Tetrodotoxin-resistant divalent action potentials in an axon of Aplysia

Activity-related calcium dynamics in lamprey motoneurons as revealed by video-rate confocal microscopy

In lamprey spinal cord, intracellular calcium ([Ca2+]i) plays a key role in mechanisms regulating neuronal activity in the segmental network for locomotion. In this report, measurements of [Ca2+]i with fluo-3 in various regions of motoneurons in the intact spinal cord were obtained on a high speed confocal microscope following electrical stimulation. Likewise, rhythmic calcium fluctuations within dendrites and axons were seen during "fictive swimming" and were directly correlated with electrical activity. Antidromic stimulation of motoneuron axons induced large calcium transients and revealed spatially restricted "hot spots," both of which required external calcium and were blocked by nickel, but not by known calcium channel antagonists. These results suggest that lamprey spinal cord axons may possess a pharmacologically novel class of calcium channel.

Action potential propagation and threshold parameters in inhomogeneous regions of squid axons

The squid giant axon was used as a model system in which to determine the independent contributions of membrane excitability and diameter changes to threshold parameters and propagation of action potentials in inhomogeneous regions. The membrane excitability of a segment of an axon was altered by changes in the bathing solution, while its effective electrical diameter was increased by the insertion of a low-resistance axial wire. In computer simulations of these experiments, similar alterations were made in the membrane's conductance and axon's diameter. The inflexions in the shapes of action potentials propagating into a region with abrupt decreases in axial resistance become more pronounced when the interval between impulses was shortened. At short intervals, propagation of the second impulse failed. In contrast, reduction of membrane excitability produced inflexion-free changes in action potential shape and allowed a close-following second impulse to pass through the inhomogeneity. A combined decrease in membrane excitability and increase in diameter of the same region exaggerated the changes in action potential shape characteristic of the diameter increase alone. Threshold parameters were obtained from 'strength-duration' excitability relationships measured by injection of current at different points along the axon. When only the membrane excitability was reduced, threshold characteristics changed smoothly from one region of the nerve to another. In contrast, lowering the internal resistance or increasing the diameter in one region of a nerve lowered the time constant of excitation and the threshold for brief (relative to rheobasic) current stimuli in the small-diameter region near the transition while raising them in the larger-diameter region.

Evidence for initiation of calcium spikes in C-cells of the rabbit nodose ganglion

In Na+-free solution or in tetrodotoxin (TTX, 10(-6) M) solution, direct electrical stimulation of nodose ganglion C-cells of the rabbit elicited regenerative action potentials. The amplitude of the action potentials generated in these neurons is dependent on the external Ca2+ concentration ([Ca2+]0). These action potentials were characterized by reduced amplitude, prolonged duration, and graded responses to changes of the stimulation intensity. Either removal of [Ca2+]0 or application of organic Ca2+-blocking agents, diltiazem (10(-5) M) or verapamil (10(-5) M), abolished these action potentials. The conduction of action potentials along the axon was blocked in Na+-free solution or by application of TTX. The present results provide evidence for the initiation of Ca2+-dependent action potentials in the soma membrane of mammalian nodose ganglia, and suggest that the Ca2+ ion plays an important role in the development of action potentials in the C-cell of rabbit nodose ganglia.

Ca-K bi-ionic action potential in squid giant axons

Under intracellular perfusion with a solution containing K+ as the sole cation species, squid giant axons were found to be capable of developing all-or-none action potentials when immersed in a medium in which CaCl2 was the only electrolyte. The adequate range of ion concentration for demonstrating this capability was mentioned. The reversal potential level measured by the voltage-clamp technique varied directly with the logarithm of the concentration of extracellular Ca-ion; the proportionality constant was close to RT/2F. The action potential observed under this Ca-K bi-ionic condition could not be suppressed by addition of tetrodotoxin or saxitoxin to the external medium. The external Ca-ion could be replaced with Co- or Mn-ion without eliminating the capability of the axons to develop action potentials. D-600 could not suppress the inward current observed under the voltage-clamp condition, but 4-aminopyridine could suppress it. The experimental findings were interpreted based on the current channel hypothesis and on the macromolecular theory.

Calcium-dependent increase in spike duration during repetitive firing of Aplysia axon in the presence of TEA

Repetitive stimulation was studied in the axon of the giant neuron, R2, of Aplysia in the presence of TEA. In 25 or 50 mM extracellular TEA, a plateau develops on the axon spike during repetitive stimulation at frequencies greater than 3/sec. The plateau in extracellular TEA is inhibited by 30 mM CoCl2 or 1 mM CdCl2, and is enhanced by raising the Ca concentration. Intracellular TEA induces a plateau on the axon spike at frequencies less than 1/30sec. This plateau increases in duration with repetitive stimulation at higher frequencies and is inhibited by 30 mM CoCl2 or 1 mM CdCl2. The increase in spike duration during repetitive firing in the presence of TEA is indicative of an increased entry of Ca during the spike train.

Propagating calcium spikes in an axon of Aplysia

1. Intracellular recordings were obtained from the axon of the giant neurone R2 of Aplysia in order to study the ionic dependency of action potentials. 2. The overshoot potential of the axon spike increases with Na concentration in the manner predicted for a Na electrode. The maximum rate of rise (Vm) is linearly related to Na concentration. The overshoot potential is insensitive to Ca concentration at Na concentrations as low as 250 mM. 3. Tetrodotoxin (TTX) or replacement of Na with Tris abolishes action potentials in the axon but not soma of R2. Addition of 4-aminopyridine to a Na-free solution permits axon spikes to be generated. These action potentials are blocked by 30 mM-Co2+, but not by TTX. The overshoot potential and Vm of these action potentials increase monotonically with Ca concentration. 4. Axonal action potentials can be generated when an equimolar concentration of Sr is substituted for all of the Ca and Mg in Na-free medium. These action potentials are abolished by 30 mM-Ca2+ or mM-Co2+, and increase with Sr concentration. 5. TTX-resistant Ba spikes can similarly be elicited in R2 axon. These action potentials are reduced by Ca, Co, or Cd, and enhanced by raising the Ba concentration. 6. The Vm of Na spikes in the absence of Ca is greater in the axon than in the soma of R2, whereas the Vm of divalent spikes is greater in the soma. 7. During repetitive stimulation the axon spikes incrase in duration. This broadening is inhibited by replacing Ca in the bath with Mn or by the addition of 30 mM-CoCl2, and is enhanced by raising the Ca concentration by 30 mM. 8. The action potention of R2 axon has a mixed Na/Ca dependency. The density of Ca current may be greater in the some than in the axon of this cell.

Properties of internally perfused, voltage-clamped, isolated nerve cell bodies

The membrane properties of isolated neurons from Helix aspersa were examined by using a new suction pipette method. The method combines internal perfusion with voltage clamp of nerve cell bodies separated from their axons. Pretreatment with enzymes such as trypsin that alter membrane function is not required. A platinized platinum wire which ruptures the soma membrane allows low resistance access directly to the cell's interior improving the time resolution under voltage clamp by two orders of magnitude. The shunt resistance of the suction pipette was 10-50 times the neuronal membrane resistance, and the series resistance of the system, which was largely due to the tip diameter, was about 10(5) omega. However, the peak clamp currents were only about 20 nA for a 60-mV voltage step so that measurements of membrane voltage were accurate to within at least 3%. Spatial control of voltage was achieved only after somal separation, and nerve cell bodies isolated in this way do not generate all-or-none action potentials. Measurements of membrane potential, membrane resistance, and membrane time constant are equivalent to those obtained using intracellular micropipettes, the customary method. With the axon attached, comparable all-or-none action potentials were also measured by either method. Complete exchange of Cs+ for K+ was accomplished by internal perfusion and allowed K+ currents to be blocked. Na+ currents could then be blocked by TTX or suppressed by Tris-substituted snail Ringer solution. Ca2+ currents could be blocked using Ni2+ and other divalent cations as well as organic Ca2+ blockers. The most favorable intracellular anion was aspartate-, and the sequence of favorability was inverted from that found in squid axon.