ISSN:0022-3034    

DOI:10.1002/neu.480200502

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractRecent studies are reviewed on the synapses of photoreceptor terminals in the first optic neuropile of the flies,MuscaandDrosophila. Afferent synaptic contacts are of uniform dimensions; they have a postsynaptic tetrad with a membrane organization of P‐face particles, resembling other inhibitory synapses. A distributed population of such contact sites forms progressively during synaptogenesis by the selective, sequential accretion of identified postsynaptic elements at the receptor terminal. The comparative anatomy of this synapse indicates that elements have also been added during its phylogeny from an ancestral dyad. All cells are homologs of those in other species of Diptera. The number of synaptic sites is regulated by both pre‐ and postsynaptic cells, in proportion to their cell surfaces; an independent size increase in the receptor terminals (procured in theDrosophilamutantgigas) produces an increase in their synaptic population. The number of sites declines with age, however, accompanied by an increase in size of those synaptic sites remaining; this occurs for both afferent and feedback photoreceptor synapses. Lastly, the number of sites changes with visual experience; the frequency of feedback synapses is larger following dark rearing during early adult life than following visual experie

ISSN:0022-3034    

DOI:10.1002/neu.480200503

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThis essay provides a brief overview of neuronal plasticity in adult invertebrate nervous systems. Our discussion focuses on the factors which influence sprouting by adult neurons, i.e., (1) the nature of the neuron itself, (2) axon integrity, (3) the presence of targets, (4) diffusible factors, and (5) ageing. Evidence that the neurites of some adult neurons exhibit a dynamic equilibrium of expansion and retraction is presented, a topic which prompted us to speculate on the significance of such plasticity in altered behavioral states. We conclude with some suggestions as to specific questions that need to be addressed by future studies in this challenging area.

ISSN:0022-3034    

DOI:10.1002/neu.480200504

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractIn an attempt to define the mechanism of synaptic specificity, we have been studying pairs of identified leech neurons isolated in tissue culture. The cultured neurons reform specific synapses when paired with appropriate partners in the absence of other cell types. In recent studies, we have examined in detail the reformation of a serotoninergic synapse between the Retzius cell and one of its targets, the pressure sensitive (P) cell. The P cellin vivoand its somain vitrohave two types of responses to serotonin (5‐HT). From voltage clamp analysis of cultured P cells, we demonstrated the parallel activation of chloride (gCls) and monovalent cation (gCations) channels coupled to distinct receptor subtypes and gated by separate second messengers. Only gClswas activated by 5‐HT released from the presynaptic Retzius cell bothin vivoandin vitro. This demonstrates the remarkable specificity of the reformation of this synapse in culture since only the correct 5‐HT receptor subtype is activated.An 80% reduction of gCationswas observed in P cells that had failed to be innervated by Retzius cells in culture, suggesting that gCationsmay be lost prior to synapse formation. Retzius cells depleted of 5‐HT also reduced gCationssin the paired P cells and incubating single P cells in 5‐HT did not reduce gCations. In addition, aldehyde‐fixed Retzius cells were able to selectively reduce gCationswhen paired with P cells. We conclude that the loss of gCationswas due to contact between the neurons. The early clearing of counter‐effective receptor subtypes may be a prelude to syna

ISSN:0022-3034    

DOI:10.1002/neu.480200505

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractDuring the initial phase of metamorphosis in the hawkmoth,Manduca sexta, persistent mechanosensory neurons expand their terminal arborizations within the CNS and evoke a reflex response in the pupa which is different than in the larva. In an effort to determine the contribution of sensory neuron modifications to the difference in reflex responses, manipulations of juvenile hormone and 20‐hydroxyecdysone were used to generate mosaic animals in which the sensory neurons were advanced or delayed developmentally with respect to the rest of the animal, including circuit components within the CNS. In the larval stage electrical stimulation of the sensory axons evokes a slow depolarization and a prolonged burst of action potentials in the ipsilateral intersegmental muscle motor neurons. By contrast, in pupal preparations the same motor neurons respond to an identical stimulus with a larger, more rapid depolarization which leads to a relatively brief, high‐frequency burst of action potentials. Motor responses on the contralateral side of the body are also altered during pupal development. In mosaic animals where larval‐like sensory neurons interact with a pupal CNS, a larval reflex response is generated. In the converse situation, pupal‐like sensory neurons interacting with a larval or prepupal CNS evoke a motor response that is typical of larvae or prepupae. We conclude, therefore, that pupal development of the sensory neurons is necessary, but not sufficient, for the production of the pupal

ISSN:0022-3034    

DOI:10.1002/neu.480200506

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractEvidence is provided that during invertebrate development synapses undergo a period of refinement during which there are changes in synaptic connectivity and specific synaptic properties. It appears that extrinsic cues such as competition and neural activity are involved in guiding these synaptic changes in invertebrates. Comparisons are made with findings in the vertebrate literature.

ISSN:0022-3034    

DOI:10.1002/neu.480200507

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractWe have used the gill‐ and siphon‐withdrawal reflex ofAplysia californicato determine the morphological basis of the prolonged changes in synaptic effectiveness that underlie long‐term habituation and sensitization. We have found that clear structural changes accompany behavioral modification and have demonstrated that these can be detected at the level of identified sensory neuron synapses, a critical site of plasticity for the short‐term forms of both types of learning. These alterations occur at two different levels of synaptic organization and include (1) changes in focal regions of synaptic membrane specialization—the number, size and vesicle complement of sensory neuron active zones are larger in sensitized animals and smaller in habituated animals compared with controls—and (2) a parallel but more dramatic and global trend involving modulation of the total number of presynaptic varicosities per sensory neuron. Quantitative analysis of the time course over which these structural alterations occur during sensitization has further demonstrated that changes in the number of varicosities and active zones persist in parallel with the behavioral retention of the memory. This increase in the number of sensory neuron synapses during long‐term sensitization inAplysiais similar to changes in the number of synapses in the mammalian brain following various forms of environmental manipulations and learning (Greenough, 1984). Therefore learning may involve a form of neuronal growth across a braod segment of the animal kingdom, thereby suggesting a role for structural synaptic plasticity during long‐term behaviora

ISSN:0022-3034    

DOI:10.1002/neu.480200508

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractIn this review we present recent evidence implicating second‐messenger systems in two forms of long‐lasting synaptic change seen at crustacean neuromuscular junctions. Crustacean motor axons are endowed with numerous terminals, each possessing many individual synapses. Some synapses appear to be quiescent or impotent, but can be recruited in response to imposed functional demands. Supernormal impulse activity leads to long‐term facilitation (LTF) which persists for many hours. During the persistent phase, additional synapses are physiologically effective, and morphological changes in synapses are seen at the ultrastructural level. Pulsatile application of serotonin, a neuromodulator, also enhances synaptic transmission, but this enhancement declines more rapidly than LTF. Elevation of intraterminal Ca2+is neither necessary nor sufficient for long‐lasting enhancement of transmission, but activation of A‐kinase is necessary. LTF is set in motion by an unknown depolarization‐dependent mechanism leading to A‐kinase activation, whereas serotonin facilitation depends for its initiation on the phosphatidylinositol system. The initial phase of serotonin facilitation may be accounted for by production of inositol triphosphate, whereas the secondary long‐lasting phase appears to require participation of both C kinase and A kinase. Neither LTF nor serotonin facilitation requires an intact neuron; both are presynaptic phenomena expressed by the nerve terminals. Brief comparison is made with long‐lasting synaptic changes

ISSN:0022-3034    

DOI:10.1002/neu.480200509

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThe crayfish opener neuromuscular preparation exhibits most of the plasticities yet described for any synpase, including facilitation, long‐term potentiation, presynaptic inhibition, and modulation. Since the presynaptic terminals and postsynaptic muscle fibers can both be intracellularly penetrated, one can now more easily examine the cellular/molecular bases for these plasticities. Data from such studies suggest that facilitation may be influenced by something other than residual free calcium and that presynaptic inhibition is produced by a conductance increase to chloride in the terminals of the excitor axon. Several drugs (ethanol, pentobarbital) have significant effects on these synaptic plasticities over concentration ranges which produce obvious behavioral effects in crayfish and mammals. Hence, this preparation should be a useful model system to determine cellular/molecular bases for various synaptic plasticities and the effects of drugs on these plasticitie

ISSN:0022-3034    

DOI:10.1002/neu.480200510

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractCrustacean motor axons innervate muscle fibers via a multiplicity of synaptic terminals which release small but variable amounts of transmitter. Differences in release performance appear to be correlated with the size of synaptic contacts and presynaptic dense bars (active zones). These structural parameters proliferate via sprouting from existing synaptic terminals and relocate to ever more distal sites during development and growth of an identified axon. Moreover, alterations in number of synaptic contacts and active zones occur in adults following stimulation or decentralization, demonstrating structural plasticity of crustacean neuromuscular synapses.

ISSN:0022-3034    

DOI:10.1002/neu.480200511

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY