Ephrin-B reverse signaling promotes structural and functional synaptic maturation in vivo

Ephrin-Eph signaling is involved in axon guidance during development, but it may also regulate synapse development after the axon has contacted the target cell. Here we report that the activation of ephrin-B reverse signaling in the developing Xenopus laevis optic tectum promotes morphological and functional maturation of retinotectal synapses. Elevation of ephrin-B signaling increased the number of retinotectal synapses and stabilized the axon arbors of retinal ganglion cells. It also enhanced basal synaptic transmission and activity-induced long-term potentiation (LTP) of retinotectal synapses. The functional effects were caused by a rapid enhancement of presynaptic glutamate release and a delayed increase in the postsynaptic glutamate responsiveness. The facilitated LTP induction occurred during the early phase of enhanced transmitter release and appeared to be causally related to the late-phase postsynaptic maturation via an NMDA receptor-dependent mechanism. This ephrin-B-dependent synapse maturation supports the notion that the ephrin/Eph protein families have multiple functions in neural development.     

Long-term potentiation in the optic tectum of rainbow trout

We examined synaptic plasticity in the optic tectum of rainbow trout by extracellular recordings. We found that the field-excitatory postsynaptic potential in the retinotectal synapses was potentiated by repetitive stimuli of 1.0 Hz for 20 s to the retinotectal afferents. The long-term potentiation (LTP) developed slowly, and was maintained for at least 2 h. Applications of an antagonist for N-methyl-D-aspartic acid (NMDA) receptors or Mg²⁺-free saline showed that activation of NMDA receptors was required to form the LTP beyond the induction period. The present findings indicate that presynaptic stimulation in the retinotectal synapses causes LTP mediated by NMDA receptors in the optic tectum of rainbow trout.     

Synaptic reorganization in the rabbit hippocampus after lesion of commissural afferents

Summary The degeneration of commissural afferents to the hippocampus in the rabbit was studied by using the Fink-Heimer degeneration method, electron microscopy, and the combined Golgi/EM technique. The stratum oriens (CA3) was selected for quantitative electron microscopic evaluation of postlesional changes since the degeneration of commissural fibers as seen in Fink-Heimer preparations was dense throughout the width of that layer. Accordingly, in electron micrographs of stratum oriens many electron-dense degenerating boutons were found after short survival times (3 and 6 days, respectively), most of them (96%) in synaptic contact with dendritic spines. In the fine structural analysis of Golgi-impregnated CA3 pyramidal cells, spines of basal dendrites were identified as postsynaptic elements of degenerating commissural afferents in stratum oriens.Three days after the lesion, the number of intact synapses/unit area was reduced in stratum oriens of CA3 to 64% of the control; 20% of the synapses were degenerating. Thus, part of the degenerated synapses had disappeared. Evidence is provided that phagocytosis of degenerated boutons still attached to fragments of dendritic spines played a role in this process.Seven weeks after the lesion, the number of intact synapses had returned to control level, suggesting reactive growth of synaptic structures. When the ratio of spine synapses versus shaft synapses was compared with controls, no change had occurred. Thus, after an initial loss of spine synapses after short survival times, new spines have been formed in parallel with ingrowth (sprouting) of neighbouring nonlesioned afferents.     

Systems-matching by degeneration

Summary The total number of optic nerve fibers of the chicken was determined at twenty sequential developmental stages from incubation day 5 to 104 days after hatching. It was found that the total number of optic nerve fibers increases from 4400 on incubation day 5 to about 4.0 million on incubation days 10 and 11. Thereafter, it decreases to a final value of about 2.4 million by incubation day 18 and remains constant from that time on until adulthood. Thus, 40 % of optic fibers degenerate.Degenerating ganglion cells in the retina are first detectable by incubation day 9. Initially degenerating cells are located mainly in the central retina, but on subsequent days they can be found predominantly in peripheral zones.It is postulated that cell death occurs because of competition for adequate arborization space. If more retinal afferent fibers arrive than tectal termination sites are available, supernumerary fibers may degenerate. By degeneration the two systems retina and optic tectum, are matched in size.     

Changes in retinotectal projection in adult xenopus following partial retinal ablation

The retinotectal projection was mapped electrophysiologically and autoradiographically after surgical removal of the nasal or temporal retinal quadrants in Xenopus. The immediate result of retinal ablation was a corresponding temporal or nasal scotoma in the visual field and the absence of part of the optic fibre projection from the tectum. Thirty to fourty days after operation the visual field scotoma still persisted but the vacated tectal area became reinnervated by collateral sprouting of optic fibres from the fringe of the residual retinal projection. It is concluded that the retinotectal connection pattern can be altered by retinal ablations in adult toads.     

Synaptic replacement in the dentate gyrus after unilateral entorhinal lesion: electron microscopic analysis of the extent of replacement of synapses by the remaining entorhinal cortex

Summary In response to a unilateral entorhinal lesion the input from the contralateral entorhinal cortex to the dentate gyrus appears to increase. We have studied this crossed projection by electron microscopy in normal animals and in animals one year or more after a unilateral entorhinal lesion. In normal animals few degenerating boutons are found after a contralateral entorhinal lesion. However, when the contralateral lesion was made one year after an ipsilateral entorhinal lesion, degenerating boutons were readily identified. The boutons were relatively few in number, but formed an abnormally large number of synaptic contacts. These results support the previous conclusion that fibres from the contralateral entorhinal cortex form additional synapses when their ipsilateral homologues are removed. However, these new cortical synapses probably account for only a small portion of those formed in response to the lesion. Thus an anatomically homologous input does not, in this case, selectively capture most of the newly available synaptic sites.     

Suprathreshold excitation of frog tectal neurons by short spike trains of single retinal ganglion cell

It has been established that coincident inputs from multiple presynaptic axons are required to achieve a suprathreshold level of excitation for the most of central neurons. The present study, however, was designed to determine whether a train of spikes of an individual retinal ganglion cell (that is, input from a single presynaptic axon) targeting a frog tectum layer F could evoke suprathreshold excitation of tectal neurons. The lungs of immobilized frog were artificially ventilated during experiments. An individual ganglion cell was electrically stimulated in the retina through a multi-channel electrode. Responses evoked in the tectum by the stimulation were recorded extracellularly from a terminal arborization of the retinotectal fiber using the carbon-fiber microelectrode. Negative and negative-positive spikes (referred to as first type population responses) and polyphasic spikes followed by excitatory synaptic potentials (referred to as second type population responses) were observed in the recordings of retinotectal activity. Usually, the population responses have ensued after the frequency facilitated first and/or second testing individual retinotectal synaptic potential and disappeared in a threshold manner with a reduction of retinotectal transmission by an application of kynurenic acid. These observations have suggested that the population responses were a consequence of a suprathreshold excitation of tectal neurons and, therefore, could serve as the sign for such an excitation. Recordings have also demonstrated that sources of the first type population responses (likely, the hillocks of axons or somas of postsynaptic neurons) lie deeper than the optic fiber layer F of the tectum, whereas sources of the second type population responses (likely, axon terminal arborizations of these postsynaptic neurons) are scattered throughout the optic fiber layers. The findings have suggested: 1) a short train of action potentials of an individual retinal ganglion cell (likely darkness, also known as 5th, detector) can excite tectal neurons to suprathreshold level; 2) tectal and perhaps, nucleus isthmi neurons that make up recurrent connection circuits to the optic fiber layers of the tectum are also activated; 3) a suprathreshold level for an individual retinotectal input is achieved primarily due to the frequency facilitation of synaptic potentials; and 4) an artificial ventilation of the lungs of immobilized frog favors the eliciting of a suprathreshold excitation of tectal neurons, demonstrating that the ventilation certainly improves the physiological condition of a frog.     

Roles of periventricular neurons in retinotectal transmission in the optic tectum

The midbrain roof is a retinorecipient region referred to as the optic tectum in lower vertebrates, and the superior colliculus in mammals. The retinal fibers projecting to the tectum transmit visual information to tectal retinorecipient neurons. Periventricular neurons are a subtype of these neurons that have their somata in the deepest layer of the teleostean tectum and apical dendrites ramifying at more superficial layers consisting of retinal fibers. The retinotectal synapses between the retinal fibers and periventricular neurons are glutamatergic, and ionotropic glutamate receptors mediate the transmission in these synapses. This transmission involves long-term potentiation, and is modulated by hormone action. Visual information processed in the periventricular neurons is transmitted to adjacent tectal cells and target nuclei of periventricular neuron axonal branches, some of which relay the visual information to other brain areas controlling behavior. We demonstrated that periventricular neurons play a principal role in visual information processing in the teleostean optic tectum; the effects of tectal output on behavior is discussed also in the present review.     

The retinotectal projections in the pigeon. an experimental optical and electron microscope study.

Optical and electron microscope studies have been made of the termination pattern of the retinotectal projection in the pigeon. The use of Golgi impregnation and orthograde transport of peroxidase concurred to show that: (i) the majority of retinotectal preterminal segments descend obliquely rather than radially through the superficial layers of the tectum opticum; (ii) terminals in Cajal's layers 2–3 and 7 arborize tangentially, terminals in layer 4 arborize diffusely in a loose way, and, in layer 5, terminal arborizations are arranged in radial columns in which terminal ramifications are tightly packed. Radioautographic labeling of the retinotectal terminals confirmed that they do not extend beyond layer 7. It demonstrated that the total thickness of the superficial tectum opticum tends to decrease from rostral to caudal, and that the density of retinotectal terminals is higher in the plexiform 3rd and 5th layers than in the 4th and 7th layers. Fink-Heimer staining of the terminal degeneration consecutive to an enucleation confirmed the results of radioautographic studies. It showed that the evolution of the silver impregnation differs from one layer to the next. This evolution is roughly parallel in the 3rd and 5th layers on the one hand, in the 4th and 7th layers on the other hand.The electron microscope study disclosed three successive stages of terminal degeneration over the whole projection area. They respectively present a dilatation of synaptic vesicles, a hyperplasia of the neurofibrillar system, and a darkening of the axoplasm. The three stages do not appear simultaneously in the 3rd and 5th layers and in the 4th and 7th layers. Comparison between the evolution of the terminal degeneration at the electron microscopic level and that of Fink-Heimer staining at the optical microscope level shows that the Fink-Heimer technique impregnates the earliest stage of degeneration (with vesicle dilatation), but not that with neurofibrillar hyperplasia.Following the resorption of retinotectal boutons, most of the postsynaptic differentiations disappear. However, some persist and become partially reoccupied by small terminals. The possible origin of these new synaptic relations is discussed.     

Aberrant axonal paths in regenerated goldfish retina and tectum opticum following intraocular injection of ouabain

Following ouabain-induced degeneration, the neural retina and the retinotectal axons regenerate. The pathways of regenerated retinal ganglion cell axons in retina and in tectum are visualized by labeling with horseradish peroxidase (HRP) applied to the optic nerve. In retina, the axons exhibit highly abnormal courses, including extensive fascicle crossing, hairpin loops and circular routes. In tectum, retinal axon fascicles are not neatly aligned in a normal fascicle fan. Instead, long and short fascicles are mixed, and take erratic routes, crossing each other and crossing the tectal equator.     

Blockade of arachidonic acid pathway induces sprouting in the adult but not in the neonatal uncrossed retinotectal projection

The uncrossed retinotectal projection of rats undergoes extensive axonal elimination and subsequent growth of axonal arbors in topographically appropriate territories within the first two/three postnatal weeks. Nitric oxide has been implicated in development and stabilization of synapses in the retinotectal pathway since blockade of nitric oxide synthesis disrupts the normal pattern of retinal innervation in subcortical nuclei. The present work investigated the role of arachidonic acid pathway in the development and maintenance of ipsilateral retinotectal axons. We also investigated the role of this retrograde messenger in the modulation of plasticity that follows retinal lesions in the opposite eye. Pigmented rats received systemic treatment with quinacrine, a phospholipase A2 inhibitor, indomethacin, a cyclooxygenase inhibitor, nordihydroguaiaretic acid, a 5-lipoxygenase inhibitor or vehicle during 4-8 days at various postnatal ages. Rats given a unilateral temporal retinal lesion were treated with either quinacrine or vehicle during the same period. For anterograde tracing of ipsilateral retinal projections, animals received intraocular injections of horseradish peroxidase. Before the third postnatal week no difference was observed in the laminar or topographic organization of the ipsilateral retinotectal projection between vehicle and treated rats in either normal or lesion conditions. After the third postnatal week, however, systemic blockade of phospholipase A2 or 5-lipoxygenase, but not cyclooxygenase induced sprouting of uncrossed axons throughout the collicular visual layers in unoperated rats. In retinal lesion groups, phospholipase A2 blockade increased the sprouting of uncrossed intact axons to the collicular surface in the same period. The results suggest that arachidonic acid or lipoxygenase metabolites play a role in the maintenance of the retinotectal synapses after the critical period and that the blockade of the arachidonic acid pathway induces reactive sprouting of retinal axons late in development.     

Fine structural studies of synaptogenesis in the superficial layers of the chick optic tectum

Summary Synaptogenesis in the superficial layers of the rostral pole of the optic tectum has been studied in the chick from embryonic day six (E6) to seven days post-hatching. Symmetrical membrane densities orpuncta adhaerentia are observed prior to the detection of synapses and throughout development. Immature synaptic contacts are observed by E7. These early synapses are primarily axodendritic; however, somatodendritic, dendrodendritic, axosomatic and axoglial synapses are also observed. The majority of these synapses have asymmetrical membrane densities and the presynaptic terminals contain clear, spherical, synaptic vesicles. Synaptic terminals containing pleomorphic vesicles and making symmetrical synaptic contacts are not commonly observed until the third week of embryonic development, and may represent the onset of inhibitory function within the tectum.Comparison of the number of synapses per unit area in control versus experimental tecta, after unilateral eye enucleations at E3, indicates that the presynaptic terminals of some synapses present at E8 are of retinal origin. It is suggested that the development of retinotectal synapses follows a rostrocaudal gradient in the tectum and corresponds to the intrinsic tectal pattern of cytoarchitectonic differentiation.     

Visual input induces long-term potentiation of developing retinotectal synapses

Early visual experience is essential in the refinement of developing neural connections. In vivo whole-cell recording from the tectum of Xenopus tadpoles showed that repetitive dimming-light stimulation applied to the contralateral eye resulted in persistent enhancement of glutamatergic inputs, but not GABAergic or glycinergic inputs, on tectal neurons. This enhancement can be attributed to potentiation of retinotectal synapses. It required spiking of postsynaptic tectal cells as well as activation of NMDA receptors, and effectively occluded long-term potentiation (LTP) of retinotectal synapses induced by direct electrical stimulation of retinal ganglion cells. Thus, LTP-like synaptic modification can be induced by natural visual inputs and may be part of the underlying mechanism for the activity-dependent refinement of developing connections.     

Synaptogenesis in the dorsal lateral geniculate nucleus of the rat

Summary Synapse formation and maturation were examined in the rat dorsal lateral geniculate nucleus (dLGN) from birth to adulthood. Examination of animals, whose ages were closely spaced in time, showed that the maturation of the synaptic organization of the nucleus takes place chiefly during the first 3 weeks of postnatal life. This period of maturation may be divided into 3 broad stages. During the first stage, which spans the first 4 days of life, there are only a few immature synapses scattered throughout the nucleus; occasionally aggregates of 3 or 4 synapses are encountered. Dendrodendritic synapses first appear at the end of this stage. The second stage, which lasts from the end of the first stage through day 8, is characterized by intensive synaptogenesis as well as extensive growth and degeneration. For the first time, large boutons resembling retinal terminals form multiple synaptic contacts with dendrites and dendritic protrusions; these synaptic arrangements are partially covered by glial processes.A feature characteristic of the developing dLGN during the first 2 postnatal weeks, and particularly during the second stage, is the presence of membrane specializations that resemble vacant postsynaptic densities. These specializations, which may be unapposed or opposite another neuronal process, decrease in frequency as the number of synapses increases. It is not known whether these densities are converted to synapses or whether they result from loss of presynaptic elements.The third stage in the process of synaptogenesis, which spans a period between days 10 and 20, is characterized by myelination and by the diminution of growth cones, degenerating profiles and vacant postsynaptic densities. There is also a very significant increase in the number and maturation of synapses including synaptic glomeruli. However, it is not until the end of this stage that synapses appear qualitatively indistinguishable from synaptic arrangements identified in adult animals.     

Contextual interaction of GABAergic circuitry with dynamic synapses

Visual context shapes human perception, yet our understanding of this phenomenon in terms of synaptic circuitry is still rudimentary. Our in vitro experiments with avian tectum reveal two distinct GABAergic pathways that mediate the spatiotemporal tectal interaction of retinal inputs. One pathway mediates postsynaptic lateral inhibition. The other pathway interacts with the synaptic depression of retinotectal synapses. Simulations of an experimentally constrained model including the two pathways reproduce the observed avian tectum wide-field neuron's sensitivity to small and moving stimuli, while being insensitive to whole-field motion.     

Ultrastructure of reorganising visual projections in half tecta of carp kept in constant light

In carp with the caudal half of the tectum cut off the projection of the whole contralateral visual field, mapped electrophysiologically, becomes compressed on to the remaining half tectum during the succeeding months. Ultrastructural study of tecta in various stages of reorganisation shows initially a profuse and widespread sprouting of unmyelinated axons in the optic nerve and external grey layers. This occurs before any compression of the projection is detectable electrophysiologically. After physiological compression is completed the bundles of unmyelinated fibres are greatly reduced and many myelinated axons are present. We conclude that compression initially involves the widespread growth of axon sprouts with the retention and myelination of a selected few when new connections are formed. Contrary to previous reports, exposure to constant light did not always delay or prevent the reorganisation of the visual projection. The widespread occurrence of optic terminal degeneration in both operated and normal tecta of fish kept in constant light suggests that light-induced retinal degeneration may have over-ridden the effect of constant light by providing vacant synaptic sites in the tectum for growing axonal sprouts.     

The primary optical projection in albino and pigmented rats]

1. We have studied by silver degeneration methods in transversal and horizontal brain series the pathways and the terminal nuclei of the primary optic system in albino and pigmented rats. 2. About 90% of the retinal axons are crossing in the Chiasma opticum. Differences between albino and pigmented rats are small. At the level of the lateral geniculate body (l.g.b.) the axons ramify. One branch is going in the optic tract or in the transversalsystem to the pretectum and tectum. The other branch goes as a collateral in the longitudinal system of the dorsal l.g.b. and ends here. 3. The optic fibers project--especially contralateral--to the following nuclei: veutral l.g.b. (lateral half), dorsal l.g.b., nucleus of the optic tract, Nc. olivaris praetectalis, Ncc. praetectales anterior et posterior and superior colliculus (laminae I-III). In the nuclei of the pretectal region are ipsilateral only a few degenerating fibers. 4. Especially in the dorsal l.g.b. but also in the superior colliculus the pigmented rats show ipsilateral an larger retinal input. 5. The ipsilateral fibers project to a special region in the dorsal l.g.b. This region is also innervated by contralateral axons. 6. In both the albino and pigmented rats the dorsal l.g.b. shows no lamination in the cytoarchitectonical picture and the ipsi- and contralateral input regions overlap. Therefore in the rat we can not speak from a laminated dorsal l.g.b. We have here--prepoperantly functional caused--a simple form of l.g.b. 7. The accessory optic system is only decussated. It includes the Fasciculus inferior tr. opt. accessorius, who projects only and direct to the median terminal nucleus and the Fasciculus superior tr. opticus accessorius. The last goes off from the optic tract at the level of the pretectal region and innervates all the 3 accessory optic nuclei. 8. In a few preparates we found references for a direct retino-hypothalamic connection (Nc. suprachiasmaticus and Nc. arcuatus hypothalami).     

成年斑马鱼视神经再生期间顶盖表面纤维层和灰质层的突触形态学研究

目的研究斑马鱼视神经再生过程中神经递质的变化,探讨神经递质和神经再生的关系。方法应用斑马鱼视神经再生模型,通过电镜技术观察顶盖表面纤维和灰质层突触的形态变化。结果视神经损伤后顶盖突触变化过程大致可分4个时期：1．视神经损伤后早期顶盖突触结构的退变;突触密度和突触小泡密度均下降,空型终末密度增大,8d时突触小泡密度降到最低水平;而空型终末密度最高。2．损伤后14d再生纤维大量进入顶盖;这一时期大核小泡和小核小泡密度都大量增加,但进入顶盖的纤维还没有或很少形成突触,14d时突触密度降到最低。3．损伤后2ld的特征是突触大量形成和圆形清亮小泡、扁平清亮小泡密度增加。4．再生晚期突触形态和功能恢复及精确化;100d时突触密度和突触小泡密度都大体恢复正常,但大核小泡密度还很高。结论兴奋性神经递质和抑制性神经递质可能对早期神经再生的启动和突触的形成起重要作用,而肽类和胺类可能对再生轴突的投射和精确化有重要作用;神经递质促进神经再生有先后顺序。     

A quantitative electron microscopic study of synaptic reorganization in the rat medial habenular nucleus after transection of the stria medullaris

Synaptic plasticity has been studied electron-microscopically in the rat medial habenular nucleus by counting the numbers of different types of synapses per unit area at various times after transection of the stria medullaris on one or both sides.Over 80% of synapses in the normal medial habenula had asymmetrical thickenings, and of these most were in contact with dendritic spines, the rest were in contact with dendritic shafts. The presynaptic terminals were complex structures frequently invaginated by slender spinules from the postsynaptic element. Each presynaptic terminal made contact with a large number of dendritic spines. The remainder of the synapses involved symmetrical contacts on dendritic shafts or neuronal somata.After transection of the ipsilateral stria medullaris the presynaptic elements of both symmetrical and asymmetrical synapses showed electron-dense degeneration. Degeneration was first seen at 24 hours after operation, reached a maximum (of 19%) at two days, and disappeared by 12 days.Non-degenerating synapses fell to around 30%, of their normal level at four days and finally recovered to around 80%, of normal by more than 100 days. Differential counts for synapses on dendritic spines, shafts and cell bodies revealed different time courses for reinnervation. Thus, synapse numbers on spines began to recover sooner than those on shafts and somata. but only the number of synapses on somata returned to almost normal (98%), whilst those on spines and shafts reached only about 70%, of normal.Membrane thickenings resembling vacant postsynaptic sites were rarely (0.3%,) found in unoperated animals. They appeared after transection of the ipsilateral stria, reached a peak of around 12%, at 4 6 days, (slightly later than the degeneration), and then fell, although not quite reaching their normal low levels even at more than 100 days.The nucleus as a whole shrank by about 17% of its normal volume. This would cause the synaptic density to increase by about 12% (calculated as³ √17²%), an effect which is not of sufficient magnitude to account for the observed recovery of synapse numbers, which involves a nearly three-fold increase —i.e. from 30% to 80% normal. The temporal coincidence between the recovery of non-degenerating synapses, and the disappearance of degenerating synapses and vacant sites favours the view that the denervated sites are reinnervated by the formation of new presynaptic terminals.Under normal circumstances the contralateral stria medullaris forms few synapses in the medial habenular nucleus (0.07%, degeneration at four days after transection). However, at long-term survivals after an ipsilateral striai lesion (when the degeneration from that lesion has been completely removed), a lesion of the contralateral stria causes appreciably more degeneration (around 2%), and a considerable fall (more than 30%,) in non-degenerating synapses. Despite the (unexplained) discrepancy between these figures, the findings still suggest that at least some of the new synapses induced after denervation by an ipsilateral strial lesion are formed by axons belonging to the contralateral stria.     

Single retinal changing contrast (third) detector elicits NMDA receptor response and higher activity level of frog tectum neuron network

The present study was designed to explore whether a discharge of a certain type of frog retinal ganglion cell [likely changing contrast (third) detector] can evoke NMDA response in frog tectum neurons and higher level of activity of tectal neuron network. Discharge of a single retinal ganglion cell was elicited by electrical stimulation of the retina. Evoked electrical activity of the tectum was recorded by the carbon-fiber microelectrode brought into the optic fiber layer F. We show that: (1) strong discharge of a frog individual retinal ganglion cell (third detector) has evoked NMDA response of tectal neurons and higher level of tectal neuron network activity characterized by prominent suprathreshold excitation of efferent neurons. Consequently, the firing of only one retinal ganglion cell (third detector) could lead to the activation of the tectobulbospinal tract and motor reaction. (2) The excitation of a retinotectal fiber of the first kind (axon of third detector) gave rise to the same effects as activation of a retinotectal fiber of the second kind (axon of fifth detector): the suprathreshold excitation of recurrent and efferent tectal neurons, the slow depolarizing potential (seen as the sNW), and the NMDA receptor activation were observed. However, stronger excitation (longer bursts of action potentials) was needed to evoke those effects in the considered case of the retinotectal input of the first kind. This difference could be attributed to the lower quantal size of neurotransmitter release in synapses of the retinotectal input of the first than second kind.