Postsynaptic control of hippocampal long-term potentiation

Long-term potentiation (LTP) in the hippocampus has the property of cooperativity, i.e. greater potentiation is produced if a larger number of afferent fibres is tetanized. The possible involvement of postsynaptic mechanisms in this process was investigated in the CA1 area of the hippocampal slice preparation. Following blockade of postsynaptic inhibition by GABA antagonists, e.g. picrotoxin, the induction of LTP was greatly facilitated. In picrotoxin-treated slices, LTP was induced in a pathway stimulated by single volleys, if these occurred in conjunction with brief tetanic activation of other afferents. This interaction operated over a short period of time (less than 50 ms) and was also present if the inputs were separated in space (cooperativity between inputs to basal and apical dendrites). LTP could be induced by pairing single volley synaptic activation and intracellularly injected depolarizing current pulses, the timing requirements being similar to those observed in the extracellular "conjunction studies". Previous studies have suggested that glutamate receptor channels of the N-methyl-D-aspartate (NMDA) type are somehow involved in LTP induction. Evidence presented here shows that activation leading to LTP evokes a potential which is sensitive to the NMDA receptor blocker 2-amino-5-phosphonovalerate (APV), indicating passage of current through NMDA receptor channels. The results suggest that hippocampal LTP depends on simultaneous presynaptic transmitter release and postsynaptic depolarization in a manner analogous to the model proposed by HEBB (1949) for associative learning. Furthermore, it is proposed that the required pre- and postsynaptic interaction is handled by the NMDA receptor channel complex, which is known to have the required voltage and transmitter sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Postsynaptic potentiation and desensitization in frog neuromuscular junction

The fractional increase in ACh responses that occurs at the beginning of each train of iontophoretically applied ACh pulses has been examined at the frog neuromuscular junction at room temperature, in the presence of active cholinesterase, during desensitization produced by a rapid sequence (every 20 s) of short (5 Hz, 5 s) iontophoretic trains of ACh. The fractional increase in ACh responses, which is used as an indicator of postsynaptic potentiation, becomes progressively greater with ACh application, often markedly (greater than 100%), although ACh responses are greatly reduced (as much as 90%) owing to desensitization. Clearly postsynaptic potentiation can exist concomitantly with desensitization. In addition, the dose-response curve is shifted to the right and its maximal response is diminished. The shift in the dose-response curve to the right, which can explain greater postsynaptic potentiation, is unlikely to be caused by accumulation of "monoligand-bound ACh receptor complexes," since experiments were done with active cholinesterase. The shift probably results from a greater number of desensitized receptors which, because of their large affinity for ACh molecules, serve as "high affinity traps." A small decrease of the maximal dose-response suggests only a small fractional decrease in the number of activable receptors, whereas a large shift to the right indicates a large fractional increase in the number of desensitized receptors. It appears that prior to ACh application only a small fraction of all receptors are desensitized. Alternatively, the shift to the right occurs because the cooperative action of ACh on receptors increases during desensitization.     

Postsynaptic neuroligin enhances presynaptic inputs at neuronal nicotinic synapses

Neuroligins are cell adhesion molecules that interact with neurexins on adjacent cells to promote glutamatergic and GABAergic synapse formation in culture. We show here that neuroligin enhances nicotinic synapses on neurons in culture, increasing synaptic input. When neuroligin is overexpressed in neurons, the extracellular domain induces presynaptic specializations in adjacent cholinergic neurons as visualized by SV2 puncta. The intracellular domain is required to translate the SV2 puncta into synaptic input as reflected by increases in the frequency of spontaneous mini-synaptic currents. The PDZ-binding motif of neuroligin is not needed for these effects. Together, the extracellular and proximal intracellular domains of neuroligin are sufficient to induce presynaptic specializations, align them over postsynaptic receptor clusters, and increase synaptic function. Manipulation of endogenous neuroligin with beta-neurexin-expressing cells confirms its presence; repressing function with dominant negative constructs and inhibitory shRNA shows that endogenous neuroligin helps confer functionality on existing nicotinic synaptic contacts. Endogenous neuroligin does not appear to be required, however, for initial formation of the contacts, suggesting that other components under these conditions can also initiate synapse formation. The results indicate that postsynaptic neuroligin is important for functional nicotinic synapses on neurons and that the effects achieved will likely depend on neuroligin levels.     

Postsynaptic mechanisms of induction of NMDA-dependent long-term post-tetanic potentiation of synaptic transmission

Experiments on hippocampus slices of rats showed that the pattern of induction of long-term post-tetanic potentiation of synaptic transmission is determined by the frequency of tetanic stimulation of Schaffer collaterals. With their high-frequency (>10/sec) stimulation, a phase of reversible increase in the amplitude of population EPSP (pEPSP) is observed within the initial 30-min-long interval; it is related to an increase in the intracellular Ca²⁺ concentration resulting from simultaneous activation of NMDA and metabotropic glutamate receptors and voltage-activated calcium channels. With the participation of calmodulin, Ca²⁺ activate Ca²⁺-calmodulin-dependent protein kinase II. The latter phosphorylates AMPA/kainate receptors (their kainate-responsive compartments), which promotes an increase in their chemosensitivity. Under conditions of low-frequency (<10/sec) tetanic stimulation of synaptic inputs, for the same reasons, an increase in the intradendritic Ca²⁺ concentration exerts no expressed influence on protein kinase II, but activates calcineurin. The latter, with the involvement of other phosphoprotein phosphatases, dephosphorylates AMPA/kainate receptors and turns some of them into the refractory state; this is expressed in a reversible depression of pEPSP. After 30 min of either high-frequency, or low-frequency stimulation, a non-decremental phase of long-term post-tetanic potentiation develops, which is related to the increase in the protein kinase C activity, phosphorylation of the AMPA-responsible compartments of AMPA/kainate receptors, their rising sensitivity, and a stable increase in the pEPSP amplitudeNeirofiziologiya/Neurophysiology, Vol. 28, No. 4/5, pp. 163–172, July–October, 1996.     

Fusion pore modulation as a presynaptic mechanism contributing to expression of long-term potentiation

Working on the idea that postsynaptic and presynaptic mechanisms of long-term potentiation (LTP) expression are not inherently mutually exclusive, we have looked for the existence and functionality of presynaptic mechanisms for augmenting transmitter release in hippocampal slices. Specifically, we asked if changes in glutamate release might contribute to the conversion of 'silent synapses' that show N-methyl-D-aspartate (NMDA) responses but no detectable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) responses, to ones that exhibit both. Here, we review experiments where NMDA receptor responses provided a bioassay of cleft glutamate concentration, using opposition between peak [glu](cleft )and a rapidly reversible antagonist, L-AP5. We discuss findings of a dramatic increase in peak [glu](cleft) upon expression of pairing-induced LTP (Choi). We present simulations with a quantitative model of glutamatergic synaptic transmission that includes modulation of the presynaptic fusion pore, realistic cleft geometry and a distributed array of postsynaptic receptors and glutamate transporters. The modelling supports the idea that changes in the dynamics of glutamate release can contribute to synaptic unsilencing. We review direct evidence from Renger et al., in accord with the modelling, that trading off the strength and duration of the glutamate transient can markedly alter AMPA receptor responses with little effect on NMDA receptor responses. An array of additional findings relevant to fusion pore modulation and its proposed contribution to LTP expression are considered.     

Recruitment of release sites underlies chemical presynaptic potentiation at hippocampal mossy fiber boutons

Synaptic plasticity is a cellular model for learning and memory. However, the expression mechanisms underlying presynaptic forms of plasticity are not well understood. Here, we investigate functional and structural correlates of presynaptic potentiation at large hippocampal mossy fiber boutons induced by the adenylyl cyclase activator forskolin. We performed 2-photon imaging of the genetically encoded glutamate sensor iGlu_u that revealed an increase in the surface area used for glutamate release at potentiated terminals. Time-gated stimulated emission depletion microscopy revealed no change in the coupling distance between P/Q-type calcium channels and release sites mapped by Munc13-1 cluster position. Finally, by high-pressure freezing and transmission electron microscopy analysis, we found a fast remodeling of synaptic ultrastructure at potentiated boutons: Synaptic vesicles dispersed in the terminal and accumulated at the active zones, while active zone density and synaptic complexity increased. We suggest that these rapid and early structural rearrangements might enable long-term increase in synaptic strength.

This study uses several high-resolution imaging techniques to investigate the structural correlates of presynaptic potentiation at hippocampal mossy fiber boutons, observing an increase in release sites and in release synchronicity accompanied by synaptic vesicle dispersion in the terminal and accumulation at release sites, but no modulation of the distance between calcium channel and release sites.     

Vesicular zinc promotes presynaptic and inhibits postsynaptic long-term potentiation of mossy fiber-CA3 synapse

The presence of zinc in glutamatergic synaptic vesicles of excitatory neurons of mammalian cerebral cortex suggests that zinc might regulate plasticity of synapses formed by these neurons. Long-term potentiation (LTP) is a form of synaptic plasticity that may underlie learning and memory. We tested the hypothesis that zinc within vesicles of mossy fibers (mf) contributes to mf-LTP, a classical form of presynaptic LTP. We synthesized an extracellular zinc chelator with selectivity and kinetic properties suitable for study of the large transient of zinc in the synaptic cleft induced by mf stimulation. We found that vesicular zinc is required for presynaptic mf-LTP. Unexpectedly, vesicular zinc also inhibits?a form of postsynaptic mf-LTP. Because the mf-CA3 synapse provides a major source of excitatory input to the hippocampus, regulating its efficacy by these dual actions, vesicular zinc is critical to proper function of hippocampal circuitry in health and disease.     

Using specificity to strategically target proteases

Proteases are a family of naturally occurring enzymes in the body whose dysregulation has been implicated in numerous diseases and cancers. Their ability to selectively and catalytically turnover substrate adds both signal amplification and functionality as parameters for the detection of disease. This review will focus on the development of activity-based methodologies to characterize proteases, and in particular, the use of positional scanning, synthetic combinatorial libraries (PS-SCL's), and substrate activity screening (SAS) assays. The use of these approaches to better understand a protease's natural substrate will be discussed as well as the technologies that emerged.     

Postsynaptic potentiation of end plate currents in the rat diaphragm at different levels of synaptic acetylcholinesterase activity

Nerve-evoked end-plate currents were recorded intracellularly in rat diaphragm when acetylcholinesterase (AChE) was intact or in the presence of galanthamine which reduces AChE activity up to 50% or 75%. Coefficient of correlation of the dependence between half-decay time and amplitude of end-plate currents during short tetanic stimulation pulse (20, 50 and 100 impulses per second) of motor nerve and the slope of this dependence were used to estimate the postsynaptic potentiation. Our results indicate that postsynaptic potentiation shows up clearly when AChE is fully active and grows significantly when AChE activity is slightly reduced. It is proposed that the modulation of the synaptic transmission can be performed by means of postsynaptic potentiation and synaptic AChE can take part in such way of modulation when varying its own activity under the influence of any endogenous factors.     

Borna disease virus blocks potentiation of presynaptic activity through inhibition of protein kinase C signaling

Infection by Borna disease virus (BDV) enables the study of the molecular mechanisms whereby a virus can persist in the central nervous system and lead to altered brain function in the absence of overt cytolysis and inflammation. This neurotropic virus infects a wide variety of vertebrates and causes behavioral diseases. The basis of BDV-induced behavioral impairment remains largely unknown. Here, we investigated whether BDV infection of neurons affected synaptic activity, by studying the rate of synaptic vesicle (SV) recycling, a good indicator of synaptic activity. Vesicular cycling was visualized in cultured hippocampal neurons synapses, using an assay based on the uptake of an antibody directed against the luminal domain of synaptotagmin I. BDV infection did not affect elementary presynaptic functioning, such as spontaneous or depolarization-induced vesicular cycling. In contrast, infection of neurons with BDV specifically blocked the enhancement of SV recycling that is observed in response to stimuli-induced synaptic potentiation, suggesting defects in long-term potentiation. Studies of signaling pathways involved in synaptic potentiation revealed that this blockade was due to a reduction of the phosphorylation by protein kinase C (PKC) of proteins that regulate SV recycling, such as myristoylated alanine-rich C kinase substrate (MARCKS) and Munc18-1/nSec1. Moreover, BDV interference with PKC-dependent phosphorylation was identified downstream of PKC activation. We also provide evidence suggesting that the BDV phosphoprotein interferes with PKC-dependent phosphorylation. Altogether, our results reveal a new mechanism by which a virus can cause synaptic dysfunction and contribute to neurobehavioral disorders.     

Postsynaptic potentiation and desensitization at the frog neuromuscular junction produced by repeated stimulation of the motor nerve

Investigations were performed on a split neuromuscular preparation of frog sartorial muscle during acetylcholinesterase inhibition. A study was made of the part played in postsynaptic potentiation (PSP) and desensitization (DS) in changes in the amplitude and time course of miniature endplate currents (MEPC) recorded directly after regular stimulation of the motor nerve at a frequency of 10 Hz for 5 or 60 sec, producing short and long series of multiquantal endplate currents (EPC) respectively. After the short train the amplitude of MEPC could hardly be distinguished from initial level, while the decay time constant (MEPC) increased by 32%, indicating PSP. Comparable but more pronounced biphasic changes occurred in the time course of endplate currents. These effects were not observed when acetylcholinesterase was uninhibited. Both PSP and DS were restored when 1×10^–6 M exogenous acetylcholine was added to the bath. The ratio between them could be changed by aprodifen — a substance which accelerates desensitization.S. V. Kurashov Medical Institute, Ministry of Public Health of the RSFSR, Kazan'. I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 645–654, September–October, 1986.     

Molecular mechanisms of target specificity during synapse formation

Neurons are connected with a high degree of specificity in neuronal circuits. Axon guidance mechanisms are responsible for directing axons to their approximate target region. It is not well understood how precise synaptic connections form between specific pre- and postsynaptic neurons within the target area. Recent analysis of a group of cell surface proteins in different systems has shed light on the diverse cellular and molecular mechanisms that generate the precise patterns of connectivity.     

Cytokines and cytokine networks target neurons to modulate long-term potentiation

Cytokines play crucial roles in the communication between brain cells including neurons and glia, as well as in the brain-periphery interactions. In the brain, cytokines modulate long-term potentiation (LTP), a cellular correlate of memory. Whether cytokines regulate LTP by direct effects on neurons or by indirect mechanisms mediated by non-neuronal cells is poorly understood. Elucidating neuron-specific effects of cytokines has been challenging because most brain cells express cytokine receptors. Moreover, cytokines commonly increase the expression of multiple cytokines in their target cells, thus increasing the complexity of brain cytokine networks even after single-cytokine challenges. Here, we review evidence on both direct and indirect-mediated modulation of LTP by cytokines. We also describe novel approaches based on neuron- and synaptosome-enriched systems to identify cytokines able to directly modulate LTP, by targeting neurons and synapses. These approaches can test multiple samples in parallel, thus allowing the study of multiple cytokines simultaneously. Hence, a cytokine networks perspective coupled with neuron-specific analysis may contribute to delineation of maps of the modulation of LTP by cytokines.     

Noradrenergic-mediated potentiation of acetylcholine release from the phrenic nerve: evidence for presynaptic alpha 1-adrenoceptor involvement

This study, conducted in the rat phrenic nerve-diaphragm preparation, was designed to establish more direct evidence that norepinephrine enhances acetylcholine (ACh) release from motor neurons and characterize the alpha-adrenoceptor type mediating this action. Norepinephrine (50 microM, alpha 1 + alpha 2 agonist) increased nerve-stimulated release by 183%, as determined by radioenzymatic assay. This effect was completely abolished by pretreatment with the alpha-adrenoceptor antagonists phentolamine (alpha 1 + alpha 2) and by WB 4101 (alpha 1) but only modestly reduced by yohimbine (alpha 2). Clonidine (alpha 2 agonist) did not enhance ACh release or nerve-stimulated muscle contractions, while phenylephrine (alpha 1 agonist) and norepinephrine increased muscle contractions up to 19.5-22.4%. These results support the hypothesis that norepinephrine increases ACh release from somatic motor nerves via a presynaptic alpha 1 interaction.     

Two distinct mechanisms target GAD67 to vesicular pathways and presynaptic clusters

The inhibitory neurotransmitter γ-amino butyric acid (GABA) is synthesized by two isoforms of the enzyme glutamic acid decarboxylase (GAD): GAD65 and GAD67. Whereas GAD67 is constitutively active and produces >90% of GABA in the central nervous system, GAD65 is transiently activated and augments GABA levels for rapid modulation of inhibitory neurotransmission. Hydrophobic lipid modifications of the GAD65 protein target it to Golgi membranes and synaptic vesicles in neuroendocrine cells. In contrast, the GAD67 protein remains hydrophilic but has been shown to acquire membrane association by heterodimerization with GAD65. Here, we identify a second mechanism that mediates robust membrane anchoring, axonal targeting, and presynaptic clustering of GAD67 but that is independent of GAD65. This mechanism is abolished by a leucine-103 to proline mutation that changes the conformation of the N-terminal domain but does not affect the GAD65-dependent membrane anchoring of GAD67. Thus two distinct mechanisms target the constitutively active GAD67 to presynaptic clusters to facilitate accumulation of GABA for rapid delivery into synapses.     

Postsynaptic potentiation of miniature endplate currents in rat diaphragm muscles. Effects of acetylcholinesterase inhibition,temperature, and curare

Miniature endplate potentials (MEPC) were recorded from rat diaphragm muscle fiber. A positive correlation was found in controls between half-decay time and amplitude of individual MEPC, an effect enhanced by acetylcholinesterase (AChE) inhibition (correlation coefficients: 0.29 and 0.49 respectively at a temperature of 28°C). Adding curare following AChE inhibition produced a reduction in the amplitude and duration of MEPC without influencing the correlation relationship between the above-mentioned parameters. This relationship declined significantly with a temperature reduction to 18°C in both the control and cases of AChE inhibition. The increase in MEPC half-decay time following AChE inhibition was greater at 28° than at 18°C; Q₁₀ equalled about two for duration of rising time as compared with around three for MEPC half-decay time. Factors determining the time course of MEPC are discussed. The findings obtained are explained by postsynaptic potential (and cooperative binding of agonists to cholinoreceptors lies at the root of this) and by the pattern of ACh diffusion at the synaptic cleft.A. A. Ukhtomskii Institute of Physiology, A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 19, No. 4, pp. 504–512, July–August, 1987.     

Long-term potentiation in hippocampal oriens interneurons: postsynaptic induction,presynaptic expression and evaluation of candidate retrograde factors

Several types of hippocampal interneurons exhibit a form of long-term potentiation (LTP) that depends on Ca²⁺-permeable AMPA receptors and group I metabotropic glutamate receptors. Several sources of evidence point to a presynaptic locus of LTP maintenance. The retrograde factor that triggers the expression of LTP remains unidentified. Here, we show that trains of action potentials in putative oriens-lacunosum-moleculare interneurons of the mouse CA1 region can induce long-lasting potentiation of stimulus-evoked excitatory postsynaptic currents that mimics LTP elicited by high-frequency afferent stimulation. We further report that blockers of nitric oxide production or TRPV1 receptors failed to prevent LTP induction. The present results add to the evidence that retrograde signalling underlies N-methyl-d-aspartate (NMDA) receptor-independent LTP in oriens interneurons, mediated by an unidentified factor.