The Hodgkin–Huxley–Katz Prize Lecture

Ca2+ -independent transmission at the central synapse formed between dorsal root ganglion and dorsal horn neurons

A central principle of synaptic transmission is that action potential-induced presynaptic neurotransmitter release occurs exclusively via Ca2+ -dependent secretion (CDS). The discovery and mechanistic investigations of Ca2+ -independent but voltage-dependent secretion (CiVDS) have demonstrated that the action potential per se is sufficient to trigger neurotransmission in the somata of primary sensory and sympathetic neurons in mammals. One key question remains, however, whether CiVDS contributes to central synaptic transmission. Here, we report, in the central transmission from presynaptic (dorsal root ganglion) to postsynaptic (spinal dorsal horn) neurons in vitro, (i) excitatory postsynaptic currents (EPSCs) are mediated by glutamate transmission through both CiVDS (up to 87%) and CDS; (ii) CiVDS-mediated EPSCs are independent of extracellular and intracellular Ca2+ ; (iii) CiVDS is faster than CDS in vesicle recycling with much less short-term depression; (iv) the fusion machinery of CiVDS includes Cav2.2 (voltage sensor) and SNARE (fusion pore). Together, an essential component of activity-induced EPSCs is mediated by CiVDS in a central synapse.

Amyloid pathology-produced unexpected modifications of calcium homeostasis in hippocampal subicular dendrites

INTRODUCTION

Alzheimer's disease (AD) is linked to neuronal calcium dyshomeostasis, which is associated with network hyperexcitability. Decreased expression of the calcium-binding protein cal- bindin-D28K (CB) might be a susceptibility factor for AD. The subiculum is affected early in AD, for unknown reasons.

METHODS

In AD, CB knock-out and control mice fluorescence Ca2+ imaging combined with patch clamp were used to characterize Ca2+ dynamics, resting Ca2+ , and Ca2+ -buffering capacity in subicular neurons. CB expression levels in wild-type and AD mice were also analyzed.

RESULTS

The subiculum and dentate gyrus of wild-type mice showed age-related decline in CB expression not observed in AD mice. Resting Ca2+ and Ca2+ -buffering capacity was increased in aged AD mice subicular dendrites. Modeling suggests that AD calcium changes can be explained by alterations of Ca2+ extrusion pumps rather than by buffers.

DISCUSSION

Overall, abnormal Ca2+ homeostasis in AD has an age dependency that comprises multiple mechanisms, including compensatory processes.

Ca2+-independent but voltage-dependent quantal catecholamine secretion (CiVDS) in the mammalian sympathetic nervous system

Action potential-induced vesicular exocytosis is considered exclusively Ca²⁺ dependent in Katz's Ca²⁺ hypothesis on synaptic transmission. This long-standing concept gets an exception following the discovery of Ca²⁺-independent but voltage-dependent secretion (CiVDS) and its molecular mechanisms in dorsal root ganglion sensory neurons. However, whether CiVDS presents only in sensory cells remains elusive. Here, by combining multiple independent recordings, we report that [1] CiVDS robustly presents in the sympathetic nervous system, including sympathetic superior cervical ganglion neurons and slice adrenal chromaffin cells, [2] uses voltage sensors of Ca²⁺ channels (N-type and novel L-type), and [3] contributes to catecholamine release in both homeostatic and fight-or-flight like states; [4] CiVDS-mediated catecholamine release is faster than that of Ca²⁺-dependent secretion at the quantal level and [5] increases Ca²⁺ currents and contractility of cardiac myocytes. Together, CiVDS presents in the sympathetic nervous system with potential physiological functions, including cardiac muscle contractility.

From single cells and single columns to cortical networks: dendritic excitability, coincidence detection and synaptic transmission in brain slices and brains

Although patch pipettes were initially designed to record extracellularly the elementary current events from muscle and neuron membranes, the whole-cell and loose cell-attached recording configurations proved to be useful tools for examination of signalling within and between nerve cells. In this Paton Prize Lecture, I will initially summarize work on electrical signalling within single neurons, describing communication between the dendritic compartments, soma and nerve terminals via forward- and backward-propagating action potentials. The newly discovered dendritic excitability endows neurons with the capacity for coincidence detection of spatially separated subthreshold inputs. When these are occurring during a time window of tens of milliseconds, this information is broadcast to other cells by the initiation of bursts of action potentials (AP bursts). The occurrence of AP bursts critically impacts signalling between neurons that are controlled by target-cell-specific transmitter release mechanisms at downstream synapses even in different terminals of the same neuron. This can, in turn, induce mechanisms that underly synaptic plasticity when AP bursts occur within a short time window, both presynaptically in terminals and postsynaptically in dendrites. A fundamental question that arises from these findings is: 'what are the possible functions of active dendritic excitability with respect to network dynamics in the intact cortex of behaving animals?' To answer this question, I highlight in this review the functional and anatomical architectures of an average cortical column in the vibrissal (whisker) field of the somatosensory cortex (vS1), with an emphasis on the functions of layer 5 thick-tufted cells (L5tt) embedded in this structure. Sensory-evoked synaptic and action potential responses of these major cortical output neurons are compared with responses in the afferent pathway, viz. the neurons in primary somatosensory thalamus and in one of their efferent targets, the secondary somatosensory thalamus. Coincidence-detection mechanisms appear to be implemented in vivo as judged from the occurrence of AP bursts. Three-dimensional reconstructions of anatomical projections suggest that inputs of several combinations of thalamocortical projections and intra- and transcolumnar connections, specifically those from infragranular layers, could trigger active dendritic mechanisms that generate AP bursts. Finally, recordings from target cells of a column reveal the importance of AP bursts for signal transfer to these cells. The observations lead to the hypothesis that in vS1 cortex, the sensory afferent sensory code is transformed, at least in part, from a rate to an interval (burst) code that broadcasts the occurrence of whisker touch to different targets of L5tt cells. In addition, the occurrence of pre- and postsynaptic AP bursts may, in the long run, alter touch representation in cortex.

Dendritic Release of Neurotransmitters

Release of neuroactive substances by exocytosis from dendrites is surprisingly widespread and is not confined to a particular class of transmitters: it occurs in multiple brain regions, and includes a range of neuropeptides, classical neurotransmitters, and signaling molecules, such as nitric oxide, carbon monoxide, ATP, and arachidonic acid. This review is focused on hypothalamic neuroendocrine cells that release vasopressin and oxytocin and midbrain neurons that release dopamine. For these two model systems, the stimuli, mechanisms, and physiological functions of dendritic release have been explored in greater detail than is yet available for other neurons and neuroactive substances. © 2017 American Physiological Society. Compr Physiol 7:235-252, 2017.

Biophysical properties of presynaptic short-term plasticity in hippocampal neurons: insights from electrophysiology, imaging and mechanistic models

Hippocampal neurons show different types of short-term plasticity (STP). Some exhibit facilitation of their synaptic responses and others depression. In this review we discuss presynaptic biophysical properties behind heterogeneity in STP in hippocampal neurons such as alterations in vesicle priming and docking, fusion, neurotransmitter filling and vesicle replenishment. We look into what types of information electrophysiology, imaging and mechanistic models have given about the time scales and relative impact of the different properties on STP with an emphasis on the use of mechanistic models as complementary tools to experimental procedures. Taken together this tells us that it is possible for a multitude of different mechanisms to underlie the same STP pattern, even though some are more important in specific cases, and that mechanistic models can be used to integrate the biophysical properties to see which mechanisms are more important in specific cases of STP.

Asynchronous Ca2+ current conducted by voltage-gated Ca2+ (CaV)-2.1 and CaV2.2 channels and its implications for asynchronous neurotransmitter release

We have identified an asynchronously activated Ca(2+) current through voltage-gated Ca(2+) (Ca(V))-2.1 and Ca(V)2.2 channels, which conduct P/Q- and N-type Ca(2+) currents that initiate neurotransmitter release. In nonneuronal cells expressing Ca(V)2.1 or Ca(V)2.2 channels and in hippocampal neurons, prolonged Ca(2+) entry activates a Ca(2+) current, I(Async), which is observed on repolarization and decays slowly with a half-time of 150-300 ms. I(Async) is not observed after L-type Ca(2+) currents of similar size conducted by Ca(V)1.2 channels. I(Async) is Ca(2+)-selective, and it is unaffected by changes in Na(+), K(+), Cl(-), or H(+) or by inhibitors of a broad range of ion channels. During trains of repetitive depolarizations, I(Async) increases in a pulse-wise manner, providing Ca(2+) entry that persists between depolarizations. In single-cultured hippocampal neurons, trains of depolarizations evoke excitatory postsynaptic currents that show facilitation followed by depression accompanied by asynchronous postsynaptic currents that increase steadily during the train in parallel with I(Async). I(Async) is much larger for slowly inactivating Ca(V)2.1 channels containing β(2a)-subunits than for rapidly inactivating channels containing β(1b)-subunits. I(Async) requires global rises in intracellular Ca(2+), because it is blocked when Ca(2+) is chelated by 10 mM EGTA in the patch pipette. Neither mutations that prevent Ca(2+) binding to calmodulin nor mutations that prevent calmodulin regulation of Ca(V)2.1 block I(Async). The rise of I(Async) during trains of stimuli, its decay after repolarization, its dependence on global increases of Ca(2+), and its enhancement by β(2a)-subunits all resemble asynchronous release, suggesting that I(Async) is a Ca(2+) source for asynchronous neurotransmission.

In vitro system capable of differentiating fast Ca2+-triggered content mixing from lipid exchange for mechanistic studies of neurotransmitter release

Understanding the molecular principles of synaptic vesicle fusion is a long-sought goal. It requires the development of a synthetic system that allows manipulations and observations not possible in vivo. Here, we report an in vitro system with reconstituted synaptic proteins that meets the long-sought goal to produce fast content release in the millisecond time regime upon Ca(2+) triggering. Our system simultaneously monitors both content and lipid exchange, and it starts from stable interacting pairs of donor and acceptor vesicles, mimicking the readily releasable pool of synaptic vesicles prior to an action potential. It differentiates between single-vesicle interaction, hemifusion, and complete fusion, the latter mimicking quantized neurotransmitter release upon exocytosis of synaptic vesicles. Prior to Ca(2+) injection, the system is in a state in which spontaneous fusion events between donor and acceptor vesicles are rare. Upon Ca(2+) injection, a rapid burst of complete fusion events emerges, followed by a biphasic decay. The present study focuses on neuronal SNAREs, the Ca(2+) sensor synaptotagmin 1, and the modulator complexin. However, other synaptic proteins could be added and their function examined. Ca(2+) triggering is cooperative, requiring the presence of synaptotagmin, whereas SNAREs alone do not produce a fast fusion burst. Manipulations of the system mimic effects observed in vivo. These results also show that neuronal SNAREs alone do not efficiently produce complete fusion, that the combination of SNAREs with synaptotagmin lowers the activation barriers to full fusion, and that complexin enhances this kinetic control.

Stochastic models for the in silico simulation of synaptic processes

Background

Research in life sciences is benefiting from a large availability of formal description techniques and analysis methodologies. These allow both the phenomena investigated to be precisely modeled and virtual experiments to be performed in silico. Such experiments may result in easier, faster, and satisfying approximations of their in vitro/vivo counterparts. A promising approach is represented by the study of biological phenomena as a collection of interactive entities through process calculi equipped with stochastic semantics. These exploit formal grounds developed in the theory of concurrency in computer science, account for the not continuous, nor discrete, nature of many phenomena, enjoy nice compositional properties and allow for simulations that have been demonstrated to be coherent with data in literature.

Results

Motivated by the need to address some aspects of the functioning of neural synapses, we have developed one such model for synaptic processes in the calyx of Held, which is a glutamatergic synapse in the auditory pathway of the mammalia. We have developed such a stochastic model starting from existing kinetic models based on ODEs of some sub-components of the synapse, integrating other data from literature and making some assumptions about non-fully understood processes. Experiments have confirmed the coherence of our model with known biological data, also validating the assumptions made. Our model overcomes some limitations of the kinetic ones and, to our knowledge, represents the first model of synaptic processes based on process calculi. The compositionality of the approach has permitted us to independently focus on tuning the models of the pre- and post- synaptic traits, and then to naturally connect them, by dealing with “interface” issues. Furthermore, we have improved the expressiveness of the model, e.g. by embedding easy control of element concentration time courses. Sensitivity analysis over several parameters of the model has provided results that may help clarify the dynamics of synaptic transmission, while experiments with the model of the complete synapse seem worth explaining short-term plasticity mechanisms.

Conclusions

Specific presynaptic and postsynaptic mechanisms can be further analysed under various conditions, for instance by studying the presynaptic behaviour under repeated activations. The level of details of the description can be refined, for instance by further specifying the neurotransmitter generation and release steps. Taking advantage of the compositionality of the approach, an enhanced model could then be composed with other neural models, designed within the same framework, in order to obtain a more detailed and comprehensive model. In the long term, we are interested, in particular, in addressing models of synaptic plasticity, i.e. activity dependent mechanisms, which are the bases of memory and learning processes.

More on the computer science side, we plan to follow some directions to improve the underlying computational model and the linguistic primitives it provides as suggested by the experiments carried out, e.g. by introducing a suitable notion of (spatial) locality.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.