Conductance-based models of neural activity produce large amounts of data that can be hard to vis... more 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. Keywords: neuronal oscillators, N a + channels, Ca ++ channels, K + channels, leak channels.
presence of an activator domain which promotes the step. This suggests that the addition of a com... more presence of an activator domain which promotes the step. This suggests that the addition of a component that binds to promoter DNA in a nonspecific fashion (that is, chromatin proteins) could under some conditions satisfy some of the assay properties suggested previously for coactivators. In a mechanistic sense, little is known about the kinetics of TFIIB binding to the TFIIDtemplate complex. The specific footprint generated by the binding of TFIID over the initiation site is not pronounced and is confined to one strand of the template 4 , so TFIIB may not be strongly bound to the template in the absence of an activator domain. Final answers to most of the questions addressed by these experiments await the purification and analysis of the specific proteins that constitute TFIIB and the other basal components. The most purified preparations of TFIIB contain a protein of relative molecular mass 32,000, and specific antiserum has been prepared 11 which reacts with pro-tein in the template TFIID-TFIIB complex. It is highly likely that a complementary DNA encoding this factor will be available shortly, and more refined studies of the interaction of the protein with the acidic activator and TFIID will then be possible. D
We studied the effects of increased sodium conductance on firing rate and gain in two populations... more We studied the effects of increased sodium conductance on firing rate and gain in two populations of conductance-based, single-compartment model neurons. The first population consisted of 1000 model neurons with differing values of seven voltage-dependent conductances. In many of these models, increasing the sodium conductance threefold unexpectedly reduced the firing rate and divisively scaled the gain at high input current. In the second population, consisting of 1000 simplified model neurons, we found that enhanced sodium conductance changed the frequencycurrent (FI) curve in two computationally distinct ways, depending on the firing rate. In these models, increased sodium conductance produced a subtractiveshiftintheFIcurveatlowfiringratesbecausetheadditionalsodiumconductanceallowedtheneurontorespondmorestronglytoequivalent input current. In contrast, at high input current, the increase in sodium conductance resulted in a divisive change in the gain because the increased conductance produced a proportionally larger activation of the delayed rectifier potassium conductance. The control and sodium-enhanced FI curves intersect at a point that delimits two regions in which the same biophysical manipulation produces two fundamentally different changes to the model neuron's computational properties. This suggests a potentially difficult problem for homeostatic regulation of intrinsic excitability.
The crustacean stomatogastric ganglion (STG) is modulated by numerous neuropeptides that are rele... more The crustacean stomatogastric ganglion (STG) is modulated by numerous neuropeptides that are released locally in the neuropil or that reach the STG as neurohormones. Using 1,5diaminonaphthalene (DAN) as a reductive screening matrix for matrix-assisted laser desorption/ ionization (MALDI) mass spectrometric profiling of disulfide bond-containing C-type allatostatin peptides followed by electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) tandem mass spectrometric (MS/MS) analysis, we identified and sequenced a novel C-type allatostatin peptide (CbAST-C1), pQIRYHQCYFNPISCF-COOH, present in the pericardial organs of the crab, Cancer borealis. Another C-type allatostatin (CbAST-C2), SYWKQCAFNAVSCFamide, was discovered using the expressed sequence tag (EST) database search strategy in both C. borealis and the lobster, Homarus americanus, and further confirmed with de novo sequencing using ESI-Q-TOF tandem MS. Electrophysiological experiments demonstrated that both CbAST-C1 and CbAST-C2 inhibited the frequency of the pyloric rhythm of the STG, in a state-dependent manner. At 10 -6 M, both peptides were only modestly effective when initial frequencies of the pyloric rhythm were >0.8 Hz, but almost completely suppressed the pyloric rhythm when applied to preparations with starting frequencies < 0.7Hz. Surprisingly, these state-dependent actions are similar to those of the structurally unrelated allatostatin A and allatostatin B families of peptides.
Proceedings of the National Academy of Sciences of the United States of America, Aug 7, 2007
Data deposition: The sequences reported in this paper have been deposited in the GenBank database... more Data deposition: The sequences reported in this paper have been deposited in the GenBank database [accession nos. DQ103254 (Cab-shal), DQ103256 (Cab-BKKCa), DQ103255 (Cabshab), DQ103257 (Cab-IH), EF089568 (Cab-para), and EF089569 (Cab-shaw)].
The neurons of the crustacean stomatogastric ganglion (STG) exhibit highly-conserved firing patte... more The neurons of the crustacean stomatogastric ganglion (STG) exhibit highly-conserved firing patterns, voltage waveforms, and circuit functions despite quantifiable animal-toanimal 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 (E rev s) of inhibitory responses evoked with focal glutamate photo-uncaging at more than 20 sites varying in their distance (100-800 µm) from the somatic recording site in the presence of TTX. Apparent E rev s 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 µm 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. .
How tightly tuned are the synaptic and intrinsic properties that give rise to neuron and circuit ... more How tightly tuned are the synaptic and intrinsic properties that give rise to neuron and circuit function? Experimental work shows that these properties vary considerably across identified neurons in different animals. Given this variability in experimental data, this review describes some of the complications of building computational models to aid in understanding how system dynamics arise from the interaction of system components. We argue that instead of trying to build a single model that captures the generic behavior of a neuron or circuit, it is beneficial to construct a population of models that captures the behavior of the population that provided the experimental data. Studying a population of models with different underlying structure and similar behaviors provides opportunities to discover unsuspected compensatory mechanisms that contribute to neuron and network function.
SummaryThe Na+channels that are important for action potentials show rapid inactivation, a state ... more SummaryThe Na+channels that are important for action potentials show rapid inactivation, a state in which they do not conduct, although the membrane potential remains depolarized1,2. Rapid inactivation is a determinant of millisecond scale phenomena, such as spike shape and refractory period. Na+channels also inactivate orders of magnitude more slowly, and therefore have impacts on excitability over much longer time scales than those of a single spike or a single inter-spike interval3-9. Here, we focus on the contribution of slow inactivation to the resilience of axonal excitability10,11when ion channels are unevenly distributed across the axonal membrane. We study models in which the voltage-gated Na+and K+channels are unevenly distributed along axons with different variances, capturing the heterogeneity that biological axons display12. In the absence of slow inactivation many conductance distributions result in spontaneous tonic activity. Faithful axonal propagation is achieved wi...
Axons reliably conduct action potentials between neurons and/or other targets. Axons have widely ... more Axons reliably conduct action potentials between neurons and/or other targets. Axons have widely variable diameters and can be myelinated or unmyelinated. Although the effect of these factors on propagation speed is well studied, how they constrain axonal resilience to high frequency spiking is incompletely understood. Maximal firing frequencies range from ~ 1 Hz to > 300 Hz across neurons, but the process by which Na/K pumps counteract Na+ influx is slow, and it is unclear the extent to which slow Na+ removal is compatible with high frequency spiking. Modeling the process of Na+ removal shows that large diameter axons are more resilient to high frequency spikes than small diameter axons, because of their slow Na+ accumulation. In myelinated axons, the myelinated compartments between nodes of Ranvier act as a ‘reservoir’ to slow Na+ accumulation and increase the reliability of axonal propagation. We now find that slowing the activation of K+ current can increase the Na+ influx ra...
A pharmacological analysis was made of the depolarizing acetylcholine (ACh) response found on the... more A pharmacological analysis was made of the depolarizing acetylcholine (ACh) response found on the gastric mill 1 muscles of the crabs Cancer pagurus, Cancer irroratus and Cancer borealis. Acetylcholine, carbamylcholine, trimethylammonium, nicotine, and dimethyl-4-phenyl-piperazinium were effective in producing contractures and depolarizations in these muscles. No response to decamethonium, suberyldicholine, acetyl-β-methylcholine, carbamyl-β-methylcholine, pilocarpine and oxotremorine could be detected. High concentrations of muscarinic agonists (10−4 to 10−3M) potentiated and prolonged the ACh iontophoretic response. When the acetylcholinesterase activity was inhibited with neostigmine, or when the response was elicited with carbamylcholine, muscarinic agonists partially inhibited the response. ACh responses were most effectively blocked by vertebrate nicotinic ganglionic antagonists, including dihydro-β-erythroidine, pempidine, and mecamylamine. α-Bungarotoxin was without effect o...
There is more to theory in biology than replicating the results of experiments – the best theory ... more There is more to theory in biology than replicating the results of experiments – the best theory papers help experimentalists to identify which of their results might be general and to plan a path through the maze of all possible future experiments.
Drosophila ether-à-go-go (eag) is the founding member of a large family of voltage-gated K+ chann... more Drosophila ether-à-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 decr...
• Neural activity is generically highly temperature sensitive • Neurons achieve temperature robus... more • Neural activity is generically highly temperature sensitive • Neurons achieve temperature robustness with highly variable conductance densities • Feedback regulation shapes variability to permit temperature robust neural activity • Robustness to global perturbations constrains cellular regulation mechanisms eTOC summary All biochemical processes, including neuronal activity, are temperature sensitive. Yet many animal species experience large temperature fluctuations. O'Leary and Marder show how a simple regulatory control mechanism can ensure temperature robust neural activity by balancing expression of multiple, temperature dependent ion channel types.
Understanding how the brain works requires a delicate balance between the appreciation of the imp... more Understanding how the brain works requires a delicate balance between the appreciation of the importance of a multitude of biological details and the ability to see beyond those details to general principles. As technological innovations vastly increase the amount of data we collect, the importance of intuition into how to analyze and treat these data may, paradoxically, become more important. This Essay is part of the "Where Next?" Series. Experimental biologists collect details. In the early days, naturalists prowled their backyards, local forests, and meadows. They traveled the Amazon River and African savannahs and collected species and categorized them. These collectors of beetles and ferns then tried to formulate hypotheses about evolutionary relationships by looking at commonalities of structure, function, and development. In those days, there was an implicit belief that the passionate acquisition of detailed information about the idiosyncrasies of individual species contained the route to understanding the general principles of life. Although today's experimental neuroscientists employ much more sophisticated methods, most retain a deep conviction that the specific properties of molecules, synapses, neurons, circuits, and connectomes are important for understanding how brains, be they small or large, work. Modern neuroscience traces much of its history to prescient physiologists, pharmacologists, and anatomists. Early anatomists such as Ramón y Cajal pioneered the use of stains to reveal the structure of neurons and to make astonishing leaps of intuition about the structure and function of brain circuits . Early physiologists and pharmacologists deduced the existence of receptors and kinetics from bioassays . Observation and reasoning from first principles led T. Graham Brown to first articulate that reciprocal inhibition in the spinal cord could underlie the generation of rhythmic movements. Cajal and Brown anticipated systems neuroscience as we know it today: understanding how the particular properties of neurons and their connections give rise to the complex and adaptive responses that allow animals to interact with each other and their worlds.
Early neuroscientists scoured the animal kingdom for the ideal preparation with which to study sp... more Early neuroscientists scoured the animal kingdom for the ideal preparation with which to study specific problems of interest. Today, non-mammalian nervous systems continue to provide ideal platforms for the study of fundamental problems in neuroscience. Indeed, the peculiarities of body plan and nervous systems that have evolved to carry out precise tasks in unique ecological niches enable investigators not only to pose specific scientific questions, but also to uncover principles that are general to all nervous systems.
Interactions among rhythmically active neuronal circuits that oscillate at different frequencies ... more Interactions among rhythmically active neuronal circuits that oscillate at different frequencies are important for generating complex behaviors, yet little is known about the underlying cellular mechanisms. We addressed this issue in the crab stomatogastric ganglion (STG), which contains two distinct but interacting circuits. These circuits generate the gastric mill rhythm (cycle period, ϳ10 sec) and the pyloric rhythm (cycle period, ϳ1 sec). When the identified modulatory projection neuron named modulatory commissural neuron 1 (MCN1) is activated, the gastric mill motor pattern is generated by interactions among MCN1 and two STG neurons [the lateral gastric (LG) neuron and interneuron 1]. We show that, during MCN1 stimulation, an identified synapse from the pyloric circuit onto the gastric mill circuit is pivotal for determining the gastric mill cycle period and the gastric-pyloric rhythm coordination. To examine the role of this intercircuit synapse, we replaced it with a computational equivalent via the dynamic-clamp technique. This enabled us to manipulate better the timing and strength of this synapse. We found this synapse to be necessary for production of the normal gastric mill cycle period. The synapse acts, during each LG neuron interburst, to boost rhythmically the influence of the modulatory input from MCN1 to LG and thereby to hasten LG neuron burst onset. The two rhythms become coordinated because LG burst onset occurs with a constant latency after the onset of the triggering pyloric input. These results indicate that intercircuit synapses can enable an oscillatory circuit to control the speed of a slower oscillatory circuit, as well as provide a mechanism for intercircuit coordination.
performed preliminary experiments that led to several of the hypotheses addressed here. Dr. Ekate... more performed preliminary experiments that led to several of the hypotheses addressed here. Dr. Ekaterina Morozova provided essential guidance pertaining to dynamic clamp experiments.
In complex nervous systems patterns of neuronal activity and measures of intrinsic neuronal excit... more In complex nervous systems patterns of neuronal activity and measures of intrinsic neuronal excitability are often used as criteria for identifying and/or classifying neurons. We asked how well identification of neurons by conventional measures of intrinsic excitability compares with a measure of neuronal excitability derived from a neuron's behavior in a dynamic clamp constructed two-cell network. We used four cell types from the crab stomatogastric ganglion: the pyloric dilator, lateral pyloric, gastric mill, and dorsal gastric neurons. Each neuron was evaluated for six conventional measures of intrinsic excitability (intrinsic properties, IPs). Additionally, each neuron was coupled by reciprocal inhibitory synapses made with the dynamic clamp to a Morris-Lecar model neuron and the resulting network was assayed for four measures of network activity (network activity properties, NAPs). We searched for linear combinations of IPs that correlated with each NAP, and combinations of NAPs that correlated with each IP. In the process we developed a method to correct for multiple correlations while searching for correlating features. When properly controlled for multiple correlations, four of the IPs were correlated with NAPs, and all four NAPs were correlated with IPs. Neurons were classified into cell types by training a linear classifier on sets of properties, or using k-medoids clustering.The IPs were modestly successful in classifying the neurons, and the NAPs were more successful. Combining the two measures did better than either measure alone, but not well enough to classify neurons with perfect accuracy, thus reiterating that electrophysiological measures of single-cell properties alone are not sufficient for reliable cell identification.
Conductance-based models of neural activity produce large amounts of data that can be hard to vis... more 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. Keywords: neuronal oscillators, N a + channels, Ca ++ channels, K + channels, leak channels.
presence of an activator domain which promotes the step. This suggests that the addition of a com... more presence of an activator domain which promotes the step. This suggests that the addition of a component that binds to promoter DNA in a nonspecific fashion (that is, chromatin proteins) could under some conditions satisfy some of the assay properties suggested previously for coactivators. In a mechanistic sense, little is known about the kinetics of TFIIB binding to the TFIIDtemplate complex. The specific footprint generated by the binding of TFIID over the initiation site is not pronounced and is confined to one strand of the template 4 , so TFIIB may not be strongly bound to the template in the absence of an activator domain. Final answers to most of the questions addressed by these experiments await the purification and analysis of the specific proteins that constitute TFIIB and the other basal components. The most purified preparations of TFIIB contain a protein of relative molecular mass 32,000, and specific antiserum has been prepared 11 which reacts with pro-tein in the template TFIID-TFIIB complex. It is highly likely that a complementary DNA encoding this factor will be available shortly, and more refined studies of the interaction of the protein with the acidic activator and TFIID will then be possible. D
We studied the effects of increased sodium conductance on firing rate and gain in two populations... more We studied the effects of increased sodium conductance on firing rate and gain in two populations of conductance-based, single-compartment model neurons. The first population consisted of 1000 model neurons with differing values of seven voltage-dependent conductances. In many of these models, increasing the sodium conductance threefold unexpectedly reduced the firing rate and divisively scaled the gain at high input current. In the second population, consisting of 1000 simplified model neurons, we found that enhanced sodium conductance changed the frequencycurrent (FI) curve in two computationally distinct ways, depending on the firing rate. In these models, increased sodium conductance produced a subtractiveshiftintheFIcurveatlowfiringratesbecausetheadditionalsodiumconductanceallowedtheneurontorespondmorestronglytoequivalent input current. In contrast, at high input current, the increase in sodium conductance resulted in a divisive change in the gain because the increased conductance produced a proportionally larger activation of the delayed rectifier potassium conductance. The control and sodium-enhanced FI curves intersect at a point that delimits two regions in which the same biophysical manipulation produces two fundamentally different changes to the model neuron's computational properties. This suggests a potentially difficult problem for homeostatic regulation of intrinsic excitability.
The crustacean stomatogastric ganglion (STG) is modulated by numerous neuropeptides that are rele... more The crustacean stomatogastric ganglion (STG) is modulated by numerous neuropeptides that are released locally in the neuropil or that reach the STG as neurohormones. Using 1,5diaminonaphthalene (DAN) as a reductive screening matrix for matrix-assisted laser desorption/ ionization (MALDI) mass spectrometric profiling of disulfide bond-containing C-type allatostatin peptides followed by electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) tandem mass spectrometric (MS/MS) analysis, we identified and sequenced a novel C-type allatostatin peptide (CbAST-C1), pQIRYHQCYFNPISCF-COOH, present in the pericardial organs of the crab, Cancer borealis. Another C-type allatostatin (CbAST-C2), SYWKQCAFNAVSCFamide, was discovered using the expressed sequence tag (EST) database search strategy in both C. borealis and the lobster, Homarus americanus, and further confirmed with de novo sequencing using ESI-Q-TOF tandem MS. Electrophysiological experiments demonstrated that both CbAST-C1 and CbAST-C2 inhibited the frequency of the pyloric rhythm of the STG, in a state-dependent manner. At 10 -6 M, both peptides were only modestly effective when initial frequencies of the pyloric rhythm were >0.8 Hz, but almost completely suppressed the pyloric rhythm when applied to preparations with starting frequencies < 0.7Hz. Surprisingly, these state-dependent actions are similar to those of the structurally unrelated allatostatin A and allatostatin B families of peptides.
Proceedings of the National Academy of Sciences of the United States of America, Aug 7, 2007
Data deposition: The sequences reported in this paper have been deposited in the GenBank database... more Data deposition: The sequences reported in this paper have been deposited in the GenBank database [accession nos. DQ103254 (Cab-shal), DQ103256 (Cab-BKKCa), DQ103255 (Cabshab), DQ103257 (Cab-IH), EF089568 (Cab-para), and EF089569 (Cab-shaw)].
The neurons of the crustacean stomatogastric ganglion (STG) exhibit highly-conserved firing patte... more The neurons of the crustacean stomatogastric ganglion (STG) exhibit highly-conserved firing patterns, voltage waveforms, and circuit functions despite quantifiable animal-toanimal 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 (E rev s) of inhibitory responses evoked with focal glutamate photo-uncaging at more than 20 sites varying in their distance (100-800 µm) from the somatic recording site in the presence of TTX. Apparent E rev s 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 µm 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. .
How tightly tuned are the synaptic and intrinsic properties that give rise to neuron and circuit ... more How tightly tuned are the synaptic and intrinsic properties that give rise to neuron and circuit function? Experimental work shows that these properties vary considerably across identified neurons in different animals. Given this variability in experimental data, this review describes some of the complications of building computational models to aid in understanding how system dynamics arise from the interaction of system components. We argue that instead of trying to build a single model that captures the generic behavior of a neuron or circuit, it is beneficial to construct a population of models that captures the behavior of the population that provided the experimental data. Studying a population of models with different underlying structure and similar behaviors provides opportunities to discover unsuspected compensatory mechanisms that contribute to neuron and network function.
SummaryThe Na+channels that are important for action potentials show rapid inactivation, a state ... more SummaryThe Na+channels that are important for action potentials show rapid inactivation, a state in which they do not conduct, although the membrane potential remains depolarized1,2. Rapid inactivation is a determinant of millisecond scale phenomena, such as spike shape and refractory period. Na+channels also inactivate orders of magnitude more slowly, and therefore have impacts on excitability over much longer time scales than those of a single spike or a single inter-spike interval3-9. Here, we focus on the contribution of slow inactivation to the resilience of axonal excitability10,11when ion channels are unevenly distributed across the axonal membrane. We study models in which the voltage-gated Na+and K+channels are unevenly distributed along axons with different variances, capturing the heterogeneity that biological axons display12. In the absence of slow inactivation many conductance distributions result in spontaneous tonic activity. Faithful axonal propagation is achieved wi...
Axons reliably conduct action potentials between neurons and/or other targets. Axons have widely ... more Axons reliably conduct action potentials between neurons and/or other targets. Axons have widely variable diameters and can be myelinated or unmyelinated. Although the effect of these factors on propagation speed is well studied, how they constrain axonal resilience to high frequency spiking is incompletely understood. Maximal firing frequencies range from ~ 1 Hz to > 300 Hz across neurons, but the process by which Na/K pumps counteract Na+ influx is slow, and it is unclear the extent to which slow Na+ removal is compatible with high frequency spiking. Modeling the process of Na+ removal shows that large diameter axons are more resilient to high frequency spikes than small diameter axons, because of their slow Na+ accumulation. In myelinated axons, the myelinated compartments between nodes of Ranvier act as a ‘reservoir’ to slow Na+ accumulation and increase the reliability of axonal propagation. We now find that slowing the activation of K+ current can increase the Na+ influx ra...
A pharmacological analysis was made of the depolarizing acetylcholine (ACh) response found on the... more A pharmacological analysis was made of the depolarizing acetylcholine (ACh) response found on the gastric mill 1 muscles of the crabs Cancer pagurus, Cancer irroratus and Cancer borealis. Acetylcholine, carbamylcholine, trimethylammonium, nicotine, and dimethyl-4-phenyl-piperazinium were effective in producing contractures and depolarizations in these muscles. No response to decamethonium, suberyldicholine, acetyl-β-methylcholine, carbamyl-β-methylcholine, pilocarpine and oxotremorine could be detected. High concentrations of muscarinic agonists (10−4 to 10−3M) potentiated and prolonged the ACh iontophoretic response. When the acetylcholinesterase activity was inhibited with neostigmine, or when the response was elicited with carbamylcholine, muscarinic agonists partially inhibited the response. ACh responses were most effectively blocked by vertebrate nicotinic ganglionic antagonists, including dihydro-β-erythroidine, pempidine, and mecamylamine. α-Bungarotoxin was without effect o...
There is more to theory in biology than replicating the results of experiments – the best theory ... more There is more to theory in biology than replicating the results of experiments – the best theory papers help experimentalists to identify which of their results might be general and to plan a path through the maze of all possible future experiments.
Drosophila ether-à-go-go (eag) is the founding member of a large family of voltage-gated K+ chann... more Drosophila ether-à-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 decr...
• Neural activity is generically highly temperature sensitive • Neurons achieve temperature robus... more • Neural activity is generically highly temperature sensitive • Neurons achieve temperature robustness with highly variable conductance densities • Feedback regulation shapes variability to permit temperature robust neural activity • Robustness to global perturbations constrains cellular regulation mechanisms eTOC summary All biochemical processes, including neuronal activity, are temperature sensitive. Yet many animal species experience large temperature fluctuations. O'Leary and Marder show how a simple regulatory control mechanism can ensure temperature robust neural activity by balancing expression of multiple, temperature dependent ion channel types.
Understanding how the brain works requires a delicate balance between the appreciation of the imp... more Understanding how the brain works requires a delicate balance between the appreciation of the importance of a multitude of biological details and the ability to see beyond those details to general principles. As technological innovations vastly increase the amount of data we collect, the importance of intuition into how to analyze and treat these data may, paradoxically, become more important. This Essay is part of the "Where Next?" Series. Experimental biologists collect details. In the early days, naturalists prowled their backyards, local forests, and meadows. They traveled the Amazon River and African savannahs and collected species and categorized them. These collectors of beetles and ferns then tried to formulate hypotheses about evolutionary relationships by looking at commonalities of structure, function, and development. In those days, there was an implicit belief that the passionate acquisition of detailed information about the idiosyncrasies of individual species contained the route to understanding the general principles of life. Although today's experimental neuroscientists employ much more sophisticated methods, most retain a deep conviction that the specific properties of molecules, synapses, neurons, circuits, and connectomes are important for understanding how brains, be they small or large, work. Modern neuroscience traces much of its history to prescient physiologists, pharmacologists, and anatomists. Early anatomists such as Ramón y Cajal pioneered the use of stains to reveal the structure of neurons and to make astonishing leaps of intuition about the structure and function of brain circuits . Early physiologists and pharmacologists deduced the existence of receptors and kinetics from bioassays . Observation and reasoning from first principles led T. Graham Brown to first articulate that reciprocal inhibition in the spinal cord could underlie the generation of rhythmic movements. Cajal and Brown anticipated systems neuroscience as we know it today: understanding how the particular properties of neurons and their connections give rise to the complex and adaptive responses that allow animals to interact with each other and their worlds.
Early neuroscientists scoured the animal kingdom for the ideal preparation with which to study sp... more Early neuroscientists scoured the animal kingdom for the ideal preparation with which to study specific problems of interest. Today, non-mammalian nervous systems continue to provide ideal platforms for the study of fundamental problems in neuroscience. Indeed, the peculiarities of body plan and nervous systems that have evolved to carry out precise tasks in unique ecological niches enable investigators not only to pose specific scientific questions, but also to uncover principles that are general to all nervous systems.
Interactions among rhythmically active neuronal circuits that oscillate at different frequencies ... more Interactions among rhythmically active neuronal circuits that oscillate at different frequencies are important for generating complex behaviors, yet little is known about the underlying cellular mechanisms. We addressed this issue in the crab stomatogastric ganglion (STG), which contains two distinct but interacting circuits. These circuits generate the gastric mill rhythm (cycle period, ϳ10 sec) and the pyloric rhythm (cycle period, ϳ1 sec). When the identified modulatory projection neuron named modulatory commissural neuron 1 (MCN1) is activated, the gastric mill motor pattern is generated by interactions among MCN1 and two STG neurons [the lateral gastric (LG) neuron and interneuron 1]. We show that, during MCN1 stimulation, an identified synapse from the pyloric circuit onto the gastric mill circuit is pivotal for determining the gastric mill cycle period and the gastric-pyloric rhythm coordination. To examine the role of this intercircuit synapse, we replaced it with a computational equivalent via the dynamic-clamp technique. This enabled us to manipulate better the timing and strength of this synapse. We found this synapse to be necessary for production of the normal gastric mill cycle period. The synapse acts, during each LG neuron interburst, to boost rhythmically the influence of the modulatory input from MCN1 to LG and thereby to hasten LG neuron burst onset. The two rhythms become coordinated because LG burst onset occurs with a constant latency after the onset of the triggering pyloric input. These results indicate that intercircuit synapses can enable an oscillatory circuit to control the speed of a slower oscillatory circuit, as well as provide a mechanism for intercircuit coordination.
performed preliminary experiments that led to several of the hypotheses addressed here. Dr. Ekate... more performed preliminary experiments that led to several of the hypotheses addressed here. Dr. Ekaterina Morozova provided essential guidance pertaining to dynamic clamp experiments.
In complex nervous systems patterns of neuronal activity and measures of intrinsic neuronal excit... more In complex nervous systems patterns of neuronal activity and measures of intrinsic neuronal excitability are often used as criteria for identifying and/or classifying neurons. We asked how well identification of neurons by conventional measures of intrinsic excitability compares with a measure of neuronal excitability derived from a neuron's behavior in a dynamic clamp constructed two-cell network. We used four cell types from the crab stomatogastric ganglion: the pyloric dilator, lateral pyloric, gastric mill, and dorsal gastric neurons. Each neuron was evaluated for six conventional measures of intrinsic excitability (intrinsic properties, IPs). Additionally, each neuron was coupled by reciprocal inhibitory synapses made with the dynamic clamp to a Morris-Lecar model neuron and the resulting network was assayed for four measures of network activity (network activity properties, NAPs). We searched for linear combinations of IPs that correlated with each NAP, and combinations of NAPs that correlated with each IP. In the process we developed a method to correct for multiple correlations while searching for correlating features. When properly controlled for multiple correlations, four of the IPs were correlated with NAPs, and all four NAPs were correlated with IPs. Neurons were classified into cell types by training a linear classifier on sets of properties, or using k-medoids clustering.The IPs were modestly successful in classifying the neurons, and the NAPs were more successful. Combining the two measures did better than either measure alone, but not well enough to classify neurons with perfect accuracy, thus reiterating that electrophysiological measures of single-cell properties alone are not sufficient for reliable cell identification.
Central-pattern-generating neural circuits function reliably throughout an animal's life, despite... more Central-pattern-generating neural circuits function reliably throughout an animal's life, despite constant molecular turnover and environmental perturbations. Fluctuations in temperature pose a problem to the nervous systems of poikilotherms because their body temperature follows the ambient temperature, thus affecting the temperature-dependent dynamics of various subcellular components that constitute neuronal circuits. In the crustacean stomatogastric nervous system, the pyloric circuit produces a triphasic rhythm comprising the output of the pyloric dilator, lateral pyloric, and pyloric constrictor neurons. In vitro, the phase relationships of these neurons are maintained over a fourfold change in pyloric frequency as temperature increases from 7°C to 23°C. To determine whether these temperature effects are also found in intact crabs, in the presence of sensory feedback and neuromodulator-rich environments, we measured the temperature dependence of the pyloric frequency and phases in vivo by implanting extracellular electrodes into Cancer borealis and Cancer pagurus and shifting tank water temperature from 11°C to 26°C. Pyloric frequency in the intact crab increased significantly with temperature (Q10 = 2-2.5), while pyloric phases were generally conserved. For a subset of the C. borealis experiments, animals were subsequently dissected and the stomatogastric ganglion subjected to a similar temperature ramp in vitro. We found that the maximal frequency attained at high temperatures in vivo is lower than it is under in vitro conditions. Our results demonstrate that, over a wide temperature range, the phases of the pyloric rhythm in vivo are generally preserved, but that the frequency range is more restricted than it is in vitro.
Histamine is a neurotransmitter with actions throughout the nervous system of vertebrates and inv... more Histamine is a neurotransmitter with actions throughout the nervous system of vertebrates and invertebrates. Nevertheless, the actions of only a few identified histamine-containing neurons have been characterized. Here, we present the actions of a histaminergic projection neuron on the rhythmically active pyloric and gastric mill circuits within the stomatogastric ganglion (STG) of the crab Cancer borealis. An antiserum generated against histamine labeled profiles throughout the C. borealis stomatogastric nervous system. Labeling occurred in several somata and neuropil within the paired commissural ganglia as well as in neuropil within the STG and at the junction of the superior oesophageal and stomatogastric nerves. The source of all histamine-like immunolabeling in the STG neuropil was one pair of neuronal somata, the previously identified inferior ventricular (IV) neurons, located in the supraoesophageal ganglion. These neurons also exhibited FLRFamide-like immunoreactivity. Activation of the IV neurons in the crab inhibited some pyloric and gastric mill neurons and, with inputs from the commissural ganglia eliminated, terminated both rhythms. Focal application of histamine had comparable effects. The actions of both applied histamine and IV neuron stimulation were blocked, reversibly, by the histamine type-2 receptor antagonist cimetidine. With the commissural ganglia connected to the STG, IV neuron stimulation elicited a longer-latency activation of commissural projection neurons which in turn modified the pyloric rhythm and activated the gastric mill rhythm. These results support the hypothesis that the histaminergic/peptidergic IV neurons are projection neurons with direct and indirect actions on the STG circuits of the crab C. borealis.
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