Journal of neurobiology最新文献

The mechanisms that guide axons through a complex cellular landscape to reach appropriate target cells are central to our understanding of neural development. Decades of work suggest that guidance information is interpreted by signaling machinery that controls the complex and dynamic cytoskeleton at the growth cone leading edge. Recent insights from the areas of signal transduction and cell biology have identified a number of key components that play central roles in this chain of command, including members of the Ena/VASP and WASP family of proteins. Although our understanding of the precise mechanism by which these proteins control actin assembly is still incomplete, these players are emerging as potential sites of integration that translate convergent signals into directional cell movement. This brief review explores some of the most recent articles on this topic.

Growth cone motility and navigation in response to extracellular signals are regulated by actin dynamics. To better understand actin involvement in these processes we determined how and in what form actin reaches growth cones, and once there, how actin assembly is regulated. A continuous supply of actin is maintained at the axon tip by slow transport, the mobile component consisting of an unassembled form of actin. Actin is co-transported with actin-binding proteins, including ADF and cofilin, structurally related proteins essential for rapid turnover of actin filaments in vivo. ADF and cofilin activity is regulated through phosphorylation by LIM kinases, downstream effectors of the Rho family of GTPases, Cdc42, Rac and Rho. Attractive and repulsive extracellular guidance cues might locally alter actin dynamics by binding specific GTPase-linked receptors, activating LIM kinases, and subsequently modulating the activity of ADF/cofilin. ADF is enriched in growth cones and is required for neurite outgrowth. In addition, signals that influence growth cone behavior alter ADF/cofilin phosphorylation, and overexpression of ADF enhances neurite outgrowth. Growth promoting effects of laminin are mimicked by expression of constitutively active Cdc42 and blocked by expression of the dominant negative Cdc42. Repulsive effects of myelin and sema3D on growth cones are blocked by expression of constitutively active Rac1 and dominant negative Rac1, respectively. Thus a series of complex pathways must exist for regulating effectors of actin dynamics. The bifurcating nature of the ADF/cofilin phosphorylation pathway may provide the integration necessary for this complex regulation.

The motor patterns produced by the stomatogastric ganglion (STG) are strongly influenced by descending modulatory inputs from anterior ganglia. With these inputs intact, in control saline, the motor patterns produced by the stomatogastric nervous system of embryonic and larval lobsters are slower and less regular than those of adult lobsters. We studied the effects of the hormonal modulator, crustacean cardioactive peptide (CCAP) on the discharge patterns of STG motor patterns in embryos, larvae, and adult Maine lobsters, Homarus americanus, with the anterior inputs present and absent. In adults, CCAP initiated robust pyloric rhythms from STGs isolated from their descending control and modulatory inputs. Likewise, CCAP initiated robust activity in isolated embryonic and larval STGs. Nonetheless, quantitative analyses revealed that the frequency and regularity of the STG motor neuron discharge seen in the presence of CCAP in isolated STGs from embryos were significantly lower than those seen late in larval life and in adults under the same conditions. In contrast, when the descending control and modulatory pathways to the STG were left intact, the embryonic and larval burst frequency seen in the presence of CCAP was increased by CCAP, whereas the burst frequency in adults was decreased by CCAP, so that in CCAP the frequencies at all stages were statistically indistinguishable. These data argue that immature embryonic motor patterns seen in the absence of CCAP are a function of immaturity in both the STG and in the descending and modulatory pathways.

Drosophila melanogaster sex peptide (SP) and Ductus ejaculatorius peptide (DUP99B) are male pheromones transferred in the seminal fluid to the female during copulation. Both reduce sexual receptivity and stimulate oviposition in females. The presence of high-affinity SP and DUP99B binding sites in the female were investigated by incubation of cryostat tissue sections with (125)I-iodinated peptides and subsequent autoradiography. We found that in adult females radiolabeled SP and DUP99B bind to peripheral nerves, the subesophageal ganglion, the cervical connective, to discrete parts of the thoracic ganglion, and to the genital tract. Weak and uniform labeling was detected in the neuropil of the brain and the thoracic ganglion. The labeling pattern in the nervous system suggests binding of the peptides to sensory afferents or glial cells. Scatchard analysis of the binding of (125)I-DUP99B to antennal nerves yielded a dissociation constant K(d) of 6.4 nM. Competition experiments with peptide fragments show that the peptides bind with their homologous C-terminal regions. Binding sites in the nervous system of females are established throughout sexual maturation. Prominent binding of the peptides to afferent nerves suggests modification of sensory input.

Neurotransmitter receptors are considered an important class of membrane proteins that are involved in plasticity-induced changes underlying learning and memory. Recent studies, which demonstrated that the mRNAs encoding for various receptor proteins are localized to specific dendritic domains, allude toward the possibility that these membrane bound molecules may be synthesized locally. However, direct evidence for the local axonal or dendritic synthesis and functional integration of receptor proteins in either vertebrates or invertebrates is still lacking. In this study, using an invertebrate model system we provide the first direct evidence that isolated axons (in the absence of the soma) can intrinsically synthesize and functionally integrate a membrane-bound receptor protein from an axonally injected mRNA. Surgically isolated axons from identified neurons were injected with mRNA encoding a G-protein-coupled conopressin receptor. Immunocytochemical and electrophysiological techniques were used to demonstrate functional integration of the receptor protein into the membrane of the isolated axon. Ultrastructural analysis of axonal compartments revealed polyribosomes, suggesting that some components of the protein synthesizing machinery are indeed present in these extrasomal compartments. Such axonal propensity to locally synthesize and functionally insert transmitter receptors may be instrumental in plasticity induced changes, for instance those that underlie learning and memory.

Female sedge warblers select males that have more complex songs as mates. This study tests two predictions concerning HVc, a telencephalic nucleus that is essential for song learning and production: first, that males with more complex songs will have a larger HVc, and second that males who pair successfully will have a larger HVc than unpaired males. Data on song composition and pairing status were collected from wild sedge warblers breeding in Hungary. We found significant positive correlations between three song attributes (repertoire size, song complexity, and song length) and the size of HVc. Males that paired successfully also had more complex songs (repertoire size and song complexity, though not song length) than males that did not. However, we find no direct evidence that males who paired successfully had a larger HVc than unpaired males. These findings are discussed in relation to the possible functions of HVc and also to current views on sexual selection and the evolution of the song control pathway.

Identified wind-sensitive giant interneurons in the cricket's cercal sensory system integrate cercal afferent signals and release an avoidance behavior. A calcium-imaging technique was applied to the giant interneurons to examine the presence of the voltage-dependent Ca(2+) channels (VDCCs) in their dendrites. We found that presynaptic stimuli to the cercal sensory nerve cords elevated the cytosolic Ca(2+) concentration ([Ca(2+)](i)) in the dendrites of the giant interneurons. The dendritic Ca(2+) rise coincided with the spike burst of the giant interneurons, and the rate of Ca(2+) rise depended on the frequency of the action potentials. These results suggest that the action potentials directly caused [Ca(2+)](i) increase. Observation of the [Ca(2+)](i) elevation induced by depolarizing current injection demonstrates the presence of the VDCCs in the dendrites. Although hyperpolarizing current injection into the giant interneuron suppressed action potential generation, EPSPs could induce no [Ca(2+)](i) increase. This result means that ligand-gated channels do not contribute to the synaptically stimulated Ca(2+) elevation. On the other hand, antidromically stimulated spikes also increased [Ca(2+)](i) in all cellular regions including the dendrites. And bath application of a mixture of Ni(2+), Co(2+), and Cd(2+) or tetrodotoxin inhibited the [Ca(2+)](i) elevation induced by the antidromic stimulation. From these findings, we suppose that the axonal spikes antidromically propagate and induce the Ca(2+) influx via VDCCs in the dendrites. The spike-dependent Ca(2+) elevation may regulate the sensory signals processing via second-messenger cascades in the giant interneurons.

Retrograde signaling from target tissues has been shown to influence many aspects of neuronal development in a number of developmental systems. In these experiments using embryonic leeches (Hirudo medicinalis), we examined how depriving a neuron of contact with its peripheral target affects the development of the cell's central arborization. We focused our attention on the motor neuron cell 3, which normally stimulates dorsal longitudinal muscle fibers to contract. At different locations in the periphery and in embryos of several different stages, we cut the nerve containing the growing axon of cell 3. This surgery led to dramatic overgrowth of cell 3's central dendritic branches, which normally accept synaptic contacts from other neurons, including the inhibitory motor neuron cell 1. When cell 3's peripheral axon was cut relatively early in development, its overgrown central branches eventually retracted. However, cells that were disrupted later in development retained their overextended branches into adulthood. In addition, if the axon was cut close to the ganglion early in development, depriving the cell of contact with any dorsal tissues, the central branches failed to retract and were instead retained into adulthood. Unlike cell 3, the central branches of cell 1, which has the same peripheral target muscles as cell 3, remained unchanged following all axotomy protocols. These results suggest that in at least some neurons contact with peripheral targets can influence development of the central processes that normally mediate synaptic contacts.

To investigate the involvement of the cell adhesion molecules L1.1, L1.2, NCAM, and tenascin-C in memory formation, zebrafish (Brachydanio rerio) were trained in an active avoidance paradigm to cross a hurdle to avoid mild electric shocks after a light signal. Application of [(14)C]deoxyglucose prior to the training session revealed an increased energy demand in the optic tectum during acquisition of the active avoidance response compared with untrained fish and with fish not learning the task (nonlearners). In situ hybridization with digoxigenin-labeled cRNA probes directed against zebrafish L1.1, L1.2, NCAM, and tenascin-C revealed an enhanced expression of L1.1 and NCAM mRNA in the optic tectum of learners 3 h after acquisition of the task compared with untrained fish, nonlearners, overtrained fish, and learners decapitated 1 or 6 h after acquisition. Levels of L1.2 mRNA were not significantly increased in the tectum 3 h after learning. Tenascin-C was neither expressed in the optic tectum of untrained fish nor in the tectum of learners. To test for a possible involvement of L1.1 in memory consolidation, antibodies were injected intracerebroventricularly 1 h after the last training trial. Two days later, injected zebrafish were tested for recall and evaluated by a retention score (RS), ranging from 1.0 for immediate recall to 0.0 indicating no savings. The average retention score of L1.1 antibody-injected fish (RS = 0. 29) was different from that of tenascin-C antibody-injected (RS = 0. 71) or uninjected fish (RS = 0.78), indicating a pivotal function of L1.1 in long-term memory formation in zebrafish.

During the appetitive phase of feeding, hungry leeches detect a prey by the integration of signals perceived by different sensory systems. Earlier reports suggested that chemical or thermal sensory stimulation of the lip was associated with increased afferent activity in cephalic nerves connecting the lip to the central nervous system. These authors further suggested that this activity was relayed to Retzius cells in segmental ganglia, which then released serotonin to initiate and control all aspects of feeding behavior. In this study, we show that chemosensory or thermal activation of the lip lasting for at least 5 min produces a distinct signal in the cephalic nerves consisting of action potentials of low amplitude. These small amplitude signals are clearly distinguishable from the large action potentials evoked by mechanosensory stimuli applied to the same area of the lip. Both types of sensory stimuli also evoke an increase in the firing frequency of the Retzius cells in segmental ganglia. However, the response recorded in the nerves and the Retzius cells during a maintained stimulus is not constant but decreases with an exponential time course. These results agree with our earlier observations on a semi-intact feeding preparation in which we showed that the firing frequency of the Retzius cell decreased as soon as the leech began to ingest its meal. Therefore, our data provide further evidence suggesting that it is unlikely that heat or chemical cues maintain the Retzius cell in an active state throughout the consummatory phase of feeding.