Early patterning of prelimbic cortical axons to the striatal patch compartment in the neonatal mouse

In humans, striato-pallido-thalamic projections are largely segregated by their origin in either the striosome-like or matrix-like compartments

Cortico-striato-thalamo-cortical (CSTC) loops are fundamental organizing units in mammalian brains. CSTCs process limbic, associative, and sensorimotor information in largely separated but interacting networks. CTSC loops pass through paired striatal compartments, striosome (aka patch) and matrix, segregated pools of medium spiny projection neurons with distinct embryologic origins, cortical/subcortical structural connectivity, susceptibility to injury, and roles in behaviors and diseases. Similarly, striatal dopamine modulates activity in striosome and matrix in opposite directions. Routing CSTCs through one compartment may be an anatomical basis for regulating discrete functions. We used differential structural connectivity, identified through probabilistic diffusion tractography, to distinguish the striatal compartments (striosome-like and matrix-like voxels) in living humans. We then mapped compartment-specific projections and quantified structural connectivity between each striatal compartment, the globus pallidus interna (GPi), and 20 thalamic nuclei in 221 healthy adults. We found that striosome-originating and matrix-originating streamlines were segregated within the GPi: striosome-like connectivity was significantly more rostral, ventral, and medial. Striato-pallido-thalamic streamline bundles that were seeded from striosome-like and matrix-like voxels transited spatially distinct portions of the white matter. Matrix-like streamlines were 5.7-fold more likely to reach the GPi, replicating animal tract-tracing studies. Striosome-like connectivity dominated in six thalamic nuclei (anteroventral, central lateral, laterodorsal, lateral posterior, mediodorsal-medial, and medial geniculate). Matrix-like connectivity dominated in seven thalamic nuclei (centromedian, parafascicular, pulvinar-anterior, pulvinar-lateral, ventral lateral-anterior, ventral lateral-posterior, ventral posterolateral). Though we mapped all thalamic nuclei independently, functionally-related nuclei were matched for compartment-level bias. We validated these results with prior thalamostriate tract tracing studies in non-human primates and other species; where reliable data was available, all agreed with our measures of structural connectivity. Matrix-like connectivity was lateralized (left > right hemisphere) in 18 thalamic nuclei, independent of handedness, diffusion protocol, sex, or whether the nucleus was striosome-dominated or matrix-dominated. Compartment-specific biases in striato-pallido-thalamic structural connectivity suggest that routing CSTC loops through striosome-like or matrix-like voxels is a fundamental mechanism for organizing and regulating brain networks. Our MRI-based assessments of striato-thalamic connectivity in humans match and extend the results of prior tract tracing studies in animals. Compartment-level characterization may improve localization of human neuropathologies and improve neurosurgical targeting in the GPi and thalamus.

Ten-m3 plays a role in the formation of thalamostriatal projections

The importance of the thalamostriatal pathway for a myriad of brain functions is becoming increasingly apparent. Little is known about the formation of this pathway in mice. Further, while Ten-m3, a member of the Ten-m/teneurin/Odz family, is implicated in the proper wiring of mature thalamostriatal projections, its developmental time course is unknown. Here, we describe the normal development of thalamostriatal projections arising from the parafascicular nucleus (PFN) and show a role for Ten-m3 in its formation. Ten-m3 is expressed in both the PFN and the striatum by embryonic day 17 (E17). By postnatal day 3 (P3), it had a patchy appearance in the striatum, overlaid on a high dorsal-low ventral expression gradient in both structures. In wild-type mice, axons from the PFN begin to innervate the striatum by E17. By P3, terminals had ramified but were not confined to any striatal subregion. By P7, the axons had begun to avoid striosomes. The first indication of clustering of thalamic terminals within the striatal matrix was also seen at this time point. The compartmental targeting and clustering of PFN projections became more apparent by P10. Analysis of Ten-m3 knockout mice showed that while the early developmental progression of the thalamostriatal pathway is conserved, by P10 differences emerged, with a loss of topographic precision and the absence of terminal clustering. No evidence of the involvement of EphA7 downstream of Ten-m3 was found. Overall, our results suggest that Ten-m3 plays a role in the consolidation and refinement of thalamic axons to a specific subregion of the striatal matrix.

Striosomes and Matrisomes: Scaffolds for Dynamic Coupling of Volition and Action

Striosomes form neurochemically specialized compartments of the striatum embedded in a large matrix made up of modules called matrisomes. Striosome-matrix architecture is multiplexed with the canonical direct-indirect organization of the striatum. Striosomal functions remain to be fully clarified, but key information is emerging. First, striosomes powerfully innervate nigral dopamine-containing neurons and can completely shut down their activity, with a following rebound excitation. Second, striosomes receive limbic and cognition-related corticostriatal afferents and are dynamically modulated in relation to value-based actions. Third, striosomes are spatially interspersed among matrisomes and interneurons and are influenced by local and global neuromodulatory and oscillatory activities. Fourth, striosomes tune engagement and the motivation to perform reinforcement learning, to manifest stereotypical behaviors, and to navigate valence conflicts and valence discriminations. We suggest that, at an algorithmic level, striosomes could serve as distributed scaffolds to provide formats of the striatal computations generated through development and refined through learning. We propose that striosomes affect subjective states. By transforming corticothalamic and other inputs to the functional formats of the striatum, they could implement state transitions in nigro-striato-nigral circuits to affect bodily and cognitive actions according to internal motives whose functions are compromised in neuropsychiatric conditions.

Dynamic proteomic and phosphoproteomic atlas of corticostriatal axons in neurodevelopment

Mammalian axonal development begins in embryonic stages and continues postnatally. After birth, axonal proteomic landscape changes rapidly, coordinated by transcription, protein turnover, and post-translational modifications. Comprehensive profiling of axonal proteomes across neurodevelopment is limited, with most studies lacking cell-type and neural circuit specificity, resulting in substantial information loss. We create a Cre-dependent APEX2 reporter mouse line and map cell-type-specific proteome of corticostriatal projections across postnatal development. We synthesize analysis frameworks to define temporal patterns of axonal proteome and phosphoproteome, identifying co-regulated proteins and phosphorylations associated with genetic risk for human brain disorders. We discover proline-directed kinases as major developmental regulators. APEX2 transgenic reporter proximity labeling offers flexible strategies for subcellular proteomics with cell type specificity in early neurodevelopment, a critical period for neuropsychiatric disease.

An MRI method for parcellating the human striatum into matrix and striosome compartments in vivo

The mammalian striatum is comprised of intermingled tissue compartments, matrix and striosome. Though indistinguishable by routine histological techniques, matrix and striosome have distinct embryologic origins, afferent/efferent connections, surface protein expression, intra-striatal location, susceptibilities to injury, and functional roles in a range of animal behaviors. Distinguishing the compartments previously required post-mortem tissue and/or genetic manipulation; we aimed to identify matrix/striosome non-invasively in living humans. We used diffusion MRI (probabilistic tractography) to identify human striatal voxels with connectivity biased towards matrix-favoring or striosome-favoring regions (determined by prior animal tract-tracing studies). Segmented striatal compartments replicated the topological segregation and somatotopic organization identified in animal matrix/striosome studies. Of brain regions mapped in prior studies, our human brain data confirmed 93% of the compartment-selective structural connectivity demonstrated in animals. Test-retest assessment on repeat scans found a voxel classification error rate of 0.14%. Fractional anisotropy was significantly higher in matrix-like voxels, while mean diffusivity did not differ between the compartments. As mapped by the Talairach human brain atlas, 460 regions were significantly biased towards either matrix or striosome. Our method allows the study of striatal compartments in human health and disease, in vivo, for the first time.

Environmental Enrichment From Birth Impacts Parvalbumin Expressing Cells and Wisteria Floribunda Agglutinin Labelled Peri-Neuronal Nets Within the Developing Murine Striatum

Environmental enrichment can dramatically affect both the development and function of neural circuits. This is accomplished, at least in part, by the regulation of inhibitory cellular networks and related extracellular matrix glycoprotein structures known as perineuronal nets. The degree to which enhanced housing can influence brain areas involved in the planning and execution of actions is not well known. We examined the effect of enriching mice from birth on parvalbumin expression and perineuronal net formation in developing and adult striatum. This input nucleus of the basal ganglia consists of topographically discernible regions that serve different functions, providing a means of simultaneously examining the influence of environmental factors on discrete, but related networks. Greater densities of striatal parvalbumin positive cells and wisteria floribunda agglutinin labelled perineuronal nets were present in enriched pups during the second postnatal week, primarily within the lateral portion of the nucleus. Housing conditions continued to have an impact into adulthood, with enriched mice exhibiting higher parvalbumin positive cell densities in both medial and lateral striatum. Curiously, no differences due to housing conditions were detected in striatal perineuronal net densities of mature animals. The degree of overlap between striatal parvalbumin expression and perineuronal net formation was also increased, suggesting that heightened neural activity associated with enrichment may have contributed to greater engagement of networks affiliated with cells that express the calcium binding protein. Brain derived neurotrophic factor, an important regulator of inhibitory network maturation, is also subtly, but significantly affected within the striatum of enriched cohorts. Together, these findings suggest that environmental enrichment can exert cell specific effects within different divisions of an area vital for the regulation of action.

Synaptic Wiring of Corticostriatal Circuits in Basal Ganglia: Insights into the Pathogenesis of Neuropsychiatric Disorders

The striatum is a key hub in the basal ganglia for processing neural information from the sensory, motor, and limbic cortices. The massive and diverse cortical inputs entering the striatum allow the basal ganglia to perform a repertoire of neurological functions ranging from basic level of motor control to high level of cognition. The heterogeneity of the corticostriatal circuits, however, also renders the system susceptible to a repertoire of neurological diseases. Clinical and animal model studies have indicated that defective development of the corticostriatal circuits is linked to various neuropsychiatric disorders, including attention-deficit hyperactivity disorder (ADHD), Tourette syndrome, obsessive-compulsive disorder (OCD), autism spectrum disorder (ASD), and schizophrenia. Importantly, many neuropsychiatric disease-risk genes have been found to form the molecular building blocks of the circuit wiring at the synaptic level. It is therefore imperative to understand how corticostriatal connectivity is established during development. Here, we review the construction during development of these corticostriatal circuits at the synaptic level, which should provide important insights into the pathogenesis of neuropsychiatric disorders related to the basal ganglia and help the development of appropriate therapies for these diseases.

Optogenetic stimulation of glutamatergic neuronal activity in the striatum enhances neurogenesis in the subventricular zone of normal and stroke mice

Neurogenesis in the subventricular zone (SVZ) of the adult brain may contribute to tissue repair after brain injuries. Whether SVZ neurogenesis can be upregulated by specific neuronal activity in vivo and promote functional recovery after stroke is largely unknown. Using the spatial and cell type specific optogenetic technique combined with multiple approaches of in vitro, ex vivo and in vivo examinations, we tested the hypothesis that glutamatergic activation in the striatum could upregulate SVZ neurogenesis in the normal and ischemic brain. In transgenic mice expressing the light-gated channelrhodopsin-2 (ChR2) channel in glutamatergic neurons, optogenetic stimulation of the glutamatergic activity in the striatum triggered glutamate release into SVZ region, evoked membrane currents, Ca²⁺ influx and increased proliferation of SVZ neuroblasts, mediated by AMPA receptor activation. In ChR2 transgenic mice subjected to focal ischemic stroke, optogenetic stimuli to the striatum started 5days after stroke for 8days not only promoted cell proliferation but also the migration of SVZ neuroblasts into the peri-infarct cortex with increased neuronal differentiation and improved long-term functional recovery. These data provide the first morphological and functional evidence showing a unique striatum-SVZ neuronal regulation via a semi-phasic synaptic mechanism that can boost neurogenic cascades and stroke recovery. The benefits from stimulating endogenous glutamatergic activity suggest a novel regenerative strategy after ischemic stroke and other brain injuries.

Foxp2 controls synaptic wiring of corticostriatal circuits and vocal communication by opposing Mef2c

Cortico-basal ganglia circuits are critical for speech and language and are implicated in autism spectrum disorder (ASD), in which language function can be severely affected. We demonstrate that in the striatum, the gene, Foxp2, negatively interacts with the synapse suppressor, Mef2C. We present causal evidence that Mef2C inhibition by Foxp2 in neonatal mouse striatum controls synaptogenesis of corticostriatal inputs and vocalization in neonates. Mef2C suppresses corticostriatal synapse formation and striatal spinogenesis, but can, itself, be repressed by Foxp2 through direct DNA binding. Foxp2 deletion de-represses Mef2C, and both intrastriatal and global decrease of Mef2C rescue vocalization and striatal spinogenesis defects of Foxp2-deletion mutants. These findings suggest that Foxp2-Mef2C signaling is critical to corticostriatal circuit formation. If found in humans, such signaling defects could contribute to a range of neurologic and neuropsychiatric disorders.

The presence of cortical neurons in striatal-cortical co-cultures alters the effects of dopamine and BDNF on medium spiny neuron dendritic development

Medium spiny neurons (MSNs) are the major striatal neuron and receive synaptic input from both glutamatergic and dopaminergic afferents. These synapses are made on MSN dendritic spines, which undergo density and morphology changes in association with numerous disease and experience-dependent states. Despite wide interest in the structure and function of mature MSNs, relatively little is known about MSN development. Furthermore, most in vitro studies of MSN development have been done in simple striatal cultures that lack any type of non-autologous synaptic input, leaving open the question of how MSN development is affected by a complex environment that includes other types of neurons, glia, and accompanying secreted and cell-associated cues. Here we characterize the development of MSNs in striatal-cortical co-culture, including quantitative morphological analysis of dendritic arborization and spine development, describing progressive changes in density and morphology of developing spines. Overall, MSN growth is much more robust in the striatal-cortical co-culture compared to striatal mono-culture. Inclusion of dopamine (DA) in the co-culture further enhances MSN dendritic arborization and spine density, but the effects of DA on dendritic branching are only significant at later times in development. In contrast, exogenous Brain Derived Neurotrophic Factor (BDNF) has only a minimal effect on MSN development in the co-culture, but significantly enhances MSN dendritic arborization in striatal mono-culture. Importantly, inhibition of NMDA receptors in the co-culture significantly enhances the effect of exogenous BDNF, suggesting that the efficacy of BDNF depends on the cellular environment. Combined, these studies identify specific periods of MSN development that may be particularly sensitive to perturbation by external factors and demonstrate the importance of studying MSN development in a complex signaling environment.

The glycoprotein Ten-m3 mediates topography and patterning of thalamostriatal projections from the parafascicular nucleus in mice

The striatum is the key input nucleus of the basal ganglia, and is implicated in motor control and learning. Despite the importance of striatal circuits, the mechanisms associated with their development are not well established. Previously, Ten-m3, a member of the Ten-m/teneurin/odz family of transmembrane glycoproteins, was found to be important in the mapping of binocular visual pathways. Here, we investigated a potential role for Ten-m3 in striatal circuit formation. In situ hybridisation revealed a patchy distribution of Ten-m3 mRNA expression superimposed on a high-dorsal to low-ventral gradient in a subregion of the striatal matrix. A survey of afferent/efferent structures associated with the matrix identified the parafascicular thalamic nucleus (PF) as a potential locus of action. Ten-m3 was also found to be expressed in a high-dorsal to low-ventral gradient in the PF, corresponding topographically to its expression in the striatum. Further, a subset of thalamic terminal clusters overlapped with Ten-m3-positive domains within the striatal matrix. Studies in wild-type (WT) and Ten-m3 knockout (KO) mice revealed no differences in overall striatal or PF structure. Thalamostriatal terminals in KOs, however, while still confined to the matrix subregion, lost their clustered appearance. Topography was also altered, with terminals from the lateral PF projecting ectopically to ventral and medial striatum, rather than remaining confined dorsolaterally as in WTs. Behaviorally, Ten-m3 KOs displayed delayed motor skill acquisition. This study demonstrates that Ten-m3 plays a key role in directing the formation of thalamostriatal circuitry, the first molecular candidate reported to regulate connectivity within this pathway.

BDNF signaling and survival of striatal neurons

The striatum, a major component of the basal ganglia, performs multiple functions including control of movement, reward, and addiction. Dysfunction and death of striatal neurons are the main causes for the motor disorders associated with Huntington’s disease (HD). Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is among factors that promote survival and proper function of this neuronal population. Here, we review recent studies showing that BDNF determines the size of the striatum by supporting survival of the immature striatal neurons at their origin, promotes maturation of striatal neurons, and facilitates establishment of striatal connections during brain development. We also examine the role of BDNF in maintaining proper function of the striatum during adulthood, summarize the mechanisms that lead to a deficiency in BDNF signaling and subsequently striatal degeneration in HD, and highlight a potential role of BDNF as a therapeutic target for HD treatment.

Enrichment from birth accelerates the functional and cellular development of a motor control area in the mouse

Background

There is strong evidence that sensory experience in early life has a profound influence on the development of sensory circuits. Very little is known, however, about the role of experience in the early development of striatal networks which regulate both motor and cognitive function. To address this, we have investigated the influence of early environmental enrichment on motor development.

Methodology/Principal Findings

Mice were raised in standard or enriched housing from birth. For animals assessed as adults, half of the mice had their rearing condition reversed at weaning to enable the examination of the effects of pre- versus post-weaning enrichment. We found that exclusively pre-weaning enrichment significantly improved performance on the Morris water maze compared to non-enriched mice. The effects of early enrichment on the emergence of motor programs were assessed by performing behavioural tests at postnatal day 10. Enriched mice traversed a significantly larger region of the test arena in an open-field test and had improved swimming ability compared to non-enriched cohorts. A potential cellular correlate of these changes was investigated using Wisteria-floribunda agglutinin (WFA) staining to mark chondroitin-sulfate proteoglycans (CSPGs). We found that the previously reported transition of CSPG staining from striosome-associated clouds to matrix-associated perineuronal nets (PNNs) is accelerated in enriched mice.

Conclusions/Significance

This is the first demonstration that the early emergence of exploratory as well as coordinated movement is sensitive to experience. These behavioural changes are correlated with an acceleration of the emergence of striatal PNNs suggesting that they may consolidate the neural circuits underlying these behaviours. Finally, we confirm that pre-weaning experience can lead to life long changes in the learning ability of mice.

Afferent islands are larger than mu-opioid receptor patch in striatum of rat pups

Dopamine afferent islands were observed in rodent caudate-putamen only during development, whereas patches with intense mu-opioid receptor (MOR) immunoreactivity were seen throughout the life. We performed direct comparison between MOR patches and dopamine islands in the caudate-putamen of rat pups, by double immunofluorescence labeling for MOR and tyrosine hydroxylase. MOR patches were included in dopamine islands at postnatal day (P) 0 to P8, although the patches occupied the same region as the islands at P12-16. Furthermore, the regions of glutamatergic afferents with intense vesicular glutamate transporter 1 and vesicular glutamate transporter 2 immunoreactivities well corresponded to those of dopamine islands at P4. These results suggest that the striatal 'afferent islands' are larger than MOR patches in the early postnatal life.

Dynamic gene and protein expression patterns of the autism-associated met receptor tyrosine kinase in the developing mouse forebrain

The establishment of appropriate neural circuitry depends on the coordination of multiple developmental events across space and time. These events include proliferation, migration, differentiation, and survival-all of which can be mediated by hepatocyte growth factor (HGF) signaling through the Met receptor tyrosine kinase. We previously found a functional promoter variant of the MET gene to be associated with autism spectrum disorder, suggesting that forebrain circuits governing social and emotional function may be especially vulnerable to developmental disruptions in HGF/Met signaling. However, little is known about the spatiotemporal distribution of Met expression in the forebrain during the development of such circuits. To advance our understanding of the neurodevelopmental influences of Met activation, we employed complementary Western blotting, in situ hybridization, and immunohistochemistry to comprehensively map Met transcript and protein expression throughout perinatal and postnatal development of the mouse forebrain. Our studies reveal complex and dynamic spatiotemporal patterns of expression during this period. Spatially, Met transcript is localized primarily to specific populations of projection neurons within the neocortex and in structures of the limbic system, including the amygdala, hippocampus, and septum. Met protein appears to be principally located in axon tracts. Temporally, peak expression of transcript and protein occurs during the second postnatal week. This period is characterized by extensive neurite outgrowth and synaptogenesis, supporting a role for the receptor in these processes. Collectively, these data suggest that Met signaling may be necessary for the appropriate wiring of forebrain circuits, with particular relevance to the social and emotional dimensions of behavior.

Rapid reversal of chondroitin sulfate proteoglycan associated staining in subcompartments of mouse neostriatum during the emergence of behaviour

Background

The neostriatum, the mouse homologue of the primate caudate/putamen, is the input nucleus for the basal ganglia, receiving both cortical and dopaminergic input to each of its sub-compartments, the striosomes and matrix. The coordinated activation of corticostriatal pathways is considered vital for motor and cognitive abilities, yet the mechanisms which underlie the generation of these circuits are unknown. The early and specific targeting of striatal subcompartments by both corticostriatal and nigrostriatal terminals suggests activity-independent mechanisms, such as axon guidance cues, may play a role in this process. Candidates include the chondroitin sulfate proteoglycan (CSPG) family of glycoproteins which have roles not only in axon guidance, but also in the maturation and stability of neural circuits where they are expressed in lattice-like perineuronal nets (PNNs).

Methodology/Principal Findings

The expression of CSPG-associated structures and PNNs with respect to neostriatal subcompartments has been examined qualitatively and quantitatively using double-labelling for Wisteria floribunda agglutinin (WFA), and the μ-opioid receptor (μOR), a marker for striosomes, at six postnatal ages in mice. We find that at the earliest ages (postnatal day (P)4 and P10), WFA-positive clusters overlap preferentially with the striosome compartment. By P14, these clusters disappear. In contrast, PNNs were first seen at P10 and continued to increase in density and spread throughout the caudate/putamen with maturation. Remarkably, the PNNs overlap almost exclusively with the neostriatal matrix.

Conclusions/Significance

This is the first description of a reversal in the distribution of CSPG associated structures, as well as the emergence and maintenance of PNNs in specific subcompartments of the neostriatum. These results suggest diverse roles for CSPGs in the formation of functional corticostriatal and nigrostriatal connectivity within the striosome and matrix compartments of the developing caudate/putamen.

Augmented D1 dopamine receptor signaling and immediate-early gene induction in adult striatum after prenatal cocaine

BACKGROUND

Prenatal exposure to cocaine can impede normal brain development, triggering a range of neuroanatomical and behavioral anomalies that are evident throughout life. Mouse models have been especially helpful in delineating neuro-teratogenic consequences after prenatal exposure to cocaine. The present study employed a mouse model to investigate alterations in D(1) dopamine receptor signaling and downstream immediate-early gene induction in the striatum of mice exposed to cocaine in utero.

METHODS

Basal, forskolin-, and D(1) receptor agonist-induced cyclic adenosine monophosphate (cAMP) levels were measured ex vivo in the adult male striatum in mice exposed to cocaine in utero. Further studies assessed cocaine-induced zif 268 and homer 1 expression in the striatum of juvenile (P15), adolescent (P36), and adult (P60) male mice.

RESULTS

The D(1) dopamine receptor agonist SKF82958 induced significantly higher levels of cAMP in adult male mice treated with cocaine in utero compared with saline control subjects. No effects of the prenatal treatment were found for cAMP formation induced by forskolin. After an acute cocaine challenge (15 mg/kg, IP), these mice showed greater induction of zif 268 and homer 1, an effect that was most robust in the medial part of the mid-level striatum and became more pronounced with increasing age.

CONCLUSIONS

Together these findings indicate abnormally enhanced D(1) receptor signal transduction in adult mice after prenatal cocaine exposure. Such changes in dopamine receptor signaling might underlie aspects of long-lasting neuro-teratogenic effects evident in some humans after in utero exposure to cocaine and identify the striatum as one target potentially vulnerable to gestational cocaine exposure.

Glutamate and Regulation of Proliferation in the Developing Mammalian Telencephalon

Increasing evidence suggests that classical neurotransmitters play an important morphogenetic role during development of the mammalian central nervous system. Using in vitro and in vivo models, we have previously identified a role for the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors in the proliferation of striatal progenitors. Here, we compare the roles of ionotropic glutamate receptors in the proliferation of either striatal or cortical progenitors. In culture, glutamate receptor activation promoted proliferation of both striatal and cortical neuroblasts. However, cortical and striatal neuroblasts responded to distinct ionotropic receptors. Cortical cultures were sensitive to AMPA/KA receptor blockade, whereas striatal neuroblast proliferation was altered by NMDA antagonists. In vivo, BrdU uptake in the proliferative ventricular zone was reduced in embryos following acute administration of ionotropic glutamate receptor antagonists. In keeping with in vitro observations, proliferation in cortical and striatal ventricular regions was reduced, respectively, by either AMPA/KA or NMDA receptor blockade. We also determined whether forebrain-derived progenitors expanded as neurospheres in the presence of growth factors show similar ionotropic glutamatergic responses. Cells in neither dorsal nor ventral telencephalon-derived neurospheres showed altered proliferation following exposure to either class of ionotropic glutamate receptor antagonist. Together, these findings suggest that glutamate influences the proliferation of forebrain neuronal progenitors, but not more primitive populations represented in multipotential progenitors expanded in vitro. The effects on neuroblast proliferation in different forebrain domains are heterogeneous and are mediated by distinct subclasses of ionotropic glutamate receptors.

Early striatal dendrite deficits followed by neuron loss with advanced age in the absence of anterograde cortical brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, modulates neuronal survival, differentiation, and synaptic function. Reduced BDNF expression in the cortex caused by mutation of the huntingtin gene has been suggested to play a role in the striatal degeneration observed in Huntington's disease. BDNF expression rises dramatically in the cortex during the first few weeks of postnatal life in mice. Previously, it has been impossible to study the specific long-term effects of BDNF absence on CNS structures because of the early postnatal lethality of BDNF-/- mice. Mice harboring a floxed BDNF gene were bred with Emx1(IREScre/+) mice to generate Emx-BDNF(KO) mice that lack cortical BDNF but are viable. Adult Emx-BDNF(KO) mice display a hindlimb clasping phenotype similar to that observed in mouse models of Huntington's disease. The striatum of postnatal Emx-BDNF(KO) mice was reduced in volume compared with controls, and the most abundant neuron type of the striatum, medium spiny neurons (MSNs), had shrunken cell somas, thinner dendrites, and fewer dendritic spines at 35 d of age. Although significant striatal neuron losses were not detected at 35 or 120 d postnatal, 35% of striatal neurons were missing in Emx-BDNF(KO) mice aged beyond 1 year. Thus, cortical BDNF, although not required for the generation or near-term survival of MSN, is necessary for normal striatal neuron dendrite morphology during the period when BDNF expression rises in the cortex. Furthermore, a long-term in vivo requirement for cortical BDNF in supporting the survival of MSNs is revealed.

Pattern of corticostriatal innervation in organotypic cocultures is dependent on the age of the cortical tissue

The patch-matrix organization of the striatum is defined by the selective expression of neuronal markers and a semisegregated pattern of afferents and efferents that develops before birth in all mammals. Differential projections from 'limbic' and 'somatomotor' cortices contribute to the selective circuitry of patch ("striosome") and matrix compartments. Organotypic cultures were used to determine the pattern of early corticostriatal innervation as a first step toward understanding the role of cortical innervation in the development of striatal patch-matrix organization. Perinatal striatum (E19-P4) was cocultured with the cortex obtained from same-age or different-age rats in the presence or absence of substantia nigra obtained from E14-15 fetuses. After 4-21 days in vitro, crystals of biocytin were placed directly onto the cortical piece to trace cortical projections into the striatal piece. Cortex obtained from fetuses (E19-22) or neonatal (P0-1) rats gave rise to a dense innervation of both prenatal and postnatal striatal slices; however, the pattern of biocytin-labeled fibers was found to be highly dependent on the age of the cortical tissue used. Cortex derived from rats between E20 and P1 gave rise to a heterogeneous distribution of fibers indicative of striatal patches when combined with striatal slices from same-age or younger (E18-19) fetuses. Cortex from E18-19 fetuses produced a homogeneous innervation even when cocultured with older striatal tissue in which the striatal patches were already present. The postnatal cortex (P2-P5) gave rise to little to no innervation of striatum of all ages. Similar findings were obtained with the use of either prelimbic or somatosensory cortex. In double- and triple-labeled cultures, the distribution of corticostriatal fibers overlapped substantially with patches of developing striatal neurons, as revealed by DARPP-32 immunocytochemistry. Dopaminergic innervation present when the substantia nigra was included in the cocultures also distributed preferentially to the developing patch compartment, but it did not substantially alter the pattern of corticostriatal innervation. These findings suggest that the cortex provides directive signals to the developing striatum rather than simply responding to the presence of patches that have already formed.

Basal EGR-1 (zif268, NGFI-A, Krox-24) expression in developing striatal patches: role of dopamine and glutamate

Egr-1 (also known as zif268, NGFI-A, or Krox 24) is an immediate-early gene of the zinc finger family that exhibits relatively high constitutive expression in the brain, as well as inducibility by seizure activity, stimulants, and salient physiological stimuli. Immunocytochemical detection of the Egr-1 protein in the developing striatum revealed that in the late prenatal and early postnatal period, Egr-1 protein was expressed selectively in patches of striatal neurons under basal conditions. Egr-1 immunoreactivity was co-expressed with known markers of striatal patch neurons, indicating that expression was greatest in the striatal patch compartment. This patchy expression of Egr-1 transitioned to a nearly homogeneous pattern of Egr-1-immunoreactive cells by postnatal day 10, at which time most striatal neurons appeared to be Egr-1-immunoreactive. The dopamine D1 antagonist SCH23390 (0.5-1.0 mg/kg) reduced Egr-1 expression during the first week postnatal, but it was no longer effective at postnatal day 10. On the other hand, the noncompetitive NMDA antagonist MK-801 (0.5-1.0 mg/kg) became more effective at reducing Egr-1 expression with age. Neonatal destruction of nigrostriatal dopamine afferents reduced the basal pattern of Egr-1 expression for 2-3 days after the lesion, but then Egr-1 expression returned. Thus, Egr-1 expression in the developing striatum appears to be driven first by dopaminergic afferents, and then later in development by excitatory glutamatergic afferents.

Long-term effects of prenatal stress on dopamine and glutamate receptors in adult rat brain

Prenatal stress greatly influences the ability of an individual to manage stressful events in adulthood. Such vulnerability may result from abnormalities in the development and integration of forebrain dopaminergic and glutamatergic projections during the prenatal period. In this study, we assessed the effects of prenatal stress on the expression of selective dopamine and glutamate receptor subtypes in the adult offsprings of rats subjected to repeated restraint stress during the last week of pregnancy. Dopamine D2-like receptors increased in dorsal frontal cortex (DFC), medial prefrontal cortex (MPC), hippocampal CA1 region and core region of nucleus accumbens (NAc) of prenatally stressed rats compared to control subjects. Glutamate NMDA receptors increased in MPC, DFC, hippocampal CA1, medial caudate-putamen, as well as in shell and core regions of NAc. Group III metabotropic glutamate receptors increased in MPC and DFC of prenatally stressed rats, but remained unchanged in all other regions examined. These results indicate that stress suffered during the gestational period has long lasting effects that extend into the adulthood of prenatally stressed offsprings. Changes in dopamine and glutamate receptor subtype levels in different forebrain regions of adult rats suggest that the development and formation of the corticostriatal and corticolimbic pathways may be permanently altered as a result of stress suffered prenatally. Maldevelopment of these pathways may provide a neurobiological substrate for the development of schizophrenia and other idiopathic psychotic disorders.

Development of striatal patch/matrix organization in organotypic co-cultures of perinatal striatum, cortex and substantia nigra

Organotypic cultures of fetal or early postnatal striatum were used to assess striatal patch formation and maintenance in the presence or absence of dopaminergic and glutamatergic influences. Vibratome-cut slices of the striatum prepared from embryonic day 19 to postnatal day 4 rat pups were maintained in static culture on clear membrane inserts in Dulbecco's modified Eagle's medium/F12 (1:1) with 20% horse serum. Some were co-cultured with embryonic day 12-16 ventral mesencephalon and/or embryonic day 19 to postnatal day 4 cortex, which produced a dense dopaminergic innervation and a modest cortical innervation. Donors of striatal and cortical tissue were previously injected with bromo-deoxyuridine (BrdU) on embryonic days 13 and 14 in order to label striatal neurons destined to populate the patch compartment of the striatum. Patches of BrdU-immunoreactive cells were maintained in organotypic cultures of late prenatal (embryonic days 20-22) or early postnatal striatum in the absence of nigral dopaminergic or cortical glutamatergic influences. In slices taken from embryonic day 19 fetuses prior to the time of in vivo patch formation, patches were observed to form after 10 days in vitro, in 39% of nigral-striatal co-cultures compared to 6% of striatal slices cultured alone or in the presence of cortex only. Patches of dopaminergic fibers, revealed by tyrosine hydroxylase immunoreactivity, were observed in the majority of nigral-striatal co-cultures. Immunostaining for the AMPA-type glutamate receptor GluR1 revealed a dense patch distribution in nearly all cultures, which developed in embryonic day 19 cultures after at least six days in vitro. These findings indicate that striatal patch/matrix organization is maintained in organotypic culture, and can be induced to form in vitro in striatal slices removed from fetuses prior to the time of in vivo patch formation. Furthermore, dopaminergic innervation from co-cultured pieces of ventral mesencephalon enhances patch formation in organotypic cultures.

In vitro biocytin injection into perinatal mouse brain: a method for tract tracing in developing tissue

Injection of biocytin provides an effective method for labeling axonal projections. Several difficulties arise when this technique is employed in fetal or early postnatal animals in vivo, including limited access to injection sites and extended post-injection survival periods. To circumvent these problems, we adapted the technique of extracellular biocytin injection for use in explanted brain hemispheres of developing mice. Briefly, entire brain hemispheres from perinatal mice (E16-P9) were removed and placed in oxygenated aCSF in a brain slice recording chamber. Following visually guided injection of biocytin (2%) into the prelimbic cortex, the brains were then incubated in oxygenated artificial cerebrospinal fluid (aCSF) for varying periods of time and then immersion-fixed in 4% paraformaldehyde and 0.5% glutaraldehyde. The next day, the brains were sectioned and processed for biocytin histochemistry using the avidin-biotin-complex method. We examined the method of injection, electrode type, time of injection, and post-injection incubation period. We found that in E16-P9 animals iontophoresis of biocytin using 8- to 12-megaohm patch clamp electrodes for a duration of 10 min provides optimal axonal labeling. Post-injection incubation times of four or more hours are sufficient for labeling fine caliber collaterals as well as axon bundles that reach distances over 3 mm. In vitro injection of biocytin into explanted brain hemispheres provides a quick and easy method for tract tracing in developing brains.