The level and integrity of synaptic input regulates dendrite structure

Molecular mechanisms that mediate dendrite morphogenesis

Neurons develop dendritic morphologies that bear cell type-specific features in dendritic field size and geometry, branch placement and density, and the types and distributions of synaptic contacts. Dendritic patterns influence the types and numbers of inputs a neuron receives, and the ways in which neural information is processed and transmitted in the circuitry. Even subtle alterations in dendritic structures can have profound consequences on neuronal function and are implicated in neurodevelopmental disorders. In this chapter, I review how growing dendrites acquire their exquisite patterns by drawing examples from diverse neuronal cell types in vertebrate and invertebrate model systems. Dendrite morphogenesis is shaped by intrinsic and extrinsic factors such as transcriptional regulators, guidance and adhesion molecules, neighboring cells and synaptic partners. I discuss molecular mechanisms that regulate dendrite morphogenesis with a focus on five aspects of dendrite patterning: (1) Dendritic cytoskeleton and cellular machineries that build the arbor; (2) Gene regulatory mechanisms; (3) Afferent cues that regulate dendritic arbor growth; (4) Space-filling strategies that optimize dendritic coverage; and (5) Molecular cues that specify dendrite wiring. Cell type-specific implementation of these patterning mechanisms produces the diversity of dendrite morphologies that wire the nervous system.

Partial Cone Loss Triggers Synapse-Specific Remodeling and Spatial Receptive Field Rearrangements in a Mature Retinal Circuit

Resilience of neural circuits has been observed in the persistence of function despite neuronal loss. In vision, acuity and sensitivity can be retained after 50% loss of cones. While neurons in the cortex can remodel after input loss, the contributions of cell-type-specific circuits to resilience are unknown. Here, we study the effects of partial cone loss in mature mouse retina where cell types and connections are known. At first-order synapses, bipolar cell dendrites remodel and synaptic proteins diminish at sites of input loss. Sites of remaining inputs preserve synaptic proteins. Second-order synapses between bipolar and ganglion cells remain stable. Functionally, ganglion cell spatio-temporal receptive fields retain center-surround structure following partial cone loss. We find evidence for slower temporal filters and expanded receptive field surrounds, derived mainly from inhibitory inputs. Surround expansion is absent in partially stimulated control retina. Results demonstrate functional resilience to input loss beyond pre-existing mechanisms in control retina.

Care et al. find that photoreceptor ablation causes structural rearrangement of bipolar cell input synapses while output synapses endure. Functionally, recipient ganglion cells show altered receptive field sizes, an effect not seen after partial stimulation of control retina, demonstrating de novo changes that occur in inhibitory circuitry after photoreceptor loss.

Activity-dependent synaptic integration and modulation of bilateral excitatory inputs in an auditory coincidence detection circuit

KEY POINTS

Binaural excitatory inputs to coincidence detection neurons in nucleus laminaris (NL) play essential roles in interaural time difference coding for sound localization. Here, we show that the two excitatory inputs are physiologically nearly completely segregated. Synaptic integration shows linear summation of EPSPs, ensuring high efficiency of coincidence detection of the bilateral excitatory inputs. We further show that the two excitatory inputs to single NL neurons are symmetrical in synaptic strength, kinetics and short-term plasticity. Modulation of the EPSCs by metabotropic glutamate receptors (mGluRs) is identical between the two excitatory inputs, maintaining balanced bilateral excitation under neuromodulatory conditions. Unilateral hearing deprivation reduces synaptic excitation and paradoxically strengthens mGluR modulation of EPSCs, suggesting activity-dependent anti-homeostatic regulation, a novel synaptic plasticity in response to sensory manipulations.

ABSTRACT

Neurons in the avian nucleus laminaris (NL) receive bilateral excitatory inputs from the cochlear nucleus magnocellularis, via morphologically symmetrical dorsal (ipsilateral) and ventral (contralateral) dendrites. Using in vitro whole-cell patch recordings in chicken brainstem slices, we investigated synaptic integration and modulation of the bilateral inputs to NL under normal and hearing deprivation conditions. We found that the two excitatory inputs onto single NL neurons were nearly completely segregated, and integration of the two inputs was linear for EPSPs. The two inputs had similar synaptic strength, kinetics and short-term plasticity. EPSCs in low but not middle and high frequency neurons were suppressed by activation of group I and II metabotropic glutamate receptors (mGluR I and II), with similar modulatory strength between the ipsilateral and contralateral inputs. Unilateral hearing deprivation by cochlea removal reduced the excitatory transmission on the deprived dendritic domain of NL. Interestingly, EPSCs evoked at the deprived domain were modulated more strongly by mGluR II than at the counterpart domain that received intact input in low frequency neurons, suggesting anti-homeostatic regulation. This was supported by a stronger expression of mGluR II protein on the deprived neuropils of NL. Under mGluR II modulation, EPSCs on the deprived input show transient synaptic facilitation, forming a striking contrast with normal hearing conditions under which pure synaptic depression is observed. These results demonstrate physiological symmetry and thus balanced bilateral excitatory inputs to NL neurons. The activity-dependent anti-homeostatic plasticity of mGluR modulation constitutes a novel mechanism regulating synaptic transmission in response to sensory input manipulations.

Proteomic analyses of nucleus laminaris identified candidate targets of the fragile X mental retardation protein

The avian nucleus laminaris (NL) is a brainstem nucleus necessary for binaural processing, analogous in structure and function to the mammalian medial superior olive. In chickens (Gallus gallus), NL is a well-studied model system for activity-dependent neural plasticity. Its neurons have bipolar extension of dendrites, which receive segregated inputs from two ears and display rapid and compartment-specific reorganization in response to unilateral changes in auditory input. More recently, fragile X mental retardation protein (FMRP), an RNA-binding protein that regulates local protein translation, has been shown to be enriched in NL dendrites, suggesting its potential role in the structural dynamics of these dendrites. To explore the molecular role of FMRP in this nucleus, we performed proteomic analysis of NL, using micro laser capture and liquid chromatography tandem mass spectrometry. We identified 657 proteins, greatly represented in pathways involved in mitochondria, translation and metabolism, consistent with high levels of activity of NL neurons. Of these, 94 are potential FMRP targets, by comparative analysis with previously proposed FMRP targets in mammals. These proteins are enriched in pathways involved in cellular growth, cellular trafficking and transmembrane transport. Immunocytochemistry verified the dendritic localization of several proteins in NL. Furthermore, we confirmed the direct interaction of FMRP with one candidate, RhoC, by in vitro RNA binding assays. In summary, we provide a database of highly expressed proteins in NL and in particular a list of potential FMRP targets, with the goal of facilitating molecular characterization of FMRP signaling in future studies.

Development of dendritic form and function

The nervous system is populated by numerous types of neurons, each bearing a dendritic arbor with a characteristic morphology. These type-specific features influence many aspects of a neuron's function, including the number and identity of presynaptic inputs and how inputs are integrated to determine firing properties. Here, we review the mechanisms that regulate the construction of cell type-specific dendrite patterns during development. We focus on four aspects of dendrite patterning that are particularly important in determining the function of the mature neuron: (a) dendrite shape, including branching pattern and geometry of the arbor; (b) dendritic arbor size;

Intense and specialized dendritic localization of the fragile X mental retardation protein in binaural brainstem neurons: a comparative study in the alligator, chicken, gerbil, and human

Neuronal dendrites are structurally and functionally dynamic in response to changes in afferent activity. The fragile X mental retardation protein (FMRP) is an mRNA binding protein that regulates activity-dependent protein synthesis and morphological dynamics of dendrites. Loss and abnormal expression of FMRP occur in fragile X syndrome (FXS) and some forms of autism spectrum disorders. To provide further understanding of how FMRP signaling regulates dendritic dynamics, we examined dendritic expression and localization of FMRP in the reptilian and avian nucleus laminaris (NL) and its mammalian analogue, the medial superior olive (MSO), in rodents and humans. NL/MSO neurons are specialized for temporal processing of low-frequency sounds for binaural hearing, which is impaired in FXS. Protein BLAST analyses first demonstrate that the FMRP amino acid sequences in the alligator and chicken are highly similar to human FMRP with identical mRNA-binding and phosphorylation sites, suggesting that FMRP functions similarly across vertebrates. Immunocytochemistry further reveals that NL/MSO neurons have very high levels of dendritic FMRP in low-frequency hearing vertebrates including alligator, chicken, gerbil, and human. Remarkably, dendritic FMRP in NL/MSO neurons often accumulates at branch points and enlarged distal tips, loci known to be critical for branch-specific dendritic arbor dynamics. These observations support an important role for FMRP in regulating dendritic properties of binaural neurons that are essential for low-frequency sound localization and auditory scene segregation, and support the relevance of studying this regulation in nonhuman vertebrates that use low frequencies in order to further understand human auditory processing disorders.

Illuminating the multifaceted roles of neurotransmission in shaping neuronal circuitry

Across the nervous system, neurons form highly stereotypic patterns of synaptic connections that are designed to serve specific functions. Mature wiring patterns are often attained upon the refinement of early, less precise connectivity. Much work has led to the prevailing view that many developing circuits are sculpted by activity-dependent competition among converging afferents, which results in the elimination of unwanted synapses and the maintenance and strengthening of desired connections. Studies of the vertebrate retina, however, have recently revealed that activity can play a role in shaping developing circuits without engaging competition among converging inputs that differ in their activity levels. Such neurotransmission-mediated processes can produce stereotypic wiring patterns by promoting selective synapse formation rather than elimination. We discuss how the influence of transmission may also be limited by circuit design and further highlight the importance of transmission beyond development in maintaining wiring specificity and synaptic organization of neural circuits.

Resolution of interaural time differences in the avian sound localization circuit-a modeling study

Interaural time differences (ITDs) are a main cue for sound localization and sound segregation. A dominant model to study ITD detection is the sound localization circuitry in the avian auditory brainstem. Neurons in nucleus laminaris (NL) receive auditory information from both ears via the avian cochlear nucleus magnocellularis (NM) and compare the relative timing of these inputs. Timing of these inputs is crucial, as ITDs in the microsecond range must be discriminated and encoded. We modeled ITD sensitivity of single NL neurons based on previously published data and determined the minimum resolvable ITD for neurons in NL. The minimum resolvable ITD is too large to allow for discrimination by single NL neurons of naturally occurring ITDs for very low frequencies. For high frequency NL neurons (>1 kHz) our calculated ITD resolutions fall well within the natural range of ITDs and approach values of below 10 μs. We show that different parts of the ITD tuning function offer different resolution in ITD coding, suggesting that information derived from both parts may be used for downstream processing. A place code may be used for sound location at frequencies above 500 Hz, but our data suggest the slope of the ITD tuning curve ought to be used for ITD discrimination by single NL neurons at the lowest frequencies. Our results provide an important measure of the necessary temporal window of binaural inputs for future studies on the mechanisms and development of neuronal computation of temporally precise information in this important system. In particular, our data establish the temporal precision needed for conduction time regulation along NM axons.

Pre-target axon sorting in the avian auditory brainstem

Topographic organization of neurons is a hallmark of brain structure. The establishment of the connections between topographically organized brain regions has attracted much experimental attention, and it is widely accepted that molecular cues guide outgrowing axons to their targets in order to construct topographic maps. In a number of systems afferent axons are organized topographically along their trajectory as well, and it has been suggested that this pre-target sorting contributes to map formation. Neurons in auditory regions of the brain are arranged according to their best frequency (BF), the sound frequency they respond to optimally. This BF changes predictably with position along the so-called tonotopic axis. In the avian auditory brainstem, the tonotopic organization of the second- and third-order auditory neurons in nucleus magnocellularis (NM) and nucleus laminaris (NL) has been well described. In this study we examine whether the decussating NM axons forming the crossed dorsal cochlear tract (XDCT) and innervating the contralateral NL are arranged in a systematic manner. We electroporated dye into cells in different frequency regions of NM to anterogradely label their axons in XDCT. The placement of dye in NM was compared to the location of labeled axons in XDCT. Our results show that NM axons in XDCT are organized in a precise tonotopic manner along the rostrocaudal axis, spanning the entire rostrocaudal extent of both the origin and target nuclei. We propose that in the avian auditory brainstem, this pretarget axon sorting contributes to tonotopic map formation in NL.

Neurodevelopmental role for VGLUT2 in pyramidal neuron plasticity, dendritic refinement, and in spatial learning

The level and integrity of glutamate transmission during critical periods of postnatal development plays an important role in the refinement of pyramidal neuron dendritic arbor, synaptic plasticity, and cognition. Presently, it is not clear how excitatory transmission via the two predominant isoforms of the vesicular glutamate transporter (VGLUT1 and VGLUT2) participate in this process. To assess a neurodevelopmental role for VGLUT2 in pyramidal neuron maturation, we generated recombinant VGLUT2 knock-out mice and inactivated VGLUT2 throughout development using Emx1-Cre(+/+) knock-in mice. We show that VGLUT2 deficiency in corticolimbic circuits results in reduced evoked glutamate transmission, release probability, and LTD at hippocampal CA3-CA1 synapses during a formative developmental period (postnatal days 11-14). In adults, we find a marked reduction in the amount of dendritic arbor across the span of the dendritic tree of CA1 pyramidal neurons and reduced long-term potentiation and levels of synaptic markers spinophilin and VGLUT1. Loss of dendritic arbor is accompanied by corresponding reductions in the number of dendritic spines, suggesting widespread alterations in synaptic connectivity. Conditional VGLUT2 knock-out mice exhibit increased open-field exploratory activity yet impaired spatial learning and memory, endophenotypes similar to those of NMDA receptor knock-down mice. Remarkably, the impairment in learning can be partially restored by selectively increasing NMDA receptor-mediated glutamate transmission in adult mice by prolonged treatment with d-serine and a d-amino acid oxidase inhibitor. Our data indicate that VGLUT2 expression is pivotal to the proper development of mature pyramidal neuronal architecture and plasticity, and that such glutamatergic deficiency leads to cognitive malfunction as observed in several neurodevelopmental psychiatric disorders.

Astrocyte-secreted factors modulate the developmental distribution of inhibitory synapses in nucleus laminaris of the avian auditory brainstem

Nucleus laminaris (NL) neurons in the avian auditory brainstem are coincidence detectors necessary for the computation of interaural time differences used in sound localization. In addition to their excitatory inputs from nucleus magnocellularis, NL neurons receive inhibitory inputs from the superior olivary nucleus (SON) that greatly improve coincidence detection in mature animals. The mechanisms that establish mature distributions of inhibitory inputs to NL are not known. We used the vesicular GABA transporter (VGAT) as a marker for inhibitory presynaptic terminals to study the development of inhibitory inputs to NL between embryonic day 9 (E9) and E17. VGAT immunofluorescent puncta were first seen sparsely in NL at E9. The density of VGAT puncta increased with development, first within the ventral NL neuropil region and subsequently throughout both the ventral and dorsal dendritic neuropil, with significantly fewer terminals in the cell body region. A large increase in density occurred between E13–15 and E16–17, at a developmental stage when astrocytes that express glial fibrillary acidic protein (GFAP) become mature. We cultured E13 brainstem slices together with astrocyte-conditioned medium (ACM) obtained from E16 brainstems and found that ACM, but not control medium, increased the density of VGAT puncta. This increase was similar to that observed during normal development. Astrocyte-secreted factors interact with the terminal ends of SON axons to increase the number of GABAergic terminals. These data suggest that factors secreted from GFAP-positive astrocytes promote maturation of inhibitory pathways in the auditory brainstem.

Stress-induced grey matter loss determined by MRI is primarily due to loss of dendrites and their synapses

Stress, unaccompanied by signs of post-traumatic stress disorder, is known to decrease grey matter volume (GMV) in the anterior cingulate cortex (ACC) and hippocampus but not the amygdala in humans. We sought to determine if this was the case in stressed mice using high-resolution magnetic resonance imaging (MRI) and to identify the cellular constituents of the grey matter that quantitatively give rise to such changes. Stressed mice showed grey matter losses of 10 and 15 % in the ACC and hippocampus, respectively but not in the amygdala or the retrosplenial granular area (RSG). Concurrently, no changes in the number or volumes of the somas of neurons, astrocytes or oligodendrocytes were detected. A loss of synaptic spine density of up to 60 % occurred on different-order dendrites in the ACC and hippocampus (CA1) but not in the amygdala or RSG. The loss of spines was accompanied by decreases in cumulative dendritic length of neurons of over 40 % in the ACC and hippocampus (CA1) giving rise to decreases in volume of dendrites of 2.6 mm(3) for the former and 0.6 mm(3) for the latter, with no change in the amygdala or RSG. These values are similar to the MRI-determined loss of GMV following stress of 3.0 and 0.8 mm(3) in ACC and hippocampus, respectively, with no changes in the amygdala or RSG. This quantitative study is the first to relate GMV changes in the cortex measured with MRI to volume changes in cellular constituents of the grey matter.

In vivo reversible regulation of dendritic patterning by afferent input in bipolar auditory neurons

Afferent input regulates neuronal dendritic patterning locally and globally through distinct mechanisms. To begin to understand these mechanisms, we differentially manipulate afferent input in vivo and assess effects on dendritic patterning of individual neurons in chicken nucleus laminaris (NL). Dendrites of NL neurons segregate into dorsal and ventral domains, receiving excitatory input from the ipsilateral and contralateral ears, respectively, via nucleus magnocellularis (NM). Blocking action potentials from one ear, by either cochlea removal or temporary treatment with tetrodotoxin (TTX), leads to rapid and significant retraction of affected NL dendrites (dorsal ipsilaterally and ventral contralaterally) within 8 h compared with the other dendrites of the same neurons. The degree of retraction is comparable with that induced by direct deafferentation resulting from transection of NM axons. Importantly, when inner ear activity is allowed to recover from TTX treatments, retracted NL dendrites regrow to their normal length within 48 h. The retraction and growth involve elimination of terminal branches and addition of new branches, respectively. Examination of changes in NL dendrites at 96 h after unilateral cochlea removal, a manipulation that induces cell loss in NM and persistent blockage of afferent excitatory action potentials, reveals a significant correlation between cell death in the ipsilateral NM and the degree of dendritic retraction in NL. These results demonstrate that presynaptic action potentials rapidly and reversibly regulate dendritic patterning of postsynaptic neurons in a compartment specific manner, whereas long-term dendritic maintenance may be regulated in a way that is correlated with the presence of silent presynaptic appositions.

Undersized dendritic arborizations in retinal ganglion cells of the rd1 mutant mouse: a paradigm of early onset photoreceptor degeneration

Retinitis pigmentosa (RP) is a family of inherited diseases causing progressive photoreceptor death. Retinal ganglion cells (RGCs) form the biological substrate for various therapeutic approaches designed to restore vision in RP individuals. Assessment of survival and preservation of RGCs in animal paradigms mimicking the human disease is of key importance for appropriate implementation of vision repair strategies. Here we studied the survival of RGCs in the rd1 mutant mouse, a known model of early onset, autosomic recessive RP, at various stages of photoreceptor degeneration. Furthermore, we analyzed the morphology of various types of RGCs using the newly generated transgenic mouse rd1/Thy1-GFP, in which the rd1 mutation is associated with green fluorescent protein (GFP) expression in a small population of different RGCs. We found excellent survival of cells at up to 1 year of age, a time at which the inner retina is known to have severely reorganized and partially degenerated. However, 50% of the cells analyzed within all RGC types exhibit an undersized dendritic tree, spanning about half of the normal area. Undersized cells are found both in adult and in very young (1-month-old) mice. This suggests that their aberrant phenotype is due to incomplete dendritic development, possibly as a consequence of altered visual input at the time of dendritic arbor refinement. These data show the importance of the timing of photoreceptor death in RGC dendritic development.

Developmental origins and architecture of Drosophila leg motoneurons

Motoneurons are key points of convergence within motor networks, acting as the "output channels" that directly control sets of muscles to maintain posture and generate movement. Here we use genetic mosaic techniques to reveal the origins and architecture of the leg motoneurons of Drosophila. We show that a small number of leg motoneurons are born in the embryo but most are generated during larval life. These postembryonic leg motoneurons are produced by five neuroblasts per hemineuromere, and each lineage generates stereotyped lineage-specific projection patterns. Two of these postembryonic neuroblasts generate solely motoneurons that are the bulk of the leg motoneurons. Within the largest lineage, lineage 15, we see distinct birth-order differences in projection patterns. A comparison of the central projections of leg motoneurons and the muscles they innervate reveals a stereotyped architecture and the existence of a myotopic map. Timeline analysis of axonal outgrowth reveals that leg motoneurons reach their sites of terminal arborization in the leg at the time when their dendrites are elaborating their subtype-specific shapes. Our findings provide a comprehensive description of the origin, development, and architecture of leg motoneurons that will aid future studies exploring the link between the assembly and organization of connectivity within the leg motor system of Drosophila.

Control of neuronal excitability by NMDA-type glutamate receptors in early developing binaural auditory neurons

Precise control of neuronal excitability in the auditory brainstem is fundamental for processing timing cues used for sound localization and signal discrimination in complex acoustic environments. In mature nucleus laminaris (NL), the first nucleus responsible for binaural processing in chickens, neuronal excitability is governed primarily by voltage-activated potassium conductances (K(VA)). High levels of K(VA) expression in NL neurons result in one or two initial action potentials (APs) in response to high-frequency synaptic activity or sustained depolarization. Here we show that during a period of synaptogenesis and circuit refinement, before hearing onset, K(VA) conductances are relatively small, in particular low-voltage-activated K(+) conductances (K(LVA)). In spite of this, neuronal output is filtered and repetitive synaptic activity generates only one or two initial APs during a train of stimuli. During this early developmental time period, synaptic NMDA-type glutamate receptors (NMDA-Rs) contain primarily the GluN2B subunit. We show that the slow decay kinetics of GluN2B-containing NMDA-Rs allows synaptic responses to summate, filtering the output of NL neurons before intrinsic properties are fully developed. Weaker Mg(2+) blockade of NMDA-Rs and ambient glutamate early in development generate a tonic NMDA-R-mediated current that sets the membrane potential at more depolarized values. Small KLVA conductances, localized in dendrites, prevent excessive depolarization caused by tonic activation of NMDA-Rs. Thus, before intrinsic properties are fully developed, NMDA-Rs control the output of NL neurons during evoked synaptic transmission.

Astrocyte-secreted factors modulate a gradient of primary dendritic arbors in nucleus laminaris of the avian auditory brainstem

Neurons in nucleus laminaris (NL) receive binaural, tonotopically matched input from nucleus magnocelluaris (NM) onto bitufted dendrites that display a gradient of dendritic arbor size. These features improve computation of interaural time differences, which are used to determine the locations of sound sources. The dendritic gradient emerges following a period of significant reorganization at embryonic day 15 (E15), which coincides with the emergence of astrocytes that express glial fibrillary acidic protein (GFAP) in the auditory brainstem. The major changes include a loss of total dendritic length, a systematic loss of primary dendrites along the tonotopic axis, and lengthening of primary dendrites on caudolateral NL neurons. Here we have tested whether astrocyte-derived molecules contribute to these changes in dendritic morphology. We used an organotypic brainstem slice preparation to perform repeated imaging of individual dye-filled NL neurons to determine the effects of astrocyte-conditioned medium (ACM) on dendritic morphology. We found that treatment with ACM induced a decrease in the number of primary dendrites in a tonotopically graded manner similar to that observed during normal development. Our data introduce a new interaction between astrocytes and neurons in the auditory brainstem and suggest that these astrocytes influence multiple aspects of auditory brainstem maturation.

Relative input strength rapidly regulates dendritic structure of chick auditory brainstem neurons

Competition between presynaptic inputs has been suggested to shape dendritic form. This hypothesis can be directly tested on bitufted, auditory neurons in chicken nucleus laminaris (NL). Each NL neuron contains two relatively symmetrical dendritic arbors; the dorsal dendrites receive excitatory glutamatergic input from the ipsilateral ear, and the ventral dendrites receive corresponding input from the contralateral ear. To assess the effect of relative synaptic strength on NL dendrites, we used single-cell electroporation; electrophysiology; and live, two-photon laser scanning microscopy to manipulate both the amount and the balance of synaptic input to the two matching sets of dendrites. With simultaneous activation, both sets of dendrites changed together, either growing or retracting over the imaging period. In contrast, stimulation of only one set of dendrites (either dorsal or ventral) resulted in the unstimulated dendrites losing total dendritic branch length, whereas the stimulated dendrites exhibited a tendency to grow. In this system, balanced input leads to balanced changes in the two sets of dendrites, but imbalanced input results in differential changes. Time-lapse imaging revealed that NL dendrites respond to differential stimulation by first decreasing the size of their unstimulated dendrites and then increasing the size of their stimulated dendrites. This result suggests that the relative activity of presynaptic neurons dynamically controls dendritic structure in NL and that dendritic real estate can rapidly be shifted from inactive inputs to active inputs.

Single-cell electroporation in Xenopus

Single-cell electroporation (SCE) is a versatile technique for delivering electrically charged macromolecules including DNA, RNA, synthetic oligonucleotides, peptides, dyes, and drugs to individual cells within intact tissues. Here, we describe methods for in vivo-targeted electroporation of single tectal neurons within the albino Xenopus laevis tadpole. Focal electroporation is achieved using a pipette electrode filled with a solution of the delivery molecules and with a tip diameter much smaller than the width of the target cell. The small tip allows for localization of an electric field, which restricts pore formation to only the individual cell in direct contact with the tip. Thus, the small tip permits focal delivery of the charged molecules within the pipette into individual cells. Factors affecting the efficiency of SCE, as well as various applications of this technique, are discussed. Particular focus is directed toward combining SCE with in vivo two-photon microscopy for three-dimensional (3D) imaging of neuron growth and cell-autonomous effects of altered protein function.

Preparation and culture of chicken auditory brainstem slices

The chicken auditory brainstem is a well-established model system that has been widely used to study the anatomy and physiology of auditory processing at discreet periods of development as well as mechanisms for temporal coding in the central nervous system. Here we present a method to prepare chicken auditory brainstem slices that can be used for acute experimental procedures or to culture organotypic slices for long-term experimental manipulations. The chicken auditory brainstem is composed of nucleus angularis, magnocellularis, laminaris and superior olive. These nuclei are responsible for binaural sound processing and single coronal slice preparations preserve the entire circuitry. Ultimately, organotypic slice cultures can provide the opportunity to manipulate several developmental parameters such as synaptic activity, expression of pre and postsynaptic components, expression of aspects controlling excitability and differential gene expression This approach can be used to broaden general knowledge about neural circuit development, refinement and maturation.

In vivo time-lapse imaging and serial section electron microscopy reveal developmental synaptic rearrangements

Dendrites, axons, and synapses are dynamic during circuit development; however, changes in microcircuit connections as branches stabilize have not been directly demonstrated. By combining in vivo time-lapse imaging of Xenopus tectal neurons with electron microscope reconstructions of imaged neurons, we report the distribution and ultrastructure of synapses on individual vertebrate neurons and relate these synaptic properties to dynamics in dendritic and axonal arbor structure over hours or days of imaging. Dynamic dendrites have a high density of immature synapses, whereas stable dendrites have sparser, mature synapses. Axons initiate contacts from multisynapse boutons on stable branches. Connections are refined by decreasing convergence from multiple inputs to postsynaptic dendrites and by decreasing divergence from multisynapse boutons to postsynaptic sites. Visual deprivation or NMDAR antagonists decreased synapse maturation and elimination, suggesting that coactive input activity promotes microcircuit development by concurrently regulating synapse elimination and maturation of remaining contacts.

Evidence of cell-nonautonomous changes in dendrite and dendritic spine morphology in the met-signaling-deficient mouse forebrain

Human genetic findings and murine neuroanatomical expression mapping have intersected to implicate Met receptor tyrosine kinase signaling in the development of forebrain circuits controlling social and emotional behaviors that are atypical in autism-spectrum disorders (ASD). To clarify roles for Met signaling during forebrain circuit development in vivo, we generated mutant mice (Emx1(Cre)/Met(fx/fx)) with an Emx1-Cre-driven deletion of signaling-competent Met in dorsal pallially derived forebrain neurons. Morphometric analyses of Lucifer yellow-injected pyramidal neurons in postnatal day 40 anterior cingulate cortex (ACC) revealed no statistically significant changes in total dendritic length but a selective reduction in apical arbor length distal to the soma in Emx1(Cre)/Met(fx/fx) neurons relative to wild type, consistent with a decrease in the total tissue volume sampled by individual arbors in the cortex. The effects on dendritic structure appear to be circuit-selective, insofar as basal arbor length was increased in Emx1(Cre)/Met(fx/fx) layer 2/3 neurons. Spine number was not altered on the Emx1(Cre)/Met(fx/fx) pyramidal cell populations studied, but spine head volume was significantly increased (∼20%). Cell-nonautonomous, circuit-level influences of Met signaling on dendritic development were confirmed by studies of medium spiny neurons (MSN), which do not express Met but receive Met-expressing corticostriatal afferents during development. Emx1(Cre)/Met(fx/fx) MSN exhibited robust increases in total arbor length (∼20%). As in the neocortex, average spine head volume was also increased (∼12%). These data demonstrate that a developmental loss of presynaptic Met receptor signaling can affect postsynaptic morphogenesis and suggest a mechanism whereby attenuated Met signaling could disrupt both local and long-range connectivity within circuits relevant to ASD.

In vivo development of dendritic orientation in wild-type and mislocalized retinal ganglion cells

Background

Many neurons in the central nervous system, including retinal ganglion cells (RGCs), possess asymmetric dendritic arbors oriented toward their presynaptic partners. How such dendritic arbors become biased during development in vivo is not well understood. Dendritic arbors may become oriented by directed outgrowth or by reorganization of an initially unbiased arbor. To distinguish between these possibilities, we imaged the dynamic behavior of zebrafish RGC dendrites during development in vivo. We then addressed how cell positioning within the retina, altered in heart-and-soul (has) mutants, affects RGC dendritic orientation.

Results

In vivo multiphoton time-lapse analysis revealed that RGC dendrites initially exhibit exploratory behavior in multiple directions but progressively become apically oriented. The lifetimes of basal and apical dendrites were generally comparable before and during the period when arbors became biased. However, with maturation, the addition and extension rates of basal dendrites were slower than those of the apical dendrites. Oriented dendritic arbors were also found in misplaced RGCs of the has retina but there was no preferred orientation amongst the population. However, has RGCs always projected dendrites toward nearby neuropil where amacrine and bipolar cell neurites also terminated. Chimera analysis showed that the abnormal dendritic organization of RGCs in the mutant was non-cell autonomous.

Conclusions

Our observations show that RGC dendritic arbors acquire an apical orientation by selective and gradual restriction of dendrite addition to the apical side of the cell body, rather than by preferential dendrite stabilization or elimination. A biased arbor emerges at a stage when many of the dendritic processes still appear exploratory. The generation of an oriented RGC dendritic arbor is likely to be determined by cell-extrinsic cues. Such cues are unlikely to be localized to the basal lamina of the inner retina, but rather may be provided by cells presynaptic to the RGCs.

Single-cell electroporation

Single-cell electroporation (SCEP) is a relatively new technique that has emerged in the last decade or so for single-cell studies. When a large enough electric field is applied to a single cell, transient nano-pores form in the cell membrane allowing molecules to be transported into and out of the cell. Unlike bulk electroporation, in which a homogenous electric field is applied to a suspension of cells, in SCEP an electric field is created locally near a single cell. Today, single-cell-level studies are at the frontier of biochemical research, and SCEP is a promising tool in such studies. In this review, we discuss pore formation based on theoretical and experimental approaches. Current SCEP techniques using microelectrodes, micropipettes, electrolyte-filled capillaries, and microfabricated devices are all thoroughly discussed for adherent and suspended cells. SCEP has been applied in in-vivo and in-vitro studies for delivery of cell-impermeant molecules such as drugs, DNA, and siRNA, and for morphological observations.

Mechanisms for adjusting interaural time differences to achieve binaural coincidence detection

Understanding binaural perception requires detailed analyses of the neural circuitry responsible for the computation of interaural time differences (ITDs). In the avian brainstem, this circuit consists of internal axonal delay lines innervating an array of coincidence detector neurons that encode external ITDs. Nucleus magnocellularis (NM) neurons project to the dorsal dendritic field of the ipsilateral nucleus laminaris (NL) and to the ventral field of the contralateral NL. Contralateral-projecting axons form a delay line system along a band of NL neurons. Binaural acoustic signals in the form of phase-locked action potentials from NM cells arrive at NL and establish a topographic map of sound source location along the azimuth. These pathways are assumed to represent a circuit similar to the Jeffress model of sound localization, establishing a place code along an isofrequency contour of NL. Three-dimensional measurements of axon lengths reveal major discrepancies with the current model; the temporal offset based on conduction length alone makes encoding of physiological ITDs impossible. However, axon diameter and distances between Nodes of Ranvier also influence signal propagation times along an axon. Our measurements of these parameters reveal that diameter and internode distance can compensate for the temporal offset inferred from axon lengths alone. Together with other recent studies, these unexpected results should inspire new thinking on the cellular biology, evolution, and plasticity of the circuitry underlying low-frequency sound localization in both birds and mammals.

A simple method for multiday imaging of slice cultures

The organotypic slice culture (Stoppini et al. A simple method for organotypic cultures of nervous tissue. 1991;37:173-182) has become the method of choice to answer a variety of questions in neuroscience. For many experiments, however, it would be beneficial to image or manipulate a slice culture repeatedly, for example, over the course of many days. We prepared organotypic slice cultures of the auditory brainstem of P3 and P4 mice and kept them in vitro for up to 4 weeks. Single cells in the auditory brainstem were transfected with plasmids expressing fluorescent proteins by way of electroporation (Haas et al. Single-cell electroporation for gene transfer in vivo. 2001;29:583-591). The culture was then placed in a chamber perfused with oxygenated ACSF and the labeled cell imaged with an inverted wide-field microscope repeatedly for multiple days, recording several time-points per day, before returning the slice to the incubator. We describe a simple method to image a slice culture preparation during the course of multiple days and over many continuous hours, without noticeable damage to the tissue or photobleaching. Our method uses a simple, inexpensive custom-built insulator constructed around the microscope to maintain controlled temperature and uses a perfusion chamber as used for in vitro slice recordings.

Differential gene expression during compensatory sprouting of dendrites in the auditory system of the cricket Gryllus bimaculatus

Neurones that lose their presynaptic partners because of injury usually retract or die. However, when the auditory interneurones of the cricket Gryllus bimaculatus are denervated, dendrites respond by growing across the midline and forming novel synapses with the opposite auditory afferents. Suppression subtractive hybridization was used to detect transcriptional changes 3 days after denervation. This is a stage at which we demonstrate robust compensatory dendritic sprouting. Whereas 49 unique candidates were down-regulated, no sufficiently up-regulated candidates were identified at this time point. Several candidates identified in this study are known to influence the translation and degradation of proteins in other systems. The potential role of these factors in the compensatory sprouting of cricket auditory interneurones in response to denervation is discussed.

Dendritic calcium channels and their activation by synaptic signals in auditory coincidence detector neurons

The avian nucleus laminaris (NL) encodes the azimuthal location of low-frequency sound sources by detecting the coincidence of binaural signals. Accurate coincidence detection requires precise developmental regulation of the lengths of the fine, bitufted dendrites that characterize neurons in NL. Such regulation has been suggested to be driven by local, synaptically mediated, dendritic signals such as Ca(2+). We examined Ca(2+) signaling through patch clamp and ion imaging experiments in slices containing nucleus laminaris from embryonic chicks. Voltage-clamp recordings of neurons located in the NL showed the presence of large Ca(2+) currents of two types, a low voltage-activated, fast inactivating Ni(2+) sensitive channel resembling mammalian T-type channels, and a high voltage-activated, slowly inactivating Cd(2+) sensitive channel. Two-photon Ca(2+) imaging showed that both channel types were concentrated on dendrites, even at their distal tips. Single action potentials triggered synaptically or by somatic current injection immediately elevated Ca(2+) throughout the entire cell. Ca(2+) signals triggered by subthreshold synaptic activity were highly localized. Thus when electrical activity is suprathreshold, Ca(2+) channels ensure that Ca(2+) rises in all dendrites, even those that are synaptically inactive.

Compartment-specific regulation of plasma membrane calcium ATPase type 2 in the chick auditory brainstem

Calcium signaling plays a role in synaptic regulation of dendritic structure, usually on the time scale of hours or days. Here we use immunocytochemistry to examine changes in expression of plasma membrane calcium ATPase type 2 (PMCA2), a high-affinity calcium efflux protein, in the chick nucleus laminaris (NL) following manipulations of synaptic inputs. Dendrites of NL neurons segregate into dorsal and ventral domains, receiving excitatory input from the ipsilateral and contralateral ears, respectively, via nucleus magnocellularis (NM). Deprivation of the contralateral projection from NM to NL leads to rapid retraction of ventral, but not the dorsal, dendrites of NL neurons. Immunocytochemistry revealed symmetric distribution of PMCA2 in two neuropil regions of normally innervated NL. Electron microscopy confirmed that PMCA2 localizes in both NM terminals and NL dendrites. As early as 30 minutes after transection of the contralateral projection from NM to NL or unilateral cochlea removal, significant decreases in PMCA2 immunoreactivity were seen in the deprived neuropil of NL compared with the other neuropil that continued to receive normal input. The rapid decrease correlated with reductions in the immunoreactivity for microtubule-associated protein 2, which affects cytoskeleton stabilization. These results suggest that PMCA2 is regulated independently in ventral and dorsal NL dendrites and/or their inputs from NM in a way that is correlated with presynaptic activity. This provides a potential mechanism by which deprivation can change calcium transport that, in turn, may be important for rapid, compartment-specific dendritic remodeling.

Subcellular topography of visually driven dendritic activity in the vertebrate visual system

Neural pathways projecting from sensory organs to higher brain centers form topographic maps in which neighbor relationships are preserved from a sending to a receiving neural population. Sensory input can generate compartmentalized electrical and biochemical activity in the dendrites of a receiving neuron. Here, we show that in the developing retinotectal projection of young Xenopus tadpoles, visually driven Ca2+ signals are topographically organized at the subcellular, dendritic scale. Functional in vivo two-photon Ca2+ imaging revealed that the sensitivity of dendritic Ca2+ signals to stimulus location in visual space is correlated with their anatomical position within the dendritic tree of individual neurons. This topographic distribution was dependent on NMDAR activation, whereas global Ca2+ signals were mediated by Ca2+ influx through dendritic, voltage-dependent Ca2+ channels. These findings suggest a framework for plasticity models that invoke local dendritic Ca2+ signaling in the elaboration of neural connectivity and dendrite-specific information storage.

Retinal ganglion cells survive and maintain normal dendritic morphology in a mouse model of inherited photoreceptor degeneration

Retinitis pigmentosa (RP), a family of inherited disorders characterized by progressive photoreceptor death, is a leading cause of blindness with no available cure. Despite the genetic heterogeneity underlying the disease, recent data on animal models show that the degeneration of photoreceptors triggers stereotyped remodeling among their postsynaptic partners. In particular, bipolar and horizontal cells might undergo dendritic atrophy and secondary death. The aim of this study was to investigate whether or not concomitant changes also occur in retinal ganglion cells (RGCs), the only retinal projection neurons to the brain and the proposed substrate for various therapeutic approaches for RP. We assessed the retention of morphology, overall architecture, and survival of RGCs in a mouse model of RP at various stages of the disease. To study the morphology of single RGCs, we generated a new mouse line by crossing Thy1-GFP-M mice (Feng et al., 2000), which express GFP (green fluorescent protein) in a small number of heterogeneous RGCs types, and rd10 mutants, a model of autosomal recessive RP, which exhibit a typical rod-cone degeneration (Chang et al., 2002). We show remarkable preservation of RGC structure, survival, and projections to higher visual centers in the time span from 3 to 9 months of life, well beyond the death of photoreceptors. Thus, unlike second-order neurons, RGCs appear as a considerably stable population of cells, potentially constituting a favorable substrate for restoring vision in RP individuals by means of electronic prostheses or direct expression of photosensitive proteins.

Connections and synaptic function in the posteroventral cochlear nucleus of deaf jerker mice

Mutations in the gene that encodes espins can cause deafness and vestibular disorders; mice that are homozygous for the autosomal recessive jerker mutation in the espin gene never hear. Extracellular injections of biocytin into the anteroventral cochlear nucleus (AVCN) revealed that although the cochlear nuclei are smaller in je/je mice, the topography in its innervation resembles that in wild-type mice. Auditory nerve fibers innervate narrow, topographically organized, "isofrequency" bands in deaf animals over the ages examined, P18-P70. The projection of tuberculoventral cells was topographic in je/je as in wild-type mice. Terminals of auditory nerve fibers in the multipolar cell area included both large and small endings, whereas in the octopus cell area they were exclusively small boutons in je/je as in wild-type mice, but end bulbs near the nerve root of je/je animals were smaller than in hearing animals. In whole-cell recordings from targets of auditory nerve fibers, octopus and T stellate cells, miniature excitatory postsynaptic currents (mEPSCs) had similar shapes as in +/+, indicating that the properties of AMPA receptors were not affected by the mutation. In je/je animals the frequency of spontaneous mEPSCs was elevated, and synaptic depression in responses to trains of shocks delivered at between 100 and 333 Hz was greater than in wild-type mice, indicating that the probability of neurotransmitter release was increased. The frequency of spontaneous mEPSCs and extent of synaptic depression were greater in octopus than in T stellate cells, in both wild-type and in je/je mice.

Rapid regulation of microtubule-associated protein 2 in dendrites of nucleus laminaris of the chick following deprivation of afferent activity

Differential innervation of segregated dendritic domains in the chick nucleus laminaris (NL), composed of third-order auditory neurons, provides a unique model to study synaptic regulation of dendritic structure. Altering the synaptic input to one dendritic domain affects the structure and length of the manipulated dendrites while leaving the other set of unmanipulated dendrites largely unchanged. Little is known about the effects of neuronal input on the cytoskeletal structure of NL dendrites and whether changes in the cytoskeleton are responsible for dendritic remodeling following manipulations of synaptic input. In this study, we investigate changes in the immunoreactivity of high-molecular weight microtubule associated protein 2 (MAP2) in NL dendrites following two different manipulations of their afferent input. Unilateral cochlea removal eliminates excitatory synaptic input to the ventral dendrites of the contralateral NL and the dorsal dendrites of the ipsilateral NL. This manipulation produced a dramatic decrease in MAP2 immunoreactivity in the deafferented dendrites. This decrease was detected as early as 3 h following the surgery, well before any degeneration of afferent axons. A similar decrease in MAP2 immunoreactivity in deafferented NL dendrites was detected following a midline transection that silences the excitatory synaptic input to the ventral dendrites on both sides of the brain. These changes were most distinct in the caudal portion of the nucleus where individual deafferented dendritic branches contained less immunoreactivity than intact dendrites. Our results suggest that the cytoskeletal protein MAP2, which is distributed in dendrites, perikarya, and postsynaptic densities, may play a role in deafferentation-induced dendritic remodeling.

Hepatocyte growth factor-induced enhancement of dendritic branching is blocked by inhibitors of N-methyl-D-aspartate receptors and calcium/calmodulin-dependent kinases

Hepatocyte growth factor (HGF) and its receptor, Met, are clustered at excitatory synapses and can enhance N-methyl-D-aspartate (NMDA) receptor current and promote formation of neurites and dendrites. In this study, we examine the effects of HGF on dendritic arborization in mature cultures of dissociated hippocampal neurons. Exogenous HGF treatment caused a dose-dependent increase in total dendritic branch tip number, total dendritic branch length, and dendritic complexity in these neurons. NMDA receptor activity has been linked to changes in dendritic structure, so we tested the effects of HGF on the dendritic arbor in the presence of DL-2-amino-5-phosphonopentanoic acid (APV), an NMDA receptor inhibitor. APV blocked the HGF-induced enhancement of the dendritic arbor in a dose-dependent manner. Similarly, pretreatment of neurons with KN62, an inhibitor of calcium-dependent kinases, suppressed changes in dendritic branching induced by HGF. These results suggest that HGF initiates Ca2+-dependent processes, so we examined the effect of HGF on intracellular calcium levels and autophosphorylation of the calcium/calmodulin-dependent protein kinase II (CaMKII). HGF caused a persistent increase in fluorescence in clusters along dendrites of neurons preloaded with the Ca2+ indicator Fluo-4. HGF treatment also enhanced autophosphorylation of CaMKII. The increases in Fluo-4 fluorescence and autophosphorylation of CaMKII were blocked by pretreatment of neurons with APV. These results indicate that HGF stimulates Ca2+ influx into dendrites through the NMDA receptor and that this effect is necessary for the changes in dendritic morphology induced by HGF.

Simulation of motoneuron morphology in three dimensions. I. Building individual dendritic trees

We have developed a computational method that accurately reproduces the three-dimensional (3D) morphology of individual dendritic trees of six cat alpha motoneurons. The first step was simulation of trees with straight branches based on the branch lengths and topology of actual trees. A second step introduced the meandering, or wandering, trajectories observed in natural dendritic branches into the straight-branch tree simulations. These two steps each required only two parameters, one extracted from the data on actual motoneuron dendrites and the other adjusted by comparing simulated and observed trees, using measurements that were independent of the model specifications (i.e., emergent properties). The results suggest that: 1) there is a somatofugal "tropism" (a bias introduced by the environment that affects the trajectory of dendritic branches) that tends to constrain the lateral expansion of alpha motoneuron dendrites; and 2) that most of the meandering of natural dendritic branches can be described by assuming that they are fractal objects with an average fractal dimension D of about 1.05. When analyzed in the same way, the dendrites of gamma motoneurons showed no evidence of a similar tropism, although they had the same fractal dimension of branch meandering.

Development of gerbil medial superior olive: integration of temporally delayed excitation and inhibition at physiological temperature

The sensitivity of medial superior olive (MSO) neurons to tens of microsecond differences in interaural temporal delay (ITD) derives in part from their membrane electrical characteristics, kinetics and timing of excitatory and inhibitory inputs, and dendrite structure. However, maturation of these physiological and structural characteristics are little studied, especially in relationship to the onset of auditory experience. We showed, using brain slices at physiological temperature, that MSO neurons exhibited sensitivity to simulated temporally delayed (TD) EPSCs (simEPSC), injected through the recording electrode, by the initial phase of hearing onset at P10, and TD sensitivity was reduced by block of low threshold potassium channels. The spike generation mechanism matured between P10 and P16 to support TD sensitivity to adult-like excitatory stimuli (1-4 ms duration) by P14. IPSP duration was shorter at physiological temperature than reported for lower temperatures, was longer than EPSP duration at young ages, but approached the duration of EPSPs by P16, when hearing thresholds neared maturity. Dendrite branching became less complex over a more restricted time frame between P10 and P12. Because many physiological and structural properties approximated mature values between P14 and P16, we studied temporal integration of simEPSCs and IPSPs at P15. Only a narrow range of relative onset times (< 1 ms) yielded responses showing sensitivity to TD. We propose that shaping of excitatory circuitry to mediate TD sensitivity can begin before airborne sound is detectable, and that inhibitory inputs having suboptimal neural delays may then be pruned by cellular mechanisms activated by sensitivity to ITD.

Rapid seasonal-like regression of the adult avian song control system

We analyzed how rapidly avian song control nuclei regress after testosterone (T) withdrawal. Regression of neuronal attributes resulting from T withdrawal has been observed in several animal models. The time course over which regression occurs is not known, however. To address this issue, we castrated adult male white-crowned sparrows and rapidly shifted them to short-day photoperiods after being held under breeding conditions (long-day photoperiod and systemic T exposure) for 3 weeks. We found that the volume of one song nucleus, HVC, regressed 22% within 12 h after T withdrawal. Changes in HVC neuron density after T withdrawal were dynamic; density increased at 12 h and then decreased by 4 days. HVC neuron number was reduced by 26% by 4 days. The volumes of Area X and the robust nucleus of the arcopallium (RA) were significantly regressed by 7 and 20 days, respectively. RA somatic area and neuronal spacing were significantly reduced by 2 days. The rapidity of HVC regression is unprecedented among vertebrate models of hormone-sensitive neural circuits. These results reveal that the rapid regression of the song control system provides a model for the important role sex steroid hormones play in mediating adult neural plasticity and in neuroprotection.

Embryonic motor neuron dendrite growth is stunted by inhibition of nitric oxide-dependent activation of soluble guanylyl cyclase and protein kinase G

We have examined the participation of a neuronal nitric oxide synthase (nNOS) signaling pathway in the elaboration of motor neuron dendrites during embryonic life. During chick embryogenesis, nNOS is expressed by interneurons that surround the motor neuron pools in the ventral horn. Pseudorabies virus tracing suggests that these cells, while juxtaposed to motor neurons are not synaptically connected to them. The downstream effectors, soluble guanylyl cyclase (sGC) and protein kinase G (PKG), are found in motor neurons as well as several other populations of spinal cord cells. To determine the functional significance of the nNOS/sGC/PKG signaling pathway, pharmacological inhibitors were applied to chick embryos and the effects on motor neuron dendrites monitored. Inhibition of nNOS activity led to a lasting reduction in the overall size and degree of branching of the dendritic tree. These alterations in dendritic architecture were also seen when the activity of sGC or PKG was blocked. Our results suggest that normal motor neuron dendrite elaboration depends, in part, on the activity-dependent generation of NO by ventral horn interneurons, which then activates sGC and PKG in motor neurons.

Afferent control of horizontal cell morphology revealed by genetic respecification of rods and cones

The first inhibitory interneurons of the retina, the horizontal cells, stratify within the outer plexiform layer, extending dendritic terminals that connect to the pedicles of cone photoreceptors and an axon terminal system contacting the spherules of rod photoreceptors. How the horizontal cells acquire this morphology is unknown, but instructive interactions with afferents are suggested to play a role in the development of synaptic circuits. Here, we show that the morphology of the axon terminal system and the dendritic field are selectively regulated by innervation from their respective afferents: genetic respecification of all cones to become rods, in Crxp-Nrl transgenic mice, produces an atrophic dendritic field yet leaves the axon terminal system largely intact. In contrast, in the retinas of Nrl-/- mice, in which the population of rod photoreceptors is respecified to adopt a cone fate, the dendritic field is hypertrophic, whereas the axon terminal system is underdeveloped. Our studies reveal that, although cell-intrinsic mechanisms drive the formation of independent dendritic versus axonal domains, the afferents play a selectively instructive role in defining their respective morphologies.

AMPA receptors regulate experience-dependent dendritic arbor growth in vivo

The size and shape of neuronal dendritic arbors affect the number and type of synaptic inputs, as well as the complexity and function of brain circuits. In the intact brain, dendritic arbor growth and the development of excitatory glutamatergic synapse are concurrent. Consequently, it has been difficult to resolve whether synaptic inputs drive dendritic arbor development. Here, we test the role of AMPA receptor (AMPAR)-mediated glutamatergic transmission in dendrite growth by expressing peptides corresponding to the intracellular C-terminal domains of AMPAR subunits GluR1 (GluR1Ct) and GluR2 (GluR2Ct) in optic tectal neurons of the Xenopus retinotectal system. These peptides significantly reduce AMPAR synaptic transmission in transfected neurons while leaving visual system circuitry intact. Daily in vivo imaging over 5 days revealed that GluR1Ct or GluR2Ct expression dramatically impaired dendrite growth, resulting in less complex arbors than controls. Time-lapse images collected at 2-h intervals over 6 h show that both GluR1Ct and GluR2Ct decrease branch lifetimes. Ultrastructural analysis indicates that synapses formed onto neurons expressing the GluRCt are less mature than synapses onto control neurons. These data suggest that the failure to form complex arbors is due to reduced stabilization of new synapses and dendritic branches. Although visual stimulation increases dendritic arbor growth rates in control tectal neurons, a weak postsynaptic response to visual experience in GluRCt-expressing cells leads to retraction of branches. These results indicate that AMPAR-mediated transmission underlies experience-dependent dendritic arbor growth by stabilizing branches, and support a competition-based model for dendrite growth.

Deafferentation induces novel axonal projections in the auditory brainstem after hearing onset

Deafferentation of neural tissue can result in cell death, morphological changes, and/or alterations in sources of innervation. These changes often occur during a limited period of development. In the auditory brainstem, the ventral cochlear nucleus (VCN) projects to the contralateral but not ipsilateral medial nucleus of the trapezoid body (MNTB). This pathway is part of a circuit that computes interaural intensity differences used in sound localization. Previous studies have shown that, after the cochlea is removed early in postnatal development, cells in the VCN on the deafferented side die, and the intact VCN innervates MNTB on both sides of the brain. These changes after cochlea removal are limited to an early postnatal period that preceeds hearing onset. In this study, we lesioned the VCN directly to evaluate plasticity in axonal pathways after hearing onset. We found that novel projections from the intact VCN to ipsilateral MNTB emerge after lesions performed as late as postnatal day 25. The morphological sequence of events is similar to that seen during the initial development of this pathway. These data suggest that plasticity in the auditory brainstem is possible when pathways are challenged with denervation of target nuclei. The results show that the opportunity for plasticity in auditory brainstem circuitry is more prolonged than previously thought and that novel pathways can form after the normal pathways are fully mature and functional. Moreover, sensitive periods for changes in individual pathways are independently regulated.