Tonic nanomolar dopamine enables an activity-dependent phase recovery mechanism that persistently alters the maximal conductance of the hyperpolarization-activated current in a rhythmically active neuron

Peptidergic Modulation of the Lobster Cardiac System Has Opposing Action on Neurons and Muscles

Modulation of neuronal networks, primarily through neuropeptides, generates variations in motor patterns that allow organisms to adapt to environmental changes or sensory inputs. Modulation is complex, with receptors for neuromodulators expressed at various locations within a nervous system; neuromodulators can thus alter muscle dynamics peripherally via the neuromuscular junction (NMJ) and the muscle itself. The neurogenic cardiac neuromuscular system of the American lobster (Homarus americanus) is a well-characterized model for investigating peptidergic modulation. Myosuppressin (pQDLDHVFLRFamide) is an endogenous peptide that interestingly decreases contraction frequency while also increasing contraction force by acting at both the lobster heart central pattern generator (CPG; the cardiac ganglion) and the periphery (cardiac muscles). Myosuppressin decreases heartbeat frequency by decreasing the burst frequency of the cardiac ganglion. Here, we investigated the remaining question, does myosuppressin exert its peripheral effects directly on the cardiac muscle, the NMJ, or both? To elucidate myosuppressin's effects on the cardiac muscle, the muscle was isolated from the CPG, and contractions were evoked using focal application of the endogenous neurotransmitter, l-glutamate, while superfusing myosuppressin over the heart. Myosuppressin increased glutamate-evoked contraction amplitude in the isolated muscle, suggesting that myosuppressin exerts its peripheral effects directly on the cardiac muscle. To examine effects on the NMJ, excitatory junction potentials were evoked by stimulating the motor nerve and recording the intracellular membrane voltage from a single muscle fiber both in control saline and in the presence of myosuppressin. Myosuppressin did not modulate the amplitude of excitatory junction potentials suggesting that myosuppressin acts directly on the muscle and not via the NMJ, to cause an increase in contraction force.

Ih block reveals separation of timescales in pyloric rhythm response to temperature changes in Cancer borealis

Motor systems operate over a range of frequencies and relative timing (phase). We studied the role of the hyperpolarization-activated inward current (Ih) in regulating these features in the pyloric rhythm of the stomatogastric ganglion (STG) of the crab, Cancer borealis, as temperature was altered from 11°C to 21°C. Under control conditions, rhythm frequency increased monotonically with temperature, while the phases of the pyloric dilator (PD), lateral pyloric (LP), and pyloric (PY) neurons remained constant. Blocking Ih with cesium (Cs+) phase advanced PD offset, LP onset, and LP offset at 11°C, and the latter two further advanced as temperature increased. In Cs+ the frequency increase with temperature diminished and the Q10 of the frequency dropped from ~1.75 to ~1.35. Unexpectedly in Cs+, the frequency dynamics became non-monotonic during temperature transitions; frequency initially dropped as temperature increased, then rose once temperature stabilized, creating a characteristic 'jag'. Interestingly, these jags persisted during temperature transitions in Cs+ when the pacemaker was isolated by picrotoxin, although the temperature-induced change in frequency recovered to control levels. Overall, these data suggest that Ih plays an important role in maintaining smooth transitory responses and persistent frequency increases by different mechanisms in the pyloric circuitry during temperature fluctuations.

Ih Block Reveals Separation of Timescales in Pyloric Rhythm Response to Temperature Changes in Cancer borealis

Motor systems operate over a range of frequencies and relative timing (phase). We studied the contribution of the hyperpolarization-activated inward current (Ih) to frequency and phase in the pyloric rhythm of the stomatogastric ganglion (STG) of the crab, Cancer borealis as temperature was altered from 11°C to 21°C. Under control conditions, the frequency of the rhythm increased monotonically with temperature, while the phases of the pyloric dilator (PD), lateral pyloric (LP), and pyloric (PY) neurons remained constant. When we blocked Ih with cesium (Cs+) PD offset, LP onset, and LP offset were all phase advanced in Cs+ at 11°C, and the latter two further advanced as temperature increased. In Cs+ the steady state increase in pyloric frequency with temperature diminished and the Q10 of the pyloric frequency dropped from ~1.75 to ~1.35. Unexpectedly in Cs+, the frequency displayed non-monotonic dynamics during temperature transitions; the frequency initially dropped as temperature increased, then rose once temperature stabilized, creating a characteristic "jag". Interestingly, these jags were still present during temperature transitions in Cs+ when the pacemaker was isolated by picrotoxin, although the temperature-induced change in frequency recovered to control levels. Overall, these data suggest that Ih plays an important role in the ability of this circuit to produce smooth transitory responses and persistent frequency increases by different mechanisms during temperature fluctuations.

Excitatory and inhibitory effects of HCN channel modulation on excitability of layer V pyramidal cells

Dendrites of cortical pyramidal cells are densely populated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, a.k.a. Ih channels. Ih channels are targeted by multiple neuromodulatory pathways, and thus are one of the key ion-channel populations regulating the pyramidal cell activity. Previous observations and theories attribute opposing effects of the Ih channels on neuronal excitability due to their mildly hyperpolarized reversal potential. These effects are difficult to measure experimentally due to the fine spatiotemporal landscape of the Ih activity in the dendrites, but computational models provide an efficient tool for studying this question in a reduced but generalizable setting. In this work, we build upon existing biophysically detailed models of thick-tufted layer V pyramidal cells and model the effects of over- and under-expression of Ih channels as well as their neuromodulation. We show that Ih channels facilitate the action potentials of layer V pyramidal cells in response to proximal dendritic stimulus while they hinder the action potentials in response to distal dendritic stimulus at the apical dendrite. We also show that the inhibitory action of the Ih channels in layer V pyramidal cells is due to the interactions between Ih channels and a hot zone of low voltage-activated Ca2+ channels at the apical dendrite. Our simulations suggest that a combination of Ih-enhancing neuromodulation at the proximal part of the apical dendrite and Ih-inhibiting modulation at the distal part of the apical dendrite can increase the layer V pyramidal excitability more than either of the two alone. Our analyses uncover the effects of Ih-channel neuromodulation of layer V pyramidal cells at a single-cell level and shed light on how these neurons integrate information and enable higher-order functions of the brain.

The dynamic range of voltage-dependent gap junction signaling is maintained by Ih-induced membrane potential depolarization

Like their chemical counterparts, electrical synapses show complex dynamics such as rectification and voltage dependence that interact with other electrical processes in neurons. The consequences arising from these interactions for the electrical behavior of the synapse, and the dynamics they create, remain largely unexplored. Using a voltage-dependent electrical synapse between a descending modulatory projection neuron (MCN1) and a motor neuron (LG) in the crustacean stomatogastric ganglion, we find that the influence of the hyperpolarization-activated inward current (Ih) is critical to the function of the electrical synapse. When we blocked Ih with CsCl, the apparent voltage dependence of the electrical synapse shifted by 18.7 mV to more hyperpolarized voltages, placing the dynamic range of the electrical synapse outside of the range of voltages used by the LG motor neuron (-60.2 mV to -44.9 mV). With dual electrode current- and voltage-clamp recordings, we demonstrate that this voltage shift is not due to a change in the properties of the gap junction itself, but is a result of a sustained effect of Ih on the presynaptic MCN1 axon terminal membrane potential. Ih-induced depolarization of the axon terminal membrane potential increased the electrical postsynaptic potentials and currents. With Ih present, the axon terminal resting membrane potential is depolarized, shifting the dynamic range of the electrical synapse toward the functional range of the motor neuron. We thus demonstrate that the function of an electrical synapse is critically influenced by a voltage-dependent ionic current (Ih).NEW & NOTEWORTHY Electrical synapses and voltage-gated ionic currents are often studied independently from one another, despite mounting evidence that their interactions can alter synaptic behavior. We show that the hyperpolarization-activated inward ionic current shifts the voltage dependence of electrical synaptic transmission through its depolarizing effect on the membrane potential, enabling it to lie within the functional membrane potential range of a motor neuron. Thus, the electrical synapse's function critically depends on the voltage-gated ionic current.

Hypothalamic Dopamine Neurons Control Sensorimotor Behavior by Modulating Brainstem Premotor Nuclei in Zebrafish

Dopamine (DA)-producing neurons are critically involved in the production of motor behaviors in multiple circuits that are conserved from basal vertebrates to mammals. Although there is increasing evidence that DA neurons in the hypothalamus play a locomotor role, their precise contributions to behavior and the circuit mechanisms by which they are achieved remain unclear. Here, we demonstrate that tyrosine-hydroxylase-2-expressing (th2+) DA neurons in the zebrafish hypothalamus fire phasic bursts of activity to acutely promote swimming and modulate audiomotor behaviors on fast timescales. Their anatomy and physiology reveal two distinct functional DA modules within the hypothalamus. The first comprises an interconnected set of cerebrospinal-fluid-contacting DA nuclei surrounding the 3^rd ventricle, which lack distal projections outside of the hypothalamus and influence locomotion through unknown means. The second includes neurons in the preoptic nucleus, which send long-range projections to targets throughout the brain, including the mid- and hindbrain, where they activate premotor circuits involved in swimming and sensorimotor integration. These data suggest a broad regulation of motor behavior by DA neurons within multiple hypothalamic nuclei and elucidate a novel functional mechanism for the preoptic DA neurons in the initiation of movement.

A Population of Interneurons Signals Changes in the Basal Concentration of Serotonin and Mediates Gain Control in the Drosophila Antennal Lobe

Serotonin (5-HT) represents a quintessential neuromodulator, having been identified in nearly all animal species [1] where it functions in cognition [2], motor control [3], and sensory processing [4]. In the olfactory circuits of flies and mice, serotonin indirectly inhibits odor responses in olfactory receptor neurons (ORNs) via GABAergic local interneurons (LNs) [5, 6]. However, the effects of 5-HT in olfaction are likely complicated, because multiple receptor subtypes are distributed throughout the olfactory bulb (OB) and antennal lobe (AL), the first layers of olfactory neuropil in mammals and insects, respectively [7]. For example, serotonin has a non-monotonic effect on odor responses in Drosophila projection neurons (PNs), where low concentrations suppress odor-evoked activity and higher concentrations boost PN responses [8]. Serotonin reaches the AL via the diffusion of paracrine 5-HT through the fly hemolymph [8] and by activation of the contralaterally projecting serotonin-immunoreactive deuterocerebral interneurons (CSDns): the only serotonergic cells that innervate the AL [9, 10]. Concentration-dependent effects could arise by either the expression of multiple 5-HT receptors (5-HTRs) on the same cells or by populations of neurons dedicated to detecting serotonin at different concentrations. Here, we identify a population of LNs that express 5-HT7Rs exclusively to detect basal concentrations of 5-HT. These LNs inhibit PNs via GABA_B receptors and mediate subtractive gain control. LNs expressing 5-HT7Rs are broadly tuned to odors and target every glomerulus in the antennal lobe. Our results demonstrate that serotonergic modulation at low concentrations targets a specific population of LNs to globally downregulate PN odor responses in the AL.

Molecular mechanisms of homeostatic plasticity in central pattern generator networks

Central pattern generator (CPG) networks rely on a balance of intrinsic and network properties to produce reliable, repeatable activity patterns. This balance is maintained by homeostatic plasticity where alterations in neuronal properties dynamically maintain appropriate neural output in the face of changing environmental conditions and perturbations. However, it remains unclear just how these neurons and networks can both monitor their ongoing activity and use this information to elicit homeostatic physiological responses to ensure robustness of output over time. Evidence exists that CPG networks use a mixed strategy of activity-dependent, activity-independent, modulator-dependent, and synaptically regulated homeostatic plasticity to achieve this critical stability. In this review, we focus on some of the current understanding of the molecular pathways and mechanisms responsible for this homeostatic plasticity in the context of central pattern generator function, with a special emphasis on some of the smaller invertebrate networks that have allowed for extensive cellular-level analyses that have brought recent insights to these questions.

Modulator-Gated, SUMOylation-Mediated, Activity-Dependent Regulation of Ionic Current Densities Contributes to Short-Term Activity Homeostasis

Neurons operate within defined activity limits, and feedback control mechanisms dynamically tune ionic currents to maintain this optimal range. This study describes a novel, rapid feedback mechanism that uses SUMOylation to continuously adjust ionic current densities according to changes in activity. Small ubiquitin-like modifier (SUMO) is a peptide that can be post-translationally conjugated to ion channels to influence their surface expression and biophysical properties. Neuronal activity can regulate the extent of protein SUMOylation. This study on the single, unambiguously identifiable lateral pyloric neuron (LP), a component of the pyloric network in the stomatogastric nervous system of male and female spiny lobsters (Panulirus interruptus), focused on dynamic SUMOylation in the context of activity homeostasis. There were four major findings: First, neuronal activity adjusted the balance between SUMO conjugation and deconjugation to continuously and bidirectionally fine-tune the densities of two opposing conductances: the hyperpolarization activated current (I_h) and the transient potassium current (I_A). Second, tonic 5 nm dopamine (DA) gated activity-dependent SUMOylation to permit and prevent activity-dependent regulation of I_h and I_A, respectively. Third, DA-gated, activity-dependent SUMOylation contributed to a feedback mechanism that restored the timing and duration of LP activity during prolonged modulation by 5 μm DA, which initially altered these and other activity features. Fourth, DA modulatory and metamoduatory (gating) effects were tailored to simultaneously alter and stabilize neuronal output. Our findings suggest that modulatory tone may select a subset of rapid activity-dependent mechanisms from a larger menu to achieve homeostasis under varying conditions.SIGNIFICANCE STATEMENT Post-translational SUMOylation of ion channel subunits controls their interactions. When subunit SUMOylation is dysregulated, conductance densities mediated by the channels are distorted, leading to nervous system disorders, such as seizures and chronic pain. Regulation of ion channel SUMOylation is poorly understood. This study demonstrated that neuronal activity can regulate SUMOylation to reconfigure ionic current densities over minutes, and this regulation was gated by tonic nanomolar dopamine. Dynamic SUMOylation was necessary to maintain specific aspects of neuronal output while the neuron was being modulated by high (5 μm) concentrations of dopamine, suggesting that the gating function may ensure neuronal homeostasis during extrinsic modulation of a circuit.

Dopamine maintains network synchrony via direct modulation of gap junctions in the crustacean cardiac ganglion

The Large Cell (LC) motor neurons of the crab cardiac ganglion have variable membrane conductance magnitudes even within the same individual, yet produce identical synchronized activity in the intact network. In a previous study we blocked a subset of K+ conductances across LCs, resulting in loss of synchronous activity (Lane et al., 2016). In this study, we hypothesized that this same variability of conductances makes LCs vulnerable to desynchronization during neuromodulation. We exposed the LCs to serotonin (5HT) and dopamine (DA) while recording simultaneously from multiple LCs. Both amines had distinct excitatory effects on LC output, but only 5HT caused desynchronized output. We further determined that DA rapidly increased gap junctional conductance. Co-application of both amines induced 5HT-like output, but waveforms remained synchronized. Furthermore, DA prevented desynchronization induced by the K+ channel blocker tetraethylammonium (TEA), suggesting that dopaminergic modulation of electrical coupling plays a protective role in maintaining network synchrony.

Three members of a peptide family are differentially distributed and elicit differential state-dependent responses in a pattern generator-effector system

C-type allatostatins (AST-Cs) are pleiotropic neuropeptides that are broadly conserved within arthropods; the presence of three AST-C isoforms, encoded by paralog genes, is common. However, these peptides are hypothesized to act through a single receptor, thereby exerting similar bioactivities within each species. We investigated this hypothesis in the American lobster, Homarus americanus, mapping the distributions of AST-C isoforms within relevant regions of the nervous system and digestive tract, and comparing their modulatory influences on the cardiac neuromuscular system. Immunohistochemistry showed that in the pericardial organ, a neuroendocrine release site, AST-C I and/or III and AST-C II are contained within distinct populations of release terminals. Moreover, AST-C I/III-like immunoreactivity was seen in midgut epithelial endocrine cells and the cardiac ganglion (CG), whereas AST-C II-like immunoreactivity was not seen in these tissues. These data suggest that AST-C I and/or III can modulate the CG both locally and hormonally; AST-C II likely acts on the CG solely as a hormonal modulator. Physiological studies demonstrated that all three AST-C isoforms can exert differential effects, including both increases and decreases, on contraction amplitude and frequency when perfused through the heart. However, in contrast to many state-dependent modulatory changes, the changes in contraction amplitude and frequency elicited by the AST-Cs were not functions of the baseline parameters. The responses to AST-C I and III, neither of which is COOH-terminally amidated, are more similar to one another than they are to the responses elicited by AST-C II, which is COOH-terminally amidated. These results suggest that the three AST-C isoforms are differentially distributed in the lobster nervous system/midgut and can elicit distinct behaviors from the cardiac neuromuscular system, with particular structural features, e.g., COOH-terminal amidation, likely important in determining the effects of the peptides. NEW & NOTEWORTHY Multiple isoforms of many peptides exert similar effects on neural circuits. In this study we show that each of the three isoforms of C-type allatostatin (AST-C) can exert differential effects, including both increases and decreases in contraction amplitude and frequency, on the lobster cardiac neuromuscular system. The distribution of effects elicited by the nonamidated isoforms AST-C I and III are more similar to one another than to the effects of the amidated AST-C II.

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.

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.

Monoaminergic tone supports conductance correlations and stabilizes activity features in pattern generating neurons of the lobster, Panulirus interruptus

Experimental and computational studies demonstrate that different sets of intrinsic and synaptic conductances can give rise to equivalent activity patterns. This is because the balance of conductances, not their absolute values, defines a given activity feature. Activity-dependent feedback mechanisms maintain neuronal conductance correlations and their corresponding activity features. This study demonstrates that tonic nM concentrations of monoamines enable slow, activity-dependent processes that can maintain a correlation between the transient potassium current (I_A) and the hyperpolarization activated current (I_h) over the long-term (i.e., regulatory change persists for hours after removal of modulator). Tonic 5 nM DA acted through an RNA interference silencing complex (RISC)- and RNA polymerase II-dependent mechanism to maintain a long-term positive correlation between I_A and I_h in the lateral pyloric neuron (LP) but not in the pyloric dilator neuron (PD). In contrast, tonic 5 nM 5HT maintained a RISC-dependent positive correlation between I_A and I_h in PD but not LP over the long-term. Tonic 5 nM OCT maintained a long-term negative correlation between I_A and I_h in PD but not LP; however, it was only revealed when RISC was inhibited. This study also demonstrated that monoaminergic tone can also preserve activity features over the long-term: the timing of LP activity, LP duty cycle and LP spike number per burst were maintained by tonic 5 nM DA. The data suggest that low-level monoaminergic tone acts through multiple slow processes to permit cell-specific, activity-dependent regulation of ionic conductances to maintain conductance correlations and their corresponding activity features over the long-term.

Animal-to-Animal Variability in Neuromodulation and Circuit Function

Each animal alive in the world is different from all other individuals, while sharing most attributes of form and function with others of the same species. Still other attributes are shared within a phylum, and still others are common to most eukaryotic organisms. All animals have mechanisms that modulate the strength of their synapses or alter the intrinsic excitability of component neurons. What animal-to-animal variability in behavior arises from differences in neuronal structure, ion channel expression, or connectivity, and what variability arises from neuromodulation of brain states? Conversely, can robust behavior be maintained despite variability in circuit components by the action of neuromodulatory inputs? These are fundamental issues relevant to all nervous systems that have been illuminated by many years of study of the small, rhythmic motor circuits found in the crustacean stomatogastric nervous system.

Tonic 5nM DA stabilizes neuronal output by enabling bidirectional activity-dependent regulation of the hyperpolarization activated current via PKA and calcineurin

Volume transmission results in phasic and tonic modulatory signals. The actions of tonic dopamine (DA) at type 1 DA receptors (D1Rs) are largely undefined. Here we show that tonic 5nM DA acts at D1Rs to stabilize neuronal output over minutes by enabling activity-dependent regulation of the hyperpolarization activated current (I _h). In the presence but not absence of 5nM DA, I _h maximal conductance (G _max) was adjusted according to changes in slow wave activity in order to maintain spike timing. Our study on the lateral pyloric neuron (LP), which undergoes rhythmic oscillations in membrane potential with depolarized plateaus, demonstrated that incremental, bi-directional changes in plateau duration produced corresponding alterations in LP I _hG _max when preparations were superfused with saline containing 5nM DA. However, when preparations were superfused with saline alone there was no linear correlation between LP I _hG_max and duty cycle. Thus, tonic nM DA modulated the capacity for activity to modulate LP I _h G _max; this exemplifies metamodulation (modulation of modulation). Pretreatment with the Ca2+-chelator, BAPTA, or the specific PKA inhibitor, PKI, prevented all changes in LP I _h in 5nM DA. Calcineurin inhibitors blocked activity-dependent changes enabled by DA and revealed a PKA-mediated, activity-independent enhancement of LP I _hG _max. These data suggested that tonic 5nM DA produced two simultaneous, PKA-dependent effects: a direct increase in LP I _h G _max and a priming event that permitted calcineurin regulation of LP I _h. The latter produced graded reductions in LP I _hG _max with increasing duty cycles. We also demonstrated that this metamodulation preserved the timing of LP’s first spike when network output was perturbed with bath-applied 4AP. In sum, 5nM DA permits slow wave activity to provide feedback that maintains spike timing, suggesting that one function of low-level, tonic modulation is to stabilize specific features of a dynamic output.

Modulation of a Single Neuron Has State-Dependent Actions on Circuit Dynamics,

We use computational models to examine the circumstances under which modulation of a single neuron can alter the dynamics of an entire circuit. We show that under some circumstances, the circuit's behavior is robust to the neuromodulation of a single hub neuron, while under other circumstances the same neuromodulatory action of a single neuron can produce a large variety of circuit outcomes.

Cover Figure

Robust circuit rhythms in small circuits arise from variable circuit components and mechanisms

Small central pattern generating circuits found in invertebrates have significant advantages for the study of the circuit mechanisms that generate brain rhythms. Experimental and computational studies of small oscillatory circuits reveal that similar rhythms can arise from disparate mechanisms. Animal-to-animal variation in the properties of single neurons and synapses may underly robust circuit performance, and can be revealed by perturbations. Neuromodulation can produce altered circuit performance but also ensure reliable circuit function.

Frank Beach Award Winner: Steroids as neuromodulators of brain circuits and behavior

Neurons communicate primarily via action potentials that transmit information on the timescale of milliseconds. Neurons also integrate information via alterations in gene transcription and protein translation that are sustained for hours to days after initiation. Positioned between these two signaling timescales are the minute-by-minute actions of neuromodulators. Over the course of minutes, the classical neuromodulators (such as serotonin, dopamine, octopamine, and norepinephrine) can alter and/or stabilize neural circuit patterning as well as behavioral states. Neuromodulators allow many flexible outputs from neural circuits and can encode information content into the firing state of neural networks. The idea that steroid molecules can operate as genuine behavioral neuromodulators - synthesized by and acting within brain circuits on a minute-by-minute timescale - has gained traction in recent years. Evidence for brain steroid synthesis at synaptic terminals has converged with evidence for the rapid actions of brain-derived steroids on neural circuits and behavior. The general principle emerging from this work is that the production of steroid hormones within brain circuits can alter their functional connectivity and shift sensory representations by enhancing their information coding. Steroids produced in the brain can therefore change the information content of neuronal networks to rapidly modulate sensory experience and sensorimotor functions.

Neuromodulation as a mechanism for the induction of repetition priming

It is becoming apparent that the activity of many neural networks is shaped by effects of endogenous neuromodulators. Modulators exert second messenger-mediated actions that persist. We consider how this may impact network function and its potential role in the induction of repetition priming (increased performance when behavior is repeated). When effects of modulators persist and modulatory substances are repeatedly released, their effects will accumulate (summate) and become more pronounced. If this enhances the ability of a network to generate a particular output, performance will improve. We review data that support this model, and consider its implications for task switching. This model predicts that priming of one type of network activity will negatively impact the rapid transition to an incompatible type.

A codimension-2 bifurcation controlling endogenous bursting activity and pulse-triggered responses of a neuron model

The dynamics of individual neurons are crucial for producing functional activity in neuronal networks. An open question is how temporal characteristics can be controlled in bursting activity and in transient neuronal responses to synaptic input. Bifurcation theory provides a framework to discover generic mechanisms addressing this question. We present a family of mechanisms organized around a global codimension-2 bifurcation. The cornerstone bifurcation is located at the intersection of the border between bursting and spiking and the border between bursting and silence. These borders correspond to the blue sky catastrophe bifurcation and the saddle-node bifurcation on an invariant circle (SNIC) curves, respectively. The cornerstone bifurcation satisfies the conditions for both the blue sky catastrophe and SNIC. The burst duration and interburst interval increase as the inverse of the square root of the difference between the corresponding bifurcation parameter and its bifurcation value. For a given set of burst duration and interburst interval, one can find the parameter values supporting these temporal characteristics. The cornerstone bifurcation also determines the responses of silent and spiking neurons. In a silent neuron with parameters close to the SNIC, a pulse of current triggers a single burst. In a spiking neuron with parameters close to the blue sky catastrophe, a pulse of current temporarily silences the neuron. These responses are stereotypical: the durations of the transient intervals-the duration of the burst and the duration of latency to spiking-are governed by the inverse-square-root laws. The mechanisms described here could be used to coordinate neuromuscular control in central pattern generators. As proof of principle, we construct small networks that control metachronal-wave motor pattern exhibited in locomotion. This pattern is determined by the phase relations of bursting neurons in a simple central pattern generator modeled by a chain of oscillators.

Dopaminergic tone regulates transient potassium current maximal conductance through a translational mechanism requiring D1Rs, cAMP/PKA, Erk and mTOR

Background

Dopamine (DA) can produce divergent effects at different time scales. DA has opposing immediate and long-term effects on the transient potassium current (I_A) within neurons of the pyloric network, in the Panulirus interruptus stomatogastric ganglion. The lateral pyloric neuron (LP) expresses type 1 DA receptors (D1Rs). A 10 min application of 5-100 μM DA decreases LP I_A by producing a decrease in I_A maximal conductance (G_max) and a depolarizing shift in I_A voltage dependence through a cAMP-Protein kinase A (PKA) dependent mechanism. Alternatively, a 1 hr application of DA (≥5 nM) generates a persistent (measured 4 hr after DA washout) increase in I_A G_max in the same neuron, through a mechanistic target of rapamycin (mTOR) dependent translational mechanism. We examined the dose, time and protein dependencies of the persistent DA effect.

Results

We found that disrupting normal modulatory tone decreased LP I_A. Addition of 500 pM-5 nM DA to the saline for 1 hr prevented this decrease, and in the case of a 5 nM DA application, the effect was sustained for >4 hrs after DA removal. To determine if increased cAMP mediated the persistent effect of 5nM DA, we applied the cAMP analog, 8-bromo-cAMP alone or with rapamycin for 1 hr, followed by wash and TEVC. 8-bromo-cAMP induced an increase in I_A G_max, which was blocked by rapamycin. Next we tested the roles of PKA and guanine exchange factor protein activated by cAMP (ePACs) in the DA-induced persistent change in I_A using the PKA specific antagonist Rp-cAMP and the ePAC specific agonist 8-pCPT-2′-O-Me-cAMP. The PKA antagonist blocked the DA induced increases in LP I_A G_max, whereas the ePAC agonist did not induce an increase in LP I_A G_max. Finally we tested whether extracellular signal regulated kinase (Erk) activity was necessary for the persistent effect by co-application of Erk antagonists PD98059 or U0126 with DA. Erk antagonism blocked the DA induced persistent increase in LP I_A.

Conclusions

These data suggest that dopaminergic tone regulates ion channel density in a concentration and time dependent manner. The D1R- PKA axis, along with Erk and mTOR are necessary for the persistent increase in LP I_A induced by high affinity D1Rs.

Comparison of two voltage-sensitive dyes and their suitability for long-term imaging of neuronal activity

One of the key approaches for studying neural network function is the simultaneous measurement of the activity of many neurons. Voltage-sensitive dyes (VSDs) simultaneously report the membrane potential of multiple neurons, but often have pharmacological and phototoxic effects on neuronal cells. Yet, to study the homeostatic processes that regulate neural network function long-term recordings of neuronal activities are required. This study aims to test the suitability of the VSDs RH795 and Di-4-ANEPPS for optically recording pattern generating neurons in the stomatogastric nervous system of crustaceans with an emphasis on long-term recordings of the pyloric central pattern generator. We demonstrate that both dyes stain pyloric neurons and determined an optimal concentration and light intensity for optical imaging. Although both dyes provided sufficient signal-to-noise ratio for measuring membrane potentials, Di-4-ANEPPS displayed a higher signal quality indicating an advantage of this dye over RH795 when small neuronal signals need to be recorded. For Di-4-ANEPPS, higher dye concentrations resulted in faster and brighter staining. Signal quality, however, only depended on excitation light strength, but not on dye concentration. RH795 showed weak and slowly developing phototoxic effects on the pyloric motor pattern as well as slow bleaching of the staining and is thus the better choice for long-term experiments. Low concentrations and low excitation intensities can be used as, in contrast to Di-4-ANEPPS, the signal-to-noise ratio was independent of excitation light strength. In summary, RH795 and Di-4-ANEPPS are suitable for optical imaging in the stomatogastric nervous system of crustaceans. They allow simultaneous recording of the membrane potential of multiple neurons with high signal quality. While Di-4-ANEPPS is better suited for short-term experiments that require high signal quality, RH795 is a better candidate for long-term experiments since it has only minor effects on the motor pattern.

Neuromodulation of neuronal circuits: back to the future

All nervous systems are subject to neuromodulation. Neuromodulators can be delivered as local hormones, as cotransmitters in projection neurons, and through the general circulation. Because neuromodulators can transform the intrinsic firing properties of circuit neurons and alter effective synaptic strength, neuromodulatory substances reconfigure neuronal circuits, often massively altering their output. Thus, the anatomical connectome provides a minimal structure and the neuromodulatory environment constructs and specifies the functional circuits that give rise to behavior.