Agrin and Synaptic Laminin Are Required to Maintain Adult Neuromuscular Junctions

As synapses form and mature the synaptic partners produce organizing molecules that regulate each other’s differentiation and ensure precise apposition of pre- and post-synaptic specializations. At the skeletal neuromuscular junction (NMJ), these molecules include agrin, a nerve-derived organizer of postsynaptic differentiation, and synaptic laminins, muscle-derived organizers of presynaptic differentiation. Both become concentrated in the synaptic cleft as the NMJ develops and are retained in adulthood. Here, we used mutant mice to ask whether these organizers are also required for synaptic maintenance. Deletion of agrin from a subset of adult motor neurons resulted in the loss of acetylcholine receptors and other components of the postsynaptic apparatus and synaptic cleft. Nerve terminals also atrophied and eventually withdrew from muscle fibers. On the other hand, mice lacking the presynaptic organizer laminin-α4 retained most of the synaptic cleft components but exhibited synaptic alterations reminiscent of those observed in aged animals. Although we detected no marked decrease in laminin or agrin levels at aged NMJs, we observed alterations in the distribution and organization of these synaptic cleft components suggesting that such changes could contribute to age-related synaptic disassembly. Together, these results demonstrate that pre- and post-synaptic organizers actively function to maintain the structure and function of adult NMJs.     

β-Catenin gain of function in muscles impairs neuromuscular junction formation

Neuromuscular junction (NMJ) formation requires proper interaction between motoneurons and muscle cells. β-Catenin is required in muscle cells for NMJ formation. To understand underlying mechanisms, we investigated the effect of β-catenin gain of function (GOF) on NMJ development. In HSA-β-cat(flox(ex3)/+) mice, which express stable β-catenin specifically in muscles, motor nerve terminals became extensively defasciculated and arborized. Ectopic muscles were observed in the diaphragm and were innervated by ectopic phrenic nerve branches. Moreover, extensive outgrowth and branching of spinal axons were evident in the GOF mice. These results indicate that increased β-catenin in muscles alters presynaptic differentiation. Postsynaptically, AChR clusters in HSA-β-cat(flox(ex3)/+) diaphragms were distributed in a wider region, suggesting that muscle β-catenin GOF disrupted the signal that restricts AChR clustering to the middle region of muscle fibers. Expression of stable β-catenin in motoneurons, however, had no effect on NMJ formation. These observations provide additional genetic evidence that pre- and postsynaptic development of the NMJ requires an intricate balance of β-catenin activity in muscles.     

Neuromuscular synaptogenesis in wild-type and mutant zebrafish

Genetic screens for synaptogenesis mutants have been performed in many organisms, but few if any have simultaneously screened for defects in pre- and postsynaptic specializations. Here, we report the results of a small-scale genetic screen, the first in vertebrates, for defects in synaptogenesis. Using zebrafish as a model system, we identified seven mutants that affect different aspects of neuromuscular synapse formation. Many of these mutant phenotypes have not been previously reported in zebrafish and are distinct from those described in other organisms. Characterization of mutant and wild-type zebrafish, from the time that motor axons first arrive at target muscles through adulthood, has provided the new information about the cellular events that occur during neuromuscular synaptogenesis. These include insights into the formation and dispersal of prepatterned AChR clusters, the relationship between motor axon elongation and synapse size, and the development of precise appositions between presynaptic clusters of synaptic vesicles in nerve terminals and postsynaptic receptor clusters. In addition, we show that the mechanisms underlying synapse formation within the myotomal muscle itself are largely independent of those that underlie synapse formation at myotendinous junctions and that the outgrowth of secondary motor axons requires at least one cue not necessary for the outgrowth of primary motor axons, while other cues are required for both. One-third of the mutants identified in this screen did not have impaired motility, suggesting that many genes involved in neuromuscular synaptogenesis were missed in large scale motility-based screens. Identification of the underlying genetic defects in these mutants will extend our understanding of the cellular and molecular mechanisms that underlie the formation and function of neuromuscular and other synapses.     

Distinct roles of muscle and motoneuron LRP4 in neuromuscular junction formation

Neuromuscular junction (NMJ) formation requires precise interaction between motoneurons and muscle fibers. LRP4 is a receptor of agrin that is thought to act in cis to stimulate MuSK in muscle fibers for postsynaptic differentiation. Here we dissected the roles of LRP4 in muscle fibers and motoneurons in NMJ formation by cell-specific mutation. Studies of muscle-specific mutants suggest that LRP4 is involved in deciding where to form AChR clusters in muscle fibers, postsynaptic differentiation, and axon terminal development. LRP4 in HEK293 cells increased synapsin or SV2 puncta in contacting axons of cocultured neurons, suggesting a synaptogenic function. Analysis of LRP4 muscle and motoneuron double mutants and mechanistic studies suggest that NMJ formation may also be regulated by LRP4 in motoneurons, which could serve as agrin's receptor in trans to induce AChR clusters. These observations uncovered distinct roles of LRP4 in motoneurons and muscles in NMJ development.     

The SAD-1 kinase regulates presynaptic vesicle clustering and axon termination

During synapse formation, presynaptic axon outgrowth is terminated, presynaptic clusters of vesicles are associated with active zone proteins, and active zones are aligned with postsynaptic neurotransmitter receptors. We report here the identification of a novel serine/threonine kinase, SAD-1, that regulates several aspects of presynaptic differentiation in C. elegans. In sad-1 mutant animals presynaptic vesicle clusters in sensory neurons and motor neurons are diffuse and disorganized. Sensory axons fail to terminate in sad-1 mutants, whereas overexpression of SAD-1 causes sensory axons to terminate prematurely. SAD-1 protein is expressed in the nervous system and localizes to synapse-rich regions of the axons. SAD-1 is related to PAR-1, a kinase that regulates cell polarity during asymmetric cell division. Overexpression of SAD-1 causes mislocalization of vesicle proteins to dendrites, suggesting that sad-1 affects axonal-dendritic polarity as well as synaptic development.     

Seeking long-term relationship: axon and target communicate to organize synaptic differentiation

Synapses form after growing axons recognize their appropriate targets. The subsequent assembly of aligned pre and postsynaptic specializations is critical for synaptic function. This highly precise apposition of presynaptic elements (i.e. active zones) to postsynaptic specializations (i.e. neurotransmitter receptor clusters) strongly suggests that communication between the axon and target is required for synaptic differentiation. What trans‐synaptic factors drive such differentiation at vertebrate synapses? First insights into the answers to this question came from studies at the neuromuscular junction (NMJ), where axon‐derived agrin and muscle‐derived laminin β2 induce post and presynaptic differentiation, respectively. Recent work has suggested that axon‐ and target‐derived factors similarly drive synaptic differentiation at central synapses. Specifically, WNT‐7a, neuroligin, synaptic cell adhesion molecule (SynCAM) and fibroblast growth factor‐22 (FGF‐22) have all been identified as target‐derived presynaptic organizers, whereas axon‐derived neuronal activity regulated pentraxin (Narp), ephrinB and neurexin reciprocally co‐ordinate postsynaptic differentiation. In addition to these axon‐ and target‐derived inducers of synaptic differentiation, factors released from glial cells have also been implicated in regulating synapse assembly. Together, these recent findings have profoundly advanced our understanding of how precise appositions are established during vertebrate nervous system development.     

Nitric oxide synthase activity is required for postsynaptic differentiation of the embryonic neuromuscular junction

Agrin, a synapse-organizing protein externalized by motor axons at the neuromuscular junction (NMJ), initiates a signaling cascade in muscle cells leading to aggregation of postsynaptic proteins, including acetylcholine receptors (AChRs). We examined whether nitric oxide synthase (NOS) activity is required for agrin-induced aggregation of postsynaptic AChRs at the embryonic NMJ in vivo and in cultured muscle cells. Inhibition of NOS reduced AChR aggregation at embryonic Xenopus NMJs by 50-90%, whereas overexpression of NOS increased AChR aggregate area 2- to 3-fold at these synapses. NOS inhibitors completely blocked agrin-induced AChR aggregation in cultured embryonic muscle cells. Application of NO donors to muscle cells induced AChR clustering in the absence of agrin. Our results indicate that NOS activity is necessary for postsynaptic differentiation of embryonic NMJs and that NOS is a likely participant in the agrin-MuSK signaling pathway of skeletal muscle cells.     

The function of p120 catenin in filopodial growth and synaptic vesicle clustering in neurons

At the developing neuromuscular junction (NMJ), physical contact between motor axons and muscle cells initiates presynaptic and postsynaptic differentiation. Using Xenopus nerve-muscle cocultures, we previously showed that innervating axons induced muscle filopodia (myopodia), which facilitated interactions between the synaptic partners and promoted NMJ formation. The myopodia were generated by nerve-released signals through muscle p120 catenin (p120ctn), a protein of the cadherin complex that modulates the activity of Rho GTPases. Because axons also extend filopodia that mediate early nerve-muscle interactions, here we test p120ctn's function in the assembly of these presynaptic processes. Overexpression of wild-type p120ctn in Xenopus spinal neurons leads to an increase in filopodial growth and synaptic vesicle (SV) clustering along axons, whereas the development of these specializations is inhibited following the expression of a p120ctn mutant lacking sequences important for regulating Rho GTPases. The p120ctn mutant also inhibits the induction of axonal filopodia and SV clusters by basic fibroblast growth factor, a muscle-derived molecule that triggers presynaptic differentiation. Of importance, introduction of the p120ctn mutant into neurons hinders NMJ formation, which is observed as a reduction in the accumulation of acetylcholine receptors at innervation sites in muscle. Our results suggest that p120ctn signaling in motor neurons promotes nerve-muscle interaction and NMJ assembly.     

What controls the position,number, size,and distribution of neuromuscular junctions on rat muscle fibers?

This review focuses on mechanisms that determine the position, number, size, and distribution of neuromuscular junctions (NMJs) on skeletal muscle fibers. Most of the data reviewed derive from studies of ectopic NMJ formation on soleus (SOL) muscle fibers in adult rats, which recapitulates essential aspects of NMJ formation in normal development. Transplanted axons induce acetylcholine receptor (AChR) aggregates, which are multiple and irregularly distributed initially but subsequently undergo massive reorganization such that one or a few winners survive and reach a certain size while the rest are eliminated (the losers). Results obtained by blocking nerve activity early and stimulating the SOL electrically show that evoked muscle impulse activity is responsible for the growth of winners to a given size and the creation of refractory zones, about 0.75 long, on each side of the winners, in which the elimination of losers occurs. Consequently, when two or more aggregates or NMJs survive on one fiber, they are, on average, at least 1.5 mm apart. Locally applied neural agrin induces comparable aggregation of AChRs and other postsynaptic proteins on denervated SOL fibers and such aggregates undergo similar activity-dependent selection for survival or elimination in refractory zones. In a dose-dependent way, neural agrin alone also induces expression of ε-AChR subunits and stabilizes AChRs to a half-life of 10 days, as found at normal NMJs. It is argued that signs of prepatterning of innervation sites by intrinsic muscle mechanisms may refer to epiphenomena that play no important role in NMJ formation. The conclusion is that neural agrin initiates and then maintains NMJs where motor axons happen to contact receptive muscle fibers and that evoked muscle impulse activity then ensures that the NMJs reach their appropriate size, efficiency and spatial distribution along each fiber.     

Identified motor terminals in Drosophila larvae show distinct differences in morphology and physiology

In Drosophila, the type I motor terminals innervating the larval ventral longitudinal muscle fibers 6 and 7 have been the most popular preparation for combining synaptic studies with genetics. We have further characterized the normal morphological and physiological properties of these motor terminals and the influence of muscle size on terminal morphology. Using dye-injection and physiological techniques, we show that the two axons supplying these terminals have different innervation patterns: axon 1 innervates only muscle fibers 6 and 7, whereas axon 2 innervates all of the ventral longitudinal muscle fibers. This difference in innervation pattern allows the two axons to be reliably identified. The terminals formed by axons 1 and 2 on muscle fibers 6 and 7 have the same number of branches; however, axon 2 terminals are approximately 30% longer than axon 1 terminals, resulting in a corresponding greater number of boutons for axon 2. The axon 1 boutons are approximately 30% wider than the axon 2 boutons. The excitatory postsynaptic potential (EPSP) produced by axon 1 is generally smaller than that produced by axon 2, although the size distributions show considerable overlap. Consistent with vertebrate studies, there is a correlation between muscle fiber size and terminal size. For a single axon, terminal area and length, the number of terminal branches, and the number of boutons are all correlated with muscle fiber size, but bouton size is not. During prolonged repetitive stimulation, axon 2 motor terminals show synaptic depression, whereas axon 1 EPSPs facilitate. The response to repetitive stimulation appears to be similar at all motor terminals of an axon.     

Coordinated motor neuron axon growth and neuromuscular synaptogenesis are promoted by CPG15 in vivo

We have used in vivo time-lapse two-photon imaging of single motor neuron axons labeled with GFP combined with labeling of presynaptic vesicle clusters and postsynaptic acetylcholine receptors in Xenopus laevis tadpoles to determine the dynamic rearrangement of individual axon branches and synaptogenesis during motor axon arbor development. Control GFP-labeled axons are highly dynamic during the period when axon arbors are elaborating. Axon branches emerge from sites of synaptic vesicle clusters. These data indicate that motor neuron axon elaboration and synaptogenesis are concurrent and iterative. We tested the role of Candidate Plasticity Gene 15 (CPG15, also known as Neuritin), an activity-regulated gene that is expressed in the developing motor neurons in this process. CPG15 expression enhances the development of motor neuron axon arbors by promoting neuromuscular synaptogenesis and by increasing the addition of new axon branches.     

Formation of the neuromuscular junction: molecules and mechanisms

The vertebrate skeletal neuromuscular junction is the site at which motor neurons communicate with their target muscle fibers. At this synapse, as at synapses throughout the nervous system, efficient and appropriate communication requires the formation and precise alignment of specializations for transmitter release in the axon terminal with those for transmitter detection in the postsynaptic cell. Classical developmental studies demonstrate that synapse formation at the neuromuscular junction is a mutually inductive event; neurons induce postsynaptic differentiation in muscle cells and myofibers induce presynaptic differentiation in motor axon terminals. More recent experiments indicate that Schwann cells, which cap axon terminals, also play an active role in the formation and maintenance of the neuromuscular junction. Here, we review recent advances in the identification of molecules mediating such inductive interactions and the mechanisms by which they produce their effects. Although our discussion concerns events at developing neuromuscular junctions, it seems likely that similar molecules and mechanisms may act at neuron–neuron synapses in the peripheral as well as the central nervous system. BioEssays 20 :819–829, 1998. © 1998 John Wiley & Sons, Inc.     

HGF induction of postsynaptic specializations at the neuromuscular junction

A critical event in the formation of vertebrate neuromuscular junctions (NMJs) is the postsynaptic clustering of acetylcholine receptors (AChRs) in muscle. AChR clustering is triggered by the activation of MuSK, a muscle-specific tyrosine kinase that is part of the functional receptor for agrin, a nerve-derived heparan sulfate proteoglycan (HSPG). At the NMJ, heparan sulfate (HS)-binding growth factors and their receptors are also localized but their involvement in postsynaptic signaling is poorly understood. In this study we found that hepatocyte growth factor (HGF), an HS-binding growth factor, surrounded muscle fibers and was localized at NMJs in rat muscle sections. In cultured Xenopus muscle cells, HGF was enriched at spontaneously occurring AChR clusters (hot spots), where HSPGs were also concentrated, and, following stimulation of muscle cells by agrin or cocultured neurons, HGF associated with newly formed AChR clusters. HGF presented locally to cultured muscle cells by latex beads induced new AChR clusters and dispersed AChR hot spots, and HGF beads also clustered phosphotyrosine, activated c-Met, and proteins of dystrophin complex; clustering of AChRs and associated proteins by HGF beads required actin polymerization. Lastly, although bath-applied HGF alone did not induce new AChR clusters, addition of HGF potentiated agrin-dependent AChR clustering in muscle. Our findings suggest that HGF promotes AChR clustering and synaptogenic signaling in muscle during NMJ development.     

Acetylcholine receptor distribution on regenerating mammalian muscle fibers at sites of mature and developing nerve-muscle junctions

When rat soleus muscles fibers regenerated after notexin-induced damage, AChRs were present at high density on the surface of the new muscle fibers at the sites of the original NMJs, even if the intact motor axons were not present during regeneration. Some AChR molecules which were labelled with R-BgTx before notexin-induced damage persisted for some days at junctional sites after new muscle fibres had regenerated. During muscle fiber degeneration, components of the muscle fiber plasma membrane appeared to remain longer in the junctional region than elsewhere. When muscles on which new "ectopic" NMJs had been forming for at least 2 weeks were damaged, AChR clusters together with sites of high AChE activity were present 2 weeks later on the regenerated muscles in the region of new NMJ formation, even if the "foreign" nerve was not intact during the period of regeneration. If ectopic NMJs had been forming for only 4 days at the time of muscle and nerve damage, neither AChR clusters nor AChE activity were detected on the regenerated muscle fibers.     

An intrinsic distinction in neuromuscular junction assembly and maintenance in different skeletal muscles

We analyzed the formation of neuromuscular junctions (NMJs) in individual muscles of the mouse embryo. Skeletal muscles can be assigned to one of two distinct classes of muscles, termed "Fast Synapsing" (FaSyn) and "Delayed Synapsing" (DeSyn) muscles, which differ significantly with respect to the initial focal clustering of postsynaptic AChRs, the timing of presynaptic maturation, and the maintenance of NMJs in young adult mice. Differences between classes were intrinsic to the muscles and manifested in the absence of innervation or agrin. Paralysis or denervation of young adult muscles resulted in disassembly of AChR clusters on DeSyn muscles, whereas those on FaSyn muscles were preserved. Our results show that postsynaptic differentiation processes intrinsic to FaSyn and DeSyn muscles influence the formation of NMJs during development and their maintenance in the adult.     

Regulation of axonal growth and neuromuscular junction formation by neuronal phosphatase and tensin homologue signaling

During the development of the vertebrate neuromuscular junction (NMJ), motor axon tips stop growing after contacting muscle and transform into presynaptic terminals that secrete the neurotransmitter acetylcholine and activate postsynaptic ACh receptors (AChRs) to trigger muscle contraction. The neuron-intrinsic signaling that retards axonal growth to facilitate stable nerve–muscle interaction and synaptogenesis is poorly understood. In this paper, we report a novel function of presynaptic signaling by phosphatase and tensin homologue (PTEN) in mediating a growth-to-synaptogenesis transition in neurons. In Xenopus nerve–muscle cocultures, axonal growth speed was halved after contact with muscle, when compared with before contact, but when cultures were exposed to the PTEN blocker bisperoxo (1,10-phenanthroline) oxovanadate, axons touching muscle grew ∼50% faster than their counterparts in control cultures. Suppression of neuronal PTEN expression using morpholinos or the forced expression of catalytically inactive PTEN in neurons also resulted in faster than normal axonal advance after contact with muscle cells. Significantly, interference with PTEN by each of these methods also led to reduced AChR clustering at innervation sites in muscle, indicating that disruption of neuronal PTEN signaling inhibited NMJ assembly. We thus propose that PTEN-dependent slowing of axonal growth enables the establishment of stable nerve–muscle contacts that develop into NMJs.     

Essential roles of the acetylcholine receptor gamma-subunit in neuromuscular synaptic patterning

Formation of the vertebrate neuromuscular junction (NMJ) takes place in a stereotypic pattern in which nerves terminate at select sarcolemmal sites often localized to the central region of the muscle fibers. Several lines of evidence indicate that the muscle fibers may initiate postsynaptic differentiation independent of the ingrowing nerves. For example, nascent acetylcholine receptors (AChRs) are pre-patterned at select regions of the muscle during the initial stage of neuromuscular synaptogenesis. It is not clear how these pre-patterned AChR clusters are assembled, and to what extent they contribute to pre- and post-synaptic differentiation during development. Here, we show that genetic deletion of the AChR gamma-subunit gene in mice leads to an absence of pre-patterned AChR clusters during initial stages of neuromuscular synaptogenesis. The absence of pre-patterned AChR clusters was associated with excessive nerve branching, increased motoneuron survival, as well as aberrant distribution of acetylcholinesterase (AChE) and rapsyn. However, clustering of muscle specific kinase (MuSK) proceeded normally in the gamma-null muscles. AChR clusters emerged at later stages owing to the expression of the AChR epsilon-subunit, but these delayed AChR clusters were broadly distributed and appeared at lower level compared with the wild-type muscles. Interestingly, despite the abnormal pattern, synaptic vesicle proteins were progressively accumulated at individual nerve terminals, and neuromuscular synapses were ultimately established in gamma-null muscles. These results demonstrate that the gamma-subunit is required for the formation of pre-patterned AChR clusters, which in turn play an essential role in determining the subsequent pattern of neuromuscular synaptogenesis.     

Mitochondrial clustering at the vertebrate neuromuscular junction during presynaptic differentiation

During vertebrate neuromuscular junction (NMJ) development, presynaptic motor axons differentiate into nerve termini enriched in synaptic vesicles (SVs). At the nerve terminal, mitochondria are also concentrated, but how mitochondria become localized at these specialized domains is poorly understood. This process was studied in cultured Xenopus spinal neurons with mitochondrion-specific probe MitoTracker and SV markers. In nerve-muscle cocultures, mitochondria were concentrated stably at sites where neurites and muscle cells formed NMJs, and mitochondria coclustered with SVs where neurites were focally stimulated by beads coated with growth factors. Labeling with a mitochondrial membrane potential-dependent probe JC-1 revealed that these synaptic mitochondria were with higher membrane potential than the extrasynaptic ones. At early stages of bead-stimulation, actin-based protrusions and microtubule fragmentation were observed in neurites at bead contact sites, suggesting the involvement of cytoskeletal dynamics and rearrangement during presynaptic differentiation. Treating the cultures with an actin polymerization blocker, latrunculin A (Ltn A), almost completely abolished the formation of actin-based protrusions and partially inhibited bead-induced mitochondrial and SV clustering, whereas the microtubule disrupting agent nocodazole was ineffective in inhibiting the clustering of mitochondria and SVs. Lastly, in contrast to Ltn A, which blocked bead-induced clustering of both mitochondria and SVs, the ser/thr phosphatase inhibitor okadaic acid inhibited SV clustering but not mitochondrial clustering. These results suggest that at developing NMJs, synaptogenic stimuli induce the clustering of mitochondria together with SVs at presynaptic terminals in an actin cytoskeleton-dependent manner and involving different intracellular signaling molecules.     

An Attempt to Account for the Diversity of Crustacean Muscles

Crustacean muscles are known to contain muscle fibers of variableproperties and to be innervated by phasic and/or tonic motoneuronswhich may possess synapses of diverse physiological properties.Frequently, phasic motor axons innervate short-sarcomere phasicmuscle fibers and tonic motor axons innervate long-sarcomeretonic muscle fibers, but some muscles receiving a single (tonic)motor axon contain both phasic and tonic muscle fibers. Althoughit is not known whether neural trophic influences are involvedin muscle differentiation, some neural trophic effects havebeen found in crustaceans, and it is reasonable to assume thatsuch influences may be involved in establishing the definitiveproperties of the muscle. Several other postulates must be made:(1) Phasic and tonic motor axons differ in their trophic effectiveness:(2) muscle fibers innervated relatively early in developmentby a tonic motor axon acquire the properties of tonic musclefibers, while those innervated later become intermediate orphasic muscle fibers; (3) the developmental stage of a growingor regenerating axon terminal plays a role in determinationof synaptic properties. Studies on regenerating limb buds supportthe hypothesis, which can account for the genesis of all observedtypes of crustacean neuromuscular system. Further experimentalwork is necessary to test the hypothesis.     

NMDA receptor activity stabilizes presynaptic retinotectal axons and postsynaptic optic tectal cell dendrites in vivo

To investigate the role of N-methyl-D-aspartate (NMDA) receptor activity in the stability of the presynaptic axon arbor and postsynaptic dendritic arbors in vivo, we took time-lapse confocal images of single DiI-labeled Xenopus retinotectal axons and optic tectal neurons in the presence and absence of the NMDA receptor antagonist, APV. Retinotectal axons or tectal neurons were imaged at 30-min intervals over 2 h, or twice over a 24-h period. Retinal axons in animals exposed to DL-APV (100 microM) showed an increase in rates of branch additions and a decrease in branch lifetimes over 2 h compared to untreated axons. Under the same experimental conditions, tectal neurons showed a decreased rate of branch tip additions and retractions. APV treatment over 24 h had no apparent effect on axon arbor morphology, but did decrease tectal cell dendritic arbor elaboration. These observations demonstrate that NMDA receptor activity in postsynaptic neurons stabilizes pre- and postsynaptic neuronal morphology in vivo.. However, when NMDA receptor activity is blocked, presynaptic retinal axons respond with increased arbor dynamics while postsynaptic tectal cell dendrites decrease arbor dynamics. Such differential responses of pre- and postsynaptic partners might increase the probability of coactive afferents converging onto a common target under conditions of lower NMDA receptor activity.