Soma potential of an interneurone controls transmitter release in a monosynaptic pathway in Aplysia

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.

Activity-dependent increases in [Ca2+]i contribute to digital-analog plasticity at a molluscan synapse

In a type of short-term plasticity that is observed in a number of systems, synaptic transmission is potentiated by depolarizing changes in the membrane potential of the presynaptic neuron before spike initiation. This digital-analog form of plasticity is graded. The more depolarized the neuron, the greater the increase in the efficacy of synaptic transmission. In a number of systems, including the system presently under investigation, this type of modulation is calcium dependent, and its graded nature is presumably a consequence of a direct relationship between the intracellular calcium concentration ([Ca²⁺]_i) and the effect on synaptic transmission. It is therefore of interest to identify factors that determine the magnitude of this type of calcium signal. We studied a synapse in Aplysia and demonstrate that there can be a contribution from currents activated during spiking. When neurons spike, there are localized increases in [Ca²⁺]_i that directly trigger neurotransmitter release. Additionally, spiking can lead to global increases in [Ca²⁺]_i that are reminiscent of those induced by subthreshold depolarization. We demonstrate that these spike-induced increases in [Ca²⁺]_i result from the activation of a current not activated by subthreshold depolarization. Importantly, they decay with a relatively slow time constant. Consequently, with repeated spiking, even at a low frequency, they readily summate to become larger than increases in [Ca²⁺]_i induced by subthreshold depolarization alone. When this occurs, global increases in [Ca²⁺]_i induced by spiking play the predominant role in determining the efficacy of synaptic transmission.NEW & NOTEWORTHY We demonstrate that spiking can induce global increases in the intracellular calcium concentration ([Ca²⁺]_i) that decay with a relatively long time constant. Consequently, summation of the calcium signal occurs even at low firing frequencies. As a result there is significant, persistent potentiation of synaptic transmission.

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.

Multitasking in Gonadotropin-Releasing Hormone Neuron Dendrites

Gonadotropin-releasing hormone (GnRH) neurons integrate synaptic information in their dendrites in order to precisely control GnRH secretion and hence fertility. Recent discoveries concerning the structure and function of GnRH neuron dendrites have shed new light on the control of GnRH neuron excitability and GnRH secretion. This work suggests that GnRH neurons have a unique projection to the median eminence that possesses both dendritic and axonal properties. We propose that this 'dendron' projection allows GnRH neurons to multitask and integrate information in ways that would not be possible in a classically envisioned axon projection.

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.

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.

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.

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.

Presynaptic modulation of transmitter release by the early outward potassium current in Aplysia

The mechanism involved in presynaptic modulation of transmitter release was studied in an identified synapse of Aplysia californica. Presynaptic hyperpolarization induces a decrease in he evoked postsynaptic potential amplitude. This is shown to be due to a reduction in the presynaptic spike amplitude during the hyperpolarization. The decreased presynaptic spike amplitude with hyperpolarization is explained s resulting from the superimposition of an early outward potassium current on the transient inward current. It is suggested that the presynaptic hyperpolarizing conditioning pulse decreases inactivation of the early outward current, which shunts the transient inward current. The superimposition of these two currents (transient inward current and the early outward current) induces a decrease in presynaptic spike amplitude, which in turn reduces the synaptic output from the terminal.

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.

Modulation of synaptic output by the transient outward potassium current in aplysia

The mechanisms involved in synaptic output modulation by presynaptic membrane potential was studied in identified Aplysia synapses, where a presynaptic hyperpolarization reduces the postsynaptic potential amplitude. The experiment reported here reveals that a presynaptic conditioning hyperpolarization induces a decreased presynaptic spike amplitude and that the reduction is due to a superimposition of a transient outward potassium current on the inward current. This is demonstrated by the external application of 4-aminopyridine which suppresses the early outward current and thus suppresses the modulation both of the presynaptic spike amplitude and the synaptic output.

Soma spike of neuroendocrine bag cells of Aplysia californica

Soma action potentials of the neuroendocrine bag cells of Aplysia californica were studied with intracellular recording and current injection. Spikes in artificial sea water (ASW) were either graded with increasing depolarizing current pulses, or had a well-defined threshold. The latter spikes typically had faster rise times with larger overshoots and hyperpolarizing afterpotentials. Repetitive stimulation led to spike potentiation (SP), manifested as an increase in overshoot amplitude and duration of successive spikes in a train. SP was usually detectable at 0.5 Hz, and maximal between 0.8 and 4 Hz. Concomitant accommodation occurred rapidly at greater than or equal to 5 Hz. The increase in spike duration during SP resulted from a progressive enhancement of an inflection on the repolarizing phase. The inflection was dependent on membrane potential; small depolarizations (5-10 mV) enhanced it; hyperpolarization (less than 35 mV) reduced it. Solutions with O--Na+ (Tris-substituted) or O--Ca2+ (1 mM EGTA) revealed mixed Na+/Ca2+ spikes with variable degrees of Na+ versus Ca2+ dominance. Cd2+, Co2+, and Mn2+ reversibly abolished the inflection on the repolarizing phase, indicating that it is Ca2+ mediated; the spike was reduced irreversibly at higher concentrations. SP was generally reduced only if the spike was severely attenuated. It is proposed that SP results primarily from a voltage- and time-dependent potassium inactivation which then unmasks a calcium current. SP may play a role in augmenting the release of egg-laying hormone.

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.

Quantal release of acetylcholine examined by current fluctuation analysis at an identified neuro-neuronal synapse of Aplysia

Quantal events have been analyzed at a neuro-neuronal synapse of Aplysia where the nature of the transmitter is established and both presynaptic and postsynaptic neurons are identified and can be voltage-clamped. Prolonged depolarizations were applied to the presynaptic neuron, which gave rise in the postsynaptic cell to a current response characterized by current fluctuations or noise. Acetylcholine was also applied ionophoretically on the same postsynaptic cell. Amplitude and time course of miniature currents and acetylcholine-activated chloride channels of the same cell were examined by using a current fluctuation analysis. It was estimated that one presynaptic spike releases about 180 quanta, each opening 500 chloride channels.