Zinc-dependent action potentials in giant neurons of the snail,Euhadra quaestia

Summary In giant neurons of subesophageal ganglion of the Japanese land snail,Euhadra quaestia Deshayes, permeation of Zn ions through Ca channels were investigated with a conventional current clamp method.All-or-none action potentials of long duration (90 to 120 sec) were evoked in 24mm Zn containing salines. The overshoots were about +10 mV and the maximum rate of rises (MRRs) was about 2.9 V/sec. The amplitudes and the MRRs of the action potentials depended on external Zn ion concentrations.The action potentials were suppressed by specific Ca-channel inhibitors such as Co²⁺, La³⁺ and Verapamil, but they were resistant to Na-channel inhibitor, tetrodotoxin, even at 30 m.It is concluded that these action potentials are generated by Zn ions permeating Ca channels in snail neuronal membrane.On the basis of Hagiwara and Takahashi's (S. Hagiwara & K. Takahashi, 1967,J. Gen. Physiol. 50:583) model of Ca channels, it is inferred that Zn ions are 5 to 10 times stronger in affinity to Ca channels than Ca ions, but 10 to 20 times less permeable.     

Disturbance of action potential generation in the mollusk giant neuron after preliminary membrane hyperpolarization

A disturbance of action potential generation in the mollusk giant neuron was investigated by intracellular recording. The anodal-breaking effect resulting from cessation of intracellular hyperpolarization was chosen as the factor evoking splitting of the action potential into its separate components. This effect was shown to be accompanied by splitting of the high-amplitude action potential into low-amplitude components. This splitting arises at a certain critical strength of the hyperpolarizing current (the disintegration threshold) and it increases with an increase in the intensity and duration of the hyperpolarizing current. Splitting of the action potential into components is regarded as disturbance of the connections between individual areas of the excitable membrane of the neuron soma. Frequent repetition of hyperpolarization leads to accumulation of the disturbances and intensifies the disintegration of the action potential into its components.     

Characteristics of apple snail giant neurons capable of generating action potentials in sodium-free solutions

The ability of apple snail giant neurons to generate action potentials in solutions that lack sodium ions is associated with the input resistance of these neurons in such a way that the higher the input resistance is, the more pronounced is this ability. Neurons in which this ability is well expressed usually exhibit low resting potential values and a slow repolarization phase. When calcium ions are replaced with barium ions, the neurons retain their excitability in a sodium-free medium for a longer period of time. Raising the calcium ion concentration to 30 µmole may exert a restorative effect on neurons that have lost their excitability in a solution that originally lacked sodium ions but contained 10 µmole of calcium ions. Increasing the calcium ion concentration to 60 µmole leads to loss of excitability, which under these conditions can be restored by means of depolarizing the neuron with an outward current. The results are discussed from the point of view of the theory of ionic conductivity of the surface membrane of neurons. It is hypothesized that the ability of the surface membrane of neurons to make use of sodium or calcium ions in generation of action potentials depends upon its permeability to potassium ions.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 100–106, January–February, 1970.     

Potential-dependent membrane sodium pump current in snail giant neurons

The effect of the membrane potential on the pump current evoked by iontophoretic injection of sodium into the neuron and the effect of the intracellular sodium ion concentration on the potential dependence of the pump current were investigated by the voltage clamp method in isolated and semi-isolated neurons ofHelix pomatia andHelix italiana. The pump current was shown to change its direction in the presence of marked hyperpolarization of the membrane (by more than −80 to −120 mV). An increase in the intracellular sodium ion concentration following injection of excess ions into the neuron increases the potential dependence of the pump current. A possible connection between passive potassium permeability and the activity of the enzymic transport mechanism for the elimination of sodium from the cell is postulated.     

Effect of sodium-free solutions on ionic currents of snail giant neurons

The ionic currents of the snail giant neurons were investigated by the voltage clamp method. The effect of sodium-free solutions on the inward and outward currents was studied. It is shown that the current entering the cells is created mainly by sodium ions. When a preparation is immersed into a solution not containing sodium ions, most neurons (tentatively neurons of type "a") "lose" the inward currents. In other neurons (tentatively of type "b") this process lasts 40 min and more. A number of peculiarities of type "b" neurons were noted. The response of the excitable membrane to conditioning polarization was also investigated. The data obtained permit the conclusion that 85–90% of the sodium-transfer system is activated in the case of a voltage clamp from the level of the resting potential.A. A. Bogomolets Institute of Physiology, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 3, pp. 314–320, May–June, 1970.     

Effect of monoidoacetic acid on electrophysiological properties of snail giant neurons

Monoiodoacetic acid (MIA) causes depolarization and a decrease in the amplitude of the action potential and resistance of the membrane, and also leads to significant changes in the potassium and sodium concentrations in the neurons of the subesophageal ganglia. All these changes are more marked with a decrease in pH of the Ringer's solution containing the inhibitor. During the action of acid Ringer's solution without an inhibitor the electrophysiological changes in the neurons develop more slowly and to a lesser degree and they are easily reversible. It is concluded that the electrophysiological changes induced in neurons by MIA are due not only to inhibition of active ion transport but also to changes in the ionic permeability of the membrane and, in particular, to an increase in sodium permeability.A. A. Bogomolets' Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 1, pp. 97–104, January–February 1972.     

Calcium entry induced by acetylcholine action on snail neurons

A study was made of excitatory and inhibitory responses elicited by acetylcholine (ACh) in neurons of the snail Eobania vermiculata. At resting potential, ACh evoked a depolarizing inward current in some neurons (D-cells) and a hyperpolarizing current in others (H-cells). The currents elicited by ACh were nonlinearly dependent on membrane potential. After either D- or H-cells were equilibrated in chloride-free isotonic calcium, ACh evoked a depolarizing inward current which reversed sign at about -55 mV. These results suggest that ACh causes an influx of Ca2+ in both types of neurons.     

Decreases of action potential amplitudes, in sodium-free and calcium-free conditions, of identifiable giant neurons of an African giant snail (Achatina fulica Férussac)--I. The right parietal ganglion

Decreases of the action potential amplitude in sodium- and calcium-free states were observed with respect to the four giant neurons, PON (periodically oscillating neuron), Tan (tonically autoactive neuron), RAPN (right anterior pallial neuron) and d-RPLN (dorsal-right parietal large neuron), identified in the right parietal ganglion of the suboesophageal ganglia of an African giant snail (Achatina fulica Férussac). The decrease of the PON action potential amplitude, caused in the sodium-free state, was observed to be 25.4 +/- 2.1% (23.0 +/- 2.0 mV), expressed by M +/- SE, while that of the calcium-free state was 35.0 +/- 2.1% (30.9 +/- 1.7 mV). Then, the two ionic dependencies of the PON action potential were estimated to be about 40-50% on sodium and 50-60% on calcium. The decrease of the TAN action potential in the sodium-free state, was observed to be 20.7 +/- 1.2% (18.8 +/- 1.3 mV), whereas that of the calcium-free state was 42.2 +/- 2.7% (39.0 +/- 2.2 mV), indicating that the two ionic dependencies were 30-40% on sodium and 60-70% on calcium. The decrease of the RAPN action potential in the sodium-free state, was 45.8 +/- 3.7% (40.3 +/- 3.1 mV), whereas that of the calcium-free state was 21.7 +/- 2.5% (17.8 +/- 2.0 mV), indicating that the two ionic dependencies were about 70% on sodium and about 30% on calcium. The decrease of the d-RPLN action potential in the sodium-free state was found to be 17.6 +/- 2.4% (15.2 +/- 1.8 mV), whereas that of the calcium-free state was 23.1 +/- 1.4% (20.8 +/- 1.4 mV), indicating that the two ionic dependencies were 40-50% on sodium and 50-60% on calcium. The action potential amplitudes of all the neurons tested were decreased in both sodium-free and calcium-free states. However, their ionic dependencies were estimated to vary from 70% on sodium (30% on calcium) to 30% on the sodium (70% on the calcium), according to the neurons tested.     

动作电位形成的机制

动作电位是短暂、快速的膜电位的变化(100 mV),在此期间,细胞膜内外的极性发生反转,即细胞膜由静息状态时的膜内为负、膜外为正转变为膜内为正而膜外为负的状态。一个单个动作电位仅包括全部兴奋细胞膜的一小部分。与分级动作电位不同的是,动作     

Neurotransmitters of giant neurons in the African giant snail, Achatina fulica Férussac. IV. Supplemental results

Following the previous works, we identified recently the twelve giant neurones in the ganglia of an African giant snail (Achatina fulica Férussac), by the pharmacological study of their sensitivities to putative neurotransmitters and derivatives, and by the morphological investigation of their axonal pathways due to the intracellular injection of Lucifer Yellow. The neurones studied were: TAN-2, TAN-3, BAPN, LPPN, LBPN and LAPN in the right parietal ganglion; RPeNLN and LPeNLN in the pedal ganglia; and d-LBAN, d-LBMN, d-LBCN and d-LBPN in the left buccal ganglion.     

Time-dependent actions of aluminates on membrane and action potentials of snail neurons

Aluminum (Al) is one of the elements, which is frequently subjected to experiments, however, the neurological observations with it are rather conflicting. The cause of this controversiality is not known but relates to some human disorders such as Alzheimer's disease and others as well. We studied the time-dependent actions of AlCl3 base solutions on resting membrane potential (Em), input resistance (Rin) and spike shape in giant neurons of the snail Helix pomatia L. at pH 7.7 and room temperature (22-24 degrees C) by use of intracellular technique. We reported significant differences in the effectiveness of the various Al solutions depending on the time of storage before use in the experiments (0, 2 and 6 days at room temperature). Freshly prepared and applied Al solutions caused a significant and dose-dependent depolarization with a concomitant decrease of Rin and the amplitude of the action potentials, but the 6 days solutions induced a hyperpolarization. Ouabain (0.1 mM) antagonized the hyperpolarization. The pH (7.1 or 7.7) and the time of the storage in combination also modified the direct membrane effects. Our experiments show that Al can induce differential membrane effects depending on the presence of various aluminum compounds. Namely, the predominate aluminum-monomer at pH 7.7 the Al(OH)4- might cause depolarization but the polynuclear aluminum complexes after polymerization of the monomers could hyperpolarize the neuronal membrane. We suppose, that the time-dependent equilibrium of various aluminum complexes plays a role in generating controversies in this field and emphasize again the importance of standardization of the experimental protocol.