Restoring lost nigrostriatal fibers in Parkinson's disease based on clinically-inspired design criteria.

Parkinson's disease is a neurodegenerative disease affecting around 10 million people worldwide. The death of dopaminergic neurons in the substantia nigra and the axonal fibers that constitute the nigrostriatal pathway leads to a loss of dopamine in the striatum that causes the motor symptoms of this disease. Traditional treatments have focused on reducing symptoms, while therapies with human fetal or stem cell-derived neurons have centered on implanting these cells in the striatum to restore its innervation. An alternative approach is pathway reconstruction, which aims to rebuild the entire structure of neurons and axonal fibers of the nigrostriatal pathway in a way that matches its anatomy and physiology. This type of repair could be more capable of reestablishing the signaling mechanisms that ensure proper dopamine release in the striatum and regulation of other motor circuit regions in the brain. In this manuscript, we conduct a review of the literature related to pathway reconstruction as a treatment for Parkinson's disease, delve into the limitations of these studies, and propose the requisite design criteria to achieve this goal at a human scale. We then present our tissue engineering-based platform to fabricate hydrogel-encased dopaminergic axon tracts in vitro for later implantation into the brain to replace and reconstruct the pathway. These tissue-engineered nigrostriatal pathways (TE-NSPs) can be characterized and optimized for cell number and phenotype, axon growth lengths and rates, and the capacity for synaptic connectivity and dopamine release. We then show original data of advances in creating these constructs matching clinical design criteria using human iPSC-derived dopaminergic neurons and a hyaluronic acid hydrogel. We conclude with a discussion of future steps that are needed to further optimize human-scale TE-NSPs and translate them into clinical products.

Coordinated Postnatal Maturation of Striatal Cholinergic Interneurons and Dopamine Release Dynamics in Mice.

Dynamic changes in motor abilities and motivated behaviors occur during the juvenile and adolescent periods. The striatum is a subcortical nucleus critical to action selection, motor learning, and reward processing. Its tonically active cholinergic interneuron (ChI) is an integral regulator of the synaptic activity of other striatal neurons, as well as afferent axonal projections of midbrain dopamine (DA) neurons; however, little is known about its development. Here, we report that ChI spontaneous activity increases during postnatal development of male and female mice, concomitant with a decreased afterhyperpolarization (AHP). We characterized the postnatal development of four currents that contribute to the spontaneous firing rate of ChIs, including ISK, IA, Ih, and INaP We demonstrated that the developmental increase in INaP drives increased ChI firing rates during the postnatal period and can be reversed by the INaP inhibitor, ranolazine. We next addressed whether immature cholinergic signaling may lead to functional differences in DA release during the juvenile period. In the adult striatum, nicotinic acetylcholine receptors (nAChRs) prevent linear summation of DA release in response to trains of high-frequency stimuli. We show that, in contrast, during the second postnatal week, DA release linearly sums with trains of high-frequency stimuli. Consistently, nAChR antagonists exert little effect on dopamine release at postnatal day (P)10, but enhance the summation of evoked DA release in mice older than postnatal day P28. Together, these results reveal that postnatal maturation of ChI activity is due primarily to enhanced INaP and identify an interaction between developing cholinergic signaling and DA neurotransmission in the juvenile striatum.SIGNIFICANCE STATEMENT Motor skills and motivated behavior develop rapidly in juvenile rodents. Recent work has highlighted processes that contribute to the postnatal maturation of striatal principal neurons during development. The functional development of the striatal cholinergic interneuron (ChI), however, has been unexplored. In this study, we tracked the ontogeny of ChI activity and cellular morphology, as well as the developmental trajectory of specific conductances that contribute to the activity of these cells. We further report a link between cholinergic signaling and dopamine (DA) release, revealing a change in the frequency-dependence of DA release during the early postnatal period that is mediated by cholinergic signaling. This study provides evidence that striatal microcircuits are dynamic during the postnatal period and that they undergo coordinated maturation.

Prefronto-Striatal Structural Connectivity Mediates Adult Age Differences in Action Selection.

In complex everyday environments, action selection is critical for optimal goal-directed behavior. This refers to the process of choosing a proper action from the range of possible alternatives. The neural mechanisms underlying action selection and how these are affected by normal aging remain to be elucidated. In the present cross-sectional study, we studied processes of effector selection during a multilimb reaction time task in a lifespan sample of healthy human adults (N = 89; 20-75 years; 48 males, 41 females). Participants were instructed to react as quickly and accurately as possible to visually cued stimuli representing single-limb or combined upper and/or lower limb motions. Diffusion MRI was used to study structural connectivity between prefrontal and striatal regions as critical nodes for action selection. Behavioral findings revealed that increasing age was associated with slowing of action selection performance. At the neural level, aging had a negative impact on prefronto-striatal connectivity. Importantly, mediation analyses revealed that the negative association between action selection performance and age was mediated by prefronto-striatal connectivity, specifically the connections between left rostral medial frontal gyrus and left nucleus accumbens as well as right frontal pole and left caudate. These results highlight the potential role of prefronto-striatal white matter decline in poorer action selection performance of older adults.SIGNIFICANCE STATEMENT As a result of enhanced life expectancy, researchers have devoted increasing attention to the study of age-related alterations in cognitive and motor functions. Here we study associations between brain structure and behavior to reveal the impact of central neural white matter changes as a function of normal aging on action selection performance. We demonstrate the critical role of a reduction in prefronto-striatal structural connectivity in accounting for action selection performance deficits in healthy older adults. Preserving this cortico-subcortical pathway may be critical for behavioral flexibility and functional independence in older age.

General Sources of Dopamine As a Potential Morphogenic Factor in the Developing Striatum of Rats.

In the striatum of rats at different stages of development, we determined the content of tyrosine hydroxylase and aromatic L-amino acids decarboxylase using double immunohistochemical labeling and estimated the expression level of their transcripts by real-time PCR. We found that, in different periods of development, there are three sources of dopamine in the striatum of rats: bienzymatic nerve fibers (throughout ontogeny), bienzymatic neurons (appear on day 18 of embryonic development), and monoenzymatic neurons (in adult animals). Dopamine, which is synthesized in the striatal neurons in the prenatal period, may function as a morphogenetic factor.

Topoisomerase 1 inhibitor topotecan delays the disease progression in a mouse model of Huntington's disease.

Huntington's disease (HD) is a dominantly inherited progressive neurodegenerative disorder caused by the accumulation of polyglutamine expanded mutant huntingtin as inclusion bodies primarily in the brain. After the discovery of the HD gene, considerable progress has been made in understanding the disease pathogenesis and multiple drug targets have been identified, even though currently there is no effective therapy. Here, we demonstrate that the treatment of topotecan, a brain-penetrating topoisomerase 1 inhibitor, to HD transgenic mouse considerably improved its motor behavioural abnormalities along with a significant extension of lifespan. Improvement of behavioural deficits are accompanied with the significant rescue of their progressively decreased body weight, brain weight and striatal volume. Interestingly, topotecan treatment also significantly reduced insoluble mutant huntingtin load in the HD mouse brain. Finally, we show that topotecan treatment to HD mouse not only inhibits the expression of transgenic mutant huntingtin, but also at the same time induces the expression of Ube3a, an ubiquitin ligase linked to the clearance of mutant huntingtin. These findings suggest that topotecan could be a potential therapeutic molecule to delay the progression of HD.

Frontostriatal White Matter Integrity Predicts Development of Delay of Gratification: A Longitudinal Study.

The ability to delay gratification increases considerably across development. Here, we test the hypothesis that this impulse control capacity is driven by increased maturation of frontostriatal circuitry using a fiber-tracking approach combined with longitudinal imaging. In total, 192 healthy volunteers between 8 and 26 years underwent diffusion tensor imaging scanning and completed a delay-discounting task twice, separated by a 2-year interval. We investigated dynamic associations between frontostriatal white matter (WM) integrity and delay of gratification skills. Moreover, we examined the predictive value of frontostriatal WM integrity for future delay of gratification skills. Results showed that delay discounting increases with age in a quadratic fashion, with greatest patience during late adolescence. Data also indicated nonlinear development of frontostriatal WM, with relative fast development during childhood and early adulthood and--on average--little change during mid-adolescence. Furthermore, the positive association between age and delay discounting was further increased in individuals with higher WM integrity of the frontostriatal tracts. Predictive analysis showed that frontostriatal WM development explained unique variance in current and future delay of gratification skills. This study adds to a descriptive relation between WM integrity and delay of gratification by showing that maturation of frontostriatal connectivity predicts changes in delay of gratification skills. These findings have implications for studies examining deviances in impulse control by showing that the developmental path between striatum and prefrontal cortex may be an important predictor for when development goes astray. SIGNIFICANCE STATEMENT: During the transition from childhood to adulthood, individuals generally show increased patience and become better in delaying gratification. The exact neural correlates of delay of gratification, however, remain poorly understood. By measuring both frontostriatal white matter (WM) integrity and delay of gratification skills at two time points, we were able to provide links for our understanding of the neural mechanisms underlying this type of impulse regulation capacity. We demonstrate that the ability to delay gratification improves between childhood and young adulthood and this improvement is predicted by the integrity of frontostriatal WM connections. This study adds to a descriptive relation between WM quality and delay of gratification by showing that maturation of frontostriatal connectivity predicts improvements in delay of gratification skills.

Sonic Hedgehog Controls the Phenotypic Fate and Therapeutic Efficacy of Grafted Neural Precursor Cells in a Model of Nigrostriatal Neurodegeneration.

The expression of soluble growth and survival promoting factors by neural precursor cells (NPCs) is suggested to be a prominent mechanism underlying the protective and regenerative effects of these cells after transplantation. Nevertheless, how and to what extent specific NPC-expressed factors contribute to therapeutic effects is not well understood. Using RNA silencing, the current study investigated the roles of two donor NPC molecules, namely glial cell-line derived neurotrophic factor (GDNF) and sonic hedgehog (SHH), in the protection of substantia nigra dopamine neurons in rats treated with 6-hydroxydopamine (6-OHDA). Analyses indicate that as opposed to the knock-down of GDNF, SHH inhibition caused a profound decline in nigrostriatal neuroprotection. Further, SHH silencing also curbed endogenous neurogenesis and the migration of host brdU+/dcx+ neural precursors into the striatum, which was present in the animals receiving control or GDNF silenced NPCs. A change in graft phenotype, mainly reflected by a reduced proportion of undifferentiated nestin+ cells, as well as a significantly greater host microglial activity, suggested an important role for these processes in the attenuation of neuroprotection and neurogenesis upon SHH silencing. Overall these studies reveal core mechanisms fundamental to grafted NPC-based therapeutic effects, and delineate the particular contributions of two graft-expressed molecules, SHH and GDNF, in mediating midbrain dopamine neuron protection, and host plasticity after NPC transplantation.

Present simple and continuous: emergence of self-regulation and contextual sophistication in adolescent decision-making.

Sophisticated, intentional decision-making is a hallmark of mature, self-aware behaviour. Although neural, psychological, interpersonal, and socioeconomic elements that contribute to such adaptive, foresighted behaviour mature and/or change throughout the life-span, here we concentrate on relevant maturational processes that take place during adolescence, a period of disproportionate developmental opportunity and risk. A brief, eclectic overview is presented of recent evidence, new challenges, and current thinking on the fundamental mechanisms that mature throughout adolescence to support adaptive, self-controlled decision-making. This is followed by a proposal for the putative contribution of frontostriatal mechanisms to the moment-to-moment assembly of evaluative heuristics that mediate increased decision-making sophistication, promoting the maturation of self-regulated behaviour through adolescence and young adulthood.

Effects of the volatile anesthetic sevoflurane on tonic GABA currents in the mouse striatum during postnatal development.

The volatile anesthetic sevoflurane, which is widely used in pediatric surgery, has proposed effects on GABAA receptor-mediated extrasynaptic tonic inhibition. In the developing striatum, medium-sized spiny projection neurons have tonic GABA currents, which function in the excitatory/inhibitory balance and maturation of striatal neural circuits. In this study, we examined the effects of sevoflurane on the tonic GABA currents of medium spiny neurons in developing striatal slices. Sevoflurane strongly increased GABAA receptor-mediated tonic conductance at postnatal days 3-35. The antagonist of the GABA transporter-1, 1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride further increased tonic GABA conductance during the application of sevoflurane, thereby increasing the total magnitude of tonic currents. Both GABA (5 µM) and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol hydrochloride, the δ-subunit-containing GABAA receptor agonist, induced tonic GABA currents in medium spiny neurons but not in cholinergic neurons. However, sevoflurane additively potentiated the tonic GABA currents in both cells. Interestingly, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol hydrochloride-sensitive neurons made a large current response to sevoflurane, indicating the contribution of the δ-subunit on sevoflurane-enhanced tonic GABA currents. Our findings suggest that sevoflurane can affect the tone of tonic GABA inhibition in a developing striatal neural network.

Resting state functional connectivity of the basal nucleus of Meynert in humans: in comparison to the ventral striatum and the effects of age.

The basal nucleus of Meynert (BNM) provides the primary cholinergic inputs to the cerebral cortex. Loss of neurons in the BNM is linked to cognitive deficits in Alzheimer's disease and other degenerative conditions. Numerous animal studies described cholinergic and non-cholinergic neuronal responses in the BNM; however, work in humans has been hampered by the difficulty of defining the BNM anatomically. Here, on the basis of a previous study that delineated the BNM of post-mortem human brains in a standard stereotaxic space, we sought to examine functional connectivity of the BNM, as compared to the nucleus accumbens (or ventral striatum, VS), in a large resting state functional magnetic resonance imaging data set. The BNM and VS shared but also showed a distinct pattern of cortical and subcortical connectivity. Compared to the VS, the BNM showed stronger positive connectivity with the putamen, pallidum, thalamus, amygdala and midbrain, as well as the anterior cingulate cortex, supplementary motor area and pre-supplementary motor area, a network of brain regions that respond to salient stimuli and orchestrate motor behavior. In contrast, compared to the BNM, the VS showed stronger positive connectivity with the ventral caudate and medial orbitofrontal cortex, areas implicated in reward processing and motivated behavior. Furthermore, the BNM and VS each showed extensive negative connectivity with visual and lateral prefrontal cortices. Together, the distinct cerebral functional connectivities support the role of the BNM in arousal, saliency responses and cognitive motor control and the VS in reward related behavior. Considering the importance of BNM in age-related cognitive decline, we explored the effects of age on BNM and VS connectivities. BNM connectivity to the visual and somatomotor cortices decreases while connectivity to subcortical structures including the midbrain, thalamus, and pallidum increases with age. These findings of age-related changes of cerebral functional connectivity of the BNM may facilitate research of the neural bases of cognitive decline in health and illness.

Ephrin-A5 deficiency alters sensorimotor and monoaminergic development.

The Eph receptors and their ligands, the ephrins, play an important role during neural development. In particular, ephrin-A5 is highly expressed in the developing nervous system in several brain regions including the olfactory bulb, frontal cortex, striatum and hypothalamus. Although a number of studies have characterized the expression of ephrin-A5 in these regions, very little is known about the functional consequences that might follow alterations in the expression of this ligand. Previously, we demonstrated that ephrin-A5 acts as a guidance molecule regulating the trajectory of the ascending midbrain dopaminergic pathways. In light of this finding and the critical role of dopamine in modulating a number of behaviors, we sought to determine whether loss of ephrin-A5 altered neurobehavioral development. Our results indicate that ephrin-A5-null mice exhibit delays in reaching developmental milestones and in the maturation of motor skills. In addition, they exhibit increased locomotor activity and reduced levels of brain monoamines. Therefore, we conclude that ephrin-A5 expression appears to be critical for proper development of central monoaminergic pathways and that its loss results in a number of neurodevelopmental abnormalities. Because alterations in monoamine function are associated with a variety of neurodevelopmental disorders, these data suggest that further study on the potential role of ephrin-A5 in such disorders is warranted.

Calbindin expression in developing striatum of zebra finches and its relation to the formation of area X.

A sexually dimorphic network of brain regions controls learning and production of song in zebra finches. How this specialized song system evolved is unknown. To start addressing this question, we focused on developmental differences between the sexes, using the expression of the calcium-binding protein calbindin (CB) during embryonic to adult stages to map out the early development of Area X, a male-specific striatal structure. We related this pattern to the expression of three transcription factors, Pax6 and Islet1 to delineate the striatal radial domains, and Nkx2.1 as a marker for cells of pallidal origin. An incipient Area X-CB+ domain became discernable at embryonic day 13 in the Islet1-ventral striatal field. This region contained many Nkx2.1-expressing cells with a morphology characteristic of migrating cells. Eight days after hatching (PHD) CB staining clearly delineated Area X. Another CB+ structure formed around PHD5 at the subpallial/pallial boundary. We call it the CB+striatal capsule (CB-StC) and discuss its relation with the previously described striatal capsule in vertebrates. The CB cell population in both Area X and CB-StC, but not in the surrounding striatum, colocalized with the striatal medium spiny neurons (MSNs) marker, D1-receptor associated signaling protein dopamine-and-cAMP-regulated phosphoprotein of 32 kDa, DARPP32. In females, CB-positive cells were also present in the rostral striatum but did not coalesce into an Area X-like structure. We discuss possible reasons for CB expression in MSNs in songbirds and mammals, but not described in chicken striatum.

Striatal development involves a switch in gene expression networks, followed by a myelination event: implications for neuropsychiatric disease.

Because abnormal development of striatal neurons is thought to be the part of pathology underlying major psychiatric illnesses, we studied the expression pattern of genes involved in striatal development and of genes comprising key striatal-specific pathways, during an active striatal maturation period, the first two postnatal weeks in rat. This period parallels human striatal development during the second trimester, when prenatal stress is though to lead to increased risk for neuropsychiatric disorders. To identify genes involved in this developmental process, we used subtractive hybridization, followed by quantitative real-time PCR, which allowed us to characterize the developmental expression of over 60 genes, many not previously known to play a role in neuromaturation. Of these 12 were novel transcripts, which did not match known genes, but which showed strict developmental expression and may play a role in striatal neurodevelopment. An additional 89 genes were identified as strong candidates for involvement in this neurodevelopmental process. We show that during the first two postnatal weeks in rat, an early gene expression network, still lacking key striatal-specific signaling pathways, is downregulated and replaced by a mature gene expression network, containing key striatal-specific genes including the dopamine D1 and D2 receptors, conferring to these neurons their functional identity. Therefore, before this developmental switch, striatal neurons lack many of their key phenotypic characteristics. This maturation process is followed by a striking rise in expression of myelination genes, indicating a striatal-specific myelination event. Such strictly controlled developmental program has the potential to be a point of susceptibility to disruption by external factors. Indeed, this period is known to be a susceptibility period in both humans and rats.

Fetal programming effects of testosterone on the reward system and behavioral approach tendencies in humans.

BACKGROUND: Sex differences are present in many neuropsychiatric conditions that affect emotion and approach-avoidance behavior. One potential mechanism underlying such observations is testosterone in early development. Although much is known about the effects of testosterone in adolescence and adulthood, little is known in humans about how testosterone in fetal development influences later neural sensitivity to valenced facial cues and approach-avoidance behavioral tendencies. METHODS: With functional magnetic resonance imaging we scanned 25 8-11-year-old children while viewing happy, fear, neutral, or scrambled faces. Fetal testosterone (FT) was measured via amniotic fluid sampled between 13 and 20 weeks gestation. Behavioral approach-avoidance tendencies were measured via parental report on the Sensitivity to Punishment and Sensitivity to Rewards questionnaire. RESULTS: Increasing FT predicted enhanced selectivity for positive compared with negatively valenced facial cues in reward-related regions such as caudate, putamen, and nucleus accumbens but not the amygdala. Statistical mediation analyses showed that increasing FT predicts increased behavioral approach tendencies by biasing caudate, putamen, and nucleus accumbens but not amygdala to be more responsive to positive compared with negatively valenced cues. In contrast, FT was not predictive of behavioral avoidance tendencies, either through direct or neurally mediated paths. CONCLUSIONS: This work suggests that testosterone in humans acts as a fetal programming mechanism on the reward system and influences behavioral approach tendencies later in life. As a mechanism influencing atypical development, FT might be important across a range of neuropsychiatric conditions that asymmetrically affect the sexes, the reward system, emotion processing, and approach behavior.

Medium spiny neurons of the neostriatal matrix exhibit specific, stereotyped changes in dendritic arborization during a critical developmental period in mice.

In mice, the matrix compartment of the striatum (caudate/putamen) undergoes major developmental changes during the second postnatal week, including the establishment of corticostriatal and nigrostriatal afferents, the maturation of parvalbumin-positive interneurons and the appearance of perineuronal nets. It is not known if any of these events influence the dendritic structure of medium spiny neurons, the principal output cells of the striatum. To determine whether any measurable changes in the dendrites of matrix medium spiny neurons occur during this important developmental period, we labeled individual cells at different time points flanking the second postnatal week. These cells exhibit distinct dendritic morphologies from the earliest postnatal time points examined. Furthermore, our data show that the dendritic arbors of these neurons change in length, branch points, diameter and tortuosity, regardless of morphological type. The increase in dendritic length is accompanied by a decrease in the number of branch points that occur in different, but consistent, parts of the dendritic arbor. All of these changes are most pronounced during the second postnatal week, coinciding with a number of developmental events considered important for consolidating circuitry within the striatal matrix. Our results further support the critical importance of this early postnatal period in striatal development.

Postnatal development of tyrosine hydroxylase mRNA-expressing neurons in mouse neostriatum.

The striatum harbors a small number of tyrosine hydroxylase (TH) mRNA-containing GABAergic neurons that express TH immunoreactivity after dopamine depletion, some of which reportedly resembled striatal medium spiny projection neurons (MSNs). To clarify whether the TH mRNA-expressing neurons were a subset of MSNs, we characterized their postnatal development of electrophysiological and morphological properties using a transgenic mouse strain expressing enhanced green fluorescent protein (EGFP) under the control of the rat TH gene promoter. At postnatal day (P)1, EGFP-TH+ neurons were present as clusters in the striatum and, thereafter, gradually scattered ventromedially by P18 without regard to the striatal compartments. They were immunonegative for calbindin, but immunopositive for enkephalin (54.5%) and dynorphin (80.0%). Whole-cell patch-clamp recordings revealed at least two distinct neuronal types, termed EGFP-TH+ Type A and B. Whereas Type B neurons were aspiny and negative for the MSN marker dopamine- and cyclic AMP-regulated phosphoprotein of 32 kDa (DARPP-32), Type A neurons constituted 75% of the EGFP+ cells, had dendritic spines (24.6%), contained DARPP-32 (73.6%) and a proportion acquired TH immunoreactivity after injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 3-nitropropionic acid. The membrane properties and N-methyl-d-aspartate : non-N-methyl-d-aspartate excitatory postsynaptic current ratio of Type A neurons were very similar to MSNs at P18. However, their resting membrane potentials and spike widths were statistically different from those of MSNs. In addition, the calbindin-like, DARPP-32-like and dynorphin B-like immunoreactivity of Type A neurons developed differently from that of MSNs in the matrix. Thus, Type A neurons closely resemble MSNs, but constitute a cell type distinct from classical MSNs.

Developmental profile of the aberrant dopamine D2 receptor response in striatal cholinergic interneurons in DYT1 dystonia.

BACKGROUND: DYT1 dystonia, a severe form of genetically determined human dystonia, exhibits reduced penetrance among carriers and begins usually during adolescence. The reasons for such age dependence and variability remain unclear. METHODS AND RESULTS: We characterized the alterations in D2 dopamine receptor (D2R) signalling in striatal cholinergic interneurons at different ages in mice overexpressing human mutant torsinA (hMT). An abnormal excitatory response to the D2R agonist quinpirole was recorded at postnatal day 14, consisting of a membrane depolarization coupled to an increase in spiking frequency, and persisted unchanged at 3 and 9 months in hMT mice, compared to mice expressing wild-type human torsinA and non-transgenic mice. This response was blocked by the D2R antagonist sulpiride and depended upon G-proteins, as it was prevented by intrapipette GDP-ß-S. Patch-clamp recordings from dissociated interneurons revealed a significant increase in the Cav2.2-mediated current fraction at all ages examined. Consistently, chelation of intracellular calcium abolished the paradoxical response to quinpirole. Finally, no gross morphological changes were observed during development. CONCLUSIONS: These results suggest that an imbalanced striatal dopaminergic/cholinergic signaling occurs early in DYT1 dystonia and persists along development, representing a susceptibility factor for symptom generation.

Tachykinin-immunoreactive neurons in developing feline neostriatum: somatodendritic morphogenesis demonstrated by combined immunohistochemistry/Golgi impregnation-gold toning.

This investigation was designed to survey and characterize the development of a key link between chemically mediated neurotransmission and neuronal cytoarchitecture in mammalian basal ganglia. Peroxidase immunohistochemical and Golgi impregnation/gold toning methods were combined to doubly label the tachykinin neuromodulator signature and somatodendritic structure of neostriatal neurons in late fetal, postnatal and adult cats. The results supported 3 conclusions of considerable significance. (1) Colocalization of immunohistochemical and Golgi impregnation/gold toning labels is a feasible, rational and productive means to identify and determine the somatodendritic morphogenesis of tachykinin neurons. (2) The application of this method to developing feline neostriatum demonstrates directly that the principal tachykinin cells are medium-sized spiny neurons, which undergo progressive growth and elaboration of cell bodies, dendritic arbors and dendritic spines during the late fetal and postnatal periods. (3) There is a strong but incomplete concordance between tachykinin and medium-sized spiny neuronal phenotypes, because a minor variant of medium-sized spiny neurons and rare subgroups of medium- and large-sized sparse spiny neurons also show the tachykinin neuromodulator signature. Taken together, these results suggest that neostriatal neurons show an early commitment to heterogeneous tachykinin phenotypes, although the full and final expression of their somatodendritic characteristics coincides with synaptogenesis.

Humanized Foxp2 specifically affects cortico-basal ganglia circuits.

It has been proposed that two amino acid substitutions in the transcription factor FOXP2 have been positively selected during human evolution and influence aspects of speech and language. Recently it was shown that when these substitutions are introduced into the endogenous Foxp2 gene of mice, they increase dendrite length and long-term depression (LTD) in medium spiny neurons of the striatum. Here we investigated if these effects are found in other brain regions. We found that neurons in the cerebral cortex, the thalamus and the striatum have increased dendrite lengths in the humanized mice whereas neurons in the amygdala and the cerebellum do not. In agreement with previous work we found increased LTD in medium spiny neurons, but did not detect alterations of synaptic plasticity in Purkinje cells. We conclude that although Foxp2 is expressed in many brain regions and has multiple roles during mammalian development, the evolutionary changes that occurred in the protein in human ancestors specifically affect brain regions that are connected via cortico-basal ganglia circuits.

Differential short-term plasticity at convergent inhibitory synapses to the substantia nigra pars reticulata.

Inhibitory projections from the striatum and globus pallidus converge onto GABAergic projection neurons of the substantia nigra pars reticulata (SNr). Based on existing structural and functional evidence, these pathways are likely to differentially regulate the firing of SNr neurons. We sought to investigate the functional differences in inhibitory striatonigral and pallidonigral traffic using whole-cell voltage clamp in brain slices with these pathways preserved. We found that striatonigral IPSCs exhibited a high degree of paired-pulse facilitation. We tracked this facilitation over development and found the facilitation as the animal aged, but stabilized by postnatal day 17 (P17), with a paired pulse ratio of 2. We also found that the recovery from facilitation accelerated over development, again, reaching a stable phenotype by P17. In contrast, pallidonigral synapses show paired-pulse depression, and this depression could be solely explained by presynaptic changes. The mean paired-pulse ratio of 0.67 did not change over development, but the recovery from depression slowed over development. Pallidonigral IPSCs were significantly faster than striatonigral IPSCs when measured at the soma. Finally, under current clamp, prolonged bursts of striatal IPSPs were able to consistently silence the pacemaker activity of nigral neurons, whereas pallidal inputs depressed, allowing nigral neurons to reinstate firing. These findings highlight the importance of differential dynamics of neurotransmitter release in regulating the circuit behavior of the basal ganglia.