Multiple interneuronal afferents to the giant cells in Aplysia

Background calcium induced by subthreshold depolarization modifies homosynaptic facilitation at a synapse in Aplysia

Some synapses show two forms of short-term plasticity, homosynaptic facilitation, and a plasticity in which the efficacy of transmission is modified by subthreshold changes in the holding potential of the presynaptic neuron. In a previous study we demonstrated a further interactive effect. We showed that depolarizing changes in the presynaptic holding potential can increase the rate at which facilitation occurs. These experiments studied synaptic transmission between an Aplysia sensory neuron (B21) and its postsynaptic follower, the motor neuron (B8). We have also shown that subthreshold depolarizations of B21 produce widespread increases in its [Ca²⁺]_i via activation of a nifedipine-sensitive current. To determine whether it is this change in ‘background’ calcium that modifies synaptic transmission we compared the facilitation observed at the B21-B8 synapse under control conditions to the facilitation observed in nifedipine. Nifedipine had a depressing effect. Other investigators studying facilitation have focused on Ca_res (i.e., the calcium that remains in a neuron after spiking). Our results indicate that facilitation can also be impacted by calcium channels opened before spiking begins.

Past and Future of Analog-Digital Modulation of Synaptic Transmission

Action potentials (APs) are generally produced in response to complex summation of excitatory and inhibitory synaptic inputs. While it is usually considered as a digital event, both the amplitude and width of the AP are significantly impacted by the context of its emission. In particular, the analog variations in subthreshold membrane potential determine the spike waveform and subsequently affect synaptic strength, leading to the so-called analog-digital modulation of synaptic transmission. We review here the numerous evidence suggesting context-dependent modulation of spike waveform, the discovery analog-digital modulation of synaptic transmission in invertebrates and its recent validation in mammals. We discuss the potential roles of analog-digital transmission in the physiology of neural networks.

Dynamic Control of Neurotransmitter Release by Presynaptic Potential

Action potentials (APs) in the mammalian brain are thought to represent the smallest unit of information transmitted by neurons to their postsynaptic targets. According to this view, neuronal signaling is all-or-none or digital. Increasing evidence suggests, however, that subthreshold changes in presynaptic membrane potential before triggering the spike also determines spike-evoked release of neurotransmitter. We discuss here how analog changes in presynaptic voltage may regulate spike-evoked release of neurotransmitter through the modulation of biophysical state of voltage-gated potassium, calcium and sodium channels in the presynaptic compartment. The contribution of this regulation has been greatly underestimated and we discuss the impact for information processing in neuronal circuits.

Presynaptic hyperpolarization induces a fast analogue modulation of spike-evoked transmission mediated by axonal sodium channels

In the mammalian brain, synaptic transmission usually depends on presynaptic action potentials (APs) in an all-or-none (or digital) manner. Recent studies suggest, however, that subthreshold depolarization in the presynaptic cell facilitates spike-evoked transmission, thus creating an analogue modulation of a digital process (or analogue–digital (AD) modulation). At most synapses, this process is slow and not ideally suited for the fast dynamics of neural networks. We show here that transmission at CA3–CA3 and L5–L5 synapses can be enhanced by brief presynaptic hyperpolarization such as an inhibitory postsynaptic potential (IPSP). Using dual soma–axon patch recordings and live imaging, we find that this hyperpolarization-induced AD facilitation (h-ADF) is due to the recovery from inactivation of Nav channels controlling AP amplitude in the axon. Incorporated in a network model, h-ADF promotes both pyramidal cell synchrony and gamma oscillations. In conclusion, cortical excitatory synapses in local circuits display hyperpolarization-induced facilitation of spike-evoked synaptic transmission that promotes network synchrony.

'Digital' spike-evoked transmission can be facilitated by slow subthreshold 'analogue' depolarisation of the presynaptic neuron. Here, the authors identify a novel, rapid form of digital-analogue facilitation in mammalian neurons whereby presynaptic hyperpolarisation enables de-inactivation of axonal Nav channels.

Modulation of spike-evoked synaptic transmission: The role of presynaptic calcium and potassium channels

Action potentials are usually considered as the smallest unit of neuronal information conveyed by presynaptic neurons to their postsynaptic target. Thus, neuronal signaling in brain circuits is all-or-none or digital. However, recent studies indicate that subthreshold analog variation in presynaptic membrane potential modulates spike-evoked transmission. The informational content of each presynaptic action potential is therefore greater than initially expected. This property constitutes a form of fast activity-dependent modulation of functional coupling. Therefore, it could have important consequences on information processing in neural networks in parallel with more classical forms of presynaptic short-term facilitation based on repetitive stimulation, modulation of presynaptic calcium or modifications of the release machinery. We discuss here how analog voltage shift in the presynaptic neuron may regulate spike-evoked release of neurotransmitter through the modulation of voltage-gated calcium and potassium channels in the axon and presynaptic terminal. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Action potential modulation in CA1 pyramidal neuron axons facilitates OLM interneuron activation in recurrent inhibitory microcircuits of rat hippocampus

Oriens-lacunosum moleculare (O-LM) interneurons in the CA1 region of the hippocampus play a key role in feedback inhibition and in the control of network activity. However, how these cells are efficiently activated in the network remains unclear. To address this question, I performed recordings from CA1 pyramidal neuron axons, the presynaptic fibers that provide feedback innervation of these interneurons. Two forms of axonal action potential (AP) modulation were identified. First, repetitive stimulation resulted in activity-dependent AP broadening. Broadening showed fast onset, with marked changes in AP shape following a single AP. Second, tonic depolarization in CA1 pyramidal neuron somata induced AP broadening in the axon, and depolarization-induced broadening summated with activity-dependent broadening. Outside-out patch recordings from CA1 pyramidal neuron axons revealed a high density of α-dendrotoxin (α-DTX)-sensitive, inactivating K⁺ channels, suggesting that K⁺ channel inactivation mechanistically contributes to AP broadening. To examine the functional consequences of axonal AP modulation for synaptic transmission, I performed paired recordings between synaptically connected CA1 pyramidal neurons and O-LM interneurons. CA1 pyramidal neuron–O-LM interneuron excitatory postsynaptic currents (EPSCs) showed facilitation during both repetitive stimulation and tonic depolarization of the presynaptic neuron. Both effects were mimicked and occluded by α-DTX, suggesting that they were mediated by K⁺ channel inactivation. Therefore, axonal AP modulation can greatly facilitate the activation of O-LM interneurons. In conclusion, modulation of AP shape in CA1 pyramidal neuron axons substantially enhances the efficacy of principal neuron–interneuron synapses, promoting the activation of O-LM interneurons in recurrent inhibitory microcircuits.

Analog modulation of spike-evoked transmission in CA3 circuits is determined by axonal Kv1.1 channels in a time-dependent manner

Synaptic transmission usually depends on action potentials (APs) in an all-or-none (digital) fashion. Recent studies indicate, however, that subthreshold presynaptic depolarization may facilitate spike-evoked transmission, thus creating an analog modulation of spike-evoked synaptic transmission, also called analog-digital (AD) synaptic facilitation. Yet, the underlying mechanisms behind this facilitation remain unclear. We show here that AD facilitation at rat CA3-CA3 synapses is time-dependent and requires long presynaptic depolarization (5-10 s) for its induction. This depolarization-induced AD facilitation (d-ADF) is blocked by the specific Kv1.1 channel blocker dendrotoxin-K. Using fast voltage-imaging of the axon, we show that somatic depolarization used for induction of d-ADF broadened the AP in the axon through inactivation of Kv1.1 channels. Somatic depolarization enhanced spike-evoked calcium signals in presynaptic terminals, but not basal calcium. In conclusion, axonal Kv1.1 channels determine glutamate release in CA3 neurons in a time-dependent manner through the control of the presynaptic spike waveform.

What are the mechanisms for analogue and digital signalling in the brain?

Synaptic transmission in the brain generally depends on action potentials. However, recent studies indicate that subthreshold variation in the presynaptic membrane potential also determines spike-evoked transmission. The informational content of each presynaptic action potential is therefore greater than initially expected. The contribution of this synaptic property, which is a fast (from 0.01 to 10 s) and state-dependent modulation of functional coupling, has been largely underestimated and could have important consequences for our understanding of information processing in neural networks. We discuss here how the membrane voltage of the presynaptic terminal might modulate neurotransmitter release by mechanisms that do not involve a change in presynaptic Ca(2+) influx.

Somatic depolarization enhances GABA release in cerebellar interneurons via a calcium/protein kinase C pathway

In cortical and hippocampal neurons, tonic somatic depolarization is partially transmitted to synaptic terminals, where it enhances transmitter release. It is not known to what extent such "analog signaling" applies to other mammalian neurons, and available evidence concerning underlying mechanisms is fragmentary and partially controversial. In this work, we investigate the presence of analog signaling in molecular layer interneurons of the rat cerebellum. GABA release was estimated by measuring autoreceptor currents in single recordings, or postsynaptic currents in paired recordings of synaptically connected neurons. We find with both assays that moderate subthreshold somatic depolarization results in enhanced GABA release. In addition, changes in the calcium concentration were investigated in the axon compartment using the calcium-sensitive dye OGB-1 (Oregon Green BAPTA-1). After a step somatic depolarization, the axonal calcium concentration and the GABA release probability rise with a common slow time course. However, the amount of calcium entry that is associated to one action potential is not affected. The slow increase in calcium concentration is inhibited by the P/Q calcium channel blocker ω-agatoxin-IVA. The protein kinase C inhibitor Ro 31-8220 (3-[3-[2,5-dihydro-4-(1-methyl-1H-indol-3-yl)-2,5-dioxo-1H-pyrrol-3-yl]-1H-indol-1-yl]propyl carbamimidothioic acid ester mesylate) did not affect the calcium concentration changes but it blocked the increase in GABA release. EGTA was a weak blocker of analog signaling, implicating a close association of protein kinase C to the site of calcium entry. We conclude that analog signaling is prominent in cerebellar interneurons and that it is triggered by a pathway involving activation of axonal P/Q channels, followed by calcium entry and local activation of protein kinase C.

Effect of presynaptic membrane potential on electrical vs. chemical synaptic transmission

The growing realization that electrical coupling is present in the mammalian brain has sparked renewed interest in determining its functional significance and contrasting it with chemical transmission. One question of interest is whether the two types of transmission can be selectively regulated, e.g., if a cell makes both types of connections can electrical transmission occur in the absence of chemical transmission? We explore this issue in an experimentally advantageous preparation. B21, the neuron we study, is an Aplysia sensory neuron involved in feeding that makes electrical and chemical connections with other identified cells. Previously we demonstrated that chemical synaptic transmission is membrane potential dependent. It occurs when B21 is centrally depolarized prior to and during peripheral activation, but does not occur if B21 is peripherally activated at its resting membrane potential. In this article we study effects of membrane potential on electrical transmission. We demonstrate that maximal potentiation occurs in different voltage ranges for the two types of transmission, with potentiation of electrical transmission occurring at more hyperpolarized potentials (i.e., requiring less central depolarization). Furthermore, we describe a physiologically relevant type of stimulus that induces both spiking and an envelope of depolarization in the somatic region of B21. This depolarization does not induce functional chemical synaptic transmission but is comparable to the depolarization needed to maximally potentiate electrical transmission. In this study we therefore characterize a situation in which electrical and chemical transmission can be selectively controlled by membrane potential.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

Two distinct mechanisms mediate potentiating effects of depolarization on synaptic transmission

Two distinct mechanisms mediate potentiating effects of depolarization on synaptic transmission. Recently there has been renewed interest in a type of plasticity in which a neuron's somatic membrane potential influences synaptic transmission. We study mechanisms that mediate this type of control at a synapse between a mechanoafferent, B21, and B8, a motor neuron that receives chemical synaptic input. Previously we demonstrated that the somatic membrane potential determines spike propagation within B21. Namely, B21 must be centrally depolarized if spikes are to propagate to an output process. We now demonstrate that this will occur with central depolarizations that are only a few millivolts. Depolarizations of this magnitude are not, however, sufficient to induce synaptic transmission to B8. B21-induced postsynaptic potentials (PSPs) are only observed if B21 is centrally depolarized by >or=10 mV. Larger depolarizations have a second impact on B21. They induce graded changes in the baseline intracellular calcium concentration that are virtually essential for the induction of chemical synaptic transmission. During motor programs, subthreshold depolarizations that increase calcium concentrations are observed during one of the two antagonistic phases of rhythmic activity. Chemical synaptic transmission from B21 to B8 is, therefore, likely to occur in a phase-dependent manner. Other neurons that receive mechanoafferent input are electrically coupled to B21. Differential control of spike propagation and chemical synaptic transmission may, therefore, represent a mechanism that permits selective control of afferent transmission to different types of neurons contacted by B21. Afferent transmission to neurons receiving chemical synaptic input will be phase specific, whereas transmission to electrically coupled followers will be phase independent.

Invertebrate preparations and their contribution to neurobiology in the second half of the 20th century

This review summarized the contribution to neurobiology achieved through the use of invertebrate preparations in the second half of the 20th century. This fascinating period was preceded by pioneers who explored a wide variety of invertebrate phyla and developed various preparations appropriate for electrophysiological studies. Their work advanced general knowledge about neuronal properties (dendritic, somatic, and axonal excitability; pre- and postsynaptic mechanisms). The study of invertebrates made it possible to identify cell bodies in different ganglia, and monitor their operation in the course of behavior. In the 1970s, the details of central neural circuits in worms, molluscs, insects, and crustaceans were characterized for the first time and well before equivalent findings were made in vertebrate preparations. The concept and nature of a central pattern generator (CPG) have been studied in detail, and the stomatogastric nervous system (STNS) is a fine example, having led to many major developments since it was first examined. The final part of the review is a discussion of recent neuroethological studies that have addressed simple cognitive functions and confirmed the utility of invertebrate models. After presenting our invertebrate "mice," the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, our conclusion, based on arguments very different from those used fifty years ago, is that invertebrate models are still essential for acquiring insight into the complexity of the brain.

Neural coding: hybrid analog and digital signalling in axons

Mammalian axons are thought to act as digital signaling devices, conveying information only by the timing and rate of all-or-none action potentials. Two recent studies now show that synaptic potentials can also spread far down the axon and influence action potential-triggered transmitter release in a graded, 'analog' manner. Axons thus encode information both about subthreshold and suprathreshold synaptic activity.

Modulation of intracortical synaptic potentials by presynaptic somatic membrane potential

Traditionally, neuronal operations in the cerebral cortex have been viewed as occurring through the interaction of synaptic potentials in the dendrite and soma, followed by the initiation of an action potential, typically in the axon. Propagation of this action potential to the synaptic terminals is widely believed to be the only form of rapid communication of information between the soma and axonal synapses, and hence to postsynaptic neurons. Here we show that the voltage fluctuations associated with dendrosomatic synaptic activity propagate significant distances along the axon, and that modest changes in the somatic membrane potential of the presynaptic neuron modulate the amplitude and duration of axonal action potentials and, through a Ca2+-dependent mechanism, the average amplitude of the postsynaptic potential evoked by these spikes. These results indicate that synaptic activity in the dendrite and soma controls not only the pattern of action potentials generated, but also the amplitude of the synaptic potentials that these action potentials initiate in local cortical circuits, resulting in synaptic transmission that is a mixture of triggered and graded (analogue) signals.

In vitro formation and activity-dependent plasticity of synapses between Helix neurons involved in the neural control of feeding and withdrawal behaviors

Short-term activity-dependent synaptic plasticity has a fundamental role in short-term memory and information processing in the nervous system. Although the neuronal circuitry controlling different behaviors of land snails of the genus Helix has been characterized in some detail, little is known about the activity-dependent plasticity of synapses between identified neurons regulating specific behavioral acts. In order to study homosynaptic activity-dependent plasticity of behaviorally relevant Helix synapses independently of heterosynaptic influences, we sought to reconstruct them in cell culture. To this aim, we first investigated in culture the factors regulating synapse formation between Helix neurons, and then we studied the short-term plasticity of in vitro-reconstructed monosynaptic connections involved in the neural control of salivary secretion and whole-body withdrawal. We found that independently of extrinsic factors, cell-cell interactions are seemingly sufficient to trigger the formation of electrical and chemical synapses, although mostly inappropriate--in their type or association--with respect to the in vivo synaptic connectivity. The presence of ganglia-derived factors in the culture medium was required for the in vitro reestablishment of the appropriate in vivo-like connectivity, by reducing the occurrence of electrical connections and promoting the formation of chemical excitatory synapses, while apparently not influencing the formation of inhibitory connections. These heat-labile factors modulated electrical and chemical synaptogenesis through distinct protein tyrosine kinase signal transduction pathways. Taking advantage of in vitro-reconstructed synapses, we have found that feeding interneuron-efferent neuron synapses and mechanosensory neuron-withdrawal interneuron synapses display multiple forms of short-term enhancement-like facilitation, augmentation and posttetanic potentiation as well as homosynaptic depression. These forms of plasticity are thought to be relevant in the regulation of Helix feeding and withdrawal behaviors by inducing dramatic activity-dependent changes in the strength of input and output synapses of high-order interneurons with a crucial role in the control of Helix behavioral hierarchy.

Spike timing-dependent serotonergic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit

Neuromodulation is often thought to have a static, gain-setting function in neural circuits. Here we report a counter example: the neuromodulatory effect of a serotonergic neuron is dependent on the interval between its spikes and those of the neuron being modulated. The serotonergic dorsal swim interneurons (DSIs) are members of the escape swim central pattern generator (CPG) in the mollusk Tritonia diomedea. DSI spike trains heterosynaptically enhanced synaptic potentials evoked by another CPG neuron, ventral swim interneuron B (VSI-B), when VSI-B action potentials occurred within 10 sec of a DSI spike train; however, if VSI-B was stimulated 20-120 sec after DSI, then the amplitude of VSI-B synaptic potentials decreased. Consistent with this, VSI-B-evoked synaptic currents exhibited a temporally biphasic and bidirectional change in amplitude after DSI stimulation. Both the DSI-evoked enhancement and decrement were occluded by serotonin and blocked by the serotonin receptor antagonist methysergide, suggesting that both phases are mediated by serotonin. In most preparations, however, bath-applied serotonin caused only a sustained enhancement of VSI-B synaptic strength. The heterosynaptic modulation interacted with short-term homosynaptic plasticity: DSI-evoked depression was offset by VSI-B homosynaptic facilitation. This caused a complicated temporal pattern of neuromodulation when DSI and VSI-B were stimulated to fire in alternating bursts to mimic the natural motor pattern: DSI strongly enhanced summated VSI-B synaptic potentials and suppressed single synaptic potentials after the cessation of the artificial motor pattern. Thus, spike timing-dependent serotonergic neuromodulatory actions can impart temporal information that may be relevant to the operation of the CPG.

Spike timing-dependent serotonergic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit

Neuromodulation is often thought to have a static, gain-setting function in neural circuits. Here we report a counter example: the neuromodulatory effect of a serotonergic neuron is dependent on the interval between its spikes and those of the neuron being modulated. The serotonergic dorsal swim interneurons (DSIs) are members of the escape swim central pattern generator (CPG) in the mollusk Tritonia diomedea. DSI spike trains heterosynaptically enhanced synaptic potentials evoked by another CPG neuron, ventral swim interneuron B (VSI-B), when VSI-B action potentials occurred within 10 sec of a DSI spike train; however, if VSI-B was stimulated 20-120 sec after DSI, then the amplitude of VSI-B synaptic potentials decreased. Consistent with this, VSI-B-evoked synaptic currents exhibited a temporally biphasic and bidirectional change in amplitude after DSI stimulation. Both the DSI-evoked enhancement and decrement were occluded by serotonin and blocked by the serotonin receptor antagonist methysergide, suggesting that both phases are mediated by serotonin. In most preparations, however, bath-applied serotonin caused only a sustained enhancement of VSI-B synaptic strength. The heterosynaptic modulation interacted with short-term homosynaptic plasticity: DSI-evoked depression was offset by VSI-B homosynaptic facilitation. This caused a complicated temporal pattern of neuromodulation when DSI and VSI-B were stimulated to fire in alternating bursts to mimic the natural motor pattern: DSI strongly enhanced summated VSI-B synaptic potentials and suppressed single synaptic potentials after the cessation of the artificial motor pattern. Thus, spike timing-dependent serotonergic neuromodulatory actions can impart temporal information that may be relevant to the operation of the CPG.

Single neuron control over a complex motor program

While there are many instances of single neurons that can drive rhythmic stimulus-elicited motor programs, such neurons have seldom been found to be necessary for motor program function. In the isolated central nervous system of the marine mollusc Tritonia diomedea, brief stimulation (1 sec) of a peripheral nerve activates an interneuronal central pattern generator that produces the long-lasting (approximately 30-60 sec) motor program underlying the animal's rhythmic escape swim. Here, we identify a single interneuron, DRI (for dorsal ramp interneuron), that (i) conveys the sensory information from this stimulus to the swim central pattern generator, (ii) elicits the swim motor program when driven with intracellular stimulation, and (iii) blocks the depolarizing "ramp" input to the central pattern generator, and consequently the motor program itself, when hyperpolarized during the nerve stimulus. Because most of the sensory information appears to be funneled through this one neuron as it enters the pattern generator, DRI presents a striking example of single neuron control over a complex motor circuit.

Quantal analysis of action of hemicholinium-3 studied at a central cholinergic synapse of Aplysia

The effects of hemicholinium-3 (HC-3) on cholinergic transmission were studied on central identified inhibitory (H-type post-synaptic cell, Cl- channels) and on excitatory (D-type post-synaptic cell, cationic channels) synapses of Aplysia californica. In the H-type post-synaptic cell, the amplitude and the decay time of miniature post-synaptic currents (m.p.s.c.s.) were calculated by statistical analysis of long duration induced post-synaptic current (l.d.i.p.s.c.) due to 3 s depolarizations of the presynaptic neurone in the presence of tetrodotoxin. On H-type receptors, with respect to acetylcholine (ACh), HC-3 acted as an agonist and a blocker whereas on D-type receptors, it acted only as a blocker. At low concentration of bath-applied HC-3, in the H-type synapse, the decay time of the evoked inhibitory post-synaptic current (i.p.s.c.) as well as that of the m.p.s.c. was lengthened. These changes were rapidly reversible by wash. The decay time of excitatory post-synaptic current (e.p.s.c.) at the D-type synapse was not affected. On the inhibitory synapse, HC-3 applied in the bath at the concentration of 10(-5) M, reduced considerably the size of the m.p.s.c.s whereas the evoked i.p.s.c.s and the l.d.i.p.s.c.s were only slightly affected pointing to an increase of the quantal content of both responses. After wash, both i.p.s.c.s and l.d.i.p.s.c.s showed a clear facilitation which persisted for several tens of minutes. The presence of presynaptic receptors was considered. Similar facilitation of e.p.s.c.s by HC-3 was observed at the D-type synapse. The comparison of the degree of depression by HC-3 of the m.p.s.c.s and of the responses to ionophoretically applied ACh, indicated that the size of the quantum was not changed. Intracellular injection of HC-3 into the presynaptic neurone of the H-type synapse led to a decrease of transmitter release which affected solely the quantal content of the responses. As the synaptic transmission could not be restored by injection of exogenous ACh into the presynaptic neurone, it was concluded that the depression of transmission was not due to a decrease of ACh synthesis.

Synaptic block of a transmitter-induced potassium conductance in Aplysia neurones

A voltage-clamp study was made of a slow excitatory post-synaptic potential (slow e.p.s.p.) that can be elicited in the medial cells of the left pleural ganglion of Aplysia californica by the firing of at least three different presynaptic neurones (labelled I, II and III). Each of these three neurones elicits other permeability changes in addition to the slow e.p.s.p., and all elements of these synaptic responses were shown to be mediated monosynaptically. The slow e.p.s.p., associated with an increase in membrane resistance, was shown to be due to a decrease in K permeability. When the slow e.p.s.p. was present spontaneously, it could be blocked by three compounds (tetraethylammonium (TEA), phenyltrimethylammonium (PTMA), or methylxylocholine (beta-TM 10], all previously shown to block the cholinergic receptor that mediates an increase in K conductance in the medial cells (see Kehoe, 1972b). Furthermore, in ganglia in which no slow e.p.s.p. was seen in response to firing of the neurones I, II, and III, such a response became manifest when agonists capable of activating the cholinergic receptor were applied (e.g. acetylcholine (ACh), carbachol, arecoline, or F2268). The slow e.p.s.p. thus appears to result from the reduction, induced by any one of three 'blocking neurones', of a cholinergically controlled K conductance. Finally, when presynaptic neurone I (the only neurone tested) was fired shortly before or during the activation of presynaptic neurone IV, previously shown to be cholinergic (Kehoe, 1972b), the K component of the cholinergic post-synaptic inhibitory potential was markedly reduced. The concentration at which a given agonist caused the manifestation of the synaptic diminution in K conductance (i.e. the slow e.p.s.p.) was found to be the same as that at which it caused a reduction in the synaptically activated, cholinergic, K-dependent conductance elicited by presynaptic neurone IV. Intracellularly injected adenosine 3',5'-cyclic monophosphate (cyclic AMP) imitated the effect of the 'blocking neurones' on the K conductance activated by bath-applied cholinomimetics. This effect was superimposed on a cyclic-AMP-induced, voltage-dependent inward current that disappeared when the cell was bathed in Na-free sea water, or when the extracellular Ca concentration was increased to 60 mM. The effect of cyclic AMP on the cholinergic K conductance remained even after this cyclic-AMP-activated inward current was eliminated.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the synaptic actions of an interneuron in the central nervous system of Tritonia

The motor program that drives the swimming behavior of the marine mollusk Tritonia diomedea is generated by three interneuronal populations in the cerebral ganglia. One of these populations, the pair of C2 neurons, is shown to also exert powerful synaptic actions upon most cells in the contralateral pedal ganglion. Intracellular staining with Co2+ showed that the C2 neurons projected to the contralateral pedal ganglion as a single unbranched axon, and nearly all contralateral pedal neurons received monosynaptic input from C2. Orthodromic stimulation of most peripheral nerves caused monosynaptic excitation of C2 by afferent sensory cells and, in some cases, monosynaptic inhibition from an unidentified source. C2 neurons produced four types of postsynaptic potential (PSP) on pedal neurons: (1) a fast, Cl- -mediated inhibition (FIPSP); (2) a fast, Na+ -mediated excitation (FEPSP); (3) a slow, K+ -mediated inhibition (SIPSP); and (4) a slow, conductance-decrease excitation (SEPSP). All four could be recorded simultaneously in some pedal neurons. The C2 neurons appear to be high-order, multiaction neurons involved in both the generation of a complex motor program and the coordination of ancillary neuronal activity.

Neurophysiological consequences of presynaptic receptor activation: changes in noradrenergic terminal excitability

Experiments were carried out to explore the view that activation of presynaptic receptors on the terminals of noradrenergic neurons is accompanied by alterations in their excitability to direct electrical stimulation. Antidromic action potentials evoked from frontal cortex of urethane anesthetized rats were recorded extracellularly from nucleus locus coeruleus. The threshold current necessary to evoke antidromic action potentials varied as a result of infusion of adrenergic agonists and antagonists into frontal cortex within 50 micrometer of the stimulating electrode. Local infusion of the alpha-adrenergic agonist clonidine produced a marked decrease in terminal excitability, while the alpha-antagonist phentolamine produced an increase in terminal excitability and was shown to reverse the effect of the agonist. Infusion of the beta-adrenergic agonist isoproterenol was without effect, although the beta-antagonist propranolol resulted in a decrease in terminal excitability. Infusions of potassium increased excitability of locus coeruleus terminals. Terminal excitability was seen to vary inversely with the rate of spontaneous or high frequency stimulation-induced firing of locus coeruleus neurons. From these observations, it may be inferred that activation or blockade of alpha-adrenergic presynaptic receptors results in changes in polarization and/or conductance of the noradrenergic synaptic endings. These results are discussed with respect to phenomena associated with the possible presynaptic regulation of neurotransmitter release.

Identification of neurons contributing to presynaptic inhibition in Aplysia californica

Electrical stimulation of the connectives presynaptically inhibits the PSP from cell L10 to the left upper quadrant cells (LUQC). The present report describes the properties of some of the individual neurons contributing to this response. Action potentials produced in a cluster of cells in the abdominal ganglion reduce the amplitude of the L10-LUQC PSP for periods greater than 30 sec. At least some of their inhibitory action is mediated by a slow hyperpolarization of L10 which results in a decreased transmitter release. In other cases, however, the inhibition is produced with no significant alteration of L10 membrane potential, indicating that additional mechanisms may also be present. The neurons producing these effects are approximately 75 microns in diameter and are located on the left ventral surface of the ganglion. They have axons in the connectives and are thus activated by stimuli previously utilized to produce presynaptic inhibition. They appear to be some of the same cells that produce a slow inhibition of ink motoneuron L14; one of these has been identified as L32. The identification of these cells allows for the further biochemical, biophysical and morphological analysis of the events underlying presynaptic inhibition.

Graded synaptic transmission between spiking neurons

Graded synaptic transmission occurs between spiking neurons of the lobster stomatogastric ganglion. In addition to eliciting spike-evoked inhibitory potentials in postsynaptic cells, these neurons also release functionally significant amounts of transmitter below the threshold for action potentials. The spikeless postsynaptic potentials grade in amplitude with presynaptic voltage and can be maintained for long periods. Graded synaptic transmission can be modulated by synaptic input to the presynaptic neuron.

Presynaptic inhibition in Aplysia involves a decrease in the Ca2+ current of the presynaptic neuron

By voltage clamping presynaptic cell L10 and using pharmacologic separation techniques, we have analyzed the specific ionic currents in the presynaptic cell that correlate with presynaptic inhibition while assaying transmitter release with intracellular recordings from postsynaptic cells. We have found that presynaptic inhibition can be elicited in conditions in which the Na+ and the various K+ channels are pharmacologically blocked and depolarizing current pulses produce only an inward Ca2+ current. Both inward currents and tail currents at and above the K+ reversal potential were always less inward during presynaptic inhibition. The changes in conductance associated with presynaptic inhibition were voltage sensitive and paralleled the voltage sensitivity of the Ca2+ channel. We therefore conclude that presynaptic inhibition is caused by a direct transmitter-mediated decreased of presynaptic Ca2+-channel conductance.

Presynaptic membrane potential affects transmitter release in an identified neuron in Aplysia by modulating the Ca2+ and K+ currents

We have examined the relationships between the modulation of transmitter release and of specific ionic currents by membrane potential in the cholinergic interneuron L10 of the abdominal ganglion of Aplysia californica. The presynaptic cell body was voltage-clamped under various pharmacological conditions and transmitter release from the terminals was assayed simultaneously by recording the synaptic potentials in the postsynaptic cell. When cell L10 was voltage-clamped from a holding potential of -60 mV in the presence of tetrodotoxin, graded transmitter release was evoked by depolarizing command pulses in the membrane voltage range (-35 mV to + 10 mV) in which the Ca(2+) current was also increasing. Depolarizing the holding potential of L10 results in increased transmitter output. Two ionic mechanisms contribute to this form of plasticity. First, depolarization inactivates some K(+) channels so that depolarizing command pulses recruit a smaller K(+) current. In unclamped cells the decreased K(+) conductance causes spike-broadening and increased influx of Ca(2+) during each spike. Second, small depolarizations around resting potential (-55 mV to -35 mV) activate a steady-state Ca(2+) current that also contributes to the modulation of transmitter release, because, even with most presynaptic K(+) currents blocked pharmacologically, depolarizing the holding potential still increases transmitter release. In contrast to the steady-state Ca(2+) current, the transient inward Ca(2+) current evoked by depolarizing clamp steps is relatively unchanged from various holding potentials.

Transmitter release: ruthenium red used to demonstrate a possible role of sialic acid containing substrates

1. The possible function of sialic acid-containing substrates (SACS) in synaptic terminals of Aplysia was studied by intracellular injection of ruthenium red and of neuraminidase. 2. Ruthenium red, a dye known to have sialic acid as a molecular target, blocked transmission irreversibly in both cholinergic (buccal ganglion) and non-cholinergic (cerebral ganglion) synapses. 3. An intracellular site of action is likely because much less ruthenium red was necessary to block transmission when it was injected intracellularly than when it was presented by bath perfusion. 4. Ca2+ spikes recorded in the presence of tetrodotoxin or in Na+-free solution were not modified by ruthenium red or neuraminidase injections or perfusions. It is therefore improbable that these substances blocked transmission by blocking voltage-dependent Ca2+ influx. 5. Strong electrotonic depolarization of a pre-synaptic interneurone in the presence of 10(-4) M-tetrodotoxin caused a sustained post-synaptic response, which was abolished by ruthenium red. This result eliminates axonal conduction block as the principal mechanism of ruthenium red action. 6. Post-synaptic responses to ionophoretically applied acetylcholine (ACh) were not modified by bath perfusion of 2 x 10(-2) M-ruthenium red. 7. Biochemical analysis of pools of [3H]ACh was performed after injection of a precursor, [3H]acetate, into an identified interneurone. Ruthenium red appeared to increase significantly the 'free' (cytoplasmic) ACh pool without any change of 'bound' (vesicular) [3H]ACh-pool. 8. A model is proposed in which SACS act as intracellular Ca2+ receptors involved in transmitter release.

Role of interganglionic synaptic connections in the control of pedal and parapodial movements in Aplysia

Locomotion in Aplysia can be elicited by food chemosensory, tactile, proprioceptive and nociceptive stimuli. The effects of these stimuli on cerebral B neurons, pleural neurons and motor activity in the foot were examined and the behavioral roles of identified synaptic connections among these neurons investigated in semi-intact preparations. The motor effects of intracellular stimulation of pleural neurons were determined. Sensory stimulation elicited spiking in the left giant cell (LGC) in quiescent preparations. When the LGC was spontaneously bursting, sensory stimulation caused decreased interburst intervals and increased burst durations. Following sensory stimulation, LGC firing was correlated with motor activity in the foot. Intracellular stimulation of the LGC evoked contractions in the foot and parapodia. Pleural neurons which produced EPSPs in the LGC and/or B neurons were excited by sensory stimuli that elicit locomotion and when driven caused contractions in the foot. Pleural neurons which inhibited the B neurons were excited by nociceptive stimuli that inhibit normal locomotion, and were inhibited by tactile and chemosensory stimulation of the tentacles which excited the B neurons and can elicit normal locomotion. Intracellular stimulation of the neurons which produced IPSPs in the B neurons evoked contractions in the posterior foot suggesting a motor function in nociceptive induced withdrawal. The synaptic connections between pleural and cerebral neurons are consistent with the proposed modulatory role of the pleural ganglion in locomotion, and may account for changes in locomotion with result from cerebro-pleural connective lesions.

Intra- and interganglionic synaptic connections in the CNS of Aplysia

Synaptic connections were found between two groups of neurons in the CNS of Aplysia, the cerebral ganglion A and B cluster neurons which are involved in the control of pedal and parapodial movements and neurons in the pleural ganglion which has been shown to modulate locomotion. The A and B neurons made synaptic connections in both the cerebral and pleural ganglia. Pleural neurons had synaptic connections among themselves and with A and B neurons. The A neurons made excitatory monosynaptic connections with the B neurons and a minimum of 6 pleural neurons including the left giant cell (LGC). All of the A neuron synapses found were excitatory. The B neurons received excitatory synaptic input from two other groups of neurons in the cerebral ganglion and both excitatory and inhibitory input from pleural neurons. The latter were identified on the basis of their synaptic connections with the LGC and A neurons. The B neurons and LGC had several common synaptic inputs. The A neurons received monosynaptic input from only 2 pleural neurons. Complex synaptic circuits between A and B neurons and pleural neurons were found. These included recurrent inhibition of B neurons by A neurons via a pleural interneuron, feedforward summation of A neuron synaptic input to the LGC, and reciprocal excitatory synaptic connections between B and the pleural neurons. The activity of the B neurons was modulated by direct inhibitory and excitatory synaptic connections from pleural neurons. The A neurons were modulated primarily by a polysynaptic pathway through the B neurons. The modulation of cerebral A and B neurons by pleural neurons is consistent with behavioral results obtained studying locomotion.

Characteristics of sodium and calcium conductance changes produced by membrane depolarization in an Aplysia neurone

1. The time course and voltage dependence of Na and Ca conductance changes produced by depolarization of the soma of the neurone R15 in the abdominal ganglion of Aplysia juliana were examined at temperatures of 10--14 degrees C. 2. During a maintained depolarization, Na currents turned on then decayed (inactivated). Inactivation was exponential with time constant tauh. Activation (after correction for inactivation) was reasonably well described by the expression G'Na(t) = G'Na (infinity) (1 - exp [-t/taum])3 over a wide range of potentials. 3. taum and tauh were both voltage dependent. In the range -20 to +40 mV, taum varied from 5 to 0.5 msec and tauh from 25 to 8 msec (13.5 degrees C). Steady-state Na conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 25--30 muS at +10 to +20 mV. Half-maximal Na conductance occurred at a membrane potential of -8 mV and from -15 to -5 mV, a 5 mV change in membrane potential produced an e-fold change in steady-state Na conductance. 4. Steady-state inactivation of Na conductance (hNa(infinity)) was voltage dependent with half-inactivation occurring at a membrane potential of -32 mV. Recovery from Na inactivation followed an exponential time course with a voltage-dependent time constant. 5. During a maintained depolarization Ca currents activated then decayed (inactivated) more slowly than Na currents. The decay was exponential with time constant tauH. The decay of Ca current was not an artifact porduced by an outward current. The amplitude of calcium tail currents, produced by voltage steps back to epsilonK at different times during the decay of ICa, decayed also with a time constant close to tauH. 6. Ca conductance (after correction for inactivation) could be described approximately by the expression G'Ca(t) = G'Ca(infinity) (1 - exp [-t/tauM])p but it was necessary to vary p from 1 to 2 at different potentials. No value of p gave as good a fit to this model as that obtained for Na currents. 7. taum and tauH were voltage dependent. In the range of potentials from 0 to +60 mV, tauM varied from 9 to 5 msec and tauH from 300 to 50 msec (13.5 degrees C). Steady-state Ca conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 10--15 muS at +30 to +40 mV. Half-maximal Ca conductance occurred at a membrane potential of +12 mV, and from +10 to +20 mV a 6 mV change in membrane potential produced an e-fold change in Ca conductance. 8. Steady-state inactivation of Ca conductance (hCa(infinity)) varied with holding potential (VH). Half-inactivation occurred with depolarization to -20 mV. At potentials more negative than -40 mV, hCa(infinity) was less than at -40 mV, i.e. hyperpolarization produced Ca 'inactivation'. 9...

Synaptic connections and functional organization in Aplysia buccal ganglia

The buccal ganglion of Aplysia contains three morpho-functional groups (A, B, and C) of large cells and two groups (s1 and s2) of small cells. The A cells evoke monoxynaptic IPSPs in the B cells. We found that s1 cells can evoke large EPSPs in the A cells, IEPSPs in the B cells, and EIIPSPs in the C cells; several s1 cells are able to evoke all three types of responses. Many s2 cells can evoke these same responses, but only in the A and B cells. Furthermore, the s cells can evoke depolarizing PSPs in other s cells; this relation is often reciprocal. All these responses may also be contralateral. Their monosynaptic nature is shown by the consistent 1:1 relationship with the presynaptic spike, and also by the effects of intracellular tetraethylammonium and of high Mg2+ concentration in the bathing medium. d-tubocurarine reversibly suppresses the I phase of the IEPSP evoked by the s cells in the B cells. All the responses evoked by the s cells undergo depression with repetition. The network formed by all these relations is outlined, and a double relationship proposed between s cells and B cells. By electrophysiological tracing of axonal pathways it is shown that the A cells send axons into the 3rd buccal nerve, the B cells into the 2nd and/or 3rd buccal nerve and in two cases into the radular nerve, and the C cells into the gastro-oesophageal nerve. Spontaneous synaptic activity of the buccal neurons appears to be formed mostly by the described PSPs. Spontaneous firing inside the isolated ganglion corresponds well to the alternate pattern of muscular contractions of the buccal mass.

Modulation of transmission at an inhibitory synapse in the central nervous system of the leech

The synaptic interactions among a group of cells in the leech C.N.S. that regulate the animal's heartbeat exhibit several remarkable features (Thompson & Stent, 1976 a, b, c). We have examined in detail the properties of the inhibitory synapse between two of these cells, the heart interneurone (HN cell) and the heart excitor motoneurone (HE cell). 1. Impulses in the presynaptic HN cell gave rise to monosynaptic i.p.s.p.s in the HE cell that were blocked by high concentrations of Mg and were reversed when the membrane potential of the post-synaptic motoneurone was hyperpolarized beyond--75 m V or when Cl was injected into the cell body. These i.p.s.p.s were chemically mediated, and involved an increase in chloride conductance. 2. In contrast to chemical synapses between sensory and motor cells in the leech C.N.S., little facilitation or depression of transmission occurred when the HN cell was stimulated at frequencies of 0.1--50 Hz. 3. Steady subthreshold depolarization of the presynaptic HN interneurone evoked a maintained hyperpolarization of the post-synaptic HE cell, indicating that currents injected into the HN cell body could spread to the terminals and cause continuous release of transmitter. 4. The size of the i.p.s.p. evoked in the HE motoneurone by an action potential in the HN interneurone varied with the resting membrane potential of the presynaptic cell. An impulse superimposed on a prolonged, subthreshold, depolarizing pulse produced a larger i.p.s.p.; conversely, prolonged hyperpolarization of the HN interneurone reduced the i.p.s.p. amplitude recorded in the HE cell. This effect was most obvious when the natural, rhythmical bursts of activity in the HN interneurone were interrupted by bathing the preparation in leech Ringer fluid containing elevated concentrations of Mg. Under these conditions a 10 mV depolarization of the HN cell increased the size of the i.p.s.p. in the HE cell approximately sixfold. Significant changes in i.p.s.p. amplitude occurred without any noticeable change in the amplitude and duration of the presynaptic action potential. With large presynaptic depolarizations, which produced the biggest i.p.s.p.s, there was some reduction in the amplitude and increase in the duration of the action potential. 5. Following a step depolarization of the presynaptic cell, the size of successive i.p.s.p.s increased with a time constant of about 1 sec. Upon repolarization the i.p.s.p.s decreased in amplitude to the original level. 6. stimulation of one HN cell also gives rise to an i.p.s.p. in its contralateral homologue (Thompson & Stent, 1976c). Trains of i.p.s.p.s produced in this way hyperpolarized at HN cell to such an extent that the size of the synaptic potential it evoked in an HE cell was reduced. 7. Thus, an HN interneurone inhibitis transmission between the contralateral HN and HE cells presynapitcally in addition to inhibiting directly the ipsilateral HE motoneurone.

Presynaptic modulation of voltage-dependent Ca2+ current: mechanism for behavioral sensitization in Aplysia californica

Behavioral sensitization of the gill-withdrawal reflex of Aplysia is the result of a prolonged increase in transmitter release from the presynaptic terminals of sensory neurons. Earlier work suggested that this presynaptic facilitation might be mediated by a serotonin-sensitive adenylate cyclase in the sensory neuron terminals. Here we present evidence that presynaptic facilitation results from a cyclic AMP-dependent increase in the calcium current that underlies action potentials in the sensory neurons. The action potentials of sensory neuron cell bodies have, in addition to a sodium current, a calcium current that is enhanced by blocking the opposing potassium current with tetraethylammonium. Under these conditions, the action potentials show a slowly repolarizing plateau that follows the Nernst potential for a calcium electrode and serves as a sensitive assay for changes in calcium current. Stimulation of the pathway that mediates sensitization, incubation with serotonin or phosphodiesterase inhibitors, or intracellular injection of cyclic AMP produces an increase in the calcium plateau in the presence of tetraethylammonium. In addition, both before and after sensitizing stimulation, the duration of the plateau potential parallels transmitter release as measured by the amplitude of monosynaptic excitatory postsynaptic potentials evoked in the motor neurons by intracellular stimulation of single sensory neurons. These results are consistent with the idea that presynaptic facilitation is caused by a cyclic AMP-mediated increase in a voltage-sensitive calcium current in sensory neuron presynaptic terminals. This synaptic action is novel in that it can produce little or no change in the resting potential, is of long duration, and exerts its influence directly on a conductance triggered by the action potential, rather than on non-voltage-sensitive conductances, as is typical of conventional synaptic actions.

Evoked depolarizing and hyperpolarizing potentials in reticulospinal axons of lamprey

1. Intracellular recordings were made from reticulospinal axons (Müller axons) in the lamprey spinal cord. Electrical stimuli applied to the spinal cord surface elicited depolarizing and hyperpolarizing 'synaptic-like' potentials in Müller axons. The physiological basis of these evoked potentials was investigated. 2. The depolarizing response was not the result of increased extracellular K, as demonstrated by the constancy of the undershoot of the axonal action potential during the depolarization, by the failure of the response to summate during repetitive stimulation and by the failure of the response amplitude to vary as predicted when the [K] of the saline was varied. 3. When the membrane potential of the axon was varied by passing current through a micro-electrode, the amplitude of the depolarizing evoked potential decreased at membrane potentials positive to the resting potential and increased up to a maximum when the axon was hyperpolarized by about 10 mV. The extrapolated 'reversal potential' for the depolarizing response was about 15 mV positive to the normal -80 mV resting potential of the axon. However, the amplitude of the response did not continue to grow with hyperpolarizations greater than 10 mV, and, thus, the response did not behave as would a normal depolarizing synaptic potential. 4. Müller axons make numerous electrical synapses with spinal motoneurones and interneurones, and this suggested that the depolarizing response might be a coupling potential. In agreement with this idea, quantitative correspondence was found between changes in the input resistance of the axon produced by the depolarizing response and the variation in the depolarizing response amplitude. Thus, although the depolarizing response mimicked in some ways the behaviour of an excitatory synaptic potential, we conclude that it is a coupling potential. 5. The hyperpolarizing response also appeared to be a coupling potential. Its amplitude was not changed by hyperpolarizing the axon up to 30 mV and was decreased by depolarizing the axon sufficiently to decrease the axon's input resistance. 6. It is proposed that both depolarizing and hyperpolarizing evoked potentials in lamprey Müller axons are a result of passive flow of current from cells activated by the spinal cord stimulus and electrically coupled to Müller axons.

Synaptic connexions and related postsynaptic pharmacology studied in the cerebral ganglion of Aplysia

Interneuronal connexions have been studied within the cerebral ganglion of Aplysia californica. Excitatory monosynaptic connexions between two groups of cells located on the dorsal surface of the ganglion near the cerebropleural connectives were analyzed in detail. The monosynapticity of these connexions was established not only by the strict one-to-one correlation between presynaptic action potential and excitatory postsynaptic response (EPSP) and the constant latency for any given cell pair, but also by the following criteria: (a) gradual change in the EPSP following tetraethylammonium injection into the presynaptic neurone, (b) sustained EPSP in the presence of a high external calcium ion concentration, (c) sensitivity of the EPSP amplitude to presynaptic polarization. Iontophoretic application of acetylcholine, 5-hydroxytryptamine and glutamate on the postsynaptic cells elicited excitatory responses in many cases. Inhibitory responses were obtained by local iontophoresis of dopamine, gamma-aminobutyric acid and occasionally also by acetylcholine. The only agent found to block the EPSP was bufotenine, which also readily blocked the 5-hydroxytryptamine response. Bufotenine was completely ineffective on the acetylcholine or glutamate excitatory responses. Of the various cholinolytics tested, none had an effect on the EPSP. Our data all point to 5-hydroxytryptamine as a transmitter in the studied synaptic connexions. However, it must be emphasized that in the absence of biochemical and histological evidence the role of the 5-hydroxytryptamine cannot be regarded as conclusive.