Dopamine D2 receptor stimulation promotes the proliferation of neural progenitor cells in adult mouse hippocampus

Adult Neurogenesis of the Medial Geniculate Body: In Vitro and Molecular Genetic Analyses Reflect the Neural Stem Cell Capacity of the Rat Auditory Thalamus over Time

Neural stem cells (NSCs) have been recently identified in the neonatal rat medial geniculate body (MGB). NSCs are characterized by three cardinal features: mitotic self-renewal, formation of progenitors, and differentiation into all neuroectodermal cell lineages. NSCs and the molecular factors affecting them are particularly interesting, as they present a potential target for treating neurologically based hearing disorders. It is unclear whether an NSC niche exists in the rat MGB up to the adult stage and which neurogenic factors are essential during maturation. The rat MGB was examined on postnatal days 8, 12, and 16, and at the adult stadium. The cardinal features of NSCs were detected in MGB cells of all age groups examined by neurosphere, passage, and differentiation assays. In addition, real-time quantitative polymerase chain reaction arrays were used to compare the mRNA levels of 84 genes relevant to NSCs and neurogenesis. In summary, cells of the MGB display the cardinal features of NSCs up to the adult stage with a decreasing NSC potential over time. Neurogenic factors with high importance for MGB neurogenesis were identified on the mRNA level. These findings should contribute to a better understanding of MGB neurogenesis and its regenerative capacity.

mRNA Abundance of Neurogenic Factors Correlates with Hearing Capacity in Auditory Brainstem Nuclei of the Rat

Neural stem cells (NSCs) have previously been described up to the adult stage in the rat cochlear nucleus (CN). A decreasing neurogenic potential was observed with critical changes around hearing onset. A better understanding of molecular factors affecting NSCs and neurogenesis is of interest as they represent potential targets to treat the cause of neurologically based hearing disorders. The role of genes affecting NSC development and neurogenesis in CN over time on hearing capacity has remained unclear. This study investigated the mRNA abundance of genes influencing NSCs and neurogenesis in rats' CN over time. The CN of rats on postnatal days 6, 12, and 24 were examined. Real-time quantitative polymerase chain reaction arrays were used to compare mRNA levels of 84 genes relevant to NSCs and neurogenesis. Age- and hearing-specific patterns of changes in mRNA abundance of neurogenically relevant genes were detected in the rat CN. Additionally, crucial neurogenic factors with significant and relevant influence on neurogenesis were identified. The results of this work should contribute to a better understanding of the molecular mechanisms underlying the neurogenesis of the auditory pathway.

Dopamine signaling impairs ROS modulation by mitochondrial hexokinase in human neural progenitor cells

Dopamine signaling has numerous roles during brain development. In addition, alterations in dopamine signaling may be also involved in the pathophysiology of psychiatric disorders. Neurodevelopment is modulated in multiple steps by reactive oxygen species (ROS), byproducts of oxidative metabolism that are signaling factors involved in proliferation, differentiation, and migration. Hexokinase (HK), when associated with the mitochondria (mt-HK), is a potent modulator of the generation of mitochondrial ROS in the brain. In the present study, we investigated whether dopamine could affect both the activity and redox function of mt-HK in human neural progenitor cells (NPCs). We found that dopamine signaling via D₁R decreases mt-HK activity and impairs ROS modulation, which is followed by an expressive release of H₂O₂ and impairment in calcium handling by the mitochondria. Nevertheless, mitochondrial respiration is not affected, suggesting specificity for dopamine on mt-HK function. In neural stem cells (NSCs) derived from induced-pluripotent stem cells (iPSCs) of schizophrenia patients, mt-HK is unable to decrease mitochondrial ROS, in contrast with NSCs derived from healthy individuals. Our data point to mitochondrial hexokinase as a novel target of dopaminergic signaling, as well as a redox modulator in human neural progenitor cells, which may be relevant to the pathophysiology of neurodevelopmental disorders such as schizophrenia.

Obligatory roles of dopamine D1 receptors in the dentate gyrus in antidepressant actions of a selective serotonin reuptake inhibitor, fluoxetine

Depression is a leading cause of disability. Current pharmacological treatment of depression is insufficient, and development of improved treatments especially for treatment-resistant depression is desired. Understanding the neurobiology of antidepressant actions may lead to development of improved therapeutic approaches. Here, we demonstrate that dopamine D1 receptors in the dentate gyrus act as a pivotal mediator of antidepressant actions in mice. Chronic administration of a selective serotonin reuptake inhibitor (SSRI), fluoxetine, increases D1 receptor expression in mature granule cells in the dentate gyrus. The increased D1 receptor signaling, in turn, contributes to the actions of chronic fluoxetine treatment, such as suppression of acute stress-evoked serotonin release, stimulation of adult neurogenesis and behavioral improvement. Importantly, under severely stressed conditions, chronic administration of a D1 receptor agonist in conjunction with fluoxetine restores the efficacy of fluoxetine actions on D1 receptor expression and behavioral responses. Thus, our results suggest that stimulation of D1 receptors in the dentate gyrus is a potential adjunctive approach to improve therapeutic efficacy of SSRI antidepressants.

Modulation of Postnatal Neurogenesis by Perinatal Asphyxia: Effect of D1 and D2 Dopamine Receptor Agonists

Perinatal asphyxia (PA) is associated to delayed cell death, affecting neurocircuitries of basal ganglia and hippocampus, and long-term neuropsychiatric disabilities. Several compensatory mechanisms have been suggested to take place, including cell proliferation and neurogenesis. There is evidence that PA can increase postnatal neurogenesis in hippocampus and subventricular zone (SVZ), modulated by dopamine, by still unclear mechanisms. We have studied here the effect of selective dopamine receptor agonists on cell death, cell proliferation and neurogenesis in organotypic cultures from control and asphyxia-exposed rats. Hippocampus and SVZ sampled at 1-3 postnatal days were cultured for 20-21 days. At day in vitro (DIV) 19, cultures were treated either with SKF38393 (10 and 100 µM, a D₁ agonist), quinpirole (10 µM, a D₂ agonist) or sulpiride (10 μM, a D₂ antagonist) + quinpirole (10 μM) and BrdU (10 μM, a mitosis marker) for 24 h. At DIV 20-21, cultures were processed for immunocytochemistry for microtubule-associated protein-2 (MAP-2, a neuronal marker), and BrdU, evaluated by confocal microscopy. Some cultures were analysed for cell viability at DIV 20-21 (LIVE/DEAD kit). PA increased cell death, cell proliferation and neurogenesis in hippocampus and SVZ cultures. The increase in cell death, but not in cell proliferation, was inhibited by both SKF38393 and quinpirole treatment. Neurogenesis was increased by quinpirole, but only in hippocampus, in cultures from both asphyxia-exposed and control-animals, effect that was antagonised by sulpiride, leading to the conclusion that dopamine modulates neurogenesis in hippocampus, mainly via D₂ receptors.

Dopamine D3 receptor activation promotes neural stem/progenitor cell proliferation through AKT and ERK1/2 pathways and expands type-B and -C cells in adult subventricular zone

The neurotransmitter dopamine acts on the subventricular zone (SVZ) to regulate both prenatal and postnatal neurogenesis, in particular through D(3) receptor (D(3) R) subtype. In this study, we explored the cellular mechanism(s) underlying D(3) R-mediated cell proliferation and tested if systemic delivery of a D(3) R agonist would induce SVZ multipotent neural stem/precursor cell (NSC/NPC) proliferation in vivo. We found that treatment with the D(3) R agonist, 7-OH-DPAT, enhances cell proliferation in a dose-dependent manner in cultured SVZ neurospheres from wild-type, but not D(3) R knock-out mice. Furthermore, D(3) R activation also stimulates S-phase and enhances mRNA and protein levels of cyclin D1 in wild-type neurospheres, a process which requires cellular Akt and ERK1/2 signaling. Moreover, chronic treatment with low dose 7-OH-DAPT in vivo increases BrdU(+) cell numbers in the adult SVZ, but this effect was not seen in D(3) R KO mice. Additionally, we probed the cell type specificity of D(3) R agonist-mediated cell proliferation. We found that in adult SVZ, GFAP(+) astrocytes, type-B GFAP(+) /nestin(+) and type-C EGF receptor (EGFR(+) )/nestin(+) cells express D(3) R mRNA, but type-A Doublecortin (Dcx)(+) neuroblasts do not. Using flow cytometry and immunofluorescence, we demonstrated that D(3) R activation increases GFAP(+) type-B and EGFR(+) type-C cell numbers, and the newly divided Dcx(+) type-A cells. However, BrdU(+) /Dcx(+) cell numbers were decreased in D(3) R KO mice compared to wildtype, suggesting that D(3) R maintains constitutive NSC/NPCs population in the adult SVZ. Overall, we demonstrate that D(3) R activation induces NSC/NPC proliferation through Akt and ERK1/2 signaling and increases the numbers of type-B and -C NSC/NPCs in the adult SVZ.

Long-term administration of the dopamine D3/2 receptor agonist pramipexole increases dopamine and serotonin neurotransmission in the male rat forebrain

BACKGROUND

Long-term administration of the dopamine (DA) D2-like (D3/2) receptor agonist pramipexole (PPX) has been previously found to desensitize D2 autoreceptors, thereby allowing a normalization of the firing of DA neurons and serotonin (5-HT)1A autoreceptors, permitting an enhancement of the spontaneous firing of 5-HT neurons. We hypothesized that PPX would increase overall DA and 5-HT neurotransmission in the forebrain as a result of these changes at the presynaptic level.

METHODS

Osmotic minipumps were implanted subcutaneously in male Sprague-Dawley rats, delivering PPX at a dose of 1 mg/kg/d for 14 days. The in vivo electrophysiologic microiontophoretic experiments were carried out in anesthetized rats.

RESULTS

The sensitivity of postsynaptic D2 receptors in the prefrontal cortex (PFC) remained unaltered following PPX administration, as indicated by the unchanged responsiveness to the microiontophoretic application of DA. Their tonic activation was, however, significantly increased by 104% compared with the control level. The sensitivity of postsynaptic 5-HT1A receptors was not altered, as indicated by the unchanged responsiveness to the microiontophoretic application of 5-HT. Similar to other antidepressant treatments, long-term PPX administration enhanced the tonic activation of 5-HT1A receptors on CA3 pyramidal neurons by 142% compared with the control level.

LIMITATIONS

The assessment of DA and 5-HT neuronal tone was restricted to the PFC and the hippocampus, respectively.

CONCLUSION

Chronic PPX administration led to a net enhancement in DA and 5-HT neurotransmission, as indicated by the increased tonic activation of postsynaptic D2 and 5-HT1A receptors in forebrain structures.

Tbx1: identification of a 22q11.2 gene as a risk factor for autism spectrum disorder in a mouse model

Although twin studies indicate clear genetic bases of autism spectrum disorder (ASD), the precise mechanisms through which genetic variations causally result in ASD are poorly understood. Individuals with 3 Mb and nested 1.5 Mb hemizygosity of the chromosome 22q11.2 represent genetically identifiable cases of ASD. However, because more than 30 genes are deleted even in the minimal deletion cases of 22q11.2 deficiency, the individual 22q11.2 gene(s) responsible for ASD remain elusive. Here, we examined the impact of constitutive heterozygosity of Tbx1, a 22q11.2 gene, on the behavioral phenotypes of ASD and characterized the regional and cellular expression of its mRNA and protein in mice. Congenic Tbx1 heterozygous (HT) mice were impaired in social interaction, ultrasonic vocalization, memory-based behavioral alternation, working memory and thigmotaxis, compared with wild-type (WT) mice. These phenotypes were not due to non-specific alterations in olfactory function, exploratory behavior, motor movement or anxiety-related behavior. Tbx1 mRNA and protein were ubiquitously expressed throughout the brains of C57BL/6J mice, but protein expression was enriched in regions that postnatally retain the capacity of neurogenesis, and in fact, postnatally proliferating cells expressed Tbx1. In postnatally derived hippocampal culture cells of C57BL/6J mice, Tbx1 levels were higher during proliferation than during differentiation, and expressed in neural progenitor cells, immature and matured neurons and glial cells. Taken together, our data suggest that Tbx1 is a gene responsible for the phenotypes of 22q11.2 hemizygosity-associated ASD possibly through its role in diverse cell types, including postnatally and prenatally generated neurons.

Pathophysiology of perinatal asphyxia: can we predict and improve individual outcomes?

Perinatal asphyxia occurs still with great incidence whenever delivery is prolonged, despite improvements in perinatal care. After asphyxia, infants can suffer from short- to long-term neurological sequelae, their severity depend upon the extent of the insult, the metabolic imbalance during the re-oxygenation period and the developmental state of the affected regions. Significant progresses in understanding of perinatal asphyxia pathophysiology have achieved. However, predictive diagnostics and personalised therapeutic interventions are still under initial development. Now the emphasis is on early non-invasive diagnosis approach, as well as, in identifying new therapeutic targets to improve individual outcomes. In this review we discuss (i) specific biomarkers for early prediction of perinatal asphyxia outcome; (ii) short and long term sequelae; (iii) neurocircuitries involved; (iv) molecular pathways; (v) neuroinflammation systems; (vi) endogenous brain rescue systems, including activation of sentinel proteins and neurogenesis; and (vii) therapeutic targets for preventing or mitigating the effects produced by asphyxia.

Destruction of dopaminergic neurons in the midbrain by 6-hydroxydopamine decreases hippocampal cell proliferation in rats: reversal by fluoxetine

Background

Non-motor symptoms (e.g., depression, anxiety, and cognitive deficits) in patients with Parkinson disease (PD) precede the onset of the motor symptoms. Although these symptoms do not respond to pharmacological dopamine replacement therapy, their precise pathological mechanisms are currently unclear. The present study was undertaken to examine whether the unilateral 6-hydroxydopamine (6-OHDA) lesion to the substantia nigra pars compacta (SNc), which represents a model of long-term dopaminergic neurotoxicity, could affect cell proliferation in the adult rat brain. Furthermore, we examined the effects of the selective serotonin reuptake inhibitor (SSRI) fluoxetine and the selective noradrenaline reuptake inhibitor maprotiline on the reduction in cell proliferation in the subgranular zone (SGZ) by the unilateral 6-OHDA lesion.

Methodology/Principal Findings

A single unilateral injection of 6-OHDA into the rat SNc resulted in an almost complete loss of tyrosine hydroxylase (TH) immunoreactivity in the striatum and SNc, as well as in reductions of TH-positive cells and fibers in the ventral tegmental area (VTA). On the other hand, an injection of vehicle alone showed no overt change in TH immunoreactivity. A unilateral 6-OHDA lesion to SNc significantly decreased cell proliferation in the SGZ ipsilateral to the 6-OHDA lesion, but not in the contralateral SGZ or the subventricular zone (SVZ), of rats. Furthermore, subchronic (14 days) administration of fluoxetine (5 mg/kg/day), but not maprotiline significantly attenuated the reduction in cell proliferation in the SGZ by unilateral 6-OHDA lesion.

Conclusions/Significance

The present study suggests that cell proliferation in the SGZ of the dentate gyrus might be, in part, under dopaminergic control by SNc and VTA, and that subchronic administration of fluoxetine reversed the reduction in cell proliferation in the SGZ by 6-OHDA. Therefore, SSRIs such as fluoxetine might be potential therapeutic drugs for non-motor symptoms as well as motor symptoms in patients with PD, which might be associated with the reduction in cell proliferation in the SGZ.

Targeting the hippocampal mossy fiber synapse for the treatment of psychiatric disorders

It is widely known that new neurons are continuously generated in the dentate gyrus of the hippocampus in the adult mammalian brain. This neurogenesis has been implicated in depression and antidepressant treatments. Recent evidence also suggests that the dentate gyrus is involved in the neuropathology and pathophysiology of schizophrenia and other related psychiatric disorders. Especially, abnormal neuronal development in the dentate gyrus may be a plausible risk factor for the diseases. The synapse made by the mossy fiber, the output fiber of the dentate gyrus, plays a critical role in regulating neuronal activity in its target CA3 area. The mossy fiber synapse is characterized by remarkable activity-dependent short-term synaptic plasticity that is established during the postnatal development and is supposed to be central to the functional role of the mossy fiber. Any defects, including developmental abnormalities, in the dentate gyrus and drugs acting on the dentate gyrus can modulate the mossy fiber-CA3 synaptic transmission, which may eventually affect hippocampal functions. In this paper, I review recent evidence for involvement of the dentate gyrus and mossy fiber synapse in psychiatric disorders and discuss potential importance of drugs targeting the mossy fiber synapse either directly or indirectly in the therapeutic treatments of psychiatric disorders.

Intracranial self-stimulation enhances neurogenesis in hippocampus of adult mice and rats

Running is known to promote neurogenesis. Besides being exercise, it results in a reward, and both of these factors might contribute to running-induced neurogenesis. However, little attention has been paid to how reward and exercise relate to neurogenesis. The present study is an attempt to determine whether a reward, in the form of intracranial self-stimulation (ICSS), influences neurogenesis in the hippocampus of adult rodents. We used bromodeoxyuridine labeling to quantify newly generated cells in mice and rats that experienced ICSS for 1 h per day for 3 days. ICSS increased the number of 5-bromodeoxyuridine (Brdu)-labeled cells in the hippocampal dentate gyrus (DG) of both species. The effect, when examined at 1 day, 1 week, and 4 weeks post-ICSS, was predominantly present in the side ipsilateral to the stimulation, although it was distributed to the contralateral side. We also found in rats that, 4 weeks after Brdu injection, surviving newborn cells in the hippocampal DG of the ICSS animals co-localized with a mature neuron marker, neuronal nuclei (NeuN), and these surviving cells in rats were double-labeled with Fos, a marker of neuronal activation, after the rats had been trained to perform a spatial task. The results demonstrate that ICSS can increase newborn neurons in the hippocampal DG that endure into maturity.

D1-like dopamine receptors regulate GABAA receptor function to modulate hippocampal neural progenitor cell proliferation

The proliferation and differentiation of neural progenitor (NP) cells can be regulated by neurotransmitters including GABA and dopamine. The present study aimed to examine how these two neurotransmitter systems interact to affect post-natal hippocampal NP cell proliferation in vitro. Mouse hippocampal NP cells express functional GABAA receptors, which upon activation led to an increase in intracellular calcium levels via the opening of L-type calcium channels. Activation of these GABAA receptors also caused a significant decrease in proliferation; an effect that required the entry of calcium through L-type calcium channels. Furthermore, while activation of D1-like dopamine receptors had no effect on proliferation, it abrogated the suppressive effects of GABAA receptor activation on proliferation. The effects of D1-like dopamine receptors are associated with a decrease in the ability of GABAA receptors to increase intracellular calcium levels, and a reduction in the surface expression of GABAA receptors. In this way, D1-like dopamine receptor activation can increase the proliferation of NP cells by preventing GABAA receptor-mediated inhibition of proliferation. These results suggest that, in conditions where NP cell proliferation is under the tonic suppression of GABA, agonists which act through D1-like dopamine receptors may increase the proliferation of neural progenitors.

Induction of cell migration of neural progenitor cells in vitro by alpha-1 adrenergic receptor and dopamine D1 receptor stimulation

The radial migration is an important process in the development of the cerebral cortex. Earlier studies have reported that classical neurotransmitters such as L-dopamine and L-adrenaline regulate the proliferation of neural progenitor cells. We examined whether L-dopamine and L-adrenaline regulate cell migration, using embryonic neural progenitor cells from mouse embryonic telencephalon in vitro. In this study, we showed that dopamine D1 agonist induces cell migration of embryonic neural progenitor cells. In addition, we have demonstrated that L-adrenaline induces cell migration of embryonic neural progenitor cells, mediated through the activation of alpha-1 adrenergic receptors. Our results suggest that alpha-1 adrenergic receptor and dopamine D1 receptor stimulations in neural progenitor cells are the important process for embryonic brain development, respectively.