Biology subjects

Marder, E.

Publications and source records attributed to Marder, E..

32 records · Page 2Linked to original sources

bioRxiv2020

Coupling between fast and slow oscillator circuits in Cancer borealis is temperature compensated

Coupled oscillatory circuits are ubiquitous in nervous systems. Given that most biological processes are temperature sensitive, it is remarkable that the neuronal circuits of poikilothermic animals can maintain coupling across a wide range of temperatures. Within the stomatogastric ganglion (STG) of the crab, Cancer borealis, the fast pyloric rhythm (~1Hz) and the slow gastric mill rhythm (~0.1Hz) are precisely coordinated at ~11{degrees}C such that there is an integer number of pyloric cycles per gastric mill cycle (integer coupling). Upon increasing temperature from 7-23{degrees}C, both oscillators showed similar temperature-dependent increases in cycle frequency, and integer coupling between the circuits was conserved. Thus, although both rhythms show temperature dependent changes in rhythm frequency, the processes that couple these circuits maintain their coordination over a wide range of temperature. Such robustness to temperature changes could be part of a toolbox of processes that enables neural circuits to maintain function despite global perturbations.

neuroscience↗

bioRxiv2019

Dynamic clamp constructed phase diagram of the Hodgkin-Huxley action potential model

Excitability - a threshold governed transient in transmembrane voltage - is a fundamental physiological process that controls the function of the heart, endocrine, muscles and neuronal tissues. The 1950s Hodgkin and Huxley explicit formulation provides a mathematical framework for understanding excitability, as the consequence of the properties of voltage-gated sodium and potassium channels. The Hodgkin-Huxley model is more sensitive to parametric variations of protein densities and kinetics than biological systems whose excitability is apparently more robust. It is generally assumed that the models sensitivity reflects missing functional relations between its parameters or other components present in biological systems. Here we experimentally construct excitable membranes using the dynamic clamp and voltage-gated potassium ionic channels (Kv1.3) expressed in Xenopus oocytes. We take advantage of a theoretically derived phase diagram, where the phenomenon of excitability is reduced to two dimensions defined as combinations of the Hodgkin-Huxley model parameters. This biological-computational hybrid enabled us to explore functional relations in the parameter space, experimentally validate the phase diagram of the Hodgkin-Huxley model, and demonstrate activity-dependence and hysteretic dynamics due to the impacts of slow inactivation kinetics. The experimental results presented here provide new in-sights into the gap between technology-guided high-dimensional descriptions, and a lower, physiological dimensionality, within which biological function is embedded.

neuroscience↗

bioRxiv2019

Homeostatic plasticity rules that compensate for cell size are susceptible to channel deletion

Neurons can increase in size dramatically during growth. In many species neurons must preserve their intrinsic dynamics and physiological function across several length scales. For example, neurons in crustacean central pattern generators generate similar activity patterns despite multiple-fold increases in their size and changes in morphology. This scale invariance hints at regulation mechanisms that compensate for size changes by somehow altering membrane currents. Using conductance-based neuron models, we asked whether simple activity-dependent feedback can maintain intrinsic voltage dynamics in a neuron as its size is varied. Despite relying only on a single sensor that measures time-averaged intracellular calcium as a proxy for activity, we found that this regulation mechanism could regulate conductance densities of ion channels, and was robust to changes in the size of the neuron. By mapping changes in cell size onto perturbations in the space of conductance densities of all channels, we show how robustness to size change coexists with sensitivity to perturbations that alter the ratios of maximum conductances of different ion channel types. Our findings suggest that biological regulation that is optimized for coping with expected perturbations such as size changes will be vulnerable to other kinds of perturbations such as channel deletions.

neuroscience↗

bioRxiv2019

Temperature compensation in a small rhythmic circuit

Temperature affects the conductances and kinetics of the ionic channels that underlie neuronal activity. Each membrane conductance has a different characteristic temperature sensitivity, which raises the question of how neurons and neuronal circuits can operate robustly over wide temperature ranges. To address this, we employed computational models of the pyloric network of crabs and lobsters. We employed a landscape optimization scheme introduced previously (Alonso and Marder, 2019) to produce multiple different models that exhibit triphasic pyloric rhythms over a range of temperatures. We use the currentscapes introduced in (Alonso and Marder, 2019) to explore the dynamics of model currents and how they change with temperature. We found that temperature changes the relative contributions of the currents to neuronal activity so that rhythmic activity smoothly slides through changes in mechanisms. Moreover, the responses of the models to extreme perturbations--such as gradually decreasing a current type--are often qualitatively different at different temperatures.

neuroscience↗

bioRxiv2019

Molecular Profiling to Infer Neuronal Cell Identity: Lessons from small ganglia of the Crab Cancer borealis

Understanding circuit organization depends on identification of cell types. Recent advances in transcriptional profiling methods have enabled classification of cell types by their gene expression. While exceptionally powerful and high throughput, the ground-truth validation of these methods is difficult: if cell type is unknown, how does one assess whether a given analysis accurately captures neuronal identity? To shed light on the capabilities and limitations of solely using transcriptional profiling for cell type classification, we performed two forms of transcriptional profiling - RNA-seq and quantitative RT-PCR, in single, unambiguously identified neurons from two small crustacean networks: the stomatogastric and cardiac ganglia. We then combined our knowledge of cell type with unbiased clustering analyses and supervised machine learning to determine how accurately functionally-defined neuron types can be classified by expression profile alone. Our results demonstrate that expression profile is able to capture neuronal identity most accurately when combined with multimodal information that allows for post-hoc grouping so analysis can proceed from a supervised perspective. Solely unsupervised clustering can lead to misidentification and an inability to distinguish between two or more cell types. Therefore, our study supports the general utility of cell identification by transcriptional profiling, but adds a caution: it is difficult or impossible to know under what conditions transcriptional profiling alone is capable of assigning cell identity. Only by combining multiple modalities of information such as physiology, morphology or innervation target can neuronal identity be unambiguously determined.\n\nSIGNIFICANCE STATEMENTSingle cell transcriptional profiling has become a widespread tool in cell identification, particularly in the nervous system, based on the notion that genomic information determines cell identity. However, many cell type classification studies are unconstrained by other cellular attributes (e.g., morphology, physiology). Here, we systematically test how accurately transcriptional profiling can assign cell identity to well-studied anatomically- and functionally-identified neurons in two small neuronal networks. While these neurons clearly possess distinct patterns of gene expression across cell types, their expression profiles are not sufficient to unambiguously confirm their identity. We suggest that true cell identity can only be determined by combining gene expression data with other cellular attributes such as innervation pattern, morphology, or physiology.

neuroscience↗

bioRxiv2019

Rapid adaptation to elevated extracellular potassium in the pyloric circuit of the crab, Cancer borealis

Elevated [K+] is often used to alter excitability in neurons and networks by shifting the potassium equilibrium potential (EK) and, consequently, the resting membrane potential. We studied the effects of increased extracellular [K+] on the well-described pyloric circuit of the crab, Cancer borealis. A 2.5-fold increase in extracellular [K+] (2.5x[K+]) depolarized Pyloric Dilator (PD) neurons and resulted in short-term loss of their normal bursting activity. This period of silence was followed within 5-10 minutes by the recovery of spiking and/or bursting activity during continued superfusion of 2.5x[K+] saline. In contrast, when PD neurons were pharmacologically isolated from pyloric presynaptic inputs, they exhibited no transient loss of spiking activity in 2.5x[K+], suggesting the presence of an acute inhibitory effect mediated by circuit interactions. Action potential threshold in PD neurons hyperpolarized during an hour-long exposure to 2.5x[K+] concurrent with the recovery of spiking and/or bursting activity. Thus, the initial loss of activity appears to be mediated by synaptic interactions within the network, but the secondary adaptation depends on changes in the intrinsic excitability of the pacemaker neurons. The complex sequence of events in the responses of pyloric neurons to elevated [K+] demonstrates that electrophysiological recordings are necessary to determine both the transient and longer-term effects of even modest alterations of K+ concentrations on neuronal activity. Significance StatementSolutions with elevated extracellular potassium are commonly used as a depolarizing stimulus. Moreover, hyperkalemia is associated with a number of disease states, including epileptic seizures and brain traumas. We studied the effects of high [K+] saline on the well-described pyloric circuit of the crab stomatogastric ganglion. A 2.5-fold increase in extracellular [K+] led to a transient loss of activity in pyloric neurons that was not due to depolarization block. This was followed by a rapid increase in excitability and concurrent recovery of spiking and rhythmic bursting activity within minutes. These results suggest that effects of high [K+] on neuronal circuits can be complex and non-stationary.

neuroscience↗

bioRxiv2018

Neural circuit robustness to acute, global physiological perturbations

Neural function depends on underlying physiological processes that are highly sensitive to physical variables such as temperature. However, some robustness to perturbations in these variables manifests at the circuit level, suggesting that circuit properties are organized to tolerate consistent changes in underlying parameters. We show that a crustacean pacemaker circuit is robust to two global perturbations - temperature and pH - that differentially alter circuit properties. Consistent with high variability in underlying circuit parameters, we find that the critical temperatures and pH values where circuit activity breaks down vary widely across animals. Despite variability in critical points the network state transitions at these critical points are consistent, implying that qualitative circuit dynamics are preserved across animals, in spite of high quantitative parameter variability. Surprisingly, robustness perturbations in pH only moderately affect temperature robustness. Thus, robustness to a global perturbation does not necessarily imply sensitivity to other global perturbations.

bioinformatics↗

bioRxiv2018

Visualization of the relative contributions of conductances in neuronal models with similar behavior and different conductance densities

Conductance-based models of neural activity produce large amounts of data that can be hard to visualize and interpret. Here we introduce two novel visualization methods to display the dynamics of the ionic currents, and to investigate how the contribution of each current changes in response to perturbation. We explored the solutions of a single compartment, conductance-based model of neural activity with seven voltage-gated ionic currents and a leak channel. We employed landscape optimization to find sets of maximal conductances that produce similar target activity and displayed the dynamics of the currents. We examined in detail six examples of a bursting model neuron that differ as much as 3-fold in the conductance densities of each of the 8 currents in the model. The maximal conductance of each current does not simply predict the importance of the current for neuronal dynamics. We then compared the effects of systematically reducing the conductances of each current for neuronal dynamics, and demonstrate that models that appear similar under starting conditions behave dramatically differently to the decreases in conductance densities. These examples provide heuristic insight into why individuals with similar behavior can nonetheless respond widely differently to perturbations.

neuroscience↗

bioRxiv2018

Xolotl: An Intuitive and Approachable Neuron and Network Simulator for Research and Teaching

Conductance-based models of neurons are used extensively in computational neuroscience. Working with these models can be challenging due to their high dimensionality and large number of parameters. Here, we present a neuron and network simulator built on a novel automatic type system that binds object-oriented code written in C++ to objects in MATLAB. Our approach builds on the tradition of uniting the speed of languages like C++ with the ease-of-use and feature-set of scientific programming languages like MATLAB. Xolotl allows for the creation and manipulation of hierarchical models with components that are named and searchable, permitting intuitive high-level programmatic control over all parts of the model. The simulators architecture allows for the interactive manipulation of any parameter in any model, and for visualizing the effects of changing that parameter immediately. Xolotl is fully featured with hundreds of ion channel models from the electrophysiological literature, and can be extended to include arbitrary conductances, synapses, and mechanisms. Several core features like bookmarking of parameters and automatic hashing of source code facilitate reproducible and auditable research. Its ease of use and rich visualization capabilities make it an attractive option in teaching environments. Finally, xolotl is written in a modular fashion, includes detailed tutorials and worked examples, and is freely available at https://github.com/sg-s/xolotl, enabling seamless integration into the workflows of other researchers.

neuroscience↗

bioRxiv2018

Two central pattern generators from the crab, Cancer borealis, respond robustly and differentially to extreme extracellular pH

Animals and their neuronal circuits must maintain function despite significant environmental fluctuations. The crab, Cancer borealis, experiences daily changes in ocean temperature and pH. Here, we describe the effects of extreme changes in extracellular pH - from pH 5.5 to 10.4 - on two central pattern generating networks, the stomatogastric and cardiac ganglia of C. borealis. Given that the physiological properties of ion channels are known to be sensitive to pH within the range tested, it is surprising that these rhythms generally remained robust from pH 6.1 to pH 8.8. Unexpectedly, the stomatogastric ganglion was more sensitive to acid while the cardiac ganglion was more sensitive to base. Considerable animal-to-animal variability was likely a consequence of similar network performance arising from variable sets of underlying conductances. Together, these results illustrate the potential difficulty in generalizing the effects of environmental perturbation across circuits, even within the same animal.\n\nAbbreviations

neuroscience↗

bioRxiv2018

Neuronal morphologies built for reliable physiology in a rhythmic motor circuit

The neurons of the crustacean stomatogastric ganglion (STG) exhibit highly-conserved firing patterns, voltage waveforms, and circuit functions despite quantifiable animal-to-animal variability in their neuronal morphologies. In recent work, we showed that one neuron type, the Gastric Mill (GM) neuron, is electrotonically compact and operates much like a single compartment, despite having thousands of branch points and a total cable length on the order of 10 mm. Here, we explore how STG neurite morphology shapes voltage signal propagation and summation in four STG neuron types. We use focal glutamate photo-uncaging in tandem with somatic intracellular recordings to examine passive electrotonic structure and voltage signal summation in the GM neuron and three additional STG neuron types: Lateral Pyloric (LP), Ventricular Dilator (VD), and Pyloric Dilator (PD) neurons. In each neuron, we measured the amplitudes and apparent reversal potentials (Erevs) of inhibitory responses evoked with focal glutamate photo-uncaging at more than 20 sites varying in their distance (100-800 m) from the somatic recording site in the presence of TTX. Apparent Erevs were relatively invariant (mean CVs = 0.04, 0.06, 0.05, and 0.08 for 5-6 GM, LP, PD, VD neurons, respectively), suggesting that all four neuron types are similarly electrotonically uniform and compact. We then characterized the directional sensitivity and arithmetic of voltage summation (with fast sequential activation of 4-6 sites) in individual STG neurites. All four neuron types showed no directional bias in voltage signal summation and linear voltage summation. We motivate these experiments with a proof-of-concept computational model that suggests the immense tapering of STG neurite diameters: from 10-20 m to sub-micron diameters at the terminal tips, may explain the uniform electrotonic structures experimentally observed and contribute to the robust nature of this central pattern-generating circuit.

neuroscience↗

bioRxiv2018

In vivo Effects of Temperature on the Heart and Pyloric Rhythms in the Crab, Cancer borealis

Summary StatementTemperature elevation increases the frequency of the heart and pyloric rhythms of the crab, Cancer borealis, but the heart rhythm has a higher critical temperature than the pyloric rhythm.\n\nAbstractThe heart and pyloric rhythms of crustaceans have been studied separately and extensively over many years. Local and hormonal neuromodulation and sensory inputs onto these central pattern generating circuits play a significant role in the animals responses to perturbations, but are usually lost or removed during in vitro studies. To examine simultaneously the in vivo motor output of the heart and pyloric rhythms, we used photoplethysmography (PPG). In the population measured (n = 49), the heart rhythm frequencies ranged from 0.3-2.3 Hz. The pyloric rhythms varied from 0.2-1.6 Hz. During multiple hour-long recordings, many animals held at control temperature showed strong inhibitory bouts in which the heart decreased in frequency or become quiescent and the pyloric rhythm also decreased in frequency. Many animals show significant coherence in frequency between the rhythms at the frequency of the heart rhythm. We measured the simultaneous responses of the rhythms to temperature ramps by heating or cooling the saline bath while recording both the heart and pyloric muscle movements. Q10s, critical temperatures (temperatures at which function is compromised), and changes in frequency were calculated for each of the rhythms tested. The heart rhythm was more robust to high temperature than the pyloric rhythm.

neuroscience↗

bioRxiv2017

Regulation of Eag by calcium/calmodulin controls presynaptic excitability in Drosophila

Drosophila ether-a-go-go (eag) is the founding member of a large family of voltage-gated K+ channels, the KCNH family, which includes Kv10, 11 and 12, (Ganetzky et al. 1999). Concurrent binding of calcium/calmodulin (Ca2+/CaM) to N-and C-terminal sites inhibits mammalian EAG1 channels at sub-micromolar Ca2+ concentrations (Schonherr et al. 2000), likely by causing pore constriction (Whicher and MacKinnon 2016). Although the Drosophila EAG channel was believed to be Ca2+-insensitive (Schonherr et al. 2000), both the N-and C-terminal sites are conserved. Here we show that Drosophila EAG is inhibited by high Ca2+ concentrations that are only present at plasma membrane Ca2+ channel microdomains. To test the role of this regulation in vivo, we engineered mutations that block CaM-binding to the major C-terminal site of the endogenous eag locus, disrupting Ca2+-dependent inhibition. eag CaMBD mutants have reduced evoked release from larval motor neuron presynaptic terminals and show decreased Ca2+ influx in stimulated adult projection neuron presynaptic terminals, consistent with an increase in K+ conductance. These results are predicted by a conductance-based multi-compartment model of the presynaptic terminal in which some fraction of EAG is localized to the Ca2+ channel microdomains that control neurotransmitter release. The reduction of release in the larval neuromuscular junction drives a compensatory increase in motor neuron somatic excitability. This misregulation of synaptic and somatic excitability has consequences for systems-level processes and leads to defects in associative memory formation in adults.\n\nNew and NoteworthyRegulation of excitability is critical to tuning the nervous system for complex behaviors. We demonstrate here that the EAG family of voltage-gated K+ channels exhibit conserved gating by Ca2+/CaM. Disruption of this inhibition in Drosophila results in decreased evoked neurotransmitter release due to truncated Ca2+ influx in presynaptic terminals. In adults, disrupted Ca2+ dynamics cripples memory formation. These data demonstrate that the biophysical details of channels have important implications for cell function and behavior.

neuroscience↗

bioRxiv2017

Circuit robustness to temperature perturbation is altered by neuromodulators

In the ocean, the crab, Cancer borealis, is subject to daily and seasonal temperature changes. Previous work, done in the presence of descending modulatory inputs, had shown that the pyloric rhythm of the crab increases in frequency as temperature increases, but maintains its characteristic phase relationships until it \"crashes\" at extreme high temperatures. To study the interaction between neuromodulators and temperature perturbations, we studied the effects of temperature on preparations from which the descending modulatory inputs were removed. Under these conditions the pyloric rhythm was destabilized. We then studied the effects of temperature on preparations in the presence of oxotremorine, proctolin, and serotonin. Oxotremorine and proctolin enhanced the robustness of the pyloric rhythm, while serotonin made the rhythm less robust. These experiments reveal considerable animal-to-animal diversity in their crash stability, consistent with the interpretation that cryptic differences in many cell and network parameters are revealed by extreme perturbations.

neuroscience↗