Cellular, synaptic, network, and modulatory mechanisms involved in rhythm generation

Multistability of bursting rhythms in a half-center oscillator and the protective effects of synaptic inhibition

For animals to meet environmental challenges, the activity patterns of specialized oscillatory neural circuits, central pattern generators (CPGs), controlling rhythmic movements like breathing and locomotion, are adjusted by neuromodulation. As a representative example, the leech heartbeat is controlled by a CPG driven by two pairs of mutually inhibitory interneurons, heart interneuron (HN) half-center oscillators (HCO). Experiments and modeling indicate that neuromodulation of HCO navigates this CPG between dysfunctional regimes by employing a co-regulating inverted relation; reducing Na+/K+ pump current and increasing hyperpolarization-activated (h-) current. Simply reducing pump activity or increasing h-current leads to either seizure-like bursting or an asymmetric bursting dysfunctional regime, respectively. Here, we demonstrate through modeling that, alongside this coregulation path, a new bursting regime emerges. Both regimes fulfill the criteria for functional bursting activity. Although the cycle periods and burst durations of these patterns are roughly the same, the new one exhibits an intra-burst spike frequency that is twice as high as the other. This finding suggests that neuromodulation could introduce additional functional regimes with higher spike frequency, and thus more effective synaptic transmission to motor neurons. We found that this new regime co-exists with the original bursting. The HCO can be switched between them by a short pulse of excitatory or inhibitory conductance. In this domain of coexisting functional patterns, an isolated cell model exhibits only one regime, a severely dysfunctional plateau-containing, seizure-like activity. This aligns with widely reported notion that deficiency of inhibition can cause seizures and other dysfunctional neural activities. We show that along the coregulation path of neuromodulation, the high excitability of the single HNs induced by myomodulin is harnessed by mutually inhibitory synaptic interactions of the HCO into the functional bursting pattern.

Reciprocally inhibitory circuits operating with distinct mechanisms are differently robust to perturbation and modulation

Reciprocal inhibition is a building block in many sensory and motor circuits. We studied the features that underly robustness in reciprocally inhibitory two neuron circuits. We used the dynamic clamp to create reciprocally inhibitory circuits from pharmacologically isolated neurons of the crab stomatogastric ganglion by injecting artificial graded synaptic (I_Syn) and hyperpolarization-activated inward (I_H) currents. There is a continuum of mechanisms in circuits that generate antiphase oscillations, with ‘release’ and ‘escape’ mechanisms at the extremes, and mixed mode oscillations between these extremes. In release, the active neuron primarily controls the off/on transitions. In escape, the inhibited neuron controls the transitions. We characterized the robustness of escape and release circuits to alterations in circuit parameters, temperature, and neuromodulation. We found that escape circuits rely on tight correlations between synaptic and H conductances to generate bursting but are resilient to temperature increase. Release circuits are robust to variations in synaptic and H conductances but fragile to temperature increase. The modulatory current (I_MI) restores oscillations in release circuits but has little effect in escape circuits. Perturbations can alter the balance of escape and release mechanisms and can create mixed mode oscillations. We conclude that the same perturbation can have dramatically different effects depending on the circuits’ mechanism of operation that may not be observable from basal circuit activity.

Comodulation of h- and Na+/K+ Pump Currents Expands the Range of Functional Bursting in a Central Pattern Generator by Navigating between Dysfunctional Regimes

Central pattern generators (CPGs), specialized oscillatory neuronal networks controlling rhythmic motor behaviors such as breathing and locomotion, must adjust their patterns of activity to a variable environment and changing behavioral goals. Neuromodulation adjusts these patterns by orchestrating changes in multiple ionic currents. In the medicinal leech, the endogenous neuromodulator myomodulin speeds up the heartbeat CPG by reducing the electrogenic Na+/K+ pump current and increasing h-current in pairs of mutually inhibitory leech heart interneurons (HNs), which form half-center oscillators (HN HCOs). Here we investigate whether the comodulation of two currents could have advantages over a single current in the control of functional bursting patterns of a CPG. We use a conductance-based biophysical model of an HN HCO to explain the experimental effects of myomodulin. We demonstrate that, in the model, comodulation of the Na+/K+ pump current and h-current expands the range of functional bursting activity by avoiding transitions into nonfunctional regimes, such as asymmetric bursting and plateau-containing seizure-like activity. We validate the model by finding parameters that reproduce temporal bursting characteristics matching experimental recordings from HN HCOs under control, three different myomodulin concentrations, and Cs+ treated conditions. The matching cases are located along the border of an asymmetric regime away from the border with more dangerous seizure-like activity. We found a simple comodulation mechanism with an inverse relation between the pump and h-currents makes a good fit of the matching cases and comprises a general mechanism for the robust and flexible control of oscillatory neuronal networks.SIGNIFICANCE STATEMENT Rhythm-generating neuronal circuits adjust their oscillatory patterns to accommodate a changing environment through neuromodulation. In different species, chemical messengers participating in such processes may target two or more membrane currents. In medicinal leeches, the neuromodulator myomodulin speeds up the heartbeat central pattern generator by reducing Na+/K+ pump current and increasing h-current. In a computational model, we show that this comodulation expands the range of central pattern generator's functional activity by navigating the circuit between dysfunctional regimes resulting in a much wider range of cycle period. This control would not be attainable by modulating only one current, emphasizing the synergy of combined effects. Given the prevalence of h-current and Na+/K+ pump current in neurons, similar comodulation mechanisms may exist across species.

Computational Modeling of Spinal Locomotor Circuitry in the Age of Molecular Genetics

Neuronal circuits in the spinal cord are essential for the control of locomotion. They integrate supraspinal commands and afferent feedback signals to produce coordinated rhythmic muscle activations necessary for stable locomotion. For several decades, computational modeling has complemented experimental studies by providing a mechanistic rationale for experimental observations and by deriving experimentally testable predictions. This symbiotic relationship between experimental and computational approaches has resulted in numerous fundamental insights. With recent advances in molecular and genetic methods, it has become possible to manipulate specific constituent elements of the spinal circuitry and relate them to locomotor behavior. This has led to computational modeling studies investigating mechanisms at the level of genetically defined neuronal populations and their interactions. We review literature on the spinal locomotor circuitry from a computational perspective. By reviewing examples leading up to and in the age of molecular genetics, we demonstrate the importance of computational modeling and its interactions with experiments. Moving forward, neuromechanical models with neuronal circuitry modeled at the level of genetically defined neuronal populations will be required to further unravel the mechanisms by which neuronal interactions lead to locomotor behavior.

The neuromuscular transform in a single segment of a segmented heart tube

Leech hearts are hybrids; they are myogenic but need entrainment by a heartbeat central pattern generator (CPG) to execute functional constriction patterns. Leech hearts are modular: two lateral segmented heart tubes running the length of the animal. Moving blood through the segmented heart tubes of leeches requires sequential constrictions, timed by a heartbeat CPG and relayed to each heart segment by likewise segmental motor neurons. The heartbeat CPG produces bilaterally asymmetric coordinations: rear-to-front peristaltic on one side and nearly synchronous on the other, periodically switching sides. We examined the neuromuscular transform of isolated heart segments in response to electrical nerve stimulation to identify the range of parameters (burst duration, intraburst pulse frequency, period) allowing the heart to constrict continuously and reliably. Constriction amplitudes increased with increasing intraburst frequencies and decreased with decreasing burst durations. Similar amplitudes were achieved with longer burst durations combined with lower frequencies or with shorter burst durations combined with higher frequencies. Long burst durations delayed relaxation, leading to summation and tetanus. The time, and its variability, between stimulus onset and time to constriction onset or to peak decreased with increasing frequency. Data previously obtained in vivo showed that the heart excitatory motor neurons fired longer bursts at lower frequencies at long periods moving to shorter bursts with higher intraburst frequencies as the period shortened. In this scenario, active constriction started earlier and the time to reach full systole shortened, allowing more time for relaxation. Relaxation time before the next motor neuron burst appears critical for maintaining constriction amplitude.NEW & NOTEWORTHY Moving blood through the segmented heart tubes of leeches requires sequential constrictions driven by motor neurons controlled by a central pattern generator. In a single heart segment, we varied stimuli to explore the neuromuscular transform. Decreasing the cycle period, e.g., to increase volume pumped over time, without altering motor burst duration and intraburst spike frequency shortens relaxation time and decreases amplitude. The likely strategy to preserve constriction amplitude is to shorten burst duration while increasing spike frequency.

Global vs. Network-Specific Regulations as the Source of Intrinsic Coactivations in Resting-State Networks

Spontaneous neural activities are endowed with specific patterning characterized by synchronizations within functionally relevant distant regions that are termed as resting-state networks (RSNs). Although the mechanisms that organize the large-scale neural systems are still largely unknown, recent studies have proposed a hypothesis that network-specific coactivations indeed emerge as the result of globally propagating neural activities with specific paths of transmission. However, the extent to which such a centralized global regulation, rather than network-specific control, contributes to the RSN synchronization remains unknown. In the present study, we investigated the contribution from each mechanism by directly identifying the global as well as local component of resting-state functional MRI (fMRI) data provided by human connectome project, using temporal independent component analysis (ICA). Based on the spatial distribution pattern, each ICA component was classified as global or local. Time lag mapping of each IC revealed several paths of global or semi-global propagations that are partially overlapping yet spatially distinct to each other. Consistent with previous studies, the time lag of global oscillation, although being less spatially homogenous than what was assumed to be, contributed to the RSN synchronization. However, an equivalent contribution was also shown on the part of the more locally confined activities that are independent to each other. While allowing the view that network-specific coactivation occurs as part of the sequences of global neural activities, these results further confirm an equally important role of the network-specific regulation for its coactivation, regardless of whether vascular artifacts contaminate the global component in fMRI measures.

Electrophysiological Evidence for Intrinsic Pacemaker Currents in Crayfish Parasol Cells

I used sharp intracellular electrodes to record from parasol cells in the semi-isolated crayfish brain to investigate pacemaker currents. Evidence for the presence of the hyperpolarization-activated inward rectifier potassium current was obtained in about half of the parasol cells examined, where strong, prolonged hyperpolarizing currents generated a slowly-rising voltage sag, and a post-hyperpolarization rebound. The amplitudes of both the sag voltage and the depolarizing rebound were dependent upon the strength of the hyperpolarizing current. The voltage sag showed a definite threshold and was non-inactivating. The voltage sag and rebound depolarization evoked by hyperpolarization were blocked by the presence of 5-10 mM Cs2+ ions, 10 mM tetraethyl ammonium chloride, and 10 mM cobalt chloride in the bathing medium, but not by the drug ZD 7288. Cs+ ions in normal saline in some cells caused a slight increase in mean resting potential and a reduction in spontaneous burst frequency. Many of the neurons expressing the hyperpolarization-activated inward potassium current also provided evidence for the presence of the transient potassium current IA, which was inferred from experimental observations of an increased latency of post-hyperpolarization response to a depolarizing step, compared to the response latency to the depolarization alone. The latency increase was reduced in the presence of 4-aminopyridine (4-AP), a specific blocker of IA. The presence of 4-AP in normal saline also induced spontaneous bursting in parasol cells. It is conjectured that, under normal physiological conditions, these two potassium currents help to regulate burst generation in parasol cells, respectively, by helping to maintain the resting membrane potential near a threshold level for burst generation, and by regulating the rate of rise of membrane depolarizing events leading to burst generation. The presence of post-burst hyperpolarization may depend upon IA channels in parasol cells.

Selection of motor programs for suppressing food intake and inducing locomotion in the Drosophila brain

Central mechanisms by which specific motor programs are selected to achieve meaningful behaviors are not well understood. Using electrophysiological recordings from pharyngeal nerves upon central activation of neurotransmitter-expressing cells, we show that distinct neuronal ensembles can regulate different feeding motor programs. In behavioral and electrophysiological experiments, activation of 20 neurons in the brain expressing the neuropeptide hugin, a homolog of mammalian neuromedin U, simultaneously suppressed the motor program for food intake while inducing the motor program for locomotion. Decreasing hugin neuropeptide levels in the neurons by RNAi prevented this action. Reducing the level of hugin neuronal activity alone did not have any effect on feeding or locomotion motor programs. Furthermore, use of promoter-specific constructs that labeled subsets of hugin neurons demonstrated that initiation of locomotion can be separated from modulation of its motor pattern. These results provide insights into a neural mechanism of how opposing motor programs can be selected in order to coordinate feeding and locomotive behaviors.

Two interconnected kernels of reciprocally inhibitory interneurons underlie alternating left-right swim motor pattern generation in the mollusk Melibe leonina

The central pattern generator (CPG) underlying the rhythmic swimming behavior of the nudibranch Melibe leonina (Mollusca, Gastropoda, Heterobranchia) has been described as a simple half-center oscillator consisting of two reciprocally inhibitory pairs of interneurons called swim interneuron 1 (Si1) and swim interneuron 2 (Si2). In this study, we identified two additional pairs of interneurons that are part of the swim CPG: swim interneuron 3 (Si3) and swim interneuron 4 (Si4). The somata of Si3 and Si4 were both located in the pedal ganglion, near that of Si2, and both had axons that projected through the pedal commissure to the contralateral pedal ganglion. These neurons fulfilled the criteria for inclusion as members of the swim CPG: 1) they fired at a fixed phase in relation to Si1 and Si2, 2) brief changes in their activity reset the motor pattern, 3) prolonged changes in their activity altered the periodicity of the motor pattern, 4) they had monosynaptic connections with each other and with Si1 and Si2, and 5) their synaptic actions helped explain the phasing of the motor pattern. The results of this study show that the motor pattern has more complex internal dynamics than a simple left/right alternation of firing; the CPG circuit appears to be composed of two kernels of reciprocally inhibitory neurons, one consisting of Si1, Si2, and the contralateral Si4 and the other consisting of Si3. These two kernels interact with each other to produce a stable rhythmic motor pattern.

The effect of octopamine on the locust stomatogastric nervous system

Octopamine (OA) is a prominent neuromodulator of invertebrate nervous systems, influencing multiple physiological processes. Among its many roles in insects are the initiation and maintenance of various rhythmic behaviors. Here, the neuromodulatory effects of OA on the components of the locust stomatogastric nervous system were studied, and one putative source of OA modulation of the system was identified. Bath application of OA was found to abolish the endogenous rhythmic output of the fully isolated frontal ganglion (FG), while stimulating motor activity of the fully isolated hypocerebral ganglion (HG). OA also induced rhythmic movements in a foregut preparation with intact HG innervation. Complex dose-dependent effects of OA on interconnected FG-HG preparations were seen: 10⁻⁵ M OA accelerated the rhythmic activity of both the HG and FG in a synchronized manner, while 10⁻⁴ M OA decreased both rhythms. Intracellular stimulation of an identified octopaminergic dorsal unpaired median neuron in the subesophageal ganglion was found to exert a similar effect on the FG motor output as that of OA application. Our findings suggest a mechanism of regulation of insect gut patterns and feeding-related behavior during stress and times of high energy demand.

Functional organization of locomotor interneurons in the ventral lumbar spinal cord of the newborn rat

Although the mammalian locomotor CPG has been localized to the lumbar spinal cord, the functional-anatomical organization of flexor and extensor interneurons has not been characterized. Here, we tested the hypothesis that flexor and extensor interneuronal networks for walking are physically segregated in the lumbar spinal cord. For this purpose, we performed optical recordings and lesion experiments from a horizontally sectioned lumbar spinal cord isolated from neonate rats. This ventral hemi spinal cord preparation produces well-organized fictive locomotion when superfused with 5-HT/NMDA. The dorsal surface of the preparation was visualized using the Ca²⁺ indicator fluo-4 AM, while simultaneously monitoring motor output at ventral roots L2 and L5. Using calcium imaging, we provided a general mapping view of the interneurons that maintained a stable phase relationship with motor output. We showed that the dorsal surface of L1 segment contains a higher density of locomotor rhythmic cells than the other segments. Moreover, L1 segment lesioning induced the most important changes in the locomotor activity in comparison with lesions at the T13 or L2 segments. However, no lesions led to selective disruption of either flexor or extensor output. In addition, this study found no evidence of functional parcellation of locomotor interneurons into flexor and extensor pools at the dorsal-ventral midline of the lumbar spinal cord of the rat.

Comparative studies provide evidence for neural reuse

Comparative studies demonstrate that homologous neural structures differ in function and that neural mechanisms underlying behavior evolved independently. A neural structure does not serve a particular function so much as it executes an algorithm on its inputs though its dynamics. Neural dynamics are altered by a neuromodulation, and species-differences in neuromodulation can account for behavioral differences.

A hyperpolarization-activated inward current alters swim frequency of the pteropod mollusk Clione limacina

The pteropod mollusk, Clione limacina, exhibits behaviorally relevant swim speed changes that occur within the context of the animal's ecology. Modulation of C. limacina swimming speed involves changes that occur at the network and cellular levels. Intracellular recordings from interneurons of the swim central pattern generator show the presence of a sag potential that is indicative of the hyperpolarization-activated inward current (I(h)). Here we provide evidence that I(h) in primary swim interneurons plays a role in C. limacina swimming speed control and may be a modulatory target. Recordings from central pattern generator swim interneurons show that hyperpolarizing current injection produces a sag potential that lasts for the duration of the hyperpolarization, a characteristic of cells possessing I(h). Following the hyperpolarizing current injection, swim interneurons also exhibit postinhibitory rebound (PIR). Serotonin enhances the sag potential of C. limacina swim interneurons while the I(h) blocker, ZD7288, reduces the sag potential. Furthermore, a negative correlation was found between the amplitude of the sag potential and latency to PIR. Because latency to PIR was previously shown to influence swimming speed, we hypothesize that I(h) has an effect on swimming speed. The I(h) blocker, ZD7288, suppresses swimming in C. limacina and inhibits serotonin-induced acceleration, evidence that supports our hypothesis.

Repetition priming of motoneuronal activity in a small motor network: intercellular and intracellular signaling

The characteristics of central pattern generator (CPG) outputs are subject to extensive modulation. Previous studies of neuromodulation largely focused on immediate actions of neuromodulators, i.e., actions that were exerted at the time when either neuromodulators were present or neuromodulatory inputs to the CPG were active. However, neuromodulatory actions are known to persist when neuromodulators are no longer present. In Aplysia, stimulation of cerebral-buccal interneuron-2 (CBI-2), which activates the feeding CPG, produces a repetition priming of motor programs. This priming is reflected in an increase of firing of motoneurons. As CBI-2 contains two neuromodulatory peptides, FCAP (feeding circuit-activating peptide) and CP2 (cerebral peptide 2), we hypothesized that repetition priming may involve persistent peptidergic neuromodulation. We find that these peptides produce priming-like effects, i.e., they increase the firing of radula-opening (B48) and radula-closing (B8) motoneurons during motor programs. Proekt et al. (2004, 2007) showed that repetition priming of neuron B8 is implemented by modulatory inputs that B8 receives from the CPG. In contrast, our current findings indicate that priming of B48 may be implemented by a direct peptidergic modulation of its intrinsic characteristics via a pathway that activates cAMP. We suggest that the direct versus indirect, i.e., CPG-dependent, repetition priming may be related to the type of input that individual motoneurons receive from the CPG. We suggest that in motoneurons that are driven by concurrent excitation-inhibition, repetition priming is indirect as it is preferentially implemented via modulation of the output of CPGs. In contrast, in motoneurons that are driven by alternating excitation-inhibition, direct modulation of motoneurons may be preferentially used.

Octopamine promotes rhythmicity but not synchrony in a bilateral pair of bursting motor neurons in the feeding circuit of Aplysia

Octopamine-like immunoreactivity was localized to a limited number (<40) of neurons in the Aplysia central nervous system, including three neurons in the paired buccal ganglia (BG) that control feeding movements. Application of octopamine (OA) to the BG circuit produced concentration-dependent (10(-8)-10(-4) mol l(-1)) modulatory actions on the spontaneous burst activity of the bilaterally paired B67 pharyngeal motor neurons (MNs). OA increased B67's burst duration and the number of impulses per burst. These effects reflected actions of OA on the intrinsic tetrodotoxin-resistant driver potential (DP) that underlies B67 bursting. In addition to its effects on B67's burst parameters, OA also increased the rate and regularity of burst timing. Although the bilaterally paired B67 MNs both exhibited rhythmic bursting in the presence of OA, they did not become synchronized. In this respect, the response to OA differed from that of dopamine, another modulator of the feeding motor network, which produces both rhythmicity and synchrony of bursting in the paired B67 neurons. It is proposed that modulators can regulate burst synchrony of MNs by exerting a dual control over their intrinsic rhythmicity and their reciprocal capacity to generate membrane potential perturbations. In this simple system, dopaminergic and octopaminergic modulation could influence whether pharyngeal contractions occur in a bilaterally synchronous or asynchronous fashion.

How does maintenance of network activity depend on endogenous dynamics of isolated neurons?

Robust activity of some networks, such as central pattern generators, suggests the existence of physiological mechanisms that maintain the most important characteristics, for example, the period and spike frequency of the pattern. Whatever these mechanisms are, they change the appropriate model parameters to or along the isomanifolds on which the characteristics of the pattern are constant, while their sensitivities to parameters may be different. Setting synaptic connections to zero at the points of isomanifolds allows for dissecting the maintenance mechanisms into components involving synaptic transmission and components involving intrinsic currents. The physiological meaning of the intrinsic current changes might be revealed by analysis of their impact on endogenous neuronal dynamics. Here, we sought answers to two questions: (1) Do parameter variations in insensitive directions (along isomanifolds) change endogenous dynamics of the network neurons? (2) Do sensitive and insensitive directions for network pattern characteristics depend on endogenous dynamics of the network neurons? We considered a leech heartbeat half-center oscillator model network and analyzed isomanifolds on which the burst period or spike frequency of the model, or both, are constant. Based on our analysis, we hypothesize that the dependence on endogenous dynamics of the isolated neurons is the stronger the more characteristics of the pattern have to be maintained. We also found that in general, the network was more flexible when it consisted of endogenously tonically spiking rather than bursting or silent neurons. Finally, we discuss the physiological implications of our findings.

State-, timing-, and pattern-dependent neuromodulation of synaptic strength by a serotonergic interneuron

Here we report that a serotonergic neuron evokes two distinct neuromodulatory actions with different state, timing, and firing pattern dependencies. These neuromodulatory actions may have important behavioral functions. In the mollusc, Tritonia diomedea, EPSCs evoked by ventral swim interneuron B (VSI) exhibited intrinsic plasticity; after a spike train, EPSC amplitude increased from a basal state to a potentiated state, which usually lasted >10 min. While the synapse was in a potentiated state, stimulation of a serotonergic dorsal swim interneuron (DSI) decreased VSI synaptic strength, returning it to a basal state. The extent of the DSI-evoked decrement was strongly correlated with the magnitude of the homosynaptic potentiation. This synaptic reset, or depotentiation, by DSI was blocked by the serotonin receptor antagonist methysergide and mimicked by a serotonin puff. In contrast to this state-dependent neuromodulatory action, we found that a previously described DSI-evoked transient enhancement of VSI synaptic strength was state-independent, producing the same multiplicative increase in EPSC amplitude regardless of whether the synapse was in a potentiated or basal state. These two actions also differed in their dependencies on the firing pattern of DSI and VSI action potentials. Results suggest that state-independent synaptic enhancement by DSI may play a short-term role during a swim motor pattern, whereas state-dependent actions may have longer-lasting consequences, resetting VSI synaptic strength after a swim bout. Thus, differences in two neuromodulatory actions at one synapse may allow a serotonergic neuron to play distinct roles at different stages of a motor pattern.

Episodic swimming behavior in the nematode C. elegans

Controlling the choice of behavioral output is a central function of the nervous system. Here we document a novel spontaneous behavioral transition in C. elegans locomotion. Upon transfer of the nematode from a solid surface into a liquid environment, swimming occurs in two phases: an initial, 1-2 h phase of continuous swimming, followed by a second phase during which swimming is episodic. During the second, episodic phase, periods of active swimming alternate in a highly regular fashion with a quiescent state lasting for several minutes. We analyzed the nature of the quiescent state and the basis for spontaneous switching between swimming and quiescence. The transition from swimming to quiescence is promoted by acetylcholine signaling and initially during quiescence body wall muscles are in a state of contraction. After the first minute, quiescent worms respond to prodding and resume swimming normally. The major command interneurons that control the locomotory circuits are not necessary for quiescence since swimming-quiescence cycling occurs after ablation of command interneurons. However, when subsets of neurons including the command interneurons are killed, the switching pattern becomes less regular, suggesting that a timer governing switching may lie within circuitry controlling motor neurons. The results show that the motor circuits have a tendency to switch spontaneously between active and inactive behavioral states. This property might be important to the animal in a uniform environment where sensory input is invariant.

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

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

Regulation and modulation of electric waveforms in gymnotiform electric fish

Weakly electric gymnotiform fish specialize in the regulation and modulation of the action potentials that make up their multi-purpose electric signals. To produce communication signals, gymnotiform fish modulate the waveforms of their electric organ discharges (EODs) over timescales spanning ten orders of magnitude within the animal's life cycle: developmental, reproductive, circadian, and behavioral. Rapid changes lasting milliseconds to seconds are the result of direct neural control of action potential firing in the electric organ. Intermediate-term changes taking minutes to hours result from the action of melanocortin peptides, the pituitary hormones that induce skin darkening and cortisol release in many vertebrates. Long-term changes in the EOD waveform taking days to weeks result from the action of sex steroids on the electrocytes in the electric organ as well as changes in the neural control structures in the brain. These long-term changes in the electric organ seem to be associated with changes in the expression of voltage-gated ion channels in two gene families. Electric organs express multiple voltage-gated sodium channel genes, at least one of which seems to be regulated by androgens. Electric organs also express multiple subunits of the shaker (Kv1) family of voltage-gated potassium channels. Expression of the Kv1 subtype has been found to vary with the duration of the waveform in the electric signal. Our increasing understanding of the mechanisms underlying precise control of electric communication signals may yield significant insights into the diversity of natural mechanisms available for modifying the performance of ion channels in excitable membranes. These mechanisms may lead to better understanding of normal function in a wide range of physiological systems and future application in treatment of disease states involving pathology of excitable membranes.

Graded inhibitory synaptic transmission between leech interneurons: assessing the roles of two kinetically distinct low-threshold Ca currents

In leeches, two pairs of reciprocally inhibitory heart interneurons that form the core oscillators of the pattern-generating network for heartbeat possess both high- and low-threshold (HVA and LVA) Ca channels. LVA Ca current has two kinetically distinct components (one rapidly activating/inactivating, ICaF, and another slowly activating/inactivating, ICaS) that mediate graded transmission, generate plateau potentials driving burst formation, and modulate spike-mediated transmission between heart interneurons. Here we used different stimulating protocols and inorganic Ca channel blockers to separate the effects of ICaF and ICaS on graded synaptic transmission and determine their interaction and relative efficacy. Ca2+ entering by ICaF channels is more efficacious in mediating release than that entering by ICaS channels. The rate of Ca2+ entry by LVA Ca channels appears to be as critical as the amount of delivered Ca2+ for synaptic transmission. LVA Ca currents and associated graded transmission were selectively blocked by 1 mM Ni2+, leaving spike-mediated transmission unaffected. Nevertheless, 1 mM Ni2+ affected homosynaptic enhancement of spike-mediated transmission that depends on background Ca2+ provided by LVA Ca channels. Ca2+ provided by both ICaF and ICaS depletes a common pool of readily releasable synaptic vesicles. The balance between availability of vesicles and Ca2+ concentration and its time course determine the strength of inhibitory transmission between heart interneurons. We argue that Ca2+ from multichannel domains arising from ICaF channels, clustered near but not directly associated with the release trigger, and Ca2+ radially diffusing from generally distributed ICaS channels interact at common release sites to mediate graded transmission.

Serotonergic enhancement of a 4-AP-sensitive current mediates the synaptic depression phase of spike timing-dependent neuromodulation

The mechanism underlying spike timing-dependent neuromodulation (STDN) was investigated in the opisthobranch mollusc Tritonia diomedea. The serotonergic dorsal swim interneurons (DSIs) dynamically modulated the synaptic output of ventral swim interneuron B (VSI); immediately after DSI stimulation, there was a potentiation of VSI synaptic strength followed by a longer-lasting synaptic depression. The potentiation phase of STDN was unaffected by spike broadening produced by the potassium channel blocker 4-aminopyridine (4-AP). In contrast, the depression phase was eliminated by 4-AP. Bath-applied serotonin (5-HT) decreased VSI spike duration and increased the magnitude of the A-current (IA), a voltage-dependent, transient, outward current. 4-AP preferentially blocked IA and prevented the spike narrowing caused by 5-HT, uncovering the full extent of 5-HT-induced synaptic potentiation. A consistent correlation was observed between IA and spike duration, but the correlation between synaptic strength and spike duration differed between preparations. Conductance-based simulations showed that the magnitude of A-current conductance could affect spike duration and gave an estimation of the change needed to produce spike narrowing. An artificial IA introduced into the VSI in the presence of 4-AP by means of the dynamic-clamp technique restored spike duration and gave a further approximation of the magnitude of modulation needed for spike narrowing. Together, these results suggest a mechanism for STDN: the DSIs release 5-HT, which causes a spike duration-independent enhancement of synaptic strength and a longer-lasting enhancement of IA that narrows the VSI spike and hence decreases VSI synaptic strength. Thus, STDN arises from the dynamics of independent intracellular signaling events.

Reconfiguration of multiple motor networks by short- and long-term actions of an identified modulatory neuron

The pyloric and gastric motor pattern-generating networks in the stomatogastric ganglion of the lobster Homarus gammarus are reconfigured into a new functional circuit by burst discharge in an identified pair of modulatory projection interneurons, originally named the pyloric suppressor (PS) neurons because of their inhibitory effects on pyloric network activity. Here we elucidate the actions of the PS neurons on individual members of the neighbouring gastric circuit, as well as describing their ability to alter synaptic coupling between the two networks. PS neuron firing has two distinct effects on gastric network activity: an initial short-lasting action mediated by transient inhibition of most gastric motoneurons, followed by a long-lasting circuit activation associated with a prolonged PS-evoked depolarization of the medial gastric (MG) motoneuron and the single network interneuron, Int1. These long-lasting effects are voltage-dependent, and experiments with hyperpolarizing current injection and photoablation suggest that excitation of both the MG neuron and Int1 is critical for PS-elicited gastric network rhythmicity. In parallel, PS neuron discharge persistently (lasting several minutes) enhances the strength of an inhibitory synaptic influence of the MG neuron on the pyloric dilator (PD)-anterior burster (AB) pacemaker neurons, thereby facilitating operational fusion of the two networks. Therefore, a single modulatory neuron may influence disparate populations of neurons via a range of very different and highly target-specific mechanisms: conventional transient synaptic drive and up- or down-modulation of membrane properties and synaptic efficacy. Moreover, distinctly different time courses of these actions allow different circuit configurations to be specified sequentially by a given modulatory input.

Myomodulin increases Ih and inhibits the NA/K pump to modulate bursting in leech heart interneurons

In the medicinal leech, a rhythmically active 14-interneuron network composes the central pattern generator for heartbeat. In two segmental ganglia, bilateral pairs of reciprocally inhibitory heart interneurons (oscillator interneurons) produce a rhythm of alternating bursts of action potentials that paces activity in the pattern-generating network. The neuropeptide myomodulin decreases the period of this bursting and increases the intraburst spike frequency when applied to isolated ganglia containing these oscillator interneurons. Myomodulin also decreases period, increases spike frequency, and increases the robustness of endogenous bursting in synaptically isolated (with bicuculline) oscillator interneurons. In voltage-clamp experiments using hyperpolarizing ramps, we identify an increase in membrane conductance elicited by myomodulin with the properties of a hyperpolarization-activated current. Voltage steps confirm that myomodulin indeed increases the maximum conductance of the hyperpolarization-activated current I(h). In similar experiments using Cs(+) to block I(h), we demonstrate that myomodulin also causes a steady offset in the ramp current that is not associated with an increase in conductance. This current offset is blocked by ouabain, indicating that myomodulin inhibits the Na/K pump. In current-clamp experiments, when I(h) is blocked with Cs(+), myomodulin decreases period and increases spike frequency of alternating bursting in synaptically connected oscillator interneurons, suggesting that inhibiting the Na/K pump modulates these burst characteristics. These observations indicate that myomodulin decreases period and increases spike frequency of endogenous bursting in synaptically isolated oscillator heart interneurons and alternating bursting of reciprocally inhibitory pairs of interneurons, at least in part, by increasing I(h) and by decreasing the Na/K pump.

FLR-4, a novel serine/threonine protein kinase, regulates defecation rhythm in Caenorhabditis elegans

The defecation behavior of the nematode Caenorhabditis elegans is controlled by a 45-s ultradian rhythm. An essential component of the clock that regulates the rhythm is the inositol trisphosphate receptor in the intestine, but other components remain to be discovered. Here, we show that the flr-4 gene, whose mutants exhibit very short defecation cycle periods, encodes a novel serine/threonine protein kinase with a carboxyl terminal hydrophobic region. The expression of functional flr-4::GFP was detected in the intestine, part of pharyngeal muscles and a pair of neurons, but expression of flr-4 in the intestine was sufficient for the wild-type phenotype. Furthermore, laser killing of the flr-4-expressing neurons did not change the defecation phenotypes of wild-type and flr-4 mutant animals. Temperature-shift experiments with a temperature-sensitive flr-4 mutant suggested that FLR-4 acts in a cell-functional rather than developmental aspect in the regulation of defecation rhythms. The function of FLR-4 was impaired by missense mutations in the kinase domain and near the hydrophobic region, where the latter allele seemed to be a weak antimorph. Thus, a novel protein kinase with a unique structural feature acts in the intestine to increase the length of defecation cycle periods.

Pharmacological characterization of ionic currents that regulate high-frequency spontaneous activity of electromotor neurons in the weakly electric fish,Apteronotus leptorhynchus

The neural circuit that controls the electric organ discharge (EOD) of the brown ghost knifefish (Apteronotus leptorhynchus) contains two spontaneous oscillators. Both pacemaker neurons in the medulla and electromotor neurons (EMNs) in the spinal cord fire spontaneously at frequencies of 500-1,000 Hz to control the EOD. These neurons continue to fire in vitro at frequencies that are highly correlated with in vivo EOD frequency. Previous studies used channel blocking drugs to pharmacologically characterize ionic currents that control high-frequency firing in pacemaker neurons. The goal of the present study was to use similar techniques to investigate ionic currents in EMNs, the other type of spontaneously active neuron in the electromotor circuit. As in pacemaker neurons, high-frequency firing of EMNs was regulated primarily by tetrodotoxin-sensitive sodium currents and by potassium currents that were sensitive to 4-aminopyridine and kappaA-conotoxin SIVA, but resistant to tetraethylammonium. EMNs, however, differed from pacemaker neurons in their sensitivity to some channel blocking drugs. Alpha-dendrotoxin, which blocks a subset of Kv1 potassium channels, increased firing rates in EMNs, but not pacemaker neurons; and the sodium channel blocker muO-conotoxin MrVIA, which reduced firing rates of pacemaker neurons, had no effect on EMNs. These results suggest that similar, but not identical, ionic currents regulate high-frequency firing in EMNs and pacemaker neurons. The differences in the ionic currents expressed in pacemaker neurons and EMNs might be related to differences in the morphology, connectivity, or function of these two cell types.

Hyperpolarization-activated, cyclic nucleotide-gated cation channels: roles in the differential electrophysiological properties of rat primary afferent neurons

The large, medium-sized, and small neurons of the dorsal root ganglion (DRG) have different functions in the processing of various senses. Hyperpolarization-activated, cyclic nucleotide-gated channels (HCN) contribute greatly to neuronal excitability. In the present study, which used whole-cell patch clamp techniques and immunohistochemical staining methods, the electrophysiological properties of DRG neurons were systematically compared, and the roles of HCN-1, -2, and -4 were examined. The main results were as follows. 1) The large neurons had significantly higher V0.5 values (membrane potential at which the HCN channels were half-activated) and shorter time constants (tau) than small or medium-sized DRG neurons. However, large DRG neurons had higher Ih density (HCN neuron current). 2) HCN-1 was found predominantly, but not exclusively, in large and medium-sized DRG neurons; HCN-2 was found in all DRG neurons; and HCN-4 was poorly visualized in all DRG neurons. HCN-1 and HCN-2 were colocalized in large and medium-sized neurons with immunostaining of adjacent sections. In the dorsal horn of the spinal cord, HCN-1, HCN-2, and HCN-4 were all expressed in laminae I-IV, although HCN-1 was not detectable in lamina II. 3) Blockade of Ih current in DRG neurons caused a significant decrease in V0.5, resting membrane potential, and repetitive firing number of action potential and a significant increase in time of rising phase of action potential. These results suggest that the different HCN channels in the three types of DRG neurons might contribute to their differential electrophysiological properties.

The physiological and morphological characteristics of interneurons caudal to the trigeminal motor nucleus in rats

In this study we have characterized the membrane properties and morphology of interneurons which lie between the caudal pole of the trigeminal motor nucleus and the rostral border of the facial motor nucleus. Previous studies suggest that many of these interneurons may participate in the genesis of rhythmical jaw movements. Saggital brainstem slices were taken from rats aged 5-8 days. Interneurons lying caudal to the trigeminal motor nucleus were visualized using near-infrared differential interference contrast (DIC) microscopy, and were recorded from using patch pipettes filled with a K-gluconate- and biocytin-based solution. The 127 neurons recorded could be categorized into three subtypes on the basis of their responses to injection of depolarizing current pulses, namely tonic firing (type I), burst firing (type II) and spike-adaptive (type III) neurons. Type I interneurons had a higher input resistance and a lower rheobase than type II neurons. All three neuron subtypes showed 'sag' of the voltage response to injection of large-amplitude hyperpolarizing current pulses, and, in addition, also showed rectification of the voltage response to injection of depolarizing current pulses, with type II neurons showing significantly greater rectification than type I neurons. The axonal arborizations were reconstructed for 44 of 63 neurons labelled with tracer. Neurons of each subtype were found to issue axon collaterals terminating in the brainstem nuclei, including the parvocellular reticular nucleus (PCRt), the trigeminal motor nucleus (Vmot), the supratrigeminal nucleus or the trigeminal mesencephalic nucleus. Twenty-five of the 43 neurons issued collaterals which terminated in the Vmot and the other brainstem nuclei. When viewed under 100x magnification, the collaterals of some interneurons were seen to give off varicosities and end-terminations which passed close to the somata of unidentified neurons in the trigeminal motor nucleus and in the area close to the interneuron soma itself. This suggests that the interneurons may make synaptic contacts both on motoneurons and also on nearby interneurons. These results provide data on the membrane properties of trigeminal interneurons and evidence for their synaptic connections both with nearby interneurons and also with motoneurons. Thus, the interneurons examined could play roles in the shaping, and possibly also in the generation, of rhythmical signals to trigeminal motoneurons.

Spike timing-dependent serotonergic neuromodulation of synaptic strength intrinsic to a central pattern generator circuit

Neuromodulation is often thought to have a static, gain-setting function in neural circuits. Here we report a counter example: the neuromodulatory effect of a serotonergic neuron is dependent on the interval between its spikes and those of the neuron being modulated. The serotonergic dorsal swim interneurons (DSIs) are members of the escape swim central pattern generator (CPG) in the mollusk Tritonia diomedea. DSI spike trains heterosynaptically enhanced synaptic potentials evoked by another CPG neuron, ventral swim interneuron B (VSI-B), when VSI-B action potentials occurred within 10 sec of a DSI spike train; however, if VSI-B was stimulated 20-120 sec after DSI, then the amplitude of VSI-B synaptic potentials decreased. Consistent with this, VSI-B-evoked synaptic currents exhibited a temporally biphasic and bidirectional change in amplitude after DSI stimulation. Both the DSI-evoked enhancement and decrement were occluded by serotonin and blocked by the serotonin receptor antagonist methysergide, suggesting that both phases are mediated by serotonin. In most preparations, however, bath-applied serotonin caused only a sustained enhancement of VSI-B synaptic strength. The heterosynaptic modulation interacted with short-term homosynaptic plasticity: DSI-evoked depression was offset by VSI-B homosynaptic facilitation. This caused a complicated temporal pattern of neuromodulation when DSI and VSI-B were stimulated to fire in alternating bursts to mimic the natural motor pattern: DSI strongly enhanced summated VSI-B synaptic potentials and suppressed single synaptic potentials after the cessation of the artificial motor pattern. Thus, spike timing-dependent serotonergic neuromodulatory actions can impart temporal information that may be relevant to the operation of the CPG.

Calcium signaling components of oscillating invertebrate neurons in vitro

We have studied the Ca(2+) dynamics of bursting-spiking neurons in the lobster stomatogastric ganglion. Neurons in this ganglion undergo spontaneous oscillations in membrane voltage with a period of 1-10 s in situ. We found that neurons isolated from the ganglion and filled with the fluorescent calcium indicator Fluo-4 show simultaneous changes of membrane potential and cytoplasmic Ca(2+) concentration ([Ca(2+)](I)). These Ca(2+) signals are highly heterogeneous both in terms of amplitude and time constants. They showed variable spatial distributions with the soma exhibiting low and slow signals, and a region in the process with large and fast signals. Ca(2+) transients in the processes are dependent on external Ca(2+) and can be blocked by Co(2+), but not other, more specific Ca(2+) current blockers. Rather, nifedipine a known Ca(2+) current blocker, affects the distribution of the Ca(2+) signal, which suggests a specific localization of Ca(2+) channels. Although the signal is not absolutely dependent on action potentials, it is greatly reduced when action potentials are blocked by tetrodotoxin. Termination of the signal depends only slightly on Ca(2+) buffering mechanisms such as mitochondria, Ca(2+)/Na(+) and Ca(2+)/H(+) exchangers. We also demonstrate the presence of caffeine-sensitive internal stores in stomatogastric ganglion cells. The store distribution is different but overlaps with the voltage-dependent distribution. The maximal caffeine-activated Ca(2+) signal is in the soma and it is smaller in the processes. Unlike the voltage-activated Ca(2+) signal this signal is not blocked by Co(2+). Nevertheless, the two types of signal interact during caffeine application. This unique spatial separation of two Ca(2+) sources may have important functional implication.

G protein signaling in a neuronal network is necessary for rhythmic motor pattern production

G protein-coupled receptors are widely recognized as playing important roles in mediating the actions of extrinsic neuromodulatory inputs to motor networks. However, the potential for their direct involvement in rhythmic motor pattern generation has received considerably less attention. Results from this study indicate that G protein signaling appears to be integral to the operation of the central pattern generator (CPG) underlying the escape swim of the mollusk Tritonia diomedea. Blocking G protein signaling in a single CPG neuron, cerebral neuron C2, with intracellular iontophoresis of the guanine nucleotide analogue guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S), prevented the production of the swim motor program. Moreover, tonic activation of G protein signaling in this neuron by iontophoresis of the GTP analogues guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and 5'-guanylyl-imidodiphosphate also inhibited motor pattern production. The possible sites of action of these guanine nucleotide analogues were examined to assess potential mechanisms by which they interfered with motor pattern production. Intracellular iontophoresis of GDP-beta-S into C2 did not affect C2 basal synaptic strength. However, it did reduce heterosynaptic facilitation of C2 synapses caused by the dorsal swim interneurons (DSIs), a set of serotonergic swim CPG neurons. In contrast, GTP-gamma-S directly enhanced C2 synaptic strength onto DFN, mimicking the neuromodulatory effect of the DSIs. GDP-beta-S, but not the GTP analogues, decreased C2 excitability, whereas both GTP analogues, but not GDP-beta-S, blocked the ability of DSI stimulation to increase C2 excitability. The decrease in C2 excitability caused by GDP-beta-S is not likely to be responsible for the inhibition of the swim motor pattern because decreasing C2 firing rate, by injecting hyperpolarizing current, did not prevent the production of the rhythmic motor pattern. Taken together, these data suggest that G protein signaling is a necessary and integral component of the escape swim CPG in Tritonia and that G protein signaling mediates DSI heterosynaptic facilitation of C2 but may not mediate the DSI-evoked enhancement of C2 excitability.

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

Rhythm generation by the pyloric motor network in the stomatogastric ganglion (STG) of the spiny lobster requires permissive neuromodulatory inputs from other central ganglia. When these inputs to the STG are suppressed by cutting the single, mainly afferent stomatogastric nerve (stn), pyloric neurons cease to burst and the network falls silent. However, as shown previously, if such a decentralized quiescent ganglion is maintained in organ culture, pyloric network rhythmicity returns after 3-4 days and, although slower, is similar to the motor pattern expressed when the stn is intact. Here we use current- and voltage-clamp, primarily of identified pyloric dilator (PD) neurons, to investigate changes in synaptic and cellular properties that underlie this transition in network behavior. Although the efficacy of chemical synapses between pyloric neurons decreases significantly (by <or=50%) after STG decentralization, the fundamental change leading to rhythm recovery occurs in the voltage-dependent properties of the neurons themselves. Whereas pyloric neurons, including the PD, lateral pyloric, and pyloric cell types, are unable to generate burst-producing membrane potential oscillations in the short-term absence of extrinsic modulatory inputs, in long-term decentralized ganglia, the same cells are able to oscillate spontaneously, even after experimental isolation in situ from all other elements in the pyloric network. In PD neurons this reacquisition of rhythmicity is associated with a net reduction in outward tetraethylammonium-sensitive ionic currents that include a delayed-rectifier type potassium current (I(Kd)) and a calcium-dependent K(+) current, I(KCa). By contrast, long-term STG decentralization caused enhancement of a hyperpolarization-activated inward current that resembles I(h). These results are consistent with the hypothesis that modulatory inputs sustain the modulation-dependent rhythmogenic character of the pyloric network by continuously regulating the balance of membrane conductances that underlie neuronal oscillation.

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.

Non-mammalian models for studying neural development and function

Early neuroscientists scoured the animal kingdom for the ideal preparation with which to study specific problems of interest. Today, non-mammalian nervous systems continue to provide ideal platforms for the study of fundamental problems in neuroscience. Indeed, the peculiarities of body plan and nervous systems that have evolved to carry out precise tasks in unique ecological niches enable investigators not only to pose specific scientific questions, but also to uncover principles that are general to all nervous systems.

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.

Central pattern generators and the control of rhythmic movements

Central pattern generators are neuronal circuits that when activated can produce rhythmic motor patterns such as walking, breathing, flying, and swimming in the absence of sensory or descending inputs that carry specific timing information. General principles of the organization of these circuits and their control by higher brain centers have come from the study of smaller circuits found in invertebrates. Recent work on vertebrates highlights the importance of neuro-modulatory control pathways in enabling spinal cord and brain stem circuits to generate meaningful motor patterns. Because rhythmic motor patterns are easily quantified and studied, central pattern generators will provide important testing grounds for understanding the effects of numerous genetic mutations on behavior. Moreover, further understanding of the modulation of spinal cord circuitry used in rhythmic behaviors should facilitate the development of new treatments to enhance recovery after spinal cord damage.

Nonlinear behavior of sinusoidally forced pyloric pacemaker neurons

Periodic current forcing was used to investigate the intrinsic dynamics of a small group of electrically coupled neurons in the pyloric central pattern generator (CPG) of the lobster. This group contains three neurons, namely the two pyloric dilator (PD) motoneurons and the anterior burster (AB) interneuron. Intracellular current injection, using sinusoidal waveforms of varying amplitude and frequency, was applied in three configurations of the pacemaker neurons: 1) the complete pacemaker group, 2) the two PDs without the AB, and 3) the AB neuron isolated from the PDs. Depending on the frequency and amplitude of the injected current, the intact pacemaker group exhibited a wide variety of nonlinear behaviors, including synchronization to the forcing, quasiperiodicity, and complex dynamics. In contrast, a single, broad 1:1 entrainment zone characterized the response of the PD neurons when isolated from the main pacemaker neuron AB. The isolated AB responded to periodic forcing in a manner similar to the complete pacemaker group, but with wider zones of synchronization. We have built an analog electronic circuit as an implementation of a modified Hindmarsh-Rose model for simulating the membrane potential activity of pyloric neurons. We subjected this electronic model neuron to the same periodic forcing as used in the biological experiments. This four-dimensional electronic model neuron reproduced the autonomous oscillatory firing patterns of biological pyloric pacemaker neurons, and it expressed the same stationary nonlinear responses to periodic forcing as its biological counterparts. This adds to our confidence in the model. These results strongly support the idea that the intact pyloric pacemaker group acts as a uniform low-dimensional deterministic nonlinear oscillator, and the regular pyloric oscillation is the outcome of cooperative behavior of strongly coupled neurons, having different dynamical and biophysical properties when isolated.

Identified serotonergic neurons in the Tritonia swim CPG activate both ionotropic and metabotropic receptors

Although G-protein-coupled (metabotropic) receptors are known to modulate the production of motor patterns, evidence from the escape swim central pattern generator (CPG) of the nudibranch mollusk, Tritonia diomedea, suggests that they might also participate in the generation of the motor pattern itself. The dorsal swim interneurons (DSIs), identified serotonergic neurons intrinsic to the Tritonia swim CPG, evoke dual component synaptic potentials onto other CPG neurons and premotor interneurons. Both the fast and slow components were previously shown to be due to serotonin (5-HT) acting at distinct postsynaptic receptors. We find that blocking or facilitating metabotropic receptors in a postsynaptic premotor interneuron differentially affects the fast and slow synaptic responses to DSI stimulation. Blocking G-protein activation by iontophoretically injecting the GDP-analogue guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) did not significantly affect the DSI-evoked fast excitatory postsynaptic potential (EPSP) but decreased the amplitude of the slow component more than 50%. Injection of the GTP analogues guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and 5'-guanylyl-imidodiphosphate, to prolong G-protein activation, had mixed effects on the fast component but increased the amplitude and duration of the slow component of the DSI-evoked response and, with repeated DSI stimulation, led to a persistent depolarization. These results indicate that the fast component of the biphasic synaptic potential evoked by a serotonergic CPG neuron onto premotor interneurons is mediated by ionotropic receptors (5-HT-gated ion channels), whereas the slow component is mediated by G-protein-coupled receptors. A similar synaptic activation of metabotropic receptors might also be found within the CPG itself, where it could exert a direct influence onto motor pattern generation.

Neural adaptation in the generation of rhythmic behavior

Motor systems can adapt rapidly to changes in external conditions and to switching of internal goals. They can also adapt slowly in response to training, alterations in the mechanics of the system, and any changes in the system resulting from injury. This article reviews the mechanisms underlying short- and long-term adaptation in rhythmic motor systems. The neuronal networks underlying the generation of rhythmic motor patterns (central pattern generators; CPGs) are extremely flexible. Neuromodulators, central commands, and afferent signals all influence the pattern produced by a CPG by altering the cellular and synaptic properties of individual neurons and the coupling between different populations of neurons. This flexibility allows the generation of a variety of motor patterns appropriate for the mechanical requirements of different forms of a behavior. The matching of motor output to mechanical requirements depends on the capacity of pattern-generating networks to adapt to slow changes in body mechanics and persistent errors in performance. Afferent feedback from body and limb proprioceptors likely plays an important role in driving these long-term adaptive processes.

Topographic organization of efferent neurons with different neurochemical characters in the cerebral ganglia of the snail Helix pomatia

This study provides a description of the organization of neurons efferent to different head areas in the cerebral ganglia of Helix pomatia, revealed by simultaneous Ni-lysine and Co-lysine back-filling of different pairs of cerebral nerves. The backfills show that labeled cerebral neurons that innervate the head areas are concentrated in seven representation foci distributed in different parts of the cerebral ganglia. Almost each head area is represented in each focus. At a gross level, the representation of the different head areas in the representation foci shows a topographic arrangement. Each focus is constituted by neurochemically different groups of neurons. All head areas are innervated by serotonin-containing fibers from a single focus (Focus 2) and by dopamine-containing fibers from Foci 1, 2, and 4. However, they are innervated by CARP and FMRFamide-containing fibers from all of the foci. The combination of retrograde labeling with 5, 6-dihydroxytriptamine induced pigment labeling of serotonin-containing neurons or with fluorescence tyrosinehydroxylase immunocytochemistry to detect dopamine-containing neurons showed that the different head areas are topographycally represented in the clusters of both the serotonin- and dopamine-containing cells. The combination of Ni-lysine backfillings from different cerebral nerves with fluorescence CARP and FMRFamide immunocytochemistry revealed that the head areas are represented also in both the CARP and FMRFamide immunoreactive groups of neurons in the different foci.

Bridging the gap - from ion channels to networks and behaviour

A major challenge for current research in neuroscience is to understand the intrinsic operation of the functional modules of the central nervous system, such as those formed by cortical columns and the neuronal networks controlling motor behaviour. Most vertebrate experimental models used in network analyses involve developing nervous systems, which are in rapid transition with regard to their cellular properties and the expression of different ion channels. Recent advances in our understanding of the cellular and circuit properties of motor networks are making it possible to decipher the mechanisms involved in vertebrate motor pattern generation.

Evolution and analysis of model CPGs for walking: II. General principles and individual variability

Are there general principles for pattern generation? We examined this question by analyzing the operation of large populations of evolved model central pattern generators (CPGs) for walking. Three populations of model CPGs were evolved, containing three, four, or five neurons. We identified six general principles. First, locomotion performance increased with the number of interneurons. Second, the top 10 three-, four-, and five-neuron CPGs could be decomposed into dynamical modules, an abstract description developed in a companion article. Third, these dynamical modules were multistable: they could be switched between multiple stable output configurations. Fourth, the rhythmic pattern generated by a CPG could be understood as a closed chain of successive destabilizations of one dynamical module by another. A combinatorial analysis enumerated the possible dynamical modular structures. Fifth, one-dimensional modules were frequently observed and, in some cases, could be assigned specific functional roles. Finally, dynamic dynamical modules, in which the modular structure itself changed over one cycle, were frequently observed. The existence of these general principles despite significant variability in both patterns of connectivity and neural parameters was explained by degeneracy in the maps from neural parameters to neural dynamics to behavior to fitness. An analysis of the biomechanical properties of the model body was essential for relating neural activity to behavior. Our studies of evolved model circuits suggest that, in the absence of other constraints, there is no compelling reason to expect neural circuits to be functionally decomposable as the number of interneurons increase. Analyzing idealized model pattern generators may be an effective methodology for gaining insights into the operation of biological pattern generators.