Characteristics of pacemaker oscillations in Aplysia neurons

Organelle calcium-derived voltage oscillations in pacemaker neurons drive the motor program for food-seeking behavior in Aplysia

The expression of motivated behaviors depends on both external and internally arising neural stimuli, yet the intrinsic releasing mechanisms for such variably occurring behaviors remain elusive. In isolated nervous system preparations of Aplysia, we have found that irregularly expressed cycles of motor output underlying food-seeking behavior arise from regular membrane potential oscillations of varying magnitude in an identified pair of interneurons (B63) in the bilateral buccal ganglia. This rhythmic signal, which is specific to the B63 cells, is generated by organelle-derived intracellular calcium fluxes that activate voltage-independent plasma membrane channels. The resulting voltage oscillation spreads throughout a subset of gap junction-coupled buccal network neurons and by triggering plateau potential-mediated bursts in B63, can initiate motor output driving food-seeking action. Thus, an atypical neuronal pacemaker mechanism, based on rhythmic intracellular calcium store release and intercellular propagation, can act as an autonomous intrinsic releaser for the occurrence of a motivated behavior.

Multi-timescale systems and fast-slow analysis

Mathematical models of biological systems often have components that vary on different timescales. This multi-timescale character can lead to problems when doing computer simulations, which can require a great deal of computer time so that the components that change on the fastest time scale can be resolved. Mathematical analysis of these multi-timescale systems can be greatly simplified by partitioning them into subsystems that evolve on different time scales. The subsystems are then analyzed semi-independently, using a technique called fast-slow analysis. In this review we describe the fast-slow analysis technique and apply it to relaxation oscillations, neuronal bursting oscillations, canard oscillations, and mixed-mode oscillations. Although these examples all involve neural systems, the technique can and has been applied to other biological, chemical, and physical systems. It is a powerful analysis method that will become even more useful in the future as new experimental techniques push forward the complexity of biological models.

Two types of burst firing in gonadotrophin-releasing hormone neurones

Gonadotrophin-releasing hormone (GnRH) neurones fire spontaneous bursts of action potentials, although little is understood about the underlying mechanisms. In the present study, we report evidence for two types of bursting/oscillation driven by different mechanisms. Properties of these different types are clarified using mathematical modelling and a recently developed active-phase/silent-phase correlation technique. The first type of GnRH neurone (1-2%) exhibits slow (∼0.05 Hz) spontaneous oscillations in membrane potential. Action potential bursts are often observed during oscillation depolarisation, although some oscillations were entirely subthreshold. Oscillations persist after blockade of fast sodium channels with tetrodotoxin (TTX) and blocking receptors for ionotropic fast synaptic transmission, indicating that they are intrinsically generated. In the second type of GnRH neurone, bursts were irregular and TTX caused a stable membrane potential. The two types of bursting cells exhibited distinct active-phase/silent-phase correlation patterns, which is suggestive of distinct mechanisms underlying the rhythms. Further studies of type 1 oscillating cells revealed that the oscillation period was not affected by current or voltage steps, although amplitude was sometimes damped. Oestradiol, an important feedback regulator of GnRH neuronal activity, acutely and markedly altered oscillations, specifically depolarising the oscillation nadir and initiating or increasing firing. Blocking calcium-activated potassium channels, which are rapidly reduced by oestradiol, had a similar effect on oscillations. Kisspeptin, a potent activator of GnRH neurones, translated the oscillation to more depolarised potentials, without altering period or amplitude. These data show that there are at least two distinct types of GnRH neurone bursting patterns with different underlying mechanisms.

Modeling interactions between electrical activity and second-messenger cascades in Aplysia neuron R15

The biophysical properties of neuron R15 in Aplysia endow it with the ability to express multiple modes of oscillatory electrical activity, such as beating and bursting. Previous modeling studies examined the ways in which membrane conductances contribute to the electrical activity of R15 and the ways in which extrinsic modulatory inputs alter the membrane conductances by biochemical cascades and influence the electrical activity. The goals of the present study were to examine the ways in which electrical activity influences the biochemical cascades and what dynamical properties emerge from the ongoing interactions between electrical activity and these cascades. The model proposed by Butera et al. in 1995 was extended to include equations for the binding of Ca(2+) to calmodulin (CaM) and the actions of Ca(2+)/CaM on both adenylyl cyclase and phosphodiesterase. Simulations indicated that levels of cAMP oscillated during bursting and that these oscillations were approximately antiphasic to the oscillations of Ca(2+). In the presence of cAMP oscillations, brief perturbations could switch the electrical activity between bursting and beating (bistability). Compared with a constant-cAMP model, oscillations of cAMP substantially expanded the range of bistability. Moreover, the integrated electrical/biochemical model simulated some early experimental results such as activity-dependent inactivation of the anomalous rectifier. The results of the present study suggest that the endogenous activity of R15 depends, in part, on interactions between electrical activity and biochemical cascades.

Models of respiratory rhythm generation in the pre-Bötzinger complex. I. Bursting pacemaker neurons

A network of oscillatory bursting neurons with excitatory coupling is hypothesized to define the primary kernel for respiratory rhythm generation in the pre-Bötzinger complex (pre-BötC) in mammals. Two minimal models of these neurons are proposed. In model 1, bursting arises via fast activation and slow inactivation of a persistent Na+ current INaP-h. In model 2, bursting arises via a fast-activating persistent Na+ current INaP and slow activation of a K+ current IKS. In both models, action potentials are generated via fast Na+ and K+ currents. The two models have few differences in parameters to facilitate a rigorous comparison of the two different burst-generating mechanisms. Both models are consistent with many of the dynamic features of electrophysiological recordings from pre-BötC oscillatory bursting neurons in vitro, including voltage-dependent activity modes (silence, bursting, and beating), a voltage-dependent burst frequency that can vary from 0.05 to >1 Hz, and a decaying spike frequency during bursting. These results are robust and persist across a wide range of parameter values for both models. However, the dynamics of model 1 are more consistent with experimental data in that the burst duration decreases as the baseline membrane potential is depolarized and the model has a relatively flat membrane potential trajectory during the interburst interval. We propose several experimental tests to demonstrate the validity of either model and to differentiate between the two mechanisms.

Dissection and reduction of a modeled bursting neuron

An 11-variable Hodgkin-Huxley type model of a bursting neuron was investigated using numerical bifurcation analysis and computer simulations. The results were applied to develop a reduced model of the underlying subthreshold oscillations (slow-wave) in membrane potential. Two different low-order models were developed: one 3-variable model, which mimicked the slow-wave of the full model in the absence of action potentials and a second 4-variable model, which included expressions accounting for the perturbational effects of action potentials on the slow-wave. The 4-variable model predicted more accurately the activity mode (bursting, beating, or silence) in response to application of extrinsic stimulus current or modulatory agents. The 4-variable model also possessed a phase-response curve that was very similar to that of the original 11-variable model. The results suggest that low-order models of bursting cells that do not consider the effects of action potentials may erroneously predict modes of activity and transient responses of the full model on which the reductions are based. These results also show that it is possible to develop low-order models that retain many of the characteristics of the activity of the higher-order system.

Properties of presympathetic neurones in the rostral ventrolateral medulla in the rat: an intracellular study "in vivo'

1. Intracellular recordings were made in pentobarbitone-anaesthetized rats from sixty-eight neurones located in the rostral ventrolateral medulla (RVLM), which responded with inhibition (latency, 33.6 +/- 9.3 ms) after stimulation of the aortic depressor nerve with short bursts of pulses. This inhibition was due to chloride- and voltage-dependent IPSPs. 2. Seventeen neurones could be excited antidromically after stimulation in the T2 spinal segment (conduction velocity 1.9-8.5 m.s-1) and were classified as RVLM presympathetic vasomotor neurones. 3. "Spontaneously' active neurones (n = 29) displayed a largely irregular pattern of firing, with no clear relationship between the level of the membrane potential and cycles of phrenic nerve activity at end-tidal CO2 < 5.0%. Cardiac cycle-related shifts of the membrane potential were not considered indicative of baroreceptor input as they could be due to movement artifacts. 4. All neurones displayed large synaptic activity (EPSPs and IPSPs, peak-to-peak amplitude > 5.0 mV). The depolarizing IPSPs observed during injection of chloride and/or negative current consisted of a phasic and a tonic component. 5. The on-going activity of these neurones resulted from synaptic inputs, with individual action potentials usually preceded by identifiable fast EPSPs. 6. No evidence was found for the presence of gradual depolarizations (autodepolarizations) between individual action potentials, and therefore under these experimental conditions the activity of RVLM presympathetic neurones did not depend on intrinsic pacemaker properties. 7. These results are consistent with the "network' hypothesis for the generation of sympathetic vasomotor tone.

C fiber generates a slow Na+ spike in the frog sciatic nerve

The present study was carried out to examine the properties of A and C fibers in the bullfrog sciatic nerves by applying several agents through perfusing solutions between stimulating and recording electrodes. The compound action potentials (CAPs) of A beta and A delta fibers were tetrodotoxin (TTX)-sensitive and were abolished in Na(+)-free solution. However, C fiber CAP was TTX-insensitive although CAP disappeared in Na(+)-free solution. Moreover, C fiber CAP was not blocked by Ca2+ channel blockers and its chronaxy (2 ms) and conduction velocity (0.70 m/s) indicate that the time constant of C fiber CAP is relatively large (2.88 ms). These suggest that a slow Na+ channel, which is TTX-resistant, contributes to C fiber action potentials.

Slow membrane currents in bursting pace-maker neurones of Tritonia

1. Membrane ionic currents in bursting pace-maker neurones of the marine mollusc Tritonia were studied in voltage-clamp experiments with emphasis on slow tail current relaxations after depolarizing pulses. 2. The slow tail current undergoes a complex transition from an initially inward current to an initially outward current as the duration of the depolarizing pulse is lengthened. It was found that the slow tail current is the sum of two separate and independent ionic currents. Methods were devised to study each current in isolation. 3. A slow inward tail current, termed IB, is activated by depolarization and decays exponentially on return to -55 mV with a time constant of 2-4 s. The voltage dependence and kinetics of IB activation were measured. Current amplitude is sensitive to removal of both Na+ and Ca2+ from the bathing medium but the current is not blocked by either tetrodotoxin (TTX) or replacement of Ca2+ by Co+. The amplitude of the current is independent of the external K+ concentration. 4. A slow outward tail current, termed IC, is also activated by depolarization. It is shown to be a K+ current whose activation results from an increase in the cytoplasmic Ca2+ concentration during depolarization. The decay of IC on repolarization requires more than 30 s to reach completion. 5. The slow rates of relaxation of IB and IC tail currents suggest that they are important determinants of the slow membrane potential variations characteristic of burst firing. IB activates more rapidly than IC during depolarization and is thought to be important for maintaining the depolarized phase of the burst cycle and for producing the depolarizing after-potential after each spike. IC activates more slowly but reaches greater amplitudes. It is thought to be important for adaptation in spike frequency during the burst, for burst termination, and for determining the duration of the interval between bursts.

Effect of ethanol on subthreshold currents of Aplysia pacemaker neurons

The subthreshold currents in bursting pacemaker neurons of the Aplysia abdominal ganglion were individually studied with the voltage clamp technique for sensitivity to 4% ethanol. The most prevalent effect of ethanol on unclamped bursting neurons was a hyperpolarization. This was shown to be due to a decrease of a voltage independent inward leakage current. Direct measurement of the Na-dependent slow inward current showed that this current was eliminated by 4% ethanol. Direct measurement of the Ca-dependent slow inward current showed that this current was substantially reduced by 4% ethanol. Injection of EGTA into cell bodies did not eliminate the ethanol-induced block of the slow inward calcium current. Thus, ethanol cannot be reducing the Ca-dependent slow inward current solely by an increase of internal calcium concentration. The effect of ethanol on voltage dependent outward current was measured by blockage of all inward current. The peak outward current was increased by ethanol. The rate of inactivation of this outward current was also increased. Calcium activated potassium current (IK(Ca)) is particularly complicated in its response to ethanol because it is dependent on both Ca and voltage for its activation. The level of IK(Ca) elicited in response to constant Ca injection was increased by ethanol treatment. The level of this current as activated by voltage clamp pulses was either increased or decreased depending on the neuron type. Ca2+ activated potassium conductance increased e-fold for a 26 mV depolarization in membrane holding potential. Ethanol decreased this voltage dependence to e-fold for a 55 mV change in potential. This result was interpreted to mean that ethanol shifted an effective Ca2+ binding site of these channels from about halfway through the membrane field to one quarter of the way across. The same theoretical approach allowed the further conclusion that ethanol caused an increased internal free calcium concentration probably by decreasing calcium binding by intracellular buffers.

Abnormal discharges and chaos in a neuronal model system

Using the mathematical model of the pacemaker neuron formulated by Chay, we have investigated the conditions in which a neuron can generate chaotic signals in response to variation in temperature, ionic compositions, chemicals, and the strength of applied depolarizing current.

Ionic requirements for membrane oscillations and their dependence on the calcium concentration in a molluscan pace-maker neurone

1. Membrane currents from the bursting pace-maker neurone R-15 of Aplysia were measured under conditions designed to simulate membrane oscillations. Changes in the absorbance of the Ca(2+)-sensitive dye arsenazo III were used to monitor changes in the free intracellular Ca(2+) concentration, [Ca](i), under these conditions. In addition, changes in the extracellular K(+), concentration [K](o) were measured with K(+)-sensitive electrodes.2. In normal external ionic conditions the depolarizing phase of pace-maker activity was associated with a slow inward current and the hyperpolarizing phase with a slow outward current.3. In cells where the early inward Na(+) current was blocked by tetrodotoxin and outward K(+) currents were suppressed by intracellular EGTA and extracellular tetraethylammonium and 4-aminopyridine, the slow inward current was significantly larger in amplitude and was suppressed by removal of external Ca(2+) or the addition of external La(3+), but not by the removal of external Na(+).4. The slow inward current was increased when [Ca](o) was raised and decreased when it was reduced in the manner expected for current flow through a Ca(2+) channel. The selectivity of the slow inward current for divalent cations was [Formula: see text].5. The slow inward current was only slightly reduced by a 10 degrees C reduction in temperature.6. In normal external and internal ionic conditions changes in dye absorbance occurred when the membrane was depolarized with slow triangular voltage ramps or long depolarizing steps within the pace-maker oscillation range. The obsorbance change, and thus the increase in Ca(2+), [Ca](i), was well correlated with the appearance of the slow inward current. Moreover, the magnitude of the slow outward current was dependent upon the change in [Ca](i).7. The slow inward current and a substantial fraction of the outward current, as well as the change in [Ca](i), were reduced appreciably by the addition of La(3+) ions (3 mM) to the external medium.8. The increase in [Ca](i) during prolonged depolarization was not affected by external tetrodotoxin or by the removal of external Na(+), but was abolished by a Ca(2+)-free external medium containing EGTA. Nevertheless, significant changes occurred in [Ca](i) during depolarization in 0.1 mM-external Ca(2+).9. In normal external and internal ionic conditions extracellular K(+), [K](o), increased during the depolarizing phase of the pace-maker cycle and decayed during the hyperpolarizing phase.10. There was a measurable increase in [K](o) during small prolonged depolarizing steps which produced a net inward current, indicating that inward and outward currents overlap under normal conditions.11. In the absence of action potential discharge, [Ca](i) increased during the depolarizing phase and decreased during the hyperpolarizing phase of the membrane oscillation.12. It is proposed that pace-maker oscillations depend upon three separate but linked systems which include a voltage-dependent Ca(2+) current, the free intracellular Ca(2+) concentration and the Ca(2+)-activated K(+) current.

Synaptic mechanisms that generate network oscillations in the absence of discrete postsynaptic potentials

Synaptic mechanisms were examined in the pyloric network of the lobster stomatogastric which generate network oscillations in the absence of discrete postsynaptic potentials (PSPs). In normal saline, the unstimulated pyloric network underwent weak bursting in only a few cells. Stimulation of the input nerve, or bath application of the input neurotransmitter dopamine, produced similar vigorous bursting in many pyloric neurons. In saline-containing tetrodotoxin (TTX) plus dopamine, action potentials and corresponding discrete PSPs were blocked, but the underlying slow wave oscillations in network neurons continued. No oscillations occurred in TTX-saline without dopamine. The generation of these nonspiking network oscillations can be explained by the interaction between two synaptic mechanisms which do not produce discrete PSPs: neurotransmitter activation of bursting pacemaker oscillations in a single network neuron, and graded inhibition between network neurons.

Phase plane description of endogenous neuronal oscillators in Aplysia

Phase plane techniques are used to describe graphically the limit cycle behavior of identified endogenous neuronal oscillators in the isolated abdominal ganglion of Aplysia. Intracellularly recorded membrane potential from a bursting neuron and its first derivative with respect to time are used as coordinates (state variables) in phase space. The derivative is either measured electronically or calculated digitally. Each trajectory in phase space represents the entire output of the bursting neuron, i.e., both the rapid action potentials and slow pacemaker potentials. Phase plane portraits are presented for the free run limit cycle before and after a change in a system parameter (applied transmembrane current) and also for phase resetting produced by direct synaptic inhibition from an identified interneuron. The complex topology of the trajectory suggests that the bursting oscillator is a higher order system. Therefore, the second time derivative is used as another state variable. This type of phase plot can help to relate biophysical and mathematical analyses.

Bifurcation and resonance in a model for bursting nerve cells

In this paper we consider a model for the phenomenon of bursting in nerve cells. Experimental evidence indicates that this phenomenon is due to the interaction of multiple conductances with very different kinetics, and the model incorporates this evidence. As a parameter is varied the model undergoes a transition between two oscillatory waveforms; a corresponding transition is observed experimentally. After establishing the periodicity of the subcritical oscillatory solution, the nature of the transition is studied. It is found to be a resonance bifurcation, with the solution branching at the critical point to another periodic solution of the same period. Using this result a comparison is made between the model and experimental observations. The model is found to predict and allow an interpretation of these observations.

The effects of calcium++ on bursting neurons. A modeling study

Many observed effects of ionized calcium on bursting pacemaker neurons may be accounted for by assuming that calcium has multiple effects on the membrane conductance mechanisms. Two models are proposed that represent extreme cases of a set of possible models for these multiple effects. Both models are a priori designed to account for directly observed phenomena, and both are found to be able to simulate a posteriori certain observed phenomena, including persistent inactivation, increasing spike width, and decreasing after-polarization. Experimental tests are proposed for the decision of validity between the set of models discussed and the null hypothesis, and for the decision of validity between the two models themselves. Extensions of the models are discussed. One of these extensions leads to a simulation of the behavior of the cell when placed in a calcium-free bathing medium.

Short-term modulation of endogenous bursting rhythms by monosynaptic inhibition in Aplysia neurons: effects of contingent stimulation

A presynaptic neuron fires a high-frequency train of spikes that produces long-lasting synaptic inhibition that modulates the bursting rhythm in a small population of endogenous bursting neurons in the left upper quadrant of the isolated abdominal ganglion of Aplysia. Single inputs decrease or increase the duration of the burst cycle as a function of the precise phase of the input (the phase response curve). Two phases of the burst cycle were used to analyze the effects of repeated contingent (phase-locked) stimulation. One contingency involved synaptic input early in the burst cycle that inhibited spikes and decreased the duration, whereas the other contingency involved input late in the cycle that increased the duration. Under both contingencies of stimulation, buildup and short-term persistence were found, however these cumulative effects were not dependent upon the phase of the burst cycle. The locus of the short-term plasticity that underlies the buildup and persistence is in the pacemaker properties of the postsynaptic cell rather than in the synapse. The plastic change appears to involve a nonspecific postinhibitory rebound that follows a single input and builds up with repetition. These results support the suggestion that endogenous rhythms of pacemaker cells can undergo plastic changes and can therefore serve as a means of short-term information storage in the nervous system. However, this neuronal circuit does not have the specificity required to mediate operant conditioning.

Metabolic regulations of the rhythmic activity in pacemaker neurons. II. Metabolically induced conversions of beating to bursting pacemaker activity in isolated Aplysia neurons

In pacemaker neurons of the sea hare Aplysia californica, isolated from their synaptic, ephaptic and humoral inputs, conversion of the regular beating to a bursting discharge pattern can be induced by certain cell metabolites. Administration of the phosphofructokinase (PFK) activator fructose-6-phosphate (F-6-P), or its nonmetabolizable analogue 1-deoxy-F-6-P, induced bursting discharges in R3, R5, R6 and R11 neurons, with spike doublets and triplets appearing transiently in the time pattern. With another PFK activitor, adenosine-5-monophosphate, only double spikes have been noted in R7, R8 and R14 neurons. Burst activity was induced also in the presence of the fructose-1,6-diphosphatase activators, citrate and 3-phosphoglycerate, in R9, R10 and R12 neurons. Cyclic 3',5'-AMP, which also activates the PFK (beside other effects on cellular metabolism), induced bursting discharges in all R3-R14 neurons. In contrast, the inhibitors of the PFK, citrate and ATP, decreased the spike activity of the bursting L3 and L6 neurons, even changing L3 neurons to the regular beating type. Among a variety of cell metabolites tested only pyruvate was able to induce a burst-like tendency in R9 neurons. The characteristic bursting patterns which appeared in the presence of the described metabolic effectors could not be duplicated by low Ca2+ and/or high K+ media nor by artificial shifts in membrane potential triggered by depolarizing and hyperpolarizing currents.

Mathematical description of a bursting pacemaker neuron by a modification of the Hodgkin-Huxley equations

Modifications based on experimental results reported in the literature are made to the Hodgkin-Huxley equations to describe the electrophysiological behavior of the Aplysia abdominal ganglion R15 cell. The system is then further modified to describe the effects with the application of the drug tetrodotoxin (TTX) to the cells' bathing medium. Methods of the qualitative theory of differential equations are used to determine the conditions necessary for such a system of equations to have an oscillatory solution. A model satisfying these conditions is shown to preduct many experimental observations of R15 cell behavior. Numerical solutions are obtained for differential equations satisfying the conditions of the model. These solutions are shown to have a form similar to that of the bursting which is characteristic of this cell, and to preduct many results of experiments conducted on this cell. The physiological implications of the model are discussed.

Studies on bursting pacemaker potential activity in molluscan neurons. I. Membrane properties and ionic contributions

Bursting pacemaker potential (BPP) activity of identified molluscan neurons has been studied using cells from Aplysia and Otala. The results presented in this paper indicate that (1) a potassium conductance mediates the hyperpolarizing phase of the BPP; (2) the BPP amplitude is directly dependent on [Na+]0; (3) BPP activity requires the presence of divalent cations and is prevented by C02+ and La3+, but not D-600; (4) the apparent increase in membrane resistance during the depolarizing phase of the Bd can be accounted for by the movement of the membrane potential along the non-linear portion of the I-V curve; and (5) non-linear I-V relations and a minimal effective membrane resistance are pre-requisite to BPP generation. Coupled with recent observations on the presence of an inward current in these cells, the results suggest that the mechanisms underlying the BPP are similar to those proposed to describe the myocardial pacemaker potential: the hyperpolarizing phase is due to activation of a potassium conductance which slowly inactivates, resulting in a gradula deplorization until a voltage-dependent inward current is activated which then leads to an increasingly rapid deplorization and initiation of the burst of spikes. It would appear that Na+ may play the major role in carrying the inward current, although a secondary role for divalent cations cannot be discounted.

Cyclic variation of potassium conductance in a burst-generating neurone in Aplysia

1. The hyperpolarization between bursts in the R 15 cell of Aplysia is accompanied by an increase in membrane slope conductance.2. The post-burst hyperpolarization can be observed with ouabain, lithium, or potassium-free solution if artificial inward current is applied. The hyperpolarization can be observed with dinitrophenol or cooling to 10 degrees C, with no injected current. Thus, the hyperpolarization apparently is not due to the cyclic activity of an electrogenic pump.3. A reversal potential for the post-burst hyperpolarization can be demonstrated by passage of inward current during the inter-burst period. The reversal of direction of the potential depends on recent occurrence of a burst.4. The reversal potential varies with external potassium concentration, but not with chloride or sodium.5. The post-burst hyperpolarization is not blocked by external tetraethylammonium at a concentration which greatly prolongs the action potentials.6. During the onset of spike blockage by, and recovery from, calcium-free+tetrodotoxin saline, the bursts of action potentials appear to be driven by endogenous waves of membrane potential.7. The hyperpolarizing phase of the waves in calcium-free+tetrodotoxin medium is accompanied by an increased slope conductance.8. A reversal potential can be demonstrated for the hyperpolarization following a wave in calcium-free+tetrodotoxin medium by applying inward current during the interwave period.9. The waves in calcium-free+tetrodotoxin medium are blocked by ouabain but can be reinstated by artificial hyperpolarization.10. The post-burst hyperpolarization and the post-wave hyperpolarization appear to result from a periodic increase in membrane conductance, primarily to potassium ions.

Tetrodotoxin desensitization in aggregates of embryonic chick heart cells

Spontaneous beating of heart-cell aggregates from 4-day chick embryos was initially blocked by 10(-5) g/ml tetrodotoxin (TTX). With continued exposure to the drug, the fraction of blocked aggregates decreased from about 80% at 15 min to about 25% at 2-3 h, at which time, beating aggregates had become desensitized to the toxin, showing no response to a fresh dose. Aggregates from 5-day hearts were more sensitive to TTX, but fewer became desensitized in its presence. Desensitization to TTX was not seen in 6- and 7-day aggregates. Inhibition of protein synthesis by cycloheximide did not affect beating or initial sensitivity to TTX of 4-day aggregates, but desensitization failed to occur. Before TTX, the mean value of maximal upstroke velocity (V(max)) of the action potentials in 4-day aggregates was 33 V/s. After desensitization V(max) was 12 V/s. Activity of desensitized aggregates in the presence of TTX was augmented by elevated calcium levels, and suppressed by presumed inhibitors of slow inward current (manganese, D600). Desensitization was reversible; upon removal of TTX 10(-5) g/ml, aggregates regained their responsiveness to a fresh dose of the drug with a 2-3 h time-course similar to that of desensitization. This was prevented by continued exposure to TTX at concentrations as low as 10(-8) g/ml. These data suggest that (a) desensitization involves a change in the mode of action-potential generating from one involving Na-specific, TTX-sensitive channels to one utilizing slower Mn-sensitive channels; (b) the process of desensitization occurs over a period of 2-3 h and is dependent upon the products of protein synthesis; and (c) desensitization is reversible after removal of TTX over a 2-3 h time-course similar to its onset.

Development of sensitivity to tetrodotoxin in beating chick embryo hearts, single cells, and aggregates

The spontaneous activity of intact embryonic heart becomes progressively more sensitive to tetrodotoxin block with increasing age of the embryo. The activity of isolated single heart cells in culture was relatively insensitive, independent of embryo age. Aggregates formed from single cells responded to tetrodotoxin in the same manner as intact hearts; aggregated cells from older hearts were sensitive.