Electrophysiological activity of tetrodotoxin on the resting membrane of the squid giant axon

A simple microfluidic platform for the partial treatment of insuspendable tissue samples with orientation control

Microfluidic devices have extensively been applied to study biological samples, including single cells. Exploiting laminar flows on a small scale, microfluidics allow for the selective and partial exposure of samples to various chemical treatments. Traditionally, suspendable samples are first flowed into formed microchannels and are allowed to adhere to the channel floor randomly with no control over sample placement or orientation, before being subjected to partial treatment. This severely limits the choice of samples and the extent of sample preparations. Here, we overcame this limit by reversing the sequence. We prepared the samples first on glass substrates. A patterned silicone slab was then placed on the substrate to form channels at an appropriate orientation with respect to the sample. We used liquid silicone rubber (LSR) as the base material. Its compliance (low elastic modulus) and its adhesion to glass offer the necessary seal to form the microchannels naturally. The applicability of the device was demonstrated by testing single axons of embryonic Drosophila motor neurons in vivo. A segment of the axons was subjected to drugs that inhibit myosin activities or block voltage-gated sodium ion channels. In response, the axons reduced the clustering of neuro-transmitter vesicles at the presynaptic terminal of neuromuscular junctions, or increased the calcium intake and underwent membrane hyperpolarization, respectively. Such fundamental studies cannot be carried out using conventional microfluidics.

Contributions of various ions to the resting and action potentials of crayfish medial giant axons

The membrane of crayfish medial giant axons is permeable at rest to ions in the rank K>Na>Ca>Cl. With K present, variation of the other ions has little or no effect, but with K absent the axon hyperpolarizes when Na is reduced or eliminated by replacement with Tris (slope ca. 30 mV/decade Na0). The hyperpolarization is independent of the presence of Cl or its absence (substitution with methanesulfonate or isethionate). The resistance increases progressively as Na is removed. These changes persist after the spike is blocked with tetrodotoxin. An increase in Ca causes depolarization (slope ca. 20 mV/decade) provided K, Na and Cl are all absent, but in the presence of Cl there is little or no change in membrane potential on increasing Ca to 150MM. The depolarization induced by Ca is associated with an increased resistance. Spike electrogenesis involves Ca activation as well as Na activation, but the after-depolarization at the end of the spike is due to a conductance increase for Ca. Two alternative equivalent circuits for the resting and active membrane are discussed.

Specific blockage of squid axon resting potassium permeability by Haliclona viridis (Porifera: Haliclonidae) toxin (HvTX)

The action of partially purified HvTX, toxin of the marine sponge H. viridis, was explored on the giant axon of the tropical squids Doryteuthis plei and Sepioteuthis sepioidea. HvTX depolarizes the nerves dose dependently. The effect occurs after blocking sodium channels with tetrodoxin (1 microM), removing external Na+, blocking electrically excitable K+ channels with 3,4-diaminopyridine (10 mM) or internal and external application of tetraethylammonium (40 mM). Ouabain (up to 10 mM) does not modify HvTX effect. The action of HvTX occurs only when it is applied to the outer phase of the nerve membrane; microinjection of the toxin into the axons lacks depolarizing effects. HvTX reduces the dependence of membrane potential on external potassium concentration. The apparent 86Rb+ permeability (pi') was measured in axons of S. sepioidea. The value of pi' in normal artificial sea water was 80 (61,96) nm/sec (median and its 95% confidence interval, n = 8) and raised to 1030 (588, 2113) nm/sec (n = 7) when the axons were depolarized to 0 mV raising external K+ to 300 mM. In axons depolarized with HvTX (10 mM external K+) to 0 mV, pi' was 88 (55, 97) nm/sec (n = 8). HvTX could not prevent (P >> 0.05) the increase in pi' induced by 300 mM K+ when the ion concentration was raised before toxin application [pi' = 660 (354, 1876) nm/sec, n = 7]. Most of the 86Rb+ permeability increase in high K+ was prevented if HvTX was added before external K+ was raised [pi' = 298 (264, 337) nm/sec, n = 8]. All the measures of pi' were carried out in solutions containing 1 microM tetrodotoxin, 1 mM 3,4-diaminopyridine and 2 mM ouabain.

Role of activity in the selection of new electrical synapses between adult Helisoma neurons

Our aim was to determine whether neural activity in the form of sodium-dependent action potentials play a role in the formation, maintenance and specificity of electrical synapses between regenerating neurons. We axotomized buccal neurons of the mollusc, Helisoma trivolvis, and placed ganglia into organ culture in the absence or presence of tetrodotoxin (TTX), a specific sodium channel blocker. Electrical coupling was measured using intracellular microelectrodes positioned within the soma of identified neurons. Neurite outgrowth was assessed by epifluorescence microscopy after filling neurons by iontophoresis with Lucifer yellow. Previous studies found that two days after axotomy transient electrical synapses form between heterologous neurons (e.g. buccal neurons 4 and 5). Five days after axotomy these transient connections disappeared and a new electrical synapse was stabilized between the paired buccal neurons 5. To determine whether blocking neural activity with TTX affected the specificity and formation of new electrical synapses, we examined electrical coupling between the heterologous neurons 4 and 5 two days after axotomy, and the paired buccal neurons 5 five days after axotomy. Our electrophysiological recordings indicated that different neurons in the buccal ganglion varied in their sensitivity to TTX (i.e. sensitivity of buccal neurons 19 greater than 5 greater than 4), but spontaneous activity was abolished in all 3 neurons by 2 x 10(-5) M TTX. Furthermore, the inhibitory effects of TTX occurred within seconds of superfusion and persisted for at least 6 days. Inhibition of activity by TTX could be reversed after superfusion with normal saline. Neurite outgrowth from axotomized neurons was not appreciably altered in the presence of TTX. Furthermore, no differences in the incidence of electrical coupling or the coupling resistance were detected between neurons 4 and 5 two days after axotomy and organ culture in the presence of TTX. However, electrical coupling between the symmetrically paired neurons 5 was elevated in the presence of TTX after 5 days. We conclude from these results that neural activity in the form of sodium-dependent action potentials does not play an important role in the formation or breaking of transient electrical synapses during neuronal regeneration in the mollusc Helisoma trivolvis.

Tetrodotoxin enhances initial neurite outgrowth from fetal rat cerebral cortex cells in vitro

The effect of two different concentrations of the sodium channel blocker tetrodotoxin on neurite outgrowth from fetal rat cerebral cortex neurons was studied in vitro. A concentration of 10(-6) M tetrodotoxin enhanced after two days in vitro: the percentage of isolated cells and reaggregates which formed neurites, the number of neurites per cell soma, and neurite elongation and subsequent branching. The effects observed after treatment with 10(-7) M tetrodotoxin were on the whole intermediate between control and 10(-6) M tetrodotoxin.

Is the K permeability of the resting membrane controlled by the excitable K channel?

To test whether or not the potassium permeability of the resting membrane is controlled by the excitable K channels (delayed rectifier), we examined changes in the Na and K permeability ratio, PNa/PK, of the squid axon before and after the excitable K channels were blocked. The blockage of the K channels was accomplished by three independent methods: internal application of tetraethylammonium, internal application of 4-aminopyridine plus Cs, and prolong internal perfusion of NaF solution. The permeability ratio was determined using two different methods: the conventional electrophysiological method and a new method based on the measurements of the hyperpolarizing effect of Na removal. We found that blocking the K channels did not cause a proportional decrease in the K permeability of the resting membrane, suggesting that the semipermeable property of the resting membrane is not determined by the excitable K channels.

A comparative study of the effects of tetrodotoxin and the removal of external Na+ on the resting potential: evidence of separate pathways for the resting and excitable Na currents in squid axon

To investigate whether the Na permeability of the resting membrane is determined predominantly by the excitable Na channel, we examined the effects of tetrodotoxin (TTX) and the complete removal of external Na+ on the resting potential. In the intact squid axon bathed in K-free artificial seawater, both TTX and the removal of Na+ produced small hyperpolarizations. The effect of Na removal, however, was larger than that of TTX. In the perfused squid axon, the hyperpolarization produced by the removal of external Na+ was greatly enhanced when the internal K concentration ([K+]i) was reduced. The effect of TTX, on the other hand, was not sensitive to the [K+]i or to the membrane potential. For [K+]i = 50 mM and [K+]o = 0, the average hyperpolarization produced by TTX was 1.2 mV, while the hyperpolarization produced by Na removal was approximately 21 mV. The difference between these two effects suggests that the majority of the resting Na current passes through pathways other than the excitable Na channel.

Comparison of glucokinase in C3H/He and C58 mice that differ in their hepatic activity

Partially purified preparations of the hepatic glucokinase from C3H/He and C58 inbred mice have been used to explore the molecular basis for the observed twofold difference in activity between the strains. The single codominant gene that appears to regulate activity, the alleles of which are designated Gka and Gkb, respectively, for the two strains, could represent a structural gene change. This now seems unlikely because the mouse enzyme, although showing small differences from rat glucokinase, appeared to be identical in the two strains with respect to thermal stability, electrophoretic mobility in agarose gels, and kinetic properties such as the apparent Km values for MgATP2- and glucose and the unique cooperative interaction with the latter substrate. The enzymes also reacted identically in a range of immunological tests (double-diffusion, immunoelectrophoresis, immune precipitation and immune inhibition assays) and ELISA immune inhibition assays indicated that the twofold difference in activity was due to a similar difference in antigenically active enzyme. Genetic control over the physiologically significant regulation of enzyme amount is therefore probable.

A new method of monitoring membrane potential in rat hippocampal slices using cyanine voltage-sensitive dyes

A novel application of voltage-sensitive dyes is described. Hippocampal slices in vitro accumulated voltage-sensitive cyanine dyes under conditions presumed to cause depolarization and hyperpolarization. Increasing extracellular potassium caused a depression of dye uptake that correlated linearly with the membrane potential calculated from the Goldman equation. Veratrine depressed dye uptake, and this effect was blocked by addition of tetrodotoxin or removal of extracellular sodium. Ouabain also depressed dye uptake. Conversely, hyperpolarizing conditions using reduced extracellular sodium caused increased dye uptake. These results support a voltage-dependent mechanism for the uptake of cyanine dyes in hippocampal slices. Application of this phenomenon as an alternative to 2-deoxyglucose autoradiography for mapping neuronal activity will be presented.

Cation movements in normal and short-term denervated rat fast twitch muscle

1. The earliest known change in rat fast muscle following denervation is a fall in resting membrane potential unaccompanied by change in membrane resistance. The present study tested the hypothesis that increased Na permeability (P(Na)) accounted for this early depolarization.2. In all experiments, rat extensor digitorum longus muscles were studied in vitro at 25 degrees C. Li uptake in vitro, used as a measure of P(Na), was greater in 1- and 2-day denervated muscles (and in 2-day denervated diaphragm) than in paired controls.3. The extra Li taken up by denervated muscle was not sequestered in an extracellular or freely exchangeable compartment, nor was it irreversibly bound.4. Measurements of resting membrane potential and of internal Na, K, and Li in Krebs solution before and 2 hr after replacement of NaCl by LiCl, were used to compute the ratios P(Na)/P(K) and P(Li)/P(K) for normal or denervated muscles. P(Na) and P(Li) were similar relative to P(K) within each class of muscle.5. Both P(Na)/P(K) and P(Li)/P(K) ratios were elevated more than twofold in denervated muscle, as were most estimates of relative P(Li) approximated by the flux equation.6. These data, and measurement of resting membrane potential of normal muscle in 1 mM external K-Krebs solution, support the view that an electrogenic Na-K pump does not substantially contribute to this potential of normal or denervated muscle, and that the early depolarization after denervation results from increased P(Na).7. The Na-K pump of denervated muscle was as sensitive to ouabain as normal muscle. An effect of ouabain on P(Na) may explain previously noted differential effects of ouabain on normal and denervated muscle.

How Gymnodinium breve red tide toxin(s) produces repetitive firing in squid axons

Partially purified toxin(s), GbTX, extracted from Gymnodinium breve red tide organisms elicits a spontaneous train of action potentials in the squid giant axon. The spikes have a shape similar to that in the normal seawater control except for an increase in the rate of recovery from the afterhyperpolarization. With this more rapid recovery, the membrane potential overshoots the resting potential and threshold, triggers another spike, and thus produces repetitive firing. Voltage-clamp studies revealed that the toxin has no effect on the normal sodium or potassium conductance changes produced by step depolarization. However, consistent with the faster recovery after an action potential, GbTX speeds recovery of the "shut-off" currents to their steady-state values after a depolarization. The most likely mechanism by which the toxin accelerates recovery after an action potential (leading to repetitive firing) is the induction of a small additional inward current which was found to be reduced by prehyperpolarization. This toxin-induced current which speeds recovery is blocked by tetrodotoxin and hence presumably flows through the sodium channel.

Pharmacologic characterization of the Na+ ionophores in L6 myotubes

We present a pharmacologic characterization of the Na+ ionophores present in L6 myotubes in vitro. Action potentials are abolished by replacement of the external Na+ by Tris. The amplitude of the action potential is generally resistant to high concentrations of tetrodotoxin (10(-5) M) and saxitoxin (10(-6 M), but the effect of these agents is highly variable. Veratridine (10(-4 M) consistently induces, as a short-term effect, a marked prolongation of the falling phase of the action potential. As a long-term effect, veratridine consistently induces a Na+-dependent reduction in the resting potential of the cell. The effects of veratridine on the action potential are not antagonized by tetrodotoxin or saxitoxin. However, the effects of veratridine on the resting potential are strongly antagonized by tetrodotoxin (10(-5) M) and fully inhibited by saxitoxin (10(-6) M). Significantly, under conditions where saxitoxin has fully inhibited the effects of veratridine on the resting potential, the myotubes are capable of generating overshooting action potentials. In contrast to their sensitivity to veratridine, L6 myotubes are insensitive to 10(-5) M alpha-dihydro-grayanotoxin-II. These results are discussed in the contexts of developmental significance and current views about Na+ ionophores.

Evidence that alpha-dihydrograyanotoxin II does not bind to the sodium gate

The basis for the ability of alpha-dihydrograyanotoxin II (alpha-2HG-II) to promote Na+ conductance in axons was sought. The apparent binding of tritiated alpha-2HG-II to neural and other preparations was studied, using equilibrium dialysis, with lobster axon membranes, Torpedo electroplax, housefly head, and rat brain, liver and kidney. In every case the "binding" was nonsaturating and was suggested to involve nonspecific partitioning into the tissue. Supporting evidence was the similarity of extent of "binding" in all tissues and its relative insensitivity to neuropharmacological agents. Alpha-2HG-II did not affect the Na+ conductance of phospholipid bilayers, nor did it permit transport of 22Na into a bulk organic phase. It was concluded that alpha-2HG-II did not bind to the sodium gate, but possibly to a sodium permease present at a frequency of less than one per mu2 of cell membrane.

The effect of intravenously administered gamma-aminobutyric acid on afferent fiber polarization

(1) The effect of intravenously administered gamma-aminobutyric acid (GABA) on afferent fiber polarization in the feline spinal cord was ascertained from fluctuations induced in the DC level of dorsal root filaments. (2) A dose-related depolarization of the filament, with a concomitant reduction in the magnitude of the dorsal root potential, was observed after 50 and 100 mg/kg GABA. (3) GABA also depolarized filaments of preparations in which interneuronal activity was suppressed by pretreatment with tetrodotoxin. Since the magnitude of the depolarization induced in these preparations was equal to that observed in nonpretreated animals, it is likely that the depolarization in the latter preparations reflects a direct effect on afferent terminals or fibers rather than an action on interneurons. (4) GABA failed to depolarize filaments in animals pretreated with bicuculline. This suggests that intravenously administered GABA interacted with receptors that are identical with or similar to those involved in neurally evoked primary afferent depolarization (PAD). (5) The direct depolarization of afferent fibers by intravenous GABA and the blockade thereof by bicuculline are characteristics compatible with those of the endogenous axo-axonic transmitter operating in pathways mediating neurally evoked PAD. These data, therefore, support the involvement of GABA at this synapse in the mammalian spinal cord.

Mechanism of nerve membrane depolarization caused by grayanotoxin I

1. The mechanism of depolarization of squid axon membranes caused by grayanotoxin I has been studied by means of internal perfusion and voltage clamp techniques.2. The depolarization induced by either internal or external application of grayanotoxin I was reversed by decreasing the external sodium concentration from 449 to 1 mm.3. No depolarization was observed when both external and internal media were devoid of sodium ions, indicating that the depolarization by grayanotoxin I in normal media is due to a specific increase in resting sodium permeability.4. The resting sodium permeability as measured by voltage clamp was increased to 1.31 x 10(-6) cm/sec by internal application of 1 x 10(-5)m grayanotoxin I, an increase by a factor of about 90.5. The apparent dissociation constant of internally applied grayanotoxin I in increasing the resting sodium permeability was estimated to be 4.12 x 10(-5)m, and the toxin interacts with the membrane receptor on a one-to-one stoichiometric basis.6. Tetrodotoxin antagonized the action of grayanotoxin I in increasing the resting sodium permeability in a non-competitive manner.