Long term changes in augmentation, potentiation, and depression of transmitter release as a function of repeated synaptic activity at the frog neuromuscular junction

Sensorimotor effects of plasticity-inducing percutaneous peripheral nerve stimulation protocols

BACKGROUND

Electrical stimulation of skin afferents can induce somatosensory plasticity in humans. Nevertheless, it is unknown if this is possible to do through percutaneous stimulation of a peripheral nerve, which will allow for regional anaesthesia interventions. Furthermore, potentiation protocols applied over mainly non-nociceptive fibers inhibit nociception in rodents, but this has not been tested in humans.

OBJECTIVE

to determine whether a protocol aiming to depress the nociceptive circuit and another aiming to potentiate non-nociceptive circuits produce regional hypoalgesia and changes in motor function, applied through percutaneous peripheral nerve stimulation (pPNS), and to assess which of them is more promising for pain relief, immediately and 24 hours after intervention.

METHODS

PT-cLF protocol aims to depress the nociceptive pathway through Pain Threshold, continuous Low Frequency stimulation and ST-bHF aims to produce potentiation of the non-nociceptive pathway, through Sensory Threshold burst stimulation at High Frequency. All subjects (n=29) went through both protocols and a control condition in a randomized and blinded crossover design.

RESULTS

Compared to control, ST-bHF induced distal hypoalgesia, towards electrical (p=0.04) and mechanical stimuli (p=0.02) and produced mechanical hypoesthesia (p=0.02). Contrarily, hypoalgesia was not observed after PT-cLF (p>0.05) but increased electrical motor threshold (p=0.04), reduced motor recruitment (p=0.03), and the subjects reported feeling reduced strength (p<0.01).

CONCLUSION

This works provides evidence that is possible to induce antinociceptive plasticity in a wide territory using pPNS. Moreover, it demonstrates for the first time in humans that a protocol aiming to produce long-term potentiation applied predominantly over non-nociceptive afferents induces hypoesthesia and hypoalgesia.

Molecular Mechanisms of Short-Term Plasticity: Role of Synapsin Phosphorylation in Augmentation and Potentiation of Spontaneous Glutamate Release

We used genetic and pharmacological approaches to identify the signaling pathways involved in augmentation and potentiation, two forms of activity dependent, short-term synaptic plasticity that enhance neurotransmitter release. Trains of presynaptic action potentials produced a robust increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs). Following the end of the stimulus, mEPSC frequency followed a bi-exponential decay back to basal levels. The time constants of decay identified these two exponential components as the decay of augmentation and potentiation, respectively. Augmentation increased mEPSC frequency by 9.3-fold, while potentiation increased mEPSC frequency by 2.4-fold. In synapsin triple-knockout (TKO) neurons, augmentation was reduced by 83% and potentiation was reduced by 74%, suggesting that synapsins are key signaling elements in both forms of plasticity. To examine the synapsin isoforms involved, we expressed individual synapsin isoforms in TKO neurons. While synapsin IIIa rescued both augmentation and potentiation, none of the other synapsin isoforms produced statistically significant amounts of rescue. To determine the involvement of protein kinases in these two forms of short-term plasticity, we examined the effects of inhibitors of protein kinases A (PKA) and C (PKC). While inhibition of PKC had little effect, PKA inhibition reduced augmentation by 76% and potentiation by 60%. Further, elevation of intracellular cAMP concentration, by either forskolin or IBMX, greatly increased mEPSC frequency and occluded the amount of augmentation and potentiation evoked by electrical stimulation. Finally, mutating a PKA phosphorylation site to non-phosphorylatable alanine largely abolished the ability of synapsin IIIa to rescue both augmentation and potentiation. Together, these results indicate that PKA activation is required for both augmentation and potentiation of spontaneous neurotransmitter release and that PKA-mediated phosphorylation of synapsin IIIa underlies both forms of presynaptic short-term plasticity.

A Well-Defined Readily Releasable Pool with Fixed Capacity for Storing Vesicles at Calyx of Held

The readily releasable pool (RRP) of vesicles is a core concept in studies of presynaptic function. However, operating principles lack consensus definition and the utility for quantitative analysis has been questioned. Here we confirm that RRPs at calyces of Held from 14 to 21 day old mice have a fixed capacity for storing vesicles that is not modulated by Ca²⁺. Discrepancies with previous studies are explained by a dynamic flow-through pool, established during heavy use, containing vesicles that are released with low probability despite being immediately releasable. Quantitative analysis ruled out a posteriori explanations for the vesicles with low release probability, such as Ca²⁺-channel inactivation, and established unexpected boundary conditions for remaining alternatives. Vesicles in the flow-through pool could be incompletely primed, in which case the full sequence of priming steps downstream of recruitment to the RRP would have an average unitary rate of at least 9/s during heavy use. Alternatively, vesicles with low and high release probability could be recruited to distinct types of release sites; in this case the timing of recruitment would be similar at the two types, and the downstream transition from recruited to fully primed would be much faster. In either case, further analysis showed that activity accelerates the upstream step where vesicles are initially recruited to the RRP. Overall, our results show that the RRP can be well defined in the mathematical sense, and support the concept that the defining mechanism is a stable group of autonomous release sites.

Short-term plasticity has a dramatic impact on the connection strength of almost every type of synapse during normal use. Some synapses enhance, some depress, and many enhance or depress depending on the recent history of use. A better understanding is needed for modeling information processing in biological circuits and for studying the molecular biology of neurotransmission. Here we show that first principles at the calyx of Held, such as whether or not a readily-releasable pool of vesicles in the presynaptic terminal has a fixed capacity for storing vesicles, are unexpectedly similar to synapse types that are used at much lower frequencies. Our study establishes new methods for studying the function of presynaptic molecules, and the results suggest that a tractable general model of short-term plasticity can capture the full computational power of dynamic synaptic modulation across a large range of synapse types and situations.

Translating neuronal activity at the synapse: presynaptic calcium sensors in short-term plasticity

The complex manner in which patterns of presynaptic neural activity are translated into short-term plasticity (STP) suggests the existence of multiple presynaptic calcium (Ca(2+)) sensors, which regulate the amplitude and time-course of STP and are the focus of this review. We describe two canonical Ca(2+)-binding protein domains (C2 domains and EF-hands) and define criteria that need to be met for a protein to qualify as a Ca(2+) sensor mediating STP. With these criteria in mind, we discuss various forms of STP and identify established and putative Ca(2+) sensors. We find that despite the multitude of proposed sensors, only three are well established in STP: Munc13, protein kinase C (PKC) and synaptotagmin-7. For putative sensors, we pinpoint open questions and potential pitfalls. Finally, we discuss how the molecular properties and modes of action of Ca(2+) sensors can explain their differential involvement in STP and shape net synaptic output.

Presynaptic α7 nicotinic acetylcholine receptors enhance hippocampal mossy fiber glutamatergic transmission via PKA activation

Nicotinic acetylcholine receptors (nAChRs) are expressed widely in the CNS, and mediate both synaptic and perisynaptic activities of endogenous cholinergic inputs and pharmacological actions of exogenous compounds (e.g., nicotine and choline). Behavioral studies indicate that nicotine improves such cognitive functions as learning and memory. However, the mechanism of nicotine's action on cognitive function remains elusive. We performed patch-clamp recordings from hippocampal CA3 pyramidal neurons to determine the effect of nicotine on mossy fiber glutamatergic synaptic transmission. We found that nicotine in combination with NS1738, an α7 nAChR-positive allosteric modulator, strongly potentiated the amplitude of evoked EPSCs (eEPSCs), and reduced the EPSC paired-pulse ratio. The action of nicotine and NS1738 was mimicked by PNU-282987 (an α7 nAChR agonist), and was absent in α7 nAChR knock-out mice. These data indicate that activation of α7 nAChRs was both necessary and sufficient to enhance the amplitude of eEPSCs. BAPTA applied postsynaptically failed to block the action of nicotine and NS1738, suggesting again a presynaptic action of the α7 nAChRs. We also observed α7 nAChR-mediated calcium rises at mossy fiber giant terminals, indicating the presence of functional α7 nAChRs at presynaptic terminals. Furthermore, the addition of PNU-282987 enhanced action potential-dependent calcium transient at these terminals. Last, the potentiating effect of PNU-282987 on eEPSCs was abolished by inhibition of protein kinase A (PKA). Our findings indicate that activation of α7 nAChRs at presynaptic sites, via a mechanism involving PKA, plays a critical role in enhancing synaptic efficiency of hippocampal mossy fiber transmission.

The number of components of enhancement contributing to short-term synaptic plasticity at the neuromuscular synapse during patterned nerve Stimulation progressively decreases as basal release probability is increased from low to normal levels by changing extracellular Ca2+

Presynaptic short-term plasticity (STP) dynamically modulates synaptic strength in a reversible manner on a timescale of milliseconds to minutes. For low basal vesicular release probability (prob0), four components of enhancement, F1 and F2 facilitation, augmentation (A), and potentiation (P), increase synaptic strength during repetitive nerve activity. For release rates that exceed the rate of replenishment of the readily releasable pool (RRP) of synaptic vesicles, depression of synaptic strength, observed as a rundown of postsynaptic potential amplitudes, can also develop. To understand the relationship between enhancement and depression at the frog (Rana pipiens) neuromuscular synapse, data obtained over a wide range of prob0 using patterned stimulation are analyzed with a hybrid model to reveal the components of STP. We find that F1, F2, A, P, and depletion of the RRP all contribute to STP during repetitive nerve activity at low prob0. As prob0 is increased by raising Ca(o)(2+) (extracellular Ca2+), specific components of enhancement no longer contribute, with first P, then A, and then F2 becoming undetectable, even though F1 continues to enhance release. For levels of prob0 that lead to appreciable depression, only F1 and depletion of the RRP contribute to STP during rundown, and for low stimulation rates, F2 can also contribute. These observations place prob0-dependent limitations on which components of enhancement contribute to STP and suggest some fundamental mechanistic differences among the components. The presented model can serve as a tool to readily characterize the components of STP over wide ranges of prob0.

Augmentation Controls the Fast Rebound From Depression at Excitatory Hippocampal Synapses

Short-term plasticity occurs at most central chemical synapses and includes both positive and negative components, but the principles governing interaction between components are largely unknown. The residual Ca2+ that persists in presynaptic terminals for several seconds after repetitive use is known to enhance neurotransmitter release under artificial, low probability of release conditions where depression is absent; this is termed augmentation. However, the full impact of augmentation under standard conditions at synapses where depression dominates is not known because of possibly complicated convolution with a variety of potential depression mechanisms. This report shows that residual Ca2+ continues to have a large enhancing impact on release at excitatory hippocampal synapses recovering from depression, including when only recently recruited vesicles are available for release. No evidence was found for gradual vesicle priming or for fast refilling of a highly releasable subdivision of the readily releasable pool (RRP). And decay of enhancement matched the clearance of residual Ca2+, thus matching the behavior of augmentation when studied in isolation. Because of incomplete RRP replenishment, synaptic strength was not typically increased above baseline when residual Ca2+ levels were highest. Instead residual Ca2+ caused single pulse release probability to rebound quickly from depression and then depress quickly during subsequent bursts of activity. Together, these observations can help resolve discrepancies in recent timing estimates of recovery from depression. Additionally, in contrast to results obtained under reduced release conditions, augmentation could be driven to a maximal level, occluding paired-pulse facilitation and other mechanisms that increase release efficiency.

Temperature-dependent shift of balance among the components of short-term plasticity in hippocampal synapses

Studies of short-term plasticity (STP) in the hippocampus, performed mostly at room temperature, have shown that small central synapses rapidly depress in response to high-frequency stimulation. This decrease in synaptic strength with synapse use places constraints on the use of STP as a dynamic filter for processing of natural high-frequency input. Here we report that, because of a strong but differential temperature dependence of STP components, the properties of STP in excitatory hippocampal synapses change dramatically with temperature. By separating the contributions of various STP processes during spike trains at different temperatures, we found a shift from dominating depression at 23 degrees C to prevailing facilitation and augmentation at 33-38 degrees C. This shift of balance among STP components resulted from a large increase in amplitudes of facilitation and augmentation (Q10 approximately 2.6 and approximately 5.1, respectively) and little change in the amplitude of depression (Q10 approximately 1.1) with temperature. These changes were accompanied by the accelerated decay of all three processes (Q10 = 3.2, 6.6, and 2.1, respectively). The balance of STP components achieved at higher temperatures greatly improved the maintenance of synaptic strength during prolonged synaptic use and had a strong effect on the processing of natural spike trains: a variable mixture of facilitated and depressed responses at 23 degrees C changed into a significantly more reproducible and depression-free filtering pattern at 33-38 degrees C. This filtering pattern was highly conserved among cells, slices, and animals, and under various physiological conditions, arguing for its physiological significance. Therefore, the fine balance among STP components, achieved only at near body temperatures, is required for the robust function of STP as a dynamic filter during natural stimulation.

Augmentation increases vesicular release probability in the presence of masking depression at the frog neuromuscular junction

Synaptic augmentation is a short-term component of synaptic plasticity that increases transmitter release during repetitive stimulation and decays thereafter with a time constant of approximately 7 sec. Augmentation has typically been observed under conditions where there is little or no depression because of depletion of synaptic vesicles from the readily releasable pool (RRP) of transmitter. We now study augmentation under conditions of pronounced depression at the frog neuromuscular junction to gain additional insight into mechanism. If augmentation reflects an increase in the size of the RRP of transmitter, then augmentation should not be present with depression. Our findings using four different experimental approaches suggested that augmentation was still present in the presence of pronounced depression: mathematical extraction of augmentation from the changes in transmitter release after repetitive stimulation, identification of augmentation with Ba2+, correction of the data for the measured depletion of the RRP, and identification of an augmentation component of residual Ca2+. We conclude that the augmentation machinery still acts to increase transmitter release when depression reduces the RRP sufficiently to mask obvious augmentation. The masked augmentation was found to increase transmitter release by increasing the probability of releasing individual vesicles from the depressed RRP, countering the effects of depression. Because augmentation and depression have similar time courses, either process can mask the other, depending on their relative magnitudes. Consequently, the apparent absence of one of the processes does not exclude that it is still contributing to short-term synaptic plasticity.

Safety margin at single neuromuscular junctions

Jitter measurement with axonal microstimulation was used to study synaptic function at 115 neuromuscular junctions (NMJs) of normal subjects at various stimulation rates. Jitter was lowest at 0.5 Hz; it increased slightly at 1, 2, and 5 Hz and remained at that level at 10 Hz (a light work load) and 20 Hz (a heavy work load); and it increased further at 50 Hz (an extreme load). This pattern was seen for the majority of the NMJs, suggesting a high safety factor of neuromuscular transmission maintained rather uniformly over a wide range of discharge rates. A proportion of the normal NMJs had relatively large jitter; these tended to show prominent facilitation as the rate was raised from 5 or 10 to 20 Hz. Similar but more dramatic facilitation improving the safety factor was seen at most NMJs in myasthenia, which was studied for comparison. Such facilitation was not found at normal NMJs with low jitter.

Sr2+ induce a release of divalent cation from internal Ca2+ store by nerve stimulation at frog neuromuscular junction

The effects of Sr2+ on the transmitter release at the frog neuromuscular junction were examined electrophysiologically. The nerve trunk was stimulated by paired pulses at various time intervals after replacing extracellular Ca2+ by Sr2+, the paired-pulse facilitation at the 10 ms interval was smaller than that at 20-30 ms intervals. Administration of several intracellular Ca2+ mobilizers decreased the paired-pulse facilitation. These results suggest that the transmitter release in Sr2+ solution is caused, at least partly, by the release of divalent cations from the intracellular stores.

Information processing with frequency-dependent synaptic connections

The efficacy of synaptic transmission between two neurons changes as a function of the history of previous activations of the synaptic connection. This history dependence can be characterized by examining the dependence of transmission on the frequency of stimulation. In this framework synaptic plasticity can also be examined in terms of changes in the frequency dependence of transmission and not merely in terms of synaptic strength which constitutes only a linear scaling mechanism. Recent work shows that the frequency dependence of transmission determines the content of information transmitted between neurons and that synaptic modifications can change the content of information transmitted. Multipatch-clamp recordings revealed that the frequency dependence of transmission is potentially unique for each synaptic connection made by a single axon and that the class of pre-postsynaptic neuron determines the class of frequency dependence (activity independent), while the unique activity relationship between any two neurons could determine the precise values of the parameters within a specific class (activity dependent). The content of information transmitted between neurons is also formalized to provide synaptic transfer functions which can be used to determine the role of the synaptic connection within a network of neurons. It is proposed that deriving synaptic transfer functions is crucial in order to understand the link between synaptic transmission and information processing within networks of neurons and to understand the link between synaptic plasticity and learning and memory.

Differential signaling via the same axon of neocortical pyramidal neurons

The nature of information stemming from a single neuron and conveyed simultaneously to several hundred target neurons is not known. Triple and quadruple neuron recordings revealed that each synaptic connection established by neocortical pyramidal neurons is potentially unique. Specifically, synaptic connections onto the same morphological class differed in the numbers and dendritic locations of synaptic contacts, their absolute synaptic strengths, as well as their rates of synaptic depression and recovery from depression. The same axon of a pyramidal neuron innervating another pyramidal neuron and an interneuron mediated frequency-dependent depression and facilitation, respectively, during high frequency discharges of presynaptic action potentials, suggesting that the different natures of the target neurons underlie qualitative differences in synaptic properties. Facilitating-type synaptic connections established by three pyramidal neurons of the same class onto a single interneuron, were all qualitatively similar with a combination of facilitation and depression mechanisms. The time courses of facilitation and depression, however, differed for these convergent connections, suggesting that different pre-postsynaptic interactions underlie quantitative differences in synaptic properties. Mathematical analysis of the transfer functions of frequency-dependent synapses revealed supra-linear, linear, and sub-linear signaling regimes in which mixtures of presynaptic rates, integrals of rates, and derivatives of rates are transferred to targets depending on the precise values of the synaptic parameters and the history of presynaptic action potential activity. Heterogeneity of synaptic transfer functions therefore allows multiple synaptic representations of the same presynaptic action potential train and suggests that these synaptic representations are regulated in a complex manner. It is therefore proposed that differential signaling is a key mechanism in neocortical information processing, which can be regulated by selective synaptic modifications.

Potential for multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses

Recent experimental evidence indicates that in the neocortex, the manner in which each synapse releases neurotransmitter in response to trains of presynaptic action potentials is potentially unique. These unique transmission characteristics arise because of a large heterogeneity in various synaptic properties that determine frequency dependence of transmission such as those governing the rates of synaptic depression and facilitation. A theoretical analysis was therefore undertaken to explore the phenomenologies of changes in the values of these synaptic parameters. The results illustrate how the change in any one of several synaptic parameters produces a distinctive effect on synaptic transmission and how these distinctive effects can point to the most likely biophysical mechanisms. These results could therefore be useful in studies of synaptic plasticity in order to obtain a full characterization of the phenomenologies of synaptic modifications and to isolate potential biophysical mechanisms. Based on this theoretical analysis and experimental data, it is proposed that there exists multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses. Finally, it is shown that the impact of changing the values of synaptic parameters depends on the values of the other parameters. This may indicate that the various mechanisms, phenomena and algorithms are interlinked in a 'synaptic plasticity code'.

Vesicle-associated proteins and calcium in nerve terminals of chick ciliary ganglia during development of facilitation

1. The developmental appearance of synaptic vesicle-associated proteins and nerve terminal calcium ([Ca2+]i) sequestering processes were determined for the chick ciliary ganglia in relation to the maturation of the different phase of increased efficacy of transmitter release following nerve impulses. The maturation phases studied were from stages 34-35, at the time of synapse formation, to stage 46 at hatching. 2. Western blots and immunohistochemical localization indicated that synaptotagmin 1 and synapsin IIa were detectable at stages 34-35 and were clearly localized at the nerve terminals by stage 37. Syntaxin was clearly localized at the nerve terminals at stage 34. 3. The relative size of the postganglionic compound action potential, used to measure the transmission efficacy through the ganglion, showed that the slope of the relationship between log efficacy and log extracellular calcium concentration ([Ca2+]o) in low [Ca2+]o was about 4 by stage 46. 4. A mature facilitatory mechanism for transmission was not present at stage 34 and did not emerge until stage 38. A mature augmentation was not present at stages 34 or 38 and was not established until stages 41-42. Post-tetanic potentiation (PTP) was not present at stage 34; it was evident at stages 37-38 and only reached maturity by stages 41-42. 5. The time course of calcium changes in the nerve terminals following trains of impulses that give rise to facilitation, augmentation and PTP was determined for different stages of development using the indicator Calcium Green-1 in the nerve terminal. The mature time course of the phases of calcium decline in the nerve terminal associated with facilitation and augmentation was observed as early as stage 38, whereas that of the PTP phase did not mature until after stage 42. 6. These results are discussed in terms of the maturation of the vesicle-associated proteins and calcium influx into the terminal following trains of impulses that give rise to the different components of increased synaptic efficacy.

Single fiber electromyography in studies of neuromuscular function

Single-fiber electromyography (SFEMG) allows precise study of the microphysiology of the human motor unit under normal conditions. The physiological parameters that can be quantified include impulse transmission along the intramuscular axon collaterals, pre- and post synaptic events at the neuromuscular junction, and muscle fiber membrane properties. This chapter illustrates some of the advantages of SFEMG in studies of neuromuscular fatigue in normal muscle, as well as in disorders of neuromuscular transmission, and conditions associated with disturbed muscle fiber depolarization-repolarization.

Changes in MEPP frequency during depression of evoked release at the frog neuromuscular junction

1. Endplate potentials (EPPs) and miniature endplate potentials (MEPPs) were recorded from frog neuromuscular junctions bathed in Ringer solutions containing normal (1.8 mM) or high (3.6 mM) Ca2+. The peptide toxin mu-conotoxin GIIIA was added to the Ringer solution to prevent muscle action potentials and contraction. 2. The nerve was stimulated with conditioning trains of 200-4800 impulses applied at 20 impulses s-1 to characterize the effects of repetitive stimulation on changes in EPP amplitude and MEPP frequency under high quantal conditions. 3. MEPP frequency was dramatically increased during and immediately following repetitive stimulation under high quantal conditions, whereas EPP amplitude was greatly depressed. There was no effect of repetitive stimulation on MEPP amplitude. 4. Following the conditioning stimulation the increase in MEPP frequency decayed back to the control level with a time course that could be described by four exponentials. The time constants of these exponentials were very similar to those that describe the components of stimulation-induced increases in EPP amplitude and MEPP frequency observed under low quantal conditions when depression is absent. 5. The results of this study indicate that depression and the components of stimulation-induced increases in release (facilitation, augmentation and potentiation) can be present at the same time, suggesting that the mechanism of depression involves different underlying factors from the mechanism(s) responsible for increases in release. They also indicate either that depression selectively affects only those quanta destined to be released in direct response to the nerve action potential, which would suggest that EPPs and MEPPs arise from different pools of transmitter, or that depression in some way affects a step in the release process involved only in evoked release, and not asynchronous (spontaneous) release.

Effect of glycerol treatment on transmitter release at the frog neuromuscular junction

Exposure of frog skeletal muscle to a Ringer solution made hyperosmotic with glycerol, followed by return of the preparation to a normal Ringer, results in an irreversible uncoupling of the mechanical activity of the muscle. This technique for preventing muscle contraction has often been used to study neuromuscular transmission under normal or high levels of release without interference from muscle contraction. Little was known, however, about the effects of glycerol treatment on the process of transmitter release. In this paper we report the results of a study in which we examined the effects of glycerol treatment on transmitter release during repetitive stimulation at the frog sartorius neuromuscular junction. We found that glycerol treatment altered the stimulation-induced changes in end-plate potential (EPP) amplitude normally observed during repetitive stimulation at this synapse. This effect, which increased progressively with time for up to 8 h following removal of glycerol, could be accounted for by presynaptic changes in the amount of transmitter released.

omega-Conotoxin reduces facilitation of transmitter release at the frog neuromuscular junction

We have examined the effects of the peptide toxin omega-conotoxin GVIA (omega-CgTx), a known calcium channel blocker, on stimulation-induced changes in end-plate potential (EPP) amplitude at the frog neuromuscular junction. We found that the addition of this toxin in submicromolar concentrations reduced both the control EPP amplitude and the increase in EPP amplitude that normally occurs during repetitive stimulation under low quantal conditions. These effects of omega-CgTx developed slowly following its addition to the bathing solution, were concentration-dependent and were essentially irreversible. The effects of omega-CgTx appeared to result from reductions in the facilitation and augmentation components of stimulation-induced increases in release. While the effects of omega-CgTx on EPP amplitude could be reversed by increasing the extracellular concentration of Ca2+, we were unable to reverse the effects of the toxin on stimulation-induced increases in EPP amplitude. Thus it appears that omega-CgTx has a dual effect on neuromuscular transmission, perhaps by acting at two different presynaptic sites.

Effects of aging on the dynamics of information processing and synaptic weight changes in the mammalian hippocampus

It is clear that the properties of LTE make it a plausible mechanism for associative information storage at some synapses in the central nervous system. While many of the factors that regulate LTE's induction and expression have been discovered and a strong case is being developed for its role in learning and memory processes, until we understand more clearly the mechanisms underlying both the expression and maintenance of LTE, an understanding of its change with age will be difficult. Judging by the progress that has been made over the past several years in uncovering some of the molecular events that are critical for LTE's expression, one may be optimistic that answers will be forthcoming reasonably soon. Of particular importance to aging mammals, such answers may provide insights into why older organisms show faster forgetting. This may have a profound impact on therapeutic strategies for memory disorders in both normal and pathological conditions of aging.

Activation of type B gamma-aminobutyric acid receptors in the intact mammalian spinal cord mimics the effects of reduced presynaptic Ca2+ influx

Intracellular recordings from mammalian spinal motoneurons in vivo show that the type B gamma-aminobutyric acid receptor agonist, L-(-)-baclofen, when administered systemically to pentobarbital-anesthetized or decerebrate unanesthetized cats decreases the amplitude of monosynaptic group Ia excitatory postsynaptic potentials (EPSPs), markedly increases tetanic and posttetanic potentiation, and reduces or abolishes synaptic depression during high-frequency synaptic activation and in the posttetanic period. These changes occur without detectable alteration in motoneuron input resistance, EPSP shape, or the invasion of action potentials into the intraspinal group Ia terminal arborizations. The baclofen-induced effects are qualitatively similar to those observed in more accessible synaptic systems when presynaptic Ca2+ influx and, concomitantly, transmitter release are reduced. Based on these and other recent findings regarding the mechanism of action of baclofen and the distribution of its receptors in the spinal cord, we suggest that L-(-)-baclofen modifies frequency modulation of Ia synaptic transmission by reducing presynaptic Ca2+ influx and the concomitant level of transmitter release from Ia afferent terminals. The drug appears to be a useful tool in studies of the ionic mechanisms that control the release of transmitter and its frequency modulation at inaccessible mammalian synapses.

Effects of 2-(4-phenylpiperidino)cyclohexanol (AH5183) and barium ions on frog neuromuscular transmission

1. By applying electrophysiological techniques such as frequency facilitation, tetanic run-down and depression, recovery from depression and post-tetanic potentiation (PTP) of the end-plate potential (EPP), the effects on frog neuromuscular transmission of 2-(4-phenylpiperidino)cyclohexanol (AH5183), a compound known to inhibit specifically the loading of newly synthesized acetylcholine (ACh) molecules into synaptic vesicles, and Ba2+, a selective activator of the augmentation phase of PTP, were investigated to elucidate whether these were related to ACh turnover. 2. Effects of AH5183 and Ba2+ ions were frequency dependent. At low frequency of stimulation, both agents showed essentially no effects on the EPPs recorded from Mg2+-blocked preparations. 3. AH5183 pivoted the log-linear frequency facilitation relation clockwise without altering the intercept on the ordinate, whereas Ba2+ ions did so counter-clockwise. As is the case with Ca2+ ions, Sr2+ ions shifted the relation upwards leaving its slope unaffected. 4. AH5183 selectively depressed the component of potentiation in PTP while the effect of Ba2+ ions was a specific increase in the augmentation phase of PTP. 5. Ba2+ ions increased the amplitude of EPPs in the late depressed phase during the tetanic run-down and depression experiment, but 4-aminopyridine and Ca2+ ions failed to do so. 6. AH5183 increased, Ba2+ ions reduced but Ca2+ ions did not change the constant of recovery from depression of the EPP measured on curarized preparations. 7. The present results suggested that mobilization of the ACh quanta readily available for release might be a common mechanism underlying both frequency facilitation and two components of PTP (augmentation and potentiation). The term 'frequency facilitation' would be more comprehensive if it were re-termed 'frequency augmentation' or 'frequency potentiation'.

Depression, recovery and facilitation of neuromuscular transmission during prolonged tetanic stimulation

The phrenic nerve of an unparalysed "cut" rat diaphragm preparation was stimulated with a rapid sequence of short tetanic trains. The amplitudes of both the first and the last (25th) endplate potentials produced by short tetanic trains progressively decreased with repeated application, but the latter diminished faster. Both relative depression and relative recovery (defined as fractional decrease and fractional recovery of endplate potential amplitudes that occur during and after each short train) became more pronounced with time. Relative facilitation (defined as fractional increase of endplate potential amplitudes observed at the beginning of each train) when initially present, persisted or increased slightly with duration of stimulation. Present results suggest that the "classic" depletion model for depression ought to be modified. They can be explained if it is assumed that during stimulation the capacity of the immediately available store to contain transmitter increases, and/or that newly formed transmitter preferentially replenishes the store of quanta immediately available for release. (An interesting consequence of the former model is that the immediately available store is replenished even when the small store behind is relatively more depleted.

Evoked release of acetylcholine from the growing embryonic neuron

An excised patch of embryonic muscle membrane was used as a probe for measuring the release of acetylcholine (AcCho) from growing spinal neurons in Xenopus cell culture. The neuron was stimulated extracellularly at the soma, and the evoked AcCho release was monitored at the growth cone, along the neurite, and near the soma. For a majority of the neurons studied, a brief suprathreshold stimulation of the soma triggered a pulse of AcCho release from the growth cone. This release showed many of the characteristics reminiscent of the transmitter release at the nerve terminal of a mature neuromuscular synapse: it occurs within a few ms following the stimulation, depends on extracellular Ca2+ concentration, and exhibits depression and potentiation during and after high-frequency stimulation, respectively. Similar evoked release was also observed only at selected points along the neurite, and prolonged suprathreshold stimulus was required to induce release from the soma. These results indicate that some of the growing spinal neurons have acquired a substantial number of AcCho molecules as well as an efficient mechanism for excitation-secretion coupling at the growth cone, ready for establishing functional contact with the target muscle cell. This notion was further supported by the finding that the evoked AcCho release is capable of inducing suprathreshold excitation of the muscle cell within the first minute following neurite-muscle contact.

Changes in post-tetanic potentiation of neuromuscular transmission in chronically stretched muscle

1. Changes in the parameters of the magnitude of PTP of m.e.p.p. frequency were examined in rat soleus muscles chronically stretched for a variety of durations (4-28 days) by leg-lengthening operations. 2. On day 4 after the operation, the magnitude of PTP in the stretched muscles obviously increased compared to the control muscles. This indicates that Ca2+-conductance in the nerve terminals increases in the stretched muscles at day 4. 3. On day 7 after the operation, there was no significant difference between the magnitudes of PTP in the control and stretched muscles. 4. After day 14 following the operation, the magnitude of PTP in the stretched muscles was smaller than that in the control muscles. This indicates that Ca2+-conductance in the nerve terminals decreases in the stretched muscles after day 14.

Post-tetanic potentiation of miniature end-plate potential frequency at neuromuscular junction of the rat soleus muscle

1. Changes in miniature end-plate potential (m.e.p.p.) frequency by repetitive nerve stimulation were examined in the rat soleus muscle. 2. The increase of m.e.p.p. frequency was induced by repetitive stimulation and persisted for several minutes after the tetanus. That is, post-tetanic potentiation (PTP) of neuromuscular transmission was first demonstrated here in the rat soleus muscle. 3. The time course of the decay of m.e.p.p. frequency after the tetanus showed a double exponential curve which consisted of a fast decaying component (augmentation) and a slow decaying component (potentiation). 4. The magnitude of PTP depended on the stimulation frequency and its duration. It increased with the increase of duration and was at its maximum at a frequency of 100 Hz. 5. No PTP was elicited by repetitive stimulation under conditions in which end-plate potential (e.p.p.) was completely suppressed, and, moreover, m.e.p.p. frequency tended to decrease after the tetanus.

Bicuculline-induced alterations of response properties in functionally identified ventroposterior thalamic neurones

Extracellular recordings of 105 neurones in the cat's somatosensory thalamus were obtained with carbon fibre-containing multibarrel micropipettes. The responses of cells to natural stimulation of cutaneous or deep structures were characterized and the responses to electrical stimulation of primary somatosensory cortex were determined. Receptive fields were mapped and the functional properties were examined before and during the microiontophoretic administration of glutamate, gamma-aminobutyric acid (GABA) and bicuculline methiodide (BMI). Modality and submodality properties of all cells tested apparently remained unchanged qualitatively, despite all pharmacological interventions. BMI lowered the response threshold of a majority of the 48 cells tested for this variable, although almost 25% responded with elevated thresholds. BMI changed the temporal properties of the responses of both thalamocortical relay neurones and of presumed interneurones. Discharges evoked by natural stimuli and by electrical stimulation of the cortex were prolonged and their pattern was altered. Decreases in the frequency of bursts of discharges were often observed with BMI, and these bursts were invariably prolonged and the interspike interval profiles were altered. Receptive field size changes were observed only in 8 of 48 neurones. For two of these, the field size decreased, while for the others there were small increases.

Electrophysiological control of ciliary motor responses in the ctenophore Pleurobrachia

Prey capture by a tentacle of the ctenophore Pleurobrachia elicits a reversal of beat direction and increase in beat frequency of comb plates in rows adjacent to the catching tentacle (Tamm and Moss 1985). These ciliary motor responses were elicited in intact animals by repetitive electrical stimulation of a tentacle or the midsubtentacular body surface with a suction electrode. An isolated split-comb row preparation allowed stable intracellular recording from comb plate cells during electrically stimulated motor responses of the comb plates, which were imaged by high-speed video microscopy. During normal beating in the absence of electrical stimulation, comb plate cells showed no changes in the resting membrane potential, which was typically about -60 mV. Trains of electrical impulses (5/s, 5 ms duration, at 5-15 V) delivered by an extracellular suction electrode elicited summing facilitating synaptic potentials which gave rise to graded regenerative responses. High K+ artificial seawater caused progressive depolarization of the polster cells which led to volleys of action potentials. Current injection (depolarizing or release from hyperpolarizing current) also elicited regenerative responses; the rate of rise and the peak amplitude were graded with intensity of stimulus current beyond a threshold value of about -40 mV. Increasing levels of subthreshold depolarization were correlated with increasing rates of beating in the normal direction. Action potentials were accompanied by laydown (upward curvature of nonbeating plates), reversed beating at high frequency, and intermediate beat patterns. TEA increased the summed depolarization elicited by pulse train stimulation, as well as the size and duration of the action potentials. TEA-enhanced single action potentials evoked a sudden arrest, laydown and brief bout of reversed beating. Dual electrode impalements showed that cells in the same comb plate ridge experienced similar but not identical electrical activity, even though all of their cilia beat synchronously. The large number of cells making up a comb plate, their highly asymmetric shape, and their complex innervation and electrical characteristics present interesting features of bioelectric control not found in other cilia.

Posttetanic potentiation in strong and weak neuromuscular junctions: physiological differences caused by a differential Ca2+-influx

We have sought physiological explanations for large differences in synaptic strength of different frog sartorius neuromuscular junctions, using the time course of posttetanic potentiation (PTP) of mEPP frequency as an indicator of the kinetics of Ca2+-metabolism in junctions of varying strengths. Results obtained in Ca2+ vs Ca2+-free/EGTA Ringers suggest that differences in the Ca2+-influx contribute to the physiological differences between strong and weak junctions.

Short-term potentiation phenomena in the rat limbic forebrain

Several types of short-term postactivation potentiation (PAP) effect were examined in limbic forebrain pathways in the chronic rat. We tested 9 different stimulation sites and 1-3 target sites for each stimulation site. All pathways showed PAP effects following activation by single electrical pulses or pulse trains. Using exponential curve fitting procedures, we found that the decay curves could be best fitted by one, or a sum of 2-3, exponential curves. On the basis of time constants, these curves fell into 4 different categories: facilitation (tau = 80 ms), augmentation (tau = 7s), potentiation 1 (tau = 70s), and potentiation 2(tau = 6.5 min). The latter component was the one most reliably generated in the chronic preparation. Frequency facilitation (facilitation during a stimulation train) was also examined and it appeared to be based upon a mechanism similar to that underlying paired pulse facilitation. Evidence is presented that facilitation and augmentation may be based on the the same mechanism. The possibility that the remaining components involve different mechanisms is discussed.

Neuromuscular transmission in crustaceans is enhanced by a sodium ionophore, monensin, and by prolonged stimulation

A sodium ionophore, monensin, was applied to crustacean neuromuscular preparations to determine whether increased intracellular sodium could lead to enhancement of transmitter release similar to that observed with prolonged stimulation. Following a single application of monensin (3-13 microM), the excitatory postsynaptic potential (e.p.s.p.) increased in amplitude by 50-800%. The increase was entirely due to a presynaptic effect that resulted in higher quantal content of transmission and increased frequency of spontaneous miniature potentials. A second application of monensin was less effective than the first. Application of monensin in calcium-free solutions led to rapid, marked enhancement of the e.p.s.p. upon restoration of the normal physiological solution, indicating that monensin can produce its effects in the absence of external calcium. Spontaneous miniature potentials often occurred more frequently in calcium-free solution after monensin had been applied. The extent of enhancement of e.p.s.p. amplitude depended on the concentration of external sodium, being smaller in solutions of low sodium. Prolonged stimulation of the motor axon usually enhanced the e.p.s.p. to a greater extent than application of monensin alone, but the time course of recovery of e.p.s.p. amplitude was similar in both cases. We conclude that the increase in e.p.s.p. amplitude promoted by monensin can be attributed to increased transmitter output resulting from influx of sodium into the nerve terminal. Increased intracellular sodium may lead to a rise in intracellular calcium ion concentration. Some features of long-term facilitation of transmitter release can be attributed to build-up of intracellular sodium during stimulation.

Long-term synaptic enhancement and short-term potentiation in rat fascia dentata act through different mechanisms

1. The component processes contributing to post-activation change in synaptic efficacy in the perforant pathway to the fascia dentata were studied in rats under sodium pentobarbitone anaesthesia.2. With low stimulus strength, which activated only a relatively small number of perforant path fibres, repetitive stimulation led to effects which had very similar characteristics to those observed at neuromuscular synapses under similar conditions. Paired shocks resulted in a short ( approximately 100 ms) facilitation superimposed on a depression, possibly due to depletion of available transmitter, which recovered more slowly ( approximately 4 s). Short trains of stimuli at 125-250 Hz led to a longer lasting increase in synaptic strength which decayed to control levels with a double exponential time course. The two exponential components behaved like augmentation and potentiation at neuromuscular synapses, with time constants at 33 degrees C of about 5 s and about 90 s respectively.3. High-intensity stimulus trains of identical frequency and duration led to an enhancement of synaptic strength which lasted for longer than 30 min.4. The paired shock depletion effect was increased in direct proportion to the amount of augmentation and potentiation present following low-intensity stimulus trains. Following high-intensity trains the paired shock depletion effect was increased by the same amount, and recovered with the same time course as following low-intensity stimulus trains, even though there remained a significant enhancement of the synaptic response.5. The results are interpreted as indicating that augmentation and potentiation are due to an increase in the probability of transmitter release whereas long-term enhancement acts through some other, as yet undetermined, mechanism. Following high-intensity stimulation all three processes are activated.

Changes in miniature endplate potential frequency during repetitive nerve stimulation in the presence of Ca2+, Ba2+, and Sr2+ at the frog neuromuscular junction

Miniature endplate potentials (MEPPs) were recorded from frog sartorious neuromuscular junctions under conditions of reduced quantal contents to study the effect of repetitive nerve stimulation on asynchronous (tonic) quantal transmitter release. MEPP frequency increased during repetitive stimulation and then decayed back to the control level after the conditioning trains. The decay of the increased MEPP frequency after 100-to 200-impulse conditioning trains can be described by four components that decayed exponentially with time constants of about 50 ms, 500 ms, 7 s, and 80 s. These time constants are similar to those for the decay of stimulation-induced changes in synchronous (phasic) transmitter release, as measured by endplate potential (EPP) amplitudes, corresponding, respectively, to the first and second components of facilitation, augmentation, and potentiation. The addition of small amounts of Ca2+ or Ba2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with Sr2+, led to a greater increase in the stimulation-induced increases in MEPP frequency. The Sr-induced increase in MEPP frequency was associated with an increase in the second component of facilitation of MEPP frequency; the Ba-induced increase with an increase in augmentation. These effects of Sr2+ and Ba2+ on stimulation-induced changes in MEPP frequency are similar to the effects of these ions on stimulation-induced changes in EPP amplitude. These ionic similarities and the similar kinetics of decay suggest that stimulation induced changes in MEPP frequency and EPP amplitude have some similar underlying mechanisms. Calculations are presented which show that a fourth power residual calcium model for stimulation-induced changes in transmitter release cannot readily account for the observation that stimulation-induced changes in MEPP frequency and EPP amplitude have similar time-courses.

The effects of calcium and magnesium ions, temperature and repetitive stimulation on inhibitory junctional transmission in smooth muscle of guinea-pig small intestine

1 The effects of calcium and magnesium ions and temperature on the peak amplitude of the nonadrenergic, noncholinergic inhibitory junction potential, evoked by a single stimulus, or paired transmural stimuli, were examined in the circular muscle of guinea-pig small intestine.2 The peak amplitude of the inhibitory junction potential (i.j.p.) could be decreased by lowering the external concentration of calcium or by raising the external magnesium concentration (at 25 degrees C).3 At 25 degrees C, the second of a pair of i.j.ps was larger than the first at short intervals (<0.2 s), but smaller at larger intervals (0.2 to 20 s). Enhancement of the second (test) response decayed exponentially with a time constant of 87 ms. Depression of the test i.j.p. was maximal at 0.4 s and then recovered exponentially with a time constant of 11 s.4 In low calcium or high magnesium solution, depression of the test i.j.p. decreased without any change in the rate of recovery from depression.5 Despite the slow rate of recovery from depression after a single conditioning response, transmitter output could be maintained during low-frequency repetitive stimulation.6 The peak amplitude of the i.j.p. increased as the temperature was raised to 35 degrees C (Q(10) = 1.5).7 In contrast to the neuromuscular junction, the depression of the second of a pair of i.j.ps decreased as the temperature was raised. At 35 degrees C the test i.j.p. was larger than the conditioning i.j.p. for most stimulus intervals (1 to 20 s).8 The results suggest that the rate of replenishment of the store of inhibitory transmitter is sensitive to both temperature and repetitive stimulation.

Stimulation-induced factors which affect augmentation and potentiation of trasmitter release at the neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog sartorius neuromuscular junctions under conditions of decreased transmitter release to study the effect of repetitive stimulation on augmentation and potentiation of transmitter release. 2. The magnitudes and time constants of decay of augmentation and potentiation were determined both following a primary conditioning train and following an identical secondary conditioning train applied from 30 to 170 sec after the primary conditioning train. 3. The magnitude of augmentation following the secondary conditioning trains was increased over that following the primary conditioning trains even though augmentation, with a time constant of decay of about 7 sec, had decayed to insignificant levels before the onset of the secondary trains. This increase in augmentation was not due to a change in its rate of decay during the secondary trains. 4. The increased magnitude of augmentation can be described as arising from an expression factor which, for conditioning trains of 200 impulses at 20/sec, has an initial magnitude of 1-6 +/- 1-2 (S.D. of observation) (the magnitude of augmentation is increased 2-6 times) and decays approximately exponentially with a time constant of 90 +/- 50 (S.D. of observation) sec. The expression factor thus decays about ten times slower than augmentation. 5. Doubling the number of impulses in the primary conditioning train from 100 to 200 led to a 2-8 +/- 1-0 (S.D. of observation) times increase in the magnitude of the expression factor, estimated by placing a 200 impulse secondary conditioning train 40 sec after the primary conditioning train. 6. The expression factor, while increasing the magnitude of augmentation, had little or no effect on the magnitude of potentiation or on trasmitter release in the absence of augmentation. The expression factor decayed about twice as slowly as potentiation. 7. The time constants characterizing the decay of potentiation were greater following the secondary conditioning trains than following the primary conditioning trains. 8. The increased time constant for the decay of potentiation can be described as arising from a time constant factor which, for conditioning trains of 200 impulses at 20/sec, has an initial magnitude of 1-2 +/- 0-7 (S.D. of observation) (the time constant of potentiation is increased 2-2 times) and decays approximately exponentially with a time constant of 130 +/- 45 (S.D. of observation) sec. The time constant factor decayed about three times slower than potentiation. 9. Doubling the number of impulses in the primary conditioning train from 100 to 200 led to a 1-6 +/- 0-8 (S.D. of observation) times increase in the magnitude of the time constant factor, estimated by placing a 200 impulse secondary conditioning train 40 sec after the primary conditioning train. 10...

Augmentation: A process that acts to increase transmitter release at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions bathed in Ringer solution containing increased Mg and decreased Ca to reduce transmitter release. Conditioning and testing stimulation was applied to the nerve to study a previously uncharacterized process which acts to increase e.p.p. amplitudes. We will refer to this process as augmentation. 2. Following repetitive stimulation augmentation decayed approximately exponentially over most of its time course with a mean time constant of about 7 sec (range 4-10 sec) which is intermediate in duration between the time constants for the decay of facilitation and potentiation. 3 . The magnitude of agumentation increased with the duration of the conditioning stimulation. Assuming a multiplicative relationship between augmentation and potentiation, values of the magnitude of augmentation ranged from 0-3 to 0-6 following 50 impulses at 20/sec to 0-5-7-8 following 600 impulses at 20/sec. (An augmentation of 0-3 and 7-8 would increase e.p.p. amplitudes 1-3 and 8-8 times, respectively.) 4. The time constant characterizing the decay of augmentation remained relatively constant as the duration of the conditioning stimulation was increased. 5. Augmentation as well as facilitation and potentiation resulted from an increase in the number of quanta of transmitter released from the nerve terminal. 6. Augmentation decayed faster at higher temperatures with a mean temperature coefficient, Q10, of about 3-8. The corresponding Q10 for the decay of potentiation was found to be about 2-4. 7. It is concluded that augmentation can be a significant factor in increasing transmitter release and will therefore have to be accounted for when studying the effects of repetitive stimulation on the function of the nerve terminal or when formulating models of transmitter release.