Membrane characteristics of bursting pacemaker neurones in Aplysia

Invertebrate preparations and their contribution to neurobiology in the second half of the 20th century

This review summarized the contribution to neurobiology achieved through the use of invertebrate preparations in the second half of the 20th century. This fascinating period was preceded by pioneers who explored a wide variety of invertebrate phyla and developed various preparations appropriate for electrophysiological studies. Their work advanced general knowledge about neuronal properties (dendritic, somatic, and axonal excitability; pre- and postsynaptic mechanisms). The study of invertebrates made it possible to identify cell bodies in different ganglia, and monitor their operation in the course of behavior. In the 1970s, the details of central neural circuits in worms, molluscs, insects, and crustaceans were characterized for the first time and well before equivalent findings were made in vertebrate preparations. The concept and nature of a central pattern generator (CPG) have been studied in detail, and the stomatogastric nervous system (STNS) is a fine example, having led to many major developments since it was first examined. The final part of the review is a discussion of recent neuroethological studies that have addressed simple cognitive functions and confirmed the utility of invertebrate models. After presenting our invertebrate "mice," the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, our conclusion, based on arguments very different from those used fifty years ago, is that invertebrate models are still essential for acquiring insight into the complexity of the brain.

Bursting responses of Lymnea neurons to microwave radiation

Microelectrode and voltage-clamp techniques were modified to record spontaneous electrical activity and ionic currents of Lymnea stagnalis neurons during exposure to a 900-MHz field in a waveguide-based apparatus. The field was pulse-modulated at repetition rates ranging from 0.5 to 110 pps, or it was applied as a continuous wave (CW). When subjected to pulsed waves (PW), rapid, burst-like changes in the firing rate of neurons occurred at SARs of a few W/kg. If the burst-like irregularity was present in the firing rate under control conditions, irradiation enhanced its probability of occurrence. The effect was dependent on modulation, but not on modulation frequency, and it had a threshold SAR near 0.5 W/kg. CW radiation had no effect on the firing rate pattern at the same SAR. Mediator-induced, current activation of acetyl-choline, dopamine, serotonin, or gamma-aminobutyric-acid receptors of the neuronal soma was not altered during CW or PW exposures and, hence, could not have been responsible for the bursting effect.

The A-type potassium current: catechol-induced blockage in snail neurons

The effects of catechol (1-12.5 mM) on membrane properties, action potential and membrane ionic currents were investigated in identified snail neurons under current- and voltage-clamp conditions. Catechol hardly influenced the resting membrane potential, or the action potential amplitude and duration, but it increased the spike voltage threshold and slightly decreased the input resistance. Catechol specifically decreased the amplitude of the potassium A-currents in a dose-dependent way (Kd = 5 mM), without significant modulation of other potassium currents. The time constants of decay of A-current increased and the steady-state activation or inactivation curve shifted to more positive potentials in the catechol solutions. The blocking effect of catechol on A-currents followed a one-to-one binding stoichiometry (nH = 0.8).

Serotonin increases an anomalously rectifying K+ current in the Aplysia neuron R15

Previous work has shown that serotonin causes an increase in K+ conductance in the identified Aplysia neuron R15. This response is mediated by cAMP-dependent protein phosphorylation. The results presented here show that the K+ channel modulated by serotonin is an anomalous or inward rectifier (designated IR) that is present in R15 together with the three other distinct K+ channels previously described for this cell. Several lines of evidence indicate that this inward rectifier is partially activated in the resting cell and is further activated by serotonin. Voltage clamp analysis of resting and serotonin-evoked membrane currents at various external K+ concentrations shows that both currents have reversal potentials close to the potassium equilibrium potential, exhibit similar dependences in magnitude on external K+ concentration, and display marked anomalous rectification. The effects of particular monovalent and divalent cations are also similar on the resting and serotonin-evoked currents. Rb+, Cs+, and Ba2+ block both currents while Tl+ can substitute for K+ as a charge carrier and channel activator in both. These properties are characteristics of anomalous rectifiers in other systems. Furthermore, measurement of the voltage dependence of inactivation for the fast transient K+ current shows that this current cannot account for the anomalously rectifying K+ conductance in R15. The inward rectifier is therefore a separate current mediated by its own channels, the activity of which can be modulated by serotonin.

Mechanisms of action of cyclic nucleotides on a bursting pacemaker and silent neuron in Aplysia

Cyclic nucleotides are believed to mediate a long-lasting synaptic hyperpolarization in the bursting pacemaker neuron, R15, and are capable of inducing bursting pacemaker activity in the usually silent metacerebral giant cell. Steady state voltage clamp techniques were used to examine the alterations of membrane characteristics produced in these different cell types by cyclic nucleotides. In both cells, IBMX, a phosphodiesterase inhibitor, increased two components: (1) voltage-dependent sodium current and (2) slope conductance believed to reflect potassium flux. The effects of 8-benzylthio-cAMP were identical to those of IBMX in the metacerebral cell. In R15, 8-benzylthio-cAMP affected only the slope conductance. These results are discussed in terms of cyclic nucleotide of bursting pacemaker activity.

Alteration of neuronal activity in response to cyclic nucleotide agents in Aplysia

Responsiveness of Aplysia neurons to agents that affect cyclic nucleotide levels was not limited to neurons exhibiting a spontaneous bursting activity pattern. Analyses of the I-V relationship elicited by triangular current ramps within cells exposed to different agents presumably causing elevated cyclic nucleotide levels showed qualitatively similar alterations. These included an increased slope conductance at more negative potentials, possibly related to anomalous rectification, and the induction of a hysteresis in response to a triangular ramp. The paired metacerebral giant cells showed induction of synchronous bursting when exposed to phosphodiesterase inhibitors, and we examined this phenomenon more closely. Classical methods to inactivate a presynaptic source did not eliminate the induction of synchronous bursting. Intracellular injection of a phosphodiesterase inhibitor into a metacerebral giant cell caused changes in the current-voltage relationship similar to those described above for other cells. Subsequent perfusion with the inhibitor caused an enhancement of these effects and the induction of bursting. The alteration of the current-voltage plot in the metacerebral cells and abdominal ganglion cells is qualitatively similar to that induced in the similarly treated bursting neuron R15, suggesting a similar mechanism of action in both burster and nonburster neurons. The implications of these results for cyclic nucleotide mediation of neuronal events are discussed.

Single cell isolation: a way to examine network interactions

A procedure for isolating identified, small neurons from snail ganglia is described. The technique allows a particular neuron, previously identified by morphological and electrophysiological characteristics, to be marked and then isolated from the ganglia. This procedure was developed to permit the detailed comparison of the electrical characteristics of a neuron before and after isolation from an intact system. An earlier description has appeared. The cell somata is marked intracellularly by the iontophoretic injection of Procion navy blue H3RS which visually differentiates the cell from other cells in the ganglion. The ganglion is then treated with a trypsin-haluronidase solution to soften the ganglion sheath, which is then removed. The cells are gently shaken to isolate them from the ganglion and then examined electrophysiologically. A comparison of membrane properties, such as action potential height, duration and rate of rise and decay was made before and after all treatments were applied to assess deleterious effect. An analysis of network properties, such as burst duration, number of spikes per burst and presynaptic activity was also performed after each phase of the procedure. No significant differences were noted after dye injection, enzyme treatment, and where appropriate, after isolation. An increase in input resistance and corresponding decrease in the slope of the steady state current--voltage plot (I--V plot) were observed after isolation of a cell. These were expected results of removing the 'load' (i.e. axon or electrical coupling) from the cell soma. This method may be applied to many other systems to study the effects of network interactions on the properties of a single cell and should therefore facilitate the analysis of neuronal networks as well as single cell properties.

Duplication of a spontaneously active neuron in Aplysia: electrical coupling and effects of a phosphodiesterase inhibitor

An abdominal ganglion from an Aplysia californica is described, in which cell R15 has anomalously duplicated. The two cells exhibited a high degree of electrical coupling, assuring functional synchrony of output in the cells, which are characterized by a complex firing pattern. Exposure of this ganglion to the phosphodiesterase inhibitor IBMX caused a more altered firing rhythm in one of the cells, as well as an enhanced inhibitory component associated with the coupling potentials between cells, resulting in a loss of synchrony between the two cells.

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.

The effects of pentylenetetrazol on molluscan neurons. II. Voltage clamp studies

The effects of pentylenetetrazol (PTZ) upon the steady and transient outward ionic currents during PTZ-induced prolonged depolarizations were investigated using voltage clamp techniques. PTZ causes a 5-35% reduction in gL and a 40-60% reduction in steady-state gK. There is also a marked reduction in the activation of gA of Connor and Stevens6 at all clamp potentials; a shortening of the time constant for the inactivation of gA; and a 10-15 mV shift in the depolarizing direction of the curve relating the steady-state inactivation of gA to membrane potential. The equilibrium potentials for both gA and gK are depolarized by 20 mV in PTZ solution. Equation and voltage clamp data for normal repetitive firing were integrated with the normal and PTZ-alered data. Solution to these equations demonstrated: (1) normal repetitive firing in response to a constant current stimulus; and (2) PTZ-altered repetitive firing that was in the direction of, and for the most part, similar to the observed behavior.

Diphenylhydantoin: action of a common anticonvulsant on bursting pacemaker cells in Aplysia

A commonly used anticonvulsant, diphenylhydantoin (Dilantin), decreases the bursting pacemaker activity in certain cells of Aplysia. Dilantin decreases this bursting activity whether it is endogenous to the cell or induced by a convulsant agent. The sodium-dependent negative resistance characteristic which is essential for bursting behavior is reduced in the presence of Dilantin.

Peptide regulation of bursting pacemaker activity in a molluscan neurosecretory cell

Vasopressin and related peptides (10(-9) to 10(-6) molar) induced bursting pacemaker potential activity and altered the current-voltage relations of the membrane in a specific molluscan neurosecretory cell. These effects long outlasted the period of application of the peptides. Sensitivity of the cell to these peptides was primarily localized on the axon hillock region. The observed effects do not resemble conductance changes evoked by conventional neurotransmitters, but rather suggest a membrane regulatory role for these peptides, and thus may be indicative of a new form of information transfer in the nervous system.

Strychnine- and pentylenetetrazol-induced changes of excitability in aplysia neurons

In Aplysia neurons isolated from their synaptic input strychnine induces doublet discharges associated in voltage clamp with a decrease in the threshold for the inward current and a reduction and delayed onset of the outward current. Pentylenetetrazol causes oscillations and bursting behavior in normally silent cells together with an increased inactivation of the delayed outward current and induced or enhanced anomalous rectification.