Differential modulation of chemical and electrical components of mixed synapses in the lobster stomatogastric ganglion

Neuropeptide modulation of bidirectional internetwork synapses

Oscillatory networks underlying rhythmic motor behaviors, and sensory and complex neural processing, are flexible, even in their neuronal composition. Neuromodulatory inputs enable neurons to switch participation between networks or participate in multiple networks simultaneously. Neuromodulation of internetwork synapses can both recruit and coordinate a switching neuron in a second network. We previously identified an example in which a neuron is recruited into dual-network activity via peptidergic modulation of intrinsic properties. We now ask whether the same neuropeptide also modulates internetwork synapses for internetwork coordination. The crab (Cancer borealis) stomatogastric nervous system contains two well-defined feeding-related networks (pyloric, food filtering, ∼1 Hz; gastric mill, food chewing, ∼0.1 Hz). The projection neuron MCN5 uses the neuropeptide Gly1-SIFamide to recruit the pyloric-only lateral posterior gastric (LPG) neuron into dual pyloric- plus gastric mill-timed bursting via modulation of LPG's intrinsic properties. Descending input is not required for a coordinated rhythm, thus intranetwork synapses between LPG and its second network must underlie coordination among these neurons. However, synapses between LPG and gastric mill neurons have not been documented. Using two-electrode voltage-clamp recordings, we found that graded synaptic currents between LPG and gastric mill neurons (lateral gastric, inferior cardiac, and dorsal gastric) were primarily negligible in saline, but were enhanced by Gly1-SIFamide. Furthermore, LPG and gastric mill neurons entrain each other during Gly1-SIFamide application, indicating bidirectional, functional connectivity. Thus, a neuropeptide mediates neuronal switching through parallel actions, modulating intrinsic properties for recruitment into a second network and as shown here, also modulating bidirectional internetwork synapses for coordination.NEW & NOTEWORTHY Neuromodulation can enable neurons to simultaneously coordinate with separate networks. Both recruitment into, and coordination with, a second network can occur via modulation of internetwork synapses. Alternatively, recruitment can occur via modulation of intrinsic ionic currents. We find that the same neuropeptide previously determined to modulate intrinsic currents also modulates bidirectional internetwork synapses that are typically ineffective. Thus, complementary modulatory peptide actions enable recruitment and coordination of a neuron into a second network.

Neuronal Switching between Single- and Dual-Network Activity via Modulation of Intrinsic Membrane Properties

Oscillatory networks underlie rhythmic behaviors (e.g., walking, chewing) and complex behaviors (e.g., memory formation, decision-making). Flexibility of oscillatory networks includes neurons switching between single- and dual-network participation, even generating oscillations at two distinct frequencies. Modulation of synaptic strength can underlie this neuronal switching. Here we ask whether switching into dual-frequency oscillations can also result from modulation of intrinsic neuronal properties. The isolated stomatogastric nervous system of male Cancer borealis crabs contains two well-characterized rhythmic feeding-related networks (pyloric, ∼1 Hz; gastric mill, ∼0.1 Hz). The identified modulatory projection neuron MCN5 causes the pyloric-only lateral posterior gastric (LPG) neuron to switch to dual pyloric/gastric mill bursting. Bath applying the MCN5 neuropeptide transmitter Gly¹-SIFamide only partly mimics the LPG switch to dual activity because of continued LP neuron inhibition of LPG. Here, we find that MCN5 uses a cotransmitter, glutamate, to inhibit LP, unlike Gly¹-SIFamide excitation of LP. Thus, we modeled the MCN5-elicited LPG switching with Gly¹-SIFamide application and LP photoinactivation. Using hyperpolarization of pyloric pacemaker neurons and gastric mill network neurons, we found that LPG pyloric-timed oscillations require rhythmic electrical synaptic input. However, LPG gastric mill-timed oscillations do not require any pyloric/gastric mill synaptic input and are voltage-dependent. Thus, we identify modulation of intrinsic properties as an additional mechanism for switching a neuron into dual-frequency activity. Instead of synaptic modulation switching a neuron into a second network as a passive follower, modulation of intrinsic properties could enable a switching neuron to become an active contributor to rhythm generation in the second network.SIGNIFICANCE STATEMENT Neuromodulation of oscillatory networks can enable network neurons to switch from single- to dual-network participation, even when two networks oscillate at distinct frequencies. We used small, well-characterized networks to determine whether modulation of synaptic strength, an identified mechanism for switching, is necessary for dual-network recruitment. We demonstrate that rhythmic electrical synaptic input is required for continued linkage with a "home" network, whereas modulation of intrinsic properties enables a neuron to generate oscillations at a second frequency. Neuromodulator-induced switches in neuronal participation between networks occur in motor, cognitive, and sensory networks. Our study highlights the importance of considering intrinsic properties as a pivotal target for enabling parallel participation of a neuron in two oscillatory networks.

On the occurrence and enigmatic functions of mixed (chemical plus electrical) synapses in the mammalian CNS

Electrical synapses with diverse configurations and functions occur at a variety of interneuronal appositions, thereby significantly expanding the physiological complexity of neuronal circuitry over that provided solely by chemical synapses. Gap junctions between apposed dendritic and somatic plasma membranes form "purely electrical" synapses that allow for electrical communication between coupled neurons. In addition, gap junctions at axon terminals synapsing on dendrites and somata allow for "mixed" (dual chemical+electrical) synaptic transmission. "Dual transmission" was first documented in the autonomic nervous system of birds, followed by its detection in the central nervous systems of fish, amphibia, and reptiles. Subsequently, mixed synapses have been detected in several locations in the mammalian CNS, where their properties and functional roles remain undetermined. Here, we review available evidence for the presence, complex structural composition, and emerging functional properties of mixed synapses in the mammalian CNS.

Serotonergic network in the subesophageal zone modulates the motor pattern for food intake in Drosophila

The functional organization of central motor circuits underlying feeding behaviors is not well understood. We have combined electrophysiological and genetic approaches to investigate the regulatory networks upstream of the motor program underlying food intake in the Drosophila larval central nervous system. We discovered that the serotonergic network of the CNS is able to set the motor rhythm frequency of pharyngeal pumping. Pharmacological experiments verified that modulation of the feeding motor pattern is based on the release of serotonin. Classical lesion and laser based cell ablation indicated that the serotonergic neurons in the subesophageal zone represent a redundant network for motor control of larval food intake.

Analysis of the dynamics of temporal relationships of neural activities using optical imaging data

The temporal relationship between the activities of neurons in biological neural systems is critically important for the correct delivery of the functionality of these systems. Fine measurement of temporal relationships of neural activities using micro-electrodes is possible but this approach is very limited due to spatial constraints in the context of physiologically valid settings of neural systems. Optical imaging with voltage-sensitive dyes or calcium dyes can provide data about the activity patterns of many neurons in physiologically valid settings, but the data is relatively noisy. Here we propose a numerical methodology for the analysis of optical neuro-imaging data that allows robust analysis of the dynamics of temporal relationships of neural activities. We provide a detailed description of the methodology and we also assess its robustness. The proposed methodology is applied to analyse the relationship between the activity patterns of PY neurons in the crab stomatogastric ganglion. We show for the first time in a physiologically valid setting that as expected on the basis of earlier results of single neuron recordings exposure to dopamine de-synchronises the activity of these neurons. We also discuss the wider implications and application of the proposed methodology.

Complicating connectomes: Electrical coupling creates parallel pathways and degenerate circuit mechanisms

Electrical coupling in circuits can produce non‐intuitive circuit dynamics, as seen in both experimental work from the crustacean stomatogastric ganglion and in computational models inspired by the connectivity in this preparation. Ambiguities in interpreting the results of electrophysiological recordings can arise if sets of pre‐ or postsynaptic neurons are electrically coupled, or if the electrical coupling exhibits some specificity (e.g. rectifying, or voltage‐dependent). Even in small circuits, electrical coupling can produce parallel pathways that can allow information to travel by monosynaptic and/or polysynaptic pathways. Consequently, similar changes in circuit dynamics can arise from entirely different underlying mechanisms. When neurons are coupled both chemically and electrically, modifying the relative strengths of the two interactions provides a mechanism for flexibility in circuit outputs. This, together with neuromodulation of gap junctions and coupled neurons is important both in developing and adult circuits. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 597–609, 2017

Electrical coupling and innexin expression in the stomatogastric ganglion of the crab Cancer borealis

Gap junctions are intercellular channels that allow for the movement of small molecules and ions between the cytoplasm of adjacent cells and form electrical synapses between neurons. In invertebrates, the gap junction proteins are coded for by the innexin family of genes. The stomatogastric ganglion (STG) in the crab Cancer borealis contains a small number of identified and electrically coupled neurons. We identified Innexin 1 (Inx1), Innexin 2 (Inx2), Innexin 3 (Inx3), Innexin 4 (Inx4), Innexin 5 (Inx5), and Innexin 6 (Inx6) members of the C. borealis innexin family. We also identified six members of the innexin family from the lobster Homarus americanus transcriptome. These innexins show significant sequence similarity to other arthropod innexins. Using in situ hybridization and reverse transcriptase-quantitative PCR (RT-qPCR), we determined that all the cells in the crab STG express multiple innexin genes. Electrophysiological recordings of coupling coefficients between identified pairs of pyloric dilator (PD) cells and PD-lateral posterior gastric (LPG) neurons show that the PD-PD electrical synapse is nonrectifying while the PD-LPG synapse is apparently strongly rectifying.

Neuromodulation of neurons and synapses

Neuromodulation underlies the flexibility of neural circuit operation and behavior. Individual neuromodulators can have divergent actions in a neuron by targeting multiple physiological mechanisms. Conversely, multiple neuromodulators may have convergent actions through overlapping targets. The divergent and convergent neuromodulator actions can be unambiguously synergistic or antagonistic, but neuromodulation often entails balanced adjustment of nonlinear membrane and synaptic properties by targeting ion channel and synaptic dynamics rather than just excitability or synaptic strength. In addition, neuromodulators can exert effects at multiple timescales, from short-term adjustments of neuron and synapse function to persistent long-term regulation. This short review summarizes some highlights of the diverse actions of neuromodulators on ion channel and synaptic properties.

Identification and developmental expression of the enzymes responsible for dopamine, histamine, octopamine and serotonin biosynthesis in the copepod crustacean Calanus finmarchicus

Neurochemicals are likely to play key roles in physiological/behavioral control in the copepod crustacean Calanus finmarchicus, the biomass dominant zooplankton for much of the North Atlantic Ocean. Previously, a de novo assembled transcriptome consisting of 206,041 unique sequences was used to characterize the peptidergic signaling systems of Calanus. Here, this assembly was mined for transcripts encoding enzymes involved in amine biosynthesis. Using known Drosophila melanogaster proteins as templates, transcripts encoding putative Calanus homologs of tryptophan-phenylalanine hydroxylase (dopamine, octopamine and serotonin biosynthesis), tyrosine hydroxylase (dopamine biosynthesis), DOPA decarboxylase (dopamine and serotonin biosynthesis), histidine decarboxylase (histamine biosynthesis), tyrosine decarboxylase (octopamine biosynthesis), tyramine β-hydroxylase (octopamine biosynthesis) and tryptophan hydroxylase (serotonin biosynthesis) were identified. Reverse BLAST and domain analyses show that the proteins deduced from these transcripts possess sequence homology to and the structural hallmarks of their respective enzyme families. Developmental profiling revealed a remarkably consistent pattern of expression for all transcripts, with the highest levels of expression typically seen in the early nauplius and early copepodite. These expression patterns suggest roles for amines during development, particularly in the metamorphic transitions from embryo to nauplius and from nauplius to copepodite. Taken collectively, the data presented here lay a strong foundation for future gene-based studies of aminergic signaling in this and other copepod species, in particular assessment of the roles they may play in developmental control.

Crustacean neuroendocrine systems and their signaling agents

Decapod crustaceans have long served as important models for the study of neuroendocrine signaling. For example, the process of neurosecretion was first formally demonstrated by using a member of this order. In this review, the major decapod neuroendocrine organs are described, as are their phylogenetic conservation and neurochemistry. In addition, recent advances in crustacean neurohormone discovery and tissue mapping are discussed, as are several recent advances in our understanding of hormonal control in this group of animals.

Effect of presynaptic membrane potential on electrical vs. chemical synaptic transmission

The growing realization that electrical coupling is present in the mammalian brain has sparked renewed interest in determining its functional significance and contrasting it with chemical transmission. One question of interest is whether the two types of transmission can be selectively regulated, e.g., if a cell makes both types of connections can electrical transmission occur in the absence of chemical transmission? We explore this issue in an experimentally advantageous preparation. B21, the neuron we study, is an Aplysia sensory neuron involved in feeding that makes electrical and chemical connections with other identified cells. Previously we demonstrated that chemical synaptic transmission is membrane potential dependent. It occurs when B21 is centrally depolarized prior to and during peripheral activation, but does not occur if B21 is peripherally activated at its resting membrane potential. In this article we study effects of membrane potential on electrical transmission. We demonstrate that maximal potentiation occurs in different voltage ranges for the two types of transmission, with potentiation of electrical transmission occurring at more hyperpolarized potentials (i.e., requiring less central depolarization). Furthermore, we describe a physiologically relevant type of stimulus that induces both spiking and an envelope of depolarization in the somatic region of B21. This depolarization does not induce functional chemical synaptic transmission but is comparable to the depolarization needed to maximally potentiate electrical transmission. In this study we therefore characterize a situation in which electrical and chemical transmission can be selectively controlled by membrane potential.

Differential modulation of synaptic strength and timing regulate synaptic efficacy in a motor network

Neuromodulators modify network output by altering neuronal firing properties and synaptic strength at multiple sites; however, the functional importance of each site is often unclear. We determined the importance of monoamine modulation of a single synapse for regulation of network cycle frequency in the oscillatory pyloric network of the lobster. The pacemaker kernel of the pyloric network receives only one chemical synaptic feedback, an inhibitory synapse from the lateral pyloric (LP) neuron to the pyloric dilator (PD) neurons, which can limit cycle frequency. We measured the effects of dopamine (DA), octopamine (Oct), and serotonin (5HT) on the strength of the LP→PD synapse and the ability of the modified synapse to regulate pyloric cycle frequency. DA and Oct strengthened, whereas 5HT weakened, LP→PD inhibition. Surprisingly, the DA-strengthened LP→PD synapse lost its ability to slow the pyloric oscillations, whereas the 5HT-weakened LP→PD synapse gained a greater influence on the oscillations. These results are explained by monoamine modulation of factors that determine the firing phase of the LP neuron in each cycle. DA acts via multiple mechanisms to phase-advance the LP neuron into the pacemaker's refractory period, where the strengthened synapse has little effect. In contrast, 5HT phase-delays LP activity into a region of greater pacemaker sensitivity to LP synaptic input. Only Oct enhanced LP regulation of cycle period simply by enhancing LP→PD synaptic strength. These results show that modulation of the strength and timing of a synaptic input can differentially affect the synapse's efficacy in the network.

Checks and balances in neuromodulation

Neuromodulators such as monoamines and peptides play important roles in activating and reconfiguring neural networks to allow behavioral flexibility. While the net effects of a neuromodulator change the network in a particular direction, careful studies of modulatory effects reveal multiple cases where a neuromodulator will activate functionally opposing mechanisms on a single neuron or synapse. This review gives examples of such opposing actions, focusing on the lobster pyloric network, and discusses their possible functional significance. One important action of opposing modulatory actions may be to stabilize the modulated state of the network, and to prevent it from being overmodulated and becoming non-functional.

Serotonin regulates electrical coupling via modulation of extrajunctional conductance: H-current

Synaptic strength can be highly variable from animal to animal within a species or over time within an individual. The process of synaptic plasticity induced by neuromodulatory agents might be unpredictable when the underlying circuits subject to modulation are themselves inherently variable. Serotonin (5-hydroxytryptomine; 5HT) and serotonergic signaling pathways are important regulators of animal behavior and are pharmacological targets in a wide range of neurological disorders. We have examined the effect of 5HT on electrical synapses possessing variable coupling strengths. While 5HT decreased electrical coupling at synapses with weak electrical connectivity, synapses with strong electrical coupling were less affected by 5HT treatment, as follows from the equations used for calculating coupling coefficients. The fact that the modulatory effect of 5HT on electrical connections was negatively correlated with the strength of electrical coupling suggests that the degree of electrical coupling within a neural network impacts subsequent neuromodulation of those synapses. Biophysical studies indicated that these effects were primarily due to 5HT-induced modulation of membrane currents that indirectly affect junctional coupling at synaptic contacts. In support of these experimental analyses, we created a simple model of coupled neurons to demonstrate that modulation of electrical coupling could be due solely to 5HT effects on H-channel conductance. Therefore, variability in the strength of electrical coupling in neural circuits can determine the pharmacological effect of this neuromodulatory agent.

Pacemaker neuron and network oscillations depend on a neuromodulator-regulated linear current

Linear leak currents have been implicated in the regulation of neuronal excitability, generation of neuronal and network oscillations, and network state transitions. Yet, few studies have directly tested the dependence of network oscillations on leak currents or explored the role of leak currents on network activity. In the oscillatory pyloric network of decapod crustaceans neuromodulatory inputs are necessary for pacemaker activity. A large subset of neuromodulators is known to activate a single voltage-gated inward current I_MI, which has been shown to regulate the rhythmic activity of the network and its pacemaker neurons. Using the dynamic clamp technique, we show that the crucial component of I_MI for the generation of oscillatory activity is only a close-to-linear portion of the current-voltage relationship. The nature of this conductance is such that the presence or the absence of neuromodulators effectively regulates the amount of leak current and the input resistance in the pacemaker neurons. When deprived of neuromodulatory inputs, pyloric oscillations are disrupted; yet, a linear reduction of the total conductance in a single neuron within the pacemaker group recovers not only the pacemaker activity in that neuron, but also leads to a recovery of oscillations in the entire pyloric network. The recovered activity produces proper frequency and phasing that is similar to that induced by neuromodulators. These results show that the passive properties of pacemaker neurons can significantly affect their capacity to generate and regulate the oscillatory activity of an entire network, and that this feature is exploited by neuromodulatory inputs.

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.

Summation of excitatory postsynaptic potentials in electrically-coupled neurones

Dendritic electrical coupling increases the number of effective synaptic inputs onto neurones by allowing the direct spread of synaptic potentials from one neurone to another. Here we studied the summation of excitatory postsynaptic potentials (EPSPs) produced locally and arriving from the coupled neurone (transjunctional) in pairs of electrically-coupled Retzius neurones of the leech. We combined paired recordings of EPSPs, the production of artificial excitatory postsynaptic potentials (APSPs) in neurone pairs with different coupling coefficients and simulations of EPSPs produced in the coupled dendrites. Summation of the EPSPs produced in the dendrites was always linear, suggesting that synchronous EPSPs are produced at two or more different pairs of coupled dendrites and not in both sides of any one gap junction. The different spatio-temporal relationships explored between pairs of EPSPs or APSPs produced three main effects. (1) Synchronous pairs of EPSPs or APSPs exhibited an elongation of their decay phase compared to single EPSPs. (2) Asymmetries in the amplitudes between the pair of EPSPs added a "hump" to the smallest EPSP. (3) Modelling the inputs near the electrical synapse or anticipating the production of the transjunctional APSP increased the amplitude of the compound EPSP. The magnitude of all these changes depended on the coupling coefficient of the neurones. We also show that the hump improves the passive conduction of EPSPs by adding low frequency components. The diverse effects of summation of local and alien EPSPs shown here endow electrically-coupled neurones with a wider repertoire of adjustable integrative possibilities.

CNQX and AMPA inhibit electrical synaptic transmission: a potential interaction between electrical and glutamatergic synapses

Electrical synapses play an important role in signaling between neurons and the synaptic connections between many neurons possess both electrical and chemical components. Although modulation of electrical synapses is frequently observed, the cellular processes that mediate such changes have not been studied as thoroughly as plasticity in chemical synapses. In the leech (Hirudo sp), the competitive AMPA receptor antagonist CNQX inhibited transmission at the rectifying electrical synapse of a mixed glutamatergic/electrical synaptic connection. This CNQX-mediated inhibition of the electrical synapse was blocked by concanavalin A (Con A) and dynamin inhibitory peptide (DIP), both of which are known to inhibit endocytosis of neurotransmitter receptors. CNQX-mediated inhibition was also blocked by pep2-SVKI (SVKI), a synthetic peptide that prevents internalization of AMPA-type glutamate receptor. AMPA itself also inhibited electrical synaptic transmission and this AMPA-mediated inhibition was partially blocked by Con A, DIP and SVKI. Low frequency stimulation induced long-term depression (LTD) in both the electrical and glutamatergic components of these synapses and this LTD was blocked by SVKI. GYKI 52466, a selective non-competitive antagonist of AMPA receptors, did not affect the electrical EPSP, although it did block the glutamatergic component of these synapses. CNQX did not affect non-rectifying electrical synapses in two different pairs of neurons. These results suggest an interaction between AMPA-type glutamate receptors and the gap junction proteins that mediate electrical synaptic transmission. This putative interaction between glutamate receptors and gap junction proteins represents a novel mechanism for regulating the strength of synaptic transmission.

Serotonin transduction cascades mediate variable changes in pyloric network cycle frequency in response to the same modulatory challenge

A fundamental question in systems biology addresses the issue of how flexibility is built into modulatory networks such that they can produce context-dependent responses. Here we examine flexibility in the serotonin (5-HT) response system that modulates the cycle frequency (cf) of a rhythmic motor output. We found that depending on the preparation, the same 5-min bath application of 5-HT to the pyloric network of the California spiny lobster, Panulirus interruptus, could produce a significant increase, decrease, or no change in steady-state cf relative to baseline. Interestingly, the mean circuit output was not significantly different among preparations prior to 5-HT application. We developed pharmacological tools to examine the preparation-to-preparation variability in the components of the 5-HT response system. We found that the 5-HT response system consisted of at least three separable components: a 5-HT(2betaPan)-like component mediated a rapid decrease followed by a sustained increase in cf; a 5-HT(1alphaPan)-like component produced a small and usually gradual increase in cf; at least one other component associated with an unknown receptor mediated a sustained decrease in cf. The magnitude of the change in cf produced by each component was highly variable, so that when summed they could produce either a net increase, decrease, or no change in cf depending on the preparation. Overall, our research demonstrates that the balance of opposing components of the 5-HT response system determines the direction and magnitude of 5-HT-induced change in steady-state cf relative to baseline.

Heterogeneous Effects of Dopamine on Highly Localized, Voltage-Induced Ca2+ Accumulation in Identified Motoneurons

Modulation of synaptic transmission is a major mechanism for the functional reconfiguration of neuronal circuits. Neurotransmitter release and, consequently, synaptic strength are regulated by intracellular Ca2+ levels in presynaptic terminals. In identified neurons of the lobster pyloric network, we studied localized, voltage-induced Ca2+ accumulation and its modulation in varicosities on distal neuritic arborizations, which have previously been shown to be sites of synaptic contacts. We previously demonstrated that dopamine (DA) weakens synaptic output from the pyloric dilator (PD) neuron and strengthens synaptic output from the lateral pyloric (LP) and pyloric constrictor (PY) neurons. Here we show that DA modifies voltage-activated Ca2+ accumulation in many varicosities in ways that are consistent with DA's effects on synaptic transmission: DA elevates Ca2+ accumulation in LP and PY varicosities and reduces Ca2+ accumulation in PD varicosities. However, in all three neuron types, we also found varicosities that were unaffected by DA. In the PY neurons, we found that DA can simultaneously increase and decrease voltage-evoked Ca2+ accumulation at different varicosities, even within the same neuron. These results suggest that regulation of Ca2+ entry is a common mechanism to regulate synaptic strength in the pyloric network. However, voltage-evoked local Ca2+ accumulation can be differentially modulated to control Ca2+-dependent processes in functionally separate varicosities of a single neuron.

Signal transmission between gap-junctionally coupled passive cables is most effective at an optimal diameter

We analyze simple morphological configurations that represent gap-junctional coupling between neuronal processes or between muscle fibers. Specifically, we use cable theory and simulations to examine the consequences of current flow from one cable to other gap-junctionally coupled passive cables. When the proximal end of the first cable is voltage clamped, the amplitude of the electrical signal in distal portions of the second cable depends on the cable diameter. However, this amplitude does not simply increase if cable diameter is increased, as expected from the larger length constant; instead, an optimal diameter exists. The optimal diameter arises because the dependency of voltage attenuation along the second cable on cable diameter follows two opposing rules. As cable diameter increases, the attenuation decreases because of a larger length constant yet increases because of a reduction in current density arising from the limiting effect of the gap junction on current flow into the second cable. The optimal diameter depends on the gap junction resistance and cable parameters. In branched cables, dependency on diameter is local and thus may serve to functionally compartmentalize branches that are coupled to other cells. Such compartmentalization may be important when periodic signals or action potentials cause the current flow across gap junctions.

Dopamine modulation of phasing of activity in a rhythmic motor network: contribution of synaptic and intrinsic modulatory actions

The phasing of neuronal activity in a rhythmic motor network is determined by a neuron's intrinsic firing properties and synaptic inputs; these could vary in their relative importance under different modulatory conditions. In the lobster pyloric network, the firing of eight follower pyloric (PY) neurons is shaped by their intrinsic rebound after pacemaker inhibition and by synaptic input from the lateral pyloric (LP) neuron, which inhibits all PY neurons and is electrically coupled to a subset of them. Under control conditions, LP inhibition is weak and has little influence on PY firing. We examined modulation that could theoretically enhance the LP's synaptic contribution to PY firing. We measured the effects of dopamine (DA) on LP-->PY synapses, driving the LP neuron with trains of realistic waveforms constructed from prerecorded control and DA LP oscillations, which differed in shape and duration. Under control conditions, chemical inhibition underwent severe depression and disappeared; in the mixed synapses, electrical coupling dominated. Switching between control and DA LP waveforms (with or without DA present) caused only subtle changes in synaptic transmission. DA markedly enhanced synaptic inhibition, reduced synaptic depression and weakened electrical coupling, reversing the sign of the mixed synapses. Despite this, removal of the LP from the intact network still had only weak effects on PY firing. DA also enhances PY intrinsic rebound properties, which still control the onset of PY firing. Thus in a rhythmic network, the functional importance of synaptic modulation can only be understood in the context of parallel modulation of intrinsic properties.

Computational model of electrically coupled, intrinsically distinct pacemaker neurons

Electrical coupling between neurons with similar properties is often studied. Nonetheless, the role of electrical coupling between neurons with widely different intrinsic properties also occurs, but is less well understood. Inspired by the pacemaker group of the crustacean pyloric network, we developed a multicompartment, conductance-based model of a small network of intrinsically distinct, electrically coupled neurons. In the pyloric network, a small intrinsically bursting neuron, through gap junctions, drives 2 larger, tonically spiking neurons to reliably burst in-phase with it. Each model neuron has 2 compartments, one responsible for spike generation and the other for producing a slow, large-amplitude oscillation. We illustrate how these compartments interact and determine the dynamics of the model neurons. Our model captures the dynamic oscillation range measured from the isolated and coupled biological neurons. At the network level, we explore the range of coupling strengths for which synchronous bursting oscillations are possible. The spatial segregation of ionic currents significantly enhances the ability of the 2 neurons to burst synchronously, and the oscillation range of the model pacemaker network depends not only on the strength of the electrical synapse but also on the identity of the neuron receiving inputs. We also compare the activity of the electrically coupled, distinct neurons with that of a network of coupled identical bursting neurons. For small to moderate coupling strengths, the network of identical elements, when receiving asymmetrical inputs, can have a smaller dynamic range of oscillation than that of its constituent neurons in isolation.

Serotonin modulates axo-axonal coupling between neurons critical for learning in the leech

S cells form a chain of electrically coupled neurons that extends the length of the leech CNS and plays a critical role in sensitization during whole-body shortening. This process requires serotonin, which acts in part by altering the pattern of activity in the S-cell network. Serotonin-containing axons and varicosities were observed in Faivre's nerve where the S-to-S-cell electrical synapses are located. To determine whether serotonin modulates these synapses, S-cell action-potential (AP) propagation was studied in a two-ganglion chain containing one electrical synapse. Suction electrodes were placed on the cut ends of the connectives to stimulate one S cell while recording the other, coupled S cell's APs. A third electrode, placed en passant, recorded the APs near the electrical synapse before they propagated through it. Low concentrations of the gap junction inhibitor octanol increased AP latency across the two-ganglion chain, and this effect was localized to the region of axon containing the electrical synapse. At higher concentrations, APs failed to propagate across the synapse. Serotonin also increased AP latency across the electrical synapse, suggesting that serotonin reduced coupling between S cells. This effect was independent of the direction of propagation and increased with the number of electrical synapses in progressively longer chains. Furthermore, serotonin modulated instantaneous AP frequency when APs were initiated in separate S cells and in a computational model of S-cell activity after mechanosensory input. Thus serotonergic modulation of S-cell electrical synapses may contribute to changes in the pattern of activity in the S-cell network.

Dopamine modulation of calcium currents in pyloric neurons of the lobster stomatogastric ganglion

We examined the dopamine (DA) modulation of calcium currents (ICa) that could contribute to the plasticity of the pyloric network in the lobster stomatogastric ganglion. Pyloric somata were voltage-clamped under conditions designed to block voltage-gated Na+, K+, and H currents. Depolarizing steps from -60 mV generated voltage-dependent, inward currents that appeared to originate in electrotonically distal, imperfectly clamped regions of the cell. These currents were blocked by Cd2+ and enhanced by Ba2+ but unaffected by Ni2+. Dopamine enhanced the peak ICa in the pyloric constrictor (PY), lateral pyloric (LP), and inferior cardiac (IC) neurons and reduced peak ICa in the ventricular dilator (VD), pyloric dilator (PD), and anterior burster (AB) neurons. All of these effects, except for the AB, are consistent with DA's excitation or inhibition of firing in the pyloric neurons. Enhancement of ICa in PY and LP neurons and reduction of ICa in VD and PD neurons are also consistent with DA-induced synaptic strength changes via modulation of presynaptic ICa. However, the reduction of ICa in AB suggests that DA's enhancement of AB transmitter release is not directly mediated through presynaptic ICa. ICa in PY and PD neurons was more sensitive to nifedipine block than in AB neurons. In addition, nifedipine blocked DA's effects on ICa in the PY and PD neurons but not in the AB neuron. Thus the contribution of specific calcium channel subtypes carrying the total ICa may vary between pyloric neuron classes, and DA may act on different calcium channel subtypes in the different pyloric neurons.

The functional consequences of changes in the strength and duration of synaptic inputs to oscillatory neurons

We studied the effect of synaptic inputs of different amplitude and duration on neural oscillators by simulating synaptic conductance pulses in a bursting conductance-based pacemaker model and by injecting artificial synaptic conductance pulses into pyloric pacemaker neurons of the lobster stomatogastric ganglion using the dynamic clamp. In the model and the biological neuron, the change in burst period caused by inhibitory and excitatory inputs of increasing strength saturated, such that synaptic inputs above a certain strength all had the same effect on the firing pattern of the oscillatory neuron. In contrast, increasing the duration of the synaptic conductance pulses always led to changes in the burst period, indicating that neural oscillators are sensitive to changes in the duration of synaptic input but are not sensitive to changes in the strength of synaptic inputs above a certain conductance. This saturation of the response to progressively stronger synaptic inputs occurs not only in bursting neurons but also in tonically spiking neurons. We identified inward currents at hyperpolarized potentials as the cause of the saturation in the model neuron. Our findings imply that activity-dependent or modulator-induced changes in synaptic strength are not necessarily accompanied by changes in the functional impact of a synapse on the timing of postsynaptic spikes or bursts.

Heterosynaptic modulation by the octopaminergic OC interneurons increases the synaptic outputs of protraction phase interneurons (SO, N1L) in the feeding system of Lymnaea stagnalis

We examined the cholinergic synapses between protraction phase interneurons (SO or N1L) and their targets (N1M interneuron, B1 motoneuron) in the buccal ganglia of the pond snail Lymnaea stagnalis. We have tested the hypothesis that the OC (octopamine-containing) interneuron, an intrinsic modulator of the feeding network, can increase the synaptic efficacy from the SO or N1L to their targets. Prestimulation of the OC interneuron, 4 s before the activation of the SO or N1L increases the strength of their output synapses by 75% (SO)-110% (N1L). The individual excitatory postsynaptic potentials evoked by SO or N1L stimulation increase in size. OC prestimulation also produces an increase in the firing rate of these presynaptic interneurons: SO 40%; N1L 33%. The facilitation lasts up to 6 s after the end of the OC burst. The enhancement of PSPs is seen at all the output synapses (both excitatory and inhibitory) of the SO and N1L interneurons. The output synapses of the non-cholinergic swallowing phase N3p interneuron are not affected, even when the same postsynaptic target is selected. The SO-->N1M, SO-->B1 and N1L-->N1M synapses are also strengthened by bath application of 1-5 microM octopamine (average increase 60%). The major effect is an increased excitability of the SO; the B1 motoneuron response to the main transmitter of the SO, acetylcholine, is unaffected. Increased synaptic outputs of the protraction phase SO and N1L interneurons is functionally significant for generation of feeding pattern in the Lymnaea CNS. Strengthening the connections of SO and N1L to the central pattern generator (N1M) interneurons enhances their ability to drive fictive feeding. Thus heterosynaptic facilitation by the octopaminergic OC interneurons in the central pattern generator network may contribute to the behavioral plasticity of feeding in the intact animal.

Cellular, synaptic and network effects of neuromodulation

All network dynamics emerge from the complex interaction between the intrinsic membrane properties of network neurons and their synaptic connections. Nervous systems contain numerous amines and neuropeptides that function to both modulate the strength of synaptic connections and the intrinsic properties of network neurons. Consequently network dynamics can be tuned and configured in different ways, as a function of the actions of neuromodulators. General principles of the organization of modulatory systems in nervous systems include: (a) many neurons and networks are multiply modulated, (b) there is extensive convergence and divergence in modulator action, and (c) some modulators may be released extrinsically to the modulated circuit, while others may be released by some of the circuit neurons themselves, and act intrinsically. Some of the computational consequences of these features of modulator action are discussed.

A small-systems approach to motor pattern generation

How neuronal networks enable animals, humans included, to make coordinated movements is a continuing goal of neuroscience research. The stomatogastric nervous system of decapod crustaceans, which contains a set of distinct but interacting motor circuits, has contributed significantly to the general principles guiding our present understanding of how rhythmic motor circuits operate at the cellular level. This results from a detailed documentation of the circuit dynamics underlying motor pattern generation in this system as well as its modulation by individual transmitters and neurons.

Amine modulation of the transient potassium current in identified cells of the lobster stomatogastric ganglion

The pyloric network of the stomatogastric ganglion of the lobster Panulirus interruptus is a model system used to understand how motor networks change their output to produce a variety of behaviors. The transient potassium current (I(A)) shapes the activity of individual pyloric neurons by affecting their rate of postinhibitory rebound and spike frequency. We used two electrode voltage clamp to study the modulatory effects of dopamine (DA), octopamine (OCT), and serotonin (5-HT) on I(A) in the anterior burster (AB), inferior cardiac (IC), and ventricular dilator (VD) neurons of the pyloric circuit. DA significantly reduced I(A) in the AB and IC neurons and shifted their voltages of activation (V(act)) and inactivation (V(inact)) in a depolarized direction. These ionic changes contribute to the depolarization and increased firing rate of the AB and IC neurons produced by DA. Likewise, 5-HT significantly reduced I(A) and shifted V(inact) in the depolarized direction in the IC neuron, consistent with 5-HT's enhancement of IC firing. None of the amines evoked significant changes in I(A) in the VD neuron, suggesting that other currents mediate the amine effects on this neuron.

Octopamine is the synaptic transmitter between identified neurons in the buccal feeding network of the pond snail lymnaea stagnalis

We report the pharmacological properties of synaptic connections from the three octopamine-containing OC interneurons to identified buccal feeding neurons in the pond snail, Lymnaea stagnalis. Intracellular stimulation of an OC interneuron evokes inhibitory postsynaptic potentials in the B3 motoneurons and N2 (d) interneurons, while the synapse between OC and N3 (phasic) interneurons has two components: an initial electrical excitation followed by chemical inhibition. All synaptic responses persist in a saline with elevated calcium and magnesium suggesting that the connections are monosynaptic. Local perfusion of 10(-4) M octopamine produces the same inhibitory membrane responses from these buccal neurons as OC stimulation. These responses also persist in high Mg(2+)/Ca(2+) saline indicating direct membrane effects. The similarities in reversal potentials for the synaptic hyperpolarization evoked on B3 neurons after OC stimulation (-89.0 mV, S.E.M.=14.1, n=10) and the octopamine response of the B3 neurons (-84.7 mV, S.E.M.=6.6, n=6) indicate that increased K(+)-conductance underlies both responses. Bath application of the octopaminergic drugs phentolamine (10(-6) M), epinastine (10(-6) M) or DCDM (10(-4) M) blocks the inhibitory synapse onto B3 or N2 neurons and the chemical component of the N3 response. They also block the octopamine-evoked inhibition of B3, N2 and N3 neurons. NC-7 (2x10(-5) M) has a hyperpolarizing agonist effect (like octopamine) on these neurons and also blocks their chemical synaptic input from the OC interneurons. These results provide pharmacological evidence that the neurotransmitter between the octopamine-immunopositive OC interneurons and its followers is octopamine. This is the first example of identified octopaminergic synaptic connections within the snail CNS.

Development of the peptidergic modulation of a rhythmic pattern generating network

The stomatogastric ganglion (STG) of adult lobsters and crabs receives dense aminergic and peptidergic projections. The neuropeptides are found in sensory neurons and in descending interneurons that modulate the output of the rhythmic central pattern generating networks in the STG. We describe the presence of these peptidergic projections in the adult Homarus americanus, and the effects of some of these neuropeptides on the motor patterns of the adult STG. We describe the developmental acquisition of these neuropeptides during embryonic and larval times and demonstrate that the immature STG networks are already sensitive to a variety of neuromodulators.

Distributed effects of dopamine modulation in the crustacean pyloric network

It is now clear that neuromodulators can reconfigure a single motor network to allow the generation of a family of related movements. Using dopamine modulation of the 14-neuron pyloric network from the crustacean stomatogastric ganglion as an example, we describe two major mechanisms by which network output is modulated. First, the baseline electrophysiological properties of the network neurons can be altered. Dopamine can affect the activity of each neuron independently. For example, DA modulates IA in nearly every neuron in the pyloric network, but in opposite directions in different cells. Furthermore, DA usually modulates combinations of ionic currents. In some cases, currents with opposing actions on cell excitability are simultaneously affected, and the net response reflects the sum of these opposing effects. Second, neuromodulators can alter the strength of synaptic interactions within the network, quantitatively "rewiring" the network. Every synapse in the network is affected by DA, with some increased and others decreased in strength. DA acts both pre- and postsynaptically to affect transmission: these actions are frequently opposing in sign, and the net response arises as the sum of these opposing actions. Finally, spike-evoked and graded transmission at the same synapse can be oppositely affected by DA. These results emphasize the distributed nature of modulation in motor networks.

Ontogenetic alteration in peptidergic expression within a stable neuronal population in lobster stomatogastric nervous system

In the adult lobster, Homarus gammarus, the stomatogastric ganglion (STG) contains two well-defined motor pattern generating networks that receive numerous modulatory peptidergic inputs from anterior ganglia. We are studying the appearance of extrinsic peptidergic inputs to these networks during ontogenesis. Neuron counts indicate that as early as 20% of development (E20) the STG neuronal population is quantitatively established. By using immunocytochemical detection of 5-bromo-2'-deoxyuridine incorporation, we found no immunopositive cells in the STG by E70. We concluded that the STG neuronal population remains quantitatively stable from mid-embryonic life until adulthood. We then investigated the ontogeny of FLRFamide- and proctolin-like peptides in the stomatogastric nervous system, from their first appearance until adulthood by using whole mount immunocytochemistry. Numerous FLRFamide-like-immunoreactive STG neuropilar ramifications were observable as early as E45 and remain thereafter. From E50 to the first larval stage, one to three STG somata stained, while somatic staining was not observed in larval stage II and subsequent stages. From E50 and thereafter, the STG neuropilar area was immunopositive for proctolin. One to two proctolinergic somata were detected in the STG of the three larval stages but were not seen in embryos, the post-larval stage or in adults. Thus, peptidergic inputs to the STG are present from mid-embryonic life. Moreover, whereas in the adult, STG neurons only contain glutamate or acetylcholine, some neurons transiently express peptidergic phenotypes during development. Although this system expresses an ontogenetic peptidergic plasticity, the STG neurons produce a single stable embryonic-larval motor output (Casasnovas and Meyrand [1995] J. Neurosci. 15:5703-5718).

Dopamine modulates graded and spike-evoked synaptic inhibition independently at single synapses in pyloric network of lobster

Bath application of dopamine (DA) modifies the rhythmic motor pattern generated by the pyloric network in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. Synaptic transmission between network members is an important target of DA action. All pyloric neurons employ both graded transmitter release and action-potential-mediated synaptic inhibition. DA was previously shown to alter the graded synaptic strength of every pyloric synapse. In this study, we compared DA's effects on action-potential-mediated and graded synaptic inhibition at output synapses of the lateral pyloric (LP) neuron. At each synapse the postsynaptic cell tested was isolated from other descending and pyloric synaptic inputs. DA caused a reduction in the size of the LP spike-evoked inhibitory postsynaptic potentials (IPSPs) in the pyloric dilator (PD) neuron. The change in IPSP size was significantly and linearly correlated with DA-induced reduction in the input resistance of the postsynaptic PD neuron. In contrast, graded inhibition, tested in the same preparations after superfusing the stomatogastric ganglion (STG) with tetrodotoxin (TTX), was consistently enhanced by DA. DA shifted the amplitude of spike-evoked IPSPs in the same direction as the alteration of the postsynaptic cell input resistance at two additional synapses tested: DA weakened the LP spike-mediated inhibition of the ventricular dilator (VD) and reduced the VD input resistance, while strengthening the LP --> pyloric constrictor (PY) synapse and increasing PY input resistance. As previously reported, graded inhibition was enhanced at these two LP output synapses. We conclude that DA can differentially modulate the spike-evoked and graded components of synapses between members of a central pattern generator network. At the synapses we studied, actions on the presynaptic cell predominate in the modulation of graded transmission, whereas effects on postsynaptic cells predominate in the regulation of spike-evoked IPSPs.

Neuromodulation of reciprocal glutamatergic inhibition between antagonistic motoneurons by 5-hydroxytryptamine (5-HT) in crayfish walking system

In an in vitro preparation of the crayfish thoracic locomotor system, paired intracellular recordings were performed from antagonistic depressor (Dep) and levator (Lev) motoneurons (MNs) that control the second joint of walking legs. Connections between these two groups of MNs consist mainly of inhibitory connections and weak electrotonic synapses. Injection of depolarizing current into a Lev MN results in a hyperpolarization in a Dep MN, and vice versa. This reciprocal glutamatergic inhibition, is not changed in the presence of the sodium channel blocker tetrodotoxin (TTX) and therefore is likely supported by a direct connection between MNs. By contrast, reciprocal inhibition is largely reduced in the presence of 5-hydroxytryptamine (5-HT; 10 microM). Direct micro-application of glutamate pressure-ejected close to an intracellularly recorded MN, evoked an inhibitory response in that MN, accompanied by a decrease of input resistance. These two effects were dramatically reduced in the presence of 5-HT. Thus 5-HT could be involved in mechanisms of dynamic reconfigurations of the neural network controlling leg movements in crayfish.

Amine modulation of glutamate responses from pyloric motor neurons in lobster stomatogastric ganglion

The amines dopamine (DA), serotonin (5-HT), and octopamine (Oct) each elicit a distinctive motor pattern from a quiescent pyloric network in the lobster stomatogastric ganglion (STG). We previously have demonstrated that these amines alter the synaptic strength at multiple, distributed sites within the pyloric network that could contribute to the amine-induced motor patterns. Here, we examined the postsynaptic contribution to these changes in synaptic strength by determining how the amines modify responses of pyloric motor neurons to glutamate (Glu), one of the network transmitters, applied iontophoretically into the STG neuropil. Dopamine reduced the Glu responses of the pyloric dilator (PD), ventricular dilator (VD), and inferior cardiac (IC) neurons and enhanced the Glu responses of the lateral pyloric (LP) and pyloric constrictor (PY) neurons. The only effect of 5-HT was to reduce the Glu response of the VD neuron. Oct enhanced the Glu responses of the LP and PY neurons but did not affect the PD, VD, and IC responses. We also examined amine effects on the depolarizing responses to iontophoresed acetylcholine (ACh) in the PD and VD and found that they paralleled the amine effects on Glu responses in these neurons. This suggests that amine modulation of PD and VD responses to Glu and ACh may be explained by general changes in the ionic conductance of these neurons. We compare our results with our earlier work describing amine effects on synaptic strength and input resistance to show that amines act at both pre- and postsynaptic sites to modify graded synaptic transmission in the pyloric network.

Dopaminergic modulation of inhibitory glutamate receptors in the lobster stomatogastric ganglion

The intrinsic rhythmicity of the spiny lobster stomatogastric ganglion (STG) is strongly influenced by the strengths of the graded synapses between identified cells within the neural network. These synaptic strengths can be powerfully influenced by chemical neuromodulators such as dopamine and serotonin. Most of the intraganglionic chemical synapses in the STG are mediated by postsynaptic inhibitory glutamate receptors (IGluRs). To determine whether or not direct effects on these IGluRs contribute to the modulation of synaptic strength, unidentified STG neurons were extracted into primary culture and the effects of these aminergic neuromodulators on the glutamate-evoked membrane current were assessed. Dopamine (100 microM) reliably and significantly reduced the whole cell slope conductance of all IGluRs tested. Serotonin (20 microM) never affected the IGlu response, although it clearly altered other cellular membrane properties. Although all identified STG neurons may not conform to these observations, the data reveal a specific dopamine-activated modulatory pathway within cultured neurons that reduces IGluR slope conductance. The relationship between IGluR modulation and net synaptic modulation in situ contributes to an emerging model in which synaptic strengths can be multiply modulated at different functional sites, yielding a complex, distributed, and state-dependent regulatory structure.

Molecular biology and electrophysiology of glutamate-gated chloride channels of invertebrates

In this chapter we summarize the available data on a novel class of ligand-gated anion channels that are gated by the neurotransmitter glutamate. Glutamate is classically thought to be a stimulatory neurotransmitter, however, studies in invertebrates have proven that glutamate also functions as an inhibitory ligand. The bulk of studies conducted in vivo have been on insects and crustaceans, where glutamate was first postulated to act on H-receptors resulting in a hyperpolarizing response to glutamate. Recently, glutamate-gated chloride channels have been cloned from several nematodes and Drosophila. The pharmacology and electrophysiological properties of these channels have been studied by expression in Xenopus oocytes. Studies on the cloned channels demonstrate that the invertebrate glutamate-gated chloride channels are the H-receptors and represent important targets for the antiparasitic avermectins.

Intrinsic and extrinsic neuromodulation of motor circuits

Neuromodulation of motor circuits by extrinsic inputs provides enormous flexibility in the production of behavior. Recent work has shown that neurons intrinsic to central pattern-generating circuits can evoke neuromodulatory effects in addition to their neurotransmitting actions. Modulatory neurons often elicit a multitude of different effects attributable to actions at different receptors and/or through the release of co-transmitters. Differences in neuromodulation between species can account for differences in behavior. Modulation of neuromodulation may provide an additional level of flexibility to motor circuits.

Presynaptic control of neurones in pattern-generating networks

Recent studies have revealed presynaptic influences on neurones that participate in rhythmic motor patterns. Although there is still little direct information about the effects of these inputs at presynaptic terminals, their functional consequences are being unraveled. These presynaptic influences gate sensory input to pattern-generating networks and locally alter the synaptic strength and/or the activity pattern of network neurones.