Long-term neuromodulatory regulation of a motor pattern-generating network: maintenance of synaptic efficacy and oscillatory properties

Resilience of circuits to environmental challenge

Animals of all kinds evolved to deal with anticipated and unanticipated changes in a variety of features in their environments. Consequently, all environmental perturbations, adaptations, and acclimation involve a myriad of factors that, together, contribute to environmental resilience. New work highlights the importance of neuromodulation in the control of environmental resilience, and illustrates that different components of the nervous system may be differentially resilient to environmental perturbations. Climate change is today pushing animals to deal with previously unanticipated environmental challenges, and therefore understanding the complex biology of adaptation and acclimation to various environmental conditions takes on new urgency.

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

Elevated potassium concentration ([K⁺]) is often used to alter excitability in neurons and networks by shifting the potassium equilibrium potential (E_K) 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⁺] (2.5×[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 min by the recovery of spiking and/or bursting activity during continued superfusion of 2.5×[K⁺] saline. In contrast, when PD neurons were pharmacologically isolated from pyloric presynaptic inputs, they exhibited no transient loss of spiking activity in 2.5×[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 2.5×[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.NEW & NOTEWORTHY Solutions with elevated extracellular potassium are commonly used as a depolarizing stimulus. We studied the effects of high potassium concentration ([K⁺]) on the pyloric circuit of the crab stomatogastric ganglion. A 2.5-fold increase in extracellular [K⁺] caused a transient loss of activity that was not due to depolarization block, followed by a rapid increase in excitability and recovery of spiking within minutes. This suggests that changing extracellular potassium can have complex and nonstationary effects on neuronal circuits.

Neuromodulation of central pattern generators and its role in the functional recovery of central pattern generator activity

Neuromodulators play an important role in how the nervous system organizes activity that results in behavior. Disruption of the normal patterns of neuromodulatory release or production is known to be related to the onset of severe pathologies such as Parkinson's disease, Rett syndrome, Alzheimer's disease, and affective disorders. Some of these pathologies involve neuronal structures that are called central pattern generators (CPGs), which are involved in the production of rhythmic activities throughout the nervous system. Here I discuss the interplay between CPGs and neuromodulatory activity, with particular emphasis on the potential role of neuromodulators in the recovery of disrupted neuronal activity. I refer to invertebrate and vertebrate model systems and some of the lessons we have learned from research on these systems and propose a few avenues for future research. I make one suggestion that may guide future research in the field: neuromodulators restrict the parameter landscape in which CPG components operate, and the removal of neuromodulators may enable a perturbed CPG in finding a new set of parameter values that can allow it to regain normal function.

Defining the Rhythmogenic Elements of Mammalian Breathing

Breathing's remarkable ability to adapt to changes in metabolic, environmental, and behavioral demands stems from a complex integration of its rhythm-generating network within the wider nervous system. Yet, this integration complicates identification of its specific rhythmogenic elements. Based on principles learned from smaller rhythmic networks of invertebrates, we define criteria that identify rhythmogenic elements of the mammalian breathing network and discuss how they interact to produce robust, dynamic breathing.

Functional Recovery of a Locomotor Network after Injury: Plasticity beyond the Central Nervous System

Many animals depend on descending information from the brain for the initiation and proper execution of locomotion. Interestingly, after injury and the loss of such inputs, locomotor function can sometimes be regained without the regrowth of central connections. In the medicinal leech, Hirudo verbana, we have shown that crawling reemerges after removal of descending inputs. Here, we studied the mechanisms underlying this return of locomotion by asking if central pattern generators (CPGs) in crawl-recovered leeches are sufficient to produce crawl-specific intersegmental coordination. From recovered animals, we treated isolated chains of ganglia with dopamine to activate the crawl CPGs (one crawl CPG per ganglion) and observed fictive crawl-like bursting in the dorsal-longitudinal-excitor motoneuron (DE-3), an established crawl-monitor neuron. However, these preparations did not exhibit crawl-specific coordination across the CPGs. Although the crawl CPGs always generated bidirectional activation of adjacent CPGs, we never observed crawl-appropriate intersegmental phase delays. Because central circuits alone were unable to organize crawl-specific coordination, we tested the coordinating role of the peripheral nervous system. In transected leeches normally destined for recovery, we removed afferent information to the anterior-most (lead) ganglion located below the nerve-cord transection site. In these dually treated animals, overt crawling was greatly delayed or prevented. After filling the peripheral nerves with Neurobiotin tracer distal to the nerve-root lesion, we found a perfect correlation between regrowth of peripheral neuronal fibers and crawl recovery. Our study establishes that during recovery after injury, crawl-specific intersegmental coordination switches to a new dependence on afferent information.

Removal of endogenous neuromodulators in a small motor network enhances responsiveness to neuromodulation

We studied the changes in sensitivity to a peptide modulator, crustacean cardioactive peptide (CCAP), as a response to loss of endogenous modulation in the stomatogastric ganglion (STG) of the crab Cancer borealis Our data demonstrate that removal of endogenous modulation for 24 h increases the response of the lateral pyloric (LP) neuron of the STG to exogenously applied CCAP. Increased responsiveness is accompanied by increases in CCAP receptor (CCAPr) mRNA levels in LP neurons, requires de novo protein synthesis, and can be prevented by coincubation for the 24-h period with exogenous CCAP. These results suggest that there is a direct feedback from loss of CCAP signaling to the production of CCAPr that increases subsequent response to the ligand. However, we also demonstrate that the modulator-evoked membrane current (I_MI) activated by CCAP is greater in magnitude after combined loss of endogenous modulation and activity compared with removal of just hormonal modulation. These results suggest that both receptor expression and an increase in the target conductance of the CCAP G protein-coupled receptor are involved in the increased response to exogenous hormone exposure following experimental loss of modulation in the STG.NEW & NOTEWORTHY The nervous system shows a tremendous amount of plasticity. More recently there has been an appreciation for compensatory actions that stabilize output in the face of perturbations to normal activity. In this study we demonstrate that neurons of the crustacean stomatogastric ganglion generate apparent compensatory responses to loss of peptide neuromodulation, adding to the repertoire of mechanisms by which the stomatogastric nervous system can regulate and stabilize its own output.

Activation mechanism of a neuromodulator-gated pacemaker ionic current

The neuromodulator-gated current (I_MI) found in the crab stomatogastric ganglion is activated by neuromodulators that are essential to induce the rhythmic activity of the pyloric network in this system. One of these neuromodulators is also known to control the correlated expression of voltage-gated ionic currents in pyloric neurons, as well as synaptic plasticity and strength. Thus understanding the mechanism by which neuromodulator receptors activate I_MI should provide insights not only into how oscillations are initiated but also into how other processes, and currents not directly activated by them, are regulated. To determine what specific signaling molecules are implicated in this process, we used a battery of agonists and antagonists of common signal transduction pathways. We found that the G protein inhibitor GDPβS and the G protein activator GTPγS significantly affect I_MI amplitude, suggesting that its activation is mediated by G proteins. Interestingly, when using the more specific G protein blocker pertussis toxin, we observed the expected inhibition of I_MI amplitude but, unexpectedly, in a calcium-dependent fashion. We also found that antagonists of calcium- and calmodulin-associated signaling significantly reduce I_MI amplitude. In contrast, we found little evidence for the role of cyclic nucleotide signaling, phospholipase C (PLC), or kinases and phosphatases, except two calmodulin-dependent kinases. In sum, these results suggest that proctolin-induced I_MI is mediated by a G protein whose pertussis toxin sensitivity is altered by external calcium concentration and appears to depend on intracellular calcium, calmodulin, and calmodulin-activated kinases. In contrast, we found no support for I_MI being mediated by PLC signaling or cyclic nucleotides.NEW & NOTEWORTHY Neuronal rhythmic activity is generated by either network-based or cell-autonomous mechanisms. In the pyloric network of decapod crustaceans, the activation of a neuromodulator-gated pacemaker current is crucial for the generation of rhythmic activity. This current is activated by several neuromodulators, including peptides and acetylcholine, presumably via metabotropic receptors. We have previously demonstrated a novel extracellular calcium-sensitive voltage-dependence mechanism of this current. We presently report that the activation mechanism depends on intracellular and extracellular calcium-sensitive components.

Homeostatic plasticity of excitability in crustacean central pattern generator networks

Plasticity of excitability can come in two general forms: changes in excitability that alter neuronal output (e.g. long-term potentiation of intrinsic excitability) or excitability changes that stabilize neuronal output (homeostatic plasticity). Here we discuss the latter form of plasticity in the context of the crustacean stomatogastric nervous system, and a second central pattern generator circuit, the cardiac ganglion. We discuss this plasticity at three levels: rapid homeostatic changes in membrane conductance, longer-term effects of neuromodulation on excitability, and the impacts of activity-dependent feedback on steady-state channel mRNA levels. We then conclude with thoughts on the implications of plasticity of excitability for variability of conductance levels across populations of motor neurons.

Recruitment of Polysynaptic Connections Underlies Functional Recovery of a Neural Circuit after Lesion

The recruitment of additional neurons to neural circuits often occurs in accordance with changing functional demands. Here we found that synaptic recruitment plays a key role in functional recovery after neural injury. Disconnection of a brain commissure in the nudibranch mollusc, Tritonia diomedea, impairs swimming behavior by eliminating particular synapses in the central pattern generator (CPG) underlying the rhythmic swim motor pattern. However, the CPG functionally recovers within a day after the lesion. The strength of a spared inhibitory synapse within the CPG from Cerebral Neuron 2 (C2) to Ventral Swim Interneuron B (VSI) determines the level of impairment caused by the lesion, which varies among individuals. In addition to this direct synaptic connection, there are polysynaptic connections from C2 and Dorsal Swim Interneurons to VSI that provide indirect excitatory drive but play only minor roles under normal conditions. After disconnecting the pedal commissure (Pedal Nerve 6), the recruitment of polysynaptic excitation became a major source of the excitatory drive to VSI. Moreover, the amount of polysynaptic recruitment, which changed over time, differed among individuals and correlated with the degree of recovery of the swim motor pattern. Thus, functional recovery was mediated by an increase in the magnitude of polysynaptic excitatory drive, compensating for the loss of direct excitation. Since the degree of susceptibility to injury corresponds to existing individual variation in the C2 to VSI synapse, the recovery relied upon the extent to which the network reorganized to incorporate additional synapses.

Acute off-target effects of neural circuit manipulations

Rapid and reversible manipulations of neural activity in behaving animals are transforming our understanding of brain function. An important assumption underlying much of this work is that evoked behavioural changes reflect the function of the manipulated circuits. We show that this assumption is problematic because it disregards indirect effects on the independent functions of downstream circuits. Transient inactivations of motor cortex in rats and nucleus interface (Nif) in songbirds severely degraded task-specific movement patterns and courtship songs, respectively, which are learned skills that recover spontaneously after permanent lesions of the same areas. We resolve this discrepancy in songbirds, showing that Nif silencing acutely affects the function of HVC, a downstream song control nucleus. Paralleling song recovery, the off-target effects resolved within days of Nif lesions, a recovery consistent with homeostatic regulation of neural activity in HVC. These results have implications for interpreting transient circuit manipulations and for understanding recovery after brain lesions.

Consequences of acute and long-term removal of neuromodulatory input on the episodic gastric rhythm of the crab Cancer borealis

For decades, the episodic gastric rhythm of the crustacean stomatogastric nervous system (STNS) has served as an important model system for understanding the generation of rhythmic motor behaviors. Here we quantitatively describe many features of the gastric rhythm of the crab Cancer borealis under several conditions. First, we analyzed spontaneous gastric rhythms produced by freshly dissected preparations of the STNS, including the cycle frequency and phase relationships among gastric units. We find that phase is relatively conserved across frequency, similar to the pyloric rhythm. We also describe relationships between these two rhythms, including a significant gastric/pyloric frequency correlation. We then performed continuous, days-long extracellular recordings of gastric activity from preparations of the STNS in which neuromodulatory inputs to the stomatogastric ganglion were left intact and also from preparations in which these modulatory inputs were cut (decentralization). This allowed us to provide quantitative descriptions of variability and phase conservation within preparations across time. For intact preparations, gastric activity was more variable than pyloric activity but remained relatively stable across 4-6 days, and many significant correlations were found between phase and frequency within animals. Decentralized preparations displayed fewer episodes of gastric activity, with altered phase relationships, lower frequencies, and reduced coordination both among gastric units and between the gastric and pyloric rhythms. Together, these results provide insight into the role of neuromodulation in episodic pattern generation and the extent of animal-to-animal variability in features of spontaneously occurring gastric rhythms.

Degradation of extracellular chondroitin sulfate delays recovery of network activity after perturbation

Chondroitin sulfate proteoglycans (CSPGs) are widely studied in vertebrate systems and are known to play a key role in development, plasticity, and regulation of cortical circuitry. The mechanistic details of this role are still elusive, but increasingly central to the investigation is the homeostatic balance between network excitation and inhibition. Studying a simpler neuronal circuit may prove advantageous for discovering the mechanistic details of the cellular effects of CSPGs. In this study we used a well-established model of homeostatic change after injury in the crab Cancer borealis to show first evidence that CSPGs are necessary for network activity homeostasis. We degraded CSPGs in the pyloric circuit of the stomatogastric ganglion with the enzyme chondroitinase ABC (chABC) and found that removal of CSPGs does not influence the ongoing rhythm of the pyloric circuit but does limit its capacity for recovery after a networkwide perturbation. Without CSPGs, the postperturbation rhythm is slower than in controls and rhythm recovery is delayed. In addition to providing a new model system for the study of CSPGs, this study suggests a wider role for CSPGs, and perhaps the extracellular matrix in general, beyond simply plastic reorganization (as observed in mammals) and into a foundational regulatory role of neural circuitry.

Silicon central pattern generators for cardiac diseases

Cardiac rhythm management devices provide therapies for both arrhythmias and resynchronisation but not heart failure, which affects millions of patients worldwide. This paper reviews recent advances in biophysics and mathematical engineering that provide a novel technological platform for addressing heart disease and enabling beat-to-beat adaptation of cardiac pacing in response to physiological feedback. The technology consists of silicon hardware central pattern generators (hCPGs) that may be trained to emulate accurately the dynamical response of biological central pattern generators (bCPGs). We discuss the limitations of present CPGs and appraise the advantages of analog over digital circuits for application in bioelectronic medicine. To test the system, we have focused on the cardio-respiratory oscillators in the medulla oblongata that modulate heart rate in phase with respiration to induce respiratory sinus arrhythmia (RSA). We describe here a novel, scalable hCPG comprising physiologically realistic (Hodgkin-Huxley type) neurones and synapses. Our hCPG comprises two neurones that antagonise each other to provide rhythmic motor drive to the vagus nerve to slow the heart. We show how recent advances in modelling allow the motor output to adapt to physiological feedback such as respiration. In rats, we report on the restoration of RSA using an hCPG that receives diaphragmatic electromyography input and use it to stimulate the vagus nerve at specific time points of the respiratory cycle to slow the heart rate. We have validated the adaptation of stimulation to alterations in respiratory rate. We demonstrate that the hCPG is tuneable in terms of the depth and timing of the RSA relative to respiratory phase. These pioneering studies will now permit an analysis of the physiological role of RSA as well as its any potential therapeutic use in cardiac disease.

QUANTITATIVE REEVALUATION OF THE EFFECTS OF SHORT- AND LONG-TERM REMOVAL OF DESCENDING MODULATORY INPUTS ON THE PYLORIC RHYTHM OF THE CRAB, CANCER BOREALIS

Neuromodulatory inputs are known to strongly influence the intrinsic excitability of individual neurons and the networks in which the targets of modulation are found. It is therefore important to understand how nervous systems respond to altered neuromodulatory environments.

The crustacean stomatogastric ganglion (STG) receives descending neuromodulatory inputs from three anterior ganglia: the paired commissural ganglia (CoGs), and the single esophageal ganglion (OG). In this paper, we provide the first detailed and quantitative analyses of the short- and long-term effects of removal of these descending inputs (decentralization) on the pyloric rhythm of the STG. Thirty minutes after decentralization, the mean frequency of the pyloric rhythm dropped from 1.20 Hz in control to 0.52 Hz. Whereas the relative phase of pyloric neuron activity was approximately constant across frequency in the controls, after decentralization this changed markedly. Nine control preparations kept for 5–6 d in vitro maintained pyloric rhythm frequencies close to their initial values. Nineteen decentralized preparations kept for 5–6 d dropped slightly in frequency from those seen at 30 min following decentralization, but then displayed stable activity over 6 d. Bouts of higher frequency activity were intermittently seen in both control and decentralized preparations, but the bouts began earlier and were more frequent in the decentralized preparations. Although the bouts may indicate that the removal of the modulatory inputs triggered changes in neuronal excitability, these changes did not produce obvious long-lasting changes in the frequency of the decentralized preparations.

Robustness of a rhythmic circuit to short- and long-term temperature changes

Recent computational and experimental work has shown that similar network performance can result from variable sets of synaptic and intrinsic properties. Because temperature is a global perturbation that differentially influences every biological process within the nervous system, one might therefore expect that individual animals would respond differently to temperature. Nonetheless, the phase relationships of the pyloric rhythm of the stomatogastric ganglion (STG) of the crab, Cancer borealis, are remarkably invariant between 7 and 23°C (Tang et al., 2010). Here, we report that, when isolated STG preparations were exposed to more extreme temperature ranges, their networks became nonrhythmic, or "crashed", in a reversible fashion. Animals were acclimated for at least 3 weeks at 7, 11, or 19°C. When networks from the acclimated animals were perturbed by acute physiologically relevant temperature ramps (11-23°C), the network frequency and phase relationships were independent of the acclimation group. At high acute temperatures (>23°C), circuits from the cold-acclimated animals produced less-regular pyloric rhythms than those from warm-acclimated animals. At high acute temperatures, phase relationships between pyloric neurons were more variable from animal to animal than at moderate acute temperatures, suggesting that individual differences across animals in intrinsic circuit parameters are revealed at high temperatures. This shows that individual and variable neuronal circuits can behave similarly in normal conditions, but their behavior may diverge when confronted with extreme external perturbations.

Rapid homeostatic plasticity of intrinsic excitability in a central pattern generator network stabilizes functional neural network output

Neurons and networks undergo a process of homeostatic plasticity that stabilizes output by integrating activity levels with network and cellular properties to counter longer-term perturbations. Here we describe a rapid compensatory interaction among a pair of potassium currents, I(A) and I(KCa), that stabilizes both intrinsic excitability and network function in the cardiac ganglion of the crab, Cancer borealis. We determined that mRNA levels in single identified neurons for the channels which encode I(A) and I(KCa) are positively correlated, yet the ionic currents themselves are negatively correlated, across a population of motor neurons. We then determined that these currents are functionally coupled; decreasing levels of either current within a neuron causes a rapid increase in the other. This functional interdependence results in homeostatic stabilization of both the individual neuronal and the network output. Furthermore, these compensatory increases are mechanistically independent, suggesting robustness in the maintenance of neural network output that is critical for survival. Together, we generate a complete model for homeostatic plasticity from mRNA to network output where rapid post-translational compensatory mechanisms acting on a reservoir of channels proteins regulated at the level of gene expression provide homeostatic stabilization of both cellular and network activity.

Restoration of descending inputs fails to rescue activity following deafferentation of a motor network

Motor networks such as the pyloric network of the stomatogastric ganglion often require descending neuromodulatory inputs to initiate, regulate, and modulate their activity and their synaptic connectivity to manifest physiologically appropriate output. Prolonged removal of these descending inputs often results in a compensatory response that alters the inputs themselves, their targets, or both. Using the pyloric network of the crab, Cancer borealis, we investigated whether isolation of motor networks would result in alterations that change the responses of these networks to restored modulatory input. We used a reversible block with isotonic sucrose to transiently alter descending inputs into the pyloric network of the crab stomatogastric ganglion. Using this method, we found that blocking neuromodulatory inputs caused a reduced ability for subsequently restored modulatory projections to appropriately generate network output. Our results suggest that this could be due to changes in activity of descending projection neurons as well as changes in sensitivity to neuromodulators of the target neurons that develop over the time course of the blockade. These findings suggest that although homeostatic plasticity may play a critical role in recovery of functional output in a deafferented motor network, the results of these compensatory changes may alter the network such that restored inputs no longer function appropriately.

Neuromodulation independently determines correlated channel expression and conductance levels in motor neurons of the stomatogastric ganglion

Neuronal identity depends on the regulated expression of numerous molecular components, especially ionic channels, which determine the electrical signature of a neuron. Such regulation depends on at least two key factors, activity itself and neuromodulatory input. Neuronal electrical activity can modify the expression of ionic currents in homeostatic or nonhomeostatic fashion. Neuromodulators typically modify activity by regulating the properties or expression levels of subsets of ionic channels. In the stomatogastric system of crustaceans, both types of regulation have been demonstrated. Furthermore, the regulation of the coordinated expression of ionic currents and the channels that carry these currents has been recently reported in diverse neuronal systems, with neuromodulators not only controlling the absolute levels of ionic current expression but also, over long periods of time, appearing to modify their correlated expression. We hypothesize that neuromodulators may regulate the correlated expression of ion channels at multiple levels and in a cell-type-dependent fashion. We report that in two identified neuronal types, three ionic currents are linearly correlated in a pairwise manner, suggesting their coexpression or direct interactions, under normal neuromodulatory conditions. In each cell, some currents remain correlated after neuromodulatory input is removed, whereas the correlations between the other pairs are either lost or altered. Interestingly, in each cell, a different suite of currents change their correlation. At the transcript level we observe distinct alterations in correlations between channel mRNA amounts, including one of the cell types lacking a correlation under normal neuromodulatory conditions and then gaining the correlation when neuromodulators are removed. Synaptic activity does not appear to contribute, with one possible exception, to the correlated expression of either ionic currents or of the transcripts that code for the respective channels. We conclude that neuromodulators regulate the correlated expression of ion channels at both the transcript and the protein levels.

Recovery of rhythmic activity in a central pattern generator: analysis of the role of neuromodulator and activity-dependent mechanisms

The pyloric network of decapods crustaceans can undergo dramatic rhythmic activity changes. Under normal conditions the network generates low frequency rhythmic activity that depends obligatorily on the presence of neuromodulatory input from the central nervous system. When this input is removed (decentralization) the rhythmic activity ceases. In the continued absence of this input, periodic activity resumes after a few hours in the form of episodic bursting across the entire network that later turns into stable rhythmic activity that is nearly indistinguishable from control (recovery). It has been proposed that an activity-dependent modification of ionic conductance levels in the pyloric pacemaker neuron drives the process of recovery of activity. Previous modeling attempts have captured some aspects of the temporal changes observed experimentally, but key features could not be reproduced. Here we examined a model in which slow activity-dependent regulation of ionic conductances and slower neuromodulator-dependent regulation of intracellular Ca(2+) concentration reproduce all the temporal features of this recovery. Key aspects of these two regulatory mechanisms are their independence and their different kinetics. We also examined the role of variability (noise) in the activity-dependent regulation pathway and observe that it can help to reduce unrealistic constraints that were otherwise required on the neuromodulator-dependent pathway. We conclude that small variations in intracellular Ca(2+) concentration, a Ca(2+) uptake regulation mechanism that is directly targeted by neuromodulator-activated signaling pathways, and variability in the Ca(2+) concentration sensing signaling pathway can account for the observed changes in neuronal activity. Our conclusions are all amenable to experimental analysis.

Geometry and dynamics of activity-dependent homeostatic regulation in neurons

To maintain activity in a functional range, neurons constantly adjust membrane excitability to changing intra- and extracellular conditions. Such activity-dependent homeostatic regulation (ADHR) is critical for normal processing of the nervous system and avoiding pathological conditions. Here, we posed a homeostatic regulation problem for the classical Morris-Lecar (ML) model. The problem was motivated by the phenomenon of the functional recovery of stomatogastric neurons in crustaceans in the absence of neuromodulation. In our study, the regulation of the ionic conductances in the ML model depended on the calcium current or the intracellular calcium concentration. We found an asymptotic solution to the problem under the assumption of slow regulation. The solution provides a full account of the regulation in the case of correlated or anticorrelated changes of the maximal conductances of the calcium and potassium currents. In particular, the solution shows how the target and parameters of the regulation determine which perturbations of the conductances can be compensated by the ADHR. In some cases, the sets of compensated initial perturbations are not convex. On the basis of our analysis we formulated specific questions for subsequent experimental and theoretical studies of ADHR.

Lesioning alters functional properties in isolated spinal cord hemisegmental networks

Hemisegmental networks produced by longitudinal lesions of the spinal cord midline are able to generate rhythmic bursting activity. This has led to the suggestion that hemisegmental networks can independently burst in the intact spinal cord. Previous analyses in the lamprey spinal cord failed to show hemisegmental bursting in NMDA. This was subsequently attributed to the failure to wait sufficient time for NMDA-evoked hemisegmental activity to recover after being abolished by the lesion, which can take tens of minutes to hours. The reason for this delay in the onset of NMDA-evoked activity was not previously addressed. We have investigated it here by examining two hypotheses: that hemisegmental networks intrinsically burst under normal conditions but that NMDA-evoked bursting was temporarily silenced by lesion-induced transmitter release; or that lesioning altered functional properties in the hemisegment that subsequently led to the development of bursting. We found no evidence to support transmitter-induced silencing of ongoing NMDA-evoked hemisegmental activity, but did find evidence for significant changes in the cellular and synaptic properties of motor neurons and premotor excitatory interneurons in lesioned hemisegmental networks. These results thus suggest that there are lesion-induced changes in functional properties in hemisegmental networks. As the interpretation of lesion studies rests on the assumption that the functional properties of hemisegmental components are not altered, further work is needed before conclusions can be made about the function of the intact system.

Model calcium sensors for network homeostasis: sensor and readout parameter analysis from a database of model neuronal networks

In activity-dependent homeostatic regulation (ADHR) of neuronal and network properties, the intracellular Ca(2+) concentration is a good candidate for sensing activity levels because it is correlated with the electrical activity of the cell. Previous ADHR models, developed with abstract activity sensors for model pyloric neurons and networks of the crustacean stomatogastric ganglion, showed that functional activity can be maintained by a regulation mechanism that senses activity levels solely from Ca(2+). At the same time, several intracellular pathways have been discovered for Ca(2+)-dependent regulation of ion channels. To generate testable predictions for dynamics of these signaling pathways, we undertook a parameter study of model Ca(2+) sensors across thousands of model pyloric networks. We found that an optimal regulation signal can be generated for 86% of model networks with a sensing mechanism that activates with a time constant of 1 ms and that inactivates within 1 s. The sensor performed robustly around this optimal point and did not need to be specific to the role of the cell. When multiple sensors with different time constants were used, coverage extended to 88% of the networks. Without changing the sensors, it extended to 95% of the networks by letting the sensors affect the readout nonlinearly. Specific to this pyloric network model, the sensor of the follower pyloric constrictor cell was more informative than the pacemaker anterior burster cell for producing a regulatory signal. Conversely, a global signal indicating network activity that was generated by summing the sensors in individual cells was less informative for regulation.

Functional recovery after lesion of a central pattern generator

In cases of neuronal injury when regeneration is restricted, functional recovery can occur through reorganization of the remaining neural circuitry. We found an example of such recovery in the central pattern generator (CPG) for the escape swim of the mollusc Tritonia diomedea. The CPG neurons are bilaterally represented and each neuron projects an axon through one of two pedal commissures. Cutting the posterior pedal commissure [pedal nerve 6 (PdN6)] in the animal or in the isolated brain caused a deficit in the swim behavior and in the fictive motor pattern, respectively, each of which recovered over the course of 20 h. Locally blocking spiking activity in PdN6 with sodium-free saline and/or tetrodotoxin disrupted the motor pattern in a reversible manner. Maintained blockade of PdN6 led to a functional recovery of the swim motor pattern similar to that observed in response to cutting the commissure. Among the CPG neurons, cerebral neuron 2 (C2) makes functional connection onto the ventral swim interneuron-B (VSI) in both pedal ganglia. Cutting or blocking PdN6 eliminated C2-evoked excitation of VSI in the pedal ganglion distal to the lesion. Associated with the recovery of the swim motor pattern, the synaptic action of C2 onto VSI in the proximal pedal ganglion changed from being predominantly inhibitory to being predominantly excitatory. These results show that the Tritonia swim CPG undergoes adaptive plasticity in response to the loss of critical synaptic connections; reversal of synaptic action in the CPG may be at least partially responsible for this functional recovery.

Modulation of stomatogastric rhythms

Neuromodulation by peptides and amines is a primary source of plasticity in the nervous system as it adapts the animal to an ever-changing environment. The crustacean stomatogastric nervous system is one of the premier systems to study neuromodulation and its effects on motor pattern generation at the cellular level. It contains the extensively modulated central pattern generators that drive the gastric mill (chewing) and pyloric (food filtering) rhythms. Neuromodulators affect all stages of neuronal processing in this system, from membrane currents and synaptic transmission in network neurons to the properties of the effector muscles. The ease with which distinct neurons are identified and their activity is recorded in this system has provided considerable insight into the mechanisms by which neuromodulators affect their target cells and modulatory neuron function. Recent evidence suggests that neuromodulators are involved in homeostatic processes and that the modulatory system itself is under modulatory control, a fascinating topic whose surface has been barely scratched. Future challenges include exploring the behavioral conditions under which these systems are activated and how their effects are regulated.

Probing the dynamics of identified neurons with a data-driven modeling approach

In controlling animal behavior the nervous system has to perform within the operational limits set by the requirements of each specific behavior. The implications for the corresponding range of suitable network, single neuron, and ion channel properties have remained elusive. In this article we approach the question of how well-constrained properties of neuronal systems may be on the neuronal level. We used large data sets of the activity of isolated invertebrate identified cells and built an accurate conductance-based model for this cell type using customized automated parameter estimation techniques. By direct inspection of the data we found that the variability of the neurons is larger when they are isolated from the circuit than when in the intact system. Furthermore, the responses of the neurons to perturbations appear to be more consistent than their autonomous behavior under stationary conditions. In the developed model, the constraints on different parameters that enforce appropriate model dynamics vary widely from some very tightly controlled parameters to others that are almost arbitrary. The model also allows predictions for the effect of blocking selected ionic currents and to prove that the origin of irregular dynamics in the neuron model is proper chaoticity and that this chaoticity is typical in an appropriate sense. Our results indicate that data driven models are useful tools for the in-depth analysis of neuronal dynamics. The better consistency of responses to perturbations, in the real neurons as well as in the model, suggests a paradigm shift away from measuring autonomous dynamics alone towards protocols of controlled perturbations. Our predictions for the impact of channel blockers on the neuronal dynamics and the proof of chaoticity underscore the wide scope of our approach.

Using complicated, wide dynamic range driving to develop models of single neurons in single recording sessions

Neuron models are typically built by measuring individually, for each membrane conductance, its parameters (e.g., half-maximal voltages) and maximal conductance value (g(max)). However, neurons have extended morphologies with nonuniform conductance distributions, whereas models generally contain at most a few compartments. Both the original conductance measurements and the models therefore unavoidably contain error due to the electrical filtering of neurons and the differential placement of conductances on them. Model parameters (typically g(max) values) are therefore generally altered by hand or brute force to match model and neuron activity. We propose an alternative method in which complicated, rapidly changing driving input is used to optimize model parameters. This method also ensures that neuron and model dynamics match across a wide dynamic range, a test not performed in most modeling. We tested this concept using leech heartbeat and generic tonically firing models and lobster stomatogastric and generic bursting models as targets and g(max) values as optimized parameters. In all four cases optimization solutions excellently matched target activity. Complicated, wide dynamic range driving thus appears to be an excellent method to characterize neuron properties in detail and to build highly accurate models. In these completely defined targets, the method found each target's 8-13 g(max) values with high accuracy, and may therefore also provide an alternative, functionally based method of defining neuron g(max) values. The method uses only standard experimental and computational techniques, could be easily extended to optimize conductance parameters other than g(max), and should be readily applicable to real neurons.

Long-term exposure to histamine induces the expression of an embryonic-like motor pattern in an adult nervous system

Neuromodulatory inputs play important roles in shaping the outputs of neural networks. While the actions of neuromodulatory substances over the short term (seconds, minutes) have been examined in detail, far less is known about the possible longer-term (hours) effects of these substances. To investigate this issue, we used the stomatogastric nervous system (STNS) of the lobster to examine the short- and long-term effects of histamine on rhythmic network activity. The application of histamine to the entire STNS had strong inhibitory effects on all three of the STNS networks, observable within minutes. In contrast, longer-term (> 1 h) application of histamine induced the expression of a single, unified rhythm involving neurons from all three networks. Selective application of histamine to different regions of the STNS demonstrated that a unified rhythm arises following the long-term application of histamine to the commissural ganglia (CoGs; modulatory centres), but not the stomatogastric ganglion (site of neural networks). Strikingly, the single rhythm observed following the long-term application of histamine to the CoGs exhibits many similarities with the single rhythm expressed by the embryonic STNS. Together, these results demonstrate that histamine has markedly different short- and long-term effects on network activity; short-term effects arising through direct actions on the networks and long-term effects mediated by actions on modulatory neurons. Furthermore, they indicate that histamine is able to induce the expression of an embryonic-like rhythm in an adult system, suggesting that long-term actions of histamine may play key roles in the development of the STNS networks.

Neuromodulators, not activity, control coordinated expression of ionic currents

Electrical activity in identical neurons across individuals is often remarkably similar and stable over long periods. However, the ionic currents that determine the electrical activity of these neurons show wide animal-to-animal amplitude variability. This seemingly random variability of individual current amplitudes may obscure mechanisms that globally reduce variability and that contribute to the generation of similar neuronal output. One such mechanism could be the coordinated regulation of ionic current expression. Studying identified neurons of the Cancer borealis pyloric network, we discovered that the removal of neuromodulatory input to this network (decentralization) was accompanied by the loss of the coordinated regulation of ionic current levels. Additionally, decentralization induced large changes in the levels of several ionic currents. The loss of coregulation and the changes in current levels were prevented by continuous exogenous application of proctolin, an endogenous neuromodulatory peptide, to the pyloric network. This peptide does not exert fast regulatory actions on any of the currents affected by decentralization. We conclude that neuromodulatory inputs to the pyloric network have a novel role in the regulation of ionic current expression. They can control, over the long term, the coordinated expression of multiple voltage-gated ionic currents that they do not acutely modulate. Our results suggest that current coregulation places constraints on neuronal intrinsic plasticity and the ability of a network to respond to perturbations. The loss of conductance coregulation may be a mechanism to facilitate the recovery of function.

Neuronal morphology and neuropil structure in the stomatogastric ganglion of the lobster, Homarus americanus

The stomatogastric nervous system (STNS) has long been used as a model system for the study of central pattern generation, neuromodulation, and network dynamics. Anatomical studies of the crustacean stomatogastric ganglion (STG) in different species have mostly been restricted to subsets of neurons and/or general structural features. For the first time, we describe the morphology of all STG neurons belonging to the two circuits that produce the well-described pyloric and gastric rhythms in the lobster, Homarus americanus. Somata sit on the dorsal and lateral surface of the STG and send a single primary neurite into the core of the neuropil, which is mostly made up of larger lower order branches. The perimeter of the neuropil consists mostly of finer higher order branches. Immunohistochemical labeling for synaptic proteins is associated with the small diameter branches. Somata positions are not constant but show preferred locations across individuals. The number of copies is constant for all neuron types except the PY and GM neurons (PY neuron number ranges from 3 to 7, and GM neuron number ranges from 6 to 9). Branch structure is largely nondichotomous, and branches can deviate substantially from cylindrical shape. Diameter changes at branch points can be as large as 20-fold. Clearly, the morphology of a specific neuron type can be quite variable from animal to animal.

Understanding Circuit Dynamics Using the Stomatogastric Nervous System of Lobsters and Crabs

Studies of the stomatogastric nervous systems of lobsters and crabs have led to numerous insights into the cellular and circuit mechanisms that generate rhythmic motor patterns. The small number of easily identifiable neurons allowed the establishment of connectivity diagrams among the neurons of the stomatogastric ganglion. We now know that (a) neuromodulatory substances reconfigure circuit dynamics by altering synaptic strength and voltage-dependent conductances and (b) individual neurons can switch among different functional circuits. Computational and experimental studies of single-neuron and network homeostatic regulation have provided insight into compensatory mechanisms that can underlie stable network performance. Many of the observations first made using the stomatogastric nervous system can be generalized to other invertebrate and vertebrate circuits.

Plasticity and stability in neuronal output via changes in intrinsic excitability: it's what's inside that counts

The nervous system faces an extremely difficult task. It must be flexible, both during development and in adult life, so that it can respond to a variety of environmental demands and produce adaptive behavior. At the same time the nervous system must be stable, so that the neural circuits that produce behavior function throughout the lifetime of the animal and that changes produced by learning endure. We are only beginning to understand how neural networks strike a balance between altering individual neurons in the name of plasticity, while maintaining long-term stability in neural system function. The balance of this plasticity and stability in neural networks undoubtedly plays a critical role in the normal functioning of the nervous system. While mechanisms of synaptic plasticity have garnered extensive study over the past three decades, it is only recently that more attention has been turned to plasticity of intrinsic excitability as a key player in neural network function. This review will focus on this emerging area of research that undoubtedly will contribute a great deal to our understanding of the functionality of the nervous system.

Variability, compensation and homeostasis in neuron and network function

Neurons in most animals live a very long time relative to the half-lives of all of the proteins that govern excitability and synaptic transmission. Consequently, homeostatic mechanisms are necessary to ensure stable neuronal and network function over an animal's lifetime. To understand how these homeostatic mechanisms might function, it is crucial to understand how tightly regulated synaptic and intrinsic properties must be for adequate network performance, and the extent to which compensatory mechanisms allow for multiple solutions to the production of similar behaviour. Here, we use examples from theoretical and experimental studies of invertebrates and vertebrates to explore several issues relevant to understanding the precision of tuning of synaptic and intrinsic currents for the operation of functional neuronal circuits.

Ionic mechanism underlying recovery of rhythmic activity in adult isolated neurons

Neurons exhibit long-term excitability changes necessary for maintaining proper cell and network activity in response to various inputs and perturbations. For instance, the adult crustacean pyloric network can spontaneously recover rhythmic activity after complete shutdown resulting from permanent removal of neuromodulatory inputs. Dissociated lobster stomatogastric ganglion (STG) neurons have been shown to spontaneously develop oscillatory activity via excitability changes. Rhythmic electrical stimulation can eliminate these oscillatory patterns in some cells. The ionic mechanisms underlying these changes are only partially understood. We used dissociated crab STG neurons to study the ionic mechanisms underlying spontaneous recovery of rhythmic activity and stimulation-induced activity changes. Similar to lobster neurons, rhythmic activity spontaneously develops in crab STG neurons. Rhythmic hyperpolarizing stimulation can eliminate, but more commonly accelerate, the emergence of stable oscillatory activity depending on Ca(2+) influx at hyperpolarized voltages. Our main finding is that upregulation of a Ca(2+) current and downregulation of a high-threshold K(+) current underlies the spontaneous homeostatic development of oscillatory activity. However, because of a nonlinear dependence on stimulus frequency, hyperpolarization-induced oscillations appear to be inconsistent with a homeostatic regulation of activity. We find no difference in the activity patterns or the underlying ionic currents involved between neurons of the fast pyloric and the slow gastric mill networks during the first 10 days in isolation. Dynamic-clamp experiments confirm that these conductance modifications can explain the observed activity changes. We conclude that spontaneous and stimulation-induced excitability changes in STG neurons can both result in intrinsic oscillatory activity via regulation of the same two conductances.

Role for the subthreshold currents ILeak and IH in the homeostatic control of excitability in neocortical somatostatin-positive inhibitory neurons

Cortical circuitry reconfigures in response to chronic (1-3 days) changes in activity levels. To understand this process, we must know the role played by inhibitory neurons because they crucially influence network properties by controlling action potential generation and synaptic integration. Using pharmacological blockade of activity in neocortical organotypic slice cultures, we examined the activity-dependent regulation of membrane excitability in a specific inhibitory neuron subtype: the somatostatin-positive (SOM+) neuron. Chronic action potential blockade (TTX, 2.5 days) resulted in increased excitability in SOM+ neurons. This result is consistent with a homeostatic process to maintain the average firing rate of SOM+ neurons at a particular level. Excitability changes were not ascribed to changing cell size or alterations in voltage-dependent sodium current. Instead, the excitability increase was largely the result of a decrease in the density of two subthreshold currents: a passive leak current (ILeak) and H-current (IH). The downregulation of these currents increased excitability mostly through a decrease in membrane input conductance. The coadaptation of ILeak and IH enabled a change in input conductance while helping to preserve membrane potential. Evidence indicated that ILeak was probably mainly mediated by K+. At earlier culture ages, this adaptation was superimposed on developmental changes, whereas at older ages, the same types of induced alterations occurred but with no developmental component. Together with other studies, these data indicate that both inhibitory and excitatory neurons increase membrane excitability with chronic reduction in activity, but through different mechanisms.

Invertebrate central pattern generation moves along

Central pattern generators (CPGs) are circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration. We summarize recent work on invertebrate CPGs which has provided new insights into how rhythmic motor patterns are produced and how they are controlled by higher-order command and modulatory interneurons.

Variable channel expression in identified single and electrically coupled neurons in different animals

It is often assumed that all neurons of the same cell type have identical intrinsic properties, both within an animal and between animals. We exploited the large size and small number of unambiguously identifiable neurons in the crab stomatogastric ganglion to test this assumption at the level of channel mRNA expression and membrane currents (measured in voltage-clamp experiments). In lateral pyloric (LP) neurons, we saw strong correlations between measured current and the abundance of Shal and BK-KCa mRNAs (encoding the Shal-family voltage-gated potassium channel and large-conductance calcium-activated potassium channel, respectively). We also saw two- to fourfold interanimal variability for three potassium currents and their mRNA expression. Measurements of channel expression in the two electrically coupled pyloric dilator (PD) neurons showed significant interanimal variability, but copy numbers for IH (encoding the hyperpolarization-activated, inward-current channel) and Shal mRNA in the two PD neurons from the same crab were similar, suggesting that the regulation of some currents may be shared in electrically coupled neurons.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

Substance P-mediated modulation of pacemaker properties in the mammalian respiratory network

Neuromodulators are integral parts of a neuronal network, and unraveling how these substances alter neuronal activity is critical for understanding how networks generate patterned activity and, ultimately, behavior. In this study, we examined the cellular mechanisms underlying the excitatory action of substance P (SP) on the respiratory network isolated in spontaneously active transverse slice preparation of mice. SP produced a slow depolarization in all recorded inspiratory pacemaker and non-pacemaker neurons. Ion exchange experiments and blockers for different ion channels suggest that the slow depolarization is caused by the activation of a low-threshold TTX-insensitive cationic current that carries mostly Na+. The SP-induced slow depolarization increased tonic discharge in non-pacemaker neurons and primarily enhanced the frequency of bursting in Cd2+-insensitive pacemaker neurons. In the Cd2+-sensitive pacemaker neuron, the burst frequency was not significantly affected, whereas burst duration and amplitude were more enhanced than in Cd2+-insensitive pacemaker neurons. In a subset of non-pacemaker neurons that produced NMDA-dependent subthreshold oscillations, SP caused the production of bursts of action potentials. We conclude that the degree of pacemaker activity in the respiratory network is not fixed but dynamically regulated by neuromodulators such as SP. This finding may have clinical implications for Rett syndrome in which SP levels along with other neuromodulators are decreased in the brainstem.

Alternative to hand-tuning conductance-based models: construction and analysis of databases of model neurons

Conventionally, the parameters of neuronal models are hand-tuned using trial-and-error searches to produce a desired behavior. Here, we present an alternative approach. We have generated a database of about 1.7 million single-compartment model neurons by independently varying 8 maximal membrane conductances based on measurements from lobster stomatogastric neurons. We classified the spontaneous electrical activity of each model neuron and its responsiveness to inputs during runtime with an adaptive algorithm and saved a reduced version of each neuron's activity pattern. Our analysis of the distribution of different activity types (silent, spiking, bursting, irregular) in the 8-dimensional conductance space indicates that the coarse grid of conductance values we chose is sufficient to capture the salient features of the distribution. The database can be searched for different combinations of neuron properties such as activity type, spike or burst frequency, resting potential, frequency-current relation, and phase-response curve. We demonstrate how the database can be screened for models that reproduce the behavior of a specific biological neuron and show that the contents of the database can give insight into the way a neuron's membrane conductances determine its activity pattern and response properties. Similar databases can be constructed to explore parameter spaces in multicompartmental models or small networks, or to examine the effects of changes in the voltage dependence of currents. In all cases, database searches can provide insight into how neuronal and network properties depend on the values of the parameters in the models.

Episodic bouts of activity accompany recovery of rhythmic output by a neuromodulator- and activity-deprived adult neural network

The pyloric rhythm of the stomatogastric ganglion of the crab, Cancer borealis, slows or stops when descending modulatory inputs are acutely removed. However, the rhythm spontaneously resumes after one or more days in the absence of neuromodulatory input. We recorded continuously for days to characterize quantitatively this recovery process. Activity bouts lasting 40-900 s began several hours after removal of neuromodulatory input and were followed by stable rhythm recovery after 1-4 days. Bout duration was not related to the intervals (0.3-800 min) between bouts. During an individual bout, the frequency rapidly increased and then decreased more slowly. Photoablation of back-filled neuromodulatory terminals in the stomatogastric ganglion (STG) neuropil had no effect on activity bouts or recovery, suggesting that these processes are intrinsic to the STG neuronal network. After removal of neuromodulatory input, the phase relationships of the components of the triphasic pyloric rhythm were altered, and then over time the phase relationships moved toward their control values. Although at low pyloric rhythm frequency the phase relationships among pyloric network neurons depended on frequency, the changes in frequency during recovery did not completely account for the change in phase seen after rhythm recovery. We suggest that activity bouts represent underlying mechanisms controlling the restructuring of the pyloric network to allow resumption of an appropriate output after removal of neuromodulatory input.

Postdecentralization plasticity of voltage-gated K+ currents in glandular sympathetic neurons in rats

This paper presents the kinetic and pharmacological properties of voltage-gated K(+) currents in anatomically identified glandular postganglionic sympathetic neurons isolated from the superior cervical ganglia in rats. The neurons were labelled by injecting the fluorescent tracer Fast Blue into the submandibular gland. The first group of neurons remained intact, i.e. innervated by the preganglionic axons until the day of current recordings (control neurons). The second group of neurons was denervated by severing the superior cervical trunk 4-6 weeks prior to current recordings (decentralized neurons). In every control and decentralized neuron three categories of voltage-dependent K(+) currents were found. (i) The I(Af) K(+) current, steady state, inactivated at hyperpolarized membrane potentials. This current was fast activated and fast time-dependently inactivated, insensitive to TEA and partially depressed by 4-AP. (ii) The I(As) K(+) current, which was steady-state inactivated at less hyperpolarized membrane potentials than I(Af). The current activation and time-dependent inactivation kinetics were slower than those of I(Af). I(As) was blocked by TEA and partially inhibited by 4-AP. (iii) The IK K(+) current did not undergo steady-state inactivation. In decentralized compared to control neurons the maximum I(Af) K(+) current density (at +50 mV) increased from 116.9 +/- 8.2 to 189.0 +/- 11.5 pA/pF, the 10-90% current rise time decreased from 2.3 to 0.7 ms and the recovery from inactivation was faster. Similarly, in decentralized compared to control neurons the maximum I(As) K(+) current density (at +50 mV) increased from 49.9 +/- 3.5 to 74.3 +/- 5.0 pA/pF, the 10-90% current rise time shortened from 29 to 16 ms and the recovery from inactivation of the current was also faster. The maximum density (at +50 mV) of I(K) in decentralized compared to control neurons decreased from 76.6 +/- 3.9 to 60.7 +/- 6.3 pA/pF. We suggest that the upregulation of voltage-gated time-dependently-inactivated K(+) currents and their faster recovery from inactivation serve to restrain the activity of glandular sympathetic neurons after decentralization.

Current compensation in neuronal homeostasis

How do neurons maintain stable intrinsic properties over long periods of time as the channels that govern excitability turn over in the membrane? In this issue of Neuron, MacLean et al. argue that homeostatic regulation of intrinsic activity can occur by an activity-independent mechanism.