Doublecortin and doublecortin-like are expressed in overlapping and non-overlapping neuronal cell population: implications for neurogenesis

We have characterized the expression of doublecortin-like (DCL), a microtubule-associated protein involved in embryonic neurogenesis that is highly homologous to doublecortin (DCX), in the adult mouse brain. To this end, we developed a DCL-specific antibody and used this to compare DCL expression with DCX. In the neurogenic regions of the adult brain like the subventricular zone (SVZ), the rostral migratory stream (RMS), the olfactory bulb (OB), and the hippocampus, DCL colocalizes with DCX in immature neuronal cell populations. In contrast to DCX, we also found high DCL expression in three other brain regions with suspected neurogenesis or neuronal plasticity. First, the radial glia-like, hypothalamic tanycytes show high DCL expression that partly colocalizes with the neural stem cell marker vimentin. Second, DCL expression is found in cells of the suprachiasmatic nucleus (SCN), which lacks expression of the adult neuron marker NeuN. Third, a novel region exhibiting DCL expression is part of the olfactory tubercle where DCL is found in the neuropil of the islands of Calleja (ICj). Our findings define DCL as a novel marker for specific aspects of adult neurogenesis, which partly overlap with DCX. In addition, we propose unique roles for DCL in adult neurogenesis and we suggest high levels of neuronal plasticity in tanycytes, SCN, and ICj.     

The involvement of dopamine D2 receptors, but not D3 or D4 receptors, in the rewarding effect of brain stimulation in the rat

To identify the subtype of dopamine receptors critically involved in the rewarding effect of brain stimulation, four dopamine antagonists were intracranially injected in 25 rats. The importance of dopamine D1 receptors had been demonstrated previously by using SCH 23390, a highly selective D1 antagonist. Rats were implanted with electrodes into the medial forebrain bundle and cannulae into either one of the following structures: the nucleus accumbens, the vicinity of the islands of Calleja, or the ventral tegmental area, all ipsilateral to the electrodes. The animals were trained to press a bar for electrical stimulation, and the frequency-response functions were plotted before and after injection of each dopamine antagonist through the cannulae. Raclopride and haloperidol, which have high affinities for D2 receptors, reduced the rewarding effect after injection into any one of the three cannula sites. Neither (+)-UH232, a selective D3 antagonist, nor clozapine, a D4 antagonist, influenced the rewarding effect. The results suggest that dopamine D2, but not D3 or D4, receptors are critically involved in producing the rewarding effect of brain stimulation.     

Dopaminergic nigrostriatal projections regulate neural precursor proliferation in the adult mouse subventricular zone

An understanding of the regulators of neurogenesis in the normal and diseased brain is necessary in order to recruit endogenously produced neural precursors for cell replacement in neurodegenerative disorders such as Parkinson's disease. The location of dopaminergic projections from the midbrain to the neostriatum and nucleus accumbens overlaps with the most active region of neurogenesis in the adult brain, the subventricular zone of the anterior lateral ventricle. This suggests that dopamine may contribute to regulation of the subventricular niche of adult neurogenesis. Here, we show in adult mice that destruction of the dopaminergic neurons in the substantia nigra and ventral tegmental area in a 6-hydroxydopamine model of Parkinson's disease reduced the number of proliferating neural precursors in the subventricular zone of the anterior lateral ventricle by approximately 40%. The effect on neural precursor proliferation correlated with the extent of dopaminergic denervation in the neighboring neostriatum. This identifies dopamine as one of the few known endogenous regulators of adult neurogenesis with implications for the potential use of endogenous neural precursors in cell replacement strategies for Parkinson's disease.     

Induction of neurogenesis in the adult rat subventricular zone and neostriatum following dopamine D3 receptor stimulation

Discrete regions of the adult CNS, including the subventricular zone (SVZ), do retain the capacity for neurogenesis. These progenitor cells may represent a potential new source of cells for replacement therapies in neuroregenerative diseases. An understanding of the microenvironmental signals regulating neurogenesis in the adult brain would facilitate the development of such therapeutic approaches. A particularly strong expression of dopamine D(3) receptor mRNA occurs in the proliferative SVZ during prenatal and early postnatal ontogeny. Although its expression diminishes following development, a restricted D(3) receptor expression persists in this region through adulthood, coincident with continued proliferation in this region. Here, we demonstrate a two-fold induction of cell proliferation (BrdU incorporation) in the SVZ and rostral migratory stream of the adult Sprague-Dawley rat brain following intrasubventricular administration of the dopamine D(3) receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) for 2 weeks. The number of BrdU-positive cells was elevated ten-fold from very low baseline levels in the neighbouring neostriatum, another region known to express D(3) receptors. These striatal BrdU-positive cells appeared within 3 days following intracerebral infusion of 7-OH-DPAT and were distributed homogeneously throughout the striatum following systemic administration. This suggests that these cells originate from resident progenitor cells rather than the SVZ. Dopamine D(3) receptor activation may serve as a proneuronal differentiation signal as 60-70% of the new cells had neuronal markers following 7-OH-DPAT infusion. These results suggest that the dopamine D(3) receptor may be a good drug target for cell replacement strategies, particularly because of the fact that its expression is almost exclusively limited to the nervous system.     

Dopamine D2 receptors in the rat brain: autoradiographic visualization using a high-affinity selective agonist ligand

The non-catechol, selective dopamine D2-agonist compound 3H-205-502 was used to localize dopamine D2 receptors by autoradiography after in vitro labeling of brain sections. The characteristics of the binding of this ligand to tissue sections were those expected from the labeling of dopamine D2 receptors. The binding of 3H-205-502 was inhibited selectively and stereospecifically by dopamine D2 agents but not by dopamine D1 compounds. The autoradiographic localization of 3H-205-502 binding sites showed high densities of dopamine D2 receptors in areas such as the glomerular layer of the olfactory bulb, the nucleus accumbens, caudate-putamen, olfactory tubercle, the lateral septum, and the islands of Calleja. Besides these dopamine-innervated areas the substantia nigra and the ventral tegmental area also showed important receptor densities. Other areas where dopamine D2 receptor binding was found were the stratum lacunosum-moleculare of the hippocampus, bands of labeling in the molecular layer of the 9th and 10th lobules of the cerebellum, and several components of the visual system. This distribution presents similarities and differences with previously reported distributions of dopamine D2 receptors visualized autoradiographically using 3H-labeled agonists and antagonists. In view of the high affinity, guanine nucleotide insensitivity, and dopamine D2 selectivity of this agonist ligand, it is suggested that dopamine D2 receptors exist in different states in different areas. 3H-205-502 appears to be a new and useful tool for the study of dopamine D2 receptors.     

Neurotransmitters couple brain activity to subventricular zone neurogenesis

Adult neurogenesis occurs in two privileged microenvironments, the hippocampal subgranular zone of the dentate gyrus and the subventricular zone (SVZ) along the lateral ventricle. This review focuses on accumulating evidence suggesting that the activity of specific brain regions or bodily states influences SVZ cell proliferation and neurogenesis. Neuromodulators such as dopamine and serotonin have been shown to have long-range effects through neuronal projections into the SVZ. Local γ-aminobutyric acid and glutamate signaling have demonstrated effects on SVZ proliferation and neurogenesis, but an extra-niche source of these neurotransmitters remains to be explored and options will be discussed. There is also accumulating evidence that diseases and bodily states such as Alzheimer's disease, seizures, sleep and pregnancy influence SVZ cell proliferation. With such complex behavior and environmentally-driven factors that control subregion-specific activity, it will become necessary to account for overlapping roles of multiple neurotransmitter systems on neurogenesis when developing cell therapies or drug treatments.     

Psychotic experiences in the general population: a twenty-year prospective community study

Adult neurogenesis is one of the most rapidly growing areas in neuroscience research and there is great interest in its potential role in the pathophysiology of psychiatric illness. In parallel with early development, adult neurogenesis occurs through the proliferation of precursor cells which migrate to specific regions and differentiate into neurons with characteristics indistinguishable from existing mature neurons. These findings have led to the re-definition of the concept of network plasticity in the adult brain, to include the formation of new neurons as well as new connections. This review examines the idea that adult neurogenesis may be disturbed in schizophrenia. We discuss evidence for abnormal mechanisms of neurogenesis and expression of developmental genes in schizophrenia, the influence of antipsychotic drugs on neurogenesis and the role of candidate schizophrenia susceptibility genes in adult neurogenesis. The recent discovery of molecular markers transiently expressed in newborn neurons within adult neurogenic brain regions could be used to probe whether neurogenesis is disturbed in schizophrenia. Adult neurogenesis could also be used as a unique tool for investigating genes involved in early brain development using post-mortem brains. This is particularly relevant for brain disorders with developmental origins such as schizophrenia.     

Intense dopamine innervation of the subventricular zone in Huntington's disease

Dopamine exerts a robust promoting effect on adult neurogenesis. Here, we report the presence of an intense dopamine (tyrosine hydroxylase immunoreactive) zone along the ventricular border of the caudate nucleus in patients with Huntington's disease, but not in age-matched controls. This thin (150-400 microm) paraventricular zone was composed of numerous small and densely packed dopamine axon varicosities and overlapped the deep layers of the subventricular zone. Immunoreactivity in the paraventricular zone was 50% higher than in adjacent striatal areas. This intense dopamine zone concurs with the striking increase of neurogenesis noted in the subventricular zone of Huntington's disease patients and indicates that dopamine might play a crucial role in intrinsic mechanisms designed to compensate for the massive striatal neuronal losses that occur in Huntington's disease.     

D1 dopamine receptors mediate neuroleptic-induced Fos expression in the islands of Calleja

Systemic injections of the selective, full, D₁ agonists A-77636 and SKF-82958 induced pronounced Fos-like immunoreactivity in the islands of Calleja in the olfactory tubercle of intact rats. Fos expression in this region could also be induced by injections of the D₂-like dopamine antagonist raclopride (0.5 mg/kg). Pretreatment with the selective D₁ dopamine antagonist SCH-23390 (0.2 mg/kg) completely abolished this response, but was without significant effect on raclopride-induced Fos expression in the dorsolateral region of the striatum. SCH-23390 was also able to prevent the atypical neuroleptic clozapine (30 mg/kg) from inducing Fos expression in the islands of Calleja. These findings demonstrate that stimulation of D₁ dopamine receptors plays an essential role in neuroleptic induction of Fos-like immunoreactivity in the islands of Calleja, but not in the dorsal striatum, and thus suggest that different mechanisms underlie neuroleptic stimulation of immediate early gene expression in these two structures. Synapse 28:154–159, 1998. © 1998 Wiley-Liss, Inc.     

Expression of vascular endothelial growth factor receptor‐3 mRNA in the rat developing forebrain and retina

Vascular endothelial growth factor receptor (VEGFR)‐3, a receptor for VEGF‐C and VEGF‐D, is expressed in neural progenitor cells, but there has been no comprehensive study of its distribution in the developing brain. Here, the temporal and cell‐specific expression of VEGFR‐3 mRNA was studied in the developing rat forebrain and eye. Expression appeared along the ventricular and subventricular zones of the lateral and third ventricles showing ongoing neurogenesis as early as embryonic day 13 but was progressively down‐regulated during development and remained in the subventricular zone and rostral migratory stream of the adult forebrain. VEGFR‐3 expression was also detectable in some differentiating and postmitotic neurons in the developing cerebral cortex, including Cajal‐Retzius cells, cortical plate neurons, and subplate neurons. Expression in the subplate increased significantly during the early postnatal period but was absent by postnatal day 14. It was also highly expressed in nonneural tissues of the eye during development, including the retinal pigment epithelium, the retinal ciliary margin, and the lens, but persisted in a subset of cells in the pigmented ciliary epithelium of the adult eye. In contrast, there was weak or undetectable expression in the early neural retina, but a subset of retinal neurons in the postnatal and mature retina showed intense signals. These unique spatiotemporal mRNA expression patterns suggest that VEGFR‐3 might mediate the regulation of both neurogenesis and adult neuronal function in the rat forebrain and eye. J. Comp. Neurol. 518:1064–1081, 2010. © 2009 Wiley‐Liss, Inc.     

Adolescent Maturation of Cortical Dopamine

Dopamine is a critical modulator of prefrontal cortical function, and it is known to be dysfunctional in schizophrenia. Current hypotheses on schizophrenia highlight developmental aspects and genetic predisposition for the disease; yet, symptom onset typically occurs during adolescence. Several aspects of prefrontal cortical circuits and their modulation by dopamine mature postnatally, as late as during adolescence. Here we review studies assessing the postnatal trajectory of dopamine control of GABA interneurons, a neuronal population that has been long suspected to be critical for schizophrenia pathophysiology. Dopamine modulation of fast-spiking interneurons changes dramatically during adolescence (postnatal day 45–50 in rats) with D2 agonists switching from being mildly inhibitory in prepubertal rats to strongly excitatory in young adult rats. In vivo recordings in adult rats reveal that deep-layer pyramidal neurons respond to endogenous DA release with suppression of firing while interneurons are activated. In adult rats with a neonatal ventral hippocampal lesion (NVHL), an extensively studied developmental model of schizophrenia, the maturation in the D2 modulation of interneuron physiology fails to occur, rendering a disinhibited prefrontal cortex. Abnormal interneuron maturation may therefore impair cognitive function in the adult animal.     

MicroRNA-124 is a subventricular zone neuronal fate determinant

New neurons are continuously generated from neural stem cells with astrocyte properties, which reside in close proximity to the ventricle in the postnatal and adult brain. In this study we found that microRNA-124 (miR-124) dictates postnatal neurogenesis in the mouse subventricular zone. Using a transgenic reporter mouse we show that miR-124 expression is initiated in the rapid amplifying progenitors and remains expressed in the resulting neurons. When we stably inhibited miR-124 in vivo, neurogenesis was blocked, leading to the appearance of ectopic cells with astrocyte characteristics in the olfactory bulb. Conversely, when we overexpressed miR-124, neural stem cells were not maintained in the subventricular zone and neurogenesis was lost. In summary, our results demonstrate that miR-124 is a neuronal fate determinant in the subventricular zone.     

D3 dopamine receptors do not regulate neurogenesis in the subventricular zone of adult mice

Testing the effects of drugs that stimulate endogenous neurogenesis in different species is important for the development of neural repair strategies in humans. We have previously shown in adult rats that a 14-day intracerebroventricular infusion of the D3 preferential agonist 7-hydroxydipropyl-amino-tetraline (7-OH-DPAT) increases BrdU labeling of neural precursors in the subventricular zone of the anterior lateral ventricle (SVZ). Here, we show that such a treatment failed to affect neurogenesis in C57Bl/6 and FVB mice, even at a high dose or when infused into the neostriatum. We confirmed that such a treatment was effective in adult rats. Moreover, D3 receptor inhibition or genetic knockout failed to affect the neurogenesis in mice. These results raise the possibilities that neurogenesis is not regulated by D3 receptors in all species and, therefore, that D3 agonists like pramipexole may not be useful to harness endogenous neurogenesis in cell replacement strategies for Parkinson's disease.     

Targeting prefrontal cortical dopamine D1 and N-methyl-D-aspartate receptor interactions in schizophrenia treatment.

The prefrontal cortex plays a principal role in higher cognition and particularly in the fast online manipulation of appropriate information to guide forthcoming behavior. Dysfunction of this process represents a main feature in the pathophysiology of schizophrenia. Both dopamine D1 and N-methyl-D-aspartate (NMDA) receptors in the prefrontal cortex play a critical role in synaptic plasticity, memory mechanisms, and cognition. Recent data have shown that D1 and NMDA receptors interact bidirectionally and may greatly influence the output of the prefrontal cortex. Hypofunction of these receptor systems in the prefrontal cortex is found in schizophrenia. This review attempts to summarize some of the latest findings on the cellular mechanisms that underlie D1-NMDA receptor interactions. These findings have provided potential therapeutic strategies that aim to functionally up-regulate D1 and/or NMDA receptor safely via selective activation of D1 receptors or coagonist activation of NMDA receptors through blockade of the glycine transporter-1.     

Chronic inhibition of nitric oxide synthesis enhances both subventricular zone neurogenesis and olfactory learning in adult mice

The ability to generate new neurons during the course of adult life is preserved in the subventricular zone of the lateral ventricles and the dentate gyrus of the hippocampus in the mammalian brain. These two regions constitute specifically regulated neurogenic niches, and provide newborn neurons involved in olfactory and spatial learning, respectively. Nitric oxide (NO) is a negative regulator of neurogenesis in the subventricular zone, whereas its role in the dentate gyrus remains controversial. Using systemic administration of NO synthase (NOS) inhibitors to chronically inhibit NO production, we increased neural precursor proliferation in the subventricular zone as well as neurogenesis in the olfactory bulb, without modifying the number of mitotic cells or the granular cell layer thickness in the dentate gyrus. The same treatment specifically improved olfactory learning performance, whereas spatial learning and memory was unchanged, thus demonstrating that olfactory memory is closely associated with the level of ongoing neurogenesis in the subventricular zone-olfactory bulb. The anatomical specificity of the NOS inhibitor actions was not due to differences in the availability of NO, as demonstrated by immunohistochemical detection of neuronal NOS and S-nitrosylated proteins in both regions. Remarkably, the distinct NO sensitivity might result from a differential expression of epidermal growth factor receptor in precursor cells in both regions, as the proliferative effect of NOS inhibitors in the subventricular zone was restricted to the cells that expressed this receptor.     

Effects of steroid hormones on neurogenesis in the hippocampus of the adult female rodent during the estrous cycle, pregnancy, lactation and aging

Adult neurogenesis exists in most mammalian species, including humans, in two main areas: the subventricular zone (new cells migrate to the olfactory bulbs) and the dentate gyrus of the hippocampus. Many factors affect neurogenesis in the hippocampus and the subventricular zone, however the focus of this review will be on factors that affect hippocampal neurogenesis, particularly in females. Sex differences are often seen in levels of hippocampal neurogenesis, and these effects are due in part to differences in circulating levels of steroid hormones such as estradiol, progesterone, and corticosterone during the estrous cycle, in response to stress, with reproduction (including pregnancy and lactation), and aging. Depletion and administration of these same steroid hormones also has marked effects on hippocampal neurogenesis in the adult female, and these effects are dependent upon reproductive status and age. The present review will focus on current research investigating how hippocampal neurogenesis is altered in the adult female rodent across the lifespan.     

Modulation of Postnatal Neurogenesis by Perinatal Asphyxia: Effect of D<Subscript>1</Subscript> and D<Subscript>2</Subscript> 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.     

Expression of prokineticins and their receptors in the adult mouse brain

Prokineticins are a pair of regulatory peptides that have been shown to play important roles in gastrointestinal motility, angiogenesis, circadian rhythms, and, recently, olfactory bulb neurogenesis. Prokineticins exert their functions via activation of two closely related G-protein-coupled receptors. Here we report a comprehensive mRNA distribution for both prokineticins (PK1 and PK2) and their receptors (PKR1 and PKR2) in the adult mouse brain with the use of in situ hybridization. PK2 mRNA is expressed in discrete regions of the brain, including suprachiasmatic nucleus, islands of Calleja and medial preoptic area, olfactory bulb, nucleus accumbens shell, hypothalamic arcuate nucleus, and amygdala. PK1 mRNA is expressed exclusively in the brainstem, with high abundance in the nucleus tractus solitarius. PKR2 mRNA is detected throughout the brain, with prominent expression in olfactory regions, cortex, thalamus and hypothalamus, septum and hippocampus, habenula, amygdala, nucleus tractus solitarius, and circumventricular organs such as subfornical organ, median eminence, and area postrema. PKR2 mRNA is also detected in mammillary nuclei, periaqueductal gray, and dorsal raphe. In contrast, PKR1 mRNA is found in fewer brain regions, with moderate expression in the olfactory regions, dentate gyrus, zona incerta, and dorsal motor vagal nucleus. Both PKR1 and PKR2 are also detected in olfactory ventricle and subventricular zone of the lateral ventricle, both of which are rich sources of neuronal precursors. These extensive expression patterns suggest that prokineticins may have a broad array of functions in the central nervous system, including circadian rhythm, neurogenesis, ingestive behavior, reproduction, and autonomic function.