Slow voltage-dependent block of sodium channels in crayfish nerve by dihydropyrazole insecticides

Previous current-clamp work has shown that dihydropyrazole insecticides block sodium channels in tonic sensory receptors and in axons depolarized by high K+ external solutions and that hyperpolarization removes the block [Pestic. Sci. 28:389-411 (1990)]. Voltage-clamp studies on internally perfused crayfish giant axons were done to confirm and extend these observations. At -100 mV dihydropyrazoles had little effect on the sodium current, but at more depolarized potentials they blocked it from either face of the membrane. The onset of block following a holding potential change or during wash-in of a dihydropyrazole was very slow, with a time constant of several minutes, and, although block could be removed with a similar time course by hyperpolarization, the effects of the insecticides could not be reversed by prolonged washing. Dihydropyrazoles did not affect delayed rectifier potassium currents in the axon. The voltage-dependent block could be described as a uniform shift of the steady state (slow) sodium inactivation (S infinity) curve in the direction of hyperpolarization, indicative of selective binding to inactivated states of the channel. Using hyperpolarizing prepulses to remove slow inactivation, block of sodium channels by dihydropyrazoles could be measured directly at holding potentials as positive as -50 mV, and it could be demonstrated that block saturated near -70 mV, consistent with a dependence on slow inactivation. The data were fit to a model tha assumes the dihydropyrazole binds to the slow-inactivated state of the channel on a one to one basis. Dissociation constants obtained from this analysis were similar to those obtained from analysis of inhibition of the binding of [benzoyl-2,5-3H]-batrachotoxinin A 20-alpha-benzoate by the same dihydropyrazoles. In axons whose fast or slow inactivation gates had been removed by N-bromoacetamide or trypsin, respectively, dihydropyrazoles still blocked sodium current, indicating that dihydropyrazoles can block the channel as well as enhance the normal slow inactivation process.

Neuropeptides from the sinus gland of the lobster Homarus americanus: Characterization of hyperglycemic peptides

In order to characterize hyperglycemic peptides from the sinus gland of the lobster, Homarus americanus, a bioassay was developed with juvenile H. gammarus. This assay was used for determining the hyperglycemic activity of peptides perified by reversed-phase high-performance liquid-chromatography, from acidic extracts of sinus gland. The major peptides are eluted in three sets of two peptides. Among them, two pairs show hyperglycemic activity when assayed on lobster; when assayed on crayfish, three peptides are active. The less hydrophobic pair consists of basic peptides (pI: 8.7), with a MW of 8633 Da., determined by fast-atom bombardment mass spectrometry. The most hydrophobic pair consists of acid peptides (pI: 5.0), with a MW of 8577 Da. Amino acid composition of the hyperglycemic peptides shows strong homologies within each pair.

Quisqualate-activated single channel currents in neuromuscular preparations of small and large crayfish

Single channel currents elicited by 1-5 mumol/l quisqualate in neuromuscular preparations in large (greater than 16 month old) and small (1-3 month old) crayfish were recorded by means of the patch-clamp technique. In preparations from large crayfish single channel currents of variable amplitude (-1 to -12 pA) were induced by quisqualate. The mean burst lengths of these currents were tau approximately equal to 1-2 ms. In the opener muscle of the first walking leg and the contractor epimeralis muscle of small crayfish the mean burst lengths of single channel currents evoked by quisqualate were prolonged by a factor of about 4 (tau approximately equal to 5 ms). Moreover, in the opener muscle of the first walking leg of small crayfish single channel currents of small amplitude (-0.5 to -2.5 pA) were preferentially evoked by quisqualate. By contrast, in the contractor epimeralis muscle of small crayfish mainly single channel currents of large amplitude (-10 to -12 pA) were elicited by quisqualate. The results suggest that at the stage of neuromuscular development characterizing the small crayfish, gating properties of excitatory postsynaptic channels are different from those in adult crayfish. Furthermore, the results obtained in the opener muscle of the first walking leg of small crayfish are consistent with those obtained previously by means of the noise analysis technique.

Studies of axon-glial cell interactions and periaxonal K+ homeostasis--II. The effect of axonal stimulation, cholinergic agents and transport inhibitors on the resistance in series with the axon membrane

The small electrical resistance in series with the axon membrane is generally modeled as the intercellular pathway for current flow through the periaxonal glial (Schwann cell) sheath. The series resistance of the medial giant axon of the crayfish, Procambarus clarkii, was found to vary with conditions known to affect the electrical properties of the periaxonal glia. Series resistance was estimated from computer analysed voltage waveforms generated by axial wire-constant current and space clamp techniques. The average series resistance for all axons was 6.2 +/- 0.5 omega cm2 (n = 128). Values ranged between 1 and 30 omega cm2. The series resistance of axons with low resting membrane resistance (less than 1500 omega cm2) increased an average of 30% when stimulated for 45 s to 7 min (50 Hz) whereas the series resistance of high membrane resistance (greater than 1500 omega cm2) axons decreased an average of 10%. Carbachol (10(-7) M) caused the series resistance of low membrane resistance axons to decrease during stimulation but had no effect on high membrane resistance axons. d-Tubocurare (10(-8) M) caused the series resistance of high membrane resistance axons to increase during stimulation but had no effect on low membrane resistance axons. Bumetanide, a Na-K-Cl cotransport inhibitor and low [K+]o, prevented the stimulation-induced increase in series resistance of low membrane resistance axons but had no effect on the high membrane resistance axons. The results suggest that the series resistance of axons varies in response to the activity of the glial K+ uptake mechanisms stimulated by the appearance of K+ in the periaxonal space during action potential generation.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of axon-glial cell interactions and periaxonal K- homeostasis--I. The influence of Na+, K+, Cl- and cholinergic agents on the membrane potential of the adaxonal glia of the crayfish medial giant axon

The ionic basis for the low (-40 mV) resting membrane potential of glial cells surrounding the giant axons of the crayfish and their hyperpolarization by cholinergic agents (to -55 mV) was studied using standard electrophysiological techniques, ionic substitutions and pharmacological agents. The resting membrane potential of the glial cell was depolarized by increasing [K+]o, but the response was not Nernstian. Na+ depletion caused a small depolarization of the glial resting membrane potential, whereas Cl- depletion resulted in a hyperpolarization comparable to that seen with carbachol at various [K+]o. Both furosemide (1 mM) and bumetanide (0.1 mM) produced an 8-10 mV hyperpolarization as compared to 15-17 mV seen with Cl- depletion or carbachol. Carbachol has no further effect on the potential following furosemide treatment or Cl- depletion. After carbachol administration or Cl- depletion the resting membrane potential of the glial cell responded to [K+]o in a more Nernstian manner. The data indicate that the low resting membrane potential of glial cells is due to a combination of a low [K+]i and an outwardly-directed (depolarizing) Cl- electrochemical gradient. Carbachol acts to decrease Cl- conductance, resulting in the hyperpolarization of the glial cell membrane and a decrease in the outwardly-directed K+ electrochemical gradient by approximately two-thirds. We hypothesize that this mechanism for modulation of the glial cell membrane potential and the K+ electrochemical gradient serves to enhance the uptake of K+ by the glial cell transport system.

Glutamate inhibitors in the crayfish neuromuscular junction

1. The effects of chlorisondamine and TI-233 on the crayfish neuromuscular junction were investigated in order to compare the action of glutamate with that of the excitatory transmitter. 2. The glutamate-induced synaptic current was inhibited by both of these two drugs. Excitatory junctional potentials were significantly reduced by chlorisondamine, whereas they were increased by TI-233. 3. It is suggested that chlorisondamine and TI-233 are powerful non-competitive antagonists for glutamate. 4. A quantum analysis of extracellular EJPs demonstrated that chlorisondamine did not possess presynaptic action in the crayfish neuromuscular junction. Chlorisondamine shortened the decay phase of extracellular EJPs, and the decay was frequently fitted by a double exponential in relatively low concentrations. 5. Semilogarithmic plots of the decay phase of the glutamate current evoked by a short glutamate pulse were nearly linear, but they shifted from linearity to some extent in the presence of chlorisondamine, showing prolongation of the glutamate current tails. 6. When TI-233 was added to the bathing solution at a concentration of 0.1 mM, the quantum content of extracellular EJPs was increased by about two times, but the average unit size was not changed. 7. There was no change in the rise time and the decay phase of the glutamate potential in the presence of TI-233. 8. Pharmacological difference between glutamate responses and EJPs was revealed in the presence of chlorisondamine and TI-233. Unless this difference can be explicated with a reasonable explanation on the glutamate transmitter hypothesis, it is difficult to confirm that glutamic acid is an excitatory transmitter at the crayfish neuromuscular junction.

Fever and antipyresis in the crayfish Cambarus bartoni

1. Normothermic Cambarus bartoni crayfish thermoregulating behaviourally in an electronic shuttlebox prefer a mean temperature of 22.1 degrees C +/- 0.5 S.E. of mean. 2. Injection of 1% saline, or addition of paracetamol to the ambient water, caused a statistically insignificant decrease in preferred temperature of 0.4 degrees C. 3. Injection of killed Aeromonas hydrophila bacteria into the abdomen caused a significant mean increase in preferred temperature of 1.8 degrees C (defined as a behavioural fever). 4. Injection of A. hydrophila, with paracetamol in the water, resulted in an insignificant increase of 0.1 degrees C above the normothermic temperature; thus paracetamol is antipyretic in crayfish, preventing fever but not altering afebrile thermoregulation. 5. These results suggest that similar mechanisms may be operating in the neuropharmacological mediation of fever in vertebrates and invertebrates.

A comparison of effects of SKF 525-A and procaine on excitation-contraction coupling in single crayfish muscle fibers

The effects of beta-diethylaminoethyldiphenylpropylacetate hydrochloride (SKF 525-A) on excitation-contraction coupling and Ca-dependent electrogenesis are compared to those of procaine. At pH 7.2, SKF 525-A and procaine occur essentially (greater that 97%) as a free base and as a cation, respectively. At this pH, SKF 525-A elicited tension development, blocked K-induced contractures and the K-induced repriming of caffeine contractions, potentiated caffeine-induced tensions, inhibited the procaine-induced spikes and twitches and, depending on the concentration, either potentiated (25-50 muM) or depressed (greater than 100 muM) the tensions associated with the graded membrane electrogenesis. At the same pH, procaine blocked the contractions elicited by SKF 525-A, by high K media, by the graded electrogenesis and by caffeine, and converted the graded membrane responses into all-or-none spikes. It is proposed that SKF 525-A as a free base 1) inhibits membrane Ca activation more effectively than it depresses K conductance and 2) is synergistic with caffeine in reducing the effectiveness of Ca sequestration by the sarcoplasmic reticulum. Procaine as a cationic molecule is thought to depress K activation more than Ca activation during depolarization and to block the release of Ca ions from the sarcoplasmic reticulum.

Purification and characterization of a new family of polypeptide neurotoxins from the heteronemertine Cerebratulus lacteus (Leidy)

Mucus secreted from the integument of the marine heteronemertine Cerebratulus lacteus (Leidy) contains two major types of polypeptide neurotoxin: the A toxins (11,000 daltons) which are lethal to a variety of animal species including mammals, and the B toxins (6,000 daltons) which appear to be selectively toxic for crustaceans. Both types of toxin were readily obtained from live worms by stimulation and collection of mucus with 1% acetic acid followed by batch adsorption of the basic polypeptide fraction upon CM-cellulose. Separate A and B toxin fractions were obtained by gel chromatography. Four toxins were purified from the B toxin fraction by CM-cellulose gradient elution chromatography. Each consists of a single polypeptide chain with NH2-terminal alanine and three (B-I) or four (B-II, B-III, B-IV) intrachain disulfide bonds. All four B toxins contain large proportions of lysine and exhibit identical gel electrophoretic mobilities at neutral pH. The B toxins lack cysteine and reducing sugar moieties. Toxin B-I differs considerably from the others in its amino acid composition and its shorter chain length (49 instead of 53 or 54 residues). The other three toxins have similar compositions, but lack proline, methionine, and phenylalanine. All four polypeptides show crustacean-selective toxicities which vary over a 30-fold range; B-II possesses the highest toxicity. As a working hypothesis it is suggested that the Cerebratulus B toxins are isotoxins which act via a common receptor present in crustacean axon membranes.

Purification from black widow spider venom of a protein factor causing the depletion of synaptic vesicles at neuromuscular junctions

The aqueous extract of the venom glands of black widow spiders was fractionated on a column of Sephadex G-200 and then on a column of DEAE-Sephadex A-50 pH 8.2. A protein fraction was obtained that caused a great increase in the frequency of occurrence of miniature end plate potentials at the frog neuromuscular junction, and caused swelling of the nerve terminals and depleted them of their vesicles. The fraction consists of a least four protein components that are similar in their molecular weights (about 130,000) and isoelectric points (ranging from pH 5.2 to 5.5) and are immunologically indistinguishable. It contains no sugar residues and has little or no lipolytic or proteolytic activity. The fraction is toxic to mice and is different from the fractions that act on houseflies, the crayfish stretch receptor and the cockroach heart. It seems pure enough to warrant a detailed study of its site and mode of action.

The effects of D600 and verapamil on action potential in the X-organ neuron of the crayfish

The effects of D600 and verapamil on the membrane activity of the X-organ neuron of the crayfish were investigated. In TTX solution, D600 and verapamil reversibly reduced the amplitude and the maximum rate of rise of the Ca-dependent action potential as well as delayed rectification. The decrease and the recovery in amplitude of the Ca spike were usually accompanied by parallel changes in delayed rectification. The threshold concentration for observable effects on the Ca spike was from 5 X 10(-6) g/ml to 1X 10(-5) g/ml for both drugs. At 1 X 10(-4) g/ml, the Ca spike was almost blocked. The dose-response relation with regard to the maximum rate of rise of the Ca spike showed no noticeable difference in effect between D600 and verapamil. By increasing the extracellular Ca concentration, the Ca spike, which had been reduced by these drugs, was restored by the delayed rectification remained to be reduced. In Mn solution, both D600 and verapamil reduced the amplitude and the maximum rate of rise of the Na-dependent action potential as well as the delayed rectification. At 1 X 10(-4) g/ml, the maximum rate of rise of the Na spike was reduced by 30 to 50% with each drug. From the experiments in the Ca and the Mn solutions, it was inferred that the decrease in K activation by these drugs was not brought about secondarily by the decrease in Ca activation but both activation were inhibited independently. It was concluded that both D600 and verapamil are the inhibitors for Ca activation in the X-organ neuron of the crayfish, although they are not Ca specific.

Neurohemal organ extracts effect the ventilation oscillator in crustaceans

Extracts of the pericardial organs of crabs injected into intact animals cause an increase in the frequency of scaphognathite beating. The active factor is not dopamine of 5-hydroxytryptamine, which are present in pericardial organs; it is probably a peptide. The factor may represent a hormonal mechanism of coordinating heart rate and ventilation.

Effect of penicillin on an excitatory synapse

When penicillin, an epileptogenic agent, was applied to the neuromuscular junctions of the superficial flexor muscles of crayfish, the excitatory junctional potential (EJP) amplitudes were increased by 50-200%. This effect of the drug was not due to changes in the passive electrical properties of the muscle cell membrane or to an increase in its chemical sensitivity to acetylcholine (ACh), the presumed transmitter at the junction studied. Inactivating the penicillin with the enzyme penicillinase, or substituting acetate for penicillin in the test solutions eliminated the effect on EJPs, showing that the penicillin ion was the active agent. Penicillin ions did decrease the frequency of spontaneous miniature EJPs and increase the amplitude or presynaptic spikes recorded extracellularly, suggesting that augmentation of EJPs may have been due to alterations at the presynaptic nerve terminals.