Synaptic transmission in the sixth ganglion of the cockroach: action of 4-aminopyridine.

1. Study was made of the action of 4-aminopyridine (5 X 10(-5) M) on synaptic transmission in the last abdominal ganglion of Periplaneta americana. The 'oil-gap' technique was used to record postsynaptic events in a single giant axon. 2. 4-AP quickly increased the 'background' of postsynaptic activity, which consisted of 'spontaneous' unitary EPSPs and IPSPs. Postsynaptic spikes were also propagated. 3. Both evoked EPSPs (stimulation of cercal nerve XI) and evoked IPSPs (stimulation of cercal nerve X) were greatly increased in amplitude although their duration (half-time) was unaltered. 4. 4-AP triggered presynaptic action potentials in the cercal nerves (recorded with external electrodes). These 'antidromic' potentials appeared singly or sometimes repetitively, especially after electrical stimulation of the cercal nerves. They were often in monosynaptic correlation with unitary EPSPs. 5. Neither the resting potential nor the postsynaptic membrane resistance was modified. 6. There were no changes in the equilibrium potentials of the ions involved in postsynaptic events. 7. The results may be essentially explained by an increase in transmitter release after 4-AP treatment, which may be partly the result of a rise in presynaptic terminal excitability, and partly the result of a lengthening of the presynaptic action potentials.     

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus.

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.     

Maintained depolarization of synaptic terminals facilitates nerve-evoked transmitter release at a crayfish neuromuscular junction

Presynaptic and postsynaptic potentials were examined by intracellular recording at a crayfish neuromuscular junction. During normal synaptic transmission, the action potentials were recorded in the terminal region of the excitatory axon and postsynaptic responses were obtained in the muscle fibers. We found that it was possible to modify the synaptic transmission by applying depolarizing or hyperpolarizing currents through the presynaptic intracellular electrode. Typically, a 7-15 mV depolarization lasting longer than 50 msec leads to a large (500%) enhancement of transmitter release, even though the preterminal action potential is reduced in amplitude. Hyperpolarization increases the amplitude of the action potential, but slightly reduces the transmitter release. These results are different from those reported for other neuromuscular synapses and the squid giant synapse, but are similar in many respects to the results reported for several invertebrate central synapses. We conclude, first, that different synapses may have markedly different responses to conditioning by membrane polarization and, secondly, that maintained low-level depolarization may induce a potentiated state in the nerve terminal, perhaps brought about by slow entry of calcium.     

Intracellular recording from the giant synapse of the squid

1. Recording with glass micropipette electrodes inserted close to the synaptic region, in the presynaptic and in the postsynaptic fibers of the giant synapse in the stellate ganglion of the squid, has been accomplished. 2. The forms of the spike and of the synaptic potential are very much like those reported earlier (Bullock, 1948) from macroelectrodes. The crest time and the rate of fall are labile and depend on the state of fatigue, though the time of initiation of the postsynaptic potential does not. 3. It is concluded after examination of both intra- and extracellular recordings that there is a real synaptic delay of the order of 1 or 2 milliseconds at 15–20°C. 4. There is sometimes a very small and sometimes no visible deflection in the intracellular postsynaptic record attributable to the presynaptic spike. It is concluded that transmission cannot be electrical. 5. The amplitude of the postsynaptic potential can be controlled over some range by the amplitude of the presynaptic potential. 6. Hyperpolarization of the postsynaptic membrane results in increase in amplitude of spikes up to 200 millivolts, in increase in the membrane potential level at which the spike flares up, but in no considerable change in the amplitude in postsynaptic potential. 7. The postsynaptic potential can add to the late falling phase and the undershoot of an antidromic spike in the postfiber but cannot add to the crest or early part of the falling phase. The earliest part of the antidromic spike during which the postsynaptic potential can add is probably a period of refractoriness to electrical shock by analogy with the properties of the axon.     

Effect of guanidine on the squid giant synapse

Effects of guanidine on pre- and postsynaptic activities in the untreated or tetrodotoxin-treated squid giant synapses were examined by externally perfusing with various concentrations (423 mM, 42 mM, 21 mM, and 4.2 mM), or by iontophoretic injection of guanidine into the presynaptic terminal. In 423 mM guanidine (Na-free), the pre- and postsynaptic spikes together with PSP were completely abolished. In concentrations of 42 mM or lower of guanidine media the following changes related to the concentration used were observed: reduction of delayed rectification of both axon membranes without significant alteration of resting membrane resistances; a few millivolts decrease in the resting membrane potentials; small decrease in amplitude of pre- and post-synaptic spikes without marked increase of spike duration; enhancement of synaptic activity as manifested by increases in the amplitude and duration of the PSP. Iontophoretically injected guanidine also reduced delayed rectification of the presynaptic membrane. Input-output relation was modified in a way similar to externally applied guanidine and an “Off-PSP” was demonstrated shortly after application of an inside positive presynaptic polarization. Thus, a comparison of the augmentation of synaptic transmission by the extracellular and intracellularly applied guanidine demonstrates that the primary effect is at the presynaptic terminal.     

Correlation of Transmitter Release with Membrane Properties of the Presynaptic Fiber of the Squid Giant Synapse

Depolarization of the presynaptic terminal by current produced a postsynaptic potential (PSP) which increased with increasing presynaptic polarization and then reached a plateau. Iontophoretic injection of tetraethylammonium ions (TEA) into the presynaptic axon near the terminal produced a prolonged presynaptic spike. The resulting PSP is increased in size and its time course closely followed that of the presynaptic spike. The presynaptic fiber no longer exhibited rectification and strong depolarizations revealed that the PSP reached a maximum with about 110 mv depolarization. Further depolarization produced a decrease in PSP amplitude and finally transmission was blocked. However, a PSP then always appeared on withdrawal of the depolarizing current. Under the conditions of these experiments, the PSP could be considered a direct measure of transmitter release. Bathing the TEA-injected synapse with concentrations of tetrodotoxin (TTX) sufficient to block spike activity in both pre- and postsynaptic axons did not greatly modify postsynaptic electrogenesis. However, doubling TTX concentration reversibly blocked PSP. Thus the permeability changes to Na and K accompanying the spike do not appear necessary for transmitter release. Some other processes related to the level of presynaptic polarization must be involved to explain the data. The inhibition of transmitter release by strong depolarizations appears to be related to Ca action. A membrane Ca current may also be necessary for normal transmitter release.     

Bilateral processing in chemical synapses with electrical 'ephaptic' feedback: a theoretical model

I have developed a detailed biophysical model of the chemical synapse which hosts voltage-dependent presynaptic ion channels and takes into account the capacitance of synaptic membranes. I find that at synapses with a relatively large cleft resistance (e.g., mossy fiber or giant calyx synapse) the rising postsynaptic current could activate, within the synaptic cleft, electrochemical phenomena that induce rapid widening of the presynaptic action potential (AP). This mechanism could boost fast Ca(2+) entry into the terminal thus increasing the probability of subsequent synaptic releases. The predicted difference in the AP waveforms generated inside and outside the synapse can explain the previously unexplained fast capacitance transient recorded in the postsynaptic cell at the giant calyx synapse. I propose therefore the mechanism of positive ephaptic feedback that acts between the postsynaptic and presynaptic cell contributing to the basal synaptic transmission at large central synapses. This mechanism could also explain the supralinear voltage dependence of EPSCs recorded at hyperpolarizing membrane potentials in low extracellular calcium concentration.     

Computation of action potential propagation and presynaptic bouton activation in terminal arborizations of different geometries.

Action potential propagation in axons with bifurcations involving short collaterals with synaptic boutons has been simulated using SPICE, a general purpose electrical circuit simulation program. The large electrical load of the boutons may lead to propagation failure at otherwise uncritical geometric ratios. Because the action potential gradually fails while approaching the branch point, the electrotonic spread of the failing action potential cannot depolarize the terminal boutons above an assumed threshold of 20 mV (Vrest = 0 mV) for the presynaptic calcium inflow, and therefore fails to evoke transmitter release even for boutons attached at short collaterals. For even shorter collaterals the terminal boutons can again be activated by the spread of passive current reflected at the sealed end of the bouton which increases the membrane potential above firing threshold. The action potential is then propagated in anterograde fashion into the main axon and may activate the terminal bouton on the other collateral. Differential activation of the synaptic boutons can be observed without repetitive activation of the main axon and with the assumption of uniform membrane properties. Axon enlargements above a critical size at branch points can increase the safety factor for propagation significantly and may serve a double function: they can act both as presynaptic boutons and as boosters, facilitating invasion of the action potential into the terminal arborizations. The architecture of the terminal arborizations has a profound effect on the activation pattern of synapses, suggesting that terminal arborizations not only distribute neural information to postsynaptic cells but may also be able to process neural information presynaptically.     

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.     

突触前α7烟碱受体对海马神经元兴奋性突触传递的调控

采用盲法膜片钳技术观察突触前烟碱受体(nicotinic acetylcholinel receptors,nAChRs)对海马脑片CAl区锥体神经元兴奋性突触传递的调控作用。结果显示,nAChRs激动剂碘化二甲基苯基哌嗪(dimethylphenyl—piperazinium iodide,DMPP)不能在CAl区锥体神经元上诱发出烟碱电流。DMPP对CAl区锥体神经元自发兴奋性突触后电流(spontaneous excitatory postsynaptic current,sEPSC)具有明显的增频和增幅作用,并呈现明显的浓度依赖关系。DMPP对微小兴奋性突触后电流(miniature excitatory postsynaptic current,mEPSC)具有增频作用,但不具有增幅作用。上述DMPP增强突触传递的作用不能被nAChRs拮抗剂美加明、六烃季铵和双氢-β-刺桐丁所阻断,但可被α-银环蛇毒素阻断。上述结果提示,海马脑片CAl区锥体神经元兴奋性突触前nAChRs含有对α-银环蛇毒素敏感的胡亚单位,其激活可增强海马CAl区锥体神经元突触前递质谷氨酸的释放,从而对兴奋性突触传递发挥调控作用。     

Activity-dependent remodeling of presynaptic inputs by postsynaptic expression of activated CaMKII

Competitive synaptic remodeling is an important feature of developmental plasticity, but the molecular mechanisms remain largely unknown. Calcium/calmodulin-dependent protein kinase II (CaMKII) can induce postsynaptic changes in synaptic strength. We show that postsynaptic CaMKII also generates structural synaptic rearrangements between cultured cortical neurons. Postsynaptic expression of activated CaMKII (T286D) increased the strength of transmission between pairs of pyramidal neuron by a factor of 4, through a modest increase in quantal amplitude and a larger increase in the number of synaptic contacts. Concurrently, T286D reduced overall excitatory synaptic density and increased the proportion of unconnected pairs. This suggests that connectivity from some synaptic partners was increased while other partners were eliminated. The enhancement of connectivity required activity and NMDA receptor activation, while the elimination did not. These data suggest that postsynaptic activation of CaMKII induces a structural remodeling of presynaptic inputs that favors the retention of active presynaptic partners.     

Synaptic interactions between nonspiking local interneurones in the terminal abdominal ganglion of the crayfish

Nonspiking local interneurones are the important premotor elements in arthropod motor control systems. We have analyzed the synaptic interactions between nonspiking interneurones in the crayfish terminal (6th) abdominal ganglion using simultaneous intracellular recordings. Only 15% of nonspiking interneurones formed bi-directional excitatory connections. In 77% of connections, however, the nonspiking interneurones showed a one-way inhibitory interaction. In these cases, the presynaptic nonspiking interneurones received excitatory synaptic inputs from the sensory afferents innervating hairs on the surface of the uropods and the postsynaptic nonspiking interneurones received inhibitory synaptic inputs that were partly mediated by the inputs to the presynaptic nonspiking interneurones. The membrane hyperpolarization of the postsynaptic nonspiking interneurones mediated by the presynaptic nonspiking interneurones was reduced in amplitude when the hyperpolarizing current was injected into the postsynaptic interneurones, or when the external bathing solution was replaced with one containing low calcium and high magnesium concentrations. The role of these interactions in the circuits controlling the movements of the terminal appendages is discussed.Abbreviations AL antero-lateral - epsp excitatory postsynaptic potential - ipsp inhibitory postsynaptic potential - PL postero-lateral     

Potentiating effect of allatostatin on transmitter quantal secretion in the mouse nerve-muscle synapse

Action of allatostatin on the spontaneous and evoked quantal acetylcholine secretion was studied for the first time in the mouse nerve-muscle synapse. End plate miniature potentials (EPMP) and miniature currents (EPMC) as well as end plate single evoked currents (SEP) were recorded in mouse semidiaphragm. Allatostatin (1 nm–1 μM) produced a dose-dependent increase of the EPMP amplitude (that reached 209% of control at 1 μM of peptide), but without affecting statistically significantly the EPMP frequency and membrane potential of muscle fibers throughout the entire range of its concentrations. The potentiating action of 1 μM peptide on the EPMP amplitude was accompanied by a rise of time of the EPMP ascent and semidrop (by 17 and 13%, respectively). Allatostatin (1 μM) caused a twofold rise of EPMC amplitude, but the time parameters of miniature postsynaptic currents did not change statistically significantly. Amplitude of SEP also increased more than twice under effect of 1 μM peptide, but the SEP quantal composition remained at the control level. On the background of allatostatin there were revealed no rise of the postsynaptic membrane input resistance (on the contrary, it decreased by 25%) and no changes of the EPMC potential-dependent amplitude and of the droptime constant that characterize cholinoreceptor conductivity. The potentiating allatostatin effect on EPMP amplitude was prevented by vesamicol (1 μM), a blocker of transport of acetylcholine into synaptic vesicles. Preliminary treatment of the nerve-muscle preparation with the inhibitor of protein kinase A (PKA) H-89 (50 nM) prevented the allatostatin-evoked EPMC amplitude increment. The obtained data allow us to suggest that allatostatin in the mouse nerve-muscle synapse acts at the presynaptic level by producing an increase of the acetylcholine quantum size due to an intraterminal cascade of reaction with participation of PKA.     

BDNF increases synapse density in dendrites of developing tectal neurons in vivo

Neuronal connections are established through a series of developmental events that involve close communication between pre- and postsynaptic neurons. In the visual system, BDNF modulates the development of neuronal connectivity by influencing presynaptic retinal ganglion cell (RGC) axons. Increasing BDNF levels in the optic tectum of Xenopus tadpoles significantly increases both axon arborization and synapse density per axon terminal within a few hours of treatment. Here, we have further explored the mechanisms by which BDNF shapes synaptic connectivity by imaging tectal neurons, the postsynaptic partners of RGCs. Individual neurons were co-labeled with DsRed2 and a GFP-tagged postsynaptic density protein (PSD95-GFP) to visualize dendritic morphology and postsynaptic specializations simultaneously in vivo. Immunoelectron microscopy confirmed that PSD95-GFP predominantly localized to ultrastructurally identified synapses. Time-lapse confocal microscopy of individual, double-labeled neurons revealed a coincident, activity-dependent mechanism of synaptogenesis and axon and dendritic arbor growth, which is differentially modulated by BDNF. Microinjection of BDNF into the optic tectum significantly increased synapse number in tectal neuron dendritic arbors within 24 hours, without significantly influencing arbor morphology. BDNF function-blocking antibodies had opposite effects. The BDNF-elicited increase in synapse number complements the previously observed increase in presynaptic sites on RGC axons. These results, together with the timescale of the response by tectal neurons, suggest that the effects of BDNF on dendritic synaptic connectivity are secondary to its effects on presynaptic RGCs. Thus, BDNF influences synaptic connectivity in multiple ways: it enhances axon arbor complexity expanding the synaptic territory of the axon, while simultaneously coordinating synapse formation and stabilization with individual postsynaptic cells.     

Synaptic localization of adrenergic disinhibition in the rat hippocampus.

Norepinephrine is an endogenous neurotransmitter that reduces synaptic inhibition onto pyramidal neurons in the hippocampus by an action at an alpha-adrenergic receptor. The physiological mechanism of this disinhibition was previously not known, except that it occurred at a site presynaptic to the inhibited pyramidal cell. In this paper we present evidence that adrenergic disinhibition is restricted to the early phase of the evoked inhibitory postsynaptic potential in area CA1 of the hippocampus. The locus of disinhibition does not appear to reside in the interneuronal terminal, axon, or cell body. Instead, adrenergic agonists appear to reduce evoked synaptic inhibition by depressing excitatory synapses that activate the interneuron.     

Presynaptic effects of biogenic amines modulating synaptic transmission between identified sensory neurons and giant interneurons in the first instar cockroach

Intracellular recording was used to investigate the modulatory effects of serotonin and octopamine on the identified synapses between filiform hair sensory afferents and giant interneurons in the first instar cockroach, Periplaneta americana. Serotonin at 10(-4) mol l(-1) to 10(-3) mol l(-1) reduced the amplitude of the lateral axon-to-ipsilateral giant interneuron 3 excitatory postsynaptic potentials. and octopamine at 10(-4) mol l(-1) increased their amplitude. Similar effects were seen on excitatory postsynaptic potentials in dorsal giant interneuron 6. Several lines of evidence suggest that both substances modulate the amplitude of excitatory postsynaptic potentials by acting presynaptically, rather than on the postsynaptic neuron. The fitting of simple binomial distributions to the postsynaptic potential amplitude histograms suggested that, for both serotonin and octopamine, the number of synaptic release sites was being modulated. Secondly, the amplitudes of miniature excitatory postsynaptic potentials recorded in the presence of tetrodotoxin were unaffected by either modulator. Finally, recordings from contralateral giant interneuron 3, which has two identifiable populations of synaptic inputs, showed that each modulator had a more pronounced effect on excitatory postsynaptic potentials evoked by the lateral axon than on those evoked by the medial axon. Immunocytochemistry confirmed that neuropilar processes containing serotonin are present in close proximity to these synapses.     

The quantal release at a neuro-neuronal synapse is regulated by the content of acetylcholine in the presynaptic cell

Transmitter release was studied with respect to the presynaptic acetylcholine (ACh) content at a central identified inhibitory synapse (Cl- conductance) of Aplysia californica. Statistical analysis of the synaptic noise evoked by sustained depolarization of the presynaptic neuron allowed us to calculate the quantal parameters of the postsynaptic responses. Loading of the presynaptic neurone with injected ACh led to an increase in the postsynaptic responses whereas the calculated miniature postsynaptic current (MPSC) was unmodified. Destruction of choline by choline oxidase either applied extracellularly and coupled to intense stimulations of the presynaptic cell or injected into the presynaptic neuron induced a depression of the postsynaptic response although the amplitude of the calculated MPSC remained constant. As the size of the MPSC, i.e. the size of the quantum, did not change in these experiments, it was concluded that the presynaptic ACh content controls the number of quanta released by a given presynaptic depolarization. As additional evidence, effects of abrupt increase in tonicity of the external medium were studied. The observed transient enhancement of the quantal content of the postsynaptic response could be attributed to an increase in the presynaptic concentration of ACh, resulting from the reduction in cellular volume.     

Long-term potentiation of neuronal glutamate transporters

Persistent, use-dependent modulation of synaptic strength has been demonstrated for fast synaptic transmission mediated by glutamate and has been hypothesized to underlie persistent behavioral changes ranging from memory to addiction. Glutamate released at synapses is sequestered by the action of excitatory amino acid transporters (EAATs) in glia and postsynaptic neurons. So, the efficacy of glutamate transporter function is crucial for regulating glutamate spillover to adjacent presynaptic and postsynaptic receptors and the consequent induction of plastic or excitotoxic processes. Here, we report that tetanic stimulation of cerebellar climbing fiber-Purkinje cell synapses results in long-term potentiation (LTP) of a climbing fiber-evoked glutamate transporter current recorded in Purkinje cells. This LTP is postsynaptically expressed and requires activation of an mGluR1/PKC cascade. Together with a simultaneously induced long-term depression (LTD) of postsynaptic AMPA receptors, this might reflect an integrated antiexcitotoxic cellular response to strong climbing fiber synaptic activation, as occurs following an ischemic episode.     

Effects of ethimizol on frog neuromuscular junction

The effects were studied of ethimizol, a substance activating memory processes, on features of synaptic transmission during experiments on frog cutaneous pectoris muscle. It was found that the presynaptic action of ethimizol consists of raising the frequency of miniature potentials, when used at a concentration of 0.5–10 mM, and modulating quantal content of synaptic transmission due to changes in binomial quantal release parameters p and n when 0.5–2 mM ethimizol was used. This substance facilitated transmission at synapses with a low initial level of transmitter release. This substance facilitated transmission at synapses with a low initial level of transmitter release. Ethimizol was also found to have a postsynaptic action, consisting of reducing amplitude at a concentration of 5–10 mM and prolonging synaptic currents and potentials when concentrations of 0.5–10 mM were used. The latter effect produced a considerable increase in the time integral of endplate potentials. The postsynaptic action of ethimizol is perhaps seen in its effects on features of postsynaptic ionic channels. The effects of ethimizol are discussed with a view to how it may act within the central nervous system as a nonspecific modulator.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 17, No. 6, pp. 757–763, November–December, 1985.     

Hepoxilin A3 blocks the release of norepinephrine from rat hippocampal slices

Hepoxilin A3 was previously shown to display neuromodulatory actions on rat hippocampal CA1 neurons, with hyperpolarization of the membrane potential, an increase in the amplitude and duration of the post-spike train after hyperpolarization and an increase in the inhibitory post synaptic potential. The present report describes new biochemical evidence of a presynaptic action of hepoxilin A3 in rat hippocampal slices prelabeled with [3H]-norepinephrine. Hepoxilin A3 on its own had a marginal effect on the release of label, but blocked release which was induced by 4-aminopyridine (4-AP). Prostaglandin E2 also behaved in a similar way. These results demonstrate that hepoxilins modulate neurotransmission in the mammalian CNS through both pre- and postsynaptic actions.