Chemical communication controls a wide range of behaviors via conserved signaling networks. Axon regeneration in response to injury is determined by the interaction between the extracellular environment and intrinsic growth potential. In this study, we investigated the role of chemical signaling in axon regeneration in Caenorhabditis elegans . We find that the enzymes involved in ascaroside pheromone biosynthesis, ACOX-1.1, ACOX-1.2, and DAF-22, participate in axon regeneration by producing a dauer-inducing ascaroside, ascr#5. We demonstrate that the chemoreceptor genes, srg-36 and srg-37 , which encode G-protein-coupled receptors for ascr#5, are required for adult-specific axon regeneration. Furthermore, the activating mutation in egl-30 encoding Gqα suppresses axon regeneration defective phenotype in acox-1.1 and srg-36 srg-37 mutants. Therefore, the ascaroside signaling system provides a unique example of a signaling molecule that regulates the regenerative pathway in the nervous system. SIGNIFICANCE S...

It is not known why neurons in the adult mammalian central nervous system fail to regenerate their axons after injury. Recently it has been reported that retinal ganglion cells (RGCs) lose their intrinsic axon growth ability around the time of birth, and this loss can be signaled by amacrine cell membranes. Concomitantly, there is an increase in the RGCs ability to grow dendrites, suggesting there may be a developmentally regulated switch from an axonal to a dendritic growth mode soon after birth. These results raise the hypothesis that failure in CNS regeneration is not only due to glial inhibition, but also because the neurons have lost their intrinsic ability to rapidly extend their axons. In order to understand the molecular basis of the irreversible developmental switch from axonal to dendritic growth mode, we are currently investigating the changes in the gene expression profile of rat RGCs during development. We compared RGC gene expression from embryonic day 17 through postnatal day 21 using Affyme...

Both vertebrate and invertebrate motor neurons can display bistable behavior, in which self-sustained tonic firing results from a brief input. Induction of the bistability is usually dependent upon activation of intrinsic conductances located in the somato-dendritic area and is sensitive to neuromodulatory action. We have observed bistable behavior in a preparation from the foregut of the crab Cancer borealis that consists of the gastric mill 4 (gm4) muscle and the nerve that innervates it. Nerve-evoked contractions of anomalously large amplitude and long duration (> 30 sec) were induced by 3 sec duration, 12.5 Hz stimulations when the stimulus voltage was above a certain threshold. Intracellular and extracellular recordings showed that the large contractions were accompanied by persistent firing of the DG neuron, the motor neuron that innervates gm4. Surprisingly, this persistent firing could only be induced by stimulating a specific region of the axon and could not be triggered by depolarizing the soma, ...

Recent evidence reveals alterations in the somatostatin (SST) subpopulation of GABAergic interneurons in the prefrontal cortex (PFC) of subjects with schizophrenia. Understanding the pathophysiological significance of these findings requires knowledge of the synaptic targets of SST-containing neurons and their role in the circuitry of the primate PFC. Thus, we used immuno-electron microscopy to examine the synaptic targets of SST-labeled axon terminals in the superficial and deep layers of macaque monkey PFC, and compared these findings to those of a different subset of interneurons that contain the calcium-binding protein calretinin (CR). Preliminary data reveal that SST-immunoreactive (IR) axon terminals exclusively form symmetric synapses. In the superficial layers, SST-IR axon terminals predominantly contact dendritic shafts (77%), with the remainder forming synapses onto dendritic spines. Similarly, in the deep layers, the primary targets of SST-IR axon terminals are dendritic shafts (90%), with dendr...

Sensorimotor cortex (SMC) activity plays an important role in promoting growth of corticospinal (CS) terminations during the entire period of postnatal CS system development (postnatal weeks, PW, 3-7) in kittens. In previous experiments we have shown that we can alter the process of activity dependent refinement of CS axon termination topography and morphology by either inactivation (Friel and Martin 2005) of SMC or by electrical stimulation of the CS tract (Salimi and Martin 2004) during the critical developmental period. Blocking activity of the CS system on one side resulted in failure of the inactivated CS tract neurons to develop dense termination. Whereas electrical stimulation promoted CS axon growth and maintained transient terminations later in development. In this study we determined if electrical stimulation of the previously inactivated CS system could rescue pyramidal tract neurons at risk of failing to secure dense spinal terminations. We inactivated the left M1 from PW 5-7 by continuous intr...

Both excitatory and inhibitory neocortical neurons spontaneously fire during active cortical states and thus both excitatory and inhibitory influences are sensed by postsynaptic neurons. Previously we reported (Crochet et al., 2005) that active neocortical states in vivo are associated with large failure rates between cortical excitatory neurons. The properties of single-axon inhibitory responses in vivo were not yet described. In this study we used multi-site (up to 4) intracellular recordings from closely located neurons in anesthetized cats in vivo to identify and describe properties of single-axon inhibitory responses. In intracellularly stained neuronal pairs the distance between inhibitory interneuron and its postsynaptic targets was 0.3 mm or shorter. Most of the time, the membrane potential of postsynaptic neurons was negative to the reversal potential of single-axon IPSPs and most of postsynaptic responses elicited by spontaneous presynaptic firing of inhibitory neurons were depolarizing. Either s...

The transcription factor NF-KappaB (NF-KB) is present in the cytosol in an inactive state, complexed with the inhibitory protein IKappaBα. Constitutive activation of NF-KB occurs in neurons of the central nervous system via phosphorylation of IKappaBα at Ser32 and Ser36 and subsequent degradation of phospho-IKappaBα. We studied this constitutive neuronal activation of NF-KB in brain sections of several mammals (mice, rats, rhesus monkeys, humans) using immunostaining with polyclonal and monoclonal antibodies directed against IKappaBα phosphorylated at Ser32 and Ser36. Strikingly, a highly selective enrichment of phospho-IKappaBα was found in the axon initial segment of all species examined. Specificity of the immunostaining was demonstrated by preincubating the anti-phospho-IKappaBα antibodies with the corresponding phospho-IKappaBα blocking peptides which completely prevented immunolabeling. The selective axonal localization of phospho-IKappaBα was confirmed by double-immunostaining with antibodies direct...

Peripheral somatosensory neurons innervate the skin and sense the environment. Whereas many studies focus on initial axon outgrowth and pathfinding, how signaling pathways contribute to maintenance of the established axon arbors and terminals within the skin is largely unknown. This question is particularly relevant to the many types of neuropathies that affect mature neuronal arbors. We show that a receptor tyrosine kinase (RTK), c-Kit, contributes to maintenance, but not initial development, of cutaneous axons in the larval zebrafish before sex determination. Downregulation of Kit signaling rapidly induced retraction of established axon terminals in the skin and a reduction in axonal density. Conversely, misexpression of c-Kit ligand in the skin in larval zebrafish induced increases in local sensory axon density, suggesting an important role for Kit signaling in cutaneous axon maintenance. We found Src family kinases (SFKs) act directly downstream to mediate Kit’s role in regulating cutaneous axon densit...

The axon initial segment (AIS) is a specialized neuronal compartment in which synaptic input is converted into action potential (AP) output. This process is supported by a diverse complement of sodium, potassium, and calcium channels (CaV). Different classes of sodium and potassium channels are scaffolded at specific sites within the AIS, conferring unique functions, but how calcium channels are functionally distributed within the AIS is unclear. Here, we use conventional two-photon laser scanning and diffraction-limited, high-speed spot two-photon imaging to resolve AP-evoked calcium dynamics in the AIS with high spatiotemporal resolution. In mouse layer 5 prefrontal pyramidal neurons, calcium influx was mediated by a mix of CaV2 and CaV3 channels that differentially localized to discrete regions. CaV3 functionally localized to produce nanodomain hotspots of calcium influx that coupled to ryanodine-sensitive stores, whereas CaV2 localized to non-hotspot regions. Thus, different pools of CaVs appear to pla...

How do myelinated axons signal to the nuclei of cells that enwrap them? The cell bodies of oligodendrocytes and Schwann cells are segregated from axons by multiple layers of bimolecular lipid leaflet and myelin proteins. Conventional signal transduction strategies would seem inadequate to the challenge without special adaptations. Wallerian degeneration provides a model to study axon-to-Schwann cell signaling in the context of nerve injury. We show a hitherto undetected rapid, but transient, activation of the receptor tyrosine kinase erbB2 in myelinating Schwann cells after sciatic nerve axotomy. Deconvolving microscopy using phosphorylation state-specific antibodies shows that erbB2 activation emanates from within the microvilli of Schwann cells, in direct contact with the axons they enwrap. To define the functional role of this transient activation, we used a small molecule antagonist of erbB2 activation (PKI166). The response of myelinating Schwann cells to axotomy is inhibited by PKI166 in vivo . Using...