Extracellular potassium and trasmitter release at the giant synapse of squid.

1. The effects of changes in extracellular K concentration, [K]0, on synaptic transmission were studied at the squid giant synapse with intracellular recording from the presynaptic terminal and post-synaptic axon. 2. The amplitudes of both the presynaptic spike and the e.p.s.p. varied inversely with [K]0. On the average, a 10 mV change in spike height was accompanied by a 3-1 mV change in e.p.s.p. amplitude. 3. The amplitude of the presynaptic spike after-hyperpolarization (AH) varied inversely with [K]0. On the average, increasing [K]0 resulted in a 20% change in e.p.s.p. amplitude per mV change in presynaptic spike AH. 4. Repetitive antidromic stimulation of the post-synaptic giant axon resulted in an exponential decline in the post-synaptic spike AH, a depolarization of the presynaptic membrane potential and a reduction in the AHs of presynaptic spikes. This suggests that the K which accumulates in the extracellular spaces around the post-synaptic axon also affects the presynaptic terminal. 5. Repetitive antidromic stimulation of the post-synaptic axon resulted in a reduction in the amplitude of e.p.s.p.s. elicted by stimulation of the presynaptic axon. The reduction in e.p.s.p. amplitude relative to the change in presynaptic spike AH was quantitatively close to the change produced by increasing [K]0, suggesting that the reduction in e.p.s.p. amplitude is due to the accumulation of extracellular K at the presynaptic terminal. 6. Repetitive stimulation of the presynaptic axon reduced the amplitudes of the e.p.s.p. and the presynaptic spike AH. On the average, a 1 mV change in presynaptic spike AH was accompanied by a 204% change in e.p.s.p. amplitude, suggesting that K accumulation may only contribute to a small extent, under these conditions, to the depression of transmitter release.     

A comparison of the presynaptic and post-synaptic actions of pentobarbitone and phenobarbitone in the neuromuscular junction of the frog.

1. Pentobarbitone or phenobarbitone, in increasing concentrations up to 0-5 mM, progressively reduced the amplitude of miniature end-plate potentials (min.e.p.p.s). Pentobarbitone was the more potent of the two barbiturates in this regard. 2. Both barbiturates produced a monotonic increase in mean quantum content of the end-plate potential (e.p.p.) with increasing concentrations up to 0-5 mM. Pentobarbitone and phenobarbitone were equally potent in their action on evoked transmitter release. 3. The effect, if any, of increasing concentrations of barbiturates on the e.p.p. amplitude was depression. Therefore, over the range of concentrations examined the enhancement of transmitter release was quantitatively less than the reduction in responsiveness of the post-synaptic membrane. 4. Because of the greater ratio of post-synaptic to presynaptic actions, pentobarbitone was more potent than phenobarbitone in reducing synaptic efficacy (e.p.p. amplitude). 5. It is concluded that the presynaptic actions of pentobarbitone and phenobarbitone contribute significantly to barbiturate-induced changes in synaptic efficacy at low levels of transmitter release in the frog neuromuscular junction.     

Inhibition of presynaptic Na(+)/K(+)-ATPase reduces readily releasable pool size at the avian end-bulb of Held synapse

A glutamatergic end-bulb synapse in the avian nucleus magnocellularis relays temporal sound information from the auditory nerve. Here, we show that presynaptic Na⁺/K⁺-ATPase (NKA) activity at this synapse contributes to the maintenance of the readily releasable pool (RRP) of vesicles, thereby preserving synaptic strength. Whole-cell voltage clamp recordings were made from chick brainstem slices to examine the effects of NKA blocker dihydroouabain (DHO) on synaptic transmission. DHO suppressed the amplitude of EPSCs in a dose-dependent manner. This suppression was caused by a decrease in the number of neurotransmitter quanta released because DHO increased the coefficient of variation of EPSC amplitude and reduced the frequency but not the amplitude of miniature EPSCs. Cumulative plots of EPSC amplitude during a stimulus train revealed that DHO reduced the RRP size without affecting vesicular release probability. DHO did not affect [Ca²⁺]_i-dependent processes, such as the paired-pulse ratio or recovery time course from the paired-pulse depression, suggesting a minimal effect on Ca²⁺ concentration in the presynaptic terminal. Using mathematical models of synaptic depression, we further demonstrated the contribution of RRP size to the synaptic strength during a high-frequency stimulus train to highlight the importance of presynaptic NKA in the auditory synapse.     

Presynaptic plasticity at two giant auditory synapses in normal and deaf mice

Large calyceal synapses are often regarded as simple relay points, built for high-fidelity and high-frequency synaptic transmission and a minimal requirement for synaptic plasticity, but this view is oversimplified. Calyceal synapses can exhibit surprising activity-dependent developmental plasticity. Here we compare basal synaptic transmission and activity-dependent plasticity at two stereotypical calyceal synapses in the auditory pathway, the endbulb and the calyx of Held. Basal synaptic transmission was more powerful at the calyx than the endbulb synapse: the amplitude of evoked AMPA receptor-mediated excitatory postsynaptic currents (eEPSCs) was significantly greater at the calyx, as were the release probability, and the number of release sites. The quantal amplitude was smaller at the calyx, consistent with the smaller amplitude of spontaneous miniature EPSCs at this synapse. High-frequency trains of stimuli revealed that the calyx had a larger readily releasable pool of vesicles (RRP), less tetanic depression and less asynchronous transmitter release. Activity-dependent synaptic plasticity was assessed in congenitally deaf mutant mice ( dn/dn ). Previously we showed that a lack of synaptic activity in deaf mice increases synaptic strength at the endbulb of Held via presynaptic mechanisms. In contrast, we have now found that deafness does not affect synaptic transmission at the calyx synapse, as eEPSC and mEPSC amplitude, release probability, number of release sites, size of RRP, tetanic depression and asynchronous release were unchanged compared to normal mice. Synaptic transmission at the calyx synapse is more powerful and has less capacity for developmental plasticity compared to the endbulb synapse.     

Adenosine A1 receptor-mediated presynaptic inhibition at the calyx of Held of immature rats

At the calyx of Held synapse in brainstem slices of 5- to 7-day-old (P5–7) rats, adenosine, or the type 1 adenosine (A₁) receptor agonist N ⁶-cyclopentyladenosine (CPA), inhibited excitatory postsynaptic currents (EPSCs) without affecting the amplitude of miniature EPSCs. The A₁ receptor antagonist 8-cyclopentyltheophylline (CPT) had no effect on the amplitude of EPSCs evoked at a low frequency, but significantly reduced the magnitude of synaptic depression caused by repetitive stimulation at 10 Hz, suggesting that endogenous adenosine is involved in the regulation of transmitter release. Adenosine inhibited presynaptic Ca²⁺ currents ( I _pCa) recorded directly from calyceal terminals, but had no effect on presynaptic K⁺ currents. When EPSCs were evoked by I _pCa during simultaneous pre- and postsynaptic recordings, the magnitude of the adenosine-induced inhibition of I _pCa fully explained that of EPSCs, suggesting that the presynaptic Ca²⁺ channel is the main target of A₁ receptors. Whereas the N-type Ca²⁺ channel blocker ω-conotoxin attenuated EPSCs, it had no effect on the magnitude of adenosine-induced inhibition of EPSCs. During postnatal development, in parallel with a decrease in the A₁ receptor immunoreactivity at the calyceal terminal, the inhibitory effect of adenosine became weaker. We conclude that presynaptic A₁ receptors at the immature calyx of Held synapse play a regulatory role in transmitter release during high frequency transmission, by inhibiting multiple types of presynaptic Ca²⁺ channels.     

Post-tetanic potentiation in the rat calyx of Held synapse

We studied synaptic plasticity in the calyx of Held synapse, an axosomatic synapse in the auditory brainstem, by making whole-cell patch clamp recordings of the principal cells innervated by the calyces in a slice preparation of 7- to 10-day-old rats. A 5 min 20 Hz stimulus train increased the amplitude of excitatory postsynaptic currents (EPSCs) on average more than twofold. The amplitude of the synaptic currents took several minutes to return to control values. The post-tetanic potentiation (PTP) was accompanied by a clear increase in the frequency, but not the amplitude, of spontaneous EPSCs, which returned to baseline more rapidly than the potentiation of evoked release. The size of the readily releasable pool of vesicles was increased by about 30%. In experiments in which presynaptic measurements of the intracellular calcium concentration were combined with postsynaptic voltage clamp recordings, PTP was accompanied by an increase in the presynaptic calcium concentration to about 210 n m . The decay of the PTP matched the decay of this increase. When the decay of the calcium transient was shortened by dialysing the terminal with EGTA, the PTP decay sped up in parallel. Our experiments suggest that PTP at the calyx of Held synapse is due to a long-lasting increase in the presynaptic calcium concentration following a tetanus, which results in an increase in the release probability of the vesicles of the readily releasable pool. Although part of the PTP can be explained by a direct activation of the calcium sensor for phasic release, other mechanisms are likely to contribute as well.     

Synaptic transfer at a vertebrate central nervous system synapse.

1. The relation between presynaptic depolarization and transmitter release was examined at a synapse between a Müller axon and a lateral interneurone in the spinal cord of the lamprey. Two micro-electrodes, one for passing current and the other for recording the resulting voltage change, were placed in the presynaptic axon; a single electrode for recording the post-synaptic potential produced by release of transmitter was placed in the post-synaptic cell. 2. When action potentials were blocked with tetrodotoxin, brief depolarizing pulses in the presynaptic fibre were as effective as the action potential had been in producing transmitter release. 3. The release process had an apparent threshold depolarization of 40-50 mV and saturated at presynaptic depolarizations of the order of 100 mV. Increasing the duration of the presynaptic pulse increased the maximum level of release. 4. Displacing the presynaptic voltage recording electrode from the position of synaptic contact toward the current passing electrode increased the apparent depolarization required to produce a given level of transmitter release. This shift in the input-output relation was consistent in magnitude with the voltage attenuation between the presynaptic recording electrode and the synapse expected from the space constant of the fibre. 5. The effect of conditioning hyperpolarization and depolarization of the presynaptic fibre on subsequent transmitter release by brief depolarizing pulses was examined. No effect was observed when the presynaptic recording electrode was in the region of synaptic contact. When the presynaptic electrode was not so positioned, conditioning effects were observed which depended on electode position and could be attributed to changes in the space constant of the presynaptic fibre. No conditioning effects were observed on transmitter release by the action potential.     

Synaptic depression related to presynaptic axon conduction block.

1. The depression of synaptic transmission, which occurs during prolonged repetitive activation, was examined in the opener muscle of the crayfish walking leg. 2. Excitatory post-synaptic potentials (e.p.s.p.s) initially facilitated but then declined to low amplitudes after about 4000 stimulus pulses had been delivered; this depression is presynaptic in origin; 3. Axon conduction blocks occured at points of bifurcation along the entire length of the presynaptic nerve. This resulted in failure of the nerve impulse to invade some branches of the terminal arborization. 4. Nerve terminal invasion failure caused either intermittent or complete inactiviation of some synaptic release sites; this was associated with depression of the post-synaptic response. 5. The statistics of transmitter release during prolonged repetitive stimulation were examined by focal extracellular recording methods. Transmitter release could be described by binomial statistics, and depression involved a drop in m, n and p. 6. The rate of spontaneous quantal release did not decrease, however, arguing against transmitter depletion. 7. It is concluded that repetitive stimulation eventually leads to depolarization of the axon membrane. This causes impulse propagation failure which reduces the number of synaptic release sites that are activated and mimics a drop in the effective stimulation rate; both effects cause synaptic depression.     

Simulation of synaptic depression, posttetanic potentiation, and presynaptic facilitation of synaptic potentials from sensory neurons mediating gill-withdrawal reflex in Aplysia

The defensive gill-withdrawal reflex in Aplysia has proven to be an attractive system for analyzing the neural mechanisms underlying simple forms of learning such as habituation, sensitization, and classic conditioning. Previous studies have shown that habituation is associated with synaptic depression and sensitization with presynaptic facilitation of transmitter release from sensory neurons mediating the reflex. The synaptic depression, in turn, is associated with a decrease in Ca2+ currents in the sensory neurons, whereas presynaptic facilitation with increased Ca2+ currents produced indirectly by a decrease in a novel serotonergic sensitive K+ current. The present work represents an initial quantitative examination of the extent to which these mechanisms account for each of these types of synaptic plasticity. To address these issues a lumped parameter mathematical model of the sensory neuron release process was constructed. Major components of this model include Ca2+-channel inactivation, Ca2+-mediated neurotransmitter release and mobilization, and readily releasable and upstream feeding pools of neurotransmitter. In the model, release of neurotransmitter has a linear function of Ca2+ concentration and is not affected directly by residual Ca2+. The model not only simulates the data of synaptic depression and recovery from depression, but also qualitatively predicts other features of neurotransmitter release that it was not designed to fit. These include features of synaptic depression with high and low levels of transmitter release, posttetanic potentiation, a steep relationship between action potential duration and transmitter release, enhanced release produced by broadening the sensory neuron action potential (presynaptic facilitation), and dramatic synaptic depression with two closely spaced tetraethylammonium (TEA) spikes. The model cannot account fully for synaptic depression with empirically observed somatic Ca2+-current kinetics. Rather a large component of synaptic depression is due to reduction to the pools of releasable neurotransmitter (depletion). In the model when spike durations are greater than 15-20 ms, spike broadening produces little facilitation. However, when spike durations are more physiological, spike broadening leads to enhanced transmitter release.     

Functional and structural changes in mammalian sympathetic neurones following interruption of their axons.

The effects of interrupting the axons of principal neurones in the superior cervical ganglion of adult guinea-pigs were studied by means of intracellular recording, and light and electron microscopy. 1. Within 72 hr of axon interruption, the amplitude of exitatory postsynaptic potentials potentials (e.p.s.p.s) recorded in principal neurons in response to maximal preganglionic stimulation declined. E.p.s.p.s were maximally reduced (by more than 70% on average) 4-7 days following interruption, and failed to bring many cells to threshold. E.p.s.p.s. recorded in nearby neurones whose axons remained intact were unaffected. 2. In ganglia in which axon interruption was achieved by means of nerve crush (thus allowing prompt regeneration), mean e.p.s.p. amplitudes began to increase again after about 1-2 weeks. One month after the initial injury many neurones had e.p.s.p.s of normal amplitude, and by 2 months affected neurones were indistinguishable from control cells. Functional peripheral connexions were re-established during the period of synaptic recovery. 3. The mean number of synapses identified electron microscopically in ganglia in which all the major efferent branches had been crushed decreased by 65-70% in parallel with synaptic depression measured by intracellular recording. However synapse counts did not return to normal levels even after 3 months. 4. During the period of maximum synaptic depression, numerous abnormal profiles which contained accumulations of vesicular and tubular organelles, vesicles, and mitochondria were observed in electron microscopic sections. Injection of horseradish peroxidase into affected neurones demonstrated dendritic swelling which probably correspond to these profiles. 5. Little or no difference was found in the electrical properties of normal neurones and neurones whose axons had been interrupted 4-7 days previously. However, the mean amplitude of spontaneously occurring synaptic potentials was reduced, and the amplitude distribution was shifted. This abnormality of the synapses which remain on affected neurones also contributes to synaptic depression. 6. Counts of neurones in normal and experimental ganglia showed that approximately half the principal cells died 1-5 weeks after crushing the major efferent brances. This finding presumably explains the failure of synapse counts to return to control levels after recovery. 7. If axons were prevented from growing back to their target organ by chronic ligation, surviving neurones whose axons were enclosed by the ligature did not generally recover normal synaptic function. Following ligation, most affected cells died within a month. 8. Thus the integrity of a principal cell's axon is necessary for the maintenance of preganglionic synaptic contacts, and ultimately for neuronal survival. The basis of neuronal recovery from the effects of axon interruption appears to be some aspect of regeneration to the peripheral target.     

Habituation of a monosynaptic response in frog spinal cord: evidence for a presynaptic mechanism.

1. Using the isolated spinal cord of bullfrogs (Rana catesbeiana), intracellular correlates of habituation-like depression of the monosynaptic response elicited in motoneurons by lateral column (LC) stimulation were investigated. The following properties of the motoneuron were compared before and after response depression produced by stimulation of the LC at 0.5/s: resting membrane potential, membrane conductance, critical firing level, and rheobasic current. No alteration was found in any of these parameters. 2. To determine whether transmitter release mechanisms were changing over trials, the LC was stimulated with pairs of stimuli separated by 6 ms presented at 0.5/s. While the amplitude of the first EPSP declined (74% of initial value), the amplitude of the second EPSP increased (111% of initial value). Facilitation ratios thus increased. 3. The following conclusions can thus be drawn: 1) habituation involves a process intrinsic to the LC-motoneuron synapse; 2) habituation is not totally mediated by receptor desensitization; 3) habituation is not mediated by a mechanism extrinsic to the LC-motoneuron synapse that depolarizes terminal endings, e.g., presynaptic inhibition or accumulation of extracellular potassium; 4) habituation is not produced by transmitter depletion. Any of these possibilities has as a necessary consequence that facilitiation ratios remain unchanged. 4. Possible mechanisms that could mediate habituation are: 1) alterations in mobilization and/or release of transmitter; 2) decreased probability of invasion of terminal branches of the presynaptic fiber by the action potential.     

Transmitter metabolism as a mechanism of synaptic plasticity: a modeling study

The nervous system adapts to experience by changes in synaptic strength. The mechanisms of synaptic plasticity include changes in the probability of transmitter release and in postsynaptic responsiveness. Experimental and neuropharmacological evidence points toward a third variable in synaptic efficacy: changes in presynaptic transmitter concentration. Several groups, including our own, have reported changes in the amplitude and frequency of postsynaptic (miniature) events indicating that alterations in transmitter content cause alterations in vesicular transmitter content and vesicle dynamics. It is, however, not a priori clear how transmitter metabolism will affect vesicular transmitter content and how this in turn will affect pre- and postsynaptic functions. We therefore have constructed a model of the presynaptic terminal incorporating vesicular transmitter loading and the presynaptic vesicle cycle. We hypothesize that the experimentally observed synaptic plasticity after changes in transmitter metabolism puts predictable restrictions on vesicle loading, cytoplasmic-vesicular transmitter concentration gradient, and on vesicular cycling or release. The results of our model depend on the specific mechanism linking presynaptic transmitter concentration to vesicular dynamics, that is, alteration of vesicle maturation or alteration of release. It also makes a difference whether differentially filled vesicles are detected and differentially processed within the terminal or whether vesicle filling acts back onto the terminal by presynaptic autoreceptors. Therefore, the model allows one to decide, at a given synapse, how transmitter metabolism is linked to presynaptic function and efficacy.     

Mechanisms underlying short-term modulation of transmitter release by presynaptic depolarization

Presynaptic terminal depolarization modulates the efficacy of transmitter release. Residual Ca²⁺ remaining after presynaptic depolarization is thought to play a critical role in facilitation of transmitter release, but its downstream mechanism remains unclear. By making simultaneous pre- and postsynaptic recordings at the rodent calyx of Held synapse, we have investigated mechanisms involved in the facilitation and depression of postsynaptic currents induced by presynaptic depolarization. In voltage-clamp experiments, cancellation of the Ca²⁺-dependent presynaptic Ca²⁺ current ( I _pCa) facilitation revealed that this mechanism can account for 50% of postsynaptic current facilitation, irrespective of intraterminal EGTA concentrations. Intraterminal EGTA, loaded at 10 m m , failed to block postsynaptic current facilitation, but additional BAPTA at 1 m m abolished it. Potassium-induced sustained depolarization of non-dialysed presynaptic terminals caused a facilitation of postsynaptic currents, superimposed on a depression, with the latter resulting from reductions in presynaptic action potential amplitude and number of releasable vesicles. We conclude that presynaptic depolarization bidirectionally modulates transmitter release, and that the residual Ca²⁺ mechanism for synaptic facilitation operates in the immediate vicinity of voltage-gated Ca²⁺ channels in the nerve terminal.     

The development of transmission at an identified molluscan synapse. I. The emergence of synaptic plasticities

1. A monosynaptic, chemical synapse exists between two identified neurons in the subesophageal ganglia of the pulmonate mollusc, Achatina fulica. The snail undergoes a direct development, i.e., there is no intervening metamorphic period. The presynaptic (V2) and postsynaptic (RPr1) cells are two of the largest neurons found in the ganglia. The development of transmission at this synapse was studied from the last one-third of embryonic life to adulthood. 2. Synaptic transmission was studied by eliciting an action potential in V2 and recording the resultant excitatory postsynaptic potential (EPSP) in RPr1. In a train of repetitive stimuli, the ratio of the mean amplitude of the second EPSP to that of the first EPSP (EPSP2/EPSP1) is always greater than 1, indicating that short-term facilitation is present at all developmental ages studied. Following the initial short-term facilitation, embryonic synapses undergo a profound synaptic depression. Postembryonically there is a progressive increase in the amount of frequency facilitation with age, suggesting that the synapse shows a developmental trend towards an increased capacity for transmitter release. 3. In contrast to the progressive growth of frequency facilitation, the amplitude of the first EPSP in a series of responses (EPSP1) is not significantly related to age. 4. When transmitter release is reduced to approximately 25% of normal levels by a low-Ca2+/high-Mg2+ saline, the synaptic depression that is observed in the younger synapses disappears and is replaced by an adult-like frequency facilitation. 5. The adult synapse displays a phenomenon similar to posttetanic potentiation, which we refer to as the "retention of frequency facilitation." If an initial train of 150 stimuli at 0.2 Hz is followed by a second, identical train after an interval of 1 h, the postsynaptic response is greater during the second train than during the first. This phenomenon only becomes apparent in the second month after hatching, indicating that this separate synaptic plasticity develops at a different rate than does frequency facilitation.     

The action of calcium on neuronal synapses in the squid

1. The isolated stellate ganglion of the squid (L. pealii) was studied with intracellular and extracellular micro-electrodes. Three or four nerve fibres in the preganglionic nerve establish synaptic relations with the giant axon in the last stellar nerve. Accordingly, 1-3 small presynaptic spikes (< 1 mV) could be recorded from within the post-synaptic axon.2. A micro-electrode was inserted in the presynaptic fibre and used to polarize and record simultaneously. In the distal (giant) synapse, hyperpolarization of the ending produced an increase in the size of the presynaptic action potential and post-synaptic potential (PSP). Depolarization had the opposite effect. These effects of polarization took more than 10 sec to develop fully, and declined with a similar time course at the end of polarization. Analogous results were obtained with two other preganglionic fibres, which make contacts in the proximal synaptic region.3. The second of a pair of preganglionic impulses evoked a PSP larger than the first. This facilitation of PSP was sometimes accompanied by a small increase in the size of the second action potential in the presynaptic axon. At some shorter intervals, the second presynaptic action potential was reduced in amplitude, but the PSP was still increased. Hyperpolarization of the presynaptic terminal increased the size of both PSPs in a pair and abolished the facilitation. With stronger hyperpolarization the second PSP was even smaller than the first.4. Removing or reducing the Ca in the bathing fluid reversibly abolished the post-synaptic response. The small presynaptic spikes remained practically unaffected. In these conditions a nerve impulse still invaded the ending and normal action potentials could be recorded from the pre-synaptic terminal. This shows that electrical coupling between pre- and post-synaptic axons is insufficient to account for synaptic transmission.5. In low-Ca solution synaptic transmission could be restored locally by extracellular ionophoretic application of Ca to a small portion of the synapse. At sensitive spots a post-synaptic current (recorded with the Ca pipette) and PSP could be detected earlier than 1 sec after commencing the application of Ca.6. Ca was ineffective when injected intracellularly into the presynaptic fibre at a spot where extracellular ionophoresis of Ca restored the PSP.7. The results indicate that synaptic transmission in the squid stellate ganglion is not electrical but due to the release of an unidentified transmitter. Release of this transmitter by the presynaptic nerve impulse requires the presence of Ca in the external medium. During the impulse Ca would combine with a ;Ca-receptor' in the membrane and initiate the reactions which lead to transmitter release. It appears that the ;Ca-receptor' is only accessible from the outside of the membrane.     

Developmental changes in short-term synaptic depression in the neonatal mouse spinal cord

We examined age-dependent changes in short-term synaptic depression of monosynaptic excitatory postsynaptic potentials (EPSPs) recorded in lumbar motoneurons in hemisected spinal cords of neonatal Swiss-Webster mice between postnatal day 2 (P2) and 12 (P12). We used four paradigms that sample the input-output dependence on stimulation history in different but complementary ways: 1) paired-pulse depression; 2) steady-state depression during constant frequency trains; 3) modulation during irregular stimulation sequences; and 4) recovery after high-frequency conditioning trains. Paired-pulse synaptic depression declined more than steady-state depression during 10-pulse trains at frequencies from 0.125 to 8 Hz in this age range. Depression during sequences of irregular stimulations that more closely mimic physiological activation also declined with postnatal age. On the other hand, the overall rate of synaptic recovery after a 4-Hz conditioning train exhibited surprisingly little change between P2 and P12. Control experiments indicated that these observations depend primarily, if not exclusively, on changes in presynaptic transmitter release. The data were examined using quantitative models that incorporate factors that have been suggested to exist at more specialized central synapses. The model that best predicted the observations included two presynaptic compartments that are depleted during activation, plus two superimposed processes that enhance transmitter release by different mechanisms. One of the latter produced rapidly-decaying enhancement of transmitter release fraction. The other mechanism indirectly enhanced the rate of renewal of one of the depleted presynaptic compartments. This model successfully predicted the constant frequency and irregular sequence data from all age groups, as well as the recovery curves following short, high-frequency tetani. The results suggest that a reduction in release fraction accounts for much of the decline in synaptic depression during early postnatal development, although changes in both enhancement processes also contribute. The time constants of resource renewal showed surprisingly little change through the first 12 days of postnatal life.     

Synaptic depression in visual cortex tissue slices: an in vitro model for cortical neuron adaptation

Synaptic depression was assessed from intracellular recordings in cortical tissue slices. Evoked postsynaptic potentials exhibited synaptic depression with an exponential or double exponential decrease (time constants: <1–30 s) in amplitude during repetitive afferent stimulation by short trains of suprathreshold stimuli. Depressed synaptic responses recovered with an exponential time course (time constants: 10 s-8 min) during presentation of similar short trains of stimuli every 5 or 10 s. Cortical cells recorded extracellularly in cat visual cortex show similar time constants of response decrement during adaptation to moving stripes. Postsynaptic voltage- or ion-regulated conductances and chloride conductances do not appear to be involved in synaptic depression. Input resistance changes and effects of injection of chloride indicate a lack of GABA_A receptor-mediated effects. Hyperpolarizing or depolarizing neurons, and pairing polarization with afferent stimulation, also did not affect synaptic depression. This distinguishes these processes from long-term depression and long-term potentiation. Our results suggest that the most likely mechanisms of synaptic depression and adaptation in cortical cells are presynaptic decrease in transmitter release and/or receptor desensitization. Short-term postsynaptic changes may also occur after synaptic depression.     

Paired individual and mean postsynaptic currents recorded in four-cell networks of Aplysia

1. Presynaptic neurons B4 and B5 of Aplysia buccal ganglia produce similar inhibitory postsynaptic currents (PSCs) in several postsynaptic follower cells. Two previous papers have characterized the variability of synaptic current amplitude and decay time both for individual PSCs and also for mean values characterizing synapses and have compared PSC amplitude and time course at different synapses sharing a common presynaptic or postsynaptic neuron. 2. To distinguish similarity in synaptic current amplitude or decay introduced by a common pre- or postsynaptic neuron from similarity because of factors common to the particular ganglion or animal, paired synapses were analyzed in four-cell networks in which each of two identified presynaptic neurons produces similar PSCs in each of two postsynaptic cells. Pairing the same synaptic data by common presynaptic or postsynaptic neuron tests if the presynaptic or postsynaptic element partially specifies a parameter; cross-pairing controls for more global factors. Paired values of peak conductance gpeak and decay time constant tau were compared for both individual sequential PSCs and for averages characterizing synapses. Analyses of individual PSCs examine processes affecting synaptic plasticity on a time scale of seconds to minutes, while average values compare more slowly varying factors. 3. Peak amplitudes were compared between individual PSCs in each of 24 paired sets. Correlations of gpeak fluctuations were significantly larger for PSCs produced by the same presynaptic neuron than for postsynaptic or cross pairings (P less than 0.05), consistent with partially correlated fluctuations in transmitter release at different presynaptic terminals. 4. Firing rates of individual presynaptic neurons were modulated to induce variability of test PSCs. These manipulations altered synaptic peak amplitudes in paired postsynaptic neurons, although not to the same degree. Manipulation of a single presynaptic neuron modulated input from that neuron alone to common postsynaptic cells without any effect on input from the paired presynaptic neuron. When fluctuations in the amplitude of gpeak were examined in runs incorporating presynaptic modulation, correlations were strong for sets of PSCs sharing a common presynaptic neuron (R = 0.87), significantly greater (P less than 0.001) than for other pairings. 5. In contrast to the partial presynaptic specification of fluctuations of individual PSCs, values of synaptic amplitude and time course averaged over 21-132 PSCs at a given synapse reflect postsynaptic determinants. Mean values of gpeak characterizing synapses paired by common postsynaptic cell are highly similar (P = 0.0001), in contrast to the lack of similarity seen when the same data are presynaptically (P = 0.11) or cross (P = 0.36) paired.(ABSTRACT TRUNCATED AT 400 WORDS)     

Physiological and morphological effects of post-ganglionic axotomy on presynaptic nerve terminals.

1. Electrophysiological and electron microscope studies were done on cells in the ciliary ganglion of chickens which had been axotomized on the day of hatching. 2. By the third day after post-ganglionic axotomy both electrical and chemical transmission through the ganglion were severely depressed; by the fifth day ganglionic transmission had disappeared. 3. Action potential initiation and conduction in axotomized cells and in their associated presynaptic nerve terminals were unimpaired 3-4 days after axotomy. 4. Depression of ganglionic transmission in 3-4 day axotomized preparations was due to a reduction in amplitude of both the excitatory post-synaptic potential (e.p.s.p.) and the electrical coupling potential in individual ganglion cells. 5. In addition to being reduced in amplitude, e.p.s.p.s in axotomized cells were more subject to fatigue during low frequency (1/sec) stimulation. 6. The reduction in e.p.s.p. amplitude was due to a reduction in both the mean quantal content of the e.p.s.p.s and the calculated depolarization produced by an individual quantum of transmitter. On the average the e.p.s.p. was reduced by a factor of about 4, the mean quantum content to about two thirds normal and the quantal size to about a third normal, compared with responses in unaxotomized cells of the same age. 7. Ultrastructural studies revealed a progressive maturation of pre-synaptic terminals in normal ganglia between 0 and 9 days after hatching. Over this period the content of synaptic vesicles and mitochondria in the terminals increased and the background matrix became more dense. 8. After axotomy these signs of maturation was abolished or reversed, particularly from the third day onward. In addition there was an increase in the number of cell sections in which no synaptic terminals were observed. 9. It was concluded that loss of synaptic transmission was due to at least three factors: a reduction in release of transmitter from presynaptic terminals, a reduction in quantal size, probably due to a loss of post-synaptic sensitivity, and a partial loss of presynaptic contact.     

Membrane ultrastructure of the giant synapse of the squidLoligo pealei

The ultrastructure of the ‘giant synapse’ of the stellate ganglion of the squid was studied with freeze-fracture and thin-sectioning techniques. A sheath of glial cells separates the pre- and post-synaptic axons. At intervals, round-topped processes of the postsynaptic axon pierce the sheath to contact the presynaptic axon. This area of synaptic contact is marked by a widened intercellular cleft containing electron-dense material and by a cluster of synaptic vesicles within the presynaptic cytoplasm. The number of synaptic vesicles in such clusters was greatly reduced by electrical stimulation of the synapse during fixation. Freeze-fracture reveals a roughly circular patch (0.3 μm diameter) of 10 nm particles on the cytoplasmic leaflet of the presynaptic membrane. A similar patch of particles lies on the external leaflet of the apposed postsynaptic membrane.The squid giant synapse thus consists of multiple small pre- and postsynaptic active zones where neurotransmitter is released from the presynaptic terminal and sensed by postsynaptic receptors. Comparison of the structure of these postsynaptic active zones with those at synapses where the transmitter or transmitter action is known suggests that the excitatory transmitter at this synapse is an amino acid.Presumptive gap junctions, marked by particles in the cytoplasmic leaflet, are found between small-diameter axons in the stellate ganglion but not at the giant synapse. Glial-cell membranes contain aggregates of particles and pits suggestive of gap junctions. The aggregates of pits are embedded within linear arrays of particles which somewhat resemble tight junctions.