Distinct synaptic dynamics of heterogeneous pacemaker neurons in an oscillatory network

Many rhythmically active networks involve heterogeneous populations of pacemaker neurons with potentially distinct synaptic outputs that can be differentially targeted by extrinsic inputs or neuromodulators, thereby increasing possible network output patterns. To understand the roles of heterogeneous pacemaker neurons, we characterized differences in synaptic output from the anterior burster (AB) and pyloric dilator (PD) neurons in the lobster pyloric network. These intrinsically distinct neurons are strongly electrically coupled, coactive, and constitute the pyloric pacemaker ensemble. During pyloric oscillations, the pacemaker neurons produce compound inhibitory synaptic connections to the follower lateral pyloric (LP) and pyloric constrictor (PY) neurons, which fire out of phase with AB/PD and with different delay times. Using pharmacological blockers, we separated the synapses originating from the AB and PD neurons and investigated their temporal dynamics. These synapses exhibited distinct short-term dynamics, depending on the presynaptic neuron type, and had different relative contributions to the total synaptic output depending on waveform shape and cycle frequency. However, paired comparisons revealed that the amplitude or dynamics of synapses from either the AB or PD neuron did not depend on the postsynaptic neuron type, LP or PY. To address the functional implications of these findings, we examined the correlation between synaptic inputs from the pacemakers and the burst onset phase of the LP and PY neurons in the ongoing pyloric rhythm. These comparisons showed that the activity of the LP and PY neurons is influenced by the peak phase and amplitude of the synaptic inputs from the pacemaker neurons.     

Electrically coupled pacemaker neurons respond differently to same physiological inputs and neurotransmitters

The two pyloric dilator (PD) motor neurons and the single anterior burster (AB) interneuron are electrically coupled and together comprise the pacemaker for the pyloric central pattern generator of the stomatogastric ganglion of the lobster, Panulirus interruptus. Previous work (31) has shown that the AB neuron is an endogenously bursting neuron, while the PD neuron is a conditional burster. In this paper the effects of physiological inputs and neurotransmitters on isolated PD neurons and AB neurons were studied using the lucifer yellow photoinactivation technique (33). Stimulation of the inferior ventricular nerve (IVN) fibers at high frequencies elicits a triphasic response in AB and PD neurons: a rapid excitatory postsynaptic potential (EPSP) followed by a slow inhibitory postsynaptic potential (IPSP), followed by an enhancement of the pacemaker slow-wave depolarizations. Photoinactivation experiments indicate that the enhancement of the slow wave is due primarily to actions of the IVN fibers on the PD neurons but not on the AB neuron. Bath-applied dopamine dramatically alters the motor output of the pyloric system. Photoinactivation experiments show that 10(-4) M dopamine increases the amplitude and frequency of the slow-wave depolarizations recorded in the AB neurons but hyperpolarizes and inhibits the PD neurons. Bath-applied serotonin increases the frequency and amplitude of the slow-wave depolarizations in the AB neuron but has no effect on PD neurons. Pilocarpine, a muscarinic cholinergic agonist, stimulates slow-wave depolarization production in both PD neurons and the AB neuron, but the waveform and frequency of the slow waves elicited are quite different. These results show that although the electrically coupled PD and AB neurons always depolarize synchronously and act together as the pacemaker for the pyloric system, they respond differently to a neuronal input and to several putative neuromodulators. Thus, despite electrical coupling sufficient to ensure synchronous activity, the PD and AB neurons can be modulated independently.     

Models of respiratory rhythm generation in the pre-B?tzinger complex. II. Populations Of coupled pacemaker neurons.

We have proposed models for the ionic basis of oscillatory bursting of respiratory pacemaker neurons in the pre-B?tzinger complex. In this paper, we investigate the frequency control and synchronization of these model neurons when coupled by excitatory amino-acid-mediated synapses and controlled by convergent synaptic inputs modeled as tonic excitation. Simulations of pairs of identical cells reveal that increasing tonic excitation increases the frequency of synchronous bursting, while increasing the strength of excitatory coupling between the neurons decreases the frequency of synchronous bursting. Low levels of coupling extend the range of values of tonic excitation where synchronous bursting is found. Simulations of a heterogeneous population of 50-500 bursting neurons reveal coupling effects similar to those found experimentally in vitro: coupling increases the mean burst duration and decreases the mean burst frequency. Burst synchronization occurred over a wide range of intrinsic frequencies (0.1-1 Hz) and even in populations where as few as 10% of the cells were intrinsically bursting. Weak coupling, extreme parameter heterogeneity, and low levels of depolarizing input could contribute to the desynchronization of the population and give rise to quasiperiodic states. The introduction of sparse coupling did not affect the burst synchrony, although it did make the interburst intervals more irregular from cycle to cycle. At a population level, both parameter heterogeneity and excitatory coupling synergistically combine to increase the dynamic input range: robust synchronous bursting persisted across a much greater range of parameter space (in terms of mean depolarizing input) than that of a single model cell. This extended dynamic range for the bursting cell population indicates that cellular heterogeneity is functionally advantageous. Our modeled system accounts for the range of intrinsic frequencies and spiking patterns of inspiratory (I) bursting cells found in the pre-B?tzinger complex in neonatal rat brain stem slices in vitro. There is a temporal dispersion in the spiking onset times of neurons in the population, predicted to be due to heterogeneity in intrinsic neuronal properties, with neurons starting to spike before (pre-I), with (I), or after (late-I) the onset of the population burst. Experimental tests for a number of the model's predictions are proposed.     

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.     

不同类型突触输入对电突触连接神经元同步化放电的调控

目的同步化放电在神经信息传递和编码过程中极为重要,如何调控神经元之间的同步化放电也一直是脑科学研究的热点之一。本文利用计算机模拟的方法,研究突触输入类型和相对强度的不同对同步化放电活动的调控作用。方法首先基于Izhikevich神经元模型,构造一个三神经元网络,然后添加不同类型的突触输入(电突触、兴奋性化学突触及抑制性化学突触),最后设定其中一个神经元为调控器,分析另外两个神经元的同步化放电活动。结果对于电突触和兴奋性化学突触输入,当两个神经元接收到的输入强度接近时,同步性会增强,反之则会减弱。而对于抑制性化学突触输入,当调控器神经元的放电频率较低时,同步性的变化与电突触和兴奋性化学突触的情况类似;当调控器神经元的频率较高时,同步化活动显著地被压制。结论电突触连接神经元之间的同步化活动会受到外界突触输入类型及相对大小的调控,为进一步探索神经系统中同步化活动的起源和调控提供了参考。     

Follower neurons in lobster (Panulirus interruptus) pyloric network regulate pacemaker period in complementary ways

Distributed neural networks (ones characterized by high levels of interconnectivity among network neurons) are not well understood. Increased insight into these systems can be obtained by perturbing network activity so as to study the functions of specific neurons not only in the network's "baseline" activity but across a range of network activities. We applied this technique to study cycle period control in the rhythmic pyloric network of the lobster, Panulirus interruptus. Pyloric rhythmicity is driven by an endogenous oscillator, the Anterior Burster (AB) neuron. Two network neurons feed back onto the pacemaker, the Lateral Pyloric (LP) neuron by inhibition and the Ventricular Dilator (VD) neuron by electrical coupling. LP and VD neuron effects on pyloric cycle period can be studied across a range of periods by altering period by injecting current into the AB neuron and functionally removing (by hyperpolarization) the LP and VD neurons from the network at each period. Within a range of pacemaker periods, the LP and VD neurons regulate period in complementary ways. LP neuron removal speeds the network and VD neuron removal slows it. Outside this range, network activity is disrupted because the LP neuron cannot follow slow periods, and the VD neuron cannot follow fast periods. These neurons thus also limit, in complementary ways, normal pyloric activity to a certain period range. These data show that follower neurons in pacemaker networks can play central roles in controlling pacemaker period and suggest that in some cases specific functions can be assigned to individual network neurons.     

The effect of hyperpolarizing inputs on the dynamics of a bursting pacemaker neuron model in the pre-Bötzinger complex

The pre-Bötzinger complex (pre-BötC), a subregion of the ventrolateral medulla involved in respiratory rhythm generation, contains intrinsically bursting pacemaker neurons. A previous study proposed Hodgkin–Huxley type minimal models for pacemaker neurons and predicted the effect of a hyperpolarizing input on the dynamics of a model under certain conditions. In this model, bursting is explained by the dynamics of a persistent sodium current. In the present study, the effect of a hyperpolarizing input on the dynamics of a model was investigated under variable conditions. It was observed that immediately after an input of sufficient intensity and duration, an increase in the maximal value of the gating variable h of a persistent sodium current was brought about by a decrease in the timing of the hyperpolarizing input. This corresponds to an observation that immediately after the input, a monotonic increase in the number of spikes in the neuron model was brought about by a decrease in the timing of the hyperpolarizing input. In addition, the dependency of burst duration immediately after the input on the timing of the hyperpolarizing input varied depending on the condition of input. The present study is the first to elucidate that the influence of hyperpolarizing inputs on the number of spikes within a burst in a pacemaker neuron model in the pre-BötC is dependent on the timing of the hyperpolarizing input and to clarify the possible mechanism of this influence, thereby facilitating a detailed understanding of the dynamics of a pacemaker neuron model in the pre-BötC.     

Dynamic control of irregular bursting in an identified neuron of an oscillatory circuit.

In the oscillatory circuits known as central pattern generators (CPGs), most synaptic connections are inhibitory. We have assessed the effects of inhibitory synaptic input on the dynamic behavior of a component neuron of the pyloric CPG in the lobster stomatogastric ganglion. Experimental perturbations were applied to the single, lateral pyloric neuron (LP), and the resulting voltage time series were analyzed using an entropy measure obtained from power spectra. When isolated from phasic inhibitory input, LP generates irregular spiking-bursting activity. Each burst begins in a relatively stereotyped manner but then evolves with exponentially increasing variability. Periodic, depolarizing current pulses are poor regulators of this activity, whereas hyperpolarizing pulses exert a strong, frequency-dependent regularizing action. Rhythmic inhibitory inputs from presynaptic pacemaker neurons also regularize the bursting. These inputs 1) reset LP to a similar state at each cycle, 2) extend and further stabilize the initial, quasi-stable phase of its bursts, and 3) at sufficiently high frequencies terminate ongoing bursts before they become unstable. The dynamic time frame for stabilization overlaps the normal frequency range of oscillations of the pyloric CPG. Thus, in this oscillatory circuit, the interaction of rhythmic inhibitory input with intrinsic burst properties affects not only the phasing, but also the dynamic stability of neural activity.     

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.     

Electrical coupling between model midbrain dopamine neurons: effects on firing pattern and synchrony

The role of gap junctions between midbrain dopamine (DA) neurons in mechanisms of firing pattern generation and synchronization has not been well characterized experimentally. We modified a multi-compartment model of DA neuron by adding a spike-generating mechanism and electrically coupling the dendrites of two such neurons through gap junctions. The burst-generating mechanism in the model neuron results from the interaction of a N-methyl-D-aspartate (NMDA)-induced current and the sodium pump. The firing patterns exhibited by the two model neurons included low frequency (2-7 Hz) spiking, high-frequency (13-20 Hz) spiking, irregular spiking, regular bursting, irregular bursting, and leader/follower bursting, depending on the parameter values used for the permeability for NMDA-induced current and the conductance for electrical coupling. All of these firing patterns have been observed in physiological neurons, but a systematic dependence of the firing pattern on the covariation of these two parameters has not been established experimentally. Our simulations indicate that electrical coupling facilitates NMDA-induced burst firing via two mechanisms. The first can be observed in a pair of identical cells. At low frequencies (low NMDA), as coupling strength was increased, only a transition from asynchronous to synchronous single-spike firing was observed. At high frequencies (high NMDA), increasing the strength of the electrical coupling in an identical pair resulted in a transition from high-frequency single-spike firing to burst firing, and further increases led to synchronous high-frequency spiking. Weak electrical coupling destabilizes the synchronous solution of the fast spiking subsystems, and in the presence of a slowly varying sodium concentration, the desynchronized spiking solution leads to bursts that are approximately in phase with spikes that are not in phase. Thus this transitional mechanism depends critically on action potential dynamics. The second mechanism for the induction of burst firing requires a heterogeneous pair that is, respectively, too depolarized and too hyperpolarized to burst. The net effect of the coupling is to bias at least one cell into an endogenously burst firing regime. In this case, action potential dynamics are not critical to the transitional mechanism. If electrical coupling is indeed more prominent in vivo due to basal level of modulation of gap junctions in vivo, these results may indicate why NMDA-induced burst firing is easier to observe in vivo as compared in vitro.     

Transient high-frequency firing in a coupled-oscillator model of the mesencephalic dopaminergic neuron

Dopaminergic neurons of the midbrain fire spontaneously at rates <10/s and ordinarily will not exceed this range even when driven with somatic current injection. When driven at higher rates, these cells undergo spike failure through depolarization block. During spontaneous bursting of dopaminergic neurons in vivo, bursts related to reward expectation in behaving animals, and bursts generated by dendritic application of N-methyl-d-aspartate (NMDA) agonists, transient firing attains rates well above this range. We suggest a way such high-frequency firing may occur in response to dendritic NMDA receptor activation. We have extended the coupled oscillator model of the dopaminergic neuron, which represents the soma and dendrites as electrically coupled compartments with different natural spiking frequencies, by addition of dendritic AMPA (voltage-independent) or NMDA (voltage-dependent) synaptic conductance. Both soma and dendrites contain a simplified version of the calcium-potassium mechanism known to be the mechanism for slow spontaneous oscillation and background firing in dopaminergic cells. The compartments differ only in diameter, and this difference is responsible for the difference in natural frequencies. We show that because of its voltage dependence, NMDA receptor activation acts to amplify the effect on the soma of the high-frequency oscillation of the dendrites, which is normally too weak to exert a large influence on the overall oscillation frequency of the neuron. During the high-frequency oscillations that result, sodium inactivation in the soma is removed rapidly after each action potential by the hyperpolarizing influence of the dendritic calcium-dependent potassium current, preventing depolarization block of the spike mechanism, and allowing high-frequency spiking.     

Mechanisms of gastric rhythm generation in the isolated stomatogastric ganglion of spiny lobsters: bursting pacemaker potentials, synaptic interactions, and muscarinic modulation.

1. The gastric central pattern generator (CPG), located in the stomatogastric ganglion (STG) of the spiny lobster (Panulirus interruptus), is nonrhythmic when deprived of neuromodulatory inputs from anterior ganglia. Leaving these inputs intact in vitro can sustain a gastric rhythm but also introduces numerous, uncontrolled and largely unknown modulatory and synaptic influences that greatly complicate analysis of this CPG. 2. Here we induced gastric rhythms in the isolated STG, by superfusing a specific modulator, the muscarinic agonist, pilocarpine. Muscarinic agents sustain vigorous gastric rhythms in the isolated STG. Our aim was to analyze the pattern-generating functions of the restricted gastric circuit, free of complicating influences from other ganglia, and under specific (muscarinic) modulation. 3. We used combinations of multiple cell hyperpolarizations, photodeletions, and synaptic blockade by picrotoxin to assess the pattern-generating role of individual gastric neurons and to study the activity of subcircuits. 4. Four identified gastric neurons [lateral gastric (LG), dorsal gastric (DG), 2 electrically coupled lateral posterior gastric (2LPGs)] acted as pattern-generating cells. They showed bursting pacemaker potentials (BPPs), i.e., plateau (or driver) potentials that underlay bursts of axonal spikes and slow, interburst depolarizing potentials that underlay repetitive burst activity. LG and DG, at least, became conditional bursters, able to burst repetitively because of intrinsic oscillations. The other gastric neurons behaved mainly as follower cells and derived their rhythmic bursting from synaptic coupling to the pattern-generator cells and from their own intrinsic (but nonoscillatory) properties. 5. The pattern-generating neurons form a novel "kernel" circuit that works by the cooperative interaction of cellular properties and synaptic connectivity. 6. This study constitutes the first complete and fully consistent analysis of pattern generation in the gastric network of the isolated STG. These mechanisms pertain to muscarinic rhythms in particular but also, we suggest, to gastric rhythm generation and CPG function in general. We suggest that 1) rhythmicity normally depends on the induction of bursty membrane properties in at least some component neurons; 2) different subcircuits can produce rhythmic patterns and may be activated by different modulators; and 3) the gastric network shares several important "building blocks" with CPGs that have been analyzed in other systems. 7. Muscarinic inputs are implicated as an important gastric regulator. We compare these responses with the reported modulatory actions of the anterior pyloric modulator (AMP), an identified, putatively cholinergic input interneuron that may act via muscarinic mechanisms.     

Suppressive control of a rhythmic central pattern generator by an identified modulatory neuron in crustacea

The activity of the 14 neuron network which organizes the pyloric motor rhythm in the stomatogastric ganglion of the lobster, Homarus gammarus, is controlled by neuromodulatory inputs which have been described as having mainly 'permissive' effects. By contrast, here we identify a neuron, the pyloric suppressor (PS) neuron which exerts a 'suppressive' effect on the pyloric activity. We show that PS neuron discharge can terminate in a long-lasting manner, spontaneous pyloric rhythmic activity. Its effect results from a direct suppression of the endogenous ability of the pyloric pacemaker neurons to produce rhythmic bursts of action potentials. Thus the output of the pyloric neuronal network appears to be finely tuned by neuromodulatory influences having opposite effects.     

A mechanism for production of phase shifts in a pattern generator

During motor activity of the pyloric system of the lobster stomatogastric ganglion, there are rhythmic alternations between activity in the pyloric dilator (PD) and pyloric (PY) motor neurons. We studied the phase relations between PD motor neuron activity and PY motor neuron activity in preparations cycling at a wide range of frequencies and after altering the activity of the PD neurons. The PY neurons fall into two classes, early (PE) and late (PL) (21), distinguished by the different phases in the pyloric cycle at which they fire. The phase at which PE neurons fired and the phase at which PL neurons fired was independent of pyloric cycle frequency over a range of frequencies from 0.5 to 2.25 Hz. The anterior burster (AB) interneuron is electrically coupled to the PD motor neurons. Together the AB and PD neurons form the pacemaker for the pyloric system. Synchronous depolarization of the AB and PD neurons evokes a complex inhibitory post-synaptic potential (IPSP) in PY neurons. This IPSP has two components: an early, AB neuron-derived component and a late, PD neuron-derived component. Deletion of the PD neurons from the pyloric circuit by photoinactivation removed the PD-evoked component of the pacemaker-evoked IPSP. This resulted in a decrease in the duration of the IPSP evoked by pacemaker depolarization and a significant advance in the firing phase of PY neurons. Bath application of dopamine was used to hyperpolarize and inhibit the PD neurons (30), causing them to release less neurotransmitter. As a consequence, the duration of the IPSP evoked by pacemaker depolarization was decreased and the firing phase of the PY neurons was significantly advanced. Stimulation of the inferior ventricular nerve (IVN) produces a slow excitation of the PD neurons (30), causing them to release more neurotransmitter. Consequently, the duration of the IPSP evoked by pacemaker depolarization was increased and the firing phase of the PY neurons was significantly retarded for several cycles of pyloric activity following IVN stimulation. Thus, modulation of the strength of PD-evoked inhibition in PY neurons is responsible for altering the firing phase of the PY neurons with respect to the pyloric pacemaker. We suggest that frequency of the pyloric output and the phase relations of the elements within the pyloric cycle can be regulated independently. The potential implications of these data for modulation of synaptic efficacy in other preparations are discussed.     

Models of respiratory rhythm generation in the pre-B?tzinger complex. III. Experimental tests of model predictions.

We used the testable predictions of mathematical models proposed by Butera et al. to evaluate cellular, synaptic, and population-level components of the hypothesis that respiratory rhythm in mammals is generated in vitro in the pre-B?tzinger complex (pre-B?tC) by a heterogeneous population of pacemaker neurons coupled by fast excitatory synapses. We prepared thin brain stem slices from neonatal rats that capture the pre-B?tC and maintain inspiratory-related motor activity in vitro. We recorded pacemaker neurons extracellularly and found: intrinsic bursting behavior that did not depend on Ca(2+) currents and persisted after blocking synaptic transmission; multistate behavior with transitions from quiescence to bursting and tonic spiking states as cellular excitability was increased via extracellular K(+) concentration ([K(+)](o)); a monotonic increase in burst frequency and decrease in burst duration with increasing [K(+)](o); heterogeneity among different cells sampled; and an increase in inspiratory burst duration and decrease in burst frequency by excitatory synaptic coupling in the respiratory network. These data affirm the basis for the network model, which is composed of heterogeneous pacemaker cells having a voltage-dependent burst-generating mechanism dominated by persistent Na(+) current (I(NaP)) and excitatory synaptic coupling that synchronizes cell activity. We investigated population-level activity in the pre-B?tC using local "macropatch" recordings and confirmed these model predictions: pre-B?tC activity preceded respiratory-related motor output by 100-400 ms, consistent with a heterogeneous pacemaker-cell population generating inspiratory rhythm in the pre-B?tC; pre-B?tC population burst amplitude decreased monotonically with increasing [K(+)](o) (while frequency increased), which can be attributed to pacemaker cell properties; and burst amplitude fluctuated from cycle to cycle after decreasing bilateral synaptic coupling surgically as predicted from stability analyses of the model. We conclude that the pacemaker cell and network models explain features of inspiratory rhythm generation in vitro.     

Inhibitory synchronization of bursting in biological neurons: dependence on synaptic time constant

Using the dynamic clamp technique, we investigated the effects of varying the time constant of mutual synaptic inhibition on the synchronization of bursting biological neurons. For this purpose, we constructed artificial half-center circuits by inserting simulated reciprocal inhibitory synapses between identified neurons of the pyloric circuit in the lobster stomatogastric ganglion. With natural synaptic interactions blocked (but modulatory inputs retained), these neurons generated independent, repetitive bursts of spikes with cycle period durations of approximately 1 s. After coupling the neurons with simulated reciprocal inhibition, we selectively varied the time constant governing the rate of synaptic activation and deactivation. At time constants 400 ms), bursts became phase-locked in a fully overlapping pattern with little or no phase lag and a shorter period. During the in-phase bursting, the higher-frequency spiking activity was not synchronized. If the circuit lacked a robust periodic burster, increasing the time constant evoked a sharp transition from out-of-phase oscillations to in-phase oscillations with associated intermittent phase-jumping. When a coupled periodic burster neuron was present (on one side of the half-center circuit), the transition was more gradual. We conclude that the magnitude and stability of phase differences between mutually inhibitory neurons varies with the ratio of burst cycle period duration to synaptic time constant and that cellular bursting (whether periodic or irregular) can adopt in-phase coordination when inhibitory synaptic currents are sufficiently slow.     

Red pigment concentrating hormone strongly enhances the strength of the feedback to the pyloric rhythm oscillator but has little effect on pyloric rhythm period

The neuropeptide, red pigment concentrating hormone (RPCH), strengthened the inhibitory synapse from the lateral pyloric (LP) neuron to the pyloric dilator (PD) neurons in the pyloric network of the stomatogastric ganglion (STG) of the lobster, Homarus americanus. RPCH produced several-fold increases in the amplitude of both action potential-mediated and non-impulse-mediated transmission that persisted for as long as the peptide remained present. Because the LP to PD synapse is the only feedback to the pacemaker kernel of the pyloric network, which consists of the electrically coupled two PD neurons and the anterior burster (AB) neuron, it might have been expected that strengthening the LP to PD synapse would increase the period of the pyloric rhythm. However, the period of the pyloric rhythm increased only transiently in RPCH, and a transient increase in cycle period was observed even when the LP neuron was hyperpolarized. Phase response curves were measured using the dynamic clamp to create artificial inhibitory inputs of variable strength and duration to the PD neurons. Synaptic conductance values seen in normal saline were ineffective at changing the pyloric period throughout the pyloric cycle. Conductances similar to those seen in 10(-6) M RPCH also did not evoke phase resets at phases when the LP neuron is typically active. Thus the dramatic effects of RPCH on synaptic strength have little role in modulation of the period of the pyloric rhythm under normal operating conditions but may help to stabilize the rhythm when the cycle period is too slow or too fast.     

Artificial electrotonic coupling affects neuronal firing patterns depending upon cellular characteristics

While there have been numerous theoretical studies indicating that electrotonic coupling via gap junctions interacts with the intrinsic characteristics of the coupled neurons to modify their electrical behaviour, little experimental evidence has been provided in coupled mammalian neurons. Using an artificial electrotonic junction, two distant uncoupled neurons were coupled through the computer, and the coupling conductance was varied. Tonically firing CA1 hippocampal pyramidal neurons reduced their spike firing frequency when coupled to thalamic or pyramidal cells, showing that the electrical coupling can be considered as a low-pass filter. The strength of coupling needed to entrain spike bursts of pyramidal neurons was considerably lower than the coupling needed to synchronize two neurons with different cellular characteristics (thalamic and pyramidal cells). Coupling promoted burst firing in a non-bursting cell if it was coupled to a spontaneously bursting neuron.These results support modelling studies that indicate a role for gap-junctional coupling in the synchronization of neuronal firing and the expression of low-frequency bursting.     

Intrinsic bursters increase the robustness of rhythm generation in an excitatory network

The pre-Botzinger complex (pBC) is a vital subcircuit of the respiratory central pattern generator. Although the existence of neurons with pacemaker-like bursting properties in this network is not questioned, their role in network rhythmogenesis is unresolved. Modeling is ideally suited to address this debate because of the ease with which biophysical parameters of individual cells and network architecture can be manipulated. We modeled the parameter variability of experimental data from pBC bursting pacemaker and nonpacemaker neurons using a modified version of our previously developed pBC neuron and network models. To investigate the role of pacemakers in networkwide rhythmogenesis, we simulated networks of these neurons and varied the fraction of the population made up of pacemakers. For each number of pacemaker neurons, we varied the amount of tonic drive to the network and measured the frequency of synchronous networkwide bursting produced. Both excitatory networks with all-to-all coupling and sparsely connected networks were explored for several levels of synaptic coupling strength. Networks containing only nonpacemakers were able to produce networkwide bursting, but with a low probability of bursting and low input and output ranges. Our results indicate that inclusion of pacemakers in an excitatory network increases robustness of the network by more than tripling the input and output ranges compared with networks containing no pacemakers. The largest increase in dynamic range occurs when the number of pacemakers in the network is greater than 20% of the population. Experimental tests of our model predictions are proposed.