire action potentials and display spontaneous excitatory postsynaptic events, indicating that these neurons are integrated into OB circuits. Interestingly, R-spondin 3, a potential ligand of Lgr5, is also expressed in the adult OB. Collectively, our data indicate that Lgr5-expressing cells in the OB are fully differentiated neurons and imply distinct roles of Lgr5 and its ligand in postmitotic cells.SIGNIFICANCE STATEMENT Lgr5 (leucine-rich repeat-containing G-protein coupled receptor 5) is a bona fide stem cell marker in many body organs. Here we report that Lgr5 is also highly expressed in the olfactory bulb (OB), the first relay station in the brain for processing odor information and one of the few neural structures that undergo continuous neurogenesis. Surprisingly, Lgr5 is not expressed in the OB stem cells, but instead in a few subtypes of terminally differentiated neurons, which are incorporated into the OB circuit. This study reveals that Lgr5⁺ cells in the brain represent a nonstem cell lineage, implying distinct roles of Lgr5 in postmitotic neurons.

Differential spatial expression of peripheral olfactory neuron-derived BACE1 induces olfactory impairment by region-specific accumulation of β-amyloid oligomer

Olfactory dysfunction is a common symptom associated with neurodegenerative diseases including Alzheimer's disease (AD). Although evidence exists to suggest that peripheral olfactory organs are involved in the olfactory dysfunction that accompanies AD pathology, the underlying mechanisms are not fully understood. As confirmed using behavioral tests, transgenic mice overexpressing a Swedish mutant form of human amyloid precursor proteins exhibited olfactory impairments prior to evidence of cognitive impairment. By measuring the expression of tyrosine hydroxylase, we observed that specific regions of the olfactory bulb (OB) in Tg2576 mice, specifically the ventral portion exhibited significant decreases in the number of dopaminergic neurons in the periglomerular regions from the early stage of AD. To confirm the direct linkage between these olfactory impairments and AD-related pathology, β-site amyloid precursor protein cleaving enzyme 1 (BACE1)-the initiating enzyme in Aβ genesis-and β-amyloid peptide (Aβ), hallmarks of AD were analyzed. We found that an increase in BACE1 expression coincided with an elevation of amyloid-β (Aβ) oligomers in the ventral region of OB. Moreover, olfactory epithelium (OE), in particular the ectoturbinate in which axons of olfactory sensory neurons (OSNs) have direct connections with the dendrites of mitral/tufted cells in the ventral part of OB, exhibited significant decreases in both thickness and cell number even at early stages. This result suggests that Aβ oligomer toxicity in the OE may have induced a decline in the number of OSNs and functional impairment of the olfactory system. We first demonstrated that disproportionate levels of regional damage in the peripheral olfactory system may be a specific symptom of AD with Aβ oligomer accumulation occurring prior to damage within the CNS. This regional damage in the olfactory system early in the progression of AD may be closely related to AD-related pathological abnormality and olfactory dysfunction found in AD patients.

Inhibitory circuits of the mammalian main olfactory bulb

Synaptic inhibition critically influences sensory processing throughout the mammalian brain, including the main olfactory bulb (MOB), the first station of sensory processing in the olfactory system. Decades of research across numerous laboratories have established a central role for granule cells (GCs), the most abundant GABAergic interneuron type in the MOB, in the precise regulation of principal mitral and tufted cell (M/TC) firing rates and synchrony through lateral and recurrent inhibitory mechanisms. In addition to GCs, however, the MOB contains a vast diversity of other GABAergic interneuron types, and recent findings suggest that, while fewer in number, these oft-ignored interneurons are just as important as GCs in shaping odor-evoked M/TC activity. Here I challenge the prevailing centrality of GCs. In this review, I first outline the specific properties of each GABAergic interneuron type in the rodent MOB, with particular emphasis placed on direct interneuron recordings and cell type-selective manipulations. On the basis of these properties, I then critically reevaluate the contribution of GCs vs. other interneuron types to the regulation of odor-evoked M/TC firing rates and synchrony via lateral, recurrent, and other inhibitory mechanisms. This analysis yields a novel model in which multiple interneuron types with distinct abundances, connectivity patterns, and physiologies complement one another to regulate M/TC activity and sensory processing.

Perception in attention deficit hyperactivity disorder

A large body of research demonstrated that individuals with attention deficit hyperactivity disorder (ADHD) suffer from various neuropsychological deficits. In contrast, less is known and only divergent evidence exists on perceptual functions of individuals with ADHD. This is problematic as neuropsychological and perceptual functions are closely interrelated and are often difficult to disentangle in behavioral assessments. This study presents the conduct and results of a systematic literature review on perceptual functions in children and adults with ADHD. This review considers studies using psychophysical methods (objective measurements) and self- and informant reports (subjective measurements). Results indicate that individuals with ADHD have altered perceptual functions in various domains as compared to typically developing individuals. Increased perceptual functions in individuals with ADHD were found with regard to olfactory detection thresholds, whereas reduced perceptual functions were evident for aspects of visual and speech perception. Moreover, individuals with ADHD were found to experience discomfort to sensory stimuli at a lower level than typically developing individuals. Alterations of perceptual functions in individuals with ADHD were shown to be moderated by various factors, such as pharmacological treatment, cognitive functions, and symptom severity. We conclude by giving implications for daily life functioning and clinical practice.

Ca2+-permeable AMPA receptors in mouse olfactory bulb astrocytes

Ca²⁺ signaling in astrocytes is considered to be mainly mediated by metabotropic receptors linked to intracellular Ca²⁺ release. However, recent studies demonstrate a significant contribution of Ca²⁺ influx to spontaneous and evoked Ca²⁺ signaling in astrocytes, suggesting that Ca²⁺ influx might account for astrocytic Ca²⁺ signaling to a greater extent than previously thought. Here, we investigated AMPA-evoked Ca²⁺ influx into olfactory bulb astrocytes in mouse brain slices using Fluo-4 and GCaMP6s, respectively. Bath application of AMPA evoked Ca²⁺ transients in periglomerular astrocytes that persisted after neuronal transmitter release was inhibited by tetrodotoxin and bafilomycin A1. Withdrawal of external Ca²⁺ suppressed AMPA-evoked Ca²⁺ transients, whereas depletion of Ca²⁺ stores had no effect. Both Ca²⁺ transients and inward currents induced by AMPA receptor activation were partly reduced by Naspm, a blocker of Ca²⁺-permeable AMPA receptors lacking the GluA2 subunit. Antibody staining revealed a strong expression of GluA1 and GluA4 and a weak expression of GluA2 in periglomerular astrocytes. Our results indicate that Naspm-sensitive, Ca²⁺-permeable AMPA receptors contribute to Ca²⁺ signaling in periglomerular astrocytes in the olfactory bulb.

Stem cell therapies in age-related neurodegenerative diseases and stroke

Aging, a complex process associated with various structural, functional and metabolic changes in the brain, is an important risk factor for neurodegenerative diseases and stroke. These diseases share similar neuropathological changes, such as the formation of misfolded proteins, oxidative stress, loss of neurons and synapses, dysfunction of the neurovascular unit (NVU), reduction of self-repair capacity, and motor and/or cognitive deficiencies. In addition to gray matter dysfunction, the plasticity and repair capacity of white matter also decrease with aging and contribute to neurodegenerative diseases. Aging not only renders patients more susceptible to these disorders, but also attenuates their self-repair capabilities. In addition, low drug responsiveness and intolerable side effects are major challenges in the prevention and treatment of senile diseases. Thus, stem cell therapies-characterized by cellular plasticity and the ability to self-renew-may be a promising strategy for aging-related brain disorders. Here, we review the common pathophysiological changes, treatments, and the promises and limitations of stem cell therapies in age-related neurodegenerative diseases and stroke.

Dopaminergic Neurones in the Main Olfactory Bulb: An Overview from an Electrophysiological Perspective

The olfactory bulb (OB), the first center processing olfactory information, is characterized by a vigorous life-long activity-dependent plasticity responsible for a variety of odor-evoked behavioral responses. It hosts the more numerous group of dopaminergic (DA) neurones in the central nervous system, cells strategically positioned at the entry of the bulbar circuitry, directly in contact with the olfactory nerve terminals, which play a key role in odor processing and in the adaptation of the bulbar network to external conditions. Here, we focus mainly on the electrophysiological properties of DA interneurones, reviewing findings concerning their excitability profiles in adulthood and in different phases of adult neurogenesis. We also discuss dynamic changes of the DA interneurones related to environmental stimuli and their possible functional implications.

Drug-induced Parkinson's disease modulates protein kinase A and Olfactory Marker Protein in the mouse olfactory bulb

Background

Olfaction is often affected in parkinsonian patients, but dopaminergic cells in the olfactory bulb are not affected by some Parkinson-inducing drugs. We investigated whether the drug MPTP produces the olfactory deficits typical of Parkinson and affects the olfactory bulb in mice.

Findings

Lesioned and control mice were tested for olfactory search, for motor and exploratory behavior. Brains and olfactory mucosa were investigated via immunohistochemistry for thyrosine hydroxylase, Olfactory Marker Protein and cyclic AMP-dependent protein kinase as an intracellular pathway involved in dopaminergic neurotransmission. MPTP induced motor impairment, but no deficit in olfactory search. Thyrosine hydroxylase did not differ in olfactory bulb, while a strong decrease was detected in substantia nigra and tegmentum of MPTP mice. Olfactory Marker Protein decreased in the olfactory bulb of MPTP mice, while a cyclic AMP-dependent protein kinase increased in the inner granular layer of MPTP mice.

Conclusions

MPTP mice do not present behavioural deficits in olfactory search, yet immunoreactivity reveals modifications in the olfactory bulb, and suggests changes in intracellular signal processing, possibly linked to neuron survival after MPTP.

Dendritic Arborization Patterns of Small Juxtaglomerular Cell Subtypes within the Rodent Olfactory Bulb

Within the glomerular layer of the rodent olfactory bulb, numerous subtypes of local interneurons contribute to early processing of incoming sensory information. Here we have investigated dopaminergic and other small local juxtaglomerular cells in rats and mice and characterized their dendritic arborization pattern with respect to individual glomeruli by fluorescent labeling via patching and reconstruction of dendrites and glomerular contours from two-photon imaging data. Dopaminergic neurons were identified in a transgenic mouse line where the expression of dopamine transporter (DAT) was labeled with GFP. Among the DAT+ cells we found a small short-axon cell (SAC) subtype featuring hitherto undescribed dendritic specializations. These densely ramifying structures clasped mostly around somata of other juxtaglomerular neurons, which were also small, non-dopaminergic and to a large extent non-GABAergic. Clasping SACs were observed also in wild-type mice and juvenile rats. In DAT+ SAC dendrites, single backpropagating action potentials evoked robust calcium entry throughout both clasping and non-clasping compartments. Besides clasping SACs, most other small neurons either corresponded to the classical periglomerular cell type (PGCs), which was never DAT+, or were undersized cells with a small dendritic tree and low excitability. Aside from the presence of clasps in SAC dendrites, many descriptors of dendritic morphology such as the number of dendrites and the extent of branching were not significantly different between clasping SACs and PGCs. However, a detailed morphometric analysis in relation to glomerular contours revealed that the dendrites of clasping SACs arborized mostly in the juxtaglomerular space and never entered more than one glomerulus (if at all), whereas most PGC dendrites were restricted to their parent glomerulus, similar to the apical tufts of mitral cells. These complementary arborization patterns might underlie a highly complementary functional connectivity. The morphometric approach may serve to differentiate also other subtypes of juxtaglomerular neurons, help to identify putative synaptic partners and thus to establish a more refined picture of glomerular network interactions during odor sensing.

Adult Born Olfactory Bulb Dopaminergic Interneurons: Molecular Determinants and Experience-Dependent Plasticity

The olfactory bulb (OB) is a highly plastic brain region involved in the early processing of olfactory information. A remarkably feature of the OB circuits in rodents is the constitutive integration of new neurons that takes place during adulthood. Newborn cells in the adult OB are mostly inhibitory interneurons belonging to chemically, morphologically and functionally heterogeneous types. Although there is general agreement that adult neurogenesis in the OB plays a key role in sensory information processing and olfaction-related plasticity, the contribution of each interneuron subtype to such functions is far to be elucidated. Here, we focus on the dopaminergic (DA) interneurons: we highlight recent findings about their morphological features and then describe the molecular factors required for the specification/differentiation and maintenance of the DA phenotype in adult born neurons. We also discuss dynamic changes of the DA interneuron population related to age, environmental stimuli and lesions, and their possible functional implications.

The Behavior of Migraine in Patients with Parkinson's Disease

Parkinson’s disease (PD) is characterized by the degeneration of dopaminergic systems in the central nervous system. In migraine it is supposed to occur hyperactivation of central dopaminergic pathways. We verified the hypothesis of improved migraine in patients who manifest PD. We evaluated 109 patients with PD over 40 years (57 men and 52 women) about the presence throughout the life of migraine, as well as the possibility of improvement in migraine after the onset of motor symptoms of PD. This group was compared to a control group of 152 people (41 men and 152 women) without PD regarding the presence of migraine and its improvement. Twenty-one patients manifested migraine in the group with PD (16 women and 5 men) in which 13 reported improvement in migraine after the onset of symptoms of PD. Among the controls, 37 interviewed had migraine history (32 women and 5 men) among which 20 showed improvement. There was no significant difference when comparing the two groups (χ²_1:0,05=0.337; P<0.382). We were unable to relate the improvement of migraine with the emergence of PD motor signs, despite the degeneration of dopaminergic pathways of the central nervous system.

MicroRNA-7 Enhances Subventricular Zone Neurogenesis by Inhibiting NLRP3/Caspase-1 Axis in Adult Neural Stem Cells

α-Synuclein (α-syn) has been recognized to induce neuroinflammation and to disturb nerve repair process in Parkinson's disease. However, the potential mechanisms underlying α-syn-induced impairment of adult neurogenesis remain unclear. In the present study, A53T mutant α--synuclein transgenic (A53T^tg/tg) mice, caspase-1 knockout mice, and A53T^tg/tg;caspase-1^-/- double transgenic mice were used to prepare adult neural stem cells (ANSCs) and to investigate inflammasome-related mechanism for α-syn-impaired neurogenesis in mouse subventricular zone (SVZ). We showed that α-syn inhibited neurogenesis in the SVZ of A53T^tg/tg mice and impaired proliferation and differentiation in ANSCs cultured in vitro, accompanied by reduced microRNA-7 (miR-7) expression levels. We further found that ANSC expressed NLRP3-containing inflammasome and α-syn activated both TLR4/NF-κB and NLRP3/caspase-1 signals in ANSCs. Either Nlrp3 knockdown or Caspase-1 knockout could attenuate the inhibition of proliferation in ANSCs induced by α-syn. Furthermore, we demonstrated that miR-7 post-transcriptionally controlled Nlrp3 expression besides targeting α-syn. Most notably, stereotactic injection of miR-7 mimics into lateral ventricles significantly inhibited NLRP3 inflammasome activation and improved adult neurogenesis in mouse SVZ. Our study provides a direct link between NLRP3 inflammasome activation and α-syn-impaired neurogenesis in the pathogenesis of α-synucleinopathies.

Intranasal administration of rotenone in mice attenuated olfactory functions through the lesion of dopaminergic neurons in the olfactory bulb

Many environmental chemicals are thought to affect brain function. It was reported that chemicals in the nasal cavity directly reach the brain through the connection between olfactory neurons and the olfactory bulb (OB). In this 'olfactory transport,' xenobiotics absorbed at the nasal mucosa reach the brain by bypassing some physical barriers and defenses, and thus olfactory transport is suspected to be a vulnerable mechanism of the brain against invasion threats of environmental chemicals. In this study, we focused on the neuronal toxicity of rotenone administered intranasally to mice. The results showed that the mice that were administered rotenone had attenuated olfactory functions. We also found that intranasally administered rotenone induced acute mitochondrial stress at the OB. The repeated administration of rotenone resulted in a decrease in the number of dopaminergic neurons, which are inhibitory interneurons in the OB. Taken together, our findings suggest that the inhalation of environmental toxins induces the neurodegeneration of cranial neurons through olfactory transport, and that olfactory dysfunction may be induced as an earliest symptom of neurodegeneration caused by inhaled neurotoxins.

Nucleotide sequence conservation of novel and established cis-regulatory sites within the tyrosine hydroxylase gene promoter

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis and its gene proximal promoter ( < 1 kb upstream from the transcription start site) is essential for regulating transcription in both the developing and adult nervous systems. Several putative regulatory elements within the TH proximal promoter have been reported, but evolutionary conservation of these elements has not been thoroughly investigated. Since many vertebrate species are used to model development, function and disorders of human catecholaminergic neurons, identifying evolutionarily conserved transcription regulatory mechanisms is a high priority. In this study, we align TH proximal promoter nucleotide sequences from several vertebrate species to identify evolutionarily conserved motifs. This analysis identified three elements (a TATA box, cyclic AMP response element (CRE) and a 5'-GGTGG-3' site) that constitute the core of an ancient vertebrate TH promoter. Focusing on only eutherian mammals, two regions of high conservation within the proximal promoter were identified: a ∼250 bp region adjacent to the transcription start site and a ∼85 bp region located approximately 350 bp further upstream. Within both regions, conservation of previously reported cis-regulatory motifs and human single nucleotide variants was evaluated. Transcription reporter assays in a TH -expressing cell line demonstrated the functionality of highly conserved motifs in the proximal promoter regions and electromobility shift assays showed that brain-region specific complexes assemble on these motifs. These studies also identified a non-canonical CRE binding (CREB) protein recognition element in the proximal promoter. Together, these studies provide a detailed analysis of evolutionary conservation within the TH promoter and identify potential cis-regulatory motifs that underlie a core set of regulatory mechanisms in mammals.

Maintenance of postmitotic neuronal cell identity

The identity of specific cell types in the nervous system is defined by the expression of neuron type-specific gene batteries. How the expression of such batteries is initiated during nervous system development has been under intensive study over the past few decades. However, comparatively little is known about how gene batteries that define the terminally differentiated state of a neuron type are maintained throughout the life of an animal. Here we provide an overview of studies in invertebrate and vertebrate model systems that have carved out the general and not commonly appreciated principle that neuronal identity is maintained in postmitotic neurons by the sustained, and often autoregulated, expression of the same transcription factors that initiate terminal differentiation in a developing organism. Disruption of postmitotic maintenance mechanisms may result in neuropsychiatric and neurodegenerative conditions.

Adult subventricular zone neural stem cells as a potential source of dopaminergic replacement neurons

Clinical trials engrafting human fetal ventral mesencephalic tissue have demonstrated, in principle, that cell replacement therapy provides substantial long-lasting improvement of motor impairments generated by Parkinson's Disease (PD). The use of fetal tissue is not practical for widespread clinical implementation of this therapy, but stem cells are a promising alternative source for obtaining replacement cells. The ideal stem cell source has yet to be established and, in this review, we discuss the potential of neural stem cells in the adult subventricular zone (SVZ) as an autologous source of replacement cells. We identify three key challenges for further developing this potential source of replacement cells: (1) improving survival of transplanted cells, (2) suppressing glial progenitor proliferation and survival, and (3) developing methods to efficiently produce dopaminergic neurons. Subventricular neural stem cells naturally produce a dopaminergic interneuron phenotype that has an apparent lack of vulnerability to PD-mediated degeneration. We also discuss whether olfactory bulb dopaminergic neurons derived from adult SVZ neural stem cells are a suitable source for cell replacement strategies.

COUP-TFI controls activity-dependent tyrosine hydroxylase expression in adult dopaminergic olfactory bulb interneurons

COUP-TFI is an orphan nuclear receptor acting as a strong transcriptional regulator in different aspects of forebrain embryonic development. In this study, we investigated COUP-TFI expression and function in the mouse olfactory bulb (OB), a highly plastic telencephalic region in which continuous integration of newly generated inhibitory interneurons occurs throughout life. OB interneurons belong to different populations that originate from distinct progenitor lineages. Here, we show that COUP-TFI is highly expressed in tyrosine hydroxylase (TH)-positive dopaminergic interneurons in the adult OB glomerular layer (GL). We found that odour deprivation, which is known to downregulate TH expression in the OB, also downregulates COUP-TFI in dopaminergic cells, indicating a possible correlation between TH- and COUP-TFI-activity-dependent action. Moreover, we demonstrate that conditional inactivation of COUP-TFI in the EMX1 lineage results in a significant reduction of both TH and ZIF268 expression in the GL. Finally, lentiviral vector-mediated COUP-TFI deletion in adult-generated interneurons confirmed that COUP-TFI acts cell-autonomously in the control of TH and ZIF268 expression. These data indicate that COUP-TFI regulates TH expression in OB cells through an activity-dependent mechanism involving ZIF268 induction and strongly argue for a maintenance rather than establishment function of COUP-TFI in dopaminergic commitment. Our study reveals a previously unknown role for COUP-TFI in the adult brain as a key regulator in the control of sensory-dependent plasticity in olfactory dopaminergic neurons.