The developing synapse: construction and modulation of synaptic structures and circuits

Target recognition at the tips of postsynaptic filopodia: accumulation and function of Capricious

While much evidence suggests that postsynaptic dynamism contributes to the formation of synapses, few studies have addressed its possible role in target selection. Do postsynaptic motile structures seek specific synaptic partner cells, as does the presynaptic growth cone? Here we studied the dynamics of myopodia, postsynaptic filopodia in Drosophila muscles, and the role of Capricious (CAPS) during the process of synaptic matchmaking. CAPS is a target recognition molecule with an extracellular domain containing leucine-rich repeat sequences. It is expressed in specific subsets of embryonic/larval body wall muscles, including muscle 12 (M12). We provide evidence that implicates the tips of myopodia as loci of initial neuromuscular recognition: (1) CAPS, expressed as a GFP-fusion protein in M12, accumulated at the tips of myopodia; and (2) simultaneous live imaging of presynaptic motoneurons and postsynaptic myopodia revealed that initial neuromuscular contacts occur at the tips of myopodia. The live imaging also showed that individual postsynaptic myopodia appear to be able to discriminate partner and non-partner presynaptic cells: whereas many myopodial contacts with the partner motoneurons are stabilized to form synapses, those with non-partner neurons are retracted. In caps mutants, or in double mutants lacking both CAPS and the closely related protein Tartan, we observed fewer contacts between myopodia of M12 and the presynaptic growth cones during the process of initial neuromuscular interaction. The nascent synaptic sites of M12 were also reduced. These results provide evidence for the sensing function of postsynaptic filopodia, and implicate Caps-mediated recognition at the tips of myopodia in synaptic matching.

Type A GABA-receptor-dependent synaptic transmission sculpts dendritic arbor structure in Xenopus tadpoles in vivo

The emergence of dendritic arbor structure in vivo depends on synaptic inputs. We tested whether inhibitory GABAergic synaptic transmission regulates Xenopus optic tectal cell dendritic arbor development in vivo by expressing a peptide corresponding to an intracellular loop (ICL) of the gamma2 subunit of type A GABA receptors (GABA(A)R), which is required to anchor GABA(A) receptors to the postsynaptic scaffold. Enhanced green fluorescent protein (EGFP)-tagged ICL (EGFP-ICL) was distributed in a punctate pattern at putative inhibitory synapses, identified by vesicular GABA transporter immunoreactive puncta. ICL expression completely blocked GABA(A)R-mediated transmission in 36% of transfected neurons and significantly reduced GABA(A)R-mediated synaptic currents relative to AMPA receptor-mediated synaptic currents in the remaining transfected neurons without altering release probability or neuronal excitability. Further analysis of ICL-expressing neurons with residual GABA(A)R-mediated inputs showed that the capacity of benzodiazepine to enhance GABAergic synaptic responses was reduced in ICL-expressing neurons, indicating that they were likely depleted of gamma2 subunit-containing GABA(A)R. Neurons expressing a mutant form of ICL were comparable to controls. In vivo time-lapse images showed that ICL-expressing neurons have more sparsely branched dendritic arbors, which expand over larger neuropil areas than EGFP-expressing control neurons. Analysis of branch dynamics indicated that ICL expression affected arbor growth by reducing rates of branch addition. Furthermore, we found that decreasing GABAergic synaptic transmission with ICL expression blocked visual experience dependent dendritic arbor structural plasticity. Our findings establish an essential role for inhibitory GABAergic synaptic transmission in the regulation of dendritic structural plasticity in Xenopus in vivo.

The regulative role of neurite mechanical tension in network development

A bewildering series of dynamical processes take part in the development of the nervous system. Neuron branching dynamics, the continuous formation and elimination of neural interconnections, are instrumental in constructing distinct neuronal networks, which are the functional building blocks of the nervous system. In this study, we investigate and validate the important regulative role of mechanical tension in determining the final morphology of neuronal networks. To single out the mechanical effect, we cultured relatively large invertebrate neurons on clean quartz surfaces. Applied to these surfaces were isolated anchoring sites consisting of carbon nanotube islands to which the cells and the neurites could mechanically attach. Inspection of branching dynamics and network wiring upon development revealed an innate selection mechanism in which one axon branch wins over another. The apparent mechanism entails the build-up of mechanical tension in developing axons. The tension is maintained by the attachment of the growth cone to the substrate or, alternatively, to the neurites of a target neuron. The induced tension promotes the stabilization of one set of axon branches while causing retraction or elimination of axon collaterals. We suggest that these findings represent a crucial, early step that precedes the formation of synapses and regulates neuronal interconnections. Mechanical tension serves as a signal for survival of the axonal branch and perhaps for the subsequent formation of synapses.

Basic principles of neuromuscular transmission

Neuromuscular transmission at the skeletal muscle occurs when a quantum of acetylcholine from the nerve ending is released and binds to the nicotinic acetylcholine receptors on the postjunctional muscle membrane. The nicotinic acetylcholine receptors on the endplate respond by opening channels for the influx of sodium ions and subsequent endplate depolarisation leads to muscle contraction. The acetylcholine immediately detaches from the receptor and is hydrolysed by acetylcholinesterase enzyme. Suxamethonium is a cholinergic agonist stimulating the muscle nicotinic acetylcholine receptors prior to causing neuromuscular block. Non-depolarising neuromuscular blocking drugs bind to the nicotinic acetylcholine receptors preventing the binding of acetylcholine. Non-depolarising neuromuscular blocking drugs also inhibit prejunctional alpha3beta2 nicotinic acetylcholine autoreceptors, which can be seen in the clinical setting as train-of-four fade. In some pathological states such as denervation, burns, immobilisation, inflammation and sepsis, there is expression of other subtypes of nicotinic acetylcholine receptors with upregulation of these receptors throughout the muscle membrane. The responses of these receptors to suxamethonium and non-depolarising neuromuscular blocking drugs are different and explain some of the aberrant responses to neuromuscular blocking drugs.

Developmental alterations in the functional properties of excitatory neocortical synapses

In the neocortex, most excitatory, glutamatergic synapses are established during the first 4-5 weeks after birth. During this period profound changes in the properties of synaptic transmission occur. Excitatory postsynaptic potentials (EPSPs) at immature synaptic connections are profoundly and progressively reduced in response to moderate to high frequency (5-100 Hz) stimulation. With maturation, this frequency-dependent depression becomes progressively weaker and may eventually transform into a weak to moderate EPSP facilitation. In parallel to changes in the short-term plasticity, a reduction in the synaptic reliability occurs at most glutamatergic neocortical synapses: immature synapses show a high probability of neurotransmitter release as indicated by their low failure rate and small EPSP amplitude variation. This high reliability is reduced in mature synapses, which show considerably higher failure rates and more variable EPSP amplitudes. During early neocortical development synaptic vesicle pools are not yet fully differentiated and their replenishment may be slow, thus resulting in EPSP amplitude depression. The decrease in the probability of neurotransmitter release may be the result of an altered Ca(2+) control in the presynaptic terminal with a reduced Ca(2+) influx and/or a higher Ca(2+) buffering capacity. This may lead to a lower synaptic reliability and a weaker short-term synaptic depression with maturation.

An activity-dependent retrograde signal induces the expression of the high-affinity choline transporter in cholinergic neurons

A well-accepted view of developing circuits is that synapses must be active to mature and persist, whereas inactive synapses remain immature and are eventually eliminated. We question this long-standing view by investigating nonfunctional cholinergic nicotinic synapses in the superior cervical ganglia (SCG) of mice with a disruption in the alpha3 nicotinic receptor (nAChR) subunit gene, a gene essential for fast synaptic transmission in sympathetic ganglia. Using imaging and electrophysiology, we show that synapses persist for at least 2-3 months without postsynaptic activity; however, the presynaptic terminals lack high-affinity choline transporters (CHTs), and as a result, they are quickly depleted of transmitter. Moreover, we demonstrate with rescue experiments that CHT is induced by signals downstream of postsynaptic activity, converting immature terminals to mature terminals capable of sustaining transmitter release in response to high-frequency or continuous firing. Importantly, postsynaptic neurons must be continually active to maintain CHT in presynaptic terminals.

Brain-derived neurotrophic factor signaling does not stimulate subventricular zone neurogenesis in adult mice and rats

In rodents, the adult subventricular zone (SVZ) generates neuroblasts which migrate to the olfactory bulb (OB) and differentiate into interneurons. Recent work suggests that the neurotrophin Brain-Derived Neurotrophic Factor (BDNF) can enhance adult SVZ neurogenesis, but the mechanism by which it acts is unknown. Here, we analyzed the role of BDNF and its receptor TrkB in adult SVZ neurogenesis. We found that TrkB is the most prominent neurotrophin receptor in the mouse SVZ, but only the truncated, kinase-negative isoform (TrkB-TR) was detected. TrkB-TR is expressed in SVZ astrocytes and ependymal cells, but not in neuroblasts. TrkB mutants have reduced SVZ proliferation and survival and fewer new OB neurons. To test whether this effect is cell-autonomous, we grafted SVZ cells from TrkB knock-out mice (TrkB-KO) into the SVZ of wild-type mice (WT). Grafted progenitors generated neuroblasts that migrated to the OB in the absence of TrkB. The survival and differentiation of granular interneurons and Calbindin(+) periglomerular interneurons seemed unaffected by the loss of TrkB, whereas dopaminergic periglomerular neurons were reduced. Intra-ventricular infusion of BDNF yielded different results depending on the animal species, having no effect on neuron production from mouse SVZ, while decreasing it in rats. Interestingly, mice and rats also differ in their expression of the neurotrophin receptor p75. Our results indicate that TrkB is not essential for adult SVZ neurogenesis and do not support the current view that delivering BDNF to the SVZ can enhance adult neurogenesis.

Physiology and biology of neuromuscular transmission in health and disease

The introduction of powerful and contemporary research techniques has allowed for an increasingly detailed understanding of neuromuscular transmission. The classic model of nerve signaling to muscle using acetylcholine has been well described. Newer discovery points toward a more complex signaling system with adaptive receptor physiology and a multifaceted action response scheme for muscle relaxants. Although adding complexity, these newer discoveries help align experimentally derived knowledge with clinical observations. In this review, new concepts relative to neuromuscular transmission in health and disease are discussed, including a detailed discussion of acetylcholine and acetylcholine receptor physiology. Recent elucidations of the pathophysiologic responses to neuromuscular injury and its clinical implications are also detailed.

Plasticity and injury in the developing brain

The child's brain is more malleable or plastic than that of adults and this accounts for the ability of children to learn new skills quickly or recovery from brain injuries. Several mechanisms contribute to this ability including overproduction and deletion of neurons and synapses, and activity-dependent stabilization of synapses. The molecular mechanisms for activity-dependent synaptic plasticity are being discovered and this is leading to a better understanding of the pathogenesis of several disorders including neurofibromatosis, tuberous sclerosis, Fragile X syndrome and Rett syndrome. Many of the same pathways involved in synaptic plasticity, such as glutamate-mediated excitation, can also mediate brain injury when the brain is exposed to stress or energy failure such as hypoxia-ischemia. Recent evidence indicates that cell death pathways activated by injury differ between males and females. This new information about the molecular pathways involved in brain plasticity and injury are leading to insights that will provide better therapies for pediatric neurological disorders.

Reduced dendritic spine density in auditory cortex of subjects with schizophrenia

We have previously identified reductions in mean pyramidal cell somal volume in deep layer 3 of BA 41 and 42 and reduced axon terminal density in deep layer 3 of BA 41. In other brain regions demonstrating similar deficits, reduced dendritic spine density has also been identified, leading us to hypothesize that dendritic spine density would also be reduced in BA 41 and 42. Because dendritic spines and their excitatory inputs are regulated in tandem, we further hypothesized that spine density would be correlated with axon terminal density. We used stereologic methods to quantify a marker of dendritic spines, spinophilin-immunoreactive (SP-IR) puncta, in deep layer 3 of BA 41 and 42 of 15 subjects with schizophrenia, each matched to a normal comparison subject. The effect of long-term haloperidol exposure on SP-IR puncta density was evaluated in nonhuman primates. SP-IR puncta density was significantly lower by 27.2% in deep layer 3 of BA 41 in the schizophrenia subjects, and by 22.2% in deep layer 3 of BA 42. In both BA 41 and 42, SP-IR puncta density was correlated with a marker of axon terminal density, but not with pyramidal cell somal volume. SP-IR puncta density did not differ between haloperidol-exposed and control monkeys. Lower SP-IR puncta density in deep layer 3 of BA 41 and 42 of subjects with schizophrenia may reflect concurrent reductions in excitatory afferent input. This may contribute to impairments in auditory sensory processing that are present in subjects with schizophrenia.

Structural plasticity controlled by calcium based correlation detection. helias@bccn.uni-freiburg.de

Hebbian learning in cortical networks during development and adulthood relies on the presence of a mechanism to detect correlation between the presynaptic and the postsynaptic spiking activity. Recently, the calcium concentration in spines was experimentally shown to be a correlation sensitive signal with the necessary properties: it is confined to the spine volume, it depends on the relative timing of pre- and postsynaptic action potentials, and it is independent of the spine's location along the dendrite. NMDA receptors are a candidate mediator for the correlation dependent calcium signal. Here, we present a quantitative model of correlation detection in synapses based on the calcium influx through NMDA receptors under realistic conditions of irregular pre- and postsynaptic spiking activity with pairwise correlation. Our analytical framework captures the interaction of the learning rule and the correlation dynamics of the neurons. We find that a simple thresholding mechanism can act as a sensitive and reliable correlation detector at physiological firing rates. Furthermore, the mechanism is sensitive to correlation among afferent synapses by cooperation and competition. In our model this mechanism controls synapse formation and elimination. We explain how synapse elimination leads to firing rate homeostasis and show that the connectivity structure is shaped by the correlations between neighboring inputs.

Bidirectional plasticity at developing climbing fiber-Purkinje neuron synapses

Climbing fibers provide one of the two major excitatory inputs to the cerebellar cortex. In an immature animal, several climbing fibers form synapses with one Purkinje neuron. During postnatal development most climbing fiber innervations with a Purkinje neuron are eliminated and only one strong fiber remains. Previous studies suggested that this pruning of surplus climbing fiber innervations depends on the neuronal activity. We hypothesized that synaptic plasticity might play a role in the maturation and refinement of such a climbing fiber projection pattern, and examined the plasticity properties of synapses between postnatal days 5 and 9 in mice. We found that a 5 Hz conditioning stimulation of climbing fibers forming relatively strong synapses with a Purkinje neuron induced long-term potentiation of the transmission accompanied by a decrease in the paired-pulse ratio of excitatory postsynaptic current amplitudes. This was suggestive of an increased probability of presynaptic release. However, the conditioning stimulation of climbing fibers forming relatively weak synapses induced long-term depression and tended to increase the paired-pulse ratio. Thus, the direction of plasticity appears to be determined by the strength of synaptic connection. Long-term depression occurred only in the conditioned climbing fiber, whereas long-term potentiation spread to unconditioned climbing fibers. A postsynaptic increase in the intracellular Ca(2+) concentration was required for long-term potentiation but not for long-term depression. These results reveal the existence of novel presynaptic plasticity at immature climbing fiber-Purkinje cell synapses, which may contribute to the maturation and refinement of the climbing fiber projection pattern.

Impaired dendritic development and synaptic formation of postnatal-born dentate gyrus granular neurons in the absence of brain-derived neurotrophic factor signaling

Neurons are continuously added to the hippocampal dentate gyrus throughout life. These neurons must develop dendritic arbors and spines by which they form synapses for making functional connections with existing neurons. The molecular mechanisms that regulate dendritic development and synaptic formation of postnatal-born granular neurons in the dentate gyrus are largely unknown. Hippocampal dentate gyrus (HDG) has been shown to express high level of brain-derived neurotrophic factor (BDNF). Here we reported that when BDNF is conditionally knockout in the postnatal-born granular neurons of the HDG, the mutant neurons exhibit aberrant morphological development with less dendritic branches, shorter dendritic length, and lower density of dendritic spines, while their primary dendrites are not obviously affected. Even though, these BDNF-deficient granular neurons develop immature dendritic spines to initiate synaptic contacts with afferent axons, they fail to develop or maintain mature spine structures. Thus, these postnatal-born neurons have fewer numbers of synapses, particularly mature synaptic spines. These results suggest that BDNF plays an important role during dendritic development, synaptic formation and synaptic maturation in postnatal-born granular neurons of the HDG in vivo.

Agrin expression during synaptogenesis induced by traumatic brain injury

Interaction between extracellular matrix proteins and regulatory proteinases can mediate synaptic integrity. Previously, we documented that matrix metalloproteinase 3 (MMP-3) expression and activity increase following traumatic brain injury (TBI). We now report protein and mRNA analysis of agrin, a MMP-3 substrate, over the time course of trauma-induced synaptogenesis. Agrin expression during the successful synaptic reorganization of unilateral entorhinal cortical lesion (UEC) was compared with expression when normal synaptogenesis fails (combined fluid percussion TBI and bilateral entorhinal lesion [BEC]). We observed that agrin protein was increased in both models at 2 and 7 days postinjury, and immuohistochemical (IHC) co-localization suggested reactive astrocytes contribute to that increase. Agrin formed defined boundaries for sprouting axons along deafferented dendrites in the UEC, but failed to do so after combined insult. Similarly, Western blot analysis revealed greater increase in UEC agrin protein relative to the combined TBI+BEC model. Both models showed increased agrin transcription at 7 days postinjury and mRNA normalization by 15 days. Attenuation of synaptic pathology with the NMDA antagonist MK-801 reduced 7-day UEC agrin transcript to a level not different from unlesioned controls. By contrast, MK-801 in the combined insult failed to significantly change 7-day agrin transcript, mRNA levels remaining elevated over uninjured sham cases. Together, these results suggest that agrin plays an important role in the sprouting phase of reactive synaptogenesis, and that both its expression and distribution are correlated with extent of successful recovery after TBI. Further, when pathogenic conditions which induce synaptic plasticity are reduced, increase in agrin mRNA is attenuated.

Neuropeptides in depression: role of VGF

The monoamine hypothesis of depression is increasingly called into question by newer theories that revolve around changes in neuronal plasticity, primarily in the hippocampus, at both the structural and the functional levels. Chronic stress negatively regulates hippocampal function while antidepressants ameliorate the effects of stress on neuronal morphology and activity. Both stress and antidepressants have been shown to affect levels of brain-derived neurotrophic factor (BDNF) whose transcription is dependent on cAMP response element binding protein (CREB). BDNF itself has antidepressant-like actions and can induce transcription of a number of molecules. One class of genes regulated by both BDNF and serotonin (5-HT) are neuropeptides including VGF (non-acryonimic) which has a novel role in depression. Neuropeptides are important modulators of neuronal function but their role in affective disorders is just emerging. Recent studies demonstrate that VGF, which is also a CREB-dependent gene, is upregulated by antidepressant drugs and voluntary exercise and is reduced in animal models of depression. VGF enhances hippocampal synaptic plasticity as well as neurogenesis in the dentate gyrus but the mechanisms of antidepressant-like actions of VGF in behavioral paradigms are not known. We summarize experimental data describing the roles of BDNF, VGF and other neuropeptides in depression and how they may be acting through the generation of new neurons and altered synaptic activity. Understanding the molecular and cellular changes that underlie the actions of neuropeptides and how these adaptations result in antidepressant-like effects will aid in developing drugs that target novel pathways for major depressive disorders.

Erythropoietin enhances hippocampal long-term potentiation and memory

BACKGROUND

Erythropoietin (EPO) improves cognition of human subjects in the clinical setting by as yet unknown mechanisms. We developed a mouse model of robust cognitive improvement by EPO to obtain the first clues of how EPO influences cognition, and how it may act on hippocampal neurons to modulate plasticity.

RESULTS

We show here that a 3-week treatment of young mice with EPO enhances long-term potentiation (LTP), a cellular correlate of learning processes in the CA1 region of the hippocampus. This treatment concomitantly alters short-term synaptic plasticity and synaptic transmission, shifting the balance of excitatory and inhibitory activity. These effects are accompanied by an improvement of hippocampus dependent memory, persisting for 3 weeks after termination of EPO injections, and are independent of changes in hematocrit. Networks of EPO-treated primary hippocampal neurons develop lower overall spiking activity but enhanced bursting in discrete neuronal assemblies. At the level of developing single neurons, EPO treatment reduces the typical increase in excitatory synaptic transmission without changing the number of synaptic boutons, consistent with prolonged functional silencing of synapses.

CONCLUSION

We conclude that EPO improves hippocampus dependent memory by modulating plasticity, synaptic connectivity and activity of memory-related neuronal networks. These mechanisms of action of EPO have to be further exploited for treating neuropsychiatric diseases.

Retinal waves in mice lacking the beta2 subunit of the nicotinic acetylcholine receptor

The structural and functional properties of the visual system are disrupted in mutant animals lacking the beta2 subunit of the nicotinic acetylcholine receptor. In particular, eye-specific retinogeniculate projections do not develop normally in these mutants. It is widely thought that the developing retinas of beta2(-/-) mutants do not manifest correlated activity, leading to the notion that retinal waves play an instructional role in the formation of eye-specific retinogeniculate projections. By multielectrode array recordings, we show here that the beta2(-/-) mutants have robust retinal waves during the formation of eye-specific projections. Unlike in WT animals, however, the mutant retinal waves are propagated by gap junctions rather than cholinergic circuitry. These results indicate that lack of retinal waves cannot account for the abnormalities that have been documented in the retinogeniculate pathway of the beta2(-/-) mutants and suggest that other factors must contribute to the deficits in the visual system that have been noted in these animals.

Neurotrophin-dependent dendritic filopodial motility: a convergence on PI3K signaling

Synapse formation requires contact between dendrites and axons. Although this process is often viewed as axon mediated, dendritic filopodia may be actively involved in mediating synaptogenic contact. Although the signaling cues underlying dendritic filopodial motility are mostly unknown, brain-derived neurotrophic factor (BDNF) increases the density of dendritic filopodia and conditional deletion of tyrosine receptor kinase B (TrkB) reduces synapse number in vivo. Here, we report that TrkB associates with dendritic growth cones and filopodia, mediates filopodial motility, and does so via the phosphoinositide 3 kinase (PI3K) pathway. We used genetic and pharmacological manipulations of mouse hippocampal neurons to assess signaling downstream of TrkB. Conditional knock-out of two downstream negative regulators of TrkB signaling, Pten (phosphatase with tensin homolog) and Nf1 (neurofibromatosis type 1), enhanced filopodial motility. This effect was PI3K-dependent and correlated with synaptic density. Phosphatidylinositol 3,4,5-trisphosphate (PIP3) was preferentially localized in filopodia and this distribution was enhanced by BDNF application. Thus, intracellular control of filopodial dynamics converged on PI3K activation and PIP3 accumulation, a cellular paradigm conserved for chemotaxis in other cell types. Our results suggest that filopodial movement is not random, but responsive to synaptic guidance molecules.

From synapses to behavior: development of a sensory-motor circuit in the leech

The development of neuronal circuits has been advanced greatly by the use of imaging techniques that reveal the activity of neurons during the period when they are constructing synapses and forming circuits. This review focuses on experiments performed in leech embryos to characterize the development of a neuronal circuit that produces a simple segmental behavior called "local bending." The experiments combined electrophysiology, anatomy, and FRET-based voltage-sensitive dyes (VSDs). The VSDs offered two major advantages in these experiments: they allowed us to record simultaneously the activity of many neurons, and unlike other imaging techniques, they revealed inhibition as well as excitation. The results indicated that connections within the circuit are formed in a predictable sequence: initially neurons in the circuit are connected by electrical synapses, forming a network that itself generates an embryonic behavior and prefigures the adult circuit; later chemical synapses, including inhibitory connections, appear, "sculpting" the circuit to generate a different, mature behavior. In this developmental process, some of the electrical connections are completely replaced by chemical synapses, others are maintained into adulthood, and still others persist and share their targets with chemical synaptic connections.

Critical period plasticity of axonal arbors of layer 2/3 pyramidal neurons in rat somatosensory cortex: layer-specific reduction of projections into deprived cortical columns

We examined the effect of whisker trimming during early postnatal development on the morphology of axonal arbors in rat somatosensory cortex. Axonal arbors from populations of layer 2/3 pyramidal neurons in the D2 column were labeled by lentivirus-mediated expression of green fluorescent protein. Axonal projection patterns were compared between untrimmed control animals and animals with all whiskers in A-, B-, and C-rows trimmed (D- and E-rows left intact) from postnatal days 7 to 15 (termed from here on DE-pairing). Control animals had approximately symmetrical horizontal projections toward C- and E-row columns in both supra- and infragranular layers. Following DE-pairing, the density of axons in supragranular layers projecting from the labeled neurons in the D2 column was higher in E- than in C-row columns. This asymmetry resulted primarily from a reduction in projection density toward the deprived C-row columns. In contrast, no change was observed in infragranular layers. The results indicate that DE-pairing during early postnatal development results in reduced axonal projection from nondeprived into deprived columns and that cortical neurons are capable of structural rearrangements at subsets of their axonal arbors.

GABA regulates excitatory synapse formation in the neocortex via NMDA receptor activation

The development of a balance between excitatory and inhibitory synapses is a critical process in the generation and maturation of functional circuits. Accumulating evidence suggests that neuronal activity plays an important role in achieving such a balance in the developing cortex, but the mechanism that regulates this process is unknown. During development, GABA, the primary inhibitory neurotransmitter in adults, excites neurons as a result of high expression of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1). Using NKCC1 RNA interference knockdown in vivo, we show that GABA-induced depolarization is necessary for proper excitatory synapse formation and dendritic development of newborn cortical neurons. Blocking NKCC1 with the diuretic bumetanide during development leads to similar persistent changes in cortical circuitry in the adult. Interestingly, expression of a voltage-independent NMDA receptor rescues the failure of NKCC1 knockdown neurons to develop excitatory AMPA transmission, indicating that GABA depolarization cooperates with NMDA receptor activation to regulate excitatory synapse formation. Our study identifies an essential role for GABA in the synaptic integration of newborn cortical neurons and suggests an activity-dependent mechanism for achieving the balance between excitation and inhibition in the developing cortex.

Drosophila ankyrin 2 is required for synaptic stability

Synaptic connections are stabilized through transsynaptic adhesion complexes that are anchored in the underlying cytoskeleton. The Drosophila neuromuscular junction (NMJs) serves as a model system to unravel genes required for the structural remodeling of synapses. In a mutagenesis screen for regulators of synaptic stability, we recovered mutations in Drosophila ankyrin 2 (ank2) affecting two giant Ank2 isoforms that are specifically expressed in the nervous system and associate with the presynaptic membrane cytoskeleton. ank2 mutant larvae show severe deficits in the stability of NMJs, resulting in a reduction in overall terminal size, withdrawal of synaptic boutons, and disassembly of presynaptic active zones. In addition, lack of Ank2 leads to disintegration of the synaptic microtubule cytoskeleton. Microtubules and microtubule-associated proteins fail to extend into distant boutons. Interestingly, Ank2 functions downstream of spectrin in the anchorage of synaptic microtubules, providing the cytoskeletal scaffold that is essential for synaptic stability.

Localized acetylcholine receptor clustering dynamics in response to microfluidic focal stimulation with agrin

Agrin is a proteoglycan secreted by the motor neuron's growing axon terminal upon contact with the muscle during embryonic development. It was long thought that agrin's role was to trigger the clustering of acetylcholine receptors (AChRs) to nascent synapse sites. However, agrin-predating, protosynaptic AChR clusters are present well before innervation in the embryo and in myotube cultures, yet no role has been conclusively ascribed to agrin. We used a microfluidic device to focally deliver agrin to protosynaptic AChR clusters in micropatterned myotube cultures. The distribution of AChRs labeled with fluorescent bungarotoxin was imaged at various time points over >24 h. We find that a 4-h focal application of agrin (100 nM) preferentially reduces AChR loss at agrin-exposed clusters by 17% relative to the agrin-deprived clusters on the same myotube. In addition, the focal application increases the addition of AChRs preferentially at the clusters by 10% relative to the agrin-exposed, noncluster areas. Taken together, these findings suggest that a focal agrin stimulus can play a key stabilizing role in the aggregation of AChRs at the early stages of synapse formation. This methodology is generally applicable to various developmental processes and cell types, including neurons and stem cells.

Dynamics underlying synaptic gain between pairs of cortical pyramidal neurons

Changes in connectivity between pairs of neurons can serve as a substrate for information storage and for experience-dependent changes in neuronal circuitry. Early in development, synaptic contacts form and break, but how these dynamics influence the connectivity between pairs of neurons is not known. Here we used time-lapse imaging to examine the synaptic interactions between pairs of cultured cortical pyramidal neurons, and found that the axon-dendrite contacts between each neuronal pair were composed of both a relatively stable and a more labile population. Under basal conditions, loss and gain of contacts within this labile population was well balanced and there was little net change in connectivity. Selectively increasing the levels of activated CaMKII in the postsynaptic neuron increased connectivity between pairs of neurons by increasing the rate of gain of new contacts without affecting the probability of contact loss, or the proportion of stable and labile contacts, and this increase required Calcium/calmodulin binding to CaMKII. Our data suggest that activating CaMKII can increase synaptic connectivity through a CaM-dependent increase in contact formation, followed by stabilization of a constant fraction of new contacts.

Effects of development and dopamine depletion on striatal NMDA receptor-mediated calcium uptake

Calcium (Ca(2+)) is the currency of N-methyl-D-aspartate (NMDA) receptor mediated signal transduction pathways involved in the modification of synaptic efficacy during regulation of excitatory inputs into the striatum. The aim of the present study was to investigate the effects of development and dopamine depletion on NMDA receptor function. NMDA receptors were stimulated by incubation of striatal sections (350 microm) in buffer containing NMDA (100 microm) for 2 min, the slices were washed and uptake of radioactively labelled calcium ((45)Ca(2+)) was measured. Dopamine depletion has been reported to result in alterations of glutamate receptor expression and upregulation of NMDA receptor activity. However, the results of the present study show that dopamine depletion does not alter NMDA-stimulated Ca(2+) uptake into rat striatal slices in vitro. Unilateral striatal dopamine depletion was achieved by infusion of 6-hydroxydopamine (6-OHDA, 13.5 microg/4.5 microl) into the medial forebrain bundle (MFB) of the left hemisphere of ten rats. NMDA-stimulated (45)Ca(2+) uptake into striata following dopamine depletion was not significantly different from NMDA-stimulated (45)Ca(2+) uptake into striata obtained from sham-operated rats. Other factors that induce changes in NMDA receptor function include development and aging. In young rats aged 7 weeks old (n = 7) and 16 weeks old (n = 6) a significant 2-3 fold decrease in striatal NMDA receptor function was observed with increasing age over the 9 week period of development. To our knowledge these are the first results to show developmental decreases of NMDA receptor function in the striatum of juvenile rats.

Fetal mechanisms in neurodevelopmental disorders

Normal development of the central nervous system depends on complex, dynamic mechanisms with multiple spatial and temporal components during gestation. Neurodevelopmental disorders may originate during fetal life from genetic as well as intrauterine and extrauterine factors that affect the fetal-maternal environment. Fetal neurodevelopment depends on cell programs, developmental trajectories, synaptic plasticity, and oligodendrocyte maturation, which are variously modifiable by factors such as stress and endocrine disruption, exposure to pesticides such as chlorpyrifos and to drugs such as terbutaline, maternal teratogenic alleles, and premature birth. Current research illustrates how altered fetal mechanisms may affect long-term physiological and behavioral functions of the central nervous system more significantly than they affect its form, and these effects may be transgenerational. This research emphasizes the diversity of such prenatal mechanisms and the need to expand our understanding of how, when altered, they may lead to disordered development, the signs of which may not appear until long after birth.

Reciprocal regulation of presynaptic and postsynaptic proteins in bipolar spiral ganglion neurons by neurotrophins

A unifying principle of sensory system organization is feature extraction by modality-specific neuronal maps in which arrays of neurons show systematically varied response properties and receptive fields. Only beginning to be understood, however, are the mechanisms by which these graded systems are established. In the peripheral auditory system, we have shown previously that the intrinsic firing features of spiral ganglion neurons are influenced by brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). We now show that is but a part of a coordinated package of neurotrophin actions that also includes effects on presynaptic and postsynaptic proteins, thus encompassing the input, transmission, and output functions of the spiral ganglion neurons. Using immunocytochemical methods, we determined that proteins targeted to opposite ends of the neuron were organized and regulated in a reciprocal manner. AMPA receptor subunits GluR2 and GluR3 were enriched in base neurons compared with their apex counterparts. This distribution pattern was enhanced by exposure to BDNF but reduced by NT-3. SNAP-25 and synaptophysin were distributed and regulated in the mirror image: enriched in the apex, enhanced by NT-3 and reduced by BDNF. Moreover, we used a novel coculture to identify potential endogenous sources of neurotrophins by showing that sensory receptors from different cochlear regions were capable of altering presynaptic and postsynaptic protein levels in these neurons. From these studies, we suggest that BDNF and NT-3, which are systematically distributed in complementary gradients, are responsible for orchestrating a comprehensive set of electrophysiological specializations along the frequency contour of the cochlea.

Multifunctional role of protein kinase C in regulating the formation and maturation of specific synapses

Target-dependent increases in axon growth and varicosities accompany the formation of functional synapses between Aplysia sensory neurons and specific postsynaptic neurons (L7 and not L11). The enhanced growth is regulated in part by a target-dependent increase in the secretion of sensorin, the sensory neuron neuropeptide. We report here that protein kinase C (PKC) activity is required for synapse formation by sensory neurons with L7 and for the target-dependent increases in sensorin synthesis and secretion. Blocking PKC activity reversibly blocked synapse formation and axon growth of sensory neurons contacting L7, but did not affect axon growth of sensory neurons contacting L11 or axon growth of the postsynaptic targets. Blocking PKC activity also blocked the target-induced increase in sensorin synthesis and secretion. Sensorin then activates additional signaling pathways required for synapse maturation and synapse-associated growth. Exogenous anti-sensorin antibody blocked target-induced activation and translocation into sensory neuron nuclei of p42/44 mitogen-activated protein kinase (MAPK), attenuated synapse maturation, and curtailed growth of sensory neurons contacting L7, but not the growth of sensory neurons contacting L11. Inhibitors of MAPK or phosphoinositide 3-kinase also attenuated synapse maturation and curtailed growth and varicosity formation of sensory neurons contacting L7, but not growth of sensory neurons contacting L11. These results suggest that PKC activity regulated by specific cell-cell interactions initiates the formation of specific synapses and the subsequent synthesis and release of a neuropeptide to activate additional signaling pathways required for synapse maturation.

Role of nitric oxide in classical conditioning of siphon withdrawal in Aplysia

Nitric oxide (NO) is thought to be involved in several forms of learning in vivo and synaptic plasticity in vitro, but very little is known about the role of NO during physiological forms of plasticity that occur during learning. We addressed that question in a simplified preparation of the Aplysia siphon-withdrawal reflex. We first used in situ hybridization to show that the identified L29 facilitator neurons express NO synthase. Furthermore, exogenous NO produced facilitation of sensory-motor neuron EPSPs, and an inhibitor of NO synthase or an NO scavenger blocked behavioral conditioning. Application of the scavenger to the ganglion or injection into a sensory neuron blocked facilitation of the EPSP and changes in the sensory-neuron membrane properties during conditioning. Injection of the scavenger into the motor neuron reduced facilitation without affecting sensory neuron membrane properties, and injection of an inhibitor of NO synthase had no effect. Postsynaptic injection of an inhibitor of exocytosis had effects similar to injection of the scavenger. However, changes in the shape of the EPSP during conditioning were not consistent with postsynaptic AMPA-like receptor insertion but were mimicked by presynaptic spike broadening. These results suggest that NO makes an important contribution during conditioning and acts directly in both the sensory and motor neurons to affect different processes of facilitation at the synapses between them. In addition, they suggest that NO does not come from either the sensory or motor neurons but rather comes from another source, perhaps the L29 interneurons.

Postnatal development of synaptic transmission in local networks of L5A pyramidal neurons in rat somatosensory cortex

The probability of synaptic transmitter release determines the spread of excitation and the possible range of computations at unitary connections. To investigate whether synaptic properties between neocortical pyramidal neurons change during the assembly period of cortical circuits, whole-cell voltage recordings were made simultaneously from two layer 5A (L5A) pyramidal neurons within the cortical columns of rat barrel cortex. We found that synaptic transmission between L5A pyramidal neurons is very reliable between 2 and 3 weeks of postnatal development with a mean unitary EPSP amplitude of approximately 1.2 mV, but becomes less efficient and fails more frequently in the more mature cortex of approximately 4 weeks of age with a mean unitary EPSP amplitude of 0.65 mV. Coefficient of variation and failure rate increase as the unitary EPSP amplitude decreases during development. The paired-pulse ratio (PPR) of synaptic efficacy at 10 Hz changes from 0.7 to 1.04. Despite the overall increase in PPR, short-term plasticity displays a large variability at 4 weeks, ranging from strong depression to strong facilitation (PPR, range 0.6-2.1), suggesting the potential for use-dependent modifications at this intracortical synapse. In conclusion, the transmitter release probability at the L5A-L5A connection is developmentally regulated in such a way that in juvenile animals excitation by single action potentials is efficiently transmitted, whereas in the more mature cortex synapses might be endowed with a diversity of filtering characteristics.

Experience-dependent modification of primary sensory synapses in the mammalian olfactory bulb

Experience-dependent changes in neural circuits have traditionally been investigated several synapses downstream of sensory input. Whether experience can alter the strength of primary sensory synapses remains mostly unknown. To address this issue, we investigated the consequences of odor deprivation on synapses made by olfactory sensory axons in the olfactory bulb of rats. Odor deprivation triggered an increase in the probability of glutamate release from olfactory sensory neuron synapses. Deprivation also increased the amplitude of quantal synaptic currents mediated by AMPA- and NMDA-type glutamate receptors, as well as the abundance of these receptors in the glomerular region. Our results demonstrate that sensory experience is capable of modulating synaptic strength at the earliest stages of information transfer between the environment and an organism. Such compensatory experience-dependent changes may represent a mechanism of sensory gain control.

Homeostatic regulation of intrinsic excitability and synaptic transmission in a developing visual circuit

One of the major challenges faced by the developing visual system is how to stably process visual information, yet at the same time remain flexible enough to accommodate growth and plasticity induced by visual experience. We find that in the Xenopus retinotectal circuit, during a period in development when the retinotectal map undergoes activity-dependent refinement and visual inputs strengthen, tectal neurons adapt their intrinsic excitability such that a stable relationship between the total level of synaptic input and tectal neuron spike output is conserved. This homeostatic balance between synaptic and intrinsic properties is maintained, in part, via regulation of voltage-gated Na+ currents, resulting in a stable neuronal input-output function. We experimentally manipulated intrinsic excitability or synapse strengthening in developing tectal neurons in vivo by electroporation of a leak K+ channel gene or a peptide that interferes with normal AMPA receptor trafficking. Both manipulations resulted in a compensatory increase in voltage-gated Na+ currents. This suggests that intrinsic neuronal properties are actively regulated as a function of the total level of neuronal activity experienced during development. We conclude that the coordinated changes between synaptic and intrinsic properties allow developing optic tectal neurons to remain within a stable dynamic range, even as the pattern and strength of visual inputs changes over development, suggesting that homeostatic regulation of intrinsic properties plays a central role in the functional development of neural circuits.

Neonatal dopamine depletion induces changes in morphogenesis and gene expression in the developing cortex

The mesocorticolimbic dopamine (DA) system is implicated in mental health disorders affecting attention, impulse inhibition and other cognitive functions. It has also been involved in the regulation of cortical morphogenesis. The present study uses focal injections of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle of BALB/c mice to examine morphological, behavioral and transcriptional responses to selective DA deficit in the fronto-parietal cortex. Mice that received injections of 6-OHDA on postnatal day 1 (PND1) showed reduction in DA levels in their cortices at PND7. Histological analysis at PND120 revealed increased fronto-cortical width, but decreased width of somatosensory parietal cortex. Open field object recognition suggested impaired response inhibition in adult mice after 6-OHDA treatment. Transcriptional analyses using 17K mouse microarrays showed that such lesions caused up-regulation of 100 genes in the cortex at PND7. Notably, among these genes are Sema3A which plays a repulsive role in axonal guidance, RhoD which inhibits dendritic growth and tubulin beta-5 microtubule subunit. In contrast, 127 genes were down-regulated, including CCT-epsilon and CCT-zeta that play roles in actin and tubulin folding. Thus, neonatal DA depletion affects transcripts involved in control of cytoskeletal formation and pathway finding, instrumental for normal differentiation and synaptogenesis. The observed gene expression changes are consistent with histological cortical and behavioral impairments in the adult mice treated with 6-OHDA on PND1. Our results point towards specific molecular targets that might be involved in disease process mediated by altered developmental DA regulation.

Relations among functional systems in behavior analysis

This paper proposes that an organism's integrated repertoire of operant behavior has the status of a biological system, similar to other biological systems, like the nervous, cardiovascular, or immune systems. Evidence from a number of sources indicates that the distinctions between biological and behavioral events is often misleading, engendering counterproductive explanatory controversy. A good deal of what is viewed as biological (often thought to be inaccessible or hypothetical) can become publicly measurable variables using currently available and developing technologies. Moreover, such endogenous variables can serve as establishing operations, discriminative stimuli, conjoint mediating events, and maintaining consequences within a functional analysis of behavior and need not lead to reductionistic explanation. I suggest that explanatory misunderstandings often arise from conflating different levels of analysis and that behavior analysis can extend its reach by identifying variables operating within a functional analysis that also serve functions in other biological systems.

HVC neural sleep activity increases with development and parallels nightly changes in song behavior

Sleep abnormalities are coexpressed with human communication disorders. Recent data from the birdsong system, the best model for human speech, indicate that sleep has a critical role in vocal learning. To understand the neural mechanisms that underlie behavioral changes during sleep, we recorded sleep activity in the song control area HVC longitudinally during song development in zebra finches. We focused on the sensorimotor phase of song learning, when the finch shapes his song behavior toward a learned tutor song model. Direct comparison of sleep activity in adults and juveniles revealed that the juvenile HVC has a lower spike rate and longer silent periods than the adult. Within individual finches, sleep silent periods decreased and spike rate increased with age. We next systematically compared neural sleep activity and song behavior. We now report that spike rate during sleep was significantly correlated with overnight changes in song behavior. Collectively, these data indicate that sleep activity in the vocal control area HVC increases with age and may affect song behavior.

GDNF and GFRalpha1 promote formation of neuronal synapses by ligand-induced cell adhesion

The establishment of synaptic connections requires precise alignment of pre- and postsynaptic terminals. The glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha1 is enriched at pre- and postsynaptic compartments in hippocampal neurons, suggesting that it has a function in synapse formation. GDNF triggered trans-homophilic binding between GFRalpha1 molecules and cell adhesion between GFRalpha1-expressing cells. This represents the first example of a cell-cell interaction being mediated by a ligand-induced cell adhesion molecule (LICAM). In the presence of GDNF, ectopic GFRalpha1 induced localized presynaptic differentiation in hippocampal neurons, as visualized by clustering of vesicular proteins and neurotransmitter transporters, and by activity-dependent vesicle recycling. Presynaptic differentiation induced by GDNF was markedly reduced in neurons lacking GFRalpha1. Gdnf mutant mice showed reduced synaptic localization of presynaptic proteins and a marked decrease in the density of presynaptic puncta, indicating a role for GDNF signaling in hippocampal synaptogenesis in vivo. We propose that GFRalpha1 functions as a LICAM to establish precise synaptic contacts and induce presynaptic differentiation.

Optical mapping of spatiotemporal emergence of functional synaptic connections in the embryonic chick olfactory pathway

In order to understand the functional maturation of the CNS, it is essential to first describe the functional maturation of sensory processing. We have approached this topic by following the ontogenetic patterning of neural circuit formation related to cranial and spinal sensory input using voltage-sensitive dye imaging. In previous studies, we have described the functional maturation of synapses in brainstem/midbrain neural circuits. Here, we elucidate the functional maturation of forebrain circuits by investigating neural networks related to the olfactory nerve (N. I) of chicken embryo. In the isolated N. I-olfactory bulb-forebrain preparation, application of electrical stimulation to N. I elicited excitatory postsynaptic potential (EPSP)-related slow optical signals in the olfactory bulb. The slow signal was mainly mediated by glutamate, and was easily fatigued with repetitive stimuli because of the immaturity of synapses in the embryonic CNS. Ontogenetically, the slow signal was detected from the 6-day embryonic stage, suggesting that functional synaptic connections between N. I and olfactory bulb emerge around this stage. In addition, from the 8-day embryonic stage, another response area was discriminated within the forebrain, which corresponded to the higher-ordered nucleus of the olfactory pathway. In comparison with our previous studies concerning the functional development of other cranial nerve-related sensory nuclei in the embryonic brainstem and midbrain, these results suggest that the olfactory pathway is functionally generated in the early stages of development when neural networks related to other visceral and somatic sensory inputs are also in the process of developing.

NeuronMetrics: software for semi-automated processing of cultured neuron images

Using primary cell culture to screen for changes in neuronal morphology requires specialized analysis software. We developed NeuronMetrics for semi-automated, quantitative analysis of two-dimensional (2D) images of fluorescently labeled cultured neurons. It skeletonizes the neuron image using two complementary image-processing techniques, capturing fine terminal neurites with high fidelity. An algorithm was devised to span wide gaps in the skeleton. NeuronMetrics uses a novel strategy based on geometric features called faces to extract a branch number estimate from complex arbors with numerous neurite-to-neurite contacts, without creating a precise, contact-free representation of the neurite arbor. It estimates total neurite length, branch number, primary neurite number, territory (the area of the convex polygon bounding the skeleton and cell body), and Polarity Index (a measure of neuronal polarity). These parameters provide fundamental information about the size and shape of neurite arbors, which are critical factors for neuronal function. NeuronMetrics streamlines optional manual tasks such as removing noise, isolating the largest primary neurite, and correcting length for self-fasciculating neurites. Numeric data are output in a single text file, readily imported into other applications for further analysis. Written as modules for ImageJ, NeuronMetrics provides practical analysis tools that are easy to use and support batch processing. Depending on the need for manual intervention, processing time for a batch of approximately 60 2D images is 1.0-2.5 h, from a folder of images to a table of numeric data. NeuronMetrics' output accelerates the quantitative detection of mutations and chemical compounds that alter neurite morphology in vitro, and will contribute to the use of cultured neurons for drug discovery.

Inflammation and depression: is there a causal connection with dementia?

Epidemiological studies show that there is a correlation between chronic depression and the likelihood of dementia in later life. There is evidence that inflammatory changes in the brain are pathological features of both depression and dementia. This suggests that an increase in inflammation-induced apoptosis, together with a reduction in the synthesis of neurotrophic factors caused by a rise in brain glucocorticoids, may play a role in the pathology of these disorders. A reduction in the neuroprotective components of the kynurenine pathway, such as kynurenic acid, and an increase in the neurodegenerative components, 3- hydroxykynurenine and quinolinic acid, contribute to the pathological changes. Such changes are postulated to cause neuronal damage and thereby predispose chronically depressed patients to dementia.

Mutant Presenilin 1 Alters Synaptic Transmission in Cultured Hippocampal Neurons

Mutations in presenilins are the major cause of familial Alzheimer disease, but the precise pathogenic mechanism by which presenilin (PS) mutations cause synaptic dysfunction leading to memory loss and neurodegeneration remains unclear. Using autaptic hippocampal cultures from transgenic mice expressing human PS1 with the A246E mutation, we demonstrate that mutant PS1 significantly depressed the amplitude of evoked alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptor-mediated synaptic currents. Analysis of the spontaneous miniature synaptic activity revealed a lower frequency of miniature currents but normal miniature amplitude. Both alterations could be rescued by the application of a gamma-secretase blocker. On the other hand, the application of synthetic soluble Abeta42 in wild-type neurons induced the PS1 mutant phenotype on synaptic strength. Together, these findings strongly suggest that the expression of mutant PS1 in cultured neurons depresses synaptic transmission by causing a physical reduction in the number of synapses. This hypothesis is consistent with morphometic and semiquantitative immunohistochemical analysis, revealing a decrease in synaptophysin-positive puncta in PS1 mutant hippocampal neurons.

The dynamic distribution of TrkB receptors before, during, and after synapse formation between cortical neurons

Although brain-derived neurotrophic factor (BDNF) potently regulates neuronal connectivity in the developing CNS, the mechanism by which BDNF influences the formation and/or maintenance of glutamatergic synapses remains unknown. Details about the subcellular localization of the BDNF receptor, TrkB, relative to synaptic and nonsynaptic proteins on excitatory neurons should provide insight into how BDNF might exert its effects during synapse formation. Here, we investigated the subcellular localization of tyrosine kinase receptor B (TrkB) relative to synaptic vesicle-associated proteins and NMDA receptors using immunocytochemistry, confocal microscopy, and time-lapse imaging in dissociated cultures of cortical neurons before, during, and after the peak of synapse formation. We find that TrkB is present in puncta on the surface and intracellularly in both dendrites and axons throughout development. Before synapse formation, some TrkB puncta in dendrites colocalize with NMDA receptors, and almost all TrkB puncta in axons colocalize with synaptic vesicle proteins. Clusters of TrkB fused to the enhanced green fluorescent protein (TrkB-EGFP) are highly mobile in both axons and dendrites. In axons, TrkB-EGFP dynamics are almost identical to vesicle-associated protein (VAMP2-EGFP), and these proteins are often transported together. Finally, surface TrkB is found in structures that actively participate in synapse formation: axonal growth cones and dendritic filopodia. Over time, surface TrkB becomes enriched at glutamatergic synapses, which contain both catalytic and truncated TrkB. These results suggest that TrkB is in the right place at the right time to play a direct role in the formation of glutamatergic synapses between cortical neurons.