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.     

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.     

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.     

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.     

Visual stimuli-induced LTD of GABAergic synapses mediated by presynaptic NMDA receptors

Local GABA (gamma-aminobutyric acid) circuits contribute to sensory experience-dependent refinement of neuronal connections in the developing nervous system, but whether GABAergic synapses themselves can be rapidly modified by sensory stimuli is largely unknown. Here we report that repetitive light stimuli or theta burst stimulation (TBS) of the optic nerve in the developing Xenopus retinotectal system induces long-term potentiation (LTP) of glutamatergic inputs but long-term depression (LTD) of GABAergic inputs to the same tectal neuron. The LTD is due to a reduction in presynaptic GABA release and requires activation of presynaptic NMDA (N-methyl-D-aspartate) receptors (NMDARs) and coincident high-level GABAergic activity. Thus, the presynaptic NMDAR may function as a coincidence detector for adjacent glutamatergic and GABAergic activities, leading to coordinated synaptic modification by sensory experience.     

A proportional but slower NMDA potentiation follows AMPA potentiation in LTP

Most excitatory glutamatergic synapses contain both AMPA and NMDA receptors, but whether these receptors are regulated together or independently during synaptic plasticity has been controversial. Although long-term potentiation (LTP) is thought to selectively enhance AMPA currents and alter the NMDA-to-AMPA ratio, this ratio is well conserved across synapses onto the same neuron. This suggests that the NMDA-to-AMPA ratio is only transiently perturbed by LTP. To test this, we induced LTP at rat neocortical synapses and recorded mixed AMPA-NMDA currents. We observed rapid LTP of AMPA currents, as well as delayed potentiation of NMDA currents that required previous AMPA potentiation. The delayed potentiation of NMDA currents restored the original NMDA-to-AMPA ratio within 2 h of LTP induction. These data suggest that recruitment of AMPA receptors to synapses eventually induces a proportional increase in NMDA current. This may ensure that LTP does not alter the relative contributions of these two receptors to synaptic transmission and information processing.     

Roles of distinct glutamate receptors in induction of anti-Hebbian long-term potentiation

Many glutamatergic synapses on interneurons involved in feedback inhibition in the CA1 region of the hippocampus exhibit an unusual form of long-term potentiation (LTP) that is induced only if presynaptic glutamate release occurs when the postsynaptic membrane potential is relatively hyperpolarized. We have named this phenomenon 'anti-Hebbian' LTP because it is prevented by postsynaptic depolarization during afferent activity, and hence its induction requirements are opposite to those of Hebbian NMDA receptor-dependent LTP. This symposium report addresses the roles of distinct glutamate receptors in the induction of anti-Hebbian LTP. Inwardly rectifying Ca²⁺-permeable AMPA receptors mediate fast glutamatergic signalling at synapses that exhibit this form of LTP, and they are highly likely to mediate the instructive signal that triggers the cascade leading to synapse strengthening. NMDA receptors, on the other hand, play no role, nor do they contribute substantially to synaptic transmission at synapses that exhibit anti-Hebbian LTP. Both kainate and group I metabotropic glutamate receptors are abundant in at least some interneurons in the feedback inhibitory circuit. Delineating the roles of kainate receptors has been hampered by sub-optimal pharmacological tools. As for group I metabotropic glutamate receptors, their role in anti-Hebbian LTP is permissive at the very least in some interneuron types, although an instructive role has been suggested in other forms of activity-dependent plasticity.     

Sequential events of degeneration and synaptic remodelling in the viper optic tectum following retinal ablation. A degeneration, radioautographic and immunocytochemical study

The ultrastructural changes taking place in the retino-recipient layers of the viper optic tectum were examined between 5 and 122 days after retinal ablation. The initial degeneration of retinotectal terminals proceeds at widely different rates and is characterized by a marked degree of polymorphism in which a number of different patterns can be discerned. In the final stages of degeneration, either both the degenerating bouton and the distal portion of the postsynaptic element are engulfed by reactive glia, or, more frequently, only the degenerating terminal is eliminated and the postsynaptic differentiation remains. The free postsynaptic differentiations are reoccupied predominantly by boutons containing pleiomorphic vesicles and which are for the most part gamma-aminobutyric acid (GABA)ergic, thus forming heterologous synapses; less frequently these sites are occupied by boutons of the ipsilateral visual contingent to form homologous synapses. These two processes, both of which depend on terminal axonal sprouting, take place within the first 3 postoperative months. They are followed by a decrease in the number of heterologous synapses and a concurrent increase in the number of homologous synapses newly formed by optic boutons generated by collateral preterminal sprouting of ipsilateral retinotectal fibres. The data suggest that partial deafferentation of the optic tectum induces a transitory GABAergic innervation of free postsynaptic sites prior to the restoration of new retinal synaptic contacts.     

Voltage-controlled plasticity at GluR2-deficient synapses onto hippocampal interneurons

High-frequency stimulation of pyramidal cell inputs to developing (P9-12) hippocampal stratum radiatum interneurons expressing GluR2-lacking, Ca(2+)-permeable AMPA receptors produces long-term depression of synaptic transmission, if N-methyl-d-aspartate (NMDA) receptors are blocked. Here we show that these same synapses display a remarkably versatile signal integration if postsynaptic NMDA receptors are activated. At synapses expressing GluR2-deficient AMPA receptors, tetanic stimulation that activates NMDA receptors triggered long-term potentiation or depression (LTP or LTD) depending on membrane potential. LTP was elicited at most synapses when the interneuron was held at -30 mV during the stimulus train but was typically prevented by postsynaptic hyperpolarization to -70 mV, by strong depolarization to 0 mV, by d-2-amino-5-phosphonovaleric acid, or by postsynaptic injection of the Ca2+ chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. At synapses with predominantly GluR2-containing AMPA receptors, repetitive stimulation did not change synaptic strength regardless of whether NMDA receptors were activated. The interactions among GluR2 expression, NMDA receptor expression, and membrane potential thus confer on hippocampal interneurons a distinctive means for differential decoding of high-frequency inputs, resulting in enhanced or depressed transmission depending on the functional state of the interneuron.     

Enhanced visual activity in vivo forms nascent synapses in the developing retinotectal projection

Patterned neural activity during development is critical for proper wiring of sensory circuits. Previous work demonstrated that exposing freely swimming Xenopus tadpoles to 4 h of enhanced visual stimulation accelerates the dendritic growth rate of optic tectal neurons in vivo. Here we test whether this same period of visual stimulation increased synaptic maturation and formation of new synapses in the retinotectal pathway. We assessed synaptic properties of stage 48 tadpoles that were exposed to a simulated-motion stimulus for 4-5 h. Based on our findings that immature retinotectal synapses have greater paired-pulse facilitation compared with more mature synapses, consistent with a lower release probability (Pr), we used a paired-pulse protocol to elicit responses selectively from nascent synapses with low Pr. Although AMPA/NMDA ratios for single and paired stimuli were the same in control tadpoles, visual stimulation caused a relative decrease in the AMPA/NMDA ratio of the paired response. We evoked retinotectal synaptic transmission in the presence of Sr(2+) to record asynchronous vesicle release. We compared evoked mEPSCs induced by single and paired stimuli and found that visual stimulation selectively enhances the amplitude and number of AMPA receptor (AMPAR)-mediated mEPSCs evoked by paired stimuli relative to those evoked by single stimuli. Together these results show that enhanced visual stimulation affects both AMPAR- and NMDAR-mediated responses in a population of synapses revealed by paired-pulse stimulation. This suggests that in vivo visual stimulation increases synapses that have a low Pr and that have properties consistent with immature synapses.     

Long-term potentiation in direct perforant path projections to the hippocampal CA3 region in vivo

The perforant path constitutes the primary projection system relaying information from the neocortex to the hippocampal formation. Long-term synaptic potentiation (LTP) in the perforant path projections to the dentate gyrus is well characterized. However, surprisingly few studies have addressed the mechanisms underlying LTP induction in the direct perforant path projections to the hippocampus. Here we investigate the role of N-methyl-D-aspartate (NMDA) and opioid receptors in the induction of LTP in monosynaptic medial and lateral perforant path projections to the CA3 region in adult pentobarbital sodium-anesthetized rats. Similar to LTP observed at the medial perforant path-dentate gyrus synapse, medial perforant path-CA3 synapses display LTP that is blocked by both local and systemic administration of the competitive NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid [(+/-)-CPP]. By contrast, LTP induced at the lateral perforant path-CA3 synapses is not blocked by either local or systemic administration of this NMDA receptor antagonist. The induction of LTP at lateral perforant path-CA3 synapses, which is blocked by the opioid receptor antagonist naloxone, is also blocked by the selective mu opioid receptor antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7)-amide (CTOP), but not the selective delta opioid receptor antagonist naltrindole (NTI). CTOP was without effect on the induction of medial perforant path-CA3 LTP. The selective sensitivity of lateral perforant path-CA3 LTP to mu-opioid receptor antagonists corresponds with the distribution of mu-opioid receptors within the stratum lacunosum-moleculare of area CA3 where perforant path projections to CA3 terminate. These data indicate that both lateral and medial perforant path projections to the CA3 region display LTP, and that LTP induction in medial and lateral perforant path-CA3 synapses are differentially sensitive to NMDA receptor and mu-opioid receptor antagonists. This suggests a role for opioid, but not NMDA receptors in the induction of LTP at lateral perforant path projections to the hippocampal formation.     

NMDA receptor antagonists block heterosynaptic long-term depression (LTD) but not long-term potentiation (LTP) in the CA3 region following lateral perforant path stimulation

High-frequency stimulation of lateral perforant path is accompanied by a heterosynaptic long-term depression (LTD) of medial perforant path synaptic responses in both the dentate gyrus and the CA3 region of the hippocampus. We reported previously that LTP induction at lateral perforant path-CA3 synapses is unaffected by NMDA antagonists. However, it is not known if heterosynaptic LTD that is observed in the CA3 region following lateral perforant path stimulation also is independent from NMDA receptors. We address this question in anesthetized adult rats using systemic administration of the competitive NMDA receptor antagonist CPP. Induction of lateral perforant path-CA3 LTP produced a sustained heterosynaptic depression of medial perforant path-CA3 responses. Systemic administration of CPP (10 mg/kg) was ineffective in blocking the induction of LTP at lateral perforant path-CA3 responses. However, heterosynaptic LTD of medial perforant path-CA3 responses was blocked completely by CPP. These data indicate that NMDA receptors are not required for the induction of lateral perforant path-CA3 LTP, but are involved in the induction of heterosynaptic LTD that accompanies lateral perforant path activity. The requirement for NMDA receptors for heterosynaptic LTD suggests one functional role of NMDA receptors at termination fields of the lateral perforant path.     

Late phase of long-term potentiation induced by co-application of N-methyl-d-aspartic acid and the antagonist of NR2B-containing N-methyl-d-aspartic acid receptors in rat hippocampus

Activation of N-methyl-d-aspartic acid (NMDA) glutamate receptors (NMDARs) is required for long-term potentiation (LTP) of excitatory synaptic transmission at hippocampal CA1 synapses, the proposed cellular mechanisms of learning and memory. We demonstrate here that a brief bath co-application of a low concentration of NMDA, an agonist of NMDARs, and the selective antagonist of NR2B-containing NMDARs, (α R, β S)-α-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidinepropanol (Ro25-6981), to hippocampal slices from young adult rats produced a slowly developing LTP persisting at least for 6 h following a transient depression of synaptic transmission in CA1 synapses. The LTP was likely to occur at postsynaptic site and was initiated by activation of NMDARs, and its development was mediated by cAMP-dependent protein kinase (PKA) activation and protein synthesis. This chemically induced LTP and the tetanus-induced late phase of LTP (L-LTP) were mutually occluding, suggesting a common expression mechanism. Thus, we have demonstrated that a brief bath co-application of NMDA with Ro25-6981 to a slice offers an alternative to electrical stimulation as a stimulation method to induce L-LTP. The chemically induced LTP did not require the low-frequency test stimulation typically used to monitor the strength of synapses during and after drug application. Thus, the LTP may occur at a large fraction of synapses in the slice and not to be confined to a small fraction of the synapses where electrical stimulation can reach and induce LTP. Therefore, this chemically induced LTP may be useful for assessing the biochemical and morphological correlates and the molecular aspects of the expression mechanism for L-LTP that has been proven to correlate to hippocampal long-term memory.     

NMDA receptor trafficking at recurrent synapses stabilizes the state of the CA3 network

Metaplasticity describes the stabilization of synaptic strength such that strong synapses are likely to remain strong while weak synapses are likely to remain weak. A potential mechanism for metaplasticity is a correlated change in both N-methyl-D-aspartate (NMDA) receptor-mediated postsynaptic conductance and synaptic strength. Synchronous activation of CA3-CA3 synapses during spontaneous bursts of population activity caused long-term potentiation (LTP) of recurrent CA3-CA3 glutamatergic synapses under control conditions and depotentiation when NMDA receptors were partially blocked by competitive antagonists. LTP was associated with a significant increase in membrane-bound NMDA receptors, whereas depotentiation was associated with a significant decrease in membrane-bound NMDA receptors. During burst activity, further depotentiation could be induced by sequential reductions in antagonist concentration, consistent with a depotentiation-associated reduction in membrane-bound NMDA receptors. The decrease in number of membrane-bound NMDA receptors associated with depotentiation reduced the probability of subsequent potentiation of weakened synapses in the face of ongoing synchronous network activity. This molecular mechanism stabilizes synaptic strength, which in turn stabilizes the state of the CA3 neuronal network, reflected in the frequency of spontaneous population bursts.     

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。     

Presynaptic nicotinic potentiation of a frog retinotectal transmission evoked by discharge of a single retina ganglion cell

It was demonstrated in our previous studies of the frog retinotectal transmission that retinotectal synaptic potentials are enhanced by a factor of 1.5 due to the tonic presynaptic nicotinic potentiation, caused by the ambient level of the acetylcholine in the frog tectum. Furthermore, the results of those studies have indicated that the mechanism of the nicotinic potentiation is only partially exploited, because the application of the cholinergic agonist had increased the retinotectal transmission more than 2 times above the level of the tonic potentiation. The purpose of the present study was to explore this additional potentiation. We have shown that: (1) Bursts of 4-10 action potentials of a frog retina ganglion cell gave rise to an increase (phasic potentiation) of the retinotectal transmission 1.4-2.2 times, depending on the burst strength, that lasted tens of seconds. (2) This increase has been mediated through the presynaptic nicotinic acetylcholine receptors activated by the endogenous acetylcholine released into the tectum during relatively strong bursts of the retina ganglion cell. (3) Two types of the nicotinic acetylcholine receptors are co-localized in the presynaptic terminals of the individual retinotectal input to the tectum layer F--high-affinity (tonic) and low-affinity (phasic) nicotinic receptors.     

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.     

Presynaptic kainate receptors impart an associative property to hippocampal mossy fiber long-term potentiation

Hippocampal mossy fiber synapses show an unusual form of long-term potentiation (LTP) that is independent of NMDA receptor activation and is expressed presynaptically. Using receptor antagonists, as well as receptor knockout mice, we found that presynaptic kainate receptors facilitate the induction of mossy fiber long-term potentiation (LTP), although they are not required for this form of LTP. Most importantly, these receptors impart an associativity to mossy fiber LTP such that activity in neighboring mossy fiber synapses, or even associational/commissural synapses, influences the threshold for inducing mossy fiber LTP. Such a mechanism greatly increases the computational power of this form of plasticity.     

Caffeine-mediated presynaptic long-term potentiation in hippocampal CA1 pyramidal neurons

We report a new form of long-term potentiation (LTP) in Schaffer collateral (SC)-CA1 pyramidal neuron synapses that originates presynaptically and does not require N-methyl-d-aspartate (NMDA) receptor activation nor increases in postsynaptic-free Ca2+. Using rat hippocampal slices, application of a brief "pulse" of caffeine in the bath evoked a nondecremental LTP (CAFLTP) of SC excitatory postsynaptic currents. An increased probability of transmitter release paralleled the CAFLTP, suggesting that it originated presynaptically. The P1 adenosine receptor antagonist 8-cyclopentyltheophylline and the P2 purinoreceptor antagonists suramin and piridoxal-5'-phosphate-azophenyl 2',4'-disulphonate blocked the CAFLTP. Inhibition of Ca2+ release from caffeine/ryanodine stores by bath-applied ryanodine inhibited the CAFLTP, but ryanodine in the pipette solution was ineffective, suggesting a presynaptic effect of ryanodine. Previous induction of the "classical" LTP did not prevent the CAFLTP, suggesting that the LTP and the CAFLTP have different underlying cellular mechanisms. The CAFLTP is insensitive to the block of NMDA receptors by 2-amino-5-phosphonopentanoic acid and to Ca2+ chelation with intracellular 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, indicating that neither postsynaptic NMDA receptors nor increases in cytosolic-free Ca2+ participate in the CAFLTP. We conclude that the CAFLTP requires the interaction of caffeine with presynaptic P1, P2 purinoreceptors, and ryanodine receptors and is caused by an increased probability of glutamate release at SC terminals.