Repetitive Presynaptic Stimulation Research Articles

Lack of support for surface diffusion of postsynaptic AMPARs in tuning synaptic transmission

Repetitive stimulation of excitatory synapses triggers molecular events required for signal transfer across neuronal synapses. It has been hypothesized that one of these molecular events, the diffusion of extrasynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPARs) (i.e., the diffusion hypothesis), is necessary to help synapses recover from paired-pulse depression. To examine this presumed role of AMPAR diffusion during repetitive presynaptic stimulation, a biophysical model based on published physiological results was developed to track the localization and gating of each AMPAR. The model demonstrates that AMPAR gating in short intervals of fewer than 100 ms is controlled by their position in relation to the glutamate release site and by their recovery from desensitization, but it is negligibly influenced by their diffusion. Therefore, these simulations failed to demonstrate a role for AMPAR diffusion in helping synapses recover from paired-pulse depression.

Long-term plasticity of glutamatergic input from the subthalamic nucleus to the entopeduncular nucleus.

The hyperdirect pathway of the basal ganglia bypasses the striatum, and delivers cortical information directly to the subthalamic nucleus (STN). In rodents, the STN excites the two output nuclei of the basal ganglia, the entopeduncular nucleus (EP) and the substantia nigra reticulata (SNr). Thus, during hyperdirect pathway activation, the STN drives EP firing inhibiting the thalamus. We hypothesized that STN activity could induce long-term changes to the STN->EP synapse. To test this hypothesis, we recorded in the whole-cell mode from neurons in the EP in acute brain slices from rats while electrically stimulating the STN. Repetitive pre-synaptic stimulation generated modest long-term depression (LTD) in the STN->EP synapse. However, pairing EP firing with STN stimulation generated robust LTD that manifested for pre-before post-as well as for post- before pre-synaptic pairing. This LTD was highly sensitive to the time difference and was not detected at a time delay of 10ms. To investigate whether post-synaptic calcium levels were important for LTD induction, we made dendritic recordings from EP neurons that revealed action potential back-propagation and dendritic calcium transients. Buffering the dendritic calcium concentration in the EP neurons with EGTA generated long term potentiation instead of LTD. Finally, mild LTD could be induced by post-synaptic activity alone that was blocked by an endocannabinoid 1 (CB1) receptor blocker. These results thus suggest there may be an adaptive mechanism for buffering the impact of the hyperdirect pathway on basal ganglia output which could contribute to the de-correlation of STN and EP firing.

Synaptic efficacy during repetitive activation of excitatory inputs in primate dorsolateral prefrontal cortex.

Neurons in the monkey dorsolateral prefrontal cortex (DLPFC) fire persistently during the delay period of working memory tasks. To determine how repetitive firing affects the efficacy of synaptic inputs to DLPFC layer 3 neurons, we examined the effects of repetitive presynaptic stimulation on the amplitude and temporal summation of EPSPs. Recordings were obtained in monkey DLPFC brain slices from regular spiking (RS) pyramidal cells and two types of interneurons, fast spiking (FS) and adapting non-pyramidal (ANP) cells. Repetitive stimulation of presynaptic axons in layer 3 caused EPSP depression in RS and FS neurons, but EPSP facilitation in ANP cells. A shorter EPSP duration produced weaker temporal summation in FS neurons compared to the other cell classes. Thus, due to the combined effects of dynamic changes in EPSP amplitude and differences in temporal summation, the effect of a presynaptic spike train differed according to the postsynaptic cell class. Similar results were obtained when recording unitary EPSPs evoked in connected pairs of presynaptic RS pyramidal cells and postsynaptic RS, FS or ANP neurons. In addition, similar differences in the efficacy of sustained inputs among cell classes were observed when delay-related firing was reproduced in vitro by stimulating inputs with the timing of spike trains recorded from the DLPFC of monkeys performing a delayed-response task. We suggest that the transition from baseline firing rates to higher frequency delay-related firing may lead to the differential activation of distinct cell populations, with corresponding significant effects on the patterns of activity in local prefrontal circuits.

Essential function of alpha-calcium/calmodulin-dependent protein kinase II in neurotransmitter release at a glutamatergic central synapse.

A significant fraction of the total calciumcalmodulin-dependent protein kinase II (CaMKII) activity in neurons is associated with synaptic connections and is present in nerve terminals, thus suggesting a role for CaMKII in neurotransmitter release. To determine whether CaMKII regulates neurotransmitter release, we generated and analyzed knockout mice in which the dominant alpha-isoform of CaMKII was specifically deleted from the presynaptic side of the CA3-CA1 hippocampal synapse. Conditional CA3 alpha-CaMKII knockout mice exhibited an unchanged basal probability of neurotransmitter release at CA3-CA1 synapses but showed a significant enhancement in the activity-dependent increase in probability of release during repetitive presynaptic stimulation, as was shown with the analysis of unitary synaptic currents. These data indicate that alpha-CaMKII serves as a negative activity-dependent regulator of neurotransmitter release at hippocampal synapses and maintains synapses in an optimal range of release probabilities necessary for normal synaptic operation.

Biophysical model of a single synaptic connection: Transmission properties are determined by the cooperation of pre- and postsynaptic mechanisms

A stochastic model of synaptic transmission has been designed on the basis of electrophysiological experiments. The model includes presynaptic mechanisms of recruitment and calcium related release of vesicles, transmitter dynamics in the cleft and postsynaptic receptor kinetics. Monte Carlo simulations of a single synaptic connection are performed and demonstrate that synapses depress during repetitive presynaptic stimulation due to depletion of presynaptic vesicles as well as receptor desensitization. Only for stimulation frequencies below 40 Hz depression is caused solely presynaptically by depletion of vesicles. It is shown that specific physiological conditions determine the frequency dependence of steady state depression currents and set limits on the range of possibly rate-coded transmission.

Role of the conserved WHXL motif in the C terminus of synaptotagmin in synaptic vesicle docking.

Synaptotagmin (Syt) I, an abundant synaptic vesicle protein, consists of one transmembrane region, two C2 domains, and a short C terminus. This protein is essential for both synaptic vesicle exocytosis and endocytosis via its C2 domains. Although the short C terminus is highly conserved among the Syt family and across species, little is known about the exact role of the conserved C terminus of Syt I. In this paper, we report a function of the Syt I C terminus in synaptic vesicle docking at the active zones. Presynaptic injection of a peptide corresponding to the C-terminal 21 amino acids of Syt I (named Syt-C) into the squid giant synapse blocked synaptic transmission without affecting the presynaptic action potential or the presynaptic Ca(2+) currents. The same procedure repeated with a mutant C-terminal peptide (Syt-CM) had no effect on synaptic transmission. Repetitive presynaptic stimulation with Syt-C produced a rapid decrease in the amplitude of the postsynaptic potentials as the synaptic block progressed, indicating that the peptide interferes with the docking step rather than the fusion step of synaptic vesicles. Electron microscopy of the synapses injected with the Syt-C peptide showed a marked decrease in the number of docked synaptic vesicles at the active zones, as compared with controls. These results indicate that Syt I is a multifunctional protein that is involved in at least three steps of synaptic vesicle cycle: docking, fusion, and reuptake of synaptic vesicles.

Presynaptic injection of syntaxin-specific antibodies blocks transmission in the squid giant synapse

A polyclonal antibody, raised against the squid ( Loligo pealei) syntaxin I, inhibited Ca 2+-dependent interaction of syntaxin with synaptotagmin C2A domain in vitro. Presynaptic injection of the anti- Loligo syntaxin IgG into the squid giant synapse blocked synaptic transmission without affecting the presynaptic action potential or the voltage-gated calcium current responsible for transmitter release. Repetitive presynaptic stimulation produced a gradual decrease in the amplitude of the postsynaptic potential as the synaptic block progressed, indicating that the antibody interferes with vesicular fusion. Confocal microscopy of the fluorescein-labelled anti- Loligo syntaxin IgG showed binding at the synaptic active zone, while ultrastructurally, an increase in synaptic vesicular numbers in synapses blocked when this antibody was observed. These results implicate syntaxin in the vesicular fusion step of transmitter release in concert with synaptotagmin.

Role of the C2A domain of synaptotagmin in transmitter release as determined by specific antibody injection into the squid giant synapse preterminal.

Squid synaptotagmin (Syt) cDNA, including its open reading frame, was cloned and polyclonal antibodies were obtained in rabbits immunized with glutathione S-transferase (GST)-Syt-C2A. Binding assays indicated that the antibody, anti-Syt-C2A, recognized squid Syt and inhibited the Ca(2+)-dependent phospholipid binding to the C2A domain. This antibody, when injected into the preterminal at the squid giant synapse, blocked transmitter release in a manner similar to that previously reported for the presynaptic injection of members of the inositol high-polyphosphate series. The block was not accompanied by any change in the presynaptic action potential or the amplitude or voltage dependence of the presynaptic Ca2+ current. The postsynaptic potential was rather insensitive to repetitive presynaptic stimulation, indicating a direct effect of the antibody on the transmitter release system. Following block of transmitter release, confocal microscopical analysis of the preterminal junction injected with rhodamine-conjugated anti-Syt-C2A demonstrated fluorescent spots at the inner surface of the presynaptic plasmalemma next to the active zones. Structural analysis of the same preparations demonstrated an accumulation of synaptic vesicles corresponding in size and distribution to the fluorescent spots demonstrated confocally. Together with the finding that such antibody prevents Ca2+ binding to a specific receptor in the C2A domain, these results indicate that Ca2+ triggers transmitter release by activating the C2A domain of Syt. We conclude that the C2A domain is directly related to the fusion of synaptic vesicles that results in transmitter release.

Antigen-induced long-term potentiation of nicotinic synaptic transmission in the superior cervical ganglion of the guinea pig

1. Recordings of evoked postganglionic compound action potentials (CAPs) evoked by preganglionic stimulation were obtained from guinea pig superior cervical ganglia (SCGs) in vitro to study the effects of specific antigen challenge on ganglionic synaptic transmission. SCGs were removed from guinea pigs actively sensitized to ovalbumin. 2. Exposing SCGs from sensitized animals to the sensitizing antigen (0.01-10 micrograms/ml) for 5 min produced a sustained increase in the magnitude of the evoked CAP unaccompanied by a change in the preganglionic volley. Nonsensitizing antigens were ineffective. Also ineffective were antigens applied to nonsensitized SCG. This persistent antigen-induced increase in synaptic transmission was designated antigen-induced long-term potentiation (LTP) (A-LTP) because its duration (> 30 min) greatly outlasted posttetanic potentiation (PTP) in this ganglion. 3. A-LTP and neurogenic LTP (N-LTP) were observed to coexist in the same ganglion; the presence of one form of synaptic plasticity did not preclude the development of the other. Both phenomena were influenced by presynaptic factors: prolonged (2 h, 40 Hz) repetitive presynaptic stimulation abolished A-LTP or N-LTP but did not affect PTP. 4. By contrast to N-LTP, which requires a brief presynaptic tetanus, A-LTP could be triggered over a wide range of presynaptic stimulation (0.016-3 Hz) or even in the absence of presynaptic stimulation. 5. The amplitude and duration of A-LTP were not significantly affected by 1) H1, H2, or H3 histamine receptor antagonists added before or after antigen challenge; 2) the presence of saturating concentrations of histamine (100-300 microM); 3) the presence of specific or nonspecific lipoxygenase inhibitors or a selective cyclooxygenase inhibitor; or 4) blockade of alpha- or beta-adrenergic receptors, 5-HT3 receptors, muscarinic receptors, or glutamate receptors, or inhibition of acetylcholinesterase or protein synthesis. 6. Our results indicate that specific antigen challenge of isolated sympathetic ganglia activates resident mast cells to release substances that initiate a novel form of synaptic plasticity, an activity-independent and long-lasting increase in synaptic efficacy.

Synaptic interactions in a newly identified excitatory synapse of Helix aspersa: concurrent enkephalinergic and GABAergic modulation

Mollusc neurons, including those from Helix aspersa, have been extensely used for studies on synaptic plasticity and for pharmacological experiments. Synaptic interactions in H. aspera are less well documented. In this work, I describe the identification and electrophysiological characteristics of a chemical synapse which is modulated by Met-Enkephalin (ME) and GABA. Low frequency repetitive presynaptic stimulation leads to a progressive amplitude reduction of the EPSPs (homosynaptic depression) of 45% after 20 stimuli and up to 80% after long-lasting repetitive stimulation (150–200 stimuli). The values of the smallest detectable EPSP after profound depression under normal neurotransmitter release levels corresponded to those obtained under high Mg 2+-low Ca 2+ conditions (0.33±0.045mV) and to the spontaneous EPSPs (0.4mV, σ 2=0.02). Recovery after homosynaptic depression could occur by withdrawing the stimulation during 25 s after 15–20 stimuli or 2 min after 150–200 stimuli. Both, ME and GABA reduced the amplitude of the evoked EPSPs and augmented homosynaptic depression although this inhibition was independent of the homosynaptic depression trend. The amplitude of the spontaneous EPSPs was not affected. Naloxone alone did not produce any effect, making it unlikely that the enkephalinergic system could be recruited during homosynaptic depression. ME also shortened the duration of the presynaptic action potential and enhanced its afterhyperpolarization. These results suggest that ME and GABA exert a presynaptic modulation.

Nicotinic and muscarinic synaptic transmission in canine intracardiac ganglion cells innervating the sinoatrial node

Nicotinic and muscarinic mediated synaptic mechanisms were investigated in isolated, canine intracardiac ganglia taken from the right atrial fat pad. Using conventional intracellular microelectrode recording techniques on 216 neurons fast, and slow synaptic potentials were evoked by single or trains of stimulation of presynaptic fibers in interganglionic nerves. By varying the stimulus intensity, single or multiple fast excitatory postsynaptic potentials (f-EPSPs) were evoked, indicating the convergence of synaptic inputs on these cells. These f-EPSPs often reached the action potential threshold, were enhanced by the acetelcholinesterase inhibitor physostigmine and were blocked by the nicotinic antagonist hexamethonium. The f-EPSPs were accompanied by a decreased input resistance and had an extrapolated reversal potential of −7.1 mV, suggesting increased conductances to more than one cation. Repetitive presynaptic stimulation evoked slow excitatory postsynaptic potentials (s-EPSPs) in 41% of the cells while slow inhibitory postsynaptic potentials (s-IPSPs) or s-IPSPs followed by s-EPSPs were evoked in 19% of the cells. All slow potentials were abolished by atropine and low Ca 2+/high Mg 2+ solutions and enhanced by physostigmine. Hexamethonium and adrenergic receptor antagonists had no effects on s-EPSP and s-IPSP. The M 1 receptor antagonist pirenzepine reversibly blocked the s-EPSP but not the s-IPSP. On the other hand, the M 2 receptor blocker 4-diphenyl-acetoxy- N-methyl piperidine methiodide (4-DAMP) had no effects on the s-EPSP. These observations suggest that s-EPSPs and s-IPSPs are mediated by distinct muscarinic by increased input resistance. These responses were decreased in amplitude by membrane hyperpolarization and either reversed polarity or declined to zero amplitude at about −80 mV, suggesting the inhibition of a potassium conductance.

Synaptic activity of myenteric neurons in tissue culture.

Intracellular recordings were made from myenteric neurons of the guinea-pig caecum grown in tissue culture. The experiments were carried out on cultures older than 15 days, in which the neurons formed ganglion-like aggregates with many interconnecting nerve fibres. Intracellular injection of horseradish peroxidase demonstrated that the neurons project processes both within and outside the aggregates. Spontaneous activity was recorded in about 50% of the cells. This activity was composed of depolarizing potentials 2-20 mV in amplitude, and in most cases 10-20 ms in duration. In some instances, the potentials occurred in short bursts. The spontaneous potentials appear to be nicotinic cholinergic excitatory postsynaptic potentials (EPSPs). Electrical stimulation of the connecting fibers elicited in the neurons nicotinic cholinergic EPSPs, since they were blocked by hexamethonium. By varying the stimulus intensity and by stimulating several fibers, it was shown that the neurons are multiple-innervated. During repetitive presynaptic stimulation at frequencies above 10 Hz, there was a progressive reduction of the EPSP amplitude. It is concluded that the cultured myenteric neurons retain many of the characteristics found in the freshly isolated preparation and may be useful for studying the synaptic properties of these cells.

Capsaicin causes release of a substance P-like peptide in guinea-pig inferior mesenteric ganglia.

The effects of capsaicin (0.5-100 microM) on neurones of the isolated inferior mesenteric ganglia (i.m.g.) of the guinea-pig were investigated by means of intracellular recording techniques. When applied to neurones of the i.m.g. that exhibited a slow non-cholinergic excitatory potential (Dun & Jiang, 1982), capsaicin caused in the large majority of these cells a long lasting depolarization accompanied by intense neuronal discharges. During and immediately following the depolarization, repetitive presynaptic stimulation consistently failed to elicit the non-cholinergic depolarization; a partial recovery was observed in relatively few neurones. The fast (nicotinic) excitatory post-synaptic potentials (e.p.s.p.s) were not suppressed by capsaicin in any of these cells. The membrane potential, input resistance and the amplitude of fast e.p.s.p.s in neurones of the i.m.g. that generated no detectable non-cholinergic depolarizations were not affected by capsaicin. Post-synaptic membrane sensitivity to exogenous application of substance P was not altered following capsaicin superfusion even though the latter effectively abolished the non-cholinergic depolarization in the same neurones. Superfusing the i.m.g. with a Ca-free Krebs solution markedly attenuated or abolished the depolarizing effect of capsaicin whereas tetrodotoxin (1 microM) was without effect. Capsaicin was without effect in a few neurones that generated a non-cholinergic depolarization; the latter was not desensitized by bath application of substance P. Capsaicin caused no appreciable effects in neurones of the bullfrog sympathetic ganglia; thus, the fast and slow post-synaptic potentials including the late slow e.p.s.p. the transmitter of which is the peptide luteinizing hormone-releasing hormone (Jan & Jan, 1982) were not affected. The results suggest that the depolarizing effect of capsaicin in neurones of the guinea-pig i.m.g. is due to a selective release of ganglionic substance P or a substance P-like peptide in a Ca-dependent manner, and that the non-cholinergic potentials elicited in capsaicin-insensitive neurones may be generated by a transmitter(s) other than substance P.

The mechanism of the anti-desensitizing action of sodium fluoride at the amphibian neuromyal junction

Sodium fluoride was shown in these laboratories to facilite neuromyal transmission and to be a unique antagonist of desensitization. Possible mechanisms of action of sodium fluoride include a receptor effect and chelating activity. This study concerns the mechanism of the effect of sodium fluoride (0.1–2 mM) on the desensitization of the amphibian neuromyal junction and the interaction between this mechanism and calcium. Desensitization was obtained either by repetitive (10–50 Hz), brief (2 msec) iontophoretic pulses of acetylcholine (ACh), prolonged (40 sec) iontophoretic or bath application of ACh, or repetitive (10–50 Hz) presynaptic stimulation. Microelectrode and voltage-clamp methods were employed. The rate constant of desensitization induced by repetitive pulses of ACh was calculated in terms of the decay of current or potential. It depended on the frequency of pulses and on calcium ion concentration. In low calcium solution, a high frequency of pulses had to be employed to induce desensitization with a measurable rate constant. Sodium fluoride slowed down the rate of desensitization, and increased the amplitude of ACh current or potential during desensitization. The effect of sodium fluoride on the desensitization rate was pronounced at normal and low calcium ion concentrations and was slight in high calcium solutions. Desensitization did not affect reversal potential whether the latter was evaluated in terms of ACh potentials or ACh currents, and whether desensitization was induced by repeated ACh pulses or ACh perfusion. Nor did sodium fluoride (0.5–2 mM) affect the reversal potential of either a normal or desensitized endplate. Desensitization prolonged the decay time of the excitatory postsynaptic current (EPC) without affecting the slope of the decay time-membrane voltage relationship; sodium fluoride (0.1–2 mM) further prolonged decay time without affecting membrane voltage-decay time relation. The relationship between EPC or ACh current on the one hand and membrane voltage on the other was studied with respect to desensitization, sodium fluoride, and calcium concentration. Desensitization induced by repetitive (30 Hz) presynaptic stimulation did not affect the linearity of EPC-membrane voltage relationship; this remained true in different calcium solutions and when desensitization was induced in the presence of sodium fluoride. However, desensitization attenuated the slope of this relationship, the effect being similar in all three calcium solutions. This lack of effect of calcium on the slope must be considered with respect to calcium dependence of ACh release and, thus, on the EPC amplitude. The attenuation by desensitization of the slope of the EPC-voltage relationship was antagonized by sodium fluoride. Different frequencies of ACh pulses had to be employed to induce desensitization in the three calcium solutions in order to elicit desensitization with measurable rate constant. Also in this case, the ACh current-membrane voltage remained linear before and following desensitization in the three concentrations of calcium. Again, the slope of the current-voltage relationship was attenuated by desensitization at all concentrations of calcium. Sodium fluoride antagonized this effect to a significant degree at all concentrations of calcium; the antagonism was more pronounced at low than at high concentration of calcium. Desensitization induced by ACh perfusion did not affect the linearity of ACh current-membrane voltage at either of the three calcium concentrations. In this case also, desensitization attenuated the slope of the relationship in a calcium-dependent manner. Again, sodium fluoride restored significantly the amplitude of ACh current, the recovery being well-nigh complete in low, and amounting to 44% in high calcium solution. The EGTA (see Methods) did not affect the linearity of EPC-voltage or ACh current-voltage relationship, but decreased its slope; EGTA further diminished the slope of these relationships after the slope was attenuated by desensitization induced by repetitive presynaptic stimulation or ACh pulses. At that time, sodium fluoride restored to a significant degree the slope of ACh current-membrane voltage relationship; thus, sodium fluoride and EGTA exerted opposite effects on the slope of this relationship. These results support the notion that both desensitization and the effect of sodium fluoride concern the receptor rather than the channel. They also show that while calcium is involved in desensitization and the action of sodium fluoride, the anti-desensitizing effect of sodium fluoride does not depend on its chelating effect.

Substance P: A Putative Sensory Transmitter in Mammalian Autonomic Ganglia

Repetitive presynaptic stimulation elicited slow membrane depolarization in neurons of inferior mesenteric ganglia from guinea pigs. This response was not blocked by cholinergic antagonists but was specifically and reversibly inhibited by a substance P analog, (D-Pro2, D-Phe7, D-Trp9)-substance P, which also depressed the depolarization induced by exogenously applied substance P. The atropine-sensitive slow excitatory and slow inhibitory postsynaptic potentials evoked in neurons of rabbit superior cervical ganglia were not affected by the substance P analog. These and previous results provide strong support for the hypothesis that substance P or a closely related peptide is the transmitter mediating the slow depolarization. The latter may represent a sensory input from the gastrointestinal tract to neurons of the prevertebral ganglia.

Transmission through distributed synapses between two giant axons of a sabellid worm

1. 1. Transmission between two giant axons of the sabellid polychaete worm, Eudistylia polymorpha, was studied with intracellular recording and polarization techniques. 2. 2. The spike potential of the giant axon in 100–115 mV and is produced when the resting potential of 60–70 mV is reduced by 10–15 mV. The giant axon had a membrane resistance of 1·5–4·5 kω.cm 2, membrane capacity of 2–3 μF/cm 2 and internal resistance of 45–65 ω.cm. 3. 3. A spike potential initiated in one axon always results in a potential in the other axon which resembles a synaptic potential and sometimes gives rise to a postsynaptic spike; the synapses are distributed along the whole length of the axon. 4. 4. The rise of the synaptic potential has a delay of 3·3–0·5 msec from the peak of the presynaptic spike when observed in the same segment of the worm. 5. 5. A progressive depression of the synaptic potential during repetitive presynaptic stimulation (fatique) in one direction dose not affect the transmission in the opposite direction. 6. 6. Presynaptic hyperpolarization results in a depression of the synaptic potential. 7. 7. Electrotonic spread from one axon to the other is found to be inadequate to explain the synaptic potential. It is therefore concluded that we have to deal with numerous synapses involving unidirectional chemical transmission, some transmitting from left to right axons and others from right to left. 8. 8. Because of the presence of the direct anastomosis in the head the distributed synapses would seem to have limited biological meaning for the usual anterior excitation. Since larger synaptic potentials are more often found as the axon approaches the caudal end of the worm the synapses might be important in case of posterior excitation.

On the integrative synaptic potentials of Onchidium nerve cell.

1. Excitatory postsynaptic potentials (e. p.s. p.'s) of the giant nerve cell system of Onchidium verruculatum were investigated by intracellular electrode technique.2. An impulse of a single presynaptic fibre was usually too small to initiate a postsynaptic spike and spacial and temporal summation was necessary to produce the postsynaptic spike.3. The increase of calcium or magnesium concentration in the external medium increased or decreased the amplitude of the e. p.s. p.respectively but the maximum amplitude of the synaptic potential reached by repetitive presynaptic stimulation was practically unchanged.4. From the above results the temporal facilitation is produced by increasing the active area of the synaptic membrane innervated by a single presynaptic fibre.

Synaptic potential in the motor giant axon of the crayfish.

Some electrical properties of the synapses between central giant axons (presynaptic) and the motor giant axon (postsynaptic) of the crayfish abdominal nerve cord have been investigated. Postsynaptic potential change in response to presynaptic volleys contains two components: a spike potential and a synaptic potential of very long time course. Amplitude of the synaptic potential is graded according to the number of active presynaptic axons. Conductance increase in the synaptic membrane endures over most of the period of potential change, and it is this rather than the "electrical time constant" of the membrane that in large measure determines the form of the synaptic potential. Temporal summation of synaptic potential occurs during repetitive presynaptic stimulation, and after such stimulation the rate of decay of synaptic potential is greatly slowed.