Twin pulse facilitation in dependence on pulse duration and calcium concentration at motor nerve terminals of crayfish and frogs

Methods of measuring activity at individual synapses: a review of techniques and the findings they have made possible

Neurons in the brain are often linked by single synaptic contacts (Gulyás et al., 1993) and the probabilistic character of synaptic activity makes it desirable to increase the resolution of physiological experiments by observing the function of the smallest possible number of synaptic terminals, ideally, one. Because they are critically important and technically difficult to resolve, several of the core questions investigated in singe-site experiments have been under study for decades (Auger and Marty, 2000). Many approaches have been taken toward the goal of measuring activity at few synapses, and consideration of the capabilities and limitations of each of these methods permits a review of the contributions each has made possible to present understanding of synaptic function. A number of methodological advances in recent years have increased resolving power. New techniques often build on previous developments and many effective approaches combine components of existing specialized methods with new technology. One theme that emerges is that synaptic properties vary among regions, reducing the utility of general questions such as whether synaptic glutamate saturates receptors or how rapidly synaptic vesicle pools are depleted. For several core questions, multiple studies using different methods have reached similar conclusions, suggesting that consensus may be emerging for some anatomic synapses.

Historical view and physiology demonstration at the NMJ of the crayfish opener muscle

Here we present some of the key important discoveries made with the opener neuromuscular (NMJ) preparation of crustaceans and illustrate that there is still much to learn from this model preparation. In understanding the history one can appreciate why even today this NMJ still offers a rich playground to address questions regarding pre- and post-synaptic function and plasticity. The viability and ease of access to the terminal for intracellular as well as extracellular electrophysiology and imaging are significant advantages. The mechanisms behind the modulation of vesicular kinetics and fusion within the high- and low-output terminals are begging for investigation. The preparation also offers a testable model system for computational assessments and manipulations to examine key variables in theoretical models of synaptic function, for example calcium dynamics during short-term facilitation. The synaptic complexity of active zone and statistical nature of quantal release is also an open area for future investigation both experimentally and computationally.

Short-term synaptic plasticity in layer II/III of the rat anterior cingulate cortex

Recent in vivo electrophysiological studies in our laboratory demonstrated medial thalamus (MT) induced short-term facilitation in the middle layers of the anterior cingulate cortex (ACC). The aim of the present study was to investigate different forms of short-term plasticity (STP) in layer II/III of the ACC in an in vitro slice preparation. Extracellular field potentials in layer II/III consisting of an early component (fAP) and a late component (fPSP) were activated by electrical stimulation of the deep layers. The fPSP and intracellularly recorded excitatory post-synaptic potential (EPSP) could be facilitated by paired-pulse stimulation at a low frequency (0.033Hz, pulse interval 20-400ms). An initial facilitation and subsequent depression were obtained when high frequency (12.5, 25 and 50Hz) tetanus stimulations were applied to the ACC slice. A post-tetanic augmentation 30s in duration was also observed. The effects of tetanic stimulation were altered in the presence of an increased or a decreased calcium concentration. Application of omega-conotoxin GVIA (CTX) in normal calcium concentration conditions decreased overall responses during tetanic stimulation similar to reducing calcium exposure. However CTX application did not increase paired-pulse facilitation (PPF) as is seen under low calcium conditions. These results indicate that calcium is involved in the formation of certain features of STP in layer II/III of the ACC and that N-type calcium channels contribute to some, but not all, components of these plastic changes. Two-site electrical stimulation testing showed that two separate presynaptic inputs can produce short-term facilitation. Our findings implicate a post-synaptic mechanism in STP in layer II/III of the ACC.

5-HT offsets homeostasis of synaptic transmission during short-term facilitation

In this study, we approach the topic of vesicle recruitment and recycling by perturbing neurotransmission at the crayfish neuromuscular junction with altered electrical activity and the presence of the neuromodulator serotonin (5-HT). After induction of short-term facilitation (STF) with stimulus pulse trains (40 Hz, 20 pulses), the amount of synaptic transmission can be maintained at a relatively constant level, producing a plateau in the amplitude of the excitatory postsynaptic potentials (EPSPs) throughout the remaining stimuli within a train of a few hundred milliseconds. With an increase in the frequency of the stimuli within a train (60 Hz, 20 pulses), an altered plateau of larger EPSP amplitudes occurs. This suggests that differential rates of vesicle recruitment can be rapidly reached and maintained. Exposure of nerve terminals to 5-HT further enhances the EPSP amplitudes to yet a higher plateau level. The effect of 5-HT is more pronounced for 40-Hz pulse trains than for 60-Hz trains. This suggests that 5-HT can recruit vesicles into the readily releasable pool (RRP) and that the recruitment is limited at higher stimulation frequencies. The attainment of a larger amplitude in the plateaus of the EPSPs at 60 Hz compared with 40 Hz also suggests that the rapid induction of STF enhances the entry of vesicles into the RRP. By direct quantal counts, mean quantal content increases linearly during STF, and 5-HT offsets the linear release. We propose that 5-HT and electrically induced recruitment of vesicles from a reserve pool to the RRP may share similar recruitment mechanisms.

Myofibrillar protein isoform expression is correlated with synaptic efficacy in slow fibres of the claw and leg opener muscles of crayfish and lobster

In the crayfish and lobster opener neuromuscular preparations of the walking legs and claws, there are regional differences in synaptic transmission even though the entire muscle is innervated by a single excitatory tonic motor neuron. The innervation of the proximal fibres produced larger excitatory postsynaptic potentials (EPSPs) than those of the central fibres. The amplitudes of the EPSPs in the distal fibres were intermediate between those of the proximal and central regions. These differences in EPSP amplitudes were correlated with differences in short-term facilitation between the three regions. When given a 10- or 20-pulse train of stimuli, the proximal fibres showed greater short-term facilitation initially, often followed by a maximization of short-term facilitation towards the end of a train. In contrast, the central fibres showed a linear increase in short-term facilitation throughout a stimulus train. The distal fibres showed intermediate short-term facilitation compared with the other two regions. Analysis of myofibrillar isoforms showed that levels of troponin-T1 (TnT1), a 55 kDa isoform expressed in slow-tonic (S2) fibres, were correlated with synaptic properties. Proximal fibres had the highest levels of TnT1, with lower levels in distal fibres; central fibres lacked TnT1, which is characteristic of slow-twitch (S1) fibres. In addition, differences in troponin-I isoforms correlated with TnT1 levels between the proximal, central and distal regions. The correlation between slow fibre phenotype and strength of innervation suggests a relationship between synaptic structure and expression of troponin isoforms.

Influence of serotonin on the kinetics of vesicular release

The mechanisms by which synaptic vesicles are transported and primed to fuse with the presynaptic membrane are important to all chemical synapses. Processes of signal transduction that affect vesicular dynamics, such as the second-messenger cascades induced by neuromodulators, are more readily addressed in assessable synaptic preparations of neuromuscular junctions in the crayfish. We assessed the effects of serotonin (5-HT) through the analysis of the latency jitter and the quantal parameters: n and p in the opener muscle of the walking leg in crayfish. There is an increase in the size of the postsynaptic currents due to more vesicles being released. Quantal analysis reveals a presynaptic mechanism by an increase in the number of vesicles being released. Latency measures show more events occur with a short latency in the presence of 5-HT. No effect on the frequency or size of spontaneous release was detected. Thus, the influence of 5-HT is presynaptic, leading to a release of more vesicles at a faster rate.

Differential facilitation of high- and low-output nerve terminals from a single motoneuron

In the crayfish opener neuromuscular preparation, regional differences in synaptic transmission are observed among the terminals of a single motoneuron. With a single stimulus, the high-output terminals of the proximal region of the muscle produce a larger excitatory postsynaptic potential than do the low-output terminals of the central region of the muscle. We tested the hypothesis that the low-output terminals exhibit more facilitation than do high-output terminals for twin-pulse, train, and continuous-stimulation paradigms. Previous studies have not employed several stimulation paradigms to induce facilitation among high- and low-output terminals of a single motoneuron. We found that the high-output terminals on the proximal fibers facilitate more than the low-output terminals on the central muscle fibers, in contrast with previous studies on similar muscles. The difference in measured facilitation is dependent on the stimulation paradigm. These results are important because ultrastructural differences between these high- and low-output terminals are known and can be used for correlation with physiological measurements. Short-term facilitation is a form of short-term memory at the synaptic level, and the processes understood at the crayfish neuromuscular junction may well be applicable to all chemical synapses.

Study of the inhibitor of the crayfish neuromuscular junction by presynaptic voltage control

The inhibitor of the crayfish opener muscle was investigated by a presynaptic voltage control method. Two microelectrodes were inserted into the inhibitor and the amplitude and duration of presynaptic depolarization were controlled by a voltage-clamp amplifier. The inhibitory postsynaptic potential (IPSP) was measured from a muscle fiber located near the presynaptic voltage electrode. Nonlinear summation of IPSP amplitudes was corrected after chloride equilibrium potential was measured. With the use of 5-ms presynaptic pulses, the depolarization-release coupling (D-R) curve constructed from IPSP peak amplitudes (IPSPcor) had a threshold of about -35 mV and reached its maximal level at -5 to -10 mV. Depolarization beyond the maximum led to a suppression of neurotransmitter release. When transmitter release during a presynaptic pulse was completely suppressed, IPSPs activated by tail current could be identified with an average synaptic delay of 2.5 ms. Transmitter secretion triggered by a calcium current activated during the 5-ms pulses (IPSPon) was also measured on the rising phase of an IPSP, at 2.5 ms after the end of the 5-ms pulses. D-R coupling plots measured from IPSPon exhibited a more pronounced suppression than that obtained from IPSPcor. The effect of presynaptic pulse duration on the level of transmitter release was analyzed. Transmitter release increased with increasing duration and was nearly saturated by 20-ms pulses depolarized to 0 mV. The following conditions were identified as necessary to obtain a consistent D-R curve with a clear suppression: 1) small animals, 3.8 cm head to tail, 2) 15 degrees C, 3) 40 mM tetraethylammonium and 1 mM 4-aminopyridine, 4) an extracellular calcium concentration of < or = 10 mM. In addition, a consistent correlation was found among the branching pattern of the inhibitor, the placement of the presynaptic electrode, and the characteristics of the D-R curves. An ideal presynaptic electrode configuration involved placing the voltage electrode in a secondary branch, approximately 100 microns from the main branch point, and placing the current electrode at the branch point. Postsynaptically, optimal recordings were obtained from muscle fibers innervated by a single branch of the inhibitor that originated from a point near the presynaptic voltage electrode. A cable-release model was constructed to evaluate the relationship between the shape of the D-R coupling curves and the space constants of the presynaptic terminals. A comparison between the model and the D-R coupling curves suggested that the space constant of an inhibitor branch on a muscle fiber is > or = 8 times longer than its actual length. Therefore the upper limit estimate of the space constant of a typical preparation is approximately 3 mm. Results reported here outline morphological and physiological conditions needed to achieve optimal control of the presynaptic branch of the crayfish inhibitor. The cable-release model quantitatively defines the extent of presynaptic voltage control.

Factors influencing the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction

1. The effects of several conditions and agents on the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction in low-Ca2+ high-Mg2+ bathing solutions were examined. 2. Twin-pulses gave two endplate potential (epps) with first (m2) and second (m2) quantal contents. The ratio of m2/m1 was taken as a measure of the degree of facilitation. 3. The mean value of this ratio was > 1. Individual ratios fluctuated widely at junctions with smaller values of m1 but were focused around 1 at junctions with larger values of m1. Thus, some populations of junctions with smaller values of m1 contributed to an increment in the mean ratio. 4. The mean ratio was virtually constant irrespective of changes in the spontaneous and evoked release of transmitter at temperatures between 20 and 36 degrees C and at external concentrations of Ca2+ from 0.4 to 0.8 mM. 5. 4-Aminopyridine(4-AP) slightly but significantly increased this ratio with increases in m1 and m2 at temperatures of 24 and 36 degrees C. Ouabain slightly but significantly reduced the ratio, with increases in m1 and m2. The steadiness of the ratio was maintained in the presence of caffeine, high K+, neomycin or omega-conotoxin irrespective of changes in m1 and m2, except in the case of omega-conotoxin. 6. Spontaneous output at 36 degrees C increased in the presence of 4-aminopyridine, ouabain, caffeine, high K+ or neomycin. 7. These results indicate that maintenance of a stable value of the ratio of m2 to m1 is a dominant feature.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitter release: Facilitation and three-dimensional diffusion of intracellular calcium

In order to account for the time courses of both evoked release and facilitation, in the framework of the Ca2+ hypothesis, Fogelson and Zucker (1985, Biophys. J. 48, 1003-1017) suggested treating diffusion of Ca2+, once it enters through the Ca2+ channels, as a three-dimensional process (three-dimensional diffusion model). This model is examined here as a refined version of the "Ca(2+)-theory" for neurotransmitter release. The three-dimensional model was suggested to account for both the time course of release and that of facilitation. As such, it has been examined here as to its ability to predict the dependence of the amplitude and time course of facilitation under various experimental conditions. It is demonstrated that the three-dimensional diffusion model predicts the time course of facilitation to be insensitive to temperature. It also predicts the amplitude and time course of facilitation to be independent of extracellular Ca2+ concentration. Moreover, it predicts that inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange does not alter facilitation. These predictions are not upheld by the experimental results. Facilitation is prolonged upon reduction in temperature. The amplitude of facilitation declines and its duration is prolonged upon increase in extracellular Ca2+ concentration. Finally, inhibition of the [Na+]o in equilibrium with [Ca2+]i exchange prolongs facilitation but does not alter the time course of evoked release after an impulse.

Mechanisms of presynaptic inhibition studied using paired-pulse facilitation

An investigation was made of the effect of presynaptic inhibition on paired-pulse facilitation (PPF) of group Ia afferent excitatory postsynaptic potentials (EPSPs). The main finding from this study was that PPF was enhanced during presynaptic inhibition of compound Ia EPSPs. This increase in PPF is identical to that seen at other synapses when the probability of transmitter release is decreased by lowering the extracellular calcium or raising the extracellular magnesium concentration, providing unequivocal evidence that presynaptic inhibition is associated with a decrease in the probability of transmitter release. Further, by analogy with the effects of reduced calcium influx on PPF at other synapses, the results support the idea that presynaptic inhibition is associated with reduced calcium influx into nerve terminals.

Inhibition of Ca2+ inflow at nerve terminals of frog muscle blocks facilitation while phasic transmitter release is still considerable

Action potentials were triggered in the motor nerve by a suction electrode and calcium currents (iCa) in the nerve terminals were measured by means of a perfused macro-patch-clamp electrode on the distal portion of the end-plates. Postsynaptic currents were blocked by adding d-tubocurarine, whereas presynaptic Na+ (iNa) and K+ (iK) currents were blocked by adding tetrodotoxin (TTX), tetraethylammonium and 3,4-diaminopyridine, respectively, to the perfusate of the electrode. The current components which could be suppressed by addition of Cd2+ to the perfusate were taken as presynaptic iCa. The observed effects on the presynaptic current components were very similar to those reported previously. If the electrode was perfused with Ringer's solution containing the blockers for iNa and iK, the same, obviously complete block of iCa was obtained by 50 and 100 microM Cd2+, an average of 96% block by 20 microM Cd2+ and 50% block by about 5 microM Cd2+. Using the same type of electrode and similar locations on motor nerve terminals, postsynaptic quantal currents and twin-pulse facilitation (Fd) were elicited by variable-duration (0.5-3 ms) depolarizing pulses. When the electrode was perfused with Ringer's solution containing TTX, 20 microM Cd2+ added to the perfusate reduced the rate of phasic release of quanta insignificantly for short depolarizing pulses and by a factor of about 10 for longer pulses. Fd was blocked almost completely. Addition of 50 microM Cd2+ to the perfusate had a greater depressive effect on release after short depolarizing pulses and reduced release after longer pulses by a factor of about 100.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium and depolarization dependence of twin-pulse facilitation of synaptic release at nerve terminals of crayfish and frog muscle

Transmitter quanta were elicited from nerve terminals of crayfish and frog muscle by depolarization pulses through a macro-patch-clamp electrode. The rates of quantal release for twin pulses and their ratio, twin pulse facilitation, Fd, were determined. When the electrode was perfused with normal Cae (13.5 mM for crayfish, 1.8 mM for frog), Fd was low for threshold depolarizations, increased to a maximum at medium depolarizations, decreased when the rate of release due to the first pulse approached saturation, and increased again for larger depolarizations. If under these conditions the superfusion of the muscle outside the electrode was changed from normal to 0 Cab and high Mgb solution, Fd increased. When the Ca concentration around the terminal, Cae, was reduced to levels at which release did not reach the saturation level for large depolarizations, Fd in dependence on depolarization did not show the minimum at higher depolarizations. The amplitude of Fd measured for large, constant depolarization pulses showed a maximum at a Cae below that of the normal solution. The maximum of Fd was much higher if the superfusion of the bath contained 0 Cab and high Mgb than when normal bathing solution was superfused. The maxima of Fd at a low value of Ca inflow are predicted by the "residual Ca" theory of facilitation, if release is influenced by a resting low internal Ca concentration, Cair, and reaches a saturation level for large Ca-inflow. It is also predicted that decreasing Cair (as in low Cab) will increase Fd.