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PubMed1983

The active guanidinium group of saxitoxin and neosaxitoxin identified by the effects of pH on their activities on squid axon.

The relative potencies of saxitoxin at pH 7.25 and 8.25 have been determined on the squid giant axon under voltage-clamp conditions or by Vmax of the propagated action potential. Of the two guanidinium groups in saxitoxin, the 7, 8, 9 group has been identified as the biologically active group. The evidence lies in the demonstration of a quantitative agreement between the relative abundance of the protonated, positively charged form of that group at pH's 7.25 and 8.25 (ratio 1.80) with the relative potencies (ratio 1.79) of the toxin. The 1, 2, 3 group is excluded by the lack of agreement between the relative abundance of the protonated form (ratio 1.00) and the relative potencies at these pH's. The 1, 2, 3 group is further excluded by the observation that neosaxitoxin is equally potent at pH 6.50 and 7.25, in spite of a difference of 6-fold in the abundance of a deprotonated hydroxyl group on N-1 which should have influenced the potency.

Animals↗
PubMed1983

Calcium-dependent 4-aminopyridine stimulation of protein phosphorylation in squid optic lobe synaptosomes.

When intact synaptosomes were incubated with [gamma-32P]ATP, maximal protein phosphorylation was attained 2 min after the start of incubation. Protein phosphorylation under basal conditions was dependent on external Ca2+, and the dominant peak of phosphorylation was a 50-kd protein. Incubation of intact synaptosomes in the presence of 3-6 mM 4-aminopyridine (4-AP) caused a markedly enhanced phosphorylation of high molecular weight proteins of 90, 100, 130, and 180 kd, with no increase in the 50 or 38 kd proteins. This effect of 4-AP was dependent on external calcium ions in the incubation medium. The 4-AP effect on the high molecular weight proteins was also found in synaptosomal plasma membranes isolated from the synaptosomes. Tetraethylammonium (TEA) ions did not produce this enhancement of phosphorylation.

4-Aminopyridine↗
PubMed1994

Chemical modification of squid axon K+ channel -SH groups with the organic mercurial compound p-hydroxymercuriphenylsulfonic acid (PHMPS).

In internally dialyzed voltage-clamped squid axons, intracellular or extracellular addition of the sulfhydryl group (-SH) specific reagent p-hydroxymercuriphenylsulfonic acid (PHMPS), causes major modifications in the magnitude and kinetic parameters of the delayed rectifier K+ current. PHMPS produces a dramatic slow-down of the macroscopic current activation kinetics with a simultaneous reduction in its amplitude. In addition, it causes a marked increase in the delay of the macroscopic current at various pre-pulse potentials (Cole-Moore shift). The main effect of PHMPS at the single channel level is a sharp decrease in the open probability (4- to 5-fold). There is, however, a small reduction in single channel conductance (20%). Gating current experiments indicate that PHMPS causes a reduction in the voltage dependence of the activation process as well as a shift of the charge/voltage relationship towards more positive potentials. This, together with an increase in the mean open time, suggests that the open state has been destabilized. The results indicate that the reaction of -SH groups with PHMPS differentially affects the gating process. All the above mentioned effects are partially reversed by either dithiotreitol or beta-mercaptoethanol, -SH group reducing agents.

Animals↗
PubMed1983

Osmotic properties of the squid giant axon and their modifications.

Volume and morphological changes of the squid giant axons in response to hyper- and hypoosmotic media were examined. In hyperosmotic media, which were made by adding sucrose or sodium chloride to the artificial seawater, the axons behaved approximately as ideal osmometers. The fraction of the osmotically inactive volume was less than 0.05. In hypoosmotic media down to half the osmolality of the artificial seawater, intact squid axons did not show significant volume increases. However, following a combined treatment with hyaluronidase and collagenase, the volume of the squid axons increased in these hypoosmotic media. A wrinkled pattern appeared on the surface of the axons while they were in hyperosmotic media containing excess NaCl or KCl. Trypsin treatment prevented appearance of this surface pattern. Furthermore, no such patterns appeared in media which were made hyperosmotic by the addition of sucrose or sodium glutamate.

Animals↗
PubMed1986

Ultrastructure of the squid axon membrane as revealed by freeze-fracture electron microscopy.

The structure of the axolemma of the squid giant axon was studied by freeze-fracture electron microscopy. Three types of preparations were examined: intact axons, axons with their Schwann cell sheaths stripped off prior to freezing, and axons with their Schwann cell sheaths chemically detached but not mechanically removed. Because of a problem of cross-fracturing, the first two types of preparations revealed very few membrane faces of the axolemma. This cross-fracturing problem, however, was eliminated when we used a complementary replication method to fracture the third type of preparation. We found that the E-face of the axon membrane was smooth relative to the P-face, which showed many prominent intramembrane particles (IMP). The diameters of the typical IMP range from 6 to 15 nm. The P-face of the adjacent Schwann cells also showed many large IMP. The sizes and heights of the Schwann-cell IMP, however, appear to be more homogeneous than the P-face axolemma.

Animals↗
PubMed1985

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.

Animals↗
PubMed1991

Phosphorylation of K+ channels in the squid giant axon. A mechanistic analysis.

Protein phosphorylation is an important mechanism in the modulation of voltage-dependent ionic channels. In squid giant axons, the potassium delayed rectifier channel is modulated by an ATP-mediated phosphorylation mechanism, producing important changes in amplitude and kinetics of the outward current. The characteristics and biophysical basis for the phosphorylation effects have been extensively studied in this preparation using macroscopic, single-channel and gating current experiments. Phosphorylation produces a shift in the voltage dependence of all voltage-dependent parameters including open probability, slow inactivation, first latency, and gating charge transferred. The locus of the effect seems to be located in a fast 20 pS channel, with characteristics of delayed rectifier, but at least another channel is phosphorylated under our experimental conditions. These results are interpreted quantitatively with a mechanistic model that explains all the data. In this model the shift in voltage dependence is produced by electrostatic interactions between the transferred phosphate and the voltage sensor of the channel.

Animals↗
PubMed1983

Origin of axoplasmic RNA in the squid giant fiber.

The origin of axoplasmic RNA in the squid giant fiber was investigated after exposure of the giant axon or of the giant fiber lobe to [3H]uridine. The occurrence of a local process of synthesis was indicated by the accumulation of labeled axoplasmic RNA in isolated axons incubated with the radioactive precursor. Similar results were obtained in vivo after injection of [3H]uridine near the stellate nerve at a sizable distance from the ganglion. Exposure of the giant fiber lobe to [3H]uridine under in vivo and in vitro conditions was followed by the appearance of labeled RNA in the axoplasm and in the axonal sheath. While the latter process is attributed to incorporation of precursor by sheath cells, a sizable fraction of the radioactive RNA accumulating in the axoplasmic is likely to originate from neuronal perikarya by a process of axonal transport.

Animals↗
PubMed1995

Characterization of squid enolase mRNA: sequence analysis, tissue distribution, and axonal localization.

Enolase is a glycolytic enzyme whose amino acid sequence is highly conserved across a wide range of animal species. In mammals, enolase is known to be a dimeric protein composed of distinct but closely related subunits: alpha (non-neuronal), beta (muscle-specific), and gamma (neuron-specific). However, little information is available on the primary sequence of enolase in invertebrates. Here we report the isolation of two overlapping cDNA clones and the putative primary structure of the enzyme from the squid (Loligo pealii) nervous system. The composite sequence of those cDNA clones is 1575 bp and contains the entire coding region (1302 bp), as well as 66 and 207 bp of 5' and 3' untranslated sequence, respectively. Cross-species comparison of enolase primary structure reveals that squid enolase shares over 70% sequence identity to vertebrate forms of the enzyme. The greatest degree of sequence similarity was manifest to the alpha isoform of the human homologue. Results of Northern analysis revealed a single 1.6 kb mRNA species, the relative abundance of which differs approximately 10-fold between various tissues. Interestingly, evidence derived from in situ hybridization and polymerase chain reaction experiments indicate that the mRNA encoding enolase is present in the squid giant axon.

Amino Acid Sequence↗
PubMed1988

Axoplasmic RNA species synthesized in the isolated squid giant axon.

Isolated squid stellate nerves and giant fiber lobes were incubated for 8 hr in Millipore filtered sea water containing [3H]uridine. The electrophoretic patterns of radioactive RNA purified from the axoplasm of the giant axon and from the giant fiber lobe (cell bodies of the giant axon) demonstrated the presence of RNA species with mobilities corresponding to tRNA and rRNA. The presence of labeled rRNAs was confirmed by the behavior of the large rRNA component (31S) which, in the squid, readily dissociates into its two constituent moyeties (17S and 20S). Comparable results were obtained with the axonal sheath and the stellate nerve. In all the electrophoretic patterns, additional species of radioactive RNA migrated between the 4S and the 20S markers, i.e. with mobilities corresponding to presumptive mRNAs. Chromatographic analysis of the purified RNAs on oligo(dT)cellulose indicated the presence of labeled poly(A)+ RNA in all tissue samples. Radioactive poly(A)+ RNA represented approximately 1% of the total labeled RNA in the axoplasm, axonal sheath and stellate nerve, but more than 2% in the giant fiber lobe. The labeled poly(A)+ RNAs of the giant fibre lobe showed a prevalence of larger species in comparison to the axonal sheath and stellate nerve. In conclusion, the axoplasmic RNAs synthesized by the isolated squid giant axon appear to include all the major classes of axoplasmic RNAs, that is rRNA, tRNA and mRNA.

Animals↗
PubMed1975

A presynaptic complex in the giant synapse of the squid.

A presynaptic complex consisting of thin lamellae associated with vesicles was found frequently in presynaptic terminals of the squid giant synapse. The lamellae, made of osmiophilic material, had an average length and width of about 10 mum and a thickness of 30 nm; they were of rectangular shape. While most frequent in the axoplasm of the most distal, and largest, terminal of the presynaptic giant axon, the lamellae were found also in smaller terminal branches of this fibre; They have not been observed in the proximal parts of the presynaptic or postsynaptic giant axons. Vesicles the size of synaptic vesicles surrounded the sides of the lamellae. The presynaptic complex resembles the synaptic ribbons in sensory cells.

Animals↗
PubMed1985

AVEC-DIC and electron microscopic analyses of axonally transported particles in cold-blocked squid giant axons.

Anterogradely and retrogradely transported membranous organelles were analysed separately by focally cooling axons (cold-blocking) for 2-4 h. Video-enhanced differential interference contrast light microscopy (AVEC-DIC) and dark field light microscopy showed that particles accumulated in large numbers on both the anterograde and the retrograde sides of the cold-block and that the accumulated particles resumed their transport when the preparation was rewarmed to 18 degrees C. The particles accumulated in files on both sides of the cold-block suggesting that particles move along linear pathways in the axoplasm. Comparisons of the results obtained by AVEC-DIC light microscopy with those obtained by electron microscopy indicate that the AVEC-DIC method is capable of detecting all of the different types of rapidly transported membranous organelles, including the smallest (35-80 nm) vesicles that move anterogradely. Electron microscopic analyses of the transported particles demonstrate that the anterogradely transported organelles are structurally distinct from those that are transported retrogradely. The anterogradely transported particles consisted of normal mitochondria and small (35-80 nm) tubulovesicular profiles. By contrast, the retrogradely transported particles were 150 nm or larger and they often contained complex membranous inclusions. The largest retrogradely transported particles appeared to be degenerating mitochondria. The results are consistent with the hypothesis that the direction of organelle movement is related to the physiological state of the organelle. That is, organelles containing newly synthesized membrane components move primarily anterogradely and organelles that contain transformed and degraded membrane components move retrogradely.

Animals↗
PubMed1977

An anatomical basis for the resistance and capacitance in series with excitable membrane of the squid giant axon.

To correlate periaxonal tissue layer resistance with Schwann cell layer anatomy, cross and longitudinal sections of giant axons of Loligo pealei were examined by transmission electron microscopy. Measurements were made of the width and frequency of mesaxonal clefts entering the Schwann cell layer from the periaxonal space and leaving the cell layer adjacent to the basal lamina. The average mesaxonal cleft width is 10.5 nm. One cm2 of the giant axon surface is enclosed by a single cell layer containing about 690 000 Schwann cells. One cm2 of axon surface has a sheath mesaxonal area of 0.002 cm2 at the periaxonal surface and 0.016 cm2 at the basal lamina, the mesaxons branching frequently as they cross the sheath. The volume of the Schwann cell layer extracellular space was estimated to be roughly 1% of the Schwann cell layer volume. Several models were used to predict the resistance R, across the Schwann cell layer. Assuming the mesaxonal clefts contain seawater, and can be lumped into volume conductors having simple geometries, then (normalized for one cm2 of axon surface) R was estimated to be between 0.4 and 0.9 omega cm2. This compares favourably with electrophysiological estimates of the periaxonal tissue resistance (current clamp value = 0.9 omega cm2 and the voltage clamp value = 1.4 omega cm2) as these electrically measured values include the resistance across the outer connective tissue layer as well as the Schwann cell layer. The value of the Schwann cell membrane capacity was estimated to be approximately 0.7 muF/cm2.

Animals↗
PubMed1993

Ultrastructure and permeability of the Schwann cell layer surrounding the giant axon of the squid.

The ultrastructure of the Schwann cell layer surrounding the giant axon of the squid Alloteuthis subulata is described, and the permeability of extracellular compartments assessed by exposure to electron-dense tracers. Morphometric analysis is used to deduce the number, size and shape of the Schwann cells, and the routes for ion flux across the Schwann cell layer. Axons (mean diameter 233 microns) were surrounded by a 1-2 microns thick layer of Schwann cells which were approximately 1 micron thick, approximately 70 microns long and approximately 23 microns wide. There were around 62,000 Schwann cells per cm2 axon surface. The outer (abaxonal) surface of the Schwann cells was invaginated, with evidence for a covering of fine Schwann cells processes; the inner (adaxonal) surface of the Schwann cells was less folded. The percentage area occupied by mesaxonal cleft openings to the axon and to the basal lamina was 0.02% and 1.09% respectively. A system of tubules, the glial tubular system, occupied 3.9% of the Schwann cell volume, and opened to both axonal and basal lamina surfaces, with more elaborate lattice-like clusters towards the basal side of the cell. Tubule openings accounted for 0.26% of the surface area facing the axon and 0.37% of the area facing the basal lamina (where there was greater clustering of openings). The electron dense tracers horseradish peroxidase, ionic lanthanum and tannic acid filled mesaxon clefts, glial tubular system and periaxonal space. If ion flux occurred via the mesaxonal clefts, a theoretical series resistance (Rsth) of > 20 omega cm2, would be predicted, whereas if it occurred via the tubular system, the figure would be < 2 omega cm2, closer to physiological estimates. The results presented show that the glial tubular system is likely to be the major route for ion flux into and across the Schwann cell layer, and for clearance of K+ from the periaxonal space during periods of axonal stimulation. The implications for K+ homeostasis in the axonal microenvironment are discussed.

Animals↗
PubMed1989

Visualization of polyribosomes in the postsynaptic area of the squid giant synapse by electron spectroscopic imaging.

Giant axons from squids (Loligo vulgaris and Loligo pealii) were studied by conventional electron microscopy and by electron spectroscopic imaging: electrons with a characteristic energy loss due to interaction with phosphorus atoms were used to image phosphorus-rich elements such as ribosomes. Patches of granular material were seen in the proximal region of giant axons, especially in the postsynaptic area of the giant synapse, but not on the presynaptic side. They were rare or absent in the more distal part of the axon. With electron spectroscopic imaging the patches emitted strong phosphorus signals, indistinguishable from signals of ribosomes in neuronal perikarya, confirming their identification as RNA-rich elements. In the part of the axon near the perikarya in the giant fibre lobe, the patches consisted of sacs of rough endoplasmic reticulum. It is suggested that patches of rough endoplasmic reticulum are transported to the synaptic region where ribosomes detach from the cisterns and aggregate into polyribosomes.

Animals↗