Estrogen contributes to seasonal plasticity of the adult avian song control system

Songbirds show dramatic neural plasticity as adults, including large-scale anatomical changes in discrete brain regions ("song control nuclei") controlling the production of singing behavior. The volumes of several song control nuclei are much larger in the breeding season than in the nonbreeding season, and these seasonal neural changes are regulated by plasma testosterone (T) levels. In many cases, the effects of T on the central nervous system are mediated by neural conversion to estradiol (E(2)) by the enzyme aromatase. The forebrain of male songbirds expresses very high levels of aromatase, in some cases adjacent to song control nuclei. We examined the effects of aromatase inhibition and estrogen treatment on song nuclei size using wild male songbirds in both the breeding and nonbreeding seasons. In breeding males, aromatase inhibition caused the volume of a telencephalic song control nucleus (HVC) to decrease, and this effect was partially rescued by concurrent estrogen replacement. In nonbreeding males, estradiol treatment caused HVC to grow to maximal spring size within 2 weeks. Overall, these data suggest that aromatization of T is an important mediator of song control system plasticity, and that estradiol has neurotrophic effects in adult male songbirds. This study demonstrates that estrogen can affect adult neural plasticity on a gross anatomical scale and is the first examination of estrogen effects on the brain of a wild animal.

Pallial origin of mitral cells in the olfactory bulbs of Xenopus

We used two developmental transcription factors, x-Eomes (T-box family) and x-Lhx5 (LIM-homeodomain family), to follow the origin and development of the olfactory bulbs in Xenopus. During embryonic and larval development, x-Eomes and x-Lhx5 were expressed in highly similar patterns, in the lateral and latero-ventral wall of the pallium. In adults, both markers were strongly and specifically expressed in mitral cells, i.e., in the projection neurons of the main and accessory olfactory bulbs. These results demonstrate the pallial origin of the olfactory projecting cells in Xenopus. Combined with previous results suggesting a subpallial origin for olfactory interneurons, these findings emphasize the dual origin of different neuronal populations in the bulbs of anamniotes, and suggest that this organization is a shared feature of tetrapods.

Disruption of postnatal progenitor migration and consequent abnormal pattern of glial distribution in the cerebrum following administration of methylmercury

Transplacental administration of methylmercury (MeHg) induces disruption of neuronal migration in the developing cerebral cortex. However, the effects of MeHg on glial progenitor migration remain unclear. To understand this, we performed double administration of MeHg and 5-bromo-2-deoxyuridine (BrdU) to neonatal rat pups on postnatal day 2 (P2), when glial cells are generated from progenitors in the subventricular zone (SVZ). Histopathological examination of a proportion of the MeHg-treated rats on P28 revealed no apparent abnormalities of cytoarchitecture or neuron count in either the primary motor or primary somatosensory cortex of the cerebrum. BrdU immunohistochemistry revealed abnormal accumulation of the labeled cells in the deeper layers of the cortices and underlying white matter of both areas, where an excessive number of astrocytes (glial fibrillary acidic protein- or S-100beta-immunolabeled cells) and oligodendrocytes (2',3'-cyclic-nucleotide 3'-phosphohydrolase-labeled cells) were located. Next, to investigate the migration of individual progenitors from the forebrain SVZ of P2 neonates, we labeled them in vivo with a retrovirus encoding green fluorescent protein (GFP), following administration of MeHg, and then examined the distribution pattern of the GFP-labeled cells in the P28 cerebrum. We found that the labeled cells developed into astrocytes and oligodendrocytes and were accumulated abnormally in the lateral white matter as well as in the adjacent deeper layer of the lateral cortex and lateral side of the striatum. Thus, exposure to MeHg in the gliogenic period induced irregular distribution of glia as a consequence of abnormal migration of the postnatal progenitors.

Sexually dimorphic neurocalcin expression in the developing zebra finch telencephalon

Differential display RT-PCR was used on RNA isolated from the zebra finch telencephalon to identify gene products potentially involved in its development, including the sexually dimorphic nuclei responsible for song learning and production. A cDNA identified only in juvenile females was cloned and sequenced. It shares homology with neurocalcin, a calcium binding protein. Northern blots indicated three neurocalcin species. A 10.6 kb transcript was present in males and most females throughout development and in adulthood. Smaller 6.2 and 3.3 kb species were detected almost exclusively in females and primarily between posthatching days 18-25. In situ hybridization, using a probe that identified all three mRNA species, indicated a broad distribution in the telencephalon of both sexes, with particularly high levels in the song nucleus RA. Across regions examined, neurocalcin expression was enhanced in females compared to males, probably reflecting the presence of the two smaller transcripts. However, within RA, neurocalcin expression was statistically equivalent between the sexes. These data indicate that calcium signaling via neurocalcin may be involved in telencephalic development, but suggest that sexually dimorphic expression of this gene exists on a level too general to specifically regulate masculine or feminine development of song control regions. Neurocalcin might: broadly influence functional differentiation, including areas that are not morphologically distinct between the sexes; be a benign consequence of general dimorphisms, such as those due to sex chromosomes; or involve a compensatory mechanism, which allows function of the juvenile female telencephalon to equal that of males, despite fundamental physiological differences.

Dlx2 progenitor migration in wild type and Nkx2.1 mutant telencephalon

The transcription factor Dlx2 is expressed widely throughout the ventral telencephalon. We have examined the in vitro and in vivo migration of Dlx2 progenitors originating from the different ganglionic eminences of both wild type and Nkx2.1 mutant animals. By examining the expression of tauLacZ targeted into the Dlx2 locus we were able to visualize the distribution of cells expressing this gene at both embryonic and postnatal stages. This analysis suggested that Dlx2-expressing cells traverse a number of characteristic migratory routes to populate both cortical and subcortical regions. We also examined how these patterns of migration were affected in Nkx2.1 mutant animals. In these mutants, the early but not late populations of Dlx2-expressing cells originating in the ventral telencephalon that migrate to the cortex are lost. This recovery may be, at least in part, a result of the late migration of Dlx2 progenitors from the caudal ganglionic eminences (CGE), which, based on our previous work, does not appear to require Nkx2.1 gene function.

Telomerase activity in the subventricular zone of adult mice

The subventricular zone (SVZ) is the most active site for the production of new neurons in the adult mouse brain. Neural stem cells in the adult SVZ give rise to neuroblasts that travel via the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into interneurons. The enzyme telomerase has been identified in other population of stem cells and is necessary for the synthesis of telomeric DNA to prevent chromosomal shortening, end-to-end fusions, and apoptosis during successive rounds of cell division. However, previous studies have failed to detect telomerase in the adult mammalian brain. Here we demonstrate that telomerase is expressed by all brain regions shortly after birth, but becomes restricted to the SVZ and olfactory bulb in the adult mouse brain. Cultures of neural precursor cells or of migratory neuroblasts purified from the SVZ were each found to possess telomerase activity. After elimination of migrating neuroblasts and immature precursor cells in vivo by treatment with cytosine-beta-D-arabinofuranoside (Ara-C), telomerase activity was still detectable in the remaining SVZ, which includes a population of neural stem cells. Following withdrawal of Ara-C, telomerase activity subsequently increased with a time course that parallels regeneration of the SVZ network and RMS. Finally, intracranial surgery alone, which has previously been shown to increase the number of cells in the SVZ, produced higher telomerase levels in the SVZ. We conclude that telomerase is active in neural precursor cells of the adult mouse and suggest that its regulation is an important parameter for cellular proliferation to occur in the mammalian brain.

Effect of age and testosterone on autumnal neurogenesis in male European starlings (Sturnus vulgaris)

The male European starling (Sturnus vulgaris) is an open-ended learner that increases its repertoire throughout life. In parallel, the volume of high vocal center (HVC) is larger in older birds than in yearlings. We labeled with the thymidine analog 5'-bromodeoxyuridine (BrdU) the cells that are generated during the fall in the brain of adult males that were 2 or more years old and in yearling males that were treated with exogenous testosterone (T) or kept intact before BrdU administration. In all subjects, the singing rate was recorded and BrdU-labeled cells were quantified in HVC, in proliferative areas of the ventricular zone (VZ) and in auditory regions. BrdU-containing cells were observed in all brain regions investigated. They were significantly more numerous in the VZ of the T-treated yearlings than in any other group. In older birds, a reduced number of labeled cells was specifically observed in the VZ close to the anterior commissure. No group difference was detected in auditory processing areas or in HVC. These data show for the first time a positive influence of T on the production of new cells at the VZ level in a male songbird and a decrease of this process with age. Furthermore, in T-treated birds, a correlation was observed between the HVC volume and the number of differentiated (round) BrdU-positive cell numbers in HVC on the one hand and song rate on another hand supporting the notion that singing activity is causally related to the T-induced growth of this song control nucleus.

Aggrecan regulates telencephalic neuronal aggregation in culture

Proteoglycans have been suggested to play roles in pattern formation in the developing central nervous system. In the chick embryo, aggrecan, a chondroitin sulfate proteoglycan, has a regionally-specific and developmentally-regulated expression profile. Telencephalic neuronal cultures, when aggregated, exhibit aggrecan expression patterns comparable to those observed in vivo. The chicken mutation nanomelia produces a truncated aggrecan species that cannot be processed further and is not secreted. Neurons from normal and nanomelic chick embryo telencephalon were scored for aggregate formation and analyzed for distribution of aggrecan protein and expression of aggrecan mRNA. Distinctly different pattern formation, with respect to aggregate size (smaller) and number (fewer) were observed in poly-L-lysine plated neuronal cultures derived from nanomelic embryos when compared to those derived from normal embryos. Significantly, the nanomelic phenotype was subsequently rescued upon addition of the brain-specific form of aggrecan. Modulation of neuronal aggregate formation was mimicked by treatment with chondroitinase ABC but not other glycanases, and was rescued by addition of chondroitin 6-sulfate to the culture media. Lastly, although broad and diffuse distribution of aggrecan among the cell aggregates in the culture paradigm was observed by immunocytochemistry, mRNA in situ hybridization revealed that only a small population of cells in the center of the aggregates was responsible for the production of the secreted aggrecan found associated with neuronal aggregates. These studies suggest a function for aggrecan as a diffusible signal in CNS histomorphogenesis.

Developmental distribution of coxsackie virus and adenovirus receptor localized in the nervous system

Mouse coxsackie virus and adenovirus receptor (mCAR), which was isolated from the nerve growth cone-enriched fraction of newborn mouse brains, is a member of immunoglobulin-super family, and functions as a homophilic adhesion molecule. We observed the expression of mCAR in embryos to adult tissues by means of immunohistochemical analysis with a peptide antibody. mCAR expression was first detected in the embryonic ectoderm in the uterus on embryonic day 6.5 (E6.5). Then it was strongly expressed in the neuroepithelium of the neural tube, the developing brain and the spinal cord from E8.5 to postnatal day 7 (P7), in the cranial motor nerves from E9.5 to E11.5, and in the optic nerve from E13.5 to P7, which agrees with periods of their respective morphogenetic peaks. This expression of mCAR decreased postnatally and was absent in adult tissues. We found that mCAR occurred in a few proliferating cells of the hippocampal dentate gyrus, the subventricular zone (SVZ) of the lateral ventricles, and the rostral migratory stream (RMS) over P21. These observations demonstrate that mCAR was expressed characteristically in the immature neuroepithelium including progenitor cells or radial cells derived from the neural tube and in immature cells in a selected germinal zone of the mature brain. Based on our findings, we propose that mCAR is involved in migration and fasciculation during a restricted period as an adhesion molecule.

Proliferation of neural and neuronal progenitors after global brain ischemia in young adult macaque monkeys

To investigate the effect of global cerebral ischemia on brain cell proliferation in young adult macaques, we infused 5-bromo-2'-deoxyuridine (BrdU), a DNA replication indicator, into monkeys subjected to ischemia or sham-operated. Subsequent quantification by BrdU immunohistochemistry revealed a significant postischemic increase in the number of BrdU-labeled cells in the hippocampal dentate gyrus, subventricular zone of the temporal horn of the lateral ventricle, and temporal neocortex. In all animals, 20-40% of the newly generated cells in the dentate gyrus and subventricular zone expressed the neural progenitor cell markers Musashi1 or Nestin. A few BrdU-positive cells in postischemic monkeys were double-stained for markers of neuronal progenitors (class III beta-tubulin, TUC4, doublecortin, or Hu), neurons (NeuN), or glia (S100beta or GFAP). Our results suggest that ischemia activates endogenous neuronal and glial precursors residing in diverse locations of the adult primate central nervous system.

Loss of Emx2 function leads to ectopic expression of Wnt1 in the developing telencephalon and cortical dysplasia

Leptomeningeal glioneuronal heterotopias are a focal type of cortical dysplasia in which neural cells migrate aberrantly into superficial layers of the cerebral cortex and meninges. These heterotopias are frequently observed as microscopic abnormalities in the brains of individuals with central nervous system (CNS) malformations and epilepsy. Previous work has demonstrated that the function of Emx2, which encodes a homeodomain transcription factor, is essential for development of the cortical preplate, which gives rise to the marginal zone and subplate. However, transcriptional targets of EMX2 during CNS development are unknown. We report that leptomeningeal glioneuronal heterotopias form in Emx2(-/-) mice that are equivalent to human lesions. Additionally, we observed ectopic expression of Wnt1 in the embryonic roofplate organizer region and dorsal telencephalon. To determine the phenotypic consequences of such Wnt1 misexpression, we deleted a putative EMX2 DNA-binding site from the Wnt1 enhancer and used this to misexpress Wnt1 in the developing murine CNS. Heterotopias were detected in transgenic mice as early as 13.5 days postcoitum, consistent with a defect of preplate development during early phases of radial neuronal migration. Furthermore, we observed diffuse abnormalities of reelin- and calretinin-positive cell populations in the marginal zone and subplate similar to those observed in Emx2-null animals. Taken together, these findings indicate that EMX2 is a direct repressor of Wnt1 expression in the developing mammalian telencephalon. They further suggest that EMX2-Wnt1 interactions are essential for normal development of preplate derivatives in the mammalian cerebral cortex.

Some glial progenitors in the neonatal subventricular zone migrate through the corpus callosum to the contralateral cerebral hemisphere

The great majority of glial cells of the mammalian forebrain are generated in the perinatal period from progenitors in the subventricular zone (SVZ). We investigated the migration of progenitors from the neonatal (postnatal day 0, P0) rat forebrain SVZ by labeling them in vivo with a green fluorescence protein (GFP) retrovirus and monitoring their movements by time-lapse video microscopy in P3 slices. We identified a small number of progenitors that migrated tangentially within the corpus callosum (CC) and crossed the midline. These cells retained a relatively uniform morphology: the leading process was extended toward the contralateral side but showed no process branching or turning away from the migratory direction. Net migration requires the elongation of the leading process and nuclear translocation, and the migrating cells in the CC showed both modes. We confirmed the presence of unmyelinated axon bundles within the P3 CC, but failed to detect any radially directed glial processes (vimentin- or GLAST-immunolabeled fibers) spanning through the CC. Confocal images showed a close proximity between neurofilament-immunolabeled axons and the leading process of the GFP-expressing progenitors in the CC. The destination of the callosal fibers was examined by applying DiI to the right cingulum; the labeled fibers ran throughout the CC and reached the left cingulate and motor areas. The distribution and final fates of the retrovirus-labeled cells were examined in P28 brains. A small proportion of the labeled cells were found in the contralateral hemisphere, where, as oligodendrocytes and astrocytes, they colonized predominantly the cortex and the underlying white matter of the cingulate and secondary motor areas. The distribution pattern appears to coincide well with the projection direction of the callosal fibers. Thus, glial progenitors migrate across the CC, presumably in conjunction with unmyelinated axons, to colonize the contralateral hemisphere.

A Golgi-Cox morphological analysis of neuronal changes induced by environmental enrichment

Exposure to an enriched environment (EE), consisting of a combination of increased exercise, social interactions and learning, has been shown to produce many positive effects in the CNS. In this study, we use a Golgi-Cox analysis to examine and dissect the role of various components of the enriched environment on two measures of neuronal growth: total cell volume and total dendritic length in four regions of the brain. In the hippocampus, CA1 and dentate gyrus cells, animals raised in an enriched environment demonstrate significant morphological change. These changes were not observed in layer V pyramidal neurons of the cerebral cortex or spiny neurons located in the striatum. To determine if one or more of the individual components of the EE were responsible for the changes in neuronal morphology, we examined mice raised with free access to exercise wheels. In these mice, no morphological changes were observed. These results suggest that changes in the CA1 and dentate gyrus morphology were a result of alterations in the animal's environment and not an increase in motor activity.

Gene expression of EXT1 and EXT2 during mouse brain development

Heparan sulfate (HS) and heparan sulfate proteoglycans (HSPGs) play significant roles in various biological processes. There is a wealth of circumstantial and experimental evidence suggesting the roles of HS in mammalian neural development. HS synthesis is governed by a series of enzymes. Among them, two enzymes, EXT1 and EXT2, catalyze polymerization of glucuronic acid and N-acetylglucosamine, the crucial step of HS synthesis. To obtain insight into the roles of HS in neural development, we examined the spatiotemporal expression patterns of EXT1 and EXT2 during mice brain development. RT-PCR analyses showed that expression of EXT1 and EXT2 peaks during early postnatal period in the cerebrum and around birth in the cerebellum. In situ hybridization revealed that in the embryonic brain, EXT1 and EXT2 were localized primarily in the neuroepithelial cells surrounding the lateral ventricles, the mesencephalic vesicle, and the fourth ventricle. In the early postnatal stage, intense expression of EXT1 and EXT2 was observed in the cerebral cortex and the hippocampus formation. In the postnatal cerebellum, expression of EXT1 and EXT2 was mainly observed in external and internal granular layers. Our results demonstrate that EXT1 and EXT2 are highly expressed in the developing brain, and that their expression is developmentally regulated, suggesting that HS is involved in various neurodevelopmental processes.

Expression of Hes6 and NeuroD in the olfactory epithelium, vomeronasal organ and non-sensory patches

Basic helix-loop-helix transcription factors NeuroD and Hes6 promote neuronal differentiation. The expression of their genes in the olfactory epithelium (OE), vomeronasal organ (VNO) and the non-sensory patches of the posterior nasal cavity of mice was examined. As detected by in situ hybridization, Hes6 was expressed in a basal progenitor layer of the embryonic OE. After birth, the expression of Hes6 was detected in a cell layer above the basement membrane, globose basal cells (GBCs). Expression of NeuroD in the embryonic OE was in agreement with that previously described; and in the postnatal OE, it was detected in cells of GBC layer and cells upper to GBCs. In the VNO, Hes6 was expressed throughout the sensory epithelium (S-VNO) at embryonic day 12, and later became restricted to a single layer of cells in the basal region of the S-VNO, where Hes5-expressing undifferentiated cells were present. NeuroD was expressed throughout the S-VNO during the embryonic stage. After birth, Hes6 and NeuroD expressions were observed in the border between the S-VNO and non-sensory VNO. Immunohistochemistry using anti-NeuroD antibody revealed that NeuroD-positive cells were still present not only at the edges but also in the center of the S-VNO until P3. These findings suggest that Hes6 and NeuroD are expressed in progenitors of chemoreceptor neurons and that the expression of Hes6 precedes that of NEUROD: Moreover, in the regenerating VNO of bulbectomized mice, NeuroD-positive cells were observed both at the edges and in the center of the S-VNO, suggesting that neuronal turnover occurred in both regions. Moreover, in the dorsal fossa of the posterior nasal cavity, several non-sensory patches are formed between postnatal (P) days 10 and 21 because of programmed death of ORNs and GBCs. During embryonic stages, the expression of Hes6 and NeuroD in the OE showed no regional differences. At P3-P7, expression of NeuroD and Hes6 disappeared in the region corresponding to the presumptive non-sensory patches. The loss of these genes may stop the differentiation and may cause apoptosis of GBCs and ORNs.

Expression of presenilin 1 and synapse-related proteins during postnatal development is not different between accelerated senescence-prone and -resistant mice

SAMP1TA/Ngs is an inbred strain of senescence-accelerated mice in which there is delayed development of cognitive functions and dendritic spine formation compared with normal control SAMR1TA//Ngs mice. It is hypothesized that abnormalities might be in the postnatal expression of synapse-related proteins in SAMP1TA/Ngs mice. Quantitative western blot analyses showed that the postnatal developmental changes in the expression of synaptophysin, post-synaptic density protein 95 and presenilin 1 in the cerebrum were similar between SAMP1TA/Ngs and SAMR1TA//Ngs mice. Therefore, the expression of synapse-related proteins was not disturbed in SAMP1TA/ Ngs mice regardless of reported abnormal numbers of dendritic spines during postnatal development. Immunohistochemical studies showed that the expression of synaptophysin in the neuropil increased postnatally with development in the same way in SAMP1TA/Ngs and SAMR1TA//Ngs mice. Presenilin 1 expression was relatively high at age 5 days in the neuropil of the cerebral cortex and decreased with postnatal development in the same way in SAMP1TA/Ngs and SAMR1TA//Ngs mice. At age 5 days the distribution of presenilin 1 was similar to the distribution of synaptophysin in that there were two separate immunoreactive patterns: a subpial band and patches in the middle layers reminiscent of barrels. These findings suggest that presenilin 1 is transiently expressed in the neuropil to induce synaptogenesis, and then its expression decreases overall.

Neurogenesis after ischaemic brain insults

Evidence for neuronal self-repair following insults to the adult brain has been scarce until very recently. Ischaemic insults have now been shown to trigger neurogenesis from neural stem cells or progenitor cells located in the dentate subgranular zone, the subventricular zone lining the lateral ventricle, and the posterior periventricle adjacent to the hippocampus. New neurons migrate to the granule cell layer or to the damaged CA1 region and striatum, where they express morphological markers characteristic of those neurons that have died. Some evidence indicates that these neurons can re-establish connections and contribute to functional recovery.

Light-dependent development of asymmetry in the ipsilateral and contralateral thalamofugal visual projections of the chick

Light-exposure of the chick embryo induces development of asymmetry in the thalamofugal visual projections to the Wulst regions of the forebrain since the embryo is turned so that it occludes its left and not its right eye. This asymmetry can be reversed by occluding the embryo's right eye and exposing its left eye to light. Here we show that three sub-regions of the thalamus (two in the dorsolateral anterior thalami (DLA) and one more caudal) have differing asymmetries of contralateral and/or ipsilateral projections. Hence the effect of asymmetrical light stimulation is regionally specific within the thalamus. Lateralised light stimulation appears to promote the development of ipsilateral projections from DLA pars dorsolateralis pars anterioris and contralateral projections from the caudal regions (the nucleus superficialis parvocellularis especially) but it may suppress the development of contralateral projections from the nucleus dorsolateralis anterior thalami pars lateralis rostralis. We also show that the light stimulation causes lateralised expression of c-fos and receptors for neurotransmitters.

CRMP-4 expression in the adult cerebral cortex and other telencephalic areas of the lizard Podarcis hispanica

The control of neuritogenesis is crucial for the development, maturation and regeneration of the nervous system. The collapsin response-mediated protein 4 (CRMP-4) is a member of a family of proteins that are involved in neuronal differentiation and axonal outgrowth. In rodents, this protein is expressed in recently generated neurons such as some granule neurons of the dentate gyrus, as well as in certain differentiated neurons undergoing neurite outgrowth or synaptogenesis during adulthood. Since CRMP-4 protein appears to be highly conserved throughout the evolutionary scale, we have used immunocytochemistry to study its distribution in the lizard cerebral cortex. We have found pronounced CRMP-4 immunolabeling in certain neurons of the medial cortex, the homologous region to the dentate gyrus, but also in the dorsal and lateral cortices. Double labeling with 5'-BrdU indicated that these medial cortex neurons were recently generated. However, it is also possible that many of these cells were not new but undergoing some kind of plasticity implicating neurite outgrowth. Similar CRMP-4-labeled neurons and processes were observed in subcortical regions as the PDVR and the nucleus sphericus. Our results show for the first time the expression of CRMP-4 in a reptile brain, where it appears to be expressed in regions where adult neurogenesis and/or neurite outgrowth occur.

Retinoic acid synthesis in the postnatal mouse brain marks distinct developmental stages and functional systems

Retinoic acid (RA) affects development and function of the brain, but little is known about how much is made locally and where it is distributed. To identify RA-sensitive neural processes, we mapped the RA-synthesizing retinaldehyde dehydrogenases (RALDHs) during postnatal brain formation of the mouse. High and stable RALDH expressions mark the basal ganglia, olfactory bulbs, hippocampus and auditory afferents as major sites of RA actions in the functional brain. During the early postnatal period, transient and very high RALDH3 expressions distinguish two developmental events: (i) the colonization of the nucleus accumbens and the olfactory bulbs by neuronal precursors and (ii) the maturation of selected parts of the cerebral cortex. In the cortex, RALDH3 is transiently activated in postmigratory layer II/III neurons during formation of their dendritic arbors and it is transported in their axons across the corpus callosum. RALDH3-expressing cortical regions include most of the limbic lobe, with strongest expression in the anterior cingulate cortex, medial and lateral secondary visual cortices, auditory cortical areas, the secondary motor cortex and some association areas. The transient cortical expression points to a brief RA-critical period during differentiation of the cortical network that serves in the coordination of sensory-motor activity with emotional and recently learned information.

Brain development, song learning and mate choice in birds: a review and experimental test of the "nutritional stress hypothesis"

The nutritional stress hypothesis explains how learned features of song, such as complexity and local dialect structure, can serve as indicators of male quality of interest to females in mate choice. The link between song and quality comes about because the brain structures underlying song learning largely develop during the first few months post-hatching. During this same period, songbirds are likely to be subject to nutritional and other stresses. Only individuals faring well in the face of stress are able to invest the resources in brain development necessary to optimize song learning. Learned features of song thus become reliable indicators of male quality, with reliability maintained by the developmental costs of song. We review the background and assumptions of the nutritional stress hypothesis, and present new experimental data demonstrating an effect of nestling nutrition on nestling growth, brain development, and song learning, providing support for a key prediction of the hypothesis.

Induction and patterning of the telencephalon in Xenopus laevis

We report an analysis of the tissue and molecular interplay involved in the early specification of the forebrain, and in particular telencephalic, regions of the Xenopus embryo. In dissection/recombination experiments, different parts of the organizer region were explanted at gastrula stage and tested for their inducing/patterning activities on either naive ectoderm or on midgastrula stage dorsal ectoderm. We show that the anterior dorsal mesendoderm of the organizer region has a weak neural inducing activity compared with the presumptive anterior notochord, but is able to pattern either neuralized stage 10.5 dorsal ectoderm or animal caps injected with BMP inhibitors to a dorsal telencephalic fate. Furthermore, we found that a subset of this tissue, the anterior dorsal endoderm, still retains this patterning activity. At least part of the dorsal telencephalic inducing activities may be reproduced by the anterior endoderm secreted molecule cerberus, but not by simple BMP inhibition, and requires the N-terminal region of cerberus that includes its Wnt-binding domain. Furthermore, we show that FGF action is both necessary and sufficient for ventral forebrain marker expression in neuralized animal caps, and possibly also required for dorsal telencephalic specification. Therefore, integration of organizer secreted molecules and of FGF, may account for patterning of the more rostral part of Xenopus CNS.

Differential neurogenesis and gliogenesis by local and migrating neural stem cells in the olfactory bulb

The rostral migratory stream (RMS) is a unique forebrain structure that provides a long-distance migratory route for the neural stem cells of the periventricular region towards the olfactory bulb (OB). The purpose of the study presented here is to examine the extent of neurogenesis and gliogenesis by the neural stem cells of different origins (periventricular vs. intrabulbar) in the OB. After the RMS had been subjected to injury, the rats received intraperitoneal injections of 5-bromodeoxyuridine (BrdU) and were further reared for 2 weeks. Neuronal and glial differentiations of the BrdU(+) cells in the olfactory bulbar granule cell (OB-GCL) and the olfactory glomerular (OB-GL) layers were examined immunohistochemically using antibodies against neuronal (NeuN, neuronal nuclei) and glial (GFAP, glial fibrillary acidic protein) markers in the OBs with injured and uninjured (control) RMS. In the completely RMS-lesioned OB, where migration of the periventricular neural stem cells was inhibited, a small number of BrdU(+) NeuN(+) cells were found in both the OB-GCL and OB-GL. The BrdU(+) NeuN(+) cells accounted for a much higher percentage of the BrdU(+) cells on the control side (OB-GCL, 36.7%; OB-GL, 8.8%) than on the completely RMS-lesioned side (OB-GCL, 3.7%; OB-GL, 0.6%). The percentage of the BrdU(+) GFAP(+) cells relative to the BrdU(+) cells did not show any major difference between the control and completely RMS-lesioned sides. This study revealed differences in neurogenesis and gliogenesis between the local and migrating neural stem cells in the OB of the adult rodent.

Subpallial dlx2-expressing cells give rise to astrocytes and oligodendrocytes in the cerebral cortex and white matter

The precise origins of postnatal subventricular zone (SVZ) cells are not known. Furthermore, the gliogenic potential of progenitors expressing Dlx genes that migrate ventrodorsally from the ganglionic eminences has not been explored in vivo. Here, we identify the embryonic origins of two distinct populations of postnatal SVZ cells: SVZ border cells, which express Zebrin II, and migratory cells in the central SVZ, which are generally devoid of Zebrin II expression (Staugaitis et al., 2001). Zebrin II is expressed by all cells of the telencephalic primordium, with its expression becoming restricted to astrocytes in the mature telencephalon. As the neuroepithelium folds during corticostriatal sulcus formation (embryonic day 13-15), a wedge of Zebrin II+ cells is created at the presumptive site of the dorsolateral SVZ. At this time, Dlx2-expressing cells and their progeny begin to migrate ventrodorsally along a medial path from the ganglionic eminences. These migratory subpallial cells invade the wedge of Zebrin II+ cells to form the central region of the SVZ. We used a Dlx2/tauLacZ knock-in to perform a short-term lineage analysis of Dlx2-expressing cells throughout SVZ formation and the postnatal peak of gliogenesis. Dlx2/tauLacZ [beta-galactosidase (beta-gal)]-expressing cells populate the central SVZ, whereas Zebrin II-expressing cells form its borders. Furthermore, beta-gal expression demonstrates a lineage relationship between Dlx2-expressing cells and glia residing in the dorsal telencephalon. We propose a model for the formation of the postnatal SVZ and demonstrate that subpallium-derived Dlx2-expressing cells give rise to astrocytes and oligodendrocytes in the white matter and cerebral cortex.