The envenomation of general physiology throughout the last century

Sack discusses the evolution of toxin research in JGP over the last 100 years.

Toxins are the poisonous products of organisms. Toxins serve vital defensive and offensive functions for those that harbor them: stinging scorpions, pesticidal plants, sanguinary snakes, fearless frogs, sliming snails, noxious newts, and smarting spiders. For physiologists, toxins are integral chemical tools that hijack life’s fundamental processes with remarkable molecular specificity. Our understanding of electrophysiological phenomena has been transformed time and time again with the help of some terrifying toxins. For this reason, studies of toxin mechanism are an important and enduring facet of The Journal of General Physiology (JGP). This Milestone in Physiology reflects on toxins studied in JGP over its first 100 years, what they have taught us, and what they have yet to reveal.

Long-term time-lapse live imaging reveals extensive cell migration during annelid regeneration

BACKGROUND

Time-lapse imaging has proven highly valuable for studying development, yielding data of much finer resolution than traditional "still-shot" studies and allowing direct examination of tissue and cell dynamics. A major challenge for time-lapse imaging of animals is keeping specimens immobile yet healthy for extended periods of time. Although this is often feasible for embryos, the difficulty of immobilizing typically motile juvenile and adult stages remains a persistent obstacle to time-lapse imaging of post-embryonic development.

RESULTS

Here we describe a new method for long-duration time-lapse imaging of adults of the small freshwater annelid Pristina leidyi and use this method to investigate its regenerative processes. Specimens are immobilized with tetrodotoxin, resulting in irreversible paralysis yet apparently normal regeneration, and mounted in agarose surrounded by culture water or halocarbon oil, to prevent dehydration but allowing gas exchange. Using this method, worms can be imaged continuously and at high spatial-temporal resolution for up to 5 days, spanning the entire regeneration process. We performed a fine-scale analysis of regeneration growth rate and characterized cell migration dynamics during early regeneration. Our studies reveal the migration of several putative cell types, including one strongly resembling published descriptions of annelid neoblasts, a cell type suggested to be migratory based on "still-shot" studies and long hypothesized to be linked to regenerative success in annelids.

CONCLUSIONS

Combining neurotoxin-based paralysis, live mounting techniques and a starvation-tolerant study system has allowed us to obtain the most extensive high-resolution longitudinal recordings of full anterior and posterior regeneration in an invertebrate, and to detect and characterize several cell types undergoing extensive migration during this process. We expect the tetrodotoxin paralysis and time-lapse imaging methods presented here to be broadly useful in studying other animals and of particular value for studying post-embryonic development.

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.

The molecular mystique of tetrodotoxin

In many respects tetrodotoxin (TTX) is the quintessential natural toxin. It is unequivocally toxic to mammals with LD(50) values for mice in the range of 10 μg/kg (intraperitoneal), 16 μg/kg (subcutaneous), and 332 μg/kg (oral) (Kao, 1966). Its biothreat status is recognized by its listing as a "Select Agent" by the US Department of Health and Human Services which includes regulated agents "determined to have the potential to pose a severe threat to both human and animal health" (http://www.selectagents.gov/). It has a well-defined cellular target (i.e., NaV channels) and pharmacological mode of action (i.e., block of nerve and muscle action potentials), and it is an indispensable chemical tool in neuroscience. It is widely distributed in marine and terrestrial ecosystems where it plays a role in the chemical ecology of predator-prey relationships and drives evolutionary selection of TTX-resistance (Hanifin, 2010; Williams, 2010; Zimmer and Ferrer, 2007). Lastly, TTX has acquired a certain mystique in scientific lore attributable to many fascinating aspects of its natural history and molecular interactions as presented in selected summary below. Additional information may be found in other excellent reviews (Fozzard and Lipkind, 2010; Kao, 1966; Lee and Ruben, 2008; Narahashi, 2001, 2008).

Neuronal control of the cardiac responses to osmotic stress in the gastropod limpet Patella caerulea

Mediterranean limpets Patella caerulea were exposed to different salinity conditions and treated with drugs interfering with neuronal control of heartbeat. Heart rate was monitored using a non-invasive method. Limpets were superfused with control (33 g l(-1)), hyposaline (0 and 10 g l(-1)) or hypersaline (56 and 66 g l(-1)) artificial seawater. Under osmotic stress the limpets showed an initial increase of heart rate, followed by acardia, particularly under hyposalinity. The tachycardia observed after exposure to 56 g l(-1) was abolished in the animals injected with a selective sodium channel blocker, tetrodotoxin (TTX), or with a serotoninergic antagonist, methysergide. Injection of TTX also partly prevented the acardia occurring at 0 g l(-1). The acardia was completely prevented after injection with atropine and benzoquinonium, two selective cholinergic antagonists. These findings indicate that cardiac responses of P. caerulea to variations in external salinity are regulated by an extrinsic neuronal control involving the serotoninergic and the cholinergic systems in the tachycardic and acardic responses, respectively.

Tetrodotoxin prevents copper-induced bradycardia in gastropod limpets

Acute exposure to waterborne copper is followed by a reduction in heart rate in gastropod limpets. In order to understand the mechanism of this effect, exposure to copper (0.25 or 0.5 mg l(-1); for 3 and 6 h) was combined with an injection of tetrodotoxin (TTX, 20 microl, 0.5 or 1 microM), a natural toxin that inhibits the propagation and transmission of impulses in excitable tissues. Experiments were performed on the Mediterranean limpet Patella caerulea, using a non-invasive method for the recording of cardiac activity. TTX did not affect the bradycardic effect of the cholinergic agonist carbachol. However, this toxin significantly antagonized the bradycardia induced by 0.25 and 0.5mg l(-1) of copper exposure and prevented the acardia observed in some limpets exposed to 0.5mg l(-1) of copper for 6h. These results are consistent with the hypothesis that the inhibitory action of copper on limpet cardiac activity involves an extrinsic, cholinergic neuronal control.

Neuronal activity regulates viral replication of herpes simplex virus type 1 in the nervous system

Herpes simplex virus types 1 and 2 (HSV-1, -2) infect and also establish latency in neurons. In the present study, the authors investigated the influence of neuronal activity on the replication of HSV-1. The results showed that the sodium channel blocker tetrodotoxin (TTX) and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) could significantly increase viral replication in primary neuronal cultures, by two- to fourfold. In contrast, KCl reduced viral production by at least 80% in the same cultures. Inhibitors of GABA(A) receptors completely abolished the effects of GABA. Intravitreously injected TTX in a mouse corneal scarification model enhanced the viral titers > 10-fold in both the trigeminal ganglia and the brain. At 2 h post infection, both TTX and GABA significantly up-regulated the levels of transcription for the viral immediate early (IE) genes ICP0, ICP4, and ICP27, as revealed by real time PCR. These results indicate that the neuronal excitation status may dictate the efficiency of HSV-1 viral replication, probably by regulating the levels of viral IE gene expression. These are the first findings connecting neuronal activity to the molecular mechanisms of HSV replication in the nervous system, which may significantly influence our view of herpesvirus infection and latency.

Fifty Years of Progress in Ion Channel Research

Fifty years ago, ion channels were but a reasonable hypothesis. I outline some major steps in transforming this idea from a plausible description of the biological assemblies responsible for controlling passive ion transport across membranes to established fact. Important electrophysiological, biochemical, molecular biological, structural, and theoretical tools are discussed in the context of the transition from studying whole cell preparations, containing many channels, to investigating single channel behavior. Six channel families are exemplified: the model peptide, gramicidin, the acetylcholine receptor, and the sodium, potassium, calcium, and chloride channels. Some questions of current interest are posed.

Spontaneous retinal activity modulates BDNF trafficking in the developing chick visual system

Both neuronal activity and neurotrophin signaling play critical roles in normal CNS development. This study examined whether spontaneous retinal activity (SRA) also governs the axonal transport of endogenous brain-derived neurotrophic factor (BDNF) protein within the developing chick visual system. In previous work, we have found that during the normal period of SRA, retinal BDNF protein levels decrease by about 50% while BDNF mRNA levels remain elevated. Here, we show that the blockade of SRA with tetrodotoxin (TTX), or the blockade of axonal transport with colchicine, both reversed the normal mismatch between retinal BDNF mRNA and protein. The axonal transport of retinal-derived BDNF in segments of the optic nerve as well as tectal-derived BDNF protein transported in segments of the optic tract were both significantly reduced after very brief periods of activity blockade. These results suggest that normal SRA plays a role in regulating the axonal transport of endogenous BDNF protein.

Enhanced metabolic activity coincides with survival and differentiation of cultured rat retinal ganglion cells exposed to glutamate

Neurotransmitters are prominent candidates for trans-cellular signals that influence the development of the CNS. The present study has examined the effect of glutamate on survival, differentiation and metabolic activity of cultured rat retinal ganglion cells at 3 days in vitro. Retinal cultures from neonatal Wistar rats were treated with glutamate for 48 h. The metabolic activity was markedly increased in the retinal ganglion cells exposed to 20 microM glutamate. This was accompanied by an enhanced survival of these neurons. The number of differentiated retinal ganglion cells as determined by microtubule-associated protein-2 labeling was significantly increased following exposure to low but not higher doses of glutamate. The effect of glutamate on the metabolic activity and differentiation was blocked by tetrodotoxin. The results of the present study shows that glutamate has a significant effect on survival, differentiation and metabolic activity. An increase in the metabolic activity indicates an enhancement in the electrical activity. Thus, our results are consistent with the hypothesis that glutamate is critically involved in the regulation of electrical activity in developing rat retinal ganglion cells.

Basal increase in c-Fos-like expression in superior colliculus of Royal College of Surgeons dystrophic rats can be abolished by intraocular injection of tetrodotoxin

In normal rats maintained in the dark, very few cells in the primary visual centers, including the superior colliculus, show Fos-like immunoreactivity. By contrast, in rats presented with flashing lights many Fos-like immunoreactivity cells are observed distributed throughout the visual centers. In the dystrophic Royal College of Surgeons rat, in which there is major loss of photoreceptors over the first 3 months of life, similar numbers of Fos-like immunoreactivity cells are seen on light presentation, but in marked contrast, cell densities in the rats maintained in the dark are many times higher than in non-dystrophic rats maintained under similar conditions. Here we show that this elevated dark response can be abolished by intravitreal injection of the sodium channel blocker tetrodotoxin, indicating that this effect results from changed retinal activity, rather than being centrally generated. We suggest that since Fos-like immunoreactivity is not usually elicited by steady state conditions, the elevated levels in the superior colliculus in these animals reflect the return of waves of activity, first seen in development coursing across the retina, but lost with photoreceptor maturation.

Digger wasp versus cricket: mechanisms underlying the total paralysis caused by the predator's venom

The data presented here describe neurophysiological experiments addressing the question of cellular mechanisms underlying the total paralysis of locomotor behavior in crickets occurring after being stung by females of the digger wasp species Liris niger. The Liris venom effects have been studied by both in vivo recordings from identified neurons of the well-described giant fiber pathway and in vitro recordings from cultured neurons isolated from the terminal ganglion of crickets. The total paralysis of the prey is characterized by a general block of action potential generation as well as by a block of synaptic transmission. Intracellular recordings from neurons in intact ganglia under single electrode voltage-clamp conditions, as well as whole-cell patch-clamp recordings from cultured cricket neurons consistently show that the block of action potential generation by the Liris venom is due to a block of voltage-gated sodium inward currents in neurons of the stung ganglia. Furthermore, our data provide evidence that the Liris venom also blocks calcium currents in identified neurosecretory neurons. On the other hand, outward currents are not affected by the Liris venom. The in vitro recordings suggest that the Liris venom contains active venom components, which, at least for the observed block of inward currents, do not require a metabolic modification. Because venom application does not affect the ACh-induced EPSPs in giant interneurons, the Liris venom does not seem to influence the postsynaptic ACh receptors. The possible pre- and postsynaptic sites of venom action and the functional consequences on synaptic transmission within the giant fiber system are discussed.

Digger wasp versus cricket: immediate actions of the predator's paralytic venom on the CNS of the prey

The females of the palaearctic digger wasp species Liris niger hunt crickets (e.g., Acheta domesticus) as food for their future brood. The wasps paralyze the prey by injecting their venom directly into each of the three thoracic ganglia and the suboesophageal ganglion. This study describes the effects produced by the Liris venom at the level of the intact prey animal (by chronic electromyogram) and at the level of a dissected preparation (by extra- and intracellular records) during the immediate action. Natural or artificial injections of the Liris venom into various ganglia revealed that: (a) The venom injection induced an about 15- to 35-s long tonical discharge of the neurons located in the stung ganglion. This discharge is usually accompanied by convulsions of the prey's limbs. (b) Subsequently, the generation and propagation of action potentials are blocked for up to 30 min (total paralysis). (c) During total paralysis, the venom blocks synaptic transmission. (d) The effects of the venom are restricted to the stung ganglion. Responses of mechanoreceptors in the legs can be recorded from the peripheral nerves of the stung ganglion during the whole period of total paralysis. (e) The neurons almost completely recover after this period. The venom does not selectively affect leg motoneurons, but affects any neuron (e.g., internerneurons or neurosecretory neurons) in any part of the central nervous system of the prey where it was released.

Neurochemical organization of the macaque retina: effect of TTX on levels and gene expression of cytochrome oxidase and nitric oxide synthase and on the immunoreactivity of Na+ K+ ATPase and NMDA receptor subunit I

The present study examined the relationship between an important energy-generating enzyme (cytochrome oxidase; CO), a key energy-consuming enzyme (Na+ K+ ATPase) and neurochemicals associated with excitatory glutamatergic synapses (NMDAR1 and neuronal nitric oxide synthase, nNOS) in the adult macaque retina. Polyclonal antibodies against neuronal nitric oxide synthase and N-methyl-D-aspartate receptor subunit I were generated for immunohistochemical examination and labeled sites not previously reported were found. We have also isolated cDNAs for cytochrome oxidase subunits III (mitochondrial-encoded) and IV (nuclear-encoded), as well as for a fragment of neuronal nitric oxide synthase, from a human cDNA library. The distributions of mRNAs of these genes were analyzed by in situ hybridization. We found that three or more of the markers examined coexisted in a number of sites: (a) In the inner segments of photoreceptors, high energy demand for maintaining the dark current was placed by Na+ K+ ATPase. This was partially met by ATP-generating enzymes such as CO. Neuronal NOS was also present there for the synthesis of NO and the cascading event leading to the generation of cGMP and the gating of channels for visual transduction. (b) Both the outer and inner plexiform layers had detectable amounts of all four markers, although the levels varied among them. This was most likely due to the presence of depolarizing glutamatergic synapses arising from photoreceptors and bipolar cells and such synaptic events were energy-demanding. The involvement of NMDA receptors and nNOS in these synaptic layers is strongly implicated in the present study. (c) All four markers were present in the majority of retinal ganglion cells, with some inherent heterogeneity related to intensity and size. Retinal ganglion cells are known to receive excitatory synapses from glutamatergic bipolar cells and are themselves highly active. The presence of both NMDAR1 and nNOS in these cells were verified in the present study and the energy demands related to these synaptic activities were necessarily high. Thus, active ion transporting functions related to synaptic or non-synaptically induced repolarization from the basis for an interrelationship between the neurochemicals/enzymes studied. Finally, (d) all four markers and the gene expression of CO and nNOS in the macaque retina were regulated by neuronal activity.

Acetylcholine release from rat brain cortical slices evoked by the fraction P4 of the venom of the spider Phoneutria nigriventer has Ca2+ and temperature independent components

Fractionation of the venom of the spider Phoneutria nigriventer revealed that it was a mixture of several neurotoxic peptides. The peptides so far characterized either inhibited or induced neurotransmitter release. These effects were mediated by Ca2+ channels or increasing Na+ permeability through voltage sensitive Na(+)-channels, respectively. The pooled toxic components (fraction P4) showed stimulatory effects on acetylcholine release from brain cortical slices. In addition, a component of the observed effects resembling that of alpha-latrotoxin was identified, which was characterized by the ability to provoke release of acetylcholine (ACh) at low temperature and in a manner independent of extracellular Ca2+ and of voltage sensitive Na(+)-channels.

Lid-suture myopia in tree shrews with retinal ganglion cell blockade

To determine whether central communication of retinal signals is necessary for the development of an experimentally induced myopia, tree shrews were exposed to monocular deprivation (MD) while the action potentials of retinal cells in the deprived eye were blocked with intravitreally injected tetrodotoxin (TTX-MD animals). TTX injections (0.6 microgram in 3 microL) and MD began about 15 days after eye opening, at the start of the susceptible period for the development of lid-suture myopia. Six injections were given, one every second day to produce 12 days of MD and TTX-blockade. Control TTX animals (TTX-open) received TTX in one eye, but not MD, on the same injection schedule and were always found to be behaviorally unresponsive to visual stimuli through the injected eye indicating that TTX blocked central communication of action potentials. Other control animals received intravitreally injected saline in either an open eye (saline-open), or an MD eye (saline-MD). A sham-injected group (sham-inj-MD) received MD and all anesthetic and surgical manipulations except for penetration of the sclera. In all groups, one eye in each animal was an untreated control. Two effects were found. All MD groups, including the TTX-MD animals, developed a significant vitreous chamber elongation in the deprived eye, indicating that an experimental myopia developed despite ganglion cell blockade. Thus, retinal mechanisms in tree shrew can detect the presence of a degraded visual image and produce an experimental myopia that does not depend on the receipt of visual messages by central neural structures. In addition, eyes in which the sclera was punctured had smaller vitreous chamber depths than comparable uninjected eyes, indicating that puncturing the sclera reduced the normal elongation. These data suggest that forces within the eye normally contribute to its expansion and may be resisted by the choroid and/or the sclera.

Membrane transport and disease

Four situations in which membrane transport is altered by disease are discussed: (a) non-specific leaks induced by poreforming agents; (b) glucose transport and cellular stress; (c) Ca2+-ATPase and hypertension; (d) Na+ channels and HSV infection.

Lack of effect of a neurotoxin from the scorpion Buthus martensi Karsch on nerve fibers of this scorpion

A neurotoxin (BmK I) was purified from the venom of the scorpion species Buthus martensi Karsch. Effects of this toxin on the excitability of the abdominal nerve fibers of the same scorpion were examined. The toxin had no effect at all on the action and resting potentials recorded intracellularly even at a concentration as high as 100 microM. A similar result was obtained through optical measurements of the action potential using a potential sensitive dye. Sea anemone toxin II (8 microM) had no effect on nerve excitability either. However, tetrodotoxin (50 nM) reversibly suppressed the action potential and grayanotoxin II (20 microM) induced a sustained depolarization of the nerve membrane which resulted in a reversible suppression of the action potential. BmK I at a concentration of 0.1 microM greatly prolonged the action potential in the crayfish giant axon. We conclude that the Na channel of nerve fibers of this scorpion is totally insensitive to the neurotoxin in this scorpion's venom.

Histochemical localization of cytochrome oxidase activity in the visual system of the tree shrew:normal patterns and the effect of retinal impulse blockage

The tree shrew Tupaia belangeri has three functional pathways (ON-center, OFF-center, and W-like cells) that arise in the retina and proceed through separate LGN laminae to separate cortical targets. To determine whether these pathways have consistent differences in activity, cytochrome oxidase (C.O.) patterns were examined in the retina, LGN, and striate cortex. In six normal tree shrews the outer and inner plexiform layers of the retina were highly reactive for C.O. A pale, vascularized cleft zone separated the a (OFF) and b (ON) inner plexiform sublaminae, which seemed about equally reactive for C.O. In the LGN, laminae 1 and 2 (ON-center cells) and laminae 4 and 5 (mostly OFF-center cells) were highly reactive for C.O. LGN lamina 3 and 6 are part of an W-like afferent pathway. Lamina 3 was distinctly paler than laminae 1, 2, 4, and 5 while lamina 6 was intermediate. In the striate cortex, layer IV was the most reactive layer. Sublayer IVb (predominantly an OFF region) was consistently more reactive than sublayer IVa (predominantly ON). The middle portion, layer IVm, was paler than either IVa or IVb. This paler region includes, but extends above and below, the cell-sparse "cleft" region. Thus, considering all three levels of the retinogeniculostriate pathway, the ON and OFF systems were equally active until they reached the striate cortex, where the OFF system appeared to be more active than the ON. The W-cell laminae in the LGN exhibited the lowest level of activity. The contribution of ganglion cell activity to these patterns was assessed by intravitreal administration of tetrodotoxin (TTX) blockade either monocularly (three animals) or binocularly (two animals). In the TTX-treated retinae, the inner plexiform a and b sublaminae were paler for C.O., although visible, and were still separated by the pale cleft. The ganglion cell layer was very pale in comparison to the normal. In the LGN, monocular TTX blockade reduced the C.O. reactivity in the ON and OFF laminae that received input from the treated eye but had little effect on the W-like cell laminae. The ipsilaterally innervated ON and OFF laminae were more affected than were the contralaterally innervated laminae. Binocular TTX treatment resulted in a decrease of C.O. activity in the binocular segment of the ON and OFF LGN laminae. In the striate cortex, the most marked changes following TTX treatment occurred in layer IV.(ABSTRACT TRUNCATED AT 400 WORDS)

Macromolecular sites for specific neurotoxins and drugs on chemosensitive synapses and electrical excitation in biological membranes

The present review deals with the molecular mechanisms and elementary phenomena underlying the activation of the voltage- and chemo-sensitive membrane macromolecules: sodium- and potassium-ion channels and nicotinic ACh receptors and their associated ion channel. To achieve an understanding of their various kinetics and conformational states, a number of novel alkaloids, BTX, HTXs, gephyrotoxins, and certain psychotomimetic drugs such as phencyclidine, and many other pharmacologically active agents have been used. Biochemical assays and various electrophysiological techniques have been used in a number of biological preparations--e.g., Torpedo membranes, brain synaptosomes, amphibian and mammalian neuromuscular preparations--to describe the action of such agents. The availability of BTX and scorpion toxins together with aconitine and veratridine as activators and TTX and STX as antagonists of the voltage-sensitive sodium channels, made possible the identification and the physiological and pharmacological characterization of these channels. These studies provided the basis for understanding the mechanisms underlying electrical excitability and culminated, more recently, in the purification and reconstitution of sodium channels from rat brain and in the successful cloning of these channels with the elucidation of their primary structure. We now know that the sodium channel has a molecular mass of 316,000 daltons, consists of five subunits, and has multiple sites for various ligands. In contrast to sodium channels, various classes of potassium channels (inward and outward rectifier potassium channels and Ca(2+)-activated potassium channels) have been described. Unlike the sodium channels, there are no known specific activators for potassium channels. However, a number of potassium channel blockers such as 4-aminopyridine, HTX, histamine, and norepinephrine have been identified which complement the varying types of potassium channels in different neurons. One class of potassium channel blockers with profound medical and social implications comprises PCP and its analogues. The blockade of the potassium-induced 86Rb+ efflux from brain cells, the resulting prolongation of muscle and nerve action potentials, and the increase in transmitter release observed with PCP and some analogues are all highly suggestive of a role for the potassium channel in the behavioral effects of these drugs and its potential involvement in schizophrenia. A number of toxic principles of both plant and animal origin played a significant role in the development of our knowledge about the nAChR.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous electrical activity induced by herpes virus infection in rat sensory neuron cultures

Dissociated cultures of rat dorsal root ganglion neurons were infected with a syncytial strain of herpes simplex virus type 1. Over 90% of neurons in infected cultures were spontaneously active and fired action potentials which, on membrane potential hyperpolarization, were replaced by depolarizing events similar to excitatory postsynaptic potentials. Amplitude analysis of these events produced populations described by the sum of several unitary events with Gaussian rather than binomial or Poisson distributions. Such spontaneous activity was blocked by tetrodotoxin but not by low calcium high magnesium solutions containing cadmium. Simultaneous recording from pairs of spontaneously active neurons revealed excitatory connexions between cells. It is suggested that virus-induced fusion of nerve cell processes induces electrical coupling between sensory neurons, and that the resulting electrical network supports spontaneous activity.

Sodium current in single myocardial mouse cells

Morphologically intact single myocardial cells of the adult mouse show a length of 132 +/- 20 microns, a width of 21 +/- 5 microns, and a height of 10 +/- 4 microns (all mean +/- SD) and are brick-like in shape. A one suction pipette method is used for voltage clamp of those single cells. The determined time constant of capacitive current tau = 35 +/- 14 microseconds is very short. Series resistance rs, membrane resistance rm, and membrane capacity Cm are calculated to be 192 +/- 48 k omega, 6.1 +/- 1.1 M omega, and 186 +/- 92 pF (all mean +/- SD), respectively. Assuming the specific unit membrane capacitance of 1 microF/cm2, a total membrane area of 1.86 X 10(-4) cm2 is determined yielding a specific membrane resistance Rm of 1,134 omega cm2. Settling time of voltage clamp is 30 microseconds. TTX-block of sodium current is described by 1:1 binding with a KD value of 1.4 X 10(-6) M. Using a reduced extracellular sodium concentration the maximum Na current is between 25 and 40 nA at voltages between -40 and -30 mV. Currents of between +20 and +30 mV reverse in an outward direction. Inward currents are approximated by a m3h model. The time constant of activation decreases from 0.7 ms at -60 mV to 0.12 ms at +20 mV. The time constant of inactivation falls from 9.1 ms at -60 mV to 0.6 ms at + 20 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The contribution of increases in extracellular potassium to primary afferent depolarization in the bullfrog spinal cord

To assess the extent to which depolarization by accumulated K+ contributes to the generation of primary afferent depolarization (PAD), the isolated bullfrog spinal cord was superfused with K+-rich Ringer solutions and the resultant dorsal root depolarizations were recorded extracellularly. Action potential blockade (with tetrodotoxin) did not reduce the K+-induced depolarization of primary afferents, indicating that the depolarization was generated locally in the region around the afferents. In this respect superfusion with K+-rich solutions adequately models the localized K+ accumulation which occurs physiologically during afferent activity. K+-induced depolarizations were decreased in the presence of 20 mM Mg2+; this effect was due to a direct decrease in the membrane response to K+ and not to blockade of K+-induced transmitter release onto primary afferents. The depolarization caused by a K+ concentration comparable to a maximum estimate of the K+ accumulating around afferent terminals following a single afferent volley was found to account for no more than about one-third of the DRP height. However, higher K+ levels, comparable to those resulting from high frequency afferent stimulation, caused large depolarizations of primary afferents, sometimes greater than the DRP amplitude. Therefore, K+-induced depolarization may contribute more significantly to PAD evoked by high frequency afferent activity.

Ion and transmitter movements during spreading cortical depression

A reaction--diffusion system of equations whose components are the extracellular concentrations of K+, Ca++, Na+, Cl-, an excitatory neurotransmitter and an inhibitory neurotransmitter, is developed in order to model the movements of these substances through various kinds of membrane in brain structures. Expressions are derived from probabilistic arguments for the conductances induced in subsynaptic membrane by transmitter substances at various concentrations, for one-way active transport rates and for an exchange pump rate. These expressions are employed in the reaction terms of the system. The meaning of the many constants is explained and, with appropriate choices for their values, the model predicts subthreshold responses to small enough local elevations of KCl or glutamate and stable propagating SD waves if the local elevations of these chemicals is sufficient. The SD waves consist of elevated K+ and transmitter substances and diminished Ca++, Na+ and Cl-, the velocity of propagation in cortex being about 0.6 mm/min. This is in the experimental range, the K+-amplitude being 17 mM relative to a baseline of 3 mM, as the model developed ignores the effects of action potentials. There is no SD response to either NaCl or GABA. The effects of no K+ and no glutamate diffusion are investigated, both being manifest as a failure in propagation of the stable SD waves. The wave solutions are analysed in terms of phase portraits. The roles of various amino acid uptake and release processes by neurons and glia are discussed, as are the complications with regard to their incorporation in a model for SD. The roles of neurons and glia are analysed and the six basic fluxes of K+ are outlined. It is postulated that under some circumstances, in cortex treated with TTX, there may be practically no transmitter release, but SD may propagate if TTX does not completely abolish gNa for nonsynaptic membrane, a corresponding system of model equations being developed. The data and ideas of K+-based and glutamate-based SD of Van Harreveld are discussed and interpreted in terms of which reaction terms are operative in the K+ equation. An appendix contains the values of the parameters used in the numerical calculations.

Effects of GABA-mimetic agents on the cat spinal cord

Muscimol, ibotenic acid, isoguvacine, 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridin-3-ol (THIP) and gamma-aminobutyric acid (GABA) injected intravenously exerted the following dose-dependent effects on the lumbosacral spinal cord of spinal cats: 1. The excitability of primary afferents and the amplitude of dorsal root reflexes were enhanced; segmental monosynaptic ventral root reflexes and dorsal root potentials as well as the spontaneous gamma-fibre activity and, to a smaller extent, the excitability of motoneurons and polysynaptic reflexes, were depressed. 2. Muscimol was the most potent compound (0.3-1 mg/kg produced significant effects), followed by ibotenic acid (3-10 mg/kg), isoguvacine and THIP (10-30 mg/kg) and GABA (100 mg/kg). 3. Most effects were reversibly antagonized by bicuculline, but not by strychnine, indicating that they were due to a specific interaction with GABA-receptors. 4. These results suggest that the systemic injection of GABA-mimetic agents affects spinal cord activities by actions related to GABA-receptors located on primary afferent endings, intrinsic spinal neurons, and motoneurons.

Toadfish poisoning (tetrodotoxin) in the cat

Two cases of tetrodotoxin (toadfish) poisoning in the cat are reported. The pharmacological and physiological actions of the toxin and treatment of this intoxication are discussed.

Development of spike potentials in skeletal muscle cells differentiated in vitro from chick embryo

The development of spike potential mechanisms during cell differentiation was studied in chick myotubes formed in vitro from trypsin-dissociated myoblasts. The spike potential and its rate of rise were measured in myotubes from 4-14 day old cultures. A depolarizing current pulse was delivered to evoke the spike potential after the steady membrane potential had been adjusted to a standard level of -80 mV in all cases. This gives the greatest maximum rate of rise of the spike potential and eliminates variation due to differences in the resting membrane potential of the myotubes. The size and maximum rate of rise of the spike potential increased significantly during the period examined. The spike potential was blocked by tetrodotoxin in almost all myotubes. These results suggest that during differentiation myotubes develop the ability to generate a spike potential due to an inward current carried by sodium ions.

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 action of a new toxin from gonyaulax tamarensis on nerve membranes

The toxin from the dinoflagellate Gonyaulax tamarensis blocks nervous conduction through a selective inhibition of the mechanism whereby the membrane undergoes an increase in permeability to sodium ions. The effect is exerted only from outside of the nerve membrane. These effects are exactly the same as those exerted by tetrodotoxin or saxitoxin.

Inhibition of the action of bradykinin and acetylcholine on paravascular pain receptors by tetrodotoxin and procaine

Intra-arterial injection of the algogens bradykinin and acetylcholine into the isolated perfused rabbit ear connected to the body by its nerve only elicit a dose-dependent reflex fall in blood pressure. Procaine and tetrodotoxin were used to investigate whether bradykinin and acetylcholine exerted their algesic effect via different types of nerve fibers. Procaine reduced the effect of bradykinin and acetylcholine to a very similar degree. Tetrodotoxin reduced the effect of bradykinin slightly more than that of acetylcholine. It is assumed that on the whole bradykinin and acetylcholine act via the same nerve fibers but bradykinin seems to have some more affinity to fibers with a fewer number of sodium channels than acetylcholine.

Observations on the efficiency of dystrophic muscle in vitro

A study of muscles of the dystrophic mouse has failed to substantiate earlier claims that these muscles were especially resistant to fatigue in vitro or that fast muscles are preferentially damaged. It has been found that the fast muscle selected for previous studies is very often unable to withstand isolation in an organ bath if it is working, and both the difficulty in removing the normal gastrocnemius muscle intact and the need to trim it surgically contribute independently toward its deterioration in vitro. The smaller dystrophic gastrocnemius muscle is less liable to excision damage, is able to satisfy its resting metabolic needs in nutrient solution, and requires no damaging dissection, but is nevertheless unable to recover normally from fatigue. Using EDL and soleus muscles which are small enough to withstand isolation in vitro, no differences are found between fatigue patterns of normal and dystrophic specimens. Responses to rest, KCl, and 2 mM caffeine are also quite similar, and the only distinguishing biomechanical characteristic we have found in dystrophic mouse muscle is a weaker contraction and a longer total twitch time.